The preparation of hydrogen by the catalytic pyrolysis of ethanol on a nickel catalyst

Article abstract of DOI:10.1134/S0036024409110089

High efficiency of a nickel catalyst on the SiO₂ support in low-temperature ethanol conversion as a method for the preparation of hydrogen was demonstrated. One mole of the alcohol was found to yield one mole of hydrogen. The catalyst studied d

Full text of DOI:10.1134/S0036024409110089

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 11, pp. 1855–1859. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.V. Lapin, A.N. Red’kin, V.S. Bezhok, A.F. Vyatkin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 11, pp. 2044–2048.
CHEMICAL KINETICS
AND CATALYSIS
The Preparation of Hydrogen by the Catalytic Pyrolysis of Ethanol
on a Nickel Catalyst
N. V. Lapin, A. N. Red’kin, V. S. Bezhok, and A. F. Vyatkin
Institute of Problems of Technology in Microelectronics and UltraꢀHighꢀPurity Materials, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
eꢀmail: lapin@ipmtꢀhpm.ac.ru
Received July 15, 2008
Abstract—High efficiency of a nickel catalyst on the SiO2 support in lowꢀtemperature ethanol conversion as
a method for the preparation of hydrogen was demonstrated. One mole of the alcohol was found to yield one
mole of hydrogen. The catalyst studied did not stimulate the methanation and shift reactions.
DOI: 10.1134/S0036024409110089
INTRODUCTION
The conversion of ethanol was studied on various
catalysts, including Ni, Co, their alloys with Cu, and
noble metals on various supports [6–11]. The waterꢀ
vapor reforming of ethanol is a strongly endothermic
reaction, and the yield of hydrogen is maximum at
high temperatures, usually higher than 600°С. The
high temperature of the process contributes to the
formation of large amounts of carbon monoxide,
which poisons anodic catalysts of fuel cells. In addiꢀ
tion, at a high conversion temperature, the problem
of reforming product cooling arises, because the
working temperature of fuel cells with polymeric
membranes is usually 80°С. At a high reforming
temperature, we encounter yet another problem,
that of the deactivation of catalysts because of carꢀ
bon precipitation in the form of graphite or even
nanotubes [5, 6].
Because of a high efficiency, high current density,
and low working temperature (usually, 80°С), fuel
cells with polymeric protonꢀconducting membranes
are currently considered one of the most promising
energy sources for various applications. They also offer
promise for decreasing gaseous discharges into the
environment.
The fuel for these cells can be hydrogen or a gasꢀ
eous mixture rich in hydrogen, which must be accuꢀ
mulated and stored or can be directly prepared in sysꢀ
tems integrated with fuel cells. Because of the absence
of suitable hydrogen accumulators and the necessary
infrastructure for its distribution, catalytic reforming
of suitable hydrocarbons or alcohols has been attractꢀ
ing ever increasing interest of researchers. Until
recently, methanol has been considered a likely candiꢀ
date for the production of hydrogen because of the
accessibility and ease of its reforming. Fairly good
results of numerous studies in this area have been pubꢀ
lished [1–5]. Methanol, however, has an important
shortcoming, it is toxic. In addition, there is the probꢀ
lem of methanol utilization because of its high chemiꢀ
cal stability.
A Cuꢀcontaining catalyst is preferable for dehydroꢀ
genation resulting in the formation of large amounts of
acetaldehyde [6]. On the other hand, Niꢀ and Coꢀ
containing catalysts reform ethanol more effectively
but cause the formation of large amounts of CH4 and
stimulate the hydrogenation of CO and CO2, which
decreases the yield of hydrogen. Activity loss because
of the deposition of carbon presents an additional
problem with these catalysts. Catalysts based on Cu
In recent years, ethanol has been attracting ever are less effective because of the oxidation of the active
increasing attention of researchers as a promising phase [6]. Noble metals have high efficiency in the
source of hydrogen for fuel cells. In addition, biologiꢀ conversion of ethanol [6], but they are expensive and
cal ethanol can be used for this purpose; biological can hardly find broad use in practice. The nature of the
ethanol has several advantages, (1) it is easily accessiꢀ catalyst support also plays a certain role in the selectivꢀ
ble, cheap, and is a renewable source of energy, (2) as ity of hydrogen formation. Acid supports such as Al2O3
distinct from methanol, it is not toxic, and (3) as disꢀ stimulate dehydration, whereas basic supports such as
tinct from natural hydrocarbons (gasoline etc.), ethaꢀ MgO contribute to dehydrogenation [9–11]. Catalysts
nol does not contain sulfurꢀcontaining impurities on such supports as CeO2 and ZrO2 exhibit the best
which are “catalytic poisons” and can poison catalysts catalytic characteristics. They have high selectivity
used in the reforming of ethanol and fuel cell electroꢀ with respect to hydrogen and low selectivity with
catalysts.
respect to undesirable side products.
1855

