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Catalysis and Kinetics

Cobalt-molybdenum single-layered nanocatalysts decorated on carbon nanotubes and the influence of preparation conditions on their hydrodesulfurization catalytic activity Jamie Whelan, Marios S Katsiotis, Samuel Stephen, Gisha Elizabeth Luckachan, Anjana Tharalekshmy, Nicoleta-Doriana Banu, Juan-Carlos Idrobo, Sokrates T. Pantelides, Radu Valentin Vladea, Ionut BANU, and Saeed M. Alhassan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01571 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Cobalt-molybdenum single-layered nanocatalysts decorated on carbon nanotubes and the influence of preparation conditions on their hydrodesulfurization catalytic activity Jamie Whelan,1,2 Marios S. Katsiotis,1,3 Samuel Stephen,1 Gisha E. Luckachan,1 Anjana Tharalekshmy,1 Nicoleta Doriana Banu,1,4 Juan-Carlos Idrobo,5 Sokrates T. Pantelides,6 Radu V. Vladea,1 Ionut Banu,1,7 Saeed M. Alhassan1* 1

Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi,

United Arab Emirates 2

Department of Chemistry, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi,

United Arab Emirates 3

TITAN Cement Company, 22Α Ηalkidos Str., P.O. 111 43, Αthens, Greece

4

Centre for Organic Chemistry “C.D. Nenitescu”, Romanian Academy, 202B Spl Independentei,

060023 Bucharest, Romania 5

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN

37831, USA.


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Department of Physics and Astronomy and Department of Electrical Engineering and

Computer Science, Vanderbilt University, Nashville, TN 37235, USA. 7

Department of Chemical and Biochemical Engineering, University Politehnica of Bucharest,

313 Spl. Independentei, sector 6, 060042 Bucharest, Romania.

ABSTRACT Hydrodesulfurization (HDS) of crude oil plays a vital role in the refining of petroleum products. With ever-increasing regulations restricting the allowable concentrations of sulfur in fuel, further research is required to produce more efficient and effective catalysts. Herein we have synthesized carbon-nanotube (CNT)-supported cobalt-molybdenum (CoMo) catalysts for HDS of dibenzothiophene (DBT) via Co-first and Mo-first sequential impregnation as well as co-impregnation. Spectroscopic analysis shows the formation of a CoMo catalyst with no free sulfided Co phase present. Additionally, CoMo catalysts are found to be predominantly single-layered nanocatalysts layered on the CNT support. Temperature programmed reduction (TPR) measurements show differences in reducing temperature of the sulfided CoMo catalysts prepared by the different methods but catalyst activities for HDS of DBT did not fully align with the TPR-predicted order. Thus, provided the reaction temperature is high enough, reducibility may not always be an adequate gauge of catalytic activity. Conversion of DBT was highest in Mo-first sequential impregnation (81.5%), followed by co-impregnation (64%) and Co-first sequential impregnation (60%) on CNT support. While these results contrast with some others regarding the order of impregnation, we propose that the preferred impregnation order is actually support-dependent, rather than an absolute quality.


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1. INTRODUCTION The existence of sulfur in fuel sources remains problematic. Its presence is known to produce a number of unwanted effects including catalyst poisoning of catalytic oxidation converters in natural gas vehicles;1 oxidation to SO2 which thereafter produces H2SO4, a major contributor to airborne particle formation;2 and recent findings suggest it plays a key role in the production of carbon nanotubes (CNT) in internal combustion engines,3 potentially ending up in the lungs of children.4 Environmental regulations limiting sulfur content in petroleum fuel products has been on the rise, with one such example that of the International Maritime Organization setting a global limit outside of Emission Control Areas of 0.50% mass/mass (m/m) sulfur in fuel oil in 2020, from a 2012 limit of 3.50% m/m.5 This has led to renewed interest in increasing the efficiency of catalysts tasked with sulfur removal. The process of sulfur removal from crude oil is termed hydrodesulfurization (HDS) and involves a hydrogen source and usually an alumina-supported catalyst comprised of molybdenum disulfide (MoS2)6 oftentimes promoted with either cobalt7 or nickel.8 The many drawbacks of alumina9 have led to research into different supports,10 one of which is carbon nanotubes (CNT).11 From finding use in diverse fields ranging from bio-technology12,

