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MgAl2O4 catalysts for the production of COx free hydrogen and multiwalled carbon nanotubes
Accepted Manuscript Title: Methane decomposition over Pd promoted Ni/MgAl2 O4 catalysts for the production of COx free hydrogen and multiwalled carbon nanotubes Author: Manoj Pudukudy Zahira Yaakob Mohd Sobri Takriff PII: DOI: Reference:
S0169-4332(15)02070-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.246 APSUSC 31193
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-3-2015 28-8-2015 29-8-2015
Please cite this article as: M. Pudukudy, Z. Yaakob, M.S. Takriff, Methane decomposition over Pd promoted Ni/MgAl2 O4 catalysts for the production of COx free hydrogen and multiwalled carbon nanotubes, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.246 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Methane decomposition over Pd promoted Ni/MgAl2O4 catalysts for the production of COx free
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hydrogen and multiwalled carbon nanotubes
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Manoj Pudukudy *, Zahira Yaakob, Mohd Sobri Takriff
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Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment,
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Universiti Kebangsaan Malaysia, Bangi, 43600, Selangor, Malaysia
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*E-mail: [email protected], [email protected]
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Tel.: +60 389216422/+60163851604; Fax: +60 389216148 Abstract
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This article reports the synthesis, characterization and catalytic performance of Pd promoted
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Ni/MgAl2O4 spinel catalysts for thermal decomposition of methane into hydrogen and carbon nanotubes.
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The as-synthesized catalysts were characterized for their structural, textural, morphological properties.
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The catalytic properties of the Ni/MgAl2O4 were found to be enhanced with Pd promoter. It effectively
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increased the surface area of the catalyst without affecting the pore parameters and improved the fine
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dispersion of NiO particles on the surface of the magnesium aluminate. The reduction temperature of
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Ni/MgAl2O4 was found to be lowered after Pd deposition. Moreover, the Pd promoted catalyst provided
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high catalytic activity and stability for methane decomposition. A maximum hydrogen yield of ~57% was
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obtained over the Pd catalyst at a reaction temperature of 700°C without deactivation for 420 minutes of
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time on stream. Moreover highly uniform, interwoven and thin multi-walled carbon nanotubes with high
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graphitization degree were deposited over the catalysts.
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Keywords: Hydrogen, Methane decomposition, Magnesium aluminate, Palladium, Multiwalled carbon
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nanotubes.
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1. Introduction
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The non-renewable fossil fuels are depleting progressively due to its excessive usage for
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applications in all over the world [1]. Therefore, it is necessary to develop an alternative fuel that can
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eliminate the energy scarcity for the nearby future. Renewable hydrogen is the only substitute for fossil
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fuel based energy [2]. Currently, the bulk production of hydrogen is based on the steam reforming and
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partial oxidation of natural gas [3]. The simultaneous production of greenhouse gases such as carbon
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oxides makes the processes less effective, by concerning the increased global warming [4]. Methane
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decomposition is an alternative route for the production of COx free hydrogen (fuel cell grade) without
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any greenhouse gas emission [5]. In addition to this, it produces nanocarbon with various morphologies,
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which can be applied as functional electrode materials for various applications [6].
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Methane decomposition is an endothermic process. Therefore, high reaction temperature is
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needed to decompose the methane. However, Ni, Co and Fe based catalysts were reported to be lower the
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reaction temperatures significantly [7-9]. Alumina, Silica Magnesia, Titania and their mixed oxides and 1 Page 1 of 24
activated carbon are the metallic supports studied in this respect [10-14]. The mixed oxide of Al2O3 and
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MgO, i.e. the ceramic magnesium aluminate (MgAl2O4) is a good refractory material and which is rarely
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used as a catalyst support for heterogeneous catalysis, especially for methane decomposition [15]. As
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reported by Nuernberg et al. [16], the MgAl2O4 spinel is well known for its special characteristics such as
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high melting point, chemical resistivity, low thermal expansion and excellent mechanical strength. In
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addition to this, it provides a good interaction with the metallic phase, which is the most important
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criterion for a metal loaded heterogeneous catalyst.
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Previously our group had reported the synthesis, characterization and application of several Ni,
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Co and Fe based mono- and bimetallic catalysts for methane decomposition [17-19]. These catalysts
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provided considerable activity for the decomposition reaction and yielded different types of nanocarbon,
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such as open tip multi-walled carbon nanotubes, irregular carbon particles, multilayer graphene sheets and
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chain/snake-like carbon nanotubes with open tips. In the present work, we report on the synthesis and
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characterization of Pd promoted Ni/MgAl2O4 spinel catalysts for methane decomposition. Moreover, the
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Pd catalysts were found to be highly active for the production of hydrogen and nanocarbon without any
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diminishing effect, compared to the previously reported unpromoted and noble metal promoted
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Ni/MgAl2O4 catalysts. Multiwalled carbon nanotubes with high graphitization degree were obtained over
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the catalysts. Also the crystalline and structural features of deposited carbon were analyzed by various
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methods including Raman spectroscopy and electron microscopic analysis. To the best of knowledge,
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there is no report on highly active and stable Pd promoted Ni/MgAl2O4 spinel catalysts for methane
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decomposition.
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2. Experimental
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2.1. Preparation of catalysts
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The catalyst support MgAl2O4 was synthesized by a previously reported sol gel method [16, 20].
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In the typical synthesis, 0.2 M Mg and Al precursor solutions were prepared by dissolving 22.8 g of
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Magnesium Ethoxide (Sigma Aldrich, 98%) and 40.8 g of Aluminium Isopropoxide (Fluka, >97% ) in
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500 ml of ethanol (HmbG chemicals, 99.98%) and isopropanol (Baker analyzed, 99.5%) respectively.
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Then, these two solutions were mixed together and heated to boiling under vigorous mechanical stirring.
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After that 150 ml of distilled water was added to it for hydrolysis. Then the formed materials were filtered
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from the alcoholic mixture and dried in an air driven oven at 120°C for overnight. The dried sample was
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then powdered and calcined at 700°C for 5 hours to obtain the magnesium aluminate spinel support. The
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Ni loaded (20%) and Pd promoted (0.4%) catalysts were prepared by impregnation method. In the typical
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procedure, weighed amount of Ni(NO3)2.6H2O (Sigma Aldrich) was dissolved in 500 ml of distilled water
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and the sol gel derived magnesium aluminate spinel support was slowly added into it. The resulting
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mixture was then evaporated gently under continuous stirring at 90°C until it converted a homogenous 2 Page 2 of 24
paste. Afterwards, the sample was transferred into a crucible and dried in an oven at 100°C for overnight
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and ground into fine powders. Finally, the powdered samples were calcined at 700°C for 5 hours to obtain
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the Ni loaded catalysts. The Pd promoted catalyst was also prepared by co-impregnation method using
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Dichloro (ethylenediamine)-palladium (II) as the Pd precursor.
