Dynamics of supported vanadium oxide catalysts from UHV to 0.1 mbar
The conditions of so-called “real catalysis” where catalysts consisting of complex mixtures of substances are exposed to pressures in a range of 1 – 100 bar are drastically different from single crystal studies conducted in an UHV environment. At elevated pressure, the catalytic surfaces restructure under reaction conditions with the extent ranging from atomic-scale reconstructions to real morphological changes and chemical transformations. This discrepancy is well known under the name pressure and materials gap in heterogeneous catalysis. To bridge these gaps various strategies are employed. As an example, we study vanadium oxides which are one of the most important catalysts in chemical industry. We investigate the dynamics of ultrathin vanadium oxide layers on a Rh(111) surface during catalytic methanol oxidation as model system. Using a variety of different imaging techniques that cover the pressure range from UHV to the 10-2 mbar range, we found a number of different selforganization phenomena connected with the redistribution of the VOx layer on the Rh surfaces. At 10-6 - 10-4 mbar VOx on Rh(111) condenses into macroscopic circular islands that undergo a coalescence process. We also observe a “breathing-like” oscillatory expansion and contraction that the islands undergo under stationary conditions. These oscillations can be explained with oxygen gradient inside the VOx islands resulting in different coexisting 2D-phases of VOx. With a newly developed near ambient pressure – low energy electron microscope (NAP-LEEM), it is shown that VOx islands disintegrate at 10-2 mbar generating turbulent dynamics.
Professor Ronald Imbihl
Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover
Callinstrasse 3A, D-30167 Hannover, Germany