Evaluation of different dielectric barrier discharge plasma configurations as an alternative technology for green C1 chemistry in the carbon dioxide reforming of methane and the direct decomposition of methanol

Article abstract of DOI:10.1021/jp100346q

Carbon dioxide reforming of methane and direct decomposition of methanol have been investigated using dielectric barrier discharges (DBD) at atmospheric pressure and reduced working temperatures. Two different plasma reactor configurations are compared and especial attention is paid to the influence of the surface roughness of the electrodes on the conversion yields in the first plasma device. The influence of different filling gap dielectric materials (i.e., A1₂O₃ or BaTiO₃) in the second packed configuration has been also evaluated. Depending on the experimental conditions of applied voltage, residence time of reactants, feed ratios, or reactor configuration, different conversion yields are achieved ranging from 20 to 80% in the case of methane and 7-45% for the carbon dioxide. The direct decomposition of methanol reaches 60-100% under similar experimental conditions. Interestingly, the selectivity toward the production of hydrogen and carbon monoxide is kept almost constant under all the experimental conditions, and the formation of longer hydrocarbon chains or coke as a byproduct is not detected. The maximum efficiency yields are observed for the packed-bed reactor configuration containing alumina for both reaction processes (～1 mol H ₂ per kilowatt hour for dry reforming of methane and ～4.5 mol H₂ per kilowatt hour for direct decomposition of methanol).

Full text of DOI:10.1021/jp100346q

J. Phys. Chem. A 2010, 114, 4009–4016
4009
Evaluation of Different Dielectric Barrier Discharge Plasma Configurations As an
Alternative Technology for Green C1 Chemistry in the Carbon Dioxide Reforming of
Methane and the Direct Decomposition of Methanol†
V´ıctor J. Rico,‡ Jose´ L. Hueso,*,‡,§,⊥ Jose´ Cotrino,‡,| and Agust´ın R. Gonza´lez-Elipe‡
Instituto de Ciencia de Materiales de SeVilla (CSIC-UniVersidad de SeVilla), Departamento de Qu´ımica
Inorga´nica, UniVersidad de SeVilla, Spain, Departamento de F´ısica Ato´mica, Molecular y Nuclear, UniVersidad
de SeVilla, Spain
ReceiVed: January 13, 2010; ReVised Manuscript ReceiVed: February 4, 2010
Carbon dioxide reforming of methane and direct decomposition of methanol have been investigated using
dielectric barrier discharges (DBD) at atmospheric pressure and reduced working temperatures. Two different
plasma reactor configurations are compared and especial attention is paid to the influence of the surface
roughness of the electrodes on the conversion yields in the first plasma device. The influence of different
filling gap dielectric materials (i.e., Al2O3 or BaTiO3) in the second packed configuration has been also
evaluated. Depending on the experimental conditions of applied voltage, residence time of reactants, feed
ratios, or reactor configuration, different conversion yields are achieved ranging from 20 to 80% in the case
of methane and 7-45% for the carbon dioxide. The direct decomposition of methanol reaches 60-100%
under similar experimental conditions. Interestingly, the selectivity toward the production of hydrogen and
carbon monoxide is kept almost constant under all the experimental conditions, and the formation of longer
hydrocarbon chains or coke as a byproduct is not detected. The maximum efficiency yields are observed for
the packed-bed reactor configuration containing alumina for both reaction processes (∼1 mol H2 per kilowatt
hour for dry reforming of methane and ∼4.5 mol H2 per kilowatt hour for direct decomposition of methanol).
1. Introduction
CO2(g) + CH4(g) f 2CO(g) + 2H2(g)
(1)
The energy demand existing nowadays is mainly fulfilled by
fossil sources such as coal, crude oil, or natural gas (the latter
mainly composed of methane and carbon dioxide in different
ratios, depending on the geographical location of the gas fields).
Despite technological advances for more efficient detection of
new fossil beds, their limited availability is contributing to the
exponential increase in the prices of the raw material. In
addition, their contribution to global warming and pollution can
not be neglected. Therefore, there is a clear need for alternative
clean fuels and greener synthesis routes. Hydrogen-based
processes represent one of the most promising and widespread
strategies currently under study. Other interesting approaches
must be related to finding more economical and environmentally
friendly uses for the current fossil feedstocks.
