Kinetically stable PtxIr100-x alloy thin films prepared by pulsed laser deposition: Oxidation of NH3 and poisoning resistance

A series of kinetically stable PtxIr100-x thin film alloys have been synthesized by crossed beam pulsed laser deposition and physically characterized by X-ray diffraction and X-ray photoelectron spectroscopy. It is shown that a metastable solid solution between Pt and Ir is formed over the whole composition range, with the lattice parameter of the fcc phase increasing steadily between 3.84 and 3.92 Å as the Pt content is increased from 0 to 100 at.%. The surface composition of all samples follows closely the bulk composition, indicating there is no surface segregation of either element. A detailed analysis of the Pt and Ir 4f core level peaks shows that the binding energy of the Pt 4f7/2 and Ir 4f7/2 core level peaks both decrease with increasing Pt concentration, from 71.6 to 71.2 eV as the bulk Pt content increases from ca 65% to 100%, and from 61.1 to 60.6 eV as the bulk Pt content increases from 0 to 90 at.%, respectively. Such behaviour was recently shown to reflect a change in the center of gravity of the occupied states, where the d-band level of both Pt and Ir shift to higher energy as the Pt bulk content is decreased. Both the surface composition and surface electronic structure are not altered following all the electrochemical analyses. The electrocatalytic properties of these PtxIt100-x alloys for the anodic oxidation of NH3 in aqueous alkaline media were investigated. Voltammetric and chronoamperometric studies on these alloys show improved poison tolerance for NH3 oxidation as compared to pure polycrystalline platinum, with optimal results observed with Pt89Ir11 alloys. Such improvement for long term stability is associated with the electronic effect and bifunctional mechanism at the PtIr surface alloys.