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Steam reforming of methanol over structured catalysts prepared by electroless deposition of Cu and Zn on anodically oxidized alumina
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Steam reforming of methanol over structured catalysts prepared by electroless deposition of Cu and Zn on anodically oxidized alumina E. Linga Reddy, Hyun Chan Lee, Dong Hyun Kim* Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
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abstract
Article history:
Steam reforming of methanol (SRM) was investigated over a series of porous structured Cu
Received 10 October 2014
eZn/g-Al2O3/Al catalysts. The porous g-Al2O3 layer was synthesized on the Al substrate
Received in revised form
through anodic oxidation in an oxalic acid solution. Cu and Zn in different molar con-
17 December 2014
centrations were loaded using electroless deposition over the prepared g-alumina support.
Accepted 21 December 2014
The synthesized catalysts were characterized using the BET, XRD, SEM, and energy
Available online 14 January 2015
dispersive X-ray analysis. The Cu metal surface area was measured by the selective chemisorption of nitrous oxide. The obtained g-Al2O3/Al (AAO) had a specific surface area
Keywords:
of 27 m2/g, and it was observed that the Al2O3 layer contained well-developed nanopores
Steam reforming
(~60 nm), making it ideal for use as a catalyst support. The results showed that the BET
Methanol
surface area of the catalyst linearly decreased with the CueZn loading. The Cu metal
CueZn/AAO
surface area increased with increasing copper concentration in the deposition bath solu-
Electroless deposition
tion. A fixed tubular reactor was designed and fabricated to evaluate the catalytic activity of the CueZn/AAO catalysts for SRM. Among the catalysts, Cu(0.06)Zn(0.06)/AAO showed 78% MeOH conversion at 350 C. MeOH conversion linearly increased with increasing electroless deposition time from 0.5 to 10 min and appeared to plateau thereafter, indicating that 10 min was the optimal loading time. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, hydrogen production from steam reforming of methanol (SRM) has been highly developed and thoroughly studied for use in fuel cells for mobile applications because of convenience in production, transport, and storage of MeOH [1e3]. The most widely used catalysts for the SRM reaction contain Cu since it has been found to be highly active and
selective for producing hydrogen and CO2, and the reaction is performed at around 200e300 C [2,4e6]. However, many studies have discussed SRM processes with a conventional fixed bed reformer, which has some disadvantages. These include a large volume of the reformer, great drop in pressure, low thermal conductivity, and its startup time being too long for it to be applied to fuel cell systems under existing conditions [7e9]. In recent years, there has been increasing public interest in energy-related issues to
* Corresponding author. Tel.: þ82 53 950 5617; fax: þ82 53 950 6615. E-mail address: [email protected] (D.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.12.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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develop more energy efficient technologies. In addition, the downsizing of systems followed by cost cuts is needed. To meet such requirements, as a different approach from that of the existing particle/powder-based catalyst reformers, a plate-type catalyst has been fabricated to decrease the reformer size and start-up time [10e14]. In this regard, a catalyst supported on a metal monolith has a higher thermal conductivity, as well as improved mechanical durability compared to a particle catalyst [15]. However, it is difficult to form a stable, strong porous catalyst layer on the metal surface due to the weak bonding between the metal and the metal oxide layer, if the metal oxide is coated on the bare metal surface [16]. An alternative to the catalyst coating is to grow a catalyst support layer on the metal surface and then prepare the catalyst on the support by impregnation or coating. An effective way of developing the catalyst support layer on the metal surface is anodic oxidation, which is an electrochemical process that develops the metal oxide over the metal surface. When anodic oxidation is applied to Al metal, it forms a porous Al2O3 layer bonded to the Al metal, which can be an effective catalyst support for the SRM reaction. Anodic oxidation of Al occurs within an electrolyte bath when Al is connected to an electric current source, used as the anode, while graphite is used as the cathode [17,18]. The development of a preparation method that loads catalytic components onto the support is highly important. The objective of the preparation method is to achieve highly efficient and strongly adhesive catalyst deposition onto the substrate. Hence, the loading of catalytic components onto the plate-type structured support requires a method other than those commonly applied for traditional systems. Wash coating [19e23], impregnation [24e27] and immersion methods [28,29] have been developed thus far for many platetype supports. From this perspective, we investigated the electroless deposition technique as a novel method for preparing a structured catalyst for the SRM reaction. A few research groups have reported the use of electroless deposition for plate-type and microtube-type reformer catalysts, including an investigation of their catalytic performance [12,13,30]. Fukuhara and Kawamorita [28] and Zhou et al. [29] have prepared a Cu-based plate-type catalysts on an Al plate by electroless plating. This catalyst has been reported to exhibit high catalytic activity for the SRM and low-temperature CO shift reaction; the main drawback of these experiments was that a bare Al plate was used as the catalyst support. A microtubetype reactor consisting of a Cu-based catalyst, prepared on the inner wall of an Al tube using electroless deposition, was studied for SRM by Fukuhara et al. [30], and the results showed that the microtube-type catalysts prepared in their study had high catalytic performance. However, all these researchers applied an electroless plating technique as the catalyst preparation method to either bare aluminum metal or pure alumina, not to the porous alumina layer formed over aluminum metal. Herein, we have prepared porous anodic Al2O3 (AAO) plates as catalytic support and then applied the method of electroless deposition to deposit Cu and Zn on the plate surface. The activity of these prepared catalysts was studied for the SRM reaction in a tubular reactor.
Experimental Preparation of AAO support AAO templates with nanopores were prepared by an anodization process using freshly prepared oxalic acid solution [31e33]. A high purity (99.999%) Al plate (10 12 cm, 380 mm thick) was first degreased with acetone for 1 h and then subjected to ultrasonic cleaning in distilled water for 10 min. The plate was then annealed at 400 C in air for 3 h, to remove mechanical stresses, and cleaned using acetone followed by distilled water. Subsequently, the plate was anodized in a 0.3 M oxalic acid solution at 40 V (direct current (DC)) at 10 C for 24 h to form a porous g-alumina support layer. A graphite plate was used as cathode against the Al plate. After the formation of the porous g-alumina layer, pore widening treatment (PWT) was carried out by dipping the plate in the same oxalic acid solution used for anodization at 30 C for 4 h. After completion of PWT, the plate was thoroughly washed in distilled water at 25 C to remove any residual oxalic acid, and then the AAO plate was calcined in air at 450 C for 4 h. Thus prepared AAO template thickness was 520 mm, and the galumina layer thickness was 80 mm on both sides of the plate. The prepared AAO plate was cut into four pieces of 5 6 cm.
