Production of Synthesis Gas from Methane Using Lattice Oxygen of

Nov 24, 2009 - Department of Chemical Engineering, and High Technology Research Center, Kansai UniVersity,. 3-3-35 Yamate, Suiat, Osaka 564-8680, ...
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Ind. Eng. Chem. Res. 2010, 49, 526–534

Production of Synthesis Gas from Methane Using Lattice Oxygen of NiO-Cr2O3-MgO Complex Oxide O. Nakayama,† N. Ikenaga,† T. Miyake,† E. Yagasaki,‡ and T. Suzuki*,† Department of Chemical Engineering, and High Technology Research Center, Kansai UniVersity, 3-3-35 Yamate, Suiat, Osaka 564-8680, Japan, and Energy Use R&D, The Kansai Electric Power Co., Inc., Nakoji, Amagasaki, Hyogo 661-0974, Japan

To produce nitrogen-free synthesis gas or hydrogen by the partial oxidation of methane using air as an oxidant, NiO-Cr2O3-MgO complex oxide was proposed as an effective one. Lattice oxygen of NiO in the complex oxide was effectively transferred to CH4 to give H2 and CO in the ratio of 2-3:1 at 700 °C. NiO was reduced to metallic Ni during the course of the reaction with CH4, and the reduced Ni was successfully oxidized to NiO with air at 700 °C. Repeated cycles of the reaction and oxidation were carried out without any loss of activity. The role of Cr2O3 seemed to weaken the Ni-O bond in NiO-MgO complex oxide, in which NiO is less reducible with CH4 at 700 °C. In addition, the surface oxidation state of Ni was increased to Ni3+ with Cr2O3. 1. Introduction

Step 2

As a clean energy source, hydrogen has attracted much attention. Since hydrogen is not a natural resource, potential hydrogen sources must be converted to pure hydrogen. This has currently been carried out by the steam reforming of hydrocarbons at a high temperature.1 As an energy-saving process, partial oxidation of CH4 (POM) has recently been developed by many researchers as a method for obtaining hydrogen.2-5 However, pure oxygen is required for POM. Accordingly, a costly and energy-consuming oxygen plant is indispensable.6,7 As an alternative process for obtaining pure hydrogen from CH4, catalytic decomposition of CH4 over Ni/SiO2 is proposed (reaction 1), and the carbon that forms could potentially be removed by introducing steam to give H2 and CO (reaction 2).8 Step 1 CH4 + catal f C/catal + 2H2

∆H0298 ) +75 kJ/mol (1)

Step 2 C/catal + H2O f catal + CO + 2H2 ∆H0298 ) +131 kJ/mol

(2)

However, such a process yields a large amount of carbon on the catalyst bed, which causes a large pressure drop between the front end and the bottom of the bed, making operation difficult in a fixed-bed reactor. In addition, if steam is used to regenerate the carbon-formed catalyst, the advantage of such a process against steam reforming would be diminished. Recently, a gas-solid reaction between methane and the lattice oxygen of oxides to give synthesis gas was proposed by Otsuka et al. (reactions 3-5).9 Step 1

CeO2 + xCH4 f CeO2-x + xCO + 2xH2

(3)

* To whom correspondence should be addressed. Fax: +81-66388-8869. Tel.: +81-6368-1121 (ext 6807). E-mail: v902360@ kansai-u.ac.jp. † Kansai University. ‡ Kansai Electric Power Co., Inc.

CeO2-x + xCO2 f CeO2 + xCO

(4)

CeO2-x + xH2O f CeO2 + xH2

(5)

Similarly, the redox cycle, between the reduction of Fe2O3 with CH4 (reaction 6) and the subsequent oxidation of iron metal with H2O or CO2 (reaction 7), has been proposed.10-16 Step 1 Step 2

Fe2O3 + 3CH4 f 2Fe + 3CO + 6H2

(6)

3Fe + 4H2O(CO2) f Fe3O4 + 4H2(CO)

(7)

These studies have shown that the lattice oxygen of iron oxide or cerium oxide exhibits high activity for CH4 oxidation to give synthesis gas, and that there is no danger of an explosion occurring with pure oxygen. However, these reoxidation reactions with H2O or CO2 are highly endothermic and consume a large amount of energy, necessitating a higher reaction temperature. When iron oxide is used, the catalysts suffer from sintering of iron and are rapidly deactivated by repeated runs in the reduction of oxide and reoxidation. Shikong et al. have reported that La0.9Sr0.1FeO3 perovskitetype catalyst shows high catalytic activity for the redox cycle between reaction 8 and reaction 9 by using air at 900 °C.17 Wei et al. have reported that CeO2 catalyst exhibits high catalytic activity in the same reaction by using air above 865 °C.18 In addition, the reaction of LaFeO3 perovskite with CH4 and oxidation of the reduced metal oxide with air has been reported to produce synthesis gas.19,20 Step 1 b CaHb + McOd f aCO + H2 + McOd-a 2

(8)

1 MOd-a + mair(O2 + 4N2) f MOm + 2mN2 2

(9)

Step 2

However, the gas-solid reaction between methane and the lattice oxygen of the oxides to yield synthesis gas is conducted at a high temperature above 865 °C, and it consumes a large amount of energy. The amount of lattice oxygen in LaFeO3 or CeO2 utilized in reaction 8 was limited to a small portion, since

