Laplante et al.
259
Fig. 1. Electron scanning micrograph of a piece of reticulated
vitreous carbon foam of 80 pores per inch (ppi).
(~450 rpm), and the catalyst powder particles were trapped
into the RVC matrix pores spontaneously (provided that no
sudden perturbation occurred); the RVC electrode was under
cathodic polarization during the agglomeration process. The
catalytic powder remained in the pores of the RVC electrode
(without any apparent loss during polarization); it also re-
mained cathodically polarized (J = 100 mA dm–2) for the
electrohydrogenation process and was in electrical contact
with the matrix (Fig. 2). With this type of RVC, the diame-
ter of the particle size should be between 15 and 43 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 given in Fig. 3. We observed that the ag-
glomeration process was successful with this specific cell,
since the process is closely dependent on the fluid dynamics.
The powder characteristics are summarized in Table 1. Note
that the average particle size refers to composite Pd –
support-particles when Pd is present.
porosity of the RVC offers a great number of large pores ac-
cessible to the composite powder particles. The electrolyte
penetrates through the RVC by forced convection, and the
electrocatalytic particles in suspension in the electrolyte are
trapped inside the pores. Recently, papers dealing with the
use of agglomerated electrodes in the dechlorination of poly-
chlorophenol and the electrocatalytic hydrogenation of
lignin showed the successful use of Pd–alumina and Rh–alu-
mina electrocatalysts (2, 5, 6).
The rate of the electrohydrogenation process is largely de-
pendant on several experimental parameters, particularly
those connected to the electrode fabrication. The present pa-
per deals with the key parameters for the successful entrap-
ment of the electrocatalytic particles by the RVC, that is: the
size of the particles, the applied current density during the
agglomeration process, and the appropriate design of the
electrochemical cell used for the agglomeration process. The
reaction under consideration is phenol electrohydrogenation.
In addition, different palladium powders supported on vari-
ous adsorbents (substrates) were considered, to show that the
adsorption of the phenol molecule may be the rate-
determining step of the ECH process for a given amount of
palladium (5% w/w).
Electrolysis
The electrolysis was carried out in a two-compartment,
jacketed glass H-cell (Fig. 3), having a Nafion-324 (E.I.
Dupont de Nemours & Co) membrane as a separator. The
cell temperature was fixed at 21°C during the electrohydro-
genation process by a circulating thermostated bath (VWR
1160A); to prevent analyte evaporation, a water-cooled con-
denser was added to the top of the cell. Afterwards, the cath-
odic compartment was filled with a phosphate buffer
(29 mL, 1 M KH2PO4 + 1 M NaOH) previously adjusted to
pH 7, and 200 mg of the composite powder was added to the
catholyte. The electrode was built-up in situ, using the previ-
ously described technique. The anodic compartment was
filled with a 1 M NaOH solution (Fisher); the counter-
electrode was a platinum mesh. Prior to the electrohydro-
genation process, 1 mL of a phenol solution in water
(25 mg mL–1) was added to the catholyte, giving a total vol-
ume of 30 mL and a phenol concentration of 8.8 mMol L–1.
Further, the electrocatalytic hydrogenation was performed
under galvanostatic control (J = 100 mA dm–2) using an
EG&G PAR model 273. All of the commercial powder parti-
cles considered in the present paper (Pd/Al2O3, Pd/BaCO3,
Pd/BaSO4 5 or 10 w/w %) were bought from Aldrich and
used as received.
Experimental
During the electrocatalytic hydrogenation process (ECH),
500 µL aliquots were withdrawn from the catholyte, further
saturated with NaCl, acidified to pH 1 with HCl (HCl concn.
10 N), and then extracted with 1 mL ethyl acetate, to be fur-
ther dried under sodium sulfate. After the completion of
ECH, both the RVC electrode and the whole cell were rinsed
with pure water. 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 ace-
tate. The organic layer was dried on sodium sulfate and fil-
tered. The filtrate was collected in a 50 mL volumetric flask
containing an external standard (ESTD, 3-methylcyclo-
hexanol). The GC analyses were carried out on a Hewlett-
Packard 5890 chromatograph equipped with an FID detector
on a 30-m-long HP-5 column. The products were then iden-
tified by comparison with the retention time of the authentic
compound, and the mass balance was obtained from the
ISTD:ESTD ratio of the corrected peak surface area.
The electrodes
The electrode was a piece of RVC foam (25 mm ×
20 mm × 6 mm, 100 pores per inch (ppi), Electrosynthesis
Co.). The electrode 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 of 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 electri-
cal contact from the electro-active part of the electrode. The
use of a nickel strip, as a contact with the RVC electrode,
should be avoided because the activity of the nickel towards
the electrocatalytic hydrogenation of ketone (12) may affect
the data.
The physical embodiment of the composite material into
the RVC occurred under moderate stirring of the catholyte
© 2003 NRC Canada