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Promotion of the inactive iron sulfide to an efficient hydrodesulfurization catalyst Hao Li, Jinjia Liu, Jiancong Li, Yongfeng Hu, Wennian Wang, Delin Yuan, Yandan Wang, Tao Yang, Lei Li, Houxiang Sun, Shenyong Ren, Xiaochun Zhu, Qiaoxia Guo, Xiaodong Wen, Yongwang Li, and Baojian Shen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 9, 2017

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ACS Catalysis

Promotion of the inactive iron sulfide to an efficient hydrodesulfurization catalyst Hao Li1‡, Jinjia Liu2,3,4‡, Jiancong Li1, Yongfeng Hu5, Wennian Wang1, Delin Yuan1, Yandan Wang1, Tao Yang1, Lei Li1, Houxiang Sun1, Shenyong Ren1, Xiaochun Zhu1, Qiaoxia Guo1, Xiaodong Wen2,3, Yongwang Li2,3, Baojian Shen1,*

1

State Key Laboratory of Heavy Oil Processing; The Key Laboratory of Catalysis of CNPC;

College of Chemical Engineering, China University of Petroleum, No. 18 Fuxue Road, Changping, Beijing 102249, China

2

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry Chinese Academy of

Sciences, No. 27 South Taoyuan Road, Taiyuan 03001, China

3

Synfuels China Co. Ltd., No. 1 Leyuan Second South Street, Huairou, Beijing 100195, China

4

University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China

5

Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK, Canada

ACS Paragon Plus Environment

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ABSTRACT

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Extensive efforts have been devoting to develop desulfurization

catalysts to effectively remove sulfur from fuel. Active phase metals including cobalt, nickel, molybdenum, and tungsten are extensively used in industry for hydrotreating/hydrodesulfurization catalysts for over 50 years. However, while it is desirable to use inexpensive materials to do the same job, it is a grand challenge. Herein, we report a Fe-based sulfide catalyst that is tuned by zinc with high activity for HDS, which shows an industrial application potential to replace industrial Mo-based catalysts. With an optimal configuration that has a Fe:Zn ratio close to 1:1, the

reaction

rate

constants

of

the

dibenzothiophene

(DBT)

and

4,6-dimethydibenzothiophene (4,6-DMDBT) HDS are increased by 9.2 and 17.4 times, respectively, compared with the sum of that on the mono-iron and zinc sulfides. HDS activity for the sterically hindered 4,6-DMDBT on the FeZn sulfide catalyst is even close to that of Co-MoS2. The experimental results indicate that the addition of Zn greatly modifies the electronic properties of iron sulfide by transferring electrons from Zn to Fe, which tunes the d band center to modulate the adsorption behavior of DBT and 4,6-DMDBT. Combined with theoretical calculations, our experiments show that the addition of Zn dramatically tunes the formation of sulfur vacancies. We propose that the formation of sulfur vacancies is the critical factor for designing highly efficient Fe-based sulfide catalysts. This study provides the design principle of low cost desulfurization catalysts for industrial refinery applications.

KEYWORDS iron sulfide; iron-zinc bimetallic sulfide; synergetic effect; sulfur vacancies;


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hydrodesulfurization

1. INTRODUCTION Combustion of transportation fuels such as diesel and gasoline has been confirmed as one of the major sources of SO2 which are notorious for air pollution and acid rain. In order to protect the environment, almost all countries have introduced strict regulations to limit the release of sulfur into the atmosphere from the usage of fuels. Therefore, the hydrotreating processes that remove sulfur-containing molecules efficiently from residue, diesel or gasoline fractions, are vital for human beings in the foreseeable future.1,2 As a crucial factor of this technique, classic Co (Ni)-Mo (W) S2-γAl2O3 catalysts have been widely used over 50 years.3 Substantial research efforts have been made towards continuous improvement of the intrinsic catalytic activity of these catalysts in the last decades. To date, Co(Ni)Mo(W)S phase has been widely accepted as the high HDS activity species, and the coordinatively unsaturated active site (CUS), which is composed of the Co, Mo and S atoms on the edges, is the crucial species that is actually active in HDS.4 By dissociating H2 (H2S) and breaking C-S bond, CUS is general agreed as direct desulfurization active sites.5 However, for hydrogenation pathway, which is very important in sterically hindered sulfur-containing molecules, its active sites still remain controversial. By using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, many researchers supposed that the metallic “brim sites” is the active sites of hydrogenation.6–8 Nevertheless, recent study proposed that the corner site, one of the three typical CUSs also plays a role in hydrogenation pathway by reduce steric hindrance from adsorbing sulfur intermediates.9 Although more deeply understanding of the


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active sites is needed, there’s no doubt that by improving Co-Mo interaction (e. g. new preparation method such as molecular approach-controlled surface chemistry process or the addition of organic additives), the catalytic performance could be promoted.10,11 However, the shortages of supplies and the environmental toxicity during mining of traditional Co, Mo, Ni, W metals limit their industrial applications.12 Iron, a cheaper and widely available transition metal, has received more and more attentions. In the pioneering work by Pecoraro and Chianelli in the early 1980s, iron sulfides could catalyze the hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT), though its activity was definitely poor.13 Similar conclusions were obtained from the researches on the alumina supported TMS.14 Meanwhile, several research groups studied the promoter effect of Fe on the HDS activity of MoS2,15,16 and a FeMoS phase,16–18 which has a similar structure with CoMoS phase, was proposed to be responsible for the weak synergetic effect of FeMo catalyst. Other combinations such as FeW19, FeV,20 and FeNi21 were also explored, it was found that Fe could incorporate easily with above secondary metals, leading to the formation of FexWyC/N and pentlandite species, and improving the dispersion of V active sites. However, in these cases, the synergetic effect between Fe and the secondary metal was not remarkable, particularly when compared against the high activity of the MoS2 or CoMoS2 catalysts due to a lack of a fundamental understanding on what is the critical factor to design Fe-based catalysts. Meanwhile, it is accepted that by introducing the promoter metals, the electronic state or morphology of catalysts can be changed.22–25 Especially the former, which can be interpreted as the redistribution of charge in a metal, or charge transfer between two metals, which can


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modify the catalytic properties via changing the mode and adsorption strength of the reaction participants.26 As a base metal like iron, zinc was found could significantly promote the catalytic performance of Pd catalysts in ester and alkene hydrogenation reactions. 27,28 In these studies, the formation of PdZn alloy was found to be the critical factor in improving catalytic activity and stability. And these improvements were credited to electronic effects caused by Zn, which leading to the weakening of the Pd–adsorbate bond. Although it was found that zinc sulfide is almost inactive in HDS reaction,14 Thomas et al. still reported a promoted effect of Zn on HDS reaction of gas oil over CoMo/Al2O3 catalyst. The increased activity was accounted for that Zn could enhance the bond between Co and Mo, and thus able to form more active CoMoS sites.29 Even in traditional CoMoS system, synergetic effect between Co and Mo could be contributed to the electronic effects between them, i. e. strong electron donation from Co to Mo leading to an enhanced metallic character and greatly weakened Mo-S bond.26 We are inspired by this to combine both inexpensive Fe and Zn to develop low-cost and high-activity catalysts for HDS. In this work, we prepared a series of zinc-promoted iron sulfide catalysts with different Zn/(Fe+Zn) atomic ratios through an incipient wetness impregnation method and followed by presulfiding procedures. We find that the addition of Zn dramatically tunes the catalytic performance of the Fe-based catalyst. When the ratio of Zn/Fe is close to unity, the highest 4,6-dimethydibenzothiophene (4,6-DMDBT) HDS catalytic activity obtained is close to that of the CoMoS2. With the combination of experimental and theoretical work, we aim to build a bridge

between

fundamental

research

and

industrial

application

on

Fe-based

hydrodesulfurization catalyst, and extract the design principle for such catalyst systems.


