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Preparation and characterization of active Ni-supported catalyst for syngas production
Accepted Manuscript Title: Preparation and characterization of active Ni-supported catalyst for syngas production Author: S. Candamano P. Frontera A. Macario F. Crea J.B. Nagy P.L. Antonucci PII: DOI: Reference:
S0263-8762(15)00044-1 http://dx.doi.org/doi:10.1016/j.cherd.2015.02.011 CHERD 1787
To appear in: Received date: Revised date: Accepted date:
8-9-2014 23-1-2015 19-2-2015
Please cite this article as: Candamano, S., Frontera, P., Macario, A., Crea, F., Nagy, J.B., Antonucci, P.L.,Preparation and characterization of active Ni-supported catalyst for syngas production, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.02.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and characterization of active Ni-supported catalyst for syngas production S. Candamano 1, P. Frontera 2, *, A. Macario 1, F.Crea 1, J. B.Nagy 1, P.L.Antonucci 2 Dept. of Environmental and Chemical Engineering DIATIC, University of Calabria, Italy: f.crea @unical.it, [email protected], [email protected]; [email protected] 2 Dept. of Civil Engineering, Energy, Environmental and Materials DICEAM , Mediterranea University of Reggio Calabria, Italy: [email protected], [email protected]
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* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: 00390965875308; Fax: 00390965875248
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Abstract
A new type of Ni-based catalyst, using geopolymer as support was prepared and characterized
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using Scanning Electron Microscopy (SEM), Temperature Programmed Reduction (TPR), N2 adsorption-desorption, Nuclear Magnetic Resonance (NMR) and Transmission Electron Microscopy (TEM).
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The catalytic performance of Ni-geopolymer catalyst was investigated in steam reforming (SR), partial oxidation (POX) and autothermal steam reforming (ATR) of ethanol to syngas, at the
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temperature of 700°C. The gaseous mixture flow was adjusted (in terms of molar ratio) for each of the three reactions: steam reforming, steam to carbon (S/C) = 2.5; partial oxidation, oxygen/carbon
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(O/C) = 0.5; and autothermal reforming, S/C = 2.5 and (O/C) = 0.5. Ni species supported on the geopolymer surface resulted to be highly active, with a complete
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ethanol conversion and with the largest amount of hydrogen ( 70 mol%) being produced under the SR conditions. In the spent catalysts, after POX reaction, some coke formation was observed by SEM and Thermogravimetric (TG) analyses. The results showed that geopolymer would be a promising, low cost and environmental friendly material for reforming reactions of ethanol. Keywords: Geopolymer; Nickel; Reforming; Coke formation; Syngas 1. Introduction
The development of alternative methods for hydrogen production, especially from renewable sources, has attracted much attention due to the expected increase in energy demand and the environmental concern related to the air pollution. In this work, we propose the use of geopolymer material as nickel support for catalysts to be employed in ethanol reforming reactions. Geopolymers have been selected also because they can be prepared as pellets and/or foams suitable for application as catalysts or supports (Landi et al., 2013), therefore overcoming some
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issues related to catalysts in powder or structural form, so ensuring very high reliability, low pressure drop and excellent structural stability. Many of these properties are required in applications, such as automotive, where harsh environments include vibrations, thermal cycling and thousands of start-ups and shut-downs.
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Furthermore, the close similarity of the geopolymer network to the zeolites framework, in relation to the accommodation of metal ions, opens the opportunity of using geopolymer materials as amorphous analogues of zeolites (Kriven et al., 2003; Bortnovsky et al., 2008). However, it must
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be highlighted that geopolymers possess some advantages in comparison to zeolites; they are prepared at room or low temperature, are mesoporous, are cheap and have good thermal chemical
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stabilities.
All these aspects open to potential interests for the synthesis of new robust catalysts for
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heterogeneous reactions. Geopolymers differ from crystalline zeolites with respect to precursor materials processing and applications and therefore, they can be regarded as an unique class of materials.
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Current methods for hydrogen production are mainly based on non-renewable fossil fuels. Renewable resources have attracted much attention as hydrogen sources to achieve the full
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environmental benefit for generating power with hydrogen. (Cho, 2004). Ethanol has been proposed as alternative fuel for the indirect internal reformer of molten
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carbonate fuel cells (IIR-MCFCs) and for solid oxide fuel cells (SOFCs) (Cavallaro et al., 2001). Ethyl alcohol has several advantages as hydrogen source, over other usable fuels: since i) it can be
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easily produced from biomass by fermentation (Deluga et al., 2004), (ii) high hydrogen content, (iii) good availability and low production costs, (iv) easy and safe handling and transportation, (v) nontoxicity and (vi) it can be distributed in a logistic net similar to the conventional fuel stations. The catalytic performance of supported metals catalysts for the different reforming conditions (steam reforming (SR), partial oxidation (POX), autothermal reforming (ATR), dry-reforming (DR)) has been previously investigated in a wide temperature range (200-750°C) on several metallic phases (Rh, Ru, Pt, Pd, Ni, Cu-Ni), supports (Al2O3, CeO2, MgO, TiO2, Y2O3, perovkites, zeolites, mesoporous materials), and metal loading (Llorca et al., 2001; Breen et al., 2002; Davda et al., 2005; Pompeo et al., 2005; Urasaki et al, 2008; Lindo et al., 2010, Frontera et al., 2010, 2012, 2013; Al-Fatesh et al.,2014; Lo Faro et al, 2014). One of the major problems of the catalytic reforming of ethanol, is linked to the long term instability of the catalyst arising from deposition of carbon due to C-C bond scission. Aside from noble metals, Ni is so far the best choice for hydrogen production by catalytic reforming of ethanol. Ni, in fact, has high activity for C-C bond and O-H bond breaking and it also has high activity for 2 Page 2 of 24
hydrogenation, facilitating H atoms to form molecular H2. However, Ni-based catalysts suffer of significant coke formation with respect to noble metal catalyst. Support also plays an important role in reforming reactions of ethanol, as it affects the dispersion of metal catalyst and may enhance catalytic activity via metal-support interaction. The support can facilitate the reforming reaction of
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ethanol promoting migration of OH- toward the metal catalyst in the presence of water vapour at high temperature (Cheekatamarla and Finnerty, 2006).
In this work, we have investigated the catalytic behavior of a geopolymer material used as nickel
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support in steam reforming (SR), partial oxidation (POX) and autothermal reforming (ATR) of ethanol, at the temperature of 700 °C, to produce syngas. Ni-supported catalysts show their highest
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performance (“useful conversion”) at temperatures higher than 650°C. In fact, according to thermodynamic predictions, the optimal conditions for hydrogen production by ethanol reforming,
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at atmospheric pressure, are reaction temperatures higher than 650°C and excess water in the feed: under these conditions, the carbon formation is thermodynamically inhibited (Garcia and Laborde, 1991, Breen et al., 2002).
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Fresh and exhaust samples were characterized by usual analytical techniques (XRD, SEM, TGA-
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DSC, TPR-H2, TPD-NH3, N2 adsorption-desorption and multinuclear NMR).
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2.1. Materials
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2. Experimental
Metakaolin having a D10 (D10= 10% weight percentage of the tested sample was equal or finer than the reported value, in microns) of 0.51 μm, a D50 (D50= 50% of the tested sample was equal or finer than the reported value, in microns) of 1.59 μm and a D90 (D90= 90% of the tested sample is equal or finer than the reported value, in microns) 9.74 μm, was purchased from Doldes Manara S.r.l.; all these values were provided by the manufacturer. Sodium silicate solution was provided by Condea Augusta S.p.A. The weight composition of metakaolin, as determined by X-ray fluorescence and sodium silicate solutions is listed in Table 1. Metal precursor (Ni(NO3)2*6 H2O) was supplied by Sigma-Aldrich.
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Table 1. Chemical composition of starting materials and geopolymer. Sodium Silicate
Geopolymer
(wt.%)
solution (wt.%)
(%molar)**
Al2O3
42.0
–
21.8
SiO2
53.2
29.5
58.8
K2O
0.3
–
–
Na2O
0.1
13.8
Fe2O3
1.5
–
TiO2
1.9
–
LOI*
1.0
H2 O
–
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17.6
0.72 1.06
–
–
56.7
–
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2.2.1. Ni-support synthesis
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2.2. Methods
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*Loss of Ignition at 955°C; **dry basis
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Metakaolin
Component
Geopolymer was synthesized by activating aluminosilicates source with an alkali activator solution (Davidovits, 1982). The activator solution, containing sodium silicate solution, sodium hydroxide and ultrapure water in the weight ratio of 3.31:1:1.96, was prepared using the following procedure. Firstly, the sodium hydroxide solution was obtained by dissolution of NaOH pellets in ultrapure water, with container kept sealed wherever possible to minimize contamination by atmospheric carbonation and prevent water evaporation. The solution was stirred until the NaOH pellets had dissolved and the solution became clear. During this process, a significant amount of heat is released. So it was allowed to cool back down to room temperature. Once cooled down it was poured into sodium silicate solution. The so obtained alkali activator solution was covered, sealed, stirred and allowed to cool back down to room temperature. Finally, the activator solution was added to metakaolin powder and the slurry was mechanically vigorously mixed for 10 minutes before transferring it to mould sealed from the atmosphere to prevent the loss of water and cured it for 24 h at 40°C. The sealed specimen was then stored at ambient temperature and pressure for two 4 Page 4 of 24
weeks to complete the curing. The monolith with a final theoretical composition, in terms of oxides, of 0.77Na2O-2.81SiO2-Al2O3-9.85H2O was then grinded, characterized and tested.
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2.2.2 Ni-catalyst preparation Nickel catalysts were prepared by incipient wet impregnation of the geopolymer support using an ethanol solution of the metal precursors Ni(NO3)2*6 H2O (Sigma-Aldrich) with the proper
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concentration to obtain a Ni content of 5 wt. %. This value is the optimum Ni loading, used in the most of hydrocarbons reforming reactions because it allows to obtain a good compromise between
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catalyst activity and coke formation (Chang et al., 1996, Wang and Lu, 1998, Breen et al., 2002). The impregnated catalyst has been dried in air at 100°C for 12 h. Prior to reaction, catalysts were in
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situ reduced under flowing hydrogen / argon (5 v/v%, 20 ml min-1) at 500°C for 2 h with a heating rate of 5 °C min-1.
