On the hydrogenation of CO and CO2 over copper/zirconia and palladium/zirconia catalysts

Abstract

Zirconia-supported hydrogenation catalysts were obtained by activation of the amorphous precursors Cu70Zr30 and Pd25Zr75 under CO2 hydrogenation conditions. Catalysts of comparable compositions prepared by co-precipitation and wet impregnation of zirconia with copper- and palladium salts, respectively, served as reference materials. The catalyst surfaces under reaction conditions were investigated by diffuse reflectance FTIR spectroscopy. Carbonates, formate, formaldehyde, methylate and methanol were identified as the pivotal surface species. The appearance and surface concentrations of these species were correlated with the presence of CO2 and CO as reactant gases, and with the formation of either methane or methanol as reaction products. Two major pathways have been identified from the experimental results. i) The reaction of CO2/H2-mixtures on Cu/zirconia and Pd/zirconia primarily yields surface formate, which is hydrogenated to methane without further observable intermediates. ii) The catalytic reaction between CO and hydrogen yields π-bonded formaldehyde, which is subsequently reduced to methylate and methanol. Interestingly, there is no observable correlation between absorbed formaldehyde or methylate on the one hand, and gas phase methane on the other hand. The reactants, CO2 and CO, can be interconverted catalytically by the water gas shift reaction. The influence of the metals on this system of coupled reactions gives rise to different product selectivities in CO2 hydrogenation reactions. On zirconia-supported palladium catalysts, surface formate is efficiently reduced to methane, which consequently appears to be the principal CO2 hydrogenation product. In contrast, there is a favorable reaction pathway on copper in which CO is reduced to methanol without C-O bond cleavage; surface formate does not participate significantly in this reaction. In CO2 hydrogenations on copper/zirconia, methanol can be obtained as the main product, from a sequence of the reverse water gas shift reaction followed by CO reduction.