γ-Al2O3 Catalysts Synthesized via Sonochemical Method

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 148 (2016) 64 – 71

4th International Conference on Process Engineering and Advanced Materials

Physicochemical Properties of Ni-Mo/γ-Al2O3 Catalysts Synthesized via Sonochemical Method Mariam Ameena,b, Mohammad Tazli Azizana,*, Anita Ramlib, Suzana Yusupc, Madiha Yasirb a Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia c Biomass Processing Lab, Centre for Biofuel and Biochemical Research (CBBR), Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar, Perak, Malaysia b

Abstract The physicochemical properties were studied to observe the influence of ultrasound irradiation on the synthesis of bimetallic solid acid catalyst (NiMo/γ-Al2O3). A set of catalyst was synthesized using conventional method and sonochemical method. The physicochemical characterizations were studied. The characterization techniques such as XRD, FESEM, EDX, TEM, BET surface area and temperature program reduction (H2-TPR) are highlighted in this manuscript. The XRD analysis revealed that with application of ultrasound irradiation, Ni-Mo particles are homogeneously distributed with the support in different crystal structures. The FESEM and TEM analysis confirmed that the nano catalyst synthesized via sonochemical method presents with average size of 15 nm i.e. smaller than the catalyst prepared via impregnation method. The BET surface area, pore size and pore volume also raised up to a certain level. H2-TPR profile showed lower reduction temperature 428 °C for sonochemically synthesized catalyst which assumed to be more active than conventionally synthesized catalyst.

© Published by byElsevier ElsevierLtd. Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016 The The Authors. Authors. Published (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016. Peer-review under responsibility of the organizing committee of ICPEAM 2016 Keywords: Nano-sized particles; sonochemical synthesis; bimetallic solid acid catalysts; physiochemical characterization

1. Introduction Bimetallic solid acid catalysts play an important role in petroleum refineries in hydrocracking and hydrogenation process [1]. The general methods for the synthesis of bimetallic solid acid catalysts are wet impregnation method, co-precipitation method and hydrothermal treatments. These solid acid catalysts on nano-scale are more active and efficient for petroleum refineries [2]. Nowadays, nanotechnology has observed the various methods for synthesis of nanoparticles. Besides the well-known techniques in preparation of nano materials synthesis, such as microwave synthesis and photochemical synthesis, the sonochemistry can also be considered as part of nanomaterial synthesis in the field of catalysis [3, 4]. Nanocatalysis research can be defined as the synthesis of the heterogeneous catalysts on nanoscale [5]. The nanocatalysis involved the synthesis of large surface to volume ratio of catalysts as compared to the * Corresponding author. Tel.: +6-005-368-7611. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016

