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Mlkovsky, R. J.; Welsz, P. B. J. Catal. 1962, 7 , 345. Mlkunov, B. I.; Yakerson, V. I.; Lafer. L. I.; Rublnshteln, A. M. Izv. Akad. Nauk SSSR, Ser. Khim. 1973, 449. Ngishl. H.; Sasakl, M.; Iwaki, T.; Hayes, K. F.; Yasunaga, T. J. Phys. Chem. 1984, 88, 5584. Oldfield. E.; Kirkpatrick, R. J. Nature (London) 1985, 227, 1537. Olson, D. H.; Kokotallo, G. T.; Lawton, S. L. J . Phys. Chem. 1981. 85, 2238. Olson, D. H.; Haag, W. 0.; Lago, R. M. J . Catal. 1980, 67,390. Olson, D. H., Mobll Research and Development Corp., private communication, 1980. Pauling, L. “The Nature of the Chemical Bond”; Cornel1 University Press: Ithaca, NY, 1945. Shihabi, D. S.;Garwood, W. E.; Chu, P.; Miale, J. N.; Lago, R . M.; Chu, C. T.-W.; Chang. C. D. J. Catai., In press. Topsoe, N. Y.; Pederson, K.; Derouane, E. G. J. Catal. 1981, 7 0 , 41.
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Received f o r review June 24, 1985 Revised manuscript received October 24, 1985 Accepted November 8, 1985
Model Studies of Cu/Ru Bimetallic Catalysts Charles H. F. Peden and D. Wayne Goodman’ Surface Science Division, Sandla Natlonal Laboratories, Albuquerque, New Mexico 87 785
The activity of a model Ru(0001) catalyst for the methanation and ethane hydrogenolysis reactions has been measured in a high pressureultrahigh vacuum surface analysis apparatus as a function of impurity (sulfur or copper) coverage. Unlike sulfur, which was found to poison the activity at quite low coverages, the role of copper was to simply block active ruthenium sites on a one-to-one basis. This latter result is in contrast to results obtained on supported Cu/Ru catalysts which show a marked reduction in activity upon addition of Cu. The discrepancy is attributed to an error in counting the active Ru surface area on the supported catalysts by selective hydrogen chemisorption techniques. I t is demonstrated here that hydrogen, once dissociated on Ru, can spill over onto Cu, making an overcount of Ru surface atoms possible.
Introduction The modification of catalytic behavior by surface impurities is of crucial importance to most catalytic processing. At present, the mechanisms responsible for surface chemical changes induced by surface additives are poorly understood. However, the current interest and activity in this area of research promise an emerging understanding of the fundamentals by which impurities alter surface chemistry. A pivotal question concerns the relative importance of steric (local) vs. electronic (extended or “long-range”) effects. A general answer to this question will critically influence the degree to which we will ultimately be able to tailor-make exceptionally efficient catalysts by fine-tuning material electronic structure. If, indeed, low surface impurity concentrations can profoundly alter the surface electronic structure and thus catalytic activity, then the possibilities for the systematic manipulation of these properties via additive selection would appear limitless. On the other hand, if steric effects dominate the mode by which surface additives alter the catalytic chemistry, a different set of considerations for catalyst alteration come into play, a set which will most certainly be more constraining than the former. In the final analysis, an understanding will include components of “electronic” and “steric” effects, the relative importance to be assessed for each reaction and its conditions. A major emphasis of our research has been in the area of addressing and partitioning the importance of these two effects in the role of surface additives in catalysis. The modification of catalytic performance by surface additives is an extremely difficult question to address experimentally (Imelik et al., 1982). For example, the interpretation of related data on dispersed catalysts is severely limited by the uncertainty concerning the structural and compositional characteristics of the active sur0196-43131861 1025-0058$01 .SO10
face. Specific surface areas cannot always be determined with adequate precision. In addition, a knowledge of the crystal orientation, the concentration and the distribution of impurity atoms, and their electronic states is generally poor. The surface concentration of the impurity may vary considerably along the catalytic bed, and the impurity may very well reside on the suport as well as the metal. Moreover, the active surface may be altered in an uncontrolled manner as a result of sintering or faceting during the reaction itself. The use of metal single crystals in catalytic reaction studies essentially eliminates the difficulties mentioned above and, to a large extent, allows the utilization of a homogeneous surface amenable to study using modern surface analytical techniques. These techniques allow detailed surface characterization regarding surface structure and composition. Carefully controlled, single-crystal catalytic surfaces are particularly suited to the study of impurity effects on catalytic behavior because of the ease with which impurity atoms can be uniformly introduced onto the surface, in many cases, with the knowledge of the impurity atom position. To date, relatively few studies have incorporated both surface science techniques with kinetics at elevated pressures (1atm). Kinetics, however, are an essential link between these kinds of model catalytic studies and the more relevant catalytic systems, establishing the crucial connection between the reaction rate parameters. Although the studies to date are few, the results appear quite promising in addressing the fundamental aspects of catalytic modification by surface additives. Impurities whose electronegativities are greater than those for transition metals generally poison a variety of catalytic reactions, particularly those involving Hzand CO. Considerable work has been invested in defining the 0 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 50