Preparation of electroless CueZn/AAO catalysts Electroless deposition is an autocatalytic chemical process used to deposit metal particles on a substrate without using an external electric power source. It relies on the generation of an electron by oxidizing a reducing agent. In the electroless plating, a uniform and homogeneous layer is formed even onto the surface of complex configuration, and the cost of the plating is also low. The process consists of the discharge of an electron by the anodic oxidation of the reducing agent which brings ðreducing agent þ OH /oxide þ H2 O þ e Þ about the cathodic reduction of the metal ion (Mnþ þ ne /M), resulting a deposition of the metal onto the substrate surface [16,34,35]. The typical Cu electroless deposition reaction is given below. Cu2þ þ 2HCHO þ 4OH /Cu0 þ H2 þ 2H2 O þ 2HCO 2 By changing the kind and the concentration of the metal ion in the plating solution, a series of catalysts of various compositions can be prepared. The procedure for preparing the CueZn/AAO catalyst by electroless deposition was as follows. All AAO specimens (5 6 cm) were rinsed in distilled water for 10 min. In order to remove the impurities and activate the surface, the Al plate was first immersed in a 3 M HCl solution. Afterwards, the specimens were rinsed with distilled water for over 10 min to prevent contamination of the deposition bath. In simultaneous deposition of Cu and Zn, the specimen was subsequently immersed in the deposition bath that contained Cu and Zn precursors at 30 C for 10 min. The Cu and Zn precursors (CuSO4$5H2O and ZnCl2) were used in 0.06 M concentrations of each, 20 mL/L HCHO (37% aqueous solution), 40 g/L KNaC4H4O6.4H2O, and 10 g/L NaOH. In the posttreatment stage, the samples were rinsed with distilled
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Table 1 e Specific surface area, Cu metal surface area, MeOH conversion, and CO2/DME selectivity of CueZn/AAO catalysts. Catalyst
AAO as prepared Cu0.02 Zn0.1/AAO Cu0.04 Zn0.08/AAO Cu0.06 Zn0.06/AAO Cu0.08 Zn0.04/AAO Cu0.1 Zn0.02/AAO Cu0.12/AAO
SBET (m2/g)
Cu metal surface area (m2/g)
Reaction temperature ( C)
27 18.9 14.6 13.1 14.3 12.8 13
e 0.89 0.96 2.7 2.65 2.91 3.1
e 170e360 170e360 170e360 170e360 170e360 170e360
water at 40 C for 20 min and dried in an oven at 80 C for 12 h [13]. To prepare the catalysts using sequential electroless deposition, the baths for Cu and Zn were prepared separately in appropriate concentrations (Cu and Zn metal concentrations were varied between 0.02 and 0.1 M and the total metal concentration in the deposition solution was fixed at 0.12 M). The specimens were immersed in the baths one after the other to prepare, Zn followed by Cu/AAO or Cu followed by Zn/ AAO catalysts.
Characterization of the support and catalyst Specific surface areas of the prepared catalysts were measured using the BET method (Micromeritics ASAP 2020). XRD patterns of the catalysts were collected on a Rigaku Powder Diffractometer (D/Max-2500) using Cu-Ka radiation in the 2q range of 10e90 . The Cu surface area was measured through N2O dissociation of the catalyst samples [36]. The catalyst (100 mg) was first reduced at 300 C for 2 h, under a 10% H2/He mixture flowing at 50 mL/min. The sample was then cooled to 80 C under He flow, and N2O pulses (100 mL) were injected over 10 min intervals using He as a carrier gas (25 mL/min). The N2 and N2O in the effluent were separated in a Porapak N column (1.8 m, 1/8 inch) at room temperature and analyzed with a TCD detector. The surface profile of the AAO templates and morphology of the CueZn loaded AAO samples were examined using a Field Emission Scanning Electron Microscope (SEM, JEOL, JSM-6500), and the elemental profiles in the same fields analyzed using Energy Dispersion X-rays (EDX, HORIBA EMAX-7000).
Conversion and selectivities at 360 C MeOH conversion (%)
CO2 selectivity (%)
DME selectivity (%)
e 34 68 74 53 40.6 45
e 40 100 100 100 85 63
e 29.9 0 0 0 7.7 18.5
Chromatography (GC, Agilent 6890N) equipped with a TCD detector.
Results and discussion Catalyst characterization The BET specific surface area (SBET) and Cu metal surface area of the samples are shown in Table 1. Before deposition the AAO template (g-Al2O3/Al) surface area was 27 m2/g and after deposition of Cu and Zn, the surface area decreased and the decrease depended on the times of deposition, indicating that Cu and Zn metal particles were well loaded in the alumina pores and lowered the surface areas. The Cu metal surface area increased with an increase of Cu metal concentration in the deposition bath and had a maximum of 3.1 and 2.91 m2/g for Cu(0.12)/AAO and Cu(0.1)Zn(0.02)/AAO, respectively. The dependence of the Cu metal surface area on the Cu content was not so strong as in the case of the Cu(0.04)Zn(0.08)/AAO (0.96 m2/g) and Cu(0.08)Zn(0.04)/AAO (2.65 m2/g) catalysts. The XRD patterns of the CueZn/AAO catalysts are shown in Fig. 1. Most of the diffraction peaks corresponded to the Cu(111) and Zn(101) phases, but those corresponding to the
SRM over the plated catalyst The SRM reaction was carried out at atmospheric pressure using a conventional tubular (1/4 inch quartz tube) flow reactor. The catalyst plate (3.5 g, 5 6 cm) was cut into pieces with a size of 2e3 mm2 and loaded into the reactor. Prior to the SRM experiments, the catalyst was reduced in a 10% H2/N2 stream (100 mL/min) at 250 C for 2 h. The reactor was then cooled down to 160 C in an N2 atmosphere before starting the reaction. Methanol and water were then bubbled into the reactor in the ratio of 2:3, and the reaction temperature was increased from 160 to 350 C at a rate of 0.7 C/min. The feed composition to the reactor was 16.66% CH3OH, 25% H2O and balance He with a total feed flow rate of 150 mL/min. The analysis of the reaction products was carried out using Gas
Fig. 1 e XRD profiles of the prepared CueZn/AAO catalysts.
