Tungsten-doped molybdenum sulfide with dominant double-layer

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Kinetics, Catalysis, and Reaction Engineering

Tungsten-doped molybdenum sulfide with dominant double-layer structure on mixed MgAl oxide for higher alcohol synthesis in CO hydrogenation Xuebin Luan, Jiaxi Yong, Xiaoping Dai, Xin Zhang, Hongyan Qiao, Yang Yang, Huihui Zhao, Wenyu Peng, and Xingliang Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01378 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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Industrial & Engineering Chemistry Research

Tungsten-doped molybdenum sulfide with dominant double-layer structure on mixed MgAl oxide for higher alcohol synthesis in CO hydrogenation

Xuebin Luan, Jiaxi Yong, Xiaoping Dai,* Xin Zhang, Hongyan Qiao, Yang Yang, Huihui Zhao, Wenyu Peng and Xingliang Huang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

* Corresponding author.

Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected]

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ABSTRACT Improving the C2+ alcohols selectivity is highly desirable for higher alcohols synthesis (HAS) in CO hydrogenation. Herein, an effective method was developed for Mo-based supported catalysts by the combination of tungsten-doping and surfactant-assisting hydrothermal strategy. The tungsten-doping enhanced the interaction between Ni and W/Mo metal species to form more Ni-MoW-S phase with tunable slab size and stacking layers, and thus promoted the chain growth of alcohol to form more higher alcohols in CO hydrogenation. The optimal K,Ni-Mo0.75W0.25/MMO-S exhibited the dominant double-layer structure (~39.0 %) and highly synergetic effects between Ni and W/Mo species, resulting in the highest total alcohol selectivity (76.1 %) and higher alcohol selectivity. This work provides a new route for tuning the morphology of MoS2/WS2 and synergetic effects between Ni and W/Mo species in supported catalysts to improve the selectivity of higher alcohols. Keywords: CO hydrogenation, higher alcohols, MoS2; tungsten-doping, Ni-MoW-S phase, synergetic effects.

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1. Introduction Higher alcohols (HA) have attracted considerable interest for their extensive applications in fuel additives, chemicals and solvents.1,2 At present, HA are mainly produced through fermentation of sugar, starch and lignocellulose as well as the process of ethylene hydration. Fischer-Tropsch processes is considered as an alternative route instead of competing with food resources and fossil fuels, which use syngas (H2, CO, and CO2) as feedstock originating from coal, biomass natural gas, etc.3–6 However, the improvement in the C2+ alcohols selectivity is highly desirable for higher alcohols synthesis (HAS). Due to the multi-functionality of catalysts needed for higher alcohols synthesis, the catalysts containing two or more active elements were required, and the synergetic effects among each active phases are crucial to enhance the catalyst activity.7 A large quantity of heterogeneous catalysts based on transition metals have been developed for HAS, including Cu-, Fe-, Rh-, Mo-based catalysts, etc.8–12 Molybdenum sulfided catalysts modified with alkali have drawn much interest owing to its high resistance to sulfur poisoning and relatively low cost,13 but the poor activity and selectivity restricted the development of Mo-based catalysts. Compared with bulk MoS2-based catalysts, supported Mo-based catalysts have been desired to enhance the activity and to improve higher alcohol selectivity using SiO2, CeO2, MgO, Al2O3, and carbon-based materials as supports.9,14–16 Among those of supports, mixed Mg/Al oxide (MMO) can tune the interaction between Mo and MMO to improve the product distribution toward higher alcohols.17,18 The special nature owned by MMO of weakening the alcohols dehydration and favoring oxygenate coupling reactions may be helpful for the high selectivity of C2+ alcohols.18 Based on the ‘Rim-Edge’ model of MoS2 catalysts, hydrogenation reactions only occurred on rim and edge sites.19,20 It has also been reported that the high dispersion of MoS2 on MMO with a proper layer number and slab size for more exposure of edge sites is favorable to enhance the higher alcohol selectivity.18,21,22 On the other hand, group VIII 3



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metals possess the potential for enhancing the activity and improving the HA selectivity by balancing the CO/H2 activation and alcohols formation. Among these elements, Ni and Co are regarded as superior additives.23 Ni species appears in two types in Mo-based catalysts denoted as NiSx with segregated particles and KNiMo-S phase with a strong interaction, where NiSx is active for the dissociation of CO and H2 for the chain initiation, while M-KMoSx is the site for CO insertion, chain propagation and alcohol formation.23,24 Recently, the addition of tungsten in NiMo catalysts showed more reducibility and a stronger interaction among metal phases than those of pristine NiMo catalysts, which facilitated both textural and chemical synergisms among the sulfide phases and yielding more synergetic NiWMoS active phases in the hydrodesulfurization and reaction.25–27 Inspired by those prominent works, it is desirable to explore the effects of tungsten-doped MoS2 catalysts in HAS. With Al2O3, SiO2, and TiO2 as support, the strong metal-support interactions generally hamper the formation of active phases during the sulfidation.28 The recent research have found that micro-emulsion method can be used to control the crystal size and dispersion of MoS2, and further enhanced their catalytic performance.

