Supported Vanadium Carbide Catalysts for Reverse Water Gas Shift and Methanol Steam Reforming: Activity, Stability, and Coking Pathways.

A series of supported vanadium carbide (VCx) catalysts were prepared, characterized, and tested for the carbon dioxide and methanol activation via the Reverse Water Gas Shift (RWGS) and Methanol Steam Reforming (MSR) reactions, respectively. Crystallite sizes of VCx ranging from 9 to 36 nm were obtained depending on the support used (γ-Al2O3, SiO2, CeO2, ZrO2 and TiO2). In both reactions, the supported catalysts exhibited superior performance compared to the bulk VCx sample. In the RWGS reaction, all catalysts showed high CO selectivity, with VCx/Al2O3 demonstrating the best performance and no significant deactivation after 100 h at 873 K. Under MSR conditions, VCx/ZrO2 achieved the highest methanol conversion. However, all catalysts suffered from significant deactivation due to coke formation, with CH4 as the main product instead of the desired H2 and CO2 from full steam reforming. Density Functional Theory (DFT) calculations revealed that methanol decomposition is more facile than CO2 decomposition on both stoichiometric VC and carbon-deficient V8C7 surfaces, particularly in the presence of carbon vacancies, leading to coke formation in the form of partially hydrogenated CxHy* species. These findings indicate that VCx catalysts are more susceptible to coking under MSR than RWGS conditions, in line with experimental observations, and highlight the critical role of the carbide surface structure and vacancy concentration in coke formation.