The catalysis of CO₂ electroreduction and related processes

The present PhD research is focused on the electrochemical reduction of CO2 to hydrocarbons. This process, coupled to renewable energy sources, such as wind and solar power, is an attractive alternative for the production of synthetic carbon neutral fuels and fine chemicals. Although many metals have been studied as catalysts for CO2 electro-reduction, copper is the only one at which hydrocarbons are produced in considerable amounts.

Hydrocarbon formation, however, requires a high overpotential (~1 V), which implies big energy loses. Furthermore, the selectivity of copper towards a particular product is low and a mixture of products is obtained. In order to improve the catalytic process, a better understanding of the factors affecting both the selectivity and the energy efficiency of this reaction is needed.

First of all, this work aimed at studying different polycrystalline copper surfaces as catalysts for the CO2 electrochemical reduction. This allowed us to explore the effect of the surface morphology on the catalytic activity of copper. Our results suggest that the presence of steps and kinks on rough copper surface favors the CO2 electrochemical reduction over the hydrogen evolution reaction.

Furthermore, on rough surfaces the formation of ethylene is enhanced over methane production. The next step has been studying the electrochemical reduction of CO2 onto Cu overlayers on Pt single crystals. The purpose this work was to study the effect of having a strained Cu surface on the efficiency and selectivity of the reaction.

Furthermore, by studding different crystal facets we can obtain information about the role of steps. Interestingly, the selectivity towards hydrocarbons on the copper overlayers is much lower than the obtained on polycrystalline copper. These results are consistent with a linear combination of Pt and Cu rather than an expanded copper surface.

Furthermore, electrochemical scanning tunneling microscopy (EC-STM) studies indicate that in the presence of CO the Cu overlayer changes from a nearly flat to a granular structure exposing part of the Pt surface. These results illustrate the importance of in situ measurements in order to gain an insight into the catalyst surface structure under reaction conditions.

This knowledge can be crucial for the understanding of the catalyst reactivity. Finally, this Thesis is focused on the formation of a Cu/Pt (111) overlayer, as well as near surface and surface alloys. These studies show that the reactivity of the Pt(111) depends on the location of the copper atoms. The presence of copper both on the first layer and in the surface alloy strengthens the interaction between Pt and its adsorbates.

In contrast, Cu atoms located in the second layer or in the near surface alloy lead to the opposite effect. These results are very useful for the rational design of catalysts for a variety of processes, as has been demonstrated in this work for the CO electrochemical oxidation.