Combining X-ray absorption and diffraction to relate structure to the activity in catalysts for CO2 valorization reactions

The development of effective catalytic processes for the conversion of CO2 into value-added chemicals or fuels, such as methanol synthesis or the dry reforming of methane (DRM) relies strongly on a rational catalyst design, which in turn requires an in-depth understanding of structure-activity relationships. Due to the inherent complexity of heterogeneous catalytic systems, an arsenal of complementary techniques is required to characterize the catalytic structure (and dynamics thereof) from the atomic-to-nanoscale (under reaction conditions). In this talk, we show how the application of combined X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) allows obtaining the oxidation state, the local and (nano)crystalline structure of the catalysts providing the basis for the formulation of structure-performance relationships in catalysts for CO2 valorization reactions. In the first example, we demonstrate how a combined operando XAS-XRD experiment allowed us to relate the evolution of the structure of In2O3 nanoparticles (NPs) to their activity for CO2 hydrogenation to methanol.[1] The experiments revealed a reductive amorphization of the In2O3−x nanocrystallites with time on stream (TOS), leading ultimately to an over-reduction of In2O3−x to (molten) In0, in a process that is linked to catalyst deactivation. When the In2O3 NPs were supported on a nanocrystalline monoclinic ZrO2 support, we observed the stabilization of the oxidation state of In via the formation of a solid solution m-ZrO2:In.[2] In the second example, we explore a Ni-Fe-based catalyst for the DRM. Combined, operando XAS-XRD experiments allowed us to probe the dynamics of Ni-Fe alloying/dealloying with the formation of FeO to explain the superior stability of the NiFe catalysts compared to a Ni-based analogue, due to a Fe-FeOx-based redox cycle.[3] In the last example, combined XAS–XRD experiments are used to shed light on the formation of Ru0 nanoparticles (ca.1 nm) via their exsolution from defective, fluorite-type Sm2RuxCe2−xO7 solid solutions. The resulting exsolved nanoparticles show a high activity and stability for the DRM.[4