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.

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.

A spinel solid solution expressed by a composition formula of Cu x Ni 1¹x Al 1.8 Cr 0.2 O 4 (0¯x¯0.2) was studied focusing on the role of Cu species in the catalytic activity toward NO reduction under a simulated three-way catalysis condition. X-ray Rietveld analysis revealed that Cu and Cr prefer to occupy the tetrahedral site and the octahedral site, respectively, whereas Ni and Al are distributed across both sites. Although NiAl 1.8 Cr 0.2 O 4 (x = 0) showed negligible catalytic activity for NO reduction, the partial replacement of Ni by Cu significantly enhanced the activity, achieving the highest activity at x = 0.05 not only for NO reduction but also for CO and C 3 H 6 oxidation. Based on the infrared spectra and pulsed reaction experiments, it was concluded that the monovalent Cu site in the tetrahedral site plays a key role in CO chemisorption, oxygen vacancy formation, and subsequent NO reduction via the Marsvan Krevelen mechanism.