CO2 conversion in a photocatalytic continuous membrane reactor

Article abstract of DOI:10.1039/c6ra06777h

The reduction of CO₂ with water by using photocatalysts is one of the most promising new methods for achieving CO₂ conversion to valuable hydrocarbons such as methanol (MeOH). In this work, prepared TiO₂-Nafion-based membranes were used in a photocatalytic membrane reactor, operated in continuous mode, for converting CO₂ to methanol. By using the membrane with the best TiO₂ distribution, a MeOH flow rate/TiO₂ weight of 45 μmol (g_catalyst h)^-1 was measured when operating at 2 bar of feed pressure. This value is higher than those reported in most of the literature data to date. Moreover, methanol production is considered as a relevant advance over the existing literature results which mostly propose CH₄ as the reaction product.

Full text of DOI:10.1039/c6ra06777h

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CO2 conversion in a photocatalytic continuous membrane reactor
M. Sellaroa, M. Bellarditab, A. Brunettia, E. Fontananovaa, L. Palmisanob, E. Driolia,c, G. Barbieria,*
Received 00th January 20xx,
Accepted 00th January 20xx
The reduction of CO2 with water by using photocatalysts is one of the most promising new methods for advancing in CO2
conversion to valuable hydrocarbons, as metanol. In this work, prepared TiO2- Nafion-based membranes were used in a
photocatalytic membrane reactor, operated in continuous, for converting CO2 in methanol. By using the membrane with
the best TiO2 distribution, a MeOH flow rate/TiO2 weight of 45 µmol (gcatalyst h)-1 was measured operating at 2 bar of feed
pressure. So far, this value results to be higher than the most of the literature data reported up to date. Moreover, MeOH
production is considered as a relevant advance over the existing literature results mostly proposing CH4 as reaction
products.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Therefore, the photoreduction of CO2 to chemicals, such as
methane and methanol, results to be very interesting. In
particular, the main interest is addressed to the production of
Introduction
One of the main responsible of global climate change is
greenhouse gases emission with ca. 36 Gton of CO2 emitted
per year by both natural sources, including decomposition and
human sources such as burning of fossil fuels like coal, oil and
natural gas, cement production, deforestation, etc.1 Today,
many efforts and, in some cases, with a good level of success,
have led to the concretization of capture processes able to
separate CO2 from the rest of the emitted gaseous streams
with a targeted level of purity, together with a minimal energy
penalty.2 However, the main hurdle remains the final
destination of these huge CO2 streams. If, from one hand,
storage results as the most likely option, on the other hand the
identification of new environmentally improved routes and
methanol, as it can be easily transported, stored and used as
gasoline-additives, as well as transformed to other useful
chemicals by means of classic technologies.5
Inoue et al.7 firstly reported about the production of HCOOH,
HCHO and trace amounts of CH3OH from the reduction of CO2
with H2O under irradiation of aqueous suspensions of
semiconductor powders such as TiO2, whereas the
photocatalytic production of CH4 from CO2 was firstly reported
by Hemminger et al..8
Nevertheless, this technology presents some difficulties
related to non-effective catalysts, low yield and selectivity.
From a thermodynamic perspective, CO2 conversion with
water into methanol and oxygen (Eq. 1) is endoergonic, the
Gibbs molar free energy being 698.7 kJ mol-1 at 298 K.
methods enabling to reduce the emissions of such
a
compound and to obtain new sustainable energy sources is a
key challenge, which would have a significant environmental
impact. To this purpose, new greener technologies were
studied and developed, especially to convert CO2 into useful
chemical species and fuels.3,4 Actually, converting CO2 to
valuable hydrocarbons seems to be one of the most recent
advances in CCU (Carbon Capture and Utilization), being one of
the best solutions to both global warming and energy lack
problems. The reduction of CO2 with water to fuels by using
photocatalysts is one of the most promising methods to be
investigated, as it represents a greener process and an
attractive route from an economic and environmental point of
view. CO2 can be converted by irradiating it with UV light at
room temperature and ambient pressure and thus solar
energy could be directly transformed and stored as chemical
energy.5,6
h
3
ν→
Equation 1
CO2 + 2H2O ← CH3OH+ O2
2
In order to improve the efficiency of the reaction, many
research efforts were directed to the development of several
new types of photocatalysts.9
However, among all the applied photocatalysts, TiO2 results
one of the most used materials for the photocatalytic
conversion of CO2 into fuels, owing to its chemical inertness,
no-photocorrosion, stability against photoirradiation, suitable
optical and electronic qualities, low cost and commercial
availability, as well as no-toxicity.10
TiO2 anatase and rutile band gaps are located at about 3.2 eV
and 3.0 eV, respectively and the best photocatalytic efficiency
can be obtained using anatase with a small admixture of rutile
(approximately 75% anatase and 25% rutile).10,11
However, in addition to the nature of the photocatalyst to be
used, one should consider also that the photocatalytic
conversion of CO2 is a surface reaction involving two important
stages: (1) CO2 adsorption to the catalyst surface and (2) CO2
decomposition under UV irradiation in the presence of
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reductants. Therefore, the mass transfer rate of CO2 and the Premkumar and Ramaraj31 prepared metal porphyrin and
catalyst surface area are other two important parameters to phtalocyanine adsorbed Nafion membranes to be used for the
be controlled to improve the photocatalytic efficiency. As a photocatalytic reduction of CO2 to formic acid. More recently,
consequence, catalyst configuration during the photochemical Pathak et al.32 immobilized TiO2 nanoparticles in porous
reaction is of high concern to increase the products yield.
