C catalysts used for mixed alcohol synthesis

Applied Catalysis A: General 220 (2001) 21–30

Effect of cobalt promoter on Co–Mo–K/C catalysts used for mixed alcohol synthesis夽 Zhongrui Li a,b , Yilu Fu a,∗ , Jun Bao b , Ming Jiang a , Tianduo Hu c , Tao Liu c , Ya-ning Xie c a

Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, PR China b National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, PR China c Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039, PR China Received 19 October 2000; received in revised form 30 March 2001; accepted 20 April 2001

Abstract The structures of sulfided Co–Mo–K/C catalysts were studied by means of X-ray diffraction (XRD), laser Raman spectra (LRS), and X-ray absorption fine structure (XAFS). Activities for alcohol synthesis via CO hydrogenation were used to characterize the catalytic performance of these catalysts. On the activated carbon support, molybdenum is mainly present as MoS2 species which shrinks with the cobalt loading, while cobalt is mainly present in the form of “Co–Mo–S” phase at the low Co loading and partly in a Co9 S8 -like structure at higher Co loading. The catalysts exhibit outstanding performance for higher alcohol synthesis due to the addition of the promotion of cobalt. The activity for alcohol formation is optimized at a Co/Mo atomic ratio of 0.5. Co species operate as a synergistic system, rather than independently from the MoS2 phase. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mo-based catalysts; Co promoter; Mixed alcohol synthesis; “Co–Mo–S” phase; Structure

1. Introduction The catalytic conversion of synthesis gas to alcohol is generally recognized as an interesting route for providing clean fuels and petrochemical feed stocks. The most promising application of the C2+ alcohol is as a blending stock for automotive fuel to meet the octane requirement resulting from legislative regulation of lead-free gasoline and to replace the MTBE to reduce the environmental pollution. Since blending 夽

This work was supported by BSRF. Corresponding author. Tel.: +86-551-3601118. E-mail address: [email protected] (Y. Fu). ∗

of methanol into gasoline raises problems of phase separation, high volatility, and lowering of calorific value, mixed alcohol is a better additive. A number of papers have investigated catalysts for the synthesis of mixed higher alcohol [1–4]. In particular, sulfided molybdenum-based catalysts have drawn special attention because of their high activity for mixed alcohol synthesis and superior sulfur-resistant property [4,5]. The activity and selectivity for alcohol formation over the sulfided Mo–K catalysts were significantly affected by the supports, the additive (such as alkali metal salts), and the reaction conditions. After several years’ work, we have succeeded in preparing a new kind of sulfided K–Mo/Al2 O3 catalyst, which showed

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 6 4 6 - 9

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Z. Li et al. / Applied Catalysis A: General 220 (2001) 21–30

relatively high activity and selectivity for mixed alcohol synthesis. Under these reaction conditions, 658 K, 14.0 MPa and 11 000 h−1 , the space time yield (STY) of mixed alcohol over sulfided Mo(16%)–KCl (atomic ratio K/Mo = 0.8)/Al2 O3 catalyst amounted to 416.7 ml/l h and alcohol selectivity was 82%, but the methanol content in the alcohol product was relatively high [4,5]. Increasing the content of C2+ alcohol is desirable from the practical point of view. Santiesteban et al. [6] have reported that the incorporation of cobalt to the alkali/MoS2 catalysts was in favor of the synthesis of C2+ alcohol. Storm [7] and Fujimoto and Oba [8] have also reported that cobalt is a necessary promoter for high STY of higher alcohol on reduced K–Mo/Al2 O3 and K–Mo/SiO2 catalysts, respectively. Cobalt-promoted Mo-based catalyst has been extensively used in the hydrodesulfurization (HDS) process, which leads to a continuous drive to clarify the structure and the related catalytic activity of the catalyst system. Some aspects of preparation–structure–activity relationship have been studied [9–15]. The generally accepted “Co–Mo–S” model of Topsoe et al. [11] describes the active phase as consisting of small MoS2 particles with Co promoter atoms decorating the edges of the MoS2 slabs. On the contrary, the EXAFS studies of Bouwens et al. [12] and extensive Mossbauer experiments by Craje et al. [13] indicated that the active sites were located on the cobalt atoms. The role of the MoS2 particles was to act as a secondary support merely to stabilize the highly dispersed Co-sulfide located on the MoS2 edges. Stabilization of the MoS2 slabs themselves was assumed to occur via Mo–O linkages to the alumina support, as proposed by several authors [14,15]. So far, the structure and location of the active sites in Co–Mo–K catalysts for alcohol formation are still unclear. However, the research of structure–activity relation is necessary for the optimized preparation of Co–Mo catalysts before it is industrialized. In the present work, we report the influence of cobalt loading on the catalytic properties for the synthesis of mixed alcohol, and the structural changes characterized by X-ray diffraction (XRD), laser Raman spectra (LRS), and extended X-ray absorption fine structure (EXAFS). The correlation is also discussed between the structural characteristics and the activity of these catalysts.

