Ethane Oxidative Dehydrogenation over Halogenated

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, Department of Applied. Chemistry, Faculty of Science, Beijing Universi...
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Ind. Eng. Chem. Res. 2002, 41, 37-45

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Ethane Oxidative Dehydrogenation over Halogenated Bi2Sr2CaCu2O8-δ Catalysts H. X. Dai,†,‡ H. He,†,§ and C. T. Au*,† Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, Department of Applied Chemistry, Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China, and College of Environmental and Energy Engineering, Beijing Polytechnic University, Beijing 100022, P. R. China

The layered oxide Bi2Sr2CaCu2O8-δ (Bi-2212) and halogenated Bi-2212 have been investigated as catalysts for the oxidative dehydrogenation of ethane to ethylene. By introducing a small amount of fluoride or chloride ions into the Bi-2212 lattice, one can enhance the catalytic performance significantly. At a temperature of 680 °C, a C2H6/O2/N2 molar ratio of 2/1/3.7, and a contact time of 1.67 × 10-4 h g mL-1, Bi2Sr2CaCu2O7.811F0.366 (Bi-2212-F) showed 70.8% C2H6 conversion, 72.5% C2H4 selectivity, and 51.3% C2H4 yield; Bi2Sr2CaCu2O7.901Cl0.394 (Bi-2212-Cl) showed 77.2% C2H6 conversion, 76.6% C2H4 selectivity, and 59.1% C2H4 yield. During 60 h of on-stream reaction at 680 °C, the two halogenated materials exhibited stable catalytic performance. We observed a remarkable reduction in deep ethylene oxidation over the halogenated catalysts. X-ray powder diffraction results indicated that Bi-2212 and the halogenated Bi-2212 oxides were single-phase and tetragonal in structure. The results of Cu and Bi oxidation state chemical analyses and X-ray photoelectron spectroscopic investigations revealed the presence of Cu3+, Cu2+, and Bi3+ in Bi-2212 and Cu+, Cu2+, Bi5+, and Bi3+ in Bi2212-F and Bi-2212-Cl. Oxygen temperature-programmed desorption and temperatureprogrammed reduction studies indicated that the halogenation of Bi-2212 promoted the activity of lattice oxygen; such a promotional effect was confirmed by the results of 18O/16O exchange experiments. We conclude that, by modifying the oxygen nonstoichiometry and copper and bismuth oxidation states via halogenation, one can convert the Bi-2212 oxide to catalysts active and selective for the oxidation of ethane to ethylene. 1. Introduction In the past decade, perovskite-type oxides (ABO3) have been investigated extensively and intensively for the catalytic oxidation of CO and HCs (hydrocarbons). These kinds of materials exhibit the characteristics of retaining the crystal structure while having the cationic sites substituted with other cations. Such isostructural substitution in the A and/or B sites by foreign ions with different oxidation states regulates the chemical valence of the B-site ions and generates structural defects such as anionic and cationic vacancies. Using this approach, one can modify the catalytic behaviors of these materials. For example, the substitution of Sr for La enhances the catalytic performance of LaMO3-δ (M ) Mn, Co, Cr, Fe) for CO oxidation,1,2 a result of the increase in concentrations of oxygen vacancies and hypervalent M ions; also, the substitution of Cu for Mn significantly improves the activity for CO oxidation, as revealed in studies of the LaMn1-xCuxO3-δ catalysts.3 Most authors believe that both the oxygen nonstoichiometry and the chemical states of the B-site ions play an important role in catalyzing the complete oxidation of CO and HCs.4 Since the discovery of the high-temperature supercon* Corresponding author: Prof. C. T. Au. Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong. Tel.: (852) 2339 7067. Fax: (852) 2339 7348. E-mail: [email protected]. † Hong Kong Baptist University. ‡ Beijing University of Chemical Technology. § Beijing Polytechnic University.

ductors (HTSCs) La2CuO4-δ,5 La2-xAxCuO4-δ (A ) Sr, Ba),6,7 and YBa2Cu3O7-δ,8 many researchers have used these compounds as catalysts for chemical reactions. The good catalytic activities of La2-xSrxCuO4-δ (for CO oxidation2 and NO decomposition9,10) and YBa2Cu3O7-δ (for CO oxidation11) have been reported and associated with the presence of oxygen nonstoichiometry and positive holes (Cu3+) that provide a favorable environment for the adsorption of NO, CO, or O2. In a study of fluoride-doped YBa2Cu3O6+x-zFz catalysts for CH4 oxidation, Lee and Ng found that the incorporation of Fions enhanced the tendency for the partial oxidation reaction.12 We envisage that the oxygen nonstoichiometry and the oxidation states of the B-site ions could be regulated by incorporating halide ions into the perovskite lattices and that the materials could be converted to catalysts suitable for the selective oxidation of ethane to ethylene. In the past two years, we have characterized and reported series of fluoride- or chloride-doped superconductive oxide catalysts such as YBa2Cu3O7-δ 13 and Ln1.85A0.15CuO4-δ (Ln ) La, Nd; A ) Sr, Ce);14 they exhibited good catalytic performance in the ODE (oxidative dehydrogenation of ethane) reaction. Recently, we turn our attention to the Bi2Sr2CaCu2O8-δ (Bi-2212) system, which exhibits a transition temperature of 80100 K.15-17 This ceramic compound contains Cu-O, BiO, and Sr-O layers. It has been considered that both oxygen nonstoichiometry and hole doping play crucial roles in the superconducting properties of the Bi-SrCa-Cu-O system.18-20 In this paper, we present a

