Visible light-enhanced photothermal CO2 hydrogenation over Pt/Al2O3 catalyst

Article abstract of DOI:10.1016/S1872-2067(19)63445-5

Light illumination has been widely used to promote activity and selectivity of traditional thermal catalysts. Nevertheless, the role of light irradiation during catalytic reactions is not well understood. In this work, Pt/Al₂O₃ prepared by wet impregnation was used for photothermal CO₂ hydrogenation, and it showed a photothermal effect. Hence, operando diffuse reflectance infrared Fourier-transform spectroscopy and density functional theory calculations were conducted on Pt/Al₂O₃ to gain insights into the reaction mechanism. The results indicated that CO desorption from Pt sites including step sites (Pt_step) or/and terrace site (Pt_terrace) is an important step during CO₂ hydrogenation to free the active Pt sites. Notably, visible light illumination and temperature affected the CO desorption in different ways. The calculated adsorption energy of CO on Pt_step and Pt_terrace sites was ?1.24 and ?1.43 eV, respectively. Hence, CO is more strongly bound to the Pt_step sites. During heating in the dark, CO preferentially desorbs from the Pt_terrace site. However, the additional light irradiation facilitates transfer of CO from the Pt_step to Pt_terrace sites and its subsequent desorption from the Pt_terrace sites, thus promoting the CO₂ hydrogenation.

Full text of DOI:10.1016/S1872-2067(19)63445-5

Chinese Journal of Catalysis 41 (2020) 286–293
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Visible light-enhanced photothermal CO2 hydrogenation over
Pt/Al2O3 catalyst
Ziyan Zhao a,b, Dmitry E. Doronkin c,d,#, Yinghao Ye b, Jan-Dierk Grunwaldt c,d, Zeai Huang a,b,
Ying Zhou a,b,*
a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China
b The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500,
Sichuan, China
c Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology, Karlsruhe (KIT), 76131 Karlsruhe, Germany
d Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Light illumination has been widely used to promote activity and selectivity of traditional thermal
catalysts. Nevertheless, the role of light irradiation during catalytic reactions is not well understood.
In this work, Pt/Al2O3 prepared by wet impregnation was used for photothermal CO2 hydrogena-
tion, and it showed a photothermal effect. Hence, operando diffuse reflectance infrared Fouri-
er-transform spectroscopy and density functional theory calculations were conducted on Pt/Al2O3
to gain insights into the reaction mechanism. The results indicated that CO desorption from Pt sites
including step sites (Ptstep) or/and terrace site (Ptterrace) is an important step during CO2 hydrogena-
tion to free the active Pt sites. Notably, visible light illumination and temperature affected the CO
desorption in different ways. The calculated adsorption energy of CO on Ptstep and Ptterrace sites was
−1.24 and −1.43 eV, respectively. Hence, CO is more strongly bound to the Ptstep sites. During heating
in the dark, CO preferentially desorbs from the Ptterrace site. However, the additional light irradiation
facilitates transfer of CO from the Ptstep to Ptterrace sites and its subsequent desorption from the Ptter-
race sites, thus promoting the CO2 hydrogenation.
Received 14 May 2019
Accepted 5 July 2019
Published 5 February 2020
Keywords:
CO2 hydrogenation
Photothermal catalysis
Pt/Al2O3
Operando diffuse reflectance infrared
Fourier transform spectroscopy
Density functional theory
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
portant strategy for improving the environment but also a
promising technology for producing renewable hydrocarbon
With the development of industry and economy, significant
attention is being paid to environmental remediation [1–4].
Emission of carbon dioxide as a primary greenhouse gas results
in global temperature increase and climate changes. Among the
numerous CO2 utilization strategies, CO2 hydrogenation has
attracted extensive attention, because it is not only an im-
fuels such as HCOOH, CH3OH, and CH4 [5–10]. However, CO2
hydrogenation requires high temperature because of the ther-
modynamic stability of CO2 [11–13].
