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

Ternary Fe x Ni y Cu 1– x – y ( x , y : molar fractions) metal nanoparticles supported on Al 2 O 3 were prepared by H 2 -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–C 3 H 6 –O 2 –H 2 O) 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 C 3 H 6 as a carboxylate on the Al 2 O 3 surface. A subsequent reaction with NO formed N 2 , which converted the carboxylate to CO and NCO species close to the perimeter of the metal–support interface. As oxidative C 3 H 6 adsorption consumes oxygen, this provides a more reductive surface environment, thereby enhancing the stability of the active metallic state.