Co-Mo-K Sulfide-Based Catalyst Promoted by Multiwalled Carbon Nanotubes for Higher Alcohol Synthesis from Syngas

CHINESE JOURNAL OF CATALYSIS Volume 27, Issue 11, November 2006 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2006, 27(11): 1019–1027.

RESEARCH PAPER

Co-Mo-K Sulfide-Based Catalyst Promoted by Multiwalled Carbon Nanotubes for Higher Alcohol Synthesis from Syngas MA Xiaoming, LIN Guodong, ZHANG Hongbin* College of Chemistry and Chemical Engineering, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, Fujian, China

Abstract: Using homemade multiwalled carbon nanotubes (CNT) as the promoter, sulfurized Co-Mo-K catalysts (denoted CoiMojKk-x%CNT) were prepared by the coprecipitation method. Their catalytic performance for the synthesis of higher alcohols from syngas was evaluated and compared with that of the CNT-free counterpart (CoiMojKk). Appropriate incorporation of a minor amount of CNT into CoiMojKk led to a significant increase in CO conversion and the selectivity for the higher alcohols. Under the reaction conditions of 5.0 MPa, 623 K, V(H2):V(CO):V(N2) = 60:30:10, and GHSV = 3600 ml/(g·h), the observed space-time-yield of total (C1–4) alcohols reached 241.5 mg/(g·h) with 21.6% of CO conversion over the Co1Mo1K0.3-10%CNT catalyst, which was 1.84 times that over the Co1Mo1K0.3 catalyst. Ethanol became the dominant product of the CO hydrogenation under the conditions mentioned above. The water-gas-shift (WGS) side reaction was inhibited to a great extent over the CNT-promoted catalyst. The results of catalyst characterization indicated that the addition of a small amount of CNT into the Co1Mo1K0.3 catalyst did not cause an obvious change in the apparent activation energy for the conversion of CO but led to an increase in the molar percentage of the catalytically active Mo species (Mo4+) in the total amount of Mo at the surface of the working catalyst. On the basis of the temperature-programmed desorption results, it could be inferred that under the conditions of the higher alcohol synthesis, there existed a considerably larger amount of reversibly adsorbed H species on the CNT-promoted catalyst, which would generate a surface microenvironment with high steady-state concentration of the adsorbed H species on the catalyst and thus increase the rate of a series of surface hydrogenation reactions. In addition, high steady-state concentration of adsorbed H species on the surface of the catalyst would effectively inhibit the WGS side reaction. These factors considerably contribute to the increase in the yield of the main product. Key Words: multiwalled carbon nanotube; promoter; cobalt; molybdenum; potassium; sulfide-based catalyst; carbon monoxide; hydrogenation; higher alcohol synthesis

Higher alcohols (C2+-alcohols) together with methanol and dimethyl ether (DME) have been considered the most important species among the coal-based clean synthetic fuels and chemical feedstocks. The higher alcohols have been confirmed to be a better and cleaner automobile fuel with high octane numbers and low emissions of NOx, ozone, CO, and aromatic vapors [1]. Recently, the use of methyl tert-butyl ether (MTBE) as the additive of oil-based fuels has been prohibited in some countries or regions because of the legal requirements with respect to environmental protection, which is greatly renewed interest in the hydrogenation of syngas to

C2+-oxygenates as gasoline blends. Higher alcohol synthesis (HAS) over MoS2-based catalysts has been extensively studied since the mid-1980s [2]. The progress in this field has considerably contributed to the better understanding of the nature of those catalytic reaction systems. Nevertheless, the existing HAS technology is still implemented on a small scale. Both the single-pass-conversion of the feed syngas and the selectivity for C2+-alcohols are relatively low. Most systems produce methanol as the main product instead of C2+-alcohols [2–6]. Development of catalysts with high efficiency and selectivity for C2+-alcohols has been one of the key

