Ind. Eng. Chem. Res. 2009, 48, 5223–5229
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Selectivity and Stability Enhancement of Iron Oxide Catalyst by Ceria Incorporation for Selective Oxidation of H2S to Sulfur D. D. Eslek Koyuncu and Sena Yasyerli* Chemical Engineering Department, Gazi UniVersity, Maltepe 06570, Ankara, Turkey
Cerium incorporated iron oxide catalysts and a pure iron oxide catalytic material were prepared by the complexation technique and their catalytic performances were investigated for selective oxidation of H2S to elemental sulfur in a fixed-bed flow reactor. Ce-Fe mixed oxide catalyst having a Fe/Ce ratio of 1/1 (2Fe-2Ce) showed complete conversion and very high sulfur selectivity in the temperature range of 200-300 °C. Sulfur selectivity and stability of iron oxide catalyst was significantly enhanced by the incorporation of cerium into the catalyst structure. It was concluded that incorporation of ceria into the catalyst structure significantly improved the redox ability of the catalyst. Introduction Environmental regulations for the sulfur content of flue gases are becoming more and more strict. Among the sulfur containing gases, hydrogen sulfide (H2S) emissions are especially important in gasification processes of fossil fuels, in integrated gasificationpower generation systems, during synthesis gas and hydrogen production, and in petroleum desulfurization processes. The Claus process is a commonly used process to convert H2S from process gases to elemental sulfur, which is a harmless and valuable product. In the first step of the conventional Claus process, H2S is partially converted to SO2 by thermal oxidation at high temperatures, and in the second step unconverted H2S is reacted with SO2 in a catalytic converter to produce elemental sulfur. Conversion of H2S to elemental sulfur is about 94-98% owing to thermodynamic limitations in the catalytic step of the Claus process.1 To achieve a permissible level of sulfur emission in a more intensified unit, the researchers have focused on a single step selective oxidation of H2S to elemental sulfur (R1). 1 1 H2S + O2 f Sn + H2O(200°C e T e 350°C) 2 n (R1) Although direct oxidation of H2S to elemental sulfur is irreversible and does not have any equilibrium limitation, side reactions, such as oxidation of the produced sulfur and complete oxidation of H2S to SO2 may cause reduction of sulfur yield in a single step oxidation process. Increase of sulfur yield in such a selective oxidation process strongly depends upon the development of new active and selective catalysts. Iron-, titanium-, chromium-, and vanadium-based catalysts were reported to have high potential in selective oxidation of H2S to elemental sulfur. Because of the poisoning effect of water for titanium-based catalysts, its removal from the tail gas was required before the H2S oxidation reaction.2 Major drawback of chromium containing catalysts was reported as their toxic nature.3 Vanadium-based catalysts are known to have high activity and good selectivity in selective oxidation reactions of hydrocarbons. In our earlier studies,4,5 it was shown that the oxidation state of vanadium in the catalysts structure had very important effect on the selectivity of H2S oxidation to elemental sulfur. Catalysts containing partially reduced vanadium in V4+ * To whom correspondence should be addressed. E-mail: syasyerli@ gazi.edu.tr. Tel.: +90 312 2317400/2509.
state were highly selective to elemental sulfur, while some amount of SO2 formation was reported with the catalysts containing V5+. A redox cycle of vanadium is proposed in the selective oxidation processes. Iron oxide is one of the oldest catalysts tested in selective oxidation of H2S. It is also a cheap catalyst. In the literature, it was reported that iron oxide had relatively high activity for H2S oxidation. However, its selectivity for elemental sulfur was reported to be quite low, because of requiring an excess amount of oxygen.6,7 Also, its fast deactivation is a major limitation. A number of researchers tried to modify iron oxide catalysts to obtain more selective and more stable catalysts. For example, iron-antimony, iron-tin;8 nickel-iron phosphates,9 β-SiC supported iron based catalyst,10 iron oxide,11 and silica supported Fe-containing catalysts12 were tried in the literature. The high temperature superclaus process based on alumina or silica supported Fe and Fe/Cr catalyst works above the sulfur dew point (>180 °C) to prevent the loss of catalytic activity due to the condensation of sulfur.10 Also, use of excess oxygen is recommended to minimize catalyst deactivation. In recent years, cerium oxide has been considered as an attractive catalyst for the oxidation reactions, because of its good redox properties and high mobility of the capping oxygen. Matsumoto13 reported that the following reversible reaction describes storing and releasing oxygen from CeO2. CeO2 T CeO2(1-x) + xO2 where 0 e x e 0.25 14,15
(R2)
In the study of Zeng et al., it was shown that CeO2 was a good candidate for high temperature H2S removal. In our recent work,5 it was also shown that a Ce-V mixed oxide catalyst containing equimolar amounts of cerium and vanadium (CeVO4) gave very high sulfur yields in the selective oxidation of H2S. Also, our recent studies with cerium incorporated manganese adsorbents prepared for high temperature H2S removal indicated that the major role of cerium oxide in these adsorbents was to improve the regenerability of the adsorbent and to form elemental sulfur during the regeneration step.16 The objective of the present study is to improve the selectivity and catalytic performance of iron oxide catalyst by incorporating cerium into catalyst structure. In this study, iron oxide and Fe-Ce mixed oxide catalysts having different Fe/Ce ratios in their structures were synthesized by the complexation method and catalytic performances of these materials were tested in
10.1021/ie8017059 CCC: $40.75 2009 American Chemical Society Published on Web 04/23/2009
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Table 1. Catalysts Physical Properties and Crystalline Phases Detected by the XRD Fe/Ce molar ratio(in solution)
BET surface area (m2/g)
porosity
crystallitea size, nm
crystalline phases
1/0 3/1 1/1
12 52 60
0.43 0.39 0.65
31.65 5.07 5.60
R-Fe2O3 amorphous + CeO2 CeO2 + amorphous
Fe-O 3Fe-1Ce 2Fe-2Ce a
Evaluated from: t ) Kλ/(B cos θ) (Sherrer equation; K ) 0.8929).