doi:10.1016/j.proeng.2016.06.496

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other methods [3]. The major outcomes for synthesis of catalysts via sonochemical method are high catalytic performance due to an increase in specific surface area, homogeneously metal dispersion, smaller particle size and large pore volume and pore size [6]. All these properties make the catalysts more outstanding for catalytic processes. It is evident from previous studies that the activity of sonochemically synthesized (Us) catalysts can be retained for a longer time compared to the conventional methods [7]. It is well defined that the application of ultrasonic irradiation would be an effective method of Ni-Mo catalyst synthesis. Ramos et al [8] investigated the effects of ultrasound irradiation on Ni, Co, Ni-Mo and Co-Mo catalysts for removal of sulfur contents from sulfur-based compounds in hydrodesulphurization (HDS) process [8]. It was investigated that the Ni, Co, Ni-Mo and Co-Mo were 5 to 6.4 times more active than CoS and MoS in HDS of thiophene [8]. Similarly, it is expected that the synthesis of bimetallic solid acid catalysts via sonochemical method would play an important role in petroleum refineries as well as bio-refineries for hydrogenation and hydrocracking process. The aim of this research is to investigate the effect of ultrasound irradiation on morphology, crystal structural, surface area, pore volume, size, particle size as well as physicochemical properties of bimetallic solid acid catalysts. 2. Material and Method The materials such as Ni (NO3).6H2O was used as Ni precursor, (NH4)6 Mo7O24.4H2O as Mo promoter and γ-Al2O3 as support for wet impregnation (WI) and ultrasound assisted synthesized (Us) solid acid nano-catalysts, all the chemicals were purchased from Merck Company, Malaysia and of analytical grade. 3. Experimental 3.1. Catalysts synthesis via wet impregnation and sonochemical methods The Fig. 1. illustrates the preparation of bimetallic solid catalysts by conventional wet impregnation method and sonochemical method. The catalysts are denoted as xNi-Mo/γ-Al2O3 (WI), where “x” is metal loading for 12wt% Ni and 3wt. % Mo, WI represents the wet impregnation method and (Us) represent the ultrasound irradiation method. The corresponding catalysts were prepared using Ni(NO3)3.6H2O (using fixed amounts of 12 wt.% of Ni) as Ni precursor and (NH4)6 Mo7O24.4H2O as Mo promoter. The wt. % of promoter is high, to study the effects of high metal loading of promoter in WI and Us methods. The aqueous solutions of (NH4)6 Mo7O24.4H2O and Ni(NO3)3.6H2O were prepared in de-ionized water separately for each formulation and these solutions were doped accordingly with continuous stirring of 350 rpm at room temperature for WI and for Us 30 s irradiation. The fixed amount of γ-Al2O3 as support was doped on aqueous solutions of metal oxides with continuous stirring for 4 h at room temperature for WI method and for 45 min at 60 ºC and 90 W with 30 s plus on and 5 s off using Q700 Sonica Sonicator for Us method. The obtained solutions were dried at the 120 oC for 12 h in dry oven and calcined at 500 oC for 16 h. All the prepared catalysts were subjected to characterization techniques.

Fig.1. Flow chart for synthesis of solid acid catalysts via wet impregnation method and sonochemical method

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3.2. Characterization Techniques To diagnose the surface metal oxides the X-ray Diffraction (XRD) patterns were recorded on the XRD analyzer at (Bruker AXS D8 Advance) using a CuKa radiation source (k ¼ 0.150595 nm) in the range 10 < 2q < 90. The scans were taken over the range of 2θ from 10 to 90°. The field emission electron microscopy (FESEM) techniques were performed to evaluate the morphology and particle size. EDX-Dot mapping analysis was supported by SUPRA 55VP/InLens, VPSE and As B for surface compositional analysis. Surface area, pore volume and pore size of both samples were determined by N 2 gas desorption and adsorption isotherm by ASAP 2020V3.04 H analyzer according to BET method. Transmission Electron Microscopy (TEM) was carried out in LIBRA 200FE to study the dispersion of metal particle on support. Temperature-programmed reduction (H2-TPR) experiment was carried out in TPDRO1100-Nr.20150313 instrument. The samples (20 mg) were heated from 20 ◦C to 900 ◦C at the rate of 5 ○C/min in H2 (pre-mixed gases, 5.06% Hydrogen in Nitrogen purchased from Linde sdn bhd Malaysia). 4. Results and Discussion Fig 2. shows the XRD patterns of (a) 12wt.%Ni-3wt.%Mo/γ-Al2O3 (12N3MUs) catalyst synthesized by using ultrasound assisted irradiation method and (b) 12wt.%Ni-3wt.%Mo/γ-Al2O3 (12N3MAWI) catalyst synthesized by using the wet impregnation method. The detailed examinations of x-ray diffraction patterns reveal that the diffraction peaks at 2θ = 67.1, 42.8, 45.7 and 37.6 is an indicative of cubic structure of γ-Al2O3 (JCPDS-00-01-1308). The peaks at 2θ= 37.3, 43.3, 63, 75.6 and 79.8 indicate the formation of NiO cubic nano-structure (JCPDS-00-73-1519) [1, 2] in both samples of WI and Us. The effect of Mo loading can be observed from comparison of XRD patterns. The corresponding peaks for Al(Mo2O4)3 appeared at 2θ= 31.0, 32.0, 35.8, 23.6, 15.9 in orthorhombic nano-structure (JCPDS-23-0764) for catalyst synthesized via wet impregnation method, while Mo2O3 peaks in ultrasounds assisted method observed with NiO that characterized the Ni and Mo monoclinic crystals are homogenously distributed on the surface of support. The FESEM and dot-mapping images in Fig. 3. also confirmed this homogenously distribution of particles. The corresponding peak for NiMoO4 observed at 2θ= 29.0, 33.0, 39.0, 22.0, 14.9, 41.0, 44.0, 45.7, 51.0, 53.0 monoclinic nano-structure (JCPDS-12-0348) [3]. It is observed that the aluminium molybdenum oxide (Al2(MoO4)3 peak intensity reduced with NiO by applying ultrasound irradiation and crystallinity of γ-Al2O3 decreased with the increase in metal loading for wet impregnation method as reported earlier [1, 4]. The ultrasound effects on Ni-Mo oxides crystallinity slightly changed the crystal structures of NiO tetragonal (JCPDS-20-0776) to monoclinic (JCPDS-37-1292) which confirmed from XRD spectra. The common x-ray diffraction peaks for NiO and Mo2O3 were found in both samples of the Us and WI almost in the same range. The more prominent changes in peaks were observed in their intensities reduced. The X-ray diffraction peaks differ in intensity of Mo2O3, NiO, Ni-Mo oxide and γ-Al2O3 that is clear from their compared XRD spectra. It can be confirmed that with the introduction of Us on high metal loading can affect the crystal structure of bimetallic solid acid catalysts like Ni-Mo/γ-Al2O3 [5].