The use of two potential greenhouse effect sources, such as
CO2 and CH4 to produce synthesis gas (syngas), a valuable
precursor for the attaining of further chemicals, represents a
highly attractive alternative.1-8,13 In past years, several catalytic
systems have been successfully developed for an optimal
performance of the reaction. Ni-based systems have been
denoted as the most suitable catalysts.5,6,8,14 Nevertheless, some
major drawbacks are still associated with this reaction, thereby
preventing its application on an industrial scale: (i) the rapid
deactivation of the catalysts due to the formation of coke,
especially in Ni-based systems; (ii) the high temperature ranges
required to achieve high conversion yields, thus meaning that
additional burning of fossil fuel is required to achieve those
high temperatures, thereby causing an additional output of CO2
and making the energy conversion process less economically
viable.
A very promising reaction is the dry reforming of methane
(DRM), also known as carbon dioxide reforming of methane
(eq 1; ∆H0 ) +247 kJ mol-1):1-12
Another attractive process is related to the conversion of low-
chain alcohols, such as methanol and ethanol, to obtain hydrogen
or syngas.5,7,15-27 They are considered excellent liquid H2 sources
with low toxicity and are attainable from the conversion of fossil
fuels or natural sources such as biomass.5,16,18 Their use,
especially in the case of methanol, is mainly intended for
hydrogen production for fuel cells and automotive applications,
and different copper-based systems have been claimed among
the most effective catalysts in the processes of direct decom-
position (eq 2, ∆H0 ) +92 kJ mol-1), partial oxidation (eq 3,
∆H0 ) -192.2 kJ mol-1), or steam reforming of methanol (eq
4, ∆H0 ) +49.4 kJ mol-1).19-21,27
† Part of the special issue “Green Chemistry in Energy Production
Symposium”.
* To whom correspondence should be addressed. Address: Instituto de
Ciencia de Materiales de Sevilla (ICMSE-CSIC), Avda Ame´rico Vespucio,
43, Isla de la Cartuja (CICIC), 41092 Sevilla, Spain. Phone: (+34)
954489500. Fax: (+34) 954460665. E-mail: jhueso@icmse.csic.es. Web:
www.sincaf-icmse.es.
‡ Instituto de Ciencia de Materiales de Sevilla.
§ Departamento de Qu´ımica Inorga´nica.
| Departamento de F´ısica Ato´mica, Molecular y Nuclear.
⊥ Present address: Department of Chemical Engineering, University of
Texas at Austin, Austin, TX.
10.1021/jp100346q  2010 American Chemical Society
Published on Web 02/25/2010

4010 J. Phys. Chem. A, Vol. 114, No. 11, 2010
CH3OH(g) f CO(g) + 2H2(g)
Rico et al.
(2)
(3)
(4)
2CH3OH(g) + O2(g) f 2CO2(g) + 4H2(g)
CH3OH(g) + H2O(g) f CO2(g) + 3H2(g)
In the present work, we aimed at presenting the use of plasma
technology as an alternative to carry out the dry reforming of
methane (eq 1) and the direct decomposition of methanol
(DDM) (eq 2) reactions working at ambient pressures and lower
temperatures than required by conventional catalytic processes.
Nonequilibrium plasmas are defined as highly excited gases
composed of electrons, ions, atoms, and molecules with different
energetic states.28,29 Cold plasmas can be generated using
different experimental setups, but dielectric-barrier-discharge
(DBD) reactors have been shown to be very promising28-31 for
several reasons: (i) DBD configurations require low input powers
to induce physical and chemical reactions within gases at
relatively low temperatures (T < 200 °C); (ii) these plasma
reactors can be operated at ambient pressure; (iii) the reactor
design facilitates the scaling-up process to operate with higher
volumes of gases; (iv) DBD devices have a well-established
technology for ozone production; and (v) they show a high
electron density and glow discharge extension.
All these characteristics have led to the systematic application
of DBDs in different environmental processes,28,31 especially in
decontamination of pollutants and energy conversion proces-
ses.1-4,9-11,29,32 Different groups have previously evaluated the
dryreformingofmethanewithlow-temperatureplasmas.1-4,9-11,29,32
The varieties of results are quite promising but at the same time
very difficult to compare, especially in terms of conversion
yields and selectivity of byproduct generated. The main reasons
attributed to the difference in the results have to do with the
great number of parameters influencing the generation of the
discharge itself, as well as the differences in the initial
concentrations and flow rates of the chemicals. Therefore, the
systematic evaluation of these different parameters and the
search for optimal reactors designs are still necessary to further
develop the application of plasmas in the DRM reaction. On
the contrary, the investigation and application of plasma
technology for the conversion of alcohols is still scarce on the
literature23-27,33,34 and deserves further research.
