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