摘要:

A. R. MILLER THE KINETICS OF CATALYTIC REACTIONS ON INHOMOGENEOUS SURFACES BY H. DE BRUIJN Received 3rd February, 1950 A theory has been developed for the kinetics of catalytic reactions on in- homogeneous surfaces in which we started from an entirely arbitrary distribution of the more or less active sites on the surface. The ammonia synthesis was used as a model process. It was assumed that the adsorption of the velocity-deter- mining reactant (in this case nitrogen) is of an atomic character and that an unrestricted migration may occur on the surface. The influence of simultaneous adsorption of the second reactant and the reaction product has been discussed. An essential factor of the theory is the occurrence of a so-called inhomogeneity factor in the reaction-rate relation. The value of this factor is determined by the frequency of those active sites which are really active under synthesis con- ditions.What active sites (characterized by their heat of adsorption) are really active depends on the temperature, total pressure and gas composition. In this paper particular attention is paid t o the influence which the total pressure and the gas composition have upon the value of the inhomogeneity factor. Thus it was possible to give an explanation for a well-known practical rule for the testing of catalysts used in high pressure synthesis. In practice it will in many instances (in the absence of important side reactions) be best t o test the activity of the catalyst for the decomposition reaction at I atm., taking care to establish a certain mixing ratio between the reaction product and the synthesis gases (in the ammonia synthesis 300 atm.a t 425" Cs corresponds to 4.6 % NHJ. The Inhomogeneity of a Catalyst Surface.-Many authors hold different views as t o the importance of the inhomogeneity of the surface.7 0 KINETICS OF CATALYTIC REACTIONS Whereas Taylor,l Cremer, 2 Emmett, Temkin, Zeldovit~h,~ Roginskii,6 Sips and Fricke 8 et al. support the opinion that this inhomogeneity of the surface is a very important factor in the kinetics of catalytic processes, other investigators such as Roberts,* Beeck,lo Volkenstein l1 and Eucken l2 advocate a different view. The latter consider the interaction between adsorbed molecules or atoms as a far more important factor. We may safely assume that there are catalysts whose surfaces may be considered as homogeneous.On account of the fact that equivalent adsorption sites are located close together, it will be necessary in this case to reckon with an interaction effect. Sometimes, however, the quantitative examination leads to unsatisfactory results. Following the method indicated by Roberts,s Rideal l3 calculated a far smaller inter- action energy than had been found in careful experiments and could only assume that the repulsion is considerably increased by the underlying layer (the catalytic surface). On the other hand it may likewise be assumed that on account of their preparation (cp. Fricke 8 ) many catalysts have a pronounced inhomo- geneous surface. Even though, owing to geometric factors, a certain type of crystal faces has the greatest catalytic activity (cp.Beecklo), it may nevertheless be expected that not all adsorption sites of this type of face are equivalent. Either as a result of crystal deformations, starting from the interior of the crystallites or owing to missing and extraneous structural elements in the surface of the crystallites, the energy of the adsorption sites will differ considerably. As a strong argument in favour of the assumption that the inhomo- geneity is an important factor, we may refer to the deformation of the molecules in chemisorption. Wright and Taylor l4 for instance proved the occurrence of each of the methane fragments CH, CH,, CH, in the chemisorption of methane on nickel. From desorption tracer experi- ments with CO, Emmett and Kummer concluded that the bond between CO and Fe is not equally strong for all adsorbed molecules. The CO molecules which have been adsorbed first prove to havs the stronger bond and in desorption these molecules are the last to leave the Fe surface. (The suggestion for this criterion for the inhomogeneity was made by Roginskii 15.) The iron catalyst used in the ammonia synthesis could be adequately used as a model for a catalyst with an inhomogeneous surface.It is very likely indeed that only the (I I I) face of the a-Fe has a great catalytic activity (Brunauer and Emmett 16) but in the adsorption of ammonia Taylor and McGeer l7 unmistakably found N, NH and NH, radicals on the surface which points to inhomogeneity. The latter fact need not seem odd when it is borne in mind that small admixtures in the magnetite used for the preparation of these catalysts are highly concentrated on 1 Taylor, Colloque sur Z’Adsorption et la Cinktique heterogene (Lyon, Sept., 1949).2 Cremer, J . Chim. Physique, 1949, 46, 411. Emmett and Kummer, cf. ref. I . Temkin, Levin Uspekki Khimii, 1948, 17, 174. 5 Zeldovitsch, Acta Physicochim., 1934, I , 961. 6 Roginskii, ibdd., 1947, 22, 61. Sips, J . Chem. Physics, 1948, 16, 490. Fricke, 2. Elektrochem., 1949, 53, 264. Roberts, Proc. Roy. SOC. A , 1935, 52, 445. lo Beeck Rev. Mod. Physics, 1945, 17, 61. l1 Volkenstein, Levin Uspekki Khimii, 1948, 17, 174. l a Eucken, 2. Elektrochem., 1949, 53, 285. l3 Rideal and Trapnell, cp. ref. I. l4 Wright and Taylor, Can. J . Res., 1949, 27, 303.l5 Roginskii, Acta Physicochim., 1946, 21, 537. l6 Brunauer and Emmett, J . Amer. Chem. Soc., 1937, 59, 310, 1553. Taylor and McGeer. Princeton Univ., 1949.H. DE BRUIJN 7 1 the surface (Brunauer and Emmett la) where they will give rise to dis- locations. A priori little can be said about the distribution of the more or less active sites. The distribution is probably of the Gaussian type (a fre- quency curve with a maximum (Fig. I)). The equipartition of sites with different adsorption energies as postulated in Temkin's l8 theory is most unlikely. FIG. I .-Different types of inhomogeneous surface. The Slowest Reaction Step.-The advantage of selecting the ammonia synthesis as a model for our discussions chiefly lies in the relative simplicity of the mechanism.Hardly any side reactions occur and all reaction steps are reversible. Nevertheless, this mechanism is still rather complicated : N,+ [ 1 [2NI ' - (4 3H,+ I: 1 c [6HI * * (2) [ZNI + PHI [2N + 6HI ' * (3) [ZNH,] 2NH3 + [ 1 ; - ( 5 ) [2N + 6H] t [i" + 4H Z", + 2H T 2NHJ (4) thcre are three adsorption processes (I), (2) and ( 5 ) , a migration process and a chemical reaction. Temkin l* supposed that with heterogeneous catalytic processes all reaction steps, except the first (adsorption of nitrogen) are in equilibrium because they proceed so rapidly. It should, however, be remembered that the velocity of a step is not solely determined by so-called intrinsic factors such as activation energy and frequency factor, but also by a factor containing fugacity terms of reactants and products which depends on conditions. As soon as the reaction has reached a point at which it deviates considerably from total equilibrium (high space velocities), the next slow step will become rate-determining.In any case, the latter can no longer be considered as being in equilibrium. For this reason we shall restrict ourselves to reactions occurring in a differential reactor in which the gas composition differs but slightly from the total equilibrium composition. REACTANT (NITROGEN) .-Ternkin suggests a molecular nitrogen ad- sorption. Having regard to the then available data this was a natural 18 Temkin and Pyzhev, Zhurn. $2. Rkim., 1939.31, 851. ATOMIC OR MOLECULAR ADSORPTION OF THE VELOCITY DETERMINING72 KINETICS OF CATALYTIC REACTIONS assumption, since Taylor and Joris had found that the exchange of the 14N-lCN isotopes on iron catalysts only proceeded at a velocity comparable to that of the N, adsorption, if hydrogen was present.have clearly proved, however, that if the experiments are carried out with sufficient care, the isotope exchange does proceed at a sufficiently high rate, even in the absence of hydrogen. On account of these results it will be more correct to regard the adsorption of nitrogen as an atomic adsorption. SIMULTANEOUS ADSORPTION PRocEssEs.-In his theory on the kinetics of the ammonia synthesis Temkin did not take into account the simul- taneous adsorption of hydrogen and ammonia. This omission is allowed as far as the adsorption of hydrogen is concerned since Brunauer and Emmett 2O have stated that under conditions of synthesis nitrogen and hydrogen are adsorbed on different types of sites.In addition, Brunaxer and Emmett found, however, that after adsorp- tion of nitrogen the hydrogen adsorption increases. When repeating these experiments in our laboratory we not only confirmed this result but moreover observed that, although the additional binding of hydrogen below - 250° C may be considered as adsorption, at higher temperatures, however, this increased adsorption is fictitious and due to the formation of ammonia. After we had made a correction for the amount of ammonia formed, the hydrogen adsorption proved to be normal The increased hydrogen adsorption, occurring in the presence of nitro- gen on the surface, may therefore be considered as a chemisorption of ammonia in the form of NH or NH, radicals (cf.also Taylor and McGeer 17). Even though at a low NH, pressure (i.e. at a low total pressure) this adsorption will be of minor importance, it is quite possible that at a high pressure part of the sites where nitrogen adsorption may occur, will be covered with ammonia radicals. Reaction Rate on Arbitrary Inhomogeneous Surfaces .-As Temkin did, we characterize the adsorption sites with the adsorption energy for nitrogen as a parameter, and as a characteristic for the catalytic surface we adopt a function Y(c). In general this function will have an arbitrary form which may possibly be derived from adsorption measure- ments (Roginskii and Todes,,l Sips 7 ) . In the first instance we shall neglect the adsorption of ammonia, and later show to how far our considerations must be altered when this adsorption is taken into account.Following Temkin’s treatment and assuming that all the processes, except the slowest, may be considered as being in equilibrium, we write Recent investigations by Emmett and Kummer which implies that the fugacity of the adsorbed nitrogen is determined by the normal total equilibrium constant K , the hydrogen fugacity and the fugacity of the ammonia eontained in the gas mixture. (In this paper the fugacities are supposed to be equivalent to the partial pressure of the gas phase components.) Using the subscript o for the most active sites, and i for an arbitrary site, it follows from the customary assumed linear relationship between activation energy E and binding energy E that where n is a constant.adsorption on sites of the i-type is (k: also includes the constant entropy factor). Ei - Eo = ~ ( E O - ~ i ) , . . ( 2 . 2 ) Assuming atomic adsorption and unrestricted migration the rate of V i = P N s ( I - ei)2kL e- [Bo + fi(&o - 80llRT . (2.3) l9 Taylor and Joris, J. Chem. Physics, 1939, 7, 893. * O Brunauer and Emmett, J . Amer. Chem. Soc., 1940, 62, 1732. 21 Roginskii and Todes, Acla Physicochim., 1947 15, 624.H. DE BKUIJN Since PG, is the fugacity of the adsorbed nitrogen, we may write 73 where a = k:/ki, i.e. the ratio between the frequency constants for adsorption and de- sorption (including the entropy factor). Integrating over all types of adsorption sites we find for the average rate of adsorption : V = K&, e - (Eo + ncD)/RT(apgJ -n .S[ I + (aP$l)fea/mT ' 8 (aP:l)f-ne*IzRT z Y(c)dc j W d C - or T' = QN2 e - (E,+neo)iRT(a~~,)--n[BR(~ - e)l--n]? ( 2 . 5 ) In an analogous manner we find for the rate of desorption : W = e - ( 8 0 + m o ) / ~ ~ ( a p g )l-*[@(I - 8)1-*12. , . (2.6) Since the reaction rate is determined by we consequently find that With the exception of this relation contains Temkin's theory. * the factor, F = [@(I - e)i--ny, . . (2.8) the same factors as the equation derived from In view of this result it is useless to examine the relation as a whole. Therefore we shall restrict ourselves to a discussion of the factor F which we have termed the inhomogeneity factor since it represents the inhomo- geneity of the catalytic surface in the rate equation.We note, however, that the entire reaction rate equation, for the interpretation of the experimental results, can be appropriately trans- formed to the equation : after substituting the efficiency 11 (defined as ) actual concentration of the reaction product concentration after establishing the equilibrium state * For the ammonia synthesis the last factor $-2n may be assumed to be practically equal to I, since (i) n differs only very slightly from 3 * If, contrary t o our assumption, the nitrogen adsorption should not be of an atomic but of a molecular character, the resulting change in the above rela- tion will be restricted t o the factor F which in that case will read [eyI - e ) y . C "74 KINETICS OF CATALYTIC REACTIONS (Temkin) and (ii) we shall only consider reaction rates in the neighbour- hood of the equilibrium state r) + I.(All constants inclusive of the temperature factors have been included in the constant h".) Inhomogeneity Factor F.-The inhomogeneity factor is in the first place determined by the value of exponent n. The latter originates from the relation between activation and binding energy. Most probably the value for n ranges between n = Q and n = I. When n = I and we are dealing with molecular adsorption of the rate-determining reactant, the inhomogeneity factor becomes equal to 0 which in a separate experi- ment may consequently be determined as a function of F': (A = rate- determining reactant). (In atomic adsorption and n = I, the value for F becomes F = @, which is not identical with N2.) The case n rn Q, which according to Temkin 22 occurs in the ammonia synthesis, is more interesting.In eqn. (2.8) for n = 3 and 8 = 4, the function F reaches its maximum and decreases rapidly with lower and higher degrees of occupation. From this it may be concluded that in the latter case the reaction rate is almost exclusively determined by those adsorption sites having an intermediate degree of occupation. Of the entire " surface energy spectrum " consequently only a narrow band (corresponding with ad- sorption sites with an intermediate degree of occupation) is active. This does not imply that under all conditions always the same energy band is active. Apart from the adsorption energy the degree of occupation depends on the fugacity PE2 and the temperature T , e.g.where We now introduce E+ under normal conditions degree of occupation of 8 for the adsorption energy of those sites which (e.g. T = 427' C, P = I atm., 71 = I ) have a - &. Adopting the new variable then for n = 3 we may write F = We have already observed that 8(I - 8) rapidly converges for values of 8 <> 1/2 or in other words that the function ~ will rapidly con- verge for energy values of Q > or < E* (Fig. 2a, 2b). So if the value of the energy distribution function Y(x), does not change too rapidly in the neighbourhood of c4(xO = 0) we may write : ea (1 3- eY2 F = Y ( x o ) ' (3.3) 22 Temkin and Kiperman, Zhuvn. $2. khim., 1947, 21, 927.H. DE BRUIJN 75 4-00 since in other words the inhomogeneity factor is consequently a direct measure of the frequency of those adsorption sites having a degree of occupation= 4.When the conditions de- viate from those mentioned above, 8 = 9 will hold for other adsorption sites, since E+ depends on the fugacity P:*. It will be seen that the value of the inhomogeneity factor F = Y(xo) will change to a greater or less extent with pressure according to the shape of the energy distribution func- tion ?P(x). Also a change in the efficiency of the synthesis 17 entails a shift of x,(e). How- ever, as long as we do not know at what 7 values deviations trom the theory will occur, the latter shift is less interesting since we are not sure whether the next slow step will also play a part. When we consider the de- composition of ammonia at I atm., for which case (in accordance with the postulated reversibility of the reaction) the general reaction rate equation also holds, we see that the FIG.~a.-Active area determining inhomo- geneity factor F. FIG. 2b.-- - - distortion of active area by curvature of #. same value of F at IOO atm. corresponds with a gas mixture containing 2-9 yo of ammonia (at 300 atm. 4-6 yo NH,, at 3000 atm. 9-2 Yo NH,). Simultaneous Adsorption.-In the introduction we pointed out that the adsorption of the reaction product cannot be ignored. The adsorption of ammonia should now be considered in the same way as, for example, the adsorption of methane on nickel described by Wright and Taylor.l4 The most active radical will be found on the most active sites, i.e.the N atoms on the most active and NH and NH, radicals on the less active sites. From this it follows that the NH and NH, radicals are adsorbed on the sites of intermediate activity which sites, as stated in the preceding paragraph, determine the reaction rate.KINETICS OF CATALYTIC REACTIONS FIG. 3. Equiv. I atm. P (atm.) q %NH,at U 3000 I 9'2 b 300 I 4'6 G I 0 0 I 2'9 d e I I I - - 0'1 The shifting of x o with respect t o its location under standard conditions if only the total pressure is modified, is indicated by arrows a, b, c. So, if the conditions are favourable for ammonia adsorption, a simul- taneous and competitive adsorption of nitrogen and ammonia will occur on the sites of intermediate activity. Part of the adsorption sites located in the " active energy band " are then blocked by the adsorbed NH and NH, radicals.I I -5 0 f 5 f10 FIG. +-Radical adsorption of ammonia. If now the equilibrium ammonia pressure is highly dependent on the total pressure as is the case in the ammonia synthesis, it is quite possible that the simultaneous adsorption of ammonia will decrease the reaction rate at high but not at low pressures. The influence of the ammonia adsorption is illustrated in Fig. 4. The occupation by nitrogen molecules has, as it were, reduced the frequency of the sites where nitrogen adsorp- tion may occur.H. DE BRUIJN 77 Now since the ammonia radicals have a polar character, the adsorption of these may be influenced by dipole interaction or polarization effects caused by polar molecules present on the surface, such as the so-called first promoters A1,0,, MgO and the like.For the same reason, however, it may also be imagined that the second promoter such as K,O, CaO may, at least in principle, act as antagonists. (At the moment the latter point is under investigation in our laboratory so that it is only mentioned briefly. ) Testing of the Activity of Catalysts.-Due to the peculiar be- haviour of the inhomogeneity factor in the reaction rate equation, it is not surprising that for the testing of catalysts used in high pressure pro- cesses the " synthesis test " a t I atm. may in many cases lead to wrong conclusions, whereas the decomposition test at I atm. will frequently yield a more accurate result. Thus at atmospheric pressure another part of the inhomogeneous catalyst (another band in the energy dis- tribution spectrum) will be active than at a pressure of 300 atm.and generally Y(E) will not have the same value for two different catalysts. FIG. 5.-Different sequence of catalysts at different pressures. At I atm. A, the most active. At 300 atm. B, the most active, It is by no means impossible that the Y(E) curves intersect (Fig. 5, catalyst A is the best at atmospheric pressure and catalyst B at a pressure of 300 atm. although the latter possesses a far smaller number of very active sites.) Moreover, it is quite possible that even if, in the absence of ammonia, the distribution functions do not intersect in the range be- tween I and 300 atm., one catalyst adsorbs the ammonia far better than another. (Fig. 6 .catalyst A, being the best a t atmospheric pressure, is far more sensitive to ammonia adsorption than catalyst B.) We think for instance of the typical behaviour of singly and doubly promoted cat a1 yst s . If, however, in performing the test, the rate of decomposition of the ammonia under normal conditions is used as a criterion for the catalytic activity and provisions have been made for the appropriate ammonia content of the gas mixture we are able : (i) to examine the same energy band of the inhomogeneous catalysts (ii) to find whether a possible simultaneous adsorption of ammonia as is active at high pressure and will exert an influence.78 KINETICS O F CATALYTIC REACTIONS Although in practice the most efficient testing procedure had been known for a long time, it was not known why it had to be performed in this way.FIG. 6.-Different sequence of catalysts a t different pressures. NH, adsorption on A. At I atm. A, the most active; a t 300 atm. B, the most active . . . effect of Concluding Remarks.-In this paper our considerations of the kinetics of catalytic reactions on inhomogeneous surfaces, are still of a qualitative nature, This is due to : (i) the uncertainty as to the shape of the distribution function Y(E) ; (ii) the uncertainty as to what extent the migration of the adsorbed nitrogen atoms and ammonia radicals can proceed without re- striction ; (iii) the lack of knowledge of the intrinsic rate factors (activation energy and frequency factor) for the next slow step ; (iv) the fact that the quantitative description of the adsorption of ammonia in the form of NH and NH, radicals cannot as yet be given. It may be that in the near future the application of the theories of Roginskii and Todes 21 and Sips 7 will enable us to obtain a better idea of the shape of the distribution function Y(E) from exact data on the chemisorption of nitrogen. Even the extent to which the migration of nitrogen atoms and am- monia radicals proceeds unrestrictedly’, as has been suggested here, is open to doubt. The great difference between the experimental activation energies of synthesis and decomposition reactions (with many catalysts about 20 kcal. after elimination of the heat of reaction) as well as the influence of the temperature on the activation energy in the decomposi- tion reaction (cf. Love, Emmett and Brunauer,23~ 24 and Chrizman z6) must presumably be ascribed to restricted migration. Knowledge of the intrinsic rate factors of the next slow step is also lacking. Both these points are being investigated in our laboratory. In conclusion it may be remarked that an extensive examination of the chemisorption of nitrogen, hydrogen and ammonia both separately and in combination (i.e. an extension of the investigation by Brunauer 23 Love and Emmett, J . Amer. Cham. SOC., 1941, 63, 3297. 24 Love and Brunauer, ibid., 1942, 64, 745. 25 Chrizman, Acta Physicochim., 1936, 4, 899.H. DE BRUIJN 79 and Emmett 20 will probably enable us to complete our knowledge of the chernisorption of ammonia. I wish to express my gratitude to my collaborators, particularly Dr. Kruyer and Mr. Zwietering. Central Laboratory, Staatsmijnen, Geleen, The Netherlands.