Herein, we have explored different aspects related to the
capability of two different DBD plasma setups in the carbon
dioxide reforming of methane and the direct decomposition of
methanol reactions to produce hydrogen and syngas as added-
value products. A systematic study of the influence of different
experimental parameters such as total flow rates, feed ratios,
morphology (roughness) and composition of the electrodes has
been carried out for the DRM reaction. In the case of methanol,
a comparison between reactor configurations and the influence
of different filling-gap dielectric materials (Al2O3 versus BaTiO3)
have been thoroughly studied. High conversions and efficiency
yields are obtained for both reactions and reactor configurations
under our experimental conditions, thereby showing a high and
promising potential of plasma technology in C1 chemistry
processes.
Figure 1. Schematic representation of the first DBD plasma config-
uration. The insets represent the different electrodes’ roughness and
morphologies under study (see text). The different parts of the reactor
and their corresponding assignation with numbers are fully described
within the text.
of the reactor to enable the study of the influence of roughness/
porosity of the electrodes in the reaction efficiencies. The
experimental arrangement used for the study is shown in Figure
1 and consists of the following parts: (1) A stainless steel
cylinder with controlled roughness in the zone of the plasma
discharge with holes for the gas inlet just at the beginning of
the discharge zone. We investigated three electrodes with
different induced morphologies. The first one was sandblasted,
thereby creating 1 µm depth irregularities; another cylinder
consisted of porous stainless steel that introduced the gases
inside the zone of the plasma discharge; and the third type of
cylinder contained 3 mm holes around the discharge area (Figure
1). (2) Inlet and circulation of gases. (3) A quartz tube acting
as a dielectric barrier. This quartz tube has an outer diameter
of 25 mm and a wall thickness of 2 mm. (4) A metal electrode
made of stainless steel with an aluminum belt externally
covering the dielectric tube. The area covered by this external
electrode defines the zone where the plasma is produced. (5)
An external furnace to maintain the reactor at temperatures
higher than 110 °C to avoid condensation of water. A critical
parameter of the production of plasma inside the reactor is the
distance between the rough surface of the inner stainless steel
cylinder and the dielectric tube. This distance lies between 1
and 3 mm. Total volume of the empty space in the reactor was
between 7 and 15 cm3.
The second DBD configuration under study has been previ-
ously defined as a packed-bed reactor.32 As a result, a more
robust and industrially scalable reactor was developed. In this
setup, the external electrode is entirely made of stainless steel.
In addition, the gap between the electrodes is filled with different
dielectric materials. The experimental setup is schematically
described in the Figure 2: (1) a stainless steel cylinder without
controlled roughness and 30 mm in diameter; (2) the inner
electrode covered with an alumina cylinder (3 mm thickness)
or, alternatively, filled with ferroelectric pellets of BaTiO3 (200
gr purchased from Alfa Aesar) on the order of 2-3 mm acting
2. Experimental Section
Two different plasma reactor configurations have been
evaluated for the DRM and DDM reactions. The first setup
consists of a DBD plasma reactor inspired by a previous design
described elsewhere.24 Herein, we present a modified version

Evaluation of Different DBD Plasma Configurations
J. Phys. Chem. A, Vol. 114, No. 11, 2010 4011
Figure 2. Schematic representation of the second packed-bed DBD
plasma configuration. The insets represent the midsection sketch of
the reactors packed with an alumina cylinder and ferroelectric BaTiO3
beads, respectively. The different parts of the reactor and their
corresponding assignation with numbers are fully described within the
text.
Figure 3. Typical voltage (black lines) and current (gray lines)
waveforms for the different reactor configurations. The applied
frequency was 900 Hz. The spectra correspond to the direct decomposi-
tion of methanol (total flow ) 5 sccm).
as dielectrics; (3) inlet and circulation of gases in downstream
mode; (4) a metal electrode made of stainless steel with 38 mm
of internal diameter (the length of this external electrode defines
the zone where the plasma is produced); and (5) an external
furnace to maintain the reactor at temperatures higher than 110
°C to avoid condensation of water. The whole setup was
protected with a Teflon cylinder.
reaction efficiency for all experimental conditions. A maximum
uncertainty of 10% was estimated for our experimental conditions.
