Effect of Mn/Cu Ratio on the Structure–Performance Relationship of Spinel-Type Mn–Cu/Al2Ox Catalysts for Methanol Steam Reforming

The development of highly active, thermally stable, and low-CO-selective catalysts is critical for practical methanol steam reforming (MSR) to produce high-purity hydrogen for fuel cell applications. In this work, a series of Mn–Cu/Al2Ox catalysts with varying Mn/Cu/Al molar ratios were synthesized via co-precipitation and systematically investigated to establish the relationship between composition, structure, and catalytic performance. XRD analysis revealed the formation of spinel-type CuAl2O4 and MnAl2O4 phases, with Mn preferentially occupying octahedral B-sites to form MnAl2O4, thereby inducing lattice distortion and inhibiting grain growth. SEM and TEM–EDS mapping confirmed uniform elemental distribution and a porous nanoscale morphology, while H2-TPR results suggested that increasing the Mn/Cu ratio strengthens Mn–Cu interactions, shifts Cu2+ reduction to higher temperatures, and enhances Cu dispersion (up to 26.11 m2/g). XPS analysis indicated that Mn doping enriches Mn3+ species and facilitates oxygen vacancy formation, which promotes water–gas shift (WGS) activity and suppresses CO formation. Catalytic testing (240–300 °C) showed that Mn2Cu2Al4Ox achieved the highest methanol conversion while maintaining low CO selectivity; in contrast, reducing the Mn/Cu ratio increased CO selectivity, detrimental to hydrogen purification. Stability tests under continuous steam exposure for 24 h demonstrated minimal activity loss (~2%) and negligible increase in CO selectivity (<1%), confirming excellent hydrothermal stability. The results indicate that tailoring the Mn/Cu ratio optimizes the balance between redox properties and metallic Cu dispersion, offering a promising route to design low-CO, durable catalysts for on-site hydrogen generation via MSR.