ISSN:0366-9033    

DOI:10.1039/DF9500800069

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

H. DE BRUIJN \ ns _ _ - _ . _ _ _ _ _ _ _ _ _ I _ _ _ _ _ _ _ _ _ _ _ - - - - 79 number of electrons delocalized at the surface. Heat of adsorption kcal. (n= I) 36 72 108 The heats of ethylene adsorption given by Beeck 1 cannot be used to evaluate n because of the unknown heat change in the side-reactions which accompany the adsorption process. I f we C,H* C2H2 COHs = 0~1 F t I D _ - _ _ - _ _ _ _ _ .--- ------ 4-thermionic work function. L-interatomic distance. 4 4, Energy - _ - - - - - - levels I -- D-energy needed to break bond of adsorbate molecule. in - _ _ _ . 1 + i i - - - - metal -GENERAL DISCUSSION 81 A potential valley of depth D approaches the metal and the electron has to pass out of it. It will do so in two stages ; it will become statistic- ally excited to the upper energy level of the metal and will then penetrate the bairier as a penetration wave.The transparency of a rectangular barrier is readily obtained by the Jeffreys approximation. Combining the penetration factor " and the statistical factor, we finally get for the rate of adsorption Kads = A T 2 exp (- (D - 4) /kT) . exp (- 2(24)*L) (1) L, the barrier width, must be estimated from X-ray values. Eqn. (I) can be modified to allow for the effect of an applied electric field, thus : (2) This expression involves two terms, one temperature-dependent, the other temperature-independent ; one varies as +*, the other as (n - $}. If D is constant, they will obviously vary in opposite directions (cf. Schwab). The work function of the metal # is dependent on the cohesive energy of the crystal.Now a portion of the crvstal elevated above the surface Kad, = A T 2 exp ([- ( D - 4) - 1/Ee8h]/kT) exp (- (zl/%+*L). level is already partialfy removed fromJ the main crystal, and since the cohesive energy is that required to remove an isolated atom from the crystal to infin- detached group will be much less than reduced. In our new model, the electron levels of the metal D - 4, but instead of penetrating a barrier of height 4 it would have to penetrate one of height t$ - AE,, ity, the mean cohesive energy of the that of the crystal. Hence 4 may be t still has to rise through the range of energy D - - - - - - - - - - where AE, is the change in cohesive energy. The lowest energy level in the metal " moves up " AE,. Calculation of AEc for any given con- figuration is very difficult, but the method of Onsager, applied by Frank s to a similar problem, may prove useful.Again, differences in the spacing of atoms in a surface plane may be reflected in the value of 4, which includes a term in I /rs (vs = radius of the Wigner sphere). Thus a wider crystallographic spacing would reduce r,, and hence 4. The overall reaction rate for a process such as we have discussed above would be given by combining our expression for the adsorption process with a kinetic expression, such as that used by Eyring and by Temkin. We get an expression of type for emission of product ; here 4E = effective work function and the suf- fixes G, p , refer to complex and product respectively. Similarly for rate of adsorption the notation being as before.The main problem, now, seems to me to be the determination of 4E for the groups of atoms which form " active centres ", but any relation between +E and the configuration of these groups would certainly be very complicated. Faraday Society Discussion, 1949, 5.82 GENERAL DISCUSSION Dr. G. Wyllie (BristoZ) (communicated) : The process described by Mr. Thomas is that of thermal ionization of an atom close to the surface rather than formation of a co-ordinate link. He appears to neglect the interaction of the resulting ion with the metallic electrons, which may be roughly approximated by the image potential, and I am not clear how this is justified. This process may be important as a stage in some catalytic reactions, but it is an excitation, not directly involved in adsorption, though it may be in some cases a preliminary to it.Mr. N. Thomas (lm$eriaZ College) (communicated) : The process as- sumed involves donation of an electron by the adsorbate to the metal, but this does not necessarily imply that the interaction between adsorbate and metal can be described only by Coulomb forces. The electron may be spatially localized to some extent in forming a co-ordinate bond. Dr. K. W. Sykes (Swansea) said : A recent theory of the catalysis of the oxidation of carbon might briefly be mentioned here, because it sug- gests a useful approach to some of the problems under discussion. It has long been known that the reactions of carbon with gases such as steam, carbon dioxide and oxygen are catalyzed by many of the metals which may be present as impurities in the carbon.All the explanations pro- posed hitherto involve an alternative reaction path in which the catalyst acts as an intermediate ; the oxidizing gas reacts with the catalyst which in turn oxidizes the carbon. Some recent experimental work has shown, however, that the reaction mechanism is unchanged by the catalysts, which merely alter the rates of certain of the individual stages. The new theory consequently deals with the general energy levels of the reacting system. A common feature of all reactions in which carbon is oxidized at high temperatures is the conversion of adsorbed oxygen into gaseous carbon monoxide; this process takes place a t some of the less firmly bound carbon atoms at the edges of graphitic lattice planes. When an oxygen atom is adsorbed at such a site, two types of distribution of the w-electrons may be distinguished : 0 0 I II I II I1 II (4 (b) Type @) is the more favourab’e for the evolution of carbon monoxide, because the carbon-carbon links which must be broken are weaker and the carbon-carbon bond approximates more closely to that in the carbon monoxide molecule.It is suggested, therefore, that the catalysts make the bond distribution less like (a) and more like ( b ) by interacting in some way with the v-electrons. shows that the necessary changes can, in fact, be produced by electron transfer between the carbon and the cata- lyst. To avoid the difficulties associated with calculations on the rather complex system under discussion, the phenoxyl radical, derived by removing a hydrogen atom from the hydroxyl group of the phenol mole- cule, serves as a simple model which contains the essential features of the real situation. The calculated r-bond orders in the neutral radical, and also in its singly charged positive and negative ions are as follows : \c&/ \C/?c/ Molecular-orbital theory 0 0 0 I 0.733 i 0.872 0.594 Radical Positive Ion Negative Ion * Long and Sykes, I d .Cong. (Nancy, 194g), J . Chim. Physique, 1950,47, 36. 5 Coulson, Trans. Faraday SOC., 1946, 42, ro6.GENERAL DISCUSSION 83 These figures indicate that the removal of an electron should result in appreciable catalysis, since in the positive ion the carbon-oxygen bond is strengthened and the adjacent carbon-carbon bonds are weakened ; the total change in the energy of these bonds is about 30 kcal.Similar calculations for the a-naphthoxyl and p-naphthoxyl radicals show that the effect is a perfectly general one. It is suggested, therefore, that this is the mode of operation of those catalysts which are able to accept an electron from the carbon. Transition metals, which can readily change from an upper to a lower valency state, appear to be good examples of this class. Although the detailed comparison of the theory with experiment has been discussed in the reference given above, one particular aspect might be emphasized here. The mobility of the n-electrons provides a means whereby catalysts situated a t various points in the carbon lattice can influence events at the active sites.This point seems to be especially interesting in view of what Prof. H. S. Taylor, and indeed other con- tributors, have said about the way in which the catalytic effect of an impurity atom may spread over the surface. The application of this type of theory to other heterogeneous reactions can now easily be appreciated. The whole of the reacting system, i.e. the adsorbed molecules and the appropriate orbitals of the catalyst, should be regarded as one large molecule, and the molecular orbital method, or, if preferred, a valence-bond approach, applied to it. That is essentially the procedure recommended by Dr. Huang and Dr. Wyllie. By varying the number of electrons placed in the orbitals of the reacting system, one may represent the effect of an alloy constituent and so deal with the type of situation discussed by Mr.Dowden, Dr. Eley, Prof. Schwab and others. It should thus be possible to combine many of the newer ideas in a consistent manner without undue elaboration. Dr. K. G. Denbigh (Cambridge) (communicated) : Under conditions where there are large temperature gradients over the surface, it seems probable that the mobility of an adsorbed layer may be appreciably en- hanced due to thermal migration, Such an effect on the surface would be analogous in two dimensions to the well-known thermal diffusion pro- cess in gases or to the Soret effect in liquids and solids. The isothermal mobility of an adsorbed layer has, of course, already been demonstrated experimentally by Volmer and others. Consider an ideal system in which the migrating species has a certain low concentration c.The flow along the x-axis due to the concentration gradient is given by 3c -D.- 3%’ which defines D , the diffusion coefficient. to the temperature gradient is given by The corresponding flux due 3T - D‘ . G- 3 X ’ which defines D’, along the x axis is the coefficient of thermal diffusion. The total flow thus given by The relative degree of ordinary diffusion and of thermal diffusion is de- pendent on the ratio D’/D, the Soret coefficient. The magnitude of this quantity is larger in condensed phases than in gases and may attain a value of I O - ~ . Its sign may be either positive or negative, according to whether the particular species tends to move towards lower or higher temperature.Although D‘ is always much smaller than D, the actual magnitude of the thermal flux may exceed the magnitude of the ordinary diffusion if the term c . 3T/3x is very much greater than bclbx.84 GENERAL DISCUSSION Turning to the case of the adsorbed layer, c is now the surface con- centration of the particular species and b T / d x is the temperature gradient in a particular direction over the surface. If a catalytic reaction is strongly exothermic and takes place on localized regions it seems probable that there will be very steep gradients of temperature. It may occur, there- fore, that the surface mobility due to the thermal effect may exceed that due to the concentration gradient and may be in the reverse direction. Other thermal diffusion effects which are to be borne in mind in the discussion of non -uniform catalysts are concerned with the migration of lattice defects or charged species within the solid itself.In principle such effects could give rise to differences of electric potential, although it is to be doubted whether they are appreciable. Dr. D. M. Young (Dundee) said : Dr. Halsey has pointed out that al- though the B.E.T. theory has been shown to be invalid, the surface areas estimated by the “ point B ” method appear to be remarkably satisfactory. He has given a number of qualitative reasons to account for this. In view of the importance of surface areas in catalysis, it seems worth while to attempt to account for the validity of the B.E.T. surface areas in a quantitative manner. The B.E.T. theory, as Halsey pointed out in 1948,~ is based on the quite untenable hypothesis that an isolated adsorbed molecule can adsorb a second molecule on top, yielding the full heat of liquefaction.It is far more likely that adsorption into the second layer takes place over square arrays of first layer molecules. In what follows we shall take essentially the B.E.T. model and insert a clause to this effect. If we confine our attention to the lower part of the isotherm we may neglect adsorption in the third and higher layers. Thus we may write where N is the number of molecules adsorbed and N, is the number of P FIG. I . 6 Halsey, J. Chem. Physics, 1948, 16, 931.GENERAL DISCUSSION 85 sites on the adsorbing surface. The ratio bl/b, is analogous to the R.E.T. c constant.This equation neglects the shielding effect of the second layer, impeding the evaporation of first-layer molecules ; for the two cases to which the equation has been applied this effect has been shown to be negligible. I have applied the equation to a lattice of IOO sites, using an arbitrary value of b,. Two values of b , have been considered, correspond- ing to values of the B.E.T. c constant of 50 and IOO (values of this order being usual for nitrogen at its boiling point). The lowest curve is the adsorption isotherm .for the first layer ; the upper curves represent the total adsorption for the two values of b, chosen. These isotherms become linear at higher pressures, as found experimentally for a number of systems. Application of the “ point B ’’ technique gives values of the monolayer capacity within 3 yo of the value required by the above equation, namely N, = 100. Point A, for both values of b,, occurs at 87 yo of the theoretical monolayer capacity.These few preliminary results show that the addition of this one refinement to the R.E.T. theory causes no substantial change in the calculated value of the monolayer capacity (see Fig. I). Dr. F. C. Tompkins (Imperial College, London) (communicated) : It is true that some of the unreality of the B.E.T. model can be removed in the manner described by Dr. Young, but the experimental fact is that v, is determined by a B.E.T. analysis of physical adsorption-chemi- sorption can in fact, as Dr. Beeck has shown, lead to spurious values. In physical adsorption the distribution can rarely, if ever, be a random (i.e. non-equilibrium) one.The important point is that on a uniform surface if c is large (> 100) the 1st layer is practically complete before 2nd layer adsorption is significant.* Adsorption proceeds therefore virtually on an adsorbate surface with a heat not largely different from the heat of liquefaction of the adsorbate, and moreover the number of possible adsorption sites in the 2nd layer (neglecting small edge effects) is practically the same as on the original surface. If therefore we confine attention to the range of p / p a t o . 3 , then as we have shown previou~ly,~ the amount in the 2nd layer is N, . p / p , (where N, =-: the number of adsorbent sites, p = equilibrium pressure, and pa = saturated vapour pressure of adsorbate) to within 10 yo irrespective of whether we adopt B.E.T.or Hiittig’s evaporation conditions, whether we assume adsorption on square or hexagonal arrays or on single molecule sites, or whether we apply a “ pure ” Langmuir mechanism on a covered layer. All these must lead to a linear portion as Dr. Young has indicated in his particular citse. Dr. D. M. Young (Dundee) (communicated) : Prof. Taylor has suggested the use of the Huttig equation for evaluating surface areas, since it describes multimolecular adsorption data over a much wider range than the B.E.T. equation. While this may be true for certain systems, as shown by the comparison of the two equations given by it is by no means general. In the accompanying diagram the results of Arnold I1 for the adsorption of nitrogen and oxygen on anatase at liquid-air tem- peratures are fitted to the two equations.In this particular case the B.E.T. describes the results over a slightly greater range than the Huttig equation. Similar comparisons using other published isotherms suggest that analytically there is little to choose between the two equations. Like Ross, we find that the Huttig equation usually gives larger values of the monolayer capacity, but rarely more than 10 yo higher than those given by the B.E.T. equation. 7 Beeck, Advances in Catalysis, Vol. I1 (Academic Press). * Halsey. this Discussion. lo Ross, J . Physic. Chem.. 1949, 53, 383. l1 Arnold, J . Avner. Chem. SOC., 1949, 71, 104. Crawford and Tompkins, Trans. Faraday Soc. (in press).86 GENERAL DISCUSSION FIG.2. Prof. D. €3. Everett (Dundee) said : I wish to mention some evidence which, while not conclusive, does imply that surfaces map not be as heterogeneous as some authors have suggested recently. The use of entropies of adsorption to examine surface heterogeneity has been sug- gested recently by Hi11,12 and an independent study of the thermodynamics of surfaces which we have carried out l3 shows that thermodynamic studies may assist in solving this problem. Essentially our procedure has been to use methods analogous to those of solution thermodynamics and to compare actual systems with a hypothetical " ideal system. For monolayer adsorption the Langmuir model of a localized non-interacting monolayer seems to be the most useful ideal reference system.For this model the partial molar entropy e nf adcnrntinn inrlnrlec n rnnfimimtinnnl tw-m - R l n - We mnv write the partial molar entropy change (from the standard gas state to the surface) as e I - el _ _ _ _ A S = AS* - Rln-- where s* is the entropy change associated with changes in translational and rotational degrees of freedom of the adsorbed molecule on adsorption from the standard gas state, together possibly with similar changes in- duced in the atoms of the surface. We have analyzed data for a large number of systems and find evidence for a linear relation between hs* and AH, the differential heat of adsorp- tion. For adsorption on charcoal at oo C this relation is identical with 12Hill, J . Chem. Physics, 1949, 17, 762. l 3 Trans. Faraday Soc., submitted for publication.GENERAL DISCUSSION 87 the Rarclay-Butler relation between heats and entropies of condensation ; similar relations appear for the H,-tungsten system at higher temperatures, and for adsorption of gases on charcoal and glass at very low temper- atures.Unfortunately in the absence of suitable published data we have not been able (except for the Ha-W system) to extend our examination to chemisorption. The detailed interpretation of these heat-entropy relationships need not concern us at the moment, but, if we attach any significance to them, (or some- thtir existence suggests strongly that the term - R In - thing very near to it) plays a fundamental role in adsorption equilibrium in the systems we have studied. This term only arises when all the ad- sorption sites are of equal energy, and so have equal probability of being occupied.This equal probability will be upset if the surface is hetero- geneous, and some sites are much more active than others: the term - R l n - will no longer appear. If interaction occurs between mole- cules so that the heat of adsorption varies with coverage on this account, the term - Rln - is still separable from the expression for the entropy.l* We believe therefore that our analysis indicates that the sur- faces we have examined are less heterogeneous (as regards variation of adsorption energy from site to site) than is often supposed. We still have to answer Halsey’s objection that the variation of with 8 is too great and of the wrong form to be accounted for by inter- action forces.I believe that this reflects our ignorance of the energetics of surface phases, especially on metals and charcoal where the orbitals of the electrons in the surface may extend over considerable areas. As stressed by Prof. Taylor an adsorbed molecule may influence a very large area of surface, and the contributions of Huang and Wyllie, and of Coulson and Baldock, represent perhaps the first steps in a more complete theory of interaction between adsorbed molecules. Dr. A. R. Miller (Cambridge) (communicated) : The linear relation which Prof. Everett has found between the experimentally determined quan- tities as* and KH, as defined by him, is of interest. Essentially, this implies that there is a linear relation between 3f/3T and 3 ( f / T ) /3( I / T ) wherefis the partial molar free energy, that is, f is defined as 3F/30 where F is the Helmholtz free energy of the adsorbed monolayer.This will be so provided f is the sum of a configurational term and an interaction term which depends linearly on T. An expression for F can be derived from the statistical theory. In fact e I - 1 - 0 6 1 - 0 where with x defined by eqn. (2.15) in my monograph on adsorption. It is very complicated, and there is no reason to suppose that it (orf) neces- sarily has the form required. It therefore appears that the observed linear relation is essentially empirical and apparently arises fortuitously. That there is such an empirical relation is, however, important. I agree with Prof. Everett that it suggests that the configurational term - R log 6/1 - 0) has a fundamental role in the adsorption equilibria considered; and that it lends support to the idea that each site for adsorption on the bare surface has the same energy, 14 Fowler and Guggenheim, Statistical Thermodynamics (C.U.P., 1939).p. 441.88 GENERAL DISCUSSION Prof. R. M . Barrer (Aberdeen) (communicated) : I wish to emphasize a point which is no doubt realized by those directly concerned in sorption studies, but which may not be appreciated by others, interested less closely in sorption phenomena. This is the diversity in meaning of the term " heterogeneity " as applied to surfaces. Thus a heterogeneity in a sorption process based on dispersion forces may operate in the op- posite Sense when sorption is due to electrostatic forces.In the figure, A, B and C are three possible positions a sorbed molecule might occupy on a rough surface. As regards dispersion forces, the affinity lies in the order A tB tC but for a heteropolar surface and electro- static farces the sequence in affinity tends to be A> B> C.15 Again, in a chemical adsorption in which the adsorbed molecule in any of the positions A, B or C forms a chemical bond with one atom of the sorbent, it does not necessarily follow that the energy of formation of the bond will differ essentially whether it is formed at position A, B, or C. One should moreover differentiate between heterogeneity in a heat of chemisorption and in an apparent or true energy of activation for chemisorption. In a research carried out some years ago16 a considerable heterogeneity was observed in the apparent energy of activation for chemi- sorption of hydrogen by partially graphitic carbon.It was suggested that the hydrogen molecule was chemisorbed as atoms, one atom on an edge carbon of a graphitic lamella and the other on an edge carbon of a second lamella just above or below the first. In partially crystalline carbon there is some disorder in the stacking of graphitic lamellae and variability in inter-laminar spacing, responsible for variations in energy of activation. But there seems no reason in such a case for the overall heat of chemisorption to differ. Indeed this heat was as nearly as could be ascertained constant. Both in kinetic and equilibrium studies there- fore some care must be taken to define the sense in which the term " heterogeneity " is employed.Dr. G. D. Halsey, Jr. (Harvard) said : The current emphasis on the role of surface impurities and surface heterogeneity, coupled with the realization that the adsorption on such surfaces may be far from equili- brium indicates that extreme caution is required in making theoretical calculations. The possibility of calculating configuration energy and rates from first principles seems remote indeed. Even the use of semi- empirical theories must wait for the complete formulation of mechanism. Thus, the comparison of catalytic activity and lattice parameter can be done only if one assumes identical mechanisms, or else isolates the step involved. We may recall that the (homogeneous) decomposition of N205 was attacked with most sophisticated theories, only to find, as Ogg has pointed out, that it proceeds by a complex mechanism. Prof.G. M . Schwab (Athens) said : A treatment of catalytic kinetics with a rearrangement velocity not small compared with desorption velocities can also be given in terms of the usual Langmuir-Hinshelwood formalism, using Bodenstein's principle of stationary intermediate con- centrations. For the simple sequence : 1 4 -A L &as T-- Aads. -+ Bads. - Bga8, 2 3 5 this leads to l5 E.g. de Bocr, Electron Emission and Adsorptioiz Phenomena (C.U.P.), p. 48. 16Barrer, Proc. Roy. Soc. A , 1935, 149, 253.GENERAL DISCUSSION 89 Assuming k , 4 k4, k,, this reduces to the usual expression for the rate- determining rearrangement, and assuming desorption to be rate-determining (A4:< k 3 , K,) to k4bApA - V = bAPA $.( k 5 / k 3 ) P B + k,/k3 f k 4 / k , ' The apparent reaction orders are about the same ( < I for A and > - I for B) in the limiting and the intermediate cases. For inhomogeneous surfaces, instead of K,, k 3 , R , integral mean values according to energy distribution functions must be introduced. The most active centres will have increased k 3 and diminished k , and k l , which shows (as K 3 always occurs as an addendum to desorption constants) that a certain com- pensation takes place, the reaction order depending on heterogeneity much less than the actual velocity. It is probably because of this com- pensation that the simple Constable treatment ( K = k , . exp (q/h)) is approximately true, although the change in the rate determining process has been neglected.Dr. G . H. Twigg (Epsom) (communicated) : In considering the hydro- genation of ethylene, Prof. Laidler states that the zeroorder with respect to ethylene may be due to measurements having been made in the neigh- bourhood of the maximum, i.e. that an insufficientrange of conditions has been covered. This is not so, however. The zero order is found over the temperature range - 80" C to + 150' C, and with any reasonable value for the heat of adsorption, a change in kinetics would have been evident. The simple Langmuir-Hinshelwood mechanism for the hydrogenation of ethylene can be excluded by the following argument. If adsorption equilibrium is established, as assumed by this theory, and reaction occurs between adsorbed ethylene and an adsorbed hydrogen atom, the rate of reaction can be expressed as Rate = k,plnp2m = K,B,B,, where p , and p , are the pressures of ethylene and hydrogen and 0, and 8, are the fractions of the surlace covered by ethylene and hydrogen.It is a simple deduction from the theory that n = I - 2e1 m = *(I - 28,). Now, since the ortho-para hydrogen conversion is inhibited during hydro- genation, we know that 8, is very close to unity, so that the order of reaction with respect to ethylene should be - I. That this is not so is evidence either that the two gases do not compete for the same surface (Laidler's mechanisms (2) and ( 3 ) ) or that adsorption equilibrium is not maintained in the way envisaged by the Langmuir-Hinshelwood scheme.The latter is now thought to be correct.,' Prof. G.-M. Schwab (Athens) said: As pointed out by Laidler and previously by Hinshelwood, the Langmuir-Hinshelwood mechanism for a bimolecular surface reaction predicts a denominator in the velocity expression of the form (I + KC,)^, provided both reactants are adsorbed on the same kind of sites. This leads to a maximum velocity at an optimal pressure &,. Besides enzyme reactions (wherein the maximum is mostly to be explained by a self-displacement of the reactant due to adsorption of two molecules on one site 18), the only clear example of this maximum is Hinshelwood's reaction, CO, + H, -+ CO + H,O over platinum. l7 Twigg, this Discussion. Schwab, Bamann, Laeverenz, 2. $kysioZ. Cksm., 1933, 215, 121.90 GENERAL DISCUSSION However, we showed lo that under pure conditions (freezing out water vapour rather than absorbing it in sulphuric acid) the maximum disappears.Thus, it has been concluded 2o that the square term in the denominator does not generally occur. This can be explained either by a Rideal mechanism, or by an adsorption on adjacent sites of the reactants. Both possibilities have been formulated by myself and Pietsch in 1928.~~ In ethylene hydrogenation we preferred the second possibility. We had observed22 that the maximum is definitely absent ; the decrease of velo- city at high ethylene concentrations is an almost irreversible poisoning and not due to reversible adsorption. We already concluded 23 from a classical kinetic treatment that a distinction of mechanisms from fre- quency factors will hardly be possible, at least as long as detailed know- ledge of the absolute active surface and ot the degrees of freedom of the adsorbed molecules is lacking.Dr. G. H. Twigg (Epsom) said: I am very interested in the results which Turkevich et al. have obtained on the reactions between ethylene and deuterium. The ability to analyze the individual hydrocarbons should add greatly to our knowledge. The result that light ethane is an early product of the reaction of ethylene and deuterium at 90°C is in agreement with some results that Rideal and I obtained on this reaction.24 We found that up to ca. 1 5 0 O C the hydrogen that was returned to the gas phase from the catalyst was largely H,. This is due to the fact that on the catalyst there is a fast reaction : CH,D I CH,-CH, D -+ LH, -+ CH,-CHD H I Ni Ni Ni which exchanges the adsorbed hydrogen atoms with those in the ethylene. The exchange reaction is not controlled by this reaction, but by the return of the adsorbed hydrogen to the gas phase so that the hydrogen present on the catalyst is mostly H, and the ethane produced by the addition of this hydrogen is correspondingly largely C,H,.Prof. K. J. Laidler (Washington) (communicated) : It is evident from the divergent views expressed by Twigg, Schwab and myself that there is still need for a great deal of further evidence with regard to the important matter of whether the rates of certain reactions fall off at higher pressures. For the ethylene-hydrogen reaction, Pease 25 found that there was such a falling-off on copper.Since at these high pressures the general evidence is that desorption occurs more readily than at lower pressures, it does not seem likely that the irreversible effects referred to by Schwab can be oc- curring here. It therefore seems to be more likely that the falling-off in this system is due to the necessity for the reacting molecules to be ad- sorbed side by side. The fact, stated by Twigg, that the reaction remains of zero-order with respect to ethylene over a wide range of temperatures is no€ con- vincing evidence that there is no falling-off at higher pressures. The ethylene pressure at the maximum has been seen to be equal to I/K, where K is the equilibrium constant for the adsorption of ethylene.Now K varies with temperature largely as .&RT, where q is the heat of ad- sorption. If q is quite small, as is the case when the surface is fairly fully covered,26 is it clear that K may vary very little with temperature. l9 Schwab and Nsicker, 2. Elektrochem., 1936, 42, 670. 2o Schwab, Advances in Catalysis, 1950, vol. 2. 21 Schwab and Pietsch, 2. physik. Chew B, 1929, I , 385. 22 Schwab and Zorn, ibid., 1936, 32, 169. 23 Schwab and Drikos, ibid., 1942, 52, 234 and ref. (3). 24 Twigg and Rideal, PYOC. Roy. SOC. A , 1939, 171, 55. 25 Pease, J . Amer. Chem. SOC., 1923, 45, 1196. 26 Rideal and Trapnell, this Discussion.GENERAL DISCUSSION Consequently the position of the maximum may vary only very slightly over a wide range of temperatures, so that zero-order kinetics will be found over such a range.An example of just this type of behaviour is to be found in the results of some work now being done by Prof. J. Weber and myself at the U.S. Naval Ordnance Laboratory. The system being studied is the ammonia- deuterium exchange on an activated iron catalyst, As the ammonia concentration is increased the rate of this reaction goes through a maximum, the falling-off at higher pressures being due to a reversible adsorption of ammonia. The position of the maximum varies only very slightly with the temperature so that rate measurements at a pressure corresponding to the maximum would give zero-order kinetics over a wide range of temperatures, and on the basis of Twigg’s argument would have been interpreted as indicating no falling-off at higher pressures.Prof. G.-M. Schwab (Athens) said : Using the same mechanism for the ethylene hydrogenation as that proposed by Eyring, we evaluated 27 from our kinetic data on Raney catalysts the adsorption energies of hydrogen and ethylene and calculated a “ true ” activation energy for the transi- tion of adsorbed ethylene and adsorbed hydrogen to adsorbed ethane of about 22 kcal. on the assumption that the directly observed values around 10.7 kcal. are lowered by the adsorption energy of hydrogen. This result agrees with Maxted’s figures for a series of other hydrogenation reactions. It would be of interest to examine whether the author’s treat- ment also accounts for the most general result of a temperature optimum of the hydrogenation velocity.Dr. G. Wyllie (Bristol) (communicated) : It map be profitable to take the consideration of the adsorption of hydrogen and oxygen on tungsten a little further. Examination of the results quoted by Miller for the accommodation coefficient of neon on tungsten surf aces bearing adsorbed oxygen shows that an increase in the accommodation coefficient by 0.05 is on the accepted interpretation ascribed to the adsorption of oxygen at sites occupying 0.08 of the surface. This requires, on the simple sup- position that meaning may be attached to the accommodation coefficient at a site, that the coefficient at these sites should rise to about 0.7. But its value for a complete second layer of oxygen is only 0.36. It seems more natural to describe the observed increase in terms of a smaller increment over something approaching quarter of the total surface, and there seems then to be a possible analogy with the results obtained by Rideal and Trapnell for the chemisorption of hydrogen on tungsten.An effect which may be expected to be common to all monatomic chemisorbed films on a metal surface is a tendency at high film densities to adopt a configuration which continues the lattice structure of the metal. The principal reason for this is that the electrons of the partially covalent chemisorption bonds obey the Pauli exclusion principle, and the forces involved are precisely analogous to those which tend to maintain a constant bond angle in e.g. the water molecule. In the adsorbed film, the effect would be expected to appear as a short-range many-body inter- action among the adsorbed atoms.It will not be very easy to estimate it theoretically, but it seems clear that it will be more important the greater the extent to which p and d-orbitals of the metal atoms partake in the chemisorp tion bond. However, on the (110) surface of a body-centred cubic crystal, a position at which an adsorbed atom would continue the crystal lattice is co-ordinated with only two nearest neighbours in the plane below it, but with four in its own plane. In a film of low density, on the other hand, it is more likely that an adsorbed atom will find a position equidistant from three nearest neighbours in the metal surface. If the interaction described in the last paragraph is important, as indeed it must be inside 27 Schwab and Zorn, 2.PhysiR. Chew. B , 1932, 19, 169.92 GENERAL DISCUSSION the metal to force it into the body-centred cubic rather than a close- packed structure, then at some density there will occur an allotropic change in the film, the energy of which will be reflected in a drop in the heat of adsorption with increasing 8. If in the experiments of Roberts and Trapnell an important fraction of the metal surface lay in (110) planes, the results might bear an inter- pretation on these lines. As Johnson has shown that appropriate treat- ment can develop the (100) faces of tungsten, on which the geometrical effect should not appear, some experimental check should be possible. Dr. J. G. M. Bremner (Billingham) (communicated) : Dr. Miller men- tions various conclusions that can be drawn from the work of Roberts but it has now 28 been shown that a reassessment of much of this work is necessary.Roberts' object in introducing the flashing procedure was to free the surface of the wire from impurities. He found this procedure reduced the heat loss from a tungsten wire immersed in a gas at low pressure and accounted for this reduction by a fall in the accommodation coefficient. That this interpretation is incorrect has been shown in a number of ways. Thus, as the effect produced by flashing is found to be independent of the temperature at which measurements are carried out, the reduction in accommodation coefficient is apparently greatest when the wire temperature approaches that of the gas. Measurements made at 5' above the gas temperature will, in consequence, respond to the Roberts treatment very much more than when working with an incre- ment of 50'.The effect is, moreover, more apparent as the gas pressure is lowered and, indeed, at a given time interval after flashing, is greatest when working in vacuo. That the Roberts effect is due mainly to the warming of the internals of the vessel in which the wire is mounted is shown by the nature of its dependence on the duration and temperature of flashing, its increase on platinizing the walls of the vessel and also by its duplication on flashing a similar wire adjacent to the one under examination. Mr. A. E. J. Eggleton and Dr. F. C. Tompkins (Imperial College, London) (communicated) : We are unable to agree with Dr. Bremner " that a reassessment of much of (Roberts') work is necessary ".We 3 x 0 FIG. 3. I.-Cooling curve of tungsten wire after flashing to 2300' C for I minute. I1 .--Cooling curve of experimental wire after flashing auxillary wire to 2300' C for 15 min. 28 Bremner, R o c . Roy. SOC. A (in press).GENERAL DISCUSSION 93 have measured the accommodation coefficient a using a W wire (0.07 mm. diam., 19 cm. long) in neon circulating at a pressure of 0-1 mm. Hg in an apparatus identical in design with that of Roberts. The wire was flashed at 2300OC for I min. and the resistance then measured using a small current of zmA. Fig. 3 (I) gives a typical curve ; after 8 min. the resistance is constant and within 0.02 yo of its original value. In measure- ments a current of 20 mA was used in order to maintain the wire at ca.20° above that of its surroundings ; there is then a slow drift consistent with increasing contamination of the surface and a obtained 30 min. after flashing without extrapolation to zero time is still below 0.1. Vari- ation of time of flashing and change in temperature excess of the wire over its surroundings had no effect on a values obtained using Roberts’ extrapolation method (see Table I). This is consistent with the conclusion that the drift is due to contamination (Roberts) but not in accord with the non-attainment of thermal equilibrium (Bremner) . TABLE I Expt. No. . Extrapolated value of a . Excess temp. (“C) of wire a =o-I . - 5 6 9 I2 27.0 36.0 16.7 I 8.0 0.059 0.05 7 0.05 I 0.05 5 Dr. Bremner kindly sent us details of his work mentioned in his remarks.In later experiments we used a central iron wire in a tube containing two auxiliary W wires which were flashed at 2 3 0 0 ~ C for 15 min. As shown in Fig. 3 (11) the resistance returns to its former value within 5 min. Using a contaminated iron wire, which had been maintained previously at 1200~ C for considerable periods and had given an evaporated iron film on the walls of the tube, we again find that there is attainment of thermal equilibrium within 5 min. and moreover there is no drift at all. This result would be expected if Roberts’ explanation were correct. We are forced, therefore, to the conclusions (i) that Roberts’ value of 0.06 for a is a true one, (ii) that his flashing procedure did clean his W wire, and (iii) that the drift is due to contamination.We are also equally con- vinced that Dr. Bremner’s explanation of his own results in his apparatus of difleerent design from that of Roberts is also correct. Dr. 0. Beeck (Euneryville, California) said : Dr. Miller quotes our values of the heat of adsorption of hydrogen on nickel as an example for an immobile film, while we quote the same values as indicative of a mobile film. This apparent discrepancy should be clarified. While Dr. Miller is correct that curves of this nature will result if molecules hit a plain surface randomly from the gas phase, this type of curve is not to be ex- pected for porous evaporated films, whose surface is entirely internal and can only be reached through pores from the outside of the film.Under these conditions a curve, showing a decrease of heat of adsorption with fraction of surface covered, can only result if the adsorbed atoms are mobile, as discussed in the paper by Beeck, Cole and Wheeler,29 where further references are quoted. Prof. J. H. de Boer (Geleen and Delft) said : The question whether ad- sorbed molecules are attached to localized sites or whether the surface spaces which they occupy are only governed by their mutual forces and their two-dimensional kinetic movements as a two-dimensional gas which condenses to a two-dimensional condensed phase, is of some importance for the estimation of surface areas by the B.E.T. method. In addition to the experimental evidence for localized sites in some cases, as mentioned by Dr. Miller, viz.the experiments of Crawford and Tompkins, I would like to mention that some 20 years ago I was studying the adsorption of 29 This Discussion.94 GENERAL DISCUSS ION various molecules on films of inorganic salts, obtained by evaporation in high vacuum, The surface area was measured by a surface reaction with alizarin and-to mention the case of CaF, films-was about 240 m.2/g. Water molecules and atoms of hydrogen occupied one crystallographic site, when adsorbed in a unimolecular layer, iodine molecules occupied two sites, fi-nitrophenol four sites.sO These molecules, though bound to definite sites, are nevertheless quite mobile over the surface and form two-dimensional condensed states which may act as a new surface for multimolecular adsorption. As, how- ever, the mutual distances differ from those in the liquid, one may not expect the heat of adsorption of the 2nd and higher layers to be equal to the heat of liquefaction.This conception leads to a slight alteration of the B.E.T. equation, mainly to the effect that the saturationpressure P o has to be replaced by a, mostly somewhat higher, pressure q. This exp ains why in practical cases the adsorption isotherm does not approach the axis p / p o = I asymptotically, but intersects it at a definite point. I hope to publish the consequences of this and other views shortly in a new book. Although the upper part of the isotherm is highly effected by this behaviour, the lower part is not and one can still use the method for an estimation of surface areas, provided one does not claim a too high degree of accuracy.We have compared the B.E.T. method with another method, based on the adsorption of fatty acids from pentane solutions. The adsorption of fatty acids can easily be worked out as a micro-method, when one estimates the change in concentrations of the fatty acid by spreading on water. Pentane or another saturated aliphatic hydrocarbon have to be used as solvents. Benzene is not allowed because of its own adsorption. Details of the method will be published late1-.~1 The smooth character of the isotherms, experimentally found in most practical cases, point to a heterogeneous character of most surfaces for physical adsorption. On a homogeneous surface step-wise adsorption would occur rather than adsorption showing a smooth isotherm.Ab- sorption spectra of adsorbed molecules point in the same direction. The fall of adsorption energies with increasing degree of occupation can partially be understood in this way. In adsorption of atoms or molecules on metal surfaces, however, one may expect a decrease of the heat of adsorption, even on smooth surfaces, if the formation of positive or negative ions plays a role. These ions decrease or increase the work function and therefore lower the heat of adsorption for further atoms. This is experimentally known from photo- electric and thermionic measurements.s2 Even if the bond of the adsorbed atom with the metal is mainly of homopolar or of metallic character, there is, nevertheless, a small dipole moment left which alters the work function gradually when the degree of occupation increases.Mr. R. S. Bradley (Leeds) (communicated) : In support of Prof. de Boer’s contention that the B.E.T. adsorption isotherm should become asymptotic at pressures greater than the saturation value, we have the fact that supersaturated vapours may be kept in vessels on the walls of which there must be a multirnolecular layer, and yet condensation t o bulk liquid does not occur. Dr. C. Kemball (Cambridge) said: Dr. de Bruijn mentioned that the existence of the methane fragments CH, CH,, CH, found by Wright 3OA survey of these studies can be found in J. H. de Boer, Electron Emission 3 l C . M. M. Houben, Thesis (Delft). 32 J. H. de Boer, Electron Emission and Adsorption Phenomena (Cambridge, and Adsorption Phenomena (Cambridge, 1935).1935).GENERAL DISCUSSION 95 and Taylor 33 in the chemisorption of methane on nickel and also of the radicals N, NH and NH2 found by Taylor and McGeer s4 in the adsorption of ammonia on an iron catalyst indicated inhomogeneity of the surface. It is important to emphasize that it is the way the radicals behave on change of temperature which may indicate heterogeneity of the surface but that their existence is mainly controlled by the composition of gas mixture admitted to the surface. Methane admitted to a nickel surface will yield a larger proportion of carbon atoms and CH radicals on the surface than if a mixture of hydrogen and methane is used. An example of this type was given by the study of the rupture of the C-C bond of ethane on a nickel catalyst both in the presence and the absence of hydro- gen.85 The results indicated that the slow step was the rupture of the C-C bond of adsorbed ethylene which is formed by the dissociative adsorption of ethane : The presence of hydrogen tended to shift the equilibrium to the left and in fact the rate of decomposition depended inversely on the 1-3 power of the hydrogen pressure.In the absence of hydrogen small amounts of ethane decomposed quantitatively according to the equation : 2C2H6 + 3CH4 + c, and this implied that a large number of radicals must the surface during the reaction. It was found that the could be removed quantitatively by hydrogen : C + zH2 3 CH4. have existed on adsorbed carbon These results indicated that a large variety of radicals can exist on the same catalyst surface but that the proportion of the different types de- pended markedly on the composition of the gas admitted.Dr. H. de Bruijn (Geleen, Netherlands) (communicated) : I quite agree with Dr. Kemball that the occurrence of different Iragments of adsorbed molecules on the surface of a catalyst as such is no proof of inhomogeneity of the surface. In my paper I probably mentioned the experiments of Wright and Taylor too briefly. They found on hydrogenation of the fragments with deuterium that each of the species required a specific temperature. The equilibrium which exists between the different species on a homogeneous suriace is disturbed as soon as one of the species is hydrogenated. On a inhomogeneous surface, however, the amount of the other species will not be altered unless one chooses a higher tem- perature. Apart from this, the strongest argument for inhomogeneity remains the result ob- tained by Emmett and Kummer in their experiments on the adsorption and desorption of labelled carbon monoxide on iron catalysts. Dr. A. F. H. Ward (Manchester) said : The question has been raised of the extent to which it may be necessary to attribute to active centres an important influence in catalysis. It would appear that a decision on this matter could be reached by a study of catalytic reactions on liquid metal surfaces. Many years ago, Williams attempted to catalyze the reduction of nitrobenzene to aniline on a liquid tin surface. He was unable to detect any reaction, although the oxides of tin catalyzed the reaction readily. One might, of course, expect some difficulty in ob- serving catalysis on a liquid surface on account of the much smaller area. However, it seems important to establish whether catalysis will take place This is what Wright and Taylor actually found. 33 Wright and Taylor, Can. J . Res., 1949, 27, 303. 34 Taylor and McGeer (Princeton Univ., 1949). 35 Kemball and Taylor, J . Amer. Chew. SOC., 1947, 70, 345.96 INTRODUCTORY PAPER at all on a liquid surface, and if so, to study kinetics on such surfaces in the detail that has been used with solid surfaces. If, in a comparison with equal areas of solid and liquid surfaces the latter gives a smaller (or zero) rate of reaction, it would imply that the catalysis must be at- tributed to some property characteristic of the solid state, not shown by the liquid, and not merely dependent on the type of metal used. This method of approach should allow a discrimination between the effects of non-uniformity of the surface and interaction between adsorbed mole- cules.

