Γ-alumina Surface Research Articles

Sodium dodecyl sulfate-coated alumina and C18 cartridge for the separation and preconcentration of cationic surfactants prior to their quantitation by spectrophotometric method

A new solid phase extraction method has been developed to the separation and preconcentration of trace amounts of cationic surfactants i.e. dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC). This method is based on the sorption of anionic surfactant (AS-) of sodium dodecyl sulfate (SDS) on the γ-alumina surface and C18 cartridge is used for preconcentration of cationic surfactants (CS+). Spectrophotometric determination method used for the determination of cationic surfactants is based on the competition between the cationic dye of methylene blue (MB+) and CS+ for association complex formation with SDS. The MB+in the ion associate complex can be quantitatively substituted by CS+ leading to an increase in the absorbance measured at 662 nm. Optimum experimental conditions for separation, preconcentration and determination of cationic surfactants have been reported. Under the selected optimal conditions, preconcentration factor of two-fold was achieved and the results showed that determination of CPC, DTAB and CTAB could be performed in concentration ranges of 1×10-5-2×10-4, 4×10-5-5×10-4 and 5×10-5-5×10-4 mol L-1, respectively. Eleven determinations of 10.0 mg mL-1 of CPC, DTAB and CTAB solutions led to RSD's of 4.3, 3.7 and 4.0%, respectively. Method was validated using a set of different samples containing cationic surfactants.

Decatungstate catalyst supported on silica and γ-alumina: Efficient photocatalytic oxidation of benzyl alcohols

Four supported catalysts with the same tungsten loading were prepared by depositing decatungstate species W 10O 4− 32, through wet impregnation, on the surface of γ-alumina and silica at different pH values. The prepared samples were characterized using BET measurements as well as XRD, UV–vis DR, and XP spectroscopies. Higher dispersion of W(VI) oxo-species was obtained in the silica-supported catalysts compared with the corresponding alumina-supported ones. Within the same support, the dispersion was higher when the impregnation pH is lower than the point of zero charge (pzc) of the support. The decatungstate anions were present mainly on the silica surface without any modification, whereas these underwent a partial depolymerization on their deposition on the γ-alumina surface. The extent of depolymerization was less in the sample prepared at pH above pzc. These findings were explained in terms of the mode of deposition of the W(VI) species from the solution onto the support surface. The photocatalytic activity of the aforementioned catalysts, concerning the photooxidation of 1-phenylethanol, depends on the fraction of the W 10O 4− 32 supported species rather than on the W(VI) dispersion. Thus, extremely high conversions have been obtained over the silica-based catalysts and also over the γ-alumina-based catalyst prepared at relatively high pH. These catalysts also are very effective in the photooxidation of a series of secondary and primary benzyl alcohols, in which benzyl ketones and benzoic acids were formed as the only or major products, respectively. The easy separation of the solid catalyst from the reaction mixture, the high activity, selectivity, and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis. Based on product analysis and kinetic data on the heterogeneous oxidation of benzyl alcohols, we suggest that a hydrogen abstraction transfer (HAT) mechanism predominates with respect to an electron transfer (ET) one in these reactions.

Interactions of methane, ethane and pentane with the (110C) surface of γ-alumina

Adsorptions of methane, ethane and pentane on the γ-alumina (110C) surface are investigated with semi-empirical (PM3) cluster calculations. It is found that the abstraction of an H atom accompanied by the formation of a C–O bond is the most favorable reaction for methane on the alumina surface. For ethane– and pentane–alumina interactions, the abstraction of two H atoms accompanied by the formation of an alkene is the most favorable reaction. The surface Al atoms help to promote the reactions, but are not directly involved in the bond formation.