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2.2.3. Catalyst characterization
Phase identification of prepared catalysts was carried out by X-Ray diffractograms, using CuKα
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time of 5 seconds per step.
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radiation in 2θ = 5°- 80° range (D2-Phaser Bruker, λ = 1.5404 Å) with step size of 0.0026 and count The morphologies of metakaolin and geopolymer were examined by a scanning electron
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microscope (SEM, FEI model Inspect). N2 adsorption-desorption isotherms were measured in order to examine the porous properties of geopolymer before and after Ni impregnation. The measurements were carried out in Micromeritics ASAP 2020 instruments. Before the analysis, all samples were pre-treated in vacuum condition at 200°C for 12 hours. Temperature programmed reduction (TPR) was carried out with a Chemisorb Micromeritics 2750 instrument, to monitor the reduction of metal oxide, under a flux of 50 cc min-1 of H2/Ar (10 vol.%) in the temperature range 25-800°C at atmospheric pressure. The NH3-TPD analysis was carried out in the conventional flow apparatus TPD/RO 1100 Thermofinningan, equipped with a thermal conductivity detector (TCD). The sample was pretreated at 450°C for 1 h under helium flow. After cooling, the sample was pretreated with a mixture of NH3-He at isothermal conditions, 100°C for 1 h in order to have only the chemisorbed ammonia. Then the sample was heated in the range 100-600°C at a rate of 10°C min-1. The TPD curve of the desorbed ammonia was elaborated in order to obtain the area and the top temperature of the desorption curve. 5 Page 5 of 24
High resolution transmission electron microscopy (HRTEM) investigation was carried out using a JEOL 2010F instrument equipped with a field emission gun, which allowed to achieve a point-topoint resolution of 0.19 nm and a resolution of 0.14 nm between lines, and a Philips CM12 electron microscopy provided with a high resolution camera. (SHIMATZU) under air flow of 50 cc/min, with heating rate of 5 °C min-1.
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The thermoanalytical measurements were performed on an automatic TG/DTA instrument Solid state 29Si-NMR and 27Al-NMR analyses were used to study the Si chemical neighbourhood Bruker Avance 500 spectrometer. For 27
Si (99.4 MHz), at 6 µs (θ = π/6) pulse was used with a
Al (130.2 MHz), at 1 µs (θ = π/12) pulse was used with a repetition
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repetition time of 40 s, for
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of metakaolin and geopolymer before nickel deposition. NMR measurements were performed on a
time of 0.2 s.
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2.2.4. Catalytic activity measurements
Catalytic activity experiments were performed at atmospheric pressure in a quartz microreactor
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(internal diameter = 4 mm) placed in a ceramic tube furnace, at a space velocity (GHSV) of 120,000 h−1. The catalyst (~50 mg), placed between quartz wool in the middle of the reactor, was diluted
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with quartz (quartz/catalyst = 1.5). The reaction temperature was monitored with a thermocouple inserted into the reactor bed through a quartz tube. The temperature of the reactor bed was kept
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constant by an electronic controller. The maximum deviation measured in the reactor bed from the nominal temperature was ~10 °C. An isocratic pump (Varian ProStar 210) connected to an
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evaporator, that was heated at 220 °C, was used to feed water in the gas stream and to control the steam to carbon ratio (S/C) in the reaction gas mixture; N2 was used as internal standard. The gas lines were heated at approximately 120 °C to prevent condensation. The microreactor was operated in a down-flow mode with the gas inlet placed at the top of the reactor. Gases were fed with properly calibrated reaction mass flow controllers (Brooks 5850S) after purification by Deoxo Gas Clean Filters (Chrompack) and Molecular Sieve Traps. Reaction products were analyzed by on-line gas chromatograph (GC Agilent 6590) having FID and TCD detectors and four columns (Alumina, Porapak Q, Haysep, Molecular Sieves (MS 5A), for their separation and detection. The catalytic performance of catalyst was investigated in order to evaluate the effect of the reaction conditions, i.e. steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR), at the temperature of 700°C.The gaseous mixture flow was adjusted (in terms of molar ratio) for each of the three reactions: steam reforming, steam to carbon (S/C) = 2.5; partial oxidation, oxygen/carbon (O/C) = 0.5; autothermal reforming, S/C = 2.5 and (O/C) = 0.5. 6 Page 6 of 24
Ethanol, dry oxygen and steam flow rates were adjusted according to the assigned values S/C = 2.5 and O/C = 0.5. Therefore, the inlet flow was fixed as in Table 2, using nitrogen as a balance. The diluted used feed contains an inert gas N2, and the analyses of outlet stream takes into account the change in the number of molecules between reactants and products, with a correction factor
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valued as: =N2OUT/N2by-pass . The outlet product percentages were valued on dry basis. Table 2: Stream of reactant and inert gases at the inlet of the catalytic reactor: H2O cc min-1
(vapour)
(steam)
SR
13.33
66.67
POX
13.33
--
ATR
13.07
65.35
O2 cc min-1
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--
N2 cc min-1
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C2H5OH cc min-1
20.00
6.66
80.00
6.53
15.05
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Reaction
In order to reduce the coke formation and in accordance with thermodynamic predictions, the
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steam/carbon (S/C) ratios used in the reforming feeds (SR and ATR) were higher than the stoichiometric ones (Garcia and Laborde, 1991). The oxygen/carbon (O/C) ratio used in the partial
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3.1. Catalyst characterization
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3. Results and Discussion
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oxidation reaction was the stoichiometric one.
Figure 1 shows the XRD patterns of metakaolin and of the synthesized geopolymer.
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Intensity [a.u.]
Figure 1. Powder XRD patterns of (a) metakaolin and (b) geopolymer
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b
10
20
30
2 th e ta
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a 40
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Metakaolin exhibits a pronounced broad hump with few peaks centered at approximately 22°
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2theta, indicating that it contains essentially amorphous silica and alumina with crystalline phases such as quartz (Duxson et al., 2007). For the synthesized geopolymer the characteristic broad hump
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centered at approximately 28° 2 is observed. It can be considered the distinguishing feature of the diffratogram of any geopolymer, and it shows strong similarity with diffractograms of zeolite precursors. All sharp peaks from crystalline phases in parent material are still present in the pattern
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of geopolymer, indicating that the crystalline phases are not involved in the geopolymerization
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reaction, but they rather behave as inactive fillers in the geopolymer binder. This is in agreement with the current understanding that only the amorphous phases in raw materials are reactive and
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involved in geopolymers reaction. On the DSC curve of geopolymer (Figure 2) there is a remarkable endothermic shoulder at around 155°C. Compared with the mass loss on the TG curve, it is clear that the endothermic effect on the DSC is due to the removal of adsorbed water from the sample. The associated mass loss is about 15% and no more mass loss occurs thereafter.
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Figure 2. TG and DSC geopolymer curves 100
4
EXO DS
2
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90
DSC signal
Weight loss [%]
95
TG 85
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80
0
0 0
100
200
300
400
500
Temperature [°C]
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20
600
700
800
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The micro-morphological features of the parent material and geopolymer obtained are compared in Figures 3a,b. Metakaolin particles are dominantly plate in shape, instead the obtained geopolymer shows a dense and continuous gel-like matrix with few clear particles or particle
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boundaries.
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Figure 3. SEM micrographs of metakaolin (a) and geopolymer (b, c) at different magnifications. a) b) c)
The 27Al NMR spectrum of geopolymer (Figure 4a) exhibits a dominant 27Al line at 59 ppm due to tetrahedral aluminum sites, as expected for a true geopolymer. The absence of the resonance line at 28 ppm due to Al(V) clearly shows that all Al(V)contained in metakaolin is consumed during the geopolymerization process. The weak 27Al NMR line at about 4 ppm is due to some presence of octahedral aluminum sites. The amount of Al(VI) calculated as a 27
percentage of the whole amount of Aluminum detected by
Al MAS NMR is around 1.23%.
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The 27Al weak resonance at 79 ppm is due to Al(OH)4 in the pores of geopolymer, since these units have 27Al chemical shift range around 80 ppm. This confirms that metakaolin upon addition of 9 Page 9 of 24
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the sodium silicate solution releases Al(OH)4 monomers as previously observed for aluminosilicate materials (Walther, 1996; Bauer and Berger, 1998; Duxson et al., 2005). The amount of Al(OH)4 calculated as a percentage of the whole amount of Aluminum detected by
27
-
Al MAS NMR is
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around 3.5%. In Figure 4b the Si NMR decomposed spectrum of geopolymer is shown, while the
m0
Si ( mAl )
]
4
(1)
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m0
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4
I [ Si Al m I Si (mAl ) 4
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lines position and their fractional areas are reported in Figure 5. The Equation 1:
where ISi(mAl) is the intensity of each line, was used in order to validate the accuracy of the decomposition, with respect to the nominal composition of geopolymer. The calculated Si/Al
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composition is 1.6, slightly above the nominal one, and this occurrence is in agreement with the presence of aluminum which is not incorporated into the matrix, as it was found in
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Al NMR
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spectrum. Any unreacted metakaolin exhibits an unchanged resonance line at around -109 ppm,
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while line at -115 ppm can be attributed to quartz impurities, as confirmed by XRD analysis.
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Figure 4. MAS NMR spectra of geopolymer (a) 27Al and (b) 29Si. a)
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b)
Figure 5. Line frequency and fraction of Q4(mAl) silicon sites used in decomposition of MAS NMR spectrum of geopolymer
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Si
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The chemical composition of geopolymer, in terms of oxides, obtained by EDS analysis performed by at least 10 measurements point, is reported in Table 1. The SiO2/Al2O3 and Na2O/Al2O3 molar ratios were found to be 2.7 and 0.8 respectively, which are close to the nominal ones. There is also a good agreement between the nominal formulations
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and the amount of Nickel after impregnation and reduction obtained by EDS, as reported in Table 3, which indicates that nickel impregnation procedure was carried out satisfactorily.
