Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis from CO2 Hydrogenation

Article abstract of DOI:10.1007/s10562-017-2192-4

Abstract: Hydrogenation of the greenhouse gas CO₂ to higher alcohols through catalysis holds great promise for resource transformation in low-carbon energy supply system, but the efficient and selective synthesis of value-added ethanol by a robust heterogeneous catalyst under relatively mild conditions remains a major challenge. Based on our previous work on Au/TiO₂ as an active and selective catalyst for ethanol synthesis, we report here that a facile photochemical route can be used for the preparation of anatase TiO₂ supported gold catalyst (Au/a-TiO₂) for efficient hydrogenation of CO₂. Compared with the conventional deposition-precipitation method requiring strong br?nsted base and flammable H₂ gas in the complicated and time-consuming process, the photochemical way for the facile preparation of supported gold catalyst shows the advantages of green and energy-saving. Of significant importance is that an impressive space-time-yield of 869.3?mmol?g_Au^?1?h^?1, high selectivity, and excellent stability can be readily attained at 200?°C and total pressure of 6?MPa. The effects of irradiation time, solvent, and metal loading or Au particle size on ethanol synthesis are systematically investigated. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2192-4

Catal Lett
DOI 10.1007/s10562-017-2192-4
Photochemical Preparation of Anatase Titania Supported Gold
Catalyst for Ethanol Synthesis from CO2 Hydrogenation
Dong Wang1,2,3,4 · Qingyuan Bi1 · Guoheng Yin1 · Peng Wang1 · Fuqiang Huang1,3,5
·
Xiaoming Xie2,3 · Mianheng Jiang2,3
Received: 22 May 2017 / Accepted: 15 September 2017
© Springer Science+Business Media, LLC 2017
Abstract Hydrogenation of the greenhouse gas CO2 to
higher alcohols through catalysis holds great promise for
resource transformation in low-carbon energy supply system,
but the efficient and selective synthesis of value-added etha-
nol by a robust heterogeneous catalyst under relatively mild
conditions remains a major challenge. Based on our previ-
ous work on Au/TiO2 as an active and selective catalyst for
ethanol synthesis, we report here that a facile photochemical
route can be used for the preparation of anatase TiO2 sup-
ported gold catalyst (Au/a-TiO2) for efficient hydrogenation
of CO2. Compared with the conventional deposition-precipi-
tation method requiring strong brønsted base and flammable
H2 gas in the complicated and time-consuming process, the
photochemical way for the facile preparation of supported
gold catalyst shows the advantages of green and energy-sav-
ing. Of significant importance is that an impressive space-
−1
time-yield of 869.3 mmol gAu h−1, high selectivity, and
excellent stability can be readily attained at 200 °C and total
pressure of 6 MPa. The effects of irradiation time, solvent,
and metal loading or Au particle size on ethanol synthesis
are systematically investigated.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2192-4) contains supplementary
material, which is available to authorized users.
* Qingyuan Bi
biqingyuan@mail.sic.ac.cn
* Fuqiang Huang
huangfq@mail.sic.ac.cn
1
State Key Laboratory of High Performance Ceramics
and Superfine Microstructure, Shanghai Institute
of Ceramics, Chinese Academy of Sciences,
Shanghai 200050, People’s Republic of China
2
State Key Laboratory of Functional Materials
for Informatics, Shanghai Institute of Microsystem
and Information Technology, Chinese Academy of Sciences,
Shanghai 200050, People’s Republic of China
3
School of Physical Science and Technology, ShanghaiTech
University, Shanghai 200031, People’s Republic of China
4
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
5
Beijing National Laboratory for Molecular Sciences
and State Key Laboratory of Rare Earth Materials Chemistry
and Applications, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871,
People’s Republic of China
Vol.:(0123456789)
1 3

D. Wang et al.
Graphical Abstract
high selectivity to HAs, and robust stability on the reaction
conditions of 200 °C and high total pressure of 8 MPa [18].
However, besides the inherent inconvenience accompanying
with the handling problems, this catalytic process requires
large amount of the indispensable solvent of complicated
1,3-dimethyl-2-imidazolidinone (DMI) [18]. Most recently,
Huang et al. reported a highly ordered Pd–Cu NPs catalyst
(Pd2Cu/TiO2) showing an unprecedented turnover frequency
of 359 h−1 and high selectivity to EtOH (92%) at 200 °C and
3.2 MPa (H2/CO2 =3/1) [19]. Therefore, development of an
efficient heterogeneous catalyst system for EtOH synthesis
from CO2 hydrogenation (Scheme 1) without or with simple
solvent under relatively mild conditions is urgently required.
