Γ-alumina Surface Research Articles

Structure and Energetics of (111) Surface of γ-Al2O3: Insights from DFT Including Periodic Boundary Approach.

The (111) surface of γ-alumina has been reexamined, and a new (111) surface model has been suggested. The local structure of this new surface of γ-alumina, (111)n, has been optimized by the density functionals along with the full electron basis sets by using periodic boundary condition. This newly modeled (111)n surface is characterized by the same stability as that of the (110) surface, and its surface energy amounts to 2.561 J/m2, only about 0.002 J/m2 larger than that of (110). Three different types of the tricoordinated Al centers have been identified on (111)n. Molecular orbital (MO) analysis and the population analysis demonstrate that one type of Al, Al(I), is a nonpaired electron center. The singly occupied MO on Al(I) center is expected to play an important role in the catalytic activities of the γ-alumina. Moreover, the neighboring Al (Al(III)) on the (111)n surface provides suitable acceptance position for the electron donating groups. The defected surfaces of (111)n are found to be having a similar stability. The detachment of Al(I) from the (111)n surface results in disappearance of the nonpaired electron centers. Meanwhile, the attachment of Al(I) on (111)n surface will produce rich nonpaired electron centers on this new surface. Therefore, this newly defined surface is expected to attract the research interests in the catalytic activities of γ-alumina.

Thermodynamic Characterization of the Hydroxyl Group on the γ-Alumina Surface by the Energy Distribution Function

Controlling water adsorption on γ-alumina, i.e., the quantity and nature of the surface hydroxyl groups, is essential for adjusting the acidic/basic properties of the surface. IR and DFT studies have shown that different OH groups, each of them characterized by its own chemical environment and adsorption properties, can be present on the surface of γ-alumina. However, quantifying this surface heterogeneity and predicting the influence of the synthesis and activation conditions is still a challenging problem. In this paper, a detailed experimental study is conducted on a γ-alumina sample obtained by thermal decomposition of a commercial boehmite. Using a thermogravimetric setup, both water-adsorption equilibrium and desorption kinetics were acquired in a large range of controlled experimental conditions (1 Pa < partial pressure of water <1400 Pa ; 100 °C < temperatures <600 °C). The energy distribution function (EDF) of water-adsorption enthalpy is evaluated on the basis of the theories developed for stron...

Role of Transient Co-Subcarbonyls in Ostwald Ripening Sintering of Cobalt Supported on γ-Alumina Surfaces

The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al2O3 surfaces have been investigated using density functional theory (DFT) with a view to elucidate possible mobile species on these surfaces, which can act as agents in the Ostwald ripening process. The two most stable alumina surfaces γ-Al2O3(100) and γ-Al2O3(110) were probed at different levels of hydration. The stability of cobalt subcarbonyl species on γ-Al2O3(100) at high partial pressure of CO (10 bar) increases with increasing number of CO ligands attached to the central cobalt atom up to Co(CO)3 but exhibits a more complex behavior on γ-Al2O3(110). The effect of the hydration level on the stability of cobalt subcarbonyls was investigated. The interpretation of the DFT results in a thermodynamic model shows that at equilibrium the main cobalt subcarbonyl species present on the alumina surface at ca. 500K in the presence of CO are Co(CO)3 and Co(CO)4, with Co(CO)3 being the dominant species on dry γ-Al2O3(100) and w...

Effect of the Metal–Support Interaction on the Adsorption of NO on Pd4/γ-Al2O3: A Density Functional Theory and Natural Bond Orbital Study

Density functional theory (B3LYP) was employed to analyze the metal–support interaction in a Pd4 cluster supported on a γ-alumina model (Al14O24H6) and its effect on the adsorption of a single NO molecule. Our results show that the Pd4–Al14O24H6 interaction leads to a reduction in the cohesion energy among palladium atoms, promoting greater dispersion on the γ-alumina surface. NO preferentially adsorbs in a tilted orientation on the palladium atom anchored on two oxygen atoms, with an adsorption energy of −25.4 kcal mol–1, which is in good agreement with the experimental result of −27.2 ± 1.4 kcal mol–1. The palladium–alumina interaction causes a significant reduction in the NO adsorption energy, suggesting the possible existence of a strong metal–support interaction (SMSI). We observe that the larger the decrease in the adsorption energy, the higher the electronic component of the metal–support interaction. NBO (natural bond orbital) calculations show that such an effect also leads to attenuation of the ...

