• Microscopically Imaging Molecular Reactions on Metal Surfaces
• Model Studies of Reactions on Metal Oxide Surfaces
• Dynamic Studies of Adsorption on Metal Surfaces

The overall objective of the research in the Madix group is to further the understanding of the catalytic reactivity of surfaces on the atomic and molecular scale. In general, heterogeneously catalyzed reactions proceed through a series of elementary steps, (1) adsorption of reactants, (2) reaction of adsorbed, rate-determining reactive intermediates, and (3) desorption of products. The rate of each of these steps depends on the nature of the surface - its geometric and electronic properties. It is then clear that the catalytic activity of a surface, as measured by the rate of rate determining step in the overall reaction, and the catalytic selectivity, as measured by the rate of reaction to form the desired product, depend in detail on the surface itself.

In this laboratory we are studying fundamental aspects of all three of these processes. We are engaged in research on both metal and metal oxide catalysts. We use modern methods of ultrahigh vacuum physics to examine reactivity on well-defined single crystal surfaces, using a variety of spectroscopic tools to follow the reactions. A brief description of each area of research follows.

• Microscopically Imaging Molecular Reactions on Metal Surfaces: We are directly observing reactions on metal surfaces using scanning tunneling microscopy (STM). We are able to follow the course of oxidation reactions, the spatial distribution of reactants during the reaction and the formation and subsequent reaction of reactive intermediates in real time or freeze-frame mode. The logo on this webpage is composed of a series of microscope images taken during the course of the oxidation of ammonia on a single crystal surface of silver, Ag(110). More details and examples of these studies can be found by linking to Microscopy.

• Model Studies of Reactions on Metal Oxide Surfaces: We are also engaged in research on catalysis by metal oxides of one type supported on a metal oxide of another; eg, V₂O₅ on TiO₂. The overall goal of this research is to create model catalyst systems, utilizing single crystal oxide surfaces, in order to provide a well defined surface structure that can lead to understanding the origin of the rather unique reactivity of these complex materials. We use a combination of photoelectron spectroscopy (XPS and UPS), scanning tunneling microscopy (STM) and temperature programmed reaction spectroscopy (TPRS) to relate the reactivity of a variety of organic molecules with the physical and electronic state of the surface. Link to Metal Oxides to learn more about our research on this topic.

• Dynamic Studies of Adsorption on Metal Surfaces: We are also developing the capability to predict the rates of adsorption of many small molecules on metal surfaces using molecular dynamics simulation techniques coupled with experiment. We can measure adsorption probabilities using molecular beam methods and develop the intermolecular potential functions that govern the interactions between molecules and surfaces within the united atom approximation - that is, treating each subunit within the molecule as an pseudoatom. For example, ethane is treated as a pseudodiatomic. With this approach we have developed the capability of predicting the adsorption rate of hydrocarbons on platinum surfaces, for example. Further information on these studies can be found by linking to Dynamics.

For those of you who may have further interest in the methods used in our research please consult our Tutorial.