In this study, graphitized carbon nitride photocatalysts were successfully prepared by using the desulfurized waste liquid extracting salt from coking plants. The NO x removal efficiency of g-C 3 N 4 synthesized using ammonium thiocyanate (NH 4 SCN), ammonium thiosulfate ((NH 4 ) 2 S 2 O 3 ) and ammonium sulfate ((NH 4 ) 2 SO 4 ) with different mass ratios were evaluated. The results showed that due to the addition of (NH 4 ) 2 S 2 O 3 and (NH 4 ) 2 SO 4 a large amount of sulfur-containing gas was released during pyrolysis, which significantly increased the specific surface area and pore volume of the photocatalysts. The release of sulfur-containing gas also resulted in the incorporation of sulfur, and due to this doping process the band gap of the photocatalyst was widened to 2.94 eV, thus enhancing its visible light response. C vacancies were formed in the hepazine ring, which increased the electron density around the N–(C) 3 bridge bond and formed an electron trapping center, thus inhibiting the recombination of charge carriers. The photocatalysts had increased O 2 adsorption capacity and increased number of amino groups on their surface. This facilitated the rapid generation and consumption of reactive oxygen species, inhibited the accumulation of NO 2 on the surface, reduced the production of NO 2 , thus improving the NO x removal efficiency of the photocatalysts. The principle of using “waste for waste control” was achieved, and a new method for nitrogen oxide removal was provided.
The desulfurized waste liquid (DWL) produced in the process of wet oxidation removal of hydrogen sulfide (H 2 S) and hydrogen cyanide (HCN) from coke oven gas is rich in ammonium thiocyanate (NH 4 SCN), ammonium thiosulfate ((NH 4 ) 2 S 2 O 3 ), and ammonium sulfate ((NH) 2 SO 4 ). Based on the research of thermal decomposition of DWL, the study on the preparation of graphitic carbon nitride (g-C 3 N 4 ) by thermal polymerization in hydrogen (H 2 ) atmosphere using the salts of DWL as precursor was carried out. Specifically, a single salt (NH 4 SCN), a double salt combination (NH 4 SCN + (NH) 2 SO 4 ) or (NH 4 SCN + (NH 4 ) 2 S 2 O 3 ), and a three salt combination (NH 4 SCN + (NH) 2 SO 4 + (NH 4 ) 2 S 2 O 3 ) were used as precursors to prepare g-C 3 N 4 . The morphology, phase composition, specific surface area, pore structure distribution, element composition, element valence distribution, and light absorption characteristics of the prepared samples were characterized. The performance of the prepared samples was evaluated by photocatalytic degradation of rhodamine B under visible light . The characterization results showed that the product (CN-MIX) obtained by the pyrolysis of three salt combination has a most fluffy and porous structure with the maximum specific surface area of 56.89 m 2 /g. Which can be attributed to the template effect of the gases produced by the pyrolysis of (NH 4 ) 2 S 2 O 3 and (NH) 2 SO 4 accompanying the deamination and polycondensation of the intermediate product from the pyrolysis of NH 4 SCN. CN-MIX has a widest band gap of 2.93eV with a more negative conduction band . After 4 h of visible light irradiation, the degradation rates of 300 mL rhodamine B solution (60 mg/L) photocatalyzed by 50 mg mono-salt product and tri-salt product catalyst were 58.27% and 96.49% respectively. This is due to the synergistic effect of the large specific surface area, abundant structural defects and the strong reduction ability of photo-generated electrons in CN-MIX. The great photocatalytic performance of CN-MIX has positive theoretical guiding significance for the preparation of highly active photocatalyst g-C 3 N 4 by using the mixed salt of desulfurized waste liquid.
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