Number Of Aluminium Atoms Research Articles

Alloy splitting of the FeGa acceptor level in dilute AlxGa1−xN

The results of conventional deep-level transient spectroscopy (DLTS) and high-resolution Laplace DLTS measurements of the FeGa(0/−) acceptor level in dilute AlxGa1−xN layers (x ≤ 0.05) grown by MOVPE technique (metal–organic vapor phase epitaxy) on native ammono-GaN substrates are analyzed and discussed. It is shown that the electron emission signal related to the FeGa acceptor level in AlxGa1−xN splits into individual components due to aluminum fluctuations in the second-nearest neighbor (2NN) shell around the FeGa impurity atoms. The calculations of the probability of finding a given number of aluminum atoms in the 2NN shell of the FeGa defect agree well with the experimental concentrations determined from Laplace DLTS peak intensities. This finding shows that in dilute AlxGa1−xN layers grown by MOVPE, aluminum and iron atoms are randomly distributed in the material. Finally, we demonstrate that the energy level of the FeGa acceptor with no Al atoms in the 2NN shell in the AlxGa1−xN samples shifts linearly with the aluminum content and the shifts are 28 and 55 meV relative to that in GaN for the samples with x = 0.025 and 0.05, respectively.

Innentitelbild: Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low‐Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation (Angew. Chem. 18/2022)

SiCl4-Behandlung bei niedrigem Druck (LPST) wird zur selektiven Relokalisierung von Aluminiumatomen in einem Zeolithgerüst in die gewünschten Positionen genutzt, wie Zhengxi Yu, Zhongmin Liu et al. in ihrem Forschungsartikel beschreiben (e202116990). Nach der Behandlung wandern die unerwünschten Gerüstaluminiumatome in den 12-MR-Kanälen eines Mordenit-Zeoliths in die gewünschten T3-Stellen, wodurch die Zahl der bevorzugten aktiven Zentren erhöht wird.

Adsorption of nitrogen oxides on Al-doped carbon nanotubes: the first principles study

The first principles calculations were performed to study the adsorption of NO/NO2 molecules on carbon nanotubes (CNTs) doped with different number of aluminum atoms. Calculations show that upon the aluminum doping, the adsorption distance is decreased while the charge density, adsorption energy and charge transfer are increased dramatically, indicating that the adsorption of NO/NO2 on CNTs could be greatly enhanced. The analysis of frontier orbit and density of states suggests that Al doping reduces the energy gap and increases the conductivity of CNTs. The density of states of Al-doped CNTs and NO/NO2 molecules are significantly improved at the Fermi level, which results in strong coupling between CNTs and NO/NO2 molecules and the increase of the adsorption of NO/NO2. In addition, results show that increasing the number of doped Al atoms lead to stronger adsorption of NO/NO2 on CNTs. The adsorption of NO2 on CNTs, on the other hand, is slightly stronger than NO in all the cases studied in this paper.

Effect of the Phase Composition of Catalysts Derived from Y-type Modified Zeolite on the Yield of C8 Hydrocarbons in the Alkylation of Isobutane by Olefins

Deviations in the activity parameters of previously synthesized catalysts derived from Y-type modified zeolite from the parameters of the basic phase composition (number of aluminum atoms in the unit cell, SiO2/Al2O3 ratio, unit cell size) are described. The results of a study of the structural changes in the crystalline phase of the zeolite upon its post-synthesis modification are presented.

Comparison of catalytic performance of synthesized EU-1 zeolite with dealuminated EU-1 zeolite for m-xylene isomerization reaction

EU-1 zeolite was synthesized with high purity by a hydrothermal method and under optimum conditions: synthesis time 72 h, temperature 200°C, and aging time 12 h. Then, the synthesized EU-1 zeolite was modified by dealumination with nitric acid and the changes of the properties such as surface area, pore volume, and Si/Al ratio were investigated. The catalytic performance of these two catalysts was studied and compared with a commercial mordenite catalyst for meta-xylene isomerization reaction in a fixed bed reactor. The results showed that the modification of catalyst with acid increases Si/Al ratio from 25 to 50 due to the removal of a number of aluminium atoms from the framework of zeolite. Also the catalyst surface area increased from 300.237 m2 g‒1 for EU-1 to 333.639 m2 g‒1 for modified EU-1. According to results, the modified Eu-1 had higher para/ortho ratio, meta-xylene conversion, and para-xylene yield than EU-1 and commercial mordenite in the meta-xylene isomerization reaction.

