Determination of basic sites on the surface of metal oxide catalysts by desorption of benzoic acid

A method has been developed for the determination of the concentration of basic sites on white and deeply colored solid porous materials, which is based on the displacement of adsorbed benzoic acid by acetic acid. The determination is rapid and gives results comparable with those obrained by the titration with benzoic acid using thymolphthalein as indicator.

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Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (-273 to >1000 °C) and pressure (UHV to ?100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman-IR, Raman-UV-vis, Raman-EPR) and the operando Raman spectroscopy methodology (e.g., Raman-MS and Raman-GC) is allowing for the establishment of direct structure-activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).     

Determination of the active surface of supported oxide catalysts on the basis of their reduction rate

A method for the determination of the active surface of supported oxide catalysts, based on measurement of their rate of reduction with carbon monoxide has been developed. The surface of silica gel-supported CuO was measured. The catalytic activity of the samples was found to be proportional to their active surface.

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Structure and nature of active sites on the surface of pyrogenic titanosilica

The structure and nature of active sites on the surface of pyrogenic titanosilica (TS) have been studied. The new acidic centers on TS surface occur with formation of titanosiloxane bridges, their formation seemed to be a result of combination of basic structural units of SiO₂ (tetrahedron) and TiO₂ (octahedron).     

Hydrotalcite-derived Co-containing mixed metal oxide catalysts for methanol incineration

The Co–Mg–Al mixed metal oxides were prepared by calcination of co-precipitated hydrotalcite-like precursors at various temperatures (600–800 °C), characterised with respect to chemical (AAS) and phase (XRD) composition, textural parameters (BET), form and aggregation of cobalt species (UV–vis-DRS) and their redox properties (H₂-TPR, cyclic voltammetry). Moreover, the process of thermal decomposition of hydrotalcite-like materials to mixed metal oxide systems was studied by thermogravimetric method combined with the analysis of gaseous decomposition products by mass spectrometry. Calcined hydrotalcite-like materials were tested as catalysts for methanol incineration. Catalytic performance of the oxides depended on cobalt content, Mg/Al ratio and calcination temperature. The catalysts with lower cobalt content, higher Mg/Al ratio and calcined at lower temperatures (600 or 700 °C) were less effective in the process of methanol incineration. In a series of the studied catalysts, the best results, with respect to high catalytic activity and selectivity to CO₂, were obtained for the mixed oxide with Co:Mg:Al molar ratio of 10:57:33 calcined at 800 °C. High activity of this catalyst was likely connected with the presence of a Co–Mg–Al spinel-type phases, containing easy reducible Co³⁺ cations, formed during high-temperature treatment of the hydrotalcite-like precursor.     

The nature of active sites in Pt promoted GaZSM-5 catalysts

Summary Activity and selectivity of Pt-promoted GaZSM-5 catalysts in the aromatization of ethane were studied. XRD indicated that high temperature treatment of the Pt promoted GaZSM-5 zeolites results in the formation of bimetallic GaPt clusters located on the inner surface of zeolites. Both experimental results and calculations suggest that the clusters containing Ga and Pt atoms stabilized in cationic positions are characterized by an increased activity and selectivity in the dehydrogenation of ethane.</o:p>     

Combined study of processes of formation of oxide and metal-oxide catalysts and the nature of their active surface

Conclusions The research studies on Al, Ca, Zn, Ni, Cu, and Cr catalysts that we have reviewed in this paper have been brought to the stage of large-scale commercialization, with major economic effects. As a result of the multiplicity of techniques used in these studies, success has been achieved in introducing many new features into the development of scientific principles for the synthesis of catalysts with preassigned properties.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 26–39, January, 1984.     

Selective oxidation of propane to acrylic acid over mixed metal oxide catalysts

The effects of metal atomic ratio, water content, oxygen content, and calcination temperature on the catalytic performances of MoVTeNbO mixed oxide catalyst system for the selective oxidation of propane to acrylic acid have been investigated and discussed. Among the catalysts studied, it was found that the MoVTeNbO catalyst calcined at a temperature of 600 ℃ showed the best performance in terms of propane conversion and selectivity for acrylic acid under an atmosphere of nitrogen. An effective MoVTeNbO oxide catalyst for propane selective oxidation to acrylic acid was obtained with a combination of a preferred metal atomic ratio （Mo1V0.31Te0.23Nb0.12）. The optimum reaction condition for the selective oxidation of propane was the molar ratio of C3H8 ：O2 ： H2O ： N2 = 4.4： 12.8 ： 15.3 ： 36.9. Under such conditions, the conversion of propane and the maximum yield of acrylic acid reached about 50% and 21%, respectively.     

A HYSCORE investigation of bimetallic titanium-vanadium microporous catalysts: elucidating the nature of the active sites

Vanadium and titanium bimetallic AlPO-5 molecular sieves have been synthesized and characterized by means of Electron Spin Echo detected EPR and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy. Direct evidence for framework substitution of redox-active Ti ions and VO(2+) units at Al sites is provided through the detection of large (31)P hyperfine couplings.     

Simulation of oxidation of carbon particles at the surface of mixed oxide catalysts

A model of catalytic oxidation of a cylindrical carbon particle was suggested. The model considers the rate of layer-by-layer burning of the particle, variation of its geometry, and remote action of the catalyst. A software for calculations was developed; the data obtained correlate well with the experimental data for catalysts based on lanthanum cesium vanadate. The kinetics of carbon oxidation on catalysts of varied nature was compared.     

