Interaction of Co<Subscript>6</Subscript> cluster with γ-alumina surface: A quantum chemical study

The interaction of Co₆ cluster with partially dehydroxylated γ-alumina surface was studied by the DFT method. Hydrogen atoms of surface hydroxyl groups can be transferred to the metal particle to form partially oxidized cobalt states. The energy characteristics of hydrogen transfer were determined and changes in the electronic structure of supported Co₆ particles were characterized. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2194–2199, October, 2005.     

Effect of the basic surface sites of carbons on the degree of dispersion of platinum catalysts prepared by H<Subscript>2</Subscript>PtCl<Subscript>6</Subscript> adsorption

In the synthesis of Pt/C catalysts via H₂PtCl₆ adsorption onto a carbon support, NH₄Cl can be formed catalytically during the reduction of the precursor with H₂ at 250°C. This compound favors the sintering of metal particles. This effect is likely due to the weakening of metal-support bonding because of NH₄Cl adsorption on the Pt surface. The sources of nitrogen and chlorine atoms are basic surface sites of the support, which contain nitrogen atoms in their structure and adsorb Cl^? ions from the precursor solution. This effect is typical of active carbons, whose surface contains chemically bound nitrogen as amino groups, and weakens as the Pt/N atomic ratio in the supported catalyst precursors is increased.     

Interaction of a Co<Subscript>6</Subscript> cluster with oxides of different nature: a quantum chemical study

Density functional theory was applied to investigate the interaction of Co₆ nanoparticle with various oxide supports including γ-Al₂O₃, silicalite, and zeolite HZSM-5. The introduction of cobalt into silicalite leads to insignificant stabilization of the metal cluster and induction of a small positive charge. The interaction of the Co₆ particle with the acid zeolite or alumina is accompanied by transfer of either a proton from the Br?nsted acid site or hydrogen atoms from terminal OH groups to the surface of the metal cluster with the formation of a hydride-like complex cation. Geometric parameters and energy characteristics of adsorption complexes of carbon monoxide molecule with Co₆ particles on different supports were calculated. For isolated particle on silicalite, “linear” adsorption is predicted. According to calculations, one can expect “angular” adsorption in the case of the acid zeolite and “two-point” adsorption (precursor of active surface carbon) in the system Co₆/γ-Al₂O₃.     

Effect of the composition of the reaction atmosphere on the thermal stability of highly dispersed gold particles on an oxide support (Au/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system)

Metal gold particles were supported onto the surface of aluminum oxide by physical vapor deposition. The effects of thermal treatments at 30?800°C both in a vacuum and in an atmosphere of O₂ (5 mbar), CO (5 mbar), or a mixture of CO + O₂ (5 mbar of each) on the samples of Au/Al₂O₃ were studied by X-ray photoelectron spectroscopy. An increase in the Au4f line intensity in the course of gold deposition was accompanied by a shift of this line toward smaller binding energy. Upon the supporting of a maximum quantity of gold, the binding energy E _b(Au4f _7/2) became smaller than the value characteristic of the bulk metal. It was hypothesized that this can be explained by the formation of negatively charged Au^δ? particles due to electron density transfer from the support to the particles of gold. In the course of the heating of Au/Al₂O₃ in a vacuum or in a reaction atmosphere, the agglomeration of small gold particles occurred; this fact manifested itself in a decrease in the atomic ratio [Au]/[Al]. In all of the atmospheres, the Au particles supported on Al₂O₃ exhibited high thermal stability; considerable changes in the ratio [Au]/[Al] were observed only at temperatures higher than 600°C.     

The activation characteristics of the decomposition of H<Subscript>2</Subscript>O<Subscript>2</Subscript> on palladium-carbon catalysts

The kinetics of catalytic decomposition of H₂O₂ on palladium-carbon catalysts with various deposited metal distributions in carrier (active carbon) porous granules was studied. The activation parameters (E _a and A ₀) of the process were calculated by the Arrhenius equation. A determining factor for the catalytic process was found to be the entropy factor (A ₀), which characterized the formation and dissociation of activated transition complexes. A quantum-chemical study of the electronic structure of palladium-carbon catalysts showed the occurrence of electron density transfer from the carbon matrix to metal clusters and collectivization of their electronic systems. This increased the donor-acceptor ability of the synthesized materials and, as a consequence, their catalytic activity.     