10.1021/ie9013474  2010 American Chemical Society Published on Web 11/24/2009

Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010

the bulk of rare earth oxides could not be fully reduced with CH4. The metal oxides may be rapidly deactivated by repeated runs of the reduction of oxide and the reoxidation of metal due to metal sintering. We have reported that Fe2O3 and Rh2O3 coloaded with Y2O3 catalyst exhibited a high CH4 conversion of 53.7% and high H2 and CO selectivities of 52.0% and 62.7%, respectively, with a high lattice oxygen utilization of 84.0% at 800 °C.21 In addition, no carbon formation was observed after the CH4 oxidation stage. The reaction and regeneration cycles by air could be repeated without any loss of activity. Thus, without using pure oxygen, H2 or synthesis gas free from nitrogen could be produced. In this cycle, the complex oxide is a reactant to provide oxygen to CH4, and in exact terminology the oxide is not a catalyst. However, without catalytic function of the oxide, only complete oxidation producing H2O and CO2 would proceed and small amounts of H2 and CO would be obtained. Therefore, hereafter we use “catalyst” and “catalytic activity” to the oxide producing syn-gas and the performance of the oxide. To carry out redox cycles at a lower temperature, nickel-oxidebased catalyst was investigated. Nickel oxide was reduced at a lower temperature than iron oxide.21 The redox properties of NiO (reactions 10 and 11) dispersed on different inorganic binders including MgO, yttria-stabilized zirconia, and Ni-Mg-Al mixed oxides have been reported.22-24 The reactions have been studied by means of thermogravimetric measurement, with CH4 and air at a constant temperature, and Ni-Mg-Al mixed oxides have been reported to exhibit excellent regenerability in cyclic use.24 Step 1

4NiO + CH4 f Ni + CO2 + 2H2O

(10)

1 Ni + air (O2 + 4N2) f NiO + 2N2 2

(11)

Step 2

This paper deals with the oxidation of CH4 using lattice oxygen of NiO in combination with various support materials and additives. We have found that NiO-Cr2O3-MgO is a superior oxide that exhibits high and constant catalytic activities for repeated reduction with CH4 and oxidation with air cycles without significant carbon deposition. 2. Experimental Section 2.1. Catalyst Preparation. The catalyst supports used in this study were SiO2 (Wako Pure Chemical Industries, Ltd.), Al2O3 (Sumitomo Chemical Co.), MgO (1000A; Ube Industries, Ltd.), TiO2 (P25; Japan Aerosil Co.), La2O3, and ZrO2 (Nacalai Tesque, Inc.). CeO2 and Y2O3 were prepared by thermal decomposition of Ce(NO3)3 · 6H2O and (CH3COO)3Y · 4H2O (Wako Pure Chemical Industries, Ltd.), respectively, at 600 °C under air for 5 h. The typical procedure was as follows. NiO-Cr2O3-MgO catalyst was prepared by an evaporation process: 9.3 g (32 mmol) of Ni(NO3)2 · 6H2O, 3.2 g (8 mmol) of Cr(NO3)3 · 9H2O and 6.4 g (25 mmol) of Mg(NO3)2 · 6H2O were dissolved in 100 mL of water, and the mixture was allowed to stand for 2 h with stirring. Then the mixture was put into a porcelain evaporating dish, and excess water was removed in an oven kept at 120 °C under occasional agitation. The solid mixture was calcined at 800 °C for 5 h in air prior to the reaction. In certain cases, Cr(NO3)3 · 9H2O was replaced with different trivalent metal nitrates or was excluded.

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The supported NiO-Cr2O3/MgO catalyst was prepared by impregnating a suspended 1.0 g (25 mmol) of MgO with a mixture of 9.3 g (32 mmol) of Ni(NO3)2 · 6H2O and 3.2 g (8 mmol) of Cr(NO3)3 · 9H2O, dissolved in 100 mL of water, then evaporating the mixture to dryness. The prepared catalyst precursor was calcined at 600 °C for 5 h in air prior to the reaction. 2.2. Catalytic Reaction. The partial oxidation of CH4 with metal oxides was carried out with a fixed-bed flow-type quartz reactor (8 mm I.D. × 350 mmL or 4 mm I.D. × 200 mmL) at a temperature range of 500-800 °C under atmospheric pressure. After placing 0.5 g of the catalyst in the reactor, the catalyst was heated to the desired temperature under Ar. The partial oxidation of CH4 was carried out under 5 mL/ min of CH4 and 20 mL/min of Ar, and reoxidation of the reduced catalyst was carried out with 5 mL/min of O2 and 20 mL/min of Ar at a temperature range of 500-800 °C. Products were analyzed with an online gas chromatograph equipped with TCD detectors using columns of Molecular Sieve 5A and Poraplot Q (PC-Chrom, M200 Chromato Analyzer) and a quadrupole mass spectrometer (Hiden Analytical Ltd. HAL201). Selectivities to CO, CO2 and carbon were calculated from the absolute amounts of produced CO, CO2 and carbon against converted CH4. H2 selectivity was calculated from the absolute amounts of produced H2 and H2O against converted CH4. 2.3. Catalyst Characterization. Temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) of the catalyst were carried out with an online quadrupole mass spectrometer fitted with the outlet of a fixed-bed quartz reactor (i.d. 4 mm × 200 mm). After 100 mg of the catalyst was placed in the reactor, 10 mL/min of CH4 and 20 mL/min of Ar were introduced. After reduction of the catalyst with CH4, in the TPO runs, O2, CO2, or H2O was introduced with 20 mL/min of Ar as a carrier gas. A mass spectrometer was used to scan the corresponding parent peaks of six compounds, namely H2, H2O, CH4, CO, CO2, and O2, within 1 s, and repeated scans were collected on a personal computer. Powder X-ray diffraction (XRD) patterns were obtained with a Shimadzu XRD-6000 using monochromatized Cu-KR radiation. Identification of crystallographic phase was obtained by the JCPDS (International Centre for Diffraction Data) database installed into the instrument. X-ray photoelectron spectroscopy (XPS) was carried out with a Jeol JPS-9000MX, using Mg KR (10 kV, 10 mA) radiation. As-prepared or oxidized samples were handled in air, and H2 reduced or reacted samples under CH4 were handled in a glovebox using a transfer vessel to preventing the sample from exposure to air. Binding energy was calibrated with Pt 5p3/2 51.7 eV and C1s 284.2 eV. Powdered samples were placed into a Pt-boat which was pasted using carbon tape onto the sample holder. Ar ion etching was performed under the following conditions: anode voltage, 50 V; accelerating voltage, 500 V; accelerating current, 8.6 mA; Ar pressure, 2 × 10-4 Pa. A field emission scanning electron microscope (FE-SEM), the Jeol model JSM6700, was employed to evaluate the catalyst surface. 3. Results and Discussion 3.1. Effects of Metal Oxide and Support Material on the Oxidation of CH4. From the H2 production behaviors of CH4-TPR runs, Al2O3-supported NiO afforded the lowest reaction temperature of 430 °C, and Co3O4 was reduced at 670 °C. Fe2O3 was not reduced below 800 °C and was reduced after 10 min at 800 °C. After the TPR run on NiO-loaded Al2O3 held

