Effect of Thermal Treatment on Hydrogen Uptake and Characteristics

Article pubs.acs.org/IECR

Effect of Thermal Treatment on Hydrogen Uptake and Characteristics of Ni‑, Co‑, and Mo-Containing Catalysts Houman Ojagh, Derek Creaser, Stefanie Tamm, Chaoquan Hu, and Louise Olsson* Competence Centre for Catalysis, Chemical Engineering, Chalmers University of Technology, SE-412096 Gothenburg, Sweden ABSTRACT: Nonsulfided alumina supported Ni, Co, Mo, NiMo, and CoMo hydrotreating catalysts were synthesized. The TEM results indicated low dispersions of the active metals for Mo, NiMo, and CoMo samples but significantly higher dispersion for the Ni sample. The effect of calcination and reduction on the hydrogen uptake capacity of the samples was investigated. The H2-chemisorption and XPS results together showed that the precalcination step had a detrimental effect on the hydrogen adsorption of the Ni sample formation of stable metal oxides. The XPS results revealed that the metal oxides of all calcined samples reacted with the alumina support to form very stable spinels. Further, the positive effects of a hydrogen atmosphere, during the reduction, on the hydrogen uptake of the samples were confirmed by H2-chemisorption measurements. Finally, the heats of adsorption (ΔH) of hydrogen for the Ni and Co samples were calculated to be 140 and 98 kJ mol−1, respectively. providing a high surface area with high packing density.29 Yet, several studies have elucidated strong interaction between alumina and metal oxides which hampers the quality of catalysts.30−32 Impregnation and coprecipitation are frequently used as preparation methods. But, the simplicity of the impregnation process has made it the method of choice on both laboratory and industrial scales.33 In addition, some studies have shown that supported catalysts prepared by impregnation methods often can attain higher degrees of reduction.34 However, despite the easy execution of the impregnation method in practical applications, this method is inherently complex. Many underlying intricate phenomena, such as the interactions between metal salts and support must properly be regulated during impregnation. Material structure has a significant impact on the performance of a catalyst. When investigating the effect of material structure, it is very important to evaluate the properties of the active metal nanoparticles. It is generally accepted that the number of available active sites and their state of dispersion (surface to volume ratio) is a decisive property that steers many catalytic reactions. Brunelle et al. indicated that to prepare supported catalysts with high dispersion of the active metal nanoparticles three important parameters, such as isoelectric point of the support, pH of the impregnating solution, and the nature of the metallic complex precursors must be regulated.35 Guevara-Lara et al. also confirmed the effect of the pH of the impregnating solution on the final dispersion.36 Many studies have previously been focused on the development of methods mainly based on adsorption and desorption of carbon monoxide (CO) or hydrogen (H2) to evaluate the size and dispersion of catalysts.37−39 Nonetheless, methods based on adsorption/desorption can only provide an average value over a large portion of the probed samples. Moreover, these methods

1. INTRODUCTION The modern chemical industry with its current status could not have survived without the decisive role of catalysts accelerating reactions.1 Heterogeneous catalysts are crucial for many chemical,2 pharmaceutical,1,3 food, and fuel4−6 production industries. New legislations continuously results in further removal of contaminants, such as sulfur and nitrogen, by hydrotreating processes with heterogeneous catalysis.7,8 Hydrotreating is also a vital upgrading process for the production of renewable fuels. For instance, the high oxygen content of biodiesel with deleterious properties giving low chemical stability is removed by hydrodeoxygenation (HDO) reactions.9−13 Therefore, considerable research has been dedicated to improve the hydrotreating catalysts by shedding additional light on their structures. Hydrotreating catalysts are often formed by nanosized noble metals, such as platinum (Pt), palladium (Pd), or base metals nickel (Ni), cobalt (Co), molybdenum (Mo), and tungsten (W), that are dispersed on high surface area metal oxides, such as alumina (Al2O3) and silica (SiO2).14−18 Noble metals have generally shown to have high activity and stability but their large scale industrial applications are restricted due to their high cost. Whereas base metals, in the form of bimetallic catalysts such as Ni−Mo, Co−Mo and Ni−W have demonstrated comparable actives to noble metals.15 Ni-based catalysts have long catalyzed very important industrial processes such as methanation of syngas and steam reforming.19−21 Supported Ni−Mo−S and Co−Mo−S on Al2O3 catalysts have found wide application in many hydrotreating processes, such as hydrodesulphurization (HDS) for the past six decades.8,22,23 There have also been a few reported hydroprocessing studies in which the nonsulfide form of these base metal catalysts were used.24−26 The toxicity, flammability and corrosivity of a sulphiding agent, such as H2S could create extremely unhealthy environment and damage the analytical equipment.27,28 Moreover, nonsulfided catalyst are of interest to avoid sulfur contamination of otherwise sulfur-free bio-oils. Alumina has been the most frequently used support owing to a number of essential and excellent properties, such as © 2015 American Chemical Society

Received: Revised: Accepted: Published: 11511

July 12, 2015 October 29, 2015 November 3, 2015 November 3, 2015 DOI: 10.1021/acs.iecr.5b02510 Ind. Eng. Chem. Res. 2015, 54, 11511−11524

Article

Industrial & Engineering Chemistry Research

Pd, Ag, Ir, and Pt. The nature of the molybdenum complex precursor ((NH4)6Mo7O24·4H2O) that was used is different compared to the other metal (Ni and Co) nitrate precursors. Also, different studies reported varying pH (2−11) to obtain optimum dispersion of Mo on alumina.49,50 Thus, to keep the synthesis condition the same, it was decided to set pH at 10.5 for all sample synthesis. After the alumina slurry was prepared, the pH was increased to 10.5 by adding dropwise a solution of ammonium hydroxide (30%), Aldrich. The slurry was left under moderate stirring for 30 min until the desired pH (10.5) was stabilized at room temperature. In the preparation of the monometallic samples (Ni, Co, and Mo), aqueous solutions of the metal salts were prepared. The concentrations of Ni, Co, and Mo in the aqueous solutions were adjusted to result in a desired metal loading of 10 wt % on each sample. Subsequently, the prepared metal salt solution was added to the alumina slurry, the pH of the final mixture was adjusted to 10.5 and then gently stirred for 1 h. When preparing bimetallic samples (NiMo and CoMo), a co-impregnation method was applied. The metal intended to have higher loading (Mo, 15%) was impregnated first, and then the mixture was stirred for 30 min at a pH of 10.5. After this, the solution containing the second metal (Ni or Co, 5%) was added. Finally, all samples were freeze-dried. Freeze-drying was followed by calcination processes, which were performed in atmospheric air at two different temperatures 400 °C for 1.5 h as a mild condition and 550 °C for 2 h as a harsher condition. Higher temperatures of calcination were not used to avoid excessive sintering of the metal particles.51 Some samples were kept uncalcined and the reasons will be fully discussed in later sections. On the basis of the calcination process, samples were further divided into three main groups. Group 1 includes those samples that were not calcined. These samples are denoted as fresh samples, for which the metal species are in the form of metal nitrates. Groups 2 and 3 contain those samples that were calcined, and therefore, the metal species are expected to be in the form of oxides. Samples in group 2 were calcined at 400 °C for 1.5 h and samples in group 3 were calcined at 550 °C for 2 h. Calcinations of the samples were performed in programmable furnaces, under air with a heating rate of 5 °C/min. 2.2. Sample Characterization. After preparation, samples were characterized by nitrogen physisorption (BET), inductively coupled plasma and sector field mass spectroscopy (ICPSFMS), hydrogen chemisorption (calorimetery), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). 2.2.1. Nitrogen Physisorption and ICP-SFMS. The physical properties of the samples, such as specific surface area, pore size, and pore volume, were evaluated by nitrogen physisorption using a TriStar 3000 gas adsorption analyzer. Prior to the experiments, approximately 300 mg of each sample was degassed (thermally dried) at 200 °C for 3 h, under vacuum. The N2-physisorption isotherms were collected at −195 °C under a reduced pressure. The specific surface area was calculated by using the Brunauer−Emmett−Teller equation (BET). The pore size was calculated by using the Barret− Joyner−Halenda equation (BJH) from the desorption isotherm. The elemental compositions of the samples were determined by inductively coupled plasma and sector field mass spectroscopy (ICP-SFMS). These measurements were done by ALS Scandinavia AB.

do not provide information about the individual dispersions and contact between metals of bimetallic catalysts. On the other hand, transmission electron microscopy (TEM) with high resolution has proven to be a direct method to probe the morphology of nanoparticles. The size distribution of both supported and active metal nanoparticles can be measured by SEM and TEM. Also, a combination of SEM and TEM methods have evidently shown to be such a pivotal system to assess the coke deposition on the spent catalysts.40,41 In industrial HDO processes, hydrogen gas is activated due to its chemisorption on the surface of catalysts. Therefore, hydrogen uptake capacity is an imperative property that affects the performance of all hydrotreatment catalysts. To evaluate the hydrogen uptake capacity, hydrogen chemisorption is the method of choice.42 Yet, many different thermal treatment steps such as calcination and reduction are involved in such catalyst preparation procedures that can influence the hydrogen uptake capacity of HDO catalysts. These steps are critical and must be understood and controlled in a reproducible manner. For instance, Bartholomew et al. investigated the chemistry involved in calcination and reduction of Ni/Al2O3 catalyst by H2-adsorption.32 However, there are to our knowledge very few studies in the literature that rigorously investigate the pretreatment and resulting properties of the base metal supported HDO catalysts. Hence, in the current study we have further investigated the influence of these critical thermal treatment steps by also including additional appropriate characterization information from methods, such as SEM, TEM, and XPS. The objective of this study was, first, to characterize nonsulfided alumina-supported Ni, Co, Mo, NiMo, and CoMo catalysts, second, to evaluate the effect of calcination and reduction treatments on the hydrogen uptake capacity of these catalyst, and finally, to evaluate the heats of adsorption of hydrogen.