28,29

For this reason, in our

previous work, surfactant-assisting method was proposed to modulate the interaction between MoS2 and support (MgAl mixed metal oxide), and thus improved the catalytic performance. Herein, an effective method to prepare Mo-based supported catalysts was developed by the combination of tungsten-doping and surfactant-assisting strategy, which significantly improve the dispersion, reducibility, and sulfidability to tune the slab size and stacking layer numbers of Mo/W-S phase. The optimal catalysts possess more accessible active phase (Ni-MoW-S) with dominant double-layer structure and strong synergistic effects between Ni-S and Ni-MoW-S, resulting in high selectivity of higher alcohol in CO hydrogenation. 2. Experimental 2.1. Preparation of MMO-supported oxidic K,Ni-MoW catalyst 4


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The mixed Mg/Al oxide (MMO) was prepared according to our previous work,22 except that the calcination time was changed to 4 h. The oxidic WMo/MMO precursors were prepared by the hydrothermal deposition with TEAB-encapsulated W/Mo precursors as follows. An aqueous solution of sodium molybdate (0.22 M) was adjusted to pH = 5 with 2.0 M HCl solution, and TEAB solution (0.15 M) was added dropwise into the above solution under vigorous stirring. Then, acidified sodium tungstate aqueous solution with pH=5 was added dropwise into the above solution. The pH was further controlled to 3.0, and the mixture was stirred for another 2 h to form suspension liquid containing TEAB-encapsulated W/Mo precursors. After that, the suspension liquid and 1.0 g MMO were transferred into a Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. The resulting MoW/MMO was centrifuged, washed with H2O, dried at 80 °C under vacuum overnight, and calcined at 500 °C for 4 h to obtain oxidic MoW/MMO catalysts. Oxidic trimetallic Ni-MoW/MMO catalysts were prepared by incipient impregnating method with a aqueous solution of nickel nitrate (Ni(NO3)2·6H2O), aging for 12 h at room temperature, and further calcination at 450 °C for 4 h. To attain oxidic K,Ni-MoW/MMO, K2CO3 was introduced by grinding Ni-MoW/MMO and K2CO3 powder, followed by calcination at 450 °C for 4 h. K,Ni-MoW/MMO with various molar ratios of Mo to W were also prepared by the same process. The resulting samples were denoted as K,Ni-Mo1-xWx/MMO, where x = 0, 0.1, 0.25, 0.5, respectively. The molar ratio of K to total Mo and W (K/(Mo+W), mol) was kept constant at 3.0, and the molar ratio of Ni to total Mo and W (Ni/(Mo+W), mol) was kept constant at 0.5. In other word, the K2CO3 and NiO contents loading in all catalysts were around 14.0 wt. % and 2.0 wt. %, respectively. K,-Mo1-xWx/MMO catalyst were also prepared by the same method as K,Ni-Mo1-xWx/MMO except the addition of Ni. 2.2. Catalyst characterization The as-prepared catalysts were characterized by powder X-ray diffraction (XRD) on a German 5



Bruker D8 Advance diffractometer using Cu Kα radiation with a voltage of 40 kV and a current of 30 mA. Data were collected over the range of 2θ from 5o to 90o at a scan rate of 13o min−1. Raman spectra were performed on a Renishaw Raman microscope with a 532 nm laser at 8 mV for the oxidation state or 1 mV for the sulfide state. X-ray photoelectron spectroscopy (XPS) analysis was acquired by a PHI5000 Versa Probe system using monochromatic Al Kα radiation at 1486.6 eV. The presulfurized catalysts were cooled down to room temperature in a He flow, grounded, and soaked in cyclohexane to prevent oxidation, and then quickly pressed onto a stainless steel sample holder in air for XPS characterization. The surface areas and pore parameters of the samples were analyzed through N2 adsorption and desorption with a Micromeritics JW-BK222 at -196 oC. Specific surface areas were calculated with the Brunauer–Emmett-Teller (BET) method, and the pore sizes were obtained from desorption branches with the Barrett-Joyner-Halenda (BJH) method. The composition of catalysts was determined by inductively coupled plasma mass spectrometry (ICP-MS). DRS UV– vis spectra were obtained from UV-Vis spectrophotometer (Hitachi U-4100) with the wavelength range 200–800 nm. The surface structure and elemental mapping of the samples were characterized by scanning electron microscopy (SEM) (FEI XL30 Sirion SEM). High-angle annular dark-field scanning TEM (HAADF-STEM) was obtained using a Tecnai G2 F20 S-Twin high-resolution transmission electron microscope (HRTEM) operating at 200 kV. The micro-structures of the MoS2 active phase were obtained through high-resolution transmission electron microscopy (HRTEM) using a F20 high-resolution transmission electron microscope. The fM was calculated to estimate the proportion of Mo and W atoms on the edges vs. the total number of Mo and W atoms by the following equation. =

