We have systematically studied the CO oxidation on various nanosized gold clusters with sizes ranging from 0.3 to 0.8 nm on the basis of density functional theory (DFT) calculations. A hitherto unreported trimolecular Langmuir-Hinshelwood (LH) mechanism is proposed, which offers new insights into the fundamental mechanism for CO oxidation on nanosized gold clusters. Specifically, we find that the coadsorbed CO molecule at a unique triangular Au(3) active site can act as a promoter for the scission of an O-O bond, leading to the spontaneous formation (due to extremely low energy barrier) of two CO(2) molecules as product. The key step to the O-O bond scission in the OCOO* intermediate is significantly accelerated due to the electrophilic attack of the coadsorbed neighboring CO molecule on the triangular Au(3) site. This new mechanism is called CO self-promoting oxidation, which can be visualized in real time from the trajectory of a Born-Oppenheimer molecular dynamics (BOMD) simulation. We also find that such CO self-promoting oxidation is quite universal, as long as the triangular Au(3) reaction site is available. This is demonstrated in two prototype metal oxide supported gold nanostructure systems: namely, Au(n)/MgO and bilayer-Au/TiO(2). The coadsorbed CO can not only serve as a promoter for its own oxidation but also promote other oxidation reactions such as styrene oxidation through expediting O-O scission on gold nanostructures.

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.

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 -SR-Au-SR- and two -SR-Au-SR-Au-SR- 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)]x (x = 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.

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.