Surface Engineering of CoMoS Nanosulfide for Hydrodeoxygenation

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Surface engineering of CoMoS nano-sulfide for hydrodeoxygenation of lignin-derived phenols to arenes Wenjing Song, Shijie Zhou, Shihua Hu, Weikun Lai, Yixin Lian, Jianqiang Wang, Weimin Yang, Meiyu Wang, Peng Wang, and Xingmao Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03402 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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ACS Catalysis

Surface engineering of CoMoS nano-sulfide for hydrodeoxygenation of lignin-derived phenols to arenes Wenjing Song † , Shijie Zhou † , Shihua Hu † , Weikun Lai* ‡ , Yixin Lian ‡ , Jianqiang Wang§, Weimin Yang§, Meiyu Wang⊥, Peng Wang⊥, Xingmao Jiang*† † School of Chemical Engineering & Pharmacy, Wuhan Institute of Technology, Wuhan 430073, P. R. China ‡

National

Engineering

Laboratory

for

Green

Chemical

Productions

of

Alcohols-ethers-esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China § SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, P.R. China ⊥ National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China Abstract We proposed a surface atom engineering strategy to obtain a well-dispersed Co incorporated MoS2 nanomaterial, which maximized the Co-Mo-S phase and achieved high activity in hydrodeoxygenation (HDO) of lignin-derived phenolic compounds. It was shown by XRD, TEM, Raman, HS-LEIS, and XPS that plenty of accessible Co-Mo-S phase were generated on catalyst surface, which could accelerate the hydrodeoxygenation (HDO) reaction. Notably, most of the Co-Mo-S phases located at the top surface of MoS2, which explained the full deoxygenation performance of the CoMoS catalyst. Moreover, among the diverse lignin-derived oxygenated compounds,

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phenolic hydroxyl HDO was structure sensitive relative to diphenyl ether over Co-doped MoS2 catalysts. Effective conversion of mixed phenols to corresponding arene such as BTX (benzene, toluene and xylene) with high yield (>85%) and stable recyclability was exhibited by using the CoMoS catalyst with high dispersed Co-Mo-S phase. Keywords Co-doped MoS2; nano-sulfide; Co-Mo-S phase; phenols; hydrodeoxygenation INTRODUCTION Because of the continued depletion of fossil fuels in coming decades, biomass-derived bio-oil has been considered as a sustainable, green and clean alternative material for the production of fuel and high-value chemical feedstock and consequently draws great attention.1, 2 Unfortunately, the bio-oil contains abundant oxygenated compounds (as high as 45 wt.% O), leading to some undesirable properties which hinder its direct utilization.3 Hence, utilizing the lignin-derived phenols in pyrolysis bio-oil via catalytic HDO is pivotal to produce renewable aromatics.4, 5 In particularly, light aromatics (such as BTX) are important chemical materials widely used in industry. For pyrolysis and catalytic deoxygenation of lignin, low BTX yield is a serious problem of the current state-of-the-art.6-8 As compared to the metal catalyst with high H2 consumption9 and incomplete elimination of oxygen,10 Mo-based sulfide has been proposed as an alternative catalyst with high performance. Co/Ni doped MoS2 catalysts have been commercially applied to hydrotreating in the processing of petroleum for decades. Such metal doping method has also been widely applied to enhance the hydrogen evolution


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ACS Catalysis

reaction (HER) over MoS2 catalyst.11 The introduction of Co/Ni atoms in MoS2 basal planes significantly impacts on its texture and structure, which promotes the activation of hetero-atoms such as sulfur, nitrogen and oxygen in organic molecules and accelerate proton adsorption.12, 13 For this reaction process, the enhanced activity can be contributed to the “Co(Ni)-Mo-S” phase.14 Previously, numerous experimental15,

