D. A. Pichugina

Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the Structure Effect on the Atomic Level

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.

Hydrocarbon Adsorption on Gold Clusters: Experiment and Quantum Chemical Modeling

Heats of adsorption Q of n-alkanes C6–C9 on ZrO2 modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C1–C3 with Aum, Aum − 1Ni (m = 3, 4, 5) clusters as well as the interaction of C1–C8 with Au20. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.

Gold complexes with

The reactions of methane with the gold complexes [Au(OH)]−, [Au(OCH3)4]−, [Au(O(CO)2O2]− and [Au(O2CH)2]+, [AuI(acac)], [AuIII(acac)2]+ (acac-acetylacetonato) were studied using the DFT/PBE method with the SBK basis set. High activation barriers were obtained for the electrophilic substitution in [Au(OH)]−, [Au(OCH3)4]−, [Au(O(CO)2O)2]-and [AuIII(acac)2]+ complexes, which excludes the possibility that these reactions might proceed under mild conditions. The reactions of the [Au(HCO2)2]+ and [AuI(acac)] complexes with methane have rather low energy barriers and proceed through the formation of an intermediate complex. The alternative mechanism of methane oxidation with a gold complex in the presence of oxygen is simulated.

A theoretical study of the activation of methane by Gold(III) homoleptic complexes

The density functional theory method with the PBE functional, SBK pseudopotential, and extended basis sets was used to study the reaction between methane and gold(III) homoleptic complexes, namely, [AuX4]− (X = Cl, Br, I, H, CN, NH2, OH, CH3, and SH), [Au(X(CY)2X)2]− (X = S, Y = H; X = Y = O), Au2Cl6, [Au(X2(CY))2]+ (X = S, Y = NH2; X = O, Y = H), and [Au(acac)2]+, with the formation of electrophic substitution products. The activation of methane under mild conditions was found to be uncharacteristic of anionic and neutral complexes. According to calculations of cationic oxygen-containing complexes, the formation of methane complexes is possible in their reactions with methane. The energy barrier to this reaction noticeably decreases because of the activation of the C-H bond in this complex. The heat effects vary widely depending on the nature of the ligand. There is, however, no obvious correlation between their values and the activation energy of the reaction.

Regularities of the retention of aminopyridines by silica gel modified with gold nanoparticles

New sorbents based on silica gel and gold nanoparticles stabilized by L-cysteine and its methyl ester are synthesized. Regularities of the retention of various substituted aminopyridines by the synthesized sorbents are examined in the normal-phase mode of high-performance liquid chromatography upon elution by a binary mobile phase (hexane-isopropanol). Quantum-chemical simulations of L-cysteine and its derivative adsorptions on the surface of a gold cluster are performed, along with simulations of subsequent adsorptions of substituted pyridines on the modified gold surface. It is shown that the Snyder-Soczewinski and Scott-Kucera displacement models can be used to describe the experimental data on the synthesized sorbents.

Quantum-chemical modeling of ethylene and acetylene adsorption on gold clusters

The interaction of ethylene and acetylene molecules with planar (2D) and nonplanar (3D) gold clusters Aun (n = 10, 12, 20) was studied by the density functional theory (DFT) method. The coordination of hydrocarbons at the vertices, edges, and fragments of the Au3 cluster was shown to form π, di-σ, and μ type complexes, respectively. The standard Gibbs energy and the C-C bond length of the hydrocarbon change during its adsorption in the series μ > di-σ > π complexes. The highest selectivity in adsorption of acetylene relative to that of ethylene was achieved on Au12 (3D) and Au20 (2D) clusters.

New separation materials based on gold nanoparticles

Purpose – Gold clusters supported on oxide surface are most promising as active sites as biochemical sensors, in optical industry and catalysis, but there is no information about application of these nanoparticles in analytical chemistry for the chromatographical separation of organic compounds. Thus, the aim of this paper is the development of new separation systems based on gold nanoparticles.Design/methodology/approach – The novel nanohybride system based on gold nanoparticles, cysteine and inorganic matrixes was synthesis. The results were obtained by the combined approach consisting quantum‐chemical calculations, direct organic synthesis and using chromatography investigation of obtained sorbent for separation of aminopyridines.Findings – The structure optimization of cysteine‐gold cluster complexes reveals that adsorbed acid greatly changes the structure of gold nanoparticle and, consequently, changes its properties. The most bond energy is calculated for L‐form 82 kcal/mol. Material based on alumin...

Quantum-chemical study of the effect of oxygen on the formation of active sites of silver clusters during the selective adsorption of hydrocarbons

Density functional theory (PBE with a modified Dirac-Coulomb-Breit Hamiltonian) is used to simulate the adsorption of hydrocarbons (C2H2, C2H4, C2H6) on the surface of a sorbent containing Ag0, Agδ+, and AgO sites. The dynamics of change in the structural characteristics of Agn (n ≤ 10) is analyzed and the adsorption of oxygen on Ag8 and Ag10 is studied to select the adsorption site model. Studying the interaction of hydrocarbons with Ag8, Ag10, Ag10+, Ag10O, and Ag10O2 clusters reveals that the presence of oxygen leads to an increase in the activation of unsaturated hydrocarbons, and the adsorption energy of C2H2 increases tenfold. It is found that the role of adsorbed oxygen is not only to form adsorption sites of hydrocarbons (Agδ+) but also to bind C2H2 and C2H4 directly to the sorbent’s surface.

Studying the adsorption and activation of benzene and chlorobenzene on Ni(12%)/Al2O3 by means of gas chromatography and quantum chemistry

In studying the surface and adsorption properties of Al2O3 and Ni(12%)/Al2O3 with respect to C6H6 and C6H5Cl, it is found that adsorbate-adsorbent interaction is stronger than adsorbate-adsorbate interaction. It is shown that the calculated isosteric heats of adsorption vary in a range of 61 to 45 kJ/mol depending on adsorption magnitude; for Ni(12%)/γ-Al2O3, as in the case of γ-Al2O3, the heat of adsorption of chlorobenzene is higher at low degrees of filling than that of benzene. According to density functional theory quantum-chemical calculations of the structures of complexes (NinC6H5Cl)z and (NinC6H6)z (n = 1, 4; z = −1, 0, +1), a nickel atom can penetrate into C6H5Cl along the C-Cl bond. It is concluded that a negative charge on nickel contributes to the efficient activation of the C-Cl bond and to an increase in the rate of desorption of benzene, a key step in the hydrodechlorination of chlorobenzene.

Activation of Oxygen on Palladium Nanocluster

A simulation of oxygen adsorption on a palladium nanocluster has been performed using density functional theory. It has been demonstrated that the formation of Pd8O2 complexes occurs according to the dissociation mechanism. The most probable centers of oxygen activation are atoms of metal with excess Pdδ− electron density. The possibility of identifying the oxygen coordination type on a Pd8 cluster using the IR spectra is discussed.