The adsorption of hydrogen on nickel catalysts. I. The effect of sintering

THE ADSORPTION OF H Y D R O G E N ON NICKEL CATALYSTS. I. THE EFFECT OF SINTERING O. Beeck, A. W. Ritchie and A. Wheeler 1 From the SheU Development Company, EmeryviUe, Calif. Received June 30, 19/~8 INTRODUCTION

Adsorption isobars of hydrogen on nickel have been studied by Benton and White (1), Maxted and Hassid (2), and Iizima (3). In all cases the .catalysts were prepared from nickel oxide by ~eduction with hydrogen, and isobars typical of "activated adsorption" were found. Maximum adsorption was observed in the room temperature region, the slow temperature-dependent adsorption being 2-4 times higher than the initial fast adsorption at liquid air temperature. On supported nickel catalysts Griffin (4) found a somewhat smaller increase. Beeck, Smith and Wheeler (5), in their work on the influence of crystal structure on the rate of ethylene hydrogenation over evaporated metal films, did not observe a large difference in the amount of adsorption of hydrogen on nickel at liquid air and at room temperatures on films which had been deposited at room temperature. Moreover, they found, in contrast to previous studies on reduced oxide catalyst, that the hydrogen adsorption on nickel films was practically instantaneous, both at room and at liquid air temperatures, indicating a very low activation energy. The upper limit of the activation energy was given by the time required to make the first measurement after admitting the gas, and was estimated to be not more than a few hundred cal./mole. The striking discrepancy between the behavior of reduced nickel oxide and evaporated nickel films has found a ready explanation through more recent experiments 2 which are presented in this paper. Briefly, hydrogen adsorption isobars at 0.1 mm. Hg pressure were measured on evaporated nickel films over the temperature range of - 196°C. to 400°C. with the main result that the shape of the isobar can be controlled at will by controlling the degree of sintering of the film. Films produced at 23°C. give fiat isobars with little increased adsorption between - 196° and 23°C., while films sintered at 200-400°C. give isobars similar to those found on Now with E. I. du Pont and Company, Experiment Station, Wilmington, Delaware. 2 Briefly reported before the Am. Assoc. Advancement Sci. Catalysis Conference at Gibson Island, June, 1946. 5O5

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A. W. RITCHIE

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A. WHEELER

nickel catalysts obtained by reduction of the oxide. The results suggest that recent experiments by Taylor and Shou-Chu Liang (6, 7) on the adsorption of hydrogen on zinc oxide may deserve reexamination in the

light 6f these findings. EXPERIMENTAL The experimental technique for producing the evaporated metal films has previously been described in detail (5). The films used in these experiments were evaporated onto a glass surface at room temperature and were subsequently heated to the desired sintering temperature. The isobars reported here were measured on films evaporated in high vacuum, A few experiments were also made with oriented films produced in 1 mm. pressure of nitrogen. For details see reference (5). I~ESULTS

In Fig. 1 are shown isobars at 0.1 ram. pressure for hydrogen on high vacuum-evaporated nickel films for films presintered in vacuum at room temperature, 200°C, and 400°C. Measurements were started at liquid nitrogen temperature ascending to the temperature of sintering and then

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Fro. 1. Sorption isobars of hydrogen at 0.1 ram. H g pressure on evaporated nickel presint~red at various temperatures. A~ Presintered at 23°C.; B: Presintered at

200°C.; C: Presintered at 400°C. (Althoughthis film was presintered in high vacuum at 400°C. for ½hr., additionalsinteringapparently occurred on reheatingin the presence of hydrogen. The dotted line indicatesthe curve to be expected with decreasing temperature if no additional sinteringhad occurred.)

