Figure 3. Simulated concentration profiles at the interface under
various electric fields. (a) Concentration profile of formate ions, (b)
concentration profile of Ru catalyst.
On the other hand, an external electric field induced current flow in
the reaction system along with degradation of the anode electrode
(immersed in the organic phase) were observed. Additionally, gas bubbles
of unknown composition were generated on the cathode. The degradation of
the electrode was confirmed by weight loss measurement (see Cases 1, 2
and 3 in Table 2). The degradation products which formed a thin layer of
black slurry at the liquid-liquid interface were characterized by XRF
analysis. The analysis showed that the slurry consisted mainly of Fe,
Ni, and Cr with a small amount of Ru. For reactions under higher
electric potentials, higher current flows and greater degradation rates
of the electrode were observed as shown in Table 2 (Cases 1, 2, and 3).
The decomposition of the electrode is consistent with galvanic
corrosion, in which hydrogen is produced at the cathode with the
corrosion occurring at the anode. At higher electric potentials,
stronger galvanic corrosion effects would be expected, leading to
greater electrode degradation. With more metal ions released from the
degradation, higher current flows would be observed due to increased
conductivity of the liquid phase. The degradation products formed at the
interface could also contribute to the deceleration of the reaction by
inhibiting mass transfer to the catalyst at the interface. At higher
electric potential, a higher degradation was observed, leading to a
higher blocking coverage of the interface and finally achieving a lower
reaction rate and conversion.
To eliminate the negative effects of electrode degradation and clarify
the inhibiting effects of high electric potential, chemically stable
titanium electrodes were employed, and the results are recorded as Cases
4, 5, and 6 in Table 2, compared to those with stainless steel
electrodes (Cases 1, 2, and 3). For experiments with different electric
potential applied, the average degradation rate of titanium electrodes
is less than 0.1% w/w, and no apparent black slurry was observed.
Although there was negligible evidence of electrode degradation of
titanium electrodes, reaction performance showed the same trend as
observed with stainless steel electrodes, when the applied electric
potential was increased. This suggests that the actual electrode
degradation plays a minimal role in changes in reaction performance,
thus strongly suggesting the controllable migration of Ru catalyst and
alkoxide ions under electrostatic force as being the main reason for the
reaction deactivation and the importance of external electric field in
controlling reaction rates. Furthermore, in spite of apparent reaction
deactivation, a constant product ((S)-1-phenylethanol)
enantioselectivity was achieved. This is further evidence that the
deactivation is most likely due to the controllable migration of
reactive species rather than catalyst decomposition.
A further set of experiments was conducted to investigate the reaction
performance when the applied electrical potential was varied over the
course of the reaction. With reference to Table 2, the data shown in the
last three rows list the results of these experiments, labelled Cases 7,
8, and 9. In case 7, after reacting for 8 h under 30 V, the electric
potential was switched to 15 V for 16 h, and the reaction yield reached
to 67.5%, which is an increase of 7.4% compared to that with solely 30
V applied for 24 h (Case 5). In case 8 and 9, 30 V or 50 V were applied
for 4 h respectively followed by 15 V applied for 20 h. In these two
cases, the yields of 1-phenylethanol increased when comparing to those
with solely 30 V or 50 V applied for 24 h. These results suggest that
the reaction could be externally controlled by simply switching the
applied electric potential over the course of the reaction.
Table 2. Summary for experiments under various positive external
electric fields.