Thiophene Hydrodesulfurization Activity Research Articles

Single-source approach to amorphous RuS2 materials supported on SBA-15 and their catalytic activity for thiophene hydrodesulfurization

Single-source approach to amorphous RuS2 materials supported on SBA-15 and their catalytic activity for thiophene hydrodesulfurization

One-step synthesis of mesoporous alumina-supported molybdenum carbide with enhanced activity for thiophene hydrodesulfurization

A new mesoporous alumina-supported molybdenum carbide (Mo2C/γ-MA) has been successfully prepared by one-step hydrolysis method without template, followed by carbonization with a mixed gas of CH4 and H2. The Mo2C particles (2.5 ± 0.3 nm) are homogeneously dispersed in the γ-alumina (γ-MA) framework and exhibit high catalytic activity and stability for thiophene hydrodesulfurization (HDS). Taking 1 wt% thiophene (3810 ppm S) as an example, the Mo2C/γ-MA exhibits 92% thiophene conversion, higher than the MoS2/γ-MA (85%), and other molybdenum carbides supported over commercial alumina and γ-MA prepared by the impregnation method. The stability test demonstrated that the Mo2C/γ-MA almost showed no deactivation during 150 h reaction, and no obvious carbon deposition and sulfurization of Mo2C species were observed over the spent Mo2C/γ-MA. The high HDS activity and stability of the Mo2C/γ-MA for HDS could be attributed to the formation of small Mo2C nanoparticles, appropriate surface acidity and the strong interaction between Mo2C species and alumina inhibiting the sulfurization of Mo2C species.

Applicability of CNT as support candidate for thiophene hydrodesulfurization and 1-octene hydrogenation catalyst

The application of carbon nanotubes (CNT) as a support candidate for hydrotreating catalysts for FCC (fluid catalytic cracking) gasoline was researched in the microreactor with a fixed bed at experimental conditions. The CNT synthesized using the AC arc discharge method was functionalized by chemical treatment with nitric acid. Scanning electron spectroscopy (SEM), X-ray diffraction (XRD) and X-photoelectron spectroscopy (XPS) of the functionalized CNT show that during the functionalization of it, HNO3 impurities unwanted for further application were removed and carbon nanostructures haveoxidized, and therefore they have oxide functional groups attached to those walls. At all the conditions observed, the Ni-Mo/CNT catalysts and Co-Mo-0.25 catalyst showed better hydrodesulfurization activities in comparison with the conventional Ni-Mo/γ-Al2O3. As shown in the XRD analysis, the Ni–W/CNT catalysts showed lowest thiophene hydrodesulfurization activity in the temperature range observed, but they showed very good hydrogenation activity on model compounds used.Catalytic performance tests of different catalysts showed that good applicability of the CNT as support candidate for the hydrotreating catalysts. It also was experimentally demonstrated that the Ni-Mo/CNT and Co-Mo/CNT can be the best hydrotreating catalysts.

Evidence of Octahedral Co–Mo–S Sites in Hydrodesulfurization Catalysts as Determined by Resonant Inelastic X-ray Scattering and X-ray Absorption Spectroscopy

Fundamental understanding of the active sites in CoMo/Al2O3 catalysts is crucial to improve the production of clean transportation fuels by hydrodesulfurization (HDS). In this work, the coordination state and number of active sites (“Co–Mo–S”) in sulfided CoMo/Al2O3 catalysts prepared with and without chelating additives were determined using in situ 1s2p resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS). Based on the evaluation of thiophene HDS activity as a function of the Co/Mo ratio, it was determined that only for Co/Mo < 0.1, cobalt is exclusively in interaction with the MoS2 phase, making these compositions ideal for identifying the nature of the active cobalt sites for hydrogenolysis. These cobalt promoter atoms were in centrosymmetric coordination with the MoS2 phase, with six sulfur ligands coordinated to each cobalt center. The octahedral coordination of cobalt in Co–Mo–S in alumina-supported HDS catalysts contrasts the tetrahedral cobalt coordination found in Co-promoted MoS2 model systems supported on gold or carbon. We attribute the difference to more disordered MoS2 structures in technical alumina-supported HDS catalysts. The number of Co–Mo–S sites depended strongly on the catalyst preparation method. Chelating additives, such as citric acid and nitrilotriacetic acid, increased the number of Co–Mo–S sites without significantly altering their intrinsic activity. These results demonstrate that the state-of-the-art structural characterization tools, such as RIXS, identify the structure of active sites in actual catalysts and that these structures may be different from model systems. Combining such characterization tools with guided catalyst design can provide an insight into the structure of amorphous, nanostructured materials, which aid in the development of improved heterogeneous catalysts. KEYWORDS:Hydrodesulfurization Co−Mo−S RIXS active site XAS