M ∑…(6n − 6) = M ∑…(3 − 3 + 1)

Where ni is the number of W/Mo atoms along one edge of a MS2 (M=Mo, W) slab, which can 6


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be obtained according to the length of MS2 slab as L = 3.2(2 − 1)Å. To examine the interaction between the metal and support, H2 temperature-programmed reduction (H2-TPR) was conducted with a homemade temperature-programmed reduction equipment. 30 mg catalyst was placed in a quartz tube, and pretreated with pure Ar flow at 40 mL min−1 at 400 °C for 30 min before cooled down to room temperature. The reduction procedure was carried out with 6 vol. % H2 in Ar from 100 to 950 °C at a heating rate of 10 °C min−1. The tail gas was analyzed using a thermal conductivity detector (TCD). 2.3 HAS in CO hydrogenation The mixture of 0.80 g catalyst and 2.4 g quartz sand was loaded into a tubular fixed-bed reactor with an internal diameter of 11 mm and a length of 500 mm. The catalysts were heated to 400 oC with heating rate of 10 oC/min for 2 h, and were pretreated under H2S/H2 (10 vol. % H2S) mixture gas with a flow of 20 mL min−1 at atmospheric pressure. The sulfided catalysts were denoted as K,Ni-Mo1-xWxS2/MMO (x = 0, 0.1, 0.25, 0.5). Then, the HAS performance of the catalysts was investigated under 350 oC at ramping rate of 10 °C/min, 5.0 MPa and 30 mL min-1 with syngas as feed gas (GHSV of 2250 mL g−1 h−1), which contained 45 vol. % CO, 45 vol. % H2, and 10 vol.% N2. N2 was introduced as the internal standard for CO conversion. The reactants and light products (hydrocarbons and CO2) were analyzed using an on-line SP-7890 gas chromatograph, which consist of a flame ionization detector (FID) and a thermal conductivity detector (TCD). The production selectivity towards all products was calculated by an internal normalization method. All oxygenates were collected to a cold trap where isopentyl alcohol (15 mL) was regarded as the absorbent. The alcohols and other oxygenates were further analyzed by an off-line SP-6890 gas chromatograph equipped with a FID (DB-WAX). The mass balance was estimated based on carbon, which were closed to within ±5% in all experiments. 3. Results and discussion 7



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3.1. Structural characterization of catalysts XRD patterns of oxidic catalysts and sulfided catalysts were shown in Figure 1. Diffraction characteristic peaks at 44o and 64o are ascribed to the MgO in all catalysts. Only weak peaks for Mo, W in K-Mo1-xWx/MMO are observed, indicating high dispersion of K, Mo, W species in MMO. With the presence of nickel, The K2NiO2 phase (JCPDF 27-0423, 2θ = 30.753o, 31.995o) and K2NiO3 phase (JCPFD 31-1056, 2θ = 29.554o, 33.026o) are identified in K,Ni-Mo1-xWx/MMO. Besides, the KMo4O6 phase (JCPDF 51-0371, 2θ = 12.960o, 20.564o, 26.098o) are also detected in samples of presence of nickel, which imply the nickel influence the state of molybdenum. In sulfided catalysts, the weak peaks of MoS2 (JCPDF 37-1492, 2θ = 32.7o, 33.5o, 39.5o, 58.3o) and WS2 (JCPDF 08-0237, 2θ =32.8o, 33.6o, 39.5o, 58.4o) are identified in K,Ni-Mo1-xWxS2/MMO, which further demonstrate the highly dispersed MoS2 and WS2 species in the support. Notably, the characteristic peaks of the (002) planes of MoS2 and WS2 at 2θ ≈ 14o are not observed, indicating the few layers of Mo1-xWxS2 in as-prepared sulfided samples. The weak peaks of KMoS2 (JCPDF 18-1064, 2θ = 9.7o, 19.3o, 32.4o) and

K2MoS4

(JCPDF

19-1001,

2θ

=

17.5o,

25.7o,

29.5o)

are

only

identified

in

K,Ni-Mo0.5W0.5S2/MMO, which is resulted from the high ratio of K to Mo and the presence of polymerized W/Mo species.

