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 physicalmixtures and a binary oxide (BiVO4) have been studied by thermal gravimetric analysis. The source oflattice 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 ofV2O5-Bi2O3 are heated at low temperatures which do not cause phase transformation of oxides, they canevolve or absorb oxygen reversibly. The reoxidation of reduced oxides proceeded much faster than theevolution of oxygen. The effective contact between two oxide phases appeared to be an important factorfor the synergy between two phases in the evolution of lattice oxygen.

Introduction

Mixed metal oxide catalysts are extensively used inmany industrial processes especially of selective oxidation.These catalysts are usually prepared by mixing the mainactive component (MoO3 or V2O5) with several kinds ofmetal oxide additives (Fe2O3, Cr2O3, CoO, P2O5, or Bi2O3).The mixed oxide systems derived from either bismuthoxide or vanadium oxide exhibit a variety of interestingphysical and chemical properties. For example, bismuthmolybdates and multicomponent oxides containing Bi2O3and MoO3 are active for the selective oxidation andammoxidations of alkenes and hydrocarbons.1-7 Bismuthvanadates,8 binary oxide of vanadium and magnesium,9,10

and mixed oxides of vanadium with molybdenum11-14 areactive for the selective oxidation of hydrogen sulfide toelemental sulfur.

Participation of lattice oxygen is widely recognized inmany selective oxidation reactions. In these reactionscatalysts give up their lattice oxygen to take part in theoxidation reaction, and the reduced catalysts can absorboxygen from the gas phase and transform it into latticeoxygen again.15 These steps constitute the most importantelements in the mechanism of selective oxidation, and

understanding the behavior of lattice oxygen is usuallycritical to understanding the overall reaction. In theselective oxidation of propylene to acrolein over Mo-Bi-containing multicomponent oxides and the subsequentoxidation of acrolein to acrylic acid over Mo-V oxides,Al’kaeva et al. suggested that bulk diffusion of oxygen isa rate-limiting step at low temperatures.6 Bettahar et al.also observed easier evolution of lattice oxygen at highertemperatures and a corresponding increase in catalyticactivities.7 In their propylene oxidation over Bi-Mo oxides,the reaction was controlled by the reoxidation step of thecatalyst at low temperatures and by the reduction step athigh temperatures. Ono et al. showed a sensitive effect ofoxygen on propenal formation in propene oxidation, whichis closely related to the extent of participation of latticeoxygen.16

Interaction between different metal oxides can promotethe 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 oxygenevolution in the mixtures of MoO3-R-Bi2O3, MoO3-R-Sb2O4, and MoO3-BiPO4. 4,5 They suggested that thelatter compounds of these pairs of oxides acted as donorsdelivering oxygen to active MoO3 in the reactions ofselective oxidation of isobutene and N-ethylformamidedehydration. A promotional evolution of lattice oxygenwas also suggested in the physical mixture of V2O5 andMoO3.17

In this paper, we have studied the behavior of latticeoxygen in physical mixtures and the binary oxide of V2O5-Bi2O3, which are potential catalysts for selective oxidationof H2S to elemental sulfur.8 Thermal gravimetric analysis(TGA) was employed to monitor evolution of lattice oxygenand absorption of gas-phase oxygen, and X-ray diffraction(XRD) was employed to follow the change in the bulk solidphase during the process.

Experimental SectionSample Preparation. Commercial vanadium oxide (V2O5;

Junsei Chem. Co. Ltd., 99.0% purity) was used after treating at450 °C for 4 h in air. Bismuth oxide (Bi2O3) was prepared by aprecipitation method with a 30 wt % aqueous ammonia solution

* To whom all correspondence should be addressed. Tel.: +82-562-279-2266. Fax.: +82-562-279-5799. E-mail: jlee@postech.ac.kr.

(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.

(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.

1109Langmuir 2000, 16, 1109-1113

10.1021/la990368d CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 12/01/1999

of bismuth nitrate hexahydrate (Bi2(NO3)3‚6H2O; Aldrich) andcalcined at 450 °C for 5 h in air. Physical mixtures of V2O5-Bi2O3were prepared by simply mixing the two oxides in a mortar indifferent mole ratios. The binary oxide, BiVO4, was preparedaccording to the following procedure. Ammonium metavanadateand bismuth nitrate hexahydrate in the required mole ratio wereput into a 3 wt % aqueous solution of nitric acid. The resultingmixture was evaporated to dryness, and the powder was furtherdried overnight at 110 °C, followed by calcination at 450 °C for4 h in air.

