Dissociative Adsorption Of Water Research Articles

Sustainable Production of Syngas from Biomass‐Derived Glycerol by Steam Reforming over Highly Stable Ni/SiC

The production of syngas was investigated by steam reforming glycerol over Ni/Al(2)O(3), Ni/CeO(2), and Ni/SiC (which have acidic, basic, and neutral properties) at temperatures below 773 K. The complete and stable conversion of glycerol with a yield (higher than 90 %) of gaseous products (mainly syngas) was achieved over Ni/SiC during a 60 h reaction, whereas the conversion of glycerol continually decreases over Ni/Al(2)O(3) (by 49.8 %) and Ni/CeO(2) (by 77.1 %). The deactivation of Ni/Al(2)O(3) and Ni/CeO(2) is mainly caused by coke deposition because of the C-C cleavage of the byproducts produced by dehydration over acidic sites and condensation over basic sites. Gaseous products with a 1.0-1.9 syngas ratio (H(2)/CO) are produced over Ni/SiC. This ratio is required for the Fischer-Tropsch synthesis. However, a syngas ratio of more than 3.0 was observed over Ni/Al(2)O(3) and Ni/CeO(2) because of the high activity of the water-gas-shift reaction. Any dissociative or associative adsorption of water on Al(2)O(3) and CeO(2) promotes a water-gas-shift reaction and produces a higher syngas ratio. H(2) and CO were mainly produced by decomposition of glycerol through dehydrogenation and decarbonylation over Ni sites. Thus, SiC promotes an intrinsic contribution of nickel (dehydrogenation, and decarbonylation) without any byproducts from the dehydration and condensation.

Structure and Stability of Hydrated β-MnO2 Surfaces

Hydration of the β-MnO2 (110), (100), and (101) surfaces is investigated using a combination of periodic density functional theory and ab initio thermodynamics. Fully hydrated surfaces are found to be significantly more stable than the stoichiometric ones up to temperatures in the range of 650 to 730 K at ambient oxygen and water partial pressures. A mixture of molecular and dissociative water adsorption is predicted to occur on the (110) and (101) surfaces, while the (100) surface does not dissociate water. Changes in surface reactivity upon water adsorption are explored via partial density of states analysis. Differences in surface relaxations and vibrational spectra are discussed and can be used to identify the type of adsorption mode.

Defects at the (1 1 0) surface of rutile TiO2 from ab initio calculations

Defect segregation of oxygen vacancies, protons, titanium vacancies, and the mutual complex between the two latter (i.e. Ruetschi type defects) at the (1 1 0) surface of rutile TiO2 has been studied through ab initio Density Functional Theory calculations (DFT). Oxygen vacancies and protons form mainly on the 2-fold coordinated oxygen sites, while titanium vacancy related defects are expected to be most prominent at the 5-fold titanium sites. All included defects are shown to be more stable at the surface than in the bulk oxide, and the relative tendency to segregate is most pronounced for oxygen vacancies. Based on the obtained defect segregation energies and previously calculated bulk defect chemistry at lower temperatures, our estimated surface defect formation energies indicate that the surface of TiO2 can be expected to be dominated by titanium vacancies, protons and their mutual complexes at lower temperatures under wet, oxidizing conditions. Finally, possible routes of dissociative water adsorption on the surface, involving the aforementioned defects are discussed.

Surface phase diagram of hematite pseudocubes in hydrous environments

Hematite nanoparticles often display the pseudocubic morphology enclosed exclusively by the (012) surface. The surface chemistry on these facets is important to understand the formation and properties of these nanoparticles in varying chemical environments. The surface is typically terminated by hydroxyl groups in water or humid atmospheres, and the various terminations differ largely in composition, structure, thermodynamic stability, and chemical reactivity. When we compare a large variety of chemical configurations, we find that three specific terminations are thermodynamically stable under aerobic conditions. The termination by singly and triply coordinated hydroxyl groups (S-,T-OH) is stable at low temperatures and in hydrous environments, the stoichiometric clean termination is stable at high temperatures and in dry environments, and the termination by doubly coordinated hydroxyl groups (D-OH) is stable under intermediate conditions. The S-,T-OH termination can convert topologically to the clean surface by dissociative adsorption of water, whereas the conversion from these two terminations to the D-OH termination requires re-organization of several atomic layers at the surface. Therefore, the surface may reversibly change between S-,T-OH and the clean surface depending on the temperature and humidity. Based on these findings we have constructed surface phase diagrams to predict the termination types in different hydrous and humid environments.

