Structure Investigation and Dibenzothiophene Hydrodesulfurization

Subscriber access provided by UNIV OF SOUTHERN QUEENSLAND

Article

Structure Investigation and Dibenzothiophene Hydrodesulfurization Properties of Fe Substituted Ni-Si Intermetallics Xiao Chen, Junhu Wang, Kaixuan Yang, Changgong Meng, Christopher T Williams, and Changhai Liang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10773 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 16, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 46

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

The Journal of Physical Chemistry

Structure Investigation and Dibenzothiophene Hydrodesulfurization Properties of Fe Substituted Ni-Si Intermetallics Xiao Chen a, Junhu Wang b, Kaixuan Yang a, Changgong Meng a, Christopher T. Williams c, and Changhai Liang a, * a

Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical

Engineering, Dalian University of Technology, Dalian 116024, China b

Mössbauer Effect Data Center& Laboratory of Catalysts and New Materials for

Aerospace, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian116023, China c

Department of Chemical Engineering, Swearingen Engineering Center, University of

South Carolina, Columbia, South Carolina 29208, United States

* To whom correspondence should be addressed: Fax: + 86-411-84986353; Email: [email protected]; Homepage: http://finechem.dlut.edu.cn/liangchanghai Running title: Structure and Hydrodesulfurization Properties of Fe-Ni-Si Intermetallics

1

ACS Paragon Plus Environment


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

Abstract: Development of highly active and sulfur-tolerant catalysts for deep hydrodesulfurization (HDS) is of great importance in petroleum refining. Here, the discovery of Fe-substituted Ni-Si intermetallic catalysts (Ni 1–x Fe x Si 2 ) that efficiently removes dibenzothiophene (DBT) by HDS is reported. The structure of the catalyst was identified through X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results showed that Fe atoms substituted Ni atoms in Ni 1–x Fe x Si 2 catalysts, which preferentially bond with Ni to form alloy and combine with Si to form silicide. The DBT activity for the Ni 1−x Fe x Si 2 has the following order: Ni 0.75 Fe 0.25 Si 2 > NiSi 2 > Ni 0.50 Fe 0.50 Si 2 ≈ Ni 0.25 Fe 0.75 Si 2 ≈ Fe-Si. The positive synergistic effect on HDS activity can be correlated to that the formation of Ni-Fe and Fe-Si bonds make the metal site to have high d-electron density, which promotes the hydrogenation activity. Comparison with the fresh and spent catalysts revealed the stability and sulfur-resistance of these catalysts. The present findings suggest that Fesubstituted Ni-Si intermetallics catalysts provide a good starting point for a new catalyst development in the HDS field.

Keywords: Intermetallic compound; Iron substitution; Nickel silicide; Mössbauer spectroscopy; Hydrodesulfurization; Sulfur tolerance

2


Page 2 of 46

Page 3 of 46

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


1. Introduction Silicon-based intermetallic compounds (metal silicides) have diverse physical and chemical properties that are both very useful and fundamentally significant for application as catalysts.1,2 In recent years, metal silicides have been shown to catalyze several types of reactions (especially hydrogenations), due to their specific crystal and electronic structures that vary significantly from that of their component metals.3-7 For instance, Schaak et al. indicated that metal silicides can be synthesized from metal nanoparticles by reacting with monophenylsilane in solution. These metal silicide nanoparticles were screened as electrocatalysts for the hydrogen evolution reaction in acidic aqueous solutions.3 Pd 2 Si@SiCN catalysts were found active for the selective hydrogenation of ketones to alcohols.4 In our previous work, nickel silicides prepared from organometallic polymer as efficient catalyst show suitable conversion of phenylacetylene and notable selectivity to styrene with 90%.5 Ni-Si intermetallics were also found to be efficient catalyst for selective hydrogenation of cinnamaldehyde.6 In addition, highly dispersed Ni-Si intermetallics supported on silica showed high activity and stability for CO methanation.7 However, their applications are not only limited to the aforementioned examples.

Thermochemical studies in the Gibbs free energy from silicides to sulfides in H 2 S atmosphere have shown that transition metal silicides present the stability and sulfur-

3



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

resistance.8 This means that transition metal silicides can be used in catalytic hydrotreating reactions involving sulfur-containing compounds. However, only few publications concerning these types of materials as hydrotreating catalysts have been available. In preliminary work, bulk nickel silicides (Ni 2 Si, NiSi, and NiSi 2 ) with an electron-deficient structure have been designed as promising hydrodesulfurization (HDS) catalysts with high activity and good sulfur tolerance.9 In addition, a nickel-rich Ni 0.75 Co 0.25 Si 2 catalyst was much more active than that of monometallic silicides (NiSi 2 and CoSi 2 ) and significantly improved the hydrogenation property, proving a synergistic effect between the components.10

Increasingly stringent regulations on allowable sulfur levels in transportation fuels are placing high demands on the sulfide-based hydroprocessing catalysts used for the removal of sulfur impurities from fossil fuel feedstocks.11, 12 Conventional CoMo- and NiMo-based catalysts, however, do not have sufficient activity to desulfurize diesel feed streams to ultra low sulfur levels under normal operating conditions due to the limitation of the active sites exposed only on the edge planes of MoS 2 .13 Therefore, a substantial research effort is now focused on exploring more active and stable catalysts for reducing the S-content of diesel to ultra low levels.

4


Page 4 of 46

Page 5 of 46

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


In initial studies it was reported that bimetallic NiFeP catalyst afford high activity in the HDS of dibenzothiophene and 4,6-dimethyldibenzothiphene. In addition, this catalyst exhibited high selectivity for the direct desulfurization pathway, which was ascribed to a ligand effect of Fe on the active Ni atoms.14-16 However, the cheap and ubiquitous iron has always been thought of as an inactive element in HDS reactions.

To further develop the understanding the solid-state and surface chemistry of NiSi 2 HDS catalysts upon substitution with Fe and their HDS properties, unsupported Fesubstituted Ni-Si intermetallics have been synthesized by a temperature programmed silicification method. The presence of Fe in the materials permits the use of Mössbauer spectroscopy to probe the solid-state structural chemistry of the catalysts. The activity of Fe substituted Ni-Si intermetallic catalysts for the deep HDS of DBT has been explored.

2. Experimental Methods 2.1. Catalyst Preparation

2.1.1. Synthesis of Fe Substituted Nickel Oxides

The Fe-substituted nickel oxide precursors were prepared by the method of precipitation. A solution composed of Ni(OAc) 2 ∙4H 2 O and FeC 2 O 4 ∙2H 2 O (Ni + Fe = 0.02 mol) dissolved in 60 mL of ethylene glycol was gradually heated to 145 oC. 200 mL of 0.2 M Na 2 CO 3 aqueous solution was added and then the slurry was further aged for 1 h under 5



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

vigorous stirring. After filtration and being washed with water, the obtained solid was dried at 120 oC overnight and calcined at 400 oC for 2 h in air.