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The N2 adsorption/desorption isotherms and the pore-size distribution of the oxidic catalysts and MMO were shown in Figure 2 and Figure S1. The isotherms exhibit a typical IV isotherm with H2 hysteresis loop, which is the characteristic of mesoporous materials. The values of the BET surface area, total pore volume and pore diameter were listed in Table S1. The incorporation of metal phases provokes the drastic loss of surface area, pore volume and narrow pore size distribution. Catalyst compositions were determined by ICP-MS in Table S2, which are same as theoretical feeds. The surface metal (W and Mo) concentration are all higher than 2 atoms/nm2 (Table S2 of the Supporting Information). Ultraviolet-visible light (UV-Vis) radiation was used to investigate the 8


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elemental distribution (Figure S2 of the Supporting Information), which show that all samples only have a band-gap energy (Eg), implying the formation of uniform metal oxidic nanostructure.31 The HAADF-STEM and SEM element mapping (Figure S3 and Figure S4 of the Supporting Information) results

also

verify

the

uniformly

distribution

of

Ni,

Mo

and

W species

on

the

K,Ni-Mo0.75W0.25S2/MMO. Raman spectra of sulfided catalysts (K,Ni-Mo1-xWxS2/MMO) were presented in Figure 3. The peaks for K,Ni-Mo1-xWxS2/MMO at about 378.2 cm-1 and 400.5 cm-1 are related to the MoS2-like E12g and A1g modes of Mo-S vibration, which are corresponding to the inplane vibration of molybdenum and sulfur atoms and out-of-plane vibration of sulfur atoms, respectively.32 The Raman mode at about 355.7 cm-1 should be ascribed to the WS2-like E12g in K,Ni-Mo1-xWxS2/MMO, which gradually increase with increasing tungsten content.33 The widened distance between E12g and A1g mode suggests the increasing number of Mo1-xWxS2 layers, implying the increasing of stacking degree of Mo1-xWxS2 in K,Ni-Mo1-xWxS2/MMO with the increase of tungsten content.32 The typical HRTEM images of K,Ni-Mo1-xWxS2/MMO were shown in Figure 4. The characteristic lattice fringes with interplanar spacing of 0.62 nm are observed in the K,Ni-Mo1-xWxS2/MMO, which should be attributed to Mo1-xWxS2. The average slab length () and stacking layer number ( !) were calculated according to the statistical data from approximately 400 slabs in 30-50 representative micrographs, and fM was calculated through dividing the number of W/Mo atoms at the edge by total numbers of W/Mo atoms.34 The results (Figure 4 and Table 1) show that the distribution of slab length and layer number much more centralize on 3~7 nm and 2~3 layers in the presence of tungsten, which is obviously different with the pristine one. Notably, when the ratio of Mo to W reaches 1, the distributions of slab length and layer number turn obviously broad, which means the presence of polymerized W/Mo phase. It is consistent with XRD results mentioned above. Furthermore, the length and stacking degree of Mo1-xWxS2 slabs gradually increase with the 9



increase of tungsten content, which is consistent with the Raman results (Figure 3). Notably, in K,Ni-Mo1W0S2/MMO, the single layer MoS2 are dominant with ~43 %, while the distribution of Mo1-xWxS2 slabs prominently centralized at double layers of Mo0.75W0.25S2 (~39 %) in K,Ni-Mo0.75W0.25/MMO-S. According to the rim-edge model, rim sites are responsible for hydrocarbon formation and single layers only possess rim sites.20 Hence, the results show that the tungsten-doping decrease successfully the amount of single layer MoS2. However, an excess of tungsten increases violently the stacking degree, resulting in the generation of polymeric W/Mo species. The significant difference in the distribution of stacking layer should impose on the reactivity trends of products. The proportion of Mo and W atoms on the edges vs. the total number of Mo and W atoms (fM) were investigated, which shows similar fM value (0.163 vs. 0.168) for K,Ni-Mo0.75W0.25S2/MMO and K,Ni-Mo1W0S2/MMO, respectively, due to the smaller proportion of length of slabs > 11 nm in K,Ni-Mo0.75W0.25S2/MMO (5.2 % vs. 7.9 %), meaning the similar ratio of edges to total W/Mo atoms, where edges were regard as active site for CO hydrogenation with proper incorporation of tungsten. 3.2 Reduction and sulfidation degree To investigate the effects of tungsten content on the reducibility of the oxidic catalysts, H2-TPR was conducted in Figure 5. H2-TPR profiles of all oxidic catalysts have low- and high- temperature peaks. The first peak can be assigned to the reduction of octahedral MO3 (M=Mo, W) from M6+ to M4+, and the second peak is ascribed to the deep reduction of octahedral MO3 from M4+ to M0+, and the third peak is assigned to the reduction of tetrahedral MO3 from M6+ to M0+.22,25,35 The presence of W in NiMo catalysts effects the reduction behavior, and as a result, the peak position and area change significantly.36 The peak position and area were shown in Table 2. With the incorporation tungsten, the H2 consumption in low-temperature region increase with the decreasing H2 consumption in high-temperature region, which imply the partial transformation from tetrahedrally 10


