【作者】 张万东；

【导师】 田宜灵； 刘炳泗；

【作者基本信息】 天津大学 ， 化学工艺， 2007， 博士

【摘要】 由于对化工能源和环境保护的实际需求,二氧化碳重整甲烷制备合成气是目前C1化学领域中颇受关注的课题。其原因在于,可以充分利用自然界中丰富的天然气和二氧化碳资源,制备具有低H₂/CO比的合成气,而低H₂/CO比的合成气是进行一系列重要化工生产的优良原料,有广泛的应用前景。为了制备高活性、高稳定性和高抗积碳性能的催化剂,在文献调研的基础上,本论文采用溶胶-凝胶法制备了一系列含有稀土氧化物的镍基催化剂,考察了稀土元素（La₂O₃或Sm₂O₃）的含量对催化剂的催化性能和物化性能的影响,并与其它制备方法进行了比较。考察了以稀土氧化物为载体对甲烷、二氧化碳的转化率和氢气、一氧化碳的选择性,及其对抗积碳的影响。同时,利用稳定的Ni/Sm₂O₃-CaO催化剂进行了动力学研究,在此基础上提出了CO₂/CH₄重整反应的机理。应用包括BET,XRD,XPS,XAES,AFM,TME,TG/DTA,H₂-TPR等多种表征手段对催化剂物化性质和积碳进行了深入的分析,并与催化剂的催化性能和反应稳定性相关联,得到了性能优良的CO₂/CH₄重整催化剂。本论文得到如下一些结论:1.分别采用溶胶-凝胶法、浸渍法、离子交换法制备了相同Ni含量的10%Ni/La-ZSM-5催化剂。在固定床反应器中,于700℃,常压下把这些催化剂用于CO₂/CH₄重整制合成气。实验结果表明,甲烷转化率增加的顺序是:溶胶-凝胶法>浸渍法>离子交换法。在稳定性测试中溶胶-凝胶法制备的催化剂稳定性最好,浸渍法催化剂在反应初期活性较高,但易失活。具有典型尖晶石结构的La₂NiO₄,通过溶胶-凝胶法被均匀地分散在具有高比表面积的ZSM-5载体上,与采用浸渍法和离子交换法制备的催化剂相比,此方法制备的催化剂,Ni的分散度高,颗粒较小,从而能提供较多的CH₄裂解活性位,并且CO₂能将CH₄解离生成的碳物种及时消除,故在高空速(GHSV = 4.8×104 ml·g^-1·h^-1)和长时间重整反应后（36 h）,催化活性一直保持不变。由溶胶-凝胶法制备的催化剂在抗积碳方面的能力也好于其它两种方法制备的催化剂。这是由于溶胶-凝胶法制备的催化剂中含有高分散和高稳定的镍粒子,同时在活性点周围,酸性的CO₂容易被La₂O₃吸附形成La₂O₂CO₃物种,而La₂O₂CO₃能解离为CO和O物种,而氧物种能够与催化剂表面的碳物种（CHx）反应生成CO,从而达到消除积碳的作用。从DTA图上可以看出,采用溶胶-凝胶法制备的催化剂上的积碳分为两种类型,其一是丝状碳,二是石墨型碳,而采用浸渍法和离子交换法催化剂上的积碳主要是石墨型碳,这可能是因为沉积在Ni颗粒上的碳原子容易进入Ni晶粒的晶格,并在晶格间扩散,没有及时被氧物种消除,使其沉积在Ni晶粒和载体的界面上,并逐渐石墨化造成的。2.采用溶胶-凝胶法制备了Ni/CaO, Ni/Sm₂O₃和不同Sm/Ca摩尔比的Ni/Sm₂O₃-CaO （Sm/Ca = 1:4,1:1和4:1）催化剂,实验结果表明,与Ni/CaO, Ni/Sm₂O₃相比,Ni/Sm₂O₃-CaO （Sm/Ca = 1:4, 1:1和4:1）催化剂具有较高的比表面积,较小的Ni晶粒及较强的抗积碳能力。XRD和HRTEM的分析结果表明适量Sm₂O₃的添加能够消除大的Ni金属颗粒,提高Ni的分散度,使反应活性有明显改善,而过量的Sm₂O₃的加入却容易导致催化剂失活。在对反应积碳的研究中发现,采用碱土载体和稀土金属氧化物作载体的催化剂在CO₂/CH₄重整过程中具有较好的抗积碳性能。反应中主要的积碳形式有两种。通过100 h在GHSV = 4.8×10⁴ ml·g^-1·h^-1,700℃,CO₂/CH₄ = 1:1,条件下的CO₂/CH₄反应中,证明Ni/Sm₂O₃-CaO （Sm/Ca = 1:4）在较长时间内也可保持稳定的催化性能（100 h）。3.相对于浸渍法制备的Ni/SL催化剂,由溶胶-凝胶法制备的Ni/SL催化剂具有较大的比表面积、适宜的孔分布和稳定的结构。由（Sm₂O₃）0.77（La₂O₃）0.23载体制备的负载镍催化剂在CO₂重整CH₄反应中表现出了很好的反应特性。在Ni/（Sm₂O₃）_0.77（La₂O₃）_0.23（sol-gel）催化剂上甲烷和二氧化碳的转化率分别是56﹪和60%,氢气和一氧化碳的选择性分别是96﹪和98%。如此高的活性主要是由于在催化剂中存在着高分散和高稳定性的镍粒子。通过XRD的试验已证明了这些高分散粒子的存在。另外,此催化剂也表现出了很好的抗积碳能力,这是由于具有碱性的（Sm₂O₃）0.77（La₂O₃）0.23稀土化合物能够吸附二氧化碳,尤其在活性点Ni0周围形成La₂O₂CO₃物种,此物种容易与碳物种（CHx）反应生成CO。结果导致由甲烷裂解的碳物种（CHx）还未转化成石墨碳,就与氧物种反应生成了一氧化碳。4.在固定床微分反应器中,考察了反应参数对Ni/Sm₂O₃-CaO（1:4）催化剂反应活性的影响。通过改变反应的温度和反应物的压力研究了Ni/Sm₂O₃-CaO（1:4）催化剂上的本征动力学行为。在600⁷00℃的反应温度范围内,原料气CH₄和CO₂的表观活化能是17.59和29.97 kcal·mol^-1,而生成物CO和H₂的活化能分别为19.82和33.2 kcal·mol^-1。由于逆水气反应,当在反应物中增加H₂的进料压力,可以提高CO的生成速率。在0-50 kPa压力范围内,甲烷和二氧化碳的压力变化都影响甲烷的消耗速率。根据实验和文献的分析,推导出了可能的基元反应,并根据L-H模型,推导出了反应本征动力学方程,二氧化碳的分解反应是在Ni/Sm₂O₃-CaO （Sm/Ca等于1:4）催化剂上进行CO₂/CH₄重整反应的速率决定步骤。更多还原

【Abstract】 Due to the practical demand for chemical industry and environment protection, a CO2 reforming of methane to syngas has attracted considerable attention in Cl chemistry field. By this reaction, natural gas and carbon dioxide, both are greenhouse gas in nature, can be transformed to syngas with low ratio of H₂/CO, which is a proper feed for many important chemical engineering processes. It presents extensive practical prospect. In order to prepare catalysts with high activity, stability and excellent resistance to carbon deposition for this reaction, based on a great deal of literature, a sol-gel technique is employed to prepare a series of Ni-based catalysts contained rare earth oxides. The effect of rare earth element （Sm₂O₃ or La₂O₃） on