2.1.2. Synthesis of Fe Substituted Ni-Si Intermetallics

Fe substituted Ni-Si intermetallics (Ni 1–x Fe x Si 2 ) were prepared by temperature program silicification using SiH 4 as silicon source. The Fe substituted nickel oxide precursors were firstly reduced in H 2 at 350 oC for 3 h. Subsequently, the reduced samples were reacted with a 10% SiH 4 /H 2 mixture (100 mL/min) at 450 oC for 15 min. They were then cooled to room temperature in H 2 (30 mL/min) and passivated in 1% O 2 /Ar overnight. The metal composition of the Ni 1–x Fe x Si 2 catalysts varied over the range 0 < x < 1.00.

2.2. Catalyst Characterization Powder X-ray diffraction (XRD) analyses of the samples were perfomed using a Rigaku D/Max-RB diffractometer with a Cu Kα monochromatized radiation source, operated at 40 kV and 100 mA. The average size of Ni 1–x Fe x Si 2 particles was evaluated by the Scherrer formula. The BET specific surface areas were measured on an Autosorb IQ surface area and pore size analyzer. Prior to the measurements, all samples were degassed completely at 200 ºC for at least 4 h.

57

Fe Mössbauer spectra were recorded on a

Topologic 500 A spectrometer with a proportional counter at room temperature. Moving 57Co(Rh) in a constant acceleration mode was used as the radioactive source. All 6


Page 6 of 46

Page 7 of 46

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


of the spectral analyses were conducted assuming a Lorentzian lineshape for computer fitting. The Doppler velocity of the spectrometer was calibrated with respect to α-Fe foil. High-resolution field emission scanning electron microscopy (FESEM) images of Ni 0.75 Fe 0.25 Si 2 were obtained with an Nova NanoSEM 450 from FEI Company, equipped with an energy-dispersive X-ray (EDX) analyzer. Transmission electron microscopy (TEM) was performed by using an FEI Tecnai G20. H 2 -temperature-programmedreduction (H 2 -TPR) analysis was conducted with a Quantachrome ChemBET TPR/TPD system with a thermal conductivity detector. About 100 mg of sample was treated in a 10% H 2 /Ar gas mixture flow at a rate of 50 mL min-1 from RT to 850 °C at 10 °C min-1. The X-ray photoelectron spectra (XPS) were obtained by an ESCALAB250 (Thermo VG, USA) spectrometer with Al Kα (1486.6 eV) radiation with a power of 150 W. All corelevel spectra were referenced as the C 1s neutral carbon peak at 284.6 eV and were deconvoluted into Gaussian component peaks.

2.3. HDS Activity Measurements Typically, the HDS of DBT experiment was performed at 3.0 MPa of H 2 in a highpressure fixed-bed continuous-flow stainless steel catalytic reactor over 0.2 g catalyst diluted with quartz sand to 5 mL. Prior to the activity test, the passivated catalysts were activated in situ with H 2 at 400 °C and atmospheric pressure for 2 h. Then, the temperature was adjusted to the reaction temperature. The liquid reactant was composed 7



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

of 0.5 wt.% octane (as internal standard), 0.3 wt.% DBT reactant, and decalin as solvent. Catalytic activities were measured at different temperatures (320-380 °C), with a flow rate of 144 mL min–1 and weight hourly space velocities (WHSV) of 21.1 h–1. The WHSV effect over Fe-substituted Ni-Si intermetallics was explored at 3 MPa H 2 and 340 oC. The reaction product composition was analyzed using a gas chromatograph (Agilent 7890A) with flame ionization detector and a HP-5 capillary column.

3. Results and Discussion 3.1. Synthesis of Fe Substituted Nickel Oxides Figure 1 shows the XRD patterns of Fe substituted nickel oxides with a range of metal compositions prepared by precipitation method. The XRD patterns of pure nickel and iron oxides present typical diffraction peaks of the bunsenite NiO phase (JCPDS # 047-1049) and maghemite γ-Fe 2 O 3 phase (JCPDS # 039-1346), respectively. The XRD pattern of low-level Fe substituted nickel oxide (Ni:Fe = 3:1) is similar to the pure nickel oxide sample, but the intensity of peaks is decreased. Further increasing the Fe content in the oxides (Ni:Fe = 1:1) results in the appearance of both a trevorite NiFe 2 O 4 phase (JCPDS # 010-0325) and bunsenite NiO phase (JCPDS # 047-1049). When the Ni:Fe molar ratio reached 1:3, the XRD peaks attributable to NiO phase completely disappeared, and only the trevorite NiFe 2 O 4 phase is confirmed. Thus, it can be deduced that the series of nickel-

8


Page 8 of 46

Page 9 of 46

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


iron oxides (NiO, NiFe 2 O 4 , and Fe 2 O 3 ) have been synthesized by changing the Ni:Fe molar ratios in the precursor. The low Fe-substituted Ni oxide may not reconstruct the crystal structure of cubic NiO.

3.2. Solid-State Structural Chemistry of Fe Substituted Ni-Si Intermetallic Catalysts Fe-substituted Ni-Si intermetallics with different stoichiometric proportion can be prepared by the temperature program silicification of the corresponding oxides. Figure 2 shows the XRD patterns of Fe-substituted Ni-Si intermetallics. After the silicification at 450 oC, it clearly shows that the diffraction peaks at 28.6o, 47.4o, 56.3o, 69.3o, 76.6o, and 88.4o reflect the NiSi 2 phase (JCPDS # 43-0989). However, the phase of iron silicide is not formed, with only the zero-valent iron and magnetite Fe 3 O 4 phases observed (Fe, JCPD # 06-0696, Fe 3 O 4 , JCPD # 19-0629). The formation of Fe 3 O 4 is attributed to the oxidation of zero-valent iron when exposed in the air. At low doping level of Fe into the Ni-Si intermetallic, excepting the NiSi 2 phase in the sample of Ni 0.75 Fe 0.25 Si 2 , the diffraction peaks at 44.3o, 51.5o, and 75.9o also appear, indicating the formation of Ni-Fe alloy (FeNi 3 , awaruite, JCPD # 38-0419). In addition, by comparison with the ICSD file (FeSi, fersilicite, JCPD # 38-1397), peaks at 45.1o, 49.7o, and 79.9o are attributed to the FeSi lattice planes (210), (211), and (321), respectively. This means that the presence of

9



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

Ni promotes the formation of iron silicide due to the Ni-catalyzed decomposition of SiH 4 .17 When the Ni:Fe molar ratio reaches 1:1, the intensity of peaks from NiSi 2 and FeSi decrease significantly but the positons of the XRD peaks assigned to the FeNi 3 phase shifted to lower diffraction angles. Meanwhile, some new peaks at 43.6o, 50.8o, and 74.7o appear, which act in accordance with the standard pattern (JCPD # 47-1405), indicating the formation of Fe 0.46 Ni 0.36 . Further increasing the content of Fe to a Ni:Fe molar ratio of 1:3, the NiSi 2 and FeSi phases disappear. The main peaks are attributed to the phase of γFe 2 O 3 (maghemite-Q, JCPD # 25-1402). The relatively weak peak at 44.6o can be attributed to NiFe (kamacite, JCPD # 37-0474). Therefore, in the Fe substituted Ni-Si intermetallics, the Fe bonds with Ni preferentially to form Ni-Fe alloys. Zero-valent iron is hardly silicified than metallic nickel due to the higher free energy, however, the presence of metallic Ni can promote the reaction of SiH 4 with iron.18-19 The related result has been evidenced by the XRD pattern in Figure S1, which shows that the formation of FeSi 2 when the reaction temperature has been increased into 550 oC during the temperature program silicification of iron oxides. Due to the formation of the cubic phases of NiSi 2 and FeSi and the deviation of ionic radius of Ni2+ (0.069 nm) and Fe2+ (0.078 nm), the Fe substitution of Ni in the Ni-Si intermetallic may influence the metal-silicon and metalmetal interactions, leading to a strong modification of the geometry and electronic structure.20 10


