MWCNTs as a catalyst in oxy-steam reforming of methanol

Article abstract of DOI:10.1039/c6ra15618e

The catalytic activity of multi-walled carbon nanotubes (MWCNTs) in oxy-steam reforming of methanol (ASRM) was investigated for the first time. We demonstrate that CNTs are a potent catalytic material for hydrogen generation in oxy-steam reforming of methanol.

Full text of DOI:10.1039/c6ra15618e

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MWCNTs as a catalyst in oxy-steam reforming of methanol
P. Mierczynskia, A. Mierczynskac, W. Maniukiewicza,T.P. Manieckia and K. Vasilevb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
both inside and on external wall of carbon nanotubes make then a
very promising material as catalyst support15
Furthermore,
nanoparticles dispersed on the support surface present active
centres easily accessible to the reactants.
However, despite of these few studies concerned with metal
catalysts supported on carbon nanotubes, the intrinsic catalytic
properties of carbon nanotubes have never been explored in the
oxy-steam reforming of methanol reaction (ASRM).
In the present work, multi-walled carbon nanotubes (MWCNTs)
synthesized by chemical vapour deposition (CVD), both as
synthesised and after purification, were investigated for their
potential as catalysts in ASRM. To the best of the knowledge of the
authors, this is the first time pristine MWCNTs containing iron
species (α-Fe2O3, Fe3C) or activated synthesized systems (containing
Fe3C, Fe3O4, Fe) were used alone as catalysts in ASRM reaction.
The aim of this work was to develop a new simple and efficient
catalysts nanosystem consisting only of MWCNTs which can be
utilized in hydrogen production from methanol. Methanol
The catalytic activity of multi-walled carbon nanotubes (MWCNTs)
in oxy-steam reforming of methanol (ASRM) was invesigated for
the fist time. We demonstrate that CNTs can present a potent
catalitic material for hydrogen generation in oxy-steam reforming
of methanol.
.
Tackling climate change, one of the greatest challenges today,
requires a broad portfolio of renewable energy sources 1-3. One
possible energy carrier is hydrogen. It offers high gravimetric energy
density and zero emission during combustion or conversion in fuel
cells4. The use of hydrogen as a fuel for transportation and
stationary applications has received much attention5. Hydrogen is
being explored for use in internal combustion (IC) engines and fuel
cell electric vehicles6, 7. Fuel cells technology is in the early stages of
commercialization and offers a more efficient hydrogen use over
hydrogen-fuelled IC engines. Hydrogen and fuel cells are often
considered as a key technology for future sustainable energy
supply. In comparison to traditional methods for electric energy
generation from coal, fuel cells have lower carbon dioxide emission
and about reduction of nitrogen oxides emission, which can have
represents
a potential carrier for the convenient storage of
hydrogen, a novel catalytic concepts for hydrogen generation from
methanol is of particular importance.
Multi-walled carbon nanotubes were synthesized by chemical
vapour deposition (CVD) at 850 °C using ferrocene as a catalyst. The
scanning electron microscopy (SEM) and transmission electron
significant influence on the environment8-10
.
In light of the potential of hydrogen as a future energy source,
novel catalyst concepts for methanol steam reforming are
extensively investigated. Recently, carbon nanotubes (CNTs) and
active carbon have attracted attention as novel catalyst support
materials11. Solymosi et al.12 studied group 8-10 metals immobilized
on active carbon and carbon nanotubes in the decomposition and
steam reforming of methanol. These catalysts suffered from low
selectivity to carbon dioxide. Lio and Yang prepared Cu/ZnO on CNT
catalysts and applied them to methanol steam reforming13. High
activity and selectivity was reported for the temperature range of
200 to 400 °C.
