Insight into the role of UV-irradiation in photothermal catalytic Fischer-Tropsch synthesis over TiO2 nanotube-supported cobalt nanoparticles

Article abstract of DOI:10.1039/c7cy02304a

To explore an efficient catalytic system with high activity and selectivity is the key to improve Fischer-Tropsch synthesis (FTS) technology and the main focus in the academic field. Herein, a photothermocatalytic system for FTS was constructed by combining thermo-active components with photosensitive supports. 20 wt% and 30 wt% Co supported on TiO₂ nanotube (TNT) catalysts were prepared through an incipient wetness impregnation method (named 20% Co/TNT and 30% Co/TNT) and were applied to a photothermocatalytic process to investigate the role of ultraviolet (UV) illumination in the traditional thermochemical FTS reaction. The results indicated that the introduction of UV light dramatically improved CO conversion, from 9.2% to 64.0% for 20% Co/TNT and from 8.6% to 17.1% for 30% Co/TNT. In addition, UV illumination gave rise to an increase in light paraffin selectivity. The improvement of CO conversion was found to be related to the photogenerated electron transfer from TNTs to active Co sites upon UV light irradiation, which caused the Co sites to have increased surface electron density and subsequently enhanced the adsorption and activation of CO molecules at Co sites. Meanwhile, the increase in light paraffin selectivity was attributed to the photo-promoted hydrogenation of olefin and the hydrogenolysis of the long-chain hydrocarbons. The present work promoted a deeper understanding of the photothermal FTS reaction mechanism and demonstrated that photothermocatalysis could have potential for enhancing FTS catalytic performance and adjusting product distribution.

Full text of DOI:10.1039/c7cy02304a

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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Insight into the Role of UV-Irradiation in Photothermal Catalytic
Fischer-Tropsch Synthesis over TiO2 Nanotube Supported Cobalt
Nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
Limin Wang, Yichi Zhang, Xiaojun Gu,* Yulong Zhang and Haiquan Su*
DOI: 10.1039/x0xx00000x
To explore an efficient catalytic system with high activity and selectivity is the key to improve Fischer-Tropsch synthesis
(FTS) technology and the main focus in academic field. Herein, a photothermocatalytic system for FTS was constituted by
combining thermo-active component with photo-sensitive support. The 20 wt% and 30 wt% Co supported TiO2 nanotube
(TNT) catalysts were prepared through incipient wetness impregnation method (named as 20% Co/TNT and 30% Co/TNT),
and were applied to photothermocatalytic process to investigate the role of ultraviolet (UV) illumination in the traditional
thermochemical FTS reaction. The results indicated that introduction of UV light dramatically improved CO conversion,
from 9.2% to 64.0% for 20% Co/TNT and from 8.6% to 17.1% for 30% Co/TNT. In addition, UV illumination gave rise to an
increase in light paraffin selectivity. The improvement of CO conversion was found to be related to the photogenerated
electrons transfer from TNT to active Co sites upon UV light irradiation, and this caused the Co sites with increased surface
electron density and subsequently enhanced the adsorption and activation of CO molecules at Co sites. While the increase
in light paraffin selectivity was attributed to the photo-promoted hydrogenation of olefin and the hydrogenolysis of the
long-chain hydrocarbons. The present work promoted a deeper understanding of the photothermal FTS reaction
mechanism and demonstrated that photothermocatalysis could be a potential for enhancing FTS catalytic performance
and adjusting product distribution.
www.rsc.org/
bed, stirred basket, etc.),4–6 and significant progresses have
been achieved. Nonetheless, all of the above attempts are
limited on the thermocatalytic domain, which is an energy-
1. Introduction
Fischer-Tropsch synthesis (FTS) is an effective heterogeneous
catalytic route to convert syngas (mixture of CO and H2) into
various hydrocarbons, including gasoline, diesel oil and other
chemicals, which has received wide-spread attention
nowadays due to both the dwindling nonrenewable energy
sources and the diversified source of syngas (e.g. natural gas,
coal and biomass).1–3 Low catalytic productivity and the broad
product distribution of FTS place severe limitations on its
attractiveness as a commercial operation. Hence, higher CO
conversion and desired selectivity are critical design criteria in
FTS. To address above issues, for decades tremendous
traditional endeavors have been devoted to the development
of FT technology, such as optimizing catalyst structure (active
metal particle size, support, promoter, etc.), tuning the
reaction parameters (temperature, pressure, space velocity,
etc.) and even designing various reactor type (fixed bed, slurry
consuming process by using an additional heater.
