Supercritical synthesis of platinum-modified titanium dioxide for solar fuel production from carbon dioxide

Article abstract of DOI:10.1016/S1872-2067(17)62766-9

This paper investigates the properties of TiO₂-based photocatalysts synthesised under supercritical conditions. Specifically, the characteristics of Pt dispersed on TiO₂ catalysts obtained in supercritical CO₂ are discusse

Full text of DOI:10.1016/S1872-2067(17)62766-9

Chinese Journal of Catalysis 38 (2017) 636–650
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
Supercritical synthesis of platinum‐modified titanium dioxide for
solar fuel production from carbon dioxide
Susana Tostón, Rafael Camarillo*, Fabiola Martínez, Carlos Jiménez, Jesusa Rincón
University of Castilla‐La Mancha, Department of Chemical Engineering, Faculty of Environmental Sciences and Biochemistry, Avda. Carlos III, s/n, 45071
Toledo, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
This paper investigates the properties of TiO2‐based photocatalysts synthesised under supercritical
conditions. Specifically, the characteristics of Pt dispersed on TiO2 catalysts obtained in supercritical
CO2 are discussed and compared with those of commercial TiO2. The photocatalytic activity of the
synthesised catalysts in the CO2 photoreduction reaction to produce solar fuel is tested. The main
conclusion of the study is that photocatalysts with better or similar features, including high surface
area, crystallisation degree, hydroxyl surface concentration, pore volume, absorbance in the visible
range and methane production rate, to those of commercial TiO2 may be produced for the reduction
of CO2 to fuel by synthesis in supercritical media.
Received 21 November 2016
Accepted 21 December 2016
Published 5 April 2017
Keywords:
Titanium dioxide
Platinum
Photocatalyst
Metal dispersion
Carbon dioxide photoreduction
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
(i.e., catalysts for solar fuel production from CO2), a process
with an enormous future potential despite being in the early
The increasing atmospheric concentration of greenhouse
gases, especially CO2, is a pressing social issue at present [1].
Different processes to capture gas from large point sources
such as the flue gases of coal, oil, natural gas and biomass pow‐
er plants have been patented in recent years [2,3]. The recov‐
ered CO2 can be either stored in natural caves or used as feed‐
stock to produce useful chemicals, especially fuel, which is the
only CO2 conversion product that may substantially lower an‐
thropogenic CO2 emissions because of its high rate of consump‐
tion. However, because the CO2 molecule is very stable, only a
few technologies for its conversion are available. In particular,
effective photocatalytic conversion of CO2 to fuel has been
demonstrated [4,5]. Thus, following this previous work, the
present study focuses on the synthesis of catalysts for the pho‐
tocatalytic conversion of CO2 gas into fuel using solar energy
stages of development.
The main drawbacks currently limiting photocatalytic CO2
reduction are low photoconversion speed and efficiency. These
problems may be overcome through the design of highly active
photocatalysts with favourable reactant adsorption, charge
separation and transport, light harvesting and CO2 activation
[6–14]. TiO2 particles show most of these features along with
non‐toxicity, high photostability, chemical inertness, environ‐
mentally friendly nature and low cost. Thus, TiO2 is a promising
material for use as a catalyst in CO2 photoreduction.
The main weakness of TiO2 is that it only uses the ultraviolet
(UV) region of the solar spectrum, which is less than 5% of the
total solar energy. Moreover, after UV irradiation of TiO2 with
an energy equal or larger than its band gap (3.2 eV), the result‐
ing photogenerated electron–hole pairs rapidly recombine.
* Corresponding author. E‐mail: rafael.camarillo@uclm.es
This work was supported by Spanish Government (Project CTM 2011‐26564), Regional Government of Castilla‐La Mancha (Project
PEII10‐0310‐5840), and Iberdrola Foundation (Research Grant in Energy and the Environment 2010/12 for Susana Tostón).
DOI: 10.1016/S1872‐2067(17)62766‐9 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 4, April 2017

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
637
However, both problems can be tackled with appropriate dis‐
persion or doping of TiO2 with either noble or transition metals
[7,9,10,12,15–17]. The dispersion method leads to the scatter‐
ing of metal particles on the TiO2 support, while doping in‐
volves the inclusion or substitution of foreign metal atoms into
the TiO2 lattice [10,18].
The most commonly ascribed effect of Pt as a dopant for
TiO2 is its ability to promote charge carrier separation because
electrons tend to accumulate on the doped Pt and holes remain
on TiO2. Pt also shifts the band edges of TiO2 to make certain
electron transfer processes more favourable in the vicinity of
the metal atom. In addition, Pt increases the electron scaveng‐
ing capability of O2, removes strongly bound intermediates,
promotes hydroxyl (OH) formation and promotes H+ reduction
to adsorbed H atoms [18]. However, the influence of Pt on TiO2
photocatalysis is not always positive because Pt atoms may act
as charge recombination centres or block active sites on TiO2.
Moreover, Pt is a good centre for hydrogen (H2) generation
from H2O; therefore, it is necessary to take measures to mini‐
mise this process [19].
thesis procedures, which usually use large amounts of organic
solvents.
The objective of this investigation is to use a supercritical
medium to synthesise a TiO2‐based catalyst with superior per‐
formance to that of a commercial semiconductor in the photo‐
catalytic reduction of CO2 to fuel molecules. Specifically, Pt dis‐
persed on TiO2 is synthesised by a hydrothermal method using
supercritical CO2. Pt(II) acetylacetonate and titanium tetrai‐
sopropoxide
(TTIP)
or
diisopropoxy
titanium
bis(acetylacetonate) (DIPBAT) are used as chemical precursors
of Pt and TiO2, respectively, with isopropyl alcohol or ethanol
as a hydrolytic agent. The synthesis involves the following pro‐
cess. Once the reagents are in the supercritical phase, precursor
decomposition occurs and alcohol decomposition provides the
necessary H2O for the hydrolysis reaction [33,34]. The end
products are Pt dispersed on TiO2 solid particles and carbona‐
ceous contaminants originating from the precursors. The de‐
compressed solvent is in gas phase, which facilitates catalyst
drying and recovery at the end of the process. To remove car‐
bon (C) contaminants from the catalyst, a calcination step is
performed after supercritical synthesis [35].
The properties of the Pt/TiO2 catalysts are determined by
usual characterisation methods, including scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
X‐ray photoelectron spectroscopy (XPS), atomic emission
spectroscopy with inductively coupled plasma (ICP‐AES), X‐ray
diffraction (XRD), N2 adsorption‐desorption measurements,
diffuse‐reflectance UV‐visible (DRUV‐vis) spectroscopy, Fourier
transform infrared (FTIR) spectroscopy and laser diffraction,
and compared with those of commercial TiO2. Their photocata‐
lytic activity in CO2 photoreduction to produce solar fuel is also
tested.
The amount of Pt added to TiO2 can also play a major role in
catalyst performance. The typical optimal Pt loading is around
1 wt% [9,18]. Higher metal contents can induce faster elec‐
tron–hole recombination and deactivate the photocatalyst
[10,20]. Table 1 summarises details of some recent Pt/TiO2
catalysts. Pt concentration is typically in the range of 0.2–5
wt%.
Many studies have focused on how metal dispersion meth‐
ods affect the photocatalytic behaviour of catalysts [10,18].
Several techniques have been used to disperse Pt atoms on
TiO2 substrates [21–25,27]. One method is co‐precipitation in
supercritical fluids because if a metal precursor is added to a
reaction medium together with a Ti precursor and a hydrolysis
agent, a metal dispersed on TiO2 catalyst can be obtained in situ
[28]. The use of supercritical fluids, mainly CO2 and H2O, for
particle generation and precipitation is attractive because of
their excellent properties [29,30]—they can diffuse through
solids like a gas and dissolve materials like a liquid—and their
ability to be adjusted by simply changing the operating param‐
eters [31,32]. Both the fluid properties and easy tuning of su‐
percritical fluids allow particle characteristics such as struc‐
ture, morphology, size and size distribution to be controlled. All
of these characteristics are very important for the final applica‐
tion of a catalyst [28]. Moreover, synthesis using supercritical
fluids is more environmentally sustainable than classical syn‐
2. Experimental
2.1. Chemicals
Various samples of Pt dispersed on TiO2 powder were syn‐
thesised by thermal hydrolysis of two different precursors
(DIPBAT and TTIP) with two different alcohols in the presence
of Pt(II) acetylacetonate using supercritical CO2 as the reaction
medium. DIPBAT (75 wt% in isopropyl alcohol), TTIP (pure)
and Pt(II) acetylacetonate (97 wt%) were provided by Sig‐
ma‐Aldrich. Analytical reagent‐grade ethanol and isopropyl
alcohol were provided by Scharlab. In all analyses, Degussa
Table 1
Overview of Pt/TiO2‐based catalysts.
Support
Pt concentration (wt%)
Method
Ref.
TiO2 anatase
TiO2 anatase
0.5–2
5
Impregnation + air drying + calcination (450 °C, 4 h) + H2 reduction
Mixture of TiO2 precursor and dopant solutions + drying + gel grinding + calcination [22]
(500 °C, 5 h)
[21]
Mesoporous TiO2 thin films
0.5–3
0.2–1
0.15
Evaporation induced self‐assembly
[23]
[24]
[25]
[20]
[26]
TiO2
TiO2
TiO2
TiO2
Sol‐gel
Sol‐gel
1
Two step hydrothermal route
Sol‐gel
0.1–0.4