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2. EXPERIMENTAL 2.1 Materials Pseudo-boehmite (surface area 428 m2 g-1, pore volume 1.11 cm3 g-1) was purchased from Hejin Juhua Aluminum Company Ltd., People’s Republic of China. Metal precursors (Fe(NO3)3·9H2O, Zn(NO3)2·6H2O, Co(NO3)2·6H2O, and (NH4)6Mo7O24·4H2O), carbon disulfide (CS2) and n-decane were obtained from Tianjin Guangfu Chemical Reagent Company Ltd., People’s Republic of China. DBT (99.0%), 4,6-DMDBT (97.0%), ZnS (97.0%), FeS2 (99.0%), FeS (95.0%) and MoS2 (99.0%) were purchased from Sigma-Aldrich. The gases employed were H2 (Beifen Gas 99.999%), N2 (Beifen Gas 99.999%), Ar (Beifen Gas 99.99%), and He (Beifen Gas 99.99%). 2.2 Supports and oxide catalysts preparation The mono Fe, Zn, and reference Mo oxide catalysts were prepared by following steps. Firstly, the pseudo-boehmite was shaped by extrusion to form cylindrical extrudates, and they were dried at 393 K for 12 h and calcined at 773 K for 4 h to obtain Al2O3 support. Secondly, a calculated amount of metal precursor was dissolved in deionized water and added onto the Al2O3 extrudates with single impregnation to achieve the desired metal oxide loadings of 8 wt. % and 20 wt. % (calculated for Fe2O3 and MoO3) for Fe and Mo, respectively, and 12.25% for Zn (calculated for ZnO), then dried at 393 K for 12 h, followed by calcination at 673 K for 4 h (donated as FeOx, MoOx, and ZnOx, respectively). A series of various Zn/(Fe+Zn) atom ratio (0.2, 0.4, 0.6, and 0.8) samples with Fe loading fixed at 8 wt. % (calculated for Fe2O3) were prepared by a two-step sequential incipient wetness


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technique (denoted as Fe0.8Zn0.2Ox, Fe0.6Zn0.4Ox, Fe0.4Zn0.6Ox, and Fe0.2Zn0.8Ox, respectively). Specifically, a Fe(NO3)3·9H2O aqueous solution was first impregnated on Al2O3 support, dried at 393 K for 12 h, and calcined at 673 K for 4 h, then the Zn(NO3)2·6H2O aqueous solution was impregnated on above Fe2O3/Al2O3, followed by the same drying and calcination procedures. The reference CoMo catalyst was also prepared by a same two-step sequential incipient wetness method (Mo first and Co second). The details of the samples, the related notations, and texture properties are summarized in Table S1.

2.3 Characterization XRD patterns were collected on a PANalytial X’Pert Powder equipped with a Cu Kα X-ray source operating at 40 kV and 40 mA. Step-sizes of 0.013 and accumulation times of 0.8–1.0 s were used during the scanning. Nitrogen physisorption measurements were performed at 77 K with a Micromeritics ASAP 2010 system. The total surface areas (SBET) were determined by the Brunauer–Emmett– Teller (BET) model methods. The total pore volumes (Vp) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.98. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher K-Alpha photoelectron spectrometer with a monochromated A1 anode. The charging effect was minimized by using a charge neutralizer. All measured values of binding energies (BE) were determined with respect to the C 1s line at 285.1 eV. In order to avoid the re-oxidation of sulfide samples, the sample wafers were prepared in a glove-box which was connected with the XPS instrument. High-resolution transmission electron micrographs were acquired using JEOL ARM200F


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transmission electron microscope with a probe equipped with a spherical aberration corrector. The operation voltage and space resolution of the microscope are 200 keV and 0.08 nm. The high-angle annular-dark-field (HAADF) images were acquired with the illumination semi-angle of 25 mrad and a probe current of 100 pA. Energy dispersive X-ray spectroscopy (EDX) line scan was performed to locate elemental distribution of Fe, Zn, and S with an SDD-type EDX detector. The attainable energy-resolution of the EDX detector is 128 eV. Fe, Zn, and S K-edges were used for elemental mapping of these species. Temperature-programmed reduction of sulfides (TPR-S) was performed using the Micromeritics Autochem 2920 apparatus. In a typical run, 50 mg of 40-60 meshes sulfide sample was loaded into the quartz tube in the glove-box to avoid oxidation. Then it was pretreated in a pure Ar flow (40 mL min-1) at 673 K for 2 h, and cooled down to 323 K. The test was conducted in flowing 10 vol. % H2 balanced by Ar (50 mL min-1), with a ramp rate of 10 K min-1 and an ending temperature of 1123 K. The H2S effluent (start mass 34) for each test was analyzed with a Hiden QGA online mass spectrometer instead of the TCD detector to eliminate the influence of impurities and H2 fluctuation. We use known amount of pure MoS2 powder as reference to perform the quantification of released H2S, according to the reported method.30 Under our reduction condition, total H2S released amount of MoS2 was in fair agreement with that required for complete reduction to Mo metal at 1100 K. The Fe K- and S K-edges XANES of the various samples were performed at SXRMB beamline, and the Zn K-edge XANES were recorded at IDEAS beamline, Canadian Light Source (Saskatoon, Canada). The SXRMB beamline used an Si(111) double crystal monochromator to cover an energy range of 2-10 keV with an energy resolution ∆E/E = 1 × 10-4.


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Ge(220) crystals were used at the IDEAS beamline. All data were collected in fluorescence and total electron yield modes at the SXRMB and in fluorescence and transmission modes at the IDEAS. Our XANES characterization was performed ex-situ. In order to avoid the re-oxidation of sulfided catalysts, the treated samples were prepared inside a glove-box by sealing the sample with Kapton tape for transmission measurement at the IDEAS, or the powder sample was spread onto the double-sided, conducting carbon tape and quickly loaded into a vacuum chamber at the SXRMB for the S and Fe K-edge measurement. The energies of Fe K- and Zn K-edges were calibrated by defining the first inflection point on the rising part of the XANES spectra for Fe and Zn reference foils. The XANES spectral normalization was performed using Athena software.