Interplay between molecular adsorption and metal–support interaction for small supported metal clusters: CO and C 2H 4 adsorption on [formula omitted]-Al 2O 3

The adsorption of CO and C 2H 4 molecules was investigated on Pd 4 clusters supported on two relevant models of γ-alumina surfaces, the nonhydroxylated (100) surface and the hydroxylated (110) surface, by means of density functional theory (DFT) calculations using periodic boundary conditions. The hollow η 3 mode is the most stable geometry for CO adsorption whatever the γ-alumina surface, whereas the preferred mode for ethylene depends on the support's surface. In particular, the π mode of ethylene is stabilized when the Pd 4 cluster is deposited on the hydroxylated (110) surface. The calculations demonstrate an important interplay between molecular adsorption and cluster–oxide interaction. The deposited cluster is not a frozen object, but it responds to the molecular adsorption by relaxing its geometry and changing its bonding with the oxide support. In a counterintuitive manner, molecular adsorption on the particle reinforces its interaction with the oxide in several cases. An energy decomposition scheme is proposed to give insight into the energy trends observed. Two key energetic contributions are put forward: substrate relaxation and metal–support interaction energies. A detailed electronic analysis before and after adsorption is proposed. The combined effect of the surface ligand groups (O and Al, or OH depending on the hydration state) and of the molecular adsorbate can produce specific synergy from electronic effects between an electron donor surface O atom and an electron acceptor CO adsorbate.

Possible Dual-Charge-Carrier Mechanism of Surface Conduction on γ-Alumina

We cast a dual-charge-carrier model of surface conductance on γ-alumina in mathematical form. We then carry out first-principles calculations for various possible atomic-scale structures of the low- and high-temperature charge-carrier interactions with the γ-alumina surface to estimate the values of the energy parameters in the dual-charge-carrier model. By comparing the values of these energy parameters as determined by first-principles calculations to those obtained by fitting the mathematical form of the dual-charge-carrier model to experimental data, new insight is gained into the nature of the charge-carrier species. The results support the hypothesis that the intrinsic hydrogen content and surface moisture of γ-alumina provide a possible explanation of the observed thermal dependence of surface conductance.

Oxidative thermolysis of Mn(acac) 3 on the surface of γ-alumina support

Precipitated γ-alumina support was decorated with Mn(acac) 3 by incipient wetness impregnation with toluene solutions containing Mn(acac) 3 in amount equivalent to loading of 0.35, 0.74, 1.38, 2.38 and 3.50 Mn(acac) 3 moleculs per nm 2 of the support. In order to evaluate the mechanism of Mn(acac) 3 interaction with the surface of γ-alumina support and subsequent transformations of the supported Mn(acac) 3 species, oxidative thermolysis of Mn(acac) 3/Al 2O 3 samples in air was studied by diffuse reflectance FTIR, thermogravimetric analysis (TG/DTG), differential thermal analysis (DTA) and XRD. It has been found out that decoration of γ-Al 2O 3 support with Mn(acac) 3 results in the formation of surface bound Mn(acac) 3− x species when Mn(acac) 3 loading does not exceed 1.38 Mn(acac) 3/nm 2. At higher Mn(acac) 3 loading the formation of the supported bulk-like Mn(acac) 3 species also occurs. The interaction of Mn(acac) 3 molecules with the support surface occurs via substitution of acetylacetonate ligand(s) with the oxygen atom of surface hydroxyl group(s) accompanied by elimination of acetylacetone molecules. The evolved acetylacetone reacts with the alumina surface that results in the formation of surface Al(acac) 3− x species. The oxidative thermolysis of Mn(acac) 3− x species on the surface of γ-alumina proceeds via partial elimination of acetylacetonate ligands and partial oxidation of the remaining ligands without destruction of their cyclic structure within 425–550 K. The complete oxidative destruction of acetylacetonate ligands takes place within 600–700 K and results in the formation of manganese oxide species on the alumina surface. The dispersed surface manganese oxide species originate upon the oxidative thermolysis of the surface bound Mn(acac) 3− x species while crystalline Mn 2O 3 phase results from the supported bulk-like Mn(acac) 3 species.

Quantum chemical and vibrational investigation of sodium exchanged γ-alumina surfaces

The sodium cation is well known as an efficient poison of gamma-alumina surface acidity. This poisoning effect has been revealed both by characterization methods and catalytic tests. In this work, we propose an accurate model of sodium exchanged gamma-alumina surfaces. On realistic models of hydroxylated gamma-alumina surfaces, the location of sodium cation is determined by the use of density functional theory (DFT) methods. For the (100) and (110) surfaces of gamma-alumina, the sodium cation is found in a solvated state within an inner solvation sphere complex. Its coordination sphere is constituted by O-mu(2), O-mu(3) and HO-mu(1) surface groups. The stretching frequency of these HO-mu(1) groups is shifted, leading to the appearance of a new band predicted and observed at about 3754 cm(-1) on the IR spectrum.