ISSN:0366-9033    

DOI:10.1039/DF9500800079

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

96 INTRODUCTORY PAPER 11. ADSORPTION AND CATALYSIS ON METALS INTRODUCTORY PAPER (i) BY ERIC K. RIDEAL We are indebted to our Spiers Memorial Lecturer, Dean H. S. Taylor, for the clear recognition that on a metallic surface gas adsorption may take place by two distinct methods-by the operation of the dispersive forces giving physical adsorption and by means of an electron switch by which a heteropolar or covalent bond is formed between adsorbent and adsorbate. It is nevertheless a matter of some difficulty to identify in any par- ticular case which type of adsorption is operating. In very many cases slow activated processes ascribed to the operation of chemical adsorption have been found to be due to the process of solution in the sub- strate requiring marked energies of activation.Again slow surface reactions have been found involving displacement and replacement of one chemisorbed species by another. The two general methods of pre- paring unquestionably clean metal surfaces for these studies are by utiliz- ing a wire or ribbon capable of being cleaned by flashing and by deposition of metallic mirrors. In the former case the surface areas are small and the building-up of surface phases is best followed by indirect means such as by observing changes in the accommodation coefficient of a rare gas, the thermionic or photoelectric work functions or even by electric capacity measurements. On mirrors the specific surfaces can be made large enough to permit of direct volumetric measurement of the adsorption very accurately up to pressures of about 10-lmm.Hg. In chemisorption we have evidence that complex molecules are frequently broken down to form surface compounds of simpler constitution, e.g. molecular hydrogen to form surface hydrides, which can be detected by the ortho-para hydrogen conversion or by ex- change with deuterium. These methods unfortunately do not permit us to find out how far a hydrogen-containing complex molecule undergoes surface decomposition in chemisorption, e.g. in the chemisorption of ammonia, whether a nitride, imide or amine is the residue. Therefore, attempts have been made to identify these residues by indirect means, e.g. by chemical attack by another gas or by direct volatilization into a mass spectrometer. Chemical reaction may proceed by interaction between two different chemisorbed species, or between a chemisorbed species and a physically adsorbed molecule, or by collision from the gas phase with a chemisorbed species.Energetically at first sight the latter processes seem preferable to the former, since reaction schemes can be set up in which bond exchange, i.e. a chemical for a physical, alone is involved with no rupture of primary bonds leaving an unsaturated metal atom but the first operation may take place if a fresh molecule of reactant displaces one of the chemisorbed re- acting species to react with its neighbour.ERIC K. RIDEAL 97 It is usual to differentiate between chemisorption and physical ad- sorption by the high heat of adsorption involved in the former process, but it must be noted that in those cases which have been examined over a sufficient range the AH - 8 curve for chemisorption falls as the surface becomes more closely packed.By some this has been attributed to surface heterogeneity. This involves the assumption of surface mobility so that redistribution can take place within the experimental time period. In many cases this does not appear to occur. Even if surface mobility occurs at high temperatures the fall in the AH - 8 curve is noted even at very low temperatures. This fall may be accounted for by assuming a progressive weakening of the bond strength between substrate and ad-atom or ad-radical as the surface fills up. If this bond is heteropolar then the dipole moment of the bond will change and the mutual interactions, i.e. the repulsive forces between the ad-atoms or ad-radicals likewise change.I can quote the following data that Bosworth and I obtained for the spreading pressures, i.e. the two-dimensional osmotic pressures for potas- sium chemisorbed on tungsten in comparison with formic acid physically adsorbed on mercury. e Molecules/sq. cm. x 1014 0.5 1'0 2'0 4'0 F (dynesjcm.) K on W 9 70 190 22 HCOOH on Hg 9 16 28 48 We note how great a contribution the dipole repulsion makes to the spreading pressure as the surface packing increases. At the same time the apparent adhesion to the tungsten decreases as the surface packing increases. This is reflected in a lowering of the heat of adsorption and an increase in the rate of evaporation-in fact the loga- rithm of the rate changes linearly with the heat of adsorption.The weakening of the bond is reflected in a decrease in the apparent dipole moment of the ad-atom. I cite some values of Langmuir for caesium on tungsten : e p x 1o18 e.s.u. 0 16.16 0.5 8.28 0.9 6.06 It is clear that as the composition of the electric double layer at the surface of the metal changes with increasing or decreasing adsorption the work involved in the transition of an electron from the internal levels to the double layer or vice-versa, likewise changes. Atomic spacing, the electron work function and the electron levels in the metal are thus three factors playing a part. These considerations imply that the heat of adsorption may fall to such low values as 8 increases that (a) surface mobility may set in at some value of 8, (b) t? may never obtain the value 8 = I even at relatively high pressures, (6) physical adsorption may set in before 0 = I and a mixed film will result, e.g.an ortho-para hydrogen conversion in the Bonhoeffer-Farkas scheme. I f at some value of 8 the bond strength to the substrate may be so weakened that some ad-atoms may rise to a mobile level, migration and chemical action may then occur. The mechanism of the chemical catalytic action may thus change between small and large values of 8. D98 INTRODUCTORY PAPER The evidence for the hydrogen-deuterium exchange in olefines taking place through the medium of the half-hydrogenated state or the organic alkyl metallic radical complex seems to be fairly convincing, whether the latter is produced by some mechanics of one of the following types : CIH, + MD -+ M .C2H, or C,H, + D or C,M, . . . D, / \ 1 / \ : M M M M M M (ii) (iii) or not. It is certain that hydrogenation of a double bond does not go via the same path. Evidence is presented that gaseous ethylene can re- move chemisorbed hydrogen to form ethane and that chemisorbed ethylene can, like other hydrogen-containing molecules, break down further on chemisorption. In reactions involving hydrogenation of double bonds the interatomic spacing of the catalyst on the various crystal facets plays a dominant role in the catalytic process. However, in ring closure and in the hydrogenation and dehydrogenation of aromatic substances we believe that it is the double bond that is attacked by two-point contact through the medium of the n- electrons.This at once raises the issue whether there are two mechanisms for the hydrogenation of ethylene, the removal of two chemisorbed hydrogen atoms suitably spaced by gaseous or van der Waals' adsorbed ethylene or the addition of van der Waals' hydrogen to chemisorbed ethylene. We must note that on the catalytically active facets of crystalline catalytic metals the spacing is suitable for two-point contact, i.e. CH 2-CH 2. M M / \ The change in carbon-carbon spacing to yield surface acetylenic complexes would not take place readily in these facets but on other crystal facets acetylenic compounds can be retained with ease and it is possible that these facets only are responsible for the dehydrogenation of ethylene. The fact that ring closure also takes place through double-bond two-point contact likewise supports the view that olefine hydrogenation involves the chemisorption of the olefine. The fact that AH varies with 8 suggests that the bonding energy to the substrate is a function of the surface covered we should thus anticipate that the energies of activation for catalytic processes which involve simple electron transfer reactions should be related to the electron work function provided that the relative changes in the bonding energies due to the degree of packing are identical €or the cases under consideration.This seems to be the case for the following in which Wansbrough-Jones and I found the following values :- Oxidation of NH, . Decomposition of NH, on . Pt w C Pt w Mo I E volts I I 2'74 0.87 0.52 3'5 1'7 1-39 4 6-35 4.48 4-31 4-E 3.61 3.61 3'79 2-85 2.78 2-97ERIC K.RIDEAL 99 In conclusion I might again draw attention to the valuable information to be gained in exploring the mechanism of gas adsorption and hetero- geneous catalysis on metals by a comparison with the thermionic and photoelectric behaviour of these metals when monolayers are present on their surfaces. Dauy-Faraday Labovatories, T h e Royal Institution, Albemarle St.. TY. I . INTRODUCTORY PAPER (ii) BY D. D. ELEY With one exception, the papers in this section deal with the activation of hydrogen and particularly the hydrogenation of ethylene. I shall first consider them from the viewpoint of reaction path, starting with the simple case of adsorption. I shall then discuss the factors underlying the fundamental mechanism of catalysis, i.e.electronic and lattice spacing factors. Thus the various papers may be referred to more than once under each heading, and where necessary, reference will be made to papers in other sections. Chemisorption of Gases on Metals.-Roberts established that the surface of a tungsten wire cleaned by flashing at 2500" K in uacuo, chemisorbed hydrogen, oxygen or nitrogen with immeasurably fast velocity even at - 183' C. The heats of adsorption on the bare surface (0 = 0) are 45,' 139 and 28 kcal./mole gas respectively, and the in- dications are that one atom occupied one metal site. Many of the results gained further support from the contact potential work of Bosworth and RideaL4* Roberts found the heats of adsorption (in particular he studied H,-W) fell off with surface covering.He indicated that certain cases of slow adsorption of gases on metals may result from displacement of im- purities. has confirmed these results in all essential respects, with evaporated metal films of area 106 times that of a wire, and in addi- tion extended them t o other gases, such as ethylene, and numerous other metals. The only case where a true sZow adsorption is known to exist is nitrogen on iron, where the activation energy is doubtless associated with the rupture or partial rupture of the triple bond.7 We had also confirmed many of Roberts' results on tungsten by using the para-ortho hydrogen conversion to detect adsorbed films and by the use of evaporated films of the metal.8. There are good grounds for believing that the chemi- sorbed atoms are held by largely covalent bonds.The study of contact potentials, and the new calculations of heat of chemisorption presented at this Discussion,lo establish this view beyond much doubt. Thus the primary chemisorbed layer of atoms is understood in general, though many points of detail await further experimental work. In the potential energy diagram given by Lennard- Jones," the height of the energy hill between Roberts, Proc. Roy. SOC. A , 1935, 152, 445 ; Some Problems in Adsorptiorz (London, 1939). Bosworth and Rideal, Physica, 1937, 4, 925. Bosworth and Rideal, Proc. Roy. SOC. A , 1937, 162, I . Bosworth, Proc. Camb. Phil. SOC., 1937, 33, 394. Beeck, this Discussion. Beeck, Cole and Wheeler, this Discussion.Eley and Rideal, Proc. Roy. SOC. A , 1941, 178, 429. Eley, ibid., 1941, 178, 452. l1 Lennard-Jones, Trans. Faraday Soc., 1932, 28, 333. Dr. Beeck Roberts, Nature, 1936, 137, 659. lo Eley, this Discussion.100 INTRODUCTORY PAPER van der Waals' and chemisorption is I kcal. or less when approached from the " gaseous " side. The paper of Rideal and Trapnell,12 however, raises the point that Roberts was concerned only with 70 yo of the surface. According to them, the heat of chemisorption on the last 30 Yo falls rapidly from 14 kcal. to 3 kcal. and this hydrogen is easily pumped off. Couper and I can confirm that easily removable hydrogen is present at ordinary pressures, and indeed the effect is mentioned in the first paper by Beeck.13 The fundamental question for the discussion may well be how far this hydrogen is held in a second layer.Mignolet lP has shown how the primary hydrogen layer on nickel has a negative contact potential, but that after- wards a second weakly bound layer is formed with a positive contact potential. Since Xe-Ni also has a positive contact potential, we may assume tkis hydrogen in the second layer is also held by van der Waals' forces. I f so, the heats of adsorption are rather larger than those usually attributed to this kind of adsorption; but we should not reject the hypothesis out of hand on this account in the absence of further theoretical work on the subject. Besides forming van der Waals' and chemisorbed layers, hydrogen may penetrate the lattice, giving true solutions such as those examined by Sieverts, or being held on intercrystalline boundaries.Long ago this was suggested as a mechanism for activated adsorption l5 and Beeck's work on sintering nickel films supports this most strongly.16 From the catalysis point of view, dissolved hydrogen may poison the catalyst as Couper and Eley l7 have shown for the parahydrogen conversion on palla- dium. Quite a number of cases of poisoning found in the literature may be tentatively ascribed to this effect. describes results on the adsorption of hydrogen on nickel powder, the pre-treatment of which is not stated. Eucken gives evidence for two kinds of adsorption, one with a peak in the isobar a t zooo K and the other at 280' K. The heat in the first case is estimated as 6 kcal./mole, which Eucken attributes to the chemisorption of an H, molecule to give two atoms side by side.The second maximum corre- sponds to the atoms having diffused apart into positions of minimum energy. Beeck, Cole and Wheeler with evaporated nickel films state that hydrogen atoms are mobile at - 183' C and that the heat of adsorption against b' curve is the same as at 23' C. The reduced powder may of course not only differ physically from an evaporated film, but may even contain dissolved hydrogen and oxygen. Evaporated films are much more likely to possess an uncontaminated surface. A comparison of results in two cases such as these may eventually yield valuable generalizations for technical catalysts, as indicated by Prof. Taylor in his opening lecture. The Parahydrogen Conversion.-The heterogeneous parahydrogen conversion l9 by the chemical (as opposed to magnetic) mechanism, is identical in rate and kinetics with the hydrogen deuteride reaction in cases known at present.The original Bonhoeffer mechanism involved dissociation and recombination of atoms in a loosely-bound chemisorbed layer, but as a result of surface interchange experiments on evaporated films of tungsten,* for this case the following mechanism was substituted. The paper by Eucken P-HE + ZM -+ zMH -+ 2M + O-H, l2 Rideal and Trapnell, this Discussion. l3 Beeck, Smith and Wheeler, Proc. Roy. SOG. A , 1940, 177, 78. l4 Mignolet, this Discussion. 16 Beeck, Ritchie and Wheeler, J . Colloid Sci., 1948, 3, 504. l7 Couper and Eley, this Discussion. Ward, Trans. Faraday Soc., 1931, 28, 339.Eucken, this Discussion. Reviewed by Eley in (a) Advances in Catalysis, Vol. I, I57 ; (b) Quart. Rev., 1949, 3, 209.D. D. ELEY I 0 1 A p-H2 molecule in the van der Waals' layer was supposed in some way to exchange an atom for one in the underlying chemisorbed layer P-H, + HW + H3W + WH + o-H~. This work was confirmed for the metal nickeLZ0 Trapnzll and Rideal, assuming the loosely-bound hydrogen on tungsten exists in the chemi- sorbed layer, once again suggest that in this case the Bonhoeffer mechanism is valid. Neither must we neglect here the suggestions of A. and L. Farkas 2 O for the metals platinum and palladium. Dr. Couper, Mr. Hulatt and myself have further unpublished work which we hope will help in the discussion of this matter. The adsorption work of Reeck, and of Rideal and Trapnell raises again the question of the determination of surface areas.The usefulness of the R.E.T. method is now beyond dispute, but undoubtedly other methods are to be sought and the paper by Maxted et al. 21 shows how the phenomena of catalyst poisoning may be brought to bear upon the problem, with most hopeful results. Hydrogenation of Ethylene.-While dispute can still occur concerning the mechanism of the parahydrogen conversion, the chances of agreement on the mechanism ot ethylene hydrogenation must seem remote at present. Let us remind ourselves of the general facts of ethylene hydrogenation as they stood in 1940 when Beeck commenced to add the large number of striking new data on films, summarized in his papers to this Discussion.The reaction had been investigated on many catalysts, particularly copper and nickel powder, produced by reduction of oxides, or wires activated by oxidation and reduction. References are given in a recent review.19b In the range 100-150~C, the reaction is first order in hydrogen and zero order in ethylene. A marked excess of ethylene has sometimes been ascribed an inhibiting effect. At 200' C and higher the rzaction is second order, i.e. first order in both gases. At room temperature Beeck reports that the reaction is first order in hydrogen and zero order in ethylene, while for butene-1 at 50° C. Twigg reports u - PH20-5pEt0.5, On transition metal catalysts, the activation energy is usually 5-10 kcal. Beeck reports E = 10.7 kcal. for many metals investigated, exceptions being tungsten and tantalum.A marked feature of the reaction is that as the tempLrature increases the activation energy falls to zero, a t 150' C for nickel catalysts, and much higher temperatures for nickel-silicon skeleton catalysts, as pointed out bv Schwab. This effect in the past has been attributed to desorption of ethylene ; but desorption of hydrogen must first be considered, since it starts at a lower temperature. Any theory advanced has to take account of these facts and also the many new facts advanced by Bzeck, of which two are outstanding: p a (a) Ethylene gas admitted to a clean metal film is strongly adsorbed, CH-CH probably as acetylenic complexes Ni/ \Ni which largely cover the surface. The hydrogen atoms are removed by further ethylene forming ethane in the gas phase.Such a pre-adsorbed film is only slowly removed on exposure to hydrogen gas. (b) A hydrogen film pre-adsorbed on nickel is rapidly removed by gaseous hydrogen giving ethane. The main modern theories in the field may be considered in chrcno- logical order. (I) THE HALF-HYDROGENATED STATE THEORY OF HORIUTI AND PoLANYI.~~-I~ this, two hydrogen atoms are added on independently 20 A. Farkas and L. Farkas, J . Amer. Chem. Soc., 1942, 64, 1594. 21 Maxted, Moon and Overgage, this Discussion. 22 Beeck, Rev. Mod. Physics, 1945, 17, 61. 23 Horiuti and Polanyi, Trans. Faraduy SOL, 1934, 30, z I 64.I02 INTRODUCTORY PAPER and successively. i.e. by opening of the double bond. The ethylene is held by I ‘ associative ” chemisorption, * * + C*H,-CH, + H + CH,CH, + fi + CH,-CH, {an asterisk indicates a chemisorption bond to a metal site).(2) THE ADDITION OF A MOLECULE OF HYDROGEN : * * (a) H, + CH,-CH, --f CH,-CH, (Twigg and Rideal 24) (b) H2 + CH,= CH --f CH,-CH, + H --f CH,CH, (Farkas 25) (c) H + CH,=CH, + H +CH,CH,. * * * * * Farkas favours dissociative adsorption, while Twigg and Rideal favour associative adsorption of ethylene. Mechanism (zu) may possibly be brought into line with (I) if we realize lSb that the simultaneous addition of zH atoms from one H, mole- cule comes to the same thing, (zc). Mechanism ( 2 ~ ) is expected to occur when the hydrocarbon is very strongly adscrbed as with ethylene, i.e. when the chemisorbed hydrogen cannot diffuse on the surface. Benzene, on the other hand, is more weakly adsorbed and the resultant high con- centration oi H favours independent H from separate H, molecules.(3) Beeck’s mechanism is that the surface is largely covered with ethylene molecules held as acetylenic complexes and only slowly removed by adsorbed hydrogen * * * * * * * 2H + CH-CH + ZH + CH,-CH, On the spaces set free Beeck visualizes two possible mechanisms. Firstly, hydrogen is chemisorbed and reacts with gaseous ethylene, this reaction determining the hydrogenation reaction * * H + CH2=CIIz + H 3 CH,-CH,. Originally suggested by Beeck for the ni kel group of metals, he now retains it only for the tungsten group. For nickel he now favours interaction between chemisorbcd hydrogen and chemisorbed ethylene, i.e. presumably some mechanism related to (I) or (zc) above.A good case has been made out for (I) or (2) on thermochemical grounds,lsb in contradiction to some earlier arguments of Beeck.22 It is now a matter of great interest that Eyring 26 and Laidler 27 both base their transition state treatments on formal schemes related to (2a), (zb) and ( Z G ) and can show that the low steric factor for the reaction calculated by Beeck (and also by Eucken l 8 from the results of Toyama) is due to loss of trans- latiom and rotations on adsorption of the reacting gases. It does, however, appear that a number 01 mechanisms must have the same calculated frequency factor, so that really we are not much further ahead. Also (as Schwab has pointed out) the treatments give a maximum rate at a definite pressure which is not found in practice.The extensive and important researches of Twigg and Rideal,24 and Farkas 25 on the exchange of deuterium atoms between deuterium mole- cules and ethylene molecules have been recently reviewed.la Twigg and Rideal showed : (I) The exchange and hydrogenation have identical kinetics on a nickel wire at 156°C. The activation energy for exchange is 18.6 kcal., and falls off in an identical fashion with increase of temperature, being at all temperatures 4-5 kcal. greater than that for the hydrogenation reaction. 24 Twigg and Rideal, Proc. Roy. SOC. A , 1939, 171, 55. 25 Farkas, Trans. Faraday Soc., 1939. 35, 906. 2B Eyring, Colburn and Zwolinski, this Discussion. 27 Laidler, this Discussion.D. D. ELEY 103 Farkas obtained similar results on platinum foil.The H, + D, reaction is effectively inhibited by the presence of the ethylene. Twigg and Rideal suggested the rate-determining step to be the first of the following reactions : * * * * * * C.HZ-CH2 + D, -+ CH,-CH,D + D --+ CH,-CHD + HD. ( 2 ) Conn and Twigg found no exchange between C,D4 and C,H, even at temperatures as high as 3603 on a Ni filament which favours zdsorption as CH,-CH, rather than CH,=CH H. The calculations of heat of chemisorption by Eley also tend to support the first kind of chemisorption. The paper by Turkevich, Bonner, Schissler and Irsa 28 brings forward similar data to Twigg, but by the mass spectrometer they can distinguish the various isotopic species. They also note that in the initial stages of reaction of I), + CzH4, light C2H, is produced.This result is the same as that of Baxendale and Warhurst 29 who found appreciable quanti- ties of light methyl elaidate produced by deuterogenation of methyl oleate on platinum black. This is very strong evidence for the occurrence of Beeck’s reaction outlined above, but it would appear from Turke- vich’s result, as we would expect, that the rate of production of C,H, soon decreases to zeio, and that this reaction plays no part in the stationary hydrogenation process. Thermochemical reasons support this view, since * * * * zCHZ=CH, -+ CHS-CH, + CH-CH has AH = + 17.2 kcal. and since A S -0 it will be thermodynamically impossible, and can only occiir at all because the acetylene is chemisorbed on the catalyst. My own tentative viewpoint is that a large part of the catalyst may be covered by the sc-called acetylenic complexes, ~urticulurly if pure ethylene is admitted to a clean catalyst, but that the catalyst settles down to a stationary state in which a fair part of the catalyst is covered by chemi- sorbed ethylene, which reacts by (zu) or (zc).It is difficult to see how (3) could give the observed kinetics, but there is clearly a need for a great deal more woik before a definite decision can be reached, and the lines of investigation should come out of this Discussion. I t might be convenient finally to raise one new suggestion at this time. The solubility of hydrogen in metals such as nickel increases strongly with increase of temperature. If hydrogen dissolved in the metal does exert a poisoning action, it might well lead to the activation energy falling off to zero.It is well known that ethylene does not desorb off nickel below zooo C, but it is exceedingly unlikely, since the reaction is first order in hydrogen gas at relevant temperatures, that desorption of this gas can affect the activation energy. If desorption were a factor, one would expect Ni-Si catalysts to have a lower temperature of maximum rate, but if solubility comes in a higher temperature. This is only a very tentative suggestion as we have no experiments to show a poisoning action of dissolved hydrogen on this system, but only on the palladium -$--H, system. Other reactions discussed include dehydrogenation, 30 and the oxidation of ammonia.31 There would seem to be no special problem in the kinetics of dehydro- genation comparable with the hydrogenation reaction, probably because the former occurs at much higher temperatures.Zawadzki’s paper reviews the impressive body of work done on the ammonia reaction, much of which is not easily available to workers in this country. 2 9 Turkevich, Bonner, Schissler and Irsa, this Discussion. 29 Baxendale and Warhurst, Trans. Faraday Soc., 1940, 36, 1186. Schwab, this Discussion. 3l Zawadzki, this Discussion.I04 IXTRODUCTORY PAPER Lattice Spacing Factor.-Beeck’s work on the increased activity of oriented films of nickel is well kn0wn.1~ He has now 3 2 developed this work showing that the actual activity is associated with an increased area of exposed (110) plane, and not merely increased orientation.This is so for ethylene hydrogenation and ethane dehydrogenation. On the other hand, there is no difference between oriented and non-oriented nickel films for benzene hydrogenation. Oiiented (110) films of platinum were less active than non-oriented films in cyclohexane dehydrogenation. This latter result Beeck associated with the Ralandin criterion of 6-point e dsorption on the octahedral ( I I I) faces. Reeck associates the maximum activity ot rhodium for ethylene hydrogenation and palladium for acetylene hydrogenation with the need for a lattice spacing in the metal to fit the substrate, but qualifies his view for metals like chromium, where he believes bond-type, in particular percentage d character plays a part, as emphasized recently by workers both in Billingham and Bristol, and discussed in the next section.Electronic Factors.-Schwab 3 0 has summarized his work on the dehydrogenation of formic acid on a wide range of alloys of monovalent with multivalent metals. He has quite convincingly demonstrated that the activation energy is proportional to the square of the electron con- centration, and argues that the rate-determining step is the loss of z electrons from the formic acid molecule into the lowest unfilled level in the metal. This is a particularly thorough and satisfying piece of work. He has also raised again the question of the linear relation between the logarithm of the frequency factor and the activation energy. Couper and Eley l1 have shown how the activation energy of the parahydrogen con- version on palladium is suddenly increased when gold is alloyed at the right concentration to fill up the holes in the d band. In other words, unpaired d electrons, presumably at the surface, are a necessasy con- dition for a stronFly bound activated complex. Dissolved hydrogen exerts a similar poisoning action. The clear cut nature of the result is probably associated with the simple nature of the system they examined. Dowden and ReynoldsI33 with a similar objective, examined Ni-Cu and Ni-Fe alloys. The general 2icture is that a high activity is associated with a partly empty d-band (not only for the hydrogenation of styrene, but also for the decomposition of methanol and formic acid). On the other hand, the decomposition of hydrogen peroxide is favoured by a full d-band, which they believe is due to the slow step being electron donation from metal to substrate 34 M + H,O, -+ M+ + HO + OH-. They have also produced evidence that in Ni-Fe alloys the catalytic activity increases with g(E), the energy density of electron levels. For one range of compositions while the number of holes in the d-band are decreasing, nevertheless there is a strong increase in hydrogenation activity in the composition region where specific heat data indicates an increase in g(E). It is quite clear thpt with the application of the relatively novel ideas and techniques outlined in these papers that the subject of metal catalysis is going to enter a long period of fruitful development. The University, Bristol. 32 Beeck and Ritchie, this Discussion. 33 Dowden and Reynolds, this Discussion. 34 Weiss, Trans. Faraday SOG., 1935, 31, 1547.