cavities of commercial Nafion membranes, soaking them in an
The immobilization of the catalyst into polymeric membranes isopropanol solution of Ti(OC3H7)4 and then immersing the
as supports and, thus the use of a membrane reactor for this obtained films in boiling water to form TiO2 nanoparticles by
type of reaction, can be an interesting and valid solution to be hydrolysis. They found out that the homogeneous dispersion
adopted. Membrane reactor use takes several advantages of the photocatalyst in Nafion thin films allowed the
such as a better exposition of catalyst to UV light to carry out photoreduction of CO2 under optically homogeneous reaction
the reaction, the tailoring of reactants and catalyst contact, conditions, with consequent improved conversion. In a typical
the reduction of catalyst aggregates formation, an easier experiment, they filled an optical cell, containing the
recovery of the catalyst which can be so simply reused, a photocatalytic film, with supercritical CO2 to a final pressure of
better control of fluid-dynamics. Moreover, polymeric 138 bar (2000 psi). After irradiation through a water filter for 5
membranes are easily handled and imply lower costs with h, the production of formic acid, together with methanol and
respect to other inorganic supports.
acetic acid was observed. Subsequently, Pathak et al.33
Up to now, many works developed photocatalytic membranes performed other catalytic tests using TiO2-loaded Nafion
with TiO2 deposited on or entrapped in membranes,12-17 membranes coated with silver metal via photolysis and in
especially for water purification or wastewater treatment in which the major reaction product was methanol.
advanced oxidation processes, for reduction reactions,18-19 and In this work, photocatalytic Nafion membranes were prepared
also some pilot-plant experiments were carried out as in the by immobilizing bare TiO2, previously synthetized from TiCl4 as
case of the PHOTOPERM® process for phenol and other precursor, into the polymeric matrix. Both the catalyst powder
organics degradation.20-22 Leong et al.23 have recently and then the photocatalytic membranes obtained were
proposed a review about the types of membrane as supports characterized by means of different techniques. Finally, the
and related photocatalytic membrane preparation and membranes were tested in order to verify their catalytic
characterization, focusing on the application of TiO2 efficiency for CO2 photoreduction with water for obtaining
photocatalytic membranes for removal of pollutants contained methanol, under UV-Vis irradiation in a continuous reactor. At
water.
the best of our knowledge, this work is the first example of
TiO2 can be successfully supported on perfluorinated ionomer photocatalytic reactor operated in continuous for CO2
membranes, taking advantage of the superior chemical photoreduction using dense mixed matrix TiO2-based Nafion
stability and the optical quality of the membrane itself. In membranes.
many cases, Nafion is the most studied perfluorinated material
and several studies were performed on the use of Nafion thin
Experimental
films or membranes as a support for metal or semiconductor
particles.24,25 It was demonstrated that Nafion can be useful
not only as a support to fix semiconductor particles but also as
Catalyst preparation
TiO2 sample was prepared by using titanium(IV) chloride (TiCl4,
Fluka 98%) as the starting material. TiCl4 was added under
stirring at room temperature to distilled water in the molar
ratio Ti/H2O 1:60 and a good dispersion was obtained. After ca.
12 h of stirring it turned in a clear solution that was boiled for
2 h under agitation. This treatment produced a milky white
TiO2 dispersion that was dried under vacuum at 323 K.
