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Infrared spectra of Cr(CO)6 adsorbed at different surface centers on gamma-alumina were investigated. The spectra of the probe complex revealed the existence of three types of strong Lewis centers and that of physisorbed Cr(CO)6. In the case of the physisorbed complex Oh symmetry was widely conserved, and addition to strong Lewis centers led to lowering of the symmetry. The assumption of C4v symmetry, caused by interaction of one CO group of the complex with a Lewis center, may explain the spectra. Approximate values for the integral extinction coefficients were determined. Copyright 1999 Academic Press.

Cu and Cu-Cr based catalysts were prepared on γ alumina support to study carbon monoxide oxidation activity and resistance to thermal and hydrothermal deactivation. The effect of modification of alumina with TiO2 or CeO2 on the overall performance of the catalyst was also investigated. In general, all the Cu catalysts were found to be more active than the corresponding Cu-Cr catalysts. Performance of the Cu and Cu-Cr catalysts prepared on various supports varies as Al2O3 Al2O3-TiO2 > Al2O3-CeO2. The pronounced improvement of ceria doped catalysts is due to its ability to store oxygen under transient conditions and also to impart stability for active components against thermal and hydrothermal deactivation.

The stresses in thin gold film deposited on a γ Al 2 O 3 substrate is investigated as a function of the deposit thickness by the bending beam method. The film morphology and structure are studied by electron microscopy and are related to the variations of the stresses during the growth. The influence of the substrate temperature and the deposition rate on the thin film stresses is considered and related to the modification of the various stages of the growth.

The structure of a-alumina (α-A12O3) is well known to be hexagonal close packed. In contrast, the structures of the so-called transition aluminas (-γ-alumina, η-alumina,…) are the subject of controversy. We report theoretical calculations which show that -γ-alumina is actually a sequence of compounds with the general formula H3mAl2−mO3. (0 ≤ m ≤ ⅓). m = ⅛ is a unique form, HAl5O8, with a perfect spinel structure. For m > 8, there are interstitial H atoms whereas for m < ⅛. there are vacancies. This picture is supported m > ⅛ by calculations of material density and proton vibrational frequencies. The results are in excellent agreement with measured values. The present new systematic approach accounts for a wide range of seemingly contradictory data and leads to the conclusion that γ-alumina behaves as a “reactive sponge” by storing and releasing water in a novel reactive way.

Adsorption of IDA and NTA and their sodium salts on a γ-alumina support has been investigated by transmission FT-IR. The results obtained imply, that the observed changes in the IR spectroscopic properties of the adsorbed chelates are mainly due to interaction of the chelate molecules with alumina hydroxyl groups.

The adsorption of some hydroxy-acids (i.e. citric, tartaric and salicylic ) on a γ-alumina support has been investigated by transmission FT-IR. The results imply that the observed changes in the IR spectroscopic properties of the supported hydroxy-acids are mainly due to interaction of the acids with alumina hydroxyl groups. The IR spectra in the C=O stretching region of the hydroxy-acids supported on alumina are quite different from those of the unsupported compounds.

The adsorption of EDTA on a modified γ-alumina support has been investigated by transmission FT-IR. The results allow the conclusion that acidic and alkaline procedures used for γ-Al2O3 modification have an insignificant influence on the EDTA adsorption.

The current impetus for examining the interaction of nitrilotriacetic acid (NTA or H 3 X, where X equals N(CH 2 COO) 3 3- ) molecules with (gamma) -alumina support by Fourier transform infrared (FTIR) spectroscopy is chiefly related to the preparation of new heterogeneous dispersed catalysts.

Metal particles in a Rh/γ-Al2O3 catalyst of differing particle size are oxidised by NO/He within 5 seconds at 313 K; rapid, highly exothermic dissociative chemisorption of NO is the initial step.

We report on conversion of nanoporous γ-alumina particles into the spinel type ZnAl2O4 without deteriorating porosity of the host material notably. The material was converted by depositing thin layers of ZnO by the atomic layer deposition (ALD) technique and annealing the sample at 800°C to facilitate a solid state reaction for formation of the spinel phase. Such particles represent robust and inert structures most suitable as host materials for catalysis and other processes where large surface area is desired. These samples were used as host materials for a red-emitting phosphor by depositing a thin layer of europium on the surface of the particles, also by the ALD technique. The europium coating was limited to the particle surface due to reaction kinetics, however a strong luminescence at 620 nm was observed. The synthesis and characterization of all these structures will be described.