ISSN:0366-9033    

DOI:10.1039/DF9500800096

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

STUDIES IN CONTACT POTENTIALS PART 1.-THE ADSORPTION OF SOME GASES ON EVAPORATED NICKEL FILMS BY J. C. P, MIGNOLET Received 2nd February, 1950 Surface potentials have been determined for a number of gases on unoriented evaporated nickel deposits. The chemical films studied are : Ni-H : - 0-345 V ; Ni-C,H, : + 1.0 V ; Ni-C,H4 : + 0.83 V. Contact potential appears to be suitable for directly studying surface reactions involved in the catalytic hydrogenation of C,H, and C,H, on nickel. Van der Waals' films of non-polar gases may exhibit considerable surface potentials : Ni-Xe : + 0.85 V ; Ni-N, : + 0.21 ; Ni-C,H, : + 0.77 V. Surface potential appears t o afford a criterion of limited applicability for distinguishing between van der Waals' adsorption and chemisorption. Surface potential (S.P.in short) has a number of qualities which make it an interesting physical quantity in the field of adsorption and catalysis on solids. It is a true surface property simply related to the electric moment of the adsorbed particles and to their concentration on the outer surface. Experimentally contact potential techniques are available which enable S.P. (or S.P. variations) to be measured in an extraordinarily wide range of conditions. It seems possible with them t o study reactions between adsorbed films and gases and to observe the films on catalysts in operation. Apparently, little has been done in this direction.1 In the case of the catalytic dissociation of H, on tungsten, Bosworth has been able to choose between two mechanisms on the basis of a contact potential determination, In a study of the catalytic para hydrogen conversion on a tungsten wire poisoned with an oxygen film, Eley and Rideal have used a contact potential technique to determine the relation between catalytic activity and concentration of the poison film.The main object of this investigation was to examine the films of H,, C,H,, C2H4 and C,H, on nickel, as a first step towards a study of the catalytic hydrogenation of C,H, and C,H4. In the course of this investigation, it has emerged that van der Waals' films of non-polar particles could ex- hibit appreciable S.P. Since this raises questions as to the interpretation of S.P., films of such typically non-polar particles as N, and Xe have also been studied. Experimental The apparatus used is shown schematically in Fig.I. The cell and the micro-introductors 4 are described elsewhere. Before starting an experiment, the apparatus is degassed thoroughly, baking the cell and trap I at 500° C in an oven, torching out the tubings and heating the filament for 2 hr. just under evaporation point. An additional precaution has been found advisable, i.e. t o pump impurities of intermediate volatility out of the traps. Otherwise, Bosworth, Proc. Carnb. Phil. Soc., 1937, 33, 394. * Eley and Rideal, Proc. Roy. SOC. A , 1941, 178, 429. 3 Mignolet, Part 11, this Discussion. Mignolet, Trans. Faraday SOC., 1949, 45, 271. D " '05I 06 ADSORPTION OF GASES ON NICKEL FILMS virtual leaks may occur and cause, for instance, variations of 0-1 t o 0 - 2 V when the condenser tube is cooled to - 196'C.These fractional distillations are carried out by temporarily warming-up traps I and 3 t o - 160°C as shown in Fig. I. Such slight warming-ups are also useful when handling ' I vapours," e.g. C,H,. Vacuum-tight tests showed that, with the cut-off shut, the pressure remained of the order of 10-6 mm. Hg for a few hours and rose overnighr: to I or 2 X I O - ~ mm. Hg. In normal working conditions, a bare nickel deposit will not exhibit any appreciable drift in contact potential for 2 hr. FIG. I .-Apparatus. F : iron cylinder enclosed in glass tube. T,, T,, T,: traps normally cooled in liquid nitrogen. TI outgassed with the cell at 500' C in an oven ; T, cooled temporarily before removing oven. C,, C, : copper tubes sealed to traps with alcohol; warmed to - 1 6 0 O C (indicated by thermocouple Th) for fractional pumping-out of impurities. Long lower part prevents temperature from rising owing to evaporation of liquid nitrogen.Three cells were used : a simple type I cell in series E, .F and G ; a type I cell of a somewhat different design in series €1, . . . N ; a type I1 cell in series 0, . . . W. The second cell incorporated two filaments-one opposite each " area "--and a screen t o meet secondary requirements enabling, for instance, evaporations t o be made without condensation on either " area ' I . The result was worse, however, because increased crowding caused the S.P. values to be about 15 yo too low. Much attention has been paid to avoiding and detecting possible variations of the auxiliary surface.In series E to N, controls were systematically made, fully using the possibilities of the two " areas " (see discussion in Part 11). These controls have shown that the surface of the auxiliary electrode-such as results from the outgassing-is extremely inert chemically. The situation is very favourable too with van der Waals' films a t low temperatures because the temperature of the auxiliary electrode remains much higher than that of the vibrating electrode. Actually, with the condenser tube in liquid nitrogen, the temperature of the auxiliary electrode in vucuo is roughly - IOO C. Of course, the temperature sinks as the gas pressure is raised, but since it reaches its final values within a few minutes, adsorption on the auxiliary surface would be de- tected.Actually, such effects as have been observed never exceed 0.02 V. They probably result from a thermal variation in the work function. The stability of the auxiliary surface being satisfactory, and since controls are a source of small errors, it was decided t o drop them after series N and choose the best possible conditions for the measurements proper. As a result, a better repro- ducibility of the S.P. values was obtained (see below). Evaporation was produced by heating a nickel filament (0.3 mm. diam.) electrically (2.8 t o 3-0 A) in vucuo. Nickel wire of two sorts were used, i.e. 99 yo Ni + 0.5 yo Co and after series M, spectro- scopically pure Ni. With the first two cells, the nickel was deposited on a nickel foil (vibrating plate) at - 196" C.With the third cell, the base was Pyrex glass cooled externally in ether. Apparently, there was no difference between the two sorts of deposits. An X-ray diffraction test * showed that the latter method yields microcrystalline unoriented deposits as stated by Beeck, Smith and Wheeler.5 With the thick opaque deposits used, no influence of thickness was apparent. Materials.-NICKEL DEPOSITS. No difference was detected. * Kindly carried out by Dr. H. Lambot and Dr. J. Toussaint, 5 Beeck, Smith and Wheeler, Proc. Roy. SOC. A , 1940, 177, 62.J. C. P. MIGNOLET 107 HYDROGEN : electrolytic ; purified by diffusion through a palladium tube. NITROGEN : from a steel cylinder ; purified by bubbling through Na,S,O, + KOH. ACETYLENE : prepared and purified according to Moser.6 ETHYLENE : prepared and purified according t o Moser (C,H,OH + H,PO,).ETHANE : prepared by the reaction C,H,MgBr + H,O ; washed with KMnO,. All three hydrocarbons were fractionated in DUCWO in the apparatus itself and sent directly into the introductors. ARGON : 99-9 % + 0.1 % N,. XENON : supplied by Matheson Co. the only impurity was krypton (0.25 yo). According to mass spectrometer analysis, Results HYDROGEN FILMS.-TWO kinds of hydrogen film have been observed. They are easily distinguished as their S.P. are of opposite signs, i.e. negative* for the chemical (atomic) film, positive for the van der Waals' film (see Fig. 2 ) . Experiment . Oa Ra Sa In vucuo ( p N 10-6 mm. Hg) . - 0-304 - 0.266 - 0.306 - 0.327 - 0.359 The S.P. for a complete chemical hydrogen film may be taken as -0.345 f 0.02 V.At - 196' C, addition of hydrogen to a nickel surface already covered with the negative film leads to the formation of a volatile positive film, as shown in Curves 2 and 3. Curve 2 was obtained with an incomplete negative film pre- pared by pumping at 20' C and cooling, and Curve 3 with a complete film pre- pared by cooling in hydrogen (50 mm. Hg) and pumping. Except for a constant difference of about 0-06 V due t o the negative film, the two curves do not differ appreciably. A maximum S.P. of + 0.1 V is obtained in both cases. This value is not accurate. It contains a possible contribution of thermal origin from the auxiliary surface (+ 0.02 V a t most). A few S.P. (in volt) of the negative film a t zoo C are : At saturation (9 > 2 mm.Hg) . - FIG. 2.-Surface potentials of hydrogen films as a function of pressure (plotted at two different pressure scales). Curve I : a t zoo C ; chemical film. Curve 2 : at - 196" C ; van der Waals' film on incomplete chemical film. Curve 3 : a t - 196" C ; van der Waals' film on complete chemical film. a : Expt. R a ; L-J A 0 : Expt. Sa. Points from Expt. Ra have been raised by 0.016V and those from Exyt. Sa, lowered by the same quantity so as to make the S.P. coincide for a complete negative film. aMoser, Die Reindaystellung zlon Gasen (F. Enke, Stuttgart, 1920), pp. 140 and 137. * Negative films increase the work function.108 ADSORPTION OF GASES ON NICKEL FILMS The negative film (somewhat incomplete) is formed first. Measurements of adsorbed volumes on larger nickel films prepared in the same way have shown that, a t about 4 x I O - ~ mm.Hg, the ratio of quantity of H, in 2nd film/quantity of 13, in 1st film is about 0.03. whereas the ratio of the corresponding S.P. is about 0.25. Therefore, the electric moment of the hydrogen particles is much higher in the van der Waals' film than in the chemical one. Preliminary experiments give evidence that the S.P. of the negative film is proportional to the amount of hydrogen adsorbed, i.e. that the electric moment is independent of the fraction of surface covered. No similar indication is available for the positive film. I n particular, i t has not been possible to determine whether the adsorption isotherm exhibits a saturation as does the S.P. isotherm. C2H2, CzHa and C2H6 Films.-The adsorption of these gases on bare nickel has not been studied in detail.The values given are only preliminary ones. Expt. Ta and Wb give 0.94 and I - I O V for the S.P. of a chemical C,H, film a t zoo C in good agreement with the value 1.046 V obtained from the displacement Ieaction, Ni-H + C,H, -+ Ni-C,H, + . . . . Expt. Va gives 0.835 V for t h e S.P. of a chemical C,H, film a t zoo C, in qualitative agreement with other less accurate values obtained either directly 01' from the reaction, Ni-H + C,H4 -+ Ni-C,H, + . . . . There seems to be little doubt that the S.P. of the C,H, film is in fact 0.2 V lnwer than that relative to C,H,. At - 183' C, C,H, and C,H, films exhibiting approximately the same S.P. as at 20' C. have also been observed. It is not clear whether they are van der Waals' or chemical films because it has not yet been possible to desorb them and restore the bare surface.With C,H,, a desorption from 0.943 t o only 0.766 V was observed during warming up (with appropriate precautions to pump out the C,H, quickly at a low temperature) to zo°C. With C,H,, desorption was apparently complete, but the surface would no longer adsorb hydrogen at 20' C. As regards C,H,, a series of adsorptions a t - 183O C gave the values -+ 0.62 ; + 0.80 and + 0.74 V. Heating in contact with C,H, to 250° C produces a variacion of - 0.3 V. A few observa- tions have been made on the reactions between adsorbed hydrogen and gaseous C,H, or C,H,, and also on the reverse reactions. They are insufficient t o justify amplification. Yet, in view of their interest as regards the possibilities of contact potentials in catalysis, it is perhaps as well to give a few indications. At room temperaLure and low pressures, C,H, and C,H, react rapidly (t < I min.) with an adsorbed H film.The measured S.P. jumps are equal to the difference in the S.P. given above for films on bare nickel, The reverse reactions are ex- tremely slow a t room temperature but rapid a t 250" C. Again, a jump of about I V is observed. Alternate displacement reactions can be repeated several times and the kinetics tvith C,H, and C,H, compared. It appears that the rates are not very different. Van der Waals' Films.-The results are collected together in Table I. The S.P. recorded are those for complete films except in those cases discussed below.Films on bare Ni are stable in vucuo, S.P. variations of about 5 yo being sometimes observed by pumping. The S.P. are often considerable, com- parable in magnitude with those obtained for chemical films. One of the most significant is perhaps that of Ni-Xe where there is no possibility of chemical interaction. The effect observed must therefore be ascribed to polarization in the field existing near the surface ; and clearly, the same cause is operating in the other cases. Incidentally, the agreement between the S.P. values for Ni-Xe (+ 0.840 and + 0.866 in Expt. Va ; + 0.849 in Expt. Wa) shows the degree of reproducibility that can be reached when no complication arises from the film itself. The case of argon shows that the observed S.P.are really due t o adsorption and not to the presence of gas between the electrodes. At - 196" C, argon is very slightly adsorbed as was shown by a special volumetric test. As should be, it gives a negligible S.P. Pre-adsorbed hydrogen increases the surface potential of nitrogen (Table I) but decreases the fraction of surface covered. The latter point is shown clearly by the isotherms of Fig. 3. Since the nickel deposit had been kept at a temperature somewhat higher than 20' during deposition, no sintcring can have occurred between isotherms I and 3 and it must be concluded that (i) only 20 yo of the hydrogen-covered nickel surface are capable of adsorbing nitrogen ; (ii) the electric moment of the ad- sorbed N, molecules is about 10 times greater on a Ni-H than on a Ni substrate. Bare nickel at - 196' C adsorbs hydrogen very rapidly (t < I min.). Addition of C,H, at zoo C has no effect.The isotherms are numbered in the order in which they were taken.J. C. P. MIGNOLET Ali surface potentials on Ni and Ni-H being positive, it is interesting to examine tne adsorption on positive pre-adsorbed films t o see whether the second film would not be negative. Results for.Ni-C,H, and Ni-C,H, show very small S.P., indicating that either there is no adsorption or that the adsorbed particles TABLE I.-SURFACE POTENTIAL OF VAN DER WAALS' FILMS ON NICKEL BARE OR COVERED WITH PRE-ADSORBED CHEMICAL FILMS Base Surface Bare nickel J J J J ,, Ni-H f , Incomplete Ni-€1 :- (S.P. = - 0.072) (S.P. = - 0.28) (S.P.= - 0.30) N i-C , H4 J l Ni-C,H, Gas N2 Ar Xe C2H6 1 , H2 C2H4 N2 , J C2H4 H2 N, Xe - 196 - 196 -183 - 196 - 183 - 196 - 190 - 196 J J I , - 183 - 196 - 196 , J P mm. Hg 1 0 - 3 3 x I O - ~ 2 x I O - ~ I I , I 0 1 0 - 6 5 x I O - ~ 2 x 1 0 - 4 2 x I O - ~ 2 x I O - ~ 6 x I O - ~ , I , I s .P. (volt) +0'2I f O . 0 1 < +0.03 +0.77 +0-85 f o - o ~ +0.80 +o.og *O.OI +0.88* +0.36 +0*45 f o . 4 1 +0.07 & 0.01 * -0'02 izo.01 0 f0'02 -0.006 Expt. Oa T a ; Va Ua Va; W a Wa R a ; Sa Kb Oa 1 , He Hf Va Ne t Value obtained with the second cell (see Experimental) and therefore 15 yo too low. are weakly polarized. In Expt. Wa, enough xenon to produce a polymolecular layer was added to a nickel surface covered with a chemical C,H, film a t - 196" C, care being taken to warm the traps to -183' C.A contact potential variation of about - 0.06 V occurred. Such small effects might result from a compensa- tion between two effects arising : one from the nickel surface, the other from the pre-adsorbed film. It is more probable that they are a result of the short range of the forces involved. In this respect, the adsorption of Xe on Ni-N, affords favourable conditions for a test, since there is, so to speak, a + 0.65 V reserve for polarization if the range is sufficient.* Discussion From the results presented here, it is clear that contact potential can be used to study the catalytic hydrogenation of ethylene and acetylene on nickel and the related surface reactions. A discussion is undoubtedly premature now but, perhaps, we can say a few words in relation to the problem of active centres. S.P., being proportional to the concentration of adsorbed particles, will not detect changes in the adsorption on sparse array of active centres.Hence, for a reaction occurring only at active centres, there will be no parallelism between the behaviour of the films observed by contact potentials and that expected for the films through which the catalytic reaction proceeds. Of course, a close parallelism does not prove that the surface is homogeneous. In the case studied here, the films seem to behave as would be anticipated from the catalytic data. The adsorption of hydrogen on nickel has been studied at room tem- perature by v. Duhn who reports a positive (with the convention of signs adopted here) S.P.varying from 0.3 V at IO-, mm. Hg to 0-7 V at 10 mm, t A negative surface potential is not excluded. The experiment has not yet been carried out. v. Duhn, Ann. Physik, 1943, 43, 37.I I O ADSORPTION OF GASES ON NICKEL FILMS Hg. Since both v. Duhn and the writer find a negative oxygen film,* a systematic error in the signs is excluded. It is suggested that part of the discrepancy results from a difference in the condition of the surfaces before contact with H,. The nickel surface was not bare in v. Duhn's ex- periments because of inadequate technique. On the other hand, v. Duhn's 0,005 Pressure 0.01 (mm. Hg) FIG. 3.-Adsorption isotherms a t - 196' C. Curve I : N, on bare nickel. Curve z : H, on bare nickel. Curve 3 : N, on nickel covered by an incomplete H film (prepared by pumping a t 20' C and cooling).measurements were made in vacuo, after the upper electrode had been in contact for some time with the gas at the pressure given. Therefore, his curves are not ordinary isotherms and indeed, it is not clear how, in most cases, the pressure could have had an effect at all. S.P. of van der Waals' films of non-polar gases have been reported previously. There must have been van der Waals' films in those examined by v. Duhn, though the distinction with chemisorption was not made. In another direction, Frost and Hurka * obtained S.P. of about 0.1 V for a few films, e.g. CCl, and C,H, on collodion. In several of the cases studied here, the effects are much greater. This raises the question of the interpretation of S.P. So far, it has been customary to interpret them in terms of chemical polar bonds, either within the adsorbed molecules or between adsorbate and metal.It is clear that induced polarization must be taken into account too. For the chemical films of C,H, or C,H, on Ni, for instance, it is able to account for more than half the effect. The writer has held the view that the second H, film is a " film in the gaps ".9 Curve 3 in Fig. 2 brings strong evidence against this view, since the film is still present although the In series E, a S.P. of -1.6 V has been found for the oxygen film on nickel at - 19" C, in satisfactory agreement with Bosworth's determination (- 1.4 V : Trans. Faraday SOC., 1939, 35, 397). Frost and Hurka, J . Amer. Chem. Soc., 1940, 62, 3335. Roberts, Some Problems on A dsorption (Cambridge University Press, London, 1939).Rideal and Trapnell, Colloque sur l'adsorption et la cine'tique hLtLrogine (Lyon, Sept., 1949). Rideal, Chem. and Ind., 1943, 62, 335.J. C. P. MIGNOLET I 1 1 gaps have been filled.1° I t is not proposed to discuss possibilities as to the seat of adsorption before experiments are made at intermediate temperatures and on oriented films. Perhaps, for the present, the main interest of the curves of Fig. z lies in another direction. We have here a striking example of the ability of S.P. to distinguish between van der Waals’ adsorption and chemi- sorption by virtue of its having a sign. Examining the meagre data of Table I from a purely empirical point of view, one will notice that the van der Waals’ films on bare nickel examined (Xe, N,, C,H,) exhibit positive S.P.The positive sign may well be a feature common to all van der Waals’ films of non-polar gases on bare metals. If this turns out to be true, a negative S.P. on a bare metal will point to chemisorption ; and perhaps, when more is known about induced polarization, it will be possible to develop the criterion €or positive films on a bare metal by taking the value of the S.P. into account. It is hoped in another paper to present additional data on induced polarization and discuss the matter in detail. I wish to express my deep gratitude to Prof. L. D’Or €or help, encour- agement and advice. I also wish to thank Messrs. J. Haesen and J. Sarlet for skilful technical help and Dr. J. Serpe and Dr. J.Pirenne for useful discussions. Laboratoire de Chimie ge’ne’rale, Lie’ge, Belgique. ADDENDuM-On the Origin and Nature of ‘‘ Induced Polarization ”.- How is it that van der Waals’ films of non-polar particles exhibit surface potentials ? The effect might be one of compression of the electric double layer at the metal surface by the adsorbed particles, without the latter being polarized, but this is very improbable. Therefore, it has to be assumed that the adsorbed particles are polarized and our problem is to understand the origin and nature of the electric field responsible for the effect. Let us first consider the films on bare metals. A model based on image iorces has been frequently used in the past to account for properties of films. Bosworth l1 has used it to calculate the heat of adsorption and electric moment of a H atom on an ideal metal.Besides the negative moment, he finds a positive moment at greater distance from the surface. Although the case treated by Bosworth is one of chemisorption, it might be thought that the explanation lies in that direction. However, this can hardly be so. With xenon, for instance, the image has nearly spherical symmetry. Therefore, the electric field produced in the xenon atom is negligible, and insufficient to polarize the latter appreciably. Clearly, the electrical field responsible for the effect must be the one existing within the electric double layer at the metal surface.* A number of theoretical studies l2 have been concerned with the electric double layer but, apparently, it has not been realized that the field near the surface may be of such magnitude as to induce important electric moments in adsorbed particles.10 Couper and Eley (unpublished) ; cited by Eley, Quart. Rev., 1949, 3, 216. l1 Bosworth, Trans. Roy. Soc. N.S.W., 1941, 74, 538. l2 Mrowka and Recknagel, Physik. Z., 1937, 38, 758. Bardeen, Physic. Rev., Huang and Wyllie, Proc. Physic. SOC., 1949, 62, 180. *Added in proof: Frost (Trans. Electrochem. Soc., 1942, 82, 259) has foreseen 1936, 49, 653. and briefly discussed this cause of induced polarization.112 ADSORPTION O F GASES ON NICKEL FILMS Consider a simplified model neglecting the atomic structure of the There is an electric double layer at the surface, due to electrons The electric field H surface. of the metal projecting a short distance outside. in empty space, within the double layer, is directed as shown in Fig.4. FIG. 4. Let now a film of non-polar particles be adsorbed. The particles are polarized with the positive charge outwards. Accordingly, the surface potential of the film is positive. Thus, the model accounts for the positive sign found for the three films studied (Ni-Xe, Ni-N,, Ni-C,H,). Moreover, the positive sign appears to be a property of the type of film considered here. Can the model also account for the order of magnitude ? Since the characteristics of the double layer of nickel are not known, we examine that point from the experimental side. The electrostatic potential V decays rapidly with increasing distance x from the surface. Let Curves I and 2 represent V against x for double layers thicker and thinner respectively than the film.Knowing the surface potential v of the film, we propose to estimate V o (see Fig. 4 ) and compare it with the work function of nickel. Provision- ally, it is assumed that the field is constant. Then ( V , - V,) and v are defined by Let us first consider a double layer thicker than the film. V o - P,=Hd . - (1) v = 4 ~ n M = 4 ~ n u h , . * (2) where d, M and u are the diameter, induced electric moment and polar- izability of the particles, and n is their number per unit surface. h repre- sents the field within the film. Hence The difference between h and H results from two effects, i.e. a depolar- ization due to the film and a polarization of the electric double layer itself by the film.The first effect is easily taken into account using, after Topping, I3 a depolarizing field given by - gMnsis. l3 Topping, Proc. Boy. SOC. A , 1927, I 14, 67 ; see also Roberts, Some Problewas in Adsorption (Cambridge University Press, 1936), p. 103J. C. P. MIGNOLET 113 The second effect arises because ( V o - V,) and H refer to the electric double layer at the bare surface. Since it is not clear at present how it could be taken into account, we neglect it and, therefore, content ourselves in deter- mining ( Y o - V,) for the film-covered surface. With this new definition, h = H - gMn3/z, * (4) or H h = - I + * ( 5 ) since M = ah. . Formula (3) becomes The same result is arrived at if the electric double layer is thinner than the film (curve 2 ) .For, while in this case, I/, is used completely ( V , = o), only part of the polarizability is effective. With the inhomogeneous field existing in the electric double layer relations ( I ) and (2) may still be used to define mean values of H and h, but formulae (4) and (6) are not strictly valid. It is, however, probably safe to use formula (8) as a first approximation. Let us tentatively apply eqn. (8) to a complete xenon film on nickel for which u = 4 x I O - * ~ ~ m . ~ ; d = 3-5 x I O - ~ cm. n = I / d 2 = 8-17 x 10-14 atom./cm.2 ; v = 0.85 V. We get v, > 1-57 V = 1-33 v. . - (9) Thus we find a quite reasonable lower estimate of the strength of the electric double layer, lower-as it should be-than the work function of nickel. In spite of the approximations made, it is felt that this result is strong evidence for the scheme developed above.In the absence of an essenti- ally different explanation, it may be considered that the positive surface potentials found for van der Waals' films of non-polar particles on bare metallic surfaces are to be ascribed to a polarization of the adsorbed particles by the field existing in the double layer at the metal surface. Accordingly, result (9) may be taken as experimental evidence that the electric double layer is just a few It is clear, therefore, that the atomic structure of the surface cannot be neglected. At the moment, it is difficult to see how important structural effects may be. Finally, a few words about double films. The results reported show important positive surface potentials for second films on negative films and small surface potentials on positive first films. The explanation seems to be somewhat as follows. Due to van der Waals' forces, the particles of the second film tend to locate themselves into depressions in the first film where they are polarized with the opposite sign. Thus, the effects produced by the metal and by the film reinforce (destroy) each other for negative (positive) first films. In the case of H, and N, on Ni-H, it seems necessary t o assume a deep penetration of the second film into the substrate. Would this imply that the hydrogen atoms of the first film are on top of the nickel atoms and not in between ? This may well be so.? * Consideration of a model treating the film as a dielectric leads t o a result practically identical with formula (7) viz. : thick. I + 8/3non312 v, - vl== L O = dv . * (7') € - I 4 P 7 where E is the dielectric constant of the film. 7 In that case, the (110) faces may well play an important part because of the large available spaces between rows of nickel atoms, which can easily ac- commodate the molecules of the second film (N, for instance ; cp. the 20 yo of surface covered a t saturation, and Couper and Eley, this Discussion).114 THE PARA-HYDROGEN CONVERSION To sum up, van der Waals' films can be used to explore the electric field at the surface of metals and used to obtain in'ormation about the electrical double layer. Moreover, they may well yield information about the structure of primary pre-adsorbed films,

