Nanometric Iridium Overlayer Catalysts for High-Turnover NH₃ Oxidation with Suppressed N₂O Formation

In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 μm thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH₃-O₂ reactions more environmentally benign due to a much lower selectivity for undesirable N₂O (<5%) than that of a Pt overlayer (∼70%) at 225 °C. Despite its small surface area, Ir/SUS exhibited promising activity as an ammonia slip catalyst according to a turnover frequency (TOF) >70-fold greater than that observed with conventional Ir nanoparticle catalysts supported on γ-Al₂O₃. We found that the high-TOF NH₃ oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O₂ present in simulated diesel exhaust. Additionally, we found that the Ir overlayer structure was thermally unstable at reaction temperatures ≥400 °C and at which point the Ir surface coverage dropped significantly; however, thermal deterioration was substantially mitigated by inserting a 250 nm thick Zr buffer layer between the Ir overlayer and the SUS foil substrate (Ir/Zr/SUS). Although N₂O formation was suppressed by NH₃ oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO₂) were readily converted to N₂ by coupling with a V₂O₅-WO₃/TiO₂ catalyst in a second reactor for selective catalytic reduction by NH₃. These results demonstrated that this tandem reactor configuration converted NH₃ to N₂ with nearly complete selectivity at a range of 200-600 °C in the presence of excess O₂ (8%) and H₂O (10%).