Cobalt-based nanoparticles as catalysts for low temperature hydrogen production by ethanol steam reforming

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 8 2 e9 1

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Cobalt-based nanoparticles as catalysts for low temperature hydrogen production by ethanol steam reforming Gabriella Garbarino a, Paola Riani b, Mattia Alberto Lucchini b, Fabio Canepa b, Shrikant Kawale c, Guido Busca a,* a

Universita` degli Studi di Genova, Dipartimento di Ingegneria Civile, Chimica e Ambientale (DICCA), Laboratorio di chimica delle superfici e catalisi, P.zzale Kennedy, 1, 16129 Genova, Italy b Universita` degli Studi di Genova, Dipartimento di Chimica e Chimica Industriale (DCCI), Via Dodecaneso, 31 16146 Genova, Italy c CNR-SPIN (SuPerconductors, oxides and other INnovative materials and devices), Corso Perrone 24, 16152 Genova, Italy

article info

abstract

Article history:

Results obtained in the synthesis, characterization and application as catalyst of cobalt

Received 7 August 2012

nanoparticles are reported. Cobalt nanoparticles were prepared via reduction method in

Received in revised form

aqueous solution. Structural characterization was carried out using X-ray diffraction (XRD),

15 October 2012

morphological studies were performed with a scanning electron microscope equipped with

Accepted 18 October 2012

a field emission gun (FE-SEM). A DC-superconducting quantum interference device

Available online 22 November 2012

“SQUID” magnetometer was used to measure the room temperature (RT) magnetic hysteresis cycle in the
5 O 5 Tesla (T) m0H magnetic field range as well as magnetization

Keywords:

as a function of temperature. This material is constituted by very small primary particles

Nanoparticles

(w2.8 nm radius) which appear amorphous to XRD and have a superparamagnetic

Cobalt nanoparticles

behaviour. However, annealing at 773 K and also utilization in the catalytic reactor at the

Ethanol steam reforming

same temperature result in XRD detectable cubic Co nanocrystals. These unsupported

Hydrogen

cobalt nanoparticles were found catalytically active in the ethanol steam reforming reac-

Catalysts

tion, producing hydrogen with 90% yield at 773 K. These nanoparticles show a better

Magnetic properties

catalytic behaviour compared to those of more conventional Co and Ni based catalysts, due to very low CO and methane production, and with moderate formation of carbonaceous materials. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The production of hydrogen from renewable sources is very desirable [1]. In fact, the associated use of bio-hydrogen, hydrogen-fuelled fuel cells and electrical engines represents in principle a completely environmentally friendly and highly efficient energy production system. Several processes are under study in this prospective. Bioethanol, i.e. ethanol

produced by sugar fermentation, is a possible source of hydrogen [2]. In fact a number of investigations are currently being undertaken to discover effective catalysts to understand the reaction mechanism and to develop industrial or pilot scale processes for Ethanol Steam Reforming (ESR). This work has been dealt with in many recent review papers [3e11]. Yet, consensus has not been reached on the best composition of catalysts for ESR, as well as on the best conditions to perform

* Corresponding author. Tel.: þ39 3292104505; fax: þ390103536024. E-mail address: [email protected] (G. Busca). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.054

