Potassium-decorated active carbon supported Co-Mo-based catalyst for water-gas shift reaction

Journal of Natural Gas Chemistry 20(2011)77–83

Potassium-decorated active carbon supported Co-Mo-based catalyst for water-gas shift reaction Yixin Lian1 ,

RuiFen Xiao2 ,

Weiping Fang1 ,

Yiquan Yang1∗

1. National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China; 2. Minnan Science and Technology Institute, Fujian Normal University, Quanzhou 362332, Fujian, China [ Manuscript received July 29, 2010; revised September 21, 2010 ]

Abstract The effect of potassium-decoration was studied on the activity of water-gas shift (WGS) reaction over the Co-Mo-based catalysts supported on active carbon (AC), which was prepared by incipient wetness co-impregnation method. The decoration of potassium on active carbon in advance enhances the activities of the CoMo-K/AC catalysts for WGS reaction. Highest activity (about 92% conversion) was obtained at 250 ◦ C for the catalyst with an optimum K2 O/AC weight ratio in the range from 0.12 to 0.15. The catalysts were characterized by TPR and EPR, and the results show that activated carbon decorated with potassium makes Co-Mo species highly dispersed, and thus easily reduced and sulfurized. XRD results show that an appropriate content of potassium-decoration on active carbon supports may favors the formation of highly dispersed Co9 S8 -type structures which are situated on the edge or a site in contact with MoS2 , K-Mo-O-S, Mo-S-K phase. Those active species are responsible for the high activity of CoMo-K/AC catalysts. Key words active carbon; potassium-decorated; active phase; Co-Mo-based catalyst; water-gas shift

1. Introduction Several investigations have been directed towards an understanding of the nature and function of the Alkali-promoted molybdenum sulfide catalysts. A. A. Andreev et al. [1] had reported that sulfurized Ni-Mo or Mo catalysts and their K-promoted derivatives exhibited a high activity in the WGS reaction; the presence of potassium affected the reducibility of Mo-based samples. Feng et al. [2] also reported that K itself is not reduced, but it may modify the reducibility of other metal compounds. After studying mono-, di- and tricomponent (K)(Ni)(Mo)/γ-Al2O3 systems subjected to watergas shift reaction with sulfur-containing synthesis gas as feed, D. Nikolova et al. [3] reported that the addition of potassium as a third component to the Ni18Mo sample (KNi18Mo) increases the activity due to the interaction between potassium and other deposited components and the formation of K-NiMo-O type surface species. They also found that presence of potassium influences the balance among the three molybdenum oxidation states. Hillerov´a et al. [4] and Hatanaka et al. [5,6] had proposed a technique to incorporate potassium into alumina support to improve hydrodesulfurization selectivity. Because oxidic and sulfurized Mo species are acidic and the ∗

alkali metal potassium modified alumina can counteract their acidity, the active species can reach high dispersion. Moreover, the catalytic performance depends, to a certain extent at least, on the used support, since the surface properties of the catalyst vary with the support [7]. It is well known that notable features of alumina supports include their ability to provide high dispersion of the active metal components. But, numerous chemical interactions exist between the amorphous alumina and transition metal oxides in the catalyst precursor. Some of the formed species are very stable and resistant to complete sulfuration. In recent years, Active carbon has been used to mitigate the support effect [8−10]. Furthermore, Active carbon has many interesting features such as the presence of micropores, high surface areas and controllable surface functionality [7]. Such advantages make it become an attractive catalyst support, and therefore it is considered to be reliable in many applications. Molybdenumbased catalysts supported on active carbon with potassium as promoter have been used for mixed alcohols synthesis, hydrocarbon synthesis and higher-alcohol synthesis [4,11−13]. However, there has been few study devoted to the potassiumpromoted Co-Mo catalysts supported on active carbon for WGS reaction.

Corresponding author. Tel: +86-592-2180361; Fax: +86-592-2180361; E-mail: [email protected]

Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60154-5

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In the present work, active carbon was decorated with K2 CO3 in advance and then used as the support of Co-Mobased catalysts for water-gas shift. The catalytic activity as a function of potassium content was investigated. The effect of active carbon with or without potassium-decoration on the structure of Co and Mo species over the oxidized and sulfurized catalysts was studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR).

