Langmuir 2000, 16, 1109-1113
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Behavior of Lattice Oxygen in Mixtures of V2O5 and Bi2O3 Moon Young Shin, Ki Suk Chung, Dong Won Hwang, Jong Shik Chung, Young Gul Kim, and Jae Sung Lee* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja Dong, Pohang 790-784, South Korea Received March 30, 1999. In Final Form: October 4, 1999 The interaction between V2O5 and Bi2O3 and the evolution processes of lattice oxygen from their physical mixtures and a binary oxide (BiVO4) have been studied by thermal gravimetric analysis. The source of lattice oxygen was Bi2O3, and it was more easily evolved from the physical mixtures especially of a V:Bi ) 1:1 mole ratio than respective single metal oxides or the binary oxide. When the physical mixtures of V2O5-Bi2O3 are heated at low temperatures which do not cause phase transformation of oxides, they can evolve or absorb oxygen reversibly. The reoxidation of reduced oxides proceeded much faster than the evolution of oxygen. The effective contact between two oxide phases appeared to be an important factor for the synergy between two phases in the evolution of lattice oxygen.
Introduction Mixed metal oxide catalysts are extensively used in many industrial processes especially of selective oxidation. These catalysts are usually prepared by mixing the main active component (MoO3 or V2O5) with several kinds of metal oxide additives (Fe2O3, Cr2O3, CoO, P2O5, or Bi2O3). The mixed oxide systems derived from either bismuth oxide or vanadium oxide exhibit a variety of interesting physical and chemical properties. For example, bismuth molybdates and multicomponent oxides containing Bi2O3 and MoO3 are active for the selective oxidation and ammoxidations of alkenes and hydrocarbons.1-7 Bismuth vanadates,8 binary oxide of vanadium and magnesium,9,10 and mixed oxides of vanadium with molybdenum11-14 are active for the selective oxidation of hydrogen sulfide to elemental sulfur. Participation of lattice oxygen is widely recognized in many selective oxidation reactions. In these reactions catalysts give up their lattice oxygen to take part in the oxidation reaction, and the reduced catalysts can absorb oxygen from the gas phase and transform it into lattice oxygen again.15 These steps constitute the most important elements in the mechanism of selective oxidation, and * To whom all correspondence should be addressed. Tel.: +82562-279-2266. Fax.: +82-562-279-5799. E-mail:
[email protected]. (1) Thomas, J. M.; Jefferson, D. A.; Millward, G. R. JEOL News 1985, 23E, 7. (2) Jefferson, D. A.; Thomas, J. M.; Uppal, M. K.; Grasselli, R. K. J. Chem. Soc., Chem. Commun. 1983, 594. (3) Sekiya, T.; Tsuzukiand, A.; Torii, Y. Mater. Res. Bull. 1985, 20, 1383. (4) Weng, L. T.; Ma, S. Y.; Ruiz, P.; Delmon, B. J. Mol. Catal. 1990, 6199. (5) Tascon, J. M. D.; Grange, P.; Delmon, B. J. Catal. 1986, 97, 287. (6) Al’kaeva, E. M.; Andrushkevich, T. V.; Ovsitser, O. Y.; Sokolovskii, V. D. Catal. Today 1995, 24, 357. (7) Bettahar, M. M.; Costentin, G.; Savary, L.; Lavalley, J. C. Appl. Catal. A 1996, 145, 1. (8) Hass, R. H.; Ward, J. W. UP 4,444,741, 1984. (9) Bouyanov, R. A.; Tsyboulesky, A. M.; Zolotovsky, B. P.; Klevtsov, D. P.; Mourine, V. I. UP 5,369,076, 1994. (10) Bouyanov, R. A.; Tsyboulesky, A. M.; Zolotovsky, B. P.; Klevtsov, D. P.; Mourine, V. I. UP 5,512,258, 1996. (11) Li, K.-T.; Huang, M.-Y.; Cheng, W.-D. Ind. Eng. Chem. Res. 1996, 35, 621. (12) Li, K.-T.; Yen, C.-S.; Shyu, N.-S. Appl. Catal. A 1997, 156, 117. (13) Li, K.-T.; Shyu, N.-S. Ind. Eng. Chem. Res. 1997, 36, 1480. (14) Li, K.-T.; Huang, M.-Y.; Cheng, W.-D. UP 5,653,953, 1997. (15) Mars, P.; Van Krevelan, D. W. Chem. Eng. Sci. 1954, 3, 41.
