Supported gold nanoparticles are promising catalysts for production of H2O2 from O2 and H2. Size, structure, and palladium doping effects play the key role in activity and selectivity of a gold catalyst. We performed a study of the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation on the atomic level, using the DFT/PBE approach with relativistic all electron basis set. The top, edge, and facet atoms of the tetrahedral Au20 cluster as well as a palladium atom of Au19Pd located on the top, edge, and facet of a tetrahedron have been considered as active sites of steps involved in H2O2 synthesis. The thermodynamic and kinetic data including Gibbs free energies and the activation Gibbs free energies were calculated for the steps determining the formation of H2O2 (H(s) + OOH(s) = H2O(2(s)), H2O(2(s)) = H2O(2(g))) and for one step decreasing the selectivity (H2O(2(s)) = OH(s) + OH(s)). Gold tends to have low activity and high selectivity in H2O2 synthesis regardless of the structure of active site. Low coordinated palladium atoms promote H2O2 formation as well as its dissociation. Pd on a facet of a cluster facilitates H2O2 production with high activity and selectivity.

Organic thiols are known to react with gold surface to form self-assembled monolayers (SAMs), which can be used to produce materials with highly attractive properties. Although the structure of various SAMs is widely investigated, some aspects of their formation still represent a matter of debate. One of these aspects is the mechanism of S-H bond dissociation in thiols upon interaction with gold. This work presents a new suggestion for this mechanism on the basis of DFT study of methanethiol interaction with a single gold atom and a Au(20) cluster. The reaction path of dissociation is found to be qualitatively independent of the model employed. However, the highest activation barrier of S-H bond dissociation on the single gold atom (12.9 kcal/mol) is considerably lower than that on the Au(20) cluster (28.9 kcal/mol), which can be attributed to the higher extent of gold unsaturation. The energy barrier of S-H cleavage decreases by 4.6 kcal/mol in the presence of the second methanethiol molecule at the same adsorption site on the model gold atom. In the case of the Au(20) cluster we have observed the phenomenon of hydrogen transfer from one methanethiol molecule to another, which allows reducing the energy barrier of dissociation by 9.1 kcal/mol. This indicates the possibility of the "relay" hydrogen transfer to be the key step of the thiol adsorption observed for the SAMs systems.

The morphology and charged state of gold clusters play a crucial role in heterogeneous catalysis. The selection and optimization of theoretical approaches are necessary for the investigation of active sites on isolated and supported gold clusters. In the present paper, a study of the potential isomers of the Au12 cluster is performed within the DFT/PBE framework using a scalar-relativistic approach. We have found Au12 to be a dynamic cluster with at least 24 isomers due to the Jahn–Teller distortion. The majority of these isomers exhibit low symmetry, resulting in the formation of low-coordinated atoms, which are discussed in terms of frontier molecular orbitals and a Hirschfeld analysis of their atomic charges. The energy difference between the most energetically stable 2D (D 3h ) and 3D (C 2v ) isomers of Au12 is small (equal to 25 kJ/mol), which is evidence of their coexistence. The influence of the support on properties of the cluster is investigated using Au12/MgO(100). The 2D isomer of Au12 can interact with the surface either in an upright position, with two (E ads/atom = 24 kJ/mol) or three atoms (E ads/atom = 25 kJ/mol); the preferred position is planar (E ads/atom = 30 kJ/mol). The small deformation energy is required to distort a dynamic structure of Au12 compared to rigid gold clusters. The 3D isomer interacts with MgO(100) with two of its atoms (E ads/atom = 24 kJ/mol). The Au–Au distances across the surface increase, whereas the Au–Au distances at an angle to the surface are compressed with respect to the distances in the free clusters. The weak adsorption energies of Au12 on MgO and the low activation barriers for gold atom migration (15 kJ/mol) between oxygen sites facilitate the diffusion of nanoparticles on the MgO surface.

