Computational studies of gold-absorbate complexes on modified oxides

Abstract

While bulk gold is known for its chemical inertness, nanosized gold clusters are active catalysts for a variety of important reactions. For some practical applications gold clusters are supported and the cluster-support interaction can modify the cluster's properties. The knowledge of this interaction can be vital for obtaining desired cluster properties. In this thesis, the adsorption of Au atoms and clusters on modified oxide surfaces is studied using density functional theory (DFT) calculations. The support effects are considered by direct analysis of the adsorbed Au and using other coadsorbates as reactivity probes. Doping the CaO(001) surface by replacing a cation with a high valence dopant such as Mo makes adsorption of electronegative species such as Au, O and O2 more exothermic. The stronger binding is accompanied with a charge transfer from the dopant to the adsorbate. An Au adatom is anionic on a doped oxide as seen from its Bader charge and zero magnetic moment. The charging of the dopant is seen from a PDOS analysis where some of the d-electrons of the dopant are transferred to the Au s-states. Further, the change of the dopant's charge can also be seen by the contraction of the nearest neighbour distances between the dopant and surrounding O anions shows the change in the dopant charge. The superoxo state of O2 on Mo doped CaO(001) is illustrated by its elongated bond length in addition to the Bader charge and magnetic moment of the molecule. A modified Born-Haber (BH) cycle was devised to estimate the effect of different physical processes on the adsorption energy on the doped oxide. The adsorption energy was split into three parts with an iono-covalent energy describing the local interactions, a redox energy accounting for the charge transfer, and a Coulomb energy for the electrostatics between a charged adsorbate and dopant. While the Coulomb energy decays with increasing adsorbate-dopant distance, the redox energy remains more exothermic due to a much shorter distance between the negative anions and the positive dopant. A similar BH cycle for Ag(001) supported MgO thin films showed that while the electrostatic stabilization from the adsorbate-support interaction decays quickly with increasing lm thickness, a compensation occurs in the redox energy becoming more exothermic, leading to more exothermic adsorption also for thicker films. Water is stabilized by electrostatic interaction with anionic Au on MgO/Ag(001); however, no stabilization occurs on bulk MgO when the Au adatom is neutral. The adsorption of water dissociation products H and OH on top of an Au adatom is more exothermic with increasing lm thickness. Isophorone physisorbs on the MgO/Ag(001) surface with energetic preference towards the MgO steps, which is due to stabilizing electrostatic interaction between the step cations and a polar O=C bond in the molecule. The enol tautomers can bind to anionic Au atoms on the surface via a similar interaction with the polar H-O bond. The keto form is not stabilized by Au due to a steric hindrance caused by C atoms near the positive end of the polar O=C bond.