【作者】 王宁；

【导师】 赵永祥； 张建平；

【作者基本信息】 山西大学 ， 物理化学， 2010， 硕士

【摘要】 CO甲烷化技术在工业中具有重要的应用价值,如富氢气体中少量CO的脱除、城市煤气通过甲烷化提高热值、F-T合成中避免甲烷生成的研究等,其中利用焦炉煤气、褐煤合成代用天然气技术等,受到学术界和产业界的高度关注,已成为煤化工发展的重要方向。高温镍基催化剂(280-450℃)是工艺气体中少量CO甲烷化过程普遍使用的催化剂,使用该催化剂存在反应设备要求高、操作成本高、能耗大、安全性低等缺陷。因此,开发对反应设备要求较缓和、操作成本低、能耗小、安全性高的低温甲烷化催化剂有重要意义。在镍基催化剂的制备过程中,通过对催化剂活性组分的调变来提高催化剂的活性,实现低温甲烷化是一条切实可行的途径,同时进一步研究活性金属的调变对催化剂催化性能的影响具有重要的理论价值和实际意义。双金属催化剂活性组分多为合金,由于合金具有特殊的电子效应和表面结构,因此双金属催化剂对催化加氢、催化裂解等反应表现出了更优良的活性和选择性,被广泛应用于多种化工生产过程。本文采用浸渍法制备了一系列Ni/γ-Al2O3、Fe/γ-Al2O、Ni-Fe/γ-Al2O3催化剂,采用流动微反装置考察了催化剂的CO甲烷化催化性能,通过N2-physisorption、XRD、H2-TPR、H2-TPD和CO-TPD等手段考察了催化剂的结构和表面性质,同时利用程序升温表面反应(TPSR)和原位漫反射红外光谱法(in-situ DRIFTS)对催化剂上CO甲烷化的反应机理进行了探讨。通过对比Ni/γ-Al2O3、Fe/γ-Al2O3单金属催化剂和Ni-Fe/γ-Al2O3双金属催化剂结构和表面性质的变化以及催化剂上CO吸附和甲烷化反应的TPSR、in-situ DRIFTS差异,给出了Ni-Fe/γ-Al2O3双金属催化剂活性明显提高的主要原因。主要结果和结论如下：1.在相同负载量条件下,与Ni/γ-Al2O3、Fe/γ-Al2O3单金属催化剂相比,6Ni-4Fe/γ-Al2O3双金属催化剂表现出最高的催化活性,CO甲烷化反应具有最低的起燃温度(180℃)和完全转化温度(220℃),该催化剂镍铁最佳组成为6Ni-4Fe。2.第二组分铁的引入促进了催化剂中活性组分的分散,但双金属催化剂的织构特征没有明显变化；第二组分铁的引入减弱了活性组分与载体间的相互作用,还原后形成Ni-Fe合金,使Ni-Fe/γ-Al2O3双金属催化剂还原后活性物种数量增多,对H2、CO的吸附能力增强,表现出高的CO甲烷化催化活性。3.通过对催化剂上CO吸附和甲烷化反应的TPSR和in-situ DRIFTS研究发现,第二组分铁的引入改变了Ni-Fe/γ-Al2O3双金属催化剂上CO甲烷化的反应历程,C-O键的断裂方式明显不同。Ni/γ-Al2O3催化剂上C-O键的断裂经由羰基氢化物-多氢羰基氢化物的途径,而Ni-Fe/γ-Al2O3催化剂上C-O键经由直接断裂表面碳物种加氢的途径。双金属催化剂中合金的形成使Ni-Fe/γ-Al2O3催化剂还原后在较低温度下形成新的甲烷化活性位,这是Ni-Fe/γ-Al2O3催化剂CO低温甲烷化活性提高的重要原因。更多还原

【Abstract】 Researchers expressed their great interests over CO methanation because of its important practical value in many ways. For example, it is used for the removal of a small amount of CO in H2-rich gases, improvement of the calorific value of city gas and the study on avoiding generation of methane in F-T synthesis. Especially, its application in synthesis of the substitute natural gas through coke oven gas or the low quality coal has become very important and attracted a great attention in academia and industry.The high-temperature Ni-based catalyst (280-450℃) is commonly used in the methanation process of a small amount of CO in process gases. Although the catalyst shows the high catalytic activity, the reaction has the equipment requirement, the high cost in operation, the high energy consumption and the low security. Low temperature methanation can provide useful information for development of methanation catalyst which has the moderate equipment requirement, low cost in operation, small energy consumption and high security. In order to increase its catalytic activity and achieve the purpose of low-temperature methanation, choosing the suitable metal or several metals in preparation process of catalyst is a practicable and feasible way. In addition, investigating the effect of metal or metals on the catalytic activity of catalyst has important theoretical and practical significance.The adding of the second metal to bimetallic catalyst, the two metals also have an interaction between the metal including the metal and support interaction, which can change the structure of the catalyst. Alloys were observed in most of the bimetalic catalysts as the active species owning to alloys have special electronic effects and surface structure. So bimetallic catalysts for catalytic hydrogenation, catalytic cracking reactions show a better activity and selectivity, and widely used in many fields of chemical production process. In this paper, a series of Ni/γ-Al2O3、Fe/γ-Al2O3、Ni-Fe/γ-Al2O3 catalysts were prepared by the wet impregnation method. The catalytic activity for CO methanation was investigated in a fixed-bed continuous-flow microreactor. Then the the structure and surface property of the catalysts were investigated via the N2-physisorption、XRD, H2-TPR, H2-TPD and CO-TPD characterizations. Meanwhile, CO methanation reaction mechanism was discussed by the TPSR technique and in-situ DRIFTS. By comparing the changes of structure and surface property of the catalysts as well as TPSR and DRIFTS differences of CO adsorption and methanation reaction over the monometal catalysts and the bimetallic catalyst, the reasons for increase of the catalytic activity of the Ni-Fe/γ-Al2O3 catalyst were given. The main results are as follows:1. From the investigation of the Ni/γ-Al2O3、Fe/γ-Al2O3 and Ni-Fe/γ-Al2O3 catalysts, the 6Ni-4Fe/γ-Al2O3 catalyst has the lowest light-off temperature (180℃) and completely conversion temperature (220℃), and shows the highest catalytic activity.2. Characterizations results show that the alloy formation in bimetallic catalyst has little effect on the texture of the catalysts, but the dispersion of active species in the 6Ni-4Fe/γ-Al2O3 catalyst is obviously promoted. In addition, the addition of the second component iron causes the formation of the Ni-Fe alloy, which decreases the interaction between active species and the support. The moderate interaction improves the dispersion degree and the reduction degree of active species, and then results in more active active species and stronger adsorption ability for H2 and CO.3. From TPSR and in-situ DRIFTS investigation of CO adsorption and methanation reaction over the catalysts, it is found that the addition of the second component iron change the CO methanation reaction mechanism on Ni-Fe/γ-Al2O3 bimetallic catalyst, the breaking of C-0 bond over on the Ni/γ-Al2O3 and 6Ni-4Fe/γ-Al2O3 catalysts is quite different. Ni/γ-Al2O3 catalyst is via multi-hydrogen carbonyl hydride but 6Ni-4Fe/γ-Al2O3 catalyst is via direct breaking in CO methanation reaction. Meanwhile, combined with the characterizations of the structure and surface properties, the formation of alloy leads to more active species, stronger adsorption ability for CO and H2 and in the presence of H2, the formation of a new active carbon species at lower temperature over the Ni-Fe/γ-Al2O3 catalyst, which is the reason for increase of the catalytic activity for CO methanation.更多还原