Nickel cobalt thiospinel nanoparticles as hydrodesulfurization catalysts

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Nickel cobalt thiospinel nanoparticles as hydrodesulfurization catalysts: the importance of cation position, structural stability and sulfur vacancy Kun Guo, Yi Ding, Jun Luo, and Zhixin Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03588 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Materials & Interfaces

Nickel cobalt thiospinel nanoparticles as hydrodesulfurization catalysts: the importance of cation position, structural stability and sulfur vacancy Kun Guoa,b, Yi Dingc, Jun Luoc, and Zhixin Yua,b* a

Department of Energy and Petroleum Engineering, University of Stavanger, 4036 Stavanger,

Norway b

The National IOR Centre of Norway, University of Stavanger, 4036 Stavanger, Norway

c

Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science

and Engineering, Tianjin University of Technology, Tianjin 300384, China

KEYWORDS. Thiospinel, hydrodesulfurization, cation position, structural stability, sulfur vacancy

ACS Paragon Plus Environment

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ABSTRACT. First-row transition metal based thiospinels are prepared via a one-pot versatile strategy and for the first time investigated as hydrodesulfurization (HDS) catalysts. XRD, TEM and XPS analysis confirm that these thiospinels consist of agglomerated nanoparticles (NPs) and contain multivalent metal cations. Among the sulfides synthesized at 230 °C, NiCo2S4 presents the highest thiophene conversion. This high intrinsic activity is found to be correlated with the normal spinel structure with Ni cations located on the tetrahedral sites and Co cations on the octahedral sites. However, the spent NiCo2S4 NPs experience phase transformation due to the relatively low synthetic temperature. Accordingly, six NiCo2S4 samples are prepared in the temperature range of 180–350 °C and their HDS activity increases monotonically with the synthetic temperature, which is attributed to the higher structural stability and more surface sulfur vacancy of the NiCo2S4 NPs prepared at higher temperatures. Notably, the NiCo2S4 NPs synthesized at 350 °C exhibit a much higher thiophene conversation of 62.9 % than the classic MoS2 catalyst (39.3 %) as well as excellent reusability. Our study suggests that the NiCo2S4 thiospinels with high activity and stability can represent a new promising class of industrial HDS catalysts.


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1. Introduction Sulfur removal is a long-standing issue in the petroleum industry, especially against the backdrop of stringent legislation and growing environmental awareness.1-2 Lower tolerance of sulfur content has been stipulated for the transportation fuels throughout the world, which poses higher requirements for the deep desulfurization processes in the oil refineries, and even necessitates the in-situ desulfurization of crude oil inside the reservoirs.3-5 Despite the adsorptive and oxidative desulfurization as complementary methods for sulfur removal, hydrodesulfurization (HDS) remains an effective technique to produce clean fuels with sulfur contents meeting the emission regulations.6-8 In the refining industry, HDS is often catalyzed by molybdenum or tungsten sulfide promoted by nickel or cobalt and supported on γ-alumina.9-11 This Ni(Co)–Mo(W)–S catalyst system has been extensively investigated and gained wide deployment in the oil refineries. However, as high HDS efficiency is required nowadays, these catalysts face new challenges due to their inadequate inherent catalytic activities and poor catalyst structures. In addition, traditional methods to prepare these catalysts usually consist of the pre-synthesis of metal oxides and post-sulfidation of the corresponding oxides, making it challenging to tailor-make and fine-tune the resulting catalyst structure. Recently, Lai et al.12 reported the one-step synthesis of a NiMoS flower structure composed of sulfide nanosheets, which exhibited excellent activities in the HDS of thiophene and 4,6-dimethyldibenzothiophene due to the high density of active sites in this flower structure. To stabilize such active sulfide species, Tang et al.10 introduced CoMo sulfides into the mesopores and micropores of mordenite nanofibers and the resulting catalyst achieved high activity and good catalyst life in the HDS of 4,6-dimethyldibenzothiophene. These studies


