CeO2–Al2O3 catalysts for the dry reforming of methane: The effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance

Applied Catalysis A: General 500 (2015) 12–22

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Ni/CeO2 –Al2 O3 catalysts for the dry reforming of methane: The effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance Igor Luisetto a,∗ , Simonetta Tuti a,b , Chiara Battocchio a,b , Sergio Lo Mastro a , Armida Sodo a a b

Department of Sciences, University of Rome “Roma Tre”, Via della Vasca Navale 79, 00146 Rome, Italy C.I.S.Di.C Center, University of Rome “Roma Tre”, Via della Vasca Navale 79, 00146 Rome, Italy

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 29 January 2015 Received in revised form 22 April 2015 Accepted 1 May 2015 Available online 11 May 2015

The catalytic performances of Ni/CeO2 –Al2 O3 catalysts for the dry reforming of CH4 (DRM) were investigated. Catalysts with different Ni dispersion and different amount of CeAlO3 species were prepared by different methods and characterized by BET, XRD, XPS, Raman, TPR and TPO techniques. Catalytic activity was studied during time on stream in the range 873–1073 K with a mixture of CH4 :CO2 :Ar = 40:40:20 vol.% and GHSV 90,000 cm3 g−1 h−1 . The intrinsic catalytic activity increased with the increasing of Ni crystallite size. Carbon was deposited as nano-fibres and graphite when catalysts worked at lower temperature, and the largest amount was found on the catalyst with the largest Ni crystallite size. The formation of graphitic deposits is limited by the presence of CeAlO3 species formed during catalyst activation. CA preparation method results particularly attractive because it allows to obtain catalysts with small Ni crystallite size and high content of CeAlO3 species, which both have a role in suppressing the carbon deposition and therefore in obtaining stable catalytic performances. © 2015 Elsevier B.V. All rights reserved.

Keywords: Ni/CeO2 –Al2 O3 Dry reforming Nickel particle size CeAlO3 Carbon deposition

1. Introduction The CO2 reforming of CH4 (Eq. (1)), also called dry reforming (DRM), has been recently recognized as an efficient way for the CH4 and CO2 valorization [1–4]. In fact, the produced syn-gas has an H2 /CO ratio equal to one suitable for the synthesis of oxygenated hydrocarbons and synthetic fuels. The feedstock that can be used for the DRM, ranges from CO2 -rich natural gas reserves to renewable biogas produced by anaerobic fermentation of waste sludge (mainly composed by CH4 and CO2 ), offering thus the possibility to enlarge their utilization and to avoid the release of the CO2 in the atmosphere. CH4 + CO2 → 2H2 + 2CO

0 H298 K

= 247 kJ mol

−1

(1)

The DRM plays also an important role on determine the electrochemical performances and the long-term stability of solid oxide fuel cells (SOFCs) fed by CH4 or biogas. In particular, the DRM occurs internally to the cell stack producing H2 used to feed the anode and it helpfully limits the carbon deposition [5–7].

∗ Corresponding author. Tel.: +39 0657333370; fax: +39 0657333390. E-mail address: [email protected] (I. Luisetto). http://dx.doi.org/10.1016/j.apcata.2015.05.004 0926-860X/© 2015 Elsevier B.V. All rights reserved.

Due to its high endothermicity the DRM has also been proposed for the energy storage and the energy transfer, for example in the conversion of the solar energy to chemical energy, which is referred to as solar reforming [8–10]. Regarding the application of the DRM for syn-gas production, to date its industrial implementation is impeded mainly by the following issues: (i) the co-occurrence of the reverse water gas shift reaction (RWGS) (Eq. (2)) that, consuming H2 , lowers the H2 /CO ratio; H2 + CO2 → CO + H2 O

0 H298

K

= 41 kJ mol−1

(2)

(ii) the catalyst deactivation and/or the reactor plugging due to carbon deposits formed by the methane cracking (Eq. (3)) and the Boudouard reaction (Eq. (4)). CH4 → C + 2H2

0 H298

K

= 75 kJ mol−1

(3)

2CO → C + CO2

0 H298

K

= −172 kJ mol−1

(4)

The DRM is operated at high temperatures (generally above 923 K) to achieve suitable CH4 and CO2 conversions. In that condition, the Boudouard reaction is thermodynamically unfavoured, the RWGS is suppressed by the low CO2 concentration, whereas, on the contrary, the methane cracking is favoured [11].

I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

Therefore, the development of catalysts, selective for the DRM, with high resistance towards carbon deposition and proper thermal stability, is pivotal for the DRM implementation and is still a challenge. Catalysts containing noble metal such as Pt, Ru and Rh show high activity and selectivity for the DRM reaction in addition to good stability towards the coke deposition [12–14]. However their high cost and low availability make them not economically competitive in comparison with other transition metal based materials. Among non-precious transition metals, Ni-based catalysts are so far the most active, but also highly prone to carbon formation, because, together with the ability to activate the C H bond, Ni has a high affinity to carbon. In the search of active and stable Ni-based catalysts, several approaches have been explored in order to suppress the carbon deposition. The addition of a second metal, such as Co, Cu and Sn, results in the formation of less C-sensitive alloys [15–17]. The Ni particle size has a strong effect on the carbon tolerance of the catalyst, and generally, particles smaller than 5 nm have low catalytic activity towards the C H cracking [11,18,19]. Therefore, the stabilization of small Ni nanoparticles at high temperatures is a promising way for the lifetime increase [20,21]. However, together with the properties of the active metal sites, the support choosing is of great importance in designing high-performance catalysts. Supports with basic character, such as MgO [22], and those containing lanthanum [23], showed limited carbon deposition during DRM because of their small number of Lewis acid sites, which are involved in carbon formation, and because the presence of La2 O2 CO3 that helps the gasification of carbonaceous deposits. Among different supports, those containing cerium seem to be the most promising in limiting the deactivation by coking and therefore they are extensively investigated. CeO2 is known for its oxygen storing capacity due to the redox couple Ce4+ /Ce3+ . Owing to this property, during dry reforming the oxygen vacancies over the CeO2 -surface may adsorb the oxygen formed by the dissociation of CO2 on Ni sites, improving the reforming activity and the gasification of coke [24]. However, the positive effect of CeO2 support on carbon removal has been reported to be particularly dependent on the catalyst preparation method which may affect Ni dispersion and metal support interaction [24,25]. The addition of ZrO2 to CeO2 yields solid solution with high oxygen mobility and thermal stability [26], therefore the Ce(1−x) Zr(x) O2 family has been applied as support for robust DRM catalysts. Several authors have investigated the effect of the Ce/Zr ratio on carbon resistance. However, the results are not conclusive [27,28]. Moreover, as for the pure CeO2 support, the catalytic performance of the transition metals supported on Ce(1−x) Zr(x) O2 are strictly dependant on the preparation methods [29]. CeO2 is used also as promoter of ␥-Al2 O3 based catalysts because of the combination of the large surface area and stability of ␥-Al2 O3 with the oxygen storage and release capability of CeO2 [30,31]. The promotion of the catalytic performances by CeO2 addition is also related with the ability to increase the Ni dispersion [32,33]. Moreover, in reducing atmosphere at high temperature, CeO2 supported on ␥-Al2 O3 reacts to form CeAlO3 -like species [34]. As reported by several authors these species play a key role in the removal of carbon residues [35]. A possible reaction mechanism has been recently proposed by Chen et al. [36]: the CeAlO3 formed during catalyst activation or in reaction condition, could react with CO2 to form CO and CeO2 (Eq. (5)). CeO2 oxidizes the CHx species located at the Nisupport boundary, precursors of carbonaceous residues, restoring the CeAlO3 sites (Eq. (6)). 2CeAlO3 + CO2 → Al2 O3 + 2CeO2 + CO

