Copper based TiO2 honeycomb monoliths for CO2 photoreduction

Article abstract of DOI:10.1039/c3cy00991b

The direct photoreduction of CO₂via catalytic conversion of copper supported on TiO₂ based monolithic structures is a means by which solar fuels can be produced. Copper based monolithic structures with varying loadings were synthesized through a sol-gel dip coating procedure and tested for CO₂ reduction with H₂O as a reductant in the gaseous phase. Results established that increased copper concentration can decrease crystalline size and promote anatase to rutile phase transformation. The coated monolithic structures were dominated by mainly Cu¹⁺ species, as confirmed by XPS while bulk characterization suggests that these species are present in the crystal lattice via substitution of Ti⁴⁺ ions with Cu¹⁺ ions. The catalytic performance of the Cu doped TiO₂ monoliths for hydrocarbon formation was found to be considerably higher when compared to pure TiO₂ under UVA or visible light irradiation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00991b

Catalysis
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Copper based TiO honeycomb monoliths
2
for CO photoreduction
2
Cite this: Catal. Sci. Technol., 2014,
4, 1631
Oluwafunmilola Ola* and M. Mercedes Maroto-Valer
2 2
The direct photoreduction of CO via catalytic conversion of copper supported on TiO based
monolithic structures is a means by which solar fuels can be produced. Copper based monolithic
structures with varying loadings were synthesized through a sol–gel dip coating procedure and tested for
2 2
CO reduction with H O as a reductant in the gaseous phase. Results established that increased copper
Received 29th November 2013,
Accepted 21st February 2014
concentration can decrease crystalline size and promote anatase to rutile phase transformation. The
1+
coated monolithic structures were dominated by mainly Cu species, as confirmed by XPS while bulk
4
+
characterization suggests that these species are present in the crystal lattice via substitution of Ti ions
DOI: 10.1039/c3cy00991b
1
+
with Cu ions. The catalytic performance of the Cu doped TiO
2
monoliths for hydrocarbon formation
www.rsc.org/catalysis
was found to be considerably higher when compared to pure TiO
2
under UVA or visible light irradiation.
efficiencies and improved light harvesting can only be achieved
through the combined use of an optimized photoreactor and
1
. Introduction
The utilization of carbon dioxide (CO
2
) for photocatalytic
photocatalyst configurations. Nishimura et al. dip coated TiO
on a silica–alumina gas separation membrane to obtain
3.5 ppmV h of CO after 336 hours, while Pathak et al. used
2
reduction driven by solar energy is a promising strategy for
producing sustainable fuels that are suitable for use in
existing energy infrastructure. Although the feasibility of
−
1
9
the hydrophilic structural cavities in Nafion-117 membrane
using titanium dioxide (TiO ) based materials for UV induced
films to host TiO coated with nanoscale silver and obtained
2
2
1
,2
photocatalysis has been demonstrated,
its visible light
methanol as the major product and formic acid as the minor
3
10
applications are limited. The tailoring of the properties of
product. Their results were reproducible even when these
titanium dioxide (TiO ) by the addition of metals that are rel-
films were reused. Cybula et al. employed a flat perforated
2
atively inexpensive and readily available for CO photoreduc-
tion systems is highly desirable. In this regard, the use of
steel or plastic tray as a support for the dispersion of TiO in
a tubular reactor designed for CO photoreduction studies.
2
2
2
1
1
4
copper species has been increasingly investigated.
They observed that the type of support used not only played a
critical role in determining the amount of immobilized cata-
lyst, but also influenced the photoconversion rate when the
same coating procedure was used. A decrease in catalyst load-
ing and methane production (from 90 ppm to 34 ppm) was
observed when the support was switched from steel to plastic
due to weaker adhesive properties of plastic compared to
those of steel.