The interest in using CNTs as catalysts or/and catalyst support
stems from the material high thermal and mechanical stability and
unique electron structure. In addition, the good conductivity of
CNTs is thought to promote a spillover effect between active phase
and support14. The interaction between the metal and support,
specific electron structure and the presence of active nanoparticles
microscopy (TEM) images (see
Fig. 1) showed that the
nanomaterial is composed of carbon nanotubes aligned
perpendicularly to the carpet basis which is contaminated with iron
oxide (III) (hematite) particles and iron carbide (cementite)
originating from the ferrocene catalyst. The SEM results showed
that nanotube length (i.e. carpet height) is in the range of 440 - 460
µm. TEM observations on dispersed sample reveal that the
nanotubes are multi-walled having outer and inner diameters of
approximately 30-60 nm, and in the 8-10 nm, respectively. The TEM
images also reviled the presence of iron-based particles of hematite
(Fe2O3) and cementite (Fe3C) (Fig. 1).
a. Lodz University of Technology, Institute of General and Ecological Chemistry
90-924
Lodz,
Poland,
Zeromskiego
116,
e-mail*-
mierczyn25@wp.pl,
pawel.mierczynski@p.lodz.pl tel: 0048-42-631-31-25, fax: 0048-42-631-31-28.
bSchool of Engineering, University of South Australia, Mawson Lakes, SA 5095, Adelaide,
Australia. e-mail - Krasimir.Vasilev@unisa.edu.au
c.
The Australian Wine Research Institute, P.O. Box 197,Glen Osmond (Adelaide), SA
5064, Australia.
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reaction performed without the catalyst at 350 °C proceeds with
low yield as evidenced by the low degree of methanol conversion
(10%). Increasing the process temperature to 400 °C significantly
increases the degree of methanol conversion up to 42% (see Fig. 2).
It is also worth noticing that in the case of the reaction carried out
at 350 °C only hydrogen and carbon dioxide as a major product of
the ASRM process were formed. While, at higher temperature of
the reaction formation of carbon monoxide was also observed.
Carbon monoxide formation proves, that the decomposition of
methanol and RWGS reaction during the ASRM process took place
simultaneously (see Fig. 4)11, 15. The lack of the CO in the product of
the ASRM reaction performed at 350 °C confirms that the reaction
proceeds slightly. In addition, the high yield of hydrogen formation
in the case of reaction performed at 400 °C confirmed the WGS
reaction which runs during the ASRM process (see Fig. 3).
When the process was conducted using a catalyst the highest
conversion of methanol was derived from MWCNTs activated in a
mixture of 5%H2-95%Ar at 500°C and demonstrated that H2 and CO2
are the major products in ASRM reaction. Besides the main
products, the formation of undesired by-products such as carbon
monoxide and DME were also observed (see Fig. 4). This result
indicates that oxy-steam reforming of methanol was the major
reaction which take place during the process. In addition, the
presence of carbon monoxide and DME in the product indicates
that the RWGS reaction and methanol decomposition run
simultaneously during the ASRM process.
The catalytic activity tests showed that the purification of
MWCNTs significantly influenced their catalytic activity, yield of
hydrogen and selectivity towards obtained products. The
purification of MWCNTs leads to decrease in methanol conversion.
The MWCNTs which were only reduced, exhibited the highest
activity in comparison to pristine MWCNTs and for these that were
purified or purified and reduced. Purified MWCNTs before and after
reduction exhibited comparable value of methanol conversion and
hydrogen yield. These results confirmed that iron species such as
Fe3O4 and metallic iron play crucial role in catalysing the ASRM
reaction. The highest activity of reduced MWCNTs is most likely due
to their highest content of iron species such as metallic iron,
cementite and magnetite phase (see Fig. 5A). The reduction process
carried out for the pristine material caused the reduction of surface
and encapsulated iron (III) oxide particles to metallic iron or/and
magnetite Fe3O4 phase (see Fig. 5B).
Fig.1 SEM (A) and TEM (B) images of multi-walled carbon nanotubes collected
from the reactor.
The presence of nanoparticles inside the tubes suggests that the
carbon nanotubes stabilize the Fe2O3 and/or Fe3C. Furthermore, the
presence of iron based particles suggests that iron (III) oxide is
bounded by strong interaction inside the tubes and that the
nanoparticles are encapsulated inside of the MWCNTs.
