Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Applied Catalysis A, General (v.307, #1)
Hydrogen adsorption and spill-over effects on H–Y and Pd-containing Y zeolites by Domenica Scarano; Silvia Bordiga; Carlo Lamberti; Gabriele Ricchiardi; Serena Bertarione; Giuseppe Spoto (pp. 3-12).
The adsorption of hydrogen on H–Y and Pd(0)–H–Y zeolites was investigated by IR (in the 300–20K temperature range) and EXAFS spectroscopies and the energy barriers for diffusion of hydrogen in the internal voids of the faujasite structure evaluated by ab initio calculations. On H–Y, H2 is adsorbed at 20K inside the super-cages giving rise to 1:1 –OH⋯H2 hydrogen bonded complexes with the Brønsted acid groups, as well as less specific interaction with the cage walls. Due to the high energy barrier for crossing the framework hexagonal apertures, H2 cannot enter the sodalite-cages. On Pd(0) containing zeolites, characterized as far the dispersion of the metal is concerned by TEM, EXAFS and IR spectroscopy of adsorbed CO, hydrogen is adsorbed in atomic form to give a metal-hydride phase; subsequent spill-over effects allow confinement of hydrogen also in the small sodalite-cages, as demonstrated by H/D isotopic substitution experiments.
Keywords: H–Y zeolites; Pd(0)–H–Y zeolites; Spill-over effect
States of H2O adsorbed on oxides: An investigation by near and mid infrared spectroscopy by Masato Takeuchi; L. Bertinetti; Gianmario Martra; Salvatore Coluccia; Masakazu Anpo (pp. 13-20).
The states of water adsorbed on fumed SiO2, Al2O3 and TiO2, from high to low hydration conditions, have been investigated by infrared spectroscopy in the mid-IR (where νasym, νsym andδH2O modes absorb), and near-IR (in particular the δ+ νasym zone) regions. The most specific information provided by spectral features in the mid-IR (MIR) were those related to the lowest hydration condition attained by outgassing at room temperature. Conversely, the δ+ νasym pattern appeared a useful source of insights on the states of water adsorbed even at the highest level of surface hydration obtained by exposing the materials to water vapour pressure at room temperature. Peculiar H-bonding environments for H2O molecules within and at the surface of adsorbed multilayers have been proposed.
Keywords: Adsorbed water; Oxides; High hydration levels; MIR; NIR
Use of nitriles as probe molecules for the accessibility of the active sites and the detection of complex interactions in zeolites through IR spectroscopy by Tania Montanari; Maria Bevilacqua; Guido Busca (pp. 21-29).
The results of infrared experiments performed on protonic zeolites (like H-FER, H-MFI, H-BEA, H-MOR and REY) and cationic zeolites such as Co–H-MOR, Li-MOR, Na-MOR, K-MOR, Cs-MOR, Na-X, Na-Y, are described. The nitrile probes AN=acetonitrile (ethanonitrile); PrN=propionitrile; IBN=isobutyronitrile (2-methyl-propionitrile); PN=pivalonitrile (2,2-dimethyl-propionitrile); DPPN=2,2-diphenyl-propionitrile; BN=benzonitrile; oTN= ortho-toluonitrile, have been used in this study. Data have been obtained concerning the location of the protonic sites in the internal cavities and on the external surface; locations in different internal cavities of multipore structures have been discriminated. Analogously, the location of cations at the external surface versus the internal cavities may be distinguished. Complex interactions with more than one alkali cations have also been documented.
Characterization and reactivity of Fe-[Al,B]MFI catalysts for benzene hydroxylation with N2O by G. Centi; S. Perathoner; R. Arrigo; G. Giordano; A. Katovic; V. Pedulà (pp. 30-41).
