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Forschungsarbeit

When molecules meet surfaces: Where does the energy go?

by Vanessa Bukas (08.07.2015)

Exothermic surface chemical reactions may easily release several electron volts of energy. Fundamental questions regarding the conversion and dissipation of this microscopically sizable amount of energy are critical in e.g. present day energy production and pollution mitigation, and yet in many cases remain unanswered. Here we promote microscopic understanding through a novel multi-scale approach which, for the first time, allows to model energy dissipation at the nanoscale from first-principles.

Due to its central role in technologically relevant processes such as heterogeneous catalysis, sensing, corrosion or epitaxy, the adsorption and dissociation of molecules on solid surfaces has been a most active field of research for many decades. When breaking things down to elementary processes on the molecular scale, however, our understanding of such phenomena remains rather limited. At present this receives even further stimulus from the basic energy science perspective which adds fundamental questions like the conversion of energy forms at interfaces to the agenda.

A particular example is the conversion of chemical energy to heat which arises as a consequence of a molecule's exothermic interaction with the surface and may easily amount to several electron volts of energy. For a prototypical model reaction like the dissociative adsorption of O2 molecules at metal surfaces, scanning-tunneling microscopy experiments have suggested the formation of so-called “hot” adatoms, i.e. dissociation fragments for which delayed heat dissipation results into a kinetic energy large enough to travel on the surface even at temperatures where thermal motion is prohibited.

In scrutinizing this intriguing proposition, and as the experimental quest to generate molecular movies of such reactions is still ongoing, theory has been challenged to elucidate a full picture of the equilibration dynamics through computationally demanding quantum mechanical (QM) treatments. The present work focuses on energy dissipated into phonons of the underlying metal (Me) substrate. We present a novel embedding scheme (QM/Me [1]) in which energy is dissipated out of a QM-described reaction zone and into a computationally undemanding, yet reliably described, extended bath.

[Bildunterschrift / Subline]: Illustration 1: Left: Snapshots along a dynamical trajectory depicting the motion of “hot” oxygen adatoms on a Pd(100) surface (color-coded to kinetic energy). Right: Profiles of kinetic energy as a function of time. Note that already within 1 picosecond after the initial O2 bond dissociation, phonons have propagated >50% of the initially released chemical energy outside the QM-described embedding cell and into the substrate bath.

In the application to O2 dissociation over Pd(100) QM/Me predicts “hot” dissociation fragments traveling ballistically over several lattice constants as a consequence of nonimmediate energy transfer to the underlying surface. The interfacial conversion of energy and its dissipation to the bulk occurs within a few picoseconds after the initial O2 bond dissociation, thus indicating that the thermalization process is not instantaneous on the time scale of the elementary process itself and clearly influences the actual adsorbate dynamics. The ensuing transient mobility thus intricately couples the elementary reaction steps of dissociation and diffusion; a notion hitherto not considered in prevalent kinetic models in catalysis.

We extend our investigation to several different systems and additionally advance our approach to encompass multiple reaction zones dynamically following the “hot” adatom motion. This provides trend understanding for different substrates and surface symmetries, while also allowing the comparison to experimentally studied systems that report long-ranged transient mobility. A detailed atomistic understanding of the influence of surface temperature and underlying phonon dynamics is thus finally in reach, paving the way into unprecedented insight towards accommodating such “hot chemistry”, for example, in our current understanding of heterogeneous catalysis and energy conversion mechanisms at interfaces in general.

References

[1] J. Meyer and K. Reuter, Angew. Chem. Int. Ed., 53, 4721 (2014).


mailto: Vanessa Bukas
Vanessa Bukas
* 1986

Wissenschaftlicher Werdegang
  • seit 11/2012
  • Promotionsstudium in Theoretscher Chemie, Technische Universität München. Dissertationsthema: Dissociation and dissipation dynamics of adsorbates at solid surfaces
  • 2010-2012
  • Elitestudiengang M.Sc. in Advanced Materials Science, Technische Universität München, Ludwig-Maximilians-Universität München, Universität Augsburg
  • 2003-2009
  • Diplom in Applied Mathematical & Physical Sciences, National Technical University of Athens, Griechenland

Preise und Auszeichnungen
  • * Computing time award in the amount of 25 million CPU-hours from the ’Gauss Center for Supercomputing’ towards large-scale molecular dynamics simulations (2013 – 2014)
  • * Funding for conference participation awarded by the ’DAAD’ towards attending the Gordon Research Conference, "Dynamics at Surfaces" in Newport RI, USA (Aug. 2013)
  • * Full scholarship awarded by the ’Alexander S. Onassis Public Benefit Foundation’ in support of postgraduate studies (2010 – 2012)
  • * Lyceum excellence awards granted by the Greek Ministry of Education for three successive years (2000 – 2003)

Ausgewählte Veröffentlichungen
  • * V.J.Bukas, A.den Dunnen, L.Jacobse, S.Wiegmann, J.Meyer, L.B.F. Juurlink and K.Reuter, "Precursor-mediated O2 dissociation on Pd(100): Implications for adsorption kinetics from dissipative dynamics ", in preparation
  • * V.J. Bukas, S. Mitra, J. Meyer and K. Reuter, “Fingerprints of energy dissipation for exothermic surface chemical reactions: O2 on Pd(100) ”, J. Chem. Phys. (2015), in press
  • * S.Söllradl, M.Greiwe, V.J.Bukas (...) and R.Niewa, "Urea-based solution combustion synthesis and prompt gamma activation analysis of orange colored, essentially nitrogen-free ZnO powders", Chem. Mat., 27(12), 4188-4195 (2015)
  • * M.Tsampodimou, V.J.Bukas, E.T.Stathopoulou, V.Gionis and G.D.Chryssikos, "Near-infrared investigation of folding sepiolite", Am. Mineral., 100, 195-202 (2014), DOI: 10.2138/am-2015- 4988
  • * V.J.Bukas, J.Meyer, M.Alducin and K.Reuter, "Ready, set and no action: A static perspective on potential energy surfaces commonly used in gas-surface dynamics", Z. Phys. Chem., 227, 1523- 1542 (2013), DOI: 10.1524/zpch.2013.0410
  • * V.J.Bukas, M.Tsampodimou, V.Gionis and G.D.Chryssikos, "Synchrnous ATR and NIR investigation of sepiolite upon drying", Vibr. Spectrosc, 68, 51-60 (2013)