Dissociative Adsorption Of Water Research Articles

Water Adsorption on AnO2 {111}, {110}, and {100} Surfaces (An = U and Pu): A Density Functional Theory + U Study

The interactions between water and the actinide oxides UO2 and PuO2 are important both fundamentally and when considering the long-term storage of spent nuclear fuel. However, experimental studies in this area are severely limited by the intense radioactivity of plutonium, and hence, we have recently begun to investigate these interactions computationally. In this paper, we report the results of plane-wave density functional theory calculations of the interaction of water with the {111}, {110}, and {100} surfaces of UO2 and PuO2, using a Hubbard-corrected potential (PBE + U) approach to account for the strongly correlated 5f electrons. We find a mix of molecular and dissociative water adsorption to be most stable on the {111} surface, whereas the fully dissociative water adsorption is most stable on the {110} and {100} surfaces, leading to a fully hydroxylated monolayer. From these results, we derive water desorption temperatures at various pressures for the different surfaces. These increase in the order...

Molecular and dissociative adsorption of water and hydrogen sulfide at perfect and defective Cu(110) surfaces.

We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of H2O and H2S at perfect and defective Cu(110) surfaces described using supercells with c(6 × 6) periodicity. The defective surface consists of a terrace surrounded by pits. We found considerable differences in adsorption modes and energies for H2O and H2S. At the defective Cu(110) surface, monomers of H2O and H2S preferentially adsorb at the terrace site and molecular adsorption of H2O is significantly more favorable than that of H2S. For dissociative adsorption however, the sulfur species are considerably more stable than the oxygen species. For monolayer (ML) coverages, there are small differences in the molecular adsorption energies for H2O and H2S. However, for the formation of 1 ML of HO and 1 ML of HS from 1 ML of H2O and 1 ML of H2S, respectively, with the release of H2(g), the differences are very large. The formation of 1 ML HO at the perfect Cu(110) surface is endoergic, while at the defective Cu(110) surface it is exoergic by -0.6 eV. For high coverages, H2S forms stacked half-monolayers that interact with each other via a complex hydrogen bond network with a strength per H2S molecule of -0.140 eV per H2S and -0.120 eV per H2S for H2S located in the underlayer and overlayer, respectively. The large distances between hydrogen bonded H2S molecules explain the preference for the formation of the two stacked half-monolayers of H2S instead of a single monolayer as it happens with H2O. Additionally, the formation of 1 ML of HS does not occur because of the spontaneous splitting of some H-S bonds resulting in surface bound HS and S and H2S molecules. Extensive surface reconstruction and relaxation accompanies adsorption of the sulfur adsorbates. Such reconstructions with outwards pull of Cu atoms can be at the origin of the weak adhesion of sulfide films that explains the release of CuS particles from copper sulfide films at copper surfaces. Overall, the surface defects here investigated induce non-linear effects in the molecular and dissociative adsorption energies of different O and S adsorbates.

Molecular and dissociative adsorption of water at a defective Cu(110) surface

We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of water at the perfect Cu(110) and at a defective Cu surface that provides a broad spectrum of adsorption sites in terms of coordination of Cu atoms, with the aim of understanding the role of surface defects in the dissociation of water molecules. The molecular adsorption of water is spontaneous at both surfaces but at the defective surface we found two stable molecular adsorption structures that differ slightly in the disposition of the O-atoms of H2O on the surface plane but differ considerably on the orientation of their H-atoms. Additionally we studied the dissociative adsorption of water accompanied with formation of H2(g). At the defective surface, starting from 1ML of molecularly adsorbed H2O, the dissociation of 0.22ML of H2O leading to 0.22ML of HO, 0.78ML H2O and H2(g) is exergonic but the dissociation of 0.44ML or more H2O molecules is endergonic. These findings are discussed in terms of the two main factors that affect the adsorption energies: the existence of exposed adatoms provides an environment that facilitates the interaction with small adsorbates leading to stronger bonds between the surface and such adsorbates; and at the same time, the limited polarizability of the defect sites causes high coverages of adsorbates that drag electron density from the surface to be unfavorable. The overall effect is that at the defective Cu(110) formation of low coverages of HO groups is more favorable than at the perfect Cu(110) while forming coverages higher than 0.44ML of HO is less favorable than at the perfect Cu(110). These effects have their origin in the extent of the polarization of the Cu–O bonds.