Compared with thermochemical reaction, photocatalysis is
one of the most sustainable, reliable, and economical and eco-
friendly solutions to solve energy and environmental issues by
using solar energy as the driving force.7 It has been extensively
studied as a promising technology for hydrogenation reaction,
including carbon dioxide hydrogenation,8,9 selective
11
hydrogenation of nitroaromatics,10, and hydrogenation of
carbonyl compounds,12,13 etc. For example, Pd/TiO2 has been
reported to show superior activity for the photocatalytic
hydrogenation of CO2 under UV illumination.14 The
architecture of CdS/H2Ti5O11 ultrathin nanobelt displayed
almost 100% yield with a selectivity of about 98% in 3 min
under visible light irradiation for converting 4-nitroaniline to p-
phenylenediamine.15 In 2011, Mul et al. have made an attempt
to photocatalytically hydrogenate CO by exposing Ti-SBA-15 to
120 W high-pressure mercury lamp irradiation.16 The result
showed that just trace amount of CH4, C2H4 and C2H6 were
produced, inferring that CO molecule cannot be effectively
activated by sole illumination possibly maybe owing to the
limited light energy utilization in photocatalytic technique. On
the basis of above results, we thought that if a photothermal
catalytic system with which thermal energy can be coupled
with photonic energy could be established to drive FTS process,
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it might lead to distinct new possibilities towards the for 48 h. The obtained white precipitate was washed with
production of renewable fuels. Our initial study and other two hydrochloric acid (pH = 1.0) until the pH value of the
group’s results demonstrated the general applicability of the suspension turned into 7.0. After filtration, the powder was
photothermocatalytic process for the synthesis of hydrocarbon washed with H2O and ethanol for several times and then dried
compounds.17–19 However, the insight into the mechanism, at 353 K. Finally, the powder was calcined at 673 K for 12 h to
such as the possible reaction pathways, the role of electrons obtain the TNT.
and holes, actual rate-determining elementary step, as well as
The TNT supported catalysts containing 20 wt% and 30 wt%
underlying synergism effects between light and thermal effects cobalt were prepared by the incipient wetness impregnation
has not been explored and needs to be deeply extensively method. TNT was impregnated by an aqueous solution of
studied.
Co(NO3)2·6H2O. After impregnation, the sample was dried at
In this context, the development of efficient catalytic active 353 K overnight and then calcined in an air flow at 673 K for 3h.
component and suitable photosensitizer is of prime The pure Co3O4 sample used in Raman test was obtained by
importance. Supported Co-based catalysts show the good calcining Co(NO3)2·6H2O powder in an air flow at 673 K for 3 h.
thermo-catalytic activity for CO hydrogenation reaction and
are more advantageous in the commercial FTS process.20,21
2.2 Characterizations
The powder X-ray diffraction (PXRD) characterization was
Titanium dioxide (TiO2), a totally environmentally benign
performed on a Panalytical X’pert Pro diffractometer with Cu
photo-sensitive semiconductor, has been regarded as the
Kα radiation (40 KV, 40 mA). The scan speed was 25.068 °/min,
common support in FTS reaction over the past decades due to
with a scanning angel ranged from 5° to 90°. H2-TPR
its strong metal-support interaction (SMSI) and better
stability.22–24 Compared with nanoparticles, TiO2 in the form of
measurements of the prepared catalysts were carried out
using a AutoChem II 2920 equipped with a TCD detector.
nanotubes should be a preferred support material for the
Ultraviolet-visible spectra were collected in the range of 200-
harsh FTS reaction condition due to its high surface area for
800 nm using a UV-3600 Shimadzu spectrometer. The textural
active site exposure, unique electronic characteristics, a
properties of the support and the corresponding catalysts
superior physical topology, as well as good heat transfer
properties.25–27
were determined by the N2 adsorption-desorption test
performed on a Micrometrics ASAP2020 analyzer. The
Based on the above considerations, in this work we
structures of samples were characterized with a FEI Tecnai G2
intentionally combined photo-sensitive TiO2 nanotube (TNT)
F20 S-Twin transmission electron microscope at 200 KV.
with thermo-active Co to constitute a bifunctional Co/TNT
Raman spectra were recorded on a LabRAM HR Evolution
catalyst. TNT as a photo-sensitive species was motivated under
Raman spectrometer with an excitation of 514 nm laser light.
