Journal of Colloid and Interface Science 447 (2015) 68–76

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Preparation of highly active and hydrothermally stable nickel catalysts Shaozhong Li, Hui Chen, Jianyi Shen ⇑ Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 3 December 2014 Accepted 29 January 2015 Available online 7 February 2015 Keywords: Supported nickel catalyst Hydrothermal treatment Microcalorimetric adsorption Hydrogenation of glucose

a b s t r a c t The 60%Ni/AlSiO catalysts were prepared by the co-precipitation method, in which AlSiO were the composite supports with different mass ratios of Al2O3 and SiO2. It was found that the catalyst 60%Ni/ AlSiO-4 with the Al2O3/SiO2 mass ratio of 4 in the support exhibited the high hydrothermal stability. The addition of proper amount of SiO2 inhibited the hydration of Al2O3 and prevented the growth of supported nickel particles during the hydrothermal treatment. The structure of the composite support in the 60%Ni/AlSiO-4 was stable and the supported nickel particles were highly dispersed. Accordingly, the hydrothermally treated catalyst maintained the high heats and uptakes for the adsorption of H2 and CO, and thus the high activity and stability for the hydrogenation of glucose to sorbitol in aqueous solution. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The use of organic solvents in many industrial processes increases the costs and causes environment pollutions [1]. Water is cheap, safe and widely available and may be used as an environmentally benign solvent [2–4]. The hydrogenation reactions are widely applied in industry [5]. The hydrogenation reactions with the solvent of water have been developed and used in industries [6–8]. For instance, sorbitol is an important chemical widely used in food, cosmetic, paper and other fine chemical industries. Although it exists in a variety of fruits, the cost of extraction is high, so that its mass production is based on the hydrogenation of glucose in aqueous solution [9–11]. Crotyl alcohol is widely used in the fields of pesticide, paint and plasticizer, and it is usually produced by the hydrogenation of crotonaldehyde in organic solvents. Attention has been attracted to the hydrogenation of

⇑ Corresponding author. Fax: +86 25 83594305. E-mail address: [email protected] (J. Shen). http://dx.doi.org/10.1016/j.jcis.2015.01.081 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

crotonaldehyde to crotyl alcohol in aqueous solution for the development of safer and greener process [12]. In addition, water would be generated in many hydrogenation reactions [13]. For example, the hydrogenation of nitrobenzene to aniline produces great amount of water [14]. Thus, the catalysts must withstand the severe hydrothermal environments for the hydrogenation reactions with water as a solvent or a product. The supports may be hydrolyzed, causing the collapse of pore structures and leading to the aggregation of supported active components and finally the deactivation of catalysts [15]. Thus, it is important to develop the highly active and hydrothermally stable catalysts. Unfortunately, the fundamental studies on such catalysts are rarely reported in open literature. Nickel catalysts are active and cheap and supported nickel catalysts are widely used in hydrogenation reactions [16,17]. Alumina is a widely used support with high surface areas and thermal stabilities [18,19]. However, the hydrothermal stability of Ni/ Al2O3 catalysts was poor, due to the easy hydration of Al2O3, leading to the significant decrease of surface areas and pore volumes and thus the aggregation of supported nickel particles and the loss

S. Li et al. / Journal of Colloid and Interface Science 447 (2015) 68–76

of catalytic activities [15]. In order to improve the hydrothermal stability of Ni/Al2O3 catalysts, it is necessary to improve the hydrothermal stability of the Al2O3 support. It is known that there are many unsaturated tetrahedral and octahedral voids in alumina [20], and the A13+ cations in these voids are highly active and easily hydrated to form AlOOH, Al(OH)3 or b-Al(OH)3 species [21]. The occupation of these voids by some suitable additives could inhibit the hydration of A13+ cations by water, improving the hydrothermal stability of alumina. Ravenelle et al. [22] prepared the Ni/c-Al2O3 and Pt/c-Al2O3 catalysts and studied the structural changes of these catalysts under hydrothermal conditions. They found that the presence of Ni and Pt particles significantly retarded the formation of boehmite. It has been reported that the Al–O–Al bonds in Al2O3 would be turned into Al–OH bonds in the hydrothermal environments. The addition of SiO2 might lead to the formation of Si–O–Si and Si–O–Al bonds, eliminating the surface vacancies, leading to the increased hydrothermal stability of alumina [23]. Kang et al. [24] prepared the Ni/Al2O3–SiO2 catalysts with different SiO2 contents by the impregnation method and showed that the exposure of Al3+ could be reduced by the presence of SiO2. The optimized content of SiO2 was 3%, with which the Ni/ Al2O3–SiO2 catalyst exhibited the best activity for the hydrogenation of 1,4-butynediol to 1,4-butanediol in aqueous solution. Since the hydrogenation activity depends on the number of surface metal atoms, it is desirable to prepare the supported metal catalysts with the high loading, reducibility and dispersion [25]. The co-precipitation method was used in this work for the preparation of supported nickel catalysts with the high loading of Ni (about 60 wt.%). The composite metal oxides (termed as AlSiO-x) with different mass ratios of Al2O3 and SiO2 (x) were used as the supports for the preparation of the Ni/AlSiO-x catalysts with the high hydrothermal stability. The techniques of H2–O2 titration, X-ray diffraction (XRD), transmission electron microscopy (TEM) and microcalorimetric adsorption were used to study the structures and surface properties of the fresh and hydrothermally treated catalysts. It was found that the catalyst Ni/AlSiO-4 exhibited the stable structure and the high active surface area and thus the high activity for the hydrogenation of glucose to sorbitol in aqueous solution. 2. Experimental 2.1. Preparation of catalysts The catalysts Ni/Al2O3, Ni/SiO2 and Ni/AlSiO-x (x = 1–5) containing about 60 wt.% Ni were prepared by the co-precipitation method. Specifically, the required amounts of Ni(NO3)2
6H2O and Al(NO3)3
9H2O were dissolved in 100 mL distilled water to form an aqueous solution (I). The required amounts of Na2SiO3
9H2O and Na2CO3 were dissolved in 100 mL distilled water to form another solution (II). The two solutions were added simultaneously into a beaker containing 400 mL distilled water at 353 K under vigorous stirring. Green precipitates formed were filtered and washed thoroughly with distilled water until pH of the filtrate reached 7. Each of the filter cakes was dispersed into 200 mL n-butanol and heated at 353 K for 12 h to remove water and then n-butanol. The samples were further dried at 393 K in an oven overnight. The hydrogenation of 1,4-butynediol to 1,4-butanediol was performed in aqueous solution in industry. The catalyst used for this process was the 17%Ni/Al2O3 prepared by the wet impregnation method [15]. In this work, a similar catalyst (17%Ni/Al2O3) was prepared for comparison by the incipient wetness impregnation method. The experimental results showed that the 60%Ni/AlSiO-4 was not superior over the 17%Ni/Al2O3 for the hydrogenation of 1,4-butynediol to 1,4-butanediol in aqueous solution. However,

