(71c) Effect of Mass Transfer on the Electrochemical Oxidation of Carboxylic Acids on Boron Doped Diamond Electrodes

Effect of
mass transfer on the electrochemical oxidation of carboxylic acids on boron
doped diamond electrodes

Electrochemical
oxidation is a promising purification method for waste water from various
industries. A powerful electrode material for this electrochemical process is boron
doped diamond (BDD): various carboxylic acids were shown to completely oxidize
to CO₂ on BDD electrodes^1,2. The oxidation of carboxylic acids
on BDD was found to take place in the potential region of oxygen evolution with
high current efficiencies. Despite the interest in this method and the use of
these electrodes, the effect of mass transfer on the oxidation of organic
compounds on BDD electrodes has not been reported. Mass transfer is an
important factor that has to be taken into account when designing a waste water
treatment process on industrial scale. The aim of our work is to study the
oxidation of carboxylic acids on boron doped diamond electrodes with a focus on
the effect of mass transfer.

Experiments were
performed in a two compartment parallel plate flow cell. The electrolytes were
pumped through the compartments at set flow rates. In the cyclic voltammograms
in Figure 1, a plateau close to the oxygen evolution region is observed in the
presence of formic acid at 2.1V (vs Ag/AgCl). Previous studies have shown that
the plateau height is proportional to formic acid concentration ², which is an indication for
mass transfer limitation . This is confirmed by our results that an increase in
flow rate increases the current density of the plateau.  The height of the
plateaus is in line with the expected limiting current densities.  The mass
transfer correlation  for the parallel plate was calculated using the
correlation³: both">.The limiting currents calculated were in agreement with the values
for the plateaus observed in the cyclic voltammograms at 0.005M. However at
higher concentrations this correlation overestimates the limiting current
density (Figure 2).

No clear differences
between the different flowrates are observed in the presence of acetic acid and
oxalic acid (Figure 1). The absence of apparent plateaus in the presence of
oxalic and acetic acid is in line with the cyclic voltammograms of previous
studies performed in batch experiments ^1,2,4.

In addition to the
cyclic voltammetry, chrono-amperometric experiments were performed in the same
flow cell. The currents observed in these measurements were generally lower
compared to the current measured in the cyclic voltammograms. This could be
explained by a poisoning effect of a reaction intermediate or oxide at the
electrode surface, which lowers the current during chrono-amperometry in the
potential domain of 1.9 to 2.3 V. This indication of electrode poisoning is
also observed in the backwards scan in the cyclic voltammogram, where it can be
seen that the current of the forward plateau is not reached. The poisoning can
be removed by scanning back to 0 V. At potentials of 2.3 V or larger the
poisoning is no longer observed and the current remains constant during the
chrono-amperometric experiments.

From Figure 3 it
is apparent that the oxidation of formic, oxalic, and acetic acid increases
with increasing potential.  Figure 4 shows the current efficiencies of the
various experiments. The number of electrons used in the calculation for these
current efficiencies were 2 for formic and oxalic acid and 8 for acetic acid.  At
2.3V the high current efficiencies obtained suggest that oxygen evolution is
suppressed at this potential. The decrease in in current efficiency at higher
potentials clearly shows that oxygen evolution increases.

Table 1. Average
current densities (A/m²) during the oxidation of formic acid
(0.005M), oxalic acid (0.01M) and acetic acid (0.005M) on BDD at various
potentials (V vs Ag/AgCl). Conditions: 0.5M Na₂SO₄, pH 2
and electrolyte flow rate: 20 L/h. The efficient current density is the total
current density multiplied by the current efficiency.

In Table 1 it can
be seen that the current density corresponding to formic acid oxidation (the
current density multiplied by the current efficiency) exceeds the previously
calculated limiting current density. The observed increase in oxidation rate
could be explained by 1) diffusion of ^●OH
radicals to the bulk of the solution or 2) an increased mixing effect induced
by the small oxygen bubbles formed at higher potentials. In the case 1) it is
assumed that the ^●OH radicals are formed on the surface of the electrode and react in
a reaction layer, as suggested in literature ⁵. However, literature is
inconsistent about whether these ^{" calibri>●}OH radicals are
adsorbed at the electrode surface or appear as free ^●OH radicals ^2,6.

In the case these ^●OH radicals remain adsorbed at the electrode surface, the oxygen bubbles
could increase the limiting current for the carboxylic acids. Previous research
has established that the generation of bubbles at the electrode surface can
significantly enhance mass transfer by affecting the boundary layer near the anode
^7,8. We attempted to calculate the
increase in limiting current density due to these bubbles using available mass
transfer correlations, but the calculated values were significantly lower than
the experimentally observed values.

In conclusion, we
have shown a way to measure the effect of flowrate on the plateau in the cyclic
voltammograms that is observed in the oxidation of formic acid on boron dope
diamond. These plateaus were not observed in the oxidation of acetic and oxalic
acid. In the chrono-amperic experiments, the oxidation of carboxylic acids is
faster at higher potentials, whereas the current efficiency decreases at higher
potentials. The high current densities at low potentials  show that the
carboxylic acids can be selectively oxidized at BDD by selecting the optimal
potential (2.3V vs Ag/AgCl) to suppress the oxygen evolution. However, we found
some indications that the oxygen evolution at high potentials might be beneficial
to increase the oxidation rate of the carboxylic acids.

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