Activity of Rh/TiO<Subscript>2</Subscript> catalysts in NaBH<Subscript>4</Subscript> hydrolysis: The effect of the interaction between RhCl<Subscript>3</Subscript> and the anatase surface during heat treatment

The reaction properties of Rh/TiO₂ sodium tetrahydroborate hydrolysis catalysts reduced directly in the reaction medium depend on the temperature at which they were calcined. Raising the calcination temperature to 300°C enhances the activity of the Rh/TiO₂ catalysts. Using diffuse reflectance electronic spectroscopy, photoacoustic IR spectroscopy, and chemical and thermal analyses, it is demonstrated that, as RhCl₃ is supported on TiO₂ (anatase), the active-component precursor interacts strongly with the support surface. The degree of this interaction increases as the calcination temperature is raised. TEM, EXAFS, and XANES data have demonstrated that the composition and structure of the rhodium complexes that form on the titanium dioxide surface during different heat treatments later determine the state of the supported rhodium particles forming in the sodium tetrahydroborate reaction medium.     

Structure,energy, and spin characteristics of La@C<Subscript>60</Subscript>-Lu@C<Subscript>60</Subscript> lanthanide endofullerenes

Calculations of the equilibrium geometric and electronic structure of lanthanide endofullerenes are presented. Two types of the Ln@C₆₀ structure are found. For endofullerenes of the first type (La@C₆₀-Dy@C₆₀), the stable position of the lanthanide atom is achieved at a distance of 0.67R from the center of fullerene (R is the fullerene radius); in endofullerenes of the second type (Ho@C₆₀-Lu@C₆₀), the character of the interaction between the lanthanide atom and fullerene changes because of the transfer of unpaired electrons from the corresponding atom to fullerene. It is found that in endofullerenes of the second type, metal atom mobility increases, and two minima of the potential energy appear, which corresponds to the Ln position in the center and at a distance of 0.5R from the center. Based on the obtained spin density distribution for La@C₆₀-Lu@C₆₀ endofullerenes, we conclude that there is spin leakage.     

XES and quantum chemical investigation of the electronic structure of phthalocyanine complexes MPcH<Subscript>16</Subscript> and MPcF<Subscript>16</Subscript> with M = Cu,Co

The X-ray spectroscopy and X-ray electron study of the electronic structure of unsubstituted phthalocyanines MPcH₁₆ and hexadecafluorophthalocyanines MPcF₁₆ with M = Cu, Co is reported. Quantum-chemical calculation of the electronic structure of these compounds was performed. The calculation was used to analyze the metal-ligand electronic interaction in the complexes. The theoretically calculated energy position and composition of the highest occupied molecular orbitals were compared with experimental data obtained from X-ray fluorescence spectra.     

Intermolecular complexes of nido-C<Subscript>2</Subscript>B<Subscript>3</Subscript>H<Subscript>7</Subscript> with HF and LiH molecules: the theoretical studies,bonding properties and natural bond orbital (NBO) analysis

The changes in stabilization energy upon the formation of intermolecular hydrogen, dihydrogen and lithium bond complexes between C₂B₃H₇, LiH and HF have been investigated using MP2 method with aug-cc-pVDZ basis set. The interaction of HF with nido-C₂B₃H₇ could occur through the formation of B–H^?δ···^+δH–F, C–H^+δ···F^?δ–H and B–C···H–F classical and non-classical hydrogen bonds. The B–C bonds in backbone of the C₂B₃H₇ as electron donor interact with σ* orbital of HF as electron acceptor. Also interaction of LiH with nido-C₂B₃H₇ resulted in B–C···Li–H and B–H···LiH lithium bonds as well as C–H···H–Li dihydrogen bond complexes. In some of these complexes, LiH interacts with B–C bonds. Results are indicating that more stable complexes belong to interaction of HF and LiH with backbone of the nido-C₂B₃H₇. The AIM and NBO methods were used to analyze the intermolecular interactions; also the electron density at the bond critical point and the charge transfer of obtained complexes were studied.     

Structures of active sites for alkane transformations over the Pt/HZSM-5 and Pt/NaZSM-5 catalysts

The structure and reactivity of Pt₆ particles in the sodium and hydrogen forms of the ZSM-5 zeolite were investigated by the DFT method. Upon adsorption on the sodium form, the interaction energy is 15 kcal mol⁻¹ and a negative charge appears on the metal cluster. In channels of the hydrogen form, the adsorption energy of the metal particle increases to 45 kcal mol⁻¹ and the oxidized states of platinum are formed. The formation of an active site in the H-form of the zeolite involves the interaction of the platinum particle with the acid site resulting in the suppression of the acidity of the support. An alternative mechanism of alkane transformations avoiding acid site participation was proposed. A possibility to envisage the direction of the transformation of alkanes adsorbed on the metal particles was shown. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1139–1144, June, 2008.     

