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Catalytic behavior of O2-pretreated Ni3(SbTe3)2 catalyst in oxidative coupling of methane
Applied Catalysis A: General 237 (2002) 91–101
Catalytic behavior of O2 -pretreated Ni3(SbTe3)2 catalyst in oxidative coupling of methane Sung Han Lee a,∗ , Jin-Seung Jung b , Jae-Uk Joo a , No-Seung Myung c , Jong Ho Jun c , Joong-Gil Choi d b
a Department of Chemistry, Yonsei University, Wonju 220-710, South Korea Department of Chemistry, Kangnung National University, Kangnung 210-320, South Korea c Department of Applied Chemistry, Konkuk University, Choongju 380-701, South Korea d Department of Chemistry, Yonsei University, Seoul 120-749, South Korea
Received 6 February 2002; received in revised form 27 April 2002; accepted 20 May 2002
Abstract Amorphous Ni3 (SbTe3 )2 was prepared from a metathesis between K3 SbTe3 and NiBr2 solution and was examined as a catalyst for the oxidative coupling of methane in a single-pass flow reactor system using on-line gas chromatography at atmospheric pressure. Catalytic reaction was performed by feeding the reaction mixture containing CH4 /O2 /He in the temperature range of 600–750 ◦ C. Although amorphous Ni3 (SbTe3 )2 showed no catalytic activity for the oxidative coupling of methane, the O2 -pretreated Ni3 (SbTe3 )2 was found to be active and selective for the reaction. Its C2 selectivities were in the range of 45–79% in the temperature range of 600–750 ◦ C. The best C2 -yield was 11% with a selectivity of 79% at 650 ◦ C. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the catalytic reaction to characterize the catalyst. Thermogravimetry (TG) analysis was performed for amorphous Ni3 (SbTe3 )2 in a flow of O2 /Ar mixture. Electrical conductivity of amorphous Ni3 (SbTe3 )2 was measured as a function of temperature in the range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm. The results suggest that NiTeO3 formed on the surface by the O2 -pretreatment of catalyst is active and is the selective phase for the oxidative coupling of methane. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous Ni3 (SbTe3 )2 ; Oxidative coupling of methane
1. Introduction Direct conversion of methane into ethylene or ethane has received a great deal of attention for the utilization of natural gas since Keller and Bhasin [1] reported the C2 -hydrocarbon production over metal oxide catalyst in 1982. It has been known that high surface basicity of a catalyst is necessary to enhance the C2 selectivity in the oxidative coupling of ∗ Corresponding author. Fax: +82-33-763-4323. E-mail address: [email protected] (S.H. Lee).
methane and that alkali promoters can enhance the basicity of the catalyst [2]. In the oxidative coupling of methane on metal oxide catalysts, most metal oxide catalysts require a reaction temperature greater than 700 ◦ C [3]. When a certain transition metal is incorporated into alkali/alkaline earth metal oxide mixtures, the reaction temperatures at which it produces C2 -hydrocarbons at high selectivity can be significantly reduced [4–6]. For instance, Ni-containing K/Ca oxide mixture shows an appreciable C2 selectivity at the reaction temperatures below 700 ◦ C [6].
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 3 1 9 - 8
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The catalytic reaction mechanisms of oxidative coupling of methane have been reported by many investigators [7–9]. It is widely accepted that active oxygen ions formed on the surface of metal oxide catalyst selectively activate methane to form methyl radicals and that the resulting methyl radicals are coupled to ethane on the catalyst surface or in the gas phase. The catalytic reaction mechanisms can vary with the catalysts because both adsorbed oxygen and lattice oxygen can be active sites for methane. To find suitable catalysts which can give high C2 -yield in the oxidative coupling of methane, a variety of solid materials have been examined as catalysts for the reaction over the past 20 years. However, relatively few metal–tellurium systems have been examined as heterogeneous catalysts for the reaction until now. In this work, we prepared Ni-containing telluride compound to examine it as a catalyst for the reaction. Amorphous nickel–antimony–tellurium system was prepared from the reaction between a Zintl material and nickel salt in solution and the catalytic activity in the oxidative coupling of methane was measured. The amorphous Ni–Sb–Te system is easily associated with gaseous oxygen, which gives a possibility of active oxygen species being formed on the surface. Elemental nickel is known to be an active catalyst in the partial oxidation of methane, producing almost exclusively carbon monoxide [10] and elemental antimony is a good promoter in mixed oxide catalysts for the selective oxidation and ammoxidation of light hydrocarbons [11–13]. In this respect, it is interesting to study the catalytic behavior of amorphous Ni3 (SbTe3 )2 in the partial oxidation of methane. Amorphous Ni3 (SbTe3 )2 prepared in this work is insoluble in polar solvents such as water, and thus, it is applicable as a catalyst to an oxidation of hydrocarbons. This paper reports the activity and selectivities of O2 -pretreated Ni3 (SbTe3 )2 catalyst for the oxidative coupling of methane and its characterization by means of X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), and dc electrical conductivity.
