Pyridine assisted CO₂ reduction to methanol at high pressure

Touhami, Dalila

Abstract

Significant research efforts have been directed towards exploring electrocatalysts for the selective reduction of CO₂ to fuels such as methanol. Bocarsly et al (Princeton University) have recently reported the use of aromatic amines (e.g. pyridine (C₅H₅N)) as electrocatalysts in aqueous electrolytes for the reduction of CO₂ at low overpotentials (50-150 mV). Importantly, the CO₂-pyridine reduction process was claimed to selectively produce methanol with Faradaic efficiencies of ~100% on p-GaP electrode and 22-30% on Pt and Pd electrodes. Moreover, the initially proposed mechanism based on a radical intermediate interaction with CO₂ as a key step toward the production of methanol was subsequently disproved. In this project, methanol formation by the CO₂-pyridine (C₅H₅N) system was assessed by conducting electrolysis under various conditions at platinum electrodes. High pressure CO₂ was used with the aim of increasing the methanol yield. In the course of the present study, the bulk electrolysis confirmed the methanol production at 1 bar and at 55bar of CO₂ in the presence of pyridine. However, the methanol yield was found to be persistently limited to sub-ppm level (<1ppm) under all conditions investigated. The observed methanol yield limitation could not be overcome by the electrode reactivation techniques used. Moreover, the methanol formation seemed unaffected by the current density or the biasing mode. This was an indication of the independence of methanol production from the charge transfer on the electrode. In agreement with these observations, analysis of the voltammetric data supported by simulation revealed that the CO₂-pyridine reduction system is mainly pyridinium assisted molecular hydrogen production under all conditions investigated. In particular, protonated pyridine (C₅H₅N) ‘pyridinium’ was confirmed to behave as a weak acid on platinum. It was found that CO₂ is merely a proton source of pyridine reprotonation via the hydration reaction followed by carbonic acid dissociation. The reprotonation reaction coupled to the electrode reaction ultimately leads to the dihydrogen production. No direct contribution of CO₂ in the reduction process was observed. The production of methanol seems to occur chemically rather than directly driven by the charge transfer on the electrode. The role of pyridine (C₅H₅N) appears to be restricted to assisting in the generation of the hydrogen necessary for the alcohol production.

Citation

Touhami, D. Pyridine assisted CO₂ reduction to methanol at high pressure. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4220932

• Thesis Type: Thesis
• Deposit Date: Nov 6, 2018
• Publicly Available Date: Feb 23, 2023
• Keywords: Engineering
• Public URL: https://hull-repository.worktribe.com/output/4220932
• Additional Information: School of Engineering and Computer Science, The University of Hull
• Award Date: Sep 1, 2015