Characterization of Molybdenum Carbides for Methane Reforming by

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J. Phys. Chem. B 2001, 105, 9124-9131

Characterization of Molybdenum Carbides for Methane Reforming by TPR, XRD, and XPS Katsuhiko Oshikawa, Masatoshi Nagai,* and Shinzo Omi Graduate School of Bio-Applications and Systems Engineering, Tokyo, UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ReceiVed: March 29, 2001; In Final Form: June 22, 2001

The relationship between various unsupported molybdenum carbides and their activity toward methane reforming at 973 K and 1 atm was studied. Unsupported molybdenum carbides catalyzed the formation of hydrogen in high selectivity, forming ethylene and ethane rather than benzene as the carbon-containing products. η-Mo3C2, which was nitrided at 973 K and subsequently carbided at 1173 K, was more active than both R-MoC1-x and β-Mo2C in methane decomposition, forming hydrogen in high selectivity. R-MoC1-x and γ-Mo2N were transformed to η-Mo3C2 in the bulk structure during methane reforming at 973 K. This transformation caused a significant increase in the turnover frequency of methane reforming. η-molybdenum carbide was also formed during CH4-TPR of γ-Mo2N at 788 K. The linear relationship between the amount of η-carbide determined through H2-TPR of the catalysts and the methane disappearance rate revealed that η-Mo3C2 is the active species for methane reforming. From the XPS analysis, Mo0 was the dominant molybdenum species for the η-Mo3C2 catalysts.

Introduction Molybdenum-based catalysts have gathered much renewed interest as catalysts with high selectivity toward aromatic compounds for methane reforming. Molybdenum oxide catalysts were reported to exhibit activity for methane conversion after an induction time of about 45 min.1,2 Molybdenum oxide species are partially reduced during the reaction to form molybdenum carbide as the catalytic center based on the formation of both carbidic carbon from XPS3-5 and the Mo-C bond from EXAFS.6 For supported molybdenum carbide catalysts, β-Mo2C on H-ZSM5 was reported to exhibit high conversion of methane and high selectivity of benzene toward this reaction.3,4,6-14 However, little attention has been paid to other molybdenum carbides such as R-MoC1-x and η-Mo3C2 for methane reforming, despite the fact that these catalysts have exhibited higher activity than β-Mo2C in methane reforming3,11,15,16 and CO2 hydrogenation.17 In this paper, the activities of the R, β, and η-molybdenum carbides are compared, focusing on the changes in molybdenum carbides during methane reforming. In a previous paper,17 the three different species of molybdenum carbides were deconvoluted from the methane peaks desorbed during the TPR of the catalysts. Furthermore, the changes in bulk structure are determined by XRD, while the formation of molybdenum carbides near the surface of the catalysts are determined by temperature-programmed reactions in methane (CH4-TPR) and hydrogen (H2-TPR). XPS is used to determine the oxidation state of the molybdenum species on the surface of the catalysts. The purpose of this study is to examine (1) the differences in catalytic activity among R-MoC1-x, β-Mo2C, and η-Mo3C2, which produce hydrogen in high selectivity, (2) the changes in the molybdenum carbide species during the reaction of the bulk by XRD and the surface by TPR and XPS, (3) the relationship between the molybdenum carbide phase measured by TPR and the activity of each molybdenum * To whom correspondence should be addressed. Tel/Fax: +81(42)3887060. E-mail: [email protected].

carbide species toward methane reforming, and (4) the distribution of the oxidation state of molybdenum on the surface of the molybdenum carbide catalysts. Experimental Section Materials and Preparation of Molybdenum Carbides. Helium and hydrogen (Tomoe Co., 99.9999%) were purified using deoxygen-traps (Oxiclear, Supelco Co.), while ammonia and 20% CH4/H2 (Takachiho Co., 99.999%) gases were used without further purification. 0.2 g of unsupported MoO3 powder (Aldrich Co., 99.9% purity) was packed on a fritted ceramic disk in a quartz microreactor (10 mm i.d.). The catalyst temperature was measured by a chromel-alumel thermocouple positioned in the center of the catalyst bed. The reactor was heated externally, with each temperature ramp controlled by a PID programmable-temperature controller, accurate to (1 K. The catalyst preparation and reaction experiments were performed in a flow microreactor system at atmospheric pressure with mass flow controllers. After calcining the MoO3 precursor in dry air at 773 K for 1 h, the catalyst was prepared by temperature-programmed reaction of the oxide. For the direct carburization, the catalyst was heated in a stream of 20% CH4/ H2 (179 mmol h-1). A temperature ramp of 60 K h-1 was utilized until the desired temperature of either 873, 923, or 973 K was reached, after which the temperature was maintained for 3 h. Another pretreatment method involved the pretreatment of the oxide in flowing ammonia before carbiding. For the nitrided/ carbided catalysts, the calcined precursor was heated in ammonia flow (179 mmol h-1) from 573 to 973 K at a rate of 60 K h-1 and this temperature was maintained for 3 h. The gas was then changed to 20% CH4/H2 (179 mmol h-1), and the catalysts were carbided at either 973 or 1173 K for 3 h. The pretreated catalysts were measured without exposure to air for the activity measurement, CO adsorption, and temperature-programmed reaction in methane and in hydrogen, because oxygen diffuses into the bulk structure of molybdenum carbides.18,19 The different pretreatment temperatures are represented by numerals in Table 1.

