Theoretical Research On Catalytic Oxidation Of Methanol At Normal Temperature

Volatile Organic Compounds（VOCs）are a major kind of air pollutants,and their harm to the environment and human health is increasingly valued by the government.Our country has issued a series of laws,regulations and industry standards to control VOCs emission.The catalytic combustion is currently the most widely used VOCs treatment technology in industry,but the catalytic combustion technology also has the disadvantages of high energy consumption and safety risks in processing high-concentration VOCs exhaust gas.Normal Temperature Catalytic Oxidation（NTCO）technology is an emerging VOCs treatment technology,which is characterized by its ability to efficiently degradeVOCs at normal temperature without the external energy input.The key of NTCO technology is the catalyst with high activity and high stability.At present,some literatures have reported the development of catalysts for catalytic oxidation of VOCs at normal temperature,but there is still a lack of investigation on its mechanism,and this article focuses on theoretical research.Based on the previous experimental research,the density functional theory was used to explore the mechanism of the catalyst in the NTCO process.The main contents of this article are as follows:（1）The establishment of Pt/Fe₂O₃ catalyst model.A reasonable model has a great influence on the accuracy of the theoretical calculation.According to the XRD and HRTEM characterization of the previous experimental exploration,this article used the Fe₂O₃（012）surface as the exposed crystal surface of the carrier,and 2×2 supercell model was established with a series of Pt_n（n=2～5）cluster on the model.By calculating the average binding energy of Pt_n cluster on the surface of the carrier,we learned the stability of the catalyst model.The average binding energies of Pt₂,Pt₃,Pt₄,Pt₅ and other clusters are 1.96 eV,0.81 eV,0.90 eV and 1.11 eV,respectively.In this paper,Pt₃/Fe₂O₃ with the lowest average binding energy was selected to simulate the catalyst surface.（2）The formation mechanism of hydroxyl radicals.Hydroxyl radical is an important intermediate in the catalytic process at normal temperature,which affects the degradation efficiency of VOCs directly.Therefore,this paper explored the formation mechanism of hydroxyl radical under the action of catalyst.According to the theoretical calculation results,ozone and water molecules have two paths for forming surface hydroxyl groups on the Pt cluster,which are marked as O_top path and O_bri path respectively.The rate control step of the O_top path is the oxygen desorption step,and this process needs to absorb 0.47 eV of energy.While the rate control step of O_bri path is the process in which water molecules form surface hydroxyl groups with oxygen atoms on the surface,and the reaction energy barrier of this process is 1.36 eV.So,the O_top path is more likely to occur in reality.In addition,the ozone in the gas phase will react with the surface hydroxyl groups to form ozone acid radicals,which are storage materials of hydroxyl radicals and can be further decomposed into gas phase hydroxyl radicals.（3）The degradation mechanism of methanol.Based on the formation mechanism of hydroxyl radicals,we further explored the oxidation mechanism of hydroxyl radicals and ozone to methanol.Because of the O-H bond of methanol molecule is relatively weak compared with C-H bond.Therefore,the calculation of methanol degradation in this paper started from the breakage of O-H bonds.Hydroxyl radicals with strong oxidability could break the bonds,seize hydrogen atoms and form water molecules.Then the hydroxyl radical gradually detaches the hydrogen atoms in the methyl group,degrades CH₃O*to CO*,and finally ozone reacted with CO*and forms CO₂.