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 8 2 e9 1

it. As reported by several authors (see the above cited reviews), Nickel catalysts are cheap and quite active, but must be used at relatively high temperature to limit deactivation by coking that occurs quite readily. A good compromise for activity, selectivity and duration may be obtained using combinations of metals and moderately basic supports. Duprez and coworkers propose the composition RhNi/Y2O3eAl2O3 as a promising catalyst to be used even with crude bioethanol at 948 K [10]. In these conditions, however, the formation of CO and of residual methane limits the production of hydrogen and gives rise to a gas that cannot be directly used on the anode of a polymer electrolyte membrane fuel cell (PEMFC). In fact, CO concentration (>10% mol/mol) is too high and Pt electrodes are poisoned. Low temperature ESR (573e673 K) is thermodynamically possible. In this case, CO2 is expected as the main C-containing reaction products, but the production of methane may be significant, with consequent lower hydrogen yields. Low temperature ESR can indeed be performed effectively over noble metal-based catalysts such as supported Pt [12,13] with limited coking, but methanation is significant. Cobalt based catalysts have also been found to be active and selective for ESR at low temperature. Studies using unsupported cobalt oxide as catalyst showed that this oxide is essentially not active in ESR, and low temperature steam reforming activity was observed when the oxide is reduced to the metal [14e16]. The situation may be different for supported or coprecipitated cobalt catalysts. Recent studies suggested [17e19] that unreduced cobalt (Co2þ species) may have an important role in ESR at least in some catalytic systems, where a very limited amount of metallic cobalt was observed which is supposed to be the reason for limited coking [20,21]. In fact, combining the catalyst with an oxide carrier favours the formation of highly dispersed cationic species which may be stabilized in high oxidation states [22]. Thus coalesced metal particles and dispersed cationic species may coexist and it is not always clear what is active in catalysis or in deactivation phenomena, or whether some synergism occurs. On the other hand, different oxide carriers have a different behaviour in dispersing metal species and redox-active carriers, such as ceria, have an effect in determining their oxidation state. Oxide phases can also participate differently in the water activation step. In very recent times a number of different formulations have been studied and proposed for Co catalysts for ESR, such as unsupported Co oxides [14e16], Co/CeO2 [19,23,24], Co/SiO2 [16,25], Co/Al2O3 [25,26], Co/CaOeAl2O3 [27], Co/MgO [26], coprecipitated [20,21] or impregnated Co/MgOeAl2O3 [26], Co/ ZnO [24,28], coprecipitated Co/ZnOeAl2O3 [18], Co/ZrO2 [27] and Co/ZrO2eZnO [29], Co/TiO2 [24], Co/talc [30], etc. Furthermore, promoting cobalt or alloying it with other metals such as Ni [18,27,31], Ru [32], Ir [24], Fe [17,28], Cu, Cr, Na [28] and K [20], is also under investigation. In practice, the optimal composition in terms of ingredients as well as of cobalt loading, particle size and morphologies, the best preparation procedure (coprecipitation, impregnation or other more sophisticated procedures), the real nature of the active phases and also details of the mechanism of reaction are still far from being wellestablished and are still a matter of discussion regarding Cocatalysed ESR.

83

Unsupported bulk metal particles are also used as catalysts in some cases, such as ammonia synthesis on iron catalysts. Previously, experiments were performed using unsupported Co3O4 as the starting material that was predominantly converted to Co metal upon reduction in the gas phase. The size of the material particles used in these studies was from some tens of nanometres to more than 100 nm. It seemed of interest to us to test unsupported cobalt nanoparticles in ESR. For this reason we prepared Co metal nanoparticles by reduction in aqueous solution, whose crystal size is of the order of a few nanometres of no more than 20 nm, and compared their behaviour with those of more conventional Co- and Ni-based catalysts, to evaluate the role of the support on this reaction.

2.

Experimental

2.1.

Preparation of cobalt nanoparticles

Co-based nanoparticles were prepared using a reduction method in aqueous solution. In a typical synthesis procedure, a controlled excess of sodium borohydride is added as reducing agent to a 10
2 M solution of CoCl2$6H2O maintained at room temperature under mechanical stirring and argon flux [33]. The addition of the reductive agent is quickly followed by the appearance of a black precipitate, confirming the formation of nanoparticles. The reaction is maintained for a further 15 min under flux and stirring and then the precipitate is collected using a permanent magnet and dried in open air.

2.2. Preparation of alumina-supported Co and Ni catalysts In order to compare the catalytic activity of cobalt nanoparticles (Co NPs) to that of conventionally prepared catalysts with a similar metal content, a 125% Co/Al2O3 catalyst has been prepared by conventional wet impregnation of Siralox 5/ 170 support (alumina with 5% (w/w) SiO2 from Sasol, 170 m2/g) using Co(NO3)2$6H2O water solution. After impregnation, drying 363 K for 24 h and calcination at 973 K for 5 h were performed. Similarly a 16% Ni/Al2O3 (100 wtNi/wtsupport) catalyst and a 125% Ni/Al2O3 (100 wtNi/wtsupport) catalyst, corresponding to 13.8 and 55.5 (100 wtNi/wtcatalyst) were synthesized using Ni nitrate hexahydrate as precursor.

