Pt nanostructures with different Rh surface entities: Impact on NH3 electro-oxidation

Ammonia electro-oxidation activity is promoted at Pt nanostructured catalysts which combine high surface area values and a high proportion of (100) surface atoms. The latter is extensively known to be the most active surface arrangement of atoms for NH3 oxidation. In the present study, Pt(100) nanostructured thin films were prepared and subsequently modified by surface functionalization with rhodium through pulsed electrodeposition. Rh surface coverage was varied by modifying the number of applied electrodeposition pulses, and was found to modify the voltammetric Hupd features. When Rh surface content was assessed by Hupd deconvolution analyses, it was revealed that Rh nucleation first occurred at (100) Pt step sites in the form of Rh(100) sub-monolayers, and subsequently grew further to form Rh multilayer species. For minute amounts of rhodium, NH3 activity is increased slightly, provided that long range (100) Pt wide domains were not shortened. The lower onset potential for NH3 oxidation in the presence of Rh surface atoms, which is shifted negatively by more than 50mV as compared to Pt, is attributed to the NH3 oxidative dehydrogenation processes at lower potential values. For similar Rh coverage (around 20%), similar NH3 activity is obtained at pseudomorphic Rh(100) adlayers and Rh 3D islands, a finding which emphasizes the non-specific NH3 activity of Rh(100) atoms.