Licentiate Thesis: Ghost in the Shell: Studies on Subsurface Oxygen in Oxide- Derived Copper Nanocube Catalysts

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.