The determination of the power density, an important factor
to associate with the efficiency and the economical viability of
the process, is not yet completely understood for these types of
systems. Plasmas in DBDs are normally characterized by a large
number of microdischarges per unit of electrode area and per
cycle, since the lifetimes of these microdischarges are in the
range of the nanoseconds.29 A typical value is about 106
An AC power supply whose voltage can be varied between
1 and 30 kV and its frequency between 750 and 6 kHz has
been used to ignite the plasma. The power source has a bipolar
configuration with two electric transformers in the output. Each
transformer is connected to each electrode in counter-phase,
providing a maximum voltage of 15 kV. The electrical
characterization of the source was made with a digital oscil-
loscope (Tektronix TDS3034B). The typical voltage and current
waveforms obtained for both reactor configurations are depicted
in Figure 3. The presence of spikes or sharp peaks in the current
curves is characteristic of microarc formation.12,24,35 These
filaments are generated when positive or negative voltages
induce a local electric field that causes the electric breakdown
of the gas throughout the surface of the electrodes.12,24,35 Hence,
the present architecture of the bipolar power source gives
remarkable characteristics to the discharge so that filaments
occur not only at the rising edge but also at the falling edge of
the applied voltage (Figure 3). Different mixtures of carbon
dioxide and methane or methanol were adjusted by mass flow
controllers (Bronkhorst). The gas line was heated at around 110
°C to avoid any condensation of water. The reaction products
were analyzed with a mass spectrometer (Sensorlab) placed
downstream of the reactor. This mass spectrometer was carefully
calibrated with mixtures of the pure gases to quantify the
microdischarges cm-2 s-1 36
. For many purposes, it is sufficient
to describe the discharge by overall quantities: the applied
frequency, f; the maximum applied voltage, Vpeak; and finally,
the minimum external voltage at which microdischarges are
observed in the discharge gap, Vig. The interesting feature of
the DBD power formula first proposed by Manley37 is that only
the peak value of the applied voltage is necessary, and not its
form.
Cdie - Cgas
P ) 4VigCdie f Vpeak
-
Vig
(5)
(
)
[
]
Cdie
Expression 5 correlates the total power, P, and the operating
frequency, f; the peak voltage Vpeak; and the capacitances of
the dielectric(s) Cdie and the discharge gap, Cgas, quantities that
characterize the electrode configuration. The discharge voltage

4012 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Rico et al.
Figure 5. Evaluation of the influence of the total flow rate on the
conversion of methanol for different reactor configurations. The
frequency applied was 900 Hz.
of the final products is not affected, and CO, H2, and some traces
of water are identified for all the experimental conditions.
In the decomposition of methanol reaction, a similar influence
on the DRM reaction is observed, and increasing the flow rates
decreases the methanol conversion. Figure 5 shows the different
conversion levels and total flows evaluated. The use of the first
DBD configuration with the sandblasted electrode yields the
maximum conversion levels (100-78%) for the widest range
of flow rates (3.5-80 cm3 min-1). A frequency of 900 Hz and
an applied voltage of 30 kV are the constant plasma parameters
for this experiment. Similar conditions were applied in the
packed-bed reactor (second DBD configuration) containing
the alumina cylinder as the dielectric material. In this case, the
conversion levels of methanol are reduced by 45% with
increasing feed rates (cf. Figure 5). Finally, the ferroelectric
packed-bed configuration with BaTiO3 beads could be evaluated
only for a maximum flow rate of 30 cm3 min-1 (the maximum
flow to keep a stable discharge), showing the lowest conversion
levels and the same detrimental effect of reducing the residence
time of the reactants within the reactor bed. In this latter case,
methanol conversions from 88 to 50% are detected upon
increasing the methanol flow rate. Interestingly, the total applied
voltage is kept within 2-3 kV in comparison with the higher
values applied for the rest of the configurations (i.e., 25-30
kV). CO and H2 were the only byproducts monitored throughout
all the experiments and for all the DBD configurations. No signs
of coke deposits or longer hydrocarbon formation were detected.
3.2. Influence of CO2/CH4 Ratios. Figure 4 shows the
evaluation of different CO2/CH4 mixing ratios in the methane
(Figure 4a) and carbon monoxide conversion yields (Figure 4b).
A working frequency of 900 Hz and a consuming voltage of
30 kV were applied for the different mixtures and DBD reactor
configurations. The input of CH4 was kept constant at 3.5 sccm
while the CO2 flow rates were increased up to 37.5 sccm.
Increasing the proportion of CO2 in the feed does not affect the
attainment of CO and H2 as main products, but the decreasing
conversion trend is observed equally for both CH4 and CO2 at
the same time and for the all the reactor configurations. A major
influence of the change of the residence time (space velocity)
due to the increment of total feed flows must be attributed
instead as discussed in the previous section. Nevertheless, the
appearance of some minority byproduct due to the variation of
compositions cannot be discarded. For instance, if CO2 is not
present in the mixture (results not shown), some carbonaceous
species start to deposit on the reactor. Conversely, when the
Figure 4. Evaluation of the conversion of (a) CH4 and (b) CO2 in the
dry reforming of methane under different feed ratios, residence times,
and electrode roughnesses. The frequency applied was 900 Hz.
