Hydroxylated Alumina Surface Research Articles

Diisopropyl Methylphosphonate and Sarin Decomposition on Pristine vs Hydroxylated Alumina Surfaces: Mechanistic Predictions from Ab Initio Molecular Dynamics

Diisopropyl Methylphosphonate and Sarin Decomposition on Pristine vs Hydroxylated Alumina Surfaces: Mechanistic Predictions from Ab Initio Molecular Dynamics

Insights into the mechanism of fluoride adsorption over different crystal phase alumina surfaces

Activated alumina is the most common adsorbent for purifying fluoride in water, however, little is known so far about the adsorption mechanisms and comparison of adsorption behaviors for F on different crystal phase alumina surfaces, which seriously obstacles the development of high-performance sorbents. Herein, employing the density functional theory approach, we have studied F adsorbed on α-Al2O3(0001), γ-Al2O3(110), and θ-Al2O3(010) surfaces. Results accentuate that the θ-Al2O3 (010) is the most reactive than ɑ-Al2O3 (0001) and γ-Al2O3 (110) for F adsorption and the high reactivity is mainly attributed to the high unsaturation level of Al atoms. Detailly, the most stable adsorption sites are top of Al1 site, bridge of Al6 and adjacent Al atom, and bridge of AlⅢ atoms for α, γ, θ-alumina, respectively. The bonding picture shows that the bonding between F and alumina surface is attributed to the hybridization between F-p orbitals and Al-s,p orbitals. In addition, the alumina surfaces are hydroxylated with water molecules when exposing to the atmosphere, exhibiting a great impact on the performance of purifying F element. Results suggest that the hydroxylated θ-Al2O3 (010) adsorbs F with the smallest adsorption energy than other hydroxylated alumina surfaces, exhibiting the lowest performance of purifying F element.

Substitutional adsorptions of chloride at grain boundary sites on hydroxylated alumina surfaces initialize localized corrosion

To understand the chloride (Cl)-induced initiation mechanism of localized corrosion of Aluminum (Al) alloys, we apply density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to investigate the interactions between Cl and hydroxylated α–Al2O3 surfaces, mainly (0001) orientation, under aqueous electrochemical conditions. Hydroxylated alumina surfaces thermodynamically stable in aqueous environments are constructed based on DFT calculations for both the single-crystal and bicrystal configurations. AIMD simulations suggest a Cl anion can only be stabilized on these surfaces by substituting a surface hydroxyl (OH) group. This substitution is thermodynamically favorable at sites on surface terminations of grain boundaries (GBs) in bicrystal configurations but not favorable at sites on single-crystal surfaces. Electronic structure analyses show that the different adsorption behaviors originate from the higher sensitivity of the Al–OH bond strength to the local coordination than its counterpart of the Al–Cl bond. The adsorbed Cl significantly increases the thermodynamic driving force for Al cation dissolution from alumina surfaces into the aqueous electrolyte, which can initiate localized corrosion.

First-Principles Study of Epoxy Resin Adhesion to Tin Oxide Surface

A density functional theory (DFT) is used to investigate the energetics of an epoxy resin adhere on a tin oxide and a hydroxylated-alumina surface within a supercell approach. Self—consistent geometry optimization is performed for models of adhesion interface, which is comprised of a fragment of epoxy resin and hydroxylated-Al2O3 (001), and SnO2(001) and (110) surface. The epoxy resin studied was simplified fragment based on diglycidyl ether of bisphenol A (DGEBA). It is found that the distance between the resin and the surface where the adhesion force is maximized is substantially the same for all models. Analysis of the energy-distance plot reveals that the fragment of DGEBA molecule adhere most strongly to the SnO2(001) surface, suggesting that the adhesion force is induced by van der Waals (vdW) interaction.

