Deciphering Dynamic Structural and Mechanistic Complexity in Cu/CeO2/ZSM-5 Catalysts for the Reverse Water-Gas Shift Reaction

We have prepared and investigated the structural and mechanistic properties of an array of active Cu/CeO2/ZSM-5 catalysts for the reverse water-gas shift (RWGS) reaction, where the roles of Cu and CeO2 as a promoter were studied under reaction conditions. The concentration of copper was varied between 0 and 10 wt %, while the concentration of CeO2 was varied between 0 and 48 wt % in several permutations of Cu/CeO2/ZSM-5 catalysts. RWGS reactivity trends revealed that all three components of the catalyst system are essential for promoting and sustaining the optimum catalytic activity. The dynamic chemical states of copper and cerium were probed using in situ X-ray absorption spectroscopy (X-ray absorption near-edge structure and extended X-ray absorption fine structure), revealing reduced copper and mostly oxidized ceria in the active state. In situ X-ray diffraction data revealed the presence of copper aggregates, as well as copper within the ZSM-5 lattice. In situ and isotopic switching diffuse reflectance infrared Fourier transform spectroscopy analyses were used to unambiguously establish copper speciation within the ZSM-5 lattice and CeO2 surface sites as participatory in the RWGS mechanism, with surface formate species on CeO2 being a reaction intermediate. Based on a robust set of real-time structural, electronic, and surface chemical data, we proposed a reaction mechanism that involves the participation of all three components of this catalyst system.