2.4 Catalytic activity evaluation The HDS activity tests of DBT or 4,6-DMDBT were carried out in a continuous flow, fixed bed stainless-steel micro-reactor (420 mm length and 8 mm inner diameter). A 1.0 g catalyst was loaded and sandwiched by two layers of quartz sand. Prior to the reaction, the catalyst was pre-sulfided in situ under 4.0 MPa and 633 K for 4 h, the sulfiding feed was 10 wt. % CS2 in cyclohexane solvent. Then the oil feed containing either 0.58 wt. % DBT or 0.67 wt. % 4,6-DMDBT (with sulfur content of 1 000 µg g-1) in a n-decane solvent was pumped into the reactor. HDS reaction was carried out under the conditions of temperature 593-613-633 K, a weight hourly space velocity (WHSV) of 8.4 h-1 to 33.2 h-1, a total pressure of 4.0 MPa, and a H2/hydrocarbon volumetric ratio of 300. After a stabilization period of 4 h, the reaction products were collected and analyzed by an Agilent7890 GC equipped with an MS 80 mass


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spectrometer. By assuming a pseudo-first-order reaction for the HDS of DBT or 4,6-DMDBT, the catalytic activity can be expressed by the Equation1:

= − ln (1 − x)

(1)

where x is the total conversion of DBT or 4,6-DMDBT, F is the molar feed rate of DBT or 4,6-DMDBT in mol h-1, m is the catalyst mass in kg, and kHDS is the rate constant of HDS in mol kg-1 h-1. The HDS performance of catalysts was also evaluated using a Dagang straight-run diesel with density 0.823 g cm-3 and sulfur content 641 µg g-1. The reaction was conducted in the same reactor used for the above model compounds. A 4.0 g catalyst was loaded to adjust the low WHSV. Before assessment, the catalyst was in-situ presulfided at 633 K for 6 h. After sulfidation, the feed was switched to diesel, the reaction was performed under conditions of 643 K temperature, 6.0 MPa total pressure, 1.0 h-1 WHSV and 700 H2/hydrocarbon volumetric ratio. The liquid products after 20 h were collected to ensure a steady state. The product was collected every 20 h and the total reaction time was 160 h. Sample analysis was carried out in a Zhonghuan RPP 2000SN sulfur–nitrogen analyzer. 2.5 Computational details In our calculations, the cubic FeS crystal structure which has PNMA symmetry was selected from Inorganic Crystal Structure Database (ICSD),31 which contains 4 Fe atoms and 4 sulfur atoms. After the optimization, the lattice constant was a=5.185 Å, b=3.317 Å, and c=5.582 Å. For different ratio FeZn sulfides crystal structure, we created a Fe16S16 supercell and replaced the Fe atoms with Zn using random search with Site Occupancy Disorder (SOD) code32 (Figure S1).


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ACS Catalysis

All our calculations were carried out by the Vienna Ab Initio Simulation Package (VASP),33,34 with the frozen-core projector-augmented wave (PAW) method.35 The generalized gradient approximation in the Perdew-Burke-Ernzerh (GGA-PBE) function was employed for the exchange-correlation energy. A cutoff energy of 400 eV was employed for the plane-wave expansion. For bulk, all the atoms were relaxed to their equilibrium positions when the charge in energy on each atom between successive steps was converged to 1.0× 10-5 eV/atom, the force on each atoms was converged to 0.01 eV/Å, a 2×3×3 k-points was used. For calculations of adsorption on surface model, FeS(101) and ZnS(100) were choose to simulate the hydrogenation (HYD) pathways. We selected three layers of atoms and created (2×3) supercells, for FeS(101) there are 72 atoms and for ZnS(100) slab, there are 60 atoms. A vacuum of layer of 15 Å was used to exclude the influence of vertical periodic images and a Monkhorst–Pack grid of (2×2×1) k-points was utilized. The atoms of upper two layers were relaxed to their equilibrium positions, meanwhile the lower one layers were fixed at their optimized bulk positions. To simulate the S vacancies, we create a S atom vacancy in the perfect crystals and the S vacancies formation energy was calculated as shown in Eq.2: = + − −

(2)

Ef, ES-V, EH2S, EH2, and E are S vacancies energy, the energy of crystal with one S atom vacancy, energy of H2S molecular, energy of free H2 molecular, and the energy of perfect crystal, respectively. For adsorption energy calculations, they were computed by subtracting the energies of the gas-phase molecular DBT/4,6-DMDBT and pure surfaces from the energy of the


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optimized adsorbates/surface complex, as shown in Eq.3 = − −

(3)

3. RESULTS AND DISCUSSIONS 3.1 Structure of FeZn bimetallic sulfide To obtain a picture for the synthesized FeZn catalysts, we used scanning transmission electron microscopy (STEM) imaging complemented by energy dispersive X-ray (EDX). The chemical distributions of the particles were investigated with EDX mapping; the representative STEM images of the sample, Fe0.4Zn0.6Sx are shown in Figure 1. From Figure 1a and Figure 1b, the particles are confirmed to contain iron, zinc, and sulfur, indicating a good mixture of the three elements. More images for other catalysts are shown in Figure S2 through Figure S4. The overlapping of Fe, Zn, and S suggests the formation of bimetallic sulfides instead of a mixture of separated iron and zinc species. Furthermore, the line scans of different samples were also performed (see Figure 1c and Figure 1d), and confirm that a relatively uniform elemental distribution at the nanoscale is obtained for this sample, as well as in other samples synthesized (Figure S5 to Figure S7). XRD patterns of FeSx, ZnSx and FeZn sulfide samples are shown in Figure S8. The XRD patterns of all samples demonstrate the existence of γ-Al2O3 (Jade 9.0 PDF No. 48-367), and ZnS phase (Jade 9.0 PDF No. 03-062-1691) in ZnSx sample. The XRD analysis of the prepared samples did not indicate the presence of any iron sulfides, since the amorphous-like structure and/or the crystallites were too small (Figure S2 to Figure S7) to generate XRD signals. However, for the relatively high Zn content samples (Zn/(Fe+Zn) ≥ 0.4), with the


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increasing Zn content, ZnS-like patterns appeared and magnified gradually. But when we take a zoomin view on 32-48° area, a slight shift (~0.1°) towards lower angle could be observed in FeZn sulfide samples, compared with ZnSx, indicating the change of the crystal structure of zinc-containing samples. We searched PDF-ICSD database and found the angle of FeZn sulfide is very similar to a ferroan sphalerite (Jade 9.0 PDF No. 01-089-4939), which has a different structure from iron sulfides or zinc sulfide. Nevertheless, more evidences are needed to confirm the existence of FeZn bimetallic sulfide, since the changes observed in XRD are unremarkable.


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Figure 1 a, HAADF-STEM image for the Fe0.4Zn0.6Sx sample. b, EDS Mapping of Fe (red), Zn (green), and S (blue). c, HAADF-STEM image for the Fe0.4Zn0.6Sx sample. d, EDS line scan of Fe (red), Zn (green), and S (blue).

