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

Abstract

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 NH 3 to NH 2 * , but higher barrier for dimerization of two NH 2 * to N 2 H 4 * . 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 NH 2 * . PtIr alloy combines the virtues of Ir and Pt exhibiting the widest active potential window and the highest peak current.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

“…[ 225 ] It is important to note that the inactive intermediate of *N can poison the catalyst surface, following the conclusions reported by experiments. [ 21,229,230 ] Another important study by Novell‐Leruth and co‐workers indicated that NH 3 and the intermediate *NH 2 were more stable on Pt(100) than on Pt(111), suggesting structure sensitivity for the AOR process at the Pt surface. [ 231 ] They also predicted that the most probable adsorption configurations are different for various intermediates such as top sites for NH 3 , bridge sites for *NH 2 , and hollow sites for *NH and *N on Rh, Pd, and Pt.…”

“…[ 236 ] Alloying Pt with other metals also generates significant benefits for improving the AOR, [ 247,257,258 ] due to electronic structure change (increase in Pt d‐electron vacancies), geometric transformation (decrease in the PtPt bond distance), and mitigating surface segregation. Currently, a variety of bimetallic PtM alloys includes Pt–Ir, [ 21,229 ] Pt–Ru, [ 259 ] Pt–Pd, [ 260 ] Pt–Rh, [ 237 ] Pt–Ni, [ 261 ] Pt–Cu, [ 262 ] and Pt–Au, [ 263 ] as well as ternary Pt alloys such as Pt–Ir–Rh, [ 110 ] Pt–Ir–Zn, [ 247 ] Pt–Pd–Rh, [ 264 ] Pt–Ir–Ni, [ 243 ] and Pt–Cu–Ru. [ 248 ]…”

“…[ 265 ] Pt–Ir alloy catalysts are the benchmark for AOR catalysis. [ 21 ] Similar to Pt, the intrinsic AOR activity of bimetallic Pt catalysts are mainly dependent on structures, compositions, and supports ( Figure ). For example, PtIr nanocubes (Figure 16b) show better AOR activity than dendrite‐like PtIr NPs (Figure 16a), further suggesting that AOR is a structure‐dependent reaction (Figure 16c).…”

“…Also, the temperature‐dependence of the AOR has been reported based on the Pt, Ir, and Pt–Ir alloy catalysts. [ 21 ] An increase in temperatures from 25 to 60 °C leads to a dramatic reduction of the onset potential of AOR and a noticeable rise in the peak current density (Figure 16d). These results indicate that, when operating at high temperatures, DAFC with PtIr catalysts can significantly reduce anodic AOR overpotential and generate high energy efficiency.…”

“…However, these critical reactions, including the oxygen evolution reaction (OER), [ 6–9 ] oxygen reduction reaction (ORR), [ 10–14 ] nitrogen reduction reaction (NRR), [ 15–17 ] and ammonia oxidation reaction (AOR), [ 18–20 ] are often kinetically sluggish and operated in oxidative/reductive potential windows, which cause significant overpotential and catalyst stability issues. Therefore, electrochemically active and stable catalysts lie at the heart of these energy technologies, [ 21,22 ] playing a primary role in utilizing renewable energy and more effectively implementing electricity in future transportation.…”