Study On Ni-based Methanation Catalysts Prepared By Solution Combustion Method

Natural gas has been considered as a promising energy source due to its advantages of low-carbon, clean, and high calorific value. However, the consumption of natural gas is growing with the speeding up of industrialization and urbanization, and the implementation of the policy of energy conservation and emissions reduction in China. The energy structure in China is poor in natural gas, but rich in coal. Producing synthetic natural gas (SNG) through syngas methanation will become an efficient way to make up for the shortage of natural gas in our Country. As for syngas methanation, catalysts are the core technique. Thus, developing highly efficient methanation catalysts is significant to relieve the energy crisis. Taking the intensely exothermic character of methantion and the cost into consideration, Ni-based catalysts are the most widely used catalyst. Based on solution combustion method (SCM), this paper focused on the effects of preparation conditions, Ti promoter, Na promoter, and Ce promoter on the textural properties, reduction properties, adsorption capacity, and catalytic performance of Ni/γ-Al2O3 catalysts. In addition, the influence of Mg, Mn, and La promoters on the textural properties and catalytic activity of Ni/γ-Al2O3 catalysts had also been studied.The effects of heating rate and solvents on catalytic performance were investigated. Ni/γ-Al2O3 catalysts were prepared by SCM in various heating rate (2 ℃-min-1,5 ℃-min-1, and 10 ℃-min-1) and different solvents (i.e., ethanol, n-propanol and glycerol). As indicated by XRD and TEM, both heating rate and solvents remarkably affected Ni particle distribution and mean particle size. Combustion process may become more intense with the increase of heating rate, resulting in Ni particle aggregating on the surface to form bigger particle size. Solvents with different exothermicity would lead to a diverse gases evolution rate and volume during combustion process, which may affect the structure of porous catalysts. The results of activity tests showed that Ni/y-Al2O3 catalysts prepared in ethanol with heating rate of 2 ℃-min-1 retained an optimal catalytic performance.The effects of solvents and Ti promoter on catalytic performance were investigated. Ti-doped Ni/γ-Al2O3 catalysts with Ti loading of 1 wt% were prepared by SCM in different solvents (i.e., ethanol, n-propanol and glycol). The results showed that solvents significantly affected the textural properties of the catalysts, and catalyst prepared in glycol (G-1Ti) exhibited better Ni particle distribution and narrower size distribution. The results of activity tests showed G-ITi retained an optimal catalytic performance. Moreover, compared with Ti-free Ni/γ-Al2O3 catalyst, G-1Ti catalyst exhibited higher activity. On analysis of characterization results from XRD and HRTEM, Ti species was found effectively restrict the formation of NiAl2O4 spinel phase, leading to a higher utilization of Ni species and thus more exposed active surfaces, which enhanced the CO adsorption capacity. In addition, adding Ti promoter could decrease the binding energy of Ni 2p3/2, facilitating the dissociation of CO on the catalyst surfaces. A series of Ti-doped catalysts with Ti contents of 1 wt%,3 wt%, and 5 wt%, respectively, were studied and discussed to obtain an optimal loading of Ti promoter. The results of CO-TPD indicated that the increase of Ti contents from 1 wt% to 3 wt% could enhance the CO adsorption capacity, in favor of CO methanaiton, while further increasing Ti contents to 5 wt% resulted in relatively lower CO adsorption capacity, possibly due to partial coverage of the active metal by excessive Ti species. The results showed that Ti-doped Ni/γ-Al2O3 catalyst with Ti content of 3 wt%(G-3Ti) possessed an optimal catalytic activity.The effect of Na promoter on catalytic performance was studied. Ni-based catalysts with different amounts of Na promoter from 2 wt% to 6 wt% were prepared by SCM in glycol and water mixture with volume ratio of 1:1, and characterized by N2 adsorption, H2-TPR, XRD, and TEM. The results of catalytic activity tests indicated that adding Na promoter could improve the catalytic activity of Ni-based catalysts at relatively higher temperature (300-340 ℃). Ni-based catalyst with Ni content of 2 wt%(G-2Na) retained an optimal catalytic performance. G-2Na possessed better Ni particle distribution, and milder metal-support interaction, which may improve the catalytic activity and stability of Ni-based catalysts. However, excess addition of Na resulted in deactivation distinctly. It may be due to the blockage of active sites.The effect of Ce promoter on catalytic performance was investigated. Ce-promoted M/γ-Al2O3 catalysts, with Ce loadings from 0 wt% to 8 wt%, were prepared by SCM in glycol, and characterized by N2 adsorption, HRTEM, XRD, XPS, CO-TPD, and TG. The results of low-temperature catalytic activity tests indicated that adding Ce could largely improve the catalytic activity. On analysis of the results from XRD and HRTEM, Ce affected NiAl2O4 spinel rather than NiO crystal. Ni-Ce-Al composite oxides formed in the catalysts. The promotional effects of Ce species were possibly attributed to the high Ni particle dispersion and the small Ni particles, which could expose more accessible active surface, and thus enhance the CO adsorption capacity. Furthermore, as evidenced by XPS, adding Ce species could decrease the binding energy of Ni 2p3,2, promoting CO dissociation, which could facilitate CO methanation. Moreover, during high-temperature stability tests, Ce-doped catalysts showed relatively higher CO conversion than Ce-free catalyst at water free condition. Based on the results of XRD and TG, the addition of Cc could enhance the tolerance against carbon deposition.The effects of Mg, Mn, and La promoters on catalytic performance were studied. Ni/y-Al2O3 catalysts with different promoters (Mg, Mn, and La) were prepared by SCM in glycol. The addition of Mg and Mn would enhance the interaction between Ni and Al, which led to more NiAl2O4 spinel existing in catalysts, and thus decreased catalytic activity. The addition of La could impede the strong interaction between Ni and Al, inhibiting the formation of NiAl2O4 spinel. Moreover, La promoter improved the distribution of Ni particles and the resultant higher CO adsorption capacity, which could facilitate CO methanation. The results of catalytic tests showed that La promoter improved the catalytic performance, while Mg and Mn promoters led to lower catalytic activity.