Effects of vanadium(V)-substitution on the oxidative properties of <Emphasis Type="Italic">α</Emphasis>-Keggin-type heteropolyanion clusters—progress in DFT theoretical studies

The redox properties of α-Keggin-type heteropolyanion clusters [XM₁₂O₄₀] ⁿ⁻ (X = Si, P; M = Mo, W) mainly depend on their constituent outer metal-oxygen cages {M₁₂O₃₆}. They act as “reservoirs”, through which the transfer and transition of electrons and protons may occur. At the atomic and molecular level, the redox properties of these clusters can be controlled and also tuned by modifying the metal M in the cages and the central heteroatom X of the clusters. Combined with relevant experimental results, this review summarizes our recent theoretical investigations of the effect of vanadium substitution on the redox properties of Keggin anion clusters. Theoretical modeling and calculation results showed that the oxidative ability of the modified species was increased by partial substitution of the cage M atoms of the Keggin clusters by vanadium atoms which have lower electronegativity. A linear correlation between the catalytic efficiency per vanadium atom and the microstructures of the vanadium(V)-substituted heteropolyanions [PV _n Mo_12−n O₄₀]⁽³⁺ⁿ⁾⁻ (n = 1−3) was established for the first time. This relationship may be suitable to interpret the catalytic behavior of the title compounds in the hydroxylation of benzene to phenol, and may also be used in understanding other reactions such as the oxidative dehydrogenation of isobutyric acid and the nitration of adamantine. The establishment of this nearly linear structure-property relationship may lay the foundations of understanding the behavior of the title compounds in homogeneous catalytic oxidation reactions, and may direct the design of future catalysts and the choice of other catalytic reactions.     

Allylic isomerization of dichlorobutenes catalyzed by iron-containing nanoparticles stabilized in polymeric matrices

Iron-containing clusters obtained by the decomposition of iron complexes in a solutionmelt of a polymeric matrix exhibit catalytic activity in the isomerization of dichlorobutenes. The activity of the clusters stabilized in the polytetrafluoroethylene and polyethylene matrices depends on the nature of the stabilizing matrix and the content of the metal in it,i. e., on the size and structure of the cluster, and substantially exceeds the activity of supported metals and powders. The clusters in the polytetrafluoroethylene matrix are more active than those in polyethylene. The dependence of the catalytic activity on the metal content has an extreme character, and for the polyethylene matrix it achieves a maximum at a metal content of ≈10%. In catalysts with this composition, the particle size increases to 4–5 nm, and the distance between them is shortened, on the average, to 10 nm, which leads to interaction of the cluster particles with each other. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 854–857, May, 2000.     

Enantioselective Homogeneous Hydrogenation of Monosubstituted Pyridines and Furans

Summary.  The first case of an enantioselective hydrogenation of monosubstituted pyridines and furans with homogeneous rhodium diphosphine catalysts with low but significant enantioselectivities and catalyst activities is reported. Best enantioselectivities (ees of 24–27%) were obtained for the hydrogenation of 2- and 3-pyridine carboxylic acid ethyl ester and 2-furan carboxylic acid with catalysts prepared in situ from [Rh(nbd)₂]BF₄ and the chiral ligands diop, binap, or ferrocenyl diphosphines of the josiphos type. Turnover numbers (ton) were in the order of 10–20, turnover frequencies (tof) usually 1–2 h⁻¹. Diphosphines giving 6- or 7-ring chelates led to higher ees than 1,2-diphosphines; otherwise, no clear correlation between ligand properties and catalytic performance was found. In some experiments black precipitates were observed at the end of the reaction, indicating the decomposition of the homogeneous catalysts for certain ligand/metal/substrate combinations. Received April 5, 2000. Accepted (revised) May 2, 2000     

Quantum chemical models and electronic structure of active centers of heterogeneous polymerization of olefins

Conclusions The peculiarities of the catalytic activity in olefin polymerizations which can find explanation in terms of the concepts suggested in this work are as follows. First, the low catalytic activity of the individual organometal compounds of group IV-VI transition metals is indicative [53] of the important role of the coordination state of the transition metal in AC, which, according to Cosse's model, must be octahedral (tetrahedral for individual metal-alkyl compounds MR_n). Second, the activity of a catalytic system depends essentially on the nature of the ligand environment of the metal in AC. The catalysts based on titanium halides display the highest activity. Third, the results of [19, 20] show that the highly active catalytic centers of homogeneous Ziegler-Natta's systems are “cation-like” Zr(IV) complexes Cp2Zr⁺-R. All these features find explanation in terms of the concept of the competitive contributions from the AC metal s and d orbitals to the active M-R bond. Thus a transition of AC environment from tetrahedral to octahedral may be compared with a change in transition metal AO hybridization:d ³s¹ (tetrahedron) ⇒d ³s¹ (octahedron). Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 2, pp. 391–404, March–April, 2000.     

