Keeping an Eye on Copper
Authors:   , de Ruiter Jim 
Published: February 2025

This PhD thesis provides an in-depth analysis of the catalytic mechanisms governing electrochemical CO₂ reduction (eCO₂RR) on copper catalysts in aqueous environments, using in situ Raman spectroscopy. Two key aspects were explored: (I) the role of electrochemical oxidation in restructuring copper and (II) the influence of electrolyte composition, pH, and cations on reaction selectivity. After a general introduction in Chapter 1, followed by a more in-depth background in Chapter 2, this PhD thesis continues with five experimental chapters. The thesis concludes with an overall summary and future perspectives in Chapter 8.

Chapters 3–5 examined the impact of chemical and electrochemical oxidation, demonstrating that anodization enhances CO₂ reduction selectivity over hydrogen evolution. Chapter 3 showed that applying oxidative pulses of +1 VRHE at low overpotentials (–0.25 to –0.35 VRHE) increased CO formation up to tenfold. Chapters 4 and 5 further revealed that electrochemical oxidation significantly boosts ethylene selectivity, with increases ranging from two- to sixfold. The underlying mechanism involves copper restructuring: anodization dissolves copper ions, which redeposit under cathodic conditions as small nanoparticles with improved active sites. These sites enhance CO formation, particularly the C-C directing low-frequency band CO (2000–2050 cm⁻¹) vibrations. Chapter 4 also identified a Raman fingerprint at 495 cm⁻¹, linked to a stabilized C-C coupling intermediate. Additionally, Chapter 5 detected new Raman signals above 500 cm⁻¹ (520–540 cm⁻¹), motivating further investigation in Chapters 6 and 7. Raman bands between 450–550 cm⁻¹ were analysed through transient experiments with varying electrolyte compositions. Chapter 6 identified a formaldehyde-related species (520–540 cm⁻¹) stabilized at elevated pH, suggesting that *CO insertion into *CHO facilitates C-C coupling, in stead of the well-established CO-CO coupling mechanism. Chapter 7 further showed that this band appeared more frequently in Li⁺-containing electrolytes, where poor pH buffering leads to increased local pH and decreased local CO₂ concentrations. The absence of sufficient CO₂ flux, hence, adsorbed CO species prevents *CHO-*CO coupling, explaining the lack of C₂ products in Li⁺ electrolytes. Under milder pH conditions, adsorbed CO is a key intermediate in C-C coupling. Raman signals at 450–495 cm⁻¹, linked to C₂+ intermediates, are more prominent under enhanced C₂+ production. Cs⁺-containing electrolytes exhibited the highest C₂ selectivity, particularly favouring ethylene over methane. The Cu-C stretching vibration (~360 cm⁻¹) blue shifts with cation size (Cs > K > Li), with Cs⁺ having the weakest hydration shell, allowing closer surface interaction and beneficially stabilized C-C directing intermediates.

The findings in this PhD Thesis provides spectroscopic insight into the catalytic mechanism behind the electrochemical reduction of CO₂ over copper catalysis. In situ Raman spectroscopy was used to identify surface species and intermediates involved in these processes. This study contributes to a better understanding of catalyst behaviour and reaction conditions, particular relevant to C₂+ product formation.