具有高水热稳定性的WO3/CeZrO2@MOx核壳结构催化剂的可控制备及对选择性催化还原NOx的研究

NOx as one of the main air pollutants, which is harmful to the environment and human health, is the current focus of the emission reduction. Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been proved to be one of the most effective methods for NOx elimination from the diesel engines, cerium zirconium mixed oxides (CeZrOx) NH3-SCR catalyst is a hot topic in the academia and industry. However, the poor hydrothermal stability of the catalyst seriously hindered its industrial application. Therefore, we propose this project: the highly stable and dispersed core-shell structure will be introduced into the preparation of catalysts; the controlled preparation of WO3/CeZrOx@MOx core-shell catalyst will be realized by using successive reduction and self-assembling methods and regulating the preparation and treatment conditions. Consequently, the acid sites and redox sites would be distributing effectively and the synergic effects between them would be promoted, as well as the hydrothermal stability. The core-shell catalysts own excellent NH3-SCR performance and high hydrothermal stability, CeZrOx as core, hydrophobic oxide (MOx) with high specific surface as shell, WO3 as promoter, will be favored. Then the NH3-SCR performances of the catalysts would be evaluated systematically before and after hydrothermal aging treatments. The acid and redox sites taking part in NH3-SCR reaction and the key intermediates will be illuminated before hydrothermal aging treatment by in situ DRIFTS, in situ UV-Vis-DRS and in situ XPS, combined with the reaction kinetics study and the catalytic performance evaluation. Subsequently, the mechanism of NH3-SCR reaction before and after hydrothermal aging treatments would be clarified, which could improve the development of the high hydrothermal stable core-shell NH3-SCR catalysts, based on the characterization and analysis of the mixture and structural properties, morphology, acidity and redox properties, etc.

氮氧化物(NOx)是我国主要大气污染物之一，严重危害环境和人体健康。氨选择性催化还原(NH3-SCR)是净化柴油车尾气NOx的有效方法，其中，铈锆复合氧化物(CeZrOx)催化剂是当前的研究热点，但该类催化剂的低水热稳定性严重阻碍了工业化应用。基于此，本项目引入高稳定和高分散的核壳结构，以CeZrOx作为核，具有高比表面的疏水氧化物(MOx)作为壳，调控制备和处理条件，通过连续还原和自主装等方法实现对WO3/CeZrOx@MOx核壳催化剂的可控制备，从而促进了酸中心和氧化还原中心的有效分布及二者协同作用，提高其水热稳定性；通过原位红外、原位紫外和原位X射线光电子能谱等技术，结合反应动力学研究和催化性能评价结果，获取参与反应的各中心和关键中间物在水热老化前后的变化情况，明确水热老化前后的反应机理；结合物化性质表征，优化该类催化剂的设计与制备，为其产业化应用提供关键技术支撑和实验参考。

NOx是我国主要大气污染物之一，严重危害环境和人体健康。NH3-SCR是净化柴油车尾气NOx的有效方法。其中，铈锆复合氧化物(CZ)催化剂因具有优异的SCR性能而成为具有应用前景的NH3-SCR催化剂之一，但该类催化剂的水热稳定性差严重阻碍了其工业应用。本项目通过新型的气泡辅助分散-多晶水热沉积 (GBD-HNHD)策略和化学溶液沉积法(CLD)等方法成功制备了以CZ为核，具有高比表面积的疏水性Al2O3和SiO2作为壳的CZ@MOx (M=Al、Si)复合氧化物，最终实现高密度高分散的WO3/CZ@MOx催化剂的可控制备。通过GBD-HNHD和 CLD法，不仅在W和CZ之间形成扩散屏障，抑制Ce2(WO4)3的生成，还可实现酸性中心和氧化还原中心的高分散及二者之间的有效协同作用，从而提高了低温活性和水热稳定性。与W/CZ相比，a. W/CZ@Al2O3的起燃温度（T50）从 212°C降低至177°C，反应温度窗口拓宽了70°C，且水热老化后NH3-SCR活性基本不降低; b. 水热老化后，W/CZ@SiO2的T50降低了近40°C，反应温度窗口拓宽了40°C；c. WO3和MoO3作为酸性助剂均可显著提高CZ的SCR活性，但MoO3与CeO2之间的强相互作用生成Ce2(MoO4)3造成表面L酸位远低于W/CZ，导致NH3吸附和NOx吸附量降低，最终W/CZ的T50比Mo/CZ的低23°C，反应温度窗口宽76°C，前者基本不生成N2O而Mo/CZ则高达150ppm。上述含W催化剂的SCR反应主要遵循L-H机理，活性之间的差异主要来自于有效酸性位和氧化还原位的不同，老化前后的差异在于两种活性位质与量的变化，但不影响主要反应机理。本项目的W/CZA催化剂已有少量取代钒基催化剂应用于国V柴油车尾气SCR催化剂中，为其后续工业化应用提供关键技术支撑和实验参考。