Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
O 20 Oberflächenreaktionen I
Zeit: Dienstag 11:15–13:15 Raum: H45
O 20.1 Di 11:15 H45
Adsorption and Oxidation of NH3 on RuO2(110) — •Yuemin
Wang, Ursula A. Paulus, Karl Jacobi, and Gerhard Ertl —
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-
14195 Berlin
The epitaxially grown RuO2(110) surface was found to exhibit high
catalytic activity for CO oxidation [1, 2]. In the present work the interaction
of NH3 with the stoichiometric and O-enriched RuO2(110) surfaces,
which the latter exposes a weakly bound atomic oxygen species on-top of
the unsaturated Ru atom (Ru-cus), is studied using high-resolution electron
energy-loss spectroscopy (HREELS) and thermal desorption spectroscopy
(TDS). At the stoichiometric RuO2(110) surface we observed
only the molecular adsorption of NH3 on Ru-cus at 85 K, whereas on
the O-enriched surface NH3 oxidation occurs with annealing to higher
temperature. The reaction products depend on the coverage of O-cus.
At low O2 exposures (below 0.3 L) NH3 reacts with O-cus to give rise
to N2 and H2O. With increasing oxygen exposure NO is produced and
becomes the main product at saturation of O-cus. The formation of other
N-containing species such as N2O and NO2 were not observed. Reaction
mechanisms for NH3 oxidation will be discussed in detail. [1] C. Y. Fan,
J. Wang, K. Jacobi, G. Ertl, J. Phys. Chem. B 106, 10058(2002). [2] J.
Wang, C. Y. Fan, K. Jacobi, G. Ertl, Surf. Sci. 481, 113 (2001).
O 20.2 Di 11:30 H45
Austrittsarbeitsänderungen während elektrochemischen Pumpens
von Festelektrolyt/Metall-Systemen am Beispiel YSZ/Pt
— •Tobias Neubrand, Sebastian Günther und Ronald Imbihl
— Institut für Physikalische Chemie und Elektrochemie, Universität
Hannover, Callinstr. 3-3a, 30167 Hannover
Bei der Erklärung des sogenannten NEMCA (Non-faradayic Electrochemical
Modification Catalytic Activity)-Effektes nimmt die durch elektrochemisches
Pumpen hervorgerufene Änderung der Austrittsarbeit eine
entscheidende Rolle ein. Allerdings finden sich hierzu experimentelle und
theoretische Arbeiten mit widersprüchlichen Ergebnissen. Aus diesem
Grund wurden Messungen mit einer Pt-YSZ-Pt Gasreferenzzelle durchgeführt,
bei der Arbeits-, Gegen- und Referenzelektrode durch Aufpinseln
und anschließendem Einsintern einer Pt-Paste aufgebracht wurden. Für
die Arbeitselektrode wurden verschiedene Pt-Pasten (Firma Engelhard
bzw. Demetron) verwendet, die unterschiedliches Verhalten beim elektrochemischen
Pumpen zeigten. Die Änderung der Austrittsarbeit wurde
mittels Kelvin-Sonde und PEEM (Photo Electron Emission Microscopy)
an Luft und unter UHV-Bedingungen untersucht. Die Kombination von
PEEM und Kelvin-Sonde erlaubt es, zwischen elektrostatischen Effekten
und adsorbatbedingter Änderung der Dipolbarriere an einer Oberfläche
zu unterscheiden. Die Ergebnisse zeigen, daß unter Reaktionsbedingungen
der elektrische Kontakt der Pt-Körner zum Teil verloren geht, was
zu elektrostatischen Aufladungen und einer Verfälschung der Austrittsarbeitsmessung
mit einer Kelvin-Sonde führt.
O 20.3 Di 11:45 H45
Adsorption and Interaction of Ethylene on stoichiometric and
oxygen rich RuO2(110) — •Ursula A. Paulus 1 , Yuemin Wang 1 ,
Hans P. Bonzel 2 , Karl Jacobi 1 , and Gerhard Ertl 1 — 1 Fritz-
Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195
Berlin — 2 Forschungszentrum Jülich, ISG3, D-52425 Jülich
The nature of the oxygen species responsible for ethylene (C2H4) oxidation
on Ag particles is still under discussion. Here we prepare stoichiometric
and oxygen rich RuO2(110) surfaces exposing either coordinatively
unsaturated Ru (Ru-cus) and oxygen (O-bridge) atoms or O-bridge
and O-cus atoms. We study the adsorption and interaction of C2H4 on
those surfaces applying high-resolution electron energy loss spectroscopy
(HREELS) and thermal desorption spectroscopy (TDS).
