Plenarvorträge - DPG-Tagungen

Oberflächenphysik Donnerstag
O 31.5 Do 12:15 H36
Selective methanol oxidation on a Cu (110) surface — •Ling
Zhou, Sebastian Guenther, and Ronald Imbihl — Institut fuer
Physikalische Chemie und Elektrochemie,Universitaet Hannover
The oxidation of methanol with oxygen to formaldehyde, CO2, H2 and
H2O on a Cu (110) surface was studied under stationary reaction conditions
in the 10 −7 - 10 −3 mbar range with rate measurements, AES,
LEED and PEEM. The catalytic activity of the surface exhibits a pronounced
peak at low temperature (≈ 480 K) and a second peak at high
temperature (≈ 850 K). The selectivity of the stationary reaction was
carefully analyzed and related the adsorbates present on the surface. A
pronounced hysteresis of the reactivity during temperature ramping can
be observed. It was shown that this hysteresis as well as the H2-formation
rate is entirely determined by structural effects. All products are formed
with first order kinetics with respect to oxygen partial pressure up to a
certain ratio. Above this ratio with an oxygen rich gas feed, high oxygen
coverage phases form inhibiting the reaction. By varying the total pressure
from 10 −7 mbar up to 10 −3 mbar a pressure dependent temperature
shift of the first reaction peak of about 20 K / decade was detected. This
finding is in agreement with experiments in the range above 0.01 mbar.
This result can be regarded as a first and successful step in bridging the
pressure gap between UHV- and high-pressure experiments.
O 31.6 Do 12:30 H36
Vibrational Analysis of the V2O5(010) Surface with and without
Oxygen Vacancies: ab-initio DFT Cluster Studies — •Christoph
Friedrich and Klaus Hermann — Fritz-Haber-Institut der Max-
Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin
While the catalytic behavior of vanadium oxide has been known and
utilized for a long time many microscopic details of oxidation reactions
at vanadium oxide surfaces are still not fully understood. V2O5 reduces
quickly when exposed to X-ray and LEED beams. It has been shown that
O 32 Nanostrukturen III
under catalytic reaction conditions oxygen vacancies are always present
on the surface and may act as active sites. From vibrational spectra obtained
e.g. from Infrared and Raman-spectroscopy as well as HREELS
measurements information about the geometry and bond strengths of
the different vibrating species can be gained. Vibrational frequencies are
particularly sensitive to changes in the local geometric and chemical environment.
This work focuses on how the vibrational spectra are modified
by the presence of different types of oxygen vacancies at the surface. In
addition, the influence of adsorbed CO on surface vibrations is discussed.
This work is supported by SFB 546 “Structure, Dynamics, and Reactivity
of Transition Metal Oxide Aggregates”.
O 31.7 Do 12:45 H36
SFG and TDS studies of methanol adsorption and decomposition
on Pd model catalysts — •Matthias Morkel, Günther
Rupprechter und Hans-Joachim Freund — Fritz-Haber-Institut
der Max-Planck-Gesellschaft, Abt. Chemische Physik, Faradayweg 4-6,
14195 Berlin
Methanol adsorption and decomposition on Pd(111) and Al2O3supported
Pd-nanoparticles were studied by sum frequency generation
(SFG) vibrational spectroscopy from ultrahigh vacuum (UHV) to 100
mbar, at temperatures up to 400 K. Under UHV, these processes were
also followed by thermal desorption spectroscopy (TDS).
The decomposition of methanol can be seperated in two reaction pathways.
A fast reaction pathway leads to formation of CO and hydrogen.
The second pathway produces carboneacious species (CHx or C) by scission
of the CO-bond of methanol. Although the reaction rate of the latter
pathway is much lower, it cannot be negelected under high-pressure. Depending
on the temperature the carbon poisoning leads to a partial or
complete deactivation of the catalyst. The binding sites of the carbon
species were characterized by SFG using CO as probe molecule. Regeneration
with oxygen was able to (partly) remove the carbon deposits.
