Mechanism Study On Anti-chlorine Poisoning Catalyst With Low-temperature Activity For Chlorinated Volatile Organic Compounds Catalytic Decomposition

Air pollution is one of the major environmental problems in China,which seriously threatens to the health of the Chinese people.In order to improve air quality,national and local governments have promulgated a series of policies,regulations and emission standards in recent years,and comprehensively tightened the emission limits of air pollutants for key industries and key sources of pollution.With the implementation of ultra-low emission technologies in thermal power,steel and other industries,the emission of particulates,nitrogen oxides,sulfur oxides and other pollutants from coal combustion has been greatly reduced.The characteristics of air pollution in China have changed from coal-smoke air pollution to compound air pollution.The atmosphere oxidization increases year by year.Photochemical smog pollution has appeared in many cities.The formation mechanism of haze has also shown new features.Chlorinated volatile organic compounds(CVOCs)as an important subspecies of the volatile organic compounds,involved in many industries,is an important precursor of fog and photochemical smog,and has strong carcinogenic,teratogenic and mutagenic effects on human body.Therefore,CVOCs has been listed as the priority of environmental pollution control by governments and regions of many countries.It is urgent to control the emission of CVOCs to improve the atmospheric environment.Catalytic removal is one of the main control technologies of CVOCs.Development of Catalyst is the core of this technology.At present,CVOCs catalytic removal catalyst is mainly faced with three major technical challenges in application,i)poor activity at low temperature and high operation cost in application,ii)various by-products and high tail gas toxicity,iii)Cl poisoning induced deactivation and low stability.To solve these challenges,in this paper,the reaction and deep oxidation mechanism of CVOCs catalytic degradation were studied systematically,the relationship between the structure and catalytic performance was investigated,the mechanism of catalyst inactivation and chlorosis resistance was revealed,the catalyst with multi-active centra was constructed by acid modification,and at last,the preparation of molding catalyst and pilot test were completed.The main contents are as follows.1.The mechanism of catalytic decomposition and deep oxidation of CVOCs were studied.A series of titanium base catalysts with different copper-cerium doping ratios were prepared.The elemental valence and oxygen species distribution on catalyst surface were analyzed.The effects of different Cu:Ce doping ratio on the surface acidity and oxidation of catalyst were studied.Combined with catalyst activity characterization results,it is demonstrated that,there are two key proeperties influenced the catalytic activity of catalyst.On the one hand,it is necessary to enrich the strong acid sites on the surface of the catalyst,to promote the adsorption of CVOCs molecules on the catalyst,and realize the activation of C-Cl bond.On the other hand,it is necessary to enhance the oxidation performance of the catalyst,promote the nucleophilic attack of oxygen species on C atoms,and promote the broken of C-Cl bond.With the in situ infrared experiment,the change of intermediate species on different catalyst surface with temperature was analyzed in detail,and the adsorption and activation mechanism of CVOCs on catalyst surface was further proved.The reaction mechanism of surface-active oxygen species promoted deep oxidation of reaction intermediate species(such as bidentate formate and bridged carbonate)was revealed.Through mentioned reaction,the catalysts products selectivity of CO₂ can be improved.2.Effect of surface-active component structure on reaction performance of catalysts.The supporting structures of V₂O₅ and Ru O₂ on the surface of anatase Ti O₂and the effect of the structure on products selectivity were studied.The results demonstrated that the high agglomeration of Ru O₂ on the surface of the Ti O₂ carrier leads to the direct decomposition of CVOCs molecules on the surface of Ti O₂,which is the main reason for the formation of organic chlorine-containing by-products.Rutile Sn_0.2Ti_0.8O₂ mixed metal oxide was prepared.The DFT theoretical calculation results proved that rutile Sn_0.2Ti_0.8O₂ shows stronger adsorption and dissociation activity to CVOCs molecules than anatase Ti O₂.The Ru O₂/Sn_0.2Ti_0.8O₂ catalyst was prepared and analyzed with XRD and TEM/STEM to illustrated the structure of this catalyst.The results proved that the Ru O₂ is highly dispersed on the surface of Sn_0.2Ti_0.8O₂ carrier.The crystal structure analysis results show that the lattice mismatch between Ru O₂ and the carrier oxide is the key factors to determined the agglomeration growth or epitaxial growth behavior of Ru O₂ on the surface of the carrier.The products distribution and stability of reactivity of Sn_0.2Ti_0.8O₂ and Ru O₂/Sn_0.2Ti_0.8O₂ catalysts were compared,combined with the results of DFT calculation,the deactivation mechanism of Sn_0.2Ti_0.8O₂ and the anti-chlorine poisoning mechanism of Ru O₂/Sn_0.2Ti_0.8O₂ were revealed.It is demonsrated that the competitive adsorption of CH₃Cl and HCl to DCM caused the fast deactivation of Sn_0.2Ti_0.8O₂,the deposition of Cl at the active sites caused the long-term deactivation of Sn_0.2Ti_0.8O₂.Ru O₂ supported over the surface of the carrier can promote the oxidation and desorption of Cl from the active sites through Deacon reaction,which can significantly improve the operational stability of the Ru O₂/Sn_0.2Ti_0.8O₂ catalyst.3.Surface acid modification enhances low temperature catalytic decomposition of CVOCs.Surface acidity property modification of Ru O₂/Sn_0.2Ti_0.8O₂ catalyst was carried out by loading W over the catalyst surface.A series of catalysts with different loading amount of W were prepared and characterized with XRD and Raman to reveal the influences of loading amount of W to the structure of WO_x.The results demonstrated that with the increasing of loading amount of W,the strcture of WO_x changed from monomer structure to polymer structure with the bond structure of(O=)₂W(-O-M)₂and O=W(-O-M)₄ changed into W-O-W and WO₃.The results of Py-IR demonstrated that the W=O and W⁶⁺over the surface of the catalysts influenced the acidity property of the catalysts.At last,the 10W-RST catalyst was proved to be with the highest catalytic activity at low temperature.4.At last,we prepared cordierite honeycomb ceramic catalyst with 10W-RST loading.The catalytic removal of dichloromethane,trichloroethylene and chlorobenzene was studied.The results demonstrated that the 10W-RST catalyst developed in this study has good catalytic removal activity at low temperature and high products selectivity to CO₂.Therefore,this catalyst has certain industrial application prospects.