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Table 1. Effect of Support of Ni (20 mol %)-Loaded Catalyst on the Partial Oxidation of Methanea product (mmol)

selectivity

entry

support

reduction temp (°C)

CH4 reacted (mmol)

H2

H2O

CO

CO2

carbon

H2

CO

H2/CO ratio

1 2 3 4 5 6 7

MgO Al2O3 SiO2 TiO2 Y2O3 La2O3 CeO2

549 560 479 600 469 613 438

0.63 2.01 5.29 2.87 1.03 0.92 2.66

1.25 3.99 10.1 5.42 2.00 1.81 5.17

0.00 0.03 0.45 0.31 0.05 0.03 0.14

0.14 0.30 0.13 0.52 0.05 0.15 0.27

0.01 0.02 0.11 0.07 0.02 0.02 0.05

0.49 1.69 5.05 2.28 0.96 0.75 2.34

100.0 99.3 95.7 94.6 97.6 98.4 97.4

22.0 14.9 2.5 18.2 4.9 16.3 10.2

8.9 13.3 77.9 11.8 40.0 12.1 19.1

a Catalyst: 100 mg, 20 mmol of Ni oxide/1 g of support, CH4/Ar ) 3/27 (mL/min). The catalyst was heated at 10 °C/min up to 800 °C and held at 800 °C for 40 min.

Table 2. Effect of Preparation Method of NiO-Loaded Catalysts on the Partial Oxidation of Methanea conversion (%) entry

catalyst

product (mmol)

CH4

lattice O

H2

H2O

0.08 1.20

CO

selectivity (%)

CO2

carbon

H2

CO

CO2

H2/CO ratio

0.08 0.73

1.04 0.40

97.6 70.7

30.1 43.4

5.2 36.7

6.8 3.3

0.11 0.03 0.07 0.43 0.47 0.05

1.02 0.87 1.12 0.19 0.28 1.14

95.5 98.2 96.6 74.6 56.8 98.0

34.0 47.0 37.5 64.4 34.3 28.1

6.2 2.0 3.8 24.7 41.5 3.0

6.0 1.5 1.4 2.4 1.8 1.1

Impregnation 8 9

NiO/MgO NiO-Cr2O3/MgO

77.7 97.6

20.0 110.2

3.27 2.89

10 11 12 13 14 15

NiO-MgO NiO-Al2O3-MgO NiO-CaO-MgO NiO-Cr2O3-MgO NiO-Fe2O3-MgO NiO-Co3O4-MgO

82.2 75.5 83.5 92.7 55.2 74.4

27.9 27.7 29.1 91.5 68.7 19.6

3.48 2.79 2.94 2.75 1.12 2.71

0.48 0.86

Evaporation Method

a

0.21 0.05 0.10 0.94 0.85 0.06

0.58 0.80 0.71 1.13 0.39 0.47

Catalyst, 0.5 g; mole ratio, Ni/additive/Mg ) 16:4:25; reaction temperature, 700 °C; reaction time, 10 min; CH4/Ar ) 5/20 (mL/min).

at 800 °C for 40 min, ca. 1.3 mmol of carbon was formed (against 0.25 mmol of NiO charged) on the catalyst; in contrast, no carbon formation was observed on Fe2O3-loaded catalyst. Although NiO reacted with CH4 at a lower temperature, the Ni catalyst had the drawback of inducing the formation of a large amount of carbon.21 Survey of optimal support material for NiO was carried out by TPR with CH4, and the results are summarized in Table 1. The initial temperature at which H2 production started (reduction temperature in Table 1) decreased in the following order: CeO2 > SiO2 > MgO > Al2O3 > TiO2 > La2O3. A large amount of carbon deposit was observed on all of the supports except the MgO. The carbon formation was ascribed to the direct decomposition of CH4 (reaction 1), where CH4 conversion much higher than that calculated for the lattice oxygen of NiO was observed. In contrast, in the MgO-loaded case, moderate CH4 conversion was obtained with a small amount of CO. Further studies were carried out with an MgO-loaded catalyst. The H2/CO ratios were very high in all of the support materials, showing that the oxidation of CH4 to CO2 and H2O did occur together with decomposition of CH4 and that reforming of CH4 with H2O and CO2 occurred to a slight extent. Quantitative analyses of product composition could not be performed, since in the TPR experiment, after complete conversion of the lattice oxygen of NiO, CH4 decomposition occurred to give a large amount of H2 (reaction 1). Therefore, it is desirable to limit the amount of CH4 to that of lattice oxygen below the stoichiometric ratio. Further study was done at a constant temperature. 3.2. Effect of Additives to NiO-MgO on the Partial Oxidation of CH4. Table 2 illustrates the effect of the preparation method and additives to NiO-loaded catalyst on the partial oxidation of CH4, at a constant temperature of 700 °C and a CH4 to NiO ratio of 0.6, where much smaller H2/CO ratios were observed. In Table 2, NiO/MgO (entry 8) and NiO-MgO (entry 10) indicate the catalysts prepared by impregnation and by the