Promoter Location on NiW/Al2O3 Sulfide Catalysts: Parallel Study by IR/CO Spectroscopy and High-Resolution STEM-HAADF Microscopy

In this work, the effect of citric acid (CA) on the structure and activity of Ni-promoted WS2 particles supported on γ-Al2O3 were investigated by HRTEM (high resolution transmission electron microscopy), HR STEM-HAADF (high resolution scanning transmission electron microscopy equipped with high angular annular dark field detector), CO adsorption followed by infrared (IR) spectroscopy, and thiophene hydrodesulfurization (HDS) test. It is shown that CA addition increases the thiophene HDS activity. This may be directly ascribed to the increase in NiWS site concentration detected by IR spectroscopy. HRTEM emphasizes that the addition of CA to NiW catalysts decreases the average size of the particles. This is confirmed by HR STEM-HAADF that even evidences the presence of nanoclusters on NiW catalysts prepared with CA. Hence, local and global analysis of the NiW samples are in good agreement to account for the increase of NiWS site concentration by CA addition. Then the contrast and interatomic distances were analyzed in the atomic-scale images obtained by HR STEM-HAADF. These measurements allow identification of the nature of the atoms in the core and edge of the sulfided particles. The metal atoms of particle core are only W ones, whereas the edges of the particles could be either fully promoted with detection of only Ni atoms or partially promoted with detection of both Ni and W atoms. Accordingly, IR spectroscopy evidenced edge sites with different promotion degrees.

Titania - Supported transition metals sulfides as photocatalysts for hydrogen production from propan-2-ol and methanol

A series of transition metals sulfides deposited on anatase titania (MSx/TiO2) were prepared by precipitation of transition metals salts with thioacetamide in aqueous medium under reflux. The solids were characterized by XRD, XPS, temperature programmed reduction and transmission electron microscopy. The properties of as obtained catalysts were compared for the photocatalytic hydrogen evolution reaction (PHER) in pure methanol and water-isopropanol mixture. The sequences of PHER activity were compared with electrochemical HER and thiophene hydrodesulfurization (HDS) activity of the corresponding sulfides prepared by the same technique. For PHER, in both alcohols the most active photocatalysts contain hydrogenating sulfides of Co and Ru. However the PHER activity does not follow the same trend as electrocatalytic HER and thiophene HDS. Some sulfides, such as HgS or CuS, show poor activity in HDS and electrocatalytic HER, but have the PHER activity comparable with that of the best samples. This difference suggests that the PHER rate is not merely related to the hydrogen activating properties of the co-catalyst, but is enhanced by the transfer of photogenerated electrons from TiO2 towards the sulfide. The ranking of the co-catalysts and the PHER activity depend also on the nature of the alcohol molecule, the overall PHER rates in water-isopropanol mixture being lower than in methanol.

Controllable thermal conversion of thiomolybdate to active few-layer MoS2 on alumina for efficient hydrodesulfurization

Sulfur removal is the most challenging task in petroleum refineries for upgrading clean fuel from crude oil. In this regard, an efficient Ni–MoS2 catalyst was prepared using ammonium tetrathiomolybdate (ATM), (NH4)2MoS4 as a precursor for hydrodesulfurization (HDS) reaction. ATM was derived by sulfiding an aqueous ammoniacal ammonium heptamolybdate and impregnated on alumina support with and without nickel promoter to form a series of MoS2/Al2O3, and Ni–MoS2/Al2O3 catalysts with controlled calcination temperature. The prepared ATM and catalysts were characterized by X-ray diffraction (XRD), Infrared spectroscopy and nitrogen adsorption–desorption. The phase transformation of ATM to MoS2 was observed by TGA, and number of MoS2 layer formation was calculated from the frequency difference between the redshift and blueshift by Raman spectrum. The catalytic properties of MoS2/Al2O3, and Ni–MoS2/Al2O3 were investigated using thiophene as the model compound for HDS reaction. The HDS rate, C4 hydrocarbon product distribution and selectivity were quantified by microreactor with refinery gas analyzer system. The prepared MoS2 catalysts (MoS and NiMoS) at 350 °C showed a better conversion rate than another catalyst due to the synergy between the promoter and few-layered MoS2 on support. Thus, ATM is a suitable candidate for the formulation of Ni–MoS2 catalyst at 350 °C for improving thiophene HDS activity.