Computational study of the adsorption of bimetallic clusters on alumina substrate

We performed computational investigation of the adsorption of bimetallic Pd3M2 (where M changes from Ag, Au, Co, Cu, Mn, Ni, Pt, and Ru) cluster on the hydroxylated alumina surface. Previously, it was shown that small silver cluster can control the rate of discharge at the cathode of lithium-oxygen battery. The gap near the fermi energy was shown to control the oxygen reduction, an important reaction for LiO2 formation. Controlling the gap would ultimately control the rate of LiO2 formation. One can vary the size of the cluster to vary the gap, however, this “knob” provides limited variance in the gap. Alloying, in combination with size variation offer a much wider control of the gap, hence the LiO2 formation. Using Density Functional Theory (DFT), we determined the most stable geometry of the bimetallic clusters Pd3M2 and calculated the binding energies of these clusters on the alumina substrate which ranges from 0.2 eV to 0.25 eV depending upon the composition of the alloy-cluster, its orientation and the adsorption site. We also find that Pd atoms bind strongly with the substrate oxygen atoms with an average short bond-length of about 2.2 Å. We explored how the gap at the Fermi level of the system varies as a function of elemental composition and the calculated gap ranges from 0 meV to 90 meV. Charges distribution using Bader analysis was also performed to probe how charges are transferred between the cluster and the substrate. These preliminary results will open the door for more systematic studies of alloy clusters of different size and stoichiometry for Li-O2 battery cathode design.

Molecular Dynamics Simulation Study of Adsorption of Bioinspired Oligomers on Alumina Surfaces.

The adsorption of small oligomers on a model metal oxide surface was studied with atomistically detailed molecular dynamics simulations. The oligomers consisted of two different repeat units: a maleimide, which contains a catechol functional group as in the dopamine residue found in marine adhesive proteins, and a methyl acrylate. A hydroxylated alumina surface was used as the model metal oxide surface. Adsorption interactions were investigated in aqueous as well as anhydrous conditions. In anhydrous conditions, the model oligomers displayed strong adsorption interactions with the surface. However, in aqueous conditions, the adsorption interactions were significantly weakened because of the competition with the water molecules for adsorption sites near the surface. Catechol functional groups in the model oligomers were found to play an important role in adsorption interactions with the alumina surface via hydrogen bonds. However, diverse adsorption properties were observed depending on compositions and sequences of two different repeat units and self-aggregations, indicating that the hydrogen bonding capability of catechol groups is not the sole factor determining adsorption properties.

Scale-up of a NiMoP/γAl2O3catalyst for the hydrotreating and mild hydrocracking of heavy gasoil

This contribution shows the acquired experience during the scale-up of a NiMoP/γAl2O3catalyst employed for the hydrotreating and mild hydrocracking of heavy gasoil. Three different strategies were adopted for preparing catalyst batches at pilot scale. They consisted on co-impregnation ofγ-alumina extrudates with aqueous solutions containing Ni and Mo salts and phosphoric acid in one or two successive steps. The textural, chemical composition, mechanical strength, metallic surface dispersion and elemental radial distribution profile properties were influenced by the impregnation procedure employed. The co-impregnation with diluted Ni, Mo and P solutions in two successive steps is the best way to prepare the catalyst. This procedure provides a catalyst that exhibits better physico-chemical properties and catalytic activity profile than the other impregnation methods investigated. Heat and mass transfer limitations became very important when preparing catalysts in large quantities. The diffusion intra-particle and extra-particle was observed influenced by the density and viscosity properties of the metallic solution, the liquid-solid contact angle, the reactivity of phosphate, polymolybdate and phosphomolybdate species with the alumina surface hydroxyl groups, the raise of temperature produced in the solid particles during the initial impregnation step and the porosity properties of the catalyst support. It was concluded that the fine control of the metal distribution on the alumina surface during the impregnation is crucial for producing highly active uniform catalysts.

Density-Functional Tight-Binding Study on the Effects of Interfacial Water in the Adhesion Force between Epoxy Resin and Alumina Surface.

Adhesion is one of the most interesting subjects in interface phenomena from the viewpoint of wide-range applications as well as basic science. Interfacial water has significant effects on coatings, adhesives, and fiber-reinforced polymer composites, often causing adhesion loss. The way of thinking based on quantum mechanics is essential for a better understanding of physical and chemical properties of adhesive interfaces. In this work, the molecular mechanism of the adhesion interaction between epoxy resin and hydroxylated alumina surface in the presence of interfacial water molecules is investigated by using density-functional tight-binding calculations. Periodic slab model calculations demonstrate that hydrogen bond is an important factor at the adhesion interface. Effects of interfacial water molecules located between epoxy resin and hydroxylated alumina surface are assessed by using a dry model without interfacial water and wet models with water layers of 3, 6, and 9 Å thicknesses. Interesting first- and second-layer structures are observed in the distribution of interfacial water molecules in the tight space between the adhesive and adherend. Energy plots with respect to the displacement of epoxy resin from the alumina surface are nicely approximated by the Morse potential. The adhesion force and stress are theoretically obtained by differentiating the potential curve with respect to the displacement of epoxy resin. Computational results show that the adhesion force and stress are significantly weakened with an increase in the thickness of interfacial water layer. Thus, interfacial water molecules have a clue as to the role of water in the loss of adhesion.

