摘要:

van der Waals Forces between Objects Covered with a Chromium Layer BY PETER H. G. M. VAN BLOKLAND AND J. THEODOOR G. OVERBEEK* Van’t Hoff Laboratory, Transitorium 3, Padualaan 8, Utrecht, The Netherlands Received 27th January, 1978 Dispersion forces between two metal objects have been measured at distances varying from 132 to 670 nm. Precautions had to be taken to eliminate electrostatic forces arising from differences in Volta potential. The distance between the metallic objects was determined by measuring the capacitance formed by the test objects. To compare experiment and theory van der Waals forces between chromium objects have been calculated numerically on the basis of the Lifshitz theory. It is shown that when the dielectric constant of chromium is described by the free electron gas model and when an absorption band is taken into account excellent agreement with experiment is obtained.In addition the validity of a number of approximate equations for calculating the force is tested. Since the first measurements of van der Waals forces much progress has been made in the determination of these forces. As examples we mention the experiments performed by Israelachvili and Tabor, Hunklinger, Geisselmann and Arnold and Rouweler and O~erbeek.~ In almost all cases the force has been measured between dielectrics such as glass, silica or mica. From a theoretical point of view van der Waals forces between metals are particularly interesting but very few data are available. There are two possible reasons for this. In the first instance the lack of transparency of metals prevents use of the interferometric determination of the separation.Moreover, it is difficult to obtain metal surfaces smooth enough for the measurement of van der Waals forces. Smooth metal surfaces can be obtained by evaporating a metal film on highly polished silica or glass surfaces. In our experiments highly polished fused silica substrates covered with a chromium film have been used. Chromium is well suited for the measurement of van der Waals forces since it is protected from corrosion by a thin oxide skin of 1 to 2 For this reason we believe that the influence of the oxide skin on the force is slight at the distances used in our measurements. In addition, chromium is a hard metal. Hardness is desirable since the vulnerable evaporated films are easily damaged by cleaning and by the removal of dust.One of the first direct measurements of van der Waals forces between (massive) metals was performed by Sparnaay.6 He measured the distance by first determining at which point repulsion was found. Here the distance was taken as zero. Next the plates were separated by a known distance. Finally the gap was narrowed over a part of this distance and the force was measured. Hunklinger used evaporated films and determined the minimum distance in a similar way but measured the change in the distance outside the metallized region of his test objects by interferometry. Recently Derjaguin et aZ.* measured van der Waals forces between crossed platinum fibers. Here too the distance of repulsion was taken as the zero distance.The limitation of these methods is the inaccuracy in the determination of the zero distance, for as a consequence of surface roughness the distance of repulsion is not necessarily the same as the average zero distance. 26372638 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS Van Silfhout and later Rouweler l o p l1 evaporated chromium films onto highly polished test objects. The layer on one of the plates was kept so thin that it remained transparent. The distance could then be measured by means of interferometry. Although Rouweler obtained results which were in accordance with the theory this method has a number of drawbacks. The optical system is rather complex. In the calculation of the distance from the measured intensity of the reflected light, optical data of the evaporated film must be taken into account.These data may depend on the evaporation technique. The oxide skin also becomes a more important part of the very thin film. Furthermore it is not clear whether the transparent layer may still be regarded as a bulk metal. The film is transparent and therefore thinner than the penetration depth of electromagnetic waves in metals. The result of this may be that the substrate makes a contribution to the force as well as the thin metal film. A difference in Volta potential between the test objects (from now on called the Volta potential) will give an electrostatic contribution to the force. Such a difference will exist not only between objects made of different metals and in electric connection but also between two objects made of the same material.Sparraaay eliminated the electrostatic force by seeing to it that no charge could flow between the test objects after he had discharged the surfaces. In this paper a new method for determining the distance will be presented. It is based on the measurement of the capacitance formed by the two conducting test objects. With this method it is possible to measure the distance between thick opaque metal layers, and to compensate for the disturbing Volta potential by applying a potential with the same value but with opposite sign. During the experiments it appeared that in spite of careful compensation for the Volta effect, other electrostatic effects influenced the measurements. Although the origin of these forces is not properly understood it has been possible to eliminate these disturbing effects too.THEORETICAL A first theory of dispersion forces between two conducting half-spaces was given by Casimir.12 For the interaction force per unit area he found 7C2tiC 240D4 F = - where ti = h/2n and h is Planck’s constant, c the speed of light in a vacuum and D the distance between the half-spaces. This relation is restricted to zero temperature and to ideally conducting half-spaces. A general expression for the dispersion forces between two homogeneous isotropic media separated by a gap of vacuum was given by Lifshitz.13 Later Dzyaloshinskii, Lifshitz and Pitaevskii l4 extended the Lifshitz theory to half-spaces (1 and 2) separated by a medium (3). Their expression for the van der Waals force is rather complicated but can be simplified when D k T/c ti < 1.The force between two half-spaces separated by a gap D now isP. H. G . M . V A N BLOKLAND A N D J . T H . G . OVERBEEK 2639 p is an integration variable, and &(it) is the dielectric constant of the media on the imaginary frequency axis. The last quantity is related to the imaginary part of the complex dielectric constant on the real frequency axis by a Kramers-Kronig relation (Landau and Lifshitz).15 D must be large in comparison with molecular dimensions. Calculation of van der Waals forces with eqn (2) requires optical data along the entire frequency range. These data are known only to a limited extent. In practice, however, approximation formulas based on a part of the spectrum can be used.For large distances (retarded forces), but not so large that the condition DkTI ck < 1 no longer holds, and for two identical objects (cl = EJ separated by a vacuum eqn (2) can be simplified to s = (&&- 1 +p2)4 (4) where Est is the static dielectric constant. With the approximation made to obtain eqn (2) the temperature has dropped out of the formula. At the distances and temperatures at which the measurements of the force, as described in this publication, were performed the omission of the influence of temperature is justified. At larger distances ( D > 1 pm) and/or at high tempera- tures this effect must be taken into consideration. For ideal metals E,, = GO. When this value is substituted into eqn (3) and (4), the limit of the Lifshitz equation for retarded forces reduces to the Casimir equation [eqn (l)] x3dp n2hc =- kc O3 I : = - - - - 16n2D4 Jo dx Sl p2(eX- 1) 240D4' Real metals however are not perfect conductors and Lifshitz l 3 has given a method for the derivation of a correction term for this non-ideal behaviour.At infrared frequencies &(it) for metals is well approximated by where e and m are charge and mass of the electron respectively and N is the number of free electrons per unit volume. 0, is known as the plasma frequency. Eqn (6) shows how the low frequency limit cst + co is approached on decreasing the frequency. When this eqn is used in eqn (2), the expression for the force at large distances becomes F = 2400 = 4 { 1 - 1 . 5 1 q 9 + + eD N .. .}. (In the original publication by Lifshitz the correction term has a different numerical value. This value has been corrected by Hargreaves).16 Eqn (6) implies that at frequencies higher than cop the metal becomes more and more transparent, so at distances shorter than the plasma wavelength A, the force between metals changes to non-retarded. The second term in eqn (8) can be seen as the first term of an expansion, describing the transition to the non-retarded force. The theory of Casimir will only give retarded forces and is therefore limited to the long distance region.2640 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS Hargreaves l6 has given a different and very simple method for correcting the result of Casimir for non-ideal conductors. Electromagnetic waves will penetrate a very short distance into a real metal.For wavelengths below 10 pm the penetration depth is given in a first approximation by In the calculation of the force Hargreaves now suggests adding one or two times the penetration depth to the real distance. When the penetration depth is added rz times (n between 1 and 2) to the distance the force will be F = F O ( D ) I D+nd N- F o ( I - Y ) where Fo is the force according to Casimir. Comparison of eqn (8) with the right hand side of eqn (10) shows that the two equations are equal at long distances when a correction of 1.34 times the penetration depth is used in the method of Hargreaves. We now return to the long distance limit of Lifshitz’ formula for non-ideal metals. If eqn (8) is used at too short a distance, the approximations introduced in obtaining this equation no longer hold and the force calculated from it will even change sign below a certain distance.For chromium where the number of free electrons per cm3 is 1.15 x the force would change sign at D = 0.27 pm. In the measurement of the van der Waals force described in this paper the force is determined at distances between 0.13 and 0.67 pm. The results of the experiments should, therefore, not be compared with eqn (8) for the short distances. Using the complete Hargreaves expression [first part of eqn (lo)] with n = 1.34 may be seen as a better approximation than eqn (9, since for large D the two equations lead to the same value for the force, but for shorter distances Hargreaves’ expression continues to increase with decreasing D.The method of Hargreaves is, of course, also limited to distances much larger than the penetration depth. For chromium this depth is already about 50nm. A comparison between theory and our experiments therefore requires the calculation of the force with the complete Lifshitz equation. Eqn (6), which has been used as the basis of the calculations performed above, is an approximation which describes the dielectric constant of many metals with reasonable accuracy in the infrared regions. At higher frequencies, where the metals become transparent, absorption bands will appear. Chromium has a band of this kind at about 600 nm and for a worthwhile comparison between the calculated and the measured force this absorption band must be taken into account. A more precise description of the dielectric constant of metals is given by (Kruppl’ and Hummel,l*> where go is the specific conductivity of the metal. The second term of eqn (1 1) is equal to the second term of eqn (4) but is now extended with a term that takes the damping of the free electrons into consideration. The right hand term stands for the absorption bands, where wi is the frequency of the ith maximum, mi”, is a measure of the intensity of the band, and gi is the halfwidth of the band (Krupp).17P.H. G . M . VAN BLOKLAND A N D J . TH. G . OVERBEEK 264 1 All terms of eqn (1 1) contribute to the measured optical data. Often the contribu- tion of the second term is still present at the frequencies where the absorption bands appear and it is difficult to obtain all terms separately.Fortunately Lenham l9 has calculated on the basis of absorption data the imaginary part of the interband (bands here in the sense of energy levels) absorptions [the third term in eqn (1 I)] separated from the intraband contributions (the second term) for chromium. -8 - 9 - 0 FIG. 1.-Imaginary part of the dielectric constant plotted against the angular frequency. Tne constants of the ith term of the sum in eqn (11) can be determined from the ith absorption band. - M I I I I I ; \ L I I I -9 -8 -7 -6 1% (O/m) FIG. 2.-van der Waals force between a flat plate and a sphere (radius of curvature is 1.00 m) for chromium. (I) Complete Lifshitz equation with eqn (6) for &(it) (no damping and absorption bands), (11) long distance limit of Casimir (slope - 3), 011) Lifshitz’ retarded limit with the correction term as in eqn (8), (IV) nonretarded limit of curve (I) (slope - 2), (V) Hargreaves’ result with a correction of 1.34 times the penetration depth [eqn (15)], (VI) complete Lifshitz equation with the absorption band also taken into account [eqn (11) with oj, wZip and gj as given in the text].I 1 i I I f These bars indicate the distance range in which the measurements have been performed.2642 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS Fig. 1 shows how the constants mi, mi”, and gr can be determined from absorption bands. It appears to be sufficient to take into account only one absorption band for chromium with the values oi = 3.0 x s - ~ and gi = 3.8 x lo1’ s-l.In fact this band consists of two overlapping bands. s-l, mi?, = 4.1 x PLANE-SPHERE CONFIGURATION Since in our experiments the force is determined between a sphere and a flat plate, the theory has to be adapted to this configuration. According to Derjaguin 2o the force between a flat plate and a sphere, the shortest distance between them being D, is where U(D) is the interaction energy per unit area between two flat plates at distance D. Applying this transformation to Casimir’s eqn (1) and to Hargreaves’ eqn (10) leads to F(D) = -2nRU(D) (1 3) n3RAc N- n3Rh( -- 3 3 . FHargr’D) = 360(D + F Z ~ ) ~ - 3600 In fig. 2 the results of the different approximate equations for the plane-sphere configuration are compared with the results of the complete Lifshitz equation for this geometry, the latter calculated by numerical integration of eqn (2).The retarded and non-retarded limits are also given. Fig. 2 shows that the approximations are acceptable for distances >400 nm (Hargreaves) or 600 nm [Lifshitz, eqn (8) adapted for the plane-sphere geometry]. MEASUREMENT OF THE DISTANCE (i) BY DETERMINATION OF THE CAPACITANCE In principle the measurement of the distance is very simple. The two conducting test objects form a capacitor, and if the relation between capacitance and distance is known for the geometry used and if the capacitance has been measured, the distance can be determined. In eqn (16) the relation between capacitance and distance is given for a capacitor formed by a flat plate and a part of a sphere. The capacitance is calculated by integration over rings that are concentric with the line of closest approach.c = 2re,R(( 1 +:) In ( R(l -COS 4) -2ne,R ln-+--- R 4 2 > D a n d + < 0 . 1 ( % 022) where (see fig. 3) R is the radius of curvature of the spherical test object, (b the angle subtended by half the diameter of the test object, D the distance at the place of closest approach and E, the permittivity of vacuum. With R = 1 m, D = 100nm and d) = 0.01 (values which agree with the experiments) eqn (17) becomes The dependence of the capacitance on the distance is approximately logarithmic and rather sensitive to the precise value of 4 (through the term In 42) and, therefore, to the tilt between the test objects, as shown in fig. 4. Moreover, the capacitance of the test objects is at most of the same order as the capacitance of the cables and stray C cz 27reoR (6.2+2 x -5 x (18)P .H . G . M . VAN BLOKLAND AND J . TH. G . OVERBEEK 2643 capacitances. These effects can be taken into account only by a calibration in which the capacitance is determined for at least one accurately known value of the distance. Eqn (16), therefore, is only suited for a relative but not for an absolute determination of the distance. FIG. 3.-Capacitor formed by a flafplate and a part of a sphere. FIG. 4.-Tilt between the test objects will influence the capacitance between them. (ii) BY DETERMINATION OF THE ELECTROSTATIC ATTRACTION When a voltage is established between the test objects the electrostatic attraction is more sensitive to the distance than the capacitance.This can easily be seen by considering a flat plate capacitor. The capacitance is related to the distance by an inverse first power law. The electrostatic attraction is related to the distance by an inverse second power law. In the plane-sphere configuration this means that the electrostatic force comes chiefly from a small area around the place of closest approach. A larger area will give the main contribution to the capacitance. The relation between force and distance for the plane-sphere configuration is R (R+D)(l-cos +) D - In 6- R(1-cos +)+D F = n&,V2 (19)2644 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS where Y is the applied voltage. The other symbols have the same meaning as in eqn (16). The force [eqn (20)] is nearly inversely proportional to the distance D and for R N 1 m, D -” 1100 nm and 4 N 0.01, rather insensitive to the precise value of 4, so only the first term of eqn (20) can be used.But the electrostatic attraction cannot be used for the determination of the distance during a measurement of the van der Waals attraction because then the electrostatic and the van der Waals contributions could not be separated. Therefore the electrostatic attraction was used to calibrate the capacitance method and then the capacitance method was used in the form where Kl and K2 were derived from a calibration curve. In order to obtain this curve a d.c. voltage of 200 mV was established between the test objects at a number of distances between 0.3 and 1 pm and for each of the distances capacitance and attraction force were measured.D was calculated with eqn (20) and C and log D were found to have a linear relation from which Kl and K2 were determined. The capacitance between the test objects is measured with an a.c. bridge, which makes it necessary to apply an a.c. voltage between the objects. This voltage must be so low (1 mV r.m.s.) that the resulting electrostatic force is negligible compared with the van der Waals force at distances at which the van der Waals force is measured. On the other hand the determination of the calibration curve must not be disturbed by the van der Waals force. The d.c. voltage used in the determination of this curve has to be chosen so high (200 mV) that the van der Waals force is negligible compared with the electrostatic force. During the determination of the calibration curve the tilt between the test objects should not be changed.The measurement of the van der Waals force must be performed with the same tilt as in the calibration curve. C=K,-K,logD (22) VOLTA POTENTIAL A difference in voltage between the test objects during the measurement of the van der Waals force leads to forces which are too high. Sparnaay indicated that such a difference may exist even between two identical metals. In order to examine how far our experiments were influenced by a difference in potential an adjustable voltage was applied between the test objects and the force was measured. According to eqn (20) the force must have a parabolic dependence on the voltage. If no Volta potential existed previously the force would be at its minimum at V = 0.In the case of a non-zero Volta potential the force has its smallest value when the Volta potential is compensated by the adjustable voltage. The force at the minimum is the van der Waals force. In fig. 5 a plot is given of the force as a function of the adjustable voltage at constant distance. The figure shows that the minimum force does not coincide with V = 0. The Volta potential between the test objects also influences the calibration curve. The effect can be demonstrated by changing the polarity of the calibration voltage. Instead of, for instance, increasing the calibration force, the Volta potential now decreases its. Since the force is proportional to the voltage squared the effect is rather strong. In fig.6 a plot is given of the electrostatic calibration force at constant distance and at constant calibration voltage as a function of the adjustable voltage. The two lines have been obtained with different polarity of the applied calibration voltage. In fact both lines are parts of the parabola given in fig. 5 if that is extended to much higher voltages. The point of intersection in fig. 6 represents the counter voltage needed to compensate the Volta potential between the test objects. The values of the Volta potential found by the two methods coincided very well.P. H. G. M. VAN BLOKLAND A N D J . TH. G . OVERBEEK 2645 In the case of a plane-sphere corhguration the electrostatic force, due to a Volta potential of 25 mV, equals the van der Waals force at a distance of 400 nm.Our measurements are performed in this distance range, so the electrostatic force, unless force x 107/N I - J I I I I I J -60 -40 -20 0 20 LO 60 80 counter vol tage /mV FIG. 5.-Force plotted against counter voltage. The force at the minimum is the van der Waals force. I I I I I f + 210 + 215 + 220 + 225 + 230 - 190 - 185 - 180 - 175 - 170 countervoltage ( f 200mV) FIG. 6.-Influence of the polarity of the calibration voItage on the electrostatic force as a function of the counter voltage. The total potential difference between the objects is the sum of the calibration voltage (+200 mV or -200 mV) and the adjustable voltage (from 0 to + 30 mv). The two lines intersect at +219 and - 181 mV. The Volta potential is therefore - 19 mV and can be compen- sated by a counter voltage of + 19 mV.2646 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS compensated, will interfere considerably with the measurement of the van der Waals force.In all measurements of the van der Waals force and also during the determina- tion of the calibration curve the Volta potential between the test objects was com- pensated by a counter potential. In fig. 7 an example of a calibration curve is given. Each measured point is the average of two measurements, each having different polarity. The Volta potential must have the same value all over the chromium layers. Otherwise, at the minimum of the force found by varying the countervoltage, an electrostatic force may still be present. This force would be proportional to the inverse distance squared.There is no indication, however, that the measurements were interfered with in this way. I , \ '+ - 7.0 - 6.5 - 6.0 log P/m> FIG. 7.-Calibration curve of the distance plotted against the capacitance between the chromium layers (including stray capacitances). The distances at the crosses have been calculated from the electrostatic forces under an applied potential difference. Between two identical metals in direct electric connection one would expect no difference in Volta potential. However, such a difference was found. At least two possible explanations can be given. Since the chromium layers are in electrical connection both layers must have the same Galvani potential. But the force is measured across a gap outside the metals. If the two layers do not have the same surface potential, for instance as a result of different oxide skins, a differenc in Volta potential is obtained across the gap.Another explanation is that when two metals, of which at least one has an oxide skin, are pressed together in order to make electric contact, the current may be transported by ions in the oxide layer. As long as very small currents are involved the oxide layer is kept intact and the layer will behave as a solid electrolyte. If, at both sides of the oxide layer, different metals are present a galvanic element is obtained. A number of these contacts had to be made to connect the chromium layers upon the test objects with the bridge. In one or more of such contacts a Galvani potential difference may appear.P. H.G . M . VAN BLOKLAND AND J . TH. G . OVERBEEK 2647 EXPERIMENTAL CHROMIUM LAYERS The chromium layers were evaporated from a tungsten boat under high vacuum conditions (better than Torr) in an Edwards 306 coater. During the deposition of the layers the thickness was determined by a film-thickness monitor (Edwards, model F.T.M. 2). After deposition the thickness of the layers was determined by means of interferometry. In the second method the thickness of the layer itself is measured directly. The first method is based on the frequency shift of an oscillating quartz crystal. This shift is due to an increase of the mass caused by the evaporated film. In fact the first method gives the mass of the evaporated film. Combination of the two methods gives the density of the film.Wolter 21 has found that when the partial oxygen pressure during evaporation is too high the film is a mixture of chromium and chromium oxide. Such a film has a density lying between those of bulk chromium and bulk chromium oxide. Our films had the density of bulk chromium and therefore will consist of chromium. THE BRIDGE The capacitance between the test objects was measured with a Schering bridge (Harris).22 Because of the low a.c. voltage a phase sensitive detector was used. The bridge tension was supplied by the internally generated voltage of the lock-in amplifier (Princeton Applied Research Corporation, type HR 8). In fig. 8 a schematic outline of the bridge circuit is given. TO DETECTOR FIG. 8.-Bridge circuit used in the determination of the distance.The circuit indicated by A serves to compensate the difference in Volta potential between the chromium layers. The circuit indicated by B serves to apply the calibration voltage ; its polarity can be reversed (not drawn in the figure). The variable 50 pF capacitor is used to compensate for the non-ideality of the bridge components and for stray admittances. MEASUREMENT OF THE FORCE The force was measured by attaching one of the two plates to a sensitive balance. The measurements were performed under high vacuum conditions. For this purpose the same 1-842648 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS method as used by van Silfhout and Rouweler 3 p lo was used. However, some important improvements have been introduced to permit the measurement of force and distance in an easier and more accurate way.These improvements include a better isolation against vibrations, better protection from dust and a better adjustment of the distance. A detailed description of the apparatus and the improvements has been given by van B l ~ k l a n d . ~ ~ Static charges on test objects made of glass or silica can give an attractive force which exceeds the van der Waals force by many times. During the measurements between these dielectrics careful precautions have to be taken to remove static charges. This is done by injecting some drops of water into the vacuum chamber. At a pressure of a few Torr the water vapour forms a film on the glass or silica surface and the static charge can flow off. After 15 min the water is removed by evacuation. With metallic and, therefore, conducting test objects the removal of static charge was necessary too.Otherwise especially at relatively large distances too high forces with a great spread between different series of measurements were found. After applying the water vapour method reproducible series in accordance with theory could be obtained. A similar result was obtained when the test objects were kept in the vacuum chamber for one week. However, after renewed cleaning the disturbing attraction was again present. The water vapour method shows that the disturbance is independent of the potential difference between the test objects. The Volta potential is not influenced by this method. No explanation for this disturbing effect can be given. RESULTS In fig.9 a graph is given of the logarithm of the force as a function of logarithm of the distance. The points in fig. 9 are derived from a number of series of measure- ments. log (Dlm) FIG. 9.-van der Waals force between a flat plate and a sphere both covered with a chromium layer of 100 nm. The radius of curvature is 1.00 m. The broken line represents the force according to the retarded limit found by Casimir. The drawn curve Lifshitz (I) gives the force when the complete Lifshitz equation is used in combination with eqn (6) and N = 1.15 x loz2 ~ r n - ~ . The drawn curve Lifshitz (11) gives the force when the absorption band is taken into account. The dotted curve gives the force when a correction of 1.34 times the penetration depth is applied to the broken curve (Har greaves’ correct ion).P.EI. G. 31. VAN BLOKLAND AND J . TH. G . OVERBEEK 2649 A series consisted of I0 to 20 measured points and for each series a calibration curve between distance and capacitance was made. A few series were measured after the water vapour method had been applied. Other series were measured after the test objects had been kept in the vacuum chamber for more than one week. Both kinds of series gave the same results within the accuracy of the measurements. The radius of curvature of the spherical test object was 1.00 m. The thickness of the chromium layer' on both test objects was 100+5'nin. The broken curve in fig. 9 represents the force based on the relation found by Casiniir [eqn (l)]. The drawn curve Lifshitz I gives the force which is calcuhted when the complete Lifshitz equation is used but the abscrption band is not taken into account (curve I in fig.2). The drawn curve Ljfshitz 11 gives the force when the absorption band of chromium is taken into account (curve VI in fig. 2). log (D/ni) FIG. 10.-van der Wads force between a flat plate and a sphere both covered with a chromium layer of 50nm. The radius of curvatwc is 1.00 111. The broken line gives the force according to the retarded limit of Casimir. Thc drawn curse gives the force when the complete Lifshitz equation has been used and the absorption band is taken into account. In fig. 10 a graph is given of the force as a function of the distance between objects both of which are covered with a chromium lajm of 50+5 nm. The radius of curvature of the sphetical'test object was 1 .OO rn.With these test objects repulsion was found at still greater distalices (260nni). The forces found with the two layer thicknesses do not show a systematic difference. DISCUSSION The description of the dielectric constant of metals given by eqii (11) and (12) is still an approximation of the exact dielectric constant. The value of o0 is in reality a function of the frequency. The influence of the damping of the free electrons represented by this term is most important at greater distances. At low frequencies which contribute most at greater distances the direct current value of Q can be taken (ao). Xn our measurements the influence of oo on the force was only a few per cent at the greatest measured distances. So the d.c.value of o0 (7.0 x 10l6 s-' N 7.75 x 104 Q-1 cm-1):: was used throughout the calculations. * In e.s.u. and SI units respectively2650 VAN DER WAALS FORCES BETWEEN CHROMIUM LAYERS The absorption band was based on empirical data. But the optical data found by experiment show some variations which may be caused by the method of measure- ment (reflection, ellipsometry) and/or by the surface treatment of the metal (hand- polished, electro-polished, evaporated film). It is also uncertain how well the optical data used in the calculations hold for our evaporated films. However the calculated force is not very sensitive to the precise shape of the absorption band and the values of the dielectric constant as calculated by us can be assumed to be a good basis for the calculation of the van der Wads force, particularly because at the smallest distances at which the van der Waals forces have been measured the increase of the force by the absorption band is only 40 %.According to eqn (9), the penetration depth for chromium is about 50 nm. This value is of the same order as the thickness of the evaporated chromium layers and might have been expected to have an influence on the force, especially for the 50 nm layer. However, the 50 nm layer was almost completely opaque to visible light, indicating that the wavelengths by which the force is mainly determined did not penetrate through the 50 nm layer. Furthermore, within the accuracy of the experi- ments no indications were found for a decrease in the force in comparison with the 100 nm layer.It will be worth while to examine how thin the chromium layer must be made before an effect of the thickness on the force can be found. So far we have not mentioned the influence of surface roughness on the force. However, van Bree et aZ.24 pointed out that surface roughness can easily increase the force by 10 to 50 %. Such an effect was indeed found by van Blokland 23* 2 s in the measurements of van der Waals forces between test objects of fused silica at short distances. Although with test objects covered with a metal film repulsion was found at fairly large distances (in general 140 nm or even more), electron micrographs hardly showed any obstacles > 30 nm. According to the correction for surface roughness for a retarded force in the plane-sphere geometry, given by van Bree et al., a root mean square surface roughness of 14 nm, which, on the basis of electron micrographs, is a reasonable value, will increase the force by 10 % at 150 nm.At greater distances the correction decreases rapidly. So at the distance range at which the van der Waals force has been measured the influence of surface roughness may be neglected. It turned out that the capacitive method for measuring the distance between the test objects, when calibrated with the electrostatic attraction, is very well suited and has a high accuracy and reproducibility. The relatively high spread in the measure- ments that can be seen in fig. 9 and 10 is mainly determined by the inaccuracy in the measurement of the force. In our experiments we worked with evaporated films but the same procedure can be used for measuring the distance between test objects made of massive metals provided surfaces can be obtained which are smooth enough.The final and most important conclusion is that at distances between 132 and 670 nm the measured force and the calculated force are in excellent agreement when the absorption band of chromium is taken into account in the calculation of the force. Surface roughness exists and it prevents measurements at small separations but has hardly any influence on the measured forces. This work was part of the research programme of the " Stichting voor Funda- menteel Onderzoek der Materie " (F.O.M.) with financial support from the " Neder- landse Organisatie voor Zuiver-Wetenschappelijk Onderzoek " (Z. W.O.). J. N. Israelachvili and D. Tabor, Proc. Roy. SOC. A, 1972, 331, 19. G. C. J. Rouweler and J. Th. G. Overbeek, Trans. Furuduy SOC., 1971, 67, 2117. * S. Hunklinger, H. Geisselmann and W. Arnold, Rev. Sci. lnstr., 1972, 43, 584.P. H . G . M. VAN BLOKLAND AND J . TH. G. OVERBEEK 265 1 S. Storp and R. Holm, Surface Sci., 1977, 68, 10. E. I. Alessandrini and V. Brusic, J. Vac. Sci. Techno/., 1972, 9, 83. M. J. Sparnaay, Physica, 1958, 24, 751. S. Hunklinger, Bestirnrnung der van der Waals-Krajie z wischen makroskopischen Korpern mit einer nerien hocheinpfndlichen Methode, Thesis (Miinchen, 1969). B. V. Derjaguin, Y. I. Rabinovich and N. V. Churaev, Nature, 1977, 265, 520. A. Van Silfhout, Proc. Korz. ned. Akad. Wetenschap. Ser. B, 1966, 69, 501. lo G. C. J. Rouweler, Measurement of van der Waals forces, Thesis (Utrecht, 1972). G. C. J. Rouweler, Chem. Phys. Letters, 1971,8,275. l2 H. B. G. Casimir, Proc. Kon. ned. Akad. Wetenschap. Ser. B, 1948,51, 793. l 3 E. M. Lifshitz, Soviet Phys. JETP, 1956,2,73. l4 I . E. Dzyaloshinskii, E. M. Lifshitz and L. P. Pitaevskii, Adt. Phys., 1961, 10, 165. L. D. Landau and E. M. Lifshitz, Lehrbuch der theoretischen Physik VIII Elektrodynamik der Kontinua (Akademie Verlag, Berlin, 3 Auflage, 1974). l6 C. M. Hargreaves, Proc. Kon. ned. Akad. Wetenschap. Ser. By 1965, 68, 231. l7 H. Krupp, Adv. Colloid Interface Sci., 1967, 1, 111. * R. E. Hummel, Optische Eigenschafren uon Metallen und Legierungen (Springer-Verlag, Berlin, Heidelberg, New York, 1971). l9 A. P. Lenham, J. Opt. SOC. Amer., 1967, 57, 473. 2o B. V. Derjaguin, Kolloid Z., 1934, 69, 155. 21 A. R. Wolter, J. Appl. Phys., 1965, 36, 2377. 22 F. K. Harris, Electrical Measurements (John Wiley, New York, 1952), chap. 15, p. 713. 23 P. H. G. M. van Blokland, Direct measurement of van der Wnalsforces, Thesis (Utrecht, 1977). 24 J. L. M. J. Van Bree, J. A. Poulis, B. J. Verhaar and K. Schram, Physica, 1974, 78, 187. 2 5 P. H. G. M. Van Elokland and J. Th. G. Overbeek, J. Colloid Interface Sci., 1978, in press.