10.1021/jp0111867 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001

Molybdenum Carbides for Methane Reforming

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TABLE 1: Physisorption and Chemisorption Data for Various Molybdenum Catalysts 2 -1 a -1 nitriding carbiding Sg (m g ) CO adsorption (µmol g ) b c b c catalyst temp (K) temp (K) b.r. a.r. b.r. a.r.

1 2 3 4 5 6

973 973

973

1173 973 973 923 873

77 88 7 3 3 92

69 136 5 2 3 147

0.5 173 4.8 17.8 31.1 320

0.20 0.14 0.50 0.65 1.76 0.65

a

Irreversible CO uptake. b Before reaction: measured after pretreatment. c After reaction: measured after reaction in methane at 973 K for 7 h on stream.

Activity Measurement. The activities of the catalysts for CH4 reforming were measured in situ using the flow reactor system at atmospheric pressure. The temperature of reaction was 973 K and the methane flow rate was 40 mmol h-1, with the space velocity amounting to 201 mmol h-1 g-cat-1. The reaction products were detected using a quadrupole mass spectrometer (Quadstar 422, Balzers Co.) connected online. Hydrogen, methane, water, ethylene, ethane, carbon dioxide, and benzene were determined from the mass-to-charge (m/z) ratios of 2, 16, 18, 28, 30, 44, and 78, respectively. Also, CO and N2 were determined from the m/z values of 12 and 14, respectively. The formation rate for each product was calculated using a calibration curve prepared by addition of a measured amount of gas. The disappearance rate of methane was calculated as mmol min-1 mol-Mo-1. The turnover frequency (TOF) was calculated as the rate of methane conversion (mol min-1 g-1) divided by the amount of CO uptake (mol g-1). The chemisorption value before reaction was used to calculate the TOF for the initial stage of the reaction, while the CO uptake measured after 7 h on stream was used to calculate the TOF at 7 h. Characterization. The X-ray powder diffraction before and after the reaction was done with a RAD II (Rigaku Electronics Co.) using Cu KR ray (30 kV, 20 mA). The XRD spectra of MoO3 (JCPDS 35-609: 2θ ) 23.3, 25.7, 27.3, 39.0°; this work: 2θ ) 23.3, 25.7, 27.3, 39.0°), MoO2 (32-671: 26.0, 37.0, 53.5°; 26.1, 36.9, 53.5°), fcc R-MoC1-x (36.7, 42.6, 62.0, 74.0°; 36.7, 42.5, 62.1, 74.1°),20 hcp β-Mo2C (35-787: 34.4, 38.0, 39.4, 61.5, 69.6, 74.6°; 34.4, 37.8, 39.3, 61.6, 69.5, 74.8°), hcp η-Mo3C2 (42-890: 36.5, 42.5, 61.4, 73.7°, 36.6, 42.1, 61.4, 73.7°), and fcc γ-Mo2N (25-1366: 37.4, 43.4, 63.1, 75.7°; 37.4, 43.4, 62.8, 75.6°) were identified. Temperature-programmed reaction in CH4 (CH4-TPR) was performed in situ in methane flow (99.999%, 40 mmol h-1) from room temperature to 1173 K at a temperature ramp of 60 K h-1. The products were measured by the quadrupole mass spectrometer connected online and were plotted against the formation temperature with one scan taken every minute. Temperature-programmed reaction experiments in hydrogen (H2-TPR) were performed using a temperature ramp of 600 K h-1 and a flow rate of 40 mmol h-1, with one scan taken per minute. The resulting desorption peaks of methane were deconvoluted using commercially available software (Origin 5.0 J, Microcal Co.), following the parameters outlined in a previous paper.17 The BET surface area and CO chemisorption of the catalysts was determined by conventional volumetric analysis (Omnisorp 100CX, BeckmannCoulter Co.). After preparation, the catalysts were cooled to room temperature and transferred to a quartz cell using a glovebox. The glovebox was pumped and back-filled with purified argon three times to prevent contamination by air. The quartz cell was connected to the adsorption instrument under

Figure 1. CH4 disappearance rates for reaction at 973 K for various unsupported Mo catalysts. 0, 1; 9, 2; O, 3; b , 4; 4, 5; 2, 6.

