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 (O_2ads → 2O_ads), O hydration (O_ads + H₂O_ads → 2OH_ads), and H₂O formation (OH_ads + H_ads → H₂O_ads) 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 density-functional 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, first-principles 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-determining-step (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.