depends on the nature of the gas mixture, the gas density, and
the width of the discharge gap. For this characterization, we
take into account the region of cycle where there is discharge
between the two electrodes. The remaining cycle is associated
with an electrode configuration where no plasma is generated.30
Depending on the peak voltage, the microdischarge activity is
limited to a certain fraction of the cycle and occurs at twice the
driving frequency. The reaction efficiencies were calculated in
terms of the power density (i.e., applied voltages and frequen-
cies) and the total flows and conversion of reactants (i.e.,
methane or methanol) expressed in moles of H2 converted per
kilowatt and per hour.9
3. Results
3.1. Influence of Total Flow Rate. The total feed flow rate
is directly associated with the residence time of the reactants
within the reactor. Figure 4a shows the influence of increasing
the total flow of CH4-CO2 in the final conversion yields of
methane. A similar trend is observed for all the DBD configura-
tions, regardless of the roughness degree of the electrodes (first
DBD configuration). Increasing the total flow rate from 7 to 40
sccm causes an overall 20% decrease in the CH4 conversion
yields (from 60 to 40% in the porous-electrode configuration
and from 40 to 20% for the blasted and hole-electrode ones).
Analogously, CO2 conversion yields are also reduced with
increasing feed rates, as depicted in Figure 4b. The selectivity

Evaluation of Different DBD Plasma Configurations
J. Phys. Chem. A, Vol. 114, No. 11, 2010 4013
Figure 6. Evaluation of the influence of the applied frequency on (a)
the conversion of CH4 and (b) the reaction efficiency expressed in moles
of H2 per kilowatt hour. The total flow of CH4 and CO2 was fixed at
6 and 12 sccm, respectively.
Figure 7. Evaluation of the influence of the applied frequency on (a)
the conversion of CH3OH and (b) the reaction efficiency expressed in
moles of H2 per kilowatt hour. The total flow of CH3OH was fixed at
20 µL for the rough configuration and at 40 µL for the packed-bed
reactor with alumina or BaTiO3.
CO2/CH4 ratio exceeds a value of 6 (Figure 4), some traces of
water are detected, thus meaning that the reverse WGS reaction
is partially taking place (eq 6; ∆H0 ) +41.2 kJ mol-1)9:
initially better (40% minimum conversion at 750 Hz). Interest-
ingly, the second packed-bed reactor containing BaTiO3 shows
an opposite trend and exhibits maximum conversion yields at
750 Hz (almost 60%). Increasing the frequency has a detrimental
effect and lowers the total conversion of methane to a minimum
of 30% at 3000 Hz, slightly improved to 40% for the maximum
frequency evaluated (Figure 6a). CO2 yields followed a conver-
sion trend similar to that described for methane but always
showing lower levels between 5 and 40%. Additionally, it must
be pointed out that the selectivity to CO and H2 is not affected
by the increasing frequency.
As shown in Figure 7a, the methanol conversion yields are
higher for all the frequencies applied and all the reactor
configurations in comparison with the DRM reaction (Figure
6a). The lower conversions are observed again for the packed-
bed reactor with the BaTiO3 perovskite. The yields keep constant
around 60% regardless of the applied frequencies. Conversely,
the increase in the discharge power enhances the methanol
conversion yield for the two other reactor configurations. The
packed-bed alumina reactor reaches 92% conversion at 4500
Hz. In the DBD rough reactor, a similar yield is obtained at
high frequencies, but the conversion levels are below 60% for
the lower frequency values (Figure 7a). As previously observed
for the DRM experiments, the selectivity to syngas (CO and
H2) is not altered by the increase in the discharge power in any
of the reactor configurations.
CO2(g) + H2(g) f CO(g) + H2O(g)
(6)
We conclude that the overall variations in the selectivity to H2
and CO as well as the effect of increasing the amount of CO2
under our experimental conditions can be considered negligible.
3.3. Influence of Discharge Power. After fixing CH4 and
CO2 flow rates to 6 and 12 sccm, respectively, a systematic
study of the effect of increasing the frequency (thus increasing
the power density according to the Manley equation, see eq 5)
in the conversion yields was carried out. Three different reactor
configurations were studied and compared: (i) the DBD con-
figuration with the rough electrode, (ii) a packed-bed configu-
ration with Al2O3, and (iii) a packed-bed reactor with BaTiO3.
In the case of the methanol decomposition reaction, similar
experimental conditions were chosen for a total flow of 5 µL
min-1
.