Atomistic potential calculations of the relative energies of γ-alumina structures as a function of tetrahedral Al distribution give an exhaustive list of configurations which is used to estimate thermodynamic probabilities, subsequent relaxations allow the influence of configurational entropy on structure to be assessed.

The effect of the chemisorbed hydroxyl groups on the thermal diffusivity of γ alumina is determined by evaluating the thermal diffusivity at various degassing temperatures and by doping it with rare earth oxide using photoacoustic technique. The thermal diffusivity is found to decrease with the increase in degassing temperature as well as with the increase in the doping concentration of rare earth oxide. This decrease has been attributed to the loss of hydroxyl ion from the γ-Al2O3.

In an era of increasing environmental awareness, stricter federal and state regulation of pollutant emissions are emerging. A major source of pollution arises from automobiles which inadvertently form gaseous products such as nitric oxide (NOx), carbon monoxide (CO), and hydrocarbons (HC). Since the early 1980's, these effluents have been converted to safer forms using a three-way catalytic converter that employs a high dispersion of rhodium and platinum particles supported on a large surface area of transitional γ-phase alumina. Unfortunately such a converter is susceptible to decreased performance over time, and this degradation has been attributed to changes in the catalyst microstructure. The nanoscaled nature of the transition metal catalysts and the submicron-scaled size of the transitional alumina necessitates the use of the high spatial resolution analyses made possible by transmission electron microscopy.

γ-Alumina, fluorinated with sulphur tetrafluoride followed by treatment with 1,1,1-trichloroethane, behaves as a catalyst for the room temperature fluorination of CH3CCl3 with anhydrous HF, giving a mixture of chlorofluorohydrocarbons.

Aromatic acetals over γ-alumina undergo rearrangement to give the corresponding esters (b) and ethers (c) in good yield. The product distribution varied unusually over the range of reaction temperatures. The effect of substituents has also been felt much in the study. Probable mechanisms have been suggested for the reaction. The catalyst has been characterized by various studies and the specific poisoning of the catalyst has been done with NH3, CO2 and H2S.

Surface basicities of various modified γ-aluminas, used as supports in the preparation of metallic catalysts, were measured by a thermometric method. This technique allows the quantification of the modification of basic sites of γ-Al2O3) after pretreatment with KOH or HNO3. For γ-Al2O3 pretreated with KOH a change occurred in the characteristics and number of the weaker basic centres, while the stronger basic centres remained unchanged. Some surface basic centres remained in γ-Al2O3 pretreated with HNO3.

The decisive role of the Kα/Kβ ratio in the mechanism of substitution is shown. At low and increased temperatures the isotope exchange follows stepwise and nonequivalent substitution mechanisms, respectively.

Adsorption isotherms on γ-alumina of hydrocarbons with up to eight carbon-atoms were investigated using the gas chromatographic method of Cremer and Huber. Under suitable conditions of flow-rate and amount of substances there was scarcely any peak broadening by diffusion. The experimental isotherms could be fitted using either the Langmuir or the Freundlich relation. Values for the constants are tabulated for temperatures between 473 and 643 °K. Heats of adsorption were directly evaluated for different C6 compounds. Increments for an approximate calculation of adsorption isotherms were calculated from the experimental values.

The apparent surface coverage by molybdena in three series of MoO 3 γ-Al 2 O 3 catalysts prepared by different methods was determined by electrophoretic migration. These methods were designed to vary the extent of clustering (polymerization) in the solutions and on the wet catalysts. The combined use of electrophoretic migration and molybdenum extraction measurements showed that in alumina-supported molybdena catalysts with content below 14 wt%, molybdenum is present in both monolayer and multilayer forms over the entire range of molybdenum loading studied, irrespective of the method of catalyst preparation used.

The infrared spectra of molybdenum hexacarbonyl adsorbed on two types of γ-alumina have been determined after initial adsorption at 25°C and after activation in flowing helium at 100°C to produce Mo(CO)3ads. Nearly fully hydroxylated alumina (HA), partially dehydroxylated alumina (PDA, pretreated at 450–500°C), and nearly fully dehydroxylated alumina (DA, pretreated at 1000°C) were employed. Reaction between physisorbed Mo(CO)6 and the surface of alumina commences at 25°C or slightly above. It is proposed that surface O2− (σ-O−) or OH− (σ-OH) add to the carbon atom of one CO ligand with consequent labilization of other CO ligands for displacement by σ-O− or σ-OH. An infrared absorption band for Mo(CO)3ads was observed on HA, PDA, and DA at near 1580 cm−1 but only weakly with HA. On HA it is proposed that Mo(CO)3ads is primarily (σ-OH)3Mo(CO)3, and on DA, species like (σ-O)3Mo(CO)2(C00-σ). On PDA, (σ-O)3 would be replaced by (σ-O)2(σ-OH) or (σ-O)(σ-OH)2.