10.1021/ie0102504 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/02/2002

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study of the catalytic performance of the halogen-free and halogenated superconducting cuprate Bi-2212 catalysts for the ODE reaction. We characterized these materials by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), O2 temperatureprogrammed desorption (TPD), temperature-programmed reduction (TPR), and 18O/16O isotope exchange, as well as chemical analyses of the halogen contents and Cu and Bi oxidation states. 2. Experimental Section The Bi-2212 catalyst was prepared by adopting a modified citrate gel method reported by Devi and Maiti.21 The appropriate amounts of Bi(NO3)3‚6H2O (Aldrich, >99%), Sr(NO3)2 (Aldrich, >99%), Ca(NO3)2‚ 4H2O (Aldrich, >99%), and Cu(NO3)2‚2.5H2O (Aldrich, >98%) were dissolved in deionized water and mixed with citric acid in amounts equimolar to the metals. By adding an ethylenediamine solution, we adjusted the pH value of the solution to 6.0-6.5 to avoid phase separation and to provide a truly homogeneous distribution of all of the cations at the atomic level. After evaporation of the mixed solution with continuous stirring at 80 °C, a viscous gel, greenish in color, was formed, and this gel in turn foamed, swelled, and selfignited to produce a brownish-yellow powder. Subsequently, the powder was pelletized, calcined at 820 °C for 24 h, and annealed at 600 °C for 48 h under an oxygen flow. The fluorination or chlorination of the wellground Bi-2212 powder was carried out in a vacuum (0.1 Torr) furnace using NH4F or NH4Cl as the halogenating reagent first at 350 °C for 10 h and then at 600 °C for 8 h.22 After halogenation, the samples were quenched to room temperature and were, in turn, ground, tabletted, crushed, and sieved to a size range of 80-100 mesh. Measurements for catalytic activities were carried out according to the approach described previously.13 The contact time and C2H6/O2/N2 molar ratio were 1.67 × 10-4 h g mL-1 and 2/1/3.7, respectively. A thermocouple was placed in the middle of the catalyst bed for the monitoring of reaction temperatures. Because the homogeneous reaction contribution becomes significant at or above 700 °C,13 we tested the catalysts within the range of 500-680 °C. The product mixture (C2H6, C2H4, CH4, CO, and CO2) was analyzed on-line by means of gas chromatography (Shimadzu 8A TCD, Porapak Q and 5A molecular sieve columns). The balances of carbon and oxygen were estimated to be 100 ( 2 and 100 ( 5%, respectively, for each run over the catalysts. The crystal structures of the catalysts were determined with an X-ray diffractometer (Huber) operating at 40 kV and 150 mA with Cu KR radiation. Because most diffraction lines appear below 65° (2θ), we recorded the XRD patterns in the 5-65° range. With reference to the powder diffraction files in the 1998 ICDD PDF Database, the crystal phases were identified. The XPS (Phi Quantum 2000) technique was used to determine the cationic and anionic core-level binding energies of the catalysts using a monochromatized Al KR X-ray source. Before the XPS measurements, the sample was placed in a self-made quartz-tube reactor that could conveniently be transferred between the sample chamber and an oven. The sample in the quartz reactor could be heated to 800 °C and exposed to gas(es) without being exposed to air. After calcination in O2 (flow rate, 20 mL min-1) at 800 °C for 1 h and cooling in O2 to room