Cage Structure Formation of Singly Doped Aluminum Cluster Cations Al<Subscript><Emphasis Type="BoldItalic">n</Emphasis></Subscript><Emphasis Type="BoldItalic">TM</Emphasis><Superscript>+</Superscript> (<Emphasis Type="BoldItalic">TM</Emphasis> = Ti,V, Cr)

Structural information on free transition metal doped aluminum clusters, Al _n TM ⁺ (TM = Ti, V, Cr), was obtained by studying their ability for argon physisorption. Systematic size (n = 5 – 35) and temperature (T = 145 – 300 K) dependent investigations reveal that bare Al _n ⁺ clusters are inert toward argon, while Al _n TM ⁺ clusters attach one argon atom up to a critical cluster size. This size is interpreted as the geometrical transition from surface-located dopant atoms to endohedrally doped aluminum clusters with the transition metal atom residing in an aluminum cage. The critical size, n _crit , is found to be surprisingly large, namely n _crit = 16 and n _crit = 19 – 21 for TM = V, Cr, and TM = Ti, respectively. Experimental cluster–argon bond dissociation energies have been derived as function of cluster size from equilibrium mass spectra and are in the 0.1–0.3 eV range.     

The role of urea in Cu–Zn–Al catalysts for methanol steam reforming

Abstract  

Impregnated Cu–Zn over Al₂O₃ exhibits high activity with the use of a lower amount of active metal relative to conventional co-precipitation catalysts. The activity of the catalyst could be enhanced by addition of urea to the metal salt solution during impregnation. The H₂ yield from Cu–Zn catalysts with urea is 42%, while the H₂ yield from catalyst without urea is only 28% in a continuous system at 250 °C and 1.2 atm. The H₂ yield of the catalyst with urea in this study could compete with that of commercial catalysts. The role of urea in the Cu–Zn catalysts was investigated. X-ray diffraction (XRD) analysis of the catalysts shows that the crystal size of CuO could be reduced by the addition of urea. The XRD diffractogram of the catalyst prior to calcination also shows the formation of NH₄NO₃, which could aid in dissociation of metal clusters. Scanning electron microscopy (SEM) images of catalysts show the size of Cu–Zn compound clusters and also their dispersion over the Al₂O₃ surface on the impregnated catalysts. The addition of urea could also yield smaller Cu–Zn compound clusters and better dispersion compared with the impregnated catalyst without urea. Such impregnated Cu–Zn catalysts with urea could be alternative novel catalysts for methanol steam reforming.     

Propane transformations on aluminum chloride—cobalt chloride clusters: a quantum chemical study

Density functional PBE/TZ2p quantum chemical calculations of activated complexes and pathways of model catalytic transformations of propane under the action of aluminum chloride-cobalt chloride ionic bimetallic complexes were carried out. The formation of an intermediate with a broken C-C bond can occur on the cationic cluster CoAlCl₄ ⁺ characterized by the strongest coordination of propane molecule. The activation barrier to the reaction is ΔG = 25.0 kcal mol⁻¹. Activation of alkane C-H bonds follows the alkyl pathway involving the formation of bimetallic alkyl complexes. The interaction of activated hydrocarbon fragments bound to transition metal atoms in cobalt-chloroaluminate clusters can result in alkane metathesis products (in this case, ethane and a polymetallic cluster containing an extendedchain alkyl radical).     

Atomic ordering in CuGeO3 during the spin-Peierls transition: Calculation of63Cu NQR frequencies

The X_α discrete variation (X_α-DV) method is used to calculate the electronic structures of the clusters modeling the spin-Peierls transition in CuGeO₃. The⁶³Cu NQR frequencies of two centers that fall within the simulated supercell are calculated. It is shown that the nature of the forbidden gap and the⁶³Cu NQR spectral parameters depend on the motion of copper atoms in the chains. Some assumptions are made to explain the fact that dimerization in chains of Cu atoms is experimentally unobservable. Translated from Zhurnal Strukturnoi Khimii, Vol. 38, No. 1, pp. 43–50, January–February, 1997.     

