Theoretical models for the electrochemical reduction of co2 on copper catalysts under working conditions
- Autores: Federico Dattila
- Directores de la Tesis: Rodrigo Antonio García Muelas (dir. tes.), Nuria Lopez Alonso (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2020
- Idioma: español
- Tribunal Calificador de la Tesis: Beatriz Roldán Cuenya (presid.), Emilio J. Palomares Gil (secret.), Marc Theodorus María Koper (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
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- Resumen
Climate change is driven by anthropogenic CO2 emissions and it is heavily affecting humanity through economical threads due to severe weather phenomena. Furthermore, the impact of global warming will increase significantly within this century, leading to potential climate refugees and loss of society’s welfare. Therefore, our future progress must go hand in hand with sustainability and the increasing energy demand must be fulfilled through exploitation of renewable sources rather than fossil fuels. A limitation to a widespread use of renewable energies is the intermittency of their sources, such as sun, wind, and hydropower. Energy storage can be a suitable solution to this issue, however current technologies, such as lithium-ion batteries, have low energy density, so they are unable to power large, energy-demanding devices. Therefore, chemical fuels are the only long-term solution. Among the processes to produce chemical fuels and plastics, electrochemical CO2 reduction is the most sought-after, since this technology leads to a net negative impact on CO2 atmospheric concentration by using carbon dioxide as a reactant. C2+ chemicals such as ethylene and ethanol are attractive CO2 reduction products due to their high market share and cost, however their electrochemical generation is nowadays prohibitive due to the high complexity of the copper-based catalysts which are normally employed. Theoretical methods must deal with this complexity to untangle mutual effect on the catalytic process and provide robust guidelines to experimental synthesis of better performing devices. Relevant factors playing a key role in CO2 reduction must be thoroughly considered, such as surface reconstruction at negative potential, modifiers as strong tethering sites, and electrolyte and cation effect.
This thesis aims at modeling copper-based catalysts under electrochemical CO2 reduction conditions through Density Functional Theory (DFT) and Ab Initio Molecular Dynamics simulations. DFT can be applied to many-body systems to investigate their electronic structures and reactivity. If coupled with basic notions on electrochemical processes and the Computational Hydrogen electrode formalism, DFT simulations provide deep insights on electrocatalytic properties of well-defined materials. Furthermore, Ab Initio Molecular Dynamics formalism allows to extend the understanding to complex systems, studying their dynamic evolution. Finally, validations are key to prove the correctness of theoretical models, therefore throughout the work I always compared theoretical outcomes to state-of-the-art experimental reports, being able to reproduce experimental observations.
As a first step, I demonstrated the correctness of the employed methodologies by benchmarking well-established experimental and theoretical evidences on copper single crystals, such as the promotion of methane on close-packed domains and copper selectivity toward ethylene on (100) facets. Then, I introduced a higher degree of complexity to these simple crystalline models, by elucidating the process of surface reconstruction of polycrystalline copper at negative applied potential. This process is fully driven by polarization, since close-packed domains cannot store the high local electronic density, therefore they are destabilized in favor on more open domains such as (100). The developed analytical theoretical model suggests that at long polarization time and high intensity copper should reconstruct toward defective sites on (100) planes, therefore a strongly polarized copper-based catalyst is expected to promote CO2 reduction to ethylene. I verified this theoretical hypothesis by synthesizing CuO-derived particles which were applied to a Gas Diffusion Electrode Configuration and lead to around 60% Faradaic efficiency toward this C2 hydrocarbon at high current density. As a further step, I studied the surface reconstruction for complex systems such as oxide-derived copper materials, where residual oxygen may generate specific surface patterns. Copper was identified to appear with three well-defined oxidation states and relevant surface ensembles were classified. These ensembles were found highly active in tethering the CO2 intermediate due to their higher polarization than crystalline copper. Furthermore, I suggested that the metastability of near-surface oxygens triggers the enhanced C2+ selectivity detected experimentally for oxide-derived copper. A near-surface oxygen site can stabilize the energy demanding CO-CO dimerization step as a deprotonated glyoxylate, whose formation is thermodynamically favored and has a negligible kinetic barrier. Finally, I contributed to the study performed by our research group on the influence of chalcogen modified copper nanoparticles on CO2 reduction selectivity. We rationalized the improved Faradaic efficiency to formate as a result of the strong tethering of CO2 and H by sulfur adatoms. This modifier opens two alternative reaction pathways to the usual reduction routes on polycrystalline copper, boosting the generation of formate. Finally, I introduced at the end of the work the relevance of cation in triggering CO2 reduction activity and its selectivity on copper. This effect can be rationalized by chemical interaction between dehydrated cations and key reaction intermediates, therefore it should be accounted in modeling as well.
Overall, the work carried out during these three years provides a new methodological approach to study complex catalysts under CO2 reduction conditions. The theoretical guidelines here identified suggests synthetic routes to maximize ethylene and ethanol production on copper-based catalysts, as demonstrated by our highly selective CuO system.