2. Experimental 2.1. Sample preparation A kind of activated carbon (AC) was used as the support (Shanghai chemicals factory, China Pharmaceuticals Inc.; S BET = 642 m2 /g, V pore = 0.35 ml/g). It was washed by nitric acid and distilled water. The oxidic Mo–K/AC samples were prepared by sequential pore volume impregnation and calcination technique. At the first step, the activated carbon was impregnated with a solution of K2 CO3 , then this was dried and calcinated at 573 K for 2 h. After that, it was further treated via pore volume impregnation with (NH4 )6 Mo7 O24 ·4H2 O and Co(NO3 )2 solutions. Next it was dried at 393 K for 12 h and calcinated at 773 K for 24 h in a nitrogen flow, and finally purified by passing through the 105 deoxy agent and 5A zeolite at a rate of 40 ml/min. The sulfided samples were obtained by heat-treating the oxidized ones in a flow of mixed CS2 /H2 gas, obtained by passing H2 through CS2 liquid at 273 K, of 30 ml/min at 673 K for 6 h. The molybdenum content in the samples, expressed as a weight ratio of MoO3 /AC, was maintained at 0.48, the atomic ratio of K/Mo was 0.8 and the atomic ratio of Co/Mo ranged from 1/16 to 1/1. A sulfided Co–K/AC catalyst (4.1 wt.% Co and 3.1 wt.% K) was prepared by pore volume impregnation with an aqueous solution of K2 CO3 and Co(NO3 )2 and followed by the same treatment as above. 2.2. Characterization methods The structure of the samples was examined by XRD with Cu K␣ radiation on a D/MAX-␥A rotatory target diffractometer (0.15418 nm, 40 kV, and 100 mA). The samples were ground into fine powder and packed into sample holders, and then were used for the measurements. Laser Raman spectra were recorded on an SPEX1403 spectrometer with a resolution of 2 cm−1 using the 488.0 nm−1 radiation beam from a SpectraPhysics-2020 argon laser. The laser beam intensity and the spectrum slit width were 100 mW and 3.5 cm−1 , respectively. The samples were pressed into pellets for the measurements. The Mo and Co K-edge spectra of EXAFS were measured on the beam-line of 4WIB at Beijing

Z. Li et al. / Applied Catalysis A: General 220 (2001) 21–30

Synchrotron Radiation Facility (BSRF) using a double-crystal Si(1 1 1) monochromator. The storage ring was operated at 2.2 GeV with a typical current of 50 mA. In order to avoid the sample thickness effect, it is required that the following conditions be satisfied (for concentrated materials): µx ≤ 1 and µd < 1, where µ is total absorption coefficient, µx the edge step, and d the particle size. All samples were ground and sieved through 400 mesh; the resultant powder was rubbed onto scotch tape. It was found that 10–20 layers of tapes were required to obtain a total µx ≈ 1. Under these conditions, the thickness effect distortions are expected to be negligible [16]. The higher X-ray harmonics were minimized by detuning the double crystal monochromator to 75% of the maximum. The measurements were carried out in transmission mode with optimized ionization chambers to measure the intensity before and behind the sample at room temperature. Data analysis was performed following a standard procedure [17]. Normalization was done by dividing the absorption intensities by the height of the absorption edge and subtracting the background by using cubic spline routines. The final EXAFS function was obtained and EXAFS data were normalized. To avoid cutoff effects, kmin and kmax were chosen in the nodes of the EXAFS functions. A statistical error data analysis is performed on an isolated part of the data obtained by an inverse Fourier transformation over a selected range in R space. Averaging the individually Fourier-filtered EXAFS data yields the statistical errors per data point in the isolated EXAFS functions. The errors for the fitted parameters are estimated to be 20% in coordination number N, 1% in distance R, 10% in the Debye–Waller factor σ 2 , and 10% in E0 . Since the back-scattering amplitudes contain an unknown static and thermal disorder and a damping due to photoelectron losses in the shells, the values of the disorder parameter σ 2 reported for the samples are given relative to those for reference compounds. 2.3. Measurements of catalytic activities Mixed alcohol synthesis was performed in a tubular downflow fixed-bed reactor system. Feed gasses were composed of CO (30%), H2 (60%), and N2 (10%) and passed through mass flow controllers. The reactor was a stainless tube of 350 mm length