The characteristics of Fe-[B,Al]MFI catalysts prepared by changing: (i) the gel composition during the hydrothermal synthesis (Si/Al and Si/B ratio) and (ii) other synthesis aspects (silica source, use of phosphoric acid to complex iron) were studied. The samples were investigated in terms of composition, structural (X-ray diffraction (XRD), Fourier transform-IR (FT-IR)) and morphological (scanning electron microscopy (SEM)) aspects and distribution of the iron species determined by UV–vis DR spectroscopy. The results were correlated with the reactivity of these samples (phenol productivity and selectivity during benzene hydroxylation with N2O). A linear relationship was observed between Si/B ratio and phenol maximum productivity. This indicates that the main factor determining the catalytic performances is the amount of B introduced in the zeolite which in turn depends on the gel composition during hydrothermal synthesis and the preparation method. The linear relationship could be interpreted considering that the role of B is related to its easier migration from framework to extra-framework position with respect to Al and therefore easier creation of defect sites during heat treatment. These defect sites are important, because during the initial stage of the catalytic reaction, iron ions migrate to these sites creating highly active iron sites responsible of the initial increase in the phenol productivity. The presence of Al in the gel influences negatively the catalytic performances, because both Al competes with B to be introduced in the MFI structure as T atom. Furthermore, the presence of Al as T atom determines a lowering of the selectivity to phenol with respect to both carbon oxides and carbonaceous deposits formation.
Keywords: Phenol; Benzene; Zeolite defects; N; 2; O; Fe/MFI; Boron; Aluminium
Stabilization of cationic gold species on Au/CeO2 catalysts under working conditions by Patricia Concepción; Silvio Carrettin; Avelino Corma (pp. 42-45).
Infrared spectroscopy of CO adsorption at low temperature has shown the presence of oxidized gold species (Au+ and Au3+) on nanocrystalline Au/CeO2 catalyst. This species are quite stable even under reduction conditions like the CO oxidation reaction. The concentration of this species decrease by increasing the pre-treatment temperature of the Au/CeO2 catalyst which is in accordance with a decrease in the catalytic activity of the sample. In accordance oxidized gold species are involved in the catalytic activity of Au/CeO2 catalyst for CO oxidation although the participation of metallic sites cannot be excluded.
Keywords: FTIR; CO adsorption; Oxidized gold; Nanocrystalline CeO; 2
The interaction of benzene with Cu+ sites in zeolites by Ewa Kukulska-Zając; Paweł Kozyra; Jerzy Datka (pp. 46-50).
As Cu+ ions in zeolites catalyse several reactions of organic molecules, we undertook IR and DFT studies of the interaction of organic molecules with Cu+. Our previous studies had shown that Cu+ was able to activate strongly the double and triple bonds in alkenes and acetylene, and less strongly the CO bond in acetone, which resulted in a distinct multiple bond weakening. The DFT calculations provided evidence that this was realized by π back donation of d electrons of Cu+ to π* antibonding orbitals of the molecules. The present study concerns the activation of the benzene molecule, which is different from previously studied ones. It is a good electron acceptor, but the LUMO orbital of benzene has only a small antibonding character, therefore the interaction with Cu+ was expected to result in a strong adsorption but only slight activation of the C–C bond. Our IR studies have shown, that the C–C bond weakening was indeed very small (Δ νC–C=13cm−1, a value 10 times less than that for alkenes). DFT calculations showed that the molecule acquired a negative charge (−0.06) indicating that π back-donation prevailed over π donation. Both IR and DFT revealed that benzene adsorption on Cu+ ions was relatively strong, which resulted in a withdrawing Cu+ from the oxygen ring. DFT provided information on the geometry of the Cu+–benzene adduct and evidenced that the interaction of the benzene molecule with Cu+ resulted in non-symmetric adsorption. The distances Cu+–C were in the range: 0.203–0.303nm. The shorter the Cu+–C distance, the more important the bond activation. The benzene molecule interacting with Cu+ was no longer flat, some hydrogens were directed towards Cu+ and others in the opposite direction.
Keywords: IR spectroscopy; DFT calculations; Cu; +; cations; Zeolites; Benzene
Heterogeneous catalysis and confinement effects by Stéphane Pariente; Philippe Trens; François Fajula; Francesco Di Renzo; Nathalie Tanchoux (pp. 51-57).