Electron paramagnetic resonance (EPR) measurements were performed by using a Bruker EMX-10/12 EPR spectrometer at room temperature. All spectra were obtained under the conditions: microwave power = 2.0 mW, microwave frequency = 9.8 GHz, amplitude frequency = 100 kHz, amplitude modulation = 6.0 pp, center field = 3500 Gauss, sweep width = 2000 Gauss and a time constant of 81.92 ms

2. Experimental

The activity measurement of the catalyst was carried out in a stainless steel continuous flow reactor filled with 1.0 ml catalyst of 0.25−0.59 mm in particle size per pass. The reaction was conducted under the conditions: total pressure = 1.0 MPa and feedstock composition CO/N2 /H2 /CO2 = 30/10/50/10 (vol%). Water was pumped with a precision injection pump into a steam generator. Generated steam was introduced into the reactor along with the gas feed flow. CO conversion was analyzed using an on-line gas chromatograph equipped with a thermal conductivity detector (a 5A molecular sieve column; column length, 2 m; internal standard, N2 ). The experimental data were recorded when the steady-state was achieved. Prior to activity test, the oxidic catalysts were activated in a flow of synthesis gas containing 1.0 vt% H2 S, with the temperature rising up from room temperature to 350 ◦ C at a heating rate of 5 ◦ C/min and maintaining at this temperature for 4 h.

2.1. Catalyst preparation Active carbon (Made in Tianing Jinhu Carbon Co., Ltd, China) of 35−60 mesh was washed several times with HNO3 (1 M) and distilled water and then dried. The BET surface area and pore volumes of the resulting AC are 750 m2 /g and 0.68 ml/g, respectively. And then, the activated carbon was impregnated with an aqueous solution of K2 CO3 , followed by drying at 100 ◦ C in air for 2 h. The potassium content, expressed as K2 O/AC weight ratio, varies as 0, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18, respectively. The prepared support was denoted as Kx /AC, wherein x = 0, 3, 6, 9, 12, 15 and 18, respectively. The oxidic CoMo-K/AC catalyst was prepared by incipient wetness co-impregnation method. In a typical preparation of the catalyst, required quantities of (NH4 )6 Mo7 O24 ·4H2 O and Co(NO3 )2 ·6H2 O were dissolved in deionized water, to which ammonia was added dropwise until the precipitate was fully dissolved under stirring to produce an impregnation solution. The next step is to immerse the Kx /AC support in the impregnating solution for 8 h, followed by drying at 100 ◦ C and calcining in a nitrogen flow of 40 ml/min at 350 ◦ C for 4 h. The molybdenum content in the catalyst, expressed as weight ratio of MoO3 /AC, is 0.08, and cobalt content, as Co/Mo atomic ratio, is 0.6. The prepared catalysts were denoted as CoMo-Kx /AC. The sulfided samples were obtained by heat-treating the oxidic precursors in a flow of synthesis gas containing 1.0 vol% H2 S. 2.2. Catalyst characterization X-ray diffraction (XRD) measurements were carried out using a PANa-lytical X’Pert Pro diffractometer with Cu Kα as radiation source, operating at 40 kV and 30 mA, in 2θ range from 10o to 80o , step size of 0.0167o. Phases present in the samples were identified by X’Pert HighScore software. Temperature-programmed reduction (TPR) were conducted in a U-shaped quartz tube embedded in a programmable furnace. 100 mg of the catalyst was pre-treated with pure He flow at 100 ◦ C for 1 h and then reduced with a gas mixture flow (5% H2 , 95% Ar, 30 ml/min−1). TPR patterns were obtained by using a recorder connected to a GC equipped with TCD in a temperature range from 100 to 700 ◦ C at a heating rate of 10 ◦ C per minute.

