Mechanistic Insight into the Styrene-Selective Oxidation on Subnanometer Gold Clusters (Au₁₆–Au₂₀, Au₂₇, Au₂₈, Au₃₀, and Au₃₂–Au₃₅): A Density Functional Theory Study

We performed a comprehensive study of the reaction mechanism of styrene-selective oxidation to benzaldehye and styrene epoxide on subnanometer gold clusters with the cluster size ranges from around 0.4 to 1.0 nm via the density functional theory (DFT) calculation. The major focuses of the current study are the intrinsic catalytic selectivity and size-dependent activities of gold clusters toward styrene oxidations. The reaction selectivity of styrene oxidation over subnanometer gold clusters, e.g., selective formation to benzaldehyde or styrene epoxide, with the presence of dioxygen as the sole oxidant or the H₂/O₂ mixture as the reactant is discussed. A new reaction channel leading to the formation of a benzaldehyde product involving the formation of a metastable four-membered ring CCOO* intermediate is proposed, which explains the recent experimental observations of a high yield of benzaldehyde on ∼1.4 nm gold clusters. The effect of the charge state of gold clusters on the reaction selectivity and reaction rate is examined. The results indicated that the reaction selectivity is not affected by the charge state of the cluster by using the Au₃₄^– cluster as a benchmark model. However, the reaction rate of styrene oxidation is significantly increased on the anionic gold clusters caused by larger O₂ adsorption energies, suggesting higher catalytic activity of anionic clusters. The mechanism of dramatic increase of product selectivity to styrene epoxide using H₂ as the additive is explored as well. We find that the major role of the H₂ additive is facilitating the dissociation of O₂ into an active O atom on subnanometer gold clusters, which leads to high selectivity to the epoxide product. This systematic study enables a quantitative assessment of the size-dependent activity and selectivity of subnanometer gold clusters toward styrene-selective oxidation

On the Mechanism of Anti-galvanic Metal Displacement Reaction between [Au₂₅(SR)₁₈]⁻ and Metal-Thiolate Complex

Metal displacement reaction is widely used for preparing alloy nanomaterials. In this study, the mechanism of anti-galvanic metal displacement reaction between the atomic precision [Au25(SC2H4Ph)18]− cluster and the metal-thiolate complexes SR-M-SR (M = Ag, Cd, and Hg) is studied based on dispersion correction density functional theory (DFT-D) calculations. The present study reveals that the metal displacement reaction of the Au25 cluster is carried out through two-stage metal diffusion including the rapid diffusion of the metal heteroatom from metal thiolate to the ligand layer of Au25 cluster and then gradual diffusion of the metal heteroatom into the icosahedral 13-atom core. The atomic charge analysis confirms that the SR group plays a crucial role. Due to the partial reducibility of SR group, it can nucleophilic attack Au atom to result in the fracture of the Au–S bond in the ligand layer and the formation of atomic vacancy on the surface of the metal core, which facilitates the metal heteroatom diffusion from the metal-SR complex to the ligand layer of gold cluster and then to the surface of gold core

Structure Prediction of Au₄₄(SR)₂₈: A Chiral Superatom Cluster

The structure of a thiolate-protected Au₄₄ cluster, [Au₄₄(SR)₂₈], is theoretically predicted via density functional theory calculations. Au₄₄(SR)₂₈ is predicted to contain a “two-shell” face-centered-cubic type of Au kernel and possess chirality. The predicted cluster structure is validated by comparison of optical absorption properties between theory and previous experiments, as well as energy evaluations. Based on the predicted cluster structure, the magic stability of Au₄₄(SR)₂₈ is understood from the superatom electronic configuration and formation of a unique double-helix superatom network inside

Electronic Structure and Spin Transport Properties of a New Class of Semiconductor Surface-Confined One-Dimensional Half-Metallic [Eu-(C_<i>n</i>H_<i>n</i>–2)]_<i>N</i> (<i>n</i> = 7–9) Sandwich Compounds and Molecular Wires: First Principle Studies

Transition-metal atom/π-conjugated ring sandwich compounds are promising candidates for application in molecular spintronics. However, a great challenge that has significantly restrained the practical application of these sandwich compounds is their fabrication on a well-characterized solid-state substrate in a controllable manner. In this work, we suggested a two-step self-assemble way to fabricate the Eu-C_<i>n</i>H_<i>n</i>–2 compounds on the hydrogen-terminated Si(100) surface and theoretically studied the geometric structure and electronic and magnetic properties. Theoretical results indicate that the silicon surface is an ideal substrate to support such kind of metal atom-encapsulated sandwich compounds as the lattice distance of silicon (100) surface is close to the inter-ring distances of freestanding gas-phase sandwich compounds. On the basis of the spin-polarized density functional theory calculations and ab initio molecular dynamics simulations, we find that the silicon surface-supported Si-[EuCh]_<i>N</i>, Si-[EuCOT]_<i>N</i>, and Si-[EuCnt]_<i>N</i> sandwich compounds all process a ferromagnetic ground state. Moreover, the cycloheptatrienyl (Ch) and cyclononatetraenyl (Cnt) Eu sandwich compounds show half-metallic properties. The calculation of electron/spin transport properties using the nonequilibrium Green’s-function method confirms that the Ch Eu sandwich compounds are excellent spin filters, and the spin filter efficiency (SFE) is independent of the cluster size (<i>N</i>), whereas the SFE of Si-[EuCOT]_<i>N</i> decreases rapidly with the increase of cluster size. The perfect half-metallic properties of these surface-supported sandwich compounds are promising for future application in spin devices. The present work suggests a way to fabricate the half-metallic sandwich compounds on a semiconductor silicon surface