(5)

Al2 O3 + 2CeO2 + CHx → CO + 2CeAlO3 + (x/2)H2

(6)

13

Despite recent advances, the role of CeO2 –Al2 O3 interaction and Ni particle size in determining the catalyst performance and stability remains open. Therefore, in the present work we studied Ni/CeO2 –Al2 O3 catalysts with different Ni crystallite size and Ce3+ /Ce4+ ratio, with the aim to better understand their interplay with regard to carbon formation and catalytic activity. Catalysts were prepared by coprecipitation, wet impregnation, sol–gel and citric acid methods, and characterized by several techniques. The effect of the preparation method on the solid-state reaction between CeO2 and ␥-Al2 O3 to form CeAlO3 was studied by XRD, XPS and H2 -TPR. Catalytic activity was studied in the range 873–1073 K with high flow rate and during time on stream. Carbonaceous deposits were characterized by XRD, Raman spectroscopy and O2 -TPO. 2. Experimental 2.1. Catalyst synthesis Catalysts with nominal composition of 10 wt.% Ni supported on 20 wt.% CeO2 promoted ␥-Al2 O3 have been prepared according to the following procedures. 2.1.1. Co-precipitation method (CP) Stoichiometric amounts of nitrate salts (Ni(NO3 )2 × 6H2 O; Ce(NO3 )3 × 6H2 O Al(NO3 )3 × 9H2 O) and ionic surfactant cetyl trimethyl ammonium bromide (CTAB) with molar ratio CTAB/(Ni2+ + Ce3+ + Al3+ ) = 0.4 were dissolved in water. The temperature was raised to 383 K and triethylamine (TEA) was rapidly added until pH 10. The precipitation of hydroxide instantaneously occurred and the obtained mixture was aged for 20 h. The precipitated solid was filtered and washed with water and ethanol, dried at 393 K overnight and calcined at 873 K for 5 h. 2.1.2. Solution excess wet impregnation method (WI) Aqueous solution of Ni(NO3 )2 × 6H2 O and Ce(NO3 )3 × 6H2 O was added to a commercial mesoporous alumina powder (Alfa Aesar) to form a slurry. The solvent was evaporated at 353 K under vigorous stirring. The impregnated alumina was further dried at 393 K overnight and calcined at 873 K for 5 h. 2.1.3. Sol gel method (SG) Stoichiometric amounts of Ni(NO3 )2 × 6H2 O, Ce(NO3 )3 × 6H2 O and aluminium-tri-sec butoxide were added to ethanol acidified with a small amount of HNO3 (65 wt.%). The solution was mixed under vigorous stirring at ambient temperature for 5 h, than the solvent was evaporated at 333 K during 48 h. The dried gel was calcined at 873 K for 5 h. 2.1.4. Citric acid method (CA) Stoichiometric amounts of nitrate salts (Ni(NO3 )2 × 6H2 O, Ce(NO3 )3 × 6H2 O and Al(NO3 )3 × 9H2 O) and citric acid monohydrate (CA) with molar ratio CA/(Ni2+ + Ce3+ + Al3+ ) = 1.5 were dissolved in water. Then NH4 OH solution (28 wt.%) was added until pH 8. The solvent was evaporated at 393 K yielding a gel, and then the temperature was increased up to about 523 K to ignite the autocombustion. The obtained dark powder was calcined at 873 K for 5 h. The chemical composition of samples was confirmed by energy dispersive X-ray analysis (EDAX) using a SEM FEI XL30. Sample powder was pressed in pellets at about 280 MPa. Several spots with size of about 10 ␮m were performed and the results are reported as average values in Table 1.

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Table 1 Chemical composition, textural properties (surface area, pore volume, main pore size) and Ni crystallite size of catalysts. Catalyst

CP WI SG CA a b

Composition (wt.%) Ni

CeO2

9.4 10.0 9.3 10.7

19.1 20.2 22.7 21.4

Surface area (m2 g−1 )

Pore volume (cm3 g−1 )

Pore size (nm)

DNi (nm)b

285 (147)a 200 (167) 71 (47) 135 (130)

1.143 (0.832)a 0.720 (0.678) 0.159 (0.120) 0.136 (0.128)

17.0 (17.0)a 9.8 (10.8) 4.1, 6.9, 11.8 (4.9, 13.5) 4.0 (3.3)

22.6 11.1 9.5 5.8

In parenthesis are reported the BET, pore volume and pore size values of the reduced catalysts. Calculated from XRD by Scherrer’s equation of the Ni (2 0 0) reflection.