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2.2. Materials characterisation
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To determine the real amount of active metals in the fresh catalysts, the metallic species were
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extracted using a hot acid solution (a mixture of HF, HCl and HNO3 (1:1:3)) at 50°C for 60 min. Then, the
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solution was diluted to pH 2 using deionized water and was analyzed by inductively coupling plasma
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(ICP) atomic absorption spectroscopy. The fresh and spent catalysts were characterized by powder X-Ray
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diffraction (XRD, Bruker D8 Focus Advance powder diffractometer), Field emission scanning electron
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microscopy (FESEM, Zeiss SUPRA 55 scanning electron microscope) and Transmission electron
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microscopy (TEM, Philips CM-12). The mean crystalline size of the samples was calculated from the full
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width at half maximum of the intense XRD peaks using Scherrer’s equation. The SEM and TEM
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photographs of the samples were obtained at an operating voltage of 3 kV and 100 kV respectively. The
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textural properties of the metallic catalysts were measured by N2 sorption analysis at 77 K in a
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Micromeritics ASAP2010 apparatus. The specific surface area of the fresh catalysts was calculated by
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Brunauer-Emmett-Teller (BET) method and the Barret-Joyner-Halenda (BJH) method was used to
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determine their pore parameters such as pore size and pore volume. The Temperature programmed
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reduction (TPR) experiments for the reduction properties of the catalysts were performed in a
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Micromeritics Autochem 2920 chemisorption analyser from room temperature to 800°C under a flow of
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20 ml/min 10 % H2/N2 gas mixture and a heating rate of 10°C per minute. The hydrogen consumption
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was monitored by a gas chromatograph equipped with a thermal conductivity detector (TCD). The Raman
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spectra of the deposited carbon over the catalysts were carried out in a WItec Raman spectrometer (Alpha
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300R) equipped with a diode Nd: YAG laser and an excitation wavelength of 532 nm from 10 to 4000
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cm−1.
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2.3 Methane decomposition experiments
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The methane decomposition experiments were carried out in a fixed bed reactor made of stainless
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steel 2520 (length of 60 cm, outer diameter of 3.1 cm and an inner diameter of 2.5 cm), heated by an
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electric muffle furnace. The weighed amount of catalyst (1g) was packed in the middle of the reactor
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(where the thermocouple is inserted to monitor the reactor temperature) using quartz wool. Then the
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catalyst was reduced in situ with a continuous flow of hydrogen (150 ml/min) for 1 h at 500°C. After
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reduction, the reactor was flashed with nitrogen to raise the reactor temperature for reaction. The catalytic
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experiments were carried out at 700°C under atmospheric pressure using undiluted methane (99.95%,
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NIG Gases, Malaysia) with a flow rate of 150 ml/min for 7 h. During the reaction, outlet gas were 3 Page 3 of 24
collected in gas bags at fixed time intervals and were analyzed by a gas chromatograph (SRI-GC 8610C)
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equipped with a Molecular sieve 5A column connected to a thermal conductivity detector (TCD) with
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helium as the carrier gas. The yield of hydrogen (%) was then calculated using calibrated data, to express
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the catalytic activity. No gases other than hydrogen were detected in the reaction products. After reaction,
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the reactor was cooled to room temperature under a constant flow of nitrogen (50 ml/min), in order to
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collect the deposited carbon over the catalysts for further characterization.
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3. Results and discussion
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The chemical composition of the fresh catalysts is shown in Table 1. It represents the actual
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quantity of nickel, palladium and the total metal present in the samples. There is a minimal variation in the nominated and actual values.
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XRD patterns of the catalysts are shown in Fig. 1, where the diffraction peak intensity of the sol
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gel derived support was found to be very weak, indicating the low crystalline quality of the magnesium
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aluminate support with the average crystalline size of 9 nm. However, in the case of Ni loaded catalysts,
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several intense diffraction peaks were observed at the 2θ values of 37.2°, 43.2°, 62.9°, 75.3° and 79.5,
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which indicates the formation of NiO (JCPDS: 01-078-0643). No peaks related to Palladium were
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observed in the Pd promoted catalyst. This is a clear evidence for the fine dispersion of Pd particles on the
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Ni loaded magnesium aluminate catalyst. Moreover, the crystallinity of NiO was not altered, after
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promoting with Pd (~32 nm).
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The surface morphology of the as-synthesized catalysts was studied by FESEM and the results
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are shown in Fig. 2. The images clearly reveal the fine coverage of NiO particles on the surface of the
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magnesium aluminate lumps. The white spots present in the images indicate the metal oxide particles.
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Certain agglomerated NiO nanostructures were observed on the unpromoted catalyst. However after
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promoting with Pd, the agglomeration of particles was found to be much reduced, indicating the enhanced
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role of Pd for the fine dispersion of NiO particles on the surface of MgAl2O4 support. The internal
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structure of the Pd promoted catalyst was also investigated by TEM analysis and the images are shown in
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Fig. 3. From the figure, it is clear that the metal oxide particles were randomly inter-aggregated. The dark
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spots shown in the TEM images represent the Pd species and which are found to be highly dispersed on
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the surface of the uniformly agglomerated metal oxide species. These observations were highly consistent
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with the XRD results.
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Textural properties of the unpromoted and Pd promoted catalysts were studied using BET/BJH
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analysis and the results are shown Table. 2. The unpromoted Ni catalyst showed a BET surface area of
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22.6 m2/g. However the surface area was found to be increased after Pd deposition. The Pd promoted
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catalyst showed much higher surface area of 29.3 m2/g. The increase of surface area of Ni catalysts
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promoted with noble metals was already reported by Profeti et al. [21]. This can be attributed to the 4 Page 4 of 24
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interactive effect of noble metals with NiO [19]. The BJH desorption cumulative pore size data shown in
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table 2, confirmed the presence of mesopores in the samples. The pore parameters such as pore size and
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pore volume seems to be not much altered, after promoting with Pd. The reduction properties of the catalysts were studied using TPR and the profiles are shown in
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Fig. 4. The unpromoted Ni/MgAl2O4 shows a broad reduction peak from 271°C to 510°C and centered at
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around 392°C, representing the reduction of aggregated NiO species, which had quite strong interaction
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with the support [22]. However after Pd deposition, the reduction peak of Ni/MgAl2O4 was shifted to
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335°C, which can be attributed to the reduction of NiO species, weakly interacted with the support, due
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the spillover effect of hydrogen by Pd deposition [23]. In addition to this, a small reduction peak was
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observed at around 268°C, which is due to the reduction of non-interacted NiO species. Similar results
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were previously reported for the Pt promoted Ni catalysts by Nuernberg et al. [20].