Gold catalysts have been extensively used in past decades
for many catalytic reactions [20–24]. It is also established
that the small Au NPs can activate stable CO2 and H2 mol-
ecules and show excellent CO-tolerant ability [25]. During
the search for a highly active and reusable catalyst for EtOH
synthesis from CO2 hydrogenation, we have recently found
that a simple Au-based catalyst comprising of ultrasmall
Au nanoclusters dispersed on anatase TiO2 (Au/a-TiO2)
with abundant oxygen vacancies prepared by using depo-
sition–precipitation (DP) method can achieve a high per-
formance for EtOH generation under practical conditions
(200 °C and 6 MPa) [26]. Although the parameters of cata-
lyst preparation and the specific reaction conditions were
systematically studied, the complicated preparation process
consumed strong brønsted base and flammable H2 gas which
are not meet the principles of green chemistry [26]. Moreo-
ver, the time of the DP procedure was too long which can
lead to the unnecessary waste of resources and energy.
Herein, we report an efficient photochemical method for
the preparation of Au/a-TiO2 catalyst for EtOH synthesis.
Compared with the conventional DP method, the photo-
chemical route is greener and more energy-saving and can
spend shorter time for catalyst preparation. Given the well
established fact that the preparation method strongly influ-
ences the performance of supported Au catalyst, the present
work aims to identify the catalytic behavior of the Au/a-TiO2
Keywords Gold catalysis · Photochemical route ·
Anatase titania · Ethanol · CO2 hydrogenation
1 Introduction
Carbon dioxide (CO2), an abundant greenhouse gas, has
been largely used to produce valuable chemicals and fuels,
including methane, formic acid, and alcohols [1–4]. The
hydrogenation of CO2 to yield value-added alcohols (espe-
cially methanol and ethanol) as one of the most important
means for CO2 conversion has been attracted ever-increasing
attentions [5–7]. Compared with methanol, ethanol (EtOH)
and other higher alcohols (HAs) possess higher energy den-
sity and show broader applications in fuel additives, neat
fuels, and raw chemical materials [8, 9]. However, the ther-
modynamic stability and chemical inertness of CO2 mole-
cule severely constrain its practical utilization. Furthermore,
the generation of EtOH from CO2 hydrogenation is much
more difficult than methanol synthesis because of the subse-
quent carbon chain growth process being involved in EtOH
formation. From the practical standpoint, the hydrogenation
of CO2 to EtOH is still poor and requires further develop-
ment to find suitable and efficient catalyst.
In spite of the tremendous efforts devoted to EtOH syn-
thesis via CO2 hydrogenation using homogeneous cata-
lysts [10–12], the use of the indispensable and air-sensitive
organic ligands or additives limits their large-scale applica-
tions. Heterogeneous catalyst can overcome the drawbacks
associated with organic complexes and shows more potential
in industry. Many solid catalysts, such as Rh/SiO2, [Rh10Se]/
TiO2, CoMoS, physically mixed Fe- and Cu-based compos-
ite, and K/Cu-Zn-Fe have been used to efficiently synthesize
EtOH from CO2 hydrogenation [13–17], but most reactions
are conducted at high temperatures (>250 °C) and the low
efficiencies need to be further improved. Han and co-workers
have demonstrated an efficient Co3O4 supported Pt nanopar-
ticles (NPs) catalyst (Pt/Co3O4) with excellent synergistic
effect exhibited a good space-time-yield (STY) of alcohols,
1 3

Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis…
Scheme 1 EtOH synthesis
from CO2 hydrogenation. Com-
HO
pounds surrounded with dotted
line are the possible products of
primary HAs
HO
Cat., solvent
CO2 + H2
HO
+
ƸP, ƸT
HO
OH
systems obtained by using photochemical route. To gain an
insight into the respective nature of photo-stimulated Au/a-
TiO2 catalysts in relation to their performance in EtOH syn-
thesis, extensive characterization by N2 adsorption, X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
high-angle annular dark-filed scanning transmission electron
microscopy (HAADF-STEM), UV–Vis diffuse reflectance
(UV–Vis DR), and diffuse reflectance infrared Fourier trans-
form (DRIFT) has been carried out.
was used instead of sole distilled water, the finally obtained
gold catalyst was denoted as Au/a-TiO2(E).
