介孔CeO_2的制备及其负载Au-Pd催化剂甲醇部分氧化制氢性能的研究
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摘要
甲醇部分氧化制氢被认为是实现车载制氢的有效途径。Pd基催化剂对该反应具有良好的催化活性,Au催化甲醇部分氧化制氢的研究也较多,Au-Pd双金属催化剂因其不同于其组成金属的性质可在甲醇部分氧化制氢反应中表现出较好的催化性能。
氧化铈是稀土氧化物系列中活性最高的氧化物之一,具有较为独特的晶体结构、较高的储氧能力和释放氧的能力、较强的氧化-还原性能(Ce~(3+)/Ce~(4+)),已被广泛应用于催化领域。将CeO_2制成介孔材料不但可提高CeO_2的比表面积,且能提供适当的孔结构,有利于反应物的扩散和催化剂活性的提高。
本文研究了介孔CeO_2的制备方法及制备条件对介孔CeO_2的比表面、孔容和孔径等织构性质的影响,并考察了介孔CeO_2载体的织构性质对Au-Pd催化剂催化性能的影响。同时还研究了活性组分Au、第二组分ZnO的引入以及焙烧温度对催化剂性能的影响。采用XRD、UV-Vis、TEM、N_2吸附、FT-IR、TG、ICP、TPR和TPD等方法对介孔CeO_2及其催化材料进行了表征。
1.采用有机小分子葡萄糖和丙烯酰胺以及铈前驱体硝酸铈铵为原料,水热法制备介孔CeO_2。结果表明,由四价硝酸铈盐制备的介孔CeO_2其比表面比采用三价硝酸铈盐时有较大增加。丙烯酰胺和葡萄糖共存时有利于提高样品的比表面,缺少丙烯酰胺或葡萄糖时所制样品的比表面均较小。水热处理时间和原料的加入方式对介孔CeO_2的织构性能也有一定的影响。所制样品能在较低温度下除去有机物,有利于保持介孔结构和获得较大的比表面。
2.分别采用表面活性剂聚乙二醇PEG4000、嵌段共聚物F127、十六烷基三甲基溴化铵(CTAB)、十六胺(HDA)和十二烷基硫酸钠(SDS)为模板剂和无机铈盐为铈前驱体制备介孔CeO_2。结果表明,由于无机铈盐水解缩聚快,不利于其在表面活性剂表面的自组装,表面活性剂的模板剂作用较弱,所得介孔CeO_2的性质主要与铈盐的价态有关,同一铈盐和不同表面活性剂制备的介孔CeO_2孔径大小较接近。
铈前驱体和沉淀温度对介孔CeO_2织构性质的影响较大。与四价铈盐合成的介孔CeO_2相比,由三价铈盐合成的介孔CeO_2平均晶粒较大、孔径较大、比表面较小。随着沉淀温度的升高,样品的孔径和孔容增大、比表面减小。这些均与无机铈盐的氢氧化物溶解度有关。
3.以F127、CTAB和HDA表面活性剂制备的m-CeO_2为载体,考察了载体织构性质对Au-Pd催化剂性能的影响。结果表明,m-CeO_2负载Au-Pd后,织构性质变化较小,催化剂的比表面、孔径大小顺序和载体一致,即Au-Pd/F4、Au-Pd/C4和Au-Pd/H4催化剂的平均孔径分别小于对应的Au-Pd/F3、Au-Pd/C3和Au-Pd/H3,比表面分别大于对应的Au-Pd/F3、Au-Pd/C3和Au-Pd/H3。Au-Pd/F3、Au-Pd/C3和Au-Pd/H3催化剂的平均孔径、孔容和比表面差别较小,按此顺序有所增加。与Au-Pd/F4、Au-Pd/C4的孔径较小不同,Au-Pd/H4具有较大的孔径。
m-CeO_2载体较大的比表面使Au-Pd/m-CeO_2催化剂的活性组分分散度提高、活性组分粒子变小、表面活性中心和碱性中心增多、H_2吸附增强,孔径小的载体不利于反应物及产物的扩散。
250-300℃反应时,AuPd/F3、AuPd/C3和AuPd/H3催化剂的活性均分别高于对应的AuPd/F4、AuPd/C4和AuPd/H4,但AuPd/H3和AuPd/H4二者的活性差别较小。AuPd/F3、AuPd/C3和AuPd/H3催化剂的活性顺序与催化剂活性组分的分散度有对应关系,这些结果表明,Au-Pd/m-CeO_2催化剂的活性并不完全是由活性组分的分散度决定,是多种因素综合作用的结果,孔径小的催化剂活性受气体扩散的影响较大。
反应温度为250-400℃时,AuPd/F和AuPd/C催化剂的H_2选择性顺序均为AuPd/F3＞AuPd/F4、AuPd/C3＞AuPd/C4。AuPd/H催化剂的H_2选择性顺序不同于AuPd/F和AuPd/C催化剂,反应温度为250-300℃时,H_2选择性顺序为AuPd/H3＞AuPd/H4;350-400℃反应时,H_2选择性顺序却为AuPd/H4＞AuPd/H3。催化剂的H_2选择性与孔径、比表面等因素有关,催化剂比表面大有利于提高活性组分分散度,使活性金属晶粒小,有利于提高H_2选择性;但催化剂孔径小不利于H_2的扩散,使H_2氧化为H_2O的几率增加。Au-Pd/m-CeO_2催化剂H_2的选择性是以上多种因素综合作用的结果。