(b)12N3MAWI (a)12N3MAUS x NiAl2O3 MoAl2O3 i Ni-MoAl2O3

Intensity (a.u)

h Al2O3

(b)

x

x xh

x

h

h h x hx

x

h

i i (a)

10

i

15

20

25

30

x

x

35

ii i 40

45

i 50

55

i

ii 60

i 65

70

75

i 80

85

Angle (2T

Fig. 2. XRD patterns of nanostructured Ni-Mo/γ-Al2O3 catalysts with 12 wt wt. % loading of Ni and 3 wt. wt % of Mo over γ-Al2O3, (a) (a). assisted method (12N3MAWI) and (b) Synthesized by the wet impregnation method (12N3MAUs).

Synthesized by the ultrasound

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FESEM and EDX Analysis. Fig. 3. shows the FESEM micrographs, EDX spectrum data and mapping of Ni-Mo/γ-Al2O3 nanocatalysts prepared using the wet impregnation method (12N3MAWI) and ultrasound assisted irradiation (12N3MAUs). The spectroscopic studies revealed the effect of ultrasound irradiation on morphology of nanocatalysts with different wt. % Ni and Mo loading over γ-Al2O3. The slight view over the Fig. 3 (a) for γ-Al2O3 possess a rough surface structure while the samples prepared by wet impregnation and ultrasound assisted show significant difference in morphology and surface structure of catalysts. The particle agglomeration is major reason for the deactivation of catalyst that can be removed by using proper synthesis method. In comparison of images Fig. 3 (b) with Fig. 3 (c), it is clear that for Ni-Mo the number of agglomeration decreased using ultrasound as compared to conventional method. The EDX and dot mapping analysis for all set of Ni-Mo/γ-Al2O3 (WI) and (Us) on same magnification shows that all the elements in nanocatalysts (Ni, Mo Al, and O) exist uniformly. The EDX spectra of Us synthesized catalysts shows that the Ni and Mo are highly intensified and evenly dispersed on the surface of support as shown in Fig. 3 (c-ii) and (c-iii) compared to the WI as shown in dot-mapping images and EDX spectra Fig. 3 (b-ii) and (b-iii). TEM analysis also supports the justification on the structural properties of nanocatalysts observed from FESEM. This observation reflects that the sonochemically-synthesized catalyst has showed better nano structural properties and it is expected to possess better catalytic activity for various applications. (a-i) FESEM image γ-Al2O3

