α-Al2O3 Surface Research Articles

Insights on the Structural and Dynamic Properties of Corundum–Water Interfaces from First-Principle Molecular Dynamics

The interaction of water with hydroxylated mineral surfaces is central to numerous natural and technological processes; yet, at the atomic level much remains to be understood about the structure and dynamics of interfacial water and H-bonding networks. This work applies first-principle molecular dynamics to examine the relationship between surface structure and interfacial water behavior at four hydroxylated corundum (α-Al2O3) surfaces. The four surfaces studied include the (001), (012), (110), and (113) surfaces; they have distinct surface functional groups (−OH, μ-OH, and μ3-OH) and topography (planar, corrugated, and two-tiered). Interfacial H-bonding was investigated in detail by analyzing atomic density 1-D profiles, angular distribution of surface O–H bonds, and the dipole moment orientation of interface H2O molecules and by calculating vibrational density of states in the high-frequency region. Our results reveal distinct differences in the interaction of interfacial water and H-bonding networks at the four surfaces. Specifically, inter- and intramolecular H-bonding depends on surface site coordination geometry, neighboring functional groups, and surface topography. Moreover, intramolecular H-bonding involving in-plane surface hydroxyl groups may persist, thus limiting intermolecular H-bonding. Equally, first solvation layer water molecules are preferentially orientated at the (001), (012), and (113) surfaces, whereas water molecules at the (110) surface show minimal preferred orientation. The distinct differences in the behavior of interfacial water and H-bonding at the four surfaces may play a substantial role in determining the reactivity of the surfaces toward processes such as ion adsorption–desorption and dissolution–precipitation.

First principles calculation of hydrogen adsorption, dissociation and diffusion on Cr/Ni/Mn doped α-Al2O3 (0001) surface

Cr, Ni and Mn are common impurities observed in Al2O3 tritium permeation barrier in experiments, to acquire influence of these metal impurities on the interaction of hydrogen isotopes with α-Al2O3 surface, energies and geometric structures of H2 adsorption, dissociation and diffusion on Cr/Ni/Mn doped α-Al2O3 (0001) surface have been investigated by first-principles. It's found that Cr and Ni have slight effect on H2 adsorption and dissociation on α-Al2O3 (0001) surface due to the similar adsorption and dissociation behavior and close activation energies before and after doping. On Mn doped α-Al2O3 (0001) surface, the adsorption energies for H2 are close to that on undoped surface, while the lowest activation energies for H2 dissociation and H atoms recombination are 0.32 eV and 0.24 eV, which are obviously lower than that for H2 dissociation on undoped α-Al2O3 (0001) surface. This finding reveals that Mn doping is favor to H2 dissociation and also beneficial to H atoms recombination on α-Al2O3 (0001) surface. The lowest activation energy is 1.36 eV for H atom diffusion from surface to bulk on Mn doped α-Al2O3 (0001) surface and obviously higher than that of H diffusion through RAFM steels measured through H2 gas evolution permeation experiment, indicating that Mn surface doping prevents H permeation in Al2O3 tritium permeation barrier from the view of diffusion activation energy. The results are constructive for optimization the preparation technique of TPBs.

High CO reactivity of cobalt oxide catalyst deposited on alumina powders by rotary chemical vapor deposition

Cobalt oxide (Co3O4) catalysts supported on α-Al2O3 were prepared by a rotary chemical vapor deposition (CVD) method. Although the specific surface area of α-Al2O3 powders was extremely low, finely dispersed nanoparticles of Co3O4 with the size of several nanometers were formed on the support under the deposition condition of 370 °C for 30–120 min. Despite the minimal Co3O4 content of 0.10–0.36 wt%, the Co3O4/α-Al2O3 exhibited the significant catalytic activity for CO oxidation; the temperatures at which the conversion of CO reached 20% (T20) were 157–189 °C and the conversion reached 100% until 300 °C. Thus, a rotary CVD method can finely control the dispersibility of Co3O4 on α-Al2O3 surface, which was related to high catalytic activity.

Synchrotron X-ray standing wave Characterization of atomic arrangement at interface between transferred graphene and α-Al2O3(0001)

Graphene is expected to be one of the most promising materials for nanoelectronics and spintronics. In most graphene-based devices, the graphene channel is placed on insulating substrates. Therefore, the study of interfacial interactions between graphene and the insulator surface is of critical importance. In this study, the vertical arrangement of graphene which is transferred on α-Al2O3(0001) has been studied by normal incidence X-ray standing wave (NIXSW) technique. The analysis of the NIXSW profile reveals that the graphene layer is located at 3.57 Å above the α-Al2O3(0001) surface, which is larger than the interlayer distance of graphite (3.356 Å). Micro-Raman spectroscopy shows that the transferred graphene has a limited spatial distribution of hole concentration. The present study shows that transferred graphene on the sapphire substrate followed by vacuum-annealing has an atomically flat surface free from residual contaminations such as organic compounds and there occurs the hole-doping in graphene.

