Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
The electrochemical reduction of CO2 has the potential to combat growing atmospheric levels of CO2, while at the same time providing a renewable feedstock of highly valuable chemicals and fuels. Amongst transition metals, silver shows one of the highest faradaic efficiencies for CO as the main reaction product.1 Oxide-derived silver (OD-Ag), like OD-Au and OD-Cu, is a very interesting CO2 reduction catalyst since it exhibits a higher activity, selectivity and stability than pure silver. A number of reasons have been introduced to explain these effects for oxide-derived metals. For OD-Au and OD-Cu, Kanan et al. showed that the OD treatment results in a large increase in grain boundaries.2 Ma et al. studied OD-Ag and OD-Cu and propose that improved selectivity is caused by the local pH effect: when CO2 is being reduced, 2 protons are consumed, leaving a higher pH at the porous surface then in the bulk electrolyte.3
When varying the surface coverage of porous OD-Ag catalyst, we noticed that a higher coverage does not necessarily give us a higher CO faradaic efficiency. This indicates that even though the local pH effect might be part of the explanation of the improved catalytic performance of OD-Ag compared to Ag, it cannot be the full explanation. We therefore used in-situ X-ray Absorption Spectroscopy (XAS) to better understand the nature of these oxide derived metal catalysts.
With Extended X-Ray Absorption Fine Structure (EXAFS) we monitored Ag, Ag2O and OD-Ag. Comparing the EXAFS spectra of OD-Ag with those of Ag, we pinpoint the exact differences in catalyst morphology on the atom- and micro-scale to see what causes OD-Ag to be so much more effective as a catalyst. We discovered a changed preferred orientation between the untreated Ag and the OD-Ag. Also, we found a different binding affinity to small molecules such as water and CO2. Based on these findings, we can rationally design a new catalyst incorporating these different properties.
1. Hori, Y. In Modern Aspects of Electrochemistry No. 42; Springer: New York, 2008; pp 89–189.
2. Verdaguer-Casadevall, A.; Kanan, M. W. et al. J. Am. Chem. Soc. 2015, 137 (31), 9808–9811.
3. Ma, M. et al. Angew. Chemie - Int. Ed. 2016, 55, 9748–9752.
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