22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Conversion of CO 2 and H 2 O by microwave plasma discharge coupled with a
catalytic reactor
G. Chen1,2, V. Georgieva1, T. Godfroid3, T. Silva2, N. Britun2, R. Snyders2,3 and M.-P. Delplancke-Ogletree1
1
4MAT, Université libre de Bruxelles, 50 av. F.D. Roosevelt, 1050 Brussels, Belgium
2
ChIPS, Université de Mons, 23 Place du Parc, 7000 Mons, Belgium
³ Materia Nova Research Center, av. N. Copernic 1, 7000 Mons, Belgium
Abstract: In this work, the conversion of CO 2 and H 2 O vapor mixture in a microwave
plasma system in presence of NiO/TiO 2 catalyst is investigated. The results show that the
CO 2 conversion depends on the crystal phase of the catalyst support. I t increases when
NiO/TiO 2 (anatase) and NiO/TiO 2 (anatase/rutile mixture) are used, while NiO/TiO 2 (rutile)
lead to lower CO 2 conversion efficiency in comparison with the plasma only assisted
dissociation.
Keywords: CO 2 /H 2 O conversion, plasma-catalysis, NiO/TiO 2 catalyst
1. Motivation
Decrease of natural reserves of fossil fuels and the
greenhouse effect from CO 2 emissions generated by
anthropomorphic activity incites searching for new
sources of fuels. The most generally useful method is the
conversion of electrical energy produced by renewable or
nuclear source into a chemical fuel. One of the promising
solution is using the electrical energy for plasma
processing CO 2 to convert it into synthetic fuels [1].
Recently, the combination of heterogeneous catalysis
and plasma activation, known as plasma-catalysis, has
attracted increasing interest. The dissociation of CO 2 was
extensively studied in the recent years [2-7]. However, in
order to produce synthetic gas (CO/H 2 ) H 2 has to be
added. The production of hydrogen is expensive as it
takes a great deal of energy to extract it from water.
Therefore, it is of practical interest to investigate the
simultaneous dissociation of CO 2 and H 2 O. Up to now,
few studies have investigated the steam reforming of CO 2
by plasma [8, 9].
In this paper, the conversion of CO 2 and H 2 O mixtures
in presence of titanium oxide supported NiO in a
surfacewave sustained microwave discharge is, to our
knowledge, investigated for the first time. The aim is to
evaluate the conversion efficiency dependence on the
catalyst properties.
containing the catalyst is connected to the end of the
quartz tube. The pressure in the discharge tube is set to
1330 Pa for all experiments. The whole system is
surrounded by a grounded aluminum grid to prevent any
leak of microwave radiation into the outer space. Pulse
duration is set to 300 µs, and the off period is set to
300 µs. A more detailed description of the microwave
set-up can be found in [9].
2. Experiments
2.1. Experimental system
Fig. 1 shows a schematic diagram of the experimental
setup. The discharge is sustained by microwave radiation
(915MHz) in a quartz tube, 14 mm in inner diameter and
24 cm long surrounded by a Plexiglas tube of 28 mm
inner diameter. The discharge tube is cooled by silicon
oil flowing between the inner and the outer tubes. CO 2
and H 2 O gas mixture is injected from the top of the
system. The water vapor is generated in a vaporization
system built by Omicron technologies.
A reactor
Fig. 1.
Schematic representation of surface-wave
microwave set-up.
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2.2. Catalyst preparation
The NiO supported on TiO 2 catalysts were prepared by
combination of impregnation and plasma treatment
methods. The precursor was Ni(NO 3 ) 2 • 6H 2 O (Merck).
TiO 2 nanocrystalline powder (Sigma-Aldrich) in two
polymorphs (anatase, rutile, and a mixture of rutile and
anatase) were used as a catalyst support. The mixture
consists of 86.5% anatase and 13.5% rutile. TiO 2 support
1
was impregnated with the aqueous solution of
Ni(NO 3 ) 2 • 6H 2 O for 24 hours and dried at 100 ℃ for
12 hours, followed by CO 2 plasma treatment of 30
minutes at a plasma power of 2000 W to form NiO/TiO 2
catalysts. The three catalysts were denoted NiO/TiO 2
(anatase), NiO/TiO 2 (rutile) and NiO/TiO 2 (mixture),
respectively.
2.3 Product analyses
The composition of the post-discharge is analyzed by a
gas chromatograph (GC) (Bruker) equipped with a carbon
molecular sieve column and a Molecular sieve 5A column
in series and connected to a thermal conductivity detector,
which allows to determine the concentration of H 2 , O 2 ,
CO and CO 2 . The conversion of CO 2 is calculated by
comparing the peak area of CO 2 obtained by the GC
before and after reaction. The conversion efficiency of
CO 2 and the yield of CO (and H 2 ) are calculated based on
the following ratios:
CO2 Conversion (%) =
Yield of CO(H2 ) =
moles of CO2 converted
moles of CO2 in feed
× 100%
moles of CO(H2 ) produced
× 100%
moles of CO2 (H2 O) in feed
3. Results and discussion
The following operating parameters are used in the
present study: CO 2 /H 2 O (90%:10%) gas mixture is
supplied at flow rate of 2 slm, and input power of
2000 W, which corresponds to a specific energy input per
molecule of 6.95 eV/molecule. Comparisons of CO 2
conversion using the surface-wave microwave with and
without catalysts as well as the effect of the titanium
oxide supported NiO catalysts are shown in Fig. 2. 5 g of
the catalysts (10 wt.% NiO/TiO 2 ) were used for each
experiment, corresponding to a space velocity of
24,000 ml/(h.g). As can be noted from Fig. 2, the CO 2
conversion was significantly enhanced when NiO/TiO 2
(mixture) is used. The CO 2 conversion increases from
43% to 66%, comparing with the plasma only experiment.