Method for studying high temperature aqueous electrochemical systems: Methanol and glycerol oxidation

A method for high purity aqueous electrochemical experiments at temperatures above the normal boiling point of water and at temperatures up to 140°C is described. A three-electrode cell in a self-pressurized glass autoclave is heated in an oil bath. Slow ramping of the temperature allows efficient acquisition of kinetic parameters such as activation energies, oxidation onset potentials and Tafel slopes by using cyclic voltammetry. The oxidation of two organic alcohols with different volatilities, methanol (high volatility) and glycerol (low volatility), are studied to demonstrate the capabilities of the method. Methanol oxidation on platinum is found to have a similar mechanism at all temperatures, with either dissociative adsorption of water or dissociative adsorption of methanol as the rate-determining step. In the case of glycerol oxidation on platinum, the mechanism changes at 110°C. At low temperatures dissociative adsorption of water or dissociative adsorption of glycerol is suggested to be the rate-determining step. At higher temperatures, a significant decrease in onset potential was observed and the glycerol is suggested to selectively oxidize to glyceraldehyde or dihydroxyacetone, with dissociative glycerol adsorption as the rate-determining step.

Water Dissociative Adsorption on NiO(111): Energetics and Structure of the Hydroxylated Surface

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ∼92 kJ/mol. Density functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorime...

Electronic structure of bulk AnO2 (An = U, Np, Pu) and water adsorption on the (111) and (110) surfaces of UO2 and PuO2 from hybrid density functional theory within the periodic electrostatic embedded cluster method

Generalised gradient approximation (PBE) and hybrid (PBE0) density functional theory (DFT) within the periodic electrostatic embedded cluster method have been used to study AnO2 bulk and surfaces (An = U, Np, Pu). The electronic structure has been investigated by examining the projected density of states (PDOS). While PBE incorrectly predicts these systems to be metallic, PBE0 finds them to be insulators, with the composition of the valence and conduction levels agreeing well with experiment. Molecular and dissociative water adsorption on the (111) and (110) surfaces of UO2 and PuO2 has been investigated, with that on the (110) surface being stronger than on the (111). Similar energies are found for molecular and dissociative adsorption on the (111) surfaces, while on the (110) there is a clear preference for dissociative adsorption. Adsorption energies and geometries on the (111) surface of UO2 are in good agreement with recent periodic DFT studies using the GGA+U approach, and our data for dissociative adsorption on the (110) surface of PuO2 match experiment rather well, especially when dispersion corrections are included.

Method for Studying High Temperature Aqueous Electrochemical Systems: A Self Pressurized Autoclave

Understanding oxidation kinetics and reaction mechanism at elevated temperatures is paramount to making efficient fuel cells that can run at temperatures higher than 100oC. We describe a method that allows for studies of aqueous electrochemistry at temperatures above the normal boiling point of water, and this system is used to study potential fuels for direct fuel fed fuel cells. Two alcohols with very different partial pressures at temperatures above 100oC are used in this study: methanol and glycerol. A self-pressurized glass autoclave heated in an oil bath is used for this purpose, and a model of the autoclave is shown in Fig. 1a. Slow ramping of the temperature allows for efficient data acquisition by using cyclic voltammetry up to 140oC, exemplified by Fig. 1b, and kinetic parameters such as onset potentials, Tafel slopes and activation energies are found and discussed for the relevant fuels. For methanol oxidation, dissociative water adsorption plays a key limiting role in the total reaction of methanol to CO2 at all temperatures. For glycerol oxidation, some partial oxidation products do not require an oxygen donor in the form of adsorbed water for their formation. This is deemed to play a fundamental role giving a large drop in overpotential at temperatures above 110oC, and making glycerol a potential fuel cell candidate at these temperatures. Therefore, a partial oxidation of glycerol in a fuel cell offers a potential method for a combined fuel cell and value-adding process for abundant glycerol. Figure 1

Kinetic investigations of the steam reforming of methanol over a Pt/In2O3/Al2O3 catalyst in microchannels