2. Experimental Amorphous Ni3 (SbTe3 )2 system was prepared by using a rapid precipitation metathesis reaction between
the Zintl material K3 SbTe3 and NiBr2 in solution, as described by Jung et al. [14]. The ternary Zintl phase K3 SbTe3 prepared from a direct combination of the elements is described as a polar metallic solid in which the bonding gives rise to a substantial amount of ionic bonding character. Therefore, K3 SbTe3 is soluble in polar solvents such as formamide and water by the reaction: K3 SbTe3 (s) → 3K + + SbTe3− 3 , and the Zintl solution allows subsequent metathesis reaction with nickel salt to form amorphous material due to the transfer of electrons from the SbTe3 3− anion to the nickel cation according to the reaction: 3Ni2+ + 2SbTe3− → Ni3 (SbTe3 )2 (s) [15]. A stoi3 chiometric quantity of K3 SbTe3 aqueous solution was added slowly while stirring the NiBr2 solution. A precipitate was formed immediately. It was then separated by solution filtration, washed with deionized water and acetone, and dried in a vacuum oven. The experimental empirical formula determined for the resultant product was Ni3.22 Sb2 Te7.18 . All manipulations were carried out in an argon-filled glovebox because of the air sensitivity of the compounds. Catalytic reaction was performed at atmospheric pressure in a fixed-bed flow reactor which was made of alumina tubing with 0.8 cm inner diameter and 20 cm length. The catalyst was held between alumina wool plugs in the middle of the reactor and the section beyond the catalyst bed in the reactor was filled with alumina beads to reduce the free space. The reactor was kept in a vertical tubular furnace. To control the reaction temperature, a K-type thermocouple sealed with an alumina tube was placed just above the catalyst loaded in the reactor. Methane and oxygen were co-fed with He diluent gas. The purity of gaseous oxygen and methane was greater than 99.9%. Gaseous reactants and products were analyzed using on-line gas chromatography equipped with a thermal conductivity detector and a flame ionization detector. A cold trap was placed at the reactor exit to remove water vapor from the gaseous mixture. Gas compositions were calculated using an external standard gas mixture. Blank test was performed over inert alumina beads in the absence of catalyst and approximately 2–4% conversion of methane to carbon oxides was obtained in the reaction temperature range of 600–750 ◦ C. The conversion of methane was calculated from the amounts of products generated and the methane introduced in the feed stream. The selectivities were calculated on
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the basis of the conversion of methane to each product and the yield was obtained from the methane conversion and the selectivity to each product. The closures on the carbon material balances were within 4%. Most of the experiments described in this paper were typically performed under the following conditions: atmospheric pressure, a 300 mg sample loading of catalyst, a reaction feed of CH4 /O2 /He = 5/1/14 cm3 /min at ambient conditions. The methane conversion and product selectivities were compared after 1 h time-on-stream. Since the present catalyst exhibited a catalytic activity for the oxidative coupling of methane only when it was treated in a flow of oxygen prior to reaction, the Ni3 (SbTe3 )2 was typically pretreated in situ at 600 ◦ C in a flow of dry O2 (10 cm3 /min) for 1 h before each activity measurement. After the O2 -treatment of catalyst, the reactor was cooled to room temperature, He gas was passed to remove oxygen gas remaining in the reactor, then the reactor temperature was ramped to the desired value using a programmable temperature controller and finally a reaction mixture was fed over the catalyst. To identify the crystalline phases present on the catalyst, XRD analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after reaction by using a Philips PW 1710 diffractometer. To investigate the oxidation state of each element on the catalyst surface, XPS analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the reaction by using VG ESCALAB spectrometer, in which all the XPS binding energies were referenced to the 83.0 eV of Au(4f7/2 ). The BET surface area of the O2 -treated Ni3 (SbTe3 )2 catalyst determined by measuring the adsorption of nitrogen at liquid nitrogen temperature was about 12 m2 /g. TG analysis was performed for amorphous Ni3 (SbTe3 )2 at a heating rate of 10 ◦ C/min in the temperature range of 25–900 ◦ C in a flow of O2 /Ar mixture. Electrical conductivity of amorphous Ni3 (SbTe3 )2 was measured in the temperature range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm by means of the four-probe method. To measure the electrical conductivity, the powdered sample was compressed in vacuum into a pellet with a diameter of 12 mm and a thickness of about 3 mm. The pellet was contacted with the Pt probes by using nickel paste and then inserted into the quartz sample container. The current was supplied by a Keithley model 220 Current Source
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and the voltage drop across the sample was measured using a Keithley model 181 Digital Nanovoltmeter. The details are described in [16].
3. Results and discussion In this work, non-pretreated amorphous Ni3 (SbTe3 )2 catalyst showed no C2 selectivity, but the O2 -pretreated Ni3 (SbTe3 )2 catalyst revealed C2 selectivity above 45% for the oxidative coupling of methane in the temperature range from 600 to 750 ◦ C. In the reaction, the major products were CO, CO2 , C2 H4 , and C2 H6 . The effect of O2 content on the methane conversion and product selectivities was investigated by varying the PO2 in the reaction mixture (CH4 /O2 /He) at a constant PCH4 of 0.20 atm under a total flow rate of 20 cm3 /min. Fig. 1 shows variations of the methane conversion and product selectivities of the O2 -pretreated Ni3 (SbTe3 )2 catalyst with the PO2 at 650 ◦ C. The C2 -yield increased from 7% at PO2 = 0.02 atm to 15% at PO2 = 0.16 atm, while the C2 selectivity decreased with increasing the PO2 . The ratio of ethylene/ethane increased from 0.8 to 1.3 as the PO2 increased from 0.02 to 0.16 atm. Fig. 2 shows the methane conversion and product selectivity when the reaction mixture (CH4 /O2 /He = 5/1/14 cm3 /min) was fed over the O2 -pretreated Ni3 (SbTe3 )2 catalyst in the temperature range from 600 to 750 ◦ C. The C2 selectivity decreased with increasing temperature and the production of CO2 largely increased, while the methane conversion increased with increasing temperature. The catalyst showed the best C2 -yield of 11% with a selectivity of 79% at 650 ◦ C, which is rather appreciable. We investigated the effect of time of contact (W/F) on the methane conversion and product selectivities by changing the catalyst weight under a total flow rate of 20 cm3 /min (CH4 /O2 /He = 5/1/14 cm3 /min). Fig. 3 represents variations of the methane conversion and product selectivity with the contact time at 650 ◦ C. The C2 selectivities were kept in the range of 70–80% in the W/F range. The ratio of ethylene/ethane increased with increasing the contact time. The result suggests that ethane is the initial product and ethylene is subsequently produced by the dehydrogenation of ethane, which fits with the results reported by other investigators on the oxidative coupling of methane over
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Fig. 1. Effect of oxygen content in reaction feed on methane conversion and selectivity of the O2 -pretreated Ni3 (SbTe3 )2 catalyst for the oxidative coupling of methane at 650 ◦ C: (䊉) CH4 conversion, (䉱) C2 H4 , () C2 H6 , (䊏) CO, (䊐) CO2 , (䉬) C3 -hydrocarbons.
metal oxide catalysts [17,18]. Fig. 4 shows variations of methane conversion, C2 selectivity, and COx selectivity with time-on-stream over a 15 h period when the reaction mixture was fed over the O2 -pretreated Ni3 (SbTe3 )2 at 650 ◦ C. The methane conversion de-
clined from 14% at 1 h time-on-stream to 5% at 15 h time-on-stream. To characterize the catalyst, TG analysis was performed for amorphous Ni3 (SbTe3 )2 , XRD and XPS analyses were performed for the O2 -treated
Fig. 2. Variations of methane conversion and product selectivity with temperature for the oxidative coupling of methane over the O2 -pretreated Ni3 (SbTe3 )2 catalyst: (䊉) CH4 conversion, (䊏) CO, (䊐) CO2 , (䉱) C2 H4 , () C2 H6 , (䉬) C3 -hydrocarbons.
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Fig. 3. Effect of contact time (W/F) on methane conversion and selectivity for the oxidative coupling of methane over the O2 -pretreated Ni3 Sb2 Te6 catalyst at 650 ◦ C: (䊉) CH4 conversion, (䊊) C2 total, (䉱) C2 H4 , () C2 H6 , (䊏) CO, (䊐) CO2 .
Fig. 4. Variations of methane conversion and product selectivity with time-on-stream for the oxidative coupling of methane over the O2 -pretreated Ni3 (SbTe3 )2 catalyst at 650 ◦ C: (䊉) CH4 conversion, (䊏) COx selectivity, (䉱) C2 selectivity.