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underline the importance of exploring new HDS catalysts with facile preparation procedures, improved structural design and high catalytic activity and stability. Lately, transition metal sulfides with spinel-type AB2S4 structure (i.e., thiospinel, A, B = Ni, Co, Fe, Cu, etc.) emerge as an important material for diverse applications such as catalysis, energy harvesting, storage and conversion, and electronics.13 In an AB2S4 unit cell, as illustrated in Figure 1, metal A and B atoms occupy the tetrahedral and octahedral sites, respectively, and their positions can shift within the cell, leading to the normal and inverse spinel structures.13-14 The multivalence nature of both A and B metals endows the thiospinel-based materials with unique redox properties, good electrical conductivity and surface-abundant sulfur vacancies.14-16 These superiorities enable the thiospinels to be high performance catalysts in many chemical reactions.13 Nevertheless, the application of thiospinels as catalysts for the HDS remains unexplored, particularly the NiCo2S4 and CoNi2S4 since Ni and Co sulfides are well-established active species for the HDS.17-19 Potential advantages of thiospinels as HDS catalysts can originate from (a) the unique combination of different metal sulfides with multivalence into single-phase sulfide with stoichiometric composition and ordered atomic arrangement, (b) the abundant sulfur vacancies at the thiospinel surface that could serve as reactive sites for sulfur abstraction, (c) the direct utilization of thiospinels to avoid the troublesome sulfidation step, (d) the feasibility of tailor-making rational structure of thiospinel catalysts, and (e) the low cost, high abundance, low toxicity and good flexibility of thiospinels fabricated with first-row transition metals. It is thus of great interest to investigate the potential application of thiospinel-based catalysts for the industrially important HDS reaction.


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Figure 1. Illustration of the spinel-type crystal structure of AB2S4 (A, B=Ni, Co, Fe, Cu, etc.). Metal A cations occupy the tetrahedral sites and Metal B cations occupy the octahedral sites. In this study, we propose for the first time the application of thiospinel nanoparticles (NPs) as a new class of effective HDS catalysts. Facile, one-pot synthesis of a series of earth-abundant transition metal-based monosulfide and thiospinel NPs is reported by reducing the metal acetylacetonate precursors with elemental sulfur in the presence of oleyamine. The versatility of this synthetic approach is validated with the successful synthesis and detailed characterization of sulfides including NiS, Co3S4, NiCo2S4, CoNi2S4, NiFe2S4, and CuCo2S4. These sulfides are then evaluated with respect to their catalytic activities in the HDS of thiophene using a batch reactor and compared with MoS2 as the benchmark catalyst. The synergistic effect between Ni and Co atoms together with their specific site occupancy in the thiospinel structure is verified. As NiCo2S4 NPs present the highest HDS activity but insufficient stability, we further synthesize NiCo2S4 NPs at six different temperatures of 180, 200, 230, 250, 300 and 350 °C to study the effect of synthetic temperature on the HDS activity of NiCo2S4 NPs. It is revealed that the


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structural stability of NiCo2S4 thiospinels is important for achieving high HDS activity. Catalyst life of the optimized NiCo2S4 catalyst is also examined for potential industrial applications. 2. Experimental 2.1 Chemicals All chemicals were purchased and used as received without further treatment. Chemicals received from Sigma-Aldrich included nickel(II) acetylacetonate (Ni(acac)2, 95%), cobalt(III) acetylacetonate (Co(acac)3, 98%), iron(III) acetylacetonate (Fe(acac)3, ≥99.9%), copper(II) acetylacetonate (Cu(acac)2, ≥99.9%), sulfur (99.998%), molybdenum(IV) disulfide nanopowder (MoS2, average particle size of 90 nm, 99%) oleylamine (OAm, technical grade, 70%), 1,2,3,4tetrahydronaphthalene (tetralin, 99%), and naphthalene (≥99%). Thiophene (≥99%), hexane (≥99%) and toluene (≥99.5%) were ordered from VWR International AS. 2.2 Synthesis of sulfide NPs All sulfide NPs, including NiS, Co3S4, NiCo2S4, CoNi2S4, NiFe2S4, and CuCo2S4, were synthesized via the modified heat up method.20-22 Detailed amounts of the metal precursors, sulfur and OAm used for the synthesis were listed in Table S1. In a typical synthetic receipt with NiCo2S4 as an example, a 250 mL round bottom three-neck flask containing 15 mL of OAm, 0.5 mmol of Ni(acac)2, 1.0 mmol of Co(acac)3 and 2.0 mmol of sulfur (Ni:Co:S in the stoichiometric ratio of 1:2:4) was placed in an oil bath preheated to 80 °C. The flask was connected with a nitrogen inlet and a reflux condenser attached with a bubbler. After purging the flask with nitrogen for at least 30 minutes, the flask was quickly transferred to another oil bath, which was thermostated at 230 °C. The reaction was held for 1 hour and the flask was then taken out from