The type of iron species e.g. hematite, metallic iron, and
magnetite that can be present in the MWCNTs may play a curial
role in oxy-steam reforming of methanol. To elucidate the role of
these iron based additives on the catalytic performance, the
synthesised MWCNTs we subjected to different treatment:
reduction at 500 °C in 5%H2-95%Ar atmosphere, purification with
16
concentrated HCl as reported by Edwards et al.
in order to
remove iron based impurities, and purification and reduction.
In order to ensure that the systems under investigation are
stable at the reaction temperature, thermal stability measurements
were performed for purified and pristine MWCNTs. Both systems
were reduced at 500 °C in a mixture 5%H2-95%Ar MWCNTs (Fig. S1
and S2, ESI†). The TG results showed that purification of MWCNTs
had a substantial influence on the nanomaterial stability. Carbon
nanotubes, which were purified in concentrated HCl solution,
exhibited higher thermal stability in comparison to pristine
MWCNTs. The presence of magnetite, minor phase of metallic iron
and cementite phases in as-synthesized nanomaterial reduced at
500°C facilitated the MWCNT decomposition and decreased the
initial temperature of decomposition rate (Fig.S2, ESI†). This
experiment also showed that the unpurified material showed total
loss of weight of 92 % which means that the MWCNTs contain
about 8% wt. of iron compounds. It is worth nothing that in the case
of MWCNTs systems purified in HCl and reduced at 500°C the total
loss of weight during the decomposition process is nearly 100%
which suggests that only small part of iron species could have
remained in the MWCNTs.
The catalytic activity of pristine, reduced, purified and purified
and reduced MWCNTs was evaluated in ASRM. In order to elucidate
the influence of the catalyst on the course of the investigated
process additional measurements without catalyst were performed
at 350 and 400 °C. The reactivity results expressed as methanol
conversion, yield of hydrogen and selectivity to carbon dioxide,
carbon monoxide and dimethyl ether (DME) are given in Figure 2, 3
and 4, respectively. The catalytic results showed that the ASRM
Fig.2 Methanol conversion for pristine MWCNTs, MWCNTs after reduction in 5%H2-
95%Ar mixture at 500 °C, after cleaning with HCl and after cleaning with HCl and
reduction in 5%H2-95%Ar mixture at 500 °C, respectively. ASRM reaction parameters:
H2O:O2:CH3OH = 1:0.4:1, weight of catalyst m = 0.1 g, p= 1bar, T = 350 °C and at 400°C
for the process performed without catalyst.
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It should be mentioned that during the purification process,
defects may have been introduced on the exterior and interior wall We observed the formation of Fe3C during CVD synthesis of
on the carbon nanotubes in locations from which the iron species MWCNTs as well as iron (III) oxide and cementite (see Figure 5A).
have been removed.
A covalent bond between metal particles and carbon nanotubes
The differences in methanol conversion can be attributed to the is formed by overlapping of pz orbitals of carbon belonging to the
structure modifications inside of the CNTs or/and change in the graphene sheets and the unfilled d orbitals of the metal. These
phase composition within the nanomaterial. Encapsulated interactions may be further enhanced by the presence of defects
nanoparticles inside the carbon nanotubes modify their electronic such as carbon vacancies originating from extra curvature of the
and magnetic properties as well as the carbon nanotubes introduced in the nanotubes wall pentagon and heptagon rings.
themselves.
The nature and strength of the interaction between metal
particles and carbon nanotubes depends on the metal. Whereas,
the strength of the interaction between metal and CNT increases
with the number of unfilled d-orbitals of the metal particle17. It is
known that metals with closed shell such as gold and palladium
exhibit very low affinity for carbon bonding. Iron, which has a few
vacant d-orbitals, exhibit high affinity and certain solubility in
carbon17. The electron structure of iron suggests that it is possible
that during the synthesis of MWCNTs iron particles may be built
into the structure of the carbon nanotubes and may even create
iron carbides (e.g. Fe3C).