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Fig. 5. TG curves of amorphous Ni3 (SbTe3 )2 in a flow of O2 /Ar gas mixture: (A) PO2 = 0.20 atm, (B) PO2 = 0.01 atm.
Ni3 (SbTe3 )2 before and after the catalytic reaction, and the electrical conductivities of amorphous Ni3 (SbTe3 )2 were measured by a four-probe technique. Fig. 5 presents the TG curves measured in flows of both O2 /Ar mixtures with the PO2 ’s of 0.01 and 0.20 atm. A weight gain occurs in the temperature range of 200–750 ◦ C and the uptake in weight is larger at PO2 = 0.20 atm than at PO2 = 0.01 atm, which means that a lot of gaseous oxygen is chemisorbed on the catalyst surface, leading to an oxidation of catalyst. In general, metal tellurides are known to be readily oxygenated in the presence of gaseous oxygen; the present result is similar to those for other metal tellurides [19–21]. The cause of the weight decrease above 750 ◦ C is mainly the vaporization of tellurium. XRD analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the reaction. The X-ray diffraction pattern of the O2 -treated Ni3 (SbTe3 )2 catalyst showed the presence of NiO, Ni3 Te2 , SbTe, and NiTeO3 phases, as presented in Fig. 6(A). The result indicates that Ni3 (SbTe3 )2 was decomposed and oxidized to form the oxides by the O2 -treatment. The remaining peaks in Fig. 6(A) belong to a crystalline Ni–Sb–Te phase of which the chemical formula and the structure are not determined in this work. NiTeO3 was decomposed into NiO and TeO2 above 750 ◦ C and the TeO2 was subsequently decomposed and evaporated, resulting in the weight
loss above 750 ◦ C, as shown in Fig. 5. X-ray analysis of the O2 -treated Ni3 (SbTe3 )2 catalyst after the reaction between methane and oxygen at 650 ◦ C for 15 h showed the presence of NiO, NiSb2 O6 , NiSb2 (OH)12 , and NiTeO3 phases as given in Fig. 6(B), indicating that both NiSb2 O6 and NiSb2 (OH)12 phases were produced through the catalytic reaction. We measured the electrical conductivity of amorphous Ni3 (SbTe3 )2 , while increasing the temperature at a heating rate of 5 ◦ C/min in the temperature range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm. Fig. 7 presents the electrical conductivities plotted as a function of temperature according to the equation, σ = A exp(−Ea /RT). As given in Fig. 7, inflection points are observed around 585 ◦ C. Namely, the electrical conductivity decreases with increasing temperature below 585 ◦ C, while the electrical conductivity increases with increasing temperature above 585 ◦ C, indicating that the specimen changes from a metallic conductor to a semiconductor around 585 ◦ C. Fig. 8 shows the Ni(3p) XPS spectra of the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the catalytic reaction. The Ni(3p3/2 ) XPS spectra of unused O2 -treated Ni3 (SbTe3 )2 catalyst present a maximum at 855.1 eV and a shoulder at 852.5 eV. The lower binding energy at 852.5 eV is due to Ni belonging to nickel telluride phase and the higher binding energy at 855.1 eV is due to Ni associated with oxygen.
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Fig. 6. X-ray diffraction patterns of the O2 -pretreated Ni3 (SbTe3 )2 catalyst (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
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Fig. 7. Electrical conductivity of amorphous Ni3 (SbTe3 )2 as a function of temperature in the range of 400–800 ◦ C: (A) PO2 = 0.20 atm, (B) PO2 = 0.01 atm.
The Ni(3p3/2 ) XPS spectra of O2 -treated Ni3 (SbTe3 )2 catalyst after the catalytic reaction at 650 ◦ C for 15 h give a maximum at 854.4 eV. Considering that the Ni(3p3/2 ) binding energy of stoichiometric NiO is observed at 853.3 eV, the higher binding energies at 855.1 and 854.4 eV suggest that nickel ions exist in a higher oxidation state on the catalyst surface. Fig. 9 shows the Te(3d) XPS spectra of the catalyst. The Te(3d5/2 ) XPS spectra of unused O2 -treated Ni3 (SbTe3 )2 catalyst give two peaks at 572.2 and 575.4 eV. The higher binding energy at 575.4 eV is due to Te associated with oxygen on the surface and the lower binding energy at 572.2 eV is due to Te belonging to nickel telluride or antimony telluride phase. The Te(3d5/2 ) XPS spectra of O2 -treated catalyst after the reaction give a maximum at 575.6 eV,
Fig. 8. Ni(3p) XPS spectra of the O2 -pretreated Ni3 (SbTe3 )2 (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
indicating that the Te ions are associated with oxygen ions. The O(1s) XPS spectra gave a maximum at 530.1 eV for the unused catalyst and a maximum at 529.9 eV for the used catalyst. The O(1s) XPS spectra are believed to be a combination of peaks corresponding to oxygen and antimony ions because the binding energies of O(1s) and Sb(3d5/2 ) are close. We could not resolve the O(1s) XPS peaks into sharp peaks. The Sb(3d3/2 ) XPS spectra showed a major peak at 539.3 eV for the unused catalyst and one at 539.6 eV for the used catalyst. It has been reported that some metal–metal and metal–metalloid alloys show superior catalytic activity in the hydrogenation of carbon oxides [22,23]. Their catalytic properties were largely influenced by the
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Fig. 9. Te(3d) XPS spectra of the O2 -pretreated Ni3 (SbTe3 )2 (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
conditions of preparation and pretreatment and their surface structures were modified by the catalytic reaction itself. For instance, Ni–Zr alloy exhibits a catalytic activity for the methanation of carbon oxides; its high activity was found to be due to the tetragonal ZrO2 formed on the catalyst surface [23]. In the oxidative coupling of methane over metal oxide catalysts, methyl radicals are produced by the abstraction of a hydrogen atom from methane chemisorbed on the active oxygen ions formed on the catalyst surface. The O2 -pretreated Ni3 (SbTe3 )2 catalyst showed an appreciable C2 selectivity in the oxidative coupling of methane, which suggests that active oxygen ions were generated on the catalyst surface by the O2 -treatment. Many investigators have reported that O− , O2 − , and O2 2− can be generated on oxygen vacancy or basic sites in the metal oxide catalysts and O− ion is responsible for the selective activation of methane in the oxidative coupling of methane [8,24]. The stability of O− ion formed on the catalyst surface is known to depend on the basicity of catalyst and the existence of suitable sites [25]. As shown in Fig. 6(A), when amorphous Ni3 (SbTe3 )2 was treated in a flow of oxygen at 600 ◦ C for 1 h, NiO and NiTeO3 were formed on the surface as
