Study On Carbon Dioxide Reduction To Methanol By Non-thermal Plasma And Its Mechanism

About 36 billion tons of CO₂ are emitted each year globally and are the main cause of global warming.Reducing it is an important strategy for converting waste into wealth and mitigating its severe impact on the environment.Converting CO₂into liquid fuels is seen as a potential way to mitigate both global warming and the energy crisis.Methanol（CH₃OH）is a vital primary feedstock in the chemical industry based on George Olah’s"methanol economy".Therefore,CO₂hydrogenation to methanol is an important way to achieve CO₂economy successfully.Non-thermal plasma（NTP）is a promising alternative technology for CO₂hydrogenation.It has an ultra-high electron temperature（1-10 e V）,but a low gas temperature（near room temperature）,which guarantees the activation of CO₂under ambient temperature and pressure.However,it is worth noting that the selectivity and energy efficiency of target products are not ideal if the low temperature plasma technology is used only,and the addition of appropriate catalyst is conducive to improving the selectivity of target products.Therefore,based on the above background,this study studies the reduction of CO₂into methanol by low temperature plasma,explores the reduction performance of CO₂under the action of single plasma,and reveals the contribution of electron-induced chemistry to CO₂conversion.In addition,a suitable catalyst was added under the action of plasma to improve the selectivity of the target product（methanol）.Meanwhile,the synergistic effect of plasma and catalyst was revealed,and the reaction mechanism of catalyst surface was proposed,which provided more perfect theoretical support for CO₂methanol production.The main research contents and conclusions of this study are as follows:（1）Characterization of CO₂ reduction by single plasmaThe discharge characteristics of DBD at different background temperatures were studied.When the background temperature was lower than 573 K,the discharge mode of DBD was typical filamentous micro-discharge mode.At normal temperature and pressure,the temperature of DBD reactor rose to about 90℃,and the main source of temperature was DBD discharge.The influence of energy density on CO₂conversion characteristics was analyzed by regulating Q_flowor P_dis.The study showed that there was a linear relationship between energy density and CO₂conversion.When the energy density is the same,the changes of the total gas flow(Q_flow)and the discharge power(P_dis)do not affect the conversion of reactants,but have a similar effect on product selectivity,and Q_flowis more sensitive to change the energy density.In addition,the effect on CO₂conversion characteristics at different temperatures is due to the change of average electron energy caused by the change of E/N value.The conversion of CO₂increases with the increase of E/N,and the contribution of electron-induced chemistry to CO₂conversion is much higher than that of thermochemistry.Unfortunately,as far as the target product（methanol）is concerned,the adjustment of discharge parameters and reaction conditions under the action of plasma alone cannot achieve considerable results.Therefore,the addition of catalyst is a key measure to improve the selectivity of methanol.（2）Study on plasma Co-catalysis of CO₂to methanol and its mechanismUsing DBD plasma and catalyst,plasma cocatalyst assisted hydrogenation of CO₂to methanol was studied.Under the same conditions,compared with the single plasma reaction,the addition of catalyst can improve the conversion rate of CO₂.Compared with the optimal catalyst Mn O_x/Zr O₂,the methanol selectivity of 8.5%is more than three times that of other catalysts.Therefore,the study of co-catalytic hydrogenation of CO₂to methanol by plasma and Mn O_x/Zr O₂catalyst was explored.When the H₂/CO₂ratio is 3,the STY value of CH₃OH is 4.5mg Me OH/(g_cat·h)and the synergistic factor is 87,which is much higher than the reported studies.The selectivity of CH₃OH over Mn O_x/Zr O₂catalyst,plasma and Zr O₂plasma is almost 0.The plasma co-catalytic Mn O_x/Zr O₂process showed good stability over 5 cycles and36 h.Mn O_x/Zr O₂indicated that Mn was uniformly distributed on Zr O₂support,and Mn²⁺was more selective in methanol than Mn³⁺and Mn⁴⁺.The reaction mechanism is studied by in-situ DRIFTS and gas FTIR spectroscopy.The results show that the key intermediates change from（bi）carbonates to HCOO and CH₃O under the action of Mn O_x/Zr O₂catalyst,and the formation route of CH₃OH is determined.Under the action of Mn O_x,the excited states of CO₂^*and CO₂⁺produced by plasma can be effectively hydrogenated into HCOO,and then further hydrogenated into CH₃O to produce CH₃OH.