Theoretical study on the adsorption and relative stability of conformers of l-ascorbic acid on [formula omitted] - alumina (100) surface

The adsorption energies (Eads) and relative stabilities of selected conformers of the most stable tautomer of l-ascorbic acid (vitamin C) on the dehydroxylated γ-alumina (100) surface were calculated in both gas phase and solvent (water) using the density functional theory (DFT) method. The selected conformers were related to the different rotational angles of OH groups of l-ascorbic acid. The conformational analysis of bare tautomer in both gas and water showed that the conformer No.20 (conf. 20) and 13 (conf. 13) with the dihedral angles of H15O10C11C9 (−73°) and H20O19C9C11 (−135°) were the most stable and unstable conformers, respectively. The performed calculations in the presence of surface showed that the interaction of the conformers with the surface changes their relative stabilities and structures in both gas phase and water. The Ead of each conformer was calculated and it was determined that conf. 8 and conf. 16 have the highest value of Ead in the gas phase (−62.56 kcal/mol) and water (−54.44 kcal/mol), respectively. The optimized structure of each conformer on the surface and the number of hydrogen bonds between it and surface along with their bond lengths were determined.

DFT study of Au adsorption on pure and Pt-decorated γ-alumina (110) surface

In this study, the adsorption properties of a Pt-decorated γ-Al2O3 (110) surface (Pt-alumina) were investigated to determine the favorable sublayer for Au adsorption using first principles calculations. After full structural relaxation of various configurations, our calculated results indicated that the lowest binding energy and the distance between the Pt atom and the nearest atoms in the alumina (O atom) were −1.87eV and 2.36Å, respectively. Finally, the adsorption energy, geometry, and electronic structure for the adsorption of an Au atom onto pure (Au-alumina) and Pt-decorated alumina (Au/Pt-alumina) were examined. The binding energy for the Au/Pt-alumina system was −3.06eV, which is more than that for the Au-alumina system (−1.40eV). Therefore, the results from our first principles calculations predict that the Au atom adsorptive capability on Pt-alumina is better than that of pure alumina.

Macroscopic and microscopic investigation of adsorption and precipitation of Zn on γ-alumina in the absence and presence of As

Contaminants zinc (Zn) and arsenate (As) often coexist in soils. However, little is known concerning the impacts of coexisting As on Zn adsorption and precipitation on soil minerals. In the present study, adsorption and precipitation of Zn on γ-alumina in the absence and presence of arsenate was investigated employing batch experiments and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Results indicated that Zn formed edge-sharing tetrahedral surface complexes at pH 5.5 and Zn-Al LDH-like (layered double hydroxide) precipitates at pH 7.0 on the surface of γ-alumina. The presence of arsenate significantly enhanced Zn sorption densities, and remarkably changed its bonding environment. At pH 5.5, SR-XRD (Synchrotron Radiation-based X-ray Diffraction) and EXAFS showed that koettigite-like precipitate were formed in the cosorption of Zn and As on γ-alumina regardless of the addition sequence of As and Zn. At pH 7.0, when Zn was preequilibrated with γ-alumina prior to the As introduction, mixed Zn-Al LDH-like and amorphous adamite-like precipitates formed. However, when Zn and As were added simultaneously, only amorphous adamite-like precipitate was observed. Zn inner-sphere complexes and surface ternary complexes γ-alumina-As-Zn were the main outcome when As was preequilibrated firstly. Zn-arsenate precipitates could significantly decrease the concentration of Zn in aqueous solution and decrease the bioavailability and mobilization of Zn in soils.