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Table 3. Support and catalyst Textural properties Ni content Sample name SBET (m2 g-1) VTot (cm3 g-1) (wt. %) Geopolymer 50 0.22 Ni-geopolymer after 5 45 0.17 impregnation Ni-geopolymer after 5 44 0.16 reduction
The textural properties (BET surface and micropore volume) of the catalyst and support were
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also summarized in Table 3. The bare support exhibits a bimodal pore size distribution (calculated by BJH method applied on desorption branch of N2 isotherm) centered at 3.8 nm and 18 nm
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respectively. A low presence (5 v/v%) of microporosity has been also observed by t-plot method. Its presence could be explained by the large volume of interconnected pores in combination with some
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level of crystallinity in alkali-activated specimens (Ho et al., 1995). The incorporation of nickel to the support affects the textural properties of the bare support.
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The impregnated and reduced catalyst show an expected and low suppression in surface area, which can be associated to pores being blocked by lower amounts of nickel incorporated on the surface, also confirmed by the reduction in the pores volume, as reported in Table 3. To study the reducibility of Ni(NO3)2 precursor species and metal-support interaction, TPR-H2 experiment were performed. TPR test was also performed on the bare support for comparison. The results showed that the support had no H2 consumption peaks indicating that it retained the oxide form as expected (Figure 6, line a).
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Hydrogen uptake [a. u.]
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Figure 6. TPR profile for (a) geopolymer support b) Ni-geopolymer catalyst
a
200
300
400
500
600
700
800
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b
Temperature [°C]
For the supported nickel sample the TPR profile showed different sets of H2 consumption peaks.
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The peaks at around 300°C can be attributed to the reduction of NO3- anion (Ho and Chou, 1996). Besides, the hydrogen consumption peaks appearing in the 390-450°C temperature region can be associated to Ni0-Ni2+ reduction (Diskin et al., 1998).
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In addition to this, the black color of the reduced material suggest the reduction of Ni-species to Ni . Likewise, the obtained H2 consumption value (0,84 mmol H2.g cat-1) is consistent with the total
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amount of nickel on the support.
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Comparing the TPR-H2 behaviour of nickel based support geopolymer with an analogous support such as Al-MCM-41 (Diskin et al., 1998) or other nickel support as Al2O3 , SiO2 (Chen et al., 1991) it results that geopolymer has a better reducibility of Ni-species which depends on the low nickel geopolymer interaction. TPR results are also confirmed by the TEM images of the catalyst after reduction at 500°C, (Figures 7a , 7b).
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Figure 7. TEM images of Nickel catalyst b)
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a)
The dark spots represent Ni species, whereas the lighter ones are those of the geopolymeric support. It can be observed that uniformly distributed nickel particles with size ranging between 10
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to 20 nm have been obtained. The diffraction peaks (not shown) corresponding to nickel metal was detected by XRD analysis (PDF card 00-004-0850). The nickel crystallite size was calculated using
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Ni (111) reflection and the Scherrer equation (Lemaitre et al.,1985). The average particle size
3.2. Catalytic activity results
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obtained is 10 nm in agreement with TEM analyses.
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The reforming process of converting ethanol to hydrogen-rich gas can be classified into: Steam reforming (SR) C2H5OH+3H2O → 2CO2+6H2
ΔH°=347,4 kJ mol-1
Partial oxidation (POX) C2H5OH+1/2 O2 → 2CO+ 3H2
ΔH°=14,1 kJ mol-1
Autothermal reforming (ATR) C2H5OH +2H2O+1/2 O2 → 2 CO2+5H2
ΔH°=-50,0 kJ mol-1
Steam reforming is an endothermic process and requires energy input to initiate reactions. Alternatively, hydrogen can be obtained by partial oxidation of ethanol, however the hydrogen selectivity is generally low. In order to enhance hydrogen production, autothermal reforming can be applied. This reforming process is a combination of SR and POX reactions, which has higher efficiency than the other two cited processes. Then the catalytic behavior of the catalyst was studied in spot tests (10 h) for the steam reforming (SR), autothermal reforming (ATR) and partial oxidation (POX) reactions, at T=700°C. The performance of the catalyst, in terms of ethanol conversion vs reaction time, at temperature of 700°C, under SR, ATR and POX conditions, was, for all the investigated processes, quite stable and almost equal to 100%. The percentages of the products result also quite stable over this catalyst 14 Page 14 of 24
during spot tests, so it is possible to use their average in order to have a term of comparison between the various reactions. From the data reported in Table 4, it is evident that the product distribution depends on the reaction type. SR in comparison to the other two processes, as expected, gives the
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highest yield of hydrogen.
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Table 4 Average percentage values of the products (dry basis) at 700°C. mol% Products of outlet stream* CH4 H2 CO CO2 C2H4 H2/CO2** SR 4.7 70.0 14.3 11.0 0.1 6.4 (3.0) POX 11.0 53.2 25.4 9.1 1.3 5.8 (-) ATR 10.3 56.1 12.0 20.7 0.9 2.7 (2.5) *All values are the media of three repeated experiments. ** Values measured in bracket theoretical/stoichiometric value
The largest amounts of hydrogen was produced under of SR and ATR conditions that, in the best case, is around 70 mol%. Instead, the POX reaction has produced the largest amount of carbon
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monoxide. Altogether, the production of syngas (H2 + CO) was always the largest under SR conditions.
Under ATR and POX conditions the formation of CH4 was significantly high, but in ATR the
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much lower CO concentration with respect to POX indicated that the produced CO was converted
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into CO2 through the water gas shift reactions. The deviation of measured H2/CO ratio with respect to the stoichiometric ratio is due to the multiple secondary reactions (i.e. water gas shift reaction,
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Bouduard reaction, dehydrogenation reaction). It is also observed that there was no presence of oxygen in the outlet stream in all reaction conditions.
Comparing ATR and SR results, a negative effect on hydrogen yield of the presence of oxygen in the feed (ATR conditions) is observed. Also the syngas (H2+CO) amount in the outlet stream is higher under SR conditions (84.3 vs. 68.1 mol%) and, correspondingly, a two-fold CO2 amount is found in the ATR reaction products as a result of total CH4 oxidation. As previously reported, (Laosiripojana et al., 2007) an O/C ratio of 0.4 (in this work O/C=0.5) positively affects hydrogen yield and prevents the formation of carbon species. Nevertheless, since in ATR the energy required by the reforming reactions is provided by exothermic oxidation reactions, thermally sustained operation is achieved, in contrast to the energy-expensive (endothermic) steam reforming mode (Gutierrez et al., 2011). Although a lot of Ni-based catalysts supported on different oxides have been previously demonstrated excellent performance, both in terms of ethanol conversion (>80%) and selectivity to 15 Page 15 of 24
hydrogen (>65%) (Contreras et al.,2014), our Ni/geopolymer catalyst has shown a stable total conversion and as an much stable hydrogen selectivity value (70%) under SR and ATR conditions. This is likely due to the absence of “noxious” coking phenomena during the reaction, as observed in micrographs shown in Figs. 8a-d.
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With regard to the carbon deposition issue, no coke was observed to form under ATR ( Figures 8a, 8b) and SR (Figures 8c, 8d) conditions. Also, no sintering of nickel particles was observed after 10 hours tests under both SR and ATR conditions. It can be also seen that the geopolymer support
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has not undergone any morphological change. Instead, after POX reaction (Figures 8e, 8f), we observed the presence of some filamentous carbon and/or carbon nanotubes (2.4 wt % from TG
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analysis).
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Figure 8. SEM micrographs of spent catalysts a) after ATR reaction (low magnification), b) after ATR reaction (high magnification), c) after SR reaction (low magnification), d) after SR reaction (high magnification), e) after POX reaction (low magnification), f) after POX reaction (high magnification). a) b)
d)
e)
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c)
f)
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The DSC curves reported in Figure 9 clearly show the absence of coke deposition for spent catalyst after ATR and SR reactions. While, for spent catalyst after POX reaction, the DSC curve presents an exothermic peak related to the combustion of carbon nanotubes at around 600°C
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Figure 9. DSC curves of spent catalysts.
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(Bethune et al., 1993, Porwal et al., 2007).
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Conversion of ethanol versus time on stream exhibits for all the reactions a stable profile. This is quite expected for SR and ATR, for which no coke was observed. For the POX reaction, however,
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the stability of catalytic activity despite the coke formed can be explained considering that the formation of filamentous carbon allows to the most part of the nickel sites to remain available for catalysis (Bartholomew and Farrauto, 1976;. Carrero et al., 2007; Carreño et al., 2009). The low tendency to coke formation can be influenced by the nature of substrate upon which the Ni active phase is dispersed (Fajardo et al, 2010 ). A strong metal-support interaction could have prevented Ni crystallites to sinter. The acidity of the support has a decisive role in this regard. However, the acid sites should not be too strong in order to avoid dehydration of molecules (i.e., ethanol to ethylene) and not be too low in order to avoid their dehydrogenation (i.e., ethanol to acetaldehyde). (Furtado et al., 2009; Sánchez-Sánchez et al., 2007; Youn et al., 2010) The acidity of geopolymer support was investigated by the TPD ammonia desorption. The TPDNH3 profile is characterized by a single and wide peak centered in the temperature range 250-350°C (result not shown) that can be attributed to moderately strong acid sites. (Youn et al., 2010). The detected amount of adsorbed ammonia was 0.54 mmol/g. Furthermore, the presence of extraframework aluminum in the geopolymer inhibits the ethanol dehydration reaction, which 18 Page 18 of 24
causes ethylene formation, hindering its further polymerization, that leads to significant carbonaceous deposits (Fatsikostas and Verykios, 2004).
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4. Conclusions
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In this work we have investigated the suitability of geopolymer material as Ni support in reforming reactions of ethanol to produce syngas. 27 Al, 29Si, NMR spectra confirm the complete formation of the geopolymeric structure. The presence of small Al(OH)4 (aq) resonance attests that metakaolin, upon addition of sodium silicate solution releases Al(OH)4 monomers. The bare support exhibits a bimodal pore size distribution centered at 3.8 nm and 18 nm, respectively, with a low presence (5 v/v%) of microporosity. The largest amount of syngas ( 84 mol%) was produced under SR conditions. In the spent catalysts , after ATR and SR reaction, no coke formation was observed by SEM and TG analyses, whereas after POX reaction some carbon (2.4 wt.%) was present after 10 hours. However, no significant deactivation phenomena was observed. The obtained results reveal the potentiality of geopolymer based catalytic materials.