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the oil bath and cooled down naturally. Both the reaction medium and the oil bath were stirred with a magnetic stir bar at 1000 rpm. Afterwards, a mixture of hexane and ethanol (v/v = 1/3) was added to extract the NPs, and centrifugation was conducted to separate them at 8000 rpm for 5 minutes. This process was repeated at least 3 times to remove residual surfactants and impurities. The final powder was collected by drying the sample in an electronic oven at 60 °C overnight. In average, a high yield of 97.4% for the NiCo2S4 NPs could be obtained. For the preparation of NiCo2S4 at different temperatures, the same synthetic procedure was used except that when the flask was transferred to the second oil bath, whose temperature was preset to 180, 200, 230 and 250 °C, respectively. Whereas, a heating mantle (Glas-Col, LLC) instead of the oil bath was used to prepare NiCo2S4 at 300 and 350 °C. Similarly, the reaction flask was wrapped in the heating mantle, of which the temperature was initially controlled to 80 °C for the dissolving and purging, and then heated up to 300 or 350 °C in 2 minutes for the reaction. 2.3 Catalyst characterization X-ray powder diffraction (XRD) was performed to obtain the crystallographic information of the samples. The powder diffraction patterns were recorded on a Bruker-AXS Microdiffractometer (D8 ADVANCE) using Cu Kα radiation source (λ = 1.5406 Å, 40 kV and 40 mA). Scanning angles for all samples were set in the 2θ range of 10−90° with a step interval of 2.25 °/min. Peaks were indexed according to the database established by Joint Committee on Powder Diffraction Standards (JCPDS). The microstructures and morphology of sulfide samples were characterized by transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV) and scanning electron microscopy (SEM, FEI Helios NanoLab 460HP, 10 kV). Selected-area electron diffraction (SAED) of the


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sample was performed on the TEM. Elemental mapping of the NiCo2S4 sample was carried out using an energy dispersive X-ray (EDX) analyzer attached to SEM. For the specimen preparation, one droplet of the sulfide suspension was dropped onto a copper grid coated with carbon film (400 mesh, TAAB) and dried in air. X-ray photoelectron spectroscopy (XPS) analysis was performed on the ESCALAB 250Xi (Thermo Scientific) XPS system utilizing a monochromatic Al Kα source (1486.6 eV). Highresolution spectra were obtained at a pass energy of 30.0 eV, a step size of 0.1 eV, and a dwell time of 500 ms per step. All spectra were referenced to the C 1s peak (284.8 eV). Nitrogen adsorption−desorption measurements were conducted at the liquid nitrogen temperature of 77 K on a Micromeritics TriStar II surface area and porosity analyzer after degassing under vacuum at 100 °C for 8 h using a sample degas system (Micromeritics VacPrep 061). Specific surface area (SSA) was calculated using the Brunauer−Emmett−Teller (BET) method. 2.4 HDS of thiophene HDS reaction of thiophene was performed in a 4564 Parr Mini Bench Top Reactor with a volume capacity of 160 mL. Typically, a 40 mL liquid mixture composed of thiophene as the sulfur-containing compound, tetralin as the hydrogen donor and hexane solvent in a volume ratio of 1:9:10, together with 100 mg of catalysts were loaded into the reactor. After being tightly sealed, the reactor was sonicated in an ultrasound bath for 10 minutes to disperse the catalysts. Hydrogen (purity 5.0, Praxair Norge AS) or nitrogen (purity 2.6, Praxair Norge AS) was used to purge the reactor for 30 minutes and then pressurize it to 11 bar. Afterwards, the reactor was heated up from room temperature to 340 °C and the reaction system was held for 5 h for all the experiments. A final pressure in the range of 55−60 bar inside the reactor was reached. During


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the whole reaction period, the stirring torque was maintained at half of the full power. After cooling down naturally, the reactor was depressurized to atmosphere pressure. Measurement of the thiophene content before and after the reaction was carried out on an Agilent 7820 Gas Chromatograph (GC) equipped with a J&W HP-5 column (length 30 m, diameter 0.25 mm, thickness 1.0 µm) and a flame ionization detector (FID). In each analysis, 600 µL sample liquid from the reactor was diluted with 600 µL toluene solvent, and this mixture was utilized to measure the thiophene concentration. Each sample test was repeated three times on the GC and the average value was taken as the thiophene concentration. The thiophene conversion was calculated according to the following equation

