MWCNT for ambient urea synthesis

Accepted Manuscript MWCNT for ambient urea synthesis Noorhana Yahya, Zia Ur Rehman, A'fza Shafie, Bilal al Qasem, Hassan Soleimani, Muhammad Irfan, Saima Qureshi PII:

S0921-4526(18)30169-8

DOI:

10.1016/j.physb.2018.03.005

Reference:

PHYSB 310766

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 4 November 2017 Revised Date:

8 February 2018

Accepted Date: 5 March 2018

Please cite this article as: N. Yahya, Z.U. Rehman, A'. Shafie, B.a. Qasem, H. Soleimani, M. Irfan, S. Qureshi, MWCNT for ambient urea synthesis, Physica B: Physics of Condensed Matter (2018), doi: 10.1016/j.physb.2018.03.005. 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.

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MWCNT for Ambient Urea Synthesis

Noorhana Yahyaa,*, Zia Ur Rehmana,*, A’fza Shafiea, Bilal al Qasema, Hassan

a

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Soleimania, Muhammad Irfana, Saima Qureshia

Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610, Perak, Malaysia

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Elsevier use only: Received date here; revised date here; accepted date here

Abstract

Entangled multiwall carbon nanotubes have been synthesized by means of the floating catalyst technique for ambient urea synthesis. MWCNT were prepared by the spray pyrolysis of ferrocene ethanol mixture at a temperature of 1200ºC and

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atmospheric pressure in the presence of N2 as carrier gas. The X-ray diffraction graph reveals the establishment of hexagonal structure of MWCNT. FE-SEM results show the formation of carbon nanotubes (CNT) with diameter ranging between 26-65 nm. The VSM hysteresis loops depicts that the saturation magnetization values for MWCNT were 1.03 emu/g because of high purity of CNT (99.5%). The nanotubes were used as catalyst for ambient urea synthesis at ambient conditions in the presence

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of unidirectional constant magnetic field. The use of lower flow rate (for better adsorption) and reaction time (to stop reverse reaction) with high magnetic field gives an increased yield of urea because of enhanced triplet harvesting (Zeeman splitting).

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The peak yield of urea, 10118 ppm was accomplished by applying 1.25 T of magnetic field and using 0.25 L/min flow rate for a reaction time of 1 minute.

*

© 2001 Elsevier Science. All rights reserved

Corresponding authors. E-mail: [email protected] (Zia Ur Rehman), [email protected] (Noorhana Yahya). Postal address: Block-20, Level-3, Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610, Perak, Malaysia Phone: +60122176094, +60149705904

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2 Keywords: MWCNT; nanocatalyst; singlet to triplet conversion; ambient urea; yield; magnetic field

1. Introduction

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The quantum size of nanomaterials promise enhanced properties compared to bulk materials [1]. Particularly, one dimensional nanomaterials are vital for catalysis due to high aspect ratio [2, 3]. In cylindrically shaped CNT structure, the aspect ratio is normally high and can reach up to 106. Such a high aspect ratio and σ-π hybridization

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in CNT contributes to better chemical reactivity, thermal and electrical conductance, mechanical strength [4] and fast electron transfer rate [5]. Due to these properties, CNT are used as a promoter and catalyst in chemical

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reactions [6-8]. The entangled structures are more active than straight and ordered CNT structures, hence can be potentially used in catalysis [6]. Furthermore, the applications of CNT are found in the field of electronics [9-11], electrical [12], electro thermal [13] and biomedical sciences [14].

Recently, research efforts have concentrated on metal-free synthesis [15]. This is because metal-based catalyst are energy-consuming, produce greenhouse gasses and waste resources due to lack of selectivity and reduction in activity which results from poisoning [16]. Hence, CNT is preferred over metal-based catalyst in order to provide

disposal [8].

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better activity [16], act as an inexhaustible resource, immune to corrosion and to offer environment-friendly

The multi wall carbon nanotubes (MWCNT) can be synthesized by various methods such as arc discharge

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method [17], laser ablation method [18] and chemical vapor deposition method [19]. The Floating Catalyst Chemical Vapor Deposition (FCCVD) method for CNT synthesis using alcohol with dissolved ferrocene [9, 20]

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is favored because of low cost and high yield [9, 10]. Changes in rate of chemical reactions under an external magnetic field attracted many researchers to study magnetically influenced rates of chemical reactions [21-24]. Studies showed that magnetic field can stimulate the rate of reaction, and the product yield of a chemical reaction [25, 26]. The changes occur because the magnetic field induce transition of singlet to triplet states of reactants which alters the reaction rates [21]. The magnetic field also restrict the mixing of singlet and triplet states and energetically split the triplet sublevels by Zeeman energy, (Ez) [27], hence rate of reaction is modified.

