Synthesis and Characterization of Ferromagnetic Nickel–Cobalt

Article pubs.acs.org/JPCC

Synthesis and Characterization of Ferromagnetic Nickel−Cobalt Silicide Catalysts with Good Sulfur Tolerance in Hydrodesulfurization of Dibenzothiophene Xiao Chen,† Xinkui Wang,† Jinghai Xiu,† Christopher T. Williams,‡ and Changhai Liang*,† †

Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China ‡ Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Preparation of highly active and excellent sulfur tolerant hydrodesulfurization (HDS) catalysts is very important for the removal the sulfur from the sulfur containing compounds in petroleum. Herein we report on the synthesis and characterization of ferromagnetic nickel−cobalt silicide (Ni1−xCoxSi2) solid solution catalysts having large surface area by the reaction of nickel cobalt oxide solid solutions with SiH4. The catalytic properties of Ni1−xCoxSi2 were investigated for HDS of dibenzothiophene (DBT). The results showed that the saturation magnetization of the Ni1−xCoxSi2 solid solutions with fluorite structure can be controlled by changing the molar ratio of Ni to Co. The nickel-rich Ni0.75Co0.25Si2 catalyst is much more active than that of monometallic silicide (NiSi2 and CoSi2) and significantly improves the hydrogenation property (31.5% HYD selectivity), proving the synergistic effect between the components. X-ray photoelectron spectroscopy (XPS) provided further evidence that the valence electron concentration of the Ni increased with increasing the Co substitution, enhancing the metal−silicon and metal−metal interactions. In addition, the Si sites in the silicides alter the metal coordination, leading to a strong modification of the electronic structure around the Fermi level of the metals. This engenders a high activity for the HDS of DBT and weakens the metal−sulfur bonds, improving the sulfur tolerance. semiconductor (CMOS) devices,13 thin film coatings,14 photovoltaics,15 thermoelectrics,16 and catalysts.17−24 Wallace et al. indicated that both untreated and oxidized alloys of Ni2Si, Ni5Si2, and Co2Si prepared by induction melting of the constituent metals exhibited high activity for the CO methanation.17,18 Nuzzo et al. reported that supported nickel silicides, prepared by exposing supported metals to volatile silane, presented high catalytic activity and selectivity for the competitive dehydrogenation and hydrogenolysis of cyclohexane.19 The catalytic activity of transition metal silicides in H2 oxidation are much higher compared to carbides.20 In our previous research, supported cobalt silicide catalysts, synthesized by metal organic chemical vapor deposition of Co(SiCl3)(CO)4, showed high catalytic activity and selectivity in naphthalene hydrogenation.21,22 Nickel silicides exhibit much higher selectivity to the intermediate products (hydrocinnamaldehyde and styrene) of the hydrogenation of cinnamaldehyde and phenylacetylene, respectively, compared to metallic nickel, which are attributed to electronic modification and isolation of active sites.23,24 However, to the best of our

1. INTRODUCTION Sulfur in transportation fuels such as diesel, gasoline, and jet fuel remains a major source of air pollution.1 The reduction of the sulfur content by hydrodesulfurization (HDS) catalysts has been a subject of intense investigation in recent years because of stringent regulations.2,3 The research and development of new HDS catalysts that are different from the classic supported metal sulfide catalysts is of great interest. Noble metals supported on zeolites have shown these catalysts are always limited in their application because of the high price of noble metals and the poisoning of active sites by the sulfur-containing compounds.4−6 Metal phosphides have shown initially highly active but easily poisoned by strong chemisorption of the sulfur-containing molecules on the metal sites and formation of stable and inactive Me−S species on the surface of catalysts.7−9 Thermochemical calculations have indicated that transition metal silicides can tolerate much higher H2S concentration than the corresponding phosphides.10 This means that transition metal silicides are potentially stable and sulfur-resistant in the catalytic reaction with sulfur-containing compounds. Metal silicides formed from the dissolution of silicon atoms into metal lattices have unusual physical and chemical properties.11,12 Such materials are currently employed for many applications including complementary metal oxide © 2012 American Chemical Society

Received: August 22, 2012 Revised: October 14, 2012 Published: November 7, 2012 24968

