Effects of SiO2-based scaffolds in TiO2 photocatalyzed CO2 reduction

Article abstract of DOI:10.1016/j.cattod.2021.08.003

CO₂ photoreduction has claimed as appealing process to upgrade a waste gas into valuable fuels or chemicals. Titanium dioxide (TiO₂) is one of the most popular material used as catalyst for this reaction, having however a poor activity. The utilization of transparent, insulating and highly porous scaffolds to support a photoactive phase has been reported as one of the possible strategies to improve the performances of this material. In this work, two silica-based materials with different porosity type and level, were involved as support for the TiO₂ and assessed in the gas-phase CO₂ photoreduction with H₂O. The morphological, structural and surface properties were then evaluated by means of different characterization techniques, aiming to correlate them with the catalytic activity and selectivity. The TiO₂-SiO₂ composites revealed a comparable activity compared to pure TiO₂, despite the low fraction of photoactive phase due to improved light harvesting and reagents adsorption on the composites. The CO₂ capture/photoconverting ability was evaluated, to explore the potentiality as multifunctional material.

Full text of DOI:10.1016/j.cattod.2021.08.003

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Effects of SiO -based scaffolds in TiO photocatalyzed CO reduction
2
2
2
a
a
a
a
a
Danny Zanardo , Giulia Forghieri , Sebastiano Tieuli , Elena Ghedini , Federica Menegazzo ,
b
c
a,*
Alessandro Di Michele , Giuseppe Cruciani , Michela Signoretto
a
CATMAT Lab, Department of Molecular Sciences and Nanosystems, Ca’ Foscari University Venice and INSTM-RU center and consortium, Via Torino 155, 30172
Venezia Mestre, Italy
b
Department of Physics and Geology, University of Perugia, Via Pascoli, 06123 Perugia, Italy
c
Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
CO2 photoreduction has claimed as appealing process to upgrade a waste gas into valuable fuels or chemicals.
Carbon dioxide
Photoreduction
Titanium dioxide
Silica
2
Titanium dioxide (TiO ) is one of the most popular material used as catalyst for this reaction, having however a
poor activity. The utilization of transparent, insulating and highly porous scaffolds to support a photoactive
phase has been reported as one of the possible strategies to improve the performances of this material. In this
work, two silica-based materials with different porosity type and level, were involved as support for the TiO
assessed in the gas-phase CO photoreduction with H O. The morphological, structural and surface properties
were then evaluated by means of different characterization techniques, aiming to correlate them with the cat-
alytic activity and selectivity. The TiO -SiO composites revealed a comparable activity compared to pure TiO
despite the low fraction of photoactive phase due to improved light harvesting and reagents adsorption on the
2
and
Scaffolding
2
2
2
2
2
,
composites. The CO
tional material.
2
capture/photoconverting ability was evaluated, to explore the potentiality as multifunc-
1
. Introduction
photoactive phase, to improve the reagents adsorption [11,12]. The
4
+
direct incorporation of Ti cations within silica matrix has observed to
provide tetrahedral Ti4 species, affording a blue-shift in the absorption
+
CO
2
chemical conversion has been addressed as appealing technol-
) into
ogies to upgrade a low-value and abundant raw material (CO
2
edge and an improved photocatalytic activity compared to bulk TiO
2
valuable carbon-based fuels or chemicals [1]. Among the proposed
processes, photocatalysis is an interesting choice due to the utilization of
very mild reaction conditions and the potential exploitation of sunlight
as energy source [2–4]. Several semiconductor (SC) materials have been
[12,13]. Despite the effectiveness of such tetrahedral species, the ab-
sorption edge blue-shift is detrimental for a wide band-gap material like
TiO
2
, because requires more energetic and harmful UV radiation to
-based nanoparticles (NPs), exhibiting the typical
make it active. TiO
2
studied for this reaction, and titanium dioxide (TiO
2
) is the most popular
absorption band edge at ca. 3–3.3 eV, have been successfully loaded
onto mesoporous silica materials such as SBA-15 [11,14] or COK-12
being abundant, cheap, photostable and non-toxic [5]. However, many
issues negatively affect this photocatalyst: (i) wide bandgap (3–3.3 eV)
which allows the exploitation of UV photons only (ca. 5% of the
incoming sunlight); (ii) fast charge carrier recombination and (iii) slow
[15], leading to an activity improvement compared to the bare TiO
2
NPs. Besides the improved reagents adsorption, the improved photon
harvesting through light scattering was also proposed as potential effect
in improving the activity [11]. In this regard, Wang et al. observed an
2
kinetics in some charge carrier surface transfer processes, i.e. CO or
proton reduction [6]. To overcome these limitations, several material
modifications have been proposed, such visible-light sensitization with
narrow bandgap SCs [7,8] or the surface functionalization with
co-catalysts to improve the reaction kinetics [9,10]. Another appealing
material design strategy is the utilization of an insulating, transparent
2 3 4
increased activity toward CO photoreduction on a g-C N photocatalyst
treated with a transparent layer of reconstructed cellulose, that boost up
the light absorption and consequent photon utilization, by means of the
light scattering phenomenon [16]. Besides the activity improvement,
the scaffold was observed to play an important role in determining the
selectivity of the photoreduction process as well. Indeed, the presence of
2
and porous material such as silica (SiO ), as scaffold supporting the
*
Corresponding author.
