Surface-Dependence of Adsorption and Its Influence on Heterogeneous Photocatalytic Reaction: A Case of Photocatalytic Degradation of Linuron on Zinc Oxide

This work systematically investigates a discrepancy in photocatalytic degradation pathway observed even when same kind of photocatalyst is used. This issue is particularly important for the degradation of toxic compound because the shifting in the pathway may result in intermediates having increased toxicity. Linuron, which is a toxic herbicide, is used as a model compound and zinc oxide (ZnO) is used as a catalyst. Two forms of ZnO having different exposing surfaces, i.e., conventional particles and nanorods are considered. It is experimentally shown and confirmed by molecular simulation that the adsorption and subsequent degradation are surface dependent. The adsorption capacity of linuron on polar surfaces of ZnO is much higher than that on non-polar surface. More importantly, alignments of linuron molecule adsorbing on different surfaces are different, leading to formation of different degradation intermediates. The degradation takes place on both aliphatic and aromatic sides when linuron adsorbs on polar surfaces whereas only the aliphatic part is attacked by hydroxyl radicals on the non-polar mixed terminated surface. The results in this work indicate that the exposing surface of the photocatalyst is the key factor in determining the degradation pathway and it should be considered in the catalyst design.