A pulsed cathodic arc-plasma deposition of a few-nanometer-thick Pt(111) overlayer on a 50 μm-thick Fe−Cr−Al metal foil produced a thin-film catalyst; this catalyst exhibited high activity for low-temperature NH 3 oxidation and was superior to that of a conventional powder catalyst (Pt/Al 2 O 3 ). A metal honeycomb that was fabricated using a metal foil catalyst successfully demonstrated a light-off performance at a practical gas hourly space velocity of 1.2 × 10 5 h −1 . Despite its nonporosity and small surface area, the Pt overlayer is a promising alternative to Pt/Al 2 O 3 for more efficient ammonia-slip catalysts that use less Pt loading; this is because the turnover frequency for the NH 3 oxidation at 200 °C is more than 180-fold greater than that achieved with Pt/Al 2 O 3 consisting of Pt nanoparticles. Although the nanometric Pt overlayer structure was thermally unstable at reaction temperatures of ≥600 °C, inserting a 250 nm-thick Zr buffer layer between the Pt overlayer and the metal foil substrate significantly mitigated the thermal deterioration. The undesired byproducts of NH 3 oxidation, NO and NO 2 , can be efficiently converted to N 2 by a selective catalytic reduction over a V 2 O 5 −WO 3 /TiO 2 catalyst in a tandem reactor system.

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%).

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