E-mail address: miky@unive.it (M. Signoretto).
https://doi.org/10.1016/j.cattod.2021.08.003
Received 15 March 2021; Received in revised form 29 June 2021; Accepted 4 August 2021
Available online 8 August 2021
0
920-5861/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: Danny Zanardo, Catalysis Today, https://doi.org/10.1016/j.cattod.2021.08.003

D. Zanardo et al.
Catalysis Today xxx (xxxx) xxx
a mesoporous scaffold was observed to enhance the formation of highly
• TiO
2
, pristine P25
reduced carbon products (CH
the moiety (hydroxyl groups) on the scaffold surface was observed to
play an important role in determining the CH vs CH OH selectivity on a
Ti-modified β-zeolite photocatalyst [12]. Tasbihi et al. observed also an
improved yield of hydrogen (H ) by supporting TiO onto a mesoporous
silica [15]. H is generally considered as a side product in CO photo-
reduction, since it consumes both photogenerated electrons and the
reductant (H O) to generate an unwanted product (H ) [18]. Thus,
several strategies have been adopted to inhibit this side reaction, either
by using a CO -rich reaction medium [19] or by surface modification
with alkaline materials (i.e. MgO) [18]. The former, despite being
widely exploited in CO photoconversion, could represent a bottleneck
4
vs CO) on Ce-promoted TiO
2
[17], while
• ST10, benchmark SiO
2
with 10 wt% of P25
• SBT10, SBA-15 with 10 wt% of P25
4
3
2
.3. Characterizations
2
2
2
2
X-ray diffraction (XRD) patterns were collected on a Bruker D8
Advance DaVinci powder diffractometer using a sealed X-ray tube
copper anode; operating conditions, 40 kV and 40 mA) and a linear
array detector (LynxEye), set to discriminate the Cu K radiation,
2
2
(
α
2
coupled with a Ni filter to completely remove the Cu Kβ component. The
samples were spun during data collection and a vertical knife was used
2
to avoid low angle air scattering contribution. Data scans were per-
formed in the 2θ range 5–90◦ with 0.02◦ step size and point-detector
in future large-scale application, because largely available as diluted
source (i.e. flue gases) [20] and an intermediate enrichment step would
equivalent counting times of 5 s/step. Quantitative phase analysis and
crystallite size determination were performed using the Rietveld method
as implemented in the TOPAS v.5 program (Bruker AXS) using the
fundamental parameters approach for line-profile fitting. The determi-
nation of the crystallite size was accomplished by the Double-Voigt
approach and calculated as volume-weighted mean column heights
be required [21]. Liu et al. reported a TiO
2
photocatalyst supported onto
in dark, thanks to the
a mesoporous alkaline scaffold, able to adsorb CO
2
alkaline support, and then reducing it upon irradiation in quite harsh
◦
conditions (T > 100 C), thus acting as hybrid capture-photoconverting
system [22]. To the best of authors knowledge, this is the only reported
work concerning
photoconversion.