1856
LAPIN et al.
1
2
3
Ar
Tsvetꢀ500
VRТ
6
5
4
Fig. 1. Scheme of experimental unit: (
troller, and ( ) chromatograph.
1) reactor, (2) furnace, (3) catalyst, (4) Patrikeev peristaltic pump, (5) temperature conꢀ
6
In this work, we are concerned with the wellꢀ
known but scarcely studied process of the lowꢀtemꢀ
perature reforming of ethanol for the preparation of
hydrogen. It is known [5–8] that lowꢀtemperature
ethanol reforming can be divided into two stages,
although the reactions can also proceed concurꢀ
rently. At the first stage, ethanol is dehydrogenated
with the formation of acetaldehyde and hydrogen.
At the second stage, acetaldehyde splits into methꢀ
ane and carbon monoxide. Subsequently, carbon
monoxide can react with water to produce hydroꢀ
The catalyst was synthesized following a proceꢀ
dure developed by us. To prepare the support, ashꢀ
less filter paper was treated with a 20% solution of
tetraethoxysilane in alcohol and then held in a desꢀ
iccator over 10% aqueous ammonia. The procedure
was repeated several times until the required weight
increase was attained. Next, the material was
annealed in air at 200°С for an hour. The temperaꢀ
ture was then increased to 700°С for 2 h. As a result,
cellulose fibers were completely burned out, and a
gen and carbon dioxide. Catalysts containing Ni material consisting of sintered porous thinꢀwalled
can stimulate this reaction [6]. In this work, we
studied the lowꢀtemperature reforming of ethanol
with the participation of the nickel catalysts earlier
developed by us for the pyrolytic synthesis of nanoꢀ
tubes [12].
SiO2 microtubes was obtained. This silicon oxide
“paper” was impregnated with a solution of nickel
nitrate and annealed at 400°С. The catalytic subꢀ
strate prepared this way had a developed surface and
good gas permeability. We used samples containing
25 wt % NiO.
EXPERIMENTAL
The first trial experiments with a water–ethanol
mixture (water : ethanol molar ratio 1 : 1) showed that,
with this catalyst, the reaction between carbon monꢀ
oxide and water (shift reaction) did not occur. For this
reason, subsequent experiments were performed only
Studies were performed on a flow unit (Fig. 1),
whose main element was a cylindrical microreactor
70 mm long with a 6 mm inside diameter. The reactor
was placed into a furnace with resistive heating and a
VRT highꢀaccuracy (1°C) temperature controller. with a mixture of the azeotropic composition, 96 wt %
Ethanol was supplied into the reactor by a Patrikeev
peristaltic pump or from a bubbler through which
argon was bubbled. The bubbler was held at room
temperature. Argon flow rate was measured by a rotꢀ
ameter and varied over the range 10–100 cm3/min.
The temperature of the reactor was varied from 50 to
425°С. The catalyst was loaded in an amount of
0.06–0.08 g (together with the support). The catalyst
bed height in the reactor was 40 mm. Ethanol flow
changed from 0.12 (bubbler) to 1.5 g/h (peristaltic
pump).
ethanol–4 wt % water. The vaporization of this mixꢀ
ture in a bubbler did not cause changes in its compoꢀ
sition, which was therefore constant during the whole
experiment. The gas phase was analyzed on a Tsvetꢀ
500 gas chromatograph with two columns 2 m long,
with molecular sieves A5 for simple gases and with
polysorbꢀ1 for ethanol, water, and acetaldehyde. The
detector was a katharometer. The IR spectra of outꢀ
going gases were recorded on a Specord M82 instruꢀ
ment.
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THE PREPARATION OF HYDROGEN BY THE CATALYTIC PYROLYSIS OF ETHANOL
1857
α, %
100
−
ln(cx/c0)
1
5
3
1
2
60
0
2
4
τ, s
Fig. 3. Dependence of the fraction of unreacted ethanol on
contact time at ( ) 375 and ( 325°С. Ethanol flow
0.12 g/h, catalyst load 0.06 g; and are the concentraꢀ
1
2)
20
c
c
0
x
tions of ethanol at the exit of the reactor and in the initial
mixture, respectively.
200
250
300
T, °C
shown in Fig. 3. We see that, in the coordinates corꢀ
responding to a firstꢀorder reaction, the required linꢀ
ear dependences are observed at 325 and 375°С. The
linear character of the dependences shows that, over
the temperature range studied, the catalytic pyrolysis
of alcohol is not limited by adsorption processes, in
the first place, by the desorption of reaction prodꢀ
ucts.
Fig. 2. Temperature dependence of the degree of ethanol
conversion ; ethanol flow 0.12 g/h, catalyst load 0.06 g.
α
RESULTS AND DISCUSSION
In [12], IR spectroscopy was used to qualitatively
study the pyrolysis of ethanol vapor on a NiO/SiO2
catalyst in a flow reactor at 550°С. It was found that,
in the absence of the catalyst, ethanol vapor passed
through the reactor and almost did not decompose. In
the presence of the catalyst, the IR spectrum of gases
at the exit of the reactor contained a band at 1725 cm–1