understanding the behavior of lattice oxygen is usually critical to understanding the overall reaction. In the selective oxidation of propylene to acrolein over Mo-Bicontaining multicomponent oxides and the subsequent oxidation of acrolein to acrylic acid over Mo-V oxides, Al’kaeva et al. suggested that bulk diffusion of oxygen is a rate-limiting step at low temperatures.6 Bettahar et al. also observed easier evolution of lattice oxygen at higher temperatures and a corresponding increase in catalytic activities.7 In their propylene oxidation over Bi-Mo oxides, the reaction was controlled by the reoxidation step of the catalyst at low temperatures and by the reduction step at high temperatures. Ono et al. showed a sensitive effect of oxygen on propenal formation in propene oxidation, which is closely related to the extent of participation of lattice oxygen.16 Interaction between different metal oxides can promote the evolution of lattice oxygen even by physical mixing.4,5,17 Delmon et al. observed a synergy in catalytic activity, which could be correlated with characteristics of oxygen evolution in the mixtures of MoO3-R-Bi2O3, MoO3R-Sb2O4, and MoO3-BiPO4. 4,5 They suggested that the latter compounds of these pairs of oxides acted as donors delivering oxygen to active MoO3 in the reactions of selective oxidation of isobutene and N-ethylformamide dehydration. A promotional evolution of lattice oxygen was also suggested in the physical mixture of V2O5 and MoO3.17 In this paper, we have studied the behavior of lattice oxygen in physical mixtures and the binary oxide of V2O5Bi2O3, which are potential catalysts for selective oxidation of H2S to elemental sulfur.8 Thermal gravimetric analysis (TGA) was employed to monitor evolution of lattice oxygen and absorption of gas-phase oxygen, and X-ray diffraction (XRD) was employed to follow the change in the bulk solid phase during the process. Experimental Section Sample Preparation. Commercial vanadium oxide (V2O5; Junsei Chem. Co. Ltd., 99.0% purity) was used after treating at 450 °C for 4 h in air. Bismuth oxide (Bi2O3) was prepared by a precipitation method with a 30 wt % aqueous ammonia solution (16) Ono, T.; Nakajo, T.; Hironaka, T. J. Chem. Soc., Faraday Trans. 1990, 86 (24), 4077. (17) Zhan, X.-L.; Xie, K.; Yu, Q.-L.; Qi, X.-B. J. Catal. 1989, 119, 249.
10.1021/la990368d CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/1999
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Figure 1. Weight loss, ∆Wt (%), in TGA experiments with flowing He for single oxides: (a) Bi2O3, (b) V2O5. The flow rate was 50 mL/min, and the weight of each sample is ca. 15 mg. The temperature was raised at a rate of 5 °C/min.
Figure 2. Temperature-programmed desorption (TPD) of pure Bi2O3 measured by mass spectrometry with flowing He. The flow rate was 50 mL/min, and the weight of each sample is 10 mg. The temperature was raised at a rate of 5 °C/min.
of bismuth nitrate hexahydrate (Bi2(NO3)3‚6H2O; Aldrich) and calcined at 450 °C for 5 h in air. Physical mixtures of V2O5-Bi2O3 were prepared by simply mixing the two oxides in a mortar in different mole ratios. The binary oxide, BiVO4, was prepared according to the following procedure. Ammonium metavanadate and bismuth nitrate hexahydrate in the required mole ratio were put into a 3 wt % aqueous solution of nitric acid. The resulting mixture was evaporated to dryness, and the powder was further dried overnight at 110 °C, followed by calcination at 450 °C for 4 h in air. Characterization. X-ray diffraction patterns of samples were obtained using a M18XHF (MAC Science Co.), which utilizes Ni-filtered Cu KR radiation (λ ) 1.5405 Å). Diffraction patterns were obtained in the range of 2θ ) 10-90° with an X-ray gun operated at 40 kV and 200 mA, using a scan rate of 4°/min (2θ). Identification of a compound was accomplished by comparison of a measured spectrum with that in JCPDS files. Mass spectra of the species generated from the catalyst were obtained using a HP 5890II GC/HP 5972 MSD. Evolution of lattice oxygen and absorption of gas-phase oxygen were monitored by TGA, which was carried out with a Perkin-Elmer TGS-2 thermobalance under flowing gases of He and O2:He ) 1:1, respectively. The flow rate was 50 mL/min, and the typical sample loading was ca. 15 mg. The temperature was raised at a rate of 5 °C/min.
