Photocatalytic reduction of nitrite to dinitrogen in aqueous suspensions of metal-loaded titanium(IV) oxide in the presence of a hole scavenger: an ensemble effect of silver and palladium co-catalysts

Nitrite (NO2-) was photocatalytically reduced to dinitrogen (N-2) in an aqueous suspension of two kinds of titanium(IV) oxide particles loaded with palladium and silver (Pd-TiO2 and Ag-TiO2) at pH 8 under irradiation of UV light in the presence of sodium oxalate as a hole scavenger. The two metal-loaded TiO2 photocatalysts had different roles in conversion of NO2- to N-2 and worked in an effective ensemble without conflict: (1) Pd-TiO2 induced photocatalytic disproportionation of NO2- to N-2 and nitrate (NO3-) and (2) Ag-TiO2 selectively reduced the thus-formed NO3- back to NO2- (partially to N-2) with oxalate acting as a hole scavenger. When Pd-TiO2 was used alone for NO3- reduction in the presence of sodium oxalate, Pd-TiO2 induced fruitless photocatalytic decomposition of oxalate to carbon dioxide and dihydrogen. The presence of Ag-TiO2 suppressed the fruitless decomposition of oxalate by Pd-TiO2 because Ag-TiO2 continuously provided NO2- in the reaction system using oxalate as a hole scavenger and Pd-TiO2 therefore only worked as a photocatalyst for disproportionation of NO2- to N-2 and NO3- as it did when used alone.