Hydrothermal treatment of gibbsite to boehmite and subsequent calcination to form alumina have been investigated. The aqueous solution of 0.5 – 1.0 M concentration alkali metal salts, such as NaNO3, KNO3 and LiNO3,accelerated boehmite formation and the gibbsite conversion to boehmite could be controlled with good reproducibility. The resulting boehmite had a good thermal resistance and γ-alumina with a BET surface area of ~100 m2 g−1 could be obtained by calcination, even at moderate temperatures of 500 – 700°C.

Reaction of γ-Al203 with CCl4 has been investigated up to about 900 K in a quartz microreactor attached to a quadrupole mass spectrometer. Surface reaction starts at about 400 K as detected by the formation of COCl2CO2 and Cl2. Volatilization of γ-Al203 by the formation of AlCl3, with a slight amount of Al2Cl6 becomes detectable above 600 K. According to the expected stoichiometry, CO2 is the only other major reaction product up to 900 K.

Hydrogen adsorption at 293–823 K, oxygen adsorption at 293 K and hydrogenoxygen titration at 293–523 K have been investigated on a 0.84% Ru/γ-Al2O3 catalyst. After correcting for adsorption on the support, the results have been found to be similar to those established earlier on Ru powder.

The influence of Na+ cations, introduced into the support from different compounds, on the formation of cobalt species on γ-alumina surface has been studied. The monitoring of the Co3O4/CoAl2O4 critical ratio has been obtained using diffuse reflectance spectroscopy (DRS). Sodium cations were deposited on the carrier surface by dry impregnation with aqueous solutions of NaNO3, NaOH and CH3COONa as well as with a solution of CH3COONa in CH3COOH. The active phase was mounted on the doped supports by pore volume impregnation with aqueous Co(NO3)2·6H2O solutions. Sodium cations catalyze the solid transformation of Co3O4 into CoAl2O4. The extent of this transformation is nearly independent of the particular sodium compound used.

The existence of three carbonaceous species during methanation on nickel catalysts are demonstrated using temperature programmed surface reaction and multiple reflectance infrared spectroscopy. The two species active at normal methanation temperatures are a surface nickel carbide and a carbon triply bonded to one nickel site. This gives rise to two possible molecular routes for methanation.

Etude de la permeabilite et de la selectivite des membranes composites constituees d'alumine-γ modifiee par de la silice

The oxidised and reduced states of MoO3Al2O3 catalysts containing up to 30 wt.% MoO3 have been studied by various chemical-physical techniques. The system can roughly be divided into two main regions centered around 15 wt.% MoO3 on the basis of the nature of the surface structures which are governed by the molybdenum concentration and the acidity of the support. Generally, increase of MoO3 leads from monolayer to multilayer deposition, and correspondingly from tetrahedral monomeric to octahedral polymeric species. In particular, oxyanionic species of the type [MoO4]2− in oxidised, and [MoO4]3− in reduced samples were detected in the low ( 15 wt.% MoO3) oxyanionic species are found as polymolybdates, as well as the oxycationic form [MoO2]2+; also part of the molybdenum is incorporated in the octahedral vacancies of the support and Al2(MoO4)3 is formed. Reduced samples give evidence for structures containing Mo(VI) and Mo(V) (reduced heteropolyacids) and the oxycationic species MoO3+ at the most acidic sites, both at the surface and in the bulk. The effect of increasing temperature is that the species described are present at lower concentration threshold limits.

Adsorption data for the impregnation of alumina with an aqueous solution of cobalt dichloride and ammonium molybdate were treated in terms of the Langmuir adsorption isotherm and compared with a mathematical model set up to describe the kinetics of simultaneous impregnation of a support by two components. The effective diffusion coefficients of the two components at 25 °C in a cylindrical particle of alumina were obtained. The validity of the model used was verified qualitatively by comparing the numerical results with the experimental time dependent concentration profiles of the active components in a catalyst particle, measured by electron microanalysis technique.

When aromatic acetals in vapour phase were passed over alumina (γ-Al2O3), the product mixture was found to contain ester, ether and aldehyde of aromatic nature. Activatedγ-Al2O3 exhibits Lewis acid-base properties at elevated temperatures. Possible mechanisms for the formation of the above products, consistent with experimental observation have been suggested.