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Acknowledgments
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The authors thank to Dr. Francesco Frusteri (CNR-ITAE, Messina, Italy) for the TEM images.
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References 1. Al-Fatesh, A.S., Naeem, M.A., Khan, W.U., Abasaeed, A.E., Fakeeha, A.H., Effect of nanosupport and type of active metal on reforming of CH4 with CO2. J Chin Chem Soc-Taip, 2014, 61(4), 461-470.
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2. Bartholomew C. H., Farrauto, R. J. Chemistry of Nickel-alumina Catalyst. J Catal, 1976, 45, 41-53.
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3. Bauer, A., Berger, G. Kaolinite and smectite dissolution rate in high molar KOH solutions at 35 and 80 °C. Appl Geochem, 1998; 13, 905–916.
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4. Bethune, D. S., Kiang, C. H., de Vries, M. S., Gorman, G., Savoy, R., Vazquez, J., Beyers, R., Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature, 1993, 363, 605-607.
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5. Bortnovsky, O., Dedecek, J., Tvaruzkova, Z., Sobalık, Z., Subrt, J. Metal ions as probes for characterization of geopolymer materials. J Am Chem Soc, 2008, 91, 3052–3057.
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6. Breen, J. P., Burch, R., Coleman, H. M. Metal-Catalyzed Steam Reforming of Ethanol in the Production of Hydrogen for Fuel Cell Applications. Appl Catal B-Environ, 2002, 39, 65–74. 7. Carreño, N.L.V., Garcia, I.T.S., Raubach, C.W., Krolows, M., Santos, C.C.G., Probst L.F.D., Fajardo, H.V. Nickel–carbon nanocomposites prepared using castor oil as precursor: A novel catalyst for ethanol steam reforming. J Power Sources, 2009, 188, 527-531.
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8. Carrero A., Calles J.A., Vizcaíno A.J. Hydrogen production by ethanol steam reforming over Cu-Ni/SBA15 supported catalysts prepared by direct synthesis and impregnation. Appl Catal A-Gen, 2007, 327, 82–94.
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9. Cavallaro, S., Mondello, N., Freni, S. Hydrogen produced from ethanol for internal reforming molten carbonate fuel cell. J Power Sources, 2001, 102, 198- 204. 10. Chang, J.-S., Park, S.-E., Chon, H. Catalytic activity and coke resistance in the carbon dioxide reforming of methane to synthesis gas over zeolite-supported Ni catalysts. Appl Catal A-Gen, 1996, 145, 111–124. 11. Cheekatamarla P. K., Finnerty C. M. Reforming catalysts for hydrogen generation in fuel cell applications. J Power Sources, 2006; 160, 490-499. 12. Chen, S.L., Zhang, H.L., Hu, J., Contescu, C., Schwarz, J.A. Effect of alumina supports on the properties of supported nickel catalysts. Appl Catal, 1991, 73, 289-312. 13. Cho, A. Hydrogen From Ethanol Goes Portable. Science, 2004, 303, 942-943. 14. Contreras J.L, Salmones J., Colín-Luna J.A., Nuño L., Quintana B.,I. Córdova I. , Zeifert B., Tapia C.,. Fuentes G.A , Catalysts for H2 production using the ethanol steam reforming (a review), Int. J. Hydrogen Energy, 2014, 39, 18835-18853. 15. Davda, R. R., Shabaker, J. W., Huber, G. W., Cortright, R. D., Dumesic, J. A. A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueous-
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phase reforming of oxygenated hydrocarbons over supported metal catalysts. Appl Catal BEnviron, 2005, 56, 171–186. 16. Davidovits J., Mineral polymers and methods for making them, US Patent 4,349,386 (1982).
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17. Deluga, G. A., Salge, J. R., Schmidt, L. D., Verykios, X. E. Renewable Hydrogen from Ethanol by Autothermal Reforming. Science, 2004, 303, 993–997. 18. Diskin A.M., Cunningham R.H., Ormerod R.M., 1998. The oxidative chemistry of methane over supported nickel catalysts. Catal Today, 1998, 46, 147-154.
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19. Duxson, P., Fernandez-Jimenez, A., Provis, J.L., Lukey, G.C., Palomo, A.,Van Deventer J.S.J. Geopolymer technology: the current state of the art. J Mater Sci, 2007; 42, 2917-2933
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20. Duxson, P., Lukey G.C., Separovic F., Van Deventer J.S.J. Effect of alkali cations on aluminum incorporation in geopolymeric gels. Ind Eng Chem Res, 2005, 44, 832-839.
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21. Fajardo H.V., Longo E., Mezalira D. Z., Nuernberg G. B., Almerindo G. I., Collasiol A., Probst L. F. D., Garcia I. T. S., Carren˜o N. L. V., Influence of support on catalytic behavior of nickel catalysts in the steam reforming of ethanol for hydrogen production, Environ Chem Lett, 2010, 8,79–85
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22. Fatsikostas A.N., Verykios, X.E. Reaction network of steam reforming of ethanol over Nibased catalysts. J Catal, 2004, 225, 439–452
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23. Frontera, P., Aloise, A., Macario, A., Antonucci, P.L., Crea, F., Giordano, G., Nagy, J.B. Bimetallic Zeolite Catalyst for CO2 Reforming of Methane. Top Catal, 2010, 53, 265-272.
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24. Frontera, P., Macario, A., Aloise, A., Antonucci, PL, Giordano, G., Nagy, J.B. Effect of support surface on methane dry-reforming catalyst preparation. Catal Today, 2013, 218-219, 18-29 25. Frontera, P., Macario, A., Aloise, A., Crea, F., Antonucci, P.L., Nagy, J.B., Frusteri, F., Giordano, G. Catalytic dry-reforming on Ni–zeolite supported catalyst. Catal Today, 2012, 179, 52-60 26. Furtado A.C., Alonso C.G., Cantão M.P., Fernandes-Machado N.R.C., Bimetallic catalysts performance during ethanol steam reforming: Influence of support materials, Int. J. Hydrogen Energy ,2009, 34,7189–7196. 27. Garcia, E.Y., Laborde, M.A. Hydrogen production by the steam reforming of ethanol: thermodynamic analysis. Int J Hydrogen Energ, 1991, 16, 307-312. 28. Gutierrez A., Karinen R., Airaksinen S., Kaila R., Krause A.O.I., Int. J. Hydrogen Energy, 2011, 36, 8967-8977. 29. Ho, S.-C., T.C. Chou. The Role of Anion in the Preparation of Nickel Catalyst Detected by TPR and FTIR Spectra. Ind Eng Chem Res, 1995, 34, 2279–2284. 30. Kriven, W.M., Bell, J.L., Gordon, M. Microstructure and microchemistry of fully-reacted geopolymers and geopolymer matrix composites. Ceram Trans, 2003, 153, 227-250.
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31. Landi, E., Medri, V., Papa, E., Dedecek, J., Klein, P., Benito, P., Vaccari, A. Alkali-bonded ceramics with hierarchical tailored porosity. Appl Clay Sci, 2013, 73, 56–64. 32. Laosiripojana N., Assabumrungrat S., Charojrochkul S., Appl. Catal. A, 2007, 327, 180-188
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33. Lemaitre J.L., Menon P.G., Delanny F., in: F. Delanny (Ed.), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1985, p. 299-301. 34. Lindo, M., Vizcaíno, A.J., Calles, J.A., Carrero, A. Ethanol steam reforming on Ni/Al-SBA15 catalysts: effect of the aluminium content. Int J Hydrogen Energ, 2010, 35, 5895-5901.
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35. Llorca, J., de la Piscina, P. R., Sales, J., Homs, N. Direct production of hydrogen from ethanolic aqueous solutions over oxide catalysts. Chem Commun, 2001, 7, 641–642.
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36. Lo Faro M., Frontera P., Antonucci P., Aricò A.S., Ni–Cu based catalysts prepared by two different methods and their catalytic activity toward the ATR of methane. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.05.014
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37. Pompeo, F., Nichio, N.N., Feretti, O.A., Resasco, D. Study of Ni catalysts on different supports to obtain synthesis gas. Int J Hydrogen Energ, 2005, 30, 1399–1405.
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38. Porwal, D., Mukhopadhyay, K., Ram, K., Mathur, G.N., 2007. Investigation of the synthesis strategy of CNTs from CCVD by thermal analysis. Thermochim Acta, 2007, 463, 53-59.
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39. Sánchez-Sánchez M.C., Navarro R.M., Fierro J.L.G., Ethanol steam reforming over Ni/MxOy–Al2O3 (M=Ce, La, Zr and Mg)catalysts: Influence of support on the hydrogen production International Journal of Hydrogen Energy, 2007, 32,1462 – 1471.
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40. Urasaki, K., Tokunaga, K., Sekine, Y., Matsukata, M., Kikuchi, E. Production of hydrogen by steam reforming of ethanol over cobalt and nickel catalysts supported on perovskite-type oxides. Catal Commun, 2008, 9, 600-604.
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41. Walther, J. V. Relation between rates of aluminosilicate mineral dissolution, pH, temperature, and surface charge. AM J SCI, 1996, 296, 693-728. 42. Wang, S., Lu, G.Q.M. CO2 reforming of methane on Ni catalysts: effects of the support phase and preparation technique. Appl Catal B-Environ, 1998,16, 269–277. 43. Youn M. H., Seo J. G., Lee H., Bang Y., Chung J. S., Song I. K., Hydrogen production by auto-thermal reforming of ethanol over nickel catalysts supported on metal oxides: Effect of support acidity, Applied Catalysis B: Environmental , 2010, 98,57–64.
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Graphical Abstract
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Highlights Environmental friendly geopolymer support for nickel catalyst has been prepared.