XRD detectable phases. According to the conductivity data in Fig. 7, Ni3 (SbTe3 )2 alters from a metallic conductor to a semiconductor around 585 ◦ C and the electrical conductivities measured at PO2 = 0.20 atm are higher than those measured at PO2 = 0.01 atm, implying that the specimen is a p-type semiconductor above 585 ◦ C. The p-type conductivity seems to originate from NiO networks, because NiO is known to be a p-type semiconductor. The activation energy calculated from the conductivity–temperature data in the temperature range of 600–800 ◦ C is 0.8 eV, such a value is in the range of 0.78–1.07 eV reported by other investigators for pure NiO [26], supporting the conclusion that the p-type conductivity arises from NiO phase. When NiO particles in the catalyst are either making contacts among themselves or are separated by very small gaps, the charge carriers can migrate from one aggregate to a neighboring one through hopping; then a p-type conductivity will be observed, as in the present result. We measured catalytic activities and selectivities of NiO, Sb2 O3 , TeO2 , and NiSb2 O6 catalysts for the oxidative coupling of methane at 700 ◦ C. In the reaction, both NiO and TeO2 catalysts did not produce C2 -hydrocarbons, Sb2 O3 catalyst showed a C2
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selectivity of 32% at a methane conversion of 8%, and NiSb2 O6 catalysts showed a C2 selectivity of 64% at a methane conversion of 15%. The results indicate that these catalysts are less active and selective than the O2 -pretreated Ni3 (SbTe3 )2 catalyst. We could not measure the catalytic activity of NiTeO3 in this work. We tried to prepare NiTeO3 phase from the oxidation of Ni–Te mixture in a flow of gaseous oxygen, but we could not obtain the monophase NiTeO3 . When Ni–Te mixture was oxidized in a flow of oxygen at 600 ◦ C, various oxides such as NiO, TeO2 , NiTe2 O5 , Ni2 Te3 O8 , and NiTeO3 were produced. According to the study on the oxidation of nickel–tellurium compounds [27], NiO is produced in the first stage of oxidation; above 450 ◦ C, various nickel–tellurium oxides are slowly formed; above 900 ◦ C, the oxides decompose, leaving NiO as the final solid product. The nickel–tellurium oxide mixture containing some nickel oxide exhibited a methane conversion of 7% with a C2 selectivity of 25% for the reaction at 650 ◦ C. In the case of Ni-containing K/Ca oxide catalyst showing an appreciable C2 selectivity in the oxidative coupling of methane, the selective phase was found to be a K/Ca/Ni oxide with some nickel in valency higher than +2 [5]. Since the formation of higher valent Ni ion can be closely related with the existence of active oxygen ions (O− ), nickel ion in valency higher than +2 is believed to be a suitable site that is able to stabilize the O− ions on the catalyst surface at high temperatures. The present catalyst showed the best C2 -yield of 11% with a selectivity of 79% at 650 ◦ C, as presented in Fig. 2. It is noted that the temperature (650 ◦ C) showing the best C2 -yield is lower than those (>700 ◦ C) observed usually for metal oxide catalyst for the oxidative coupling of methane. In Fig. 6, the XRD analysis of the O2 -treated Ni3 (SbTe3 )2 catalyst after the catalytic reaction shows the presence of NiSb2 O6 and NiSb2 (OH)12 phases in the catalyst. The XRD signal intensities of both NiO and NiTeO3 were somewhat reduced after the catalytic reaction. The results indicate that some NiTeO3 is decomposed, leaving NiO through the catalytic reaction, and then some NiO reacts with antimony oxide to form NiSb2 O6 phase. The NiSb2 O6 phase reacts with H2 O(g) being produced from the catalytic reaction to form NiSb2 (OH)12 . Therefore, the decrease in the methane conversion and C2 selectivity
along with time-on-stream in Fig. 3 seems to be due to the formation of nickel–antimony oxide and nickel–antimony hydroxide. Although the nature of active oxygen species associated with the catalyst surface and the mechanism of this reaction remain to be further studied, the experimental results suggest that the selective phase on the O2 -treated Ni3 (SbTe3 )2 catalyst is NiTeO3 containing some higher valent Ni ions and tellurium ions may promote the formation of higher valent Ni ions. Acknowledgements The authors are grateful for financial support from Korea Research Foundation Grant (KRF-2000-042D00056). References [1] G.E. Keller, M.M. Bhasin, J. Catal. 73 (1982) 9. [2] T. Ito, J.-X. Wang, C.-H. Lin, J.H. Lunsford, J. Am. Chem. Soc. 107 (1985) 5062. [3] J.S. Lee, S.T. Oyama, Catal. Rev. Sci. Eng. 30 (1988) 249. [4] Y. Amenomiya, V.I. Birss, M. Goledzionwsk, J. Guluszk, A.R. Sanger, Catal. Rev. Sci. Eng. 32 (1990) 163. [5] K.M. Dooley, S.-Y. Chen, J.R.H. Ross, J. Catal. 145 (1994) 402. [6] P. Pereira, S.H. Lee, G.A. Somorjai, H. Henemann, Catal. Lett. 6 (1990) 255. [7] K. Otsuka, K. Jinno, A. Morikawa, J. Catal. 100 (1986) 353. [8] S. Lacombe, C. Geartet, C. Mirodatos, J. Catal. 151 (1994) 439. [9] K.D. Campbell, H. Zhang, J.H. Lunsford, J. Phys. Chem. 92 (1988) 750. [10] R.K. Grasselli, Catal. Today 49 (1999) 141. [11] J. Nilsson, A.R. Land-Canovas, S. Hansen, A. Anderson, J. Catal. 186 (1999) 442. [12] H.-W. Zanthoff, S.A. Buchhloz, Catal. Lett. 49 (1997) 213. [13] H. Liu, H. Imoto, T. Shido, Y. Iwasawa, J. Catal. 200 (2001) 69. [14] J.-S. Jung, L. Ren, C.J. O’Connor, J. Mater. Chem. 2 (1992) 829. [15] B. Wu, L. Ren, C.J. O’Connor, J. Tang, J.-S. Jung, J. Ferre, J.-P. Jamet, J. Mater. Res. 9 (1994) 909. [16] W.R. Runyan, Semiconductor Measurements and Instrumentation, McGraw-Hill, New York, 1975. [17] E. Iwamatsu, K.-I. Alka, J. Catal. 117 (1989) 416. [18] S. Ahmed, J.B. Moffat, Appl. Catal. 54 (1989) 241. [19] P. Arun, A.G. Vedeshwar, Mater. Res. Bull. 34 (1999) 203. [20] S.B. Elvy, P.A. Williams, A.N. Buckley, Surf. Interface Anal. 24 (1996) 641. [21] J.-I. Kong, J.-S. Jung, J.-G. Choi, S.-H. Lee, Appl. Catal. A 204 (2000) 241.
