G Model
CATTOD-10474; No. of Pages6
ARTICLE IN PRESS
ˇ
T. Cizˇmar et al. / Catalysis Today xxx (2016) xxx–xxx
2
2. Experimental methods
tomultiplier tube was 56 or 78. For each irradiation time, at least
2.1. Synthesis of Cu-modified TiO2–SiO2 photocatalysts
Oxidation of TPA to HTPA under UV/vis irradiation can be
described by zero-order kinetics, (rate constant k1). For the sub-
sequent degradation of HTPA, the pseudo-first order kinetics (rate
constant k2) is proposed [13],
Sol–gel method was used to synthesize new copper-modified
and 3 mol%), using titanium tetraisopropoxide (TTIP), tetraethyl
ortosilicate (TEOS) and colloidal SiO2, and copper acetlyacetonate
(Cu(acac)2) as Ti, Si and Cu sources. We have revised a formerly pro-
posed synthesis method [3]. For the TiO2–SiO2 colloidal solution we
prepared silica binder solution from 1.11 mL tetraethyl ortosilicate
(TEOS, Acros Organics), 1.7 mL colloidal SiO2 Levasil (Obermeier)
200/30% aqueous solution, 30 L HCl (32 wt%, J.T. Baker) to catal-
yse TEOS hydrolysis, and after 1 h of mixing 5 mL 1-propanol (Fluka)
was added. The obtained product contained 11.5 wt% of SiO2. The
TiO2 sol was prepared by dissolving titanium tetraisopropoxide
(TTIP, Acros Organics) (15 mL) in absolute ethanol (2.5 mL). In
the first step, double deionized water (45 mL) and 70% perchlo-
ric acid (1 mL) were mixed separately. This solution was then
added to the TTIP solution drop-wise, under reflux and heating,
where exothermic reaction of uncontrolled hydrolysis and conden-
sation of TTIP took place, gaining white precipitate of hydrated
amorphous TiO2. After heating and refluxing for 48 h, a stable
translucent TiO2 sol was obtained. The TiO2–SiO2 sol (denoted TS)
was obtained by adding silica binder solution (3 mL) to TiO2 sol
(3 mL) The sol was further diluted with 4 mL double deionized
water and organic solvents (6 mL of 1-propanol and 19.5 mL of
2-propoxyethanol). Copper cations were added by direct incorpo-
ration during the sol–gel synthesis. Cu(acac)2 (200 mg) dissolved
in 2-propoxyethanol (150 mL), used as a source of copper, was
added (600 L, 18 mL) into the TiO2–SiO2 sol. All TiO2–SiO2 sols
(unmodified and Cu-modified) were dried at 150 ◦C for 1 h, to
obtain unmodified TiO2–SiO2 and Cu-modified TiO2–SiO2 catalysts
with two different dopant copper concentrations (0.1 and 3 mol%),
denoted as TS 150, TS 0.1Cu 150, TS 3Cu 150, respectively. In addi-
tion, another set of photocatalyst samples (TS 500, TS 0.1Cu 500,
TS 3Cu 500) was prepared from the dried photocatalysts, by calci-
nation of the dried powders at 500 ◦C for 1 h.
d HTPA
[
]
= k1 − k2 HTPA
(1)
[
]
dt
which predicts the following dependence of the concentrations of
HTPA in model aqueous middles containing both TPA and the tested
photocatalytic samples on the duration of their light illumination:
k1
·t
2
HTPA =
)
(2)
[
]
k2
Measurements of HTPA concentrations were performed in the
time interval from 0 to 20 min. Due to a small value of the rate
constant k2 (t « 1/k2), we can approximate the exponential term
in Eq. (2) with a linear approximation (1-k2t) and model the time
dependence of the HTPA concentration with a linear function:
HTPA = k1 · t
(3)
[
]
The formation rate constant of HTPA (k1) is used to compare
photocatalytic activity of the Cu-modified photocatalysts with an
unmodified one.
2.3. XRD analysis
The crystal structures of unmodified and Cu-modified TiO2–SiO2
samples were investigated by X-ray diffraction (MiniFlex Bench-
top 300/600, 150) using Cu K␣ irradiation from 10 to 80◦ at a scan
rate of 2◦/min. Quantitative phase composition analysis was per-
formed using Rietveld refinement method by the High Score Plus
software. The crystallite size was determined from XRD pattern,
using Scherrer formula:
d = (0.9ꢀ/ˇcosꢁ),
(4)
where d is the crystallite size in nm, ꢀ is the wavelength of X-ray in
Å (1.5418 Å), ˇ is the full width of diffraction peaks at half maxima
(FWHM) in radians, and ꢁ is the Bragg angle.
2.2. Photocatalytic activity
2.4. N2-physisorption
From the alkaline stock solution of TPA [14], with the concen-
tration of 130 mg/L that was prepared by using 2 × 10−3 M NaOH,
working solution of TPA (100 mL, 83 mg/L) was freshly made prior
to photocatalytic experiments. 25 mL of TPA and 10 mg of photocat-
alyst were mixed in 25 mL of double deionized water and stirred
under sunlight irradiation. Samples (1 mL) of the water solution
were taken from the reactor at different UV irradiation times (0 min,
3 min, 6 min, 10 min, and 20 min) and centrifuged (1300 min−1) for
3 min. A fixed volume (159 L) of the solution was then sampled
with an automatic pipette, and transferred into microliter plate
wells (microliter plate with 96 wells, flat bottom, black) for flu-
orescence measurements. Photocatalytic tests were carried out in
solar simulator (Suntest XLS+, Atlas, USA) chamber with a simu-
lated solar irradiation source (Xenon lamp), using daylight filter
(300–800 nm), at UV light flux of 750 W/m2. During irradiation
in the presence of the photocatalyst the TPA is decomposed and
highly fluorescent 2-hydroxyterephthalic acid (HTPA) is formed
as an intermediate oxidation product. Fluorescence measurements
were performed using a microplate reader in the fluorescence mode
(Infinite F200 Microplate reader, Tecan, Switzerland). The wave-
length of the excitation light was 320 nm (filter bandwidth: 25 nm)
and emission was measured at 430 nm (filter bandwidth: 35 nm).
The instrument was operating in top mode with 25 reads per well,
with 20 s integration time. The amplification factor for the pho-
Nitrogen adsorption measurements were performed at 77 K
using a Tristar 3000 Micromeritics volumetric adsorption analyser.
vacuum for 2 h at 473 K in the port of the adsorption analyser.
The BET specific surface area [15] was calculated from adsorption
data in a relative pressure range from 0.05 to 0.25. The total pore
volume was estimated on the basis of the amount of adsorbed nitro-
gen at a relative pressure of 97% [16]. The pore size distributions
(PSDs) were calculated from nitrogen adsorption data using the
Barrett–Joyner–Halenda (BJH) method [17]. The maxima on the PSD
data plots were considered as the primary mesopore diameters for
given samples.
2.5. XAS
The local atomic structure and the chemical state of copper
cations in Cu-modified TiO2–SiO2 photocatalyst were analysed by
X-ray absorption spectroscopy. Cu K-edge absorption spectra of the
prepared photocatalyst samples, and the spectra of a 0.005 M solu-
tion Cu(acac)2 in 2-propoxyethanol used as source of copper, were
recorded at room temperature in transmission and fluorescence
detection mode at the XAFS beamline of the ELETTRA synchrotron
radiation facility in Trieste, Italy, and at P65 beamline of PETRA
ˇ
Please cite this article in press as: T. Cizˇmar, et al., Correlations between photocatalytic activity and chemical structure of Cu-modified