mechanism by which this category of impurities alters the chemisorptive and catalytic properties of several transition-metal surfaces. The effect of preadsorbed electronegative atoms, C1, S, and P, on the adsorption-desorption of CO and H2 on Ni(100) has been extensively studied (Kiskinova and Goodman, 1981a) by using thermally programmed desorption (TPD), low-energy electron diffraction (LEED), and Auger Spectroscopy (AES). It has been found that the presence of electronegative atoms causes a reduction of the adsorption rate, the adsorption bond strength, and the capacity of the Ni(100) surface for CO and H2adsorption. The poisoning effect becomes more prominent with increasing electronegativityof the preadsorbed atoms. Adsorption of C1, S, and P on nickel causes a reduction in CO adsorption and a shift of the CO TPD peak maxima to a lower temperature. The effects of P, however, are much less pronounced than for C1 or S. Similarly, C1, S, and P also cause a reduction of hydrogen adsorption and a shift of the H2TPD peak maxima to a lower temperature (Kiskinova and Goodman, 1981a). The extent of this effect increases in the sequence P, S, C1. The similarity in the atomic radii of C1, S, and P (0.99, 1.04, and 1.10 A, respectively) suggests a relationship between electronegativity and the poisoning of chemisorptive properties by these surface impurities. Related studies (Kiskinova and Goodman, 1981b) have been carried out in the presence of C and N. These impurities have the same electronegativities as S and C1: 2.5 and 3.0, respectively. The comparison between the results for C and N and those for S and C1 is entirely consistent with the interpretation that electronegativity effects dominate poisoning of chemisorption by surface impurities with close atomic size, occupying the same a>mption sites. In the case of adsorbed impurities with the same electronegativity but with different atomic radii (S and C, C1 and N), the effect becomes less pronounced with decreasing atomic radius. Particularly noteworthy in the above studies is the general observation that those impurities strongly electronegative with respect to nickel, C1, N, and S, modify the chemisorptive behavior far more strongly than would result from a simple siteblocking model. The initial effects of these impurities suggest that a single impurity successfully poisons as many as ten nickel atoms, a result supporting an interaction that is long range and electronic in nature. However, such an interpretation has been questioned (Madix et al., 1983). Kinetic studies have been carried out for several reactions as a function of surface coverage over single crystals of nickel (Goodman and Kiskinova, 1981; Peebles et al., 1983; Goodman, 1984a; Goodman, 1984b), rhodium (Goodman, 1984c), and ruthenium (Peden and Goodman, 1985). A result common to these studies is that sulfur effectively poisons catalytic activity a t coverages less than 10% of the surface metal concentration (or 0.1 monolayer). The major conclusion of the above studies is that electronic effects, rather than ensemble requirements, dominate the catalytic poisoning mechanism. Recent studies (Goodman, 1984c; Peden and Goodman, 1985; Yates et al., 1985; Goodman and Peden, 1985; Goodman et al., 1985) have addressed more directly the component of surface metal ensembles in the attenuation of reaction rates by impurities such as sulfur. Most extensively studies is the Cu/Ru bimetallic system which is the subject of this discussion.
Experimental Section The studies to be discussed were carried out utilizing the specialized apparatus which has been discussed in
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Figure 1. Arrhenius plot of the rate of methanation over a clean and sulfided Ru(0001) catalyst. Sulfur coverages (0,) are expressed as fractional monolayers.
detail elsewhere (Goodman et al., 1980; Goodman, 1984d). This device consists of two distinct regions, a surface analysis chamber and a microcatalytic reactor. The custom-built reactor, contiguous to the surface analysis chamber, employs a retraction bellows that supports the metal crystal and allows translation of the catalyst in vacuo from the reactor to the surface analysis region. Both regions are of ultrahigh-vacuum construction, bakeable, and capable of ultimate pressures of less than 10-lotorr. The catalyst samples are mounted on tungsten leads and heated resistively. The reactor is operated in a batch mode with sampling subsequent to reaction into an on-line gas chromatograph. Analysis is via a flame ionization detector.
Results and Discussion Figure 1is an Arrhenius plot of the methanation reaction rate over a Ru(0001) catalyst precovered with various quantities of sulfur. Three things are noteworthy in this graph. First, small amounts of sulfur (