(a-ii) mapping γ-Al2O3

(a-iii) EDX-Spectra γ-Al2O3

(b-i) FESEM image 12N3MAWI

(b-ii) mapping 12N3MAWI

(b-iii) EDX-Spectra 12N3MAWI

(c-i) FESEM image -12N3MAUs

(c-ii) mapping 12N3MAUs

(c-iii) EDX-Spectra 12N3MAUs

Fig. 3. FESEM images of Ni-Mo/γ-Al2O3 prepared by using the wet impregnation method (NMAWI) and ultrasound assisted (NMAUs). FESEM images of (ai-iii) represents the image for γ-Al2O3 at 100.00 kx with mapping and EDX spectra; [(b-i-iii) at 5.0 kx magnification] represents 12wt.%Ni-3wt.%Mo/γ-Al2O3 (WI) with mapping and EDX spectra; [(c-i-iii) at 5.0 kx magnification] represents 12wt.%Ni-3wt.% Mo/γ-Al2O3 (Us) with mapping and EDX spectra.

Specific Surface Area (SBET): For (SSA) the Brunauer-Emmett-Teller (BET) equation was used to determine the specific surface area of both samples synthesized via both sonochemical (Us) and wet impregnation (WI) methods as shown in Table 1. The surface area of support (γ-Alumina), the catalysts that are prepared by wet impregnation method (WI) which is 12%Ni-3%Mo/γ-Al2O3 and that

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catalysts prepared by ultrasound assisted method 12%Ni-3%Mo/γ-Al2O3 (Us) are reported as 134 m2/g, 91.58 m2/g and 91.04 m2/g respectively. The general trend was observed where, there is decline in a specific surface area with increase in Ni loading on support with loading of Mo for (WI) and (US) methods. The nitrogen adsorption at standard temperature and pressure was measured as shown in Fig. 4. The graphs show the mesoporous structure of both catalysts. It is reported that the metal loading leads to blockage of pores due to low metal dispersion over support and results in specific surface area of catalyst to be reduced. The similar behavior was observed in both methods as, the surface area of bimetallic Ni-Mo/γ-Al2O3 was found to be almost similar. The increase in Mo loading reduced the surface area. Overall, no further change was observed in surface area of bimetallic catalyst formulations. The general trend is observed i.e. by increasing the metal loading, the pore size and the pore volume decreased in the conventional method. However, in the ultrasound irradiation method the pore size and the pore volume slightly increased as shown in Table 1. This slight change may be due to change in crystal structure with effect of ultrasound irradiations. As ultrasound irradiation produced high frequency of waves that create cavitation inside the molecules ultimately effects the materials structures. Using the influential Us radiations, molecules experience speedy reaction at these frequencies, 20 kHz-10 MHz. These Us radiations produce nearly some sort of physicochemical variations on the surface of materials by means of high cavitation [3]. The increment in pore size and pore volume also effects on the activity of the catalysts to generate the active sites on the surface of support with metal oxides. (b) 12N3MAWI

100

100

90

90 Quantity Adsorbed (cm³/g STP)

Quantity Adsorbed (cm³/g STP)

(a) 12N3MAUs

80 70 60 50 40 30 20

80 70 60 50 40 30 20 10

10

0

0 0

0.5

1

1.5

0

0.5

1

1.5

Relative Pressure (P/Po)

Relative Pressure (P/Po)

Fig. 4. The Nitrogen adsorption (cm3/g) for specific surface area of (a) 12N3MAUs; (b) 12N3MAWI at standard temperature pressure. Table 1: The Surface are BET, pore size and pore volume of bimetallic solid acid catalysts synthesized via WI and Us method. Samples

codes

Specific Surface area (SBET)

