Catalysts often undergo changes in structure and composition when they are exposed to the CO2 hydrogenation reaction environment. Such in situ phase changes can completely alter the activity, selectivity, and stability of the catalyst. For this reason, tracking the dynamic structural and compositional evolution of a catalyst over the course of a reaction is essential for understanding catalyst performance. In this talk, I will discuss two examples wherein dynamic changes to the catalyst structure and composition significantly alter CO2 hydrogenation activity and selectivity. In the first system, we found that the crystal phase of In2O3 transitions from a hexagonal phase (h-In2O3) to a cubic phase (c-In2O3) during the reverse water-gas-shift reaction (ACS Catal. 2020, 10, 5, 3264–3273). In situ x-ray diffraction (XRD) demonstrated that the phase transformation is induced by the gas phase reactants, where h-In2O3 is first reduced by H2 and then is oxidized by CO2 to form the c-In2O3 phase. Computations with density functional theory (DFT) demonstrated that the higher activity of the c-In2O3 phase could be attributed to a favorable arrangement of surface oxygen atoms that facilitates dissociative H2 adsorption. In the second system, we found that iron catalysts undergo drastic changes in composition during CO2 hydrogenation (Sci. Adv. 2022, 8, 5, abm3620). Using a combination of in situ characterization techniques and DFT calculations, we showed that metallic Fe first is carburized to Fe3C and then to Fe5C2. H2O produced as the by-product of CO2 hydrogenation then oxidizes the iron carbide to form a Fe3O4 phase. At steady state, the catalyst particle forms a core-shell Fe3O4@(Fe5C2+Fe3O4) structure, where the specific surface composition is highly sensitive to the reaction conditions (i.e., to the balance of carburization and oxidation). Controlling the surface composition is essential, as the Fe5C2-rich surface yields the highest CO2 conversion rate, C2+ selectivity, and chain growth factor. Overall, these studies illustrate the importance of in situ structural evolution during CO2 hydrogenation.

Implications of in situ catalyst phase transformations during CO2 hydrogenation

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