Zeit: Donnerstag 11:15–13:15 Raum: H38
O 32.1 Do 11:15 H38
STM, TDS and LEED examinations of trimesic acid on single
crystal surfaces — •Lorenz Kampschulte, Robert Kraus,
Stefan Griessl, and Wolfgang Heckl — Department für Geound
Umweltwissenschaften, LMU München, Theresienstr. 41, 80333
München, www.nano.geo.uni-muenchen.de
The adsorption of trimesic acid (TMA) on single crystal surfaces was
studied by STM (Scanning Tunnelling Microscopy), TDS (Thermal Desorption
Spectroscopy) and LEED (Low Energy Electron Diffraction).
Trimesic acid consists of a benzene ring with three carboxylic groups
in symmetric 1,3,5-positions. Three different single crystals were used as
substrates: graphite(0001), silver(111) and gold(111).
The measurements were done under Ultra High Vacuum (UHV) conditions,
the trimesic acid was evaporated on different substrates using an
effusion cell. The preparation of a TMA monolayer was observed and adjusted
with thermal desorption spectroscopy. From these measurements
it was possible to deduce the binding conditions. In all cases characteristic
periodically arranged structures could be demonstrated. In the
monolayer regime the structure is characterized by non-dense-packing of
molecules on the surface. It was shown that, depending on the substrate
material and the preparation method, different network structures could
be assembled. These networks are induced by directed hydrogen bonding
(self assembly), forcing the organic molecules to build a two-dimensional
grid architecture with small cavities.
O 32.2 Do 11:30 H38
Self-organization of Au-nanowires on two-dimensional tungsten
carbide — •A. Varykhalov, O. Rader, and W. Gudat — BESSY,
12489 Berlin
A thermally induced formation of well-ordered arrays of Au nanowires
on top of the carbon-induced 15 × 3 reconstruction of W(110) has been
discovered with STM and LEED. STM measurements reveal the formation
of pairs of nanowires showing a continuous, dense character of
the first nanowire and a cluster-like periodic assembly in the internal
structure of the second one. Band mapping with angle-resolved photoemission
confirmes one-dimensional anisotropy of the electronic structure
of the system. Based on quantum size effects observed in photoemission
from the valence band of the clean 15 × 3 surface carbide, a crystallographic
model of the substrate relaxation is proposed as explanation for
the nanowire geometry. In addition, large chemical shifts in the W4f
line permit an element-specific analysis of the self-organized nanowire
formation and reveal the possible role of Au-W surface alloying.
O 32.3 Do 11:45 H38
Epitaxial growth of transition metals on the Ir (100) - (5 ×
1) surface — •Chiara Giovanardi, Andreas Klein, Andreas
Schmidt, Lutz Hammer, and Klaus Heinz — Lehrstuhl für
Festkörperphysik, Universität Erlangen- Nürnberg, Staudtstr. 7,
D-91058 Erlangen
The early growth stages of transition metals (Fe, Ni, Co) on the quasihexagonally
reconstructed Ir(100)-(5×1) surface have been studied by
Scanning Tunnelling Microscopy (STM) and quantitative Low Energy
Electron Diffraction (LEED). The periodic corrugation of the reconstructed
Ir surface rules the growth of the metal by promoting the diffusion
of atoms within the one dimensional troughs of the surface layer.
At sub-monolayer coverage, linear chains are therefore formed with their
length increasing with increasing substrate temperature during deposition.
Laterally, the chains are mostly diatomic, but for Ni and Co also
triple and quadruple chains are observed. Recently published DFT calculations
[1] claim that Fe atoms reside in short bridge sites, while our
experimentally based LEED analisys favours the long bridge position, as
directly imaged by STM in the case of Ni chains, too. With increasing
coverage, the hexagonal reconstruction is locally lifted. This is prior to
the chain phase covering the whole surface and a surface compound (e.
g. Fe4Ir ) forms.
[1] D. Spiˇsák and J. Hafner, Surf. Sci. 546 (2003) 47
O 32.4 Do 12:00 H38
Temperature dependence of surface morphology at grazing
incidence ion bombardement on Pt(111) — Henri Hansen,
•Sebastian Messlinger, Celia Polop, and Thomas Michely —
I. physikalisches Institut, RWTH-Aachen, Postfach, 52056 Aachen