Photocatalytic disproportionation of nitrite to dinitrogen and nitrate in an aqueous suspension of metal-loaded titanium(IV) oxide nanoparticles

Photocatalytic reaction of a nitrite ion in aqueous suspensions of bare and metal-loaded TiO2 particles was examined without electron and hole scavengers under irradiation of UV light. In the bare TiO2 system, disproportionation of NO2- to N-2 (or N2O) and NO3- with nitrogen balance (NB) and redox balance (ROB) close to unity within experimental errors was observed, although the reaction was slow. Palladium (Pd)-loaded TiO2 particles exhibited an extraordinarily large rate of disproportionation of NO2- in their aqueous suspension, i.e. NO2- was almost completely converted to N-2 (or N2O) and NO3- even after only 3 h of photoirradiation, both the values of NB and ROB being close to unity. This result suggests that Pd loaded on TiO2 particles acted as storage sites for photogenerated electrons and effectively transferred the electrons to NO2- and, therefore, that the reduction process in the photocatalytic disproportionation of NO2- was accelerated by Pd loaded on TiO2. Effects of the amount of Pd and pH of the suspension on the reaction rate were also examined.