The aim of this study is to highlight the impact of confinement effects in catalysis for reactants confined in small mesopores. Double bond isomerization of 1-hexene at 343K has been chosen as model reaction and mesostructured aluminosilicates of the MCM-41 type with the very same chemical composition (Si/Al=400±20) and varying pore sizes (2.3–9.3nm) were used as catalysts. Adsorption isotherms of 1-hexene over analogous purely siliceous MCM-41 materials were measured at 343K in order to determine the energetics of the adsorption process as well as levels of pores filling. Enthalpies of adsorption at zero coverage amounted to −55 to −47kJmol−1. The isosteric heats of condensation were found significantly larger than the heat of condensation of 1-hexene in the bulk and increased as the pore size decreased. Over the fresh catalysts, the smallest pore materials proved the most active and the differences in activity correlate well with the sorption energetics. Over the deactivated catalysts, a sharp maximum in activity is observed for the material featuring pores 3.7nm in diameter as the result of a compensation effect between diffusion limitations and confinement activity enhancement.
Keywords: Catalysis; Confinement; Heterogeneous; Mesoporous; n; -Hexene
CO adsorption on the surface of MgO(001) thin films by Martin Sterrer; Thomas Risse; Hans-Joachim Freund (pp. 58-61).
We report infrared reflection–absorption and thermal desorption spectra of CO adsorbed at low temperature (30K) on the surface of MgO(001) thin films. The high similarity of the observed IR bands as compared to data reported for MgO(100) single crystals and sintered MgO particles gives evidence for the identical surface morphology of the different substrates. CO adsorption series taken at constant temperature/variable pressure, variable temperature/constant pressure, as well as isotope mixing experiments reveal almost no diffusion of CO on MgO(001) at low temperature, but high adsorbate mobility at slightly elevated temperature.
Keywords: MgO(0; 0; 1) thin films; CO adsorption; Infrared spectroscopy; Thermal desorption
Effects of ion-exchange on the photoreactivity of ETS-10 by Yuni K. Krisnandi; Russell F. Howe (pp. 62-69).
This paper describes an investigation by EPR spectroscopy and FTIR spectroscopy of the reactivity of ETS-10 photocatalysts subjected to different ion exchange treatments. Trapped photoelectrons can be detected only in ion-exchanged samples which have been damaged as a result of the ion exchange procedure. UV-irradiation of such samples in the presence of adsorbed organic molecules that scavenge photogenerated holes forms Ti3+ ions, which transfer the electron to oxygen to form superoxide O2− on subsequent addition of O2. Superoxide ions are also formed when damaged samples are irradiated in O2. As-synthesised (undamaged) samples of ETS-10 are not photo-reduced when irradiated in the presence of adsorbed organic molecules, but do catalyse a novel photo-polymerisation of ethene; the activity for this reaction depends on the level of stacking faults in the ETS-10. Photo-oxidation of ethene to adsorbed carbonyl compounds occurs most extensively in ion exchanged samples; this is attributed to the generation of photocatalytic sites on the external surface of the ETS-10 as a result of the damage caused by ion exchange.
Keywords: ETS-10; Photocatalysis; Photoreduction; Ethene oxidation
Transalkylation of polynuclear aromatics with diisopropylbenzene over zeolite catalysts by N.S. Nesterenko; A.S. Kuznetsov; S.E. Timoshin; F. Fajula; I.I. Ivanova (pp. 70-77).
Transalkylation of naphthalene and biphenyl with p-diisopropylbenzene has been studied over Y, Beta, mordenite and ZSM-12 zeolites. The main products of naphthalene or biphenyl interaction with p-diisopropylbenzene were mono- and diisopropylsubstituted naphthalenes or biphenyls, cumene and benzene. Polyalkylated binuclear aromatic compounds were observed only in small amounts. Among the zeolites studied, Beta catalyst was the most active, while mordenite showed the highest shape selectivity towards β,β′-diisopropylnaphthalenes and p, p′-diisopropylbiphenyls. Dealumination of mordenite led to the creation of a secondary micro/mesopore structure, providing for easier transport of the alkylaromatic compounds inside the pores and higher acid sites accessibility. Both factors led to a significant improvement of the catalytic activity, stability and selectivity. The highest conversions of naphthalene (78%) and biphenyl (57%) were obtained over mordenite dealuminated by calcination at 750°C followed by leaching with methanesulfonic acid, while the highest selectivities to β,β′-diisopropylnaphthalene (90%) and 4,4′-diisopropylbiphenyl (79%) were observed over calcined mordenite treated with oxalic acid.
