STRUCTURE AND CATALYTIC ACTIVITY OF ALUMINA SUPPORTED PLATINUM COBALT BIMETALLIC CATALYSTS .3. EFFECT OF TREATMENT ON THE INTERFACE LAYER

The structure of the interface between metal and alumina in a series of Pt1-xCox/Al2O3 bimetallic samples has been investigated by X-ray photoelectron spectroscopy (XPS) after calcination in oxygen at 770 K with subsequent in situ reduction in hydrogen at 770 K. On all calcined samples platinum was found in the Pt(4+) valence state as an oxide species. Subsequent treatment in H2 at 770 K resulted only in a partial reduction of ionic platinum into the zerovalent state. The remaining platinum found in the Pt(2+) form is strong evidence of metal-support interaction affecting the platinum-alumina system. After calcination Co3O4 and the cobalt surface phase (CSP), depending upon the cobalt content of the samples, were found to be the predominant species. Starting from a pure cobalt sample (x(Co) = 1.0), the initial decrease in dispersion of the oxidized cobalt phases is interpreted by the effect of platinum (or chlorine). On these bimetallic samples, the dispersion of the Co3O4 phase forming three-dimensional particles was decreased, which also contributes to the enhanced cobalt reducibility. After reduction on the samples with low cobalt content, formation of CoPt3 bimetallic particles was clearly demonstrated. Along with this result, a prediction of the possible catalytic behavior of Pt1-xCox/Al2O3 catalysts of various surface compositions is discussed. The results are compared with those obtained earlier in the case of treatments at 570 K (J. Phys. Chem. 1991, 95, 798).