UV illumination to generate the reactive electrons and holes,
X-ray photoelectron spectra were collected on a Thermo Fisher
while cobalt as a thermocatalyst loaded on the surface of TNT
Scientific ESCALAB 250Xi photo electron spectrometer. CO-TPD
could weaken C-O bonds and couple C-C bonds. Considering
measurements of the samples were conducted using the same
the adsorption and activation of CO are favored over the metal
nanoparticles with higher electron density,28–30 it can be
apparatus that was used for TPR to analyze the CO adsorption
ability on the used catalysts after Fischer-Tropsch synthesis
envisioned that under UV light irradiation the photogenerated
reaction for approximately 55 h under UV irradiation and in
electrons of TiO2 would be transferred to active Co sites due to
the dark. For TPD testing, 90 mg of the used catalyst was
the difference in work function between metal and
packed in a quartz U-tube, flushed with Ar for 30 min at 373 K
semiconductor, leading to the electron density enrichment of
and then cooled down to 323 K slowly. Subsequently, the
active Co sites and the resulting superior catalytic performance
samples were exposed in CO gas flow at 323 K for 60 min, and
on CO hydrogenation under photothermocatalytic condition.
then Ar was introduced to allow physisorbed CO to desorb and
Thus a photothermocatalytic process for driving FTS reaction
to clear the apparatus of gas phase CO. After there was no CO
over TNT supported cobalt catalysts with different loadings
signal detected in the trail gas, the sample was heated from
was explored, and the influence of UV illumination on the
323 K to 1173 K at a rate of 10 K/min while the thermal
activity and the product distribution during the FTS reaction
conductivity of the sample tube effluent was monitored. H2-O2
were investigated by some control experiments, in-situ Raman
titrations were also carried out in 2920 II Micromeritics system
and other characterizations. A possible reaction mechanism
with
a thermal conductivity detector to measure the
was tentatively proposed.
dispersion of cobalt in Co/TNT catalysts. Before the
measurements, the samples were completely reduced by 10
vol.% H2/Ar at 885 K and subsequently preoxidized in O2 at 373
K and outgassed in an Ar flow at 373 K for 2 h; then, pulses of
H2 were sent to the sample until complete saturation was
reached. Operando Raman spectra were obtained in an in situ
reaction cell unit to test the chemisorption of CO on sample
under different excitation. Before measurements, the 20%
Co/TNT catalyst were completely reduced in situ at 885 K
under atmospheric pressure with a stream of hydrogen. The
2. Experimental
2.1 Catalyst preparation
TNT was prepared by a hydrothermal method according to
previous reports.31 Typically, 1.25 g of anatase TiO2 powder
was dispersed into NaOH solution (8 M, 125 mL). The
suspension was transferred into a Teflon-lined container (200
mL) sealed in a stainless steel autoclave and was kept at 423 K
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4
temperature was then cooled to 298 K, after which the catalyst
was then exposed to high purity CO (>99.99%) for 60 min.
Raman spectra were recorded with a 633 nm light laser in the
range between 1750 and 2200 cm−1. For testing CO
chemisorption under UV light irradiation, a 325 nm light laser
was selected to excite Raman spectra.
ꢀ ꢁ
SC % =100 ꢃ ꢄ SC ( %)
5+
i
i=1
Where A represents corresponding peak area and n is the
number of moles.
2.3 Catalytic activity test
3. Results and discussion
Hydrogenation of CO was conducted with a self-designed
apparatus consisting of a stainless reaction chamber with a
quartz window in the middle for light irradiation (As shown in
Fig. SI) . The volume of the chamber was 2.4 L, in which the
catalyst bed could be both heated and irradiated. Catalyst
sample (1.2 g) was dispersed in 10.0 mL ethanol with the aid of
an ultrasonic bath. Subsequently, the obtained suspension was
coated on ten pieces of quartz (290 mm×65 mm×1.6 mm),
then the quartz was perpendicularly inserted in the edge of
stainless steel reactor. In order to inspect the influence of the
illumination on catalytic activity and selectivity, all the samples
(including TNT support and Co/TNT catalysts) were tested
under three different conditions: photocatalytic process (with
only UV irradiation), thermocatalytic process (with heat under
dark), and photothermocatalytic process (with both heat and
UV irradiation), respectively. Prior to the reaction, the
chamber was evacuated by a mechanical pump. Then, the
3.1 Characterization of catalysts
The crystallographic structure and phases of the synthesized
TNT and fresh Co/TNT catalysts were determined by PXRD as
illustrated in Fig. 1. Diffraction lines assigned to anatase TiO2
(PDF#:21-1272) and Co3O4 (PDF#:42-1467) were observed from
the PXRD patterns. Peaks of other cobalt compounds such as
CoO and Co-titanate were not detected. Co3O4 has a spinel-
type structure, in which oxygen atoms are arranged in fcc and
Co2+ and Co3+ cations are located in tetrahedral and octahedral
coordination, respectively.32 Moreover, it was found that the
diffraction intensities of anatase TiO2 dramatically decreased
due to the decrease of TiO2 ratio when increasing cobalt
loading from 20 wt% to 30 wt%. The crystallite sizes of Co3O4
estimated from the most intense diffraction line at 2θ = 36.8°
using the Scherrer equation were shown in Table 1. Table 1
also summarized the metal dispersion evaluated from H2-O2
titration. As shown, a better dispersion was obtained in 20%
Co/TNT compared with that in 30% Co/TNT, which was in
agreement with the crystallite diameter obtained from XRD
estimation.
o
catalyst was reduced for 10 h at 360 C under pure hydrogen
flowing at 28 ml/min at atmospheric pressure. After the
o
treatment, the reactor was cooled down to 50 C. Then the
syngas(CO/H2/Ar=20:10:1 WSHV=1400 mL/h·g) was introduced
into the reactor and the pressure was increased to 2.0 MPa.
