Study On The Pyrolysis Characteristics Of Main Salts Of Desulfurization Waste Liquid And Photocatalytic Performance Of The Prepared G-C₃N₄

Visible light-responsive non-metallic organic semiconductor graphite phase carbon nitride g-C₃N₄ is stacked by a graphene-like lamellar structure containingπ-conjugated system,while the graphene-like lamellar structure is formed by alternate bonding of C and N elements.g-C₃N₄ has such a suitable energy band of 2.7 e V under ideal conditions that can effectively absorb visible light.g-C₃N₄ has good stability,and easy to synthesize,is a photocatalytic material with excellent physical and chemical properties,which has received widespread attention.In the study of using the waste heat of the riser to pyrolyze the desulfurization waste liquid,it was found that g-C₃N₄ is produced during the pyrolysis of the main salts（ammonium thiocyanate NH₄SCN,ammonium thiosulfate（NH₄）₂S₂O₃,and ammonium sulfate（NH₄）₂SO₄）of the desulfurization waste liquid.However,the pyrolysis behavior of the above salts in a reducing atmosphere is not clear,the role distribution and interaction of mixed salts in the process of pyrolysis to g-C₃N₄are unclear,the influence of main components in gas（H₂,CH₄ and CO）on the formation of g-C₃N₄ by NH₄SCN is also without report.Aiming at these problems,this paper carried out the research.The main results and conclusions of the research are as follows:1.Study on pyrolysis characteristics of main salts of desulfurization waste liquid（1）During the entire pyrolysis process of NH₄SCN in an inert atmosphere（Ar）or reducing（H₂）atmosphere,with the increase of pyrolysis temperature,NH₄SCN successively undergoes two crystal transformations at 90-105°C and114-127°C,which are from monoclinic crystal to orthorhombic crystal,and from orthorhombic crystal to the square crystal phase.When the temperature rises to 135-167°C（Ar）and 144-167°C（H₂）,NH₄SCN melts and ionizes,and decomposes to form isothiocyanate HNCS and NH₃.And partial of HNCS and NH₃ recombine to generate thiourea SC（NH₂）₂.This stage was accompanied by sublimation of NH₄SCN and dehydration moisture removal,resulting in a mass loss of approximately 2.97%（Ar）and 3.2%（H₂）.In the temperature range of 167-350°C（Ar）and 167-265°C（H₂）,the main reactions accompanying the isomerization of NH₄SCN are the formation and decomposition polymerization of guanidine salt H₂N（C=NH）NH₂,the thermal weight loss reached 84.53%（Ar）and 87.52%（H₂）.The main generation routes of guanidine salt are the action of NH₄SCN and HNCS,the reaction of SC（NH₂）₂ with HNCS,and SC（NH₂）₂ interacts with NH₃,accompanied by the formation of H₂S,CS₂ and other products.The guanidine salt decomposes at about 230°C to form cyanamide NH₂CN.NH₂CN rapidly polymerizes to form dimer dicyandiamide and trimer melamine.At the same time,guanidine also undergoes deamination polymerization to generate dicyandiamide and melamine,accompanied by the massive release of NH₃ and CS₂.The above polymer undergoes deamination polycondensation above 350°C to produce highly polymerized graphite phase carbon nitride g-C₃N₄;above 600°C,g-C₃N₄ gradually decomposes.The pyrolysis process of NH₄SCN in H₂ is faster,which is shown by the lower final temperature of pyrolysis and the lower temperature range of gas-phase product formation.This is a result of the high thermal conductivity of H₂ and the effective promotion of pyrolysis by active hydrogen that may be formed at high temperatures.（2）The pyrolysis of（NH₄）₂S₂O₃ under Ar atmosphere occurred sequentially at 128-252°C,252-315°C and 315-400°C.The weight loss was57.55%,18.37%,and 22.25%,and the cumulative weight loss was 98.17%.Decomposition under H₂ atmosphere occurs sequentially at 139-243°C,243-306°C,and 306-394°C,with weight loss of 56.91%,18.05%,and 24.19%,respectively,with a cumulative total of 98.41%.The gas-phase products produced in the first stage of the decomposition of（NH₄）₂S₂O₃ mainly include NH₃,SO₂,H₂O and H₂S.At this stage,the conversion of（NH₄）₂S₂O₃ to sulfamic acid SO₃NH₃ will also occur.The gas-phase products corresponding to the pyrolysis of（NH₄）₂S₂O₃ in the second stage are mainly NH₃.The main gas phase products in the third stage is SO₂.