ON THE MECHANISM OF CO AND CO2 HYDROGENATION REACTIONS ON ZIRCONIA-SUPPORTED CATALYSTS - A DIFFUSE REFLECTANCE FTIR STUDY .2. SURFACE SPECIES ON COPPER ZIRCONIA CATALYSTS - IMPLICATIONS FOR METHANOL SYNTHESIS SELECTIVITY

Hydrogenation reactions of CO and CO2 over copper/zirconia catalysts obtained from an amorphous Cu70Zr30 precursor, as well as catalysts prepared by coprecipitation and sequential precipitation of copper and zirconium hydroxides, were investigated by diffuse reflectance FTIR spectroscopy. Surface species and reaction products are compared with those on palladium/zirconia catalyst systems, for which results have been reported in Part I of this study. Two major pathways were identified from the observed correlations of surface species and gas phase products. (i) The rapid adsorption of CO2 followed by reduction yields surface formate, which is efficiently reduced to methane without further observable intermediates. (ii) The adsorption of CO in the presence of hydrogen yields pi-bonded formaldehyde, which is reduced to methylate and finally methanol. These two reaction systems are connected, as CO2 and CO can be interconverted on the catalyst surface by the water-gas shift reaction. While the principal types of surface species are similar in Pd/ZrO2 and Cu/ZrO2 catalyst systems, there are large differences in the selectivities towards methane and methanol, respectively. Apparently the amorphous zirconia support mainly provides adsorption sites for the surface reaction intermediates. The activity of the metal and its interface, on the other hand, is the decisive factor in determining the principle hydrogenation product. Palladium is very active in reducing surface formate to methane; as a consequence of the continuous desorption of methane, CO2 is replenished on the surface by the water-gas shift reaction, and the product distribution is biased towards the principal CO2 hydrogenation product, i.e. methane. With copper, on the other hand, formate hydrogenation is less effective. In contrast, there is a favorable reaction pathway on copper in which adsorbed CO is reduced to methanol without C-O bond cleavage. The reverse water-gas shift equilibrium is biased towards CO on the copper surface, and methanol can be obtained as the principle CO2 hydrogenation product.