Figure 6a shows how the increase in the frequency from 750
to 4500 Hz supposes a systematic increase in methane conver-
sion for the DBD rough configuration and the packed-alumina
reactor. However, the influence of power is shown more
pronounced in the DBD rough configuration with an increasing
methane conversion yield from 20% at the lowest frequency of
750 Hz up to 75% for 4500 Hz. In the alumina reactor, methane
conversion rises with the input power to similar levels, but it is

4014 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Rico et al.
3.4. Reaction Efficiencies. Another important parameter to
consider is the efficiency of the process and the energetic balance
for the conversion of reactants to obtain valuable products. In
our case, we have referred the reactor efficiency to the power
necessary to convert methane or methanol into hydrogen
(expressed in terms of moles of H2 per kilowatt hour). The
determination of the energy input is done through the Manley
equation29,30,37 and the I-V curves (Figure 3) corresponding to
each reactor configuration. Figure 6b shows how the reactor
efficiencies in the dry reforming of methane are really dependent
on the type of plasma reactor and the input frequency. Both
packed-bed configurations follow a similar trend and show lower
efficiencies with increasing input powers. The DBD rough
configuration shows the opposite effect and slightly raises the
reaction efficiency from 0.05 to 0.2 mol H2/kWh when increas-
ing the applied frequency from 750 to 4500 Hz. Nevertheless,
these values are far from those obtained for the packed-bed
reactors, especially at the lower applied frequencies (∼0.5 mol
H2/kWh for the reactor containing BaTiO3 and 0.7 mol H2/kWh
for the alumina reactor).
In the case of the direct methanol decomposition, the reaction
efficiency trends observed for each reactor configuration are
quite similar to the DRM experiments (see Figure 7b). However,
the efficiency values are much higher than those obtained for
the methane reforming reaction. Again, it is shown how the
DBD rough configuration yields the less energetic favorable
values with 0.5-0.7 mol H2/kWh. The packed bed with BaTiO3
reaches an energy efficiency of ∼2 mol H2/kWh, which slightly
diminishes to 1.5 when the frequency rises. Similarly, the other
packed-bed reactor containing alumina reaches 4.5 mol H2/kWh
values of energetic efficiency reduced to 1.8 mol H2/kWh at
the maximum input power (Figure 7b). Even under the least
favorable conditions, the conversion of methanol into hydrogen
is at least 3 times more efficient than the corresponding
conversion of methane under the same reactor and plasma
conditions (Figure 6 b and 7b).
authors for the remediation of volatile organic compounds
(VOCs) in the presence of plasmas and porous alumina or
silica.39-43 Nevertheless, a scalable version of this porous DBD
prototype would not be economically viable due to the prohibi-
tive cost of fabrication of the electrode with that specific
morphology. In addition, the high applied voltages (close to 30
kV) and the relatively low reaction efficiencies obtained (<0.2
mol H2/kWh) made the DBD configuration less energetically
competitive than other systems available in the literature.1,9
These economical burdens were the main motivation to explore
a different DBD reactor approach based on a packed-bed
configuration.32
Built on low cost and easy-to-assembly components, a more
robust, manageable, and industrial scalable reactor could be
developed (cf. Figure 2). Figure 6 accounts for the different
conversion yields of methane and reaction efficiencies obtained
in the DRM reaction when comparing one of the first DBD
configurations (rough electrode) with the packed-bed versions
filled with a porous alumina cylinder or BaTiO3 beads,
respectively (see insets in Figure 2). Higher conversion yields
were obtained for the new configurations, especially at low
frequency values of 750 Hz (cf. Figure 6a). Although the
conversion levels for the rough first DBD configuration were
comparable or even better with increasing power density, the
main differences were definitely observed in terms of reaction
efficiencies (Figure 6b). Values of 0.5 mol H2/kWh (or ∼0.15
mmol H2/kJ) and 0.7 mol H2/kWh (or ∼0.20 mmol H2/kJ) were
achieved for the BaTiO3 and the alumina-packed reactors,
respectively. This meant an order of magnitude higher than the
rough DBD reactor under the same experimental conditions and
similar efficiency levels compared with other systems.1,9 Like-
wise, the applied voltage in the BaTiO3 reactor was within 2-3
kV, whereas the other configurations oscillated around 30 kV,
yielding the most energetic competitive configuration. Thus, the
better conversion and efficiency results obtained with the
alumina reactor should be attributed to certain additional
catalytic abilities of the dielectric material itself, as has been
previously claimed for other plasma-catalytic systems.39-43