ISSN:0300-9599    

DOI:10.1039/F19787402637

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Reactions of n-Butenes on Palladium Films Evidence for n-Allylic Species BY MARC J. LEDOUX* AND FRANCOIS G. GAULT Laboratoire de Catalyse, UniversitC Louis Pasteur, Strasbourg, France AND ALAIN BOUCHY AND GEORGES ROUSSY Laboratoire de Chimie Thiorique, Universite Nancy I, Villers-les-Nancy, France Received 27th February, 1978 The contact reactions of all three n-butenes have been investigated on palladium films in the presence of perdeuteropropene or deuterium, using jointly mass spectrometry and microwave spectroscopy. Conclusive. evidence has been found for an allylic mechanism in exchange and double-bond migration. Part of the exchanged molecules rcsult from dissociativc vinylic-type adsorption and isomerization also takes place according to an associative Horiuti-Polanyi mechanism or by direct processes without deuterium incorporation.A proposal was made which correlates the various reaction mechanisms with various types of surface' metal atoms, each having a different coordination number. Since the introduction of n-allylic species into heterogeneous catalysis 9 there has been controversy about their existence and significance for hydrocarbon reactions on metal surfaces. The exchange distribution patterns of a number of cyclic hydro- carbons can be accounted for by this intermediate, but only by assuming that n-allylic species can convert into n-olefinic species in two different ways, reacting either with adsorbed hydrogen atoms or with molecular hydrogen coming from the gas p h a ~ e . l ' ~ While it was soon recognized that n-allylic complexes are very coinmon in organometallic cheniistry and convert easily into n-olefinic complexes by insertion into a metal-hydrogen bond, the reaction of molecular hydrogen with n-allylic species to form n-bonded olefins has never been observed in homogeneous catalysis.On the other hand, an alternative mechanism, the roll-over mechanism, based on adsorbed species doubly attached to the metal by three-centre bonds, has been this mechanism could account for all the observed exchange distribution patterns, as well as the n-allylic mechanism. Later on, a key compound that allowed one to distinghish between the roll-over mechanism and a n-allylic mechanism with transoid stddition of molecular hydrogen was investigated for exchange. The results unambiguously favoured the former mechanism? While the question of the involvement of n-allylic species in the exchange of cycloalkaiies remained unsettled, some illdirect proofs were provided that z-allylic species might phy an important role in exchange and isoinerization of o i e i h 011 pa1ladiuix8* in the presence of 2652 Moreover it was shown that on iron,loiM .J . LEDOUX, F . G . GAULT, A . BOUCHY AND G . ROUSSY 2653 perdeuteropropene, most of the reactions of butenes could not be accounted for by the classical Horiuti-Polanyi mechanism ; no fewer than five reaction mechanisms could be identified, among which are a vinylic-type dissociative mechanism for exchange (the Farkas mechanism), direct cis-trans isomerization, and double-bond migration without deuterium incorporation. Because olefin reactions on metals are so complex, it is illusory to characterize the various mechanisms in detail by simply looking at the deuterium distributions as obtained by mass spectrometer measurements.Location of the deuterium in the reaction products is required and that is better achieved by microwave spectroscopy. This technique was first introduced into catalytic study by Hirota, studying the exchange of propene on various metal catalysts.14* l 5 However, interpretation of the results might be difficult in that case, since no distinction can be made between the exchanged and the isomerized molecules. For this reason, we decided to reinvestigate the reaction mechanisms of olefins on metals, by combining mass spectrometry and microwave spectroscopy and choosing butenes as model The present paper is devoted to reactions on palladium to decide whether or not n-allylic species may be formed on this metal.On the other hand, an attempt has recently been made to correlate the various reactions of olefins with various types of sites on a metal surface.l* To each reaction mechanism was associated a superficial metal atom with specific configuration and coordination number. If such a theory were correct, one should observe, when modifying the roughness of the surface, an effect on the relative contribution of the various reaction paths. A second aim of this work, then, was to determine the effect on the reaction mechanisms of variation in the conditions of film preparation. EXPERIMENTAL CATALYSTS Palladium films of different weights were prepared by evaporating, under a vacuum of 10-6-10-7 Torr, onto a glass vessel maintained at 0°C (films of type I) or 470°C (films of type 11), a specpure filament of palladium (diam.0.2 m) tightly wound on a 15 cm long wire of tungsten (diam 0.3 mm). Before film preparation, the reaction vessel was outgassed at 500°C for at least three hours and the filament heated for 30 min at the limit of evaporation, to eliminate dissolved and adsorbed gases. MATERIALS The three n-butenes, Fluka puriss, were used without further purification. We checked their purity (>99.995 %) by gas chromatography before each experiment. Deuterium, from Air Liquide Co., was purified by diffusion through a palladium thimble. Its isotopic purity was 99.4 %.Perdeuteropropene, from Merck, Sharpe and Dohme, was purified by semipreparative gas chromatography. Mass spectrometer measurements revealed an isotopic purity of 99.1 %. APPARATUS AND PROCEDURE We used a conventional static Pyrex apparatus like that previously described byKembal1,l but all the valves, taps and joints were of Teflon or Viton without any grease and the dead volumes were as small as possible to avoid loss of products. The mixture of but-Zene and perdeuteropropene (or deuterium), prepared in a separate part of the apparatus, was introduced on a fresh film at 0°C in the case of C3De and at -84°C in the case of deuterium. In the presence of C3D6, the total conversion was kept as low as possible in order to avoid consecutive reactions.2654 REACTIONS OF n-BUTENES ON PALLADIUM FILMS ANALYSIS CHROMATOGRAPHY The products were separated and purified on a semi-preparative gas-chromatograph operating under a pressure of 170 Torr and equipped with a 5-m, $in.column of 30% dimethylsulpholane deposited on fire-brick. The working temperature was - 30°C and a catharometer was used as the detector. Sicce the deuteromolecules are slightly separated by gas-liquid chromatography according to their deuterium content, several careful con- secutive chromatographies were made to avoid any loss of product; each moiecule was obtained with a purity >99%. MASS SPECTROMETRY The various deuterated but-2-enes were analysed with a Varian CH7 mass spectrometer. 70V electrons were used to ionize the molecules. The usual corrections were made for naturally occurring isotopes, and for the first four C-H or C-D fragmentations.For these latter corrections, we used the method developed by Gault and Kemball,20 introducing the correct values for C-D fragmentations, as obtained from the mass spectrum of C4Ds molecules. MICROWAVE SPECTROSCOPY The microwave analysis of [2Hl]b~t-l -enes and ci~-[~H~]but-2-enes has been described previou~1y.l~ The position of the last non-exchanged hydrogen atom in [2H,]but-l-enes was also determined. To do this, we had to assign some transitions for [2H7]but-l-ene isomers, whose rotational spectra had not previously been studied. The assignment of the transitions of [1,1 ,3,3,4,4,4-2H7]but-1-ene, which is reported in table 1, was obtained TABLE 1 .-TRANSITIONS OF [I , I ,3,3,4,4,4-'H,]BUT-l-ENE.transition 414-+515 5 1 4 ' 6 6 1 5 5 1 5 3 6 1 6 calculated observed frequency/MHz 32 785.3 39 902.0 39 341.5 frequencylMHz 32 781.6 39 896.2 39 335.9 after we had predicted the spectrum using the method of Nosberger 21 and available data from the following molecules : [2Ho], cis-['Hl], tran~-[~H~], [2-2M1], [3-*Hl], [l ,3-2H2], [2,3-2H2], [4-'H 1], [5-'H l]but- 1 -enes, [2HO] but- 1 -ene 1 3C and [ 'Hs] but-1 -em. The 1as t two were prepared specifically for this purpose. The [2Hs]bUt-l-ene was prepared using the method of Larson et aZ.,22 and the but-l-ene-l-13C obtained from 13CH31 and propion- aldehyde by the Grignard reaction. The microwave study will be fully described in a separate publication. RESULTS PALLADIUM FILMS CONDENSED AT 0°C All three butenes were reacted at 0°C in the presence of perdeuteropropene, each time on a fresh palladium film that had been condensed at the reaction temperature (films of type I).The deuterium distributions of the various reaction products and the location of the label in some specific deuterated molecules (hyperfine distributions) are given in table 2. All these distributions may be considered as initial or quasi- initial, except the one obtained for the exchanged but-1-ene. Since readsorption and isotopic dilution, in this case could have scrambled the labelling of the [2H,] species, an experiment was made at smaller conversion. The corresponding fine and hyperfine distributions, reported in brackets, show that such a scrambling does not occur.M .J . LEDOUX, F. G . GAULT, A . BOUCHY AND G . ROUSSY 2655 REACTIONS OF BUT- 1 -ENE The exchange of but-1-ene, faster than isomerization, yielded mainly the mono- deuterated molecule [2H,], but also more extensively deuterated molecules in decreasing amounts up to [2H7]. The microwave analysis showed that the deuterium in the [2Hl] molecule was located mainly on carbon atom 3 (21 60 %) and 1 ( ~ 4 0 %), with position 1 -trans largely predominant over 1 -cis. The deuterium distributions in both isomers, trans-but-2-ene and cis-but-2-ene were very similar and included all the deuteromolecules from [2H0] to [2H,], in decreasing amounts. [2Ho] and [2H,] molecules were the major products (60 %), and the microwave analysis of the [2H,]-cis-but-2-ene showed that all the deuterium was located on the same carbon atom 1.TABLE 2.-REACTIONS OF BUTENES IN THE PRESENCE OF C3D6 ON PALLADIUM FILMS CONDENSED AT 0°C (TYPE-I FILM) reacting hydrocarbon but-1-ene temp. time O'C 5 min CSD 6/C& ratio 15.4 weight of film 70.3 mg exch. /isom. 2.6 products B1 trans B2 trans B2* cis B2 (calculated) conversion 83.3 % 5.6 % 11.1 2 isom. ratio by m.s. t cis B2/trans B2 = 2 [ZH 01 57.2 (79.2) 29.1 0.3 37.3 12H 11 26.4 (16.3) 24.3 2.1 27.4 t2&1 8.9 (3.2) 13.4 6.3 13.5 kZHd 3.6 (1.2) 11.9 10.7 9.1 IZH41 2.1 10.7 10.7 6.3 12Hsl 1.2 7.1 7.1 4.0 [2H61 0.5 2.9 2.75 1.9 PH71 0.1 0.6 0.6 0.5 t2Hsl - 0.05 0.05 0.05 ~#1/100 0.73 1.87 1.42 microwave B 1 -[zHl] ci~-B2-[2H 11 analysis trans-but-Zene 16.25 81.9 mg 0.8 0.5"C 10 min B1 trans B2 cis B2 2.5 % 95.9 % 1.4 % 2 96.6 9.2 2.5 1.0 24.6 4 0.3 4.5 5 0.2 2.2 4 0.2 3.2 5 0.4 5.9 16.5 1.1 15.4 39 0.2 22.3 22.5 0.05 12.6 6.18 0.13 4.32 B1-[2H7] cis-B2-[zH1] cis-but-2-ene 0°C 5 min 12.4 26.8 m g 2 B1 trans B2 cis B2 4.6 % 2.0 % 93.4 % Bl/trans B2 = 2.3 1.7 24.6 86.7 8.1 31.0 7.0 12.8 5.3 2.0 14.0 4.1 1.3 13.6 5.2 1 .o 15.9 8.2 0.8 16.6 11.1 0.8 13.3 8.3 0.3 3.4 2.1 0.1 4.29 2.57 0.31 cis-B2-[zH 11 --- I;;\ /--- 71 * Contribution of the rc-allylic mechanism. t Mass spectrometry.R E A C TI 0 N S 0 F franS-B U T - 2 - EN E A N D CiS-B U T - 2 - EN E Both exchange and isomerization were slower for trans-but-2-ene than for but-1 -ene and there was less exchange than isomerization. Two pronounced maxima at C2H1] and [2H6] appeared in the deuterium distribu- tion of the exchanged trans-but-2-ene, while the deuterium distributions of the isomers, but- 1 -ene and cis-but-2-ene, were very different.The main deuteromolecules obtained by double-bond migration were the [2H6], [2H7] and ['HJ but-I-enes ; ['H7] was predominant and the last non-exchanged hydrogen atom in this molecule was located on carbon atom 2. The deuterium distributions of the cis-but-2-ene extended up to [2H8] with two maxima at [2H,] and ['H7] ; the deuterium atom in the mono- deuterated cis-but-2-ene was located exclusively on carbon atom 2. The exchange and isomerization rates of cis-but-2-ene lay between those of2656 REACTIONS OF n-BUTENES ON PALLADIUM FILMS but- 1 -ene and trans-but-2-eneY with an exchange to isomerization ratio very similar to that obtained for but-1-ene.The monodeuterated species [2Hl] was the major exchange product, but the deuterium distribution extended up to [2H,] with a sharp break after [2H6]. In the [2Hl] molecule, about 70 % of the label was located on carbon 2 (or 3) and 30 % on carbon 1 (or 4). The deuterium distributions of the two isomers, but-1 -ene and trans-but-2-ene, were very similar to the distributions of the but-1-ene and cis-but-2-ene obtained by isomerization of trans-but-2-ene, except for the second maximum which lay at [2H6] instead of [2H7]. The shift towards the less deuterated species in the deuterium TABLE 3.-REACTION OF BUTENES IN THE PRESENCE OF C3D6 ON A PALLADIUM FILM CONDENSED AT 470°C (TYPE-IT FILM) reacting hydrocarbon temperature and time C,D6/C4& weight of film exch./isom. product conversion isom. ratio m.s.* [2H,] 12H11 L2H2I [2H31 E”H41 f2H51 12H71 [2H81 [ 2 ~ 6 1 4/1m deuterium content microwave analysis cis-but-2-ene 0°C 10 min 16.4 43.5 mg 0.2 B1 trans B2 cis B2 1.0 % 1.1 % 97.9 % Bl/trans B2 = 0.9 11.0 34.3 16.3 8.0 8.7 9.2 7.4 4.2 0.9 27.8 99.5 56.6 0.5 3.2 2.5 - 2.6 3.3 - 2.4 - 1.2 - 0.4 I - 2.53 1.23 0.005 but- 1-ene 0°C 15.7 65.6 mg 1 trans B2 B1 94.9 % 2.8 % cisltrans = 0.8 94.9 44.9 3.8 33.4 0.7 9.1 0.3 6.5 0.2 3.8 0.1 1.7 - 0.5 - 0.1 - 0.1 0.07 0.99 B 1 -[2H 11 7 10 min cis B2 2.3 % 48.9 34.4 7.8 4.6 2.6 1.2 0.4 0.1 - 0.88 * Mass spectrometry distributions, as well as the decrease in the deuterium content of each isomer, might be due to higher isotopic dilution when passing from the trans-but-2-ene to the cis-but-2-ene experiment.Indeed, the number of hydrogen atoms released on the surface was 0.54 per molecule in the reaction of cis-but-2-ene instead of 0.34 in the reaction of trans-but-2-ene. PALLADIUM FILMS CONDENSED AT 470°C Some reactions were effected on palladium films deposited by evaporation on a glass substrate maintained at 470°C (type I1 films). The deuterium distributions of the reaction products obtained from cis-but-2-ene and but-1-ene at 0°C in the presence of C3D6 are reported in table 3. In the case of but-1-ene, both isomerization and exchange reactions were strongly affected by the thermal treatment, the latter muchM. J . LEDOUX, F . G . GAULT, A . BOUCHY AND G . ROUSSY 2657 more than the former. A value of 1 was obtained for the exchange to isomerization ratio, instead of 2.6 on films of type I.Similarly, the deuterium content of either isomer was much lower than on films of type I : 0.99 and 0.83 for trans-but-2-eiie and cis-but-2-ene instead of 1.87 and 1.42, respectively. The hyperfine distribution of the [2H1]-but-l-ene was also very different from the one obtained on type-I films. The label was located not only on carbon 1 (27 %) and 3 (54 %), but also on carbon 2 (20 3/). TABLE %-REACTIONS OF fl-BUTENES IN THE PRESENCE OF D2 ON PALLADIUM FILMS CONDENSED AT 470°C (TYPE-11 FILM) reacting product trans-but-2-ene cis- but-2-ene temperature and time - 84°C 2 min 30 s - 84OC 2 min 30 s DzIC~HS ratio weight of film product conversion ms.* [2HO] m 1 1 I”H21 m 3 1 E”H41 I”H51 12H61 12H71 I ” b 1 E2H91 [“HI01 4/100 deuterium content microwave analysis B1 1.7 % 7.5 15.8 20.3 19.1 15.2 10.8 6.5 3.4 1.4 10.9 34.5 mg trans B2 cis B2 68.9 % 18.9 % 44.4 11.7 11.8 30.4 12.0 15.5 9.5 12,.8 7.7 10.4 5.9 7.8 4.3 5.8 2.9 3.9 1.3 1.7 3.33 1.81 2.56 ci~-B24’Ht 11 butane 16.5 % 1.1 6.9 15.0 15.9 15.2 13.3 10.7 8.1 6.1 4.8 2.8 4.53 B1 5.5 % 5.4 11.5 15.7 16.0 14.9 13.3 10.9 8.3 4.0 - - 9.9 9.8 mg trans B2 cis B2 butane 50.4 % 33.0 % 11.1 % 6.3 40.0 0.0 19.1 10.2 3.1 12.6 9.8 8.7 11.7 8.4 10.2 11.1 7.6 11.0 11.2 7.3 11.6 11.7 7.1 12.1 10.6 6.1 13.0 5.7 3.2 13.8 - - 11.0 - I 5.4 3.72 3.70 2.33 5.80 B1 -[”Hi] ci~B2-I ’Hi] ?q / 34 * Mass spectrometry, In the case of cis-but-2-eneY exchange and double-bond migration were much more decreased than cis-trans isomerization, with an exchange to isomerization ratio of 0.2, instead of 2 on type-I films, and a but-l-ene/tram-but-2-ene ratio of 0.9 instead of 2.3.The deuterium content of the but-1-ene isomer was decreased (2.5 instead of 4.3), and the monodeuterated species became predominant in the deuterium distribution, although a secondary maximum at I2H5] was still present. The deuterium pattern of the tramisomer was very similar to the one obtained on type-I films, with [2Ho] and [2H,] predominant, but the perdeuterated molecules [2M,]-[2H,] were in much smaller amounts (15.6 as coinpared with 44.3). Some experiments were done at -84°C on type-I1 films in the presence of deuterium. The reactions were very fast and deuteration to butane took place to some extent (11-17 %).Although the readsorption processes could no longer be neglected, the deuterium distributions of the various products, reported in table 4, are reminiscent of the distributions obtained in the presence of perdeuteropropene.2658 REACTIONS OF n-BUTENES ON PALLADIUM FILMS On account of the larger amounts of material available, the microwave analysis of some monodeuterated species were made, which could not be done in the previous experiments. ~is-[2-~H,]but-2-ene is the only monodeuterated species obtained from trans-but-2-ene. In the [2Hl]but-l-ene obtained from trans-but-2-ene, all the deuterium was located on carbon 3. The specificity of the labelling is all the more significant as the hyperfine distribution of the exchanged close to equilibrium.DISCUSSION EXCHANGE (TYPE-I FILMS) EXCHANGE OF BUT- 1 -ENE cis-[*Hl]but-2-ene is very The hyperfine distribution of the [2H 1] species allows the Horiuti-Polanyi mechanism for exchange to be ruled out. (1) If the alkyl-alkene reversal were operative, the [3-2Hl]but-l-ene could only be explained by assuming two consecutive steps and very high isotopic dilution, [2-2Hl]but-2-enes should then be observed simultaneously, but they are not : SCHEME 1 (2) Since the classical Horiuti-Polanyi mechanism involves as intermediate a half- hydrogenated state, with both hydrogen atoms of the CHzD group equivalent, equal amounts of cis-[ 1-2Hl] and trans-[ 1-2Hl] species should be observed, whereas the labelling of carbon 1 in but-1-ene is actually highly dissymmetrical. H SCHEME 2 (3) According to the Horiuti-Polanyi mechanism, the observed very poor exchange- ability of the hydrogen atom located on carbon 2 would imply that adsorbed sec-butyl species are much more reactive than n-butyl species.Although differences have been observed between the reactivities of these two 24 (most often the n-butyl form is more reactive), the differences are never pronounced enough to explain the very high observed [1-2H,]/[2-2H,] ratio. The present results thus force us to conclude that the associative Horiuti-Polanyi mechanism does not operate for but- 1 -ene exchange and that dissociative adsorption takes place. In a previous paper,18 it was suggested that metal atoms of high coordination number (isolated adatoms or corner atoms) could promote dissociative adsorption.In these sites, C, the presence, at least, of three free valencies allows exchange according to a vinylic or an allylic mechanism. We believe that both exchange mechanism operate concurrently on palladium films. The allylic-type mechanism,M. J . LEDOUX, F . G . GAULT, A . BOUCHY AND G . ROUSSY 2659 first proposed by molecules (scheme could account for 1 Rooney and Webb,25 would explain the formation of [3-2Hl] 3). The vinylic-type mechanism, first introduced by Farkas et d.,' :he dissymmetrical labelling of positions cis-1 and trans-1 (scheme 4). SCHEME 3 H-M-0 - 1 ..c & ~is-[l-~Hi]but-l-ene H-M-D SCHEME 4 According to the Farkas mechanism, there is no reason for having equal amounts of tran~-[l-~H~] and ci~-[l-~H,] molecules.On the contrary, a difference in reactivity is expected between the two hydrogen atoms of carbon 1, cis-1 and trans-1, on account of the steric interaction between the substituent and the carbon-metal bond in the resulting a-vinylic adsorbed species. I \- + -Fee -c + I ,Fe, .I. + i -- -Pd- L/ .. . . . -cc + I -Pd- FIG. 1 .---Free energy diagrams showing differences in dissociative adsorption of but-1-ene between iron and palladium. The very small amount of [2-2Hl]but-l-ene could also be due to a vinylic-type mechanism of exchange. [2-2Hl]but-l-ene was the major product in the exchange of but-1-ene on iron l6 and this metal is very suitable for vinylic dissociative adsorption,2660 REACTIOKS OF n-RUTENES ON PALLADIUM FILMS as shown by the very fast and exclusive exchange of the three vinylic hydrogeil atoms of but-l-ene.'* The difference in behaviour between iron and palladiuix, where dissociative adsorption of but-l-ene involves carbon atoms 2 and 1, respectively, could be explained by the free energy diagrams represented in fig.1. In these diagrams, we assume that the free energy (F.E.) levels of the activaied co:-i.iyicxes corresponding to the three adsorbed species, although arranged in the same order on both metals, are considerably raised in the case of iron relative to the F.E. levels of allylic mechanism associative mechanism FIG. 2.-Mechanisms for formation of ~is-[l-~H~]but-2-ene and [l-2Hl]but-l-ene, tram-[ 2-2Ii.11 but-2-ene associative mechanism r._-_-_.__---_____----~------------- FIG. 3.-Mechanisiii of formation of ~is-[2-~H~]but-2-ene. the reactants.That would explain why on iron the exchange at carbon 1 could not be detected in the [21-Il]b~it-l-er,e iiiolecufes. In the case of paIladium, the very poor exchange at carbon 2 could be exp!ained if desorption is thc ratedetermining step and involves too high a free energy o f activation. EXCHANGE OF CiS-BUT-2-ENE The results obtained in the case of cis-but-2-ene confirin well the existence of two dissociative mechanisms for exchange, allylic and vinylic, responsible for thc two reaction products, [ 1 -2H and [2-2H,]cis-buf-2-ene, respectively. We believe that ei~-[l-~H~]but-2-ene is obtained by an allylic mechanism. It is difficult indeed to accaunt for this species by the classical Horiuti-Polanyi mechanism, except by assuming isotopic dilution and a number of consecutive steps which should also provide [1-2Hl]but-l-ene : this species is not observed in the reaction products (fig.2). Nor can the major exchange product, .ci~-[2-~H~]but-2-ene be explained byM. J . LEDOUX, F . G . GAULT, A . BOUCHY A N D G. ROUSSY 266 1 a simple Horiuti-Polanyi mechanism, unless one assumes at least two consecutive alkyl-alkene reversals (fig. 3). Since formation of tran~-[~H~]but-2-ene involves only one of these steps, one should observe more trans-[2Hl]isomer than exchanged ~ i s - [ ~ H ~ ] . The reverse was true : the amount of ~is-[~H,]but-2-ene obtained by exchange (4.5 %) was much larger than the amount of tran~-[~HJbut-2-ene obtained by cis-trans isomerization (0.6 %).We believe, therefore, that ~is-[2-~H,]but-2-ene is formed by a vinylic type dissociative mechanism. If the vinylic but-2-enyl adsorbed species is placed in the free energy diagrams at a level very close to the internal but-1-enyl species and if the F.E. level of the reacting cis-but-2-ene is much below the level of but-1-ene, one would expect that, on iron, cis-but-2-ene exchange would be much slower than but-1-ene exchange. Indeed, the exchange of cis-but-2-ene was almost non-existent on iron, and that is a good confir- mation of the above proposals. 0 2 + + A2 FIG. 4.-Energy diagram for double bond migration in but-1-ene. DOUBLE-BOND MIGRATION (TYPE-I FILMS) BUT-2-ENE -+ BUT- 1 -ENE ISOMERIZATION The characteristic deuterium distribution of the but- 1-enes obtained from truns- but-2-ene included three highly deuterated species [2H6], and [2H8].This distribution should be related to that of the [2H6]- [2H,]cis-but-2-enes and to that of multiply exchanged trans-but-2-enes with a pronounced maximum at C2H6]. We believe that all these highly deuterated molecules are obtained by an allylic mechanism involving several interconversions between n-olefinic and n-allylic species. Such a mechanism was proposed previously to explain the complete exchange of cycloalkanes on palladium 1* and was rejected on the basis of a single e~periment.~ As will be shown, the labelling of the C2H7]but-l-ene and the [’H6] maximum in the exchange pattern of trans-but-2-ene are definite proofs of the existence of such a mechanism in the case of olejn reactions on palladium.The break after [2H6] in the distribution of truns-but-2-enes, very similar to the break after [2H,] in the exchange pattern of 1,2 dimethylcyclopentene, implies a very fast exchange of all allylic hydrogens, best explained by very fast interconversions2462 REACTIONS OF n-BUTENES ON PALLADIUM FILMS between allylic species and n-olefinic species having the same but-2-ene structure ; in this process the conversion into olefinic adsorbed species with but-1-ene structure is a difficult step. In order to explain double-bond migration and the deuterium pattern of but-1-ene, then, we have to admit that two different asymmetrical n-allylic species exist on the surface, interconverting through a transition state which could be a symmetrical n-allylic species.In fig. 4, species A, represents an asymmetrical allylic species where thep, orbitals of carbons 2 and 3 interact more strongly with the CT metal orbitals than does the p , orbital of carbon 1. Such a species would then part 1 y retain the cis- but -2-ene configuration. In species A,, conversely, the C, and C2 pz orbitals interact more strongly with the metal than does the C3 pz orbital. The high energy barrier between species A, and A2, which corresponds to the symmetrical n-allylic species, prevents fast inter- conversions between the two n-olefinic species 0, and 02. But-2-ene -+ but-1-ene isomerization may thus be represented by the following succession of steps (fig. 5). 4 D3 M s k w [ Hs]-cis-but-2- ene 4 HD D3 4 ki=-!-b-&+D~~ D U ?JJ [1,1 .3,3,4,4,4-2H7]but-1-ene * -.-.4 - *2 H H H D H D H D 2 H 02 Al 01 *I 01 A! A2 O2 A2 A i Oi A i (1) Fast interconversions between O2 and A, exchange the six hydrogen atoms of (2) Species A2, quintuply deuterated, converts into species Al. (3) Interconversions between A, and O1 exchange a sixth and a seventh hydrogen atom on carbon 3. Desorption at this stage yields C2H7]but-l-ene with the last hydrogen remaining on carbon 2. (4) In order to form [2H8]bUt-l-ene, two more interconversions between A,-type and A2-type allylic species are required. Interconversions between .n-olefinic and n-allylic species, 0 1 and Ai, should be faster than olefin desorption, since the amounts of [1,1,2,3,4,4,4-2H7]but-l-ene are negligible. Consequently, [2H5] and [2H6]bUt-l-eneS derive from adsorbed trans- but-2-enes that have exchanged only part of their methyl hydrogens.(5) Free rotation around the C2-C3 bond in O1 and reversal to adsorbed cis- but-2-ene could account for the ci~-[~H,]- [2H8]but-2-enes. The above mechanism thus explains the distribution of deuterium in but-I-ene, including the location of the deuterium atoms on the [,H,] species, the multiple exchange of trans-but-2-ene, and the highly deuterated cis-but-2-enes. FIG. 5.-Steps in but-2-ene-tbut-1-ene isomerization. the methyl groups. Desorption at this stage yields tran~-[~H,]but-2-ene. BUT- 1 -ENE 4 BUT-2-ENE ISOMERIZATION Since double-bond migration is accompanied by extensive exchange of but- 1-ene, the but-2-ene deuterium distributions are strongly altered by isotopic dilution.To take this effect into account, the deuterium patterns of the more highly deuteratedM. J . LEDOUX, F . G . GAULT, A . BOUCHY AND G . ROUSSY 2663 but-2-ene isomers have been recalculated, using as true initial distribution that of the but-1-ene obtained from trans-but-2-ene and using several values of the deuterium mole fraction x . The best fit is obtained with x = 0.51, and the corresponding distributions, calculated by adjusting [2H4] and [2H5], are reported in the fourth column of table 2. For the [2H,]-[2H,] species, excellent agreement with the observed values was obtained. One may consider, then, that these molecules, and also most of [2H3] and half of [2H2], are obtained by the same allylic mechanism Al as proposed above for the but-2-ene + but-1-ene isomerization.However, part of the trans- but-2-ene distribution, including E2H0] = 28.8, ['H1] = 22.2, [2H,] = 7.2 and [2H3] = 1.2, was obtained by some other process. ,y/-- ;?I-.-. \f/ -. [2Ho] M-rr M-H M 4 FIG. 6.-Mechanism accounting for [2Ho]-[zH3] distribution. A combination of an intramolecular hydrogen shift and the Horiuti-Polanyi mechanism could account for the above [2Ho]-[2H,] distribution. 1-2 double-bond migration takes place almost exclusively without deuterium incorporation on iron films and was interpreted by a sigmatropic mechanism.' ' A non-repetitive Horiuti- Polanyi mechanism accounts for the [ 2H 1] species obtained by cis-trans isomerization on most metals.ll However, an alternative mechanism could be proposed, accounting for the whole [2Ho]-[2H3] distribution.Such a mechanism, A2, basically of the same nature as the allylic mechanism Al, would involve, instead of C sites, B sites with only two free valencies available. A decreasing [2Ho]-[2H3] deuterium distribution is to be expected according to fig. 6 , provided that replacement of adsorbed hydrogen by adsorbed deuterium is as fast as or slower than olefin desorption. CiS-tranS I S 0 M ERI Z A TI ON The distribution of deuterium in the cis- or trans-isomer obtained in this reaction includes, besides the highly deuterated species discussed previously, [2H,] and [2Hl] molecules. Direct cis-trans isomerization, without deuterium incorporation, is commonly observed on transition metals.l1. 26 The mechanism for this reaction involves a weakening of the C2-C3 bond and free rotation, either by some electron transfer to the metal [scheme 5(b)] or by partial hydrogen displacement [scheme 5(a)].The latter mechanism is suggested by the result of nb initio calculations 29 showing that, during the course of a sigmatropic migration, the migrating hydrogen interacts with carbon 2 before any bonding with carbon 3 becomes significant. It was proposed that highly coordinated surface atoms, the ones of low Miller index faces, with one single free valency available, were responsible for these reactions (scheme 5a). Isomeric2664 REACTIONS OF n-BUTENES ON PALLADIUM FILMS monodeutero-cis-but-2-ene, as shown by microwave analysis, retains all the label on carbon 2 or 3, which is consistent with a simple Horiuti-Polanyi mechanism. The break after [,H1] in the deuterium distribution suggests that, in contrast to what is conventionally believed, very few alkyl-alkene reversals take place before desorption.That three consecutive steps are required to form traizs-[*H,], instead of one step to form [2H,], makes the break more pronounced than it would be in the case of the but-2-enes obtained from but-1 -ene by double-bond migration. In the latter case, 9 M free rotation L SCHEME S(u).-Signiatropic mechanism. SCHEME S(b).-Charge transfer to the metal, I Dv ci~-[~Hi]but-2-ene M-H double bond migration SCHEME 6 -___________-_______---.------------- two and one steps are required to form the di- and mono-deuterobut-2-enes respec- tively (scheme 6). Thus formation of [,H,]- [2H,]but-2-enes from but-1-ene may be simply explained by the classical Horiuti-Polanyi mechanism and does not require the introduction of an additional allylic mechanism A,.As already pointed out,18 it is believed that the associative mechanism requires sites with two free valencies available (sites B) such as the sites arising at the intersection of two-secant low-index faces (edge atoms) (scheme 7). SCHEME 7M. J. LEDOUX, F. G. GAULT, A. BOUCHY AND G. ROUSSY 2665 GENERAL DISCUSSION REACTIONS ON FILMS CONDENSED AT 470°C A combination of mass spectrometric and microwave analyses allows one to characterize a number of reactions, which may be classified as follows : (A) direct cis-trans isomerization and sigmatropic double-bond migration ; (B) Horiuti- Polanyi mechanism for cis-truns isomerization and double-bond migration.This associative mechanism yields mainly singly deuterated molecules ; (C) dissociative mechanisms including a vinylic adsorption operative for exchange, and an allylic adsorption involved in exchange and in double-bond migration with extensive deuterium incorporation. FIG. 7. According to the number of ligands required in the various reaction mechanisms, these three classes of reactions were related to three types of sites A, B and C , differently coordinated and having respectively one, two, or three free valencies available. Some of these sites may accommodate two different species. Allylic and vinylic adsorptions for instance, compete on C sites. If one assumes that the free energy level of the activated complexes corresponding to the allylic species (dotted line in fig.1) are placed above the ones corresponding to but-2-enyl species and below the ones corresponding to but-1-enyl species, the inversion of the ratio between allylic- type and vinylic-type exchange when passing from but-1-ene to but-2-ene could be explained. One advantage of the above definition of the active sites A, 33 and C is that they may be identified with the normal low-index surface atoms (A sites), or with some of the defects arising in a rough surface : edge atoms (B sites) corner or adatoms (C sites) (fig. 7). If such an identification is correct, any modification of the metal surface should modify the relative amounts of sites A, B arid C, and consequently the relative contributions of the various mechanisms.In this paper, we give an example of such a surface modification when palladium films are deposited on a substrate maintained at high temperature. One might expect that, under such conditions of film preparation, C sites and type C reactions will be drastically suppressed. Comparison between the deuterium distributions obtained on films condensed at 0 and 470°C (tables 2 and 3) shows that this was indeed the case. Ali the highly deuterateci molecules obtained by allylic-type isomerization were2666 REACTIONS OF n-3UTENES ON PALLADIUM FILMS drastically reduced : ['H3]- ['HJbut-1 -enes and trans-but-2-ene from cis-but-2-ene and [2H3]- [2H,]but-2-enes from but-1-ene. Similarly, both allylic- and vinylic- type exchange of but-1-ene and but-2-ene were strongly decreased.Lastly, the very high cisltrans ratio in but-1-ene isomerization (2) and the but-1-eneltrans ratio in cis-but-2-ene isomerization (2.3) on normal films, which could have been explained by the preferential syn-configuration of the .n-allylic species,27* dropped drastically to 0.8 and 0.9, respectively, on treated films; this finding, too is consistent with the disappearance of C sites and type-C reactions. On films condensed at 470°C, the major remaining reactions, then, are those occurring on A and B sites : direct cis-trans isomerization and isomerization according to a sigmatropic and Horiuti-Polanyi mechanism. The latter mechanism even became significant for exchange, accounting for the [2-2H,] species in the hyperfine distribution of but-1-enes, whereas it was negligible on normal rough palladium films.Another possible explanation of the results obtained on films type 11, suggested by one of the referees, is a selective contamination of the surface by carbon deposit during film preparation. However, it was checked by using Leed and Auger that a clean palladium surface [either oriented (1 11) or polycrystalline] does not retain a significant amount of carbon when exposed to hydrocarbon or CO contaminants at temperature lower than 600°C.30 J. J. Rooney, F. G. Gault and C. Kernball, Proc. Chem. SOC., 1960,407. F. G. Gault, J. J. Rooney and C. Kemball, J. Catalysis, 1962, 1, 255. J. J. Rooney, J. Catalysis, 1963, 2, 53. L. Hilaire, G. Maire and F. G. Gault, Bull. SOC. chim. France, 1967, 886. R. L. Burwell and K. Schrage, J. Amer. Chem. SOC., 1966,88,4549. J. A. Roth, B. Geller and R. L. Burwell Jr., J. Res. Inst. Catalysis Hokkaido Unio., 1968, 16, 221. H. A. Quinn, J. H. Graham, M. A. McKervey and J. J. Rooney, J. Catalysis, 1971, 22, 35. L. Hilaire and F. G. Gault, J. Catalysis, 1971, 20, 267. R. Touroude, L. Hilaire and F. G. Gault, J. Catalysis, 1974, 32, 279. lo R. Touroude and F. G. Gault, J. Catalysis, 1974, 32,288. l 1 R. Touroude and F. G. Gault, J. Catalysis, 1974, 32,294. l2 J. Horiuti and M. Polanyi, Nature, 1933, 132, 819 and Trans. Faraday Soc., 1934, 30, 663. l 3 A. Farkas, L. Farkas and E. K. Rideal, Proc. Roy. Soc. A , 1934, 146, 630. l4 K. Hirota and Y. Hironaka, Bull. Chem. SOC. Japan, 1966, 39, 2638 and J. Catalysis, 1965, l5 T. Ueda, J. Hara, K. Hirota, S . Teratani and N. Yoshida, 2. phys. Chem. (Frankfurt), 1969, l 6 M. Ledoux, F. G. Gault, J. J. Masini and G. Roussy, J.C.S. Chem. Comm., 1975, 1034. l7 F. G. Gault, M. Ledoux, J. J. Masini and G. Roussy, Proc. VIth Int. Congr. on Catalysis (The Chemical SOC., London), p. 469, vol. 1. l 8 M. J. Ledoux, Nouv. J. Chim., 1978, 2, 9. l 9 C. Kemball, Proc. Roy. SOC. A, 1951,207,539. 2o F. G. Gault and C. Kemball, Trans. Faraday SOC., 1961, 57, 1781. 21 P. Nosserger, A. Bauder and Hs. H. Gunthard, Chem. Phys., 1973, 1,418. 2 2 J. G. Larson, J. W. Hightower and W. Keith Hall, J. Org. Chem., 1966, 31, 1225. 23 T. Ueda, Proc. Vth Int. Congr. on Cutalysls, Miami, 1972, 1, 431. 24 E. Hirota, M. Ito and T. Weda, Proc. VIth Int. Congr. on Catalysis (The Chemical SOC., London) 2 5 J. J. Rooney and G. Webb, J. Catalysis, 1964, 3,488. 26 M. J. Ledoux, Thisse Etat (Strasbourg, 1977). 27 G. C. Bond and M. Hellier, J. Catalysis, 1965, 4, 1. 28 R. Guisnet, G. Perot and R. Maurel, J. Chim. phys., 1972, 69,1059. 29 J. P. Grima, F. Choplin and G. Kaufniann, J. Organomet. Chem., 1976, 124, 315. 30 P. Legare, Y. Holl and G. Maire, personal communication. 4, 602. 64,64. p. 518, vol. 1. (PAPER 81360)

