Dissociative Chemisorption Of Water Research Articles

Water sorption on pyrochlore – Niobium hydration and calcium susceptibility to leaching unraveled by DFT simulations

Hydration of pyrochlore is a preponderant step during its flotation well before any other surface-reagent interaction to take place. Water adsorption on low-index pyrochlore surfaces is investigated by Density Functional Theory (DFT) simulation, with a focus on the mechanisms of water interaction with surface sites and their coordination. As far as the pyrochlore (100) and (111) surfaces favor the associative physisorption of water, the dissociative chemisorption of water on the (110) surface is by far the most energetically favorable. The affinity of pyrochlore surface to water increases from −81.4 (111), −102.7 (100) to −331.6 kJ/mol (110) leading to decreasing hydrophilic surfaces: (110) ≫ (100) > (111). Water adsorption on the Nb sites located on the (100) surface is unlikely due to the full-coordination state of the exposed Nb. Moreover, the dissociative adsorption of water, driven by a lack of coordination on the (110) surface, would be at the origin of the selective (or non-congruent) dissolution of pyrochlore calcium by weakening its innate bonds. These primary results on the relationship between water and the atomic topology of pyrochlore cleavage planes will help guide the choice of aqueous chemical reagents to be used to address the challenges of this mineral's flotation.

Effect of Surface Chemistry and Crystallographic Parameters of TiO2 Anatase Nanocrystals on Photocatalytic Degradation of Bisphenol A

The photocatalytic activity of a series of anatase TiO2 materials with different amounts of exposed (001) facets (i.e., 12% (TiO2-1), 38% (TiO2-3), and 63% (TiO2-3)) was tested in a batch slurry reactor towards liquid-phase bisphenol A (BPA, c0(BPA) = 10 mg/L, ccat. = 125 mg/L) degradation. Photo-electrochemical and photo-luminescence measurements revealed that with the increasing amount of exposed anatase (001) facets, the catalysts generate more electron-hole pairs and OH∙ radicals that participate in the photocatalytic mineralization of pollutants dissolved in water. In the initial stages of BPA degradation, a correlation between % exposure of (001) facets and catalytic activity was developed, which was in good agreement with the findings of the photo-electrochemical and photo-luminescence measurements. TiO2-1 and TiO2-3 solids achieved 100% BPA removal after 80 min in comparison to the TiO2-2 sample. Adsorption of BPA degradation products onto the TiO2-2 catalyst surface was found to have a detrimental effect on the photocatalytic performance in the last stage of the reaction course. Consequently, the global extent of BPA mineralization decreased with the increasing exposure of anatase (001) facets. The major contribution to the enhanced reactivity of TiO2 anatase (001) surface is the Brønsted acidity resulting from dissociative chemisorption of water on a surface as indicated by FTIR, TPD, and MAS NMR analyses.

The dissociative chemisorption of water on Ni(111): Mode- and bond-selective chemistry on metal surfaces.

A fully quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore the dissociative chemisorption of H2O, HOD, and D2O on Ni(111). For this late barrier system, excitation of both the bending and stretching modes significantly enhances dissociative sticking. The vibrational efficacies vary somewhat from mode-to-mode but are all relatively close to one, in contrast to methane dissociation, where the behavior is less statistical. Similar to methane dissociation, the motion of lattice atoms near the dissociating molecule can significantly modify the height of the barrier to dissociation, leading to a strong variation in dissociative sticking with substrate temperature. Given a rescaling of the barrier height, our results are in reasonable agreement with measurements of the dissociative sticking of D2O on Ni(111), for both laser-excited molecules with one or two quanta of excitation in the antisymmetric stretch and in the absence of laser excitation. Even without laser excitation, the beam contains vibrationally excited molecules populated at the experimental source temperature, and these make significant contributions to the sticking probability. At high collision energies, above the adiabatic barrier heights, our results correlate with these barrier heights and mode softening effects. At lower energies, dissociative sticking occurs primarily via vibrationally nonadiabatic pathways. We find a preference for O-H over O-D bond cleavage for ground state HOD molecules at all but the highest collision energies, and excitation of the O-H stretch gives close to 100% O-H selectivity at lower energies. Excitation of the O-D stretch gives a lower O-D cleavage selectivity, as the interaction with the surface leads to energy transfer from the O-D stretch into the O-H bond, when mode softening makes these vibrations nearly degenerate.

