ACS Catalysis
Research Article
mortar. Two calcination procedures were used. In this first
method, the sample was heated under static air in a muffle
furnace to 550 °C (2 °C·min‑1) for 6 h. In the second method,
the sample was heated in a quartz tube inside a tubular oven to
550 °C (2 °C·min‑1) first in a N2-flow (3 h) and subsequently in
an air-flow (3 h) (Figure S1). Gas flow rates of 20 mL·min‑1 were
employed.
We also checked if the air-flow parameter affected the
outcome of catalytic activity since we used static air (muffle-
furnace) in the first calcination method and an N2 and air flow in
the second method. However, no effect of the air-flow parameter
was observed.
4.2. Characterization Techniques. Powder XRD (PXRD)
patterns were recorded on a high-throughput STOE Stadi P
Combi diffractometer in the transmission mode equipped with a
Cu Kα1 source and IP-PSD detector. To calculate the crystallite
size of SnO2 present in the samples, the Debye−Scherrer
equation was used
gas-phase pyridine, 2,6-lutidine, or deuterated acetonitrile. 2,6-
Lutidine was adsorbed at 50 °C, by exposing the sample to 8
mbar of the probe molecule until saturation. Spectra were
recorded at 150 °C in vacuo after 15 min of equilibration, and 3.4
cm·μmol−1 was used as the integrated molar extinction
coefficient for calculations of LA density at 1612 cm−1.96 For
pyridine adsorption, the samples were subjected to 20 mbar of
the probe until saturation at 50 °C. Spectra were recorded at 150
°C in vacuo after equilibration for 40 min. 1.42 cm·μmol−1 was
used as the integrated molar extinction coefficient for
calculations of LA density at 1451 cm−1.59 Deuterated
acetonitrile was adsorbed at 30 °C. The samples were exposed
to sequential doses of 4 mbar of probe molecule until saturation,
which was evidenced when gas-phase CD3CN (2267 cm−1)
appeared. Once saturated, a spectrum was recorded at 30 °C
after 30 s in vacuo. The spectra were deconvoluted in open Sn
sites (2316 cm−1), closed Sn sites (2308 cm−1), SnO2 (2287
cm−1), silanol groups (2275 cm−1), and physisorbed or gas-
phase CD3CN (2265 cm−1) with OMNIC software. All peaks
were fitted using mixed Gaussian−Lorentzian peaks (50/50)
except for the peak corresponding to physisorbed CD3CN,
which was fitted using a fully Lorentzian (0/100) peak. For all
deconvolutions, a peak center variation of 3 cm−1 was allowed,
and FWHM values ranged from 5− to 20 cm−1.22 For open and
closed Sn sites, integrated molar extinction coefficients of 1.04
and 2.04, respectively, were used.59
kλ
β cos θ
d =
(10)
with d the crystallite size, λ the wavelength of the X-ray radiation
(Cu Kα1 = 0.15406 nm), constant k of 0.9, β is the full width at
half-maximum height in radians (FWHM) and θ is the
diffraction angle in radians. To estimate the average crystallite
size of SnO2, the two most intense, indexed peaks, (101) and
(211) at 33.9 and 52.3°, respectively, were used.44,45
TGA of the samples was performed on TA Instruments TGA
Q500. About 8 mg of the ground Sn(II) acetate and deAlβ
mixture (with a final Sn-content of 5 wt %) was heated at 2 °C·
min−1 to 550 °C in O2 or N2 and was kept isothermal for 6 h (6 h
in O2 or 3 h in N2 followed by 3 h in O2) at a flow rate of 90 mL·
min−1.
For all probe molecules, spectra of unloaded materials were
recorded as reference spectra. The number of LA sites titrated by
the probe molecules was calculated using the following equation.
integrated peak area (cm−1
)
Awafer (cm2)
m (g)
LA density (μmol g−1) =
×
ε (cm μmol−1
)
(11)
TPDE MS analysis was performed in a horizontal home-made
oven with temperature control, coupled to a Pfeiffer OmniStar
Mass spectrometer. Sn(II) acetate and deAlβ (5 wt % Sn) were
ground for 10 min and 200 mg was pelletized and placed inside
the quartz tube. The sample was heated at 2 °C·min−1 in 20 mL·
min−1 of an inert atmosphere (He-flow) or oxidizing flow (O2-
flow) to 550 °C. m/z of 18, 44, 45, and 58 was tracked with the
MS to follow the production of water, CO2, acetic acid, and
acetone, respectively.
N2 sorption measurements were performed using a Micro-
meritics Instruments Tristar 3000 at 77 K. Samples were
degassed under N2 flow at 300 °C for 6 h prior to measurement.
The relative nitrogen pressure was varied between 0.01 and 0.99
(p/p0). The specific surface area was calculated using the
Brunauer−Emmett−Teller theory, and pore volumes were
calculated by the t-plot method.
DRUV−vis measurements were performed in quartz U-tubes
equipped with a window. The samples were dried at 550 °C for 1
h in an air flow inside the tubes and measured, after cooling, on
an Agilent Cary 5000 spectrophotometer. Dried barium sulfate
was used as the diffuse reflectance standard.
Elemental analysis was performed using an ICP-AES
(PerkinElmer Optima 3300 DV) with signals for Sn, Al, and
Si at 189.9, 238.2, and 251.6 nm, respectively. Before ICP-AES,
the samples were decomposed with lithium metaborate at 1000
°C and dissolved in 5% HCl in Milli-Q water.
with Awafer and m the area and the mass of the dry wafer,
respectively, and ε the molar extinction coefficient.
XPS measurements were carried out on a SSI X probe
spectrometer (model SSI 100, Surface Science Laboratories,
Mountain View, CA) equipped with a monochromatized Al Kα
radiation (1486 eV). The sample powders, pressed in small
stainless troughs of 4 mm diameter, were placed on an insulating
home-made ceramic carousel. The pressure in the analysis
chamber was around 10−6 Pa. The analyzed area was ∼1.4 mm2,
and the pass energy was set at 150 eV. The Si 2p peak of silicon
was fixed to 103.5 eV to set the binding energy scale. Data
treatment was performed with the CasaXPS program (Casa
Software Ltd, UK); spectra were decomposed with the least
squares fitting routine provided by the software with a Gaussian/
Lorentzian (85/15) product function and after baseline was
subtracted.
119
̈
Sn Mossbauer spectra of the powder samples were
collected at RT and 77 K in transmission geometry, using a
̈
constant-acceleration Mossbauer spectrometer equipped with a
Ca 119mSnO3 source kept at RT and a variable-temperature
(Thor Cryogenics) liquid nitrogen bath cryostat. The
spectrometer was calibrated with metallic iron at room
temperature and 77 K. Analyses of the spectra were performed
by the IMSG code using Lorentzian-type lines.97 The sample
treated in inert conditions was stored, sealed, and transported in
̈
a N2-atmosphere, and the corresponding Mossbauer sample
FTIR measurements were performed on a Nicolet 6700
spectrometer equipped with a DTGS detector. Samples were
pressed into self-supporting wafers and degassed at 400 °C in
vacuo for 1 h before measurements. LA sites were analyzed with
holder was prepared and sealed in a glovebox under a N2-
atmosphere prior to its measurement.
4.3. Catalytic Reactions. BVO reactions were performed in
magnetically stirred thick-walled glass reactors, capped with a
5994
ACS Catal. 2021, 11, 5984−5998
Peeters, Elise
