Surface Properties Of Alumina Research Articles

Structure evolution of porous alumina during aging process and its support effect on PtSn/Al2O3 catalyzed propane dehydrogenation

Precise control of porous alumina microstructures is always challenging due to the diversity and complexity of the growth process-related alumina crystalline phase and surface properties. In this work, we established a bottom-up synthesis route with a combination of rapid precipitation at high supersaturation and high-temperature aging in terms of enhancing particle aggregation kinetics. Surprisingly, we discovered that there were two distinct growth stages of alumina precursor (ammonium aluminum carbonate hydroxide) during the aging procedure, that is, oriented attachment at the early stage of crystallization and subsequent Oswald ripening. High-temperature aging can effectively strengthen the aggregation kinetics between the particles and produce large mesoporous (∼16 nm) and a remarkable number of defect sites with the exposure of up to 14 % unsaturated pentacoordinate aluminum ions. Owing to the presence of strong metal-support interactions, the alumina-supported PtSn catalyst exhibited excellent catalytic activity in the propane dehydrogenation reaction with propylene formation with 99 % selectivity and superior stability with a low deactivation rate of 0.007 h−1 over 24 h. These results suggested that it was feasible and instructive to achieve precise control of the structure and surface properties of alumina by manipulating the aggregation process of the initial particles during the aging stage.

Effects of alumina phases on the structure and performance of VOx/Al2O3 catalysts in non-oxidative propane dehydrogenation

Abstract Supported vanadium oxides are promising in propane non-oxidative dehydrogenation and the activity of which shows clear support effects. The present study reports the effects of alumina polymorphs on the structure and performances of VOx-based catalysts. VOx/Al2O3 catalysts with the same V surface density were prepared over four different alumina supports (γ-Al2O3, δ-Al2O3, θ-Al2O3, θ,α-Al2O3) obtained by varying the calcination temperature of boehmite. The alumina calcination temperature was shown to affect the surface acid properties and VOx-support interactions and make it possible to tune the activity and coking behavior of catalysts. Weakening the VOx-support interactions due to the change in alumina surface properties was found critical to improve the intrinsic activity of catalysts. The coking behavior of catalysts was shown to depend on both total acid sites and VOx polymerization degree. This study may provide basic principles to tune the performance of VOx-based catalysts for non-oxidative dehydrogenation.

Effect of heat treatment on the interfacial bonding between SiC coating and alumina plate

ABSTRACTIn this paper, the SiC coating was deposited on an alumina substrate by a radio frequency magnetron sputtering method using a-SiC (99.999%) target. The produced SiC–Al2O3 structure was heat treated in air through a range of temperatures. The effects of heat treatment temperature and duration on the surface roughness, interfacial bonding and surface hardness were investigated using experimental means such as optical surface profilometer, scanning electron microscope, energy dispersive X-ray analysis and hardness analyser. The results indicated that the sputtered SiC coating is an effective approach to alter the surface properties of alumina, through significantly increasing the surface roughness and hardness. Furthermore, the heat treatment did not result in decomposition of the surface coating. Appropriate heat treatment was found to improve the interfacial bonding between the sputtering material and substrate via enhanced species diffusion.

Features of Surface Structures of Alumina and Titanium Dioxide Nanoparticles Produced Using Different Synthesis Methods

A comparative study of the scope and surface properties of alumina (Al2O3) and titanium dioxide (TiO2) nanoparticles, synthesized using different methods, was carried out using Fourier-transform infrared spectroscopy (FTIR), ultraviolet UV-Vis diffuse reflection spectroscopy (UV-Vis DRS), and Raman spectroscopy, as well as X-ray diffraction methods. It is shown that the differences in the synthesis methods can change the surface properties of the nanoparticles, while maintaining the phase composition of the material. The nanoparticles of each material are shown to exhibit unexpected properties. In particular, the special luminescence characteristics of TiO2, a photon-energy shift from the rutile region into that region typical for the anatase, and a significant difference in the Lewis center concentration values for the alumina γ-phase were observed. This variation in the properties indicates the necessity to involve a wider range of analysis techniques and the importance of precisely characterizing the surface properties. To identify those nanoparticle functional properties that determine their interactions with other materials, a comprehensive study of their phase compositions and surface properties must be completed.

