| Carbon monoxide methanation not only has been widely used in these aspects such as hydrogen preparation in synthesis of ammonia and petroleum chemicals, but also has important applications in other many fields. For example, hydrogen purification in fuel cells; improvement of the calorific value of city gas and the study on avoiding generation of methane in F-T synthesis and so on. Especially in recent years, the synthesis of the substitute natural gas through the methanation of coke oven gas or the gasification and methanation of the low quality coal has attracted a great deal of attention in aeademia and industry fields. Meanwhile, in the theoretical research aspect, CO methanation is ofen used as a probe reaction to study the methanation catalyst beeause it is a simple reaction system.Among the catalysts for CO methanation, noble-metal Ru catalytic activity is the highest, but its expensive price makes its application limited. However, Ni-based catalyst is widely used for the removal of a small amount of CO in hydrogen-rich gas because of its higher catalytic activity and lower price. In order to improve the catalytic performance of catalyst, adding suitable promoters in catalyst has been considered as an effective way. Alkali metals, alkaline earth metals and rare earth metal oxides are ofen used as catalyst promoters. Specially, CeO2is widely favored in the catalytic field because of its excellent storage and supply oxygen capacity, oxidation-reduction quality and the characteristics of dispersing the surface active component, etc.SiO2aerogel has been used as an excellent catalyst carrier because of its small size, high specific surface area, high porosity and other properties. Our previous studies suggested that the Ni/SiO2catalyst possessed higher catalytic activity for CO methanation. Therefore, in this paper, a series of Ni/SiO2and Ni-M/SiO2(M=Ce、Co、 Cu) catalysts were prepared by incipient-wetness impregnation method, and the catalytic activity was investigated. The effects of preparation conditions (including different Ni/Ce ratios, Ce loading, impregnation orders, etc.) on the CO methanation activity of the Ni-Ce/SiO2catalyst were also studied. On this basis, the structure and surface properties of the catalysts were investigated by N2-physisorption, XRD, H2-TPR, TPD and TPSR techniques and so on, and the effect of the addition of Ce promoter on the absorption and desorption behaviors of the methanation reactants and product was discussed thoroughly. At last, the influence of technological conditions (such as space velocity, reaction temperature, CO content in feed gas) on the catalytic activity of the Ni-Ce/SiO2catalyst, and heat-resistant stability and lifetime were investigated. The main results were as follows:1. The addition of Ce promoter remarkably enhanced the catalytic activity of Ni/SiO2catalyst for CO methanation. The catalytic activity of the7wt%Ni-2wt%Ce/SiO2was higher than that of the7wt%Ni/SiO2and13wt%Ni/SiO2, and its light-off temperature and the lowest complete conversion temperature was190℃and230℃, respectively.2. The structure and surface properties of the7wt%Ni/SiO2and7wt%Ni-2wt%Ce/SiO2were characterized. The results showed that the interaction was generated among NiO, CeO2and SiO2due to the additon of Ce, which did not significantly change the texture characteristics of the catalyst, but markedly promoted the dispersion of the active Ni species, decreased the interaction between the active component and support, increased the active surface areas. Furthermore, new moderate intensity CO adsorption sites were formed on the surface of the7wt%Ni-2wt%Ce/SiO2catalyst. As a result, the catalyst owned higher adsorption and activation capacity toward CO, and exhibited higher catalytic activity for CO methanation.3. In the study of the technological conditions, heat-resistant stability and lifetime on the7wt%Ni-2wt%Ce/SiO2catalyst, we found that in the course of space velocity from3000h-1to10000h-1, CO conversion rate slightly decreased; In the range of100-640℃, the catalyst had wider reaction activity temperature range; The catalyst possessed higher heat-resistant stability after reaction at550℃, and its activity was not declined in about240h lifetime test experiment under the conditions of240℃,7000h-1,1%CO. |
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