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exposure to 100% Ar for 30 min to remove residual O2, the sample was ready for pulse flow H2 titration. At room temperature, the adsorbed atomic oxygen reacts rapidly with the 10% H2/balance Ar pulse to form H2O and replace the adsorbed oxygen atom with atomic hydrogen. Hydrogen consumption was quantitatively determined by means of a high sensitivity thermal conductivity detector below the sample cell. Hydrogen pulses were continued until no further uptake of H2 was observed.29 The magnetization was characterized by a superconducting quantum interference (SQUID, MPMSXL5) magnetometer with a maximum field of 50 kOe. Magnetic parameters such as saturation magnetization (Ms), coercive force (Hc), and residual magnetization (Mr) were measured. The morphology and structural composition of Ni0.75Co0.25Si2 were characterized and analyzed by high resolution field emission scanning electron microscopy (FESEM, Nova NanoSEM 450 from FEI Co.), equipped with an energy-dispersive X-ray (EDX) analyzer. Transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) were performed by using a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV after placing a drop of ethanol solution contained powder samples on carbon-coated Cu grids. The composition analysis was performed by using the highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) at a JEOL 2200FS STEM/ TEM instrument operating at 200 kV. Surface compositions were investigated by X-ray photoelectron spectroscopy (XPS) employing an ESCALAB250 (Thermo VG, USA) spectrometer with Al Kα (1486.6 eV) radiation with a power of 150 W. Survey and individual highresolution spectra were recorded with a pass energy of 50 eV. Ni 2p, Co 2p, Si 2s, and S 2s lines were monitored. All core− level spectra were referenced as the C 1s neutral carbon peak at 284.6 eV and were deconvoluted into Gaussian component peaks. HDS Activity Measurements. The HDS of DBT was performed in a high-pressure fixed bed continuous flow stainless steel catalytic reactor. The reaction temperature was measured with an interior placed thermocouple in direct contact with the catalyst bed. The liquid reactant was composed of 0.5 wt % octane (as internal standard), 0.3 wt % DBT reactant, and decalin as solvent. Prior to the activity test, the passivated catalysts (0.2 g, diluted with quartz sand to 5 mL) were activated in situ with H2 at 400 °C and atmospheric pressure for 2 h. Catalytic activities were measured at different temperatures (320−380 °C), under 3.0 MPa of H2, with a flow rate of 144 mL min−1 and with weight hourly space velocities (WHSV) of 21.1 h−1. The reaction product composition was analyzed using a gas chromatograph (GC 7890F) with flame ionization detector and a SE-54/52 capillary column.

knowledge, the metal silicides as novel HDS catalyst have not yet been reported so far. Taking into consideration the synergistic effect of sulfided NiMo and bimetallic phosphides catalysts (Co0.02Ni2P, FexNi2−xPy) in HDS,25−27 we might expect to observe the similar phenomenon in bimetallic silicides. In our work, nickel cobalt bimetallic silicide (Ni1−xCoxSi2) catalysts had successfully been synthesized by a direct silicification method at relatively low temperature and atmospheric pressure. The Nirich Ni1−xCoxSi2 catalyst exhibited excellent desulfurizing properties in HDS of DBT. A synergistic effect is found for the promoted metal silicides. In addition, the high sulfur tolerance of nickel−cobalt silicides had been studied in depth.

2. EXPERIMENTAL SECTION Catalyst Preparation. Bimetallic nickel−cobalt silicides (Ni1−xCoxSi2) were prepared by the reaction of a solid solution of nickel and cobalt oxides with SiH4.28 The metal oxides solid solution precursors were prepared by the method of precipitation. 0.02 mol of metal acetate tetrahydrate mixture (Me(OAC)2·4H2O, Me = Ni + Co) was dissolved in 60 mL of ethylene glycol, and the mixture was gradually heated to 140 °C. 200 mL of aqueous 0.2 M Na2CO3 solution was added, and then the slurry was further aged for 1 h under vigorous stirring. After filtration and being washed with water, the solid obtained was dried at 100 °C overnight and calcined at 400 °C for 2 h in air. The resulting solids were subsequently reduced in H2 at a flow rate of 30 sccm in a temperature-controlled manner from room temperature (RT) to 350 °C and kept at 350 °C for another 3 h. The reduced samples were reacted with a 10% SiH4/H2 mixture (100 sccm) at 450 °C for 15 min. They were then cooled to the RT in H2 (30 sccm) and passivated in 1% O2/Ar overnight. The metal composition of the Ni1−xCoxSi2 catalysts varied over the range 0 < x < 1.00. Catalyst Characterization. X-ray diffraction (XRD) analyses of the samples were carried out using a Rigaku D/ Max-RB diffractometer with a Cu Kα monochromatized radiation source, operated at 40 kV and 100 mA. The XRD patterns were compared with the calculated patterns obtained from the Inorganic Crystal Structure Database (ICSD) using Jade6.0 software. Analysis of the Ni, Co, and Si contents of Ni1−xCoxSi2 catalysts was carried out on an inductively coupled plasma optical emission spectrometer (ICP-OES) after digestion in hydrofluoric acid. Nitrogen adsorption and desorption isotherms were constructed using the multipoint method at −196 °C and were measured using a Micrometrics 2020. Prior to the measurements, all samples were degassed completely at 200 °C in a vacuum of 10−3 Torr for at least 4 h. Surface areas were calculated from the linear part of the Brunauer−Emmet−Teller (BET) plot. H2 chemisorption and H2−O2 pulse titration of Ni1−xCoxSi2 were performed using a Micromeritics Autochem II 2920 automated chemisorption analyzer. Prior to measurement, ∼0.05 g samples were in situ reduced in pure H2 at 400 °C for 2 h and then cooled down to room temperature at Ar. After pretreatment, 10% H2/Ar pluses were injected and the H2 uptake was measured using a TCD. H2 pluses repeatedly injected until the response from the detector showed no further H2 uptake. During the H2−O2 titration, the sample was exposed to 10% O2/balance Ar for 30 min to saturate the Ni1−xCoxSi2 surface with adsorbed atomic oxygen. Following