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The kinetic curves of methane decomposition as a function of time on stream (TOS) over the
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unpromoted and Pd promoted Ni/MgAl2O4 catalysts are shown in Fig. 5. It can be seen that, there is a
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significant difference in activity of both of the catalysts. However, the two catalysts provided high
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catalytic activity for methane decomposition. No gaseous products other than hydrogen were observed
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over the catalysts during the whole period of reaction. As shown in Fig. 5, the Ni/MgAl2O4 catalyst
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provided slight low efficiency than the Pd promoted catalyst. The Pd promoted catalyst showed a
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moderate activity in the initial stage of reaction (46% hydrogen yield) and it acquired a maximum
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hydrogen yield of 57 % within 30 minutes of time on stream. After that the activity remained more or less
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same (an average of 50 %) for 180 minutes. Then its activity was started to decrease with increasing time
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on stream until 420 minutes. This can be attributed to accumulation of nanocarbon over the active sites of
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the catalyst. However, in the case of unpromoted catalyst, a highest hydrogen yield of around 49% was
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reached within 30 minutes of time on stream and after that its activity began to decrease and reached to
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~20% at the end of 420 minutes. However, it is worth to mention that the Pd promoted catalyst was more
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active than the unpromoted catalyst, even after 420 minutes of reaction without deactivation. The high
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catalytic activity and stability of the Pd promoted catalyst is mainly attributed to existance of large
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amount of non-interacting NiO particles on the magnesium aluminate surface with high surface area. In
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addition to this, the finely dispersed NiO particles on the surface support can be easily reduced after Pd
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deposition (Fig. 4), providing more active nickel sites for the rapid adsorption and solubility of methane
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molecules on the catalyst and which further allows the faster methane decomposition. The low activity of
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the unpromoted catalyst can be attributed to the reduced surface area and the strong metal-support
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interaction (MSI) of aggregated nickel oxides with the magnesium aluminate support.
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Previously Nuernberg et al. [16] performed the methane decomposition over 20% Ni/MgAl2O4
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catalysts and studied the effect of methane dilution, catalyst reduction temperature and reduction time. As 5 Page 5 of 24
per their report, a maximum methane conversion of 37% was obtained in the initial period of reaction
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with 7:1 molar ratio of N2:CH4 at 550°C with a catalyst reduction temperature of 700°C for 1h. With
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increase in the concentration of methane (1:3), the catalytic activity was decreased to less than 18%. This
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is due to the unavailability of sufficient catalytic sites for methane molecules. Furthermore, the catalyst
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was completely deactivated after 240 minutes of time on stream with an exception of the reaction with a
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reduction temperature of 550°C for 1 h, where the activity remained more or less same (~12%). Once
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more Nuernberg et al. [24] studied the reaction over 20% Ni loaded MgAl2O4 spinel particles prepared via
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a metal chitosan complexation route and produced COx free hydrogen and multiwalled carbon nanotubes.
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A maximum methane conversion of ~37% was obtained in the initial stage with 7:1 molar ratio of N2:CH4
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at 550°C with a catalyst reduction temperature of 700°C for 1h and it was found that after 180 minutes of
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time on stream, the catalyst was deactivated completely. Again Nuernberg et al. [20] studied the methane
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decomposition over Pt promoted 15% Ni/MgAl2O4 catalysts and reported that the addition of a small
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amount of Pt promoted the catalytic efficiency of Ni/MgAl2O4 catalyst for the formation of hydrogen and
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multiwalled carbon nanotubes. The 0.1% Pt loaded catalyst showed a maximum methane conversion of
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14% in the initial stage of reaction using 7:1 molar ratio of N2: CH4 feed at 550°C and the activity was
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further dropped to 8% with increase in the concentration of methane (1:3). However, a maximum
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methane conversion of 45% was observed over the catalyst when the reaction temperature was increased
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from 550°C to 700°C under the experimental conditions of N2:CH4 of 7:1 and the reduction temperature
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of 700°C. Like the unsupported one, the Pt promoted catalyst was also deactivated rapidly with a short
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duration of time on stream (240 min). Thus from the above discussion, it is clear that the Pd promoted
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Ni/MgAl2O4 catalysts exhibits significant activity with the aforementioned catalysts.
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A set of Pd promoted catalysts were already reported for methane decomposition [25]. Takenaka
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et al. [26] studied the promoting effects of Pd on the Ni catalyst and reported that the addition of Pd
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promoter reduced the deactivation rate of Ni/SiO2 catalyst. Moreover, a bulk amount of carbon was
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deposited over the catalyst. The increased carbon yield and the improved morphology of the nanocarbon
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over the Pd-promoted catalyst were attributed to the formation of Pd–Ni alloys. Again Takenaka et al.
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[27] reported the methane decomposition over monometallic Ni/SiO2, Pd/SiO2 and bimetallic Ni-Pd/SiO2
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catalysts. According to their report, the addition of Pd to the Ni/SiO2 catalyst enhanced the catalytic
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activity and stability due to the formation of increased number of facets on the Ni–Pd bimetallic alloys
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after the carbon precipitation. Besides, they studied the methane decomposition over Ni-, Fe- and Cu-
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based alumina catalysts and their Pd-based bimetallic catalysts. A maximum methane conversion of 35–
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50 mol % was obtained over Pd-M/Al2O3 (M - Fe, Co, Ni, Cu, Ag, and Rh) catalysts in the temperature
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range of 700–900°C and space velocity of 160 l/h g. However, the methane conversion was reduced to 5-
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10 % during the course of reaction with a streaming time of 3–10 h [28]. Ogihara et al. [29] studied 6 Page 6 of 24
methane decomposition over alumina supported Pd-promoted Ni, Co, Rh, and Fe based bimetallic
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catalysts. 94% of hydrogen yield was obtained by the Pd-Co/Al2O3 bimetallic catalyst at 850°C. Prasad et
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al. [30] studied the thermal decomposition of methane over Pd-promoted (5 and 10%) activated carbon.
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Per their report, high catalytic activity and stability was shown by the 10% Pd-promoted catalyst at 850°C
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with a volume hourly space velocity of 1.62 l/h g. This is due to the presence of Pd crystals on the
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catalyst.
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Fig. 6 shows the XRD patterns of the spent catalysts. The highly intense diffraction peak at the
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2θ value of 26.1° with (002) plane, represents the formation of crystalline carbon with high graphitization
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degree. The diffraction peaks centered at the 2θ values of 44.4°, 51.7° and 76.3° were attributed to the
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formation of metallic phase of Ni by the reduction of NiO after the reaction. All of the other diffraction
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peaks were attributed to the presence of magnesium aluminate support. The calculated interlayer d-
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spacing of deposited carbon (2θ = ~26°) was found to be 0.3372 nm and 0.3364 nm for the unpromoted
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and Pd promoted catalysts respectively and the values are quite close to the reported value of graphitic
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layers (0.3354 nm), indicating their crystalline quality [31, 32].
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The external morphology of the spent catalysts was studied by FESEM and the images at
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different magnifications are shown in Fig. 7. From the figures, it is clear that the surface of the catalysts
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was fully covered with bulk amount of highly interwoven carbon nanotubes. The carbon nanotubes were
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found to be uniform, thin and long. Due to the interweaving of nanotubes, its actual length determination
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is found to be quite tiresome and the length goes to several micrometers. The high magnified image of a
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single nanotube is shown in Fig. 7(d) and it shows that the nanotube is very thin, with a total diameter of
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~30-35 nm. The TEM photographs shown in Fig. 8 support this observation and found that the carbon
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nanotubes were multi wall in nature. The wall thickness was measured to be ~16 nm with the internal
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channel space of around 4-5 nm. The dark spots shown in Fig 8(a, b) represents the presence of Ni/Pd
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metal species.