2.2. Catalyst characterization 2.2.1. BET N2 adsorption–desorption isotherms were obtained at 77 K using a Micromeritics Gemini V apparatus. The surface area was calculated by the Brunauer–Emmett–Teller (BET) method in the equilibrium pressure interval 0.05 < P/P◦ < 0.5. The pore size distribution was obtained from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method and the total pore volume was calculated from the maximum adsorption point at P/P◦ = 0.99. Prior to the N2 adsorption, the sample was treated at 623 K in flowing He in order to remove adsorbed molecules. 2.2.2. XRD Powder X-ray diffraction patterns were collected using a Scintag ˚ source and X1 diffractometer equipped with a Cu K␣ ( = 1.5418 A) the Brag-Brentano – configuration in the 10–70 2 range, with 0.05◦ step size and 3 s acquisition time. The Ni0 and CeO2 crystallite size were estimated by Scherrer’s equation from the Ni (2 0 0) and CeO2 (1 1 1) reflection, respectively. 2.2.3. H2 -TPR Temperature programmed reduction (TPR) experiments were performed by a Thermo Scientific TPDRO1100 flow apparatus. Sample (0.050 g) was pre-treated in a flow 5% O2 /He mixture (20 cm3 min−1 ) at 573 K for 30 min. The TPR was conducted flowing a 5% H2 /Ar mixture (10 cm3 min−1 ) starting at 313 K and heating up to 1273 or 1073 K, with a rate of 10 K min−1 . The H2 consumption was measured by a TCD detector, calibrated by the reduction of a known amount of CuO (99.99% purity from Sigma Aldrich). Before flowing into the TCD detector, the H2 O generated in the reduction was removed by a trap. 2.2.4. O2 -TPO Temperature programmed oxidation (TPO) of the used catalysts were performed in the same apparatus used for the TPR experiments. Prior to TPO analysis, the used catalyst was carefully homogenized in an agate mortar. About 0.010 g was ramped from 313 up to 1073 K, with a rate of 10 K min−1 flowing 5% O2 /He mixture (50 cm3 min−1 ). Before flowing into the TCD detector, the H2 O and CO2 generated by the oxidation were removed by a trap. In identical experimental condition, CO was not observed by GC analysis of the exhaust stream gas. 2.2.5. XPS XPS analysis was performed in an instrument of our own design and construction, consisting of a preparation and an analysis UHV chamber, equipped with a 150 mm mean radius hemispherical electron analyser with a four-elements lens system with a 16channel detector giving a total instrumental resolution of 1,0 eV as measured at the Ag 3d5/2 core level. Al K␣ non-monochromatised X-ray radiation (h = 1486.6 eV) was used for acquiring core level spectra of all samples (C1s, Ce3d, Ni2p, Al2p and O1s). The spectra were energy referenced to the C1s signal of aliphatic C atoms

having a binding energy BE = 285.00 eV, due to surface contamination, as expected for XPS measurements performed on solid samples exposed to air. Atomic ratios were calculated from peak intensities by using Scofield’s cross-section values and calculated  factors [37]. Curve-fitting analysis of the C1s, Ce3d, Ni2p, Al2p and O1s spectra was performed using Gaussian profiles as fitting functions, after subtraction of a Shirley-type background [38]. 2.2.6. Raman Raman measurements were performed by using a Labram Micro-Raman spectrometer by Horiba, equipped with a He–Ne laser sources at 632.8 nm (nominal output power 18 mW). The illumination and collecting optics of the system consists in a microscope in confocal configuration. The system achieves the high contrast required for the rejection of the elastically scattered component by an edge filter. The backscattered light is dispersed by a 1800 line/mm grating and the Raman signal is detected by a Peltier cooled (203 K) 1024 × 256 pixel CCD detector. Nominal spectral resolution was about 1 cm−1 . Spectral acquisitions (3 accumulations, 30 s each, in the range 1000–2800 cm−1 ) were performed with a long distance 20× objective (N.A. = 0.35). 2.3. Catalytic activity The catalytic activity was measured in a fixed-bed quartz reactor at atmospheric pressure connected to a flow apparatus equipped with mass flow controllers. The reactor was specially designed to remove the inner part containing the catalyst bed. Sample (0.050 g) was reduced in situ with 50% H2 /Ar flow (30 cm3 min−1 ) increasing the reactor temperature from RT up to 1073 K with a ramp of 10 K min−1 and isothermally kept at this temperature for 1 h. The dry reforming of methane was studied with a mixture of CH4 :CO2 :Ar = 40:40:20 vol.% and flow rate of 75 cm3 min−1 (GHSV 90,000 cm3 g−1 h−1 ). After reduction of the catalyst at 1073 K, the reactor was cooled to 873 K and purged with Ar flow (15 cm3 min−1 ) for 15 min, then the gas flow was switched to the reactant mixture. The catalytic run was performed in the temperature range from 873 to 1073 K with 50 K temperature increments. Each temperature step was maintained for 5 h. Reaction stream was analyzed on line, at regular times, by a Agilent 7820 gas chromatograph equipped with a Molecular Sieve X13 (for the H2 , Ar, CO, CH4 separation) a Hayesep Q (for CO2 separation) columns and a TCD detector. After each catalytic run at specified temperature, the catalyst was cooled to RT in Ar flow (15 cm3 min−1 ) and the catalyst bed was weighted in order to verify the formation of massive carbon. Then, the catalyst was warmed up to the following temperature step in Ar flow (15 cm3 min−1 ) thus preventing the oxidation of catalyst surface due to the exposure to ambient atmosphere. CH4 and CO2 percent conversions (Xi %) were calculated according to Eq. (7) using Ar as internal standard.

