CO adsorption and oxidation on bimetallic Pt/Ru(0001) surfaces - a combined STM and TPD/TPR study

The results of a comparative STM and TPD/TPR study on the adsorption and oxidation of CO on pure Ru(0001) and on structurally and compositionally well-defined, bimetallic Pt/Ru(0001) surfaces are presented. The defect structure and local surface composition of the bimetallic substrates, which are produced by epitaxial growth of monolayer Pt islands or by Pt deposition and subsequent surface alloying, are characterized by high-resolution STM images with chemical contrast. TPD and TPR experiments for CO adsorption/oxidation on these surfaces show a distinct lowering of the CO adsorption energy on the bimetallic surfaces with respect to the pure Ru(0001) and Pt(lll) surfaces, with the onset of CO desorption already at 230 K. The reduction in adsorption energy is attributed to an electronic modification of the deposit metal due to interaction with the Ru(0001) substrate. The bimetallic surface alloy catalyzes CO oxidation under UHV conditions. The reduced temperature for CO2 desorption as compared to Pt(lll) indicates an even lower barrier than on the latter surface, whereas the pure Ru(0001) surface is inert under these conditions. The results are discussed with respect to the superior CO tolerance of bimetallic PtRu catalysts in low-temperature fuel cells as compared to monometallic Pt catalysts, leading to a mechanistic explanation of that phenomenon distinctly different from previous ideas. (C) 1998 Elsevier Science B.V. All rights reserved.