Effective photocatalytic reduction of nitrate to ammonia in an aqueous suspension of metal-loaded titanium(IV) oxide particles in the presence of oxalic acid

Photocatalytic reduction of nitrate ion (NO3−) in an aqueous suspension of metal-loaded titanium(IV) oxide (TiO2) was examined in the presence of oxalic acid (OA) as a hole scavenger. Conversion of NO3− into ammonia (NH3) competed with hydrogen liberation, and the NH3 production selectivity increased with the order of loaded metal, (Pt, Pd, Co) < (Ni, Au) < (Ag, Cu), which was attributable to the efficiency of reduction of protons by photogenerated electrons at the loaded metal, i.e., hydrogen overvoltage of the loaded metal. TiO2 powder loaded with Cu showed higher NH3 yield and selectivity as well as higher efficiency of OA consumption. TiO2 with in situ deposited Cu gave results comparable to those of a Cu pre-loaded photocatalyst.