The kinetics of one-component and two-component impregnation of γ-alumina (prepared from aluminium trichloride) with aqueous solutions of cobalt nitrate and ammonium molybdate were studied and treated in terms of a mathematical model accounting for the simultaneous diffusion and adsorption of the active components. The time-dependent radial concentration profiles of the components across the cylindrical support particle were obtained. The profiles of the cobalt-to-molybdenum atomic ratio were derived from the results. The conclusions are applicable to the development of procedures for the preparation of hydrodesulphuration catalysts for the working of crude oil fractions.

The limiting isosteric heats of adsorption on γ-alumina were determined by gas-solid chromatography in the temperature range 150 -225 °C for the compounds: CCl4 (9.0 kcal/mol), CHCl3 (10.2), CH2Cl2 (9.9), CH3-CCl3 (8.2), CH2Cl-CHCl2, (8.2 and 9.6 according to adsorption position) . The results are discussed with regard to possible orientations of the adsorbed molecules on the solid surface as well as to the interaction of the electric surface field with the adsorbate molecular dipoles.

Oxygen interaction with 0.5% Ru/Al2O3 has been studied by the static adsorption and TPD methods. Irreversible adsorption and the existence of a surface molecular species of oxygen (Edes=22 kcal/mol) at Tads=−110 to +25°C have been found. At Tads ≥ 85 °C, Ru/Al2O3 oxidation begins with the formation of two more (molecular and atomic) oxygen species with Edes=5 and 107 kcal/mol, respectively.

γ-Alumina whiskers were grown from copper doped alumina gel solution at room temperature. X-ray diffraction measurement indicate a cubic structure in agreement with our earlier results on powder.

The thermodesorption of oxygen from 0.1–5 wt.% Pd/Al2O3 catalysts has been studied. After gas adsorption at 300°C four types of oxygen complexes have been found with Edes equal to 5.7–7.7, 45.2, 67.6–71.2 and 71.2 kcal/mol, respectively.

The mechanism of interaction between molybdenum(VI) as aqueous solutions of ammonium paramolybdate and γ-alumina, has been studied in transient conditions by pulse technique, using combined differential refractive index and u.v. absorption detectors. In acidic environment, molybdenum(VI) species are irreversibly adsorbed through an anionic exchange mechanism with liberation of ammonium salts. In a neutral environment, molybdenum(VI) species are only partially irreversibly adsorbed onto γ-alumina through a protonic transfer mechanism with liberation of basic species. The depletion of H3O+ due to adsorption of molybdenum(VI) from neutral solution, provides an explanation of the observed mutual enhancement of cobalt(II) and molybdenum(VI) adsorption on alumina. The general result presented in this paper can be summarized by stating that the amount of irreversibly adsorbed molybdenum(VI) is related to the amount of available basic sites present onto alumina. Untersuchung der Wechselwirkung von Molybdan(VI)-Ionen mit γ-Aluminiumoxid durch Pulstechnik Der Mechanismus der Wechselwirkung von Molybdan(VI), in Form wasriger Ammoniumparamolybdat-Losungen, mit γ-Aluminiumoxid wurde mittels der Pulstechnik unter kombinierter Anwendung des differentiellen Brechungsindexes und der UV-Absorption untersucht. In saurer Umgebung wird MoVI irreversibel nach einem anionischen Austauschmechanismus und Freisetzung von Ammoniumsalzen adsorbiert. In neutraler Umgebung wird MoVI nur teilweise irreversibel am γ-Al2O3 nach einem Protonen-Transfermechanismus und unter Freisetzung basischer Spezies adsorbiert. Der Verbrauch von H3O+ auf Grund der Adsorption von MoVI aus neutraler Losung gibt eine Erklarung fur die beobachtete gemeinsame Verstarkung der Kobalt(II) und Molybdan(VI)-Adsorption an γ-Al2O3. Das allgemeine Ergebnis ist, das die Menge des irreversible adsorbierten MoVI in Beziehung zur Menge der verfugbaren basischen Zentren am Aluminiumoxid steht.

Transport characteristics of four porous samples with bidisperse or broad monodisperse pore structure were determined by combination of diffusion and permeation measurements with simple gases and compared with results obtained from diffusion of toluene or α,α,α-trifluorotoluene in cyclohexane in liquid phase. From comparison of both types of results it followed that all pores are decisive for the rate of diffusional transport in liquids, whereas only the wide transport pores are significant in gas diffusion.