No abstract

The interaction of gold clusters Au 10 of different structural and charge states with various hydrocarbons was studied by the PBE density functional method. Saturated hydrocarbons interact weakly with the neutral cluster Au 10 , for charged Au 10 + the alkane-cluster bond energies increase threefold. Unsaturated hydrocarbons interact with cluster surface more strongly than saturated hydrocarbons, while coordination to the benzene ring is possible for aromatic compounds PhC 2 H, PhC 2 H 3 , and PhC 2 H 5 . The low coordinative gold atoms located on the peaks and edges of the cluster are the active adsorption site of the cluster. The appearance of a positive charge on the cluster leads to a greater increase in the hydrocarbon-gold cluster bond energy than the transition from the planar 2D structure to the three dimensional (3D) structure of the neutral cluster.

Au, NiO, and NiO/Au clusters of 2.5-16 nm, supported on Al 2 O 3 , ZrO 2 , TiO 2 , and ZnO, were studied in the purification of ethene feedstock from ethyne by hydrogenation at 357 K. The Au, NiO, and NiO/Au catalysts possessed 100 % selectivity to ethene. As the size of NiO clusters decreased from 7 to 3 nm, the turnover frequency (TOF) decreased from 812-1,023 to 276 h −1 . In contrast with NiO, Au activity increased with decreasing particle size. NiO/Au catalysts possessed higher stability and activity in comparison with Au and NiO catalysts. The synergistic gain on NiO/Au clusters (SG) calculated as TOF NiO/Au -TOF Au -TOF NiO was 1,466; 1,147; 563; and 569 h −1 for NiO/Au/Al 2 O 3, NiO/Au/TiO 2 , NiO/Au/ZnO, and NiO/Au/ ZrO 2 , respectively. The reasons of the observed catalytic trends and the origin of the most active and selective sites are discussed.

This study aims to identify the role of the various electronic states of gold in the catalytic behavior of Au/MxOy/TiO2 (where MxOy are Fe2O3 or MgO) for the liquid phase oxidation of n-octanol, under mild conditions. For this purpose, Au/MxOy/TiO2 catalysts were prepared by deposition-precipitation with urea, varying the gold content (0.5 or 4 wt.%) and pretreatment conditions (H2 or O2), and characterized by low temperature nitrogen adsorption-desorption, X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDX), scanning transmission electron microscopy-high angle annular dark field (STEM HAADF), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy of CO adsorption, temperature-programmable desorption (TPD) of ammonia and carbon dioxide, and X-ray photoelectron spectroscopy (XPS). Three states of gold were identified on the surface of the catalysts, Au0, Au1+ and Au3+, and their ratio determined the catalysts performance. Based on a comparison of catalytic and spectroscopic results, it may be concluded that Au+ was the active site state, while Au0 had negative effect, due to a partial blocking of Au0 by solvent. Au3+ also inhibited the oxidation process, due to the strong adsorption of the solvent and/or water formed during the reaction. Density functional theory (DFT) simulations confirmed these suggestions. The dependence of selectivity on the ratio of Brønsted acid centers to Brønsted basic centers was revealed.

The efficiency of Au/TiO2 based catalysts in 1-phenylethanol oxidation was investigated. The role of support modifiers (La2O3 or CeO2), influence of gold loading (0.5% or 4%) and redox pretreatment atmosphere, catalyst recyclability, effect of oxidant: tert-butyl hydroperoxide (TBHP) or O2, as well as the optimization of experimental parameters of the reaction conditions in the oxidation of this alcohol were studied and compared with previous studies on 1-octanol oxidation. Samples were characterized by temperature-programmed oxygen desorption (O2-TPD) method. X-ray photoelectron spectroscopy (XPS) measurements were carried out for used catalysts to find out the reason for deactivation in 1-phenylethanol oxidation. The best catalytic characteristics were shown by catalysts modified with La2O3, regardless of the alcohol and the type of oxidant. When O2 was used, the catalysts with 0.5% Au, after oxidative pretreatment, showed the highest activity in both reactions. The most active catalysts in 1-phenylethanol oxidation with TBHP were those with 4% Au and the H2 treatment, while under the same reaction conditions, 0.5% Au and O2 treatment were beneficial in 1-octanol oxidation. Despite the different chemical nature of the substrates, it seems likely that Au+(Auδ+) act as the active sites in both oxidative reactions. Density functional theory (DFT) simulations confirmed that the gold cationic sites play an essential role in 1-phenylethanol adsorption.