Synthesis of Pt–Ir catalysts by coelectrodeposition: Application to ammonia electrooxidation in alkaline media

The electrochemical activity for ammonia oxidation of Pt(x)-Ir(1-x) electrodes prepared by coelectrodeposition on carbon substrate was investigated by cyclic voltammetry and potentiostatic methods. Morphologies of deposits were observed by field emission scanning electron microscopy and their chemical composition and crystallographic structure were analyzed by X-ray photoelectron spectroscopy and X-ray diffraction. It is demonstrated that the atomic composition of the catalysts is strongly influenced by the deposition conditions (e.g. relative concentration of the metallic complexes and deposition potential). Ammonia oxidation was investigated in alkaline media for concentrations varying from 0.005 to 0.1 M. Cyclic voltammetry experiments revealed an increase of activity for the Pt(90)–Ir(10) electrode in comparison with Pt at low NH3 concentration (<0.02 M). Moreover all the Pt(x)–Ir(1−x) electrodes showed improved catalytic properties at low oxidation potentials (−0.55 to −0.4 V vs. Ag/AgCl) with respect to pure Pt. Finally, potentiostatic experiments indicated that an increase of the Ir content led to weaker poisoning effect of the resulting electrode despite that Ir does not seem to display a significant activity for the electrochemical oxidation of ammonia.