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Fig. 3 e SEM image of a cross section of a AAO template (a) Before electrodeposition (b) after Cu and Zn electroless deposition (Cu(0.06)Zn(0.06)/AAO), which shows some CueZn catalyst particles are deposited in the pore column of the AAO template.
Fig. 2 e SEM images for AAO support before and after Cu and Zn electroless deposition. (a) AAO template after 24 h anodization, (b) AAO template after pore widening treatment, (c) Cu(0.06)Zn(0.06)/AAO catalyst surface.
Cu(200) and Cu(220) phases were also present. XRD peaks for CuO and ZnO were not detected in any of the samples, most probably due to incorporation of the Cu and Zn metal particles in the spinel lattice. With an increase in Cu loading from 0.02 to 0.12, the diffraction lines from the spinel phase became more intense. Scanning microphotographs of the AAO template and Cu(0.06)Zn(0.06)/AAO catalyst surfaces are shown in
Fig. 2 (a)e(c). Prior to the pore-widening step, the pore width is around 30e40 nm (Fig. 2(a)). With the subsequent pore widening treatment, it has increased to around 60 nm (Fig. 2(b)). Many studies have been done to optimize the pore parameters of AAO. It has been previously reported that the pore wall distance and pore diameter of the AAO can be controlled by changing different anodization parameters such as current density, anodization time, electrolyte temperature and acid concentration [31,33,37]. The diameter of the pores can be adjusted by wet chemical etching without changing the pore density, which is known as “the pore-widening process”. This process involves dissolution of the alumina barrier layer (pore wall) in an acid solution, during which the pore wall distances decreases and as a result, the pore diameter increases. The homogenous loading of CueZn catalyst particles over the AAO template is shown in Fig. 2(c). From the top view of sample SEM image shows that the deposited CueZn catalyst particles covered the pores. Additionally, although the size of deposited CueZn particle was not identical, the particles were of a diameter near or below 100 nm. The SEM image
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Fig. 4 e EDX analysis of the Cu(0.06)Zn(0.06)/AAO catalyst prepared using electroless deposition.
of a cross section of a AAO template before and after the electroless deposition (Cu(0.06)Zn(0.06)/AAO catalyst) is shown in Fig. 3 which confirms the deposition of CueZn catalyst particles in the pores of alumina throughout the template. Elemental peaks detected by EDX for the prepared Cu(0.06)Zn(0.06)/AAO catalyst are shown in Fig. 4. The weight percent (wt%) of the elements detected on the surface layer of AAO-supported catalysts by EDX analysis are indicated in Table 2. As seen in the table, the wt% of the elements changed with the molar concentration of the Cu and Zn precursors in the deposition bath solution; namely, increasing the Cu concentration in the deposition bath solution leads to a higher loading of Cu onto the AAO surface. As a trait of the CueZn/ AAO catalyst, white deposits of Cu and Zn particles (which can be seen in Fig. 2(c) for Cu(0.06)Zn(0.06)/AAO catalyst) appeared in a large number of spots on the surface during drying of the catalyst in air after deposition. We confirmed that the
elemental concentrations of the Cu and Zn over AAO were mostly uniform throughout the catalyst surface.
Effect of sequential and simultaneous deposition on MeOH conversion A catalyst whose catalytic components are based on a combination of Cu, Zn, and alumina exhibits a high performance
Table 2 e Elemental analysis using EDX of the plated surface layers of CueZn/AAO catalysts prepared in different concentrations. Catalyst
Cu0.02 Zn0.1/AAO Cu0.04 Zn0.08/AAO Cu0.06 Zn0.06/AAO Cu0.08 Zn0.04/AAO Cu0.1 Zn0.02/AAO Cu0.12/AAO
Concentration (wt%) (balance oxygen) Cu
Zn
Al
2.13 5.41 7.99 8.62 9.72 12.78
9.05 8.63 8.39 6.01 3.61 0
41.8 40.32 39.27 38.36 42.08 45.89
Fig. 5 e MeOH conversion versus temperature for the catalysts prepared using sequential and simultaneous electroless deposition.
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lower activities for the sequentially deposited catalysts may have been due to the first metal deposition covering the entire AAO surface leaving no empty pores for the second metal. Furthermore, sufficient deposition of the Cu component could not be attained if Cu is plated on the Zn-plated surface or vice versa.
Effect of the Cu and Zn concentrations
for SRM [2,5,13,38]. For the preparation of a plate-type catalyst by electroless deposition, we intended to produce such a catalyst system on an Al substrate. In the first part of this study, MeOH conversion was measured over the catalysts that were prepared through sequential and simultaneous deposition of Cu and Zn on the AAO substrate in the temperatures between 160 and 350 C, and the results are shown in Fig. 5. Amongst the three catalysts, simultaneous deposition of CueZn/AAO showed better conversion at 350 C (74%) than the catalysts prepared through Zn deposition followed by Cu/ AAO (38%) and Cu deposition followed by Zn/AAO (44%). Thus, it indicated that the CueZn/AAO catalyst made from the simultaneous deposition is more active for MeOH conversion than that synthesized using the sequential deposition. The
The MeOH reforming characteristics of the Cu-based catalysts prepared through simultaneous electroless deposition are presented in Fig. 6. The Cu and Zn metal concentrations were varied between 0.02 and 0.1 M and the total metal concentration in the deposition solution was fixed at 0.12 M. The prepared Cu-based catalyst over the structured porous support showed high reforming activity. The reaction started at 160 C and a conversion of 74% was observed over Cu(0.06)Zn(0.06)/AAO catalyst at 350 C. For the Cu(0.1)Zn(0.02)/ AAO, Cu(0.02)Zn(0.1)/AAO, Cu(0.08)Zn(0.04)/AAO and Cu0.12/AAO catalysts, the conversion was low when compared with the Cu(0.06)Zn(0.06)/AAO and Cu(0.04)Zn(0.08)/AAO catalysts, and it was concluded that an equimolar mixture of CueZn metals over AAO showed the highest reforming activity. Selectivity for carbon dioxide (Fig. 7) was high in the reaction temperature range and it was 100% for Cu(0.06)Zn(0.06)/AAO, Cu(0.04)Zn(0.08)/AAO, and Cu(0.08)Zn(0.04)/AAO catalysts at all temperature ranges. For the catalysts with very low Cu content (Cu(0.02)Zn(0.1)/AAO), very high Cu content (Cu(0.1)Zn(0.02)/ AAO), and with pure Cu (Cu(0.12)/AAO), we observed slightly lower CO2 selectivity. The formation of DME with respect to temperature is shown in Fig. 8. The results indicated that DME formation was observed only for the Cu(0.02)Zn(0.1)/AAO, Cu(0.1)Zn(0.02)/AAO and Cu(0.12)/AAO catalysts. The main advantage of plated catalyst preparation is the formation of the CueZn spinel, which is thought to be one of the factors that bring about improved catalytic activity. Hence, the plated catalyst with an equimolar concentration of metal
Fig. 7 e Effect of Cu and Zn content in CueZn/AAO catalysts on CO2 selectivity versus temperature during SRM.