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Physico-chemical characterization of geopolymer material has been carried out. Ni-geopolymer catalyst showed high activity in ethanol reforming reactions.
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In the spent catalysts very low coke formation was observed
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S0263-8762(15)00044-1 http://dx.doi.org/doi:10.1016/j.cherd.2015.02.011 CHERD 1787
To appear in: Received date: Revised date: Accepted date:
8-9-2014 23-1-2015 19-2-2015
Please cite this article as: Candamano, S., Frontera, P., Macario, A., Crea, F., Nagy, J.B., Antonucci, P.L.,Preparation and characterization of active Ni-supported catalyst for syngas production, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.02.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and characterization of active Ni-supported catalyst for syngas production S. Candamano 1, P. Frontera 2, *, A. Macario 1, F.Crea 1, J. B.Nagy 1, P.L.Antonucci 2 Dept. of Environmental and Chemical Engineering DIATIC, University of Calabria, Italy: f.crea @unical.it, [email protected], [email protected]; [email protected] 2 Dept. of Civil Engineering, Energy, Environmental and Materials DICEAM , Mediterranea University of Reggio Calabria, Italy: [email protected], [email protected]
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* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: 00390965875308; Fax: 00390965875248
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Abstract
A new type of Ni-based catalyst, using geopolymer as support was prepared and characterized
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using Scanning Electron Microscopy (SEM), Temperature Programmed Reduction (TPR), N2 adsorption-desorption, Nuclear Magnetic Resonance (NMR) and Transmission Electron Microscopy (TEM).
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The catalytic performance of Ni-geopolymer catalyst was investigated in steam reforming (SR), partial oxidation (POX) and autothermal steam reforming (ATR) of ethanol to syngas, at the
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temperature of 700°C. The gaseous mixture flow was adjusted (in terms of molar ratio) for each of the three reactions: steam reforming, steam to carbon (S/C) = 2.5; partial oxidation, oxygen/carbon
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(O/C) = 0.5; and autothermal reforming, S/C = 2.5 and (O/C) = 0.5. Ni species supported on the geopolymer surface resulted to be highly active, with a complete
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ethanol conversion and with the largest amount of hydrogen ( 70 mol%) being produced under the SR conditions. In the spent catalysts, after POX reaction, some coke formation was observed by SEM and Thermogravimetric (TG) analyses. The results showed that geopolymer would be a promising, low cost and environmental friendly material for reforming reactions of ethanol. Keywords: Geopolymer; Nickel; Reforming; Coke formation; Syngas 1. Introduction
The development of alternative methods for hydrogen production, especially from renewable sources, has attracted much attention due to the expected increase in energy demand and the environmental concern related to the air pollution. In this work, we propose the use of geopolymer material as nickel support for catalysts to be employed in ethanol reforming reactions. Geopolymers have been selected also because they can be prepared as pellets and/or foams suitable for application as catalysts or supports (Landi et al., 2013), therefore overcoming some
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issues related to catalysts in powder or structural form, so ensuring very high reliability, low pressure drop and excellent structural stability. Many of these properties are required in applications, such as automotive, where harsh environments include vibrations, thermal cycling and thousands of start-ups and shut-downs.
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Furthermore, the close similarity of the geopolymer network to the zeolites framework, in relation to the accommodation of metal ions, opens the opportunity of using geopolymer materials as amorphous analogues of zeolites (Kriven et al., 2003; Bortnovsky et al., 2008). However, it must
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be highlighted that geopolymers possess some advantages in comparison to zeolites; they are prepared at room or low temperature, are mesoporous, are cheap and have good thermal chemical
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stabilities.
All these aspects open to potential interests for the synthesis of new robust catalysts for
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heterogeneous reactions. Geopolymers differ from crystalline zeolites with respect to precursor materials processing and applications and therefore, they can be regarded as an unique class of materials.
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Current methods for hydrogen production are mainly based on non-renewable fossil fuels. Renewable resources have attracted much attention as hydrogen sources to achieve the full
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environmental benefit for generating power with hydrogen. (Cho, 2004). Ethanol has been proposed as alternative fuel for the indirect internal reformer of molten
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carbonate fuel cells (IIR-MCFCs) and for solid oxide fuel cells (SOFCs) (Cavallaro et al., 2001). Ethyl alcohol has several advantages as hydrogen source, over other usable fuels: since i) it can be
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easily produced from biomass by fermentation (Deluga et al., 2004), (ii) high hydrogen content, (iii) good availability and low production costs, (iv) easy and safe handling and transportation, (v) nontoxicity and (vi) it can be distributed in a logistic net similar to the conventional fuel stations. The catalytic performance of supported metals catalysts for the different reforming conditions (steam reforming (SR), partial oxidation (POX), autothermal reforming (ATR), dry-reforming (DR)) has been previously investigated in a wide temperature range (200-750°C) on several metallic phases (Rh, Ru, Pt, Pd, Ni, Cu-Ni), supports (Al2O3, CeO2, MgO, TiO2, Y2O3, perovkites, zeolites, mesoporous materials), and metal loading (Llorca et al., 2001; Breen et al., 2002; Davda et al., 2005; Pompeo et al., 2005; Urasaki et al, 2008; Lindo et al., 2010, Frontera et al., 2010, 2012, 2013; Al-Fatesh et al.,2014; Lo Faro et al, 2014). One of the major problems of the catalytic reforming of ethanol, is linked to the long term instability of the catalyst arising from deposition of carbon due to C-C bond scission. Aside from noble metals, Ni is so far the best choice for hydrogen production by catalytic reforming of ethanol. Ni, in fact, has high activity for C-C bond and O-H bond breaking and it also has high activity for 2 Page 2 of 24
hydrogenation, facilitating H atoms to form molecular H2. However, Ni-based catalysts suffer of significant coke formation with respect to noble metal catalyst. Support also plays an important role in reforming reactions of ethanol, as it affects the dispersion of metal catalyst and may enhance catalytic activity via metal-support interaction. The support can facilitate the reforming reaction of
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ethanol promoting migration of OH- toward the metal catalyst in the presence of water vapour at high temperature (Cheekatamarla and Finnerty, 2006).
In this work, we have investigated the catalytic behavior of a geopolymer material used as nickel
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support in steam reforming (SR), partial oxidation (POX) and autothermal reforming (ATR) of ethanol, at the temperature of 700 °C, to produce syngas. Ni-supported catalysts show their highest
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performance (“useful conversion”) at temperatures higher than 650°C. In fact, according to thermodynamic predictions, the optimal conditions for hydrogen production by ethanol reforming,
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at atmospheric pressure, are reaction temperatures higher than 650°C and excess water in the feed: under these conditions, the carbon formation is thermodynamically inhibited (Garcia and Laborde, 1991, Breen et al., 2002).
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Fresh and exhaust samples were characterized by usual analytical techniques (XRD, SEM, TGA-
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DSC, TPR-H2, TPD-NH3, N2 adsorption-desorption and multinuclear NMR).
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2.1. Materials
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2. Experimental
Metakaolin having a D10 (D10= 10% weight percentage of the tested sample was equal or finer than the reported value, in microns) of 0.51 μm, a D50 (D50= 50% of the tested sample was equal or finer than the reported value, in microns) of 1.59 μm and a D90 (D90= 90% of the tested sample is equal or finer than the reported value, in microns) 9.74 μm, was purchased from Doldes Manara S.r.l.; all these values were provided by the manufacturer. Sodium silicate solution was provided by Condea Augusta S.p.A. The weight composition of metakaolin, as determined by X-ray fluorescence and sodium silicate solutions is listed in Table 1. Metal precursor (Ni(NO3)2*6 H2O) was supplied by Sigma-Aldrich.
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Table 1. Chemical composition of starting materials and geopolymer. Sodium Silicate
Geopolymer
(wt.%)
solution (wt.%)
(%molar)**
Al2O3
42.0
–
21.8
SiO2
53.2
29.5
58.8
K2O
0.3
–
–
Na2O
0.1
13.8
Fe2O3
1.5
–
TiO2
1.9
–
LOI*
1.0
H2 O
–
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17.6
0.72 1.06
–
–
56.7
–
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2.2.1. Ni-support synthesis
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2.2. Methods
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*Loss of Ignition at 955°C; **dry basis
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Metakaolin
Component
Geopolymer was synthesized by activating aluminosilicates source with an alkali activator solution (Davidovits, 1982). The activator solution, containing sodium silicate solution, sodium hydroxide and ultrapure water in the weight ratio of 3.31:1:1.96, was prepared using the following procedure. Firstly, the sodium hydroxide solution was obtained by dissolution of NaOH pellets in ultrapure water, with container kept sealed wherever possible to minimize contamination by atmospheric carbonation and prevent water evaporation. The solution was stirred until the NaOH pellets had dissolved and the solution became clear. During this process, a significant amount of heat is released. So it was allowed to cool back down to room temperature. Once cooled down it was poured into sodium silicate solution. The so obtained alkali activator solution was covered, sealed, stirred and allowed to cool back down to room temperature. Finally, the activator solution was added to metakaolin powder and the slurry was mechanically vigorously mixed for 10 minutes before transferring it to mould sealed from the atmosphere to prevent the loss of water and cured it for 24 h at 40°C. The sealed specimen was then stored at ambient temperature and pressure for two 4 Page 4 of 24
weeks to complete the curing. The monolith with a final theoretical composition, in terms of oxides, of 0.77Na2O-2.81SiO2-Al2O3-9.85H2O was then grinded, characterized and tested.
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2.2.2 Ni-catalyst preparation Nickel catalysts were prepared by incipient wet impregnation of the geopolymer support using an ethanol solution of the metal precursors Ni(NO3)2*6 H2O (Sigma-Aldrich) with the proper
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concentration to obtain a Ni content of 5 wt. %. This value is the optimum Ni loading, used in the most of hydrocarbons reforming reactions because it allows to obtain a good compromise between
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catalyst activity and coke formation (Chang et al., 1996, Wang and Lu, 1998, Breen et al., 2002). The impregnated catalyst has been dried in air at 100°C for 12 h. Prior to reaction, catalysts were in
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situ reduced under flowing hydrogen / argon (5 v/v%, 20 ml min-1) at 500°C for 2 h with a heating rate of 5 °C min-1.
