Study On Preparation Of Mesoporous CeO₂ And Hydrogen Production Performance By Partial Oxidation Of Methanol Over Mesoporous CeO₂ Supported Au-Pd Catalysts

Partial oxidation of methanol has been suggested as a suitable route for hydrogen production from methanol.The supported Pd-based catalyst is reported to be active for partial oxidation of methanol to produce hydrogen.And there are also many researches about Au-based catalyst for hydrogen production from methanol or ethanol. Au-Pd bimetallic catalyst may be shows better performance in the methanol partial oxidation reaction because of its property different from that of components.Ceria is one of a series of rare earth oxides with highest activity.It has been widely used as an additive or support of catalysts because of its unique crystal structure,high oxygen storage capacity(OSC) and strong redox properties. Preparation of ceria with mesoporous structure can not only increase surface area but also offer appropriate pore structure,which favors diffusion of reactant and product as well as enhancement of catalytic activity.In this paper,the effect of preparation method and factors of mesoporous CeO₂ on its texture properties and the effect of support texture properties on catalytic properties of corresponding catalysts were investigated.Moreover the influence of active component Au and the second component ZnO as well as calcination temperature on catalyst performance were also studied.Physical and chemical properties,structure of mesoporous CeO₂ and Au-Pd catalysts were characterized by means of XRD,UV-Vis,TEM,N₂-adsorption,FT-IR,TPR,and TPD etc.1.Mesoporous ceria with high specific surface area were prepared using glucose, acrylamide and ceric nitrate via hydrothermal method.Results indicate that transition of cerium ion valence fromâ…¢toâ…£causes higher surface area of mesoporous ceria. Furthermore,glucose and acrylamide coexisting in the synthesis is essential to obtain samples with high surface areas.In the absence of glucose or acrylamide,surface areas of samples are lower.In addition,the properties of mesoporous ceria are also found to be influenced by hydrothermal reaction time and adding sequence of raw materials.Non-surfactant organic compounds can be removed at lower temperature, which facilitates to maintain mesoporous structure and obtain higher surface area.2.Mesoporous ceria were prepared with different surfactants PEG4000,F127, CTAB,SDS and HDA,respectively.The results reveal surfactant less affects pore size of mesoporous CeO₂ because of quickly hydrolysis and condensation of inorganic cerium salt,which is adverse to its self-assembly on surfactant surface.The properties of prepared mesoporous CeO₂ mainly reflects nature of inorganic cerium salt and the pore size of mesoporous ceria synthesized from the same cerium salt is similar.Precipitation temperature and cerium precursor have large effect on texture properties of mesoporous CeO₂,as a result of their impact on the solubility of hydrous cerium hydroxide.Ceric nitrate as cerium precursor is conducive to preparation of mesoporous CeO₂ with smaller grain size,pore size and higher surface area.Moreover Pore size and volume of samples increase and surface area decrease with the precipitation temperature increasing.3.Mesoporous CeO₂ prepared from different surfactants F127,CTAB and HDA, respectively were used as supports for Au-Pd catalysts.And the influence of texture properties of supports on catalytic properties of Au-Pd/m-CeO₂ catalysts was studied. The results show that the texture properties change little after Au-Pd is supported and the sequence of surface area,pore size of catalysts is the same as that of supports.The average pore size of Au-Pd/F4,Au-Pd/C4 and Au-Pd/H4 is smaller than corresponding Au-Pd/F3,Au-Pd/C3 and Au-Pd/H3,respectively.And the surface area of Au-Pd/F4,Au-Pd/C4 and Au-Pd/H4 is higher than that of Au-Pd/F3,Au-Pd/C3 and Au-Pd/H3,respectively.The pore size,pore volume and surface area of Au-Pd/F3, Au-Pd/C3 and Au-Pd/H3 are similar and slightly increase in the sequence.The impact of texture properties of support on catalysts is mainly manifested in the following aspects.Catalyst with higher surface area reveals higher dispersion and smaller size of active components,more superficial active sites and base sites,as well as stronger adsorption of H₂.And smaller pore