ISSN:0366-9033    

DOI:10.1039/DF9500800105

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

114 THE PARA-HYDROGEN CONVERSION THE MECHANISM AND TEMPERATURE HYDROGEN CONVERSION COEFFICIENT OF THE PARA- BY E. K. RIDEAL AND B. M. W. TRAPNELL Received 2nd February, 1950 The results of a volumetric study of the chemisorption of hydrogen by evaporated tungsten films are summarized. These indicate that the probable mechanism of the parahydrogen conversion by tungsten surfaces is the con- densation and re-evaporation of chemisorbed gas. The significance of the temperature coefficient of the conversion rate is discussed. Using a tungsten filament as adsorbent, which was cleaned by flashing, and following the uptake of hydrogen volumetrically, thermally and by measurement of the change in neon accommodation coefficient on ad- sorption, Roberts made important discoveries concerning the nature of chemisorption processes.Two of the results that he claimed have particularly influenced theories of the parahydrogen conversion and hydrogen-deuterium exchange. Firstly, that the chemisorbed layer was complete at pressures of 3 x I O - ~ mm. Hg at 0" C. Secondly, that evapor- ation of hydrogen from the chemisorbed layer took place at an immeas- urably slow rate until temperatures of over 400' C had been reached. On the other hand, the parahydrogen conversion proceeds readily through a chemical mechanism at tungsten surfaces at room temperatures and even at liquid air temperatures., Rideal therefore rejected the mechanism of reaction suggested by Bonhoeffer and F a r k a ~ , ~ zW + f i . H, + zWH + zW + 0 . H, . * (1) and postulated that reaction proceeds through interaction of a chemi- sorbed atom and a molecule held by van der Waals' forces either in a second layer or in a so-called gap site p . H , + W H - t H W + o .H , . (2) Later work by Eley5 showed that deuterium, chemisorbed by an evaporated tungsten film, could exchange with gas-phase hydrogen even at 77" K. If the chemisorbed deuterium was unable t o evaporate below 400' C, the exchange must have been described by eqn. ( 2 ) . Eley also disproved a suggestion by Farkas that the hydrogen was converted by the mechanism of eqn. ( I ) , but only on a very small number of lattice sites, too few to be detected by Roberts' technique. Roberts, Proc. Roy. Soc. A , 1935, 152, 445. Rideal, Proc. Camb. Phil. Sac., 1939, 35, 130. 2 Eley and Rideal, ibid., 1941, 178, 429.* Bonhoeffer and Farkas, 2. Physik. Chem. B , 1931, 12, 231. 6 Eley, Proc. Roy. Soc. A , 1941, 178, 452. 6 Farkas, Trans. Faraday SOC., 1939, 35, 943.E. K. RIDEAL AND B. M. W. TRAPNELL Roberts,' and Beeck 7 who has as a result of a study of the adsorption by evaporated films reached similar conclusions to those of Roberts, agree that the heat of chemisorption can fall to values of about 15,000 cal. as the layer becomes more densely packed. For such heats, a measure of reversibility for the adsorption would be expected at room temper- atures ; irreversibility up to 400' C, as Roberts claimed, would certainly not be expected. Moreover, reflection shows that Roberts' statement that the adsorbed layer only became complete at the finite pressure of 3 x I O - ~ mm.at room temperatures itself implies that the adsorption is to a degree reversible under these conditions. It was therefore decided to investigate the adsorption volumetrically using evaporated metal films to decide under what circumstances and to what extent the chemisorption is reversible, and to consider how the present views of the parahydrogen conversion and hydrogen deuterium exchange would have to be modified. The Chemisorption of Hydrogen by Evaporated Tungsten Films .- The experimental study covers the temperature range oo C to - 183" C and has been made at pressures up to I O - ~ mm. I t will form the sub- stance of a forthcoming publication, and has given the following results. (i) For smaller adsorbed amounts, the equilibrium gas pressure is immeasurably low, as found by Roberts.When, however, the adsorbed layer has passed a certain density of packing, the equilibrium pressure rises to sensible values, so that reversible adsorptions are observed, with isothermal heats falling from 14,000 cal. to some 3,000 cal. for the largest adsorbed amounts. Heats of van der Waals' adsorption of hydrogen do not seem to exceed 2,000 cal., so that these adsorptions must be of the chemical type. (ii) Confirmation of this conclusion was obtained in the following Areas of evaDorated metal films may be controlled and repeated. manner. area of a given f;lm type has been mide by and determination of ;he measurement of both the oxygen and carbon mon- oxide chemisorptions. The relative amounts of oxy- gen chemisorption, carbon monoxide chemisorption and saturation hydrogen adsorption on films of equal area have been found to be very nearly 1/2/1.This is taken as proof that the reversible hydrogen adsorptions de- scribed above essentially refer to a first-layer process, and are chemi- sorptions. (iii) From the results, rough values of fractional surface coverages under various conditions of temperature and pressure have been obtained. b 90 -782 0 O C -36*c FIG. I. These are plotted as isotherms in Fig. I. It is seen that the coverage varies extremely slowly with temperature and pressure. This is believed to be the reason for Roberts' failure to detect these phenomena with his accommodation coefficient technique. Beeck, Rev. Mod. Physics, 1945, 17, 61.I 16 THE PARA-HYDROGEN CONVERSION The Mechanism of the Parahydrogen Conversion at Tungsten Surfaces.-The heat of adsorption of the process 2W + H, --f zWH falls to far lower values than hitherto believed, so that the adsorption is partly reversible, even at - 183" C.This result caused us to reconsider the mechanism of the parahydrogen conversion. The kinetics of the conversion have been investigated in detail by Eley and Rideal.a The features are as follows : (i) The energy of activation, as measured by a temperature coefficient of reaction velocity, is low. Reported values vary between 1,000 and 3,800 cal., the average value of those obtained by Eley and Rideal being 1,950 cal. (ii) The conversion at constant volume is always first order. (iii) Representing the rate of reaction as k p , k is found to depend on pressure, and between - 78" C and - IOOO C and at pressures between I and 20 mm., kfi = const.pn, where n varies between 0-1 and 0-5. the number of molecules reacting per second per sq. cm. of catalyst (iv) At pressures around I mm., and between oo C and - 1 9 6 O C, = 2-6 x IoZOe-E/ET. These characteristics of the reaction may then be calculated from the adsorption data, assuming the mechanism of eqn. (I), and compared with the experimental values. Now the adsorbed layer is mobile in the presence of a finite equilibrium gas pressure if only because molecules are continu- ally evaporating and condensing, thereby enabling the pattern of the adsorbed phase to alter and achieve a state of minimum energy. In this case, the expression for the rate of condensation of molecular hydrogen as atoms on pairs of adjacent sites is given by the expression - ( 3 ) (1 - 0) I + E' R = mp/dznmkT .- where 0 is the fractional surface coverage, u the condensation coefficient and E is given by the equation where rl = e-VIRT, V being the energy of repulsion of atoms adsorbed on adjacent sites.Assuming the mechanism of eqn. ( I ) to be correct, the rate of conversion can then be calculated from the rate of condensation, and using Lennard- Jones and Devonshire's 9 value of 0.3 for the condensation coefficient, and a value 1200 cal. for V , it is found that (i) the temperature coefficient of the condensation rate gives an (ii) the order at constant volume is unity, and at varying pressures (iii) the calculated absolute rate of conversion agrees with the experi- 'These results make it probable that the conversion proceeds by the path of eqn. (I) activation energy of some 1800 cal.; less than unity ; mental. Peierls, Proc. Camb. Phil. SOC., 1936, 32, 471. Lennard-Jones and Devonshire, Proc. Roy. SOC. A , 1936, 156, 6.E. K. RIDEAL AND B. M. W. TRAPNELL Experiments performed with evaporated nickel films ha.ve shown that the adsorption of hydrogen is very similar t o that described for tungsten, the equilibrium gas pressure being appreciable when the surface coverage exceeds a certain value at room temperatures and below. Calorimetric work by Beebe and his collaborators l1 has yielded a similar result for the surfaces of iron and of chromic oxide, for hydrogen is chemi- sorbed on both with a heat change of some 5000 cal.With these cata- lysts it may be concluded that the parahydrogen conversion and hydrogen deuterium exchange may also proceed by the condensation and re- evaporation of chemisorbed hydrogen, and it is probable that the mechan- ism will prove to be general. The Temperature Coefficient of Velocity of the Conversion.-The activation energy of adsorption is at most a few hundred calories. The activation energy of desorption is therefore effectively equal t o the heat of adsorption. On the basis of eqn. (I) the true energy of activation of the conversion must hence be the heat of adsorption. This is very much greater than that derived from a measurement of the temperature co- efficient of the velocity of the conversion.For example, under conditions where the heat of adsorption is 11,000 cal., the activation energy derived from the temperature coefficient is only 16 yo of this figure. Now, the temperature coefficient of reaction is, from eqn. (3), drnl (1 - 4) * (1 + %)/(I - &) (1 + €1) where the suffixes I and 2 refer t o the two temperatures of measurement. The temperature coefficient therefore gives an apparent energy E which is determined by the change of 8 with T , and as indicated in Fig. I , this is very small. The reason is that in the particular region of surface coverage studied, the heat of adsorption is falling very rapidly as the surface fills. This may be seen in a general way by the following con- sideration. Imagine a surface 70 yo covered by an adsorbate under given conditions of temperature and pressure, and then let its temperature be lowered.If the heat of adsorption AH is independent of surface coverage, let the drop in temperature increase the surface coverage t o 80 yo. If, however, the heat falls very rapidly with increasing coverage, cooling the adsorbate, though causing condensation of gas, will cause less than in the above case as the condensation itself has the effect of weaken- ing the binding of the adsorbate. The coverage will therefore not in- crease to such a high value as 80 yo. This is what is happening in the above case, and is responsible for the great disparity between E and AH. E has no relation t o the true activation energy, being determined by the manner AH changes with increasing 8.It is certainly no measure of catalytic power. The following two points arise from these considerations. (i) The activation energy of a heterogeneous reaction is invariably obtained from a temperature coefficient of velocity at constant pressure. It has been shown the figure derived in this way may be quite unrelated t o the energetics of the surface processes involved in reaction. A true energy of activation is only definitely obtained if the temperature co- efficient is measured a t constant surface coverage of reactants, and this is not, of course, experimentally attainable. (ii) Smith and Taylor la have measured the quantity E a t different temperature ranges of working on zinc oxide surfaces, and have found considerable variations. Between 143 and 178" K, E has the value 600 cal. In the range 195 to 373OK, E is roughly constant, but with the value 7000 f 2000 cal. Above 373OK it is larger, and except for the interval 405 to 430° K, where E = 0, has the rough value 12,000 cal. lo Beebe and Dowden, J . Amer. Chem. SOC., 1938, 60, 2912. l1 Beebe and Stevens, ibid., 1940, 62, 2134. l2 Smith and Taylor, ibid., 1938, 60, 362.118 HYDROGENATION CATALYSTS These results were cited as proof of the marked heterogeneity of the surface for chemisorption. The above analysis shows that the results may be due to varying rates of fall in 8 with increasing temperature, and it must first be decided whether these differences, and the differing rates of change of AH with fl which they indicate, require the concept of a non-uniform surface. The Royal Institution, London, W.I. 21 Albernarle Street,

ISSN:0366-9033    

DOI:10.1039/DF9500800114

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

118 HYDROGENATION CATALYSTS HYDROGENATION CATALYSTS BY 0. BEECK Received 7th February, 1950 The rate of the hydrogenation of ethylene over evaporated porous metal films of the transition elements is strongly depending on the heat of adsorption of ethylene, which (except for much higher absolute values) parallels that of hydrogen. When plotted against the d-character of the metallic single bond (Pauling), both the heats of adsorption of ethylene (or hydrogen) and the rates of hydrogenation fall on smooth curves, such that the greater the d-character, the lower are the heats of adsorption and the higher the rates. Since the d-character is closely related to the atomic distances in the crystal lattice, especially in Group VIII elements, the dependence of catalytic activity on crystal parameter is largely, but not solely, a consequence of the nature of the metallic bond.Depending on the heat of adsorption for ethylene, initial irreversible poisoning by acetylenic adsorption complexes, formed by self-hydrogenation of ethylene, renders the surface more or less inactive for hydrogenation. On the remaining part of the surface the overall rate of reaction is determined by the reaction of adsorbed ethylene (non-dissociative) with adsorbed hydrogen with an apparent activation energy of 10.7 kcal. for metals of Group VIII. Strong evidence is presented that a third type of a fast reaction with low activation energy is also involved. This reaction is thought not to necessitate empty crystallographic sites for the separate adsorption of ethylene, but of being able to form the activated state on a surface already covered with hydrogen.Related phenomena, such as the cracking of ethane in the presence of hydrogen, the hydrogenation of acetylene, and the reaction of preadsorbed ethylene and acetylene with hydrogen, are also discussed. A systematic study of the catalytic hydrogenation of ethylene and other hydrocarbons on evaporated porous metal films has been carried out by the author and his collaborators during the past years. The experimental observations, which are briefly reviewed and discussed here, illustrate the surprising complexity of this apparently simple reaction, and demonstrate the difficulty of arriving at any simple theory of catalytic activity in terms of any one kinetic scheme or any one property of the catalyst surface.It will be shown how chemical properties (electronic configuration) and the resultant physical properties (crystal parameter) of the catalyst surface influence the rate of reaction. with oriented and unoriented films suggested t h a t t h e (110) plane is much more active as a catalyst €or this reaction than any other simple plane of metallic Beeck, Smith and Wheeler, Proc. Boy. SOC. A , 1940, 177, 62. The following mechanism for the hydrogenation is suggested. Earlier results published by Beeck, Wheeler and Smith0. BEECK nickel. Direct calorimetric measurements of the heats of adsorption have also been made.2 Recently, new and direct experimental proof was obtained by Beeck and Ritchie that gas evaporated nickel films, showing (110) orientation parallel to the backing, actually expose (110) planes to the gas phase. By determining the surface through low-temperature adsorption using the B.E.T. method and dividing the surface area obtained by the number of hydrogen atoms adsorbed on the same surface, the crystallographic site of the oriented film was found to be 8.65 Hi2 in surprisingly good agree- ment with the X-ray value of 8-70 Hi2.Correspondingly the area of the site of unoriented films was found to be 6-17 Hiz which again agrees well with the X-ray value for the (100) plane of 6-15 A2. The latter agreement may be fortuitous, however, since the average size of the three major planes is 5-84 A2, very’ close to the value for the (100) plane. If, therefore, a number of planes along the cube edges were (110) planes and another number at the corners were (111) planes, an overall value close to that of the (100) plane would be expected.Beeck, Wheeler and Smith1 have found that (110) oriented nickel films have at least five times the specific activity of oriented films for the hydrogenation of ethylene. In continuation of this work it was of great interest to investigate how far crystal geometry considerations could be used to explain the greatly different activities of other metals for this reaction. It was expected that metals with different chemical properties would show differences in activity for reasons of their specific electronic configurations which in turn must be related to the crystal geometry in the sense that the former is the cause of the latter; This statement needs the further qualification that the catalyst must be able to chemisorb hydrogen and that in th.e absence of this prerequisite crystal geometry considerations become meaningless.(Pure copper, for instance, will not chemisorb hydrogen although its crystal parameter is very close to that of nickel; it also will not produce porous films except when evaporated at very low temperature.) Aside from these considerations, however, the distance at which two hydrogen atoms are chemisorbed should in itself for purely geometrical reasons affect the rate at which hydrogen is adsorbed and the rate at which the ethylene molecule picks up two hydrogen atoms from the surface especially if this reaction takes place without the adsorption of ethylene itself as will become apparent later.Beeck and Wheeler investigated the rate of hydrogenation of ethylene over evaporated films of iron, platinum, palladium, rhodium, chromium, tungsten and tantalum, in addition to nickel, The results of this in- vestigation have been published already in a preliminary report by Beeck in which Fig. 5 shows the logarithm (used merely to accommodate the data) of the specific velocity constant against the crystal parameter of the various metals investigated. A smooth curve was obtained showing a maximum for rhodium at 3-75 %i, this metal being several hundred times more active than nickel and 104 times more active than tungsten, based on the same hydrogen adsorption. In order to investigate the influence of the electronic structure of the different metals, Beeck, Cole and Wheeler measured the heats of adsorp- tion of hydrogen and ethylene (and other gases) on evaporated metal films.Of immediate interest is a comparison of oriented and unoriented nickel films. The results of this investigation have been briefly reported in Fig. 6 and 7,4 and Fig. 1 4 , ~ and are also shown in Fig. I of this paper without the experimental points. The heats of adsorption at 23OC for both Beeck, Cole and Wheeler, this Discussion. Beeck, Rev. Mod. Physics, 1945, 17, 61. Beeck, Advances in Catalysis, Vol. I1 (Academic Press, New York, 1950). 3 Beeck and Kitchie, this Discussion.120 HYDROGENATION CATALYSTS hydrogen and ethylene are the same for the two film types (with perhaps the heat of adsorption of hydrogen on oriented films 2 to 3 yo greater than on unoriented films).The heat of adsorption of hydrogen is 31 kcal. for the sparsely covered surface and decreases to 15 kcal. for the com- pletely covered surface. The conclusion was reached that this decrease is due to interaction of adsorbed atoms via the crystal lattice and that hydrogen atoms are mobile at 23OC in the sense that they can easily migrate from site to site, except perhaps for tungsten where mobility seems to be somewhat impaired. At - 183O C hydrogen is still mobile on nickel and is adsorbed with the same heat of adsorption as at room temperature, while on iron at - 183" C it is immobile, with a constant incremental heat of adsorption (average value of the heat of adsorption at 23' C) which drops to very low values when the surface is completely covered.For further information the reader is referred to ref. 5 where the adsorption of hydrogen on metal films is discussed in great detail. -- 50 - 20 I Oi 2 0; 4 Oi 6 0; a 1.q Fraclion o f surface covered FIG. he heats of adsorption of hydrogen. * The heat of adsorption of ethylene at 23OC decreases rapidly from 58 kcal. to lower values with increasing adsorption. When ethylene is adsorbed at 23" C, only about 5 x 1o18 molecules per IOO mg. of unoriented film (to be multiplied by 2 for oriented films) will be adsorbed practically instantaneously and irreversibly. On addition of more ethylene, ethane will appear in the gas phase and when 12-5 x I O ~ * molecules are adsorbed the same number of ethane molecules will be in the gas phase, leaving an adsorbate of acetylenic nature on the surface.This process is still very fast, in agreement with the immeasurably fast rate when adsorbed hydrogen reacts with ethylene from the gas phase. Apparently ethylene adsorption leads to dissociation of the ethylene molecule into two hydrogen atoms and an acetylenic residue, which, however, as will be mentioned later, does not behave like acetylene itself. Upon adding more ethylene the process of self-hydrogenation will proceed more slowly, and the com- position of the adsorbate for very large admissions (80 x 1o18 molecules per IOO mg.) becomes CnHo.4n. The molecules in the adsorbate also polymerize, as will be discussed later. Fig. I shows the heats of adsorption of hydrogen on Rh, Ni, Fe and W as a function of fraction of surface covered where the adsorption at * Subsequent re-evaluation has shown that the initial value for Ta is likely t o be at least as high as th3t for W- see also Fig.3.0. BEECK I 2 1 0.1 mm. pressure is taken to represent total coverage. Tantalum showed strong exothermic absorption into the structure, with 860 x 1olS mole- cules of hydrogen adsorbed and absorbed at 1.3 mm. The original curve for tantalum shows a clear break where adsorption is complete and this point was used for complete coverage. Fig. 2 shows the heats of adsorp- tion cf ethylene as a functiou of number of molecules adsorbed for IOO mg. of film. An especially careful study was made of the rate of reaction and of reaction products when hydrogen from the gas phase reacts with ethylene that is pre-adsorbed on oriented and unoriented nickel films.Reaction All films were high vacuum evaporated. rates a i d products are identical for bAh types of film. At 2 3 O C, 20 yo of the adsorbed residue can be reacted off in I hr. The product is about go yo saturated polymers (C, to C, and higher) and 10 Yo ethane. Similar experiments with rhodium at z3O C yield 60 Yo of the adsorbate in I min. (and nearly IOO yo with long contact times), the product being primarily ethane with only a few per cent. polymer. When ethylene is pre-adsorbed on nickel, the initial rate of hydro- genation at oo C is decreased by 60 yo ; when pre-adsorbed on tan- talum or tungsten, the rate is de- creased by factors of ten and five, respectively ; however, a decrease of only a few per cent.is noted when ethylene is pre-adsorbed on rhodium or platinum. Pre-adsorbed acetylene cannot be hydrogenated off nickel at 23" C but 40 Yo of pre- adsorbed acetylene can be hydro- genated off rhodium in I hr. It has been shown in ref. 4 (p. 69, FIG. 2.-Heats of adsorption of ethyl- ene. Fig. 9) that the apparent activation energy €or the hydrogenation of ethylene over nickel films is 10.7 kcal. and identical for (110) oriented and non-oriented films, although the activity of the former is a t least 5 times that of the latter. In the figure referred to, the rate values for the two film types were adjusted to be identical at oo C for easy comparison. It is of particular interest that the activation energy for the same reaction over rhodium was found t o be 10.7 kcal.also, although the rate over this metal is several hundred times faster than over nickel. In contrast to this, the activation energy over tungsten was found to be only 2.4 kcal. although the rate over tungsten is 20 times lower than over non-oriented nickel. However, the initial rate over tungsten when an equal molar gas mixture is admitted is 10 times faster than the final rate which is strictly first order and which was used for comparison with other metals. The initial rate (about 20-30 sec. after admitting the gas mixture) was therefore 0-1 times that of non- oriented nickel film9. Another series of experiments (Beeck, Ritchie and Wheeler) was concerned with the cracking of ethane on evaporated metal films a t about 20o-25o0C.If pure ethane is admitted to a nickel film, the rate of this reaction decreases to zero within a short time with only a very small amount of the ethane having reacted. With equal molar mixtures of ethane and hydrogen the reaction C,H, + H, -+ 2CH, proceeds on nickel with zero order and an activation energy of 48-4 kcal. The rate is 6 times faster Tantalum is simi1a.r in its behaviour.I 2 2 HYDROGENATION CATALYSTS on (110) oriented than on unoriented nickel films. The rate over rhodium is 60,000 times faster than over unoriented nickel films. If residues are formed on both nickel and rhodium by admitting ethane to the films at 2 2 5 O C , these residues can be removed by hydrogen from the gas phase with methane formation only, the rate being much faster in th? case of rhodium.Residues thus formed cannot be removed by hydrogen at 23' C, except for a very small amount of methane formation in case of rhodium. If residues formed on both metals at 2 3 O C are heated to 2 2 5 O C , hydrogen and a little methane evaporate from the nickel film and methane with a little hydrogen evaporate from the rhodium film. When the remaining xesidues are contacted with hydrogen at 225' C, methane only is formed. Some experiments were also carried out (Beeck and Ritchie) to investi- gate the influence of crystal geometry on benzene hydrogenation and cyclohexane dehydrogenation. Both oriented and unoriented nickel films were equally active for the benzene hydrogenation at 58" C. The rates were very low (independent of the benzene pressuxe and Fropor- tional to the o-44th power of hydrogen pressure) and hydrogenation pro- ceeds most likely through the adsorbed state of benzene.The apparent activation energy was 8.7 kcal. Nickel was found to be 2.3 times more active than iron. Cyclohexane dehydrogenation proceeds fast at 325O C over unoriented platinum films, partially (110) oriented films being 10 times less active. An activation of 9-3 kcal. was found for this reaction, which in its first half is first order with respect to cyclcbexane but is increasingly poisoned by benzene in the second half. Beeck and Wheeler have also carried out experiments on acetylene hydrogenation with the result that iron and tungsten were found to be inactive at 23' C; nickel was mildly active ( 2 x 102 to 2 x 103 less active than for ethylene hydrogenation), rhcdium and platinum were 10 times more active than nickel, and palladium 20 times more active.Maximum activity is found for palladium and not for rhodium as in the case of ethylene. The activation energy for acetylene hydrogenation on all four metals was 6 to 7 kcal. A thorough kinetic study was made of the acetylene hydrogenation also in the presence of ethylene where selective hydrogenation of acetylene takes place. Discussion.-It has been suggested already (Beeck 4, that the overall rate of hydrogenation is proportional to the fraction of surface not covered by adsorbed ethylene and to the ethylene pressure itself. This not only accounts for the observed kinetics : but offers a satisfactory explanation for all other experimental facts.According to this mechanism the fast process is the reaction of adsorbed hydrogen with ethylene from the gas phase without adsorption of the ethylene itself. The high rate of this process is substantiated by the experimental fact that the reaction of pre-adsorbed hydrogen with ethylene a t very low pressure and at - 79' C is practically instantaneous. At this temperature ethylene does not self-hydrogenate. The slow and rate-determining process is the removal of adsorbed ethylene from the surface by hydrogenation with adsorbed hydrogen. The rate of this reaction will, under steady state conditions, determine the surface avail- able for the fast reaction. The initial step of poisoning the surface with ethylene proceeds most likely through the adsorption of ethylene on two crystallographic sites but does not lead to formation of acetylenic complexes except when four adjacent sites are empty to accommodate the two hydrogen atoms.The heat of adsorption of ethylene on two sites is not directly known but must be relatively low, perhaps of the order cf 10 to 15 kcal. The heat of adsorption of hydrogen on a well-covered0. BEECK I 33 surface may be taken as 15 to 20 kcal. Reaction between the two re- actants with an activation energy of 10.7 kcal. may reasonably be ex- pected. Once ethylene adsorption has led to the formation of acetylenic complexes and to polymerization, the surface is irreversibly poiscned since the rate of removal of these materials by hydrogen was found to be very low.This explains why surfaces initially contacted with ethylene (and successive runs over the same catalyst) have lower activities. In the case of tungsten with its high heat of adsorption for ethylene the poisoning process is actually observable and proceeds apparently to a point where the formation of acetylenic complexes becomes impossible due to the fact that four adjacent sites are no longer available. Under these conditions the surface available becomes constant and the rate of removal of adsorbed ethylene is no longer rate determining. The rate is now primarily’ determined by the fast reaction of adsorbed hydrogen with ethylene fiom the gas phase. The low activation energy of the fast reaction should therefore prevail. An intermediate stage must, of course, be expected.This mechanism not only explains satisfactorily the low activation energy of the slow reaction on tungsten but also represents an indirect proof for existence of the fast hydrogenation reaction. In order to explain the (at least) five-fold activity (based on the same hydrogen adsorption) of (I 10) oriented nickel films over unoriented films, both having the same activation energy, it must either be concluded that the rate-determining surface reaction between adsorbed hydrogzn and adsorbed ethylene has a higher frequency factor for geometrical reasons or that the fraction of surface initially poisoned by acetylenic complexes and polymers is larger for unoriented films due to the higher metal atom density in the surface of these films, which may facilitate the formation of these complexes and of polymers. It is possible, of course, that both factors are involved.However, the latter factor, i.e, the formation of acetylenic complexes and polymer formation, seems to outweigh the former when different metals are compared since i t was shown that the heats of adsorption of ethylene differ largely, rhodium having the lowest heat, the least polymer formation, and the fastest rate for removal of adsorbed residues by hydrogen. In fact, if the specific activities of the different metals studied axe plotted against the heats of adsorption of either hydrogen or ethylene (Figs, I and z ) , it is seen that the lower the heats of adsorption, the higher are the activities. The tenfold activity for the dehydrogenation of cyclohexane by un- oriented platinum films in comparison with partially (I 10) oriented films could be explained on geometrical grounds following the hypothesis of Balandi~~,~ according to which the simultaneous removal of 6 hydrogen atoms from the cyclohexane molecule is facilitated by the octahedral faces (111 planes) of face-centred cubic lattices.It appears likely that the influenc,e of surface geometiy should be particularly large for this reaction and the observed kinetics favour the dehydrogenation reaction proper to be rate determining. The six-fold activity of (110) oriented nickel films for the cracking of ethane in the presence of hydrogen at 225O C in comparison to unoriented films is of particular interest since in this zero-order reaction the late must be determined by the famation of methane on the surface from radicals produced by ethane.The activation energy for hydrogenation of these radicals, which is equal to that of the over-all reaction, was found to be about 48 kcal., almost five times greater than the activation energy for the removal of adsorbed ethylene. This shows clearly that ethylene cannot have been adsorbed as a tru? radical but has formed partial bonds only. This conclusion is in agreement with the fact that acetylenic complexes formed by the adsorption of ethylene behave chemically differently from adsorbed acetylene. It does not seem likely that the rate of methane formation from a singly -bonded radical should be increased materially by the wider spacing in the (110) plane especially since theHYDROGENATION CATALYSTS (110) plane is the plane of highest surface energy (although this is only borne out by a marginal increase of the heat of adsorption of hydrogen, almost within the limits of experimental error).Conversely it will be morz difficult to remove CH,, CH and C radicals from the denser planes since multiple bonds of carbon with the metal atoms in the surface are more likely to be formed, which would lead to a preferential poisoning of unoriented films. It is of interest to note that palladium is more active for the hydro- genation of acetylene than rhodium, although the reverse is true for ethylene. In this case the gecmetrical factor seems to outweigh the over-all heat of adsorption picture, probably because the bond-forming character of the two metals is very nearly the same.I t has been pointed out repeatedly by the author that geometiic con- siderations are meaningless in catalysis unless the prerequisite of adsorp- tion is fulfilled. However, if the surface is capable of making chemical bonds with the adsorbate, the distance of the atoms in the smface must influence the activation energy in cases such as the adsorptive dissociation of hydrogen,sg 7? the hydrogenation of the double bond, polymerization on the surface, etc. Both hydrogen adsorption and hydrogenation of gas phase ethylene by adsorbed hydrogen have extIemely low activation energies and are not rate determining. Small differences in heat of ad- sorption of hydrogen cannot therefoie influence the rates.Rate deter- mining in the broadest sense is the poisoning of the surface by adsorbed ethylene and its degradation products. This poisoning depends in general on the ability of the metal surface to make strong bonds with carbon. The simplest and most direct way to m2asure the energy of these bonds is by measuring the heats of adsorption of the ethylene or acetylene. Fig. z shows how the heat of adsorption increases from rhodium to tantalum, this incIease corresponding to a decrease in catalytic activity by a factor 104. The heats of adsorption of hydIogen and ethylene on chromium (not shown in the Figures) are almost identical with those for tungsten and correspondingly the catalytic activities are nearly the same. This is of interest because chromium, which has a lattice spacing almost identical to one of the spacings in iron, did not fall on the smooth curve of aciivity against lattice distance (ref.4 Fig. 5) showing clearly that the nature of the chemical bond can outweigh completely any geo- metrical effect. The apparent correlation of catalytic activity with the interatomic spacing of different metals may well a r k from two causes, of which the electronic configuration is the more important one and is reflected by the crystal parameters of the metals. The geometrical factors can outweigh the purely chemical factors, however, when different planes of the same metal are considered, in which case the resulting differ- ences in chemical bonding of the adsorbate on the surface have their cause in crystal geometry. The next step in any attempt ta further elucidate the phenomenon of heterogeneous catalysis must involve the correlation of heats of ad- sorption (strength of surface bcnds) with the nature of the chemical bond in metals, intermetallic compounds, oxides, etc.This was pointed out appropriately in a recent note where work by Dowden and Reynolds, Eley, Maxted, and Anderson is briefly reported. In the absence of a general theoretical picture of surface states, Beeck et al. (this work was started in 1940) have preferred to measure heats of adsorption rather than to derive them from theoretical considerations, although the time is probably ripe now to do both, as the encouraging results reported would indicate, both in the light of the older band theory of metals and in the newer resonating-valence-bond theory (Pauling) .According to Belandin, 2. physik. Chem. B, 1929, 2, 289 ; 1929, 3, 167. Sherman and Eyring, J . Amer. Chem. Soc., 1932, 54, 2661. 13 Okamoto, Horiuti and Hirota, Sci. Pap. Inst. Phys. Chern. Res., Tokyo, 1936, 29, 223. Nature, 1949, 164, 5c.0. BEECK 125 Pauling,'O the number of empty atomic orbitals decreases as the d-band is filled. Assuming that the surface states are, qualitatively at least, related to the electron configuration of the bulk of the metal, one may expect that metals with a large d-character of their metallic bond will have less orbitals available for bonding with the adsorbate than those with a small d-character. This is borne out by the measured heats of adsorption of hydrogen and especially of ethylene on the series of metals investigated where the heat of adsorption decreases with the increasing d-character of the bond.This was correctly anticipated by Dowden and Reynolds and was independently and indirectly suggested by Boudarr,ll who relates the d-character of the metallic bond directly t o the activities ieported by Bzeck et u Z . ~ without having had knowledge of the cor- relation between activity and heats of adsorption of ethylene fiist reported in this paper. The discrepancy in the relation of d-character and activity which Boudart points out for tungsten does not apply, however, to the heats of adsorption measurements which were shown in this paper to be almost identical for tungsten and chromium. The short ~ e p o r t , ~ as well as a more recent letter by Coupr and Eley,l2 takes the view of minimizing the importance of crystal parameter con- siderations.While this is possibly true for the ortho-para hydrogen ccn- version (if adsorption of the hydrogen molecule by a single surface atom of palladium is sufficient for this conversion), th? crystal parameter can certainly not be neglected when dissociation (or partial dissociation) of a hydrogen molecule through a two-point adsorption is an apparently necessary prerequisite for hydrogenation at low temperature. already in 1940 that pure copper films do not adsorb hydrogen at - 183°C (at which temperature they could be pre- pared with a large surface) and do not show any activity at room tem- perature or higher unless they were prepared by evaporation in hydrogen at - 183" C, and then only to the extent of I O - ~ of the activity of nickel films for the same weight.This was taken as proof that pure copper does not chemisorb hydrogen. It is quite plausible therefore that if copper and nickel atoms were to occupy alternate positions in the surface lattice, the dissociative adsorption of hydrogen could be impaired. If in addition the valency electrons of copper fill up the holes in the $-band of nickel, both processes may have t o be considered simultaneously, and this may well be the reason why in the work of Dowden and Reynolds 9 30 t o 40 atomic per cent. of copper decrease the activity of the copper-nickel alloy to very low values while the magnetic measurements show that 60 atomic per cent. are necessary to fill u p the 3d-band of nickel.Such considerations are of course not necessary when activation through the formation of a singly-bonded activated complex is sufficient as for instance in the isomeiization and deuterium exchange of hydrocarbons catalyzed by aluminium chloride or bromide, or sulphuric acid l3 where the activated complex (carbonium ion type) is formed by a single bond. A good indication that even hydrogen atoms are using d-character bond energy for adsorption on metals is shown by the decrease of heats of adscrption as a function of fraction of surface covered, especially with metals which have metallic bonds of a low d-character. The hydrogen bonding energy and the bonding energy in general of these metal surfaces can be lowered substantially by pre-adsorption of nitrogen.Such a '' nitrided " surface is in principle equivalent to an intermetallic com- pound or alloy but is more easily prepared with evaporated metal films. As an example, Fig. 3 in the paper by Beeck et nZ.14 shows the heat It was pointed out lo PVOC. ROY. SOC. A , 1949, 196, 343. l1 J . Amer. Chem. SOC. (in press). l2 Nature, 1949, 164, 578. 13Beeck, Otvos, Stevenson and Wagner, J . Chem. Physics, 1949, 17, 418 and 419 ; 1948, 16, 225 and 745. lQ This Discussion.I 26 HYDROGENATION CATALYSTS of adsorption at room temperature of nitrogen on tantalum and the heat of adsorption of hydrogen on a nitrogen-covered surface. The initial heat of adsorption of hydrogen on the nitrogen-covered surface (27 kcal.) can be compared with that of pure tantalum in Fig.I of the same paper (45 kcal.). Shell Development Company, California. Emery ville, ADDENDuM.-Additional Figures (3 to 8) were presented at the Discussion. Fig. 4 and 5 show the heats of adsorption (initial heats, sparsely covered surface) for ethylene and hydrogen respectively plotted against the logarithm of the velocity constant of the hydrogenation reaction. Of particular interest is the large increase in rate from iron to rhodium for the relatively small decrease in heat of adsorption. This fact is borne out FIG. 3. more :strongly when the heats of adsorption of ethylene are plotted against yo d-character of the metallic single bond (Fig. 5 ) . Noteworthy is the fact that a much smoother curve is obtained in this correlaticn as well as in the plot of yo d-character against log k shown in Fig.7. The value for tungsten shown in the Figure refers to the usually body-centred cubic structure ( a = 3.15 A), whereas tungsten evaporated on to a cold surface will crystallize in a face-centred cubic form lS (a = 4-17 A). This consideration brings the points for tungsten in Fig. 6 and 7 back on the curve. Of further interest is the fact that the point for oriented nickel falls definitely outside the smooth curve, indicating that this parameter effect is a purely geometrical one, especially since the d-character of the metallic bonds of the surface atoms could not be larger in this plane, but only smaller, a fact that is borne out by the slightly higher heats of adsorp6on for hydrogen on the (110) plane. Since the d-character of the metallic bond is closely associated with the distance between nearest neighbours (especially in Group VIII elements), it is not surprising that a plot of log k of the hydrogenation reaction against crystal parameter (the long distances were chosen since they appear to be operative in this reaction for geometrical reasons) reflects both the measured heats of adsorption (total bond energies) as well as the yo d-character as shown in Fig. 8, where the yo d-character value for tungsten falls actually on the curve as explained above. A further interesting correlation was suggested by Dr. G. C. A. Schuit of the Koninklij ke/Shell-Laboratorium, Amsterdam, in this Discussion. lS Private communication by F. H. Horn, General Electric Co., Schenectady.0. BEECK I 2 7 13E 0 N I * 105 LL 0 v, 0 LL 0 I- 75 a a W I 4 5 4e I" 40 IL 0 cn n a 35 LL 0 I- W a 30 25 1 1 L -3.0 -1.0 0. -2.0 -4.0 LOG lo k (HYDROGENATION OF C2H4) Pd 80-F I 1 I I I -4.0 -3.0 - 2 .o -1.0 0. LOG,, k (HYDROGENATION OF C2H4) ow FIG. 4. FIG. 5. FIG. 6. % d CHARACTER OF METALLIC BOND (PAULING)I 28 THE EXISTENCE OF ACTIVE CENTRES F I G . 7. I I I I -4.0 -3.0 - 2 . 0 - 1.0 ( LOG 10 k (HYDROGENATION OF G 2 H 4 I 50 a~ 4 0 % ~ 0 30 5 I F I G . 8. W NI Pd Fe W I To I Rh iPt 'Cr I To 6.0 I I I I I I I I 3 .O 4 0 5.0