ISSN:0300-9599    

DOI:10.1039/F19787402652

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Thermodynamic Study of Dilute Aqueous Solutions of Organic Compounds Part 5.-Open-chain Saturated Bifunctional Compounds BY SERGIO CABANI* AND VINCENZO MOLLICA Istituto di Chimica Fisica, Universiti di Pisa, Pisa, Italy AND LUCIANO LEPORI Laboratorio di Chimica Quantistica ed Energetica Molecolare del C.N.R., Pisa, Italy Received 2 1 st March, 1978 Values have been determined at 25°C of the changes in the free energy, enthalpy and entropy corresponding to the process of transfer from the ideal gas state to dilute aqueous solution for ethylene- diamine, 2-methoxyethylamine7 3-methoxypropylamine, 1,2-dirnethoxymethane and four 2-alkoxy- ethanols (methoxy to n-butoxy). These data have been used to calculate the variations in the thermodynamic functions of hydration for the hypothetical process of introducing a Y group (Y = 0, NH) into a monofunctional RX compound (X = 0, NH, NH2, OH), either by breaking a C-C or a C-H bond. Evidence of strong interactions between hydrophilic centres in bifunctional compounds emerges.The strength and nature (entropic or enthalpic) of these interactions depend on both the type of functional groups and their relative distance. Studies on the thermodynamic hydration functions of organic molecules have been mostly devoted to monofunctional compounds and few AX: ( X = G, H and S ) values are known for polyfunctional compounds. In fact, as far as we know, the complete set of the hydration functions is limited to some cyclic diamines, diethers and arnin0ethers.l. For other polyfunctional systems only enthalpies or only free energies of hydration have been determined.In this paper values of free energy, enthalpy and entropy are reported for the transfer process from the ideal gas state to dilute aqueous solution at 25°C for some open-chain bifunctional compounds having as functional centres the amino, the ethereal oxygen or the alcoholic hydroxyl group. EXPERIMENTAL MATERIALS All organic compounds were of the purest grade (> 99 % purity) available commercially. 2-Methoxyethylamine, 3 -methoxypropylamine, ethylenediamine and I72-dimethoxyethane were purified as previously described. 5 * The n-alkoxyethanols were fractionally distilled at atmospheric pressure after prolonged refluxing over calcium hydride. In all the cases the middle fraction was retained after checking that the purity of the compounds by g.1.c.was better than 99.5 %. The water used in all the experiments was first deionized and then distilled from alkaline KMn04. APPARATUS AND MEASUREMENTS The heats of solution and vapour pressures of the pure substances, as well as of their aqueous solutions, were measured using the calorimetric and vapour pressure apparai us described in a previous paper.' Particular care was taken in deaerating the solutions by repeated freezing under high vacuum. 26672668 THERMODYNAMICS O T AQUGOUS ORGANIC COMPOUNDS The activity coefficients at infinite dilution, f , , of the investigated corripounds were obtained by extrapolating the termf' to zero concentration of solute where Pobs Is the total vapour pressure of the solution containing a molar fraction x, of solute and x, of water,'and p,: and pi are the vapour pressures of pure water and solute, respectively.After measuring the vapour pressure P&s, the solute conceritration xs of the aqueous solution was determined. by freezing point depression measurements in the case of alkoxy compounds, and by HC1 iitration in the case of an-rines. Generally, five measure- ments were carried out for each substance in the concentration range xs = 0.005 to 0.1. The thermochemical calorie (4.184 J) has been used :hrougliout this paper. RESULTS Table 1 gives the experimental data at 25"C, i.e., vapour pressures of pure liquid substances (p:), activity coefficients of solutes at Lnfinite dilution (fa), as well as the enthalpy changes associated with the transfer from the liquid to dilute aqueous TABLE 1 .-VAPOUR PRESSURES, LIMITING ACTIVITY COEFFICIENTS IN AQUEOUS SOLUTION, STANDARD ENTHALPIES OF SOLUTION AND VAPORIZATION AT 25'C substance ethylenediamine 2methoxyet hylamirie 3 -met hoxy propylamine 1 ,2-dimet hoxyet hane 2-methoxyethanol 2-ethoxyethanol 2-propoxye than01 2-butoxyethaiiol p P/mm tig 12.78$- 0.03 50.74k 0.02 14.3250.01 68.59+ 0.06 9,.43 & 0.02 5.32+ 0.02 2.32F0.01 0.93&0.01 f G 3 0.22f 0.07 0.3 3 -t.0.03 0.61 to.10 4.3 I & 0.22 1.21 &On 10 3.80f0.24 8.84k0.77 28.7 t 2 . 5 -AH,Olkcal mol-1 7.44f O.Ola 6.02+ O.0Zb G.76k 0.04& 5.49+_0.Olc 3.65 & 0.0Ie 4.34+0.01e 4.26&O.0le 4.08&0.0le AH;/kcal mol-1 10.92$-0.02a 9.16$-0.Olc 10.52k 0.01 8.70f 0.01f 10.80f0.01g 11.52f 0.01s 12.46& 0.02s 13.53 & 0.01Q 0 Ref.(8). b Determined by adding a known amoun: of pure amine to the calorimeter charged with watcr madc alkaline with 0.01 ni01 dm-3 NaOH to prevent hydrolysis. In the concentration range examined (0.01-0.03 mol d n r 3 ) no concentration depcndence of AHs was observed. C Calcu- lated from vapour pressure data of the pure coinpouiid at different temperatures through : AH: = T(RT/pi-kB'- Vi) dpi/dT. The liquid molar voiunie, Vl = 87 CJil' niol-', was evaluated from literature density data; the second virial coefficient B' = -700 cm3 inol-' was estimated 2s described in ref. (1) ; the derivative dp:/dT was obtained from the Aiitoine equation, log$ = A -B/(ti- C), employed to dcscribe pressure-temperature ("C) data in the range 5-45°C.The values of the parameters OC the Anroine equation are: A = 7,1935rtl0.054, B = 1334.40k26.6, C = 215.1401 2.45. (1 Calculated as in (c) using the values : VI = 102 cni3 mol-1 ; B' = - 761 cni3 niol-L ; A = 7.2573 fi0.049 ; B = 1438.84+23.5 ; C = 210.822 & 1.96. e Ref. (3). f'Ref. (9). B Ref. (10). TABLE 2.-sTANDAW) THERMODYNAMIC FUNCTIONS OF HYDRATION AT 25OC (kcdl lllOl-l)'' substance ethylenediamine, 2-inethoxyeth ylaniine 3-methoxypropy lanline lY2-dimethoxyethane %met 11 oxyethanol 2-ethoxyet hariol 2-propoxyethano I 2-but oxyethanol -AG; 3.32+ 0.2 2.27+0.05 2.65+ 0.1 0.56_f0.03 2.49+ 0.05 2.33 3- 0.05 2.14$- 0.05 1.992 0.05 -AH: 1 8.36 & 0.02b 15.1 813 0.02 17.28-t 0.04 14.18f 0 . 0 1 C 14.45+ 0 . 0 1 C 15.86k 0.02c 16.723- 0.02" 17.60+0~Olc - TAS; 15.043-0.2 12.91 3- 0.05 14.63& 0.11 13.62k0.03 11.965 0.05 13.53+0.05 14.58+0.05 15.61 k0.05 0 Standard states : ideal gas at 1 atm, hypothetical ideal aqueous solution at unit molar fraction of solute.b Ref. (8). C Ref. (3).S. CABANI, V . MOLLICA AND L. LEPORI 2669 solution (AH:) or to ideal gas phase (AH;). Most enthalpies given in table 1 are calorimetric data from other authors.3* The values of AH: and AH: for 2- rnethoxyethylamine and 3-methoxypropylamine have been determined by us using calorimetric and vapour pressure measurements, respectively. In table 2 the standard thermodynamic functions of hydration (AX,", X = G, H, S ) are reported for the transfer of one mole of solute from the ideal gas state at 1 atm to the hypothetical ideal aqueous solution at unit molar fraction of solute.The cal- culation of AG: and AH: has been performed using the equations : AG; = AG,"+RT In p," (2) AH," = AH:-AH:. (3) In eqn (2), AG: is the standard free energy of solution related tof, by: AG; = RTlnf,, while p i is the vapour pressure, in atm, of the pure compound. The TAS; values have been calculated from the thermodynamic relation : TAS; = AH,-AG,". (4) DISCUSSION In fig. 1 a comparison is made of the thermodynamic hydration functions for n-butane and the saturated open-chain monofunctional and bifunctional compounds with four carbon atoms which have as functional centres ethereal oxygen, the amino group and the hydroxyl alcoholic group. The most striking feature is that the en- tropies of hydration are almost the same for every compound, whereas AH: and 4 - I, 0 - - - 4 - 3 5 - 8 - 3 Q -12- d 8 24 -16 - -20 - FIG.1 .-Standard thermodynamic functions of hydration at 25°C for open chain saturated organic compounds having four carbon atoms : m, AG; ; A, AH; ; 0, TASh". Data for n-butane from ref. (ll), for diethylether from ref. (l), for n-butylamine from ref. (12) and (13), for n-butanol from ref. (12) and (14). consequently AG; values depend on the presence and the number of hydrophilic centres. Moreover, the AG; and AH: values change more on going from hydro- carbon to monofunctional compound than in the process monofunctional + bi- functional compound. This fact suggests that additivity group rules are not valid for the thermodynamic functions of hydration, i.e., the hydrophilic centres interact with each other.2570 THERMODYNAMICS OF AQUEOUS ORGANIC COMPOUNDS In order to obtain information about these interactions between polar centres in bifunctional compounds, we considered the hypothetical process of adding a hydro- philic group Y to a hydrocarbon or a monofunctional RX molecule either by breaking a C-C bond according to -C-C- -+ -C-Y-C- (M process) or by breaking a C-H bond according to -C-H -+ -C-Y-H ( p process).TABLE 3 .-VARIATIONS IN THE STANDARD THERMODYNAMIC FUNCTIONS OF HYDRATION (kcal md-') FOR THE HYPOTHETICAL ADDITION PROCESS OF A Y GROUP TO A HYDROCARBON OR TO A MONOFUNCTIONAL RX SATURATED ORGANIC COMPOUND 1 2 3 4 5 6 7 8e 9 10 11 12 13 14 15 36 17 18 19 20 u process --C-c-+-~--y-~-- n-butanea -+ diethyletherb n-propylaminec n-butylaminec + 3-methoxypropylainine n-propanold + 2-methoxyethanol n-butanold 4 2-ethoxyethanol n-pentanold 4 2-propoxyethanol n-hexanold + 2- but oxye t hanol diethylethefi -+ 1,2-dimethoxyethane tetrahydrofuraneb + 1,4-dioxanb pyrrolidinef -+ morpholineg n-butane" -+ diethylamineh cy clohexane' -+ hexamethyleneimi nef tetrahydrofuraneb -+ morpholineg pyrrolidinef 3 piperazineg -+ 2-me t hoxy e thy lam ine B process -C-H -+ -C-Y-H n-butane" -+ n-butanold cyclohexanei -+ cyclohexanol' diethyletherb + 2-ethoxyethanol ethaneu -+ ethylaminec propaneu + n-propylaminec e thylamineC -+ ethylenediamiiie 0 -3.70 0 -2.15 0 -2.63 0 -1.92 0 -1.88 0 -1.93 0 -1.89 0 -3.21 0 -1.60 0 -1.69 NH -6.14 NH -6.13 NH -3.71 NH --1.89 -4.59 -0.87 -1.86 0.30 -3.17 -0.56 -0.72 1.19 -1.14 0.72 -1.25 0.66 -1.78 0.10 - 3.38 -0.17 -0.17 1.43 -1.46 0.29 -9.35 -3.38 -8.38 -2.25 -5.30 -1.60 -6.41 -4.51 - 0 -4.80 -8.51 -1.69 = 0 -6.70 -8.92 -2.22 0 0 -4.98 -5.06 -0.08 - - NH -6.34 -8.19 -1.85 = NH -6.35 -7.95 -1.60 NH2 NH -3.09 -5.45 -2.37 a Ref.(11). Ref. (1). CThe AH; value was taken from ref. (12); the A G i value was calcu- lated from the data reported in ref. (13). dThe AHi value was taken from ref. (12) ; the AGhO value was calculated from the data reported in ref. (14). e This process is not strictly comparable to the others, because it also involves a strong change in the structure of hydrocarbon chain. f Ref. (7). Ref. (2). h The AH; value was taken from ref. (15) ; the AGL value was calculated from the data reported in ref.(13). i The AGL value was taken from ref. (16); the AH; value from ref. (17). IIRef. (18). Table 3 lists the changes in the thermodynamic functions of hydration associated with the operations described above. Data relative to cyclic compounds previously studied As far as the free energies of hydration are concerned, the gain in thermodynamic stability in the process monofunctional 4 bifunctional molecule is smaller with respect to that of the corresponding process hydrocarbon -+ monofunctional com- pound (cf., e.g., process 1 with 5,15 with 17, and 18 with 20). Moreover, the decrease in AG; which accompanies the formation of a bifunctional compound from a satur- ated RX compound depends to a minor extent on the nature of the functional group are also included in the table.S .CABANI, V . MOLLICA AND L . LEPORI 267 1 X already present, as it can be ascertained by comparing the 6AGi values for amino- ethers, alkoxyethanols and diethers (cf. processes 2 to 8, 13 and 14). The pattern outlined above of the 6AGi quantities originates from a compensation between the 6AHi and 6TASL terms. These latter quantities, more than 6AGg terms do, depend not only on the type of the X group in the monofunctional RX molecule (cf. processes 2 to 8, 9-10, 13-14), but also on the distance between the X and Y centres (cf. processes 2 with 3) as well as on the different chain length of alkoxy groups in the alkoxyethanol series (cf. processes 4 to 7). As far as the nature of the hydrophilic centres is concerned, it is noteworthy that the gain in thermodynamic stability accompanying the introduction of an ethereal oxygen is generally due to both the entropy and enthalpy terms (cf.processes 2, 4-7,9,10). In contrast, the AG; decrease associated with the introduction of a nitrogen centre is exclusively due to the enthalpic effects which overwhelm the unfavourable entropy changes (cf. processes 13, 14, 20). The analysis given above, although mostly limited to the case in which Y is an ethereal oxygen, reveals that a complicated though rational trend in the thermo- dynamic hydration functions of bifunctional saturated compounds arises from the interaction between the hydrophylic centres, according to their nature and mutual distance. This is particularly true for the enthalpies and entropies of hydration which are more sensitive, with respect to other thermodynamic properties of organic compounds in water, to the water-solute interactions.Nevertheless, in order to have data more significantly correlated to the effects of water-solute interactions in mono- and bifunctional compounds, it would be advisable to review and rationalize the hydration thermodynamic functions known so far. We are grateful to the Consiglio Nazionale delle Ricerche of Italy for financial support. S. Cabani, G. Conti and L. Lepori, Trans. Faraday SOC., 1971, 67, 1943. S. Cabani, G. Conti, D. Giannessi and L. Lepori, J. C. S. Faraday I, 1975, 71, 1154. K. Kusano, J. Suurkuusk and I. Wadso, J. Chem. Thermodynamics, 1973, 5, 757. J. Hine and P. K. Mookerjee, J. Org. Chem., 1975, 40, 292. S. Cabani, V. Mollica, L. Lepori and S. T. Lobo, J. Phys. Chem., 1977, 81,987. L. Lepori and V. Mollica, J. Chem. Eng. Data, 1978,23,65. S . Cabani, G. Conti and L. Lepori, Trans. Faraday SOC., 1971, 67, 1933. P. Paoletti, personal communication. K. Kusano and I. Wadso, Acta Chem. Scand., 1970,24,2037. E. Wilhelm, R. Battino and R. I. Wilcock, Chem. Reo., 1977,77,219. l o K. Kusano and I. Wadso, Acta Chem. Scand., 1971, 25, 219. l 2 J. Konicek and I. Wadso, Acta Chem. Scand., 1971,25, 1541. l 3 A. 0. Christie and D. J. Crisp, J. Appl. Chem., 1967, 17, 11. l4 J. A. V. Butler, Trans. Farachy SOC., 1937,33,229 ; J. A. V. Butler, C. N. Ramchandani and l 5 F. Franks and €3. Watson, Trans. Faraday SOC., 1969, 65,2339. l 6 R. D. C. Cramer 111, J. Amev. Chem. SOC., 1977, 91, 5408. l 7 S. J. Gill, N. F. Nichols and I. Wadso, J. Chem. Thermodynamics, 1976,8,445. l 8 S . Cabani, G. Conti, V. Mollica and L. Lepori, J. C. S. Favaday I, 1975,71, 1943. D. W. Thomson, J. Chem. SOC., 1935,280; 1936, 1171. (PAPER 81537)

ISSN:0300-9599    

DOI:10.1039/F19787402667

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Studies of Reactions of Atoms in a Discharge Flow Stirred Reactor Part 3.-The 0 + H, + 0, System B Y IAN M. CAMPBELL," JOHN s. ROGERSON AND BRIAN J. HANDYt School of Chemistry, The University, Leeds LS2 9JT Received 12th April, 1978 By addition of H2 to O(3P) atoms in N2 carrier, a partial conversion of 0 atoms to H atoms was achieved through the reactions O+H2 -+ OH+H (1 1 O+OH +02+H (2) in a discharge-flow stirred reactor at 425 K. Addition of small amounts of CO (c 5 %) generated bluish 0 + CO chemiluminescence, the intensity of which at entry and exit ports was measured to establish 0 atom decay rates. When O2 (< 1 %) was added, H02 radicals were formed in situ by reaction H+02+M+HOz+M (9) H+H02 + 20H (10) O+HO2 + OH+02 (11) H+HO2 +H2+02 (22) H+HO2 H20+0 (23) and the 0 atom decay rate was accelerated due to subsequent reactions and reaction (2).The parallel steps to reaction (10) consume H atoms and reaction (23) produces 0 atoms so decreasing the catalytic rate of removal of 0 atoms. From the variation of the catalytic rate as a function of [H]/[O] ratios in the range 0.16 to 3.46, a value of k9(M = N2) = (1.2k0.2) x 10" dm6 mok2 s-' at 425 K was obtained, together with a placing of the ratio kll/(k10+k22+k23) (i.e. the relative reactivity of H02 with 0 and H atoms) in the range 0.2 to 0.5 at 425 K. The H02 radical has long been recognized as an important species in combustion and explosion phenomena and more recently as a significant component of strato- spheric chemistry. The reactions of H02 with O(3P) and H(2S) atoms play major roles in such situations.For example Donahue et a2.l have pointed out the critical role of reaction of H02 with O(3P) atoms in determining the depletion effects of chlorofluorocarbons upon the stratospheric ozone layer, but these can only be quantified if rate constants are available for all the elementary reactions concerned. we have described the operation of the discharge-flow stirred reactor, when it was used to synthesize HNO and HCO radicals by in situ three body combination reactions (H + NO/CO + M) and thence to measure their relative reactivities with H(2S) and O(3P) atoms. In this study 0, is added to the 0 + H2 + N2 system to synthesize H02 by combination and thence to establish the ratio of rate ?Present address : Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP.In Parts 1 and 2,29 2672I . M. CAMPBELL, J . S. ROGERSON AND B. J . HANDY 2673 parameters for this subsequent reactions with H and 0 atoms, which has not been measured directly before. As in the previous work 2 * the method is based upon the catalytic consumption of 0 atoms, here induced by the presence of H02 in the reactor. EXPERIMENTAL The general procedures used in this work were similar to those used previously.2* The central feature was a Pyrex sphere of internal volume 0.54 dm3, internally-coated with syrupy phosphoric acid to inhibit wall recombination of atoms. The entry and exit tubings were non-penetrating and in-line : the entry tubing had inset jets J1 and J2, the latter just upstream of the observation point L1, as shown in fig.1 of Part 1.. Between these two jets was a sidearm through which CO and O2 were added as required. At the upstream jet, Jl, N(4S) atoms in the discharged N2 were titrated with NO, so generating known concentra- tions of O(3P) atoms according to the stoichiometric and rapid reaction represented by the equation N(~S)+NO -+ N~+O(~P). The disappearance of visible emissions, associated with N+ N, N+ 0. or O+ NO combina- tions, in the tubing below J1 marked the equivalence point of the titration. The intensities of the bluish O+ CO chemiluminescence were measured at the observation points L1 and L2, at the entry and exit of the sphere respectively, using an RCA 1P28 photomultiplier viewing through an Oriel Optics G-774-3550 coloured glass filter as b e f ~ r e .~ The photomultiplier signals, displayed on a Pye Scalamp galvanometer, were proportional to [O] and had been intercalibrated to provide the oxygen atom decay parameter { - A[O]/([O]At)) as described b e f ~ r e , ~ where A[O] is the difference between the 0 atom concentrations at entry and exit, [O] here is the uniform 0 atom concentration in the sphere and At is the residence time of the gases in the sphere. Calibrated flowrates of H2 (<lo % of total flowrate) were added through the sidearms of the sphere. This and the other gases were purified and delivered as described Total pressures were in the range 0.2 to 0.5 kPa, measured using a silicone oil manometer balanced against running vacuum. Total flowrates were < 100 pmol s-' and the ratio of residence to diffusion time was always >5 to ensure stirred flow operation.2 The sphere was enclosed in an insulated box fitted with a heater and air circulating arrangement and the temperature used, 425 K, was measured using a thermocouple junction in contact with the external wall of the sphere.RESULTS AND DISCUSSION The temperature of 425 K was chosen since this allowed greatest flexibility in generating [H]/[O] ratios in the range 0.1 to 4 without H2 becoming a more than minor component in the stirred flow reactor. By varying the H2 flowrates in the range 0.4 to 7 pmol s-l, the reactions 0+H2 + OH+H (1) OfOH 3 0 2 f H (2) [with reaction (1) rate determining and pseudo-first order] achieved the partial conversion of O(3P) to H(2S) to an extent dependent only on [HJ.The presence of the small amounts of CO induced slight competition for OH radicals through the reaction (reaction numbering scheme continues from parts 1 and 2).** Our recent measure- ment of k2/kL8 = 260120 at 425 K allowed us to calculate values of a parameter CO+OH 3 COz+H (18)2674 O+H,+O, SYSTEM r = k,[O]/(k,[O]+k18[CO]) used in the analysis since [CO]/[O] was known. cycle of reactions studied in Part 2 The I-I+CO+M 3 HCO+M (19) H+HCO -+ H2+C0 (20) O+HCO 3 CO+QH O+HCO -+ C02+H was of little significance under our present conditions with [CO]/[O] < 40 but a small correction was applied using the values of k19, k20/k21 and k21a/k21b measured previ~usly.~ The reaction HCO+O,-+ CO+HO, has a rate constant only - 3 % of kal additional competition for OH radicals induced by reaction was corrected for on occasions of any significance by incorporation of a term kl2[H2] (21a)}(21) (2 1 b) and was, therefore, insignificant.The small OH+H, -+ H,O+H (12) into the denominator of r as b e f ~ r e . ~ The addition of O2 to the O+H2+N2 the decay parameter { - [AO]/([O]At)} as significant reactions in this respect are H+O,+M 3 H+HO2 3 O+HO2 system leads to a linear enhancement of was shown in fig. 3 of Part 1.2 The with OH radicals reacting in (2) subsequently. possible pathways, the others being Reaction (10) is one of three parallel H+HO2 -+ H2+02 H+H02 -+ H 2 0 + 0 . Reaction (22) exerts no influence on the 0 atom decay rate while reaction (23) will decrease it. But reactions (22) and (23) consume H atoms and so will decrease the [H]/[O] ratio compared to that in the system without 0, addition.The ozone- forming reaction will also occur in the system and will be followed by the reaction However, the rate constant k24 is only around 1 x lo8 dm6 mo1-2 s-l [ref. (6)] as opposed to k9 being around 1 x 1O1O dm6 mo1-2 s-I (see later), both for M = N2 at 425 K. Accordingly since [H]/[O] 0.1 in this work, reaction (24) can only make a minor contribution (< 10 %) to catalysed 0 atom decay in comparison with reaction (9). Thus a small correction can be made based on the known value of kz4 without introducing significant uncertainty. Analysis of the above mechanism yields the expression for the decay parameter O+02+M -+ 0 3 + M (24) H+03 4 OH+-,.(25) (ii)1 . M. CAMPBELL, J . S . ROGEKSON AND B . J . HANDY 2675 The subscript c denotes that the decay parameter has been corrected for the small effects of reactions (19), (20), (21) and (24) and G is evidently the gradient of a plot of (- A[O]/[O]At), against [O,]. The [H]/[O] ratio in the reactor is given by after correction for the minor effects of reactions (19)-(21). The [H]/[O] ratio for a particular experimental determination of G was taken as an average calculated by an iterative procedure applied to eqn (iii). Literature values and our own results (see later) suggested kg (M = N,) - 1 x 1O1O dm6 mo1-2 s-I at 425 K. Also existing literature values (see later) suggested 2 kl - klo + k22 +k23. These values were applied first, the resultant [H]/[O] was applied to eqn (ii) to generate an improved set of rate constants for reapplication in eqn (iii).The values of [H]/[O] converged rapidly to a constant value largely because the denominator term in eqn (iii) never exceeded 1.15 under our conditions ; it was particularly close to unity for the most crucial experiments with the lowest [H]/[O] ratios. For the main analysis of results it was useful to define two ratio parameters; RH = (k,o-k,3/2r)/(klo+k2,+k23) and R, = kll/(klo+k22+k23), the latter then expressing the relative reactivity of HO, radicals with 0 and H atoms. Eqn (ii) may then be expressed Under our conditions of [CO]/[O] < 40, the value of r was - 0.9 and thus 1 + Y and 2r are almost equal. The error involved in replacement of both by the average between the two, roughly weighted in one direction or the other depending on the value of [O]/[H), was considerably less than the experimental uncertainty.Thus division of the left hand side of eqn (iv) by this average value (- 1.85) then yields an apparent value of kg denoted by k,(app). From the measurements of G from the apparently linear plots of (- A[O]/[O]At), against [O,], when 0, was < 1 % of the total gases, a set of values of k9(app) was obtained for 0.16 < [H]/[O] < 3.46 and these are plotted in fig. 1. Strong upward curvature is evident at lower [H]/[O] ratios, reflecting the effect of reaction (11) between 0 and H02 in producing maximum efficiency of consumption of 0 atoms per H02 formed in the reactor. Above [H]/[O] = 1, it is clear that only about one half of the maximum efficiency is achieved by the then dominant reactions (lo), (22) and (23) between H and H02, which reflects the production of OH radicals in reaction (1 0), subsequently reacting with 0 atoms in reaction (2).We have attempted to fit the form of fig. 1 using sets of values of kg, RH and Ro. In the first place the highest values of kg(app) suggest that k g cannot be less than - 1 x lo1' dm6 mok2 s-l. Moreover realistic extrapolation of the trend of lc,(app) with decreasing [H]/[O] suggests an upper limit of 1.4 x 1010 dm6 mok2 s-'. At the highest values of [H]/[O] shown in fig. 1, the range of kg(app) is encompassed by (6.0k0.5) x lo9 dm6 mo1-2 s-l, which will closely approximate to kg RH. RH is, therefore, indicated to be 0.5+0.1 by combination with the above range of kg.The two full curves in fig. 1 appear to offer reasonable fits to the data : curve (a) is produced with values of kg = 1.2 x 1O1O dm6 mok2 s-l, RH = 0.45 and Ro = 0.25, while curve (b) is based on kg = 1.05 x 1 O 1 O dm6 mol-1 s-I, RH = 0.50 and R, = 0.50. However, the two dashed curves do not appear to be able to fit the data adequately.2676 O+H,+02 SYSTEM' Curve (c) is based on k9 = 1.4 x lO1O dm6 in01-~ s-l, RH = 0.4 and Ro = 0.1, while curve (d) has k9 = 9.5 x lo9 dm6 mol-2 s-l, RH = 0.6 and Ro = 0.6. Both (c) and (d) fail to fit adequately at the left side of the figure, passing below the entire set of highest values of k,(app). Curves (a) and (b) indicate R, to be in the range 0.2 to 0.5 at 425 K, with the marginally better fit of (a) suggesting a median value of 0.3.[HI/[Ol FIG. 1.-Plot of k9(app) against [H3/[0] showing calculated curves which attempt to fit the variation. The full lines are considered acceptable, the dashed lines unacceptable; for the parameters used, see text. Kurylo and Wong and Davies have made independent measurements of kg(M = N2) at 298 K and 220 K or 226 K using flash-photolysis resonance-fluores- cence techniques. Their individual results are in extremely close agreement [l0-l0kg/ dm6 mo1-2 s-l = 1.92 and 1.92 (298 K) and 3.16 (226 K) and 3.03 (220 K)]. A straightforward Arrhenius extrapolation to 425 K predicts k9 = 1.1 x 1O1O dm6 mo1-2 s-l, which must lend strong support to our above analysis yielding k9 = (1.2k0.2) x 1O1O dm6 rn01-~ s-l.There is considerable uncertainty in the literature on both relative and absolute values of the rate constants for H atom reaction with HOz radicals, klo, kzz and k23, and only one measurement of the rate constant for 0 atom reaction with H02, kll. A recent evaluation has taken into account all available kinetic data to generate Arrhenius expressions for the rate constants ; the resulting predictions for 425 K are k,, = 2.5 x 1O'O (2), k,, = 1.1 x lo1* (2.5) and k,, = 1.5 x !OIO (>3) dm3 mol-l s-l, with the uncertainty factors given in the brackets. The middle values combineI . M. CAMPBELL, J . S . ROGERSON AND B . J . HANDY 2677 to produce RH - 0.33, in reasonable agreement with our result of 0.5 kO.1 considering the scale of uncertainty in the absolute values.The range of uncertainty in the literature relative results can be illustrated by the values for the ratio k22/(k10 +k23) of 0.51 k0.21 and 1.63k0.30 lo which were derived in two discharge flow studies at room temperature; the middle values yield upper limits (assuming k23 = 0) for RH of 0.67 and 0.38 respectively. of kl = (2.1 & 0.8) x 1O1O dm3 mol-l s-l at 293 K was obtained using laser magnetic resonance detection of HO, in a fast flow system. The magnitude of all these rate constants for H02 destruction suggests that their temperature dependences will be small. Simple combination of the middle values given above (ignoring the different temperatures) leads to a prediction of Ro N 0.4, uncertain to a factor of 2.5 and unlikely to vary by more than a factor of 2 if all the temperature coefficients could be taken into account.Thus our placing of R, in the range 0.2 to 0.5 at 425 K is quite reasonable and considerably improves our knowledge of this ratio parameter. At the same time since the rate constants concerned are not much less than the collision frequencies, it might be expected that Ro would be related to the inverse ratio of the square roots of the reduced masses of 0 + H02 and H+ H02 collision complexes. This is in fact 0.3 which may lend support to our measured value. The recent and only direct measurement T. M. Donahue, R. J. Cicerone, S. C. Liu and W. L. Chameides, Geophys. Res. Letters, 1976, 3, 105. I. M. Campbell and B. J. Handy, J.C.S. Farahy I, 1975,71,2097. I. M. Campbell and B. J. Handy, J.C.S. Faraday I, 1978,74, 316. I. M. Campbell and B. J. Handy, Chem. Phys. Letters, 1977, 47,475. N. Washida, R. I. Martinez and K. D. Bayes, 2. Naturforsch., 1974, 29a, 251. Chemical Kinetic and Photochemical Data for Modelling Atmospheric Chemistry, ed. R. F Hampson and D. Garvin (N.B.S. Technical Note), no. 866, 1975. M. J. Kurylo, .I. Phys. Chem., 1972, 76, 3518. W. Wong and D. D. Davis, cited in ref. (7) as private communication. M. A. A. Clyne and B. A. Thrush, Proc. Roy. SOC. A, 1963, 275, 559. l o A. A. Westenberg and N. de Haas, J. Phys. Chem., 1972, 76, 1586. l 1 J. P. Burrows, G. W. Harris and B. A. Thrush, Nature, 1977, 267, 233. (PAPER 8/696)