Fig. 8 e Effect of Cu and Zn content in CueZn/AAO catalysts on DME selectivity versus temperature during SRM.
Fig. 6 e Effect of Cu and Zn content in CueZn/AAO catalysts on MeOH conversion versus temperature during SRM. Feed: 150 ml/min (25 mL/min MeOH, 37.5 mL/min H2O and balance N2).
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Fig. 9 e Effect of Cu and Zn deposition time on MeOH conversion versus temperature during SRM. Feed: 150 mL/ min (25 mL/min MeOH, 37.5 mL/min H2O and balance N2).
particles forms a better CueZn spinel over the AAO, leading to higher MeOH conversion and high selectivity to CO2.
Effect of deposition time A series of Cu(0.06)Zn(0.06)/AAO catalysts were prepared through simultaneous electroless deposition by varying the deposition time between 0.5 and 15 min (0.5, 1, 2, 3, 4, 5, 10, 15 min). The corresponding MeOH conversions are shown in Fig. 9. The maximum conversion of 74% was observed at 350 C for both catalysts with 10 and 15 min deposition time, while the remaining catalysts prepared with lower deposition times gave lower MeOH conversions. Therefore, by increasing the deposition time from 0.5 to 10 min, the conversion was increased, and after 10 min of deposition there was negligible effect on MeOH conversion, which indicated that the optimal loading time was 10 min. The SBET values (Table 3) indicated that the deposition of Cu and Zn metal particles over AAO template increased with increasing the deposition time, resulting in lower SBET due to metal deposition into the AAO micropores. This was also confirmed from the Cu and Zn wt% profiles, obtained from EDX elemental analysis, and the wt%
of the metals indicated in Table 3, which shows the Cu and Zn content on the AAO as a function of deposition time. After 0.5 min of deposition, the Cu and Zn wt% was low (0.86 and 1.01, respectively), but after 10 min, the values had increased (7.99 and 8.39, respectively). However, after 15 min, there was little improvement in metal content (8.04 and 8.4, respectively). Evidently, the content of Cu and Zn increased rapidly during the initial stages of deposition, whereas the rate of deposition decreased with increasing deposition time. This suggests that, until a certain loading time, the Cu and Zn metal particles were deposited homogeneously into the AAO pores, and after the maximum distribution of metal particles, further loading time resulted in clusters of CueZn agglomerates and retarded further deposition. The pores of AAO blocked owing to the deposition of CueZn metal particles. The CO2 selectivities for the catalysts with different deposition times are shown in Fig. 10. Higher deposition times resulted in 100% CO2 selectivity, indicating that the AAO surface was well covered by Cu and Zn metal particles. The effect of Cu and Zn deposition time on DME selectivity is shown in Fig. 11. Lower catalyst deposition times resulted in higher DME selectivity, which may have been due to Cu and Zn metal particles not being evenly deposited on the entire AAO surface, exposing a high amount of acidic sites of g-Al2O3. From the experimental data, it seems that the selectivity to CO2 was high and no CO was present in the products. Therefore, the Cu-based catalysts formed on the Al substrate were highly active and selective for the SRM reaction.
Conclusions The catalytic activity of CueZn/AAO for SRM was investigated using a tubular flow reactor. Cu and Zn were deposited over the prepared AAO support using electroless deposition. The catalyst and support were characterized by means of BET, SEM, EDX, XRD, and N2O dissociation studies. The electroless deposition conditions affected the reforming activity of the AAO-supported catalyst; optimizing
Table 3 e Effect of deposition time on Cu and Zn content and surface area of the catalysts. Deposition time (min) 0.5 1 2 3 4 5 10 15
Cu content (wt%)
Zn content (wt%)
Surface area (m2/g)
0.86 2.58 3.27 4.13 5.58 6.21 7.99 8.04
1.01 2.64 3.25 4.64 5.29 6.57 8.39 8.4
25.5 23.27 18.27 16.71 16 14.32 13.1 12.58
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Fig. 10 e Effect of Cu and Zn deposition time on CO2 selectivity versus temperature during SRM.
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Fig. 11 e Effect of Cu and Zn deposition time on DME selectivity versus temperature during SRM.
the process for simultaneous rather than sequential deposition can lead to an improvement in catalytic activity for the SRM reaction. Therefore, the effect of Cu and Zn metal concentration on AAO played an important role in improving MeOH conversion. Among the catalysts examined, Cu(0.06)Zn(0.06)/AAO showed the best MeOH conversion. The effect of CueZn deposition time on AAO also played an important role in improving MeOH conversion and CO2/DME selectivities, due to the variation in the Cu and Zn content over AAO. MeOH conversion linearly increased with increasing electroless deposition time from 0.5 to 10 min and appeared to plateau thereafter, indicating that 10 min was the optimal loading time. It is then noted that, the use of electroless deposition to prepare CueZn/AAO catalysts for SRM provides a useful technique for synthesizing highly functional metallic catalytic systems. The main advantage of a plated catalyst is effective heat exchange and the formation of the CueZn spinel, which brings about the improvement in catalytic activity. The plated catalyst made from an equimolar concentration of metal solution forms a better CueZn spinel over AAO and leads to higher MeOH conversion and high CO2 selectivity.
