Selective control and rate enhancement of reactions involving catalytic reduction of organohalides and reduced form of myoglobulin in microemulsions
G. N. Kamau and B. Munge
Department of Chemistry, Box U-60, University of Connecticut,
Storrs,
CT 06269-3060, USA; Department of Chemistry, University of Nairobi,
P.O. Box 30197, Nairobi, Kenya
Abstract: Myoglobin (horse heart, Mb) adsorbed on carbon electrodes
or in solution at platinum electrodes was used for electrocatalytic
reduction of trans-1,2-dibromocyclohexane
(DBCH) and trichloroacetic acid (TCA) in a bicontinuous microemulsion,
prepared from sodium didodecyl sulfate (SDS) or hexadecyltrimethylammonium
bromide (CTAB), tetradecane, pentanol, and water. Carbon electrodes
(glassy carbon, GC, and pyrolytic graphite, PG) exhibited adsorption
peaks, whereas platinum (Pt) portrayed diffusion-controlled peaks. Electrode
reduction of Mb had E0' at 0.179 ± 0.013 V at GC/SDS, 0.189
± 0.015 at
GC/CTAB, and 0.562 ± 0.018 at Pt/SDS and 0.611 ±
0.008 at Pt/CTAB media.Current efficiencies for electrocatalytic reduction
of TCA were higher than that for reduction of dibromocyclohexane. For
both substrates, dibromocyclohexane and trichhloroacetic acid, a
lowering of overpotential of at least 1.3 V was achieved. Overall, the
Pt electrode exhibited higher catalytic efficiencies compared to carbon
electrodes, suggesting higher surface concentration and unhindered orientation
of the substrate on the Pt electrode. For DBCH, catalytic efficiencies
decreased with scan rate, at carbon electrodes as expected for homogeneous
solutions, whereas an average value of 2.22 ± 0.42 in SDS and
3.29 ± 0.76 in CTAB
microemulsion was obtained at Pt electrodes at a scan rate range of
15 to 3000 mVs1. Pseudo-first-order rate constants (k')
for the rate-determining step (rds) involving reaction of organohalides
with catalyst had an average value of 41 ±1 M1
s1 for reduction of
DBCH/SDS and 4.8 ± 1.0 ×102 M1 s1 for reduction
of TCA/SDS. Moreover, rate constants were higher in CTAB microemulsion,
giving values of 1.5 ±0.016 ×102 and 2.75 ±0.67
×
103 M1 s1 for DBCH and TCA, respectively. Positive interfacial
charge of the microemulsion conduits influenced catalytic reaction of
catalyst and the substrate. Under the conditions of the experiment,
no hydrogen evolution was observed at Pt electrodes.
*Plenary lectures presented at the Inaugural Conference for the Southern and Eastern Africa Network of Analytical Chemists (SEANAC), Gaborone, Botswana, 7-10 July 2003. Other presentations are published in this issue, pp. 697-888.
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