Surface-Dependent Activation of Model α-Al2 O3 -Supported P-Doped Hydrotreating Catalysts Prepared by Spin Coating.

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS2 hydrotreating catalysts supported on α-Al2 O3 single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al2 O3 crystal orientations, C(0001), A(11 0), M(10 0) and R(1 02) that exhibit different speciations of surface -OH. 31 P and 95 Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS2 phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 0)>A(11 0), R(1 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al2 O3 surface orientations from the C(0001) to the R(1 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 0), R(1 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS2 . The surface-dependent phosphorus doping on model α-Al2 O3 supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al2 O3 carrier.

Density Functional Theory and Thermodynamics Modeling of Inner-Sphere Oxyanion Adsorption on the Hydroxylated α-Al2O3(001) Surface.

The inner-sphere adsorption of AsO43-, PO43-, and SO42- on the hydroxylated α-Al2O3(001) surface was modeled with the goal of adapting a density functional theory (DFT) and thermodynamics framework for calculating the adsorption energetics. While DFT is a reliable method for predicting various properties of solids, including crystalline materials comprised of hundreds (or even thousands) of atoms, adding aqueous energetics in heterogeneous systems poses steep challenges for modeling. This is in part due to the fact that environmentally relevant variations in the chemical surroundings cannot be captured atomistically without increasing the system size beyond tractable limits. The DFT + thermodynamics approach to this conundrum is to combine the DFT total energies with tabulated solution-phase data and Nernst-based corrective terms to incorporate experimentally tunable parameters such as concentration. Central to this approach is the design of thermodynamic cycles that partition the overall reaction (here, inner-sphere adsorption proceeding via ligand exchange) into elementary steps that can either be fully calculated or for which tabulated data are available. The ultimate goal is to develop a modeling framework that takes into account subtleties of the substrate (such as adsorption-induced surface relaxation) and energies associated with the aqueous environment such that adsorption at mineral-water interfaces can be reliably predicted, allowing for comparisons in the denticity and protonation state of the adsorbing species. Based on the relative amount of experimental information available for AsO43-, PO43-, and SO42- adsorbates and the well-characterized hydroxylated α-Al2O3(001) surface, these systems are chosen to form a basis for assessing the model predictions. We discuss how the DFT + thermodynamics results are in line with the experimental information about the oxyanion sorption behavior. Additionally, a vibrational analysis was conducted for the charge-neutral oxyanion complexes and is compared to the available experimental findings to discern the inner-sphere adsorption phonon modes. The DFT + thermodynamics framework used here is readily extendable to other chemical processes at solid-liquid interfaces, and we discuss future directions for modeling surface processes at mineral-water and environmental interfaces.

The compatibility between environmentally friendly insulation gas C4F7N and α-Al2O3 (0001) surface: Theoretical and experimental insights

In this paper, the compatibility between C4F7N and α-Al2O3 (0001) surface has been evaluated based on the density functional theory (DFT). The adsorption energy, total charge transfer, states analysis, and electron density difference were obtained to analyze the interaction behavior of C4F7N on α-Al2O3 (0001) surface. The results demonstrated a chemisorption between C4F7N and α-Al2O3 (0001) surface. Moreover, the pathway of C4F7N dissociated on α-Al2O3 (0001) surface after the adsorption was determined. However, the activation energy required for the dissociation is too large to achieve, which means that the dissociation of C4F7N on α-Al2O3 surface is difficult to occur. Additionally, the thermal compatibility between α-Al2O3 and C4F7N were verified by experiments, which shows a certain compatibility between C4F7N and α-Al2O3 (0001) under up to 170 ℃. The results demonstrate that α-Al2O3-based materials can be used as insulators in C4F7N-filled electrical equipment.