Pressure-driven water flow through hydrophilic alumina nanomembranes

We present an experimental study that focuses on pressure-driven flow of distilled water through γ alumina membranes with 5, 10 and 20 nm pore radii. The nanopore geometry, pore size and porosity are characterized using scanning electron microscopy images taken pre- and post-flow experiments. Comparisons of these images have shown reduction in the pore size, which is attributed to precipitation of hydroxyl groups on alumina surfaces. Measured flowrates compared with the Hagen–Poiseuille flow relations consistently predict 2.2 nm reductions in the pore size for three different membranes. This behavior can be explained by the formation of a thick stick layer of water molecules over hydroxylated alumina surfaces, evidenced by water droplet contact angle measurements that exhibit increased hydrophilicity of alumina surfaces. Other possible effects of the mismatch between theory and experiments such as unaccounted pressure losses in the system or the streaming potential effects were also considered, but shown to be negligible for current experimental conditions.

Molecular Dynamics Simulations of Adsorption of Catechol and Related Phenolic Compounds to Alumina Surfaces

We performed atomistically detailed molecular dynamics simulations to study adsorption behaviors of catechol, which is a key functional group in marine bioadhesives, to two different alumina surfaces in both anhydrous and aqueous conditions. In anhydrous conditions, without competing interactions from water molecules, catechol adsorbed to both hydroxylated and nonhydroxylated alumina surfaces. In aqueous conditions, catechol and several analogous phenolic compounds displaced water molecules and were strongly attracted to the nonhydroxylated alumina surface, which is more hydrophobic. When comparing the phenolic moieties near the hydroxylated alumina surface in aqueous conditions, the catechol molecules displayed the strongest adsorptions mainly through cooperative hydrogen bonding interactions of two neighboring hydroxyl groups with the surface hydroxyl groups of alumina as evidenced by the longer hydrogen bonding lifetimes and the larger number of adsorbed molecules near the surface. Insights gained from...

Fast photo-catalytic degradation of pyridine in nano aluminum oxide suspension systems

UV light can degrade pyridine to ammonia nitrogen at room temperature. The decomposition of pyridine under UV irradiation with the help of aluminum oxide powders was rarely studied. While with the assist of alumina, fast photo-catalytic degradation of pyridine to 100 percentage ammonia nitrogen was realized within an hour. The promising promotion phenomena were confirmed in opening other nitrogen heterocyclic compounds (NHCs) such as quinoline and 2,2′-bipyridine. Scanning electron microscopy images showed the alumina powder was about 100 nm in size. X-ray diffraction results indicated that the sample was mainly a-Al 20 3. Specific surface area of the sample was about 280 m 2/g determined by BET method. The optimum dosages of catalysts and effects of pH value were also tested. There was no clear absorbance of the alumina sample showed in the range of 200-420 nm. It is believed that the interaction between pyridine and alumina surface hydroxyl caused by chemisorptions weakened the carbon-nitrogen bond and led to the promotion of the decomposition.

The reactivity of quaternary ammonium- versus potassium- fluorides supported on metal oxides: paving the way to an instantaneous detoxification of chemical warfare agents

