Mechanistische Untersuchungen der Pyridin-katalysierten Reduktion von CO2 an Pt-Elektroden: Eine elektrochemische Surface Science Studie
Festkörper- und Oberflächenchemie, Materialsynthese
Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
Zusammenfassung der Projektergebnisse
The starting point of this project were reports that the pyridine catalyzed reduction of CO2 on some metal electrodes, in particular on Pt and Pd electrodes, yields methanol as reduction product with faradaic efficiencies of up to 30 % and only smaller amounts of other CO2 reduction products (formic acid and traces of formaldehyde). Several reduction mechanisms involving a pyridinium ion were proposed in the literature; however, none of them seemed to be conclusive and literature reports on divergent reactivity existed. The objective of this study was to uncover the mechanism of CO2 reduction to methanol on Pt electrodes and to clarify the origin of the discrepancies in the literature. All the mentioned studies had been performed in a near-neutral pH electrolyte, which can easily lead to an increase of the local pH in front of the electrode. As a consequence, pyridine and HCO3- become the predominant species next to the electrode instead of the pyridinium ion and CO2. With a carefully designed electrolysis setup, we could overcome the problem of the local pH shift. This allowed us to demonstrate that pyridinium ions are completely hydrogenated to piperidine/ piperidinium under mild reductive conditions, i.e. close to 0 VRHE, both in the absence and presence of CO2. In contrast, we found that pyridine was electrochemically stable. Furthermore, we could not detect any methanol or other CO2 reduction reaction products under conditions where pyridinium existed at the electrode. We could thus rule out that protonated pyridine acts as a catalyst for the CO2 electroreduction reaction. Using Pt(111) single crystal electrodes, we studied the interaction of pyridine/pyridinium with the Pt surface in the H-UPD region. We could show that pyridine adsorbs on the Pt surface and squeezes the H-UPD to a smaller potential interval just positive of 0 VRHE compared to the pyridine free electrolyte. We attributed this to the adsorption of the positively charged pyridinium ion, which disturbs ad-/desorption of positively charged protons. Subsequent electrolysis studies under various electrolysis conditions did not yield signs that CO2 was reduced at the electrode. At this point, we had to conclude that the literature results were not reproducible with the given information, and that the discussed mechansims for electroreduction of CO2 to methanol through pyridinium ions has to be discarded. However, we had discovered in our group that a certain preparation method of Cu2O resulted in a Cu electrode which could reduce CO2 to methanol. Due to the difficulties with the Pt/pyridin system and these very promising first results towards methanol formation from electrochemical reduction of CO2, we concentrated in the remaining time of the project on the investigation of CO2 reduction on these Cu2O-derived Cu (Cu-OD) electrodes. The Cu-OD catalyst was electrochemically synthesized by sequential depositions of Cu and Cu2O, leading to a high surface area with a roughness factor r ≈ 30. Scanning electron microscopy images revealed that the electrode had a fractal-like, self-similar structure and XPS analysis suggested that the structure is stabilized by incorporated Na+ ions, which were, however, shown to leak from the electrode at negative potentials (in a K+-based electrolyte). The reduction of CO2 was studied in a CO2-saturated, 0.5 M K2CO3 electrolyte. At -0.7VRHE, the reduction current densities obtained were in the order of 1 mA cm-2 normalized to the ECSA, i.e. 30 mA cm-2 normalized to the geometric electrode area. Liquid product analysis was performed by 1H NMR. At -0.7VRHE, ethanol and methanol were the main products, with faradaic efficiencies above 30% for both alcohols. This high selectivity toward alcohol formation combined with the high activity could be tentatively linked to a combination of roughness and morphology, or, in other words, a combination of the arrangement of the Cu atoms on the atomic scale and of the topography on the mesoscopic scale.