Photothermal catalysis, a combination of photoexcitation
with thermal energy to trigger chemical reactions, significantly
improves in the activity and selectivity of catalytic reactions
* Corresponding author. Tel: +86-28-83037401; Fax: +86-28-83037406; E-mail: yzhou@swpu.edu.cn
# Corresponding author. Tel: +49-721-60848090; Fax: +49-721-60844820; E-mail: dmitry.doronkin@kit.edu
This work was supported by the National Natural Science Foundation of China (U1862111, U1232119), Sichuan Provincial International Cooperation
Project (2017HH0030), the Innovative Research Team of Sichuan Province (2016TD0011).
DOI: S1872-2067(19)63445-5 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 2, February 2020

Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–283
287
and the stability of the catalysts [5,14–16]. Therefore, light ir-
radiation has been utilized to promote diverse thermal catalytic
reactions such as CO oxidation [17], propylene oxidation [18],
Fischer-Tropsch synthesis [19], reverse water gas shift (RWGS)
reaction [20], and CO2 hydrogenation [21]. Light irradiation can
significantly improve the catalytic performance of these reac-
tions. For instance, in our previous work, we found that light
irradiation promoted CO oxidation through O2 excitation [22].
Wang et al. [19] reported that photo-induced back donation of
electrons from Co to CO facilitated CO activation and dissocia-
tion. In addition, visible light irradiation increased RWGS activ-
ity of Au nanoparticles by 1300% by decreasing the activation
energy from 47 to 35 kJ/mol [20]. For CO2 hydrogenation,
light-induced electron transfer from Rh to anti-bonding orbitals
of a reaction intermediate resulted in efficient CH4 generation
[15]. CO desorption was regarded as the rate-limiting step
during CO2 hydrogenation [23]. However, the effects of light
and temperature on this step are still not understood in depth.
In this work, CO2 hydrogenation was conducted over
Pt/Al2O3 catalyst. Operando diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) was used to investigate CO
desorption on Pt sites during the photothermal CO2 hydrogena-
tion. The reaction pathway is discussed to understand the
mechanism of the photothermal catalysis.
1000 ppm CO2 and 6000 ppm H2 in He were dosed into the
reactor (Harrick Praying Mantis™ high temperature reaction
chamber with a flat cover and a quartz window, Scheme 1, left)
at 110 mL/min. Schott KL 2500 LCD light source with an inten-
sity of 710 mW/cm2 was used as the light source. CO and CO2
concentrations were detected by a Hartmann & Braun Uras 10E
detector. The photo-enhancement was calculated as follows:
Photo-enhancement = (Ratephotothermal − Ratethermal)/Ratethermal
.
OmniStar GSD-320 quadrupole mass spectrometer (Pfeiffer
Vacuum) was used to detect CO (m/z = 28), CO2 (m/z = 44),
CH3OH (m/z = 31), HCOOH (m/z = 46), and CH4 (m/z = 16).
For operando DRIFTS, the gas composition was kept the
same as that for catalytic tests. The spectra were measured on a
VERTEX 70 FTIR spectrometer (Bruker) equipped with a mer-
cury cadmium telluride detector. Harrick in situ cell with a
dome cover with two KBr windows and a glass window was
used as the reactor (Scheme 1, right). The catalyst was pre-
treated in 5% H2/He at a flow rate of 100 mL/min at 200 °C for
6 h. The spectra of the pre-reduced catalysts at 30 °C in pure He
gas were used as the background spectra. The reacting systems
were stabilized for 30 min in the absence of light at room tem-
perature, and then DRIFTS spectra were collected (without
light) at 30, 80, 120, 160, and 200 °C. After the spectrum was
obtained at each temperature, the catalyst was irradiated for
10 min. Then, it was flushed with He, and the spectrum was
recorded again. At each temperature, the catalyst was flushed
with He gas for 10 and 20 min, and the additional spectra were
collected. The IR spectra were recorded from 4000 to 700 cm‒1
with 4 cm‒1 resolution and by averaging over 100 scans.
2. Experimental
2.1. Catalyst synthesis
Pt/Al2O3 catalysts were prepared by the wet impregnation
method using aqueous H2PtCl6, as described in our previous
work [24]. γ-Al2O3 (10 nm) was bought from Aladdin Co., Ltd.