 Xi (%) = 100 ×

1−

0 Ci · CAr

Ci0 · CAr

 (7)

I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

15

0 are the inlet concentrations of the reactants where Ci0 and CAr (i = CH4 or CO2 ) and Ar respectively and Ci and CAr are the corresponding outlet concentrations. The site time yield (STY) of hydrogen was calculated according to Eq. (8)

STYH2 = 2 ·

0 F 0 · CCH · XCH4 4

22.414 · NNi

(8)

0 is the inlet where F0 is the inlet flow of reactants (in L s−1 ), CCH 4 concentration of methane in the reactants mixture, XCH4 is the initial CH4 conversion at 1073 K, 22.414 is the volume of one mole of gas at standard condition (L mol−1 ) and NNi is the number of moles of the Ni active sites. The number of moles of the Ni active sites NNi was calculated according to Eq. (9)

NNi =

g · WNi · DNi MNi

(9)

where g is the mass of catalyst, WNi is the weight fraction of Ni in the sample as determined by EDAX, MNi is the molar mass of Ni (58.71 g mol−1 ), DNi is the Ni0 dispersion. The nickel dispersion DNi was estimated by the Vannice method [39,40] (Eq. (10)): DM = 6 × 107

VM 1 · AM d(nm)

(10)

where DM is the metal dispersion, VM is the bulk atomic volume of the metal (cm3 ), AM is the atomic area (cm2 ), and d is the metal crystallite size (nm) from XRD. The rate of carbon formation rc (h−1 ) was calculated according the Eq. (11): rC (h−1 ) =

m gcat. h

(11)

where m represents the difference between the mass of the catalyst bed at the start and at the end of the time on stream test at a specified temperature, gcat is the mass of the freshly charged catalyst (0.050 g) and h is the time in hours of the catalytic step run (5 h). 3. Results and discussions 3.1. Structural and textural characterization The XRD patterns of samples calcined at 873 K are reported in Fig. 1A. The CA sample showed a broad and weak XRD peak at about 2 = 33◦ indicative of the amorphous phase. The other samples showed peaks of fluorite CeO2 (JCPDS 81-0792) with different crystallinity, beside those of ␥-Al2 O3 . In particular the largest CeO2 crystallite size of 7.0 nm was observed in WI sample, while in the other samples, CeO2 was intimately dispersed in the ␥-Al2 O3 skeleton as smaller and less crystalline particles. Indeed the CeO2 crystallite size was 3.5 nm for SG and 4.8 nm for CP samples; moreover the peaks of CeO2 of the SG sample were much less intense suggesting that CeO2 was partially amorphous. Peaks corresponding to NiO cubic phase (JCPDS 78-0643) were observed only in SG sample. The NiO absence in the other specimens was likely due to the low size of particles or to the formation of NiAl2 O4 spinel. The latter species is hard to distinguish from the ␥-Al2 O3 phase since most of their diffraction lines overlap. The XRD patterns of samples reduced at 1073 K are reported in Fig. 1B. All samples showed peaks at about 44.8◦ and 51.8◦ assigned to the (1 0 0) and (2 0 0) reflections of cubic Ni0 (JCPDS 87-0712) originated from the reduction of NiO and Ni2+ -species. The crystallite size of Ni0 , calculated by Scherrer’s equation, ranges between

Fig. 1. XRD patterns of the catalysts calcined at 873 K (A) and reduced at 1073 K (B): CA (a); SG (b); WI (c); CP (d).

5.8 nm and 22.6, increasing in the sample order CA < SG ≈ WI < CP (Table 1). In CA sample no other peaks were observed, indicating that the oxide lattice remained amorphous even after reduction. In the other samples ␥-Al2 O3 , CeO2 and CeAlO3 phases were detected. In particular, peaks of cubic CeO2 (at 28.5◦ ; 33.0◦ ; 47.5◦ ) were clearly observed on WI and SG samples and barely detected on CP sample, moreover peaks of CeAlO3 (at 23.6◦ ; 33.5◦ ; 41.5◦ ; 60.0◦ ) were observed only on SG sample. The Ce2 O3 , formed by H2 reduction of CeO2 (Eq. (12)), was not observed due to the rapid oxidation of Ce3+ to Ce4+ upon exposure to ambient atmosphere. The disappearance of CeO2 in CP sample and the presence of CeAlO3 peaks in SG sample were due to the solid-state reaction between Ce2 O3 and ␥-Al2 O3 (Eq. (13)), generally observed above 873 K under reducing condition. However on WI sample the absence of modifications of CeO2 phase suggested that the large and well crystallized CeO2 particles were only partially reduced to Ce2 O3 in H2 at 1073 K, and that a very small fraction of Ce3+ has been

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I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

incorporated into alumina as CeAlO3 species. These results agree with the literature data. In fact Shyu et al. [34] reported that highly dispersed CeO2 nanoparticles supported on Al2 O3 have become CeAlO3 above 873 K and agglomerated CeO2 particles have been reduced only at temperature higher than 1073 K. 2CeO2 + H2 → Ce2 O3 + H2 O

(12)

Ce2 O3 + Al2 O3 → 2CeAlO3

(13)

The textural properties (BET surface area, pore volume and main pore size) of calcined and reduced catalysts are summarized in Table 1. According to the IUPAC classification [41], calcined CP, WI and CA catalysts belonged to IV type isotherms, characteristic of mesoporous materials, whereas the calcined SG sample showed a composite isotherm between type IV and type II, indicating the presence of mesoporous and macroporous structures (Fig. S1A in the Supporting information). The hysteresis loop of SG samples was H3-type, characteristic of aggregate particles with no uniform size and shape, whereas on the other samples was H1-type, indicating the presence of cylindrical mesopores. The PSD curve analysis (Fig. S1B in the Supporting information) showed that CA sample had the most uniform pore texture with small primary pore width of 4.0 nm. The SG sample showed a broad pore size distribution mainly in the mesoporous region (<50 nm) with a primary pore width of 4.1 nm, with shoulders at 6.9 and 11.8 nm, and a broad and weak peak centred at 50 nm, suggesting the presence of some macropore. The WI and CP samples showed a broad pore size distribution with large mesopore of 9.8 and 17.0 nm, respectively. The BET surface area ranges from 71 to 285 m2 g−1 depending on the preparation method. The large surface area of CP sample was due to the use of surfactant in the synthesis whereas the low surface area of SG sample was attributed to the rapid hydrolysis and condensation of the alkoxide precursor. The N2 adsorption–desorption isotherms of reduced catalysts showed the same IUPAC classification of calcined catalysts, suggesting that the mesoporous structure was mostly retained after the thermal treatment at 1073 K. PSD analysis of CA sample showed the shrinkage of pore size; on the contrary a slight increase of pore size was observed for WI and SG samples. The latter showed a bimodal distribution in the mesoporous region confirming its heterogeneity. The CP sample did not change the primary pore size, however its pore distribution become narrow. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.05. 004. 3.2. XPS To get a deeper insight on the electronic and chemical properties at the surface of the samples, XPS studies have been performed. With this aim, the C1s, Al2p, Ni2p and Ce3d core level spectra have been collected and analyzed. The core level binding energy (BE) and the full width at half-maxima (FWHM) were analyzed with particular attention to the Ni2p and Ce3d signals components, which are of major interest for rationalizing the observed catalytic activity. The BE, FWHM and atomic percent values observed for calcined and reduced catalysts are collected in Table 1S in the Supporting Information. Al2p core level spectra were also investigated. In agreement with the literature, the individual BE positions for the Al2p signal contribution strongly depend on the preparation condition of the samples, thus Al O associated peaks are found in quite wide BE ranges. Al2p3/2 BE is observed between 71.8 and 74.2 eV (Table 1S). As an example, Al2p spectrum of CA is reported in Fig. 2c. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.05. 004

