羟基锡酸盐表面受阻路易斯酸碱对构建及光催化还原CO2机制研究

Great significance of tailoring effective sites on surface of semiconductor-based photocatalysts to promote reaction reactivity of inert CO2 molecule in thermodynamics is for efficient CO2 reclamation. Frustrated Lewis acid-base pairs are well demonstrated for efficient molecular activation. However, big challenges still remain to find an efficient route to construct the frustrated Lewis acid-base pairs on inorganic materials. In this project, photocatalysts of hydroxystannates are intended to be synthesized, and frustrated Lewis pairs are more likely garnered by formation of the coordinately unsaturated surface metal ions (Lewis acid sites) over oxidation of surface hydroxyls (Lewis base sites) by photohenerated holes of hydroxystannates. Activation of CO2 into carbonate species could be achieved by the frustrated Lewis pairs in the presence of unoccupied orbital of Lewis acid and lone electron pairs of Lewis base, thus decreasing reaction barriers and improving conversion efficiency. The main research contents are as follows: (1) Analyzing the mechanisms for formation of the frustrated Lewis pairs and CO2 activation. (2) Revealing inner rules of CO2 activation between the surface chemistry of hydroxystannates and the frustrated Lewis pairs. (3) Exploring effective routes for improving CO2 reduction with the help of the frustrated Lewis pairs. This project is respected, from aspect of molecular activation, to provide valuble guidance for developing high performance photocatalyst and making progressive understanding of reaction mechanism for the CO2 photoreduction.

在半导体光催化材料表面构建有效的催化活化位点，以提高热力学惰性CO2分子的反应活性，对推进CO2资源化具有重要意义。受阻路易斯酸碱对具有较强的活化分子能力，然而，在无机材料表面合理构建受阻路易斯酸碱对依然面临巨大挑战。本项目拟以羟基锡酸盐为研究基础，提出利用光生空穴腐蚀晶格羟基（路易斯碱）生成配位不饱和的金属阳离子（路易斯酸）以构建受阻路易斯酸碱对。路易斯酸提供空轨道，路易斯碱提供孤对电子，将CO2活化为碳酸盐类物质，从而降低反应能垒，提高还原效率。主要研究内容包括：（1）解析受阻路易斯酸碱对的形成规律与CO2活化机制；（2）揭示羟基锡酸盐的表界面特性与受阻路易斯酸碱对活化CO2之间的内在联系；（3）提出提高受阻路易斯酸碱对改善CO2还原性能的有效方法。本课题旨从分子活化方面，为开发具有高性能的光催化剂，以及揭示光催化CO2还原反应机理提供一定的理论指导。

针对光催化CO2还原反应动力学缓慢、反应能垒高等限制因素，本项目利用光生空穴氧化羟基锡酸盐的表面晶格羟基构建全固体受阻路易斯酸碱对，使其有效吸附活化惰性CO2分子，实现反应性能的显著提升。研究发现的相关机理具体如下：羟基锡酸盐表面的Sn-OH羟基容易被光生空穴氧化产生配位不饱和Sn离子（路易斯酸），在晶格束缚作用下，其能耦合毗邻的晶格羟基（路易斯碱）形成受阻的路易斯酸碱对，从而促进惰性CO2分子的氢化反应活性，有效提升了CO（主要产物）和CH4产量。酸碱位点间较小的空间距离（位阻效应）对提升CO2还原性能有着重要影响。此外，热空穴的引入会丰富路易斯酸位点（如ZnSn(OH)6中的Zn离子），而受阻路易斯酸碱对的引入对促进CO2还原C-C耦合产C2+产物同样有着重要的作用。本项目拓展了受阻路易斯酸碱对在无机材料体系中的应用，同时为开发高性能的光催化材料及揭示CO2还原反应机理提供了一定的理论指导。