Briefly, 500 mg γ-Al2O3 was dispersed in a mixture of 5 mL
H2PtCl6 (2 mg/mL) solution and 30 mL water to achieve 2 wt%
Pt loading on γ-Al2O3. Then, the suspension was stirred for 6 h
at 27 °C. The product was centrifuged and dried at 65 °C for 24
h. Pt/TiO2 was prepared in the same way using hydrothermally
synthesized anatase TiO2 support, as used in our previous work
[22] and described in the Supporting Information.
2.4. Thermodynamic calculation
Equilibrium concentrations of species obtained during CO2
hydrogenation were calculated by HSC chemistry 7 [25]. The
ratio of CO2/H2 was 1/6, and CHOOH, CH3OH, CO, H2O, and CH4
were originally considered as products. Including CH4 in the
calculation led to the conclusion that is the preferred product
under thermodynamic control, with an equilibrium concentra-
2.2. Characterization
X-ray diffraction (XRD) patterns were recorded on a PANa-
lytical X’pert diffractometer using Cu Kα radiation operating at
40 mA and 40 kV. Transmission electron microscopy (TEM)
was carried out on an FEI Tecnai G2 20 microscope operating
at 200 kV. UV-vis spectra were recorded on a Shimadzu 2600
UV-vis spectrophotometer. Raman spectra were obtained on a
Horiba LabRAM HR Evolution Raman spectrometer with a 532
nm laser.
2.3. Evaluation of CO2 hydrogenation and operando DRIFTS
Sieved powder catalysts (50 mg, 100‒200 μm) were used
for the tests. The catalysts were pretreated in 5% H2/He (100
mL/min) at 200 °C for 6 h. The temperature of the catalyst bed
was calibrated by an IR camera. For evaluating the activity,
Scheme 1. Scheme of the setup for evaluating the CO2 hydrogenation
activity and performing the operando DRIFTS.

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Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–293
tion close to 100% among all the carbon-containing species at
all the studied temperatures [26]. Since only a small amount of
CH4 was detected experimentally, its production was limited by
kinetics. Therefore, it was excluded from further thermody-
namic analysis.
conversion and CO production rate. For example, the CO2 con-
version rate in the presence of at 120 °C was about 50
μmol/(g·min), while it was only 10 μmol/(g·min) in the dark.
The CO production rate was ca. 0 μmol·g-1min-1 at 120 °C in the
dark and about 15 μmol/(g·min) under light irradiation (Fig.
1(b)). The production rates at other temperatures were also
significantly promoted by light. Moreover, light irradiation de-
creased the temperature required for CO generation on
Pt/Al2O3. The CO production rate at 120 °C under light was
close to the thermally activated CO production rate at 250 °C,
and a similar trend was also observed for CO2 conversion rates.
Photo enhancement was used to evaluate the effect of light on
the thermally driven reaction. The enhancement reached the
maximum value at 80 °C, which was about 9 times that of the
corresponding purely thermally driven CO2 conversion rate. As
the temperature increased, the thermally induced rate further
increased, while the photo-induced enhancement decreased,
which may be because of mass-transfer limitations [23].
Simultaneously, the temperature increase due to light irra-
diation should also be taken into consideration. In our previous
work, although the light intensity was 972 mW/cm2, light irra-
diation increased the surface temperature by no more than 60
°C [22]. In this work, at a lower light intensity of 710 mW/cm2,
both CO2 conversion and CO production rates over Pt/Al2O3 at
80 °C were much higher than the rates at 160 °C in the dark.
Therefore, Pt/Al2O3 exhibited typical synergistic effect of pho-
toexcitation and thermal energy. Since the γ-Al2O3 support used
here is a typical insulator and cannot not take part in the elec-
tron transfer process under light irradiation, it is evident that
the active sites for CO2 hydrogenation are provided by Pt.