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In this research, a unidirectional constant magnetic field is used to enhance catalytic activity of CNT at ambient conditions. ambient urea was synthesized by triplet harvesting. This was achieved using H2, N2, CO2 mixture and CNT as catalyst in the presence of applied magnetic field. The novelty of work lies in study of the simultaneous effect of these parameters on urea yield at ambient conditions. The objective of the current work is

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to study the simultaneous effect of strength of magnetic field, flow rate of mixture of reactant gases and duration of reaction to enhance urea yield. In the first set of experiments, only strength of magnetic field was varied,

Behnken design.

2. Methodology

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2.1. Adsorption of gases on CNT

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whereas in second step the flow rates, magnetic field strength and duration of reaction were varied under Box-

Calculation of adsorption energy of reactant gasses (H2, N2 and CO2) at the surface of CNT were conducted using adsorption locator module of material studio 6.0. To run simulations for adsorption energy, the structures of

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gas molecules were built and subsequently geometrically optimized using BFGS algorithm of CASTEP [28]. The structure of CNT was retrieved from material studio structures library. Then, adsorption locator was used for

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adsorption of gases on CNT [29]. DMOL3 modules were used to calculate the density of states and band structure. The, binding energy, electronic parameters and energy components were also calculated using DMOL3 module of material studio 6.0.

2.2. Synthesis and characterization of CNT nanocatalysts

The FC-CVD process, used by many researchers

was applied to synthesize carbon nanotubes using a

commercial ferrocene (Fe/C 1:10 ) catalyst from Aldrich with a purity of 98% [20, 30-35]. Whereas thiophene

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with a purity of 99% (Aldrich) was used as a promoter [36-39]. Ethanol with 99% purity (HmbG chemicals) was used as feedstock to synthesize CNT [20, 40-42]. The reason for using ethanol is to produce pure CNT free

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from amorphous carbon due to etching effect of OH radical [43]. To prepare the feedstock catalyst mixture, 0.33 gram of ferrocene, 100 ml of ethanol and 1.2 ml of thiophene were sonicated for 8 minutes for complete dissolution. The CVD system similar to Orbaek et al. [30] with some modification was utilized. In the first step of CNT synthesis, CVD system was flushed with nitrogen gas for an hour to create an inert environment. In the

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second step, a continuous, controlled injection (2 ml/min) of feedstock catalyst mixture and carrier gas (2 L/min) were maintained at the pre-heater (100oC). Afterwards, these vapors were carried by nitrogen carrier gas to a hot

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furnace (1200oC) [44]. Consequently, the catalyst decomposed forming the nanoparticle of iron agglomerated to form larger diameter nanoparticles allowing CNT to grow [45]. Floating catalyst in the hot furnace produces aerogel, which is the first stage of CNT formation. The nucleated CNT is deposited on the walls of hot furnace. The reason for using substrate free approach is to achieve higher yield due to higher volume of reactor compared to the substrate[46, 47]. Afterwards, the CNT aerogel was transferred to the collection box in an inert N2

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environment and sprayed with acetone. The process was terminated after 1 hours and the sample was collected. The CNT was sonicated in acetone ethanol mixture to disperse it for further analysis. Analytical methods were used to probe different features of the CNT catalyst. Field emission scanning electron microscope (FE-SEM, Carl

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Zeiss AG-SUPRA 55VP, Germany) supported with Energy Dispersive X-ray (EDX) and High-Resolution Transmission electron microscopy (HRTEM, Carl Zeiss AG-LIBRA 200FE, Germany) was used to investigate the surface morphology of the CNT. The phase analysis of the CNT was acquired by Powder X-ray Diffractometer

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(XRD, Model: XPert3 Powder, PANalytical). To obtain the magnetic properties of CNT, Vibrating Sample Magnetometer (VSM, Lakeshore-7400, USA) was used.