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2.2.3. Catalyst characterization
Phase identification of prepared catalysts was carried out by X-Ray diffractograms, using CuKα
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time of 5 seconds per step.
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radiation in 2θ = 5°- 80° range (D2-Phaser Bruker, λ = 1.5404 Å) with step size of 0.0026 and count The morphologies of metakaolin and geopolymer were examined by a scanning electron
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microscope (SEM, FEI model Inspect). N2 adsorption-desorption isotherms were measured in order to examine the porous properties of geopolymer before and after Ni impregnation. The measurements were carried out in Micromeritics ASAP 2020 instruments. Before the analysis, all samples were pre-treated in vacuum condition at 200°C for 12 hours. Temperature programmed reduction (TPR) was carried out with a Chemisorb Micromeritics 2750 instrument, to monitor the reduction of metal oxide, under a flux of 50 cc min-1 of H2/Ar (10 vol.%) in the temperature range 25-800°C at atmospheric pressure. The NH3-TPD analysis was carried out in the conventional flow apparatus TPD/RO 1100 Thermofinningan, equipped with a thermal conductivity detector (TCD). The sample was pretreated at 450°C for 1 h under helium flow. After cooling, the sample was pretreated with a mixture of NH3-He at isothermal conditions, 100°C for 1 h in order to have only the chemisorbed ammonia. Then the sample was heated in the range 100-600°C at a rate of 10°C min-1. The TPD curve of the desorbed ammonia was elaborated in order to obtain the area and the top temperature of the desorption curve. 5 Page 5 of 24
High resolution transmission electron microscopy (HRTEM) investigation was carried out using a JEOL 2010F instrument equipped with a field emission gun, which allowed to achieve a point-topoint resolution of 0.19 nm and a resolution of 0.14 nm between lines, and a Philips CM12 electron microscopy provided with a high resolution camera. (SHIMATZU) under air flow of 50 cc/min, with heating rate of 5 °C min-1.
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The thermoanalytical measurements were performed on an automatic TG/DTA instrument Solid state 29Si-NMR and 27Al-NMR analyses were used to study the Si chemical neighbourhood Bruker Avance 500 spectrometer. For 27
Si (99.4 MHz), at 6 µs (θ = π/6) pulse was used with a
Al (130.2 MHz), at 1 µs (θ = π/12) pulse was used with a repetition
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repetition time of 40 s, for
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of metakaolin and geopolymer before nickel deposition. NMR measurements were performed on a
time of 0.2 s.
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2.2.4. Catalytic activity measurements
Catalytic activity experiments were performed at atmospheric pressure in a quartz microreactor
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(internal diameter = 4 mm) placed in a ceramic tube furnace, at a space velocity (GHSV) of 120,000 h−1. The catalyst (~50 mg), placed between quartz wool in the middle of the reactor, was diluted
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with quartz (quartz/catalyst = 1.5). The reaction temperature was monitored with a thermocouple inserted into the reactor bed through a quartz tube. The temperature of the reactor bed was kept
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constant by an electronic controller. The maximum deviation measured in the reactor bed from the nominal temperature was ~10 °C. An isocratic pump (Varian ProStar 210) connected to an
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evaporator, that was heated at 220 °C, was used to feed water in the gas stream and to control the steam to carbon ratio (S/C) in the reaction gas mixture; N2 was used as internal standard. The gas lines were heated at approximately 120 °C to prevent condensation. The microreactor was operated in a down-flow mode with the gas inlet placed at the top of the reactor. Gases were fed with properly calibrated reaction mass flow controllers (Brooks 5850S) after purification by Deoxo Gas Clean Filters (Chrompack) and Molecular Sieve Traps. Reaction products were analyzed by on-line gas chromatograph (GC Agilent 6590) having FID and TCD detectors and four columns (Alumina, Porapak Q, Haysep, Molecular Sieves (MS 5A), for their separation and detection. The catalytic performance of catalyst was investigated in order to evaluate the effect of the reaction conditions, i.e. steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR), at the temperature of 700°C.The gaseous mixture flow was adjusted (in terms of molar ratio) for each of the three reactions: steam reforming, steam to carbon (S/C) = 2.5; partial oxidation, oxygen/carbon (O/C) = 0.5; autothermal reforming, S/C = 2.5 and (O/C) = 0.5. 6 Page 6 of 24
Ethanol, dry oxygen and steam flow rates were adjusted according to the assigned values S/C = 2.5 and O/C = 0.5. Therefore, the inlet flow was fixed as in Table 2, using nitrogen as a balance. The diluted used feed contains an inert gas N2, and the analyses of outlet stream takes into account the change in the number of molecules between reactants and products, with a correction factor
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valued as: =N2OUT/N2by-pass . The outlet product percentages were valued on dry basis. Table 2: Stream of reactant and inert gases at the inlet of the catalytic reactor: H2O cc min-1
(vapour)
(steam)
SR
13.33
66.67
POX
13.33
--
ATR
13.07
65.35
O2 cc min-1
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--
N2 cc min-1
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C2H5OH cc min-1
20.00
6.66
80.00
6.53
15.05
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Reaction
In order to reduce the coke formation and in accordance with thermodynamic predictions, the
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steam/carbon (S/C) ratios used in the reforming feeds (SR and ATR) were higher than the stoichiometric ones (Garcia and Laborde, 1991). The oxygen/carbon (O/C) ratio used in the partial
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3.1. Catalyst characterization
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3. Results and Discussion
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oxidation reaction was the stoichiometric one.
Figure 1 shows the XRD patterns of metakaolin and of the synthesized geopolymer.
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Intensity [a.u.]
Figure 1. Powder XRD patterns of (a) metakaolin and (b) geopolymer
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b
10
20
30
2 th e ta
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a 40
50
Metakaolin exhibits a pronounced broad hump with few peaks centered at approximately 22°
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2theta, indicating that it contains essentially amorphous silica and alumina with crystalline phases such as quartz (Duxson et al., 2007). For the synthesized geopolymer the characteristic broad hump
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centered at approximately 28° 2 is observed. It can be considered the distinguishing feature of the diffratogram of any geopolymer, and it shows strong similarity with diffractograms of zeolite precursors. All sharp peaks from crystalline phases in parent material are still present in the pattern
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of geopolymer, indicating that the crystalline phases are not involved in the geopolymerization
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reaction, but they rather behave as inactive fillers in the geopolymer binder. This is in agreement with the current understanding that only the amorphous phases in raw materials are reactive and
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involved in geopolymers reaction. On the DSC curve of geopolymer (Figure 2) there is a remarkable endothermic shoulder at around 155°C. Compared with the mass loss on the TG curve, it is clear that the endothermic effect on the DSC is due to the removal of adsorbed water from the sample. The associated mass loss is about 15% and no more mass loss occurs thereafter.
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Figure 2. TG and DSC geopolymer curves 100
4
EXO DS
2
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DSC signal
Weight loss [%]
95
TG 85
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0
0 0
100
200
300
400
500
Temperature [°C]
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20
600
700
800
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The micro-morphological features of the parent material and geopolymer obtained are compared in Figures 3a,b. Metakaolin particles are dominantly plate in shape, instead the obtained geopolymer shows a dense and continuous gel-like matrix with few clear particles or particle
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boundaries.
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Figure 3. SEM micrographs of metakaolin (a) and geopolymer (b, c) at different magnifications. a) b) c)
The 27Al NMR spectrum of geopolymer (Figure 4a) exhibits a dominant 27Al line at 59 ppm due to tetrahedral aluminum sites, as expected for a true geopolymer. The absence of the resonance line at 28 ppm due to Al(V) clearly shows that all Al(V)contained in metakaolin is consumed during the geopolymerization process. The weak 27Al NMR line at about 4 ppm is due to some presence of octahedral aluminum sites. The amount of Al(VI) calculated as a 27
percentage of the whole amount of Aluminum detected by
Al MAS NMR is around 1.23%.
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The 27Al weak resonance at 79 ppm is due to Al(OH)4 in the pores of geopolymer, since these units have 27Al chemical shift range around 80 ppm. This confirms that metakaolin upon addition of 9 Page 9 of 24
-
the sodium silicate solution releases Al(OH)4 monomers as previously observed for aluminosilicate materials (Walther, 1996; Bauer and Berger, 1998; Duxson et al., 2005). The amount of Al(OH)4 calculated as a percentage of the whole amount of Aluminum detected by
27
-
Al MAS NMR is
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around 3.5%. In Figure 4b the Si NMR decomposed spectrum of geopolymer is shown, while the
m0
Si ( mAl )
]
4
(1)
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m0
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4
I [ Si Al m I Si (mAl ) 4
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lines position and their fractional areas are reported in Figure 5. The Equation 1:
where ISi(mAl) is the intensity of each line, was used in order to validate the accuracy of the decomposition, with respect to the nominal composition of geopolymer. The calculated Si/Al
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composition is 1.6, slightly above the nominal one, and this occurrence is in agreement with the presence of aluminum which is not incorporated into the matrix, as it was found in
27
Al NMR
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spectrum. Any unreacted metakaolin exhibits an unchanged resonance line at around -109 ppm,
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while line at -115 ppm can be attributed to quartz impurities, as confirmed by XRD analysis.
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Figure 4. MAS NMR spectra of geopolymer (a) 27Al and (b) 29Si. a)
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b)
Figure 5. Line frequency and fraction of Q4(mAl) silicon sites used in decomposition of MAS NMR spectrum of geopolymer
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Si
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The chemical composition of geopolymer, in terms of oxides, obtained by EDS analysis performed by at least 10 measurements point, is reported in Table 1. The SiO2/Al2O3 and Na2O/Al2O3 molar ratios were found to be 2.7 and 0.8 respectively, which are close to the nominal ones. There is also a good agreement between the nominal formulations
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and the amount of Nickel after impregnation and reduction obtained by EDS, as reported in Table 3, which indicates that nickel impregnation procedure was carried out satisfactorily.