2.2 Catalyst Characterization
The Brunauer–Emmett–Teller (BET) surface areas of the
prepared catalysts were determined by adsorption–desorption
of nitrogen at − 196 °C, using a Micromeritics TriStar 3000
equipment. Sample degassing was carried out at 300 °C prior
to acquiring the adsorption isotherm. The metal loadings of
the catalysts were measured by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) using iCAP 6300
spectrometer. XRD information of the samples was carried
out on a German Bruker D8 Advance X-ray diffractometer
using nickel filtered Cu Kα radiation at 40 kV and 40 mA.
XPS data were recorded with a Perkin Elmer PHI 5000C sys-
tem equipped with a hemispherical electron energy analyzer.
The spectrometer was operated at 15 kV and 20 mA, and a
magnesium anode (Mg Kα, hν=1253.6 eV) was used. The
2 Experimental
2.1 Catalyst Preparation
All chemicals were supplied by Aladdin and used with-
out further purification. Anatase TiO2 (a-TiO2) was pre-
pared using a hydrothermal method [27] with titanium bis
(ammonium lactate) dihydroxide aqueous solution (TALH,
50%) and urea (99%). Typically, 10 mL TALH and 90 mL
0.1 M urea were mixed under stirring at room temperature.
The resultant solution was then transferred into a Teflon-
lined stainless autoclave, which was sealed and placed in an
electric oven held at 160 °C for 24 h. The precipitate was
separated by filtration and washed thoroughly with distilled
water. After freeze-drying, the solid was obtained and cal-
cined in air at 400 °C for 3 h with a ramp of 2 °C min−1.
A photochemical procedure [28] was employed to prepare
the Au/a-TiO2 samples by illuminating for different times with
a 300 W Xe lamp (Aulight CEL-HX, Beijing) and the power
of the light source is calibrated to AM 1.5 by a NREL-cali-
brated Si cell (Oriel 91,150). Briefly, 0.5 g a-TiO2 powder was
dispersed in 200 mL distilled water and a certain amount of
HAuCl4 (e.g., 0.25 mM) was added to the suspension. After
stirring at room temperature for 3 h, the photodeposition was
performed under light irradiation for 5 s to 30 min. The Au/a-
TiO2 catalyst was separated by filtration, washed several times
with distilled water, and freeze-dried completely. Once the
mixed solution of 150 mL distilled water and 50 mL ethanol
Au/a-TiO2(E)
Au/a-TiO2
a-TiO2
JCPDS 21-1272
10
20
30
40
50
60
70
80
2Theta (Degree)
Fig. 1 XRD patterns of a-TiO2, Au/a-TiO2, and Au/a-TiO2(E) cata-
lysts
1 3

D. Wang et al.
C 1s line (284.6 eV) was used as the reference to calibrate
the binding energies (BE). HAADF-STEM images were
obtained on a JEM 2100F electron microscope operating at
200 kV equipped with a field-emission-gun (FEG) and using
an energy dispersive X-ray (EDX) unit (Si(Li) detector) and a
HAADF detector. The samples for electron microscopy were
prepared by grinding and subsequent dispersing the powder
in ethanol and applying a drop of very dilute suspension on
carbon-coated grids. The size distribution of the metal NPs
was determined by measuring about 200 random particles on
the images. The dispersion (D) of Au NPs can be estimated
according to the relationship between the D and particle size
as the following Eq. (1) [23, 29, 30], where d means the aver-
age size of Au NPs.
UV–Vis DR spectra of the solids diluted in BaSO4 were
recorded at room temperature on a Shimadzu UV 2401PC
Spectrometer equipped with an integrating sphere and using
BaSO4 as reference. The DRIFT experiments were carried out
on a Thermo Nicolet 6700 instrument equipped with a MCT
detector and Harrick diffuse reflectance accessory. Spectra
were obtained on the apparatus loaded with 50 mg of catalyst.
Prior to the DRIFT test, sample was subjected to the pretreat-
ment with Helium flow at 200 °C for removing any other gases
and moisture. Each spectrum was obtained in Helium flow at
room temperature and by subtracting the background (base
spectrum) of the unpretreated sample.
