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Selective Oxidation of Hydrogen Sulfide to Sulfur over LaVO4 Catalyst: Promotional Effect of Antimony Oxide Addition Kuo-Tseng Li* and Chen-Hwa Huang Department of Chemical Engineering, Tunghai UniVersity, Taichung, Taiwan, ROC
The promoting effect of antimony oxide addition on the catalytic performances of LaVO4 catalyst for catalyzing the selective oxidation of hydrogen sulfide to sulfur was studied. The multiphase La-V-Sb catalysts were characterized by temperature programmed reduction, X-ray diffraction, and Brunauer-Emmett-Teller techniques. A synergistic effect between the mixed metal oxides was observed which strongly increased the catalyst selectivity and the sulfur yield (the maximum yield increased from 88.5% for LaVO4 to 100% for La-V-Sb when weight hourly space velocity was 0.0244 mol of H2S/((g of catalyst)/h)). The best activity was observed for the La-V-Sb catalyst with V/Sb atomic ratio ) 1.0. The improvements of catalytic performances were partly ascribed to the formation of SbVO4 species. The La-V-Sb catalysts showed a much wider temperature window for obtaining 100% sulfur yield than V-Sb and Mg-V-Sb catalysts. This was explained in terms of the better isolation of the active sites (caused by the large La3+ size) and the easier desorption of the elemental sulfur (caused by the lanthanum basicity). Introduction Hydrogen sulfide is a byproduct of many industrial operations, such as hydrodesulfurization of crude oil, coal, and natural gas.1 It is usually converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. Due to thermodynamic limitation, the sulfur recovery offered by a Claus process is limited in practice to about 97%. Because of the strict air pollution regulations, a variety of Claus tail gas treatment (TGT) processes have been developed to increase the total sulfur-recovery efficiency.2 Conventional Claus TGT processes are wet processes, which use liquid to absorb the remaining hydrogen sulfide in the tail gas. Removing the last percentages of H2S by means of these conventional Claus TGT processes is relatively expensive. Comprimo’s Super-Claus process3 and Mobil’s direct-oxidation process4 are two dry Claus TGT processes. Both comprise a step of recovering elemental sulfur from Claus tail gas by selective oxidation of hydrogen sulfide using the following catalytic reaction:
H2S + 1/2O2 f (1/n)Sn + H2O (n ) 6 f 8)
(1)
The catalyst used by Comprimo’s Super-Claus process is R-alumina supported iron oxide/chromium oxide, and the catalyst used by Mobil’s direct-oxidation process is a TiO2based catalyst. Research continues to obtain a more active catalyst and to exclude chromium oxide as an additive in the active phase because of the toxic nature of chromium-containing materials.5,6 Reaction 1 is irreversible so the maximum recovery of hydrogen sulfide can be obtained. However, SO2 can also be generated simultaneously due to the following side reactions:7
H2S + 3/2O2 f SO2 + H2O
(2)
(1/n)Sn + O2 f SO2
(3)
(3/n)Sn + 2H2O S 2H2S + SO2
(4)
Therefore, an optimum catalyst should be able to maximize the sulfur yield and to minimize the sulfur dioxide generation.
