# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 15 Carbon Nanofiber-Based Catalysts  
As in the case of nanotubes, the main advantages of carbon nanofiber-based catalysts are related to the high thermal conductivity of the latter, chemical inertness, and the lack of ink-bottle pores where elemental sulfur may settle [93]. Furthermore, the presence of pores as microcavities between nanofibers increases the sulfur capacity of the material. Using CNT for H\({}_{2}\)S selective oxidation at high temperatures (>180 \({}^{\circ}\)C) has been investigated more widely than CNT-based catalysts. The latter are more promising from the standpoint of using a high excesses of oxygen to stoichiometry [94]. Herewith, the catalytic characteristics might be quite different depending on the nature of the initial catalyst over which the synthesis of nanofibers was carried out. When water is absent, nanofibers produced over a Fe-Ni catalyst [95] with a structure of multilayered CNT have the highest selectivity for sulfur. The selectivity for sulfur is maintained at a level of 90%, whereas H\({}_{2}\)S conversion degree decreases to 65% after 25 h of the reaction. The most highly active CNT samples were obtained using a Ni-Cu catalyst. After 25 h of reaction, hydrogen sulfide conversion degree and selectivity for sulfur were 95% and 70%, respectively. Compared to those species, nanofibers grown on Ni-catalyst displayed low activity because of sulfur deposits. In order to improve catalytic characteristics, these fibers were modified by treatment with HNO\({}_{3}\) or NH\({}_{3}\)[91]. As determined, acid treatment improved catalyst stability and selectivity for sulfur due to the the partial removal of nickel from CNF. In contrast, ammonia treatment reduced selectivity. As noted, the presence of 40% of water vapor improved the characteristics of the procedure, achieving a conversion degree of 70% and a selectivity of 89%. [PERSON] and co-authors [96] investigated the process kinetics of selective hydrogen sulfide oxidation over CNT. The proposed kinetic model matched well with experimental results across a broad temperature range (155-250 \({}^{\circ}\)C) with hydrogen sulfide, oxygen, and water vapor concentrations of 0.5-2 vol.% and 0.25-10 vol.%, and 0-35 vol.%, respectively. The findings may be used when modeling processes and reactor designs for H\({}_{2}\)S selective oxidation using nanofiber-based catalysts. [PERSON] and co-authors [97] systematically investigated H\({}_{2}\)S selective oxidation over acrylonitrile-derived CNF impregnated with Na\({}_{2}\)CO\({}_{3}\). The capacity for sulfur over these catalysts was shown to be 0.10-0.81 g of H\({}_{2}\)S/g. First of all, the pore structure affected the sulfur capacity. Additionally, unlike other nitrogen-doped carbon materials, the concentration of nitrogenated functional groups almost did not affect the characteristics of the H\({}_{2}\)S oxidation process. As demonstrated by analysis data, the prevalent product, i.e., elemental sulfur, was deposited in larger pores, whereas H\({}_{2}\)SO\({}_{4}\) was generated in smaller ones. The effect of temperature and water on H\({}_{2}\)S selective oxidation over CNF-based macroscopic catalysts was analyzed by [PERSON] and co-authors [98]. Carbon nanofibers were grown over a graphite fiber substrate. The active phase was NiS\({}_{2}\). The catalyst demonstrated very high selectivity and stability at 60 \({}^{\circ}\)C owing to its stability to sulfur deposits removed therefrom through the presence of water and the hydrophobic properties of the catalyst. The efficiency of H\({}_{2}\)S removal using catalysts based on new nanocarbon materials, i.e., CNT and CNF, was shown to be much higher, and material doping with nitrogen improved the purification process characteristics to a greater extent. This research demonstrates that carbon materials are highly efficient during direct H\({}_{2}\)S oxidation and the sorption of sulfur compounds. [PERSON] and co-authors [99] described the synthesis and study of a catalyst for the catalytic oxidation of H\({}_{2}\)S to S at room temperature.