（3）The pyrolysis of（NH₄）₂SO₄ in Ar atmosphere occurs at 234-313°C,313-360°C and 360-459°C with the corresponding weightlessness of 9.60%,9.47%and 79.43%,the total weightlessness was 98.49%.When the pyrolysis atmosphere is H₂,the three stages of（NH₄）₂SO₄ pyrolysis occur at 238-313°C,313-352°C,and 352-451°C.The corresponding weightlessness rates at each stage were 10.42%,7.54%and 80.43%,and the total weightlessness was98.39%.The first stage of the decomposition of（NH₄）₂SO₄ is the dehydration and deamination.The products are NH₄SO₃NH₂ and（NH₄）₃H（SO₄）₂,respectively.The second stage is the deamination of（NH₄）₂SO₄ to produce NH₄HSO₄.With the partial decomposition of NH₄SO₃NH₂ and（NH₄）₃H（SO₄）₂,the decomposition products are mainly NH₃,SO₂,H₂O,and there may be N₂,O₂ and SO₃ generated.The last stage is mainly the dehydration and decomposition of NH₄HSO₄.（4）The pyrolysis of mixed salts under Ar atmosphere mainly occurs in three stages of 85-150°C,150-273°C and 273-392°C.The weightlessness in each stage was 11.11%,69.28%and 17.47%,the cumulative weightlessness was 97.87%,and the temperature continued to rise to 600°C,and the weightlessness reached 99.23%.Each stage reached the maximum weight loss rate at 133°C,225°C and 369°C.When the pyrolysis atmosphere is H₂,pyrolysis of mixed salts mainly occurs at 83-149°C,149-289°C and289-400°C,with weight loss of 11.98%,72.92%and 14.04%,respectively,and cumulative weight loss of 98.94%.The maximum weightlessness rate of each stage was reached at 133°C,221°C and 371°C in sequence.When heated to506°C,the total weight loss is 99.99%.The thermal weight loss characteristics of the mixed salts in the two atmospheres are obviously similar.The source of weightlessness in the first stage is mainly the decomposition of（NH₄）₂S₂O₃and the loss of intermediate products during the isomerization of NH₄SCN.The second stage of mixed salt pyrolysis is the co-pyrolysis of NH₄SCN and（NH₄）₂S₂O₃,and the pyrolysis of NH₄SCN is basically completed,（NH₄）₂SO₄melts but there is almost no decomposition in the stage.The third stage of mixed salt pyrolysis corresponds to the coupling of the decomposition of（NH₄）₂S₂O₃ intermediate product and the deamination and dehydration reaction of（NH₄）₂SO₄.The pyrolysis above 400°C is mainly due to the deamination of（NH₄）₂SO₄ and the dehydration and decomposition of NH₄HSO₄.2.Effect of thermal polymerization preparation conditions on the degradation of rhodamine B catalyzed by g-C₃N₄（1）g-C₃N₄ prepared with ammonium thiocyanate（AT）,thiourea（T）,dicyandiamide（D）and melamine（M）as precursors has a similar lamellar stacking structure.The AT and T products exhibit specific surface areas of65.66 m²·g^-1 and 54.74 m²·g^-1,which are significantly greater than the 23.94m²·g^-1 and 24.50 m²·g^-1 of D and M products.Compared with the products obtained with D and M,the products obtained by pyrolysis of AT and T have stronger redox capacity.Moreover,g-C₃N₄ prepared by T and AT has stronger photo-generated carrier separation and transfer ability.The g-C₃N₄ prepared with D and M as precursors showed similar adsorption characteristics for Rh B and photocatalytic Rh B degradation capacity.The kinetic constant k of the product obtained by AT in the degradation of Rh B reaction is 4 times the k value of the sample obtained by M,which is the result of a large specific surface area,strong redox properties of photo-generated carriers,and the ability of strong photo-generated carriers to migrate.（2）During the preparation of g-C₃N₄ using ammonium thiocyanate as a precursor,as the thermal polymerization temperature increases from 450°C to550°C,the more sufficient the pyrolysis,the more obvious the effect of the gas phase product on the sample’s pore expansion.The resulting product block becomes smaller,the pore structure gradually forms and increases,the sheet structure appears and becomes thinner,and the product specific surface area and pore volume gradually increase.Increasing the reaction temperature helps the resulting sample to form a smaller and thinner porous nanosheet structure.As the pyrolysis temperature increases,the resulting sample layer spacing decreases and the material crystallinity increases.Among the products obtained by pyrolysis with final temperatures of 450°C,500°C and 550°C,the proportion of C in the presence of N=CN in triazines decreased gradually;the proportion of graphite carbon or amorphous carbon decreased.Among the different forms of N,the proportion of N-（C）₃ gradually decreases,while the proportion of C=N-C is increased respectively.As the thermal polymerization temperature increases,the material may graphitize under the action of H₂ and high temperature.At the same time,the bridged N atoms are converted to C=N-C,so that the degree of polymerization of the material is improved.In addition,the decrease in the proportion of N=C-N and the increase in the proportion of C=N-C in the triazine together indicate the formation of C defects in the material.As the pyrolysis temperature gradually increased,the adsorption characteristics and the photocatalytic Rh B degradation of the resulting catalyst gradually increased.