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Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
2θ/( o
)
Fig. 1. SEM image (left), XRD pattern (middle) and FTIR spectrum (right) of commercial P‐25 catalyst.
P‐25 (Evonik) TiO2 powder was used as a reference. Some
characteristics of this commercial catalyst are shown in Fig. 1
for comparison with those of the catalysts synthesised in this
work. Supercritical CO2 (purity >99.9%, Carburos Metálicos
S.A.) was used as received.
Pt concentrations in the range of 1–3 wt%. Catalyst synthesis
was performed for 2 h once the experimental conditions were
reached. All syntheses were repeated three times. After the
synthesised catalysts were removed from the reactor, they
were dried at 105 °C for 12 h and then calcined at 400 °C for 6 h
to remove any C contaminants from the catalysts [35].
2.2. Synthesis of catalysts
2.3. Catalyst characterisation
The experimental set‐up depicted schematically in Fig. 2
was used to synthesise photocatalysts in supercritical CO2 in
discontinuous mode. The set‐up consisted of a high pressure
pump (Thar SFC, P‐series) preceded by a thermostated bath
(Selecta, Frigiterm‐30) and followed by a high pressure synthe‐
sis reactor (DEMEDE Engineering & Research, 100 mL). To
synthesise the catalysts, the alcohol and Ti and Pt precursors
were first added to the reactor. The system was sealed and then
CO2 was pumped into the reactor after being cooled in the
thermostated bath. The high pressure pump and an electric
resistor were used to reach and maintain the operating pres‐
sure and temperature in the reactor. After synthesis, the system
was depressurised by opening valve V3 and the catalyst was
collected from the reactor. Further details of this procedure are
given elsewhere [34].
Catalyst synthesis was conducted at a pressure of 20 ± 0.2
MPa and temperature of 300 ± 5 °C using a molar ratio of 28
mmol alcohol/mmol precursor except for in the case of the
ethanol/DIPBAT combination of reactants. Given that DIPBAT
is a solution in isopropyl alcohol, the molar ratio used in this
case was 28 mmol ethanol/mmol DIPBAT, although the test
was actually performed in an alcohol mixture [33]. An appro‐
priate amount of Pt(II) acetylacetonate was added to give final
The TiO2‐based catalysts were characterised by different
techniques. The real percentage of Pt was measured by ICP‐AES
(Varian, Liberty Sequential). The detection limit of the ICP
spectrometer was 20 ppb Pt. The particle size and external
morphology of the particles were observed by SEM using a
scanning electron microscope (JEOL, 6490 LV). TEM and
high‐resolution TEM were measured with a JEOL 2100 TEM
operating at 200 kV equipped with a side‐entry double‐tilt (±
25°) sample holder and energy‐dispersive spectroscopy detec‐
tor (Oxford Link). XPS was conducted using an ultra‐
high‐vacuum Specs Phoibos‐150 electron spectrometer. The
spectra were obtained with a photon energy of 1486.6 eV (Al
anode). All binding energies were referenced to the C 1s peak
originating from surface adventitious C at 284.6 eV. The crys‐
tallinity and crystalline phase of the catalysts were determined
by powder XRD using an X‐ray diffractometer (Philips, X´Pert
MPD). The crystallite sizes of the TiO2 photocatalysts were es‐
timated via the Scherrer equation using the peak at 2 = 25.4°.
The specific surface area of the powders was evaluated using a
Brunauer‐Emmett‐Teller (BET) area analyser (Micromeritics,
ASAP 2020). DRUV‐vis spectra of all catalysts were obtained on
a DRUV‐vis spectrophotometer (JASCO, V650). Absorbance
thresholds and band gap energies (Eg) were calculated from
these spectra. FTIR spectra of all samples were obtained with a
FTIR attenuated total reflectance spectrometer (Thermo Ni‐
colet, Avatar 370 FT‐IR). Particle size and particle size distribu‐
tion were calculated using a laser scattering particle size dis‐
tribution analyser (Malvern, Mastersizer 2000).
2.4. Photocatalytic reaction tests
The photocatalytic activities of the catalysts in photocata‐
lytic CO2 reduction experiments were assessed using an ex‐
perimental set‐up consisting of a stainless steel chamber (50
cm3) (Fig. 3) with valves for evacuation, gas introduction and
Fig. 2. Experimental system used to synthesise TiO2‐based catalysts.
connection to
a gas chromatography (GC) system, an

