Theoretical And Experimental Studies On The Catalytic Conversion Of CO₂

Catalytic conversion of CO₂ into valuable chemicals will help to neutralize the CO₂ emissions into the atmosphere, and thereby alleviate the greenhouse effect caused by CO₂. The main purpose of our present work is to design and prepare highly efficient catalysts for the catalytic conversion of CO₂. Firstly, using density functional theory method (DFT), we detailedly studied the interaction of CO₂ with solid oxides and oxide-supported metal catalysts. And then, on the basis of the results from the theoretical study and our previous experiments, we applied a plasma technique to prepare a coke resistant Ni/SiO₂ catalyst for CO₂ reforming of methane.On the dryγ-Al₂O₃ surfaces, the O–Al bridge sites were found to be energetically favorable for CO₂ adsorption. The strongest binding with an adsorption energy of 0.80 eV occurs at the O3c–Al5c bridge site of the dryγ-Al₂O₃(100). On the hydroxylatedγ-Al₂O₃(110) andγ-Al₂O₃(100), a bicarbonate species was formed by protonating the adsorbed CO₂ with the protons from neighboring hydroxyl groups. The activation barrier for forming the bicarbonate species is about 0.50 eV. Theoretical calculation results indicated that, over theγ-Al₂O₃-supported metal catalyst, the interface betweenγ-Al₂O₃ and the metal particles is the most favorable site for CO₂ adsorption and activation. Increasing the area of the interface between the oxide support and the metal particles is expected to improve the adsorption and activation of CO₂ on the catalysts, and thereby benefit the conversion of CO₂. In practice, increasing the dispersion of the metal particles and maintaining smaller size of the supported metal particles will maximize the interface available for CO₂ adsorption.In the previous experiments of our group, we prepared a Ni/γ-Al₂O₃ catalyst for the CO₂ reforming of methane using a novel plasma technique. It was found that, on the plasma-prepared Ni/γ-Al₂O₃, the size of the Ni particles is smaller and the dispersion of the Ni particles is higher, compared with the Ni/γ-Al₂O₃ prepared with the conventional impregnation method. Moreover, the plasma-prepared Ni/γ-Al₂O₃ exhibites a higher coke resistance in the CO₂ reforming of methane. Based on the results from our present theoretical calculations and our previous experiments, we applied the plasma technique to prepare a Ni/SiO₂ catalyst for the CO₂ reforming of methane in the present work. The plasma treatment decreases the size of the Ni particles and significantly improves the Ni dispersion. The plasma-prepared Ni/SiO₂ catalyst exhibits comparable activity to the Ni/SiO₂ catalyst prepared by the conventional impregnation method, but the coke resistance of the former is higher. Over the plasma-prepared Ni/SiO₂, coke formation from methane decomposition is suppressed. Moreover, the methane-derived carbon on the plasma-prepared Ni/SiO₂ is more reactive toward CO₂. Therefore, the methane-derived carbon is more easily eliminated by CO₂ on the plasma-prepared Ni/SiO₂. The smaller Ni particle size and higher Ni particle dispersion led to a larger interface between Ni particles and the support on the plasma-prepared Ni/SiO₂. According to our calculation results, a larger interface between Ni particles and the oxide support will promote the CO₂ adsorption and activation on the catalyst, and thereby make the reaction between CO₂ and the methane-derived carbon easier.