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coordinated W/Mo species to octahedrally coordinated W/Mo species.35 This can be attribute tungsten-doping effects the reduction behavior of the NiMo catalysts.35 With the incorporation of tungsten, the peak shifts to higher temperature, which is similar with Huirache’s report.36 The Ni-Mo0.75W0.25/MMO exhibits the highest temperature of the first peak. But with further increasing the tungsten content, the temperature of the first peak is even lower than without the tungsten-doping, which should be ascribed to the easy reduction of large size octahedrally coordinated polymetalates.25,35,37 It is supported by high stacking degree of K,Ni-Mo0.5W0.5S2/MMO in the HRTEM image (Figure 4D). The Mo 3d and W 4f XPS spectra of K,Ni-MoxW1-xS2/MMO catalyst were shown in Figure 6A-B and Figure S5. The binding energies (BE) of the Mo 3d5/2 of Mo4+ (MoS2), Mo5+ (MoOxSy) and Mo6+ (MoO3) are about 229.0, 230.5 and 232.8 eV, respectively.35,38,39 The BE of the W 3d7/2 of W4+ (WS2), W5+ (WOxSy) and W6+ (WO3) are about 32.2, 32.9 and 36.1 eV, respectively.20,25,39 The negative shift of Mo 3d5/2 and W 3d7/2 is due to the electron transfer from Ni to W/Mo, which depends on the strength of interaction between W/Mo and Ni.40 The partial replacement of molybdenum by tungsten enhance the promoting effect of Ni, which is further demonstrated by Ni 2p.38,40 The Ni 2p spectrum of sulfided catalysts were shown in Figure 6C-F, in which the peaks at around 853.5, 855.1, and 856.6 eV can be attributed to Ni sulfides, Ni-MoW-S phases and Ni oxides, respectively.38 The Ni-MoW-S phases are generated due to Ni atoms neighboring to Mo and W atoms with electron transfer from Ni to adjacent W/Mo atoms. The weak peak at 856.6 eV reveal a rather low content of Ni oxide over all the sulfided catalysts. With the addition of tungsten, the proportion of Ni-MoW-S phases gradually increase (Table 1), signifying the improving interaction of Ni and W/Mo to form K-Ni-MoW-S phase, which is responsible for CO insertion, alcohol formation and chain growth.20,38 The high proportion of K-Ni-MoW-S phase are helpful in the synthesis of higher alcohols. The sulfidation degree of metal species (Msulfidation) is usually defined as the ratio 11



M4+ to the sum of M4+, M5+ and M6+, i.e., Msulfidation = M4+/(M4+ + M5+ + M6+).25 The values of Msulfidation were displayed in Table 1. The incorporation of tungsten would cause the decrease of total metal sulfidation degree. The low sulfidation degree of tungsten on all samples shows that the sulfidation of tungsten is difficult under the condition.25,36 The K,Ni-Mo0.75W0.25S2/MMO has the highest sulfidation degree of molybdenum owing to the vast transformation of tetrahedrally coordinated W/Mo species into octahedrally coordinated ones, which is consistent with the H2-TPR characterization. Notably, the K,Ni-Mo0.5W0.5/MMO with the most octahedrally coordinated W/Mo species displays low sulfidation degree of molybdenum, which could be relevant with the presence of Mo6+ in new K2MoS4 phase.30 3.3 HAS performance The catalytic performance of K,Ni-MoxW1-xS2/MMO were shown in Table 3 and Figure 7, and the distribution of products were demonstrated in Figure 7. The CO conversion shows the trend of slight decrease with increasing of tungsten content, because of the decreasing amount of active site. With the increase of tungsten, the selectivity of alcohols exhibits a volcano-shaped curve. The alcohols selectivity significantly reach to the highest value of 76.1% in K,Ni-Mo0.75W0.25S2/MMO, which result from the synergistic effects of K-Ni-MoW-S and the proper distribution of layers.21,42 CO2 selectivity (19.2%) is the highest in K,Ni-Mo1W0S2/MMO due to the facilitated water-gas shift reaction over MoS2-based catalysts,43,44 which are suppressed by the partial replacement of molybdenum with tungsten. From the distribution for alcohols and linear hydrocarbons (Figure 7), chain growth possibility enhance gradually, which are also explained to the increasing density of K-Ni-MoW-S sites among total Mo1-xWxS2 sites. The distinct deviation of distribution for alcohols from Anderson-Schulz-Flory (ASF) distribution are observed (Figure S6 of the Supporting Information) because of Mo/K/MMO domains and oxygenate coupling reactions which are favorable to occur in those catalysts supported on basic materials like MMO and MgO.18 In order to 12


