Chemistry Reference
In-Depth Information
dynamically favourable reactions. This can be
achieved in many cases by the appropriate choice of
the counter-electrode reaction, e.g. the reduction of
oxygen and the oxidation of hydrogen gas. The
application of these reactions reduces potential
by-products at the counter-electrode and reduces
energy consumption for the cell, thereby reducing
the waste generation at the power plant. The reduc-
tion of potential waste products can be achieved by
utilising both reactions in the cell in what are called
paired syntheses.
5.4 Paired synthesis
Paired electrochemical syntheses are processes in
which both the anode and cathode reactions simul-
taneously contribute to the formation of the final
products. The classic example is the simultaneous
production of chlorine and sodium hydroxide in the
chloralkali industry. Paired electrosynthesis can be
classified generally in terms of:
(1)
The generation of two distinct products from two
reagents.
(2)
The generation of one product by coupling of
anode and cathode reactions.
(3)
The generation of one common product from
the anodic and cathodic reactions of two
reagents, e.g. glyoxylic acid from glyoxal and
oxalic acid [17].
(4)
The generation of one product in which the
intermediate species is formed by reaction at
the counter-electrode, e.g. the formation 1,2-
butanone from 2,3-butanediol [18].
(5)
The generation of two products from one
reagent, e.g. sorbitol and gluconate from glucose
[19].
It is interesting to consider examples of type 1 and
type 2 paired electrosyntheses.
Fig. 19.11
Electrochemical cell for the generation of ozone.
The use of arsine gas (AsH
3
) is essential in several
applications in the electronics industry. Arsine is an
extremely toxic material and stringent safety require-
ments are needed in its storage and use. These con-
cerns have promoted the use of on-site, on-demand
supply of arsine to eliminate the need for storage.
One recently developed method [16] is based on the
electrochemical reduction of arsenic to arsine:
As + 3 H
+
+ 3 e
-
= AsH
3
The generation of arsine in alkaline solution (1 mol
dm
-3
NaOH) from high-purity (99.999%) arsenic
cathodes can be carried out with current efficiencies
in the range of 95-97%. The only other product is
hydrogen gas. This production can be achieved over
two orders of magnitude of current density and thus
high arsine gas concentrations are produced at vari-
able rates for use in the manufacture of InGaAs
materials.
Organic chemical production
Paired synthesis can play a potential role in organic
chemical production. A recent example is the syn-
thesis of phthalide and 4-
t
-butyl-benzaldehyde at
BASF (Germany) using their capillary gap cell tech-
nology [20].
5.3 Influence of counter-electrode
The manufacture of dinitrogen pentoxide
The reduction of waste can be brought about gener-
ally by the improvement in efficiency in the use of
electrical energy by, for example, the use of thermo-
An electrochemical process for the production of
dinitrogen pentoxide (N
2
O
5
) from nitric acid has