2.3. Catalyst activity measurements

3. Results 3.1. Catalyst performance for WGS reaction Figure 1 shows the WGS activities of the CoMo-Kx /AC catalysts as a function of potassium content. The potassiumfree catalyst, CoMo-K0 /AC, exhibits low activity for the WGS reaction. However, the activities of CoMo-K6 /AC, CoMoK9 /AC and CoMo-K12 /AC catalysts increased obviously with the increase of K2 O/AC weight ratio in the range from 0.06 to 0.12. The optimum potassium loadings, which is expressed as K2 O/AC weight ratio, were 0.12, and 0.15 on CoMo-K12 /AC and CoMo-K15 /AC catalysts, respectively. In those cases, CO conversions for the reaction are close to their equilibrium values. It was observed that the activities of CoMo-Kx /AC catalysts decrease in the following order: CoMoK15 /AC ≈ CoMo-K12/AC > CoMo-K9 /AC  CoMo-K6 /AC > CoMoK0 /AC. Such a trend of effect is similar under the conditions of the steam to gas ratio = 0.3, 0.6 and 1.0, respectively. Figure 2 illustrates the activities of the CoMo-Kx/AC catalysts as a function of reaction temperature. It can be found that the activities of CoMo-Kx/AC catalysts decreased in the following order: CoMo-K15/AC ≈ CoMo-K12 /AC > CoMoK9 /AC  CoMo-K6 /AC > CoMo-K0 /AC. Based on the conversion of CO shown in Figure 1, it can be found that the potassium addition into the Co-Mo/AC catalysts can increase their activity for CO. However, it can also be seen from Figure 2 that the conversions of CO over all Co-Mo-Kx/AC cat-

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alysts decrease linearly with the increase of reaction temperature. Furthermore, The catalysts with high potassiumcontent, namely CoMo-K15/AC, CoMo-K12/AC and CoMoK9 /AC, have a comparatively large decrease in reactivity in high temperature range. These results indicated that the deactivation of CoMo-Kx/AC catalysts for WGS reaction is related to the potassium content. In this work, we considered that the main reason for the loss of activity is that the mobility of alkali promoter in the catalysts affects adversely the stability of the catalysts. At relatively higher operation temperature, the potassium loss takes place more easily [14].

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low potassium content is loaded on active carbon, such as the CoMo-K3 /AC catalysts, no other evident diffraction peak different from those present over CoMo-K0 /AC was detected, indicating that the interaction between potassium and Mo species was relatively weak. As a result, potassium could not interact fully with molybdenum to form K-Mo-O species or other new phases [17]. With the increase of K2 O/AC weight ratio, the addition of K destroys the uniformity in distribution of Co and Mo species. Some new phases, such as Co3 O4 (2θ = 32.6o, 36.8o; Ref code: 00-042-1467), and several KMo-O species, such as K2 Mo3 O10 (2θ = 16.7o, 19.0o, 24.8o, Ref code: 00-027-0416), K2 Mo2 O7 (2θ = 21.4o, 31.8o and 32.6o; Ref code: 00-035-0422), were detected [15,16], while the diffraction intensity of CoMoO4 phase decreases monotonically. This indicates that the interaction increases between K and Mo species with the increase of potassium loadings over active carbon. However, for CoMo-K18 /AC catalysts, the peaks at 2θ = 23.8o , 29.7o and 30.8o (Ref code: 00-024-0880) for K2 MoO4 boost up, this probably means that during the impregnation, a part of the MoO2− 4 ions is transformed into K2 MoO4 on the surface of active carbon decorated with high quantity of potassium [18].

Figure 1. CO conversion over CoMo-Kx /AC catalyst as a function of potassium content. Reaction conditions: 1.0 MPa, GHSV = 3000 h−1 , reaction temperature = 250 ◦ C

Figure 2. CO conversion over CoMo-Kx /AC catalyst as a function of reaction temperature under the conditions of 1.0 MPa, steam to gas ratio = 0.6 and GHSV = 5000 h−1

3.2. XRD characterization of the catalysts Figure 3 shows the XRD patterns for the oxidic catalysts CoMo-Kx /AC. Compared with the pure active carbon support, some new peaks (2θ = 21.3o, 23.5o and 26.8o, Ref code: 00015-0439) were observed over the oxidic catalysts of CoMoK0 /AC, which can be assigned to CoMoO4 [15,16]. When

Figure 3. XRD patterns of the oxidic catalysts with various potassium contents

The XRD patterns of the sulfurized state CoMo-Kx /AC catalysts are shown in Figure 4. After sulfuration of the catalyst CoMo-K0 /AC, on which the diffraction peaks observed in the oxidized CoMo-K0/AC catalysts completely disappeared. Similarly, no obvious diffraction patterns, besides those of the support, were observed on the sulfurized CoMo-K3 /AC catalysts. For the sulfurized catalyst CoMoK6 /AC, new diffraction patterns at 2θ = 30.5o and 39.7o (Ref Code: 00-017-0744) appeared, which can be attributed to MoS2 species [19,20]. There were also some weak diffraction peaks at 2θ = 31.4o,