Characterization. X-ray diffraction patterns of samples wereobtained using a M18XHF (MAC Science Co.), which utilizesNi-filtered Cu KR radiation (λ ) 1.5405 Å). Diffraction patternswere obtained in the range of 2θ ) 10-90° with an X-ray gunoperated at 40 kV and 200 mA, using a scan rate of 4°/min (2θ).Identification of a compound was accomplished by comparisonof a measured spectrum with that in JCPDS files. Mass spectraof the species generated from the catalyst were obtained usinga HP 5890II GC/HP 5972 MSD. Evolution of lattice oxygen andabsorption of gas-phase oxygen were monitored by TGA, whichwas carried out with a Perkin-Elmer TGS-2 thermobalance underflowing gases of He and O2:He ) 1:1, respectively. The flow ratewas 50 mL/min, and the typical sample loading was ca. 15 mg.The temperature was raised at a rate of 5 °C/min.

Results and Discussion

Oxygen Evolution from the Single Metal Oxides.The XRD patterns of prepared V2O5, Bi2O3, and BiVO4were measured, and the phases of V2O5 and Bi2O3 werefound 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 Heas a function of temperature for pure V2O5 and Bi2O3. Itcan be seen that no weight loss is observed for pure V2O5up to 650 °C and a small weight loss is observed at about700 °C. The small amount of weight loss is probably dueto volatilization of V2O5, which has a melting point of 690°C. For pure Bi2O3, there are weight losses at about 470and 540 °C. To know whether this weight loss is due tothe loss of water originating from the remaining precursor,bismuth hydroxide, after incomplete calcination or becauseof the loss of oxygen from the sample, temperature-programmed desorption (TPD) was performed under Hewith mass spectroscopy, and the result is shown in Figure2. No H2O was detected up to 620 °C, and all the weightloss was due to the oxygen generated from the catalyst

itself. The evolution pattern of oxygen corresponded wellto the TGA result shown in Figure 1. Bi2O3 has fourrecognized solid phases (R, â, γ, and δ), and these phasesundergo polymorphic reversible or irreversible transitionsover the temperature range between 450 and 800 °C. Thereis an irreversible phase transition over the temperaturerange of 450-550 °C from â-Bi2O3 (metastable tetragonal)to R-Bi2O3 (monoclinic or pseudo-orthorombic).18 The samephase change is observed from XRD patterns shown inFigure 3. Fresh Bi2O3 composed of â and γ forms haschanged to the R form upon treatment at 620 °C. Thoughthere is no direct evidence that the oxygen evolution andthe phase transition of Bi2O3 are indeed related exceptthat they are taking place at approximately the sametemperature, probably the metastable â form containsmore oxygen than the more stable R form and two kindsof evolution processes might be involved.

Oxygen Evolution from Physical Mixtures and theBinary Oxide. Figure 4 shows the weight loss in TGA

(18) Samsonov, G. V. The Oxide Handbook, 2nd ed.; Plenum DataCompany: New York, 1981; Chapter 1, p 16.

Figure 1. Weight loss, ∆Wt (%), in TGA experiments withflowing He for single oxides: (a) Bi2O3, (b) V2O5. The flow ratewas 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 pureBi2O3 measured by mass spectrometry with flowing He. Theflow rate was 50 mL/min, and the weight of each sample is 10mg. 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 flowingHe. Flow rate of He is 50 mL/min.

1110 Langmuir, Vol. 16, No. 3, 2000 Shin et al.

experiments in He as a function of temperature for physicalmixtures (V:Bi ) 1:4, 1:1, and 4:1 mole ratio) and thebinary oxide, BiVO4. It can be seen that the oxygen inBiVO4 evolves above 500 °C (Figure 4d) but the amountof evolved oxygen is small compared with the physicalmixtures. For the case of physical mixtures, the mixtureof V:Bi ) 1:1 (Figure 4b) shows a substantially lowertemperature of oxygen evolution compared to othermixtures and BiVO4. The mixture of V:Bi ) 1:1 generatesoxygen from about 320 °C while the other mixtures startfromabout420°C. This trend is evident from thederivativecurves of weight loss for all of the samples shown in Figure5. The mixture of V:Bi ) 1:1 (Figure 5e) is the only samplewhich shows a substantial oxygen loss below 420 °C.

