Nanometric Iridium Overlayer Catalysts for High-Turnover NH(3) Oxidation with Suppressed N(2)O Formation

[Image: see text] 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 NH(3)–O(2) reactions more environmentally benign due to a much lower selectivity for undesirable N(2)O (<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-Al(2)O(3). We found that the high-TOF NH(3) oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O(2) 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 N(2)O formation was suppressed by NH(3) oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO(2)) were readily converted to N(2) by coupling with a V(2)O(5)–WO(3)/TiO(2) catalyst in a second reactor for selective catalytic reduction by NH(3). These results demonstrated that this tandem reactor configuration converted NH(3) to N(2) with nearly complete selectivity at a range of 200–600 °C in the presence of excess O(2) (8%) and H(2)O (10%).