22
2 Comprehensive Electrochemistry of Tc …
Na 2 SO 4 ; pH = 2) was also studied by Schrebler et al. (2001). They concluded that
this process starts with a two-dimensional progressive nucleation (PN2D), followed
by a 3D progressive nucleation under diffusional control (PN3D dif ) and, for a sufficiently long time, the process follows a progressive nucleation mechanism under
charge transfer control (PN3D CT ). Unfortunately, there is lack of similar studies
on nucleation and growth of metallic technetium. One may expect, however, that a
similar mechanism should be applicable also for Tc.
A critical review on the electroreduction of perrhenate ions in acidic media was
published in 2003 (Méndez et al. 2003). The authors concluded that the final products
of this process strongly depend on the concentration of the acid in the solution. More
recent works of Szabó and Bakos (2004) show that a nonsoluble ReO 2 is the main
product of the perrhenates reduction in diluted H 2 SO 4 (e.g. 0.1 M H 2 SO 4 ). This
aspect of the rhenium chemistry is somewhat similar to the chemical behavior of
technetium because TcO 2 , i.e., an analog to ReO 2 , is considered as the main product
of the TcO
−
4 reduction. Szabó and Bakos reported that Re 2 O 5 precipitate is formed
as a non-soluble deposit in concentrated H 2 SO 4 solutions (12 M H 2 SO 4 ). On the
other hand, electrogeneration of Tc 2 O 5 has not been confirmed experimentally so
far. Lawler et al. (2018) discussed the possibility of generation of so called “tech
red,” i.e., technetium species more volatile than Tc 2 O 7 . They assumed that the most
probable identity of this mysterious form of technetium is Tc 2 O 5 generated as a
product of oxidation of technetium dioxide by oxygen in the presence of water at
250 °C. TcO
3+ , which was described in detail for the first time by Poineau et al.
(2013), is the form of Tc(V) observed in aqueous solutions at room temperature.
Tc(V) may exist in various forms both in acidic and alkaline solutions and such
behavior is different from what is observed for Mn and Re which species with +
V oxidation state are stable only under very limited conditions. Thus, Mn(V) is
observed only in concentrated alkaline solutions in contrast to Re(V), which can be
generated in concentrated acidic solutions. These observations indicate periodicity
of changes within the manganese group.
Some authors (e.g. Pihlar 1979) pointed out that technetium trioxide, TcO 3 , can be
generated on the electrodes during the pertechnetates reduction in acidic solutions.
This behavior would make technetium similar to rhenium although TcO 3 is expected
to be much less stable than ReO 3 . The formation of rhenium(VI) oxide, ReO 3 , as
a result of the electrochemical reduction of perrhenates on a gold surface, has been
confirmed by Schrebler et al. (2005) using the quartz microbalance technique. These
authors found that electroreduction of the perrhenates in 0.1 M Na 2 SO 4 + H 2 SO 4
solution with pH of 2 at potentials before the onset of hydrogen evolution reaction
(HER) leads to formation of not only rhenium (VI) oxide but also other rhenium oxide
forms, i.e., ReO 2 and Re 2 O 3 . A scheme of these processes is shown in Fig. 2.9.
Szabó and Bakos suggested (2000) that in strongly acidic media rhenium(III) is
produced as a result of rhenium dioxide disproportionation. Interestingly, respective
disproportionation reactions are not observed for technetium dioxide. Although early
works (e.g. Mazzocchin et al. 1974) suggested that Tc 2 O 3 may play a role in reduction
of technetium at lower potentials, there is lack of experimental evidence, which shows
that Tc may be electroreduced exactly to Tc 2 O 3 .
2 Comprehensive Electrochemistry of Tc …
Na 2 SO 4 ; pH = 2) was also studied by Schrebler et al. (2001). They concluded that
this process starts with a two-dimensional progressive nucleation (PN2D), followed
by a 3D progressive nucleation under diffusional control (PN3D dif ) and, for a sufficiently long time, the process follows a progressive nucleation mechanism under
charge transfer control (PN3D CT ). Unfortunately, there is lack of similar studies
on nucleation and growth of metallic technetium. One may expect, however, that a
similar mechanism should be applicable also for Tc.
A critical review on the electroreduction of perrhenate ions in acidic media was
published in 2003 (Méndez et al. 2003). The authors concluded that the final products
of this process strongly depend on the concentration of the acid in the solution. More
recent works of Szabó and Bakos (2004) show that a nonsoluble ReO 2 is the main
product of the perrhenates reduction in diluted H 2 SO 4 (e.g. 0.1 M H 2 SO 4 ). This
aspect of the rhenium chemistry is somewhat similar to the chemical behavior of
technetium because TcO 2 , i.e., an analog to ReO 2 , is considered as the main product
of the TcO
−
4 reduction. Szabó and Bakos reported that Re 2 O 5 precipitate is formed
as a non-soluble deposit in concentrated H 2 SO 4 solutions (12 M H 2 SO 4 ). On the
other hand, electrogeneration of Tc 2 O 5 has not been confirmed experimentally so
far. Lawler et al. (2018) discussed the possibility of generation of so called “tech
red,” i.e., technetium species more volatile than Tc 2 O 7 . They assumed that the most
probable identity of this mysterious form of technetium is Tc 2 O 5 generated as a
product of oxidation of technetium dioxide by oxygen in the presence of water at
250 °C. TcO
3+ , which was described in detail for the first time by Poineau et al.
(2013), is the form of Tc(V) observed in aqueous solutions at room temperature.
Tc(V) may exist in various forms both in acidic and alkaline solutions and such
behavior is different from what is observed for Mn and Re which species with +
V oxidation state are stable only under very limited conditions. Thus, Mn(V) is
observed only in concentrated alkaline solutions in contrast to Re(V), which can be
generated in concentrated acidic solutions. These observations indicate periodicity
of changes within the manganese group.
Some authors (e.g. Pihlar 1979) pointed out that technetium trioxide, TcO 3 , can be
generated on the electrodes during the pertechnetates reduction in acidic solutions.
This behavior would make technetium similar to rhenium although TcO 3 is expected
to be much less stable than ReO 3 . The formation of rhenium(VI) oxide, ReO 3 , as
a result of the electrochemical reduction of perrhenates on a gold surface, has been
confirmed by Schrebler et al. (2005) using the quartz microbalance technique. These
authors found that electroreduction of the perrhenates in 0.1 M Na 2 SO 4 + H 2 SO 4
solution with pH of 2 at potentials before the onset of hydrogen evolution reaction
(HER) leads to formation of not only rhenium (VI) oxide but also other rhenium oxide
forms, i.e., ReO 2 and Re 2 O 3 . A scheme of these processes is shown in Fig. 2.9.
Szabó and Bakos suggested (2000) that in strongly acidic media rhenium(III) is
produced as a result of rhenium dioxide disproportionation. Interestingly, respective
disproportionation reactions are not observed for technetium dioxide. Although early
works (e.g. Mazzocchin et al. 1974) suggested that Tc 2 O 3 may play a role in reduction
of technetium at lower potentials, there is lack of experimental evidence, which shows
that Tc may be electroreduced exactly to Tc 2 O 3 .