Studying the dynamics of model catalysts
A catalyst is a material that increases the rate of a reaction without being consumed. Frequently, such catalysts consist of metal nanoparticles or clusters on an oxide support. At reaction temperatures (typically a few 100°C) and under reactive gas atmospheres, a catalyst becomes highly dynamic, continuously changing its structure and thus active sites. In our group in Munich, we use video-rate scanning tunneling microscopy (STM) to monitor the cluster geometry and stability during a reaction and track mobile species on the surface (employing our recently developed FastSTM module). We can thus monitor equilibrium processes on ms to s time scales but also non-equilibrium processes, co-existing species and rare events. The next step in our research is to conduct video-rate STM under reactive gas mixtures in a NAP-STM which will allow us to observe surface dynamic processes in situ with atomic resolution, at elevated temperatures and pressures.
The surface dynamical processes that we investigate include
- temperature- and adsorbate-dependent stability of size-selected clusters on a support,
- intrinsic dynamics of support materials (e.g. oxide surfaces) at reaction temperatures,
- cluster-support interplay during a reaction, such as reactant spill-over from active sites onto an inert support and vice versa, and
- intrinsic cluster dynamics, changing between structural isomers (so-called fluxionality).
A. Bourgund, B.A.J. Lechner, M. Meier, C. Franchini, G.S. Parkinson, U. Heiz, F. Esch, “Influence of local defects on the dynamics of O-H bond breaking and formation on a magnetite surface", J. Phys. Chem. C, 123, 19742-19747 (2019).
- B.A.J. Lechner, F. Knoller, A. Bourgund, U. Heiz, F. Esch, “A microscopy approach to investigating the energetics of small supported metal clusters", J. Phys. Chem. C, 122, 22569-22576 (2019).
- B.A.J. Lechner, X. Feng, P.J. Feibelman, J.I. Cerdà, M. Salmeron, “Scanning tunneling microscopy study of the structure and interaction between carbon monoxide and hydrogen on the Ru(0001) surface", J. Phys. Chem. B, 122, 649-656 (2017).
- P. Rotter, B.A.J. Lechner, A. Morherr, D.M. Chisnall, D. Ward, A.P. Jardine, J. Ellis, W. Allison, B. Eckhardt, G. Witte, “Coupling between diffusion and orientation of pentacene molecules on an organic surface", Nat. Mater. 15, 397-400 (2016).
- B.A.J. Lechner, Y. Kim, P.J. Feibelman, G. Henkelman, H. Kang, M. Salmeron, “Solvation and reaction of ammonia in molecularly thin water films", J. Phys. Chem. C 119, 23052 (2015).
- B.A.J. Lechner, H. Hedgeland, J. Ellis, W. Allison, M. Sacchi, S.J. Jenkins, B.J. Hinch, “Quantum influences in the diffusive motion of pyrrole/Cu(111)", Angew. Chem. Int. Ed. 52, 5085-5088 (2013).
Observing surface reactions under ambient conditions
To complement the real space information from STM with information on the chemical state of metal clusters and support materials under reaction conditions, we use near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) at synchrotrons. While the size and composition of catalysts can be controlled to a high degree using chemical synthesis methods, a non-zero size distribution and some ligands at the particle surface typically remain. Instead, we prepare samples with our laser evaporation source here in Munich, which produces truly monodisperse, ligand-free metal clusters. This way, we can compare the catalytic behavior of particles just one atom apart in size.
B. Zugic, L. Wang, C. Heine, D.N. Zakharov, B.A.J. Lechner, E.A. Stach, J. Biener, M. Salmeron, R.J. Madix, C.M. Friend, “Dynamic Restructuring Drives Catalytic Activity on Nanoporous Gold-Silver Alloy Catalysts", Nat. Mater., 16, 558-564 (2017).
- C. Heine, B.A.J. Lechner, H. Bluhm, M. Salmeron, “Recycling of CO2: Probing the chemical state of the Ni(111) surface during the methanation reaction with ambient pressure x-ray photoelectron spectroscopy", J. Am. Chem. Soc., 138, 13246-13252 (2016).
- C. Heine, B. Eren, B.A.J. Lechner, M. Salmeron, “A Study of the O/Ag(111) system with scanning tunneling microscopy and x-ray photoelectron spectroscopy at ambient pressures", Surface Science 652, 51-57 (2016).
List of Publications
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