Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy Elucidate the CO₂ Promotion on a Commercial Methanol Synthesis Catalyst

Optimal amounts of CO₂ are added to syngas to boost the methanol synthesis rate on Cu-ZnO-Al₂O₃ in the industrial process. The reason for CO₂ promotion is not sufficiently understood at the particle level due to the catalyst complexity and the high demands of characterization under true reaction conditions. Herein, we applied operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy on a commercial catalyst to gain insights into its morphology and surface chemistry. These studies unveiled that Cu and ZnO agglomerate and ZnO particles flatten under CO/H₂ and/or CO₂/H₂. Under the optimal CO/CO₂/H₂ mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H₂O byproduct, enhancing the water-gas shift activity and thus the methanol production. Our results provide a rationale to the CO₂ promotion emphasizing the importance of advanced analytical methods to establish structure–performance relations in heterogeneous catalysis.