characteristic of aldehydes, a doublet at 2140 cm–1 of
carbon monoxide, and intense sharp peaks at 3020 and
1305 cm–1 corresponding to methane. In addition, an
intense band of carbon dioxide appeared at 2350 cm–1
in the spectrum of outgoing gases.
Changes in the contents of all the observed reaction
products in the gas phase depending on pyrolysis temꢀ
perature are shown in Fig. 4. We see that the yields of
H2 and CH4 and the degree of ethanol conversion
increase as the temperature grows; the yields of H2 and
CH4 are maximum at 400–425°С. The contents of the
main gas phase components (hydrogen, methane, and
carbon monoxide) are close to each other (of about
33 vol %). It follows from [5] that the final ethanol
conversion products over the temperature range studꢀ
ied are hydrogen, methane, and carbon monoxide.
This means that the volume concentrations of these
products should be equal, and this is what is observed
in experiments to within measurement errors. The
shift reaction (between carbon monoxide and water) is
not noticeable. Nor is the formation of carbon dioxide
necessarily produced in the shift reaction observed.
Calculations show that a decrease in the concentraꢀ
tion of water present in rectificate in a concentration
of 10 mol % is completely explained by an increase in
the volume of the gas phase (three moles of products
are obtained from one mole of ethanol). The methaꢀ
nation reaction is not observed too, although,
The pyrolysis of ethanol at 550°С is accompanied
by intense deposition of carbon fibrous nanomaterial
(up to 100% with respect to the catalyst weight). The
deposition of carbon deactivates the catalyst, and
pyrolysis is rapidly decelerated. The rate of carbon
deposition decreases sharply as the temperature lowers
(almost to zero at 350°С). This allows the service life
of the catalyst to be increased substantially. The special
features of the pyrolysis of ethanol vapor on the cataꢀ
lyst prepared as described above at a temperature
below 400°С were quantitatively studied by chromaꢀ
tography.
The degrees of alcohol conversion at various temꢀ
peratures are shown in Fig. 2. We see that the pyrolysis according to [13], it can occur over the temperature
of ethanol begins at as low as 200°С. A sharp increase range studied.
in the fraction of reacted alcohol is observed at 300°С
and, at 350°С, almost complete reforming of ethanol
occurs.
,
The content of acetaldehyde in the gas phase was
low and continuously decreased. Intermediate prodꢀ
ucts are characterized by the presence of a maximum
The order of the reaction with respect to ethanol is of the temperature curve. In the reaction studied, aceꢀ
of interest. Typical dependences of the logarithm of taldehyde rapidly decomposed into methane and carꢀ
the fraction of unreacted alcohol on the time during bon monoxide; a maximum could be present at lower
which the reaction mixture was in the reactor are temperatures. At higher reaction temperatures (above
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1858
LAPIN et al.
x, vol %
cC H OH
2 5
40
30
20
10
0
80
60
40
20
0
1
2
4
3
5
6
200
250
300
, °C
350
400
150
T
Fig. 4. Temperature dependences of vapor phase composition
x: (1) hydrogen, (2) methane, (3) carbon monoxide, (4) ethanol,
) water, and ( ) acetaldehyde. Ethanol flow 1.5 g/h, catalyst load 0.08 g.
(5
6
425°С), the concentration of CO decreased. This can
be explained by the disproportionation of CO into carꢀ
bon and carbon dioxide. The deposition of elemental
carbon was observed visually as blackening of catalyst
plates.
C2H5OH = H2 + C2H4O,
C2H4O = CH4 + CO,
2CO = C + CO2.
(1)
(2)
(3)
The temperature dependence of the rate constant
for the dehydrogenation of ethanol is shown in Fig. 5.
The apparent activation energy of the process is
44 kJ/mol, which is much less than the activation
energy of the lowꢀtemperature waterꢀvapor reforming
of ethanol (149 kJ/mol [5]).
CONCLUSIONS
To summarize, the nickel catalyst earlier developed
for the pyrolysis of ethanol vapor was found to be fairly
active in the reforming of the alcohol at comparatively
low temperatures. A comparison of the efficiency with
respect to hydrogen of the process studied with that of
waterꢀvapor conversion shows that, in the two reacꢀ
tions, approximately equal liquid reagent volumes are
required for the preparation of one mole of hydrogen.
At the same time, lowꢀtemperature reforming is
accompanied by the release of large amounts of carbon
monoxide and methane. The use of hydrogen in a fuel
cell can require a fairly complex procedure for purifyꢀ
ing it, especially from carbon monoxide.
An analysis of the experimental data obtained in
this work shows that the catalytic pyrolysis of ethanol
occurs as a sequence of the reactions
ln
2
k
0
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