Received date: 2006-05-08. * Corresponding author. Tel/Fax: +86-592-2184591; E-mail: [email protected] Foundation item: Supported by the National Basic Research Program of China (2005CB221403) and the National Natural Science Foundation of China (20473063 and 20590364). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

objectives of research. Multiwalled carbon nanotubes (CNT) are a novel type of nano-carbon material. This new form of carbon, first discovered in 1991 by Iijima [7] in carbon soot made by an arc-discharge method, is structurally similar to the hollow graphite fiber, except that it has a very high degree of structural perfection. This type of carbon nanotubes possess highly graphitized tube wall, nanosized channel, and sp2-C-constructed surface. They display exceptionally high mechanical strength, high thermal/electrical conductivity, medium to high specific surface area, and excellent performance for adsorption and spillover of hydrogen. Recently, the use of CNT as a novel nano-carbon support or promoter of catalysts has drawn increased attention [8, 9]. Catalytic applications include selective hydrogenation of α, β-unsaturated aldehydes [10], hydrofomylation of alkenes [11], selective dehydrogenation [12], selective oxidation [13], ammonia synthesis [14], Fischer–Tropsch synthesis [15], methanol synthesis [16], higher alcohol synthesis [17], etc. The catalytic studies conducted so far on CNT-based systems have shown promising results in terms of activity and selectivity. In this article, the authors report the development of the CNT-promoted Co-Mo-K sulfide-based catalyst, which is highly active for HAS from syngas. The catalyst displayed much better performance for the highly effective and selective formation of C2+-alcohols as compared to the CNT-free counterpart. These results shed some light on understanding the nature of the promoting action by the CNT and the prospect of developing highly active sulfur-resistant catalysts for HAS.

1 Experimental 1.1 Preparation of the catalysts CNT samples were synthesized using the catalytic method reported previously [18]. The freshly prepared CNT was treated with boiling nitric acid for 8 h followed by rinsing with deionized water, once more acid treating/water rinsing, and then drying at 383 K under a N2 atmosphere. Open-end CNT with hydrophilic surface was then obtained. A series of CNT-promoted Co-Mo-K catalysts, denoted CoiMojKk-x%CNT (where x% represents the mass fraction of CNT), were prepared by coprecipitation and impregnation methods. Two aqueous solutions containing calculated amounts of Co(NO3)2·6H2O and (NH4)6Mo7O24·4H2O (all of AR grade), respectively, were simultaneously added dropwise into a Pyrex flask containing a calculated amount of CNT at 358 K with vigorous stirring. The amount added was adjusted to maintain a constant pH of approximately 5. The precipitate was continuously stirred for 4 h at 358 K followed by cooling to room temperature, aging overnight, and then filtering. The filter cake (precipitate) was repeatedly rinsed with deionized water until the filtrate was neutral, then dried at 383 K for 4 h,

and calcined at 773 K for 4 h under a N2 atmosphere. After cooling to room temperature, the sample was impregnated with a calculated amount of potassium carbonate aqueous solution by the conventional incipient wetness method, then dried at 383 K, and calcined at 673 K under a N2 atmosphere for 4 h. Thus the oxide precursors of the CNT-promoted Co-Mo-K catalysts were obtained. A CNT-free oxide precursor of the Co-Mo-K catalyst (denoted CoiMojKk) was prepared in a similar way. All samples of catalyst precursors were pressed, crushed, and sieved to 40–80 mesh for the evaluation of activity. 1.2 Catalyst evaluation Performance of the catalysts for the HAS from syngas was evaluated in a fixed-bed continuous flow reactor and gas chromatograph (GC) combination system. A total of 0.80 g of catalyst sample was used for each test. Prior to the reaction, the sample of the oxide precursor was pre-reduced/sulfurized in situ in a 95% H2/5% CS2 gaseous mixture stream at 0.1 MPa and 1800 ml/(g·h). The sulfurization temperature was programmed from room temperature to 673 K, kept at 673 K for 6 h, and then decreased to the desired temperature for the catalyst test. The HAS reaction was carried out in a steady state under the reaction conditions of 573–623 K, 2.0–5.0 MPa, V(H2):V(CO):V(N2) = 45:45:10, and GHSV = 2400– 3600 ml/(g·h). The reactants and products were determined using an online gas chromatograph (GC) (Model GC-950) equipped with dual detectors (a thermal conductivity detector and a flame ionization detector) and dual columns filled with carbon molecular sieve (TDX-01) and 5% Carbowax 20M/Carbopack B, respectively. The former column (2.0 m length) was used for the analysis of N2 (as an internal standard), CO, and CO2, and the latter (60 cm length) was used for the analysis of C1–4-alkanes, C1–4-alcohols, and other oxygenates. CO conversion was determined through an internal standard, and the carbon-based selectivity for the carbon-containing products (including alcohols, alkanes, and other oxygenates) was calculated using an internal normalization method. 1.3 Characterization of the catalysts Transmission electron microscope (TEM) observations were carried out using a Technai F30 (Philips, The Netherlands) or an H-600 (Hitachi, Japan) electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Quantum 2000 Scanning ESCA Microprobe system (PHI, USA) with Al Kα radiation (15 kV, 25 W, hν = 1486.6 eV) under ultrahigh vacuum (1 × 10–7 Pa), and the binding energies were calibrated internally by the carbon deposit C 1s (Eb = 284.6 eV). Specific surface area (SSA) was determined by N2 adsorption using a Micromeritics Tris-