2+
Several researchers have reported that Cu species are the
active sites on Cu based TiO catalysts for the degradation
2
5
6
of rhodamine B, photocatalytic water splitting and CO2
7,8
reduction. For CO reduction studies, it is generally accepted
2
that CuO can trap photoexcited electrons from the conduction
band of TiO2 and these trapped elections can participate in
reduction reactions with the surface adsorbed species thus
1
preventing electron–hole recombination. Furthermore, previ-
The interconnected three-dimensional structures like the
honeycomb monolith containing parallel straight channels
have been exploited for industrial processes due to its poten-
tially high surface to volume ratio, ease of scale-up through
an increase of its dimensions and channels, and control of
structural parameters (i.e. pore volume, pore size and surface
ous studies have established that the addition of copper can
improve visible light absorption and efficiency of TiO ; however,
2
little is known about the effect of these materials on supports
i.e. monoliths for the photocatalytic reduction of CO2.
Many researchers have focused on ways of anchoring
photocatalysts onto supports since high photoconversion
1
2,13
area) etc.
Photocatalytic studies conducted using a mono-
lith as a support have identified low light utilization efficiency,
due to little or no light absorption in the pores or channels of
the honeycomb monolith. Not all immobilised photocatalysts
may be activated due to limited light distribution arising from
the catalyst coated on the outer surface absorbing most of
Centre for Innovation in Carbon Capture and Storage (CICCS), School of Engineering
and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
E-mail: O.O.Ola@hw.ac.uk; Fax: +44 (0)131 451 3180;
Tel: +44 (0)131 451 4737
1
4
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1
5
the light. The light intensity also decays along the opaque
Hiltonbrooks X-ray powder diffractometer with a Philips PW
1050 goniometer and a proportional detector. Nickel filtered
Cu Kα radiation was used, operating at 20 mA and 40 kV with
a scan range of 5–65 (2θ), a scan speed of 2 degrees (2θ) per
minute and a step size of 0.05. The morphology of the nano-
particles was studied by transmission electron microscopy
(TEM) using a JEOL 2100F instrument at an acceleration volt-
age of 200 kV. A Quanta 600 model equipped with an energy
dispersive X-ray (EDX) system was used to perform quantita-
tive analysis and observe the morphology of the catalysts at
the voltages of 25 kV and 30 kV, respectively. Specific surface
16
channels of the monolith. More recently, it has been reported
that the drawbacks of limited light penetration and efficiency
of CO reduction can be improved by threading channels of
2
2
,17,18
monolithic structures with optical fibres.
Comparison of
the slurry reactor system with the monolith system demon-
strated that higher conversion and quantum efficiency can be
achieved when the monolith was employed as a catalyst
carrier. This was attributed to the combined advantages of
the higher geometrical internal surface area of the monolith
and the elimination of uneven light distribution via the optical
fibres. Accordingly, experimental analyses using copper based
nanomaterials immobilized onto monolithic structures
1
8
area measurements were estimated from N
2
adsorption–
desorption isotherms at 77 K that were measured using a
ChemBET TPR/TPD analyzer connected to a linear mass flow
controller/gas blender. The porosity and pore size distribu-
tions of the monoliths were characterized using a mercury
(Hg) porosimetry analyzer (Micromeritics Autopore IV 9520
V1.05) with Hg pressure in the range of 0.7–275 790 kPa.
The elemental ratios of the metals contained within the
nanoparticles were quantified using the Varian Vista MPX
ICP-OES (inductively coupled plasma optical emission spec-
troscopy) system that used an echelle polychromator with a
mega-pixel CCD detector. Prior to sample injection, approxi-
threaded with optical fibres for CO reduction were conducted.
2
Detailed characterization techniques were employed in order
to investigate the effect of copper doping on the physico-
chemical properties of TiO2 based monoliths and correlate
2
these properties to CO photoconversion.