argon atmosphere. Before measuring the CO uptake, the sample was first evacuated at 10-2 Pa at 653 K and pretreated in hydrogen at 653 K for 2 h and 673 K for 1 h. After pretreatment, the catalyst was degassed at 10-2 Pa, 673 K for 2 h, then slowly cooled to room temperature in a vacuum. The amount of irreversible CO uptake was obtained from the difference between the two isotherms which were extrapolated to zero pressure. The oxidation states of the molybdenum atoms on the catalyst surface were determined by XPS using a spectrometer (Shimadzu ESCA3200) with monochromatic MgKR exciting radiation (8 kV, 30 mA) at 5 × 10-4 Pa. To avoid contamination of the surface species by oxygen, the catalyst was transferred to the XPS chamber without exposure to air using a glovebox. The catalyst was removed from the reactor under argon atmosphere in a glovebox and secured onto the XPS holder with carbon tape, and no argon etching was procured. Both the Table and the Figure indicate the XPS spectra without argon etching. The Mo 3d envelope was deconvoluted to obtain the oxidation state distribution of 0 to +6 for Mo. The baseline of this envelope was determined by the Shirley method because the χ2 correction value of the Shirley method was about 40% smaller than that of the linear method, and each peak was simulated using a combined 50% Lorentian/50% Gaussian function. The Mo 3d5/2 and Mo 3d3/2 peaks were separated by 3.1 eV with peak ratios of 3/2. The binding energies of the Mo 3d5/2 were set as thus, with the full width at half-maximum (fwhm) values in parentheses, according to XPS studies published previously.5,10,21-25 The binding energy values (Mo 3d5/2) contain a (0.1 eV error, while the fwhms have a (0.15 eV error. Mo0: 227.6 (1.8); Mo2+: 228.2 (2.1); Mo3+: 238.8 (2.1); Mo4+: 229.9 (2.2); Mo5+: 231.8 (2.3); and Mo6+: 233.1 eV (2.4 eV). The C1s peak was deconvoluted for the carbidic carbon peak at 283.0 eV (fwhm 1.4 eV) identified in the literature (282.826,27 and 282.9 eV28) as being due to the presence of carbidic carbon and the graphitic carbon peak at 284.5 eV (1.6-1.9 eV) (284.524,25 and 284.6 eV4). Furthermore, the XPS N 1s binding energy of the samples at 397 ( 0.3 eV (2.0 eV) was used to calculate the nitrogen content in the samples on the basis of a new determination method30 for the deconvolution of Mo 3p3/2 and N 1s in the region of 390 and 410 eV. Results and Discussion Activities of Various Molybdenum Carbides. The main products for methane reforming at 973 K were hydrogen, ethylene, and ethane, with benzene formed in trace amounts. The disappearance rates of methane at 973 K for the catalysts prepared by various pretreatment methods are shown in Figure 1 and Table 2. Catalyst 1 exhibited the highest disappearance rate for methane decomposition at 2 min on stream, followed

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TABLE 2: Activity Data for Various Molybdenum Catalysts reaction ratea H2 formation rate C2 formation ratec C6H6 formation rate conversion (%) (mmol min-1 mol-Mo-1) TOFb (min-1) (mmol min-1 mol-Mo-1) (µmol min-1 mol-Mo-1) (nmol min-1 mol-Mo-1) catalyst

2 min

7h

2 min

7h

2 min

7h

1 2 3 4 5 6

6.7 4.9 4.1 7.2 5.6 9.4

0.5 0.4 0.4 0.3 0.5 0.4

32.2 23.8 19.9 35.1 27.1 45.2

2.57 2.01 2.00 1.66 2.19 1.96

448 89.0 1.0 100 28.8 27.8 13.7 17.7 6.0 8.6 1.0 20.9

2 min

7h

2 min

7h

2 min

7h

59.3 46.0 33.0 60.0 52.4 78.3

4.50 4.40 3.87 2.86 5.37 4.10

2.85 4.01 2.31 4.09 5.54 6.25

18.9 9.19 6.79 18.7 1.89 2.78

2.6 1.1 1.8 1.2 1.1 0.8

1.4 1.3 0.7 1.8 1.4 1.0

a Disappearance rate of methane. b Turnover frequency was calculated by dividing the reaction rate by the CO uptake. c Sum of ethane and ethylene formation rates.

by catalyst 3. These two catalysts decayed slowly over a 4-h period. Catalyst 1 had the highest methane decomposition rate at 7 h on stream of 2.57 mmol min-1 mol-Mo-1 (conversion: 0.5%) followed by catalyst 5 with 2.19 mmol min-1 mol-Mo-1 (0.5%). Catalysts 2, 4, 5, and 6 decayed rapidly during the first 30 min on stream, reaching a steady-state value at 3 h on stream. Catalyst 5 had the highest formation rate of hydrogen at 7 h on stream, 5.37 mmol min-1 mol-Mo-1, while 1 and 4 had high rates of formation for C2 products (ethylene and ethane) of nearly 19 µmol min-1 mol-Mo-1 at 7 h on stream. Of the various catalysts, catalyst 1 maintained the activity longest for methane reforming, producing the most hydrogen. Benzene formation over the unsupported catalysts was less than 10-6 the amount of H2 formed. This result indicated that unsupported Mo2C did not catalyze the formation of benzene but instead formed hydrogen. Recent studies have found that H-ZSM5-supported molybdenum carbide catalysts have high selectivity toward the formation of benzene and naphthalene. As a result, the H-ZSM5 support is especially high in the selectivity toward the formation of benzene during methane reforming, exhibiting over 90% selectivity.7,9-14,31,32 The BET surface area (S8) and the amount of CO chemisorption of the catalysts are shown in Table 1. Catalysts 1, 2, and 6 exhibited surface areas one order larger than those of 3, 4, and 5. After the reaction, the surface areas decreased for the catalysts with the exception of catalysts 2 and 6, which increased from 88 to 136 m2 g-1 and 92 to 147 m2 g-1, respectively. The nitriding/carbiding procedure increases the surface area of molybdenum carbide catalysts due to the topotactic synthesis of molybdenum carbides from γ-Mo2N.20,28,33,34 The formation of additional micropores of 0.7-nm diameter led to an increase in surface area. Catalyst 1 decreased from 77 to 69 m2 g-1, possibly due to the formation of carbonaceous carbon, discussed later. The CO chemisorption was reduced drastically after 7 h reaction for all catalysts. This is probably due to the agglomeration of molybdenum carbides during the reaction and the blockage of pores or active sites by accumulation of carbonaceous carbon formed as a byproduct of the reaction.35 In terms of the TOF in Table 2, catalyst 1 exhibited extremely high activity for CH4 decomposition (448 min-1) after 2 min, while after 7 h on stream catalyst, 2 exhibited a greater TOF than the other catalysts at 100 min-1. The TOF values of catalysts 2 and 6 for methane decomposition increased during the reaction from 1.0 to 100 and 20.9 min-1, respectively, although the TOF of 1 decreased from 448 to 89 min-1 from 2 min to 7 h. The increase in the TOFs of 2 and 6 suggests the transformation of the surface species to more active species during the reaction. Since η-Mo3C2 (by XRD) had high TOF (21 and 89-448 min-1) and β-Mo2C had low TOF (6-29 min-1), η-molybdenum carbide was discriminated from β-molybdenum carbide. Structural Changes Resulting from CH4 Reaction. The XRD patterns of the various catalysts before and after the