Ni2p spectra are made complicated by the presence of high BE satellites adjacent to the main peaks; as an example, CA Ni2p spectrum is reported in Fig. 2a. However, by following a peak fitting procedure, a single pair of spin–orbit components was individuated for all calcined samples. The spin–orbit component of higher intensity 2p3/2 was taken as reference, and with the exception of SG sample, was found at about 856 eV, typical of oxidized nickel. This value was slightly higher than that reported for bulk NiO, characterized by a BE of about 854 eV, and was ascribed to the presence of Ni2+ in strong interaction with the support [42]. SG sample showed an intermediate situation with a BE value of 855 eV suggesting the presence of Ni2+ in weak interaction with the support. The above assignments were supported by the XRD analysis detecting bulk NiO only on SG sample. For reduced samples, a second contribution was observed at lower BE values (Ni2p3/2 = 853.0–853.5 eV), indicative of metallic Ni [25]. However, the spectral contribution arising by Ni(OH)2 on Ni particle surface [43] was still intense (about 50% of total Ni2p signal), due to the sample exposure to the atmosphere during preparation and prior the introduction into the UHV XPS measurement chamber. Ce3d core level spectra were widely and deeply investigated because Ce3+ ions in CeAlO3 species, formed under reaction condition, are involved in the CO2 dissociative adsorption (Eq. (5)). This step has been proposed to enhance the conversion of CHx intermediates at the metal support interface, avoiding their accumulation as carbon (Eq. (6)). Therefore it is expected that samples with higher Ce3+ amount are the most efficient. Ce3d spectra of all samples are reported in Fig. 2b. Ce3d spectra were extremely complicated, due to the different components arising by Ce3+ and Ce4+ ions and by Ni2p1/2 satellites superimposed to the first peaks. By following a peak-fitting procedure, five spin orbit pairs related to Ce3d were individuated, and the resulting components were associated to different ions by comparison with literature data [34,44]. The small peak observed at higher BE values (nearly 916 eV BE), that is not observed in pure CeAlO3 reference samples [34], is the 3d3/2 spin orbit component associated to the higher BE 3d5/2 peak (in purple in Fig. 2b) and can be associated to the presence of Ce4+ ions [44]. By estimating the intensity percent of this peak, it was possible to compare Ce4+ amounts in the calcined and reduced samples. The obtained values, indicative for oxidized Ce4+ cerium amount trend, are collected in Table 2. On calcined samples, the intensity of the Ce4+ 3d3/2 peak was very similar to that of pure CeO2 for CP, WI and SG, whereas it was particularly less intense for CA. After reduction at 1073 K the intensity of the peak lowered, indicating that CeO2 was partially reduced to CeAlO3 -like species, which are stable in the atmosphere during preparation and prior the introduction into the UHV XPS measurement chamber. The reduction extent depended by the preparation method. More in detail it is possible to distinguish the reduced samples in two types: WI and SG with high Ce4+ content, namely less reducible, and CA and CP with low Ce4+ content, namely more reducible. 3.3. Reducibility From the literature it is known that CeO2 supported on ␥Al2 O3 is reduced in three main temperature regions. Peaks at low temperature (≈773–873 K) correspond to the reduction of surface Ce4+ of small ceria crystallites. Peaks at intermediate temperature (≈900–1100 K) correspond to the reduction of large and bulk ceria crystallites. Peaks at high temperature (>1100 K) correspond to the reduction Ce4+ → Ce3+ of bulk related to the CeAlO3 formation. The position of these peaks is strongly dependent from the CeO2 loading, the interaction with ␥-Al2 O3 and the particle size [32,34,45,46]. Regarding Ni2+ supported on ␥-Al2 O3 , four species with increasing reduction temperatures have been recognized in the literature: bulk NiO, scarcely interacting with ␥-Al2 O3

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Fig. 2. XPS core level spectra of (a) Ni2p (sample CA); (b) Ce3d (from top to bottom: sample CP, WI, SG, CA. The spectral component associated with Ce3d5/2 signal of Ce4+ is in purple); (c) Al2p (sample CA). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(Ni-␣ species), reduced in the temperature range of pure NiO (573–753 K); NiO interacting with the support (Ni-␤1 species), like bi-dimensional NiO–AlOx monolayer, reduced at mild temperature (773–873 K); non-stoichiometric spinel in strong interaction with the surface (Ni-␤2 species) and bulk NiAl2 O4 (Ni-␥ species), both reduced at high temperatures (873–1153 K) [47–49]. The H2 -TPR profiles of samples reduced up to 1273 K are reported in Fig. 3. Several overlapped peaks corresponding to reduction processes described above were observed. All samples showed two weak peaks below 750 K ascribed to the reduction of Ni-␣ and of Ce4+ located at the surface of CeO2 nanoparticles [50].

