Preparation Of Ba-Cu-Ce Oxide Catalysts And NO_x-assisted Catalytic Soot Oxidation Mechanism

Three transition metal-cerium mixed oxides （MnO_x-CeO₂, CoO_x-CeO₂and CuO_x-CeO₂） were prepared by a sol-gel method. Then these mixed oxideswere loaded with Ba（Ac）₂by a wetness impregnation method and werefollowed by a calcination to obtain Ba-TM-Ce （TM=Mn, Co and Cu） sootoxidation catalysts. The soot oxidation activities of the barium-loaded andbarium-free samples were tested in a NO/O2atmosphere. Barium loadingenabled the catalysts to store a great amount of NO_xwhich was then releasedbetween350°C and380°C to assist soot catalytic oxidation, so that the sootignition temperature was greatly decreased.Further investigations were carried out overwith Ba-Cu-Ce catalysts.Samples with various barium loading amounts （the molar ratio ofBa/（Cu+Ce）=0%,3%,6%,10%and15%, respectively） were synthesized. Allthe samples were treated in air flow with10%H₂O under800oC for10h toobtain the hydrothermally aged samples. The catalysts were characterized bymeans of XRD, BET and Raman to obtain were performed for the samples’structural and textural information.; H2-TPR was appliedused to determine theredox properties.; the soot oxidation activity were tested under different NOconcentrations were tested. The results showed that barium existed mainly inthe form of BaCO₃on the fresh Ba-Cu-Ce. The redox property of catalystswere weakened to some extent by the coverage of barium species weakenedthe catalysts’ redox activity, which reduce the soot oxidation activity in theO2atmosphere. However, barium markedly enhanced the NO_xstorage abilityof catalysts at low temperatures, and released NO_xfor assisting the sootoxidation, resulting into a superior catalytic performance of Ba-Cu-Ce in theNO/O2atmosphere, comparing to Cu-Ce, especially under low NOconcentration. Meanwhile, the introduction of barium inhibited the crystallinesintering of CuO_x-CeO₂, so as to improve the thermal stability of the mixedoxides catalysts. The optimum Ba loading ratio was6%¹0%, with the best soot oxidation performance amount both fresh and hydrothermally agedsamples.NO-TPO, NO_x-TPD and in-situ DRIFTS were carried out to exploreforbetter understanding of the NO_xoxidation and NO_xabsorption behaviors onthe catalysts, and to help uncover the mechanism of NO_x-assisted sootoxidation on Ba-Cu-Ce in combination with the soot oxidation activity results.It wais suggested that the oxidative absorption of NO on CuO_x-CeO₂promoted transformation of bulk BaCO₃to Ba（NO₃）₂, endowing Ba-Cu-Cewith a strong NO_xstorage capacity at low temperaturesbility. During thesoot-TPO process with under heat transfer limitations, the slow soot oxidationwere ignited by the NO2derived both from surface nitrates decomposition anddirect NO oxidation firstly began the slow rate soot oxidation. Theexothermic reaction of soot oxidation by NO2helped to initiate thedecomposition of bulk Ba（NO₃）₂to release abundant NO_x, accerelating thesoot oxidation rate greatly. As a result, the extensive combustion of soot withO2occured, so that the soot was eliminated within a short time.