2.3 Catalytic Activity Test
1.0 (nm)
D (% ) =
× 100%
The experiments were carried out in high-pressure stain-
less autoclave reactor (Parr Instrument Co., 4790, 50 mL).
(1)
d (nm)
Fig. 2 HAADF-STEM images and Au particle size distributions of a, b Au/a-TiO2 and c, d Au/a-TiO2(E). The selected Au NPs are marked with
arrows in (a) and (c)
1 3

Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis…
Typically, 5 mL solvent of N,N-dimethylformamide (DMF)
and a known amount of catalyst were placed in the auto-
clave. The autoclave was sealed and flushed many times with
0.5 MPa CO2 to remove the air in the reactor, then CO2 and
H2 gaseous mixtures were charged. The stirrer (800 rpm)
was started until the desired temperature was reached. After
a certain time, the autoclave was placed in cool water and the
gas was carefully released. The gaseous mixture was ana-
lyzed using a gas chromatograph Agilent 7820A equipped
with a TDX-01 column connected to a thermal conductivity
detector. A known amount of internal standard 1,4-dioxane
was added into the aqueous product in autoclave. The reac-
tion mixture was transferred into a centrifuge tube and the
solid catalyst was separated by centrifugation. The prod-
uct solution was quantitatively analyzed using a gas chro-
matograph Agilent 7820A equipped with a HP-5 capillary
column connected to a flame ionization detector. Identifi-
cation of the products was performed by using a GC-MS
spectrometer. For all the experimental data, the total carbon
balance was >95% and the experiments were repeated for
three times to ensure the repeatability of the results. For the
catalyst recycling experiment, the centrifuged catalysts were
collected and washed with distilled water several times, fol-
lowed by freeze-drying. The STY and selectivity to EtOH
were calculated using Eqs. (2–5):
(a)
4f5/2 86.9 eV (Au0)
4f7/2 83.4 eV (Au0)
90
88
86
84
82
Binding energy (eV)
(b)
4f7/2 83.4 eV (Au0)
4f5/2 87.0 eV (Au0)
Generated EtOH (mmol)
STY (mmol g−1 h−1) =
Total Au mass (g) × time (h)
(2)
Generated EtOH (mmol)
STYsurface (mmol g−1 h−1) =
Surface Au mass (g) × time (h)
90
88
86
84
82
Binding energy (eV)
(3)
Fig. 3 XPS data of a Au/a-TiO2 and b Au/a-TiO2(E) catalysts
Surface Au mass (g) = Total Au mass (g) × D (% )
(4)
Generated EtOH (mmol)
SEtOH =
× 100%
(5)
Generated products (mmol)
Au/a-TiO2(E)
Au/a-TiO2
3 Results and Discussion
3.1 Structural Characterization and Surface Properties
The a-TiO2 material with abundant oxygen vacancies and
noticeable specific surface area of 65 m2 g−1 (Fig. S1) is
favourable for the dispersion of the active Au NPs [26].
At the start of this work, we prepared the Au/a-TiO2 and
Au/a-TiO2(E) with the actual Au loading of 0.3 wt% using
photochemical method on light irradiation for 3 min. The
well-defined crystal of anatase phase and the structure of
supported gold catalysts are determined by XRD, and the
corresponding results are shown in Fig. 1. a-TiO2 exhib-
its a series characteristic diffraction peaks at 25.3°, 37.9°,
Ti4+-OH
3430
4000
3800
3600
3400
3200
3000
Wavenumber (cm-1)
Fig. 4 DRIFT spectra of Au/a-TiO2 and Au/a-TiO2(E) catalysts
1 3

D. Wang et al.
48.1°, 53.9°, 55.1°, and 62.7°, which are assigned to anatase
phase (JCPDS 21-1272) [31]. It can be seen from Fig. 1 that
the XRD patterns of these supported Au catalysts are very
similar with a-TiO2 material. There are no visible changes
on structure after Au loading, and the absence of any Au-
containing phases indicates the low Au loading and high
dispersion of the Au species.
ca. 87.0 eV for Au 4f5/2 and 83.4 eV for Au 4f7/2 clearly
demonstrate that there is exclusive metallic gold on the sur-
face of Au/a-TiO2 and Au/a-TiO2(E) samples (Fig. 3) [32].
