One step citric acid-assisted synthesis of Mn-Ce mixed oxides and their application to diesel soot combustion

Abstract

Mn-Ce catalysts were prepared by a one-step synthesis method using citric acid, and were characterized by X-ray diffraction, CO temperature-programmed reduction, laser Raman spectroscopy and X-ray photoelectron spectroscopy. These catalysts were applied to the catalytic combustion of diesel soot in the presence of nitric oxide. A remarkable influence of the ratio of precursors and the method of synthesis on the physicochemical properties and subsequent catalytic activity was evidenced. The mixed oxides had higher activities than the individual ones, showing a synergistic effect whereby the catalyst with Mn/(Mn + Ce) = 0.1 had the lowest temperature of the maximum combustion rate. Mechanical mixtures of oxides were also prepared in order to better understand the synergistic effect observed. The main TPO peak was located at a similar temperature for both methods of synthesis. However, the TPO profiles of the catalysts synthesized by mechanical mixing had a shoulder at around 420 °C, which was more evident for catalysts prepared with lower Mn contents. This fact could be related to the coexistence of different MnOx species, as demonstrated by XRD, which exhibited different catalytic performances on oxidation reactions. The formation of a solid solution between Mn and Ce oxides for a ratio of Mn/(Mn + Ce) < 0.25 was observed, which enhanced the Ce4+ ↔ Ce3+ redox process. It was observed that both the coexistence of Mn2O3 and Mn3O4 at high concentrations and the formation of a solid solution strongly favored the oxidation of diesel soot, obtaining lower values of maximum combustion rate temperature.