Page 10 of 46

Page 11 of 46

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


The specific surface area of the Fe substituted Ni-Si intermetallics (NiSi 2 , Ni 0.75 Fe 0.25 Si 2 , Ni 0.50 Fe 0.50 Si 2 , Ni 0.25 Fe 0.75 Si 2 , and Fe-Si) are found to be 53, 54, 21, 36, and 88 m2/g, respectively (as shown in Table 1). Although the surface areas of these intermetallics should be further improved, those are the largest surface area of unsupported silicides reported in the literature to date. The nitrogen adsorption-desorption isotherms of the Fe-substituted Ni-Si intermetallics are shown in Figure 3. The shape of the isotherms indicates that they can be classified as type IV. In addition, the presented adsorption–desorption hysteresis loop of Fe-substituted Ni-Si intermetallics may be classified as H1 according to the IUPAC classification, which is characteristic for solids made by aggregates or agglomerates of spheroidal particles.21 The total pore volume and the average pore size are also listed in Table 1. The Fe-substituted Ni-Si intermetallics have higher pore volume and pore size than pure Ni-Si intermetallic compounds, which means substituting the Ni atoms by the Fe atoms leads to lattice expansion that enlarges the pores.

57

Fe Mössbauer spectroscopy is a well-known tool for clarifying the chemical state and

relative amount of iron species in catalytic materials.22 Room temperature 57Fe Mössbauer spectra of samples of Ni 0.75 Fe 0.25 Si 2 , Ni 0.50 Fe 0.50 Si 2 , Ni 0.25 Fe 0.75 Si 2 , and Fe-Si are shown in Figure 4. The related hyperfine interaction parameters are summarized in Table 2. In the Mössbauer spectrum of Ni 0.75 Fe 0.25 Si 2 , one resolved doublet, one singlet, and one 11



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

sextuplet can be easily identified. The IS (isomer shift) value is considered the most important Mössbauer parameter that is influenced by the chemical state of iron. The doublet having 57Fe Mössbauer parameters of IS = 0.27 mm/s, QS (quadrupole splitting) = 0.43 mm/s accounted for about 56.4% and is attributed to FeSi,23 while the sextuplet with IS = 0.02 mm/s, QS = 0.05 mm/s, and H = 27.3 T could be attributed to FeNi 3 ,24 which accounted for 33.1% of the total spectral area. The singlet with the IS = −0.03 mm/s can be ascribed to cubic silicide like FeSi 2 , which accounted for 10.5%.25 The cubic structure can be expected because of zero quadrupole splitting which indicates spherical symmetry in the surroundings of the iron atoms. The theoretical calculations based on electron transport model confirmed the structure of nickel silicide changed when Fe atoms with higher d-electron counts substituted Ni atoms, which distorted the six rings in the structure and is traced to the formation of Ni-Fe bonds.20 Therefore, Fe atoms occupy the relative sites in the Ni 0.75 Fe 0.25 Si 2 material, which preferentially bond with Ni to form alloy and combine with Si to form silicide.

Similarly, for the sample Ni 0.50 Fe 0.50 Si 2 , the Mössbauer spectrum is fitted into two doublets for FeSi and ferrosilite (FeSiO 3 ) and two sextets for FeNi 3 and Fe 0.64 Ni 0.36 . The doublet with IS = 0.42 mm/s and QS = 2.49 mm/s can be assigned to trace FeSiO 3 (only accounted for 2.6%).26-27 The formation of ferrosilite in the Fe substituted Ni-Si intermetallics can be attributed to the uncompleted silicification of iron oxides or the 12


Page 12 of 46

Page 13 of 46

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


oxidation of the formed iron silicide. Due to the Fe:Ni molar ratio increasing, the account of Ni-rich Fe-Ni alloy decreased to 20.0%, while the Fe-rich Fe-Ni alloy (Fe 0.64 Ni 0.36 ) accounted for 35.7%, having the 57Fe Mössbauer parameters of IS = -0.06 mm/s, QS = 0.08 mm/s, and H = 30.4 T.

For the Ni 0.25 Fe 0.75 Si 2 sample, the Mössbauer spectrum could be fitted with three sextets and two doublets. One sextuplet associated with Fe 0.64 Ni 0.36 accounted for 28.4% of the total spectral area, while the two sextuplets with IS = 0.29 mm/s, QS = -0.03 mm/s, and H = 47.8 T and IS = 0.63 mm/s, QS = -0.35 mm/s, and H = 44.4 T are assigned to the two types of FeNi. In addition, two resolved doublets can be easily indentified, one attributed to FeSiO 3 and the other to Fe 2 O 3 . The presence of Fe 2 O 3 can be attributed to the low degree of silicification of zero-valent Fe due to the decreasing content of Ni, which results in Fe oxididation by air during the passivation. This result is similar with the XRD data.

When the bulk Fe was treated in SiH 4 /H 2 at 450 oC to form Fe-Si, the Mössbauer spectrum could also be fitted into two doublets and three sextuplets. The sextuplet with IS = 0.01 mm/s, QS = 0 mm/s could be attributed to Feo, which accounted for 22.4% of the total spectral area. The other two sextuplets can be identified to Fe3+ in the tetrahedral site of spinel magnetite and the Fe atoms located in the octahedral site.28 The value of the

13



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

mean isomer shift and hyperfine fields are similar to the bulk Fe 3 O 4 , which results from the partial oxidation of zero-valent Fe.29 The two doublets could again be ascribed to Fe 2 O 3 and Si doped Fe 2 O 3 . The doublet with IS = 0.73 mm/s and QS = 1.08 mm/s is consistent with the reported Fe3+ value at six-coordinated sites in silicates and oxides.30 During the silicifiaction process, the Si atoms are doped into the lattice of Fe 2 O 3 and formed Fe-Si-O bonds.

