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M. Gutierrez et al. / Electrochimica Acta 55 (2010) 5831–5839
The hydrogen permeation reaction was carried out and the
a two–compartment cell separated by Pd sheet and palladized Pd
(Pd/Pd black) sheet electrodes. The catalysts were characterized by
XRD and SEM.
products were analyzed by GC and identified using the product
of chemical hydrogenation as standard. Only one product was
observed, which was identified as benzylacetone (71%). The ben-
zalacetone remains in 29%.
XRD studies show that there are no peaks corresponding to
hydride formation and the peaks corresponding to palladium oxide
are not observed. SEM analysis of the Pd/Pd black samples shows
a three-dimensional growth of Pd black deposits with a crystallite
size of about 1 m. After the hydrogenation reaction, the attack
to the surface is also observed. The current efficiency for hydro-
genation reaction increases when the current density for water
electrolysis decreases and depends on the initial chalcone concen-
The results obtained during the catalytic hydrogenation reac-
tion of benzalacetophenone and benzalacetone on Pt/Pt catalyst
show that ketone hydrogenated, unsaturated alcohol and saturated
alcohol are obtained as products. On the other hand the hydroge-
nation of these chalcones using permeation hydrogen lead to the
ketone hydrogenated as only product. The hydrogen reaction is
performed on palladium and Pd/Pd black catalysts and also is a
ling the hydrogenation process with the current value of hydrogen
generation.
There are several studies of the selective hydrogenation of the
␣,-unsaturated carbonyl compounds in relation with the compe-
titive C C and C O adsorption [10–13] and it is generally assumed
that the reactive bond is the one involved in chemisorption on the
surface. Therefore the problem of selective hydrogenation can be
reduced in a first rough approximation to determine the electronic
factors which control the adsorption mode of the ␣,-unsaturated
carbonyl compound.
Benzalacetone has similar structure than cinnamaldehyde
and chalcone. All of them contain at least a benzene ring, a
carbon–carbon double bond and a conjugated carbonyl group.
Cinnamaldehyde is one of the most experimentally studied unsa-
turated aldehydes and it is used as model for theoretical and
experimental studies to the selective hydrogenation of the ␣,-
unsaturated carbonyl compound. Aldaz et al. [13] the influence
of the shape/surface structure of Pt nanoparticles on the selec-
been studied. Pt(1 1 1)/C showed higher selectivities to unsatura-
ted alcohol than Pt(1 0 0)/C and Pt/C samples. Delbeq et al. done
a very elucidating paper to explain the behavior of platinum and
palladium metal towards the hydrogenation of ␣,-unsaturated
aldehydes [42]. Their argumentation rest on the hypothesis that
the double bond which is first hydrogenated is the one invol-
ved in the chemisorption on the surface. The preferred adsorption
modes allow one to explain the selectivity observed with different
geometry molecules They informed that phenyl ring of the cinna-
maldehyde has a non-negligible stabilizing interaction with the
surface which explains that the adsorption geometries through
the C C bond are more stable than those through the C O bond.
This is present when the four electron repulsions between the sur-
face and the molecule is small and this is the typical case for a
selective hydrogenation of the C O bond of a ␣, ethylenic alde-
hyde. Carbonyl groups are usually hydrogenated over platinum
while hydrogenation has not been reported over palladium for ali-
phatic aldehydes [43,44].
tration. It is over 90% at the concentration of 10 mmol L−1
.
The reduction products were identified by GC chromatogra-
phy, UV–vis absorption spectroscopy and NMR spectroscopy. The
ketone hydrogenated, unsaturated and saturated alcohols are
obtained when the catalytic hydrogenation is performed using Pt/Pt
electrode. The hydrogen permeating method lead to the ketone
hydrogenated as single product because palladium catalyst is a very
good catalyst for the C C hydrogenations, but a very bad catalyst
for hydrogenation of carbonyl compounds. In addition, the rate of
hydrogenation can be easily controlled by the electrolytic current,
which is one of the advantages of that method.
The hydrogen absorption and diffusion into and through a
palladium membrane electrode has been studied by using an ac
impedance method. We considered the permeable boundary condi-
tion for hydrogen permeation and indirect model for har that is
hydrogen entering the metal through an adsorbed state. The Keq
,
Rct and DH values are in agreement with the values informed by Lim
and Pyun which worked with permeation cell, where the anodic
side was subjected to a constant anodic potential which hydro-
gen atoms oxidized to H+ keeping the activity of hydrogen equal to
zero. The impedance results obtained in the present paper would
indicate that the hydrogen permeated trough the membrane is
consumed by the chalcone during the hydrogenation process kee-
ping the hydrogen activity almost zero in the outer side of the Pd
membrane.
Acknowledgements
We acknowledge the financial support of CONICET and CICYT-
UNSE.
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The hydrogenation of the benzalacetophenone and benzala-
cetone was developed using permeating hydrogen through the
palladium membrane electrode. The reaction was carried out using