Precursor-mediated adsorption of oxygen on the (111) surfaces of platinum-group metals

Current status of ab initio quantum chemistry study for oxygen electroreduction on fuel cell catalysts

“…The study of oxygen adsorption on Ni(1 1 1) [40,41] illustrated that two precursor states exist similar to adsorption on Pt(1 1 1). However, the most striking difference between Ni and Pt was the much stronger binding affinity of the precursor on Ni than on Pt, combined with a much more pronounced preference for adsorption of the precursor in one of the hollows rather than on the bridge.…”

“…Eichler et al [41] further studied oxygen adsorption on Pd(1 1 1). They showed that oxygen adsorption on Pd(1 1 1) was more like Pt(1 1 1) than Ni (1 1 1).…”

Near Neutrality of an Oxygen Molecule Adsorbed on a Pt(111) Surface

“…The formal charge assignments 1À and 2À were based on magnetic moments, vibrational frequencies and the shape of the charge difference density Á ½Ptð111Þ þ O 2 À ½Ptð111Þ À ½O 2 for these two O Ã 2 precursors. Since the vertical distance between O Ã 2 and surface is about 2 Å [7,8], these charge assignments would indicate large induced dipoles, defined as the difference in supercell total dipole before and after O 2 adsorption. However, this contradicts Hyman and Medlin's DFT study of oxygen molecule and atom adsorption on Pt(111) surface, where it was found that the induced electric dipole moments are very small (0.07 and 0:04 # Ae for O Ã 2 and O Ã , respectively) [9].…”

“…In this model, all the adsorbates such as O Ã 2 , OOH Ã , O Ã , and OH Ã are assumed to be charge neutral, so the 4 electron transfers always occur concurrently with the 4 proton (hydronium) transfers from the electrolyte; i.e., all electron transfers are proton-coupled (PCET) [13]. Freeenergy landscapes of the electrochemical ORR as a function of the electrode potential V were obtained, with the underlying assumption that the adsorption free energies of reaction intermediates are unaffected by V. This means very small electric dipoles induced by O Ã 2 , OOH Ã , O Ã , and OH Ã in the surface normal direction, synonymous with the near-neutrality of these adsorbates, which needs to be justified in view of the conflicting reports [5,[7][8][9].…”

“…Eichler and Hafner used density functional theory (DFT) calculation to study O 2 adsorption and identified superoxide O À 2 as a paramagnetic chemisorbed precursor at the bridge site of Pt(111) surface, and peroxide O 2À 2 as a nonmagnetic precursor at the fcc hollow site, illustrated in Fig. 1(a) [7,8]. The formal charge assignments 1À and 2À were based on magnetic moments, vibrational frequencies and the shape of the charge difference density Á ½Ptð111Þ þ O 2 À ½Ptð111Þ À ½O 2 for these two O Ã 2 precursors.…”