The morphology of the as-prepared Ni 0.75 Fe 0.25 Si 2 sample was imaged using SEM, as shown in Figure 5. The representative image shows flower-like structures deposited by irregular sheet. The corresponding EDX spectrum verified the elemental Ni, Fe, and Si presented in the nanostructure, for which the atomic percentage are approximately 37.85%, 10.99%, and 51.16%, respectively. The Ni:Fe atomic ratio is in accordance with the stoichiometry of Ni 0.75 Fe 0.25 Si 2 , which means that some Ni sites are uniformly substituted by Fe. The representative low-magnification TEM image of the Ni 0.75 Fe 0.25 Si 2 sample is shown in Figure 5c. It can be seen that the Ni 0.75 Fe 0.25 Si 2 is uniformly dispersed with the particle size of 11 nm, which is close to the value obtained from the XRD result. The HRTEM image (as shown the inset in Figure 5c) presents the lattice spacing of the sample as 0.1825 nm, which is close to the 0.1916 nm value for NiSi 2 (220).31 The deviation further evidence that the Ni sites of Ni-Si intermetallic have been substituted by Fe to some extent, fine-tuning the crystal structure. 14


Page 14 of 46

Page 15 of 46

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


H 2 -TPR is a powerful tool for studying the nature and reducibility of the Fe-substituted Ni-Si intermetallics catalysts. Figure 6 shows the H 2 -TPR profiles for the passivated Fesubstituted Ni-Si intermetallic catalysts. For the NiSi 2 sample, there is a peak at ca. 200300 °C that is tentatively assigned to the reduction of oxygen species adsorbed on nickel silicide. These results are similar to those of the TPR of CoSi/SBA-15.32 The peak shifts to the higher temperature (300-450 oC) upon increasing the substituted Fe atoms, probably due to the strong interaction of Fe-Ni and Fe-Si. In addition, the intensities of relatively low temperature reduction peaks (200-450 oC) are weakened with increasing the iron content. However, a broad band with two overlapping peaks appeared at the high temperature for the Fe-Si and Ni 0.25 Fe 0.75 Si 2 sample, which correspond to the reduction of hematite (Fe 2 O 3 ) to magnetite (Fe 3 O 4 ) at ca. 650 oC and magnetite to Fe0 at ca. 760 oC.33 The result is further confirmation that iron is not significantly silicified into Fe-Si intermetallics at 450 oC and transforms into iron oxides during passivation.

3.3. HDS Properties of Ni 1–x Fe x Si 2 Catalysts The Fe-substituted Ni-Si intermetallics catalysts were tested for dibenzothiophene (DBT) HDS. The reaction network for DBT HDS is complex and includes the direct desulfurization pathway (DDS), and the hydrogenation pathway (HYD).34 The reaction network shown in Scheme 1 is consistent with the hydrocarbon products identified via

15



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

GC-MS of the liquid reactor effluent at the different reaction temperatures and verified contact times. Figure 7 shows the DBT HDS activity and selectivity to products as a function of reaction temperature (300-380 oC) for Fe-substituted Ni-Si intermetallics. All of the catalysts exhibited increased DBT HDS activity with increasing temperature (as shown in Figure 7a). The DBT activity for the Ni 1−x Fe x Si 2 has the following order: Ni 0.75 Fe 0.25 Si 2 > NiSi 2 > Ni 0.50 Fe 0.50 Si 2 ≈ Ni 0.25 Fe 0.75 Si 2 ≈ Fe-Si. For understanding the HDS activity of Fe-Si in depth, the DBT HDS over FeSi 2 has been tested, as shown in Figure S2. The conversion of DBT is lower than 10% and biphenyl (BP) is the unique product, which verifies that Fe-Si is not the active site in the HDS reaction. The nickelrich Ni 0.75 Fe 0.25 Si 2 catalyst has the highest activity. With regard to the selectivity of the different products, two main products are detected: BP and cyclohexylbenzene (CHB). As shown in Figure 7b, BP is formed in greater proportions in all cases, which present more than 80%, indicating that the DDS pathway is the dominant pathway for S removal from the DBT.

The DBT HDS activities of the Fe-substituted Ni-Si intermetallics catalysts are plotted as a function of the Ni metal fraction (Ni/(Ni+Fe)) in Figure 8a. Starting from the Fe-Si catalyst, the HDS activities are low up to the Ni 0.50 Fe 0.50 Si 2 catalyst, but beyond this composition, the activity increases as the catalyst become Ni-rich. The Ni 0.75 Fe 0.25 Si 2 catalyst exhibited an activity (48.7 nmol/g/s) higher than that of NiSi 2 catalyst (32.9 16


Page 16 of 46

Page 17 of 46

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


nmol/g/s). The DBT HDS results are consistent with those reported previously for Ni 1x Co x Si 2

catalysts.10 A Fe-Si catalyst had a very low DBT HDS activity (as did a CoSi 2

catalyst), but the substitution of a small amount of Fe into nickel silicide yielded catalysts (Ni 0.75 Fe 0.25 Si 2 ) that had DBT HDS activities (ca. 50%) higher than that of a NiSi 2 catalyst. These results indicate that new sites are created, causing a positive synergistic effect on HDS activity due to the addition of a second metal to nickel silicide. These observations are consistent with those studies by Smith and Bussell, who observed that unsupported Co 0.08 Ni 2 P and Fe 0.03 Ni 1.97 P 2 catalysts had significantly higher activity than that of a Ni 2 P catalyst for HDS of DBT.14,35 Based on the Mössbauer spectroscopy results for the iron-containing catalysts, the high HDS activity can be correlated to the modified nickel sites in the NiSi 2 due to the formation of Ni-Fe and Fe-Si bonds. An increase in the valence electron concentration through the Fe substitution weakens the Si–Si interactions but enhances the metal-silicon and metal-metal interactions, which may enhance the π adsorption of DBT, promoting the hydrogenation activity.36

The selectivity results at steady state also showed remarkable dependency on compositions. The product selectivity for Ni-rich Ni 0.75 Fe 0.25 Si 2 catalysts shows preference for BP (82.0%), but to a less extent than that observed for the Fe-Si (88.7%) and NiSi 2 (91.8%) catalysts. The substitution of Fe for Ni decreases the selectivity for the DDS route, but greatly increases the selectivity for the HYD. This result can be associated 17



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

with the formation of FeNi bimetallic alloys, which have been previously described and have demonstrated improved catalytic hydrogenation performance when compared with pure Ni or Fe based catalysts.37-38

As shown in Figure 8b, the conversion of DBT over Fe-substituted Ni-Si intermetallic catalysts increases with the increase of contact time in the HDS of DBT at 3 MPa H 2 and 340 °C. Typically, HDS conversion over Ni 0.75 Fe 0.25 Si 2 catalyst reaches from 32.5% to 75.8% with increasing the contact time from 1.26 min to 7.58 min. Among five catalysts, the nickel-rich Ni 0.75 Fe 0.25 Si 2 catalyst with the major phase NiSi 2 and FeNi alloy shows the highest conversion. Combined with the phase state analysis results of XRD and Mössbauer spectra, it can be ascertained that the formation of FeNi bimetallic alloy in the Fe-substituted Ni-Si intermetallic dramatically promoted the HDS activity due to the alteration of the electronic structure by the chemical bonding between Ni and Fe. In the Fe substituted Ni-Si intermetallics, the Ni and Fe sites possess small positive charges. Density functional theory calculations have shown that the replacement of Ni atoms in NiSi 2 with Fe atoms increases the d electron density of metal near the Fermi level because of an electron transfer from Ni to Fe.15,39 As proposed by Ueckert et al., the metal site with a high electron density favors the formation of the π back bond between the aromatic ring and metal sites, which promotes the hydrogenation of the aromatic ring.40 This may be one reason for the higher activity of the Ni-rich nickel-iron bimetallic silicides. 18


Page 18 of 46

Page 19 of 46

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


Figure 9 shows the product distribution and selectivitiy of the HDS of DBT versus the contact time at 3 MPa H 2 , and 340 oC over the Ni 0.75 Fe 0.25 Si 2 catalyst. It is worth noting that the HDS of DBT over Ni 0.75 Fe 0.25 Si 2 forms two main reaction products BP, CHB and two other trace products (tetrahydrodibenzothiophene (THDBT), bicyclohexyl (BCH)). The selectivity of the main product BP decreased slowly from 85% at the low contact time to 80% at high contact time. The second most abundant reaction product was CHB, with the increased selectivity from 12% to 19% as increasing the contact time. The result indicated that the hydrogenation of BP occurred over the bimetallic silicide catalyst. In addition, the selectivity to THDBT decreased from 3% at low contact time to a negligible amount at high contact time. This indicates that the THDBT was further hydrogenated to the CHB with the increased contact time. At high contact time, traces of BCH were produced, meaning that CHB was further hydrogenated to the BCH over Ni 0.75 Fe 0.25 Si 2 catalyst. This result further confirmed that the substitution of Fe for the Ni atoms in the Ni 1−x Fe x Si 2 significantly improved hydrogenation activity.

