Carbon dioxide hydrogenation over Au/ZrO2 catalysts from amorphous precursors: catalytic reaction mechanism

Abstract

An active catalyst for carbon dioxide hydrogenation is obtained by exposing an amorphous Au25Zr75 alloy to CO2 hydrogenation conditions. During this in situ activation, metallic gold particles of 8.5 nm mean size are formed, and the zirconium component of the catalyst is oxidized to ZrO2. For comparison, a further Au/ZrO2 catalyst was synthesized by coprecipitation, followed by calcination of the amorphous precipitate. The calcination step strongly enhances the activity of the catalyst; gold segregation and zirconia crystallization are found to occur in this process. The structural and chemical changes are characterized by gas adsorption, X-ray diffraction and thermal analysis.The main products of CO2 hydrogenation over these catalysts, as identified by gas chromatography, are methanol and CO. To investigate the reaction mechanism, diffuse reflectance FTIR spectroscopy has been used. Observed surface species are correlated with the formation of gas-phase products. Adsorption of CO2–H2 results in rapid formation of formate as the primary surface intermediate; two types of formate species are clearly detected on the coprecipitated catalyst, and are assigned by means of formic acid adsorption experiments. CO formation from CO2 appears to proceed via surface carbonate, in a surface reaction that corresponds to a ‘basic variant’ of the reverse water-gas-shift reaction. The CO formed in this process is, in turn, the starting point for a series of surface hydrogenation steps that yield π-bonded formaldehyde, surface-bound methylate and finally methanol. This sequence of reactions is confirmed by separate CO–H2 adsorption experiments.