temperature, the sample was thermally treated in helium (20 mL min-1) at the desired temperature for 1 h and then quenched (using liquid nitrogen) in helium to room temperature. After the respective treatments, the sample was transferred to the spectrometer in a transparent glove bag (Instruments for Research and Industry, Cheltenham, PA) filled with helium to avoid exposure to air. Finally, the sample was outgassed in a vacuum (10-5 Torr) for 0.5 h and introduced into the ultrahigh-vacuum chamber (3 × 10-9 Torr) for recording. The C 1s line at 284.6 eV was taken as the reference for binding energy calibration. The specific surface areas of the catalysts were measured using the BET method on a Nova 1200 apparatus. We performed 18O2 pulse experiments at various temperatures to investigate the activity of surface lattice oxygen of the catalysts. A catalyst sample (0.2 g) was placed in a microreactor and was thermally treated at 740 °C in helium (flow rate, 20 mL min-1) for 1 h. Then, pulses of 18O2 were introduced continuously into the system at a desired temperature until no change in pulse size could be observed (usually after about 15 pulses). The effluent was analyzed on-line with a HP G1800A mass spectrometer. The pulse size was 50.0 µL (at 25 °C, 1 atm), and helium (HKO, >99.995%) was the carrier gas. The procedures for the O2-TPD and TPR experiments and the analysis of halogen contents were the same as those described before.13,23 The temperature ranges were from room temperature to 950 °C for TPD and to 800 °C for TPR. The amounts of O2 desorbed from the catalysts were quantified by calibrating the peak areas against that of a standard pulse. The experimental error is estimated to be (0.05% for the halogen analysis. The chemical analyses of the Cu3+ and Bi5+ contents were performed according to the method described by Oku and co-workers.24 The procedure for Cu+ analysis was the same as that described previously.13 The experimental errors of the approaches were estimated to be (0.5%. 3. Results 3.1. Catalyst Compositions, Crystal Structures, and Surface Areas. Figure 1 shows the XRD patterns of Bi-2212, Bi2Sr2CaCu2O8-δFσ (Bi-2212-F), and Bi2Sr2CaCu2O8-δClσ (Bi-2212-Cl). The patterns of the used (after 60 h of on-stream ODE reaction) halogenated catalysts are also included. By comparing the XRD patterns (Figure 1) of these oxides and halo-oxides with the ICDD PDF Database data for Bi-2212 (Nos. 46-0545 and 47-0713), we deduce that the Bi-2212, Bi-2212-F, and Bi-2212-Cl catalysts were single-phase and tetragonal in structure. Table 1 shows the physical properties of the halogen-free and halogenated catalysts. The δ values were calculated on the basis of the assumption of electroneutrality. In Bi-2212, there was an excess amount of oxygen (δ ) -0.058); according to the results of the chemical analyses, copper existed in tri- and bivalencies, whereas bismuth existed in trivalency. In Bi-2212-F and Bi-2212-Cl, Cu+, Cu2+, Bi3+, and Bi5+ ions were present; the contents of Cu+ and Bi5+, as well as the amount of incorporated halogen, in the chlorinated catalyst were higher than those in the fluorinated catalyst. The surface area of the halogen-free catalyst was slightly larger than those of the halogenated catalysts. The results indicate that the incorporation of

Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002 39 Table 1. Crystal Structures, Compositions, and Surface Areas of Catalysts catalyst

phase composition

Cu3+ or Cu+ contenta (mol %)

Bi5+ contenta (mol %)

F or Cl content (wt %)

δ

σ

surface area (m2 g-1)

Bi2Sr2CaCu2O8-δ Bi2Sr2CaCu2O8-δFσ Bi2Sr2CaCu2O8-δClσ

tetragonal tetragonal tetragonal

5.8 9.8b 12.6b

0 4.6 11.2

0.78 (0.77)c 1.55 (1.53)c

-0.058 0.189 0.099

0.366 0.394

5.3 5.1 5.0

a Cu3+, Cu+, and Bi5+ contents were calculated on the basis of the assumption that only Cu3+ and Cu2+, Cu2+ and Cu+, and Bi5+ and Bi3+ ions were present in the samples, respectively. b Cu+ content. c Values in parentheses were obtained after 60 h of on-stream ODE reaction.

Figure 1. XRD patterns of (a) Bi-2212, (b, b′) Bi-2212-F, and (c, c′) Bi-2212-Cl catalysts. Patterns b′ and c′ of the halide-doped samples were recorded after 60 h of on-stream reaction.

F- (or Cl-) ions into Bi-2212 results in changes in the Cu3+ (or Cu+) and Bi5+ (or Bi3+) concentrations and in the oxygen nonstoichiometry. It could also be observed that the changes in crystal structure of Bi-2212-F (or Bi-2212-Cl) before and after 60 h of on-stream reaction were not significant. 3.2. Catalytic Performance. Figure 2 shows the catalytic performances of Bi-2212, Bi-2212-F, and Bi2212-Cl as functions of the reaction temperature. For a halogen-free catalyst (Figure 2a), only small changes in C2H6 conversion, C2H4 and COx selectivities, and C2H4 yield occurred below 660 °C; above 660 °C, the C2H6 conversion, C2H4 selectivity, and C2H4 yield increased significantly, but the COx selectivity decreased markedly. Throughout the temperature range examined, the O2 conversion increased monotonically, and the CH4 selectivity was 0. Although the profiles of the C2H6 and O2 conversions; C2H4, CH4, and COx selectivities; and C2H4 yield were rather similar over Bi-2212-F and Bi2212-Cl, the onset temperatures where significant changes occurred were different, being 600 °C for the fluorinated catalyst (Figure 2b) and 540 °C for the chlorinated catalyst (Figure 2c).

Figure 2. Catalytic performances of (a) Bi-2212, (b) Bi-2212-F, and (c) Bi-2212-Cl as functions of reaction temperature at 1.67 × 10-4 g mL-1. (9) C2H6 conversion, ([) C2H4 selectivity, (2) C2H4 yield, (b) COx selectivity, (×) CH4 selectivity, and (O) O2 conversion.