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
639
Fig. 3. Details of the photocatalytic reactor. (a) 50‐cm3 stainless steel chamber; (b) O‐ring‐sealed quartz window; (c) filter with catalyst sample; (d)
Air Mass 1.5 Global filter; (e) Xe arc lamp.
O‐ring‐sealed quartz window at the top to admit radiation from
a Xe arc lamp (Oriel, 450 W) with an Air Mass 1.5 Global filter
and a dew point transmitter and different manometers to
measure relative humidity and pressure, respectively, during
the reaction. A bubbler containing deionised H2O was posi‐
tioned before the reaction chamber. The beam of the lamp was
diverged with a collimator with the aim of having the inside of
the reaction chamber at the same irradiance as that of the sun
(100 mW/cm2) to simulate solar radiation. It is possible that
this irradiance decreased slightly with the use of the lamp, but
this should have been negligible according to the manufacturer.
Additional information can be found in a previous study [34].
Most experiments described in this paper were performed
with a H2O vapour/CO2 ratio of 2:7, although other ratios were
also assessed (2:20 and 2:1). The H2O vapour/CO2 ratio was
increased by using the same amount of H2O vapour and de‐
creasing the amount of CO2. However, to operate the photore‐
actor at constant pressure, appropriate quantities of an inert
gas (He) were fed into the system. We operated the system in
this way because if we tried to increase the H2O vapour/CO2
ratio by increasing the partial pressure of H2O vapour while
keeping the CO2 amount constant, the H2O vapour saturation
pressure was reached and H2O condensed. Other variables like
catalyst weight (34.6–75.5 mg) and reaction time (3–4 h) were
also evaluated. The initial absolute pressure in the reactor was
1.07 bar in all experiments.
In previous experiments with Degussa‐P25 conducted at a
higher H2O/CO2 ratio (2 g H2O/g CO2) than in the present work,
CH4, CO and H2 production rates were 0.025, 1.233 and 0.005
µmol g–1 h–1, respectively. Therefore, the amount of H2 gener‐
ated during the experiments described in the present work
should be less than 0.005 µmol g–1 h–1.
3. Results and discussion
3.1. Synthesis yield
The yield is the ratio between the moles of catalyst pro‐
duced during synthesis with supercritical CO2 and the moles of
precursor used, taking into account that the synthesis reaction
involves a 1:1 ratio for both precursors (1 mol precursor : 1
mol product). The yields obtained are presented in Table 2.
Reported yields are the average of triplicate experiments. The
two highest yields (85.5% and 72.0%) were achieved when
TTIP was used. Furthermore, the average yield attained in the
six experiments performed with TTIP was higher than the av‐
erage yield of the six runs carried out with DIPBAT (66.5% with
TTIP vs. 62.4% with DIPBAT). In principle, these results could
be attributed to the higher thermal and kinetic stability of
DIPBAT compared with that of TTIP. TTIP is easily hydrolysa‐
ble, even at ambient temperature, in the presence of humidity
because it has four isopropoxy ligands coordinated to its Ti
To quantitatively and qualitatively measure the different
species in the reaction chamber, a GC analytical method was
developed. The gas chromatograph (Agilent GC 7890A) pos‐
sessed two thermal conductivity (TCD) detectors and a flame
ionisation detector (FID) with a methaniser, which made it
possible to determine gases such as CO, CO2, CH4, light hydro‐
carbons (C1–C7), light alcohols, ethers and ketones. H2 could
not be analysed in the system because of technical limitations.
Specifically, because He was used to increase the H2O va‐
pour/CO2 ratio in the photocatalytic reactor, H2 and He were
both present in the gas stream leaving the photoreactor. These
gases could not be quantified separately because the signals for
both He and H2 were detected in the same channel and with
similar retention times. Nevertheless, according to the litera‐
ture [36], in all the experiments performed in this work, the H2
produced should be negligible.
Table 2
Synthesis yields of different catalysts.
Precursor
(mmol)
TiO2
(mmol) (%)
Yield
Combination
TTIP‐isopropyl alcohol‐1%Pt
TTIP‐isopropyl alcohol‐2%Pt
TTIP‐isopropyl alcohol‐3%Pt
TTIP‐ethanol‐1%Pt
4.889 ± 0.003 2.8 ± 0.4 57 ± 8
4.889 ± 0.003 3.1 ± 0.4 63 ± 8
4.889 ± 0.003 3.2 ± 0.4 66 ± 8
4.889 ± 0.003 4.2 ± 0.4 86 ± 8
4.889 ± 0.003 2.7 ± 0.4 55 ± 8
4.889 ± 0.003 3.5 ± 0.4 72 ± 8
5.078 ± 0.002 2.9 ± 0.4 57 ± 8
5.078 ± 0.002 3.0 ± 0.4 59 ± 8
5.078 ± 0.002 3.3 ± 0.4 65 ± 8
5.078 ± 0.002 3.1 ± 0.4 61 ± 8
5.078 ± 0.002 3.4 ± 0.4 67 ± 8
5.078 ± 0.002 3.5 ± 0.4 69 ± 8
TTIP‐ethanol‐2%Pt
TTIP‐ethanol‐3%Pt
DIPBAT‐isopropyl alcohol‐1%Pt
DIPBAT‐isopropyl alcohol‐2%Pt
DIPBAT‐isopropyl alcohol‐3%Pt
DIPBAT‐ethanol‐1%Pt
DIPBAT‐ethanol‐2%Pt
DIPBAT‐ethanol‐3%Pt

640
Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
centre [33]. DIPBAT has two of these groups substituted by two
acetylacetonate ligands, which are more thermally and kinet‐
ically stable than isopropoxy ligands. Thus, hydrolysis of
DIPBAT is more difficult than that of TTIP but it allows better
reaction control.
Some agglomeration was observed as the Pt loading increased,
especially when ethanol was used as the hydrolysis agent.
However, this phenomenon was largely avoided by calcination
of the synthesised catalysts. It should also be highlighted that
the particle sizes and shapes are similar to those obtained by
other groups using supercritical fluid synthesis [33,38] and
classical methods like chemical vapour deposition [39].
Regarding the effect of particle size and morphology on cat‐
alyst performance, a previous study [40] showed that photo‐
catalytic activity tended to decrease with increasing particle
size because specific surface area was lowered. Likewise, it has
been demonstrated that anatase nanoparticles with polyhe‐
dral‐like shapes are more active than those with rounded or
spherical shapes [41].
Representative TEM images of TTIP‐isopropyl alcohol‐1%Pt
and TTIP‐isopropyl alcohol‐3%Pt samples are provided in Fig.
5. In both cases, small Pt particles were uniformly dispersed
over the crystalline TiO2 particles. The size distribution of Pt is
relatively narrow for both catalysts, with average particle di‐
ameters of 2.2 nm for the sample with 1% Pt and 3.5 nm for
that with 3% Pt. This size range is consistent with that reported
by Semlali et al. [23] for mesoporous Pt/TiO2 thin films synthe‐
sised by evaporation‐induced self‐assembly with Pt loadings of
0.5–3 wt%.
In contrast, synthesis experiments with ethanol as a hydro‐
lytic agent gave slightly higher yields than those with isopropyl
alcohol (71.2% vs. 61.8% when TTIP was used as the precursor
and 64.7% vs. 60.1% in the case of DIPBAT). This is probably
caused by the higher polarity of ethanol than isopropyl alcohol.
Alonso et al. [33] also reported that isopropyl alcohol led to a
higher degree of C contamination in catalysts, as by‐product
elimination deteriorated because of their lower solubility in the
less polar alcohol. For this reason, all the catalysts (even the
reference commercial one) were calcined at 400 °C for 6 h be‐
fore conducting photocatalytic experiments. The calcination
temperature was limited by the transition between anatase and
rutile phases, which takes place at about 700 °C [37]. This cal‐
cination step will have important consequences for the proper‐
ties of the catalysts discussed below. Regarding the effect of
metal loading on yields, it can be seen that this variable does
not have a clear influence on the results.
3.2. SEM, TEM, XPS and ICP‐AES analyses
The Pt concentration of the samples was measured by
ICP‐AES. In all cases, the total amount of Pt was incorporated
into the final product during the synthesis of the Pt/TiO2 cata‐
lyst within the interval assayed (1–3 wt% Pt).
With respect to XPS analyses, Fig. 6 shows the XPS survey
spectrum of TTIP‐ethanol‐1%Pt. The sample contains Ti, O, C
and Pt, with sharp photoelectron peaks appearing at binding
energies of 458.4 eV (Ti 2p), 529.6 eV (O 1s), 284.6 eV (C 1s)
and 68–78 eV (Pt 4f). The Ti 2p and O 1s peaks originated from
TiO2.
The estimated ratio between the Pt 4f and Ti 3s peak inten‐
sities at 61.8 eV leads to a relative Pt:Ti concentration of 0.6%
on the TiO2 surface, which is compatible with the expected
stoichiometry in the final product (taking into account the er‐
ror when predicting stoichiometry from XPS data).
The morphology of the synthesised Pt/TiO2 particles after
calcination is shown in Fig. 4. Spherical particles were obtained
when DIPBAT was used as a precursor. The SEM images reveal
these particles were well‐defined spheres with diameters of
about 4–5 µm. These spherical particles are slightly bigger than
those of commercial TiO2 (Fig. 1) and reported by Alonso et al.
[33], who obtained particles with diameters of 270 ± 125 nm
using TTIP and 200 ± 100 nm using DIPBAT under synthesis
conditions similar to those in this work (20 MPa and 300 °C).
Because particle size depends on reaction temperature, the
larger particles obtained here could be caused by temperature
fluctuation in the reactor (300 ± 5 °C); i.e., higher temperatures
(305 °C) led to larger particles. In the case of TTIP, larger poly‐
hedral‐like particles were obtained than when using DIPBAT,
especially when ethanol was used as the hydrolysis agent.
The C peak is attributed to the residual C in the sample. The
(a)
(b)
(c)
(d)
(e)
(f)
(h)
(g)
Fig. 4. SEM images of synthesised catalysts after calcination. (a) TTIP‐isopropyl alcohol‐1%Pt; (b) TTIP‐ethanol‐1%Pt; (c) TTIP‐isopropyl alco‐
hol‐3%Pt; (d) TTIP‐ethanol‐3%Pt; (e) DIPBAT‐isopropyl alcohol‐1%Pt; (f) DIPBAT‐ethanol‐1%Pt; (g) DIPBAT‐isopropyl alcohol‐3%Pt; (h)
DIPBAT‐ethanol‐3%Pt.