2.5. Computational details
Density functional theory (DFT) calculations were per-
formed using DMol3 software package based on Materials Stu-
dio software. The electron exchange and correlation were ap-
proximated by generalized gradient approximation (GGA) with
the Perdew-Burke-Ernzerhof (PBE) functional. Dispersion cor-
rection by TS (DFT-D) was used to describe the van der Waals
interaction. The valence electron configurations were 2s22p2
for C, 2s22p4 for O, and 5d96s1 for Pt. As for the Monkhorst-Pack
grid k-point in the Brillouin-zone, a 5 × 5 × 1 k-point was used
for geometry optimization. For convergence threshold, the total
energy of the system, maxforce, and displacement tolerances
were set to 1×10‒5 Ha, 0.02 Ha/Å, and 0.05 Å, respectively. A
vacuum space of 15 Å was set between the slabs in order to
avoid interaction.
The adsorption energy (Ead) of adsorbates was calculated as
follows:
Ead = ECO+surface − Esurface − ECO
(1)
Here, ECO+surface is the total energy of Pt surface with CO, Esur-
is the total energy of the clean surface, and ECO is the total
face
energy of CO in the gas phase. The CO molecule was optimized
in a three-dimensional (3D) box of a = b = c = 15 Å.
3. Results and discussion
To investigate the effect of the support, the alumina support
was replaced by a typical photocatalyst, TiO2. As shown in Fig.
S1(a), CO2 conversion rate over Pt/TiO2 increased from 10 to
250 μmol/(g·min) when the temperature was increased from
80 to 400 °C. These rates were also significantly promoted by
light irradiation. For example, the CO2 conversion rate was 50
μmol/(g·min) at 80 °C under light irradiation, while it was only
about 10 μmol/(g·min) in the dark. Moreover, the light-induced
CO2 conversion rate, 50 μmol/(g·min), was identical to the rate
at 250 °C in the dark. The CO2 conversion and CO production
rates over Pt/TiO2 were similar to those of Pt/Al2O3. The
slightly higher activity of Pt/TiO2 may be due to the strong in-
3.1. Photothermal effects
The CO2 conversion rate and CO production rate during CO2
hydrogenation over Pt/Al2O3 at different temperatures are
depicted in Fig. 1. Fig. 1(a) shows that during the thermal ca-
talysis, CO2 conversion rate increased from
0 to 10
μmol/(g·min) when the temperature changed from 80 to 120
°C and reached up to about 225 μmol/(g·min) at 400 °C. No CO
was detected until the temperature reached 200 °C (Fig. 1(b)).
Additional light irradiation significantly promoted both CO2
10
90
80
70
60
50
40
30
20
10
0
14
12
10
8
(a)
(b)
Thermal
Photo and Thermal
250
Thermal
Photo and Thermal
8
6
4
2
0
200
150
100
50
6
4
2
0
0
100
150
200
250
300
350
400
100
150
200
250
300
350
400
T / C
T / C
Fig. 1. CO2 (a) conversion and CO production rates (b) in photothermal CO2 hydrogenation on Pt/Al2O3.

Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–283
289
Fig. 2. (a) CO2 conversion and CO production rates of Pt/Al2O3 induced by light irradiation (after subtracting the corresponding thermally induced
rates). (b) Ion currents of the products of CO2 hydrogenation over Pt/Al2O3 measured at different temperatures in the dark and under light irradiation.
teraction between the Pt and TiO2 support, resulting in the
higher dispersion of Pt and inhibition of sintering [24]. Obvi-
ously, both the catalysts exhibited very small differences in the
catalytic activity. Besides, Al2O3 as a support with high surface
area facilitates high dispersion of Pt even without strong met-
al-support interaction. Moreover, with the wide band gap, it
cannot be excited by light, thereby allowing to decouple the
photoexcitation of Pt species and photocatalytic effect of titania.
Thus, the following investigations of the mechanism of CO2
hydrogenation on Pt sites were conducted on Pt/Al2O3.
heating resulted in the corresponding increase in CH4 and H2O
signals, while CO2 and HCOOH signals decreased. These results
indicate that CO2 and H2 were gradually converted into CH4 and
H2O, while HCOOH may have been consumed as the reaction
intermediate.
Thermodynamic equilibrium calculation for the used gas
feed (molar ratio CO2:H2 = 1:6) is shown in Fig. 3. Three ranges
can be observed during heating process. Below 120 °C, CH3OH
is the thermodynamically preferred product of CO2 hydrogena-
tion (CH4, if included in the calculation, would be the most
thermodynamically stable in the entire temperature range).