A kinetic study of methanol steam reforming over bimetallic Pt/In2O3/Al2O3 catalyst was carried out. The kinetic measurements were performed in a microstructured monolithic reactor with an external recycle free of temperature and concentration gradients. By the help of residence time distribution measurements it could be verified that the reactor showed the behaviour of an ideal continuous stirred tank reactor (CSTR). The absence of external and internal concentration gradients could be proven by corresponding experiments and theoretical diagnostic criteria.The kinetic measurements performed by variation of the reactant inlet partial pressures revealed that in the temperature range from 310°C to 355°C the molar rate of methanol consumption mainly depends on the methanol partial pressure, especially at higher temperatures, whereas there is only minor dependence on the water partial pressure. Carbon dioxide has no inhibiting effect, whereas hydrogen showed a weak inhibiting effect.Two power laws and three Langmuir-Hinshelwood rate equations were created for the modelling of the kinetic data. Power laws could not be fitted to the measured values. Therefore the uses of Langmuir-Hinshelwood rate laws with temperature dependent sorption constants are inevitable for the modelling. The model discrimination revealed that the rate law derived from a mechanism, which assumes the dehydrogenation of an adsorbed methoxy-species as rate determining step, described the measured kinetic data second best. Optimum agreement between observed and predicted molar rates of methanol consumption was obtained when applying a Langmuir-Hinshelwood rate law assuming dissociative methanol and molecular water adsorption on the catalyst surface. Dissociative adsorption of water and methanol at the same active site may be excluded. This leads to a better evaluation of the models that assume molecular water adsorption at the same site where methanol ties or the ones with no participation of water in the rate determining step (RDS) under discrimination.

Dissociative adsorption of H2O on LiCoO2 (00l) surfaces: Co reduction induced by electron transfer from intrinsic defects.

Understanding the mechanism of the interaction of lithium ion conductors with water is crucial for both fundamental and technological points of view. Despite the generally accepted fact that water is one of main sources of the degradation of Li-ion recharge batteries, the physicochemical processes occurring at the water-lithium ion conductor interface are not fully understood. By using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K- and Co L- X-ray absorption near edge structure (XANES), we evidence that H2O is dissociatively adsorbed on LiCoO2 thin film at room temperature resulting in the formation of OH groups and the accumulation of the negative charge at the surface accompanied by electron transfer to the initial empty Co3d (eg (*)) state. By considering the experimentally obtained energy diagram of the ionic conductor and water, direct charge transfer is not favorable due to a high difference in the chemical potential of the ionic conductor and electronic levels of the molecule. Here, we develop the model for the dissociative water adsorption which explains the electron transfer to LiCoO2 by using the atomistic approach. The model takes into account the intrinsic defects found on the surface (<2 nm depth) by using the depth resolved photoemission experiments and can be explored to other layered transition metal oxides to interpret the interaction of water with the surface of ionic conductors.

Facile Synthesis and Reaction Mechanism of Oxygen Evolution on RuO2, IrO2, and RuO2@IrO­2 Core-Shell Nanocatalysts