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
641
(b)
(a)
(c)
(d)
Particle size (nm)
Particle size (nm)
Fig. 5. Representative TEM images (a,b) and relative estimated Pt particle size distributions (c,d) of Pt/TiO2 catalysts. (a,c) TTIP‐isopropyl alco‐
hol‐1%Pt; (b,d) TTIP‐isopropyl alcohol‐3%Pt.
low intensity of this peak is indicative of the ready ability of the
calcination process to remove precursor and alcohol C‐based
waste species. This peak could also arise from adventitious
hydrocarbons originating from the XPS instrument itself [42].
The results of SEM, TEM, XPS and ICP‐AES indicate that,
from the viewpoint of morphology, the catalysts with higher
photocatalytic activity will be those obtained from the
TTIP‐ethanol‐Pt reactant combination, because they have pol‐
yhedral shapes. However, focusing on particle size, catalysts
synthesised from the TTIP‐isopropyl alcohol‐Pt combination
may also display high catalytic activity.
compound. Knowledge about the crystallinity of a catalyst is
relevant because photocatalytic performance usually increases
with crystallinity, which is usually attributed to the removal of
dangling bonds and distorted lattice structure acting as recom‐
bination sites as crystallinity increases. Nevertheless, high
crystallinity may also lead to decreases of surface OH coverage
and total surface area and, as a consequence, poorer catalytic
behaviour [18].
XRD patterns of the catalysts obtained from the different
precursor‐alcohol combinations are presented in Fig. 7. The
TTIP‐isopropyl alcohol‐Pt catalysts show the highest crystallin‐
ity (highest peak height and resolution) of the sample, very
close to that of the commercial catalyst (Fig. 1). In contrast,
catalysts from the other combinations exhibit poorer crystal‐
linity, even after calcination, which usually enhances this pa‐
rameter [35]. Additionally, the loading percentage of Pt does
not seem to markedly affect the crystallinity of the catalysts.
Regarding crystal phase, anatase TiO2 displays diffraction
peaks at 2θ of 25.4°, 37.8°, 48.5°, 54.0°, 55.4°, 62.9°, 68.9°, 70.3°
and 75.2° [43,44]. According to the patterns in Fig. 7, this allo‐
tropic phase (anatase) is the most common in the catalysts
synthesised in this work. In fact, no diffraction peaks originat‐
ing from any other crystal phase of TiO2 were observed in the
XRD patterns of synthesised catalysts, indicating the formation
of pure anatase TiO2. Considering the crystal phase of the cata‐
lysts before calcination, only the anatase phase was detected, as
was the case in a similar study [33] with the same reagents and
under the same synthesis conditions (20 MPa and 300 °C).
3.3. XRD analysis
XRD is a unique method to determine the crystallinity of a
Fig. 6. XPS survey spectrum of the TTIP‐ethanol‐1%Pt catalyst.

642
Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
(a)
(b)
2θ/( o )
2θ/( o )
(c)
(d)
2θ/( o )
2θ/( o )
Fig. 7. XRD patterns of synthesised catalysts. (a) TTIP‐isopropyl alcohol‐Pt; (b) TTIP‐ethanol‐Pt; (c) DIPBAT‐isopropyl alcohol‐Pt; (d)
DIPBAT‐ethanol‐Pt.
Metal loading does not affect the resulting crystal phase, be‐
cause similar catalysts obtained in the absence of metal show
only the anatase phase in their XRD patterns [45]. However,
this was not the case for the commercial catalyst, because TiO2
P‐25 from Evonik (formerly Degussa) is a mixture of 80% ana‐
tase and 20% rutile phases [46]. In this case, additional peaks
at 27.5°, 36.9° and 41.4° ascribed to rutile TiO2 are observed
(Fig. 1). The absence of the rutile phase in the synthesised cat‐
alysts also means that the calcination to remove all organic
contaminants is conducted at an appropriate temperature that
avoids phase change [47].
This result is important because anatase TiO2 is inherently
more photoactive than rutile as a result of both its solid‐state
properties (better light absorption and charge transport) and
surface properties (larger response to charge trapping and
transfer and superior chemical response to the adsorbates in‐
volved in electron transfer reactions) [18]. Fig. 7 does not con‐
tain any peaks attributable to Pt, which may be caused by the
low content and high dispersity of the loaded Pt atoms [44].
Crystallite size (D) is a measure of single‐crystal size, and
therefore it can be interpreted as an indicator of the crystalline
quality of catalyst particles. This crystal parameter has the
largest effect on photocatalysis from the viewpoint of
light–material interactions including photon absorption,
charge‐carrier generation and dynamics, and surface trapping
[37]. D, which is the size of particles in the direction vertical to
the corresponding lattice plane, can be determined from XRD
line broadening measurements using the Scherrer equation
[48]:
D ≈ 0.9λ/βcosθ
(1)
in this equation, λ is the X‐ray wavelength (0.1541 nm), β the
full width at half‐maximum intensity (rad) and θ is half of the
diffraction peak angle (approximately 12.7° for the <101> ana‐
tase crystal facet).
Comparison of the data in Table 3 shows that the synthe‐
sised catalysts are smaller than the commercial catalyst (19.97
nm) with the sole exception of the TTIP‐isopropyl alcohol‐Pt
combination (D = 21.25–22.77 nm). The smallest catalyst is
TTIP‐ethanol‐Pt (9.65–9.98 nm). All D values reported here are
compatible with those found by other groups [38,49]. It should
also be highlighted that, according to Table 3, the smaller parti‐
cles generally have higher surface areas, although this rela‐
tionship does not hold for the combination DIPBAT‐ethanol‐Pt.
This discrepancy may be because the Scherrer equation is not
completely adequate to estimate D of catalysts, despite being
the most popular method for this purpose. That is, when using
the Scherrer equation to calculate D from XRD data, D are as‐
sumed to be those of a coherently diffracting domain, which is
not necessarily the same as particle size. The Scherrer equation
attributes XRD peak broadening exclusively to crystal size, and
does not take into account that crystalline defects also cause
line broadening.