Although HCOOH is thermodynamically less stable than the
other products, it was experimentally detected, and its concen-
tration decreased with increasing temperature. In the range of
120 to 180 °C, CO2 is the most thermodynamically stable com-
ponent; hence, its reduction is not favored in this temperature
range, as also confirmed experimentally from the low conver-
sion rates. When the temperature was above 180 °C, CO2 con-
version rate gradually increased, and CO production (reverse
water gas shift reaction) was favored, which was consistent
with the experimental change in CO2 conversion and CO pro-
duction rates shown in Fig. 3(a). As shown in Fig. S2, the ion
currents of reactants and products obtained from the experi-
mental results exhibited similar trends in the calculation. Thus,
3.2. Products of CO2 hydrogenation
It is clear from Fig. 2(a) that the CO2 conversion rate of
Pt/Al2O3 increased from 32 to 75 μmol/(g·min) and then de-
creased to 30 μmol/(g·min) as the temperature increased from
80 to 400 °C. Meanwhile, CO production rate increased from 8
to 25 μmol/(g·min) and then decreased to 15 μmol/(g·min).
Obviously, the consumed CO2 was not entirely converted into
CO. To evaluate the composition of CO2 hydrogenation prod-
ucts, the outlet gas was analyzed by mass spectrometry (Fig.
2(b)), and the ion currents of some reactants and products are
summarized in Fig. S2. Once CO2 and H2 feed was switched on,
higher H2O, CH4, and HCOOH background was detected. Further
6
5
4
3
2
1
0
1.0
(a)
(b)
0.8
0.6
0.4
0.2
0.0
H2
CO2
H2O
CH3OH
CO
HCOOH
50
100 150 200 250 300 350 400
50
100 150 200 250 300 350 400
T / C
T / C
Fig. 3. Relative thermodynamic stabilities of species in the system including CO2 (initial concentration 1), H2 (initial concentration 6), and CO, HCOOH,
CH3OH, H2O (initial concentrations of these four species were 0). (a) Temperature dependent equilibrium concentrations of all the considered reac-
tants and products except CH4; (b) Enlarged view of the trends of CO2, CO, CH3OH, and H2O from (a).

290
Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–293
Table 1
Assignments of peaks in IR spectra during CO2 hydrogenation.
Wavenumber/cm‒1
2326–2362
2058–2046
2038–2007
2007
Assignment
CO2
CO Ptterrace
CO Ptstep
CO on Ptstep
Bridged CO
‒OH
Refs.
[30]
[31,32]
[31] [32]
[32] [ 33]
[31]
1760
1625
[32,34]
Fig. 4. XRD patterns (a) and a representative HRTEM image (b) of
Pt/Al2O3 (with a histogram showing the size distribution of Pt nanopar-
ticles).
3.4. Operando DRIFTS
Operando DRIFT spectra obtained during the photothermal
catalytic CO2 hydrogenation are shown in Fig. 5, and the as-
signments of IR peaks are summarized in Table 1. The maxi-
mum temperature to obtain a reasonable signal-to-noise ratio
was 200 °C.
the thermodynamic calculation agrees well with the experi-
mental results, indicating that CO2 hydrogenation is not limited
by kinetics (unlike the CH4 pathway).