Earlier work from our laboratory showed that several rare earth orthovanadates had better sulfur yield than vanadium oxide alone for the selective oxidation of hydrogen sulfide,8 and their sulfur yield decreased in the order CeVO4 > YVO4 > SmVO4 > LaVO4. Sulfur yields of these rare earth orthovanadates were not high enough and their sulfur selectivities were sensitive to temperature change. It is therefore desired to develop catalysts with a higher sulfur yield and with a wider range of operation temperature, which can easily withstand inadvertent plant upsets. In this work, selective oxidation of hydrogen sulfide to sulfur was carried out over LaVO4 with antimony oxide as an additive. It was found that the La-V-Sb catalysts exhibited 100% sulfur yield in a wide range of temperature, which was much better than that of LaVO4 alone and also exceeded those of V-Sb and Mg-V-Sb catalysts reported before.9,10 Experimental Section Catalyst Preparation and Characterization. LaVO4 was prepared by citrate method.11,12 The starting materials were La2O3 (ACROS Organics, Belgium) and NH4VO3 (Showa Chemicals, Tokyo). The main preparation steps were as follows: (i) preparation of an aqueous solution of La3+ and V5+ with an atomic ratio of La/V ) 1/1; (ii) addition of citric acid (from Showa) such that the molar number of anions (COO-) was equal to that of cations (La3+ and V5+); (iii) evaporation of the obtained solution at 80 °C to obtain a solid; (iv) decomposition of the obtained solid at 400 °C for 24 h and finally calcination of the sample at 550 °C for 6 h. For the purpose of comparisons, pure V2O5 was prepared by the same procedure listed above, except that La3+ was not added in step i. R-Sb2O4 was prepared by calcination of Sb2O3 (RiedeldeHaen, Germany) in air at 500 °C for 20 h. La-V-Sb (or V-Sb) catalysts were prepared by mechanically mixing R-Sb2O4 with LaVO4 (or V2O5) in n-pentane, followed by evaporation in a vacuum (at 60 °C), drying (at 80 °C for 12 h), and calcination (at 600 °C for 144 h). Crystal structures of the catalysts were analyzed by X-ray diffraction crystallography on a Shimadzu XRD-6000 diffractometer with Cu KR radiation. Catalyst surface areas were determined by nitrogen adsorption with a Micromeritics Brunauer-
10.1021/ie060384n CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2006
Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006 7097 Table 1. Catalytic Properties of La-V-Sb Catalysts for Selective Oxidation of Hydrogen Sulfide temp (°C) 200 210 220 230 240 250 260 270 Figure 1. Effect of reaction temperature on sulfur yield for LaVO4 alone (curve a) and for La-V-Sb samples with Sb/V atomic ratio ) 0.56 (curve b), 1.0 (curve c), and 1.67 (curve d). The weight hourly space velocity was 0.0244 mol of H2S/((g of catalyst)/h).
Emmett-Teller (BET) surface area analyzer (Model Gemini). Catalyst reducibility was studied with temperature-programmed reduction (TPR) method, which was conducted using 0.15 g of catalyst in a stream of 10% hydrogen in argon and with a heating rate of 10 °C/min. The microscopic aspect of the catalysts was examined under a scanning electron microscope (TOPCON ABT-32). Reaction Studies. Selective oxidation of hydrogen sulfide to elemental sulfur was carried out in a continuous flow reactor containing 0.2 g of catalyst. Before the catalytic studies, catalysts were pretreated in an environment of 9 vol. % hydrogen sulfide at 250 °C for 8 h. Reaction conditions were as follows: H2S/ O2/N2 (diluting gas) ) 1/5/94 (vol.); total feed flow rate ) 200 mL/min; total pressure ) 1 atm, which corresponded to the weight hourly space velocity of 0.0244 mol of H2S/((g of catalyst)/h); reaction temperature ) 200-340 °C. The data at 200 °C were taken 14 h after the catalyst pretreatment stage, and the data at 340 °C were taken at 28 h after the catalyst pretreatment stage. After the data at 340 °C were taken, the reactor temperature was decreased to 200 °C, and the data were taken at this temperature again. Experimental results confirmed good reproducibility was achieved when the same reaction temperature was used, which indicated that the reaction reached the steady state during the tests of catalytic properties. The gas products were dried and analyzed by a gas chromatograph using a 9 m long Porapak Q column. Reaction conversion was defined as (moles of hydrogen sulfide reacted)/ (moles of hydrogen sulfide fed) × 100%. Sulfur selectivity was calculated as (moles of hydrogen sulfide reacted - moles of sulfur dioxide produced)/(moles of hydrogen sulfide reacted) × 100%. Sulfur yield was defined as conversion times selectivity. Results and Discussion Oxidation of Hydrogen Sulfide. Three La-V-Sb catalysts were used for catalyzing the selective oxidation of hydrogen sulfide to sulfur, which had Sb/V atomic ratios of 0.56, 1.0, and 1.67. For comparisons, LaVO4 alone and R-Sb2O4 alone were also used for catalyzing the same reaction. Figure 1 presents sulfur yield as a function of reaction temperature for LaVO4 alone (curve a) and for three La-VSb samples (curves b-d). The maximum sulfur yield was 88.5% for LaVO4 alone and reached 100% for La-V-Sb catalysts.