16

and theoretical works17,

18

has provided an

atomistic description of Co(Ni)-Mo-S active phase and shown that the Co-Mo-S phase contains Co atoms located at MoS2 edge. After the introduction of Co, obvious enhancement in the HDO performance of MoS2 has been made.19, 20 Wang et al.21 demonstrated that despite of the surface area decreased after the introduction of Co, the MoS2 catalysts exhibited an promoted HDO activity and less hydrogen consumption. Romero et al.20 reported that the addition of Ni and Co increased the HDO rate of 2-ethylphenol. Compared with the

non-promoted

MoS2,

the

Co-Mo-S

phase

enhanced

the

direct

deoxygenation (DDO) pathway in guaiacol HDO.22 Such information is important for the understanding of the relevance of Co-doped MoS2 to the lignin-derived phenols conversion processes. For the catalyst, note that although the synthesis of chemically Co/Ni doped MoS2 has been reported by different methods,23 the Co/Ni sulfide species were simultaneously formed in the sample because of the distinct different precipitation rate between metal ions and S2- ion.24, 25 Unfortunately, the formed large Co/Ni sulfide crystals made no distinctive contributions to the HDS/HDO activity of MoS2.26 Hence, it would be most important to highly disperse Co/Ni atoms on MoS2 surface, generating abundant Co-Mo-S active phase. A typical strategy to synthesis transition metal doped MoS2 materials is based on


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hydrothermal method by engineering the composition of surface atoms with various precursors and solvents. Xia et al.27 prepared transition metal ions doped MoS2 using (NH4)2MoS4 and corresponding metal ion as precursors and DMF as solution. Defect-rich CoMoS nanosheets were obtained through hydrothermal synthesis using (NH4)6Mo7O24 and Co(NO3)2.28 Moreover, due to different formation routes, CoS2/MoS2 catalyst was prepared by the sequential hydrothermal.26

Recently,

a

Co-substituted

S

site

method

with

thiourea-coordinate Co adsorbed on the MoS2 molecular sheet was demonstrated to display high performance for 4-methylphenol HDO.29 Moreover, single atoms could been inserted in MoS2 layers by substitution of Mo sites through a one-pot method using CS2 as sulfurizing reagent.30 Herein, developing a simple and efficient approach to synthesize high-dispersed Co doped MoS2 nanomaterial will attract wide interest in catalytic applications. In this study, we propose a surface atomic engineering strategy to construct Co-doped MoS2, the crucial aspect of this strategy is to minimize the presence of cobalt sulfide and form highly dispersed Co-Mo-S phase. For this purpose, a simple self-induced method to effectively regulate the Co-substituted S sites and subsequent generation of Co-Mo-S phase was reported. The Co-Mo-S phase with atomically dispersed Co on surface displayed outstanding performance in lignin-derived phenols HDO. In addition, this work will be an important aspect to the relationship between different sulfide phases and phenols HDO performance.

RESULTS AND DISCUSSION Characterization of the catalysts


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The cobalt incorporated MoS2 nanoflowers are prepared via a two-step hydrothermal process, as shown in Scheme 1. MoS2 nanoflowers were obtained by autoclaving Mo precursor, elemental sulfur and hydrazine at 180 oC. After that, a certain amount of MoS2 nanoflowers and Co precursor were subjected to ultrasonic dispersion in DI water, and once more hydrothermal at 120 oC for 24 h to facilitate the adsorption of cobalt on MoS2 layer edges, finally resulting in highly dispersed CoMoS nanoflowers. The introduction of hydrazine was to undergo a generation of S vacancies for the Co incorporation and regulated the ion exchange for the subsequent Co-Mo-S phase growth.31-33 The MoS2, Co9S8 and Co9S8-CoMoS catalysts were synthesized in the same hydrothermal procedure. Besides, after mechanical mixing Co9S8 and MoS2, we obtained Co9S8-MoS2, as presented in Scheme 1. To better understanding the different phased for Co-doped MoS2 catalyst, structure and property of the as-synthesized samples have been characterized in detail.