3.3. Structure, morphology, and optical properties
Fig. 5(a) shows that after the exposure of Pt/Al2O3 to CO2
and H2 at 30 °C, peaks emerged at 2326–2362 and 1625 cm‒1
,
Pt was not detected in the XRD patterns (Fig. 4(a)), and all
the diffraction peaks were attributed to γ-Al2O3
(PDF#16-0394). TEM images (Fig. 4(b)) showed Pt nanoparti-
cles with a diameter of 0.9 ± 0.2 nm (shown in Fig. S3) depos-
ited on Al2O3. The crystalline interplanar spacings of 0.226 and
0.195 nm were indexed to {111} and {200} facets of Pt, while
the spacing of 0.289 nm was attributed to {117} facet of
γ-Al2O3. As reflected in the UV-vis spectra (Fig. S4), Pt deposi-
tion promoted the light adsorption ability of Al2O3 in the range
200–600 nm [27]. Strong interaction at the Pt/TiO2 interface
was evidenced for Pt/TiO2 [24]. Raman spectra showed a signal
located at 145 cm-1 attributable to the Eg(1) peak of anatase
(spectrum given in the supporting information, Fig. S5). After
Pt deposition, this peak shifted to 151 cm‒1, which indicated the
generation of Pt-Ovancancy-Ti3+ at the interface between Pt and
TiO2 [28]. Hong et al. [29] reported that Pt-Ovancancy-Ti3+ be-
haved as an adsorption site for the reaction intermediates such
as CO generated from HCOOH decomposition. This increased
the residence time of surface intermediates, resulting in the
promoted activity. It might be also the reason that Pt/TiO2 (Fig.
S1) exhibited higher activity than Pt/Al2O3 (Fig. 1).
which were attributed to CO2 and -OH on Pt [30,32,34]. These
signals became weaker, and the peaks at 2007 and 1760 cm‒1
corresponding to carbonyl and CO evolved as the temperature
increased from 30 to 200 °C [31–33]. No CO production was
observed when the temperature was increased to 120 °C, (Fig.
S2(d)), which was in agreement with thermodynamics (Fig.
3(b)). On the other hand, HCOOH decreased (Fig. S2(e)), while
the intensity of carbonyl band at 2007 cm‒1 (Fig. 5(a)) did not
increase. Increase in the catalyst temperature resulted in high-
er intensity of the carbonyl band at 2007 cm‒1. Considering the
gradual decrease in HCOOH concentration in the effluent (Fig.
S2(e)) and the increase in CO concentration (Figs. 1–3), the
higher band intensity was attributed to the CO produced on
Ptstep site [31,32].
Under light illumination below 80 °C, the intensity of the
band at 1624 cm‒1 (surface -OH) decreased (Fig. 5(b)). The
signals in the range 2326–2362 cm‒1 (CO2) slightly increased,
while the intensity of the peaks at 2007 and 1760 cm‒1 (CO)
obviously rose [30–33]. Besides, CO2 and HCOOH ion currents
decreased, as shown in Fig. S2(a) and Fig. S2(e). These changes
indicate that CO2 was preferably adsorbed on Pt and then
formed small amount of CO under light irradiation. At temper-
atures above 120 °C, CO generation was promoted by temper-
30C
30C_light
80C
80C_light
120C
120C_light
160C
160C_light
200C
(a)
(b)
2038
2007
1760
2017
200C
1625
200C
160C
1760
2331
2358
2019
2326
2362
1625
160C
120C
2007
120C
80C
30C
2007
200C_light
80C
30C
2344
2346
1400 24002300
2400 2300
2000
1800
1600
2000
1800
1600
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 5. Operando DRIFT spectra of Pt/Al2O3 during thermal (a) and photothermal (b) CO2 hydrogenation.

Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–283
291
ature, and CO molecules gradually accumulated on Ptstep sites in
the absence of light. When the light was turned on at the re-
spective temperatures, CO signals increased significantly, indi-
cating the enhanced CO generation (Fig. 5(b)). This was in ac-
cordance with the increased concentration of CO in the gas
phase (Fig. 1(b) and Fig. S2(d)). It was worth noting that the CO
signal at 2007 cm‒1 was blue shifted once the light was turned
on, indicating the weakened interaction of CO molecules with
Ptstep sites.
To further investigate the interaction between CO and Pt, He
was used to flush the catalyst surface to observe the intermedi-
ate species adsorbed during the photothermal CO2 hydrogena-
tion (Fig. 6). CO on Ptstep sites (2038–2007 cm‒1 [31,32]) was
not visible after He flushing. However, CO on Ptterrace sites was
detected in the range 2058–2046 cm‒1, which was different
from the thermal catalysis shown in Fig. 5(a) [31,32]. This in-
dicates that light and temperature have different effects on CO
adsorption. The adsorption of CO on Ptstep sites is much strong-
er compared with that on Ptterrace sites, the desorption from
which was found to occur at 77–177 °C [35]. As shown in Fig.