2.3.

Characterization techniques

2.3.1.

FE-SEM

The scanning electron microscope ZEISS SUPRA 40 VP, with a field emission gun, was used to study the morphology of all the prepared catalysts. This instrument is equipped with a high sensitivity “InLens” secondary electrons detector and with an EDX microanalysis OXFORD “INCA Energie 450  3”. Samples for SEM analysis were suspended in ethanol and exposed to ultrasonic vibrations to decrease the aggregation. A drop of the resultant mixture was finally laid on a Lacey Carbon copper grid.

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2.3.2.

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XRD

X-ray diffraction patterns on dried nanoparticles, annealed at 773 K for 4 days, were carried out by using a vertical powder diffractometer X’Pert with Cu Ka radiation (l ¼ 0.15406 nm). The patterns were usually collected in the 25e100 2q range with a step of 0.03 and a counting time for each step of 12 s. Powder patterns were indexed by comparing experimental results to the data reported in the Pearson’s Crystal Data database [34].

2.3.3.

Magnetic measurements

DC magnetization was performed in a DC-superconducting quantum interference device “SQUID” magnetometer (MPMS magnetic properties measurement system, Quantum Design). The room temperature (RT) magnetic hysteresis cycle was obtained in the
5 O 5 T m0H magnetic field range. The thermal dependence of the magnetisation (H ¼ 400 Oe, 5e300 K range) was obtained following the zero field cooling (ZFC)efield cooling (FC) procedure. In a ZFC process the sample is cooled to the lowest temperature in absence of a magnetic field. At that temperature the field is switched on and the magnetization is recorded as a function of T. In an FC procedure the same magnetic field of the ZFC is applied before cooling down the sample from RT to 5 K; then, with the magnetic field already switched on, the magnetisation is measured as a function of T.

Fig. 1 e FE-SEM image of as-prepared Co nanoparticles. Fisher), in order to have a precise identification of the compounds. The conversion of the reactants is defined as follows: Xreactant ¼

Catalytic measurements

The catalytic experiments were carried out in a flow fixed-bed tubular glass reactor, loaded with 44 mg of catalyst mixed with 440 mg of silica glass particles (60e70 mesh sieved). Experiments were performed with a fixed bed tubular reactor fed with 6 Water/Ethanol molar ratio in He 41.6% carrier with GHSV ¼ 51,700 h
1. Product analysis was performed with a gas-chromatograph Agilent 4890 equipped with a Varian capillary column “Molsieve 5A/Porabond Q Tandem” and TCD and FID detectors in series. Between them a nickel catalyst tube was employed to reduce CO to CH4. Products analysis was also performed on GC/MS (FOCUS and ISQ from Thermo

(1)

while selectivity to C-product i is defined as: Si ¼

2.4.

nreact:in
nreact:out nreact:in

ni ni ðnreact:in
nreact:out Þ

(2)

where ni is the number of moles in compound i, and ni is the ratio of stoichiometric reaction coefficients. The hydrogen yield is defined as nH2 out =6  nethanol in .

3.

Results and discussion

3.1.

Characterization of cobalt nanoparticles

FE-SEM images of the as-prepared Co NPs are reported in Fig. 1. Their size distribution ranges from a few nm to a maximum of 20 nm. It may be noticed that the biggest NP is

Fig. 2 e XRD patterns of the Co-based catalysts before and after reaction.

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85

Fig. 3 e Room temperature magnetic hysteresis cycle of Co-based nanoparticles. The continuous line is the Langevin fit as reported in the text. In inset a) the radii size distribution function obtained from the same fit is shown; the vertical line, corresponding to the critical radius (rc [ 15 nm) emphasizes the boundary between the superparamagnetic and ferromagnetic regimes. In inset b) the ZFCeFC magnetisation measurements performed with an applied magnetic field of 400 Oe is reported.

