Al2O3 catalysts for enhanced hydrogen production

international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Ethanol CO2 reforming on La2O3 and CeO2promoted Cu/Al2O3 catalysts for enhanced hydrogen production Mohd-Nasir Nor Shafiqah a, Hai Nguyen Tran b, Trinh Duy Nguyen c, Pham T.T. Phuong d, Bawadi Abdullah e, Su Shiung Lam f, Phuong Nguyen-Tri g, Ravinder Kumar h, Sonil Nanda i, Dai-Viet N. Vo c,* a

Faculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300, Gambang, Kuantan, Pahang, Malaysia b Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City, 700000, Viet Nam c Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City, 755414, Viet Nam d Institute of Chemical Technology, Vietnam Academy of Science and Technology, 1 Mac Dinh Chi Str., Dist. 1, Ho Chi Minh City, Viet Nam e Biomass Processing Laboratory, Centre for Biofuel and Biochemical Research, Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia f Pyrolysis Technology Research Group, Eastern Corridor Renewable Energy Group, School of Ocean Engineering, Universiti Malaysia Terengganu, 21030, Kuala Nerus, Terengganu, Malaysia g Department of Chemistry, University of Montreal, Montreal, Quebec, Canada h Department of Environmental Sciences, Macquarie University, Sydney, NSW, 2109, Australia i Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada

highlights

graphical abstract


3%Lae10%Cu/Al2O3 exhibited high C2H5OH (87.6%) and CO2 (55.1%) conversions.
Promoter

addition

reduced

C2H5OH activation energy from 53.29 to 47.05 kJ mol1.
CeO2 and La2O3 hindered carbon deposition from 40.04% to 27.55%.

article info

abstract

Article history:

3%Ce- and 3%La-promoted 10%Cu/Al2O3 catalysts were synthesized via a sequential

Received 12 April 2019

incipient wetness impregnation approach and implemented for ethanol CO2 reforming

Received in revised form

(ECR) at 948e1023 K and stoichiometric feed ratio. CeO2 and La2O3 promoters reduced CuO

22 September 2019

crystallite size from 32.4 to 27.4 nm due to diluting impact and enhanced the degree of

* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (D.-V.N. Vo). https://doi.org/10.1016/j.ijhydene.2019.10.024 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

2

international journal of hydrogen energy xxx (xxxx) xxx

Accepted 2 October 2019

reduction of CuO / Cu0. Irrespective of reaction temperature, 3%Lae10%Cu/Al2O3

Available online xxx

exhibited the highest reactant conversions, H2 and CO yields followed by 3%Cee10% Cu/Al2O3 and 10%Cu/Al2O3. The greatest C2H5OH and CO2 conversions of 87.6% and 55.1%,

Keywords:

respectively were observed on 3%Lae10%Cu/Al2O3 at 1023 K whereas for all catalysts,

Ethanol CO2 reforming

H2/CO ratios varying from 1.46 to 1.91 were preferred as feedstocks for Fischer-Tropsch

CeO2

synthesis. Activation energy for C2H5OH consumption was also reduced with promoter

La2O3

addition from 53.29 to 47.05 kJ mol1. The thorough CuO / Cu0 reduction by H2 activation

Cu-based catalysts

was evident and the Cu0 active phase was resistant to re-oxidation during ECR for all

Syngas

samples. Promoters addition reduced considerably the total carbon deposition from 40.04%

Hydrogen

to 27.55% and greatly suppressed non-active graphite formation from 26.94% to 4.20% because of their basic character and cycling redox enhancement. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The extensive dependence on non-renewable fossil fuels to fulfil rising energy demands in industrial production and transportation leads to non-energy security and environmental concerns such as greenhouse gas emissions, global warming and air pollution. Hence, exploring environmentally friendly and sustainable alternatives for petroleum resources is highly desired. Synthesis gas (so-called as syngas, a CO and H2 mixture) appears as an alluring solution since it can be implemented as an intermediate feedstock for a wide range of downstream green fuels production (via Fischer-Tropsch synthesis [1e3] and methanol generation [4]) and other vital petrochemicals such as methyl tert-butyl ether (MTBE) and dimethyl ether (DME) [5]. Apart from being used as a buildingblock reactant, separated H2 can be employed as a green and standalone fuel for combustion and fuel cells [6,7]. Although methane steam reforming, methane partial oxidation and methane autothermal reforming are presently the most common and economical processes for large-scale syngas production [8,9], the major setbacks of these technologies are the utilization of unsustainable natural gas and significant CO2 emission, a main factor causing global warming. Ethanol CO2 reforming, ECR (see Eq. (1)) is regarded as an eco-friendly approach for syngas production and has recently received substantially industrial and academic attentions because it consumes unwanted CO2 greenhouse gas and utilizes a sustainable, low-cost and non-toxic ethanol feedstock [10] to yield more valuable products. The other benefits of ECR are that ethanol is a high hydrogen-containing compound and can be easily derived from renewable biomass resources [11e13]. C2 H5 OH þ CO2 / 3CO þ 3H2

