S. Hermans et al. / Applied Catalysis A: General 474 (2014) 263–271
265
reactant (VWR) and titrated with NaOH. All solutions were pre-
pared with decarbonated water (freshly distilled and flushed with
nitrogen) and kept under nitrogen.
To titrate the acidic functions, 0.5 g carbon were suspended in
50 mL base (NaHCO3 0.05 mol L−1, Na2CO3 0.025 mol L−1 or NaOH
0.05 mol L−1). After 24 h stirring, the carbon is filtered out and
aliquots are titrated rapidly by HCl 0.025 mol L−1. The indicators
were phenolphthalein for NaOH and bromothymol blue (upon boil-
ing) in the cases of NaHCO3 and Na2CO3.
are provided elsewhere [30]. At the end of the test, the catalyst is
separated from the reaction mixture by filtration.
The GC chromatograph was an Agilent Technologies 6890 N
fitted with a WCOT fused silica Chrompack CP-WAX 52 CB capil-
lary column (l = 50 m, internal diameter = 0.53 mm, thickness of the
film = 2 m). The vector gas was He (50 kPa) and the temperature
of the injector and detector was kept at 250 ◦C. The samples were
automatically injected via a split–splitless injector with a split flow
of 20 mL min−1. The temperature program for analysis was as fol-
lows: 160 ◦C for 5 min, heating to 240 ◦C (10 ◦C min−1), isotherm at
240 ◦C for 2 min. The signal received by a FID detector was directly
interpreted by a computer using the HP Chemstation software. The
area of each peak was converted into concentration by applying the
response factor with respect to the internal standard.
The basic functions were titrated by suspending 0.5 g carbon in
50 mL HCl 0.05 mol L−1. After 24 h stirring, the filtrate is titrated
with NaOH 0.025 mol L−1 with phenolphthalein as indicator.
2.3. Synthesis of Pd precursor
The specific activity measured can be expressed either by the
conversion rate of MNP or by the rate of formation of TBA. In
this study, the catalysts are compared on the basis of the former,
because an apparent zero-order is always observed, hence the slope
of the curve MNP consumption versus time is proportional to the
catalyst amount. The presented results are normalized with respect
the MNP consumption rate is expressed in mmol min−1 g−1. Some
TBHA intermediate was detected but always in very small amounts,
and with no variation from one catalyst to the other. The 2-methyl-
2-nitrosopropane (MNoP) intermediate does not accumulate in the
reaction medium: it is known to be very reactive [31].
The [Pd(OAc)2(Et2NH)2] complex was synthesized following
a procedure from the literature [29]. Yield: 81%; IR: ꢀ(COO)sym.
1374 cm−1, ꢀ(COO)asym. 1599 cm−1; EA: Exp. C 38.89, H 7.69, N
7.52%, Calc. C 38.87, H 7.61, N 7.56%; TGA: Exp. 70%, Calc. 71% (Pd
residue).
2.4. Grafting of Pd precursor on functionalized SX+ carbon
The amount of Pd introduced in solution corresponds to a the-
oretical 5 wt.% loading in the final catalyst which was verified by
atomic absorption analysis of the grafting filtrates at the end of the
procedure. In a typical experiment, 0.17 g [Pd(OAc)2(Et2NH)2] com-
plex are dissolved in 50 mL distilled water. This solution is added
to 0.95 g carbon powder (functionalized or not). After 24 h stirring
at room temperature, the solid sample is recuperated by filtration,
washed with distilled water and dried at 50 ◦C under vacuum. The
filtrates with the washings are poured in a 100 mL volumetric flask.
2.7. Instrumental
Infrared (IR) spectra of functionalized carbon samples were
recorded on a FT-IR Bruker Equinox 55 spectrophotometer in
absorbance mode from KBr disks prepared from 500 mg KBr for
1 mg carbon sample and dried overnight at 110 ◦C. For metallic
complexes, spectra were recorded in transmittance, using KBr disks
containing 1 wt.% sample.
Thermogravimetric analyses (TGA) were carried out on a
TGA/DSC combined SDTA 851e apparatus from Mettler-Toledo. The
samples (∼5 mg) were placed in 70 L alumina containers, and sub-
mitted to a 10 ◦C min−1 heating ramp under 100 mL min−1 nitrogen
flow.
2.5. Thermal activation to form supported nanoparticles from the
Pd grafted samples
The catalysts were activated in order to remove the ligands and
reduce PdII into Pd0 to produce supported Pd nanoparticles as active
phase by heating in a Carbolite tubular oven (type STF 16/450)
under nitrogen flow at 200 ◦C for 4 h. The samples were contained
as thin layers in porcelain combustion boats.
The amounts of metal present in solution were determined
by atomic absorption (AA) on a Perkin-Elmer 3110 spectrometer
equipped with a flame atomizer. Calibration curves were estab-
2.6. Catalytic tests
lished with Pd standard solutions of concentration 1–10 mg L−1
.
C
H
N elemental analyses (EA) were carried out by the micro-
steel Parr 4521 reactor fitted with a temperature control device
and a mechanical stirrer fixed at 2500 rpm. In these conditions,
extra-granular diffusion limitations are avoided and kinetic regime
is guaranteed [30]. 350 mL methanol (Fluka, p.a. >99.8%), a known
quantity (about 0.3 g) of catalyst and 3 mL 1-propanol (chromato-
graphic internal standard, Acros, p.a. 99+%) were poured in the
reactor and degassed (15 min) under nitrogen. A pretreatment was
applied to the catalyst at 80 ◦C for 1 h, under 17 bar of H2 to clean
the surface that might have been passivated by handling under air
after activation and avoid consumption of hydrogen by the catalyst
itself during the first stages of catalysis. The reactant (MNP, Alfa
Aesar, 99%) was dissolved in 25 mL methanol and degassed (under
N2) before being added in the reactor via a pressurized addition
ampoule together with a last portion of 25 mL degassed methanol
without opening the reactor to the air. The initial reactant concen-
tration is thus 0.1515 mol L−1. During catalytic reaction, the reactor
was kept under a pressure of about 12 bar H2 and a temperature
of 60 ◦C. Aliquots of the reaction mixture are taken at regular time
intervals and analyzed by gas chromatography (GC). Further details
analysis service of University College London, UK.
The metal loading in solids was determined, after digestion
overnight in a mixture of strong acids, by ICP-OES, by the Medac
Ltd. Company (UK). Oxygen content (direct measure, not differ-
ence) was also determined by Medac on a Thermo FlashEA1112
elemental analyzer, after pyrolysis of the sample under catalytic
conditions that produces oxygen bound as carbon monoxide.
X-ray Photoelectron Spectroscopy (XPS) was carried out at
room temperature on a SSI-X-probe (SSX-100/206) spectrome-
ter from Fisons connected to a computer running the S-probe
software. The samples were fixed by double-face isolating sticky
tape on small brass troughs and placed on a ceramic rotating
sample holder (Macor®, Switzerland), capped by a Ni grid on a
3 mm O-ring to avoid charge effects. The absence of signal aris-
ing from underlying sticky tape was verified. During analysis,
the created surface charges were neutralized by a floodgun of
energy fixed at 8 eV. The pressure in the analysis compartment
was 10−6 Pa. The data were interpreted with the CasaXPS soft-
ware. The photopeaks were calibrated with reference to the C C,H
component which was fixed at 284.8 eV. The peaks were decom-
posed into a sum of Gaussian/Lorentzian (85/15) after subtraction