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Catalytic behavior of O2 -pretreated Ni3(SbTe3)2 catalyst in oxidative coupling of methane Sung Han Lee a,∗ , Jin-Seung Jung b , Jae-Uk Joo a , No-Seung Myung c , Jong Ho Jun c , Joong-Gil Choi d b
a Department of Chemistry, Yonsei University, Wonju 220-710, South Korea Department of Chemistry, Kangnung National University, Kangnung 210-320, South Korea c Department of Applied Chemistry, Konkuk University, Choongju 380-701, South Korea d Department of Chemistry, Yonsei University, Seoul 120-749, South Korea
Received 6 February 2002; received in revised form 27 April 2002; accepted 20 May 2002
Abstract Amorphous Ni3 (SbTe3 )2 was prepared from a metathesis between K3 SbTe3 and NiBr2 solution and was examined as a catalyst for the oxidative coupling of methane in a single-pass flow reactor system using on-line gas chromatography at atmospheric pressure. Catalytic reaction was performed by feeding the reaction mixture containing CH4 /O2 /He in the temperature range of 600–750 ◦ C. Although amorphous Ni3 (SbTe3 )2 showed no catalytic activity for the oxidative coupling of methane, the O2 -pretreated Ni3 (SbTe3 )2 was found to be active and selective for the reaction. Its C2 selectivities were in the range of 45–79% in the temperature range of 600–750 ◦ C. The best C2 -yield was 11% with a selectivity of 79% at 650 ◦ C. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the catalytic reaction to characterize the catalyst. Thermogravimetry (TG) analysis was performed for amorphous Ni3 (SbTe3 )2 in a flow of O2 /Ar mixture. Electrical conductivity of amorphous Ni3 (SbTe3 )2 was measured as a function of temperature in the range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm. The results suggest that NiTeO3 formed on the surface by the O2 -pretreatment of catalyst is active and is the selective phase for the oxidative coupling of methane. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous Ni3 (SbTe3 )2 ; Oxidative coupling of methane
1. Introduction Direct conversion of methane into ethylene or ethane has received a great deal of attention for the utilization of natural gas since Keller and Bhasin [1] reported the C2 -hydrocarbon production over metal oxide catalyst in 1982. It has been known that high surface basicity of a catalyst is necessary to enhance the C2 selectivity in the oxidative coupling of ∗ Corresponding author. Fax: +82-33-763-4323. E-mail address: [email protected] (S.H. Lee).
methane and that alkali promoters can enhance the basicity of the catalyst [2]. In the oxidative coupling of methane on metal oxide catalysts, most metal oxide catalysts require a reaction temperature greater than 700 ◦ C [3]. When a certain transition metal is incorporated into alkali/alkaline earth metal oxide mixtures, the reaction temperatures at which it produces C2 -hydrocarbons at high selectivity can be significantly reduced [4–6]. For instance, Ni-containing K/Ca oxide mixture shows an appreciable C2 selectivity at the reaction temperatures below 700 ◦ C [6].
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The catalytic reaction mechanisms of oxidative coupling of methane have been reported by many investigators [7–9]. It is widely accepted that active oxygen ions formed on the surface of metal oxide catalyst selectively activate methane to form methyl radicals and that the resulting methyl radicals are coupled to ethane on the catalyst surface or in the gas phase. The catalytic reaction mechanisms can vary with the catalysts because both adsorbed oxygen and lattice oxygen can be active sites for methane. To find suitable catalysts which can give high C2 -yield in the oxidative coupling of methane, a variety of solid materials have been examined as catalysts for the reaction over the past 20 years. However, relatively few metal–tellurium systems have been examined as heterogeneous catalysts for the reaction until now. In this work, we prepared Ni-containing telluride compound to examine it as a catalyst for the reaction. Amorphous nickel–antimony–tellurium system was prepared from the reaction between a Zintl material and nickel salt in solution and the catalytic activity in the oxidative coupling of methane was measured. The amorphous Ni–Sb–Te system is easily associated with gaseous oxygen, which gives a possibility of active oxygen species being formed on the surface. Elemental nickel is known to be an active catalyst in the partial oxidation of methane, producing almost exclusively carbon monoxide [10] and elemental antimony is a good promoter in mixed oxide catalysts for the selective oxidation and ammoxidation of light hydrocarbons [11–13]. In this respect, it is interesting to study the catalytic behavior of amorphous Ni3 (SbTe3 )2 in the partial oxidation of methane. Amorphous Ni3 (SbTe3 )2 prepared in this work is insoluble in polar solvents such as water, and thus, it is applicable as a catalyst to an oxidation of hydrocarbons. This paper reports the activity and selectivities of O2 -pretreated Ni3 (SbTe3 )2 catalyst for the oxidative coupling of methane and its characterization by means of X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), and dc electrical conductivity.
2. Experimental Amorphous Ni3 (SbTe3 )2 system was prepared by using a rapid precipitation metathesis reaction between
the Zintl material K3 SbTe3 and NiBr2 in solution, as described by Jung et al. [14]. The ternary Zintl phase K3 SbTe3 prepared from a direct combination of the elements is described as a polar metallic solid in which the bonding gives rise to a substantial amount of ionic bonding character. Therefore, K3 SbTe3 is soluble in polar solvents such as formamide and water by the reaction: K3 SbTe3 (s) → 3K + + SbTe3− 3 , and the Zintl solution allows subsequent metathesis reaction with nickel salt to form amorphous material due to the transfer of electrons from the SbTe3 3− anion to the nickel cation according to the reaction: 3Ni2+ + 2SbTe3− → Ni3 (SbTe3 )2 (s) [15]. A stoi3 chiometric quantity of K3 SbTe3 aqueous solution was added slowly while stirring the NiBr2 solution. A precipitate was formed immediately. It was then separated by solution filtration, washed with deionized water and acetone, and dried in a vacuum oven. The experimental empirical formula determined for the resultant product was Ni3.22 Sb2 Te7.18 . All manipulations were carried out in an argon-filled glovebox because of the air sensitivity of the compounds. Catalytic reaction was performed at atmospheric pressure in a fixed-bed flow reactor which was made of alumina tubing with 0.8 cm inner diameter and 20 cm length. The catalyst was held between alumina wool plugs in the middle of the reactor and the section beyond the catalyst bed in the reactor was filled with alumina beads to reduce the free space. The reactor was kept in a vertical tubular furnace. To control the reaction temperature, a K-type thermocouple sealed with an alumina tube was placed just above the catalyst loaded in the reactor. Methane and oxygen were co-fed with He diluent gas. The purity of gaseous oxygen and methane was greater than 99.9%. Gaseous reactants and products were analyzed using on-line gas chromatography equipped with a thermal conductivity detector and a flame ionization detector. A cold trap was placed at the reactor exit to remove water vapor from the gaseous mixture. Gas compositions were calculated using an external standard gas mixture. Blank test was performed over inert alumina beads in the absence of catalyst and approximately 2–4% conversion of methane to carbon oxides was obtained in the reaction temperature range of 600–750 ◦ C. The conversion of methane was calculated from the amounts of products generated and the methane introduced in the feed stream. The selectivities were calculated on