Modeling the active centers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> and V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> supported catalysts. DFT theoretical analysis of optical properties

Within the framework of the density functional theory (DFT), the electronic structure of monooxodioxovanadium functional groups in tetrahedral coordination, which model the active centers (ACs) of fine supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂, has been analyzed. The optimal structures of three ACs as possible models of monomeric and polymeric oxovanadium forms on the carriers with low vanadium content were determined. The modified DFT method involving the time dependence of Kohn-Sham equation (TDDFT) was used for the adopted AC models to calculate the energies of the excited states, and optical spectra of the absorption in 25000–60000 cm^?1 region were reconstructed on their base. The spectrum in this region is due to O → V charge transfer. The features of electronic spectra with the charge transfer for V₂O₅/SiO₂ and V₂O₅/TiO₂ catalysts and the vibrational spectra of three AC models corresponding to the monomeric and dimeric oxovanadium forms of the supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂ were defined. The detailed interpretation of normal vibration frequencies is given. The frequencies typical of the monomeric and dimeric oxovanadium forms on the carrier surface were identified.     

Effect of the Composition of the Reaction Atmosphere on the Thermal Stability of Highly Dispersed Gold Particles on an Oxide Support (Au/Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> System)

Gold metal particles were vacuum deposited onto the iron oxide surface and the effect of thermal treatment on the resulting Au/Fe₂O₃ samples at 30–800°C in vacuum and reaction media with different chemical compositions, i.e., 5 mbar of O₂, 5 mbar of CO, and 5 mbar of CO + 5 mbar of O₂, was studied by X-ray photoelectron spectroscopy. During the gold deposition, the increase in the intensity of the Au4f line was shown to be accompanied by its shift toward lower binding energies, which is due to the increase in the particle size of gold. Starting from a certain amount of deposited gold, the binding energy E_b(Au4f_7/2) reaches the value typical of bulk gold metal. During the heating of Au/Fe₂O₃ in vacuum or a reaction medium, fine gold particles agglomerate into coarser ones, which is manifested in the decrease in the relative [Au]/[Fe] atomic ratio. The tendency of gold particles toward agglomeration depends on the composition of the reaction medium: the process proceeds most efficiently in the CO + O₂ mixture and less intensively in CO, and the highest stability was observed upon treatment in O₂. It is assumed that the decrease in the [Au]/[Fe] atomic ratio during the thermal treatment of Au/Fe₂O₃ in CO + O₂, CO, and vacuum could be not only due to agglomeration but also due to the encapsulation of the gold particles by the reduced fragments of the support.     

Analysis of the Interaction Between the C<Subscript>20</Subscript> Cage and <Emphasis Type="Italic">cis</Emphasis>-Ptcl<Subscript>2</Subscript>(NH<Subscript>3</Subscript>)<Subscript>2</Subscript>: A DFT Investigation of the Solvent Effect,Structures, Properties,and Topologies

This study investigates the interaction between C²⁰ and the cis-PtCl₂(NH₃)₂ complex using MPW1PW91 quantum chemical calculations in gas and solution phases. Two interaction modes between C²⁰ and the cis-PtCl₂(NH₃)₂ complex are considered: I-isomer (η₂-C²⁰) and II-isomer (η₁-C²⁰). It also determines the effects of the solvent polarity on the dipole moment, electronic spatial extent (ESE), structural parameters, and frontier orbital energies of two possible isomers of the C²⁰…cis-PtCl₂(NH₃)₂ complex. The bonding interaction between C²⁰ and the cis-PtCl₂(NH₃)₂ complex was examined through energy decomposition analysis (EDA). The metal–ligand bonds are evaluated using the percentage composition of the specific groups of frontier orbitals. The quantum theory of atoms in molecules (QTAIM) analysis is applied to assess the Pt–C bonds within the complex. Finally, the Pt–C spin-spin coupling constants are calculated using the gauge independent atomic orbital (GIAO) method.     

Catechol reactivity: Synthesis of dopamine derivatives substituted at the 6-position

Dopamine is a ubiquitous neurotransmitter essential in the proper functioning of the human body. In addition to this critical role, the catecholamine core has shown utility as a scaffold for numerous drugs and in other applications, like metal detection and adhesive materials. Substituents at the 6-position of dopamine’s catechol core can modulate its stereoelectronic properties, the acidity of its phenolic hydroxyl groups, and the overall hydrophobicity of the molecule. Herein, we report the synthesis of a series of four novel dopamine analogues substituted at the 6-position of catechol core. The ¹H NMR chemical shift of the aromatic proton meta to the substituent correlated strongly with the Hammett σ_m constant, confirming the electronic properties of substituents.     