Theoretical study of perfluorodecyltrimethoxysilane and polyethylene glycol adsorption/dissociation reactions on dry and hydrated Al2O3(0 0 0 1) surface

FAS-17 (CF3(CF2)7(CH2)2Si(OCH3)3) and PEG (H(OCH2CH2)nOH) were always used for modifying surfaces for superhydrophobicity and superhydrophilicity, respectively. In order to understand the wetting property and durability of aluminum surface, the interactions between FAS-17/PEG and surface were investigated. The adsorption/dissociation mechanism of FAS-17/PEG on Al-terminated α-Al2O3(0001) surface have been studied with the DFT method. The 1,2-dissociation products of FAS-17 and PEG are more stable than the 1,4-dissociation products on the dry surface. The products on the dry surface are more stable than on the hydroxylated surface, which explains the phenomenon that the durability of superhydrophobicity decreased dramatically in wet environment. The dissociated FAS-17 molecules are stable on the 1,2-hydroxylated surface when the coverage is up to 50%. A layer of air appears to be trapped at the surface/modifier interface which is prerequisite for superhydrophobicity.

Preparation, characterization and application of Al2O3 nanoparticles for the protection of boiler steel tubes from high temperature corrosion

In this study, α-Al2O3 nanoparticles were prepared using a simple sol-gel method. Synthesized nanoparticles with NiCrAlY alloy were used as coating layer for high temperature corrosion control of boiler tubes in absence and presence of fuel ash. Thermal plasma technique was used for coating process. Corrosion rates of austenitic stainless steel tubes were evaluated via weight loss technique as a function of temperature. Prepared α-Al2O3 nanoparticles and surface morphology of austenitic stainless steel tubes were characterized by XRD, SEM and EDS analyses. The results show that the corrosion rate increases with increasing temperatures, in absence and presence of coating. Maximum coating efficiency was 82 and 88% in absence and presence of fuel ash respectively.

Catalytic degradation of ethylene oxide over Ag/α-Al2O3

Ethylene oxide (EO) degradation over Ag/α-Al2O3 catalysts involves bifunctional pathways, whereby α-Al2O3 surfaces catalyze hydration, isomerization, dimerization, and oxidation while Ag surfaces facilitate combustion and isomerization of ethylene oxide. EO degradation reactions run over Ag/α-Al2O3, α-Al2O3, and intrapellet mixtures thereof with co-feeds of O2 and oxygenate products show that Ag surfaces are more reactive than α-Al2O3 and that oxygenate products generated over α-Al2O3 combust over Ag much more (>30x) readily than EO. These observations suggest that additives which mitigate surface hydration of α-Al2O3 and combustion activity of Ag will most effectively mitigate EO degradation.

First-principles insights of hydrogen diffusion dynamics at the α-Al2O3 (0001) surface

Hydrogen diffusion dynamics in the α-Al2O3 is very important because it associates directly with the performance of α-Al2O3 as the tritium permeation barrier (TPB) material in the tritium handling systems. In this work, the kinetics of hydrogen diffusion at the α-Al2O3 (0001) surface and hydrogen permeation into the α-Al2O3 bulk have been systematically studied by using the first-principles calculations. Results show that the vacancy defect can reduce the hydrogen migration barrier, while the Fe-doping defect can enhance the hydrogen migration barriers. This result is due to the dopant Fe showing bivalent instead of trivalent, which leads to the formation of a negatively charged center due to the substitution of divalent Fe for trivalent Al. In addition, results show quantitatively that the outer three oxygen layers (9 atomic layers) play a key role in hydrogen resistance. Hydrogen permeation from the outmost surface to the subsurface is the rate-determining step, and the corresponding energy barrier is about 3.327 eV.

Investigation on adsorption of Ar and N2 on α-Al2O3(0001) surface from first-principles calculations

To better understand the interaction between purification gas media (Ar and N2) and the main solid inclusion α-Al2O3 in aluminum melt, first-principle calculation was employed to investigate the adsorption energy, bonding, electronic characteristics, and density of states of Ar and N2 adsorption on α-Al2O3(0001) surface. We determined that the adsorption stability of N2 is much higher than Ar, it is thus speculated N2 can serve as a better refiner to remove α-Al2O3 from molten aluminum. Also, it is found FCC sites are calculated to be energetically preferred for adsorption in these two cases. Besides, it is confirmed Ar and N2 both have obvious bonding effect at the Top site, where N2 has a large charge transfer at the Top site and forms covalent bonds. In particular, the adsorption of Ar and N2 is found mainly owing to the hybridization of Ar-3p and Al-3p orbitals, and the hybridization of N2-p and Al-p orbitals. On the other hand, with Ar adsorbed at hcp or N2 adsorbed at top site of α-Al2O3(0001) surface, the conductivity of α-Al2O3 was enhanced, and the adverse effects of α-Al2O3 to the corrosion resistance of aluminum was promoted.