The particular characteristics of the active sites of MoS2, WS2 catalysts in thiophene hydrodesulfurization

It was reported in earlier studies that a linear correlation existed between the total sulfur exchange capacity and thiophene hydrodesulfurization activity of different, alumina- or silica-supported molybdenum- or tungsten-sulfide based porous catalysts promoted with different metals: Co, Ni, Pt and Pd. The aim of this study was to clear up how general the linear correlation is, whether that correlation is generally applicable, whether it shows a particular character of the active sites in thiophene hydrodesulfurization (HDS). Industrial catalysts are more complex than typical model catalysts so the extents of thiophene HDS were determined on six industrially manufactured catalysts available today (combinations of Mo, W and Ni supported on alumina and of one NiW on silica-alumina) and compared to their sulfur uptake, and radiosulfur exchange capacity at 623 K. The majority of these catalyst samples also contained some phosphorus in different amounts. The results of the cyclohexane dehydrogenation as a model reaction indicated the absence of metallic nickel on the surface of the catalysts. A linear correlation was observed between the sulfur exchange capacity and HDS activity of these catalysts (with the exception of the sample supported on silica-alumina) similar to that observed previously.

Kinetics of thiophene hydrodesulfurization over a supported Mo–Co–Ni catalyst

In this study, the kinetics of thiophene (TH) hydrodesulfurization (HDS) over the Mo–Co–Ni-supported catalyst was investigated. Trimetallic catalyst was synthesized by pore volume impregnation and the metal loadings were 11.5 wt % Mo, 2 wt % Co, and 2 wt % Ni. A large surface area of 243 m 2 /g and a relatively large pore volume of 0.34 cm 3 /g for the fresh Mo–Co–Ni-supported catalyst indicate a good accessibility to the catalytic centers for the HDS reaction. The acid strength distribution of the fresh and spent catalysts, as well as for the support, was determined by thermal desorption of diethylamine (DEA) with increase in temperature from 20 to 600 °C. The weak acid centers are obtained within a temperature range between 160 and 300 °C, followed by medium acid sites up to 440 °C. The strong acid centers are revealed above 440 °C. We found a higher content of weak acid centers for fresh and spent catalysts as well as alumina as compared to medium and strong acid sites. The catalyst stability in terms of conversion as a function of time on stream in a fixed bed flow reactor was examined and almost no loss in the catalyst activity was observed. Consequently, this fact demonstrated superior activity of the Mo–Co–Ni-based catalyst for TH HDS. The activity tests by varying the temperature from 200 to 275 °C and pressure from 30 to 60 bar with various space velocities of 1–4 h −1 were investigated. A Langmuir–Hinshelwood model was used to analyze the kinetic data and to derive activation energy and adsorption parameters for TH HDS. The effect of temperature, pressure, and liquid hourly space velocity on the TH HDS activity was studied.

Effect of high pressure sulfidation on the morphology and reactivity of MoS2 slabs on MoS2/Al2O3 catalyst prepared with citric acid

MoS2/Al2O3 catalysts were prepared by a simultaneous impregnation method with citric acid as chelating agent in order to investigate the effect of high pressure sulfidation on the morphology and reactivity of MoS2 slabs by infrared spectroscopy of CO adsorption (IR/CO), transmission electron microscopy, thiophene hydrodesulfurization (HDS) and 2,6-dimethylaniline hydrodenitrogenation (HDN) reactions. It is found that the MoS2 slabs are more truncated when sulfided at higher pressure, exposing larger S-edge/M-edge ratio. The thiophene HDS activity of MoS2/Al2O3 catalysts was significantly improved by high pressure sulfidation. The MoS2 edges formed after high pressure sulfidation are very stable toward H2-treatment, while the same H2-treatment can create sulfur vacancies on both M-edge and S-edge of the 0.1MPa sulfided MoS2 slabs. Meanwhile, parallel between the IR/CO results and HDN test suggests that the S-edge favors the C–N bond scission whereas the M-edge benefits the hydrogenation in HDN reaction.

