전이금속산화물 촉매를 이용한 SO₂/H₂S의 선택적 제거

Abstract

Sulfur compounds SO₂ and H₂S from stationary sources, such as coal-fired power plants and petroleum refineries, are major components of atmospheric pollution. Direct reduction and oxidation of SO₂ and H₂S to elemental sulfur over catalysts, extremely beneficial, as they produces a salable product without any solid waste to dispose of. The objectives of this study are to explore a more efficient catalytic system for the reactions, and to investigate the correlation of the physicochemical properties of the transition metal oxides catalytic system and their activities for the removal of sulfur compounds. Especially, the acid-base and redox behaviors and the effect of calcined temperature were studied and the mechanistic were performed on these catalysts. A series of Nb-Fe mixed oxides, SnO₂ and Co₃O₄ catalysts were tested for the removal of sulfur compounds. Compared with single oxides Nb₂O5 and Fe₂O₃, Nb-Fe mixed oxides showed a decrease in the particle size and an increase in the surface area. With increasing niobia loading, the amounts of both acidic and basic sites were greatly increased, but more significantly for acid sites. The catalytic activity for reduction of SO₂ to elemental sulfur by CO, was improved by introducing niobia into iron oxide and the catalyst with a Nb/Fe atomic ratio of 1/1, which has the acid/base ratio of 14.8, showed the highest activity. Strong synergistic phenomena in catalytic activity and selectivity were observed for the Nb-Fe mixed oxides. The Nb-Fe(atomic ratio of 1/1) binary mixed oxide was heat-treated at temperatures between from 550 to 950 ℃, and was studied for physicochemical properties and catalytic behavior. The mixed oxide was shown to be amorphous up to 650 ℃, where it crystallized in the form of FeNbO₄ with the decrease of specific surface area. It was observed that in the case of Nb-Fe mixed oxides, redox properties were greatly improved and new basic features were created resulting in a rather satisfactory mild oxidation catalyst. On the other hand, the surface reactivity of Nb-Fe mixed oxides depended on the treatment temperature. The treatment at higher temperatures improved both redox and basic properties. Particularly, in the Nb-Fe mixed oxide calcined at 650 ℃ showed the highest activity in the reaction conditions. While sulfur selectivity was decreased with increasing the CO/SO₂ molar ratio of the reactant, the maximum yield to sulfur was obtained in the stoichiometric ratio of 2. Active sites in the active catalyst appeared in iron sulfide of FeS₂ and selective reduction of SO₂ by CO, followed by the COS intermediate mechanism. Sulfur dioxide conversion and selectivity to sulfur were closely related to the strength of the basicity and to the redox ability of the surfaces. The mesoporous Nb-Fe mixed oxides with different Nb-Fe ratios were prepared by short-chain amine (hexylamine) templating method via co-precipitation process, and found to be an efficient catalyst for the selective oxidation of H₂S to elemental sulfur. Compared with the conventional coprecipitated Nb-Fe mixed oxide, the mesoporous oxide presented higher yields (ca. 83 % at 200 ℃) of elemental sulfur. The high catalytic activity and good stability is believed to be due to the high surface area and the relatively thick pore wall of mesoporous Nb-Fe mixed oxide. The H₂S oxidation reaction was also carried out with Nb-Fe nanoparticles supported on Al₂O₃, SiO₂, TiO₂, and ZrO₂. The activity of the catalysts was shown to be strong with the nature of the support, whereas the selectivity to sulfur was not apparently affected. Compared with the effects of various supports, the activity increased in order to bulk Nb-Fe(atomic ratio of 1/1)〈 SiO₂ 〈 ZrO₂ 〈 Al₂O₃ 〈 TiO₂, indicating that moderately basic surface and high sulfur tolerance are desired. SnO₂/TiO₂ and Co₃O₄/TiO₂, catalysts were found to exist with a strong synergistic promotional effects in the reduction of SO₂ by CO, when a cobalt or tin component was mixed with TiO₂. Addition of active transition metals such as tin or cobalt to the TiO₂ not only increased the catalytic activity, but also suppressed the formation of side-product COS. While Co₃O₄ contacted with TiO₂ was transformed to CoS₂ under reaction conditions of this study, the bulk structure of SnO₂ was not significantly affected by the introduction of TiO₂. The observed effects of activation can be interpreted in terms of a mechanism ; the sulfurization of Co₃O₄ is necessary to produce COS intermediate by the reaction between CoS₂ and CO, whereas SnO₂ by itself is active for the formation of COS due to its high oxygen vacancy concentration and mobility properties. It is proposed that the COS intermediate is transformed into elemental sulfur by reacting with SO₂ over TiO₂.