3. RESULTS AND DISCUSSION Synthesis of Nickel−Cobalt Oxides. The XRD patterns of nickel−cobalt oxides solid solutions obtained by the method of precipitation are shown in Figure 1. It clearly shows the diffraction peaks at 37.2°, 43.3, 62.9°, 75.4°, and 79.4° reflect the pure cubic phase NiO (JCPDS no. 47-1049), while the Ni:Co molar ratios reached 1:1, the position of the XRD peaks due to NiO shifted to higher 2θ. Meanwhile, some new peaks at 31.0° and 58.9° appear. The peaks position act in accordance with the standard ICSD file no. 40-1191, meaning the 24969

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phases were identified by comparison with ICSD files (NiSi2, cubic, no. 43-0989; CoSi2, cubic no. 38-1449). Those peaks at 28.6°, 47.4°, 56.3°, 69.3°, 76.6°, and 88.4° are attributed to the NiSi2 lattice planes (111), (220), (311), (400), (331), and (422), respectively. On the other hand, those peaks at 28.8°, 47.9°, 56.9°, 70.1°, 77.5°, and 89.4° are attributed to the CoSi2 lattice planes (111), (220), (311), (400), (331), and (422), respectively. A cubic phase with CaF2 structure was observed for both NiSi2 and CoSi2 (as shown in Figure 2). In the case of the Ni1−xCoxSi2 solid solutions, all of the XRD peaks positions due to NiSi2 shifted to higher 2θ values as the Co concentration in the mixed silicides increased. The unit cell dimension, a, was determined from the observed d-spacing for the (220) plane at around 48.0° by using the formula for a cubic lattice: a = d(h2 + k2 + l2)1/2, where h, k, and l are the Miller indices. One can notice a lattice parameter (inset of Figure 2) decrease with increasing Co, consistent with the fact that the ionic radius of Co2+ (0.54 Å) is lower than that of Ni2+ (0.69 Å).32 The result corresponds with Vergard’s law and is similar to that of the formation of CexFe1−xO2 and Fe1−xCoxSi solid solutions.33,34 It can be presumed that some Co atoms are substituted for Ni atoms in the Ni1−xCoxSi2 solid solution. Because both the NiSi2 and CoSi2 are cubic phases with CaF2 structure, the above results also suggest the formation of Ni1−xCoxSi2 solid solutions with the CaF2 structure. Based on calculations using the Scherrer equation, the NixCo1−xSi2 catalysts had crystallite sizes in the range 19−23 nm with an average of 21 nm (Table 1). The nominal and actual compositions of the as-prepared Ni1−xCoxSi2 catalysts are listed in Table 1. Both the actual and the surface compositions are similar to the nominal compositions. There is no surface segregation like some other metal oxides solid solution. The BET surface area and hydrogen (H2) chemisorption capacities of the Ni1−xCoxSi2 catalysts are also listed in Table 1. The BET surface areas of NiSi2, Ni0.75Co0.25Si2, Ni0.50Co0.50Si2, Ni0.25Co0.75Si2, and CoSi2 are 20, 33, 32, 39, and 8 m2/g, respectively. The Ni1−xCoxSi2 solid solutions have higher surface areas than the pure Ni and Co silicide, which is the highest surface area reported in the literature so far. Traditional metal silicides inherited from the microelectronic industry had low surface area ( NiSi 2 > CoSi 2 > Ni 0.25 Co 0.75 Si 2 > Ni0.50Co0.50Si2. With regard to the selectivities of the different products, two main products are detected: biphenyl (BP) and cyclohexylbenzene (CHB). As shown in Figure 8, BP is formed in greater proportions in all cases. Except for the Ni0.75Co0.25Si2 catalyst, which exhibited a BP selectivity of ca. 71%, the other Ni1−xCoxSi2 solid solution catalysts present more than 80%, indicating that the direct desulfurization (DDS) pathway is the dominant pathway for S removal from the DBT. Activity with Contact Time. With the purpose of determining the product distribution with the contact time on stream, Figure 9 shows the catalytic activities of Ni1−xCoxSi2 catalysts versus the contact time in the HDS of DBT at 3 MPa H2 and 340 °C. The HDS conversion represents the yield of all desulfurized hydrocarbon compounds. HDS conversion reached 77% for Ni0.75Co0.25Si2 at high contact time, 52% for NiSi2, and 26% for CoSi2. Over the monometallic silicide catalysts (NiSi2 and CoSi2), the conversion of DBT was much lower than it was over Ni0.75Co0.25Si2 catalyst. Thus, a few Co atoms substituted for Ni in the Ni−Co bimetallic silicide dramatically improved the HDS activity. The HDS of DBT over NiSi2 produces the three reaction species (Figure S2): biphenyl (BP) as the DDS product and tetrahydrodibenzothiophene (THDBT) and cyclohexylbenzene (CHB) as the intermediate and final products of the HYD 24973