2. EXPERIMENTAL METHODS 2.1. Sample Preparation. A group of base metal catalysts, both monometallic and bimetallic, such as nickel (Ni), cobalt (Co), nickel−molybdenum (NiMo), and cobalt−molybdenum (CoMo), supported on a commercial γ-Al2O3 [Puralox SCCa 150/200, Sasol] were synthesized by an impregnation method. In impregnation, the porous support is impregnated by a solution, often an aqueous solution, of the metal salt (metal oxide precursor), then the mixture is dried.33,43 Drying is often followed by thermal treatment (calcination), typically at a temperature range between 400 and 600 °C, to decompose the metal salt into their metal oxide form.36,44−48 The aqueous solutions of the metal oxide precursors were prepared by using [Ni(NO3)2·6H2O, Sigma-Aldrich], [Co(NO3)2·6H2O, SigmaAldrich] and [(NH4)6Mo7O24·4H2O, Sigma-Aldrich]. The γAl2O3 support was previously calcined at 550 °C for 2h, under air with a heating rate of 5 °C/min. In each preparation, the first step was to prepare a slurry of alumina with excess of water. Two grams of the γ-Al2O3 was mixed with 5 mL of deionized water and left under moderate stirring. According to previous studies,35,46 the cationic amine complexes of metals, such as [Ni(NH3)x2+] and [Co(NH3)x2+], formed by adding ammonium hydroxide to the metal nitrate solutions, will be highly dispersed on a γ-Al2O3 support in the alkaline region with pH between 9 and 12. This optimum pH region is reported to be related to the metals situated in groups 8 and 1b in the periodic table including: Co, Ni, Cu, Ru, Rh, 11512

DOI: 10.1021/acs.iecr.5b02510 Ind. Eng. Chem. Res. 2015, 54, 11511−11524

Article

Industrial & Engineering Chemistry Research Table 1. Properties and the Measured Metal Contents of the Samples at Different Calcination Temperatures samples Ni (wt %) Co (wt %) Mo (wt %)

γAl2O3

Ni/γ-Al2O3 (10%, Ni)a

Mo/γ-Al2O3 (10%, Mo)a

NiMo/γ-Al2O3 (5%, Ni and 15% Mo)a

9.1

calcination temperature (°C) surface area (m2/g, BET) pore diameter (nm, BET) pore volume (cm3/g, BJH)

CoMo/γ-Al2O3 (5%, Co and 15% Mo)a

5.3 8.9

γ-Al2O3

a

Co/γ-Al2O3 (10%, Co)a

550 197 8.0 0.49

Ni/γ-Al2O3 (10%, Ni)a 400 172 6.5 0.40

550 174 7.2 0.42

10.4 Co/γ-Al2O3 (10%, Co)a 400 170 6.7 0.41

550 171 7.6 0.44

15.1 Mo/γ-Al2O3 (10%, Mo)a 400 182 6.3 0.39

550 188 6.5 0.40

NiMo/γ-Al2O3 (5%, Ni and 15% Mo)a 400 147 6.4 0.31

550 146 6.5 0.32

8.1 12.3 CoMo/γ-Al2O3 (5%, Co and 15% Mo)a 400 138 6.8 0.31

550 140 7.1 0.32

Nominal metal concentration by synthesis method.

2.2.2. Hydrogen Chemisorption. The effects of calcination and reduction on the hydrogen uptake capacity of the samples and the heat of hydrogen adsorption (ΔH) were studied in hydrogen chemisorption experiments. The experimental setup contained a manifold of gas mass flow controllers (MFC, Bronkhorst), a differential scanning calorimeter (DSC, Setaram Sensys), and a mass spectrometer (MS, Hiden Analytical HPR 20). The desired inlet gas flow and composition was obtained from the gas manifold prior to the DSC. In the DSC, the heat released due to H2 chemisorption was detected. The DSC consisted of two vertical quartz tubes used for reference and sampling, respectively. The reference tube was kept empty but the sampling tube contained sample which was placed on a sintered quartz bed located in the middle of the tube. Subsequently, the outlet gas compositions were analyzed by the MS. The experimental details are presented below. Prior to all measurements, the samples, 100 mg of each, were pretreated (degreened) by a flow of 8% H2 in Ar at 500 °C for 15 min following heating from 25 °C at a rate of 10 °C/min. The total flow used was 400 mL/min, of which 20 mL/min was passed over the sample and the rest was sent to ventilation in order to improve the transient gas transitions. Pretreatment was followed by the reduction treatments which were performed at 450 °C. Three sets of experiments were performed which all followed the same structure. The samples were first degreened, according to the procedure described above. Thereafter, the samples were reduced with different flows of H2 and different durations (see below), and then cooled in Ar to 80 °C. Finally, the samples were exposed to 100 ppm of H2 in Ar at 80 °C for 1 h. In the first set of experiments, the effect of the initial calcination treatment was analyzed. In these experiments, all samples (calcined and noncalcined) were reduced in a flow of 15% H2 in Ar for 12 h. In the second set of experiments, the effect of hydrogen concentration on the reduction of the samples was analyzed. For these experiments, the samples were reduced by three different concentrations of H2 (5%, 10%, and 15%) in Ar for 6 h In the third set of experiments, the effect of the length of reduction time was assessed. For these experiments, the samples were exposed to the same flow of hydrogen (15% in Ar) for different durations (3, 6, and 12 h) for the reductions of the samples. 2.2.3. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The scanning electron microscopy (SEM) measurements were conducted to study the topology of the powder samples by using a Zeiss Ultra 55 FEG SEM microscope combined with an X-ray energy dispersive

spectrometry (XEDS) system. The images were formed by secondary electrons (SEs) for all measurements. The transmission electron microscopy (TEM) studies were performed to obtain quantitative overviews on size and shape of metal species on the samples. For the TEM measurements, all samples were physically grinded in a mortar into very small particles. Then, the particles were suspended in ethanol, and subsequently they were collected on carbon coated copper grids. For SEM, in contrast to the TEM measurements, the sample powders were intact. Prior to all SEM and TEM measurements, the samples were pretreated (cleaned) in diluted oxygen plasma to remove possible carbon contaminations. The TEM investigation was done using an FEI Titan 80−300 microscope equipped with a field emission gun (FEG), a probe Cs corrector and a Gatan image filter (GIF) Tridium. This instrument operated at an acceleration voltage of 300 kV. The scanning TEM (STEM) annular dark field (ADF) images were acquired with a probe convergent angle of 19.3 mard. The inner and outer collection angle for ADF imaging is 54 mard and 270 mard, respectively. 2.2.4. X-ray Photoelectron Spectroscopy (XPS). The oxidation states and the chemical compositions of the samples were assessed by the X-ray photoelectron spectroscopy (XPS) studies which were conducted by using a PerkinElmer PHI 5000C ESCA system equipped with an EDS elemental mapping system. To start with, all samples were pretreated in a flow of 8% H2 in Ar at 500 °C for 15 min and some samples were reduced with the same procedure presented in the hydrogen chemisorption experimental methods. Subsequently, samples were placed on carbon rubber pads which were situated on a sample holder. When the pressure of the main chamber was dropped to 1.2 × 10−8, the sample holder was shifted to the ultrahigh vacuum chamber. The XPS spectra were collected using a monochromatic Al Kα source with a binding energy of 1486.6 eV. A 90° angle between the X-ray source and the detected photoelectrons was used for all measurements. Sample charge neutralization was done on all samples. The O 1s peak from the alumina with a binding energy of 531.0 eV was taken as reference for all obtained spectra. It should be noted that the C 1s energy, which is often used as a reference could not be used in this study because of the presence of different surface dipoles in the samples.