The reactions of the chemical warfare agents (CWAs) 2,2'-dichloroethyl sulfide (HD), O-ethyl S-2-(diisopropylamino)-ethyl methylphosphonothioate (VX) and isopropyl methylphosphonofluoridate (GB) with various metal oxide-supported quaternary ammonium fluorides (QAF) and/or potassium fluoride (KF) reagents are described. These active sorbents, which were prepared by a modified procedure, include alumina, silica and titania, enriched with "available" (not bound to the surface) fluoride ions. Alumina-based fluoride reagents were found to be more active than their silica or titania counterparts. QAF/Al(2)O(3) reagents, compared to KF/Al(2)O(3), exhibit an exceptional reactivity toward HD, as demonstrated both in reaction rates and product identity. For example, with TBAF, t(1/2) is 15 min for the formation of the elimination product divinyl sulfide (DVS), while with KF, t(1/2) is 10 h for the formation of the hydrolysis product thiodiglycol (TDG). On the other hand, both sorbents reacted similarly against the nerve agents GB or VX. In order to increase the "available" fluoride content on the solid surface, the mixed active sorbent TBAF/KF/Al(2)O(3) (20/20/60) was developed. On this powder, all three CWAs were degraded instantaneously at the low loading of 1 wt% (t(1/2) < 2 min) and rapidly at the higher loadings of 5-10 wt% (t(1/2) of minutes scale). We assume that the relatively large amount of inorganic fluoride (KF) acts synergistically as a reservoir for the more reactive organic fluorides (TBAF). Moreover, the alumina surface hydroxyl groups may also operate as a water reservoir for the hydrolysis of VX or GB. Therefore, TBAF/KF/Al(2)O(3) might be considered as a promising destructive sorbent for CWAs.

Small gold species at hydroxylated alumina surfaces. A computational study using embedded-cluster models of α-Al 2O 3(0001)

We calculated equilibrium structures for adsorption complexes of gold monomers, dimers, and trimers on a α-Al 2O 3(0001) model surface, partially covered by μ 1 and μ 3 hydroxyl groups. We applied a scalar-relativistic gradient-corrected density functional method to cluster models of the support that were embedded in an elastic polarizable environment. The most stable structures, with calculated adsorption energies in the range 0.81–1.74 eV, feature coordination bonds to surface μ 1-OH group and are 0.24–0.79 eV more stable than the corresponding Au n complexes on a dehydrated surface. Isomeric rearrangements of the most stable complexes are hindered by barriers of 0.65–1.08 eV.

Reactivity of a Hydroxylated Alumina Surface in the Presence of NO Diluted in N2: A PM-IRRAS in Situ Investigation

The interaction of NO (900 ppm NO/N2, 2 × 105 Pa, RT) with a hydroxylated alumina surface was investigated by in situ polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). The resulting spectra reveal two distinct periods, namely 0-1 day and 1-13 days of exposure. During the first period, NO exposure primarily leads to the formation of nitrites (1523, 1470, 1330, 1235, and 1130 cm-1), HONO (1235 cm-1), and perhaps also some nitrates (1650-1620 cm-1) and hyponitrites (1340 cm-1). After one day of exposure, the transformation of nitrites into nitrates is demonstrated by the decrease in intensity of the band at 1235 cm-1, attributed to the ν1 stretching vibration of bridging bidentate nitrites and also by the presence of an isosbestic point at 1260 cm-1. Various nitrate species including monodentate, bidentate, bridged, and water-solvated nitrates (1600-1280 cm-1) are then formed on the surface. The stability of the formed ad-NOx species during N2 purges, NO/N2, O2 or H2O/O2 exposures was also investigated in detail in this study. These experiments revealed an equilibrium at the surface between water-solvated and oxide-coordinated nitrates.

An ab initio analysis of adsorption and diffusion of silver atoms on alumina surfaces

The adsorption and diffusion of silver adatoms on a variety of different surface terminations of alumina was studied with density functional theory. Both Al and O terminated surfaces of α-Al 2O 3(0 0 0 1), as well as hydroxylated versions of these surfaces, were examined. The results indicate that Ag bonds weakly to the Al terminated surface and has low barriers for diffusion. This suggests that the diffusion of Ag on the alumina terminated surface is very rapid. Ag bonds much more strongly, however, to the O-terminated surface which results in higher diffusion barriers. Calculations for Ag on hydroxylated surfaces indicate that the presence of water serves to lower barriers to diffusion for Ag atoms as compared to the oxygen terminated surface but decreases diffusion as compared to the aluminum terminated surface. The results described herein help to provide a more detailed understanding of the observed surface wetting of other transition metals upon hydroxylated alumina surfaces.