To provide better understanding of the surface structure of the FeZn catalysts, XPS characterization was performed. Figure 2a presents the Fe 2p XPS spectra of different Zn/(Fe+Zn) atomic ratio sulfide catalysts. For the mono-iron sulfide sample (FeSx), two separate 2p3/2 peaks which have BE values of 710.6 and 706.9 eV could be observed, which are consistent with the positions of FeS and FeS2,36,37 respectively, suggesting a full sulfidation of the surface Fe. With the addition of zinc, a new peak at 709.0-709.2 eV BE


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ACS Catalysis

appears. Meanwhile, the features corresponding to FeS drastically decrease in intensity. With an increase in the concentration of Zn, the intensity of the new peak increases progressively, suggesting that the addition of zinc greatly modifies the chemical state of Fe. Karroua et al. reported similar phenomenon in a sulfided FeMo catalyst,18 the authors designated a 708.9-709.1 eV BE feature to a FeMoS structure as result of interaction between Fe and MoS2. Meanwhile, if the new Fe species observed is due to the interaction between Fe and Zn, the chemical state of Zn is also expected to be modified. Indeed, a double peak could be observed in the Zn 2p2/3 XPS spectra of Zn/(Fe+Zn) catalysts (Figure 2b), especially for low Zn/(Fe+Zn) atomic ratio samples, i.e. 0.2 (Fe0.8Zn0.2Sx). The lower BE peak at 1022.0 eV can be assigned the ZnS phase38 (spectrum of sample ZnSx), but the 1022.5 eV peak is different from the possible zinc species such as ZnSO4 (1021.3 eV), Zn (1021.3 eV) and ZnAl2O4 (1022.0 eV)38. In addition, the higher zinc content leads to an overlapping of ZnS (1022.0 eV) peak and the new peak around 1022.5 eV. In the high zinc content sample Fe0.2Zn0.8Sx, ZnS becomes the predominant species, but a blue shift is still present, which could be distinguished from ZnS, indicating that the iron simultaneously influences the state of zinc owing to the formation of FeZn bimetallic sulfide, as observed in STEM.


ACS Catalysis

a

710.6 eV

Intensity (cps)

Fe 2p

706.9 eV Fe0.2Zn0.8Sx Fe0.4Zn0.6Sx Fe0.6Zn0.4Sx Fe0.8Zn0.2Sx FeSx

740

730

720

710

700

690

Binding Energy (ev)

b

Zn 2p3/2

1022.5 eV

1022.0 eV

ZnSx

Intensity (cps)

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

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Fe0.2Zn0.8Sx Fe0.4Zn0.6Sx Fe0.6Zn0.4Sx Fe0.8Zn0.2Sx

1032 1030 1028 1026 1024 1022 1020 1018 1016 1014 1012

Binding Energy (ev) Figure 2 a, Fe 2p XPS spectra of various catalysts. b, Zn 2p2/3 XPS spectra of various catalysts.


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Normalized absorption (a.u.)

1.2

a

Fe K-edge

0.8

0.4

Fe0.2Zn0.8Sx Fe0.4Zn0.6Sx Fe0.6Zn0.4Sx Fe0.8Zn0.2Sx

0.0

FeSx

7100

7110

7120

7130

7140

7150

7160

1st Derivative of Absorption (a.u.)

Energy (eV)

0.16

b

Fe K-edge FeSx

Fe0.4Zn0.6Sx

0.08

0.00

7100

7110

7120

7130

7140

7150

Energy (eV)


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1.6

c

Zn K-edge

1.2

0.8 Fe0.8Zn0.2Sx

0.4

Fe0.6Zn0.4Sx Fe0.4Zn0.6Sx

0.0

Fe0.2Zn0.8Sx ZnSx

9650

9660

9670

9680

9690

9700

Energy (eV)

Figure 3 a, Fe K-edge XANES spectra of various catalysts. b, The 1st derivative of the Fe K-edge XANES spectra for FeSx and Fe0.4Zn0.6Sx samples c, Zn K-edge XANES spectra of various catalysts.


ACS Catalysis

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The XANES results of Fe K- and Zn K-edge also further confirm the interaction between Zn and Fe. From Figure 3a, all four FeZn catalysts containing bimetallic sulfide exhibit observable differences in the shape compared to that of pure Fe sample. Furthermore, the 2.0 eV shifting of the first derivative of the Fe K-edge XANES for Fe0.4Zn0.6Sx (Figure 3b) compared with FeSx sample also indicates the different Fe chemical state of catalysts containing Zn. However, one can also argue that the shift of energy may be caused by the formation of more reduced mono-iron sulfides species. Thus, we compared the spectra of Fe0.6Zn0.4Sx, FeS2, and FeS (Figure S9), the last two are the classic reference compounds of S rich and Fe rich (FeS1-x) sulfides. As shown in Figure S9, Fe0.6Zn0.4Sx shows the lowest energy, which is even significantly lower than FeS. Therefore, we propose that the energy shift in FeZn samples are not the simple sum of different reduced mono-Fe sulfides. In order to take further understanding of this bimetallic sulfide, we tested the Fe K-edge XANES of chalcopyrite (CuFeS2), a Cu-Fe bimetallic sulfide, which has a photon energy lower than FeS239 (Figure S9). It’s not difficult to understand this phenomenon since Cu has a fully filled 3d orbitals (3d10), the electrons could transfer form Cu to Fe, which has half-filled d orbitals, and leading to a lower energy value shift of Fe K-edge.40 It is likely that the electron-rich state is more profound in the Fe of the FeZn bimetallic sulfide, since its Fe K-edge position is much lower than that of CuFeS2. And this could be related to the strong interaction between Fe and Zn (transfer of Zn 3d electrons to Fe orbitals). Meanwhile, a progressive change can be seen from Figure 3a, where the photon energies shift in the order of Fe0.2Zn0.8Sx < Fe0.4Zn0.6Sx < Fe0.6Zn0.4Sx < Fe0.8Zn0.2Sx, indicating that the more Zn, the more negative


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ACS Catalysis

charge on Fe. At the same time, for the Zn K-edge XANES (Figure 3c), all FeZn samples exhibit higher white lines compared with the pure Zn sample (ZnSx), which can be interpreted as the Zn in FeZn samples having a more positive charge. Since all the spectra were normalized to unity, we can compare the edge jump intensity directly, with an order of peak area: Fe0.8Zn0.2Sx > Fe0.6Zn0.4Sx > Fe0.4Zn0.6Sx > Fe0.2Zn0.8Sx, indicating that the more Zn content, the lower relative positive charges on zinc. In addition, from the computed Bader charge based on DFT (Table S2), with adding Zn, the delocalized electrons can be transferred from Zn to Fe, making the Fe electron rich. The XANES results on Fe and Zn indicate a direct interaction between Fe and Zn and the charge transfer effect, instead of a simple physical mixture of Fe and Zn sulfides. After evaluating metals, we performed a temperature programmed reduction of sulfides (TPR-S) to identify different Sx species. There are three regions related to different sulfur species marked in Figure 4, such as the low temperature region from 350 K to 650 K, the medium temperature region from 650 to 900 K, and the high temperature region (above 900 K). Pure Fe sample (FeSx) shows no peak in region I, but with overlapping double peaks in region II, including a peak at about 700 K and another about 770 K, represent two different Sx species bonded with iron. Compared with reference compounds spectra of FeS2 and FeS (Figure S10), also in agreement with the XPS results, we propose that they could be assigned to the reduction of FeS and FeS2, respectively. It is worth noting that the temperature of these two released peaks are lower than bulk FeS and FeS2, this is due to the high concentration of defects of the surface Al2O3, as observed in other research.41 Similarly, in the pure Zn sample (ZnSx), the dominate H2S release in region II (with a temperature of 800 K) is caused by the


