The electrocatalytic carbon dioxide (CO 2 )reduction reaction (CO 2 RR) into hydrocarbons is apromising approach for greenhouse gas mitigation, but many details of this dynamic reaction remain elusive.Here,time-resolved surface-enhanced Raman spectroscopy( TR-SERS) is employed to successfully monitor the dynamics of CO 2 RR intermediates and Cu surfaces with sub-second time resolution. Anodic treatment at 1.55 Vvs. RHE and subsequent surface oxide reduction (below À0.4 Vv s. RHE) induced roughening of the Cu electrode surface,w hich resulted in hotspots for TR-SERS,e nhanced time resolution (down to % 0.7 s) and fourfold improved CO 2 RR efficiency toward ethylene.W ithT R-SERS,t he initial restructuring of the Cu surface was followed (< 7s), after which astable surface surrounded by increased local alkalinity was formed. Our measurements revealed that ahighly dynamic CO intermediate,w ith ac haracteristic vibration below 2060 cm À1 ,isrelated to CÀCcoupling and ethylene production (À0.9 Vv s. RHE), whereas lower cathodic bias (À0.7 Vv s. RHE) resulted in gaseous CO production from isolated and static CO surface species with adistinct vibration at 2092 cm À1 .

Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) techniques exhibit highly localized chemical sensitivity, making them ideal for studying chemical reactions, including processes at catalytic surfaces. Catalyst structures, adsorbates, and reaction intermediates can be observed in low quantities at hot spots where electromagnetic fields are the strongest, providing ample opportunities to elucidate reaction mechanisms. Moreover, under ideal measurement conditions, it can even be used to trigger chemical reactions. However, factors such as substrate instability and insufficient signal enhancement still limit the applicability of SERS and TERS in the field of catalysis. By the use of sophisticated colloidal synthesis methods and advanced techniques, such as shell-isolated nanoparticle-enhanced Raman spectroscopy, these challenges could be overcome.

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Oxide-derived copper electrodes have displayed a boost in activity and selectivity toward valuable base chemicals in the electrochemical carbon dioxide reduction reaction (CO2RR), but the exact interplay between the dynamic restructuring of copper oxide electrodes and their activity and selectivity is not fully understood. In this work, we have utilized time-resolved surface-enhanced Raman spectroscopy (TR-SERS) to study the dynamic restructuring of the copper (oxide) electrode surface and the adsorption of reaction intermediates during cyclic voltammetry (CV) and pulsed electrolysis (PE). By coupling the electrochemical data to the spectral features in TR-SERS, we study the dynamic activation of and reactions on the electrode surface and find that CO2 is already activated to carbon monoxide (CO) during PE (10% Faradaic efficiency, 1% under static applied potential) at low overpotentials (−0.35 VRHE). PE at varying cathodic bias on different timescales revealed that stochastic CO is dominant directly after the cathodic bias onset, whereas no CO intermediates were observed after prolonged application of low overpotentials. An increase in cathodic bias (−0.55 VRHE) resulted in the formation of static adsorbed CO intermediates, while the overall contribution of stochastic CO decreased. We attribute the low-overpotential CO2-to-CO activation to a combination of selective Cu(111) facet exposure, partially oxidized surfaces during PE, and the formation of copper-carbonate-hydroxide complex intermediates during the anodic pulses. This work sheds light on the restructuring of oxide-derived copper electrodes and low-overpotential CO formation and highlights the power of the combination of electrochemistry and time-resolved vibrational spectroscopy to elucidate CO2RR mechanisms.

Raman spectroscopy is known as a powerful technique for solid catalyst characterization as it provides vibrational fingerprints of (metal) oxides, reactants, and products. It can even become a strong surface‐sensitive technique by implementing shell‐isolated surface‐enhanced Raman spectroscopy (SHINERS). Au@TiO2 and Au@SiO2 shell‐isolated nanoparticles (SHINs) of various sizes were therefore prepared for the purpose of studying heterogeneous catalysis and the effect of metal oxide coating. Both SiO2‐ and TiO2‐SHINs are effective SHINERS substrates and thermally stable up to 400 °C. Nano‐sized Ru and Rh hydrogenation catalysts were assembled over the SHINs by wet impregnation of aqueous RuCl3 and RhCl3. The substrates were implemented to study CO adsorption and hydrogenation under in situ conditions at various temperatures to illustrate the differences between catalysts and shell materials with SHINERS. This work demonstrates the potential of SHINS for in situ characterization studies in a wide range of catalytic reactions.