2.3. Synthesis

of Ambient Urea

Urea was produced using CNT nanocatalyst at ambient conditions under the influence of an external constant magnetic field to increase catalytic activity and triplet conversion. The reactor used for urea synthesis is similar to

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the previous research work [48, 49]. Carbon dioxide, nitrogen and hydrogen gases were mixed in a section of reactor and passed over CNT placed inside the two poles of DC electromagnets producing magnetic field. The

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urea synthesis takes place at ambient conditions of temperature and pressure. In the first set of experiments 0.1, 0.4, 0.7, 1.0, 1.3 and 1.6 T of magnetic field and 0.6 L/min of H2, 02 L/min of N2 and 0.2 L/min of CO2 flow rates were used based on molar ratio of reactants for urea synthesis. The urea was collected for half hour by diluting in acetonitrile. It was then quantified using Agilent Cary 630 Fourier Transform Infrared (FTIR)

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Spectrometer. In the second set of experiments, flow rate between 0.25 to 2.5 L/min, magnetic field from 0 to 2T

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and exposure time from 1 min to 60 min were considered using Box-Behnken Design.

3. Results and Discussion

3.1. Adsorption Locator calculations and CASTEP simulation

Figure 1 shows the iso-surface which reflects the Fermi surface of molecules after adsorption of gases. Red,

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green, pink and violet color were used for CNT, nitrogen, carbon dioxide and hydrogen atoms, respectively. The blue color mesh represents the iso-surface equivalent to Fermi surfaces of respective molecules. The Iso-surface depicts the orientation and interference of wave functions of H2, N2 and CO2 molecules with each other and with

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the inner wall of carbon nanotube. The interference of wave function of molecules with each other is shown. Furthermore, the interaction of combined molecular wave function of gas molecules with the wall of CNT is also

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evident, indicating physisorption of gases. It also implies that these gases can be stored together inside the walls of CNT.

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Fig. 1. Iso surface of CNT after adsorption of gases

Figure 2 shows the band structure of CNT after the adsorption of gases. The down spin (blue) and up spin (red) lines are closely packed together across the ground energy which establishes a band gap of zero electron volts with more density of state and electronic levels. Fermi level (zero electron volt energy) is totally filled with

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electronic energy level hence zero band gap. The electronic band structure of SWCNT 10 unit cells after adsorption of gases reveals that the energy states are numerous and closely packed below the Fermi level. The continuous band structure below the Fermi level is the main cause of zero band gap energy, enhance electron

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transfer between sorbate and sorbent, and overlapping of orbitals.

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Fig. 2. Band Structure of SWNT (6,6) 10 units after adsorption of gases.

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Figure 3(a, b) portrays the variation in partial and total density of states of s, p, d and f because of adsorption of reactant gases through the dotted line (Fermi level). To explore the region across Fermi level, integrated density of states graph was plotted. The main contributor to the energy orbital states above the Fermi level comes

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from p orbital. Which is followed by d and s-orbitals respectively. Even though most of the energy Density Of State (DOS) occupied by s and d-orbital are below the Fermi level. The DOS before and after adsorption follow

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the same trend.

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(a)

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(b)

Fig. 3. Partial density of states of SWNT (6,6) 10 units (a) before adsorption (b) after adsorption.

The IDOS of CNT and CNT + adsorbed gases were obtained and then plotted as a single graph to study the contribution of individual orbitals to the enhancement of IDOS at Fermi level due to adsorption of reactant gases at CNT. Figure 4(a) highlights the total integrated DOS of CNT before and after adsorption of gases. The graph portrays the effect of the adsorption of gases on the total integrated density of states of CNT. Where, the black

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lines correspond to CNT and the red line belongs to CNT with adsorbed gas molecules. The graph depicts that the integrated DOS becomes higher after adsorption of gases.

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In both cases the density of states was existing for both below (-1.0 to 0.0) and above Fermi level (0.0 to +1.0). Throughout the adsorption of gases, the reorganization of electrons occurs between the partially filled orbitals of sorbent and sorbate. The integrated density of state at Fermi level increased from 1438 to 1476 due to adsorption of H2, N2 and CO2 molecule on 240 carbon atoms of CNT. This net increase of 38 states is only

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because of adsorption and the interaction of wave functions of sorbate and sorbent, hence the new states are formed. The enhancement in density of states indicates that adsorption of gases on CNT had enhanced it activity

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for chemical reaction.

Figure 4(b) showcases the integrated DOS of s, p and d sub-orbitals of CNT after adsorption of gases which is higher than its counterpart before adsorption of gases. The graph reveals the consequence of the adsorption of gases on each of the suborbital (s, p, d) of CNT. The black, red and blue lines correspond to partial DOS of s, p and d orbitals of CNT whereas turquoise, pink and greenish yellow line belongs to CNT with adsorbed gas

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molecules. The integrated DOS of CNT shows non zero states of d-orbital before adsorption of gases, which is in line with previous results of partial density of states of pristine armchair CNT [50]. There is a net increase (28 and 23 state) in DOS of p and s orbital after adsorption of gases which increases magnetic moment and hence

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makes CNT more active for chemical reaction. The increase in DOS of p and s orbital indicates electronic rearrangements and the interaction of p and s orbital wave functions with wave functions of adsorbed gases.

