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

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 tetrabutylammonium 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 H-2 evolution and an 8-fold increase in O-2 evolution over the bulk phase under the same conditions. The sacrificial electron donor methanol improves H-2 production by 2-3 orders of magnitude to 20-30 mmol of H-2/h/g, while the electron acceptor AgNO3 increases O-2 production to 400 mu mol of O-2/h/g. Rates for H-2 and O-2 evolution further depend on the presence of cocatalysts (Pt or IrOx) and, in the case of HI, inversely on their particle size. To rationalize these findings and the increased activity of the nanoscale particles, we propose a kinetic model for photocatalysis with semiconductor particles. The model calculates the electronic rate of the catalysts as a product of terms for charge generation, charge and mass transport, chemical conversion, and charge recombination. The analysis shows that the activity of the catalysts is limited mainly by the kinetics of the redox reactions and by the rate of charge transport to the water-catalyst interface. Mass transport in the solution phase does not play a major role, and neither does surface charge recombination.