Figure 3 shows the catalytic performances of Bi2212-F and Bi-2212-Cl during 60 h of on-stream reaction at 680 °C and 1.67 × 10-4 h g mL-1. It is observed that only slight changes in the C2H6 conversion and C2H4 selectivity occurred. In other words, the two catalysts are stable for the ODE reaction. Table 2 summarizes the catalytic performances of Bi2212, Bi-2212-F, and Bi-2212-Cl for the oxidation of C2H6 or C2H4 at 680 °C. One can see that the introduction of F- or Cl- ions into the layered oxide lattice results in a significant drop in the C2H4 conversion and substantial increases in the C2H6 conversion and CO/ CO2 ratio. This indicates that the halogenation of Bi2212 could reduce C2H4 deep oxidation considerably. 3.3. XPS Studies. Shown in Figure 4 are the Bi 4f, Cu 2p3/2, and O 1s XPS spectra of the Bi-2212, Bi-2212F, and Bi-2212-Cl samples that had been treated in

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Figure 3. Lifetime studies of (a) Bi-2212-F and (b) Bi-2212-Cl during 60 h of on-stream reaction time at 680 °C and 1.67 × 10-4 h g mL-1. (9) C2H6 conversion and ([) C2H4 selectivity. Table 2. Catalytic Performances of Bi-2212, Bi-2212-F, and Bi-2212-Cl for the Oxidation of Ethane and Ethylene at a Temperature of 680 °C and a Contact Time of 1.67 × 10-4 h g mL-1 oxidation of C2H4 a

oxidation of C2H6

catalyst

C2H4 conversion (%)

CO/CO2 ratio

C2H6 conversion (%)

C2H4 selectivity (%)

Bi-2212 Bi-2212-F Bi-2212-Cl

48.9 12.6 9.9

1/15.7 1/3.6 1/1.7

40.8 70.8 77.2

45.8 72.5 76.6

a

At a C2H4/O2/N2 molar ratio of 2/1/3.7.

helium at different temperatures and quenched in the same atmosphere to room temperature. For the Bi-2212 sample treated at 240 °C (Figure 4Ia), two quite symmetric peaks at ca. 158.5 and 163.7 eV (binding energy, BE), corresponding to the characteristic Bi 4f7/2 and Bi 4f5/2 signals of Bi3+, were recorded; further treatments at 700 or 800 °C did not bring about any significant changes in the spectral profiles (Figure 4Ib and Ic). For the halogenated samples (Figure 4Ia′-Ic′ and Ia′′-Ic′′), however, each of the two spin-orbit structures showed a broadening at the high-BE side and could be resolved into two components: 158.5 and 159.7 eV, and 163.7 and 164.9 eV, respectively. The signals at BE ) 158.5 and 163.7 eV were due to Bi3+, and we assign the components at BE ) 159.7 and 164.9 eV to Bi5+.25 With the increase in treatment temperature, the intensities of the Bi5+ signals decreased, disappearing at a treatment temperature of 740 °C. Compared to the Bi5+ signal intensity observed over Bi-2212-F (Figure 4Ia′), that observed over Bi-2212-Cl (Figure 4Ia′′) was higher, indicating that higher amounts of Bi5+ were present in the Cl-doped sample than in the fluoridedoped one. In addition to the shake-up satellite peaks at BE ) 940-945 eV (Isat/Imain ≈ 0.36, caused by charge transfer from neighboring oxygen ligands into an empty d state of Cu2+), the Cu 2p3/2 spectrum of the undoped sample treated at 240 °C (Figure 4IIa) showed a resolvable structure in the range of 932-935 eV, with one peak at ca. 933.5 eV and the other at ca. 935.0 eV. The former is attributable to Cu2+, and the latter to