∆C=

C0 -C1 ×100% C0

where ߂‫ ܥ‬is the thiophene conversion ratio; ‫ܥ‬଴ and ‫ܥ‬ଵ were the thiophene concentration before and after the HDS reaction, respectively. Two sets of experiments were conducted to study the effect of hydrogen donor. One was carried out by charging the reactor with tetralin and pressurizing it with nitrogen (purity 2.6, Praxair Norge AS). The other was carried out by charging the reactor without tetralin but pressurizing it with hydrogen. Reusability of the NiCo2S4 catalyst was evaluated by using the same catalyst for five consecutive HDS reactions. The same procedure was followed unless otherwise indicated. After each HDS test, the NiCo2S4 catalyst was recovered from the solution by centrifugation (8000 rpm, 5 min), washed twice with a mixture of hexane and ethanol, and dried in an electronic oven at 60 °C


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overnight. A new solution containing thiophene, tetralin and hexane with the same ratio as mentioned above was then added for each consecutive test. 3. Results and Discussion 3.1 Material characterization For the synthesis of metal monosulfides and thiospinels, metal acetylacetonates and elemental sulfur with specific stoichiometric ratios were dissolved in OAm at 80 °C to form a homogeneous solution, which was then heated up to 230 °C for 1 h. The targeted metal sulfides were obtained directly as the reaction products. This one-pot synthetic strategy can be well extended to a wide spectrum of sulfides by simply varying the type of metal precursors. In this study, six sulfides, i.e., NiS, Co3S4, NiCo2S4, CoNi2S4, NiFe2S4, and CuCo2S4, were prepared using this strategy. Commercial MoS2 nanopowder with an average particle size of 90 nm is used as a benchmark HDS catalyst. A representative TEM image of the as-prepared NiCo2S4 is shown in Figure 2a, indicating that the NiCo2S4 product consists of agglomerated NPs with sizes below 10 nm. TEM images of the NiS, Co3S4 and CoNi2S4 samples are displayed in Figures 2b-2d. The NiS product (Figure 2b) is composed of well-defined NPs with larger particle sizes and wider size distribution than that of NiCo2S4, whereas the poor image contrast of Co3S4 product (Figure 2c) suggests its quasiamorphous nature. The CoNi2S4 product (Figure 2d) also consists of agglomerated NPs and exhibits similar morphology as the NiCo2S4. The SAED of NiS (inlet of Figure 2b) manifests a spot pattern and the spots can be readily assigned to the crystal planes of hexagonal NiS, which agrees with the XRD result (Figure S1a). In contrast, the NiCo2S4, CoNi2S4 and Co3S4 show ring SAED patterns (inlet of Figure 2a, c and d), evidencing their low crystallinity. In particular, two


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ill-defined diffraction rings are observed for Co3S4, which further verify the poor crystallinity of Co3S4 (Figure S1b). A typical SEM image of the NiCo2S4, shown in Figure 2e, reveals that these NiCo2S4 NPs form large aggregates in submicron scale, resulting in an accidented surface. Elemental mapping of a 3µm × 3µm area, as highlighted in Figure 2e, and the corresponding EDX spectra (Figure 2i) demonstrate that the NPs contain uniformly distributed elements of Ni (Figure 2f), Co (Figure 2g), and S (Figure 2h). The atomic ratio of Ni:Co:S, as listed in inlet of Figure 2i, is calculated to be approximately 1:2:4, which is consistent with the stoichiometric composition of NiCo2S4.


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Figure 2. TEM images of the as-prepared (a) NiCo2S4, (b) NiS, (c) Co3S4, and (d) CoNi2S4 NPs. Inlets are the corresponding SAED patterns. SEM image (e) of the as-prepared NiCo2S4 NPs, and the corresponding elemental mapping of (f) Ni, (g) Co, and (h) S of the red dash area selected from (e), EDX spectra (i) and atomic percentage (inlet).


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The powder XRD patterns for the sulfides are acquired to identify their crystal phases. Figure 3a shows the XRD pattern for the NiCo2S4 NPs. All the characteristic peaks can be indexed to the cubic spinel-type NiCo2S4 phase, corresponding to the JCPDS No. 20-0782. The broad and weak peaks imply relatively small crystal sizes, which is in accordance with the result from TEM characterization. XRD patterns for the NiS, Co3S4 and CoNi2S4 NPs are shown in Figure S1. Their diffraction peaks conform to the JCPDS No. 75-0613, No. 47-1738 and No. 24-0334, respectively, indicating the formation of hexagonal NiS, cubic Co3S4 and cubic CoNi2S4 phases. The diffraction peak intensities, which correlate to the crystal size, also agree well with the TEM observations. It is found that the crystallinity of the NiS, Co3S4, CoNi2S4, and NiCo2S4 decreases with increasing Co content in the sulfide, which could be due to the strong coordinating effect between OAm and cobalt cation, hindering the formation of crystalline sulfides.23-25 Notice that the characteristic peaks of NiCo2S4 NPs are close to that of CoNi2S4 as they have the same crystal structure and cations. It is widely accepted that the NiCo2S4 thiospinel, which can be derived by substituting the Co2+ of Co3S4 with Ni2+, has the normal spinel structure, whereas the CoNi2S4 thiospinel has the inverse spinel structure.13,

