碱金属催化碳烟燃烧反应的普适作用机制研究

The soot particulate, especially PM2.5, emitted from diesel vehicles have led to the severe gray haze over large area of China in this year. Due to the small particle size, PM2.5 is easily inbreathed into the respiratory tract of human, resulting in respiratory and cardiovascular diseases. In the methods to control the emission of soot particulate, catalytic combustion is the most economical and most effective one. For soot combustion, alkali metals containing materials behave excellent activities amongst. However, the explanation for the catalytic roles of alkali metals are still in dispute. In this project, the nature of catalysis for alkali metal catalyzing soot combustion will be revealed based on the electronic properties of alkali metals. The study will be begun from the research on the reaction mechanism. The alkali metals (Na, K, Rb and Cs) supported metal oxides will be tested to catalyze the reaction of soot with O2, in which both apparent and inherent activities of alkali metals will be obtained. Before, during, after the reactions, the structure characterization and surface analysis of both catalysts and soot will be in situ or ex situ performed, and thus the active sites of alkali metal and reaction intermediates will be distinguished. Besides, the reaction rout of O2 will be tracked. Based on these results, the universal reaction mechanism for alkali metal catalyzing soot combustion will be deduced. Furthermore, the unique electronic properties of alkali metals will be related to the data of structure and activities of catalysts. Finally, the nature of catalytic roles of alkali metal will be revealed based on the calculation of electronic structure. The finding of this project will provide instruction to design the catalysts with high activity for soot combustion, which would be important for the control of the emission of PM2.5.

柴油车碳烟颗粒物-尤其是PM2.5-的排放导致了我国今年大面积的严重雾霾。由于粒径较小，PM2.5极易进入人的呼吸道，引发各种呼吸系统和心血管疾病。控制碳烟排放的最经济和最有效的方式是催化燃烧。在各类碳烟燃烧催化剂中，廉价的碱金属材料表现出优异的活性。但是，对碱金属催化作用的解释仍存在争议。本项目从研究反应机理出发，基于碱金属的电子特性揭示其催化作用本质。以Na、K、Rb、Cs等碱金属负载的金属氧化物为催化剂，考察其催化碳烟与O2反应的表观活性和内在活性。对反应前、中、后的催化剂和碳烟进行原位或离线的结构表征以及表面分析，识别碱金属活性位和反应中间物种，跟踪O2分子，推导碱金属催化碳烟燃烧的普适反应机理。在此基础上，归纳结构和活性数据，关联碱金属的电子特性，通过电子结构计算揭示碱金属的催化作用本质。本项目的研究成果可为高活性碳烟燃烧催化剂的设计提供依据，对有效控制PM2.5排放有重要意义。

柴油车碳烟颗粒物-尤其是PM2.5-的排放导致了我国近几年大面积的严重雾霾。由于粒径较小，PM2.5极易进入人的呼吸道，引发各种呼吸系统和心血管疾病。控制碳烟排放的最经济和最有效的方式是催化燃烧。在各类碳烟燃烧催化剂中，廉价的碱金属材料表现出优异的活性。但是，对碱金属催化作用的解释仍存在争议。本项目以Na、K、Rb、Cs等碱金属负载的金属氧化物(MgO、Al2O3、CeO2、ZrO2)为催化剂，评价了其催化碳烟与O2反应的表观活性和内在活性，并对反应前、中、后的催化剂和碳烟进行原位或离线的结构表征以及表面分析，识别反应活性位为碱金属离子，其普遍的反应中间物种为烯酮物种。结合18O同位素示踪实验，推导碱金属催化的氧转移机理：O2 分子被碱金属离子活化，然后溢流到碳烟表面形成的烯酮物种，而烯酮物种又被活化氧进一步氧化为CO2。同时，碱金属电负性与其内在活性存在正相关性，结合UPS表征和以及半经验分子轨道计算结果证实了碱金属催化的电子转移机理：在碱金属离子静电场的作用下，碳烟内部电子迁移至边缘，提高了边缘碳原子与亲电物种（氧气和活性氧）的反应活性。进一步地，本项目以碱金属氯化物为碳烟燃烧反应的模型催化剂，结合Raman和XPS表征以及DFT计算，揭示了氧转移机理和电子转移机理的统一性，即碳烟与碱金属离子存在静电势作用，提升了两者之间的静电势，驱动了电子从碳烟向碱金属离子的转移，然后活化氧气分子，以活性氧的形式转移到碳烟表面，完成碳烟的氧化。本项目的研究成果可为高活性碳烟燃烧催化剂的设计提供依据，对有效控制PM2.5排放有重要意义。