Figure 3. Structural change of Bi2O3 with heating treatment: (a) fresh Bi2O3, (b) Bi2O3 heated to 620 °C for 4 h with flowing He. Flow rate of He is 50 mL/min.
Results and Discussion Oxygen Evolution from the Single Metal Oxides. The XRD patterns of prepared V2O5, Bi2O3, and BiVO4 were measured, and the phases of V2O5 and Bi2O3 were found to be pure. It should be noted that binary oxide, BiVO4, does not contain any single phase of V2O5 or Bi2O3. Figure 1 shows the weight loss in TGA experiments in He as a function of temperature for pure V2O5 and Bi2O3. It can be seen that no weight loss is observed for pure V2O5 up to 650 °C and a small weight loss is observed at about 700 °C. The small amount of weight loss is probably due to volatilization of V2O5, which has a melting point of 690 °C. For pure Bi2O3, there are weight losses at about 470 and 540 °C. To know whether this weight loss is due to the loss of water originating from the remaining precursor, bismuth hydroxide, after incomplete calcination or because of the loss of oxygen from the sample, temperatureprogrammed desorption (TPD) was performed under He with mass spectroscopy, and the result is shown in Figure 2. No H2O was detected up to 620 °C, and all the weight loss was due to the oxygen generated from the catalyst
itself. The evolution pattern of oxygen corresponded well to the TGA result shown in Figure 1. Bi2O3 has four recognized solid phases (R, β, γ, and δ), and these phases undergo polymorphic reversible or irreversible transitions over the temperature range between 450 and 800 °C. There is an irreversible phase transition over the temperature range of 450-550 °C from β-Bi2O3 (metastable tetragonal) to R-Bi2O3 (monoclinic or pseudo-orthorombic).18 The same phase change is observed from XRD patterns shown in Figure 3. Fresh Bi2O3 composed of β and γ forms has changed to the R form upon treatment at 620 °C. Though there is no direct evidence that the oxygen evolution and the phase transition of Bi2O3 are indeed related except that they are taking place at approximately the same temperature, probably the metastable β form contains more oxygen than the more stable R form and two kinds of evolution processes might be involved. Oxygen Evolution from Physical Mixtures and the Binary Oxide. Figure 4 shows the weight loss in TGA (18) Samsonov, G. V. The Oxide Handbook, 2nd ed.; Plenum Data Company: New York, 1981; Chapter 1, p 16.
Behavior of Latice Oxygen in Mixtures of V2O5 and Bi3O3
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Figure 4. Weight loss, ∆Wt (%), in TGA experiments with flowing He for physical mixtures and binary oxide: (a) V2O5: Bi2O3 (1:4), (b) V2O5:Bi2O3 (1:1), (c) V2O5:Bi2O3 (4:1), (d) BiVO4. The numbers in parentheses denote mole ratios. The flow rate was 50 mL/min, and the weight of each sample is ca. 15 mg. The temperature was raised at a rate of 5 °C/min.
Figure 6. Structural changes of the V2O5:Bi2O3 (1:1) physical mixture at different temperatures: (a) 400, (b) 500, and (c) 650 °C. The sample was treated 2 h at each temperature in the flowing condition of He.
Figure 5. Derivative curves of weight loss, ∆Wt (%), in TGA experiments with flowing He for various samples: (a) V2O5, (b) Bi2O3, (c) BiVO4, (d) V2O5:Bi2O3 (1:4), (e) V2O5:Bi2O3 (1:1), (f) V2O5:Bi2O3 (4:1). The numbers in parentheses denote mole ratios.