Scheme 1 Diagram presentation for the synthesis of cobalt doped MoS2 nanohybrids. Figure 1 shows electron micrographs of the as-prepared MoS2, Co9S8-CoMoS and CoMoS catalysts. In all cases, the nano-sulfides exhibit a flower shape with a size


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about 200 nm. The hydrothermal synthesized sulfides with the nanoflower morphology are assembled from nanosheets. The type IV feature with noticeable hysteresis are observed on the as-prepared Co-doped MoS2 and MoS2, revealing the mesoporous nature of these catalysts (Figure S1). The CoMoS catalyst possesses a surface area of 128.9 m2/g and a pore volume of 0.31 cm3/g. As shown in Table S1, there is no considerable difference in texture properties noted among these samples. High resolution TEM images provide further insight into interior structure. As seen in Figure 1c, the obvious crystal lattices with interplanar distance of 0.64 and 0.30 nm correspond fairly to (002) planes of MoS2 and (311) planes of Co9S8, respectively. For the Co9S8-CoMoS sample, Co9S8 and MoS2 phase inevitably co-exist with direct hydrothermal sulfidation of Co and Mo, while the Co9S8 particles are always larger than 10 nm.34,

35

As a contrast, no corresponding Co sulfide form on the CoMoS

sample after the separated introduction of Co into MoS2 nanoflower. The lattices with interplanar distance of 0.16 and 0.27 nm observed in Figure 1f correspond to (110) and (100) planes of MoS2, respectively. Meanwhile, the observed bright rings in the corresponding selected area electron diffraction (SAED) pattern (Figure 1f, inset) can be indexed to (002), (100) and (110) planes of 2H-MoS2 by using comparative d spacings method and indicate the polycrystalline nature of the sample.


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Figure 1 (a) SEM image of the as-prepared MoS2 nanoflowers, TEM and HRTEM images of the (b, c) Co9S8-CoMoS and (d, e, f) CoMoS catalyst. The inset of (f) shows the corresponding selective area electron diffraction pattern.

Figure 2 (a) XRD patterns, (b) Raman spectra for the MoS2, Co9S8-MoS2, Co9S8-CoMoS and CoMoS catalysts. XRD patterns of the Co-doped MoS2 catalysts present three diffraction peaks at 2θ = 14o, 33o and 59o, attributing to the (002), (100) and (110) planes of hexagonal MoS2 (JCPDS 37-1492), respectively. The results suggest that MoS2 crystal structure fully retained after incorporated by Co atoms. It is noted that there appear some


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diffraction peaks at 2θ = 29 o, 47o and 51o, corresponding to Co9S8 crystal (JCPDS 19-0364), both on the samples prepared via direct hydrothermal and mechanical mixing.36 In comparison with previous literature that reported isolated MoS2 and Co sulfide coexisted in the hydrothermal sample,37, 38 the characteristic diffraction peaks of Co species (such as Co, Co9S8) have not been observed on the CoMoS sample, indicating that most of the Co were highly dispersed on the surface of MoS2 and accommodated in the “Co-Mo-S” phase. Furthermore, Raman spectroscopy was carried out to investigate the chemical state of the Co-doped MoS2, as shown in Figure 2b. Several characteristic Raman peaks arises (

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ACS Catalysis

Co9S8-CoMoS > Co9S8-MoS2 > MoS2. Among these compounds, the 4-O-5 bond in diphenyl ether has been identified as the strongest aryl ether bond with a bond dissociation enthalpy of 314 kJ mol-1,49 it is a challenge to cleavage the ether bonds under H2 atmosphere without subsequent hydrogenation of aromatic ring. Hence, the cleavage of Caryl-O-Caryl bond usually requires harsh conditions with a specific catalyst. For the CoMoS catalyst, the reactivity of these typical C-O bonds in decreasing order was: Caryl-OH bond in p-cresol, Caryl-OMe bond in anisole and Caryl-O-Caryl bond in diphenyl ether. Moreover, benzene or toluene is final product at these reaction conditions. HDO reaction with high hydrogen efficiency is concluded by low saturated hydrocarbon selectivity.50, 51 When the CoMoS catalyst is used, the arenes yield increases to over 98 % at 4 h for all the compounds, which exhibits a high-efficiency cleavage of C-O bands. Additionally, as shown in Figure S4, the reaction solution at 4 h still remain more phenol as an intermediate on the Co9S8-CoMoS and Co9S8-MoS2 as compared to that on CoMoS catalyst. These results indicate a high efficiency in both phenolic C-O bond and ether bond cleavage over the CoMoS catalyst. As a consequence, it seems imperative to take into account the conversion of guaiacol. OH H3C

H2

CoMoS

Sel.