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

tar-3000 (Carlo Erba) system. Tests of H2 temperature-programmed reduction (TPR) and H2 temperature-programmed desorption (TPD) of the catalysts were carried out in a fixed-bed continuous flow reactor or an adsorption–desorption system. A KOH column and a 3A-zeolite molecular sieve column were installed in sequence at the reactor exit to remove water vapor produced by the reduction of the metallic oxide components of the catalysts. The temperature ramp rate was 10 K/min. The change in hydrogen signals was monitored using an online GC (Shimadzu GC-8A) with a thermal conductivity detector. For the TPR measurements, 10 mg of the oxide precursor of the catalyst was used. The sample was first flushed by an Ar (of 99.999% purity) stream at 573 K for 90 min to clean its surface and then cooled to room temperature followed by switching to a N2-carried 5% H2 gaseous mixture as the reducing gas (40 ml/min) to start the TPR measurement from 293 to 723 K. For the TPD test, 50 mg of the catalyst sample was used each time. Prior to the test, the oxide precursor of the catalyst was pre-reduced/sulfurized in situ in a H2-carried 5% CS2 gaseous mixture stream at 0.1 MPa and 1800 ml/(g·h). The reduced/sulfurized sample was then flushed by an Ar stream at 573 K for 30 min followed by cooling to 433 K and switching to a H2 (of 99.999% purity) stream (40 ml/min). It was then cooled to room temperature and maintained at this temperature for 2 h. Subsequently, the sample was flushed by an Ar stream at room temperature until the stable baseline of GC appeared, and the TPD measurement was then carried out from 293 to 823 K.

Fig. 1 Results of higher alcohol synthesis (HAS) over sulfurized Co1Mo1K0.3-x%CNT catalysts with varying CNT addition (1) X(CO), (2) S(C2+-alc), (3) S(C1–4-alc), (4) Y(C1–4-alc) (Reaction conditions: 2.0 MPa, 603 K, V(H2):V(CO):V(N2) = 45:45:10, GHSV = 2400 ml/(g·h), 18 h.)

tion upon the Co/Mo molar ratio was investigated for a series of CoxMo1K0.3-10%CNT catalysts. Fig. 2 shows that the catalyst with the Co/Mo molar ratio of 1 had the best catalytic performance. The yield of C1–4-alcohols over the Co1Mo1K0.310%CNT catalyst reached 11.0%, while the yield of C1–4-alcohols over the other four catalysts were 9.8%, 5.9%, 5.4%, and 4.7%, respectively.

2 Results and discussion 2.1 Optimization of catalyst composition The HAS reaction over a series of Co1Mo1K0.3-x%CNT catalysts with the same Co:Mo:K molar ratio of 1:1:0.3 and varying amount of CNT addition was investigated, and the results are shown in Fig. 1. The observed carbon-containing products included CO hydrogenation products C1–4-alcohols and C1–3-alkanes, and CO2 yielded from the water–gas shift (WGS) side reaction (CO + H2O = CO2 + H2). The catalyst with the addition of 10% CNT showed the best catalytic performance. The yield of C1–4-alcohols reached 11.0% over this catalyst, while the yield of C1–4-alcohols over the other five catalysts were 9.8%, 8.3%, 7.9%, 7.3%, and 6.1%, respectively. It is well known that CO hydrogenation over the existing alkali (K2CO3)-promoted MoS2-based catalysts produces methanol as the main product instead of C2+-alcohols [2–6]. The incorporation of a proper amount of cobalt into the catalyst systems can increase the CO conversion and the selectivity for C2+-alcohols [19]. On the basis of the optimization of the amount of added CNT, the dependence of the HAS reac-

Fig. 2 Results of HAS over sulfurized CoxMo1K0.3-10%CNT catalysts with varying Co/Mo molar ratio (1) X(CO), (2) S(C2+-alc), (3) S(C1–4-alc), (4) Y(C1–4-alc) (Reaction conditions are the same as in Fig. 1.)