280 290 300 310 320
conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%) conversion (%) selectivity (%)
LaVO4 alone
Sb/La ) 0.56
Sb/La ) 1.0
Sb/La ) 1.67
86.9 100 88.5 100 93.0 90.4 94.4 66.6 96.3 49.9 97.8 41.0 100 36.4 100 11.9
61.2 100 63.9 100 81.0 100 87.2 100 93.7 100 100 100 100 100 100 100 100 100 100 100 100 100 100 94.0 100 56.5
71.6 100 84.7 100 93.4 100 95.2 100 98.4 100 100 100 100 100 100 100 100 100 100 100 100 100 100 80.8
45.3 100 61.0 100 76.5 100 85.3 100 92.2 100 95.4 100 97.5 100 98.5 100 98.8 100 100 100 100 100 100 100 100 30.8
Sb2O4 alone
23.6 100
36.0 100 39.9 100 55.6 93.9 69.0 90.1
Table 2. Catalyst Surface Area surface area (m2/g) catalyst
before reaction
after reaction
LaVO4 alone La-V-Sb (Sb/V atomic ratio ) 0.56) La-V-Sb (Sb/V atomic ratio ) 1.0) La-V-Sb (Sb/V atomic ratio ) 1.67) R-Sb2O4 alone
6.34 1.43 1.46 2.3 0.50
1.82 1.28 0.83 2.89 0.42
To minimize the emissions of both H2S and SO2, the maximum sulfur yield is the most important criterion for evaluating catalyst performance in the selective oxidation of hydrogen sulfide because sulfur yield ) H2S conversion × sulfur selectivity. Hence, the results in Figure 1 indicate that the catalytic performances of La-V-Sb samples were superior to that of LaVO4 alone, and La-V-Sb catalysts exhibited strong synergistic behavior in the catalytic performance for hydrogen sulfide oxidation. The most important characteristic of the La-V-Sb samples is shown in curves b and c, which exhibit 100% sulfur yield over a very wide temperature range (250-300 °C). This characteristic makes these two La-V-Sb samples (with Sb/V atomic ratio ) 0.56 and 1) very suitable for industrial use to catalyze the oxidation of hydrogen sulfide to sulfur, because they can easily withstand inadvertent plant upsets. Table 1 presents H2S conversion and sulfur selectivity for the selective oxidation of hydrogen sulfide over LaVO4 alone, three La-V-Sb samples, and Sb2O4 alone. Among these catalysts, LaVO4 alone had the highest conversion. On LaVO4, H2S should be activated by the V5+ site of LaVO4 because it is known that only vanadium was reducible in LaVO4.13 Due to its resonance structure (V5+dO T 4+V•-O•), the oxygen of this site has a partial radical character14 and therefore can easily abstract hydrogen from an approaching H2S molecule, thereby activating the molecule. It is also known that V5+ is able to activate and dehydrogenate propane to propene15 and O2- can react with H2S to form HS-HO2.16 In Table 1, the high conversion of LaVO4 alone should be due to its high surface area. As shown in Table 2, the surface area of the used LaVO4 was 1.82 m2/g, which was higher than
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Figure 2. Effect of Sb/V atomic ratio on the reaction rate per unit area (reaction temperature ) 200 °C).
Figure 3. Relationships between sulfur yield and reaction temperature for (a) V2O5 alone, (b) V-Sb catalyst with Sb2O4/V2O5 weight ratio ) 1/3. The weight hourly space velocity was 0.0244 mol of H2S/((g of catalyst)/ h).
