Berlin 2015 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 6: Catalysis
O 6.2: Vortrag
Montag, 16. März 2015, 10:45–11:00, MA 043
Travelling Vanadium Oxide Islands in a Catalytic Reaction — •Bernhard von Boehn, Martin Hesse, and Ronald Imbihl — Institut für Physikalische Chemie und Elektrochemie, Leibniz-Universität Hannover, Callinstr. 3 – 3a, D-30167 Hannover, Germany
Upon evaporation of a submonolayer coverage of vanadium in O2 onto a Rh(111) surface one obtains well ordered two-dimensional V-oxide islands which we use as model catalysts for the partial oxidation of methanol to formaldehyde [1]. With photoemission electron microscopy we observe that under reaction conditions in the 10−4 mbar range the initially homogeneous VOx film (θV=0.2) transforms upon heating at 560 ∘C into a quasi-stationary macroscopic concentration pattern. This pattern consists of equidistant parallel stripes of VOx surrounded by nearly bare metal surface. Above 700 ∘C the stripe pattern changes into a spot pattern of circular VOx islands with 20 – 200 µm diameter. Due to attractive interactions the islands approach each other with a velocity up to a few µm/s depending on distance. The islands coalesce, afterwards their shape becomes rapidly circular again indicating a high line tension at the island boundaries. The islands only move under reaction conditions. A mechanism is proposed which is based on the chemical equilibrium of polymerization of individual VOx clusters into macroscopic VOx islands. Since the poly-/depolymerisation equilibrium is controlled by oxygen coverage, oxygen gradients surrounding the VOx islands under reaction conditions can cause the movement.
[1] J. Schoiswohl, S. Surnev, and F. P. Netzer, Topics in Catalysis 36 (2005) 91.