Photocatalytic oxidation of urea on TiO 2 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⁻.

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