A NiMoS flower-like structure with self-assembled nanosheets as high-performance hydrodesulfurization catalysts.

Uniform 3D NiMoS nanoflowers with self-assembled nanosheets were successfully synthesized via a simple hydrothermal growth method using cheap and nontoxic elemental sulfur as sulfur sources. The structure and morphology of the nanomaterials were characterized by SEM, TEM, XRD, Raman and XPS analyses, revealing that the NiMoS nanoflowers were composed of ultrathin nanosheets with a thickness of approximately 6-12 nm. The HRTEM results indicate that the curve/short MoS2 slabs on the nanosheets possess the characteristics of dislocations, distortions and discontinuity, which suggests a defect-rich structure, resulting in the exposure of additional Ni-Mo-S edge sites. The obtained NiMoS nanoflowers exhibited an excellent activity for thiophene hydrodesulfurization (HDS) and 4,6-dimethyldibenzothiophene deep HDS due to their high density of active sites. The outstanding HDS performance suggests that these NiMoS composites with a unique flower-like nanostructure could be useful as promising catalysts for deep desulfurization of fuel oils.

Ultrasound Assisted Synthesis and Physicochemical Characterizations of Fluorine-Modified CoMo/Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; Nanocatalysts Used for Hydrodesulfurization of Thiophene

A series of CoMo/F-Al2O3 nanocatalysts with different fluorine loadings of 0, 0.4, 0.8 and 1 wt% were successfully synthesized by a sonochemical method and used for catalytic hydrodesulfurization (HDS) of thiophene. The nanocatalysts were characterized with X-ray diffraction analysis (XRD), Field Emission Scanning Electron Microscopy (FESEM), BET nitrogen adsorption Brunauer-Emmett-Teller (BET) analysis, Fourier Transform Infrared Spectroscopy (FTIR) and temperature programmed desorption of ammonia (TPD-NH3) techniques. The XRD results showed a high dispersion of the molybdenum species on the γ-Al2O3 support due to the application of sonochemical method along with fluorine addition. The CoMo/F-Al2O3 nanocatalysts had a particle size less than 100 nm. The specific surface area of the samples was slightly decreased with increasing the fluorine content. The FTIR results confirmed that with the increase of fluorine amount in the CoMo/Al2O3 nanocatalysts, it would generate more active sites. Moreover, the TPD results showed that the fluorinated nanocatalysts had higher acidity than the promoter-free nanocatalysts, because of the formation of new strong acid sites on the γ-Al2O3 support by the promoter. The catalytic activity for thiophene HDS reaction was investigated in a stirred slurry-tank reactor in the atmospheric pressure to determine the effect of fluorine amount on the nanocatalyst performance. The results of thiophene HDS reaction showed that the fluorinated nanocatalysts were more active than fluorine-free nanocatalysts and consequently, they were able to remove nearly 100% of thiophene in the initial solution. Furthermore, the nanocatalyst with the fluorine content of 0.8 wt% had the highest activity in HDS of thiophene. However, further addition of fluorine led to decrease in catalytic activity which could be attributed to the agglomerated particles formed on the nanocatalyst surface.

Sulfidability and thiophene hydrodesulfurization activity of supported NiMo carbides

Ni–Mo carbides supported on activated carbon were synthesized by carbothermal hydrogen reduction and the effect of the sulfidability on the thiophene hydrodesulfurization catalytic activity was studied. The X-ray diffraction patterns of Ni–Mo carbides showed the presence of β-Mo2C and NiC when the atomic ratio AR=Ni/(Ni+Mo) was between 0.25 and 0.75, while for AR=1, it only was detected metallic Ni. The X-ray photoelectron spectroscopy results showed the distribution of different surface species in the passivated catalysts: Moδ+ (0≤δ≤2), Mo4+, Mo6+, Niδ+ and Ni2+. After sulfiding the carbides were modified in their surface and catalytic activity.

