Mixed-metal aerogels for catalytic converter applications: A closer look

Vehicles that employ an internal combustion engine produce harmful gases including  carbon monoxide, nitrogen oxides (NOx) and unburned hydrocarbons. Current catalytic converters utilize the precious group metals (PGMs) platinum, palladium, and rhodium, as three-way catalysts for the oxidation of CO and unburned hydrocarbons and the reduction of NOx; however, these metals come with high economic and environmental costs. Catalytic aerogels show promise as less expensive alternatives to PGM-based catalytic converters. Aerogels’ high surface area and high thermal stability, which arise from the nanoporous structure of the aerogel, are attractive properties for high-performance heterogeneous catalysis. The current work involves the synthesis of mixed-transition-metal aerogels and characterization of these aerogels using various forms of spectroscopy. Wet ceria-copper-alumina (CeCuAl) gels were formed using epoxide assisted gelation with aluminum chloride hexahydrate, copper(II) nitrate trihydrate, and cerium(III) chloride heptahydrate salts used as starting materials. Alumina wet gels doped with rhodium and palladium nanoparticles were synthesized as well. Wet gels were converted into aerogels using a patented rapid supercritical extraction (RSCE) method. Characterization of the CeCuAl and PGM-doped aerogels as prepared, following heat-treatment and following catalytic testing was performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and powder X-ray diffraction (XRD). The catalytic ability of heat-treated CeCuAl and PGM-doped aerogels were assessed using an in-house-constructed testbed and simulated automotive exhaust. As expected, the materials have a predominately noncrystalline structure; however, small microcrystals were observed by SEM/EDS after heat-treating aerogels to 800˚C for 24 hours in air. The PGM aerogels contained small aggregates of PGM nanoparticles that were observed along the surface. Evidence of crystallization is further supported by XRD data: copper oxide and ceria structures are observed in heat-treated and catalytically tested samples and rhodium(IV) oxide and palladium(II) oxide were observed in heat-treated samples. CeCuAl aerogels show three-way catalytic activity, albeit with light-off temperatures (<250˚C for CO, 300˚C for propene, 350˚C for NO) that are higher than those of commercial PGM-based catalytic converters. PGM doped aerogels show relatively lower light-off temperatures than the CeCuAl aerogels (200˚C for CO, 250˚C for propene, 300˚C for NO). Continued work on PGM doped aerogels at varying concentrations is being investigated.