Figure 2. XRD patterns of unsupported molybdenum carbide catalysts. 0, R-MoC1-x; 4, γ-Mo2N; O, η-Mo3C2; 9, β-Mo2C. (A) 2 before reaction. (B) 2 after reaction. (C) 6 before reaction. (D) 6 after reaction. (E) 1 before reaction. (F) 1 after reaction. (G) 5, 4, 3 before reaction. (H) 5, 4, 3 after reaction.

reaction are shown in Figure 2. Catalysts 1, 2, and 6 gave XRD patterns of η-Mo3C2 (2B), R-MoC1-x (2A), and γ-Mo2N (2C), respectively. After reaction, these carbides were all transformed to η-Mo3C2 (2D, E, and F). Thus, a significant structural transformation is induced by the CH4 reaction for the nitrided (6) and nitrided/carbided (1 and 2) catalysts. The nitriding pretreatment greatly increased the TOF and carbiding further promoted an increase in the TOF. The increase in activity for 2 and 6 during the reaction is explained by this transformation of molybdenum carbide species from β-Mo2C to η-Mo3C2. Severe carbiding at 1173 K probably caused the decrease in the TOF of 1. Still, the TOF of 1, equivalent to that of 2, was 3 times that of the catalysts prepared by direct carbiding in the methane/hydrogen mixture. Sintering and agglomeration of molybdenum carbides probably occurred for catalyst 1 during the transformation to η-Mo3C2, causing a decrease in the CO uptake. Because the β-Mo2C structure of 3, 4, and 5 did not change during the reaction, the TOF values were of the same order, unlike 2 and 6 which increased in TOF during the reaction. Thus, the increase in the activity for methane decomposition is due to the transformation of the molybdenum carbide to the active η-Mo3C2. The nitriding/carbiding procedure formed a catalyst with higher activity than the catalyst prepared by direct carburization. Temperature-Programmed Reaction in CH4 (CH4-TPR). CH4-TPR was performed to determine the changes in the carbide structure of 1, 3, 6, and MoO3 during methane reforming and is shown in Figure 3. The reaction products were hydrogen, water, benzene, and carbon monoxide for 1, 3, and MoO3, and nitrogen gas for 6. The MoO3 sample showed a large peak at 1020 K for hydrogen (Figure 3A) and water formation (3B), together with a small desorption peak for carbon monoxide (3C)

Molybdenum Carbides for Methane Reforming

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TABLE 3: Deconvolution of CH4 Desorption from TPR of Various Molybdenum Carbides CH4 desorption from TPR (µmol g-1) XRD pattern catalyst

b. r.a

a. r.b

1 2 3 4 5 6

η-Mo3C2 R-MoC1-x β-Mo2C β-Mo2C β-Mo2C γ-Mo2N

η-Mo3C2 η-Mo3C2 β-Mo2C β-Mo2C β-Mo2C η-Mo3C2

a

b. R-MoC1-x

a. r.b

β-Mo2C

Pyrc

η-Mo3C2

Grd

123 84 40 13

444

217 68 13

19 25 577 433 4

315 24

r.a R-MoC1-x

β-Mo2C

Pyrc

η-Mo3C2

Grd

2 13 16 4

47 32 127 426 63 74

335 212 270 157 241 194

705 98 63 87 93 212

Before reaction. b After reaction. c Pyrolytic carbon. d Graphitic carbon.

Figure 3. Desorption of various products during CH4-TPR of MoO3 (;), 3 (+), 6 (-‚-), and 1 (‚‚‚) catalysts. (A) H2 (B) H2O (C) CO (N2 for 6) (D) C6H6. Points a, b, and c: MoO3; d, e, and f: 6.

and benzene (3D). The XRD of the MoO3 catalyst at temperatures of 990, 1020, and 1193 K (points a, b, and c, respectively, in Figure 3D) during CH4-TPR revealed that the crystal structure changed from MoO3 (4A) to β-Mo2C (4C). MoO2 and MoO3 were observed at 1020 K (4C) but the bulk structure was β-Mo2C at 1193 K. A broad desorption peak for 6 was detected for H2 at 864 K (3A) and for N2 at 860 K (3C). This was due to the desorption of NHx species present on the surface of this catalyst.36 Although the formation of benzene was smaller than H2 on the order of 10-5, the formation peak of benzene indicated a change in the catalyst surface during the CH4-TPR. The bulk structure of 6 was examined before and after benzene formation with a maximum at 788 K. The XRD pattern before the formation of benzene in the CH4-TPR taken at 723 K (point d in Figure 3D) indicated the γ-Mo2N structure (4D), whereas the pattern showed the formation of η-Mo3C2 (4E) after the CH4TPR. At 788 K (point e in Figure 3D), the peak temperature of the benzene formation showed that the bulk retained the original γ-Mo2N structure. Further heating of the catalyst induced the formation of the η-Mo3C2 phase, but this was observed only above temperatures of 1000 K (point f in Figure 3D), well above the first observed benzene peak for this catalyst. Catalysts 1 and 3 showed the formation of benzene at 704 K, lower than that for MoO3 and 6. The peaks of benzene formation at 1020 K for MoO3 and 788 K for 6 indicated the formation of molybdenum carbide on the surface, although the XRD patterns

