Unveiling the mysteries of catalytic processes of CO₂ hydrogenation: a journey through in situ/operando spectroscopy

Hydrogenating CO₂ is acknowledged as a structure-sensitive reaction, where structural changes of catalysts profoundly influence the product distribution and catalysts activity. However, traditional characterization techniques (so-called ex situ) fail to accurate and authentic detection of the structure of catalysts and reaction pathway changes when the catalyst is in the working state. To address this issue, in situ/operando spectroscopies have been developed to elucidate the structure-activity relationships of catalysts in the thermo-catalytic CO₂ hydrogenation process, advancing our understanding and guiding the design of catalysts. In particular, Raman spectroscopy and X-ray absorption spectroscopy (XAS) can be used to monitor the real-time changes of catalyst structures, elucidating the metal (or oxide)-support interaction, identifying active sites and investigating the causes of catalyst deactivation. Fourier-transform infrared spectroscopy (FTIR) can be employed to track the evolution of surface intermediates during the reaction, inferring plausible reaction mechanisms. Combined with steady-state isotope transient kinetic analysis (SSITKA) experiments, it provides valuable insights into the kinetics information of surface species. In this review, we summarize the up-to-date advances in these spectroscopic techniques and delineate potential future trends in the CO₂ reduction.