可调谐可见光和频光谱研究甲醇在TiO2(110)表面的光催化反应

TiO2 is one of the most studied materials due to its versatile applications in photocatalytic splitting of water and photodegradation of organic pollutants. Methanol/TiO2 is a prototype for the photocatalytic oxidation of organic contaminants on TiO2 because methanol is the simplest alcohol molecule. However, deep insight into the photocatalytic reaction mechanism of methanol requires more effort. Several controversial questions will be addressed in this proposal. First, since methoxy is considered as the photoactive form of methanol on TiO2 surface, it becomes critical issues whether methanol adsorbs intactly or dissociatively on the surface, to what extent the methanol molecule can spontaneously dissociated into methoxy, and how the photo excitation induces the dissociation of methanol. Second, the details of stepwise photooxidation of methoxy need to be clarified and the identification of the possible intermediate products, such as formaldehyde, methyl formate, require direct spectroscopic evidences. Here, we will study the adsorption and photocatalytic reactions of methanol on TiO2 by sum frequency generation (SFG) spectroscopy, which is capable of probing the surface molecules selectively with sub-monolayer sensitivity. In addition, we will introduce the tunable visible light into the SFG setup, which can generally provide higher detection sensitivity and more importantly the opportunity to probe the electronic structures of the surface. Moreover, the photon-in and photon-out detection mode of SFG coupled with a home-developed vacuum chamber allows us to study the photocatalytic mechanism from ultrahigh vacuum to high pressure conditions, and to fill the pressure gap between model research and the realistic application. This project will deepen our understanding of photocatalytic process on a microscopic level and improve its applied research.

TiO2因在光催化分解水以及光催化降解有机物等方面的应用成为研究最为广泛的材料之一。作为最简单的醇，甲醇/TiO2成为研究有机污染物光催化氧化的模型体系。但目前，甲醇在TiO2表面的吸附状态和反应机理还存在诸多争议。首先，甲氧基是表面具有光反应活性的物种，因而甲醇在表面能否发生，或在多大程度上发生自发解离吸附，以及光照如何诱导甲醇解离等成为关键问题。其次，甲氧基分步氧化机理以及可能产生的甲醛等产物需要直接光谱证据。我们拟利用具有界面选择性的和频光谱方法来研究单晶TiO2表面的光催化过程，并利用可调谐可见光来提高检测灵敏度和得到表面电子态结构信息。还将利用和频光谱光进光出的特点，结合自行研制的适用于从超高真空到高压环境检测的真空腔体，研究不同气压环境下的光催化反应机理，填补模型催化和实际催化之间的“压力断层”。本项目将大大加深对TiO2表面光催化反应机理的理解，促进光催化的应用研究。

本课题拟利用可调谐波长的和频振动光谱来研究水及有机物分子在TiO2表面光催化降解反应的机理。作为最简单的醇，甲醇/TiO2成为研究有机污染物光催化氧化的模型体系。在项目支持下，我们在固定可见光波长的和频光谱基础上搭建的可见光波长可调谐的和频光谱检测系统，并观察到TiO2体系的波长依赖关系。我们比较了650nm作为可见光与800nm作为可见光，与CH波段红外发生界面和频作用时，前者信号要比后者强一个数量级。初步判断是由于TiO2表面态共振造成的和频信号增强。这个结果展现了可调谐可见光和频光谱作为材料表面电子态结构的潜力，而表面电子态结构是材料领域，特别是凝聚态领域十分重要的科学问题。