My Research

Studying the dynamics of model catalysts

At reaction temperatures (typically a few 100°C), a catalyst surface becomes dynamic and adsorbates, reaction intermediates as well as catalyst particles (clusters) can move across the support. During my Ph.D., I used helium spin-echo (HeSE) spectroscopy to investigate the steady state diffusion of small molecular adsorbates on pico- to nanosecond time scales. In our group in Munich, we use scanning tunneling microscopy (STM) to monitor the cluster geometry and stability during a reaction and track mobile species on the surface. While the time resolution of STM is limited compared to HeSE, it brings the significant advantage that we can thus monitor non-equilibrium processes, co-existing species and rare events. Furthermore, using the recently developed FastSTM module, we can measure two to three orders of magnitude faster than with conventional STM instruments, reaching video-rate scan frequencies. This allows us to observe surface dynamic processes in situ with atomic resolution, even at elevated temperatures.

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

Our current focus lies on Pt clusters on the magnetite (001) surface. Here, we are particularly interested in the role of surface defects on catalyst reactivity and in transport limitations when studying simple test reactions.

Select publications:

  • 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 clutsers and support materials, we use x-ray photoelectron spectroscopy (XPS). Specifically, we use ambient pressure XPS (APXPS) to observe chemical changes in situ during a reaction. 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 even with an averaging technique such as XPS. Since size-selected cluster samples intrinsically have a low coverage (to avoid coalescence into dimers, trimers, etc.), we need a highly sensitive technique. Therefore, we travel to synchrotrons (typically to the Advanced Light Source in Berkeley and recently also to the Diamond Light Source in England) to achieve the required resolution and signal intensity.

Select publications:

  • 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

See my Google Scholar profile