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hydrogenation of Sx in ZnS (Figure S10), we suggest that the reduction in region III is due to the reduction of large ZnS particles-like species in the catalyst. This is corroborated by the increase of the peak in region III as the Zn loading is increased, indicating the increasing amount of bulk ZnS. From Figure 4, Region I only appears when Zn is added, which reaches the maximum in the sample of Fe0.4Zn0.6Sx. Hydrogenation of this Sx species occurs at the temperature (about 600 K) which is significantly lower than in the case of either FeSx or ZnSx. Above phenomena indicating a different kind of Sx species is formed when zinc was introduced. At least 100-200 K temperature difference between it and mono-iron or zinc sulfides suggests the state of this Sx species is rather different, i.e. the temperature of the peak maximum in region I shows that the chemisorption of S on the bimetallic sulfide is weaker than that on the monometallic Fe and Zn species. Similar results were also obtained which indicates the formation of FeMoS, CoMoS and NiMoS.18

41 42

In the studies of Co promoted MoS2 HDS catalyst, it was

concluded that reducing the surface sulfur species at low temperature could create coordinatively unsaturated sites (CUS) or so-called sulfur vacancies. More “Co–Mo–S” structures will generate more sulfur vacancies, resulting in a higher HDS activity.26,43–45 From our results, it is concluded that the sulfur vacancies are easily created in Region I, which is demonstrated to be responsible for the HDS activity in the studies of MoS2-based catalysts. Thus, the hydrogenation of Sx in Region I not only serves as an indicator for the formation of a Fe-Zn bimetallic sulfide species, but also to create sulfur vacancies.


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Region I

Region II

Region III

Fe0.2Zn0.8Sx Fe0.4Zn0.6Sx Fe0.6Zn0.4Sx Fe0.8Zn0.2Sx ZnSx

MS signal (a.u.)

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ACS Catalysis

FeSx

400

600

800

1000

Temperature (K) Figure 4 TPR-S-MS spectra of various sulfided Fe-Zn catalysts.

The S K-edge XANES further confirms this special sulfur species. From Figure 5a, in the pure iron catalyst, the features of FeS and FeS2 are visible (peaks around 2469 eV and 2471 eV), indicating a mixture of FeS and FeS2 (Figure S11). The pure Zn sample also manifests ZnS features. The peak at 2482 eV is a result of the S 1s electron to the triply degenerate unoccupied 6t2 orbital of sulfate,46,47 this sulfate peak is likely due to the surface oxidation of the samples during sample transfer and other handlings. We should note the sulfate peak is higher in FeZn catalysts, but the percentage of sulfate in all samples are relatively very low, since the transition probability of sulfate is much higher than in the reduced sulfur species because the S in sulfate has six vacant states. Notably, a significant change can be observed in samples with various Zn content: with the increasing amount of Zn, a new peak at about 2470.3 eV gradually appears with the decreasing FeS peak intensity, thus,


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a double peak structure is formed in all FeZn samples. Furthermore, this structure is most profound in Fe0.4Zn0.6Sx. It is reasonable to assign this signal to the easily reduced new Sx in TPR-S characterization. Thus, the S K-edge XANES spectra of a reduced Fe0.4Zn0.6Sx sample (named Fe0.4Zn0.6Sx-R, see Figure 5b) confirms the conclusion: the 2470.3 eV signal is significantly weaker than other Sx species, indicating the formation of S vacancies in FeZn catalysts is inevitable under reaction conditions.

Table 1 H2S evolution in TPR-S of various sulfide samples H2S evolution (10-3 mol H2S/g catalyst) Total

Region I

H2S evolution percentage of Region I (%)

FeSx

1.2

-

-

Fe0.8Zn0.2Sx

1.4

0.05

3.8

Fe0.6Zn0.4Sx

1.7

0.16

9.4

Fe0.4Zn0.6Sx

2.8

0.60

21.2

Fe0.2Zn0.8Sx

4.4

0.58

13.2

ZnSx

0.9

-

-

Catalysts

Furthermore, as shown in Table 1, the total H2S released amount is gradually increasing as the zinc content increased, since the total loading amount of metals is increased. Meanwhile, the H2S evolution percentage increases and reaches highest to 21.2 % in Fe0.4Zn0.6Sx sample and then decreases, this means the Fe0.4Zn0.6Sx catalyst released maximum amount of H2S, which is even higher than the highest metal loading amount sample, Fe0.2Zn0.8Sx. Above results indicate the reduction of Sx in Region I of Fe0.4Zn0.6Sx may generate more sulfur vacancies than other samples.


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a 4.0

2-

3.5

ZnS

SO4

2470.3 eV

FeSx ZnSx

3.0

Fe0.8Zn0.2Sx

2.5

Fe0.6Zn0.4Sx Fe0.4Zn0.6Sx

2.0

Fe0.2Zn0.8Sx

FeS

1.5 1.0 0.5 0.0

FeS2

2466 2468 2470 2472

2460

2480

2500

Energy (eV)

b Normalized absorption (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

ACS Catalysis

3.0 2.5

2470.3 eV

2.0 1.5 1.0 0.5 0.0

Fe0.4Zn0.6Sx

2466 2468 2470 2472

2450

Fe0.4Zn0.6Sx-R 2460

2470

2480

2490

2500

Energy (eV) Figure 5 a, S K-edge XANES spectra of various catalysts. b, XANES spectra for Fe0.4Zn0.6Sx and reduced Fe0.4Zn0.6Sx sample (named Fe0.4Zn0.6Sx-R) recorded at S K-edge. The reduction was performed ex-situ in home-made equipment with quartz tube. Catalyst loading was 100 mg, 10 vol. % H2 balanced by Ar (50 mL min-1) as reduction atmospheric, with a ramp rate of 10 K min-1 and an ending temperature of 573 K, the reaction time was 1 h.


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3.2 Catalytic activity From the above discussion, a Fe-Zn bimetallic sulfide, which is distinctly different from mono-metallic iron or zinc sulfides, is confirmed. Catalytic behavior of mono- and bimetallic Fe-Zn sulfide catalysts was investigated in HDS of DBT and 4,6-DMDBT, respectively. These two S-containing aromatic molecules were selected because they are representative for diesel fuel and, in addition, they have quite different reactivities toward two possible HDS pathways. The addition of Zn greatly changes the HDS behavior for these two model compounds, which will be discussed in following sections. 3.2.1 HDS of DBT The typical GC chromatograph of DBT HDS products is presented in Figure S12. According to molecular weight (MW) provided by MS, it’s not difficult to determine the HDS reaction products of DBT mainly include biphenyl (BIP), phenylcyclohexane (PCH), dicyclohexane (DIC), hydrogenated S-containing molecules tetrahydro-DBT (THDBT) and hexahydro-DBT (HHDBT), some isomer products mainly as benzylcyclopentane (Benzyl-CP) and cyclopentyl-cyclohexyl-methane (CP-CH-methane). In all cases, no other compounds were observed, the products on all of catalysts resemble that of reference MoS2 sample, it’s not difficult to understand this result since Fe, Zn and Mo sulfides are all transition metal sulfides (TMS), and may have a similar reaction mechanism. Figure 6a and Table 2 show DBT HDS activities on various catalysts at 633 K. Not surprisingly, FeSx exhibits low activity, which has a kHDS of only one-seventeenth compared with MoS2, the pure zinc catalyst (ZnSx) is almost inactive. Similar results were reported by Pecoraro and Quartararo et al..13,14 Noticeably, FeZn catalysts show strong synergetic effects and much higher activities than


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either the Fe or Zn catalysts, which has never been reported in previous studies. In particular, for the optimal Fe0.4Zn0.6Sx catalyst, DBT conversion reaches 88.3%, which is much higher than that of FeSx and ZnSx catalysts, at 17.8% and 3.7%, respectively. DBT conversion at 593 K and 613 K could be seen in Figure 6b, which also indicates a strong synergetic effect between Fe and Zn.