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further clarify tungsten-doping enhanced interaction between Ni and W/Mo, the reaction performance catalysts without nickel (K,-MoxW1-xS2/MMO) were investigated as comparison, as shown in Table S3. The results indicated that CO conversion decrease slightly with the incorporation of tungsten. Furthermore, the hydrocarbon selectivity increase slightly, and the alcohol selectivity as well as CO2 selectivity decrease negligibly. Methanol distribution distinctly deviate from ASF distribution, and chain growth possibility almost keep consistence by the incorporation of tungsten (Figure S6 of the Supporting Information). After the introduction of Ni, the enhanced interaction between Ni and W/Mo significantly improve activity, alcohol selectivity and products distribution. 3.4 Structure-performance relationships in K,Ni-Mo1-xWxS2/MMO catalysts The structure and HAS performance were correlated for gaining insight into the change of performance in HAS on K,Ni-Mo1-xWxS2/MMO catalysts. The CO conversion decrease slightly with the increase of tungsten content due to the decreasing value of fM and low sulfidation degree of tungsten. The TPR characterization have unraveled that the moderate amount of tungsten in K,Ni-Mo0.75W0.25/MMO can make the proper transformation from tetrahedrally coordinated W/Mo species to octahedrally coordinated W/Mo species (Figure 5). After further sulfidation, the K,Ni-Mo0.75W0.25S2/MMO exhibits a highest sulfidation degree (Table 1), and provide more active site for CO hydrogenation. Nevertheless, the CO conversion of K,Ni-Mo0.75W0.25S2/MMO remains lower than K,Ni-Mo1W0S2/MMO, which is mainly due to the low sulfidation degree of tungsten. The XPS characterization also demonstrates the higher proportion of Ni-MoW-S phase and decreasing ratios of Ni-S to Ni-MoW-S with more tungsten loading (Figure 6 and Table 1). The higher amount of tungsten also make negative effect about CO conversion due to the unbalanced ratio of Ni-S to Ni-MoW-S (Table 3, S3 and Figure 7A). The more Ni-MoW-S sites imply the stronger abilities for higher alcohols synthesis and chain growth through enhanced CO insertion.22,41 However, K,Ni-Mo0.5W0.5S2/MMO with the most proportion about K-Ni-MoW-S sites does not 13



display highest selectivity to alcohols, but possesses strongest chain growth possibility (Figure 7C). TPR results show the presence of polymeric W/Mo species on K,Ni-Mo0.5W0.5/MMO (Figure 5). Meanwhile, the distinctly high Mo/W-S stacking degree with only 27.1 % double layer were found on K,Ni-Mo0.5W0.5S2/MMO in HRTEM, and the domain size turns to dominant triple layers from double layers (Figure 4E-F), which result in the decreased selectivity toward alcohols on K,Ni-Mo0.5W0.5S2/MMO (Table 3). Taborga et al. observed the similar trend over K/MoS2, where the total alcohol selectivity reduce with the decreasing fraction of double layers.21 Notably, the selectivity of hydrocarbon decrease with an decreasing percentage of single layers in the catalysts, but the decreasing percentage of double layers also hinder the alcohols synthesis.21 The rim-edge model reveals that, hydrocarbon can be formed at rim sites, and single-layer metal sulfides only possess rim sites; edge sites are responsible for alcohols formation, and the decrease of double-layer metal sulfides also means the decrease of edge sites.20 Although the proportion about Ni-MoW-S phase of K,Ni-Mo0.75W0.25S2/MMO is not the highest, the most double-layer Mo1-xWxS2 (~ 39.0 %) are found in K,Ni-Mo0.75W0.25S2/MMO, which mainly contribute to alcohols synthesis, and result in the highest alcohols selectivity. The above results suggest that the synergetic effects of K-Ni-MoW-S phase and MoS2/WS2 stacking degree improve the synthesis of alcohols, as shown in Figure 8. The stacking degree of MoS2/WS2 slabs and proper Ni-MoW-S phase in K,Ni-Mo1-xWxS2/MMO can be tuned by controlling the incorporation of tungsten. As a result, the more suitable stacking of MoS2/WS2 slabs and high proportion Ni-MoW-S phase, and the balance of Ni-S and Ni-MoW-S sites in K,Ni-Mo0.75W0.25S2/MMO are critical for the improvement of HAS activity and alcohol selectivity.19,21,22 4. Conclusions A series of Mo1-xWxS2 catalysts were prepared by the combination of tungsten-doping and surfactant-assisting method. The synergetic effects of K-Ni-MoW-S phase and Mo1-xWxS2 stacking 14