experiments in He as a function of temperature for physical mixtures (V:Bi ) 1:4, 1:1, and 4:1 mole ratio) and the binary oxide, BiVO4. It can be seen that the oxygen in BiVO4 evolves above 500 °C (Figure 4d) but the amount of evolved oxygen is small compared with the physical mixtures. For the case of physical mixtures, the mixture of V:Bi ) 1:1 (Figure 4b) shows a substantially lower temperature of oxygen evolution compared to other mixtures and BiVO4. The mixture of V:Bi ) 1:1 generates oxygen from about 320 °C while the other mixtures start from about 420 °C. This trend is evident from the derivative curves of weight loss for all of the samples shown in Figure 5. The mixture of V:Bi ) 1:1 (Figure 5e) is the only sample which shows a substantial oxygen loss below 420 °C. BET surface areas of all samples are shown in Table 1 and compared with arithmetic areas for the mixtures. BiVO4 had a somewhat higher surface area than Bi2O3, and measured surface areas of mixtures were consistent
Table 1. Surface Areas of Single, Binary, and Mixture Oxides catalyst (mole ratio)
surface area (m2/g)
arithmetic surface area of mixtures (m2/g)
Bi2O3 V2O5 BiVO4 V2O5:Bi2O3 (1:4) V2O5:Bi2O3 (1:1) V2O5:Bi2O3 (4:1)
3.14 6.80 3.64 3.99 4.64 5.68
3.87 4.97 6.07
with calculated arithmetic areas. These indicate that grinding does not lead to an increase in the surface area of any component, and the behavior of oxygen evolution has little to do with the surface area of each component. The behavior of V2O5 in our V2O5-Bi2O3 system is different from that of V2O5 in the V2O5-MoO3 system reported by Zhan et al.17 They showed that the amount of absorbing or evolving oxygen depends strongly on V2O5 content in both physical mixtures and the binary oxide of V2O5MoO3. Phase Change During Oxygen Evolution. When the lattice oxygen is involved in the reaction, oxygen absorption from the gas phase of O2 is also important. The rates of oxygen evolution and absorption control the overall rate of the redox catalytic reaction. If the irreversible phase change of Bi2O3 above 440 °C serves as a negative factor in the oxygen absorption, this could also impose some restriction on the application of the physical mixture of V2O5 and Bi2O3 to a real catalytic oxidation reaction. To show the phase stability of the physical mixture as a function of temperature, the mixture of V:Bi ) 1:1 was heated for 2 h in He at 400, 500, and 650 °C, respectively. XRD patterns in Figure 6 following these heat treatments show that the physical mixture still maintains the phase of each oxide at 400 °C. Binary oxide, BiVO4, is formed in a small amount at 500 °C, and finally the main phase becomes BiVO4 at 650 °C. As shown in Figure 3, the transition from the tetragonal to the monoclinic phase of Bi2O3 was observed when Bi2O3 alone was heated. This is not the case for the physical mixtures. Instead, the stable binary phase of BiVO4 is formed at high temperatures. Figure 7 shows the weight change of a V:Bi ) 1:1 mixture in TGA experiments carried out in He up to 650 °C with increasing temperatures and subsequently in O2:He )
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Figure 7. Weight changes, ∆Wt (%), of the V2O5:Bi2O3 (1:1) physical mixture in a TGA experiment heated to 650 °C: (a) O2 evolution, heated to 650 °C; (b) O2 absorption, subsequently after part a. Flowing gases were He for O2 evolution and O2:He ) 1:1 for O2 absorption. The flow rate was 50 mL/min, and the weight of each sample is 15 mg. The temperature was raised at a rate of 5 °C/min.
Figure 8. Weight changes, ∆Wt (%), of the V2O5:Bi2O3 (1:1) physical mixture in a TGA experiment heated to 420 °C: (a) O2 evolution, heated to 420 °C; (b) O2 absorption, subsequently after part a. Flowing gases were He for O2 evolution and O2:He ) 1:1 for O2 absorption. The flow rate was 50 mL/min, and the weight of each sample is 15 mg. The temperature was raised at a rate of 5 °C/min.
