No abstract

Catalysis by gold supported on reducible oxides has been extensively studied, yet issues such as the nature of the catalytic site and the role of the reducible support remain fiercely debated topics. Here we present ab initio molecular dynamics simulations of an unprecedented dynamic single-atom catalytic mechanism for the oxidation of carbon monoxide by ceriasupported gold clusters. The reported dynamic single-atom catalytic mechanism results from the ability of the gold cation to strongly couple with the redox properties of the ceria in a synergistic manner, thereby lowering the energy of redox reactions. The gold cation can break away from the gold nanoparticle to catalyse carbon monoxide oxidation, adjacent to the metal/oxide interface and subsequently reintegrate back into the nanoparticle after the reaction is completed. Our study highlights the importance of the dynamic creation of active sites under reaction conditions and their essential role in catalysis.

The structure of self-assembled monolayers (SAMs) of long-chain alkyl sulfides on gold(111) has been resolved by density functional theory-based molecular dynamics simulations and grazing incidence x-ray diffraction for hexanethiol and methylthiol. The analysis of molecular dynamics trajectories and the relative energies of possible SAM structures suggest a competition between SAM ordering, driven by the lateral van der Waals interaction between alkyl chains, and disordering of interfacial Au atoms, driven by the sulfur-gold interaction. We found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first gold surface layer contains gold atom vacancies (which are partially redistributed over different sites) as well as gold adatoms that are laterally bound to two sulfur atoms.

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.

Defect states arise from unpaired electrons which are created by point defects. Within the framework of DFT+U, the defect state energy location can be directly correlated to the choice of the U parameter. As noted in the main text, low U values produce defect states at the bottom of the conduction band, while larger U values push the defect energies into the gap. A comparison of several defects and their location relative to the conduction band is shown in Figure S.1. The plot indicates that the defect state location varies nearly linearly as a function of U, regardless of the type of defect (i.e. HO b or O V ).

We describe a DFT þ U study of the (110) rutile surface with oxygen vacancies (O v 's). Oxygen vacancies leave behind two excess unpaired electrons per O v , leading formally to the formation of two Ti 3þ ions. We investigate the location of the Ti 3þ ions within the first three surface layers. In total, we obtained 49 unique solutions of possible Ti 3þ pairs, to examine the stability of all Ti types (e.g., five-coordinated surface Ti, six-coordinated surface Ti, subsurface sites, etc.). Our results show that subsurface sites are preferred but that many configurations are close in energy, within up to 0.3À0.4 eV of each other. In contrast to findings in previous work, we show that sites directly adjacent to the O v 's are unstable. Analysis of our results shows that the two Ti 3þ ions within a pair behave independently of each other, as there are little electronic interactions between the excess electrons associated with these sites. We also examined the migration of Ti 3þ sites from the surface into the bulk and find the surface locations to be preferred by ∼0.5 eV relative to the bulk. Our systematic results provide a comprehensive picture of excess electrons that indicates that they are not trapped or localized at specific sites but are distributed across several sites due to nearly degenerate Ti 3þ states.

Hydroxyls on a TiO 2 surface and photoinduced e -polarons give rise to excess charges, the electronic structure of which is critical to the fundamental understanding of their role in the reactivity of surface absorbates and other photochemical processes. In this paper, we report on a DFT+U characterization of the electronic structure of one excess electron in bare and singly hydroxylated rutile (110) surfaces. The excess electron has the electronic structure of a small polaron with its spin density and associated lattice distortion localized around a single site. Calculations indicate that the most stable Ti trapping site in both bare and hydroxylated surfaces resides in the first subsurface layer under the Ti 5c row. However, trapping energy differences between several Ti sites are within 0.2 eV, indicating that the Boltzmann population of these sites is significant at room temperature and that the excess electron will appear as fractionally occupying several sites. On the basis of earlier calculations, the activation barrier for electron hopping from site to site is small (<0.1 eV). The stability ordering of the different Ti sites is very similar for the bare and hydroxylated surface, suggesting that the hydroxyl only weakly perturbs the surface electronic structure.

Car-Parrinello molecular dynamics simulations demonstrate that pulling a single thiolate molecule anchored on a stepped gold surface does not preferentially break the sulfur-gold chemical bond. Instead, it is found that this process leads to the formation of a monoatomic gold nanowire, followed by breaking a gold-gold bond with a rupture force of about 1.2 nN. The simulations also indicate that previous single-molecule thiolate-gold and gold-gold rupture experiments both probe the same phenomenon, namely, the breaking of a gold-gold bond within a gold nanowire.