Mechanistic Study On Active Oxygen Species In The Catalytic Oxidation Of Carbon Monoxide And Formaldehyde

Carbon monoxide(CO)and formaldehyde(HCHO)are two typical air pollutants,whose emission reduction is of great significance to solve the problems of environmental pollution.Both CO and HCHO can be controlled economically and efficiently through catalytic oxidation.Revealing the heterogeneous oxidation mechanism of CO and HCHO on the catalyst surface has important guiding significance for the development of high-efficiency catalysts.However,the research on the reaction mechanism of CO and HCHO is still in the exploratory stage,especially that the understanding of the evolution law of active oxygen species(AOSs)in the oxidation reaction cycle is still insufficient.Therefore,with the help of quantum chemistry tools based on density functional theory(DFT),non-noble metal catalysts with good application prospects and high economy were selected.The role of AOSs in the catalytic oxidation cycle of CO and HCHO over the catalysts were deeply studied and analyzed at the atomic level in this thesis,which coule provide important theoretical guiding significance and practical value for the development of high-efficiency applied catalysts for the removal of CO and HCHO.This subject mainly forcuses on the following two aspects:Firstly,although the transition metal modified selective catalytic reduction(SCR)denitration catalysts show excellent performance in the synergistic removal of multiple pollutants,the mechanism of CO oxidation on their surfaces is still not clear.Hence,two vanadium titanium based selective catalytic reduction(SCR)denitration catalyst models were constructed to reveal the possible pathway of CO oxidation and the evolution mechanism of AOSs in the oxidation process,i.e.,V2O5/TiO2(001)and V2O5-WO3/TiO2(001).It indicates that V=O is the active site.The whole oxidation cycle of CO on two SCR catalysts is mainly composed of two oxidation stages.Firstly,CO reacts with the terminal lattice oxygen(the O atom of V=O site)on the catalyst surface,which belongs to the Mars-van Krevelen mechanism and is the rate determining step(RDS)of the whole CO oxidation process.Meanwhile,an oxygen vacancy is formed on the the catalyst surface.Subsequently,the atomic oxygen(Oe)formed by the adsorbed O2 at the Ov site is consumed by the oxidation reaction of another CO,following the Eley-Rideal mechanism.The energy barriers of the CO oxidation on V2O5/TiO2 surface are lower than those on V2O5-WO3/TiO2,indicating that WO3 doping inhibits CO oxidation.Based on the previous research,we further carried out the research on the oxidation mechanism of V2O5/TiO2 modified by transition metals(Mo,Fe and Co).It is found that the CO oxidation mechanism on the surface of Mo-V2O5/TiO2 is similar to that of V2O5-WO3/TiO2.which is different from that of Fe(or Co)modified SCR catalyst.The active site on the surface of Fe or(Co)modified SCR catalyst is the structure of Fe-O(or Co-O).The lattice oxygen(oxygen of Fe-O or Co-O)and the atomic oxygen formed at the active site play a key role in the whole CO catalytic oxidation cycle.Compared with W and Mo doped catalysts,CO and Fe doping can significantly reduce the energy barrier of RDS in the whole CO oxidation process,indicating that Co and Fe can improve the CO oxidation activity of V2O5/TiO2 catalyst.The main reasons is that Co and Fe doping is conducive to enhance the surface charge transfer and provide AOSs with high activity on the the modified SCR catalyst surfaces.Secondly,non-metallic modification is also an efficient regulation strategy of catalyst for CO oxidation.In this text,the earth-abundant and low-cost iron pyrite minerals(FeS2)was selected as catalyst support and modified with non-metallic nitrogen(N)atoms.All possible reaction pathways of CO oxidation and the evolution mechanism of AOSs on FeS2 and N-doped FeS2(N-FeS2)were considered.The results show that two kinds of active oxygen species are observed on the catalyst surfaces,including the adsorbed O2 and atomic O.The CO first prefers to react with the atomic O formed by O2 dissociation(FeS2)and the adsorbed O2(N-FeS).Comparing to FeS2,the energy barriers of CO oxidation over N-FeS2 are lower,which indicates that N doping can improve the activity for CO oxidation.The analyses of electronic structure evidence that N doping can modify the electronic density of the adjacent Fe atoms and thus form favorable electronic configurations for O2 adsorption,which facilitates O2 activation and the succeeding CO oxidation.The whole favorable CO oxidation cycle for N-FeS2 consists of three stages:O2 adsorption→first CO oxidation→second CO oxidation.Then.based on the excellent low-temperature performance of Mn-based catalyst,DFT calculations were performed to reveal the roles of the active oxygen species in the CO oxidation and the interactions between the gaseous molecules(CO,O2,CO2.etc.)and the active sites over Mn/TiO2 catalyst.The results show that the Ti5a-O2b-Mn5c(TOM)coordination structure is the active center for the chemisorption of CO and O2.The dominant cycles were proposed over the TOM structure,which consist of three reaction stages,i.e.,first CO oxidation→surface re-oxidation→second CO oxidation.Two kinds of active oxygen species,i.e.,the lattice oxygen of TOM and the two atomic oxygen formed during the surface re-oxidation process.play critical roles in the dominant CO oxidation cycles.The energy barriers of the dominant oxidation cycles are lower than 50 kJ/mol,which explain well the high lowtemperature activity of Mn/TiO2 catalyst for CO abatement.Finally,based on the previous understanding of the surface properties of Mn/TiO2 catalyst,we further explored the effect of Ov in the process of HCHO catalytic oxidation on Mn/TiO2 catalyst.The results show that the presence of Ov can significantly reduce the energy barrier of HCHO oxidation on the surface of Mn/TiO2(the highest energy barriers of HCHO in the surface oxidation reaction of the perfect and defective Mn/TiO2 are 247.96 and 62.54 kJ/mol,respectively).The main reason is that the existence of Ov can change the electron density distribution of the active center and thus form favorable electronic configurations for O2 adsorption and activation(dissociated into atomic O),which facilitates C-H bond rupture and the subsequent complete oxidation of HCHO.