PGM-free metal oxide nanoarray forests for water-promoted low-temperature soot oxidation

Abstract

In this study, PGM free metal oxide based nanorod array (nanoarray) forests have been conformally integrated onto the millimeter-sized channel walls and microscale pores of SiC diesel particulate filter (DPF) substrates using scalable microwave-assisted hydrothermal and dip-coating methods. High-efficiency and robust low temperature soot oxidation is successfully achieved using such a new type of cDPFs as a result of i) promoted tight soot-catalyst contact through spontaneous soot deposits in-between nanorods, ii) highly dispersed loading of LaSrCoO3 perovskite oxidation catalysts onto ZnO nanorod arrays, effectively mitigating active site blockage by soot deposition. The light-off temperature of soot oxidation was lowered to 250 °C in oxygen, a 250 °C decrease compared to that of the commercial Pt based catalyst, due to the abundant oxygen vacancies enabled by the LaSrCoO3/ZnO (LSCO/ZnO) nanorod array catalysts. The performance of the catalyst was further enhanced to initiate the soot oxidation at ∼150 °C with co-fed gas of NO, due to the LSCO catalyzed NO2 formation and its favorable oxidation with soot at low temperature. Furthermore, the nanoarray supported perovskite catalyst shows excellent stability and activity after 120 h of hydrothermal aging at 650 °C. Soot oxidation Tmax is lowered from 542 °C to 510 °C under 6 % water steam condition due to the formation and promotion effect of oxidative -OH species. In situ DRIFTS study reveals that such -OH species form at various catalyst surfaces and interfaces, pointing to a potentially generic water promotion effect for solid/solid interfacial catalytic oxidation under humidity condition. Finally, the mechanistic soot oxidation pathways under O2, NO2, and H2O feeds are proposed for understanding the low-temperature soot oxidation behaviors over the nanoarray supported perovskite catalysts.