The effect of K/Mo molar ratio on the HAS reaction was investigated over a series of Co1Mo1Kx-10%CNT catalysts. Over the K-free system, the main products were C1–3-alkanes, while over the Co1Mo1Kx-10%CNT system (Fig. 3), the yield

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

Fig. 3 Results of HAS over sulfurized Co1Mo1Kx-10%CNT catalysts with varying K/Mo molar ratio (1) X(CO), (2) S(C2+-alc), (3) S(C1–4-alc), (4) Y(C1–4-alc) (Reaction conditions: 5.0 MPa, 603 K, V(H2):V(CO):V(N2) = 45:45:10, GHSV = 3600 ml/(g·h), 18 h.)

of C1–4-alcohols increased with the increase in the K doping amount. After reaching the maximum at K/Mo = 0.3, it decreased with the further increase in the K doping amount. Hence the optimal K/Mo molar ratio was 0.3. 2.2 Reactivity of HAS over the CNT-promoted catalysts To evaluate the performance of the catalysts under working conditions with a higher extent of reaction, the HAS reaction from syngas was carried out at a high pressure of 5.0 MPa and GHSV of 3600 ml/(g·h). Fig. 4 shows the results of the comparative assay of the HAS reaction over the sulfurized Co1Mo1K0.3-10%CNT and Co1Mo1K0.3 catalysts. Similar behavior was observed on the two catalysts, i.e., CO conversion and selectivity for C2+-alcohols and C1–3-alkanes increased with increasing reaction temperature, while the selectivity for both methanol and total alcohols decreased. However, there was an obvious difference in reactivity between the two catalysts toward the HAS. Over the CNT-promoted catalyst, CO conversion reached 21.6% at 623 K with the selectivity for total alcohols and C2+-alcohols higher than that over the CNT-free counterparts. Fig. 5 shows the product distribution of HAS over the CNT-promoted catalyst and the CNT-free counterpart. Over the sulfurized Co1Mo1K0.3-10%CNT catalyst under the reaction conditions of 5.0 MPa and 623 K, the selectivity for total alcohols and C2–4-alcohols reached 61.5% and 41.3%, respectively, which were higher than those over Co1Mo1K0.3. The main product of CO hydrogenation was ethanol (with a selectivity of 27.3%) on the former, while it was methane (with a selectivity of 27.7%) on the latter.

Fig. 4 Results of HAS over the sulfurized Co1Mo1K0.3-10%CNT (a) and Co1Mo1K0.3 (b) catalysts (1) X(CO), (2) S(MeOH), (3) S(EtOH), (4) S(C2+-alc), (5) S(C1–4-alc), (6) S(C1–3-HC), (7) S(CO2) (Reaction conditions are the same as in Fig. 3 except the reaction temperature.)

It was experimentally found that the incorporation of a proper amount of CNT into the Co1Mo1K0.3 catalyst not only enhanced the CO conversion and C2+-alcohol selectivity but also inhibited the WGS side reaction to a considerable extent. The selectivity for the by-product CO2 over the Co1Mo1K0.310%CNT catalyst at 573–633 K was maintained at 16.5%–12.5%, while the selectivity for CO2 over Co1Mo1K0.3 under the same reaction conditions reached as high as 33.9%–28.5%, two times that of the former. Table 1 shows the space-time-yield (STY) of HAS over the CNT-promoted Co-Mo-K sulfide catalyst and the CNT-free counterpart. At the reaction temperature of 623 K, STY of the total alcohols reached 241.5 mg/(g·h), which is higher than that (131.0 mg/(g·h)) on the CNT-free counterpart. The C2+-alcohols/MeOH ratio for the former was 1.39 (C-based selectivity ratio); therefore, such mixed alcohol products show good prospects to be used as gasoline additives or even alternative automobile fuels.

