Surface Composition and Chemical Behavior of Supported Single-Atom Alloy (SAA) Nanoparticles under Catalytic Conditions

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With the support of the Chemical Catalysis program in the Division of Chemistry, Professor Francisco Zaera of the University of California, Riverside is studying the chemical behavior of so-called single atom alloy (SAA) catalysts. Catalysis is used extensively in industry to manufacture chemicals, and transition metals are in many instances the key components of the catalysts used. To tune the properties of metal catalysts to optimize catalytic performance, alloys of two or more metals are sometimes used. In recent years, much interest has developed in using a particular version of metal alloys, SAAs, as catalysts where a small amount of one metal, e.g. platinum (Pt) in the Zaera lab, is dispersed on nanoparticles (NPs) of a second (Cu) to add specific new functionality. In the case of hydrocarbon hydrogenations, a family of important catalytic processes where hydrogen atoms (H) are added to carbon-carbon or carbon-oxygen multiple bonds, research using model systems has suggested that the Pt atoms are needed to dissociate H2 to provide the required H atoms. However, preliminary studies under realistic catalytic conditions have indicated that the behavior of the single atoms may be more nuanced. This project focuses on characterizing the nature of the surfaces of SAA catalysts under catalytic conditions to better understand the behavior of the minority (Pt) and majority (Cu) metals and their contributions to hydrogenation catalysis to improve catalytic performance. The knowledge derived from this work is aimed to help tune the selectivity of processes for the industrial production of fine chemicals. The knowledge derivied from this work will also be useful for educational purposes, by providing data to illustrate basic principles in kinetics, catalysis, and NP synthesis in undergraduate and graduate classes. Collaborations with Latin American research groups are being forged, and student participation from groups underrepresented in research is being pursued.In these studies, Professor Francisco Zaera of the University of California, Riverside is studying the surface composition and chemical behavior of supported SAA nanoparticles (NPs) under catalytic conditions. The main objective of this project is to characterize SAA catalysts in situ or under operando conditions (while carrying out catalytic processes) to bridge the pressure and materials gaps and better understand their working surface chemistry. Two hypotheses underpin this proposal: (1) that the minority metal (Pt) may diffuse into the bulk under reaction conditions and influence catalysis in a remote way (a direct consequence of the pressure gap); and (2) that such environment-induced metal diffusion may occur under different conditions in supported NPs vs. in bulk materials (a direct manifestation of the materials gap). Three parallel experimental approaches are being followed to test the key hypotheses. First, kinetic measurements under catalytic conditions are being performed on model systems by using a so-called "high-pressure cell" and complemented and correlated with results from in situ characterization of the surface using reflection-absorption infrared spectroscopy (RAIRS). Second, synthetic methods are being developed to make well mixed diluted alloys and core-shell bimetallic NPs to investigate the possible diffusion and intermixing of the metals under reaction conditions. Third, catalysts are being characterized under catalytic conditions in situ and under operando conditions by X-ray absorption spectroscopy (XAS) to evaluate changes in the coordination sphere around the minority atoms, one way to determine their presence on the surface vs. inside the bulk, and by infrared absorption spectroscopy (IR), to extract information about the adsorption mode of the reactants and to titrate the minority atoms available at the surface under catalytic conditions. The proposal combines studies with model systems, new catalyst synthesis methodology, in situ spectroscopic studies of catalysts, and quantum mechanics calculations to answer a critical question; namely, the effect of the pressure and materials gaps on SAA performance, with wide-ranging implications to the field of catalysis.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.