Study On The Synthesis Of Porous Carbon Materials And Their Adsorption Performance For NO And CO₂

At present,air pollution(especially NO_x and CO₂ emission)is becoming more and more serious,which is paid attention by the government.Porous carbon materials are widely used in the treatment of air pollutants because of their stable properties,low cost,large specific surface area,adjustable pore size and functional groups.However,owing to the disordered structure and different pore sizes of traditional porous carbon materials,the single variable and mechanism of gases adsorption are still equivocal.Ordered mesoporous carbon can improve the shortcomings of traditional carbon materials due to its ordered structure and adjustable pore size.In this research,ordered mesoporous carbon CMK-3 and C-FDU-15 and their modified materials were prepared,and their adsorption properties for(NO+O₂)were tested.At the same time,the determining factors and mechanism of their(NO+O₂)adsorption were discussed.Moreover,biochar has been widely concerned for its low cost,simple process and excellent performance in CO₂ adsorption.Therefore,in this research,oxygen rich functional group corn activated carbon was prepared.and the influence of activation temperature and KOH dosage on CO₂ adsorption was studied.In addition,the mechanism of CO₂ adsorption on biochar was discussed.(1)CMK-3 and C-FDU-15 samples were synthesized using hard-templating and evaporation-induced self-assembly(EISA)methods,respectively.NO adsorption was investigated on CMK-3,C-FDU-15 as well as commercial activated carbon(AC).The results show that the CMK-3 and C-FDU-15 possessed ordered and uniform structures.The co-adsorption capacity of NO and O₂ by CMK-3 and C-FDU-15 was greater than that by AC significantly.There were two main adsorption species on CMK-3 and C-FDU-15:nitrite(NO₂^-)and nitrate(NO₃^-).NO₂^-is converted to NO₃^-easily.However,the adsorption species were more complex on AC,with NO₃^-being the main species.Moreover,CMK-3 and C-FDU-15 exhibit excellent regeneration efficiency.(2)The alkali metal(M=Na,K,Rb,and Cs)-modified C-FDU-15(M-C-FDU-15(x);x was the M/C-FDU-15 molar ratio,and equal to 0.01-0.03)samples were prepared through an in-situ process.The sequence of(NO+O₂)adsorption performance was Na-C-FDU-15(0.01)>K-C-FDU-15(0.01)>C-FDU-15>Rb-C-FDU-15(0.01)>Cs-C-FDU-15(0.01).The alkali metal was uniformly distributed in C-FDU-15 and its doping enhanced the amount of the basic sites in the sample.Further study on the optimum loading amount of alkali metal Na showed that the optimal Na/C-FDU-15 molar ratio was 0.02.Moreover,the results showed(NO+O₂)were chemically adsorbed through the adsorption kinetics.In three stages of film diffusion,intraparticle diffusion,and final equilibrium,which controlled the adsorption rate.In situ FTIR showed that the adsorption of(NO+O₂)was dominated by physically adsorbed NO₂ and chemically adsorbed NO₂^-and NO₃^-species.There were three key factors of enhancing the(NO+O₂)adsorption capacity of C-FDU-15 due to alkali metal doping:the first factor was the increasing of surface area and pore volume of the sample,the second one was the enhancement in amount of the active sites in the sample,and the third one was the smaller alkali metal ionic radius in the sample.(3)To improve the removal capacity of(NO+O₂)effectively,the alkaline earth metal-doped C-FDU-15(A-C-FDU-15(0.001)(A=Mg,Ca,Sr and Ba))and Mg-C-FDU-15(x)(x=0.001-0.003)samples were prepared.The results showed that Mg-C-FDU-15(0.001)sample had the highest(NO+O₂)adsorption capacity(106.2mg/g)among all A-C-FDU-15(0.001)samples,and x=0.002 was the best loading amount in Mg-C-FDU-15(x)samples.The species of alkaline earth metals and basic sites were important factors determining the adsorption of(NO+O₂)on the A-C-FDU-15(x)samples.The(NO+O₂)adsorption on the samples was mainly chemical adsorption through the adsorption kinetics.There were two main pathways of(NO+O₂)adsorption:one way was first the conversion of NO and O₂ to NO₂ and then part of NO₂ was converted to NO₂^-and NO₃^-;and the other way was the direct oxidation of NO to NO₂^-and NO₃^-.(4)The effects of(NO+O₂)adsorption on K-CMK-3(x)and Ba-CMK-3(x)samples were studied at low temperature.The(NO+O₂)adsorption performance of the samples was improved significantly,and K-CMK-3(15)(143.8 mg/g)and Ba-CMK-3(10)(108.1 mg/g)samples had the excellent(NO+O₂)adsorption performance,respectively.The results showed that the structure and the number of basic sites of the sample were the main factors affecting the(NO+O₂)adsorption.The adsorption of(NO+O₂)on K-CMK-3(x)and Ba-CMK-3(x)samples was mainly chemical adsorption through adsorption kinetics.The adsorption speed limit steps in K-CMK-3(x)samples could be divided into film diffusion,intraparticle diffusion,and final equilibrium,while the adsorption speed limit step in Ba-CMK-3(x)sample was the film diffusion.The adsorption mechanism of K-CMK-3(x)and Ba-CMK-3(x)sample is similar.There were two main adsorption pathways during the process of(NO+O₂)adsorption:one pathway was the conversion of NO and O₂ to a large number of NO₂,in which part of NO₂ was weakly adsorbed on the surface hydroxyl groups,and the other part of NO₂ was disproportionated to NO,NO₂^-,and NO₃^-;and the other pathway was first the direct oxidization of NO to NO₂^-by the oxygen functional groups in carbon and then some of the formed NO₂^-species were converted to NO₃^-.In addition,the K-CMK-3(x)and Ba-CMK-3(x)samples exhibit excellent regeneration efficiency(BE%).However,Both BE%of K-CMK-3(x)and Ba-CMK-3(x)samples decreased with the increase of K and Ba loading.(5)Biochar(CKC-X-Y,CKC is the biochar precursor;X is the activation temperature;Y is the KOH:CKC mass ratio,and equal to 2,3,4 and 5)was prepared from corn granules by KOH activation method.The texture of CKC-X-Y was affected by the amount of activator and activation temperature.Among the CKC-X-Y samples,the CKC-900-3 sample had the best texture,and its specific surface area and pore volume could reach 1943 m~2/g and 1.10 cm~3/g,respectively.The CKC-900-4 sample had the largest CO₂ adsorption capacity in the CKC-X-Y samples,which could be reached 3.63 mmol/g at 25 ^oC.The results indicated that the activated corn porous biochar with high specific surface area and abundant oxygen functional groups had high CO₂ capture in this research.The CKC-X-Y samples had good CO₂/N₂selectivity,good recyclability and easy regeneration.Through the kinetic and thermodynamic analysis of CO₂ adsorption experiment,it was found that the adsorption of CO₂ by biochar CKC-X-Y included physical adsorption and chemical adsorption,and physical adsorption played a major role.Further study on the adsorption mechanism confirmed that the CO₂ adsorption was mainly physical adsorption,and the chemical adsorption mainly occurred on the hydroxyl group of the sample.