Study The Morphology Effect Of Mn-based Catalyst On Automobile Exhaust Puification

Diesel and gasoline engines are widely used as automobile engines because of their excellent power output system,good economy and stability.However,the soot and volatile organic compounds（VOCs）,the main pollutants in automobile exhaust,have seriously threatened the ecological environment and human health.Catalytic purification technology is the most effective means to eliminate soot and VOCs.The key to the application lies in the development of catalysts,achieving the transformation of soot and VOCs into CO₂ at low temperature.Therefore,the high-efficiency catalysts R&D,using in the purification of exhaust,has become a challenging topic in the field of environmental engineering,and also has important practical significance.In my work,Mn-based（Mn O_x:Mn₃O₄ and Mn₂O₃）were the research objects.By tailoring the Mn O_x morphology to selectively expose the highly active crystal facets,the catalytic performances were evaluated via soot and low-concentration ethanol combustion as the model reaction.By using various characterization techniques,the relationship between the morphology and reactivity of manganese-based catalysts and the catalytic mechanism of high activity crystal facets were investigated thoroughly,and new ideas were provided for the catalysts design and the catalytic technology establishment.The detail are as follows:（1）Various shapes（hexagonal nanoplate,octahedral,and nanoparticle）of nanoscale Mn₃O₄ have been prepared via hydrothermal and co-precipitation methods,respectively,and evaluated for their performance on diesel soot combustion for the first time.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 T_m of 407.7^oC and S^m_CO2 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 XRD,BET,H₂-TPR,FE-SEM,HR-TEM,XPS,soot-TPR,and NO/NO+O₂-TPSR.The different shapes controlled by the preparation method are found to influence the extent of exposed Mn₃O₄crystal 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 E_a obtained from three samples ranked as the order of（112）<（101）<（220）facets in reverse order of activity data,confirming the 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.In loose contact mode,the Mn₃O₄-HNS catalytic activity was evaluated under various gas conditions.Furthermore,the best performing Mn₃O₄-HNS displayed the good stability via recycling test.（2）A series ofα-Mn₂O₃ catalysts with different morphologies（cubic,truncated octahedral>octahedral）were successfully prepared via hydrothermal method and has been investigated for catalyzing low-concentration ethanol total oxidation for the first time,the order of activity at high gas hourly space velocity:cubic>truncated octahedral>octahedral,which proves that the reaction is structurally sensitive.Reaction conditions:ethanol 600 ppm/20 vol%O₂/N₂,space velocity is 192000 m L/（g·h）,the temperature at which cubicα-Mn₂O₃（α-Mn₂O₃-C）completely converted the ethanol was 190 ^oC,and acetaldehyde as the main intermediate product.HR-TEM proved that the cubic,truncated octahedral,octahedral exposed（001）,（001）&（111）and（111）crystal facets respectively.As revealed by various physicochemical characterization techniques such as H₂-TPR,XPS,ethanol-TPD and CO-TPSR,the excellent reactivity overα-Mn₂O₃-C is originated with the exposed facets.Compared with（111）,（001）facets has higher redox properties,abundant Mn⁴⁺and actived oxygen species,which is conducive to enhancing the ethanol oxidation efficiency.Moreover,the kinetic study performed the order E_a:（001）<（111）facets,which provided that the key factor determining the reactivity is the high energy facets rather than the specific surface area.The effect of space velocity,ethanol concentration and water on catalyst performance were also investigated.Under the reaction condition containing 6 vol.%H₂O,α-Mn₂O₃-C was tested continuously for 50 hours and presented excellent stability.