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the loading of nickel and thus the active Ni surface area were low in the 17%Ni/Al2O3, so that it might not be active for other hydrogenation reactions (for example, the hydrogenation of glucose to sorbitol) in aqueous solution. The catalysts were reduced in flowing H2 at 723 K for 2 h. After cooled down to room temperature, they were transferred under the protection of flowing H2 into a 100 mL Teflon-lined stainlesssteel autoclave containing 40 mL distilled water. After the autoclave was sealed, it was placed in an oven and heated for 8 h at 363, 393 and 423 K, respectively. The samples were then filtered and dried in an oven at 393 K for 12 h. The hydrothermal temperatures applied were added as the suffix to name the hydrothermally treated samples. For example, the catalyst Ni/ AlSiO-4 hydrothermally treated at 363 K for 8 h was termed as Ni/AlSiO-4-363. 2.2. Characterization of catalysts The adsorption of H2 and O2 was carried out in a home-made volumetric apparatus. The catalyst was reduced in H2 at 723 K for 2 h and evacuated at 723 K for 1 h before the measurements. The adsorption of H2 was performed at room temperature. After the adsorption of H2, the sample was heated to 673 K at a rate of 10 K/min and evacuated at the temperature for 1 h. The adsorption of O2 was then performed at 673 K. The uptakes of H2 and O2 were obtained by extrapolating the coverage of corresponding isotherms to P = 0. The degree of reduction (reducibility), dispersion, average particle size and active surface area of supported nickel were calculated based on the amount of H2 and O2 adsorbed and the loading of nickel. The chemical compositions of catalysts were measured by an ARL-9800 X-ray fluorescence spectrometer (XRF). XRD patterns were collected in ambient atmosphere by an X-ray diffractometer (Shimadzu XRD-6000) with Cu Ka radiation (k = 1.5408 Å) generated at 40 kV and 30 mA. Diffraction intensities were recorded from 10° to 80° at a rate of 7°/min. TEM measurements were carried out using a JEOL electron microscope (JEM-2010) with an accelerating voltage of 200 keV. The surface areas and pore sizes were measured by the Micromeritics Gemini V 2380 autosorption analyzer. Experiments were performed at 77.3 K using N2 as an adsorbate. The samples were degassed in flowing nitrogen at 573 K for 3 h before the measurements. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method, and the pore size distributions were obtained by the Barrett–Joyner–Halenda (BJH) method according to the desorption branches. Temperature-programmed reduction (TPR) measurements were performed by using a quartz U-tube reactor loaded with about 20 mg of a dried sample in 4.84% H2/N2 (v/v) at a flow rate of 60 mL/min. The temperature was raised from 303 to 1173 K with a programmed rate of 10 K/min. Microcalorimetric measurements for the adsorption of H2 and CO were performed using a Tian-Calvet type C-80 microcalorimeter (Setaram, France), connected to a glass vacuum system equipped with a Baratron capacitance manometer (USA) for the pressure measurements and gas handling. Prior to the microcalorimetric measurements, about 0.1–0.2 g of a catalyst were typically reduced in H2 at 723 K for 2 h followed by the evacuation at the same temperature for 1 h. The reduced catalysts were passivated overnight in N2 containing about 1% O2 before they were characterized by XRF, XRD, TEM and BET-BJH methods. The hydrothermally treated catalysts could be directly characterized by these techniques, but must be re-reduced in H2 at 723 K before they were measured by the adsorptions and catalytic reactions.