3.5. Characterization of HDS-tested Catalysts

The effect of Fe-substituted Ni-Si intermetallics on the structural stability was determined from measurements of surface chemistry and phase state in the spent catalyst, using XPS and XRD, respectively. Figure 10 shows XPS spectra of the fresh and spent Fe19



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

substituted Ni-Si intermetallic catalysts including Ni 2p, Fe 2p, and S 2p core level spectra. For the Ni 2p 3/2 core level of the fresh NiSi 2 catalyst, the peak at the bonding energies of 853.8 eV is assigned to NiSi 2 intermetallic compound, which are higher than that of metallic Ni (852.3 eV), indicating that the covalent interactions between Ni and Si lead to a slight transfer of electron density from the metal to silicon.41 The peak due to the lessintense asymmetric tail for the nickel silicide nearby 856.6 eV is present. A slight shift of the Ni 2p 3/2 binding energy to 853.0 eV for the Ni 0.75 Fe 0.25 Si 2 was observed, which indicated that the substitution of Fe enhanced the metal−metal interactions. After the HDS reaction, there is no appearance of new peak for the signal of Ni 2p 3/2, which clearly suggests that the Fe substituted Ni-Si intermetallic presented high sulfur tolerance. However, a slight shift of the Ni 2p to high binding energy was observed, which can be assigned to the adsorption of sulfur-containing compound or oxygen species on the surface of catalysts lead to the variation of metal electron structure.42

From the Fe 2p spectra in Figure 10b, the peak attributed to the Fe-O phase at ca. 710.2 eV is presented in the Fe-Si sample. It is deduced that zero-valent iron is not significantly silicified into iron silicides at the silicification temperature 450 oC but oxidized into iron oxides due to the passivation process. However, both the peak due to the Fe-O phase and the peak assigned to Fe-Si phase at ca. 706.8 eV are present in the Ni 0.75 Fe 0.25 Si 2

20


Page 20 of 46

Page 21 of 46

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


sample.43 It was therefore further confirmed that nickel promoted the silicification reaction. After the HDS process, the signals of Fe 2p 3/2 do not show significant variation.

As shown in Figure 10c, the S 2p signals from NiS (162.8 eV) and FeS (161.4 eV) for the spent catalysts have not been observed. Only a broad peak at ca. 170 eV is shown, which can be attributed to the adsorbed H 2 S on the surface of catalysts or the oxidation of the sulfur species retained on the surface. The result is similar to that of the surface of Ni 2 P particles after the HDS reaction.44-45 The chemical bond between metal and silicon weakens the interaction of metal with sulfur, improving the sulfur tolerance capacity.

Figure 11 shows the XRD patterns of spent Fe- substituted Ni-Si intermetallics. The structure of NiSi 2 and Ni 0.75 Fe 0.25 Si 2 catalysts are almost invariant after the HDS activity test, which further indicated that Fe-substituted Ni-Si intermetallics can efficiently resist sulfur incorporation during the HDS reaction. Some of the silicon electrons occupy the partly empty nickel or iron d orbital, which allows silicon to bond with free electron pairs of e.g. sulfur or oxygen and protects the metal from sulfur poisoning or oxidation.46 For the Fe-Si sample, the pattern of the sample after the HDS process resembles the pattern of the fresh sample. However, there are only peaks attributed to Feo for the spent Fe-Si sample and the peaks due to the Fe 2 O 3 disappeared. The zero-valent iron was surprisingly not sulfided or oxidized during the HDS process. It can be assumed that the metallic Fe

21



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

atoms were coated by SiO 2 overlayers, hindering the transformation.29 These results clearly suggest that the Fe-substituted Ni-Si intermetallics are efficient and sulfur-tolerant hydrotreating catalysts in petroleum processing technologies.

4. Conclusions Fe-substituted Ni-Si intermetallics as new catalytic materials have been tested for the first time for the HDS of DBT. Cheap and ubiquitous Fe substituted Ni atoms to form Nirich Ni 1-x Fe x Si 2 that significantly improved the hydrogenation activity, which indicated the presence of a synergistic effect between the two metals. Analysis of Mössbauer spectroscopy measurements and XRD results revealed Fe atoms occupied the relative site in the Ni 0.75 Fe 0.25 Si 2 material and preferentially bond with Ni to form alloy and combine with Si to form silicide. This led to a strong modification of geometry and electronic structure of the metals, allowing a high activity for the HDS of DBT. The trend in HDS activities observed for the Ni 1-x Fe x Si 2 catalysts is similar to that reported previously for Ni 1-x Co x Si 2 catalysts. High sulfur resistance was also observed for these catalysts. These findings will lay the scientific basis on the controllable synthesis of new bimetallic silicide catalysts and their applications in the field of catalytic HDS.

22


Page 22 of 46

Page 23 of 46

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


ACKNOWLEDGEMENTS We gratefully acknowledge financial support provided by the National Natural Science Foundation of China (21373038 and 21403026), the China Postdoctoral Science Foundation (2015T80255 and 2014M551068), and the Natural Science Foundation of Liaoning Province in China (2015021014).

SUPPORTING INFORMATION XRD pattern of FeSi 2 and product distribution of the HDS of DBT versus reaction temperature and contact time over FeSi 2 catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1) Schmitt, A. L.; Higgins, J. M.; Szczech, J. R.; Jin, S. Synthesis and Applications of Metal Silicide Nanowires. J. Mater. Chem. 2010, 20, 223-235. (2) Chen, X.; Zhao, A. Q.; Shao, Z. F.; Li, C.; Williams, C. T.; Liang, C.H. Synthesis and Catalytic Properties for Phenylacetylene Hydrogenation of Silicide Modified Nickel Catalysts. J. Phys. Chem. C 2010, 114, 16525-16533. (3) McEnaney, J. M.; Schaak, R. E. Solution Synthesis of Metal Silicide Nanoparticles. Inorg. Chem. 2015, 54, 707-709.

