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

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

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

El ferro, la llaminadura i el verí dels bacteris

El ferro és vital per a gairebé tots els organismes vius, atès el paper crucial que té en nombrosos processos biològics. Però, en concentracions inadequades, aquest element és un verí per a qualsevol cèl·lula. Per això, els éssers vius disposen de mecanismes que en controlen la concentració. Així, els vertebrats tenen sistemes que segresten ferro per evitar-ne l'efecte deleteri. A més, el segrest té un altre efecte positiu per a aquests animals, ja que el ferro no estarà disponible per al desenvolupament dels bacteris patògens que els puguin infectar. Com a resposta, els patògens bacterians han desenvolupat diverses estratègies de captura del ferro segrestat

Cationic motion in dehydrated zeolites

Les ions échangeables dans les zéolithes X et Y déshydratées, sont localisés sur plusieurs sites, différents entre eux du point de vue coordination et énergie potentielle. Le nombre de sites vides est du même ordre de grandeur que le nombre de sites occupés. Il en suit que la migration des cations est corrélée. La relaxation diélectrique est décrite par un moment dipolaire par cubo-octahèdre. La grandeur de œ moment dépend de la composition cationique des cubo-octahèdres. La conductivité ionique est due au mouvement des cations dans les supercages. Le mouvement corrélé demande un facteur de corrélation de conductivité ionique dans l'équation de Nernst-Einstein à part du facteur de corrélation géométrique.The exchangeable cations in dehydrated zeolites, types X and Y, are located on several energetically and coordinatively different sites. The number of vacant sites is of the same order of magnitude as the number of occupied sites. As a result, the motions of the cations as evidenced by the dielectric relaxation and electrical conductivity, are highly correlated. The dielectric relaxation can be described in terms of a dipole moment per cubooctahedron, the magnitude of which depends on the number of cations in the cubooctahedron. The ionic conduction due to the movement of the cations in the supercages, requires the introduction of an ionic conductivity correlation factor besides the classical geometrical correlation factor in the Nernst-Einstein equation

Electronic spectroscopies

Diffuse reflectance spectroscopy (DRS) in the ultraviolet, visible and near-infrared region is a versatile spectroscopic technique, as both d-d and charge transfer transitions of supported TMI can be probed. One of the advantages of electronic spectroscopy is that the obtained information is directly chemical since the outer shell electrons of the TMI are probed and provide information about the oxidation state and coordination environment of TMI on surfaces. Furthermore, the DRS technique can be used under in situ conditions and is quantitative under well-defined conditions. The main disadvantage is that diffuse reflectance spectra are complex, and usually encompasses several broad and overlapping bands. To avoid biased spectral analysis, chemometrical techniques need to be employed. The chapter starts with a short overview of the principles of DRS spectroscopy. In the first two sections, theoretical and practical aspects of DRS, together with an introduction to crystal field, ligand field and molecular orbital theory are given. The third section will focus on selected examples in order to illustrate the potential and limitations of DRS spectroscopy for unravelling the chemistry of TMI on surfaces