Α-alumina Surface Research Articles

Thermal decomposition of PFOA: Influence of reactor and reaction conditions on product formation

The thermal decomposition of perfluorooctanoic acid (PFOA) was studied in an α-alumina flow reactor between 400 and 1000 °C. Experimental results indicate that the thermal decomposition of PFOA is initiated at 400 °C and that at temperatures below 600 °C, the primary products are perfluoroheptene (C7F14), CO2, and HF, together with minor amounts of perfluoroheptanoyl fluoride (C7F14O) and CO. These results contradict quantum chemical calculations that predict HF, CO, and C7F14O to be the major products formed via a lactone intermediate. Through quantum chemical calculations, we discovered that PFOA can be physisorbed onto an alumina nanocluster and can release C7F14 and CO2. Adding more α-alumina surface area confirmed a significant increase in CO2 concentration, supporting the postulate that an α-alumina surface impacts the thermal decomposition of PFOA. Temperatures above 600 °C were found to result in an increased yield of HF and CO and fluorocarbons.

Effects of pH on Ice Nucleation by the α-Alumina (0001) Surface

Effects of pH on Ice Nucleation by the α-Alumina (0001) Surface

Fate of Water Molecules on (11-20) and (1-102) α-Alumina Surfaces: 2D Periodic Self-Consistent Charge-Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics Study

Numerous studies on the adsorption and dissociation of water molecules on α-alumina surfaces, both experimentally and theoretically, have been presented within decades. However, the dynamical properties of water at the solid–liquid interface are still a challenging problem. To explain the dynamics of the adsorption and dissociation of water molecules on α-alumina surfaces, mainly (11-20) and (1-102) faces, we carried out a two-dimensional periodic self-consistent charge density-functional tight-binding/molecular mechanics molecular dynamics study. In this study, we used the same approach previously applied to the (0001) α-alumina surface. The adsorption of single, dimer, and monolayer water molecules was employed to study the adsorption and dissociation of water molecules at (11-20) and (1-102) α-alumina surfaces. The single water system showed that no significant changes occurred on both surfaces, which indicates that no water dissociation was observed. Dimer water molecules on both surfaces were dissociated to form new OsHw groups following a 1-4 dissociation channel. Monolayer water molecules showed dissociation that occurred on both surfaces. Protonation analysis showed that both surfaces form stable OH groups without attempting to form water molecules. At the 100 ps simulation period, for the (11-20) α-alumina surfaces, up to 1.93 OH groups/nm2 were formed. The (1-102) α-alumina surface showed higher reactivity with a formation rate of up to 5.0 OH groups/nm2. By contrast, 2.1 OH groups/nm2 were formed on the (0001) α-alumina surface. The formation of OH groups on the (11-20) and (1-102) α-alumina surfaces occurred in the equilibration step and continued during the sampling step. In studying these three surfaces using a similar approach, the (1-102) α-alumina surface is the most reactive with water.

Surface orientation dependent interaction of cobalt (II) precursors with alpha-alumina

The performances of alumina-supported cobalt catalysts strongly depend on Co/alumina interactions, that are poorly understood. In the present work, α-alumina surfaces are used as models of polycrystalline support, to unravel their orientation dependent behavior by GI-EXAFS. The CoII/α-alumina interactions in the dried state are investigated, with a resolution of the parallel versus perpendicular deposit formation. DFT calculations shed light on the existence of specific grafting modes. EXAFS shows that Co(OH)2 forms parallel to the R(11¯02), A(112¯0) and M(101¯0) surfaces, whatever the contact time (from 1 h to 6 days). However, a specific behavior of the C(0001) surface is observed. Whereas after one hour, the same parallel Co(OH)2 is observed, at longer times it disappears while species exhibiting long then short Co-Co distances appear. The latter is assigned to epitaxial grafting of CoII leading to a mixed phase, that is possible parallel to the C surface due to surface symmetry.

Surface recombination in the direct simulation Monte Carlo method

This work is aimed at the development of surface chemistry models for the Direct Simulation Monte Carlo (DSMC) method applicable to non-equilibrium high-temperature flows about reentry vehicles. Probabilities of adsorption and Eley-Rideal recombination dependent on individual properties of each particular molecule and frequencies of desorption and Langmuir-Hinshelwood recombination are determined from macroscopic reaction rate data. Various macroscopic finite-rate surface reaction sets are used for the construction of the DSMC surface recombination models for the reaction cured glass and α-alumina surfaces. The models are implemented in the SMILE++ software system for DSMC computations, and detailed verification of the code is performed. The proposed approach is used to study the effects of surface recombination on the aerothermodynamics of a blunt body at high-altitude reentry conditions.

