Adsorption behavior and mechanism of ultra-low concentration sulfur compounds in natural gas on Cu-impregnated activated carbon

Sulfur compounds critically deteriorate the performance of catalysts and fuel cells in H2 production and natural gas applications. This study aims to elucidate the adsorption characteristics of low-concentration tetrahydrothiophene (THT) and tert-butyl mercaptan (TBM) in CH4 (4.5 ppmv) on Cu-impregnated activated carbon (Cu/AC) by analyzing the state of the Cu and sulfur species during dynamic tests of the 1st desulfurization, regeneration and 2nd desulfurization. Physical adsorption was dominant in THT removal, enabling facile regeneration by the N2 thermal treatment at 100 °C. TBM removal was primarily carried out by the chemical adsorption utilizing CuO, Cu2O, and oxygen on the AC. During desulfurization, CuO acts as a catalyst, an adsorption site, and an oxygen donor. CuO was actively consumed during the initial stage, leading to the complete removal of the TBM. During desulfurization, oxidation to form organic sulfur and copper sulfonate was the primary reaction in the presence of sufficient amounts of CuO and oxygen on the AC. As the oxygen content of AC was consumed with subsequent desulfurization, reactive adsorption on Cu2O to form thiolate became one of the major reactions. Owing to the strong chemisorption of sulfur compounds, the recovery of the removal efficiency was limited by N2 thermal regeneration. However, oxidative regeneration at 250 °C using dilute O2 flow effectively removed chemisorbed organic sulfur and thiolate. Because oxygen consumption was accompanied by both desulfurization and regeneration reactions, this method successfully restored the desulfurization performance by recovering CuO and oxygen on the AC surface and maintaining well-dispersed Cu.