OCH3

+ H3C

H3C 100

OH +

H2

O

CH3 +

0

Sel.

0.9

0.4

OH +

H2 CoMoS

CoMoS

98.7


Sel.

2.0

98.0

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Figure 6 Time courses of (a) p-cresol; (b) anisole; and (c) diphenyl ether conversion over the MoS2, Co9S8-MoS2, Co9S8-CoMoS and CoMoS catalyst. (Reaction conditions: 20 mg catalyst, 5 mmol reactant, 300 oC, 4 MPa H2. The selectivity of 4 h reaction time over the CoMoS catalyst is displayed with the reaction pathways.) Guaiacol consists of three different types of C-O bonds, including Caryl-OMe, Cmethyl-OAr, and Caryl-OH. The products distribution of guaiacol conversion over time were measured, as shown in Figure 7. As reaction time increasing, guaiacol concentration decreases and disappears in 4 h and 6 h on the CoMoS and Co9S8-CoMoS catalyst, respectively. The amount of intermediates, such as phenol and anisole, increase first and then decrease gradually. Besides, oxygen-free products like benzene and toluene increase finally.22 This indicates the occurrence of multi-step deoxygenation. It seems surprising that over 98 % yield of benzene and toluene can be obtained on the high-dispersed CoMoS catalyst. Regarding to this phenomenon, the amount of phenol improves distinctly with increasing reaction time and peaks at approximately 1 h for the CoMoS catalyst, which is much shorter than the time for the Co9S8-CoMoS catalyst (about 4 h). These results suggest that the hydrogenolysis of Caryl-OMe bond in guaiacol happen firstly during the reaction and faster on the CoMoS catalyst. Additionally, the intermediate of phenol is obtained primarily, and after accumulating to a certain amount, phenol concentration decreases and is totally deoxygenated to benzene until 5 h on the CoMoS catalyst. While the guaiacol conversion is relatively low and phenol is still presented as major product over the Co9S8-CoMoS catalyst at 5 h (Figure 7b), which indicates an incomplete reaction.19


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ACS Catalysis

To clearly illustrate the reaction mechanism on Co-doped MoS2 catalyst, rate constants of the individual phenol, anisole and guaiacol HDO are calculated based on pseudo-first order reaction and shown in Figure 7c along with their associated error. In this study, a higher rate constant of phenol is reached compared to that of anisole, which suggests that the cleavage of Caryl-OH is easier than that of Caryl-OMe. However, there is a noticeable difference between individual phenol, anisole and guaiacol HDO reactions. In guaiacol HDO, Caryl-OMe bond breaking is the favorable reaction pathways with phenol as main intermediates and very few anisole detected over the catalyst. Additionally, it is surprising that similar rate constants for anisole and guaiacol are displayed both over the CoMoS and Co9S8-CoMoS catalyst, implying that guaiacol HDO depends on the cleavage of Caryl-OMe. Besides, based on the analysis of product distribution, the phenol disappears faster on CoMoS catalyst compared with Co9S8-CoMoS catalyst. This can be explained by the almost three-fold rate constant of phenol conversion on CoMoS than that on Co9S8-CoMoS catalyst. In this case, a possible reaction pathway has been proposed. The conversion of guaiacol to arenes involves two main routes: the hydrogenolysis of Caryl-OMe bond and the direct HDO of Caryl-OH bond in guaiacol. The second stage comes up the HDO of phenolic bonds, along with transmethylation of Cmethyl-OAr group. Remarkably, there is almost no side product obtained during the conversion, such as ring hydrogenation. The proposed pathway is considerably distinct from that on noble metal catalysts, in which aromatic rings preferentially hydrogenated and then deoxygenation.52 Because of the difference, the as-prepared CoMoS catalyst results in a higher aromatics yield compared with noble metal catalysts.