Experimental and theoretical studies of hydroxyl-induced magnetism in TiO nanoclusters

A main challenge in understanding the defect ferromagnetism in dilute magnetic oxides is the direct experimental verification of the presence of a particular kind of defect and distinguishing its magnetic contributions from other defects. The magnetic effect of hydroxyls on TiO nanoclusters has been studied by measuring the evolution of the magnetic moment as a function of moisture exposure time, which increases the hydroxyl concentration. Our combined experiment and density-functional theory (DFT) calculations show that as dissociative water adsorption transforms oxygen vacancies into hydroxyls, the magnetic moment shows a significant increase. DFT calculations show that the magnetic moment created by hydroxyls arises from 3d orbitals of neighboring Ti sites predominantly from the top and second monolayers. The two nonequivalent hydroxyls contribute differently to the magnetic moment, which decreases as the separation of hydroxyls increases. This work illustrates the essential interplay among defect structure, local structural relaxation, charge redistribution, and magnetism. The microscopic differentiation and clarification of the specific roles of each kind of intrinsic defect is critical for the future applications of dilute magnetic oxides in spintronic or other multifunctional materials.

Density functional theory study of theγ-MnOOH (010) surface: Response to oxygen and water partial pressures and temperature

Ab initio thermodynamics was combined with density functional theory calculations to identify stable \ensuremath{\gamma}-MnOOH (010) surface terminations in response to varying oxygen and water partial pressures. Within the range of accessible oxygen chemical potentials, reduced manganese atoms are not thermodynamically stable at the surface. Oxidation of the surface by addition of oxygen is favorable at oxygen chemical potentials typically found in experiments. Entropy drives the removal of H${}_{2}$ from the stoichiometric surface above 603 K under ambient conditions, in close agreement with the experimental decomposition temperature of 573 K. Molecular adsorption of water at half-monolayer and monolayer coverages is highly exothermic and significantly lowers the surface free energy of the clean surface. Dissociative adsorption of water is only possible at monolayer coverage, where it is stabilized by the formation of a hydrogen-bonding network on the surface. The most thermodynamically stable surfaces are oxidized surfaces, but the stoichiometric and fully hydrated surfaces may be accessible in experiments due to slow oxidation kinetics of the surface.

Acid-base properties of the surfaces of Cd x Hg1–x Te solid solutions

The acid-base properties of actual CdTe and CdHgTe surfaces were investigated by means of non-aqueous conductometric titration for pHpzc determination, mechanochemistry, and UV spectroscopy. It was shown that the alkaline nature of their surface was due to the dissociative adsorption of atmospheric water flowing on the coordination-saturated surface atoms of cadmium with vacancy defects. The adsorbed water was involved in the formation of tellurium oxide TeO2 and H2TeO3 tellurous acid in the dispersion of CdTe and CdHgTe.

Reactivity Descriptors for Borohydride Interaction with Metal Surfaces

First-principles density functional theory calculations were performed to study the adsorption of borohydride (BH4–) on close-packed transition-metal surfaces, M(111) (M = Au, Pt, Ir, Os, Ag, Pd, Rh, Ru). A correlation between the relative adsorption energies of BH4ad and the d-band center of the metals is established. In terms of the adsorbate configuration, both molecular (BH4ad) and dissociated (BH4-y,ad + yHad, y = 1, ...,3) structures are possible regardless of the adsorption energy value. On Os, Rh, and Ru surfaces, molecular (i.e., undissociative) adsorption is preferred despite the strong surface binding energy of BH4ad. Orbital-specific analysis of the bonding, points to the role of the dzz and dyz states of the surface metal atoms in determining the final BH4ad configuration on all metals. However, in the presence of H2O molecules, the preference for strong molecular adsorption may be lost because of BH4ad–H2Oad interaction. Using the coadsorption on Os(111) of BH4ad and H2Oad with and without the presence of OHad (generated either by electrosorption of OH– or dissociative water adsorption), the origins of the adsorbate–adsorbate and adsorbate–metal interactions are discussed. Electronic factors to predict the BH4ad conformation on metal catalysts in water environment are proposed.

Understanding the interaction of water with anatase TiO 2 (101) surface from density functional theory calculations

The behavior of water molecule on anatase TiO 2 (101) surface has been investigated by density functional theory calculations. The primary purpose of this Letter is to clarify the distinctions between molecular adsorption and dissociative adsorption of water on anatase TiO 2 (101) surface. By analyzing interaction potential forms and bonding mechanism, it is found that the dipole interaction is the crucial factor for water adsorption on anatase TiO 2 (101) surface. The adiabatic potential energy surface calculations indicate that the on-surface diffusion of water molecule is anisotropy: its diffusion energy barrier along [010] direction is smaller than that of along [ 11 1 ¯ ] / [ 1 1 ¯ 1 ¯ ] direction.

