Bimetallic PtSn catalyst for selective CO oxidation in H-2-rich gases at low temperatures

Kinetic measurements on preferential CO oxidation in a H-2-rich atmosphere (PROX) over a bimetallic, carbon supported PtSn catalyst reveal a high acti vity and selectivity already at low temperatures (0-80 degreesC), superior to a commercial Pt/Al2O3 system. The selectivity, though steadily decreasin g with temperature, is remarkably high, 85% at low temperatures around 0-20 degreesC, and even at 120 degreesC it is, at 45%, still higher than that o f standard Pt catalysts. The observation that CO desorption is not rate lim iting and that the selectivity decreases with increasing temperature, can b e explained in a mechanistic model involving separation of the reactant ads orption sites (bifunctional surface), with competing CO and hydrogen adsorp tion on Pt sites/areas and oxygen adsorption predominantly on Sn sites and SnOx islands on/adjacent to the active PtSn particles. The reaction takes p lace in a bifunctional way at the perimeter of these islands or by invoking a spill-over process. This model is supported by CO temperature-programmed desorption (TPD), in situ diffuse reflectance IR Fourier transform spectro scopy (DRIFTS), and x-ray photon spectroscopy (XPS) measurements, which ind icate that under reaction conditions the surface CO coverage on the metalli c particles is high, but decreases with temperature, and that only part of the Sn is reduced, included in PtSn alloy particles, while another part is in an oxidic state, forming SnOx islands on and presumably also beside the active particles. Its excellent performance makes PtSn an interesting catal yst for fuel gas purification in low temperature polymer electrolyte membra ne fuel cell technology (PEM-FC).