Effects of alkaline earth metals on the surface, structure, and reactivity of α-alumina

A series of strontium- and barium-doped alumina samples were prepared by hydrolysis, in neutral medium, starting from commercial Al2O3, SrCO3, and BaCO3 materials. The precursors thus obtained were calcined under air at 700 °C; then, the bulk and surface properties of the resulting mixed oxides were characterized by nitrogen physisorption, X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), thermogravimetry (TGA), and differential thermal analysis (DTA). Contrary to SrCO3, an addition of BaCO3 to α-Al2O3 increases slightly the specific surface area. XRD patterns essentially reveal the characteristic reflections assigned to α-Al2O3. In agreement with TGA and XRD analysis, strontium and barium carbonates remain after calcination at 700 °C, their decomposition starting above 800 °C. Let us note that this decomposition occurs more readily on AlSr-100 than on AlBa-100 with no apparent relationship with the evolution observed on the specific surface areas. H2-TPR experiments underline a significant bulk reduction of barium and strontium carbonates taking place significantly above 900 °C with similar trend noticed during TGA regarding their thermal decomposition. However, the most relevant observation is related to a sharp enhancement of the reducibility of AlSr-y with the appearance two reduction ranges highlighting the existence of different types of interactions with strontium and the alumina substrate.

A Case Study on Metal-Ceramic Interfaces: Wetting of Alumina by Molten Aluminum

Interfacial behavior of Al and α-Al2O3 are investigated via molecular dynamic simulation (MD) employing reactive force fields parameterized for Al and Al2O3. The main result of this work is elucidating the wetting behavior and interface chemistry of molten aluminum on the α-Alumina (0001) surface through MD simulations. Wetting and interface chemistry are studied at 8 different temperatures from 700 to 1400 K for four different droplet sizes: with 16, 24, 32 and 40 Å diameters. Chemical reactions are observed at all temperatures and sizes in addition to diffusion between droplet and substrate atoms into each other during the wetting process. To define the level of wetting, we characterized contact angles of aluminum droplets on alumina substrates for all temperatures and sizes by using a method developed by Hautman and Klein. Chemical reactions are more extensive for the small droplets (16 and 24 Å) due to their larger surface to volume ratio in comparison to the larger droplets (32, and 40 Å) of droplets.

Study of first electronic transition and hydrogen bonding state of ultra-thin water layer of nanometer thickness on an α-alumina surface by far-ultraviolet spectroscopy

The first electronic transition (A˜←X˜) and the hydrogen bonding state of an ultra-thin water layer of nanometer thickness between two α-alumina surfaces (0.5–20nm) were studied using far-ultraviolet (FUV) spectroscopy in the wavelength range 140–180nm. The ultra-thin water layer of nanometer thickness was prepared by squeezing a water droplet (~1μL) between a highly polished α-alumina prism and an α-alumina plate using a high pressure clamp (~4.7MPa), and the FUV spectra of the water layer at different thicknesses were measured using the attenuated total reflection method. As the water layer became thinner, the A˜←X˜ bands were gradually shifted to higher or lower energy relative to that of bulk water; at thicknesses smaller than 4nm, these shifts were substantial (0.1–0.2eV) in either case. The FUV spectra of the water layer with thickness <4nm indicate the formation of structured ice-like hydrogen bond (H-bond) layers for the higher energy shifts or the formation of slightly weaker H-bond layers as compared to those in the bulk liquid state for lower energy shifts. In either case, the H-bond structure of bulk liquid water is nearly lost at thicknesses below 4nm, because of steric hydration forces between the α-alumina surfaces.

