Cr-Fe-Ni-Cu Quaternary Nanostructure as a Substitute for Precious Metals in Automotive Three-Way Catalysts

The replacement of precious metals (Rh, Pd, and Pt) in three-way catalysts with inexpensive and earth-abundant metal alternatives is an ongoing challenge. In this research, we examined various quaternary metal catalysts by selecting from six 3d transition metals, i.e., Cr, Mn, Fe, Co, Ni, and Cu, equimolar amounts (0.1 mol each), which were prepared on the Al₂O₃ support (1 mol Al) using H₂ reduction treatment at 900 °C. Among 15 combinations, the best catalytic performance was achieved by the CrFeNiCu system. Light-off of NO-CO-C₃H₆-O₂-H₂O mixtures proceeded at the lowest temperature of ≤200 °C for CO, ≤300 °C for C₃H₆, and ≤400 °C for NO when the molar fraction of Cr in Cr Fe_0.1Ni_0.1Cu_0.1 was around x = 0.1. The activity for CO/C₃H₆ oxidation was superior to that of reference Pt/Al₂O₃ catalysts but was less active for NO reduction. The structural analysis using scanning transmission electron microscopy and X-ray absorption spectroscopy showed that the as-prepared catalyst consisted of FeNiCu alloy nanoparticles dispersed on the Cr₂O₃-Al₂O₃ support. However, the structural change occurred under a catalytic reaction atmosphere, i.e., producing NiCu alloy nanoparticles dispersed on a NiFe₂O₄ moiety and Cr₂O₃-Al₂O₃ support. The oxidation of CO/C₃H₆ can be significantly enhanced in the presence of Cr oxide, resulting in a faster decrease in O₂ concentration and thus regenerating the NiCu metallic surface, which is active for NO reduction to N₂.