Experimental upper bound on phosphate radical production in TiO2 photocatalytic transformations in the presence of phosphate ions
作者:Marta A. Brusa、Mar�a A. Grela
DOI:10.1039/b302296j
日期:——
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.
利用 5,5-二甲基-1-吡咯啉-N-氧化物(DMPO)的自旋捕获技术研究了磷酸自由基在二氧化钛光催化体系中的参与情况。首先在 DMPO 的存在下,通过对 pH 值为 4.0 的过氧化二磷酸水溶液进行紫外光解,合成了一种持久的 (HO)2PO2-DMPO˙ 加合物(aN = 14.6 G、aHβ = 12.4 G、aHγ = 1.1 G 和 aP = 0.5 G)。当[DMPO] = 0.3 mM 时,磷自由基的捕获效率η = 35%。在异构体系中,当[DMPO] = 0.3 mM 和 pH = 4.0 时,钛白粉溶胶在连续紫外线照射下的 HO-DMPO˙ 氨氧自由基生成速率会逐渐受到抑制,[(HO)2PO2-] 增加到 0.1 mM 以上。然而,在本研究探讨的整个条件范围内(0.1 ≤ [DMPO]/mM ≤ 1 mM,0.1 ≤ [(HO)2PO2-]/mM ≤ 100 mM),均未发现 (HO)2PO2-DMPO˙ 的迹象。根据上述实验,我们估算出磷酸盐负载 TiO2 粒子中 (HO)2PO2˙ 生成的量子产率的保守上限为 Φ = 2.0 × 10-4。本文讨论了对这一结果的分析及其对在磷酸盐存在下 TiO2 光催化氧化有机化合物机理的影响。