Studies On Effects Of S-containing Groups On Electrochemical Oxygen Reduction To Hydrogen Peroxide Production Over Carbon-based Catalysts

As an efficient and environment-friendly oxidant,hydrogen peroxide,has been widely used in many fields,such as environmental protection and chemical synthesis.At present,the preparation of hydrogen peroxide by electrochemical oxygen reduction through the two-electron pathway using high-performance catalysts is considered to be a very promising production alternative.Metal-free carbon-based catalysts are currently a hot spot in the research of oxygen reduction reaction catalysts and expected to promote the industrialization process of electrochemical hydrogen peroxide preparation.Usually,the electronic structure of carbon materials can be changed by the doping of heteroatom,thus affecting its catalytic selectivity.In this paper,sulfur and carbon nanotubes have been used as heteroatom and carriers,respectively,to prepare sulfur-doped carbon nanotubes with different interactions between sulfur and oxygen,and the structure-activity relationship between sulfur doping and its catalytic performance of oxygen reduction to generate hydrogen peroxide has been explored.This research has laid a foundation for the design and optimization of sulfur-doped carbon catalysts.First,the original carbon nanotubes have been functionalized in different ways（oxidation,sulfur doping）,the performance of the catalysts have been tested in an alkaline electrolyte.According to the results,after oxidation treatment,the selectivity of carbon nanotubes catalyzed oxygen reduction to produce H₂O₂ has increased,and the selectivity of the carbon nanotubes which have been doped with sulfur after oxidation has further increased to 79.87%.These have indicated that the doping of sulfur element could effectively improve the H₂O₂ selectivity of the catalysts.To further explore the influence of sulfur groups on the surface of carbon nanotubes,the sulfur content of carbon nanotubes has been changed to control the S-containing groups.The results of electrochemical performance test have shown that S-OCNTs-4 with moderate sulfur content has had the highest H₂O₂ selectivity（79.87%）and good stability.Combined with the characterization results,it has been found that the H₂O₂ selectivity of sulfur-doped carbon nanotubes has been positively correlated with the ratio of sulfur to oxygen atoms of S-containing groups on the surface（S/O）,while the catalytic activity was negatively correlated with the S/O.In the comparison of the oxygen reduction performance of sulfur-doped carbon nanotubes with different surface properties（carboxylation,hydroxylation）,it has been found that sulfur-doped hydroxylated carbon nanotubes have higher S/O and H₂O₂ selectivity than that of the sulfur-doped carboxylated carbon nanotubes.S-OCNTs-4,one of the sulfur-doped carbon nanotubes,has been prepared as a gas diffusion electrode,and the concentration of H₂O₂obtained after 120 min of electrolysis in the alkaline electrolyte of the gas diffusion electrode has been 45.17 mg?L^-1.After S-OCNTS-4 was treated with polyethylene oxide,the concentration of H₂O₂ obtained by the gas diffusion electrode electrolysis under the same conditions has been 61.62 mg?L^-1,and the current efficiency has been increased to 50.71%.This gas diffusion electrode is expected to be applied in Electro-Fenton to treat organic pollutants.