Furthermore, the DRIFT band of 3430 cm−1, as depicted
in Fig. 4, assigned to the stretching vibration of H-bound
OH groups in Ti4+–OH (oxygen defects) indicates that Au/a-
TiO2 exhibits more oxygen vacancies than Au/a-TiO2(E) and
thus shows stronger metal-support interactions in Au–a-TiO2
interface [26]. It is widely demonstrated that the defects like
oxygen vacancies can act as reactive perimeter sites at the
solid–solid interface (e.g., Au–a-TiO2), and they can also
in turn provide numerous highly mobile oxygen-containing
species being able to react with CO2 [33, 34].
HAADF-STEM images in Fig. 2 and STEM-EDS map-
pings in Fig. S2 show that the Au NPs have been formed and
homogeneously anchored on the a-TiO2 surface. And the cat-
alyst made in the solvent of water possesses slightly smaller
Au NPs than that in the mixed solvent of EtOH-water sys-
tem (2.0 vs. 2.6 nm). Meanwhile, the binding energies of
Fig. 5 HAADF-STEM images of Au/a-TiO2 catalysts prepared on the irradiation time of a 5 s, b 1 min, c 10 min, and d 30 min. The selected
Au NPs are marked with arrows
1 3

Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis…
To further explore the effect of light irradiation and sol-
vent on supported gold catalyst, we prepared the series of
Au/a-TiO2 and Au/a-TiO2(E) samples by changing the irradi-
ation time ranging from 5 s to 30 min. Irradiation time shows
no visible effects on structure of gold catalysts (Fig. S3),
but can significantly affect the Au particle size, as shown
in Figs. 5 and 6. At the initial period of 5 s to 1 min, many
small nuclei generated, and then aggregated to form Au
NPs. Particle size of Au/a-TiO2 is smaller than Au/a-TiO2(E)
in the irradiation time of within 3 min. When the irradia-
tion time was prolonged further, the particles with larger
diameters were obtained, for example 8.6 and 7.2 nm at the
irradiation time of 30 min for Au/a-TiO2 and Au/a-TiO2(E),
respectively. The UV–Vis spectra in Fig. 7 clearly show
the apparent absorption peaks around 540 nm in the visible
region for Au/a-TiO2 and Au/a-TiO2(E) catalysts due to the
surface plasmon resonance of Au NPs [35]. The loading of
Au NPs can also affect the shape of absorption peak remark-
ably. For Au/a-TiO2 samples, the peak width increases with
irradiation time>3 min, probably due to the larger diameter
of Au NPs. However, the peak width of Au/a-TiO2(E) cata-
lysts is insensitive to the prolongation of irradiation time.
EtOH as a solvent is beneficial for the transfer of photogen-
erated electrons [36], which can induce the formation of
Au NPs in shorter time (< 1 min) relative to that in sole
water during preparation process. Furthermore, numerous
Fig. 6 HAADF-STEM images of Au/a-TiO2(E) catalysts prepared on the irradiation time of a 5 s, b 1 min, c 10 min, and d 30 min. The selected
Au NPs are marked with arrows
1 3

D. Wang et al.
1000
800
600
400
200
0
(a)
(a)
a-TiO2
Au/a-TiO2, 5 s
Au/a-TiO2, 30 s
Au/a-TiO2, 1 min
Au/a-TiO2, 3 min
Au/a-TiO2, 10 min
Au/a-TiO2, 30 min
5 s
30 s 1 min
Irradiation time
3 min
10 min 30 min
300
400
500
600
700
Wavelength (nm)
(b)
(b)
800
600
400
200
0
a-TiO2
Au/a-TiO2(E), 5 s
Au/a-TiO2(E), 30 s
Au/a-TiO2(E), 1 min
Au/a-TiO2(E), 3 min
Au/a-TiO2(E), 10 min
Au/a-TiO2(E), 30 min
5 s
3 min
10 min 30 min
30 s 1 min
Irradiation time
300
400
500
600
700
Wavelength (nm)
Fig. 8 Catalytic activities of a Au/a-TiO2 and b Au/a-TiO2(E) pre-
pared on different irradiation time. Reaction conditions: 100 mg cata-
lyst, 5 mL DMF, 45 bar H2 and 15 bar CO2 at 25 °C, 200 °C, 10 h.
The standard deviation error bars are shown
Fig. 7 UV–Vis spectra of a Au/a-TiO2 and b Au/a-TiO2(E) prepared
on different irradiation time
dissolved oxygen in EtOH solution can consume the pho-
togenerated electrons with the irradiation time of more than
3 min, thus preventing the accumulation of the electrons and
the subsequent growth of Au NPs [37].