Dynamics of water dissociative chemisorption on Ni(111): effects of impact sites and incident angles.

The dissociative chemisorption of water on rigid Ni(111) is investigated using a quasiclassical trajectory method on a nine-dimensional global potential energy surface based on a faithful permutation invariant fit of ∼25 000 density functional theory points. This full-dimensional model not only confirms the validity of our earlier reduced-dimensional model with 6 degrees of freedom, but also allows the examination of the influence of impact sites and incident angles. It is shown that the reactivity depends on the site of impact in a complex fashion controlled by the topography of the potential energy surface rather than the barrier height alone. In addition, the reaction is promoted by momenta both parallel and perpendicular to the surface, as predicted by the recently proposed sudden vector projection model.

Prediction of Mode Specificity, Bond Selectivity, Normal Scaling, and Surface Lattice Effects in Water Dissociative Chemisorption on Several Metal Surfaces Using the Sudden Vector Projection Model

Dissociative chemisorption of water is a key step in many heterogeneous catalytic processes such as water–gas shift and steam reformation. As a result, a better understanding of the mechanism and dynamics of these processes is important for developing a predictive model of catalysis. In this work, we use the recently proposed Sudden Vector Projection (SVP) model to predict mode specificity, bond selectivity, normal scaling behavior, and surface lattice effects in water dissociative chemisorption on Ni(111), Cu(111), Pt(111), and Pt(110)-(1 × 2), based on direct plane-wave density functional theory calculations. While mode specificity and bond selectivity are similar on these surfaces, the SVP model predicts significant differences in the reaction promoting effects of kinetic energies in the surface normal and along the surface plane, signifying the different characters of the transition state on these metal surfaces. Furthermore, the involvement of surface atoms is shown, which predicts significant surface temperature effects.

Effects of reactant internal excitation and orientation on dissociative chemisorption of H2O on Cu(111): Quasi-seven-dimensional quantum dynamics on a refined potential energy surface

To understand the influence of reactant internal excitation and orientation on the dissociative chemisorption of water on Cu(111), a quasi-seven-dimensional quantum dynamics study has been carried out on a refined potential energy surface (PES). The new PES was modified in the asymptotic region to allow an accurate characterization of the H(2)O ro-vibrational levels. The mode selectivity of the reaction was reexamined on the new PES and found to be consistent with our earlier work. To rationalize the observed mode selectivity, a vibrationally adiabatic reaction path model was determined on this PES. Furthermore, the reactivity for various rotationally excited H(2)O was investigated. It is shown that even low rotational excitation in H(2)O can either enhance or inhibit the reaction and the reactivity depends on the orientation of the impinging molecule.

Effect of water and ammonia on surface species formed during NOx storage–reduction cycles over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts

The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.

Water interactions with micronized lunar surrogates JSC‐1A and albite under ultra‐high vacuum with application to lunar observations

Interactions of molecular water with two lunar regolith surrogates (micronized JSC‐1A and albite) were examined using temperature program desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy. TPD revealed water desorption during initial heating to 750 K under ultrahigh vacuum and diffuse reflectance infrared Fourier transform spectroscopy indicated possible water formation via recombinative desorption of native hydroxyls above 425 ± 25 K. Dissociative chemisorption of water (i.e., formation of surface hydroxyl sites) was not observed on laboratory time scales after controlled dosing of samples (initially heated above 750 K) with 0.2–500 L exposures of water. However, preheated samples of both types of surrogates were found to have a distribution of molecular water chemisorption sites, with albite having at least twice as many as the JSC‐1A samples by mass. A fit to the TPD data yields a distribution function of desorption activation energies ranging from ~ 0.45 to 1.2 eV. Using the fitted distribution function as an initial condition, the TPD process was simulated on the time scale of a lunation.

