Formation of nano-rod structures in manganese-rich ceria–manganese mixed oxides and their soot oxidation activity

Abstract

EDTA-citrate synthesized manganese-rich ceria–manganese oxide catalysts (Ce1−xMnxO2−δ; where x = 1 to 0.6, termed CM100, CM90, CM80, CM70, CM60, CM50 and CM40 respectively) were obtained after calcination at 600 °C/5 h, characterized and tested for soot oxidation activity. Mn3O4 and Mn2O3 phases were observed in the XRD spectra; with the increase in the ceria concentration, cubic fluorite phase formation was observed from CM80 to CM40. BET and BJH analyses revealed that the pore diameter and volume had a meaningful impact on the soot oxidation activity, as seen for CM80 which had a BET surface area of 31 m2/g, pore diameter of 21.119 nm, and a pore volume of 0.1637 cc/g. The formation of the Mn3O4 peak in all the synthesized catalysts and ceria’s cubic fluorite F2g peak (452 cm −1) were observed from CM80 to CM40 from Raman spectral analysis. FESEM analysis of all the CMX samples revealed an intriguing aspect, the appearance of nano-rods. These nano-rods were observed in the HRTEM imaging as well. The high-resolution imaging and SAED analysis proved that the nano-rod structures were primarily due to the α-MnO 2 phase. The mixture of agglomerates and nano-rods profoundly affected the soot oxidation activity of the synthesized samples. CM70 had a minimum T50 of 389 °C among the CMX catalysts. The activation energy was reckoned with the Ozawa and KAS methods. CM50, with 131 kJ/mol for Ozawa and 127 kJ/mol for the KAS method, had the lowest values among the chosen catalysts. Synergistic interactions between ceria and the manganese oxides, considerable specific surface area, pore diameter, pore volume, substantially higher exposed (220) and (200) planes and the formation of the nano-rods helped to lower the activation energy for soot oxidation activity and consequently a reduction in the soot oxidation temperature of CM50 to CM70. The concentration of the nano-rods rendered a considerable role in reducing the T50 temperatures of the CMX catalysts.