Fig.3 Yield of hydrogen production for pristine MWCNTs, MWCNTs after reduction in
5%H2-95%Ar mixture at 500 °C, after cleaning with HCl and after cleaning with HCl and
reduction in 5%H2-95%Ar mixture at 500 °C, respectively. ASRM reaction parameters:
H2O:O2:CH3OH = 1:0.4:1, weight of catalyst m = 0.1 g, p= 1bar, T = 350 °C and at 400°C
for the process performed without catalyst.
Fig.5. 5A) XRD measurements for A) pristine MWCNTs and B) pristine MWCNTs
after reduction at 500 °C in 5%H2-95 %Ar mixture and reaction, 5B) TPR
profiles of pristine MWCNT and MWCNT cleaned with HCl solution.
The presence of metal particles inside of the CNT close to such
defects can generate stronger interactions compared metal particle
on the outer surface of the CNT. Metal particles inside the carbon
nanotubes are decorated with layers of graphene sheets at the CNT
side growth, the result of strong interactions between the metal
particles and the first graphene sheet17. The geometrical structure
of CNTs can be also affected by the interaction with another CNT or
a metal surface. Additionally, the surface van der Waals forces
could influence on MWCNTs structure18 and can induce a decrease
of MWCNTs diameter up to 30%. This is supported by a study of
Muthaswami et al.19 reporting that some chemical and physical
Fig.4 Selectivity towards CO, CO2 and DME formation in ASRM reaction for pristine
MWCNTs, MWCNTs after reduction in 5%H2-95%Ar mixture at 500 °C, after cleaning
properties of carbon nanotubes are induced by the defects created
as a result of encapsulated of metal particles or by defects located
at the tip or inside of the CNT.
with HCl and after cleaning with HCl and reduction in 5%H2-95%Ar mixture at 500 °C,
respectively. ASRM reaction parameters: H2O:O2:CH3OH = 1:0.4:1, weight of catalyst m
= 0.1 g, p= 1bar, T = 350 °C and at 400°C for the process performed without catalyst.
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The catalytic tests performed for all catalysts at 350 °C showed these spectra were presented in Figure 6. The system, which was
that the highest hydrogen yield in ASRM had catalytic materials purified in concentrated HCl solution, exhibited lower intensity
which were cleaned or cleaned and reduced before activity test. bands assigned to surface species observed during the
Those systems also showed the lowest selectivity to CO2 and the decomposition of methanol what can be easily seen on the
highest selectivity to CO formation. The selectivity to DME was also difference spectrum.
measured (see Fig. 4). The selectivity results showed that the lowest
selectivity to DME close to zero per cent exhibited catalyst which
undergone cleaning processes. This suggests that iron species may
have significant influence on DME formation during the reaction.
The activity results confirmed that ASRM proceeds easier on
reduced MWCNT containing metallic iron and magnetite. Carbon
nanotubes could promote electron transfer in the reduction process
and destabilize the α-Fe2O3 bonds and facilitate the reduction
process of hematite to metallic iron and magnetite which enhances
the catalytic activity. Cleaning of the nanotubes with HCl may
introduce structural defects inside of the tube, which had significant
influence on the catalytic activity of MWCNT in ASRM20. Gayathri et
al.21 studied hydrogen adsorption in carbon nanotubes defects and
reported that defects in different types and sizes carbon nanotubes
have an important contribution to the adsorption mechanism.
These defects affect on hydrogen storage capacity by CNTs. These
reports support the hypothesis that CNTs purified in HCl, can adsorb
hydrogen which facilitate the ASRM.
