Catalytic CO oxidation over pumice supported Pd-Ag catalysts

Results on the catalytic oxidation of CO at 523K over bimetallic Pd-Ag catalysts supported on pumice are reported and
compared with those obtained from the monometallic Pd and Ag catalysts. The catalysts were prepared by organometallic
precursors and were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) after different
treatments with air and with hydrogen. The activity of the catalysts decreases with increasing Ag/Pd atomic ratio, the
monometallic palladium exhibiting the highest activity. Furthermore, the treatment with air and with hydrogen always results
in a better catalytic performance. On the basis of the structural characterization by XRD and the surface atomic composition
obtained from XPS measurements, Ag has tendency to segregate at the surface of the catalysts, except 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 the most active situation represented by clean particles of pure palladium next to dispersed silver particles.