evaporation method, respectively. The NiO-MgO catalyst afforded slightly higher CO selectivity than the NiO/MgO catalyst, in addition to producing slightly higher CH4 and lattice oxygen conversions with nearly the same amounts of carbon. The XRD pattern of NiO-MgO exhibited diffraction peaks ascribed to NiMgO2 complex oxide. To improve the catalytic activity of NiO-MgO catalyst, the effect of the addition of third components to the NiO-MgO catalyst on the partial oxidation of CH4 was examined. Among the additive oxides examined, as seen in entry 13, Cr2O3 afforded the highest CH4 conversion and CO selectivity. In addition, the smallest amount of carbon formation was observed over NiO-Cr2O3-MgO. Other metal oxides such as Al2O3, Fe2O3, and Co3O4 decreased CH4 conversion and increased carbon formation (entries 11, 14, and 15). Co-impregnation of Cr2O3 into NiO/MgO also afforded a higher CH4 conversion of 97.6% (entry 9) and CO selectivity with a smaller amount of carbon formation than with the NiO-MgO catalyst. In contrast, the selectivity of CO was lower than that of NiO-Cr2O3-MgO catalyst. Complex oxides prepared by the evaporation method seem to give a higher CO selectivity than that obtained with the impregnation method. The XRD pattern of NiO-Cr2O3-MgO showed the diffraction peaks of NiMgO2, NiCr2O4, and MgCr2O4 of complex oxides. This finding is discussed later. 3.3. Effects of Support of NiO-Cr2O3 Catalyst on the Partial Oxidation of CH4. Table 3 shows the effect of support of the NiO-Cr2O3-loaded catalyst on the partial oxidation of CH4. All the catalysts afforded ca. 90% CH4 conversions and small amounts of carbon. The order of the activities evaluated by the amounts of H2 and CO was MgO > Y2O3, SiO2, CeO2 > La2O3, indicating that the NiO-Cr2O3-MgO catalyst exhibited the highest H2 and CO selectivities among the metal oxides examined. Without support, however, NiO-Cr2O3 catalyzed complete oxidation of CH4, and showed very low H2 and CO selectivities, indicating that supports are essential for this reaction.

Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 Table 3. Effect of Support of NiO-Cr2O3-Loaded Catalysts on the Partial Oxidation of Methane at 700 °C product (mmol)

529

a

selectivity (%)

entry

support

convn (%) CH4

H2

H2O

CO

CO2

carbon

H2

CO

CO2

H2/CO ratio

16 17 18 19 20 21

MgO SiO2 CeO2 Y2O3 La2O3 none

92.7 94.4 91.5 83.4 91.7 93.0

2.75 2.74 2.15 2.43 2.07 1.17

0.94 1.15 2.32 1.10 2.04 3.06

1.13 0.92 0.85 1.04 0.53 0.24

0.43 0.71 1.04 0.61 0.86 1.49

0.19 0.13 0.23 0.07 0.48 0.37

74.6 70.5 48.1 68.7 50.4 27.6

64.4 52.1 40.0 60.6 28.5 11.4

24.7 40.6 49.3 35.5 45.8 70.8

2.4 3.0 2.5 2.3 3.9 4.9

a

Catalyst, 0.5 g; mole ratio, Ni/Cr/support ) 16:4:25; reaction temperature, 700 °C; reaction time, 10 min; CH4 /Ar ) 5/20 (mL/min).

Figure 1. Effect of Ni-loading level of NiO-Cr2O3-MgO catalyst on the performance of POM. Catalyst, 0.5 g; molar ratio, Cr/Mg ) 4:25; reaction temperature, 700 °C; reaction time, 10 min; CH4/Ar ) 5/20 (mL/min).

Figure 2. Effect of Cr2O3 composition in the NiO-Cr2O3-MgO catalyst on the performance of POM. Catalyst, 0.5 g; molar ratio, Ni/Mg ) 32:25; reaction temperature, 700 °C; reaction time, 10 min; CH4/Ar ) 5/20 (mL/ min).

