Crystal facet-dependent reactivity of α-Mn2O3 microcrystalline catalyst for soot combustion

Abstract

In this work, a series of α-Mn2O3 micro crystals with different morphologies and crystal shapes were successfully prepared by hydrothermal method and has been investigated as potential soot combustion catalysts for the first time. The activity data showed that the soot combustion efficiency were markedly affected by the shape of the prepared α-Mn2O3 catalysts, among which their activity ranked as α-Mn2O3-cubic>α-Mn2O3-truncated octahedral>α-Mn2O3-octahedra. As revealed by various physicochemical characterization techniques such as XRD, SEM, HR-TEM, FT-IR, BET, H2-TPR, O2-TPD, NO- and NO+O2-TPSR, the enhanced activity as well as selectivity over α-Mn2O3-cubic is originated with the nature of the exposed α-Mn2O3 (001) surface facets, on which the higher concentration of low-coordinated surface oxygen sites facilitates the oxygen activation and improves surface redox properties, thereby accelerating the formation of the crucial intermediate i.e., NO2 formation by NO oxidation and thus promoting the overall soot combustion efficiency. Moreover, the kinetic study performed under isothermal condition provided solid evidence and proof to support that it is the exposed crystal facet, rather than surface area, to be critical to determine the catalytic performance of soot combustion. Under loose contact condition, soot combustion efficiency over best performing α-Mn2O3-cubic catalyst was further enhanced in the presence of water, CO and hydrocarbons, which are essential components in real diesel exhaust. Furthermore, the cubic α-Mn2O3 displayed excellent durability against structural collapse upon repeated recycling test and accelerated deactivation test, demonstrating its promise for the practical use in diesel soot combustion.