13

and composite

materials14 to supports for heterogeneous catalysis,15, 16 a major benefit for their use as catalyst supports under elevated temperature conditions is their high thermal conductivity (up to 3000 W/mK).17 In a direct comparative study of CNT and γ-Al2O3 and their effects on activity of CoMo HDS in sulfur recovery, Pour et al. found that the interaction of CNT with the active metal


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catalyst resulted in a lowering of the reduction temperature of the active species, which translates, all other things being equal, to lower operational costs for HDS reactions.18 While many studies can be found in the literature showing excellent HDS activity,19 the effect of impregnation is generally not the focus with typical impregnation procedures being either sequential impregnation (either Mo or Co first) or co-impregnation (Mo and Co together). Farag compared adding Co first vs. co- impregnation with Mo and observed higher decomposition of dibenzothiophene (DBT) of the former, attributed to improved dispersion of the catalysts on the active carbon support.20 Impregnation order isn’t the only critical factor; the support itself also plays a key role. Rana et al. presented their findings on Co, Mo and EDTA impregnated in different sequences on supports with distinct isoelectric points (γ-Al2O3, SiO2, Al2O3–MgO – the latter being the most active) for HDS.21 The effect of the support was attributed to the observed delay of Co sulfidation temperature with respect to Mo, and was found to be significantly more important than the order of impregnation. On phosphate-containing hexagonal mesoporous silica, Nava et al. observed that co-impregnation of phosphate-containing Co and Mo resulted in better performing catalysts for deep HDS of DBT than sequential impregnation (Mo first).22 This improvement was attributed to a better dispersion of metals as well as a larger stacking degree of MoS2. On USY zeolites, using degree of acidity as a measure for catalytic activity, Nugrahaningtyas et al. found that sequential impregnation first with Mo then with Co yielded greater activity than sequential impregnation with Co first, followed by co-impregnation.23 In the case of other Co-Mo catalyzed reactions, Ferrari et al. discussed the preferential deposition of Co and Mo on the surface of active carbon particles for hydrodeoxygenation reactions.24 The


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differences in catalytic activity were attributed to the location of the metal components and their interaction with the support; Co exhibits preferential deposition on the surface while Mo at the interior of the grains. With sequential impregnation, it was found that adding Mo first resulted in the migration of Mo towards the outer surface, providing higher exposure to the reactants and thus improved activity. Clearly supports play a key role, as does the chosen impregnation method. Herein we have chosen CNT as the support due to its suitability25 and looked at Co and Mo sequential impregnation as well as co-impregnation to see if the order plays a role and their impact on HDS of DBT. Additionally, we aim to see if the resulting reducibility of the bi-metallic catalyst is a useful indicator of catalytic activity.


