溶液pH对羟基苯硫酚同分异构体脱羟基反应影响的SERS研究

金属纳米结构因表面等离激元(SPR)而产生光学增强和催化效应已成为表面科学研究热点之一。SPR和电化学联用可以诱导催化一些非常规反应，并且不同pH值电解质溶液可改变表面吸附分子的存在形式，影响SPR光催化反应。以羟基苯硫酚的同分异构体为探针，采用电化学表面增强拉曼光谱(SERS)研究了取代基羟基位置、溶液pH值等对其在银电极表面吸附和SPR催化反应行为。结果表明，不同羟基取代基位置的羟基苯硫酚SPR催化脱羟基反应对溶液pH值的敏感程度不同，邻羟基苯硫酚(OHTP)的C—O键谱峰强度的变化与溶液pH值相关，其O端更易与金属作用而吸附在表面，且随pH增大而增强。对羟基苯硫酚(PHTP)在碱性条件下被完全抑制的脱羟基反应在间羟基苯硫酚(MHTP)和OHTP中均可发生。MHTP在中性(pH 7)溶液中SPR催化脱羟基反应效率最高，约为酸性(pH 2)的1.36倍，碱性(pH 12)的2.70倍。OHTP在碱性(pH 12)溶液中SPR催化脱羟基反应效率最高，约为酸性(pH 2)的13.71倍，中性(pH 7)的4.95倍。SPR催化脱羟基主要源于非去质子化条件以及形成Ag—O键这两种途径。酸性条件下MHTP及OHTP的脱羟基反应主要是未去质子化的羟基反应，碱性条件主要因去质子化后形成Ag—O键所致。中性条件下，两种贡献同时发生。对MHTP而言，由于位阻效应仅部分分子去质子化后形成Ag—O键而促进SPR催化脱羟基，因此pH 7溶液中两种效应的同时作用导致催化效率最高。对于OHTP分子，去质子化状态的O端更易与电极表面发生作用，且pH升高羟基呈现的去质子化程度更加彻底，更有利于发生脱羟基反应，在pH 12溶液中脱羟基反应主要由于形成Ag—O键，其效率亦最高。同分异构体结构以及介质酸碱度对SPR催化脱羟基反应的研究对于拓宽SPR催化反应类型及从分子水平解析其机理具有重要意义。

Surface-Enhanced Raman Spectroscopic Investigation on the Effect of Solution pH on Dehydroxylation of Hydroxythiophenol Isomers

The optical enhancement effect and catalytic activities produced by surface plasmon resonance (SPR) of metallic nanostructures have become one of the hot fields in surface scientific research. The SPR and electrochemical control combine to induce and catalyze the unconventional reactions, and electrolyte solutions with different pH values affect the SPR photocatalytic reaction by changing the adsorption form of surface adsorbed molecules. In this study, the adsorption and reaction behaviors of the isomers of hydroxythiophenol were served as probes modified on the Ag electrode and were investigated by the combination of electrochemistry and surface-enhanced Raman spectroscopy (SERS). The results revealed that the SPR-catalyzed dehydroxylation reaction of hydroxythiophenol with different hydroxyl substituent positions exhibited different sensitivity to the pH value of the solution. The C—O bond peak intensity of o-hydroxythiophenol (OHTP) was related to the pH value of the solution. The O end was easier to interact with metal and adsorb on the surface, and it improved with increased pH. Under alkaline conditions, the dehydroxylation reaction of p-hydroxythiophenol (PHTP) was completely inhibited, and it could occur in both meta-hydroxythiophenol (MHTP) and OHTP. MHTP held the highest SPR catalytic dehydroxylation reaction efficiency in neutral (pH 7) solution, which was about 1.36 times that of acidic (pH 2) and 2.70 times that of alkaline (pH 12). OHTP exhibits the highest reaction efficiency in alkaline (pH 12) solution, which was about 13.71 times that of acidic (pH 2) and 4.95 times that of neutral (pH 7). SPR-catalyzed dehydroxylation was mainly contributed by two approaches non-deprotonation conditions and Ag—O formation. The dehydroxylation reaction of MHTP and OHTP under acidic conditions was mainly due to the undeprotonated hydroxyl reaction, and the formation of Ag—O mainly caused the alkaline conditions after deprotonation. Under neutral conditions, both contributions occurred simultaneously. For MHTP, due to the steric hindrance, only part of the molecules was deprotonated to form Ag—O, which promoted the catalytic dehydroxylation of SPR. Therefore, the simultaneous action of the two effects in the pH 7 solution led to the highest catalytic efficiency. For OHTP molecules, the O terminal in the deprotonated state was more likely to interact with the electrode surface, and the degree of deprotonation of the hydroxyl group as the pH increases was more thorough, more conducive to the dehydroxylation reaction. The dehydroxylation reaction in the pH 12 solution mainly occurred in Ag—O, where the efficiency was the highest. The study of the isomer structure and the pH of the medium on the SPR-catalyzed dehydroxylation reaction was of great significance for broadening the types of SPR catalytic reactions and analyzing the mechanism at the molecular level.