（3）When the visible light catalyst g-C₃N₄ was prepared under the H₂atmosphere with NH₄SCN as the precursor,with the polymerization constant temperature time extended from 1 h to 5 h,the interval of the graphite phase carbon nitride layer obtained continued to shrink,and the crystallinity of the resulting sample increase.The increase in the proportion of amorphous C and the decrease in the proportion of N=C-N indicate the formation of C defects,which can be attributed to the corrosive effect of H₂ on the sample.With the extension of the constant temperature time of thermal polymerization,the specific surface area of the resulting product increased from 24.05 m²·g^-1 to65.88 m²·g^-1,and the catalyst showed a gradually enhanced adsorption capacity and the Rh B catalytic degradation rate.（4）Among the products prepared by pyrolysis of NH₄SCN in different atmospheres,the interlayer spacing of products obtained in H₂ and CH₄ is smaller than that in Ar and CO,which may be the result of hydrogen bonding in the material structure.H₂ atmosphere is more conducive to the transition from bridging N to C=N-C and the formation of C defects in the ring.The product laminar structure of Ar atmosphere is relatively large and there are obvious nitrogen vacancies in the ring.CH₄ converts more bridged nitrogen to NH_x,producing more terminal carbons and terminal amino groups,causing fragmentation of the lamellar structure.The inner structure of the CO atmosphere product is relatively complete and has a large lamellar structure.The band gaps of the products obtained in Ar,CO,and CH₄ atmospheres are basically the same,all about 2.70 e V,which are much smaller than those obtained in the H₂ atmosphere（2.98 e V）.The kinetic constant of the product obtained in the H₂ atmosphere is 3.8 times that of the product in the CH₄ and CO atmosphere,and 2.38 times that of the product in the Ar atmosphere.This is a result of the combined effects of excellent specific surface area,defect states,and enhanced redox capability caused by wide band gap.3.Preparation of g-C₃N₄ by co-pyrolysis of desulfurization waste liquid mixed salt and its photocatalytic performance（1）After ATS and AS are added to the precursor ammonium thiocyanate（AT）,a large amount of gas phase product release,which can effectively promote the formation and expansion of the pore structure during the polymerization of g-C₃N₄.The specific surface area of the product obtained by co-pyrolysis of the three salts is 4.42 times of CN-AT.The addition and co-addition of ATS or AS are beneficial to the distribution of larger mesopores in the product,which can effectively promote the absorption of light by the sample.The increased specific surface area of the material can provide more active sites for the reaction and enhance photocatalysis active.（2）Compared with CN-AT,the proportions of N-C=N form C in CN-ATAS,CN-ATATS and CN-MIX are significantly higher.The percentages of C atoms connected to-H or-NH_x decreased.The addition of ammonium thiosulfate and ammonium sulfate during the preparation of g-C₃N₄ by thermal polymerization of ammonium thiocyanate is beneficial for the interconnection of the hepazine ring structure.The proportion of N in the form of N-（C）₃ in CN-AT,CN-ATAS,CN-ATATS and CN-MIX has gradually decreases,the proportion of g-C₃N₄ framework C=NC sp²-hybrid nitrogen gradually increases.It is believed that the addition of ammonium sulfate and ammonium thiosulfate can lead to the generation of N vacancies between triazines.Compared with CN-AT,the remaining samples have more negative valence and conduction bands,that is,stronger reducing and weaker oxidation.（3）The adsorption effect of four photocatalysts of CN-AT,CN-ATAS,CN-ATATS,and CN-MIX on Rh B is about 5%,and the degradation rates of catalytic Rh B after 4 hours of visible light irradiation are 58.27%,78.22%,71.23%,and 96.49%respectively,CN-MIX performed best.This is due to the synergistic effect of enhanced specific surface area,structural defects,and strong reduction of photogenerated electrons.The active species that play major role in the degradation process of photocatalytic Rh B are holes and superoxide radicals·O₂￣,and the migration and separation of carriers are not directly related to the enhancement of photocatalytic activity.