Cm(III) sorption onto γ-Al2O3: New insight into sorption mechanisms by time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure

Sorption of trivalent Cm onto γ-Al2O3is investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The pH dependent sorption of Cm(III) is followed at constant ionic strength of 0.1 M NaClO4under argon atmosphere in the pH range between 3.5 and 13.2 at a total Cm(III) concentration of 2.5×10-7mol/L. Three different surface-sorbed Cm(III) species appearing at different pH are identified by their emission spectra with peak maxima at 600.6 nm, 602.5 nm, and 605.7 nm using an excitation wavelength of λex=396.6 nm. No variation of the emission decay is observed for the three sorbed species (constant lifetimes of 110 μs). The pH dependent species distribution obtained from peak deconvolution of emission spectra is in agreement with earlier Eu/Cm sorption studies and sorbed species are assigned to >Al−O−Cm(OH)x(2-x)(H2O)5-xinner-sphere complexes. Indirect Cm excitationviaenergy transfer from the solid to the metal ion at λex=355 nm yields emission bands significantly shifted compared to spectra obtained at λex=396.6 nm. We interpret this observation as a hint for the existence of Cm inner-sphere complexes bound to different γ-Al2O3surface sites. The exact nature of those species is presently not known, but they likely involve Cm interaction with Al(OH)3phases at the solid surface which may even dominate the solid surface chemistry. Complementary EXAFS measurements are performed with Lu and Gd, taken as chemical homologues of the trivalent actinides. EXAFS for Gd/Lu(III) sorbed onto γ-Al2O3at pH=6 identifies a single oxygen coordination shell with a high degree of disorder. For Gd this shell consists of 9±1.5 atoms at a 2.45±0.02 Å distance. A coordination number of 7±1.5 and a Lu−O distance of 2.28±0.02 Å is found in the Lu-samples. Those values are quite close to what is known for the respective aquo ions. The large asymmetry in the coordination shell, however, suggests inner-sphere complexation. This is in agreement with TRLFS results showing clearly the existence of inner-sphere surface complexes where the first coordination shell of the trivalent metal ions consists of 5H2O/OH-entities. Together with EXAFS data, a metal ion coordination to four oxygen atoms belonging to the γ-alumina surface is concluded. Both spectroscopic studies indicate a certain variability of surface binding sites for trivalent metal ions.

γ-Alumina-supported boron trifluoride: Catalysis, radiotracer studies and computations

The irreversible adsorption of boron trifluoride on calcined γ-alumina and amorphous chromia, in both cases at room temperature, has been studied using [ 18F]-labelled BF 3. Although the resulting γ-alumina surface has some catalytic activity for the room temperature fluorination by anhydrous HF of CH 3CCl 3 under static conditions, its activity is far lower than that of γ-alumina, which has been fluorinated with SF 4, nominally at room temperature. A possible explanation for the observed behaviour is given.

Stabilization of gamma alumina slurry for chemical–mechanical polishing of copper

Stabilization of γ-alumina suspension for chemical–mechanical polishing (CMP) of copper was investigated. Citric acid and poly(acrylic acid) (PAA) ( M w = 5000 ) were used as dispersant. The stability of suspension was evaluated from the changes in viscosity, particle size and zeta potential. It appears that metastable γ-alumina mainly due to its high specific surface area and to the presence of aluminol groups on its surface is progressively transformed to bayerite ( β-Al(OH) 3) by hydration procedure. Citric acid molecules were adsorbed onto γ-alumina surface effectively and exhibited the excellent hydration inhibition effect. Although citrate–alumina surface complexes give barrier to the flocculation, the repulsion potential is based mainly on the electrostatic repulsion, thereby steric hindrance caused by the adsorption of these small molecules is very weak. The electrosteric repulsion, which provides more effective dispersion stability than electrostatic repulsion force, can be expected by using polyelectrolyte such as PAA; however, adsorbed layers of PAA onto solid/liquid interface are loosely formed. Therefore, a large amount of PAA was required to inhibit the surface hydration of γ-alumina suspension, thereby the excess addition of PAA decreased the electrosteric repulsion and re-bridging of the dispersant between particles caused an increase in suspension viscosity. Therefore, synergistic effect can be expected in mixed dispersant system of citric acid and PAA, since small citric acid molecules are adsorbed faster than PAA, inhibiting the progress of surface hydration, and then adsorbed PAA layers exhibit the effective electrosteric repulsion interaction between particles with a small amount compared with PAA alone. It was revealed that the γ-alumina slurry dispersed by mixed dispersant exhibited the improved removal rate of Cu layer by CMP polishing test.

