Studies On CO₂ Reforming Of Methane To Syngas Over Ni-based Catalysts Modified By Rare Earth Oxide

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.