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
643
surface area results are undoubtedly related to the crystallinity
of the synthesised catalysts. Thus, samples with high crystallin‐
ity, such as those synthesised from TTIP and isopropyl alcohol
show smaller surface areas (Table 3). In contrast, materials
with poor crystallinity (like the catalysts obtained using
DIPBAT) display increased porosity and surface area. Similar
findings have been reported in the literature for other nano‐
particles synthesised in supercritical CO2 [33,50]. Although it is
not shown in Table 3, a slight decrease (<5%) in specific sur‐
face area with increasing Pt loading was observed for all pre‐
cursor/alcohol combinations. This finding may be related to the
deposition of Pt0 nanoparticles [26].
The mean pore size in the catalysts was estimated using the
Barrett‐Joyner‐Halenda model, while the Harkins‐Jura equation
was used to determine the thickness of the adsorbed N2 layer
from the adsorption data assuming cylindrical pore geometry.
As shown in Table 3, all samples had mesoporous structures,
which can enhance the rate of gaseous photocatalytic reactions
because of the rapid diffusion of gas molecules within meso‐
pores. According to the above results, if only surface area and
pore volume are taken into account, the ideal particles for pho‐
tocatalysis should be those obtained from the combinations of
TTIP‐ethanol‐Pt and DIPBAT‐isopropyl alcohol‐Pt.
Table 3
Surface properties and crystallite sizes of commercial and synthesised
catalysts.
Pore Mean pore
D
SBET
Catalyst
volume
(cm3/g)
0.39
size
(nm)
12.5
9.5
(nm) (m2/g)
P‐25
20
23
21
21
10
10
10
13
11
11
11
11
11
63
51
TTIP‐isopropyl alcohol‐1%Pt
TTIP‐isopropyl alcohol‐2%Pt
TTIP‐isopropyl alcohol‐3%Pt
TTIP‐ethanol‐1%Pt
TTIP‐ethanol‐2%Pt
TTIP‐ethanol‐3%Pt
DIPBAT‐isopropyl alcohol‐1%Pt
DIPBAT‐isopropyl alcohol‐2%Pt
DIPBAT‐isopropyl alcohol‐3%Pt
DIPBAT‐ethanol‐1%Pt
DIPBAT‐ethanol‐2%Pt
DIPBAT‐ethanol‐3%Pt
0.22
140
140
96
0.48
0.46
0.40
7.0
6.6
6.2
Neither the Ti precursor nor the percentage of Pt loading
strongly influenced D when ethanol was used as the hydrolytic
agent. In contrast, the catalysts synthesised with isopropyl al‐
cohol displayed D that depended on the precursor; larger D was
obtained with TTIP than with DIPBAT. Likewise, neither the
alcohol type nor the Pt loading affected D when DIPBAT was
used as the Ti precursor. In contrast, when TTIP was used, D
obtained with isopropyl alcohol were larger than those with
ethanol. In principle, these experimental findings should be
related to the nature of the reactants (TTIP is hydrolysed more
easily than DIPBAT and isopropyl alcohol is less polar than
ethanol). However, we are still looking for a convincing expla‐
nation for our results.
According to the XRD analyses, TTIP‐isopropyl alcohol‐Pt
should be the combination with the highest photocatalytic ac‐
tivity, because the formation of highly crystallised anatase with
larger D might facilitate the transfer of photoelectrons, which
could lower the probability of the recombination of photoin‐
duced electrons and holes. However, as we shall see in the next
section, this synthesis combination also led to small specific
surface areas.
3.5. DRUV‐vis analysis
The DRUV‐vis spectra of the synthesised catalysts generally
shifted toward the visible range compared with that of the ref‐
erence commercial catalyst (Fig. 8). These results are of great
interest because the emission spectrum of the Xe lamp used in
the photocatalytic experiments to mimic the solar spectrum
exhibited high irradiance in the visible range. The reason for
using this lamp was to gauge the possibility of using sunlight as
the energy source in future research.
Many groups have examined how loading TiO2 with differ‐
ent cations shifts its absorption into the visible region [16‐18].
The metal atoms act as a sink for photoexcited electrons, which
enhances charge separation efficiency [51]. This increase in
visible‐light absorption is also associated with the formation of
oxygen vacancies/Ti3+ species [11]. In this study, the absorb‐
ance in the visible range increased by up to ten times for
TTIP‐ethanol‐Pt (Fig. 8(b)), DIPBAT‐isopropyl alcohol‐Pt (Fig.
8(c)) and DIPBAT‐ethanol‐Pt (Fig. 8(d)) compared with that of
the commercial catalyst. In the case of TTIP‐isopropyl alco‐
hol‐Pt, visible light absorption increased by seven times (Fig.
8(a)).
Regarding the effect of the Pt loading percentage in the cat‐
alyst, Fig. 8 reveals that the absorbance in the visible range is
higher for the Pt/TiO2 catalysts than for commercial TiO2.
Moreover, the absorbance intensity increased with the metal
concentration for catalysts synthesized with both Ti precursors
[26]. Absorption thresholds and Eg of the synthesised catalysts
were calculated from the UV‐vis spectra in Fig. 8, and are pre‐
sented in Table 4. Each absorption threshold was obtained
from the intersection of the x‐axis and a line tangent to the ab‐
sorption curve where the maximum slope is found [52]. A shift
towards the visible region of the absorption threshold of met‐
3.4. BET analysis
The specific surface area (m2/g) is a parameter commonly
used to determine the type and properties of a material re‐
garding adsorption, heterogeneous catalysis and surface reac‐
tions. Several methods have been developed to measure the
specific surface area of materials; the BET N2 adsorption pro‐
cedure is one of the most widely used. In this work, we used the
BET method to determine the specific surface areas of the cat‐
alysts.
The synthesised Pt/TiO2 catalysts displayed type‐IV adsorp‐
tion isotherms with one hysteresis loop at a relative pressure
range of 0.5–0.8. The measured specific surface areas for the
catalysts are listed in Table 3. The samples synthesised from
TTIP possessed smaller average surface areas than those ob‐
tained from DIPBAT. With respect to the alcohol used in the
catalyst synthesis, the results depend on the Ti precursor. The