the catalytic performance and physicochemical property of catalysts was investigated. The comparison was done between sol-gel and other preparation methods. The effect of rare earth oxide as a support on the conversion of CH₄ and CO2, the selectivity of H₂ and CO and the resistance to carbon deposition over catalysts was investigated. In the meantime, the kinetic behavior of the CO₂/CH₄ reforming reaction over high stable Ni/Sm₂O₃-CaO catalyst was investigated. A mechanism of the CO₂/CH₄ reforming has been proposed based on the experimental results and report in literature. Many characterization techniques, such as BET, XRD, XPS, XAES, AFM, HRTME（EDX）, TG/DTA, H₂-TPR were used to analyze the physicochemical property of the catalysts and the behavior of carbon deposition. The obtained results were correlated with catalytic performance and reaction stability. We have obtained the catalyst with high catalytic performance for CO₂/CH₄ reforming. The main conclusions are as follows:（1） The catalysts of 10% Ni/La-ZSM-5 with same loading amount of Ni were prepared by means of the sol-gel, incipient-wetness impregnation and ion-exchange methods, respectively. The prepared catalysts were used to produce syngas from CO2 and CH₄ at 700℃under normal pressure in a fix-bed reactor. The order on conversion of CH₄ is as follows:sol-gel>imp.>ion-exchange. Additionally, the catalyst prepared by a sol-gel technique exhibited higher performance than those prepared by impregnation or ion-exchange method, whereas the catalyst prepared by impregnation showed rather high activity at the beginning of the reaction but resulted in deactivation easily. The La₂NiO₄ catalyst had a typical spinel structure. By means of a sol-gel method La₂NiO₄ were uniformly dispersed on a ZSM-5 support. By comparison with the impregnation and ion-exchange method, the La₂NiO₄/ZSM-5 catalyst prepared by a sol-gel method, exhibited high dispersion and small Ni particles, which can provide more active sites for CH₄ decomposition and CO₂ was able to eliminate the carbon species generated from CH₄ decomposition. At the GHSV = 4.8×10⁴ ml·g^-1·h^-1, the catalytic activity was changeless during long-time reforming. Concerning resistance to carbon deposition, the catalyst prepared by a sol-gel technique exhibited higher performance than those prepared by other method. This is due to the formation of highly dispersed and stable Ni particles in the former, meanwhile, on the adjacent Ni sites, the La₂O₃ can adsorb CO₂ to form the La₂O₂CO₃ species, the La₂O₂CO₃ species decomposed into CO and O species, which react with accumulated carbon （CHx） on catalyst surface to produce CO. In addition, TG/DTA analysis indicated that at least two kinds of carbon depositions （filamentous whisker carbon and graphitic carbon） were formed on the catalyst prepared by sol-gel method, whereas only one kind of carbon deposition, graphitic carbon, was observed on the catalyst prepared by impregnation and ion-exchange methods. This is due to the lack of enough oxygen species to react with CHx in the latter, and CHx may further decompose into coke species, which penetrated into the Ni lattice and diffusion through the metal lattice, and final, the coke species gradually changed to graphitic carbon.