Electroless Ni–P coated on graphite as catalyst for the electro-oxidation of dextrose in alkali solution

Graphite particles were coated with Ni–P by electroless deposition using a conventional bath consisting of a nickel salt and hypophosphite. After 15 min of electroless deposition, the graphite particles were covered with 10 wt% nickel and 0.7–1.0 wt% phosphorus as analysed by wet chemical method. Surface morphology was studied by scanning electron microscopy (SEM). Electrochemical characterisation for the catalytic activity was done by cyclic voltammetry. Pure Ni powder and electroless Ni–P coated on graphite were used as catalysts for the electro-oxidation of dextrose (1.8 × 10⁻³ to 4.5 × 10⁻³ M) in 0.1 M KOH solution. Comparative studies revealed that electroless Ni–P coated on graphite particles acted as a better catalyst than pure Ni powder for catalytic reaction.     

Cobalt salts of decamolybdodicobaltic acid as precursors of the highly reactive type II CoMoS phase in hydrorefining catalysts

Catalysts have been synthesized using the Anderson polyoxometalates (POMs) (NH₄)₄[Ni(OH)₆Mo₆O₁₈] (NiMo₆POM), (NH₄)₆[Co₂Mo₁₀O₃₈H₄] · 7H₂O (Co₂Mo₁₀POM), and H₆[Co₂Mo₁₀O₃₈H₄] (Co₂Mo₁₀HPA) as the precursors and hydrogen peroxide as the solvent. The catalysts have been characterized by low-temperature nitrogen adsorption, XPS, and HRTEM. Their catalytic properties have been tested in thiophene hydrodesulfurization and in the hydrodesulfurization and hydrogenation of components of diesel oil. The active phase of the catalysts synthesized using the POMs is the type II CoMoS phase in which the mean plate length is 3.6–3.9 nm and the mean number of MoS₂ plate per plate packet is 1.8–2.0. Use of hydrogen peroxide provides an efficient means to reduce the proportion of Co²⁺ promoter atoms surrounded by oxygen in the case of an impregnating solution containing both an ammonium salt of a heteropoly acid and a Co²⁺ salt. In the catalysts synthesized using cobalt salts of Co₂Mo₁₀HPA, the support surface contains the multilayer type II CoMoS phase and cobalt sulfides. These catalyst show high catalytic properties in thiophene hydrogenolysis and diesel oil hydrorefining. Models are suggested for the catalysts synthesized using Anderson POMs.     

Post-modification of metal-organic framework for improved CO2 photoreduction efficiency

Utilizing metal-organic frameworks (MOFs) to design photocatalysts for CO₂ reduction catalysts is an excellent idea but currently restricted by the relatively low activity. Enhancing CO₂ affinity and tuning the oxidation state of metal clusters in MOFs might be a solution to improve the catalytic performance. Herein, the Cl-bridge atoms in the metal clusters of a cobalt MOF were easily exchanged with OH^?, which simultaneously oxidized a portion of Co(II) to Co(III) and resulted in a much enhanced photocatalytic activity for CO₂ reduction. In contrast, the original framework does not exhibit such superior activity. Comprehensive characterizations on their physicochemical properties revealed that the introduction of hydroxyl group not only greatly increases the CO₂ affinity but also alters the oxidation state of metal clusters, resulting in significantly improved photocatalytic activities for CO₂ reduction. This work provides important insight into the design of efficient photocatalysts.     

Computational modeling of amorphous silica. 2. Modeling the initial structures. Aerosil

A qualitative algorithm for constructing large clusters of aerosil model structures is used. According to this algorithm and the classification of amorphous silicas, aerosil is classified with tectosilicas, which are characterized by close packing of silicon-oxygen tetrahedra. Two quantitative algorithms for constructing large close-packed clusters are proposed. The structures of the clusters having from 10 to 24 silicon atoms, completely optimized by quantum chemical methods, were obtained. Small, medium, and large clusters for modeling the local and collective properties of atomic and functional group packing in the aerosil structure are distinguished. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 3, pp. 16–26, May–June, 1994. Translated by O. Kharlamova     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Sol gel derived Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> as catalysts for methane combustion: effect of zirconium loading