4.采用沉积沉淀法制备Pd/CeO_2、Au-Pd/CeO_2和Au/CeO_2,考察了Au的引入对Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。结果表明,Pd/CeO_2催化剂中Pd对H_2的吸附强,不利于活性中心再生,生成的H_2易被进一步氧化为H_2O,使H_2选择性下降。Au/CeO_2反应活性较低,低温时具有较高的H_2选择性,但400℃时H_2选择性下降较快。
Au-Pd/CeO_2双金属催化剂表现出较高的催化活性和较高的H_2产率,这是由于Au和Pd的相互作用减少了Pd对反应产物H_2的吸附,有利于活性中心再生,且减小了H_2的深度氧化,同时Au和Pd相互作用使金粒子更稳定、不易聚集,有利于高温时H_2选择性的稳定。
5.考察了焙烧温度对Au-Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。结果表明,焙烧温度升高使Au-Pd/CeO_2催化剂的比表面、孔容略微减小,此外PdO和表面CeO_2的还原峰温有所降低,低温还原峰面积减小,H_2吸附量减小,可能是由于随着焙烧温度的升高,金前驱体分解产生的金属态金逐渐增多,金钯前驱体相互作用有所减弱,Pd-CeO_2相互作用加强,同时金属态金有所聚集、覆盖了少量钯表面。
550℃焙烧的催化剂活性高于300℃焙烧和未焙烧的催化剂,可能与焙烧温度升高使Pd-CeO_2相互作用增强、有利于反应有关。反应温度较低时550℃焙烧的催化剂具有较高的H_2选择性,反应温度较高时未焙烧的催化剂H_2选择性较高。
6.采用浸渍法制备ZnO-CeO_2,考察了第二组分ZnO的引入对Au-Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。Au-Pd/ZnO-CeO_2催化剂的TPR中约200℃时开始有部分ZnO被还原,CO-IR中吸收峰移向低频,这些结果表明Au-Pd/ZnO-CeO_2催化剂中Pd和Zn之间发生了相互作用。
ZnO的引入虽然降低了Au-Pd/CeO_2催化剂的活性,但提高了催化剂H_2选择性和降低了CO选择性,可能是由于Pd和Zn之间发生了相互作用,抑制了Pd的甲醇分解活性,有利于H_2O与甲醇或其脱氢中间体反应转化为H_2和CO_2,从而提高了H_2选择性、降低了CO选择性。
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 CeO_2 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 CeO_2 and Au-Pd catalysts were characterized by means of XRD,UV-Vis,TEM,N_2-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 fromⅢtoⅣcauses 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 CeO_2 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 CeO_2 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 CeO_2,as a result of their impact on the solubility of hydrous cerium hydroxide.Ceric nitrate as cerium precursor is conducive to preparation of mesoporous CeO_2 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 CeO_2 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-CeO_2 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 H_2.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-CeO_2 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 H_2 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 H_2 selectivity of AuPd/H is different from that of AuPd/F and AuPd/C.H_2 selectivity of AuPd/H is AuPd/H3＞AuPd/H4 within 250-300℃and AuPd/H4＞AuPd/H3 within 350-400℃.