a
stabilizing agent for semiconductor microcrystalline
colloids.26 Nafion is constituted by an extremely hydrophobic
perfluorinated hydrocarbon backbone and several side-chains
with fixed sulfonic end groups able to interact with
charged/polar species by electrostatic interactions and
hydrogen bonds.27 In this way the polymer conjugates a high
stability under quite harsh conditions, including UV irradiation,
with a high affinity for charged/polar catalyst, as well as it
offers a functional microstructured environment that can have
a positive influence on the transition states and reaction
kinetics for the formation of polar products.28 Moreover, as it
is also reported in literature, no change of the band gap are
expected when TiO2 is incorporated in the Nafion.19
Catalyst characterization
XRD pattern of the powder was recorded at room temperature
by an Ital Structures APD 2000 powder diffractometer using
the CuK-
sizes was evaluated by means of the Scherrer equation:
Kλ/(β cos θ), where is the crystallite size, λ is the wavelength
α radiation and 2θ scan rate of 2°/min. The crystallite
Φ
=
Miyoshi et al.26 prepared TiO2 microcrystallites in Nafion,
adding an alcoholic Nafion solution to TiO2 colloids. The
obtained TiO2/Nafion in wet form was then used for
photodecomposition of acetic acid into CH4 and CO2. In many
cases, TiO2 is incorporated in Nafion commercial membranes
by soaking them in a solution of Ti-precursor and then treating
Φ
of the X-ray radiation (0.154 nm), K is usually taken as 0.89, β
is the peak width at half maximum height after subtraction of
the equipment broadening and θ=12.65° for TiO2 anatase and
θ=13.70° for TiO2 rutile. The phase content (%) was calculated
using the formula:
ꢀꢁ = ꢂꢁ/(ꢃꢄꢂꢄ + ꢂꢁ )
the Ti-loaded films for obtaining the formation of TiO2
particles.24,
29, 30
As regards CO2 conversion, in 1997,
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Table 1. Membrane preparation conditions
Membrane
1
2
3
4
Solvent in solution, wt%
Polymer in solution, wt%
Catalyst in solution, wt%
97.80
2.173
0.027
97.72
2.172
0.109
97.83
2.174
0
Solvent
MeOH:H2O (50:50 wt%)
1.2
EtOH:H2O (50:50 wt%)
1.2
EtOH:H2O (50:50 wt%)
5
MeOH:H2O (50:50 wt%)
0
Catalyst in membrane, wt%
Where WR indicates the content of Rutile, AA and AR the The catalyst and polymer dispersion obtained was then casted
integrated intensities of Anatase (101) and Rutile (110) peaks, in the Petri dish and the solvent evaporation was carried out in
respectively, and KA is a coefficient equal to 0.88434.
the climatic chamber. In the case of blank Membrane 4, the
The specific surface area of the sample was calculated in a same procedure was followed but without catalyst dispersion.
Flow Sorb 2300 apparatus (Micromeritics) by using the single- For membrane 1 and 4 the temperature of the climatic
point BET method. The sample was degassed for 0.5 h at 250 chamber was 60±4; for Membrane 2 and 3 was 68±4. For all
°C prior to the measurement. SEM observations were obtained the membranes samples prepared the relative humidity of the
using a Philips XL30 ESEM microscope, operating at 25 kV on climatic chamber was fixed at 12±5% and the solution volume
specimens upon which a thin layer of gold was evaporated.
cast in the Petri dish was selected in order to have an initial
UV-Vis spectra of the photocatalyst were obtained by diffuse liquid layer thickness of 5 mm.
reflectance spectroscopy (DRS) using a Shimadzu UV-2401 PC The membrane surface exposed to air during evaporation step
instrument. BaSO4 was used as a reference sample and the was indicated as UP; whereas the surface in contact with the
spectra were recorded in the range 200-800 nm. Band gap Petri dish was indicated as DOWN. After solvent evaporation,
value was determined by plotting the modified Kubelka-Munk both the two types of membranes (photocatalytic and blank)
function, [F(R’∞)hν]1/2, versus the energy of the exciting light.
underwent heat treatment at 120°C for residual solvent
removal. Then the flat sheet membranes obtained were
detached from the Petri dish with a little water quantity and
then dried at room temperature. The mean membrane
thickness was of 75±5 micrometers.
Membranes preparation
NafionTM (Fig. 1) 5 wt% solution was purchased by Quintech
e.K. – Brennstoffzellen Technologie (Germany). Methanol and
ethanol were purchased from VWR Prolabo Chemicals (USA).
Distilled water was used as a co-solvent for the membranes
preparation. The flat sheet Nafion membranes were prepared
by using the casting and solvent evaporation technique. Two
types of membrane were prepared: a bare Nafion membrane
(0 wt% of catalyst) and photocatalytic Nafion membranes,
containing the TiO2 catalyst. In the general procedure adopted,
as a first step, the polymer contained in the commercial 5 wt%
solution was recovered by solvent evaporation at 80°C under
magnetic stirring. Then the polymeric solution for the
membranes preparation was obtained by adding to the
recovered polymer the solvent mixture (MeOH:H2O or
EtOH:H2O, 50:50 wt%, Table 1) under magnetic stirring and at
room temperature.
Membranes characterization
The obtained membranes were characterized by different
techniques. The cross-section and surface morphology was
observed by scanning electron microscopy (SEM) using a FEI
Quanta 200 Philips SEM instrument. Cross-sections were
prepared by fracturing the membrane in liquid nitrogen. The
samples were “metallized” with graphite. The distribution of
heavy elements into the catalytic membranes was observed by
using the imaging of a back-scattered electron (BSE) in
addition to the secondary electron (SE) detectors.
A
Perkin Elmer Spectrum One was utilized for FT-IR
spectroscopy analyses in attenuated total reflectance (ATR) of
both UP and DOWN membrane surfaces. The diffuse
reflectance UV-Vis spectra were recorded with a Perkin Elmer
LAMBDA 650 spectrophotometer operating with a 60 mm
Integrating Sphere in a wave length range between 250 and
800 nm. The correspondent reflectance spectra were
processed and reported as absorbance spectra in Kubelka-
Munk units.
For the photocatalytic membranes preparation (Membrane 1,
2 and 3), after a complete polymer dissolution, the catalyst
was added to the obtained solution and the resulting
dispersion was left under stirring for 1 hour more. Then it was
sonicated for 30 minutes more in order to favour the
homogenization.