Pore size (nm)

(m²/g)

Pore volume (cm³/g)

γ-Al2O3

A

134

6.90

0.235

12%Ni-3%Mo/γ-Al2O3 (WI)

12N3MAWI

91.589

5.71

0.130

12%Ni-3%Mo/γ-Al2O3 (Us)

12N3MAUs

91.043

5.99

0.136

Transmission Electron Microscopy (TEM): studies were carried out for both sets of catalysts (WI) and (Us). Fig. 5. covers the major particle distribution area for (a) 12N3MAWI, (b) for 12N3MAUs, while (c) and (d) represent statistically count and standard deviation for a measure of particle size for 12N3MAWI and for 12N3MAUs respectively. The statistically counted particle presents the maximum and minimum particle size for all catalysts prepared by (WI) and (Us) methods. Maximum particle size 33 nm and the minimum is 1 nm for the catalysts prepared from ultrasound irradiation method (Us). Similarly, the average particle size for US is 15.88 nm and considerable numbers particle size are 99.99% in the range of 1-33 nm. While the TEM analysis of catalysts synthesized by WI shows that maximum particle size 37 nm and the minimum is 2.04 nm. Similarly, the average particle size for WI is 14.27 nm and considerable numbers of particle size are 99.99% in the range of 1-80 nm. The more dispersed phase of nanoparticles are present in catalysts prepared by ultrasound assisted irradiation method. Nanocatalysts study is more favorable in green technology due to smaller particle size and

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reactive materials for the production of various sustainable and renewable fuels. For example biodiesel, bioethanol, biogas, and green diesel [6].The synthesis of nanocatalysts is also cost effective, the total cost of metal loading and energy consumption can be reduced by the application of the ultrasound assisted method in bimetallic active metal catalysts. Hence, the sonochemical method is more effective route towards the synthesis of active metal catalysts with reduced metal particle size and particle scattering to increase the properties of nanocatalysts with reduced amount of metal loading. (a) 12N3MAWI

(c) 12N3MAWI 12 Count Mean Min Max

Frequency

10 8 6

100 14.27 nm 2.04 nm 37 nm

SD

9.27

4 2 0 Particle size (nm)

Count Mean Min Max

(d) 12N3MAUs (b) 12N3MAUs

100 nm

92 15.88 nm 1.00 nm 33 nm

SD

8

8.98

7

Frequency

6 5 4 3 2 1

100 nm

33

31

29

27

25

23

21

19

17

15

13

9

11

7

5

3

1

0 Particle size (nm)

Fig. 5. Presents the TEM images of catalysts prepared by wet impregnation (WI) method and sonochemical method (Us). TEM images (a) for 12N3MAWI; (b) for12N3MAUs; statically count particle distribution area (c) for 12N3MAWI; (d) for 12N3MAUs.

H2-Temperature Programmed Reduction (H2-TPR): The effect of ultrasound irradiation on H2-TPR observed for 12N3MAWI and 12N3MAUs. The reduction peaks exhibited on two reduction temperature at 472 ◦C and 477 ◦C for (12N3MAUs) and (12N3MAWI) respectively in Fig. 6. The shoulder peaks appeared in the range of 499-540◦C for both catalysts, second reduction peak appeared at 842◦C and 825◦C for (12N3MAUs) and (12N3MAWI) respectively. The former peaks appeared due to reduction of NiO and MoO3. More phenomena that are interesting to observe on lower reduction temperature of Ni-Mo/γ-Al2O3 (Us synthesized) compared to synthesize via WI. This lower reduction temperature suggested the weak interaction of Ni-Mo with support [7]. Moreover, the hydrogen spilled over Ni to Mo that promoted the reduction rate of MoO 3 [8]. The ultrasound irradiation lowers the reduction temperature [9]. The shift of reduction peak in the higher temperature in WI synthesized catalysts is an indication of bigger particle size of Ni-Mo [9]. It is well defined that nano particles easily reduced at lower temperature and more active than those reduced at higher temperature [10]. It is reported that the catalysts with lower reduction temperature give high activity and high selectivity for hydrodeoxygenation of model compounds. Isomerization is more commonly found in catalysts having a high reduction temperature [8]. It is expected that ultrasound synthesized catalysts are more selective due to lower reduction temperature for hydrodeoxygenation and catalysts synthesized via the conventional method give more conversion and isomers in hydrodeoxygenation.