3. RESULTS AND DISCUSSIONS 3.1. Catalyst Characterization. The physical properties and the metal contents of the samples were measured by using N2-adsorption and inductively coupled plasma and sector field mass spectroscopy (ICP-SFMS), respectively (Table 1). The measured metal contents (by ICP-SFMS) generally agreed well 11513


Article

Industrial & Engineering Chemistry Research with their nominal values from their preparation. The measured surface area of the calcined γ-Al2O3 support (197 m2/g) is relatively high and the average pore diameter stays in the mesoporous region (defined by IUPAC). It is evident that the additions of metal species onto the support have resulted in a reduced surface area and pore size of the samples, indicating that some pores may have been blocked. Also, the increase in calcination temperature from 400 to 550 °C did not have any significant effect on the physical properties of the samples. However, the influence of the textural changes on the hydrogen uptake capacity of the samples were expected to be less significant compared to other treatment variables such as calcination temperature which will be discussed later. As stated previously, samples were divided into three main groups based on the calcination process. Figure 1 shows a photograph of the prepared samples. The color of uncalcined samples originated from the impregnated

Figure 2. SEM images of (a) γ-Al2O3, calcined at 550 °C and (b) Ni/ Al2O3, calcined at 400 °C.

The calcined Ni/Al2O3 sample (Figure 2b) however contained a considerable portion of very small and fractured particles, less than 10 μm in size, with irregular shapes distributed among the larger particles. As shown by the representative selection of SEM results in Figure 3, a distribution of these small and irregular shape particles were observed in all calcined catalysts (monometallic, bimetallic and calcined at 400 and 550 °C), but not on the alumina. A selection of SEM results including micrographs of the alumina supported Ni sample calcined at 550 °C (a), alumina supported Co sample, calcined at 400 °C (b) and alumina supported CoMo sample, calcined at 550 °C (c) are shown in Figure 3. It is possible that the impregnation of the alumina caused fracturing of the particles. This hypothesis will be discussed further in a later section. The chemical mapping performed by EDS spectroscopy on different locations of the calcined samples showed variations in the metal concentrations between large and small (fractured) particles. The results of the local SEM/EDS elemental analysis of these samples are presented in Table 2. The analyzed areas are marked as box 2 for the smaller and inhomogeneously shaped particles, and box 1 for the larger and more homogeneously shaped particles. The elemental analysis results (Table 2) showed a much higher metal/aluminum ratio for the smaller particles (area 2) in all three cases. On the basis of the results from the SEM micrographs and the local SEM/EDS elemental analysis of the differently shaped particles, the fractured particles could be formed during two different processes: either during the impregnation of the alumina with the metal precursors or during the calcination of the samples. The second possibility would imply that, the metal oxides of the samples have interacted with the alumina support to form new stable spinel phases such as NiAl2O4. The effects of the temperature of calcination on the individual interactions of NiO−Al2O3 and CoO-Al2O3 that resulted in formation of spinels such as NiAl2O4 and CoAl2O4 have previously been reported.52−54 The solid−solid interactions between cobalt and molybdenum oxides which result in formation of cobalt molybdate crystals (Co3O4 + 3MoO3 → 3CoMoO4 + (1/2) O2 at 350 °C), and their individual interaction with Al2O3 to form Al2(MoO4)3 (at 750 °C) and Co2Al2O4 (at 1000 °C) were also previously reported.55,56 Shaheen et al. concluded that formation of a CoMoO4 and Al2(MoO4)3 phases are dominant at temperatures below 750 °C compare to formation of a Co2Al2O4 phase which forms at temperatures exceeding 1000 °C.56 Interestingly, the SEM/EDS elemental analysis of the CoMo sample showed that the metal−support interactions can mainly be attributed to Mo−Al since the cobalt concentrations of the probed areas (1 and 2) are nearly identical. Also, the presence of the CoMoO4 phase cannot be confirmed here. The SEM/

Figure 1. Color variations of the samples because of calcination.

metal salts. However, a shift in the apparent colors of the calcined samples (400 and 550 °C) is very notable. The color variations of differently calcined samples are most likely caused by the presence of different metal oxides, which were in different oxidation states. The oxidation states of the samples were further examined by using XPS and the results are presented in sections 3.2.1, 3.2.2, and 3.2.3. Samples were further characterized by SEM and TEM measurements. The topology of the powder samples was studied by SEM measurements. Figure 2 shows a SEM image of the calcined alumina sample in panel a and the supported Ni on alumina sample, calcined at 400 °C in panel b. The average particle size of the alumina sample was calculated to be 143 μm (55 particles counted), and the particles were found to have deformed spherical shapes. The average particle size of the Ni/ Al2O3 sample was calculated to be 138 μm from nearly 60 particles, thus similar to the alumina particles. 11514


Article


Figure 3. SEM micrograph of Ni/Al2O3 calcined at 550 °C (a), Co/Al2O3 calcined at 400 °C (b), and CoMo/Al2O3 calcined at 550 °C (c).

Table 2. Elemental SEM/EDS Analysis of Ni Calcined at 550°C, Co Calcined at 400°C, and Mo Samples sample Ni/Al2O3 10 wt % Ni (calcined at 550 °C) Co/Al2O3 10 wt % Co (calcined at 400 °C) CoMo/Al2O3 5 wt % Co, 15 wt %,Mo (calcined at 550 °C)

probe area

C (wt %)

O (wt %)

Al (wt %)

Ni (wt %)

1 2 1 2 1

9.7 8.7 3.7 3.5 13

48 42.1 51.6 34.8 24

37.6 32.4 33 13.8 36.5

4.6 16.8

2

6.3

32.3

23.7

Co (wt %)

Mo (wt %)

11.6 47.8 7.2

17.9

6.1

31.6

metal/alumina ratio 0.12 0.52 0.35 3.4 Co/Al = 0.19 Mo/Al = 0.49 Co/Al = 0.25 Mo/Al = 1.3

The analyzed areas are marked as box 2 for the very small particles (∼10 μm), box 1 and box 3 for the larger particles (20−150 μm). From the SEM micrograph shown in Figure 4, it can be observed that the distribution of the small particles (≤10 μm), in both uncalcined samples, are reduced in comparison to the calcined samples (Figure 3). The high metal/alumina ratios were only detected for the analyzed area 2 (very small particles (∼10 μm)). However, the detected concentrations of alumina for the area 2 were very small (1.4% and 1.1%) and close to the experimental error involved in the measurements. Therefore, we propose that the small particles (∼10 μm) are predominately the nonimpregnated dried salt precursors and not the spinel phase that were distributed in the calcined samples (Figure 3). The presence of spinel phases will be further discussed in relation to the XPS measurements in sections 3.2.1, 3.2.2, and 3.2.3. The morphologies of the samples alongside their structures were observed by TEM analysis. Figure 5a−f shows the STEM micrographs of selected samples: γ-Al2O3, Ni/Al2O3, Co/Al2O3, Mo/Al2O3, NiMo/Al2O3, and CoMo/Al2O3. The γ-Al2O3 sample was calcined at 550 °C, and the rest were calcined at 400 °C. The STEM micrograph of the calcined γ-Al2O3 sample (Figure 5a) is presented for the purpose of easier visual verification of the metal oxide particles on the other supported samples which appear as bright and sparkly spots. The metallic

EDS elemental analysis were carried out on all samples (monometallic, bimetallic and calcined at 400 and 550 °C) and a higher metal/aluminum ratio was detected on the small (fractured) particles. To validate the hypothesis of calcination causing spinel phases formation, which are seen as small fractured particles in Figure 3, two additional SEM measurements on two uncalcined samples (Co and NiMo) were performed. The impregnated species are expected to be in the form of metal nitrates. Figure 4 shows an SEM image of the uncalcined alumina supported

Figure 4. SEM micrograph of uncalcined Co/Al2O3 (a), and uncalcined NiMo/Al2O3 (b).

Co sample in panel (a) and the uncalcined alumina supported NiMo in panel (b). The results of the local SEM/EDS elemental analysis of these samples are presented in Table 3.