formed by agglomerations of a few tiny nanoparticles. Their sizes were determined by magnetic measurements. XRD analysis of the as-prepared Co NPs (not shown here) shows that the material is essentially amorphous, without any well-defined peak. After annealing at 773 K (which is the maximum temperature used in the catalytic experiment, see below) for 4 days (Fig. 2) the material appears to be formed by cubic cobalt (cF4-Cu type) which is in fact the stable phase for

cobalt metal at high temperature (above 696 K). Moreover, several broad peaks appear in this diffraction pattern which may be attributed to a poorly crystalline phase of residual Co2B (Fig. 2). Fig. 3 shows the RT hysteresis cycle (open symbols) of the as-prepared Co-based NPs, normalized to the saturation magnetisation (Ms). The cycle is completely anhysteretic with no detectable coercive field: this behaviour is the typical

Fig. 4 e XRD patterns of 125% Ni/Al2O3 catalyst before and after reaction.

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Table 1 e Ethanol conversion, C-product selectivities and hydrogen yield on cobalt nanoparticles. T Furnace [K] 523 573 673 773 673 573 523

X C2H6O

X H2O

S CH4

S CO

S CO2

S CH2CH2

S CH3CH3

S HCHO

S CH3COH

S (C2H5)2O

Y H2

0.00 0.49 0.99 1.00 1.00 0.31 0.02

0.00 0.02 0.18 0.43 0.31 0.04 0.00

0.00 0.05 0.15 0.06 0.22 0.03 0.00

0.00 0.06 0.05 0.08 0.09 0.02 0.00

0.00 0.09 0.44 0.86 0.67 0.25 0.00

0.00 0.01 0.04 0.00 0.02 0.00 0.00

0.00 0.00 0.09 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.00 0.78 0.22 0.00 0.00 0.70 1.00

0.00 0.00 0.01 0.00 0.00 0.00 0.00

0.00 0.12 0.44 0.90 0.66 0.11 0.00

fingerprint of magnetic nanoparticles in the superparamagnetic regime. The experimental data were very well fitted using the Langevin theory for superparamagnetic nanoparticles, taking into account a log normal distribution of the NPs due to size dispersion effects already present in any real system, as reported in eq. (3)

MðH; rÞ ¼

  Zb  VNP ðrÞ$1:606$mB $H coth VCo $kB $T a  VCo $kB $T
$f ðrÞ$dr VNP ðrÞ$1:606$mB $H

ð3Þ

where r is the radius of the NP, VNP(r) is the volume of the nanoparticle, 1.606$mB is the magnetic moment (in Bohr magnetons) per Co atom, VCo is the volume of the Co atom, kB is the Boltzmann constant, H is the magnetic field, T the temperature, a and b are the integral limits (in our fit a ¼ 0.1 nm and b ¼ 30 nm). f(r) is the log normal distribution of radii, currently adopted in this kind of analysis, given by eq. (4)

2

 2 3 r 6ln rm 7 1 pffiffiffiffiffiffi :exp4 f ðrÞ ¼ 5 2$s2 r$s$ 2p

(4)

Here rm is the mean radius and s the distribution width. The fit is reported in the figure as a continuous line. From the fit we obtained rm ¼ 2.9 nm and s ¼ 0.9. The radii size distribution function obtained from the fit is reported in inset a) of the figure. In the same inset, the critical radius for superparamagnetism in Co-based nanoparticles (rcr y 15 nm) is fixed as a continuous vertical line: it is evident that more than 95% of the nanoparticles are superparamagnetic at RT, i.e. with a radius of less than 15 nm. The ZFCeFC curves are reported in inset b) of the same figure. The maximum in the ZFC curve is defined as the

blocking temperature (TB), i.e. the temperature at which, for a given value of the magnetic field, a passage from a free rotation of the magnetic moment of the NP (superparamagnetism) to the blocked state (ferromagnetism) occurs. In the framework of the Ne´el relaxation theory [35], using TB ¼ 120 K and taking into account the time constant of the SQUID magnetometer (102 s), we obtain a mean radius rm ¼ 2.7 nm, in very good agreement with the calculated value from the Langevin equation. In conclusion as prepared Co NPs are constituted by very small particles (w2.8 nm radius) which, due to their very small size, appear amorphous to XRD and have a superparamagnetic behaviour. However annealing at 773 K results in Co cubic crystals detected by means of X-ray diffractometer.

3.2.

