Investigation On The Catalytic Removal Of NO By CO Over Activated Semi-Coke Based Catalysts

Coal is a significant fossil energy in China,and approximately 70%of the coal is used through combustion.However,along with the coal combustion,a large amount of NO_x would be produced.As reported,Nitric oxides?NO_x?contributes to the major air pollutants.They have threaten the ecological environment with urban smog,acid rain,ozone depletion and photochemical smog.Among the NO_x removal technologies,the selective catalytic reduction of NO_x by NH₃?NH₃-SCR?prove to be the most efficient process for the abatement of NO emitted from power plants.However,this process suffers from some disadvantages,e.g.,NH₃ leaks,which could induce the large amount of ammonia nitrogen in the desulfurization wastewater,and the leaked NH₃ could also evolves in the natural atmosphere,contributing to the major factor of the haze in China.Therefore,researchers focus on the development of the substituted process of NH₃-SCR recent years.Based on three-way catalysts technology?TWC?,the catalytic removal of NO by CO?CO-deNO?have demonstrated significant potential for the substitution,owing to the fact that CO is cost-effective,can be easily produced,and cannot generate solid carbon deposits.The coal-based porous adsorption material have been used for exhaust purification in power plants for many years.The removal of SO₂ by activated coke could obtain an efficiency of 90%or above.Our group has focused on the investigation of SO₂ removal by coal-based activated semi-coke,and obtained amount of theories for this process.In the investigating progress,it is found that the major of the pyrolysis gas from the coke production is CO.Considering that CO-deNO can substitute the NH₃-SCR,this research proposes that coal based coke is first activated by chemical reagents,and then loaded with some transition metal oxides.These semi-coke based materials would play as the catalysts for NO reduction by CO.This process conform to the goal of Green Economy and Circular Economy,because that all the materials can be obtained in power plants,except the little amount of metals.Hence,in this paper,we investigate the feasibility of the activated semi-coke based catalysts for NO removal by CO,the influence of flue gas content for NO+CO reaction over the catalysts,the optimizing mola ratio of the active metal and the mechanism of NO+CO reaction over activated semi-coke based catalysts,and the water or SO₂ resistance of the catalysts in the catalytic removal of NO.These investigation have provided sufficient support for the industrial application of activated semi-coke based catalysts.Semi-coke was activated by nitric acid,and then loaded with metal oxides by a hydrothermal method for the reduction of NO by CO.The catalysts were proved to be highly efficient with the optimal Fe?NO3?2 solution concentration of 20 wt%and the activities at low temperature were improved by the addition of Co,La and Ce.The doping of Co into the catalysts could obtain the highest efficiency and the temperature at 90%conversion decreased by 95 ?,compared with the sample loaded with only FeO_x.The characteristic analysis demonstrated that the larger specific surface,the high fraction of chemisorbed oxygen and the formation of CoFe₂O₄ were the key factor in this catalytic reaction.Besides,cobalt oxides could facilitate the adsorption and dissociation of NO.It is also found that the NO+CO reaction over activated semi-coke based catalysts could be influenced by many factors,therefore,the study about influence factors was carried out.The excellent efficiency NO reduction by CO could be achieved for oxygen-free conditions.Excess CO could enhance NO reduction,implying no competition for the active sites.However,NO+CO reaction is strongly inhibited by the presence of oxygen,due the oxidation of active metal sites.If oxygen is in excess,CO will be rapidly consumed,and then CO-carbon reaction will be predominant.Oxygen could promote the NO-carbon reaction due to the formation of surface oxygen compounds.But these cases are strongly undesired since there will be serious carbon support consumption.Therefore,NO reduction by CO is only effective when there is no or little oxygen presented in the flue gas.Oxygen preferentially oxidizes CO than carbon support.If minor oxygen exists in the flue gas,carbon consumption could be avoided by feeding excess CO.But the consumption and cost of the excess CO should be evaluated first.Another method to achieve NO+CO reaction in oxygen-rich conditions is to restrict the temperature in a narrow range around 200?-250?.Because in this temperature range,CO involved reactions are already active while reactions relevant to carbon support are not activated yet.But it should be noted that significant N₂O could be produced in a temperature range of 200-250 ?.In order to obtain a higher NO conversion and lower N₂O selectivity in the temperature range of 200-250 ?,this paper studied the optimization of the loaded metal oxides,and the NO+CO reaction mechanism over semi-coke based catalysts.The activity test demonstrated that when the molar ratio of Fe:Co was 4:1,the NO conversion could reach to above 95%at 200 ?.Moreover,N₂O selectivity was closed to 0 at this temperature.According to the characteristic analysis,it is established that there were Surface synergistic oxygen vacancies?SSOVs?at this molar ratio.In SSOVs,metals with a higher electronegativity played the redox role,while metals with a lower electronegativity served to assist for NO+CO reaction.Furthermore,in the Fe-?-Co structure,Co species were the catalytic sites for nitrate oxides,while Fe species were the catalytic sites for CO.The reaction pathway investigation implied that at low temperatures?<200 ??,coordinated carbonates reacted with coordinated nitro species,affording CO₂ and N₂O.However,at higher temperatures?>200 ??,adsorbed NO was transformed to coordinated nitrates,which would react with carbonates,affording N2 and CO₂.H2O and SO₂ were unavoidable components of the flue gas,the poisonous mechanism study of water and SO₂ was also performed in this paper.It is found that water could be competitively adsorbed on the catalyst surface with NO,which would decrease the NO conversion.S02 could also be adsorbed by the porous carrier,thereby transforming to sulfites,causing reversible deactivation.The interaction between SO₂ and water produced sulfates,causing irreversible deactivation.To improve the water and SO₂ resistance of the ASC-based catalysts,we doping other components onto the ASC.SiO₂ was loaded onto ASC by a wet-impregnation method to promote the water tolerance while Ce was doped into Fe-Co metal oxides to improve S02 resistance.SiO₂ on the surface of ASC repulse both water and NO molecules.However,this repulsive force is smaller for NO molecules.As for the SO₂ tolerance mechanism,the generation of Ce₂?SO₄?₃ could protect the active metal from the poison of SO₂ poisoning.In addition this sulfate could alleviate the interaction of surface sulfates and nitrates,which would promote the reaction of NO+CO.