Correlation between the Pt2+/Pt4+ ratio and the catalytic activity for theCO oxidation of Ba-12[BaxPt3-x]Pt6O27 (0 <= x <= 3)

A variety of perovskite-type mixed oxides corresponding to the solid soluti on Ba-12[BaxPt3-x]Pt6O27 (0 less than or equal to x less than or equal to 3 ) was studied by X-ray photoelectron spectroscopy (XPS). Pt was found to be in two valence states, Pt2+ and Pt4+ and the quantity of Pt2+ decreases wh en x increases. The intrinsic activity, for catalytic CO oxidation, was fou nd to be dependent on x. At low temperature (below 170 degrees C), the high est activity is obtained for Ba12Pt9O27 (x = 0) and the activity decreased with increasing x. The apparent activation energies increased, from 80 to 1 10 kJ.mol(-1), when x increased; for a sample of metallic platinum catalyst studied in the same conditions, a value of 120 kJ.mol(-1) was found. The p artial reaction order/O-2 (in the range of 0.64 to 0.90) increased with x w hile the partial order/CO (in the range of -0.34 to -0.70) decreased when x increased. Moreover, for the oxide containing only Pt4+ (Ba15Pt6O27) the r eaction orders were similar to those determined for the metallic platinum. Hence, there is undoubtedly a correlation between the Pt2+/Pt4+ ratio in th ese oxides and the catalytic activity for CO oxidation. This can be explain ed by assuming that CO chemisorption on Pt2+ is weaker than on Pt4+, which implies a less inhibitor effect of CO at low temperature. (C) 2000 Elsevier Science Ltd.