Multicomponent Spinel Oxide Solid Solutions: A Possible Alternative to Platinum Group Metal Three-Way Catalysts

Single-phase quaternary spinel solid solutions, Cu_0.05Ni_0.95Al_yCr_2–yO₄ (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 high-temperature 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% H₂O) simulating real TWC conditions. Comprehensive structural analyses via X-ray absorption fine structure and X-ray 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 m² g^–1 (y = 0) to 36 m² g^–1 (y = 1.8), which is a common feature of the NiAl₂O₄-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–H₂O and C₃H₆–O₂ 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_xNi_1–xCr₂O₄ (0 ≤ x ≤ 1.0) solid solutions are less stable and decompose because of the low equilibrium O₂ pressure.