Surface activity structure of Mo-based catalyst and its characteristics of soot oxidation in diesel engine

Abstract: The diesel particulate filter (DPF) with the trapping has become an effective and broadly used measure. However, the trapped particulates adhering to the carrier channel easily block the exhaust channel, and then consequently affect the power performance and fuel economy of the diesel engine. Catalytic technology can effectively reduce the temperature for soot combustion by coating the active catalytic material on the channel wall of DPF. The molybdenum catalyst shows excellent catalytic performance in many research fields. The high catalytic activity of molybdenum catalysts is mainly related to the mobility of the active species in the reaction condition, or by the surface migration or by the melting of molybdenum catalyst, so this mobility could improve the contact between reactants and catalyst. Thus, according to that idea, in a loose contact situation between soot and catalyst, the migration of the active component across the soot surface could play an important role in the oxidation reaction during trap regeneration. In order to thoroughly study the surface activity structure phase of the supported molybdenum catalyst and its' function mechanism on soot oxidation, nano-scale MoO3/TiO2 catalysts for soot oxidation were prepared by loading different amounts (5%, 10%, 20% and 40%) of MoO3 on the TiO2 particles through an impregnation method. The structures and physico-chemical properties for those catalysts were characterized using BET, Fourier transform infrared spectra (FT-IR), X-Ray diffraction (XRD) and a Scanning electron microscope (SEM). Printex-U carbon black has been often used in the research field of catalytic oxidation of diesel soot. The practical engine soot and Printex-U carbon black have more characteristics in common such as components and structures. Besides, the reproducibility of the test seemed satisfactory, too. Printex-U carbon black was chosen to replace the practical engine soot. The catalytic activity of molybdenum catalysts for soot oxidation was evaluated using a TG/DTA analyzer. Based on the Starink method, the catalytic oxidation process of soot was quantitatively analyzed. The results showed that, for the limitation of dispersion threshold value, the structure of active MoO3 was amorphous or micro crystalline at the low molybdenum loading. When the MoO3 loading rate was more than 10%, the significant MoO3 orthorhombic crystals appeared on the catalyst surface. The light sheet and octahedral crystal structure with close texture appeared in the MoO3 catalyst surface. The functional groups Mo=O were stable in all samples, and its content increased with the increase in molybdenum loading rate. Under a loose contact state, MoO3 characterized with low melting point, still showed good catalytic effect. With the increase in MoO3 loading rate, TG and DTG curve moved to the low temperature area, the characteristic temperatures of soot oxidation were decreased, and the catalytic activity of catalyst was increased. Conversely, the specific surface area and pore volume of the molybdenum catalyst were decreased. However, the catalytic activity was not limited by the specific surface area and threshold effect, and it was closely bound up with the content of the active component MoO3. Among all samples, the catalyst loaded by 40% MoO3 exhibited the highest catalytic activity for soot oxidation. The ignition temperature, the temperature of weight loss peak and the burnout temperature of soot oxidation were reduced about 84.4, 122.6 and 122.7℃ respectively, as compared with the non-catalyzed particulates. The order of the activation energy for soot oxidation worked out by the Starink method was soot < Mo:Ti (mass ratio 5:95) < Mo:Ti (mass ratio 10:90) < Mo:Ti (mass ratio 20:80) < Mo:Ti (mass ratio 40:60). Thereby, with the increase in the MoO3 loading rate, the catalytic activity of MoO3/TiO2 catalyst was increased.