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Figure 9. Conversion of DBT versus the contact time at 3 MPa H2 and 340 °C over NiSi2, CoSi2, and Ni0.75Co0.25Si2 catalysts.

route. The main product was BP, with a selectivity of 100% at the low contact time and 89% at the high contact time. Moreover, the formation of BP seemed to occur easily because its yield increased steeply with increasing the contact time. THDBT as an intermediate was present in a very small amount probably because of its slow formation and high reactivity under the experimental conditions. CHB was the second major product, whose selectivity increased from 0 at low time to 11% at high contact time. The high selectivity of the desulfurized products (almost 100%) indicates that the removal of sulfur from DBT and from the sulfur-containing intermediates is rapid over NiSi2. Furthermore, over CoSi2 the most abundant reaction product was BP (Figure S3), whose selectivity is above 96%. DBT almost completely reacted via the DDS route and only 4% reacted via the HYD pathway at the high contact time over CoSi2. The hydrogenated sulfur-containing intermediates were present in small quantities (lower 1% at our reaction condition). BP formed very rapidly over Ni0.75Co0.25Si2 catalyst and was the major product (Figure 10), as it was over the monometallic silicide catalysts. Its selectivity decreased slowly from 78% at low contact time to 68% at high contact time, indicating that hydrogenation of BP occurred over the bimetallic silicide catalyst. Initially, 80% DBT converted through the DDS route and 20% converted through the HYD route. The second most abundant reaction product was CHB, with increasing the selectivity from 14% to 30% as increasing the contact time. THDBT was observed in very small amounts. The selectivity decreased from 8% at low contact time to only 1% at high contact time, indicating the THDBT was further hydrogenated to the CHB with the increasing contact time. Comparing among the three catalysts, it is further demonstrated that the Ni-rich Ni1−xCoxSi2 improved the hydrogenation property due to the synergistic effect. Surface Chemistry of HDS-Tested Catalysts. To investigate the stability and the surface properties of the Ni1−xCoxSi2 solid solutions, XPS analysis was performed for the spent catalysts (Figure 11). After HDS reaction, the signals of Ni 2p3/2 and Co 2p3/2 do not show significant variation. From the Ni 2p spectra in Figure 11a, the peak attributed to the Ni− Si phase at ca. 854.8 eV and the peak due to less-intense asymmetric tail for the nickel silicide nearby 857.0 eV are still present. With increasing amount of Co in the Ni1−xCoxSi2 catalysts, the Ni 2p signal shifted from 854.0 to 853.6 eV. In Figure 11b, the intensity of Co 2p attributed to Co−Si phase at

