New catalysts are needed more than ever. This is because our society
is facing an inevitable transition away from fossil fuels as a source of
energy and carbon-based commodity chemicals. Carbon derived from fossil
fuels is used to manufacture many products in everyday life, including fuels,
plastic, and medicine. New sources of the elements needed by society will
have to take the place of fossil fuels. Most likely, this will include extracting
hydrogen from water as well as capture and conversion of carbon dioxide
to precursor chemicals. The energy for these processes must likewise be derived
from sustainable sources. With regard to this, efficient energy storage
and release of sustainable, intermittent energy is needed.
The success of all of the above scenarios highly depends on the ability to
catalyze the chemical reactions involved. Without catalysis, the cost of the
energy involved in these processes will become insurmountable. Unfortunately,
suggesting a suitable material to use as the catalyst is not straightforward.
The aim of this thesis is to show that a new class of materials, namely
high-entropy alloys, provides a way to design new materials with desired and
highly tunable properties.
High-entropy alloys are metallic materials containing five or more metals
in about the same proportions. The number of ways to combine metals
from the periodic table and form variants of these materials is exceedingly
large. This is both an advantage and a complicating factor of high-entropy
alloys. While the complexity associated with predicting the catalytic properties
is increased compared to the pure metals, it is precisely this complexity
that allows for unconventional properties to show.
This thesis describes how successful modeling of the surfaces of high-entropy
alloys can be achieved. It also shows how models can assist in solving the
problem of suggesting catalysts for two of the reactions alluded to above,
namely (1) the conversion of hydrogen and oxygen to water for the release
of stored intermittent energy, and (2) the conversion of carbon dioxide into
useful chemicals. Moreover, a highly transferable methodology for tuning
the compositions of high-entropy alloys favorably is demonstrated.
There still remain investigations to be done before high-entropy alloys can
be upscaled to large-scale use. In particular, a satisfactory theory remains to
be constructed that allows for the simultaneous prediction of catalytic activity,
long-term stability, as well as chemical selectivity of high-entropy alloys.
This theory should at least be valid for the cornerstone chemical reactions
needed to realize a sustainable future.