Surface Gradient Sn Doping of Copper Enables Adsorption-Controlled Electrohydrogenation of Biomass-Derived Aldehydes

Electrochemical hydrogenation of biomass-derived aldehydes is a sustainable alternative to thermochemical routes, yet its efficiency is often limited by competitive hydrogen evolution and poorly defined surface-adsorption relationships on nonprecious metal catalysts. Here, we report a copper core/surface gradient tin-doping strategy that enables adsorption-controlled electrohydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF). Unlike conventional bulk alloying, gradient Sn enrichment selectively tailors the Cu surface electronic structure, stabilizing carbon-centered intermediates while suppressing hydrogen adsorption. The optimized CuSn_0.18 catalyst achieves a BHMF Faradaic efficiency of 81.4% with excellent stability and scalability in both H-cell and membrane-electrode assembly configurations. By integrating Pb underpotential deposition with operando spectroscopy, we establish a quantitative structure-activity framework correlating Sn surface coverage, active site density, and electronic modulation with catalytic performance. This work demonstrates surface-specific dopant engineering as a general strategy for rationally controlling selectivity in electrocatalytic biomass valorization.