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In the normal spinel structure of

NiCo2S4, Ni cations occupy the tetrahedral sites while Co cations occupy the octahedral sites. In addition, XRD pattern of the MoS2 nanopowder in Figure S2 confirms its hexagonal 2H–MoS2 phase.


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Figure 3. (a) XRD pattern of the as-prepared NiCo2S4 NPs. High-resolution XPS spectra of the (b) Ni 2p, (c) Co 2p, and (d) S 2p regions for the NiCo2S4 NPs. High-resolution XPS is conducted to investigate the surface species and chemical states of elements of the NiCo2S4 NPs. By using Gaussian fitting, the Ni 2p spectrum (Figure 3b) can be resolved into two spin-orbit doublets, i.e., Ni 2p1/2 and Ni 2p3/2, and two shakeup satellite peaks (indicated as “sat.”). The peaks at binding energy of 853.1 and 870.4 eV correspond to the Ni2+, while the peaks at binding energy of 856.4 and 874.1 eV belong to the Ni3+, indicating the coexistence of Ni2+ and Ni3+. In the Co 2p spectrum (Figure 3c), two spin-orbit doublets, corresponding to Co 2p1/2 and Co 2p3/2, and two shakeup satellite peaks (indicated as “sat.”) are obtained. The peaks at binding energy of 778.8 and 794.0 eV are indexed to the Co3+, while the


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peaks at binding energy of 781.5 and 797.5 eV are ascribed to the Co2+. The S 2p spectrum is shown in Figure 3d, in which the peaks at binding energy of 161.6 and 162.8 eV are attributed to S 2p1/2 and S 2p3/2, respectively. These XPS results reveal that the NiCo2S4 NPs contain Ni2+, Ni3+, Co2+, Co3+, and S2−, in good agreement with that reported in the literature.26-30 XPS spectra of NiS, Co3S4 and CoNi2S4 are illustrated in Figures S3-S5. The resulting elements can be confirmed and the metal cations show similar multivalence states of Ni2+, Ni3+, Co2+, and Co3+. Note that Ni3+ is also detected for the NiS NPs, which could be explained by the partial oxidation of surface Ni2+ when exposed to air.31 The surface elemental contents in atomic percentage from the XPS analysis, as listed in Table S2, are all consistent with the stoichiometric ratios of the four sulfides. SSAs of the four sulfides are measured by the nitrogen adsorption−desorption method. Figure S6 shows the nitrogen adsorption−desorption isotherms of NiS, Co3S4, CoNi2S4, and NiCo2S4 NPs. All the isotherms exhibit type III characteristic according to the IUPAC classification. No obvious hysteresis loops for these sulfides are observed, indicating a non-porous nature, which coincides with the fact that these sulfides are comprised of aggregated NPs. By using the BET method, the SSAs are calculated to be 65.09, 213.78, 135.91 and 159.45 m2/g for the NiS, Co3S4, CoNi2S4, and NiCo2S4 NPs, respectively. 3.2 HDS activity of the sulfides The catalytic activity of NiS, Co3S4, CoNi2S4, NiCo2S4, and MoS2 for the HDS reaction using thiophene as a sulfur-containing model compound is evaluated in a batch reactor. A mixture solution of thiophene, tetralin and hexane in the volume ratio of 1:9:10 is loaded in the reactor together with 100 mg of sulfide catalysts. Tetralin serves as the liquid hydrogen donor and