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37.3o and 47.3o (Ref Code: 00-003-0631), which were related to the second phase of CoSx such as Co9 S8 [20,21]. For sulfurized catalysts CoMo-K9/AC, CoMo-K12 /AC and CoMoK15 /AC, with increasing the K2 O/AC weight ratio, most of the diffraction peaks of the K-Mo-O species observed in the oxidic samples disappeared; meanwhile, some weak diffraction peaks were observed, which were assigned to K-Mo-O-S species, i.e. K2 MoO2 S2 , K2 MoOS3 , and K-Mo-S species, i.e. K2 MoS4 [19−21], and MoS2 diffraction peaks boosted up with the increase in potassium loading. This indicates that oxygen atoms in the sample were mainly substituted by sulfur atoms; and three-dimensional MoS2 micro-crystallites were formed on the activated carbon surface. MoS2 and sintered Co9 S8 was observed from sulfurized catalysts supported on activated carbon in our work; and K-Mo-O-S and K-Mo-S peak intensities also became strong. However, with the incorporation of potassium, for the sulfurized catalyst of CoMo-K18 /AC, new diffraction patterns were observed, which were assigned to K2 SO4 species (2θ = 23.8o , 29.7o , 30.8o (Ref code: 00-003-0608)) [18,22], while MoS2 , K-MoS and Co9 S8 diffraction peaks weakened, indicating that the aggregated K-Mo-O species were only partly sulfurized and reduced during the sulfuration. These results could be related to the fact that the excessive potassium added in the CoMoK18 /AC catalysts resulted in the aggregation of Mo species and made the reduction and sulfuration of Mo difficult [23]. The formation of K2 SO4 indicates that the sulfur lability on the surface of samples increases, being resulted from the oxidation of a part of the adsorbed H2 S or low valent sulfur because the oxidizing agents such as H2 O and CO2 are present in the system [3,23].

with different potassium contents are shown in Figure 5. It can be seen that each of all the CoMo-Kx /AC catalysts exhibits a H2 consumption peak in the range from 430 ◦ C to 542 ◦ C along with a shoulder peak in the range from 565 ◦ C to 660 ◦ C. Both of the two peaks can be a superposition of more than one peak due to existance of various Mo species, which correspond to the facile reduction of octahedral Mo6+ to Mo4+ and the hard reduction of tetrahedral Mo6+ to Mo4+ , respectively [24,25]. From Figure 5, it can be seen that the potassium-containing CoMo-Kx /AC catalysts have quite different TPR profiles from that of the potassiumfree CoMo-K0 /AC. The temperature, at which the first reduction peak appeared, shifts downwards from 542 ◦ C to 430 ◦ C with the increase of potassium loadings, suggesting that the reduction of Mo6+ to Mo4+ becomes easier after potassium addition. These results indicate that the addition of K greatly promotes the formation of octahedral Mo species reducible at low-temperature, thus the degree of dispersion of Mo supported on potassium-decorated active carbon is increased with the increase of potassium contents. These are in line with the results of Geng et al. [26]. They observed two kinds of Mo oxide species on SiO2 , one is a well-dispersed monolayer of octahedral Mo species, and the other is tetrahedral Mo species in the form of double or multiple layers. The former was easier to be reduced than the latter. Based on these results, the amount of monolayer Mo oxide on potassium-containing CoMo-Kx/AC catalysts is increased with increasing the potassium content from 3 to 12 wt%, while it remains almost constant with further increasing in potassium loading. The addition of 12 wt% K2 O does not increase the consumption of H2 and lower significantly the reduction temperature. This indicates that the formation of monolayer MoO3 on the support surface completes when the potassium loading is up to 12 wt%,

Figure 4. XRD patterns of the sulfided catalysts with various potassium contents

3.3. TPR characterization of the catalysts H2 -TPR profiles of the oxidic CoMo-Kx /AC catalysts

Figure 5. H2 -TPR profiles of the oxidic catalysts with various potassium contents

