F. Martinez-Espinar et al. / Journal of Catalysis 354 (2017) 113–127
125
a Finnigan MAT 900S (EB-Trap Geometry) Syringes pump Model
22.
the values for the rhodium that is the element of interest for these
analyses.
Solid state 13C{1H}-Cross Polarization- magic Angle Spinning
(CPMAS) experiments were performed on a BRUKER Avance III
spectrometer operating at a magnetic field of 9.4 T and equipped
with a double channel 4.0 mm MAS probe. The powder materials
were packed into 4 mm ZrO2 rotors and were sealed with tight fit-
ting Kel-F caps. Sample spinning was set to 12 kHz in all experi-
ments. Chemical shifts are reported in parts per million (ppm)
externally referenced to adamantane (CH2 peak set to 38.5 ppm).
Cross polarization time was set to 2500 ms and performed with
a radio-field strength of 83 kHz and a 1H ramp pulse was used
(ramp70100 in Bruker nomenclature). 1H-decoupling was per-
formed using SPINAL-64 pulse scheme. The recovery delay was
set to 1 s and overall experimental time was set from 12 to 24 h
by varying the number of scans depending on the sample sensitiv-
ity. Spectra were acquired at 20°C controlled by a BRUKER BCU
unit.
Transmission Electron Microscopy (TEM) analysis were per-
formed at the ‘‘Unitat de Microscopia dels Serveis Cientificotècnics
de la Universitat Rovira i Virgili” (TEM-SCAN) in Tarragona with a
Zeiss 10 CA electron microscope operating at 100 kV with resolu-
tion of 3 Å. The particles size distributions were determined by a
manual analysis of enlarged images. At least 300 particles on a
given grid were measured in order to obtain a statistical size distri-
bution and a mean diameter. For High Resolution Transmission
Electron Microscopy (HRTEM) and Scanning Electron Microscopy
(SEM) a probe-corrected, cold-FEG JEOL ARM microscope equipped
with a centurio EDX detector operated at 200 keV was used.
X-ray diffraction (XRD) measurements were made using a Sie-
mens D5000 difractometer (Bragg Brentano parafocusing geometry
and vertical h–h goniometer) fitted with a curved graphite dif-
fracted beam monochromator, incident and diffracted beam Soller
slits, a 0.06° receiving slit and scintillation counter as a detector.
The angular 2h diffraction range was between 26 and 95°. The data
were collected with an angular step of 0.05° at 16s per step and
sample rotation. A low background Si(5 1 0) wafer was used as
Thermogravimetric (TGA) analysis was carried out in the fur-
nace of a Mettler Toledo TGA/SDTA851 instrument. As a typical
TGA experiment, 132 mg of NPs were placed in the simple holder
in the furnace and the material was heated up at a rate of 10 °C/
min in N2, while the weight was recorded continuously from
30 °C to 900 °C. The weight loss of the organic part and the metal
were used to calculate an approximate number of ligands coordi-
nated to the metal surface. The ligand loss was attributed to the
weight loss observed between 150 and 900 °C. For the calculation,
the molecular weight of the corresponding ligands and the metal,
and the number of metal atoms at the surface from TEM data were
taken into account.
Infrared spectroscopy (IR) analysis was performed by the prepa-
ration of samples as KBr pellets. The nanoparticles were used with-
out any preparation step, mixed and crushed with dry KBr in the
glove box before the preparation of the pellet. For CO coordination
studies, Rh nanoparticles were introduced in a Fischer Porter bottle
and were pressurised with 3 bar of H2 in solid state for 5 h. After
this period of time, the H2 gas was evacuated under vacuum for
10 min. The Fischer Porter bottle was then pressurised with
1 atm of CO for 16 h. Then, the gas was evacuated under vacuum
for 15 min and IR spectroscopy samples were prepared as KBr pel-
lets in the glove box.
