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Joshua Vincent1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Heterogeneous catalysts accelerate chemical reactions by reducing the required reaction activation energy. The specific locations on the catalyst at which the activation energy is lowest – the so-called active sites – are poorly understood because catalytically relevant atomic structures only emerge under reaction conditions. Even with in situ TEM, determining atomic-level structure-activity relationships is difficult still due to the large number of surface structures forming dynamically during catalysis.

Discerning catalytically relevant structures from spectators may be possible by studying supported metal systems in which the active sites are localized to the metal-support interface. The rate of CO oxidation over Pt/CeO2 has been shown to depend strongly on the perimeter length of the metal-support interface [1]. The interface facilitates the reaction through a Mars van Krevelen mechanism, whereby the CeO2 lattice provides oxygen to oxidize CO to CO2. Interfacial structures that enhance oxygen transfer, for example, through strain, may improve catalytic activity. However, at present there is no experimental data on the atomic structures that comprise the Pt/CeO2 interface during catalysis.

Here, we seek to establish atomic-level structure-activity relationships by identifying and characterizing the active sites for CO oxidation over nanostructured Pt/CeO2. Nanostructured CeO2 cubes will be synthesized and loaded with 2 wt. % Pt by a photodeposition technique [2]. A quartz tube microreactor coupled to a gas chromatograph will be used to determine turn-over frequencies and activation energies. The ex situ reactor data will inform the reaction space explored during in situ TEM experiments. Atomic structures forming at and in the vicinity of the Pt/CeO2 interface during CO oxidation will be visualized with aberration-corrected environmental TEM (AC-ETEM). The observed structures will be correlated with the catalyst’s overall activity through in situ mass spectrometry. Detectable conversions will be achieved through a unique porous microfiber pellet TEM specimen technique [3]. Dynamic particle and interfacial restructuring will be quantified by evaluating changes in particle morphology and interfacial lattice coherency. The observed structures will inform theorists simulating catalyst behavior and experimentalists studying other supported-metal systems. Also, understanding structure-activity relationships in this system will facilitate the engineering of highly active catalysts for the energy and environmental remediation reactions where Pt/CeO2 is indispensably used.

[1] Cargnello, M. et al; Science 341, 771-773 (2013)
[2] Vincent, J. L. et al; Microscopy and Microanalysis 23(S1), 966-967 (2017)
[3] Miller, B. K. et al; Ultramicroscopy 156, 18-22 (2015)
[4] We gratefully acknowledge the support of NSF grant CBET-1604971 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.

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