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hexane functions as the solvent. This mixture solution gives a relatively high sulfur content in percentage level that is close to the sulfur content of crude oils.32-33 The reactor is pressurized with hydrogen gas and heated up to 340 °C for 5 h, which are typical reaction conditions for industrial HDS processes. Figure 4a shows the thiophene conversion after the HDS without catalyst and with NiS, Co3S4, CoNi2S4, NiCo2S4, and MoS2. The blank test gives a very low conversion of 1.9 %, indicating that thiophene is thermally stable. When NiS NPs are employed as the HDS catalyst, a thiophene conversion of 10.4 % is achieved. Co3S4 catalyst delivers a much higher thiophene conversion of 24.3 %, which might be attributed to its larger SSA or higher intrinsic HDS activity than that of NiS. Thiophene conversion for CoNi2S4 NPs (19.0 %) is between that for NiS and Co3S4, implying that Co sulfide might play a more important role in the HDS than Ni sulfide. Interestingly, NiCo2S4 NPs give a high thiophene conversion of 36.0 %, close to that of MoS2 (39.3 %) as the benchmark HDS catalyst. Note that the SSA of NiCo2S4 is almost the same as that of CoNi2S4 and even smaller than that of Co3S4. Moreover, the composition-dependent activity trend is also not correlated to the crystallinity of these sulfides. Previous research has demonstrated that CoMo sulfide catalysts possess higher HDS activity than NiMo sulfides due to the higher desulfurization activity of the former, whereas NiMo sulfide catalysts show higher hydrogenation activity than CoMo sulfide, which should be resulted from the different Ni and Co promoters.34-35 Combination of Ni and Co sulfides as the thiospinel could bring synergistic effect to enhance both the hydrogenation and desulfurization activity. In addition, the HDS reaction involves electron donation and transfer, the facile redox behavior of multivalent cation in the thiospinel may play a positive role in the HDS activity.36-38 It is thus speculated that the activity enhancement exhibited by NiCo2S4 should originate from its


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normal spinel structure with Ni and Co cations located on the tetrahedral and octahedral sites, respectively.

Figure 4. (a) Thiophene conversion of the HDS reaction catalyzed without catalyst and with NiS, Co3S4, CoNi2S4, NiCo2S4, and MoS2. (b) GC chromatogram peaks corresponding to thiophene and naphthalene for the initial solution and the HDS reaction products catalyzed by NiS, Co3S4, CoNi2S4, and NiCo2S4 NPs. The liquid products after the catalytic HDS reaction are analyzed with a GC to understand the reaction mechanism. Individual pure compound is first examined to establish the retention time, as shown in Figure S7. Figure 4b presents the chromatogram peaks corresponding to thiophene and naphthalene for the initial solution and the liquid products after the HDS reaction catalyzed by NiS, Co3S4, CoNi2S4, NiCo2S4, and MoS2. It is observed that naphthalene is a new product after the HDS, which should be due to the dehydrogenation of tetralin.39 Following the increasing thiophene conversion in the order of NiS < CoNi2S4 < Co3S4 < NiCo2S4 < MoS2, the chromatogram peak of thiophene decreases while the peak of naphthalene increases, manifesting that naphthalene is a by-product in the HDS of thiophene. Furthermore, as both tetralin and hydrogen gas can act as the hydrogen donor for HDS, their individual roles are studied by


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comparing different catalytic reactions in the presence of tetralin and hydrogen, tetralin and nitrogen, and hydrogen only. Figure S8 presents the resulting thiophene conversion under these conditions. Thiophene is appreciably converted when either tetralin or hydrogen alone is added, demonstrating that both of them contribute to the HDS as effective hydrogen donor. Accordingly, the main mechanism of the HDS reaction is derived and illustrated in Figure S9, in which thiophene is converted to gaseous C4 compounds and hydrogen sulfide gas, tetralin is dehydrogenated to naphthalene, and both hydrogen gas and tetralin serve as hydrogen donors. To clarify the effect of normal spinel structure of NiCo2S4 with Ni cations located on the tetrahedral sites and Co cations on the octahedral sites, we further prepare NiFe2S4 and CuCo2S4 thiospinels via the same synthetic method as NiCo2S4. Ni cations of the NiFe2S4 thiospinel occupy the tetrahedral sites, whereas Co cations of the CuCo2S4 thiospinel are located at the octahedral sites. Besides, Fe and Cu sulfides are reported to be barely active for the HDS.40-42 These factors enable us to rationally study the individual effect of Ni and Co cations in the spinel structure of NiCo2S4. XRD patterns of the as-prepared NiFe2S4 and CuCo2S4 products, as shown in Figure 5a, confirm the formation of cubic thiospinel structure, which again highlights the versatility of this one-pot synthetic strategy. Figure 5b shows the thiophene conversion of HDS catalyzed by the four different thiospinels. NiFe2S4 and CuCo2S4 containing either Ni or Co cations exhibit lower thiophene conversion than CoNi2S4 and NiCo2S4, indicating that the differently positioned Ni and Co cations of NiCo2S4 should have synergistic structural effect in enhancing their HDS catalytic activity.


