Effect of carbonate addition on the photocatalytic decomposition of liquid water over a ZrO2 catalyst

The stoichiometric photodecomposition of water into H-2 and O-2 proceeds over ZrO2 powder suspended in aqueous solutions in the absence of loaded metals such as Pt, RuO2 and NiOx. The activity is affected significantly by the pressure of the reaction system, the types of additive and the pH of the solution. Specifically, the addition of carbonate, such as NaHCO3 and Na2CO3, leads to a remarkable increase in the activity and stability of the gas evolution rate vs, the reactor pressure. The surface of ZrO2 was covered with carbonate species by the addition of carbonate salt. It is thought that carbonate species on ZrO2 play an important role in the desorption of O-2 via the carbonate radical. In contrast, the promotion effect of platinum loading on water splitting is negligible. This behaviour can be explained in terms of the backward reaction (H2O formation from H-2 and O-2) on Pt and the barrier height of the semiconductor-metal junction at which electrons in the conduction band migrate. Furthermore, it is concluded that the suppression of the backward reaction on Pt is essential for overall water splitting into H-2 and O-2, and this is one of the important functions of NaHCO3 and Na2CO3. Pt on ZrO2 in carbonate solutions exists as an oxide; therefore the effect of the backward reaction is suppressed.