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the basis of the conversion of methane to each product and the yield was obtained from the methane conversion and the selectivity to each product. The closures on the carbon material balances were within 4%. Most of the experiments described in this paper were typically performed under the following conditions: atmospheric pressure, a 300 mg sample loading of catalyst, a reaction feed of CH4 /O2 /He = 5/1/14 cm3 /min at ambient conditions. The methane conversion and product selectivities were compared after 1 h time-on-stream. Since the present catalyst exhibited a catalytic activity for the oxidative coupling of methane only when it was treated in a flow of oxygen prior to reaction, the Ni3 (SbTe3 )2 was typically pretreated in situ at 600 ◦ C in a flow of dry O2 (10 cm3 /min) for 1 h before each activity measurement. After the O2 -treatment of catalyst, the reactor was cooled to room temperature, He gas was passed to remove oxygen gas remaining in the reactor, then the reactor temperature was ramped to the desired value using a programmable temperature controller and finally a reaction mixture was fed over the catalyst. To identify the crystalline phases present on the catalyst, XRD analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after reaction by using a Philips PW 1710 diffractometer. To investigate the oxidation state of each element on the catalyst surface, XPS analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the reaction by using VG ESCALAB spectrometer, in which all the XPS binding energies were referenced to the 83.0 eV of Au(4f7/2 ). The BET surface area of the O2 -treated Ni3 (SbTe3 )2 catalyst determined by measuring the adsorption of nitrogen at liquid nitrogen temperature was about 12 m2 /g. TG analysis was performed for amorphous Ni3 (SbTe3 )2 at a heating rate of 10 ◦ C/min in the temperature range of 25–900 ◦ C in a flow of O2 /Ar mixture. Electrical conductivity of amorphous Ni3 (SbTe3 )2 was measured in the temperature range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm by means of the four-probe method. To measure the electrical conductivity, the powdered sample was compressed in vacuum into a pellet with a diameter of 12 mm and a thickness of about 3 mm. The pellet was contacted with the Pt probes by using nickel paste and then inserted into the quartz sample container. The current was supplied by a Keithley model 220 Current Source
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and the voltage drop across the sample was measured using a Keithley model 181 Digital Nanovoltmeter. The details are described in [16].
3. Results and discussion In this work, non-pretreated amorphous Ni3 (SbTe3 )2 catalyst showed no C2 selectivity, but the O2 -pretreated Ni3 (SbTe3 )2 catalyst revealed C2 selectivity above 45% for the oxidative coupling of methane in the temperature range from 600 to 750 ◦ C. In the reaction, the major products were CO, CO2 , C2 H4 , and C2 H6 . The effect of O2 content on the methane conversion and product selectivities was investigated by varying the PO2 in the reaction mixture (CH4 /O2 /He) at a constant PCH4 of 0.20 atm under a total flow rate of 20 cm3 /min. Fig. 1 shows variations of the methane conversion and product selectivities of the O2 -pretreated Ni3 (SbTe3 )2 catalyst with the PO2 at 650 ◦ C. The C2 -yield increased from 7% at PO2 = 0.02 atm to 15% at PO2 = 0.16 atm, while the C2 selectivity decreased with increasing the PO2 . The ratio of ethylene/ethane increased from 0.8 to 1.3 as the PO2 increased from 0.02 to 0.16 atm. Fig. 2 shows the methane conversion and product selectivity when the reaction mixture (CH4 /O2 /He = 5/1/14 cm3 /min) was fed over the O2 -pretreated Ni3 (SbTe3 )2 catalyst in the temperature range from 600 to 750 ◦ C. The C2 selectivity decreased with increasing temperature and the production of CO2 largely increased, while the methane conversion increased with increasing temperature. The catalyst showed the best C2 -yield of 11% with a selectivity of 79% at 650 ◦ C, which is rather appreciable. We investigated the effect of time of contact (W/F) on the methane conversion and product selectivities by changing the catalyst weight under a total flow rate of 20 cm3 /min (CH4 /O2 /He = 5/1/14 cm3 /min). Fig. 3 represents variations of the methane conversion and product selectivity with the contact time at 650 ◦ C. The C2 selectivities were kept in the range of 70–80% in the W/F range. The ratio of ethylene/ethane increased with increasing the contact time. The result suggests that ethane is the initial product and ethylene is subsequently produced by the dehydrogenation of ethane, which fits with the results reported by other investigators on the oxidative coupling of methane over
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Fig. 1. Effect of oxygen content in reaction feed on methane conversion and selectivity of the O2 -pretreated Ni3 (SbTe3 )2 catalyst for the oxidative coupling of methane at 650 ◦ C: (䊉) CH4 conversion, (䉱) C2 H4 , () C2 H6 , (䊏) CO, (䊐) CO2 , (䉬) C3 -hydrocarbons.
metal oxide catalysts [17,18]. Fig. 4 shows variations of methane conversion, C2 selectivity, and COx selectivity with time-on-stream over a 15 h period when the reaction mixture was fed over the O2 -pretreated Ni3 (SbTe3 )2 at 650 ◦ C. The methane conversion de-
clined from 14% at 1 h time-on-stream to 5% at 15 h time-on-stream. To characterize the catalyst, TG analysis was performed for amorphous Ni3 (SbTe3 )2 , XRD and XPS analyses were performed for the O2 -treated
Fig. 2. Variations of methane conversion and product selectivity with temperature for the oxidative coupling of methane over the O2 -pretreated Ni3 (SbTe3 )2 catalyst: (䊉) CH4 conversion, (䊏) CO, (䊐) CO2 , (䉱) C2 H4 , () C2 H6 , (䉬) C3 -hydrocarbons.
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Fig. 3. Effect of contact time (W/F) on methane conversion and selectivity for the oxidative coupling of methane over the O2 -pretreated Ni3 Sb2 Te6 catalyst at 650 ◦ C: (䊉) CH4 conversion, (䊊) C2 total, (䉱) C2 H4 , () C2 H6 , (䊏) CO, (䊐) CO2 .
Fig. 4. Variations of methane conversion and product selectivity with time-on-stream for the oxidative coupling of methane over the O2 -pretreated Ni3 (SbTe3 )2 catalyst at 650 ◦ C: (䊉) CH4 conversion, (䊏) COx selectivity, (䉱) C2 selectivity.
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Fig. 5. TG curves of amorphous Ni3 (SbTe3 )2 in a flow of O2 /Ar gas mixture: (A) PO2 = 0.20 atm, (B) PO2 = 0.01 atm.
