Catalytic activity of Pt38 in the oxygen reduction reaction from first-principles simulations

The activity of truncated octahedral Pt38 clusters as a catalyst in the oxygen reduction reaction (ORR) is investigated via first-principles simulations. Three catalytic steps: O2 dissociation (O2ads -> 2Oads), O hydration (Oads + H2Oads -> 2OHads), and H2O formation (OHads + Hads -> H2Oads) are considered, in which all reactant species are co-adsorbed on the Pt38 cluster according to a Langmuir-Hinshelwood mechanism. The minimum structures and saddle points for these different steps are then calculated at the densityfunctional theory (DFT) level using a gradient-corrected exchange-correlation (xc-)functional and taking into account the effect of the solvent via a self-consistent continuum solvation model. Moreover, firstprinciples molecular dynamics (AIMD) simulations in which the H2O solvent is explicitly described are performed to explore dynamic phenomena such as fast hydrogen transfer via meta-stable hydronium-type configurations and their possible role in ORR reaction paths. By comparing the present findings with previous results on the Pt(111) surface, it is shown that in such a nanometer-size cluster the rate-determiningstep (rds) corresponds to H2O formation, at variance with the extended surface in which O hydration was rate-determining, and that the overall reaction barrier is actually increased with respect to the extended system. This is in agreement with and rationalizes experimental results showing a decrease of ORR catalytic activity in the nanometer-size cluster range.