(618e) Influence of Rhodium Nuclearity Embedded in Metal Oxide Frameworks for CO2 hydrogenation

As global concerns over carbon mitigation increase, means to transform carbon dioxide (CO₂) have received increasing attention. CO₂ hydrogenation can not only mitigate CO₂ emission but also produce valuable chemicals. Due to the thermodynamic stability of CO₂, the reaction requires catalysts with good activity, selectivity, stability with certain pressure and temperature. Rh, with the highest reactivity, allows CO₂ hydrogenation with much lower temperature and pressure relative to base transition metal catalysts. Furthermore, metal oxide frameworks (MOxFs) serve to isolate metal centers via high surface area, tunable structure, and easy accommodation of secondary elements. However, a detailed study of the structure-function relationship for CO₂ hydrogenation over highly controlled Rh sites in MOxFs is lacking.

In this study, we investigated CO₂ hydrogenation with Rh on two different MOxFs, octahedral layered structure (OL1) and octahedral molecular sieve structure (OMS2), to determine the influence of different MOxFs. Furthermore, we utilized three secondary elements, V, Zn, and Na. We found that Rh-Na-OL1 provided the highest CH₄ selectivity of 95% around 250 °C. Generally, OL1 catalysts gave higher CH₄ selectivity compared to OMS2 catalysts, while OMS2 catalysts showed initially lower reactivity but better stability over 48-hours stability test. To determine the structure and dispersion of the catalysts, temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and XAFS were utilized. XAFS showed the Rh sites existed initially as Rh²⁺ under ambient conditions. To understand the local structure change surrounding Rh atoms during reaction, in-situ XAFS was performed, showing a change from Rh²⁺ towards Rh¹⁺-Rh⁰. By leveraging the structural information from in-situ XAFS and the surface species via in-situ DRIFTS, all at reaction conditions, we are poised to gain a strong understanding of the structure-function relationship for Rh catalysts with controlled nuclearity and discern the kinetic and mechanistic influence of secondary metal addition for CO₂ hydrogenation.