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H2-TPR Profile

12N3MAWI 12N3MAUs

300

428

477 ◦C

250 Hydrogen consumption (a.u)

◦C

499-540

200 150 825-842◦C

100 50 0 300

400

500

600

700

800

900

Temparature (◦C) Fig. 6. H2-TPR profile for 12N3MAWI and 12N3MAUs.

5. Conclusion The XRD patterns revealed that catalysts synthesized via sonochemical formed in nano-size with different crystalline structure. The addition of Mo metal loading to ultrasound irradiation method, effect on the crystal structure in variable shapes. FESEM images, mapping and EDX analysis showed that nanocatalysts contained particles in the range of mesoporous structure. The morphology of catalysts has slightly changed with the use of ultrasound irradiation where Ni particles having more pores on the support than Mo and showed more interaction between Ni and support. The introduction of ultrasound irradiation increased the pore volume and pore size. The increased in pore size and pore volume may increase the activity of catalysts. These catalysts will be test for deoxygenation of some oxygenated compounds for further studies. The high adsorption can increase more interaction of reactants on the surface of catalysts. Similarly, the particle size reduced (1-33 nm) with an average particle size of 15.88 nm. The drop in reduction temperature for sonochemically-synthesized catalysts also supported the higher activity of catalysts. Based on the characterization results, the conclusion is that the ultrasound irradiation method has more significance over conventional method. As ultrasound irradiation increased the surface area, pore size, pore volume with an increase of metal loading and also helped to reduce the particle size up to 1 nm. Acknowledgement The authors would like to acknowledge the Yayasan Universiti Teknologi PETRONAS, Perak, Malaysia for funding this project (YUTP-0153AA-A91) and to thankful to Universiti Teknologi PETRONAS for providing Graduate Assistantship Program for Ph.D. scholars for funding their research work. References [1] Y. Vafaeian, M. Haghighi, and S. Aghamohammadi, Ultrasound assisted dispersion of different amount of Ni over ZSM-5 used as nanostructured catalyst for hydrogen production via CO 2 reforming of methane, Energy Convers. Manage. 76 (2013) 1093-1103. [2] N. Rahemi, M. Haghighi, A.A. Babaluo, M.F. Jafari, and S. Khorram, Non-thermal plasma assisted synthesis and physicochemical characterizations of Co and Cu doped Ni/Al2O3 nanocatalysts used for dry reforming of methane, International Journal of Hydrogen Energy 38 (2013) 16048-16061. [3] A. Morsali and A. Panjehpour, Ultrasonic-assisted synthesis of nano-structured lead (II) coordination polymers as precursors for preparation of lead (II) oxide nanoparticles, Inorganica Chimica Acta 391 (2012) 210-217. [4] P. Estifaee, M. Haghighi, N. Mohammadi, and F. Rahmani, CO oxidation over sonochemically synthesized Pd–Cu/Al2O3 nanocatalyst used in hydrogen purification: Effect of Pd loading and ultrasound irradiation time, Ultrasonics Sonochemistry 21 (2014) 1155-1165. [5] R.R. Ramos, C. Bolívar, J. Castillo, J. Hung, and C.E. Scott, Ultrasound assisted synthesis of nanometric Ni, Co, NiMo and CoMo HDS catalysts, Catalysis Today 133 (2008) 277-281.

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