Study On The Promoting Effect Of Boron Doping Of Carbon Aerogels On Ammonia Selective Catalytic Reduction Of NO At Low Temperature

Ammonia selective catalytic reduction（NH₃-SCR）has become a widely used flue gas denitration technology at home and abroad due to its high NO_xremoval efficiency,easy control of reaction conditions,and low energy consumption.At present,the commercial SCR catalysts are mainly V₂O₅/Ti O₂and V₂O₅-WO₃/Ti O₂,but this catalytic system has disadvantages such as high operating temperature and the toxicity of vanadium species.There is an urgent need at home and abroad to develop a low-temperature denitration catalyst with high efficiency,low energy consumption,and environment-friendly to tackle these problems.Carbon-based catalysts have become potential low-temperature denitrification catalysts at a low price,large specific surface area,chemical stability,controllable pore structure,low-temperature activity,and environmental friendliness.Among them,carbon aerogel is considered a new type of porous carbon material.Due to its rich pore structure,it has good application value as a catalyst carrier in SCR denitration.In this paper,carbon aerogel is used as a catalyst,and the catalytic active sites of carbon aerogel are controlled by doping boron atoms（B）.The introduced boron atoms into carbon materials can increase the number of hole charge carriers and improve the adsorption capability for reactants.At the same time,boron doping can improve the oxidation resistance and the structural stability of carbon materials.Combining density functional theory（DFT）theoretical calculations,in-depth exploration of the catalytic mechanism of NH₃-SCR among BC3,BC2O,and BCO2 species provides a theoretical basis for the structural design,optimization,and application of high-performance boron-doped carbon aerogel SCR denitration catalysts.The specific research content and main conclusions are as follows:（1）A series of boron-doped carbon aerogels（B-CA-x）denitration catalysts were prepared by using resorcinol,catechol,and formaldehyde as the carbon precursors and phenylboronic acid as the boron source.With the help of commonly used characterization methods,the relationship between the surface structure,chemical properties,and denitrification performance of the boron-doped carbon aerogel catalyst was investigated.After the evaluation of catalytic activity,mesoporous B-CA-16 is the best catalyst.Kinetic studies show that under dry conditions,the activation energy（53.9 k J/mol）of carbon aerogel（CA）without doping boron is higher than that of B-CA-16（23.78 k J/mol）.The activation energy of B-CA-16 with 5 vol%water vapor added is 21.13 k J/mol,which explains that the catalytic efficiency is increased by 0.5%at 120°C with 5 vol%water.Through testing the stability and water resistance and sulfur resistance,it was found that the stability of the B-CA-16 catalyst needs to be improved,but the water resistance performance is excellent.XPS test reveals that its three boron species,BC3,BC2O,and BCO2,play a key role in catalyzing the reduction of NO by NH₃.From the NH₃-TPD-MS analysis,it is found that boron doping provides more acidic and active sites on the catalyst surface,which is more conducive to NH₃adsorption,while also effectively improving the catalyst’s NH₃reduction ability.（2）The BC3/BC2O/BCO2 graphene model was constructed by the Gauss 09 software,and then the B3LYP density functional theory in the software was used to adsorb O₂,NO,and NH₃on the model graphene with different active sites of BC3/BC2O/BCO2.The BC3 site is the most active for the dissociation of O₂molecules,and its adsorption energy is 13.362k J/mol.Combined with the Mulliken charge distribution,it can be seen that after O₂molecules dissociate and adsorb on the active site of BC3 and stable B-O and C-O bonds are formed.At the same time,the charge on the catalyst surface transfers to the direction of O atoms,which promotes the rapid generation of active oxygen species.BC2O adsorbs NO most actively,with an adsorption energy of-102.830 k J/mol.From the perspective of adsorption height,Mulliken charge q,and bond length,BC2O chemically adsorbs NO,and the nitrogen atom and the edge carbon adjacent to the boron form a stable N-C structure.NH₃tends to be adsorbed on the active site of BCO2 due to its electron-donating properties,its adsorption energy is-15.257 k J/mol.Compared with the B acid site,the L acid site has a stronger ability to capture NH₃.Mulliken charge distribution analysis shows that NH₃transfers electrons to the surface of the catalyst during the adsorption of BCO2 active sites,which is beneficial to the generation of NH₃active species.（3）According to the exhaust NO₂,N₂O detection,and in-situ infrared diffuse reflection of the B-CA-16 catalyst in different atmospheres,the reaction pathway of the catalyst in NH₃-SCR was explored.By comparing the conversion rate of B-CA-16 catalyst in different atmospheres,it is found that the introduction of O₂can increase the conversion rate of NO,while the introduction of NH₃will further increase the conversion rate of NO.In-depth analysis of the contents of NO₂and N₂O in the exhaust gas under different atmospheres shows that the formation of N₂O is directly caused by the formation and dissociation of NO dimers.In-situ DRIFTS analysis reveals that the SCR reaction of the boron-doped carbon aerogel（B-CA-16）catalyst mainly follows the L-H mechanism,where is NH₃（g）and NO（g）in the presence of oxygen are adsorbed on the adjacent active sites on the catalyst surface,NH₃exists in two forms（-NH₂and proton NH₄⁺）,and the resulting active species will combine with nitrogen oxides（NO（g）,NO（a）,NO₂（a））and N₂O₃（a）)react to form N₂and H₂O.（4）Another B-doped carbon aerogel was prepared by the sol-gel method using boric acid as a boron source,which is activated by CO₂and used as a denitration catalyst.The NO_xconversion rate of the B-CA-800_CO2catalyst is lower than B-doped carbon aerogel（B-CA）from phenylboronic acid at low temperature.However,the intrinsic catalytic activity of B-CA-800_CO2catalyst is higher at 40-100°C by TOF comparison.It is further proved that B-CA-800_CO2with a low boron content of 1.05 at%has the highest NO conversion efficiency,which is consistent with the calculation result of Ea(7.93 k J/mol^-1).The two intermediate NOx species identified by in-situ DRIFTS on both B-doped carbon aerogels are the dimers（NONO,N₂O₃and N₂O₄）.