In order to further clarify the catalytic activity of MWCNT, the
surface adsorbed species formed during methanol decomposition
over differently treated MWCNT (reduced only, purified and
Fig.6 FTIR spectra of adsorbed species taken after reduction at 500 °C and exposure to
a 1 vol.% methanol-argon mixture at 350 and 400 °C of the following catalytic systems
a) pristine MWCNT b) MWCNT cleaned with HCl c) the difference spectrum a-c.
purified and reduced) were investigated by FTIR spectroscopy. IR
spectra of adsorbed species taken after exposure of MWCNT only
This result provides an explanation of the differences in reactivity of
after reduction and after cleaning and reduction process collected
purified and pristine CNTs in oxy-steam reforming of methanol. The
at 350 and 400 °C are shown in Figure S3 ( Fig.S3, ESI†). The IR
lower intensity of surface species in the case of MWCNTs after
spectra recorded for nano-system reduced at 500 °C showed
purification and reduction attributed to gaseous CO2, formate,
several characteristic bands. The visible bands located at 830, 930,
methoxy and carbonate species explain the lower methanol
1020, 1060, 1080, 1370, 1390, 1450, 1500, 1590, 1640, 1740, 2000,
conversion and CO2 selectivity in ASRM process.
2040, 2180, 2320, 2360, 3580, 3615, 3640 3670, 3700 cm-1 were
In summary, this report demonstrates the potential of CNTs to
observed both for the cleaned and none cleaned samples. The
serve as a potent catalyst in hydrogen generation. We have
visible bands located at 1370 cm-1, 1390 cm-1 and 1590 cm-1 were
confirmed that the Fe3O4, metallic iron and defects formed by
attributed to symmetric O-C-O stretching, CH deformation mode
encapsulated or removal of metal particles play significant role in
and asymmetric stretching of the O-C-O band intensity of formates
the oxy-steam reforming of methanol. TG results indicated that the
presence of iron nanoparticles inside the carbon nanotubes
species 22, 23. In addition to the foregoing formate surface species,
the bands assigned to the metoxy species can be found on the IR
facilitates their decomposition. The reduced MWCNTs exhibited
spectrum at a following positioned: 1060 cm-1 and 1450 cm-1.
excellent performance in ASRM and showed the highest methanol
Carbonate species were also detected visible at the following
conversion efficiency at 350 °C and the highest selectivity to CO2. In
wavelengths: 830 cm-1, 1020-1090 cm-1 (1020 cm-1, 1080 cm-1),
parallel reduced MWCNTs also showed the lowest selectivity
1570–1440 cm-1 (1500 cm-1). In addition the band situated at 1636
towards the formation of CO. This phenomenon is explained by the
cm−1 is attributed to C=C stretching, while the IR band situated at
presence of metallic iron and magnetite phases, which have
1740 cm−1 is attributed to C=O stretching. Whereas, the bands
significant influence on the catalytic activity of MWCNTs in ASRM.
situated at 2000 and 2040 cm-1 were assigned to carbonyl species
This report points out to novel unexplored opportunities in
due to CO adsorption24
. Moreover, on the IR spectrum the
designing simple but effective catalytic systems for hydrogen
generation.
appropriate bands seen at 2180 cm-1 attributed to gaseous CO and
gaseous CO2 assigned to the bands positioned at 2360 and 2320 cm-
1 were also detected during the exposure of the investigated
systems to a 1 vol.% methanol in an argon mixture25. The IR spectra
collected during decomposition of methanol carried out at 350 and
400 °C showed also several bands located in the wavenumber range
3700 - 3580 cm-1 (3580 cm-1, 3615 cm-1, 3640 cm-1, 3670 cm-1, 3700
cm-1) which were attributed to the free, non-associated -OH
Experimental: Multi-walled carbon nanotubes were synthesized by
catalytic decomposition of hydrocarbon gas using chemical vapor
deposition (CVD) method as described previously27, 28. Briefly, the
MWNTs were home-made by pyrolyzing 5 wt.% of ferrocene in
toluene mixture at 850°C in an Ar atmosphere. As-produced
material consists of carpet-like structures containing highly oriented
nanotubes of uniform diameter. The MWCNTs were cleaned twice
using concentrated HCl solution for 12 h. Through this process most
of the iron containing particles was removed from the MWCNTs. In
the next step the MWCNTs were washed with deionized water until
the pH was around 7, filtered and dried at 120 °C for 6 h. The TPR-
H2 measurements were carried out in an automatic TPR system
stretching frequencies present in the molecule of alcohol 26
.