3.4. Effects of NiO-Loading Level of NiO-Cr2O3MgO Catalyst on the Partial Oxidation of CH4. Figure 1 shows the effect of the NiO-loading level of NiO-Cr2O3-MgO catalyst on the partial oxidation of CH4 when the Cr/Mg mole ratio is held constant at 4/25. Without Ni loading, Cr2O3-MgO alone showed very low CH4 conversion, indicating that NiO on Cr2O3-MgO is essential for this reaction. Conversion of CH4 and lattice oxygen of NiO increased with an increase in the Ni-loading level. The highest CO selectivity was obtained on NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) catalyst. In contrast, NiO (21.7 mol %)-Cr2O3 (10.9 mol %)-MgO(balance) catalyst exhibited a high H2 selectivity; however, a large amount of carbon formation with a low CO selectivity was observed. This finding can be attributed to the smaller amount of lattice oxygen of NiO against the CH4 feed, resulting in CH4 decomposition (reaction 1). With an increase in Ni-loading level, relative amounts of Cr2O3 and MgO against NiO decreased, and the reaction of CH4 with lattice oxygen of NiO proceeded to promote complete oxidation of CH4. 3.5. Effects of Cr2O3-Loading Level of NiO-Cr2O3MgO Catalyst on the Partial Oxidation of CH4. Figure 2 shows the effect of the Cr2O3-loading level of the NiO-Cr2O3MgO catalyst on the partial oxidation of CH4 with the Ni/Mg mole ratio held constant at 32/25. Without Cr2O3 loading, NiO-MgO alone exhibited a very low CO selectivity with a large amount of deposited carbon, indicating that Cr2O3 on NiO-MgO is essential for the production of CO. The lattice oxygen of NiO was not effectively utilized to oxidize carbon below 3.4 mol % of the Cr2O3-loading level. However, at Cr2O3loading levels above 3.4 mol %, the reaction path drastically changed from CH4 decomposition to the partial oxidation of CH4. The highest CO selectivity was obtained on the NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO catalyst. Increasing Cr2O3

to 6.6 mol % is essential for high and constant activity in the repeated runs. 3.6. Effects of Reaction Conditions for NiO-Cr2O3MgO Catalyst on the Partial Oxidation of CH4. Table 4 shows the effect of the reaction temperature on the partial oxidation of CH4 when NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) catalyst is used. With an increase in the reaction temperature, conversions of CH4 and lattice oxygen of NiO increased, and nearly 90% CH4 conversion was obtained above 700 °C. At the higher temperatures of 750 and 800 °C (entries 25, 26); however, pronounced complete oxidation proceeded to give H2O and CO2 with higher selectivities, indicating that at a higher temperature, the lattice oxygen of NiO would rapidly be consumed to give H2O and CO2. Consequently, the slower reforming reaction of H2O and CO2 with unreacted CH4 could not proceed sufficiently. Figure 3 shows the effect of reaction time on the partial oxidation of CH4 with NiO-Cr2O3-MgO catalyst. With an increase in the reaction time, the selectivity to synthesis gas increased, and the highest CO selectivity was obtained at the reaction time of 9 min (CH4/NiO ) 1.8/4.0 mmol/mmol). After the run longer than 10 min, a gradual increase in the deposited carbon was observed with an increase in the H2 selectivity and a decrease in the CO selectivity. This indicates that in the initial stage of the reaction, the reaction of CH4 with abundant NiO proceeds to give H2O and CO2, and after the formation of metallic Ni, the reforming of CH4 effectively occurs. 3.7. Regeneration of Used NiO-Cr2O3/MgO Catalyst with Ar-O2 Mixed Gas. Figure 4 shows the product formation behavior of three successive reaction-and-regeneration cycles at a constant temperature of 700 °C. With the introduction of CH4 (CH4/NiO ) 1.6 mmol/4.0 mmol), a rapid response of H2 was observed, followed by an increase in the CO formation rate and a gradual decrease in the unreacted CH4 concentrations.

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Table 4. Effect of Reaction Temperature on the Partial Oxidation of Methane over NiO(52.6)-Cr2O3(6.6)-MgOa conversion (%)

product (mmol)

selectivity (%)

entry

reaction temp(°C)

CH4

lattice O

H2

H2O

CO

CO2

carbon

H2

CO

CO2

H2/CO ratio

22 23 24 25 26

600 650 700 750 800

40.0 76.9 92.6 96.8 98.0

20.0 44.8 72.0 90.2 101.4

1.56 2.82 2.92 2.58 2.28

0.10 0.26 0.66 1.11 1.48

0.52 1.09 1.29 1.16 0.98

0.09 0.22 0.47 0.67 0.80

0.28 0.31 0.20 0.20 0.29

93.7 91.5 81.6 69.9 60.7

58.3 67.1 66.0 57.3 47.4

10.1 13.7 23.8 32.9 38.8

3.0 2.6 2.3 2.2 2.3

a

Catalyst, 0.5 g; reaction temperature ) re-oxidation temperature; reaction time, 10 min; re-oxidation time, 10 min; CH4 /Ar ) 5/20 (mL/min).

Figure 3. Effect of reaction time on the performance of POM. Catalyst, NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance), 0.5 g; reaction temperature, 700 °C; CH4/Ar ) 5/20 (mL/min).

Figure 4. Change in the product formation rates in the POM and oxidation of the reacted catalyst. Catalyst, NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance), 0.5 g; reaction temperature, 700 °C; reoxidation temperature, 700 °C; reaction time, 8 min; reoxidation time, 8 min; CH4 or O2/Ar ) 5/20 (mL/min); CH4/NiO ) 1.6/4.0 mmol/mmol; O2/NiO ) 1.6/ 4.0 mmol/mmol.

After CH4 was swept off, O2-Ar (O2/Ar ) 1/4 mol/mol) mixed gas was introduced. Here, detection of the effluent was made possible by a mass spectrometer, and air could not be utilized to detect CO. No response of O2 was observed just after the O2 introduction, and after ca. 5 min, O2 gradually began to appear and reached the original concentration with small responses to CO and CO2. This indicates that oxidation of metallic nickel occurred first, and then a trace amount of carbon on the catalyst was oxidized. The same reaction behavior was observed in the second and the third reaction stages with CH4, indicating that oxidation of the reduced catalyst with O2 for 8 min is sufficient to complete regeneration. The fact that only trace amounts of CO and CO2

Figure 5. Effect of repeated reaction with CH4 and reoxidation cycles of NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance). Catalyst, 0.5 g; reaction temperature, 700 °C; reoxidation temperature, 700 °C; reaction time, 10 min; reoxidation time, 10 min; CH4 or O2/Ar ) 5/20 (mL/min); CH4/NiO ) 1.6/4.0 mmol/mmol; O2/NiO ) 1.6/4.0 mmol/mmol.