Keywords: Transalkylation; Diisopropylbenzene; 2,6-Diisopropylnaphthalene; 4,4′-Diisopropylbiphenyl; Dealuminated mordenites
Surfactant-promoted novel synthesis of supported metallic Cu nanoparticles active for selective dehydrogenation of methanol by Mizuki Tada; Rajaram Bal; Seitaro Namba; Yasuhiro Iwasawa (pp. 78-84).
We have found that a surfactant cetyltrimethylammonium bromide promoted the formation of metallic Cu nanoparticles on several metal oxides under hydrothermal synthesis conditions without any additive reductants. The Cu nanoclusters and oxide supports were characterized by means of XRD, XRF, XPS, XAFS, BET, SEM, and TEM. We have also found that the supported metallic Cu nanoparticles were active for the selective dehydrogenation of methanol to produce formaldehyde and hydrogen with 100% H2 selectivity.
Keywords: Cetyltrimethylammonium bromide; Cu nanoparticles; Hydrothermal synthesis; Methanol dehydrogenation
SCR of NO by propene over nanoscale LaMn1− xCu xO3 perovskites by Runduo Zhang; Adrian Villanueva; Houshang Alamdari; Serge Kaliaguine (pp. 85-97).
Nanoscale LaMn1− xCu xO3 perovskites with high specific surface areas were prepared by reactive grinding and characterized by N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H2-temperature programmed reduction (TPR), O2-, NO+O2- and C3H6-temperature programmed desorption (TPD) and NO+O2-temperature programmed surface reduction (TPSR) under C3H6/He flow. The samples were then submitted to activity tests in the selective catalytic reduction (SCR) of NO by C3H6 with or without O2. The catalytic performances over unsubstituted LaMnO3 is observed with maximum N2 yield of 62% and a C3H6 conversion of 80% at 550°C at a space velocity of 50,000h−1 (3000ppm NO, 3000ppm C3H6, 1% O2 in helium). The N2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N2 yield of 86% at 500°C at 20% Mn substitution by Cu. The content of α-oxygen over lanthanum manganite is enhanced by Cu substitution, but the opposite occurs for excess oxygen. The better performance of Cu-substituted samples is likely to correspond to the facility in the formation of adsorbed nitrate species via the oxidation of NO by α-oxygen in addition to the intrinsic effect of Cu in NO transformation. However, the excessive α-oxygen content observed over LaCo0.8Cu0.2O3 accelerated the unselective hydrocarbon oxidation and suppressed the formation of organo nitrogen compounds, which led to a poor N2 yield with respect to Mn-based perovskites. A mechanism involving the formation of an organic nitrogen intermediate, which further converts into N2, CO2 and H2O via isocyanate, was proposed. The gas phase oxygen acts as a promoter when its concentration is lower than 1000ppm because of the promotion of nitrate formation and organo nitrogen compounds transformation. O2 acts however as an inhibitor when its concentration is higher than 5000ppm due to the heavily unselective combustion of C3H6 by O2, in the reaction of NO and C3H6 over LaMn0.8Cu0.2O3 at 400°C.
Keywords: SCR of NO; Propene; Reactive grinding; High surface area; Mn-based; Perovskite; TPR; TPD
Use of pyridine CH(D) vibrations for the study of Lewis acidity of metal oxides by Arnaud Travert; Alexandre Vimont; Azziz Sahibed-Dine; Marco Daturi; Jean-Claude Lavalley (pp. 98-107).
C–H and C–D stretching vibrations of h5- and d5-pyridine adsorbed on various metal oxides have been investigated by infrared spectroscopy. Experimental spectra were further compared with density functional (DFT) calculations of the interaction of pyridine with cluster models. Both νCH(D) frequencies and intensities are sensitive to the adsorption mode and allow one to distinguish between H-bonded, protonated and coordinated species. In the case of coordinated species, an increase in ν(CH) frequencies is observed and it could be correlated with the ν8a and ν19b frequency shifts which are commonly used to assess acid strength of metal oxides. It is found that the study of ν(CH) modes is more reliable than that of the ν8a and ν19b modes to distinguish H-bonded from weakly coordinated species. An example of such an application is given for the cases of CeO2, MgO and CaO.