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and 6 mm inside diameter. It is housed in an electric furnace controlled by a temperature controller. The effluent gas was cooled to 273 K and separated into gas and liquid phases at high pressure. The gaseous products were directly analyzed on a chromatograph through a sampling valve, and the liquid ones were collected for a proper period with their volume and weight measured and subsequently analyzed on the same chromatograph by injection. CO, CO2 , and H2 in the gas and H2 O in liquid products were analyzed on a 2 m TDX-01 column by using a thermal conductivity detector with argon as carrier gas. The hydrocarbons, alcohol, and other oxygenated compounds were analyzed on a 2 m Porapak Q column by using a hydrogen flame detector with N2 as carrier gas. The composition of hydrocarbons was calculated using 1.04% CH4 as standard gas, while that of the mixed alcohol was calculated directly from the peak areas by using a standard liquid of mixed alcohol. Under the present experimental conditions, only a trace amount of CO2 could be detected by thermal conductivity detector, and thus the activity mentioned below would be referred to as CO2 -free. 3. Results and discussion 3.1. XRD The XRD patterns of the oxidic samples are shown in Fig. 1; they indicate several phases as expected. For the activated carbon-supported cobalt-free sample (i.e. Co/Mo = 0 which was not shown in Fig. 1, see [18]), the diffraction peaks can mainly be assigned to K2 Mo2 O7 species with d values of 0.618, 0.330, 0.300, 0.290, 0.267, 0.223, and 0.158 nm according to JCPDS card 21-663. When cobalt is incorporated, there appear many diffraction peaks of new Mo species such as K2 Mo3 O10 (d = 0.322, 0.686, 0.347 nm, JCPDS card 21-99), K0.3 MoO3 (d = 0.316, 0.340 nm, JCPDS card 35-444), and K2 Mo4 O13 (d = 0.316, 0.686 nm, JCPDS card 28-777). Also, CoMoO4 phase (d = 0.380, 0.347, 0.331, 0.322, 0.316 nm, JCPDS card 21-868) and CoMoO3 phase (d = 0.495, 0.369, 0.249, 0.241 nm, JCPDS card 21-869) also exist on the samples with low Co loading (such as Co/Mo = 1/16). With the increase in Co loading, the diffraction intensity of K2 Mo2 O7 phase decreases monotonically,

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Z. Li et al. / Applied Catalysis A: General 220 (2001) 21–30

Fig. 1. XRD patterns of oxidic Co–Mo–K/AC samples with different Co/Mo atomic ratios.

while those of CoMoO3 and CoMoO4 phases have no notable change. In the sample with Co/Mo = 1/1, the cobalt is mainly transformed into CoMoO4 phase, while the diffraction intensity of molybdenum species is very weak except for that of KMoO4 (d = 0.369, 0.564, 0.289 nm, JCPDS card 21-1293). The XRD patterns of sulfided samples are quite different from those of oxidic ones. After sulfidation, except for a tiny amount of K–Mo–O species (e.g. K2.66 MoO4 , d = 0.298, 0.288, 0.223, 0.241 nm, JCPDS card 21-1000), most of the diffraction peaks of the (K–)Mo–O species observed in the oxidic samples are removed (Fig. 2); in the meanwhile, MoS2 diffraction peaks (d = 0.609, 0.289, 0.274, 0.158, 0.154 nm, JCPDS card 6-97) appear and shrink with the increase in Co loading. This indicates that oxygen atoms in the sample are mainly substituted by sulfide, three-dimensional MoS2 micro-crystallite is formed on activated carbon surface. With the incorporation of cobalt, bulk Co9 S8 particles (d = 0.176, 0.298, 0.191 nm, JCPDS card 19-364) grow on the support at higher cobalt content (Co/Mo ratio > 0.5). 3.2. LRS Raman spectra of the oxidic samples are shown in Fig. 3. Before cobalt was incorporated into Mo–K/AC sample [18], the strong vibration peaks located around

Fig. 2. XRD patterns of sulfided Co–Mo–K/AC samples.

