THE ADSORPTION OF HYDROGEN BY NICKEL POISONED WITH CARBON MONOXIDE* BY T. A. WHITE AND ARTHUR F. BENTON
Introduction
It has been recognized for some years that the reactions which occur a t the surface of a solid catalyst are intimately connected with the nature and extent of the adsorptions of the reacting substances and products by the catalyst. While all gaseous reactions involve two or more gases, adsorption studies have largely been concerned with one gas a t a time. I n order to have a complete picture of the process occurring a t the solid surface, all the gases involved should be considered simultaneously. This has been done to some extent with catalytically inactive materials, and to a lesser extent with active catalysts. One of the most interesting facts concerning the catalytic activity of a substance is the inhibiting action which a small amount of a foreign gas can have on the activity of the catalyst. The inhibitory action of carbon monoxide on various metallic catalysts’ has been known for a long time. Pease and Stewart2 have studied the poisoning effect of carbon monoxide on the hydrogen-ethylene reaction with a copper catalyst. I n studying the effect of small amounts of carbon monoxide on the adsorption of hydrogen and ethylene by copper, Griffin3 found that adsorptions were increased a t lower pressures and decreased at higher pressures. Since nickel is a very active hydrogenation catalyst, it appeared to us that it would throw considerable light on the adsorption problem if the effect of carbon monoxide on the hydrogen adsorption of nickel was studied. It is known that carbon monoxide is a marked poison to a nickel ~ a t a l y s t . By ~ making such a study we hoped, also, to see if the effect Griffin observed was peculiar to copper or if it was a property of all catalysts. Experimental Method and Apparatus The initial resuits were obtained with the usual type of apparatus, which has been described by Pease;j a new apparatus, in which stop-cocks are largely eliminated, was employed in obtaining the later results. The details of apparatus, procedure, materials, constant temperature baths and free space determinations have previously been described.6 * Contribution from the Cobb Chemical Laboratory, University of Virginia, S o . 70 2
4 5
Henry: Phil. Mag., 65, 269 (1825). Pease and Stewart: J. Am. Chem. Soc., 47, 1235 (1925). Griffin: J. Am. Chem. SOC., 49, 2136 (1927). Green: “Indust,rid Catalysis,” p. 305, Pease: J. Am. Chem. Soc., 45, I 197 (1923). Benton and White: J. Am. Chem. SOC.,52, 2325 (1930).
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The small amounts of carbon monoxide that were used were always placed in the adsorption bulb, which was a t the required temperature, half an hour before the regular run was started. Between runs the adsorbent was evacuated for about two hours, a t 200' in the case of the first adsorbent, and a t 250' for the second of the two used. The first adsorbent, which was the same as the first adsorbent already used by US,^ weighed 23.68 g., and the second weighed 23.03 g. Both were prepared from the nitrate as previously described. Helium, assumed to be unadsorbed, was used as a reference gas. Carbon monoxide was prepared by the reaction of formic acid with sulphuric acid and purified by passage over soda-lime and phosphorus pentoxide.
0 Run 23
8un 2f A Run 2 5 x Run 2 6
Q
0
/OO
PO0
300
400
500
600
700
FIG.I Adsorption of hydrogen by nickel poisoned with carhon monoxide a t 0'. Run 23, hydrogen with no CO; Run 24,hydrogen with 0.038 cc. CO; Run 2j, hydrogen with no CO; Run 26, hydrogen with 0.038 cc. CO.
The small amounts of carbon monoxide were measured in two ways, both of which were essentially the same. I n the earlier apparatus, it was possible to measure volumes as small as 0.038 C.C.by knowing the volume of the bore of a given stop-cock and filling it with carbon monoxide a t a known pressure and temperature. I n the later form of apparatus, the volumes of carbon monoxide were measured in the calibrated capillary tube,6 F, at a known pressure and temperature.