Influence of pH on the Stability of Alumina and Silica Nanosuspension Produced by Wet Grinding

Effects of suspension pH on stability and surface properties of alumina and silica nanosuspension were investigated. Nanosuspensions of alumina and silica used in the present study were produced by wet grinding in stirred media mill. The electrostatic stabilization of two types of nanosuspension was conducted by zeta potential and turbiscan analysis as a function of pH. The long-term stability analysis of alumina and silica nanosuspension was investigated using an optical analyzer. It was observed that particles gradually aggregated and settled with storage time and can be easily redispersed by mere shaking of suspensions under desirable pH conditions. Transmission electron microscopy images obtained at different suspension pH for both alumina and silica nanoparticles show that the state of aggregation is highly dependent on pH and found to affect particles morphology.

Modifying the functional cover of the γ-Al2O3 surface using organic salts of aluminum

A method is suggested for modifying the surface properties of alumina without changing its chemical composition. The sorption of aluminum complexes with organic acid ligands on the γ-Al2O3 surface is reported. The thermal decomposition of the adsorbed oxalate complexes yields supported aluminum oxide compounds on the surface of the initial support. This modifies the functional cover of the γ-Al2O3 surface, altering the proportions of different types of surface hydroxyl groups, reducing their total number, and lowering the concentration of weak Lewis acid sites.

SIZE OF BOEHMITE NANOPARTICLES BY TEM IMAGES ANALYSIS

Transition aluminas, and especially the gamma type, are largely used as catalyst supports in refining and petrochemicals. Most studies focus on properties resulting from material texture and casting (specific surface, porous volume, pore shape and diameter). However, surface properties of alumina should be considered as well, as the catalytic activity is tightly related to the structure of exposed crystalline faces. As γ alumina results from controlled thermal treatment of boehmite γ-AlOOH by a topotactic transformation, the nature of exposed crystalline planes is related to the starting material. Therefore, the synthesis of the oxihydroxide γ-AlOOH, and especially size and shape of these particles, is critical in determining the relevant surface properties. Unlike often aggregated alumina, boehmite nanoparticles can be observed by TEM. Analysis of these TEM images can be performed to estimate the size of the boehmite nanoparticles. Information about morphology of the nanoparticles is obtained by the analysis of the covariance, modeling micrographs by a dilution model.

Laser gas assisted nitriding of alumina surfaces

The laser control melting can be applied to improve the structural and tribological properties of alumina surfaces. In this case, the laser remelting provides a homogeneous structure while the assisting gas diffuses into molten layer forming the nitride compounds in the irradiated region. Since the process is rapid, local heating with controllable depth and cost effective, the laser remelting provides several advantages over the conventional surface treatment techniques. In the present study, laser melting and gas assisted nitriding of alumina pellets are carried out. Morphological and metallurgical changes after the laser treatment process are examined using scanning electron microscopy (SEM), optical microscopy and X-ray diffraction (XRD), and indentation tests. It is found that two regions are formed in the laser irradiated zone. The first region below the surface is dense and composes of α-Al2O3 and AlN while in the second region, which is below the first region, randomly stacked lamellae structure is observed.

Gelification process to prepare phosphate modified alumina: Study on structure and surface properties

P-modified alumina was prepared through gelification with phosphate compounds introduced during the formation of alumina gel. Common characterizations such as XRD, BET and IR were employed to investigate the effect of phosphate dopant on the structure and surface properties of alumina. The results showed that the addition of phosphate could effectively improve thermal stability and textural properties of alumina. For G 5 sample, the α-phase formation temperature is enhanced up to 1573 K and the surface area retains 85 m 2 g −1 as well as the pore volume 0.356 cm 3 g −1 after calcination at 1473 K for 3 h. Moreover, the surface area and pore volume both have increasing trends with the increasing phosphorous contents in alumina after calcinations at the same temperature. There is also an increasing trend of lattice constants of P-modified aluminas with the increase of phosphorous content in alumina. Considering the defective spinel structure of γ-alumina, it can thus be deduced that some of the P 5+ ions might occupy the vacancy of cationic site and interrupted the track of Al 3+ ions transfer from tetrahedral to octahedral in the processes of sintering and transformation.

Determination of intrinsic equilibrium constants at alumina/electrolyte interface

Intrinsic ionization and complexation constants at an alumina/electrolyte interface were studied by the site binding model, while the sorption of alkali cations from aqueous solutions was interpreted by the triple-layer model. The surface properties of alumina were investigated by the potentiometric acid-base titrationmethod. The point of zero charge (pHpzc) of alumina obtained by thismethod was found to be 7.2. The obtained mean values of the intrinsic protonation and ionization constants of the surface hydroxyl groups and the intrinsic surface complexation constant, in different electrolytes, are pKinta1 = 4.4, pKinta2 = 9.6 and pKintM+ = 9.5 respectively.