196, 320, 335, 861, 887, and 930 cm−1 are contributed from different Mo–O vibration modes [19] of K2 Mo2 O7 species as detected by XRD. On the contained cobalt samples, there appear some new bands; the bands at 810, 873, 930 cm−1 and a broad band at 356 cm−1 may be assigned to Mo–O–Co stretching vibrations in CoMoO4 species [20–23]. With increasing Co loading, the magnitudes of these peaks become weaker, which may be caused by a new kind of Raman non-active cobalt species covering over the former species.

Z. Li et al. / Applied Catalysis A: General 220 (2001) 21–30

Fig. 3. LRS of oxidc Co–Mo–K/AC samples.

After sulfidation, the Raman patterns (Fig. 4) of these samples were different from that of oxidic ones. Only weak peaks at 377 and 406 cm−1 of characteristic bands of MoS2 were found on the cobalt-free sample. When cobalt is incorporated, new bands (810, 873, and 930 cm−1 at high Co loading) appear which can be assigned to Co–Mo–O species (like CoMoO4 ). However, the XRD results show that cobalt mainly exists in the form of Co9 S8 , which may indicate that some species like “Co–Mo–S” phase on the surface of catalyst become easily converted into CoMoO4 -like species in air. 3.3. EXAFS The Mo and Co K-edge absorption spectra of the model compound and the sulfided Co–Mo–K/AC

Fig. 4. LRS of sulfided Co–Mo–K/AC samples.

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Fig. 5. Mo and Co K-edge absorption spectra for sulfided Co–Mo–K/AC samples.

samples are shown in Fig. 5. It can be seen that all the spectra display high signal-to-noise ratios. Some differences in features of the spectra can be distinguished. The envelopes of the spectra of all the samples indicate that the amplitudes decrease as a function of energy. The EXAFS functions k3 χ (k) of Mo and Co K-edge of sulfided Co–Mo–K/AC samples are show in Fig. 6. In these curves, the signals above 125 nm−1 are cutoff due to high noise. The magnitude of the Fourier transforms (FT) of k3 χ (k) (k = 17–121 nm−1 ) of the samples together with the MoS2 standard compound are shown in Fig. 7. MoS2 is of hexagonal structure with one molybdenum atom surrounded by six sulfur atoms at a distance of 0.241 nm and six molybdenum neighbors at 0.316 nm. In the FT of the samples, two peaks were observed at 0.201 and 0.283 nm. They were located at almost the same positions as those for MoS2 standard compound, indicating that the local structure of the sulfided molybdenum species is similar to that of MoS2 . In comparison with those of MoS2 however, the magnitudes of Fourier transforms of the Mo–S and Mo–Mo coordination are significantly decreased compared to those of MoS2 , demonstrating that the sizes of the supported MoS2 -like species are smaller. Besides, the ratio of the magnitude of Mo–Mo shell to that of Mo–S shell falls with the increase of Co loading, indicating that MoS2 crystallite sizes shrink with the increase of Co loading. Phase shifts and back-scattering amplitudes extracted from the spectra of MoS2 standard compound were used to calculate the structural parameters of

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Fig. 6. Mo and Co K-edge EXAFS of k3 χ (k) for the samples — left: (a) MoS2 ; (b) Co/Mo = 1/1; (c) Co/Mo = 1/2; (d) Co/Mo = 1/8; right: (a) Co9 S8 ; (b) Co/Mo = 1/1; (c) Co/Mo = 1/2; (d) Co/Mo = 1/8.

the samples. The fitting results of the molybdenum coordination shells for the samples are presented in Table 1. For clarity, the crystallographic data, coordination numbers, and distances of MoS2 standard compounds are also collected in Table 1. No clear indication of a Mo–Co coordination (RMo–Co = 0.28 nm) could be observed in Mo K-edge XAFS spectra. This might be due to the fact that for this catalyst, the Mo–Co coordination number is quite low and the Mo–Co coordination is overlapped by the strong Mo–Mo coordinatiion shell from the near coordination distance.