23



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

(4) Zaheer, M.; Motz, G.; Kempe, R. The Generation of Palladium Silicide Nanoalloy Particles in a SiCN Matrix and Their Catalytic Applications. J. Mater. Chem. 2011, 21, 18825-18831. (5) Yang, K. X.; Chen, X.; Guan, J. C.; Liang, C. H. Nickel Silicides Prepared from Organometallic Polymer as Efficient Catalyst towards Hydrogenation of Phenylacetylene. Catal. Today 2015, 246, 176-183. (6) Chen, X.; Li, M.; Guan, J. C.; Wang, X. K.; Williams, C. T.; Liang, C. H. Nickel– Silicon Intermetallics with Enhanced Selectivity in Hydrogenation Reactions of Cinnamaldehyde and Phenylacetylene. Ind. Eng. Chem. Res. 2012, 51, 3604-3611. (7) Chen, X.; Jin, J. H.; Sha, G.Y.; Li, C.; Zhang, B. S.; Su, D. S.; Williams, C. T.; Liang, C. H. Silicon–Nickel Intermetallic Compounds Supported on Silica as a Highly Efficient Catalyst for CO Methanation. Catal. Sci. Technol. 2014, 4, 53-61. (8) Levy, R. B. in Advanced Materials in Catalysis, eds. Burton, J. J.; Garten, R. L. Academic Press, New York, 1977. (9) Chen, X.; Liu, X.; Wang, L.; Li, M.; Williams, C. T.; Liang, C.H. High Sulfur Tolerance of Ni–Si Intermetallics as Hydrodesulfurization Catalysts. RSC Adv. 2013, 3, 1728-1731. (10) Chen, X.; Wang, X. K.; Xiu, J. H.; Williams, C. T.; Liang, C. H. Synthesis and Characterization of Ferromagnetic Nickel–Cobalt Silicide Catalysts with Good Sulfur 24


Page 24 of 46

Page 25 of 46

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


Tolerance in Hydrodesulfurization of Dibenzothiophene. J. Phys. Chem. C 2012, 116, 24968-24976. (11) Srivastava, V. C. An Evaluation of Desulfurization Technologies for Sulfur Removal from Liquid Fuels. RSC Adv. 2012, 2, 759-783. (12) Vasudevan, P. T.; Fierro, J. L. G. A Review of Deep Hydrodesulfurization Catalysis. Catal. Rev.: Sci. Eng. 1996, 38, 161-188. (13) Pawelec, B.; Navarro, R. M.; Campos-Martin, J. M.; Fierro, J. L. G. Towards Near Zero-Sulfur Liquid Fuels: A Perspective Review. Catal. Sci. Technol. 2011, 1, 23-42. (14) Gaudette, A. F.; Burns, A. W.; Hayes, J. R.; Smith, M. C.; Bowker, R. H.; Seda, T.; Bussell, M. E. Mössbauer Spectroscopy Investigation and Hydrodesulfurization Properties of Iron–Nickel Phosphide Catalysts. J. Catal. 2010, 272, 18-27. (15) Oyama, S. T.; Zhao, H.; Freund, H.; Asakurad, K.; Włodarczyk, R.; Sierka, M. Unprecedented Selectivity to the Direct Desulfurization (DDS) Pathway in a Highly Active FeNi Bimetallic Phosphide Catalyst. J. Catal. 2012, 285, 1-5. (16) Zhao, H.; Oyama, S. T.; Freund, H.; Włodarczyk, R.; Sierka, M. Nature of Active Sites in Ni 2 P Hydrotreating Catalysts as Probed by Iron Substitution. Appl. Catal. B: Environ. 2015, 164, 204-216.

25



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

(17) Chen, X.; Guan, J. C.; Sha, G. Y.; Gao, Z. M.; Williams, C. T.; Liang, C. H. Preparation and Magnetic Properties of Single Phase Ni 2 Si by Reverse Rochow Reaction. RSC Adv. 2014, 4, 653-659. (18) Zhang, H.; Li, F.; Liu, C.; Cheng, H. The Facile Synthesis of Nickel Silicide Nanobelts and Nanosheets and Their Application in Electrochemical Energy Storage. Nanotechnol. 2008, 19, 165606-165613. (19) Tuan, H.; Lee, D. C.; Hanrath, T.; Korgel, B. A. Catalytic Solid-Phase Seeding of Silicon Nanowires by Nickel Nanocrystals in Organic Solvents. Nano Lett. 2005, 5, 681684. (20) Landrum, G. A.; Hoffmann, R.; Evers, J.; Boysen, H. The TiNiSi Family of Compounds: Structure and Bonding. Inorg. Chem. 1998, 37, 5754-5763. (21) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity. Pure Appl. Chem. 1985, 57, 603-916. (22) Liu, K.; Wang, A.; Zhang, W.; Wang, J.; Huang, Y.; Shen, J.; Zhang, T. Quasi-in situ 57Fe Mössbauer Spectroscopic Study: Quantitative Correlation between Fe2+ and H 2 Concentration for PROX over Ir−Fe/SiO 2 Catalyst. J. Phys. Chem. C 2010, 114, 85338541. 26


Page 26 of 46

Page 27 of 46

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


(23) Cranshaw, T. E.; Johnson, C. E.; Ridout, M. S. The Mössbauer and NMR Spectra of FeSi Alloys. Phys. Lett. 1966, 21, 481-483. (24) Zeifert, B. H.; Salmones, J.; Hernández, J. A.; Reynoso, R.; Nava, N.; Reguera, E.; Cabañas-Moreno, J. G.; Aguilar-Ríos, G. X-Ray Diffraction and Mössbauer Characterization of Raney Fe-Ni Catalysts. J. Radioanal. Nucl. Chem. 2000, 245, 637-639. (25) Vondráček, M.; Dudr, V.; Tsud, N.; Lejček, P.; Cháb, V.; Prince, K. C.; Matolín, V.; Schneeweiss, O. Surface Segregation in FeSi Alloys. Surf. Sci. 2006, 600, 4108-4112. (26) Abdu, Y. A.; Ericsson, T. Mössbauer Spectroscopy, X-Ray Diffraction, and Electron Microprobe Analysis of the New Haifa Meteorite. Meteoritics Planet. Sci. 1997, 32, 373375. (27) Cuttler, A. H. The Behavior of a Synthetic 57Fe Doped Kaolin: Mössbauer and Electron Paramagnetic Resonance Studies. Clay Miner. 1980, 15, 429-444. (28) Mitra, A.; Mohapatra, J.; Meena, S. S.; Tomy, C. V.; Aslam, M. Verwey Transition in Ultrasmall-Sized Octahedral Fe 3 O 4 Nanoparticles. J. Phys. Chem. C 2014, 118, 1935619362. (29) Li, M.; Chen, X.; Guan, J. C.; Wang, X. K.; Wang, J. H.; Williams, C. T.; Liang, C. H. A Facile and Novel Approach to Magnetic Fe@SiO 2 and FeSi 2 @SiO 2 Nanoparticles. J. Mater. Chem. 2012, 22, 609-616.