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2. MATERIALS AND METHODS 2.1 Materials and chemicals The following chemicals and gases were used as purchased: ammonium heptamolybdate tetrahydrate (AHM) (Merck, 99.97%); cobalt (II) nitrate hexahydrate (CoN) (Aldrich, 98+%); multi-walled carbon nanotubes (CNT) (Nanocyl 7000); dibenzothiophene (DBT) (Aldrich, 98%); decahydronaphthalene (decalin, mixture of cis and trans) (Aldrich, 98%); poly-butyl sulfide (SulfrZol 54, Lubrizol); hydrogen gas in helium (Air Products, 99.992%); nitrogen gas (Air Products, 99.995%). 2.2 Preparation of Catalysts The following aqueous stock solutions were prepared: (a) AHM (1.50 g, 1.21 x 10-2 M); (b) CoN (0.803 g, 2.76 x 10-2 M); and (c) CoN + AHM (1.50 g Mo + 0.803 g Co). Catalysts were prepared according to the following incipient wetness impregnation method: a metal solution (20.0 cm3) was added dropwise to 1.00 g of CNT spread evenly onto a petri dish to ensure complete coverage; the mixture was then sonicated, left to stand for 2 min, then dried on a heating mantle (50 oC); these steps were repeated for any other additions. The dried mixture was then calcined at 350 oC (5 oC/min) for 2 hr. The suffix designation “Mo” means impregnation only with Mo; “Co” is impregnation only with Co; “Mo_Co” indicates sequential impregnation with Mo first, followed by Co; “Co_Mo” indicates sequential impregnation with Co first, then Mo; and “CoMo” indicates co-impregnation of Co and Mo together. Sample catalysts are prefixed with “CNT”. For background purposes, CNT, solutions (a), (b) and (c) above were separately dried and calcined giving “CNT”, “Mo”, “Co” and “CoMo”.


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2.3 Sulfidation All catalysts were sulfided according to the following procedure: initially, the oxide catalyst was degassed under vacuum for 2 hr at 200 oC. Poly butyl sulfide (PBS, 2.0 cm3), containing 54% sulfur by weight, was added to a 100 cm3 batch reactor. The degassed catalyst (0.250 g) was added onto the PBS in an aluminum boat in the reactor at room temperature. The reactor was then sealed, flushed with N2 three times, flushed with H2 three times, filled with H2 gas (375 psig), then heated to 300 oC at 10 oC/min and left for 16 hr with a final pressure achieved of 720 psig. After the required time elapsed, the reactor was left to cool naturally to 50 oC and the sulfided catalyst was collected and used immediately for characterization and/or activity measurements (in a separate batch reactor); this procedure was identical for each catalyst. The designation S_CNT_Mo denotes sulfided CNT_Mo, with the prefix (S_) used for all sulfided material. 2.4 Catalyst Characterization Thermogravimetric analysis – differential thermal analysis (TGA-DTA) was performed using a LINSEIS STA PT1600 simultaneous system coupled with a Pfeiffer mass spectrometer (MS). All measurements were carried out with a heating rate of 10 oC/min under air atmosphere (flow rate 50 cm3/min). Temperature programmed reduction (TPR) measurements were carried out using a ChemBET Pulsar (Quantachrome) under continuous H2 flow (5% H2 in He) in a quartz cell at 10 oC/min with a thermal conductivity detector. A weighted sample (catalyst/control) was placed into a quartz cell under flowing H2 and left for 1 hr to ensure baseline stabilization. Once the signal had stabilized, the analysis was performed by heating the sample to 600 oC at 10 oC/min.


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Textural characterization was carried out on pre-sulfided catalysts from N2 sorption at -196 oC (Quantachrome Autosorb-1), with BET surface area (SBET) and average pore diameter (APD) determined from the resulting isotherm; prior to analysis each sample was degassed under vacuum at 300 oC for 2 hr. Samples for inductively coupled plasma mass spectrometry (ICP-MS) were initially digested into an aqueous solution of high purity HNO3 using an industrial grade microwave oven (Questron Qlab 8000). Digested samples were diluted and analyzed using a Perkin Elmer DRCe ICP-MS. Quantitative analysis was carried out with the instrument pre-calibrated with known standards across a wide mass range. Control standards were run along with the unknown samples to ensure accuracy of analysis data. X-Ray Diffraction (XRD) was performed with a Panalytical Powder diffractometer (X’Pert PRO) using CuKα radiation (45 kV, 40 mA) in the 2θ range 10 to 80o with a step time of 0.01 s. Diffuse reflectance infrared spectroscopy (DRIFT) spectra of all samples were obtained in a Bruker VERTEX 70 spectrometer equipped with Praying Mantis, KBr windows and DLTGs detector (Deuterated L-Alanine Doped Triglycine Sulfate). Samples were prepared by diluting with KBr (300:1). IR acquisitions between 4000 - 500 cm-1 were carried out using OPUS software at 8 cm-1 resolution with 32 scans. Raman spectra were recorded using a LabRAM HR spectrophotometer (Horiba Jobin Yvon) with CCD detector interfaced with an Olympus BX41 microscope (50x objective lens). Samples were excited with a 514 nm laser (50 mW) and a grating with 1200 lines/mm was used. Raman shifts were calibrated using Si wafer (521 cm-1).