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Figure 5. (a) XRD patterns of the as-prepared NiFe2S4 and CuCo2S4 thiospinels. (b) Thiophene conversion of the HDS reaction catalyzed by CoNi2S4, NiCo2S4, CuCo2S4, and NiFe2S4 thiospinels. 3.3 Effect of synthetic temperature As the synthetic temperature (230 °C) of NiCo2S4 NPs is lower than the HDS reaction temperature (340 °C), the structural stability of thiospinel NPs is evaluated. Figure 6a shows the XRD patterns of the fresh and spent NiCo2S4 catalysts. After the HDS reaction, newly appeared characteristic peaks indexed to a Co9S8 phase (JCPDS No. 86-2273) are observed for the spent NiCo2S4 catalyst. The formation of Co9S8 phase reveals the deficient structural stability of NiCo2S4 thiospinels. This instability can be ascribed to the lower synthetic temperature relative to the HDS reaction temperature, which induces the surface segregation of Co atoms to form Co9S8 with S. The resulting Co9S8-rich surface is apparently detrimental to the HDS activity of thiospinels.


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Figure 6. (a) XRD patterns of NiCo2S4 prepared at 230 °C before and after the HDS reaction. (b) XRD patterns of the NiCo2S4 prepared at six temperatures. (c) Thiophene conversion of the HDS reaction catalyzed by NiCo2S4 prepared at six temperatures. (d) XRD patterns of the NiCo2S4 prepared at 350 °C before, after 1st and after 5th HDS reaction. (e) Content of surface sulfur vacancy expressed by the ratio of sulfur to metal content for NiCo2S4 prepared at six temperatures. The horizontal line at 1.33 is the stoichiometric value of NiCo2S4. (f) Thiophene


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conversion of the HDS reaction catalyzed by NiCo2S4 prepared at 350 °C for five consecutive cycles. To further elucidate the effect of preparation temperature or essentially the structural stability on the HDS activity, we compare the catalytic performance of NiCo2S4 NPs prepared at six different temperatures of 180, 200, 230, 250, 300, and 350 °C. Their XRD patterns in Figure 6b reveal that along with the increase of synthetic temperature, the resulting NiCo2S4 products present slightly higher crystallinity. The similarity of peak intensities can be ascribed to the strong binding strength of OAm surfactants to the NP surface, controlling the crystal growth. High resolution XPS of the Ni 2p, Co 2p and S 2p spectra for the six NiCo2S4 are presented in Figure S10-S12. It is confirmed that all the thiospinels contain similar multivalent elements. However, their SSAs, as indicated in Figure S13, increase firstly and then decrease as the synthetic temperature is elevated. Figure 6c displays the thiophene conversion catalyzed by the six different NiCo2S4 NPs. It can be seen that NiCo2S4 NPs synthesized at higher temperatures present higher thiophene conversions in the temperature range of 180–350 °C. Results from the corresponding chromatogram peaks of thiophene and naphthalene (Figure S14) also agree with this monotonic trend. Particularly, when the synthetic temperature is 350 °C, higher than the HDS reaction temperature of 340 °C, the highest thiophene conversion of 62.9 % is achieved, even higher than that of MoS2 (39.3 %). Figure 6d shows the XRD patterns of fresh and spent NiCo2S4 prepared at 350 °C. The thiospinel crystal phase remains unchanged, indicating the high structural stability. In comparison, XRD patterns in Figure S15 show that NiCo2S4 prepared at low temperatures all experience phase transformation with the formation of Co9S8. Furthermore, surface sulfur vacancy is considered as it can contribute as the active sites for sulfur abstraction in thiophene during HDS. The XRD characterization (Figure 6b) has