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2.3. Catalytic tests The hydrogenation reactions were performed in a stainless steel fixed-bed reactor with the inner diameter of 10 mm. The catalysts were pelletized, crushed, and sieved to a fraction of size 40–60 meshes, with which the internal diffusion limitation for the reaction was eliminated [10]. The catalysts were loaded in the middle of the vertical trickle-bed reactor, sandwiched by quartz sands of 60–80 meshes. The catalysts were reduced in flowing H2 at 723 K for 2 h. The aqueous solution of glucose (10 wt.%) was pumped into the reactor with the liquid flow rates from 2 to 72 mL/h and flowed downward with H2 through the packed catalyst bed. The reaction was mainly performed at 4 MPa and 373 K. The products were collected and analyzed by an HPLC (Agilent 1100) with a refractive index detector (RID). An Agilent Zorbax-NH2 column (250 mm  4.6 mm  5 lm) was used with mixed solution of acetonitrile and water (v/v = 4:1) as the mobile phase (1 mL/min). The turnover frequency (TOF) was calculated by dividing the number of molecules converted per second by the number of active nickel atoms determined by H2 adsorption.

The effect of hydrothermal temperature was investigated in order to understand further the hydrothermal stabilities of these catalysts. The results are shown in Fig. 2. It is seen that the uptakes of H2 decreased significantly with the increase of hydrothermal temperatures for all the catalysts studied. However, the catalyst Ni/AlSiO-4 decreased the least. After the hydrothermal treatment at 423 K, the Ni/AlSiO-4-423 still exhibited the H2 uptake of 567 lmol/g, indicating that the catalyst was more hydrothermally stable than the other catalysts studied in this work. In contrast, the uptakes of H2 on the Ni/Al2O3-423 and Ni/SiO2-423 were only 253 and 27 lmol/g, respectively. Figs. 3 and 4 show the N2 adsorption–desorption isotherms (A) and corresponding pore size distributions (B) for the fresh and hydrothermally treated catalysts. These isotherms were typical of IV type with H3 hysteresis loops, characteristic of mesopores. The pore sizes were widely distributed for the fresh and hydrothermally treated catalysts. The surface areas, pore diameters and pore

The number of surface active sites in nickel catalysts can be determined by the adsorption of H2 [26,27]. Fig. 1 shows the uptakes for the adsorption of H2 on the fresh and hydrothermally treated catalysts at 363 K. It is seen that the mass ratio of Al2O3/ SiO2 in the support affected the uptakes of H2 on fresh catalysts. The 60%Ni/AlSiO-3 exhibited the highest uptake of H2 (1007 lmol/g) in the 60%Ni/AlSiO catalysts studied in this work, which was higher than those on the fresh 60%Ni/Al2O3 and 60%Ni/SiO2 (791 and 998 lmol/g, respectively). These values were much higher than that on the 17%Ni/Al2O3 (56 lmol/g) prepared by the wet impregnation method. After the hydrothermal treatment at 363 K for 8 h, the uptakes of H2 for all the catalysts studied were significantly decreased (Fig. 1). The uptake of H2 on the Ni/Al2O3-363 was measured to be 640 lmol/g, which was significantly higher than that on the Ni/SiO2-363 (262 lmol/g), indicating that Ni/Al2O3 was more stable than Ni/SiO2 upon the hydrothermal treatment. The uptakes of H2 on the Ni/AlSiO-3-363 and Ni/AlSiO-4-363 were measured to be 775 and 764 lmol/g, respectively, higher than that on the Ni/ Al2O3-363 (640 lmol/g), indicating that the Ni/AlSiO catalysts were more hydrothermally stable than the Ni/Al2O3.

Ni/Al2O3 Ni/AlSiO-3 Ni/AlSiO-4 Ni/AlSiO-5

800

600

400

200 Fresh

363

393

423

Hydrothermal temperature (K) Fig. 2. Effect of hydrothermal temperature on the H2 uptakes for the Ni/Al2O3 and Ni/AlSiO catalysts.

3

3.1. Effect of hydrothermal treatment on the textural and structural properties

Quantity adsorbed (cm /g STP)

3. Results and discussion

H 2 uptake (μmol g -1)

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Ni/Al2 O3

m(Al2 O3 ) : m(SiO2 ) Fig. 1. Effect of Al2O3/SiO2 mass ratio on the H2 uptakes for the fresh and hydrothermally treated (at 363 K) catalysts.

0

10

20 30 Pore diameter (nm)

40

Fig. 3. N2 adsorption–desorption isotherms (A) and pore size distributions (B) for the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 reduced in H2 at 723 K.

71

400

(A)

Table 2 Physical properties of hydrothermally treated (at 363 K) catalysts Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363 after the reduction in H2 at 723 K.

Ni/Al2 O 3 -363 Ni/AlSiO-4-363 Ni/SiO 2 -363

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S. Li et al. / Journal of Colloid and Interface Science 447 (2015) 68–76

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0.5

Ni/SiO 2 -363

0.4 0.3 0.2 0.1 0

10

20

30

40

Pore diameter (nm) Fig. 4. N2 adsorption–desorption isotherms (A) and pore size distributions (B) for the hydrothermally treated (at 363 K) catalysts Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363 without the further reduction.