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and the excessive potassium loadings in catalysts, such as those in CoMoK15/AC and CoMo-K18/AC, would result in the aggregation of Mo species. It is noticeable that the H2 consumption peak of Co oxide did not individually appear in the profile. This is in accord with the results of L. Kaluˇza et al [24]. They reported that the TPR curves arising from CoMo catalysts were always similar to those of Mo sample alone and the reduction peaks of Co oxides were suppressed. 3.4. Catalysts characterization by EPR Figure 6 shows the EPR spectra of the sulfurized CoMoKx /AC catalysts with different potassium contents. Two signals (g1 and g2 ) were detected. The resonant signal of g1 over the CoMo-K0/AC catalysts is ascribed to the oxoMo5+ species [27,28]. Compared with the potassium-free CoMo-K0 /AC catalyst, the relative intensities of g1 signals over CoMo-K6 /AC and CoMo-K9 /AC have no significant difference, suggesting that part of the oxidized Mo species over the sulfurized CoMo-Kx/AC catalysts with low potassium content still existed after sulfuration at 350 ◦ C. Interestingly, the g1 signals over CoMoK12/AC and CoMo-K15/AC, which can be ascribed to the surface oxo-Mo5+ species, did not appear in this study. It is probable that the MoO3 species are monolayer-like dispersion on the K12 /AC and K15 /AC supports, resulting in the terminal oxygen ligands easily in contact with H2 S in the sulfuration [3]. These features are in agreement with the results of TPR, that is, monolayer Mo is more easily reduced in the presence of potassium. However, the g1 signal was observed again over CoMo-K18 /AC, indicating that the excessive addition of potassium in the CoMoK18 /AC catalysts resulted in the aggregation of Mo species and made the reduction and sulfuration of Mo difficult. This fact was also reported by Tatsumi and DeCanio et al. [29,30]. They found that the addition of K did not always enhance the reduction of Mo at low-temperature. Another signal at g2 is an overlapping signal, which appears as a result of the co-existence of oxysulfo-Mo species and trace amounts of paramagnetic sulfur species [3,31,32]. It was observed that the presence of the potassium additives promotes an increase in the relative intensity of g2 signal, i.e. an increase of the oxo-Mo5+ species at the expense of S-containing structure, and it is associated with the extent of sulfuration of the molybdenum components. It must be noted that more Mo5+ ions are located in an oxysulfo-surrounding species with the addition of potassium, indicating that potassium stabilizes the Mo species in a +5 valence state after sulfuration, which is consistent with the result reported by R. B. Watson et al. [33]. This was also associated with the fact that potassium acting as an electron donor enhances the electron density on Mo6+ , and thus promotes the reduction and sulfuration of Mo species. Meanwhile, the interaction between Mo species and activated carbon decorated with potassium makes Mo species highly dispersed and activated as well as easily reduced and sulfurized as seen from the peaks appearing at the lower reduction temperatures in our TPR experiments.

Figure 6. EPR profiles of the sulfurized catalysts with various potassium contents

4. Discussion Curves in the Figure 1 show that with the increases of potassium content, the percentage of CO conversion over the catalysts increases rapidly and goes through a maximum, nearly close to the equilibrium value. It is worth noting that the addition of potassium enhances the activity, but affects adversely the stability of Co-Mo based catalysts. Curves in the Figure 2 show that the active carbon-supported Co-Mo based catalysts decorated with high contents of potassium (K2 O/AC weight ratio >0.09) exhibit higher deactivation of activities in a higher reaction temperature range than those decorated with low content of potassium or potassium-free. Such a fact suggests that potassium plays a key role on the CoMo-Kx /AC catalysts. According to the reaction path of the WGS reactions occurring on MoS2 -based catalysts reported by Li et al. [34], the idealized way of realizing the WGS reactions is that CO is undissociated, and H2 O is dissociated into H2 and Oad which reacts with COad to form CO2 . Thus, the only way to modify the MoS2 catalyst is to introduce different kinds and quantities of promoters and carriers to meet the requirement for the WGS reaction. The Co species over Co-Mo based catalysts has certain promoting effect by accelerating the oxygen and sulfur exchange during sulfuration and formation of the active phase CoMoS. Meanwhile potassium is believed to possess a synergetic action in WGSR catalyst. Potassium is a wellknown effective electronic promoter; as an electron donor, it can increase the electron density in MoS2 and increase the bond strength of the adsorbed CO, and thus prevent the CO dissociation. Furthermore, potassium has a very strong ability to adsorb H2 O and to start the oxidation reaction quickly. When potassium was added to Co-Mo based catalyst to form a doubly promoted catalyst, its overall catalytic activity was increased greatly. These obtained results show that the decoration of activated carbon with potassium in advance has a remarkable promoting function on the activities of the Co-

