【作者】 许宏图；

【导师】 刘源； 罗坚；

【作者基本信息】 天津大学 ， 化学工程， 2020， 硕士

【摘要】 自工业时代以来,化石燃料的大规模使用直接导致了大量的二氧化碳排放,所引发的气候变化——温室效应愈加严重。在近年来提出的各种解决二氧化碳释放问题的概念或技术方法中,CO₂甲烷化被认为是最有效的方法之一。目前,应用于该反应的镍基催化剂也存在两个挑战:低温下活性较差和高温下金属镍纳米粒子易烧结。为了有效提高催化剂的低温活性和抗烧结能力,分别从载体组成、活性金属的颗粒尺寸及分散度等方面优化了催化剂的结构。采用共沉淀法制备了一系列Ce_xZr_1-xO₂固溶体和Y₂O₃-ZrO₂复合氧化物作为催化剂载体,然后采用柠檬酸络合法将Ni负载于制得的载体上,还原后得到一系列负载于大比表面积载体的具有小尺寸Ni纳米颗粒的且高度分散的负载型Ni基催化剂。利用X射线衍射（XRD）、N₂吸附-脱附（N₂-BET）、程序升温脱附（TPD）、程序升温还原（TPR）等技术对催化剂进行表征,在常压微型固定床反应器上测试了催化剂的CO₂甲烷化性能,考察了载体以及活性组分镍含量对催化剂催化性能的影响。在Ni/Ce_xZr_1-xO₂催化剂上,考察了活性组分Ni负载量和载体中Ce/Zr比对催化剂结构和CO₂甲烷化催化性能的影响。当Ni负载量为12 wt%时,12%Ni/Ce_0.25Zr_0.75O₂催化剂在200℃、常压和空速为15,000 mL·g^-1·h^-1条件下,CO₂转化率达74%,CH₄选择性为100%。300 h的稳定性测试结果显示,与Ni/ZrO₂相比,12%Ni/Ce_0.25Zr_0.75O₂催化剂具有更好的抗烧结性能。催化剂的优异活性归因于采用了新的负载方法——柠檬酸络合法负载活性组分镍,该法实现了镍的高分散。实验结果表明:随着Ce⁴⁺部分取代Zr⁴⁺形成Ce_xZr_1-xO₂固溶体有利于增强活性组分Ni和载体之间的相互作用,显著提高了Ni基催化剂的抗烧结性能。同时,载体中Ce的掺杂提高了催化剂的储释氧能力以及Ni的分散度,因此,12%Ni/Ce_0.25Zr_0.75O₂催化剂表现出了优异的催化性能,以及良好的抗积碳和抗烧结性能。在Ni/Y₂O₃-ZrO₂催化剂上,考察了载体中Y/Zr比对催化剂结构和CO₂甲烷化催化性能的影响,实验结果发现我们制备的Y掺杂量为3%的12%Ni/Y₂O₃-ZrO₂催化剂具备较好的低温催化性能,在200℃、常压和空速为15,000 mL·g^-1·h^-1条件下,CO₂转化率达60%,CH₄选择性为100%。实验结果表明,随着ZrO₂中Y₂O₃掺杂量的增加,立方相或四方相ZrO₂逐渐增多,而单斜相随着Y₂O₃掺杂量的增加而减少。Y₂O₃-ZrO₂复合氧化物的形成,使催化剂中Ni O纳米颗粒与载体之间相互作用明显增强,显著提高了催化剂的CO₂解离吸附能力,从而提高该系列催化剂的低温催化性能。更多还原

【Abstract】 Since the industrial era,the large-scale use of fossil fuels has directly led to a large amount of carbon dioxide emissions,and the climate change-greenhouse effect-caused by it has become more and more serious.Among the various concepts or technical methods proposed in recent years to solve the problem of carbon dioxide release,CO₂methanation is considered to be one of the most effective methods.At present,there are two challenges in the application of nickel-based catalysts to this reaction:poor activity at low temperature and easy sintering of metallic nickel nanoparticles at high temperature.In order to effectively improve the low-temperature activity and anti-sintering ability of the catalyst,the structure of the catalyst was optimized in terms of carrier composition,particle size and dispersity of active metals,respectively.In this paper,a series of Ce_xZr_1-xO₂solid solutions and Y₂O₃-ZrO₂composite oxides were prepared by coprecipitation method as catalyst supports.Then,Ni was loaded on the prepared carrier by citric acid method.And after reduction,a series of highly dispersed supported Ni-based catalysts with small-size Ni nanoparticles supported on carriers with large specific surface area are obtained.The catalysts were characterized by X-ray diffraction（XRD）,N₂adsorption-desorption（N₂-BET）,temperature programmed adsorption（TPD）,temperature programmed reduction（TPR）,etc.The methanation performance of the catalyst CO₂was tested on a micro fixed bed reactor at atmospheric pressure,to investigate the effects of carrier and nickel content on the catalytic performance of the catalyst.The effects of Ni loading and Ce/Zr ratio on the structure of Ni/Ce_xZr_1-xO₂catalyst and the catalytic performance of CO₂methanation were investigated.When the Ni loading is 12 wt.%,the CO₂conversion and CH₄selectivity of the12%Ni/Ce_0.25Zr_0.75O₂catalyst are 74%and 100%respectively at 200℃,atmospheric pressure and space velocity of 15,000 mL·g^-1·h^-1.The stability test results for 300 h showed that 12%Ni/Ce_0.25Zr_0.75O₂catalyst had better sintering resistance than Ni/ZrO₂catalyst.The excellent activity of the catalyst was attributed to the adoption of a new loading method-citric acid complexation method to load the active component nickel,which realized high dispersion of nickel.The experimental results show that as Ce⁴⁺partially replaces Zr⁴⁺to form Ce_xZr_1-xO₂solid solution,the crystal form of ZrO₂changed from monoclinic phase to tetragonal phase,and the ceria-zirconia solid solution carrier formed is conducive to enhancing the interaction between active component Ni and the carrier.Therefore,the sintering resistance of the Ni-based catalyst was remarkably improved.The doping of Ce in the carrier improved the oxygen storage,release capacity of the catalyst and the dispersion degree of Ni.Therefore,the 12%Ni/Ce_0.25Zr_0.75O₂catalyst showd excellent catalytic performance,as well as good carbon deposition resistance and sintering resistance.The effects of Y/Zr ratio on the structure of Ni/Y₂O₃-ZrO₂catalyst and the catalytic performance of CO₂methanation were investigated.The 12%Ni/Y₂O₃-ZrO₂catalyst with 3%Y doping has good catalytic performance at low temperature.Under the conditions of 200℃,atmospheric pressure and space velocity of 15,000 mL·g^-1·h^-1,the conversion rate of CO₂is 60%and the selectivity of CH₄is 100%.The experimental results show that with the increase of Y₂O₃doping amount in ZrO₂,cubic phase or tetragonal phase ZrO₂gradually increases,while monoclinic phase decreases with the increase of Y₂O₃doping amount.The formation of Y₂O₃-ZrO₂composite oxide significantly enhances the interaction between Ni nanoparticles and carriers in the catalyst,and significantly improves the dissociation adsorption capacity of CO₂.Thus the low-temperature catalytic performance of the series of catalysts was improved.更多还原