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 degrees 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 degrees C for CO, <300 degrees C for C3H6, and <400 degrees C for NO when the molar fraction of Cr in CrxFe0.1Ni0.1Cu0.1 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.

Automated summary

In this research, various quaternary metal catalysts were investigated for the replacement of precious metals in three-way catalysts. The CrFeNiCu system demonstrated the best catalytic performance, achieving a lower light-off temperature for CO, C3H6, and NO compared to a reference catalyst. Structural analysis revealed that the catalyst underwent structural changes during the catalytic reaction. The presence of Cr oxide significantly enhanced CO/C3H6 oxidation and promoted the regeneration of the active NiCu metallic surface for NO reduction.