Table 3. Elemental SEM/EDS Analysis of Co/Al2O3 (Uncalcined) and NiMo/Al2O3 (Uncalcined) Samples sample Co/Al2O3 10 wt % Co (uncalcined)

NiMo/Al2O3 5 wt % Co and 15 wt % Mo (uncalcined)

probe area

N (wt %)

C (wt %)

O (wt %)

Al (wt %)

1 2 3 1

5.9 6.2 5.3 3.1

4.2 9.9 3.6 7.7

46.1 35.2 47.2 51.6

37.6 1.4 38.1 33

3.3

9.9

2

3.9

8.8

34.8

1.1

23.2

25.9

3

3.8

6.1

43.1

31.4

4.5

11.1

11515

Ni (wt %)

Co (wt %)

Mo (wt %)

6.2 47.3 5.8

metal/alumina ratio Co/Al = 0.16 Co/Al = 33.7 Co/Al = 0.15 Ni/Al = 0.1 Mo/Al = 0.49 Ni/Al = 13.6 Mo/Al = 14.9 Ni/Al = 0.14 Mo/Al = 0.35


Article


would negatively affect the dispersions of the impregnated active metals.35 Unlike Ni and Co samples that were formed by their nitrate precursors, the Mo part of the molybdenum based samples were synthesized from ammonium molybdate precursor. It seems that setting the pH at 10.5 has positively affected the dispersion of both Ni and Co samples, and negatively affected the dispersion of both molybdenum and molybdenum based catalysts (NiMo and CoMo). Accordingly, the low dispersion of all three Mo based samples (Figure 5) may have been due to the pH of the impregnating solutions which was set in a basic region (10.5). Another possible reason for the detection of very large metal particles in the Mo based catalysts may be due to the reported formation and growth of the CoMoO4, NiMoO4, and Al2(MoO4)3 crystalline phases during the sample calcination at temperatures above 350 °C.56 The possible formation of the Al2(MoO4)3 crystalline phase was previously assessed in the SEM measurements (Figures 3 and 4). Therefore, both effects, the pH of the impregnating solutions and the temperature of calcination, may influence the size of metal particles in the Mo based catalysts. Figure 6 presents the XEDS results of the NiMo/Al2O3 sample. The XEDS spectra confirmed the presence of both Ni

Figure 5. HAADF-STEM micrographs of (a) γ-Al2O3, (b) Ni/Al2O3, (c) Co/Al2O3, (d) Mo/Al2O3, (e) NiMo/Al2O3, and (f) CoMo/Al2O3.

nanoparticles were confirmed by using X-ray energy dispersive spectrometry (XEDS) and electron energy loss spectroscopy (EELS) analysis. The particle sizes of the metal oxides of the supported samples were calculated by averaging over a number of particles from the STEM images. The averaged particle sizes of the Ni and Co nanoparticles (Figure 5b, c, respectively) were calculated, by manual image examination, to be 4.6 (55 particles counted) and 18.9 nm (39 particles counted), respectively. The Ni nanoparticles seemed approximately spherical with more uniform distribution over the support than the single and more locally grouped Co nanoparticles. When running the TEM measurements on the prepared molybdenum sample, only a few and larger Mo nanoparticles were identified (Figure 5c). The Mo particles seemed to be agglomerated to form larger particles of about 124 nm in size (11 particles counted), which is indicative of a low dispersion of Mo on the sample. The NiMo and CoMo nanoparticles are shown in Figure 5e, f. The NiMo nanoparticles seemed to be either in the form of single small particles or clusters of different sizes, whereas CoMo nanoparticles were found to be in form of single and marginally larger separate particles but with varying sizes. The averaged particle sizes of NiMo and CoMo nanoparticles were calculated to be 125 (28 particles counted) and 153 nm (7 particles counted), respectively. The results showed that the average particle sizes of the metal species for Mo, NiMo, and CoMo samples were very large. In other words, the dispersions of the active metals were low. As previously mentioned, the isoelectric point of the support, in this case alumina, depends on the pH of the impregnating solutions and the nature of the precursors. Choosing an inappropriate region for the pH results in weak interactions between the metal salt and support which in turn

Figure 6. HAADF-STEM micrograph analyses of NiMo/Al2O3 calcined at 400 °C (on the left) and the corresponding XEDS spectra (on the right).

and Mo elements on larger particles (e.g., boxes 1 and 2) but small (≤10 nm) single Ni particles were also traced in the sample (e.g., box 3). The XEDS results for CoMo (not shown here) confirmed both Co and Mo elements on larger particles but the presence of single Co particles was not observed. Furthermore, the TEM micrographs of NiMo and CoMo particles (Figure 5e, f) showed homogeneous and uniform patterns of these bimetallic particles. The homogeneity in distribution of both Ni and Mo elements on the NiMo particles were also confirmed by using drift corrected spectrum image scanning system (not shown here). A uniform distribution of Co on Mo was similarly observed for all analyzed CoMo particles. These findings may be considered as a valid argument for alloy formations of Ni−Mo and Co−Mo in NiMo/Al2O3 and CoMo/Al2O3 samples, respectively. However, the formation of chemical bonding between those aforementioned elements was not confirmed in this study. 3.2. Hydrogen Chemisorption. The hydrogen chemisorption studies were performed to measure the hydrogen uptake capacity of the samples, which is an indication of their reduced states. In these experiments, the calcination and 11516


Article


spectra based on the O 1s peak at 531.0 eV. The center of Ni 2p3/2 peaks are positioned in a region of 853.4−855.2 eV with apparent satellite features with approximately 6 eV higher than the main photoemissions. This would clearly indicate the presence of Ni2+ in all samples.57 The Ni 2p3/2 peaks of all three samples were further investigated and deconvoluted by fitting a Gaussian−Lorentzian function with a linear background (Figure 8, right panel) around the binding energies of 852.7 eV for Ni°(metallic), 853.3 and 853.5 eV for Ni2+ (NiO), and 855.8 eV for Ni3+ (Ni2O3).45 It is worth noting that two peak positions were considered for the Ni2+ since it has been reported that the peak shape of Ni2+ is in form of a double peak.58 As previously stated, it has been reported that the binding energy of 855.8 eV also corresponds to a Ni spinel (nickel aluminate, NiAl2O4). The peak positions and areas were optimized until the standard deviation (χ2) stabilized to a minimum at 1.72. The deconvolution of the Ni 2p3/2 peak of uncalcined Ni sample revealed three peaks with binding energies of 852.7 eV, 853.8 and 854.3 eV, respectively. These binding energies can be attributed to Ni° and Ni2+ sites since the differences between the measured and reported binding energies are minimal. It can clearly be observed that the uncalcined Ni sample mainly contained the metallic form of Ni (Ni°) and a very small fraction of the oxide form (Ni2+). The deconvoluted Ni 2p3/2 peak of the sample calcined at 400 °C contained four peaks with binding energies of 852.7 eV, 853.4 eV, 854.1 and 855.6 eV. These peaks can be attributed to Ni°, Ni2+, and Ni3+/ NiAl2O4 sites, respectively. The fraction of Ni° sites gradually decreased while the fraction of Ni2+ and particularly Ni3+/ NiAl2O4 increased because of the precalcination treatment. The deconvolution results of the Ni 2p3/2 peak of the sample calcined at 550 °C showed that this sample mainly contained the oxide forms of Ni sites, Ni2+ and Ni3+/NiAl2O4. These results are indicative of a detrimental effect of a precalcination treatment on the reducibility of Ni samples. Also, as previously stated by Bartholomew et al. a high temperature calcination treatment (in air) favors formation and growth of very stable spinel crystals such as Ni3+/NiAl2O4 that are very difficult to reduce.32To summarize, the XPS measurements revealed that reducing the uncalcined Ni/Al2O3 sample resulted in large amount of metallic nickel (Figure 8). This is consistent with the hydrogen chemisorption measurements (Figure 7), where the largest hydrogen uptake occurred on this sample. Another interesting parameter to be studied was the effect of H2 concentration, during the reduction treatment, on the eventual reduced state of the samples. The results of these sets of experiments are shown in Figure 9. All three Ni samples were not previously calcined (group 1) but pretreated by a flow of 8% H2 in Ar at 500 °C for 15 min. The pretreatment was followed by a reduction step for 6 h but with different hydrogen concentrations in argon (5%, 10%, and 15% H2/Ar) at 450 °C. Finally the samples were exposed to 100 ppm of H2 in Ar for 1 h at 80 °C after cooling in Ar only. From both DSC (in top panel) and MS (in bottom panel) results, it can be observed that the H2 uptake of the Ni sample reduced by the lowest concentration of hydrogen (5%), is almost zero. In other words, the sample has not been reduced to any measurable extent. On the other hand, the Ni samples that were subjected to the reduction treatments with higher hydrogen concentrations (10% and 15%) seemed to be partially reduced because of their clear heat evolution peaks and H2 adsorptions. Yet, the amount of the hydrogen adsorption on the sample that was treated

reduction treatment conditions were varied and then the hydrogen uptake and reduced state of the sample was assessed by hydrogen chemisorption and XPS measurements, respectively. Finally, the heat of adsorption (ΔH) of hydrogen was determined. 3.2.1. Reducibility of Ni/Al2O3. To evaluate the effect of calcination on the reducibility, samples from each of the three main groups 1, 2, and 3 (not precalcined, calcined at 400 °C and calcined at 550 °C) were first pretreated by a flow of 8% H2 in Ar at 500 °C for 15 min. Then, they were reduced for 12 h by 15% H2/Ar at 450 °C and finally exposed to 100 ppm of hydrogen in argon at 80 °C for 1 h. It is worth noting that after the impregnation step, the deposited metal nitrates on the uncalcined samples (group 1) were attempted to be converted directly to their metallic state by this reduction treatment. But the deposited metal nitrates on the calcined samples (group 2 and 3) were first decomposed to oxide forms, during the calcination treatments, and then the reduction treatment was intended to convert them to their metallic state. Figure 7 presents the hydrogen chemisorption measurements from three Ni/Al2O3 samples, with the same Ni loadings but different calcination conditions.