Adsorption of 1-hexene on γ-alumina (110C)

Adsorption of 1-hexene on the γ-alumina (110C) surface is investigated with semi-empirical (PM3) cluster calculations. It is found that on the Al–O terminated surface, H-abstraction to surface oxygen from the hexene allylic position is the most favorable reaction, and is facilitated by C–Al interaction. Hexene interactions with surface aluminum atoms are purely repulsive. Except for the pure H-abstraction, chemisorption occurs through interactions of C and H with surface oxygen atoms, which is typically an endothermic process but is most favorable when an H is abstracted from the hexene allylic position accompanied by the formation of a C–O bond, and becomes exothermic when there is an associated transfer of a surface H. On the oxygen-terminated surface, numerous different types of H atoms on hexene can be abstracted by surface oxygen when they come sufficiently close to the surface and these reactions are exothermic. The energy barriers to these different H-abstraction processes are all in the range 10–15 kcal/mol, with those for the dehydrogenation of a terminal H farthest from the double bond or an allylic H being slightly lower than those for abstracting H from other positions. The comparable energy barriers for abstraction of numerous different kinds of H provide a possible explanation for the experimentally observed plethora of products.

Synthesis of Highly Porous Aluminas Mediated by Cationic Surfactant: Structural and Textural Properties

The cationic surfactant cetyltrimetylammonium bromide was used to synthesize mesostructured γ–Al2O3. The effects of the surfactant concentration and of the sol aging (at 95 °C for 24 h) were studied by x-ray powder diffraction, nuclear magnetic resonance, transmission electron microscopy, and analysis of the low-temperature nitrogen adsorption-desorption isotherms. Mesostructured alumina with wormhole morphology and amorphous walls was obtained through the precipitation by ammonium hydroxide of a 0.1 M aluminum nitrate aqueous solution in presence of 0.1 M surfactant. The pore size was smaller than 5 nm. After digesting the milky suspension under atmospheric pressure at 95 °C, a crystallized boehmite-surfactant phase, with fiber morphology, is formed which at 550 and 700 °C is transformed into a highly porous γ−Al2O3. A similar evolution was observed using 0.01 M CTAB solution and aging. Pore volume up to 1.1 cm3/g and pore size up to 16 nm were obtained. Without surfactant, the same aging treatment led to aggregated fibers: the pore size is less than 8 nm and the pore volume is smaller than 0.6 cm3/g. The γ-alumina surface area is determined mainly by the organization generated by the surfactant and to a lesser extent by the boehmite precursor particle size. From the point of view of catalyst preparation, the surfactant at the concentration of 0.01 M in 0.1 M aluminum nitrate and the aging treatment in solution play a beneficial role.

Interaction of Co6 cluster with γ-alumina surface: A quantum chemical study

The interaction of Co6 cluster with partially dehydroxylated γ-alumina surface was studied by the DFT method. Hydrogen atoms of surface hydroxyl groups can be transferred to the metal particle to form partially oxidized cobalt states. The energy characteristics of hydrogen transfer were determined and changes in the electronic structure of supported Co6 particles were characterized.

Cobalt oxide/γ-alumina catalysts prepared by equilibrium deposition filtration: The influence of the initial cobalt concentration on the structure of the oxide phase and the activity for complete benzene oxidation

