# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 9 Modifications of the Claus Process
### Chemism of the Process of Direct Catalytic Oxidation of Hydrogen Sulfide
However, the most promising systems are individual metal oxides or mixtures of transition metal oxides due to their apparent advantages, i.e., high mechanical strength, thermal stability, and relative cheapness. It should be noted that oxides are used both in a bulk state and in the supported form. This is confirmed by the fact that all commercial processes for sulfur production from H\({}_{2}\)S through its direct catalytic oxidation, such as Catasulf\({}^{\circledgtrgtr}\) of BASF (Ludwigshafen, Germany), BSR/SELECTOX\({}^{\circledgtr}\) of Unocal Company (California, CA, USA), Modop\({}^{\circledgtr}\) of Mobil Oil (Panama City, Panama), etc., are based on the application of heterogeneous multicomponent oxide catalysts. **13. Activated Carbon. Catalysts Based on Activated Carbon**  
As demonstrated above, active carbon (AC) simultaneously acts as the adsorbent of hydrogen sulfide and the catalyst for the oxidation of the latter to sulfur, as H\({}_{2}\)S is transformed into sulfur which accumulates in AC pores upon purification. Microporous ACs have been well investigated as adsorbents/catalysts for the periodical partial oxidation of hydrogen sulfide at temperatures below 150 \({}^{\circ}\)C [56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66]. They demonstrate high activity and selectivity under these conditions. As shown, a relatively large volume of large pores is required for the oxidation process to occur, whereas smaller  
Figure 5: The dependence of the equilibrium sulfur yield on the temperature in the reaction of direct oxidation of hydrogen sulfide at atmospheric pressure (adapted from [51]). pores serve for adsorptive desulfurization processes. Elemental sulfur is mainly accumulated in pores <12 A, and initially in small and later in larger ones. It has been shown that there is maximum adsorption in cases where the pore size is maximally close to the size of the adsorbent molecules [67; 68]. Hence, efficient carbon materials should have the optimal pore structure with a good volume of both micro- and meso- pores, and a relatively narrow pore size distribution to ensure high selectivity for sulfur. Nevertheless, the complete picture of the effect of AC pore structure on H\({}_{2}\)S selective oxidation is not quite clear yet. [PERSON] and co-authors [69] investigated the effect of water vapor on adsorbent efficiency. It was found that a relative moisture content of about 20% facilitates an enhanced reaction rate. The reaction rate drops dramatically when the moisture content is decreased, and less significantly when increased. It is assumed that HS\({}^{-}\) ions are generated in water, being readily oxidized to sulfur with oxygen. Surface chemistry has a significant effect on catalyst efficiency; therefore, AC-basedorbents/catalysts undergo modification with various reagents, such as metal salts [70] and alkaline [63; 64; 71] or oxidative (permanganate) additives [62], by the introduction of heteroatoms, such as nitrogen, oxygen, and phosphorus [72], and also by thermal treatment and controlled surface oxidation [73; 74; 75]. When AC is treated with nitric acid, oxygenated groups (C=O, C-O, and C-O-) are generated. Modified ACs contain charged oxygen particles, have higher catalytic activities, and may oxidize to 1.9 g of H\({}_{2}\)S per g of catalyst, which is much higher than literature data for carbon catalysts [58; 62; 76; 77]. The dynamic adsorption capacity of AC is reduced, as high temperatures decrease adsorption efficiency and selectivity for sulfur because COS and SO\({}_{2}\) are formed. In order to improve the capacity for sulfur and catalytic activity at high temperatures, AC modified with metal oxides is used [62]. At 180 \({}^{\circ}\)C and in the absence of water vapor, catalytic activity is varied in series, i.e., Mn/AC > Cu/Ac > Fe/AC > Ce/AC > Co/AC, being reduced to between 142 mg and 6 mg of H\({}_{2}\)S/g for Mn/AC and V/AC, respectively. The major reaction product is elemental sulfur, which forms on active sites (carrier and coal micropores).