ISSN:0300-9599    

DOI:10.1039/F19787402672

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Reactions of Propyne and Propadiene on Magnesium Films Part 1 .-Self- hydrogenation BY YVONNE GAULT* Institut de Chimie, UniversitC Louis Pasteur, 67000 Strasbourg, France Received 27th June, 1977 The self-hydrogenation of propyne and propadiene to propene on evaporated magnesium films has been studied at 373 and 423 K. The species retained in the solid state in the course of the reaction were then desorbed by deuterium oxide and characterized as deuterated hydrocarbons. The structures of these hydrocarbons (mainly propyne and propene) and the variations in their deuterium distributions with temperature and contact time of the previous self-hydrogenation reaction are consistent with a mechanism involving two parallel processes : (a) dehydrogenation of propyne and propadiene into metalated propyne CH3-C=C-Mg- (deuterolysed to ['HJpropyne), stable at 373 K but rapidly further dehydrogenated at 423 K to the magnesium carbide Mg2C3 (deuterolysed to [2H41propyne) ; (b) two-step hydrogenation of the reagent to propene, via half-hydrogenated intermediates, stable at 373 K and deuterolysed to [2Hl]propene.Little is known about interactions of hydrocarbons with non-transition metals in heterogeneous conditions. However, reactions of ethylene, acetylene and propyne at the surface of liquid alkali metals ' 9 and self-hydrogenation of alkynes and dienes on magnesium and other metal films have been reported, suggesting that non- transition metals are able to react at least with some hydrocarbons. The present paper describes more detailed investigations of the mechanisms of self-hydrogenation of propyne and propadiene on magnesium films.EXPERIMENTAL MATERIAL A N D PRODUCTS Magnesium from Johnson-Matthey of 99.997 % metallic purity was used. Propyne and propadiene (Matheson) were purified by preparative g.1.c. Tetradeuteropropyne (isotopic purity 98.7 %) was provided by Mr. A. Janin (University of Caen). Mono- deuteropropyne CH3-bCD containing 1 0 % C3H4 was prepared by exchanging the acidic H of [2Ho]propyne with NaOD at room temperature ; no isomerization was detected under these conditions. Deuterium oxide and perdeuteropropene (isotopic purities 99.7 and 99 % respectively) were obtained from Merck, Sharpe and Dohme. Light propene was purified by g.1.c. APPARATUS A N D PROCEDURE A grease-free static apparatus was used, attaining vacua of N m-2 provided by a mercury diffusion pump.Magnesium films of 35 to 70 mg and %YO0 cmz geometrical area were evaporated at 273 K from Mg chips held in a tungsten spiral on the inner walls of a cylindrical reaction vessel ( z 300 cm3). The surface area of magnesium films, as determined by krypton physisorption measurements at 77 K following the procedure of Kaganer? was found to be roughly the same as the geometrical area ( ~ 5 . 1 0 ~ ~ atoms cm-2). 1 Torr (133.3 N m-2, lOI9 molecules) of the reacting hydrocarbon was usually introduced on the freshly evaporated film at the reaction temperature (373 or 423 K). Connection of the reaction vessel, through an adjustable leak, to a quadrupole mass spectrometer allowing fast 2678Y .GAULT 2679 repeated scanning, was used for continuous analysis of the gaseous reaction mixture (C3H4, C3H6 and Hz). In the deuterolysis experiments, the reaction was stopped at the desired stage by rapid cooling to room temperature and the gaseous products removed for further analysis. After evacuation of the reaction vessel during 30-60 min at room temperature, 0.2 cm3 of outgased deuterium oxide was added to the reacted film at 77 K and allowed to react for 120-150 min at 300 K. Deuterium formed from unreacted magnesium was then eliminated and the hydrocarbon mixture removed for gas-chromatographic analysis and separation, and mass-spectrometric analysis. ANALYSIS (1) For continuous analysis of the gaseous reaction mixture (quadrupole mass- spectrometer, ionization energy 70 eV), the parent peaks at m/e 40 and 42 were used.The ions at mass m/e = 40 were then corrected for the contribution of propene, and the number of ions at m/e = 42 multiplied by the appropriate sensitivity factor, determined before each experiment. (2) The gas-chromatographic analysis of the reaction products was effected on a dimethyl- sulpholane column. The deuterated hydrocarbons arising from deuterolysis were isolated on the same column. Propene and propane were collected together on DMS and further separated on a silica column. The chromatographic purity of the various molecules was checked before and after mass-spectrometric analysis. (3) The deuterated molecules were analysed with a Varian-Matt CH7 mass spectrometer operating under high resolution conditions (m/Am = 3500).Positive ion mass spectra (parent peaks) were obtained using an ionization energy of 70 eV. The deuterium distribu- tion of each deuteromolecule was obtained after corrections of the mass spectra for the natural abundance of I3C, and fragmentation corrections made on a statistical basis using the fragmentation patterns of the corresponding light and heavy molecules (determined before each analysis). Deutero-cis-butenes distributions were calculated according to the fragmentation patterns established by Touroude and Gault .5 RESULTS GASEOUS REACTION PRODUCTS Reactions of propyne or propadiene on magnesium films result in the formation of various gaseous products, which are : (i) at any temperature (273-423 K), the isomer of the reacting hydrocarbon and propene ; (ii) at 373-423 K, propane and c6 hydrocarbons.At 423 K, small amounts of hydrogen were also detected. (a) ISOMER OF THE REACTING HYDROCARBON Propyne-propadiene interconversion is fast in the range of temperature investigated (273-423 K). The equilibrium compositions are usually reached in <20 min at any temperature. They include 90 % propyne at 300K, 87 % at 373 K, 85 % at 423 K, according to the results of Cordes and Giinzler.6 (b) PROPENE The rate of propene formation at 273 or 300 K is too slow and irreproducible to allow accurate studies ; therefore the present study was carried out systematically at 373 and 423 K. Fig. 1 shows the variations with time of the gas phase concentrations of propyne and propene. Whereas propyne exhibits a sharp initial decrease, propene appears in the gas phase after an induction period of a few minutes. This figure does not reflect the stoichiometry of the over-all reaction, since the C6 hydrocarbons were not 1-852680 PROPYNE AND PROPADIENE REACTIONS ON Mg taken into account, but suggests that under the conditions used propyne is preferen- tially adsorbed.Increasing the pressure of propyne has no marked influence, while the rates of propyne disappearance and of propene evolution are film weight- and t emperat ure-dependent . 15 W z 2 2 $10 a w .3 I 5 - 4 - 3 - 80 60 40 20 20 40 60 80 - - time/min FIG. 1.-Production of propene from propyne. The composition of the gas phase is expressed as mol percent of propyne and propene. The initial pressure of propyne is 133.3 N m-z ( X l O I 9 molecules). A Propyne, A propene (reaction temperature 373 K, film weight 38 mg) ; 0 propyne, 0- propene (reaction temperature 423 K, film weight 38 mg) ; Cl propyne, propene (reaction temperature 423 K, film weight 66 mg).I 1 I 1 I I 20 40 60 80 100 timelmin F~~.&.-Variations with time of the ratio ANC~H~IANC~H~, where A N c ~ H ~ is the decrease in the number of molecules of gaseous C3H4, and a N ~ 2 ~ 1 - r ~ the related increase in the number of molecules of gaseous C3Hs. A Reaction temperature 373 K, film weight 38 mg ; 0 reaction temperature 423 K, film weight 35 mg ; 0 reaction temperature 423 K, film weight 59 mg ; 0 reaction tempera- ture 423 K, film weight 70 mg.Y , GAULT 2681 Similar curves were obtained for the reaction of propadiene.However, because of the very close fragmentation patterns of both C3H4 isomers, they do not give information about the propyne-propadiene composition as a function of time. Fig. 2 reports the variations with time of the ratio between the number of C3H4 molecules which have disappeared (ANc3H4) and the number of C3H6 molecules evolved in the gas phase (ANc3Ha). The shapes of the curves are temperature- dependent, whereas good reproducibility is observed at a given temperature whatever the weight of the film might be. After a sharp initial decrease, the curves reach more or less rapidly a stationary value, close to 3 at 423 K and 4 at 373 K; this means that after an induction period, 3 or 4 molecules of the reagent are consumed per molecule of propene evolved.(C) PROPANE Propane is not detected at low reaction temperatures, but accounts roughly for one-tenth of the propene at 373-423 K. It arises, presumably, from a weak self- hydrogenation of propene on magnesium, as shown by control experiments with propene. (d) c6 HYDROCARBONS These are not detected at low temperatures. They are branched hexens and hexadiens (mainly 2,3-dimethyl-l-butene and 2-methyl-l-pentene), amounting to 5-40 % of the propene, according to temperature and contact time. In addition, tiny amounts of other hydrocarbons (i.e. isobutene, isopentenes), assigned to the cracking of the C 6 hydrocarbons, were occasionally detected. (4) HYDROGEN Slight amounts of hydrogen were present in the reaction mixtures at 423 K.Since identical amounts of hydrogen are evolved from a fresh magnesium film heated at the same temperature, they are believed to arise from gases dissolved in the bulk metal and not completely evacuated during film evaporation. Indeed, only traces of D, and HD are obtained from tetradeuteropropyne at 423 K. Moreover, dissolved hydrogen does not participate in propene formation, since the reaction of a 20/1 mixture of deuterium and [2Ho]propyne gives pure [2Ho]propene. DEUTEROLYSIS EXPERIMENTS Addition of water or deuterium oxide on the film after reaction results in the desorption of a mixture of hydrocarbons. Chromatographic analysis of some hydro- or deutero-lysates are reported in table 1. It is seen that (i) very similar mixtures are formed from propyne and propadiene ; (ii) C3 hydrocarbons account for 75-90 % of the total products; (iii) depending on the temperature of the previous reaction, either propyne or propene is the mean product.Besides 2,3-dimethyl-l-butene (2,3DMlB), other c6 are present in trace amounts. Use of deuterium oxide results in formation of deuterated hydrocarbons. Deuterium distributions of propyne and propene are reported in tables 2 and 3. Depending on the conditions of the previous reaction, the distribution pattern of deuteropropynes has a maximum at C2Hl], a maximum at [,H4], or two maxima at [,HI] and [2H4], while the corresponding maxima in deuteropropenes are [2H,], [,€I6] or [2H1] and [,H6]. Propane, when present in sufficient amounts (previous reaction at 423 K) was collected and analysed: it was found in every case to be2682 PROPYNE AND PROPADIENE REACTIONS ON Mg mainly [2H,].No attempts were made to determine the deuterium contents of acetylene and propadiene on account of their low concentrations. 2,3 dimethyl-l- butene will be described in our next paper. TABLE 1 .-PRODUCT DISTRIBUTIONS IN HYDRO- AND DEUTERO-LYSATES OF REACTED MAGNESIUM FILMS (GAS-CHROMATOGRAPHIC DATA) expt. no. 439 497 460 396 506 reagent propyne propadiene propyne propadiene propyne previous 373K 373K 423 K 423 K 423 K reaction 1 90 min 90 min 90 min 30 min 180 rnin products % propane 3 3.7 14.6 17.1 17.3 propene 43.6 47.9 23.4 19.4 11.8 acetylene 0.9 0.8 1.7 1.9 4.8 propadiene 3.3 - 1.3 3.3 1.8 ProPYne 31.7 31.1 39.2 49.4 45.4 23DMlB 20.8 16.5 19.6 8.9 18.8 TABLE 2.-oBSERVED DISTRIBUTIONS OF DEUTEROPROPYNES IN DEUTEROLYSATES expt.no. 500 439 497. 460 396 429 506 reagent propync propyne propadiene propyne propadiene propadiene propyne reaction cond. 373 K 373K 373K 423 K 423.K 423 K 423 K 20min 90min 90min 20min 30mm 180 rnin 180 rnin % (a) (b) 31.7 31.1 39.2 49.4 (b) 45.4 [2H*I - 7.7 8.4 7.7 0.2 2.4 [2H11 58.9 54.6 43 45 38.5 12.8 15.3 IZH21 5.3 11.1 11.7 9.7 10.7 14.3 8.6 L2&1 5.8 11.8 4.5 4.1 22.9 29.8 13 w 4 1 30 14.9 32.3 33.4 27.6 40.7 63 a '' % " refers to the percent of total propyne among total deuterolysis products, as determined - by g.1.c. (cf. table 1) ; b not determined. TABLE 3 .-OBSERVED DISTRIBUTIONS OF DEUTEROPROPENES IN DEUTEROLYSATES expt. no. 500 503 439 497 460 506 reagent propyne propadiene propyne propadiene propyne propyne reaction cond.373 K 373.K 373 K 373 K 423 K 423 K 20 mm 20 mln 90min 90min 20min 180rmn % (4 (b) 33 43.6 47.9 23.4 11.8 12 11.7 19.3 - 12.9 - [2H11 58.2 51 47.4 51.4 48.2 13.4 [%I 7 17.3 10.8 22 6.3 15.2 [%I 9.3 5 9.5 5.7 4.4 4.6 rH41 4.2 1.1 2.1 4.9 1.8 3.6 12H51 2.3 1.2 5.7 3 2.7 13.4 [2H61 7 12.6 5.1 13.1 23.6 49.7 a " % " refers to the percentage of total propene among total deuterolysis products, as determined "Hol by g.1.c. (cf. table 1) ; b not determined. CONTROL EXPERIMENTS A number of control experiments were carried out in order to check the validity of the results reported in tables 1-3. (a) Hydrolysis of fresh Mg films yields traces of propyne and acetylene together with the corresponding olefins. These hydrocarbons undoubtedly arise from traces of Mg carbides contained in the metal.However, the amounts of hydrocarbonY . GAULT 2683 formed in this way are x500 to 1000 times smaller than the ones desorbed by hydrolysis of the reacted films, and therefore do not perturb the results. (b) In the absence of a Mg film, heavy propyne C3D4 exchanges one single D atom (presumably the acidic one) with H even at room temperature, most probably with traces of water remaining adsorbed on the glass, as is suggested by enhanced exchange in the presence of added water. Neither isomerization nor hydrogenation occurs in the absence of a Mg film in the range of temperature investigated (300-423 K). The following experiments [ (c)-(g)] were effected under the same experimental conditions (room temperature, 120-150 min) as in the deuterolysis of the reacted films : (c) In the presence of a Mg film previously isolated and heated for 1 h at 423 K, light propyne C3H4 exchanges one single H atom with deuterium oxide.Isomeriza- tion and deuteration into propene occur simultaneously. Propyne (68 % of the TABLE 4.-EXCHANGE OF LIGHT AND HEAVY PROPYNES AT 298 K ON MAGNESIUM FILM AFTER PREVIOUS REACTION OF LIGHT PROPYNE FOR 1 h AT 423 K initial composition final composition 46 14.5 - 23.4 - 18.2 [2H31 2.4 30.8 E"H41 51.6 13.1 ['Hal ['&I ['H21 TABLE 5.-EXCHANGE OF A MIXTURE OF LIGHT AND HEAVY PROPYNES AT 298 K WITH DEUTERIUM OXIDE ON MAGNESIUM FILM PREVIOUSLY HEATED FOR 1 h AT 423 K initial composition final composition 52.1 11.8 - 26.4 ['Hd ['I311 17.1 15.9 ['Hi] 45.6 28.5 reaction mixture) was recovered as 16 % [2H,] and 84 % [2Hl], while propene (27 % of the reaction mixture) was found to be mainly [2H2].The total absence of [2H3] propene suggests that deuteration of [2H,] propyne is faster than its exchange. (a) Under the same conditions, ['H0] propene remains unchanged and unex- changed. (e) On a Mg film previously reacted with propyne at 423 K (reacted Mg films are unsuitable for self-hydrogenation of fresh doses of reagent), a synthetic mixture of C3H4 and C3D4 interchanges all H and D (table 4), showing that the exchange is not limited to the H and D in acidic positions. (f) Under the same conditions, no exchange occurs between C3H6 and C3D6. (9) On a Mg film previously isolated and heated for 1 h at 423 K, a mixture of C3H4 and C3D4 was converted, in the presence of deuterium oxide, into a mixture of all deuteropropynes with 2 maxima at [l2H1] and [2H4] (table 5), as expected from superposition of (i) exchange of [2Ho] propyne with D20 to [2Hl], (ii) interchange of light species with [2H4].The resulting mixture reproduced approximately some of the propyne distributions reported in table 2. EXPERIMENTS USING LABELLED PROPYNES (a) Self-hydrogenation of monodeuteropropyne CH3-C=CD was investigated at 373 K (1 h) and at 423 K (3 h). The distributions of the alkynes before and after2684 PKOPYNE AND PROPADIENE REACTIONS ON Mg reaction and of the resulting propenes are given in table 6 . The residual gaseous reagents were recovered with some change in the H-D distributions, involving both a slight decrease in the total D/H ratio, and a slight scrambling of the H and D atoms.The resulting propenes were mainly [2H1], E2H2] and [2H3]. Species I2H0] was not detected, and species ['H4] and [2H5], which accounted together for < I % of the total propenes, are not reported in table 6. The observed distributions are compared with those calculated by assuming the random addition on C3H3D of D-H pools of 53-47 and 36-64 for the propenes formed at 373 and 423 K, respectively. TABLE 6.-sELF-HYDROGENATION OF CH3-CsCD (a) self-hydrogenation at 373 K (1 h) propyne propyne propene propene (miQa1) (final) (obs) (calc) 9 20.8 - - 91 73.5 19.1 22.1 - 4.7 55.6 49.8 - 1 25.3 28.1 - - 0.29 0.27 0.52 0.52 (6) self-hydrogenation at 423 K (3 h) propyne propyne propene propene (mtial) (final) (obs) (calc) C2H0] 10.5 31.1 - - ['Hi] 89.5 60.7 39.9 41.1 [2H21 - 6 48.5 46 [%I - 2.1 11.6 12.9 ['&I - I - - - - [%I L2H61 - - D/H 0.31 0.24 0.40 0.40 (b) Reaction of a 1 : 1 mixture of heavy propyne C3D4 and light 2-butyne C4H6 on a magnesium film for 45 min at 423 K was followed by isolation and mass- spectrometrical analysis of the residual 2-butyne and of the cis-2-butene (75 % of the total butenes) arising from the latter.Deuterium distributions of both C4 hydro- carbons are reported in table 7. More than 95 % of the 2-butyne was recovered as [2Ho], whereas cis-2-butene exhibited a much higher deuterium content, the main species being E2H0], C2HJ and ['H2] in increasing amounts. TABLE 7.-oBSERVED DISTRIBUTIONS OF RESIDUAL 2-BUTYNE AND Cis-2 BUTENE AFTER REACTION OF A MIXTURE OF f2&]PROPYNE AND ['H&2*BUTYNE ON A MAGNESIUM FILM (45 min AT 423 K) t2Hol I*Hd 12H21 PH31 PH4J PHs1 [*H6] PH71 I2HsI - - 2-butyne 95.5 3.3 0.9 0.3 cisZbutene 6.9 33.6 55.7 2 0.8 0.5 0.3 - - DISCUSSION As shown in fig.1, propene accounts for only a minor part of the reacted propyne or propadiene. Since the C6 hydrocarbons account at most for 40 % of the propeneY . GAULT 2685 (after long reactions at 423 K), an appreciable fraction of the reagent therefore remains on the magnesium fdm, probably as species including C-Mg bonds. As shown in table 1, these species are recovered after deuterolysis mainly as C3 hydrocarbons. Their structures and the variations in their deuterium distributions with temperature and contact time of the previous reaction (tables 2 and 3) support the general reaction scheme suggested below (scheme 1) in which part of the reagent is dehydrogenated and another part hydrogenated to propene.The dehydrogenation and hydrogenation processes will be discussed in turn. The additional hydrodimeriza- tion reaction, giving mainly branched hexenes, will be developed further in a future paper. + I + 1': H CH3 I C 111 CH2=CH-CH2 + CH2=C--CH3 c + H I I I I * * * I I * 1 1 Mg2C3 + 3H * (SCHEME 1) DEHYDROGENATION As shown in table 2, propyne in the deuterolysates consists mainly of species C2H1] and E2H4], in various ratios, depending on the conditions of the previous reaction. Since species [2H2] and [2H3] may result from exchange between lighter and heavier species, as shown by control experiments, they will not be further discussed.[2H4] propyne, which predominates after long contact times at 423 K, may be assigned with high probability to the magnesium carbide Mg,C3 , whose hydrolysis is known to supply mainly propyne with small amounts of propadiene.8-f0 The magnesium carbide Mg2C3 is the expected product of complete dehydrogenation of propyne or propadiene. Parallel formation of small amounts of the other magnesium carbide MgC2 is suggested by the presence of acetylene in the hydro- or deutero- 1 ysates. O Partial deuteration of [2H,]propyne into [2H6]propene and [2H,]propane in the course of deuterolysis is suggested by the observed correlation between the deuterium contents in the three C3 hydrocarbons : high contents in all three perdeuteromolecules were obtained after long reactions at 423 K, while the yields of [2H4]pr~pyne, f2H,]propene and total propane were low after the reactions at 373 K.2686 PROPYNE A N D PROPADIENE REACTIONS ON Mg Analogous formation of the alkali metal carbides Na,C, or K2Cz has been suggested by Pulham et al." to account for the stoichiometry of the reactions of acetylene and ethylene respectively on sodium and potassium surfaces, whereas reaction of propyne at 383 K on sodium (which does not form a carbide similar to Mg,C,) was assumed to yield metalated propyne CH,-C-CNa as sole solid product, as in the corresponding reaction in liquid ammonia.There is no report of isolation of magnesium salts of 1-alkynes. However [2H,]propyne, which predominates after all reactions at 373 K and after short reactions at 423 K, could be assigned to the deuterolysis product of dissociatively adsorbed or metalated propyne CH3-C-C-Mg--.Such a species would account for the very close compositions of the hydro- or deutero-lysates obtained from propyne and propadiene. One may assume indeed that dehydrogenation of propadiene on magnesium involves its conversion into metalated propyne, in the same way as allenic hydrocarbons transpose into the corresponding metalated 1 -alkynes in the course of their reactions with sodium or sodium amide,ll* l 2 or at the surface of non-transition metal oxides.' The formation of this dissociatively adsorbed propyne, stable at 373 K, could be, at 423 K, the first step in the rapid dehydrogenation process whose the final step is the carbide Mg,C,.However, this interpretation is weakened by the easy exchange with deuterium oxide of the acidic H atom of propyne : one could object that [2H,]propyne in the deuterolysates may arise from the exchange of [2H,]propyne. Self-hydrogenation of CH3-C=CD was investigated in order to obtain evidence for the occurrence of a metalation process. This molecule indeed is the best model to distinguish between acidic and methylic hydrogen atoms, and if, under some specific conditions, metalation does occur, not followed by carbide formation, one should observe then the exclusive participation of the acidic hydrogen atoms in the self-hydrogenation process. Although C3H3D was partly exchanged in the course of the reactions, as shown by the distributions of the recovered propynes, the distributions of the propenes obtained at 373 and 423 K are best interpreted by assuming the statistical additions on C3H3D of D-H pools corresponding to 53 and 36 % of D respectively (table 6).Were the hydrogenation process to involve exclusively the acidic D atom of C3H3D, only [2M3]propene should be observed. On the other hand, the participation, resulting from carbide formation, of all the H and D atoms would yield a randomized distribution of propene corresponding to the addition of a 25-75 D-H pool. The observed results correspond therefore to intermediate but unequal situations : at 373 K the acidic D atom is the major, but not the only, source of hydrogenating atoms ; at 423 K the hydrogen source is much more indiscriminate.These labelling experiments provide therefore a good support for a progressive dehydrogenation process as represented in scheme 1. One could discuss at this stage whether the formation of carbide and metalated species is a surface reaction or involves the bulk metal. Although the accurate material balance could not be established, one can estimate that at 423 K roughly 50 % of the reacting molecules are recovered as gaseous C3 and C6 hydrocarbons, and hence that an equivalent number (m 5 x 10' *) is retained as carbonaceous or hydrocarbonaceous residues. Since the number of superficial magnesium atoms, as determined by krypton physisorption measurements, is m these estimates suggest that = 50 magnesium layers are probably involved for metal- ation and carbide formation.Therefore the dehydrogenation reaction may be considered not as a surfaceY. GAULT 2687 reaction, but as a solid state reaction, and it is also tempting to consider the hydrogena- tion process as a true chemical reaction between magnesium hydride and propyne or propadiene. HYDROGENATION The hydrogen atoms released by the dehydrogenation process do not recombine and evolve as molecular hydrogen; therefore, they may be supposed to be temporarily retained as labile Mg-H species and to act for hydrogenation. The Mg-H bond of magnesium hydride has been shown to hydrogenate a variety of organic compounds, among which are unsaturated hydrocarbons,14* l5 so that the over- all self-hydrogenation process may be considered as the hydrogenation of a part of the reagent by magnesium hydride produced by the dehydrogenation of the other part.These H atoms can be transferred onto molecules different from the parent ones, as shown by the cross-reaction between [2H,]propyne and [2Ho]-2-butyne, in which the deuterium atoms arising from the first are transferred onto the second, which is directly converted into deutero-butenes without previous exchange (table 7). On magnesium, as on transition metals, the hydrogenation of alkynes and allenes to alkenes probably involves the stepwise addition of two H atoms with the inter- mediacy of half-hydrogenated states. * Two-step addition of H atoms onto propyne and propadiene to form propene is strongly suggested by the presence of [2H,]propene in the deuterolysates. This species, which cannot result from any exchange or deuteration, is best explained as resulting from the deuterolysis of half-hydrogenated states of propyne or propadiene, C3H5-Mg-, which may be considered also as insertion adducts of propyne or propadiene into Mg-H bonds.Whereas the half-hydrogenated states involved in catalytic hydrogenation on transition metals are highly unstable, a number of stable a-alkenyl metal complexes resulting from insertion of multiple C-C bonds into M-H bonds have been isolated. l Concerning the half-hydrogenated species (or insertion adducts) formed on magnesium films, their life-time appears to be long enough to allow their characteriza- tion by deuterolysis in the form of [2H,]propene.Moreover, results reported in table 3 suggest that at 373 K, in the course of the hydrogenation process, a large fraction of the reaction molecules remains frozen out on the magnesium film as half-hydrogenated species. That suggests that the addition of the second hydrogen atom required to form propene from the half- hydrogenated states occurs more slowly than the addition of the first one. The different rates of addition of the two hydrogen atoms could also explain the induction period required for the appearance of propene in the gas phase as shown in fig. 1 . GENERAL REACTION SCHEME The overall results related in this paper are consistent with the general scheme 1, where two additional assumptions are included : (a) existence of a direct hydrogena- tion pathway from propadiene to propene, (b) existence of two different half- hydrogenated species of propyne and propadiene, arising from the addition of one hydrogen atom either on the central or on the terminal carbon atom of the reacting hydrocarbons.Although no result in the present work argues for these assumptions, the first is suggested by the fact that self-hydrogenation of 2-butyne on magnesium films produces mainly cis-Zbutene, while 1-butyne gives 1-butene and 1,2-butadiene a mixture of2688 PROPYNE AND PROPADIENE REACTIONS ON Mg 1- and cis-2-butenes, showing that the structures of the olefins formed reflect the primitive structure of the reacting hydrocarbon rather than the equilibrium composi- tion of the C4H6 isomers.20 The second assumption, previously suggested by Yoshida and Hirota for propyne,17 is supported by the carbon skeletons of the main C6 hydrocarbons (2,3-dimethyl-l-butene and 2-methyl-l-pentene), for which the half-hydrogenated species of propyne and propadiene are the most consistent precursors.CONCLUSION The present work suggests that, under specific conditions, some reactions may occur on magnesium films which are not observed with magnesium when conventional procedures and solvents are used. Moreover, it shows that magnesium films, although not able to activate molecular hydrogen or deuterium, are able to use hydrogen or deuterium atoms produced in situ t o hydrogenate alkynes or dienes to alkenes. The observed reactions of propyne and propadiene on magnesium films may be paralleled with the following : (a) reduction of alkynes and allenes by alkali metals, which give the corresponding alkene and the metalated 1-alkyne ; (b) hydrogenation of unsaturated hydrocarbons on transition metal catalysts, which occur with the intermediacy of half-hydrogenated states ; (c) insertion of alkynes into metal- hydrogen bonds, in homogeneous catalysis, which give a-alkenyl adducts.We are indebted to Mr. A. Janin (University of Caen) for gift of a sample of tetradeuteropropyne, and to Mr. J. J. Ehrhardt (Centre de Cinetique Physique et Chimiqwe, C.N.R.S., Nancy), for help in krypton physisorption measurements. C. C. Addison, M. R. Hobdell and R. J. Pulham, J. Chem. SOC. A, 1971, 1704, 1708. G. Parry and R. J. Pulham, J.C.S. Dalton, 1975, 2576. Y . Gault, J.C.S. Chem. Comm., 1973, 478. M. G. Kaganer, Zhur. jiz. Khim., 1959, 33,2202. R. Touroude and IF. G. Gault, J. Catalysis, 1974, 32, 294. J. F. Cordes and H. Gunzler, Chem. Ber., 1959, 92, 1055. M. Hock, Bull. SOC. chim. France, 1963, 1422. R. C. Lord and P. Venkateswarlu, J. Phys. Chem., 1952, 20, 1237. J. F. Cordes and K. Wintersberger, 2. Naturforsch. 6, 1957, 12, 136. lo W. H. C. Rueggeberg, J. Amer. Chem. SOC., 1943, 65, 602. l1 (a) A. Faworskii, J. Prukt. Chem., 1888, 37,417 ; (6) A. Behal, Bull. SOC. Chim. France, 1888, l2 (a) M. Bourguel, Ann. Chim., 1925, HI, 205, 345 ; (6) M. Bouis, Ann. Chim., 1928, Tx, 459. l3 (a) J. Saussey, J . Lamotte, J. C. Lavalley and N. Sheppard, J. Chim. phys., 1975, 71, 818; l4 W. K. Henle and E. J. Smutny, U.S. Patent 3, 666, 416 (30/5/72); Chem. Abs., 1972, 77, 50, 629. (b) J. Saussey, J. C . Lavalley and N. Sheppard, J. Chim. phys., 1977, 74, 329. 64156~. L. H. Slaugh, J. Org. Clem., 1967,32, 108. l6 E. F. Meyer and R. L. Burwell, J. Amer. Chem. SOC., 1963,85,2881. l7 N. Yoshida and K. Hirota, Bull. Chem. SOC. Japan, 1975, 48, 184. (a) J. Grant, R. B. Moyes, R. G. Oliver and P. B. Wells, J. Catalysis, 1976, 42,213 ; (6) R. G. Oliver and P. B. Wells, J. Catalysis, 1977, 47, 364. l9 See for example : (a) K. Ziegler, Angew. Chem., 1956, 68, 721 ; (b) H. C. Brown, Organic Synthesis via Boranes (Wiley, N.Y., 1975), p. 10 ; (c) J. Trocha-Grimshaw and H. B. Henbest, Chem. Comm., 1968, 757; (d) J. A. Labinger and J. Schwartz, J. Amer. Chem. Soc., 1975, 97,1596 ; (e) D. W. Hart, T. F. Blackburn and J. Schwartz, J. Amer. Chem. Soc., 1975,97,79. 2o Y. Gault, unpublished results. (PAPER 7/1120)

