A density functional theory (DFT) study on reduced partition function ratios of oxygen species adsorbed on a Pt 19 cluster and oxygen isotope effects.

A density functional theory (DFT) computation on oxygen species adsorbed on platinum (Pt) catalyst surfaces has been carried out to elucidate oxygen isotope fractionation observed at the cathode of a polymer electrolyte membrane fuel cell (PEMFC). The Pt(111) catalyst surface was modelled by a Pt ₁₉ cluster, and O, OH, OHH, OO, OOH, OHOH and HOHOH were assumed to be the oxygen species adsorbed on the Pt(111) surface. The oxygen isotope reduced partition function ratios (RPFRs) of the adsorbed species were calculated using the vibrational frequencies obtained by normal mode analyses performed on the optimized structures. Various oxygen isotope exchange equilibria among the adsorbed oxygen species and oxygen and water molecules in the gas phase were examined using their RPFRs. Experimental observation that the lighter ¹⁶ O is enriched in water molecules exhausted from the cathode is explainable in a satisfactory manner by assuming oxygen isotope exchange equilibria of O ₂ molecule with O, OH, OO and OOH adsorbed on the Pt(111) surface that appear in the first half of the conversion reaction from O ₂ to H ₂ O and those of H ₂ O molecule with the adsorbed oxygen species, OHH, OHOH and HOHOH, formed in the latter half of the conversion reaction.