Preparation And Hydrophobic Modification Of Mn-based Catalytic Materials And Its Application In Selective Catalytic Reduction Of NO_x At Low Temperature

The catalyst is a special functional material whose phase structure and surface properties determine the degree and stability of the catalytic activity for the reaction.If the research results of hydrophobic and superhydrophobic materials can be combined,conventional catalytic materials can be given hydrophobic or superhydrophobic properties,which inhibits the adsorption of water on the catalyst surface,reduces the water pressure near the active center,promotes the desorption of water molecules and keeps water away from the active center of catalyst.It can be predicted that the hydrophobic or superhydrophobic catalyst will show more and superior performance than the conventional catalyst in the reaction containing water in the reactant,product or reaction medium.Therefore,the research and development of hydrophobic or superhydrophobic catalytic materials have vital theoretical significance and application value.NO_x is one of the main pollutants in the atmosphere,and is the main culprit of ecological pollution such as acid rain,haze,and photochemical pollution.Its main sources are from fixed source devices such as coal-fired power generation and boiler combustion.Ammonia Selective Catalytic Reduction（NH₃-SCR）of NO_x at low-temperature has significant advantages such as high denitration efficiency and low operating costs,and is the most economical and suitable choice to solve NO_x pollution.Among them,the Mn-based catalyst is considered to be the most promising low-temperature NH₃-SCR catalyst,which is expected to achieve large-scale industrial applications.The noticeable problem now facing is that water has a strong inhibitory effect on the low-temperature denitration activity of manganese-based catalysts,resulting in poor resistance of manganese-based catalysts to H₂O,decreased activity and prone to continuous activity decline.Therefore,the research and development of a low-temperature denitrification catalyst with its own hydrophobicity and good water resistance is expected to solve the industrial demand for SCR denitrification catalysts with low-temperature,high-efficiency and stable performance.The successful application of this technology contains huge economic and environmental benefits.This thesis mainly includes the following aspects:1.A series of Mn O_xpure manganese-based catalysts were prepared by facile thermal decomposition and redox methods.Through the test of NO_x removal activity under low temperature conditions,the manganese-based denitrification catalyst Mn（MST）prepared by starch reduction with potassium permanganate has the best performance.Through BET,XRD,XPS,IR and SEM tests,it is found that the pure manganese-based catalyst Mn（MST）has an amorphous structure,with a specific surface area of 175.4 m²/g,surface manganese mainly exists in the form of Mn²⁺and Mn³⁺,and there is a high proportion of surface chemisorbed oxygen（O_β）.The NO_x removal efficiency is 100%at 80～200℃.2.A titanium-aluminum composite catalyst carrier was prepared by doping titaniainto alumina.Then,a series of Mn O_x supported manganese-based catalysts were prepared by the equal impregnation method.Through the test of NO_x removal activity under low temperature conditions,a manganese-based denitration catalyst Mn10%（AM/TA）with 10%manganese content prepared by manganese acetate as the precursor,calcined at 500°C,was found to have the best performance.Through BET,XRD,XPS,IR,SEM tests,it is found that the active components of the supported manganese-based catalyst Mn10%（AM/TA）are highly dispersed with a specific surface area of 193.2 m²/g,surface manganese mainly exists in the form of Mn²⁺and Mn³⁺,and there is a high proportion of surface chemisorbed oxygen（O_β）.The NO_xremoval efficiency is 100%at 140～200℃.3.Based on the self-made pure manganese-based catalyst Mn（MST）and the supported manganese-based catalyst Mn10%（AM/TA）,a series of hydrophobic PTFE-modified manganese-based low-temperature denitration catalysts were prepared through introducing different proportions of hydrophobic component PTFE into the catalyst matrix by means of dry and wet mixing methods.It is shown that the pure manganese-based catalyst Mn（WM）-20%PTFE prepared by wet mixing method is hydrophobic with a water contact angle of 139°.Through BET,XRD,XPS,IR,SEM tests,it is found that the specific surface area of Mn（WM）-20%PTFE hydrophobic manganese-based catalyst is 167.7 m²/g.PTFE modifier has a chemical combination with the surface of Mn-based catalyst,which makes PTFE re-dispersed and evenly distributed on the surface,thus producing excellent hydrophobic performance.The maximum activity loss of Mn（WM）-20%PTFE was 4.3%in the water content range of 0～20%.The NO_x removal efficiency is 100%at 160～200℃.4.Based on the self-made pure manganese-based catalyst Mn（MST）and the supported manganese-based catalyst Mn10%（AM/TA）,a series of hydrophobic aliphatic amine-modified manganese-based low-temperature denitration catalysts were prepared by introducing different types of hydrophobic aliphatic amine groups into the catalyst matrix by solvothermal method.On this basis,a series of hydrophobic manganese-based low-temperature denitrification catalysts were prepared by direct solvothermal synthesis using manganese acetate as the precursor.It is shown that the supported manganese-based catalyst Mn/TA（STR）-DA prepared by solvothermal modification is superhydrophobic with a water contact angle of 154°.Through BET,XRD,XPS,IR,SEM tests,it is found that the specific surface area of Mn/TA（STR）-DA superhydrophobic manganese-based catalyst is 187.3 m²/g.The maximum activity loss of Mn/TA（STR）-DA was 4.8%in the water content range of0～20%.The NO_x removal efficiency is above 90%at 120～160℃.5.The reaction mechanism and kinetics of four kinds of manganese-based catalysts were studied in a fixed-bed reactor.The results showed that:（1）the order of NO reaction before and after hydrophobic modification was 0.64 and 0.70,respectively,indicating that the reaction followed both E-R and L-H mechanism,but the L-H mechanism was the main one.The activation energy of the pure manganese catalyst before and after hydrophobic modification is 17.0 k J/mol and 26.6 k J/mol respectively,which is obviously lower than the activation energy reported in literature（38 k J/mol）.It proves that the prepared hydrophobic catalyst not only has excellent water resistance,but also has relatively higher SCR catalytic activity;（2）The order of NO reaction before and after hydrophobic modification is 0.81 and0.89,respectively,indicating that the reaction follows both E-R mechanism and L-H mechanism,but the E-R mechanism is dominant.The activation energies of pure manganese catalysts before and after hydrophobic modification are 33.5 k J/mol and 37.6 k J/mol,respectively,which are slightly lower than those reported in literature（38 k J/mol）,indicating that the prepared superhydrophobic catalysts not only have excellent water resistance,but also have high SCR catalytic activity.The method and kinetic parameters obtained in this paper can be used as reference for subsequent research and it accumulate data for industrial application.