Font Size: a A A

Study On NH3-SCR Reaction And Deactivation Mechanism On Molecular Sieve Supported Metal Oxide Catalysts

Posted on:2023-03-02Degree:MasterType:Thesis
Country:ChinaCandidate:T ZhangFull Text:PDF
GTID:2531306794493854Subject:Chemical engineering
Abstract/Summary:PDF Full Text Request
Nitrogen oxides(NOx)are receiving increasing attention as air pollutants.Selective catalytic reduction is considered to be the most effective method to control NOx.Cu/ZSM-5 catalysts are widely used in NOxremoval due to their good hydrothermal aging resistance and excellent NH3-selective catalytic reduction(NH3-SCR)activity.However,SO2and H2O in the flue gas environment will lead to deactivation of the catalyst.There are many experimental studies on the deactivation of Cu/ZSM-5 catalysts,but few theoretical studies.In this paper,based on the B3LYP method in density functional theory,the theoretical calculation of the NH3-SCR reaction and deactivation mechanism on Cu/ZSM-5 catalysts is carried out.The NH3-SCR reaction process on Cu/ZSM-5 includes the oxidation part where Cu+is oxidized to Cu2+and the reduction part where Cu2+is reduced to Cu+,which plays a catalytic role through the change of the valence state of Cu ions,and NH3reduces NO under the action of the catalyst is N2.For the Cu/ZSM-5 catalyst,two active sites,Z-[Cu]+and Z-[Cu2+(OH)]+,were constructed,and the NH3,NO,O2,and NO2on the two active sites were calculated respectively.The results show that the adsorption of O2is the most stable on the Z-[Cu]+site,which is the initial step of the standard NH3-SCR reaction.The oxidation of NH3-SCR partially follows the L-H reaction mechanism,where the adsorbed NH3reacts with the adsorbed[Cu-NO2]species on the Z-[Cu]+site to form the[-ONNH]intermediate,which is subsequently decomposed into N2forms Z-[Cu2+(OH)]+active sites,and the rate-controlling step of the oxidation part is the generation of nitrite species[Cu-NO2],and the energy barrier to be overcome is 105.08 k J/mol.The reduction part follows the ER reaction mechanism.Adsorbed NH3reacts with gaseous NO to generate[-NH2NO]intermediate.The rate-controlling step of the reduction part is the formation of[Cu-NH3]to[Cu-NH2],which needs to overcome the energy barrier of 91.97 k J/mol.Both the oxidation part and the reduction part participate in the reaction of NH3and NO,and both N2and H2O are generated.The NO2generated by the oxidation part directly participates in the SCR reaction,and the energy barrier of NO2participating in the reaction to form the intermediate[Cu-NO2]is lower,and a fast SCR reaction occurs at this time.The rate-determining step in the decomposition of by-product N2O is the formation of Z-[Cu O-N2O]+→Z-[Cu-O2-N2]+.In order to reduce the formation of N2O,we can find ways to reduce the reaction energy barrier of the rate-controlling step in the decomposition of N2O and accelerate the decomposition of N2O.The mechanism of deactivation revealed that SO2was more easily adsorbed at Z-[Cu]+(Lewis acid sites)sites,while H2O molecules were more easily adsorbed at Z-[Cu2+(OH)]+(Br(?)nsted acid sites).When SO2exists alone,Cu-SO4will be generated on the active site,and the formation of copper sulfate not only prevents NO from participating in the SCR reaction,but also reduces the number of active sites.In a humid environment,H2O,SO2,and NH3produce synergistic deactivation to generate NH4HSO3and NH4HSO4species.Comparing the adsorption energy and reaction energy barrier,in a humid environment,the catalyst surface is more likely to generate NH4HSO4species,and the generated NH4HSO4species cover the catalyst surface,preventing Further adsorption of other gases and subsequent SCR reactions proceed.By calculating the deactivation behavior on the surface of Mn/ZSM-5and Ce/ZSM-5 catalysts,Mn and Ce can act as sacrificial sites.When SO2and H2O are encountered,the formation of sulfate first occurs on Mn and Ce,our true active site Cu is protected,thereby improving the resistance to sulfur deactivation on the surface of the Cu/ZSM-5 catalyst.
Keywords/Search Tags:DFT, Cu/ZSM-5, NH3-SCR, reaction mechanism, SO2、H2O, deactivation mechanism
PDF Full Text Request
Related items