Under the applied conditions (ultra-high vacuum, no continuous reactant
supply) C2H4 adsorbs molecularly on the stoichiometric RuO2(110)
surface at 85 K and desorbs again molecularly at 320 K. Warming up the
sample to 260 K leads to changes in the bond strength. A rearrangement
from π-bonded to σ-bonded is suggested.
On the oxygen rich RuO2(110) surface a complete oxidation of C2H4
to CO2 and water is observed. Warming up the sample between 85 K and
500 K HREEL spectra indicate the presence of several reaction intermediates,
which are discussed here.
[1] J. Wang et al., J. Phys. Chem. B. 106 (2002) 3422.
O 20.4 Di 12:00 H45
CO oxidation at RuO2(110) studied by first-principles kinetic
Monte-Carlo simulations — •Karsten Reuter 1,2 , Daan
Frenkel 2 , and Matthias Scheffler 1 — 1 Fritz-Haber-Institut,
Berlin — 2 AMOLF, Amsterdam
A quantitative modeling of catalytic activity requires a high accuracy
in the description of each of the manifold of involved elementary processes,
as well as a correct statistical mechanics treatment of the interplay
between them. We attempt such a modeling by first-principles kinetic
Monte-Carlo simulations, i.e. using rates derived from density-functional
theory as input. Modeling the CO oxidation over RuO2(110) we obtain
a full (T, pO2 , pCO) turnover frequency diagram, which agrees with available
experimental data. The surface configurations actuating catalysis
are found to be strongly influenced by kinetic effects, most prominently
under gas phase conditions where the system is close to a surface phase
transition. The highest steady-state turnover rates are obtained for the
resulting phase coexistence at the surface, which enables specific reaction
mechanisms that do not operate at other temperature and pressure
conditions including ultra-high vacuum.
O 20.5 Di 12:15 H45
Depolarisation of the rotational alignment. — •M. Rutkowski
and H. Zacharias — Physikalisches Institut, Westfälische Wilhelm-
Universität, Wilhelm-Klemm Str. 10, 48149 Münster, Germany
Up to now experimental investigations of the rotational alignment of
molecules both in the gas phase and desorbing from surfaces are performed
under the assumption that the rotational alignment is nonvarying
with time. However, early theoretical investigations for atoms [1]
predict a strong time dependence, caused by the interaction of the nuclear
spin with the electronic angular momentum. In this talk a corresponding
model for the rotational angular momentum of molecules is applied to
D2, H2 and NO molecules. The calculations of the rotational alignment
of these molecules yield strong periodic oscillations, which significantly
change both in amplitude and periodicity with the rotational quantum
number.
[1] U. Fano, J.H. Macek, Rev. mod. Phys., 45, 553 (1973).
O 20.6 Di 12:30 H45
Dehydrogenation of Methanol on Rhodium/Vanadium Surface
Alloys and the influence of preadsorbed oxygen — •Georg
Krenn, Klaus D. Rendulic, and Robert Schennach — Institute
of Solid State Physics, Graz University of Technology, Austria
The dehydrogenation of methanol on metal surfaces has regained interest
recently due to the development of methanol powered fuel cells.
Dehydrogenation of methanol on Rh(111) leads to the formation of carbon
monoxide and hydrogen only. The reaction products are the same
on the Rh(111)/V alloy surfaces, but the reaction kinetics differ significantly.
On a Rh(111)/V subsurface alloy and on a Rh(111)/V islands
surface the reaction probabilities stay high up to about 400 K and 550
K, respectively, in contrast to about 200 K on Rh(111). On the Rh(111)
surface with V islands a decrease in the reaction probability occurs due
to buildup of carbon layers on the V islands. Preadsorption of oxygen
on the three surfaces reverses the trend from the oxygen free surfaces, so
that the Rh(111) surface shows the highest activity. With preadsorbed
oxygen water and carbon dioxide are found as additional reaction products.
The difference between the oxygen covered surfaces and the oxygen
free surfaces is discussed in terms of two different initial reaction steps
on the surface. With preadsorbed oxygen a proton transfer reaction from
the OH group of methanol to adsorbed oxygen seems to be the first reaction
step, while on the oxygen free surface conventional dehydrogenation
takes place.
O 20.7 Di 12:45 H45
Mixed quantum classical simulation of DIET processes —
•Christian Bach and Axel Groß — Physik-Department T30,
Technische Universität München, 85747 Garching
We present a mixed quantum-classical method for the simulation of
laser-induced desorption processes at surfaces. In this method, the nuclear
motion is described classically while the electrons are treated quan-