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

Fig. 5 Product distribution of HAS over the sulfurized catalysts (a) Co1Mo1K0.3-10%CNT, (b) Co1Mo1K0.3 (Reaction conditions are the same as in Fig. 3 except T = 623 K.) Table 1 Catalyst

T/K

Co1Mo1K0.3-10%CNT

Co1Mo1K0.3

STY of HAS over the CNT-promoted CoMoK-sulfide catalyst and the CNT-free counterpart STY/(mg/(g·h)) MeOH

EtOH

PrOH

BuOH

C2+-alc

C1–4-alc

C1–3-HC

CO2

613

106.3

84.8

20.4

12.4

117.6

223.8

46.2

84.6

623

101.2

98.0

25.2

17.2

140.3

241.5

62.0

93.5

633

77.8

123.4

37.4

23.1

183.9

261.6

98.6

107.5

613

66.3

41.2

11.1

3.1

55.4

121.7

38.5

133.9

623

62.0

50.2

15.0

3.8

69.0

131.0

57.9

160.2

633

52.0

52.3

20.0

6.2

78.6

130.6

72.9

172.2

Reaction conditions are the same as in Fig. 3 except the reaction temperature.

Fig. 6 shows the operation stability of the Co1Mo1K0.310%CNT catalyst for HAS lasting 100 h. After 24 h of the initial stage of the reaction, the catalyst attained a stable operating state, with a CO conversion and selectivities for the total alcohols and C2+-alcohols being 22.4%, 69.3%, and 45.0%,

respectively. The catalyst did not show any obvious deactivation after reaction for 100 h. The apparent activation energy (Ea) of the HAS reaction was measured and the results are illustrated as the Arrhenius plots in Fig. 7. The Ea of the Co1Mo1K0.3-10%CNT catalyst was 74.3 kJ/mol, which is quite similar to the Ea value of 80.2 kJ/mol observed on Co1Mo1K0.3. This indicated that the incorporation of a proper amount of CNT into the Co-Mo-K sulfide catalyst did not cause an obvious change in Ea for the HAS reaction, implying that the addition of small amount of CNT into Co1Mo1K0.3 did not alter the major reaction pathway of CO hydrogenation. 2.3 Characterization of the CNT-promoted catalysts

Fig. 6 Operation stability of HAS over the sulfurized Co1Mo1K0.3-10%CNT catalyst lasting 100 h (1) X(CO), (2) S(C1–4-alc) (Reaction conditions are the same as in Fig. 3 except T = 623 K.)

It is quite evident that the considerably good performance of the CNT-promoted catalyst for HAS from syngas is closely related to the unique structures and properties of the CNT promoter. Fig. 8(a) shows the TEM image of the CNT. Previous characterization studies [18] have shown that this type of CNT was a herringbone-type multiwalled carbon nanotubes with outer diameters of 10–50 nm, inner diameters of 3–5 nm, and N2-BET specific surface area of about 140 m2/g. These

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

Fig. 7 Arrhenius plots of HAS over the sulfurized catalysts (1) Co1Mo1K0.3-10%CNT, (2) Co1Mo1K0.3 (Reaction conditions: 2.0 MPa, V(H2):V(CO):V(N2) = 45:45:10, GHSV = 6000 ml/(g·h), 18 h.)

nanotubes were constructed by the superposition of several graphene layer facets, which were tilted at a certain angle with respect to the axis of the central hollow nanofiber, as if a number of cones were placed one on the top of the other one [18]. It was shown by elemental analysis and O2 tempera-

Fig. 8 TEM images of the CNT (a) and the Co1Mo1K0.3-10%CNT catalyst (b)