2
a multifunctional system for CO capture and
2
based on integral breadths of peaks. N physisorption analyses were
performed using a Micromeritics TriStar II Plus analyser, recording the
So far, most attention has been paid to use of high-surface area
mesoporous transparent materials due to their effectiveness as scaffolds
◦
adsorption–desorption isotherms at – 196 C. All samples were previ-
ously outgassed at 200 C for 2 h. The surface area was evaluated using
◦
2
in improving the CO photoconversion efficiency. However, to the best
the standard BET equation [25]. The morphology and composition were
examined by Field Emission Electron Scanning Microscopy (FE-SEM)
LEO 1525 ZEISS. Elemental composition and chemical mapping were
determined using a Bruker Quantax EDS. The samples were deposited on
adhesive carbon tape and metallized with chromium. The morphology
was also evaluated by a Transmission Electron Miscroscopy (TEM) JEOL
of our knowledge, no macroporous materials have been tested yet and,
in particular, a comparison between the effect of a macro- and meso-
porous scaffold on the photocatalytic performances of TiO
2
has not been
reported yet.
Through this work, a benchmark TiO
different SiO -based scaffolds: a crystalline silica support with macro-
porosity and a material with ordered distribution of mesopores defined
by amorphous walls (SBA-15). Gas-phase CO photoreduction activity
2
material was supported on two
2
3
010-UHR instrument operating at 300 kV and equipped with a LaB
6
filament. Digital micrographs were acquired by a Gatan (2k × 2k)-pixel
Ultrascan1000 CCD camera. The sample were dry dispersed onto Cu
grids coated with lacey carbon before analysis. The FTIR spectra were
collected through a Perkin Elmer Spectrum one spectrophotometer in
2
and selectivity of the synthetized materials were then compared and
correlated to their physical-chemical properties. Finally, such materials
were assessed as hybrid CO
conditions.
2
capture-photoconverting systems in mild
–
1
the 4000–400 cm range, dispersing the powders in a KBr pellet. The
UV–vis spectra were measured by diffuse reflectance spectroscopy (DRS)
by a Cary100 UV–vis spectrophotometer equipped with an integrating
2
. Experimental part
sphere, using a BaSO
4
as internal reference, in the 200–800 nm range
–
1
and 600 nm∙s
scan rate. The spectra were plotted through the
2
.1. Materials
Kubelka-Munk function [26], where R∞ is the reflectance of an infinite
thick layer:
The following materials were used as-received: benchmark TiO
P25, Evonik), SiO (quartz, size > 230 mesh, Sigma Aldrich), tetrae-
thylortosilicate (TEOS, assay 98%, Sigma Aldrich), triblock copolymer
Pluronic P123, EO20-PO70-EO20, Sigma Aldrich), isopropanol (assay
2
2
(
1 ꢀ R )
∞
(
2
f(R
∞
) =
The bandgap (E
2R
∞
(
g
) value was determined through the Tauc relation
9
9.8%, Sigma Aldrich) and aqueous HCl (assay 37%, Sigma Aldrich).
1/2
[
27], plotting (f(R
∞
)h
ν
)
vs E (eV). The temperature programmed
desorption (TPD) analyses were carried out in a lab-made equipment.
◦
The sample was placed in quartz reactor firstly outgassed at 300 C in He
2
2
.2. Catalyst preparation
–
1
◦
flow (40 mL∙min ), then heated from r.t. to 500 C (heating rate
–
1
◦
1
0 C∙min ), analysing the gas composition through a Gow-mac TCD
.2.1. SBA-15 synthesis
2
detector. The CO -TPD analysis were carried out through the same
SBA-15 was prepared according to a previous reported method [23].
procedure but exposing the outgassed sample to a flow of pure CO
to the TPD analysis.
2
prior
The template (P123) was dissolved in aqueous HCl and TEOS was then
slowly added as silica precursor. The mixture was stirred at r.t. for 24 h
◦
and then aged in a sealed Teflon vessel at 95 C for 42 h. The obtained
◦
solid was then washed with deionized water, filtered, dried at 110 C for
2.4. Photocatalytic tests
◦
1
8 h and finally annealed at 550 C for 6 h in air flow (50 mL/min).