644
Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
(b)
(a)
λ/nm
λ/nm
(d)
(c)
λ/nm
λ/nm
Fig. 8. DRUV‐vis spectra of commercial and synthesised catalysts. (a) TTIP‐isopropyl alcohol‐Pt; (b) TTIP‐ethanol‐Pt; (c) DIPBAT‐isopropyl alcohol‐Pt
(d) DIPBAT‐ethanol‐Pt (black lines: commercial, dashed lines 1% Pt, grey lines 2% Pt, dotted lines 3% Pt).
al/TiO2 photocatalysts has been often taken as direct evidence
for enhancement of their photocatalytic activity under solar or
visible irradiation [52].
ꢂ ∙ ꢃ
ꢄ
(2)
ꢀg ꢁ
The absorption threshold of the commercial catalyst was
407 nm, whereas those of the synthesised catalysts were higher
(415–483 nm) (Table 4). The combination that gave the highest
The band gap is the void region that extends from the top of
the filled valence band to the bottom of the vacant conduction
band in a semiconductor [53]. The band gap of a semiconductor
catalyst defines the amount of photons that are available for
quantum conversion. Eg can be calculated from equation 2,
where h is the Planck constant (4.13566733 × 10–15 eV·s), c is
the speed of light (3 × 105 km/s) and λ is absorption threshold
(nm) [54].
mean
value
was
TTIP‐ethanol‐Pt,
even
though
DIPBAT‐ethanol‐3%Pt displayed the highest individual value.
The absorbance intensity in the visible range is reflected by the
colour of the samples. Thus, as shown in Fig. 9, the samples
with weak absorbance in the visible range (P‐25) are white,
those with a moderate absorbance are grey (TTIP‐isopropyl
alcohol‐1%Pt), and those with the strongest visible absorption
(TTIP‐ethanol‐1%Pt) are brown. It should be noted that Fig. 9
depicts the photographs of the catalysts after use in the photo‐
catalytic reaction, although their colour was similar before re‐
action. Another interesting observation in Fig. 8 is that only one
absorption edge can be detected in the DRUV‐vis spectra, indi‐
cating that the catalysts behave as a single phase rather than a
mixture of compounds with different absorption thresholds
[52]. Moreover, there is a linear correlation between the ab‐
sorption threshold and the amount of Pt in the catalysts.
Table 4
Absorption threshold and band gap energies of commercial and syn‐
thesised catalysts.
Absorption threshold Band gap
Catalyst
(nm)
407
415
427
436
453
457
457
416
430
471
426
447
483
(eV)
P‐25
3.048
2.990
2.906
2.846
2.739
2.715
2.715
2.982
2.885
2.834
2.812
2.776
2.669
TTIP‐isopropyl alcohol‐1%Pt
TTIP‐isopropyl alcohol‐2%Pt
TTIP‐isopropyl alcohol‐3%Pt
TTIP‐ethanol‐1%Pt
TTIP‐ethanol‐2%Pt
TTIP‐ethanol‐3%Pt
DIPBAT‐isopropyl alcohol‐1%Pt
DIPBAT‐isopropyl alcohol‐2%Pt
DIPBAT‐isopropyl alcohol‐3%Pt
DIPBAT‐ethanol‐1%Pt
DIPBAT‐ethanol‐2%Pt
DIPBAT‐ethanol‐3%Pt
Similarly, while Eg of the commercial catalyst was 3.048 eV
(pure anatase: 3.2 eV), all the synthesised catalysts showed Eg
in the range of 2.669–2.990 eV. This means that the energy
required to excite the synthesised catalysts is lowered because
extra levels are introduced in the gap region below the conduc‐
tion band of TiO2 [6,15]. This decrease in excitation energy

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
645
(a)
(c)
(b)
Fig. 9. Photographs of filters with catalysts after photocatalytic experiments. (a) Commercial; (b) TTIP‐isopropyl alcohol‐1%Pt; (c)
TTIP‐ethanol‐1%Pt.
allows more efficient use of the visible spectrum by the Pt/TiO2
catalysts [15,55].
have a strong influence on other previously mentioned param‐
eters such as surface area and porosity [33,38,50], which de‐
termine the efficiency of photocatalysis because they provide a
contacting surface between catalyst, light and reactants.
Regarding mean particle size, the mean size of the commer‐
cial catalyst (3 μm) is considerably smaller than those of the
synthesised catalysts; for example, 8.75 μm for
TTIP‐ethanol‐1%Pt. These results are in accordance with the
SEM images of the synthesised catalysts. The particle size dis‐
tributions of the synthesised catalysts are wide; only 50% of
particles are around 5 μm in size, whereas the commercial cat‐
alyst has 72% of particles with sizes in the range from 0.6–5
μm.
Considering the effects of the reagents used on particle size,
the influence of both precursors on particle size is smaller than
those of the alcohol employed and metal loading. Thus, the
introduction of ethanol and a larger amount of Pt facilitate in‐
creased particle sizes and aggregation, as indicated in SEM
measurements. These results agree with those reported by
Alonso et al. [33]. Decreasing particle size leads to increased
surface energy, lattice distortion/strain and changed surface
dangling bond population, which means that TiO2 surface
properties improve as particle size decreases [18]. In other
words, if only surface properties are considered, the commer‐
cial catalyst should show higher photocatalytic activity than the
synthesised catalysts.
Notably, the combinations with the lowest mean band gap
value are TTIP‐ethanol‐Pt (2.723 eV) and DIPBAT‐ethanol‐Pt
(2.752 eV). This variable decreased less than 5% as Pt loading
increased from 1 to 3 wt%. For this reason, all the photocata‐
lytic experiments were performed using catalysts with a Pt
loading of 1 wt%. If these results are compared with those from
catalysts synthesised in the absence of Pt [45], the adsorption
threshold and Eg of every Pt/TiO2 catalyst are higher and
smaller, respectively, than those of the corresponding catalyst
without Pt. Thus, the introduction of Pt has a positive influence
on the light harvesting ability of TiO2. In short, the absorbance
of the synthesised catalysts is increased in the visible range
compared with that of P‐25, especially when ethanol is used as
the hydrolysis agent.
3.6. FTIR analysis
The FTIR spectrum of the commercial P‐25 catalyst (Fig. 10)
only shows a peak around 690 cm–1, which corresponds to
Ti–O–Ti bonds [56].
The FTIR spectra of the synthesised catalysts are generally
similar to that of the commercial catalyst (Figure 10). Some of
the catalysts show a small peak around 1625 cm–1 and/or a
wide band centred at 3000 cm–1, which are attributed to
stretching vibrations of OH groups and H2O on the catalyst sur‐
face [57,58]. In principle, as explained above, the presence of
these species usually favours the photocatalytic process [18].
The presence of OH groups (Brønsted acid sites) is beneficial
for the reduction of CO2, because they lead to efficient charge
separation and transfer to the TiO2 surface [11]. The above‐
mentioned peaks mainly appear in catalysts that use ethanol as
3.8. Photocatalytic activity in CO2 reduction
The majority of photocatalytic CO2 reduction studies have
been performed in liquid media using H2O as the solvent.
However, one important issue when studying this process is
the physical state in which it is carried out. For example, alt‐
hough CO2 photoreduction in liquid H2O is relatively simple,
certain problems arise because of both the low solubility of CO2
in H2O and the existence of competing reactions that consume
holes and electrons, leading to the formation of H2 and H2O2 at
the expense of CO2 reduction products (methanol, CH4, CO, etc.)
[59]. Furthermore, if CO2 reduction is performed in liquid H2O,
CO2 is in the form of carbonates or bicarbonates, which are
more difficult to reduce than CO2 itself [6]. Thus, it has been
the hydrolysis agent,
especially
the combination
TTIP‐ethanol‐Pt. In the case of DIPBAT‐ethanol‐Pt, these peaks
disappear when the Pt content exceeds 1 wt%.
3.7. Particle size and particle size distribution
Particle size distribution and mean particle size are two
important parameters of particulate materials. Both variables