Cu3+.26 An increase in the treatment temperature caused the Cu3+ signals to decrease in intensity and to ultimately disappear at a treatment temperature of 740 °C (Figure 4IIb and IIc). After halogenation, however, the samples treated at 240 °C exhibited a spin-orbit feature that could be resolved into two components, one at ca. 932.6 eV and the other at ca. 933.5 eV, attributable to Cu+ and Cu2+,26 respectively; we observed an Isat/Imain ratio of ca. 0.28. Also, the intensity of the Cu+ signal for Bi-2212-Cl (Figure 4IIa′′) was higher than that for Bi-2212-F (Figure 4IIa′), indicating a larger amount of Cu+ in the former than in the latter. After treatments at 740 °C (Figure 4IIb′ and IIb′′), the signals for Cu+ increased in intensity, and the Isat/Imain ratios decreased. When the treatment temperature was raised to 800 °C, the Cu+ signal observed over the F-doped sample disappeared (Figure 4IIc′), whereas that over the Cl-doped sample increased slightly in intensity (Figure 4IIc′′); the Isat/Imain ratios decreased over both halogenated samples. The results of the chemical analyses of the Bi and Cu oxidation states support the above assignments. The changes in the bismuth and copper valence states with the thermal treatment temperature could be associated with the reduction of these cations because of the desorption of oxygen at elevated temperatures. Furthermore, the BEs of Sr 3d (132.3 and 134.1 eV), Ca 2p3/2 (345.2 and 346.1 eV), F 1s (683.5 eV), and Cl 2p (198.5 eV) were almost unaltered throughout the temperature range of thermal treatment. As shown in the O 1s XPS profiles, two well-resolved peaks were present at ca. 528.3 and 531.2 eV for the Bi-2212 sample treated at 240 °C (Figure 4IIIa); when the treatment temperature was raised to 740 (Figure 4IIIb) or 800 °C (Figure 4IIIc), the component at ca. 531.2 eV disappeared. For the Bi-2212-F and Bi-2212-Cl samples treated at temperatures varying from 240 to 800 °C, however, only one peak at ca. 530.4 eV was observed (Figure 4IIIa′-IIIc′ and IIIa′′-IIIc′′). Because all of the treated samples were not exposed to air before XPS measurements, it is reasonable to assign the signal at 528.3-530.4 eV to surface lattice oxygen27 and that at ca. 531.2 eV to extrastoichiometric oxygen.25,28,29 3.4. O2-TPD and TPR Studies. Figure 5 shows the O2-TPD and TPR profiles of Bi-2212, Bi-2212-F, and Bi2212-Cl. Three desorption peaks were found, one at ca. 240 °C (1.5 µmol gcat-1) and the other two at ca. 798 and 908 °C (a total of 32.8 µmol gcat-1), in the Bi-2212 profile (Figure 5Ia); the former peak can be attributed to the excessive oxygen, the latter two to lattice oxygen. In the Bi-2212-F profile (Figure 5Ib), two desorption peaks at ca. 774 and 878 °C (a total of 59.6 µmol gcat-1) were recorded. In the Bi-2212-Cl profile (Figure 5Ic), one large peak was present at ca. 734 °C, with a shoulder at temperatures above 800 °C; the total amount of desorbed oxygen was 56.4 µmol gcat-1. The O2-TPD peaks above 700 °C are due to the desorption of lattice oxygen. Such an assignment is substantiated by the O 1s XPS spectra recorded after treatments at 740 and 800 °C (Figure 4IIIb, IIIc, IIIb′, IIIc′, IIIb′′, and IIIc′′). In the TPR profiles, one can observe three reduction bands at ca. 210, 562, and 775 °C over the undoped sample (Figure 5IIa); two reduction bands at ca. 628 and 703 °C over the F-doped sample (Figure 5IIb); and only one reduction band at ca. 612 °C over the Cl-doped sample (Figure 5IIc). 3.5. 18O/16O Isotope Exchange Studies. Figure 6 shows the results of 18O/16O exchange at temperatures

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Figure 4. XPS spectra of (I) Bi 4f, (II) Cu 2p3/2, and (III) O 1s of the (a-c) Bi-2212, (a′-c′) Bi-2212-F, and (a′′-c′′) Bi-2212-Cl samples treated in a helium flow of 20 mL min-1 at (a, a′, a′′) 240, (b, b′, b′′) 740, and (c, c′, c′′) 800 °C for 1 h.

Figure 5. (I) O2-TPD and (II) TPR profiles of (a) Bi-2212, (b) Bi-2212-F, and (c) Bi-2212-Cl.

varying from 500 to 800 °C over Bi-2212, Bi-2212-F, and Bi-2212-Cl. The data were recorded at the 15th 18O2 pulse where a steady state had been reached. Over the halogen-free catalyst (Figure 6a), with the increase in temperature, the concentration of 18O2 decreased, whereas those of 18O16O and 16O2 increased gradually to reach the highest concentrations of 12.6 and 25.1%, respectively, at 800 °C. Significant changes in the

isotope dioxygen concentrations occurred above 620 °C over the fluorinated catalyst (Figure 6b) and above 560 °C over the chlorinated catalyst (Figure 6c); a maximal 18O16O concentration of 25.6% occurred at 760 °C over Bi-2212-F and of 29.9% at 740 °C over Bi-2212-Cl. A further increase in temperature to 800 °C resulted in a decrease in the 18O16O concentration and a substantial increase in the 16O2 concentration.

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Figure 6. Distribution of isotopic dioxygen in the 18O/16O exchange experiments performed over (a) Bi-2212, (b) Bi-2212-F, and (c) Bi-2212-Cl (all three were pretreated in helium at 800 °C for 1 h) at various temperatures. (b) 18O2, (0) 18O16O, and (O) 16O2.