Fig. 8 shows the FT of the Co K-edge EXAFS of sulfided Co–Mo–K/AC samples. For comparison, the FT of Co9 S8 model compound is also shown in Fig. 6 [24]. The Co9 S8 is of particular interest in this study, since it is the thermodynamically most stable cobalt sulfide compound under the reaction conditions. Co9 S8 in its unit cell contains eight cobalt atoms in a distorted tetrahedron of sulfur atoms and one cobalt atom in a regular octahedron of sulfur atoms. The crystallographic data of Co9 S8 are also collected in Table 2. The three separate Co–S coordinations in Co9 S8 (see [24]) exhibit in the FT spectrum

Fig. 7. Fourier transforms of k3 χ (k) for the crystalline MoS2 and the samples.

Fig. 8. Fourier transforms of k3 χ (k) for the crystalline Co9 S8 and the samples.

Z. Li et al. / Applied Catalysis A: General 220 (2001) 21–30

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Table 1 Coordination structure parameters of MoS2 and Mo atoms from the Fourier-filtered data for the sulfided samples σ 2 × 10−6 (nm2 )a

E0 (eV)b

Sample (Co/Mo)

Bond

R (nm)

N

1/1

Mo–S Mo–Mo

0.240 0.316

3.97 2.49

7 18

1.20 −0.94

1/2

Mo–S Mo–Mo

0.241 0.313

3.71 2.51

−10 16

1.00 4.02

1/4

Mo–S Mo–Mo

0.242 0.316

4.33 2.53

5 6

−0.43 −1.57

1/8

Mo–S Mo–Mo

0.241 0.316

4.46 3.12

3 6

−0.47 −1.57

0

Mo–S Mo–Mo

0.241 0.316

4.51 3.24

0 0

0.52 1.60

MoS2

Mo–S Mo–Mo

0.241 0.316

6.0 6.0

a b

Relative Debye–Waller factor of the sample to that of the standard compounds. Correction of the inner potentials of the samples based upon those of the crystalline MoS2 .

as one overall Co–S coordination (0.221 nm). Likewise, the two separate Co–Co coordinations at about 0.35 nm exhibit as one overall Co–Co(2) coordination (indexed (2) to differentiate it from the first Co–Co coordination at 0.250 nm). Regarding the known crystallographic distances, the first peak in the radial distribution function (RDF) of Co9 S8 has to be attributed

to combined Co–S and Co–Co(1) coordinations and the second peak only to the Co–Co(2) coordination (Fourier transforms of EXAFS data give peaks which are displaced to lower R values due to the phase shift). With regard to the spectra of Co–Mo–K/AC samples, it can be seen that their first peaks are slightly shifted to lower R values. In general, however, the higher

Table 2 Coordination structure parameters of Co9 S8 and Co atoms from the Fourier-filtered data for the sulfided samples Sample (Co/Mo)

Bond

R (nm)

N

σ 2 × 10−6 (nm2 )a

1/1

Co–S Co–Co(1) Co–Co(2) Co–Mo

0.220 0.251 0.354 0.279

4.5 2.5 1.2 0.8

31 15 5 46

3.0 −6.5 7.1 −6.5

1/2

Co–S Co–Co(1) Co–Co(2) Co–Mo

0.217 0.249 0.351 0.280

4.1 2.2 0.7 0.9

35 19 12 53

4.2 −2.1 13.2 7.4

1/8

Co–S Co–Co(1) Co–Co(2) Co–Mo

0.214 0.245 0.354 0.278

3.3 1.2 0.3 0.5

43 27 21 61

4.3 2.3 13.3 28.5

Co9 S8

Co–S Co–Co(1) Co–Co(2)

0.221 0.251 0.354

4.8 2.7 2.7

a b

Relative Debye–Waller factor of the sample to that of the standard compounds. Correction of the inner potentials of the samples based upon those of the crystalline Co9 S8 .

E0 (eV)b

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Fig. 9. Imaginary part of Fourier transforms for the crystalline Co9 S8 and the samples.