1

ðDHrxn ¼ 296:7 kJ mol Þ

activity and stability for 70 h at 973 K [14]. Da Silva et al. used Rh/CeO2 for ECR at 1073 K and observed great C2H5OH and CO2 conversions of about 99% and 88%, respectively [15]. Supported transition metals including Ni [16,17], Co [18] and Cu [19,20], recently appeared as appealing catalysts for ECR because of their abundant availability, cost-effectiveness, great capability for CeC bond rupture and relatively equivalent performance to precious metals [21e23]. Recently, Cao et al. employed a facile co-precipitation approach to prepare Cu/Ce0.8Zr0.2O2 for ECR and reported an excellent catalytic stability for 90 h because of strong interaction between metal and support [19]. The influence of support types, namely, CeO2, ZrO2 and CeO2eZrO2 on Cu catalyst during ECR was also examined by Cao et al. and they found that CuCeZr was the best catalyst in terms of C2H5OH conversion [20]. Nevertheless, Cu-based catalysts tend to be deteriorated at high reaction temperature initiated by the excessive amounts of carbon formation on Cu metal active site [24]. One of the most efficient approaches for suppressing carbonaceous deposits is promoter implementation. However, there is currently no available study regarding the promotional effect on Cu-based catalysts in terms of reducing carbon deposition for ECR. On the other hand, lanthanide metal oxides, namely, La2O3 and CeO2 reportedly prevented carbonaceous species formation during reforming processes including ethanol steam reforming [25] and methane CO2 reforming [26] owing to their excellent oxygen storage-release capability and redox attributes. These dopants could also benefit the performance of supported Cu catalyst in ECR. Hence, the aims of this paper are to determine the impact of La2O3 and CeO2 promoters on Cu/Al2O3 at varied reaction temperatures in terms of catalytic attributes and performance.

(1)

The noble metal catalysts, namely, Ir [14] and Rh [15] supported on CeO2 were investigated for ECR and exhibited high reactant conversions and stability with time-on-stream although they are not appropriate for industrial scale production due to their high cost and low abundance. Hou et al. found that the strong IreCeO2 interaction could inhibit Ir sintering and carbon deposits, thereby enhancing catalytic

Experimental Catalyst preparation 10%Cu/Al2O3 was prepared by incipient wetness impregnation (IWI) technique, whereas 3%Ce- and 3%La-promoted 10% Cu/Al2O3 samples were synthesized using sequential IWI

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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method. The g-Al2O3 (Puralox TH 100/150, Sasol, Hamburg, Germany) support was calcined in Carbolite furnace (Bemaford, Sheffield, UK) for 6 h at 1023 K and 5 K min1. For the IWI procedure, the calcined g-Al2O3 was soaked in the sufficiently measured volume of Cu(NO3)2.3H2O (Sigma-Aldrich, St. Louis, Missouri, US) aqueous solution. Thereafter, the mixture was ¨ CHI R-200 rotary evaporator (BU ¨ CHI Laborstirred in a BU technik AG, Switzerland) at 323 K for 3 h. After being subsequently dried overnight in UFB-500 oven (Memmert GmbH, Schwabach, Germany) at 393 K, solid sample was further calcined in air for 5 h, 5 K min1 at 873 K. 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 were prepared via implementation of the above-mentioned IWI method, in which as-prepared 10% Cu/Al2O3 powder was further impregnated with calculated quantities of corresponding Ce(NO3)3.6H2O and La(NO3)3.6H2O (Merck KGaA, Darmstadt, Germany) promoter solutions. All catalysts were crushed using pestle and mortar followed by sieving to small and uniform particle size (125e160 mm) before packing inside the catalytic reactor. For comparison purpose, 10%Cu/CeO2 and 10%Cu/La2O3 were also synthesized by an analogous method previously employed for 10%Cu/Al2O3. Additionally, CeO2 and La2O3 supports were pretreated at the same conditions for g-Al2O3.

Catalyst characterization techniques Textural features for catalysts such as Brunauer-EmmettTeller (BET) surface area, mean pore diameter and total pore volume were quantified in a Micromeritics ASAP-2020 apparatus (Norcross, Georgia, US) using experimental data from N2 adsorption-desorption isotherms at 77 K. Before analysis, the specimen was thermally pretreated for 1 h with flowing N2 at 573 K in order to confiscate any moisture or adsorbed impurity molecules on sample surface. The Micromeritics V.3.04 software was used to interpret the above-mentioned physical attributes. X-ray diffraction (XRD) was implemented to examine crystalline phases and quantify mean crystallite size of the catalysts. XRD was conducted by means of monochromatic Cu X-ray (wavelength, l ¼ 1.5418
A at 15 mA and 30 kV) radiation on Rigaku Miniflex II (Akishima-shi, Tokyo, Japan) unit. The powder XRD patterns at 2q ¼ 3 e80 were recorded with a step increment of 0.02 whereas the scan speed of 1 min1 was used for each analysis. The XRD data were analyzed with the aid of Match! software version 3.6.2. Hydrogen temperature-programmed reduction (H2-TPR) was conducted on a Micromeritics AutoChem II-2920 (Norcross, Georgia, US) chemisorption system to scrutinize the relationship between catalytic reducibility and temperature. After heat pretreatment to remove moisture and impurities for 30 min at 373 K with flowing He (50 ml min1), sample (roughly 0.1 g) layered by quartz wool in a quartz U-tube was reduced under 10%H2/Ar (50 ml min1) atmosphere at 373e1173 K with 10 K min1 heating rate. Before cooling to ambient temperature under N2, the sample was maintained at 1173 K for 30 min to assure thorough reduction. CO2 temperature-programmed desorption analyses (CO2-TPD) for basicity were conducted on a Micromeritics AutoChem II-2920 (Norcross, Georgia, US) chemisorption system. The sample was pretreated to remove moisture by

