Enhanced Mechanism Of SO₂/NO Catalytic Oxidation Based On Coal-based Amorphous Carbon

Industrial emissions of SO₂ and NO_x are important incentives for environmental problems such as smog.Efficient desulfurization and denitrification have always been the focus of research in the field of energy and environment.Wet desulfurization and NH₃-SCR denitration based on vanadium,tungsten and titanium are mainstream desulfurization and denitration processes,but they consume a large amount of high-quality limestone and precious metals every year,and produce low-quality gypsum and waste catalysts that are difficult to dispose,facing the dilemma of sustainable development.The SO₂/NO catalytic oxidation technology based on coal based activated coke has the advantages of resource utilization of pollutants,harmless circulation of adsorbent and low cost,and is considered as one of the technical options to replace the current desulfurization and denitrification process.However,coal based activated coke has typical amorphous structure characteristics,and the multi-scale structure of base,microcrystalline and pore is interdependent.The catalytic oxidation mechanism is unclear,and the reaction activity is low and the stability is poor.In this paper,the density functional theory calculation is used to extract the amorphous structure characteristics of coal based activated coke,analyze the catalytic oxidation path of SO₂/NO surface,and explore the theoretical screening criteria of the catalytic oxidation active sites on the amorphous carbon surface;Based on the theoretical calculation results,the preparation strategies of high-performance amorphous carbon based catalysts were developed.Through theoretical calculation and experimental verification,it lays a foundation for the development of an efficient SO₂/NO removal method based on amorphous carbon.Firstly,an amorphous carbon cluster model containing typical defect structures such as unsaturated carbon atoms and basic functional groups is constructed,and the surface catalytic oxidation mechanism is analyzed through the systematic calculation and analysis of the elementary steps involved in SO₂ catalytic oxidation.The results show that during the catalytic oxidation of SO₂,O₂ can undergo non dissociative chemisorption on the surface of heteroatom doped carbon clusters and be activated into C-O-O structures with high oxidation activity.C-O-O structure is directly used as an oxidant to oxidize two SO₂ molecules to SO₃ at a lower energy barrier(41.1 and 62.4 k J mol^-1)to achieve continuous catalytic oxidation of SO₂.In order to guide the directional design of coal based amorphous carbon-based catalysts,and further aim at the size effect of the base surface,the high-throughput density functional theory is used to calculate,analyze the distribution rule of catalytic oxidation active sites and the judgment method,so as to establish the theoretical screening criteria of highly active amorphous carbon based surface.The results show that the optimal active sites of the finite size amorphous carbon clusters are not necessarily the carbon atoms near the heteroatoms,but also distributed at the edge far away from the heteroatoms.Through correlation analysis of activation O₂ characteristics and local electronic structure parameters at each point,it is found that the electrophilic Fukui function can accurately describe the off site distribution of active sites and can be used as a descriptor to determine the local active sites of catalytic oxidation.Based on the electrophilic Fukui function descriptor,the effects of oxygen functional groups and amorphous carbon edge structure on catalytic oxidation were studied.It was found that the catalytic oxidation activity of amorphous carbon clusters depends on both functional groups and edge types.Compared with other oxygen doped carbon clusters,the carbon clusters with cyclic ether embedded at the edge of K-region have the best catalytic oxidation activity,and the free energy barrier of critical step is only 89.3 k J mol^-1.Under the guidance of theoretical calculation results,cyclic ether functional groups were directionally grafted into commercial activated coke through heat induced esterification and dehydration condensation of primary oxygen-containing functional groups,which improved its desulfurization performance by 2 times.Based on the above research,this paper analyzes the enhancement mechanism of SO₂ catalytic oxidation by water in flue gas using transition state search combined with atom medium molecule（AIM）theory.It was found that the H transfer mediated by adsorbed water significantly regulated the activation of O ₂,which reduced the energy barrier of H extraction from chemisorbed O₂ to hydroperoxide by 30 k J mol^-1,and increased the reaction rate constant by nearly two orders of magnitude.Inspired by this phenomenon,we further investigated the regulatory effect of adsorbed water mediated H transfer on SO₂ oxidation path.It was found that water mediated H transfer changed the types of oxidation products,thereby weakening the interaction between oxidation products and active sites from-132.6 k J mol^-1 to-29.2 k J mol^-1,avoiding the poisoning of oxidation products on active sites and promoting the continuous reaction.The above mechanisms and screening criteria of active sites were extended to NO catalytic oxidation.Using Fukui function descriptor and reaction path calculation,the strengthening mechanism and method of amorphous carbon on NO catalytic oxidation were studied.It is found that the electrophilic Fukui function of nitrogen doped system has a good correlation with the free energy of chemical adsorption of reactants,which can be used as a descriptor to accurately determine the active site of NO catalytic oxidation.Using this descriptor,it was found that graphite nitrogen,compared with pyridine nitrogen and pyrrole nitrogen,had a stronger role in promoting the activation process of O₂,reducing the activation energy barrier of O₂ from 156.2 k J mol^-1 to 86.8 k J mol^-1.Based on the theoretical calculation results,nitrogen doped graded porous activated coke was prepared based on one-step catalytic activation,showing excellent NO catalytic oxidation performance:the NO removal efficiency was maintained at 70%within 60 minutes,about twice that of commercial microporous activated coke.