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Raman spectroscopy of the deposited carbon was studied to evaluate their crystallinity and
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graphitization degree. Two distinct Raman bands were observed in the spectrum of each sample as shown
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in Fig. 9. The first band centered at ~1345 cm−1 (D band) was attributed to the structural imperfections of
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graphitic carbon [33], since no amorphous carbon was detected in the XRD patterns (Fig. 6). The second
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band centered at ~ 1570 cm−1 (G band) was assigned to the in plane carbon-carbon stretching vibration of
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the graphitic layers, indicative of the formation of graphitized multiwall carbon nanotubes [34]. The
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graphitization degree and crystalline quality of the deposited carbon over the catalysts was assessed in
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terms of their peak intensity ratio of D band and G band (ID/IG ratio). The ID/IG value was calculated to be
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1.13 and 1.01 for the unpromoted and Pd promoted Ni/MgAl2O4 catalysts respectively. The values were
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found to be very small and close to unity, indicating their superior crystalline quality and better 7 Page 7 of 24
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graphitization degree. However, much higher graphitization degree and crystallinity was observed for the
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carbon nanotubes deposited over the Pd promoted catalyst.
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4. Conclusion In summary, the Pd promoted Ni/MgAl2O4 spinel catalysts were synthesized, characterized and
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its catalytic activity was evaluated for the methane decomposition. The promoter, palladium is found to
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have an enhanced impact on the catalytic efficiency of Ni/MgAl2O4. The surface area of Ni/MgAl2O4 was
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considerably increased after Pd deposition and it further improved the fine dispersion of NiO particles on
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the surface of MgAl2O4 without particle aggregation. A maximum hydrogen yield of ~57% was obtained
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over the Pd promoted catalyst with high stability for 420 minutes of time on stream. The characterization
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of the deposited nanocarbon indicated the formation of multiwalled carbon nanotubes with high
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graphitization degree and crystallinity.
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Acknowledgements
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This project is financially supported by Universiti Kebangsaan Malaysia - Yayasan Sime Darby (YSD),
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under grants PKT 6/2012 and KK-2014-014. The authors would like to acknowledge FST and CRIM,
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UKM for the analysis of samples. Institut Teknologi Maju (ITMA), Universiti Putra Malaysia (UPM) is
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acknowledged for providing the Raman analysis.
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208 (2012) 409-414.
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[17] M. Pudukudy, Z. Yaakob, Z.S. Akmal, Direct decomposition of methane over SBA-15 supported Ni,
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Co and Fe based bimetallic catalysts, Appl. Surf. Sci. 330 (2015) 418–430.
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[18] M. Pudukudy, Z. Yaakob, Methane decomposition over Ni, Co and Fe based monometallic catalysts
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supported on sol–gel derived SiO2 microflakes, Chem. Eng. J. 262 (2015) 1009–1021.
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[19] M. Pudukudy, Z. Yaakob, Z.S. Akmal, Direct decomposition of methane over Pd promoted Ni/SBA-
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15 catalysts, Appl. Surf. Sci. 353 (2015) 127–136.
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[20] G.D.B. Nuernberg, H.V. Fajardo, E.L. Foletto, S.M.H. Probst, N.L.V. Carreño, L.F.D. Probst, J.
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Barrault, Methane conversion to hydrogen and nanotubes on Pt/Ni catalysts supported over spinel
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MgAl2O4, Catal. Today 176 (2011) 465- 469.
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[21] L.P.R. Profeti, J.A.C. Dias, J. M. Assaf, El. M. Assaf, Hydrogen production by steam reforming of
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ethanol over Ni-based catalysts promoted with noble metals, J Power Sources 190 (2009) 525-533.
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[22] Z. Beatriz, A. Miguel, Valenzuela , P. Jorge, T.G. Enelio, Effect of Ca, Ce or K oxide addition on the
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activity of Ni/SiO2 catalysts for the methane decomposition reaction, Int J Hydrogen Energy 35 (2010)
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12091-12097.
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[23] E.L. Foletto, R.W. Alves, S.L. Jahn, Preparation of Ni/Pt catalysts supported on spinel (MgAl2O4)
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for methane reforming, J Power Sources 161 (2006) 531-534.
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[24] G.B. Nuernberg, E.L. Foletto, L.F.D. Probst, N.L.V. Carreño, M.A. Moreira, MgAl2O4 spinel
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particles prepared by metal–chitosan complexation route and used as catalyst support for direct
8
decomposition of methane, J. Mol. Catal. A: Chem 370 (2013) 22–27.
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[25] Z. Jiang, B. Wang, T. Fang, Adsorption and dehydrogenation mechanism of methane on clean and
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oxygen-covered Pd (1 0 0) surfaces: A DFT study, Appl. Surf. Sci. 320 (2014) 256–262.
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[26] S. Takenaka, Y. Shigeta, E. Tanabe, K. Otsuka, Methane decomposition into hydrogen and carbon
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nanofibers over supported Pd-Ni catalysts, J Catal 220 (2003) 468–477
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[27] S. Takenaka, Y. Shigeta, E. Tanabe, K. Otsuka, Methane decomposition into hydrogen and carbon
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nanofibers over supported Pd-Ni catalysts: characterization of the catalysts during the reaction, J. Phys.
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Chem. B. 108 (2004) 7656–7664.
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[28] S. Takenaka, Y. Shigeta, K. Otsuka, Supported Ni-Pd catalysts active for methane decomposition
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into hydrogen and carbon nano fibers, Chem. Lett. 32 (2003) 26–27.
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[29] H. Ogihara, S. Takenaka, I. Yamanaka, E. Tanabe, A. Genseki, K. Otsuka, Formation of highly
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concentrated hydrogen through methane decomposition over Pd-based alloy catalysts, J. Catal. 238
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(2006) 353–360.
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[30] J.S. Prasad, V. Dhand, V. Himabindu, Y. Anjaneyulu, Production of hydrogen and carbon nanofibers
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through the decomposition of methane over activated carbon supported Pd catalysts, Int. J. Hydrogen
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Energy 35 (2010) 10977–10983.
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[31] A.E. Awadallah, A.A. Aboul-Enein, D.S. El-Desouki, A.K. Aboul-Gheit, Catalytic thermal
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decomposition of methane to COx-free hydrogen and carbon nano-tubes over MgO supported bimetallic
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group VIII catalysts, Appl. Surf. Sci. 296 (2014) 100–107.
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[32] A.E. Awadallah, W. Ahmed, M.R. Noor El-Din, A.A. Aboul-Enein, Novel aluminosilicate hollow
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sphere as a catalyst support for methane decomposition to COx-free hydrogen production, Appl. Surf. Sci.
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287 (2013) 415-422.
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[33] N.P. Ivleva, A. Messerer, X. Yang, R. Niessner, U. Pöschl, Raman Microspectroscopic Analysis of
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Changes in the Chemical Structure and Reactivity of Soot in a Diesel Exhaust After treatment Model
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System, Environ. Sci. Technol. 41 (2007) 3702–3707.