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The maximum number of states belongs to p orbital. In carbon electronic configuration, the p orbital

C = 1s , 2 s , 2 p is the outermost, but the C–C bonds of pristine CNT involve sp2 hybridization. The increase in 2

2

2

the integrated density of states for s, p and d after the adsorption of gases as compared to before adsorption was 24, 28, 3 states, respectively. Here the increase in integrated DOS for p is more as compared to s and d orbital because p is the outermost orbital and it interacts more with adsorbed atoms. Increase in s orbital’s integrated density of states also reveals that this orbital is involved in the interaction with adsorbed molecules. Whereas,

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negligible increase in low lying d-orbital’s IDOS indicates no significant contribution in the Fermi region. The contribution of f state was zero in case of CNT, therefore it was omitted.

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The increased density of states (partial and total) at the Fermi level after adsorption of gases (Figure 4) shows the availability of more energy states. When these increased states are subjected to external magnetic field, splitting of energy levels occurs due to Zeeman interactions. Hence singlet to triplet conversion, involved in

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ambient urea synthesis can be increased.

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(b)

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(a)

Fig. 4. Integrated density of states (DOS) before and after adsorption of gases on CNT (a) total (sum) (b) partial density of state of sub-orbitals where (s, p, d) represent before and (s’, p’, d’) after adsorption of gases.

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Table 1 explains the total energy for complete system, adsorption energy and adsorption rate of the reactant gases (N2, H2 and CO2) over CNT catalyst.

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The rate of adsorption energy, (dEa/dNi) of CO2 molecule is -13.45 kcal/mol followed by N2 and H2 molecules -8.32 kcal/mol and -2.57 kcal/mol, respectively. CO2 molecules have preferential adsorption and minimum adsorption (dEa/dNi) energy compared to the other gases which makes it more likely form ambient urea.

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The negative value of average adsorption energy shows that the system emit energy and does not require heat to conduct the reaction. Furthermore, the system is stable after adsorption. It means that this process can take

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place without requirement of external heat at room temperature which is a validation of urea formation. Total

Adsorption

energy

energy

(kcal/mol)

(kcal/mol)

-24.18

(kcal/mol)

(N2)

(H2)

(CO2)

-8.32

-2.57

-13.45

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-19.74

dEa/dNi

Table 1

Adsorption energy evaluation of hydrogen, nitrogen and carbon dioxide gases over CNT nanocatalyst

Simulation was also used to further evaluate the energy components, binding energy and orbitals of molecules

nanocatalyst.

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adsorbed over CNT surfaces which provides further comprehension of the effect of adsorption of gases on the

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Simulations were also performed to understand the change in binding energy, total number of electrons, spins and orbitals that gives deep analysis of activity of catalyst for adsorption. The results of these calculations performed before and after the adsorption of gases are summarized in Table 2. The value of binding energy increased due to adsorption of one molecules each of H2, N2 and CO2 on CNT molecule (consisting of 240 carbon atoms). The number of electrons also increased by 38 due to contribution from the 3 gas molecules adsorbed. An increase in total number of valence orbital is observed, which resulted from the interaction of wave functions of

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gas molecules with the CNT inner walls (Figure 1). The increase in valence orbital is an indication of occurrence of adsorption process. Furthermore, the binding energy reflects the amount of energy that keep the atoms together

CNT. CNT

CNT(6,6) 10 unit cell

(6,6) 10

after adsorption of

unit cell

gases (CO2, H2, N2)

-

-1862.59

1827.98 1440

1478

Spin up =Spin down

709

728

480

484

3360

3440

electrons Empty orbitals

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Total number of electrons

Total number of valence

Table 2

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orbital

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Binding energy (eV)

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in the form of a molecule. Hence change in binding energy clearly indicate the adsorption of some molecule to

Comparison of electronic parameters and orbitals before after adsorption for CNT (6,6) 10 repeat units having 240 carbon atoms

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3.2. Morphological and structural properties of the nanocatalysts

One of the most important characterization methods in evaluating the characteristic of CNT is by

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microstructure investigation as depicted in Figure 5. Figure 5 portrays the FE-SEM micrographs of the CNT nanocatalyst samples obtained by the spray pyrolysis of ethanol ferrocene mixture. It is obvious from the micrograph that the CNT diameter range from 24-65 nm. In CNT, large diameter filaments (48.77 nm) were also observed together with large fused particles. The material mostly consist of large tubes. The micrograph also shows that the diameters of the CNT were not equal, but were uniform along their individual lengths. Iron nanoparticles were found at their tips. A similar morphology

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of CNT were observed by some authors [51-53]. The amount iron nanoparticles could be decreased by reducing the ferrocene content in feedstock catalyst mixture. To verify the composition of the synthesized CNT, FESEM-

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EDX analysis was done. As shown in the micrograph, globular particles found at the tops of the nanotubes are the

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Fe nanoparticles.