ISSN:0366-9033    

DOI:10.1039/DF9500800118

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

I 28 THE EXISTENCE OF ACTIVE CENTRES ON THE EXISTENCE OF ACTIVE CENTRES IN CHEMICAL ADSORPTION AND CONTACT CATALYSIS BY A. EUCKEN Received 25th January, 1950 The problem as to whether chemisorption and catalytic reaction occur over the whole available surface of a catalyst or only at active centres is discussed. The apparent heterogeneity arising from mutual interaction of adsorbed par- ticles and the possibility of different states of the adsorbate on the surface are considered in detail for the adsorption of hydrogen on highly activated nickel. It is concluded that the supposed effect of active defects should be rejected and that the essential part is played by the active intermediate states of the adsorbed reactant.A. EUCKEN A final answer has not yet been given to the question as to whether chemical adsorption or catalytic reaction occurs on the whole available surface of the solid adsorbent or catalyst, or only at certain places (active centres).It is, however, doubtful having regard to the complexity of this field, whether this question can ever be completely answered. It should therefore be noted that the conclusions obtained here depend on a limited number of facts, and that different conclusions might be obtained with different types of adsorption processes or catalytic reactions. I t is advisable to consider the existence of active centres in chemical adsorption separately from contact catalysis since, contrary to a view held earlier, it appears that the chemical adsorption of any particle is frequently, but by no means always, connected with an increase of its chemical reactivity (for this reason the term " activated adsorption ", which is frequently used, is not considered suitable).Chemical Adsorption.-Two cases of the inhomogeneity of solid surfaces in regard to chemical adsorption can a priori be distinguished. (i) The heat of adsorption and also the activation energy often varies considerably with coverage, and this has led to the conception of the existence of a continuous spectrum of adsorption centres of different activity, which might be identified with the lattice defects of the contact surface. (ii) On heating, the adsorption isobar frequently shows two maxima which, following H. S. Taylor, are considered as an indication of the existence of two (homogeneous) regions, whose adsorption capacity differs from that of the bulk.No details are given as to how this comes about. We can consider these two regions as crystal planes having different arrangements. Fundamentally, however, at least with the ionic type of adsorbent, it is possible to consider this difference as being determined by the individual lattice units, for example, the one as an adsorption on the cations, and the other on the anions. An alternative interpreta- tion of these two adsorption effects which will apply to monatomic ad- sorbents (pure metals) and in part also to salts (oxides, etc.), can be pro- posed. (i) THE SURFACE IS PERFECTLY HOMOGENEOUS IN REGARD TO ITS ADSORPTION PROPERTIES.-The apparent inhomogeneity (the existence of a spectrum of active centres) then arises from the influence of the ad- sorbed particles on each other, since a particle is less tightly held in the immediate environment of another adsorbed particle than on a completely clean surface.l If we assume this kind of mutual interaction2 it is clear that the heat of adsorption continually decreases with increasing coverage of the surface.(ii) DIFFERENT STATES OF THE ADSORBATE CAN EXIST ON THE HOMO- GENEOUS SURFACE, i.e. the steps in the adsorption isobars are not associ- ated with the regions of different activity of the surface. These two theories will be discussed in detail for the adsorption of hydrogen on highly active nickel. Two maxima occur with heating, the first being not strongly marked and only observed at relatively low pressures (approximately I O - ~ mm.Hg). I t can be shown thermodynamically for the adsorption equilibrium above 273' K that the hydrogen is essentially atomically bound to the surface. 'For example in the chemical adsorption of hydrogen atoms on a metal (e.g. Ni) the interaction between adsorbed hydrogen atoms occurs in the following way. As the metal surface possesses a certain electron affinity, the hydrogen atoms become positively polarized, and consequently there is coulombic repulsion between them. Furthermore as more hydrogen atoms become available as electron donors, the electron affinity of the metal surface becomes satisfied. At the same time the binding energy of the individual hydrogen atoms decreases. a Recently this point of view has been expressed by Beeck (Rev.Mod. Physics, 1945, 17, 61) ; cf. also Herington and Rideal (Trans. Faraday Soc., 1g44,40, 505). The shape of a typical adsorption isobar is shown in Fig. I. E1 30 THE EXISTENCE O F ACTIVE CENTRES Thus the first maximum of the adsorption isobar can be ascribed to an adsorption of molecules, similar in type to an atomic adsorption of a chem- ical nature, since physical (van der Waals') adsorption is only observed at much lower temperatures (in the region of liquid hydrogen). The heat of adsorption is estimated to be about 6 kcal.,/mole, i.e, it is relatively small, so that on heating, desorption occurs at about zooo K, and at 273' K the fraction of the surface covered by molecules falls to about I yo. However, one would have to assume, not only at this temperature but also at higher temperatures, that the hydrogen is primarily adsorbed as molecules.The adsorbed molecules subsequently dissociate more or less rapidly into atoms, which are initially closely packed but later diffuse FIG. I. Isobars of adsorption velocities (schematically). FIG. 2 . H, adsorption isobars at 10-2 mm. Hg on 28.8 g. Ni powder. Desorption graphs. Expt' T6)Adsorption at oo C on completely pure Ni surface. 0 Expt. 34 I A Expt. 35. 0 Expt. 33. Previous covering at 100' C after adsorption a t oo C . Adsorption at 100' C. Adsorption graphs. 0 Expt. 33 from 200 to 100' C. -0 Expt. 36 from 100' to oo C. apart (slowly at the lower temperatures and more rapidly at higher temperatures), since according to the above consideration a uniform distribution of the atoms over the whole surface corresponds to minimum potential energy.IA. EUCKEN The shape of the isobar on subsequent cooling is abnormal. Contrary to a number of other investigators, a smaller amount of material is ad- sorbed in the range 260' K to 360' K on subsequent cooling, than on heat- ing (Fig. I and 2 ) . The effect becomes clearer on comparing the two velocity isotherms at oo C (Fig. 3 ) . Here, the adsorption was in some cases on pure nickel and in others on a nickel preparation which had been previously covered with hydrogen at 100' and zooo C respectively, FIG. 3. 0 Expt. 28 O- Expt' 31 Adsorption without previous covering. 0 ' I EXPt. 37 A Expt. 29. Adsorption after previous covering at 100' C. 0 Expt. 32. Adsorption after previous covering at 200' C.Isotherms of adsorption velocities at oo C (23.0 g. Ni). This result can hardly be explained by the assumption of two separate regions (independent of each other), since in such circumstance there is no reason to expect previous covering to cause a decrease in the ad- sorption velocity. On the contrary, it can be regarded as a Poisoning of the total surface by the adsorbed atomic hydrogen. On the basis of the assumption previously made, we can visualise this effect as follows. Since the atoms adsorbed at rooo C are almost uniformly distributed over the surrace, then during cooling only those places with relatively small adsorption activity are available for adsorption. Moreover the dissociation and diffusion of the newly-formed atoms will be greatly hindered by atoms already present in stable positions, so that establish- ment of the final stable state, which is characterized by the most uniform distribution of hydrogen atoms over the total surface, is made much more difficult than on a surface initially completely unoccupied. One might expect to find similar effects during the adsorption of hydrogen on other metals ; in the experiments of Emmett and Harkness (1935) adsorption isobars with two maxima are obtained for the system Fe-H,.Unfortunately, in the isobars as yet determined for other metals and gases, the decreasing temperature branches are lacking in almost every case. The results obtained with the Ni-H, system will probably not be directly comparable with those obtained with salt-type catalysts, as con- ditions with these vary considerably from case to case.In Fig. 4 the full line shows an isobar recently obtained for the adsorption of hydrogen on WS,. The quantities of gas chemically adsorbed (near saturation) are approximately the same as for physical adsorption ; therefore the total surface is active in the chemical adsorption. Evidently two processes are superimposed which correspond to the dotted lines I and z in the graph. It is probable that the first graph corresponds to a chemical (atomic) adsorption and the second, to a solution of hydrogen. The1 32 THE EXISTENCE OF ACTIVE CENTRES heat of adsorption (approximately I 7 kcal. /mole) changes relatively little with coverage, at least within the region where measurements could be made.The influence of the adsorbed particles on each other is in this case smaller than for a metallic surface. A number of other salt-type solid surfaces show analogous behaviour to that of the metal and WS,, as here again the physically and chemically active surfaces practically coincide. One of the few exceptions is y-Al,O, for which only about 10 yo of the physically active surface is shown to be reactive in the adsorption FIG. 4. H, adsorption isobar of H, on WS, at TOO mm. Hg. of steam and alcohols. Here the lattice defects must be responsible for the preferred chemical adsorption. This can easily be understood as the lattice of y-Al,O, is known to be of the spinel type in which the Mg2+ ions are replaced by an equivalent number of A13+ ions ; to main- tain electroneutrality 33 yo of the original sites occupied by Mg2+ ions (a total of about 10 yo of the cation lattice sites of the spinel) must remain unoccupied. In the environment of such " holes " there are regions of excess negative charge, which can now interact with the hydrogen atoms of the hydroxyl groups of water and the alcohols, and thus cause strong adsorption.Apparently we are concerned here with a reinforced hydrogen- bridge binding. CONTACT CATALYsIs.-We shall confine ourselves to the discussion of a typical example of low temperature catalysis, since in high temper- ature catalysis (e.g. the catalytic combination of H atoms to molecules on metals) there is no doubt that the total surface is catalytically active. However, it may at first appear as though in low temperature catalysis, e .g . the hydrogenation of unsaturated hydrocarbons in the region of of oo C, the chemical change occurs only on a negligibly small fraction of the contact surface, i.e. as if the assumption of the existence of active centres cannot be avoided. As an example we may quote an experiment by Toyama (1937) and its interpretation. It deals with the hydrogenation of ethylene on the surface of a Ni powder at a pressure of about I mm. Hg. The reaction is zero order with respect to ethylene, and first order with respect to hydrogen. Taking the reaction velocity as -- dn -(--- no. of moles a value of the order of I x 10-l~ per cm., of the contact surface was ob- tained. Assuming that every H, molecule from the gas phase, colliding dt .p ~ ~ : ~ ~ ~ ) sec. mm. Hg. ern..)'A. EUCKEN I 3 3 with sufficient energy with the ethylene adsorbed on the surface leads t o a reaction, an activation energy of 5000 cal./mole and a value of 2.5 x 10-7 for dn/dt are obtained. The collision factor is approximately 4 x 10-6, which may be taken to mean that only a very small part of the total surface is catalytically active. This and similar results support the view that the reaction only occurs on a relatively few small centres of the surface ; but the problem can by no means be regarded as finally solved. Some experiments under- taken with a view to a solution of the problem showed first of all t h a t ethylene a t normal pressures has a strong poisoning effect on the reaction. Starting with a carefully cleaned surface and allowing the reactants to react simultaneously at low pressures (of about I O - ~ to 10-1 mm.Hg), values IOO times greater are obtained for the reaction velocity. In order t o remove the possibility of any poisoning effect remaining even at low pressures, an attempt was made to replace the ethylene by a hydrocarbon the poisoning effect of which would be much less. It was shown that cyclohexene served this purpose, and was also very suitable for use in other contact catalytic hydrogenation experiments. The real clue to the understanding of the processes occurring in the low-temperature hydrogenation on Ni was found in experiments with a, relatively large amount of solid, on which one of the reactants was adsorbed; the other was subsequently added and allowed to react from the gas phase, If cyclohexene was adsorbed, and then hydrogen allowed to react in this manner, only a negligibly small amount of C6Hl, was formed.If, however the hydrogen was adsorbed and the cyclohexene subsequently added, a considerable change was obtained. This surprisingly depended to a. great extent upon the time interval between adsorption of the hydrogen and addition of the C6H,,. It was also of importance whether the hydrogen was adsorbed a t oo C or 100' C. I TABLE HYDROGENATION EXPERIMENTS. SURFACE PREVIOUSLY COVERED WITH HYDROGEN I Ni powder (670 mg.) ; reaction temperature : oo C I 5 5 5 5 I5 5 5 5 2 Expt. No. I 2 3 4 5 6 7 8 9* Relative Covering of Surface with H, 0.200 0.172 0'143 0.404 0.400 0.286 0.286 0.306 0.310 Temp.of Previous Covering (" C) 0 0 0 0 0 I00 I00 0 -44 Time between Covering and Start of Reaction Duration of Reaction (min.) Yield of CIHlZ (%I 86 80 I 7 45 66 I9 35 68 I 2 * At - 46' C for reaction temperature. The results are summarized in Table I. It can be seen that the atomically adsorbed hydrogen, distributed over the whole surface, is apparently not capable of reaction. The fact that the reactivity of the hydrogen adsorbed a t oo C for a short duration of adsorption is rather large but decreases rapidly with time, may mean that the adsorbed molecules cause the reaction, prior to their dissociation and subsequent diffusion as atoms. A more detailed quantitative calculation of the reaction kinetics indicates that the adsorbed molecules as such most 9 Pease (1923) was the first t o point this out.I 3 4 THE EXISTENCE O F ACTIVE CENTRES probably do not determine the kinetics of the reaction.Finally we may suppose that a characteristic maximum of the reactivity is reached im- mediately after their dissociation i.e. in a state where two hydrogen atoms are still close together or form a H-atom pair. On the basis of this picture it is possible to calculate the course of the catalytic hydrogenation of C6HIo from the gas phase on Ni, if we assume that the change of coverage per unit time O,, is proportional to the number of H atom pairs and to the number per sec. colliding with C6H10 molecules from the gas phase. The adsorption velocity of the hydrogen also enters into the process, but for this a sufficiently accurate empirical expression is available.In the final expression for the reaction velocities we have to include the collision factor, the activation energy and the ratio A / v of the active contact surface to the reaction volume. Taking all the experiments into account, a collision factor of about 0-1 was obtained, which is of a reason- able order of magnitude. The activation energy was calculated in the usual manner from the temperature dependence of the reaction velocity. The ratio A / v can be estimated with an accuracy of at least & 50 yo, as the effective surface was always determined by physical adsorption experiments. The experiments were carried out with a series of different hTi surfaces (nickel-powder from carbonate and formate, nickel-foil, Raney-Nickel from NiA1, and Nisi,, evaporated nickel films) at pressures of I O - ~ to 10-1 mm.Hg. The observed variation of the reaction velocity with time was on the whole given satisfactorily by the final equation; as a rule, however, it was found that the reaction velocity decreased rather more rapidly within the individual series of experiments than demanded by the equation. This can be explained by the fact that a small amount of adsorbed hydrogen is not used up during the reaction but distributes itself over the surface in the atomic form and thus poisons it. It is remarkable to find that the final expression remained quantitatively valid for all the Ni preparations employed, provided they had not been previously heated above 280° C.l One gains the impression therefore, that on pure nickel planes irre- spective of the method of preparation, the same hydrogenation mechanism always occurs, and thus the supposed effect of active defects, which should necessarily show some difference in the compounds prepared differently, must be rejected.This must not, however, be taken to mean that at very instant the total surface is catalytically active. It seems more reasonable to regard the sites instantaneously occupied by H-atom pairs as temporary centres of catalytic activity, their position on the surface being subject to con- tinuous change. From the above, the function of the surface covered with these temporary centres, OH,, may easily be calculated ; in our experiments values between 5 x 10-3 and I x 10-1 (maximum) accord- ing to the magnitude of the ratios of pH,@,, were obtained. In fact, at any instant, only a relatively small fraction of the surface is taking part in the reaction, whereas the major part of the stably adsorbed reactants (C6HIo and H atoms) covering the surface, are not at all, or only to a very small extent, capable of reaction. It may be expected that similar conditions exist for other low temperature catalytic processes and that an essential part is played by the active intermediate states of the adsorbed reactants.* Institut fur Physikalische, Chemie der Universitat Gottingen, Gottingen, Biirgerstrasse 50, Germany. * The results communicated here are given in detail in Naturwiss., 1949, 36, 74 ; 2. Elektrochem., 1949, 53, 285, and 1950, 54, Heft 2.

