Enhancing the Reactivity of Cu/Al2O3 for Methanol Steam Reforming through adding CrOx: Unraveling Reaction Pathways and the Mechanism for Improvement

Abstract

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%CrO_x/Al₂O₃ catalysts were synthesized. The optimal Cu-7%CrO_x/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 g_cat^–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 CrO_x, 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%CrO_x/Al₂O₃ compared to Cu/Al₂O₃.