Cationic doping of MgO surfaces to build corrosion protection in Mg alloys

Reactivity of MgO surfaces towards dissociative adsorption of water is crucial in determining the protective performance of oxide films formed on top of Mg alloy surfaces. Engineers and scientists have repeatedly reported that heavy metal contamination could significantly affect the oxide film stability towards water. We performed first-principles analysis on MgO surfaces adsorbed with water molecules in the presence of six different types of cationic impurities – Zn 2+, Al 3+, Y 3+, Mn 4+, Zr 4+, and Ce 4+ – to assess the effect of commonly present metal impurities in MgO films on the dissociative adsorption of water. The trend in the adsorption energies with different metal impurities shows that water is less likely to dissociate when cationic impurities with high oxidation states such as Mn 4+, Zr 4+, and Ce 4+ are present. Charge transfer from the adsorbed species to the impurity atom is identified as the source of the trend. Increased dissociation barrier for water adsorbed on the same doped surfaces indicates that the incorporation of such impurities will effectively block the dissolution of oxide films and potentially enhance the film's resistivity to water.

Stoichiometry-Dependent Chemical Activity of Supported MgO(100) Films

Here, we show that the stoichiometry and, consequently, the chemical activity toward hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depend on the O(2) partial pressure during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.

An ONIOM and DFT study of water and ammonia adsorption on anatase TiO2 (001) cluster

AbstractDensity functional theory (DFT) calculations at ONIOM DFT B3LYP/ 6‐31G**‐MD/UFF level are employed to study molecular and dissociative water and ammonia adsorption on anatase TiO2 (001) surface represented by partially relaxed Ti20O35 ONIOM cluster. DFT calculations indicate that water molecule is dissociated on anatase TiO2 (001) surface by a nonactivated process with an exothermic relative energy difference of 58.12 kcal/mol. Dissociation of ammonia molecule on the same surface is energetically more favorable than molecular adsorption of ammonia (−37.17 kcal/mol vs. −23.28 kcal/mol). The vibration frequency values also are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO2 (001) surface. The computed adsorption energy and vibration frequency values are comparable with the values reported in the literature. Finally, several thermodynamical properties (ΔH°, ΔS°, and ΔG°) are calculated for temperatures corresponding to the experimental studies. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

A Step toward the Wet Surface Chemistry of Glycine and Alanine on Cu{110}: Destabilization and Decomposition in the Presence of Near-Ambient Water Vapor

The coadsorption of water with organic molecules under near-ambient pressure and temperature conditions opens up new reaction pathways on model catalyst surfaces that are not accessible in conventional ultrahigh-vacuum surface-science experiments. The surface chemistry of glycine and alanine at the water-exposed Cu{110} interface was studied in situ using ambient-pressure photoemission and X-ray absorption spectroscopy techniques. At water pressures above 10(-5) Torr a significant pressure-dependent decrease in the temperature for dissociative desorption was observed for both amino acids, accompanied by the appearance of a new CN intermediate, which is not observed for lower pressures. The most likely reaction mechanisms involve dehydrogenation induced by O and/or OH surface species resulting from the dissociative adsorption of water. The linear relationship between the inverse decomposition temperature and the logarithm of water pressure enables determination of the activation energy for the surface reaction, between 213 and 232 kJ/mol, and a prediction of the decomposition temperature at the solid-liquid interface by extrapolating toward the equilibrium vapor pressure. Such experiments near the equilibrium vapor pressure provide important information about elementary surface processes at the solid-liquid interface, which can be retrieved neither under ultrahigh vacuum conditions nor from interfaces immersed in a solution.

Effect of water adsorption on the structural and optical properties of silicon and titanium dioxide nanoclusters

The formation of the structure and infrared (IR) spectra of (SiO2)n (n = 1–4) and (TiO2)n (n = 1–3) clusters with an increase in their size n has been simulated using the B3LYP/6-31G* method of the density functional theory. The effect that water adsorption has on the structural and optical properties of silicon and titanium dioxide nanoclusters has been studied by this method as well. The calculated IR absorption bands for the structures of the (SiO2)n and (TiO2)n clusters are in good agreement with the bands observed in the IR spectra of the SiO2 and TiO2 films. It has been shown that the dissociative adsorption of the water occurs at the silicon centers, while the molecular form of adsorption is more energetically favorable at the active titanium centers. As a result of H2O adsorption, the electronic structure of TiO2 undergoes a transition from the semiconductor state to the dielectric one. The traces of water adsorbed at the titanium centers can be identified by the IR absorption spectra of samples due to a multiple (up to two orders of magnitude) increase in the intensities of the H2O molecule vibrations within the adsorption complex.