ISSN:0366-9033    

DOI:10.1039/DF9500800128

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

THE RELATIONSHIP BETWEEN SENSITIVITY TO POISONING AND CATALYTIC SURFACE BY E. B. MAXTED, I<. L. MOON AND E. OVERGAGE Received 6th February, 1950 The expected inverse variation of sensitivity t o catalyst poisoning with the surface area of a catalyst has been confirmed experimentally for platinum hydrogenation catalysts poisoned with methyl sulphide, a relationship of the type : eels, = tczsz, in which a is a measure of the sensitivity and s is a measure of the surface area, being obtained. This relationship apparently holds even when the variation in the area of a constant weight of platinum is induced by sintering. A characteristic value for the product c(s has been obtained which should make possible the calculation of the surface area of platinum catalysts in general by sensitivity tests carried out on small quantities of platinum.In connection with measurements of the specific catalytic activities of metals of the platinum and palladium groups, it became necessary to try to develop a method by means of which the relative surface area of finely divided metal catalysts could be measured with amounts of catalyst which are too small for a determination of the surface by Brunauer-Emmett low-temperature adsorption isotherms. This pre- liminary work has been done with platinum, which-unlike some of the rarer metals of the group-was available in sufficient quantities also for area determinations by the low-temperature adsorption method. The main object of the present work has been to examine the degree to which the sensitivity of a catalyst to poisoning, which can be determined with a fraction of a gram of material, can be used as a measure of the surface of catalysts.It would be expected on first principles that the sensitivity of a catalyst to poisoning would be inversely proportional to the surface area, this inverse relationship being derivable both for a catalyst having a cata- lytically uniform surface and for a catalyst containing points or areas of special activity, provided that the ratio of specially active area to the total surface is not changed by the method used for varying the surface area. In the present work the surface area of a platinum hydrogenation catalyst has been varied in two ways : firstly, by using different weights of the same catalyst and, secondly, by using a constant weight of a second stock of platinum, the surface area of which was varied by sintering.In the latter case, it was necessary to determine the relative surface independently by the Rrunauer-Emmett method. Experimental The catalysts consisted throughout of platinum black which had been made by the reduction of chloroplatinic acid with alkaline formate solution. The measurements of the sensitivity t o poisoning were made by the general method which has already been described in previous papers and which is applicable either for the determination of the relative toxicity of different poisons or, by using a standard poison, for the determination of the relative sensitivity of different catalysts. Methyl sulphide was used as the poison; and the standard charge taken for each hydrogenation test consisted of 10 ml.of a 10 yo Maxted and Evans, J . Chem. Soc., 1937, 603, 1004 and later papers. I351 36 SENSITIVITY AND CATALYTIC SURFACE solution of cyclohexene in cyclohexane, together with known amounts of platinum and of methyl sulphide. The hydrogenation reaction, which was carried out in a shaker under standardized conditions a t 30°, was, as is usual, of zero order ; and the approximately uniform rate of absorption of hydrogen, in any test, up to the stage at which the cyclohexene approaches saturation, made possible a direct and accurate reading of the hydrogenation velocity, which was taken as representing the activity of the catalyst. On plotting the hydrogenation rates against the poison content, the usual type of graphs consisting of a linear or very nearly linear portion followed by a region of inflexion were obtained, the course of the main linear portion being expressible by an equation of the type : k , = k , ( ~ - ac), in which k , is the orginal unpoisoned activity, k , is the activity in the presence of a concentration c of the poison and cc is the poisoning ~oefficient.~ It will be seen that cc represents the fraction of the original activity which is suppressed by each unit of poison present and that the values of a, for a series of catalysts poisoned by the same poison under similar conditions, can be used as a measure of the relative sensitivity of the catalysts to poisoning.Sensitivity Tests Involving a Variation of the Surface Area by Varying the Amount of Catalyst.-This is the simplest method of studying the variation of the sensitivity with the surface, in that the surface area is obviously directly proportional t o the weight of catalyst used and accordingly no independent determination of the relative surface, within the series, is necessary.In the present paper, for reasons of space, a summary only of the results obtained can be given ; but a more detailed account of the work is in course of p~blication.~ The graphs for various weights of the stock of platinum used for this series (Platinum A) are given in Curves I to IV of Fig. I. From these, the correspond- ing values of a can be calculated and are given in Table I. If the sensitivity varies inversely with the surface area of the catalyst (which is proportional, within the series, to the weight of the platinum used), the product as, where s is the relative surface, should be constant.The experimentally observed degree of this constancy is shown in the last column of the Table. This constancy also follows directly from the geometry of Fig. I, since the initial linear portions of all the graphs are approximately parallel to one another. 3Maxted and Stone, J . Chem. Soc., 1934, 2 G . Maxted, Trans. Faraday SOC., 1945, 41, 406. Maxted and Overgage (in course of publication).E. B. MAXTED, K. L. MOON AXD E. OVERGAGE 137 Wt. g. 1 Relative Surface 1 Area, s TABLE I Platinum A as x 10-6 Activity (R, in cm.3 Hz per rnin.) ar ID-' 0.025 0.0375 0.05 0.0625 1.0 1'5 2.5 2'0 18.7 27'4 37'1 45'7 0.90 I 0.599 0'435 0'355 0.90 0.90 0.87 0.89 Sensitivity Tests Involving a Variation in the Surface Area by Sintering .-This work was carried out by reducing the surface area of a fresh stock of platinum black (Platinum B) by sintering a t 400-410~. The change in the sur- face area per unit of weight caused by the sintering was followed by means of Brunauer-Emmett isotherms and was compared with the corresponding change in the sensitivity t o poisoning by hydrogenation tests on samples of the platinum taken before and after sintering. The measurements of surface area were made with nitrogen at - 18+3', helium being used as the calibration gas. The isotherms obtained on plotting the nitrogen adsorption against the resulting pressure were of the normal Brunauer-Emmett type 5~ 6 having a long linear portion, by the extrapolaton of which to zero pressure a value could be obtained for the volume of nitrogen required t o form a monolayer and consequently for the surface area.This zero-pressure extrapolation method was adopted for its simplicity in place of using the more complicated Brunauer-Emmett-Teller equation.' It is per- haps open to some doubt whether the figure obtained by any method of extra- polation corresponds very exactly to the adsorption of a monolayer, although Emmett, in a recent critical review * of the reliability of all the proposed extra- polation methods (including the use of the Brunauer-Emmett-Teller equation), states, " It must be admitted that this zero pressure extrapolation method certainly will yield areas which are of the correct order of magnitude and which are probably capable of yielding fairly reliable relative areas for various finely divided solids ".The results for the relative surface of the platinum before and after sinteIing are contained in Table 11, from which it will be seen that about one-third of the original surface area has been lost by the heat treatment. State of Platinum Unsint ered Sintered . TABLE I1 Platinum B I-- I Relative Surface Area of Pt per Gram I 0.68 The corresponding graphs for 0.05 g. of the unsintered and sintered platinum are given in Curves V and VI of Fig. I . It will be seen that these curves agree, from the standpoint of their position and slope, with what would be expected if they had been based on poisoning tests carried out with catalysts of the first series having an equivalent unpoisoned activity, in spite of the fact that Curves 5 Emmett and Brunauer, J .Amer. Chem. Soc., 1937, 59, 1553. 6 Brunauer and Emmett, ibid., 1935, 57, 1754. 7 Brunauer, Emmett and Teller, zbzd., 1938, 60, 309. * Emmet, Advances in Catalysis (Academic Press, New York, 1948)~ Vol. I, P* 75. E "138 SENSITIVITY AND CATALYTIC SURFACE V and VI have been obtained with a different stock of platinum and that the change in the surface area has been induced by sintering ; and it is evident that all the graphs of Fig. I are closely interconnected and that some simple relation- ship connects all these platinum catalysts, irrespective of whether they have been drawn from different stocks or whether the surface has been sintered or not.From Curves V and VI the relative sensitivity to poisoning of the unsin- tered and of the sintered Platinum B can be calculated as before : further, on the basis of the low-temperature isotherms, approximate values for the surface areas of the amount of Platinum B (0.05 g.) used in the poisoning tests can also be inserted. These are included in Table 111. Unsintered . Sintered . TABLE I11 0.05 g. Platinum R 0.134 0.59 o.og17 0.40 Surface Area (cm.3 N2 N.T.P.) Uupoisoned Activity k (cm.3 H2 per min) 33-41 22’22 [Sensitivity a x 10-6 0’459 0.680 as x1o-6 (s in m.2) 0.271 0.272 From the above figures, al/a2 = 0.67, s2/sl = 0.68, and k2/k1. = 0.67, in which the subscripts refer respectively to the unsintered and to the sintered platinum. .Accordingly, within the limit of the accuracy of the measurements, %=5=& 012 s1 kl’ this inverse variation of the sensitivity with the surface area, and the direct variation of the rate of working with the surface, being similar to the results obtained in the first series with Platinum A.Discussion If all the curves of Fig. I can be treated as a connected series, it should be possible, by applying the relationship, alsl = a2s2, to calculate the surface area of all the catalysts used in the Figure, provided that the area is known in at least one case. This area in m.2 is known in two cases, namely for the 0.05 g. of unsintered Platinum used in Curve V (s = 0.59 m.2) and for the 0.05 g. of sintered Platinum (s = 0.40 m.2) used in Curve VI : furthermore both of these catalysts give approximately identical values (0.271 x 1o6 and 0.272 x I O ~ ) for the sensitivity-area product, which can be used as one side of the as equation, the surface area of any of the catalysts of Table I being obtained by dividing this product by the value of a for the catalyst in question.It can in this way be shown that the surface area of Platinum A is about 12.1 m.”g. This conversion of the arbitrary scale of relative surface used in Table I into areas expressed in m.2 gives Table IV, in which the areas of all the catalysts used in both series are consequently expressed on a directly comparable scale. The above conversion could also, in principle, have been carried out by using the alternative surface-to-rate relationship, slK = s,Kl, but this method is less accurate in practice than the use of the as relationship, in that it is influenced more by any traces of impurities, the effects of which tend to cancel out in the ratio k,/k,, which determines a, since K O and K , are affected proportionately : further, the as relationship is of special interest since a has a direct coverage component due to the poison.The product as for the figures in the last two columns of the Table is, of course, in each case 0-27 x I O ~ , which-since it was given by two in- dependent and direct measurements of surface on two rather widelyE. B. MAXTED, K. L. MOON AND E. OVERGAGE 139 different catalysts, i.e. both for the unsintered and for the sintered Platinum B-seems to be a characteristic sensitivity-area product, which can be used for calculating the surface area in m.2, not only of Platinum A, but also of other platinum catalysts, merely by measuring the value of the sensitivity a of the catalyst in question and dividing the above TABLE IV ~ ~~ Platinum Stock Wt.of Pt (8.) 1 Sensitivity, a x 10-0 1 Surface Areas (m.*) A . B (sintered) . A . B (unsintered) . A . A . 0.025 0.050 0.0375 0.050 0.050 0.0625 oag01 0,680 0.599 0.459 0'435 0'355 0.30 0.40 0.46 0.59 0.61 0.76 characteristic product by the observed value for a. In more general terms, it should be possible, after an initial calibration of a hydrogenation apparatus, for a single catalyst by means of a single low-temperature isotherm, subsequently t o use the apparatus for the determination of the area in m.2 of any other specimen of platinum catalyst without the necessity for further Brunauer-Emmett isotherms.Rate of Working of the Platinum Surface.-Finally, the rate of working per unit of area of the various platinum surfaces used may be considered briefly. If the two series are combined by expressing the areas in m.2, as in Table IV, Fig. z is obtained on plotting these areas against 0 Surface urea i n mf FIG. 2. the unpoisoned catalytic activities KO. It will be seen that the rate of working of unit surface area in all the catalysts taken is approximately, although not exactly, the same in both series, the slight difference being, however, possibly due t o a change in the cyclohexane stock from Series I t o Series 11, since, as has already been stated, the values for the actual velocities of hydrogenation are found in practice t o be far more sensitive t o such changes than the ratio of velocities used for calculating a.From the results given in the Figure, each square metre of platinum surface in Series I works at a rate corresponding to the insertion of about 61 ~ r n . ~140 AMMONIA OXIDATION AND ANALOGOUS REACTIONS H, per min. into cyclohexene. In Series I1 the corresponding rate of fixation of hydrogen was 57 cm.s per min. in each case, in spite of the fact that in one case the surface of the platinum had been sintered. The very close correspondence of the rate of working per square metre of this sintered and unsintered platinum-for which the comparison of surface area was not dependent on a conversion factor but on direct measurements by low-temperature isotherms-as well as the degree of the agreement of the rate of working by each square metre of platinum catalysts generally, are obviously of interest in connection with the nature, and with the resistance to sintering, of active points or of specially active crystal planes of the surfaces of Pt catalysts and woIk is in pIogress to investigate this aspect more systematically. One of the authors (E. B. M.) wishes to acknowledge the help given by a Leverhulme Research Fellowship in the carrying out of the above work. Department of Chemistry, University of Bristol.

ISSN:0366-9033    

DOI:10.1039/DF9500800135

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

140 AMMONIA OXIDATION AND ANALOGOUS REACTIONS THE MECHANISM OF AMMONIA OXIDATION AND CERTAIN ANALOGOUS REACTIONS BY J. ZAWADZKI Received 30th January, 1950 The characteristic feature of the reaction of catalytic ammonia oxidation is its great velocity ; in favourable conditions nearly every one of the NH, mole- cules striking the surface of the platinum catalyst gives NO as a final product. The attainment of the high efficiency of the oxidation to the desired product depends on the velocities of a number of successive and simultaneous reactions, which take place in the system NH,-0,-catalyst, as well as on the suppressing of the reactions between reagents, intermediate and final products of reaction before and after the catalyst. From experiments on various catalysts (Pt, oxides) under such conditions that N,O appears as one of the main products a scheme of the most probable mechanism of reaction is given.Certain assumptions are made concerning the elementary processes and the chemisorption of 0, on the catalyst which proceeds with great velocity. 1. Preliminary Remarks. - Recent experiments on the ammonia oxidation 1-4 at temperatures below 550’ clearly proved that N,O was one of the main products of reaction on oxide catalysts and on platinum. The results of these experiments enabled us to make a step towards a better understanding of the mechanism of this reaction (cp.”). was critically reviewed in 1922-1927.’-~~9 l2 that N,O was present in the products it was shown that : The extensive experimental material collected by several authors Although it was not known 1 Zawadzki, Roczniki Chem., 1948, 22, 220.* Tuszynski, ibid., 1949, 23, 397. Winnicki, ibid., 1949, 23, 388. Ostwald, Berg.-u. Hiittenman. Rundschau, 1906, 3, 71. 6 Gmelin, Handbzrch der anorganischen Chemie (Stickstoff), 1936, 639. 7 Zawadzki and Wolmer, Roczniki Chem., 1922, 2, 158. Zawadzki and Lichtensteinowa, ibid., 1926, 6, 824. Zawadzki and Narkiewicz, ibid., 1927, 7, 369. l o Andrussow, 2. angew. Chem., 1926, 39, 321 ; 1927, 40, 166. Marczewska, ibid., 1949, 23, 406.J. ZAWADZKI 141 (i) It is necessary to distinguish between the reactions which occur on the catalyst itself and those which take place before or after the catalyst bed. (ii) There is no optimal temperature for a given catalyst. The greater the gas velocity the higher the temperatwe at which the maximum yield is obtained.(iii) Ccnsequently we can obtain l o high yields of NO even at IZOOO, provided the time of contact is sufficiently short. (iv) In connection with (i) it was pointed out lo that at low flow rates a countercurrent diffusion of gases was an important factor. The pro- cesses which took place either before or after the catalyst bed were demonstrated by carrying out the oxidation in a platinum capillary tube.11 The final products of ammonia oxidation are N, and W,O and NO is an intermediate (non-equilibrium) product, which under suitable con- ditions can be recovered with high yields. In the oxidation we have to distinguish between successive and simultaneous reactions. Only one of the simultaneous reactions gives the desired product ; others (e.g.2NH, H, + 3H,, or reactions of NH, with intermediate or final pro- ducts) are detrimental to high yields of NO. Another source of loss is the successive reactions in which finaI products take part (e.g. 2N0 N, + O,, or reactions of NO and NO, with NH,, etc.). In these elementary processes there are involved not only the original reagents (NH ,, 0,) and final products, but also various intermediate products and by-products (e.g. NH,OH, NH, HNO, HNO,, N,O, etc.). By considering the results of the latest experiments (where high propor- tions of N,O were formed at low temperatures) it is possible to explain the nature of losses occurring in the NH, oxidation on platinum as well as on oxide catalysts. (a) AT HIGH TEMPERATURES.(i) The velocity of decomposition of NH, to N, and H, at sufficiently high temperatures is comparable to that of its oxidation to NO. The extent of loss is thus controlled by temperature and not by the rate of flow of gases, except when this is too low when there are losses due to NO decomposition. These rise with increasing temperature and de- creasing rate of flow. (ii) Losses are developed also when unconverted ammonia passes through the catalyst bed and beyond it due to reaction with NO and other intermediate products. Too large or uneven mesh of the Pt gauze catalyst or too high a rate of flow may be responsible for the passage of unconverted ammonia. (iii) When the rate of flow is too low the reaction before the catalyst bed causes additional 10sses.~~ lo (i) N,O is present in considerable proportions in the reaction products.The amount depends on temperature and rate of flow. The formation of N,O accounts for the decrease in yield of NO. The higher the temperature and the longer the time of contact, the greater is the proportion of decomposed N,O. (ii) When the rate of flow is low the losses before the catalyst bed are caused by the countercurrent diffusion. (If, e.g. NO passes into the space before the catalyst bed, it is oxidized to NO,, which reacts with NH, giving free nitrogen through formation of NH,NO,). (iii) There are losses on the catalyst itself when conditions are such that the first intermediate product of reaction (e.g. NH) is formed slowly and can react with undecomposed ammonia, the product being free nitrogen.Nitrogen is also produced when NH, reacts with inter- mediate products formed in the next stage of the process. (iv) When the rate of flow is high, the unchanged NH, can pass beyond the catalyst bed, where analogous reactions take place, although probably they proceed at a lower rate. The course of these reactions depends on the space available and tem- perature. 2. Proposed Schemes of Mechanism of the Reaction.-To explain the mechanism of ammonia oxidation we have also t o consider the HCN oxidation, and the reaction of NO with hydrogen. At first it was assumed 11 Andrussow, Ber., 1927, 60, 2005. (b) AT TEMPERATURES BELOW 5 0 0 ~ . N,O decomposes easily into N, and 0,.142 AMMONIA OXIDATION AND ANALOGOUS REACTIONS that NH, and 0, decomposed to atoms and that the yield of NO depended on a proportion of the N atoms combining with 0 atoms.Only that part of the hypothesis concerned with the role of atomic oxygen is true. Contrary t o Andrussows’ lo* l1 (nitroxyl) hypothesis the formation of HNO is not an intermediate reaction ; it is a side reaction, which is re- sponsible for formation of N,O in the final products. The weak point of this hypothesis is the assumption that a reaction of NH, with molecular oxygen proceeds on Pt surfaces. The work of Langmuir,l6 Cassel and Gliickauf l7 and others definitely proved that the adsorption of atomic oxygen does occur and that it is a fundamental process in oxidation of CO or H, on platinum. In Langmuir’s experiments each molecule of CO or H, striking the platinum surface with adsorbed atomic oxygen enters the reaction.Similarly in the NH, oxidation, nearly every NH, molecule, which strikes the catalyst surface, gives ultimately a molecule of NO, provided the conditions are favourable. In a process of such a high velocity as that of NH, oxidation to NO we cannot assume the prim ary reaction to be NH, + 0,, as the heat of activation would have to be considerable. In our studies on NO decompoFition on platinum we have postulated a stronger inhibiting action of atomic oxygen formed in the reaction itself than of molecular oxygen introduced t o reacting mixture. Numerous experiments carried out a t low pressures and at high temperatures by Bodenstein and Krauss 21 and their collaborators, and also investigations on various reactions of hydroxylamine, have led Bodenstein 149 to the following reaction mechanism : NH, + 0 = NH,OH.. (1) NH20H + 0, = HNO, + H,O . - (4 HNO, + 0, = HNO, . * ( 2 4 HNO, = NO + 0, + OH . * (2b) 2 0 H = H 2 0 + O . * (3) NH,O + 0 = HNO + H,O . - (4) zHNO = N,O + H20 . * (44 HNO, + NH, = zH,O + N, . . ( 2 4 HNO + NH,OH = zH,O + N, ( 4 4 Reactions leading to NO : Reactions leading to N,O : Reactions leading to N, : and the decomposition of NO and N,O. reaction is endothermic *O as Bodenstein admits. with oxides such as MnO, or Fe,O, are appreciably endothermic. haskoj promyshlennosti, 1933, 10, 37. However, we consider Rodenstein’s hypothesis unfounded. Thiis the NHsads. + Oads.Pt = NHZOHads. Similarly, the reaction of NH, l2 Adadurow, Proizwodstwo azotnoj kistoty, Leningrad, I 934 ; Zurna‘i‘ chimic- Bodenstein, 2.angew. Chem., 1927, 40, 174. l4 Bodenstein and Buttner, Trab. IX Congr. I n t . Quim. Pura Aplicata. Madrzd, 1934, 3, 475 : Bodenstein, Tqans. Electrochem. SOC., 1937, 71, 353 ; Helv. chim. Acta. 1935. 18, 758. l5 Nagel, 2. Elektrochem., 1930, 36, 754. l6 Langmuir, Trans. Faraday Soc., 1922, 17, 607, 621. l7 Cassel and Gluckauf, Z . physik. Chem., 1930, 9, 435 ; 1932, 17, 380 ; 18, l8 Zawadzki and Badzynski, Roczniki Chem., 1931, 1 1 , 15. Zawadzki and Is Bodenstein, 2. Elektrochem., 1941, 47, 517. * O Bichowsky and Rossini, The Thermochemistry of the Chemical Substances 347 ; 19328 19, 47. Perlinski, Compt. rend., 1934, 158, zoo. (Reinhold Publishing Corp., New York, 1936).J. ZAWADZKT J 43 It is clear that an endothermic reaction cannot be the first step in a process, in which practically every impact of NH, molecule with the catalyst surface is effective, because the heat of activation] being at least equal to the endothermic heat of reaction, should be equal t o or very nearly zero ; this does not agree with calculations.Yields of NH,O and HNO, in Bodenstein’s l4 and Krauss’ 2 1 ~ 23 experi- ments rise with increasing temperatures (from 105oOto 1350’). The yield a t 1050’ is still comparatively low and would be lower at temperatures below IOOO*. Bodenstein 22 admits that if platinum used had not been heated to a very high temperature, hydroxylamine would not have been found in the reaction products. On the other hand under certain conditions it is possible t o obtain high yields (well over g o yo) of NO from NH,, even a t temperatures slightly higher than 500°, and that the reaction proceeds with a very high velocity.Hydroxylamine has never been discovered except under special conditions a t very high temperature, and it is well known, that hydroxyl- amine is unstable well below 500’. The combination of NH, with atomic oxygen may result in a transition state or activated complex (catalyst . 0 . NH,). In Bodenstein’s and Krauss’ experiments this activated com- plex probably gives hydroxylamine, which is then desorbed from the catalyst. The higher the temperature, the greater is the probability of formation and desorption of NH,OH. At lower temperatures it is probable that the activated complex gives NH which stays on the surface of the catalyst] and H,O which is set free.We shall now discuss the imide theory and thereby t r y t o explain the results of ourselves and other authors for the NH, oxidation on Pt and on oxide catalysts. Several of the facts can be explained by both the hydroxylamine and the imide theory ; however, using the latter, the explanation is simpler and, what is more important, it is in agreement with energy calculations. AUTHOR’S SCHEME OF REACTION MECHANISM. NH, + 0 = NH + HzO . (1) NH+O,=HKO, . - (2) HNO,=NO+OH . * 624 20H = H,O + O . . (2a’) zHNO, = NO + NO, + H20 . - (2b) Reactions leading to NO : or1 or reactions (2a), (zb), (3) of Bodenstein’s scheme. Reactions leading to N,O : N H + O = H N O . - (3) zHNO = N,O + H,O . - (34 NH + NH = N, + H, * ( 4 4 NH + HNO = N, + H,O* .- ( 5 ) z N O = N , + O , . - (7) Reactions leading to N, : NH + NH, = N,H, and decomposition of N,H, . (4b) ( 6 ) (8) NH, + HNO, = N, + 2H,O (decomposition of NH4N0, ) zNzO = 2N2 + 0,, also reactions between NH, and NO,. . 21 Krauss, 2. physik. Chem. B, 1938, 39, 83. 2 2 Bodenstein, 2. Elektrochem., 1935, 41, 466. 23 Krauss and Neuhaus, 2. Physzk. Chem. B, 1941, 50, 323. NH, + HNO = N,H, + H,O and N,H, + 1/2 0, = N, + H,O. * Andrussow gives also :144 AMMONIA OXIDATION AND ANALOGOUS REACTIONS All these reactions proceed on the catalyst surface; in the gas phase probably only the reaction of NH, with NO, takes place. The regeneration of atomic oxygen (2a') (of this scheme or (3) of Bodenstein's) gives, at best, half of the amount of 0 necessary for re- action (I), and the rest must then be supplied by the dissociation of 0, on Pt, i.e.0, = 20ads. The following arguments have been proposed in favour of the imide as the first product of oxidation : (i) An unstable NH is found in several reactions in which N, and H,, or compounds of nitrogen with hydrogen, take part. The characteristic spectrum of NH has been found and its occurrence has been proved by optical methods. 24 (For imide formation in the catalytic decomposition of NH ,, see Mittasch and Frankenburger, 25 Frankenburger and Hodler, 2* Christiansen 27 and others.) (ii) Hydrazine has been identified among the products of various reactions in which NH, takes part. It is probably formed by the reaction, NH, + NH = N,H,.*'* ,'* (iii) Raschig has explained the occurrence of N, and H, in a ratio of I / I and also of N,H, in ammonia-oxygen flames (with NH, in excess) by assuming NH as an intermediate product. (iv) Hofmann and Korpium 30 have similarly found N, and H, in a ratio of I/I, when carrying out the oxidation of NH, in excess in presence of a fine copper wire.Hydrazine was identified on Ni-NaOH catalysts. In my opinion the first elementary process in the NH, oxidation on platinum or oxide catalysts is the reaction of NH, with atomic oxygen : NH, + Oads. = NH + H20 2o AH (kcal,) : 11 21.7 -39'9 57.8. Assuming gaseous NH to be the product, it follows that the reaction is endothermic (AH = 8.8 kcal.). If, however, we take into account the heat of adsorption of NH on platinum, the result is different.This heat of adsorption is undoubtedly much higher than the heat of adsorption of NH, or NH,OH, and is probably of the order found in the adsorption of atoms such as 0, H or N. (The heat of adsorption of 0 on Pt calcu- lated from the data of Cassel and Gluckauf = 80.8 kca1.l') However, it follows that the heat of adsorption of NH on Pt is higher than 8.8 kcal., and that the reaction is exothermic. The heat of ad- sorption of NH undoubtedly greatly exceeds this limiting value, i.e. the reaction of NH, with adsorbed oxygen is highly exothermic. It is difficult to calculate the energetics for all the elementary processes proposed in the scheme because of the lack of thermochemical data. A rough estimate indicates that the NH, oxidation is composed either of exothermic or of slightly endothermic elementary processes.Hence the assumption of low heats of activation of all elementary processes involved is justified. 3. Discussion of Experimental Data in Terms of the Proposed Scheme of Reaction.-We shall now discuss the results of experiments reported in the present and previous papers, and of other authors,l5' 23* 25 which were carried out at temperatures below 6 0 0 ~ . Our experiments, 24 Gaydon, Spectroscopy and Combustion Theory (Chapman Hall, London, 25 Mittasch and Frankenburger, 2. Elektrochenz., 1929, 35, 927. ** Frankenburger and Hodler, Trans. Faraday Soc., 1932, 28, 289. 27 Christiansen, Det Kgl. Danske Videns. Selsk. Mat. fys. Medd., 1935, 13, 12, 2s Bredig, Koenig and Wagner, 2. physik.Chem., 1928, 139, 211. 29 Raschig, 2. angew. Chem., 1927, 40, 1183 ; 192S, 41, 265. 30 Hofman and Korpium, Bey., 1929, 62, 3001. '94')J 7'1 '572 '73.J. ZAWADZKI I45 the Iesults of which are given in Fig. I, 2 and 3, have been performed in manner analogous to that described by Tuszynski Information on experiments concerning Fig. 4 and 5 can be found in a paper by Winni~ki.~ In Fig. 4 has been plotted a curve for constant temperatures from the data obtained in experiments performed a t various temperatures and a t constant flow rates. and Marczewska. ruo - % .50 FIG. catalyst : Pt gauze 3600 mesh per cm.2, double-folded. Incoming gases : 10 yo NH,+go yo air Flow rates : :lo" 6 l./hr. ----- -- . N,O I* l./hr* . NO FIG. 2.-Catalyst : Pt gauze 3600 mesh per cm.*, double-folded.Incoming gases : 10 Yo NH,+go yo air, - . . - -2; N, - - - - - _ In experiments recently carried out by Kepinski and Witczynski (Fig. 6, 7 and S), NO, NO, and NH, were absorbed in concentrated H,SO, as described in a paper by Zawadzki and Wolmer.7 Special care was146 AMMONIA OXIDATION AND ANALOGOUS REACTIONS given to the complete removal of NO and NO,. From the gases deprived of NO, NO, and NH, nitrous oxide was removed by cooling in liquid air. ,+--------- /r / u, .50 ,L-.<. ,*, ' \ -46- rc' \ -.. \ 0200 80 0 0 0 , % 0 ,' / 0 FIG. 4.-Catalyst : 35-58 yo CuO + 65.62 yo MnO,. Incoming gases : 10 yo NH, + go yo 0,. N2O N, - . . - - N O - - - - The method of estimation of X,O by the change of volume after passing over P t is far from perfect.Some N,O, though small in amount, is decomposed to NO + N, and the NO is then oxidized to NO,. In our latest experiments this error did not appreciably exceed I yo. (Detailed data will be published by Kepinski a t a later date.)J. ZAWADZKI I 4 7 These results can be explained in terms of the reaction mechanism, given above. NH (formed by (I)), which is adsorbed on the catalyst, can react either with another NH radical, or NH, molecule ((4a) and ( q b ) ) , or with atomic oxygen ((3), and further conversion to N,O), or FIG. 5.-Catalyst : 79.45 % Fe20, f 11-53 yo Bi203 + 7-21 % MnO,. Incoming gases : 10 yo NH, + 90 yo air. N2. A IOO l./hr., 0 200 l./hr., a 300 l./hr. even with HNO ( 5 ) , or with molecular oxygen ( 2 ) . This last reaction, giving NO as final product, undoubtedly has the greatest heat of activa- tion due t o the necessity of loosening the bond between oxygen atoms in a 0, molecule in order to form HNO,.This heat of activation is N20, - - NO, --- FIG. 6.-Catalyst : Bi,03. Incoming gases : 10 %NH, + 90 yo air. N20, -. -NO, - - - NH ,. x 10 l./hr., 0 20 l./hr. different for various catalysts and depends on their structures. This accounts for the different temperatures at which this reaction starts and the various rates of reaction observed a t the same temperature for various catalysts. It follows that the formation of NO will be favoured but reaction (3) diminished by increasing temperature.148 AMMONIA OXIDATION AND ANALOGOUS REACTIONS There are several reasons why the yield of NO increases with increasing rates of flow.Naturally the decomposition of NO to elements is negligible, but at low rates of flow there is a possibility of a counter-current diffusion and consequently of a reaction of NO2 with NH, which takes place before the catalyst bed and, at low temperatures, even in the first part of the bed. This leads to loss of NO already formed. . % \ :500 , ..& /..--6 D..' \ /'.4 270 300 3h.; ./400 ~ % 5QQ d,./ Temp. C /' .' FIG. 8.-Catalyst : Pt gauze 1039 mesh per cm.2, triple-folded. Incoming gases : 10 yo NHs-t-90 yo 0,. --- N20, - . - NO, -- -NH3. x 4 l./hr. 0 16 l./hr. Moreover, there are other factors connected with the fact that the course of reaction is not uniform on the whole length of the catalyst bed. I f we suppose the catalyst bed to be divided into a number of sections of equal width in the direction of gas flow, the reaction may practically cease after the gases have passed the first section, provided the rate of flow is sufflciently low.A two-fold increase of the rate of flow may resultJ. ZAWADZKI I49 in reaction proceeding also in the second section, etc. Further sections of the bed are then inactive and only at high temperatures are they responsible for partial decomposition of previously formed NO. The migration of adsorbed NH, molecules on the catalyst surface (Volmer 31) could not alone explain these observations. Reagents are being consumed on their way through the catalyst bed : in the first part of it, more NH, and 0, molecules strike a given area of catalyst surface in unit of time than in further parts where the reacting gases contain less NH, and 0,.The amounts of NH, and 0, consumed, as well as the amounts of NH, etc. produced, then gradually diminish as the gases flow through the bed. Accordingly the concentration of adsorbed NH is greatest a t the beginning. This exerts an influence on the velocity of reactions since the processes are exothermic and the amount of heat evolved at the beginning is greater than in the rest of the bed. The temperature therefore should be higher than that measured after the bed. This has been well illustrated in several experiments.l* 8 p The higher the rate of flow, the greater the difference is. Now, as the yield of NO is strongly increased by increasing temperature, the influence exerted by flow rate on temperature prevailing in the bed constitutes another reason why the high rates of flow favour the good yields of NO.The occurrence of N, in the products before the formation of NO is ascribed to reaction ( 6 ) . Possibly another reason is the hypothetical reaction : NH + HNO, = N,O + H,O. At high temperatwes, when NH, and NH are consumed rapidly, these reactions play no part at all, but they must be considered when unconverted NH, is present or when NH slowly reacts to HNO,. have not found it using Bi,O,, our own experiments, the results of which are shown on Fig. 6, prove that yields of N,O on this catalyst are not insignificant. The main cause is a rapid decomposition of N,O, which takes place even below this temperature, and which was shown to be present in experiments on catalysts such as MnO,, Mn,O, and Ni0,23 by Kepinski on CuO + MnO,, and by many authors on platinum.s With increasing temperature and constant rate of flow the yield of N,O first rises to a maximum, then falls to zero.The difference between P t and oxide catalysts is qualitative rather than quantitative. The yield diminishes at higher temperatures : because of (i) the decomposition of N,O, and (ii) the increasing rates of reactions leading to NO. The rising part of the yield against temperature plot is explained by reaction (3). and especially by (3a), which has a definite heat of activation though smaller no doubt than that of reaction ( 2 ) . As a result of (3), comparatively small amounts of HNO are formed at low temperatures, since the chance of (3a) occurring is less a t higher temperatures.This favours (5) and also a reaction between unconverted NH, and HNO giving N,. This is an additional reason for the appearance of free nitrogen as a reaction product at the lowest temperatures. The differences in the action of various oxide catalysts constitute an important argument that the process of N,O formation goes through A BaO, catalyst 23 which decomposes to BaO and oxygen a t compara- tively low temperatures, gives N,O as almost the sole product of NH, oxidation. Reaction proceeds in the came direction and with nearly equal yields, independently of whether the usual mixture of NH, and 0, or one of NH, with N, is used. BaO too gives high amounts of N,O and, as atomic oxygen is available in smaller quantities, large propor- tions of free nitrogen are formed.All the oxides which easily undergo conversion to lower stages of oxidation give high yields of N,O, as atomic Adhikari and Felman, ibid., 1928, 131, 34. N,O has been found on all catalysts. Although Krauss and Neuhaus N,O appears only at temperatures up to about 550~. stages ( I ) , (3) and ( 3 4 . 31 Volmer and Adhikari, 2. physik. Chem., 1926, I 19. Volmer, Trans. Faraday SOC., 1932, 28, 359.150 AMMONIA OXIDATION AND ANALOGOUS REACTIONS oxygen is here available for reactions (I) and (3). Especially interesting are experiments with Ni0.23 In NiO lattice an excess of oxygen is present and an excess proportionality of N20 to amount of oxygen has been found. The examination of changes of N,O yield with varying rates of flow on different catalysts is important for better understanding of the mechanism of reaction.There are considerable differences in the action of Pt and oxides, though some of the latter approach Pt in their behaviour. The results of our experiments on oxide catalysts, where one can expect high amounts of atomic oxygen to be available, are shown in Fig. 4, 5 and 7. Analogous results were obtained by Winnicki * and Nagel.lS These experiments prove that the yields of N20, within the wide range of temperatures under consideration, increase with diminishing time of contact. This can be explained by an increased rate of deswption of N,O, and by the fact that in the first part cf the catalyst bed, where the main portion of NH, reacts, we have to deal with high concentrations of NH on the catalyst only when the rates of flow are high.Because atomic oxygen is available in considerable proportions, the concentration of NH on the catalyst surface constitutes the rate-controlling factor. Deviations observed in Fig. 4 at very short times of contact can be due to the true temperature in the first part of the bed being higher than that actually read on a milivoltmeter. In Fig. 7, we have also to note that reactions (3a) and (3) are not so rapid as reaction (I), and that consequently, when rates of flow are high, conditions can arise such that the influence of a reaction of NH with HNO on the final result is considerable. When there is less atomic oxygen available, 9s with Bi,03, the de- pendence of N,O yield is similar to that observed on Pt.On Pt the yield of N20 a t constant temperature decisively diminishes with increasing flcw rates (Fig. I, 2, 3, 8). In this case oxygen is only adsorbed on the catalyst surface, probably forming a unimolecular layer. The regener- ation of atomic oxygen which can take place only by surface adsorption, is then much more difficult than with oxides, where the typical reaction is MeO, = Me0 + 0. From the proposed scheme it follows that the atcmic oxygen is necessary for a very rapid reaction (I), as well as for the slower reaction ( 3 ) . When small proportions of oxygen are available the chances of N20 formation diminish. This explains why we cannot obtain as high yields of N20 on Pt or Bi203 as on the oxides of the type Fe,O or MnOz, which easily undergo regeneration in oxidizing atmosphere.It can be seen from the scheme that half of the atomic oxygen needed for reaction (I) is regenerated in reaction leading from HNO, to NO. This is independent whether we consider the mechanism to be represented by eqn. (zu) and (za’), or by ( z b ) , or even by ( z a ) , ( z b ) , (3) of Bodenstein’s scheme. The remaining atomic oxygen has to be supplied either by way of adsorption of 0, from the gas phase, or by oxidation of a lower oxide foImed in reaction (I). The situation is worse in reactions leading to N,O. The atomic oxygen is not regenerated at all, nor is it in reactions which give N,. In addition we must have atomic oxygen not only for reaction (I), but also for (3), i.e. twice as much as when NO is the product.The higher the rate of flow of gases through the catalyst bed, the more ammonia reacts in unit time on a given area of catalyst surface, and the more atomic oxygen is needed for reaction (I). When it is available only in limited amounts, as with Pt, it is primarily consumed by reaction (I), because it is more rapid than (3) and because it precedes it. We see then, that by high flow rates, unfavourable conditions for (3) and (3a) arise. On the contrary, favourable ones arise for (2), especially at high temperatures, as molecular oxygen is always available. When the flow is low, chances of formation of HNO and N20 are higher, because less atomic oxygen is necessary, particularly at low temperatures. Free nitrogen appearing in NH, oxidation may be formed in reactions (4a) and (qb), as well as in (5) to (8).J.ZAWADZKI 151 An assumption that imide is the first intermediate product of reaction explains the oxidation of HCN and also the reduction of NO by excess hydrogen up to 90 yo yield. Neumann and Manke 33 in work on HCN oxidation consider NH to be an intermediate product, and some of our experiments lead to the same conclusion. When HCN is oxidized on Pt and oxygen is available in insufficient proportion?, we find in the " post-reaction " gases considerable quantities of NH, and carbon is deposited in the catalyst bed. When HCN with N, is passed through the Pt gauze in absence of oxygen, (CN), is in the reaction products. These facts are explained by the following reactions : 2HCN = (CN), + H2 ; 3HCN + 0, = NH + H2 + zCO + N, + C NH + H, = NH,.Evidently the normal oxidation of HCN by oxygen adsorbed on Pt gives a complex HCNO which immediately decomposes into NHaas. + CO. In the reduction of NO,]. 32 when hydrogen is used in excess, a film of atomic hydrogen is formed on Pt, and this reacts as follows : N Hads. NHads. NHads. + H2 = NH, HzO + Hads. 0 Hsds. OH - OH + H, I1 + 4. The Effect of Temperature on the Rate of Reaction.-The oxidation of NH, to NO is rapid in spite of its complicated course. Reactions such as the catalytic synthesis of ammonia proceed a t a much lower rate. This needs a n explanation as does a sudden increase of the velocity which occurs on Pt within a small range of temperatures. Study- ing NH, oxidation on oxide catalysts, we found in numerous experiments that the difference of temperatures a t which reaction starts and a t which NH, has practically completely disappeared is from about IOOO to 400' or more (cp.also 2 3 ~ 16). This difference on Pt does not generally exceed 50' (e.g. Fig. 6, 8). With the oxide catalyst the increase of rate with temperature can be roughly explained in terms of the factor e -*/Rr ( A = heat of activation), but with platinum it is more difflcult. It is also difficult a t first to understand a high velocity of adsorption of 0, as atomic oxygen on platinum. Now in the formation of CaCO, from CaO and CO,, or CdCO, from CdO and CO, etc., the process proceeds without activation a t pressures not far from the equilibrium pressure.34 In the reversed process (endothermic) the heat of activation is equal to the heat of reaction.When conditions are chosen so that the interface F, at which reaction occurs, remains practically constant, then K from 'u = hF(p - P o ) is independent of ternperat~re.,~~ 36 There is, however, a powerful influence of a comparatively small change of temperature when F does not remain constant because of the formation of nuclei i.e. new centres of reaction, when the system is far from equilibrium (high values of p - fro) and the temperature is high.S5 When a two-dimensional nucleus has been formed, the building-up of a layer of the new phase goes on rapidly (each of the CO, molecules striking the solid phase reacts with CaO to form CaCO,). This is a mechanism analogous to that of a chain reaction and is a result of deformation connected with the liberation of energy in an exothermic process. A point situated near the place where reaction has just occurred is ready to take up another C 0 2 molecule without hindrance. This may be explained as follows. 32 Andrussow, Ber., 1927, 60, 536. 33 Neumann and Manke, 2. Etaktrochem., 1929, 35, 751. 34 Zawadzki and Bretsznajder, ibid., 1935, 41, 215. Zawadzki and Bretsznajder, Trans. Faraday SOC., 1938, 34, 951 ; 2. 36 Zawadzki, Fetskrift Tillagnad I. Arvid Hedvatl, Goteborg, 1948, 611 ; physik. Chem. B, 1933, 22, 79. Roczniki Chem., 1934, 14, 823 ; 1938, 18, 892.1 5 2 HYDROGENATION OF ETHYLENE A CO, molecule reaches this point not only directly from the gas phase, but possibly also as one of loosely adsorbed CO, molecules, which move over the surface (Volmer 31). In ammonia oxidation it is prcbably easier for atomic oxygen to move over an oxide catalyst surface than over a platinum one and consequently the regeneration of atomic oxygen would be easier. On Pt there is a chemosorption of oxygen which can, by analogy, be considered as the formation of two-dimensional crystals, The points of the surface from which oxygen atoms are released, as result of a number of exothermic processes, can easily take up more oxygen from the gas phase and this chemosorption does not need any activation energy. This is a qualitative hypothesis and is not yet definitely proved. I think, however, that a more precise analysis of this hypothesis can con- tribute to the explanation of the enormous velocity of such processes as the NH, oxidation on Pt, as well as of the rapid increase of velocity within a small range of temperature. Laboratory of Inorganic Technology, Institute of Technology, Warsaw.