2
. Experimental
.1 Preparation of copper based TiO monoliths
2
2
A series of Cu doped TiO
.2–2 wt% were prepared by the sol–gel method (Fig. 1). The
monoliths were pre-coated with the SiO sol prior to dip-
coating in the Cu–TiO sol. As shown in Fig. 1, the Cu–TiO sol
2
monoliths within the range of
0
2
mately 25 mg of Cu–TiO samples were digested in a mixture
2
of 5 ml of H SO and 0.5 ml of HClO . The solution was then
2
4
4
2
2
made up to 100 ml in deionised water. X-ray photoelectron
spectroscopy (XPS) was performed on the nanoparticles using
a Kratos AXIS ULTRA instrument with a mono-chromated Al
Kα X-ray source (1486.69 eV) operated at a 15 mA emission cur-
rent and a 12 kV anode potential (180 W). High resolution
scans were taken for 5 or 10 minutes each over the appropriate
regions for the photoelectron peaks with a step of 0.1 eV and a
pass energy of 20 eV. Wide/survey scans over the full energy
range B.E. of 1400–5 eV were performed on each sample at a
pass energy of 80 eV. The wide scans were used to estimate
quantification of each element present based on the peak
areas using the CASAXPS software with Kratos sensitivity
factors. The high resolution scans were charge corrected to the
main C 1s peak = 285 eV and used to determine the chemical
states of the elements detected. Spectral fitting was performed
using the CasaXPS software with a line shape based on a
Gaussian/Lorentzian mix of 70: 30 (GL30). The band gap,
threshold wavelength and the absorbance of ultraviolet light as
a function of the transmittance were measured using the diffu-
sive reflective ultraviolet-visible spectrophotometer (Varian
Cary 300). The band gap energy of the samples were calculated
was synthesized by adding a mixture of titanium(IV) butoxide
and n-butanol to calculated amounts of copper(II) chloride
dihydrate (CuCl
quently, polyethylene glycol (PEG) solution was added to the
metal loaded sol and stirred for 6 hours. The pre-coated SiO
monoliths were then dip-coated in the resulting Cu–TiO sol for
0 minutes. The Cu–TiO coated monoliths and the remaining
2 2
·2H O) dissolved in 14 ml of acetic acid. Subse-
2
2
3
2
sol were dried and calcined in a furnace at 150 °C and 500 °C,
18
respectively. This procedure is detailed in previous work.
2
.2 Photocatalyst characterization
Detailed information about the crystallographic structure of
the sample showing the integrated intensity, peak positions,
planes and unit cell parameters was obtained using a
g
using E = hc/λ where h, c and λ represent the Planck's
constant, velocity of light and wavelength, respectively.
2.3 Photoreduction of CO2
The photocatalytic reduction of CO2 under UVA or visible
light was conducted in a cylindrical Pyrex glass reactor with a
3
volume of 216 cm . The catalyst coated ceramic honeycomb
monoliths with 177 channels were threaded with optical
fibres to ensure light distribution within the internal channels
2
Fig. 1 Sol–gel procedure for Cu–TiO monoliths.
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of the monolith. The humidifier was connected before the gas
inlet, while the temperature and pressure were monitored via
a type T thermocouple and a pressure gauge, respectively,
connected by 1/8′′ fittings after the product outlet. Light was
irradiated into the side of the reactor by a light guide, with
the illumination system being either a 200 W mercury lamp
or a 500 W halogen lamp, with light intensities of 33.42 and
These results suggest that the addition of Cu causes the
gradual transformation of anatase to rutile with increasing
metal concentration. The lattice constants (a & c) of Cu–TiO
2
monoliths calculated based on the anatase (101) diffraction
peaks, as listed in Table 1, increase with a higher doping
amount, when compared to the lattice parameters of TiO
2
(a = 3.7892 Å, c = 9.4803 Å). Lattice parameter measurements
were repeated thrice for verifying reproducibility. The stan-
dard error of the lattice parameter measurement via XRD is
within the range of ±0.05–0.28%. The lattice parameter of
these Cu doped TiO2 monoliths increases as the crystallite
size of anatase decreases.
−2
6
8.35 mW cm respectively.
After performing a leak test with helium (He) gas, ultra
pure CO (Air Products, 99.9995%) gas saturated with water
2
vapour was bubbled into the reactor for 1 hour at a flow rate
−
1
of 4 ml min . Subsequently, the light source was turned on
and readings were taken after 4 hours. The flow of CO satu-
rated with water vapour was continuous throughout the reac-
tion. The H O content in the feed was 50 ml and the
2
pressure was maintained at 1 bar for every experimental run.