Calcium phosphate growth on sintered α-alumina treated with piranha solution using a wet-chemical procedure

Two alumina samples, a high surface powder γ-alumina and a sintered α-alumina have been studied as substrates for apatite growth. In the first case, the initial stage was the calcium adsorption in an alkaline solution, followed by phosphate uptake. On the other hand, the α-alumina was firstly subjected to an acid piranha treatment at 80°C for four hours, before alternate immersions on calcium and phosphate solutions. Next, the alumina samples were immersed repeatedly in a glycine buffer, pH 10.2, and then in tris buffer solutions, pH 7.4 at 32°C. The resultant solids were studied by means of XRD, TEM or SEM. Calcium phosphate growth was observed in both cases. An almost uniform and continuous layer was formed on the sintered α-alumina. The results suggest that pretreatment with piranha solution seems to be a fast and efficient alternative to bioactivate the α-alumina surface.

First-principles study on adsorption structure and electronic state of stanene on α-alumina surface

The adsorption structure and electronic state of stanene on an α-Al2O3(0001) 1×1 surface were investigated by first-principles calculations. The variation in the electronic state of the adsorbed stanene from that of the free-standing one increased with the stanene–alumina distance, because the strength of the stanene–alumina interaction increased with the distance. The band splitting induced by the Rashba effect was observed in the electronic band structures. It was observed from the band structures with spin–orbit interactions that the degrees of band-gap opening due to the spin–orbit interactions were much lower than that due to the interaction between stanene and the α-alumina surface. By population analyses for chemical bonds, we revealed that the electronic state of stanene on the α-alumina surface was affected by Sn–O bonds with antibonding nature.

Adsorption of substituted benzoic acids onto α-Al2O3 surface in mixed-adsorbate mode: 2,4-Dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid and 1,2,4-benzenetricarboxylic acid (trimellitic acid)

Influence of position and type of functional groups (COOH and phenolic OH groups) in a benzene ring on the adsorption of three natural organic matter (NOM) analogues e.g. 2,4-dihydroxybenzoic acid (2,4-DHBA), 2,6-dihydroxybenzoic acid (2,6-DHBA) and trimellitic acid (1,2,4-benzenetricarboxylic acid) onto α-alumina surface are explored in terms of adsorption kinetics and isotherms in single- and mixed-adsorbate mode. Focusing at the positional functional groups, adsorption profile of 2,4-DHBA and 2,6-DHBA is compared in single- and mixed-adsorbate mode. 2,4-DHBA produces higher adsorption density than 2,6-DHBA in both single- and mixed-adsorbate mode, governing factor is the ortho-para position of phenoilc OH group in 2,4-DHBA for higher adsorption density. Concerning type of functional groups, adsorption profile of 2,4-DHBA is also compared with trimellitic acid. Here again 2,4-DHBA produces higher adsorption density than trimellitic acid in single-adsorbate mode. In contrast, in mixed-adsorbate mode both adsorbates exhibit roughly equal adsorption densities indicating competition of co-operative nature, which is also supported by their activation energy values in mixed-adsorbate mode. Nevertheless, there exists a competition for the same surface sites of α-alumina that results in reduction of adsorption densities in mixed-adsorbate mode than that of the single-adsorbate mode.

Understanding the Fate of Insensitive Munitions Compounds: Computational Study on Adsorption of Nitroguanidine (NQ) and 1,1-Diamino-2,2-dinitroethylene (FOX7) on Pristine and Al-Hydroxylated α-Alumina Surfaces

Plane-wave density functional theory (DFT) level of investigation was performed on the adsorption of two insensitive munition compounds, nitroguanidine (NQ) and 1,1-diamino-2,2-dinitroethylene (FOX7), on the (0001) surfaces of (4 × 4) Al-terminated α-alumina (α-Al2O3) and its Al-hydroxylated form. Surface hydroxylation was obtained by using 16 water molecules. Full geometry optimizations were performed using the vdW-DF2 van der Waals functional. The interaction of electron with core was accounted using the Vanderbilt ultrasoft pseudopotentials. The charge-density difference maps were performed to characterize the nature of munitions adsorption on alumina surfaces. It was revealed that both of the munition compounds will be adsorbed in parallel orientations on alumina surfaces. It was found that deformation energy correction would be required for the reliable interaction energy prediction on pristine alumina surface. On the pristine surface adsorption will be characterized by the covalent type of interacti...

