Promoting diesel soot combustion efficiency by tailoring the shapes and crystal facets of nanoscale Mn^sub 3^O^sub 4

Various shapes (hexagonal nanoplate, octahedral, and nanoparticle) of nanoscale Mn3O4 have been successfully prepared via facile hydrothermal and co-precipitation methods, respectively, and evaluated for their performance on diesel soot combustion. The catalytic performance of hexagonal nanoplates Mn3O4 (Mn3O4-HNS) is distinguished from the samples with other shapes by its superior activity on soot combustion with Tm of 407.7 °C and SmCO2 of 99.1% at the gas hourly space velocity of 9990 h−1 with the feed composition of 2500 ppm NO/5 vol.%O2/N2 under loose contact mode. The physicochemical properties of Mn3O4 were systematically examined by different characterization techniques, including XRD, FTIR, BET, H2-TPR, FE-SEM, HR-TEM, XPS, soot-TPR, and NO/NO + O2-TPSR. The different crystal shapes controlled by the preparation method are found to influence the extent of exposed Mn3O4 crystal facet, as determined by HR-TEM, which, in turn, was found to correlate with the catalytic performance. The kinetic study of soot combustion over these samples further revealed that the Ea obtained from three samples ranked as the order of (112) < (101) < (220) facets in reverse order of activity data, confirming the crystal-facet dependent reactivity. In view of the difference in catalytic activity and their physicochemical properties as estimated by various techniques, the superior catalytic performance of Mn3O4-HNS for diesel soot combustion was originated from its exposed (112) facet in association with its good lower temperature reducibility, abundant surface Mn4+ and active oxygen species, and the enhanced NO oxidation capability. Furthermore, the best performing Mn3O4-HNS displayed the good stability through recycling test. Our work may provide an important insight on the design strategies to develop the high-efficient soot combustion catalysts under the working conditions of the diesel engines.