Molecular structure of hexatomic heteronuclear (AlFe) metal clusters as determined by the DFT quantum-chemical calculation

Geometric parameters of molecular structures of hexatomic heteronuclear (AlFe) clusters with various number of aluminum atoms (Al2Fe4 and Al3Fe3) were calculated using the density functional hybrid method in the OPBE/TZVP approximation. It was found that the first of these clusters is able to exist in nine and the second in twenty structural modifications substantially differing from each other in their stability and geometric parameters. The values of bond lengths, planar and dihedral (torsion) angles for some of these modifications are given.

Magnetic moment of iron atoms in Fe-Al body-centered cubic alloys as a function of the nearest environment

The Fe-Al systems in the concentration range from 29 to 44 at. % Al are investigated in terms of the density functional theory. It is shown that, in the system under consideration, there can exist three magnetic states with close energies. Two of these three magnetic states have collinear magnetic moments (the ferromagnetic and antiferromagnetic states), and the third is a spin-spiral state. In collinear magnetic structures, the local magnetic moments are determined by the nearest chemical environment and, in the antiferromagnetic state, the iron atoms surrounded by a large number of aluminum atoms in their environment have a negative magnetic moment. The results obtained substantiate the applicability of modified models of the Jaccarino-Walker type for the interpretation of the experimental data obtained for Fe-Al alloys. The results of the calculations also indicate a significant role of Stoner excitations in the formation of magnetic order in these alloys.

The role of aluminium and silicon in the setting chemistry of glass ionomer cements

A model of the setting chemistry of glass-ionomer cements (GICs) is proposed based on 27Al and 29Si solid state nuclear magnetic resonance spectroscopy data on three GICs. All the precursor glasses are found to contain three aluminium species viz.: four, five and six-coordinate aluminium environments as well as four-bridging silicate tetrahedra. Upon cement formation, Al3+ ions in the glass are leached out from the surface layer of the glass. On entering the cement matrix, these ions adopt six-coordination and crosslink the polymer chains as part of the setting reaction. The remaining four-coordinate aluminium is distributed between two species: one in the inert core of the glass particles; and a second, less concentrated, in the surface layer of the glass particles, modified by the curing reactions. There is some evidence for residual five and six coordinate aluminium species in the final cement in some of the systems. In the case of the silicate tetrahedra, the curing reactions result in a decrease in the number of aluminium atoms in the second coordination sphere, with a subsequent recondensation of silicate network of the glass.

The Structural and Electronic Properties of Tin-Based Heteroatom Clusters Studied by the Density Functional Theory

The focus of this study is on compound clusters and, due to the existence of many phases with different structural properties, tin-based materials have been chosen as the reference case. The clusters considered below are of two types: in the first case the clusters have the skeleton of the pure tin clusters and are doped with oxygen and aluminum atoms with composition SnxYywith Y = Al, O, x = 1, 10 and y = 1, 2. In the second case the clusters have a rutile lattice with a columnar or a spherical shape and a size up to 80 atoms and are doped with a number of aluminum atoms up to 20. The calculations are based on the Density Functional Theory (DFT) and the results describe the cluster structure, its binding energy and the density of states (DOS). The general indication of the calculations is that the additive coordinates outside, rather than inside, the pristine skeleton with the formation of hybrid bonds with properties similar to the ones of the host atoms. Also conspicuous effects of hybridization are observed in the electronic structure and, due to these effects, the binding energy may decrease with respect to the one in the undoped clusters.

Nature, Structure and Strength of the Acidic Sites of Amorphous Silica Alumina: An IR and NMR Study

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Brønsted acidity.

Investigation on the nature of the adsorption sites of pyrrole in alkali-exchanged zeolite y by nuclear magnetic resonance in combination with infrared spectroscopy.