After that, the catalyst was slowly heated to 220 °C. During the
photothermocatalytic process, the sample was irradiated by
UV light (produced by a 500 W Hg lamp) from the top of the
reactor via
a
cylindrical quartz window. Meanwhile,
integration of circulating thermostated water through an outer
jacket around the lamp was carried out to prevent the heat
generated by the lamp from reaching the reactor. The reaction
products were continuously removed from the reactor. The
heavy hydrocarbons and most of the water were condensed in
o
o
two traps kept at 120 C and 0.5 C, respectively, and the tail
gas (C1–C4 hydrocarbons, CO2, Ar and unconverted CO) was
analyzed online by gas chromatograph (GC-2014C, Shimadzu)
equipped with two detectors (a TCD and a FID). Ar in the feed
gas was used as internal standard to calculate CO conversion,
and the CO conversion and product selectivity parameters are
defined as:
Fig. 1 PXRD patterns of TNT support and fresh Co/TNT
catalysts.
ꢁ
ꢀ
ꢁ
CO conversion (%)= ꢀ
out ×100
ACO/AAr inꢂ ACO/AAr
ꢀ
ꢁin
ACO/AAr
Representative TEM images of the TNT support and the
Co/TNT catalysts were shown in Fig. 2. The results showed that
the TiO2 support had mainly one-dimensional nanotubular
morphology with open ends, featuring 200–400 nm of length
and ca.5–6 and 8–10 nm of inner and outer diameter,
respectively. From Fig. 2d and 2g, it could be clearly seen that
the Co3O4 nanoparticles were dispersed on the surface of TNT.
n
co2,out
S
(%)=
×100
co2
nco × CO conversion
in
nproduct
× carbon number
out
Sc (%)=
×100 (i=1, 2, 3, 4）
i
nco × CO conversionꢂnco
in
2,out
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Table 1 Physicochemical properties of as-prepared TNT and Co/TNT catalysts.
dCo3O4
(nm)
Dispersion
(%)
BET area
(m2/g)
Pore diameter
(nm)
Total pore volume
Sample
(cm3/g)
TNT
–
–
182
138
133
17.5
3.4
1.01
0.82
0.30
20% Co/TNT
30% Co/TNT
12.6
16.3
2.18
1.76
3.4
Dispersion (%): cobalt metal dispersion obtained from H2-O2 titration
The sizes of Co3O4 nanoparticles were in the range of 8 nm–12
nm for 20% Co/TNT and 11–16 nm for 30% Co/TNT, which
were roughly in agreement with the crystallite sizes estimated
from XRD patterns. From the TEM and the data in Table 1, it
could be concluded that the extent of cobalt oxide aggregation
for the 30% Co/TNT catalyst was more serious than that for
the 20% Co/TNT. Meanwhile, Fig. 2e and Fig. 2h displayed the
HRTEM images of 20% Co/TNT and 30% Co/TNT respectively.
The clear fringe spacing values of 2.86 Å and 2.06 Å were
corresponding to (220) and (400) planes of Co3O4, respectively,
which was in accordance with XRD phase identification. The
corresponding energy-dispersive X-ray spectroscopy (EDS) for
each sample was also tested, clearly showing the expected
metal element signals (Fig. 2c, 2f, and 2i).
The BET surface area, total pore volume, and average pore
diameter of the TNT support and freshly prepared Co/TNT
catalysts had been characterized. The results were presented
in Table 1. Surface area and pore volume significantly
decreased after cobalt loaded on TiO2, suggesting that the
cobalt oxide particles were deposited into the pores or blocked
the pore channels of the TNT support.33 Adding 20 wt% and 30
wt% of cobalt decreased the BET surface area from 182 to 138
m2/g and 182 to 133 m2/g, respectively. In addition, the pore
volume decreased from 1.01 to 0.82 cm3/g for 20 wt% Co and
from 1.01 to 0.30 cm3/g for 30 wt% Co. These values indicated
that pore blockage for 30% Co/TNT was higher than that for 20%
Co/TNT.