Thiourea Derivatives as Potent Inhibitors of Aluminum Corrosion: Atomic-Level Insight into Adsorption and Inhibition Mechanisms

Suitable corrosion inhibitors are of prime importance in order to prevent degradation of surfaces by oxidizing chemicals. In this work we studied ten symmetrical thiourea derivatives on aluminum and their efficacy in preventing oxidation by hydrochloric acid computationally and experimentally. We carried out DFT calculations of the inhibitors in both tautomer forms adsorbed on an aluminum-terminated α-alumina surface, focusing on the structure and energetics of adsorption as well as electronic properties. Chemisorption is dominated by electron transfer from the inhibitor S atom toward the surface as well as into the first few layers of the solid. We find that the aggregated amount of transferred charge is an important parameter of the system that correlates with the inhibition efficiency as determined with potentiodynamic measurements. The measurements indicate that the thiourea derivatives are cathodic-type inhibitors, which hinder the reduction of protons and thus indirectly the surface oxidation. This ...

Characterization of water dissociation on α-Al2O3(11[combining macron]02): theory and experiment.

The interaction of water with α-alumina (i.e. α-Al2O3) surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with α-Al2O3 surfaces other than the (0001) are extremely limited. Here we characterize the interaction of water (D2O) with a well defined α-Al2O3(11[combining macron]02) surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1-4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schrödinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1500 cm(-1). Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable.

Candidate Source of Flux Noise in SQUIDs: Adsorbed Oxygen Molecules.

A major obstacle to using superconducting quantum interference devices (SQUIDs) as qubits is flux noise. We propose that the heretofore mysterious spins producing flux noise could be O_{2} molecules adsorbed on the surface. Using density functional theory calculations, we find that an O_{2} molecule adsorbed on an α-alumina surface has a magnetic moment of ~1.8 μ_{B}. The spin is oriented perpendicular to the axis of the O-O bond, the barrier to spin rotations is about 10 mK. Monte Carlo simulations of ferromagnetically coupled, anisotropic XY spins on a square lattice find 1/f magnetization noise, consistent with flux noise in Al SQUIDs.

Influence of alkane chain length on adsorption on an α-alumina surface by MD simulations

Plasma surface techniques provide both an efficient and ecological tool for the functionalization of surfaces. Hence, a proper understanding of the plasma–surface interactions of precursors and radicals during the deposition process is of great importance. Especially during the initial deposition process, the deposition of molecules and fragments is difficult to investigate by experimental techniques and import insights can be obtained by molecular dynamics simulations. In this work, the reactive force field developed by the group of Kieffer at the University of Michigan was used to study the adsorption of single linear alkane chains on an α-alumina surface. The chain length was changed from 6 backbone carbon atoms to 16 carbon atoms, the deposition energy from 0.01 to 10eV and the incidence angle from 0° to 60° with respect to the surface normal. Results show that the adsorption depends a lot on the ratio of deposition energy to alkane chain length and the incidence angle. More grazing incidence reduces the adsorption probability and a low ratio of energy to chain length increases it.

Attachment of ribonucleotides on α-alumina as a function of pH, ionic strength, and surface loading.

The interactions between nucleic acids and mineral surfaces have been the focus of many studies in environmental sciences, in biomedicine, as well as in origin of life studies for the prebiotic formation of biopolymers. However, few studies have focused on a wide range of environmental conditions and the likely modes of attachment. Here we investigated the adsorption of ribonucleotides onto α-alumina surfaces over a wide range of pH, ionic strength, and ligand-to-solid ratio, by both an experimental and a theoretical approach. The adsorption of ribonucleotides is strongly affected by pH, with a maximum adsorption at pH values around 5. Alumina adsorbs high amounts of nucleotides >2 μmol/m(2). We used the extended triple-layer model (ETLM) to predict the speciation of the surface complexes formed as well as the stoichiometry and equilibrium constants. We propose the formation of two surface species: a monodentate inner-sphere complex, dominant at pH <7, and a bidentate outer-sphere complex, dominant at higher pH. Both complexes would involve interactions between the negatively charged phosphate group and the positively charged surface of alumina. Our results provide a better understanding of how nucleic acids attach to mineral surfaces under varying environmental conditions. Moreover, the predicted configuration of nucleotide surface species, bound via the phosphate group, could have implications for the abiotic formation of nucleic acids in the context of the origin of life.