Multinuclear solid-state NMR and infrared spectroscopy have been applied to investigate the host-guest interactions and the nature of the adsorption sites of pyrrole on alkali-exchanged zeolites Y (LiNaY, NaY, KNaY, and CsNaY). The presence of pyrrole provokes changes in the MAS NMR spectra of (23)Na, (7)Li, and (133)Cs to a degree dependent upon the amount adsorbed. The decrease in the quadrupolar coupling constant for (23)Na as well as the shift for (7)Li and (133)Cs signals are attributed to the interaction of the cation with the pyrrole ring system. The adsorption of pyrrole induces the displacement of cations located at SI' and SII sites toward the supercage to bind the guest molecules. In this way, the distribution of the cations at nonframework sites depends on the amount of adsorbate in the zeolite. At low loadings, pyrrole molecules bind preferentially to more electropositive cation in partially exchanged zeolites Y. Quantitative analysis by (1)H NMR shows that the cation-pyrrole complexes formed possess a stoichiometry of 1:1. The origin of the basic site heterogeneity, evidenced by the presence of several components in the -NH infrared stretching band, is investigated assuming that the heterocycle of pyrrole interacts with cations at SII sites in the supercage and the -NH group forms a hydrogen bond with a basic oxygen atom placed in the framework six-member ring. Making use of the information derived from NMR, it is concluded that the main source of basic site heterogeneity comes from the number of aluminum atoms in the six-member rings of the SII site where the alkaline cation is located.

Effect of Framework Charge Density on Catalytic Activity of Copper Loaded Molecular Sieves of Chabazite Structure in Nitrogen(II) Oxide Decomposition

NO decomposition over Cu-loaded molecular sieves of chabazite structure with different framework negative charge density was investigated. Cu-ZnAlPO-34 with framework charge density (Al + P)/Zn = 15 exhibited high and stable catalytic activity in NO decomposition while Cu-chabazite with Si/Al = 2.7 was inactive. This evidences that the number of aluminum atoms balancing Cu ions in cationic sites controls the catalytic activity of Cu ion. Ions balanced by single framework aluminum atoms exhibit high activity in NO decomposition while ions balanced by two framework aluminum atoms are inactive. Topology of cationic sites is not the limiting parameter for NO decomposition over Cu ions located in molecular sieves.

Cluster models for Brønsted acid centres in faujasites

Cluster models for Brønsted acid centres in zeolites are proposed which would be suitable for intermediate quantum chemical calculations. Our extended models for different environments of framework hydroxyls in faujasites include explicitly the second cationic coordination sphere of the bridging unit and are based upon real atoms as saturators of dangling bonds at cluster boundaries. For such models MNDO-type investigations with geometry optimization of the entire cluster could be undertaken. The adequacy of the proposed cluster models is checked on the basis of deprotonisation energies and their direct dependence on the number of aluminium atoms in the neighbourhood of the bridging hydroxyl.

Influence of the Si/Al ratio and ZSM-5 crystallite size on the methylation of toluene

The alkylation of toluene with methanol over ZSM-5 was studied using crystallites of different dimensions and Si/Al ratios. The products of the reaction consisted of 70–80% xylenes. Some benzene and ethyltoluene are also formed. The formation of para-xylene was strongly influenced by the number of aluminium atoms per unit cell of the zeolite and the gel composition utilized in its preparation. For the conditions studied here, no relationship between the crystallite dimensions and the para-selectivity was observed.

Quarternary ammonium cation effects on the crystallization of zeolites of the offretite-erionite family: Part III. 29Si n.m.r., 27Al n.m.r., and mid-i.r. characterization

Infrared spectra in the framework absorption region reflect the various degrees of offretite and erionite character of several zeolites belonging to the offretite-erionite family. I.r. spectra of materials rich in offretite contained an absorption band in the 600–610 cm −1 region that is absent in erionite. Spectra of materials rich in erionite contained bands (at 410–425 cm −1, 550–555 cm −1, and 665–685 cm −1) that are absent in offretite. Thus, i.r. spectroscopy is diagnostic in detecting the presence of these two structures. In the 29Si MAS n.m.r. spectra of these zeolites, the dispersion of the chemical shift due to crystallographic site inequivalence is approximately equal to the shift due to the different number of aluminium atoms in the first coordination sphere (∼ 5 ppm). The Si/Al ratios calculated from solid-state n.m.r. have been found to agree only semiquantitatively with those obtained from chemical analyses. This result indicates that the assumption of aluminium a toms randomly distributed over two nonequivalent sites present in a 2:1 ratio in offretite is not valid. The effects of calcination temperature on dealumination is documented by n.m.r. spectra of both 29Si and 27Al.

