Preparation, Characterization And Catalytic Activity Of Ni-Mn/γ-Al₂O₃ Catalyst For CO₂ Methanation

Carbon dioxide methanation technology not only makes positive contribution to carbon resources recycling and environment protection, but also has important applications in many other aspects. These applications include removing trace CO2 and CO in the preparation process of ammonia, syngas and pure hydrogen; carbon dioxide removal and oxygen regeneration in the cabin of submarine and spacecraft; gas chromatography analysis for detecting whether purity permanent gas conform to the state standards and for researching plant photosynthesis, etc. Especially, its application in synthesis of the substitute natural gas technology through coke oven gas or low quality coal attracts great attention in academia and industry, and has become very important direction of coal chemical industry.Compared with carbon monoxide, carbon dioxide methanation has lower activation energy, lower reaction temperature and higher selectivity. Moreover, thermodynamics and kinetics are more favorable than other CO2 hydrogenation reactions. Due to the simplicity of the reaction system, CO2 methanation is often used as a probe reaction for the theoretical research of hydrogenation catalysts. The core of CO2 methanation is catalyst. In the carbon dioxide methanation catalyst system, nickel-based catalyst has been widely studied by researchers because of its low-cost, convenient source, high activity and good selectivity.The addition of the second metal to bicomponent catalyst results in the interaction between two metals, in addition to the metal and support interaction. Thus the catalyst structure, texture and catalytic properties will be affected. Bicomponent catalysts can show better activity and selectivity in catalytic hydrogenation and catalytic cracking reactions. They are also widely used in many fields of chemical production process. Transition metal manganese (Mn) has much variable d electronic structure, which shows special effect on hydrogenation reaction, so Mn served as an additive is widely favored by researchers. In this paper, a series of Ni/γ-Al2O3, Ni-M/γ-Al2O3 (M= Ca, Ce, Mn) catalysts were prepared by the wet impregnation method. The catalytic activity for CO2 methanation was investigated in a fixed-bed continuous-flow microreactor. Then the effect of preparation parameters (including loading, impregnation method, calcination temperature, reduction temperature) on the CO2 methanation activity of Ni-Mn/γ-Al2O3 catalyst has been studied, and the optimal preparation parameters have been selected. The catalysts structure and surface property were investigated via the N2-physisorption, XRD,TEM, H2-TPR, H2-TPD, CO2-TPD and CO2/H2-TPSR characterizations. The influence of Mn on the methanation reactants and products adsorption and desorption behaviors had been made deep discussion. By comparing the changes of structure and surface property as well as differences of CO2-TPD and CO2/H2-TPSR over the catalysts, reasons were given for increasing the catalytic activity of Ni-Mn/y-Al2O3 catalyst. Lastly, the impact of technological conditions on the Ni-Mn/γ-Al2O3 catalytic performance had been investigated, such as space velocity, CO2 content of feed gas, etc. The catalyst heat-resistant stability and lifetime were also studied in the end. The main results and conclusions are as follows:1. Under the same nickel loading, all the catalysts with additives of Ca, Ce, Mn show higher CO2 methanation activity than Ni/y-Al2O3 catalyst, and the catalytic performance of Ni-Mn/γ-Al2O3 is more remarkable. The Mn loading has little effect on the CO2 methanation catalytic activity. The Ni-Mn/y-Al2O3 catalyst calcined at 400℃and reduced at 400℃shows good methanation activity. The Ni-Mn/γ-Al2O3 displays higher activity prepared by co-impregnation than that of prepared by step-by-step impregnation.2. The addition of Mn does not change significantly the texture characteristics of the catalyst, but significantly promotes the dispersion of the catalyst active component. The interaction between the metal and the carrier is weakened and the catalyst active surface area is increased. So the number of active species of Ni-Mn/γ-A12O3 is increased after reduction, both H2 and CO2 adsorption capacity are significantly increased too. 3. Through the CO2-TPD and CO2/H2-TPSR characterizations, we can find that the addition of Mn does not change the type of catalyst surface active center, suggesting that the pathways of methanation reaction may not be changed. The moderate strength adsorption sites which desorption temperature is at about 180℃are the CO2 methanation activity centers. The metal dispersion is improved and the reduction temperature is decreased. Futhermore the moderate CO2 adsorption sites and active surface area of the catalyst are increased. As a result, the Ni-Mn/γ-Al2O3 shows good performance on CO2 methanation.4. The research of technological conditions, heat-resistant stability and lifetime about 10Ni-2.5Mn/γ-Al2O3 catalyst has found that, space velocity has obvious influence on catalytic activity and the suitable space velocity of CO2 methanation is about 5000 h-1 in the range of 2000 to 10000 h-1. The lower CO2 content of feed gas, the lower the complete conversion temperature of methanation. The catalyst has good heat-resistant stability and its activity does not decline in 200 h lifetime experiment.