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again descending to liquid nitrogen temperature. Curve A shows the hydrogen isobar for a film sintered at 23°C. Starting at -196°C. the adsorption increases as the temperature is raised, and decreases slightly again at room temperature. When the film is cooled back t o - - 196°C., the adsorption increases again slightly. The effect is much more pronounced in films Sintered at 200°C., as shown in Curve B. InCreasing the temperature from - 1 9 6 ° to 200 ° and decreasing it again to -196°C. shows a final ~/dsorption which is larger by a factor of 3 over the initial adsorption, which was decreased by a factor of 10. The ascending branch of the curve shows a pronounced maximum near 23°C. The points on the ascending branch of the curve are not true equilibrium points. It is, therefore, believed that with longer waiting time this maximum would have occurred at somewhat lower temperature. While, for instance, at -78°C., enough time was allotted to obtain most of the slow adsorption which took place at this temperature, the rate of adsorption became ultimately so slow that it did not seem justifiable to jeopardize the experiment by lengthy waiting. At -196°C. the adsorption is practically instantaneous. On the descending branch of curves, adsorption was found to be fast at all temperatures, and equilibrium was obtained within a few minutes. It is obvious that we are dealing here with two distinctively different types of "sorption." The word sorption is used advisedly, because we shall show in the following that the slow type of sorption is not "adsorption" in the customary sense of the word. There are several compelling reasons why this slow sorption must be regarded as absorption or solution into the interior of the structure. 1. It is seen in Fig. 1 that the difference between the adsorption at 196°C., as obtained initially, and the sorption found at - 196°C. after obtaining the isobars decreases little with temperature of sintering, while the initial adsorption at - 1 9 6 ° decreases rapidly with temperature of sintering, films sintered at 23°C. having about 10 times the adsorption of films sintered at 200°C. and about 175 times the adsorption of films sintered at 400°C. Since the sorption is given in molecules per weight of film, the slow sorption appears to be dependent on the weight of the film rather than on the surface available for fast adsorption. This was also borne out by comparing isobars for metal films evaporated in vacuum and in lmm. of nitrogen. As was reported previously (5), the films produced in 1 mm. of nitrogen were oriented and had a surface for fast adsorption twice as large as the nonoriented high vacuum films for the same film weight. The small slow adsorption, however, was found to be equal for both films, indicating a weight effect rather than a relation to the fast adsorption. 2. It was already shown previuosly (5) that for oriented and unoriented films sintered at higher temperatures the CO adsorption at room -

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O. BEECK, A. W. RITCHIE AND A. WHEELER

temperature decreased proportionally to the activity for hydrogenation of ethylene. More recent experiments in these laboratories have shown that the surfaces of films sintered at various temperatures and measured by the Brunauer-Emmett-Teller method using van der Waals' adsorption of krypton at -196°C. decrease also proportionally to the decrease in activity for hydrogenation of ethylene at room temperature. Thus, b o t h the relative chemisorption of CO at room temperature and the van der WaalS adsorption of krypton are measures for the surface available for hydrogenation of ethylene. The same is true for the fast adsorption of hydrogen at -196°C., so that also the relative hydrogen adsorption at -196°C. is a direct measure of the surface available for the activity of sintered films. In Fig. 2 data for catalytic activity, CO adsorption at 1.0

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Fro. 2. Correlation of catalytic activity with adsorption and the surface area. (The ordinate represents fraction of respective values obtained for films sintered at 23°C.) © Catalytic activity for hydrogenation of ethylene [] Fast CO adsorption at 23°C. V Fast H2 adsorption at -196°C. O Surface by B.E.T. method using krypton at -- 196°C.