3.2. General procedure for the synthesis of Rh NPs stabilised by IPr
In a typical procedure, the [Rh(N
3-(C3H5)3] (250 mg, 0.22 mmol)
was placed into a Fischer-Porter reactor and dissolved in 230 ml of
dry and deoxygenated THF by three freeze-pump cycles. The
resulting yellow solution was cooled at -110 °C (acetone/N2 bath)
and a solution of 20 ml of THF containing the appropriate amount
of equivalents of the IPr carbene was added into the reactor. The
Fischer-Porter reactor was then pressurized under 4 bar H2 and
stirred for 30 min at room temperature. The solution was then
heated to 35 °C and stirred at this temperature during 16 h. The
initial yellow solution became black after 1 h. A small amount (2
drops approx.) of the solution was deposited under an Argon atmo-
sphere on a carbon-covered copper grid for transmission electron
microscopy analysis. The rest of the solution was evaporated to
dryness. Precipitation and washings with pentane (3 ꢁ 15 ml)
was then carried out, obtaining a black powder. (Yield: ca.
150 mg, 98%). A similar procedure was followed for the synthesis
of the 13C-Rh NPs in similar yields.
Sample holder. Cuk radiation was obtained from a copper X-ray
a
tube operated at 40 kV and 30 mA.
Wide-angle X-ray scattering (WAXS) was performed at CEMES-
CNRS. Samples were sealed in 1 mm diameter Lindemann glass
capillaries. The samples were irradiated with graphite-
monochromatized molybdenum K
a (0.071069) radiation and the
X-ray intensity scattered measurements were performed using a
dedicated two-axis diffractometer. Radial distribution functions
(RDF) were obtained after Fourier Transformation of the reduced
intensity functions.
X-ray photoelectron spectroscopy (XPS) analysis were per-
formed at the ‘‘Centres Científics i Tecnològics de la Universitat
de Barcelona” (CCiT UB) in a PHI 5500 Multitechnique System
(from Physical Electronics) with a monochromatic X-Raysource
(Aluminium Kalfa line of 1486.6 eV energy and 350 W), placed per-
pendicular to the Analyser axis and calibrated using the 3d5/2 line
of Ag with a full width at half maximum (FWHM) of 0.8 eV. The
analysed area was a circle of 0.8 mm diameter, and the selected
resolution for the spectra was 187.5 eV of Pass Energy and
0.8 eV/step fort the general spectra and 23.5 eV of Pass Energy
and 0.1 eV/step for the spectra of the different elements in the
depth profile spectra. A low energy electron gun (<10 eV) was used
in order to discharge the surface when necessary. All measure-
ments were performed in an ultra high vacuum (UHV) chamber
pressure between 5 ꢁ 1039 and 2 ꢁ 1038 torr. For this analysis,
the data processing was carried out with the program CasaXPS. Ini-
tially, the general spectrum of the different binding energies
observed for this sample was analysed and was used to calibrate
the following calculations. This calibration was performed using
3.3. General procedure for the catalytic hydrogenation reactions
Autoclave Par 477 equipped with PID control temperature and
reservoir for kinetic measurements and HEL 24 Cat reactor for sub-
strate scope were used as reactors for the hydrogenation reactions.
In a typical experiment, the autoclave was charged in the glove-
box with the desired Rh NPs (1.25 or 0.625 mol%; the catalyst con-
centration was calculated based on the total number of metallic Rh
atoms in the surface of the NPs) and the substrate (0.124 M) in
THF. Molecular hydrogen was then introduced until the desired
pressure was reached and the reaction was stirred for the desired
reaction time at the selected temperature. At the end of the reac-
tion, the autoclave was depressurised and the solution was filtered
through silica for subsequent analysis by GC. The conversion and
selectivities for each reaction product were determined by GC-
FID on an Agilent Technologies 7890A spectrometer, with a HP-5
column (30m ꢁ 0.25 mm ꢁ 0.25
lm) using undecane as internal
standard. TOF was defined as moles of products per mol Rh at
the surface of the NPs per hour.