In agreement with XRD and XPS analysis, SG sample showed Ni␣ peak with the highest intensity. CP sample showed a large and asymmetric peak with maximum at 1090 K attributed to the reduction of Ni-␤2 and Ni-␥ species, and a very weak peak at 811 K due to the reduction of Ni-␤1 . WI sample showed a main and broad peak at 1090 K ascribed to the reduction of Ni-␤2 and Ni-␥, a shoulder at 846 K assigned to the reduction of Ni-␤1 , and a sharp peak at 1187 K due to the reduction Ce4+ → Ce3+ with formation of CeAlO3 . SG catalyst showed a similar TPR profile with broad and overlapped peaks at 864, 930 and 1043 K assigned to the reduction of Ni-␤1 , Ni-␤2 , Ni-␥ species, respectively, and a sharp peak at 1168 K indicative of

Table 2 XPS Ce4+ diagnostic component percent (depth: 1–4 nm max); hydrogen consumption in TPR experiments and Ce3+ percentage in calcined catalysts. Catalyst

CP WI SG CA

XPS analysis

TPR analysis

Ce4+ diagnostic peak %

H2 consumption (mmol g−1 ) a

% Ce3+ d b

c

Calcined

Reduced at 1073 K

Theoretical

TPR 1273 K

TPR 1073 K

19.0 26.7 23.0 9.3

2.5 10.0 14.9 1.6

2.17 2.29 2.24 2.45

2.04 2.19 2.17 2.13

1.94 1.70 1.65 2.12

5.85 4.27 2.65 12.9

a Theoretical H2 consumption for the reduction of Ni and Ce assuming as reduction reactions Ni2+ + H2 → Ni0 + 2H+ and 2Ce4+ + H2 → 2Ce3+ + 2H+ . Ni and Ce content by EDAX analysis used in calculation. b Hydrogen consumption in TPR experiment up to 1273 K. c Hydrogen consumption in TPR experiment up to 1073 K. d Percentage of Ce3+ calculated according to Eq. (14) (see text) using the hydrogen consumption in TPR experiment up to 1273 K.

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analysis of the Ce3d core level. In fact, CA sample showed the lowest intensity of the peak at 916 eV BE assigned to Ce4+ . With the aim of studying the effect of the activation treatment used in the DRM reaction on the oxidation state of the sample, H2 -TPR experiments were also conducted up to 1073 K and then maintained in isothermal step. During the isothermal step at 1073 K, the hydrogen consumption rapidly decreased to the baseline (Fig. S2 in the supporting information). The hydrogen consumption (Table 2) of CA and CP samples was comparable to that observed in the H2 -TPR up to 1273 K, conversely, that of WI and SG samples was significantly lower. Since the reduction temperature of Ni-␥ species (the most difficult to reduce) in WI and SG samples was similar or lower than that observed in CP and CA (Fig. 3), and taking into account the very similar hydrogen consumption in TPR up to 1273 K and up to 1073 K of CP and CA samples, it is reasonable to conclude that the Ni2+ species in all samples were completely reduced. Therefore, the lower hydrogen consumption in reduction up to 1073 K of WI and SG samples may be attributed to the lower reducibility of Ce4+ species, in agreement with XRD and XPS analysis. Indeed XRD diffraction lines of CeO2 were still observed on WI and SG samples after reduction up to 1073 K, whereas they almost disappeared on CP catalyst (Fig. 1B). Moreover, XPS analysis of samples reduced at 1073 K, showed that the intensities of the Ce4+ peaks of WI and SG samples were approximately six times higher than those observed on the other specimens, indicating larger amount of unreduced CeO2 (Table 2). Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.05. 004 3.4. Catalytic activity

Fig. 3. H2 -TPR profiles of catalysts calcined at 873 K: CA (a); SG (b); WI (c); CP (d).

the CeAlO3 formation. CA sample showed a main reduction peak at 930 K attributed to the reduction of Ni-␤1 and Ni-␤2 and a less intense peak at 1123 K assigned to the reduction of Ni-␥. Among the catalysts, CA has the highest intensity of Ni-␤1 and Ni-␤2 peaks. In all samples the weak peak of bulk CeO2 reduction was not distinguished because superimposed to Ni reduction. Moreover in CP and CA samples also the high temperature peak of CeAlO3 formation is not observed. However, as reported in the literature, the CeAlO3 peak position may depends from many features like CeO2 loading, particle size and support interaction. Furthermore, the quantitative analysis of the hydrogen consumption (see below) suggests that the reduction of both cerium species occurred. These considerations let us to claim that also CeAlO3 peak in CP and CA catalysts was masked by Ni reduction. The hydrogen consumptions are reported in Table 2. The theoretical hydrogen consumption for the Ni2+ → Ni0 and Ce4+ → Ce3+ reduction is calculated from the chemical composition. The experimental hydrogen consumption was slightly lower than the theoretical one for CP, WI and SG catalysts whereas it was significantly lower for CA sample. Assuming that the Ni2+ species were completely reduced to Ni0 , the difference between the theoretical and experimental hydrogen consumption ([H2 ]) suggested that part of cerium was present as Ce3+ after synthesis and the corresponding amount, was calculated as percentage according to Eq. (14), where Ce(EDAX) is the cerium content obtained by EDAX. %Ce3+ = 100

[H2 ] 2 · Ce(EDAX)

(14)

The CA sample showed the largest amount of Ce3+ followed by CP, WI and SG samples. This finding was in line with the XPS

The methane and carbon dioxide conversions as a function of temperature and time on stream are reported in Fig. 4a and b. Being endothermic, the DRM reaction is thermodynamically and kinetically favoured at high temperatures, therefore, the conversions increased with the increasing of temperature in the studied range 873–1073 K. The CO2 conversions were higher than methane conversions at all temperatures, indicating the occurrence of the side reverse water gas shift reaction (RWGS). Both methane and CO2 conversions slightly decreased during 5 h of time on stream. The deactivation was mild at high temperatures, for example at 1073 K only WI catalyst showed a 4% deactivation, whereas on CP and CA catalysts the conversions remained almost constant and on SG catalyst they slightly increased. The catalytic activity strongly depended on the preparation method. At 873 K the activity followed the sample order SG < WI < CA < CP whereas at 1073 K the order changed in WI < SG  CA < CP. In particular, at 1073 K the highest methane conversion was obtained for CP and CA catalysts with similar values of 75% and 70%, respectively. The H2 /CO ratio as a function of temperature and time on stream is reported in Fig. 5. Consistently with the RWGS occurrence, the H2 /CO ratio was lower than unity at all temperatures. However, its values increased with the increasing of temperature, suggesting a better selectivity towards the DRM at high temperatures. The highest H2 /CO ratio value of ≈0.9 was observed for the most active CP and CA catalysts at 1073 K. The mechanism of DRM reaction has been widely investigated [51–55]: CH4 dissociates over Ni0 sites leaving reactive carbon atom C*. This carbon atom may be converted to CO by reacting with oxygen deriving from CO2 activation on Ni0 or on support sites. The dehydrogenation of CH4 on Ni0 sites has been recognized as the rate determining step for the DRM reaction. In order to compare the specific DRM activity of the catalysts, the STY of hydrogen at 1073 K was calculated (Table 3). In the DRM the only reactant containing hydrogen is CH4 , however the H2 produced by the DRM may further react with CO2 by the

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19

Table 3 Ni dispersion, active Ni site, STY and oxygen consumption of used catalysts. Catalyst

DNi a

Ni sites (mol × 106 )b

STY (s−1 )

CP WI SG CA

2.9 5.9 6.9 11.4

2.3 5.1 5.5 10.4

16.0 5.3 4.7 3.2

a b

Oxygen consumption (mmol g−1 ) C␣

C␤

74.33 9.60 – –

8.73 46.45 27.67 7.48

Ni dispersion calculated by the Vannice method (Eq. (10)). Active Ni sites calculated as DNi × Ni mol.