Fig. 2 TEM images and the corresponding EDS patterns of TNT
and fresh Co/TNT catalysts: (a, b, c) TNT; (d, e, f) 20% Co/TNT;
(g, h, i) 30% Co/TNT.
Optical properties are primarily important in connection
with the photocatalytic activity. Fig. 3 showed UV-visible
absorption spectra of the samples. Pristine TNT had UV-
absorption band in the range of 200-385 nm. The Co3O4 loaded
surface caused strong absorption in the visible region. Broad
absorption ranging from 400–750 nm was attributed to the d–
d transitions of Co2+ and Co3+ ions located in the tetrahedral
and octahedral coordination spheres.34
Fig. 3 UV-vis absorption spectra of TNT support and fresh
Co/TNT catalysts.
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The reduction behaviour of the Co/TNT catalysts was
studied by H2-TPR, as displayed in Fig. 4. The TPR profiles of
the catalysts were characterized by three main reduction
o
o
stages the first peak (192 C–347 C) was associated with the
o
reduction of Co3O4 to CoO, while the second one (347 C–612
oC) corresponded to the reduction of CoO to metallic cobalt,35,
36
o
and the third peak (> 790 C) was the reduction of cobalt
species with strong Co-TiO2 interaction, as previously reported
in literatures.37 It should be noted that the area under the TPR
peaks was proportional to the amount of H2 consumed by
cobalt oxide during the reduction process. The area under the
TPR peaks for 30% Co/TNT was approximately 0.5 times larger
than that for 20% Co/TNT catalyst.
Fig.
4
H2 consumption profiles during temperature
Fig. 5 XPS spectra of fresh Co/TNT samples with different Co
programmed reduction for the freshly synthesized Co/TNT
catalysts
loadings: (a) Ti 2p and (b) Co 2p.
The surface chemical composition and electronic state of Ti
and Co for TNT support and as-prepared Co/TNT catalysts
were determined by XPS. In the Ti 2p XPS spectra (Fig. 5a), two
strong peaks of Ti 2p3/2 and Ti 2p1/2 centered at approximately
458.6 and 464.4 eV, respectively. The shapes and energy
positions were nearly the same for the catalysts and pure TNT
sample, which agreed well with the TiO2 values reported.38 Co
2p XPS spectra were shown in Fig. 5b. Two main peaks
containing 2p3/2 state and 2p1/2 state with two shake-up
satellite peaks were observed from the Co photoelectron
spectra of catalysts, and the binding energy between the Co
2p1/2 and Co 2p3/2 peak was about 15.6 eV, corresponding to
the characteristic peaks of the standard Co3O4.39,40
Furthermore, the results exhibited that binding energies of Co
2p3/2 and Co 2p1/2 in 20% Co/TNT slightly shifted to lower
values compared to those in 30% Co/TNT, indicating that the
electronic density of Co ions in the 20% Co /TNT catalyst was
higher than that in 30% Co/TNT catalyst. This might be due to
strong interaction between cobalt oxide and support.41
Fig. 6 Raman spectra of TNT support and fresh Co/TNT
catalysts, and pure Co3O4 sample (inset).
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Fig. 6 showed the laser Raman spectra of the TNT support, introduction of the UV light, the conversions of CO increased
fresh Co/TNT catalysts and pure Co3O4 samples. The results over all Co/TNT catalysts. For 20% Co/TNT catalyst, the CO
showed that the vibration bands of anatase TiO2 were located conversion was 9.2% in dark reaction, which was lower than
at 150.4 cm−1 (Eg), 204.2 cm−1 (Eg), 404.7 cm−1 (B1g), 521.7 cm−1 that of other Co-based catalysts reported in literatures. Its low
(A1g), and 646.8 cm−1 (Eg).42, 43 After Co3O4 loading, the Raman activity was attributed to only partial reduction from cobalt
vibration peaks of anatase TiO2 were blue-shifted, and such oxide to Co0 after in situ pretreatment at 360 oC under H2 flow,
shift could be attributed to an interaction between TiO2 and slight aggregation of metal nanoparticles because of high
cobalt species and disorder in the oxygen sublattice.44 New loading, and intrinsic feature of anatase TNT support.47, 48
vibration bands appeared at 196.5 cm-1, 477.5 cm-1, 519.7 cm-1 While it surprisingly increased to 63.9% with UV irradiation
and 685.9 cm-1 could be assigned to F2g, Eg, F2g, and A1g of under the same temperature and pressure. Therefore, the
Co3O4, respectively,45,46 which was in accordance with the light-enhanced activity is 54.7%. This result indicated that the
spectrum of pure Co3O4 sample (inset in Fig. 6).