Figure 7. Effect of calcination on the reducibility of Ni/Al2O3 (10 wt %) samples. The top panel shows the heat signal from the DSC and the sample temperature during the adsorption. The bottom panel presents the corresponding concentration signal measured by MS.

The thermogram signals of H2 adsorption measured by the DSC are presented in the top panel and the corresponding hydrogen concentrations measured by the MS are shown in the bottom panel. On the basis of these results, it is evident that the hydrogen uptake of the sample calcined at 400 °C prior to reduction was considerably higher compared to the sample precalcined at 550 °C. Also, the highest hydrogen uptake occurred on the Ni sample which was not previously calcined (fresh sample) but instead the precursor was decomposed during the reduction treatment in a flow of hydrogen. The difference in the hydrogen uptakes may be explained by the oxidation states of the Ni sites. Therefore, as a useful heuristic, X-ray photoelectron spectroscopy (XPS) measurements were preformed to further investigate the different states of Ni sites caused by the initial calcination of the samples. The XPS spectra for all three samples are shown in Figure 8. The left panel presents the Ni 2p core level spectra including clear peaks of Ni 2p1/2, Ni 2p3/2 and their shake-ups satellites. The charging effects on the samples were compensated for by shifting all 11517


Article


Figure 8. Left panel shows the XPS spectra of three Ni/Al2O3 samples for binding energies between 890 and 840 eV, which contain the Ni 2p1/2 and Ni 2p3/2 and their satellite peak positions. The right panel shows the deconvolution of the Ni 2p3/2 peaks.

Figure 10. Effect of reduction period length on Ni/Al2O3 samples. Top panel shows the heat signal from the DSC and the sample temperature. Bottom panel presents the corresponding MS concentration signal.

Figure 9. Effect of H2 concentration on the reduction of uncalcined Ni/Al2O3 samples. The top panel shows the heat signal from DSC and the sample temperature during the adsorption. Bottom panel presents the corresponding MS concentration signal. Samples were reduced at 450 °C for 6 h.

clear difference in the H2 adsorptions of the samples reduced at different time lengths can be noted here. The largest amount of hydrogen was adsorbed over the sample with the longest reduction period (12 h). Also, the XPS results for the Ni sample, uncalcined and reduced for 12 h (Figure 8) showed that after 12 h reduction, almost all of the Ni converted to metallic nickel (Ni°). Therefore, no major increase in reduction would be expected beyond a 12 h reduction treatment. The H2 uptake of the sample reduced for 6 h dropped to nearly half of the H2 uptake of the 12 h reduced sample and as previously shown (Figure 9), the percentage of reduction for this sample was only 47%. The smallest H2 uptake was adsorbed on the sample reduced for 3 h. The reduction treatment with 3 h reduction duration correspond to a marginal reduction of approximately 12% (The contribution of any H2 physisorption was neglected). The results presented in Figure 10 showed that another important factor for reducing the Ni species to their metallic state is the length of exposure to hydrogen.

(reduced) by the highest hydrogen concentration was nearly as twice large as the one treated with 10% hydrogen. The reduction treatments with 10% and 15% hydrogen concentration correspond to H/Ni ratios of approximately 0.24 and 0.47, respectively. (The contribution of any H2 physisorption was neglected.) In other words, the percentages of reduction for the samples treated with 10% and 15% hydrogen concentration were 24% and 47%, respectively. These results clearly confirm how significant the hydrogen concentration is during the reduction of the Ni samples. Figure 10 shows the results of the heat flow and the corresponding H2 uptakes of three Ni samples from the same Ni batch of group 1 (uncalcined). All three samples were reduced at 450 °C with 15% H2/Ar but with the different durations of 3, 6, and 12 h. Then the samples were exposed to 100 ppm of H2 in Ar for 1 h at 80 °C after cooling in Ar only. A 11518


Article

Industrial & Engineering Chemistry Research 3.2.2. Reducibility of Co/Al2O3. The results of hydrogen chemisorption and XPS experiments of Co samples that were subjected to the same reduction but different calcination treatments are presented in Figure 11 and Figure 12. Figure 11

shows the complete thermogram signals detected by the DSC in the top panel. For easy comparisons, the heat peak region of all three samples is magnified in the bottom panel. The MS signals (see the complementary information) showed that the H2 uptakes over the Co samples, unlike the Ni samples, were very small with no significant differences among them. The hydrogen chemisorption results may indicate that the precalcination step had little or very minor effect on the reducibility of the prepared Co samples. In order to further investigate the Co sites and to validate the hydrogen chemisorption results, XPS measurements were conducted. The Co 2p core level spectrum includes Co 2p1/2, Co 2p3/2, and shake-up satellites for all three reduced (450 °C) samples that were previously treated (uncalcined, calcined at 400 and 550 °C) are shown in Figure 12. The Co 2p3/2 peak position of all three Co samples are marked with the dotted lines at 779.1, 780, and 780.4 eV, respectively. These binding energies of Co 2p3/2 peaks are centered in a region of 779−781 eV, which is close to the binding energies of CoO and CoAl2O4/Co3O4 spinel.57 Moreover, it is widely accepted that the Co 2p3/2 peak in metallic cobalt has an asymmetric shape with no obvious satellites but when oxidized, it has obvious satellite features. For instance, some references reported that Co2+ contains a Co 2p3/2 satellite peak at 786 eV.59,60 As it is shown in Figure 12, all three Co 2p3/2 peaks have satellites with binding energies around 786 eV. This is another indication of the presence of oxide forms. For further examination, the Co 2p core level including Co 2p1/2, Co 2p3/2, and shake-up satellites were deconvoluted with the same procedure previously presented for the Ni/Al2O3 sample. The deconvolution results are presented in Figure 13. The deconvolution results of the Co 2p3/2 peaks of the uncalcined sample (Figure 13, left) showed that the Co sites were mostly in form of oxide CoO (780 eV), but a small fraction of metallic cobalt sites Co° (778.5 eV) were also detected. It is very noticeable that the majority of the Co sites in both previously calcined samples (400 and 550 °C, Figure 13, middle and right) were in form of oxide (Co2+). Also, small fractions of metallic cobalt sites (Co°) and CoAl2O4/Co3O4 spinel were detected in both calcined samples. In other words, the uncalcined sample was only marginally reduced. The results of reduction treatments of supported cobalt catalysts has been documented in a number of studies.61−63 For instance, Jongsomjit et al. showed results of a two-stage reduction for supported cobalt catalysts. First, a low temperature (∼300 °C) conversion of Co3O4 to CoO (Co3O4 → 3CoO + (1/2)O2), (decomposition of the impregnated Co(NO3)2 formed the Co3O4 species). Second, a high temperature (∼600 °C) conversion of CoO to metallic cobalt (CoO → Co).61 Lin et al. also measured the activation energies needed for those two reduction steps as 94.43 kJ mol−1 for

Figure 11. Effect of calcination on the reducibility of Co/Al2O3 (10%) samples. The top panel shows the thermographs from the DSC and the sample temperature during the adsorption. The bottom panel represents magnified region of the heat peaks.

Figure 12. XPS spectra for three Co/Al2O3 samples for binding energies between 890 and 840 eV, which contain the Co 2p1/2, Co 2p3/2, and their satellite peak positions. The Co 2p3/2 and their shakeup satellite peaks are marked with the dotted lines.

