Speakers
Description
Copper (Cu) is an efficient catalyst towards CO$_{2}$ electrocatalytic reduction (CO$_{2}$RR). Strikingly, the identification of the responsible surface site(s) leading to the observed activity and/or selectivity still remains debatable. A way to address this is by controlled surface modification of model metallic Cu single crystals (SCs) and compare it with CO$_{2}$RR activity. Two pursued approaches are: (a) electrochemical oxidation of the surface, and (b) decorating steps with a promoter metal (Ag). The pre-oxidized copper surface enhances the activity and selectivity towards C$_{2}$ and C$_{3}$ products (ethylene, ethanol and propanol)$^1$ but the reason is still unclear. Contrastingly, decoration of the promotor (Ag) allows for local enhancement of CO formation, thereby enhancing C$_{2+}$ products, but a controlled surface site investigation is still lacking.$^2$ Thus, to gain better understanding, we utilize the SMART spectromicroscope at BESSY-II,$^3$ combining surface sensitive tools (XPS, NEXAFS, LEED, XPEEM and LEEM) to investigate electrochemically oxidized and Ag-decorated Cu SCs, before and after quasi in situ CO$_{2}$RR.
First, in order to induce changes in the Cu oxidation state, different potential pulse sequences between oxidizing and reducing conditions were applied. This approach has been proven in the past to enhance Cu selectivity towards C$_{2+}$ products.$^4$ Nevertheless, roughening of the surface, understood previously as a secondary effect of pulsing,$^5$ can be correlated with (n10) facets formation, identified by correlating microscopy with diffraction data.
In the second approach, Ag is decorated on Cu(100) by PVD and followed by LEEM and LEED in real time. Preferential step decoration occurs and is explored for different sub-monolayer coverages (0.10, 0.30 and 0.50 ML), exhibiting a c(10$\times$2) Ag overlayer structure at > 0.4 ML coverage. The overall CO$_{2}$RR current density increased upon increasing the Ag coverage, reinforcing the expected promotion effect. NEXAFS shows no change in the Cu oxidation state upon Ag deposition, however, surface oxygen species are observed post electrochemistry. A detailed XPS depth profile suggests plausible reordering between Ag and Cu post CO$_{2}$RR.
References
1. Mistry, H., Varela, A. S., Bonifacio, C. S., Zegkinoglou, I., Sinev, I.; Choi, Y. W., Kisslinger, K., Stach, E. A., Yang, J. C.; Strasser, P., Roldan Cuenya, B. Nat. Commun. 7, 12123 (2016).
2. Herzog, A., Bergmann, A., Sang Jeon, H., Timoshenko, J., Kühl, S., Rettenmaier, C., Lopez Luna, M., Haase, F., Roldan Cuenya, B. Angew. Chem. Int. Ed. 60, 7426 (2021).
3. Schmidt T., et al., Ultramicroscopy 110, 1358 (2010).
4. Arán-Ais, R. M., Scholten, F., Kunze, S., Rizo, R., Roldan Cuenya, B. Nature Energy 5, 317-325 (2020).
5. Scholten, F., Nguyen, K.-L., Bruce, J.P., Heyde, M., Roldan Cuenya, B., Angew. Chem. Int. Ed. 60, 19169–19175 (2021).
| Abstract Number (department-wise) | ISC 07 |
|---|---|
| Department | ISC (Roldán) |
