Carbon monoxide oxidation over unreduced and reduced palladium chloride, cupric chloride, and their mixtures dispersed on alumina and carbon

CO oxidation by O{dollar}sb2{dollar} and H{dollar}sb2{dollar}O was studied over unreduced PdCl{dollar}sb2{dollar}, CuCl{dollar}sb2{dollar}, and PdCl{dollar}sb2{dollar}-CuCl{dollar}sb2{dollar} dispersed on alumina or carbon. IR spectroscopy was employed to identify PdCl{dollar}sb2{dollar}CO, (PdClCO){dollar}sb{lcub}rm n{rcub}{dollar}, CuClCO, and small amounts of CO chemisorbed on metallic Pd particles prior to and under reaction conditions with O{dollar}sb2{dollar} and H{dollar}sb2{dollar}O. The proposed reaction model for monometallic catalysts has water, when present, reacting with PdClCO, but in its absence O{dollar}sb2{dollar} reacts with PdClCO (or CuClCO) to produce CO{dollar}sb2{dollar}. The highest specific activity for CO oxidation by O{dollar}sb2{dollar} occurs with unreduced PdCl{dollar}sb2{dollar}-CuCl{dollar}sb2{dollar} catalysts, even compared to the reduced bimetallic catalysts, and it increases 10- to 1000-fold when H{dollar}sb2{dollar}O vapor is present giving high activity at 300K. The water both reacts with PdClCO and dissolves the Pd and Cu precursor compounds in a very thin, two-dimensional aqueous layer on the support surface, and a carbon black support gave the most active catalyst of all. Both O{dollar}sb2{dollar} and H{dollar}sb2{dollar}O are oxidants when H{dollar}sb2{dollar}O is present, and the principal role of Cu is the reoxidation of Pd{dollar}spcirc{dollar} when H{dollar}sb2{dollar}O vapor is present, but in its absence a reactive Pd-Cu carbonyl intermediate may exist.; CO oxidation over these same catalysts after prereduction was characterized by in situ IR spectroscopic/kinetic measurements. On Pd/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} a model invoking a reaction between compressed islands of oxygen and CO applies below 400K, but a Langmuir-Hinshelwood model assuming randomly adsorbed oxygen atoms and CO molecules applies near 450K. Oxygen adsorption becomes the rate determining step at low O{dollar}sb2{dollar} pressures. For Cu/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, IR measurements verified a Langmuir-Hinshelwood reaction between adsorbed CO molecules and O atoms, and CO adsorption was inhibited by chemisorbed oxygen. Over Pd-Cu/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} catalysts containing 9% Pd and 91% Cu, IR spectra showed little CO adsorbed on surface Pd{dollar}sb{lcub}rm s{rcub}{dollar} atoms initially, but a band near 1990 cm{dollar}sp{lcub}-1{rcub}{dollar} formed under reaction conditions. A kinetic study showed that Pd dominates at low CO pressures and Cu is the principal contributor at higher pressures, but no synergistic rate enhancement occurred.