Environmental Engineering Reference
In-Depth Information
2.4.3 Electrochemical Incineration of Water-Soluble Organic Compounds
Organic compounds can be disposed by oxidation with air at high temperatures (incin-
eration) [156]. However, contaminants present at low concentration in water have to be
concentrated or removed from water before incineration. Electrochemical oxidation is able
to oxidize organics at ambient temperature in water, providing that some amount of salts
is present to give enough electrical conductivity [157]. This is usually the case with waste-
water. One of the most effective catalysts for electrooxidation of organics is Pt, which is
able to break the C—H bond present in most organic compounds [158]. Pure metal is quite
expensive precluding its use in pure form [159]. Additionally, the electrochemical catalysis
depends directly on the active surface area of the catalyst. Pt nanoparticles have large sur-
face areas, allowing the use of small amounts of metal. However, use in electrodes requires
that they be supported on a conductive material. Nanoporous carbons have both large
surface area and good electrical conductivity. Therefore, they could be used to support
the nanoparticles and conduct the oxidation current. However, Pt is poisoned by species
present in the solution or produced during oxidation such as CO [160]. The addition of
other metals (e.g., Ru) allows oxidation of the poisons on the Pt surfaces, maintaining the
electrocatalyst activity [161]. The additional metal can be deposited together with Pt if the
nanoparticle formation is made
in situ
by chemical reduction.
Previously, our group has investigated the electrooxidation of methanol at mesoporous
Pt and Pt-Ru electrodes [162]. We have deposited PtRu nanoparticles inside a nanoporous
hierarchical carbon (1PSC) by reduction of metal chlorides (
PtCl
6
−
, RuCl
3
) with formic acid.
Small (2-4 nm) PtRu nanoparticles are evenly distributed inside the carbon (Figure 2.21)
[105].
The electrodes are tested for the electrooxidation of methanol:
CH
3
OH + H
2
O = CO
2
+ 6 H
+
+ 6 e
−
(V)
While the main purpose is the electrochemical destruction of the organics, it should
be borne in mind that the oxidation half reaction has to be coupled with a reduction half
reaction, such as
1.5 O
2
+ 6 H
+
+ 6 e
−
= 3 H
2
O
(VI)
If the oxygen is provided by air, the whole electrochemical device operates as a fuel cell,
producing electrical energy instead of consuming it. This is quite interesting in a waste
management system. However, it requires an eficient oxygen reduction electrode.
The response of the electrode to a potential step from 0.05 V
RHE
(a potential where meth-
anol oxidation is negligible) to 0.55 V
RHE
(a potential where methanol is completely oxi-
dized) can be observed in Figure 2.22. The current density reaches 220 μA/c m
2
after 600 s of
polarization. This value is higher than that measured for commercial catalysts consisting
of PtRu nanoparticles supported on conventional carbon. Assuming that three adsorption
sites are required for each methanol molecule to be oxidized, the measured current value
implies that the whole catalyst surface is being renewed every 6 s. As a criterion of metal
utilization, the current can also be expressed in terms of mass activity. With this purpose,
the obtained current is divided by the total mass of the metal present at the electrode. This
value is shown in the right axis of Figure 2.22. The obtained value (black triangle) after 600 s
of polarization (120 Ag
−1
at 0.55 V
RHE
) reveals that the catalysts consist of well-dispersed,
small PtRu nanoparticles and a low degree of agglomeration. The gray circle shows the
