4
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Can. J. Chem. Vol. 85, 2007
Scheme 1. Reaction scheme for the electrocatalytic hydrogenation of octanal to octan-1-ol.
3
0 mL of 0.25 mol/L supporting electrolyte (acetate or bu-
Fig. 1. ECH of octanal (5.2 mmol/L) over finely divided Pd ()
and 10% Pd/alumina (᭹) catalysts. Supporting electrolyte:
0.25 mol/L acetate buffer solution (pH 5) in aq. ethanol (50%
H2O v/v); I = 10 mA.
tyrate buffer solution) in aq. ethanol (variable compositions
of H O + CH CH OH). The pH was previously adjusted to 5
with 10 mol/L NaOH. The catalyst (200 mg of 10% Pd/alu-
mina or finely divided Pd) was added to the cathodic com-
partment solution and dynamically circulated through the
RVC cathode. The anodic compartment was filled with
2
3
2
1
mol/L solution of acid buffer in water. All the electrolyses
were carried out under galvanostatic conditions (I = 10 mA
or 20 mA) using an Agilent galvanostat/potentiostat (6634 B
model). The RVC cathode was polarized under an applied
constant current (10 or 20 mA); the corresponding charge
being equal to 15 °C. The coulometer was reset after polar-
ization; octanal was added in the cathodic compartment (2.6
or 5.2 mmol/L final concentration, depending on the experi-
mental conditions), and 0.5 mL aliquot of mixed solution
was immediately removed as reference sample (0 °C) and
treated as described in the next section.
Products analyses
The progress of the reaction was monitored by GC. Quan-
titative analyses were performed on an Agilent 6890 Series
chromatograph, equipped with an FID detector and HP-
Innowax (crosslinked polyethylene glycol) capillary column
version) than the non-supported. The same effect was ob-
served in the ECH and CH of other organic compounds (2,
(
50 m × 0.22 mm × 0.4 µm). Aliquots of 0.5 mL were with-
drawn from the electrochemical reactor at different times of
electrolysis and were treated as follows: sample was filtered
on Millex-HN syringe-driven filter (0.45 µm, Millipore); at
8
, 11). The poor yield of hydrogenation using finely divided
Pd catalyst is explained by the absence of the adsorption
sites located on the catalyst support (alumina in the case of
Pd/alumina catalyst) where the organic target molecule is
adsorbed (8).
0
.4 mL of filtered solution, 0.1 mL external standard
(
cyclohexanol) was added, and the mixture was injected in
the GC. The conversion percentages presented in this study
are determined from the GC yields. Both octanal and octan-
The relative rates of these processes are not only affected
by the adsorption of target species, but also by the current
intensity, the polarity of solvent, or the electrolyte nature, as
discussed further. We have verified the influence of current
intensity on the ECH of octanal over Pd/alumina catalyst in
acidic aq. ethanol. One expects to have a decrease in the
ECH efficiency because hydrogen evolution reactions are fa-
vored. As observed in Fig. 2, the higher the current intensity,
the lower the ECH efficiency, which indicates that concur-
rent hydrogen evolution is more significant for a current in-
tensity of 20 mA. In addition to the kinetic aspect, the
current intensity influences the cell flow, by the formation of
many cavities of hydrogen gas, as observed experimentally
at high intensity of current. To limit the current loss, associ-
ated with the hydrogen-evolution reaction, the current must
be adjusted to optimize the ECH reaction, while preserving a
suitable reaction time; this is why all the reactions were car-
ried out using a current intensity of 10 mA.
1
-ol were determined by GC. Kinetic curves, represented as
relative concentrations vs. charge consumed, are reproduc-
ible with a standard deviation of ±3.1%.
Results and discussion
Adsorbed hydrogen is generated at the palladium surface
by the electrochemical reduction of water. The brief electri-
cal contact between the RVC cathode and the Pd/alumina
particles, or finely divided Pd particles, is enough to charge
the metallic aggregates and to produce the atomic hydrogen,
which is implied in the hydrogenation process (10). The
ECH efficiency, defined as the amount of transformed target
species vs. the cumulative charge passed, is determined by
the competition among the hydrogenation of the unsaturated
compound (Scheme 1), hydrogen evolution, and, in some
cases, the direct reduction of the substrate on the electrode
surface. Figure 1 presents the remaining fraction of octanal
for the ECH over 10% Pd/alumina and finely divided Pd cat-
alysts under identical experimental conditions. Clearly, the
supported catalyst is much more effective (nearly 100% con-
Another parameter, which can affect the ECH process, is
the supporting electrolyte. It was demonstrated earlier that
the acid used as supporting electrolyte intervenes in the ECH
process by the change in the surface properties of the cata-
lyst matrix (8), i.e., by fixation at the alumina surface. This
©
2007 NRC Canada