Figure 13. XPS spectra of the Co 2p core level spectrum for the samples: Not calcined but reduced (left), calcined at 400 °C and reduced (middle), and calcined at 550 °C and reduced (right). The deconvoluted peaks are marked with dotted lines. 11519


Article


Figure 14. XPS spectra of the Co 2p core level spectrum for the samples: calcined at 400 °C and not reduced (left) and calcined at 550 °C and not reduced (right). The deconvoluted peaks are marked with dotted lines.

reduction of Co3O4 to CoO and 82.97 kJ mol−1 for reduction of CoO to Co (metallic).63 However, in this study, to assess the effect of calcination treatment, only a one step reduction (450 °C) was performed for all the catalysts. Yet, in comparison to the previous reduction results for the Ni catalysts, particularly group 1 and group 2, the Co samples were hardly reduced. It is very noticeable that the majority of the Co sites in all three cobalt samples were still in form of CoO (Figure 13). On the basis of the above results, it is very likely that the one-step reduction treatment (450 °C) was only sufficient to overcome the first activation barrier (and result in conversion of Co3O4 to CoO). In other words the uncalcined sample was only marginally reduced which is in line with the low hydrogen uptake observed for the sample (Figure 11). Nonetheless, some detrimental effect of a high calcination temperature on the cobalt samples were observed. Figure 14 presents the deconvolution results of the Co 2p3/2 peaks of two calcined samples. Both samples were from the same cobalt batch (same loadings), calcined differently (400 and 550 °C) and not reduced. The peak intensities of CoO and CoAl2O4/ Co3O4 (spinel) sites of the sample calcined at 400 °C (left) are almost equal which indicates nearly equal portions of CoO and CoAl2O4/Co3O4 in this sample. In contrast, the peak intensity of the CoAl2O4/Co3O4 (spinel) phase of the sample calcined at 550 °C is significantly higher than the intensity of CoO. In other words, the sample reduced at 550 °C contains a higher portion of spinel phase. It seems that a higher temperature of calcination stimulated the growth of the CoAl2O4/Co3O4 crystalline phase which is generally more difficult to reduce. 3.2.3. Reducibility of Bimetallic Samples. The reducibility of the Mo/Al2O3 sample together with its combination with Ni and Co was tested at different reducing conditions. Two selected sets of results from the hydrogen adsorption measurements are shown in Figures 15 and 16. The previous hydrogen adsorption results of Ni and Co samples are also presented here to ease comparison between single and bimetallic samples. All samples used in these two sets of experiments were chosen from group 1, the uncalcined samples. After pretreatment by a flow of 8% H2 in Ar at 500 °C for 15 min, the samples were reduced for 12 h by 15% H2/Ar at 450 °C, cooling in Ar only to 80 °C and then exposed to 100 ppm of hydrogen in argon at 80 °C. The Mo samples did not show any hydrogen adsorption and the following XPS measurements confirmed the presences of MoO3 with binding energies of 232.4 eV (Not shown here). The NiMo sample though displayed a small amount of H2 uptake.

Figure 15. Effect of combining Ni and Mo in NiMo/Al2O3 sample. Top panel shows the heat signal from the DSC and the sample temperature. Bottom panel represents the corresponding MS concentration signal (samples are uncalcined, reduced 12 h by 15% H2/Ar at 450 °C).

Figure 16. Effect of combining Co and Mo in CoMo/Al2O3 sample. Top panel shows the heat signal from the DSC and the sample temperature. Bottom panel presents the corresponding MS concentration signal (Samples are uncalcined, reduced 12 h by 15% H2/Ar at 450 °C).

The XPS results for the Ni presented in Figure 17. The including Ni 2p1/2, Ni 2p3/2, (Figure 17 on the left) were 11520

sites in the NiMo sample is Ni 2p core level spectrum and their shake-up satellites deconvoluted with the same DOI: 10.1021/acs.iecr.5b02510 Ind. Eng. Chem. Res. 2015, 54, 11511−11524

Article


Figure 17. XPS spectra of the Ni 2p core level spectrum (on the left) and the Mo 3d core level spectrum (on the right) in the uncalcined and reduced NiMo/Al2O3 sample. The deconvoluted peaks are marked with dotted lines.

Figure 18. XPS spectra of Co 2p (on the left) and the Mo 3d (on the right). The deconvoluted peaks are marked with dotted lines.

metallic form, explaining why no adsorption was observed for the CoMo sample (Figure 16). 3.2.4. Calorimetric Measurements. To develop a kinetic model for HDO, it is very important to measure the heat of adsorption of hydrogen on the catalyst. The heats of adsorption (ΔH) for the Ni and Co samples were determined from the micro calorimetric results shown in Figures 7 and 11. The heats of adsorption measurements were based on the regions where the heat signals reached a constant plateau and simultaneously a total uptake of all H2 was observed using MS. This is a method that was previously developed by our group and used for ammonia adsorption on zeolite.68 The heat of adsorption for the two prepared Ni samples, uncalcined and calcined at 400 °C, were determined to be −140 kJ mol−1 and this value is close to the previously reported adsorption enthalpies for hydrogen on nickel69,70 which were −151 kJ mol−1 and −130 kJ mol−1 (for low surface coverage). The heat of hydrogen adsorption of all three cobalt samples, calcined and uncalcined, was determined with the same procedure and found to be −98 kJ mol−1. This value is also close to the previously reported adsorption enthalpies for hydrogen on cobalt which was −105 kJ mol−1.71

procedure previously presented for the Ni/Al2O3 sample. The Ni 2p3/2 peak showed three peaks with binding energies of 852, 854.5, and 856 eV which can be attributed to Ni° (metallic), Ni2+ and NiMoO4 sites, respectively.45 The Ni 2p3/2 shake-up satellite together with the Ni 2p1/2 peak and Ni 2p1/2 satellite confirmed the presence of Ni2+ with the binding energy of 860 eV, 872.9 and 879.8 eV.64 The Ni 2p1/2 peak also confirmed the presence of Ni° (metallic) sites with the binding energy of 870.1 eV.65 The deconvoluted results for the Mo 3d core level spectrum including Mo 3d3/2 and Mo 3d5/2 peaks is presented in Figure 17 (on the right). The Mo 3d5/2 peak showed two peaks with binding energies of 232.6 eV and 233.3 which can be attributed to Al2(MoO 4) 3/NiMoO4 and MoO3 phases, respectively.57,66 Therefore, the low hydrogen adsorption on the NiMo sample (Figure 15) is likely only due to the presence of a small portion of the Ni° (metallic) sites for H2 adsorption. The XPS results for the Co and Mo sites in the CoMo sample is presented in Figure 18. The Co 2p core level spectrum including Co 2p1/2, Co 2p3/2, and shake-up satellites were deconvoluted (Figure 18 on the left). The deconvoluted Co 2p3/2 peak showed two peaks with binding energies of 780.7 and 782.8 eV which can be attributed to Co2+ (CoO) and CoMoO4 respectively.57,67 Moreover, a Co 2p3/2 satellite peak with the binding energy of 786 eV confirmed the presence of the Co2+ species. The deconvoluted Mo 3d core level spectrum including Mo 3d3/2 and Mo 3d5/2 also confirmed that Mo was in the form of MoO3 and Al2(MoO4)3/CoMoO4 with the binding energies of 232.5 and 233.4 eV respectively45,52 (Figure 18 on the right). None of the Co and Mo species were in

4. CONCLUSIONS In this study, nonsulfided alumina supported Ni, Co, Mo, NiMo, and CoMo hydrotreating catalysts were synthesized by an impregnation method. The effect of calcination temperature and time/concentration of hydrogen during the reduction was studied. In addition, detailed characterization of the materials was performed. The main conclusions are as follows: (1) 11521