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[34] A.E. Awadallah, A.A.A. Enein, A.K. Aboul-Gheit, Effect of progressive Co loading on commercial
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Co-Mo/Al2O3 catalyst for natural gas decomposition to COx-free hydrogen production and carbon
3
nanotubes, Energy Convers. Manage. 77 (2014) 143-151.
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Figure captions
2
Fig. 1- XRD patterns of the synthesized catalysts
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Fig. 2- FESEM images of the catalysts (a, b) Ni/MgAl2O4 and (c, d) 0.4 % Pd promoted Ni/MgAl2O4
4
Fig. 3- TEM images of the 0.4% Pd promoted Ni/MgAl2O4 catalyst
5
Fig. 4- TPR profiles of the catalysts (a) Ni/MgAl2O4 and (b) 0.4 % Pd promoted Ni/MgAl2O4
6
Fig. 5- Kinetic curves of methane decomposition over the catalysts
7
Fig. 6- XRD patterns of the nanocarbon over (a) Ni/MgAl2O4 and (b) 0.4 % Pd promoted Ni/MgAl2O4
8
Fig. 7- FESEM photographs of the nanocarbon deposited over the catalysts
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Fig. 8- TEM images of the deposited multiwalled carbon nanotubes
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Fig. 9- Raman spectra of the nanocarbon over (a) Ni/MgAl2O4 and (b) 0.4% Pd promoted Ni/MgAl2O4
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Graphical Abstract
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Enhanced catalytic activity of the Pd promoted Ni/MgAl2O4 catalysts for CH4 decomposition into H2 & C
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Highlights Synthesis and characterization of Pd promoted Ni/MgAl2O4 catalysts for methane decomposition
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Enhanced role of Pd in the properties and catalytic activities of Ni/MgAl2O4 catalysts
4
Production of COx free hydrogen and multi-walled carbon nanotubes
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Crystalline and structural characterization of multiwalled carbon nanotubes
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Tables
2 Table 1- Real amount of Ni and Pd in the prepared catalysts (ICP analysis) Catalyst
Ni (wt.%)
Pd (wt.%)
Ni/MgAl2O4
15.18
-
Pd-Ni/MgAl2O4
16.63
0.31
15.18 16.94
cr
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Total metal (wt.%)
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Table 2- BET/BJH results of the as-prepared catalysts
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Pore size
Pore volume
2
(m /g)
(nm)
(cm3/g)
Ni/MgAl2O4
22.6
23.6
0.152
Pd-Ni/MgAl2O4
29.3
22.8
0.139
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Surface area
Catalyst
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S0169-4332(15)02070-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.246 APSUSC 31193
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-3-2015 28-8-2015 29-8-2015
Please cite this article as: M. Pudukudy, Z. Yaakob, M.S. Takriff, Methane decomposition over Pd promoted Ni/MgAl2 O4 catalysts for the production of COx free hydrogen and multiwalled carbon nanotubes, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.246 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Methane decomposition over Pd promoted Ni/MgAl2O4 catalysts for the production of COx free
2
hydrogen and multiwalled carbon nanotubes
3
Manoj Pudukudy *, Zahira Yaakob, Mohd Sobri Takriff
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Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment,
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Universiti Kebangsaan Malaysia, Bangi, 43600, Selangor, Malaysia
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*E-mail: [email protected], [email protected]
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Tel.: +60 389216422/+60163851604; Fax: +60 389216148 Abstract
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This article reports the synthesis, characterization and catalytic performance of Pd promoted
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Ni/MgAl2O4 spinel catalysts for thermal decomposition of methane into hydrogen and carbon nanotubes.
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The as-synthesized catalysts were characterized for their structural, textural, morphological properties.
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The catalytic properties of the Ni/MgAl2O4 were found to be enhanced with Pd promoter. It effectively
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increased the surface area of the catalyst without affecting the pore parameters and improved the fine
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dispersion of NiO particles on the surface of the magnesium aluminate. The reduction temperature of
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Ni/MgAl2O4 was found to be lowered after Pd deposition. Moreover, the Pd promoted catalyst provided
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high catalytic activity and stability for methane decomposition. A maximum hydrogen yield of ~57% was
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obtained over the Pd catalyst at a reaction temperature of 700°C without deactivation for 420 minutes of
18
time on stream. Moreover highly uniform, interwoven and thin multi-walled carbon nanotubes with high
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graphitization degree were deposited over the catalysts.
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Keywords: Hydrogen, Methane decomposition, Magnesium aluminate, Palladium, Multiwalled carbon
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nanotubes.
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1. Introduction
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The non-renewable fossil fuels are depleting progressively due to its excessive usage for
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applications in all over the world [1]. Therefore, it is necessary to develop an alternative fuel that can
25
eliminate the energy scarcity for the nearby future. Renewable hydrogen is the only substitute for fossil
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fuel based energy [2]. Currently, the bulk production of hydrogen is based on the steam reforming and
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partial oxidation of natural gas [3]. The simultaneous production of greenhouse gases such as carbon
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oxides makes the processes less effective, by concerning the increased global warming [4]. Methane
29
decomposition is an alternative route for the production of COx free hydrogen (fuel cell grade) without
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any greenhouse gas emission [5]. In addition to this, it produces nanocarbon with various morphologies,
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which can be applied as functional electrode materials for various applications [6].
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Methane decomposition is an endothermic process. Therefore, high reaction temperature is
33
needed to decompose the methane. However, Ni, Co and Fe based catalysts were reported to be lower the
34
reaction temperatures significantly [7-9]. Alumina, Silica Magnesia, Titania and their mixed oxides and 1 Page 1 of 24
activated carbon are the metallic supports studied in this respect [10-14]. The mixed oxide of Al2O3 and
2
MgO, i.e. the ceramic magnesium aluminate (MgAl2O4) is a good refractory material and which is rarely
3
used as a catalyst support for heterogeneous catalysis, especially for methane decomposition [15]. As
4
reported by Nuernberg et al. [16], the MgAl2O4 spinel is well known for its special characteristics such as
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high melting point, chemical resistivity, low thermal expansion and excellent mechanical strength. In
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addition to this, it provides a good interaction with the metallic phase, which is the most important
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criterion for a metal loaded heterogeneous catalyst.
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Previously our group had reported the synthesis, characterization and application of several Ni,
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Co and Fe based mono- and bimetallic catalysts for methane decomposition [17-19]. These catalysts
10
provided considerable activity for the decomposition reaction and yielded different types of nanocarbon,
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such as open tip multi-walled carbon nanotubes, irregular carbon particles, multilayer graphene sheets and
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chain/snake-like carbon nanotubes with open tips. In the present work, we report on the synthesis and
13
characterization of Pd promoted Ni/MgAl2O4 spinel catalysts for methane decomposition. Moreover, the
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Pd catalysts were found to be highly active for the production of hydrogen and nanocarbon without any
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diminishing effect, compared to the previously reported unpromoted and noble metal promoted
16
Ni/MgAl2O4 catalysts. Multiwalled carbon nanotubes with high graphitization degree were obtained over
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the catalysts. Also the crystalline and structural features of deposited carbon were analyzed by various
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methods including Raman spectroscopy and electron microscopic analysis. To the best of knowledge,
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there is no report on highly active and stable Pd promoted Ni/MgAl2O4 spinel catalysts for methane
20
decomposition.