Figure 10. Product selectivities of the HDS of DBT versus the contact time at 3 MPa H2 and 340 °C over the Ni0.75Co0.25Si2 catalyst.

ca. 782.1 eV decreased with decreasing of Co content in the Ni1−xCoxSi2 solid solutions. Another peak at 778.2 eV may be due to the zerovalent Co in the Ni1−xCoxSi2 solid solutions, which is surprisingly not been sulfided or oxidized after HDS reaction. It assumed that metallic Co atoms were coated by SiO2 overlays, hindering the transformation. The Ni1−xCoxSi2 also showed a stable Ni surface and Co surface after HDS reaction with no appearance of new peaks or detectable shift of the Ni 2p signal and Co 2p signal, respectively. Those results clearly suggest that the formation of Ni1−xCoxSi2 solid solutions strengthen the sulfur tolerance remarkably. The Si 2s signal analyses after HDS reaction is shown in the Figure 11c. The signal centered at ca. 154.0 eV is attributed to SiO2 arising from the oxidation of the surface of the spent catalysts. The Si 2s due to the Ni1−xCoxSi2 phases located at ca. 150.7 eV is noticeable in the spent catalysts. Because of the surface oxidation layer of the spent catalysts increasing, the intensity of Si 2s assigned to the Ni1−xCoxSi2 phases decreased. From the S 2p signals for the spent catalysts in Figure 11d, only a signal at 169.5 eV is shown. According to ref 45, the S 2p band at 169.9 eV on a H2S-passivated Ni2P/MCM-41 catalyst was due to the oxidation of the sulfur species retained on the surface, indicating a sulfur-associated surface reconstruction. In addition, Cecilia et al.46 have also reported partial oxidation of a sulfur species such as NiPxSy occurs on the surface of Ni2P particles after the HDS reaction. Therefore, the S 2p signals in our study may due to Ni1−xCoxSiySZ formed on the surface of Ni1−xCoxSi2 after the HDS reaction. The formation of sulfur species Ni1−xCoxSiySZ may imply that trace silicon is segregated and partial oxidized to SiO2, which further hindered the 24974

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Figure 11. XPS of (a) Ni 2p region, (b) Co 2p region, (c) Si 2s region, and (d) S 2p region of Ni1−xCoxSi2 spent catalysts after HDS reaction.

The Ni-rich Ni1−xCoxSi2 catalysts were very active, much more than those of NiSi2 and CoSi2 catalysts, which indicated the presence of a synergistic effect between the two metals. In addition, the Si sites in the silicide play an important role in the HDS reaction. The interaction between metals and silicon in the Ni1−xCoxSi2 solid solutions leads to a strong modification of electronic structure around the Fermi level of metals, which allows a high activity for the HDS of DBT. The number of metal active sites present on the surface decreases due to the coordination of Si, which weakens the metal−sulfur bonds and improves the sulfur tolerance. The findings provide bimetallic silicides as novel HDS catalyst and open up new ideas for development of HDS catalysts with high catalytic activity as well as efficient resistance to sulfur incorporation.

transformation of Ni1−xCoxSi2 solid solutions in the HDS reaction, improving the sulfur tolerance. The S 2p from NiS (162.8 eV) and CoS (161.9 or 162.6 eV) have not been observed, indicating that the Ni1−xCoxSi2 solid solutions prevented the system from deactivation induced by high coverage of strongly bound S that reduce the number of metal active sites present on the surface. Hence, the Si sites in the Ni1−xCoxSi2 solid solutions not only allow a high activity for the dissociation of DBT and molecular hydrogen because of the charge transfer from metal to silicon but also weaken the interaction of metal with sulfur. This means the Ni1−xCoxSi2 solid solutions can effectively and stably remove the sulfur from the sulfur-containing compounds in petroleum.

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4. CONCLUSIONS Mesostructured nickel−cobalt silicide solid solutions with high activity toward HDS and good sulfur tolerance have been successfully synthesized by using direct silicification method. All of nickel cobalt bimetallic silicide materials were cubic phases with CaF2 structure and ferromagnetic at room temperature.

ASSOCIATED CONTENT

S Supporting Information *

N2 adsorption−desorption isotherms of Ni1−xCoxSi2 solid solutions catalysts and product selectivities of the HDS of DBT versus the contact time over NiSi2 and CoSi2 catalyst. 24975

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AUTHOR INFORMATION

Corresponding Author

*Fax +86-411-84986353; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21073023 and 20906008) and the Fundamental Research Funds for the Central Universities (DUT12YQ03).

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