4. Discussion
Another important aspect systematically observed in the DRM
reaction under our experimental conditions was that CO2
rendered lower conversion percentages than methane. In all
cases, the values were ∼20% below the conversion levels
achieved for the hydrocarbon coreactant. This phenomenon has
been similarly described by numerous authors.1-4,9-13,44-51 Bo
et al.44 explained this difference on the basis of the higher
dissociation energy of CO2 (bond dissociation energy ) 5.52
eV) in comparison with that of CH4 (bond dissociation energy
) 4.55 eV). Similarly, Wang et al.13 determined that the
ionization energy required for CO2 was higher than for CH4
(13.8 and 12.9 eV, respectively); thus, the former was more
stable and less reactive within the plasma discharge. Finally,
other authors have claimed the existence of a competitive
reaction to form CO2: (OH* + CO* f CO2 + H*).44,52 Anyway,
the methane conversion yields showed no significant dependence
on increasing the feed percentage of CO2 (Figure 4a). Further-
more, even under experimental conditions when CH4 was
predominant, no sign of carbonaceous deposits or longer
hydrocarbon chain formations was detected, in comparison with
other plasma configurations.9-13,46,51-54 The selectivity toward
CO + H2 formation was kept almost constant, and only small
traces of water formation, probably due to reverse-WGS (eq
6), were identified with increasing CO2 contents. This same trend
was also shown by Song et al. in their noncatalytic plasma
reactor.46,49 Methane dissociates under the influence of the
4.1. Dry Reforming of Methane (DRM). One key param-
eter under study in the present work has been the systematic
evaluation of different reactor configurations and their contribu-
tion to the overall reaction efficiencies and conversion yields
for the carbon dioxide reforming of methane. Furthermore,
specific attention has been paid to a novel aspect such as the
influence of the surface morphology of the inner electrodes.
Previous studies presented by Choi et al.38 suggested better
plasma homogeneity after coating the electrodes with ZnO/Al2O3
rough films. Preliminary results in our group also envisioned
that providing some roughness to the initially flat inner electrode
was an important parameter to take into account.24 Herein, after
comparing three electrodes with different surface textures (see
the Experimental Section), we have been able to confirm the
better performance of the reactor in comparison with the
smoother electrode version working under identical experimental
conditions.24 Moreover, we observed that the best conversion
yields were achieved with the most porous electrode (cf. Figure
4). Up to 60% of methane conversion was obtained, almost 20%
higher than the electrode with 3 mm holes or the sandblasted
one (Figure 4a). A possible explanation for the better reaction
yields observed in the porous DBD reactor might be attributed
to the better immobilization of gas molecules within the
intraporous network of the electrode, thereby creating additional
active sites for the interaction with short-lived excited inter-
mediates. This hypothesis has already been proposed by other

Evaluation of Different DBD Plasma Configurations
J. Phys. Chem. A, Vol. 114, No. 11, 2010 4015
reactive plasma conditions, yielding carbonaceous particles,
higher hydrocarbons, or both.11,46,50,52-56 In our opinion, the main
factors preventing the formation of these nondesired byproducts
have to do with (i) the role of CO2 to scavenge any coke
formation and produce CO as the only byproduct (differs from
O2 as oxidant agent, since does not generate gas phase radical
reactions and oxygenate intermediate species so easily),9,46 (ii)
the absence of a catalyst within the discharge to increase the
selectivity toward C2 species,2,3,10,46,51 and (iii) the use of a
bipolar configuration source that duplicates the formation of
filament discharges at both the falling and rising edges of the
applied voltage.24,48,49
4.2. Direct Decomposition of Methanol (DDM). As previ-
ously pointed out in the Introduction, alcohols in general and
methanol in particular represent one of the most promising
alternatives to fossil fuels as cleaner energy sources.18 Hydrogen
can be easily stored and handled in the form of liquid alcohol,
thereby avoiding the use of high-pressure conditions, risks of
leakage and explosions, and exhaustive storage and safety
protocols.5,16,18,20,57 In addition, part of these alcohols can be
obtained from natural resources (i.e., biomass).5,16,18,57 Although
great efforts are being made to develop efficient catalytic
processes for the reforming or direct decomposition of
alcohols,7,8,14,19-23,58 the application of plasma discharges for this
purpose is still surprisingly scarce in the literature.23-27,33,34
rates. Both configurations required voltage values (25-30 kV)
identical to those previously described for the DRM reaction.