2
The CO photoconversion reactivity tests were carried out in gas-
2
.2.2. TiO
2
-SiO
2
composites synthesis
composites were prepared by incipient wetness
phase using a flat-type glass photoreactor [10]. The photocatalyst was
immobilized onto the irradiated side of the reactor by suspending it in
isopropanol and evaporating the solvent within the photoreactor. A
The TiO
-SiO
2 2
impregnation [24]. An appropriate amount of P25 (benchmark TiO
was dispersed in isopropanol and sonicated for 1 h. The mixture was
then impregnated on both benchmark SiO and synthetized SBA-15,
. The composites were finally air-dried at
2
)
CO
2
/H
2
O mixture (13.3 molar ratio) was produced by bubbling CO
2
◦
2
(99.9%) through a milli Q water-filled bubbler kept at 40 C. A
medium-pressure Hg lamp (125 W, Helios Italquartz) with a 365 nm
loading a 10 wt% TiO
2
–
2
◦
1
10 C for 18 h. The samples were labelled as follow:
main emission line was used as light source, with a 60 W∙m
light
2

D. Zanardo et al.
Catalysis Today xxx (xxxx) xxx
Fig. 1. (a) Physisorption isotherms of SBA-15 (dashed line) vs SBT10 (straight line); (b) physisorption isotherms of SiO
2
(dashed line) vs ST10 (straight line).
intensity, controlled with a Delta Ohm HD 2302.0 photo-radiometer and
a LP 471 probe. The reaction was carried out, after getting a stable
◦
CO
2
/H
2
O mixture, in static conditions for 6 h, at 70 C (heat from the
lamp) and atmospheric pressure. The reaction mixtures were analysed
through a 6890 HP gas chromatographer, equipped with a Porapak Q
packed column, a TCD detector and an automatic sampling valve. Each
reaction was repeated twice. The CO
were performed by running the reaction in the same conditions above
reported but using a CO -free He/H O mixture (13.3 molar ratio) as
reaction medium. The used catalyst was then left on a 10 mol. % CO /He
stream for 18 h in dark, before re-using it for the reaction with the
He/H O mixture under irradiation. This procedure was repeated twice.
The turnover frequency (TOF) for CH , H or O was calculated as
follow:
2
capture-photoconverting tests
2
2
2
2
4
2
2
mol
P
TOF = m
CAT
∙
τ
2
Fig. 2. XRD diffractogram of pure SiO scaffold. Crystal phases: quartz (Q),
where mol
P
are the produced moles of CH
4
, H
2
or O
2
, mCAT the photo-
) to a
microcline (M).
catalyst mass and
τ
the reaction time. The reaction selectivity (S
P
given reduction product (P), was calculated as follow:
isotherm of a mesoporous material, and the presence of uniform cylin-
drical shaped pores by the sharp hysteresis loop (Fig. 1a, dashed line).
The morphology of pores can be observed more in details by TEM an-
alyses, where uniform and well-aligned cylindrical pores were found
mol
P
S
P
= mol
H
2
+ molCH4
where molH2 and molCH4 are the produced moles of CH
4 2
and H ,
(
2
Fig. S3b and S3c). Upon loading TiO on SBA-15, the physisorption
respectively.
hysteresis changes (Fig. 1a, straight line) and the SSA slightly decreases
2
–
1
2 1
–
from 866 m ∙g to 802 m ∙g , suggesting a comparable porosity of
3
. Results and discussion
.1. Photocatalysts characterization
The bare TiO is a crystalline material composed of a mixture of
2
the two materials. On the contrary, the pristine SiO showed a type III
hysteresis (Fig. 1b, dashed line), related to macroporous materials with
very low SSA, which cannot be quantified by the BET model. Upon
3
loading TiO
2
, a sharp hysteresis close to P/P0 = 1 appeared (Fig. 1b,
2
straight line), pointing out the appearance of some mesopores, ascrib-
anatase (89%) and rutile (11%), as evidenced by XRD reported in
Fig. S1a, with an averaged crystallite size of 18 nm. The physisorption
analysis (Fig. S1b) highlights the macro-mesoporous nature of such
2
able to the loading of TiO NPs.