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degree were proposed to tune the synthesis of alcohols in CO hydrogenation. The proper replacement of molybdenum with tungsten successfully promote the synergetic effects between Ni and W/Mo species to improve the reducibility and sulfidation degree by weakening the interactions between metal species and support, thus provide more active sites for alcohol formation. The optimal K,Ni-Mo0.75W0.25S2/MMO possess highest proportion of double layer Mo1-xWxS2 and highest sulfidation degree and strong synergetic effects between Ni and W/Mo species, resulting in the highest total alcohol selectivity. This work provides a novel strategy for tuning the morphology of Mo1-xWxS2 phase and metal-support interaction between Ni and W/Mo species in supported catalysts to improve selectivity of higher alcohols. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Nitrogen adsorption–desorption isotherms, pore size distributions, ICP-MS, UV-vis DRS spectra of oxidic catalysts, HAADF-STEM images and element mappings, FESEM images and correspond element mappings of K,Ni-Mo0.75W0.25S2/MMO, Mo3d XPS spectra, ASF distribution of different samples, and carbon monoxide hydrogenation performance on K-Mo1-xWxS2/MMO (PDF). AUTHOR INFORMATION Corresponding Author E–mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgments The authors acknowledge the financial supports from the NSFC (Nos. 21576288 and U1662104). Reference 15



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1984, 87 (2), 482–496.

21



Figures

A

• K2NiO2

♦ KMo4O6 ♥ K2NiO3 ♦

♥•

Intensity (a.u.)

K,Ni-Mo1W0/MMO K,Ni-Mo0.9W0.1/MMO K,Ni-Mo0.75W0.25/MMO K,Ni-Mo0.5W0.5/MMO K-Mo1W0/MMO K-Mo0.9W0.1/MMO K-Mo0.75W0.25/MMO K-Mo0.5W0.5/MMO MMO

20

40

60

80

2θ (degree)

B

♦ Mo1-xWxS2 ♦♦

∗ KMoS2

♦ ♦

•K

MoS4

2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

K,Ni-Mo1W0S2/MMO

K,Ni-Mo0.9W0.1S2/MMO

∗


•


10

20

30

40

50

60

70

80

90

2θ (degree)

Figure 1 XRD patterns of (A) oxidic catalysts, (B) sulfided catalysts.

22


Page 23 of 33

140

A 120

K,Ni-Mo1W0/MMO

100

K,Ni-Mo0.9W0.1/MMO

B

Pore Volume (cm3/g·nm)

Quantity Adsorbed (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K,Ni-Mo0.75W0.25/MMO

80 K,Ni-Mo0.5W0.5/MMO

60 40 20

K,Ni-Mo1W0/MMO K,Ni-Mo0.9W0.1/MMO K,Ni-Mo0.75W0.25/MMO K,Ni-Mo0.5W0.5/MMO

0 0.0

0.2

0.4

0.6

0.8

1.0

0

Relative pressure (P/P0)

2

4

6

8

10

12

14

16

18

20

Pore diameter (nm)

Figure 2. (A) N2 adsorption-desorption isotherms, (B) pore size distribution oxidic catalysts.

23



K,Ni-Mo1W0S2/MMO

A1g

K,Ni-Mo0.9W0.1S2/MMO K,Ni-Mo0.75W0.25S2/MMO

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

K,Ni-Mo0.5W0.5S2/MMO MoS2-like E12g WS2-like E12g

320

340

360

380

400

420

440

Raman shift (cm-1) Figure 3 Raman spectra of K,Ni-Mo1-xWxS2/MMO.

24


Page 24 of 33

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Figure 4 HRTEM images of (A) K,Ni-Mo1W0S2/MMO, (B) K,Ni-Mo0.9W0.1S2/MMO, (C) K,Ni-Mo0.75W0.25S2/MMO, (D) K,Ni-Mo0.5W0.5S2/MMO, and distributions of (E) lengths and (F) layers numbers of MS2 slabs on K,Ni-Mo1-xWxS2/MMO.

25



-0.1

6+

4+

6+

0

M to M of octahedral W/Mo species 4+ 0 M to M of octahedral W/Mo species M to M of tetrahedral W/Mo species

TCD Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.0

K,Ni-Mo1W0/MMO

0.1

K,Ni-Mo0.9W0.1/MMO

0.2

0.3

100

K,Ni-Mo0.75W0.25/MMO

K,Ni-Mo0.5W0.5/MMO

200

300

400

500

600

700

800

900

o

Temperature( C) Figure 5 Temperature-programmed reduction profiles of K,Ni-Mo1-xWx/MMO.

26


Page 26 of 33

Page 27 of 33

B

Mo4+

5+

Mo

S2s

224

226

228

230

232

W4+

Intensity (a.u.)

Intensity (a.u.)

A

234

236

W5+ W6+

30

32

Binding Energy (ev)

D NiS

850

852

854

856

858

860

848

850

852

854

F

850

852

854

856

858

860

858

860

Ni-M-S

Intensity (a.u.)

Ni-M-S

856

40

Binding Energy (ev)

E

848

38

Ni-M-S

Binding Energy (ev)

NiS

36

NiS

Intensity (a.u.)