Article


(6) Cepeda, E. A.; Calvo, B. Sunflower oil hydrogenation: Study using response surface methodology. J. Food Eng. 2008, 89, 370−374. (7) Qian, W. H.; Yamada, S.; Ishihara, A.; Ichinoseki, M.; Kabe, T. Methods of activating catalysts for hydrodesulfurization of light gas oil (Part 2) - Catalytic properties of CoMo/Al2O3 presulfided by polysulfides for deep and ultra-deep hydrodesulfurization of light gas oil. Sekiyu Gakkaishi 2001, 44, 225−231. (8) Sotelo-Boyas, R.; Liu, Y. Y.; Minowa, T. Renewable Diesel Production from the Hydrotreating of Rapeseed Oil with Pt/Zeolite and NiMo/Al2O3 Catalysts. Ind. Eng. Chem. Res. 2011, 50, 2791− 2799. (9) Lestari, S.; Maki-Arvela, P.; Simakova, I.; Beltramini, J.; Lu, G. Q. M.; Murzin, D. Y. Catalytic Deoxygenation of Stearic Acid and Palmitic Acid in Semibatch Mode. Catal. Lett. 2009, 130, 48−51. (10) Maki-Arvela, P.; Rozmyslowicz, B.; Lestari, S.; Simakova, O.; Eranen, K.; Salmi, T.; Murzin, D. Y. Catalytic Deoxygenation of Tall Oil Fatty Acid over Palladium Supported on Mesoporous Carbon. Energy Fuels 2011, 25, 2815−2825. (11) Rozmyslowicz, B.; Maki-Arvela, P.; Lestari, S.; Simakova, O. A.; Eranen, K.; Simakova, I. L.; Murzin, D. Y.; Salmi, T. O. Catalytic Deoxygenation of Tall Oil Fatty Acids Over a Palladium-Mesoporous Carbon Catalyst: A New Source of Biofuels. Top. Catal. 2010, 53, 1274−1277. (12) Furimsky, E. Catalytic hydrodeoxygenation. Appl. Catal., A 2000, 199, 147−190. (13) Ryymin, E. M.; Honkela, M. L.; Viljava, T. R.; Krause, A. O. I. Competitive reactions and mechanisms in the simultaneous HDO of phenol and methyl heptanoate over sulphided NiMo/gamma-Al2O3. Appl. Catal., A 2010, 389, 114−121. (14) Lugo-Jose, Y. K.; Monnier, J. R.; Heyden, A.; Williams, C. T. Hydrodeoxygenation of propanoic acid over silica-supported palladium: effect of metal particle size. Catal. Sci. Technol. 2014, 4, 3909− 3916. (15) Zuo, H.; Liu, Q.; Wang, T.; Ma, L.; Zhang, Q.; Zhang, Q. Hydrodeoxygenation of Methyl Palmitate over Supported Ni Catalysts for Diesel-like Fuel Production. Energy Fuels 2012, 26, 3747−3755. (16) Hancsok, J.; Kasza, T.; Kovacs, S.; Solymosi, P.; Hollo, A. Production of bioparaffins by the catalytic hydrogenation of natural triglycerides. J. Cleaner Prod. 2012, 34, 76−81. (17) Sanna, A.; Vispute, T. P.; Huber, G. W. Hydrodeoxygenation of the aqueous fraction of bio-oil with Ru/C and Pt/C catalysts. Appl. Catal., B 2015, 165, 446−456. (18) Li, K. L.; Wang, R. J.; Chen, J. X. Hydrodeoxygenation of Anisole over Silica-Supported Ni2P, MoP, and NiMoP Catalysts. Energy Fuels 2011, 25, 854−863. (19) Liu, Z. H.; Chu, B. Z.; Zhai, X. L.; Jin, Y.; Cheng, Y. Total methanation of syngas to synthetic natural gas over Ni catalyst in a micro-channel reactor. Fuel 2012, 95, 599−605. (20) Fatsikostas, A. N.; Verykios, X. E. Reaction network of steam reforming of ethanol over Ni-based catalysts. J. Catal. 2004, 225, 439− 452. (21) Liu, C. J.; Ye, J. Y.; Jiang, J. J.; Pan, Y. X. Progresses in the Preparation of Coke Resistant Ni-based Catalyst for Steam and CO2 Reforming of Methane. ChemCatChem 2011, 3, 529−541. (22) Costa, V.; Guichard, B.; Digne, M.; Legens, C.; Lecour, P.; Marchand, K.; Raybaud, P.; Krebs, E.; Geantet, C. A rational interpretation of improved catalytic performances of additiveimpregnated dried CoMo hydrotreating catalysts: a combined theoretical and experimental study. Catal. Sci. Technol. 2013, 3, 140−151. (23) Ma, X. L.; Sakanishi, K. Y.; Mochida, I. Hydrodesulfurisation reactivities of various sulfur-compounds in diesel fuel. Ind. Eng. Chem. Res. 1994, 33, 218−222. (24) Chen, N.; Gong, S.; Qian, E. W. Effect of reduction temperature of NiMoO3-x/SAPO-11 on its catalytic activity in hydrodeoxygenation of methyl laurate. Appl. Catal., B 2015, 174−175, 253−263. (25) Zhou, M.; Tian, L.; Niu, L.; Li, C.; Xiao, G.; Xiao, R. Upgrading of liquid fuel from fast pyrolysis of biomass over modified Ni/CNT catalysts. Fuel Process. Technol. 2014, 126, 12−18.

Results from hydrogen adsorption experiments showed that the noncalcined Ni/Al2O3 sample was much easier to reduce compared to the calcined samples. In addition, higher calcination temperature (550 versus 400 °C) made the reduction even more difficult. The XPS results confirmed that the noncalcined sample contained significantly larger amount of Ni° compared to the calcined samples. In addition, formation of a nickel−alumina spinel was found for the calcined samples, which was larger for the sample precalcined at 550 compared to 400 °C. To conclude, it is advantageous to treat the Ni samples directly with hydrogen, without prior calcination, to avoid spinel formation and increase the reducibility of the material. (2) The degree of reduction for the uncalcined Ni was increased when the reduction time was increased. Also, the concentration of hydrogen during the reduction was shown to have a strong impact on the reducibility of the Ni samples. (3) The Co, Mo, NiMo, and CoMo samples were difficult to reduce. The calcined Mo samples did not possess a measurable hydrogen uptake and the XPS result showed that the Mo species were in form of oxidized phases. The hydrogen uptake capacity of the Mo sample was shown to be slightly improved when promoted by nickel. But, this was likely due to hydrogen uptake on small nickel particles. Small hydrogen adsorption was found for the Co sample, but nonetheless, Co had no apparent promotional effect on the hydrogen capacity of the CoMo sample. (4) Calorimetric measurements were conducted to determine the heat of adsorption of hydrogen (ΔH) on Ni and Co samples and resulted in ΔH of −140 and −98 kJ mol−1, respectively.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +46 31 772 4390. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We would like to acknowledge Formas (Contract: 239-20121584) for the financial support. In addition we acknowledge the help from Stefan Gustafsson and Lunjie Zeng from the division of Applied Physics of Chalmers University of Technology with the SEM and TEM analysis.

(1) Chorkendorff, I.; Niemantsverdriet, J. W. Concept of Modern Catalysis and Kinetics; Wiley-VCH Verlag GmbH & Co.: Weinheim, Federal Republic of Germany, 2003; p 452. (2) Climent, M. j.; Corma, A.; Ibborra, S. Heterogeneous Catalysts for the One-Pot Synthesis of Chemicals and Fine. Chem. Rev. 2010, 111, 1072−1133. (3) Choi, J.; Lee, H.; Choi, Y.; Kim, S.; Lee, S.; Lee, S.; Choi, W.; Lee, J. Heterogeneous photocatalytic treatment of pharmaceutical micropollutants: Effects of wastewater effluent matrix and catalyst modifications. Appl. Catal., B 2014, 147, 8−16. (4) Chakraborty, R.; Gupta, A. K.; Chowdhury, R. Conversion of slaughterhouse and poultry farm animal fats and wastes to biodiesel: Parametric sensitivity and fuel quality assessment. Renewable Sustainable Energy Rev. 2014, 29, 120−134. (5) Saadi, S.; Ariffin, A. A.; Ghazali, H. M.; Miskandar, M. S.; Abdulkarim, S. M.; Boo, H. C. Effect of Blending and Emulsification on Thermal Behavior, Solid Fat Content, and Microstructure Properties of Palm Oil-Based Margarine Fats. J. Food Sci. 2011, 76, C21−C30. 11522


