Study On The Preparation Of Nano-mesoporous Ceria Materials And Their Characteristics And Catalytic Performances

Ceria has been widely used as a promoter and support of the catalysts because ofits unique redox properties and high oxygen storage capability. However, the lowsurface area makes this support less desirable in catalytic field because of low metaldispersion. Synthesizing mesoporous ceria (m-CeO₂) materials can increase theirsurface areas effectively. Polyoxyethylene dodecylether(Brij35) posses many virtuessuch as low price, innocuity, high stability and low dosage, and is easily to be wipedoff.In this paper, m-CeO₂ materials were synthesized using Brij35, Brij35 andCTAB or Brij35 and PEG20000 as templates, respectively. The influences of pH,adding sequences of NH₃·H₂O and (NH₄)₂Ce(NO₃)₆, kinds of templates, synthesizingtemperature, ageing temperature, molar ratio of template to ceria source andcalcination temperature on textural properties of m-CeO₂ were investigated.Ru/m-CeO₂ and Ru/m-CeO₂/Al₂O₃ catalysts were prepared by the impregnatingmethod, and methanol stream reforming were uesed as the probe reaction.Physico-chemical properties, structure of the m-CeO₂ and Ruobased catalysts werecharacterized by means of XRD, TEM, BET, DTA, FT-IR and TPR1. m-CeO₂-B_CN prepared with Brij35 as template. The m-CeO₂-B_CN with higherspecific surface areas and pore volumes were synthesized by adding NH₃·H₂O to themixed solutions of (NH₄)₂Ce(NO₃)₆ and Brij35, pH 9～10 and ageing periods 2d. Theresult of heat treatment show that the surfactant in m-CeO₂-B_CN can be removedeasily at low temterature (450℃).2. m-CeO₂-BC and m-CeO₂-BP were prepared with mixed-template of Brij35and CTAB or PEG20000, respectively. The results showed that the m-CeO₂-BC havehigher surface areas and pore volumes. The surface area, pore volume and averagepore diameter of m-CeO₂-BC prepared with molar ratio of Brij35:CTAB at 3:1 are102m².g^-1, 0.16cm³.g^-1 and 5.05nm, respectively. The catalytic activity order of Rucatalysts for methanol stream reforming is Ru/m-CeO₂-BC＞Ru/m-CeO₂-BP, which isclosely correlation with their surface areas and pore volumes.3. The synthesizing temperature and ageing temperature have some influence on the characteristics of m-CeO₂-BC, m-CeO₂-BC prepared with higher synthesizingtemperature has widely pore distribution and lower specific surface area. Thecatalytic activity of Ru/m-CeO₂-BC (m-CeO₂-BC synthesied at 40℃) is higher thanthat Ru/m-CeO₂-BC (m-CeO₂-BC synthesied at 70℃). Nona-particles reunite easilyand forming incomplete crystalline structure at low ageing temperature, finemesoporous structure of m-CeO₂-BC was formed at ageing temperature of 60℃.4. The molar ratio of mixed templates to ceria source can influence theformation of m-CeO₂, and the m-CeO₂ prepared with the molar ratio ofn_Brij35+CTAB:n_{(NH4)2Ce(NO3)6}=2:1 has higher surface area and pore volume.5. The influences of adding m-CeO₂ into Al₂O₃ on catalytic activity of Ru-basedcatalysts were discussed. With increasing the m-CeO₂ content, the catalytic activitiesof Ru-based catalysts increasing for the methanol stream reforming. The addition ofm-CeO₂ can effectively inhibit Ru₂O₃ incorporating into the crystal lattice of Al₂O₃,and decrease the correlation function of Ru₂O₃ and Al₂O₃. Among the catalysts, S_m-CeO2＜S_m-ceO2/Al2O3, Ru/m-CeO₂ catalyst have the higher catalytic activity and H₂selectivity than them of Ru/m-CeO₂/Al₂O₃ catalysts,which indicates that theCeO₂/Al₂O₃ supports are disvantage for the methanol stream reforming in spite ofhigher surface areas.