With NiO/TiO 2 (anatase), the CO 2 conversion initially
increases and reaches stable values around 60% with the
catalyst activation. The results show that coupling plasma
with catalysts allows modifying the conversion efficiency
of CO 2 . This can result from different phenomena. The
CO 2 can be decomposed on the catalyst surface which
explains the improvement observed. It is also possible
that water decomposes on the catalyst. The H 2 formed by
this reaction can react with the CO 2 to produce CO and
water through the reverse water gas shift reaction
(WGSR). On the other hand, NiO/TiO 2 (rutile) catalyst
did not affect CO 2 conversion efficiency.
Fig. 3 shows the X-ray powder diffraction patterns of
NiO supported on TiO 2 catalysts in different crystal
phases. The XRD patterns confirmed the crystal structure
of these three catalysts. The crystallite sizes of catalysts
were estimated from the XRD patterns according to the
2
Scherrer’s formula, and the data are listed in Table 1. The
crystallite size of NiO/TiO 2 (rutile) is 51 nm. However, a
Fig. 2. Comparison of CO 2 conversion efficiencies in
pure plasma and the combined effects of plasma and
catalyst in CO 2 /H 2 O mixture.
small crystallite size was noticed for the other two
catalysts. Very weak diffraction peaks observed in the
diffraction pattern in the diffraction pattern of NiO/TiO 2
(anatase) catalyst at 2θ = 43.5, correspond well with the
(200) crystal planes of face-centered cubic NiO. In
addition, no NiO diffraction peaks could be observed
from the XRD measurements of NiO/TiO 2 (mixture)
catalyst, suggesting that the nickel oxide is either highly
dispersed on the support or the crystallite size is below the
XRD resolution limit.
Fig. 3. XRD patterns of TiO 2 supported NiO catalysts:
b) NiO/TiO 2 (rutile); and
a) NiO/TiO 2 (anatase;
c) NiO/TiO 2 (mixture).
Table 1 also compares the measured BET surface area,
porosity and total pore area. NiO/TiO 2 (rutile) had the
lowest BET surface area (m2/g), almost one quarter as
much as that of NiO/TiO 2 (mixture) and one fifth as much
as that of NiO/TiO 2 (anatase). The porosities of the three
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catalysts were similar. However, higher total pore areas
were observed for NiO/TiO 2 (anatase) and NiO/TiO 2
(mixture) catalysts.
Table 1. Properties of TiO 2 supported NiO catalysts.
catalyst
Property
Crystallite size
(nm)
Porosity (%)
Total pore Area
(m2/g)
BET surface area
(m2/g)
NiO/TiO 2
(Anatase)
NiO/TiO 2
(Rutile)
NiO/TiO 2
(mixture)
17
51
20
44.1
51.5
43
9.3
58.4
58.6
58
12
49
Fig. 4a-c shows the scanning electron microscope
images of TiO 2 supported NiO catalysts. As shown in
Fig. 4, smaller particle and uniform size distribution can
be observed for the NiO/TiO 2 (anatase) and NiO/TiO 2
(mixture) catalysts. This observation is in agreement with
the XRD results.
However, comparing them, the
NiO/TiO 2 (rutile) catalysts exhibited larger particle, and
particles agglomerated to form block-like crystal.
use in the microwave reactor by state-of-the art tools such
as X-ray photoelectron spectroscopy (XPS), Raman
spectroscopy, Time of Flight Secondary Ion Mass
Spectrometry (ToF-SIMS) and CO 2 Temperature
Program Desorption (CO 2 -TPD). The corresponding
research is ongoing and further studies regarding this
subject will be reported in future.
4. Acknowledgments
This research is carried out in the framework of the
network on Physical Chemistry of Plasma Surface
Interactions - Interuniversity Attraction Poles phase VII
project (http://psi-iap7.ulb.ac.be/), supported by BELSPO.
NB is a post-doctoral researcher of the Fonds National
de la Recherche Scientifique (FNRS), Belgium.
5. References
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Fig. 4. SEM micrographs of TiO 2 supported NiO
catalysts: a) NiO/TiO 2 (anatase); b) NiO/TiO 2 (rutile);
and c) NiO/TiO 2 (mixture).
The higher activity observed in the plasma treatment
using NiO on TiO 2 (anatase) and (mixture) catalysts may
be linked to the formation of well-dispersed NiO particles,
large surface area and small crystallite size. The
fundamental mechanisms of reactions are still unknown
and, therefore, further investigations are necessary.
To get a better understanding of the relationship
between the plasma-catalyst interactions and synergistic
effect of plasma-catalysis from both a chemical and
physical perspective, characterization of the different
catalysts before and after their utilization must be done.
Particularly, the chemical composition and morphology of
the catalysts will be characterized before and after their
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