Table 1 Physical properties of fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 after reduction in H2 at 723 K. Catalyst

Ni/Al2O3

Ni/AlSiO-4

Ni/SiO2

Loading (wt.%) SBET (m2/g) Pore size (nm) Vpore (cm3/g) H2 uptake (lmol/g) O2 uptake (lmol/g) Reducibility (%) Dispersion (%) DNi (nm) SNi (m2/g)

59.1 282 12.63 1.23 791 2989 68 26.5 3.8 62

61.4 333 9.02 0.97 915 3665 83.4 25 4.0 72

62.1 324 5.67 0.54 998 3867 88 25.8 3.9 78

volumes of the fresh and hydrothermally treated catalysts were given in Tables 1 and 2. By comparing these data, it is seen that the surface areas, pore diameters and pore volumes of the catalysts were all decreased upon the hydrothermal treatment. However, the hydrothermally treated catalysts still exhibited the quite high surface areas and quite large pore sizes and pore volumes, indicating that the changes of textural properties were not the essential factors determining the dispersion of nickel particles on the hydrothermally treated catalysts. Table 1 summarizes the composition and textural properties as well as the reducibility, dispersion, size of nickel particles and active surface area of the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 reduced at 723 K. The loading of Ni measured by XRF was close to the value added (60%). The surface areas were measured to be 282, 333 and 324 m2/g with the average pore sizes of 12.63, 9.02 and 5.67 nm for the Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2, respectively. The pores were larger in Ni/Al2O3 than in Ni/SiO2

Catalyst

Ni/Al2O3-363

Ni/AlSiO-4-363

Ni/SiO2-363

Loading (wt.%) SBET (m2/g) Pore size (nm) Vpore (cm3/g) H2 uptake (lmol/g) O2 uptake (lmol/g) Reducibility (%) Dispersion (%) DNi (nm) SNi (m2/g)

58.7 251 7.67 0.51 640 3417 77.8 18.7 5.4 50

60.3 315 6.3 0.62 764 3907 88.9 19.6 5.2 60

56.6 294 7.2 0.55 262 1486 90.7 6.6 15.4 21

and the addition of SiO2 in Ni/Al2O3 resulted in the smaller pores in Ni/AlSiO than in Ni/Al2O3. According to the uptakes of H2 and O2, the dispersion (percentage of nickel on the surface in the reduced nickel) and reducibility (percentage of nickel reduced) of supported nickel in the catalysts could be derived. The reducibility of nickel was calculated to be about 68%, 83.4% and 88%, respectively, in the Ni/Al2O3, Ni/AlSiO4 and Ni/SiO2 reduced at 723 K, indicating the stronger interaction of nickel with Al2O3 than SiO2. Thus, the addition of SiO2 resulted in the higher reducibility of Ni/AlSiO than Ni/Al2O3. Since the dispersions of Ni in these catalysts were similar (25–26.5%), the catalyst (Ni/SiO2) with the higher reducibility possessed the higher active Ni surface area (78 m2/g), although the active Ni surface areas of Ni/Al2O3 and Ni/AlSiO-4 were also quite high (62 and 72 m2/g, respectively). According to the data of reducibility and dispersion, the average diameters of metallic Ni particles were estimated to be about 3.8, 4.0 and 3.9 nm in the fresh Ni/Al2O3, Ni/ AlSiO-4 and Ni/SiO2, respectively. Table 2 summarizes the physical properties of the Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363 hydrothermally treated at 363 K. It is seen that the textural properties (surface area, pore diameter and pore volume) of the hydrothermally treated catalysts were changed as compared to those of fresh ones, indicating that the chemical reactions occurred on the surfaces of supports during the hydrothermal treatment. However, the surface area of Ni/ AlSiO-4-363 was still high (315 m2/g) and the decrease of surface area of Ni/AlSiO-4 upon the hydrothermal treatment was not much (from 333 to 315 m2/g), indicating the highly hydrothermal stability of the AlSiO-4 support. In addition, the hydrothermal treatment increased the reducibility of the catalysts, while decreased the dispersion of nickel significantly, indicating that the hydrothermal treatment weakened the interaction of nickel with supports and led to the aggregation and growth of supported nickel particles. In particular, the dispersion of nickel in the Ni/SiO2-363 was greatly decreased to 6.6% from 25.85% (in Ni/SiO2) upon the hydrothermal treatment, with the simultaneous decrease of the active Ni surface area from 78 to 21 m2/g and the substantial increase of the average size of Ni particles from 3.9 to 15.4 nm, indicating the weak hydrothermal stability of the Ni/SiO2 catalyst. In contrast, the hydrothermal stability of the Ni/Al2O3 and Ni/AlSiO-4 was high, and the average sizes of metallic Ni particles were increased only to 5.4 and 5.2 nm, respectively. In these two catalysts, the Ni/AlSiO-4-363 exhibited the higher reducibility and dispersion than the Ni/ Al2O3-363, and thus the Ni/AlSiO-4-363 possessed the higher active Ni surface area (60 m2/g) than the Ni/Al2O3-363 (50 m2/g), indicating the higher hydrothermal stability of Ni/AlSiO-4 than Ni/Al2O3. Fig. 5 shows the XRD patterns for the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 reduced in H2 at 723 K and the corresponding hydrothermally treated catalysts Ni/Al2O3-363, Ni/

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(A)

 



Intensity (a.u.)