H_2 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 H_2 selectivity.However smaller pore size of catalyst is against to H_2 diffusion out from pore to gas bulk and H_2 is easily further oxidized to H_2O.H_2 selectivity of catalyst is the comprehensive results of these factors.
4.Au/CeO_2,Au-Pd/CeO_2 and Pd/CeO_2 catalysts were prepared by deposition-precipitation (DP) method,and their activities and H_2 selectivity for partial oxidation of methanol were evaluated.H_2 adsorption on Pd/CeO_2 is strong, unfavorable to regeneration of the active site,causing its activity is equal to Au-Pd/CeO_2 with lower Pd content.Furthermore strongly adsorbed H_2 on Pd/CeO_2 easily causes depth oxidation of H_2 to H_2O and consequently decrease of H_2 selectivity.Au/CeO_2 shows low reactivity.Its H_2 selectivity is high at lower temperature but significantly reduces at 400℃.In comparison to Au/CeO_2 and Pd/CeO_2 catalysts,Au-Pd/CeO_2 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/CeO_2 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 CeO_2 are both slightly shifted toward lower temperatures,and the areas of hydrogen reduction at the lower temperature as well as H_2 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 CeO_2 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 CeO_2 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 H_2 selectivity while at higher temperature the uncalcined catalyst shows higher H_2 selectivity.
6.ZnO-CeO_2 was prepared by impregnation method.The influence of ZnO on the performance of Au-Pd/CeO_2 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-CeO_2 catalyst.These reveal interaction between Pd and Zn.
Although Au-Pd/ZnO-CeO_2 exhibits relatively lower methanol conversion,it shows higher H_2 selectivity and lower CO selectivity than Au-Pd/CeO_2.It may results from interaction between Pd and Zn,which restrains methanol decomposition of Pd and favors H_2O react with methanol or dehydrogenated intermediate to formation of CO_2 and H_2,consequently increasing H_2 selectivity and decreasing CO selectivity of Au-Pd/ZnO-CeO_2 catalyst.
氧化铈是稀土氧化物系列中活性最高的氧化物之一,具有较为独特的晶体结构、较高的储氧能力和释放氧的能力、较强的氧化-还原性能(Ce~(3+)/Ce~(4+)),已被广泛应用于催化领域。将CeO_2制成介孔材料不但可提高CeO_2的比表面积,且能提供适当的孔结构,有利于反应物的扩散和催化剂活性的提高。
本文研究了介孔CeO_2的制备方法及制备条件对介孔CeO_2的比表面、孔容和孔径等织构性质的影响,并考察了介孔CeO_2载体的织构性质对Au-Pd催化剂催化性能的影响。同时还研究了活性组分Au、第二组分ZnO的引入以及焙烧温度对催化剂性能的影响。采用XRD、UV-Vis、TEM、N_2吸附、FT-IR、TG、ICP、TPR和TPD等方法对介孔CeO_2及其催化材料进行了表征。
1.采用有机小分子葡萄糖和丙烯酰胺以及铈前驱体硝酸铈铵为原料,水热法制备介孔CeO_2。结果表明,由四价硝酸铈盐制备的介孔CeO_2其比表面比采用三价硝酸铈盐时有较大增加。丙烯酰胺和葡萄糖共存时有利于提高样品的比表面,缺少丙烯酰胺或葡萄糖时所制样品的比表面均较小。水热处理时间和原料的加入方式对介孔CeO_2的织构性能也有一定的影响。所制样品能在较低温度下除去有机物,有利于保持介孔结构和获得较大的比表面。
