NSF-GACR: Atoms to nanoparticles to atoms - predicting evolving catalyst activity under inherently transient conditions

金属交换沸石是常用的催化剂，用于从化石燃料和生物质碳氢化合物转化到二氧化碳捕获和减少柴油发动机尾气排放的各种应用。在反应条件下，描述其中许多反应的活性中心仍然是难以捉摸的，因此限制了催化剂设计的改进。根据反应条件的不同，离子交换金属原子、溶剂化离子、二聚体、三聚体、小团簇和纳米颗粒都被称为活性催化中心。此外，对于某些反应，这些物种中的几个可能共存。该项目的总体目标是与捷克斯洛伐克布拉格化学与技术大学(UCTP)进行国际合作，以表征和预测在反应条件下催化剂/表面动态变化过程中含金属沸石中活性催化物种的类型和数量的变化，从而评估活性变化的分子来源。由此产生的见解将导致改进的催化剂设计，从而能够更有效地使用昂贵的贵金属，提高催化过程的能源效率，并增加催化剂的耐用性和投产时间。该项目将开发和使用方法来量化沸石中不同类型的铜(Cu)和钯(Pd)，以及它们如何受到典型反应环境条件和温度的影响。这两种金属与SSZ和BEA沸石一起被选中，因为它们用作选择性催化还原(SCR)催化剂和被动NOx吸附器(PNA)，这是排放控制催化的关键技术。探测分子反应以及红外和X射线光谱技术将被用来分离单独的铜和钯部分。例如，探针反应包括多核铜中心的CO滴定，离子交换铜的NO+NH3滴定，以及NO吸附滴定离子交换Pd中心。将研究可能引起活性物种变化的各种反应条件。这些因素包括温度、氧化与还原混合气体以及水含量。此外，该项目还将调查接触二氧化硫(SO2)--一种常见的催化剂毒物--如何影响活性物种的变化。除了确定活动站点之外，该项目还将检查活动站点的动态变化可逆的程度。确定促进可逆变化的操作条件至关重要，因为可逆性允许催化剂再生，从而延长催化剂寿命。识别和量化单个活性中心类型的实验数据，以及这些数量如何随环境变化，将被用于建立预测的动力学SCR和PNA模型，其中包括单个活性中心浓度作为气体组成、温度和时间的函数的演化。除了技术方面，该项目还包括为研究生和本科生提供的研究培训机会，以及美国和捷克研究人员与他们的研究生进行合作交流的机会。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Metal-exchanged zeolites are common catalysts, used in applications ranging from fossil fuel and biomass hydrocarbon transformations, to CO2 capture, and mitigation of diesel engine exhaust emissions. Describing the active sites for many of these reactions, under reaction conditions, remains elusive, thus limiting improvements in catalyst design. Depending on the reaction conditions, ion-exchanged metal atoms, solvated ions, dimers, trimers, small clusters, and nanoparticles have all been invoked as active catalytic sites. Further, for some reactions, several of these species may co-exist. The overarching goal of the project - an international collaboration with the University of Chemistry and Technology in Prague, Czechoslovakia (UCTP) - is to characterize and predict changes in the type and amount of active catalytic species in metal-containing zeolites during dynamic catalyst/surface changes under reaction conditions, and thereby assess the molecular origin for changes in activity. The resulting insights will lead to improved catalyst designs enabling more efficient use of expensive noble metals, improved energy efficiency of catalytic processes, and increases in catalyst durability and time-on-stream. This project will develop and use methodologies to quantify different types of copper (Cu) and palladium (Pd) species in zeolites, and how they are impacted by typical reaction environment conditions and temperatures. These two metals were chosen, along with SSZ and BEA zeolites, due to their use as selective catalytic reduction (SCR) catalysts and passive NOx adsorbers (PNA), critical technologies in emissions control catalysis. Probe molecule reactions, as well as infrared and X-ray spectroscopic techniques, will be used to isolate individual Cu and Pd moieties. Example probe reactions include CO titration of multinuclear Cu sites, NO+NH3 titration of ion-exchanged Cu, and NO adsorption to titrate ion-exchanged Pd sites. Various reaction conditions will be studied that are likely to induce changes in active species. Those include temperature, oxidizing vs. reducing gas mixtures, and water content. In addition, the project will investigate how exposure to sulfur dioxide (SO2) - a common catalyst poison - influences changes in active species. Beyond active site identification, the project will examine the extent to which the dynamic changes in active sites are reversible. Identification of operating conditions that promote reversible changes are critical, as reversibility allows catalyst regeneration, thus extending catalyst lifetimes. Experimental data identifying and quantifying individual types of active sites, and how those quantities change with environment, will be used to build predictive kinetic SCR and PNA models that include evolution of individual active site concentration as a function of gas composition, temperature, and time. Beyond the technical aspects, the project includes research training opportunities for both graduate and undergraduate students and opportunities for collaborative exchange between American and Czech researchers and their graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.