Products extracted from the outlet of the gas-phase
photoreactor were analyzed using a mass spectrometer (MS,
Hiden Analytical) equipped with a capillary, quadrupole mass
analyser (HAL 201-RC) and Faraday/Secondary electron multi-
plier (SEM) detectors. Prior to every photocatalytic experi-
ment, blank reactions were performed to confirm product
The high resolution (HR) TEM images of 1 wt% Cu–TiO2
using different magnifications illustrated in Fig. 3a show
aggregates of spherical nanocrystals with varying sizes from
5–27 nm. The SEM-EDS (energy dispersive spectroscopy)
micrograph of the 1 wt% Cu–TiO2 monolith presented in
Fig. 3b confirms the presence of Cu, with the morphological
features of the samples remaining unchanged by doping. The
2
2
thickness of the 1 wt% Cu–TiO film measured by SEM was
up to 0.32 μm on the surface of the monolith. The pore size
distribution of the 1 wt% Cu–TiO2 monolith measured by
mercury porosimetry is illustrated in Fig. 4. The porosity and
2
formation was due to CO photoreduction.
total intrusion volume for the 1 wt% Cu–TiO
2
sample were
−
1
35.04% and 0.17 mL g while the pore size distribution was
3
. Results and discussion
within the macropore range with the average pore diameter
being 250 Å.
3
.1 Textural properties of supported Cu catalysts
As shown in Fig. 2, the XRD diffraction patterns of TiO2
monoliths doped with different concentrations of copper con-
sist mainly of two diffraction phases of anatase (A) and rutile (R).
The rutile phase was detected in these samples at peak
positions of 27.4° and 36.1° after calcination at 500 °C. The
The BET specific surface area of the Cu–TiO based mono-
2
2
−1
liths was within the range of 34.77–88.96 m g (Table 1),
and the standard error of these measurements is within the
range of +0.02–0.5%. An increase in specific surface area of
2
TiO occurs with an increase in the Cu loading.
crystallite size of all doped Cu–TiO
lated from the Scherrer equation was within the range of
6.37–19.12 nm (Table 1). As the crystallite size of anatase
2
based monoliths calcu-
3.2 ICP-OES and XPS analyses
1
decreased, an increase in rutile content was observed with
Table 1 lists the quantitative analysis results calculated from
increased metal concentration, with the 2 wt% Cu–TiO sam-
ICP-OES. The ICP-OES analysis of Cu–TiO based monoliths
2
2
ple showing the maximum growth of rutile nuclei. This is
due to the ability of Cu in enhancing the particle sintering
process i.e. accelerating densification and grain growth, and
demonstrated that Cu was present in the TiO2 matrix. The
bulk elemental ratios of the samples are in agreement with
the elemental concentration present in the precursor and
show an increasing trend with increased metal concentra-
tion. This suggests that the added metals were primarily
1
9
thus promoting mineral phase transformation.
4
+
located in the crystal lattice via substitution of the Ti ions
2
+
with Cu ions.
2
The high resolution XPS spectra of Cu 2p of Cu–TiO mono-
liths are presented in Fig. 5. The Cu 2p3/2 and 2p1/2 peaks
formed doublets by peak fitting suggesting that the chemical
1
+
2+ 20,21
state is mainly Cu with small amounts of Cu .
It has
been reported that Cu may be reduced under the X-ray beam
2
2
during XPS analysis. In the work presented here, three sets of
scans were collected for each sample on three different areas.
The experiment times were about 1 hour per area analysed
with a monochromated source which has lower X-ray flux at
the sample than at a conventional ‘flood’ source. Nevertheless,
it is possible that the Cu oxidation state started out before
analysis proceeded, but analysis of auger peaks of Cu was not
Fig. 2 XRD pattern of TiO
: 0.2 wt% Cu–TiO ; 3: 0.5 wt% Cu–TiO
Cu–TiO , 6: 2 wt% Cu–TiO , anatase (A) and rutile (R)).