Characterization of alumina supported catalysts

The XRD pattern of the Siralox support shows the peaks which are typical of a transition alumina and very weak at the scale used in Figs. 2 and 4. They correspond to a tetragonal structure such as d-Al2O3 or g0 -Al2O3 [36]. In the patterns of the 125% Co/ Al2O3 (Fig. 2) and the 125% Ni/Al2O3 (Fig. 4) catalysts characteristic peaks of the Co3O4 (spinel cF56) and NiO phases (rock salt or periclase structure, cF8) appear, respectively. The analysis of the crystal size of the cobalt and nickel oxide phases using the Scherrer calculation gives rise to values of about 45 nm for the most intense peak (311) of the Co3O4 phase and of about 36 nm for the most intense peak (200) of the NiO phase.

3.3.

Catalytic ESR results

The results of ESR catalytic Tables 1e4. Over Co-based ethanol is practically zero acetaldehyde is already

experiments are summarized in NPs (Table 1) the conversion of at 523 K. At 573 K conversion, significant being the largely

Table 2 e Ethanol conversion, C-product selectivities and hydrogen yield on 125% Co/Al2O3. T Furnace [K] X C2H6O X H2O S CH4 S CO S CO2 S CH2CH2 S CH3CH3 S HCHO S CH3COH S (C2H5)2O S CH3COO(CH2CH3) Y H2 523 573 673 773 673 573 523

0.02 0.09 0.57 0.70 1.00 0.37 0.15

0.00
0.01
0.06 0.24 0.30 0.00 0.00

0.00 0.00 0.00 0.20 0.23 0.04 0.05

0.00 0.00 0.00 0.09 0.12 0.06 0.06

0.00 0.00 0.00 0.71 0.65 0.03 0.02

0.00 0.09 0.43 0.00 0.00 0.01 0.00

0.00 0.00 0.00 0.00 0.01 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.67 0.37 0.21 0.00 0.00 0.70 0.75

0.33 0.54 0.36 0.00 0.00 0.08 0.05

0.00 0.00 0.01 0.00 0.00 0.07 0.05

0.00 0.01 0.02 0.49 0.64 0.07 0.03

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Table 3 e Ethanol conversion, C-product selectivities and hydrogen yield on 125% Ni/Al2O3. T Furnace [K] X C2H6O X H2O S CH4 S CO S CO2 S CH2CH2S CH3CH3 S CH3OH S CH3COH S (C2H5)2O S CH3COO(CH2CH3) Y H2 523 573 673 773 673 573 523

0.01 0.05 0.46 1.00 1.00 0.31 0.10

0.00 0.00
0.04 0.34 0.19 0.00 0.00

0.00 0.00 0.00 0.21 0.46 0.08 0.04

0.00 0.00 0.00 0.08 0.02 0.09 0.05

0.00 0.02 0.00 0.72 0.52 0.04 0.02

0.00 0.06 0.36 0.00 0.00 0.02 0.00

predominant product. At 673 K conversion is almost total and steam reforming is the main reaction occurring. In fact CO2 is the main C-containing product but acetaldehyde is still produced with significant selectivity (22%) and methane, ethane and ethylene are also formed besides CO. At 773 K, instead, steam reforming is essentially complete, with very small amounts of methane and CO as secondary products. Thus hydrogen yield t is 90% and selectivity to CO2 is 86%. By further decreasing the reaction temperature to 673 K conversion is still total but selectivities to CO2 and hydrogen decrease in favour of selectivity to methane and ethylene. However, the catalyst is now more active and selective than it was at this temperature during the increasing temperature step. This suggests that the catalyst may undergo some kind of “conditioning” on stream. A further decrease of the reaction temperature gives rise to the disappearance of steam reforming activity, reproducing more or less the behaviour of the fresh catalyst at the same reaction temperature upon increasing it. ESR does not actually occur over 125% Co/Al2O3 catalyst (Table 2) at 523 K, 573 K and 673 K during increasing temperature experiment, with the formation of acetaldehyde, ethylene and diethylether. Uncovered alumina could participate in the catalysis producing ethylene and diethylether. Steam reforming activity starts to be predominant over 125% Co/Al2O3 catalyst at 773 K (selectivity to CO2 > 70%), but the conversion is still largely incomplete and methane selectivity is also significant. Thus the hydrogen yield is low (49%). The behaviour of the catalyst significantly improves in the next step, where the temperature has been decreased to 673 K. In these conditions, the conversion of ethanol is complete and selectivity to CO2 is still very high (65%), with moderate selectivity to methane and CO. Thus the hydrogen yield of 64% is obtained. By further lowering the reaction temperature, the steam reforming activity almost vanishes and conversion strongly decreases, while acetaldehyde becomes the most abundant product. However, it is worth noting that the behaviour of the catalyst is different at 523 and 573 K than the catalyst in the increasing temperature experiment: conversion as well as selectivity to acetaldehyde are both much higher. This suggests that during the experiment the catalyst is probably modified by reduction. ESR does not occur at 523 K, 573 K and 673 K during the increasing temperature experiment also over 125% Ni/Al2O3 catalyst (Table 3), with the formation again of acetaldehyde, ethylene and diethylether, in which uncovered alumina could participate. Steam reforming activity is predominant at 773 K (selectivity to CO2 >70%) and the conversion is