Effect of catalyst precursor and its pretreatment on the amount of β-Pd hydride phase and HDS activity of Pd-Pt/silica-alumina

The effect of catalyst precursor and pretreatment on the content of β-Pd hydride (PdHx) and activity of the reduced bimetallic Pd-Pt catalysts in hydrodesulfurization (HDS) of thiophene and benzothiophene was studied. The catalysts prepared by co-impregnation of mesoporous silica–alumina (MSA) by different metal compounds were characterized by nitrogen, hydrogen, CO adsorption, chemical analyses, electron-probe microanalysis (EPMA) and temperature-programmed hydride decomposition (TPHD). It was found that metal precursor and its pretreatment greatly affected the properties of the catalysts. The most active catalysts were prepared from Pd acetate and Pt(NH3)4(OH)2. Strong Brønsted acidity of the support led to the highly active and sulfur tolerant catalysts. About 20% of the active phase surface remained in the metallic state after thiophene HDS. The reduced Pd-Pt/MSA catalysts absorbed hydrogen in the form of PdHx phase. Its amount was greatly affected by the precursor, its pretreatment and overall metal dispersion. A relation between thiophene HDS activity and the amount of hydrogen present in the PdHx was observed within the whole series of Pd-Pt catalysts. This shows that the hydrogen accumulated in the bulk of Pd participated in the reaction along with the hydrogen activated on the catalyst surface and positively contributed to the overall thiophene transformation over Pd-Pt/MSA catalysts.

Early stage deactivation of heavy crude oil hydroprocessing catalysts

Four different CoMo catalysts supported by alumina, alumina–titania, alumina–silica and carbon were used to study the early stage of deactivation. Hydrodemetallization (HDM) and hydrodesulfurization (HDS) activities of these catalysts have been tested in high pressure, high temperature micro-plant by using heavy crude oil as feed. Thiophene activity of the spent catalysts was compared with the fresh and regenerated catalysts. The results show that the alumina and alumina–titania supported CoMo catalysts exhibit high and stable performance for both HDM and HDS activities. Though the initial activities of alumina–silica supported CoMo catalyst are high, the activities decrease rapidly with time-on-stream. Having higher acidic sites may cause this rapid deactivation. Thermogravimetric analysis results also support that the nature of deposited carbon on this catalyst is different from the coke deposited on the other three catalysts. SEM-EDX results show that vanadium sulfides are more preferably deposited at the outer surface of carbon catalyst. Thiophene HDS activity of fresh, spent and regenerated results suggest that the deactivation of alumina catalyst is cause of coke deposition whereas both metal sulfides and coke depositions are responsible for CoMo/Al2O3–TiO2 and CoMo/Al2O3–SiO2 catalysts deactivation, particularly at the early stage of hydroprocessing of the heavy oil.

Effect of preparation of Pd and Pd–Pt catalysts from acid leached silica–alumina on their activity in HDS of thiophene and benzothiophene

Mesoporous silica–alumina (MSA) modified by post-synthesis acid leaching was studied as a support for Pd and Pd–Pt catalysts. Activity of catalysts was evaluated in hydrodesulfurization (HDS) of model compounds (thiophene and benzothiophene). The leaching decreased the Al2O3 content of MSA from 52 to 9wt.%. This treatment mainly removed the non-acidic extra-framework Aloct species, exposed the Brønsted acidic sites and increased the BET surface area by 50%. The higher acidity and surface area improved the activities of Pd catalysts in HDS of thiophene and benzothiophene. Pd(OAc)2 deposited on the leached MSA gave the best monometallic Pd catalyst. Two bimetallic Pd–Pt catalysts were prepared by co-impregnation of the leached MSA with Pd(OAc)2+Pt(NH3)4(OH)2 and Pd(OAc)2+H2PtCl6. The bimetallic catalyst prepared from Pt(NH3)4(OH)2 showed significant promotional effect and exhibited the highest activity in HDS of thiophene and benzothiophene.