1(b), a small amount of CO evolved at temperature below 200
°C in the dark; hence, the generated CO was probably desorbed
from Ptterrace sites, and the corresponding signals could not be
observed in the IR spectra (Fig. 5(a)). On the other hand, the
adsorption of CO was stronger on the undercoordinated Ptstep
sites, resulting in clearly visible IR bands. Even when the tem-
perature reached 200 °C, the CO produced accumulated on
Ptstep sites, as reported previously [36]. Thus, the observed CO
on Ptterrace (Fig. 6) did not originate during the thermal catalysis
but during light irradiation.
that light and temperature affected CO desorption in different
ways. The formed CO was preferably desorbed from Ptterrace
sites during heating, while its distribution seem to be altered
under additional light irradiation.
Compared with the terrace sites, metal step sites are more
coordinatively unsaturated, which is beneficial for CO adsorp-
tion [40,41]. To further reveal the interaction between CO and
Pt, energies of CO adsorption on Ptterrace and Ptstep sites were
calculated by DFT. Adsorption geometries and the correspond-
ing energies are shown in Fig. 7. The Pt-C distances for CO ad-
sorbed on Ptterrace and Ptstep sites were 2.01 and 1.98 Å, and the
adsorption energies were −1.24 and −1.43 eV, respectively,
which were similar to the reported values of −1.39 and −1.42
eV in the previous work [42]. This indicates that CO is more
strongly adsorbed on Ptstep sites than on Ptterrace sites, con-
sistent with the IR spectra [43,44]. Fig. 5(b) and Fig. 6 show
that light irradiation weakens the interaction between CO and
Ptstep sites. However, even after He flushing, CO on Ptterrace site
with weaker bonding was detected during the photothermal
process in the range 30–200 °C, while the CO on Ptstep site was
not visible. This indicates that CO was desorbed from Ptstep
sites, while the CO produced accumulated on Ptterrace. Addition-
ally, Lawrenz et al. [45] reported that laser light could induce
the spillover of CO from Ptstep to Ptterrace sites. Thus, upon light
irradiation, the CO produced first transfers from Ptstep sites to
Ptterrace sites and then desorbs from Ptterrace sites.
Based on the results mentioned above, the main reactions
occurring during CO2 hydrogenation are summarized in Eqs.
(2–5), and the mechanism of light- and temperature-induced
activation of CO desorption from Pt during CO2 hydrogenation
is summarized in Fig. 8. CO2 and H2 are spontaneously con-
verted to HCOOH because of the low activation energy [46].
Subsequently, HCOOH is decomposed to *CO adsorbed on Ptter-
3.5. Effects of light and temperature on CO2 desorption
According to the mechanism of CO2 hydrogenation [37], CO
desorption is regarded as an important step, with activation
energy of CO desorption from Pt and Pd being much higher
than that from other metals [23]. At low temperatures, the ac-
tive sites on metal surface are usually covered by CO, which
poisons or blocks the sites [38,39]. Hence, CO desorption from
the active sites requires the sites to be free for catalyzing the
CO2 hydrogenation. The operando DRIFTS results indicated
(Eads = ‒1.24 eV) and Ptstep (Eads = ‒1.43 eV) sites. CO de-
race
sorption is regarded as a crucial step during CO2 hydrogenation
to free the active, low-coordinated Pt step sites. Considering the
weaker interaction between CO and Ptterrace sites, the CO gener-
ated preferably desorbed from Ptterrace site in the thermally
activated CO2 hydrogenation process [36]. On the contrary,
additional light irradiation induced CO desorption from Ptstep
sites (with possible spillover to Ptterrace sites), as evidenced by
operando DRIFTS. As a result, the interaction between CO and
Pt site became weaker, and further thermally activated CO de-
sorption from Ptterrace site was facilitated. Hence, we suggest
that this, in turn, promotes CO2 hydrogenation [45,47]. This
should be further substantiated in future studies.
