Enhancing the Reactivity of Cu/Al₂O₃ for Methanol Steam Reforming through adding CrO_<i>x</i>: Unraveling Reaction Pathways and the Mechanism for Improvement

Copper-based catalysts are widely utilized for methanol steam reforming (MSR) reactions. However, improving their performance and achieving a deeper understanding of the reaction mechanism remain significant challenges. Herein, a series of Cu-y%CrOx/Al2O3 catalysts were synthesized. The optimal Cu-7%CrOx/Al2O3 catalyst achieved a high CH3OH conversion of 93.2%, a low CO selectivity of 0.16%, and a competitive hydrogen production rate of 1142.7 mmol gcat–1 h–1 at 260 °C with a weight space velocity of 14.6 h–1, significantly outperforming the Cu/Al2O3 catalyst. Combined in situ spectroscopy and surface reaction experiments revealed that the MSR reaction on both catalysts predominantly followed the HCOO* pathway. This involves the dehydrogenation of CH3OH to CH3O*, followed by oxidation to HCOO*, and then decomposition to produce H2 and CO2, with the conversion of CH3O* to HCOO* being the rate-determining step (RDS). The steam acted as a promoter for the conversions of CH3O* and HCOO*. A small amount of formaldehyde (HCHO) derived from CH3O* dehydrogenation tends to dissociate, forming the byproduct CO rather than converting to HCOO*. Due to the promoting effect of CrOx, improved Cu dispersion, the Cu+/Cu0 ratio of around 1.0, and increased active oxygen species facilitate the RDS of CH3O* to HCOO* and the oxidation of CO, leading to an enhanced hydrogen production rate and CO2 selectivity on Cu-7%CrOx/Al2O3 compared to Cu/Al2O3.