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Figure 7 Composition of reaction mixture vs. reaction time for guaiacol HDO on the (a) CoMoS and (b) Co9S8-CoMoS catalyst, (c) corresponding pseudo-first-order reaction rate constant and reaction pathway. Reaction conditions: guaiacol (5 mmol), catalyst (20 mg), decalin (20 mL), 4 MPa H2, 300 oC.) After measuring the proportion of Co-Mo-S phase on those Co-doped MoS2 catalysts by XPS (Table S1), we correlate the initial HDO reaction rates with the proportion of Co-Mo-S phase in Figure 8. Obviously, increasing the Co-Mo-S proportion results in an increase in the HDO activity both for p-cresol, guaiacol and diphenyl ether. This is consistent with previous study by Antonio,53 which indicated that the concentration of Co-Mo-S phase in various materials mainly determined HDS activity. Hence, the proportions of Co-Mo-S phase dictate the catalytic behavior. Additionally, an exponential growth of HDO reaction rates in those three different biomass-derived oxygenates is displayed, instead of a linear growth. This can be ascribed to the spatial distribution of Co-Mo-S phases, where most of Co-Mo-S phases locate on the top surface of MoS2. On the other hand, different exponents defined by the exponential growth are demonstrated, indicating the significant difference on the growth of HDO rate. That is, p-cresol, guaiacol and diphenyl ether show distinct reaction sensitivity to Co-Mo-S active site. Therefore, as indicated in Figure 8, the hydrodeoxygenation of phenolic hydroxyl on Co-doped MoS2


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ACS Catalysis

catalyst is the most structure sensitive. Meanwhile, the cleavage of Caryl-O-Caryl is relatively less structure sensitive than other two compounds. As is well known, a “steric hindrance” effect with a large molecule dibenzothiophene is present over unsupported catalysts in the HDS reaction, so it should be suitable for the diphenyl ether HDO reaction.54, 55

Figure 8 Comparison of intrinsic activities obtained for p-cresol, guaiacol and diphenyl ether HDO with respect to Co-Mo-S phase proportion determined by XPS for the Co-doped MoS2 catalysts. Selective hydrodeoxygenation of substituted phenols To explore the scope of application of this Co-doped MoS2 with high dispersed Co-Mo-S phase, the HDO of representative substituted phenolic compound was further studied, which were generally depolymerized from lignin, and the reaction results are showed in Table 1. The substrate conversion and arenes yield decrease with the growth of carbon atoms.56 In most cases, the CoMoS catalysts exhibit high selectivities (> 90%) towards aromatics, with a complete conversion of lignin-derived phenols. Compared with phenol and p-cresol, the time for full deoxygenation of ether compound to product arenes is longer. Besides, the HDO of catechol is more difficult due to the poor solubility of substrates in decalin.57 For guaiacol HDO,


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yields of 85.6 % benzene and 10.4 % toluene are achieved because of the transmethylation of -CH3 with benzene ring. Comparatively, when 4-methyl- or 4-ethyl-substituted guaiacol is used as substrates, a prolonged reaction time is necessary for a maximum conversion and a high aromatic yield, while the yields of aromatic hydrocarbons are lowered respectively to 93.5 % and 84.8 %, indicating that the rates for cleaving the phenolic-OH and Caryl-OMe of phenolic compounds are reduced. This is consistent with the experimental result which toluene yield (86.7 %) is lower than ethylbenzene (70.4 %) under the same reaction conditions. It takes minimum 6 h to convert more than 90 % of syringol, and the arenes yield can reach the level of 90 % extending reaction time to 12 h. The reusability of the high-dispersed CoMoS catalyst is investigated for the HDO of mixed phenolic compounds to arenes, as plotted in Figure 9. A mixture of five kinds of phenolic compound (anisole, guaiacol, syringol, etc.) are catalytic hydrodeoxygenated to obtain aromatic hydrocarbons including C6 benzene (74.6 % yield), C7 toluene (0.1 % yield) and C8 xylene (25.3 % yield) at 300 oC with 8 h, as shown in Figure S5. Overall, the catalyst still maintains its activity for the HDO of mixed phenols, giving mixed phenols conversion of over 95 % during all the recycle runs. The compositions of reaction products are mainly BTX. However, the proportion of toluene and phenols increased while that of benzene and xylene decreased accordingly with the increase of cycle number. No indication of catalyst significant deactivation was observed, which distinguished the CoMoS catalyst from Pt/Al2O3 catalyst Ni2P/SiO2 catalyst

59

58

and

that exhibited a decreasing m-cresol conversion of ∼15 %


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ACS Catalysis

in 4 h at 250 oC and a decreasing guaiacol conversion of ∼40 % in 10 h at 300 oC,

respectively.