Density Functional Theory Investigation into the B and Ga Doped Clean and Water Covered γ-Alumina Surfaces

The structures and energies of the B and Ga incorporated γ-alumina surface as well as the adsorption of water are investigated using dispersion corrected density functional theory. The results show that the substitution of surface Al atom by B atom is not so favored as Ga atom. The substitution reaction prefers to occur at the tricoordinated A(4) sites. However, the substitution reaction becomes less thermodynamically favored when more Al atoms are substituted by B and Ga atoms on the surface. Moreover, the substitution of bulk Al atoms is not so favored as the Al atoms by B and Ga on the surface. The γ-alumina surface is found to have stronger adsorption ability for water than the B and Ga incorporated surface. The total adsorption energy increases as water coverage increases, while the stepwise adsorption energy decreases. The studies show the coverage of water at 7.5 H2O/nm2 (five H2O molecules per unit cell) can fully cover the active sites and the further water molecule could only be physically adsorbed on the surface.

Influence of bacterial extracellular polymeric substances on the sorption of Zn on γ-alumina: A combination of FTIR and EXAFS studies

Extracellular polymeric substances (EPS) isolated from bacteria, are abound of functional groups which can react with metals and consequently influence the immobilization of metals. In this study, we combined with Zn K-edge Extended X-ray Absorption Fine Structure (EXAFS), Fourier Transform Infrared (FTIR) spectroscopy, and High-Resolution Transmission Electron Microscopy (HRTEM) techniques to study the effects of EPS isolated from Bacillus subtilis and Pseudomonas putida on Zn sorption on γ-alumina. The results revealed that Zn sorption on aluminum oxide was pH-dependent and significantly influenced by bacterial EPS. At pH 7.5, Zn sorbed on γ-alumina was in the form of Zn-Al layered doubled hydroxide (LDH) precipitates, whereas at pH 5.5, Zn sorbed on γ-alumina was as a Zn-Al bidentate mononuclear surface complex. The amount of sorbed Zn at pH 7.5 was 1.3–3.7 times higher than that at pH 5.5. However, in the presence of 2 g L−1 EPS, regardless of pH conditions and EPS source, Zn + EPS + γ-alumina ternary complex was formed on the surface of γ-alumina, which resulted in decreased Zn sorption (reduced by 8.4–67.8%) at pH 7.5 and enhanced Zn sorption (increased by 10.0–124.7%) at pH 5.5. The FTIR and EXAFS spectra demonstrated that both the carboxyl and phosphoryl moieties of EPS were crucial in this process. These findings highlight EPS effects on Zn interacts with γ-alumina.

Tribochemical Degradation of Polytetrafluoroethylene Catalyzed by Copper and Aluminum Surfaces

The tribochemical reaction of polytetrafluoroethylene (PTFE) resin sliding against a metallic surface was investigated for a purpose of improving its tribological performance. The tribochemical reaction between PTFE and aluminum surface has been investigated experimentally and theoretically [Onodera et al., J. Phys. Chem. C 2015, 119, 15954−15962]. One of the important results is that the wear and self-lubrication properties can be controlled by the catalytic activity of the oxidized aluminum surfaces with different crystal structures (α- and γ-alumina). In particular, the amount of wear was higher on the γ-alumina surface because it easily activates tribochemical degradation of PTFE and suppresses the formation of transfer film (a necessary phenomenon for reducing wear). Accordingly, for controlling catalytic activity a copper surface was tested as a model surface exhibiting a weaker reactivity than an aluminum surface. A thermogravimetric analysis proved that the copper surface showed a weaker catalytic...

Adsorption of copper on a γ-alumina support

The interaction of a Cu atom with two model γ-alumina surfaces was examined using configuration interaction theory. These two surfaces are centered on Td and Oh coordinated Al atoms near the surface, with both surfaces fully hydroxylated. Copper was found to bind to non-hydrogenated oxygen atoms in the surface by 25kcal/mol, with the possibility that an under coordinated aluminum site may bind copper much more strongly. Vacancies formed by the removal of hydrogen atoms, or by removal of OH groups, were found to bind Cu by values ranging from 40 to 98kcal/mol. The interaction of a planar Cu7 particle with an exposed Td coordinated Al atom at an OH vacancy site was found to lead to decomposition of the particle and partial absorption of the Cu into the alumina surface.