In order to better visualize the differences in the absorption
species taken after reduction at 500°C in a 5%H2-95%Ar mixture and
exposure of the pristine MWCNT and MWCNT cleaned with HCl to a
1 vol.% methanol-argon mixture at 350 °C the IR spectra of the
adsorbed species for both systems and difference spectrum of
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(Altamira AMI-1) in the temperature range 25-900 °C with linear
heating rate 10 °C min-1 (weight about 0.1 g, reductiong mixture 5%
H2-95% Ar). Thermo-gravimetric TG method, equipped with
differential thermal analysis DTA device MOM Budapest
derivatograph (type 34-27T 1000 °C) was used for temperature
programmed decomposition of carbon nanotubes in air
atmosphere. Room temperature powder X-ray diffraction patterns
were collected using a PANalytical X’Pert Pro MPD diffractometer in
Bragg-Brentano reflecting geometry. Copper CuKα radiation from a
sealed tube was utilized. Data were collected in the range of 5-90°
2θ with step 0.0167° and exposure time per step of 27 s. Due to the
fact that raw diffraction data contain some noise, the background
during the analysis was subtracted using a Sonneveld, E.J. and
Visser algorithm. The data was then smoothed using a cubic
polynomial. All calculations were performed with X'Pert HighScore
Plus computer program. Infrared spectra were recorded with a
IRTracer-100 FTIR (Shimadzu) spectrometer equipped with a liquid
nitrogen cooled MCT detector. Before analysis the sample was
reduced at 500 °C in a 5%H2-95%Ar stream (50 cm3 min-1). A
resolution of 4.0 cm−1 was used throughout the investigation. 128
scans were taken to achieve a satisfactory signal to noise ratio. The
background spectrum was collected at 100 °C after the reduction
process. Then the reducing mixture was shifted to a mixture of
approximately 1 vol.% CH3OH in argon stream and spectra were
collected at 350 and 400 °C. The decomposition process involved
exposure of the reduced catalysts to 1 vol. % CH3OH in argon
stream flowing at 50 cm3 min-1 for 30 min at 350 and 400 °C under
atmospheric pressure. The catalytic activity in the oxy-steam
reforming of methanol reaction was measured in a quartz flow
Notes and references
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microreactor. About 0.1
g of catalyst was loaded into a
microreactor for the catalytic measurements. The reaction
conditions were as follows: reaction temperature 350 °C, ratio of
CH3OH:H2O:O2 = 1:1:0.4, flow rate 31.5 cm3 min-1, atmospheric
pressure. Methanol vapour or water vapour were introduced by
bubbling argon buffer gas through two saturators filled with
methanol or water, respectively. Whereas, oxygen was added using
a mixture of 5%O2-95%Ar to the stream directly before the reaction
to avoid undesirable interaction of the reactants in the saturator.
The catalytic activities are averages of three measurements and
were determined after 2 h of time on-stream. The product stream
was analyzed online using three gas chromatographs. Material
balances on carbon were calculated to verify the obtained results.
CH3OH conversion and H2 yield was calculated by the following
equations:
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13.
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CH3OH conv. = [(FCH3OH in – FCH3OH out) / (FCH3OH in)] x 100
%;
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yield of H2: YH2 = (FH2 / (FCH3OH in – FCH3OH out));
SCO2 = (FCO2/(∑F carbon containing product)) x 100 %;
SCO = (FCO/( ∑F carbon containing product)) x 100 %;
SDME= (FDME/( ∑F carbon containing product)) x 100 %;
where: FH2 = the molar flow rate of H2 (mol min-1), FCO = the
molar flow rate of CO (mol min-1), FCO2 = the molar flow rate
of CO2 (mol min-1), FDME = the molar flow rate of dimethyl ether
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