Electrooxidation of Ammonia at Tuned (100)Pt Surfaces by using Epitaxial Thin Films

PtxNi100-x thin films (72x100, 10-25nm thick) were deposited on MgO (100) substrates by using pulsed laser deposition. As shown from X-ray diffraction analysis, the formation of a Pt(Ni) solid solution was observed over the Ni range investigated. All PtNi-based films showed epitaxial growth in the (100) direction at 350 degrees C, which is in contrast to pure Pt films, where a Ni seeding layer was required to obtain epitaxial deposits at this temperature. X-ray photoelectron spectroscopy depth-profile analyses showed a Ni enrichment at the PtNi/MgO interface, which may be at the origin of the epitaxial growth in the alloys. After immersion in acidic media, Ni atoms are totally dissolved from the first ten atomic layers of the PtNi films, forming pure Pt electrodes. On the basis of underpotential-deposited hydrogen electrochemical analyses, a (100) preferential surface orientation of the crystallites originating from epitaxial growth was confirmed on both Pt-seeded and PtxNi100-x films. It was shown that the fraction of (100) terraces and terrace edge sites are a determining factor in the electrooxidation of NH3. The highest electrocatalytic activity was observed with the Pt-seeded film.