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Fig. 5. FESEM micrograph of the CNT (magnification =100 kx).

Figure 6 shows the EDX result of the CNT sample. It reveals the qualitative elemental compositions of the

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sample. EDX is proof that the Fe nanoparticles were well-dispersed. The graph clearly reveals that the synthesized nanocatalyst surface has purity of 93.85% of C whereas the weight percentage of impurities were

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0.13% of Fe, 5.69 % of O2 and 0.32% of S. These impurities arise from ferrocene, ethanol and thiophene respectively.

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Fig. 6. EDX spectra and composition of CNT.

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3.3. Magnetic properties of CNT nanocatalyst

The magnetic properties of CNT nanocatalysts were measured using Lakeshore 7400 series model (up to 3T). The zero net magnetic moment and zero net spin of carbon may lead to diamagnetic behavior of pure carbon structures. But impurity atoms of Fe contribute to some magnetic properties in encapsulated CNT. Figure 7 shows the hysteresis loops plotted using DC magnetization versus applied magnetic field H up to 2 T −2

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measured at room temperature. A relatively low residual magnetization Mrem = 1.9 × 10 , with a coercive force of Hc = 295 Gauss, and the saturation magnetization Ms = 1.08 emu/g was observed. These CNT can be classified as soft magnetic materials due to low coercivity value. The minuteness of the coercive field illustrates

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small domain wall sizes of CNT [54]. Both remnant magnetization, Mrem and saturation magnetization, Ms of the synthesized CNT measured at room temperature are relatively low compared to raw or purified CNT reported

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in the literature measured at 4.5K [55] due to very high purity. The low magnetization reveals that the iron impurities are small and the CNT are mostly clean [56]. The sample prepared has a saturation magnetization of 1.08 (emu/g).

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Fig. 7. VSM hysteresis loops of the CNT.

X-ray diffraction (XRD) analysis was used to probe the crystal structure of nanocatalyst. Figure 8 shows the XRD patterns of the prepared CNT nanocatalyst. For a typical XRD pattern of CNT prepared using

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ethanol/ferrocene mixture, graphite peak exist at 2θ = 26.24o and is intense on (002) with d002 spacing greater than 0.34 nm [34]. As perceived from Figure 8, single phase of hexagonal graphite-2H (JCPDS 75-1621) was observed. Furthermore, three diffraction peaks of carbon at 2 thetas were observed for the dominant planes of

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hexagonal graphite (002), (101) and (102) at 2θ = 26.07o, 43.93o and 50.35o degree respectively [JCPDS no. 411487] [57, 58]. The peaks observed around 26o belongs to (0 0 2) diffractions of graphite [JCPDS 75–1621],

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indicates that the hexagonal graphite structure was well retained [57]. The diffraction pattern also match closely with hexagonal CNT structure [JCPDS 00-058-1638] [59]. The d-spacing of (002) plane of hexagonal carbon nanotube was 0.344 nm, whereas for hexagonal graphite planes (101) and (102) the plane spacing was 0.202 nm, 0.181 nm respectively. No other clear peaks induced by catalyst can be detected in the XRD pattern. The strong and broad peak at 26o designates the highly graphitic nature of the CNT produced. The (0 0 2) peaks in XRD spectrum revealed that the CNT were not well aligned. For well aligned straight nanotubes, (002) peaks vanish

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[60]. The high intensity of peak represents low degree of alignment of CNT in the bundle. The (0 0 2) peak is the dominant peak and its intensity is higher than the intensity of other peaks due to entangled structure of carbon

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nanotubes. In summary, the diffraction peaks, and plane confirm the formation of non-straight CNT. a = 2.4700

Fig. 8. XRD patterns of as synthesized CNT nanocatalyst.

α

= 90°,

β

= 90°,

γ

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Å, b = 2.4700 Å, c = 6.7900 Å,

= 120°]. The synthesized CNT was in the form of mesh

for this feedstock catalyst concentration.

Figure 9 shows the HRTEM images of a CNT.

