G Model
APCATA-15554; No. of Pages9
ARTICLE IN PRESS
A. Beltram et al. / Applied Catalysis A: General xxx (2015) xxx–xxx
2
the most studied phase — anatase and rutile — as examples, the
electronic band structure. Anatase has a larger band gap of rutile
(3.20 vs 3.02 eV), resulting in slightly higher conduction band edge
energy and electron Fermi level [22,23]. Due the lower density, the
Ti–Ti distance in anatase is larger than in rutile, resulting in more
localized 3d states [61,62]. Generally, anatase has larger surface
area than rutile, favoring adsorption of molecules and increas-
ing the activity because of the higher number of active sites.
Finally, the smaller crystallite size of anatase improves the effi-
brookite, another polymorph of TiO2, attracted great attention in
the last years. Particular advantage has been reported by the use
of brookite-containing materials in H2 production from aqueous
solution containing methanol [63–65] or ethanol [66]. These results
have been related with the larger band gap of brookite (3.3–3.4 eV),
lying the edge of its conduction band to higher potential with
respect to anatase [65].
The morphology of the composite materials was investigated
by high resolution TEM (HR-TEM) measurements, performed on
a TEM JEOL 2010-FEG microscope with an acceleration voltage of
200 kV and with 0.19 nm spatial resolution at Scherzer defocus
conditions. In order to obtain accurate particle size distribu-
tion of the photodeposited Pt nanoparticles, High-Angle Annular
Dark-Field (HAADF)—Scanning Transmission Electron Microscopy
(STEM) technique was carried out using an electron probe of 0.5 nm
and a diffraction camera length of 120 cm. HAADF-STEM allows to
obtain Z-contrat images and easily identify heavy metals on light
support. At least 150 particles were counted to obtain the parti-
cle size distribution. The samples collected after the catalytic tests
were washed with a water/ethanol solution, dried and dispersed
onto lacey carbon grid.
N2 physisorption measurements at the liquid nitrogen temper-
ature was performed using a Micrometrics ASAP 2020 automatic
analyzer. The samples were degassed in vacuum at 120 ◦C for 12 h
prior to analysis.
In this study, we employed a thermal hydrolysis process to
prepare crystalline TiO2 materials with different phase compo-
sition, tuned changing systematically the synthesis conditions.
Adopting the same reaction conditions (160 ◦C, 24 h) and the same
Ti precursor (titanium (IV) bis(ammonium lactate) dihydroxide
— Ti(NH4C3H4O3)2(OH)2) and changing the urea concentration
employed for hydrolysis (0–7.0 M), the phase composition can be
tuned depending on the urea/Ti molar ratio, ranging from rutile
to anatase and finally pure brookite, through the formation of the
biphasic composites (rutile/anatase and anatase/brookite). These
materials have been tested for photocatalytic H2 production under
simulated sunlight employing renewable raw materials as sacrifi-
cial agents: ethanol and glycerol.
2.3. Photocatalytic tests
The activity of the prepared materials was evaluated in terms of
hydrogen production by photoreforming of two alcohols of relevant
environmental interest present in aqueous solution: ethanol at 50%
v/v and glycerol 1 M.
In a Teflon-lined photoreactor described elsewhere [68], 50 mg
of the calcined materials were suspended into 80 mL of alcohol
solution containing 100 L of an aqueous solution of Pt(NO3)2 (1 mg
Pt mL−1). The amount of the catalyst has been optimized follow-
ing the indication recently reported by Kisch and Bahnemann [69].
Before switching on the lamp, the reactor was thermostated at
25 ◦C and purged from the air with Ar flow of 15 mL min−1 for at
least 40 min. The photoreactor was illuminated with a Lot-Oriel
Solar Simulator equipped with a 150 W Xe lamp and an Atmo-
spheric Edge Filter with a cut-off at 300 nm. This results in a surface
power density of ∼25 mW cm−2 in the UV range (300–400 nm) and
∼180 mW cm−2 in the visible range (400–100 nm), approaching the
conditions used in a solar concentrator. During the initial part of
the photocatalytic experiments, Pt nanoparticles were loaded as
co-catalysts on the surface of the TiO2 materials, reaching a final
loading of 0.2 wt%.
The on-line detection of volatile products was carried out using
an Agilent 7890A Gas Chromatograph equipped with two analyt-
ical lines. A 10 way-two loops injection valve was employed for
injection during on-line analysis of the gaseous products. A Car-
boxen 1010 PLOT (Supelco, 30 m × 0.53 mm ID, 30 m film) column
followed by a Thermal Conductivity Detector (TCD) was used for
gaseous products quantification using Ar as carrier and a DB-225ms
column (J&W, 60 m × 0.32 mm ID, 20 m film) using He as carrier
followed by a mass spectrometer (MS) HP 5975C was employed for
the detection of the volatile organic compounds.
After photocatalytic runs, the catalysts were collected by filtra-
tion on a 0.45 m PVDF Millipore membrane in order to analyze
the Pt nanoparticles distribution on the materials while the solu-
tions recovered were analyzed by GC/MS to detect the by-products
accumulated in the liquid phase. For a semi-quantitative analy-
sis, 1-butanol and 1-hexanol were used as internal standards in
samples from photoreforming of ethanol and glycerol, respectively.
Possible leaching of Pt during photocatalytic runs was checked
by Graphitic Furnace—Atomic Absorption Spectroscopy (GF-AAS)
using a Thermo M series AA spectrometer equipped with a GF95Z
Zeeman Furnace and a FS95 Furnace Autosampler (Thermo Electron
Corporation, Cambridge, UK). Pt detection limit at the analytical
wavelength of 266.0 nm was 10 g/L. A five-point standard curve
(in the range of 10–1000 g/L) was used for the analytical measure-
ments. The correlation coefficient of the standard curve was at least
2. Experimental
2.1. Catalyst preparation
TiO2 nanomaterials with different phase composition were syn-
thesized by hydrothermal treatment from commercial titanium
(IV) bis(ammonium lactate) dihydroxide Ti(NH4C3H4O3)2(OH)2
aqueous solution (50 wt%, Sigma–Aldrich) in the presence of urea,
adapting the procedure reported by Zhao et al. [67]. A 40 mL Teflon-
lined autoclave was charged with 1.5 mL of the Ti based precursor
and 13.5 mL of urea solution. The concentration of urea solution
was systematically increased from 0 to 7.0 M, in order to obtain
urea/Ti molar ratios in the range 0 – 44.5. After careful mixing, the
autoclave was heated at 160 ◦C for 24 h in a convection oven. The
product is obtained as a precipitate with a color changing from
slightly yellow to white as the urea/Ti molar ratio increases. The
precipitate was collected by centrifugation, washed several times
with bi-distilled water and finally dried at 80 ◦C overnight. The
materials were subjected to calcination at 400 ◦C for 3 h in order to
remove any organic contaminants deriving from partial decompo-
sition of the lactate precursor, obtaining white solids. Hereafter, the
obtained TiO2 materials are labeled as T-X, where X is the urea/Ti
molar ratio.
2.2. Materials characterization
X-ray diffraction (XRD) patterns were collected on
a
Philips X’Pert diffractometer using a monochromatized Cu K␣
(ꢁ = 0.154 nm) X-ray source in the range 10◦ < 2ꢂ < 100◦ and data
were analyzed using the PowderCell 2.0 software. Mean crystal-
lite sizes were calculated applying the Scherrer equation to the
principal reflection of each phase.