3

purging He flow for 60 min at 773 K. Then, specimen was reduced in flowing 10%H2/N2 (50 ml min1) at 973 K for 30 min. The sample was subsequently cooled down to 423 K under N2 flow and maintained at this temperature for 30 min followed by flushing with CO2 flow for 1 h. Helium gas was purged into U-tube to remove excess CO2 gas for 30 min followed by heating process to 1073 K with 10 K min1 for CO2 desorption measurement by thermal conductivity detector. The same flow rate of 50 ml min1 was used for all stages. Temperature-programmed oxidation (TPO) was applied for measuring carbon deposition and inspecting carbon type formed on used catalysts. TPO analyses were performed on thermogravimetric TGA Q500 unit (TA Instruments, New Castle, Delaware, US). Sample powder (about 6.5 mg) placed on ceramic crucible was initially dehydrated by exposure to purging N2 for 0.5 h at 373 K. Amounts of carbonaceous species were quantified by heating up sample under 20%O2/N2 at 373e1023 K with 10 K min1 ramping rate followed by 30 min of isothermal oxidation in the aforementioned oxidizing mixture. To investigate the morphology of deposited carbon and catalysts after ECR, high-resolution transmission electron microscopy, HRTEM (EM-002B, TOPCOM, Tokyo, Japan) was employed with acceleration voltage at 200 kV. A droplet of ethanol-dispersed samples was poured on to Cu micro-grid layered with a thin and holey carbon film. Prior to HRTEM measurement, ethanol solvent was evaporated in vacuum.

Ethanol CO2 reforming procedure ECR was performed in a fixed-bed rig (stainless steel SS316, outer diameter: 3/8 inch and length: 17 inches) positioned in tubular furnace at 4 different temperatures (including 948, 973, 998 and 1023 K) and stoichiometric feed ratio, FCO2 : FC2 H5 OH : FN2 ¼ 1 : 1 : 3 (Fi: molar flow rate, mol s1 and N2: diluent gas). A small amount of quartz wool was used to fix 0.1 g of sample in the middle of tubular rig. Before ECR reaction was initiated, mounted sample was activated in situ by 50% H2/N2 reducing agent (50 ml min1) at 973 K for 2 h. The highly precise syringe pump (KellyMed KL-602, Beijing, China) was employed to inject ethanol reactant into reactor whereas diluent gas (N2) and CO2 were monitored through mass flow controllers (Alicat Scientific, Tucson, Arizona, US). For 8 h of reaction time, total flow rate was sustained at 70 ml min1 and 1 gas hourly space velocity, GHSV was 42 L g1 cat h . Gas chromatograph (Agilent GC 6890, Agilent, Santa Clara, California, US) was used to determine the molar composition of reactants and products implementing thermal conductivity detector (TCD). In order to verify the accuracy of GC analysis and experimental works, carbon balance was estimated with a small error bar of 4.5e5.7%. The blank test was also conducted for ECR without catalyst presence at 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1. As seen in Fig. S1 (Supplementary data), C2H5OH and CO2 conversions had relatively low values of about 14.31% and 7.80%, correspondingly in the absence of catalyst during ECR due to unavoidable C2H5OH decomposition at high temperature. Nevertheless, Fig. S2 (see Supplementary data) shows C2H5OH and CO2 conversions were significantly enhanced up to 53.19% and 37.23%,

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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respectively with 10%Cu/Al2O3 presence. Hence, the improvement of ECR catalytic performance by using Cu-based catalysts in this study was credited to the intrinsically physiochemical attributes of utilized catalysts.

Result and discussion Textural properties Table 1 represents BET surface area and features of porous structure, namely, total pore volume and average pore diameter for all fresh samples. N2-physisorption analyses on all three catalysts show mild changes in BET area according to this trend; 10%Cu/Al2O3 (98.5 m2 g1) > 3%Cee10%Cu/Al2O3 (95.7 m2 g1) > 3%Lae10%Cu/Al2O3 (93.4 m2 g1). The inevitable and trivial drop in BET area and total pore volume evidenced with Ce or La promotion could be indicative of successful metal distribution on catalyst surface with minimal pore blockage.

X-ray diffraction analyses Fig. 1 displays XRD analyses for g-Al2O3 and catalysts. Apart from implementing database of the Joint Committee on Powder Diffraction Standards (JCPDS) for clarifying all XRD patterns [28] the XRD spectrum of calcined g-Al2O3 was also conducted and employed as a reference for interpretation. As illustrated in Fig. 1, diffraction peaks for g-Al2O3 were recorded at 32.67 , 37.34 , 45.65 and 67.02 (JCPDS sheet No. 04e0858) for all specimens [28]. Additionally, CuO phase arising from Cu(NO3)2 decomposition in air-calcination was detected at 2q of 32.53 , 35.54 , 38.75 , 48.75 , 58.36 , 61.57 and 75.28 (JCPDS sheet No. 41e0254) for every catalyst [29]. Remarkably, the representative peaks of CuAl2O4 species (2q ¼ 31.50 , 56.32 , 59.76 , 65.81 and 77.60 based on JCPDS sheet No. 71e0966) [30] were not observed in all patterns. The absence of this undesirable phase could benefit catalytic

Table 1 e Textural properties of 10%Cu/Al2O3, 3%Cee10% Cu/Al2O3 and 3%Lae10%Cu/Al2O3. Sample

10%Cu/ Al2O3 3%Ce e10% Cu/ Al2O3 3%La e10% Cu/ Al2O3 a b

c

BET Total pore surface volume area (cm3 g1)a (m2 g1)

Average pore diameter (nm)b

Average crystallite size of CuO, dCuO (nm)c

98.5

0.78

22.6

32.4

95.7

0.76

23.8

29.8

93.4

0.75

24.1

27.4

Acquired at p/p0 ¼ 0.99. Calculated using Barret-Joyner-Halenda (BJH) desorption technique. Computed using Scherrer equation [27] for the highest CuO peak at 2q ¼ 35.5 .

