Aqueous Glycerol\nto Glyceric Acid and Green Hydrogen\nby Visible-Light-Driven Photocatalysis with Ni/Co(PO<sub>4</sub>)<sub>2</sub>‑TiO<sub>2</sub>: Parallel Utilization of Holes and\nElectrons

Replacing the kinetically sluggish oxygen evolution reaction\n(OER)\nwith the oxidation of an abundantly available organic molecule to\nvalue-added product(s) (VAPs) at low voltage along with the hydrogen\nevolution reaction (HER) is a big challenge in water splitting, either\nby electrolysis or sunlight-driven photocatalysis. Glycerol oxidation\nto a VAP is kinetically fast, compared to an OER, and offers hope\nto enhance sunlight-driven water splitting to hydrogen by the concurrent\nutilization of holes and electrons. Mixed bimetal phosphates of Co\nand Ni (Co<sub><i>x</i></sub>Ni<sub><i>y</i></sub>(PO<sub>4</sub>)<sub>2</sub> (CoNiP)) with different Co:Ni ratios\n(10:0, 7:3, 5:5, 3:7, and 0:10) were integrated with TiO<sub>2</sub> to generate final photocatalyst composites (<i>x</i> wt\n% CoNiP with TiO<sub>2</sub>) and employed for concurrent photocatalytic\nHER and glycerol oxidation. Irrespective of the weight ratios of CoNiP\nand TiO<sub>2</sub>, any TiO<sub>2</sub>–CoNiP composite showed\nbetter photocatalytic activity for the HER and glycerol oxidation\ncompared to virgin TiO<sub>2</sub>. The highest HER as well as selectively\ngenerated glyceric acid yield was observed to be 54 and 67 mmol/g,\nrespectively, after 25 h of reaction under 1 sun conditions with TiO<sub>2</sub>–CoNiP-5:5. An increase in catalytic activity can be\nattributed to the formation of p–n heterojunctions of the constituent\ncomponent along with uniform distribution of CoNiP to effectively\nutilize the charge carriers for redox reactions. Highly selective\noxidation of glycerol to glyceric acid (85%), along with other minor\nproducts, is also demonstrated, which offers further scope to use\nsolar light to generate VAPs in a sustainable manner. A simple comparison\nof H<sub>2</sub> yield and all oxidized products together indicates\nthe better utilization of holes for the latter, and hence, there is\nscope to increase HER and possibly the whole photocatalytic activity.