(226b) ­Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage and Implications for CO Oxidation Structure Sensitivity

Authors: 
Kale, M., California Institute of Technology
Avanesian, T., University of California, Riverside
Dai, S., University of California, Irvine
Graham, G. W., Unversity of Michigan
Pan, X., University of California-Irvine
Christopher, P., University of California, Santa Barbara
Atomic-scale insights into how different geometric configurations of surface atoms on supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at the atomic scale is critical for understanding the structure sensitivity of various chemical reactions. Elucidation of surface structures of high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions. In addition, a lack of agreement between theoretical structure sensitivity and correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions, as has been the case for CO oxidation on Pt nanoparticles, precludes the correct determination of the active site under reaction conditions.

We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by in-situ TEM and quantitatively with sample-averaged measurements of surface atom configurations by in-situ quantitative infrared (IR) spectroscopy, all at identical saturation adsorbate coverage. This has recently been demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic converter.1 Through our correlated approach, it was observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact. In addition, we apply the same in-situ quantitative IR technique during CO oxidation on various sizes of Pt nanoparticles to measure the concentration of CO bound to under-coordinated (UC) and well-coordinated (WC) Pt sites and relate the turnover frequency of CO oxidation to the concentration of UC and WC Pt sites.2 We have shown that CO oxidation on Pt nanoparticle surfaces is structure sensitive, where WC Pt atoms are the active site, but that this effect is masked by the observed adsorbate-induced reconstructions observed under reaction conditions. Our results provide a complete description of the observed trends in the structure sensitivity of CO oxidation on Pt nanoparticles of various sizes, in agreement with experimental observations and theoretical predictions.

References:

(1) Avanesian, T.; Dai, S.; Kale, M. J.; Graham, G. W.; Pan, X.; Christopher, P. J. Am. Chem. Soc. 2017, 139, 4551.

(2) Kale, M. J.; Christopher, P. ACS Catal. 2016, 6, 5599.

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