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Figure 9 (a) shows a low magnification (50kx) of a single CNT. Furthermore, multiple encapsulations were also observed as indicated by black dots, mostly at the tips of carbon nanotubes. Figure 9 (b) shows a High

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Resolution (630 kx) TEM image of a CNT. HRTEM image was taken to observe the morphology of CNT. The alignment of graphitic planes in CNT was observed and the fringe spacing measured was 0.35 nm, which is in closer agreement to the inter-planar spacing of the (002) graphitic plane and concurs with previous studies [34]. The wall spacing of MWCNT ranges from 0.342 to 0.375 nm, and depends on the number of walls and diameter [61]. These results confirm the hexagonal structure of CNT as it exhibits characteristic planes of a hexagonal graphite. The results agree and confirm the XRD results. Furthermore, the number of walls and tube diameter

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were analyzed. HRTEM image confirmed the CNT were multiwall, and the calculated external diameter and number of wall were 17.92 nm and 15, respectively.

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(a)

Fig. 9. (a) TEM images of a single CNT nanocatalyst at magnification 50 kx (b) HRTEM images of the CNT.

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3.4. Synthesis of Ambient Urea at Different Magnetic Field Strength

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FTIR technique was utilized to determine quantity of the ambient urea. The intensity of absorbance was

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measured for experimentally prepared urea and then compared with absorbance of pure commercial urea. The

Fig. 10. FTIR plots of urea synthesized at 0.1 to 1.6 T.

calibration curve for urea obtained from previous work [48] establish a relationship between urea yield (ppm) and

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corresponding absorbance. Figure 10 shows the FTIR absorption peaks for seven samples (ambient urea prepared at 0.1 T to 1.6T). From the figure, the absorbance of the samples changed in the ranges (3300-3700 cm−1), (16001700 cm−1) and (700-1200 cm−1). The infrared absorption bands of urea with frequencies (3333-3436), (1659-

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1676), 1630, 1595, (1464-1492), (1000-1014) and 793 (cm−1), corresponding to NH(stretching), CO(stretching), NH2, NH(bending), CN(bending), CN(stretching), and OCNN groups, respectively, agree with experimental sample spectra [62, 63]. Within the range, 1660 cm-1 to 1690 cm-1 the height of the peak was proportional to the applied magnetic field (0.1 to 2.0 T) during urea synthesis using magnetic induction. This depicts an increase in concentration of ambient urea with application of external magnetic field. This region of interest was zoomed and presented in Figure 10. Which confirms that the urea concentration is affected by magnetic field strength.

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The quantity of urea in the solution was calculated and plotted against the magnetic field. The urea yields were

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calculated from FTIR results. The urea yield variation versus magnetic field strength is shown in the Figure 11. From the graph, it is obvious that the yield varied linearly with B. Equation (1) obtained from curve fitting

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(R2= 0.94) of the data represents the linear relationship of yield to the magnetic field.

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Fig. 11. Plots of urea yield versus applied magnetic field.

Yield = 789(Magnetic field strength) + 218 (1)

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The maximum urea yield 1565 ppm was achieved using magnetic field of 1.6 T. Further experiments are

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required to evaluate the role of magnetic field, flow rate and reaction time on urea yield.

3.5. Optimization of Ambient Urea

The CNTs synthesized using CVD method was used as catalyst to synthesize ambient urea. Optimum urea yield was determined by optimizing the total flow rate (A), magnetic induction (B) and reaction time (C).

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3.5.1. Experimental design

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The response surface methodology is a statistical design of experiments to study the relationships between the factors and the response. The design is also intended to calculate the optimum value of factors corresponding to the highest (optimum) value of urea yield. In RSM, the Box-Behnken [64] was applied to optimize the magnetic induction (Tesla), the flow rate of reactant gasses (L/min) and reaction time (minutes) to optimize urea yield. The

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high, low and middle values ranges chosen were 1, 30.5, 60 min for reaction time, a mixture of reactant gasses flow rate of 0.05, 0.275, 0.5 L/min and magnetic induction of 0, 1.25, 2.5 Tesla. A total of fourteen experimental

a

second

order

model

was

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experiments,

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trials were conducted with 2 center points based on the experimental design (Table 3). In this scheme of selected

with

two

factor

interaction.

