Molybdenum carbide catalyst for the reduction of CO₂ to CO: surface science aspects by NAPPES and catalysis studies

Carbon dioxide is a greenhouse gas, and needs to be converted into one of the useful feedstocks, such as carbon monoxide and methanol. We demonstrate the reduction of CO₂ with H₂ as a reducing agent, via a reverse water gas shift (RWGS) reaction, by using a potential and low cost Mo₂C catalyst. Mo₂C was evaluated for CO₂ hydrogenation at ambient pressure as a function of temperature, and CO₂ : H₂ ratio at a gas hourly space velocity (GHSV) of 20 000 h^-1. It is demonstrated that the Mo₂C catalyst with 1 : 3 ratio of CO₂ : H₂ is highly active (58% CO₂ conversion) and selective (62%) towards CO at 723 K at ambient pressure. Both properties (basicity and redox properties) and high catalytic activity observed with Mo₂C around 700 K correlate well and indicate a strong synergy among them towards CO₂ activation. X-ray diffraction and Raman analysis show that the Mo₂C catalyst remains in the β-Mo₂C form before and after the reaction. The mechanistic aspects of the RWGS reaction were determined by near-ambient pressure X-ray photoelectron spectroscopy (NAPXPS) with in situ generated Mo₂C from carburization of Mo-metal foil. NAPXPS measurements were carried out at near ambient pressure (0.1 mbar) and various temperatures. Throughout the reaction, no significant changes in the Mo²⁺ oxidation state (of Mo₂C) were observed indicating that the catalyst is highly stable; C and O 1s spectral results indicate the oxycarbide species as an active intermediate for RWGS. A good correlation is observed between catalytic activity from atmospheric pressure reactors and the electronic structure details derived from NAPXPS results, which establishes the structure-activity correlation.