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Transmission Electron Microscopy (TEM) was performed using a FEI Tecnai G20 with a 0.11 nm point resolution, operated at 200 kV. The microscope is coupled with Energy Dispersion Spectroscopy (EDS) X-ray analysis, allowing for determination of elemental composition. Samples were prepared by dispersing approximately ~2 mg of CNT supported catalyst in 25 cm3 cyclohexane (99.99%) by sonication at room temperature for 15 s. A drop of each suspension was deposited on 400 mesh copper grids covered with thin amorphous film (lacey carbon), and were immediately inserted in the TEM holder to avoid specimen contamination. The surface of the specimen was studied using low beam intensity to locate areas of interest and avoid specimen degradation. Prior to image acquisition, focusing and astigmatism correction were performed in proximity to the area of interest. Following image acquisition, EDS spectra were collected using a probe size roughly equal to image dimensions. Scanning transmission electron microscopy (STEM) experiments were performed in an aberration-corrected Nion UltraSTEM 10026, operated at acceleration voltage of 60 kv and 100 kV. The electron microscope is equipped with a cold field emission electron source and a corrector of 3rd and 5th order aberrations. The convergence semi-angle of the incident probe was set to ~30 mrad, the high-angle annular dark field (HAADF) images. The electron energy loss spectroscopy (EELS) experiments were performed using a collection angle of 54 mrad.

2.5 Catalyst Activity Measurement Activity was determined as conversion of DBT based on initial DBT concentration. The activity measurements were carried out as follows: freshly synthesized CNT-supported catalyst (0.200 g) was transferred to a 100 cm3 batch reactor immediately after preparation. 25 cm3 of


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feedstock containing 1% DBT in decalin was then added. The reactor was sealed, flushed with N2 three times and the reactor heated to 300 oC (10 oC/min) under stirring (1,000 rpm). Upon reaching the desired temperature, H2 (500 psig, 3.45 MPa) was injected into the reactor (time zero). After 3 hr, heating and stirring was stopped and the reactor allowed to cool naturally to room temperature. A sample of the reaction mixture was then removed, centrifuged, and the supernatant was diluted with decalin (10:1). The concentration of DBT was measured using a GC-MS (Agilent 6890N with an 5973MSD, Column HP5-MS 30 m x 0.250 mm x 0.25 um, maximum temperature 320 oC). Conversion of DBT was based on the decrease in signal from the GC-MS compared to a blank measurement performed at same conditions without the presence of catalyst.


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3. RESULTS AND DISCUSSION 3.1 Thermal and textural properties Results of TGA (mass loss given as % of total at 700 oC), DTA (CNT oxidation peak maximum), Brunauer–Emmett–Teller (BET) specific surface area (SBET) and average pore diameter (APD, acquired from desorption branch) of the catalysts are given in Table 1. As expected, impregnation of CNT with metal results in a decrease in SBET and the increase in APD indicates that the catalyst material preferentially occupies the smaller pores. Increased metal loading, as expected, left a larger residual mass compared to CNT without impregnation following thermal treatment of the catalyst material in air (i.e. burning). The percent mass loss at 700 oC (Table 1 and selected catalysts shown in Figure 1) shows reduced carbon content in samples impregnated with catalyst. CNT are known to burn in air at temperatures between 500 and 800 oC;27 in a similar fashion, our samples displayed an almost constant mass reading after 700 oC (Figure 1). DTA results show a decrease in oxidation temperature of CNT following metal impregnation, indicating thermal treatment of CNT in air is catalyzed by both Mo and Mo-Co (Table 1), also clearly evident from the TGA plots in Figure 1. Importantly, the overall mass of catalyst material loaded onto CNT is consistent across all three bi-metallic catalysts. Subtle differences in SBET may be indicative of a variation in dispersion depending on impregnation order.