The diffraction pattern of the two scaffolds revealed some more
differences. SBA-15 is an amorphous silica material exhibiting a
0
material due to the hysteresis close to P/P = 1, with an average specific
2
–
1
nanometer-sized ordered pores structure [23]. SiO
was found to be a crystalline material composed mainly of quartz (SiO
0.4%) with a small amount of feldspar microcline (KAlSi , 9.6%) as
composites, due to the mild thermal
phase composition and crystallite size were
2
, on the contrary,
surface area (SSA) of 50 m ∙g . Regarding the morphology, it is well
known that this kind of material is composed of roughly spherical NPs of
ca. 20–40 nm in size [28]. The optical properties, analysed through
UV–vis DRS, revealed an absorption band edge below 400 nm (Fig. S2a),
2
,
9
3 8
O
impurity (Fig. 2). On SiO
2
-TiO
2
◦
treatment (110 C), the TiO
2
completely lying in the UV region, and an assessed bandgap (E
.05 eV (Fig. S2b), in agreement with literature reports [29].
The SiO -TiO composites were prepared by loading 10 wt% of TiO
on the two silica-based support, SiO and SBA-15, aiming to compare the
effect of these scaffolds on TiO physical-chemical properties and pho-
tocatalytic performances. The SiO -TiO composites revealed a notable
difference in term of SSA. The bare SBA-15 exhibited a typical type IV
g
) value of
supposed to be comparable with the pristine material, as
high-temperature thermal treatment are usually required to modify
these properties [30].
3
2
2
2
2
The morphology was examined through scanning electron micro-
scopy (SEM). ST10 is composed of big micrometer-sized quartz particles
2
2
2
(
Fig. 3a), decorated on the surface by the TiO
2
NPs (Fig. 3c), as
surface
observable by Ti elemental mapping too (Fig. 3e). The TiO
2
3

D. Zanardo et al.
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Fig. 3. (a), (b), (c) SEM images of ST10; (d) Si elemental mapping; (e) Ti elemental mapping; (f) EDX spectrum.
Fig. 4. (a), (b), (c) SEM images of SBT10; (d) Si elemental mapping; (e) Ti elemental mapping; (f) EDX spectrum.
decoration can be further confirmed comparing the pristine SiO
2
that,
scaffolds are completely transparent in the absorption region of TiO
(dashed lines in Fig. S4). This suggest the transparent scaffolds to aid the
TiO photon absorption by improving their availability within the ma-
2
contrary to rough ST10, revealed a relatively smooth surface (Fig. S4). K
and Al were found through the EDX analysis (Fig. 3f), supporting the
presence of potassium aluminosilicate (microcline) observed by XRD
analysis.
2
terial thanks to the light scattering phenomenon.
The FTIR spectrum in Fig. 5, revealed three characteristic signal for
On the SBT10, micrometre-sized bean-shaped particles of the scaf-
fold, can be recognized (Figs. 4a and 6b). As for ST10, the SBA-15 sur-
the bare TiO
stretching
2
(black line) namely the hydroxyl or adsorbed water
–
1
(
ν
O
–
H
)
at 3380 cm
1630 cm [33], and the Ti-O bond vibration (
These features becomes less apparent as overlapped by the signals
belonging to bare SiO and SBA-15 scaffolds. On silica-based material,
the most relevant signals are the adsorbed water and silanols (Si OH)
) [35] and bending vibration
) at 1630 cm , plus a strong band due to Si-O-Si stretching vi-
[32] and vibration (δ
O
–
H
)
at
–
1
–
1
face is observed to be decorated by agglomerates of TiO
and 6c), also confirmed by the elemental mapping (Fig. 4e).
On both the SiO -TiO composite (ST10 and SBT10), the TiO
2
NPs (Figs. 4b
ν
Ti O
–
) at 790 cm [34].
2
2
2
NPs
as high-
2
morphology was supposed to be comparable with the pure TiO
2
–
–
1
◦
temperature (T > 400 C) [30] or hydrothermal [31] treatments, known
to induce morphological alternation on the material, were not involved
to prepare the composites.
stretching vibrations at 3425 cm
(ν
O H
–
–
1
(δ
brations (
the relative intensity of
SBT10, supporting the larger and hydroxyl-rich surface of these sample
compared to SiO and ST10 macroporous materials.