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.01 0.00

1.00 0.47 0.27 0.00 0.00 0.64 0.83

0.00 0.40 0.36 0.00 0.00 0.11 0.06

0.00 0.04 0.00 0.00 0.00 0.01 0.00

0.00 0.01 0.02 0.70 0.38 0.06 0.02

complete. Significant methane selectivity (22%) and production of CO (8%) limit hydrogen yield to 70%. Also the 125% Ni/ Al2O3 catalyst appears to be “conditioned” on stream, and its activity in the decreasing temperature experiment at 673 K is better than in the increasing one. In this case ethanol conversion is still complete but selectivity to methane is very high (46%) thus significantly limiting the hydrogen yield. By further lowering the reaction temperature, the steam reforming activity almost vanishes and conversion strongly decreases, while acetaldehyde becomes the most abundant product. To have a more significant comparison, we also performed experiments with a less charged (16% Ni wt/wtsupport) Ni/Al2O3 catalyst prepared with the same support and the same procedure. The loading of this material is in a more common range, and is similar to that needed to cover the whole alumina surface with a monolayer of Ni oxide species. The data reported in Table 4 show that these catalysts are still not active at 773 K for the ESR. In fact, even if ethanol conversion is complete at this temperature, water is not converted but produced, the main product being ethylene (66% selectivity), the result of ethanol dehydration, with acetaldehyde and acetone as the main byproducts. Only at 973 K do these reactions vanish and steam reforming is almost complete. However, methane selectivity is relatively high over this catalyst even at this high temperature and CO selectivity is also higher than over Cobased catalysts. The comparison of the results obtained with the two Ni/Al2O3 catalysts shows how relevant the role of the amount of Ni on the catalyst is, and how much higher the catalytic activity of the most loaded catalyst is. Table 5 shows the composition of the mixture coming out from the reactor, filled with Co NPs, 125% Co/Al2O3 and 125% Ni/Al2O3 at 773 K, compared with those expected with thermodynamics calculations at the same temperature. The composition of the gas coming out in the experiment with 125% Ni/Al2O3 is similar to that expected from a complete thermodynamics calculation while that relative to the experiment with Co NPs is similar to that calculated excluding methane as a possible product. Over 125% Co/Al2O3 at this temperature conversion is still kinetically limited. This suggests that over highly charged Ni catalysts the reaction comes to the thermodynamic equilibrium while over Co catalysts the methane forming reactions (COx methanation and ethanol decomposition) are kinetically inhibited. Furthermore, all cases show that the CO2/CO ratio in the products is higher than that expected from thermodynamics, suggesting that either these compounds are produced independently from each other (and the production of CO is kinetically limited), or CO is formed from CO2 by reverse water

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0.01 0.03 0.04 0.10 0.34 0.63 0.54 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.12 0.36 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.07 0.00 0.00 0.68 0.49 0.21 0.15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.01 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02

