Due to its high reducibility and oxygen storage capacity, ceria (CeO2) is extensively used as an active support for platinum-group metals in fuel-cell electrodes. Here we combine DFT+U simulations with ab-initio molecular dynamics, metadynamics, and umbrella sampling methods to provide insight into the surface chemistry opened by novel ceria catalysts with ultra-lowPt loading (1). The calculations are used to characterize the structure and the reactivity of the active sites in a wide range of compositions and environments, from ideal surfaces to complex nano-structured catalysts in the presence of liquid water at finite temperature. We start with the hydroxylation and reduction of ceria surfaces driven by molecular H2, and demonstrate the so-far unrecognized entropic effects on the reaction mechanism and kinetics. Our metadynamics calculations show that finite temperature effects alter the reaction mechanism, change the nature of the rate-limiting transition state, and decrease the activation temperatures for H2 dissociation by more than 25% (Fig. 1) (2). The study of supported sub-nm Pt clusters reveals that cluster morphology plays an important role in the thermodynamics and kinetics of catalytically relevant surface processes such as cluster mobility, charge transfers at the metal-oxide interface, reverse oxygen spillover, and oxygen vacancy formation (3,4). Our thermodynamics analysis predicts the dispersion and stabilization of specific Pt2+ and Pt4+ species on ceria surfaces in realistic reaction conditions. We demonstrate the key role played by surface line defects and investigate the different reactivity of these sites towards water, hydrogen and methanol oxidation (5,6). Finally, the distinct effects of liquid water, often present at realistic electrodes, on the surface chemistry of Pt-ceria catalysts are characterized on the basis of ab-initio molecular dynamics simulations. FP7-NMP-2012 project chipCAT under Contract No. 310191 References (1) www.chipcat.eu (2) Negreiros, F. R.; Farnesi Camellone, M.; and Fabris, S. submitted. (3) Ghosh, P.; Farnesi Camellone, M.; and Fabris, S. J. Phys. Chem. Lett., 2013, 4 (14), 2256–2263 (4) Negreiros, F. R.; and Fabris, S. J. Phys. Chem. C, 2014, 118 (36), 21014–21020. (5) Szabova, L.; Tateyama, Y.; Matolin, V.; Fabris, S. J. Phys. Chem. C, 2015, 119 (5), 2537–2544. (6) Tran, N.-D.; Farnesi Camellone, M. and Fabris, S. submitted.

Modeling the structure and reactivity of ceria electrodes from ideal to realistic reaction environments

250th National Meeting (2015)

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