High‐Pressure Cell Design for Modulation Excitation Spectroscopy: Application to Mechanistic Analysis of CO 2 Hydrogenation to Methanol

Modulation excitation spectroscopy (MES) is a powerful method to provide information on active species and sites in catalytic reactions. We present the design of a diffuse‐reflectance IR Fourier transform spectroscopy (DRIFTS) cell, which is suitable for MES but also applicable to other in situ and operando DRIFTS studies on catalytic materials. The cell is characterized by a low void volume to allow for sufficiently fast gas exchange during MES experiments, even at high gas pressures. The potential of the cell is illustrated for the mechanistic analysis of CO 2 hydrogenation over a ceria‐supported copper catalyst (Cu/CeO 2 ) at 10 bar using ME‐DRIFTS combined with phase‐sensitive detection (PSD). Using MES/PSD key intermediates of methanol formation could be identified, that is, carbonates and formates, as well as methoxy groups, which are immediate precursors of methanol. By comparison of the mechanistic behavior of Cu/CeO 2 with bare ceria, the crucial role of copper for the reaction toward methanol was demonstrated. The presented DRIFTS cell offers high versatility for steady‐state and transient spectroscopic analysis under in situ/operando conditions, providing detailed mechanistic information, including the identification of intermediates during surface reactions over catalytic materials, facilitating their rational design.