Operando Spectroscopic Monitoring of Active Species in CO₂ Hydrogenation at Elevated Pressure and Temperature: Steady-State versus Transient Analysis

Operando infrared spectroscopy is an invaluable tool to provide a deep insight into underlying mechanisms in heterogeneous catalysis. Recent advances in spectroscopic techniques enabled operando analysis under elevated pressure and temperature. In this study we compared a conventional steady-state IR analysis with a transient IR analysis by modulation excitation spectroscopy (MES) using a model reaction, CO₂ hydrogenation to methanol at 30 bar and 250 °C. The steady-state IR spectra provide information about surface properties of catalysts while the transient IR spectra are a powerful tool to unveil the kinetics of surface species involved in catalytic cycles and therefore to determine rate-limiting step. Operando MES-IR spectroscopy evidenced that under the reaction conditions (30 bar, 250 °C) hydrogenation of surface methoxy species (CH₃O−) is the rate-limiting step for the Cu–Zn based catalyst. We herein demonstrate that comparative analysis of both steady-state and transient spectra at elevated pressure provides a solid understanding of surface processes, allowing a rational catalyst design.