Electroviscous Effects in Ceramic Nanofiltration Membranes.

Membrane permeability and salt rejection of a γ-alumina nanofiltration membrane were studied and modeled for different salt solutions. Salt rejection was predicted by using the Donnan-steric pore model, in which the extended Nernst-Planck equation was applied to predict ion transport through the pores. The solvent flux was modeled by using the Hagen-Poiseuille equation by introducing electroviscosity instead of bulk viscosity. γ-Alumina particles were used for ζ-potential measurements. The ζ-potential measurements show that monovalent ions did not adsorb on the γ-alumina surface, whereas divalent ions were highly adsorbed. Thus, for divalent ions, the model was modified, owing to pore shrinkage caused by ion adsorption. The ζ-potential lowered the membrane permeability, especially for membranes with a pore radius lower than 3 nm, a ζ-potential higher than 20 mV, and an ionic strength lower than 0.01 m. The rejection model showed that, for a pore radius lower than 3 nm and for solutions with ionic strengths lower than 0.01 m, there is an optimum ζ-potential for rejection, because of the concurrent effects of electromigration and convection. Hence, the model can be used as a prediction tool to optimize membrane perm-selectivity by designing a specific pore size and surface charge for application at specific ionic strengths and pH levels.

Preparation of activated ceria and its desulfurization performance

Activated ceria (CeO2/γ-Al2O3) prepared by impregnation was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and hydrogen temperature-programmed reduction (TPR). The desulfurization of the activated ceria was investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TG). The results showed that ceria could be highly dispersed or crystallized on the surface of γ-alumina. The reduction temperatures of 0.1CeO2/γ-Al2O3, 0.45CeO2/γ-Al2O3, and CeO2 ranged from 250°C to 470°C, 330°C to 550°C, and 350°C to 550°C, respectively. The reduction peak temperature of 0.45CeO2/γ-Al2O3 was higher than that of 0.1CeO2/γ-Al2O3, which was consistent with the reduction temperature of CeO2. O2 participated in the reaction between ceria and sulfur dioxide. The desulfurization product was cerium(III) sulfate. The intensity of the hydroxyl band decreased with the formation of sulfate species.

Ab Initio Investigation of Surface Chemistry of Alumina ALD on Hydroxylated γ-Alumina Surface

Atomic layer deposition (ALD) of alumina using trimethyl aluminum (TMA) and water on amorphous alumina is analyzed using periodic-dispersion corrected DFT calculations. The energetics of the investigated reactions suggest that monomethyl aluminum (MMA) is the most abundant reaction intermediate at ALD operating conditions. The dominant reaction path toward the methylation of the surface is found to be adsorption of TMA on bridge oxygen via Lewis acid–base complex formation followed by ligand exchange reactions (LERs) with hydroxyls and surface water in its vicinity. Further adsorption and LERs of TMA leads to a saturated methylated surface (∼6.4 CH3 nm–2) which is in agreement with experimental observations and infrared spectra. The surface restructuring that is observed in almost all the reactions investigated seems to play an important role in the formation of conformal alumina films.

A periodic density functional theory calculation: The structure of isolated copper(I) oxide species on γ-Al2O3 (1 1 0) surface and its adsorption ability toward thiophene and benzene

A periodic density functional theory approach was applied to explore the isolated copper(I) species on γ-Al2O3 (110) surface. Several potential Cu(I) centers located variously on the (110) surface of γ-alumina were modeled by attaching a Cu+ cation and removing one H+ ions from the surface hydroxyl groups. In these structures, the copper is 2-, or 3-fold bonded to the hydrated surface. The adsorption of thiophene and benzene on the structures was investigated. It was found that the coordinating environment and energy varied when isolated Cu+ located at different surface vacant sites, which resulted in different reactivity toward thiophene and benzene. And thiophene could be adsorbed preferentially on the surface supported isolated Cu species comparing to benzene. The main adsorption modes of thiophene were η1 and η2 modes, and the later was more energetically favorable.