27



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

(30) Hafner, S. S.; Huckenholz, H. G. Mössbauer Spectrum of Synthetic Ferri-diopside. Nature 1971, 233, 9-11. (31) Langkau, S.; Wagn er, G.; Kloess, G.; Heuer, M. TEM Analysis of (Ni, Fe)Si 2 Precipitates in Si. Phys. Status Solidi A 2010, 207, 1832–1844. (32) Zhao, A.Q.; Zhang, X. F.; Chen, X.; Guan, J. C.; Liang, C. H. Cobalt Silicide Nanoparticles in Mesoporous Silica as Efficient Naphthalene Hydrogenation Catalysts by Chemical Vapor Deposition. J. Phys. Chem. C 2010, 114, 3962-3967. (33) Voskoboinikov, T. V.; Chen, H.; Sachtler, W. M. H. On the Nature of Active Sites in Fe/ZSM-5 Catalysts for NO x Abatement. Appl. Catal. B: Environ. 1998, 19, 279–287. (34) Egorova, M.; Prins, R. Hydrodesulfurization of Dibenzothiophene and 4, 6Dimethyldibenzothiophene Over Sulfided NiMo/γ-Al 2 O 3 , CoMo/γ-Al 2 O 3 , and Mo/γAl 2 O 3 Catalysts. J. Catal. 2004, 225, 417–427. (35) Abu, I. I.; Smith, K. J. The Effect of Cobalt Addition to Bulk MoP and Ni 2 P Catalysts for the Hydrodesulfurization of 4, 6-Dimethyldibenzothiophene. J. Catal. 2006, 241, 356–366. (36) Yao, J.; Lyutyy, P.; Mozharivskyj, Y. Crystal Structure, Coloring Problem and Magnetism of Gd 5-x Zr x Si 4 . Dalton. Trans. 2011, 40, 4275−4283. (37) Gallezot, P.; Cerino, P. J.; Blanc, B.; Flèche, G.; Fuertes, P. Glucose Hydrogenation on Promoted Raney Nickel Catalysts. J. Catal. 1994, 146, 93–102. 28


Page 28 of 46

Page 29 of 46

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


(38) Sitthisa, S.; An, W.; Resasco, D. E. Selective Conversion of Furfural to Methylfuran Over Silica Supported NiFe Bimetallic Catalysts. J. Catal. 2011, 284, 90–101. (39) Rodriguez, J. A.; Kim, J. Y.; Hanson, J. C.; Sawhill, S. J.; Bussell, M. E. Physical and Chemical Properties of MoP, Ni 2 P, and MoNiP Hydrodesulfurization Catalysts: TimeResolved X-Ray Diffraction, Density Functional, and Hydrodesulfurization Activity Studies. J. Phys. Chem. B 2003, 107, 6276–6285. (40) Ueckert, T.; Lamber, R.; Jaeger, N. I.; Schubert, U. Strong Metal Support Interactions in a Ni/SiO 2 Catalyst Prepared via Sol-Gel Synthesis. Appl. Catal., A 1997, 155, 75– 85. (41) García-Me ́ndez, M.; Galvan, D. H.; Posada-Amarillas, A.; Farías, M. H. Experimental and Theoretical Study of the Electronic Properties of CoSi 2 and NiSi 2 . Appl. Surf. Sci. 2004, 230, 386−392. (42) Liu, J.; Lu, J. P.; Chu, P. W.; Blakely, J. M. Effect of Sulfur on Oxygen Adsorption and Oxidation of the Ni(111) Surface. J. Vac. Sci. Technol. A 1992, 10, 2355. (43) Shabanova, I. N.; Trapeznikov, V. A. A Study of the Electronic Structure of Fe 3 C, Fe 3 Al and Fe 3 Si by X-Ray Photoelectron Spectroscopy. J. Electron Spectrosc. Relat. Phenom. 1975, 6, 297. (44) Duan, X. P.; Teng, Y.; Wang, A. J.; Kogan, V. M.; Li, X.; Wang, Y. Role of Sulfur in Hydrotreating Catalysis Over Nickel Phosphide. J. Catal. 2009, 261, 232−240.

29



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

(45) Cecilia, J. A.; Infantes-Molina, A.; Rodríguez-Castellón, E.; Jiménez-López, A. The Influence of the Support on the Formation of Ni 2 P Based Catalysts by a New Synthetic Approach: Study of the Catalytic Activity in the Hydrodesulfurization of Dibenzothiophene. J. Phys. Chem. C 2009, 113, 17032−17044. (46) Lewandowski, M. Hydrotreating Activity of Bulk NiB Alloy in Model Reaction of Hydrodesulfurization 4, 6-Dimethyldibenzothiophene. Appl. Catal. B: Environ. 2014, 160-161, 10-21.

30


Page 30 of 46

Page 31 of 46

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


Tables Table 1. Physicochemical Properties of Fe Substituted Ni-Si Intermetallics. Surface Area

Total Pore Volume

Average Pore Size

m2/g

cm3/g

nm

NiSi 2

53

0.15

15.4

Ni 0.75 Fe 0.25 Si 2

54

0.29

21.2

Ni 0.50 Fe 0.50 Si 2

21

0.20

38.7

Ni 0.25 Fe 0.75 Si 2

36

0.23

25.3

Fe-Si

88

0.57

26.0

sample

31



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

Table 2.

57

Fe Mössbauer Parameters of the Fe Substituted Ni-Si Intermetallics.

Iron sites IS/mm s-1 QS/mm s-1 H/T RI/% LW/mm s-1 FeSi 0.27 0.43 56.4 0.70 Ni 0.75 Fe 0.25 Si 2 FeSi 2 , Cubic -0.03 10.5 0.42 FeNi 3 0.02 0.05 27.3 33.1 0.77 Fe 0.64 Ni 0.36 -0.06 -0.18 30.4 35.7 0.68 FeSi 0.26 0.52 41.7 0.55 Ni 0.50 Fe 0.50 Si 2 FeSiO 3 0.42 2.29 2.6 0.22 FeNi 3 0.19 0.34 30.1 20.0 0.59 FeNi, Taemite 0.29 -0.03 47.8 33.6 0.66 FeNi, Kamacite 0.63 -0.35 44.4 23.6 1.30 Ni 0.25 Fe 0.75 Si 2 Fe 0.64 Ni 0.36 -0.14 -0.12 30.9 28.4 1.56 FeSiO 3 0.37 2.33 4.9 0.46 Fe 2 O 3 0.35 0.93 9.5 0.61 Fe 2 O 3 0.36 0.88 27.5 0.71 Si doped Fe 2 O 3 0.73 1.08 24.5 1.25 Fe-Si Feo 0.01 -0.01 32.8 22.4 0.33 Fe 3 O 4 A Tet 0.29 -0.02 48.3 5.8 0.27 Fe 3 O 4 B Oct 0.65 0 44.8 19.9 0.85 a Isomer shift (IS) is refered to α-Fe foil at room temperature. Uncertainty is ±5% of the reported value. Sample