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2. Experimental
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2.1. Preparation of catalysts
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The catalyst support MgAl2O4 was synthesized by a previously reported sol gel method [16, 20].
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In the typical synthesis, 0.2 M Mg and Al precursor solutions were prepared by dissolving 22.8 g of
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Magnesium Ethoxide (Sigma Aldrich, 98%) and 40.8 g of Aluminium Isopropoxide (Fluka, >97% ) in
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500 ml of ethanol (HmbG chemicals, 99.98%) and isopropanol (Baker analyzed, 99.5%) respectively.
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Then, these two solutions were mixed together and heated to boiling under vigorous mechanical stirring.
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After that 150 ml of distilled water was added to it for hydrolysis. Then the formed materials were filtered
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from the alcoholic mixture and dried in an air driven oven at 120°C for overnight. The dried sample was
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then powdered and calcined at 700°C for 5 hours to obtain the magnesium aluminate spinel support. The
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Ni loaded (20%) and Pd promoted (0.4%) catalysts were prepared by impregnation method. In the typical
32
procedure, weighed amount of Ni(NO3)2.6H2O (Sigma Aldrich) was dissolved in 500 ml of distilled water
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and the sol gel derived magnesium aluminate spinel support was slowly added into it. The resulting
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mixture was then evaporated gently under continuous stirring at 90°C until it converted a homogenous 2 Page 2 of 24
paste. Afterwards, the sample was transferred into a crucible and dried in an oven at 100°C for overnight
2
and ground into fine powders. Finally, the powdered samples were calcined at 700°C for 5 hours to obtain
3
the Ni loaded catalysts. The Pd promoted catalyst was also prepared by co-impregnation method using
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Dichloro (ethylenediamine)-palladium (II) as the Pd precursor.
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2.2. Materials characterisation
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To determine the real amount of active metals in the fresh catalysts, the metallic species were
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extracted using a hot acid solution (a mixture of HF, HCl and HNO3 (1:1:3)) at 50°C for 60 min. Then, the
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solution was diluted to pH 2 using deionized water and was analyzed by inductively coupling plasma
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(ICP) atomic absorption spectroscopy. The fresh and spent catalysts were characterized by powder X-Ray
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diffraction (XRD, Bruker D8 Focus Advance powder diffractometer), Field emission scanning electron
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microscopy (FESEM, Zeiss SUPRA 55 scanning electron microscope) and Transmission electron
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microscopy (TEM, Philips CM-12). The mean crystalline size of the samples was calculated from the full
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width at half maximum of the intense XRD peaks using Scherrer’s equation. The SEM and TEM
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photographs of the samples were obtained at an operating voltage of 3 kV and 100 kV respectively. The
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textural properties of the metallic catalysts were measured by N2 sorption analysis at 77 K in a
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Micromeritics ASAP2010 apparatus. The specific surface area of the fresh catalysts was calculated by
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Brunauer-Emmett-Teller (BET) method and the Barret-Joyner-Halenda (BJH) method was used to
18
determine their pore parameters such as pore size and pore volume. The Temperature programmed
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reduction (TPR) experiments for the reduction properties of the catalysts were performed in a
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Micromeritics Autochem 2920 chemisorption analyser from room temperature to 800°C under a flow of
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20 ml/min 10 % H2/N2 gas mixture and a heating rate of 10°C per minute. The hydrogen consumption
22
was monitored by a gas chromatograph equipped with a thermal conductivity detector (TCD). The Raman
23
spectra of the deposited carbon over the catalysts were carried out in a WItec Raman spectrometer (Alpha
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300R) equipped with a diode Nd: YAG laser and an excitation wavelength of 532 nm from 10 to 4000
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cm−1.
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2.3 Methane decomposition experiments
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The methane decomposition experiments were carried out in a fixed bed reactor made of stainless
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steel 2520 (length of 60 cm, outer diameter of 3.1 cm and an inner diameter of 2.5 cm), heated by an
29
electric muffle furnace. The weighed amount of catalyst (1g) was packed in the middle of the reactor
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(where the thermocouple is inserted to monitor the reactor temperature) using quartz wool. Then the
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catalyst was reduced in situ with a continuous flow of hydrogen (150 ml/min) for 1 h at 500°C. After
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reduction, the reactor was flashed with nitrogen to raise the reactor temperature for reaction. The catalytic
33
experiments were carried out at 700°C under atmospheric pressure using undiluted methane (99.95%,
34
NIG Gases, Malaysia) with a flow rate of 150 ml/min for 7 h. During the reaction, outlet gas were 3 Page 3 of 24
collected in gas bags at fixed time intervals and were analyzed by a gas chromatograph (SRI-GC 8610C)
2
equipped with a Molecular sieve 5A column connected to a thermal conductivity detector (TCD) with
3
helium as the carrier gas. The yield of hydrogen (%) was then calculated using calibrated data, to express
4
the catalytic activity. No gases other than hydrogen were detected in the reaction products. After reaction,
5
the reactor was cooled to room temperature under a constant flow of nitrogen (50 ml/min), in order to
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collect the deposited carbon over the catalysts for further characterization.
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3. Results and discussion
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quantity of nickel, palladium and the total metal present in the samples. There is a minimal variation in the nominated and actual values.
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XRD patterns of the catalysts are shown in Fig. 1, where the diffraction peak intensity of the sol
12
gel derived support was found to be very weak, indicating the low crystalline quality of the magnesium
13
aluminate support with the average crystalline size of 9 nm. However, in the case of Ni loaded catalysts,
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several intense diffraction peaks were observed at the 2θ values of 37.2°, 43.2°, 62.9°, 75.3° and 79.5,
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which indicates the formation of NiO (JCPDS: 01-078-0643). No peaks related to Palladium were
16
observed in the Pd promoted catalyst. This is a clear evidence for the fine dispersion of Pd particles on the
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Ni loaded magnesium aluminate catalyst. Moreover, the crystallinity of NiO was not altered, after
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promoting with Pd (~32 nm).
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The surface morphology of the as-synthesized catalysts was studied by FESEM and the results
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are shown in Fig. 2. The images clearly reveal the fine coverage of NiO particles on the surface of the
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magnesium aluminate lumps. The white spots present in the images indicate the metal oxide particles.
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Certain agglomerated NiO nanostructures were observed on the unpromoted catalyst. However after
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promoting with Pd, the agglomeration of particles was found to be much reduced, indicating the enhanced
24
role of Pd for the fine dispersion of NiO particles on the surface of MgAl2O4 support. The internal
25
structure of the Pd promoted catalyst was also investigated by TEM analysis and the images are shown in
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Fig. 3. From the figure, it is clear that the metal oxide particles were randomly inter-aggregated. The dark
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spots shown in the TEM images represent the Pd species and which are found to be highly dispersed on
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the surface of the uniformly agglomerated metal oxide species. These observations were highly consistent
29
with the XRD results.