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina.

Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant γ-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the γ-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40 °C. The catalyst activity can be regenerated by oxidation at 700 °C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.

γ-Alumina modification with long chain carboxylic acid surface nanocrystals for biocompatible polysulfone nanocomposites.

High performance polysulfone/γ-alumina biocompatible nanocomposites are reported for the first time and the effects of γ-alumina surface modification are explored. We show that some fatty acids chemisorb over the surface of γ-alumina forming nanosized self-assembled structures. These structures present thermal transitions at high temperatures, 100 °C higher than the melting temperatures of the pure acids, and are further shifted about 50 °C in the presence of polysulfone. The chemistry involved in the chemisorption is mild and green meeting the stringent bio sanitary protocols for biocompatible devices. It has been found that the self-assembled structures increase mechanical strength by about 20% despite the foreseeable lack of strong particle-matrix interactions, which manifests as small variations in both the glass transition temperature and the Young's modulus. Electron microscopy observation of fractured surfaces has revealed that some acids induce an extended region of influence around the nanoparticles and this fact has been used to explain the enhancement of mechanical strength.

Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View

The nature of γ-alumina (γ-Al2O3) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated 27Al NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of γ-Al2O3. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (CQ) values (20–34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic “defect sites”, and gradual lowering to bulklike CQ values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local...

Adsorption/combustion-type gas sensors employing mesoporous γ-alumina loaded with core(Au)/shell(Pd) nanoparticles synthesized by sonochemical reduction

Adsorption/combustion-type gas sensors employing mesoporous γ-alumina (mp-Al2O3) powders loaded with core(Au)/shell(Pd) nanoparticles, which were synthesized by sonochemical reduction, (n(Au/Pd)/mp-Al2O3, n: the amount of Au/Pd nanoparticles loaded, (0.1–1.0wt%), Au:Pd=1:4 in weight) have been fabricated and their sensing properties to ethanol, toluene and n-hexane have been investigated in air. The n(Au/Pd)/mp-Al2O3 sensors showed larger responses to these target gases at lower temperatures than those of sensors fabricated with mp-Al2O3 powders loaded with Au and Pd by general impregnation technique (n(Au-Pd)/mp-Al2O3 sensors, Au:Pd=1:4 in weight), especially when the amount of Au and Pd loaded onto the mp-Al2O3 powders was reduced (≤0.5wt%). These sensing properties seem to be largely dependent on the morphological and compositional characteristics of Au and Pd loaded on the γ-alumina surface. In addition, the n(Au/Pd)/mp-Al2O3 sensors showed much larger response to ethanol than those to toluene and n-hexane, probably because of the difference in the magnitude of polarity (dipole moment and dielectric constant) of their molecules. However, the sensors obviously detected even 10ppm n-hexane with the extremely low dipole moment and dielectric constant. These results promise that the n(Au/Pd)/mp-Al2O3 sensors can detect the lower concentration of these gases as well as other VOCs.

Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over CoMo Catalysts Supported on γ-Alumina with Different Morphology

Nanostructured γ-alumina with two different morphologies (rod-like and cube-like) was used as support for CoMo hydrodesulfurization catalyst. Both γ-aluminas were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide precursor, which was synthesized by a convenient hydrothermal method at two pH values. Fourier transform infrared spectroscopy of prydine adsorption, thermogravimetric analysis, and 27Al magic angle spinning (MAS) NMR showed that the rod-like γ-alumina exhibited a lower acidity than the cube-like γ-alumina. The result of X-ray diffraction and temperature-programmed reduction indicated that CoMo oxidic catalysts supported on the rod-like γ-alumina presented higher reducibility compared to those of cube-like γ-alumina, because more β-CoMoO4 was formed on the surface of the rod-like γ-alumina than that of the cube-like γ-alumina. After sulfidation, a large stack with slightly longer MoS2 slabs was formed on the rod-like γ-alumina supports, thereby creating a catalyst with hi...