Expermental Results Fig. I shows a series of isotherms for Sample I . While the same effect was obtained with other samples of nickel, none of them gave such a marked increase in adsorption, all along the isotherm, when carbon monoxide was used. This increase could only be obtained with a fresh sample of nickel which had not previously been exposed to carbon monoxide. It was impossible to be sure that the carbon monoxide was pumped off after a given run. For example, Run z j checked Run 2 3 , both of which were made with pure hydrogen; and Run 26 checked Run 2 4 , in each of which 0.038 C.C.of carbon monoxide had been taken up by the adsorbent. But Run 27, which
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T. A. WHITE AND ARTHUR F. BENTON
was supposed to be a pure hydrogen run, checked Runs 24 and 26, indicating that the carbon monoxide might have remained adsorbed in spite of the evacuation following Run 26. This same type of thing was encountered with Sample z ; an increase was obtained at first, but could not be checked in subsequent runs. Larger amounts of carbon monoxide, close to a tenth of a cubic centimeter, produce no marked effect on the hydrogen adsorption. The isotherms of pure hydrogen and those with approximately 0.1 C.C. of carbon monoxide present lie so close to each other that it is doubtful if the differences exceed the experimental error. However, it may be that there is a slight increase in the hydrogen adsorption all along the isotherm when this quantity of carbon monoxide is used.
FIG.2 Adsorption of hydrogen by nickel poisoned -71th carbon monoxide at 0'. Rim 8, hydrogen with 0.3 cc. CO; Run g, hydrogen uith 0.8 cc. CO; Run IO, hydrogen u i t h 1.6 cc. CO.
The isotherms given in Fig. z shorn the effect of larger amounts of carbon monoxide on the hydrogen adsorptions of Sample 2 . Run 9 agrees very closely with the pure hydrogen isotherm. Runs 1-7 were made with small amounts of carbon monoxide, and checked satisfactorily the results obtained with Sample I . It will be noticed that larger amounts of carbon monoxide produce no striking changes in the type of isotherm or in the amounts adsorbed. However, the rate a t which equilibrium is reached in these last three cases is slover the greater the amount of carbon monoxide employed. These experiments were extended to lower temperatures, at - 183', but no quantitative results could be obtained. While each isotherm determined at that temperature was probably as accurate as those obtained a t oo, it was impossible to obtain satisfactory checks. On the other hand, it may be stated qualitatively that the hydrogen adsorptions, in the presence of carbon monoxide a t -183', are decreased by an amount which is approximately equal to the amount of carbon monoxide already present. Due to the fact that carbon monoxide reacts with nickel, a complete isotherm was never made with that gas at 0'. The amount of carbon monox-
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ide adsorbed a t - 183' is far in excess of the amount of hydrogen adsorbed a t that temperature. Some qualitative experiments indicate that the amount of carbon monoxide adsorbed a t oo is a t least equal to, and probably greater than, the amount of hydrogen adsorbed a t the same temperature and pressure.
Discussion of Results
It will be noticed at once that the results differ considerably from those obtained with copper by Griffin.3 On the other hand, a t least one point is common to both copper and nickel; namely, a very small amount of carbon monoxide increases the hydrogen adsorption. In the case of copper the increase is noted only at low pressures, for the poisoned isotherm crosses the pure hydrogen isotherm a t relatively low pressures; but with nickel the increase can be observed a t all pressures up to one at,mosphere, when very small amounts of carbon monoxide are employed. Since it takes only a very small amount of carbon monoxide to cause the increased adsorption, one would hardly expect that such a few molecules could of themselves attract and hold so many more hydrogen molecules. In explaining this increased adsorption, then, it must be assumed that the carbon monoxide changes the surface activity of the adsorbent in some way so as to make more areas available for hydrogen adsorption. There are three ways by which this may be accomplished. Either the carbon monoxide can increase the activity of the already most active areas, or it can stimulate those areas which do not function until higher pressures, or it can create entirely new areas. I n view of t'he fact that only small amounts of carbon monoxide give this effect, the first of the above three ways seems most reasonable to us, because the most active areas have the carbon monoxide adsorbed on them. Therefore, it may be assumed that the increased adsorption is due t o an enlargement of active areas, which are produced by having very small amounts of carbon monoxide adsorbed on them. Griffin explained the decrease of hydrogen adsorption a t higher pressures on copper, poisoned with carbon monoxide, by assuming that the carbon monoxide prevented the solution of hydrogen in copper. No decrease in hydrogen adsorption with nickel was observed. However, since the isotherms shown in Fig. I draw together a t higher pressures, it seems reasonable to assume that a t some pressure greater than atmospheric pressure the two isotherms may cross. This drawing together from 1.0 C.C. a t 40 mm. pressure to 0.8 C.C. at 7 0 0 mm. pressure could be explained by assuming a slight amount, 0.2 c.c., of solubility of hydrogen in nickel. I n view of the results of Sieverts' this amount does not seem unreasonable. With the exception of the rate with which equilibrium is reached, it would be difficult to distinguish between the isotherms obtained when larger amounts of carbon monoxide are employed from a pure hydrogen isotherm. Since there seems to be practically no solubility of hydrogen, and in view of the same hydrogen adsorption with or without carbon monoxide present, we must 7
Sieverts: Z. physik. Chem. 60, 170 (1907).