Acknowledgements This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (2009-0093819).
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Steam reforming of methanol over structured catalysts prepared by electroless deposition of Cu and Zn on anodically oxidized alumina E. Linga Reddy, Hyun Chan Lee, Dong Hyun Kim* Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
article info
abstract
Article history:
Steam reforming of methanol (SRM) was investigated over a series of porous structured Cu
Received 10 October 2014
eZn/g-Al2O3/Al catalysts. The porous g-Al2O3 layer was synthesized on the Al substrate
Received in revised form
through anodic oxidation in an oxalic acid solution. Cu and Zn in different molar con-
17 December 2014
centrations were loaded using electroless deposition over the prepared g-alumina support.
Accepted 21 December 2014
The synthesized catalysts were characterized using the BET, XRD, SEM, and energy
Available online 14 January 2015
dispersive X-ray analysis. The Cu metal surface area was measured by the selective chemisorption of nitrous oxide. The obtained g-Al2O3/Al (AAO) had a specific surface area
Keywords:
of 27 m2/g, and it was observed that the Al2O3 layer contained well-developed nanopores
Steam reforming
(~60 nm), making it ideal for use as a catalyst support. The results showed that the BET
Methanol
surface area of the catalyst linearly decreased with the CueZn loading. The Cu metal
CueZn/AAO
surface area increased with increasing copper concentration in the deposition bath solu-
Electroless deposition
tion. A fixed tubular reactor was designed and fabricated to evaluate the catalytic activity of the CueZn/AAO catalysts for SRM. Among the catalysts, Cu(0.06)Zn(0.06)/AAO showed 78% MeOH conversion at 350 C. MeOH conversion linearly increased with increasing electroless deposition time from 0.5 to 10 min and appeared to plateau thereafter, indicating that 10 min was the optimal loading time. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, hydrogen production from steam reforming of methanol (SRM) has been highly developed and thoroughly studied for use in fuel cells for mobile applications because of convenience in production, transport, and storage of MeOH [1e3]. The most widely used catalysts for the SRM reaction contain Cu since it has been found to be highly active and
selective for producing hydrogen and CO2, and the reaction is performed at around 200e300 C [2,4e6]. However, many studies have discussed SRM processes with a conventional fixed bed reformer, which has some disadvantages. These include a large volume of the reformer, great drop in pressure, low thermal conductivity, and its startup time being too long for it to be applied to fuel cell systems under existing conditions [7e9]. In recent years, there has been increasing public interest in energy-related issues to
* Corresponding author. Tel.: þ82 53 950 5617; fax: þ82 53 950 6615. E-mail address: [email protected] (D.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.12.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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develop more energy efficient technologies. In addition, the downsizing of systems followed by cost cuts is needed. To meet such requirements, as a different approach from that of the existing particle/powder-based catalyst reformers, a plate-type catalyst has been fabricated to decrease the reformer size and start-up time [10e14]. In this regard, a catalyst supported on a metal monolith has a higher thermal conductivity, as well as improved mechanical durability compared to a particle catalyst [15]. However, it is difficult to form a stable, strong porous catalyst layer on the metal surface due to the weak bonding between the metal and the metal oxide layer, if the metal oxide is coated on the bare metal surface [16]. An alternative to the catalyst coating is to grow a catalyst support layer on the metal surface and then prepare the catalyst on the support by impregnation or coating. An effective way of developing the catalyst support layer on the metal surface is anodic oxidation, which is an electrochemical process that develops the metal oxide over the metal surface. When anodic oxidation is applied to Al metal, it forms a porous Al2O3 layer bonded to the Al metal, which can be an effective catalyst support for the SRM reaction. Anodic oxidation of Al occurs within an electrolyte bath when Al is connected to an electric current source, used as the anode, while graphite is used as the cathode [17,18]. The development of a preparation method that loads catalytic components onto the support is highly important. The objective of the preparation method is to achieve highly efficient and strongly adhesive catalyst deposition onto the substrate. Hence, the loading of catalytic components onto the plate-type structured support requires a method other than those commonly applied for traditional systems. Wash coating [19e23], impregnation [24e27] and immersion methods [28,29] have been developed thus far for many platetype supports. From this perspective, we investigated the electroless deposition technique as a novel method for preparing a structured catalyst for the SRM reaction. A few research groups have reported the use of electroless deposition for plate-type and microtube-type reformer catalysts, including an investigation of their catalytic performance [12,13,30]. Fukuhara and Kawamorita [28] and Zhou et al. [29] have prepared a Cu-based plate-type catalysts on an Al plate by electroless plating. This catalyst has been reported to exhibit high catalytic activity for the SRM and low-temperature CO shift reaction; the main drawback of these experiments was that a bare Al plate was used as the catalyst support. A microtubetype reactor consisting of a Cu-based catalyst, prepared on the inner wall of an Al tube using electroless deposition, was studied for SRM by Fukuhara et al. [30], and the results showed that the microtube-type catalysts prepared in their study had high catalytic performance. However, all these researchers applied an electroless plating technique as the catalyst preparation method to either bare aluminum metal or pure alumina, not to the porous alumina layer formed over aluminum metal. Herein, we have prepared porous anodic Al2O3 (AAO) plates as catalytic support and then applied the method of electroless deposition to deposit Cu and Zn on the plate surface. The activity of these prepared catalysts was studied for the SRM reaction in a tubular reactor.
Experimental Preparation of AAO support AAO templates with nanopores were prepared by an anodization process using freshly prepared oxalic acid solution [31e33]. A high purity (99.999%) Al plate (10 12 cm, 380 mm thick) was first degreased with acetone for 1 h and then subjected to ultrasonic cleaning in distilled water for 10 min. The plate was then annealed at 400 C in air for 3 h, to remove mechanical stresses, and cleaned using acetone followed by distilled water. Subsequently, the plate was anodized in a 0.3 M oxalic acid solution at 40 V (direct current (DC)) at 10 C for 24 h to form a porous g-alumina support layer. A graphite plate was used as cathode against the Al plate. After the formation of the porous g-alumina layer, pore widening treatment (PWT) was carried out by dipping the plate in the same oxalic acid solution used for anodization at 30 C for 4 h. After completion of PWT, the plate was thoroughly washed in distilled water at 25 C to remove any residual oxalic acid, and then the AAO plate was calcined in air at 450 C for 4 h. Thus prepared AAO template thickness was 520 mm, and the galumina layer thickness was 80 mm on both sides of the plate. The prepared AAO plate was cut into four pieces of 5 6 cm.