In the present work we studied the influence of the initial concentration of the impregnating solution used for mounting Co(II) species on the γ-alumina surface by equilibrium deposition filtration method (edf) on the physicochemical properties and the catalytic activity of the “cobalt oxide”/γ-alumina catalysts. The complete oxidation of benzene has been used as a model reaction. Two series of catalysts (edf-X-A and edf-X-B) of varying Co content (X: up to 21 wt.% Co) were prepared using the above-mentioned method and tested at various temperatures in the range 200–300 °C using a fixed-bed reactor. In the first series (A) various Co loadings were obtained by varying the initial Co(II) concentration of the impregnating solution. In the second series (B) the corresponding Co loadings were obtained by using the impregnating solution used for the preparation of the catalyst of A series with the maximum Co(II) content and varying the impregnation time. All catalysts were characterized using various techniques, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), X-ray powder analysis (XRD), nitrogen adsorption (BET) and temperature-programmed reduction (TPR). It was found that the initial Co(II) concentration of the impregnating solution used for depositing the corresponding species on the γ-alumina surface by edf influences the catalytic activity of the “cobalt oxide”/γ-alumina catalysts with respect to the complete oxidation of benzene. The increase of the initial Co(II) concentration of the impregnating solution brings about a change in the composition of the deposited phase formed in the impregnation step by decreasing the ratio “Co(II) surface inner-sphere complexes/surface Co(II) precipitates”. Upon calcination, the Co(II) surface inner-sphere complexes are transformed to well-dispersed “cobalt oxide” phase strongly interacting with the support surface while the surface Co(II) precipitates are transformed to Co 3O 4 crystallites loosely interacting with the support surface. The former phase is responsible for the relatively high dispersion of Co observed in the A series of catalysts but it is rather inactive, while the latter exhibits lower dispersion but higher activity. Thus, the edf catalysts prepared using high initial Co(II) concentration exhibited higher catalytic activity than the corresponding ones prepared using low initial Co(II) concentration.

Kinetics of Adsorption of the Cobalt Ions on the “Electrolytic Solution/γ-Alumina” Interface

In the present work we studied, for the first time, the kinetics of adsorption of the Co(H(2)O)(6)(2+) species on the "electrolytic solution/gamma-Al(2)O(3)" interface at pH = 7 and 25 degrees C for a very broad range of Co(II) surface concentrations ranged from 0.03 to 6 theoretical Co(H(2)O)(6)(2+) surface layers. Moreover, we studied the surface dissolution of gamma-alumina in the presence of the Co(H(2)O)(6)(2+) ions in the impregnating solution, the contribution of the Co(II) desorption on the whole deposition process and the deposition isotherm. It was found that under the conditions where the deposition has taken place, the dissolution of the gamma-alumina surface is negligible even in the presence of the Co(H(2)O)(6)(2+) species in the impregnating solution. It was, moreover, inferred that the Co(II) desorption does not participate significantly to the whole deposition process. It was found that the deposition kinetics may be described by the following kinetic expression r(Co,bulk) = k'C(Co,bulk)(2), which relates the rate of disappearance of the Co(H(2)O)(6)(2+) ions from the impregnating solution, r(Co,bulk,) with their concentration C(Co,bulk). This kinetic expression may be derived assuming the following deposition scheme: nS + 2[Co(H(2)O)(6)(2+)] --> S(n) - [Co(H(2)O)(x,x)(<)(6)(2+)](2), where S represents the surface reception sites. The above expressions indicated that two Co(H(2)O)(6)(2+) ions are involved, from the side of the interface, in the reaction with the reception sites. It seems probable that the deposition step involves the simultaneous adsorption and dimerization of the two interfacial Co(H(2)O)(6)(2+) ions through (hydr)oxobridges. On the other hand, the sigmoidal form of the deposition isotherm and the dependence of the apparent rate constant, k', on the interfacial Co(II) concentration suggested that the already deposited Co(II) species may be involved in the reception sites, S, promoting the adsorption and resulting to the formation of multinuclear complexes and Co(II) surface precipitates. Finally, reasonable interface potential values for oxides were determined for the first time using kinetic results.

Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation

In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.Three series of catalysts of varying Co content (up to 21wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–pvi≫pvi≫edf, nta–pvi≫edf≈pvi and edf>nta–pvi>pvi.The catalysts prepared were characterized using various techniques (BET, UV–vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on Co3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned Co3O4 crystallites: the ratio ‘amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step’ closely related with the ratio ‘amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step’ as well as the composition of the precipitated phase.The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)62+·2NO3−. Upon calcination it is transformed into loosely bounded Co3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts.The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800°C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content.The application of the nta–pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(II)–nta]− and [Co(II)–2nta]4− complexes, in which the Co(H2O)62+ complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta–pvi catalysts a precipitated phase containing the [Co(II)–nta]−·NH4+(or H+) and [Co(II)–2nta]4−·4NH4+ (or 4H+) complex salts predominates. Upon calcination these are transformed into Co3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity.

