In Situ Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: Kinetics of the Reduction Process with Carbon Monoxide

In situ diffuse reflectance spectra of supported chromium oxide catalysts are investigated for the first time at elevated temperatures under controlled reaction conditions using a specially designed diffuse reflection attachment. The obtained results are compared and discussed with those obtained by the classical diffuse reflectance spectroscopy technique. A novel method for studying the reduction kinetics of supported transition metal oxides is proposed. In the case of Cr(VI), the reduction is faster on silica than on alumina. A kinetic model is developed to explain the kinetics. It consists of the activation of CO by adsorption followed by the reduction of Cr^6+ with formation of surface carboxylates

Zeolite-Encapsulated Copper(II) Amino Acid Complexes: Synthesis, Spectroscopy, and Catalysis

The spectroscopic properties and catalytic behavior of Cu(AA)n m+ complexes (AA ) amino acid (glycine, lysine, histidine, alanine, serine, proline, tyrosine, phenylalanine, glutamine, glutamic acid, cysteine, tryptophan, leucine, and arginine)) in faujasite-type zeolites have been investigated. Successful immobilization was achieved by a simple cation exchange procedure with aqueous solutions of preformed Cu(AA)n m+ complexes. The best ion exchange results were obtained with lysine, arginine, proline (at pH ) 10), and histidine (at pH = 7.3) as ligands and with a AA:Cu 2+ ratio of 5. The internal surface and pore volume are drastically reduced by the uptake of the Cu(AA)n m+ complexes, and no precipitation of Cu(AA)n m+ crystals was observed by scanning electron microscopy. Both observations suggest the location of the complexes in the supercages of the faujasite-type zeolites. The composition of the first coordination sphere around Cu 2+ can be designed from NNNN to NOOO by varying the type of amino acid. A free coordination site is available for catalysis, and the oxidation of alcohols, alkanes, and alkenes with peroxides was observed at low temperatures

Characterization of Al2O3-Supported Manganese Oxides by Electron Spin Resonance and Diffuse Reflectance Spectroscopy

Alumina-supported manganese oxides, used as catalysts for the selective catalytic reduction of NO, were characterized by combined electron spin resonance and diffuse reflectance spectroscopies. Upon impregnation of the acetate precursor solution, the [Mn(H2O)6]^2+ complex interacts strongly with surface hydroxyls of the y-Al2O3. Evidence was obtained that this anchoring reaction proceeds at a Mn/OH = 1/2 ratio up to 4.5 wt % Mn loading, leading to a highly dispersed oxidic manganese layer. At higher loadings, the precursor complex is deposited on the surface concurrently. Upon drying at 383 K, part of the manganese is oxidized to higher oxidation states (Mn^3+ and Mn^4+ ), while a further increase in (average) oxidation state takes place upon calcination at 573 K. After calcination, the manganese species are present as a mixture of Mn^2+ ,Mn^3+ , and Mn^4+ . At low loadings (<1 wt %), approximately equal amounts of these three oxidation states are present, whereas Mn 3+ becomes the predominant species at higher loadings. ESR reveals that at low loadings, almost all the manganese is present as isolated species, while at 4.5 wt % Mn loading, still more than 70% of the manganese is isolated. The decrease of the fraction of isolated manganese species at higher loadings is accompanied by a decreased selectivity toward N2 production in the selective catalytic reduction of NO. The fraction Mn^2+ is present in an axially distorted octahedral coordination

Fluorescence resonance energy transfer between organic dyes adsorbed onto nano-clay and Langmuir-Blodgett (LB) films

In this communication we investigate two dyes N,N' -dioctadecyl thiacyanine perchlorate (NK) and octadecyl rhodamine B chloride (RhB) in Langmuir and Langmuir-Blodgett (LB) films with or with out a synthetic clay laponite. Observed changes in isotherms of RhB in absence and presence of nano-clay platelets indicate the incorporation of clay platelets onto RhB-clay hybrid films. AFM image confirms the incorporation of clay in hybrid films. FRET was observed in clay dispersion and LB films with and without clay. Efficiency of energy transfer was maximum in LB films with clay.Comment: 15 pages 5 figures, 1 tabl

Supported Vanadium Oxide Catalysts: Quantitative Spectroscopy, Preferential Adsorption of V^4+/5+, and Al2O3 Coating of Zeolite Y

A series of supported vanadium oxide catalysts were prepared by the incipient wetness method as a function of the support composition (Al2O3, SiO2, and USY), the metal oxide loading (0-1 wt %), and the impregnation salt (vanadyl sulfate and ammonium vanadate). These catalysts have been studied by combined DRS-ESR spectroscopies in order to quantify the amount of V^4+ and V^5+ and to unravel their coordination geometries. These spectroscopic fingerprints have been used to study the preferential adsorption of V^4+/5+ ions on SiO2, Al2O3, and USY. Both V^4+ and V^5+ were preferentially adsorbed on Al2O3 and showed a much smaller pref-erence for USY and SiO2. The observed preference orders are discussed in relation with the properties of the support. In addition, a novel method is proposed to coat the external surface of USY with a thin film of Al2O3. The method is based on the deposition of USY with the so-called Keggin ion, [Al13O4(OH)24(H2O)12]7+ , which is too big to enter the USY channels or pores. The obtained Al2O3/USY material showed a preferential adsorption of V^4+ onto the Al2O3 film, suggesting that this method could be useful for vanadium passivation of FCC catalysts

Cu-ZSM-5: A biomimetic inorganic model for methane oxidation

The present work highlights recent advances in elucidating the methane oxidation mechanism of inorganic Cu-ZSM-5 biomimic and in identifying the reactive intermediates that are involved. Such molecular understanding is important in view of upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu-containing oxidation enzymes