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.
Multicomponent 3d transition-metal nanoparticles supported on Al 2 O 3 were prepared using a complex polymerization process and a post H 2 -reduction treatment at 900 °C. Catalysts in a binary system were divided into two groups: single-phase alloys (NiCu and FeNi) and immiscible two-phase mixtures (FeCu and CoCu), whereas ternary (FeNiCu and CoNiCu) and quaternary (FeCoNiCu) catalysts produced single-alloy nanoparticles. The ternary and quaternary alloy catalysts achieved high NO reduction in a stoichiometric NO−CO−C 3 H 6 −O 2 reaction under wet conditions (5% H 2 O), which simulates automotive three-way catalysis (TWC). In contrast, the activity of unary and binary systems of these metal elements significantly deteriorated in the presence of H 2 O. Cu-based metal catalysts are efficient for NO reduction, but they are easily deactivated by oxidation to less active oxides in the presence of O 2 and/or H 2 O. The superiority of the multinary alloy catalysts is a result of the higher stability and regenerability of the metallic Cu species. Therefore, increasing the number of metal elements in alloy nanoparticles can provide a phase stabilization against oxidation under TWC conditions.
Cu supported on Al2O3, prepared by impregnation, was thermally aged at different temperatures, and the influence of thermal aging on the local structure, redox behavior of Cu, and catalytic activity for a stoichiometric NO–CO–C3H6–O2 reaction was investigated. Crystalline CuO was mainly formed on Al2O3 after thermal aging at ≤700 °C, whereas aging at higher temperatures induced Cu2+ incorporation into tetrahedral (Td-Cu2+) rather than octahedral (Oh-Cu2+) sites of γ-Al2O3. Despite its lower surface area, thermally aged Cu/Al2O3 with Td-Cu2+ sites showed higher catalytic performance for the stoichiometric NO–CO–C3H6–O2 reaction compared with the as-prepared catalyst, especially for NO reduction. Td-Cu2+ was reduced to Td-Cu+ during reaction with CO and/or C3H6, and NO could be reduced in subsequent reoxidation of Td-Cu+ to Td-Cu2+ by NO. This redox behavior is more probable on Td-Cu rather than crystalline CuO, resulting in enhancement of NO reduction during the three-way catalyst reaction.
Single-phase quaternary spinel solid solutions, Cu 0.05 Ni 0.95 Al y Cr 2−y O 4 (0 ≤ y ≤ 2.0), were prepared over the whole range of y by a polymerized complex method to study as platinum group metal-free three-way catalysts (TWC). Most conventional binary and/or ternary spinel oxides lose their NO reduction activity in the presence of water vapor and/or after hightemperature aging. In contrast, the present quaternary system with y = 1.8, which was aged at 900 °C for 25 h, preserved high activity even under a wet gas stream (5% H 2 O) simulating real TWC conditions. Comprehensive structural analyses via X-ray absorption fine structure and Xray Rietveld analysis showed that, in the quaternary system, Cu and Cr prefer to occupy the tetrahedral site and the octahedral site, respectively, whereas Ni and Al are distributed across both sites. The partial replacement of Cr by Al increased the specific surface area from 7 m 2 g −1 (y = 0) to 36 m 2 g −1 (y = 1.8), which is a common feature of the NiAl 2 O 4 -based spinel platform. The replacement also yielded monovalent Cu on the surface, which plays a key role in the catalytic NO reduction via the Mars−van Krevelen mechanism. Cr and Ni are beneficial for promoting CO−H 2 O and C 3 H 6 −O 2 reactions, respectively. A positive synergy between these different functionalities arising from each metal element affords high NO reduction activity under a wet gas stream. Furthermore, single-phase quaternary spinel solid solutions seem to provide an entropy-mediated phase-stabilization effect under stoichiometric TWC conditions where ternary Cu x Ni 1−x Cr 2 O 4 (0 ≤ x ≤ 1.0) solid solutions are less stable and decompose because of the low equilibrium O 2 pressure.
A platinum-group-metal-free catalyst comprising Fe-Ni alloy nanoparticles on a γ-Al2O3 support was investigated for use in three-way catalytic converters, with particular attention being paid to its NO reduction activity. The catalyst showed activity for the simultaneous removal of NO, CO, and C3H6 in the stoichiometric NO-CO-C3H6-O2 reaction. Low-oxidation-state Fe sites were found to be effective for NO reduction, while their oxidation by this reaction induced catalyst deactivation. Ni atoms adjacent to the low-oxidation-state Fe atoms were found to stabilize them by catalyzing the consumption of the O atoms in the nanoparticles for CO oxidation, which indirectly promoted further NO reduction.
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