Ni3 (SbTe3 )2 before and after the catalytic reaction, and the electrical conductivities of amorphous Ni3 (SbTe3 )2 were measured by a four-probe technique. Fig. 5 presents the TG curves measured in flows of both O2 /Ar mixtures with the PO2 ’s of 0.01 and 0.20 atm. A weight gain occurs in the temperature range of 200–750 ◦ C and the uptake in weight is larger at PO2 = 0.20 atm than at PO2 = 0.01 atm, which means that a lot of gaseous oxygen is chemisorbed on the catalyst surface, leading to an oxidation of catalyst. In general, metal tellurides are known to be readily oxygenated in the presence of gaseous oxygen; the present result is similar to those for other metal tellurides [19–21]. The cause of the weight decrease above 750 ◦ C is mainly the vaporization of tellurium. XRD analysis was performed for the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the reaction. The X-ray diffraction pattern of the O2 -treated Ni3 (SbTe3 )2 catalyst showed the presence of NiO, Ni3 Te2 , SbTe, and NiTeO3 phases, as presented in Fig. 6(A). The result indicates that Ni3 (SbTe3 )2 was decomposed and oxidized to form the oxides by the O2 -treatment. The remaining peaks in Fig. 6(A) belong to a crystalline Ni–Sb–Te phase of which the chemical formula and the structure are not determined in this work. NiTeO3 was decomposed into NiO and TeO2 above 750 ◦ C and the TeO2 was subsequently decomposed and evaporated, resulting in the weight
loss above 750 ◦ C, as shown in Fig. 5. X-ray analysis of the O2 -treated Ni3 (SbTe3 )2 catalyst after the reaction between methane and oxygen at 650 ◦ C for 15 h showed the presence of NiO, NiSb2 O6 , NiSb2 (OH)12 , and NiTeO3 phases as given in Fig. 6(B), indicating that both NiSb2 O6 and NiSb2 (OH)12 phases were produced through the catalytic reaction. We measured the electrical conductivity of amorphous Ni3 (SbTe3 )2 , while increasing the temperature at a heating rate of 5 ◦ C/min in the temperature range of 400–800 ◦ C at both PO2 ’s of 0.01 and 0.20 atm. Fig. 7 presents the electrical conductivities plotted as a function of temperature according to the equation, σ = A exp(−Ea /RT). As given in Fig. 7, inflection points are observed around 585 ◦ C. Namely, the electrical conductivity decreases with increasing temperature below 585 ◦ C, while the electrical conductivity increases with increasing temperature above 585 ◦ C, indicating that the specimen changes from a metallic conductor to a semiconductor around 585 ◦ C. Fig. 8 shows the Ni(3p) XPS spectra of the O2 -treated Ni3 (SbTe3 )2 catalyst before and after the catalytic reaction. The Ni(3p3/2 ) XPS spectra of unused O2 -treated Ni3 (SbTe3 )2 catalyst present a maximum at 855.1 eV and a shoulder at 852.5 eV. The lower binding energy at 852.5 eV is due to Ni belonging to nickel telluride phase and the higher binding energy at 855.1 eV is due to Ni associated with oxygen.
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Fig. 6. X-ray diffraction patterns of the O2 -pretreated Ni3 (SbTe3 )2 catalyst (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
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Fig. 7. Electrical conductivity of amorphous Ni3 (SbTe3 )2 as a function of temperature in the range of 400–800 ◦ C: (A) PO2 = 0.20 atm, (B) PO2 = 0.01 atm.
The Ni(3p3/2 ) XPS spectra of O2 -treated Ni3 (SbTe3 )2 catalyst after the catalytic reaction at 650 ◦ C for 15 h give a maximum at 854.4 eV. Considering that the Ni(3p3/2 ) binding energy of stoichiometric NiO is observed at 853.3 eV, the higher binding energies at 855.1 and 854.4 eV suggest that nickel ions exist in a higher oxidation state on the catalyst surface. Fig. 9 shows the Te(3d) XPS spectra of the catalyst. The Te(3d5/2 ) XPS spectra of unused O2 -treated Ni3 (SbTe3 )2 catalyst give two peaks at 572.2 and 575.4 eV. The higher binding energy at 575.4 eV is due to Te associated with oxygen on the surface and the lower binding energy at 572.2 eV is due to Te belonging to nickel telluride or antimony telluride phase. The Te(3d5/2 ) XPS spectra of O2 -treated catalyst after the reaction give a maximum at 575.6 eV,
Fig. 8. Ni(3p) XPS spectra of the O2 -pretreated Ni3 (SbTe3 )2 (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
indicating that the Te ions are associated with oxygen ions. The O(1s) XPS spectra gave a maximum at 530.1 eV for the unused catalyst and a maximum at 529.9 eV for the used catalyst. The O(1s) XPS spectra are believed to be a combination of peaks corresponding to oxygen and antimony ions because the binding energies of O(1s) and Sb(3d5/2 ) are close. We could not resolve the O(1s) XPS peaks into sharp peaks. The Sb(3d3/2 ) XPS spectra showed a major peak at 539.3 eV for the unused catalyst and one at 539.6 eV for the used catalyst. It has been reported that some metal–metal and metal–metalloid alloys show superior catalytic activity in the hydrogenation of carbon oxides [22,23]. Their catalytic properties were largely influenced by the
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Fig. 9. Te(3d) XPS spectra of the O2 -pretreated Ni3 (SbTe3 )2 (A) before and (B) after the reaction between methane and oxygen at 650 ◦ C for 15 h.