Table 1 The difference in the lignin-derived phenols conversion over the CoMoS catalyst. Product yiled (%) Substrate

R

t (h)

Phenols (%) R=CH3,CH2CH3

OH

2

100

~

~

~

3

~

100

~

~

4

98.7

0.4

~

0.9

4

98.0

~

~

2.0

OH

7

99.2

~

~

0.8

OH

5

85.6

10.4

~

4.0

6

0.2

86.7

6.6

6.5

6

~

0.6

84.2

15.2

12

62.7

17.0

11.6

8.7

OH

OCH3

O

OH

OCH3

OCH3 OH

OCH3 OH

OCH3 OH

OCH3

Reaction conditions: substance (5 mmol), CoMoS catalyst (20 mg), decalin (20 ml), 4.0 MPa H2, 300 oC.


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Figure 9 Recycling experiment for the mixed phenolic compounds HDO on the CoMoS catalyst after 8 h.

CONCLUSIONS A surface atom engineering strategy was employed to design an efficient CoMoS nano-sulfide for the HDO of lignin-derived phenols to aromatics. This self-induced method effectively regulated the Co-substituted S sites, led to a well incorporation of Co atoms onto the top surface of MoS2 nanosheets maintaining its original morphology, followed by successful generation of accessible Co-Mo-S phase. The results reported here may provide inspiration for the surface engineering of transition metal doped MoS2 nanomaterials. The as-prepared CoMoS nano-sulfide achieved a remarkable improvement in the HDO performance. For the diverse lignin-derived oxygenated compound, the deoxygenation of pheglic hydroxyl over Co-doped MoS2 catalyst was structure sensitive relative to diphenyl ether. Additionally, the Co-Mo-S phase was mostly distributed on the top surface of MoS2, which result in a full deoxygenation performance over the CoMoS nano-sulfide. Further catalytic application demonstrated that the CoMoS nano-sulfide also exhibited high yield (>85%) and stable recyclability in selective conversion the phenolic compounds to their corresponding fully deoxygenated aromatics such as BTX.


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ACS Catalysis

EXPERIMENTAL Reagents and materials: All reagent-grade chemicals were employed without further purification. Co(NO3)2·6H2O, (NH4)6Mo7O24·4H2O, phenolic compounds (phenol, p-cresol, guaiacol, anisole, diphenyl ether, 2-methoxy-4-methylphenol, syringol and 2-methoxy-4-ethylphenol) were obtained from Shanghai Aladdin Reagent. Elemental sulfur, decalin and N2H4·H2O (85 wt% in water) were purchased from Sinopharm Chemical Reagent. Preparation of Catalysts The MoS2 nanoflowers sample was prepared via a previously reported hydrothermal procedure.60 High-dispersed Co doping MoS2 catalysts were prepared by Co-substituted S sites on the MoS2 nanomaterials with hydrothermal method. Typically, 50 mg MoS2 nanoflowers was added into 59 mL Co(NO3)2·6H2O (46 mg) aqueous solution (Co/Mo = 0.5, molar ratio). An ultrasonic treatment of the mixture was carried out for 10 min, after that, 1 mL of hydrazine hydrate was dissolved in the solution. Then, the mixture was transferred into a 100 mL Teflon-lined autoclave and then kept at given temperatures (120 oC) for 24 h. The resultant black precipitate was filtered out, washed for several times with water and absolute ethanol, and then dried overnight in a vacuum oven at 80 oC. This high-dispersed CoMo nano-sulfide was designated as CoMoS. For comparison, the one-pot hydrothermal synthesized sample with Co and Mo precursors was also prepared by the same procedure (denoted as Co9S8-CoMoS), as well as the individual Co9S8 sample. The mechanical mixed and ground sample of Co9S8 and MoS2 was denoted as Co9S8-MoS2.