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Magnetic

Time

Urea

of reactant

Induction

C-(Minutes)

Concentration

gases

B-(Tesla)

(ppm)

A-(L/min) 2.50

30.50

9508

0.25

1.25

1.00

10118

1.40

2.50

60.00

3379

1.40

1.25

20.50

4521

0.25

1.25

60.00

3048

1.40

0.00

1.00

5702

1.40

2.50

1.40

1.25

1.40

0.00

2.50

0.00

0.25

0.00

2.50

1.25

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3102

30.50

1173

60.00

2984

30.50

3942

30.50

2091

60.00

5938

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Table 3

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0.25

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Flow rate

2.50

2.50

30.50

1856

2.50

1.25

1.00

488

Effect of flow rate, magnetic induction and reaction time on urea yield

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Table 3 depicts the concentration of urea obtained by FTIR for samples from each of the experimental runs. The maximum value of urea yield (10118 ppm) is achieved when the strength of the magnetic field was 1.25 T,

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the flow rate was 0.25 L/min and time was 1 min. This is higher than the urea yield obtained using hematite as catalyst ( 5000 ppm) [49].

Factors and the response were numerically optimized by setting the targets individually. The numerical factors such as flow rate, magnetic induction and time were set by choosing option ‘in range’ whereas yield was set to ‘maximize’, to get the maximum response factor. A number of the optimized suggested solutions were created to get the optimized response.

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3.5.2. Analysis of Variance

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Analysis of Variance (ANOVA) tool was applied to investigate the significance of the parameters and design. It is useful to determine significant terms (linear and interaction) to ensure that the model give close approximation of real system. From ANOVA (Table 4) it is clear that urea yield is affected by the following factors; flow rate and time. The F-value of Model implies that the model is significant. The probability value (p-values) for A, AB

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and AC are less than 0.05 indicate that the factors are significant. The model obtained to predict urea yield is

Source

Sum of squares

Model

8.696×107

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given by Equations 2 in terms of symbolic factors when CNT nanocatalysts were used.

Df

Mean square

F value

p-value Prob.> F

6

1.449×107

5.28

0.0230

1

1.966×10

7

7.17

0.0317

1.221×10

6

0.45

0.5260

1

2.062×10

6

0.75

0.4146

1.966×10

7

1.221×10

6

C-time

2.062×10

6

AB

2.258×107

1

2.258×107

8.23

0.0240

AC

3.919×107

1

3.919×107

14.29

0.0069

BC

2.244×106

1

2.244×106

0.82

0.3958

1.920×10

7

7

2.742×10

6

1.360×10

7

6

2.266×106

0.40

0.8331

5.602×10

6

1

6

1.602×10

8

13

Residual Lack of Fit Pure Error

5.602×10

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Cor Total

1

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B-Magnetic Induction

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A-flow rate

Table 4

Analysis of Variance for urea yield

Yield =+4132.14-1567.55 ×A+390.70 ×B -507.72×C-2375.87×A×B +3130.23 × A×C+748.94 ×B×C

(2)

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where A- flow rate (L/min)

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B-Magnetic field strength (Tesla) C-time (min)

The Equation (2) is the expression for prediction of yield using the input parameter. The result of general

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regression model indicates that the response was negatively influenced by flow rate (A), and the time (C) as well as moderate interaction factors (A×B). Nevertheless, the magnetic induction (B) and the interaction term (A×C)

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had positive influence on the urea yield.

3.5.3. The diagnostic plots

The diagnostic graph was constructed to assess the design and to guarantee that the statistical expectations

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match with analysis data. Moreover, the diagnostic plot was studied to check the acceptable model.

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Figure 12 express the normality graph of the residuals for the response. The straight line mostly touches the residuals, shows the normal distribution of residuals. The graph depicts that the fitted line was close to the

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observed values.

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Fig. 12. The normal percentage probability plot of residuals and for the urea yield.

Figure 13 represents the plot of actual and predicted values for ambient urea yield. In this graph the solid line

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and square boxes represents the predicted and experimental values of the response respectively. The experimental

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values

lie

in

close

proximity

to

the

line,

endorsing

the

precision

of

the

model.

Fig. 13. The predicted versus actual value plot.

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(actual)

Figure 14 represents a plots of predicted urea yield and residuals. The diagram depicts that area between +3.00

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to -3.00 cover all points, confirming that variance is constant and it is a suitable model. Therefore, the predicted model can be used to explain the correlation between variables and experimental response data.

One factor analysis was performed to study the relationship between individual factors (magnetic field, time

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and flow rate) and the response (Urea yield). Figure 15(a) shows the relationship of combined flow rate of reactant gases and the urea yield. From the plot, it is obvious that urea yield decreases with the increase in flow

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rate of gases passing over the CNT catalyst in the reactor. Figure 15(b) represents the increase in urea yield depends on applied magnetic field strength. Likewise, Figure 15(c) depict that the urea yield decrease by increasing reaction time. In all three graphs of one-factor analysis, an approximately direct relationship was observed with the urea yield. Whereas, Figure 15(d) depicts the interaction of the three factors. The effect of two factors on urea yield was predicted in terms of 2D counter plots and 3D response surface plots through the model.

