Uncovering the Pressure-Dependent Mechanism of CO2 Hydrogenation to Methanol on Ga-Promoted Cu/ZrO2 Using Operando Modulation-Excitation DRIFTS.

Abstract

The synthesis of methanol via CO2 hydrogenation is attracting significant interest, with Cu-based catalysts currently leading this promising approach. Incorporating Ga and Zr promoters further enhances catalyst performance by suppressing the competing reverse water-gas shift (RWGS) reaction. However, their precise mechanistic roles and the identities of key reaction intermediates remain debated, which may be the key for catalyst design and process optimization. In this study, we extend operando modulation-excitation spectroscopy coupled with diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry (ME-DRIFTS-MS) to investigate CO2 hydrogenation over Ga-promoted Cu/ZrO2 under varying industrially relevant pressures up to 50 bar. Our results indicate that methanol formation proceeds predominately via the formate pathway with formate (HCOO*) and methoxy (CH3O*) as pivotal intermediates. Additionally, we demonstrate that the rate-determining step is strongly dependent on the pressure and temperature, ultimately dictated by the local abundance of adsorbed hydrogen (H*) and gaseous H2O. Ga facilitates hydrogen adsorption, accelerating HCOO* hydrogenation to CH3O* and preventing its decomposition to CO. Notably, CH3O* conversion to CH3OH occurs via a water-assisted pathway rather than direct hydrogenation, explaining previously unclear correlation between Cu dispersion and catalytic activity. These mechanistic insights highlight the potential of optimizing reaction conditions─especially lower operating temperatures and controlled water cofeed─to significantly enhance methanol selectivity over Cu-based CO2 hydrogenation catalysts.