174
A. Philippaerts et al. / Journal of Catalysis 270 (2010) 172–184
Table 2
Although unwanted diffraction contrast inevitably occurs with this
technique, use of bright-field TEM was required for the tomogra-
phy images as prolonged STEM illumination can damage soft por-
ous materials [28].
X-ray photoelectron spectroscopy (XPS) measurements were
performed with a Perkin–Elmer PHI ESCA 5500 system, using
Composition of solutions used for ZSM-5 synthesis in microwave (MW).
Sample
Si/Al
Na/Al
MW-40
MW-80
40
80
24.0
8.0
10.0
33.0
15.0
2.5
MW-100A
MW-100B
MW-150
MW-250A
MW-250B
MW-500
100
100
150
250
250
500
monochromatic 450 W Al K
a radiation. Despite the low Pt concen-
tration, both the Pt4f and the weaker Pt4d signals were clearly vis-
ible. As the more intense Pt4f photolines coincide with the Al2p
peak, the weaker but isolated Pt4d photolines around 315 eV were
used to calculate atomic Pt/Si ratios. The oxidation state of Pt is
normally extracted from the position of the strongest photoline,
viz. Pt4f. The binding energies of the Pt4f7/2 for various oxidation
states are well documented, viz. 71, 74 and 74.5–75 eV for Pt0,
Pt2+, and Pt4+, respectively [29]. Because of the overlap of Pt4f7/2
with Al2p, the distinct Pt4d5/2 photoline was used to calculate
the position of the overlapped Pt4f7/2 line. Reference measure-
ments on Pt foil were used to determine the exact distance be-
82.6
4.8
(ICP-AA). In case of normal ion-exchange, no Pt could be detected,
while very low Pt levels (<0.10 mg/L) were measured after CIE.
Pt-loaded catalysts are activated by a successive calcination and
reduction at 350 and 500 °C, respectively. Prior to activation, the
dry powders were compressed, crushed, and sieved, the 0.25–
0.50 mm fraction being retained for further use. Calcination was
conducted under flowing oxygen (120 mL/g/min), whereas reduc-
tion was done under flowing hydrogen (120 mL/g/min) applying
heating rates of 0.3 °C/min and 0.4 °C/min, respectively. After calci-
nation, the catalysts were cooled to room temperature under flow-
ing nitrogen. After reduction, the acid sites formed by the
reduction of Pt2+ to Pt0 were neutralized in a flow of 5% dry ammo-
nia in N2.
tween the two Pt lines (
X-ray diffraction (XRD) measurements were performed on a
STOE Stadi P instrument in transmission mode using Cu K radia-
DEb = 243.6 0.1 eV).
a
tion. On one hand, these measurements were done in order to con-
firm the zeolite structure and crystallinity after zeolite synthesis
and catalyst preparation. On the other hand, the diffractograms
were used to check for possible sintering of Pt, which can be eval-
uated from the Pt diffraction peaks at 2h = 39.6 (111), 2h = 45.9
(200) and 2h = 67.3 (220).
Platinum dispersions were determined using CO-chemisorp-
tion. Catalyst pellets loaded in a tubular reactor were reduced
according to the pretreatment procedures described previously
and cooled down to room temperature under flowing He. For the
In the notation of the Pt ion-exchanged samples, the amount
and nature of the occluded metal was preceding the zeolite sample
notation, while the Si/Al-ratio is given in brackets. A sample de-
noted as 0.5Pt/Na–ZSM-5(138) was loaded with 0.5 wt.% of Pt
and has a Si/Al ratio of 138.
titration of the Pt surface, pulses of 5 lL of pure CO were added
2.4. Catalyst characterization
to a He flow of 10 mL/min at an interval of 2 min. The CO concen-
tration in the outlet stream was followed continuously via ion
monitoring at m/e = 28 with a Pfeiffer Omnistar quadrupole mass
spectrometer. For the calculation of the dispersion, adsorption of
1 CO per accessible Pt atom was assumed. The size of Pt particles,
dPt, was derived from the Pt dispersion, DPt, assuming a cubic par-
ticle shape [30]:
SEM images of gold-coated samples were taken with a Philips
XL series XL 30 FEG, the energy of the incident electron beam being
30 keV.
For transmission electron microscopy (TEM) investigation, the
catalyst samples were prepared by suspending the powder materi-
als in methanol and subsequently placing a few drops of the sus-
pension on holey carbon-coated Cu-grids.
Bright-field TEM, high-resolution TEM, and electron diffraction
experiments were performed on a Philips CM20 operated at
200 kV. Particle size distributions of the Pt particles were mea-
sured from the obtained electron micrographs assuming a spheri-
cal morphology of the particles, averaging over 50 particles. EDX
analysis was performed using an Oxford EDX detector and Inca
analysis software.
Electron tomography experiments were performed on a JEOL
3000F TEM-STEM microscope operated at 300 kV and equipped
with a ꢁ70° to +70° tomography tilt stage and holder. Images for
tomographic reconstruction were taken using a 2° interval, over
the largest possible angle, viz. 132°. A reference image at 0° tilt
was taken before and after image acquisition, to ensure changes
in the sample structure were absent due to beam damage during
acquisition. Tomographic reconstruction was performed using
the TOM Tomography Toolbox [27]. High-angle annular dark field
scanning transmission electron microscope (STEM) images were
taken using a Technai G2 microscope operated at 200 kV at a nom-
inal spot size of 0.2 nm. The HAADF inner collection semi-angle
was 90 mrad. For all techniques, low-intensity beam conditions
(lowest possible magnification, low beam intensity and long expo-
sure times) were used as much as possible to minimize the elec-
tron dose and possible beam damage of the supported metal
particles [28]. The images for the tomographic acquisition were ta-
ken in bright-field TEM instead of the HAADF-STEM mode.
ꢀ
ꢁ
1
DPt
dPt ¼ 0:821
In situ UV–vis–NIR spectra in the diffuse reflectance mode (DRS)
of the series of Pt/ZSM-5 samples oxidized at increasing tempera-
ture were recorded on a Varian Cary 5 UV–vis–NIR spectrophotom-
eter. Sample pellets of 0.25–0.5 mm were brought into quartz flow
cells, equipped with a suprasil window for DRS measurements.
After calcination at different temperatures in a pure oxygen flow
of 120 mL/g/min at a heating rate of 0.3 °C/min, the samples were
cooled under He and scanned at RT. The spectrum of reference
white BaSO4 was subtracted from all Pt/ZSM-5 spectra.
Room temperature chromatographic adsorption experiments in
the HPLC mode were conducted with hexane as the mobile phase
using a column (4.6 ꢂ 45.0 mm) packed with zeolite pellets in
the Na-form but devoid of Pt, previously dried at 450 °C. Twenty
microliters of the adsorbate (MO or ME) was injected in the mobile
phase and passed over the zeolite column. Concentration changes
at the column outlet were monitored with a refractive index (RI)
detector. The response curve was analyzed by the method of mo-
ments [31], the first moment or mean retention time of the pulse
being identical to the adsorption equilibrium constant, K, which re-
flects the affinity of the sorbate compound for the adsorbent con-
cerned. A high value for K implies that the adsorbate shows a
significantly higher affinity for the adsorbent than for the mobile
phase. The ratio of the adsorption equilibrium constants of ME
and MO (KME/KMO) corresponds to the selectivity ratio or separa-