Co loading samples (like Co/Mo = 1) exhibit identical features to those of the Co9 S8 compound. The more important to mention is that the samples show an additional peak at about 0.28 nm (corrected value, see Table 2) which is not present in the Co9 S8 model compound and, consequently, should be ascribed to additional Mo back-scatters, i.e. the Co–Mo coordination [24]. It means that a Co–Mo–S species on the samples is formed. For the sample with atomic ratio Co/Mo below 1/8, the peak at 0.35 nm, i.e. Co–Co(2) shell which belong to Co9 S8 , could not be determined. It is suggested that in the lower Co loading samples, Co might be present in the form of Co–Mo–S phase rather than as Co9 S8 crystallites. Some more important information can be obtained from the imaginary part of the Fourier transformation. Fig. 9 exhibits the imaginary part of FT of Co9 S8 model compound and sulfided Co–Mo–K/AC samples in a small R range (R = 0.55 nm). The Co–S and Co–Co(1) coordinations can be clearly separated in Fig. 9; they cannot be distinguished in Fig. 8. It can be seen that the heights of Co–S coordination peaks only have small change, whereas the heights of Co–Co(1) coordination peaks rise with the increase of Co loading. This phenomenon can be well explained by the model Co–Mo–S species; the cobalt atom prefers to substitute some Mo atoms at the edge of MoS2 -like species.

The coordination parameters of Co atom in samples were calculated and are listed in Table 2. The back-scattering amplitude and phase shift functions were obtained by using the model Co9 S8 for Co–S and Co–Co contributions, and CoMoO4 for Co–Mo contribution constructed with the program FEFF7 [25]. The three nearest Co–S coordinations in the model compound were fitted by using one overall Co–S coordination. The results should be compared with the average Co–S coordination distance and coordination number obtained from the crystallographic data (Table 2). 3.4. Mixed alcohol synthesis The effects of Co loading on the activity and selectivity of the carbon monoxide–hydrogen reactions after an induction period of initial 16 h are given in Table 3. Over the cobalt-free sample, the activity towards alcohol formation is rather low, methanol being the main product. With the incorporation of cobalt promoter, the activity for alcohol formation (alcohol STY) remarkably increases and gets optimized at a Co/Mo ratio of 0.5, which is about three times that over cobalt-free sample. The CO conversion increases monotonically and C2+ alcohol proportion (MeOH/ C2+ OH) in the product is also improved. This suggests that the incorporation of cobalt into the Mo–K/ AC sample promotes the conversion of synthesis gas

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Table 3 Performance of alcohol synthesis over sulfided Co–Mo–K/AC samplesa Sample (Co/Mo)

CO conversion (%)

0 1/16 1/8 1/4 1/2 1/1

6.4 8.7 9.5 11.7 14.3 14.5

a

Alcohol selectivity (C %)

Cn OH selectivity (C %) MeOH

EtOH

PrOH

BuOH

MeOH C2+ OH

Alcohol STY (ml/kg/h)

38.3 39.3 40.1 41.9 46.6 27.4

19.5 19.6 19.6 18.7 19.9 11.1

13.9 16.1 14.1 13.2 17.3 10.6

4.0 5.2 6.8 6.9 6.3 4.2

0.9 0.4 0.7 1.1 3.1 1.4

1.04 0.90 0.91 0.88 0.74 0.68

69.1 95.6 126.5 150.2 198.5 107.6

Reaction conditions: 603 K, 5.0 MPa, 4800 h−1 , H2 /CO = 2 (v/v).

to higher alcohol. Cobalt is a significant promoter for high STY of higher alcohol on activated carbonsupported sulfided K–Mo catalysts.