（2） A sol-gel method was employed to prepare Ni/CaO, Ni/Sm₂O₃ and a series of Ni/Sm₂O₃-CaO （the molar ratio of Sm/Ca is 1:4, 1:1 and 4:1,respectively） catalysts dispersed uniformly. The Ni/Sm₂O₃-CaO （Sm/Ca is 1:4, 1:1 and 4:1） catalysts exhibited the high surface areas, carbon deposition resistance and the small Ni particles compare with the Ni/CaO and Ni/Sm₂O₃ catalysts. X-ray diffraction （XRD） and high resolution transmission electron microscopy （HRTEM） analyses reveal that appropriate addition of Sm₂O₃ can suppress the formation of large nickel particle and produce the highly dispersed nickel species, and consequently, improves the catalytic activity, whereas excess addition of Sm₂O₃ reduces the catalyst activity. It is found that the resistance to carbon deposition can be greatly improved with the alkaline earth and rare earth oxides as the supports at CO₂/CH₄ reforming. Long-term experiments were carried out, showing the excellent stability of the Ni/Sm₂O₃-CaO （Sm/Ca is 1:4） in the range of 100 h.（3） The Ni/（Sm₂O₃）0.77（La₂O₃）0.23 （sol-gel） catalyst prepared by the sol-gel technique showed large specific surface area, appropriate pore distribution and stable structure compared to Ni/（Sm₂O₃）0.77（La₂O₃）0.23 prepared by impregnation method. （Sm₂O₃）0.77（La₂O₃）0.23 as a support gives good performances for CO₂/CH₄ reforming reaction. The CH₄ and CO₂ conversion were 56 and 60% respectively, and the H₂ and CO selectivity were 96 and 98% over the Ni/（Sm₂O₃）0.77（La₂O₃）0.23 （sol-gel） catalyst. The high activity attributed to the high dispersion of the nickel particles. There exists highly dispersion of the nickel particles has been proved by XRD technique. In the meantime, it also showed the excellent resistance to carbon deposition. This is due to the rare earth oxides, （Sm₂O₃）0.77（La₂O₃）0.23 with basicity, which is favorable for CO₂ adorption on the adjacent Ni sites to form La₂O₂CO₃ species. As a consequence, the active carbon species （CHx） can be removed rapidly by the O species before converting to graphitic carbon.（4） The effect of reaction parameters on the catalytic activity of Ni/Sm₂O₃-CaO （Sm/Ca is 1:4） catalyst for CO₂ reforming of CH₄ was studied. The kinetic behavior of Ni/Sm₂O₃-CaO （Sm/Ca is 1:4） catalyst in the reforming reaction was investigated as a functions of temperature and partial pressures of CH₄ and CO₂. The apparent activation energy for CH₄ and CO₂ consumption, and H₂ and CO production were 17.59, 29.97, 33.2 and 19.82 kcal·mol^-1, respectively, in the range of 600–700℃. An increase of the H₂ partial pressure leads to a continuous enhancement of the rate of CO formation, due to the simultaneous occurrence of the water–gas shift reaction. The variation of CH₄ and CO₂ partial pressures have strong influence on the rate of methane consumption in the pressure range of 2–48 kPa, respectively. A reactive mechanism of the CO₂/CH₄ reforming was proposed based on the experimental results and some reviews. Based on this mechanism, a kinetic model was developed. The activation of CO₂ to form CO and O is suggested to be the rate-determining step for the CO₂/ CH₄ reforming over Ni/Sm₂O₃-CaO （Sm/Ca is 1:4） catalyst.更多还原