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Can. J. Chem. Vol. 82, 2004
lysts (5–7). ECH is another way to hydrogenate organic
compounds with high efficiency. The more convenient ap-
proach is to carry out the hydrogenation process using elec-
tricity to generate in situ atomic hydrogen at room
temperature and under normal pressure.
It is well established in the literature (8, 9) that the gener-
ation of atomic hydrogen (Hads) and molecular hydrogen
(H2) on metal (M) in solutions can proceed through any of
the following reactions:
of a nickel strip as a contact with the RVC electrode should
be avoided because the nickel activity towards the electro-
catalytic hydrogenation of ketone may affect the data (18).
The physical entrapment of the composite material in the
RVC foam occurred under moderate stirring of the catholyte
(~450 rpm), and the catalyst powder particles were trapped
into the RVC matrix pores spontaneously. The catalytic pow-
der remained in the pores of the RVC electrode cathodically
polarized (j = 100 mA/dm²) for the electrohydrogenation
process and was in electrical contact with the matrix (17).
With such an RVC, the average diameter of the particle is
12.4 µm. The entrapment process is completed within 2 h,
up to a charge of 50 C. The dimensions of the H-cell and
those related to the other parts are described in a previous
paper (17).
[1]
[2]
[3]
H2O + M + e– ꢀ MHads + OH–
(Volmer reaction)
H2O + MHads + e– ꢀ M + H2 + OH–
(Heyrovsky reaction)
Electrolysis
2MHads ꢀ 2M + H2
The electrolysis was carried out in a two-compartment
jacketed glass H-cell having a Nafion-117 (E.I. Dupont de
Nemours and Co.) membrane as a separator. The cell tem-
perature was fixed at 21 °C during the electrocatalytic hy-
drogenation process by a circulating thermostated bath
(VWR 1160A). To prevent analyte evaporation, a water-cooled
condenser was added to the top of the cell. Afterwards, the
cathodic compartment was filled with a phosphate buffer
(29 mL, 1 mol/L KH2PO4 + 1 mol/L NaOH) previously ad-
justed to pH 7 and 200 mg of the composite powder was
added to the catholyte. The electrode was built-up in situ
with the previously described technique. The anodic com-
partment was filled with a 1 mol/L NaOH solution (Fisher);
the counter-electrode was a platinum mesh. Prior to the
electrocatalytic hydrogenation process, 1 mL of a phenol so-
lution in water (25 mg/mL) was added to the catholyte giv-
ing a total volume of 30 mL and a phenol concentration of
8.8 × 10–3 mol/L. The electrocatalytic hydrogenation was
performed under galvanostatic control ( j = 1 mA/cm²) using
EG&G PAR model 273 for 25 h (450 C at 5 mA). The com-
mercial powder catalyst considered in the present paper
(Pd/Al2O3, 5%) was provided by Aldrich and used as re-
ceived.
During the electrocatalytic hydrogenation process (ECH),
aliquots of 500 µL were withdrawn from the catholyte, satu-
rated with NaCl, and acidified to pH 1 with HCl (conc. HCl,
10 mol/L), and then extracted with 1 mL ethyl acetate and
dried under sodium sulfate. After the completion of ECH,
both the RVC electrode and the whole cell were rinsed with
pure water. An internal standard (ISTD, 2-cyclohexen-1-one)
was added to the cell solution; further, the extraction was
carried out twice (2 × 20 mL) with distilled ethyl acetate.
The organic layer was dried on sodium sulfate and filtered.
The filtrate was collected in a 50 mL volumetric flask con-
taining an external standard (ESTD, 3-methylcyclohexanol).
The GC analyses were carried out using a Hewlett-Packard
6890 chromatograph equipped with a FID detector on a
30 m HP-5 column. The products were identified by com-
parison with the retention time of the authentic compound
and the mass balance was obtained from the ISTD/ESTD ra-
tio of the corrected peak surface area. In accordance to the
ISTD/ESTD ratio, a recovery percentage greater than 85%
was observed.
(Tafel reaction)
The efficiency of the electrocatalytic hydrogenation of
phenol is determined by the preceding reactions, the adsorp-
tion and desorption power of the substrate toward the or-
ganic molecules, and other properties as well as the nature
of the electrode, the current density, the pH, the solvent
composition (6). Heyrovsky and Tafel reactions are
competive reactions for the hydrogenation of organic com-
pounds.
Recent developments in the preparation of catalytic elec-
trode materials having high efficiency and long term stabil-
ity have raised new interest in electrocatalytic hydrogenation
(6, 7, 10–12). There are several methods to produce compos-
ite materials with a metallic matrix: powder metallurgy (13),
metal plasma spraying (14), chemical and physical vapour
deposition (15), liquid metal forging (15), and electroco-
deposition (16).
In this paper, the effect of the porosity of the RVC foam
vs. the stirring speed on the efficiency of commercial
Pd/Al2O3 5% catalysts for the hydrogenation of phenol was
investigated. The experimental results were compared with a
computer simulation of the hydrodynamic behaviour of cata-
lyst particles for a RVC matrix of various porosity in the
electrochemical cell described in a previous paper (17).
Experimental
The electrodes
The electrochemical cell used in this paper has been pre-
viously described (17) and is shown in Fig. 1. The electrode
was made from the entrapment of catalytic powder particles
in a piece of reticulated vitreous carbon (RVC) matrix. The
RVC matrix was a piece of foam (25 mm × 20 mm × 6 mm,
30 pores per inch (ppi) or 100 ppi, Electrosynthesis Co.).
The conducting matrix was mounted by inserting a glass rod
(OD 5 ~ 6 mm, ID 3.5 mm) in the horizontal axis of a piece
of RVC (20 mm × 40 mm × 6 mm). The excess RVC was
then removed and a copper wire was inserted into the RVC
to be further cemented with silver epoxy (Epoxy Technol-
ogy). Finally, the electrical contact zone on the RVC matrix
was glued to the glass rod with epoxy to isolate the electrical
contact from the electroactive part of the electrode. The use
© 2004 NRC Canada