Adsorption of HCl on Single-Crystal α-Al2O3 (0001) Surface: A DFT Study

DFT study of mercury adsorption on Al2O3 with presence of HCl

mercury emission control from flue gas is a crucial issue for environment protection. Alumina is an important alkali metal oxide for mercury adsorption in particulate, meanwhile is the potential adsorbent for mercury removal. The cognition on mercury heterogeneous reaction mechanism with alumina in presence of hydrogen chloride is inadequate. In this work, the DFT calculation was applied to detect mercury's chlorides adsorption on α-Al2O3 (001) surface, the Bader charge analysis was used to estimate electron transfer and the transition state theory was used to clarify reaction pathway and energy barrier, besides, the kinetic analysis based on Gibbs free energy was conducted to study the impact of temperature on chemical reaction. The results show that Hg can be captured by weak chemisorption on α-Al2O3 (001) surface with the adsorption energy of -56.37 kJ/mol, HgCl, HgCl2 are intensively bonded on surface with adsorption energies of -276.90 kJ/mol and -231.87 kJ/mol, the surface unsaturated Al and O atoms are the active sites. Charge transfer and PDOS analysis prove that the forming of covalent bonding is responsible for Hg species adsorption. Two possible reaction pathways of Hg oxidization to HgCl2 are discussed, in which a smaller energy barrier of 0.1 eV implies the dominant pathway 1 via Eley-Rideal mechanism: two adsorbed HCl molecules dissociate on surface and then react with one Hg atom. High temperature can promote the reaction rate constants of pathway 1 and 2, but is only favorable for reducing energy barrier of pathway 2.

Density functional theory study on the formation mechanism of CaClOH in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is defined as a kind of hazardous waste because of its high levels of multiple pollutants. The main component of MSWI fly ash is CaClOH, and the characteristics have not achieved consensus. And density functional theory (DFT) was used to calculate the formation process of CaClOH in this study, which mainly included HCl adsorption on CaO (0 0 1) surface and Ca(OH)2 (0 0 1) surface and the surface reaction process. The reaction mechanism was investigated. The results showed that the maximum adsorption energies of HCl on CaO and Ca(OH)2 surfaces reached - 195.17 kJ/mol and - 83.48 kJ/mol, respectively, representing strong chemisorption. The chemisorption process was shown as the adsorption of H atom on O site, and the adsorption capacity was reflected in the adsorption range of O site. The significant electron density overlap between O site and H atom meant that a new chemical bond formed, which made the adsorption structure stable. The adsorption energy of multi-HCl adsorption on the crystal surfaces was not proportional to the number of HCl molecule, indicating that the adsorption processes were influenced by each other. After surface reaction, the H-Cl bond was broken completely, and the structure of CaO and Ca(OH)2 changed to new structures. According to transition state (TS) search, the formation of CaClOH had a higher priority, easier than that of CaCl2, explaining the presence of CaClOH in fly ash. The study provides helpful information for the solidification treatment of fly ash.

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 Al_Ⅲ 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.

Density Functional Theory Study of Ethylene Carbonate Adsorption on the (0001) Surface of Aluminum Oxide α-Al2O3

Surface coating is one of the techniques used to improve the electrochemical performance and enhance the resistance against decomposition of cathode materials in lithium-ion batteries. Despite several experimental studies addressing the surface coating of secondary Li-ion batteries using α-Al₂O₃, the reactivity of the material toward the electrolyte components is not yet fully understood. Here, we have employed calculations based on the density functional theory to investigate the adsorption of the organic solvent ethylene carbonate (EC) on the major α-Al₂O₃(0001) surface. During adsorption of a single EC molecule, it was found that it prefers to bind parallel to the surface through its carboxyl oxygen. As the surface coverage (θ) was increased up to a monolayer, we observed larger adsorption energies per EC molecule (E _ads/N _EC) for parallel interactions and a reduction for perpendicular interactions. We also noted that increasing the surface coverage with both parallel and perpendicularly interacting EC molecules led to a decrease of the surface free energies and hence increased stability of the α-Al₂O₃(0001) surface. Despite the larger E _ads/N _EC observed when the molecule was placed parallel to the surface, minimal charge transfer was calculated for single EC interactions and at higher surface coverages. The simulated scanning tunneling microscopy images are also presented for a clean corundum α-Al₂O₃ surface and after adsorption with different coverages of parallel and perpendicularly placed EC molecules.