was kept as 1%. In our earlier study,18 sulfidation experiments were repeated at different flow rates (space times). Results of these experiments had shown that the change of flow rate did not have any effect on reaction rate parameters, indicating negligible external mass transfer resistance. Mass flow controllers are used for each gas stream. These flow rates were also confirmed by soap flow meters measurements. The effluent stream was continuously analyzed by an FT-IR (Perkin-Elmer, containing a flow gas cell) spectrophotometer connected online to the exit of the reactor. A sulfur condenser was placed between the reactor and the FT-IR, to collect elemental sulfur. Temperature of the sulfur condenser was kept at 100 °C to allow condensation of sulfur. Details of the analysis procedure and the reaction system were similar to the ones reported in the earlier study of Yasyerli et al.18 Instantaneous fractional conversion of H2S and sulfur selectivity were defined as Figure 1. XRD patterns of iron oxide and cerium incorporated iron oxide catalysts: (a) Fe-O; (b) 3Fe-1Ce; (c) 2Fe-2Ce.
selective oxidation of H2S to elemental sulfur at different O2/ H2S ratios and reaction temperatures.
fractional conversion ) S selectivity )
(H2S)in - (H2S)out (H2S)in
(H2S)in - (H2S)out - (SO2)out (H2S)in - (H2S)out
(E1) (E2)
Experimental Details
Results and Discussions
Catalyst Preparation and Characterization. In this study, an iron oxide catalyst and two Fe-Ce mixed oxide catalysts containing different ratios of Fe/Ce (with atomic ratios of 3/1 and 2/2) were synthesized by the complexation method. Details of this method are described in the earlier publications.17-19 In the synthesis procedure, iron(III)-nitrate-9-hydrate and cerium(III) nitrate hexahydrate were used as the iron and cerium sources, respectively. Citric acid monohydrate was used as the complexation agent. The solution, containing equimolar quantities of the metal ions and citric acid, was evaporated by continuous stirring in a temperature range between 60 and 70 °C for about 24 h, during which viscosity of the solution was increased. This viscous solution was then further dehydrated in an oven at about 75 °C. The solid foam thus formed was then calcined at 550 °C, for 8 h.20 At the calcination step which was carried out in oxidation atmosphere, Ce3+ might be oxidized to Ce4+ and CeO2 will be produced. The catalytic materials synthesized in this work were characterized by BET (Quanta chrome), XRD (Philips PW 1840 employing a Cu KR radiation source), and SEM (JEOL), mercury porosimeter (Quanta chrome), TPR (Quanta chrome), EDS (JEOL, JSM-6400), and XPS (SPECS) measurements. Catalytic Tests. H2S selective oxidation experiments were carried out in a fixed-bed quartz reactor having a 0.6 cm inside diameter. The reactor packed with 0.2 g of catalyst was prepared by putting quartz wool supports from both ends and then the reactor was placed into a tubular furnace. Experiments were carried out with O2/H2S ratios, ranging between 0 and 2, in a temperature range of 200-300 °C. Reactions carried out in the presence and absence of oxygen are called as oxidation and sulfidation runs, respectively. In all experiments, total flow rate of the gas mixture, balancing with helium, was 100 cm3/min (measured at 25 °C; GHSV: 21000 h-1) and H2S concentration
Characterization of the Fresh Catalysts. BET surface area values, porosities, and detected crystalline phases of synthesized materials are given in Table 1. BET surface area values and the porosities of metal oxide catalysts increase with a decrease of Fe/Ce mole ratio in the catalyst structure. Results showed that the catalyst prepared with an equimolar mixture of Fe and Ce had the highest surface area and porosity. XRD patterns of the catalysts prepared in this work are given in Figure 1. XRD patterns of pure Fe-O catalyst showed that this metal oxide is in Fe2O3 crystalline form. The 3Fe-1Ce catalyst is found to be highly amorphous with some CeO2 (cerianite) phase. 2Fe-2Ce mixed oxide catalyst also has the characteristic peaks of CeO2. There is no peak corresponding to iron oxide in the XRD patterns of both of the mixed oxide catalysts containing cerium. These results indicated that iron oxide in the structure of mixed metal oxides had small particles (