Ni  Al 2 O 3 SiO 2



 













Ni/AlSiO-4 





10

Ni/Al 2 O 3



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50



60

Ni/SiO2 

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Ni/Al 2 O3 -363

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30









10

Ni  Al(OH) 3 SiO 2  Al 2 O3

The hydration of Al2O3 was a complicated process and various hydration products might be formed [29]. Although XRD results did not show any phase changes of SiO2 in the Ni/SiO2 upon the hydrothermal treatment, the hydration of SiO2 surface for the formation of surface hydroxyl groups (Si–OH) was highly possible [30], which might weaken the interaction of nickel particles with the surface of SiO2, leading to the greatly increased sizes of Ni particles in the Ni/SiO2-363. It is apparent that the Ni/SiO2 was not hydrothermally stable. On the other hand, the Ni/AlSiO-4 showed the high stability upon the hydrothermal treatment. The presence of SiO2 inhibited the hydration of Al2O3 while the presence of Al2O3 stabilized the dispersion of supported Ni particles. Thus, the Ni/ AlSiO-4-363 remained the phase of support as well as the high dispersion of supported Ni particles. The occupation of unsaturated voids in Al2O3 by SiO2 might explain the reason why the support AlSiO was highly stable upon the hydrothermal treatment. Fig. 6 shows the TEM images of fresh catalysts Ni/Al2O3, Ni/ AlSiO-4 and Ni/SiO2 reduced at 723 K. It is seen that the Ni particles

Ni/AlSiO-4-363 



Ni/SiO 2 -363

40 50 60 2 θ (degree)



70

80

Fig. 5. XRD patterns for the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 reduced in H2 at 723 K (A) and the hydrothermally treated catalysts Ni/Al2O3-363, Ni/AlSiO4-363 and Ni/SiO2-363 without the further reduction (B).

AlSiO-4-363 and Ni/SiO2-363 without the further reduction. The diffraction peaks around 44.5°, 51.9° and 76.4° in Fig. 5A could be assigned to (1 1 1), (2 0 0) and (2 2 2) lattice planes of FCC Ni [28]. The diffraction peaks of metallic Ni in the catalysts were broad, indicating the small nickel particles in these catalysts. The average sizes of nickel particles were estimated to be 3.6, 3.9 and 3.8 nm, respectively, in the Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 according to the Scherrer equation, agreeing well with the results obtained by the adsorption of H2 and O2. Fig. 5B shows the XRD patterns for the Ni/Al2O3-363, Ni/AlSiO4-363 and Ni/SiO2-363 hydrothermally treated at 363 K. The sharp diffraction peaks around 18.9°, 20.4°, 28.0°, 40.6° and 53.3° for Al(OH)3 were observed in the Ni/Al2O3-363 (JCPDS 20-0011). The diffraction peaks for metallic Ni were still present, and the intensities of these diffraction peaks were not changed significantly. According to the diffraction peaks of Ni(1 1 1) and Scherrer equation, the average size of nickel particles was estimated to be about 5.5 nm in the Ni/Al2O3-363, ageing well with that (5.4 nm) determined by the adsorption of H2 and O2. The diffraction peaks of metallic Ni in the Ni/SiO2-363 after the hydrothermal treatment at 363 K were sharp, and the average size of nickel particles was estimated to be 31.5 nm, which was significantly larger than that (15.4 nm) determined by the adsorption of H2 and O2. This might be due to the inhomogeneous distribution of particle sizes in the Ni/SiO2-363. While the intensities of XRD peaks might be mainly contributed from the large nickel particles, the H2 uptake might be mainly contributed from the small nickel particles in the sample. The XRD pattern of the Ni/AlSiO-4-363 was similar to that of Ni/ AlSiO-4. No Al(OH)3 was observed in Ni/AlSiO-4-363. The average size of Ni particle sizes in the Ni/AlSiO-4-363 was calculated to be 4.8 nm by the Scherrer equation, agreeing well with that (5.2 nm) determined by the adsorption of H2 and O2. The XRD results above indicated the hydration of Al2O3 support for the formation of Al(OH)3 during the hydrothermal treatment.

Fig. 6. TEM images of the fresh catalysts Ni/Al2O3 (A), Ni/AlSiO-4 (B) and Ni/SiO2 (C) reduced in H2 at 723 K and then passivated.

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Consumption of H2 (a.u.)

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(A)

670 710 Ni/Al2O3

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700 Ni/SiO2

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Temperature (K)

Consumption of H2 (a.u.)

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570

Ni/Al2O3-363 510 Ni/AlSiO-4-363 820 560 520 Ni/SiO2-363

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Temperature (K) Fig. 8. TPR profiles of the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 (A) and the hydrothermally treated catalysts Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363 (B).