4. Discussion 4.1. Enhancement of Catalytic Performance. From Figure 2, one can observe that, with the introduction of halide ions into the layered cuprates, the C2H6 conversion, C2H4 selectivity, and C2H4 yield increased significantly. On the basis of similar specific surface areas (Table 1), we conclude that the catalytic performances follow the order of Bi-2212-Cl > Bi-2212-F . Bi-2212. As discussed previously,13 the amount of oxygen consumed in the ODE reaction is smaller than that consumed in the deep ethane oxidation reactions; at similar oxygen conversions (above 90%), the C2H6 conversion and C2H4 selectivity can increase simultaneously if the ethane deep oxidation is reduced. By using 13C-labeled C H and C H , Lunsford and co-workers30 2 4 2 6 revealed that C2H4 was the dominant source for COx formation at or above 650 °C. This implies that the C2H4 selectivity would be improved if the deep oxidation of C2H4 could be reduced or suppressed. As shown in Table 2, halide doping results in a remarkable decrease in the C2H4 conversion and a considerable increase in the CO/ CO2 ratio in the oxidation of ethylene. This phenomenon indicates that, by incorporating the halide ions into the Bi-2212 lattice, one can reduce the deep oxidation of C2H4 and thus enhance the C2H4 selectivity in the ODE reaction. It should be noted that no CH4 was formed over the Bi-2212 catalyst throughout the whole tem-

perature range of examination, whereas over the Bi2212-F and Bi-2212-Cl catalysts, the CH4 selectivity increased with the increase in reaction temperature (Figure 2). The generation of methane might follow two possible routes: ethane decomposition in the gas phase and a heterogeneous pathway involving an ethylperoxy intermediate.31 The ethylperoxy reacts with surface oxygen species to form CH4 and HCO2, and the latter is further oxidized to COx. This mechanism also explains the enhancement in CO/CO2 ratio with the increase in CH4 selectivity over the halogenated catalysts (Table 2). After examining the stability limit of a Bi-2212 phase, Rubin et al.32 reported that this material is stable below 800 °C. The existence of a single phase of tetragonal layered structure and the fact that no significant structural changes occurred after use (60 h of on-stream ODE reaction) imply that the Bi-2212-F and Bi-2212Cl (Figure 1) halo-oxides are thermally stable in the temperature range investigated in the present work. The halogen contents of the fresh and used catalysts were rather similar (Table 1). Lifetime studies demonstrated that the halide-intercalated catalysts were stable over a period of 60 h. These results suggest that both Bi-2212-F and Bi-2212-Cl are good and durable catalysts for the ODE reaction. 4.2. Formation of Structure Defects. It has generally been accepted that the structural defects of perovskites have an important role to play in the catalysis of HC oxidation reactions. For Cu-substituted LaMnO3-δ perovskite, its structure transforms from an oxidative nonstoichiometry (δ < 0) to a reductive one (δ > 0) with an increase in Cu content.33,34 Similar situations arise for the Bi-2212 material. Most researchers believe that extrastoichiometric oxygen is present in Bi-2212. As shown in Table 1, the oxygen nonstoichiometric amount (δ) of the halogen-free catalyst was -0.058. The distribution of the excess oxygen between the Bi-O and Cu-O layers depends on the conditions of thermal treatment during catalyst preparation.35 Because the Bi-2212 oxide of the present work was annealed in O2 at 600 °C for 48 h, Cu3+, but not Bi5+, ions were present in it, as suggested by the outcomes of the copper and bismuth oxidation state chemical analyses (Table 1) and the XPS measurements (Figure 4I), in accordance with the results of Bobylev et al.35 and Huong et al.36 Because only a limited amount of F- or Cl- ions was incorporated into the layered cuprate lattice, the crystal structures remained unaltered after halogenation, as implied by the XRD results (Figure 1). The incorporation of halide ions, however, directly influences the concentrations of Cu3+ and/or Bi5+ ions, a result of charge compensation. After investigating the crystal structures of a number of halide-doped perovskite HTSCs, researchers37-40 have come to believe that the changing of oxygen nonstoichiometry is an important factor for the presence of halide ions. The intercalation of halogen, such as bromine,41 iodine,42 or both bromine and iodine,43 into the Bi-2212 lattice has been investigated intensively. The intercalated halogen species locate between the BiO-BiO double layers, as confirmed by the results of transmission electron microscopic studies,44,45 and exist in ionized forms (Br-, I- or I3-).41,46,47 In our previous studies,13,14 we have discussed the location of the halide ions in a perovskite. The entrance of halide ions into oxygen vacancies or interstitial sites would lead to an increase in the Cu3+ and/or Bi5+ content. In contrast, if