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Fig. 14. Graph of residuals against predicted response.

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Fig. 15. Graphs of effect of individual factors (a) flow rate, (b) magnetic field, (c) duration of reaction conduction, and interaction of factor (d) flow rate, magnetic field and time decency on urea yield.

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Figure 16(a, b) represents response surfaces and contour graphs, respectively for the relationship of the response (urea yield) and the two factors (magnetic field and flow rate) using CNT as catalyst. The urea yield lies above 4000 ppm for all magnetic fields up to 30 min reaction time. Beyond that it drops drastically for magnetic field below 1.2 Tesla and remain high for field above that as seen in Figure 16 (c, d). This is due to increase in the reverse direction of ammonia reaction [65-74], that is, more decomposition of product molecules to reactant specie takes place due to reversible nature of the reaction, hence the urea yield decreases. From the graphs it is

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clear that maximum yield was around 10118 ppm (red region) attained using 0.25 L/min of total flow rate

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(N2=0.05, H2=0.15, 0.05=CO2) of reactant gases, magnetic field of 1.25 Tesla and 1.00-minute time.

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Fig. 16. Graphs of effect of individual factors (a, b) reaction time and flow rate, (c, d) magnetic induction and flow rate on urea yield.

This high yield at the lower flow rate, high magnetic field and shorter reaction time may be credited to improved interaction of gases with catalyst, interconversion of spins under local inhomogeneous magnetic fields or due to different g values corresponding to the reactant gases [75]. The key for increase of urea yield is the

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conversion of singlet states of reactant gases to the triplet. The flipping of electron spins to parallel loosen the bonding force due to repulsion between gas atoms in a molecule which leads to bond weakening and ultimately

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bond breakage. For singlet to be converted to triplet in catalyst, local inhomogeneity of magnetic fields in the catalyst must be present [76, 77]. In this work the CNT nanocatalyst were used under static magnetic field which can be regarded as homogeneous on the reactor dimension. Nevertheless, the magnetic field can be assumed to be inhomogeneous on the individual scale of nanotube because of the diameter and number of walls difference,

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length difference, entanglement, open and close ends, encapsulation and purity of CNT. More induced magnetic field is expected in encapsulated caps compared to clean CNT, due to high induced field and high electron

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density in iron based material [78] . The more entangled CNT has higher inhomogeneity in directionality and lower magnitude of induced magnetic field and vice versa. Therefore, entangled CNT results in dissimilar direction and magnitude of induced magnetic fields, hence the net magnetic field strength is compromised [79]. During the triplet formation, the magnitude of applied magnetic field play a vital role in restricting mixing of these states with singlet states [75]. The wave functions of reactant gases (H2, N2 and CO2) and CNT nanocatalyst

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interact causing weakening of bonds or breakage due to interference effects [80]. In heterogeneous reaction, surface area, pore volume and pore size play vital role in reactivity of catalyst, and catalytic activity is directly related to surface area of catalyst [81]. The Nano size MWCNT produced using CVD with 15 walls has surface

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area (65.62 m2/g) [82], pore volume (0.39 cm3/g) and pore size (25.43 nm) which is higher than 6 m2/g surface

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area of non-porous magnetite [83]. Therefore, using high activity MWCNT nanocatalyst improved response.

4. Conclusion

The MWCNT nanocatalyst was successfully synthesized. Generally, the MWCNT were 26-65 nm in diameter. The influential factors effecting urea yield were the flow rate of reactant gases, strength of magnetic field and reaction time. Two sets of experiments were performed and the difference between their flow rate, magnitude of magnetic field and reaction time was 750 ml/min, 0.35 T and 29 min respectively. In first experiment, the maximum urea yield was 1565 ppm. Whereas in second set of experiments, the maximum yield was 10118 ppm.

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The percentage increase in the ambient urea yield is 546%. The highest urea yield was achieved at 1.25 T magnetic field, 0.25 L/min flow rate and 1 min using CNT as catalyst. The reason for such a drastic increase in

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urea yield was due to enhanced triplet states at these conditions.

Acknowledgments

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The research work has been carried out with the financial support of graduate assistance scheme (cost center 0153AB-C71) of Universiti Technologi PETRONAS and one Baja project of long research grant scheme

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