U. Caudillo-Flores et al.
MolecularCatalysisxxx(xxxx)xxx–xxx
gas and liquid phases, indicating the pros and cons of each process.
corresponding methanol content using a saturator at controlled tem-
perature. Photocatalytic experiments were carried out under UV-A light
irradiation (Sylvania F6WBLT-65; 6 W). Four fluorescent lamps sym-
metrically positioned outside the photo-reactor were used (see config-
uration at supporting information section, Figure S4). Reaction rates for
hydrogen production were evaluated under steady-state conditions,
typically achieved after ca. 3 h from the irradiation starting.
In both liquid and gas phase reactions and for selected samples we
carried out experiments with different catalyst concentration,
Methanol/Water ratio and irradiation intensity, using 3 levels for each
factor according to a Box–Behnken design [36]. Full details of factors
and levels are summarized in the supporting information section. In
both set-ups, the reaction rate and selectivity were analyzed using an
on-line Mass spectrometry (Onmistart 300), gas (TCD/FID detection
using HP-PLOT-Q/HP-Innowax columns and an Agilent 6890 appa-
ratus) and liquid chromatography (xD8-C18/5 microm, 4.6 x 150 mm
Agilent HPLC Column Eclipse and a Varian Pro Star 230 apparatus).
Quantum efficiency is defined in this work, according to the IUPAC
recommendation [29], as the ratio of the number of molecules reacting
by the number of photon interacting with the sample (Eq. 1).
Experimental section
The Nb-doped titania having a 3 mol. % (cationic basis) was syn-
thesized from titanium butoxide in a microwave reaction followed by
spraying dry and using a recipe previously reported [21]. Noble metals
were incorporated through a chemical reduction method using H2PtCl6
(Aldrich) and/or PdNO3 (Aldrich) and NaBH4 (Aldrich) as reducing
agent (M: NaBH4 = 1:5). The nomenclature used to identify the mate-
rials is: 3NbTi for the support, 3NbTi/Pt, 3NbTi/Pd, or 3NbTi/Pt-Pd
(x,y) where (x,y) is the Pt:Pd atomic ratio for the corresponding cata-
lysts. Samples contain a constant 0.4 mol. % of (total) noble metal
content within error as measured using atomic emission with inductive
coupled plasma (ICP-AES) using an Optima 3300DV Perkin Elmer
spectrometer).
XRD profiles of the samples were obtained using a Policristal X’Pert
Pro PANalytical diffractometer using Ni-filtered Cu Kα radiation with a
0.02° step. The particle size and strain were estimated using XRD using
the Williamson–Hall formalism [30]. Fitting of XRD profiles was carried
out using the Von Dreele approach to the Le Bail method [31]. Anatase
to rutile weight ratio was calculated using the method of Spurr and
Mayers [32]. The BET surface areas were measured by nitrogen phy-
sisorption (Micromeritics ASAP 2010). UV − vis diffuse-reflectance
spectroscopy experiments were performed on a Shimadzu UV2100 ap-
paratus using Teflon as a reference and the results presented as Ku-
belka-Munk transform [33]. Band gap analysis for the titania (anatase)
indirect gap semiconductor was done following standard procedures;
e.g. plotting (hva)n (n=1/2 or 2 for indirect or direct semiconductor;
hv = excitation energy, a = absorption coefficient) vs. energy and ob-
taining the corresponding intersection of the linear fit with the baseline
[34]. XPS data were recorded on 4 × 4 mm2 pellets, 0.5 mm thick,
prepared by slightly pressing the powered materials which were out-
gassed in the prechamber of the instrument at room temperature up to a
pressure < 2×10−8 Torr remove chemisorbed water from their sur-
faces. The SPECS spectrometer main chamber, working at a pres-
sure < 10−9 Torr, was equipped with a PHOIBOS 150 multichannel
hemispherical electron analyzer with a dual X-ray source working with
Al Kα (hv = 1486.2 eV) at 120 W, 20 mA using C 1s as energy reference
for adventitious carbon (284.6 eV). Surface chemical compositions were
estimated from XP-spectra, by calculating the integral of each peak
after subtraction of the “S-shaped” Shirley-type background [35] using
the appropriate experimental sensitivity factors and the CASA-XPS
(version 2.3.15) software.