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Table 3. Support and catalyst Textural properties Ni content Sample name SBET (m2 g-1) VTot (cm3 g-1) (wt. %) Geopolymer 50 0.22 Ni-geopolymer after 5 45 0.17 impregnation Ni-geopolymer after 5 44 0.16 reduction
The textural properties (BET surface and micropore volume) of the catalyst and support were
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also summarized in Table 3. The bare support exhibits a bimodal pore size distribution (calculated by BJH method applied on desorption branch of N2 isotherm) centered at 3.8 nm and 18 nm
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respectively. A low presence (5 v/v%) of microporosity has been also observed by t-plot method. Its presence could be explained by the large volume of interconnected pores in combination with some
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level of crystallinity in alkali-activated specimens (Ho et al., 1995). The incorporation of nickel to the support affects the textural properties of the bare support.
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The impregnated and reduced catalyst show an expected and low suppression in surface area, which can be associated to pores being blocked by lower amounts of nickel incorporated on the surface, also confirmed by the reduction in the pores volume, as reported in Table 3. To study the reducibility of Ni(NO3)2 precursor species and metal-support interaction, TPR-H2 experiment were performed. TPR test was also performed on the bare support for comparison. The results showed that the support had no H2 consumption peaks indicating that it retained the oxide form as expected (Figure 6, line a).
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Hydrogen uptake [a. u.]
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Figure 6. TPR profile for (a) geopolymer support b) Ni-geopolymer catalyst
a
200
300
400
500
600
700
800
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b
Temperature [°C]
For the supported nickel sample the TPR profile showed different sets of H2 consumption peaks.
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The peaks at around 300°C can be attributed to the reduction of NO3- anion (Ho and Chou, 1996). Besides, the hydrogen consumption peaks appearing in the 390-450°C temperature region can be associated to Ni0-Ni2+ reduction (Diskin et al., 1998).
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In addition to this, the black color of the reduced material suggest the reduction of Ni-species to Ni . Likewise, the obtained H2 consumption value (0,84 mmol H2.g cat-1) is consistent with the total
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amount of nickel on the support.
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Comparing the TPR-H2 behaviour of nickel based support geopolymer with an analogous support such as Al-MCM-41 (Diskin et al., 1998) or other nickel support as Al2O3 , SiO2 (Chen et al., 1991) it results that geopolymer has a better reducibility of Ni-species which depends on the low nickel geopolymer interaction. TPR results are also confirmed by the TEM images of the catalyst after reduction at 500°C, (Figures 7a , 7b).
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Figure 7. TEM images of Nickel catalyst b)
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a)
The dark spots represent Ni species, whereas the lighter ones are those of the geopolymeric support. It can be observed that uniformly distributed nickel particles with size ranging between 10
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to 20 nm have been obtained. The diffraction peaks (not shown) corresponding to nickel metal was detected by XRD analysis (PDF card 00-004-0850). The nickel crystallite size was calculated using
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Ni (111) reflection and the Scherrer equation (Lemaitre et al.,1985). The average particle size
3.2. Catalytic activity results
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obtained is 10 nm in agreement with TEM analyses.
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The reforming process of converting ethanol to hydrogen-rich gas can be classified into: Steam reforming (SR) C2H5OH+3H2O → 2CO2+6H2
ΔH°=347,4 kJ mol-1
Partial oxidation (POX) C2H5OH+1/2 O2 → 2CO+ 3H2
ΔH°=14,1 kJ mol-1
Autothermal reforming (ATR) C2H5OH +2H2O+1/2 O2 → 2 CO2+5H2
ΔH°=-50,0 kJ mol-1
Steam reforming is an endothermic process and requires energy input to initiate reactions. Alternatively, hydrogen can be obtained by partial oxidation of ethanol, however the hydrogen selectivity is generally low. In order to enhance hydrogen production, autothermal reforming can be applied. This reforming process is a combination of SR and POX reactions, which has higher efficiency than the other two cited processes. Then the catalytic behavior of the catalyst was studied in spot tests (10 h) for the steam reforming (SR), autothermal reforming (ATR) and partial oxidation (POX) reactions, at T=700°C. The performance of the catalyst, in terms of ethanol conversion vs reaction time, at temperature of 700°C, under SR, ATR and POX conditions, was, for all the investigated processes, quite stable and almost equal to 100%. The percentages of the products result also quite stable over this catalyst 14 Page 14 of 24
during spot tests, so it is possible to use their average in order to have a term of comparison between the various reactions. From the data reported in Table 4, it is evident that the product distribution depends on the reaction type. SR in comparison to the other two processes, as expected, gives the
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highest yield of hydrogen.
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Table 4 Average percentage values of the products (dry basis) at 700°C. mol% Products of outlet stream* CH4 H2 CO CO2 C2H4 H2/CO2** SR 4.7 70.0 14.3 11.0 0.1 6.4 (3.0) POX 11.0 53.2 25.4 9.1 1.3 5.8 (-) ATR 10.3 56.1 12.0 20.7 0.9 2.7 (2.5) *All values are the media of three repeated experiments. ** Values measured in bracket theoretical/stoichiometric value
The largest amounts of hydrogen was produced under of SR and ATR conditions that, in the best case, is around 70 mol%. Instead, the POX reaction has produced the largest amount of carbon
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monoxide. Altogether, the production of syngas (H2 + CO) was always the largest under SR conditions.
Under ATR and POX conditions the formation of CH4 was significantly high, but in ATR the
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much lower CO concentration with respect to POX indicated that the produced CO was converted
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into CO2 through the water gas shift reactions. The deviation of measured H2/CO ratio with respect to the stoichiometric ratio is due to the multiple secondary reactions (i.e. water gas shift reaction,
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Bouduard reaction, dehydrogenation reaction). It is also observed that there was no presence of oxygen in the outlet stream in all reaction conditions.
Comparing ATR and SR results, a negative effect on hydrogen yield of the presence of oxygen in the feed (ATR conditions) is observed. Also the syngas (H2+CO) amount in the outlet stream is higher under SR conditions (84.3 vs. 68.1 mol%) and, correspondingly, a two-fold CO2 amount is found in the ATR reaction products as a result of total CH4 oxidation. As previously reported, (Laosiripojana et al., 2007) an O/C ratio of 0.4 (in this work O/C=0.5) positively affects hydrogen yield and prevents the formation of carbon species. Nevertheless, since in ATR the energy required by the reforming reactions is provided by exothermic oxidation reactions, thermally sustained operation is achieved, in contrast to the energy-expensive (endothermic) steam reforming mode (Gutierrez et al., 2011). Although a lot of Ni-based catalysts supported on different oxides have been previously demonstrated excellent performance, both in terms of ethanol conversion (>80%) and selectivity to 15 Page 15 of 24
hydrogen (>65%) (Contreras et al.,2014), our Ni/geopolymer catalyst has shown a stable total conversion and as an much stable hydrogen selectivity value (70%) under SR and ATR conditions. This is likely due to the absence of “noxious” coking phenomena during the reaction, as observed in micrographs shown in Figs. 8a-d.
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With regard to the carbon deposition issue, no coke was observed to form under ATR ( Figures 8a, 8b) and SR (Figures 8c, 8d) conditions. Also, no sintering of nickel particles was observed after 10 hours tests under both SR and ATR conditions. It can be also seen that the geopolymer support
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has not undergone any morphological change. Instead, after POX reaction (Figures 8e, 8f), we observed the presence of some filamentous carbon and/or carbon nanotubes (2.4 wt % from TG
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analysis).
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Figure 8. SEM micrographs of spent catalysts a) after ATR reaction (low magnification), b) after ATR reaction (high magnification), c) after SR reaction (low magnification), d) after SR reaction (high magnification), e) after POX reaction (low magnification), f) after POX reaction (high magnification). a) b)
d)
e)
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c)
f)
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The DSC curves reported in Figure 9 clearly show the absence of coke deposition for spent catalyst after ATR and SR reactions. While, for spent catalyst after POX reaction, the DSC curve presents an exothermic peak related to the combustion of carbon nanotubes at around 600°C
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Figure 9. DSC curves of spent catalysts.
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(Bethune et al., 1993, Porwal et al., 2007).
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Conversion of ethanol versus time on stream exhibits for all the reactions a stable profile. This is quite expected for SR and ATR, for which no coke was observed. For the POX reaction, however,
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the stability of catalytic activity despite the coke formed can be explained considering that the formation of filamentous carbon allows to the most part of the nickel sites to remain available for catalysis (Bartholomew and Farrauto, 1976;. Carrero et al., 2007; Carreño et al., 2009). The low tendency to coke formation can be influenced by the nature of substrate upon which the Ni active phase is dispersed (Fajardo et al, 2010 ). A strong metal-support interaction could have prevented Ni crystallites to sinter. The acidity of the support has a decisive role in this regard. However, the acid sites should not be too strong in order to avoid dehydration of molecules (i.e., ethanol to ethylene) and not be too low in order to avoid their dehydrogenation (i.e., ethanol to acetaldehyde). (Furtado et al., 2009; Sánchez-Sánchez et al., 2007; Youn et al., 2010) The acidity of geopolymer support was investigated by the TPD ammonia desorption. The TPDNH3 profile is characterized by a single and wide peak centered in the temperature range 250-350°C (result not shown) that can be attributed to moderately strong acid sites. (Youn et al., 2010). The detected amount of adsorbed ammonia was 0.54 mmol/g. Furthermore, the presence of extraframework aluminum in the geopolymer inhibits the ethanol dehydration reaction, which 18 Page 18 of 24
causes ethylene formation, hindering its further polymerization, that leads to significant carbonaceous deposits (Fatsikostas and Verykios, 2004).
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4. Conclusions
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In this work we have investigated the suitability of geopolymer material as Ni support in reforming reactions of ethanol to produce syngas. 27 Al, 29Si, NMR spectra confirm the complete formation of the geopolymeric structure. The presence of small Al(OH)4 (aq) resonance attests that metakaolin, upon addition of sodium silicate solution releases Al(OH)4 monomers. The bare support exhibits a bimodal pore size distribution centered at 3.8 nm and 18 nm, respectively, with a low presence (5 v/v%) of microporosity. The largest amount of syngas ( 84 mol%) was produced under SR conditions. In the spent catalysts , after ATR and SR reaction, no coke formation was observed by SEM and TG analyses, whereas after POX reaction some carbon (2.4 wt.%) was present after 10 hours. However, no significant deactivation phenomena was observed. The obtained results reveal the potentiality of geopolymer based catalytic materials.
