Experimental upper bound on phosphate radical production in TiO2 photocatalytic transformations in the presence of phosphate ions

The spin trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to investigate the involvement of phosphate radicals in TiO2-photocatalyzed systems. A persistent (HO)2PO2–DMPO˙ adduct (aN = 14.6 G, aHβ = 12.4 G, aHγ = 1.1 G and aP = 0.5 G) was first synthesized by UV photolysis of aqueous peroxodiphosphate solutions, at pH 4.0, in the presence of DMPO. The trapping efficiency of the phosphate radicals, η, determined at [DMPO] = 0.3 mM was η = 35%. In the heterogeneous system, the rate of HO–DMPO˙ aminoxyl radical generation under continuous UV irradiation of TiO2 sols, at [DMPO] = 0.3 mM and pH = 4.0, becomes gradually inhibited increasing [(HO)2PO2−] above 0.1 mM. However, no evidence of (HO)2PO2–DMPO˙ could be obtained in the whole range of conditions explored in this work (0.1 ≤ [DMPO]/mM ≤ 1 mM, 0.1 ≤ [(HO)2PO2−]/mM ≤ 100 mM). Based on the above experiments, we estimated a conservative upper limit for the quantum yield of (HO)2PO2˙ production, Φ = 2.0 × 10−4, in phosphate loaded TiO2 particles. The analysis of this result and its implications for the mechanism of the TiO2 photocatalytic oxidations of organic compounds in the presence of phosphate are discussed. Fil: Brusa, Marta Adelina. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Grela, Maria Alejandra. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina