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 x Fe0.1Ni0.1Cu0.1 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.

Nanometric Iridium Overlayer Catalysts for High-Turnover NH3 Oxidation with Suppressed N2O Formation.

In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 µm thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH3-O2 reactions more environmentally benign due to a much lower selectivity for undesirable N2O (<5%) than that of a Pt overlayer (∼70%) at 225 °C. Despite its small surface area, Ir/SUS exhibited promising activity as an ammonia slip catalyst according to a turnover frequency (TOF) >70-fold greater than that observed with conventional Ir nanoparticle catalysts supported on γ-Al2O3. We found that the high-TOF NH3 oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O2 present in simulated diesel exhaust. Additionally, we found that the Ir overlayer structure was thermally unstable at reaction temperatures ≥400 °C and at which point the Ir surface coverage dropped significantly; however, thermal deterioration was substantially mitigated by inserting a 250 nm thick Zr buffer layer between the Ir overlayer and the SUS foil substrate (Ir/Zr/SUS). Although N2O formation was suppressed by NH3 oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO2) were readily converted to N2 by coupling with a V2O5-WO3/TiO2 catalyst in a second reactor for selective catalytic reduction by NH3. These results demonstrated that this tandem reactor configuration converted NH3 to N2 with nearly complete selectivity at a range of 200-600 °C in the presence of excess O2 (8%) and H2O (10%).