Combination of Mixed Metal Oxides with Cu-Zeolite for Enhanced Soot Oxidation on an SCRoF

A push for more stringent emissions regulations has resulted in larger, increasingly complex aftertreatment solutions. In particular, oxides of nitrogen (NO_X) and particulate matter (PM) have been controlled using two separate systems, selective catalytic reduction (SCR) and the catalyze diesel particulate filter (CDPF), or the functionality has been combined into a single device producing the SCR on filter (SCRoF). The SCRoF forgoes beneficial NO₂ production present in the CDPF to avoid NH₃ oxidation which occurs when using platinum group metals (PGM) for oxidation. In this study, mixed-metal oxides are shown to oxidize NO to NO₂ without appreciable NH₃ oxidation. This selectivity leads to enhanced performance when combined with a typical Cu-zeolite catalyst. Ce was co-precipitated with Zr, La, Sn and Pr at different ratios and doped with Cu or Fe. 0.5% Fe on Ce_0.8Zr_0.2O_X and Ce_0.75Pr_0.25O_X oxidized 30% of incoming NO at 350 °C while oxidizing < 5% of the NH₃ in a synthetic exhaust environment, demonstrating possible formulations that allow enhanced NO₂ derived soot oxidation compatible with an SCRoF. The selected formulations were washcoated onto state-of-the-art filter substrates and soot loaded using a realistic burner-based method to achieve representative soot-catalyst contacting within the filter. When combined with a typical Cu-zeolite catalyst, the mixed metal oxides increased both soot oxidation and NO_X conversion relative to a standard SCRoF. The catalyst samples were characterized by x-ray diffraction, surface area measurements, x-ray fluorescence and scanning electron microscopy with energy dispersive X-ray spectroscopy.