Fig. 1 e XRD analyses of fresh calcined specimens including (a) g-Al2O3, (b) 10%Cu/Al2O3, (c) 3%Cee10%Cu/ Al2O3 and (d) 3%Lae10%Cu/Al2O3. performance in terms of facilitated reduction temperature and lessening loss of active sites due to inactive CuAl2O4 phase formation. In addition, for 3%Cee10%Cu/Al2O3, the characteristic peak shown at 2q ¼ 28.57 belonged to CeO2 form (JCPDS sheet No. 34e0394) similar to the findings by Yang et al. (2010) [26]. The detected CeO2 phase could deduce that Ce(NO3)3 promoter precursor was eventually decomposed to Ce2O3 phase which was later oxidized to CeO2 throughout air calcination process. However, the diffractive peaks belonging to La2O3 phase such as 2q ¼ 29.87 and 53.42 (JCPDS sheet No. 83e1355) [31] were not identified on 3%Lae10%Cu/Al2O3 surface as illustrated in Fig. 1(d). This is most likely due to the intrinsic feature of La2O3 form normally existing as highly dispersed La2O3 species outside XRD detection range [32,33]. The average crystallite size, dCuO of CuO phase was approximated based on the highest CuO peak at 2q ¼ 35.5 via the usage of Scherrer equation (Eq. (2)) [27]. dCuO ðnmÞ ¼

0:94l b cos q

(2)

where l stands for wavelength and b represents the line broadening at half maximum intensity whereas q denotes the Bragg angle. The mean CuO crystallite size for 10%Cu/Al2O3 is 32.4 nm comparable with findings from other studies [34] while for 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3, the dCuO is about 29.8 nm and 27.4 nm, respectively as summarized in Table 1. The decline in crystallite size for promoted samples could be attributed to the diluting impact arising from CeO2 and La2O3 particles which isolated and inhibited CuO particles from agglomerate.

H2-TPR analyses Catalyst reducibility examined by H2-TPR runs is shown in Fig. 2. Two peaks (a and b) were apparently noticed for 10%Cu/ Al2O3 and 3%Cee10%Cu/Al2O3 but only notable peak b for 3% Lae10%Cu/Al2O3 was observed as seen in Fig. 2(c). For 10%Cu/ Al2O3 and 3%Cee10%Cu/Al2O3, the first peak a, found at low

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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result in the highest H2 uptake during H2-TPR on 3%Lae10% Cu/Al2O3 [35]. The rise in H2 uptake with promotion is indicative of increasing reduction degree and growing amounts of active Cu0 metallic form.

CO2-TPD analyses

Fig. 2 e H2-TPR analyses for (a) 10%Cu/Al2O3, (b) 3%Cee10% Cu/Al2O3 and (c) 3%Lae10%Cu/Al2O3.

temperature from 450 K to 474 K, indicated the reduction of surface and bulk-like CuO particles (owning weak interaction with support) to Cu0 metallic form (see Eq. (3)) [35,36]. The second peak b located at greater temperature within 482e502 K was assigned to the reduction of small and greatly scattered CuO nanoparticles strongly interacting with Al2O3 support. The reduction temperature of metal oxides was reportedly reliant on particle size and metal-support interaction degree [37,38]. Fig. 2 shows that intensity of peak a reduced with Ce addition and it disappeared on 3%Lae10%Cu/ Al2O3. This phenomenon could be ascribed to decreasing CuO crystallite size from 32.4 to 27.4 nm (see Table 1) with promoter addition, thereby increasing metal-support interaction degree. The temperature location of peak b also shifted towards elevated temperature with dopant addition, thereby confirming the rising interaction between metal and support. Beyond 525 K, the TCD signals seemed to be unchanged with no detectable peaks. This observation suggests that catalysts were completely reduced at temperature higher than 525 K. CuO þ H2 /Cu þ H2 O

(3)

As summarized in Table 2, H2 uptake during H2-TPR grew from 0.243 to 0.293 mmol H2 gcat1 with added promoters in the following trend; 10%Cu/Al2O3 < Ce-doped < La-doped specimens. The rising H2 uptake with Ce and La promoters was induced by the basic feature of dopants capable of giving electrons and hence increasing electron density on CuO particles. Thus, the surplus electron density could ease the H2 reduction process [39,40]. Apart from electron-donated ability, hydrogen spillover effect arising from La2O3 promoter could

The basicity nature of catalysts was evaluated using CO2-TPD. As seen in Fig. S3 (Supplementary data), two CO2 desorption peaks I and II were observed for three catalysts and indicated the presence of corresponding weak and strong basic sites [41]. The intensity of these peaks was enlarged with CeO2 and La2O3 addition owing to the basic character of abovementioned promoters. As summarized in Table S1 (Supplementary data), the total CO2 uptake for all samples follows this order; 10%Cu/Al2O3 (1.544 mmol CO2 gcat1) < 3% Cee10%Cu/Al2O3 (1.981 mmol CO2 gcat1) < 3%Lae10%Cu/Al2O3 (2.566 CO2 gcat1). The enhanced basic nature of promoted catalysts could improve deposited carbon oxidation through CO2 gasification and hence increasing ECR activity.

