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,