2
monoliths with different Cu loadings (1: TiO
2
,
2
2
2
, 4: 1 wt% Cu–TiO , 5: 1.5 wt%
2
2
2
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Table 1 Physicochemical properties of the Cu–TiO
2
based photocatalysts
Lattice parameters
a
Crystallite size (nm)/phase content (%)
ICP-OES
b
2
−1
Photocatalysts
TiO
Anatase
Rutile
a (Å)
c (Å)
S
BET (m g
)
Cu (wt%)
Band gap (eV)
2
12.99 (96.50)
23.12 (96.70)
19.12 (96.00)
18.69 (94.60)
18.29 (90.80)
16.37 (89.10)
4.91 (3.50)
4.79 (3.30)
5.97 (4.00)
8.51 (5.40)
8.77 (9.20)
25.51 (10.90)
3.7892
3.8039
3.8166
3.8186
3.8206
3.8295
9.4803
9.4899
9.4943
9.4981
9.507
52.50
34.77
37.52
48.22
71.34
88.96
0.00
0.25
0.53
0.99
1.60
2.03
3.08
3.02
2.96
2.82
2.74
2.61
0
0
1
1
2
.2 wt% Cu–TiO
.5 wt% Cu–TiO
.0 wt% Cu–TiO
.5 wt% Cu–TiO
.0 wt% Cu–TiO
2
2
2
2
2
9.5242
a
b
Estimated using the Scherrer equation on the (101) diffraction peak of anatase TiO
2
.
BET surface area.
2
Fig. 3 TEM (A) and SEM-EDS (B) micrographs of 1 wt% Cu–TiO .
Fig. 4 Pore size distribution measured by mercury porosimetry of the
wt% Cu–TiO monolith showing the cumulative intrusion (A) and
differential intrusion volume (B).
Fig. 5 XPS spectra of Cu 2p of Cu–TiO
2 2
monoliths A) 0.5 wt% Cu–TiO ,
1
2
B) 1 wt% Cu–TiO C) 1.5 wt% Cu–TiO D) 2 wt% Cu–TiO .
2
2
2
possible as scans were not recorded over sufficiently long expo-
sure time to confirm reduction.
These satellite peaks which have been reported to be respon-
sible for the shakeup transitions by ligand to metal 3d charge
2
+
1+
The intensity of the characteristic satellite peak for Cu
transfer cannot be found in metallic Cu and Cu species,
2
3
observed at 942.3 eV increased with the increasing Cu con-
due to their completely filled 3d shells. The satellite peaks
4
centration (Fig. 5). Colon et al. reported that the key differ-
were observed at 941.7 eV and 942.5 eV for 1 wt% Cu/N–TiO2
1
+
2+
ence between Cu and Cu species was the prominent
and 1 wt% Cu–TiO samples calcined at 600 °C. Liu et al. also
2
2
3
satellite peak present on the high binding energy sides.
observed satellite peaks at 942.2 eV and 942.4 eV for 1 wt%
1634 | Catal. Sci. Technol., 2014, 4, 1631–1637
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and 5 wt% Cu–TiO samples prepared by simple precipita-
tion, respectively. The XPS spectra of binding energies for Ti
photoreduction under UVA and visible light irradiation
(Fig. 6). As shown in Fig. 7, several products such as hydro-
gen, methanol, acetaldehyde and ethanol were formed after 4
hours of light irradiation. The CO2 reduction experiments
were repeated thrice, with the production rates averaged and
the standard deviations reported in Fig. 7. The product rates
steadily increase with an increase in the metal concentration
2
4
2
p were observed at 458.8 eV and 464.6 eV which correspond
4
+
24,25
to Ti in TiO2.
literature, where Ti peaks were observed at 457.7 eV and
These results are in agreement with the
4
+
2
4
4
63.4 eV for the 1 wt% Cu–10 wt% I–TiO
2
sample.
The XPS spectra of the O 1s region suggest that oxygen
exists in three forms on the sample surface with the binding
energies of 529.5, 530.1 and 531.7 eV. The main peak appears
at 529.9 eV and can be assigned to the bulk oxygen bound on
to give the optimal ratio of 0.5 wt% Cu–TiO for the internally
2
illuminated monolith photoreactor systems under either UVA
or visible light irradiation, after which reduced product rates
were observed for the subsequent higher doping ratios.
TiO . This value is consistent with the value of 530.1 eV
2
2
1
reported in the literature for anatase TiO
29.5 eV probably corresponds to the O 1s peak of CuO
while the other peak at 531.7 eV can be attributed to surface
2
.
The peak at
Hydrogen and methanol were favourably produced; with max-
2
6
−1 −1
5
imum product rates of 12.55 μmol g
h
and 3.92 μmol
cat
−
1
−1
gcat h , respectively under UVA (Fig. 7(I)) and 3.73 μmol
−
25,27
−1 −1
−1 −1
adsorbed components of the hydroxyl (OH ) group.
g
cat
h
and 0.23 μmol gcat h , respectively under visible
light irradiation (Fig. 7(II)). The higher hydrocarbon evolu-
tion rate observed when the monolith was used as a catalyst
carrier was due to the improved light distribution in the
internally illuminated monolith photoreactor system.