Fig. 5. H2 /CO ratio as a function of time on stream under different reaction temperatures:  CA;  SG;  WI; ♦ CP. Reaction condition: catalyst loading 0.050 g, reactant mixture composition CH4 :CO2 :Ar = 40:40:20 vol.%, GHSV 90,000 cm3 g−1 h−1 .

Fig. 4. Methane conversion (A) and carbon dioxide conversion (B), as a function of time on stream under different reaction temperatures:  CA;  SG;  WI; ♦ CP. Reaction condition: catalyst loading 0.050 g, reactant mixture composition CH4 :CO2 :Ar = 40:40:20 vol.%, GHSV 90,000 cm3 g−1 h−1 .

RWGS. Therefore, in order to exclude the H2 consumption by RWGS reaction, the H2 produced was calculated from the methane consumption. The STY increased with the increasing of Ni crystallite size. This behaviour is consistent with the observations of several authors [18,56] that reported the increase of TOF with Ni particle size. Moreover it is known that the rate of methane dehydrogenation depends on Ni particle size [57]. Thus, the trend of specific DRM activity observed in the present study, was primarily due to the different ability of Ni nanoparticles to activate the C-H bond, which

is mainly related to the particle size. On the other hand, small Ni crystallite size provides a high number of active sites; therefore the sample CA, despite having the lowest STY, showed nearly the same activity of the CP sample having the highest STY value. Differences in H2 /CO ratio for the catalysts reflected the differences in the catalytic activity and the lower reactants conversion yields a lower H2 /CO ratio. In fact the RWGS, which is responsible for the low H2 /CO ratio, is reported to be near equilibrium in the 700–850 K temperature range [4,58]. At high temperature the DRM is favoured and the CH4 and CO2 reactants are converted efficiently. Therefore the less amount of CO2 available for the RWGS gives H2 /CO ratio closer to unity as expected when only the DRM reaction occurs. Coke may be formed during DRM reaction by the conversion of accumulated C* atoms to less reactive carbon species, which may encapsulate the surface of Ni0 or may dissolve in the nickel crystallite. The dissolved carbon diffuses through the nickel to nucleate at the metal-support interface forming carbon filaments [59]. The carbon formation rate (rc ) and the amount of accumulated carbon during 5 h of time of stream at different temperatures are reported in Fig. 6a and b. On all catalysts the highest rc was observed at 873 K. The rc abruptly decreased with the increasing of temperature up to 973 K and it decreased less or do not vary at higher temperature. The high rc observed on our catalysts at low temperature is attributed to a low rate of the C* intermediates oxidation and to the occurrence of the CO disproportionation reaction. Moreover, CP catalyst showed a slight increase of rc above 973 K, suggesting that on this catalyst the C* was gasified slower in comparison with the other catalysts. The rc increased in the following sample order CA  SG < WI  CP, namely it increased with Ni0 crystallite

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Fig. 7. XRD patterns of the used catalysts: CA (a); SG (b); WI (c); CP (d).

carbon (JCPDS 41-1487) in addition to phases observed on the freshly reduced catalysts. The peak intensity was in agreement with the carbon amount weighted at the end of the catalytic test, being the highest for CP and the lowest for CA. Furthermore, the Ni0 crystallite size on used samples was nearly the same than on reduced samples indicating that during the catalytic cycle, the metal sintering was minimal. Raman spectroscopy was used to investigate the order of the deposited carbon on the used Ni catalysts (Fig. 8). For each sample, at least three Raman spectra were collected in different areas to assess the homogeneity of the investigated material. All the spectra collected on the same sample showed exactly the same features thus confirming the homogeneity of the carbon deposits. Raman spectra of CP, SG and WI samples display two main bands at 1325 (D band) and at 1576 cm−1 (G band), a shoulder at 1590 cm−1 (D band) and a less intense band at 2642 cm−1 (G band). Raman spectra of CA sample shows two large bands at 1320 (D band) and 1593 cm−1

Fig. 6. Carbon formation rate at different temperatures (A), and the corresponding cumulative carbon deposition (B), each temperature lasted for 5 h: CA (red), SG (blue), WI (black), CP (green) catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

size, confirming the structure-sensitive nature of methane decomposition reported by other authors [19]. The mass of carbon deposited on the surface, increased with the increasing of temperature reaching a plateau (Fig. 6b). On all samples the much greater amount of carbon was accumulated at 873 K; at higher temperature the carbon increased by a small amount, suggesting a better oxidation of C* carbon. The amount of deposited carbon strongly depends on catalyst preparation method being nearly negligible on CA and about 13 times greater on CP catalyst. In order to shed light on the possible reasons of the stability of catalysts having different amount of coke, the deposited carbon was investigated by XRD, Raman and TPO analysis. 3.5. Characterizations of used catalysts The XRD patterns of the catalysts after catalytic tests (Fig. 7) showed a broad peak at about 2 = 26.1◦ corresponding to graphitic

Fig. 8. Raman spectra of the used catalysts: CA (a); SG (b); WI (c); CP (d).

I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

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quantity of C␤ compared to samples CP and CA, with high Ce3+ content. Despite this positive effect, coke deposition is mainly driven by the nickel crystallite size, in fact CP catalyst, having the largest Ni crystallite, showed the greater carbon amount. It is worth to note that CP catalyst showed high activity and stability despite of the greater carbon content. This behaviour may be explained considering the nature of the carbon deposits. In fact, the filamentous carbon C␣ does not result in fast deactivation because does not encapsulate Ni0 sites that remain accessible to the reactants. 4. Conclusions

Fig. 9. O2 -TPO profiles of the used catalysts: CA (a); SG (b); WI (c); CP (d).

