Catalytic CO oxidation over pumice supported Pd-Ag catalysts

Results on the catalytic oxidation of CO at 523 K over bimetallic Pd-Ag cat alysts supported on pumice are reported and compared with those obtained fr om the monometallic Pd and Ag catalysts. The catalysts were prepared by org anometallic precursors and were characterized by X-ray diffraction (XRD) an d X-ray photoelectron spectroscopy (XPS) after different treatments with ai r and with hydrogen. The activity of the catalysts decreases with increasin g Ag/Pd atomic ratio,. the monometallic palladium exhibiting the highest ac tivity. Furthermore, the treatment with air and with hydrogen always result s in a better catalytic performance. On the basis of the structural charact erization by XRD and the surface atomic composition obtained from XPS measu rements, Ag has tendency to segregate at the surface of the catalysts. exce pt in air environment at 623 K. In this case, an inward diffusion of silver atoms occurs. By hydrogen treatment at high temperature. silver reappears at the surface with further increase of the catalytic activity. The results are explained in terms of rearrangement of the bimetallic particles with t he most active situation represented by clean particles of pure palladium n ext to dispersed silver particles. (C) 2001 Elsevier Science B.V. All right s reserved.