Conversely, the second packed-bed reactor with BaTiO3
required only 2 kV and reached 50% under the least favorable
conditions (i.e., minimum residence time). Hence, when the
systematic influence of increasing the applied frequency (the
power density) was studied, this latter reactor gave the most
stable and reproducible results and maintained similar methanol
conversion percentages (around 60%) (Figure 7a) and energy
efficiencies over 1.5 mol H2/kWh (Figure 7b) (better than any
result with the DRM reaction; see the previous section). The
rough DBD and the packed-bed with alumina reactors exhibited
a noticeable increase in the conversion levels with increasing
power discharge, almost reaching total conversion levels (Figure
7a). Nevertheless, in terms of efficiency, the first DBD config-
uration barely reached 0.5 mol H2/kWh, whereas the latter
showed the best results for every applied frequency. The high
applied voltage values made the reactor less efficient with the
increasing power density (Figure 7b), but even under those
experimental conditions, efficiency levels oscillated between 4.5
and 1.8 mol H2 per kWh. As previously pointed out, the high
specific surface area of alumina can be providing additional
reaction sites and behaving as a catalyst within the reactor
discharge.39,40
The overall higher conversion yields and reaction efficiencies
found in comparison with the dry reforming of methane reaction
must be associated with the lower bond dissociation energy for
methanol25 (4.16 eV) and the oxidizing contribution from the
hydroxyl radicals released under the influence of the plasma
discharge. The presence of OH* intermediates has been proved
to be beneficial for the reforming of hydrocarbons and carbon-
aceous species.23,24,27,60 Nevertheless, in our opinion, future
efforts in this relatively unexplored field should be focused on
the development of a versatile reactor that permits a better
control of the stoichiometry of the final byproduct. In this regard,
recent results by Rico et al.27 have demonstrated that the
combination of the BaTiO3 packed reactor with certain amounts
of Cu-Mn catalysts can increase the selectivity of the final
product CO into CO2. This result can foster the extrapolation
to the fuel cells and the proton exchange membrane fields where
carbon monoxide would act as a poison for the Pt electrode.
One of the most remarkable results obtained within the frame
of the present work has consisted of the attainment of high
conversion yields of methanol into hydrogen and carbon
monoxide (Figures 5 and 7) without the formation of other
byproducts such as carbon deposits or higher hydrocarbons.
Preliminary results in our group showed that the reforming of
ethanol by a microwave-assisted surfatron launcher yielded
carbon deposits on the walls and acetylene (C2H2) in the absence
of a second oxidant agent such as water.23 Nevertheless, local
heating effects generated by the microwave irradiation could
be inducing the formation of these carbon deposits.
According to this, our first reforming experiments with
alcohols using a DBD configuration were made with the partial
addition of water to prevent the formation of coke.24 To our
surprise, all the experiments made subsequently with the
configurations described in this work could be initially carried
out in the absence of water without the formation of carbon-
aceous particles. Therefore, direct decomposition of methanol
(see eq 2), a reaction that is thermodynamically unfavorable,
was instead plausible under the nonequilibrium energetic
conditions of plasmas. Initially, we attributed this fact to the
rough morphology of the new inner electrode in the first DBD
configuration. However, after some preliminary attempts re-
forming methane alone (results not shown) in which carbon
started to deposit, we finally came up with the conclusion that
the formation of hydroxyl radicals (OH*) from the direct
decomposition of methanol was preventing the formation of
carbonaceous species.23,25,59-61
5. Conclusions
Different DBD plasma configurations were investigated for
the dry reforming of methane and the direct decomposition of
methanol reactions. In the first reactor, the roughness of the
inner electrodes was demonstrated to play an important role on
the conversion and efficiency levels of methane. However, the
best results were obtained for the packed-bed configuration
based on a more robust, manageable and scalable design and
more efficiency, especially at lower frequency inputs. On the
other hand, the decomposition of methanol yielded higher
conversion percentages and better energy efficiencies for the
production of hydrogen than the reforming of methane and
constituted a novel and promising reaction in which the
synergetic combination of plasmas and catalysts might play a
decisive role in the final application for hydrogen production
and fuel cells. The use of a bipolar source also facilitated an
elevated selectivity of the byproduct toward CO and H2 and
the absence of carbon deposits or higher hydrocarbons.
In terms of a comparison of methanol conversion yields for
the different plasma DBD configurations under study, better
results were obtained with the first DBD device and the rough
electrode (Figure 5). In addition, the detrimental effect of
reducing the residence times (i.e., increasing flow rates) was
less pronounced for this configuration and enabled up to 80%
conversion levels working with an applied frequency of 900
Hz and feed rates of 80 sccm. The packed-bed reactor with
alumina rendered quite similar results, but the maximum
conversion capacity was kept below 60% for the higher flow
Acknowledgment. The authors thank the Spanish Ministry
of Education and Science for financial support. Part of this work
has been carried out within a collaboration project with the
company Hynergreen (Seville, Spain).

4016 J. Phys. Chem. A, Vol. 114, No. 11, 2010
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