dispersions of 26 to 58 percent. The catalysts with platinum dispersions of 46 and 58 percent exhibit pronounced surface peaks at 1.089 kG/MHz. The ratio of the area of the surface peak to the total area of the NMR line should be equal to the ratio of surface atoms to total atoms in the platinum clusters. In general, there was fair agreement between the value of this ratio obtained from the NMR data and the value obtained from hydrogen chemisorption data. In the determination of the area of the surface peak in cases where only the low-field side of the peak was well resolved from the line, the high-field side was drawn by assuming the peak was symmetric at about 1.089 kG/MHz.

When the adsorbed species are removed from the catalysts by a cleanup procedure involving alternate treatments with flowing hydrogen and oxygen at 573 K, followed by evacuation, the surface peak disappears from the NMR spectrum. The surface platinum atoms are then metallic, since their conduction electrons are no longer tied up in chemisorption bonds. The observation of the resonance for surface platinum atoms is therefore dependent on the presence of adsorbed species with which they form chemical bonds.

BIMETALLIC CATALYSTS

Bimetallic catalysts have played an important role in heterogeneous catalysis. They have been used extensively for fundamental investigations and have had a major technological impact, especially in the petroleum industry.23

Metal Alloys as Catalysts

A complicating feature in catalytic studies on metal alloys is the possibility of a difference between surface and bulk compositions. Evidence for such a difference in the case of nickel-copper (Ni-Cu) alloys is based on the observation that strong H2 chemisorption does not occur on copper. The addition of only a few percent of Cu to Ni decreases the amount of strongly chemisorbed H2 severalfold (Figure 8), an indication that the concentration of Cu in the surface is much greater than in the bulk.24 Similar results have been obtained by several different groups of investigators,2426 and the findings are consistent with the results of studies of surface composition by Auger electron spectroscopy.27 An important factor in determining surface composition is the nature of the gas in contact with the surface of an alloy. Thus, for nickel-gold (Ni-Au) alloys, Au concentrates in the surface in an inert atmosphere, whereas Ni is the predominant surface component in the presence of O2.28 If the interaction of a gas with one of the components is sufficiently strong and selective, the surface tends to become enriched in that particular component.

The emphasis in early studies on alloy catalysis was on the activity for a



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