Ni-M-S

Intensity (a.u.) 848

34

Binding Energy (ev)

C

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


858

860

NiO

NiS

848

850

852

854

856

Binding Energy (ev)

Binding Energy (ev)

Figure 6 (A) Mo3d, (B) W4f XPS spectra of the K,Ni-Mo0.75W0.25S2/MMO, Ni2p XPS spectra of (C) K,Ni-Mo1W0S2/MMO, (D) K,Ni-Mo0.9W0.1S2/MMO, (E) K,Ni-Mo0.75W0.25S2/MMO, (F) K,Ni-Mo0.5W0.5S2/MMO.

27


20

100

Conversion Alcohols Selectivity

Conversion (%)

A 16

80

12

60

8

40

4

20

Carbon distribution for hydrocarbon (%)

0

Carbon distribution for alcohol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

Alcohols Selectivity (%)


0

NiK,

W Mo 1

100

O MM S 2/

0

NiK,

O MM S 2/

W 0.1 .9 Mo 0

O MM S 2/

O MM S 2/

5 W 0.5 W 0.2 o 0.5 .75 -M Mo 0 i ,Ni N , K K

C 4H

B

C 3H

80

C 2H C 1H

60 40 20 0

O O O MM MO MM MM S 2/ /M S 2/ S 2/ S 2 .25 .5 0 W 0 W0 W 0.1 o 1W o 0.5 o 0.75 o 0.9 -M -M -M -M Ni Ni Ni Ni K, K, K, K,

100

C

C4OH C3OH

80

C2OH C1OH

60 40 20 0

NiK,

W Mo 1

O O O MM MM MM S 2/ S 2/ S 2/ .5 .25 0 0 W 0.1 W W .5 o 0.9 o 0.75 Mo 0 -M -M Ni NiNi K, K, K,

MM S 2/

0

O

Figure 7 (A) Error bar of conversion and alcohols selectivity, carbon distribution of (B) hydrocarbons, and (C) alcohols in K,Ni-Mo1-xWxS2/MMO.

28


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Figure 8 Dependence of hydrocarbon selectivity on the proportion of single layer and Ni-S phase (A), alcohol selectivity on the proportion of single layer and Ni-S phase (B) in K,Ni–Mo1-xWxS2/MMO catalysts.

29



Page 30 of 33

Tables

Table 1 XPS and HRTEM characterization of the K,Ni-Mo1-xWxS2/MMO catalysts. Sulfidation degree

MS2 morphology parameter

Samples

fNi-MoW-S d

Ni-S/Ni-

Mosulfidation/%

Wsulfidation/%

"a

!a

fMb

f2c (%)

K,Ni-Mo1W0S2/MMO

84.4

-

5.68

1.90

0.168

34.5

31.44

2.18


82.0

27.8

5.76

2.06

0.158

35.1

33.22

2.01


90.1

41.9

6.17

2.21

0.163

39.0

56.60

0.76


72.4

36.3

7.41

3.02

0.133

27.1

76.01

0.20

a

(%)

MoW-Se

Average slab length and stacking layer number attained by statistical analyses from different

HRTEM micrographs. b

Mo1-xWxS2 dispersion calculated by dividing the total number of Mo and W atoms at the edge

surface by the total number of Mo and W atoms. c

Proportion of double Mo1-xWxS2 layers from HRTEM micrographs.

d

Proportion of Ni-MoW-S phase from XPS.

e

Area ratio of Ni-S to Ni-MoW-S phase from XPS.

30


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Table 2 TPR characterization of the K,Ni-Mo1-xWxS2/MMO catalysts.

Catalysts

M6+ to M4+ of octahedral MO3

M4+ to M0 of octahedral MO3

M6+ to M0 of tetrahedral MO3

Peak position/oC

Peak area

Peak position/oC

Peak area

Peak position/oC

Peak area

K,Ni-Mo1W0S2/MMO

612.6

4.5

700.9

2.2

781.7

6.8


620.6

5.2

713.5

3.1

782.5

4.4


646.1

5.9

710.8

4.1

782.2

2.9


609.8

6.2

716.7

4.5

771.8

1.7

31



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Table 3 Carbon monoxide hydrogenation on K,Ni-Mo1-xWxS2/MMO. Select. (%)

Hydrocarbo

CO2

n select.

Select.

(%)

(%)

Con.

SCOH

Samples (%)

Alcohols

/SCH

Other Oxygenates

K,Ni-Mo1W0S2/MMO

8.5

62.3

3.7

14.9

19.2

4.1


6.2

66.5

4.8

9.9

18.7

6.7


7.4

76.1

4.4

9.0

10.5

9.3


5.8

71.5

5.1

10.0

13.4

7.1

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Table of Contents

33


Tungsten-doped molybdenum sulfide with dominant double-layer

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