were different, as shown before. Concerning the desorption of water (3B), a small desorption peak was observed for 6 but was hardly observed for 1 and 3. In Figure 3C, the desorption of carbon monoxide was observed only in trace amounts for 1, 3, and 6. The absence of these oxygen species in the desorption peaks of 1 and 3 indicates that molybdenum oxides or oxycarbides are not present in these catalysts. No conformational change resulted through the CH4-TPR of 3 (4C, β-Mo2C) or 1 (4E, η-Mo3C2). Determination of Molybdenum Carbide Species by H2TPR. H2-TPR was utilized to differentiate the carbide species formed on the surface of the MoO3 and 6 during CH4-TPR and methane reforming. The desorption of methane was monitored and deconvoluted,11 revealing the presence of R-MoC1-x at 798 K, β-Mo2C at 886 K, and η-Mo3C2 at 1082 K, along with two other carbon species, i.e., pyrolytic carbon at 960 K and graphitic carbon at 1166 K. The results of the deconvolution of the methane peaks are given in Table 3, along with the XRD patterns taken before and after the reaction in methane. Catalyst 1 before the reaction, with the bulk structure of η-Mo3C2, had the largest amount of η-Mo3C2 (123 µmol g-1) and the highest TOF of 448 min-1. A drastic change in the carbide composition due to H2-TPR was observed for the catalysts after the reaction. The amount of η-Mo3C2 increased for all catalysts, while the amounts of R-MoC1-x and β-Mo2C decreased, indicating that the carbide species on the surface of the catalysts changed to η-Mo3C2 during the methane reaction. The H2-TPR spectra of 2 and 4 are compared before and after the reaction (Figure 5). Catalyst 2 before the reaction had a large amount of desorption for carbon attributed to R-MoC1-x (315 µmol g-1) followed by η-Mo3C2 (84 µmol g-1) and pyrolytic carbon (25 µmol g-1). Catalyst 2 exhibited predominantly an R-MoC1-x character before the reaction, as was evident from the large peak for R-MoC1-x at 798 K and from the R-MoC1-x pattern in the XRD spectrum of this catalyst (2A). In the H2-TPR spectrum of 2 after 15 min on stream as shown in Figure 5B, R-MoC1-x decreased while η-Mo3C2 (1082 K) increased. This result indicates that R-MoC1-x started to transform to η-Mo3C2 within 15 min. After 7 h into the reaction (5C), the surface of 2 was predominantly η-Mo3C2. Thus, it was observed by both TPR and XRD that the carbide species of 2 changed from R-MoC1-x to η-Mo3C2 during the CH4 reaction at 973 K. The extra carbon needed for the insertion of carbon to form η-Mo3C2 from R-MoC1-x and β-Mo2C may be provided from the methane of the feed. Another possibility would be from the carbidic, pyrolytic, or graphitic carbon. In Figure 5D, the η-Mo3C2 peak for 4 increased after methane reforming, although roughly 8% of the carbide (13 µmol g-1) remained β-Mo2C (5E). However, the XRD spectrum of 4 after the reaction showed no change in the β-Mo2C structure. This is most likely due to the difference between the bulk structure and surface, with the increase in the η-Mo3C2 on the surface. The amount of η-carbide increased

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Figure 6. Desorption of water during H2-TPR of molybdenum carbide catalysts before reaction. (;), 5; (‚‚‚), 4; (---), 3.

TABLE 4: Amount of H2O Desorption from TPR of Various Molybdenum Carbides H2O desorbed (µmol g-1)

Figure 4. XRD patterns of unsupported molybdenum carbide catalysts during CH4-TPR. b, MoO3; 2, MoO2; 9, β-Mo2C; 4, γ-Mo2N; O, η-Mo3C2. (A) MoO3 before reaction. (B) MoO3 after reaction to 1020 K. (C) MoO3 after reaction to 1193 K and 3 before and after reaction. (D) γ-Mo2N after reaction to 788 K. (E) γ-Mo2N after reaction to 1193 K.

Figure 5. Peak deconvolution of methane desorption peaks from H2TPR. (A) Catalyst 2 before reaction. (B) Catalyst 2 after reaction for 15 min. (C) Catalyst 2 after reaction for 7 h. (D) Catalyst 4 before reaction. (E) Catalyst 4 after reaction for 7 h.

3-fold for 1 and 2, while for 3, 4, 5, and 6, the amount of η-Mo3C2 increased more than 6 times. The broad peak at temperatures above 1100 K was due to the desorption of graphitic carbon. The amount of graphitic carbon on 1 increased greatly, from 444 to 705 µmol g-1, which probably caused the rapid decrease in the methane decomposition activity for 1. Catalyst 6 was also covered with a significant amount of graphitic carbon after reaction, at 212 µmol g-1, so that the activity for methane decomposition was relatively low, despite the fact that this catalyst was transformed to the catalytically active η-Mo3C2. Concerning the mass balance of carbon after reaction, the sum of C2 products and benzene formed accounts for only 0.01% of the carbon. The H2-TPR titrates only 13% of the total carbon while the remainder consists of graphite, which was measured from temperature-programmed oxidation of the catalyst. Thus, during the methane reforming, most of the carbon

catalyst

a

b. r.

a. r.