Ind. Eng. Chem. Res., Vol. 41, No. 1, 2002 43

the halide ions entered into the positions originally occupied by lattice oxide ions, then the oxidation states of nearby copper and/or bismuth would drop. Fuertes and co-workers48 reported that the filling of oxygen vacancies via the oxidation of Bi6Sr8-xCa3+xO22 (-0.5 e x e 1.7) at 650 °C would result in the formation of a double perovskite (Sr2-xCax)Bi1.4Ca0.6O6 (0 e x e 0.5) that showed 92.5% Bi5+. It should be noted that, compared to the conversion process of Cu2+ to Cu3+, those of Cu2+ to Cu+ and Bi3+ to Bi5+ are relatively easier. Furthermore, Martin and Lee49 pointed out that the Bi-O layers are only partially occupied with oxygen and account for the variable incorporation of oxygen. Working on a series of undoped and Pb(Ba)-doped TlSr2Ca2Cu3OyFx HTSC materials using the XPS technique, Hamdan and co-workers50 identified the sites of fluorine incorporated in this layered oxide; they concluded that most of the F- ions replaced the O2- ions in the Cu-O planes, causing the valence of copper to decrease. Taking into account the presence of Cu+ and Bi5+ in the halogenated Bi-2212 catalysts (Table 1 and Figure 4), as well as considering the requirement for charge balance, one can deduce that (i) certain amount of the halide ions occupies the oxygen vacancies and/or diffuses into the interstitial spacings between the BiO-BiO double layers (bringing about the formation of Bi5+) and (ii) the rest displace lattice oxide ions in the Cu-O layers (leading to the generation of Cu+). 4.3. Promotion of Lattice Oxygen Activity. The incorporation of halide ions into Bi-2212 results in the removal of the extrastoichiometric oxygen, causing (i) the low-temperature oxygen desorption peak (in Figure 5Ia) and reduction band (in Figure 5IIa) to disappear, (ii) the temperatures for the desorption (Figure 5Ib and Ic) and reduction (Figure 5IIb and IIc) of lattice oxygen to decrease, and (iii) the amounts of lattice oxygen desorbed to increase (Figure 5Ib and Ic). In other words, the inclusion of halide ions in the Bi-2212 lattice promotes the activity of the lattice oxygen. The XPS results revealed that the O 1s BEs (ca. 530.4 eV) of the lattice oxygen in Bi-2212-F and Bi-2212-Cl were 2.1 eV higher than that (528.3 eV) in Bi-2212 (Figure 4). For Bi-2212-F, this is understandable because F is more electronegative than O (electronegativity: F, 3.98; O, 3.44). In addition to a change in carrier number of the host material resulting from the charge transfer between guest atoms and the host material, chloride incorporation would induce an expansion of lattice because Cl- (1.81 Å) is larger than O2- (1.40 Å) in size. Similar effects of halide doping have been observed in studies of Br- or I-incorporated Bi-221241,51 and in our previous investigations.13,14 This means that the presence of F or Cl in the layered oxide lattice weakens the cation-oxygen bonds, making the lattice oxygen more active. Therefore, halogenation enhances the activity of lattice oxygen in Bi-2212. Generally speaking, lattice oxygen in a perovskite oxide is responsible for the selective oxidation of HCs. As shown in Figure 5I, the onset temperatures for lattice oxygen desorption decreased in the order of ca. 560 °C (Bi-2212-Cl) < ca. 650 °C (Bi-2212-F) < ca. 740 °C (Bi-2212), coinciding with the sequence of the drop in C2H4 selectivity (Figure 2). The results provide supporting evidence for the function of lattice oxygen in the ODE reaction. The high activity of the lattice oxygen in the halideincorporated catalysts is confirmed by the results of the 18O/16O exchange studies (Figure 6). The process of

oxygen isotope exchange can proceed via the following steps 18

O2 (g) + 16O (*) f 18O16O (g) + 18O (*)

(1)

18

O16O (g) + 16O (*) f 16O2 (g) + 18O (*)

(2)

18

18

O2 (g) + 216O (*) f 16O2 (g) + 218O (*)

O16O (g) + 216O (*) f 16O2 (g) + 18O (*) + 16O (*) (4) 18

18

(3)

O2 (g) + 16O2 (g) f 218O16O (g)

(5)

O16O (g) + 218O (*) f 18O2 (g) + 18O (*) + 16O (*) (6)

where g denotes the gas phase and *, an adsorbed or lattice oxygen. Because the catalysts had been purged with helium at 740 °C for 1 h before the pulsing of 18O2 [this treatment guarantees the removal of adsorbed oxygen species, as illustrated in the O2-TPD studies (Figure 5I)], one can disregard the existence of adsorbed 16O species on the surfaces or at the oxygen vacancies of the catalysts; i.e., the 16O atoms in the desorbed 18O16O and 16O originated entirely from the lattice 2 oxygen of the catalysts. By comparing the relative concentrations of 18O2, 18O16O, and 16O2, one can deduce that the 18O/16O exchange processes proceed (i) via steps 1-6 below 680 °C and via steps 1-5 above 680 °C over Bi-2212; (ii) via steps 1-6 below 620 °C, via steps 1-5 between 620 and 760 °C, and via steps 1-4 above 760 °C over Bi-2212-F; and (iii) via steps 1-6 below 560 °C, via steps 1-5 between 560 and 740 °C, and via steps 1-4 above 740 °C over Bi-2212-Cl. Compared to Bi-2212 (Figure 6a), Bi-2212-F and Bi2212-Cl showed much higher concentrations of 18O16O and 16O2 at 760 or 740 °C (Figure 6b and c), indicating that the incorporation of F- or Cl- ions promoted the activity of the lattice oxygen in the Bi-Sr-Ca-Cu-O oxide. The facile infiltration of 18O into the layered oxide lattice and the rapid replenishment of lattice oxygen according to the sequence e-