conditions of preparation and pretreatment and their surface structures were modified by the catalytic reaction itself. For instance, Ni–Zr alloy exhibits a catalytic activity for the methanation of carbon oxides; its high activity was found to be due to the tetragonal ZrO2 formed on the catalyst surface [23]. In the oxidative coupling of methane over metal oxide catalysts, methyl radicals are produced by the abstraction of a hydrogen atom from methane chemisorbed on the active oxygen ions formed on the catalyst surface. The O2 -pretreated Ni3 (SbTe3 )2 catalyst showed an appreciable C2 selectivity in the oxidative coupling of methane, which suggests that active oxygen ions were generated on the catalyst surface by the O2 -treatment. Many investigators have reported that O− , O2 − , and O2 2− can be generated on oxygen vacancy or basic sites in the metal oxide catalysts and O− ion is responsible for the selective activation of methane in the oxidative coupling of methane [8,24]. The stability of O− ion formed on the catalyst surface is known to depend on the basicity of catalyst and the existence of suitable sites [25]. As shown in Fig. 6(A), when amorphous Ni3 (SbTe3 )2 was treated in a flow of oxygen at 600 ◦ C for 1 h, NiO and NiTeO3 were formed on the surface as
XRD detectable phases. According to the conductivity data in Fig. 7, Ni3 (SbTe3 )2 alters from a metallic conductor to a semiconductor around 585 ◦ C and the electrical conductivities measured at PO2 = 0.20 atm are higher than those measured at PO2 = 0.01 atm, implying that the specimen is a p-type semiconductor above 585 ◦ C. The p-type conductivity seems to originate from NiO networks, because NiO is known to be a p-type semiconductor. The activation energy calculated from the conductivity–temperature data in the temperature range of 600–800 ◦ C is 0.8 eV, such a value is in the range of 0.78–1.07 eV reported by other investigators for pure NiO [26], supporting the conclusion that the p-type conductivity arises from NiO phase. When NiO particles in the catalyst are either making contacts among themselves or are separated by very small gaps, the charge carriers can migrate from one aggregate to a neighboring one through hopping; then a p-type conductivity will be observed, as in the present result. We measured catalytic activities and selectivities of NiO, Sb2 O3 , TeO2 , and NiSb2 O6 catalysts for the oxidative coupling of methane at 700 ◦ C. In the reaction, both NiO and TeO2 catalysts did not produce C2 -hydrocarbons, Sb2 O3 catalyst showed a C2
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selectivity of 32% at a methane conversion of 8%, and NiSb2 O6 catalysts showed a C2 selectivity of 64% at a methane conversion of 15%. The results indicate that these catalysts are less active and selective than the O2 -pretreated Ni3 (SbTe3 )2 catalyst. We could not measure the catalytic activity of NiTeO3 in this work. We tried to prepare NiTeO3 phase from the oxidation of Ni–Te mixture in a flow of gaseous oxygen, but we could not obtain the monophase NiTeO3 . When Ni–Te mixture was oxidized in a flow of oxygen at 600 ◦ C, various oxides such as NiO, TeO2 , NiTe2 O5 , Ni2 Te3 O8 , and NiTeO3 were produced. According to the study on the oxidation of nickel–tellurium compounds [27], NiO is produced in the first stage of oxidation; above 450 ◦ C, various nickel–tellurium oxides are slowly formed; above 900 ◦ C, the oxides decompose, leaving NiO as the final solid product. The nickel–tellurium oxide mixture containing some nickel oxide exhibited a methane conversion of 7% with a C2 selectivity of 25% for the reaction at 650 ◦ C. In the case of Ni-containing K/Ca oxide catalyst showing an appreciable C2 selectivity in the oxidative coupling of methane, the selective phase was found to be a K/Ca/Ni oxide with some nickel in valency higher than +2 [5]. Since the formation of higher valent Ni ion can be closely related with the existence of active oxygen ions (O− ), nickel ion in valency higher than +2 is believed to be a suitable site that is able to stabilize the O− ions on the catalyst surface at high temperatures. The present catalyst showed the best C2 -yield of 11% with a selectivity of 79% at 650 ◦ C, as presented in Fig. 2. It is noted that the temperature (650 ◦ C) showing the best C2 -yield is lower than those (>700 ◦ C) observed usually for metal oxide catalyst for the oxidative coupling of methane. In Fig. 6, the XRD analysis of the O2 -treated Ni3 (SbTe3 )2 catalyst after the catalytic reaction shows the presence of NiSb2 O6 and NiSb2 (OH)12 phases in the catalyst. The XRD signal intensities of both NiO and NiTeO3 were somewhat reduced after the catalytic reaction. The results indicate that some NiTeO3 is decomposed, leaving NiO through the catalytic reaction, and then some NiO reacts with antimony oxide to form NiSb2 O6 phase. The NiSb2 O6 phase reacts with H2 O(g) being produced from the catalytic reaction to form NiSb2 (OH)12 . Therefore, the decrease in the methane conversion and C2 selectivity
along with time-on-stream in Fig. 3 seems to be due to the formation of nickel–antimony oxide and nickel–antimony hydroxide. Although the nature of active oxygen species associated with the catalyst surface and the mechanism of this reaction remain to be further studied, the experimental results suggest that the selective phase on the O2 -treated Ni3 (SbTe3 )2 catalyst is NiTeO3 containing some higher valent Ni ions and tellurium ions may promote the formation of higher valent Ni ions. Acknowledgements The authors are grateful for financial support from Korea Research Foundation Grant (KRF-2000-042D00056). References [1] G.E. Keller, M.M. Bhasin, J. Catal. 73 (1982) 9. [2] T. Ito, J.-X. Wang, C.-H. Lin, J.H. Lunsford, J. Am. Chem. Soc. 107 (1985) 5062. [3] J.S. Lee, S.T. Oyama, Catal. Rev. Sci. Eng. 30 (1988) 249. [4] Y. Amenomiya, V.I. Birss, M. Goledzionwsk, J. Guluszk, A.R. Sanger, Catal. Rev. Sci. Eng. 32 (1990) 163. [5] K.M. Dooley, S.-Y. Chen, J.R.H. Ross, J. Catal. 145 (1994) 402. [6] P. Pereira, S.H. Lee, G.A. Somorjai, H. Henemann, Catal. Lett. 6 (1990) 255. [7] K. Otsuka, K. Jinno, A. Morikawa, J. Catal. 100 (1986) 353. [8] S. Lacombe, C. Geartet, C. Mirodatos, J. Catal. 151 (1994) 439. [9] K.D. Campbell, H. Zhang, J.H. Lunsford, J. Phys. Chem. 92 (1988) 750. [10] R.K. Grasselli, Catal. Today 49 (1999) 141. [11] J. Nilsson, A.R. Land-Canovas, S. Hansen, A. Anderson, J. Catal. 186 (1999) 442. [12] H.-W. Zanthoff, S.A. Buchhloz, Catal. Lett. 49 (1997) 213. [13] H. Liu, H. Imoto, T. Shido, Y. Iwasawa, J. Catal. 200 (2001) 69. [14] J.-S. Jung, L. Ren, C.J. O’Connor, J. Mater. Chem. 2 (1992) 829. [15] B. Wu, L. Ren, C.J. O’Connor, J. Tang, J.-S. Jung, J. Ferre, J.-P. Jamet, J. Mater. Res. 9 (1994) 909. [16] W.R. Runyan, Semiconductor Measurements and Instrumentation, McGraw-Hill, New York, 1975. [17] E. Iwamatsu, K.-I. Alka, J. Catal. 117 (1989) 416. [18] S. Ahmed, J.B. Moffat, Appl. Catal. 54 (1989) 241. [19] P. Arun, A.G. Vedeshwar, Mater. Res. Bull. 34 (1999) 203. [20] S.B. Elvy, P.A. Williams, A.N. Buckley, Surf. Interface Anal. 24 (1996) 641. [21] J.-I. Kong, J.-S. Jung, J.-G. Choi, S.-H. Lee, Appl. Catal. A 204 (2000) 241.
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