The Study Of Methane Reforming With Carbon Dioxide Over Co- And Ni- Based Catalysts

With the gradual depletion of the oil and coal resources, the consumption of natural gas in primary energy is significantly increasing. Methane is the main component of natural gas. Converting methane into higher value chemicals and liquid fuels becomes an important route for the efficient use of natural gas resources. On the other hand, there has been increasing global concern over the rise in carbon dioxide emissions from different sources into the atmosphere during the past decade. In the future, atmospheric carbon dioxide levels are expected to increase even further, due to ongoing burning of fossil fuels for heating, power generation and transportation. China is the largest producer of CO2 emissions in the world and is facing major pressure regarding the reduction of CO2 emissions. The dry reforming of methane with carbon dioxide (the DRM process) is a very attractive reaction in terms of the utilization of natural gas and CO2. However, catalyst deactivation in DRM can occur by sintering, coking, loss of the active species and so on. Extensive studies have been dedicated to the development of active and coke-resistant catalysts for the DRM process.In this thesis, two main kinds of catalysts, the Ni-and Co- based catalysts, are employed for the DRM process. Investigation into supports, active metal, preparation methods and promoters has been conducted, with the aim of improving the coke resistance of catalysts. In the hydrotalcite Co/Mg-Al catalysts, it was found that the catalyst with the (Mg+Co)/Al molar ratio of 3 has the smallest particle size of active metal and the largest amount of active sites. The interaction between Ni and Rh promotes the reducibility of Ni particles and decreases the Ni particle size. Also, a series of Ni-SiO2 samples was synthesized by the evaporation induced self-assembly method which leads to homogeneous incorporation of nickel species into the mesoporous silica matrix under low Ni content. A strong anchoring effect was suggested to account for the remaining of small Ni particle size and the improved catalytic performance. Higher Ni metal dispersions and larger amount of surface oxygen species were obtained with the incorporation of Ce cations. The heterogeneous catalysts we synthesized exhibit high activity and stability in the DRM process. The detailed research work and results are the follows.A series of Co/Mg-Al oxide samples, CoMgAl-x (x:the molar ratio of (Mg+Co)/Al in the range of 1-5), were prepared by the self-combustion method followed by H2 reduction. The reduced CoMgAl-x samples mainly consist of solid solution and spinel phases with cobalt particles. In CoMgAl-3 sample the average Co crystallite size is around 14.6 nm, which is the smallest among all CoMgAl-x samples. According to the H2-TPR results, a larger amount of CO3O4 exists on the CoMgAl-3 sample, reflecting the high content of the active sites. The CoMgAl-3 sample has the optimal catalytic activity and stability. The high performance may be attributed to the smaller size of active metal particles, the stable cobalt species within Mg-Al oxide matrix, and the increase of the active sites. Carbon deposits on the catalysts after reaction mostly consist of filamentous carbon.The highly ordered hexagonal mesoporous silica (SBA-15) supported Ni catalysts with different nickel content were prepared for the DRM process. The catalyst with 10 wt.% Ni has high activity for the reaction. Thus, we further synthesized the Rh-Ni/SBA-15 catalysts with different Rh/Ni weight ratio. The addition of a small amount of Rh promotes the reducibility of Ni particles and decreases the Ni particle size. In comparison with bare Ni-based catalyst, the Rh-Ni bimetallic catalysts show high activity and superior stability in the dry reforming. The total amount of carbon deposition over the used 0.02Rh-Ni/SBA-15 catalyst was 14.7% in comparison with 27.3% over the used Ni/SBA-15 catalyst. The deposited carbon is originated from CH4 decomposition and CO disproportionation. It has been shown that the rate of carbon removal depends on the Ni particle size in the catalyst. The Rh-Ni catalyst with smaller Ni particle size inhibits the carbon formation and exhibits high efficiency in the removal of coke.The conventional impregnation method may cause blocking pore, uneven dispersion of metal and relatively larger metal particle size. In this study, mesoporous Ni-SiO2 samples with different nickel content (3.1-13.2%) were co-synthesized via the evaporation induced self-assembly (EISA) method. Their catalytic activity was tested in the DRM process. It is postulated that the anchoring effect between metallic Ni particles and Ni ions distributed in the silica matrix is responsible for the stabilization of the metallic Ni nanoparticles. Unconfined metallic clusters can freely migrate and aggregate to produce larger particles, but the anchored clusters almost retain their initial size as a result of the constraint of the anchoring sites.The 6.7%Ni-SiO2 catalyst with highly dispersed small nickel particles exhibit excellent catalytic activity and long-term stability. The carbon deposition on this catalyst is only 7.4% after 30 h reaction. A strong anchoring effect between metallic nickel particles and unreduced nickel ions in the silica matrix is believed to account for the remaining of small Ni particle size and the improved catalytic performance. During the 30 h stability test, there was almost no significant activity loss over 6.7%Ni-SiO2, whereas a significant deactivation over 6.7%Ni/meso-SiO2 were observed.It has been pointed out that CeO2 could promote the performance via improving the metal dispersion and metal-support interaction, which effectively prevented the metal sintering. Meanwhile, the large oxygen storage ability of CeO2 was beneficial to the carbon elimination and the inhibition of carbon deposition. A series of Ce-Ni-SiO2 samples with different cerium content and fixed nickel content of 6.7% was synthesized by the EISA method, which leads to the homogeneous incorporation of cerium and nickel species into the mesoporous silica matrix. Ce is added to the catalyst to reduce the amount of isolated nickel oxide. Highly dispersed Ni metal particles with 2-4 nm of particle size may be obtained with the incorporation of Ce cations. There is no significant deactivation in the stability test over the Ce-Ni-SiO2 catalysts. After 60 h reaction, the total amount of carbon deposition over the used 1.2%Ce-Ni-SiO2 catalyst is only 5.5% in comparison with 10.8% over the used Ni-SiO2 catalyst. This high stability is attributed to the formation of well-dispersed Ni particles due to the anchoring effect of Ce species as well as the ability of Ce species to increase the surface oxygen species.