Figure 6. FE-SEM images of NiO(32)-Cr2O3(8)-MgO: (a) fresh oxide, calcined at 800 °C, (b) after 10 reaction and reoxidation cycles. Reaction, 700 °C, 8 min; reoxidation, 700 °C, 8 min.

were observed in the oxidation stage clearly shows that carbon formation during the reaction stage is negligible. The reaction and regeneration cycles were repeated further, and the results are shown in Figure 5. The catalyst exhibited constant CH4 conversion with constant H2 and CO selectivities without carbon formation. In addition, the surface area of the catalyst was nearly constant before and after the 10th reaction (from 13.2 to 14.0 m2/g). From FE-SEM images of fresh and regenerated catalysts (Figure 6), fine particles of ca. 86 nm in size were observed. No significant increase in the particle sizes was seen before and after the 10th reoxidation cycle, indicating that the sintering of oxides or agglomeration of particles hardly occurred. Figure 7 shows the changes in the reaction products during the reaction and oxidation stages when smaller and larger amounts of CH4 against lattice oxygen (CH4/NiO ) 0.8 /4.0 and 2.5/4.0 mmol/mmol) were supplied as compared to the runs shown in Figure 4, together with the changes in the catalyst

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Figure 7. Change in the product formation rates and catalyst-bed temperature over NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance). Catalyst, 0.5 g; reaction temperature, 700 °C; reoxidation temperature, 700 °C; CH4 or O2/ Ar ) 5/20 (mL/min). (a) Reaction time, 4 min; reoxidation time, 6 min; CH4/NiO ) 0.8/4.0 mmol/mmol. (b) Reaction time, 12 min; reoxidation time, 12 min; CH4/NiO ) 2.5/4.0 mmol/mmol.

bed temperature. During the reaction stages (Figures 7a,b), a decrease in the catalyst bed temperature of ca. 10 °C was observed, indicating that the heat of exothermic CH4 oxidation was almost compensated for the endothermic reforming reactions. The response of H2 exhibited a constant value in the 12min run (Figure 7b). In contrast, the response of CO exhibited a maximum after 5 min of reaction and then decreased gradually. This shows that in the late stage the amount of lattice oxygen of NiO decreased to react with CH4. Consequently, the response of CO showed maxima, and by contrast the formation of H2 proceeded via reaction 1. After CH4 and product gases were swept off by Ar, an Ar-O2 mixture was introduced to regenerate the catalyst. A large and rapid increase in the catalyst bed temperature was observed without the leaking of O2 into the effluent. The bed temperature increased from the prefixed furnace temperature to the maximum of 720 °C, and it decreased to the furnace temperature following the O2 response. No CO or CO2 formation was observed, indicating that O2 was consumed to oxidize reduced Ni to NiO. Similar behavior was observed in the 12-min run (Figure 7b), in which the amount of feed CH4 increased to the half of stoichiometric amount of lattice oxygen (CH4/NiO ) 2.5 mmol/4.0 mmol). Even below the stoichiometric CH4/NiO ratio, a certain amount of nickel oxide was reduced to metallic nickel, leading to CH4 decomposition to give H2 and carbon (reaction 1). Carbon formation was confirmed by the responses of CO and CO2 in the oxidation stage together with the temperature increase in the catalyst bed. Here we observed simultaneous oxidation of the carbon and metallic Ni of the catalyst. 3.8. Catalyst Characterization. 3.8.1. XRD. To confirm the above-mentioned redox cycles between NiO and metallic Ni, XRD patterns of as-prepared and reacted catalysts were examined. Figure 8 shows the X-ray diffraction patterns of the as-prepared (NiO (49.3 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) Figure 8a) catalyst, that after the reaction with CH4 for 10 min (Figure 8b), the catalyst after the reduction with H2 for 60 min (Figure 8c) and that after the 10th reaction-regeneration cycle (Figure 8d). In the as-prepared and regenerated catalysts, diffraction peaks ascribed to MgO and composite oxide phases (NiO, NiO/MgO, NiMgO2-MgO solid solution (circle); NiCr2O4 or MgCr2O4 (square)) were observed. After the reaction

Figure 8. XRD patterns of NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) catalyst: (a) as-prepared, (b) after CH4 flow 10 min at 700 °C, (c) after H2 flow 60 min at 700 °C, (d) catalyst after the 10th reoxidation cycle.