(D band). The G and G bands are related to the stretching vibration in the aromatic layers of graphite (in-plane displacement of carbon atoms in the hexagonal sheet) and they are the only bands present in perfect crystalline graphite [60–62]. The D and D bands are defect bands that appear when disorder is introduced into graphite structure and they have been assigned to the non-zone centred phonons associated to the disorder-induced vibration of C C bond [63–65]. The relative intensity ratio of D and G bands (ID /IG ) can be used to investigate the graphitization of carbon and the degree of disorder in the structure [66]. An ID /IG ratio near zero indicates high order whereas a ratio around 1 reveals high disorder due to abundant defects in the graphite structure. In our case, ID /IG it is 1.4 for CP and WI samples and 1.3 for SG sample, indicating that graphitic carbon deposits have a high degree of disorder. The absence of G and G’ bands on CA sample indicate that the small amount of carbon deposit, revealed by XRD, has a much higher degree of disorder. O2 -TPO experiments were carried out with the aim to study the reactivity of coke with the oxygen. TPO profiles are reported in Fig. 9 and the corresponding oxygen consumption in Table 3. TPO profiles showed two peaks of oxygen consumption with maximum at about 813 and 945 K, assigned to the oxidation of the C␣ and C␤ carbon species, respectively. In agreement with literature data [20], the C␣ species, corresponding to filamentous graphitic carbon, were oxidized at lower temperature in comparison with the C␤ species, which are more stable moss like graphitic carbon deposits. The C␣ species were the main on CP sample, whereas they were found in low amount on WI and were not observed on the other samples. On the other hand, the C␤ species were the main on sample WI and the only carbon species on SG and CA. TPO analysis showed that the reactivity of carbon deposits depended by their morphology more then their disorder degree. In fact, despite the similar ID /IG ratio observed on CP, WI and SG, they present different distribution of C␣ and C␤ species. The amounts of deposited carbon estimated from the consumption of oxygen in TPO experiment were in good agreement with the increase of the weight of samples at the end of the catalytic test, and with the intensity of XRD peak assigned to graphite. It is interesting to note that on samples CP and CA the content of the C␤ carbon species were similar, suggesting that the nature of carbonaceous residues is also influenced by the presence of CeAlO3 . Indeed samples WI and SG, with low Ce3+ content, have a greater

The dry reforming of methane was studied on catalysts with 10 wt.% Ni and 20 wt.% CeO2 supported on ␥-Al2 O3 having different Ni crystallite size, Ce3+ /Ce4+ ratio and textural properties. The textural properties and thermal stability of catalysts depend on the preparation method. The highest stability of surface area and pore size, is showed by the sample prepared using the citric acid method that, allowing the mixing of cations at atomic level, yields a Ni Ce Al Ox mixed oxide with amorphous phase. By using the co-precipitation and citric acid methods, Al O Ce bonds are formed and CeO2−x results well dispersed within ␥-Al2 O3 lattice. CeO2−x , strongly interacting with ␥-Al2 O3 , presents greater amount of Ce3+ in comparison with segregated CeO2 . The specific catalytic activity for DRM increased with the increasing of Ni crystallite size, regardless of the textural properties and of the Ce3+ /Ce4+ amount. Carbon deposition is limited by operating at temperature higher than 1023 K on catalysts having Ni crystallites smaller than ≈10 nm. The amount of less reactive carbonaceous deposits, graphitic moss carbon, is lower on catalysts containing great amount of Ce3+ stabilized as CeAlO3 like species. The citric acid method results particularly attractive because allows to obtain catalysts with very small Ni crystallite size (5.8 nm) and high content of CeAlO3 , that have a beneficial role in increasing the rate of carbon gasification. The most active catalysts CA and CP are stable during time on stream: because CA catalyst accumulates a low amount of coke, whereas CP accumulates mainly filamentous carbon which does not encapsulate Ni0 active sites, causing only a slightly decrease of the catalytic activity. However excessive carbon accumulation may results in the reactor plugging and in the pressure increase, therefore the catalyst prepared by citric acid method, showing comparable performances with minor carbon deposition, results the most suitable catalyst for long term operation. References [1] M.-S. Fan, A.Z. Abdullah, S. Bhatia, ChemSusChem 4 (2011) 1643–1653. [2] T.V. Choudhary, V.R. Choudhary, Angew. Chem., Int. Ed. Engl. 47 (2008) 1828–1847. [3] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Philos. Trans. A Math. Phys. Eng. Sci. 368 (2010) 3343–3364. [4] M.C.J. Bradford, M.A. Vannice, Catal. Rev. 41 (1999) 1–42. [5] I. Luisetto, E. Di Bartolomeo, A. D’Epifanio, S. Licoccia, J. Electrochem. Soc. 158 (2011) B1368–B1372. [6] J. Kirtley, A. Singh, D. Halat, T. Oswell, J.M. Hill, R.A. Walker, J. Phys. Chem. C 117 (2013) 25908–25916. [7] M. Pillai, Y. Lin, H. Zhu, R.J. Kee, S.A. Barnett, J. Power Sour. 195 (2010) 271–279. [8] C. Agrafiotis, H. von Storch, M. Roeb, C. Sattler, Renew. Sustain. Energy Rev. 29 (2014) 656–682. [9] T. Kodama, A. Kiyama, T. Moriyama, T. Yokoyama, K.I. Shimizu, H. Andou, N. Satou, Energy Fuels 17 (2003) 914–921. [10] N. Gokon, Y. Yamawaki, D. Nakazawa, T. Kodama, Int. J. Hydrog. Energy 35 (2010) 7441–7453. [11] Y. Li, D. Li, G. Wang, Catal. Today 162 (2011) 1–48. ´ J. Batista, A. Pintar, Int. J. Hydrog. Energy 37 (2012) 2699–2707. [12] P. Djinovic, [13] J. Chen, C. Yao, Y. Zhao, P. Jia, Int. J. Hydrog. Energy 35 (2010) 1630–1642. [14] S. Gaur, D.J. Haynes, J.J. Spivey, Appl. Catal. A 403 (2011) 142–151.

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