MODEL STUDIES OF CESIUM PROMOTERS IN WATER GAS SHIFT CATALYSTS - CS/CU(110)

The kinetics and the mechanism of the water-gas shift (WGS) reaction have been studied over a Cu(110) surface containing vapor-deposited Cs adatoms. The optimum rate occurs for a Cs coverage of about one cesium atom per four Cu surface atoms. Postreaction surface analysis (XPS, AES, TDS) proves that this Cs is almost immediately converted to a surface complex containing cesium, oxygen, and carbon under reaction conditions. This complex has TDS, XPS, and AES features similar to a surface cesium carbonate, CS · CO3,a (x = 1.5 to 2.0). The kinetics of the elementary reaction steps leading to the formation and decomposition of this carbonate are also studied. These steps are in rapid equilibrium under reaction conditions. The electronic effects of the Cs, on the elementary steps of the WGS reaction are also investigated. Cesium causes an acceleration in water dissociation on the surface and in the conversion of surface oxygen to C02 by CO. The former reaction is rate-limiting with no Cs present, whereas the latter partially controls the rate on the optimally promoted surface. © 1992.