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Figure 1: TGA of selected catalysts, CNT_Mo and CNT_Mo_Co, compared to CNT.

Results from ICP-MS and EDS show actual Mo:Co mole ratios of 7:1 for all Mo-Co catalysts, a slight deviation of the theoretical impregnated ratio of ca. 5:1. Overall, consistency of mass of loaded metals from TGA analysis and Mo-Co ratios from ICP-MS were found across multiple trials.


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Table 1: Thermal and textural properties of catalysts. Mass loss at 700 oC (%)

Oxidation Temp (oC)

SBET

CNT

80

547

392

7.4

CNT_Mo

76

489

190

22

CNT_Co

76

486

195

20

CNT_Mo_Co

69

473

167

20

CNT_Co_Mo

70

473

181

20

CNT_CoMo

70

474

204

17

Sample Name

2

-1 *

APD (nm)*

(m g )

*

Sulfur decomposition occurred during degassing thus these measurements are conducted on the oxide form.

3.2 XRD analysis XRD of the samples (Figure 2) show poorly crystalline MoS2 loaded on CNT. The presence of CNT phase is confirmed by the 002 (2θ = 26o) and 100 (2θ = 43o) peaks.28 For MoS2 three peaks are observed, 002 (2θ = 13o), 100 (2θ = 33o) and 110 (2θ = 58o), with the very broad nature of the 002 peak highly indicative of few or singly layered MoS2 slabs.29 Isolated Co peaks were not observed in any Co-impregnated catalysts, implying one or both of the following: Co is very well dispersed and/or some other complex has formed.


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Figure 2: XRD spectra of CNT sulfided and impregnated with Mo and Co. a) CNT, b) S_CNT_Co, c) S_CNT_Mo, d) S_CNT_CoMo, e) S_CNT_Co_Mo, f) S_CNT_Mo_Co.

3.3 DRIFT and Raman FT-IR analysis of sulfided background materials (Figure 3a) shows the evolution of a new peak (legend symbol *) for what is attributed to the CoMoS phase (423 cm-1). It can clearly be seen that there are several peaks which are in the CoMoS sample that are also present from MoS2, which is expected given the larger relative concentration of Mo in the samples (385 and 370 cm1

). Sulfided catalysts (Figure 3b) show significant peak overlap, making it difficult to adequately

distinguish individual peaks (given presence of the support and its impurities), although it can be seen that for CNT-supported CoMoS catalysts, there are peaks indicating the presence of all


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sulfided material. Similar to FT-IR spectra of sulfided background materials and catalysts, the Raman spectra (Figure 4a and b) also show the presence of new peaks which are attributed to the CoMoS phase (366 and 874 cm-1), with again the presence of all sulfided material also present on CNT-supported catalysts.


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Figure 3: FT-IR spectra of (a) sulfided background material S_Cal Mo, S_Cal_Co and S_Cal_CoMo; and (b) is FT-IR of sulfided catalysts; * indicates new peak distinct from calcined material (i.e. metal oxides).


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Figure 4: Raman spectra of (a) sulfided background material S_Cal Mo, S_Cal_Co and S_Cal_CoMo; and (b) is Raman of sulfided catalysts; * indicates new peaks distinct from calcined material (i.e. metal oxides).

