Speakers
Description
Catalysts have the ability of improving the kinetics of a given chemical reaction by providing alternative reaction paths with lower activation energies, thus affecting the selectivity of a chemical reaction.$^1$ Their activity, selectivity, and lifetime depend on their morphology, substrate, chemical composition, and oxidation state.$^2$ Particularly, bimetallic nanoparticles (NPs) are often interesting candidate materials because they typically exhibit enhanced reactivity compared to bulk or their monometallic counterparts.$^3$ However, the evolution of their structure, composition, and oxidation state under reaction conditions must be considered when discussing their application. We propose the use of two well-defined systems to study fundamental aspects such as the stability and segregation trends of the catalytic materials under reaction conditions.
On the one hand, we combine local microscopy (LEEM), diffraction (LEED), and spectroscopy (XPEEM/NEXAFS) techniques to identify possible phase redistribution on Ni/Cu(100) samples used as model systems in quasi in situ experiments performed under CO$_2$ hydrogenation conditions in a NAP (mbar) reactor. It has been shown that the surface composition of CuNi NPs changes as a function of the gas phase composition in similar conditions to those used in our study, and that the product distribution (methane vs. methanol) is strongly affected by it.$^4$ With our approach, we can establish correlations between the chemical nature of the different surface components in the bimetallic system and the composition of the reactive environments (CO$_2+$H$_2$ and CO$_2+$CO$+$H$_2$).
On the other hand, it is widely accepted that the oxidation state and morphological changes of Cu-based NPs can have an impact on the product distribution of many catalytic reactions, such as methanol steam reforming5 and CO$_2$ electrocatalytic reduction. In order to study these effects, Cu NPs supported on SiO$_2$@Si(100) were used in our studies, where initially oxidized NPs were subjected to consecutive annealing steps in UHV. For each NP, we tracked the evolution of their chemical composition, oxidation state (XPEEM, NEXAFS, XPS), and morphology (LEEM) in situ. We were able to identify parameters that can influence the activation energy of the thermal reduction from CuO to Cu$_2$O. Moreover, local decrease in the silica thickness and a change in the oxidation state appear to facilitate the reduction of the NPs. Furthermore, CuNi NPs were used as a catalyst for the dry reforming of methane, where we studied the changes of the catalyst at different reaction temperatures under NAP conditions.
References
1. J.K. Norskov, F. Studt, F. Abild-Pedersen, T. Bligaard, in: “Fundamental Concepts in Heterogeneous Catalysis.”, Wiley-VCH, New Jersey, 2014.
2. B. Roldan Cuenya, Thin Solid Films 518, 3127–3150 (2010).
3. M. Sankar, N. Dimitratos, P. J. Miedziak, P. P. Wells, C. J. Kiely, G. J. Hutchings, Chem. Soc. Rev. 41, 8099 (2012).
4. I. Zegkinoglou, L. Pielsticker, Z.-K. Han, N. J. Divins, D. Kordus, Y.-T. Chen, C. Escudero, V. Pérez-Dieste, B. Zhu, Y. Gao, B. Roldan Cuenya, J. Phys. Chem. C 123, 8421 (2019).
5. S. Sá, H. Silva, L. Brandão, J. Sousa, A. Mendes, Applied Catalysis B: Environmental 99, 43–57 (2010).
| Abstract Number (department-wise) | ISC 16 |
|---|---|
| Department | ISC (Roldán) |