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Textural properties of the unpromoted and Pd promoted catalysts were studied using BET/BJH
31
analysis and the results are shown Table. 2. The unpromoted Ni catalyst showed a BET surface area of
32
22.6 m2/g. However the surface area was found to be increased after Pd deposition. The Pd promoted
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catalyst showed much higher surface area of 29.3 m2/g. The increase of surface area of Ni catalysts
34
promoted with noble metals was already reported by Profeti et al. [21]. This can be attributed to the 4 Page 4 of 24
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interactive effect of noble metals with NiO [19]. The BJH desorption cumulative pore size data shown in
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table 2, confirmed the presence of mesopores in the samples. The pore parameters such as pore size and
3
pore volume seems to be not much altered, after promoting with Pd. The reduction properties of the catalysts were studied using TPR and the profiles are shown in
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Fig. 4. The unpromoted Ni/MgAl2O4 shows a broad reduction peak from 271°C to 510°C and centered at
6
around 392°C, representing the reduction of aggregated NiO species, which had quite strong interaction
7
with the support [22]. However after Pd deposition, the reduction peak of Ni/MgAl2O4 was shifted to
8
335°C, which can be attributed to the reduction of NiO species, weakly interacted with the support, due
9
the spillover effect of hydrogen by Pd deposition [23]. In addition to this, a small reduction peak was
10
observed at around 268°C, which is due to the reduction of non-interacted NiO species. Similar results
11
were previously reported for the Pt promoted Ni catalysts by Nuernberg et al. [20].
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The kinetic curves of methane decomposition as a function of time on stream (TOS) over the
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unpromoted and Pd promoted Ni/MgAl2O4 catalysts are shown in Fig. 5. It can be seen that, there is a
14
significant difference in activity of both of the catalysts. However, the two catalysts provided high
15
catalytic activity for methane decomposition. No gaseous products other than hydrogen were observed
16
over the catalysts during the whole period of reaction. As shown in Fig. 5, the Ni/MgAl2O4 catalyst
17
provided slight low efficiency than the Pd promoted catalyst. The Pd promoted catalyst showed a
18
moderate activity in the initial stage of reaction (46% hydrogen yield) and it acquired a maximum
19
hydrogen yield of 57 % within 30 minutes of time on stream. After that the activity remained more or less
20
same (an average of 50 %) for 180 minutes. Then its activity was started to decrease with increasing time
21
on stream until 420 minutes. This can be attributed to accumulation of nanocarbon over the active sites of
22
the catalyst. However, in the case of unpromoted catalyst, a highest hydrogen yield of around 49% was
23
reached within 30 minutes of time on stream and after that its activity began to decrease and reached to
24
~20% at the end of 420 minutes. However, it is worth to mention that the Pd promoted catalyst was more
25
active than the unpromoted catalyst, even after 420 minutes of reaction without deactivation. The high
26
catalytic activity and stability of the Pd promoted catalyst is mainly attributed to existance of large
27
amount of non-interacting NiO particles on the magnesium aluminate surface with high surface area. In
28
addition to this, the finely dispersed NiO particles on the surface support can be easily reduced after Pd
29
deposition (Fig. 4), providing more active nickel sites for the rapid adsorption and solubility of methane
30
molecules on the catalyst and which further allows the faster methane decomposition. The low activity of
31
the unpromoted catalyst can be attributed to the reduced surface area and the strong metal-support
32
interaction (MSI) of aggregated nickel oxides with the magnesium aluminate support.
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Previously Nuernberg et al. [16] performed the methane decomposition over 20% Ni/MgAl2O4
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catalysts and studied the effect of methane dilution, catalyst reduction temperature and reduction time. As 5 Page 5 of 24
per their report, a maximum methane conversion of 37% was obtained in the initial period of reaction
2
with 7:1 molar ratio of N2:CH4 at 550°C with a catalyst reduction temperature of 700°C for 1h. With
3
increase in the concentration of methane (1:3), the catalytic activity was decreased to less than 18%. This
4
is due to the unavailability of sufficient catalytic sites for methane molecules. Furthermore, the catalyst
5
was completely deactivated after 240 minutes of time on stream with an exception of the reaction with a
6
reduction temperature of 550°C for 1 h, where the activity remained more or less same (~12%). Once
7
more Nuernberg et al. [24] studied the reaction over 20% Ni loaded MgAl2O4 spinel particles prepared via
8
a metal chitosan complexation route and produced COx free hydrogen and multiwalled carbon nanotubes.
9
A maximum methane conversion of ~37% was obtained in the initial stage with 7:1 molar ratio of N2:CH4
10
at 550°C with a catalyst reduction temperature of 700°C for 1h and it was found that after 180 minutes of
11
time on stream, the catalyst was deactivated completely. Again Nuernberg et al. [20] studied the methane
12
decomposition over Pt promoted 15% Ni/MgAl2O4 catalysts and reported that the addition of a small
13
amount of Pt promoted the catalytic efficiency of Ni/MgAl2O4 catalyst for the formation of hydrogen and
14
multiwalled carbon nanotubes. The 0.1% Pt loaded catalyst showed a maximum methane conversion of
15
14% in the initial stage of reaction using 7:1 molar ratio of N2: CH4 feed at 550°C and the activity was
16
further dropped to 8% with increase in the concentration of methane (1:3). However, a maximum
17
methane conversion of 45% was observed over the catalyst when the reaction temperature was increased
18
from 550°C to 700°C under the experimental conditions of N2:CH4 of 7:1 and the reduction temperature
19
of 700°C. Like the unsupported one, the Pt promoted catalyst was also deactivated rapidly with a short
20
duration of time on stream (240 min). Thus from the above discussion, it is clear that the Pd promoted
21
Ni/MgAl2O4 catalysts exhibits significant activity with the aforementioned catalysts.
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A set of Pd promoted catalysts were already reported for methane decomposition [25]. Takenaka
23
et al. [26] studied the promoting effects of Pd on the Ni catalyst and reported that the addition of Pd
24
promoter reduced the deactivation rate of Ni/SiO2 catalyst. Moreover, a bulk amount of carbon was
25
deposited over the catalyst. The increased carbon yield and the improved morphology of the nanocarbon
26
over the Pd-promoted catalyst were attributed to the formation of Pd–Ni alloys. Again Takenaka et al.
27
[27] reported the methane decomposition over monometallic Ni/SiO2, Pd/SiO2 and bimetallic Ni-Pd/SiO2
28
catalysts. According to their report, the addition of Pd to the Ni/SiO2 catalyst enhanced the catalytic
29
activity and stability due to the formation of increased number of facets on the Ni–Pd bimetallic alloys
30
after the carbon precipitation. Besides, they studied the methane decomposition over Ni-, Fe- and Cu-
31
based alumina catalysts and their Pd-based bimetallic catalysts. A maximum methane conversion of 35–
32
50 mol % was obtained over Pd-M/Al2O3 (M - Fe, Co, Ni, Cu, Ag, and Rh) catalysts in the temperature
33
range of 700–900°C and space velocity of 160 l/h g. However, the methane conversion was reduced to 5-
34
10 % during the course of reaction with a streaming time of 3–10 h [28]. Ogihara et al. [29] studied 6 Page 6 of 24
methane decomposition over alumina supported Pd-promoted Ni, Co, Rh, and Fe based bimetallic
2
catalysts. 94% of hydrogen yield was obtained by the Pd-Co/Al2O3 bimetallic catalyst at 850°C. Prasad et
3
al. [30] studied the thermal decomposition of methane over Pd-promoted (5 and 10%) activated carbon.
