No abstract

The adsorption and dissociation of O2 on the Pt(111) surface in both the absence and the presence of the hydrated proton were investigated using ab initio DFT calculations to evaluate the role of the proton in the initial steps of the Pt-catalyzed oxygen reduction reaction (ORR) in acid solutions. The results from geometric optimization and electronic structure and minimum energy path calculations indicated that, although in both cases, a t-b-t configured chemisorption state serves as the most stable molecular precursor for the dissociation of O2, the formation of this precursor state and its dissociation are substantially altered in the presence of the hydrated proton. The interactions of O2 with the hydrated proton inhibit the formation of the t-b-t precursor state but facilitate its dissociation. In the presence of the hydrated proton, the t-b-t molecular chemisorption of O2 is preceded by a metastable end-on chemisorption state that is protonated while the t-b-t state itself is not protonated. That is, the chemisorption of O2 on Pt in acid solution may undergo a sequential protonation and deprotonation process. It is also shown that the transformation from the end-on state to the t-b-t state is nearly a nonactivated process with the reaction energy larger in amount than the activation energy required for the subsequent dissociation. The formation of the end-on state via a proton-coupled electron-transfer process is, therefore, identified as the rate-determining step in the adsorption and dissociation processes of O2 on the Pt(111) surface in acidic media. The present calculation results may provide a link between the long disputed Damjanovic’s view and the Yeager’s view on the mechanism of the initial steps in ORR.

The research results have great significance for the in depth understanding of the selectivity of the CO2 electrochemical reduction process and the sensitivity of the Cu surface structure.

DFT geometry optimization and minimum energy path calculations were used to investigate the mechanisms of oxygen reduction reaction (ORR) on the Pd(111) and Pt(111) surfaces, including the adsorption and dissociation of O2 molecule and the protonation of dissociated adsorbates. The results indicated that in the presence of a hydrated proton ORR on the Pd(111) surface proceeded through the adsorption and dissociation of O2 molecule, whereas ORR on the Pt(111) surface may involve in parallel the adsorption and dissociation of O2 molecule as well as the formation and dissociation of OOH species. During the entire four-electron ORR, the protonation of adsorbed O atom to form OH is the rate-determining step (rds) on both of the Pd(111) and Pt(111) surfaces. Such a finding about the rds of ORR can well explain why Pt- and Pd-based catalysts that more weakly bind atomic oxygen have better ORR activity. Comparison of the ORR mechanisms on the Pd(111) and Pt(111) surfaces revealed that the adsorption and dissociation processes of O2 molecule more easily occurred on the Pt(111) surface and that the serial protonation of the dissociative product to form H2O molecule also more easily occurred on the Pt(111) surface than on the Pd(111) surface. Therefore, the difference between the catalytic activities for ORR between both metals was large, explaining why Pt can serve as ORR electrocatalysts and the inexpensive Pd cannot.