Preparation of electroless CueZn/AAO catalysts Electroless deposition is an autocatalytic chemical process used to deposit metal particles on a substrate without using an external electric power source. It relies on the generation of an electron by oxidizing a reducing agent. In the electroless plating, a uniform and homogeneous layer is formed even onto the surface of complex configuration, and the cost of the plating is also low. The process consists of the discharge of an electron by the anodic oxidation of the reducing agent which brings ðreducing agent þ OH /oxide þ H2 O þ e Þ about the cathodic reduction of the metal ion (Mnþ þ ne /M), resulting a deposition of the metal onto the substrate surface [16,34,35]. The typical Cu electroless deposition reaction is given below. Cu2þ þ 2HCHO þ 4OH /Cu0 þ H2 þ 2H2 O þ 2HCO 2 By changing the kind and the concentration of the metal ion in the plating solution, a series of catalysts of various compositions can be prepared. The procedure for preparing the CueZn/AAO catalyst by electroless deposition was as follows. All AAO specimens (5 6 cm) were rinsed in distilled water for 10 min. In order to remove the impurities and activate the surface, the Al plate was first immersed in a 3 M HCl solution. Afterwards, the specimens were rinsed with distilled water for over 10 min to prevent contamination of the deposition bath. In simultaneous deposition of Cu and Zn, the specimen was subsequently immersed in the deposition bath that contained Cu and Zn precursors at 30 C for 10 min. The Cu and Zn precursors (CuSO4$5H2O and ZnCl2) were used in 0.06 M concentrations of each, 20 mL/L HCHO (37% aqueous solution), 40 g/L KNaC4H4O6.4H2O, and 10 g/L NaOH. In the posttreatment stage, the samples were rinsed with distilled
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Table 1 e Specific surface area, Cu metal surface area, MeOH conversion, and CO2/DME selectivity of CueZn/AAO catalysts. Catalyst
AAO as prepared Cu0.02 Zn0.1/AAO Cu0.04 Zn0.08/AAO Cu0.06 Zn0.06/AAO Cu0.08 Zn0.04/AAO Cu0.1 Zn0.02/AAO Cu0.12/AAO
SBET (m2/g)
Cu metal surface area (m2/g)
Reaction temperature ( C)
27 18.9 14.6 13.1 14.3 12.8 13
e 0.89 0.96 2.7 2.65 2.91 3.1
e 170e360 170e360 170e360 170e360 170e360 170e360
water at 40 C for 20 min and dried in an oven at 80 C for 12 h [13]. To prepare the catalysts using sequential electroless deposition, the baths for Cu and Zn were prepared separately in appropriate concentrations (Cu and Zn metal concentrations were varied between 0.02 and 0.1 M and the total metal concentration in the deposition solution was fixed at 0.12 M). The specimens were immersed in the baths one after the other to prepare, Zn followed by Cu/AAO or Cu followed by Zn/ AAO catalysts.
Characterization of the support and catalyst Specific surface areas of the prepared catalysts were measured using the BET method (Micromeritics ASAP 2020). XRD patterns of the catalysts were collected on a Rigaku Powder Diffractometer (D/Max-2500) using Cu-Ka radiation in the 2q range of 10e90 . The Cu surface area was measured through N2O dissociation of the catalyst samples [36]. The catalyst (100 mg) was first reduced at 300 C for 2 h, under a 10% H2/He mixture flowing at 50 mL/min. The sample was then cooled to 80 C under He flow, and N2O pulses (100 mL) were injected over 10 min intervals using He as a carrier gas (25 mL/min). The N2 and N2O in the effluent were separated in a Porapak N column (1.8 m, 1/8 inch) at room temperature and analyzed with a TCD detector. The surface profile of the AAO templates and morphology of the CueZn loaded AAO samples were examined using a Field Emission Scanning Electron Microscope (SEM, JEOL, JSM-6500), and the elemental profiles in the same fields analyzed using Energy Dispersion X-rays (EDX, HORIBA EMAX-7000).
Conversion and selectivities at 360 C MeOH conversion (%)
CO2 selectivity (%)
DME selectivity (%)
e 34 68 74 53 40.6 45
e 40 100 100 100 85 63
e 29.9 0 0 0 7.7 18.5
Chromatography (GC, Agilent 6890N) equipped with a TCD detector.
Results and discussion Catalyst characterization The BET specific surface area (SBET) and Cu metal surface area of the samples are shown in Table 1. Before deposition the AAO template (g-Al2O3/Al) surface area was 27 m2/g and after deposition of Cu and Zn, the surface area decreased and the decrease depended on the times of deposition, indicating that Cu and Zn metal particles were well loaded in the alumina pores and lowered the surface areas. The Cu metal surface area increased with an increase of Cu metal concentration in the deposition bath and had a maximum of 3.1 and 2.91 m2/g for Cu(0.12)/AAO and Cu(0.1)Zn(0.02)/AAO, respectively. The dependence of the Cu metal surface area on the Cu content was not so strong as in the case of the Cu(0.04)Zn(0.08)/AAO (0.96 m2/g) and Cu(0.08)Zn(0.04)/AAO (2.65 m2/g) catalysts. The XRD patterns of the CueZn/AAO catalysts are shown in Fig. 1. Most of the diffraction peaks corresponded to the Cu(111) and Zn(101) phases, but those corresponding to the
SRM over the plated catalyst The SRM reaction was carried out at atmospheric pressure using a conventional tubular (1/4 inch quartz tube) flow reactor. The catalyst plate (3.5 g, 5 6 cm) was cut into pieces with a size of 2e3 mm2 and loaded into the reactor. Prior to the SRM experiments, the catalyst was reduced in a 10% H2/N2 stream (100 mL/min) at 250 C for 2 h. The reactor was then cooled down to 160 C in an N2 atmosphere before starting the reaction. Methanol and water were then bubbled into the reactor in the ratio of 2:3, and the reaction temperature was increased from 160 to 350 C at a rate of 0.7 C/min. The feed composition to the reactor was 16.66% CH3OH, 25% H2O and balance He with a total feed flow rate of 150 mL/min. The analysis of the reaction products was carried out using Gas
Fig. 1 e XRD profiles of the prepared CueZn/AAO catalysts.