The Nucleation and Growth Mechanism of Thiolate-Protected Au Nanoclusters

The understanding of the evolution mechanism of thiolate-protected Au nanoclusters from the homoleptic Au­(I)-SR clusters to core-stacked ones is crucial for the synthesis of novel thiolated Au clusters. In this work, the global search for a series of “intermediate” Au_<i>m</i>(SR)_<i>n</i> clusters with <i>m</i> and <i>n</i> ranging from 5 to 12 was implemented by combining basin hopping algorithm, genetic algorithm, and density functional theory (DFT) calculations. Most of Au_<i>m</i>(SR)_<i>n</i> clusters possess the core–shell structure. Specifically, some typical topologies, such as tetrahedral Au₄, triangular bipyramid Au₅, octahedral Au₆, and vertex-shared Au₇, are found to be dominant within the inner core of various clusters. Along with the increase in the number of gold atoms and thiolates, the preliminary nucleation and growth processes of both inner-core and staple-motif protecting units are grouped into three kinds of size evolution routes, i.e., core growth, core dissolution, and staple-motif growth, respectively. Some metastable isomers may also play an important role in the evolution of clusters. The core structures in the lowest-lying isomers and some metastable isomers are similar to the intact or part of the cores found in experimentally detected species. Both the lowest-lying and metastable intermediate clusters may serve as the building block for the further growth. These results rationalize the preliminary nucleation in the “reduction growth” stage, shedding light on the size-evolution mechanism of RS-AuNPs

Semiring Chemistry of Au₂₅(SR)₁₈: Fragmentation Pathway and Catalytic Active site

The semiring chemistry of the Au₂₅(SR)₁₈, particularly its fragmentation mechanism and catalytic active site, is explored using density functional theory (DFT) calculations. Our calculations show that the magically stable fragmental cluster, Au₂₁(SR)₁₄^–, as detected in several mass spectrometry (MS) measurements of fragmentation of the Au₂₅(SR)₁₈^–, contains a quasi-icosahedral Au₁₃-core fully protected by four <i>-SR-Au-SR-</i> and two <i>-SR-Au-SR-Au-SR-</i> staple motifs. A stepwise fragmentation mechanism of the semiring staple motifs on the surface of Au₂₅(SR)₁₈^– is proposed for the first time. Initially, the Au₂₅(SR)₁₈^– transforms into a metastable structure with all staple motifs binding with two neighboring vertex Au-atoms of the Au-core upon energy uptake. Subsequently, a ‘step-by-step’ detachment and transfer of [Au­(SR)]_<i>x</i> (<i>x</i> = 1–4) units occurs, which leads to the formation of highly stable products including Au₂₁(SR)₁₄^– and a cyclic [Au­(SR)]₄ unit. The continued fragmentation of Au₂₁(SR)₁₄^– to Au₁₇(SR)₁₀^– is observed as well, which shows same stepwise fragmentation mechanism. The proposed mechanism well explains the favorable formation of Au₂₁(SR)₁₄^– and Au₁₇(SR)₁₀^– from Au₂₅(SR)₁₈^– as observed from experimental abundance. Taking the Au₂₁(SR)₁₄ and its parent cluster Au₂₅(SR)₁₈ as the benchmark model systems, the catalytic active site of the thiolate protected gold clusters toward the styrene oxidation and the associated reaction mechanism are further investigated. We show that the Au atom in the staple motifs is the major active site for the styrene oxidation in presence of TBHP as oxidant or initiator. The Au atom in the staple motifs can change from Au­(I) (bicoordinated) to Au­(III) (tetracoordinated). The O₂ activation is achieved during this process

Density Functional Theory (DFT) Studies of CO Oxidation over Nanoporous Gold: Effects of Residual Ag and CO Self-Promoting Oxidation