with CH4 for 10 min (Figure 8b), in addition to the above phases, diffraction peaks of metallic nickel (triangle) could be detected. These results clearly indicate that the reduction of NiO in the complex oxide phase to Ni occurred to give H2 and CO in the reaction with CH4. After the reduction with H2 for 60 min (Figure 8c) a very similar pattern was observed, although the relative intensity of the peak at 43.3° (NiO containing MgO phases (circle)) decreased as compared to that observed after the reaction with CH4. Complete recovery of the XRD pattern after the 10th oxidation cycle is in agreement with the results obtained in the runs shown in Figures 4 and 7. 3.8.2. XPS. Depth profiles of the as-prepared and H2-reduced NiO-Cr2O3-MgO catalysts were obtained by XPS measurement. XPS profiles of the H2-reduced catalyst were employed to elucidate changes in the catalyst that occur after the reaction with CH4, since after this reaction, carbon deposition onto the surface is inevitable. Depth profiles of the as-prepared catalyst are shown in Figure 9. On the surface of the catalyst, binding energies (BE) of all the oxide species were unusually high, indicating that complex oxides of three elements may exist.25-32 After Ar ion sputtering of 1 min, except for the Ni species, BEs of Cr and Mg decreased to the literature values of Cr2O3 and MgO, respectively. The intensities of the peaks ascribed to Cr and Mg oxide decreased with an increase in the sputtering time. This shows that both of the less abundant oxides (Cr and Mg) are enriched on the surface of the catalyst. In contrast, even after 1 min of sputtering, a peak of the Ni species having a higher BE than that of NiO was observed at 855.8 eV, together with a peak ascribed to NiO (853.5 eV). The peak at 855.8 eV could be assigned to the Ni3+ species.33 The relative intensity of the peak having a higher BE decreased with increasing sputtering time. After 30 min of sputtering, the peak at 855.8 eV disappeared. On the surface of the as-prepared catalyst, the BE of Cr6+ species was observed at 578.8 eV skewed to the lower BE side, and this signal disappeared after 1 min of sputtering, indicating

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Figure 9. X-ray photoelectron spectra of Ni, Cr, and Mg species in NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) catalyst: (a) as-prepared; (b) after H2 reduction at 700 °C for 1 h; (c) NiO-MgO; (d) NiO-Cr2O3; (e) Cr2O3-MgO. Literature values: Ni(2p3/2): NiOx, 858.5 eV; Ni3+, 856.0 eV; Ni2+, 853.5 eV; Ni, 852.9 eV. Cr(2p3/2): CrOx, 580-575; Cr6+, 578.8 eV; Cr3+, 576.8 eV; Cr, 573.8 eV. Mg(2s): MgOx, 51.8 eV; MgO, 49.5 eV.

that higher valence state oxides of Cr could be associated with Ni and Mg. The unusually high BE of the Mg species also exhibited the same behavior in the sputtering. The depth profiles of Ni, Cr, and Mg species after H2 reduction, which would correspond to the catalyst after the reaction with CH4, are shown in Figure 9b. On the surface of the reduced catalyst, peaks of Ni, Cr, and Mg again showed higher BEs. The relative intensity of Ni species was low. In contrast to the fresh catalyst, the Ni species showed a single peak assigned to metallic Ni species after 1 min of sputtering. After 1 min of sputtering a peak ascribed to Cr2O3 appeared, and with a further increase in the sputtering time, the intensity of the peak of Cr oxide decreased, and after 10 min of sputtering a broad peak assigned to Cr3+ appeared at 573.8 eV. Again, MgO showed a peak having a higher BE of 51.8 eV, but after 1 min of sputtering a peak assignable to MgO appeared at 49.5 eV. These results indicate that on the surface of the mixed catalyst, relatively stable higher valence state complex oxides of Ni, Cr, and Mg seem to exist. To understand the formation of complex oxide, binary oxides were prepared, and XP spectra were measured. They are shown in Figure 9c-e. The result that Ni species in the NiO-MgO showed a peak at 856.0 eV suggests that this peak can be safely assigned to NiO-MgO in the ternary complex oxide (probably in the form of Ni3+). In the XPS of NiO-Cr2O3 binary oxide, the BE of NiO appeared at 857.5 eV, which closely corresponds to that observed in the asprepared NiO-Cr2O3-MgO catalyst before sputtering. In the spectra of Cr species of the binary oxides, the presence of Ni-Cr6+ complex oxide was confirmed by the peak at 578.8 eV. In the spectra of MgO species, the higher BE peak of Mg was only observed in the mixed oxide of Cr with Mg. Therefore, in all the XPS spectra, higher BE species of Ni, Cr, and Mg were ascribed to the ternary complex oxides having Cr6+ species.

Figure 10. XPS depth profile of respective species obtained by Ar+ sputtering for the as-prepared and after-H2-reduced NiO(52.6 mol %)-Cr2O3(6.6 mol %)-MgO(balance) catalyst.

The relative abundances of Ni, Cr, and Mg in the depth profile are shown in Figure 10a,b. In these calculations, raw spectral data were taken without correction of photoionization crosssection. Therefore, the results did not reflect the exact elemental composition and only exhibited the relative intensities of the counted photoelectrons. On the surface of the as-prepared and H2-reduced catalyst, a smaller amount of Ni species exists, but it is rich in Cr species. The relative amount of Ni species increased inside the catalyst, and that of Cr species correspondingly decreased. The lower concentration of Ni species on the surface of the catalyst may be partly responsible for the smaller amount of carbon with this ternary catalyst. The existence of Cr6+ evidenced by the peak at 578.8 eV was confirmed by the broad absorption at 260-300 nm in the diffuse reflectance UV-vis spectra of the catalyst. 3.9. TPR Profile of Complex Oxides. Figure 11 shows the TPR profiles of several NiO-loaded catalysts. A bulk NiO (100 mesh under) prepared by the decomposition of Ni(NO3)2 was reduced below 400 °C with H2, and NiO on MgO was reduced above 700 °C with a very weak response up to 1000 °C, indicating a strong interaction between NiO and MgO. This finding is in good agreement with the result that a low lattice oxygen conversion of NiO-MgO was observed (Table 1 entry 1). The initial reduction temperature of NiO-Cr2O3-MgO decreased to 500 °C. These results indicate that the dramatic improvement in the redox behavior of NiO-MgO by the addition of Cr2O3 seems to be partly a result of the prevention of the formation of a less reducible NiMgO2 complex oxide. Otsuka et al. reported the promoting effect of the doping of Cr2O3 into Fe3O4 in the reduction of Fe3O4 with CH4, and they proposed that doped Cr2O3 facilitates the breaking of the Fe-O bond by penetrating interstatially into the oxide.11 In our NiO-MgO complex oxide case, such effect may prevent the formation of less reactive NiO-MgO complex oxide or solid solution. In addition, the