Photocatalytic reaction measurements
The photocatalytic membranes were utilized in CO2
photoreduction with H2O as the reducing agent.
A medium-high mercury vapour lamp with emittance from 360
nm (UVA) to 600 nm, (Zs lamp by Helios Italquarz, Milan) was
used for irradiating the membranes. The runs were carried out
by placing the membranes into a flat sheet membrane module
equipped with a quartz window, allowing the UV radiation to
Fig. 1. Nafion molecular structure
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Table 2. Operating conditions for reaction measurements
CO2 (or Ar) flow rate, mL(STP) min-1
20
0.079
5:1
H2Oliquid, mL min-1
H2O : CO2 (or Ar) molar ratio
Feed Pressure – Permeate Pressure, bar
Temperature, °C
2
45±5
Both the retentate and permeate streams outgoing the reactor
were condensed by means of an ice bath (0°C). Then the
incondensable species in both cases were sent to bubble soap
flow meters, in order to evaluate the correspondent flow rate.
Moreover, the composition of these streams was measured by
an Agilent Technologies 7890A gas chromatograph with TCD
(HP-PLOT and Molsieve columns). The condensate
components of the retentate and permeate were also
periodically sampled and analysed by means of an Agilent
Technologies 6890N gas chromatograph with FID (HP-5
column).
Fig. 2. Scheme of the experimental apparatus
get to the catalytic membrane surface (active membrane area
19.2 cm2).
Ionic species were determined by ionic chromatography using
a Dionex DX 120 instrument equipped with an Ion-Pac AS14
4mm column (250 mm long, Dionex). The eluent was an
aqueous solution of NaHCO3 (8 mM) and Na2CO3 (1 mM). Each
membrane was characterized for 15 hours, allowing, anyway,
to get the desired information. No catalytic and blank
measures were carried out during the night and, thus, for
longer times. In the case of blank reaction measurements, the
liquid samples withdrawn were subjected to TOC (total organic
carbon) measurements, by means of a TOC-VCSN Shimadzu
analyser, to evaluate the possible presence of organic
contaminants in polymer solution, residual solvent or Nafion
fractions at low molecular weight. TOC was measured only
during blank (no-reaction; Ar+H2O as feed) tests confirming
the stability under irradiation of Nafion membranes since its
values decreased with the time.
The membrane module was placed in an UV exposition
chamber where a stream of CO2 was continuously fed by
means of a mass flow controller. A water stream was also fed
by means of an HPLC pump. The H2O:CO2 feed molar ratio was
5:1. The trans-membrane pressure difference was regulated by
a back pressure controller and set at 2 bar. Fig. 2 shows the
scheme of the experimental apparatus. The membrane reactor
consists mainly of three parts: the feed/retentate chamber,
the permeate volume and the catalyst loaded membrane. The
two reactor chambers can be considered as lumped
parameters systems since no concentration gradient of any
chemical species is expected owing to also the low conversion
of this specific reaction. Inside the membrane, species
concentration gradients along the membrane thickness, even
though very small, are expected owing to permeation and
reaction.
If not otherwise specified, the membrane was placed into the
module exposing toward the quartz window (i.e. the retentate
side) its UP surface.In the first case, the membrane was placed
into the module exposing to the retentate side its UP surface;
in the second one the DOWN (or “Petri side”) surface, richer in
catalyst, was exposed. Membrane 2 and 3 were tested just
exposing their UP surfaces to reactants and UV light.
The reaction performance was evaluated through MeOH yield
and flow rate/TiO2 weight calculated accordingly to Eq. 2 and
3, respectively.
molmin−1
molmin−1
MeOHOUT flowrate
Equation 2
MeOHyield=
,
CO2 feedflowrate
mol min-1
g
MeOHtot flow rate
Catalyst mass
Equation 3
MeOH production rate =
,
Results and discussion
Photocatalyst characterization
Before the photocatalytic experiments with CO2 as substrate,
all of the membranes were subjected to “blank reaction”
measurements. In this case, together with H2O, an argon
stream was fed into the reaction module instead of CO2, in
continuous for 8-12 h in the same operating conditions chosen
for the catalytic experiments including UV-Vis irradiation
(Table 2). The aim of this procedure was to clean the
membrane from residuals of solvent and other low molecular
weight organics eventually present in the polymer solution
that could be released during the reaction test contaminating
the reaction mixture.
The diffraction pattern of TiO2 sample (Fig. 3a) identifies a
mixture of the anatase and rutile polymorphs with a slight
degree of crystallinity owed to the low synthesis temperature.
Fig. 4 shows the diffuse reflectance spectra of the prepared
sample: the strong absorption in the range 300–380 nm
corresponds to the charge transfer process from O 2p to Ti 3d.
TiO2 is an indirect semiconductor so that its band gap energy
can be determined from the tangent lines to the plots of the
modified Kubelka-Munk function, [F(R’∞)hν]1/2, versus the
energy of the exciting light, as shown in the inset of the Fig..
Some features of the TiO2 sample are listed in Table 3.