Article

Industrial & Engineering Chemistry Research (26) de Haan, R.; Joorst, G.; Mokoena, E.; Nicolaides, C. P. Nonsulfided nickel supported on silicated alumina as catalyst for the hydrocracking of n-hexadecane and of iron-based Fischer−Tropsch wax. Appl. Catal., A 2007, 327, 247−254. (27) Liu, J.; Fan, K.; Tian, W.; Liu, C.; Rong, L. Hydroprocessing of Jatropha oil over NiMoCe/Al2O3 catalyst. Int. J. Hydrogen Energy 2012, 37, 17731−17737. (28) Dasgupta, N. P.; Mack, J. F.; Langston, M. C.; Bousetta, A.; Prinz, F. B. Design of an atomic layer deposition reactor for hydrogen sulfide compatibility. Rev. Sci. Instrum. 2010, 81, 044102. (29) Laurenti, D.; Phung-Ngoc, B.; Roukoss, C.; Devers, E.; Marchand, K.; Massin, L.; Lemaitre, L.; Legens, C.; Quoineaud, A.A.; Vrinat, M. Intrinsic potential of alumina-supported CoMo catalysts in HDS: Comparison between γc, γT, and δ-alumina. J. Catal. 2013, 297, 165−175. (30) Nikulshin, P. A.; Salnikov, V. A.; Mozhaev, A. V.; Minaev, P. P.; Kogan, V. M.; Pimerzin, A. A. Relationship between active phase morphology and catalytic properties of the carbon alumina-supported Co(Ni)Mo catalysts in HDS and HYD reactions. J. Catal. 2014, 309, 386−396. (31) Sun, M. Y.; Nicosia, D.; Prins, R. The effects of fluorine, phosphate and chelating agents on hydrotreating catalysts and catalysis. Catal. Today 2003, 86, 173−189. (32) Bartholomew, C. H.; Farrauto, R. J. Chemistry of nickel-alumina catalysts. J. Catal. 1976, 45, 41−53. (33) van Dillen, A. J.; Terorde, R.; Lensveld, D. J.; Geus, J. W.; de Jong, K. P. Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes. J. Catal. 2003, 216, 257−264. (34) Holm, V. C. F.; Clark, A. Reduction studies on supported metal oxide catalysts. J. Catal. 1968, 11, 305−316. (35) Brunelle, J. P. Preparation of catalysts by metallic complex adsorption on mineral oxides. Pure Appl. Chem. 1978, 50, 1211−1229. (36) Guevara-Lara, A.; Bacaud, R.; Vrinat, M. Highly active NiMo/ TiO2-Al2O3 catalysts: Influence of the preparation and the activation conditions on the catalytic activity. Appl. Catal., A 2007, 328, 99−108. (37) Ramos, A. L. D.; Alves, P. D.; Aranda, D. A. G.; Schmal, M. Characterization of carbon supported palladium catalysts: inference of electronic and particle size effects using reaction probes. Appl. Catal., A 2004, 277, 71−81. (38) Al-Daous, M. A.; Ali, S. A. Deep desulfurization of gas oil over NiMo catalysts supported on alumina−zirconia composites. Fuel 2012, 97, 662−669. (39) Chary, K.; Srikanth, C. Selective Hydrogenation of Nitrobenzene to Aniline over Ru/SBA-15 Catalysts. Catal. Lett. 2009, 128, 164−170. (40) Rossetti, I.; Gallo, A.; Dal Santo, V.; Bianchi, C. L.; Nichele, V.; Signoretto, M.; Finocchio, E.; Ramis, G.; Di Michele, A. Nickel Catalysts Supported Over TiO2, SiO2 and ZrO2 for the Steam Reforming of Glycerol. ChemCatChem 2013, 5, 294−306. (41) Daza, C. E.; Gallego, J.; Mondragon, F.; Moreno, S.; Molina, R. High stability of Ce-promoted Ni/Mg-Al catalysts derived from hydrotalcites in dry reforming of methane. Fuel 2010, 89, 592−603. (42) Jones, R. D.; Bartholomew, C. H. Improved flow technique for measurement of hydrogen chemisorption on metal catalysts. Appl. Catal. 1988, 39, 77−88. (43) Wang, H.; Yao, Z.; Zhan, X.; Wu, Y.; Li, M. Preparation of highly dispersed W/ZrO2-Al2O3 hydrodesulfurization catalysts at high WO3 loading via a microwave hydrothermal method. Fuel 2015, 158, 918−926. (44) Li, G.; Hu, L.; Hill, J. M. Comparison of reducibility and stability of alumina-supported Ni catalysts prepared by impregnation and coprecipitation. Appl. Catal., A 2006, 301, 16−24. (45) Zdrazil, M. MgO-supported Mo, CoMo and NiMo sulfide hydrotreating catalysts. Catal. Today 2003, 86, 151−171. (46) Li, G. H.; Hu, L. J.; Hill, J. M. Comparison of reducibility and stability of alumina-supported Ni catalysts prepared by impregnation and co-precipitation. Appl. Catal., A 2006, 301, 16−24.

(47) Cordero, R. L.; Llambias, F. J. G.; Agudo, A. L. Temperatueprogrammed reduction and zeta potential studies of the structure of MoO3/Al2O3 and MoO3/SiO2 catalysts -effect of the impregnation pH and molybdenum loading. Appl. Catal. 1991, 74, 125−136. (48) Ferdous, D.; Dalai, A. K.; Adjaye, J. A series of NiMo/ A1(2)O(3) catalysts containing boron and phosphorus Part I. Synthesis and characterization. Appl. Catal., A 2004, 260, 137−151. (49) Spojakina, A.; Kostova, N.; Vit, Z.; Zdrazil, M. MoO3/Al2O3 catalyst: Comparison of catalysts prepared by new slurry impregnation with molybdic acid with conventional samples. Polym. J. Chem. 2003, 77, 767−778. (50) López Cordero, R.; López Agudo, A. Effect of water extraction on the surface properties of Mo/Al2O3 and NiMo/Al2O3 hydrotreating catalysts. Appl. Catal., A 2000, 202, 23−35. (51) Sehested, J.; Gelten, J. A. P.; Remediakis, I. N.; Bengaard, H.; Nørskov, J. K. Sintering of nickel steam-reforming catalysts: effects of temperature and steam and hydrogen pressures. J. Catal. 2004, 223, 432−443. (52) Chen, Y. G.; Ren, J. Conversion of methane and carbon dioxide into synthesis gas over alumina-supported nickel catalysts. Effect of NiAl2O3 interactions. Catal. Lett. 1994, 29, 39−48. (53) Sahli, N.; Petit, C.; Roger, A. C.; Kiennemann, A.; Libs, S.; Bettahar, M. M. Ni catalysts from NiAl2O4 spinel for CO2 reforming of methane. Catal. Today 2006, 113, 187−193. (54) Liotta, L. F.; Pantaleo, G.; Macaluso, A.; Di Carlo, G.; Deganello, G. CoOx catalysts supported on alumina and alumina-baria: influence of the support on the cobalt species and their activity in NO reduction by C3H6 in lean conditions. Appl. Catal., A 2003, 245, 167− 177. (55) Zahran, A. A.; Shaheen, W. M.; El-Shobaky, G. A. Surface and catalytic properties of MoO3/Al2O3 system doped with Co3O4. Mater. Res. Bull. 2005, 40, 1065−1080. (56) Shaheen, W. M.; Abd El Maksod, I. H. Thermal solid−solid interactions and physicochemical properties of Co3O4/MoO3 system treated with Al2O3. J. Alloys Compd. 2009, 475, 874−880. (57) Moulder, J. F.; William, F. S.; Sobel, P. E.; Bomber, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (58) Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S.; McIntyre, N. S. New interpretations of XPS spectra of nickel metal and oxides. Surf. Sci. 2006, 600, 1771−1779. (59) Sexton, B. A.; Hughes, A. E.; Turney, T. W. An XPS and TPR study of the reduction of promoted cobalt-kieselguhr Fischer−Tropsch catalysts. J. Catal. 1986, 97, 390−406. (60) Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717−2730. (61) Jongsomjit, B.; Panpranot, J.; Goodwin, J. G., Jr Co-Support Compound Formation in Alumina-Supported Cobalt Catalysts. J. Catal. 2001, 204, 98−109. (62) Bechara, R.; Balloy, D.; Dauphin, J. Y.; Grimblot, J. Influence of the characteristics of gamma-aluminas on the dispersion and the reducibility of supported cobalt catalysts. Chem. Mater. 1999, 11, 1703−1711. (63) Lin, H.-Y.; Chen, Y.-W. The mechanism of reduction of cobalt by hydrogen. Mater. Chem. Phys. 2004, 85, 171−175. (64) Lian, K. K.; Kirk, D. W.; Thorpe, S. J. Investigation of a 2-state tafel phenomenon for the oxygen evolution reaction on an amorphous Ni-Co alloy. J. Electrochem. Soc. 1995, 142, 3704−3712. (65) Hsu, L. S.; Williams, R. S. Electronic-structure study of the NiGa and the Ni-In intermetallic compounds using X-ray photoemission spectroscop. J. Phys. Chem. Solids 1994, 55, 305−312. (66) Devries, J. E.; Yao, H. C.; Baird, R. J.; Gandhi, H. S. Characterization of molybdenum-platinum catalysts supported on γalumina by X-ray photoelectron spectroscopy. J. Catal. 1983, 84, 8− 14. (67) Kasztelan, S.; Grimblot, J.; Bonnelle, J. P.; Payen, E.; Toulhoat, H.; Jacquin, Y. Preparation of Co-Mo-γAl2O3 and Ni-Mo-γAl2O3 11523


Article

Industrial & Engineering Chemistry Research catalysts by ph regulation of molybdenum solution. characterization of supported species and hydrogenation activities. Appl. Catal. 1983, 7, 91−112. (68) Wilken, N.; Kamasamudram, K.; Currier, N. W.; Li, J. H.; Yezerets, A.; Olsson, L. Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications. Catal. Today 2010, 151, 237−243. (69) Beeck, O. Catalysis and the Adsorption of Hydrogen on Metal Catalysts. Adv. Catal. 1950, 2, 151−195. (70) Anderson, J. R. Structure of Metallic Catalysts. Academic Press: London, 1975. (71) Zowtiak, J. M.; Bartholomew, C. H. The kinetics of H2 adsorption on and desorption from cobalt and the effects of support thereon. J. Catal. 1983, 83, 107−120.

11524


Effect of Thermal Treatment on Hydrogen Uptake and Characteristics

Recommend Documents