Ethanol CO2 reforming evaluation The temperature influence on ECR performance was investigated from 948 K to 1023 K at stoichiometric feed ratio, FCO2 : FC2 H5 OH ¼ 1 : 1. As seen in Fig. 3, both reactant conversions increased substantially with rising temperature because of endothermic ECR nature [42]. This behavior is also in agreement with findings from Cao et al. in the study of ECR using Cu supported on single and mixed metal oxides, namely, CeO2, ZrO2 and CeO2eZrO2 [20]. For all temperature used, 3%Lae10%Cu/Al2O3 exhibited the highest C2H5OH and CO2 conversions among three catalysts. In particular, C2H5OH conversion increased from 55.4% to 87.6% with temperature increment from 948 to 1023 K whereas the improvement in CO2 conversion was observed from 38.5% to 55.1% on 3% Lae10%Cu/Al2O3. The sequence of both reactant conversions followed the order; 3%Lae10%Cu/Al2O3 > 3%Cee10%Cu/Al2O3 > 10%Cu/ Al2O3 and remained unchanged for all temperature. Notably, this trend was also the sequence of H2 uptake from TPR measurements as summarized in Table 2. Thus, the enhancing reduction extent and quantities of active Cu0 metallic particles with Ce and La dopants could result in rising reactant conversions. Bahari et al. also reported that CeeNi/ Al2O3 achieved higher ethanol conversion compared to unpromoted counterpart because of its greater reduction degree [43]. The improvement in catalytic activity for promoted catalysts could be also credited to the basic character of La and Ce promoters accelerating CO2 adsorption on catalyst surface

Table 2 e Summary of reduction peak temperature and H2 consumption during H2-TPR. Catalyst

10%Cu/Al2O3 3%Cee10%Cu/Al2O3 3%Lae10%Cu/Al2O3

Reduction temperature (K) a

b

450 474 e

482 495 502

H2 consumption (mmol H2 gcat1)

0.243 0.279 0.293

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Fig. 3 e Temperature effect on C2H5OH and CO2 conversions of 10%Cu/Al2O3, 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 at FCO2 : FC2 H5 OH ¼ 1 : 1.

[26,44]. In fact, the sequence of basic site concentration or CO2 uptake is also parallel to the trend of reactant conversions (see Table S1 in Supplementary data). The apparent activation energy derived from the Arrhenius plot is widely used in reforming processes to justify the reactiveness of catalysts without the requirement of associated kinetic models [45,46]. As seen in Fig. 4, the plots of ethanol consumption rate against reciprocal of reaction temperature show a linear relationship with correlation

coefficient (R2) of about 0.99 (Table 3). Hence, the apparent activation energy of ethanol, Ea was estimated as 53.29 kJ mol1 on 10%Cu/Al2O3. It was reduced with promoter addition from 53.29 to 47.32 kJ mol1 (for 3%Cee10%Cu/Al2O3) and 47.05 kJ mol1 (for 3%Lae10%Cu/Al2O3) as summarized in Table 3. The computed apparent ethanol activation energy within 47.32e53.29 kJ mol1 is significantly lower than that of other ECR studies using nano-Ni/SiO2eAl2O3 (97.87 kJ mol1) [47]. Notably, the order of Ea for C2H5OH consumption is parallel to the trend of C2H5OH conversion (i.e., 3%Lae10%Cu/ Al2O3 > 3%Cee10%Cu/Al2O3 > 10%Cu/Al2O3) as shown in Fig. 3. This observation could further explain the catalytic improvement of Cu catalyst with promoter addition. The effect of temperature on H2, CO and CH4 yields is presented in Fig. 5. The yield of these gaseous products increased substantially with rising temperature within

Table 3 e Summary of pre-exponential factor and apparent activation energy for C2H5OH conversion from ECR. Catalyst

Fig. 4 e Arrhenius plots for estimating apparent ethanol activation energy of 10%Cu/Al2O3, 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3.

10%Cu/ Al2O3 3%Cee10% Cu/Al2O3 3%Lae10% Cu/Al2O3

Pre-exponential factor, A (s1)

Apparent activation energy, Ea (kJ mol1)

R2

0.024

53.29

0.99

0.014

47.32

0.99

0.037

47.05

0.99

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Fig. 5 e Temperature effect on H2, CO and CH4 yields of 10%Cu/Al2O3, 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 at FCO2 : FC2 H5 OH ¼ 1 : 1.

