Activity–Composition Relationships of Fe–Ni–Cu Ternary Nanoparticles Supported on Al₂O₃ as Three-Way Catalysts for NO Reduction

Ternary FexNiyCu1–x–y (x, y: molar fractions) metal nanoparticles supported on Al2O3 were prepared by H2-reduction treatment at 900 °C and investigated as three-way catalysts free of precious metals (Rh, Pd, and Pt). As-prepared nanoparticles consisted of nearly homogeneous alloys, whereas their surfaces were partially oxidized. Further oxidation occurred upon exposure to a reaction gas mixture (NO–CO–C3H6–O2–H2O) at lower temperatures of ≤400 °C. At higher temperatures, however, Cu and Ni regenerated the active metallic states and reconstructed alloy nanoparticles, whereas most Fe was fully oxidized and formed a spinel-like Fe–Ni oxide phase. A contour map analysis revealed that the NO reduction activity and the fraction of metallic states were strongly dependent on the metal composition (x and y). Among the compositions investigated in the ternary system, near-equimolar catalysts (0.2 ≤ x ≤ 0.33, 0.2 ≤ y ≤ 0.33) exhibited the highest activity and the highest fraction of metallic states. The regenerability and stability of metallic-state Cu and Ni species were improved by the copresence of Fe–Ni oxide, which was found to promote the oxidative adsorption of C3H6 as a carboxylate on the Al2O3 surface. A subsequent reaction with NO formed N2, which converted the carboxylate to CO and NCO species close to the perimeter of the metal–support interface. As oxidative C3H6 adsorption consumes oxygen, this provides a more reductive surface environment, thereby enhancing the stability of the active metallic state.