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Towards the discovery of efficient CO2 electroreduction catalysts: well-defined RuOx and MoSx nano catalysts

Periodic Reporting for period 1 - CO2-RR-MODCAT (Towards the discovery of efficient CO2 electroreduction catalysts: well-defined RuOx and MoSx nano catalysts)

Reporting period: 2016-05-01 to 2018-04-30

Electrochemical devices – such as fuel cells in electric vehicles and electrolysers – are progressively entering our daily lives. They also have the potential to revolutionize the production of bulk chemicals and fuels, by converting CO2 into valuable compounds through the direct use of renewable electricity. Indeed, one could imagine operating an electrochemical cell that converts CO2 to syngas (a mixture of CO and H2) or ethanol through the electro-oxidation of water and the simultaneous electro-reduction of CO2. This ideal cell would be directly powered by renewable electricity. As a matter of fact, the integration of renewable energy sources (e.g. wind and solar) into our current energy, transportation and chemical sectors represents a formidable societal, scientific and technological challenge.

In order to enable this technology, major breakthroughs are necessary to discover efficient electrode materials, i.e. electrocatalysts, capable to accelerate target reactions. In the frame of the MSCA fellowship I investigated two promising classes of materials that can be utilized for this technology. In the first project I explored the application of RuO2-based electrodes to produce methanol from CO2. In the second project I investigated the application of gold electrodes for the formation of CO from CO2. The overall goal was to develop new, active and selective functional materials on the basis of fundamental insight obtained studying and investigating model systems (electrodes with well-defined surface features).

A thorough analysis of RuO2 based electrodes showed that these electrodes bind CO2 too strongly. The further reduction of CO2 to methanol was found to be impossible, in contrast with what reported in past literature studies and what suggested by theoretical predictions. At any rate, these “negative” results offer important guidelines to improve our understanding of these complex surfaces.
Important new findings were instead achieved through the study of gold surfaces. The nature of the catalytically active sites was clearly identified. It was confirmed that the reaction rate on steps-rich surfaces is ca. one order of magnitude faster than on atomically flat surfaces. Noteworthy, it was observed how the parallel competing reaction does not follow the same trend. This knowledge will be crucial to design application-relevant catalysts with optimal density of active sites and therefore with improved activity towards the target product.
Project I: Investigation of RuO2 electrodes for the electro-catalytic reduction of CO2

Several independent literature reports identified RuO2-based electrodes as potential game-changer catalysts in the CO2 electro-reduction. Indeed, theoretical and experimental studies suggested that methanol was a major reaction product.
Several types of electrodes (commercial particles, thermally prepared films) were investigated. All the catalysts tested and the several experimental conditions explored in these studies (mixed oxides, temperature, types of electrolytes) gave hydrogen as the only product detected under CO2 reduction conditions. Methanol and other oxygenates could not be detected under any conditions. As such, the experiments carried out in the CO2-RR-MODCAT project could not confirm the past reports. A series of experiments showed that CO2 and CO bind strongly to the RuO2 surface acting as poisonous species. Further reduction of CO2 to other products was not possible.
We anticipate that the reason behind the impossibility to confirm past literature reports may be found in the presence of impurities in the reaction media and/or to insufficient attention paid to the identification of the reaction products.

Project II (WP-2) – structure sensitivity in the electro-catalytic reduction of CO2 with gold catalysts

Gold is one of the most active known electrocatalysts capable to produce CO at low over-potentials and with high selectivity. Even so, little progress has been made over the first reports with bulk gold polycrystalline electrodes published in the 1980s. Many strategies have been recently proposed to further enhance its performance. However, the knowledge of how the atomistic structure of the catalyst surface influences reaction rates and selectivity remains a very important missing fundamental insight.
A thorough experimental investigation of gold electrodes having well-defined surface orientations was carried out in order to address these open questions. Atomically flat electrodes were compared to more open and steps-rich surfaces. The electrochemical reduction of CO2 to CO was found to exhibit a pronounced dependence on the structure of the surface: the formation of CO measured with the most active catalysts (defects-rich) is ca. 20 fold higher than the one measured with atomically flat electrodes. These findings suggest that steps and under-coordinated surface sites provide significantly faster turnover rates and improved binding energies for the reaction intermediates.
A set of additional experiments was carried out in order to confirm the crucial role of under-coordinated sites. A specific method was developed to selectively poison the active sites with inert foreign (Pb) atoms. Remarkably, if these defects are covered by inactive Pb atoms the CO current density is negligible. This confirmed that the reaction yielding CO from CO2 takes place predominantly at under-coordinated and defect surface sites.
The insights emerged from this study will provide pivotal guidelines for future theoretical investigations, offering valuable indications for mechanistic and kinetics analyses. In particular, the findings could foster the following research directions:

1) A better understanding of oxide-based electrodes. The current study highlighted how the knowledge and description of these materials is not accurate, as the theoretical predictions cannot be verified by the experiments. The results obtained within the CO2-MOD-CAT project should be used to re-align the first-principles calculations. In turn, a better theoretical description could provide guidelines for the discovery of efficient catalysts, for instance via specific surface modifications.

2) An improved description of the reaction kinetics – often a major challenge for several computational approaches – can be achieved making use of the data collected with gold electrodes. Indeed, results obtained with model systems (i.e. single crystals) are often the best stepping stones for computational studies.

On a general note, this study highlighted the importance of benchmark test and measuring protocols for this particular set of reactions. Indeed, these aspects are crucial in this field of research where the events occurring at the surface are often very sensitive to the presence of foreign atoms or accidental impurities and the reaction products are produced in minute quantities.
partial current density towards CO registered with Au single crystals electrodes