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demonstrated that NiCo2S4 prepared at different temperatures present the same bulk structure. Nevertheless, the surface atomic contents of Ni, Co and S from the XPS analysis show large variations, as listed in Table S3. The density of surface sulfur vacancy, as expressed by the ratio of S content to metal content, for the six NiCo2S4 is thus calculated and illustrated in Figure 6e. For NiCo2S4 NPs prepared at relatively low temperatures of 180 and 200 °C, the sulfur content is above the theoretical value of 1.33, indicating a sulfur-rich surface of the thiospinels and the probable segregation to form Co9S8. Higher synthetic temperature of 230 and 250 °C results in a sulfur content close to 1.33. Importantly, when further increasing the temperature to 300 and 350 °C, the sulfur content falls below the theoretical value, demonstrating that the surface of the resulting NiCo2S4 thiospinels is enriched with sulfur vacancy sites. This temperature effect on the surface sulfur content could be ascribed to the synthesis of NiCo2S4. Dissolution of elemental sulfur, which often forms octatomic rings with the chemical formula of S8, in OAm leads to the formation of polysulfide anions.43-45 At low synthetic temperature of 180 or 200 °C, the polysulfide anions combine with metal cations that are weakly coordinated by the OAm molecules and the ion diffusion is relatively slow, leading to the formation of NiCo2S4 thiospinel with a sulfur-rich surface. Increasing the synthetic temperature to 230 or 250 °C effectively facilitates the ion recombination with the formation of NiCo2S4 in the stoichiometric composition. When the synthetic temperature is further elevated to 300 or 350 °C, the coordinating effect of nitrogen in OAm becomes strong enough to compete with the sulfidation process, preventing part of the surface metal from being vulcanized. After surfactant removal, the resulting NiCo2S4 sample would have a surface enriched with sulfur vacancy. As such, it is inferred that the synthetic temperature of NiCo2S4 NPs can greatly affect the structural stability and the density of surface sulfur vacancy of the resulting NPs and thus the HDS activity.


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3.4 Catalyst reusability The catalyst life of HDS catalysts is another important factor to be considered for potential industrial applications. To this end, we have recycled the NiCo2S4 NPs synthesized at 350 °C for five consecutive HDS reactions. Figure 6f shows the thiophene conversion for five HDS runs. After five cycles, a high thiophene conversion of 60.7 % is retained with only a slight activity loss of 3.5 %, suggesting a remarkable reusability of the NiCo2S4 catalysts. XRD pattern of the spent NiCo2S4 catalyst after five runs, as shown in Figure 6d, is compared with that of the NiCo2S4 catalyst before and after first cycle. The similar peak intensities again demonstrate the excellent structural stability of NiCo2S4 catalyst synthesized at 350 °C. These results prove that the NiCo2S4 thiospinel NPs with high stability and activity are promising catalysts for industrial HDS processes. Conclusions Monosulfide and thiospinel NPs, including NiS, Co3S4, CoNi2S4, NiCo2S4, NiFe2S4, and CuCo2S4, are successfully prepared via a one-pot versatile strategy at 230 °C. Among these sulfides, NiCo2S4 NPs present the highest thiophene conversion in a batch reactor. The intrinsic activity is attributed to the normal thiospinel structure with Ni and Co cations occupying the tetrahedral and octahedral sites, respectively. NiCo2S4 thiospinels prepared at low temperatures face phase transformation during the HDS reaction, whereas high synthetic temperature endows the NiCo2S4 thiospinels with high structural stability and abundant surface sulfur vacancy, leading to remarkable activity and reusability, even better than the traditional MoS2 catalyst. Our study highlights the importance of structural stability and surface composition for the potential


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application of thiospinel NPs as HDS catalyst. Future study of supported NiCo2S4 catalysts could be an effective way to further enhance their catalytic performance for the HDS processes. ASSOCIATED CONTENT Supporting Information. Detailed amounts in the synthesis (Table S1), XPS elemental contents of the sulfides (Table S2 and S3), XRD patterns (Figure S1-S2) and XPS spectra (Figure S3-S5) of the sulfides, N2 adsorption-desorption isotherms (Figure S6 and S13), GC chromatograms (Figure S7 and S14), effect of hydrogen donor (Figure S8), schematic reaction pathway (Figure S9), XPS spectra of Ni 2p (Figure S10), Co 2p (Figure S11) and S 2p (Figure S12) for NiCo2S4, and XRD patterns (Figure S15) of NiCo2S4. AUTHOR INFORMATION Corresponding Author * Tel.: +47 51 83 22 38; Fax: +47 51 83 20 50; E-mail: [email protected]. ACKNOWLEDGMENT The authors thank Prof. Vidar F. Hansen and Dr. Wakshum M. Tucho, University of Stavanger, for the TEM characterization. The authors also acknowledge the Research Council of Norway and the industrial partners of The National IOR Centre of Norway for the financial support. REFERENCES (1) Dou, S. Y.; Wang, R. Recent Advances in New Catalysts for Fuel Oil Desulfurization. Curr. Org. Chem. 2017, 21 (11), 1019−1036.


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Nickel cobalt thiospinel nanoparticles as hydrodesulfurization catalysts

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