Ionic conduction in LiI–α,γ-alumina: molecular dynamics study

Molecular dynamics simulations were performed on LiI–α,γ-alumina. The LiI–α,γ-alumina systems were constructed by sandwiching a LiI crystal between respectively two α- and two γ-alumina crystals. Two interfaces were formed by the LiI (001), (002) and the α-alumina (0001) and γ-alumina (001) planes. Structural, vibrational and transport properties of the Li + ions in the bulk as well as at the interfaces were investigated. A fraction of Li + ions migrated towards the alumina surfaces occupying tetrahedral sites near the interfaces. An increase in the ionic mobility was observed which was mainly caused by the diffusion along the interfaces. The density of Li + ions near the α-alumina surfaces was larger than at the γ-alumina surfaces. The charged layer composed by the Li + ions on the alumina surfaces results in a net enhanced electric field perpendicular to the interfaces. The migration of Li + ions to the alumina surfaces leads to a higher concentration of vacancies in the LiI crystal and gives therefore an increased ionic mobility. The mechanism of diffusion was the same for the LiI–α-alumina interface as well as the LiI–γ-alumina interface. This explains the fact that the activation energy was the same, 0.38 eV. The relatively larger fraction of Li + ions which has moved to the α-alumina surfaces leads to a higher pre-exponential factor for the LiI–α-alumina interfaces than for the LiI–γ-alumina interfaces.

Factors responsible for the activity of alumina surfaces in the catalytic epoxidation of cis-cyclooctene with aqueous H 2O 2

The structural, textural, and chemical properties as well as the hydrophilicity of four different sol–gel aluminas were determined by different methods and compared with their activity in the catalytic epoxidation of cis-cyclooctene with aqueous H 2O 2. It was found that the hydrophilicity of the γ-alumina surface is the principal factor responsible for the difference in catalytic activity. The hydrophilicity of γ-alumina should be sufficient to guarantee a fast reaction with hydrogen peroxide on the surface but should not to be too high in order to allow the approach of olefin to the active sites.

Use of DFT to achieve a rational understanding of acid–basic properties of γ-alumina surfaces

In a recent priority communication [M. Digne et al., J. Catal. 211 (2002) 1], we proposed the first ab initio constructed models of γ-alumina surfaces. Using the same density-functional approach, we investigate in further detail the acid–basic properties of the three relevant γ-alumina (100), (110), and (111) surfaces, taking into account the temperature-dependent hydroxyl surface coverages. The simulations, compared fruitfully with many available experimental data, enable us to solve the challenging assignment of the OH-stretching frequencies, as obtained from infrared (IR) spectroscopy. The precise nature of the acid surface sites (concentrations and strengths) is also determined. The acid strengths are quantified by simulating the adsorption of relevant probe molecules such as CO and pyridine in correlation with surface electronic properties. These results seriously challenge the historical model of a defective spinel for γ-alumina and establish the basis for a more rigorous description of the acid–basic properties of γ-alumina.

The heterogeneous reaction of HNO3on mineral dust and γ-alumina surfaces: a combined Knudsen cell and DRIFTS study

The kinetics of the interaction of HNO3 with natural mineral dusts (Saharan, Chinese and Arizona dust) and γ-alumina surfaces has been studied at T = 298 K using a Knudsen cell reactor coupled to a mass spectrometer for kinetic studies on the gas uptake as well as DRIFT spectroscopy for studies of the surface products. The uptake of HNO3 on each surface was found to be fast with initial uptake coefficients of γ ≈ 0.1. Nitrate and water have been identified as reaction products. The formation rate of surface nitrate, however, was slower with uptake coefficients ranging from 8 × 10−3 < γ < 5.4 × 10−2 depending on the chemical nature of the surface. From these results a two-step mechanism for the reactive uptake of HNO3 onto mineral oxides, consisting of fast adsorption of gas-phase HNO3 followed by a slower surface reaction of HNO3(ads) with surface OH-groups is inferred. Furthermore, it was found that the reactivity of mineral dust towards HNO3 decreased with increasing exposure. However, the original reactivity could be recovered by water exposure and may therefore be conserved under atmospheric conditions.