ISSN:0300-9599    

DOI:10.1039/F19787402678

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Oxidation of Sulphur Dioxide in Aerosol Droplets, Catalysed by Manganous Sulphate B Y PETER W. CAINSt AND MICHAEL D. CARABINE* Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2AY Received 4th July, 1977 Experiments are described in which the changes in size distribution of water droplets in an aerosol were measured when they were exposed to humid air dosed with sulphur dioxide, such that manganese- catalysed oxidation occurred in the droplets. The size changes have been compared with calculations (a) of the growth induced by reaction and subsequent water condensation, and (6) of the changes produced by coagulation. The results are in good agreement when the most recently revised values of the rate constants of controlling steps are used.The initial rate of oxidation is related to the overall concentrations of SO2 and manganese in the aerosol. The conditions in the experiments were chosen to be similar to those a few metres from the chimney mouth in a typical industrial stack plume. Both coagulation and growth contributed significantly, and sulphuric acid production was substantially complete in 10-20 min. Light scattering size analysis was successfully applied to these rather polydisperse aerosols (cro - 0.5) of small modal diameter (UM < 0.3 pm). The object of the experiments described in this paper was to observe the rates of size change in aqueous aerosol droplets with diameters in the range 0.1 to 1 pm, as a consequence of their being exposed to humid air containing sulphur dioxide.The droplets were generated from dilute solutions of manganous sulphate and hence the oxidation of sulphur dioxide to sulphuric acid was catalytically enhanced. The results of the experiments were then compared with calculations using a model based on homogeneous liquid-phase oxidation, together with growth by condensation of water, with liquid-vapour equilibrium maintained at all stages, and with coagulation of droplets according to the theories of Smoluchowski. Conditions such as the droplet number concentrations, the initial sulphur dioxide content and humidity of the air, and the pressure and temperature, were chosen to resemble those in a stack-plume from a power-station burning sulphur-containing fuel. as a rapid route for sulphate production and for gas-to-particle conversion in such plumes ; in the present experiments the content of manganese (a particularly active catalyst) was matched to some values observed in the gases arising from ~oal-burning.~'~ Metal-catalysed oxidation in droplets is regarded 1* MODEL OF DROPLET GROWTH ARISING FROM ABSORPTION AND OXIDATION OF The possible rate-limiting processes for the growth of the droplets in this system would be (a) diffusion of SO2 to the droplets, (b) oxidation of SOz at the surface or within the droplets, (c) diffusion within the droplets of the sulphuric acid and the water and (d) diffusion of water vapour to the droplets.It has been predicted 7 Present address U.K.A.E.A., Harwell, Oxfordshire OX1 1 O M . 26892690 CATALYSED OXIDATION OF so2 I N AEROSOLS theoretically 6 s ' that the liquid and gas-phase diffusion processes (c) and (d) will reach completion in about 5 x s respectively, when droplet diameters are around 0.5 pm. Because liquid phase oxidation has been shown in experiments with bulk solutions to be complete only in times of around thirty minutes, processes (c) and (d) are considered unlikely to be rate-determining in the small droplet case.Process (a) may be eliminated by analogy and, therefore, we postulate that oxidation within the droplets is rate-limiting. There is some evidence that homogeneous reaction rate and liquid-phase diffusion are both important with droplets up to 1 mm in diameter. Proceeding with this hypothesis of rate control by the homogeneous reaction, the concentrations of all components will be uniform, at any given time, throughout the liquid phase of the aerosol, maintaining equilibrium with the gas-phase concentrations.The droplet sizes considered here are invariably too large for curvature to produce variations in equilibrium vapour pressure, and hence in concentration, from droplet to droplet. The condensed phase may, therefore, be considered as a homogeneous reaction volume for the purposes of determining the rates of increase in droplet volume (u). If this rate is denoted by dvldt, then and 2 x dv - = u l ( t ) dt where I ( t ) is a function of time only, defined by this equation. As to the distribution of sizes, discussion will be restricted to a log normal distribution function of the Z.O.L.D. type described by Espenscheid et al.,'O which has been found to be a good description of the size distributions of aerosols generated experimentally.llp l 2 The parameters of this function are aM the modal diameter, and co a measure of the spread of the distribution.Integration of eqn (1) leads to the result that the time dependence of sizes in the Z.O.L.D. distribution is given by the value of o0 remaining constant. Droplet diameter changes may be calculated from eqn (2), but first the reaction kinetics and the vapour-liquid equilibria of the system, which together determine I(t), are to be considered. KINETICS OF THE REACTION I N SOLUTION The reaction mechanism chosen for the kinetic calculations is essentially that of Bassett and Parker,8 in which there are four main stages : (1) Formation of a complex ion between Mn2+ and some anionic species derived from SO2 (e.g., SO$-, HSO; or S20g-) Mn2+ +2SOZ- + { Mn(S03)2}2-. (2) The addition to this complex ion of 02, taken to be present in some hydrated or complexed state, { Mn(S0,)2}2- + O2 + 02(Mn(S03), j2-.(3) Rearrangement, considered to be comparatively rapid, 02{ Mn(S03),j2- + {Mn(S0,)2)2-. (4) Release of the sulphate product, with sulphite ions displacing it from the complex formed in stage (3).P . W. CAINS AND M. D. CARABINE 269 1 This mechanism has been used to explain most of the qualitative features observed in experiments with bulk solutions, including the suppression, in the case of Mn2+, of the dithionate ion which is produced with some other catalytic cations.* Support for the mechanism comes mainly from the variations in the reaction rates and in the final products when various catalyst salts are used at various concentrations; as far as identification of the complex intermediates is concerned, the extraction in ether from the system of a compound of empirical formula MnS04.SO2 has been reported,13 but no evidence of application of modern analytical techniques (such as e.s.r. spectroscopy) has been found in the literature. Matteson, Stober and Luther l4 attempted to model the reaction kinetics using a scheme which is basically similar to that above, but which is expressed in terms of complexes containing neutral SO2 and SO3 molecules, as opposed to the (in our view more likely) anionic ligands of the first mechanism. The stages proposed are as follows : Mn2+ + SO2 + (MnS02}2+ (1) ki k2 k3 2(MnS02}2++02 + {(MnS02)202}2+ k4 k s ((MnS02)202}2+ + 2(MnSO3I2+ k5' k6 k7 (3) (4) (MnS03)2++H20 + Mn2++HSOi+H+.The most fundamental simplification in their analysis was the assumption that the rate limitation of the reaction is due to stages (1) and (4), and the scheme is thus equivalent kinetically to that of Bassett and Parker. For the low concentrations of SO2 considered, the concentration of oxygen is relatively high, making the rate of stage (2) unimportant. It is convenient to use the following parameters defined by Matteson et al. for their analysis. The total concentration of intermediates (moles per unit volume liquid) was expressed as the sum (XI, = (MnSO,)?' ++((MnSO2),O2)2,+ + {MnSO,);' and the relative concentration of each was assumed to remain constant as reaction proceeds.Two pseudo rate constants were defined as follows k2 (MnS O2 12' k6(MnS03)i+ i H 2 0 ) L k2. = kgf = (XI, (XI, RESULTS OF CALCULATIONS AND CHOICE OF RATE CONSTANTS In the present approach to the problem the mechanism of Matteson et al. has been adopted, and numerical solutions of their eqn (15), (35) and (36) have been evaluated giving the variation with time of the concentrations of SO2, H2S04 and intermediate X. The initial condition was (X}, = 0 ; two distinct sets of values of the rate constants were used, with contrasting results. The first set of values of k l , k2', kgP and k7 were those explicitly derived by Matte- son et al., namely k,/mol-l dm3 min-l k2 ./min-l k6./min-l k7/mol-2 dm6 m i d The results using these values are exemplified, for a given set of starting conditions, in fig.1. 2 . 4 ~ 105 10 0.22 352692 CATALYSED OXIDATION OF so2 I N AEROSOLS Secondly, a set of values was selected as follows : k6# and k7 were obtained from other theoretical work,15 kl was evaluated by fitting the data of Johnstone and Coughanowr to the equation -- d{S02'v = kl{MnS04)& dt where (SO,}, is the gas-phase concentration (moles per unit volume in gas phase), and the subscript zero denotes the value of (MnSO,), at zero time. The latter is valid for (MnSO,), < Hsoz x (S02)v,o, where Hso2 is a Henry's Law constant. Finally kzt was evaluated from eqn (21) of Matteson et al., since kl, k69, Ks and Hsoz are all known. 64 62 d d -e 60 0 .- Y E 58 Y 0 g 5 6 8 2 54 timelmin FIG.1.-Solutions to kinetics of oxidation using rate constants of Matteson et aZ.I4 and eqn (15). (35) and (36) of their paper. -- -, SOz concentration/v.p.m. ;-, H2S04concentration/mol dm-3 ; - - - -, intermediates concentration (X}L/mol dm-3. { MnZ+}L.O = 0.35 mol dm-3 ; (S0~)v.o = 63 v.p.m. ; V = 1.6 cm3 m-3: I I I I 1 I I - 2.5 60 - - 2.0 - 1.0 - 0.5 O r I I 1 I I ! I 0 4 a 12 16 20 24 28 32 t ime/min FIG. 2.-Solutions to kinetics of oxidation using rate equations and initial conditions as in fig. 1 but with revised rate constants.P . W. CAINS AND M. D. CARABINE 2693 In the same units as above, the values are kl k2 k6 ' k7 3.54x 104 6.18 2.03 1.06 x and the two sets differ most significantly in the ratio of k6' to k,, which determines the liberation rate of oxidised Svl from the complex intermediate. The results using these values, exemplified in fig.2, show all the trends observed by earlier experimenters :l6? that is to say the time scale of significant change is seen to be in tens of minutes; there is a gradual decrease in the gas-phase SO2 concentration, an increase in sulphuric acid solution concentration to values in the range 1 to 2 mol dm-3 and an initial increase in the concentration of the intermediate, followed by a slow decrease. These kinetic predictions are much more satisfactory in all respects than those illustrated in fig. 1 ; the revised values of the rate constants are evidently better. yielded kl = 2.73 x lo4 mol-1 dm3 min-l, but use of this value made no significant difference to the calculations.) (Alternative experimental data of Coughanowr and Krause CALCULATION OF DROPLET GROWTH INDUCED BY CHEMICAL REACTION Rigorous calculation of the increase in volume of the liquid phase with the accre- tion of SO2, oxygen and water vapour and with the formation of sulphuric acid requires the evaluation of the volumes of mixing of the mixtures involved.It is sufficiently accurate in the range of concentrations encountered here to consider these to be negligible and to treat the liquid as an ideal mixture. The reaction schemes for the manganese-catalysed oxidation, considered above, form the basis for the computation of growth of the exposed droplets. Initial values appropriate to those in the experiments were chosen for (a) the number concentration of aerosol droplets, (b) the concentration of MnS04 in the droplets, (c) Z.O.L.D.parameters which had been experimentally measured on the (dry) MnSO, content of sampled droplets, and (d) the gas phase concentration of SO2. From these values, the initial volume fraction V(0) of the liquid phase of the aerosol, and the initial equilibrium water vapour pressure may be calculated, giving a complete set of initial conditions. The growth was then calculated by evaluating the yield of the reaction in a time increment At (typically 0.1 min), and deriving the value of the equilibrium vapour pressure for the new liquid phase concentrations. The vapour pressure change was then converted to a change in Vfrom a total water balance on the vapour-liquid system.For convenience, this calculation of condensation and concentration was iterated.l Using the ideality assumptions stated above the increase in liquid volume fraction due to the transfer of SO2 and oxygen to the liquid was then calculated, and added to the increase due to water condensation to give a total volume increment AV in the time increment At. The growth function I(t) was then evaluated in the form of the finite difference approximation 1 AV V(t) At' I ( t ) = -- (3) The total water content of the system in both phases was used as a check during the st ep-w ise calculation. The equilibrium vapour pressures over solutions of MnSO, and H2S04 were c a l ~ u l a t e d , ~ ~ using the method of Low,2o to obtain water activities from tabulated values 21 of the mean ionic activity coefficients of the species of interest.The theory only strictly applies to solutions of single electrolytes, and the situation in this case2694 CATALYSED OXIDATION OF so2 I N AEROSOLS is further complicated by the presence of a common sulphate ion. Again, an idealised approach has been adopted, with the water activity of the solution of mixed electrolyte assumed to equal the product of the water activities that would be derived for each electrolyte individually. Although theoretically unsatisfactory, it is not anticipated that gross errors will occur as a result of this approximation. The variation with time of the droplet growth function I(t), and the volumetric increase of the droplets are illustrated in fig. 3, for a calculation using the same initial aerosol conditions as in fig.1 and 2. In table 2, column 4, are shown the mass mean diameters, Z3, at various times, resulting from further calculations using eqn (2) and the relation ii3 = aM exp (2.50;). The initial conditions were appropriate to the experiments described below, viz. aM = 0.2 pm, o0 = 0.32, (MnS04}L,0 = 0.4 mol dm-3, (SO,) v,o = 63 v.p.m., V(0) = 1.1 x cm3 m-3. 1.20 7 2x10-2 3 5 - 1.15 2 M ,./---- 0 0 4 8 12 16 20 24 28 32 time/min FIG. 3.-Function of droplet growth, I(t), (solid curve) and the fractional volumetric growth (broken curve) in the computation depicted in fig. 2. EXPERIMENTAL A block diagram of the apparatus is given in fig. 4. A stream of aqueous manganous sulphate aerosol (4.9 dm3 min-I) is mixed with an air stream (0.1 dm3 min-’) containing the required SO2 concentration, and introduced into a series of tubular reaction vessels (4 to 5 cm dim.).The aerosol-SO2 contact time before sampling and measurement could be chosen by connecting up to twenty-five 2-m long vessels in series by means of glass U-bends (radius 6 cm). The aerosol generator was of the dispersion type,lg in which a stream of humidified, filtered air atomises a bulk solution of manganous sulphate. The SOz- containing stream was prepared by introducing slugs of “ reagent grade ” SO2 into a humidi- fied, filtered air stream by means of a Wosthoff dosing valve, and dispersing to a uniform concentration by passage through ballast volumes.22 The integrity of this system was checked by SO2 absorption and titrimetric determination.The apparatus containing the aerosol and the S02/air mixture was constructed largely of glass, with greaseless cone-and-socket joints, and ball-and-socket joints sealed by PTFE-coated O-rings. Polyethylene tubing was used for flexible connections, being more suitable than plasticised PVC. S02-containing gas streams were allowed to pass through polyethylene tubing for 1 h prior to use for experimentation, to allow any adsorption/ desorption processes to eq~ilibrate.~~ Care was taken in the design of the apparatus to avoid turbulence in particle-containing streams.P . W. CAINS A N D M. D . CARABINE 2695 Measurements were made of the relative huinidity of the gas streams at the mixer outlet, with and without the S02-containing stream.The values, in excess of 96 %, exceeded the equilibrium vapour pressure over a saturated solution of manganous sulphate (94.3 %). Experiments were designed as follows : (i) to measure, in the absence of SO2, the size distribution and number concentration of the manganous sulphate residues obtained by evaporation of the solution aerosol droplets ; (ii) to measure, with and without SO2 present, the size distributions of the aerosol droplets in suspension, after a series of aerosol residence times ; (iii) to measure the changes in gas-phase SO2 concentration after a series of aerosol- SO2 contact times. The information in (i) was obtained by sampling the aerosol with a reciprocating-head thermal precipitator, and examining the dried residue by electron micrography.Samples of aerosol (60 cm3) were drawn from the stream at a rate of 7 cm3 min-l through the thermal precipitator head, the construction l9 of which closely resembles that described by Drum- m ~ n d . ~ ~ Its use ensured that a homogeneous sample could be isolated on the grid. The FIG. 4.-Schematic diagram of the flow-apparatus: G, aerosol generator; S, supply of SOZ- containing gas ; Mx, aerosol/gas mixing vessel ; TV, tubular residence vessels ; THM, temperature and humidity meter ; TP, thermal precipitator ; LS, light scattering photometer ; GS, gas sampling. The numerals denote points where connections can be made. 400 mesh copper sampling grids were coated with carbon, about 10 nm thick, by a flotation method. The samples were dried in an oven at 105°C for 1 h to ensure conversion of the residues to the monohydrate phase MnS04 .H20. The appearance of the circular deposits on the transmission electron micrographs was consistent with the formation of amorphous rn~nohydrate,~~ and no evidence of crystallinity could be detected. Additionally, some samples were shadowed with Au/Pd to determine the ratio of the thickness of the deposits to the diameter of their circular (transmission) cross-section. A value of this “shape factor” of 0.27, with a standard deviation of 0.077, was obtained from a sample of 174 particles. The residues on the micrographs obtained by the transmission method were subsequently counted and sized to give their size distribution, and the number concentration of the original droplets. The size distributions of the solution aerosol droplets, [i.e., (ii) referred to above], were obtained by the light-scattering technique of Carabine and Moore,2 using linearly polarised incident radiation with the electric vector perpendicular to the scattering plane.The size distributions encountered in this work were more polydisperse than those considered by Carabine and Moore ; early electron microscopic measurements had indicated a small modal diameter (< 0.4 pm) and a large spread parameter (ao 2 0.4). The performance of the light scattering data inversion procedure was consequently extensively tested for these conditions. The results indicate that providing (a) the light-scattering intensities obtained are not too inaccurate, ( E + 3 %), (b) sufficient angles (12 or 13) have been sampled, includ- ing some in the forward direction with the scattering angle < 40°, and (c) the “ initial guess ”2696 CATALYSED OXIDATION OF so2 I N AEROSOLS values of a M and oo supplied do not deviate grossly from the expected values, the results obtained will be acceptable.The aerosol was passed through the light scattering apparatus by containment in a nitrogen gas " sheath " in an arrangement similar to that employed by Carabine and Maddock.6 Finally, the gas phase concentration of SOz was analysed using a flame photometric detector. The results were not particularly useful owing to the relatively small quantities of SOz that would be expected to be absorbed by the aerosol, and the relatively large amounts absorbed by the liquid deposited in the tubular vessels during the course of an experiment.RESULTS FACTORS OTHER THAN so2 ABSORPTION AND OXIDATION AFFECTING AEROSOL SIZE DISTRIBUTION The technique of thermal precipitation, followed by electron micrography of the dried droplets was applied (a) to the generator outlet to assess the aerosol at source, and (b) to the (unreacted) aerosol after various residence times in the absence of the SO,-containing stream. An example of the size distribution of the residues deter- n ' I 1 I 1 1 1 diameter /pm FIG. 5.-Distribution of sizes of dried residues (MnS04. HzO) from droplets at aerosol generator outlet, counted from electron micrographs. The curve is the fitted Z.O.L.D. with aM = 0.06 pm, uo = 0.73. Aerosol flowrate = 4.0 dm min-I ; MnS04 concentration = 0.09 mol dm-3.mined from such micrographs is illustrated in fig. 5, together with the fitted Z.O.L.D. distribution function. Except in cases of high residence times where the particle number concentrations were lower (table l), the histograms were based on samples of 500-1000 particles. It is not profitable to quantify the fit of the data to the Z.O.L.D. function with such small samples,lg but the agreement typified by fig. 5 is sufficient to validate the assumption of a Z.O.L.D. in inverting the light scattering data. The confident assignment of Z.O.L.D. distribution parameters to size data as obtained above would require the deposition and sizing of very large numbers of particles (> lo4 for a standard error of % 5 %). In particular, the values of the spread parameter o0 obtained by this method were large compared with light scattering results.1g However, evaluation of the mass mean diameter from such data is more feasible and results obtained suggested that the MnSO, concentration in the aerosol droplets was in the range 0.07-0.49 mol dm-3.The data were insufficient to determine a significant variation of mass mean diameter with concentration of the generator solutions, in the range 0.09 to 0.41 mol dnr3. Experiments with a more dilute solution (0.02 mol dm-3) yielded a lower residue particle size. The light scattering technique was used to analyse the aerosol droplet size dis- tribution in the absence of the SO,-containing stream. Results, given in table 1, columns 5 to 8, showed good consistency.Over the range of residence times used,P. W. CAINS AND M. D. CARABINE 2697 particle coagulation is likely to account for significant changes in size distribution and number concentration. It is convenient to discuss size changes in terms of the particle mass mean diameter. Predictions of the changes in size and number con- centration have been made,lg based on the classical model of Brownian coagula- t i ~ n , ~ ~ ’ 2 8 using the computational method of Willis et ~ 1 . ~ ~ The values obtained from these calculations are given in table 1, columns 4 and 2. The agreement between the calculated and experimental values of the mass mean diameter is very satisfactory. The experimental number concentrations, determined by thermal precipitation and electron micrography, are relatively lower, and decline more rapidly, than the calculated values.This indicates that some removal of particles from the suspended phase was occurring, and the appearance of a “ condensate ” inside the upstream parts of the residence vessels after long periods of running tended to con- firm this. TABLE AEROSOL PROPERTIES AS A FUNCTION OF RESIDENCE TIME IN THE ABSENCE OF SOz size measurements droplet number light-scattering size measurements with S02-introduction stream in operation, concentration mass mean in the absence of the S02- /cm-3 diameter introduction stream but with zero SO2 estimated content aerosol estimated from residence from experimental, coagulation no. of Z.O.L.D. parameters mass mean no. of mass mean time coagulation by electron theory data sets from fitted function diameter data sets diameter /min theorya micrography /pm inverted &pm uo /pm inverted /pm 0.96 1.9 x 106 5.5 x 105 0.2Sb - - 2.6 1.7X106 5.1x105 0.29 4 0.22 0.32 0.29 - - 6.0 1.5x106 1.6XlO5 0.31 4 0.23 0.33 0.313 5 0.27 11.0 1.3~106 1.3xlOS 0.33 5 0.25 0.32 0.325 6 0.30 21.0 9 .o ~ 1 0 5 9.1 x 104 0.37 3 0.26 0.33 0.345 5 0.36 a Postulated initial value = 2.6X 106 cm-3. - - - - hO.01 &0.03 f0.01 rtO.01 *0.03 &0.004 *0.02 10.006 L-0.004 kO.004 *0.02 10.002 *0.005 rt0.002 rt0.02 Postulated initial value. In the last column of table 1 are the results of size analysis (light scattering) of aerosols which had been contacted with the carrier gas brought from the SO2- introduction equipment but with no SO2 present. These experiments were under- taken because the maximum relative humidity obtainable in the SO,-inducing stream was about 55-60%.This would cause a reduction in the humidity of the aerosol stream, and may have resulted in droplet shrinkage. These results are less reliable than those in column 8, being based on only 12 light scattering data points instead of the usual 14 or 15. The results at a residence time of 11.0 min are believed to be the most reliable.lg The results indicate that the size change brought about by the lower humidity in the SO2 introduction stream is less than the detection limit of the light scattering apparatus. EXPERIMENTAL OBSERVATIONS USING AEROSOLS I N THE PRESENCE OF so2 A simple, qualitative test was initially carried out to establish that the acid- producing oxidation reaction was occurring.Membrane filters (Millipore VS) were used to extract the droplets from the aerosol over a run of about 30 min (Le., from about 150 dm3 of aerosol) and after the samples had been taken into aqueous solution, the pH and response to acid BaCl, solution were compared with those of control samples. The results were positive : this incidentally confirmed that at least some of the particles present in the system were solution droplets, since solid2698 CATALYSED OXIDATION OF so2 I N AEROSOLS MnSO, cannot promote the oxidation. This was corroborated by the observed humidity values. The initial SO2 concentration (63 v.p.m.) is a value which might be expected 30 at some 10-15 m from a power-station stack exit, if an inverse square law model for dispersion 31 is assumed.The droplet mass mean diameters measured by light scattering with varying contact times are summarised in table 2. Those in column 2 were derived from scattered light intensities which had been averaged over four different experiments. TABLE 2.-LIGHT SCATTERING DETERMINATION OF PARTICLE SIZE AFTER VARIOUS RESIDENCE TIMES IN THE PRESENCE OF so2 ((MnS04)L,0 = 0.16 mol dm-3 ; (SO2)v,o = 63 v.p.m.) residence time lmin particle size (mass mean diameter) from averaged light scattering intensities lclm 0.69 0.26 0.96 0.24 2 . 6 0.25 6.0 0.27 11.0 0.27 21 .o 0.38 particle size (mass mean diameter) from selection of inversion results I m 0.26 0.24 0.25 0.30 0.37 0.39 theoretically predicted diameter I m 0.26 0.27 0.28 0.30 0.32 0.35 TABLE 3 .-TYPICAL RESULTS FROM LIGHT SCATTERING INVERSION PROCEDURE FOR AEROSOL IN THE PRESENCE OF so2 residence time = 6.0 min CZM, “‘0 values used Z.O.L.D.parameters no. of light to initiate data from fitted (inverted) mass mean scattering inversion procedure function diameter run no. angles used alr/pm “‘0 a d p m “‘0 /Pm 10.1 12 0.2 0.1 10.2 12 0 . 2 0.1 10.3 12 0.2 0.1 10.4 13 0.2 0.1 averaged 13 0.2 data 0.1 0.7 0.5 0.7 0.5 0.7 0.5 0.7 0.5 0.7 0.5 0.904* 0.050 0.051 0.050 0.064 0.065 0.140 0.140 0.111 0.111 0.139* 0.702 0.727 0.728 0.694 0.691 0.552 0.552 0.597 0.597 0.95* 0.17 0.19 0.19 0.21 0.21 0.30 0.30 0.27 0.27 Because the light scattering method can be subject to quite large and identifiable errors in data recording and in inversion, we take the view l9 that some selected results, shown in column 3, are more reliable than those based on averages.To exemplify the selection criteria used, four experiments for the 6 min contact time are detailed in table 3. The following points justify selection of 10.4 as the most reliable. The asterisked results from one inversion of run 10.1 are clearly caused by a local minimum problem in the minimisation procedure 32 ; error contours confirm this. The values of aM around 0.05 to 0.06 ,urn can be rejected because they are as low as the modal diameters of dried residues ( c . 5 fig. 5), and because the aerosol is un- questionably one composed of liquid droplets; the data in these runs 10.1 to 10.3 are intrinsically less reliable than in 10.4 because in the latter a thirteenth limiting forward angle of scatter was usable, whereas in 10.1 to 10.3 at this angle there was excessive noise originating in temporal variations in concentration of droplets at the upper extreme in size.P .W. CAINS AND M . D . CARABINE 2699 DISCUSSION The experimental analysis by light scattering has been successful in measuring the variation in size of droplets with residence time in the flow system under the conditions chosen-ambient pressure and temperature (2 1 "C), SO, concentration 63 v.p.m., relative humidity in excess of 96 %, MnS04 concentration in the range 0.07-0.49 mol dm-3 (or 0.24-1.7 mg m-3 gas phase), in droplets of 0.25 to 0.40 pm mass mean diameter, at number concentrations of lo5 to lo6 ~ m - ~ . The analysis by thermal precipitation and electron microscopy clearly indicated depletion of particulate material, particularly in the upstream parts of the apparatus (column 3, table 1).The loss by sedimentation of particulate material of the order of size dealt with in this work should be minimal; an aerosol consisting of 2.6 x lo6 ~ m - ~ of 1 pm particles should sediment out only 8 x lo3 ~ m - ~ particles in conditions analogous to those of the 21.0 min residence time e~perirnent.~,. 33 However, if regions of local supersaturation are created in the presence of particles, the latter will act as nuclei for the condensation of excess vapour present. These particles thus become enlarged and their removal by sedimentation may become marked. It was not possible to produce experimental evidence for this theory, but com- paratively small temperature fluctuations might have produced this effect.TABLE 4.-cOMPARISON OF PERCENTAGE PARTICLE GROWTHS MEASURED WITH THOSE PREDICTED FROM THEORY experimental particle growth experimental particle growth theoretical particle growth in absence of SO2 in presence of SO2 due to SO2 reaction (Le., coagulation) residence mass mean mass mean mass mean timelmin diam./pm % change diam./pm % change diam./pm % change - - 0.69 0.26 - 0.26 0.96 0.24 -7 0.27 1 2.6 0.25 - 3 0.28 6 0.29 - 6.0 0.30 16 0.30 13 0.31 7 11.0 0.37 41 0.32 21 0.33 14 21.0 0.39 50 0.35 32 0.35 21 - The results of the analysis of SO2 in the gas phase, although self-consistent, were not reliable as a measure of the uptake of SO2 in the droplets, since it appears that much more was absorbed in the liquid deposited on the walls of the apparatus.It is clear that acidity measurement on collected droplets would have been a preferable method of detecting the uptake, which is predicted only to be about 1 to 2 parts in 63. However, it can be anticipated that such analyses would be subject to well-known difficultie~.~~? 3 5 Separation of the effects on particle size distribution of SO,-induced growth and of coagulation is impossible in these experiments, since they occur on similar time scales, and has not been achieved in the theoretical predictions above. However, some progress can be made as follows in comparing the results with theory. As shown above (table l), the coagulation in the absence of SO, is consistent with the simple Brownian model.In table 4 the absolute sizes and percentage growths from the light scattering measurements are compared directly with the growth theory calculations (presented previously in table 2, column 4). Overall, there is fair agreement, and at high residence times the particle size is larger than the growth theory predicts. Also repeated here are the mass mean diameter values observed in the absence of SO,, (from table 1). It may be concluded that the effects of growth2700 CATALYSED OXIDATION OF so2 I N AEROSOLS are likely to be discriminated only at longer residence times (6 to 21 min). This is due to the limited accuracy of the light scattering technique-it is not within its scope to detect the small changes in modal diameter which would result from growth at shorter times.lg It should be noted that, since the volume fraction of particles in the experimental aerosol is 2 orders of magnitude lower than the values used to obtain fig.3, it is understandable that the growth is considerably greater than the values illustrated there. At residence times > 6 min, the margin by which the measurements (columns 2 and 3) exceed the growth theory predictions (columns 4 and 5) may be attributed to the simultaneous and coupled coagulation in the real system. RATES A N D YIELDS PREDICTED BY THE CALCULATIONS Experimental values of droplet growth in tables 2 and 4 provide evidence that the assumption of homogeneous reaction control is justified, and that the kinetics of the reaction in the situation of the experiments may be described by the rate constants given.TABLE s.-cALCULATED RATES OF so2 REACTION IN AEROSOL DROPLETS (MEASURED BY FALL IN GAS PHASE CONCENTRATION IN FIRST 3.5 min), COMPARED WITH INITIAL VALUES OF VARIOUS PARAMETERS rate volume fraction -A{SOz}v/v.p.m. min-1 V/cm3 m-3 {SO*}v/v.p.m. {Mnz+)L/mol dm-3 At {Mnz+JLx V calculation A 1.6 63 0.35 5 calculation B 0.7 63 0.35 3 calculation C 1.6 63 0.14 2.5 ratio A/C 1 1 2.5 2 ratio A/B 2.3 1 1 1.7 ratio B/C 0.43 1 2.5 1.1 (fig. 2) 0.56 0.25 0.22 2.5 2.2 1.1 It is clear that the use of the model of Matteson et al. has not lecl to realistic resulis when their rate constant values are used (fig. 1). With the revised rate constants, acid concentrations in the range 1 to 2 mol dm-3 are predicted (fig.2,3), in agreement with those quoted by other authors for the manganese-catalysed oxidation at equiva- lent sulphur dioxide concentration^.^^ 16* l 7 The revised values of kgr and k7 are also numerically consistent with the stability constant for the system 33 Mn2++SO$- + Mn SO4. In these calculations, as in those of other authors 14* l 5 no attempt was made to allow for the reduction in sulphur dioxide solubility as the acid concentration in- creased. A reduction in solubility of x 60 % has been observed for H2S04 of molality 0.09.36* 37 Calculations using different initial conditions showed the same qualitative features as those of fig. 2. In table 5 the respective rates of SO2 removal (using the rough measure of the decrement in the first 3.5 min) are compared with the various initial values of parameters V (volume fraction), (Mn2+IL, and the product (Mn2+}L x V.It is clear by inspection of the last three rows that the rate is deter- mined in part by the latter product. This enables us to emphasise that the complex reaction mechanism simplifies, as far as initial rate is concerned, to control by the first step. for The resulting simple model is one of a series described recently ransfer of a vapour to a droplet in which it reacts.P. W. CAINS AND M. 1). CARABINE 2701 For this case, if an involatile solute, B, reacts in a second order reaction with a volatile one, A, the rate law to be expected for the decrease in A per unit volume of aerosol is --- d'Ah - k(A)vHA(B),V. dt Denoting with subscript T the total concentration in both phases, then to the approxi- mation that {A}, = {AIV, and since -- - d'Ah dt - kLA)THA{B)T.The analogue in the reaction system considered here is -~ d{So21v = k{ SO,),H,,,( Mn2 + ILV. dt The total SO2 and Mn2+ concentrations per unit volume of aerosol emerge as determining the initial rate of sulphur dioxide removal. Against this background Ananth has made a useful attempt to compare results of various experiments, expressed in acid produced (m mol min-l) per mass of catalyst present (g Mn). In that review, the results of Matteson et al. are misrepresented because of an error in the time interval chosen and should be 0.476, 0.457 in these units. The results of our calculations are in the range 1 to 5 m mol min-' 8-l ; these are crude approxima- tions because rates are averaged over 5 to 30 min, and the values near 5 are subject to less error.As regards the sizes of droplets in which such acid may be located, determination of the size distribution by (a) coagulation and (b) reaction induced growth depends, as described above, on the number concentration and volume fraction of liquid: (i) with initially low values of number concentration (e.g., 2 or 3 x lo5 cm-3) and of V (e.g., cm3 m-3) growth will predominate as the process determining the size. (ii) If both have moderate values, (e.g., lo6 ~ m - ~ , cm3 m-3, as in experiments described here), both growth and coagulation will contribute. (iii) If both have initially high values (e.g., lo7 ~ m - ~ , 2 cm3 m-3 as in the calculations presented here, and in those by Willis et aZ.),29 coagulation will predominate.The authors thank the S . R. C . for a grant to P. W. C., and Prof. A. R. Ubbelohde for his interest and help. ' E. R. Gerhard and H. F. Johnstone, Ind. andEng. Chem., 1955,47,972. J. Freiberg, Atm. Em., 1975, 9, 661. K. P. Ananth, J. P. Galeski, F. I. Honea, EPA 1976, Rept. No. 600/2-76-257. P. M. Foster, Atm. Env., 1969, 3, 157. W. K. Poole and D. R. Johnston, Res. Tri. Inst. Rep., 1969, A.U. 229. M. D. Carabine and J. E. L. Maddock, Arm. Enu., 1976, 10,735. M. K. Azarniouch, A. J. Bobkowicz, N. E. Cooke and E. J. Farkas, Canad. J. Chem. Eng., 1973, 51, 590. €3. F. Johnstone and D. R. Coughanowr, Ind. and Eng. Chem., 1958, 50,1169. lo W. F. Espenscheid, M. Kerker and E. MatijeviC, J. Phys. Chem., 1964, 68, 3093. l 1 M. J. Matteson and W. Stober, J. Colloid Interface Sci., 1967, 23, 203. l 2 M. D. Carabine, J. E. L. Maddock and A. P. Moore, Nature (Phys. Sci.), 1971, 231, 18. l3 L. I. Kastanov and C . A. Guljanskaja, Zhur. obshchei Khim., 1936, 6, 227. l4 M. J. Matteson, W. Stober and H. Luther, Ind. and Eng. Chem. (Fundamentals), 1969, 8, 677. * H. Bassett and W. G. Parker, J. Chem. Soc., 1951, 1540.2702 CATALYSED OXIDATION OF so2 I N AEROSOLS R. A. Wadden, J. E. Quon and H. M. Hulbert, Atm. Em., 1974,8,1009. l6 H. F. Johnstone, Ind. and Eng. Cizem., 1931, 23, 559. l7 R. L. Copson and J. W. Payne, Ind. and Eng. Chem., 1933,25,909. '* D. R. Coughanowr and F. E. Krause, Ind. and Eng. Chem. (Fundamentals), 1965, 4, 61. 2o R. D. H. Low, J. Arm. Sci., 1969, 26, 608. 21 R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworth, London, 1959). 22 H. D. Axelrod, J. B. Pate, W. R. Barchet and J. P. Lodge, Atm. Em., 1970,4, 209. 23 R. L. Byers and J. W. Davies, J. Air. Polln Contr. Assoc., 1970, 20, 236. 24 D. G. Drummond (ed.), J. Roy. Micros. SOC., 1950, 70, chap. 4. 2 5 Inorganic and Theoretical Chemistry XII, ed. J. W. Mellor (Longmans Green, London, 26 M. D. Carabine and A. P. Moore, Faraday Symp. Chem. Sac., 1973,7, 176. 27 M. v. Smoluchowski, Phys. Z., 1916, 17, 557, 585 ; 2. phys. Chem. (Leipzig), 1918, 92, 129. 28 H. Miiller, Kolloidchem. Beih., 1928, 27, 223. 29 E. Willis, M. Kerker and E. Matijevic, J . Colloid Interface Sci., 1967, 23, 182. 30 R. J. Bibbero and I. G. Young, Systems Approach to Air Pollutioiz (Wiley, N.Y., 1974). 31 R. Scorer, Air Pollution (Pergamon Press, Oxford, 1968). 32 A. P. Moore, Ph.D. Thesis (Univ. London, 1974). 33 Aerosol Science, ed. C. N. Davies (Academic Press, N.Y., 1966). 34 R. E. Lee and J. Wagman, Amer. Ind. Hyg. Assoc. J., 1966, 27, 268. 35 J. D. Husar, R. B. Husar, E. S. Macias, W. E. Wilson, J. L. Durham, W. K. Shepherd and 36 L. G. Sillkn and A. E. Martell, Chern. SOC. Spec. Pub. (Chem. SOC., London, 1964), no. 17, p. 37 D. K. Oestreich, EPA 1976, Rept. No. 600/2-76-279. P. W. Cains, Ph.D. Thesis (Univ. London, 1975). 1932), p. 401. J. A. Anderson, Arm. Em., 1976, 10, 591. 240. (PAPER 7/1154)

