# Direct Selective Oxidation of Hydrogen Sulfide: Laboratory, Pilot and Industrial Tests
## 18 Transition Metal Oxides
Among the nanocrystals, CeO\({}_{2}\) nanorods with well-defined [110] and [100] crystal facets exhibits superb catalytic activity and sulfur selectivity. The high reactivity for H\({}_{2}\)S selective oxidation is attributed to the high concentration of surface oxygen vacancies which are beneficial for the conversion of lattice oxygen to active oxygen species. Besides, the presence of hierarchically porous structure of CeO\({}_{2}\) nanorods hinders the formation of SO\({}_{2}\) and sulfate, ensuring good sulfur selectivity and catalyst stability. Through a combined approach of density-functional theory (DFT) calculations and in situ DRIFTS investigation, the plausible reaction mechanism and nature of active sites for H\({}_{2}\)S selective oxidation over CeO\({}_{2}\) catalysts have been revealed. Thus, morphology engineering can be one of the effective methods in boosting the H\({}_{2}\)S conversion [114]. The comparison of catalytic activity and selectivity, taking into account the stability of oxides, allowed [PERSON] and co-authors [115] to find that iron oxide is the most effective catalyst for the reaction of oxidation of hydrogen sulfide to elementary sulfur. Bulk iron oxide catalysts demonstrate high activity at 250 \({}^{\circ}\)C, providing almost 100% conversion of hydrogen sulfide at sufficiently high selectivity [112]. It was also shown that the method of iron oxide preparation does not significantly affect the conversion of hydrogen sulfide and the selectivity of the process. The effects of additives of K, Cr, Ag, Ti, V, Mn, and anions: Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\) in the amount of 1-5% to the initial catalyst on catalytic properties were investigated. In particular, it was demonstrated that the introduction of chromium ions leads to a decrease in the activity, and vanadium ions and SO\({}_{4}\)\({}^{2-}\) to a decrease in the selectivity. A particular feature of the studied catalysts is their ability to oxidize hydrogen sulfide in the presence of hydrocarbons of natural gas without their involvement in the reaction. A significant advantage of these catalysts is their ability to selectively oxidize hydrogen sulfide under an over stoichiometric O\({}_{2}\)/H\({}_{2}\)S ratio. An increase in the selectivity of the iron oxide catalyst at elevated temperatures can be achieved by deposition of the active component on an alumina support. The most active catalyst at a temperature of 300 \({}^{\circ}\)C is the catalyst of the following composition: 0.5 wt.% Fe\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)[116]. 34A mixed catalytic system was investigated as a catalyst for partial oxidation of hydrogen sulfide, which consists of iron and titanium oxides (anatase modification). It is proposed to use this system in the process of two-stage oxidation of hydrogen sulfide (at the concentrations of hydrogen sulfide over 5 vol.%). When conducting the process in a temperature range of 200-300 \({}^{\circ}\)C, it is possible to remove hydrogen sulfide in the form of elementary sulfur with an efficiency close to 98-99% [117]. It is necessary to emphasize the work creating a catalyst for the direct oxidation of hydrogen sulfide made in VEG-Gasinstitut and the University of Utrecht (The Netherlands) under the general scientific leadership of Professor [PERSON] [118; 119; 120]. The overall goal of these works is to create an effective and highly selective catalyst for the direct oxidation of hydrogen sulfide by using alumina with a low specific surface area (\(\alpha\)-modification). It was experimentally shown (Figure 7) that the selectivity of the iron oxide supported on to \(\alpha\)-alumina had higher selectivity than the catalyst on \(\gamma\)-alumina even in conditions of a significant excess of oxygen in the reaction mixture. The authors' explanation of the observed results is as follows.