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Table 5. Change of Standard Enthalpy of the Reactions

Figure 11. Temperature programmed reduction profiles of NiO in several mixed oxides. Response of m/z ) 18 was plotted against temperature. Catalyst, 0.1 g, (b) Ni/Cr ) 32: 8, (c) Ni/Cr/Mg ) 32:8:25, (d) Ni/Mg ) 32:25; H2/Ar ) 20/10 (mL/min). Heating rate: 10 °C/min. Catalysts were prepared by evaporation process. Bulk NiO was prepared by calcinations of Ni(NO3)2 at 800 °C for 5 h.

reaction formula

∆H2980 (KJ/mol)

reduction of NiO

CH4 + NiO / 2H2 + CO + Ni CH4 / C + 2H2

205 75

oxidation of Ni

Ni + 1/2O2 f NiO Ni + H2O / NiO + H2 Ni + CO2 / NiO + CO C + 1/2O2 f CO C + H2O f CO + H2 C + CO2 / 2CO

–241 1 43 –111 131 173

is interpreted that a decrease in the CO2 concentration corresponded to the increase of response of CO formation. If carbon on the surface was reacted with CO2 the response of CO should reach an amount of twice as much of decrease in CO2 response. After TPO runs with H2O and CO2, oxidation with O2 at 800 °C was carried out, and the amount of oxygen utilized to oxidize the remaining metallic Ni was calculated. About 58.3 and 57.7% of Ni were oxidized to NiO with H2O and CO2, respectively, in the TPO with the respective oxidant. As shown in Table 5, endothermic oxidation with H2O afforded H2, but it proceeded completely at high temperatures above 800 °C. Reoxidation with CO2 to give CO is also endothermic and would also occur at a higher temperature. From these results, it was found that the use of O2 (air) as an oxidant is the best choice for producing syngas using the lattice oxygen of the NiO-Cr2O3-MgO catalyst. If more than two reactors are installed, alternative reactions with CH4 and air, nitrogen-free synthesis gas could be continuously obtained by using air as an oxidant. In addition, the heat required for the reaction with CH4 may be recovered from the heat generated in the oxidation stage. 4. Conclusions

Figure 12. Temperature programmed oxidation profiles of reduced NiO (52.6 mol %)-Cr2O3 (6.6 mol %)-MgO (balance) catalyst with different oxidants. Catalyst: 0.5 g; heating rate, 10 °C/min. Pretreatment: catalyst was reduced with CH4 for 12 min at 700 °C, and then cooled to room temperature. (a) O2/Ar ) 10/20 (mL/min), (b) H2O/Ar ) 10/20 (mL/min), (c) CO2/Ar ) 10/20 (mL/min).

doped Cr2O3 formed less reducible NiCr2O4 and MgCr2O4 binary oxides, and has resistance to sintering in CH4 runs.13,14 3.10. TPO of Reduced NiO-Cr2O3-MgO catalyst. Figure 12 shows the TPO profiles of reduced NiO-Cr2O3-MgO with 4.5 mmol of CH4 over 4.0 mmol of NiO at 700 °C using different oxidants. Nearly 80% of NiO in the catalyst was calculated to be reduced to Ni in the reaction with CH4. In the oxidation with O2, oxidation of metallic Ni occurred at 300-360 °C as evidenced by the decrease in the O2 response. A small and broad appearance of CO2 was observed at 500-650 °C due to the oxidation of carbon on the catalyst formed during the reaction with methane. Metallic Ni was oxidized above 500 °C in the regeneration by H2O accompanied by the oxidation of carbon species above 550 °C. In the case of CO2, it is difficult to differentiate whether Ni was oxidized with CO2 or carbon was oxidized to give CO. As seen in Figure 11c, CO formation was observed above 500 °C, and probably a small portion (surface) of metallic Ni could be oxidized. The reason for this

Mixed oxide of NiO-Cr2O3-MgO produced H2 and CO in the reaction with CH4 at 700 °C in high yields, and reduced nickel could be regenerated by air (here we used Ar/O2 ) 4:1). The result of the addition of Cr2O3 to the mixed oxide was to prevent the formation of less reducible NiMgO2 binary oxide and to decrease the temperature of the reaction of CH4 with NiO; thus, without using pure oxygen, H2 or synthesis gas free from nitrogen could effectively be produced. Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research (20560722) and MEXT.HAITEKU (20072011). Note Added after ASAP Publication: After this paper was published ASAP November 24, 2009, a correction was made to the author affiliations; the corrected version was reposted November 30, 2009. Literature Cited (1) Rostrup-Nilsen, J. R. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 1. (2) Ashcroft, T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Selective oxidation of methane to synthesis gas-using transition-metal catalysts. Nature 1990, 344, 319. (3) D.A. Hickman, D. A.; Haupfear, E. A.; Schmidt, L. D. Synthesis gas formation by direct oxidation of methane over Rh monoliths. Catal. Lett. 1993, 17, 223. (4) Yan, Q. G.; Wu, T. H.; Weng, W. Z.; Toghiani, H.; Toghiani, R. K.; Wan, H. L.; C.U. Pittman, C. U., Jr. Partial oxidation of methane to H2 and CO over Rh/SiO2 and Ru/SiO2 catalysts. J. Catal. 2004, 226, 247.

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ReceiVed for reView August 27, 2009 ReVised manuscript receiVed October 29, 2009 Accepted November 4, 2009 IE9013474