UV irradiation could drastically enhance the intrinsic catalytic
ability of Co/TNT for FTS. The enhanced activity was mainly
attributed to the presence of illumination which facilitated CO
dissociation. In the thermocatalytic process, TiO2 just played a
role of support to disperse and stabilize the active metal
species; while in the photothermocatalytic process, TiO2 was
not only a support but also an efficient photococatalyst. Upon
UV excitation of TiO2, the electrons in VB will be excited to CB,
leaving holes in VB. Because the work function of a metal is
usually larger than that of semiconductor TiO2,49 the most
commonly invoked mechanism for metal cocatalysts on
photoactive TiO2 is that the metal sites behave as electron
acceptors, resulting in an increase in the electron density on
cobalt surface sites. The holes migrate to the TiO2 surface and
probably could be trapped by the reduced species in system,
including water,50, 51 hydrocarbon products,52 CO or H2,53 while
the concrete route is very complex and not clear, because
some products or intermediates are unstable or difficult to
detect and quantify. When CO molecules were adsorbed on Co
active sites, the back donation of electrons from Co d orbital to
the π* orbital of CO molecules could weaken the C–O strength
and thus activated and dissociated CO molecules, which was a
critical step reported in the literature that largely determined
the activity of FTS reaction.17,54,55
3.2 Catalytic performance and mechanism
The CO hydrogenation performance was examined under the
pressure of 2.0 MPa for 55 h. The results of photocatalysis,
thermocatalysis and photothermocatalysis of TNT support and
Co/TNT with two different Co loadings for FTS at the steady-
state regime after 40 h on stream were shown in Table 2 and
Table S2. Some control experiments were also carried out. It
was easy to see that the sole UV irradiation was unable to
catalyze CO hydrogenation for TNT or Co/TNT catalyst,
revealing that the temperature was required to obtain kinetics
for overcoming activation barrier. Even if the temperature
o
reached 220 C, compared with that over the cobalt loaded
catalysts, the CO conversion over the TNT was very low and
less than 2% whether under UV illumination or not (as shown
in Table S2), indicating that Fischer-Tropsch active metal
component (Co in this work) indeed played an indispensable
role in activating and subsequently dissociating CO, and
further initiating carbon chain growth. Moreover, it was
noteworthy that the photocatalysis significantly affected the
activity and product selectivity. The CO conversion as function
of time on stream under UV light irradiation and in the dark
was presented in Fig. 7 for the cobalt loaded catalysts. The
catalytic activity became stable after 25 h. With the
Table 2 Performance of x% Co/TNT (x = 20, 30) for FTS under photothermocatalytic condition and thermocatalytic condition.
CO
CO2
Hydrocarbon Selectivity
Distribution in C2-C4
Catalyst
Condition
Conversion
（%）
Selectivity
（%）
（%）
（%）
CH4
─
C2-C4
C5+
─
Paraffin
Olefin
─
30 OC +UV light
─
─
─
─
220 O
C
9.2
63.9
─
21.6
17.3
─
24.7
34.6
─
33.6
22.7
─
41.7
42.7
─
89.0
98.6
─
11.0
1.4
─
20% Co/TNT
220 OC+UV light
30 OC +UV light
220 O
220 OC+UV light
C
8.6
17.9
23.1
36.2
40.7
83.0
17.0
2.3
30% Co/TNT
17.1
8.4
31.5
32.5
36.0
97.7
Reaction conditions: CO/H2 = 1/2, flow rate = 28 ml min-1, pressure of 2.0 MPa, temperature of 493 K,
catalyst (1.2 g). The hydrocarbon selectivities were normalized with the exception of CO2.
6 | J. Name., 2012, 00, 1-3
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than catalyst structure. This photo-assisted CO conversion
phenomenon was also observed over 30% Co/TNT. The CO
coneversion of 17.9% and 8.4% were obtained under UV light
irradiation and in the dark, respectively. The light-enhanced
activity was 9.5%, which was smaller than that over 20%
Co/TNT. Such results might be caused by the different
amounts of photogenerated electrons transferred from TiO2 to
active metal nanoparticles, which could result in the
adsorption and activation of different amounts of CO
molecules.
Fig. 7 Plots of CO conversion against time on stream under two
catalytic conditions for 20% Co/TNT and 30% Co/TNT (where
TC
and
PTC
represent
thermocatalysis
and
photothermocatalysis, respectively).
In order to verify the weakening effect of Co on C–O bond in
CO molecules, we further investigated the CO adsorption
behavior over 20% Co/TNT under UV irradiation (325 nm laser
excitation) and visible irradiation (633 nm laser excitation) by
in situ Raman, respectively. As seen in Fig. 8, the peaks at 2140
cm-1 could be attributed to gaseous carbon monoxide and the
peaks at about 1917 cm-1 was due to the bridged CO adsorbed
56, 57
Fig. 8 In situ Raman spectra of adsorbing CO over H2-reduced
on cobalt metals.