We report a systematic study of CO oxidation mechanism over nanoporous (NPG) using the density functional theory (DFT). In the study, the (111) and (100) flat planes that were identified as the most abundant in the nanoporous gold are mimicked by Ag_<i>x</i>@Au-(111) and Ag_<i>x</i>@Au-(100) slabs (<i>x</i> = 1 – 3). A total of 50 reaction pathways are examined at different active sites. A simplified microkinetics model termed the Sabatier analysis, which is built on the adsorption energies and activation barriers, is used to evaluate the reaction rate of different reaction pathways. Our theoretical results indicate that the Au-kink sites joining the (111) and (100) flat planes are the major active sites. The residual Ag atoms in the Au-kink site promote the adsorption of O₂ species and hence increase the reaction rate of CO oxidation. Besides the discussion of the Ag-impurity effect, we also propose that the nearby coadsorbed CO at Au steps can promote the dissociation of OCOO* reaction intermediate significantly via an electrophilic attack process, which is denoted as a trimolecular CO self-promoting oxidation mechanism. The trimolecular route has reduced reaction steps and higher reaction rate in comparison to the conventional bimolecular reaction mechanism

Thiol Ligand-Induced Transformation of Au₃₈(SC₂H₄Ph)₂₄ to Au₃₆(SPh‑<i>t</i>‑Bu)₂₄

We report a disproportionation mechanism identified in the transformation of rod-like biicosahedral Au₃₈(SCH₂CH₂Ph)₂₄ to tetrahedral Au₃₆(TBBT)₂₄ nanoclusters. Time-dependent mass spectrometry and optical spectroscopy analyses unambiguously map out the detailed size-conversion pathway. The ligand exchange of Au₃₈(SCH₂CH₂Ph)₂₄ with bulkier 4-<i>tert</i>-butylbenzenethiol (TBBT) until a certain extent starts to trigger structural distortion of the initial biicosahedral Au₃₈(SCH₂CH₂Ph)₂₄ structure, leading to the release of two Au atoms and eventually the Au₃₆(TBBT)₂₄ nanocluster with a tetrahedral structure, in which process the number of ligands is interestingly preserved. The other product of the disproportionation process, <i>i</i>.<i>e</i>., Au₄₀(TBBT)_<i>m</i>+2(SCH₂CH₂Ph)_{24–<i>m</i>}, was concurrently observed as an intermediate, which was the result of addition of two Au atoms and two TBBT ligands to Au₃₈(TBBT)_<i>m</i>(SCH₂CH₂Ph)_{24–<i>m</i>}. The reaction kinetics on the Au₃₈(SCH₂CH₂Ph)₂₄ to Au₃₆(TBBT)₂₄ conversion process was also performed, and the activation energies of the structural distortion and disproportionation steps were estimated to be 76 and 94 kJ/mol, respectively. The optical absorption features of Au₃₆(TBBT)₂₄ are interpreted on the basis of density functional theory simulations

Exploring the Effects of Crystal Facet Orientation at the Heterojunction Interface on Charge Separation for Photoanodes

As one of the most effective strategies to promote the spatial separation of charges, constructing heterojunction has received extensive attention in recent years. However, it remains unclear whether the crystal facet orientation (CFO) at the heterojunction interface is contributory to charge separation. Herein, three types of TiO2/CdS heterojunction films with different CFOs at the heterojunction interface were produced by adjusting the CdS CFO through in situ conversion. Among them, the TiO2/CdS film with a mixed CdS CFO showed the maximum photocurrent density and charge separation efficiency. In contrast, the TiO2/CdS film with a uniform CdS (100) (CdS-100) performed worst. According to the results of experimentation and DFT calculation, these three types of TiO2/CdS films varied significantly in electron transport time. This is attributable to the different Fermi levels of CdS CFO and the formation of different built-in electric fields upon coupling with TiO2. The rise in the Fermi level of CdS can increase the driving force required for charge migration at the heterojunction interface. Additionally, a stronger built-in electric field is conducive to charge separation. To sum up, these results highlight the significant impact of CFO at the heterojunction interface on charge separation

Density Functional Theory Studies on Structure, Ligand Exchange, and Optical Properties of Ligand-Protected Gold Nanoclusters: Thiolate versus Selenolate

Atomically precise thiolate-protected Au nanoclusters (NCs), i.e. Au_<i>m</i>(SR)_<i>n</i>, have attracted intensive research interest during the past few years. Recently, the synthesis and isolation of selenolate-protected gold clusters (Au_<i>m</i>(SeR)_<i>n</i>) via the ligand exchange of thiolate with selenol were achieved, which demonstrated identical compositions to those of thiolate-protected Au NCs. In this study, we perform a comprehensive theoretical study on the structure, electronic structure, and electronic optical absorption properties of 11 selenolate-protected gold clusters on the basis of density functional theory (DFT) calculations. Our results propose that the selenolate-protected Au NCs with framework structure identical to the thiolated ones are stable local minima. The ligand effect is proposed to understand the distinct geometrical structures of Au₂₄(SeR)₂₀ and Au₂₄(SR)₂₀ NCs. In addition, the optical absorption properties of thiolate- and selenolate-protected Au NCs are compared via the time-dependent density functional theory (TD-DFT). The results indicate that two types of Au NCs possess similar shape of electronic optical absorption spectra and electronic structure. The excitation wavelength dependent intermolecular electron transfer between the Au₂₅(ER)⁻ (E = S and Se) and O₂ is revealed as well