Study On CO₂Reduction Driving By Low Temperature Plasma

With the consumption of resources and the adjustment of the energy structure in the world, the development of new resources and alternative resources become the hotspot of research. Since CO2is the main greenhouse gas and the most valuable C1resource, its utilization and reasonable development have great significance on environmental pollution and greenhouse effect control, as well as the comprehensive utilization of carbon resources. Due to the highly thermodynamic stability and kinetic inert of CO2molecules, conventional CO2conversion process is usually activated at high temperature or needs extra input of light, electricity or other energy, in companion with the deactivation of the catalyst. Therefore, most attentions have been paid for effective catalysts preparation and exploring new means to improve the activation of CO2. Plasma, especially the low temperature plasma, as a kind of unconventional methods in preparation of catalyst, transformation of greenhouse gases and other fields has been drew more and more attention. Further more, it has the good prospects for development.Catalyst preparation methods have great influence on catalyst activity, so it is very important to choose the befitting preparation method to improve the activity of catalyst. In this paper, the TiO2catalysts supported with activated alumina were prepared by an improved sol-gel and impregnation method, and a series of copper-doped catalysts were prepared by orthogonal experiment with plasma treatment. The doped catalysts under various conditions were characterized by means of X-ray diffraction and scanning electron microscopy; and the catalytic activity was tested by experiments of CO2reduction by synergistic effect of plasma and catalysts. The results showed that the optimum conditions of preparation of doped TiO2Catalysts were: Ti(OBu)4:ethanol:acetic acid:water=1:4:0.3:0.4(volume ratio); pH=3; Cu%=0.80%(wt%); calcination temperature was500℃; stirring time was1h. The conversion rate of CO2may be up to35.04%. The results showed that the Cu-TiO2/Al2O3catalysts effectively improved the catalytic activity of TiO2nanoparticles to improve the utilization of CO2.Cold plasma was also used for the preparation of Cu/MoO3-Al2O3catalysts. It takes only10minutes using the plasma assisted calcination-reduction process, so it is more time-saving than the traditional way in which it takes2h for calcination and2h for high temperature reduction process. The performances of catalysts prepared via direct plasma treatment and via plasma reduction after calculation were investigated in comparison. XRD results showed that catalysts via direct plasma treatment had smaller grain size and better dispersion. Plasma treatment process for catalyst preparation could enhance the dispersion of catalyst active component. BET characterization results showed that catalysts via direct plasma treatment had smaller pore diameter and larger specific surface area. The catalysts prepared by direct plasma process had larger specific surface area, which was good for enhancement of the catalytic activity in CO2hydrogenation reaction to some extent. CO2-TPD characterization results showed that CO2desorption peaks of catalysts via direct plasma treatment were larger than that of catalysts via plasma reduction, and CO2desorption peaks of catalysts with high CuO load were larger than that of catalysts with low CuO load. So presumably, catalysts via direct plasma treatment had more active sites.In addition, the plasma reaction system is a complex reaction system, and study the reaction mechanism is helpful to improve the understanding of the reaction system. In this paper, the mechanism of catalyst preparation with plasma and the reaction mechanism of plasma system were studied. The mechanism of CO2hydrogenation was investigated in the gas phase by means of Density Functional Theory (DFT) methods of quantum chemistry and the possible reaction pathways had been discussed. Firstly, the geometries of various stationary point such as reactants, transition states, intermediates and products had been determined with B3LYP/6-311++**level. Based on the optimized geometries, the frequencies of all species were calculated and the transition states were confirmed, then the single energies of stationary point along the pathway were also calculated at CCSD/6-311G++**. The results indicated that there were two pathways in the reaction, and the main reaction was CO2â†'COâ†'H2COâ†'CH3OH.