40

43.5

28

30 13.9

10 8.8

M o

2

x

S

0. 8 S x

n

0. 6 S x

n

0. 2 Z

Fe

Fe

0. 4 S x

0. 4 Z

n

x

8 Z n 0. 2 S Fe x 0. 6 Z

0.

40

39.3

37.4 36.7

30

24.6

20 10 4.97.1 0

2

S M o

n

n

0. 6 S x

n

0. 2 Z

Fe

0. 4 Z

0.

n

0. 8 Z

Fe

2 S x

x

S Fe

0. 4 S x

0.71.6

2

x

S

56.7

54.5

53 48.4

50

S M o

Zn

0. 8 S x

n

0. 6 S x

Fe

0. 2 Z

n

0. 4 Z

n

0. 4 S x

Fe

x

Fe S

0. 8 Z n 0. Fe 2 S 0. x 6 Z

0

60

n

20

66.9

0. 6 Z

40

74.9

593 K 613 K

70

Fe

4,6-DMDBT conversion (%)

60

Fe

Catalysts

80

HH(DH)DMDBT 4-MDBT Unidentified

Catalysts

Fe

Fe

d

D1

S

2.63.5

Fe S

2

x

M oS

0. 8 S x

Zn S

0. 2 Z

n

x

0. 6 S x

S

Conversion DMBIPs DMPCHs DMDICs THDMDBT

n

0. 4

Fe

n

80

0. 4 Z

x

Fe S

0.

Fe

100

48.3 38.9 37.5

0

0

c

55

50

20

62.3

60.1

60

Zn

20

70

67.8

x

40

79

593 K 613 K

S

60

80

0. 8 S Z x

b

HHDBT Benzyl-CP CP-CH-methane

Fe

80

Conversion BIP PCH DIC THDBT

DBT conversion (%)

100

8 Z n 0. Fe 2 S 0. x 6 Z

Conversion & Selectivity (%)

a

Conversion & Selectivity (%)

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Catalysts

Figure 6 Catalytic activities of various catalysts. a, Conversion and product distribution in HDS of DBT at 633 K temperature. b, Conversion of DBT at different temperatures. c, Conversion and product distribution in HDS of 4,6-DMDBT at 633 K temperature. d, Conversion of 4,6-DMDBT at different temperatures. Catalyst loading = 1.0 g, total pressure = 4.0 MPa, WHSV = 8.4 h-1 (in conversion evaluation) or 0.5 h-1 to 16.8 h-1 (to obtain


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products selectivity at about 50 % conversion, since the activity of ZnSx is too low, the BIP selectivity on ZnSx was obtained at about 20% DBT/4,6-DMDBT conversion), the oil feed containing either 0.58 wt. % DBT or 0.67 wt. % 4,6-DMDBT (with sulfur content of 1 000 µg g-1) in a n-decane solvent, H2/HC = 300 v/v.

Table 2 The catalytic performances of various catalysts in DBT and 4,6-DMDBT HDS reaction Catalysts DBT FeSx Fe0.8Zn0.2Sx Fe0.6Zn0.4Sx Fe0.4Zn0.6Sx Fe0.2Zn0.8Sx ZnSx MoS2 4,6-DMDBT FeSx Fe0.8Zn0.2Sx Fe0.6Zn0.4Sx Fe0.4Zn0.6Sx Fe0.2Zn0.8Sx ZnSx MoS2

Conv. (%)a

BIP kHDS (Sel.)b

SEFc

kHYDd

SEFc

kDDSe

SEFc

kHYD/kDDS

17.8 53.6 79.2 88.3 74.7 3.7 96.3

61.0 37.2 34.5 29.6 26.7 59.0 48.1

5.1 20.1 41.2 56.2 36.0 1.0 86.4

3.8 7.4 9.2 4.6 -

2.0 12.6 27.0 39.6 26.4 0.4 44.8

6.1 12.4 16.5 8.6 -

3.1 7.5 14.2 16.6 9.6 0.6 41.6

2.3 4.2 4.5 2.7 -

0.6 1.7 1.9 2.4 2.8 0.7 1.1

10.2 49.7 70.1 90.1 61.3 2.5 77.0

-

2.8 18.1 31.8 60.9 25.0 0.7 38.7

6.2 10.2 17.4 5.4 -

-

-

-

-

-

a

Reaction conditions: catalyst loading = 1.0 g, reaction temperature = 633 K, total pressure = 4.0 MPa, WHSV = 8.4 h-1, the oil feed containing either 0.58 wt. % DBT or 0.67 wt. % 4,6-DMDBT (with sulfur content of 1 000 µg g-1) in a n-decane solvent, H2/HC = 300 v/v. b Determined at about 50% of DBT conversion by changing the WHSV (since the activity of ZnSx is too low, the BIP selectivity on ZnSx was obtained at about 20% DBT conversion). c Synergetic effect factors, defined as the total, HYD or DDS activity rate of Fe-Zn sulfides catalysts / the sum of corresponding activity rate of mono-Fe and Zn. d kHYD=kHDS·selectivity of (1-BIP). e kDDS=kHDS-kHYD, the dimensions of kHDS, kHYD and kDDS are all mol kg-1 h-1.

It is widely accepted that HDS of DBT on traditional MoS2 based catalysts can occur by the two parallel reaction routes48–50 (Figure 7, the structures and acronyms of the main


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ACS Catalysis

compounds involved in the present work are listed in Table S3): One is the direct desulfurization route (DDS), where sulfur elimination takes place via hydrogenolysis of C–S bonds, mainly yielding BIP, whereas another pathway is the hydrogenation pathway (HYD), desulfurization occurs after hydrogenation of one of the aromatic rings of the DBT molecule, yielding first tetra- and hexa-hydrogenated intermediates and then phenylcyclohexane and dicyclohexane.48,51–53

Figure 7 HDS reaction scheme of DBT.

However, to our knowledge, no detailed research of iron-based sulfides DBT HDS reaction scheme has been studied, thus it’s essential to estimate whether BIP, which is certainly the DDS product, could be easily hydrogenated to PCH or not. A confirmation test was performed, the results can be seen in Table 3.


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Table 3 Conversions and products distributions of BIP hydrogenation Catalysts

Fe0.4Zn0.6Sx

MoS2

BIP conversion (%)

6.6

4.7

PCH

88.8

91.2

DIC

-

-

Benzyl-CP and CP-CH-methane

11.2

8.2

Products selectivity (%)

Reaction conditions: catalyst loading = 1.0 g, reaction temperature = 633 K, total pressure = 4.0 MPa, WHSV = 8.4 h-1, the oil feed containing 1.0 wt. % BIP in a n-decane solvent, H2/HC = 300 v/v.