3
.3 Diffuse reflectance UV-Vis spectra of the Cu–TiO monoliths
2
The UV-Vis diffuse reflectance spectra of pure TiO2 and
Cu-monoliths at various loading ratios are shown in Fig. 6.
The absorption spectra of the resulting Cu-based TiO
photocatalysts showed an increased shift in the visible light
with increased Cu loading concentration in comparison with
pure TiO . The band gap energies of these catalysts were
2
within the range of 2.61–3.02 eV. The lowest band gap energy
2
3
.5 Correlation between catalyst characterization
and yields of photoconversion
The improved photoreduction activity demonstrated by the
0
.5 wt% Cu–TiO2 coated monolith in UV and visible light
regions compared to pure TiO can be attributed to the incor-
poration of Cu ions into the TiO matrix and the good
2
was observed with the 2 wt% Cu–TiO sample which is consis-
2
1+
2
tent with the literature, where increasing metal loadings
bi-crystallized TiO structure (i.e. crystallite phase of anatase
2
results in a shift in the absorption edges of the TiO based
2
4,5,7,23
with a small percentage of rutile). Phase transformation can
be facilitated by substitutional dopants when cations enter the
anatase lattice and cause an increase in the level of oxygen
samples.
The defects created in the TiO2 network and
crystalline structure is responsible for change in band gap
7
energy. The absorption edge between 400–600 nm can be
30
vacancies through valence or reduction/oxidation effects.
attributed to the presence of surface defects created during
2
8
annealing along with the crystallization of the rutile phase.
2
9
Sahu and Biswas also observed increased absorption with
the increasing Cu concentration. The change in light absorp-
2+
1+
tion was attributed to the incorporation of Cu ions into TiO
2
4
+
2+
crystal lattice via the substitution of Ti by Cu atoms. The
increased Cu concentration was also reported to increase
oxygen vacancies due to the charge compensation effect.
2+
3
.4 Photocatalytic reduction of CO2
The photocatalytic activities of the Cu-based TiO monoliths
2
threaded with optical fibres were evaluated for CO
2
Fig. 7 Effect of Cu doping on the product rate using the monolith
as a catalyst carrier under UVA (I) and visible (II) irradiation A) TiO
B) 0.2 wt% Cu–TiO , C) 0.5 wt% Cu–TiO , D) 1 wt% Cu–TiO E) 1.5 wt%
Cu–TiO F) 2 wt% Cu–TiO
2
,
2
2
2
Fig. 6 UV-Vis diffuse reflectance spectra of Cu–TiO
2
monoliths.
2
2
.
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1
+
4+
1+
Since the ionic radius of Cu is similar to Ti , results
from XRD confirm that the probability of substitutional
doping occuring is high i.e. these metal ions occupying the
lattice points of Ti. The decrease in the crystalline size with
increased Cu loading and lattice expansion observed in the
diffraction patterns of the Cu doped samples explain the
peak broadening observed which is associated with substitu-
tional doping.
After the optimal doping ratio of Cu was exceeded within
the series of synthesized catalysts (0.5 wt%), reduced
photoactivity was observed. This result could be due to the
coverage of the surface of TiO2 with increased metal ions
which inhibited interfacial charge transfer due to the insuffi-
cient amount of light energy available for activation of all of
the catalyst particles. These results are in agreement with
3
6
Li et al. where the copper dopant below or above the opti-
mum value of 0.5 wt% resulted in reduced production rates.
The decrease in production rates at lower doping ratios below
the optimum value was attributed to low Cu concentration
while reduced catalytic activity at higher loadings was attrib-
uted to excess Cu species acting as recombination centres for
photogenerated electrons and holes. When the doping con-
Nair et al. reported that cations with oxidation states of 3+
or lower tend to increase the oxygen vacancies in the lattice
of TiO if placed within the lattice points. This increased
concentration causes the subsequent rearrangement of atoms
and reorganization of the structure for the rutile phase in the
lattice of TiO through the substitution of Ti with cations.
