Interaction of CO, NO and NO/CO over Pd/γ-Al2O3 and Pd-WOx/γ-Al2O3 catalysts

The interaction of NO, CO and NO/CO mixtures over low loaded Pd/gamma-Al2O3 and Pd-WOchi/gamma-Al2O3 catalysts was investigated by a combination of FTIR, TPR, chemisorption and catalytic studies. NO and CO interaction with the alumina support gives rise to adsorbed nitrates and carbonates in addition to species associated with the palladium surface. The characterization work demonstrated the presence of a Pd-W interaction that facilitates the formation of reduced W species, makes the Pd more resistant to oxidation and decreases the intensity and shifts the location of the CO and NO adsorption bands to higher frequencies, indicating a weaker bond of these molecules with the I'd surface. These results suggest that an important redistribution of charge takes place in the valence d-band of palladium when the metal particles are in contact with reduced W species. Strong isocyanates bands are formed from NO/CO mixtures on both Pd/gamma-Al2O3 and Pd-WOchi/gamma-Al2O3 catalysts but a different temperature dependence of the band intensity due to the influence of W is observed; maximum isocyanate formation that occurs at 548 K on Pd/gamma-Al2O3 is shifted to 623 K on Pd-WOchi/gamma-Al2O3. Catalytic tests revealed that the presence of reduced W promotes the: NO dissociation reaction on Pd acting as an "oxygen scavenger". However, at comparable levels of conversion, the Pd-W catalyst enhances the formation of N2O and has a detrimental effect on the NO + CO reaction. From the FTIR results and the catalytic test the formation and reactivity of surface isocyanate species and gas phase N2O were found to be closely related. (C) 2002 Elsevier Science B.V. All rights reserved.