2 × r (mol m−ns−1
)
ηq (%) = 100 ×
<ea > (Einstein m−ns−1
)
(1)
This equation takes into account that the transfer of two electrons is
required to reduce protons and thus to produce one H2 molecule. The
reaction rate of hydrogen production (r in Eq. 1) is measured in liquid
and gas phase reactors under pseudo-steady state conditions, as de-
tailed previously. To determine the denominator, we obtain the solu-
tion of the radiative transfer equation (RTE) in the heterogeneous re-
actor(s) for both liquid (calculation of the so-called local volumetric
rate of photon absorption; ea with n superindex in Eq. 1 equal to 3) and
gas (calculation of the so-called local superficial rate of photon ab-
sorption; ea with n superindex in Eq. 1 equal to 2) phase processes. In
Eq. 1 we used the volume or surface average values of the ea observable
parameter. The RTEs for liquid (Eq. 2) and gas phase (Eq. 3) measured
the variation of intensity (associated to a beam of x-rays at wavelength
λ in the direction of a solid angle vector , Ω_ ) through a direction of the
space (s).
dIλ,Ω_ (_x)
ds
σλ
4π
= −κλ Iλ,Ω_ (_x) − σλ Iλ,Ω_ (_x) +
= 0
p(Ω_ ′ → Ω_ )Iλ,Ω_ ′dΩ_
∫
′
Ω =4π
(2)
dIλ,Ω_ (x)
_
(3)
ds
Where κλ is the absorption coefficient; κλ is the dispersión coefficient;
and p(Ω_ ′ → Ω_ ) is the scattering phase measured with the Henyey and
Greenstein phase function, as usually carried out for titania samples
[37]. The justification of Eqs. 2 and 3 as well as details of the mathe-
matical procedure to calculate the denominator of Eq. 1 are presented
in the supporting information section and summarized in relevant
publication [14,29,37,38]. The specific catalyst(s) optical properties
involved in the determination of the ea parameter at the liquid/gas
phase reactions are also presented (Figures S3 and S9) in the mentioned
supporting information section. Results of the ea parameter for liquid
and gas phase processes and representative samples are summarized in
Figure S10.
Regarding photocatalytic measurements at liquid medium, they
were carried out using a batch pyrex (cutting absorption edge at ca.
300 nm) reactor as depicted in Figure S1 of the Supporting Information
section. The reactor contains a x:y (v/v) MeOH/H2O mixture medium
maintained at a constant temperature (20
1 °C). The catalyst sus-
pension (0.5 g L−1) was first degassed with an Ar stream for around
20 min. Subsequently, the Ar flow was settled down to 10 mL min−1
and stabilized before reaction. Ar is used as carrier to displace reaction
gases from the reactor to the detection system. The solution inside a
reactor was irradiated using a Hg-Xe lamp (500 W) and dichroic filters
(LOT Quantum Design) allowing exposure of the catalysis to the UV
(280–400 nm) wavelength range. The reaction rates for hydrogen pro-
duction were evaluated at 2–3 h from the start of the irradiation, where
a pseudo-stationary situation is reached.
Finally, to measure the absorption capability of the solids in both
reactors we will also report the so-called the light absorption efficiency
ƞA which relates the number of photon effectively absorbed by the
catalyst with the total number of incident photons [39]. This observable
is calculated as Eq. 4 for liquid phase and Eq. 5 for gas phase reaction.
Gas-phase reaction was carried in a continuous flow annular pho-
toreactor containing ca. 0.4 mg cm−2 of photocatalyst as a thin layer
coating on a pyrex tube. Gas and liquid phase use the same amount of
catalyst per experiment. The scheme of the reactor is shown in Figure
S4 of the supporting information section. After degassing the line with
Ar, the flow was settled down to 10 mL min-1 and stabilized before
reaction. A x:y (v/v) MeOH/H2O mixture was introduced in an Ar
carrier stream. Water was injected with a syringe pump and the
<ea
>
(Einstein m−3s−1) × V (m3)
ηA (%) = 100 ×
ηA (%) = 100 ×
q (Einstein m−2s−1) × A (m2)
(4)
(5)
<ea
>
(Einstein m−−12s−1
)
q (Einstein m−2s
)
2