# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 14 Catalysts Based on Carbon Nanotubes  
Nanocarbon materials, i.e., carbon nanotubes (CNT) and carbon nanofibers (CNF), have recently attracted considerable research attention [84; 85; 86]. In particular, due to their lack of microporosity, diverse structures (the outer or inner diameter and the number of graphene layers), and rich surface chemistry (heteroatoms and structural defects), they are more promising than microporous AC, in which quite a few micropores substantially increase the role of diffusion. In particular, the tubular morphology of CNT ensures a special reactivity with liquid and gas reagents when passing through small tubes. For example, the so-called confinement effect [84] should be mentioned. Moreover, the chemical inertness of CNT species avoids problems of sulfation. Metal oxides, alkaline agents, and heteroatoms are often used to modify CNT. According to the data of [PERSON] and co-authors [87], Ni\({}_{2}\)S-modified CNT has a high capacity for sulfur (1.8 g of H\({}_{2}\)S per catalyst) in a trickle-bed reactor. Active sites of Ni\({}_{2}\)S are located inside the tube due to the confinement effect, and condensed water acts as the conveyor track, transferring elemental sulfur from the inner graphene layers to the outer ones in multilayer CNT, from where it is desorbed from the active phase. This mechanism ensures a high rate for hydrogen sulfide removal without any deactivation for 70 h. A substantial free catalyst volume makes it possible to save the resulting sulfur. However, because of the hydrophobic properties of Na\({}_{2}\)S/CNT, condensed water is required to maintain high activity, which complicates reactor design and production. Multiwalled CNT modified with Na\({}_{2}\)CO\({}_{3}\) also make it possible to achieve capacity values of 1.86 g of H\({}_{2}\)S/g catalyst at a low temperature (30 \({}^{\circ}\)C), which is approximately four times higher than commercial AC (0.48 g of H\({}_{2}\)S/g catalyst) [88]. As in the case of NiS\({}_{2}\)/CNT, a high capacity for sulfur is ensured by the presence of a large free volume formed by voids between CNT aggregates. In addition, introducing Na\({}_{2}\)CO\({}_{3}\) increases the hydrophilicity and alkaline properties of CNT as an adsorbent. Alkaline properties promote the sorption and dissociation of H\({}_{2}\)S into HS\({}^{-}\) ions in the aqueous layer. The gradual deactivation of the catalyst is linked to a decrease in pH upon sodium sulfate formation and the blocking of catalyst pores with sulfur. Hydrogen sulfide oxidation over multiwalled CNT decorated with tungsten sulfide was investigated in [89; 90]. The metal content in the catalysts was 4.7-4.9%. The catalyst activity was examined compared to WS\({}_{2}\)/AC and WS\({}_{2}\) catalysts using single-walled CNT under the following conditions: 5000 ppmv of H\({}_{2}\)S, 20% of water vapor, a volumetric flow rate of 5000 h\({}^{-1}\), O\({}_{2}\)/H\({}_{2}\)S = 2, and a temperature of 60 \({}^{\circ}\)C. As shown, the catalyst over multiwalled tubes displayed the highest activity. The catalyst activity has been shown to increase with increased metal content but to cease when the latter is over 15%. When the volumetric flow rate is increased, the conversion degree naturally decreases. Upon an increase in temperature to between 70 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, there is a high degree of conversion of hydrogen sulfide (at 180 \({}^{\circ}\)C), i.e., close to 10%, which is stable for 10 h, in contrast to the process performed at lower temperatures. This is related to the fact that sulfur is removed from catalyst pores more quickly at a high temperature, i.e., close to the melting point. Macroscopical nitrogen-doped CNT (N-CNT) were developed by [PERSON] [91] for hydrogen sulfide oxidation at high temperatures (>180 \({}^{\circ}\)C) with heavy mass flow rates, WHSV = 0.2 \(-\) 1.2 h\({}^{-1}\). As demonstrated, H\({}_{2}\)S conversion increases with nitrogen content, which is associated with a simultaneous increase in the concentration of active oxygen sites.