Fig. 7. TEM images of the catalysts Ni/Al2O3-363 (A), Ni/AlSiO-4-363 (B) and Ni/ SiO2-363 (C and D) hydrothermally treated at 363 K (without the further reduction).

in these catalysts were uniformly distributed with average sizes of about 4 nm. The TEM images of hydrothermally treated catalysts Ni/Al2O3363, Ni/AlSiO-4-363 and Ni/SiO2-363 are shown in Fig. 7. It is seen that the Ni particles in the Ni/Al2O3-363 and Ni/AlSiO-4-363 were uniformly distributed with average sizes of about 6 and 5 nm, respectively. Thus, the growth of Ni particles in these two catalysts was not significant as compared to those in the fresh ones. In contrast, Fig. 7C and D show that there were large Ni particles in the Ni/SiO2-363 and some of them might be as large as 500 nm. At the same time, small Ni particles were also observed in the Ni/ SiO2-363. The highly heterogeneous distribution of sizes of Ni particles in the Ni/SiO2-363 might account for the inconsistent measurements of particle sizes by XRD and H2 adsorption. Fig. 8A shows the TPR profiles of fresh catalysts dried at 393 K. Two reduction peaks were observed for the Ni/Al2O3 and Ni/SiO2, corresponding to the reduction of Ni2+ interacting differently with the supports. The temperatures of the two reduction peaks were 670 and 710 K for the Ni/Al2O3, while those for the Ni/SiO2 were 640 and 700 K, indicating that the interaction of Ni2+ with SiO2 was weaker than that of Ni2+ with Al2O3, in agreement with the results of H2–O2 titrations. On the other hand, the Ni/AlSiO-4 exhibited only one reduction peak around 700 K, indicating that the addition of SiO2 weakened the interaction of Ni2+ with Al2O3. In addition, these TPR peaks were broad and extended to high temperatures, indicating the existence of some Ni2+ cations that were difficult to reduce. These Ni2+ cations must be highly dispersed in the lattices of Al2O3 and SiO2 and the species like nickel aluminate and silicate might be formed.

S. Li et al. / Journal of Colloid and Interface Science 447 (2015) 68–76

100

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Differential heat (KJ/mol)

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20 0

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H2 coverage ( μmol/g)

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(B) 100

(B)

Ni/Al2O3-423 Ni/AlSiO-4-423 Ni/SiO2-423

100 80 60 40 20

0 0

200

400

600

800

1000

CO coverage (μ mol/g) Fig. 9. Differential heat versus coverage for the adsorption of H2 at 308 K (A) and of CO at 300 K (B) on the fresh catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 reduced in H2 at 723 K.

Fig. 8B shows the TPR profiles of hydrothermally treated catalysts Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363. These catalysts were pre-reduced in H2 at 723 K, hydrothermally treated at 363 K and then dried at 393 K. As compared to the fresh ones, these catalysts exhibited significantly lower temperatures of the main reduction peaks around 570, 510 and 560 K, respectively, for the Ni/Al2O3-363, Ni/AlSiO-4-363 and Ni/SiO2-363. These peaks were from the reduction of oxidized layers of metallic Ni particles formed during the pre-reduction. These oxidized species were not strongly interacted with the supports and thus were easily reduced.

3.2. Effect of hydrothermal treatment on the surface properties The adsorption of H2 and CO was sensitive to the changes of electron densities on metal surfaces [31,32]. Fig. 9 shows the results of microcalorimetric adsorption of H2 and CO on the fresh catalysts reduced at 723 K. The initial heats for the adsorption of H2 were measured to be 87, 86 and 84 kJ/mol, while those for the adsorption of CO were found to be 110, 106 and 104 kJ/mol, respectively, on the Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2. These results suggested that the support affected the bond strengths of H and CO adsorbed on Ni. The bonds Ni–H and Ni–CO seemed stronger on Ni supported on Al2O3 than on SiO2. Fig. 10 shows the results of microcalorimetric adsorption of H2 and CO on the hydrothermally treated catalysts Ni/Al2O3-423, Ni/ AlSiO-4-423 and Ni/SiO2-423. The initial heats for the adsorption of H2 were measured to be 81, 84 and 42 kJ/mol on the Ni/Al2O3423, Ni/AlSiO-4-423 and Ni/SiO2-423, respectively, which were lower than the values on the fresh catalysts, indicating the changes of chemical properties of Ni surfaces upon the hydrothermal treatment. However, the change of initial heats for the adsorption of H2 on the Ni/AlSiO-4 and Ni/AlSiO-4-423 was not significant (from 86 to 84 kJ/mol), probably owing to that the change of support was

0 0

100

200

300

400

CO coverage (μmol/g) Fig. 10. Differential heat versus coverage for the adsorption of H2 at 308 K (A) and of CO at 300 K (B) on the hydrothermally treated catalysts Ni/Al2O3-423, Ni/AlSiO4-423 and Ni/SiO2-423 re-reduced in H2 at 723 K.

