Removal Mechanism Of Low Concentration H₂S By Surface Basic Functional Groups On Carbon Materials

Carbon-material catalysts have extensive research and application foundation.In the field of catalytic oxidation of hydrogen sulfide,carbon materials are widely studied by scholars at home and abroad because of their low cost,developed pore structure,low catalytic oxidation temperature and high reaction activity.There are two important problems involved in catalytic oxidation of hydrogen sulfide?H₂S?by carbon materials,one is the removal of low concentration H₂S,the other is the unclear ability of different functional groups in controling the speed of H₂S oxidation.The further study of the functional groups on carbon material surface is helpful to reveal the catalytic oxidation of H₂S.It is also beneficial for more definite modification of carbon materials and design of low-temperature and efficient desulfurizers.Therefore,in this paper,activated carbon?AC?materials,which were rich in surface functional groups,were used;effects of different modification methods on desulfurization were compared;effects of different reaction parameters were compared;and optimal desulfurization-agent modification was obtained,finally achieving continuous desulfurization.Based on this,the relationship between the change content of surface functional group and kinetic reaction constant was studied.The different contributions of different surface functional groups to the catalytic oxidation of H₂S were obtained,providing a theoretical basis for understanding the removal of H₂S by carbon materials.The main conclusions are as follows:?1?Alkaline functional groups played an important role in enhancing desulfurization of activated carbon materials.In the solid phase modification,the fly ash modified samples showed a high-efficiency desulfurization effect.The total desulfurization amount was 19.70 mmol.g^-1in 60 min,while the pure activated carbon sample was only 17.73 mmol.g^-1,which increased the desulfurization ability by11.11%.Besides,the modification of potassium hydroxide and sodium hydroxide only increased 6.09%and 3.44%.Compared with solid modification,the desulphurization advantage of ammonia modified sample was more obvious.After the reaction for 180 minutes,55.56%H₂S was still removed,while the unmodified sample only removed 22.22%.Desulphurization ability of the modified sample was almost 2.5 times than that of the unmodified sample.Besides,the modified sample still showed high desulfurization which maintained at 40-50%,when removed ultra-low concentration of H₂S at high altitude velocity.Comparing the modified desulfurizers with different methods,the basic functional groups were increased to varying degrees,and consistent with the desulfurization effect.For example,the total basic functional groups of the samples after ammonia modification was 0.6086 mmol.g^-1.This amount was almost twice as that of the unmodified sample(0.3028 mmol.g^-1),indicating that the increase of the basic functional group plays an important role in the improvement of desulfurization performance.?2?The reason for continuous catalytic oxidation of H₂S was the self-outflow of elemental sulfur.By adjusting the reaction parameters,desulfurization reaction was found to be a continuous catalytic oxidation reaction during aerobic and at 150^oC.55.56%of H₂S was still removed after the reaction for 180 minutes.XRD and XPS were used to characterize the desulfurization products.The product was found to be mainly elemental sulfur and accounted for 43.58%of the total sulfur elements.Boehm titration results showed that the total amount of basic functional groups on surface of the samples were 0.5593,0.5593,0.5674 and 0.4927 mmol.g^-1,respectively,after reaction of 5,10,30 and 180 minutes.These functional groups generated a structure similar to-CH-S,which was characterized by FTIR,after reacting with H₂S.It was precisely because of the presence of this intermediate that high-efficiency desulfurization can be maintained.Elemental sulfur melts out of the pores,leaving the active sites exposed again,ensuring the reaction continues.?3?Compared with free radical,specific surface area and pore size distribution,alkaline functional groups played a decisive role in improving the catalytic activity.EPR spectra showed that the free electron number of the samples pyrolyzed below 700^oC was almost constant.The free radicals even could not be detected in modified samples with best desulphurization,which proved that free radicals on surface of carbon materials played a limited role in catalytic oxidation of H₂S.Comparing the specific surface area and iodine value of different modified samples,physical adsorption of the materials had little effect on the desulfurization.For example,the specific surface area and iodine value of ammonia modified samples were 1376.010 m^2.g^-1,757.55 mg.g^-1,while those of pure activated carbon samples were 1510.249 m^2.g^-1,796.14 mg.g^-1.The specific surface area and iodine value of the modified samples were decreased in varying degrees,indicating that physical adsorption of the modified samples was also reduced.In summary,these results demonstrated that the surface functional groups of carbon materials played a decisive role in the catalytic oxidation of H₂S.?4?Pyrone group played a significant role in catalytic oxidation of H₂S.Based on the rich alkaline groups in ammonia modified materials,the removal efficiency of H₂S and content of various functional groups were compared.Desulfurization of modified materials were improved in different degrees.Contents of benzoquinone and pyranone were 0.0382 and 0.2709 mmol.g^-1in ammonia modified samples,whlie they were 0.0790 and 0.0469 mmol.g^-1in activated carbon.The kinetic reaction constants of different modified samples in desulfurization were fitted.The average kinetic reaction order of the modified samples was found to be reduced to varying degrees.For example,the average reaction order of ammonia-modified samples was 0.8735,while that of the activated carbon was 1.1518.A correlation was established between the change in kinetic reaction order and functional group content.As a result,both acidic functional groups and phenolic hydroxyl groups could inhibit desulfurization,and the inhibition effect of acid functional groups was more obvious.The positive correlation coefficient was 5.4526,while the phenolic hydroxyl group was only0.3720.Moveover benzoquinone and pyrone group accelerated the desulfurization,and the effect of pyrone was better.The contribution of pyrone content to reaction order was negative correlation coefficient-1.3030,which was 1.375 times bigger than that of quinone?-0.9477?.