ture-programmed oxidation (TPO) measurements that the contents of elemental carbon and graphitized carbon were ≥ 99% and > 90%, respectively, in the purified CNT. The test of H2 temperature-programmed hydrogenation (TPH) showed that the temperature required for initiating the hydrogenation reaction of CNT with H2 was ≥ 773 K, indicating that this type of CNT was stable in a H2 atmosphere at the reaction temperatures for HAS [17]. Fig. 8(b) shows the TEM image of the oxide precursor of the Co1Mo1K0.3-10%CNT catalyst. It can be seen that the pre-oxidized CNT evenly dispersed into Co1Mo1K0.3. Fig. 9 shows the H2-TPR profiles of the Co1Mo1K0.310%CNT and Co1Mo1K0.3 catalyst precursors. For the former, H2 reduction was initiated at 448 K, reached the peak at 623 K, and was completed at 708 K. The TPR profile was broad and asymmetric, most likely involving the partial overlap of multiple peaks that corresponded to the continual multistep single-electron reduction of CoOx and MoOy components. For the latter, the H2 reduction was initiated at 648 K, reached the peak at 685 K, and was completed at 713 K with the total area intensity of the TPR profile taken in the range of 423–723 K (corresponding to a certain amount of H2 consumed) to be merely 27% of the former. In view of the fact that the Co1Mo1K0.3 content of the CNT-containing catalyst was 90% (mass fraction) of the CNT-free counterpart, it can be estimated that the specific amount of H2 consumed (i.e., the amount of H2 consumed due to the reduction of unit mass of Co1Mo1K0.3) of the CNT-containing catalyst was 4.1 times that the CNT-free counterpart. The high specific amount of H2 consumed indicated the high fraction of the CoxMoy species reducible to lower valence in the total CoxMoy amount. Thus, it could be inferred that the reducibility of Co1Mo1K0.310%CNT was much higher than that of Co1Mo1K0.3, implying that more CoxMoy species on the former were reduced to lower valence states that are catalytically active in the working catalyst. Therefore, the specific activity of the catalyst for CO hydrogenation was enhanced.

Fig. 9 H2-TPR profiles of the catalysts (oxide precursor) (1) Co1Mo1K0.3-10%CNT, (2) Co1Mo1K0.3

MA Miaoming et al. / Chinese Journal of Catalysis, 2006, 27(11): 1019–1027

Fig. 10 shows the XPS spectra of the working catalysts of Co1Mo1K0.3-10%CNT and Co1Mo1K0.3. There was little difference between the two catalysts with respect to the position and shape of their Co 2p XPS peaks. The Co 2p3/2 and Co 2p1/2 peaks appeared at 779.0 and 795.0 eV, respectively, with their area intensity ratio being about 2. These values are characteristics of the Co sulfide species interacting with Mo species to form Co–Mo–S composite species [20]. Unlike the Co 2p XPS spectra, there are some differences between the two catalysts with respect to the position and shape as well as the relative intensity of the peaks associated with the Mo species.

According to Ref. [21] and assuming Eb(Mo 3d3/2) – Eb(Mo 3d5/2) = 3.1 eV and intensity ratio of peaks Mo 3d5/2/Mo 3d3/2 = 1.5 for each Mon+ species in the same valence state, analysis and fitting of these Mo 3d XPS spectra were carried out. The results (Fig. 10 and Table 2) showed that, under the reductive atmosphere containing H2, CO, etc., most of the Mon+ was reduced to lower valence: the majority in Mo4+, minority in Mo5+, and few in Mo6+. This was analogous to the case of coexistence of Mo4+ (major) and Mo5+ (minor) in the related systems [22]. From the results in Table 2, it can be seen that the molar fraction of the Mo4+ species in the total Mo amount at the surface of the working Co1Mo1K0.3-10%CNT reached 88.1%, which is 1.14 times that of Co1Mo1K0.3 (77.3%). It is generally considered that low valence Mon+ species (mainly Mo4+) at the surface of the working catalyst are catalytically active sites that are responsible for adsorption activation, hydrogenolysis, or dissociation-hydrogenation of CO to form metallic carbene followed by CO insertion and chain growth [23]. The higher the concentration of Mo4+ species at the surface of the working catalyst, the higher the catalyst activity. In comparison with the CNT-free Co1Mo1K0.3 catalyst, the higher concentration of Mo4+ species at the surface of the working Co1Mo1K0.3-10%CNT catalyst resulted from its higher reducibility. This is one of the important factors that lead to a significant increase in the specific activity of the catalyst for CO hydrogenation. 2.4 H2-TPD of the working catalyst and the nature of promoter action of CNT Recently, there has been an increasing interest in the use of nanostructured carbon materials (especially CNT, carbon nanofibers, and mechanically milled graphite) as hydrogen sorbents. It was demonstrated by Ishikawa et al. [24] that graphitized carbon black surfaces were capable of rapidly equilibrating H2/D2 mixture. A dissociation rate of 4.2 × 10−7 mol/(s·(m2-ASA)) was measured at ambient temperatures and pressures. The ASA (active surface area) was described in terms of atoms located at edge positions on the graphite basal plane and was determined on the basis of the amount of oxygen capable of chemisorbing at these sites, irrespective of the nature of the carbon material under investigation. Our previous H2-TPD investigation [25] showed that hydrogen adsorption on CNT can occur at ambient temperatures and pressures, and the desorbed product was almost exclusively H2 at tem-

