CO2 Reduction to Methanol on Au/CeO2 Catalysts: Mechanistic Insights from Activation/Deactivation and SSITKA Measurements

Aiming at a mechanistic understanding of the methanol (MeOH) synthesis from CO2/H-2 over Au/CeO2 catalysts and the activation/deactivation of these catalysts, we have investigated these processes by a combination of kinetic measurements, time-resolved in situ diffuse reflectance Fourier transform infrared (FTIR) spectroscopy (DRIFTS) measurements, and structural characterization by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). Kinetic measurements indicated a rapid activation phase, followed by a continuous slow deactivation. A faster deactivation of CO formation (reverse water-gas shift reaction) compared to that of methanol formation results in an increasing selectivity toward MeOH formation with time on stream. The activation of the catalyst is attributed to a rapid initial reduction of the support (formation of O vacancies). Since based on STEM imaging and XRD measurements sintering of Au nanoparticles is negligible, the subsequent deactivation is attributed to the slow buildup of site-blocking adsorbates, specifically surface carbonates, and/or over-reduction of the catalyst. This is supported also by the reversible nature of the deactivation upon recalcination in O-2/N-2. Steady-state isotopic transient kinetic analysis (SSITKA) measurements, following the buildup/decay of adsorbed formate and methoxy species by DRIFTS upon changing from a CO2/H-2 to a CO2/D-2 mixture and back under steady-state conditions, indicate that surface formate species are reaction intermediates in the dominant reaction pathway for CO2 hydrogenation to methanol, with the calculated rates of formation/decay comparable to the rate of methanol formation. The consequences of these results for the mechanistic understanding of this reaction are discussed.