When excited with 633 nm laser for
20% Co/TNT under different laser excitation conditions.
which TNT fairly had no absorption, the 20% Co/TNT only
exhibited an apparent peak at 2140 cm-1, indicating the
bridged-type adsorbed CO was negligible, which would result
in lower CO conversion under thermocatalytic condition as
listed in Table 2. While with the introduction of 325 nm UV
laser excitation for which TNT had a very strong absorption, a
low-frequency peak at about 1917 cm-1 assigned to bridged CO
adsorbed on cobalt metal was also observed besides gaseous
CO.58 According to Weaver and coworkers, adsorption at
bridge bonded sites generally need a long exposure time even
from nearly saturated CO solutions.59 This indicated that the
introduction of UV light does promote the adsorption of CO at
Co sites. As reported by many researchers, the bridge-type CO
is much more active for the FT reaction due to a weaker C–O
bond that can be more easily dissociated to carbon and oxygen
atoms,60,61 thus, high CO conversion was obtained under UV
illumination in this work. Concerning that UV illumination is
more likely to change the catalyst structure and hence resulted
in different activities and selectivities, the physical, chemical
and photoelectric properties of spent 20% Co/TNT composite
after the test of photothermocatalytic and thermocatalytic FTS
were thoroughly characterized by PXRD (Fig. S2), XPS (Fig. S3),
TEM (Fig. S4), Raman spectra (Fig. S5), BET technique (Fig. S6
and Table S1), and UV-vis absorption spectra (Fig. S7). No
obvious difference was found between two conditions in such
kinds of characterization results, thus the much higher
Fig. 9 CO conversion as a function of temperature for non-
illuminated 20% Co/TNT catalysts.
Additionally, we demonstrated the thermocatalytic FTS at
different temperatures for 20% Co/TNT catalyst. Fig. 9
revealed a linear increase of the CO conversion upon elevating
the temperature. It was observed that the CO conversion at
220 oC/UV irradiation was comparable to that at 298 oC in dark
condition. In other words, for achieving the same CO
conversion, light-assisted thermocatalysis could be operated at
photothermocatalytic
activity
compared
to
the
thermocatalytic activity originated from other factors rather
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lower temperatures than their conventional counterparts that
use only thermal stimulus. Thus, photothermocatalysis process
is an energy-saving process compared to thermocatalytic
process.
With respect to the product selectivity, as displayed in Table
2, the CO2 selectivity under irradiation was lower than that
without irradiation for all the catalysts, suggesting that water
gas shift (WGS) reaction was restrained in photothermal
condition. The decrease of the WGS reaction activity was most
probably due to the decrease in water partial pressure caused
by the oxidation of water by the photogenerated holes.62 As
for the product distribution within the hydrocarbon fraction,
the high selectivity to short chain paraffins in thermocatalytic
process was dependent on the the character of apparatus. The
big volume (2.4 L) increases the residence time of formed FT
products, probably resulting in that olefins were readsorbed
on the active sites and initiate secondary reactions to produce
CH4 and C2-C4 paraffins. The CH4 selectivity increased from
24.7% to 34.6% for 20% Co/TNT and from 23.1% to 31.5% for
30% Co/TNT at the expense of C2-C4 formation or C5+
formation under illumination, which might be caused by the
photohydrogenolysis of long-chain hydrocarbons or direct
hydrogenation of surface CHx intermediates into lower
hydrocarbons under UV irradiation. Such hydrocarbons
hydrogenolysis reaction under light irradiation has been
previously documented using TiO2 63, 64and Ni-52518 as catalyst,
respectively. In addition, another pathway that favoring CHx
intermediates hydrogenated to CH4 instead of chain growth is
also possible due to excess H2 dissociated on the catalyst
surface under UV irradiation. Besides, the effect of illumination
on paraffin/olefin distribution in the range of C2-C4 was Fig. 10 C2-C4 paraffin/olefin distribution under two catalytic
presented in Fig. 10. It was notable that for 20% Co/TNT the conditions for Co/TNT catalysts: (a) 20% Co/TNT; (b) 30%
alkanes represented 98.6% of the whole C2-C4 hydrocarbons Co/TNT.
under illumination, which was 9.6% higher than its
corresponding value in the dark. Correspondingly, olefin metals took part in the hydrogenation reaction as a active
decreased by 9.6% from 11.0% to 1.4%. Similar variation component, resulting in partial olefin hydrogenation.
