Full Papers
1
Pd NPs protected with G0: H NMR (400.1 MHz, CDCl , 298 K), d=
a gaseous mixture of DME and H O in a 1:6 molar ratio (S/C=3),
3
2
1
.03–0.45 (br, CH ), 0.26–0.29 ppm (br, SiMe ); IR (KBr): n˜ =2962
and the catalysts were tested at 673, 721, 773, and 823 K for 2 h at
each temperature. Stability tests were performed with the same re-
2
3
À1
(
n(CÀH)), 1413, 1261, 1095, 1020, 860, 801 (n(SiÀC)), 688 cm ; TGA:
weight loss 26.9%.
action mixture at 823 K for 48 h. The feed load was W/F=
À1
0
.2 gcatalyst sNmLgas , which accounted for a gas hourly space veloc-
1
Pd NPs protected with G1: H NMR (400.1 MHz, CDCl , 298 K), d=
1
4
À1
3
ity (GHSV) of 4.210 h . The composition of the gaseous effluent
stream was evaluated quantitatively on-line by using a micro GC
.70–1.20 (br, CH ), 0.85–0.46 (br, SiCH ), 0.11–0.20 ppm (br, SiMe );
2 2 3
IR: n˜ =3427, 2943 (n(CÀH)), 1622, 1420, 1261, 1095, 1022, 863, 801
(
Agilent 3000A) equipped with MS 5A, Plot U, and Stabilwax capil-
lary columns and TCD detectors. The products of the reaction were
exclusively H , CO , CO, CH , and CH OH. Outlet molar flow rates
were calculated from the measured composition by GC and using
as an internal standard. DME conversion (cDME) was calculated as
À1
(n(SiÀC)), 683 cm ; TGA: weight loss 48.4%.
1
Pd NPs protected with G2: H NMR (400.1 MHz, CDCl , 298 K): d=
1
2
2
4
3
3
.63–1.22 (br, CH ), 0.69–0.46 (br, CH ), 0.15–0.10 ppm (br, SiMe );
2 2 3
IR (KBr): n˜ =2954, 2912 (n(CÀH)), 1634, 1415, 1248, 1079, 1023, 863,
N
2
À1
8
35 (n(SiÀC)), 693 cm ; TGA: weight loss 69.1%.
cDME =100nDME,conv/2nDME,in, in which nDME,conv represents the number
of moles of DME converted measured as the sum of moles of CO2,
CO, CH , and CH OH at the reactor outlet and n is the initial
The Pd/Al O catalysts were prepared by impregnation from tolu-
ene solutions that contained Pd nanoparticles protected with the
different carbosilane dendrons followed by calcination at 773 K for
3
2
3
4
3
DME,in
number of moles of DME. Product selectivity (S) was calculated on
i
a dry basis as S =100(n/Sn), in which n is the number of moles
À1
i
i
i
i
i
h (2 Kmin ). These catalysts are denoted as Pd-G0, Pd-G1, and
of product i. The yield of each product (Y) was obtained as Y =
i
i
Pd-G2, in which G0, G1, and G2 correspond to the zero generation
G0), the first generation (G1), and the second generation (G2) of
cDME Si/100.
(
dendrimers.
Acknowledgements
Catalyst characterization
This work has been funded through grant MINECO ENE2012-
36368. J.L. is a Serra Hfflnter Fellow and is grateful to ICREA Aca-
demia program. E.R. is grateful to Generalitat de Catalunya for
a FI PhD grant.
HRTEM was performed by using a JEOL JEM 2010F electron micro-
scope equipped with a field-emission source at an accelerating
voltage of 200 kV. For the thiol- and dendrimer-protected nanopar-
ticles, the sols were dropped directly onto carbon-coated grids. For
the Pd/Al O catalysts, powders were suspended in methanol
2
3
under ultrasonic treatment before they were deposited on holey
carbon-coated grids. The point-to-point resolution achieved was
Keywords: dendrimers · heterogeneous catalysis · hydrogen ·
palladium · supported catalysts
0
.19 nm, and the resolution between lines was 0.14 nm. A mini-
mum of 200 particles were measured in each sample for particle
size determination. The size limit for the detection of nanoparticles
on the support was ꢀ1 nm. The average particle diameter was cal-
culated from the mean diameter frequency distribution with the
formula: d=Snd/Sn, in which n is the number of particles with
[2] T. H. Fleisch, A. Basu, M. J. Gradassi, J. G. Masin, Stud. Surf. Sci. Catal.
1
997, 107, 117–125.
[
[
i
i
i
i
particle diameter d in a certain range. XPS was performed by using
i
a SPECS system equipped with an Al anode XR50 source that oper-
ated at 150 mW and a Phoibos 150 MCD-9 detector. The pass
energy of the hemispherical analyzer was set at 25 eV and the
energy step was set at 0.1 eV. The BE values were referenced to
the C1s peak at 284.8 eV. In situ experiments were performed
under dynamic conditions in an adjacent chamber at atmospheric
pressure equipped with a mass spectrometer to monitor the reac-
tion and an IR lamp to heat the sample. The sample was trans-
ferred under ultra-high vacuum between the in situ chamber and
the analysis chamber. Gases were accurately dosed into the in situ
chamber by using mass flow controllers, and the temperature was
[
[
[7] H. Idriss, M. Scott, J. Llorca, S. C. Chan, W. Chiu, P. Y. Sheng, A. Yee, M. A.
[
8] J. Llorca, V. C. Corberµn, N. J. Divins, R. O. Fraile, E. Taboada in Renewable
Hydrogen Technologies (Eds.: L. M. Gandía, G. Arzamendi, P. M. DiØguez),
Elsevier, Amsterdam 2013, pp. 135–169.
[
measured by using a K-type thermocouple in contact with the
À1
sample holder. TPO (10 vol% O in Ar, 30 mLmin ) was performed
2
by using a Catalyst Analyzer BELCAT-M (BEL Japan, Inc.) equipped
with a thermal conductivity detector (TCD). The evolution of gases
was monitored by MS by using a Cirrus spectrometer from MKS
spectra products equipped with a multiplier detector.
[
[
[
Catalytic tests
[
[
The DME steam reforming reaction evaluation was accomplished
at 673–823 K and atmospheric pressure by using a lab-scale set up.
Before reaction, samples were activated at 573 K for 1 h in 10% H2/
N . The temperature of the reactor was increased up to 673 K
2
under N and, at this temperature, the inert gas was replaced by
2
ChemCatChem 2015, 7, 2179 – 2187
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