γ-Al2O3 Substrate Research Articles

First-Principles Analysis of NOx Adsorption on Anhydrous γ-Al2O3 Surfaces

The interaction of nitrogen oxides NOx (x = 1−3) with γ-Al2O3 has been investigated using first-principles density functional theory calculations. NO and NO2 weakly physisorb on the clean, dehydrated (100) and (110) surfaces of γ-Al2O3, whereas the adsorption of the NO3 radical is rather strong. Only the basic-like O-down adsorption configurations were found to be stable. The interaction between NOx and γ-Al2O3 can be described as a surface-mediated electron transfer process. For single NOx adsorption, greater electron transfer from the surface to the adsorbate (negatively charged) yields stronger interaction between NOx and the surface. The adsorption of four combinations of NOx + NOy (x = 1−3, y = 2, 3) pairs on the (100) and the (110) facets of γ-Al2O3 were investigated. Except for the NO2 + NO2 pair, a strong cooperative effect that substantially enhances the stability of NOx on both γ-Al2O3 surfaces was found. This cooperative effect consists of surface-mediated electron transfer processes resulting in a favorable electrostatic interaction between two adsorbed NOx species. The NO+δNO3−δ pair was found to be the thermodynamically most stable state among the coadsorbed NOx + NOy pairs on both γ-Al2O3 surfaces. The results are used to analyze the experimentally observed NOx evolution during temperature programmed desorption from NO2-saturated γ-Al2O3 substrates.

Characterization of surface and bulk nitrates of γ-Al2O3–supported alkaline earth oxides using density functional theory

"Surface" and "bulk" nitrates formed on a series of alkaline earth oxides (AEOs), AE(NO3)2, were investigated using first-principles density functional theory calculations. The formation of these surface and bulk nitrates was modeled by the adsorption of NO2+NO3 pairs on gamma-Al2O3-supported monomeric AEOs (MgO, CaO, SrO, and BaO) and on the extended AEO(001) surfaces, respectively. The calculated vibrational frequencies of the surface and bulk nitrates based on our proposed models are in good agreement with experimental measurements of AEO/gamma-Al2O3 materials after prolonged NO2 exposure. This indicates that experimentally observed "surface" nitrates are most likely formed with isolated two-dimensional (including monomeric) AEO clusters on the gamma-Al2O3 substrate, while the "bulk" nitrates are formed on exposed (including (001)) surfaces (and likely in the bulk as well) of large three-dimensional AEO particles supported on the gamma-Al2O3 substrate. Also in line with the experiments, our calculations show that the low and high frequency components of the vibrations for both surface and bulk nitrates are systematically red-shifted with the increasing basicity and cationic size of the AEOs. The adsorption strengths of NO2+NO3 pairs are nearly the same for the series of alumina-supported monomeric AEOs, while the adsorption strengths of NO2+NO3 pairs on the AEO surfaces increase in the order of MgO<CaO<SrO approximately BaO. Compared to the NO2+NO3 pair that only interacts with monomeric AEOs, the stability of NO2+NO3 pairs which interact with both the monomeric AEO and the gamma-Al2O3 substrate is enhanced by about 0.5 eV.

Development and characterization of silica-based membranes for hydrogen separation

The method of chemical vapor deposition (CVD) in the counter current configuration was employed in the present study for the development of composite silica membranes. The experiments were carried out in a horizontal CVD reactor under controlled temperature conditions and at various reaction times and differential pressures across the substrate sides. Tetraethylorthosilicate (TEOS) and ozone were used as deposition precursors. Two types of substrates were employed: a porous Vycor tube and an alumina (γ-Al2O3) nanofiltration (NF) tube. Measurements with a novel mercury intrusion technique showed that significant reduction of the initial pore size of the γ-Al2O3 substrates was achieved, which reached 76% in the cases of extended silica deposition. Additionally, by appropriately interpreting the Knudsen type O2 permeance results, acquired during the CVD treatment of Vycor tubes, a pore radius reduction even down to the 30% of the initial value was concluded. The permeance of Η2 and other gases (Ηe, Ν2, Αr, CO2) on the developed membranes was measured in a home-made apparatus. The separation capability of the composite membranes was determined by calculating the selectivity of hydrogen over helium, nitrogen, argon and carbon dioxide.