32


Page 32 of 46

Page 33 of 46

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


Scheme and Figure Captions Scheme 1. Reaction network of HDS of DBT over the Fe substituted Ni-Si intermetallics. Figure 1. XRD patterns for Fe substituted nickel oxides having a range of compositions. Figure 2. XRD patterns of Fe substituted Ni-Si intermetallics. Figure 3. N 2 adsorption-desorption isotherms of Ni 1-x Fe x Si 2 intermetallic catalysts. Figure 4. 57Fe Mössbauer spectra of Fe substituted Ni-Si intermetallics. Figure 5. a) SEM image of the Fe substituted Ni-Si intermetallic (Ni 0.75 Fe 0.25 Si 2 ) and b) the corresponding EDX spectrum indicated by box in the SEM image. c) TEM image and HRTEM image (insert) of Ni 0.75 Fe 0.25 Si 2 sample. Figure 6. H 2 -TPR profiles of Fe substituted Ni-Si intermetallics. Figure 7. a) DBT HDS activity and b) selectivity versus reaction temperature for Fe substituted Ni-Si intermetallics. Figure 8. a) DBT HDS activity and product selectivity versus nominal Ni metal fraction for Ni 1−x Fe x Si 2 catalysts. b) Conversion of DBT versus the contact time at 3 MPa H 2 and 340 oC over Fe substituted Ni-Si intermetallics. Figure 9. Product distribution and selectivity of the HDS of DBT versus the contact time at 3 MPa H 2 and 340 oC over the Ni 0.75 Fe 0.25 Si 2 catalyst. Figure 10. XPS spectra for the fresh and spent Fe substituted Ni-Si intermetallics: a) Ni 2p core level spectra, b) Fe 2p core level spectra, c) S 2p core level spectra. Figure 11. XRD patterns for the fresh and spent NiSi 2 , Ni 0.75 Fe 0.25 Si 2 , and Fe-Si catalysts after HDS test.

33



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

DDS

S

Page 34 of 46

BP

HYD CHB

DBT

BCH

S THDBT

Scheme 1. Reaction network of HDS of DBT over the Fe substituted Ni-Si intermetallics.

34


Page 35 of 46

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


Figure 1. XRD patterns for Fe substituted nickel oxides having a range of compositions.

35



450 oC silicification

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 36 of 46

NiSi2 Ni0.75Fe0.25Si2 Ni0.50Fe0.50Si2 Ni0.25Fe0.75Si2 Fe-Si

10

20

30

40

50

60

70

80

2Theta (deg.)

Figure 2. XRD patterns of Fe substituted Ni-Si intermetallics.

36


90

Page 37 of 46

400 350 300

Volume@STP(cc/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


250 200 150


100 50 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Relative Pressure, P/P0

Figure 3. N 2 adsorption-desorption isotherms of Fe substituted Ni-Si intermetallics catalysts.

37



Ni0.75Fe0.25Si2

FeSi2, Cubic FeSi

-10

-8

-6

-4

-2

0

2

4

6

8

Ni0.50Fe0.50Si2

FeNi3

Relative Transmittance


FeNi3

10

Fe0.64Ni0.36 FeSiO3 FeSi

-10

-8

-6

-4

Velocity (mm/s) Ni0.25Fe0.75Si2

2

4

6

8

10

Fe-Si

Fe3O4 ATet


Fe0.64Ni0.36 FeNi, Kamacite FeNi, Taenite

-8

0

Fe3O4 BOct

FeSiO3

-10

-2

Velocity (mm/s)

Fe2O3


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 38 of 46

-6

-4

-2

0

2

4

6

8

10

Feo Si doped Fe2O3 Fe2O3

-10

-8

-6

-4

Velocity (mm/s)

-2

0

2

Velocity (mm/s)

Figure 4. 57Fe Mössbauer spectra of Fe substituted Ni-Si intermetallics.

38


4

6

8

10

Page 39 of 46

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


Figure 5. a) SEM image of the Fe substituted Ni-Si intermetallic (Ni 0.75 Fe 0.25 Si 2 ) and b) the corresponding EDX spectrum indicated by box in the SEM image. c) TEM image and HRTEM image (insert) of Ni 0.75 Fe 0.25 Si 2 sample.

39



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

Figure 6. H 2 -TPR profiles of Fe substituted Ni-Si intermetallics.

40


Page 40 of 46

60 50 40

(b) 100


30 20 10 0

300

320 340 360 Temperature (oC)

380

90 80 70 60 50 40 30 20 10 0

{

300

100 90 80 70 60 50 40 30 20 10 0

CHB Selectivity (%)

70

BP Selectivity (%)

(a) 80 DBT HDS Activity (nmol/g/s)

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


320 340 360 Temperature (oC)

{

Page 41 of 46

380

Figure 7. a) DBT HDS activity and b) selectivity versus reaction temperature for Fe substituted Ni-Si intermetallics.

41



100

30

80

BP CHB BCH

60

20

40

10

20

0

0

100 90 80

DBT Conversion (%)

40

(b) Product Selectivity (%)

(a) 50 DBT HDS Activity (nmol/g/s)

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

70 60

Page 42 of 46


50 40 30 20 10

0.75 1.00 0.25 0.50 0.00 Nickel Metal Fraction (Ni/(Ni+Fe))

0 0

1

2

3 4 5 6 Contact time (min)

7

8

Figure 8. a) DBT HDS activity and product selectivity versus nominal Ni metal fraction for Ni 1−x Fe x Si 2 catalysts. b) Conversion of DBT versus the contact time at 3 MPa H 2 and 340 oC over Fe substituted Ni-Si intermetallics.

42


Page 43 of 46

(a) 100 90 80 70 60

(b)

Ni0.75Fe0.25Si2

DBT THDBT BP CHB BCH

100

50 40 30

70 60 40 30 20

10

10 1

2

3

4

5

6

7

8

THDBT BP CHB BCH

50

20 0 0

Ni0.75Fe0.25Si2

90 80

Selectivity (%)

Product distribution (%)

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

1

2

3

4

5

6

7

8

Contact time (min)

Contact time (min)

Figure 9. Product distribution and selectivity of the HDS of DBT versus the contact time at 3 MPa H 2 and 340 oC over the Ni 0.75 Fe 0.25 Si 2 catalyst.

43



(a)

spent

Ni0.75Fe0.25Si2

(b) Fe 2p

Ni 2p

(c)

Fe-Si

spent

Fe-Si

S 2p Fe-O

spent

NiSi2

Ni0.75Fe0.25Si2

spent

Fe-O

Intensity (a. u.)

fresh

fresh

Intensity (a. u.)

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 44 of 46

Ni0.75Fe0.25Si2

NiSi2

fresh Fe-Si

fresh

866 864 862 860 858 856 854 852 850 848

735 730 725 720 715 710 705 700

Binding Energy (eV)

Binding Energy (eV)

178 176 174 172 170 168 166 164 162

Binding Energy (eV)

Figure 10. XPS spectra for the fresh and spent Fe substituted Ni-Si intermetallics: a) Ni 2p core level spectra, b) Fe 2p core level spectra, c) S 2p core level spectra.

44


Page 45 of 46

Fe-Si spent fresh

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


Ni0.75Fe0.25Si2

spent

fresh NiSi2 spent fresh 10

20

30

40

50

60

70

80

90

2Theta (deg.) Figure 11. XRD patterns for the fresh and spent NiSi 2 , Ni 0.75 Fe 0.25 Si 2 , and Fe-Si catalysts after HDS test.

45



TOC

60 DBT HDS Activity (nmol/g/s)

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 46 of 46

Ni0.75Fe0.25Si2

50

BP

40

HDS DBT

30

CHB

20 10 0

NiSi2

Fe-Si

0.75 1.00 0.25 0.50 0.00 Nickel Metal Fraction (Ni/(Ni+Fe))

46


Structure Investigation and Dibenzothiophene Hydrodesulfurization

Recommend Documents