100
800
80
3.2 Catalytic Hydrogenation of CO2 for EtOH
Synthesis
600
60
40
20
400
The catalytic properties of the Au/a-TiO2 and Au/a-TiO2(E)
catalysts prepared on different irradiation time for EtOH
synthesis from CO2 hydrogenation were evaluated on the
optimal conditions of 45 bar H2 and 15 bar CO2 in DMF
at 200 °C, which were demonstrated in our previous work
[26]. With the irradiation time increasing, Au/a-TiO2 cata-
lyst exhibits the enhanced activities and achieves the highest
STY of 869.3 mmol gAu−1 h−1 at 3 min, as shown in Fig. 8a;
however, the prolonged irradiation time of 10–30 min are
found to give a negative effect on hydrogenation kinetics
200
0
0
1
2
3
4
5
6
Durability times
Fig. 9 Recycling of Au/a-TiO2 catalyst for EtOH synthesis from CO2
hydrogenation. Reaction conditions: 100 mg catalyst, 5 mL DMF,
45 bar H2 and 15 bar CO2 at 25 °C, 200 °C, 10 h
1 3

Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis…
mainly due to the increased Au NPs. Noted that the maxi-
mum activity is comparable with the best ever-reported
results for EtOH synthesis from CO2 hydrogenation on the
identical conditions [18, 19, 26]. The Au/a-TiO2 catalysts
also show excellent selectivity to the target product of EtOH
(>99%). The intrinsically electronic/geometric structure of
Au NPs, suitable surface acid-base properties of a-TiO2
support, and the strong Au–a-TiO2 interactions endow the
Au/a-TiO2 catalyst with thermal catalytic functions of high
activity and exceptional EtOH selectivity. On the other hand,
Au/a-TiO2(E) catalysts present the similar tendency as Au/a-
TiO2 on hydrogenation activity, but with the highest rate of
3.3 Effect of Au Loading on EtOH Synthesis
It is well known that the size of Au NPs can significantly
affect their catalytic behaviors in many reactions [23, 38,
39]. It was found that the size of Au NPs in Au/a-TiO2 cata-
lysts could be easily regulated by changing the concentration
of the Au precursor from 0.05 to 0.25 mM with the deposi-
tion efficiency of 75–80% and/or the Au loading in the range
of 0.3 to 1.5 wt% using photochemical method on irradiation
time of 3 min. The initial HAuCl4 solution with higher con-
centration or higher Au amount in the solution can accelerate
the adsorption of Au-containing species on the underlying
a-TiO2 support during the stirring and subsequently improve
the surface density of Au atoms in the preparation process
[38]. The larger Au NPs can be formed with instaneous
nucleation and continuous growth due to the accumulation
of Au atoms. There are no visible changes on the structure
with Au loading, as shown in Fig. 10, and the absence of any
Au-containing phases suggests the high dispersion of the Au
species even in high loadings. The particle size of Au/a-TiO2
catalysts increases from 2.0 0.2 to 6.9 0.4 nm with the Au
loading of 0.3 to 1.5 wt% (Figs. 2a, 11; Table 1).
−1
797.8 mmol gAu h−1 at the irradiation time of 1 min, as
depicted in Fig. 8b. The increased Au NPs in Au/a-TiO2(E)
can also lead to the reduced performance. Compared with
Au/a-TiO2(E), the Au/a-TiO2 catalysts show slightly higher
performance for EtOH synthesis (e.g., the STYs of the cata-
lysts prepared on the irradiation time of 3 min) attributed to
the stronger metal-support interactions in Au/a-TiO2 sample,
which are consistent well with the DRIFT results demon-
strated in Fig. 4. The possible reaction pathway for EtOH
synthesis via CO2 hydrogenation over Au/a-TiO2 solid was
proposed, as shown in Scheme S1. In the process of EtOH
generation, the CO2 was firstly hydrogenated to –CH3 spe-
cies, and then followed by CO insertion to form the inter-
mediates of –CH3CO, which were further hydrogenated into
ethanol and subsequently desorbed from the catalyst surface
[13, 17, 18, 26].