3.4 TEM measurements TEM characterization indicates the formation of MoS2 nanolayers attached on the surface of the nanotubes (Figure 5), supporting the aforementioned XRD observation of few/single layered MoS2. Image analysis indicates that MoS2 nanolayers attached on CNT are predominantly single layered and range in size between 2 to 15 nm in length; no evidence of bulk material was observed. Furthermore, no isolated Co or CoMoS particles were observed, a phenomenon that has been mentioned before for this type of sample.30 EDS spectra indicate that Co is present on the MoS2 nanolayers (not shown here), confirmed through high spatial resolution EELS experiments.


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Figure 5: TEM of a) S_CNT_Mo_Co and b) S_CNT_Co_Mo. Arrows show single layered catalyst material.

Figures 6a) and b) contain HAADF images of typical CNT covered by MoS2 nanolayers; the corresponding averaged EEL spectrum is shown in Figure 6c. A change of slope is observable at approximately 750 eV and ending at about 800 eV. The change of slope is an indication that Co, which has a spectroscopy signature at the same energy range, is present in the MoS2 nanolayers. Several spectra were obtained from similar MoS2 nanolayers in other samples and the Co signal was always observed.

Figure 6: (a) and (b) HAADF images of a typical CNT covered by nanolayers of MoS2 (S_CNT_Mo_Co). (c) Average EEL spectrum of the highlighted red region shown in (b); the spectrum shows a change of slope and increase of intensity in the same energy range as where the Co L-edge is located, indicating the presence of Co atoms in the MoS2 nanolayers.

The characterization evidence put forth strongly indicates that Co is predominantly bound to Mo, not isolated as separate particles. All CoMo catalysts present an almost identical appearance


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when viewed with TEM, with the main structural component observed being single layered MoS2.

3.5 TPR analysis TPR measures the temperature at which H2(g) is consumed in the reactor and is therefore a measure of the reducibility of the HDS catalysts. A TPR plot of S_CNT_Mo shows the Mo reduction peak at 299 oC (Figure 7), while impregnation with Co decreases the maximum temperature of this peak. This decrease depends on choice of either sequential or coimpregnation, with sequential impregnation leading to lower reduction temperatures for S_CNT_Co_Mo (288 oC) and S_CNT_Mo_Co (278 oC), and co-impregnation resulting in the lowest reducing temperature for S_CNT_CoMo (272 oC). What is noteworthy is the fact that depending on the method of metal loading be it sequential (Mo or Co first) or co-impregnation, there are differences in reducibility of the sulfided form, which should conceivably carry over into their HDS activities. On this basis we would expect a catalytic activity order of S_CNT_Mo < S_CNT_Co_Mo < S_CNT_Mo_Co < S_CNT_CoMo, thus co-impregnation would appear to produce the most active catalyst based on reduction temperature.


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Figure 7: TPR of sulfided catalysts. Plots have been smoothed using Adjacent Averaging (20 points).

3.6 Catalytic activity As background, S_CNT was tested for HDS of DBT with no conversion observed while for all CoMo catalysts the well-known promoting ability of Co for MoS2-based HDS is clearly evident compared to the control S_CNT_Mo (Figure 8). Catalytic activity was found to increase in the order of S_CNT_Mo < S_CNT_Co_Mo ≈ S_CNT_CoMo < S_CNT_Mo_Co, which differs from the TPR-predicted catalyst of S_CNT_CoMo based on reducing temperature. These results contrast with those of Farag, who observed greater catalytic activity for Co-first sequential impregnation on active carbon support.20 However, the nature of the carbon support itself may


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play a larger role than previously thought. CNT and active carbon are known to have different adsorptive capacities for dyes31 as well as DBT32 and this adsorption behavior may be a decisive factor. Additionally, the method of sulfidation between the two studies varies, as do the activity measurement conditions, all of which could potentially explain discrepancies. Furthermore our results contrast with those of Nava et al. who observed Mo-first sequential impregnation yielded a less active catalyst supported on phosphate-containing hexagonal mesoporous silica.22 However, they appear to agree with findings on USY zeolites by Nugrahaningtyas et al.23 These findings lead us to propose a support-dependency on the order of impregnation, rather than a one-size-fits-all approach.

