Temperature-Dependent Kinetics and Reaction Mechanism of Ammonia Oxidation on Pt, Ir, and PtIr Alloy Catalysts

We report here a kinetic study of ammonia oxidation reaction (AOR) on carbon supported Pt, Ir, and PtIr (1:1) alloy catalysts using gas diffusion electrodes in 1 M KOH solution at temperatures up to 60°C. Ammonia concentration was kept constant by letting Ar gas bubbling through concentrated ammonia solution before entering the cell. At 0.5 V versus reversible hydrogen electrode, the currents normalized to the mass of platinum group metals are in the order of PtIr > Ir > Pt. Compared to Pt, Ir exhibited higher activity enhancement with increasing temperature, lower onset potential, and lower peak current. In correlation with previous theoretical studies, these differences are ascribed to Ir having lower activation barrier for the first one-electron deprotonation of NH3 to NH2*, but higher barrier for dimerization of two NH2* to N2H4*. The AOR current peaks at high potentials because the rate is limited by the potential-independent dimerization and formation of inactive N* that blocks the active surface. Below peak potentials, gradual deactivation occurs due to the accumulation of NH* that is harder to be dimerized than NH2*. PtIr alloy combines the virtues of Ir and Pt exhibiting the widest active potential window and the highest peak current.