Probing Atomic Scale Structure and Catalytic Properties of Cobalt Oxide Model Catalysts

Cobalt oxides are known to be active catalysts for a number of chemical reactions, but very little is known
about the atomic scale processes responsible for the activity. The research presented in this thesis is focused
on obtaining an atomic scale understanding of the chemistry of well-characterized cobalt oxide model
catalyst surfaces consisting of pristine and defective CoO and Co3O4 thin films with the (111) and (100)
terminations supported by Ag(100), Ir(100), and Au(111) single crystal surfaces. The structure and the
adsorption properties of probe molecules onto these cobalt oxide model catalyst surfaces are studied under
ultra-high vacuum conditions using the interplay of X-ray photoemission spectroscopy (XPS), scanning
tunneling microscopy (STM), and low energy electron diffraction (LEED). Further, high pressure XPS
(HPXPS) is used to study the stability and phase transitions of the cobalt oxide model catalysts in more
realistic gas environments. As a side project to the work on cobalt oxide thin films the thesis gives a
comprehensive spectroscopic picture of Ir(100) surface reconstructions and molecular adsorption onto these
surfaces.
The adsorption experiments of H2, CO, CO2, and H2O probe molecules give a detailed picture of the surface
chemistry of Co oxide surfaces and it is demonstrated that Co ions naturally found on the surface of
Co3O4(111) and Co3O4(100) thin films or artificially created on the CoO(111) surface are extremely
important for chemical properties of the surface. Water dissociation, carbonate formation, weak adsorption
of CO and CO2 are examples of processes that only take place in the presence of Co surface ions. The work
at more realistic gas pressures in the mbar regime demonstrates that Co oxide thin films should be seen as
dynamic films that easily change phase between the CoO and Co3O4 structure in response to the gas
composition.
To summarize, the work presented in this thesis is important for the fundamental understanding of cobalt
oxide surfaces and their catalytic properties, and hopefully, this fundamental understanding can be used to
develop new and better cobalt oxide based catalysts.

Catalysts are used to produce a large fraction of the materials we use in our modern society. A
very famous example is the highly efficient catalysts that are used to fix nitrogen from the air into
artificial fertilizer salts. Without this catalytic process, it is difficult to imagine that we could feed
the current population of earth. Artificial fertilizers are, however, not the only product that uses a
catalyst for its production. In fact, almost all products produced in the chemical industry such as
plastic materials, paints, coating materials, gasoline, drugs, etc. use catalysts for their production.
Catalysts are also used extensively for cleaning of exhaust gas from power plants, trucks, and cars.
As an example, the catalyst in a car convert carbon monoxide gas (CO) to non-toxic carbon dioxide
(CO2). Unfortunately, the catalyst in the car is built partly from very expensive metals such as
platinum and palladium.
As discussed above catalysts are used extensively both for the production of modern materials and
for reducing the amount of toxic chemicals we release into our environment. Most of the catalyst
materials we use today have been found by trial and error methods and knowledge of why and how
the chemical process take place on the catalyst material is therefore often very limited or missing
fully.
The goal of the present work has been to improve our understanding of chemical processes taking
place on cobalt oxide based catalysts. Instead of studying real and complex cobalt oxide catalyst
materials we have studied thin and highly idealized cobalt oxide films. Using these highly idealized
model systems of the real catalysts we studied chemical processes at the atomic scale level. One
important take home message of the studies is that single cobalt atoms found on the surface are
essential for the function of the catalysts surface and in particular for how it interact with gas
molecules.
Hopefully, the present fundamental work on cobalt oxide catalysts can be used to develop new and
better catalysts of this material. Furthermore, the work adds knowledge to our general
understanding of metal oxide films and their catalytic applications.