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(a)
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(b)
in the polymer solution (Membrane 2 and 3), a better
dispersion of the catalyst in the Nafion membrane was
observed (Fig. 6) owing to the higher capacity of the ethanol to
disperse the catalyst in comparison to methanol.
Fig. 6 shows the SEM images of Membrane 2 cross section,
both in SE and BSE mode (Fig. 6a and Fig. 6b, respectively). SE
SEM image shows that, also in this case, the membrane is
characterized by no cavities and some small catalyst
agglomerates present through the membrane thickness. This
was confirmed by BSE image, in which it is also possible to
appreciate a relevant improvement of catalyst distribution
with respect to Membrane 1, containing the same TiO2
amount, despite a residual partial segregation on DOWN
surface can be noticed. Fig. 7 shows BSE images of Membrane
3; in particular, Fig. 7b allows many catalyst agglomerates to
be observed and again the partial catalyst segregation
occurred at DOWN surface (Fig. 7b-c). This could be owed to
an excessive amount of TiO2 with respect to the polymeric
matrix, causing its sedimentation during the solvent
evaporation step of membrane formation. Also in this case, no
visible cavities are present through the membrane thickness.
The SEM images of the bare polymeric Nafion membrane not
reported here, show that the membrane appears to be
characterized by the absence of visible cavities. FT-IR
spectroscopy analyses were carried out on both membrane
surfaces.
Fig. 3. (a) XRD diffraction pattern of the TiO2 powder.
A=Anatase, R=Rutile. (b) SEM images of unsupported TiO2.
Table 3. Some properties of TiO2 catalyst
Phases
Phase
percentage
[%]
Specific
Surface
Area
Band
gap
[eV]
Crystallite
size
[nm]
[m2g-1]
Anatase
Rutile
60
40
12.8
2.8
54
3.00
100
80
60
40
20
0
8
6
4
2
0
2.7 2.9 3.1 3.3 3.5 3.7
hν
Fig. 5. Membrane 1 BSE SEM images (Mag: a. 1000x; b. 2500x;
c. 5000x)
200
300
400
500
600
700
800
λ (nm)
Fig. 4. Diffuse reflectance spectra of TiO2 sample. Inset: plot of
the square root of the modified Kubelka-Munck function vs.
the energy of the absorbed light.
The specific surface area is 54 m2 g-1, the particle sizes were
12.8 and 2.8 nm for anatase and rutile, respectively, and the
band-gap value 3.00 eV. SEM micrographs (not shown for the
sake of brevity) indicated that the TiO2 sample presented
irregular shapes and consisted of aggregates of particles
whose size was ca. 60 nm (Fig.3b).
Fig. 6. Membrane 2 cross section SEM images (Mag: a. 2500x;
b. 2500x)
Membranes morphological and chemical characterization
Fig. 5 shows SEM images of Membrane 1. As it can be seen
from Fig. 5b, many agglomerates of catalyst appearing as
white spots are present through the membrane thickness,
especially concentrated at the bottom surface, and no cavities
are visible at this resolution. The presence of a higher
concentration of catalyst in the DOWN surface with respect to
the UP surface is related to sedimentation phenomena during
the MeOH:H2O solvent evaporation step of the polymeric
solution. On the contrary, using EtOH:H2O mixture as solvent
Fig. 7. Membrane 3 BSE SEM images (Mag: a. 3000x; b. 3000x;
c. 200x)
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Table 4. CO2, CH4 and H2 permeances of Membrane 1 at 25°C
and different RH.
Permeance, dm3(STP) m h-1 m-2 bar-1
RH, %
0
CO2
4.41
2
CH4
6.96
1.6
H2
9.19
3.4
100
Fig. 8. FT-IR (ATR) spectra of UP membrane surface (blu lines)
and DOWN membrane surface (red lines)
To further confirm the integrity of the membranes,
permeation measurements with single gas were also carried
out in saturated conditions.
Photocatalytic reaction measurements
Table 4 summarizes the results for Membrane 1 which
resulted to be dense and highly permeable to CO2. The
presence of water vapour reduced the permeance of all gases;
however, CO2 permeances remained high. Nafion membranes
are well known in literature as proton transport26,27
membranes used in PEMFC with really good chemical stability.
Nafion has hydrophilic domains favouring membrane
hydration that, coupled with the high permeance of CO2,
assures the accessibility of reactants to the whole catalyst
dispersed in the membrane.
Different measurements were carried out in order to verify the
photocatalytic activity of the membranes incorporating the
catalyst. In all of them the error bar calculated was below 5%.
Firstly, two photocatalytic experiments were performed
testing Membrane 1 and varying the membrane side exposed
to CO2/H2O feed stream and UV irradiation and as it was
observed by means of the SEM and IR characterization the
membrane had not symmetric catalyst distribution (Fig. 5b and
c and Fig.8a). Table 5 reports the photocatalytic experiments
results relative to Membrane 1. In the first test performed, the
membrane was placed into the module exposing its UP surface
to the feed stream and UV radiation; whereas, in the second
one the DOWN side of the membrane, richer in catalyst, was
exposed. In both cases the photocatalytic reaction led to the
evolution of CH3OH as product, in the condensed aqueous
phase. No CO and CH4 were detected (the set up detection
limit being 100 ppm).