Effect of surface defects on biosensing properties of TiO 2 nanotube arrays

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti 3+ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H 2O 2) using an amperometric method. Cyclic voltammetry results and UV–Vis absorption spectra presented that with an increase of Ti 3+ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti 3+ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm 2), faster electron transfer rate (1.34 × 10 −3 cm/s) and the best response sensitivity (88.5 μA/mM l −1).

An ONIOM and DFT study of water adsorption on rutile TiO2 (110) cluster

AbstractDensity functional theory (DFT) calculations performed at ONIOM DFT B3LYP/6‐31G**‐MD/UFF level are employed to study molecular and dissociative water adsorption on rutile TiO2 (110) surface represented by partially relaxed Ti25O37 ONIOM cluster. DFT calculations indicate that dissociative water adsorption is not favorable because of high activation barrier (23.2 kcal/mol). The adsorption energy and vibration frequency of both molecularly and dissociatively adsorbed water molecule on rutile TiO2 (110) surface compare well with the values reported in the literature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H 2O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH) 2 upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H 2O, OH and Ox thickness changes as a function of relative humidity.

Molecular and dissociative adsorption of water at low-index V 2O 5 surfaces: DFT studies using cluster surface models

The adsorption of water in both the molecular and dissociative manners at low-index (0 1 0), (1 0 0) and (0 0 1) surfaces of V 2O 5 is examined by DFT methods using an embedded cluster model approach. The calculations showed that on saturated basal net planes ((0 1 0)V 2O 5), water is stabilized only in molecular form as a result of weak electrostatic interactions, and it may migrate through the surface. At unsaturated side planes ((1 0 0) and (0 0 1) surfaces), both the molecular and dissociative adsorption of water is possible. Molecular stabilization takes place at V centres via coordinative covalent bonds. On the (0 0 1) surface, dissociative adsorption is associated with a moderate energy barrier near bridging oxygen centres and with a very low barrier close to bridging oxygen sites connected to V atoms from different “valley”-like atomic layers. On the (1 0 0) net plane, water undergoes dissociation near the singly and doubly coordinated oxygen sites, although the process is probable only close to the twofold coordinated centre because proximity to vanadyl oxygen sites is accompanied by a high energy barrier.

Direct Visualization of Water-Induced Relocation of Au Atoms from Oxygen Vacancies on a TiO2(110) Surface

Variable-temperature scanning tunneling microscopy (STM) is used to show that Au1+ deposited onto a TiO2(110)-(1 × 1) surface under soft-landing conditions at 600 K results in a surface decorated with isolated gold atoms bound to oxygen vacancies. This result is in sharp contrast to the large, sintered islands which form from Au1+ deposited onto a hydroxylated TiO2(110)-(1 × 1) surface under soft-landing conditions at 300 K. The position of the isolated Au atoms prepared by deposition at 600 K changes from directly above the bridging oxygen rows to directly above 5c-Ti atoms when the substrate is allowed to cool from 600 to 300 K. The binding site of the Au atoms returns to directly over the bridging oxygen rows when the temperature is returned to 600 K, indicating that this process is reversible. We attribute the change in binding site to a competition between the Au atom and an adsorbed water molecule for an oxygen vacancy on the reduced TiO2 surface. Using density functional theory (DFT), we show that dissociative adsorption of water occurs at an oxygen vacancy occupied by an Au atom, displaces the Au atom, and forms a stable OH−Au−TiO 2 complex on the surface.

Experimental evidence for mixed dissociative and molecular adsorption of water on a rutileTiO2(110)surface without oxygen vacancies

We present evidence for mixed molecular and dissociative water adsorption at monolayer coverage on a rutile TiO2(110) surface free from oxygen vacancies using synchrotron radiation photoemission. At monolayer coverage the OH:H2O ratio is close to 0.5 and reducing the coverage by heating yields an increased OH:H2O ratio. At room temperature neither species originating from the monolayer on the defect-free surface can be detected. The OH species of the monolayer hence recombines and leaves the surface at much lower temperatures than OH formed by water dissociation on oxygen vacancies.