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Mo-Kx /AC catalysts for WGS reaction. Thus, It can be seen from Figure 1 and Figure 2 that the CoMo-K12 /AC and CoMoK15 /AC catalysts present the highest activity at 250 ◦ C (about 92% CO conversion), while the activity of the catalysts with low potassium content and potassium-free is very low. Generally, Al2 O3 and MgO-Al2 O3 are used for WGS catalysts, while active carbon support possesses advantages over oxide supports for some reactions because of its inactive surface. This could lead to a weak interaction between the support and the active components, thus preferentially forming active phases and make them more effective [35]. However, those peculiarities were not present in our observations. As shown in Figure 1 and Figure 2, the activity of the catalysts with potassium-free, Co-Mo-K0/AC, is very low. This means that the catalysts with potassium-free, Co-Mo-K0/AC, is not fit to be used for WGS reaction. However, the decoration of potassium on active carbon in advance enhances the activities of the Co-Mo-Kx/AC catalysts for WGS reaction, and their relative reactivities can be adjusted by introducing different quantities of potassium. As the K2 O/AC weight ratio is increased, the activity increases. And the optimum CO conversion is reached when the K2 O/AC weight ratios lie in the range from 0.12 to 0.15. This probably means that potassium decoration on the AC support has a strong modifying effect on its specific surface, leading to differences in the structure of Co-Mo species and the active phase formation on the active carbon support. This result could be interpreted by the results of our TPR, XRD and EPR experiments. Curves in the Figure 5 show that the the reducibility of the Mo species over the Co-Mo-Kx/AC catalysts is greatly affected by the amount of K loading. The results of TPR show that the presence of potassium on active carbon enhances the formation of low-temperature reducible octahedral Mo species existing in the form of well-dispersed monolayer. This means that Mo oxide over the Co-Mo-Kx/AC catalysts can be more easily reduced at low temperatures than that over the catalysts without the decoration of K. Meanwhile, the formation of monolayer MoO3 on the support surface enhances the contact of the terminal oxygen ligand with H2 S in the sulfuration. This result brings about an increase in the relative intensity of oxysulfo-Mo species and decreases the amounts of oxo-Mo5+ species, which are shown in Figure 6. Thus, activated carbon decorated with potassium makes Mo species highly dispersed and activated as well as easily reduced and sulfurized, resulting in the high activity of the the CoMoK12 /AC and CoMo-K15/AC catalysts. On the other hand,the catalytic activity was also determined by the behavior of active phases. The activities of CoMo-Kx/AC catalysts decrease in the following order: CoMo-K15/AC ≈ CoMo-K12 /AC > CoMo-K9 /AC  CoMoK6 /AC > CoMo-K0 /AC. This can be accounted for by the behavior of active phases present over the CoMo-Kx/AC catalysts. Li et al. [34] had reported that the primary and secondary active phases are Co-Mo-S-K and Mo-S-K for WGSR, excluding the carrier. However, the formation of active phase could also be affected remarkably by the surface properties of the carrier. The effect, which caused by the active car-