2.分别采用表面活性剂聚乙二醇PEG4000、嵌段共聚物F127、十六烷基三甲基溴化铵(CTAB)、十六胺(HDA)和十二烷基硫酸钠(SDS)为模板剂和无机铈盐为铈前驱体制备介孔CeO_2。结果表明,由于无机铈盐水解缩聚快,不利于其在表面活性剂表面的自组装,表面活性剂的模板剂作用较弱,所得介孔CeO_2的性质主要与铈盐的价态有关,同一铈盐和不同表面活性剂制备的介孔CeO_2孔径大小较接近。
铈前驱体和沉淀温度对介孔CeO_2织构性质的影响较大。与四价铈盐合成的介孔CeO_2相比,由三价铈盐合成的介孔CeO_2平均晶粒较大、孔径较大、比表面较小。随着沉淀温度的升高,样品的孔径和孔容增大、比表面减小。这些均与无机铈盐的氢氧化物溶解度有关。
3.以F127、CTAB和HDA表面活性剂制备的m-CeO_2为载体,考察了载体织构性质对Au-Pd催化剂性能的影响。结果表明,m-CeO_2负载Au-Pd后,织构性质变化较小,催化剂的比表面、孔径大小顺序和载体一致,即Au-Pd/F4、Au-Pd/C4和Au-Pd/H4催化剂的平均孔径分别小于对应的Au-Pd/F3、Au-Pd/C3和Au-Pd/H3,比表面分别大于对应的Au-Pd/F3、Au-Pd/C3和Au-Pd/H3。Au-Pd/F3、Au-Pd/C3和Au-Pd/H3催化剂的平均孔径、孔容和比表面差别较小,按此顺序有所增加。与Au-Pd/F4、Au-Pd/C4的孔径较小不同,Au-Pd/H4具有较大的孔径。
m-CeO_2载体较大的比表面使Au-Pd/m-CeO_2催化剂的活性组分分散度提高、活性组分粒子变小、表面活性中心和碱性中心增多、H_2吸附增强,孔径小的载体不利于反应物及产物的扩散。
250-300℃反应时,AuPd/F3、AuPd/C3和AuPd/H3催化剂的活性均分别高于对应的AuPd/F4、AuPd/C4和AuPd/H4,但AuPd/H3和AuPd/H4二者的活性差别较小。AuPd/F3、AuPd/C3和AuPd/H3催化剂的活性顺序与催化剂活性组分的分散度有对应关系,这些结果表明,Au-Pd/m-CeO_2催化剂的活性并不完全是由活性组分的分散度决定,是多种因素综合作用的结果,孔径小的催化剂活性受气体扩散的影响较大。
反应温度为250-400℃时,AuPd/F和AuPd/C催化剂的H_2选择性顺序均为AuPd/F3＞AuPd/F4、AuPd/C3＞AuPd/C4。AuPd/H催化剂的H_2选择性顺序不同于AuPd/F和AuPd/C催化剂,反应温度为250-300℃时,H_2选择性顺序为AuPd/H3＞AuPd/H4;350-400℃反应时,H_2选择性顺序却为AuPd/H4＞AuPd/H3。催化剂的H_2选择性与孔径、比表面等因素有关,催化剂比表面大有利于提高活性组分分散度,使活性金属晶粒小,有利于提高H_2选择性;但催化剂孔径小不利于H_2的扩散,使H_2氧化为H_2O的几率增加。Au-Pd/m-CeO_2催化剂H_2的选择性是以上多种因素综合作用的结果。
4.采用沉积沉淀法制备Pd/CeO_2、Au-Pd/CeO_2和Au/CeO_2,考察了Au的引入对Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。结果表明,Pd/CeO_2催化剂中Pd对H_2的吸附强,不利于活性中心再生,生成的H_2易被进一步氧化为H_2O,使H_2选择性下降。Au/CeO_2反应活性较低,低温时具有较高的H_2选择性,但400℃时H_2选择性下降较快。
Au-Pd/CeO_2双金属催化剂表现出较高的催化活性和较高的H_2产率,这是由于Au和Pd的相互作用减少了Pd对反应产物H_2的吸附,有利于活性中心再生,且减小了H_2的深度氧化,同时Au和Pd相互作用使金粒子更稳定、不易聚集,有利于高温时H_2选择性的稳定。
5.考察了焙烧温度对Au-Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。结果表明,焙烧温度升高使Au-Pd/CeO_2催化剂的比表面、孔容略微减小,此外PdO和表面CeO_2的还原峰温有所降低,低温还原峰面积减小,H_2吸附量减小,可能是由于随着焙烧温度的升高,金前驱体分解产生的金属态金逐渐增多,金钯前驱体相互作用有所减弱,Pd-CeO_2相互作用加强,同时金属态金有所聚集、覆盖了少量钯表面。
550℃焙烧的催化剂活性高于300℃焙烧和未焙烧的催化剂,可能与焙烧温度升高使Pd-CeO_2相互作用增强、有利于反应有关。反应温度较低时550℃焙烧的催化剂具有较高的H_2选择性,反应温度较高时未焙烧的催化剂H_2选择性较高。
6.采用浸渍法制备ZnO-CeO_2,考察了第二组分ZnO的引入对Au-Pd/CeO_2催化剂甲醇部分氧化制氢性能的影响。Au-Pd/ZnO-CeO_2催化剂的TPR中约200℃时开始有部分ZnO被还原,CO-IR中吸收峰移向低频,这些结果表明Au-Pd/ZnO-CeO_2催化剂中Pd和Zn之间发生了相互作用。
ZnO的引入虽然降低了Au-Pd/CeO_2催化剂的活性,但提高了催化剂H_2选择性和降低了CO选择性,可能是由于Pd和Zn之间发生了相互作用,抑制了Pd的甲醇分解活性,有利于H_2O与甲醇或其脱氢中间体反应转化为H_2和CO_2,从而提高了H_2选择性、降低了CO选择性。
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 CeO_2 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 CeO_2 and Au-Pd catalysts were characterized by means of XRD,UV-Vis,TEM,N_2-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 fromⅢtoⅣcauses 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 CeO_2 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 CeO_2 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 CeO_2,as a result of their impact on the solubility of hydrous cerium hydroxide.Ceric nitrate as cerium precursor is conducive to preparation of mesoporous CeO_2 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 CeO_2 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-CeO_2 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 H_2.