the surface areas of the used La-V-Sb catalysts with Sb/V atomic ratios of 0.56 and 1.0. Figure 2 compares the reaction rates per unit area for four catalysts (the rates were calculated from the conversion data in Table 1 and the used catalyst surface area data in Table 2), which shows that La-V-Sb catalyst with Sb/V atomic ratio of 1.0 had the highest activity (higher than LaVO4 alone). Sb2O4 alone (shown in the last column of Table 1) exhibited very low activity for catalyzing the oxidation of hydrogen sulfide to sulfur, which had H2S conversion of only 40% at the reaction temperature of 300 °C. The low activity of Sb2O4 alone should be mainly due to its low surface area (0.42 m2/g, as shown in Table 2) and due to the fact that Sb2O4 could not be easily reoxidized by gaseous oxygen to Sb5+ oxide.17 To study the role of lanthanum in La-V-Sb catalysts, V-Sb binary oxide with R-Sb2O4/V2O5 weight ratio ) 1/3 (the weight ratio was the same as that in the La-V-Sb sample with Sb/V atomic ratio of 0.56) was prepared followed the same procedure used for preparing La-V-Sb catalysts and was used to catalyze the oxidation of H2S to sulfur. Figure 3 shows the sulfur yield as a function of reaction temperature for (a) V2O5 alone and (b) V-Sb. The maximum sulfur yield was 99% for the V-Sb catalyst (shown in curve b of Figure 3), which was significantly higher than that of V2O5 alone (V2O5 alone had a maximum sulfur yield of 80%, as shown in curve a of Figure 3). The results indicate that R-Sb2O4 addition into V2O5 catalyst also improved the catalyst selectivity and the maximum sulfur yield. The difference in catalytic performances between the La-V-Sb catalyst (curve b in Figure 1) and the corresponding V-Sb catalyst (curve b in Figure 3) is very significant. The sulfur
yield of the V-Sb catalyst decreased rapidly after 230 °C, but the sulfur yield of the corresponding La-V-Sb catalyst remained constant at 100% in the temperature range of 250300 °C. The results suggest that the performance of the LaV-Sb catalyst was much less sensitive to the temperature change compared to that of the V-Sb catalyst. That is, La improved the sulfur selectivity and thermal stability, and the La-V-Sb catalyst had a much wider operating temperature range than the V-Sb catalyst. Grabowski et al.15 studied the effect of alkali metal additives to V2O5/TiO2 catalysts on the catalytic performances in oxidative dehydrogenation of propane and found that the yield and selectivity to propylene increased in the order: RbVTi > KVTi > LiKTi > VTi. Their effect was ascribed to the decrease in the heat of the propylene adsorption, which was due to the increase in basicity and the decrease in acidity (V5+ is a strong acid site) on the promoted catalysts. It is known that lanthanum oxide had high basicity18 and the addition of La atom to γ-Al2O3 poisoned strong Lewis acid sites on γ-Al2O3 and converted them to new Lewis acid sites with medium strength.19 Therefore, the better sulfur yield and selectivity obtained here for La-V-Sb (compared to V-Sb) can be ascribed to the decrease in the heat of sulfur adsorption (i.e., sulfur desorption became easier and therefore less sulfur was oxidized further to sulfur dioxide), which was caused by the increase in basicity and the decrease in acidity on the La-V-Sb catalysts. Our earlier work10 on Mg-V-Sb samples showed that the best Mg-V-Sb catalyst (MgV2O6 + R-Sb2O4) exhibited 100% sulfur yield in a very narrow temperature range (less than 10 °C wide). Therefore, the La-V-Sb catalysts studied here are superior to the Mg-V-Sb catalysts studied earlier because the La-V-Sb catalysts could exhibit 100% sulfur yield in a much wider temperature range (60 °C wide). The concept of site isolation, developed in early 1960s,20 can be used here to explain the better selectivity of La-V-Sb catalyst (compared to Mg-V-Sb catalyst). For hydrocarbon oxidation, it is known that vicinal V moieties would lead to undesirable overoxidation of the hydrocarbons and result in waste formation.21 It is therefore necessary to add an additional element to isolate the active phase in order to achieve the desirable selectivity. The ionic radius of La3+ is 1.061Å,13 which is significantly larger than that of Mg2+ (ionic radius of Mg2+ ) 0.72 Å 22). On the basis of the site isolation hypothesis, LaV-Sb catalysts should have better site isolation capability than Mg-V-Sb catalyst because La3+ has larger size than Mg2+, which results in the better performances for the La-V-Sb catalysts. Temperature-Programmed Reduction Studies. La-V-Sb catalyst reducibility was measured using a temperatureprogrammed reduction method with hydrogen as the reductant, and the results for the catalyst with Sb/V atomic ratio of 1.0 are shown in Figure 4 (curve b). For comparisons, TPR profiles of LaVO4 and the corresponding V-Sb catalyst (with Sb/V atomic ratio of 1.0) are also shown in Figure 4 (curves a and c, respectively). The TPR profile for LaVO4 alone (profile a in Figure 4) exhibits two reduction peaks with peak temperatures at 600 (small peak) and 840 °C (much larger peak), respectively, which is similar to that reported by Varma et al.23 They also observed two reduction peaks (a small peak at 550 °C and a much larger peak at 800 °C) for LaVO4. The major peak in Figure 4a should be due to the reduction of La-O-V (i.e., the reduction of VO43tetrahedrons in LaVO4), while the small peak might be due to the reduction of the residual V-O-V.