e-

O2 (g) 98 O2- (ads) 98 O22- (ads) f 2e-

2O- (ads) 98 2O2- (ads) f 2O2-lattice (7) demonstrate that the lattice oxygens in the Bi-2212-F and Bi-2212-Cl catalysts are highly active. As discussed in section 4.2, the coexistence of Cu+/Cu2+ and Bi3+/Bi5+ couples in Bi-2212-F and Bi-2212-Cl, respectively, is favorable for the oxygen cycle during the ODE reaction. With the release of electrons in the Cu+ f Cu2+ and Bi3+ f Bi5+ processes, the lattice oxygen consumed in ethane oxidation could be replenished rather effectively via eq 7. As revealed in the XPS studies (Figure 4), the oxidation states of copper and bismuth decreased with the evolution of oxygen from the catalysts at and above 740 °C, implying that the two metal ions are indeed ready to release electrons. Therefore, we suggest that the halogenation of Bi-2212 modifies the oxygen nonstoichiometry and the copper and bismuth oxidation states, thus enhancing lattice oxygen activity, which

44

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accounts for the improvement in catalytic performance of the layered oxide materials in the ODE reaction. 5. Conclusions The halogenation of Bi-2212 reduced C2H4 deep oxidation remarkably. The chlorinated catalyst performed better than the fluorinated one. At 680 °C and 1.67 × 10-4 h g mL-1 with a C2H6/O2/N2 ratio of 2/1/ 3.7, Bi-2212-F gave 70.8% C2H6 conversion, 72.5% C2H4 selectivity, and 51.3% C2H4 yield; Bi-2212-Cl gave 77.2% C2H6 conversion, 76.6% C2H4 selectivity, and 59.1% C2H4 yield. Within a 60-h period of on-stream reaction at 680 °C, the two halogenated catalysts exhibited sustainable performances. The results of the XRD studies indicate that the halogen-free Bi-2212 and halogenated Bi2Sr2CaCu2O8-δXσ (X ) F, Cl) are singlephase and tetragonal in structure. The results of XPS and Cu and Bi oxidation state studies indicated that, for Bi-2212-F and Bi-2212-Cl, copper exists in uni- and bivalencies, whereas bismuth exists in tri- and pentavalencies. The results of O2-TPD, TPR, and 18O/16O isotope exchange studies confirm the promotional effects of halide doping on the activity of the lattice oxygen. Judging from the above outcomes, we conclude that, through halogenation, one can increase the lattice oxygen activity and, hence, enhance the catalytic performance of the Bi-Sr-Ca-Cu-O materials for ethaneselective oxidation. Acknowledgment The work described above was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, P. R. China (Project HKBU 2015/99P). We thank Dr. K. W. Wong of the Physics Department, Chinese University of Hong Kong, for performing the XPS investigations. Literature Cited (1) Nitadori, T.; Kurihara, S.; Misono, M. Catalytic Properties of La1-xA′xMnO3 (A′ ) Sr, Ce, Hf). J. Catal. 1986, 98, 221. (2) Rajadurai, S.; Carberry, J. J.; Li, B.; Alcock, C. B. Catalytic Oxidation of Carbon Monoxide over Superconducting Lanthanum Strontium Copper Oxide (La2-xSrxCuO4-δ) Systems between 373 and 523 K. J. Catal. 1991, 131, 582. (3) Yasuda, H.; Fujiwara, Y.; Mizuno, N.; Misono, M. Oxidation of Carbon Monoxide on LaMn1-xCuxO3 Perovskite-Type Mixed Oxides. J. Chem. Soc., Faraday Trans. 1994, 90, 1183. (4) Seiyama, T. Total Oxidation of Hydrocarbons on Perovskite Oxides. In Properties and Applications of Perovskite-Type Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker: New York, 1993; p 215. Viswanathan, B. CO Oxidation and NO Reduction on Perovskite Oxides. In Properties and Applications of PerovskiteType Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker: New York, 1993; p 271. (5) Bednorz, J. G.; Mu¨eller, K. A. Possible High Tc Superconductivity in the Barium-Lanthanum-Copper-Oxygen System. Z. Phys. B: Condens. Matter 1986, 64, 189. (6) Cava, R. J.; van Dover, R. B.; Batlogg, B.; Rietman, E. A. Bulk Superconductivity at 36 K in Lanthanum Strontium Copper Oxide (La1.8Sr0.2CuO4). Phys. Rev. Lett. 1987, 58, 408. (7) Alp, E. E.; Shenoy, G. K.; Hinks, D. G.; Capone, D. W., II; Soderholm, L.; Schuttler, H. B. Guo, J.; Ellis, D. E.; Montano, P. A.; Ramanathan, M. Determination of Valence of Copper in Superconducting Lanthanum Strontium Barium Copper Oxide (La2-x(Sr, Ba)xCuO4). Phys. Rev. B: Condens. Matter. 1987, 35, 7199. (8) Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.; Chu, C. W. Superconductivity at 93 K in a New Mixed-Phase Yttrium-Barium-

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Received for review March 19, 2001 Revised manuscript received October 18, 2001 Accepted October 18, 2001 IE0102504