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Acknowledgments
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The authors thank to Dr. Francesco Frusteri (CNR-ITAE, Messina, Italy) for the TEM images.
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References 1. Al-Fatesh, A.S., Naeem, M.A., Khan, W.U., Abasaeed, A.E., Fakeeha, A.H., Effect of nanosupport and type of active metal on reforming of CH4 with CO2. J Chin Chem Soc-Taip, 2014, 61(4), 461-470.
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2. Bartholomew C. H., Farrauto, R. J. Chemistry of Nickel-alumina Catalyst. J Catal, 1976, 45, 41-53.
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3. Bauer, A., Berger, G. Kaolinite and smectite dissolution rate in high molar KOH solutions at 35 and 80 °C. Appl Geochem, 1998; 13, 905–916.
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4. Bethune, D. S., Kiang, C. H., de Vries, M. S., Gorman, G., Savoy, R., Vazquez, J., Beyers, R., Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature, 1993, 363, 605-607.
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5. Bortnovsky, O., Dedecek, J., Tvaruzkova, Z., Sobalık, Z., Subrt, J. Metal ions as probes for characterization of geopolymer materials. J Am Chem Soc, 2008, 91, 3052–3057.
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6. Breen, J. P., Burch, R., Coleman, H. M. Metal-Catalyzed Steam Reforming of Ethanol in the Production of Hydrogen for Fuel Cell Applications. Appl Catal B-Environ, 2002, 39, 65–74. 7. Carreño, N.L.V., Garcia, I.T.S., Raubach, C.W., Krolows, M., Santos, C.C.G., Probst L.F.D., Fajardo, H.V. Nickel–carbon nanocomposites prepared using castor oil as precursor: A novel catalyst for ethanol steam reforming. J Power Sources, 2009, 188, 527-531.
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8. Carrero A., Calles J.A., Vizcaíno A.J. Hydrogen production by ethanol steam reforming over Cu-Ni/SBA15 supported catalysts prepared by direct synthesis and impregnation. Appl Catal A-Gen, 2007, 327, 82–94.
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9. Cavallaro, S., Mondello, N., Freni, S. Hydrogen produced from ethanol for internal reforming molten carbonate fuel cell. J Power Sources, 2001, 102, 198- 204. 10. Chang, J.-S., Park, S.-E., Chon, H. Catalytic activity and coke resistance in the carbon dioxide reforming of methane to synthesis gas over zeolite-supported Ni catalysts. Appl Catal A-Gen, 1996, 145, 111–124. 11. Cheekatamarla P. K., Finnerty C. M. Reforming catalysts for hydrogen generation in fuel cell applications. J Power Sources, 2006; 160, 490-499. 12. Chen, S.L., Zhang, H.L., Hu, J., Contescu, C., Schwarz, J.A. Effect of alumina supports on the properties of supported nickel catalysts. Appl Catal, 1991, 73, 289-312. 13. Cho, A. Hydrogen From Ethanol Goes Portable. Science, 2004, 303, 942-943. 14. Contreras J.L, Salmones J., Colín-Luna J.A., Nuño L., Quintana B.,I. Córdova I. , Zeifert B., Tapia C.,. Fuentes G.A , Catalysts for H2 production using the ethanol steam reforming (a review), Int. J. Hydrogen Energy, 2014, 39, 18835-18853. 15. Davda, R. R., Shabaker, J. W., Huber, G. W., Cortright, R. D., Dumesic, J. A. A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueous-
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phase reforming of oxygenated hydrocarbons over supported metal catalysts. Appl Catal BEnviron, 2005, 56, 171–186. 16. Davidovits J., Mineral polymers and methods for making them, US Patent 4,349,386 (1982).
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17. Deluga, G. A., Salge, J. R., Schmidt, L. D., Verykios, X. E. Renewable Hydrogen from Ethanol by Autothermal Reforming. Science, 2004, 303, 993–997. 18. Diskin A.M., Cunningham R.H., Ormerod R.M., 1998. The oxidative chemistry of methane over supported nickel catalysts. Catal Today, 1998, 46, 147-154.
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19. Duxson, P., Fernandez-Jimenez, A., Provis, J.L., Lukey, G.C., Palomo, A.,Van Deventer J.S.J. Geopolymer technology: the current state of the art. J Mater Sci, 2007; 42, 2917-2933
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20. Duxson, P., Lukey G.C., Separovic F., Van Deventer J.S.J. Effect of alkali cations on aluminum incorporation in geopolymeric gels. Ind Eng Chem Res, 2005, 44, 832-839.
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21. Fajardo H.V., Longo E., Mezalira D. Z., Nuernberg G. B., Almerindo G. I., Collasiol A., Probst L. F. D., Garcia I. T. S., Carren˜o N. L. V., Influence of support on catalytic behavior of nickel catalysts in the steam reforming of ethanol for hydrogen production, Environ Chem Lett, 2010, 8,79–85
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22. Fatsikostas A.N., Verykios, X.E. Reaction network of steam reforming of ethanol over Nibased catalysts. J Catal, 2004, 225, 439–452
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23. Frontera, P., Aloise, A., Macario, A., Antonucci, P.L., Crea, F., Giordano, G., Nagy, J.B. Bimetallic Zeolite Catalyst for CO2 Reforming of Methane. Top Catal, 2010, 53, 265-272.
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24. Frontera, P., Macario, A., Aloise, A., Antonucci, PL, Giordano, G., Nagy, J.B. Effect of support surface on methane dry-reforming catalyst preparation. Catal Today, 2013, 218-219, 18-29 25. Frontera, P., Macario, A., Aloise, A., Crea, F., Antonucci, P.L., Nagy, J.B., Frusteri, F., Giordano, G. Catalytic dry-reforming on Ni–zeolite supported catalyst. Catal Today, 2012, 179, 52-60 26. Furtado A.C., Alonso C.G., Cantão M.P., Fernandes-Machado N.R.C., Bimetallic catalysts performance during ethanol steam reforming: Influence of support materials, Int. J. Hydrogen Energy ,2009, 34,7189–7196. 27. Garcia, E.Y., Laborde, M.A. Hydrogen production by the steam reforming of ethanol: thermodynamic analysis. Int J Hydrogen Energ, 1991, 16, 307-312. 28. Gutierrez A., Karinen R., Airaksinen S., Kaila R., Krause A.O.I., Int. J. Hydrogen Energy, 2011, 36, 8967-8977. 29. Ho, S.-C., T.C. Chou. The Role of Anion in the Preparation of Nickel Catalyst Detected by TPR and FTIR Spectra. Ind Eng Chem Res, 1995, 34, 2279–2284. 30. Kriven, W.M., Bell, J.L., Gordon, M. Microstructure and microchemistry of fully-reacted geopolymers and geopolymer matrix composites. Ceram Trans, 2003, 153, 227-250.
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31. Landi, E., Medri, V., Papa, E., Dedecek, J., Klein, P., Benito, P., Vaccari, A. Alkali-bonded ceramics with hierarchical tailored porosity. Appl Clay Sci, 2013, 73, 56–64. 32. Laosiripojana N., Assabumrungrat S., Charojrochkul S., Appl. Catal. A, 2007, 327, 180-188
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33. Lemaitre J.L., Menon P.G., Delanny F., in: F. Delanny (Ed.), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1985, p. 299-301. 34. Lindo, M., Vizcaíno, A.J., Calles, J.A., Carrero, A. Ethanol steam reforming on Ni/Al-SBA15 catalysts: effect of the aluminium content. Int J Hydrogen Energ, 2010, 35, 5895-5901.
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35. Llorca, J., de la Piscina, P. R., Sales, J., Homs, N. Direct production of hydrogen from ethanolic aqueous solutions over oxide catalysts. Chem Commun, 2001, 7, 641–642.
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36. Lo Faro M., Frontera P., Antonucci P., Aricò A.S., Ni–Cu based catalysts prepared by two different methods and their catalytic activity toward the ATR of methane. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.05.014
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37. Pompeo, F., Nichio, N.N., Feretti, O.A., Resasco, D. Study of Ni catalysts on different supports to obtain synthesis gas. Int J Hydrogen Energ, 2005, 30, 1399–1405.
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38. Porwal, D., Mukhopadhyay, K., Ram, K., Mathur, G.N., 2007. Investigation of the synthesis strategy of CNTs from CCVD by thermal analysis. Thermochim Acta, 2007, 463, 53-59.
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39. Sánchez-Sánchez M.C., Navarro R.M., Fierro J.L.G., Ethanol steam reforming over Ni/MxOy–Al2O3 (M=Ce, La, Zr and Mg)catalysts: Influence of support on the hydrogen production International Journal of Hydrogen Energy, 2007, 32,1462 – 1471.
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40. Urasaki, K., Tokunaga, K., Sekine, Y., Matsukata, M., Kikuchi, E. Production of hydrogen by steam reforming of ethanol over cobalt and nickel catalysts supported on perovskite-type oxides. Catal Commun, 2008, 9, 600-604.
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41. Walther, J. V. Relation between rates of aluminosilicate mineral dissolution, pH, temperature, and surface charge. AM J SCI, 1996, 296, 693-728. 42. Wang, S., Lu, G.Q.M. CO2 reforming of methane on Ni catalysts: effects of the support phase and preparation technique. Appl Catal B-Environ, 1998,16, 269–277. 43. Youn M. H., Seo J. G., Lee H., Bang Y., Chung J. S., Song I. K., Hydrogen production by auto-thermal reforming of ethanol over nickel catalysts supported on metal oxides: Effect of support acidity, Applied Catalysis B: Environmental , 2010, 98,57–64.
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Graphical Abstract
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Highlights Environmental friendly geopolymer support for nickel catalyst has been prepared.
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Physico-chemical characterization of geopolymer material has been carried out. Ni-geopolymer catalyst showed high activity in ethanol reforming reactions.
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In the spent catalysts very low coke formation was observed
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