Electrochemical oxidation of ammonia on carbon-supported bi-metallic PtM (M = Ir, Pd, SnOx) nanoparticles

Ammonia electro-oxidation was studied in alkaline solution on carbon-supported Pt and bimetallic Pt_yM_1−y (M = Pd, Ir, SnO_x and y = 70, 50 at.%) nanoparticles. Catalysts were synthesized using the modified polyol method and deposited on carbon, resulting in 20 wt.% of metal loading. Particle size, structure and surface composition of the particles were investigated using TEM, XRD and XPS. Mean size of PtM bi-metallic nanoparticles varied between 2.0 and 4.7 nm, depending on the second metal (M). XRD revealed the structure of all bi-metallic particles to be face-centered cubic and confirmed alloy formation for Pt_yPd_1−y (y = 70, 50 at.%) and Pt₇Ir₃nanoparticles, as well as partial alloying between Pt and SnO_x. Electrochemical behaviour of ammonia on Pt and PtM nanoparticles is comparable to that expected for bulk Pt and PtM alloys. Addition of Pd to Pt at the nanoscale decreased the onset potential of ammonia oxidation if compared to pure platinum nanoparticles; however stability of the catalyst was poor. For Pt₇(SnO_x)₃, current densities were similar to Pt, whereas catalyst stability against deactivation was improved. It is found that carbon supported Pt₇Ir₃ nanoparticles combine good catalytic activity with enhanced stability for ammonia electro-oxidation. Electronic effect generated between two metals in the bimetallic nanoparticles might be responsible for increase in the catalytic activity of Pd- and Ir-containing catalysts, causing weakening of the adsorption strength of poisonous N_ads intermediate.