CH3CH2OH / C2H4 þ H2O

(5)

that is mainly catalysed by alumina support, as quite obvious indeed and confirmed elsewhere [38], and also evident by the activity of the Ni16 catalyst that shows high ethylene selectivity and where the alumina support is likely to be largely exposed. However, we observe here that ethylene may also be produced to a small extent over alumina-free bulk cobalt, thereby showing that metals may have some activity too for this thermodynamically favoured reaction. A purely thermal reaction (without catalyst) does not give rise to ethylene at such a low temperature. Further more important points concern acetaldehyde and methane production. In previous studies with different Nibased catalysts [37] we found methane production above that corresponding to thermodynamic methanation/steam reforming equilibrium,

CH3CHO / CH4 þ CO

This reaction being supposed to mainly occur with the intermediacy of surface acetate species formed by oxidation of adsorbed acetaldehyde [37]:
CH3 CHO þ 2
OH/CH3 COO
ðadsÞ þ OHðadsÞ þ H2

(8)

2

CH3 COO
ðadsÞ þ OHðadsÞ /CH4 þ CO2 þ OðadsÞ

(9)

0.16 0.04 0.01 0.05 0.20 0.30 0.23 0.02 0.02 0.01 0.04 0.22 0.56 0.56 0.00 0.01 0.00 0.02 0.10 0.12 0.18

(6)

0.00 0.00
0.09
0.08 0.07 0.22 0.17

CH4 þ H2O $ CO þ 3H2

0.04 0.20 0.78 1.00 1.00 1.00 1.00

0.02 0.06 0.70 0.66 0.33 0.01 0.02

gas shift, as already found previously [37], and this step is kinetically limited. In conclusion, the behaviour of cobalt nanoparticles is exceptionally good, allowing a 90% hydrogen yield at 773 K which is mainly due to the low production of methane. Also at 673 K in the decreased temperature experiments, where the catalysts appear to be “conditioned”, the behaviour of cobalt NPs is better than that of Ni-based catalysts that produce more methane. In these conditions, it is comparable with that of Co/Al2O3. The results presented here in effect show that catalysts where Co is present essentially in a form of metal nanoparticles, both bulk or supported on alumina, present very good activity in ESR at low temperature (673 K and 773 K) and in particular a very low production of CO and methane compared with results reported in literature obtained using both Co [17,20] and noble metal [12,13] catalysts. From the mechanistic point of view, the data reported here give us some new insight. One concerns the production of ethylene

thus suggesting that a way to methane would exist other than from methanation, such as decomposition of ethanol or of some other intermediate. The data obtained on the Ni125 catalyst seem to support the idea that methane can be formed from acetaldehyde:

523 573 673 773 873 973 1023

T Furnace [K] X Ethanol X H2O S CH4 S CO S CO2 S CH2CH2 S CH3CH3 S HCHO S CH3OH S CH3CHO S CH3COCH3 S (CH3CH2)2O S CH3COOH S CH3COO(CH2CH3) Y H2

Table 4 e Ethanol conversion, C-product selectivities and hydrogen yield on 16% Ni/Al2O3.

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(7)

High acetaldehyde selectivities are found either at low temperature, when steam reforming is still very slow, or when excess water in the feed is not large [37]. This suggests that adsorbed acetaldehyde may actually be an intermediate in the

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Table 5 e Comparison of ESR product compositions and thermodynamic equilibria calculations at 773 K. Component

Xi,in

Xi,thermodynamics CH4 production not allowed

Xi,out Co NPs

Xi,out Co125

Xi,thermodynamics CH4 production allowed

Xi,out Ni125

Ethanol H 2O HE CO2 CO H2 Methane Ethylene Ethane Acetaldehyde Acetone

0.08 0.50 0.42 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.000 0.220 0.312 0.094 0.031 0.343 0.000 0.000 0.000 0.000 0.000