30C
80C
2046
120C
160C
200C
200C
160C
1760
2325
2058
120C
80C
2362
30C
1625
1600
2400 2300
2000
1800
1400
Wavenumber / cm-1
Fig. 6. Operando DRIFT spectra of Pt/Al2O3 after 20 min of flushing
Fig. 7. Adsorption energies and Pt-C distances for CO adsorbed on (a)
with He gas after turning off light.
Ptterrace and (b) Ptstep
.

292
Ziyan Zhao et al. / Chinese Journal of Catalysis 41 (2020) 286–293
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Graphical Abstract
Chin. J. Catal., 2020, 41: 286–293 doi: S1872-2067(19)63445-5
Visible light-enhanced photothermal CO2 hydrogenation over Pt/Al2O3
catalyst
Ziyan Zhao, Dmitry E. Doronkin *, Yinghao Ye, Jan-DierkGrunwaldt,
Zeai Huang, Ying Zhou *
Southwest Petroleum University, China; Karlsruhe Institute of Technology,
Germany
Light illumination and temperature affected CO coverage during CO2 hy-
drogenation in different ways, offering a new pathway for improving the
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光照对Pt/Al2O3光热CO2加氢反应的增强作用
赵梓俨a,b, Dmitry E. Doronkin c,#, 叶英浩b, Jan-Dierk Grunwaldt c,d, 黄泽皑a,b, 周 莹a,b,*
a西南石油大学油气藏地质及开发工程国家重点实验室, 四川成都610500, 中国
b西南石油大学材料科学与工程学院新能源材料及技术研究中心, 四川成都610500, 中国
c卡尔斯鲁厄理工学院化学技术与高分子化学研究所, 卡尔斯鲁厄76131, 德国
d卡尔斯鲁厄理工学院催化研究与技术研究所, 卡尔斯鲁厄76344, 德国
摘要: 在传统热催化材料的研究领域中, 光照技术已经得到了广泛的应用, 从而使传统热催化剂的催化反应活性和选择性
得到优化. 然而, 在光热协同催化反应过程中, 光照因素对催化反应过程的影响尚未得到很好地研究和理解. 本文通过浸
渍法制得Pt/Al2O3催化剂, 并应用于光热协同催化CO2加氢反应. 结果证明, 在光热协同CO2加氢催化反应中, Pt/Al2O3催化
剂表现出光热协同效应. 本文结合原位漫反射红外光谱(operando DRIFTS)和密度泛函理论计算(DFT)对光照因素对该催
化反应过程的作用机制进行了进一步深入研究. 结果表明, CO气体分子从Pt纳米颗粒上的脱附过程为CO2加氢反应的重要
步骤; CO气体分子在Pt纳米颗粒上脱附的位置包含台阶位置(Ptstep)和平台位置(Ptterrace). 结果表明, 反应过程中CO气体分子
从Pt表面的脱附有利于催化剂暴露出Pt反应活性位点. 值得注意的是, 在光热协同催化CO2加氢反应过程中, 光照和温度因
素对CO气体分子的脱附过程具有不同影响. 吸附能的计算结果证明, CO气体分子吸附在Ptstep和Ptterrace上的吸附能分别为
-1.24和-1.43 eV. 由此可见, CO气体分子与Pt纳米颗粒上的Ptstep吸附位点之间相互作用更强. 在无光照作用的条件下对催
化剂进行加热, CO气体分子更容易从Ptterrace吸附位点发生脱附; 但是在对应的温度下加入光照作用后, 吸附在Ptstep位点上
的CO气体分子会先转移到Ptterrace吸附位点上, 随后脱附, 从而促进CO2加氢反应的进行.
关键词: 二氧化碳加氢; 光热催化; 铂/三氧化二铝; 原位漫反射红外光谱; 密度泛函理论
收稿日期: 2019-05-14. 接受日期: 2019-07-05. 出版日期: 2020-02-05.
*通讯联系人. 电话: (028)83037401; 传真: (028)83037406; 电子信箱: yzhou@swpu.edu.cn
#通讯联系人. 电话: +49-721-60848090; 传真: +49-721-60844820; 电子信箱: dmitry.doronkin@kit.edu
基金来源: 国家自然科学基金(U1862111, U1232119); 四川省国际合作项目(2017HH3030); 四川省创新团队(2016TD00 11).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).