A series of Pd/Al₂O₃–ZrO₂ materials have been prepared via sol gel method as an attractive route to obtain more homogeneous binary oxides Al₂O₃–ZrO₂. A Zr loading between 2 and 15 wt% was used to investigate the Zr promotion of Pd/Al₂O₃ materials. The prepared catalysts were calcined at two different temperatures. Very interesting results have been obtained at low zirconium content. A small amount of Zr is seen to be sufficient to stabilize the activity and to obtain good catalytic performances with developed textural properties compared to conventional catalysts used to oxidize methane. The increase of the zirconium loading is seen to decrease the catalytic activity may be due to the development of tetragonal zirconia phase detected by XRD. Similar effect has been observed after heating catalysts at high temperatures. A loss in BET surface area and in metal dispersion has been also observed for zirconium rich catalysts. A contradictory effect on textural and structural properties is seen after their calcination at 700 °C.     

Towards Atomically Precise Supported Catalysts from Monolayer-Protected Clusters: The Critical Role of the Support

Controlling the size and uniformity of metal clusters with atomic precision is essential for fine-tuning their catalytic properties, however for clusters deposited on supports, such control is challenging. Here, by combining X-ray absorption spectroscopy and density functional theory calculations, it is shown that supports play a crucial role in the evolution of monolayer-protected clusters into catalysts. Based on the acidic nature of the support, cluster-support interactions lead either to fragmentation of the cluster into isolated Au–ligand species or ligand-free metallic Au⁰ clusters. On Lewis acidic supports that bind metals strongly, the latter transformation occurs while preserving the original size of the metal cluster, as demonstrated for various Au_n sizes. These findings underline the role of the support in the design of supported catalysts and represent an important step toward the synthesis of atomically precise supported nanomaterials with tailored physico-chemical properties.     

Effect of silicon compounds on the pyrolysis of propane-butane hydrocarbon mixture

The effect of silicon-containing catalysts on the pyrolysis of propane-butane hydrocarbon mixture in a flow system was studied in the temperature range 500–850°C, the rate of the gas mixture flow 50–100 ml min⁻¹, contact time 0.1–12.0 s, and the value of the heterogeneity factor 0.1–2.1×10⁷ cm⁻¹. The catalytic activity of different systems under similar conditions was compared, and the influence of various factors on the yield of ethylene and propylene was studied. The most active silicon-containing catalyst for the pyrolysis of propane-butane hydrocarbon mixture was found.     

Dependence of the catalytic activity of Ag/Al2O3 on the silver concentration in the selective reduction of NOx with n-hexane in the presence of H2

The activity of 0.25–5% Ag/Al₂O₃ catalysts in the selective catalytic reduction of nitrogen oxides with n-hexane under the conditions of promotion with a small amount of H₂ was studied. It was found that, upon the introduction of ∼1000 ppm of H₂ into the reaction mixture, the Ag/Al₂O₃ samples containing 1–2% Ag exhibited optimum activity and selectivity. It was established that, in the presence of 1000 ppm of H₂, the rate of the selective catalytic reduction of NO _x was higher by a factor of 10–13, and the onset temperature of the reaction was lower by approximately 100°C. It was found by X-ray photoelectron spectroscopy, temperature-programmed reduction, and UV spectroscopy that the high activity of 1–2% Ag/Al₂O₃ catalysts was due to the presence of small Ag _n ^δ+ and Ag _m ⁰ clusters on their surface. A decrease in the concentration of Ag below the optimum value resulted in the predominance of an inactive ionic form on the catalyst surfaces. As the concentration of Ag was increased (>2%), large particles of Ag₂O and Ag⁰, which facilitate the oxidation of n-C₆H₁₄, were formed to lead to a decrease in selectivity and in the degree of reduction of nitrogen oxides.     