Nanocrystalline gold and gold-palladium alloy oxidation catalysts: a personal reflection on the nature of the active sites

The discoveries that supported gold nanoparticles are exceptionally efficient catalysts for a whole range of reactions is one of the most surprising revelations in the recent history of chemistry. For many years gold was considered too noble to be chemically interesting. However, when divided to the nano-scale, small gold nanoparticles comprising relatively few atoms, are very efficient in catalysing a range of redox and chemical synthesis reactions. In this perspectives article some of the recent research in this rapidly evolving field is highlighted, and the nature of the active site for oxidation reactions is commented on.     

Copper hydroxide catalysts for selective oxidation: Quantum chemical study of active sites

The Cluster models of the copper hydroxide structure were constructed to investigate the oxidation of organic substrates containing unsaturated C-C bonds in the Cu(OH)₂/Н₂О₂ catalytic system. The B3LYP density functional calculations of the activation barriers for ethylene epoxidation by the mono- and binuclear Cu^II hydroperoxo complexes corroborated the possibility of non-radical oxygen transfer from the terminal CuOOH hydroperoxo group. The activation barriers presented are relatively high compared to the barriers in the range of 12-15 kcal/mol calculated at the same computational level for Ti^IV hydroperoxo intermediates and Re^VII bisperoxo complexes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of the chemical nature of the oxide support on selectivity of iron catalysts for carbon monoxide hydrogenation

The influence of the nature of the oxide support on selectivity of Fe catalysts in Fischer-Tropsch synthesis at atmospheric pressure has been studied. It was found that selectivity towards lower olefins increases with increasing basicity of the oxide support. It is suggested that this effect is due to participation of basic oxide sites in hydrogen-atom abstraction from the surface metal-alkyl complex.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 3, pp. 152–157, May–June, 1994.The authors are grateful to V. V. Pryanikova, M. G. Petrosyan, and A. M. Puzii for assistance with individual experiments.     

Effect of the chemical nature of metal oxide on adhesion to the polyacrylate matrix in filled nanocomposites

The morphology, particle size, and thermochemical properties of the surface of oxides Al₂O₃, NiO, TiO₂, ZnO, and ZrO₂ obtained by the wire electroexplosion method were studied. The nanoparticles are spherical, with a mean diameter of 54–86 nm depending on the nature of the oxide. The hydrophilicity of the surface of metal oxide nanopowders was found to change in the series NiO-ZrO₂-TiO₂-ZnO-Al₂O₃. Nanocomposites with widely varied compositions were obtained from butyl methacrylate copolymer with 5 wt % methacrylic acid and the oxides under study. The enthalpies of dissolution of the composites in chloroform were determined by Calvet calorimetry. The enthalpies of copolymer mixing with oxides were calculated using the thermochemical cycle. The limiting enthalpies of copolymer adhesion to the oxide surface were calculated from the thermochemical data. The limiting adhesion enthalpy was shown to be negative for all oxides under study; these values decreased in magnitude as the surface hydrophilicity increased. The results were analyzed from the viewpoint of balance between the specific and dispersion interactions at the interface.     

Determination of lead oxide in and on the surface of high-purity lead metal

Summary A method has been developed for the determination of a minute amount of lead oxide in and on high-purity lead metal. The lead metal is shaken with 0.5% (w/v) ammonium acetate solution and sodium tetrahydroborate in nitrogen atmosphere. The metallic lead is unaffected while the lead oxide is dissolved in the aqueous solution and can be determined by atomic absorption spectrometry. The procedure is repeated in the presence of mercury. In this case lead is converted to amalgam and total lead oxide is dissolved and determined. The lead oxide contained in the interior of the sample is obtained by the difference. Lead oxide in commercial lead metals of various forms has been determined by this method. It is simple and fast, a minute amount of lead oxide can be determined precisely with high reproducibility (recovery is 98–100%).

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Bestimmung von Bleioxid in und auf der Oberfläche von hochreinem Blei

Zusammenfassung Die Bleiprobe wird mit 0,5 %iger Ammoniumacetatlösung und NaBH₄ unter Stickstoff geschüttelt, wobei sich das an der Oberfläche befindliche Bleioxid löst und in der Lösung durch AHS bestimmt werden kann. Metallisches Blei wird durch diese Behandlung nicht angegriffen. Eine weitere Metallprobe wird dann in Gegenwart von Quecksilber ebenso behandelt, wobei Bleiamalgam gebildet wird und das gesamte Oxid in Lösung geht. Der Oxidgehalt in der Probe ergibt sich aus der Differenz der beiden Werte. Verschiedenartige Bleiproben wurden mit diesem Verfahren untersucht (Reproduzierbarkeit 98–100%).

     

Catalytic performances and correlations with metal oxide band gaps of metal-tungsten mixed oxide catalysts in propane oxydehydrogenation

Summary Tungsten-based catalysts of the form X wt.% Na₂WO₄/support (where X = 5, 10, 50 or 75 and the support is either γ-Al₂O₃ or SiO₂) were prepared by impregnation method and tested for oxidative dehydrogenation of propane. The effects of Na₂WO₄ loadings on catalyst performances were more pronounced on γ-Al₂O₃ - supported sample. Individual first-row transition metals (V, Cr, Mn, Co, Ni or Zn) were doped on NaWSi. CoWSi exhibited the best improvement, with conversion of 10.7% and selectivity to propylene of 52.9% at 450^oC. The metal oxide band gaps were correlated with catalyst performances. At a constant propane conversion, strong correlations were observed between the band gaps and respective CO and CO₂ yields. This offers insight into relationship between the species conductivities and the products distributions.