Based on this, an increase in the concentration of oxygen
vacancies will occur, which will enhance the nucleation pro-
cess (i.e. anatase to rutile transformation) as also observed in
1
9
2
4
+
5,30
2
2
+
37
tent of Cu exceeded 5 wt%, Tian et al. recorded a decrease
in photocatalytic activity due to electron hole recombination.
3
8
According to Schiavello, photoreactivity can be negatively
influenced by either a high concentration of metallic islands
on the semiconductor surface or an enhancement of their
size. When this occurs, reduced surface illumination of cata-
lysts and increased recombination rate are observed.
1
9
this study.
The phase transformation of Cu based TiO samples with
2
increased metal concentration observed in this study were
probably enhanced due to increased concentration of oxygen
vacancies which simultaneously increased atomic mobility.
Sahu and Biswas reported that the addition of metal dop-
ants can alter the crystal phase of TiO , with the degree of
mineral phase transition being dependent on the metal type
and the concentration. This same phenomenon was observed
by Nair et al., where increased enhancement was observed
2
9
4
. Conclusions
2
The photocatalytic reduction of CO2 over Cu–TiO2 coated
monolithic structures threaded with optical fibres was
conducted under UV and visible light irradiation. The copper
species present in the substitutional sites of the TiO matrix
were found to modify the crystalline and optical properties of
1
9
over CuO doped TiO samples compared to NiO doped TiO .
2
2
2
3
2
Colon et al. also observed lower anatase content with
increased Cu concentration due to the higher amounts of
dopants favouring the rutilization process. The influence of
these substitutional ions is further confirmed by the change
in light absorption properties and the electronic structure of
the metal loaded TiO samples observed in the UV-Vis spectra
2
when compared to pure TiO . According to Li et al.,
electronic states introduced by substitutional metal ions on
the bottom of the conduction band edge of TiO cause the
formation of a new higher unoccupied molecular orbital.
This molecular orbital narrows the band gap; as also found
in this study (Fig. 6) and thus influences photon absorption.
The synergistic effect between the two crystalline phases in
the Cu based samples could also be another plausible reason
for improved activity. Improved charge separation and high
reactivity at the anatase to rutile interface occur during
electron transfer from rutile to anatase at this interface where
defect sites with unique charge trapping and adsorption prop-
1
+
TiO
2
. Cu was identified as the primary Cu species which
facilitated multi electron reactions and thus improved the
1
+
efficiency of CO
2
photoreduction. The increase in the Cu
concentration facilitated the anatase to rutile transformation
2
1+
4+
due to the substitution of Cu by Ti in the TiO structure.
3
1
2
Upon UV and visible light irradiation, the Cu doped
photocatalysts exhibited improved activity compared to pure
TiO at optimal doping ratios. The decline in the production
rate observed upon increased Cu concentration was proba-
bly due to the coverage of the surface of TiO with excess
metal particles. This inhibited interfacial charge transfer was
due to the insufficient amount of light energy available for
activation of the catalyst particles. More importantly, the
improved conversion efficiency was probably due to improved
charge separation at the anatase to rutile interface and the
2
2
1
+
2
1
+
presence of Cu species serving as electron traps which
suppressed electron–hole recombination.
3
2,33
33
34
erties can be created.
Bouras et al. and Zhang et al.
reported that electron hole recombination can be retarded
through the creation of energy wells and surface anatase/rutile
phase junctions which serve as electron traps formed from
the lower band gap of rutile thus facilitating charge separation
and increasing the lifetime of photogenerated electrons and
holes. The presence of mixed crystalline phases of titania (i.e.
anatase and rutile) has also been reported to show improved
photocatalytic activity due to the synergistic effect derived
Acknowledgements
We acknowledge the financial support from the School of
Engineering and Physical Sciences and the CICCS (EPSRC
grant EP/F012098/2) at Heriot-Watt University. M. Maroto-Valer
is grateful for the support from the Leverhulme Trust
(Philip Leverhulme Prize). We thank Emily Smith, University
of Nottingham, for XPS analysis.
3
5
from better charge separation and high surface area.
1636 | Catal. Sci. Technol., 2014, 4, 1631–1637
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