BET surface areas of all samples are shown in Table 1and compared with arithmetic areas for the mixtures.BiVO4 had a somewhat higher surface area than Bi2O3,and measured surface areas of mixtures were consistent

with calculated arithmetic areas. These indicate thatgrinding does not lead to an increase in the surface areaof any component, and the behavior of oxygen evolutionhas little to do with the surface area of each component.The behavior of V2O5 in our V2O5-Bi2O3 system is differentfrom that of V2O5 in the V2O5-MoO3 system reported byZhan et al.17 They showed that the amount of absorbingor evolving oxygen depends strongly on V2O5 content inboth physical mixtures and the binary oxide of V2O5-MoO3.

Phase Change During Oxygen Evolution. Whenthe lattice oxygen is involved in the reaction, oxygenabsorption from the gas phase of O2 is also important. Therates of oxygen evolution and absorption control the overallrate of the redox catalytic reaction. If the irreversible phasechange of Bi2O3 above 440 °C serves as a negative factorin the oxygen absorption, this could also impose somerestriction on the application of the physical mixture ofV2O5 and Bi2O3 to a real catalytic oxidation reaction. Toshow the phase stability of the physical mixture as afunction of temperature, the mixture of V:Bi ) 1:1 washeated for 2 h in He at 400, 500, and 650 °C, respectively.XRD patterns in Figure 6 following these heat treatmentsshow that the physical mixture still maintains the phaseof each oxide at 400 °C. Binary oxide, BiVO4, is formed ina small amount at 500 °C, and finally the main phasebecomes BiVO4 at 650 °C. As shown in Figure 3, thetransition from the tetragonal to the monoclinic phase ofBi2O3 was observed when Bi2O3 alone was heated. Thisis not the case for the physical mixtures. Instead, the stablebinary phase of BiVO4 is formed at high temperatures.

Figure 7 shows the weight change of a V:Bi ) 1:1 mixturein TGA experiments carried out in He up to 650 °C withincreasing temperatures and subsequently in O2:He )

Figure 4. Weight loss, ∆Wt (%), in TGA experiments withflowing 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 ratewas 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 5. Derivative curves of weight loss, ∆Wt (%), in TGAexperiments 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 moleratios.

Figure 6. Structural changes of the V2O5:Bi2O3 (1:1) physicalmixture at different temperatures: (a) 400, (b) 500, and (c) 650°C. The sample was treated 2 h at each temperature in theflowing condition of He.

Table 1. Surface Areas of Single, Binary, and MixtureOxides

catalyst(mole ratio)

surface area(m2/g)

arithmetic surface areaof mixtures (m2/g)

Bi2O3 3.14V2O5 6.80BiVO4 3.64V2O5:Bi2O3 (1:4) 3.99 3.87V2O5:Bi2O3 (1:1) 4.64 4.97V2O5:Bi2O3 (4:1) 5.68 6.07

Behavior of Latice Oxygen in Mixtures of V2O5 and Bi3O3 Langmuir, Vol. 16, No. 3, 2000 1111

1:1 from 50 up to 650 °C. It can be seen that the amountof oxygen absorption is very small (about 0.2 wt %) in thismode of experiment because of the formation of stableBiVO4 above 500 °C.

Repeated Cycles of Oxygen Evolution and Ab-sorption. As shown, it is hard for the binary oxide BiVO4to evolve or absorb oxygen compared to the physicalmixtures of V2O5 and Bi2O3. A TGA experiment to showrepeated cycles of oxygen evolution and absorption wasperformed up to 420 °C, where BiVO4 is not formed fromthe physical mixture of V2O5 and Bi2O3. Figure 8 showsthe weight change of a V:Bi ) 1:1 mixture from TGAexperiment, which was carried out in He up to 420 °Cwith increasing temperatures and subsequently in O2:He) 1:1 from 50 to 420 °C. The observed weight change was

only from oxygen, and the process of oxygen evolutionand absorption came observed three times for the samesample under the above experimental conditions, althoughhere only one such circle is shown. Thus, it is establishedthat a V:Bi ) 1:1 mixture can reversibly absorb or evolveoxygen at temperatures below 420 °C. If the content ofoxygen in the mixture is lower than that of the saturationvalue, the mixture can absorb oxygen from the gas phaseand transform it into lattice oxygen. This mixture couldalso evolve the absorbed oxygen, readily making the wholeprocess reversible. The major fraction of oxygen absorp-tion of reduced sample occurred at low temperatures below50 °C, which was the initial temperature of TGA experi-ments.