size of carder is adverse to diffusion of reactant and product.At low temperature(250～300℃),Au-Pd/F3,Au-Pd/C3 and Au-Pd/H3 exhibits higher catalytic activity than Au-Pd/F4,Au-Pd/C4 and Au-Pd/H4,respectively,in which the activity difference of Au-Pd/H3 and AuPd/H4 is smaller.These catalysts' activities don't present the consistent relations with the dispersion of active components.But Au-Pd/F3,Au-Pd/C3 and Au-Pd/H3 have the corresponding relationships with the dispersion of active components.These results indicate that catalytic activity of Au-Pd/m-CeO₂ is not completely decided by the dispersion of active components.It is a combined result of many factors,in which gas diffusional limitation has large impact on activity of catalysts with small pore size.The order of H₂ selectivity of AuPd/F and AuPd/C within reaction temperature of 250-400℃is AuPd/F3＞AuPd/F4 and AuPd/C3＞AuPd/C4.And the sequence of H₂ selectivity of AuPd/H is different from that of AuPd/F and AuPd/C.H₂ selectivity of AuPd/H is AuPd/H3＞AuPd/H4 within 250-300℃and AuPd/H4＞AuPd/H3 within 350-400℃.H₂ selectivity of catalyst is also related to its pore size and surface area.Higher surface area is beneficial to increase metal dispersion,decrease metal grain size, favorable to enhancement of H₂ selectivity.However smaller pore size of catalyst is against to H₂ diffusion out from pore to gas bulk and H₂ is easily further oxidized to H₂O.H₂ selectivity of catalyst is the comprehensive results of these factors.4.Au/CeO₂,Au-Pd/CeO₂ and Pd/CeO₂ catalysts were prepared by deposition-precipitation (DP) method,and their activities and H₂ selectivity for partial oxidation of methanol were evaluated.H₂ adsorption on Pd/CeO₂ is strong, unfavorable to regeneration of the active site,causing its activity is equal to Au-Pd/CeO₂ with lower Pd content.Furthermore strongly adsorbed H₂ on Pd/CeO₂ easily causes depth oxidation of H₂ to H₂O and consequently decrease of H₂ selectivity.Au/CeO₂ shows low reactivity.Its H₂ selectivity is high at lower temperature but significantly reduces at 400℃.In comparison to Au/CeO₂ and Pd/CeO₂ catalysts,Au-Pd/CeO₂ catalyst shows high activity and hydrogen yield for partial oxidation of methanol reaction,due to the synergetic interaction between gold and palladium species,which suppresses aggregate of Au particles and decreases adsorption of hydrogen and hydrogen deep oxidation.5.Calcination temperature on the performance of Au-Pd/CeO₂ catalysts for partial oxidation of methanol has been evaluated.With the increase of calcination temperature,surface area and pore volume of catalysts decrease a little.In addition, the reduction peaks for PdO and surface CeO₂ are both slightly shifted toward lower temperatures,and the areas of hydrogen reduction at the lower temperature as well as H₂ adsorption amount decrease.It is because that with the increase of temperature, the amount of metal gold decomposed from its precursor increases,the interaction between the precursors of gold and palladium is weakened while the interaction between palladium and CeO₂ is reinforced.At the same time,aggregate degree of metal gold increases and covers a small amount of palladium surface.Activity of catalyst calcined at 550℃is higher than that of the other two.It is probably that interaction between palladium and CeO₂ becomes stronger with the increase of calcination temperature,which is in favor of reaction process.At lower temperature the catalyst calcined at 550℃exhibits higher H₂ selectivity while at higher temperature the uncalcined catalyst shows higher H₂ selectivity.6.ZnO-CeO₂ was prepared by impregnation method.The influence of ZnO on the performance of Au-Pd/CeO₂ catalysts for partial oxidation of methanol has been evaluated.TPR reveals that some of ZnO can be initially reduced at about 200℃.And CO-IR shows that the position of the bands is shifted to lower frequencies for Au-Pd/ZnO-CeO₂ catalyst.These reveal interaction between Pd and Zn.Although Au-Pd/ZnO-CeO₂ exhibits relatively lower methanol conversion,it shows higher H₂ selectivity and lower CO selectivity than Au-Pd/CeO₂.It may results from interaction between Pd and Zn,which restrains methanol decomposition of Pd and favors H₂O react with methanol or dehydrogenated intermediate to formation of CO₂ and H₂,consequently increasing H₂ selectivity and decreasing CO selectivity of Au-Pd/ZnO-CeO₂ catalyst.