Licentiate Thesis: Ghost in the Shell: Studies on Subsurface Oxygen in Oxide- Derived Copper Nanocube Catalysts
by
Chang Liu(Stockholm University, Department of Physics)
→
Europe/Stockholm
FA32
FA32
Description
With the passage of time and the advancement of our industrial civilization, environmental
concerns have become more and more recognized since the 1990s. Carbon dioxide reduction
reactions are capable of converting carbon dioxide into valuable hydrocarbons and reducing
the carbon emission from the combustion of fossil fuels. This is a promising direction for
sustainable energy resources given that the scarcity of fossil fuels is becoming more
threatening to the survival of mankind. In recent years, oxide-derived metal nanostructures
have been synthesized and show unique catalytic features. Recently, Sloan et al. synthesized a
novel oxide-derived copper nanocube structure, which showed a high selectivity toward
ethylene over methane and low overpotentials. In this work, the presence of subsurface
oxygen in the catalyst surface is tested with density functional theory (DFT) calculations, as a
complement to experimental x-ray photoelectron spectroscopy. Due to limitations on the scale
of modeling with DFT, the results indicate a very low stability of subsurface oxygen, which
give rise to a question if subsurface oxygen would be stable with a reasonably large cluster
model. Self-consistent charge density functional tight binding (SCC-DFTB) is adopted to
investigate a nanocube model. In this model, a manually reduced cuprious oxide nanocube is
constructed and investigated. Subsurface oxygen atoms close to facets are found to be more
stable inside. A higher degree of disorder is proposed to be the cause of this difference in
stabilizing subsurface oxygen atoms between the slab and nanocube models. The presence of
subsurface oxygen enhances the adsorption of CO on the Cu(100) surface, increasing the
likelihood for adsorbed CO molecules to dimerize, which is the rate determining step for
ethylene production on Cu(100) under low-overpotential conditions. With subsurface
electronegative atoms such as oxygen or fluorine, it is also found that the d-band scaling
relation could be broken.