646
Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
Fig. 10. FTIR spectra of synthesised catalysts (solid lines) and P‐25 (dotted lines). (a) TTIP‐ethanol‐1%Pt; (b) TTIP‐isopropyl alcohol‐1%Pt; (c)
TTIP‐ethanol‐2%Pt; (d) DIPBAT‐isopropyl alcohol‐1%Pt; (e) TTIP‐ethanol‐3%Pt; (f) DIPBAT‐ethanol‐1%Pt.
found that when liquid H2O is used as the reaction medium,
organic yields are low [59]. To avoid such liquid‐phase reaction
problems, some authors have proposed working under
gas‐phase conditions [35,55]. Moreover, this would provide an
additional advantage because in the reactions with H2O vapour,
the photocatalyst is immobilised, which simplifies the separa‐
tion of products from the catalyst. For all these reasons, we
decided to test the activity of the synthesised catalysts by per‐
forming CO2 photoreduction in the gas phase. Different series
of blank experiments ((a): in the absence of photocatalyst, (b):
in the dark and (c): only with wet He (in the absence of CO2))
were performed, after which no reaction product containing C
was detected, except for CO2 in series (a) and (b) (data not
shown). Table 5 presents the results obtained with commercial
and synthesised catalysts in the presence of light and CO2.
Table 5 reveals that the main reaction products were CH4
and CO, a result consistent with one of the popular possible
reduction mechanisms described in the literature [49] and with
experimental results from other authors. For example, Yui et al.
[60] reported that the main product from CO2 photoconversion
was CO and that substantial quantities of CH4 were obtained
only after introducing a metal onto TiO2. Mao et al. [44] later
confirmed this finding, reporting that CH4 was the main con‐
version product of CO2 photoreduction using Pt dispersed on
TiO2 as a catalyst.
Four preliminary tests were carried out with commercial
P‐25 as the catalyst to select the operating conditions (amount
of catalyst and H2O vapour/CO2 ratio) for the photocatalytic
experiments with synthesised Pt dispersed on TiO2 catalysts
(Table 5). These tests were also intended to gather data to
compare the performance of commercial TiO2 and Pt/TiO2 cat‐
alysts. The parameter used to assess catalyst performance was
the rate of product formation. For CO2 photoreduction, this
parameter is generally expressed as the amount of product
(µmol) divided by the time it took to accumulate (h) and the
amount of catalyst used (g) [49]. In the assays with commercial
TiO2, Table 5 shows that the CH4 and CO production rates were
0.033–0.078 and 1.497–2.014 µmol g–1 h–1, respectively. The
activity of P‐25 may be attributed to the complementary effects
of anatase and rutile phases, in which the interfaces between
the phases could play a major role in catalysis [49].
The effect of H2O vapour/CO2 ratio on photocatalyst activity
was investigated using about 35 g of TiO2 and H2O vapour/CO2
ratios of 2:1, 2:7 and 2:20. Table 5 reveals that the highest total
Table 5
Results for photocatalytic reduction of CO2.
Catalyst weight
(mg)
35.6
H2O/CO2
(mol/mol)
2/1
2/7
2/20
2/7
Reaction time
Production rates
(µmol g–1 h–1) CH4 (CO)
0.033 (1.497)
0.075 (2.014)
0.078 (1.875)
0.033 (1.887)
0.135 (0.109)
0.245 (0.058)
0.164 (0.132)
0.172 (0.096)
0.140 (0.167)
0.131 (0.069)
Catalyst
Pt loading (wt%)
(h)
3
3
3
3
3
3
3
4
P‐25
P‐25
P‐25
P‐25
0
0
0
0
1
1
1
1
1
1
35.5
35.2
75.5
34.6
39.6
35.7
36.9
39.3
TTIP‐isopropyl alcohol
TTIP‐ethanol
2/7
2/7
2/7
2/7
2/7
2/7
DIPBAT‐isopropyl alcohol
DIPBAT‐isopropyl alcohol
DIPBAT‐ethanol
DIPBAT‐ethanol
Light source: Xe arc lamp (450 W).
3
4
37.1

Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
647
rates (0.075 µmol CH4 g–1 h–1 and 2.014 µmol CO g–1 h–1) were
obtained at the intermediate ratio (2:7). This result agrees with
those reported in the literature because, although it is com‐
monly believed that CO2 reactivity increases as the proportion
of H2O is raised, it has also been reported that excess H2O could
inhibit the reaction [61]. The optimum H2O/CO2 ratio obtained
in this work (2:7) is considerably lower than that reported by
Anpo et al. [61] for CO2 photoconversion in liquid H2O (5:1).
This result is very interesting because it indicates that the reac‐
tion products are far less dilute when the photoreduction pro‐
cess is performed in the gas phase (i.e., using H2O vapour in‐
stead of liquid H2O).
Considering the effect of catalyst weight, the experiments
performed at a H2O/CO2 ratio of 2:7 with a greater weight of
catalyst (75.5 g vs. 35.5 g) did not necessarily lead to the best
photocatalytic performance. This may be explained by consid‐
ering the well‐known fact that formation rate increases linearly
with catalyst concentration only up to a certain value at which
decreased light penetration and increased scattering occur,
limiting formation rate [49].
Regarding mean pore size, although small pores result in a
large surface area, mesopores (as in our case: 7–10 nm) are
always preferable to allow diffusion of reactant and product
molecules [18]. A narrow Eg is also desired because it improves
light absorption and charge carrier generation [7–10,62]. Fi‐
nally, polyhedral particle shapes show higher activity than
rounded particles because of their higher density of catalyti‐
cally active sites, although this has not yet been fully explained
[41].
The catalyst showing the second highest CH4 production
rate was DIPBAT‐isopropyl alcohol‐1%Pt, which also exhibited
high surface area. Alonso and colleagues also obtained good
results with this catalyst in the oxidation of methyl orange in
aqueous solutions [38]. According to our results, surface area
seems to be the most influential property on the catalyst activ‐
ity in reduction of CO2 to fuel molecules. This assertion is fur‐
ther supported by the fact that the catalyst exhibiting favoura‐
ble values of other properties (TTIP‐isopropyl alcohol‐1%Pt),
like small particle size and high crystallinity (Table 6), did not
display a high CH4 production rate.
Regarding the effect of Pt on TiO2 photocatalytic perfor‐
mance, for the experiments carried out with catalyst weights of
35–40 mg, a H2O vapour/CO2 ratio of 2:7 and operating times
of 3–4 h, the CH4 production rates were about 3.2 times higher
using the Pt/TiO2 catalysts than the commercial catalyst. The
highest performance was obtained for the combination
TTIP‐ethanol‐1%Pt when the operating time was 3 h. It should
be noted that this catalyst had already shown superior charac‐
teristics, as illustrated in Table 6, which orders the catalysts
used in the photocatalytic experiments from best to worst ac‐
cording to the value of the property analysed. In particular,
Table 6 reveals that the characteristics that make this catalyst
better than the others are its morphology (polyhedral), narrow
Eg, the presence of surface OH groups, high surface area and
relatively large mean pore size.
Regarding the influence of these properties of catalysts on
the CO2 photocatalysis, it should be noted that the higher the
surface area is, the greater the number of reactant molecules
adsorbed and the more quickly the electrons can reach them.
This results in a lower recombination probability and higher
product yield [10,18]. In addition, the presence of basic species
(OH groups) on the catalyst surface leads to stronger interac‐
tions with CO2 and higher uptake of this gas [10,11,18], so the
larger the surface area is, the higher the catalyst activity will be.
These results also clearly show that the addition of Pt to the
TiO2 catalyst markedly improves the photoactivity of the re‐
sulting catalyst in CH4 production [44,60]. If the photocatalytic
activities of Pt/TiO2 are compared with those of TiO2 photo‐
catalysts obtained from the same Ti precursors and hydrolysis
agents under the same synthesis conditions in supercritical CO2
[45], we can see that there is an increase in CH4/CO ratio for
the Pt/TiO2 catalysts (0.84–4.22) compared with those of their
counterparts lacking Pt (0.03–0.30) (Table 7).
The CO production rate of the synthesised catalysts was
about 90% lower than that of the commercial catalyst, even
that of the synthesized catalyst with the highest CO formation
rate (DIPBAT‐ethanol‐1%Pt). This observation cannot be ex‐
plained only by the increase in the production of CH4. The pho‐
tochemical H2O‐gas shift reaction to produce H2 and CO2 from
CO and H2O may have occurred, lowering the yield of CO [60].
This reaction is often observed in the presence of noble met‐
al/TiO2 catalysts [60,63]. However, this does not seem a satis‐
factory explanation in these experiments given the low H2O
vapour/CO2 ratio used [36]. Regarding the reaction time, Table
5 reveals that extending the photocatalytic reaction from 3 to 4
h did not substantially increase the CH4 production rate. Ac‐
cording to Habisreutinger et al. [49], this saturation of the
product formation curve may be caused by strong adsorption
of the O2 produced in CO2 photolysis, which may block reaction
sites.
Table 6
Ordering of catalysts used in photocatalytic tests.
Table 8 compares CH4 and CO production rates reported in
recent studies for photocatalytic reduction of CO2 with H2O
vapour as the reducing agent. The values are scarcely compa‐
rable because of the many variables are involved in the CO2
reduction process. Generally, the CH4 production rates ob‐
tained in this work are of the order of those reported for metal
dispersed on TiO2 particles, TiO2 in the absence of metal, TiO2
nanotubes and even metal/TiO2 over optical fibres.
Catalyst property
Photocatalytic activity
Particle size
Morphology
Crystallinity
Desired value Ordering of catalysts
High
Small
Polyhedral
High
A>B>C>D
D>A>B>C
A>D>C>B
D>B>A>C
A>C>B>D
A>C>B>D
A>B>C>D
D>A>B>C
Band gap energy
Presence of surface OH groups
Surface area
Low
High
High
Mean pore size
High
Nevertheless, we are aware that there is still room for im‐
provement. On the one hand, some investigators have reported
higher CH4 production rates [7,8,11,13,14,70]; on the other
A: TTIP‐ethanol‐1%Pt; B: DIPBAT‐isopropyl alcohol‐1%Pt; C: DIPBAT
‐ethanol‐1%Pt; D: TTIP‐isopropyl alcohol‐1%Pt.