Planar metal–insulator–semiconductor type field emitter fabricated on an epitaxial Al/Al2O3/Si (111) structure

A planar metal–insulator–semiconductor (MIS)-type field emitter using epitaxial γ-Al2O3 (111)/Si (111) structure was successively fabricated and field-emission phenomena were observed. Planar MIS-type field emitters using epitaxially grown Al2O3 were fabricated by molecular beam epitaxy (MBE) method and characteristics of these emitters were evaluated. It was confirmed that the Al2O3 possessed good crystalline quality and surface morphology. The breakdown field of the epitaxial Al2O3 layer was able to increase in situ annealing at the substrate temperature of 750 °C. From experimental measurements of diode current, conduction band offset (ΔEc) value between the epitaxial γ-Al2O3 and Si (111) substrate was calculated to be 2.5 eV. Fowler–Nordheim (F–N) tunneling was observed from the 8-nm-thick epitaxial Al2O3 film. Emission characteristics were obtained from the 10-nm-thick epitaxial Al2O3 insulator with a 20-nm-thick Al gate electrode. The maximum transfer ratio of emission current was confirmed to be 1%.

Using EPES as an easy method in model catalysts research: Comparative study of alumina supported Rh particle growth and reconstruction

The goal of the presented work is to show that elastic peak electron spectroscopy (EPES) represents a quick and easy method to characterise the metal particle population in situ and to contribute by this way to the effort to check the morphology of non-continuous metal films, e.g. model catalysts.Formation and reconstruction of Rh deposited on polycrystalline γ-Al2O3 substrates were checked by means of Auger electron spectroscopy (AES) and EPES. Rh deposits were evaporated step-by-step in situ. AES and EPES spectra were explored to follow a growth mode and a temperature stimulated reconstruction of the deposited Rh film. Morphological parameters of the studied samples were estimated using the obtained AES and EPES spectra. These parameters were compared to the particle size and density determined by means of transmission electron microscopy (TEM). In the case of reconstructed Rh deposits, the TEM results were compared to those ones obtained on non-reconstructed reference samples. All obtained results are discussed to verify the possibility to determine the sample morphology by means of EPES.

Growth of Pt Crystallites Supported on γ-Al2O3 Studied by Atomic Force Microscopy

The growth behavior of Pt crystallites vacuum-deposited on flat γ- and α-Al2O3 substrates was investigated by atomic force microscopy (AFM). In this study, we fabricated a thin layer of γ-Al2O3 on a planar sapphire (single crystalline α-Al2O3) substrate by Al+- and O+- ion implantation and annealing at 900 °C. AFM observation revealed that γ- and α-Al2O3 substrates have a negligible roughness compared with the size of the supported Pt crystallites deposited on them. The crystal structure was identified as a γ(111) plane for the γ-Al2O3 substrate, but an α(0001) plane for the sapphire substrate by Rutherford backscattering spectroscopy-channeling analysis (RBS-C) and X-ray diffraction analysis (XRD). We then observed the surface morphology of Pt crystallites on γ- and α-Al2O3 substrates, which were fabricated by sputtering of a Pt target. The as-deposited Pt thin film changed into Pt crystallites dispersed on the substrate with heat treatment at 800 °C in an oxidative atmosphere. It was found that the grow...

The influence of an intermediate MoO3 layer on the solid state reaction of cobalt oxide withγ-Al2O3

In view of the importance for CoxOy,-MoO3/γ-Al2O3 hydrodesulphurization (HDS) catalysts, the reactivity of cobalt oxide layers towards cobalt aluminate formation was investigated on both MoO3-covered and bare γ-Al2O3 substrates. Co3O4/MoO3/γ-Al2O3 and Co3O4/γ-Al2O3 systems were prepared by vapour-deposition of MoO3 (12 × 1015 Mo atoms/cm2) and Co (400 × 1015 Co atoms/cm2) layers onto a γ-Al2O3 substrate, followed by oxidation of the Co layer to Co3O4. After annealing at 800°C for 40 h, the interfacial reaction to cobalt aluminate was assessed using Rutherford backscattering spectrometry. The presence of molybdenum oxide appeared to enhance cobalt aluminate formation. The Mo atoms, which spread out over the entire cobalt-containing layer, presumably caused a high defect density, which explains the observed higher reaction rate. The amount of MoO3 was much too low to stabilize all cobalt atoms by cobalt molybdate formation.