ISSN:0300-9599    

DOI:10.1039/F19787402689

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Tin Oxide Surfaces .Part 9.l-Infrared Study of the Adsorption of CO, NO and CO+NO Mixtures on Tin@) Oxide Gels Containing Ion-exchanged CrlII, Mn", Fe"', Co", Ni" and CuI1 BY PHILIP G. HARRISON" AND EDWARD W. THORNTON Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 9th November, 1977 Infrared transmission spectroscopy has been used to study the adsorption of CO, NO and CO + NO mixtures on tin(Iv) oxide gels containing CrlIr, Mn", FelI1, CoII, Nil1 and CuII as ion-exchanged cations. Exposure to CO results in the formation of linear physisorbed CO species exhibiting a single absorption band in the range 2200-2180cm-I for all the oxidised gels (except the FelIr- exchanged sample). The increase in absorption frequency above that of the gas phase value (2143 cm-l) is rationalised by considering the strong electric field due to the transition metal ion, and it was concluded that the carbon monoxide is adsorbed perpendicular to the surface, probably via carbon, at a cationic transition metal site, except for CulI exchanged gel which was bonded via oxygen.Bands due to bidentate carbonate complexes associated with transition metal sites were also observed for the oxidised Mn", FelI1 and Coll samples. In contrast, exposure of CO-reduced Mn", FerI1, CoI1 and NiI1 samples to CO+Oz mixtures resulted in the formation of unidentate carbonate complexes bound to transition metal ion sites. Nitric oxide is chemisorbed on all gels except the Mnrl-exchanged sample, but the nature of the chemisorbed species varies.All the samples catalysed the CO-NO reaction, and physisorbed C02 was present in the Mn", FelI1, ColI and NilI samples as well as physisorbed N20 in the case of Mn". All samples showed spectra due to carbonate species consistent with a redox mechanism for the CO-NO reaction catalysed by these oxides. Additionally, those oxides with exchanged cations which adsorb NO from CO + NO mixtures (CoII, NiiI and FerlI) show a marked selectivity for reduction of NO to N20, whereas the Cu"- exchanged gel, which preferentially adsorbs CO, exhibits a similar selectivity for reduction to Nz. Systems based on tin(1v) oxide have found extensive application as heterogeneous catalysts. Tin(1v) oxide itself has been shown to catalyse the oxidation of CO by O2 and N20,3 and is also highly selective for the formation of acrylonitrile in the ammoxidation of ~ r o p e n e , ~ and its activity towards oxidation of CO may be enhanced by incorporation of palladium in the catalyst.' Previously we have studied adsorption phenomena of oxides of carbon and nitrogen on Sn02+0.55 CuO, an efficient low-temperature catalyst for the oxidation of CO by and in particular elucidated the mechanism of formation of surface isocyanate species in this reaction. In this paper, we report a more general study of the adsorption of CO, NO and CO+NO mixtures on tin(1v) oxide gels containing Cr"', Mn", Fe"', Co", N P and Cu" as ion-exchanged cations.EXPERIMENTAL The apparatus and the preparation of carbon dioxide, carbon monoxide, nitrous oxide and nitric oxide have been described previously.5* Tin(1v) oxide gel samples containing Crr117 MnII, FeIII, Co", Nirl and CuII as ion-exchanged cations were prepared according to the procedures of Donaldson et aL8-10 All the gel samples were prepared to have SnOz .0.25MxOy stoichiometry after dehydration. Discs suitable for infrared experiments were prepared according to the procedure for tin(1v) oxide? and had a '' thickness " of 27032704 TIN OXIDE SURFACES w 12 mg cm-2. The infrared spectrometer ' and the Mossbauer spectrometer l1 have been described previously. Iron-57 Mossbauer spectra were recorded at ambient temperatures against a 57Co/Rh source (Radiochemical Centre, Amersham). Isomer shifts are quoted relative to sodium nitroprusside. Conditions of sample thermal pretreatment, exposure to the adsorbent, and spectra are described in the Results and Discussion section. RESULTS AND DISCUSSION CARBON MONOXIDE ADSORPTION The infrared bands produced during CO adsorption are summarised in table 1, and the 2000-2400 cm-l regions of the absorption spectra are shown in fig.1. A linear physisorbed CO species was observed for the oxidised samples except the FeIII-exchanged gel. In the case of the Cr"'-exchanged sample, this band could only be observed for a freshly evacuated (600-673 K) disc. After oxygen or carbon monoxide treatment in this temperature range, the CrlI1, Mn", ColI and Nil1-exchanged oxides having the linear CO species exhibited a single absorption band at 2200-2180 cm-l, which is above the gas phase frequency (2143 cm-I).The bands were all readily removed by evacuation at 320 K. Two distinct explanations have been advanced for the observation of absorption bands above the gas phase frequency. The first of these is due to Gardner and Petrucci,I2 who attributed absorption bands of CO in the range 2143 to 2184 cm-' to partially charged CO+ species [v(CO+) = 2184cm-I in the vapour phase].12 TABLE 1 .-CARBON MONOXIDE ADSORPTION adsorbent adsorbate CrIII on Sn02 CO CO or cos02 reduced CrQJ + SiOz with CO co with C2H4 30 co Cr203 + A1& CO 32 co MnlI on Sn02 co 31 Mn02 33 FelI1 on Sn02 co c0+02 co co co c0+02 pretreatment conditions evacuated at 673 K O2 or CO at 673 K O2 at 673 K CO at 623 K CO at 623 K O2 at 623 K O2 at 693 K CO at 623 K CO at 623 K i.r. bandslcm-1 assignments 2195 physisorbed CO no apparent adsorption 21 89 + 2184 2H9+ 2202) 3 bands 21 50-2250) 2200 2200 1590,1320,1020 1500,1355,1060 1580,1350 1590,1305,1030 1480,1370,1070 physisorbed CO physisorbed CO COf physisorbed CO bidentate Cog- bonded to a Mn site no apparent adsorption unidentate Cog- bonded to a Mn site unidentate COZ- bidentate Cog- bonded to an Fe site no apparent adsorption unidentate COi- bonded to an Fe siteP. G .HARRISON AND E. W . THORNTON 2705 adsorbent CoII on Sn02 c0304 34 Co oxide+ SiOz 35 NiIr on Sn02 Ni oxide+Si02 35 CuII on Sn02 Cu+SiOz 36 " oxidised Cu " 37 CUO 37 adsorbate co co co+oz co co co co c0+02 co co co CO co TABLE 1 .-contd. 0 2 at 638 K 21 80 physisorbed CO 1560,1320,1030 bidentate CO3- pretreatment conditions i.r.bandslcm-1 assignments bonded to a Co site no apparent adsorption bonded to a Co site 1545,1324,1134 bidentate CO at 638 K CO at 638 K 1500,1353,1055 unidentate Cog- 2070 coordinated CO carbonate at 623 K with Mnc decomposed nitrate 21 30 (20 associated 21 80 CO associated with oxide no apparent adsorption bonded to a Ni site decomposed nitrate 21 15 CO associated 21 80 CO associated O2 at 638 K 2185 physisorbed CO CO at 638 K CO at 638 K 1550, 1350 unidentate COj- at 623 K with Mn+ with oxide O2 at 638 K 2121 physisorbed CO 2140 coordinated CO 2170 CO+ 2113 coordinated CO 21 35 coordinated CO Observations of frequencies above 2184 cm-l can then be attributed to the removal of a second electron to form C02+,13 or (less likely) to a decrease in the anharmonicity of the CO+ vibration : where m,x, is zero for a completely harmonic vibration.14* l5 The postulation of CO+ and C02+ species have been made largely without regard to the electron donor- acceptor properties of the adsorbents, and Taylor and Amberg have pointed out that CO adsorption on ZnO was inconsistent with an electron transfer-adsorption mechanism in the formation of CO+ or C02+.16 The alternative explanation is one which has not been widely considered by surface chemists, but is excellently expounded by Hush and Williams l7 who considered the effect of strong electric fields on the equilibrium nuclear configuration, vibrational frequency and vibrational transition intensity of CO and analysed existing results for the carbon monoxide molecule in axial fields.A change in frequency accompanying physisorption does not necessarily imply a change in the CO stretching force constant (a change in stretching force constant is the basis of Gardner and Petrucci's explanation),12 and for a diatomic molecule adsorbed perpendicular to the surface, with no change in stretching force constant, the observed frequency will always be higher than for the free molecule.An estimate V-/CIII-' = 0, - 2wexe + 3 . 2 ~ e y e2706 TIN OXIDE SURFACES of this has been obtained for where it was found that an increase in the linking force constant from 5 to 40 % of that for the CO vibration, without any change in the CO force constant, would increase v(C0) by 18-1 84 cm-l. For adsorption perpen- dicular to an ionic surface, it was concluded that (1) a negative shift of CO stretching frequency is evidence for adsorption via carbon at an anionic site or via oxygen at a cationic site, and (2) a large positive shift in CO stretching frequency is evidence for adsorption via oxygen at an anionic site or via carbon at a cationic site.Adsorption of a molecule parallel to the surface should result in only a very small change in CO stretching frequency. The first case corresponds to a negative electric field, the second to a positive electric field with the CO bond direction taken as the positive z axis. It may be concluded, therefore, that, in the present study, the 2180-2200 cm-l bands are due to carbon monoxide adsorbed perpendicular to the surface probably via carbon at cationic sites. Fig. 2 shows a plot of the observed infrared stretching frequency versus the effective field at the centre of the CO molecule for CO coordinated to the bare cations : or : (ii) Qe .1 M - - - - C=O (+ve electric field) t- R -+ Qe 3- M - - - - 0-C t- R -+ (- ve electric field) where Qe is the metal charge and the effective field is given by Qe/4nR2, where R is the distance from the metal nucleus to the centre of mass of the CO molecule [approxi- mated by the metal ionic radius +-& CO collision diameter (a = 3.7 A)].In order to obtain a reasonable fit to the data, it was necessary to assume that manganese was present as MnIII in the oxygen treated samples and that copper was present as Cu', and exerted a negative electric field [case (ii) above]. The tabulated data (including other disregarded alternatives) are presented in table 2.Note that the Mn" exchanged discs turned completely black on treatment with oxygen at 673 K, but the point for a MnIV surface species lies at a high relative field of 0.254 a.u. The units of effective electric field are a.u., but the scale would probably be better expressed in arbitrary units, since the scale of surrounding electric fields has not been estimated. The shape of the curve is qualitatively the same as the curve found in ref. (17) for the same range of axial field strengths. When the transition-metal-exchanged tin (IV) oxide discs were reduced with CO at 2600 K, they showed no tendency to physisorb or chemisorb carbon monoxide at 320 K. The oxidised Mn", FerlI or Co" samples showed medium intensity absorption bands due to bidentate carbonate complexes during CO adsorption [v,,(COg-) -v,(CO$-) = 280, 285 and 240 cm-1 respectively], these complexes being associated with the transition metal ions rather than the tin ions.During exposure of CO-reduced samples to CO+O, mixtures at 320 K, intense bands were formed for the Mn", Fe1I1 and Co" samples, and weak bands were formed in the spectrum of the Ni'I-exchanged disc due to the presence of unidentate carbonate complexes associated with the transition metal components [v,,(C0~-)-~,(C0~-) = 145, 110, 147 and 200 cm-l for Co", Fe"', Mn" and Ni", respectively]. The bands formed on Fe"' and Mn" exchanged discs were identical to those observed during COz + O2 adsorptionP. G . HARRISON AND E . W . THORNTON 2707 wavenumber /cm--l FIG.1.-Infrared spectra of CO adsorbed on transition metal ion-exchanged tin(1v) oxide gel samples at 320 K. The transition metal, activation temperature [evacuation only for (4), evacuation and oxygen treatment for the rest] and final pressure of CO were (1) MnII, 673 K, 2.0 kN m-2 ; (2) NiII, 638 K, 655 N m-2 ; (3) CoII, 638 K, 1.33 k N m-2 ; (4) CrIII, 588 K, 1.33 k N m-2 ; (5) CuI*, 510 K, 1.33 k N m-2. (CU2') / 7 2100 1 1 - 0.1 0 0 .I 0.2 electric field strength1a.u. FIG. 2.-Plot of stretching frequency of CO molecule against electric field strength at the CO molecule generated by the adsorbate transition metal ion. 1 a.u. = 51.475 VA-l. See text.2708 TIN OXIDE SURFACES on CO-reduced samples, and the carbonate on the Ni" disc is assigned to a unidentate carbonate despite the large separation of the symmetric and antisymmetric C-0 stretching bands because COz adsorption on an oxidised sample produces a bidentate carbonate with v,,(CO,) = 1600 cm-1 and v,(CO,) = 1300 cm-l, AT = 300 cm-I. None of the spectra below 2000 cm-l for CO or CO + 0, adsorption are illustrated. TABLE 2.-vALuES OF THE ELECTRIC FIELDS AT PHYSISORBED CARBON MONOXIDE DUE TO TRANSITION METAL IONS exchanged ionic radius cation valenc y /A ' 8 RIAa R2/.&z (Qe/4nR2)/a.u.bj v(CO)/crn-l ( C P 2 0.89 2.74 7.51 0.102 2195) Cr1IE 3 0.63 2.48 6.15 0.186 21 95 MnI1 3 0.66 2.51 6.30 0.182 2200 (COII 3 0.63 2.48 6.15 0.186 2 180) COI' 2 0.72 2.57 6.61 0.116 2180 NilJ 2 0.69 2.54 6.45 0.119 2185 (CU" 2 0.72 2.57 6.61 -0.116 2121) CUII 1 0.96 2.81 7.90 - 0.048 2121 0 - - - - 2143 ( M n l I 2 0.80 2.65 7.02 0.109 2200) gas a o(C0) = 3.7 8, ; b 1 a.u.= 51.475 V A-'. *...a- -/ 2000 1800 1600 wavenumber/cm-' FIG. 3.-Infrared spectra of nitric oxide adsorbed on transition metal ion-exchanged tin(1v) oxide gel samples. The metal ion, conditions of activation and final NO pressure at 320 K were (1) CrrIr, evacuation at 588 K, 3.33 kN m-2; (2) FeIn, O2 at 623 K, 800 N m-2; (3) FerI1, CO at 623 K, 800 N m-' ; ,(4) CdI, 0 2 at 638 K, 133 N m-2 ; (5) Co", CO at 638 K, 1.6 k N m-' ; (6) Nilr, O2 at 638 K, 665 N m-2 ; (7) Nirr, CO at 638 K, 665 N m-2 ; (8) Curl, O2 at1643 K, 4.0 kN nr2.P. G . HARRISON AND E . W. THORNTON 2709 NITRIC OXIDE ADSORPTION The infrared bands due to adsorbed NO are shown in fig.3 and summarised in table 3. Attempts were made to plot stretching frequencies against effective axial electric field, as had been done for the spectra of adsorbed CO (fig. 2), but no satis- factory plots could be obtained. This implies that no single mode of bonding to the surface was involved in all the cases considered. TABLE ~.-NITUC OXIDE ADSORPTION adsor bent adsorbate pretreatment conditions i.r. bandslcm-1 assignments C P on SnO, NO MnIJ on SnO, NO FelI1 on SnO, NO NO CoII on SnO, NO NO Nil1 on Sn02 NO NO Cu11 on Sn02 NO evacuated at 643 K 1910,1740 uncertain O2 at 623 K 1832 Fe3+ c : NO CO at 683 K 1770 Fe3+ t : NO 1720 Fe2+ c : NO all no apparent adsorption C 0 2 at 638 K 1874,1785 CO" (NO), CO at 638 K 1840,1765 CO? (NO)2 i O2 at 633 K 1870 Ni=N=O CO at 633 K 1850 uncertain O2 at 641 K 1880 Cu=N=O + The infrared spectra of adsorbed nitric oxide have been studied extensively by Terenin et aZ.,19-22 and reviewed recently by Shelef and K ~ r n r n e r .~ ~ The types of bonding proposed by these authors are listed in table 4. TABLE 4.-METAL-NITRIC OXIDE BONDING TYPES TOGETHER WITH V ( N 0 ) FREQUENCY RANGES/Cm-' bond type notation type of bonding v ( N 0 ) frequency range I -1 NO' purely ionic 21 00-2400 N 0 I11 -1 : NO+ I1 -3 . . . 11.1 coordinative ionic 1900-2100 IV -]=N+=O double bond ionic 1800-1900 V ]:NO coordinative 1700-1 870 VI 1-N-O covalent 1600-1 800 VII +I NO- purely ionic 1500-1700 Comparing the results in table 4 with the present study, it is apparent that no purely ionic forms were present during NO adsorption on the transition-metal- exchanged tin@) oxide samples.On the above classification, the 1910 cm-l band on the freshly evacuated Cr"'-exchanged oxide can be assigned to a coordinative ionic species, I1 or 111. The 1740 cm-l band observed in the same spectrum is in a similar position to bands observed on other chromium-containing materials. A band in this position has been reported for a chromium-containing zeolite, where it was attributed to Cr+(NO)+, a nitrosyl complex with Cr" as the adsorption site.24 Alternatively, several studies have reported pairs of bands at about 1740-50 and 1870-1900 cm-l which have been attributed to the (NO), dimer,25 to adsorption on Cr" or Cr"' sites of pairs of NO and to pairs of adsorption species on CrlI1 and Crv sites.27 The nature of the adsorption site is, therefore, in doubt.2710 TIN OXIDE SURFACES The adsorption of nitric oxide on cobalt oxides has not been studied by infrared spectroscopy, but Roev and Alekseev 21 have studied the infrared spectrum of nitric oxide adsorbed on " COO +A1203 ", " COO + Si02" and " COO + Si02 .A1203 " samples. On '' CoO+A1203 " the spectrum consisted of two absorption bands at 1795 and 1840cm-I. This does not really resemble the spectra recorded in the present experiments where bands are found at 1874 and 1785 cm-' for the oxidised sample, and at 1840 and 1765 cm-1 for the CO-reduced sample. Both sets of bands behaved in concert during adsorption and desorption, and they resemble the bands reported by Zecchina et aZ.26 for pairs of NO molecules coordinated to chromium ions having two vacancies in their coordination shell.The oxidation state of the cobalt ions cannot, however, be established. 2000 I800 1600 wavenumber 1crn-l FIG. 4.-Effect of pretreatment conditions on the spectrum of nitric oxide adsorbed on Fel" exchanged tin(iv) oxide gel. Spectra recorded at 320 K in 800 N n r 2 NO. (1) Disc activated by CO at 623 K. Sample the activated by : (2) O2 at 623 K ; (3) O2 at 623 K followed by CO at 320 K and evacuation ; (4) evacuation at 693 K followed by CO at 320 K and evacuation ; (5) evacuation at 693 K followed by CO at 428 K and evacuation at 663 K. The intense 1870 cm-l band of NO adsorbed on oxidised Ni"-exchanged tin(1v) oxide resembles the spectrum of NO recorded during previous studies of nickel- containing oxide It can be assigned to a complexed NO+ species, the double bond ionic (IV), species Ni+==N=O, where the adsorption site is a Ni" cation with one vacancy in its coordination sphere, although a distinction between the species IV, V and VI is nearly impossible from infrared data alone.The band at 1880 cm-I observed in the spectrum of NO adsorbed on the Cu"-exchanged sample may be assigned to a similar surface Cu+-N-0 species. The infrared spectrum of NO adsorbed on Fe"'-exchanged tin@) oxide is shown in fig. 4 for various pretreatments of the oxide disc. As the disc was reduced by exposure to CO at higher temperatures, the 1832 cm-l band weakened and shifted continuously to 1770cm-', and a medium intensity band appeared at 1720cm-l 24 + +271 1 after CO treatment at 623 K.The 1720 cm-' band was removed by evacuation at 320 K (fig. 5), but the other band was more resistant to evacuation. The sites responsible for the 1720 cm-I band were destroyed by preadsoi-ption of oxygen at 320 K, and the 1720 cm-i band could be removed by oxygen following a brief period of evacuation to remove gaseous NO. The 1832 cm-I band was stable to oxygen. These results resemble the results of Poling and Eischens 28 for iron+iron oxide supported on silica. These authors observed a band at 1820 cm-l attributed to Fe-N=O on a reduced surface, and proposed Fe2+ as the adsorption site since chemisorption could be detected on Fe304 (1830 cm-l) but not on Fe,03. However, if that is the case, then the formulation of the surface species as Fe2+ : NO+ must be wrong since it implies that the adsorption site is Fe3+.It may, however, be more reasonable to assign the bands in the present study to coordinative, type V, NO molecules, the 1832 (shifting to 1770) cm-' bands being due to NO coordinated to Fe"' in an increasingly perturbed environment, the 1720 cm-l being due to NO coordinated to Fe" sites. P . G . HARRISON A N D E . W. THORNTON wavenurn ber /cm-l FIG. 5.-Ordinate expanded infrared spectra on nitric oxide adsorbed on FelI1 exchanged tin(1v) oxide gel. Sample activated in CO at 623 K. (1) During exposure to NO (2.66 kN m-', 320 K). After subsequent evacuation at 320 K for (2) 15 min ; (3) 20 min ; (4) 0.5 h ; (5) 1 h ; (6) 1.5 h. The oxidation state of the iron in the sample under study was determined by Mossbauer spectroscopy.The room temperature spectrum showed only resonances due to iron in the 3+ oxidation state, both for the dried, freshly precipitated gel (I.S. = 0.40 mm s-l, Q.S. = 0.61 mm s-l) and after oxidation with oxygen at 633 K (I.§. = 0.38 mm s-l, Q.S. = 0.94 mm s-l). On highly oxidised samples, spectra appeared below 1700 cm-I due to nitrate ions adsorbed on the Mn", Fe"' and Co" exchanged tin(1v) oxide discs. The spectra were weak and will not be discussed further. 1-862712 TIN OXIDE SURFACES ADSORPTION OF CO+NO MIXTURES Infrared spectra recorded during the adsorption of CO+NO mixtures on Mn", Fe"', Co" and Ni" exchanged tin(1v) oxide are given in table 5. The infrared spectra were recorded after exposure to the gas mixture at 320 and 473 K.Some general conclusions can be drawn from the experiments. All the discs were effective in catalysing the CO +NO reaction, since C02 and N20 were readily detectable in the gas phase. Physisorbed C 0 2 was present on the Mn", Fe"', Co" and Ni" samples and, in addition, the Mn" sample showed physisorbed N 2 0 also.* Adsorbed NO TABLE 5.-ADSORPTION OF co + NO MIXTURES carbonatelcm-1 adsorbed exchanged P(C0) P(N0) v(C0) 0 0 ) cation stagea /kNm-Z /kNm-Z /cm-l /cm-1 v4 *1 Av CO2 NzO MdT A 2.39 - 2200 - 1580 1312 268 - - B 2.39 2.00 2200 - 1570 1312 258 - - E - - 1530 1335 195 2340 2238,1240 FelI1 A 2.26 - I - 1575 1310 265 - - B 2.26 1.33 - 1830 1550 1335 215 2340 - D I 1812 1470 1360 110 2340 - CO" A 1.73 - 2175 - 1560 1320 240 - - B 1.73 0.93 - 1874 1550 1330 220 - - 1784 D - 1870, 1520 1340 180 2340 - 1780 B 1.46 1.67 - 1870 1600 1300 300 2340 - D - 1865 1550 1350 200 2340 - CU'I A 6.65 - 2110 - 1520 1345 175 - - B 6.65 1.33 2110 - 1500 1350 150 - - D 2105 - 1480 1360 120 - - C - - 1550 1330 220 2340 2238,1240 - - - - - NiI1 A 1.46 - 2185 - a A = after CO addition at 320 K ; B = 10 min after NO addition to gas phase ; C = after heating at 473 K for 10 min ; D = after heating at 473 K for 30 min ; E = after heating at 473 K for 60 min.displaced adsorbed CO completely for the Co" and Ni" samples, adsorbed CO was displaced for the Mn" sample, and adsorbed NO appeared on the FelIr sample. All the samples showed spectra due to carbonate ions, but none due to nitrate products.The carbonate spectra changed during the course of the reaction from spectra typical of bidentate carbonates adsorbed on freshly oxidised samples {separation of v4[v,, (COO)] and v,[v,(COO)] -200-300 cm-l) to spectra more typical of unidentate carbonates (Av = 100-200 cm-l) produced by the CO + O2 or C02 + O2 reactions on CO-reduced samples. This may indicate that the samples underwent some reduction during the reaction. The results of experiments on the adsorption of CO+NO mixtures on CO-reduced samples were not comparable with the experiments for CO+02 or CO2+O, mixtures on similar samples. Admission of a CO+NO mixture to CO-reduced samples produced new infrared bands only for the FeIII exchanged oxide. These occurred at 1780 and 1720 cm-', due to adsorbed NO and at 1500 and 1350 cm-l, due to a unidentate carbonate.This confirms that reaction * Oxidised Mn", Fe"1, CO" and Ni" exchanged gels physisorb COz linearly, exhibiting bands at 2340 cm-l close to the gas phase frequency. Similarly, linear physisorbed NzO exhibits bands at 2238 and 1236 cm-' on all the oxidised gels (except for the Cu"-exchanged sample).P. G . HARRISON AND E. W. THORNTON 2713 is with a reactive oxygen species for CO + 0, and CO, + O2 adsorption on reduced Mn", FelIr and CO" exchanged tin@) oxide. The appearance of carbonate spectra is consistent with a catalyst redox mechanism for the CO +NO reaction catalysed by these oxides, which is the mechanism indicated by kinetic experiment^.^^ The catalytic activity increases in the order Mn" < SnO, < Al"' < Ce"' < CrIII < Co" < UOZ1I < Ni" < FeIII < Cu" for SnO, .0.1 M,O, gels, and it is noticeable that those exchanged cations which adsorb one of the reactants on the surface (CO", Ni" and FeIII adsorb NO, Cu" adsorbs CO) give the highest reactivity. It is also significant that the Co", Ni" and FerI1 exchanged gels show a marked selectivity for reduction of NO to N20 whereas the Cu" exchanged gel shows a marked selectivity for reduction to N2, reflecting the change in mechanism upon change of NO to CO in the adsorbed layer. We thank the S.R.C. and the International Tin Research Institute for support in the form of a CASE Award (to E. W. T.). Part 8. P. G. Harrison and E. W. Thornton, J.C.S. Faraday I, 2604. M. J. Fuller and M. E.Warwick, J. Catalysis, 1973, 29,441. M. J. Fuller and M. E. Warwick, J. Catalysis, 1975, 39, 412. J. E. Germain and R. Perez, Bull. SOC. chim. France, 1975, 735. G. C. Bond, L. R. Molloy and M. J. Fuller, J.C.S. Chem. Comm., 1975, 796. M. J. Fuller and M. E. Warwick, J. Catalysis, 1976, 42, 418. ' P. G. Harrison and E. W. Thornton, J.C.S. Faraday Z, 1975,71, 461. * J. D. Donaldson and M. J. Fuller, J. Inorg. Nuclear Chem., 1968,30,1083. J. D. Donaldson, M. J. Fuller and J. W. Price, J. Znorg. Nuclear Chem., 1968,30, 2841. lo J. D. Donaldson and M. J. Fuller, J. Inorg. Nuclear Chem., 1970,32, 1703. P. F. R. Ewings and P. G. Harrison, Inorg. Chim. Acta, 1976,18, 165. I2 R. A. Gardner and R. H. Petrucci, J. Amer. Chem. SOC., 1960, 82, 5051. l3 R. A. Gardner and R.H. Petrucci, XVIIIth Congr. Int. Union Pure and Appl. Chem. (Montreal, l4 N. S . Bayliss and A. L. G. Rees, J. Chem. Phys., 1940, 8, 377. l5 G. E. Ewing and G. C. Pimentel, J. Chem. Phys., 1961,35,925. l6 J. H. Taylor and C. H. Amberg, Canad. J. Chem., 1961,39, 535. l7 N. S. Hush and M. L. Williams, J. Mol. Spectr., 1974, 50, 349. l8 Handbook of Chemistry and Physics (C.R.C. Press, Cleveland, Ohio, 53rd edn, 1972). I9 A. N. Terenin and L. M. Roev, Actes du Deuxiime CongrGs Int. de Cutalyse (Technip, Paris, 2o L. M. Roev and A. N. Terenin, Opt. Spektr., 1959, 7, 759. 21 L. M. Roev and A. V. Alekseev, Elementary Photoprocesses in Molecules, ed. B. S. Neporent 22 A. N. Terenin and A. Alekseev, J. Catalysis, 1965, 4,440. 23 M. Shelev and J. T. Kummer, Chem. Eng. Progr. Symp., 1971,67,74. 24 C. Naccache and Y. Ben Taarit, J.C.S. Faraday I, 1973,69,1475. 25 E. L. Kugler, R. J. Kokes and J. W. Gryder, J. Catalysis, 1975, 36, 142. 26 A. Zecchina, E. Carrone and C. Morterrs, J. Phys. Chem., 1975,79, 978. 27 D. D. Eley, C. H. Rochester and M. S. Scurrell, J.C.S. Faraday I, 1973,69,660. 2 8 C. W. Poling and R. P. Eischena, J. Electrochem. SOC., 1966, 113,218. 29 M. J. Fuller and M. E. Warwick, personal communication. 30 D. D. Eley, C. H. Rochester and M. S. Scurrell, J. Catalysis, 1973, 29, 20. 31 J. B. Peri, J. Phys. Chem., 1974, 78, 588. 32 L. H. Little and C. H. Ambery, Canad. J. Chem., 1962,40,1997. 33 A. A. Davydov, Yu. M. Shchekochikhin and N. P. Keir, Kinetika i Kataliz, 1970, 23, 136. 34 A. J. Goodsel, J. Catalysis, 1973, 30, 175. 35 L. C. Ferrera and E. C. Lsisagang, J. S. Afiican Chem. Inst., 1970, 23, 136. 36 J. W. London and A. T. Bell, J. Catalysis, 1973, 31, 32. 37 H. C. Tomkins and R. G. Greenler, Surface Sci., 1971, 28, 194. 1961), paper no. B1-30. 1961), vol. 2 ; and references therein. (Consultants Bureau, New York, 1968). (PAPER 7/1975)