Alumina-supported AlPO-18 membranes for CO2/CH4 separation

Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular α-alumina supports in a single hydrothermal synthesis step. The AlPO-18 crystals and membranes were characterized by SEM, XRD, adsorption and permeation measurements. Single-gas permeances of CO2, N2, CH4 and i-C4H10 decreased with increased kinetic diameter, but CO2 permeated faster than the smaller H2, because CO2 adsorbs more strongly. Eleven AlPO-18 membranes had an average CO2 permeance of (1.8±0.5)×10−7mol/(m2sPa) and average CO2/CH4 separation selectivities of 101±16 at room temperature and a pressure drop of 0.1MPa for the equimolar CO2/CH4 mixture. The membrane was also selective (selectivity=31) at 2MPa feed pressure and had a CO2 permeance of 0.47×10−7mol/(m2sPa) [CO2 flux of 15kg/(m2h)]. The CO2/CH4 selectivities decreased as temperature and pressure drop increased. The membranes stability was investigated for different calcination times, calcination temperature ramps, and following exposures to water vapor and liquid.

Plane-wave density functional theory investigation of adsorption of 2,4,6-trinitrotoluene on Al-hydroxylated (0001) surface of (4 × 4) α-alumina.

This article reports the results of the theoretical investigation of adsorption of 2,4,6-trinitrotoluene (TNT) on Al-hydroxylated (0001) surface of (4 × 4) α-alumina (α-Al2O3) using plane-wave Density Functional Theory. Sixteen water molecules were used to hydroxylate the alumina surface. The Perdew-Burke-Ernzerhof functional and the recently developed van der Waals functional (vdW-DF2) were used. The interaction of electron with core was accounted using the Vanderbilt ultrasoft pseudopotentials. It was found that hydroxylation has significant influence on the geometry of alumina and such changes are prominent up to few layers from the surface. Particularly, due to the Al-hydroxylation the oxygen layers are decomposed into sublayers and such partitioning becomes progressively weaker for interior layers. Moreover, the nature of TNT adsorption interaction is changed from covalent type on the pristine alumina surface to hydrogen-bonding interaction on the Al-hydroxylated alumina surface. TNT in parallel orientation forms several hydrogen bonds compared to that in the perpendicular orientation with hydroxyl groups of the Al-hydroxylated alumina surface. Therefore, the parallel orientation will be present in the adsorption of TNT on Al-hydroxylated (0001) surface of α-alumina. Further, the vdW-DF2 van der Waals functional was found to be most suitable and should be used for such surface adsorption investigation.

Change in organic molecule adhesion on α-alumina (sapphire) with change in NaCl and CaCl2 solution salinity.

We investigated the adhesion of two functional groups to α-alumina as a model for the adsorption of organic molecules on clay minerals. Interactions between organic compounds and clay minerals play an important role in processes such as drinking water treatment, remediation of contaminated soil, oil recovery, and fabricating complicated nanomaterials, and there have been claims that organic compound-clay mineral interaction created the ordering that is necessary for the genesis of life. In many organisms, interaction between organic molecules and biominerals makes it possible to control the growth of bones, teeth, and shells. Adhesion of carboxylic acid, -COO(H), and pyridine, -C5H5N(H(+)), on the {0001} plane of α-alumina wafers has been investigated with atomic force microscopy (AFM) in chemical force mapping (CFM) mode. Both functional groups adhered to α-alumina in deionized water at pH < 5, and adhesion decreased as NaCl or CaCl2 concentration increased. X-ray photoelectron spectroscopy (XPS) showed that Na(+) and Ca(2+) adsorbed to the α-alumina surface at pH < 5, decreasing surface interaction with the carboxylic acid and pyridine groups. We interpret the results as evidence that the tips adhere to alumina through hydrogen bonding when only water is present. In solutions containing NaCl and CaCl2, cations are adsorbed but Cl(-) is not. When NaCl solutions are replaced by CaCl2, Ca(2+) replaces Na(+), but rinsing with ultrapure deionized water (pH 5.6) could not restore the original protonated surface. The results demonstrate that the alumina surface at pH 3 has a higher affinity for inorganic cations than for -COO(H) or -C5H5N(H(+)), in spite of the known positive surface charge of α-alumina {0001} wafers. These results demonstrate that solution salinity plays an important role in surface properties, controlling surface tension (i.e., contact angle) and adsorption affinity on α-alumina and, by analogy, on clay minerals.