0.000 0.215 0.317 0.109 0.011 0.341 0.007 0.000 0.000 0.000 0.000

0.021 0.322 0.351 0.070 0.009 0.208 0.020 0.000 0.000 0.000 0.000

0.000 0.304 0.339 0.080 0.012 0.221 0.043 0.000 0.000 0.000 0.000

0.000 0.263 0.329 0.095 0.010 0.277 0.027 0.000 0.000 0.000 0.000

steam reforming process, its evolution being favoured by excess water that should result in a more oxidized catalyst. By increasing temperature, reactions (7) and (9) may be responsible for excess methane production. Heavily charged Ni catalysts working in excess water (as shown here) produce methane amounts corresponding to the thermodynamic equilibrium even at low temperature. Over cobalt catalysts, instead, methane is well below the thermodynamic amount. This suggests that over highly loaded Nickel catalysts with excess water, methane may be formed by decomposition, but steam reformed efficiently. The low amount observed on cobalt may be due to the different path taken in decomposing (or oxidizing) surface acetate species, possibly due (as reported previously [18]) to a more efficient activation of water. This means that cobalt would be more oxidized by water at the surface and should consequently be more active in the acetates’ oxidation step: 2þ 2

CH3 COO
ðadsÞ þ OHðadsÞ þ 2CoðsurfÞ þ OðsurfÞ /2CO2 þ 2H2 þ 2Co

(10)

3.4.

Characterization of spent catalysts

XRD patterns of the spent catalytic materials are reported in Figs. 2 and 4. In this case the silica glass material used for diluting the bed is also present, and is responsible for the broad diffraction signal which is most intense in the range

2q ¼ 20 e30 . Spent Co NPs clearly present the peaks of cubic cobalt, thus showing that they undergo a similar change to that undergone by treating at 773 K for 4 days. XRD shows very broad peaks that cannot allow a reliable calculation of the crystal size, which is however in the range lower than 20 nm. Also the XRD pattern of the spent 125% Co/Al2O3 catalyst (Fig. 2) presents broad and weak peaks of cubic cobalt, together with those of the alumina support. Instead, the peaks of the Co3O4 spinel have fully disappeared. Similarly, the XRD pattern of the spent 125% Ni/Al2O3 catalyst (Fig. 4) presents the peaks of the metastable hexagonal Nickel (hP2-Mg type) stabilized by carbon impurities, together with those of the alumina support and with residual peaks of the NiO precursor. The FE-SEM micrograph of the spent catalytic Co-based NPs shows (see Fig. 5) aggregation phenomena (up to w40 nm) of the single Co NPs, partially coalesced to each other. The comparison of the crystal size evaluated by XRD and that resulting from FE-SEM microscopy suggests that, also in the spent catalysts, the main particles appearing in the images are indeed aggregates of smaller crystals. Moreover, it is possible to observe the appearance of carbon nanotubes that contribute to decrease the activity of the Co NPs by covering them. By comparing the FE-SEM images of the two Co-based spent catalysts, it is evident (see Fig. 5) that the 125% Co/Al2O3 spent catalyst shows much more carbonaceous materials than Co NPs.

Fig. 5 e FE-SEM images of the spent catalytic Co-based NPs (left side) and of Co125 exhaust catalyst (right side).

90

4.

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Conclusions

Unsupported cobalt nanoparticles have been prepared by a reduction method in aqueous solution and are constituted by very small primary particles (w2.8 nm radius) that form bigger spherical aggregates appearing amorphous to XRD and superparamagnetic. After annealing at 773 K or after ESR catalytic experiments, Co-based materials crystallize into a cubic structure (Cu-type), the structure type stable at high temperature. They still retain very low crystal size, evaluated well below 20 nm from XRD peak width. Such Co NPs are very active in converting ethanol in the presence of steam into CO2 and hydrogen at low temperature with short time on streams (few hours), with a very limited amount of CO and CH4. It seems to be slightly more efficient than highly charged a 125% Co/Al2O3 catalyst prepared by impregnation. The performances are even better than those reported in the literature obtained with “low temperature ESR catalysts”, constituted by supported noble metals, at least in terms of lower methane and CO production. In both cases the spent catalysts are clearly mostly reduced in the form of cubic cobalt, although the existence of unreduced species at the surface or in amorphous material cannot be excluded. Cobalt based materials are more efficient than analogous Ni-based materials in producing much less methane, which is supposed to be due to the higher activity of cobalt to oxidize acetate surface intermediates. All materials produce carbonaceous species, particularly in the form of nanotubes, that could be taken as evidence of a possible deactivation in more extended experiments. However, the production of carbonaceous materials on Co NPs is definitely smaller than over alumina supported Co.

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