CPM Seminar

Scanning Tunneling Microscopy studies of model catalysts

Peter Thostrup

Department of Physics and Astronomy &
Interdisciplinary Nanoscience Center (iNANO)
University of Aarhus

1. Data from well-controlled surface-science studies are routinely being used to interpret and understand the behaviour of heterogeneous catalysts. The so-called surface science approach is used to extrapolate data obtained under ultrahigh vacuum (UHV) and at low temperature to real industrial conditions, where catalytic reactions typically proceed under elevated pressures and temperatures. While this approach has proven successful in many cases, a number of recent studies seem to invalidate and thus fundamentally undermine current practice. This on-going discussion has led to the identification of a number of so-called gaps between surface science and industrial catalysis, e.g. the pressure gap, the temperature gap and the structure gap.

   I will focus on the pressure gap and through example demonstrate both qualitative and quantitative correspondence between studies performed under UHV and elevated pressures, i.e. effectively bridge the pressure gap. The low and high-pressure studies are performed using the same technique, Scanning Tunneling Microscopy.

2. Heterogeneous catalysts typically consist of nano-sized metal clusters supported on a metal oxide. Conventional wisdom reduced the support material to a passive role whose only purpose is to maintain high dispersion, thus maximizing the surface area, of the active part. Recent studies however indicate that the support material plays an active role in number of steps in the overall catalytic cycle. Much interest is thus currently targeted at the chemistry of oxide surfaces.

   I will discuss an important model system for low-temperature carbon monoxide oxidation consisting of gold clusters supported on titanium dioxide (TiO₂). By the use of STM, we have unravelled a number of important atomistic mechanisms for the interaction of water and dioxygen with reduced TiO₂.