Structure and property correlation for Ag deposition on α-Al 2O 3—a first principle study

The nature of bonding at the interface between deposited silver and (0 0 1) surface of α-Al 2O 3 for both Al-terminated and OH-terminated has been investigated using a periodic ab initio method. Substantial inter-planar relaxations within the alumina were found at both the interfaces and the bulk. The periodic calculation with Ag deposition shows that 10% of Ag loading on alumina results maximum stability. Now, this is known that, the clean alumina surface only exists at UHV condition and normally the alumina surface prefers to stay hydroxylated. We have therefore compared the silver bonding over hydroxylated alumina surface and confirmed the fact that the hydroxylated surface binds silver weakly in comparison to the clean surface and it recommends that the silver cluster over the hydroxylated surface begins to join in to form three-dimensional nuclei. The deposited Ag forms a cluster on top of the alumina surface. The Ag atomic packing was monitored to rationalize the role of packing on activity of silver. Three low-index Ag surfaces (1 0 0), (1 1 0) and (1 1 1) are investigated via the ab initio density functional calculations with ultrasoft potentials. We have monitored the relation between Ag atomic packing and its electronic properties. The results show that the structural and electronic property of Ag deposited on alumina surface depends significantly on atomic packing. Ag(1 1 0) over clean alumina surface shows highest surface energy and smallest work function, whereas for the OH-terminated surface it is the Ag(1 1 1). The results are discussed in view of the existing experimental data and models of metal–oxide interface.

Piperidine Adsorption on Hydrated α-Alumina (0001) Surface Studied by Vibrational Sum Frequency Generation Spectroscopy

The adsorption of piperidine vapor on the hydrated alumina (alpha-Al2O3, corundum) (0001) surface was investigated using vibrational broad bandwidth and scanning sum frequency generation (SFG) spectroscopy. The interfacial vibrational signature in the C-H stretching region of piperidine at the alumina (0001) surface is shown to be a sensitive spectroscopic probe revealing the adsorption mechanism. The neat piperidine surface, aqueous piperidine surface, and aqueous piperidium chloride surface were also investigated in the C-H stretching region by SFG to establish vibrational reference frequencies. After piperidine adsorption, piperidine vapor was removed and piperidine was found to be chemisorbed onto the alumina (0001) surface through protonation by surface hydroxyl groups. The O-H stretching region of the alumina surface before and after piperidine adsorption was also investigated, and the results revealed the decrease of the surface number density of alumina surface hydroxyl groups.

Simulating the atomic layer deposition of alumina from first principles

Atomic Layer Deposition (ALD) is a chemical vapour deposition technique that is suitable for the controlled growth of thin, conformal oxide films. Insulating layers of alumina (Al2O3) are fabricated by ALD for electroluminescent flat-screen-displays, memory capacitors and read/write heads. Successful precursors for alumina ALD are trimethylaluminium (TMA) and water. Various models for the mechanism of alumina ALD have been proposed but definitive evidence for the surface intermediates is lacking. We therefore use density functional theory to investigate the atomic-scale structure and reactivity of bare and hydroxylated alumina surfaces with respect to TMA. The competition between dissociative chemisorption and molecular desorption is quantified. The calculations show that TMA adsorbs and dissociates at both O and OH sites on the surface, but that these lead to different ALD efficiencies. In this way we clarify how the degree of surface hydroxylation and hence the substrate temperature affects the ALD growth rate.

Temperature gradients and frictional energy dissipation in the sliding of hydroxylated α-alumina surfaces

The dynamics of friction and energy dissipation of a small nanoscale hydroxylated alumina surface sliding across a large identical surface were investigated by molecular dynamics (MD) simulations. The energy released into both the sliding and stationary surfaces and the energy transfer from the sliding interface to the bulk were investigated by monitoring the temperature of the surfaces and their sublayers. Steady state temperature gradients are found for the sublayers of each surface as a result of the balance between the heat released from the sliding friction and heat transfer to the bulk. The MD simulations show periodic friction energy release from the interface to the surfaces, giving rise to periodic changes in the temperature of the surface sublayers. This temperature depends on the sliding velocity. The periodicity in the friction energy release arises from the periodic unit cell structures of the surfaces. Atomic-level vibrational motions and interdigitation of the surface atoms give rise to the frictional energy of the sliding surfaces. Velocity distributions are determined for atoms within different surface layers and they become more Boltzmann-like the further the layers are from the interface and if the sliding is fast and not in the stick–slip regime. Tribological properties for continuous sliding across the same regime of the bottom surface, by using periodic boundary conditions, differ from those for sliding across a large bottom surface.

The influence of OH groups on the growth of rhodium on alumina: a model study

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.