Coffee Waste Biochar-Supported Copper Electrocatalyst for Biomass Depolymerization: Insights into Surface Dynamics and Catalyst Restructuring

This work introduces a nanostructured copper catalyst supported on biochar derived from spent coffee grounds, designed for the electrochemical depolymerization of lignin in aqueous solutions. Copper nanoparticles were synthesized in situ and subsequently converted via pyrolysis, resulting in copper nanoparticles with a predominantly metallic Cu core and a Cu(I)-enriched surface. Combined XRD, XPS, and STEM analyses revealed copper nanoparticles with a Cu-rich metallic interior and partial surface oxidation to Cu(I). Operando X-ray absorption spectroscopy was employed to probe the Cu(I) surface and assess its stability under the applied potential. Reusability tests demonstrate sustained product selectivity over four catalytic cycles, with only minor structural alterations detected in the fourth run. DFT calculations revealed mixed dispersion and covalent lignin-surface interactions, with arene groups preferentially oriented parallel to the interface at low coverage and perpendicular to the interface at high coverage. This study presents the first nanoscopic copper-based catalyst for lignin depolymerization, providing fundamental insights into the dynamic Cu/Cu2O interface and highlighting its gradual reduction and sustained activity in electrochemical lignin valorization. In contrast to previous studies on bulk Cu electrodes for electrochemical reductive lignin depolymerization, we observe significant surface restructuring of nanoparticles, leading to larger nanostructured copper agglomerates as active electrocatalytic species.