Fig. 10 XPS spectra of the functioning catalysts (a) Co1Mo1K0.3-10%CNT, (b) Co1Mo1K0.3

Table 2 Binding energies and relative contents (molar percentage) of the Mon+ species with different valence states at the surface of the functioning catalysts Catalyst

Eb of Mo 3d5/2 (eV) Mo4+

Mo5+

Relative content (%) Mo6+

Mo4+

Mo5+

Mo6+

Co1Mo1K0.3-10%CNT

228.9

229.6

232.0

88.1

9.3

2.6

Co1Mo1K0.3

228.8

229.4

232.0

77.3

15.5

7.2

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peratures lower than 723 K and included CH4, C2H4, and C2H2 in addition to H2 at temperatures of 773 K and above. This implies that H2 adsorption on CNT may have two forms, associative (molecular state) and dissociative (atomic state), as demonstrated by the Raman spectroscopic study of the H2/CNT adsorption system by the authors [26]. Fig. 11 shows the H2-TPD profiles of the prereduced Co1Mo1K0.3-10% CNT and Co1Mo1K0.3 catalysts. Overall, each profile contained a low-temperature peak (peak I) and a high-temperature peak (peak II). The low-temperature peaks resulted from the desorption of weakly adsorbed H species, most probably molecularly adsorbed hydrogen. The high-temperature peaks were attributed to the desorption of strongly adsorbed H species, perhaps dissociatively chemisorbed hydrogen.

On the basis of the above results, it could be inferred that a considerable amount of reversibly adsorbed H species existed on the working CNT-promoted catalyst, which generated a surface microenvironment with high steady-state concentration of H adspecies. Those active H adspecies could be easily transferred to CoiMojKk active sites via the CNT-promoted hydrogen spillover and thus increased the rate of the surface hydrogenation reactions. This is very similar to the cases in the synthesis of methanol and higher alcohols from syngas over the CNT-promoted catalysts [16, 17]. Moreover, the high steady-state concentration of H adspecies at the surface of the catalyst considerably inhibited the WGS side reaction, which would also contribute to the increase in the product yields.

3 Conclusions The present study indicated that CNT can serve as an excellent promoter of the CoiMojKk sulfide catalyst for HAS from syngas. The incorporation of a proper amount of CNT into the CoiMojKk sulfide catalyst significantly increases the CO conversion and the selectivity for C2+-alcohols. The excellent performance of CNT toward the adsorption and activation of H2 may play a significant role in enhancing the catalyst activity, improving the HAS selectivity, and inhibiting the WGS side reaction. For better understanding the mechanism of the promoting effect of the CNT, further studies, especially in situ characterization of reaction intermediates under the HAS reaction conditions, are needed.

References Fig. 11 H2-TPD profiles of the catalysts (1) Co1Mo1K0.3-10%CNT, (2) Co1Mo1K0.3

[1] Chianelli R R, Lyons J E, Mills G A. Catal Today, 1994, 22(2): 361

The relative area intensities of peak I and peak II for these catalyst samples were calculated, and the results are listed in Table 3. It is believed that at the reaction temperatures for HAS (573–633 K for the present study), the concentration of hydrogen adspecies associated with peak I was very low, and most of hydrogen adspecies at the surface of the working catalysts was those corresponding to peak II. Therefore, it could be inferred that those strongly chemisorbed H species were closely associated with the reaction activity of HAS. The ratio of the relative area intensities of peak II for the two catalysts was 100/22 (see Table 3), which was in line with the HAS reaction activity observed over the two catalysts.

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Table 3 Relative area intensity of peaks I and II in H2-TPD profiles for the catalysts Catalyst

Relative area intensity

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Peak I

Peak II

Co1Mo1K0.3-10%CNT

36

100

Co1Mo1K0.3

33

22

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