In situ spectroscopic Study of the Electrochemical Reduction of CO2 on single facet Copper Nanoparticles

Abstract

Converting CO2 into useful chemicals and fuels with renewably generated electricity is of great technological and fundamental interest. Copper stands out as an electrocatalyst for this electrochemical reduction, as it is the only metal that can induce C-C coupling and therefore can reduce CO2 into a variety of valuable C2+ hydrocarbons. The product distribution of these hydrocarbons depends on a lot of factors, of which the applied potential and the exposed surface facets have the most profound influence. Generally speaking it is known that the (100) facet produces more ethylene, while the (111) facet produces more methane. The exact reaction mechanisms and their differences on the different facets remain under debate. In this thesis research we use in situ Raman spectroscopy and in silico DFT calculations to study the differences between the electrochemical reduction of CO2 (eCO2RR) on copper nano cubes and octahedrons, which have (100) and (111) facets exposed respectively. We found that, different to polycrystalline Cu (pc-Cu), linear CO (COL) is not stabilized on the (100) or (111) facets under working conditions. This lead to the conclusion that the C-C coupling mechanism of the eCO2RR on both facets, probably involves at least one multiple bound CO species, which are stabilized on the (100) and (111) surface. The 460, 500 and 535 cm–1 peaks were observed on both facets and their origin was explored, but no intermediate could be assigned to these peaks with certainty. A novel find for these peaks is their negative Stark tuning rate, which has not yet been observed in literature. The direction of this tuning rate suggests that the intermediate that is the origin of these peaks possess both a σ-orbital as HOMO and a π*-orbital as LUMO, similar to CO. It was also shown that Stark tuning rates can be calculated using DFT-calculations. This type of calculation may be used in the future as a tool in determining the origin of the peaks in the 500 region, further elucidating the eCO2RR mechanism.