Behavior of Lattice Oxygen. In many selectiveoxidation reactions catalyzed by metal oxides, the role oflattice oxygen has a paramount importance. In the well-known Mars-van Krevelan mechanism,15 a substratereacts first with lattice oxygen of catalyst and then reducedcatalyst is reoxidized by gas-phase molecular oxygen. Inmany cases, these two functions take place on differentcomponents of a multicomponent catalyst.4,5,15,16 Hence,facile donation of lattice oxygen and its replenishmentfrom gas-phase molecular oxygen is an essential require-ment for active and selective oxidation catalysts.

As is more clearly seen in Figure 5, the evolution ofoxygen from Bi2O3 occurs in two or three stages when itexists alone or in a physical mixture. Compared to XRDpatterns shown in Figures 3 and 6, the first stage oxygenevolution occurring below 420 °C for the mixture of V:Bi) 1:1 does not accompany the phase change of initialoxides. The second and third oxygen evolutions at highertemperatures involving a larger amount of oxygen bringabout a change in the bulk phase. As demonstrated inFigures 7 and 8, the first stage oxygen evolution isreversible while the second and third stages are irrevers-ible. Hence, the low-temperature oxygen evolution appearsto involve oxygen that could be easily removable withoutdestroying the initial crystal structure of Bi2O3. The oxygencould be the one located close to the surface. The initialsurface area of Bi2O3 is 3.14 m2 g-1, and the oxygen sitedensity of an ideal Bi2O3 surface is 1.474 × 10-5 mol m-2

(8.876 × 1018 atoms m-2). The initial surface area of V2O5is 6.80 m2 g-1, and the oxygen site density of an ideal V2O5surface is 2.242 × 10-5 mol m-2 (1.350 × 1019 atoms m-2).Though it is impossible to assign that one oxide is solelyresponsible for the evolution of oxygen, the amount ofoxygen removed during its first stage evolution (5.536 ×10-6 mol of O) corresponds to 11 layers considering onlyBi2O3 and 9 layers considering only V2O5. The result clearlyshows that the lattice oxygen is involved because adsorbedoxygen alone does not account for such an amount.

The oxygen involved in the first stage oxygen evolutionmust be the one involved in catalytic oxidation reactions.Most of the lost oxygen is replenished by reacting withgas-phase oxygen at temperatures below 50 °C (Figure 8).The reduced oxide, still maintaining the crystal structureof Bi2O3 but full of oxygen defects, must be very reactiveto molecular oxygen. Thus, reoxidation of the reducedcatalyst would not constitute a slow elementary step incatalytic reaction cycles with these oxides.

Among three physical mixtures of V2O5 and Bi2O3 testedin this work, the 1:1 mixture gives off the largest amountof oxygen at the lowest temperature, as shown in Figure5. The amounts of samples used in TPR experiments areequal; hence, the largest amount of oxygen evolution fromthe 1:1 mixture below 420 °C without any phase changereflects the best promotional effect of oxygen evolution inthis sample. Furthermore, oxygen evolution from this

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, subsequentlyafter 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 theweight of each sample is 15 mg. The temperature was raisedat 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, subsequentlyafter 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 theweight of each sample is 15 mg. The temperature was raisedat a rate of 5 °C/min.

1112 Langmuir, Vol. 16, No. 3, 2000 Shin et al.

sample occurs at temperatures as low as 300 °C. Becausethe surface areas of each oxide are fixed, the bestperformance of this sample may be understood by themost effective contact between V2O5 and Bi2O3 phases. Itis not surprising that the interfacial contact between twophases is important for a physical mixture of two phasesshowing a synergistic effect as observed in this work.

ConclusionThe interaction between V2O5 and Bi2O3 can promote

the evolution of lattice oxygen. The lattice oxygen is moreeasily evolved from the physical mixtures especially inthe 1:1 mole ratio than the single metal oxides and the

binary oxide, BiVO4. When a physical mixture of V2O5-Bi2O3 is heated at low temperatures, which do not causethe phase transformation of the oxides, the mixture canevolve or absorb oxygen reversibly. The reoxidation ofreduced oxides proceeds much faster than the evolutionof oxygen. The effective contact between V2O5 and Bi2O3appears to be an important factor for the synergy betweentwo phases in the evolution of lattice oxygen.

Acknowledgment. The authors appreciate financialsupport of the Research Center for Catalytic Technologyof Pohang University of Science and Technology.

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