Physicochemical and surface properties of alumina modified with rare earth oxides, III. Dispersion of supported platinum

The effect of various amounts of lanthanides on the dispersion of Pt on Al2O3 — Ln2O3 (where Ln = La, Ce) has been studied by hydrogen chemisorption in a pulse chromatographic system.

Semi-empirical methods as a tool in solid-state chemistry

A new method for performing periodic calculations using the MNDO, AM1 and PM3 semi-empirical methods, with treatment of the long range interactions by lattice summation techniques, has been devised and implemented. The performance of the existing parametrisations are examined for the bulk and surface properties of alumina, the prediction of the relative stability of silica polymorphs, and for the modelling of ice crystals. While the results for the more ionic materials suggest that some reparametrisation will be needed for solid-state problems, the method shows promise for molecular crystals, particularly those where hydrogen bonding is the dominant interaction.

Effect of ammonium salts on alumina surface properties

Aluminium hydroxides were precipitated from sodium aluminate using ammonium salt solutions. The effect of the kind of ammonium salt used for the aluminium hydroxide precipitation on the surface properties of the related alumina was investigated. It was stated that kind of ammonium salt affects the aluminium hydroxide phase composition as well as the Lewis acid site surface concentration and the concentration of anionic OH − groups; the specific acidity was measured by means of TPD NH 3 and the specific rate of cyclohexene isomerization. DRS spectra of the Mo catalyst supported on investigated aluminas as well as the HDS activity vary with the kind of ammonium salt. The obtained results are explained on the basis of processes both during polycondensation of aquo-ions Al III and during ageing of amorphous hydrogel of aluminium hydroxide. Results are discussed on the basis of the literature.

Studies of physicochemical and surface properties of alumina modified with rare earth oxides

Results are presented of investigations of the porous structure of the Al2O3-La2O3 systems containing up to 10 wt% of the La modifying oxide prepared by the coprecipitation method. Pore size distribution was determined on the basis of low temperature nitrogen adsorption measurements and mercury intrusion porosimetry. The changes in pore size distribution of different forms of Al2O3-La2O3 are discussed in relation to the concentration of the lanthanum oxide.

Effects of Titanium Doping on Surface Properties of Alumina

AbstractControlled-geometry pore structures were introduced into undoped and Ti-doped sapphire using microfabrication techniques, and subsequently transferred to internal sapphire-sapphire and sapphire-polycrystalline alumina interfaces via hot pressing. The high-temperature evolution of several different types of structures was examined, in order to isolate different surface properties and evolution processes which are of interest during sintering. A study of pore channel breakup showed that Ti strongly stabilizes channels oriented in the sapphire [1100] direction, suggesting a significant alteration of the Wulff plot. In another study, the equilibration rate of isolated pores was enhanced in Ti-doped sapphire; as glassy phases are unlikely in this material, a solid-state diffusion mechanism for this enhancement is suggested. The evolution of pore channels with pre-existent perturbations was also studied, and the results of these studies are presented.

Electronic, vibrational and chemical properties of alumina surfaces

Electronic, vibrational and chemical properties of alumina surfaces

Characterization of alumina modified with sulfate and phosphate

The acidity and surface properties of alumina modified with sulfate (A1-S) and phosphate (Al-P) were characterized by IR, NH3TPD and 1-butene isomerization reaction. The Point of Zero Charge (PZC) of modified alumina and the amount of Mo anion adsorbed on modified alumina were measured. The PZC of modified alumina and the amount of Mo anion adsorbed on modified alumina decreased with increasing modifier content. Non-linear relationship between the PZC and Mo uptake in modified alumina was observed in Al-S and Al-P. The bonding mode of sulfate on alumina changed with increasing sulfate content. The Bronsted acid site appeared in alumina modified with sulfate of 10 wt%.

Surface acidity and isomerization activity of different types of aluminas

The effect of different preparation methods of the aluminium hydroxide on the surface properties of alumina has been investigated. The alumina prepared from aluminium chloride was found to be the most acidic. Four times lower was the acidity of one alumina sample prepared by thermal decomposition. A satisfactory correlation has been found between the total acidity (H0≤3.3) and the conversion of cyclohexene to methylcyclopentenes.

“Superacid” alumina catalysts. Al2O3−P2O5 and Al2O3−AlCl3 catalysts for the isomerization and cracking of n-hexane

The effect of P2O5 and AlCl3 on the surface properties of alumina was studied. The sublimation of P2O5 and AlCl3 onto alumina surface results in the formation of superacid sites (H0≤−13.7) of the Lewis type, which catalyze the isomerization and cracking of n-hexane at 373 K.