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assume that either the carbon monoxide increases the hydrogen adsorbing areas by the amount of carbon monoxide present in each case, or the hydrogen is adsorbed on top of the carbon monoxide as a second layer. I n view of the fact that amounts of carbon monoxide in the neighborhood of 0.1C.C. produce only a very slight increase, if any, in adsorption, we believe that, when larger amounts of carbon monoxide are used, the hydrogen is adsorbed either directly as a second layer, or indirectly through the carbon monoxide as some sort of a surface complex. One might expect this second layer to be taken up slowly, and hence anticipate the much slower, observed adsorption. I n conclusion, then, the inhibiting action of carbon monoxide on the hydrogen adsorption of nickel may be pictured in some such manner as this. Extremely small amounts of carbon monoxide produce a slight increase in the hydrogen adsorption; slightly larger amounts, in the neighborhood of 0.04 C.C. , produce a maximum increase in the hydrogen adsorption; larger amounts, up to 1.0 c.c, produce decreasingly smaller increases in the adsorption; and still larger amounts of carbon monoxide produce decreases in the hydrogen adsorption. The inhibiting action of carbon monoxide towards the adsorption of hydrogen by nickel might be likened to that group of drugs which have an optimum dose for stimulating body activity, but which produce death in larger amounts. It should be stated, however, that the catalytic activity of a reduced metal does not follow such a course. It has frequently been pointed out that the relative amounts of gas adsorbed a t ordinary pressures by different solids can not be used as a measure of catalytic activity. It has been assumed that only the gas adsorbed at very low pressures is sufficiently activated for catalytic reactions to occur. This hypothesis implies that the more firmly the gas is held, the greater is the degree of activation produced thereby. From the present work and the similar studies of Griffin on copper it might have been expected that carbon monoxide, the presence of which, in very small amount, increases the adsorption of hydrogen a t low pressures, would act as a promoter of the activity of the metal, rather than as a poison. Since the reverse is the case, it seems necessary to conclude that the hydrogen, though more strongly adsorbed in the presence of carbon monoxide, is less activated. This must mean that the valence of the hydrogen is largely saturated by the formation of a rather stable surface complex with the carbon monoxide. The poison acts, not by preventing the adsorption of hydrogen on the active areas of the catalyst, but rather by forming a surface compound with those hydrogen molecules which would otherwise have been activated. Depending on the stability of this compound under the given conditions, the action of the poison is more or less pronounced. It should be noted, however, that while the above situation may be fairly general, i t can not be universal, since Pease8 has shown that the adsorption of hydrogen by copper is markedly decreased by poisoning with a small quantity of mercury. a
Pease: J. Am. Chem. SOC., 45,
2296 (1923).
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In absence of additional data it would be idle to speculate on the relation between the observations here reported and the peculiarities of copper and nickel as catalysts for the various possible reactions between hydrogen and carbon monoxide. Summary A study of the effect of both small and moderately large amounts of carbon monoxide on the hydrogen adsorptions of two samples of reduced nickel has been made a t oo and a t -183'. At o', 0.038 C.C. of carbon monoxide increases the hydrogen adsorption a t all pressures up to one atmosphere; amounts in the neighborhoodof 0.1C.C. seem to produce only a very slight increase, if any, in the adsorption, and amounts from 0.3 C.C. to 1.6 C.C. have scarcely any effect on the amount of adsorption but decrease the rate of adsorption. The qualitative result at - 183' is that the amount of hydrogen adsorption, a t all pressures up to one atmosphere, is decreased by an amount approximately equal to the amount of carbon monoxide employed. An effort is made to reconcile these results with the known poisoning action of carbon monoxide on the catalytic activity of nickel in hydrogenation reactions. C m c e r s i t y , Vtrganaa.