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Figure 4. TPR profiles of (a) LaVO4 alone, (b) the La-V-Sb sample with Sb/V atomic ratio of 1.0, and (c) the V-Sb sample with Sb/V atomic ratio of 1.0.
The TPR profile of the La-V-Sb catalyst (curve b) is clearly the superimposition of the TPR curves of LaVO4 (curve a) and VSbO4 (curve c). Two reduction peaks at 600 and 760 °C were obtained in curve c, indicating two kinds of redox centers. It has been shown that SbVO4 has two redox couples, Sb5+/Sb3+ and V4+/V3+.24 For propane ammoxidation on vanadium antimonate, it was observed that both vanadium and antimony on the catalyst surface were reduced during the catalytic tests.17 Barbaro et al.25 carried out a H2-TPR study (2% H2 in nitrogen, flow rate ) 120 cm3/min, and temperature rising rate ) 10 °C/min) for a SbVO4 sample prepared via a solid-state reaction method and observed that the TPR spectrum had two distinct peaks at 666 and 714 °C. The difference in TPR peak temperatures between Figure 4c and the results of Barbaro et al. should be due to the difference in the conditions used for TPR. It is also known that TPR profiles depend strongly on the reductant used. Casagrande et al.26 found that NH3-TPR peak temperature was 330 °C lower than H2-TPR peak temperature for a TiO2-supported V2O5 catalyst, probably due to the fact that the N-H bond is weaker than the H-H bond. The bond strength of H-S is also weaker than the H-H bond, and it was proposed that proton transfer from H2S to O2- had a small or negligible energy barrier.16 Therefore, the temperature needed for catalyst reduction by H2S (shown in Figure 1) is much lower than the temperature needed for catalyst reduction by H2 (shown in Figure 4). For the ammoxiation of propane to acrylonitrile, it was proposed27 that V5+ was able to activate and dehydrogenate propane to propene, but the total oxidation of intermediate and acrylonitrile also occurred on the V5+ site, due to a too strong bonding of the adsorbed hydrocarbon. Therefore, the strong acid sites as V5+ have to be avoided, and the formation of the catalytically active and selective V3+/V4+-Sb-O phase is desired. On the basis of the same principle and the TPR results shown in Figure 4, it is also suggested that the strong acid sites as V5+ should be avoided for H2S oxidation (because of a too strong bonding of the elemental sulfur on V5+), and the more selective V3+/V4+-Sb-O phase is desired to improve the sulfur selectivity. XRD Studies. Figure 5 shows the powder X-ray diffraction spectra of the fresh catalysts for (a) LaVO4 alone, (b) the LaV-Sb sample with Sb/V atomic ratio ) 0.56, and (c) the LaV-Sb sample with Sb/V atomic ratio ) 1.0. Pattern a indicates that LaVO4 was in its monoclinic form because all the reflections in pattern a are consistent with those of monoclinic LaVO4
Figure 5. X-ray diffraction patterns of the fresh catalysts for (a) LaVO4 alone, (b) the La-V-Sb sample with Sb/V atomic ratio ) 0.56, and (c) La-V-Sb sample with Sb/V atomic ratio ) 1.0. (/: SbVO4).