948e1023 K. In comparison among catalysts, all product yields followed an analogous trend to the above order of reactant conversions (viz., 3%La-doped > 3%Ce-doped > 10%Cu/Al2O3) irrespective of reaction temperature. Jankhah et al. investigated ECR process using a carbon steel catalyst and reported that increasing H2 and CO yields were provoked by the rising secondary endothermic CH4 dry reforming [48]. Additionally, a recent study on ECR over LaeNi/Al2O3 by Bahari et al. proposed an overall two-stage ECR process in which H2 and CO were yielded from both C2H5OH decomposition (Eq. (4)) and CO2 reforming of intermediate methane (Eq. (5)) [40]. 1

C2 H5 OH / CH4 þ CO þ H2 ðDH298 K ¼ 49:6 kJ mol Þ 1

CH4 þ CO2 /2CO þ 2H2 ðDH298 K ¼ 247:0 kJ mol Þ

(4) (5)

Based on the above two-step ECR procedure, CH4 yield should be decreased with rising reaction temperature because of growing endothermic CH4 dry reforming (Eq. (5)).

Nevertheless, in this study, an increase in CH4 yield with temperature for all catalysts (Fig. 5) was evident indicating that C2H5OH decomposition rate was superior to the rate of subsequent CH4 dry reforming. 3%Lae10%Cu/Al2O3 achieved the highest CH4 yield (17.8%) at 1023 K followed by 3%Cee10% Cu/Al2O3 (12.5%) and 10%Cu/Al2O3 (10.8%). Fig. 6 displays the influence of temperature on CH4/CO and H2/CO ratios for all samples. The ratio of CH4/CO is an indicator for the reactiveness of methane dry reforming side reaction (Eq. (5)). It is supposedly declined with rising reaction temperature because of its endothermic characteristic. However, the CH4/CO ratio increased considerably with temperature further (Fig. 6) confirming that the rate for CO2 reforming of methane was lower than that of ethanol decomposition (Eq. (4)). As shown in Fig. 6, H2/CO ratio also enhanced with temperature and H2/CO ratio ranges for 3%Lae10%Cu/Al2O3, 3% Cee10%Cu/Al2O3 and 10%Cu/Al2O3 are 1.76e1.91, 1.56e1.68 and 1.46e1.56, respectively. The superior H2/CO ratio to

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Fig. 6 e Temperature effect on H2/CO ratio and CH4/CO ratio of 10%Cu/Al2O3, 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 at FCO2 : FC2 H5 OH ¼ 1 : 1.

stoichiometric value of 1 (Eq. (1)) and its increasing value with temperature increment could be attributed to the coexistence of ethanol dehydrogenation endothermic in nature during ECR [16]. The resulting H2/CO values ranging from 1.5 to 1.9 relying on catalyst and reaction temperature employed are appropriate for yielding long-chained hydrocarbons from Fischer-Tropsch synthesis [1,3]. In order to properly determine the effect of CeO2 and La2O3 promoters on Al2O3 and justify the worth of using Al2O3-supported Cu-based catalysts, 10%Cu/CeO2 and 10% Cu/La2O3 synthesized via the same procedure as 10%Cu/ Al2O3 were also tested for ECR at stoichiometric feed ratio and 1023 K. As seen in Fig. S4 (Supplementary data), the trend for reactant conversions and product yields is given as: 10% Cu/Al2O3 < 10%Cu/CeO2 < 10%Cu/La2O3 < 3%Cee10%Cu/ Al2O3 < 3%Lae10%Cu/Al2O3. Although the usage of Al2O3 support is not as efficient as CeO2 and La2O3, possessing redox and basic characters [25,26], the addition of CeO2 or La2O3 as promoters to 10%Cu/Al2O3 yielded a greater ECR performance than that of 10%Cu/CeO2 and 10%Cu/La2O3. Thus, in view of economics, it is more efficient and worthy to employ Al2O3 support with a small amount of CeO2 or La2O3 promoter for Cu-based catalysts. For further verification and assessment of Cu-based catalysts employed in this work, their catalytic performance is compared with both commonly transitional and precious metal catalysts recently reported in literature. Table 4 shows

that in this work, 10%Cu/Al2O3 had higher C2H5OH (36.4%) and CO2 (22.6%) conversions than those of 10%Ni/Al2O3 [43] and 10%Co/Al2O3 [18] at the same operating conditions. Although the C2H5OH conversion of 10%Cu/Al2O3 was lower than that of 15%Cu/ZrO2 [20], the 15%Cu/ZrO2 has a greater Cu loading and ECR was conducted with 4 times lower GHSV (10 L gcat1 h1). Notably, 3%Lae10%Cu/Al2O3 exhibited a relatively comparable C2H5OH conversion to noble metal catalysts (see Table 4) even though a significantly higher GHSV (42 L gcat1 h1) was employed in this study. Hence, 3%Lae10%Cu/Al2O3 could be regarded as a promising and potential catalyst for noble metal substitution in industrial ECR applications from an economical point of view.

Effect of La loadings As La-promoted catalyst was identified as the best catalyst compared to other samples in terms of reactant conversions and product yields, the effect of La promoter loadings (0 to 5 wt%La) on 10%Cu/Al2O3 was further inspected. Fig. S5 (see Supplementary data) shows C2H5OH and CO2 conversions of catalysts with different La loadings. Both reactant conversions increased substantially with rising La content from 0 to 3% most likely due to the enhancement of redox cycle and basic attribute of La2O3 species capable of oxidizing carbonaceous deposits from catalysts surface and thus, increasing catalytic activity [25,26]. Nevertheless, reactant conversions declined

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Table 4 e Comparison of catalytic performance for ECR on transitional and noble metal catalysts in literature. Catalysts

Noble metal catalyst 1.9%Ir/Ce0.75Zr0.25O2 1%Rh/CeO2 Unpromoted catalyst 15%Cu/ZrO2 10%Ni/Al2O3 10%Co/Al2O3 Catalyst employed 10%Cu/Al2O3 3%Cee10%Cu/Al2O3 3%Lae10%Cu/Al2O3 a

Operating Conditions

Conversion (%)

H2/CO ratio

References

T (K)

C2H5OH:CO2 ratio

GHSV (L gcat1 h1)

C2H5OH

CO2

973 973

1:1 1:1

10 18

73.9 46.8

n.m.a 11.0

1.1 n.m.