not observed in this catalyst upon the hydrothermal treatment. On the other hand, the initial heats for the adsorption of H2 on the Ni/SiO2 and Ni/SiO2-423 changed a lot (from 84 to 42 kJ/mol), probably owing to the change of support and/or to the greatly increased sizes of Ni particles upon the hydrothermal treatment. The weaker adsorption of H2 on Ni might lead to the lower hydrogenation activity of the catalyst Ni/SiO2-423. The initial heats for the adsorption of CO on the Ni/Al2O3-423, Ni/AlSiO-4-423 and Ni/SiO2-423 were measured to be 102, 106 and 97 kJ/mol, respectively. Interestingly, the initial heat was the same for the adsorption of CO on the Ni/AlSiO-4 and Ni/AlSiO-4423, indicating that the hydrothermal stability of Ni/AlSiO-4 was high and the hydrothermal treatment did not bring about the change of surface electron density of supported Ni. On the other hand, the initial heats for the adsorption of CO on the Ni/Al2O3423 and Ni/SiO2-423 were significantly decreased as compared to those on the corresponding fresh ones, indicating the changes of surface properties of supported Ni due to the changes of supports Al2O3 and SiO2 upon the hydrothermal treatment. 3.3. Hydrogenation of glucose to sorbitol in aqueous solution Fig. 11 shows the results of hydrogenation of 10% glucose in aqueous solution over the catalyst Ni/AlSiO-4. No by-products were detected and the selectivity was 100% at different reaction temperatures. The conversion of glucose increased with reaction temperature until 373 K, beyond which the conversion of glucose remained 100%. The catalysts Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2 as well as the 17%Ni/Al2O3 prepared by the impregnation method were compared for the hydrogenation of 10% glucose in aqueous solution at 373 K and 4 MPa. The results are shown in Fig. 12. At the WHSV of 1 h 1, the conversion of glucose reached 100% over the Ni/Al2O3, Ni/AlSiO-4 and Ni/SiO2, while it was only 74% over the 17%Ni/

75

Conversion and selectivity (%)

S. Li et al. / Journal of Colloid and Interface Science 447 (2015) 68–76

100

100

(A)

Ni/Al2O3-423 Ni/AlSiO-4-423 Ni/SiO2-423

Conversion (%)

80

80 Conversion Selectivity

60 40 20

60 333

373

353

393

0

413

1

Temperature (K)

4

8

12

16

20

WHSV (h-1)

Fig. 11. Conversion and selectivity of glucose at different temperatures on the Ni/ AlSiO-4 reduced in H2 at 723 K. Other reaction conditions: WHSV = 1 h 1 and P = 4 MPa.

0.012

(B)

100

(A)

Conversion (%)

80

20

Ni/Al 2 O3

0.002

Ni/AlSiO-4-423 Ni/SiO2 -423

Ni/AlSiO-4 Ni/SiO 2

0.000

0 4

8

12

16

20

24

0.006 Ni/Al2O3 -423

17% Ni/Al 2 O3

1

0.008

0.004

60 40

TOF (s-1)

0.010

28

32

36

WHSV (h -1)

1

4

12 WHSV (h 1) 8

16

20

Fig. 13. Conversion (A) and TOF (B) of glucose at different WHSV on the catalysts Ni/Al2O3-423 Ni/AlSiO-4-423 and Ni/SiO2-423 reduced in H2 at 723 K. Other reaction conditions: T = 373 K and P = 4 MPa.

0.020

(B)

TOF (s-1)

0.016 0.012 0.008

Ni/Al2O3 Ni/AlSiO-4 Ni/SiO2

0.004 0.000 1

4

8

12

16

20

24

28

32

36

WHSV (h-1) Fig. 12. Conversion (A) and TOF (B) of glucose at different WHSV on the catalysts Ni/Al2O3, Ni/AlSiO-4, Ni/SiO2 and 17%Ni/Al2O3 reduced in H2 at 723 K. Other reaction conditions: T = 373 K and P = 4 MPa.

Al2O3. The conversion of glucose decreased fast with the increase of WHSV over the Ni/SiO2 and 17%Ni/Al2O3, while it was decreased much slower with WHSV over the Ni/Al2O3 and Ni/AlSiO-4, indicating that the Ni/Al2O3 and Ni/AlSiO-4 were significantly more active than the Ni/SiO2 and 17%Ni/Al2O3. The low activity of Ni/ SiO2 at the high WHSV might be due to its low hydrothermal stability that caused the rapidly increased sizes of supported Ni particles during the hydrogenation of glucose in aqueous solution. The Ni/Al2O3 and 17%Ni/Al2O3 had the same support with different loadings of Ni. The Ni/Al2O3 had the much higher loading of Ni and much higher active Ni surface area than the 17%Ni/Al2O3, and thus the much higher activity than 17%Ni/Al2O3 for the hydrogenation of glucose in aqueous solution. According to the uptakes of H2, the turnover frequencies (TOF) for the conversion of glucose could be calculated. Fig. 12B shows the results. It is seen that the TOF values for the conversion of glucose over the Ni/Al2O3 and Ni/AlSiO-4 increased with WHSV and