(JCPDS No. 50-0367).28 For La-V-Sb samples (patterns b and c), the characteristic peaks indicate that the LaVO4 phase was accompanied by R-Sb2O4 (with a major peak at 2θ ) 29°), and the formation of a new phase was detected. The appearance of the new peak at 2θ ) 27.4° (peak marked with /) indicates the formation of a new compoundsSbVO4. On the basis of the XRD results, the coexistence of SbVO4 and R-Sb2O4 in the LaV-Sb catalysts might also contribute to the increase of sulfur yield, as reported in Figure 1. For propane ammoxidation, the V-antimonate based catalysts have been thoroughly studied structurally, kinetically, and mechanically by several research groups.29 Electronically, the addition of Sb lowers the oxidation state of vanadium; structurally, it interposes itself between V-O-V chains and thus helps to isolate V-O moieties from each other (site isolation at work). When excess antimony is present, the presence of suprasurface antimony sites can be postulated on top of the SbVO4 active phase and it was proposed that the coexistence of Sb2O4 with SbVO4 could decrease the extent of surface reduction,17 and thus improved the product selectivity. The result is more selective catalyst. The previously proposed theories for hydrocarbon oxidation can also be applied to explain the results obtained here for H2S oxidation. That is, the sulfur selectivity improvement with Sb addition was due to the decrease of the vanadium oxidation state (i.e., the formation of VSbO4), the better isolation of the active sites, and the decrease of the extent of surface reduction. Conclusions La-V-Sb catalysts were prepared by solid-state reaction between lanthanum orthovanadate and antimony oxide. The catalysts were studied for the selective oxidation of hydrogen sulfide to elemental sulfur. A synergism in catalytic performances was observed for the ternary oxide of lanthanumvanadium-antimony, which significantly improved catalyst selectivity and sulfur yield. The best performance was achieved for the catalyst with Sb/V atomic ratio ) 1, which had better activity than LaVO4 alone. The La-V-Sb catalysts had a wide “temperature window” for obtaining 100% sulfur yield when weight hourly space velocity was 0.0244 mol of H2S/((g of catalyst)/h), which was much wider than those of Mg-V-Sb
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and V-Sb catalysts. The better performances of the La-VSb catalysts were ascribed to the basicity and the larger size of the La3+ ion. TPR and XRD results indicated that SbVO4 was formed in the La-V-Sb catalysts. Therefore, Sb addition decreased the vanadium oxidation state and improved the site isolation effect, which resulted in the better sulfur selectivity. Acknowledgment The authors gratefully acknowledge the National Science Council of the Republic of China for financial support (Grant No. NSC-90-2214-E-029-002). Literature Cited (1) Duncan, M. P. Sulfur Compounds. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant, M., Eds.; Wiley: New York, 1997; Vol. 23, p 276. (2) Capone, M. Sulfur Removal and Recovery. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant, M., Eds.; Wiley: New York, 1997; Vol. 23, p 432. (3) Lagas, J. A.; Borsboom, J.; Berben, P. H. Selective Oxidation Catalyst Improve Claus Process. Oil Gas J. 1988, 86, 68. (4) Ketter, R.; Liermann, N. New Claus Tail-gas Process Proved in German Operation. Oil Gas J. 1988, 86, 63. (5) Pieplu, A.; Saur O.; Lavalley, J. C. Claus Catalysis and H2S Selective Oxidation. Catal. ReV.-Sci. Eng. 1998, 40, 409. (6) Billie, J. P.; Gary, W. L. Chromium Compounds. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant M., Eds.; Wiley: New York, 1993; Vol. 6, p 284. (7) Terorde, R. F. A. A.; Van den Brink, P. J.; Visser, L. M.; Van Dillen A. J.; Geus, J. W. Selective Oxidation of Hydrogen Sulfide to Elemental Sulfur Using Iron Oxide Catalysts on Various Supports. Catal. Today 1993, 17, 217. (8) Li, K. T.; Chi, Z. H. Selective Oxidation of Hydrogen Sulfide on Rare Earth Orthovanadates and Magnesium Vanadates. Appl. Catal., A 2001, 206, 197. (9) Li, K. T.; Shyu, N. S. Catalytic Oxidation of Hydrogen Sulfide to Sulfur on Vanadium Antimonate. Ind. Eng. Chem. Res. 1997, 36, 1480. (10) Li, K. T.; Chi, Z. H. Effect of Antimony Oxide on Magnesium Vanadates for the Selective Oxidation of Hydrogen Sulfide to Sulfur. Appl. Catal., B 2001, 31, 173. (11) Courty, P.; Ajot, H.; Marcilly, C.; Delmon, B. Oxides, Mixed or in Solid-Solution, Highly Dispersed Obtained by Thermal-Decomposition of Amorphous Precursors. Powder Technol. 1973, 7, 21. (12) Au, C. T.; Zhang, W. D.; Wan, H. L. Preparation and Characterization of Rare Earth Orthovanadates for Propane Oxidative Dehydrogenation. Catal. Lett. 1996, 37, 241. (13) Varma, S.; Wani, B. N.; Gupta, N. M. Synthesis, Characterization, and Redox Behavior of Mixed Orthovanadates La1-xCexVO4. Mater. Res. Bull. 2002, 37, 2117.