tendency was observed for 30% Co/TNT catalyst. To further Interestingly, UV irradiation significantly increased the
o
investigate the origin of the observed change of paraffin/olefin formation rate of alkane at 220 C, and the concentration of
o
distribution in the range of C2-C4 during illumination, alkane produced at 220 C with the UV irradiation was 1.32
additional experiments were conducted as follows: a standard times larger than that produced at 220oC without the UV
gas mixture (H2: 85%, C2H4: 5%, C3H6: 5%, C4H8: 5%) was irradiation. Schumacher et al. have discovered that H2
introduced to reactor as initial reactants to achieve a pressure molecules can be dissociated into two neutral atoms under
of 2.0 MPa, then the reaction system was kept in the absence intense light irradiation.65 Combined our experimental results
of catalyst for 20 h under three different specified reaction with the relative reports together, when employing UV
o
o
conditions (25 C with UV light radiation, 220 C without light irradiation, we thought that the conversion from olefin to
o
irradiation and 220 C with UV light irradiation). The reactants paraffin was promoted by hydrogen atoms which were
and products were quantified online by the FID detector of dissociatively produced from H2 molecules, accompanying the
Shimadzu GC-2014. The experimental results were shown in activation of C=C bond. Subsequently, the hydrogen atom
Fig. 11 and Table 3. It was obvious that no concentration jumped to the olefin molecule to form corresponding alkanes.
o
changes could be observed for various components at 25 C A possible reaction mechanism is proposed as follow,
o
with UV light radiation. When heating the system to 220 C, schematically. Thus, the hydrogenation reaction of alkene to
the concentration of all alkenes (C2H4, C3H6, C4H8) decreased, alkane was promoted by UV illumination even without
and correspondingly, some alkanes (C2H6, C3H8 and C4H10) catalyst. Therefore, the photochemical reaction was one of the
appeared, indicating alkenes were converted into alkanes by important paths for light alkane formation in the gaseous
hydrogenation. However, the reactor we used is made of range. Combining the previous analysis, it was evident that UV
0Cr18Ni9 stainless steel which contains 18.00
～20.00% Cr and illumination in FTS would favor hydrogenation of olefin and
hydrogenolysis of the long-chain hydrocarbons, leading to the
8.00～10.50% Ni, and it is most possible that these transition
8 | J. Name., 2012, 00, 1-3
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higher selectivity toward the lighter paraffin products
(methane, paraffin in C2–C4).
4. Conclusions
In summary, TNT supported cobalt catalyst was successfully
applied as a model system to deduce the contribution of UV
illumination to the overall Fischer–Tropsch activity and
selectivity. From comparison of the performance of
photothermocatalytic and solely thermocatalytic FTS, it was
concluded that photocatalysis can significantly improve the
catalytic performance of FTS. The enhanced CO conversion
efficiency could be attributed to the photogenerated electrons
transfer from TiO2 to active Co site, which caused the Co sites
with increased surface electron density and subsequently
enhanced the adsorption and activation of CO molecules at Co
sites. In addition, UV illumination caused an increase of lower
molecular weight paraffin and a decrease of CO2 formation in
the FTS product distribution. The increase in light paraffin
selectivity was caused by photo-promoted hydrogenation of
olefin and hydrogenolysis of the long-chain hydrocarbons,
while the decrease of CO2 selectivity was probably related to
the photogenerated holes which restrained WGS reaction. Our
approach provided a simple and energy-saving clue to enhance
FTS performance and modulate product selectivity. Through
optimizing the proper illumination condition (such as
wavelength, light intensity), it is possible to gain desirable
activity and product distribution in the future.
metal (Ni,Fe)
220 oC
H2
2H
R1CHꢅCHR2 (R1, R2: H or alkyl)
R1CHꢅCHR2 + 2H
Fig. 11 Concentrations of C2–C4 alkenes, alkanes, in the
absence of catalyst after 20 h under different conditions. The
initial concentrations were 5% C2H4, 5% C3H6, 5% C4H8 and 85%
H2, respectively. Total pressure: 2.0 MPa.
Table 3 The respective hydrocarbon concentrations under different conditions.
Concentration (%)
Condition
C2H4
5.00
1.76
4.96
0.10
C2H6
0
C3H6
5.00
3.81
5.00
1.36
C3H8
0
C4H8
5.00
0.80
5.01
0.28
C4H10
0
25 oC
220 oC
25 oC
No illumination
UV illumination
4.68
0.06
7.83
2.14
0.02
5.30
6.06
0.04
3.75
220 oC
4
5
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Acknowledgements
This work was supported by the financial support from the
National Natural Science Foundation of China (grant no.
21461017), the specialized research fund for the doctoral
program of higher education (grant no. 20131501110001) and
Prairie Excellence Innovation and Entrepreneurial Team of
Inner Mongolia (grant no. 201201).
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