Catalytic Oxidation of Low-Concentration Methylmercaptan by Persulfate Activation with Nanoconfined Ni@NCNTs

Abstract

Methylmercaptan (CH₃SH) has a low olfactory threshold with significant toxicity and corrosiveness. The nickel-nanoparticle-confined nitrogen-doped carbon nanotube (Ni@NCNT) is a potential candidate for advanced oxidation process (AOP) catalysts, because the good electrical conductivity of Ni can combine the advantages of Ni and NCNT. In this study, the large surface area, multistage pore structure, high nitrogen content, and uniformly dispersed nickel nanoparticles confined in the nitrogen-doped carbon nanotube of Ni@NCNT were conducive to increasing the electron transfer capacity, improving mass transfer, and enriching active sites. Ni@NCNTs were used for the efficient activation of peroxydisulfate (PDS) and mineralization of the gas-phase low-concentration CH₃SH. In a continuous-flow system with a PDS concentration of 0.5 g L^–1 and Ni@NCNT-800-g concentration of 0.15 g L^–1, 92% of CH₃SH was stably degraded within 180 min; almost no byproduct such as dimethyl disulfide was produced, and most sulfur species were completely oxidized to SO₄^2–. This process has a wide pH range, low Ni leaching rates, and good reusability. The mechanism study showed that Ni(0), Ni(II), pyridine N, graphite N, and C═O as active sites promoted the generation of the active species. The activation pathway of the PDS catalyst includes both free-radical and non-free-radical processes. The generation of a single oxygen atom and the excellent electron transfer performance of Ni@NCNT played important roles in the nonradical process. This study develops an effective strategy for the deep degradation of gas-phase low-concentration sulfur-containing volatile organic compounds and provides a pathway for the synthesis of highly efficient metal-nitrogen-carbon catalysts for other environmental applications.