O H
–
–
1
ν
–
O Si
–
O
) at 1080 cm [36]. The most relevant difference is
, being greater for SBA-15 and
The optical absorption properties of TiO
upon loading onto both SiO or SBA-15 (continuous lines in Fig. S5),
despite the low amount of photoactive phase (10 wt%), while the two
2
were almost unchanged
ν
O H
–
/
O Si O
– –
ν
2
2
4

D. Zanardo et al.
Catalysis Today xxx (xxxx) xxx
contribution, as isotope labelling of CO
2
can lead to misleading results
due to the conversion of surface impurities even after cleaning
pre-treatments [38], reaction tests using only H O as reactants (blank
test) were performed. As reported in Fig. S7, all the samples produced
some CH even in a CO -free reaction medium, pointing out the presence
of contaminants. No H was detected in these tests. However, on TiO
and SBT10 the activity was doubled in presence of CO , suggesting its
actual reduction, while on ST10 surface impurities was likely to be
mainly converted. To further assess the interaction with CO , after the
st reaction run with H O, the used catalyst was exposed to a flow of CO
10 mol. %) in dark conditions, and then recycled in the photoreduction
with H O. While the pristine TiO exhibited no activity, suggesting a too
weak interaction with CO , both ST10 and SBT10 retained some activity
2
4
2
2
2
2
2
1
2
2
(
2
2
2
–
1 1
–
(
ca. 0.3
μ
mol∙g ∙h ) in the following two recycles (Fig. 7, 2nd and
3
rd). This outcomes suggests both catalysts to better interact with CO
2
rather than TiO
presence of H
2
, retaining this gas and then photoconverting it in
2
2
O. Despite the lower photoactivity within a CO -free re-
action medium, these composites are active as multifunctional mate-
2 2 2
Fig. 5. FTIR spectra of TiO , TiO -SiO composites (continuous lines) and the
rials, able to adsorb CO
2
in dark, and then to photoreduce it upon
bare silica scaffolds (dashed lines).
◦
irradiation in mild conditions (T < 100 C). Furthermore, these mate-
rials were observed to have a good stability after 18 h of UVA irradia-
The surface composition was further analyzed by TPD. As reported in
◦
4
tion, due to the almost constant yield of CH in the 2nd and 3rd recycle
Fig. S6, ST10 revealed a half peak at ca. 380–400 C, which was found
regardless the sample was or not treated with a CO flow (lower spectra).
2
tests (Fig. 7). Characterizations on the spent catalysts, however, were
not possible because the low amount of used catalyst (10 mg).
In the ST10 composite, the above-mentioned impurities contributing
The following drop of the signal, observed on both ST10 and SBT10, was
ascribed to the decomposition of TiO which interferes with the TPD
2
signal acquisition. The half-peak peak observed on ST10 was supposed
to arise from the decomposition of surface alkaline carbonates [37] and
not detected by the FTIR because probably too weak and/or overlapped
with other signals. Despite further characterization would be required to
better identify the nature of such species, they are supposed to be
potassium-based compounds due to the presence of a potassium source
(
2
feldspar) as impurity in SiO .
3
.2. Reactivity tests
2 4 2
The CO photoreduction tests yielded only H , CH and O as
2
detectable products, while CO, usually reported as a reaction product in
solid-gas photocatalytic reaction [15,17], was not found in this study.
The pristine and supported TiO
ferences concerning both the activity and the selectivity. As reported in
Fig. 6a, the CH yield was almost constant for TiO
2
revealed some more interesting dif-
4
2
and ST10 (ca. 0.7–0.8
–
1 1
–
μ
μ
mol∙g ∙h ), while it was more than doubled on SBT10 (2.2
mol∙g ∙h 1). TiO
1
– –
2
2
and SBT10 yielded H as the main reduction
product with a comparable selectivity, while the hydrogen evolution
reaction (HER) was suppressed on ST10 (Fig. 6b).
Fig. 7. Reaction tests and recycles with a CO
catalyst was exposed to a dark CO flow prior to be re-used in the 2nd and
rd cycles.
2
-free reaction medium. The used
2
The interference of organic impurities on the obtained CH
4
and H
2
is
3
a well-known issue in CO
2
photoreduction [4,34]. To exclude their
Fig. 6. (a) TOF of the detected photoreduction products; (b) selectivity (%) of the photoreduction products.
5

D. Zanardo et al.
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Fig. 8. Proposed mechanism for photocatalytic activity enhancement of (a) ST10, where the straight arrows represent the UV rays and (b) SBT10, where the dashed
arrows represent the CO flow.
