In this work,the reaction mechanism of photocatalytic oxidation of sacrificial ethanol during water-splitting process by titanium dioxide (TiO2) has been studied.The pure futile TiO2 or mixed-phase structure titania (P25) was employed as the typical photocatalyst in ethanol oxidation.The as-obtained results showed that the formation of 2,3-butanediol over TiO2 in heterogeneous systems is mainly due to the photochemical reaction proceeded between acetaldehyde molecule and ethanol molecule instead of the direct coupling of α-hydroxyethyl radicals.This is different from the early work claimed that the fundamental process to produce 2,3-butanediol is based on the direct coupling of α-hydroxyethyl radicals generated by TiO2 oxidation.The photochemical reaction between acetaldehyde molecule and ethanol molecule to form 2,3-butanediol can also occur when the concentration of the solid catalyst was reduced to certain degree if using P25 as catalyst in heterogeneous model,and the selectivity of 2,3-butanediol would change from ca.60％ to 0％ when enlarging the concentration of P25 step by step.However,the selectivity of 2,3-butanediol is relatively invariable when the concentration of catalyst was changed if using rutile as photocatalyst.We thought that the distinct diffusing behaviors for mobile ·OHf and surface bound ·OHs generated on different titania can explain the varied selectivity when the solid concentration of TiO2 changed.The generation and diffusion of,OH from the surface of P25 (80％ anatase) to bulk solution is a key process to inhibit the direct coupling of α-hydroxyethyl radicals to produce acetaldehyde or further overoxidation products,and the reaction zone of ·OHf depends on the concentration of P25.For the case of rutile TiO2 promoted reaction,the lack of mobile ·OHf on rutile TiO2 makes the photochemical reaction between acetaldehyde molecule and ethanol molecule more facile to occur in bulk solution since the surface bound ·OHs can only have chance to attack the surface adsorbed substrates.This may be an important reason to explain why the selectivity of 2,3-butanediol in ethanol oxidation was not influenced significantly by the variation of rutile TiO2 concentration.All the results regarding ethanol transformation during photocatalytic process achieved here cast some light on the mechanistic understanding of the reactions proceeded on the surface of solid catalyst in heterogeneous model and in the bulk solution when both catalytic step and photochemical step existed simultaneously.%光催化分解水的产氢体系往往采用小分子醇作为电子供体,本工作以纯金红石相与混晶结构P25 (80％锐钛矿与20％金红石相)二氧化钛为光催化剂,研究光解水过程中牺牲剂乙醇的氧化路径与反应机理.研究结果表明,无论是金红石相还是锐钛矿相为主的P25二氧化钛,其无氧条件下光催化氧化乙醇的主要终端产物均为乙醛,而不是2,3-丁二醇,2,3-丁二醇产物主要由醇醛特定波长范围内的有机光化学反应偶联产生.另外,不同晶相二氧化钛氧化乙醇至2,3-丁二醇的选择性差异可能主要由非均相固液界面所产生羟基自由基迁移至溶液体系的距离不同所致,进而与有机光化学反应产生的·CH(OH)CH3自由基发生不同路径与频率的猝灭反应.本研究对乙醇无氧光催化反应步骤的探索与解析可为小分子醇参与的醇类重整反应提供机理借鉴与设计思路.
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