Formation of Environmentally Persistent Free Radicals on α-Al2O3

Metal oxides exhibit catalytic activity for the formation of environmentally persistent free radicals (EPFRs). Here, we investigate, via first-principles calculations, the activity of alumina α-Al₂O₃(0001) surface toward formation of phenolic EPFRs, under conditions relevant to cooling down zones of combustion systems. We show that, molecular adsorption of phenol on α-Al₂O₃(0001) entails binding energies in the range of -202 kJ/mol to -127 kJ/mol. The dehydroxylated alumina catalyzes the conversion of phenol into its phenolate moiety with a modest activation energy of 48 kJ/mol. Kinetic rate parameters, established over the temperature range of 300 to 1000 K, confirm the formation of the phenolate as the preferred pathways for the adsorption of phenol on alumina surfaces, corroborating the role of particulate matter in the cooling down zone of combustion systems in the generation of EFPRs.

A DFT study on carbon monoxide adsorption onto hydroxylated α-Al₂O₃(0001) surfaces

The adsorption of CO onto the hydroxylated α-Al2O3(0001) surface was studied using density functional theory (DFT). Dissociated adsorption of water was found to be stable, with an adsorption energy (Ea) of 1.62 eV at θ(water) = 0.75. The most stable hydroxylation form on the clean surface was found to be in the 1-2 dissociation configuration, where the OH group binds to a surface Al ion and the H ion binds to one of the three equivalent surface O ions. The adsorption energy of CO was found to be dependent on the degree of pre-hydroxylation of the surface as well as on the CO coverage. The highest adsorption energy of CO was found when θ(CO) = 0.25 on a pre-hydroxylated surface with θ(water) = 0.25; Ea = 0.57 eV. The adsorption energy of CO decreased upon increasing the degree of pre-hydroxylation. The vibrational frequency of ν(CO) was also computed and in all cases it was blue shifted with respect to gas-phase CO. The shift, Δν, decreased with increasing CO coverage but increased with increasing surface hydroxylation. A comparison with available experimental work is discussed.

Electronic properties and reactivity of simulated Fe(3+) and Cr(3+) substituted α-Al(2)O(3) (0001) surface

Metal oxide based minerals naturally contain transition metal impurities isomorphically substituted into the structure that can alter the structural and electronic properties as well as the reactivity of these metal oxides. Natural α-Al(2)O(3) (corundum) can contain up to 9.17% (w/w) Fe(2)O(3) and 1.81% (w/w) of Cr(2)O(3.) Here we report on changes in the structural and electronic properties of undoped and doped α-Al(2)O(3) (0001) surfaces using periodic density functional theory (DFT) methods with spin unrestricted B3LYP functional and a local atomic basis set. Both structural and electronic properties are altered upon doping. Implications for doping effects on photochemical processes are discussed.As metal oxides are major components of the environment, including atmospheric mineral aerosol, DFT was also used to study the effect of transition metal impurities on gas/surface interactions of a model acidic atmospheric gas molecule, carbon monoxide (CO). The theoretical results indicated that the presence of Fe(3+) and Cr(3+) impurities substituted on the outer layer of natural corundum surfaces reduces the propensity toward CO adsorption relative to the undoped surface. However, CO-surface interactions resemble that of bulk α-Al(2)O(3) when the impurity is substituted below the first surface layer. The presence and location of the mineral dopant was found to significantly alter the structural and electronic properties and gas/surface interactions studied here.

Joint density-functional theory: ab initio study of Cr2O3 surface chemistry in solution

After introducing a new form of density-functional theory for the ab initio description of electronic systems in contact with a molecular liquid environment, we present the first detailed study of the impact of a solvent on the surface chemistry of Cr(2)O(3), the passivating layer of stainless steel alloys. In comparison to a vacuum, we predict that the presence of water has little impact on the adsorption of chloride ions to the oxygen-terminated surface but has a dramatic effect on the binding of hydrogen to that surface. These results indicate that the dielectric screening properties of water are important to the passivating effects of the oxygen-terminated surface.

First-Principles Calculations of the Adsorption of Nitromethane and 1,1-Diamino-2,2-dinitroethylene (FOX-7) Molecules on the α-Al2O3(0001) Surface

First-principles calculations based on density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of nitromethane (NM) and 1,1-diamino-2,2-dinitroethylene (FOX-7) molecules on the basal plane of an alpha-Al(2)O(3) crystal. The calculations employ a (2 x 2) supercell slab model and 3D periodic boundary conditions. On the basis of these calculations, we have determined that both NM and FOX-7 molecules can adsorb nondissociatively on the surface with the most stable adsorption configurations parallel to the surface. The binding energies are in the range 25.3-26.0 kcal/mol for NM and 35.6-48.3 kcal/mol for FOX-7 depending on the relative molecular orientation and the surface sites. The minimum energy pathways for NM dissociation have been determined, and a low-energy pathway leading to H-atom elimination with formation of adsorbed CH(2)NO(2) and hydroxyl species has been identified. Additional calculations have focused on adsorption properties of aci-nitromethane tautomers and on description of the energetic pathways connecting adsorbed nitromethane molecule with these tautomers.

Piperidine adsorption on hydrated alpha-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.