Li, Yong
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Item-typ:Veröffentlichung, Theoretical Insights into Catalysis on Nanoporous Gold from Ab Initio Molecular Dynamics(2020-10-06); ; ; Nanoporous gold (np-Au) has recently emerged as a highly selective catalyst, potentially suited to environmentally friendly and low-temperature applications. In contrast to the more extensively studied gold nanoparticle catalysts, the mechanistic understanding of catalytic processes on pristine and oxide-coated np-Au is far less developed. Quantum chemical methods, in particular, those based on density functional theory (DFT) can be used successfully to achieve a mechanistic understanding at the microscopic level, which is needed to optimize materials based on np-Au with respect to their catalytic properties. In this thesis, I summarize my computational work devoted to a deeper insight into the chemistry and physics of np-Au as a catalyst. Modern surface science has revealed that a catalyst is not a rigid body but undergoes rapid (sometimes irreversible) dynamic changes during chemical processes occurring on its surface. Whereas many theoretical studies still use an oversimplified model of a metal catalyst as a rigid, clean, and perfect surface, my PhD work examines dynamic processes occurring on the surface of np-Au in response to changes of the chemical environment, such as oxygen-induced surface restructuring, the formation of surface oxygen chain structures, and elementary catalytic reactions on np-Au, by using ab initio molecular dynamics (AIMD) simulations. In addition to AIMD simulations, traditional “static” DFT computations have been performed to verify minima and transition states in the reaction energy landscape and to construct reaction energy diagrams. The main results of the publications comprising the foundation of this thesis can be summarized as follows. Although np-Au consists of almost pure gold, silver atoms are also present on the surface as residues of the preparation process. Therefore, its surface chemistry turns out to be more complex than anticipated. Interactions between Au atoms and O atoms pre-adsorbed and/or generated during catalysis and the involvement of Ag impurities result in complex surface dynamics. First of all, with regard to pristine np-Au, the theoretical studies reveal that surface O atoms dynamically form one- and two-dimensional –(Au–O)– chains on a stepped Au(321) surface and lead to surface restructuring. In contrast, no chain formation has been found on Au(111), pointing to higher structural flexibility and propensity for restructuring of the stepped surface. Furthermore, our study predicts migration of subsurface Ag atoms to the surface, i.e. adsorbate-induced Ag surface segregation, in the presence of adsorbed atomic oxygen. Second, my thesis addresses the physics and chemistry of np-Au material functionalized with cerium oxide. To probe the reactivity of these systems and the involvement of inherent particle-support interactions towards CO oxidation, AIMD simulations and static DFT computations were carried out. As models, Ce10O20/19 NPs supported on thermodynamically stable Au(111) and on the stepped (rough) Au(321) surface were employed. For the ceria/Au(111) system, the simulations revealed the preference of a Mars-van-Krevelen type of reaction mechanism, in which a CO molecule first reacts with a lattice O atom of ceria rather than with an activated O22- surface species, forming CO2 and leaving an O vacancy behind. This vacancy becomes subsequently refilled by an O atom which diffuses from the site of the reaction of O2 with another CO molecule (at the gold-ceria perimeter). My studies also revealed that, in contrast, CO adsorption on the stepped Au(321) surface (in proximity to the ceria nanoparticle) may lead to the dynamic extraction of Au atoms from the surface resulting in Au-CO carbonyl species, which may subsequently diffuse on the Au surface and react with lattice O of ceria to CO2 with very low activation energy. After the reaction step, the extracted bare Au atom attaches to a step on the Au surface. As this is not the original site, this part of the catalytic cycle leads to a rearrangement of the surface structure. The second part of the cycle is likely to be the same as found for the ceria/Au(111) system. Finally, this thesis discusses the correlation between two types of catalysts that are “inverse” with respect to each other, namely, Au nanoparticles deposited on ceria and ceria nanoparticles deposited on Au (being the focus of this thesis), with respect to all aspects relevant for the surface reactivity, such as surface dynamics, charge transfer between the gold and the oxide phases, and the mechanism of CO oxidationDissertation448 345 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics(American Chemical Society, 2020-01-31); ; ; 15 60 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Assessment of PBE+U and HSE06 methods and determination of optimal parameter U for the structural and energetic properties of rare earth oxides(American Institute of Physics, 2020-10-27); ; ; Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO2(111) and Nd2O3(0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV–3 eV is also found to be the optimal range of U for the reaction energies of bulk La2O3, Ce2O3, Nd2O3, Er2O3, and Ho2O3. U = 1 eV gives the best results for Pr2O3, Pm2O3, Eu2O3, Tm2O3, and Lu2O3, whereas Gd2O3 could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd2O3(0001) and CeO2(111), although larger U values are required to obtain sufficient localization of 4f electrons.Wissenschaftlicher ArtikelBand:153Heft:1647 34 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Transient Au–CO Complexes Promote the Activity of an Inverse Ceria/Gold Catalyst: An Insight from Ab Initio Molecular DynamicsTo probe particle–support interactions and their mechanistic role for catalytic CO oxidation on nanoporous gold (np-Au) coated with ceria nanoparticles, we carried out ab initio molecular dynamics (AIMD) simulations and standard density functional theory (static DFT) computations. To this end, we studied ceria clusters (Ce10O20/19) supported on a Au(321) surface exhibiting a high density of steps and kinks. Our theoretical model represents the structurally inverse situation compared to more commonly studied ceria-supported Au nanoparticle systems. In agreement with previous results for Au(111), we find that reduced (Ce10O19) as well as stoichiometric (Ce10O20) ceria nanoparticles transfer electrons to the Au(321) support. This charge transfer (particularly strong in the case of Ce10O19) reflecting a strong chemical interaction between ceria and Au is probably responsible for the stabilization of np-Au against thermal coarsening experimentally observed upon deposition of oxide nanoparticles. The adsorption energies of the ceria cluster on Au(321) are more negative than on the Au(111) surface by around ∼0.5 eV. AIMD simulations were employed to study the mechanism of catalytic CO oxidation with O2 for the ceria/Au(321) system. We found that a CO molecule adsorbed near the ceria/gold perimeter interface can extract a Au atom from the surface in the form of a mobile linear Au–CO complex, which results in a very low activation energy when this species reacts with lattice O to CO2. The released bare Au adatom subsequently attaches to a step edge of the gold surface, leading to a dynamic restructuring of the Au support. Next, an activated O2 molecule adsorbed at a perimeter site between ceria and Au reacts with a second CO molecule to CO2 and an adsorbed O atom, which eventually fills the vacancy site created in the first half of the cycle. As compared to ceria particles supported on Au(111), the reactivity is enhanced as a new low-energy mechanism is enabled, revealing the positive impact of the stepped structure of Au(321).Wissenschaftlicher ArtikelBand:125Heft:4848 48