Aluminum clusters: ionization thresholds and reactivity toward deuterium, water, oxygen, methanol, methane, and carbon monoxide

Using the pulsed cluster beam flow reactor technique, the authors have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule they observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as O/sub 2/ > CH/sub 3/OH > CO > D/sub 2/O > D/sub 2/ > CH/sub 4/ with CH/sub 4/ showing no reaction under these experimental conditions. In addition, they are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.

Collision induced dissociation of aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2): Influence of electronic structure on stability

The collision induced dissociation of size selected aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2) by argon has been studied using a low energy ion beam apparatus. The main product from AlnO+ is Al+n−2. Aln−2O+ and Al+n−4 are observed from AlnO+2. These products can be accounted for by loss of Al2O (which is very strongly bound) from the clusters. The experimental results were used to bracket the Aln/O interaction energies at 8.0±1.0 eV for Al+6 and 7.5±1.0 eV for Al+19. These large interaction energies indicate that oxygen is multiply bonded to the clusters. The collision induced dissociation cross sections show a strong cluster size dependence. Minima in the cross sections as a function of cluster size are ascribed to particularly stable clusters. The number of aluminum atoms in the particularly stable AlnO+m cluster increases with the addition of oxygen atoms. The stability of these clusters can be accounted for by the electronic shell model if it is assumed that binding oxygen atoms to the clusters removes electrons from the cluster orbitals so that more aluminum atoms are required to close the electronic shells.

Factors affecting the stability and selectivity of zeolites in the conversion of methanol to hydrocarbons. Communication 2. Effect of dealuminization

1. Thermochemical treatment of Y and ZSM zeolites with silicon tetrachloride vapor leads to the loss, first, of aluminum atoms from cation positions and, second, of aluminum atoms from the silicic acid framework of the zeolite. 2. Active centers which have aluminum atoms in cation positions in their composition lead, to a considerable extent, to hydrogen transfer reactions in the intermediate, unsaturated hydrocarbons. 3. The stability of the catalytic activity of zeolites depends on the number of aluminum atoms in cation positions. As the number of such atoms increases, the stability is reduced. 4. Nonuniform distribution of aluminum atoms along the zeolite crystal increases the probability of the formation of fragments of the type [(SiO)4−nSi(OAl)n]n−-Al3+ or [(SiO)4−nSi(OAl)n]n−Al3+(OH−...H+), which can act as centers for multipoint interaction with molecules of the reactants.

The influence of second-neighbor aluminums on the isotropic chemical shift of silicon-29 in a zeolite environment

Linear relationships between the magnitudes of the /sup 29/Si isotropic chemical shifts and the number of aluminum atoms in FAU and LTA framework materials indicate second-neighbor effects and provide an alternative method of determining framework compositions from /sup 29/Si NMR spectra. Mean values for the change in chemical shift per added aluminum atom are 6.28 and 0.70 ppm for first- and second-neighbor sites, respectively. 35 references, 2 figures, 2 tables.

X-ray investigations of laves phase intermetallic compounds in the Gd 1−xEr xAl 2 system for 0 ⩽ x ⩽ 0.6

The influence of the substitution of gadolinium atoms for erbium atoms in polycrystalline Gd 1− x Er x Al 2 Laves phase compounds was investigated. X-ray diffraction studies showed that all the samples crystallize with a cubic structure of the MgCu 2 type (C15). The lattice constants of the compounds satisfy Vegard's law for the whole range of x. Partial disorder of the crystal structure was found, i.e. the aluminium atoms partly exchange with the gadolinium and erbium atoms. The crystallographic ordering parameter y (defined as the number of aluminium atoms in the 8a sites) is equal to zero (“ideal ordering”) only for GdAl 2. It was observed that the parameter y increases with increasing x up to a content of about 30 mol.% ErAl 2. For this compound and for those with higher contents of ErA1 2 the exchange of the rare earth (gadolinium and erbium) and aluminium between the 8a and 16d sites is a maximum and equals about 1 atom per elementary cell.