[49] [50]

973 973 973

1:1 1:1 1:1

10 42 42

67.8 29.2 16.2

n.m. 15.3 n.m.

n.m. 1.4 n.m.

[20] [43] [18]

973 973 973

1:1 1:1 1:1

42 42 42

36.4 51.5 64.5

22.6 32.1 41.6

1.5 1.6 1.8

This work This work This work

Not mentioned.

Fig. 7 e XRD analyses of (a) fresh 10%Cu/Al2O3, (b) spent 10%Cu/Al2O3, (c) spent 3%Cee10%Cu/Al2O3 and (d) spent 3% Lae10%Cu/Al2O3 after ECR reactions at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1.

Fig. 8 e TPO analyses for spent 10%Cu/Al2O3, 3%Cee10% Cu/Al2O3 and 3%Lae10%Cu/Al2O3 after ECR reactions at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1.

beyond 3%La content probably because of lower metal dispersion at excessive promoter loadings in agreement with findings from Bahari et al. [51].

X-ray diffraction analyses of spent catalysts In order to verify the stability of active phases in catalysts after ECR, XRD measurements of spent catalysts collected from ECR reactions at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1 were conducted. As seen in Fig. 7, all XRD patterns recorded a small peak at 2q ¼ 26.14 attributed to graphite (JCPDS sheet No. 75e0444) [28]. Graphitic carbon formation on used catalysts seems to be inevitable because of the thermodynamically favored ethanol decomposition at high temperature [52,53]. In comparison amongst XRD profiles for origin and spent samples, all characteristic CuO peaks disappeared after ECR and three new peaks with high intensity were detected at 2q ¼ 43.29 , 51.24 and 74.08 for all spent catalysts. These typical peaks belonged to Cu0 metallic phase (JCPDS sheet No. 04e0836) produced during H2 activation [19,54]. The presence

Fig. 9 e Weight loss during TPO runs on spent 10%Cu/ Al2O3, 3%Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 obtained from ECR at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1.

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Table 5 e The weight loss of spent 10%Cu/Al2O3, 3% Cee10%Cu/Al2O3 and 3%Lae10%Cu/Al2O3 during TPO analyses. Catalyst

10%Cu/ Al2O3 3%Cee10% Cu/Al2O3 3%Lae10% Cu/Al2O3

Weight loss (%) Peak P1 (Amorphous carbon, %)

Peak P2 (Graphitic carbon, %)

Total weight loss (%)

13.10

26.94

40.04

15.69

19.28

34.97

23.35

4.20

27.55

of Cu0 form and absence of CuO phase on used catalysts could deduce that CuO phase was reduced entirely to Cu0 metallic form during H2 activation and this Cu0 active phase resisted to re-oxidation even in the CO2-oxidizing environment.

Temperature-programmed oxidation As deposited carbon was detected in XRD measurements of used catalysts, the total amount of carbon formation was further quantified via TPO analyses for selective used samples collected from ECR reaction at 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1. The derivative weight and weight loss profiles for spent catalysts are illustrated in Fig. 8 and Fig. 9, respectively. As shown in Fig. 8, two discrete peaks were noticed at 620e790 K (labelled as P1) and 790e930 K (peak P2) for all catalysts. These peaks are indicative of carbon elimination by oxygen. Based on the oxidation temperature, the low temperature peak P1 represented the oxidation of reactive amorphous carbon, which could not be identified via XRD measurements (see Fig. 7) due to its non-crystalline structure whereas the second peak P2 at higher temperature was ascribed to the oxidation of less reactive graphite [55]. As seen in the weight loss profiles (Fig. 9) of used catalysts, two stages of weight reduction belonging to peaks P1 and P2

Fig. 10 e HRTEM images of (a) spent 10%Cu/Al2O3, (b) spent 3%Cee10%Cu/Al2O3 and (c) spent 3%Lae10%Cu/Al2O3 after ECR reactions at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1. Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

