Nanometric Iridium Overlayer Catalysts for High-Turnover NH Oxidation with Suppressed NO 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 NH-O reactions more environmentally benign due to a much lower selectivity for undesirable NO (<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-AlO. We found that the high-TOF NH oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O 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 NO formation was suppressed by NH oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO) were readily converted to N by coupling with a VO-WO/TiO catalyst in a second reactor for selective catalytic reduction by NH. These results demonstrated that this tandem reactor configuration converted NH to N with nearly complete selectivity at a range of 200-600 °C in the presence of excess O (8%) and HO (10%).