【作者】 李彤；

【导师】 李永丹； 林跃生；

【作者基本信息】 天津大学 ， 工业催化， 2007， 博士

【摘要】 合成了一系列具有不同化学组成的六铝酸盐催化材料,并以其作为活性组分,以FeCrAlloy作为载体,制备了金属基整体式催化剂,考察了该催化材料的稳定性及在甲烷催化燃烧反应中的活性。首先考察了镜面层的组成对六铝酸盐的稳定性和甲烷催化燃烧活性的影响,以Sr2+、Ca2+、K+部分取代LaMnAl11O19六铝酸盐中的镜面阳离子La3+,合成了La1-xAxMnAl11O19(A= Sr2+、Ca2+、K+; x=0.2, 0.4, 0.8)系列六铝酸盐。当x=0.8时不能得到单一晶相的六铝酸盐,从而降低了材料的比表面积,Ca系列催化剂具有更高的甲烷催化燃烧活性,当x=0.2时活性最高,T10%=723K。镜面层的修饰对晶粒的生长有很大的抑制作用,Ca2+的部分取代使六铝酸盐晶粒变薄,提高了材料的抗烧结性能,使比表面积增大,并且使活性Mn3+的含量增加。这两方面的综合作用使催化活性显著提高。用Mg2+对LaMnAl11O19六铝酸盐进行修饰,合成了一系列Mg2+晶格取代Mn2+和以氧化物形式掺杂的样品,并与CeO2和ZrO2掺杂的样品进行了对比。在Mg2+部分取代的系列样品LaMn1-xMgxAl11O19中,当x=0.3时活性最高,T10%=704 K。以Mg2+部分取代Mn2+抑制了晶粒沿[001]方向的生长,使样品保持较大的比表面积。Mg2+的部分取代使Mn3+的量增多,并提高了Mn3+的氧化还原性能,同时促进了晶格氧的移动能力。这提高了催化剂的活性,使反应的活化能降低。Mg2+掺杂的样品在1373K煅烧后得到的是六铝酸盐与尖晶石的混合物,由于具有更多的晶格氧,因此比CeO2和ZrO2掺杂的样品具有更高的催化活性。Mg2+同时掺杂和取代的样品具有最高的催化活性,T10%=700 K。以不同方法制备了LaMnAl11O19/FeCrAl整体式催化剂,实验结果表明,制备方法对催化剂的形貌、氧化还原性能和催化活性有较大的影响。超声共沉淀法制得的粉末催化剂具有最大的比表面积,并且沉淀过程中施加超声波辐照使得到的产品具有更多的晶格氧,同时使Mn3+的还原性能增强,从而提高了样品的催化活性,T10%=730 K。前驱体的制备方法对整体催化剂的活性有较大的影响。以超声共沉淀法制备的粉末催化剂作为前驱体,用浸涂法制备的整体型催化剂具有最高的活性,甲烷起燃温度最低,但是该样品的粘附稳定性较差。以溶胶喷雾-分解法制备的整体式催化剂具有更高的粘附稳定性,超声振荡1 h重量损失低于20%,热冲击10次重量损失低于10%。以LaMnAl11O19和LaMn0.7Mg0.3Al11O19两种六铝酸盐作为活性组分,以FeCrAlloy为载体,分别以Al2O3和La2O3作为过渡层,采用dip-coating方法制备了金属基整体型催化剂,考察了过渡层对金属基整体催化剂稳定性和甲烷燃烧活性的影响。结果表明,使用过渡层可以改善整体式催化剂的稳定性和活性,以Al2O3作为过渡层可以提高整体式催化剂的粘附稳定性,以La2O3作为过渡层的样品活性更高。过渡层的存在可能降低了活性组分与载体之间的相互作用,从而有利于活性组分充分发挥催化性能。更多还原

【Abstract】 Hexaaluminate powder with different composition was synthesized and metallic monolithic catalysts for methane combustion were prepared by coating the hexaaluminate, acting as the active components, on a FeCrAlloy support.The effect of substitution to the mirror plane ions on the thermal stability and catalytic activity was investigated. Mirror plane cation substituted La1-xAxMnAl11O19, with A= Sr2+、Ca2+、K+; x=0.2, 0.4, 0.8, hexaaluminate catalysts were synthesized by co-precipitation. For the samples with x=0.8 diffraction peaks other than the hexaaluminates structure were observed and the surface areas were decreased. It was also found that the sample Ca0.2La0.8MnAl11O19 possesses the best catalytic activity with T10%=723 K and the highest BET surface areas 26.1 m2/g after calcination at 1373 K. The TEM micrographs revealed that the substitution of La3+ by Ca2+ led to a thinner hexaaluminate planer crystal which improved the sintering resistance and increased the surface area. This substitution increased also the amount of reactive Mn3+ species and the reducibility of the hexaaluminate. The enhanced catalytic activity was attributed to the combined effects of higher specific surface area and Mn3+ content.Mg substitution to the Mn in the spinel blocks was examined. The physico-chemical properties and catalytic activity of the Mg doped samples were investigated and compared with the CeO2 and ZrO2 doped samples. Among the Mg-substituted ones a sample with a composition LaMn0.7Mg0.3Al11O19 has the best activity, with T10%=704 K. XRD and BET analysis suggests that the partial substitution by magnesium suppresses effectively the crystal growth along [001] direction, and retains a larger surface area than the non substituted samples. Although the increase in activity is partly attributable to the high surface area, O2-TPD and H2-TPR results confirm that the substitution affects also the oxygen adsorption property and the oxidation state of Mn ions in the hexaaluminate lattice. The reducibility, the amount of Mn3+ species and the lattice oxygen mobility are also improved. Excess Mg was incorporated into the hexaaluminate with a stoicoimetry LaMn0.7Mg0.3Al11O19. Mixtures of hexaaluminate and MgAl2O4 spinel is obtained after calcination the samples at 1373 K for 5 h. Mg-doping increases further the amount of lattice oxygen and results in a higher catalytic activity than the CeO2 and ZrO2 doped samples. A sample was found very active with T10%=700 K.LaMnAl11O19/FeCrAl monolithic catalysts were prepared by different methods and characterized by XRD, BET, TEM, hydrogen TPR and oxygen TPD. The results show that a sample prepared by ultrasonic co-precipitation possesses the highest BET surface area. Ultrasonic radiation during precipitation leads to an increase of the number of crystal oxygen, improves reducibility of Mn3+ and enhance the catalytic activity. Preparation method of powder samples influence the activity of the monolithic samples. Among the monolithic catalysts the sample prepared by dip-coating, through the powder prepared by ultrasonic co-precipitation as precursor, has the best activity, but the adhesion strength of this sample is poor. The catalyst prepared by spray-pyrolysis of sol possesses the best adhesion strength. The weight loss is lower than 20% during 1 h ultrasonic treatment and than 10% after 10 times of thermal treatment.LaMnAl11O19 and LaMn0.7Mg0.3Al11O19 catalysts were supported onto commercial FeCrAlloy foil, with Al2O3 and La2O3 as a primer. Different active phase/primer combinations were tested. The primers and the active phase were deposited by a dip-coating technique. The effects of primer on the stability and catalytic activity of monolithic catalyst were investigated. The results show that presence of the primer can improve the stability and activity of the monolithic catalyst. With Al2O3 as the primer the catalyst possesses higher adhesion strength, while with La2O3 as primer a higher activity can be obtained. The presence of the primer may reduce the interaction of hexaaluminate and metal support and consequently favor the catalytic activity.更多还原