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Fig. 3 e SEM image of a cross section of a AAO template (a) Before electrodeposition (b) after Cu and Zn electroless deposition (Cu(0.06)Zn(0.06)/AAO), which shows some CueZn catalyst particles are deposited in the pore column of the AAO template.
Fig. 2 e SEM images for AAO support before and after Cu and Zn electroless deposition. (a) AAO template after 24 h anodization, (b) AAO template after pore widening treatment, (c) Cu(0.06)Zn(0.06)/AAO catalyst surface.
Cu(200) and Cu(220) phases were also present. XRD peaks for CuO and ZnO were not detected in any of the samples, most probably due to incorporation of the Cu and Zn metal particles in the spinel lattice. With an increase in Cu loading from 0.02 to 0.12, the diffraction lines from the spinel phase became more intense. Scanning microphotographs of the AAO template and Cu(0.06)Zn(0.06)/AAO catalyst surfaces are shown in
Fig. 2 (a)e(c). Prior to the pore-widening step, the pore width is around 30e40 nm (Fig. 2(a)). With the subsequent pore widening treatment, it has increased to around 60 nm (Fig. 2(b)). Many studies have been done to optimize the pore parameters of AAO. It has been previously reported that the pore wall distance and pore diameter of the AAO can be controlled by changing different anodization parameters such as current density, anodization time, electrolyte temperature and acid concentration [31,33,37]. The diameter of the pores can be adjusted by wet chemical etching without changing the pore density, which is known as “the pore-widening process”. This process involves dissolution of the alumina barrier layer (pore wall) in an acid solution, during which the pore wall distances decreases and as a result, the pore diameter increases. The homogenous loading of CueZn catalyst particles over the AAO template is shown in Fig. 2(c). From the top view of sample SEM image shows that the deposited CueZn catalyst particles covered the pores. Additionally, although the size of deposited CueZn particle was not identical, the particles were of a diameter near or below 100 nm. The SEM image
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Fig. 4 e EDX analysis of the Cu(0.06)Zn(0.06)/AAO catalyst prepared using electroless deposition.
of a cross section of a AAO template before and after the electroless deposition (Cu(0.06)Zn(0.06)/AAO catalyst) is shown in Fig. 3 which confirms the deposition of CueZn catalyst particles in the pores of alumina throughout the template. Elemental peaks detected by EDX for the prepared Cu(0.06)Zn(0.06)/AAO catalyst are shown in Fig. 4. The weight percent (wt%) of the elements detected on the surface layer of AAO-supported catalysts by EDX analysis are indicated in Table 2. As seen in the table, the wt% of the elements changed with the molar concentration of the Cu and Zn precursors in the deposition bath solution; namely, increasing the Cu concentration in the deposition bath solution leads to a higher loading of Cu onto the AAO surface. As a trait of the CueZn/ AAO catalyst, white deposits of Cu and Zn particles (which can be seen in Fig. 2(c) for Cu(0.06)Zn(0.06)/AAO catalyst) appeared in a large number of spots on the surface during drying of the catalyst in air after deposition. We confirmed that the
elemental concentrations of the Cu and Zn over AAO were mostly uniform throughout the catalyst surface.
Effect of sequential and simultaneous deposition on MeOH conversion A catalyst whose catalytic components are based on a combination of Cu, Zn, and alumina exhibits a high performance
Table 2 e Elemental analysis using EDX of the plated surface layers of CueZn/AAO catalysts prepared in different concentrations. Catalyst
Cu0.02 Zn0.1/AAO Cu0.04 Zn0.08/AAO Cu0.06 Zn0.06/AAO Cu0.08 Zn0.04/AAO Cu0.1 Zn0.02/AAO Cu0.12/AAO
Concentration (wt%) (balance oxygen) Cu
Zn
Al
2.13 5.41 7.99 8.62 9.72 12.78
9.05 8.63 8.39 6.01 3.61 0
41.8 40.32 39.27 38.36 42.08 45.89
Fig. 5 e MeOH conversion versus temperature for the catalysts prepared using sequential and simultaneous electroless deposition.
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lower activities for the sequentially deposited catalysts may have been due to the first metal deposition covering the entire AAO surface leaving no empty pores for the second metal. Furthermore, sufficient deposition of the Cu component could not be attained if Cu is plated on the Zn-plated surface or vice versa.
Effect of the Cu and Zn concentrations
for SRM [2,5,13,38]. For the preparation of a plate-type catalyst by electroless deposition, we intended to produce such a catalyst system on an Al substrate. In the first part of this study, MeOH conversion was measured over the catalysts that were prepared through sequential and simultaneous deposition of Cu and Zn on the AAO substrate in the temperatures between 160 and 350 C, and the results are shown in Fig. 5. Amongst the three catalysts, simultaneous deposition of CueZn/AAO showed better conversion at 350 C (74%) than the catalysts prepared through Zn deposition followed by Cu/ AAO (38%) and Cu deposition followed by Zn/AAO (44%). Thus, it indicated that the CueZn/AAO catalyst made from the simultaneous deposition is more active for MeOH conversion than that synthesized using the sequential deposition. The
The MeOH reforming characteristics of the Cu-based catalysts prepared through simultaneous electroless deposition are presented in Fig. 6. The Cu and Zn metal concentrations were varied between 0.02 and 0.1 M and the total metal concentration in the deposition solution was fixed at 0.12 M. The prepared Cu-based catalyst over the structured porous support showed high reforming activity. The reaction started at 160 C and a conversion of 74% was observed over Cu(0.06)Zn(0.06)/AAO catalyst at 350 C. For the Cu(0.1)Zn(0.02)/ AAO, Cu(0.02)Zn(0.1)/AAO, Cu(0.08)Zn(0.04)/AAO and Cu0.12/AAO catalysts, the conversion was low when compared with the Cu(0.06)Zn(0.06)/AAO and Cu(0.04)Zn(0.08)/AAO catalysts, and it was concluded that an equimolar mixture of CueZn metals over AAO showed the highest reforming activity. Selectivity for carbon dioxide (Fig. 7) was high in the reaction temperature range and it was 100% for Cu(0.06)Zn(0.06)/AAO, Cu(0.04)Zn(0.08)/AAO, and Cu(0.08)Zn(0.04)/AAO catalysts at all temperature ranges. For the catalysts with very low Cu content (Cu(0.02)Zn(0.1)/AAO), very high Cu content (Cu(0.1)Zn(0.02)/ AAO), and with pure Cu (Cu(0.12)/AAO), we observed slightly lower CO2 selectivity. The formation of DME with respect to temperature is shown in Fig. 8. The results indicated that DME formation was observed only for the Cu(0.02)Zn(0.1)/AAO, Cu(0.1)Zn(0.02)/AAO and Cu(0.12)/AAO catalysts. The main advantage of plated catalyst preparation is the formation of the CueZn spinel, which is thought to be one of the factors that bring about improved catalytic activity. Hence, the plated catalyst with an equimolar concentration of metal
Fig. 7 e Effect of Cu and Zn content in CueZn/AAO catalysts on CO2 selectivity versus temperature during SRM.