Keywords: Pyridine; Metal oxides; Lewis acidity; Infrared; DFT
On the interaction of light amines and alcohols with alkali metal exchanged X zeolites by R. Schenkel; R. Olindo; J. Kornatowski; J.A. Lercher (pp. 108-117).
Adsorption of ammonia, methylamine and 1-propylamine on alkali cation exchanged X zeolites was investigated using in situ IR spectroscopy and compared to sorption of water, ethanol and 1-propanol. Amines and alcohols formed similar primary adsorption complexes in the supercages of alkali cation exchanged zeolite X. Three elements contribute to the bonding, i.e., (i) the polar sorbate interacts with alkali metal cations via the lone electron pair of its electronegative atom, (ii) the protons of the functional groups of the sorbate form hydrogen bonds with the negatively charged oxygen atoms of the framework, and (iii) the hydrogen of the alkyl groups interact with the lattice oxygen. Amines form only primary adsorption complexes and do not exhibit sorbate–sorbate interactions as observed for alcohols.
Keywords: Alkali cation exchanged zeolite X; Alcohol adsorption; Amine adsorption; IR-spectroscopy
Theoretical investigation of site-specific characteristics of CO adsorption complexes in the Li+-FER zeolite by P. Nachtigall; R. Bulánek (pp. 118-127).
The coordination of extraframework Li+ cations in FER and the formation of mono- and dicarbonyl complexes in the Li+-FER zeolite were investigated using a periodic density function theory (DFT) model. The Li+ cations strongly prefer the coordination on top of five- or six-member rings on the channel wall. The Li+ cation is always coordinated to at least three framework oxygen atoms. Calculated CO stretching frequencies are in excellent agreement with experimental results [Microporous Mesoporous Mater. 34 (2000) 67]; thus, the interpretation of experimental data at the atomic scale level is proposed. The dicarbonyl complexes can be readily formed on Li+ cations coordinated in the eight-member ring entrance window of perpendicular channel. The differences in IR spectra of CO adsorbed on Li+-FER and Li+-MFI zeolites can be understood based on the theoretical investigation. The IR band at 2193cm−1 observed for CO/Li+-MFI system but missing in the IR spectra of CO/Li+-FER is due to the Li+ sites on the channel intersection where Li+ is coordinated only to two oxygen atoms of AlO4 tetrahedron. While such sites are populated in Li+-MFI they are not significantly populated in Li+-FER. Therefore, the differences in vibrational dynamics of CO adsorbed on Li+ in FER and MFI are due to the differences in Li+ coordination in these frameworks.
Keywords: IR spectroscopy; FER; MFI; Zeolite; Carbon monoxide; Periodic model; DFT; Adsorption
Amorphous aluminosilicate catalysts for hydroxyalkylation of aniline and phenol by C. Perego; A. de Angelis; A. Carati; C. Flego; R. Millini; C. Rizzo; G. Bellussi (pp. 128-136).
Amorphous aluminosilicates with controlled porosity in the region of micro-pores (ERS-8, SA) and meso-pores (MCM-41, HMS and MSA) have been compared as catalysts in the hydroxyalkylation of phenol with acetone and of aniline with formaldehyde. The comparison has also included two commercial silica–alumina gels and H-Beta.In the hydroxylakylation of phenol with acetone to bisphenol A (BPA), the catalysts have been compared by batch test in terms of activity and selectivity. In the hydroxylakylation of aniline with formaldehyde, also the catalyst life has been investigated. As a general behavior, mesoporous aluminosilicates have evidenced better catalytic activity, selectivity and longer catalyst life than both microporous ones and commercial silica–alumina gels. In the hydroxyalkylation of aniline to methylenedianiline (MDA), mesoporous MSA have shown similar performances of H-Beta in term of MDA yields, but lower catalyst life.
Keywords: Amorphous; Aluminosilicates; Catalysts
Analysis of structural transformations during the synthesis of a MoVTeNb mixed oxide catalyst by P. Beato; A. Blume; F. Girgsdies; R.E. Jentoft; R. Schlögl; O. Timpe; A. Trunschke; G. Weinberg; Q. Basher; F.A. Hamid; S.B.A. Hamid; E. Omar; L. Mohd Salim (pp. 137-147).