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Catalyst Characterization X-ray powder diffraction (XRD) patterns were taken on a Rigaku Ultima IV powder diffractometer with a Cu Kα radiation source (λ = 1.5406 Å, 40 kV, 100 mA). SEM images were acquired using a field emission scanning electron microscopy (SIGMA Zeiss, Germany). Textural properties were characterized by nitrogen adsorption-desorption using a Micromeritics Tristar 3020 instrument at liquid nitrogen temperature. Raman spectra were collected on a Renishaw Invia Raman spectrometer randomly, using a Spectra-Physics Excelsior CW solid state laser (λ = 532 nm) as excitation source. Transmission electron microscopy (TEM) images and SAED patterns were acquired at an accelerating voltage of 300 kV (FEI Tecnai 30, Philips). STEM experiment was performed with a FEI Titan Cubed G2 60-300 aberration corrected S/TEM operated at 60 kV accelerating voltage and a convergent semi-angle of 22 mrad. Electron energy-loss spectra (EELS) were recorded using a Gatan 966 EEL spectrometer (collection aperture, 2.5 mm; camera length, 29.5 mm). X-ray absorption fine structure (XAFS) spectra at Co K-edge were recorded in transmission mode, using a Si (111) double-crystal monochromator and BL14W1 beamline (3.5 GeV, maximum current of 260 mA) from Shanghai Synchrotron Radiation Facility (SSRF). Co foil with edge energy of 7709 eV was measured for energy calibration. All spectra were taken in ambient conditions and aligned in X-ray energy first. IFEFFIT package was used for X-ray

absorption

coefficient

data

analysis.61-63

The

averaged

and

background-removed spectra were then normalized using Athena61 program. X-ray photoelectron spectroscopy (XPS, PHI Quantum-2000) was operated under ultrahigh vacuum (2 × 10-9 mbar). After charging by C 1s peak (284.6


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ACS Catalysis

eV), the corresponding spectra were fitted by a combination of the Gaussian and Lorentzian functions with a XPSPEAK software. X-ray fluorescence spectroscopy (XRF) measurements were performed on a S8 TIGER (BRUKER) instrument at 60 kV and 50 mA. High-sensitivity low-energy ion scattering (HS-LEIS) spectra were collected on an Ion-TOF Qtac100 instrument equipped with helium as the ion source (kinetic energy, 3 keV; ion flux, 6000 pAm-2; spot size, 2 mm × 2 mm). Catalyst Activity Measurement The HDO tests of phenols (phenol, p-cresol, anisole or diphenyl ether) were carried out in a 50 ml stainless steel autoclave. Typically, 5 mmol of reactant, 20 mg of catalyst and 20 ml decalin solvent were introduced into the reactor. After the air was removed by repeatedly purging with nitrogen gas, the reactor was heated to 300 oC with a stirring speed of 1000 rpm and then pressurized up to 4.0 MPa with H2. H2 pressure remained stable in the whole reaction. A small volume of liquid product was collected at regular time intervals. Quantitative analysis of liquid products was carried out by gas chromatography (flame ionization detector; capillary column, HP-5). The conversion and selectivity for each experiment were evaluated as follows. HDO kinetic constant was evaluated by assuming pseudo-first order reaction with respect to phenolic compound concentration,29, 64, 65 considering that excessive hydrogen was fed to keep its partial pressure constant: Conversion  mol %  

moles of phenolic model consumed 100% moles of phenolic model initially charged

(1)

moles of product ( A) 100% moles of reacted phenolic model

(2)

Selectivity  A, mol %  


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ln 1  conv.  kt

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(3)

where conv. is the conversion of phenolic compound, k and t represent the rate constant (min−1) and reaction time (min), respectively. The rate constants are generally determined as slopes by linear fitting the different -ln(1-conv.) vs. t. All R2 values for linear fit are >0.98 within low conversion (

Surface Engineering of CoMoS Nanosulfide for Hydrodeoxygenation

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