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

The synthesis of methanol via CO 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 modulation-excitation spectroscopy coupled with diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry (ME-DRIFTS-MS) to investigate CO hydrogenation over Ga-promoted Cu/ZrO 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 (CHO*) 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 HO. Ga facilitates hydrogen adsorption, accelerating HCOO* hydrogenation to CHO* and preventing its decomposition to CO. Notably, CHO* conversion to CHOH 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 CO hydrogenation catalysts.