The BIP hydrogenation conversion on Fe0.4Zn0.6Sx is as low as that on MoS2, thus we conclude all products except BIP, should be treated as HYD reaction products, and the use of “the selectivity of (1- BIP)/ selectivity of BIP”50 as the reference of HYD/DDS routes ratio is reasonable, as well as the definition of kDDS and kHYD. Based on the analysis above, the DBT HDS mechanism could be discussed. As shown in Figure 6a and Table 2, after the Zn adding, the selectivity of BIP decreased (see the red dotted line in Figure 6a), while the HYD/DDS ratio (or kHYD/kDDS, see Table 2) gradually increasing from 0.6 for FeSx, where the DBT reaction likely went through the DDS route, to 2.4 in Fe0.4Zn0.6Sx, where the HYD pathway dominated. In short, the addition of Zn greatly promotes DBT HDS activity via an extraordinary preference of the HYD route. The advantages of this property will be demonstrated on the HDS of 4,6-DMDBT. 3.2.2 HDS of 4,6-DMDBT The typical GC chromatograph of DBT HDS products is also presented in Figure S13. The conversions of 4,6-DMDBT over FeSx and ZnSx under 633 K were only 10.2 % and 2.5 % (Figure 6c), respectively, which were significantly lower than those of DBT at 17.8 % and


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ACS Catalysis

3.7 %, respectively. This result is in line with the studies of the conventional MoS2 based catalysts,49,50,54,55 since the HDS of 4,6-DMDBT is much more difficult than DBT. After the addition of Zn, an even more significant synergetic effect than in DBT case was observed, i.e. Fe0.4Z0.6Sx has a synergy factor of 17.4 compared with the sum of FeSx and ZnSx (Table 2), and was even much higher than that of MoS2 (90.1% vs. 77% 4,6-DMDBT conversion). In addition, the 4,6-DMDBT conversions on FeZn samples just decrease slightly and is very close to their DBT conversions. Especially, over the Fe0.4Z0.6Sx catalyst, the conversion of 4,6-DMDBT was even as higher as 90.1 %, while that of DBT was 88.3 %, this result is really unusual in consideration of the low reaction activity of methyl substitute DBTs. By carefully examining the products distribution as shown in Figure 6c and Table 2, we found that the selectivity of DDS products DMBIPs decreased with the increased Zn content, instead, the accepted HYD products DMPCH and DMDICs increased. Thus, we conclude that the addition of Zn significantly changes the route preference of DBT and 4,6-DMDBT, and the reaction rate of HYD was greatly promoted. Meanwhile, since the sterically hindrance effect of the methyl substitute DBT, HDS of 4,6-DMDBT occurs preferentially via the HYD route, thus the synergetic effect was more profound in the HDS of 4,6-DMDBT. Such promoting effect was also obtained in HDS evaluation at 593 K and 613 K (Figure 6d). Wang et al.50 reported a similar deep hydrodesulfurization V-Co-Mo/TiO2-ZrO2 catalyst, which shows much higher 4,6-DMDBT HDS conversion than that of DBT (90.4% to 70.9%) under the same reaction conditions. The authors contributed this result to the binary V-Mo sulfides which could induce active sites to facilitate the HYD route. However, it is known that industrial catalysts promoted by Co/Ni show much higher


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activity than MoS2.56 HDS conversions comparison between FeZn and Co-MoS2 catalysts (Table S4) shows that at lower weight hourly space velocity (8.4 h-1), the conversions of DBT and 4,6-DMDBT are close to that of Co-MoS2. Since the reactant conversions of reference catalyst have reaching ~100% under such conditions, in order to make proper comparison, we adjusted the WHSV of reference catalyst to obtain HDS rate constants at moderate conversions. The results are shown in Table S5. The DBT HDS rate constant of Fe0.4Zn0.6Sx is comparable with that of MoS2, but much lower than that of Co-MoS2. Interestingly, for 4,6-DMDBT HDS reaction, the rate constants of FeZn sulfide are relatively close to those of Co-MoS2, i. e. under 633 K, 4,6-DMDBT HDS rate constants on Fe0.4Zn0.6Sx and Co-MoS2 are 60.8 mol kg-1 h-1and 75.3 mol kg-1 h-1, respectively. We attribute the result to the high hydrogenation activity of FeZn catalysts and significant difference between DBT and 4,6-DMDBT activities over traditional CoMo catalysts. Meanwhile, it’s worth noting the current FeZn catalyst is not fully optimized, thus we propose that the FeZn catalyst shows a potential to replace the industrial catalyst in some process after screening and tuning. 3.2.3 Activity evaluation of real straight-run diesel With the real straight run diesel as feedstock, the activity of FeSx, ZnSx, Fe0.4Zn0.6Sx and MoS2 was also evaluated (Figure 8). For the 160 hours running period, The HDS rate of Fe0.4Zn0.6Sx is maintained at ~90%, with the sulfur content in the products much lower than MoS2 catalyst (Table S5), indicating the advantageous HDS performance of FeZn catalyst. For pure iron catalyst, just about 25 % sulfur was removed, the HDS rate of pure zinc sample was extremely low (6.2-7.5 %). The above evaluation results of real diesel HDS are consistent with the assessment of model S-containing compounds, which again confirmed the strong


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synergetic effect between iron and zinc. We also performed XRD and XANES characterizations of used Fe0.4Zn0.6Sx catalyst to determine the possible structure changes after 160 h HDS reaction. The results (Figure S14 and Figure S15) show no observed differences between fresh sulfided FeZn catalyst and used sample. It means this bimetallic sulfide structure is still intact after the reaction.

Fe0.4Zn0.6Sx 90

Sulfur removal (%)

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

ACS Catalysis

MoS2 80

20

FeSx

10

ZnSx

0 40

60

80

100

120

140

160

Time on stream (h)

Figure 8 Sulfur removal in the diesel products with time on stream over Fe0.4Zn0.6Sx, MoS2, FeSx and ZnSx catalysts. Catalyst loading = 4.0 g, reaction temperature = 643 K, total pressure = 6.0 MPa, WHSV = 1.0 h-1, reaction feed = Dagang straight-run diesel with sulfur content 641 µg g-1, H2/HC = 700 v/v.

3.3 Role of zinc in the synergetic catalysis Based on the above experimental results, it is clear that the dopped zinc plays an important role in catalytic performances of FeZn sulfide. To have a thorough understanding of this system from an atomic level, we carried out first-principle calculations to study the


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effects of zinc on structural, electronic and even catalytic properties of this bimetallic sulfide catalysts. We find that the crystal structure of FeZn sulfide after optimization is different from pure FeSx and ZnSx even with the addition of a small amount of Zn, just as shown in Figure S1. It indicates that significant reconstructions occur and new crystal phases appear, which is consistent with XPS characterization (Figure 2). To date, many research groups have proposed that in hydrodesulfurization reactions, the removal of sulfur from catalysts occurs before the hydrodesulfurization can take place.6,57–59 In other words, the removal ability of sulfur has a close relationship with catalytic performance. Recently, Ramos and Chianelli26 reported the Co-MoS2 system, and proposed that a strong electron donation from Co to Mo remarkably weakens Mo-S bond and facilitates the formation of coordinatively unsaturated sites. Lacroix et al.60 found that BIP hydrogenation activity of Mo-promoted vanadium sulfide was higher than unpromoted MoS2. They explained that in the MoV sulfide, the electrons transfer from V to surrounding Mo ion may modify the V-S bond. In our study, the lower Fe photon energies and higher Zn whitelines of FeZn sulfide catalysts than pure Fe and Zn sulfide samples, as observed in the XANES spectra (see Figure 3a and Figure3c), implies the significant electron transfer from Zn to Fe. In addition, the formation of bimetallic sulfide, which has different sulfur species hydrogenation behavior, is demonstrated by STEM, XPS, XANES and TPR-S. Indeed, our TPR-S and S K-edge XANES experiments indicate that a special Sx species which could be easily hydrogenated to form sulfur vacancies under hydrogen atmosphere exists in FeZn samples. Further quantitative results (Table 1) of TPR-S experiments demonstrate that the released amount of H2S in Region I is proportional to the HDS activity; it means that the more