ISSN:0300-9599    

DOI:10.1039/F19787402703

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Chlorine Kinetic Isotope Effects Thermal Decomposition of 1 -Chloroethane and Evaluation of Possible Models of Activated Complex BY ALLAN MACCOLL* AND MARGARET N. MRUZEK Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H OAJ Received 23rd November, 1977 Chlorine kinetic isotope effects have been investigated for the pyrolysis of 1-chloroethane in a static system in the temperature range 395482°C. The temperature dependence of the kinetic isotope effects has been determined. The mass spectrometric isotope ratio analysis was made on the hydrogen chloride produced. A model for the chlorine involvement in the four-centre activated complex is advanced and various alternative geometries are evaluated in terms of heavy-atom approximation and first-order high temperature kinetic isotope effects.Best agreement with the experimentally determined values of kJ5/k3’ is given by a model of the activated complex which involves chlorine participation in the reaction coordinate with three degrees of freedom. The effect of the individual geometric parameters that includes the C-CI stretching, the C-C-CI bending and the C-CH3 stretching and their combinatioc, is evaluated. The kinetics of the thermal deconiposition of chloroalkanes in the gas phase has been the subject of many systematic studies and reviews.lm8 Despite different opinions on the nature of the activated complex all authors are agreed that in a seasoned vessel dehydrohalogenation follows first-order kinetics and proceeds by a unimolecular mechanism. Two recent reviews have dealt with this topic in great detail.** A range of possible models of the activated complex for the pyrolysis of chloroalkanes has been considered.They vary from those with a large carbonium ion character, where the C-Cl bond breaking is well advanced in comparison to other bond changes, to those involving a four-membered ring with simultaneous C-C1 and C-H bond rupture and H-Cl and C=C bond formation with a significant degree of charge separation. * Previous studies of heavy atom isotope effects have emphasised that they can provide unique mechanistic information about the nature of the activated complex. l-l and is now well established. The magnitudes of the kinetic isotope effects are known to be related to vibrational changes in bonds involving the isotopic atom in proceeding from the reactant to the activated complex and consequently connected with structural changes.With modern computer techniques it is possible to evaluate the magnitude of the isotope effects that can be expected from various models of the activated complex and correlate them with the experimental results. The aim of this study was to determine the participation of chlorine atom in the activated complex in gas phase elimination reactions by measuring the relative k3 5/k37 ratios and their temperature dependence. The chlorine kinetic isotope effects were particularly suited for the purpose of the activated complex study, since unimolecular dehydrohalogenation reactions involve reduction in force constants associated with the C-Cl bond and therefore an appreciable isotope effect might be expected for The theory was pioneered by Bigeleisen 2714A .MACCOLL AND M. N . MRUZEK 2715 the isotopic substitution on either carbon or halogen. Blades and his co-workers l6 have determined the HID intramolecular isotope effects in the pyrolysis of ethyl chloride but their method might include also the secondary isotope effects, which, being temperature independent, could have a significant influence on the magnitude at the reaction temperat~re.~. l2 The H/D isotope effects determined for the decomposition of chemically activated ethyl chloride in the gas phase were interpreted in terms of four-centre activated complex with a considerable C-H bond extension.” EXPERIMENTAL KINETICS Ethyl chloride was pyrolysed in the temperature range 395482°C using the static method.* The reaction vessel was seasoned with a carbonaceous coating in order to prevent surface reactions. The extent of reaction was determined from the total pressure increase as registered by all-glass pressure gauge and recorded on chart paper. ISOTOPE RATIO DETERMINATION The isotopic analyses were based on hydrogen chloride produced, which was the isotopically substituted product of decomposition. The chloride ion was precipitated with solution of silver nitrate and the dried silver chloride was converted into methyl chloride with MeI, using a modified sealed tube method of Hill and Fry.lS The standard sample of methyl chloride was prepared from hydrogen chloride collected after the total decomposition of ethyl chloride.It was necessary to adopt this method since hydrogen chloride produced is not suitable for direct measurement on the mass spectrometer. The ratios of C137/C135 in methyl chloride samples were measured with an isotope ratio twin collector, double inlet mass spectrometer MS 20 (G.E.C.-A.E.I.). The stability and the reproducibility was checked by determining the zero enrichment on a commerical standard sample of methyl chloride. The reproducibility obtained on four series (10 measurements of 10 samples) was 0.0039 %. The results of the individual zero enrichments were -0.0051+0.0044, - 0.0029+ 0.0040, - 0.00452 0.0029 and - 0.0020+ 0.0041 %. Therefore the mass spectro- metric system was operating satisfactorily. The measured pair of isotopic ion beams was m/e = 52 (CH3C137) and m/e = 50 (CH3C13’).CALCULATIONS All calculations were performed on an IBM System 360/6 and written in FORTRAN LV programming language. The program used for the rigid rotor approximation calculations was divided into five major subroutines. Initially the basic geometric input data and the internal coordinate values were entered and the principal moments of inertia and the MMI factors were calculated. Then the vibrational frequencies and temperatures were entered and the individual terms (VP, EXC, ZPE) evaluated. Finally all the calculations associated with the isotope effects and their temperature dependences were performed. RESULTS EXPERIMENTAL ISOTOPE EFFECTS The ratio of isotopic rate constants was calculated according to the relation derived previously k3’ In [I - f S’] k37 - In [l - r f S ’ ] - where r = R,/R,, R being the ratio of heavy isotope to the lighter isotope in the product at the fraction of reaction f, or at the complete decomposition, a.The factor S’ = [(l +I?,)/( 1 + R,)] is a correction term and can be estimated with sufficient2716 CHLORINE KINETIC ISOTOPE EFFECTS accuracy either from the literature or by the measurement of ratios of the relevant isotopic intensities. For a reaction involving heavy atoms, where the difference in the isotopic ratios is relatively small, the correction term S' is very close to unity. Chlorine kinetic isotope effects in the pyrolysis of I-chloroethane were determined at three temperatures. The fraction of reaction f was determined manometrically.The value Y which is the quotient of the isotopic ratios for the sample R, and the standard R, was averaged from 10 measurements. The values of the isotopic ratios r, fraction of reaction& and calculated k35/1;37 are listed in table 1. TABLE 1 .-CHLORINE KINETIC ISOTOPE EFFECTS IN THE THERMAL DECOMPOSITION OF 1 -CHLORO- ETHANE f 481.6 r temperaturejOC 440.9 r k35/k37 f 394.9 r 0.10 0.99859 1.00149 0.10 0.99852 1.00157 0.10 0.99851 1.00158 0.25 0.99871 1 .OO14g 0.25 0.99869 1.00152 0.10 0.99848 1.00161 0.25 0.99869 1.00152 0.25 0.998'74 1 .OOMg 0.25 0.99866 1.00154 0.50 0.99894 1 .OO153 0.50 0.99891 1.00157 0.40 0.99893 1 .OO157 0.75 0.99908 1.001 56 0.40 0.99866 1.00164 mean k35/k37 = 1.00151 mean k35/k37 = 1.00155 mean k35/k37 = 1.00159 & 0.00002 k ~ .o o ~ o ~ * 0.0000~ The magnitudes of chlorine kinetic isotope effects are small and the dependence on temperature is slight, although significant. The isotope effects studied on the chlorine leaving group appear to be primary, with normal dependence on temperature : their magnitudes increase as the temperature falls. The calculated values of k35/k37 appear to be insensitive to the fraction of decomposition f. Plots of In (k35/k37) against 1/T2 give straight lines. THEORETICAL ANALYSIS The theoretical calculations are based on Bigeleisen's heavy atom approximations for the reduced ratios of partition functions.20 The abbreviated forms of individual contributing factors were introduced by Wolfsberg and Stern.21 The RRA (the rigid rotor approximation)13 is the isotopic rate coiistant ratio on the harmonic oscillator model multiplied by the ratios of symmetry numbers and transmission coefficients The MMI factor is the contribution from rotational and translational partition functions, the EXC factor includes the product of vibrational excitations and the ZPE involves the differences in the vibrational zero-point energies in reactants and the activated complexes.With the use of the Teller-Redlich product rule 22 which relates the vibrational frequencies, principal moments of inertia and masses of nuclei in isotopically substituted molecules, an alternative expression for the isotope effects of the form is obtained where the VP factor is the product of ratios of isotopically different vibrational frequencies and vz/vg is the ratio of imaginary frequencies along the path of decomposition.At infinite temperature this ratio is the limiting value for the kinetic isotope effect and can be determined by the extrapolation of the plot of In (k35/k37) against 1/T2 to zero. In order to evaluate the individual factors contributing to the expression for the rigid rotor approximation, it is necessary to know the geometries and the vibrational RRA = MMI x ZPE x EXC. RRA = (v$/v%) VP x ZPE x EXCA . MACCOLL A N D M. N . MRUZEK 2717 frequencies of the reactants and the activated complexes for both isotopic species. Lack of sufficient information in the spectroscopic literature about isotopic shifts in individual vibrational frequencies necessitated the use of some related approxima- tions.The individual isotopic bands are not usually reported, either because of low resolution or large band broadening. In the case of primary alkyl chlorides where the chlorine isotopic splittings have been determined, only those concerning the C-Cl stretching modes have been MODEL OF THE ACTIVATED COMPLEX On the assumption that the thermal activation of alkyl chlorides resulting in HCl elimination is a unimolecular reaction proceeding through a single molecular step, the activated complex involves a four-centre cyclic structure. As the reaction progresses, the C-Cl and the C-H bonds are gradually increasing in length whilst the C-C bond is shortening and the H-C1 bond is forming. The true geometry of the activated complex might be visualised as some intermediate combination of all four processes.The reaction can be written schematically as \ / \ / \ / C- + C===C +HCl. -c-c- .-* -c=== A planar ring structure of the activated complex with relatively weak interactions between the H and C1 atoms has been used in our calculations. The advantage of such a cyclic structure is that the vibrational modes of the activated complex are similar to those of the reactant molecules and therefore the assignment of the individual frequencies is less difficult. Considering two isotopically different mole- cules, a further simplification can be used, namely that only the vibrational modes with appreciable isotopic shifts have to be included, since in the relative ratios the contributions of the other modes cancel themselves out. The hydrogen vibrations are assumed not to be affected by chlorine isotopic substitution according to the high-frequency separation approximation.In order to develop a model of the activated complex that would reproduce the experimental data to some degree of accuracy a special emphasis was placed on the influence of the relative changes in geometry on the MMI terms. The motions of heavy atoms are represented in the ground state of chloroethane by C-C1 and C-CH3 stretchings and by a C-C-Cl bending and therefore these three geometric parameters were considered to change on going to the activated complex. The effect of individual variations is shown in fig. 1. The relative percentage increase in the C-C1 bond length and the reduction in the C-C-C1 angle were taken from the ground state equilibrium values.The minimum C-CH3 interatomic distance that was considered considered as an extreme was that of the C=C double bond of ethylene. Since the initial assumption was that the model should resemble the reactant more than the products, the relative changes of geometric parameters were considered to be of the order of only a few percent. A set of models was generated as shown in table 2, where either one, two or all three of these internal coordinates were varied. The computer program was set up to evaluate the individual moments of inertia, their products and isotopic ratios of each model and to calculate the MMI factors. The set of Cartesian coordinates was determined for each atom in the ground state equilibrium configuration, using the structural parameters reported by Schwendemann2718 CHLORINE KINETIC ISOTOPE EFFECTS and The molecule was orientated in such a way that the x and y principal axes were in the plane of symmetry and the z axis was perpendicular to them.The same set of Cartesian coordinates was used for the evaluation of the principal moments of inertia for the C13' substituted molecule, since the substitution of heavier isotope changes the C-Cl bond length by <0.01 nm. Sets of Cartesian coordinates for E E I 1.0040 1.0030 1.0020 1.0010 1 / 1.0010 1.0000 0.9990 0.9980 I I f l l I 1 0 20 40 60 80 100 % change in reaction coordinate FIG. 1 .-Dependence of the MMI terms on reaction coordinate : (a) C-C-Cl bend ; (b) C-CH3 stretch ; (c) C-Cl stretch.-_ TABLE 2.-RELATIVE VARIATIONS IN THE INTERNAL COORDINATES FROM THE GROUND STATE FOR MODELS OF THE ACTIVATED COMPLEX AND THE MMI TERMS model Cl-Cl! % C-C-Cl/ % C-CHSI % MMI 1 2 3 4 5 6 7 8 9 10 11 12 13 0.5 3.0 5.0 0.5 2.0 2.0 3 .O 3 .O 3 .O 4.0 4.0 4.0 4.0 0.0 0.0 0.0 2.7 2.7 5 .O 0.7 2.7 5.0 5.0 2.7 5 .O 0.0 0.0 0.0 0.0 0.0 3.0 5 .O 1 .o 4.0 7.0 1 .o 3 .O 5 .O 5 .O 0.999 96 0.999 75 0.999 59 0.999 91 0.999 86 0.999 87 0.999 76 0.999 86 0.999 84 0.999 68 0.999 70 0.999 70 0.999 80 selected models of the activated complex were determined in a similar way by varying the individual internal coordinates according to table 2. The values of the MMI terms (table 2) consist only of the ratios of the products of moments of inertia in the ground state to those of the activated complexes, since the ratio of masses in both states is equal to unity.The MMI factors are not dependent on temperature. VIBRATIONAL MOTIONS OF THE ACTIVATED COMPLEX Three vibrational motions of the ground state ethyl chloride that exhibit a significant dependence on chlorine isotopic mass are the C-C1 and the C-CH3 stretchings and the C-C-Cl bending mode. The largest isotopic splitting has beenA . MACCOLL AND M. N. MRUZEK 2719 observed 23 for the C-C1 stretching where the frequency assigned to chlorine 37 is 4.6cm-l lower than that for chlorine 35. The other isotopic splittings for the C-C-Cl bending and the C-CH3 stretching frequencies are 1.5 and 0.5 cm-', respectively. These motions were considered to undergo significant changes in the activated complex.Since other frequencies were assumed not to be isotopically dependent on the mass of chlorine atom, the model calculations type A were using only those three frequencies and their changes in the activated complex. For model calculations type B we have extended the number of isotopically dependent frequencies according to a recent p u b l i c a t i ~ n . ~ ~ The other vibrational frequencies that appear to have a small isotopic dependence on chlorine are the CH2 wag (0.3 cm-l), the CH, twist (0.4 cm-') and the CH, rock (0.3 cm-l). The frequencies in the activated complex have been estimated by use of the rules of Pauling and Badger.26 These proved to be a valuable guide to the extensions of the internal coordinates, since according to them small changes in the ground state bond length cause considerable changes in the vibrational frequencies.The preliminary calculations showed (fig. 1) that the changes in internal coordinates cannot be larger than a few percent. The values of the isotopic splittings in the activated complex were estimated in the first approximation according to the values reported for 2-chloro-2-methylpropane and I -chlorobutane.2 DISCUSSION Chlorine kinetic isotope effects in the thermal decomposition of 1-chloroethane are relatively small and they exhibit normal dependence on temperature.* In order to limit the great number of possible models the calculated values of the rigid rotor approximation (RRA) were compared with the experimental results. The number of models was further reduced by comparison with the temperature dependence values, which proved to be a very valuable indicator.Some values of the RRA were in a very good agreement with the experimental data at one temperature but they could not be included as possible models of the activated complex, since the discrepancies at other temperatures were too large. Although the four-centre activated complex appears to be rather a complicated model, the theoretical calcula- tions on some models using the assumptions mentioned above could provide reason- able agreement with the experimental results. Since the ratios of masses in the assumed models are identical with reactants, the only contribution to the MMI terms comes from relative products of moments of inertia. The effect of systematic changes in individual geometric parameters on the MMI is shown in fig.1. The increase in the C-Cl bond length has the greatest effect on moments on inertia. Beyond a value corresponding to an extension of 100 % of the ground state internal coordinate, the curve approaches the asymptotic limit. The influence of the C-C-Cl bending coordinate on MMI is in the opposite direction to those of the C-Cl and C-CH3 stretching and appears to be a more effective geometrical parameter than the C-CH3 stretching mode in adjusting the model of the activated complex. The plots of calculated values of the RRA terms in their dependence on relative changes in the individual internal coordinates (fig. 2) show similar relationships to those for the MMI factors. The values of the RRA for the C-Cl stretching and also the temperature dependence are much higher (24.2 x than those determined experimentally (8.0 x The high ratio of the decomposition frequency would require extremely large isotopic splitting in the reaction coordinate and therefore * They are the smallest in the series of five chloroalkanes that were measured.272720 CHLORINE KINETIC ISOTOPE EFFECTS the simple extension of the C-Cl bond is not likely to account for the chlorine kinetic isotope effects.The C-CH3 distance appears to be a considerably less sensitive parameter as regards values of the RRA and also the temperature dependence is much smaller ( 3 . 0 ~ in comparison with the experimental data. The C-C-Cl bending motion increases the values of the RRA considerably over a very small change in the angle but the temperature dependence remains low (4.2 x 1.0080 C (b} 1.0070 1.0060 1 1 .m o y 1.0030 id 1.0020 1.0010 k) 1.0000 0 20 40 60 80 100 % change in reaction coordinate FIG. 2.-Dependence of the RRA values on reaction coordinate : (a) C-C-Cl bend ; (b) C-CI stretch ; (c) C-CH3 stretch ; (d) expeiimental k35/k37. TABLE 3.-RRA OUTPUT FOR MODELS OF THE ACTIVATED COMPLEX OF ETHYL CHLORIDE AT 481.6"C model A vfs/v& VP EXC ZPE VPXEXCXZPE RRAa d i f f ~ 1 0 5 b 1 2 3 4 5 6 7 8 9 10 11 12 13 1.006 93 1.006 71 1.006 55 1.000 81 1.001 10 1.001 47 0.999 91 1.001 12 1.002 06 1.001 18 1.000 96 1.001 29 1.001 43 0.993 08 0.993 08 0.993 08 0.999 09 0.998 76 0.998 41 0.999 85 0.998 74 0.997 79 0.998 49 0.998 74 0.998 41 0.998 37 1.003 46 1.004 39 1.003 46 1.004 39 1.003 46 1.004 39 0.999 89 1.001 35 1.000 03 1.001 60 1.000 37 1.001 59 0.998 97 1.001 64 1.000 05 1.001 59 1.000 93 1.001 64 1.OOO 30 1.001 59 1.000 05 1.001 59 1.Ooo 37 1.001 59 1.000 67 1.001 69 1.OOO 89 1.007 83 24.2 1.000 89 1.007 62 24.2 1.000 89 1.007 46 24.3 1.000 34 1.001 16 9.3 1.000 39 1.001 50 10.4 1.00037 1.001 83 10.0 1.000 46 1.000 37 12.3 1.000 39 1.001 51 10.6 1.000 35 1.002 41 9.7 1.00039 1.001 58 11.1 1.00039 1.001 35 10.5 1.OOO 37 1.001 66 10.0 1.00067 1.002 16 11.7 a Experimental value of k3'/k3' = 1.00151 +0.00002. b (RRA at 394.9"C-RRA at 481.6"C), experimental difference = 8 x As none of the variations in the internal coordinates can bring the calculated RRA in agreement with the experimental results, it is necessary to study the effect of combination of these modes (table 2).Since the MMI and the VP terms are independent of temperature, the contributions to the values of the RRA as a function of temperature come from the vibrationalA. MACCOLL AND M. N. MRUZBK 2721 modes, namely from the EXC and ZPE terms. Their magnitudes change with temperature but in opposite directions, with the ZPE term increasing about twice as fast as EXC as the temperature is lowered. The change in the EXC factor is relatively minor but it reduces the effect of the ZPE factor, so the total temperature dependence on their product, which is the real contribution to the ratio of reduced partition functions, is small. The values of the products of VP x ZPE x EXC for all selected models vary between 1.000 35 and 1.000 89 (table 3).From the selected set of models of the activated complex that were examined in the theoretical analysis, two models that agree best with the experimentally observed chlorine kinetic isotope effects and their temperature dependence, appear to be numbers 5 and 8. The values of k35/k37 and the RRA values for both models are TABLE 4.-cALCULATED VALUES OF THE RRA OF MODELS 5 AND 8 COMPARED WITH THE EXPERIMENTAL VALUES temperature/% model 5A model 8A model 8B experimental 4 8 1 . 6 1.001 50 1.001 51 1.001 51 1.001 51 440.9 1.001 55 1.001 55 1.001 55 1.001 5 5 3 9 4 . 9 1.001 60 1.001 6 1 1.001 60 1.001 59 compared in table 4. The geometrical parameters are close for both models (table 5 ) and the values of the MMI. terms are identical (0.999 86).Model 5 requires the C-Cl internal coordinate to be extended by 2 %, the C-CW3 bond to be shortened by 3 % and the C-C-C1 angle to be decreased by 2.7 % from the ground state equilibrium values. For model 8 the C-Cl bond is increased by 3 % and the C-CH3 bond is decreased by 4 %. The C-C-C1 angle decrease is the same in both cases. The association of the decomposition frequency with the C-C1 stretching in the model calculation of mixed modes necessitates higher isotopic splittings for the C-C-Cl bending than the C-CH3 stretching frequencies (table 6). This phenomena remained unchanged when other isotopically dependent frequencies were included. TABLE 5.-INTERNAL COORDINATES FOR CHLORINE PARTICIPATION IN THE ACTIVATED COMPLEX ground state model 5 model 8 0.1788 0 .1 8 2 4 0 . 1 8 4 2 m - C H h m 0 . 1 5 2 0 0 . 1 4 7 4 0 . 1 4 5 9 L c--c-Cl/deg 1 1 1 . 3 108.3 108.3 Y c c l /nm The temperature dependence for model B with more vibrational motions appears to make the range smaller and to improve the agreement with the experimental data, but the effect is not very significant. The values of the temperature dependent terms (ZPE and EXC) are also very similar for both types as listed in table 7. The effective frequency for model A with fewer frequencies is M 80 % of the ground state value. The derecase to 74 % for model B might be the reason for the smaller temperature range since the ZPE is the factor which is basically affected by the sum of the real iso- topic shift changes in the models of the activated complexes.It can be observed from fig. 3 that the isotopic splittings have a significant influence on the calculated values of the RRA. The increase in from 3.5 to 4.3 cm-1 produces the linear drop of the RRA from 1.002 57 to 1.000 38. The slope is about twice the value ( - 2.74 x for any of the other vibrational motions (- 1.33 x for the C-CH3 stretch). So it appears possible to adjust the value of the RRA by varying the isotopic splitting for the bending coordinate in the activated complex.2722 CHLORINE KINETIC ISOTOPE EFFECTS The theoretical analysis of models of the activated complex shows that the use of the reduced partition function ratios, together with the heavy atom approximation, provide satisfactory agreement with the experimental results. The chlorine motion in the activated complex generates changes in the other vibrational modes, with the significant contribution coming from the C-C-C1 bending and C-CH3 stretching.All changes in geometrical parameters are only a few percent from the ground state TABLE 6.-FREQUENCIES AND ISOTOPIC SPLITTINGS FOR MODEL 8 OF ETHYL CHLORIDE/Cm-* ground state model 8A model 8B EtCW Av EtC135 Avf EtC135 Av* - - 664.9 4.6 - - vc-CI VC-CHL, 974.0 0.5 1053.9 1.2 1053.9 0.74 vc-c-CI 336.0 1.5 389.8 3.7 389.8 3.85 VCH2 wag 1287.0 0.3 1426.0 1.2 VCH2twist 1245.0 0.4 1224.4 0.24 VCHzrock 785.0 0.3 1070.8 0.1 TABLE 7.-cOMPUTER OUTPUT FOR MODELS 8A AND 8B AT 481.6"C MMI v & M VP EXC ZPE ZPE X EXC model 8A 0.999 86 1.001 12 0.998 74 1.OOO 05 1.001 59 1.001 64 model 8B 0.999 86 1.001 21 0.998 65 1.00005 1.001 61 1.001 65 values, Model calculations reveal that it is possible to achieve satisfactory agreement with the experimentally observed values of kinetic isotope effects by varying the isotopic splittings in those reaction coordinates that have low vibrational frequencies. The temperature dependence seems to be influenced by the changes in reaction coordinates with higher vibrational frequencies.This effect is so strong that the 1.0022 1.0020 1 .OO I8 d '. 0°16 Fr: 1 0014 1.0012 1. 0010 1.00081 . 1 1 t I I I I 1 - 0 1.0 2.0 3 .O 4.0 cm-' stretch ; (c) CH2 twist ; (d) CH2 rock ; (e) C-C-Cl bend ; (f) experimental k3'/k3'. FIG. 3.-Dependence of the RRA terms on isotopic splittings at 394.9"C : (a) CH3 wag ; (b) C-C increase in the shift of the CH, wag by more than five times its ground state value results eventually in the inverse dependence of the RRA on temperature.The chlorine kinetic isotope effects measured on pyrolysis of 1-chloropropane, 1 -chloro- butane, 2-chloropropane and 2-chloro-2-methylpropane, where the temperature dependence is much more significant than in the case of 1-chloroethane, are in betterA . MACCOLL AND M . N. MRUZER 2723 agreement with the theoretical calculations for model 8 than mode1 5.27 Since the individual factors do not reflect the value of kinetic isotope effects consideration of the complete equation is essential in the evaluation of theoretical models of the activated complexes. We thank the S.R.C. for a grant for the purchase of the M520 (A.E.I.) isotope ratio mass spectrometer.D. H. R. Barton and K. E. Howlett, J. Chem. Soc., 1949, 165. K. E. Howlett, J. Chem. Sac., 1952, 3695. W. Tsang, J. Chem. Phys., 1964,41,2487. H. Hartmann, H. G. Bosche and H. Heydtmann, 2, phys. Chem. (Frankfurt), 1964, 42, 329. D. H. R. Barton, A. J. Head and R. S. Williams, J. Chem. Soc., 1951,2039. H. Heydtmann and G. Knck, 2. phys. Chem. (Frunkfurt), 1961,28,85. W. H. Saunders Jr and A. F. Cockerill, Mechanisms of Elimination RLactions (Wiley-Inter- science, New York, 1973). ' K. A. Holbrook and A. R. Marsh, Trans. Faraday Soc., 1967,63, 643. ' A. Maccoll, Chern. Rev., 1969, 69, 33. lo K. C. Kim and D. W. Setser, J. Phys. Chem., 1974,78,2166. IIA. Fry, Isotope Eflects in Chemical Reactions, ed. C. J. Collins and N. S. Bowman (Van Nostrand-Reinhold, New York, 1970). l 2 A. Fry, Chem. SOC. Rev., 1972,1,163. "A. Maccoll, Ann. Rep. Chem. Soc. A, 1975, 77. l4 J. Bigeleisen and M. Wolfsberg, Adv. Chem. Phys., 1958, 1, 15. J. Bigeleisen, J. Chem. Phys., 1949, 17, 675. l6 A. T. Blades, P. W. Gilderson and M. G. H. Wallbridge, Canud. J. Chem., 1962, 40, 1526. l 7 K. Dees and D. W. Setser, J. Chem. Phys., 1968,49,1193. '' J. W. Hill and A. Fry, J. Amer. Chem. SOC., 1962,84,2763. J. R. Christie, W. D. Johnson, A. G. Loudon, A. Maccoll and M. N. Mruzek, J.C.S. Faraday I, 1975,71, 1937. 2o Isotope Efleects in Chemical Reactions, ed. C . J. Ccllins and N. S. Bowman (Van Nostrand- Reinhold, New York, 1970), chap. 6. M. Wolfsberg and M. J. Stern, Pure Appl. Chem., 1964, 8,225. 1955), p. 184. 2 2 E. B. Wilson, J. C. Decius and P. C. Cross, Molecular Vibrations (McGraw Hill, New York, 23 R. C. Williams and J. W. Taylor, J. Amer. Chem. Suc., 1973, 95, 1710. 24 R. H. Schwendeman and B. D. Jacobs, J. Chem. Phys., 1961,36,1245. 2 5 R. L. Julian and J. W. Taylor, J. Amer. Chem. SOC., 1976,98,5238. 26 H. S. Johnston, Gas Phase Reaction Rate Theory (Ronald Press, 1966), p. 81. 27 M. N. Mruzek, Ph.D. Thesis (University of London, 1976). (PAPER 7/2055)

