On-line Electrode Dissolution Monitoring during Organic Electrosynthesis: Direct Evidence of Electrode Dissolution during Kolbe Electrolysis

Abstract

Electrode dissolution was monitored in real-time during Kolbe electrolysis along with the characteristic products. The fast determination of appropriate reaction conditions in electro-organic chemistry enables the minimization of electrode degradation while keeping an eye on the optimal formation rate and distribution of products. Herein, essential parameters influencing the dissolution of the electrode material platinum in a Kolbe electrolysis were pinpointed. The formation of reaction products and soluble platinum species were monitored during potentiodynamic and potentiostatic experiments using an electroanalytical flow cell coupled to two different mass spectrometers. The approach opens new vistas in the field of electro-organic chemistry because it enables precise and quick quantification of dissolved metals during electrosynthesis, also involving electrode materials other than platinum. Furthermore, it draws attention to the vital topic of electrode stability in electro-organic synthesis, which becomes increasingly important for the implementation of green chemical processes utilizing renewable energy.

Keywords: Kolbe electrolysis; Pt dissolution; electrochemistry; electrode stability; online electrochemical mass spectrometer.

Figure 1

(a) Kolbe electrolysis in the course of a chronoamperometric measurement at 3 V vs. Fc/Fc⁺ for 3 min followed by a potential step to −0.5 V vs. Fc/Fc⁺ for 10 min in methanol containing 1 mol L⁻¹ acetic acid and 0.1 mol L⁻¹ LiOH (left), NEt₃ (middle), and NEt₃+0.1 mol L⁻¹ H₂O (right), respectively. Applied potential (black), measured current density (gray), transient Pt dissolution (orange), H₂: m/z=2 (green), C₂H₆: m/z=27 (red), CO₂: m/z=44 (blue). (b) Total dissolved amounts of Pt at the potentials of 1 (blue), 2 (red), and 3 V (green) for a 3 min step followed by 10 min at reductive potentials in LiOH (left), NEt₃ (middle), and NEt₃+H₂O (right).

Figure 2

Kolbe electrolysis in the course of chronoamperometric measurements at 3 V vs. Fc/Fc⁺ for varying anodic polarization times (10, 5, 3, 1 min) followed by a potential step to −0.5 V vs. Fc/Fc⁺ for 10 min in methanol containing 1 mol L⁻¹ acetic acid and (a) NEt₃ and (b) NEt₃+0.1 mol L⁻¹ H₂O, respectively. Applied potential (black), measured current density (gray), transient Pt dissolution (orange), H₂: m/z=2(green), C₂H₆: m/z=27 (red), CO₂: m/z=44 (blue). (c) Total dissolved amounts of Pt as the function of the duration of the anodic potential step at 3 V.

Figure 3

CVs recorded between −0.5 and 3 V vs. Fc/Fc⁺ in methanol containing 1 mol L⁻¹ acetic acid and 0.1 mol L⁻¹ LiOH (left), 0.1 mol L⁻¹ NEt₃ (middle), 0.1 mol L⁻¹ NEt₃+0.1 mol L⁻¹ H₂O (right) with a scan rate of 10 mV s⁻¹. Applied potential (black), measured current density (gray), transient Pt dissolution (orange), H2: m/z 2(green), C₂H₆: m/z 27 (red), CO₂: m/z 44 (blue). The potential values in the graph refer to the onset potentials of the Pt dissolution, the Kolbe product formation, and the Pt dissolution associated with Pt−O species, respectively.

Figure 4

Measurement setup for on‐line monitoring of electrode dissolution and simultaneous gaseous product detection during electrosynthesis reactions.