Photocatalytic Water Splitting with Suspended Calcium Niobium Oxides: Why Nanoscale is Better than Bulk − A Kinetic Analysis

ABSTRACT: The layered Dion−Jacobson phase KCa2Nb3O10 is known to catalyze photochemical water reduction and oxidation under UV light in the presence of sacrificial agents. The same reactions are catalyzed by tetrabutyl-ammonium hydroxide-supported HCa2Nb3O10 nanosheets obtained by chemical exfoliation of the parent phase. Here we describe a factorial study into the effects of nanoscaling, sacrificial charge donors, cocatalysts, and cocatalyst deposition conditions on the activity of these catalysts. In water, nanoscaling leads to a 16-fold increase in H2 evolution and an 8-fold increase in O2 evolution over the bulk phase under the same conditions. The sacrificial electron donor meth-anol improves H2 production by 2−3 orders of magnitude to 20−30 mmol of H2/h/g, while the electron acceptor AgNO3 increases O2 production to 400 μmol of O2/h/g. Rates for H2 and O2 evolution further depend on the presence of cocatalysts (Pt or IrOx) and, in the case of H2, inversely on their particle size. To rationalize these findings and the increased activity of the nano-scale particles, we propose a kinetic model for photocatalysis with semiconductor particles. The model calculates the electroni