Effect of the functionalizing of carbon nanotubes on the electrodeposited catalysts’ structure and catalytic properties

The structure and hydrophilic-hydrophobic properties of functionalized single-wall carbon nanotubes are studied by the standard porosimetry method. It is shown that the functionalized nanotubes have highly hydrophilic surface; at that the summary surface area measured “by octane” decreased, as a result of the functionalizing, due to the blocking of the nanotubes’ inner channels by the functional groups located at the nanotubes’ ends. The nanotubes’ capacitive properties are studied; their charging-discharging curves appeared being highly reversible, unlike those of other carbonaceous materials. Catalytic properties of the functionalized nanotubes are studied, with particular tendency toward their using as a carrier of platinum catalysts for the methanol oxidation and oxygen electroreduction reactions. When minor amounts (5–10 μg cm⁻²) of platinum or platinum-ruthenium alloy are deposited onto the nanotubes’ hydrophilic surface, uniform layer of the catalyst is formed, with specific surface area up to 150–300 m² g⁻¹; high current of the methanol oxidation or oxygen electroreduction is observed at these catalysts. When the catalyst deposit mass increased, its specific surface area decreased, as well as the specific current of the reactions occurring thereon. When the current is related to the electrochemically active unit surface, the catalytic activity is nearly the same both for different catalyst mass deposited onto the nanotubes and the same catalyst mass at different carbonaceous carriers.     

Preparation of supported metal catalysts starting from hydrotalcites as the precursors and their improvements by adopting “memory effect”

Hydrotalcite-like compounds (HTlcs) can be used as the catalysts as it is since they contain various transition metal cations as the catalytically active species well dispersed on the basic support materials. Moreover, increasing numbers of the applications of HTlcs after the heat treatment have been found since the oxides with very small crystal size, stable to thermal treatments, are obtained after the calcination. The oxides possess interesting properties such as high surface area, basic properties and further form small and thermally stable metal crystallites by reduction. Moreover, the calcined oxides show a unique property, i.e., “memory effect,” which allows the reconstitution of the original hydrotalcite structure. We have developed the catalytic applications of hydrotalcites as it is and moreover the mixed oxides derived from hydrotalcites for various catalytic reactions, i.e., oxidation, dehydrogenation and reforming of hydrocarbons, and even for the reforming of methanol and the CO shift reaction. Aerobic oxidation of alcohols, Baeyer−Villiger oxidation of ketones and O₃ oxidation of oxalic acid have been successfully carried out with the Mg−Al hydrotalcites containing Ni, Fe and Cu, respectively, as the catalysts in liquid phase. In the O₃ oxidation of oxalic acid, the catalytic activity was enhanced by the “memory effect,” i.e., Mg(Cu)–Al hydrotaclite was reconstituted on the surface of Mg(Cu,Al)O periclase particles and oxalic acid was incorporated as anions in the hydrotalcite layer, resulting in an enhanced oxidation of oxalic acid. As the catalysts in the vapor phase reactions, Mg/Fe/Al mixed oxides prepared from Mg–Al(Fe) hydrotalcites and effectively catalyzed the dehydrogenation of ethylbenzene. Supported Ni metal catalysts have been prepared from Mg(Ni)–Al hydrotalcites and successfully used in the steam reforming and the oxidative reforming of methane and propane. Moreover, the Ni catalysts have been improved by combining a trace amount of noble metals by adopting the “memory effect” and used in the production of hydrogen for the PEFC under the daily startup and shutdown operation. Also starting from aurichalcite or hydrotalcite precursor as the precursor, Cu/Zn/Al catalysts with high Cu metal surface area have been prepared and successfully applied in the steam reforming of methanol and dimethyl ether, and moreover in the CO shift reaction.     

Mechanism Insights into Second-Order Nonlinear Optical Responses of Anionic Metal Clusters

We present the first-principle calculations on the electronic excitations and second-order properties in solution phase of two typical inorganic trinuclear anionic clusters, [MoCu₂S₄(SPh)₂]²⁻ and [Mo₂CuS₄]¹⁻(edt)₂(PPh₃) (edt=1,2-ethanedithiolato) in the framework of density functional theory (DFT). The computed excitation energies are in good agreement with the outcome of the measurements. The predicted values of the molecular quadratic hyperpolarizabilities are of the comparable order of those of the typical organometallic chromophores. We demonstrate the significant contributions to the second-order responses from the charge transfers between the metal centers (MMCT) which are ascribed to the direct metal–metal bonding interactions in these two charged clusters. This meaningful ligand-independent mechanism for the second-order response largely relates to metal–metal bonding strength, and the understanding will benefit to the future design of the new-generation molecular based nonlinear optical materials and optoelectronic devices by means of the conscious tuning of metal–metal interactions and metal-core structures of inorganic polynuclear clusters.