Enhancing dissociative chemisorption of H 2 O on Cu(111) via vibrational excitation

The dissociative chemisorption of water is an important step in many heterogeneous catalytic processes. Here, the mode selectivity of this process was examined quantum mechanically on a realistic potential energy surface determined by fitting planewave density functional calculations spanning a large configuration space. The quantum dynamics of the surface reaction were characterized by a six-dimensional model including all important internal coordinates of H(2)O and its distance to the surface. It was found that excitations in all three vibrational modes are capable of enhancing reactivity more effectively than increasing translational energy, consistent with the "late" transition state in the reaction path.

A shortcut for evaluating activities of TiO2 facets: water dissociative chemisorption on TiO2-B (100) and (001)

By means of density functional theory (DFT) calculations, we study the water adsorption behavior on two common surfaces, (001) and (100) TiO(2)-B, which maintains the monoclinic structure as high as approximately 550 degrees C or higher in ambient conditions. The two surfaces show totally different activity for water dissociation. The dissociative chemisorption of water on TiO(2)-B (100) is identified at both submonolayer and monolayer coverages, which indicates considerable reactivity. In contrast, the non-dissociative molecular adsorption of water is the most stable state on TiO(2)-B (001) which suggests no special activity. Furthermore, we compare the structural features of different surfaces with diverse crystal structures, such as rutile, anatase, brookite, TiO(2)-B etc. Keeping a close eye on the exposed atoms on the surface, we conclude a more general criterion for a quick evaluation of reactivities of different TiO(2) surfaces merely based on local surface structure features.

Dissociative Chemisorption of Water onto Silica Surfaces and Formation of Hydronium Ions

Dissociative Chemisorption of Water onto Silica Surfaces and Formation of Hydronium Ions

Dissociative Chemisorption of Water onto Silica Surfaces and Formation of Hydronium Ions

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionDissociative Chemisorption of Water onto Silica Surfaces and Formation of Hydronium Ions and S. H. GarofaliniCite this: J. Phys. Chem. C 2008, 112, 14, 5694Publication Date (Web):March 13, 2008Publication History Published online13 March 2008Published inissue 1 April 2008https://doi.org/10.1021/jp8015134Copyright © 2008 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views424Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (5 KB) Get e-Alerts Get e-Alerts

DISSOCIATED WATER ON Si(100): RELATION BETWEEN STM TOPOGRAPH AND ACTUAL GEOMETRY

We have modeled the dissociative chemisorption of water on the Si (100)-(2×1) surface using a generalized gradient approximation of density functional theory and a periodic slab model of the surface. For the energetically favorable structures, scanning tunneling microscope topographs of the filled states are simulated. These exhibit distinctively dark characteristics where water "islands" are formed, in agreement with experimental findings. In addition, they indicate that the hydrogen-atom and hydroxyl-radical adducts display somewhat different contrasts. Furthermore, in the case of a partial saturation of a Si dimer a prominent brightness is predicted for the unsaturated Si atoms if their dimer-forming counterparts are saturated by hydroxyl species, while in the case of hydrogen saturation the contrast is rather dim.

A DFT periodic study of the vanadyl pyrophosphate (1 0 0) surface

The unique ability of the vanadyl pyrophosphate (1 0 0) surface to activate n-butane and then selectively oxidise the hydrocarbon to maleic anhydride was investigated using modern quantum chemical methods. Bulk (VO) 2P 2O 7, together with stoichiometric and phosphorus-enriched (1 0 0) surfaces, were analysed using periodic density functional theory calculations. Also simulated was surface ionic relaxation from bulk geometry, and surface hydration. Density of states (DOS) plots show that, whether stoichiometric or phosphorus-enriched, bulk terminated or relaxed, bare or hydrated, local covalent reactivity at the (1 0 0) surface is controlled by vanadium species. Terminal P–O oxygen species are the most nucleophilic surface oxygens, as indicated by their predominance of sub-vanadium high-lying valence band levels. A periodic treatment of (VO) 2P 2O 7(1 0 0) hence gives results qualitatively identical to those obtained from earlier cluster calculations. Simulation of surface ionic relaxation shows that in-plane P–O–V oxygens may also be involved in rupture of substrate C–H bonds for mild oxidation, while surface hydration calculations indicate that dissociative chemisorption of water may play a key role in perpetuation of the selective oxidation cycle.