(14) Grasselli, R. K.; Buttrey, D. J.; DeSanto, P.; Burrington, J. D.; Lugmair, C. G.; Volpe, A. F.; Weingand, T. Active Centers in Mo-V-NbTe-O-X (Amm)oxidation Catalysts. Catal. Toady 2004, 91-92, 251. (15) Grabowski, R.; Grzybowska, B.; Kozlowska, A.; Stoczynski, J.; Wcisto, K.; Barbaux, Y. Effect of Alkali Metals Additives to V2O5/TiO2 Catalyst on Physicochemical Properties and Catalytic Performance in Oxidative Dehydrogenation of Propane. Top. Catal. 1996, 3, 277. (16) Bell, A. J.; Wright, T. G. Experimental and Theoretical Studies on the Complex formed between H2S and O2-. J. Phys. Chem. A 2004, 108, 10486. (17) Centi, G.; Perathoner, S. Modification of the Surface Reactivity of Vanadium Antimonate Catalysts during Catalytic Propane Ammoxidation. Appl. Catal., A 1995, 124, 317. (18) Kus, S.; Otremba, M.; Taniewski, M. The Catalytic Performance in Oxidative Coupling of Methane and the Surface Basicity of La2O3, Nd2O3, ZrO2 and Nb2O5. Fuel 2003, 82, 1331. (19) Yamamoto, T.; Hastui, T.; Matsuyama, T.; Tanaka, T.; Funabiki, T. Structures and Acid-Base Properties of La/Al2O3sRole of La Addition to Enhance Thermal Stability of γ-Al2O3. Chem. Mater. 2003, 15, 4830. (20) Callahan, J. L.; Grasselli, R. K. A Selective Factor in Vapor-Phase Hydrocarbon Oxidation Catalysis. AIChE J. 1963, 9, 755. (21) Grasselli, R. K. Ammoxidation. In Handbook of Heterogeneous Catalysis; Ertl, G., Knoezinger H., Weitkamp, J., Eds.; Wiley: New York, 1997; Vol. 5, p 2302. (22) Shriver, D. F.; Arkins, P.; Langford, C. H. Inorganic Chemistry; Freeman: New York, 1994; p 37. (23) Varma, S.; Wani, B. N.; Gupta, N. M. Redox Behavior and Catalytic Activity of La-Fe-V-O Mixed Oxides. Appl. Catal., A 2003, 241, 341. (24) Hansen, S.; Stahl, K.; Nilsson, R.; Anderson, A. The CrystalStructures of Sb0.92V0.92O4, Determined by Neutron and Dual Wavelength X-Ray-Powder Diffraction. J. Solid State Chem. 1993, 102, 340. (25) Barbaro, A.; Larrondo, S.; Duhalde, S.; Amadeo, N. Effect of Titanium-Doping on the Properties of Vanadium Antimonate Catalysts. Appl. Catal., A 2000, 193, 277. (26) Casagrande, L.; Lietti, L.; Nova, I.; Forzatti, P.; Baiker, A. SCR of NO by NH3 over TiO2-Supported V2O5-MoO3 Catalysts: Reactivity and Redox Behavior. Appl. Catal., B 1999, 22, 63. (27) Zanthoff, H. W.; Schaefer, S.; Wolf, G.-U. Ammoxidation of Propane over Modified V-Sb-Al-OxidessThe Role of Basicity and Redox Properties of the Catalysts. Appl. Catal., A 1997, 164, 105. (28) Jia, C. J.; Sun, L. D.; You, L. P.; Jiang, X. C.; Luo, F.; Pang, Y. C.; Yan, C. H. Selective Synthesis of Monazite- and Zircon-Type LaVO4 Nanocrystals. J. Phys. Chem. B 2005, 109, 3284. (29) Grasselli, R. K. Advances and Future Trends in Selective Oxidation and Ammoxidation Catalysts. Catal. Today 1999, 49, 141.
ReceiVed for reView March 28, 2006 ReVised manuscript receiVed August 19, 2006 Accepted August 21, 2006 IE060384N
