| Biomass gasification is one of the thriving techniques for biomass utilization, because the produced gas can be used to generate electricity, and synthesize liquid fuels (methanol and Fischer-Tropsch oil). However, in the gasification process, a high content of tar is produced which is composed of condensable aromatics. Tar plugs the gasification process piping, damages combustion motor, harms the living environment and human health, and decreases the recoverable. Therefore, it has become a choke point for the utilization of biomass-derived gas. Many methoods were employed to remove tar; comparatively, catalytic reforming of tar compounds is of great advantage because it converts tar into the useful syngas. Due to CO2concentration in the raw gas from biomass gasification can reach10vol%or higher, so the reforming with CO2is proposed as a meaningful way. By using CO2reforming, tar is converted into syngas, as well as the emission of CO2is minimized. A key problem in the reforming is how to obtain efficient catalysts with high stabilities. In this dissertation, great efforts were devoted to investigate the main reasons which influence the catalytic performance of Ni-based catalysts. Toluene is selected as a model compound of tar because it has a big share in tar. A fluidized-bed reactor is employed as the reforming reactor to obtain a uniform distribution of heat and pressure. The main findings are as follows:SiO2, ZrO2, α-Al2O3, γ-Al2O3, and MgO supported Ni catalysts were prepared by the impregnation method, and tested in CO2reforming of toluene. It was found that the acidity-basicity of support and the interaction between support and Ni play crucial roles in influencing the catalytic performance. H2-TPR and XRD results showed that a weak interaction between Ni and support presented in both Ni/SiO2and Ni/ZrO2catalyst, and the main Ni species was the free NiO which is reducible. During reduction, the metallic Ni from the reduction of free NiO sintered easily, which caused the activity loss and the formation of the inert coke which cannot the oxidized by CO2at the reforming temperature. A strong metal-support interaction (SMSI) was detected in Ni/α-Al2O3, Ni/γ-Al2O3and Ni/MgO; it made the Ni species hard to reduce, but brought a high dispersion of metallic Ni and a small Ni particle size after reduction. The highest conversion of toluene (-100%) at650℃was achieved on Ni/MgO catalyst in which a NiO-MgO solid solution was formed. XRD and O2-TGA results indicated, the main reason for the deactivation was coking, and the formation of coke was dependent on the Ni particle size and the acid-basic feather of support. A big Ni particle size tended to form the inert coke, whereas, a small Ni particle size favored the removable coke. As for Ni/y-Al2O3, although it had a small Ni particle size, a lot of H-rich coke was formed because the acidity ofy-Al2O3; and the coke covered the metallic Ni obviously in TEM pictures, which caused the deactivation of Ni/γ-Al2O3catalyst. As for Ni/MgO, a high concentration of CO2was achieved on catalyst surface because of the basicity of MgO, which was useful to suppress coke.In the experimental study of Ni/MgO at different reaction temperature, reduction temperature and the reactant content for CO2reforming of toluene, it was found the initial conversion of toluene over Ni/MgO was nearly linearly related to the amount of surface Ni in catalyst. At a high reduction temperature, on one hand, more reduced Ni species can be reduced; on the other hand, bigger Ni particles were yielded because of the sintering of metallic Ni which caused a loss of surface Ni sites, as a result, the conversion of toluene became worse. By analyzing the difference in catalytic activities and coke formation at different reforming temperature and different reactant content, it was inferred that CO2reforming of toluene contains the following two reactions, the first is the dissociation of toluene at metallic Ni which produces the carbon intermediates, and the second is the oxidization of carbon intermediates by CO2; if the reaction rate of the second reaction is slower than that for the first reaction, coke is formed, and the coke was recognized as the poly-aromatic compounds by Raman spectroscopy. At a low CO2/toluene mole ratio, a high toluene feed rate, or a low reaction temperature, the activity of Ni/MgO became worse with more coke. This observation suggested the activation of carbon intermediates with the oxidizing CO2species might be the rate-determining step in the reforming process. By changing the Ni loading, calcination temperature and preparation method, the distribution of different Ni species and interaction between Ni species and MgO in Ni/MgO were investigated. XPS, Raman spectrum and H2-TPR results revealed that the reduction of NiO just occurred at the outermost layer of catalyst, and the NiO-MgO solid solution in bulk cannot be reduced below800℃. At700℃, the reducible Ni species mainly contained the free NiO and NiO-MgO at the outermost layer of catalyst. Moreover, the metallic Ni from the reduction of free NiO interacted weakly with MgO, so it sintered easily, forming a big Ni particle size, as a result, the activity of Ni/MgO was worse. So it can be concluded that increasing the reducible NiO species, especially the non-free NiO is the key to obtain a good catalytic performance.MnO was used to modify Ni/MgO catalyst by different method to investigate the role of Mn species in CO2reforming of toluene. It was found that the preparation method has a significant impact on the structure and the activity of catalysts. When MnO was added by using manganese nitrate as precursor, Mg6MnO8was formed which not only weakened the basicity of MgO support but also covered the surface NiO, making NiO more difficult to reduce. However, when MnO was added by calcination of the mixture of MnO2and MgO, Mn2O3was the main species. NiO can be loaded on either MgO or Mn species, yielding more the surface Ni. The H2-chemisorption results showed the highest amount of surface Ni was obtained on this catalyst in which the support was the mixture of10%MnO2and MgO. As a result, the conversion of toluene can reach90%at570℃on this catalyst, whereas, it was just about72%on Ni/MgO.In the study of Co/MgO and Co-Ni/MgO catalysts, it was found the reduction degree of Co/MgO was smaller than Ni/MgO, but more surface metallic sites were obtained, comparing with Ni/MgO catalyst. Therefore, Co/MgO showed a higher conversion of toluene. However, Co/MgO catalyst was not stable at a low Co loading (<12wt%) because of the oxidization of metallic Co. Due to Ni/MgO has a goof anti-oxidization capacity, a bimetallic Co-Ni/MgO catalyst was prepared by the co-impregnation method to improve the stability of Co/MgO. By H2-TPR and H2chemisorption characterizations, a synergistic effect between Co and Ni species was detected, which brought more active metallic particles than Co/MgO and Ni/MgO with a similar metal loading. In the reforming test at570℃, the initial conversion of toluene on5%Co-5%Ni/MgO can reach88%with a stability of89%. |
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