Figure 8: DBT % conversion of CNT-supported sulfided catalysts.


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In all CNT-supported catalysts there was excellent reproducibility in catalyst activity with more variability with Co-first (> ± 6%) vs. Mo-first (< ± 1%) sequential impregnation. In the present study it is seen that the order of impregnation plays a decisive role in determining catalytic activity of CNT-supported Co-Mo catalysts (60 vs 81.5% for S_CNT_Co_Mo

and

S_CNT_Mo_Co, respectively). This study highlights the need for further work towards understanding exactly the origin of the support effects and how these can be tailored for maximizing catalytic activity. Additionally, fine-tuning synthetic procedures to produce more efficient catalysts for hydrodesulfurization is also possible; a more thorough understanding of the placement of Co on brim and/or edge sites of MoS2 and how this can be controlled synthetically could lead to further enhancements of the catalytic activity without having to resort to expensive rare earth metals, and this will only come about with more careful and systematic studies on catalyst synthesis. Moreover, it would appear from a comparison of TPR and catalyst activity measurements that the catalyst reducing temperature may not be an effective predictor of catalyst activity, providing the reaction temperature is high enough. While this study has looked at refinery catalysts, it should be pointed out that new research is looking into in situ processes of upgrading and recovery of oil, i.e. occurring in the well itself. One such example is that of aquathermolysis of heavy oil33,34 and the use of nanoparticles for heavy oil in situ recovery35 has received some attention of late. Hashemi et al. have highlighted the opportunities as well as limitations and challenges of this process36 and it is an exciting new


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research area. It is conceivable that the preparation method for these new catalysts could also play a significant role in their catalytic activity.

4. CONCLUSIONS Herein we have synthesized CNT-supported CoMo catalysts for HDS of DBT via Co-first and Mo-first sequential impregnation as well as co-impregnation. Spectroscopic analysis showed that no free sulfided Co phase was present and that CoMo catalysts are predominantly single-layered nanocatalysts on the CNT support. TPR measurements showed differences in reducing temperature of the sulfided CoMo catalysts prepared by the different methods but catalyst activities for HDS of DBT did not fully align with the TPR-predicted order. Conversion of DBT was highest in Mo-first sequential impregnation (81.5%), followed co-impregnation (64%) and Co-first sequential impregnation (60%). While these results contrast with some others regarding the order of impregnation, we propose that the preferred impregnation order is supportdependent, rather than an absolute quality.

AUTHOR INFORMATION Corresponding Author * corresponding author: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Abu Dhabi Oil Refining Company (TAKREER); Department of Chemical Engineering, Khalifa University of Science and Technology (The Petroleum Institute); U.S. Department of Energy ACKNOWLEDGMENT The authors would like to acknowledge financial support for this project from the Abu Dhabi Oil Refining Company (TAKREER) and Department of Chemical Engineering, Khalifa University of Science and Technology (The Petroleum Institute). STEM experiments were conducted as part of a user proposal through ORNL’s Center for Nanophase Materials Sciences, which is a U.S. Department of Energy, Office of Science User Facility (JCI). The work was also supported by the U.S. Department of Energy grant DE-FG02-09ER46554 (STP).

ABBREVIATIONS AHM, ammonium heptamolybdate tetrahydrate; APD, average pore diameter; CNT, carbon nanotubes; DBT, dibenzothiophene; DRIFT, diffuse reflectance infrared spectroscopy; DTA, differential thermal analysis; EDS, energy dispersion spectroscopy; EELS, electron energy loss spectroscopy; GC-MS, gas-chromatography – mass spectrometry; HAADF, high-angle annular dark field; HDS, hydrodesulfurization; ICP-MS, inductively coupled plasma – mass spectrometry; PBS, poly butyl sulfide; SBET, Brunauer–Emmett–Teller specific surface area; STEM, scanning transmission electron microscopy; TGA, thermogravimetric analysis; TPR,


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