reached constants of about 0.0190 and 0.0137 s 1, respectively. Thus, the surface Ni atoms in Ni/Al2O3 seemed more active than those in Ni/AlSiO-4, probably owing to that the initial heats were higher on the Ni/Al2O3 than on the Ni/AiSiO-4 for the adsorption of H2 and CO. The TOF for the conversion of glucose over the Ni/ SiO2 increased first and then decreased with WHSV, with the maximum of 0.0060 s 1. This phenomenon must be caused by the fast deactivation of the Ni/SiO2 catalyst due to the fast aggregation of supported Ni particles during the hydrogenation of glucose in aqueous solution. Thus, the TOF activities of Ni for the hydrogenation of glucose in aqueous solution followed the order Ni/Al2O3 > Ni/AlSiO-4 > Ni/SiO2, agreeing well with the order of Ni–H and Ni– CO bond strengths for these catalysts. The hydrothermally treated catalysts Ni/Al2O3-423, Ni/AlSiO-4423 and Ni/SiO2-423 were compared for the hydrogenation of 10% glucose in aqueous solution at 373 K and 4 MPa. Fig. 13 shows the results. At the WHSV of 1 h 1, the conversion of glucose was 97% over the Ni/AlSiO-4-423, higher than 87% over the Ni/Al2O3-423. The conversion of glucose over the Ni/SiO2-423 was quite low (only 26%) at the WHSV of 1 h 1. The conversion of glucose decreased with the increase of WHSV over the three catalysts. At the WHSV of 20 h 1, the conversion of glucose was 43% over the Ni/AlSiO-4423, significantly higher than 17% over the Ni/Al2O3-423. Thus, the Ni/AlSiO-4-423 was significantly more active than the Ni/ Al2O3-423 for the hydrogenation of glucose in aqueous solution. Fig. 13B shows that the TOF values for the conversion of glucose increased with WHSV over the three catalysts. At the low WHSV, the TOF was lower over the Ni/AlSiO-4-423 than those over the Ni/Al2O3-423 and Ni/SiO2-423. This was because that the conversions of glucose were relatively high at the low WHSV over the three catalysts and that the Ni/AlSiO-4-423 had significantly more active sites than the Ni/Al2O3-423 and Ni/SiO2-423. Thus, the TOF values were only meaningful when measured at the high WHSV.

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At the high WHSV, the TOF values reached constants over the Ni/ AlSiO-4-423 (0.0117 s 1), Ni/Al2O3-423 (0.0109 s 1) and Ni/SiO2423 (0.006 s 1). Thus, the activities of hydrogenation of glucose in aqueous solution over these catalysts were decreased as compared to those of fresh ones. However, the activity was still high over the Ni/AlSiO-4-423, indicating the highly hydrothermal stability of this catalyst. In fact, the heats for the adsorption of H2 and CO on the hydrothermally treated Ni/AlSiO-4-423 remained high. Thus, the high activity of Ni/AlSiO-4-423 could be attributed to the presence of more and highly active Ni sites during the hydrogenation of glucose in aqueous solution.

(5) The activity of Ni/SiO2 was low while those of Ni/Al2O3 and Ni/AlSiO-4 were high for the hydrogenation of glucose in aqueous solution. In particular, the Ni/AlSiO-4-423 hydrothermally treated at 423 K maintained the high density of active sites and the high intrinsic activity of these active sites for the hydrogenation of glucose in aqueous solution.

Acknowledgments

4. Conclusions

Financial supports from NSFC (21273105), MSTC (2013AA031703), NSFJC (BK20140596) and the fundamental research funds for central universities are acknowledged.

The following conclusions may be drawn from the above results obtained in this study:

References

(1) The catalysts Ni/Al2O3, Ni/AlSiO and Ni/SiO2 with the high loading of Ni (60%) were prepared by the co-precipitation method, and the uptakes of H2 were measured to be 791, 1007 and 998 lmol/g, respectively, on the catalysts reduced in H2 at 723 K. As compared to Ni/Al2O3, the addition of SiO2 into Al2O3 enhanced the H2 uptake of Ni/AlSiO. (2) The catalyst Ni/AlSiO-4 with the Al2O3/SiO2 mass ratio of 4 was found to exhibit the best hydrothermal stability. After the hydrothermal treatment at 423 K for 8 h, the resulted catalyst Ni/AlSiO-4-423 still possessed the high H2 uptake (567 lmol/g), while the hydrothermally treated Ni/Al2O3423 and Ni/SiO2-423 had the H2 uptakes of only 253 and 27 lmol/g, respectively. (3) The hydrothermal treatments decreased the surface areas, pore volumes and pore sizes of above catalysts. However, the surface areas, pore volumes and pore sizes of hydrothermally treated catalysts remained high, indicating that these textural properties were not the main factors affecting the activities of hydrothermally treated catalysts. (4) The results of XRD and TEM showed that the Ni particles in the hydrothermally treated Ni/Al2O3 remained small (about 5.4 nm), while crystalline Al(OH)3 was formed from the hydration of Al2O3, leading to the change of surface properties of supported Ni with the decreased heats for the adsorption of H2 and CO. On the other hand, the Ni particles in the hydrothermally treated Ni/SiO2 were greatly grown up to 15–30 nm, with the significantly decreased heats for the adsorption of H2 and CO. In contrast, no Al(OH)3 was formed and the Ni particles remained small (about 5.2 nm) in the hydrothermally treated Ni/AlSiO-4. The heats for the adsorption of H2 and CO were also relatively high for the hydrothermally treated Ni/AlSiO-4, which might be an important factor for it to maintain the high activity for the hydrogenation reaction.

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