bon with the decorated-potassium in advance and the Co-Mo species, was considered in the present work. The effects of the decorated-K on the properties of supported Co-Mo-based catalysts have been investigated by XRD and EPR experiments, which verified that some new phase consisted of K, S, Mo, Co and O are formed on the surface of catalysts supported on activated carbon decorated with K in advance. In oxidic state, cobalt and molybdenum mainly present as CoMoO4 phases in the catalysts with low potassium content or potassiumfree. With increasing the content of potassium, the formation of K-Mo-O and K-Co-Mo-O species is favorable because of the strong interaction between K and Mo species. On the other hand, the weak interaction between cobalt and K-MoO species may lead to the formation of Co3 O4 species. After sulfuration, molybdenum mainly presents as MoS2 , Mo-SK and Co-Mo-S-K species, while cobalt in the form of “CoMo-S” phase at low content of potassium or potassium-free. With the increase of potassium contents, most of the K-MoO species in the oxidic catalysts are transformed to K-MoO-S species and K-Mo-S species, respectively. As the interaction between potassium and molybdenum is enhanced, the absence of a strong Co-Mo interaction induces the sulfidation of the Co atom alone, resulting in the formation of Co9 S8 type phase which are situated on an edge or site in contact with MoS2 , K-Mo-O-S, K-Mo-S phase and also present in CoMo-S or Co-Mo-S-K structure. The amount of new phases increases with increasing K contents until the weight ratio of K2 O/AC reaches 0.12. It seems that the higher activities of CoMo-K9 /AC, CoMo-K12/AC and CoMo-K15/AC should also be related to the presence of Co9 S8 -MoS2 , Co9 S8 -K-MoO-S and Co9 S8 -K-Mo-S structures. In this case it may be expected that those structures would also be valid for the active phase formation, and would result in the highest activity of the catalysts. Very similar results were reported by M. W. J. Craj´e et al. [36], and they reported that for the CoMo/AC catalysts sulfurized under atmospheric-pressure conditions, the Co sulfurize species continuously sintered; the highly dispersed Co9 S8 type structures were stabilized at the edges of the MoS2 crystallites. However, formation of the Co-Mo-S phase and sintered Co9 S8 were never observed for sulfurized catalysts supported on activated carbon under atmospheric pressure. This difference was related to the decoration of potassium on the surface of active carbon. As the potassium loading increases, the interaction between potassium and molybdenum is enhanced. The weak interaction of the MoS2 particles with Co leads to the formation of relatively large MoS2 , K-Mo-O-S and K-Mo-S slabs. The large particles have few edge positions to accommodate all Co atoms, resulting in the sintering of Co9 S8 [37]. The absence of a strong Co-Mo interaction induced separate sulfuration of Co atoms during activation under atmospheric pressure, and the resulting Co sulfide species were located at a site in contact with MoS2 , K-Mo-O-S and K-Mo-S [20,38]. All these results suggested that the addition of K effectively inhibited the formation of separate cobalt sulfide and simultaneously enhanced the formation of K-Mo-O-S and K-Mo-S phases.

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5. Conclusions The decoration of activated carbon with potassium in advance has a remarkable promoting function on the activities of the catalysts for WGS reaction. With the increase of potassium content, the percentage of CO conversion over the catalysts increases rapidly and the highest conversion is obtained when the K2 O/AC weight ratio is in the range from 0.12 to 0.15. Therefore, potassium on activated carbon is believed to bring about a synergetic action for WGS reaction as the result of its three main effects. Potassium itself as a promoter can enhance the activities of the CoMo-Kx /AC catalysts. Potassium as a decorator on the surface of active carbon, modifies the properties of support, and thus promotes the formation of low-temperature reducible octahedral Mo species. This kind of Mo species existed in the form of well-dispersed monolayer, which makes Mo species reduced and sulfurized easily. A strong K-Mo interaction leads to the presence of Co atoms both in a Co9 S8 -like structure and in a Co-Mo-S-K structure over the CoMo-Kx/AC catalyst with high potassium contents. The Co9 S8 -like species are situated on an edge or a site in contact with MoS2 , K-Mo-O-S and K-Mo-S phase, which are related to the active phases for WGSR reaction. References [1] Andreev A A, Kafedjiysky V J, Edreva-Kardjieva R M. Appl Catal A, 1999, 179(1-2): 223 [2] Feng L J, Li X G, Dadyburjor D B, Kugler E L. J Catal, 2000, 190(1): 1 [3] Nikolova D, Edreva-Kardjieva R, Gouliev G, Grozeva T, Tzvetkov P. Appl Catal A, 2006, 297 (2): 135 [4] Hillerov´a E, V´ıt Z, Zdraˇzil M. Appl Catal A, 1994, 118 (2): 111 [5] Hatanaka S, Miyama T, Seki H, Hikita S. EP 0736589. 1996 [6] Hatanaka S, Sadakane O, Hikita S, Miyama T. US 5853570. 1998 [7] Farag H. J Colloid and Interface Sci, 2002, 254(2): 316 [8] Farag H, Whitehurst D D, Sakanishi K, Mochida I. Catal Today, 1999, 50(1): 9 [9] Dong K M, Ma X M, Zhang H B, Lin G D, J Natur Gas Chem, 2006, 15(1): 28 [10] Whitehurst D D, Isoda T, Mochida I. Adv Catal, 1998, 42(1-2): 345 [11] Li Z R, Fu Y L, Jiang M, Hu T D, Liu T, Xie Y N. J Catal, 2001, 199(2): 155

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