Roles of Ru on PtRu Surface in Electrocatalytic Ammonia Oxidation

Ammonia is both an efficient hydrogen carrier and a carbon-free fuel for direct use in fuel cells. Nevertheless, a poor understanding of the ammonia oxidation reaction (AOR) limits the development of efficient catalysts. Considering the scarcity of studies on Ru surfaces, this work compared AOR over PtRu/C, Pt/C, Ru/C, and RuO₂/C catalysts using high-surface area electrodes, together with online electrochemical mass spectrometry (OLEMS) and ion chromatography (IC) for detecting the gaseous and solution products, respectively. This approach allowed us to determine the reaction potentials of seven gaseous products over PtRu/C and Pt/C: N2, NO, N2H4, NH2OH, HN3, N2O, and NO2. The onset potential of N2 over PtRu/C was 100 mV lower than that over Pt/C. Although Ru/C and RuO2/C showed low AOR catalytic activity, small amounts of N2, NO, HN3, and N2O were still detected, with AOR onset at 0.30 V and 0.95 V for Ru/C and RuO2/C, respectively. These results suggest that the shift in AOR onset for PtRu/C is due to the presence of metallic Ru. The role of RuO2 was revealed at high potentials on Ru-based surfaces, with enhanced generation of oxygenated products in solution (NO2– and NO3–). Moreover, N₃⁻ was detected and quantified for the first time, suggesting a new approach for azide generation. The study's findings provide important mechanistic insights into the electrochemical behavior of Ru-based catalysts during ammonia oxidation, advancing the fundamental understanding of AOR and guiding the design of more efficient catalysts for ammonia fuel cells and the production of high-value-added products.