23°C., surface area by the B.E.T. method using krypton at -196°C., and the fast hydrogen adsorption at -196°C. are plotted against the temperature at which the various films were sintered. All quantities are taken as unity for films sintered at 23°C. DISCUSSION

The experiments reported indicate that the previously observed slow "adsorption" of hydrogen on nickel is not adsorption but is "sorption"

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of hydrogen into positions inaccessible to CO, Kr, and the fast hydrogen adsorption at -196°C. and also inaccessible to ethylene. While the slow sorption of hydrogen may be due to solution in the crystal lattice, the process is probably more complex since the effect is about 1000 times larger than the reported solution of hydrogen in bulk nickel (8). One may speculate that the effect is due to sorption of hydrogen into the interfaces between the crystallites which form the larger particles of the sintered structure. The hydrogen atoms may be small enough to penetrate into these interfaces, whereas molecules such as ethylene and carbon monoxide are unable to assume these positions. If this latter explanation were right, it would remain-to be explained why the interfaces in t h e agglomerate of crystallites do not increase more rapidly with sintering than the experiments indicate. It is also possible that the reported solubility data may IO.C

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Fro. 3. Sorption isotherms on reduced nickel catalyst plotted from the data of Maxted and Hassid (2). (Catalyst was d e g ~ s e d a t 250°C before making.measurements). H2 adsorption a t 10.3 mm~ when catalyst was contacted with H~ at - 190°C. and then evacuated prior to raising the temperature.

have been vitiated by sorption of the type reported in these experiments, and it is therefore of little value to speculate further whether the phenomenon is one of true solution or of a pseudo-solution into regions of crystal imperfection. The hitherto unexplained discrepancy between the behavior of evaporated metal films and nickel catalysts prepared by reduction from the oxide are satisfactorily explained by the present experiments. The earlier work of Maxted and Hassid (2) had as its main objective the measurement of the slow adsorption of hydrogen. Had Maxted and Hassid plotted their data in the form in which we have plotted them in Fig. 3, it

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o. BEECK, A. W. RITCttIE AND A. WHEELER

may have become evident much earlier that the "activated adsorption" of hydrogen on nickel is actually a sorption of hydrogen into the interior of the structure. It is seen from Fig. 3 that their results show great similarity to those obtained with sintered evaporated nickel films. SUMMARY

1. Measurements of isobars for the adsorption of hydrogen at 0.1 mm. pressure on evaporated nickel films sintered at various temperatures show that the ratio of slowly sorbed hydrogen to rapidly adsorbed hydrogen increases with the sintering temperature of the film. 2. It has been shown that the slow "activated adsorption" of hydrogen on nickel, which has been previously observed by several investigators, is not adsorption but is slow sorption of hydrogen into the interior of the metal structure. 3. Sites which are occupied by hydrogen which is slowly sorbed into the interior of the metal structure are not accessible to chemisorption of carbon monoxide or van der Waals' adsorption of krypton, nor are they accessible to ethylene. 4. The very fast adsorption (chemisorption) of hydrogen at liquid nitrogen temperature is a true measure of the surface available for chemisorption of CO and for van der Waals' adsorption of krypton and for surface reactions such as the hydrogenation of ethylene. 5. The experiments and conclusions by Taylor and Shou-Chu Liang (6, 7) regarding "The Heterogeneity of Catalytic Surfaces for Chemisorption," parts I and II, may deserve reexamination in the light of the present experiments because of the possibility that their observed slow adsorption is also not adsorption but sorption into the crystal structure and thus noncontributing to catalysis. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

BENTON, A. F., AN]) WHITE, T. A., J. Am. Chem. Soc. 52, 2325 (1930). MAXTED, E. B., AN]) HASSID,N. J., J. Chem. Soc. 1932, 1532. IIZIMA,S., Rev. Phys. Chem. Japan 12, 83 (1938). GRIFFIN, C: W., J. Am. Chem. Soc. 61, 270 (1939). BEECK, O., SMITH, A. E., AND WHEELER, A., Proc. Roy. Soc. London 177, 62 (1940). TAYLOR,H. S., AND SHou-CHu LIANG, J. Am. Chem. Soc. 69, 1306 (1947). TAYLOR,H. S., AND SHou-CHu LI•Ne, ibid. 69, 2989 (1947). ARMBRUSTER,M. H., ibid. 65, 1043 (1943).