Ab initio simulation of water interaction with the (100) surface of pyrite

Car–Parrinello simulations have been performed to study the interaction of water with pyrite (100) surface. The stability and the structural and electronic properties of both the molecular and dissociative adsorptions have been addressed. We found a very strong preference for molecular adsorption on the surface iron sites, in agreement with experiment. The dissociative chemisorption of water is energetically disfavored and is even locally unstable; the dissociated fragments transform back to the stable molecular form in a short molecular dynamics run. The calculations revealed that hydrogen bonding plays an important role in the stabilization of the adsorbed water for both the molecular and the dissociative states. We have shown that water forms a coordinative covalent bond with the surface iron atoms by donating electron to the empty iron dz2 orbitals which are the lowest empty states on the clean surface. At full coverage, the sulfur 3p states thus become the lowest available empty states and therefore the subject of possible electron-transfer reactions.

Reaction of water with the (100) and (111) surfaces of Fe 3O 4

We have examined changes in the electronic structure of magnetite (100) and (111) surfaces after reaction with water vapor ( p(H 2O) ranging from 10 −9 to 9 Torr) and liquid water at 298 K using chemical shifts in the O 1s core level photoelectron spectra obtained with a synchrotron radiation source. The surfaces were prepared in ultra-high vacuum (UHV) from natural magnetite crystals. We found that the vapor pressure, p(H 2O), at which water first reacts with magnetite is similar for the two surfaces (≤10 −5 Torr for 3 min exposures, corresponding to doses of ≤1.8×10 3 Langmuirs) and is consistent with a small sticking coefficient. This reaction is manifested in the O 1s spectra by the growth of a shoulder at about 1.5 eV lower kinetic energy than the main lattice oxygen feature. We attributed this new feature to hydroxyl groups resulting from dissociative chemisorption of water on the magnetite surfaces, initially on defect sites. p(H 2O) for the onset of an extensive hydroxylation reaction is ≈10 −3 Torr (3 min dose or ≈1.8×10 5 Langmuirs). Magnetite (100) and (111) surfaces exposed to higher p(H 2O) react more extensively, with hydroxylation extending several layers (≈8 Å) deep into the bulk. A comparison of O KVV Auger K-edge absorption spectra of water vapor-exposed magnetite (100) and (111) surfaces with the corresponding total yield spectra of goethite (α-FeOOH), limonite (FeOOH· nH 2O), and hematite (α-Fe 2O 3) clearly shows that the reaction product on the magnetite surfaces is not goethite, limonite or hematite. In addition, similarity of the Fe L 3M 23M 23 Auger yield L-edge absorption spectra before and after exposure of the magnetite (111) surface to liquid water indicates that the oxidation state of iron is unchanged. We also measured O 1s chemical shifts on magnetite (111) surfaces which had been immersed in liquid water. Surprisingly, these immersion experiments resulted in smaller chemical shifts and lower intensities of hydroxyl features relative to magnetite samples exposed to the highest water vapor pressures (10 Torr for 3 min or 1.8×10 9 Langmuirs) in our dosing experiments. To assess the thermal stability of the hydroxylated surfaces, we conducted a series of stepwise annealing experiments to 700°C. These annealing experiments indicate that once the magnetite surface is hydroxylated it is extremely difficult to thermally clean without Ar + sputtering.