Fig.
8 shows the spectra relative to the photocatalytic
membranes (Membrane 1, 2 and 3). All the signals in the range
between 970 and 1400 cm-1 can be related to the Nafion
structure. In particular, the bands appearing at 970-983 cm-1
are attributed to C
1060 cm-1 can be related to the symmetric stretching of
In the range between 1400 and 1100 cm-1 the asymmetric
stretching bands of SO3 should be found, but they are
covered from the more intense bands of CF2 stretching,
̶
O
̶
C stretching vibrations; the band at
∼
̶
SO3 ̶
̶
̶
No significant differences in MeOH yield and MeOH flow
rate/TiO2 weight were observed by performing the
photocatalytic reduction of CO2 exposing the two different
surfaces. These results could be attributed to the fact that the
Nafion polymer matrix is transparent to UV irradiation and
therefore not only the superficial layer participates to the
reaction, but also the inner layers. It has to be noticed that
both CO2 conversion and methanol yield have very low values;
however, they results among the best by a comparison with
the literature. In a second step, Membrane 2 was used to
verify if the better catalyst distribution inside the bulk of the
polymeric matrix (observed with SEM characterization, Fig. 6)
could improve the performance of the photocatalytic
membrane itself. In this case, the reaction measurements
were carried out just exposing the UP surface as it was
observed that the membrane surface exposed to retentate
side does not influence the catalytic performances. One of the
main problems related to the photocatalytic reduction of CO2
is that it is often doubted that the products obtained could be
developed from carbon impurities present in the reaction
system in the first stage. In order to be sure that the products
detected were not owing to impurities present in the
photocatalytic membrane, the membrane was firstly subjected
to “blank” reactions, with UV irradiation of Ar and H2O stream
in absence of CO2, according to the operating conditions
reported in Table 2. When performing the blank tests, some
MeOH amounts were detected in the withdrawn samples (Fig.
10a). However, both MeOH wt% (not reported here) and
“apparent” MeOH flow rate/TiO2 weight (Fig. 10a) showed a
clear decrease with run-time. This path should
̶
visible in the spectra.35 Moreover, it can be seen that OH band
intensity is higher for the DOWN surface spectrum with
respect to the UP surface. This trend could be attributed to a
higher TiO2 concentration at the DOWN surface (Petri side), as
observed in the SEM images (Fig. 5, Fig. 7).
As far as Membrane 3 is concerned (Fig. 8c), the signal relative
to the OH band has a very different intensity for the UP and
DOWN surface. This is certainly attributed to
a high
segregation of the catalyst at the DOWN surface of the
membrane, because of the excessive TiO2 amount. UV-Vis
diffuse reflectance spectroscopy was performed on Membrane
4 and 1 in order to make a comparison between the bare
Nafion membrane and that containing the photocatalyst and
to verify its structural integrity when embedded inside the
polymeric membrane. Membrane 4 gave a flat absorbance
spectrum reported in Kubelka-Munk units (Fig. 9b) and
obtained by processing the UV-Vis diffuse reflectance
spectrum. It shows no absorbance maximum and it is
characterized by a constant value of about 5 Kubelka-Munk
units, owing to scattering phenomena. Kubelka-Munk
absorbance spectrum relative to Membrane 1 (Fig. 9a),
containing 1.2 wt% of TiO2, presents an absorbance maximum
of about 13 units in the range 280-300 nm, despite the catalyst
segregation. This demonstrates that the catalyst maintains its
structural integrity also when embedded inside the Nafion
matrix, which in turn was proved to be transparent to UV
radiation in the wavelength range considered, as the spectrum
was recorded analysing the UP surface.
6 | J. Name., 2016, 00, 1-3
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Fig. 9. Absorbance spectrum reported in Kubelka-Munk units
Fig. 10. Photocatalytic experiments results using Membrane 2
Table 5 – Reaction test results relative to Membrane 1.
Membrane surface exposed
DOWN
UP
Reaction time, min
435
460
MeOH concentration, wt%
1.20 10-5
1.34 10-5
MeOH yield, %
2.07 10-5
2.30 10-5
MeOH flow rate/TiO2
0.092
0.10
weight, µmol gcatalyst1 min-1
indicate that some organic contaminants, probably owed to
solvent residuals, were present in the polymeric matrix of the
membrane giving the target product. Actually, the TOC
measurements performed on the samples collected confirmed
the presence of organics in amounts decreasing with run-time.