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-CeO_2 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 H_2 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 H_2 selectivity of AuPd/H is different from that of AuPd/F and AuPd/C.H_2 selectivity of AuPd/H is AuPd/H3＞AuPd/H4 within 250-300℃and AuPd/H4＞AuPd/H3 within 350-400℃.
H_2 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 H_2 selectivity.However smaller pore size of catalyst is against to H_2 diffusion out from pore to gas bulk and H_2 is easily further oxidized to H_2O.H_2 selectivity of catalyst is the comprehensive results of these factors.
4.Au/CeO_2,Au-Pd/CeO_2 and Pd/CeO_2 catalysts were prepared by deposition-precipitation (DP) method,and their activities and H_2 selectivity for partial oxidation of methanol were evaluated.H_2 adsorption on Pd/CeO_2 is strong, unfavorable to regeneration of the active site,causing its activity is equal to Au-Pd/CeO_2 with lower Pd content.Furthermore strongly adsorbed H_2 on Pd/CeO_2 easily causes depth oxidation of H_2 to H_2O and consequently decrease of H_2 selectivity.Au/CeO_2 shows low reactivity.Its H_2 selectivity is high at lower temperature but significantly reduces at 400℃.In comparison to Au/CeO_2 and Pd/CeO_2 catalysts,Au-Pd/CeO_2 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/CeO_2 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 CeO_2 are both slightly shifted toward lower temperatures,and the areas of hydrogen reduction at the lower temperature as well as H_2 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 CeO_2 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 CeO_2 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 H_2 selectivity while at higher temperature the uncalcined catalyst shows higher H_2 selectivity.
6.ZnO-CeO_2 was prepared by impregnation method.The influence of ZnO on the performance of Au-Pd/CeO_2 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-CeO_2 catalyst.These reveal interaction between Pd and Zn.
Although Au-Pd/ZnO-CeO_2 exhibits relatively lower methanol conversion,it shows higher H_2 selectivity and lower CO selectivity than Au-Pd/CeO_2.It may results from interaction between Pd and Zn,which restrains methanol decomposition of Pd and favors H_2O react with methanol or dehydrogenated intermediate to formation of CO_2 and H_2,consequently increasing H_2 selectivity and decreasing CO selectivity of Au-Pd/ZnO-CeO_2 catalyst.
引文
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