Fig. 8 e Effect of Cu and Zn content in CueZn/AAO catalysts on DME selectivity versus temperature during SRM.
Fig. 6 e Effect of Cu and Zn content in CueZn/AAO catalysts on MeOH conversion versus temperature during SRM. Feed: 150 ml/min (25 mL/min MeOH, 37.5 mL/min H2O and balance N2).
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Fig. 9 e Effect of Cu and Zn deposition time on MeOH conversion versus temperature during SRM. Feed: 150 mL/ min (25 mL/min MeOH, 37.5 mL/min H2O and balance N2).
particles forms a better CueZn spinel over the AAO, leading to higher MeOH conversion and high selectivity to CO2.
Effect of deposition time A series of Cu(0.06)Zn(0.06)/AAO catalysts were prepared through simultaneous electroless deposition by varying the deposition time between 0.5 and 15 min (0.5, 1, 2, 3, 4, 5, 10, 15 min). The corresponding MeOH conversions are shown in Fig. 9. The maximum conversion of 74% was observed at 350 C for both catalysts with 10 and 15 min deposition time, while the remaining catalysts prepared with lower deposition times gave lower MeOH conversions. Therefore, by increasing the deposition time from 0.5 to 10 min, the conversion was increased, and after 10 min of deposition there was negligible effect on MeOH conversion, which indicated that the optimal loading time was 10 min. The SBET values (Table 3) indicated that the deposition of Cu and Zn metal particles over AAO template increased with increasing the deposition time, resulting in lower SBET due to metal deposition into the AAO micropores. This was also confirmed from the Cu and Zn wt% profiles, obtained from EDX elemental analysis, and the wt%
of the metals indicated in Table 3, which shows the Cu and Zn content on the AAO as a function of deposition time. After 0.5 min of deposition, the Cu and Zn wt% was low (0.86 and 1.01, respectively), but after 10 min, the values had increased (7.99 and 8.39, respectively). However, after 15 min, there was little improvement in metal content (8.04 and 8.4, respectively). Evidently, the content of Cu and Zn increased rapidly during the initial stages of deposition, whereas the rate of deposition decreased with increasing deposition time. This suggests that, until a certain loading time, the Cu and Zn metal particles were deposited homogeneously into the AAO pores, and after the maximum distribution of metal particles, further loading time resulted in clusters of CueZn agglomerates and retarded further deposition. The pores of AAO blocked owing to the deposition of CueZn metal particles. The CO2 selectivities for the catalysts with different deposition times are shown in Fig. 10. Higher deposition times resulted in 100% CO2 selectivity, indicating that the AAO surface was well covered by Cu and Zn metal particles. The effect of Cu and Zn deposition time on DME selectivity is shown in Fig. 11. Lower catalyst deposition times resulted in higher DME selectivity, which may have been due to Cu and Zn metal particles not being evenly deposited on the entire AAO surface, exposing a high amount of acidic sites of g-Al2O3. From the experimental data, it seems that the selectivity to CO2 was high and no CO was present in the products. Therefore, the Cu-based catalysts formed on the Al substrate were highly active and selective for the SRM reaction.
Conclusions The catalytic activity of CueZn/AAO for SRM was investigated using a tubular flow reactor. Cu and Zn were deposited over the prepared AAO support using electroless deposition. The catalyst and support were characterized by means of BET, SEM, EDX, XRD, and N2O dissociation studies. The electroless deposition conditions affected the reforming activity of the AAO-supported catalyst; optimizing
Table 3 e Effect of deposition time on Cu and Zn content and surface area of the catalysts. Deposition time (min) 0.5 1 2 3 4 5 10 15
Cu content (wt%)
Zn content (wt%)
Surface area (m2/g)
0.86 2.58 3.27 4.13 5.58 6.21 7.99 8.04
1.01 2.64 3.25 4.64 5.29 6.57 8.39 8.4
25.5 23.27 18.27 16.71 16 14.32 13.1 12.58
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Fig. 10 e Effect of Cu and Zn deposition time on CO2 selectivity versus temperature during SRM.
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Fig. 11 e Effect of Cu and Zn deposition time on DME selectivity versus temperature during SRM.
the process for simultaneous rather than sequential deposition can lead to an improvement in catalytic activity for the SRM reaction. Therefore, the effect of Cu and Zn metal concentration on AAO played an important role in improving MeOH conversion. Among the catalysts examined, Cu(0.06)Zn(0.06)/AAO showed the best MeOH conversion. The effect of CueZn deposition time on AAO also played an important role in improving MeOH conversion and CO2/DME selectivities, due to the variation in the Cu and Zn content over AAO. MeOH conversion linearly increased with increasing electroless deposition time from 0.5 to 10 min and appeared to plateau thereafter, indicating that 10 min was the optimal loading time. It is then noted that, the use of electroless deposition to prepare CueZn/AAO catalysts for SRM provides a useful technique for synthesizing highly functional metallic catalytic systems. The main advantage of a plated catalyst is effective heat exchange and the formation of the CueZn spinel, which brings about the improvement in catalytic activity. The plated catalyst made from an equimolar concentration of metal solution forms a better CueZn spinel over AAO and leads to higher MeOH conversion and high CO2 selectivity.
Acknowledgements This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (2009-0093819).
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