CO<sub>2</sub>Reduction to Methanol on Au/CeO<sub>2</sub>Catalysts: Mechanistic Insights from Activation/Deactivation and SSITKA Measurements
Azita Rezvani, Ali M. Abdel‐Mageed, Tamao Ishida, Toru Murayama, Magdalena Parlińska‐Wojtan, R. Jürgen Behm
Abstract
Aiming at a mechanistic understanding of the methanol (MeOH) synthesis from CO2/H2 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 O2/N2. 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/H2 to a CO2/D2 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.