4
Per their report, high catalytic activity and stability was shown by the 10% Pd-promoted catalyst at 850°C
5
with a volume hourly space velocity of 1.62 l/h g. This is due to the presence of Pd crystals on the
6
catalyst.
ip t
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Fig. 6 shows the XRD patterns of the spent catalysts. The highly intense diffraction peak at the
8
2θ value of 26.1° with (002) plane, represents the formation of crystalline carbon with high graphitization
9
degree. The diffraction peaks centered at the 2θ values of 44.4°, 51.7° and 76.3° were attributed to the
10
formation of metallic phase of Ni by the reduction of NiO after the reaction. All of the other diffraction
11
peaks were attributed to the presence of magnesium aluminate support. The calculated interlayer d-
12
spacing of deposited carbon (2θ = ~26°) was found to be 0.3372 nm and 0.3364 nm for the unpromoted
13
and Pd promoted catalysts respectively and the values are quite close to the reported value of graphitic
14
layers (0.3354 nm), indicating their crystalline quality [31, 32].
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The external morphology of the spent catalysts was studied by FESEM and the images at
16
different magnifications are shown in Fig. 7. From the figures, it is clear that the surface of the catalysts
17
was fully covered with bulk amount of highly interwoven carbon nanotubes. The carbon nanotubes were
18
found to be uniform, thin and long. Due to the interweaving of nanotubes, its actual length determination
19
is found to be quite tiresome and the length goes to several micrometers. The high magnified image of a
20
single nanotube is shown in Fig. 7(d) and it shows that the nanotube is very thin, with a total diameter of
21
~30-35 nm. The TEM photographs shown in Fig. 8 support this observation and found that the carbon
22
nanotubes were multi wall in nature. The wall thickness was measured to be ~16 nm with the internal
23
channel space of around 4-5 nm. The dark spots shown in Fig 8(a, b) represents the presence of Ni/Pd
24
metal species.
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Raman spectroscopy of the deposited carbon was studied to evaluate their crystallinity and
26
graphitization degree. Two distinct Raman bands were observed in the spectrum of each sample as shown
27
in Fig. 9. The first band centered at ~1345 cm−1 (D band) was attributed to the structural imperfections of
28
graphitic carbon [33], since no amorphous carbon was detected in the XRD patterns (Fig. 6). The second
29
band centered at ~ 1570 cm−1 (G band) was assigned to the in plane carbon-carbon stretching vibration of
30
the graphitic layers, indicative of the formation of graphitized multiwall carbon nanotubes [34]. The
31
graphitization degree and crystalline quality of the deposited carbon over the catalysts was assessed in
32
terms of their peak intensity ratio of D band and G band (ID/IG ratio). The ID/IG value was calculated to be
33
1.13 and 1.01 for the unpromoted and Pd promoted Ni/MgAl2O4 catalysts respectively. The values were
34
found to be very small and close to unity, indicating their superior crystalline quality and better 7 Page 7 of 24
1
graphitization degree. However, much higher graphitization degree and crystallinity was observed for the
2
carbon nanotubes deposited over the Pd promoted catalyst.
3
4. Conclusion In summary, the Pd promoted Ni/MgAl2O4 spinel catalysts were synthesized, characterized and
5
its catalytic activity was evaluated for the methane decomposition. The promoter, palladium is found to
6
have an enhanced impact on the catalytic efficiency of Ni/MgAl2O4. The surface area of Ni/MgAl2O4 was
7
considerably increased after Pd deposition and it further improved the fine dispersion of NiO particles on
8
the surface of MgAl2O4 without particle aggregation. A maximum hydrogen yield of ~57% was obtained
9
over the Pd promoted catalyst with high stability for 420 minutes of time on stream. The characterization
10
of the deposited nanocarbon indicated the formation of multiwalled carbon nanotubes with high
11
graphitization degree and crystallinity.
12
Acknowledgements
13
This project is financially supported by Universiti Kebangsaan Malaysia - Yayasan Sime Darby (YSD),
14
under grants PKT 6/2012 and KK-2014-014. The authors would like to acknowledge FST and CRIM,
15
UKM for the analysis of samples. Institut Teknologi Maju (ITMA), Universiti Putra Malaysia (UPM) is
16
acknowledged for providing the Raman analysis.
17
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Figure captions
2
Fig. 1- XRD patterns of the synthesized catalysts
3
Fig. 2- FESEM images of the catalysts (a, b) Ni/MgAl2O4 and (c, d) 0.4 % Pd promoted Ni/MgAl2O4
4
Fig. 3- TEM images of the 0.4% Pd promoted Ni/MgAl2O4 catalyst
5
Fig. 4- TPR profiles of the catalysts (a) Ni/MgAl2O4 and (b) 0.4 % Pd promoted Ni/MgAl2O4
6
Fig. 5- Kinetic curves of methane decomposition over the catalysts
7
Fig. 6- XRD patterns of the nanocarbon over (a) Ni/MgAl2O4 and (b) 0.4 % Pd promoted Ni/MgAl2O4
8
Fig. 7- FESEM photographs of the nanocarbon deposited over the catalysts
9
Fig. 8- TEM images of the deposited multiwalled carbon nanotubes
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Fig. 9- Raman spectra of the nanocarbon over (a) Ni/MgAl2O4 and (b) 0.4% Pd promoted Ni/MgAl2O4
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Graphical Abstract
2
Enhanced catalytic activity of the Pd promoted Ni/MgAl2O4 catalysts for CH4 decomposition into H2 & C
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Highlights Synthesis and characterization of Pd promoted Ni/MgAl2O4 catalysts for methane decomposition
3
Enhanced role of Pd in the properties and catalytic activities of Ni/MgAl2O4 catalysts
4
Production of COx free hydrogen and multi-walled carbon nanotubes
5
Crystalline and structural characterization of multiwalled carbon nanotubes
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Tables
2 Table 1- Real amount of Ni and Pd in the prepared catalysts (ICP analysis) Catalyst
Ni (wt.%)
Pd (wt.%)
Ni/MgAl2O4
15.18
-
Pd-Ni/MgAl2O4
16.63
0.31
15.18 16.94
cr
4
Total metal (wt.%)
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Table 2- BET/BJH results of the as-prepared catalysts
5
Pore size
Pore volume
2
(m /g)
(nm)
(cm3/g)
Ni/MgAl2O4
22.6
23.6
0.152
Pd-Ni/MgAl2O4
29.3
22.8
0.139
an
Surface area
Catalyst
M
6
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