Infrared Spectroscopic Study of the Formation of Reactive Silica by Pyrolysis in Vacuo of a Trimethylsiloxylated Sample

The pyrolysis of a trimethylsiloxylated silica (TMS-silica) in vacuo up to 750 °C gives a “reactive silica” with surface species and properties similar to those previously reported by degassing a methoxylated silica above 600 °C. Both surface silanes absorbing around 2290 cm-1 and very stable Si-C⋮CH groups appear. A dissociative chemisorption of water on the reactive silica at 25 °C, leading to both SiH and SiOH groups, involves silylene centers (SiO)2Si: with a bicoordinated silicon atom. A concentration of about 0.13 sorption centers nm-2 is estimated, which is a value higher than on pyrolyzed ex-methoxylated silica. The SiH stretching range in the infrared spectra of TMS-silica displays a larger variety of intermediate SiH sites, at increasing temperatures, than for a methoxylated silica. In addition, the pyrolysis of TMS-silica creates intermediate aliphatic species absorbing at 2975 and 2986 cm-1, correlated to the intensity decrease of the νaCH3 band of the TMS group at 2964 cm-1. Similar intermediate components, assigned to Si(CH3)2 and SiCH3 groups, were observed by oxidation of TMS-silica in air, although at much lower temperatures than in vacuo. A SiCH3 bond is thus attributed to the previously unidentified species which absorbs at 2983 cm-1 during the last stages of the pyrolysis of a methoxylated silica.

Structure and reactivity of iron oxide surfaces

Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure–reactivity correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observed on all oxides that expose metal cations in their topmost layers, whereas purely oxygen-terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reaction, which is performed over iron oxide based catalysts, we find a decreasing chemisorption strength of the reaction product molecule styrene, if compared to ethylbenzene, when going from Fe3O4(111) over α-Fe2O3(0001) to KFexOy(111). Extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for coadsorption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemistry does not change across the pressure gap and that the model systems simulate technical styrene synthesis catalysts in a realistic way.

Classification of reactive sites on the surface of polycrystalline tin dioxide

A new method is introduced for study of the nature of reactive sites on oxide surfaces, which is derived from the study of gas sensors and is based on the correlation between combustion catalysis and the effects of gases on electrical conductivity at elevated temperature. Chemical modification of the surface, using a gaseous siloxane to react progressively with the surface sites, is employed to discriminate and classify different types of site. Both the electrical response and the combustion rate are altered by the titration of the surface sites against the siloxane. The method comprises the sequential application of test gas (hexamethyldisiloxane, HMDS, at low concentration in air) and probe gas (methane, water vapour), monitoring methane combustion rate, resistivity in pure, dry air and electrical response to methane and water vapour, as a function of HMDS dose. The measurements require the material to be fabricated into a gas sensor device having an array of electrodes designed to probe conductivity variations throughout the porous ceramic body, which allows the combustion rate and the electrical response to be obtained simultaneously. Four different types of reactive surface site have thereby been distinguished on polycrystalline SnO2: an electrically neutral oxygen site which catalyses methane and HMDS combustion, two electrically charged oxygen sites which respectively mediate methane response and dissociative chemisorption of water, and a site for molecular chemisorption of water. By comparison with other literature, the neutral site which catalyses combustion could be attributed to a coordinatively unsaturated lattice oxygen. The two different electrically charged sites could be associated with oxygen adsorbed on or at two different types of surface oxygen vacancy associated with reduced (Sn2+) surface cations.

Hydroxylation and crystallization of electropolished titanium surface

Surface properties and dissociative chemisorption of water on titanium-oxide surfaces are of particular interest for the biocompatibility of this material. Scanning force microscopy images of electropolished titanium samples and of thin films of titanium evaporated under vacuum show a similar topography with a grain size of 30 nm. Mass spectrometer thermal desorption spectroscopy displays two peaks for e/m = 18, at 380 and 520 K corresponding to adsorbed water molecules and hydroxyl groups, respectively. Auger spectra show the segregation toward the surface of chlorine from the metal-oxide interface upon heating at 720 K and of sulphur from the bulk after annealing at 950 K. The oxygen diffusion from the surface toward the bulk during the heating process induces a metallic behaviour of the surface layer which is revealed by tunneling spectroscopy: the semiconducting gap present in d I/d V curves for the air-exposed sample vanishes upon annealing. STM images of the annealed surface show the presence of tiny crystallites of a few nm in size.