Electronic Oxide–Metal Strong Interaction (EOMSI)

Strong metal–support interaction of supported metal catalysts is an important concept to describe the effect of metal–support interactions on the structures and catalytic performances of supported metal particles. By using an example of CeO_x adlayers supported on Ag nanocrystals, herein a concept of electronic oxide–metal strong interaction (EOMSI) is put forward; this interaction significantly affects the electronic structures of oxide adlayers through metal-to-oxide charge transfer. The EOMSI can stabilize oxide adlayers in a low oxidation state under ambient conditions, which individually are not stable; moreover, the oxide adlayers experiencing the EOMSI are resistant to high-temperature oxidation in air to a certain extent. Such an EOMSI concept helps to generalize the strong influence of oxide–metal interactions on the structures and catalytic performance of oxide/metal inverse catalysts, which have been attracting increasing attention.     

Influence of ZrO<Subscript>5</Subscript> treatment temperature on the interaction with titanium tetrachloride

The data on phase and chemical transformations of nanosized zirconium dioxide upon annealing at 25–1300°С are presented. The in situ interaction of titanium tetrachloride with nanosized zirconia annealed at 200–800°С has been studied. The revealed regularities of the change of titanium content and the Cl/Ti ratio in the chemisorbed groups have confirmed that TiCl₄ predominantly reacts with zirconia treated at up to 400°С via the hydroxyl groups to yield the TiCl_4–n fragments. In the case of zirconia treated at higher temperature, the interaction with TiCl₄ involves the coordination-unsaturated Zr⁺ and Zr–O^– centers as well.     

Hydration and proton transport in solid solutions based on Ba<Subscript>2</Subscript>CaWO<Subscript>6</Subscript>

Hydrated alkaline-earth metal tungstates Ba₄Ca_{2 + x} W_{2 ? x} O_{12 ? 2x} with perovskite structure were studied by the thermogravimetry, ¹H NMR, IR, and Raman spectroscopy methods. Electrical conductivity and transfer numbers were measured with varying T, \(p_{O_2 } \) and \(p_{H_2 O} \). The solid solutions are capable of reversibly intercalating water and can exhibit high-temperature proton transport. The localization of protons on oxygen results in the appearance of energetically nonequivalent OH groups; a small fraction of protons are present in the form of H₂O and H₃O⁺.     

Predicting H2S Oxidative Dehydrogenation over Graphene Oxides from First Principles

Spin-polarized periodic density functional theory was performed to characterize H₂S adsorption and dissociation on graphene oxides (GO) surface. The comprehensive reaction network of H₂S oxidation with epoxy and hydroxyl groups of GO was discussed. It is shown that the reduction reaction is mainly governed by epoxide ring opening and hydroxyl hydrogenation which is initiated by H transfer from H₂S or its derivatives. Furthermore, the presence of another OH group at the opposite side relative to the adsorbed H₂S activates the oxygen group to facilitate epoxide ring opening and hydroxyl hydrogenation. For H₂S interaction with -O and -OH groups adsorption on each side of graphene, the pathway is a favorable reaction path by the introduction of intermediate states, the predicted energy barriers are 3.2 and 10.4 kcal/mol, respectively, the second H transfer is the rate-determining step in the whole reaction process. In addition, our calculations suggest that both epoxy and hydroxyl groups can enhance the binding of S to the C-C bonds and the effect of hydroxyl group is more local than that of the epoxy.     

Structures and electronic properties of C<Subscript>12</Subscript>Si<Subscript>8</Subscript>X<Subscript>8</Subscript> (X = H,F, and Cl)

The structural stabilities and electronic properties of C₁₂Si₈X₈ where X = H, F, and Cl are probed on the basis of density functional theory at the B3LYP/6-311++G**//B3LYP/6-31+G* level. Vibrational frequency calculations show that all the systems are true minima. The infrared spectra of the most stable C₁₂Si₈X₈ molecules are simulated to assist further experimental characterization. The functionalized structures and energy gaps between the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, have been systematically investigated. It seems that C₁₂Si₈H₈ has more stability against electronic excitations via increasing the HOMO–LUMO gap comparing with C₁₂Si₈Cl₈ and C₁₂Si₈F₈. High charge transfer on the surfaces of our stable compounds, provokes further investigations on their possible application for hydrogen storage. The addition reaction energies of C₁₂Si₈X₈ are high exothermic, and C₁₂Si₈F₈ is more thermodynamically accessible.