First principles investigation of hydrogen adsorption and dissociation on Fe doped α-Al2O3 (0001) surface

Energies and geometric structures of H2 molecule adsorption and dissociation on Fe doped α-Al2O3 (0001) surface have been investigated by the first-principles calculation since Fe is a common impurity in Al2O3 tritium permeation barrier. It’s found that Fe prefers to substitute the surface Al site and the adsorption of H2 on Fe doped α-Al2O3 (0001) surface is physisorption. The adsorption energies and dissociation activation energies of H2 absorbed on Fe doped α-Al2O3 (0001) surface are slight lower than that of H2 on undoped α-Al2O3 (0001) surface. The activation energies of H atoms recombination are 0.65 eV and 0.53 eV and obviously smaller than the values of 0.92 eV and 1.03 eV for H atoms recombination on undoped α-Al2O3 (0001). Therefore, Fe surface doping has slight effect on the reactivity for H2 adsorption and diffusion on α-Al2O3 (0001) surface, but is beneficial to α-Al2O3 as a tritium barrier from the view of H atoms surface recombination.

Growth mechanism of house-of-cards aggregates of alumina platelets containing Na2O–B2O3–SiO2 glass flux

Porous alumina bodies, intended for use as heat-insulating refractory materials, were fabricated by a high-temperature evaporation method and characterized. A series of flux systems was used by adding a third component to Na2O–B2O3 glass in addition to boric acid and sodium carbonate. When SiO2 was added as the third component, the primary alumina particles grew anisotropically, forming a plate-like shape, and the house-of-cards structure was self-organized. The anisotropic growth of alumina platelets was promoted by the solid solution of Si4+ ions in the flux on the α-Al2O3 surface. Furthermore, the bonding between the alumina platelets was strengthened by the high-SiO2-concentration flux. Our typical alumina body had a porosity of 71.5%, a compressive strength of 3.7 MPa, a shrinkage rate of 2.6% when reheated at 1700 °C, and a thermal conductivity of 0.24 W m−1•K−1 at 1000 °C. Thus, the present alumina bodies are expected to find application as high-performance heat-insulating refractory materials.

Competitive reactions of SO2 and acetic acid on α-Al2O3 and CaCO3 particles

Heterogeneous reactions between gaseous pollutants and mineral particles have gradually become a research hotspot in the field of atmospheric chemistry. In this paper, competitive reactions between SO2 and acetic acid on the surface of α-Al2O3 and CaCO3 particles were studied by the diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) technique in dark and dry conditions. At the same time, the temporary evolution of the integrated absorbance of acetate and sulfite was investigated to further understand the interaction of SO2 and acetic acid on the mineral particles. On the surface of α-Al2O3 particles, acetate and sulfite can compete for surface-active sites, resulting in a decrease in the total amount of acetates. In dark and dry conditions, the effect of acetic acid on SO2 cannot be obtained by the DRIFTS method. On the surface of CaCO3 particles, SO2 can have a competitive impact on acetic acid by grabbing active sites, leading to a slight decrease of the amount of acetates. The heterogeneous reaction of SO2 can be impeded by coexisting acetic acid, resulting in a drastic reduction of the number of sulfites. It can be seen that the formation mechanisms of acetate and sulfite on the surface of different mineral particles in the atmosphere are different, which provides a variety of ideas and possibilities for the formation of related inorganic and organic salts in the atmosphere.

Adsorption and decomposition of SF6 molecule on α-Al2O3 (0 0 0 1) surface: a DFT study

Based on the first principle, the interaction process between SF6 molecule and α-Al2O3 (0 0 0 1) surface was calculated. The results show that, under five adsorption sites, SF6 can form a relatively stable chemical adsorption at the O-3 site on α-Al2O3 (0 0 0 1) surface, while the other sites mainly showed physical adsorption processes. At O-3 site, 0.664 e electrons were transferred from α-Al2O3 to the SF6 molecule, and DOS analysis indicates that there was a strong electron orbital interaction between S and F atoms in SF6 and Al and O atoms on α-Al2O3 surface. After the adsorption, the molecule structure of SF6 changed significantly, the S–F bonds were elongated and partially broken, and the whole SF6 showed a tendency to be close to the surface of α-Al2O3. The energy barrier for SF6 decomposition was 0.147 eV. The results show the adsorption and decomposition process of SF6 molecules on the surface of α-Al2O3, which proves that α-Al2O3 has certain catalytic properties. The results indicate a simulation support for the efficient and harmless treatment of SF6 gas with Al2O3 catalysts.