1 2 3 4 5 6

5 18 90 725 5625 261

3 5 15 99 121 31

a

b

O/Mo a

a. r.b

0.00 0.00 0.01 0.10 0.81 0.04

0.00 0.00 0.00 0.01 0.02 0.00

b. r.

Before reaction. b After reaction.

is deposited onto the catalyst surface in the form of graphite. Returning to the CH4-TPR, the H2-TPR spectra of the MoO3 and 6 at the peak of benzene formation (Figure 3D) showed 15 µmol g-1 of β-Mo2C and 9 µmol g-1 of R-MoC1-x, respectively, indicating the formation of molybdenum carbide on the surface of the catalyst. The desorption of water during H2-TPR before reaction is shown in Figure 6 and Table 4. For 1 and 2, the desorption of water was hardly detected before reaction. Catalysts 3, 4, 5, and 6 showed H2O desorption at above 1000 K. Catalyst 5 exhibited the largest desorption of water at 5.6 mmol g-1 at 1110 K, although the XRD suggested a slight formation of MoO2 and molybdenum oxycarbide. The amount of desorption progressively decreased as the carbiding temperature was increased. Catalysts 5, 4, and 6 before reaction had O/Mo ratios of 0.81, 0.10, and 0.04, respectively, indicating that oxygen still remained after preparation of these catalysts. The desorption of other oxygen-containing species such as CO and CO2 were not observed, with the exception of 5 before the reaction which desorbed 8 µmol g-1 of CO, This indicated the presence of oxygen remaining in 5, although no oxides or oxycarbides37 were detected by XRD. The O/Mo ratios for 5, 6, and 4 after the reaction decreased to 0.02, 0.01, and 0.0, respectively, indicating that MoO2 and molybdenum oxycarbide were transformed to the molybdenum carbides. Most Active Molybdenum Carbide Species for Methane Reforming. To determine the relationship between the carbidic phase and the activity of the catalysts toward methane decomposition, each molybdenum carbide species from H2-TPR was paired with the corresponding CH4 disappearance rates and H2 formation rates. The catalytic activity at 7 h on stream clearly showed a correlation with the amounts of η-Mo3C2 and the rates of CH4 disappearance and H2 formation, as evident in Figure 7A, whereas the amounts of β-Mo2C did not (7B). The formation rates of C2 products or benzene were not related to amounts of either carbide. Furthermore, no positive relationship was found between the amounts of β-Mo2C and η-Mo3C2 before the reaction and the initial rates of either CH4 disappearance, H2, C2 products, or benzene formation, probably due to the

Molybdenum Carbides for Methane Reforming

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9129

TABLE 5: Mass Balance of Carbon Species for Catalyst 3 after Reaction reactants (mmol g-1) catalyst

CH4 reacted

3

18.7

a

a

β-Mo2C 3.47

products (mmol g-1) b

C2 products

c

CH4 desorptiond

CO + CO2 desorptione

0.462

17.5

0.029

b

ratio of products/reactantsf 0.81

c

d

CH4 reaction at 973 K. Stoichiometric value of 1 g of Mo2C. C2 products formed during CH4 reaction at 973 K. Total desorption from H2-TPR. e Total desorption from TPO. f Sum of products divided by sum of reactants.

TABLE 6: Distribution of Oxidation States of Molybdenuma before and after Reaction by XPS before reaction (%)

after reaction (%)

catalyst

Mo6+

Mo5+

Mo4+

Mo3+

Mo2+ b

Mo0 b,c

Mo6+

Mo5+

Mo4+

Mo3+

Mo2+ b

Mo0 b

1 2 3 4 5 6

1.5 1.8 6.3 4.8 0.5 1.5

5.7 7.1 12.3 27.5 9.7 23.7

12.3 12.0 7.3 13.5 15.2 17.6

12.3 17.5 18.3 10.5 18.8 14.2

6.7 8.0 1.1 7.0 11.8 5.6

55.9 53.0 54.1 36.2 33.4 36.8

0.1 0.2 3.8 1.7 0.3 1.2

3.1 13.2 9.5 9.5 7.8 12.3

9.6 13.3 8.1 9.2 6.3 6.5

13.8 7.0 7.1 13.8 15.1 12.0

0.4 9.5 6.6 14.0 7.7 1.0

72.6 56.5 64.2 55.1 62.2 66.5

a No etching was procured and the Shirley method was used to determine the baseline for the Mo 3d spectra. Binding energy (Mo 3d ): Mo0 5/2 , 227.6 eV (fwhm, 1.8 eV); Mo2+ , 228.2 (2.1); Mo3+, 228.8 (2.1); Mo4+, 229.9 (2.2); Mo5+ , 231.8 (2.3); Mo6+, 233.1 (2.4). Binding energy (C 1s, carbidic carbon): 282.5 eV (1); 283.0 (2); 283.0 (3); 283.0 (4); 282.6 (5); 283.0 (6) before reaction; 283.0 (1); 282.8 (2); 283.0 (3); 283.0 (4); 283.0 (5); 283.0 (6) after reaction. b Moδ+ is Moδ+ (δ ) 0-+0.3) for the oxidation state between Mo0 and Mo2+. c For Mo 3d5/2 spectra of 1 before reaction in Ar etching for 2 min; 60.2% for Mo0, 5.0% for Mo2+, 11.9% for Mo3+, 10.8% for Mo4+, 7.2% for Mo5+, and 4.5% for Mo6+.