2
to the CH
taminants. Indeed, as suggested both by TPD analyses and the recycle
tests with a dark CO flow, potassium-based alkaline surface impurities
was supposed to afford a good interaction of CO with the scaffold by
forming surface-adsorbed species such as carbonates (CO
species were then photoreduced into CH , regardless the presence or not
of free CO in the reaction medium. Furthermore, the presence of
alkaline impurities that aids the CO adsorption is suggested to depress
4
production can be only partially ascribed to organic con-
mesoporous amorphous silica was involved in the composite, still using
10 wt% of photoactive phase, the overall activity was doubled likely due
2
to the improved reagent adsorption. Nevertheless, H
probably due to a competitive H O adsorption on the hydrophilic silica
surface. Finally, both the composites were also observed to retain some
CO from a diluted flow in dark, and then photoconvert it upon irradi-
ation in a CO -free reaction medium. Despite a lower activity if
compared to the CO -rich reaction system, this behaviour makes the
composites active as multifunctional CO capture/photoconverting
materials, allowing the exploitation of diluted CO streams. The capture
2
was detected
2
2
2
3
–
). Such
4
2
2
2
2
2
the HER side reaction as well: addition of basic promoters on semi-
conductor photocatalysts is a well-known strategy to decrease the
competition of HER [18], here played by the scaffold itself. Anyway, a
more detailed identification of the nature and role of these impurities
would requires further studies. Besides that, ST10 revealed a compara-
2
2
ability was ascribed to different mechanism, namely to alkaline surface
impurities on the macroporous scaffold, and the large SSA in the case of
the mesoporous one.
2
ble activity with pure TiO , despite containing only 10 wt% of active
phase and having a very low SSA. The presence of the silica-based
scaffold is supposed to act by “trapping” the light, by multiple reflec-
tion within the quartz microcrystals, and thus allowing more incoming
CRediT authorship contribution statement
Danny Zanardo: Investigation. Giulia Forghieri: Investigation.
Sebastiano Tieuli: Investigation. Elena Ghedini: Data curation. Fed-
erica Menegazzo: Writing – original draft. Alessandro Di Michele:
Investigation. Giuseppe Cruciani: Investigation. Michela Signoretto:
Supervision.
2
photons to be absorbed by the TiO on the surface (Fig. 8a).
In the SBT10 composite the above-mentioned mechanism could play
somehow a role, however the main reason of the enhanced catalytic
2
activity, which exceed that of pure TiO despite containing only 10 wt%
of photoactive phase, was supposed to be the large SSA. Indeed, the good
adsorption properties of this mesoporous silica [17,35], are supposed to
make it a huge reagent reservoir, adsorbed on its surface and then
Declaration of Competing Interest
converted by the TiO
and the weak interaction of CO
36], are supposed to induce a competitive absorption of H
reaction, which boost up the HER in this composite. Nevertheless,
despite the weak bonding with CO , the large SSA allows a small amount
of CO to be trapped within the pores upon exposing the catalyst to the
dark CO flow, and then photoconverted when irradiated with H O.
Further understanding on how to improve the activity and selectivity
of these composites, could be achieved by assessing the optimum TiO
2
NPs (Fig. 8b). The hydrophilicity of SBA-15 [24]
onto non-functionalized silica surface
O during the
2
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[
2
2
2
Acknowledgments
2
2
The MIUR (Italian ministry for education, university and research) is
gratefully acknowledged for the financial support (doctoral scholar-
ships) of the inter-university Ph.D. program of University of Trieste and
University Ca’ Foscari Venice.
2
loading (here just 10 wt% was used) or loading alkaline species such as
alkali metal (bi)carbonates, especially on the best-performing meso-
porous silica scaffolds (SBA-15). These evolutions, however, are out of
the aim of this work.
Appendix A. Supporting information
4
. Conclusion
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.cattod.2021.08.003.
Through this work, two different silica-based materials exhibiting
different morphological-structural properties were assessed as scaffolds
for TiO in the CO photoreduction with H O. Exploiting a macroporous
crystalline silica, the overall activity of the composite was comparable
with the pure TiO , despite containing one tenth of the photoactive
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