This work presents a detailed investigation of the preparation routine for the multi-metal oxide Mo1V0.30Te0.23Nb0.125O x used as catalyst for the selective oxidation of propane to acrylic acid. In situ Raman spectroscopy on the initial aqueous polyoxometalate solution prepared from ammonium heptamolybdate, ammonium metavanadate and hexaoxotelluric acid reveals the coexistence of Anderson-type anions [TeM6O24] n−, M=Mo, V; n≥6 and protonated decavanadate species [H xV10O28](6− x)−. Raman analysis showed that the monomeric motif of the Anderson-type tellurate is preserved after addition of the Nb precursor and the subsequent spray-drying process. Calcination of the X-ray amorphous spray-dried material in air at 548K seems to be the essential step, leading to a re-arrangement of the tellurate building blocks, generating nanocrystalline precursors of the phases finally established during treatment in helium at 873K.
Keywords: MoVTeNb mixed oxide; Preparation; Raman; Phase structure; M1; M2; Catalyst; Selective oxidation; Propane; Acrylic acid
The combination of propane partial oxidation and of WGS reaction in a single catalytic bed, and the self-adapting catalytic properties of vanadium oxide catalyst by Nicola Ballarini; Andrea Battisti; Fabrizio Cavani; Antonio Cericola; Carlotta Cortelli; Maria Ferrari; Ferruccio Trifirò; Philippe Arpentinier (pp. 148-155).
The gas-phase oxidation of propane catalyzed by unsupported vanadium oxide was studied under the reaction conditions that are usually employed for the oxidative dehydrogenation of alkanes. It was found that considerable amounts of H2 are produced above the temperature at which oxygen is totally consumed. Hydrogen formed in part by propane dehydrogenation to propylene and by coke formation, in part by the water–gas-shift reaction, in the fraction of catalytic bed that operated in the absence of gas-phase oxygen. V2O5 catalyzed the oxidation of propane to propylene, CO, CO2 and H2O in the first part of the bed, whereas reduced vanadium oxides (VO2, V2O3) formed in the anaerobic section of the reactor and catalyzed the reaction between CO and H2O to produce H2 and CO2. It was also found that the selectivity to oxygenates (acetic acid mainly) and to propylene at low temperature was affected by the vanadium oxide type: while V2O5 was poorly selective to these compounds, higher selectivity was reached with V2O3. The latter oxide was also more active than V2O5 in propane oxidation. The role of ionic oxygen in V2O5 was studied by feeding propane in the absence of gas-phase oxygen and following the non-steady catalytic performance along with the increasing time-on-stream. Fully oxidized vanadium oxide was very active but non-selective to propylene; the progressive reduction of V2O5 led to a decrease of conversion and to an increase of the selectivity to the olefin, due to the higher contribution of dehydrogenation with respect to oxidative dehydrogenation.
Keywords: Gas-phase oxidation; Vanadium oxide; Water–gas-shift reaction; Propane oxidehydrogenation; Alkanes oxidation
Nature of active sites in decane-SCR-NO x and NO decomposition over Cu-ZSM-5 zeolites by Jiřà DÄ›deÄ?ek; Libor ÄŒapek; Blanka Wichterlová (pp. 156-164).
Decane-SCR-NO x in the presence of water vapor and NO decomposition were investigated over Cu ions exchanged in ZSM-5 zeolites with various Si/Al and Al distribution. The local environment of the Cu ions in cationic sites was characterized by the combination of29Si MAS NMR, Vis–NIR spectroscopy of Cu(II) ions, Cu(I) Vis emission spectroscopy and UV–Vis spectroscopy of Co(II) ions as probes for the presence of Al pairs in the framework. The Cu(II) ions coordinated to four framework oxygens of the α- or β-type cationic sites containing Al-O-(Si-O)2-Al sequences represent the most active sites in decane-SCR-NO x. On contrary, the Cu(I) ions coordinated to two or three framework oxygens and located at the channel intersection adjacent to the single Al atoms in the α- or β-site control predominantly reaction of NO decomposition.
Keywords: NO decomposition; SCR-NO; x; Cu-ZSM-5; Al distribution