Figure 7. CH4 disappearance rates and H2 formation rates as a function of the amount of (A) η-Mo3C2 and (B) β-Mo2C as determined from H2-TPR. 0, CH4 disappearance rates; b, H2 formation rates.

transformation of the R-MoC1-x and β-Mo2C to η-Mo3C2 during the reaction. Thus, methane is decomposed on the η-Mo3C2 forming H2 as the main product. Catalysts 1 and 2 exhibited the highest activity for methane decomposition at steady state, because of the large amount of η-Mo3C2 on the surface of this catalyst. Thus, η-Mo3C2 is the active species for methane reforming, forming hydrogen selectively. Methane decomposes on the η-Mo3C2 sites, dissociating to CHx species and hydrogen. H2 desorbs from the surface, leaving the CHx species adsorbed on the surface with the value of x varying from 0 to 3.38 About 0.01% of these adsorbed CHx species desorb from the surface as ethylene and ethane, while a smaller fraction of the CHx species contributes to the formation of η-Mo3C2 from R-MoC1-x and β-Mo2C. Roughly 99% of the CHx species form carbon-

Figure 8. Deconvoluted Mo 3d spectra for 1. (A) After reaction in methane for 7 h at 973 K. (B) before reaction.

aceous species as evident from H2-TPR and TPO (Table 5). In the case of H-ZSM5 supported molybdenum carbides, the support enhances the formation of benzene from these CHx species, explaining the high selectivity toward benzene formation.3,9,27,39 Distribution of Molybdenum Oxidation States for Molybdenum Carbides. The distribution of the oxidation states for molybdenum in the catalysts was determined by deconvolution of the Mo 3d envelope measured by XPS. The deconvoluted Mo 3d spectra for catalyst 1 before and after the reaction are shown in Figure 8. The distribution of the Mo oxidation states for various catalysts is shown in Table 6, before and after the reaction for the various molybdenum carbides. The dominant species for all molybdenum carbides before reaction was Mo0, ranging from 33% for 5 to 56% for 1. Mo2+ ion ranged from

9130 J. Phys. Chem. B, Vol. 105, No. 38, 2001 TABLE 7: Surface Composition of Molybdenum Carbides as Determined by XPS MoOxNyCz catalyst

before reaction

after reaction

1 2 3 4 5 6

MoO0.0N0.0C0.5 MoO0.1N0.1C0.3 MoO0.3N0.0C0.5 MoO0.7N0.0C0.4 MoO1.3N0.0C0.3 MoO0.2N0.4C0.0

MoO0.0N0.0C0.6 MoO0.0N0.0C0.5 MoO0.0N0.0C0.3 MoO0.0N0.0C0.4 MoO0.1N0.0C0.5 MoO0.0N0.3C0.6

1% (3) to 12% (5). The percentage of Mo0 increased after the reaction in methane, indicating that the molybdenum species of the catalysts are reduced at 973 K in methane atmosphere. This increase in Mo0 most likely explains the increase in the TOF of these two catalysts. For all the catalysts, the percentage of Mo6+ after the reaction was less than 4%. The variance in the oxidation state of molybdenum (Mo0∼Mo6+) probably resulted from the coexistence of R-MoC1-x, β-Mo2C, and η-Mo3C2 in varied amounts. Also, both Mo-Mo and Mo-C bonds were present in the molybdenum carbides, explaining the variation in the oxidation states of molybdenum. The distribution of the molybdenum carbides was nearly the same, regardless of R-MoC1-x, β-Mo2C, or η-Mo3C2, with Mo0 (227.6 eV) being the dominant species for each catalyst (carbidic carbon, 283.5 eV) with a low distribution of Mo2+ (228.2 eV). It could not be differentiated between the R, β, and η carbide phases by the Mo oxidation states of XPS. Solymosi et al.3 and Lunsford et al.4 have assigned the peak at 227.9 eV (carbidic carbon, 283.83 and 282.74 eV) to Mo2C, and also Clair et al.40 and Kaltcher and Tysoe41 assigned the peaks at 228.1 (carbidic carbon, 283.2 eV) and 228.5 eV (carbidic carbon, 283.5 eV), respectively, without deconvolution to Mo0 and Mo2+. In our additional XPS analysis, metallic molybdenum particles were not probably formed on the molybdenum carbides, because the distribution of Mo and carbidic carbon increased in an Ar etching for 2 min. Therefore, molybdenum atom of molybdenum carbides was detected at 227.6 eV (carbidic carbon, 283.5 eV) as the same as the Mo 3d binding energy of metallic molybdenum. Furthermore, there may be another oxidation state existing between these two values. Chen et al.42 have observed through EXAFS that the charge of Mo is +0.2. Due to the difficulty in deconvoluting Mo0.2+ from XPS, the Mo 3d peak was deconvoluted into mostly Mo0 with varying amount of Mo2+. Through the XPS results, the probable species of molybdenum carbide was Moδ+ (δ+ ) 0∼+0.3 for the distribution of Mo0 and Mo2+ in Table 6). The surface compositions of the catalysts were determined by XPS, as shown in Table 7. The C/Mo ratio of β-Mo2C (3, 4, and 5) before reaction increased from 0.3 to 0.5 with the increasing carbiding temperature from 873 to 973 K. The C/Mo ratios of 0.4 and 0.3 of 4 and 5 before reaction were quite good characteristics of the mixture of R-MoC1-x, β-Mo2C, and η-Mo3C2. Concerning the oxygen content of the catalyts before reaction, the nitrided/carbided catalysts have less surface oxygen species that do the samples prepared by carbiding. Comparing 3, 4, and 5, the oxygen content decreased as the carbiding temperature was increased. This resulted in a catalyst which was more thoroughly carbided (MoO0.0C0.5 for 1 compared to MoO0.3C0.5 for 3). After reaction, the oxygen species were depleted from the catalyst surface. Moreover, the ratios of O/Mo by XPS were more than those by TPR before reaction but after reaction the O/Mo ratios were the same. The C/Mo ratios of 1, 2, and 6 after reaction were 0.5-0.6 by XPS, while the composition of η-Mo3C2 (e.g. C/Mo ) 0.67) was determined