Vibrational spectra of dissociatively adsorbed D2O on Al-terminated α-Al2O3(0001) surfaces from ab initio molecular dynamics.

Water can adsorb molecularly or dissociatively onto different sites of metal oxide surfaces. These adsorption sites can be disentangled using surface-sensitive vibrational spectroscopy. Here, we model Vibrational Sum Frequency (VSF) spectra for various forms of dissociated, deuterated water on a reconstructed, Al-terminated α-Al2O3(0001) surface at submonolayer coverages (the so-called 1-2, 1-4, and 1-4' modes). Using an efficient scheme based on velocity-velocity autocorrelation functions, we go beyond previous normal mode analyses by including anharmonicity, mode coupling, and thermal surface motion in the framework of ab initio molecular dynamics. In this way, we calculate vibrational density of states curves, infrared, and VSF spectra. Comparing computed VSF spectra with measured ones, we find that relative frequencies of resonances are in quite good agreement and linewidths are reasonably well represented, while VSF intensities coincide not well. We argue that intensities are sensitively affected by local interactions and thermal fluctuations, even at such low coverage, while absolute peak positions strongly depend on the choice of the electronic structure method and on the appropriate inclusion of anharmonicity.

Comparison of interactions between aluminium hydride oxide surfaces and three energetic plasticizers: DFT calculations

Abstract α- and γ-alumina (α-Al2O3 and γ-Al2O3) are used to model the surface oxide layer of AlH3 to investigate the micro-mechanism for the interactions of three energetic plasticizers: Bis-2,2-dinitropropyl formal (BDNPF), N-butyl-N-(2-nitroxyethyl) nitramine (BuNENA) and nitroglycerin (NG) with AlH3 surface oxidation layers. First principle density functional theory (DFT) calculations with all-electron double numerical basis functions (DND) are employed. It is found that three energetic plasticizers can be strongly adsorbed by the oxide film of AlH3 and release a large amount of heat. Moreover, BuNENA and NG containing -ONO2 groups easily undergo barrierless decomposition reactions on the α-Al2O3 (0001) surface and spontaneously produces NO2 radicals, causing significant changes in the structures of BuNENA and NG molecules as well as of the α-Al2O3 (0001) surface. Furthermore, NG can also undergo a barrierless decomposition reaction on the γ-Al2O3 (110) surface. However, BuNENA does not decompose on the γ-Al2O3 (110) surface, and it is not easy for BDNPF to decompose on either α-Al2O3 (0001) or γ-Al2O3 (110) surfaces; hence, it remains relatively stable. According to these results, it is suggested that the order of compatibility among the three energetic plasticizers with AlH3 is BDNPF > BuNENA > NG.

Atomistic Simulation of Impurities Segregation to Free Surfaces of α-Al2O3

Segregation of di- and trivalent impurities to the (00.1), (01.2) and (11.3) surfaces of α-Al2O3 has been investigated using molecular mechanics simulation. In this work, the focus has been on segregation energy calculations depending on impurity size, distance to the surface, and surface coverage. It has been shown the anisotropic segregation of dopants. The segregation energy to the most stable (00.1) surface has the lowest absolute value for all investigated impurities. For trivalent impurities at the (01.2) surface the dependence of the segregation energy on surface coverage has the minimum, which corresponds to the ordered arrangement of dopants. At the (11.3) surface the multilayer segregation has been observed, whereas at the (00.1) and (01.2) surfaces the model of monolayer segregation is acceptable in most studied cases.

Adsorption and dissociation of water molecules at the α-Al2O3(0001) surface: A 2-dimensional hybrid self-consistent charge density functional based tight-binding/molecular mechanics molecular dynamics (2D SCC-DFTB/MM MD) simulation study

Interfacial adsorption and dissociation of water molecules at the α-Al2O3(0001) surface were studied using a novel hybrid 2D-periodic Self-Consistent Charge Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics (SCC-DFTB/MM MD) simulation strategy. The (0001) surface of α-Al2O3 (C-plane) was considered to assess the structural and dynamical behavior of adsorbed water at the interface region. In-plane distribution profiles of oxygen and hydrogen with respect to the non-periodic z-coordinate provide detailed information on the ordering of monolayer adsorbed water molecules at the interface. Dissociation of water molecules occured on the picosecond time scale, indicated by the formation of an additional plane-H peak located at 0.056 nm. Along simulation 4.2 OH groups per nm2 were formed at the α-Al2O3(0001) surface.