Novel MCr2O4/TiO2 (M = Zn, Ni) heterojunctions for enhanced selectivity in biomass photoreforming

Abstract

The photoreforming of biomass is of growing interest for the simultaneous formation of hydrogen and valueadded products. In this work, H2 production under UV irradiation from aqueous solutions of glycerol and glucose was investigated for the first time over MCr2O4/P25 (M = Zn, Ni) composites. The conversion of the two substrates used as sacrificial agents was also monitored. ZnCr2O4 and NiCr2O4 spinels were synthesized via a nitrate route and coupled with commercial TiO2-P25 simply by ball milling, with varying MCr2O4 loadings to optimize the performance. The materials were characterized by XRD, SEM-EDS, Raman, UV-Vis DRS, PL, and photoelectrochemical analysis. The composites showed enhanced H2 production compared to bare components, with optimal performance observed for 3%ZnCr2O4/P25 and 5%NiCr2O4/P25, yielding 0.23 and 0.15 mM H2 from glycerol, and 0.12 and 0.08 mM from glucose, respectively. Glycerol oxidation showed slightly improved conversion but a marked increase in selectivity toward 1,3-dihydroxyacetone (51% and 82% for Zn and Ni-based composites, respectively), with suppressed formation of glyceraldehyde and glycolic acid. In contrast, glucose conversion remained largely unchanged, although selectivity toward formic acid was maintained. Control experiments in pure water confirmed the absence of H2 evolution, highlighting the crucial role of the sacrificial agent, which also enables the valorization of photogenerated holes to form high-value-added products.