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

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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.