ISSN:0300-9599    

DOI:10.1039/F19787402714

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Chemical Processes at Clean { loTO] ZnO Surfaces Part 1 .-Thermal Production of Surface Defects BY MINO GREEN" AND IMENTS R. LAUKS Department of Electrical Engineering, Imperial College, Exhibition Road, London SW7 2BT Received 24th November, 1977 Oxygen loss from single crystal ZnO { lOiO} surfaces has been investigated using the method of thermally programmed desorption (t.p.d.). The t.p.d. parameters (peak area, shape, width at half height and temperature of the maximum) have been analysed in terms of O2 and Zn loss kinetics from the outermost surface atoms of the crystal only. A reaction mechanism is proposed in which both positive surface oxygen vacancies and neutral surface step sites are formed, and in which the reaction kinetics are dependent upon the fractional concentration of both kinds of surface defect (6 and O,,).The maximum value of 8 is 4 % and 6,, is -20 %. There have been many studies of the chemical and physical properties of zinc oxide surfaces, and the present position has been thoroughly reviewed. 1-3 However, these studies have left a great deal unresolved, and if the objective is to obtain a deeper knowledge of the elementary processes occurring at the surface of the solid, then it is necessary that well-defined single crystal surfaces should be the starting point of any investigation. A well-defined surface is taken to mean one for which the surface composition, surface crystallography and surface state type and levels is known. Of the few papers on clean (lOT0) ZnO single crystal surfaces 4-8 bearing on this study, those by Gopel 7* are the most relevant.It is reported that clean (lOT0) surfaces are unreconstructed but do have split LEED spots, typical of terraces, and that surface vacancies of oxygen are formed. This paper, the first in a series of studies on well-defined ZnO single crystal needles in which -99.9 % of the exposed surface is the non-polar (IOiO), is concerned with determining the extent and kinetics of surface defect (oxygen vacancies) formation. The technique employed is thermally programmed desorption (t.p.d.). Halpern and Germain have used t.p.d. to investigate various oxides and it was their work which led to our adopting this technique, though the analysis of the data has had to be very much extended. Work carried out on Q16/018 exchange and N20 decomposition is reported in Parts 2 and 3.1° EXPERIMENTAL ZnO CRYSTALS AND SURFACE CLEANING PROCEDURE The ZnO used in this study consisted of a mass of single crystal needles of radius-to-length ranging between (5 pm to 10 mm) to (250 pm to 14 mm), most of them being in the smaller range.The needles have a regular hexagonal cross-section normal to the c-axis; the exposed planes parallel to the c-axis were found (by an X-ray rocking method) to be (1OiO) and are termed the prism planes. The hexagonal end faces of the needles, the polar planes, 2724M . GREEN AND I . R . LAUKS 2725 constitute xO.1 % of the exposed surface area. Scanning electron microscopy shows no gross surface defects. The crystals were of high purity, the greatest impurity concentrations being Ca(l), Fe(0.5), Mg(0.4), Si(5) and A1(1.5), the bracketted numbers being parts per million.The “ as-grown ” crystals had resistivities E 10 Q cm. The surface area of the crystal masses used in this study was ~ 2 5 0 cm2, as determined by gas adsorption using Kr and the B.E.T. method of analysis. The surface cleaning procedure and analysis of surface impurity concentration have been described e1sewhere.l APPARATUS The apparatus is shown in block form in fig. 1. It consists of three chambers : the gas source chamber ; the reaction chamber plus leak valves 1 and 2 ; and the analysis chamber plus pump. The essential features of the apparatus are as follows. SOURCE CHAMBER This was a metal system except for the Pyrex glass gas flasks containing O2 or N20 (research grade) and the thermistor gauge, capable of being pumped down with sorption and ion-pump to z 5 x lo-’ Torr.The high pressure thermistor gauge l2 was previously calibrated (for O2 or N20) using a McLeod gauge. Leakvalve2 Leakvalvel (conductance (conductance K2,tm3s?) K Jcm’s“) a ThiVtllktOr gLKIge Ion purrp Mass 1 (speed Skrn’s ‘1 spectmeter 1 Ion wmp I ! I Analysts chamber Reaction chamber Source chamber (volume v,/cm3) (volume v,/cm’ (volume v,lcm’) (pressure f./torr) (pressure P,/ torr) (pressure P.ltwr) FIG. 1.-Schematic diagram of the apparatus. REACTION CHAMBER This was an all-quartz vessel (shown in fig. 2) and connected to the leak valves via stainless steel connectors. The O-rings were prebaked Viton. Leak valve 1 (Vacuum Accessories) was stainless steel with a bellows mounted micro-grooved pin construction.The conductance calibration was checked in the usual way for both O2 and N,O and was accurately known down to cm3 s-l. Leak valve 2 (Nupro) was a stainless steel 10 turn needle valve with calibrated conductance in the range 10-2-5.5 x lo-’ cm3 s-l. ANALYSIS CHAMBER AND ION-PUMP These were all stainless steel. The ion-pump speed was calibrated over the pressure range 10-8-10-6 Torr (-constant to 7 %) and found to be (1.25k0.05) lo3 cm3 s-l for 0 2 and (1.5Of0.05) lo3 cm3 s-l for N2. A mass spectrometer (A.E.1.-MS10) was used for analysis. The calibration was frequently checked and found to correspond to the makers’ quoted values, namely 48 pA Torr-l for N2 and 28 pA Torr-I for 02. The entire system was baked before a run, However, it was found that the freshly baked analysis chamber walls had a finite pumping speed and so it was necessary to allow2726 ZnO SURFACES the system to stand for several hours in order to saturate this unwanted effect.The back- ground spectrum of the analysis plus reaction chambers was HzO+ (8 x CO+/N; (8.8 x The background spectrum of the reaction chamber was obtained by measuring the decrease in pressures with leak valve 2 closed and calculating the reaction chamber pressures which produced the difference in the two sets of readings : the valves so obtained were HzO+ (1.7~ lo-’), CO+/N; (3 x and CO; (4.5~ The background spectrum of the source chamber, to which another mass spectrometer was attached, was HzO+ (4x CO+/NZ.(1.5 x O2 (4x and CO, (1 x Bracketted quantities above are pressures in Torr. and CO; (1.3 x L. v 2 (z F o - ring anion--+ H 1 cm. u - Temperature distance programmer 2 FIG. 2.-Schematic diagram of the reaction chamber. - ___ FURNACE AND REACTION CHAMBER Fig. 2 shows the arrangement of reaction chamber, furnace aiid thermocouples, together with coiitroller and recorder. The total volume of the chamber, including that of the leak valves was 5.80 cm3. The tube furnace was wound so as to achieve a flat temperature profile (k2 K) at furnace temperatures in the range 300-1100 K, as shown in the figure. The furnace was powered by a temperature programmer (Stanton Redcroft) capable of linear heating, or cooling, rates of 0.017 to 1.7Ks-l, or thermostatic control up to 1300K.T/C 3 was the programmer sensing element. T/C 1 and T/C 2 measured the temperature around the ZnO crystal mass, and were given good thermal contact to the outer silica walls by means of a silver paste. At a heating rate of 0.4 I<; s-I, T2 was 5 K greater than TI : at lower rates temperature differences were 2 K and at a rate of 1.7 K s-’ the difference was 8 K. T/C 1 was taken as the temperature of the ZnO mass, since the thermocouple well was immersed in the dense mass of ZnO needles. SYSTEM RESPONSE The various system parameters such as pressures, volumes, conductances and pump speed are referred to in fig. 1. Over a wide range of operating conditions the time lagM . GREEN AND I . R. LAUKS 2727 between a pressure change in the reaction chamber and that measured in the analysis chamber reduces to the simple expression l3 For our values, Vr = 5.8 cni3, V, = 1100 cm3 and S = 1250 cm3 s-l and a typical K2 value of 3 x 10-1 gives z = 20 s.Under conditions of t.p.d. used extensively in this work leak valve 2 is wide open and z is 11.2 s. This means that a gas wave, called " a peak ", will be distorted by the system response. An analysis l3 shows the true peak height to be reduced - 5 % and the maximum temperature of the peak to be increased -2 K (this is partly cancelled by the thermal lag mentioned above). TYPICAL T.P.D. RUN A run consists of two parts : first pretreatment of the clean ZnO crystal mass followed by quenching ; and secondly a t.p.d. in which the evolution of oxygen as a function of time is followed.Pretreatment consists of initially heating the ZnO mass in vacuum at - 1100 K for 15 min. This serves to remove any carbon recontamination which may have occurred on standing and leaves the ZnO surface depleted in oxygen. The ZnO is then cooled to the specific pretreatment temperature required (in the range 850-1000 K) and exposed to a particular I40 120 100 080 063 040 M 020 300 360 ;GO 340 timelmin FIG. 3.-Oxygen t.p.d.s after various pretreatments at 963 K for 1 h in oxygen. Poz = (a) 0.360, (b) 0.194, (c) 0.062, ( d ) 0.370, (e) 0.051, (f) 0.045, (9) 0.039.2728 ZnO SURFACES O2 pressure (under flow conditions) in the range 0-0.5 Torr for 1 h. After this time leak valve 1 is closed and 2 is turned fully open and the furnace is switched off. It takes about 1 min to cool to 750 K.These conditions are such as to constitute quenching of the surface, corresponding to the chosen temperature and pressure of pretreatment. The t.p.d. run is next carried out starting from some known low temperature, e.g. 700 K, with the linear heating rate p set, for all runs reported here, at 0.4 IS s-'. p is set to become zero at 1100 K. A range of t.p.d. curves is shown in fig. 3 of the next section. O2 was the only species detected by the mass spectrometer and H20, CO, COr, etc., remained at the initial background level through a t.p.d. run. RESULTS Fig. 3 shows a typical set of t.p.d. peaks for various conditions of pretreatment. Almost invariably a single well-defined peak occurs, in which case the peak shape, defined as the ratio of the areas either side of a vertical drawn through the maximum of the peak, is found to be (1.86+0.2) : 1.Occasionally a doublet is observed, and this feature is discussed later. 0*04< Po2 /Torr FIG. 4.-Oxygen t.p.d. peak area, Ae, and fractional surface oxygen vacancy concentration, 8, against oxygen pretreatment pressure, Po2, at various tempzratures of pretreatment : (0) 993, ( 0 ) 979, (+) 963, (0) 923, m) 888 K ; the solid lines are the best fits through experimental points, the broken lines are theoretically computed isotherms. The area of a peak is a measure of the net amount of O2 evolved from the ZnO, and is expressed in atoms of oxygen evolved per surface lattice oxygen atom, i.e. as the fraction At?. A0 depended upon the O2 pretreatment conditions of temperature and pressure, and these results are summarised in fig.4.M. GREEN AND I . R. LAUKS 2729 The interrelation between t.p.d. features is shown in fig. 5 and 6. Thus fig. 5 shows the relation between maximum peak temperature, Tp, and peak area, AO, for a comprehensive set of runs, while fig. 6 shows Tp against AT., the peak width at half height. 0-035 0.030 I 0 0020 d 0 015 0 010 0 GO5 920 940 960 980 1000 1020 1040 TP/K FIG. 5.-Experimental variation of peak area, AO, with peak maximum temperature, Tp. The solid line is the theoretically computed variation. 920 940 960 980 1000 1020 1040 1060 TPIK FIG. 6.-Experimental variation of peak width at half height, AT+ with peak maximum temperature, Tp. The solid line is the theoretically computed variation.2730 ZnO SURFACES range 0.017 to 0.4 K s-l.There is no variation in the above characteristic parameters as /3 is varied over the DISCUSSION SOURCE OF EVOLVED OXYGEN When ZnO is heated to 1100 K the observed oxygen loss or gain is, in all iniportaiit senses in this study, only a loss or gain from the outermost layer of the crystal, i.e. from the surface lattice. This is to be expected from the existing thermodynamic data on ZnO taken together with our surface-to-volume ratio. The equilibrium oxygen vacancy concentration is given by l4 [Yo] = 1.1 x lo2’ P;: exp (-2,25/kT)/~m-~ (1) where the energy is in eV and Poz is the oxygen pressure in Torr. Using this relation and assuming complete equilibrium through a crystal, which in fact is highly unrealistic, we can compare the difference in the amount of oxygen (atoms) which should come from crystals pretreated at 0.36 Torr and crystals pretreated at 0.039 Torr corresponding to (a) and (9) in fig.3. For our experimental ZnO volume of 0.08 cm3 and for a temperature of 963 K, AIVo] is 1.3 x 10l2, while the difference in the amounts of oxygen obtained by t.p.d. experiments is 3.7 x 10”. There is direct experimental verification of the negative predictions made using eqn (1) based on 016/018 isotope exchange. Briefly, since this is discussed fully in Part 2,1° a known amount of 0 l 8 is incorporated into the ZnO. Some of this 0l8 is subsequently abstracted from the ZnO, by t.p.d., and the amount by which it has become diluted with 0l6 measured. The observed dilution is only consistent with equilibration with the surface lattice.Finally, the isotope exchange experiments show that the oxygen of a t.p.d. cannot be associated with oxygen adsorbed on a passive surface. Thus if we were simply dealing with chemisorbed oxygen, where there was no exchange with the surface lattice, we would expect, upon desorption, to obtain exactly that amount of OI8 which had been adsorbed. This is shown not to be the case, 0 l 8 being distributed among the atoms of the surface lattice. SURFACE LATTICE STOICHIOMETRY When ZnO is pretreated in O2 at the lowest temperature (888 K) and highest oxygen pressure (0.44 Torr) shown in fig. 4, it has its highest oxygen content and, by inference, its lowest oxygen surface vacancy concentration. The 888 K isotherm is almost saturated at the higher pressures and likewise the 0.44 Torr isobar is almost flat at the low temperature end: it is deduced that we are therefore close, in this pretreatment condition, to maximum oxygen content of the surface.By extra- polation, it is estimated that, in the limit, another = 15 % oxygen would be taken up. Such a surface would be relatively free of vacancies (i.e. 5 10l2 cm-2) and when subjected to t.p.d. would yield net oxygen loss corresponding to -4 % surface vacancies (i.e. -2.4 x 1013 cm-2). This conclusion is supported by the resistivity against temperature and pressure measurements carried out by Arghiropoulos and Teichner l5 on sintered ZnO powders. The above observations allow us, within fairly narrow limits, to estimate the fractional surface lattice vacancy concentration shown as 8 on fig.4. The scatter in the isotherms is attributed (see later) to variations in surface heterogeneity associated with the history of pretreatment.M. GREEN A N D I . R . LAUKS 273 1 REACTION MECHANISMS In proposing a reaction scheme it was necessary to note that a peak could not be formulated for the 0-4 % oxygen loss from a surface lattice, without at the same time having some limiting mechanism. For this we propose a parallel zinc loss (experi- mentally observed), and consequently the observed oxygen at the mass spectrometer is the difference in the amounts of oxygen and zinc lost from the ZnO. Two possible reaction schemes are formulated, the first of which, the vacancy-pair mechanism, follows immediately.Oxygen evolution : K t "0: $ "A . . .o- k2 slow k3 TV,+ . . . 0- +s(n)O: + O,(g) +T,+ . . . +2'pe- "6 . . . "V,+ + 2'v; and parallel zinc loss : k4 +SPe- + zn(g)+'O: k5 224s) + 02(g) + 2ZnO(s). (3) (4) The symbolism used above is the system due to Kroger and Vink16 : is oxygen on an oxygen site in the surface lattice layer (superscript s) ; is a vacant surface lattice oxygen site with effective charge + 1 ; . . . 0- is a surface (i.e. on top of the surface lattice) 0- adsorbed species adjacent to a TVof species ; s(n)Oj: is a surface lattice oxygen next to an oxygen vacancy; . . . is a nearest neighbour pair of vacancies; spe- is a quasi-free electron in the conduction band confined to a surface space charge region within the ZnO. The second mechanism differs from the above in the oxygen evolution branch after the first pseudo-equilibrium step, eqn (2), i.e.k; 'V; . . . 0- -+ 'VA + 'Pe- + O(ads) O(ads)+"V,f . . . 0- -+ 02(g)+'V~+spe-. slow k; Here O(ads) symbolises an adsorbed oxygen atom, which is taken to move a limited distance over the surface to react with a surface oxygen ion. Since t.p.d. kinetics alone do not allow a choice to be made between the two postulated mechanisms, but Gopel's work,8 in which no desorbed oxygen atoms are observed, and 016/0'8 exchange studies (Part 2) appear to point to the first of the above schemes, the rest of this discussion is confined to the vacancy-pair mechanism. The first step, eqn (2), is taken to precede the rate determining step and is in pseudo-equilibrium. The rate determining step, eqn (3), is the abstraction of a nearest neighbour lattice oxygen by the surface 0- to yield O2 gas, leaving a charged surface lattice oxygen vacancy pair and two compensating space charge electrons. The succeeding step, eqn (4), is rapid decomposition of the vacancy pair. The parallel zinc loss reaction is written in the form shown because it is assumed that only sZn; adjacent to VV,* can be lost to the gas phase and that the resulting missing ZnO has simply lowered the surface lattice by one layer at that two atom site, hence the creation of a "0: in eqn (9, see fig.10. Finally the zinc vapour condenses on the2732 ZnO SURFACES cooler parts of the In a large reaction mass-spectrometer reaction vessel, as Zn(s), where it reacts with oxygen to give ZnO.chamber O2 and Zn can be determined separately in a line-of-sight ' before surface reaction as has been shown by GopeL8 REACTION KINETICS The observed net rate of O2 evolution in a t.p.d. is given by and, for the evolved zinc, which is rapidly oxidised as condensed zinc atoms, whence eqn (9) can be rewritten as k, c" V,' ] rPe-] = 2 k5 Po, [Zn( s)] 2, (10) The net rate of formation is given by and with the pre-rate determining step in pseudo-equilibrium, i.e., p,'. . . o-] c"03 Kl = and the rapid post rate determining step giving we obtain k3rVO+ . . . 'V;] = k2[3'2 . . . O-]~'")O~], which by reference to eqn (11) shows, as it should, that dpV,'] - 2dPo2 dt dt * --- Since ["V,'] does not exceed 4 %, ["03 can be taken as constant and equal to the number of surface oxygen atoms in the (1OiO) ZnO surface.Rewriting eqn (15) using the fractional vacancy concentration, 8 = ["V,']/c"O,"], we obtain RATE EQUATIONS Eqn (17) can be written d0 + + - = j 2 - j 4 . dt Using a; for the activity of species i in phase a and adopting absolute rate theory, (19) kT h t j , = F4 -a+,, a:- exp ( - A f i * / k T )M . GREEN AND I . R . LAUKS 2733 where I;, k, T and h have their usual meaning and AFY* is the standard molecular free energy of formation of the activated complex in the slow step, eqn (5). Since the surface electron activity is related to the bulk electron activity in the ZnO, where 4' is the surface potential as defined in fig. 7 and e is the electronic charge. Eqn (19) becomes (20) a:- = a:- e-&'IkT Now A&':* can be written as the free energy difference between transition and initial state, namely which simplifies to where P4(O < p4 < 1) is related to the fractional charge on the transition state, z = -e, and AfiG8=o, is the standard free energy when the space charge potential is zero.AC' = AF:&=0)+(84- l)z$'-e@ (23) Substituting eqn (23) in eqn (21) gives Charge free Space charge bulk I region - 1- I FIG. 7.-Energy level diagram for ZnO including surface space charge region. Thus, as has been shown by Green,17 it is variations in bulk electron activity and not surface activity which is compensated by q5s changes which affect the reaction rate. But @ does affect the free energy of the charged species on the surface, e.g. the surface vacancies.For the forward reaction where a:(,,, is taken as unity. 0,2734 ZnO SURFACES Combining eqn (24) and (25) in eqn (17) SPACE CHARGE, #'AGAINST 8 The surface oxygen vacancy is a shallow singly charged donor state ~ 0 . 0 5 eV below the conduction band edge,8 hence VV,' is the state of the oxygen vacancy taken throughout this work. Since any PV,'] of the order of 10l2 cm-' or more will give rise to a degenerate carrier distribution at the surface and will also cause the surface state energy level to broaden into a band overlapping the conduction band edge, we use degenerate statistics throughout and may ignore the effects of variations in c& on the position of the Fermi level at the surface (with respect to E,, cf., fig. 7). Furthermore, at- is taken in this work to be constant and close to an at- value corresponding to EF near the conduction band edge.Using the semiconductor relations given by Seiwatz and Green for a degenerate space charge region, we obtain Taking pz = Q4 = 3, which is the situation corresponding to a symmetrical barrier, and is what is generally done in the absence of detailed knowledge, substituting eqn (27) into eqn (24) and also substituting for K,, i.e. - z p = eos = -3.25 0°*8[eV. (27) Kl = exp (- AFy[kT) (28) This rate equation can be restated in terms of enthalpies of activation by incor- porating the entropy term into the pre-exponential, which is now primed, to give 1- r H Y * ---620°*8] dtl - = x ' e x p - r p * 1,2 + kT 3.258°*8 d t Y'8 exp- (30) SURFACE HETEROGENEITY Eqn (30) predicts a peak in the oxygen t.p.d.with the correct shape and half width, but does not yield the observed Tp against A0 variation. We therefore need to seek a second term that alters the enthalpy of activation as a function of 0. We postulate oxygen loss from a heterogeneous surface where the extent of heterogeneity is dependent upon 8 ; that this heterogeneity is most likely surface steps is discussed later. We call this term g(0). The rate eqn (30) now becomesM . GREEN AND I . R. LAUKS 273 5 since (d6/dT) = P-'(de/dt). Eqn (31), and similar equations, are solved by numer- ical methods1 Y' = 8 x 10ls s-', AH?,; = 3.47, AH:* = 3.83 eV. These are to be compared in the case of AH?,; with 3.52k0.1 and 3.61 eV obtained by Gopel and Halpern and Germain respectively.Gopel reports a value of 3.47k0.1 eV for AH:*. In order to obtain the best value for g(6), eqn (31), with one trial function of g(0) against 6 inserted, is solved by an iterative technique. A family of curves, d0/dT against T, for different values of O0 is obtained. These are compared with the experimental set of t.p.d. curves. This procedure is repeated with different values of the g(0) against 8 function until the fit is good. The values of the kinetic parameters obtained were, X' = 5.2 x 5 0 i 2 - 0.08 004- 0 20 1 - I 0 16 0 0.01 0.02 0.03 0.04 8 FIG. 8.-Change in the activation energy of the surface oxygen loss reaction due to surface heterogeneity. (a) g(8) (t.p.d.); (b) g(8) (steady state); (c) 8ss(o) = 0 at e0 = 0, kTln {1+8ss exp (0.35 eV/kT) + I]) ; (d) Oss(o) = 0 at Bo = 0, kT In { 1 + BSs[exp (0.25 eV/kT) - 11 ).The g(0) function is shown in fig. 8, and from it a family of theoretical t.p.d. peaks has been computed for different values of 807 the vacancy concentration at the start of a t.p.d. run as set by the pre-treatment. This is shownin fig. 9. Acomparison of these curves with the experimental t.p.d. data is made by observing the agreement between the peak characteristics, namely peak shape, A0 and AT+ against Tp. The A6 against Tp relationship, shown in fig. 5 has predicted the correct variation, namely a decrease in Tp of over 100 K with decreasing A6. The experimental AT+ against Tp relationship is compared with the predicted relation in fig. 6. Peak shape for a large sample of experimental t.p.d.s is in the ratio (1.8640.2) : 1 and for the theoreti- cally generated peaks the ratio is (2.08 kO.2) : 1, showing agreement within experi- mental error.STEP SITES The likely physical origin of the g(0) term in the rate expression emerges upon closer examination of the reaction mechanism. When a zinc atom is lost froin the surface lattice according to the reaction svVf,+spe- + zn(g)+SOi2736 ZnO SURFACES 3 TIK FIG. 9.-Theoretically computed oxygen t.p.d.s. (a) do = 0, Ad = 0.025 ; (b) 60 = 0.01, A8 = 0.01475; (c) 4, = 0.015, Ae = 0.01 ; (d) do = 0.035, Ad = 5x (e) 60 = 0.0425, Ae = 4.9 x lo-' ; (f) Bo = 0.0428, A0 = 3.3 x lo-' ; (9) 60 = 0.0438, A6 = 1.1 x ; (h) do = 0.038, Ad = 2~ ci> eo = 0.0405, A0 = 4.6~ 10-4; (k) do = ( i ) eo = 0.0395, Ad = 9 .8 ~ 0.041, A0 = 2.9 x t Surface atoms ('AX, Bulk atoms ( bAc) FIG. 10.-Schematic diagram of the surface zinc loss reaction from the 1070 surface.M . GREEN AND I . R. LAUKS 2737 not only is a sub-surface lattice oxygen atom revealed but the surface oxygen atom next to the evaporated zinc is now in a new environment, i.e. at a step site (4. schematic diagram of the zinc loss reaction of fig. 10). It is likely that the enthalpy of activation of oxygen loss from such a site will be less than that for a perfect surface lattice oxygen site. It is proposed that the equilibrium constant of step 1 of the reaction sequence is thus different for a step site oxygen atom; i.e. an oxygen atom at a lattice step is more likely to move onto the surface. T/K FIG. 11.-Theoretically computed oxygen t.p.d.s with different values of initial surface vacancy concentration, Bo : (a), (6) and (c) = 0, (d) = 0.015 and surface step site concentration, Bsso : (a) 0.05, (b) 0.017, (c) 0, (d) 0.017.Now if a fraction O,, of all surface lattice oxygen sites are at steps, i.e. (1-O,,) is the fraction of perfect lattice sites, and AE is the extent by which the free energy of formation of 'V; . . . 0- is reduced when formed from a step site oxygen atom, eqn (3 1) becomes, Y'O - [A=* - 1 .620°.'] kT exp- This equation can be solved by iterative techniques using different initial values of 60 and O,,,, i.e., values of vacancy concentration and step site concentration at the start of the t.p.d., as set by the pretreatment conditions, and a trial value of AE, to obtain2738 ZnO SURFACES a family of dO/dT against T t.p.d. curves.Fig. 11 shows such a set of curves (for AE = 0.25 eV). In the case where 8, = 0 and O,,, = 0, a peak is obtained whose maximum, Tp, occurs at 1040 K, and whose area, A@, is -0.025, identical with the peak obtained by solution of the rate equation using the g(8) function. The values against 8 for the kT In [l +O,,[exp (0.25/kT)- l)] term, with 0,,, = 0 and 0, = 0 obtained from the iterative technique are shown in fig. 8. There is a close corres- pondence between it and the optimum g(8) function obtained earlier. For compari- son the expression with AE = 0.35 eV is also shown. In effect, then, g(0) can be replaced by the more specific relation shown above, with the arbitrarily found relation between OSs and 8, and AE = 0.3 eV.The set of t.p.d. peaks computed earlier with the g(8) function (fig. 9) which were a very close fit to experiment, can equally well be computed using the full rate expression shown in eqn (32) with the condition that Oo = 0 and 8,,, = 0. (It is shown elsewhere l 3 that kTln (1 +Qss[exp (AE/kT)- l]} is relatively insensitive to variations in T over the temperature range of the t.p.d. peak, and that 8,, = f ( 0 ) , so that the full expression can be treated to a good approximation as a function of 8 alone, i.e. equivalent to g(0)). Throughout the above discussion it has been assumed that at O0 = 0, g(0) and 8,, = 0. However, it is possible that, for some pretreatment conditions, 8,, is not zero at 8, = 0. The effect of a non-zero O,,, on the computed t.p.d.is shown in fig. 11. As 8,,, increases the observed peak shifts to lower temperatures, broadens and increases in area. Notice that with the simpler theory a maximum peak area of A8 = 0.025 is observed, whereas when O,,, is allowed to take non-zero values A8 can approach 0.04, i.e. the maximum experimentally estimated value of the vacancy concentration. The change in Tp and AT+ with changing 8,,, may well explain the scatter on the A8 and AT+ against Tp curves (fig. 5 and 6). Furthermore, the only induced heterogeneity considered is the step site but, of course, variations on this theme are possible, indeed likely. A more detailed theoretical study is planned, but the above is regarded as a reasonable step on the way to a detailed understanding. For the condition 8, = 0.02 and O,,, = 0 the computed rate expression yields a doublet in the t.p.d., cf.fig. 10. Such a doublet is occasionally observed in the experimentally obtained t.p.d. peaks, cf. fig. 3. e AND e,, AS A FUNCTION OF PRETREATMENT The area under the experimental peak yields the vacancy concentration pre- vailing at the steady-state pretreatment conditions, as is shown on fig. 4. In considering the relation of 8 to Poz we obtain, Inserting those variations of the rate constants arising from changes in enthalpy with 8 and g(e), as indicated in fig. 12, eqn (33) becomes, where ge(8) now defines the variation in the activation enthalpy of oxygen loss due to step sites at equilibrium. K is the overall equilibrium constant for the reaction and C is a temperature dependent constant.Note that when 8 and s,(@ are small eqn (34) reduced to the form 2'0; + 2"; + O,(g) + 2'Pe- (35) p0, = i q e 2 . (36)M . GREEN AND I . R. LAUKS 2739 I 1 c1 U 9 0 Q reaction coordinate FIG. 12.-Potential energy diagram for (a) the surface oxygen loss reaction, and (6) the surface zinc loss reaction.2740 zllo SURFACES At T, a steady state temperature, when Po, = 0 and 8 = 0.04, eqn (34) gives 1 c = -exp-[ 1 4.87eo.* - g,(e = 0.04) 0.04 kT (37) and eqn (34) can now be evaluated for various values of 8 over the range 0 to 0.04. The necessary ge(8) against 8 function is obtained by adjusting ge(8) for one chosen isotherm until the correct shape is obtained. Kis then chosen to give the correct mag- nitude to the isotherm.This g,(8) is then used, without change, to generate all the other isotherms and K values. The comparison of the isotherms so obtained with the experimental isotherms is shown in fig. 4. The K values obtained are used in the expression d l n K AH" d(l/T) k where AHo is the standard enthalpy of oxygen gas loss from the ZnO surface, eqn (35). The variation of In K with 1/T is shown in fig. 13, for which a value of AHo = (1.94 k 0.24) eV is obtained. This is to be compared with a value of 2.35 eV obtained by Boreskov, Popovskii and Sazanov.20 The ge(@ function required to reproduce the experimentally observed S-shaped isotherm is shown in fig. 9. This function has the same general form as the g(8) function required for the temperature transient kinetic data.The physical reason for the observed S-shape of the isotherm is that at high 8 (low Po,) there is an enhanced rate of surface oxygen loss from the step sites, and the steady state value of 8 at this low Po, is greater than that predicted by eqn (36). Using this value of ge(8) and the relationship ge(8) = kT In { 1 + B,,[exp (AEIkT) - 11) at T = 1000 K and AE = 0.30, values of 8,, against 8 can be obtained. At 8 = 0 to 0.01, 8,, z 0 ; as 8 increases 8,, increases and approaches a constant value of 8,, = 0.28 as 8 approaches 0.04. This value of 8,, is to be compared with that obtained by Gopel and Ne~enfeldt.~ From observation of LEED reflex splitting they report the formation of regular step arrays on a vacuum treated (1 100 K, 4 min) {lOTO) ZnO surface.These 2.6 x cm high steps occur perpendicular to the c-axis with an average spacing of 41 x cm. This leads to a value for OSs of xO.125. - = -- CONCLUSION The main conclusions from this work are : that an initially stoichiometric and near ideal {lOTO) ZnO surface at 900 K, when subjected to a linear temperature rise up to 1100 K, will lose x20 % ZnO (as 0, and Zn). The difference in the loss rates yields a peak in O2 t.p.d. and results in a final surface containing x 4 % TV., and ~ 2 0 % step sites. Surfaces with lower 8 and O,, can be obtained in a controllable manner. The oxygen loss rate has been shown to depend upon both 8 and OSs. A quantitative knowledge of both 8 and 8,, constitutes a considerable improvement in the definition of the nature of the solid surface and should advance our detailed understanding of chemical reactions on ZnO. A yet more detailed analysis, in which other types of surface defect are considered, should eventually be carried out, when coupled with more extensive experimental data. Clearly the techniques discussed above may be applied to other oxide systems. l C. G. Scott and C. E. Reed, Surface Physics of Phosphors and Semiconductors, ed. C . G. Scott and C. E. Reed (Academic Press, London, 1975), p. 411. P. Roussel and S. J. Teichner, Catalysis Rev., 1972, 6, 133.M. GREEN AND I . R. LAUKS 2741 G. Heiland, E. Mollwo and F. Stockmann, Solid State Physics, ed. F. Seitz and D. Turnbull (Academic Press, New York, 1959), vol. 8, p. 193. S. C. Chang and P. Mark, Surface Sci., 1974, 45,721. A. R. Lubinsky, C. B. Duke, S. C. Chang, B. W. Lee and P. Mark, J. Vac. Sci. Techn., 1976, 13, 189. W. Gopel, Ber. Bunsenges phys. Chem., 1976, 80,481. ' W. Gopel and G. Neuenfeldt, Surface Sci., 1976, 55, 362. W. Gopel, Surface Sci., 1977, 62, 165. B. Halpern and J. E. Germain, J. Catalysis, 1975, 37, 44. l o M. Green, and I. R. Lauks, to be published. l 1 M. Green and I. R. Lauks, Surface Sci., 1978, 71, 735. l 2 M. Green and M. J. Lee, J. Sci. Inst., 1966,43,948. l3 I. Lauks, Ph.D. Thesis (University of London, 1977). l4 F. A. Kroger, The Chemistry of Imperfect Crystals (North-Holland, Amsterdam, 1974), vol. 2. l 5 B. M. Arghiropoulos and S. J. Teichner, J. CataZysis, 1964, 3, 477. l6 F. A. Kroger and H. J. Vink, Solid State Physics, ed. F. Seitz and D. Turnbull (Academic Press, New York, 1956), vol. 3, p. 307. l7 M. Green, J. Chem. Phys., 1959,31,200. R. Seiwatz and M. Green, J. Appl. Phys., 1958,29,1034. E . Kreyszig, Advanced Engineering Mathematics (John Wiley, New York, 3rd edn, 1972). 'O G. K. Boreskov, V. V. Popovskii and V. A. Sazanov, Proc. 4th Int. Congr. Catalysis (Moscow 1968), 1971, vol. 1, p. 439. (PAPER 7/2070)

ISSN:0300-9599    

DOI:10.1039/F19787402724

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Electrical Conductivities of Shock-compressed Solutions of KI in Organic Solvents B Y SEFTON D. HAMANN* AND MAX LtNTON CSIRO Applied Chemistry Laboratories, G.P.O. Box 433 1, Melbourne, Victoria 3001, Australia Received 14th February, 1978 We have measured the electrical conductivities of solutions of KI in methanol, formamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and acetone, under compression by shock waves with pressures of the order of 10GPa. The results indicate that acetone and NMF become more viscous in the shocked state and the DMF and DMSO become very much more so. The shock-wave conductivities of formamide, NMF, DMF and DMSO are consistent with static conductivities measured at much lower pressures. Transparency experiments showed no evidence of freezing in DMF and acetone at shock pressures between 1 and 10GPa.We have previously described measurements of the electrical conductivities of water 9 and aqueous solutions of strong electrolyte^,^ compressed by explosively driven shock waves. The initial conditions of pressure, temperature and density in the shocked states were usually in the ranges 5-13 GPa (1 GPa = lo4 bar E 9869 atm), 500-1 100 K and 1.48-1.73 g ~ m - ~ , respectively. Rather surprisingly, the results showed that the mobilities of dissolved ions (other than H+ and OH-) are little affected by shock compression. By inference, the viscosity of water is also little affe~ted,~ and we suggested 39 that the reason is that, although the pressure jump in a shock front tends to raise the vis~osity,~ the accompanying temperature jump has an opposite and compensating effect.The aim of the present work has been to see whether a similar compensation occurs in nm-aqueous solutions. To that end, we have measured the shock-wave conductivities of both the pure solvents and of solutions of a strong electrolyte, KI, in methanol, formamide, N-methylformamide (NMF), N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO) and acetone. For comparison, we have also measured the influence of lower static pressures on the conductivities of KI in formamide, NMF, DMF and DMSO at 25, 35 and 45°C. In addition, we have examined the transparency of shocked acetone and DMF, in the hope of establishing whether or not the liquids undergo partial freezing in the s hock-compressed state.EXPERIMENTAL The solvents were purified by standard methods and dried over molecular sieves. The technique employed in the shock wave conductivity measurements was the same as that used earlier for aqueous solution^.^ The conductivity cells were of the type designated E,2 with the thickness I of the electrolyte layer adjusted to 1 mm. Before shock compres- sion, the solutions were at ambient temperature and pressure, In the transparency experiments, shock waves in the bulk liquids were viewed transversely by back illumination from a long-duration argon flash source, in the geometry described 2742S . D . HAMANN A N D M. LINTON 2743 by David and Ewald.' They were photographed by a high-speed cine camera, which took a total of 60 frames at 0 .2 7 ~ s intervals and which followed the shock waves from their launching until they had travelled about 6 cm. The static conductivity measurements were made in a temperature-controlled Teflon cell 8 g and the change in cell constant under pressure was calculated from the known compressibility of Teflon. lo- RESULTS SHOCK CONDUCTIVITIES A few representative oscillograms are shown in fig. 1-5, where the numbering and lettering of the traces are the same as previ~usly.~ The conductivities 1c of the solution in the pre-shocked (A-B) and shocked (B-F) states were calculated from the traces in the manner described earlier,3 allowance being made for the effect of the shock compression of the cell on its cell constant. 4 2ps I+ time FIG. 1.-Oscillogram for formamide at a shock pressure of 9.6 GPa.FIG. 2.-Oscillogram for a 0.1 mol dm-3 solution of formamide in water at a shock pressure of 9.6 GPa. 1-872744 SHOCK CONDUCTIVITIES The pressure of the shocked liquids was not measured directly, but can be assumed to have been close to the corresponding shock-wave pressure in the surrounding polyethylene cell : the two would certainly have become equal after a few reverberations in the thin liquid layer, that is, after about 0.5 p s . The value of the pressure quoted in this paper is that in the polyethylene cell at the time when the wave reached the top of the liquid layer.2 The temperature of the shocked liquids is unknown, but was probably in the range 500-1500°C. Y +[ 2PS I+- time FIG, 3.-Oscillogram for a 0.1 mol dm-3 solution of KI in formamide at a shock pressure of 7.3 GPa.-4 2E.Ls I+ time FIG. 4.-Oscillogram for a 0.1 mol dm-3 solution of KI in DMF at a shock pressure of 7.3 GPa. METHANOL The shock conductivity of pure methanol was examined some years ago by David and Hamann,I2 who found that it behaves rather like the conductivity of water,'. in that K rises steeply from a low initial value to a relatively very high value immediately behind the front of a strong shock wave. For instance, a 10 GPa wave causes K toS . D. HAMANN A N D M. LINTON 2745 jump from less than to about 0-l cm-l. David and Hamann attributed this jump to an enhancement, at the high pressure and temperature of the shock wave, of the self-ionization of methanol : The conductivity subsequently decays in the tail of the wave as the pressure drops and the ions recombine : in the particular geometry of the experiments, the duration of the conductivity pulse at half-peak height is about 0.5 p.The present measurements have been made on two solutions of KI in methanol at concentrations of 0.1 and 0.8 mol dm-3 * and at 11.6 and 7.3 GPa, respectively. 2CH30H f. CH3OH; + CH30-. (1) 0 2 1 1 -4 2P.s I+ time FIG. 5.-Oscillogram for a 0.06 mol dm-3 solution of KI in acetone at a shock pressure of 7.3 GPa . The oscillograms show both short initial pulses ( ~ 0 . 5 p s ) due to the temporary self-ionization of methanol and later, protracted, periods ( N" 5 ps) of increased conductivity attributable to the dissolved KI. The oscillograms at the two pressures resemble those for aqueous solutions of KC1, shown in fig.3 and 2 of ref. (3), respec- tively, and can be interpreted similarly. FORMAMIDE We examined the behaviour of pure forrnamide at four different shock pressures. The oscillogram in fig. 1, for 9.6 GPa, shows that strongly shocked formamide develops quite a high conductivity. The con 'uctivity IC is very low initially (A-B), but when the shock wave arrives there is a fast rise to C, where K = 0.045 R-l cm-I, followed by a slower and protracted rise to E, where K = 0.167 0-l cm-I. Corres- ponding values measured in the other three experiments at lower pressures are listed in table 1. We interpret this behaviour as follows. First, the fast initial rise has the character- istics of conductivity arising from self-ionization of the liquid, enhanced by the high pressure and temperature in the shock wave.David and Hamann l2 observed similar sharp conductivity pulses in other shocked organic liquids in cases where their autoprotolysis constants were > 10-1 mo12 dm-6 at normal temperature and * The concentrations quoted in this paper are those of the pre-shodced solutions at 25°C and atmospheric pressure.2746 SHOCK CONDUCTIVITIES pressure. The autoprotolysis constant of formamide l3 is 1.6 x 1O-l' mo12 dm-', and presumably relates to the reaction 2HCONH, + HCONH; +HCONH-. (2) Second, the subsequent protracted rise of conductivity from C to E indicates the formation of new stable ionic species which, unlike the very reactive ions formed by reaction (2), survive the release of pressure and temperature in the decaying tail of the shock wave.The whole behaviour between C and F, and after F, is in fact remarkably like the behaviour of a shocked solution of KCl in water [see fig. 4 of ref. (3)]. We suggest that these new ions are ammonium and formate, produced by thermal- and pressure-induced decomposition of forinamide : (3) This suggestion is supported by some recovery experiments in which we found that formamide that had been recovered after shock compression to about 10 GPa smelt strongly of ammonia and gave a mass spectrum containing peaks for formic acid, which, in the presence of excess ammonia, must have existed in solution as ammonium formate. We also found that an analogous reaction occurs when a solution of forma- mide in water is shock-compressed : (4) This reaction (4) is simply the reverse of the well-known thermal decomposition of ammonium formate into formamide and water at low pressures.The oscillogram in fig. 2 for an aqueous solution of formamide shows a sharp pulse C, arising from enhanced autoprotolysis of the water,3 followed by a sustained residual conductivity E-F due to the ammonium formate. A similarly shocked aqueous solution of urea, H2NCONHz, gives no residual conductivity. 2HCONH2 + NHZ + HCO? + HCN. HCONH, + H20 -+ NH: + HCOT. TABLE 1 .-ELECTRICAL CONDUCTIVITY OF SHOCK-COMPRESSED FORMAMIDE conductivity1 shock pressure/ sd-1 cm-1 at: GPa Ca Ea 9.6 0.045 0.167 8.4 0.008 0.027 7.9 0.002 0.007 7.3 O.OWb 0.002 a The points C and E are indicated on the oscillogram in fig.1. b This conductivity was below the limit of measurement, 0.0005 i 2 - I cm-'. We carried out two experiments on solutions of KI in formamide, one at a con- centration of 0.1 mol dm-3 and the other at 0.3 mol dm-3, both at a shock pressure of 7.3 GPa, which is low enough to prevent the formamide conductivity (0-0.002 R-' cm-I, see table 1) from masking the KI conductivity (0.003-0.013 0-l cm-l). Fig. 3 shows the results for the 0.1 mol dm-3 solution. It will be seen that the arrival of the shock wave causes a temporary drop in current between B and D, but, when correction is made for the change of cell constant caused by compression of the liquid, the conductivity is almost unchanged in that region. Subsequently the conductivity increases to E as the liquid expands and remains irreversibly heated.3 The behaviour of the 0.3 rnol dm-3 solution is very similar.N-METHYLFORMAMIDE (NMF) 0-l cm-l) at a shock pressure of 9.6 GPa. Unlike formamide, pure NMF gives no detectable conductivity (i.e. K < 0.0005S . D . HAMANN AND M . LINTON 2747 A 0.1 mol dm-3 solution of KI in NMF, shocked to 7.3 GPa, gives an oscillogram very much like that for the formamide solution in fig. 3. The conductivity is reduced by a factor of 0.9 immediately behind the shock front but rises to twice its initial value in the subsequent expansion phase. N,WD I MET H Y L F o RM A M I D E ( D M F) Pure DMF also gives no detectable conductivity in 7.3 or 9.6 GPa shock waves. Fig. 4 shows the oscillogram for a 0.1 mol dm-3 solution of KI in DMF com- pressed by a 7.3 GPa shock wave.The initial conductivity A-B drops to " zero " (i.e. IC < 0.0005 C2-l cm-I) immediately behind the shock front, €3-D, but later rises slowly to slightly more than its original value, at E. TABLE 2 .-RELATIVE CONDUCTIVITIES OF SOLUTIONS OF KI UNDER STATIC COMPRESSION solvent temp./"(= KO/R-~ cm-la formamide 25 0.002 35 35 45 NMF 25 0.003 86 35 45 DMF 25 0.005 21 35 45 DMSO 25 0.003 02 35 45 KlKOQ pressurelGPa 0.000 0.100 0.200 1.000 0.739 0.568 1.390 1.054 0.806 1.690 1.312 1.038 1.000 0.715 0.528 1.181 0.858 0.642 1.350 0.997 0.764 1.OOO 0.704 0.518 1.155 0.833 0.628 1.298 0.960 0.729 1.205 0.758 1.401 0.914 0.636 1.000 b Throughout, the concentration of KI was 0.1 rnol dm-3 at 25°C and atmospheric pressure a K~ denotes the conductivity at 25°C and atmospheric pressure (= 0.0001 GPa).b Measurements for DMSO solutions were limited by freezing of the solvent at the higher pressures at 25 and 35°C. DIMETHY L s u L PHOXIDE ( DMS 0) A shock wave of 7.3 GPa produces no detectable conductivity in pure DMSO. However, a shock of the same pressure reduces the conductivity of a 0.1 mol dm-3 solution of KI in DMSO to " zero ", initially, and gives an oscillogram very similar to the one on fig. 4. ACETONE Although pure acetone develops a slight conductivity, rather slowly, in 10 GPa shock waves l4 (the conductivity may arise from decomposition products),1s it has no measurable conductivity at 7.3 GPa. Fig. 5 shows an oscillogram for a 0.06mol dm-3 solution of KI in acetone, compressed by a 7.3 GPa shock wave. The initial effect of the shock is to reduce the conductivity, at B-D, to about half its value in the pre-shocked state A-B.TRANSPARENCY EXPERIMENTS The transparency experiments were carried out on DMF and acetone, and followed the shock waves from their launching, at initial pressures of 13 and 11 GPa respec- tively, until they had travelled 6 cm into the liquids and their pressures had dropped to2748 SHOCK CONDUCTIVITIES ~1 GPa. At no stage was there any sign of opacity or partial opacity behind the shock fronts. STATIC CONDUCTIVITIES The results of the static measurenients are listed in table 2 in the form of the ratio of the measured conductivity IC to that of the same solution at 25°C and atmospheric pressure I C ~ . They all relate to solutions having a concentration of 0.1 mol dm- of KI at 25°C and atmospheric pressure.DISCUSSION In this work we are interested principally in the conductivities K , of shocked solutions of strong electrolytes in regions immediately behind the fronts of strong shock waves, that is, in the regions B-D in the diagrams. The behaviour we have observed there can be summarized as follows : (i) for solutions in water,3 methanol and formamide, K, is close to the conductivity K~ of the preshocked solution at normal temperature and pressure, (ii) for solutions in NMF, K , is about 0.9rio, (iii) for solutions in acetone, IC, is about O . ~ K ~ , (iv) for solutions in DMF and DMSO, K , is too small to measure and is certainly less than one tenth of I C ~ . In general, the shock-induced changes in 7c could arise either from changes in the mobilities of the K+ and I- ions or, if the KI were not fully dissociated, from changes in their concentrations.Earlier conductivity measurements at normal temperature and pressure have shown that KI is completely dissociated in methanol,16 forma- mide,17 NMF,I* DMF 1 9 9 2o and DMS0,21 but is partly associated as ion pairs in acetone.22 There are theoretical and experimental reasons for believing that dissociated salts remain dissociated under shock compression and ion pairs tend to dissociate, and it follows that the changes in IC for the first five solvents reflect changes in ionic mobility, whereas the change for acetone probably includes a contribution from enhanced dissociation. On that basis we interpret (i) as indicating that the mobilities of dissolved ions in water, methanol and formamide are not much affected by the shock conditions, while (ii) indicates that the mobilities in NMF are measurably reduced and (iv) shows that the mobilities in DMF and DMSO are very much reduced.In acetone, the mobilities probably drop by more than the 50 % decrease shown by the conductivity (iii). Since the transparency experiments gave no signs of partial freezing in shocked DMF and acetone, we can presume that the reduction of ionic mobilities in these solvents (and probably also in NMF and DMSO) arises from increases in their vi~cosities.~ For these liquids, the tendency of the high shock pressure to raise the viscosity evidently outweighs the tendency of the high temperature to lower it.Finally, for comparison with the above results for strong shock waves, we can use the static data in table 2 to estimate the response of some of the solutions to weak shock compression. From the p-v-T behaviour 23 and specific heat of DMF, we calculate that a 0.2 GPa shock wave would raise the temperature by 22.7"C, so that I C , / K ~ would be close to the value K ] K ~ = 0.764 at 45°C and 0.2 GPa. The corres- ponding values of IC/IC, for the other solvents in table 2 (whose p-v-T behaviour has not been measured, but is probably similar to that of DMF) decrease in the order formamide > NMF > DMF > DMSO. This is the order in which the strong shock conductivities I C , / K ~ decrease (except that the very low values of I C , / K ~ for DMF and DMSO are indistinguishable) and so the static and shock-wave results are consistent, to that extent, in spite of the large difference in their pressures.S .D . HAMANN AND M . LINTON 2749 We are indebted to W. Connick, D. J. Pinson and M. Wolfson of the Materials Research Laboratories, Australian Department of Defence, for taking the shock- wave photographs, and to IS. G. Carey for help with experimental work. H. G. David and S . D. Hamann, Trans. Faruduy SOC., 1959,55,72. S. D. Hamann and M. Linton, Truns. Furaduy SOC., 1966, 62,2234. S . D. Hamann and M. Linton, Trans. Furaduy Soc., 1969, 65,2186. S. D. Hamann and M. Linton, J . Appl. Phys., 1969, 40,913. P. W. Bridgman, Proc. Amer. Acud. Arts Sci., 1926, 61, 57. J. A. Riddick and W. B. Bunger, Organic Solvents (Wiley-Interscience, New York, 1970). H. G. David and A. H. Ewald, Aust. J . Appl. Sci., 1950, 11, 317. S. D. Hamann and W. Straws, Trans. Fui-uday Soc., 1955, 51, 1684. F. H. Fisher, J. Phys. Chem., 1962, 66, 1607. lo C . E. Weir, J . Res. Nut. Bur. Stand., 1954, 53, 245. l 1 R. 1. Beecroft and C . A. Swenson, J. Appl. Plzys., 1959,30, 1793. l2 H. G. David and S. D. Hamann, Trans. Furuday SOC., 1960,56, 1043. l3 F. H. Verhoek, J . Amer. Chem. SOC., 1936,58,2577. l 4 S. D. Hamann, Advances in H&h Pressure Research, ed. R. S . Bradley (Academic Press, lS 0. B. Yakusheva, V. V. Yakushev and A. N. Dremin, High Temp. High Pres., 1971, 3, 261. London, 1966), vol. 1, chap. 2. C. W. Davies, Ion Association (Butterworth, London, 1962), p. 96. L. R. Dawson, E. D. Wilhoit and P. G. Sears, J. Amer. Chem. Soc., 1957, 79, 5906. l a C. M. French and K. H. GIover, Trans. Fkruduy SOC., 1955, 51, 1418. l 9 J. E. Prue and P. J. Sherrington, Trans. Furaduy SIC., 1961, 57, 1795. 2o D. P. A m and P. G. Sears, J. Phys. Chem., 1955,59, 16. 21 P. G. Sears, G. R. Lester and L. R. Dawson, J . Phys. Chem., 1956,60,1433. 22 M. B. Reynolds and C . A. Kraus, J. Amer. Chern. Soc., 1948,70, 1709. " S. B. Brummer, J . Chem. Phys., 1965, 42, 1636. (PAPER 8/263)

ISSN:0300-9599    

DOI:10.1039/F19787402742

出版商:RSC    

年代:1978

数据来源: RSC