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H2S released, the higher catalytic activity the catalysts has. Based on this, we further created S vacancies in a series in FeZn sulfide catalysts and computed the S vacancies formation energies (S-v FE) utilizing the DFT approach. The results are shown in Figure 9 (left). One can see that when Zn/Fe ≤ 1, the S vacancies formation energy is decreasing with the increasing zinc content. Whilst when Zn content is higher than Fe, the S-v FE is increasing. When Zn/Fe is about 1.0, the S-v FE is in the bottom of the valley, and the HDS activity close to the maximum. As mentioned in catalytic activity section, FeZn catalysts were found effective in catalyzing 4,6-DMDBT conversion by mainly promoting the HYD reaction pathway. Meanwhile, based on our experimental and calculation results, S vacancies play a key role in the HDS reaction. However, it is generally considered that in traditional HDS catalysts (for example Co-MoS2), S vacancies are the active sites for DDS reaction, while for the active sites of HYD, it is still in debate.3 Thus, the issue is how do S vacancies in FeZnS system promote the reaction activity of HYD pathway. According to published previous studies,61 the effect of vacancies (e.g. oxygen vacancies) on adsorption and reaction mechanism has been widely investigated. Thus, we infer that sulfur vacancies may influence the adsorption of 4,6-DMDBT molecule in current work. We further calculated the effect of S vacancies on adsorption of 4,6-DMDBT, the results are shown in Supporting Information (Table S7 and Figure S16) , which show that the more S vacancies, the easier the 4,6-DMDBT accept electrons from surface. Thus, we propose that moderate S vacancies are benefit to 4,6-DMDBT adsorption. Meanwhile, the S vacancies will be the active sites for the following C-S cleavage after the adsorption-hydrogenation reaction of 4,6-DMDBT.


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Figure 9 Relationship between HDS activity and sulfur vacancies formation energy and the position of Fe d band center. The columns are the conversion of DBT and 4,6-DMDBT on different catalysts samples at 633 K. The left red curve is S vacancies formation energy (S-v FE), and the right plotted blue curve is the position of Fe d band center (ɛ).

The review of V-based bimetallic sulfides hydrotreating catalysts by Hubaut62 contributed the synergetic effect of V and transition metal to a mechanism based on a charge transfer leading to change in the energy levels of electronic bands. Since the d band model has proven to be particularly useful to understand the trends of activity in transition metal catalysts,63–65 we calculated the positions of the Fe d band center (ɛd) in pure FeSx, ZnSx, and several FeZn sulfide samples, as shown in Figure 9 (right). According to the DFT results, it is revealed that the participation of Zn has a significant effect on the position of the Fe d band center. One can see that with the increasing of Zn proportion, the position of the Fe d band center increase, and has a summit when Zn/Fe is close to 1.0. The position of the d band center presents a good volcano-curve relationship with zinc content, which perfectly interprets the catalytic performance difference of samples. In addition, the changes in the d band center propose that the addition of zinc might tune the adsorption of DBT or 4,6-DMDBT on substrates, as confirmed by the calculations on adsorption behavior of these two representative sulfur compounds (see Figure S17). For both DBT and 4,6-DMDBT, the


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flat adsorption energies on zinc containing catalysts are higher than on the pure FeSx catalyst, which implies that the zinc addition may facilitate the flat adsorption of sulfur-containing molecules. A direct catalytic consequence of this adsorption mode is to facilitate the HYD reaction of above sulfur compounds, especially stericallyally hindered 4,6-DMDBT.

4. CONCLUSIONS In the work, we reported the strong synergetic effect of Fe and Zn in the HDS of DBT and 4,6-DMDBT. The addition of zinc simultaneously promotes the DDS, especially the HYD route. Via the latter, HDS activity for the sterically hindered 4,6-DMDBT on the FeZn sulfide catalyst is even close to that of Co-MoS2. Further characterizations and experiments confirmed the interaction between iron and zinc species, and the formation of bimetallic FeZn sulfides, instead of a simple mixture of iron and zinc sulfides. A special Sx species which is easily hydrogenated was distinguished. By combining experiments and DFT calculations, the synergetic effect was attributed to both the generation of more sulfur vacancies and the modification of Fe electronic properties and the promotion of flat adsorption of sulfur-containing molecules by zinc addition. This new finding could be useful in design of novel inexpensive desulfurization catalysts for potential industrial refinery application.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Notions and physical properties of catalysts, structures of different sulfides, HAADF-STEM images, EDS mappings, and EDS line scans of catalysts, XRD patterns of various samples,


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XANES spectra for Fe0.4Zn0.6Sx and references recorded at Fe K-edge, the Bader charge analysis of Fe, Zn and S in series of catalysts, TPR-S-MS profiles and S K-edge XANES spectra of references, the compounds involved in present work, HDS activities of catalysts, stability of FeZn sulfide catalyst, DFT calculations on the adsorption of sulfur-containing molecules.

AUTHOR INFORMATION

Corresponding Author ∗

Corresponding author

E-mail address: [email protected]

Tel &Fax: +86 10 89733369 Author Contributions Baojian Shen, Hao Li and Xiaodong Wen designed the experiments. Hao Li and Jiancong Li prepared the catalyst samples and analysed the results. Hao Li, Jiancong Li, Lei Li, Houxiang Sun, Yandan Wang, Wennian Wang, Delin Yuan and Qiaoxia Guo carried out the catalytic activity evaluation. Xiaodong Wen, Jinjia Liu, and Tao Yang performed the DFT calculations and analysed the results. Yongfeng Hu performed the XANES experiments and analysed the results. Baojian Shen, Hao Li, Xiaodong Wen and Yongwang Li conceived the experiments and wrote the manuscript. All authors discussed the results and commented on the manuscript. ‡These authors contributed equally.

Funding Sources


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This work received financial support from the National Natural Science Foundation of China (U1462202, 21176251, 21473003, 21222306, 21473229 and 91545121) and MOST ‘‘973’’ Project of China (2012CB215001, 2011CB201402 and 2013CB933100) and PetroChina.

ACKNOWLEDGMENT The authors wish to express their gratitude to Qunfeng Xiao, Aimee Maclennan, Dongniu Wang and David Muir of Canadian Light Source Inc. in Saskatoon, SK for their assistance on the XANES analysis. We also thank Qing Peng form Rensselaer Polytechnic Institute and Albert K. Dearden from Berea College for their efforts with article writing. Xiao-Dong Wen also acknowledges National Thousand Young Talents Program of China, Hundred-Talent Program of Chinese Academy of Sciences and Shanxi Hundred-Talent Program. The computational resources for the project were supplied by the Tianhe-2 in Lvliang, Shanxi and National Supercomputing Center in Shenzhen.

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