Photocatalytic Conversion of Volatile Fatty Acids into H₂ and C 1 –C 6 Alkanes over Cu 2 O/BiPO 4 p-n Heterojunctions: Insights into Radical Intermediate Coupling

Abstract Photocatalytic conversion of biomass-derived volatile fatty acids (VFAs) into H 2 and hydrocarbons remains challenging, largely because the product distribution is governed by the selective coupling of alkyl radical intermediates generated via the photo-Kolbe reaction. Herein, we synthesized a series of Cu 2 O/BiPO 4 p-n heterojunction composites with varying Cu 2 O loadings (1–15 wt%) and evaluated their performance for converting acetic (HAc), propionic (HPr), and butyric acid (HBu) into H 2 and C 1 –C 6 alkanes under UV irradiation. The p-n heterojunction is proposed to facilitate charge separation and to influence the oxidative power of photogenerated holes, thereby affecting the decarboxylation of carboxylates to generate alkyl radical intermediates (·CH 3 , ·CH 2 CH 3 , ·CH 2 CH 2 CH 3 ), whose subsequent coupling pathways dictate the product selectivity. The optimized CBP-9 (9 wt% Cu 2 O) delivered a 2.49-fold higher total H 2 /alkane yield than pristine BiPO 4 from HAc, with 85.71% total organic carbon removal within 4 h. For HPr and HBu, the yields of ethane/butane and methane/propane were enhanced by 2.27- and 1.67-fold, respectively. Mechanistic studies based on photoluminescence, transient photocurrent, and electrochemical impedance spectroscopy indicate that the p-n junction promotes spatial charge separation and improves stability. The catalyst retained > 85% of its initial activity after five cycles. This work provides insights into radical intermediate coupling in the photocatalytic conversion of VFAs over Cu 2 O/BiPO 4 heterojunctions, demonstrating the potential for directly converting wet biomass residues into H 2 -enriched fuel gases.