1:1 from 50 up to 650 °C. It can be seen that the amount of oxygen absorption is very small (about 0.2 wt %) in this mode of experiment because of the formation of stable BiVO4 above 500 °C. Repeated Cycles of Oxygen Evolution and Absorption. As shown, it is hard for the binary oxide BiVO4 to evolve or absorb oxygen compared to the physical mixtures of V2O5 and Bi2O3. A TGA experiment to show repeated cycles of oxygen evolution and absorption was performed up to 420 °C, where BiVO4 is not formed from the physical mixture of V2O5 and Bi2O3. Figure 8 shows the weight change of a V:Bi ) 1:1 mixture from TGA experiment, which was carried out in He up to 420 °C with increasing temperatures and subsequently in O2:He ) 1:1 from 50 to 420 °C. The observed weight change was
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only from oxygen, and the process of oxygen evolution and absorption came observed three times for the same sample under the above experimental conditions, although here only one such circle is shown. Thus, it is established that a V:Bi ) 1:1 mixture can reversibly absorb or evolve oxygen at temperatures below 420 °C. If the content of oxygen in the mixture is lower than that of the saturation value, the mixture can absorb oxygen from the gas phase and transform it into lattice oxygen. This mixture could also evolve the absorbed oxygen, readily making the whole process reversible. The major fraction of oxygen absorption of reduced sample occurred at low temperatures below 50 °C, which was the initial temperature of TGA experiments. Behavior of Lattice Oxygen. In many selective oxidation reactions catalyzed by metal oxides, the role of lattice oxygen has a paramount importance. In the wellknown Mars-van Krevelan mechanism,15 a substrate reacts first with lattice oxygen of catalyst and then reduced catalyst is reoxidized by gas-phase molecular oxygen. In many cases, these two functions take place on different components of a multicomponent catalyst.4,5,15,16 Hence, facile donation of lattice oxygen and its replenishment from gas-phase molecular oxygen is an essential requirement for active and selective oxidation catalysts. As is more clearly seen in Figure 5, the evolution of oxygen from Bi2O3 occurs in two or three stages when it exists alone or in a physical mixture. Compared to XRD patterns shown in Figures 3 and 6, the first stage oxygen evolution occurring below 420 °C for the mixture of V:Bi ) 1:1 does not accompany the phase change of initial oxides. The second and third oxygen evolutions at higher temperatures involving a larger amount of oxygen bring about a change in the bulk phase. As demonstrated in Figures 7 and 8, the first stage oxygen evolution is reversible while the second and third stages are irreversible. Hence, the low-temperature oxygen evolution appears to involve oxygen that could be easily removable without destroying the initial crystal structure of Bi2O3. The oxygen could be the one located close to the surface. The initial surface area of Bi2O3 is 3.14 m2 g-1, and the oxygen site density of an ideal Bi2O3 surface is 1.474 × 10-5 mol m-2 (8.876 × 1018 atoms m-2). The initial surface area of V2O5 is 6.80 m2 g-1, and the oxygen site density of an ideal V2O5 surface is 2.242 × 10-5 mol m-2 (1.350 × 1019 atoms m-2). Though it is impossible to assign that one oxide is solely responsible for the evolution of oxygen, the amount of oxygen removed during its first stage evolution (5.536 × 10-6 mol of O) corresponds to 11 layers considering only Bi2O3 and 9 layers considering only V2O5. The result clearly shows that the lattice oxygen is involved because adsorbed oxygen alone does not account for such an amount. The oxygen involved in the first stage oxygen evolution must be the one involved in catalytic oxidation reactions. Most of the lost oxygen is replenished by reacting with gas-phase oxygen at temperatures below 50 °C (Figure 8). The reduced oxide, still maintaining the crystal structure of Bi2O3 but full of oxygen defects, must be very reactive to molecular oxygen. Thus, reoxidation of the reduced catalyst would not constitute a slow elementary step in catalytic reaction cycles with these oxides. Among three physical mixtures of V2O5 and Bi2O3 tested in this work, the 1:1 mixture gives off the largest amount of oxygen at the lowest temperature, as shown in Figure 5. The amounts of samples used in TPR experiments are equal; hence, the largest amount of oxygen evolution from the 1:1 mixture below 420 °C without any phase change reflects the best promotional effect of oxygen evolution in this sample. Furthermore, oxygen evolution from this
Behavior of Latice Oxygen in Mixtures of V2O5 and Bi3O3
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sample occurs at temperatures as low as 300 °C. Because the surface areas of each oxide are fixed, the best performance of this sample may be understood by the most effective contact between V2O5 and Bi2O3 phases. It is not surprising that the interfacial contact between two phases is important for a physical mixture of two phases showing a synergistic effect as observed in this work.
binary oxide, BiVO4. When a physical mixture of V2O5Bi2O3 is heated at low temperatures, which do not cause the phase transformation of the oxides, the mixture can evolve or absorb oxygen reversibly. The reoxidation of reduced oxides proceeds much faster than the evolution of oxygen. The effective contact between V2O5 and Bi2O3 appears to be an important factor for the synergy between two phases in the evolution of lattice oxygen.
Conclusion The interaction between V2O5 and Bi2O3 can promote the evolution of lattice oxygen. The lattice oxygen is more easily evolved from the physical mixtures especially in the 1:1 mole ratio than the single metal oxides and the
Acknowledgment. The authors appreciate financial support of the Research Center for Catalytic Technology of Pohang University of Science and Technology. LA990368D