Table 1 lists the effect of Au loading on the particle
size of Au/a-TiO2 catalyst and further on the activity
for EtOH synthesis. It can be seen that the higher load-
ing of gold element, the larger size of the Au NPs. With
the increase of Au NPs, the Au/a-TiO2 catalyst exhib-
its decreased performance for EtOH synthesis markedly
−1
Having established that Au/a-TiO2 as a reliable and effi-
cient catalyst for EtOH synthesis from CO2 hydrogenation
under relatively mild conditions, we further investigated the
reusability of the Au catalyst. As shown in Fig. 9, Au/a-TiO2
is recoverable by simple treatment and proven robust even
after six reuses. Noteworthy, the STY of EtOH generation
in the 2nd run is only slightly lower than the fresh cata-
lyst in the 1st run and keeps constant in the later recycling.
Moreover, there is no significant loss of the selectivity to the
target EtOH in the whole recycling experiments. Au/a-TiO2
catalyst was removed from the reaction system after 5 h.
Further processing of the filtrate on the reaction conditions
for another 5 h did not result in any conversion. In addition,
ICP-AES analysis of the filtrate indicates the content of Au
or Ti leached is below the detection limit. These data con-
firm that the Au-catalyzed hydrogenation reaction occurs
on the solid surface and the heterogeneous catalysis nature
of Au/a-TiO2 catalyst. Besides Au/a-TiO2, the Au/a-TiO2(E)
catalyst also exhibits excellent stability (Fig. S4). Compared
with the fresh catalysts, there is no significant change on
morphology and no aggregation of Au NPs for the used
counterparts (Figs. S5 and S6). These results suggest that
a-TiO2 supported Au NPs are very robust for EtOH synthesis
from CO2 hydrogenation.
from the STY of 869.3 mmol gAu h−1 for 2.0 0.2 nm
−1
(0.3 wt%) to 393.4 mmol gAu h−1 for 4.7 0.3 nm
(1.2 wt%), as demonstrated in Table 1 and Fig. 12. The
catalytic ability of 6.9 0.4 nm Au NPs with loading
of 1.5 wt% supported on a-TiO2 is the lowest, showing
−1
the STY of only 272.8 mmol gAu h−1 on the identical
1.5 wt%
1.2 wt%
0.9 wt%
0.6 wt%
0.3 wt%
10
20
30
40
50
60
70
80
2Theta (Degree)
Fig. 10 XRD patterns of Au/a-TiO2 catalysts with different Au load-
ings
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D. Wang et al.
Fig. 11 HAADF-STEM images of Au/a-TiO2 catalysts with Au loading of a 0.6, b 0.9, c 1.2, and d 1.5 wt%. The selected Au NPs are marked
with arrows
conditions. Note that these Au catalysts show no signifi-
cant loss on EtOH selectivity. Interestingly, there is an
intrinsic consistency of the correlation on the Au load-
ing or the size effect of Au NPs between hydrogenation
performance and the surface gold active sites, as depicted
in Fig. 12. The difference from the apparent activity
of EtOH generation (STY) and the intrinsic activity
(STYsurface) is ascribed to the different amount of active
sites in calculation. It is widely accepted that the hetero-
geneous catalytic reactions are occurred on the surface or
interface of solid samples and only surface metal species
are the actual active sites [3, 23, 30]. The intrinsic activity
in EtOH synthesis indicates the origin of gold catalysis
in small molecule (e.g., CO2) activation and transforma-
tion. These results suggest that the Au-catalyzed EtOH
synthesis from CO2 hydrogenation is a typical particle-
size-dependent reaction.
4 Conclusions
We have demonstrated that an efficient anatase titania sup-
ported Au NPs catalyst was prepared using photochemical
route for EtOH synthesis from CO2 hydrogenation under
relatively mind conditions. Compared with the conven-
tional deposition–precipitation method requiring strong
1 3

Photochemical Preparation of Anatase Titania Supported Gold Catalyst for Ethanol Synthesis…
Compliance with Ethical Standards
Table 1 Effect of Au loading on Au/a-TiO2
catalyst for EtOH syn-
thesis
Conflict of interest The authors declare that they have no conflict
Entry Au
Average size
Dispersion (%) STY
(mmol gAu−1 h−1
of interests.
loading of Au NPs
)
(wt%)a
(nm)b
1
2
3
4
5
0.3
0.6
0.9
1.2
1.5
2.0 0.2
2.4 0.2
2.9 0.3
4.7 0.3
6.9 0.4
50.0
41.7
34.5
21.3
14.5
869.3
751.4
618.1
393.4
272.8
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