international journal of hydrogen energy xxx (xxxx) xxx

were evident for all samples. Sample weight was unchanged at temperature greater than 930 K indicating the thorough gasification of carbonaceous species. As summarized in Table 5, the total amount of deposited carbon was reduced by adding promoters in the order of 10%Cu/Al2O3 (40.04%) > 3% Cee10%Cu/Al2O3 (34.97%) > 3%Lae10%Cu/Al2O3 (27.55%). Based on the carbon amount, the rate of carbon deposition during ECR was also estimated for each catalyst and follows the sequence: 10%Cu/Al2O3 (9.03  107 gcarbon gcat1 s1) > 3% Cee10%Cu/Al2O3 (7.88  107 gcarbon gcat1 s1) > 3%Lae10%Cu/ Al2O3 (6.22  107 gcarbon gcat1 s1). Although carbon deposits were unavoidable in ECR owing to thermodynamically preferred ethanol and intermediate methane decomposition side reactions, these carbonaceous species were reduced substantially with promoter utilization. The carbon suppression could be attributed to multi-benefit effects of Ce and La promoters, namely, decreasing crystallite size, enhancing basic nature and redox cycling character [56]. Carbon deposition was widely reported as a structure sensitive reaction and the rate of carbon formation increased considerably with rising crystallite size [57,58]. Table 1 shows that CuO crystallite size was decreased with promoter addition in the sequence; 10%Cu/Al2O3 > Ce-doped > La-doped catalysts, thereby contributing to carbon reduction. The basic feature of CeO2 and La2O3 could also contribute to carbon resistance since it enhanced CO2 adsorption which in turn oxidized carbonaceous species on catalyst surface [56]. Additionally, the redox cycles of La2O3/La2O2CO3 (see Eqs. (6) and (7)) [59] and CeO2/Ce2O3 (see Eqs. (8) and (9)) [60,61] could gasify carbon species (CxHy) to CO and H2 gases. The reduction in carbon deposition on catalyst surface because of promoters could further explain the improvement of catalytic activity of promoted catalysts for ECR as reported in Fig. 3. La2 O3 þ CO2 /La2 O2 CO3

(6)

y xLa2 O2 CO3 þ Cx Hy /xLa2 O3 þ 2xCO þ H2 2

(7)

y 2xCeO2 þ Cx Hy /xCe2 O3 þ xCO þ H2 2

(8)

Ce2 O3 þ CO2 /2CeO2 þ CO

(9)

As seen in Table 5, promoter not only reduced total carbon deposition but also declined non-active graphitic carbon formation from 26.94% to 4.20% and facilitated the yield of reactive amorphous carbon from 13.10% to 23.35%. The increasing amount of amorphous carbon was also attributed to decreasing CuO crystallite size on promoted catalysts since the small crystallite size possessed high carbonaceous saturation concentration and low driving force of carbon diffusion [62]. This observation is also in line with other studies [40,44,63].

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filament-shaped carbon and graphitic carbon in agreement with aforesaid TPO measurements (cf. Table 5). Based on the microimages from HRTEM measurements, the amount of graphite on unpromoted catalyst (Fig. 10(a)) was visibly reduced with Ce addition (Fig. 10(b)). It was further decreased on spent 3%Lae10%Cu/Al2O3 (Fig. 10(c)). The quantity of amorphous carbon nanofilaments (CNFs) was, however, apparently increased with the addition of promoters in line with the results from TPO runs as summarized in Table 5. Nevertheless, these CNFs would not result in detrimental effect on catalytic activity as they did not encapsulate Cu active metal and grew up from catalyst surface. The presence of CNFs was in line with other ECR studies on various Co, Ni and carbon steel catalysts [16,53,64]. Cao et al. also reported that during ECR on Cu/CeeZr, the unavoidably formed filamentous carbon did not severely deteriorate catalytic performance due to its isolation from Cu surface [20].

Conclusions The IWI and sequential IWI approaches were employed to prepare the corresponding unpromoted and promoted catalysts. The BET surface area of these catalysts was about 93.4e98.5 m2 g1 whereas the calculated average CuO crystallite size declined from 32.4 to 27.4 nm with La2O3 and CeO2 additions because of the diluting effect. Promoter addition enhanced metal-support interaction as evidenced in H2-TPR analyses. The increasing H2 uptake with promoters during H2-TPR was indicative of enhancing degree for CuO / Cu0 reduction. Both reactant conversions increased substantially with rising temperature from 948 to 1023 K for all specimens due to ECR endothermic nature. Promoter addition improved catalytic activity and product yield in the order of 3%Lae10% Cu/Al2O3 > 3%Cee10%Cu/Al2O3 > 10%Cu/Al2O3 for all temperatures while the opposite trend was observed for the C2H5OH activation energy varying within 53.29 to 47.05 kJ mol1. The existence of concurrent endothermic ethanol dehydrogenation in ECR increased H2/CO ratio from 1.46 to 1.91 suitable for Fischer-Tropsch synthesis. XRD measurements of spent catalysts proved that Cu0 active phase was maintained during ECR and catalysts resisted to re-oxidation in CO2-containing feedstock. Although carbonaceous formation was detected on spent catalysts by XRD, TPO and HRTEM measurements, the significant decline in total carbon deposition from 40.04% to 27.55% and non-active graphite formation from 26.94% to 4.20% was achieved by CeO2 and La2O3 addition. The carbon suppression was attributed to the basic character, and cycling redox properties of promoters.

Acknowledgements High-resolution transmission electron microscopy analyses Fig. 10 illustrates HRTEM images for all spent catalysts after 8 h of ECR reactions at T ¼ 1023 K and FCO2 : FC2 H5 OH ¼ 1 : 1 . All spent catalysts were obviously covered by amorphous

The authors appreciate the financial assistance from Universiti Malaysia Pahang (UMP), Malaysia (via UMP Research Grant Scheme, RDU 170326). Ms. Shafiqah is also thankful to the Master Research Scheme (MRS) conferred by UMP. Dr.

Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024

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Quang Duc Truong (Tohoku University, Japan) was greatly acknowledged for conducting HRTEM measurements of our samples.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.024.

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Please cite this article as: Shafiqah M-NN et al., Ethanol CO2 reforming on La2O3 and CeO2-promoted Cu/Al2O3 catalysts for enhanced hydrogen production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.024