ISSN:0366-9033    

DOI:10.1039/DF9500800140

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

HYDROGENATION OF ETHYLENE THE MECHANISM OF THE CATALYTIC HYDROGENATION OF ETHYLENE BY G. H. TWIGG Received 1st May, I950 Ethylene has been hydrogenated on a nickel catalyst at -78" C with (a) a mixture of H, + D,, and (b) the equilibrium mixture of H, + HD + D,. Infra-red spectroscopic examination showed that the mixtures of ethanes pro- duced in the two experiments were identical, and were different from an equi- molar mixture of C,H, and C,H4D,. This is direct proof that in hydrogenation the hydrogen is first dissociated into atoms on the catalyst. This result and the existing knowledge concerning the hydrogenation and exchange reactions are reviewed, and a mechanism for hydrogenation is pro- posed which accounts for all the known facts. and in particular for the temperature dependence of the energy of activation.Hydrogen is not adsorbed directly on the catalyst, but only through reaction with a chemisorbed ethylene mole- cule t o form an adsorbed ethyl radical and an adsorbed hydrogen atom ; hydro- genation takes place through the addition of a further hydrogen atom t o the ethyl radical, while exchange is controlled by the reverse of the first step. A further fast reversible reaction occurs in exchange between adsorbed ethylene and a hydrogen atom to form an adsorbed ethyl radical. The mechanism of the hydrogenation of ethylenic double bonds on metal catalysts such as nickel has been the subject of much investigation. The determination of the kinetics alone has not thrown much light on this problem, but the discovery of the exchange reaction between ethylene and deuterium 1 appeared t o offer a fresh approach. Horiuti and Polanyi 2 proposed the following reaction schemes : CH, CH,-cH, + H + H 1 I L2 CH, - CH,--CH,; Ni Ni - H 1 Ni the first reaction and its reverse gives rise to exchange, whilst the addition of a second hydrogen atom to the " half-hydrogenated state " completes 1 Farkas, Farkas and RideaI, Proc. Roy.SOC. A , 1934, 146, 630. Horiuti and Polanyi, Trans. Faraday SOC., 1934, 30, 1164.G. H. TWIGG I 5 3 the hydrogenation. Farkas 1 9 proposed different mechanisms, and suggested that the first step in exchange was loss of a hydrogen atom from the ethylene, and that hydrogenation was independent of exchange and proceeded through the simultaneous addition of a pair of hydrogen atoms. and with higher olefines,6 and of the migration of the double bond in butene-1 which occurs simultaneously 7 with exchange, has provided a great deal of evidence that the exchange reaction did in fact proceed by the " associ- ative " mechanism of Horiuti and Polanyi.The difference in order of reaction between exchange and hydrogenation predicted by Horiuti and Polanyi did not, however, materialize, and the fact that both reactions were of the same kinetic order, and yet had different energies of activation, led Twigg and Rideal4 to suggest that in both reactions the rate-determin- ing step was reaction of a van der Waals' adsorbed hydrogen molecule with a chemisorbed ethylene molecule t o produce on the one hand an ad- sorbed ethyl radical and an adsorbed hydrogen atom (exchange), and on the other hand a non-adsorbed ethane molecule (hydrogenation). Farkas a pointed out that in many cases cis-addition of hydrogen took place, even though the product was not the more stable.Greenhalgh and P ~ l a n y i , ~ however, showed that this was not proof of the simultaneous addition of a pair of hydrogen atoms, since the attachment of the ethylene molecule to the catalyst prior to the formation of the half-hydrogenated state automatically means that addition of single atoms would occur in the cis-position. It was realized that it would be possible to distinguish between these two mechanisms by a direct experiment in which ethylene was hydrogen- ated with a mixture of H, and D,. The addition of single atoms would produce ethane of composition whereas the simultaneous addition of a pair of atoms would produce a mixture of A fuller investigation of the exchange reacticn with ethylene 4 7 $CH3-CH3 + $CH,--CH,D + $CH,D--CH,D, BCH3-CHa + 6CH2D-CH2D.These experiments were begun in 1938, but the analytical technique -infra-red spectroscopy-was not then sufficiently developed to dis- tinguish easily these two mixtures. The development of automatically recording spectrometers of good resolution has enabled this experiment t o be made. Experimental Hydrogenation must be carried out with a minimum of exchange, which means as low a temperature as possible. A supported catalyst was therefore used made by impregnating Kieselguhr pellets with nickel nitrate, igniting and i hen reducing in hydrogen a t 300' C.The rest of the apparatus used was similar to that described previously.* With 7 g. of this catalyst in a reaction vessel of 400 ml. volume, the half life of a typical hydrogenation was about 2 hr. at Cylinder ethylene was given three single-plate distillations to remove high and low boiling impurities. C,H, was prepared by reacting C,H, + H, at -78" C for 15 hr. C2H4D2 was prepared by reacting C2H4 + D, at -78" C. The relative importance of exchange with respect to hydrogenation declines as the temperature is lowered, and at -37" C on a similar catalyst it was shown that after the addition of deuterium to a sample of ethylene only 2 yo of the hydrogen of the ethylene had been exchanged. The C2H4D, obtained should thus have been of over 95 yo purity.- 78" c. Farkas and Farkas, J . Amer. Chem. SOC., 1938, 60, 22. * Twigg and Rideal, Proc. Roy. Soc. A , 1939, 171, 55. Conn and Twigg, ibid., 1939, 171, 70. Twigg, Trans. Faruday SOC., 1939, 35, 934. Twigg, PYOC. Roy. SOC. A , 1941, 178, 106. Farkas, Trans. Faraduy Soc.. 1939, 35, 906. Greenhalgh and Polanyi, ibid., 1939, 35, 520.I 5 1 HYDROGENATION O F ETHYLENE Two experiments were carried out : EXPT. 11. C2H4 + H, + D,. The catalyst was treated with H, at -78" C for 6 hr. and the H, pumped off. To prevent the reaction H, + D, + zHD occurring, the ethylene (107 mm.) was admitted first to the reaction vessel, and an additional 120 mm. of a 50-50 mixture of H, + D, added. Reaction was allowed t o proceed for 18 hr., and the ethane produced was separated from residual hydrogen by passage through a trap cooled in liquid N,.This experiment was carried out t o obtain a product similar to that anticipated if addition proceeds atom by atom. A 50-50 mixture of H, + D, (120 mm.) was admitted to the catalyst at -78" C after the previous experiment. The reaction vessel was allowed t o reach room temperature and kept there for 6 hr. before cooling to -78°C. Ethylene (107 mm.) was added and reaction allowed to proceed ior 16 hr. The product was separated as before. EXPT. 12. C,H, + H, + HD + D,. Analysis .-Infra-red spectra were recorded on a Perkin-Elmer Spectro- meter Model IZB, at medium resolving power in the region 650-4000 cm.-l and a t the highest resolving power over the range 670-890 and 1027-1450 cm.-l.The cell length was 10 cm., and the pressure 700 &- 5 mm. Fig. I and z are photographs of relevant parts of the records obtained for (a) a mixture of 50 yo C,HG + 50 yo C,H,D,, ( b ) products of Expt. 11, and (c) products of Expt. 12. i ;v frequency €xperimenl~~ FIG. I . - I O Z ~ - I ~ ~ O cm.-1. (a) 50 % CH3-CH3+50 % CH,D-CH,D ; (b) from Expt. 11 ; (c) from Expt. 12. Infra-red spectra of deuterated ethanes, Note: The arrows mark the same wavelength on the other two spectra. It is very clear that the ethanes from Expt. 11 and 12 are identical and different from the mixture of C,H, and C,H4D,. The identity was complete over the whole spectral range. In order t o check that the reaction H, + D, zHD was inhibited during the hydrogenation, the ortho-para hydrogen conversion was examined.Ethylene (48.9 mm.) was admitted first to the reaction vessel at - 78" C, and f i - H, (75.5 mm.) added. Samples were removed at various times, ethylene and ethane separated, and the hydrogen analyzed in a coiled-coil type microthermal con- ductivity gauge lo immersed in liquid N,. The results are shown in the table. No residual ethylene was observed (< 0.1 yo). Bolland and Melville, Trans. Faraday Soc., 1937, 33, 1316.G. H. TWIGG I 5 5 Time (min.) Residual Ethylene yo 73 60 38.5 5'8 0'0 Conversion Y O _- I 2 5 I3 92 The half-life of the conversion in the absence of ethylene was 13 min. FIG. 2.-670-890 cm. -'. Infra-red spectra of deuterated ethanes, (a) 50 % CH3-CH,+5o % CH,D-CH,D ; (b) from Expt. 11 ; (c) from Espt. 12. Note: The arrows mark the same wave- lengths on the other two spectra.Discussion The experiments show unequivocally that addition of hydrogen to the double bond does not take place in a single act, but that the hydrogen molecule is first split into atoms which then add one at a time. The previous work has shown that exchange proceeds through attach- ment of ethylene to the catalyst at two points and the formation of the half-hydrogenated state. In considering the mechanism of hydrogen- ation, the following salient facts will be taken into account : (a) The order of reaction is identical in exchange and hydrogenation, the rate being given by pEZH, x pE2 from - 78" C to + 150° C, where n is unity or slightly less.156 HYDROGENATION OF ETHYLENE (b) The energy of activation for exchange is greater than for hydro- genation, (G) The energy of activation for both exchange and hydrogenation decreases with increasing temperature above about 100’ C, although the order of reaction is unchanged and the reaction H, + D, + zHD is still inhibited.(d) The ortho-para conversion and the reaction H, + D, + 2HD are inhibited by ethylene except in so far as they proceed via exchange with the ethylene. (e) The hydrogen returned to the gas phase during exchange between ethylene and deuterium is very largely H, molecules. (f) Hydrogen is dissociated into atoms before hydrogenation. Fact (d) means that the available surface is almost entirely covered with ethylene. The fact that the reactions are of zero and not negative order with respect to ethylene then means11 either that the two gases do not compete for the same surface or that adsorption equilibrium by the Langmuir mechanism, is not achieved.The previous mechanism pro- posed for hydrogenation,4 and that of Beeck,’, take the former alternative. Beeck’s mechanism, that gaseous ethylene reacts with adsorbed hydrogen, does, with certain assumptions, fit the kinetics, but is unsatisfactory in that it does not explain the exchange reaction, and particularly the close connection between the tMJo reactions. We are left, therefore, with the second alternative, that adsorption equilibrium is not maintained, and it is now proposed that up to 1 5 0 O C the reaction H, -+ 2Ki-H is of no significance because the free surface available is too small ; in other words, the adsorption of the ethylene is much faster than that of the hydrogen on the available bare surface.Also, in accordance with (e) above, the interchange of ethylene between the gas and the catalvst is much faster than that of the hydrogen. The present experiments have shown that hydrogen therefore adsorbed, before hydrogenation, and proposed : (1) - C2H4 *- CH,--CH, (2) I I Ni Ni 4 H, CH3 ( 3 ) 1 7) must be dissociated, and the following scheme is ~ . I CH,-CH, + CH, H ---+ CH3-CH3 Li hl IT. Ni Ni (1) I I 4 93 9, Ethylene is adsorbed by opening of the double bond. The adsorption of the hydrogen takes place by reaction of a van der Waals’ adsorbed molecule with an adsorbed ethylene molecule. Since exchange and hydrogenation have different energies of activation, this step (3) cannot be the rate determining one.Since both reactions have identical kinetics, the rate-determining step must proceed from the same adsorbed state, and therefore reaction (4) controls exchange while reaction (7) controls hydrogenation. Because the hydrogen returned to the gas phase during exchange between ethylene and deuterium is largely H,, and reactions (3) and (4) would only give HD, we must add another and faster reaction CH3 (5) I CH,-CH, H kH, Ni Ni Ni I (6) i Ni I 1 l1 Twigg, l2 Beeck, this Discussion (comment on Prof. Laidler’s paper). Rev. M o d . Physics, 1945, 17, 61 ; this Discussion.G. H. TWIGG I 5 7 These steps can be shown to explain all the features of the exchange and hydrogenation reactions. If the fractions of the surface covered by the different species are as indicated, steady-state equations can be set up, the solutions to which give The expression for O1 is more complex, but is in agreement with the fact that 8, is close to unity for the temperature range considered. Then : Rate of hydrogenation Rate of exchange These exmessions show both reactions = k 4 8 2 8 3 , to be of the same kinetic order.i.e., rateLproportional to the first power of the hydrogen pressure, and independent of the ethylene przssure (since 8, = I). The term pH2 should strictly be replaced by the concentration of hydrogen in the van der Waals' layer, so that at low temperatures the order of reaction is reduced slightly below unity, The ratio, excha.nge/hydrogenation = k 4 / k 7 and therefore the differ- ence in energies of activation of the two reactions (E4 - E7) should be constant over the entire temperature range, as was found.4 The above expressions for the rates of reaction now provide an ex- planation of the long-known fact that the energy of activation for hydro- genation,13~ 14 and for exchange, decreases with increasing temperature above ca.goo C, and even becomes negative. Previous explanations I 4 have attributed the phenomenon to desorption of the ethylene, but it has been shown that this does not occur until temperatures above 150' C. There are two extreme cases : Hydrogenation faster than exchange, i.e. (a) Low TEMPERATURES. k7 > k4. Rate of hydrogenation = k3pH281, EH = E 3 ; Ex = E3 + E4 - E7. - P E 2 8 1 , k 3 k 4 Rate of exchange - k7 -- (b) HIGH TEMPERATURES : Exchange faster than hydrogenation, i.e.k4 > k , . Rate of hydrogenation = -. pH2e,, E , = E, + E, - E , ; k4 Rate of exchange = k 3 p & e 1 , = E 3 * On proceeding from low to high temperatures, both energies will decline by the term E, - E,. 13 Rideal, J . Cham. Soc., 1922, 121, 309. Tucholski and Rideal, ibid., 1935, Maxted and Moon, ibid., 1935, 1190. l4 Schwab, 2. physik. Chenz. A , 1934, 171, 421. zur Strassen, ibid., 1934, 1701. 169, 81.HYDROGENATION OF ETHYLENE Taking the value previously determined 6 for Ex - EH = 9.0 kcal. = E, - E,, and taking E , = 11 kcal., and noting that rate of exchange = rate of hydrogenation at goo C, a curve was constructed of log rate of hydrogen- ation against I/T. It showed the typical behaviour associated with this reaction and the apparent energies of activation at different temperatures were as tabulated. 10-4 9.8 7'3 3'4 2'5 At temperatures above about 15ooC, it has been shown that de- sorption of ethylene begins to set in, and the apparent energy of activation will decline further, and become negative.There is, however, a partial compensation as the uncovering of the surface now permits direct entry of the hydrogen (H, --f zNi-H). Numerical values for the individual energies of activation can be estimated. Making the not unreasonable assumption that the heats evolved (x) in the following two reactions are equal : CH, I CH, - CH, + H, -+ CH, Ni Ni H + x kcal. I I I I FH3 Ni Ni I CH, + H, --+ H + C,H, + x kcal. Ni Ni I I and taking the heats of adsorption for hydrogen (17 kcal.) and ethylene (36 kcal.) calculated by Eley,15 one finds x = E, - E , = 7 kcal., and E , - E, = 10 kcal. Using Beeck's values for heats of adsorption on covered surfaces, x = 11 kcal. and E, - E, = I kcal. Assuming E , = 11 kcal. from low-temperature hydrogenation, E , = 18 kcal. (Eley) or zz kcal. (Beeck), and E , = 9 kcal. (Eley) or 13 kcal. (Beeck). It should be possible to estimate E, and E, from data on double bond migration, since this reaction is mainly effected by the faster processes (5) and (6) above. On this scheme, Rate of double bond migration = k,8, However, experiments showed that all three reactions were of the same order (Pi, . ptutene) and that the olefine did not completely cover the surface.16 The above equations cannot therefore be applied, since it is probable that direct adsorption of the hydrogen was taking place. 15 Eley, this Discussion. l6 Twigg and Rideal, Trans. Faraday SOL, 1940, 36, 533.G. H. TWIGG I59 I am pleased to record my indebtedness to Messrs. A. R. Philpotts and W. Thain who carried out the infra-red spectroscopic analyses. My thanks are also due to Mr. R. D. Richards for assistance with the experi- ments, and to the Directors of the Distillers’ Company Limited for per- mission to publish this work. The Distillers’ Company Limited, Research and Development Department, Great Burgh, Epsorn, Surrey.

ISSN:0366-9033    

DOI:10.1039/DF9500800152

出版商:RSC    

年代:1950

数据来源: RSC