The correspondent weight percentage resulted to be very low
(Fig. 10b) but anyway almost two orders of magnitude higher
than that of the detected methanol. However, when switching
the feeding gas from Ar to CO2, real MeOH flow rate/TiO2
weight suddenly increased, reaching a maximum of 0.73 µmol
Fig. 11. Photocatalytic experiments results using Membrane 3
-1
gcatalyst min-1. The increasing trend of MeOH flow rate/TiO2
weight during reaction and its decreasing in blank tests are
indexes of methanol production by CO2 conversion. In
addition, this value resulted to be over 7 times higher with
respect to that obtained with Membrane 1, confirming that
the better catalyst distribution characterizing Membrane 2 had
Fig. 12. Photocatalytic experiments results using Membrane 4
As it can be observed in Fig. 12, MeOH flow rate in the samples
and TOC weight percentage reduced with time. In particular
MeOH content decreased to zero after 750 minutes of
irradiation, even after switching the feed from Ar to CO2.
a
positive effect on the catalytic performance of the
membrane itself. Membrane 3 was tested following the
previous procedure, which is feeding Ar until MeOH was not
longer measured. Then, CO2 replaced Ar in the feed and real
MeOH production was observed. MeOH concentration
remained almost constant with respect to the test carried out
by using Membrane 2, but MeOH flow rate/TiO2 weight
showed a significant decrease (Fig. 11a), being about 3 times
lower. This behaviour could be explained by considering that
the increase of the amount of catalyst embedded inside the
membrane, caused segregation and this phenomenon could
have negatively influenced the performances of the catalytic
membrane itself. Actually, catalyst aggregation could have not
allowed an effective interaction between TiO2 particles and UV
light. Moreover MeOH, in the presence of a higher amount of
TiO2 in the Membrane 3, could react. Also in this case, the TOC
content in the samples resulted to be much higher with
respect to MeOH content (Fig. 11b). The same tests were also
carried out on Membrane 4, which is to say on the bare
Nafion, in order to prove that the polymeric matrix does not
possess photocatalytic activity by itself.
This path proves that Nafion does not possess any catalytic
activity and probably, as Membrane 4 was prepared by using
MeOH as the co-solvent, a small residual MeOH amount were
released by the membrane during the occurrence of the test.
Fig. 13 summarizes the results obtained relative to MeOH flow
rate/TiO2 weight, considering both the quality distribution of
catalyst and its content inside the membrane. At equal TiO2
amount (1.2 wt% in the membrane), a better distribution,
relative to Membrane 2 (labelled in Fig. 13 as “well dispersed”)
as proved by means of SEM imaging (Fig. 6), resulted to
increase the MeOH flow rate/TiO2 weight with respect to
Membrane 1. In this last case, the only difference was owed to
the partial catalyst deposition (Fig. 5). Increasing the catalyst
amount to 5 wt% produced again its partial segregation (Fig. 7)
and caused a significant decrease of the photocatalytic
performances of the membrane. The visible pinky background
highlights the transparency of Membrane 2 owing to the good
catalyst dispersion. On the contrary, the other membranes did
not show similar transparency owing to catalyst segregation.
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of TiO2 and H2O, can take place in different stages according to
the reactions below reported (adapted from literature data):
TiO2 + hν
→ e- + h+
H2O +2h+ → 0.5O2 + 2H+
CO2 + 2H+ + 2e- → HCOOH
HCOOH + 2H+ + 2e- → HCHO + H2O
HCHO + 2H+ + 2e- → CH3OH
The circumstance that in our system formic acid and
formaldehydes are not detected, or are present in low
amounts in the case of HCOOH with Membrane 2, can be
explained by the fast reaction rate of these compounds to
form MeOH or by the direct methanol formation. Moreover,
the continuous removal of methanol from reaction volume
avoids its oxidation.
As overall consideration it can be seen that the yield of
methanol and thus CO2 conversion are very low, if evaluated
as absolute values, even if resulted among the best in
literature. It has to be considered that this process is at a very
early stage of development and it needs more and more
efforts to make it profitable. However, it remains highly
promising and attractive as it has the great advantage of being
completely green using CO2 and water as reactants exploiting
sunlight as energy source to produce liquid fuels.
Fig. 13. MeOH flow rate/TiO2 weight as a function of catalyst
content percentage in the polymeric membrane.
Consequently, the partial catalyst deposition on DOWN surface
of the membrane and the presence of aggregates seem to be
crucial parameters for the membrane efficiency and some
strategies should be attempted in order to improve the
catalyst distribution also at higher content. Such strategies
could comprise the reduction of membrane formation time by
accelerating solvent evaporation (e.g. higher temperature or
gas sweeping), TiO2 functionalization with surfactants or use of
different polymeric material.
Nevertheless, even with the current conditions used, MeOH
flow rate/TiO2 weight resulted to be comparable or even much
higher with respect to that found in other works present in
literature (Table 6), the best results being obtained by Pathak
et al.32 by immobilizing TiO2 into commercial Nafion
membranes and by using liquid CO2 in supercritical conditions
as feed.
In the present work a very high MeOH flow rate/TiO2 weight
was obtained under mild operating conditions (atmospheric
pressure, room temperature and gaseous CO2 as feed); the
best results were observed by using Membrane 2.
The photocatalytic CO2 reduction mechanism is very complex
and still unclear. Many hypotheses were advanced on the
formation of the various products deriving from CO2.36-38
Conclusions
In this work, photocatalytic CO2 conversion to methanol was
carried out in a continuous membrane reactor with a TiO2-
based membrane irradiated by UV light.