Oshikawa et al. by XRD. Also, η-Mo3C2 was mainly determined by TPR. Thus, the XPS analysis showed a slight small amount of the C/Mo ratio, compared to the TPR and XRD results. TPR may measure the subsurface composition while XPS showed the surface composition for carbon deficient43 molybdenum carbides. Conclusions The relationship between various molybdenum carbide species and their activity toward methane reforming was studied under the reaction conditions of 973 K and 1 atm. Unsupported molybdenum carbide catalysts produced hydrogen in high selectivity, forming ethylene and ethane rather than benzene as the carbon-containing products. The η-Mo3C2 catalysts, nitrided at 973 K and subsequently carbided at 1173 K, were higher in methane decomposition activity than both R-MoC1-x and β-Mo2C. Catalysts 2 and 6 showed R-MoC1-x and γ-Mo2N structures, respectively, before the reaction. However, as the reaction proceeded, the bulk structure of these two catalysts changed to η-Mo3C2, increasing the activities greatly. Carbide formation was observed at 1020 K for MoO3 and 788 K for γ-Mo2N during CH4-TPR. Based on the correlation of the amount of η-Mo3C2 and the methane disappearance rate, η-Mo3C2 is the active species for methane reforming over molybdenum carbide catalysts. Mo0 was the dominant molybdenum species for the η-Mo3C2 catalysts. References and Notes (1) Xu, Y.; Liu, S.; Wang, L.; Xie, M.; Guo, X. Catal. Lett. 1995, 30, 135. (2) Solymosi, F.; Erdo¨helyi, A.; Szo¨ke, A. Catal. Lett. 1995, 32, 43. (3) Solymosi, F.; Csere´nyi, J.; Szo¨ke, A.; Ba´nsa´gi, T.; Oszko´, A. J. Catal. 1997, 165, 150. (4) Wang, D.; Lunsford, J. H.; Rosynek, M. P. J. Catal. 1997, 169, 347. (5) Weckhuysen, B. M.; Rosynek, M. P.; Lunsford, J. H. Catal. Lett. 1998, 52, 31. (6) Liu, S.; Wang, L.; Ohnishi, R.; Ichikawa, M. J. Catal. 1999, 181, 175. (7) Wang, L.; Tao, L.; Xie, M.; Xu, G.; Huang, J.; Xu, Y. Catal. Lett. 1993, 21, 35. (8) Chen, L.; Lin, L.; Xu, Z.; Li, X.; Zhang, T. J. Catal. 1995, 157, 190. (9) Ma, D.; Shu, Y.; Cheng, M.; Xu, Y.; Bao, X. J. Catal. 2000, 194, 105. (10) Xu, Y.; Liu, W.; Wong, S.-T.; Wang, L.; Guo, X. Catal. Lett. 1996, 40, 207. (11) Solymosi, F.; Szo¨ke, A.; Csere´nyi, J. Catal. Lett. 1996, 39, 157. (12) Liu, S.; Dong, Q.; Ohnishi, R.; Ichikawa, M. J. Chem. Soc., Chem. Commun. 1997, 1455. (13) Solymosi, F.; Szo¨ke, A. Stud. Surf. Sci. Catal. 1998, 119, 355. (14) Borry, III, R. W.; Kim, Y.-H.; Huffsmith, A.; Reimer, J. A.; Iglesia, E. J. Phys. Chem. B 1999, 103, 5787. (15) Tsuji, M.; Miyao, T.; Naito, S. Catal. Lett. 2000, 69, 195. (16) Claridge, J. B.; York, A. P. E.; Brungs, A. J.; Marquez-Alvarez, C.; Sloan, J.; Tsang, S. C.; Green, M. L. H. J. Catal. 1998, 180, 85. (17) Nagai, M.; Oshikawa, K.; Kurakami, T.; Miyao, T.; Omi, S. J. Catal. 1998, 180, 14. (18) Leary, K. J.; Michaels, J. N.; Stacy, A. M. J. Catal. 1986, 101, 301. (19) Leary, K. J.; Michaels, J. N.; Stacy, A. M. J. Catal. 1987, 107, 393. (20) Lee, J. S.; Volpe, L.; Ribeiro, F. H.; Boudart, M. J. Catal. 1988, 112, 44. (21) Aegerter, P. A.; Quigley, W. W. C.; Simpson, G. J.; Ziegler, D. D.; Logan, J. W.; McCrea, K. R.; Glazier, S.; Bussell, M. E. J. Catal. 1996, 164, 109. (22) Weckhuysen, B. M.; Wang, D.; Rosynek, M. P.; Lunsford, J. H. J. Catal. 1998, 175, 347. (23) Yamada, M.; Yasumaru, J.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1991, 95, 7037. (24) Quincy, B. R.; Houalla, M.; Proctor, A.; Hercules, D. M. J. Phys. Chem. 1990, 94, 1520.

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