648
Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
Table 7
Comparison of the photocatalytic activities of synthesised catalysts with and without Pt.
Catalyst
Pt loading (wt%) Catalyst weight (mg) H2O/CO2 (mol/mol) Reaction time (h)
CH4 /CO (mol/mol)
P‐25
0
1
1
1
1
0
0
0
0
35.5
34.6
39.6
35.7
39.3
41.3
31.9
28.9
42.0
2/7
2/7
2/7
2/7
2/7
2/7
2/7
2/7
2/7
3
3
3
3
3
3
3
3
3
0.04
1.24
4.22
1.24
0.84
0.03
0.16
0.30
0.26
TTIP‐isopropyl alcohol
TTIP‐ethanol
DIPBAT‐isopropyl alcohol
DIPBAT‐ethanol
TTIP‐isopropyl alcohol
TTIP‐ethanol
DIPBAT‐isopropyl alcohol
DIPBAT‐ethanol
Table 8
Summary of reported CH4 and CO production rates.
Production rates (µmol g–1 h–1)
Catalyst
Metal
–
Reagent
Light source
Ref.
CH4 (CO)
0.11
TiO2 anchored on Vycor glass
CO2
H2O
CO2
H2O
CO2
H2O
CO2
H2O
CO2
H2O
CO2
H2O
CO2
H2O
75 W Hg lamp
[64]
Quartz wool immersed in P‐25
suspension
TiO2 pellets
–
UV lamp
3 germicidal UVC lamps (4.8 W), 24 h
75 W daylight lamp
0.1 (< 0.1)
0.22 (< 0.16)
0.3
[65]
[66]
[67]
[55]
[68]
[69]
–
TiO2 on glass beads
P‐25
0.25 wt% Pt
–
100 W high pressure Hg lamp
Sunlight
0.2
TiO2‐SiO2‐acac/optical fiber
Titania nanotubes
0.5 wt% Cu‐0.5 wt% Fe
–
0.279
100 W high pressure Hg lamp
0.3
hand, actual conversion efficiencies reported here and in pre‐
vious studies are still quite low and should be increased by
orders of magnitude before the CO2 photoreduction process
can be used in practical applications. Because of this, we are
currently attempting to further enhance the photocatalytic
activity of the TiO2‐based materials reported in this work.
Specifically, to increase the selectivity of the catalysts for the
productions of CH4 and methanol, three strategies are being
followed: The use of TiO2‐based catalysts with enhanced geom‐
etries: TiO2 nanofibres and TiO2 nanotubes with and without
metal modification [7,13]. The addition of non‐metal elements
to TiO2 [8]. The synthesis of composite photocatalysts combin‐
ing TiO2 and reduced graphene oxide [9,10].
with one of the possible reduction mechanisms described in the
literature. The CH4 production rates of the catalysts synthe‐
sised in this work were clearly improved compared with that of
a commercial catalyst and better or similar to those reported in
the literature for catalysts synthesised with more expensive
and laborious procedures.
To sum up, photochemical conversion of CO2 constitutes not
only an innovative technique to reduce greenhouse gas emis‐
sion, but also a potential alternative to the depletion of fossil
fuel resources. Although it is clear that the catalysts synthesised
to date for CO2 reduction to fuel cannot yet be commercially
applied, we expect that further developments will allow them
to be in the near future. To achieve this goal, further work on
photocatalyst design must combine the improvement in solar
light response with the control of the efficiency and selectivity
of the CO2 photoreduction process.
4. Conclusions
Photocatalysts with similar or better features, like high sur‐
face area, large pore volume, high crystallisation degree and
high OH concentration, than those of commercial TiO2 can be
obtained by supercritical media synthesis. This method is more
environmentally friendly and scalable than traditional tech‐
niques used to fabricate TiO2‐based photocatalysts. In addition,
it is possible to improve the performance of catalysts by shift‐
ing their absorption spectra to the visible region if they are
loaded with Pt at concentrations between 1 and 3 wt.%.
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Susana Tostón et al. / Chinese Journal of Catalysis 38 (2017) 636–650
649
Graphical Abstract
Chin. J. Catal., 2017, 38: 636–650 doi: 10.1016/S1872‐2067(17)62766‐9
Supercritical synthesis of platinum‐modified titanium dioxide for solar fuel production from carbon dioxide
Susana Tostón, Rafael Camarillo*, Fabiola Martínez, Carlos Jiménez, Jesusa Rincón
University of Castilla‐La Mancha, Spain
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