Various photocatalytic membranes were prepared embedding
TiO2 catalyst inside a polymeric Nafion matrix. A good
distribution of catalyst was achieved by choosing an
appropriate co-solvent for the preparation of the polymeric
solution (e.g. EtOH) at a chosen catalyst amount of 1.2 wt%.
By using the membrane 2, the one with the best TiO2
distribution, no CH4 nor CO formation were observed whereas
a MeOH flow rate/TiO2 weight of 45 µmol (gcatalyst h)-1 was
obtained under mild experimental conditions. According to
authors knowledge, such a value is a relevant advance over the
existing literature as it is higher than most of the data, also
using supercritical CO2, reported up to date.
Moreover, it was observed that the catalytic performances
were strictly related to the membrane preparation, as the
higher methanol flow rate/TiO2 weight was achieved by using
the photocatalytic membrane (Membrane 2) with the best
catalyst distribution even if with a lower content (Membrane
3). Comparing the results with bare Nafion membrane, it was
demonstrated that the polymeric matrix does not possess any
photocatalytic activity by itself and, thus, the whole methanol
produced has to be attributed to the presence of catalyst
dispersed in the Nafion matrix.
Generally the compounds formed during the gaseous
photocatalytic CO2 reduction were CO and CH4
39,40
whilst
formic acid, formaldehyde and methanol were mainly
observed during liquid phase runs.41-44
Published papers39,43,44 dealing with batch reactors operating
in similar conditions for CO2 photo-oxidation over TiO2 catalyst
showed methane as the main reaction product and trace of
formic acid. It is worth of note that, differently from what can
be found in most of the literature, in this work neither CH4 nor
CO were detected, methanol being the main product obtained.
This can probably be attributed to the synergic effect of the
use of membranes inside which the photocatalyst was
embedded and a continuous flow mode reactor to carry out
the reaction of CO2 and H2O. Actually, avoiding the use of a
batch reactor, the substrate undergoes a lower degree of
reduction as fresh CO2 is continuously fed into the system and
the produced methanol is continuously removed from the
catalytic sites reducing the possibility to have over oxidation
phenomena.39 Ion chromatography analyses, performed on
selected samples, revealed that traces amount of formic acid
were formed in the presence of Membrane 2. Methanol
formation is a multi-electronic process which, in the presence
The present work demonstrates that the use of photocatalytic
Nafion membranes has a wide range of improvement and
could be a promising candidate for an advanced route of CO2
conversion in CH3OH.
8 | J. Name., 2016, 00, 1-3
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Table 6. Comparison of results reported in literature
CH4, CO, etc. flow
MeOH flow rate/catalyst weight
Reactor
configuration
rate/catalyst weight
µmol (gcatalyst h)-1
8 CH4
Catalyst
Ref.
µmol (gcatalyst h)-1
45
46
47
TiO2/Y-zeolite anchored on Vycor glass
Ti-mesoporous zeolite
Batch
Batch
Batch
5
3.5
7.5 CH4
Ti-b(OH) and Ti-b(F)
5.9
1 CH4
0.78 for TiO2
19.75 for Cu/TiO2
0.45
48
TiO2 and 2% Cu/TiO2 in NaOH solution
Batch
-
49
5
Cu/TiO2 film supported on optical-fiber
Cu/TiO2 and Ag/TiO2 film supported on optical-fiber
TiO2 anatase
Continuous
Continuous
Batch
-
-
4.12
6
0.075
0.40 CH4
3.15 CO;
2.13 CH4
50
TiO2 polymorphs
Continuous
-
11
43
TiO2 polymorphs
Continuous
Batch
-
2.1 CO
phtalocyanines/TiO2
-
56
26 HCOOH
38 HCOOH
6 CH3COOH
Traces of HCOOH
(Membrane 2)
TiO2 nanoparticles in porous cavities of commercial
Nafion membranes
32
Continuous
Continuous
(using supercritical CO2 as feed)
45 (Membrane 2)
12.6 (Membrane 3)
TiO2 nanoparticles in Nafion membrane
This work
15 H. Choi, A.C. Sofranko and D.D. Dionysiou, Advanced
Functional Materials, 2006, 16, 1067-1074.
Acknowledgements
The research project PON 01_02257 “FotoRiduCO2
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Materials and Interfaces, 2010, 2, 1912-1917.
Photoconversion of CO2 to methanol fuel”, co-funded by MiUR
(Ministry of University Research of Italy) with Decreto 930/RIC
09-11-2011 in the framework the PON “Ricerca e competitività
2007-2013”, is gratefully acknowledged
17 C.
Pandiyarajan,
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Textual abstract
Metanol was produced by CO2 photocatalyc reduction over by TiO2- Nafion-based membranes by
using a continuos flow membrane reactor. A MeOH flow rate/TiO2 weight of 45 µmol (gcatalyst h)-1
was measured operating at 2 bar of feed pressure. So far, this value results to be higher than the
most of the literature data reported up to date. Moreover, MeOH production is considered as a
relevant advance over the existing literature results mostly proposing CH4 as reaction products.
Graphycal Abstract