4. Discussion It is valuable to outline the cobalt status present in carbon-supported sulfided Co–Mo–K catalysts. The Co K-edge Fourier transformation spectra of all samples clearly revealed the contribution of Co–Mo coordination, which may indicate the existence of “Co–Mo–S” phase. Furthermore, in the Mo K-edge Fourier transformation spectra, the magnitude of Mo–S shell almost does not change with Co loading, while that of Mo–Mo shell shrinks with it. This phenomenon can also be explained by the displacement of cobalt from the edge Mo atoms and the formation of “Co–Mo–S” phase, as many authors have suggested [11,26]. For the higher Co loading samples, both XRD and EXAFS results demonstrate the existence of Co9 S8 -like crystallines. In brief, it is concluded that Co atoms mainly exist in the form of “Co–Mo–S” phase on the lower Co loading sample, and are partly present in a Co9 S8 -like structure and partly in a “Co–Mo–S” structure in higher Co loading samples. To focus our attention to the function of cobalt species in alcohol synthesis, whether they operate independently from the intercalated MoS2 phase (physical mixture) or as a synergistic system, we also tested the catalytic performance of the Co–K/AC catalyst (in which Co exists in the form of Co9 S8 crystallites) in the reaction of CO hydrogenation. The results demonstrated that only poor amounts of C1 –C4 alkanes could be formed and no alcohol was produced over it, which is not listed in Table 2. On the basis of this result, we

conclude that these Co species do not operate independently from the MoS2 phase, but as a synergistic system, this is to say that Co9 S8 crystallites on the activated carbon surface are not the active species for alcohol formation from CO hydrogenation. It is quite different for HDS reaction in which Co9 S8 is almost as active as “Co–Mo–S” species [27]. It is important to note that on carbon, a contribution of Co9 S8 to the catalytic activity for hydrocarbon formation cannot be ruled out at the higher Co/Mo ratios. Taking a page from HDS of the Co–Mo catalyst, one might speculate about the activity and selectivity for alcohol formation on special “Co–Mo–S” sites. However, it is still a matter of debate. “Co–Mo–S” phase as the catalytic active species of hydrogenation treatment has been investigated for many years. Topsoe et al. [28], by means of a combined in situ Mossbauer spectroscopy and thiophene HDS activity study, have produced evidence for the formation of a Co–Mo–S phase in unsupported and alumina-supported sulfided Co–Mo catalysts; they describe the environment of the cobalt ions in terms of octahedral or trigonal prismatic holes. In Co–Mo/Al2 O3 , the Co–Mo–S phase, supposedly present as single S–Mo–S slabs with cobalt occupying Mo sites, was found to be preferentially formed at low Co contents, whereas Co9 S8 formation occurred only at Co/Mo ratios > 0.4. It was concluded that the promoting effect of cobalt is associated with the presence of the Co–Mo–S phase and not with the presence of Co9 S8 . In their study of sulfided Co–Mo/Al2 O3 catalysts, Candia et al. [29] distinguished between two types of “Co–Mo–S” species: type I and type II. Type I “Co–Mo–S” is supposed to have an interaction with the alumina support, while type II is almost free of interactions with the alumina support. According

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to these authors, the intrinsic activity (per Co atom present as “Co–Mo–S”) is much higher for type II than for type I “Co–Mo–S”. In the case of active carbon as support, Topsoe [30] observed that the “Co–Mo–S” structures are only weakly bound to the support surface, and hence concluded that the type II “Co–Mo–S” very much resemble the carbon-supported “Co–Mo–S” structures. By comparison with sulfided Co–Mo/Al2 O3 catalysts (4) and with the results of this work, we concluded that “Co–Mo–S” on activated carbon support should be the type II “Co–Mo–S” species.

5. Conclusion Carbon, being an inert support material, seems very useful in studies of the true catalytic properties of well-dispersed poorly crystallized metal sulfides. In oxidic state, molybdenum is partly present as K–Mo–O species and cobalt phases are mainly present as CoMoO4 and CoMoO3 with low Co loading, possibly due to the reducibility of active carbon at high preparation temperature, and CoMoO4 with high Co loading. After sulfidation, molybdenum is mainly present as MoS2 species, while cobalt in the form of “Co–S–Mo” phase at the low Co loading and as both “Co–S–Mo” species and Co9 S8 crystallites with higher Co loading. The carbon-supported catalysts exhibit outstanding performance for alcohol synthesis with the promotion of cobalt. The activity for alcohol formation was optimized at a Co/Mo atomic ratio of 0.5. Co species operate as a synergistic system, rather than independently from the intercalated MoS2 phase.

Acknowledgements The provision of funds by National Natural Science Foundation of China (no. 29773042) and the supply of EXAFS experimental facility by BSRF are gratefully acknowledged. References [1] R.G. Herman, Catal. Today 55 (2000) 233–245.

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