Photocatalytic oxidation of urea on TiO2 in water and urine: mechanism, product distribution, and effect of surface platinization.

Abstract

The photocatalytic oxidation of urea on TiO2 in water was compared with that in urine. Despite the presence of other organic compounds in urine, the oxidation efficiency of urea on TiO2 in urine was higher than that in water. This enhanced oxidation of urea in urine is ascribed to the higher production of •OH (primary oxidant for urea degradation) by the adsorption of PO43- (one constituent of urine) on the TiO2 surface. Among the various anions in urine, only PO43- was adsorbed on the surface of TiO2. Both the production of •OH and the oxidation of urea were enhanced in the presence of PO43-. These results indicate that the enhanced •OH production by in situ surface phosphorylation is the reason for the increased oxidation of urea in urine. Surface platinization of TiO2 enhanced the oxidation of urea in water. However, the oxidation efficiency of urea on Pt/TiO2 in urine was lower than that in water. This behavior is due to the adsorption of PO43- and SO42- in urine on Pt deposits, which inhibits the adsorption of oxygen and the interfacial electron transfer to oxygen. The product distribution (i.e., the molar ratio of NO3- to NH4+) in water was different from that in urine because the negatively charged surface of TiO2 in urine attracts the positively charged area of carbamic acid (intermediate) and encourages its decomposition into NH4+ and not into NO3-.