Sulfur exchange capacity and thiophene hydrodesulfurization activity of sulfided molybdena-alumina catalysts promoted by nickel

The capacity of sulfur exchange (i.e. S TE—the total number of exchangeable sulfur atoms) has been determined at 673 K in a circulation system with H 2S (partial pressure ∼2.5 and 25 kPa) for five sulfided catalyst samples: molybdena-alumina (Mo12), NiO/Al 2O 3 (Ni12) and three Ni-promoted molybdena-alumina with different Ni:Mo ratios [NiMo(0.X)]. A linear correlation has been found between the S TE values and thiophene hydrodesulfurization activity of the catalyst samples, containing Mo. The S TE values have been determined also in exchange between catalyst sulfur and thiophene, for three samples: Mo12, Ni12, and NiMo(0.35) for comparison of the S-exchange affinity of S in thiophene with that in H 2S. The isotope exchange capacity of sulfur bound to the catalyst ( 35S cat) with sulfur in H 2S was substantially higher than that with sulfur in thiophene, but the sequence of the S TE values was similar for the three samples, i.e., NiMo(0.35) > Mo12 > Ni12.

Effect of Preparation Methods and Content of Boron on Hydrotreating Catalytic Activity

The effects of boron content and preparation methods are studied in this present investigation. Three different methods are employed for the preparation of catalysts. The catalysts are characterized by pore size distribution (PSD), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thiophene hydrodesulfurization (HDS), cumene hydrocracking (HC), gas oil HDS and Maya heavy crude oil HDS, and hydrodemetallization (HDM) activities are tested. The results show that the specific surface area and PSD are not changed with B loading. The enrichment of metal distribution on the alumina surface is observed from SEM experiments for the 0.6B–NiMo catalyst. More staging of MoS2 active phases is noticed in the B catalyst than the B-free catalyst. It can be stated that boron forms a monolayer on the alumina surface and reduces the metal–support interaction. It facilitates the formation of more staging of active phases. The thiophene HDS activity slightly increases with boron loading up to 0.6 wt % B, and after that, the activity becomes constant or slightly decreases. The cumene cracking of the NiMo–B catalyst shows the highest activity. The overall activity of boron catalysts was found to be the same or marginally higher than that of the boron-free catalyst. The presence of boron may help to reduce catalyst deactivation during hydrotreating of heavy crude oil.

The Support Effect on Hydrogenolysis of Thiophene and Gas–Oil

Results are reported on the support effect on the catalytic activity in thiophene hydrodesulfurization (HDS) of sulfided Ni-Mo catalysts supported on pure niobia, mixed oxides of Nb2O5-TiO2 prepared by sol-gel method, and Nb2O5/TiO2 and Nb2O5/Al2O3 prepared by surface deposition. The prepared samples were characterized using N2 adsorption at −196°C, X-ray diffraction (XRD), and temperature-programmed reduction (TPR) techniques. This study showed activity variation as a function of support composition. The activity of niobia-rich catalysts was no longer promoted by the synergy between Ni and Mo. The absence of synergy between molybdenum and nickel on niobia can be explained by the strong interaction of each metal with niobia at the expense of interaction with each other. It was found that 5 wt% Nb2O5/TiO2-supported catalyst was the better catalyst for thiophene HDS. It was shown that by means of an adequate support design it is possible to significantly increase the functionalities of HDS catalysts. Semiconducting supports like TiO2 can improve the HDS activity by exerting electronic effects on the active phase, helping in this way the formation of sulfur vacancies. The 5 wt% Nb2O5/TiO2 was also tested at high pressure with gas oil feedstock. It is observed with the hydrogeolysis of sulfur compounds against time-on-stream that the activity of this catalyst decreases fast with time.

ChemInform Abstract: The Role of Ni in Sulfided Carbon-Supported Ni-Mo Hydrodesulfurization Catalysts.

Abstract The thiophene hydrodesulfurization activities of Ni and NiMo sulfide catalysts supported on activated carbon were measured at atmospheric pressure and the catalyst structures were studied by means of X-ray photoelectron spectroscopy, dynamic oxygen chemisorption, and chemical sulfur analysis. The Ni/C catalysts contained a Ni 3 S 2 -like phase which, due to the impregnation method, was considerably enriched in the outer shell of the carbon support grains. Both for the Ni/C and NiMo/C catalysts no correlation was observed between the dynamic oxygen chemisorption and the catalytic activity and for the Ni/C catalysts indications for full oxidation of the Ni 3 S 2 particles were obtained. The specific catalytic activity per Ni atom was lower in the Ni/C catalysts than in the NiMo/C catalysts. Since the local Ni structure in these catalysts is different, however, it proved not possible to decide whether the Ni atoms in NiMo/C catalysts are the actual active sites or not.