Core-shell bimetallic nanoparticles are effective in enhancing activity and durability of catalysts for oxygen reduction and hydrogen oxidation reactions in fuel cells. It is less clear about using this approach to develop advanced metal oxide nanocatalysts for oxygen evolution reaction (OER), partly due to insufficient understanding of the more complicated OER that involves multiple intermediates adsorbed on both metal and oxygen active sites. In this study, we deduced a phase diagram of the dominant adsorbates based on experimental results and density functional theory (DFT) calculations. We prepared RuO2 and RuO2@IrO2core-shell nanocatalysts via a facile method using carbon support as a template without and with H2IrCl6 as the Ir precursor. During calcinations in air for 1 h, Ru catalyzes a complete burning out of the carbon, which facilitates the formation of small dioxide nanoparticles. The catalysts’ weights after syntheses were consistent with the masses of pure metal dioxide - no cleaning procedure needed. We compare voltammetry signatures (Fig. 1) and polarization curves (Fig. 2) of the two synthesized and commercial IrO2 catalysts. The currents were normalized to the oxide surface area calculated from the average particle sizes and catalyst loadings. The integrated charges between 0.4 and 1.23 V are all close to 1e per metal atom for the rutile (110) surface, supporting a 1e surface reaction over a broad potential range. RuO2 exhibited higher current, but larger Tafel slope at high potentials than IrO2 did, consistent with literature results.1 The OER activity increased on RuO2@IrO2core-shell nanocatalyst from that of IrO2 while the Tafel slope remained similar to that of IrO2, which bodes well for performance in water electrolyzers operating at ~1.8 V. To gain fundamental understanding of the OER mechanism on these most active catalysts, we derived a kinetic equation using a simplified model with one major adsorbed intermediate, *O, and two activation barriers (See Fig. 2 caption). Connecting the fitted free energies with the most stable adsorbates determined by DFT calculations for metal oxidation state from 3 to 7, we deduced the phase diagrams. The one for IrO2 is shown in Fig. 3b. Water dissociative adsorption is highly facile over the coordination unsaturated site (cus) of metal and bridge oxygen (Fig. 3a), resulting in an adsorption phase of *OH H*. Ir oxidation state remains 4 because changes by +2 for each *O and -1 for each *H cancel out. In the OER potential region above 1.4 V, metal oxidation state rises with proton removal - first from adsorbed OH to the *O *H phase and then to the *O phase. It is followed by second water dissociation occurring over the *O-covered site and bridge oxygen site with one proton removed to the *OO *H phase. Below 1.2 V, *OH desorbes from Mcus, yielding a *H phase with H adsorbed on bridge oxygen site. In situX-ray absorption near edge spectra (XANES) support a change of metal oxidation state from 4 to 3 with decreasing potential from 1.3 V to 0.5 V (Fig. 3c). Comparison of DFT-calculated and experimentally-derived free energy diagrams for the OER will be presented to shed lights on how a RuO2 core enhances the OER activity on the IrO2 shell. Acknowledgements This research was supported by the US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division under contract DE-SC0012704. References (1) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-horn, Y. J. Phys. Chem. Lett. 2012, 3, 399. Figure 1

Catalytic water dissociation by greigite Fe3S4 surfaces: density functional theory study.

The iron sulfide mineral greigite, Fe3S4, has shown promising capability as a hydrogenating catalyst, in particular in the reduction of carbon dioxide to produce small organic molecules under mild conditions. We employed density functional theory calculations to investigate the {001},{011} and {111} surfaces of this iron thiospinel material, as well as the production of hydrogen ad-atoms from the dissociation of water molecules on the surfaces. We systematically analysed the adsorption geometries and the electronic structure of both bare and hydroxylated surfaces. The sulfide surfaces presented a higher flexibility than the isomorphic oxide magnetite, Fe3O4, allowing perpendicular movement of the cations above or below the top atomic sulfur layer. We considered both molecular and dissociative water adsorption processes, and have shown that molecular adsorption is the predominant state on these surfaces from both a thermodynamic and kinetic point of view. We considered a second molecule of water which stabilizes the system mainly by H-bonds, although the dissociation process remains thermodynamically unfavourable. We noted, however, synergistic adsorption effects on the Fe3S4{001} owing to the presence of hydroxyl groups. We concluded that, in contrast to Fe3O4, molecular adsorption of water is clearly preferred on greigite surfaces.

Influence of Environmental Humidity on the Wear and Friction of a Silica/Silicon Tribopair Lubricated with a Hydrophilic Ionic Liquid.

In this study, the tribological behavior of silica/silicon surfaces lubricated with the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM] EtSO4) was investigated. Tests were carried out in the presence of either humid air (45-55% relative humidity) or in a nitrogen atmosphere, and the results were compared with those obtained using pure water as a lubricant. The cross-sectional analysis of the contact area performed by focused-ion-beam scanning electron microscopy indicated the presence of cracks in the subsurface region, showing that brittle fracture contributed to wear. Sliding promoted the formation of a third body, the presence of which was indicated by optical and secondary electron microscopy. X-ray photoelectron spectroscopy showed that the third body was mostly composed of silicon oxides. The accumulation of the debris was controlled by the presence of water: in the presence of a nitrogen atmosphere, particles were trapped between the sliding surfaces, whereas in the case of humid air, the debris was progressively removed from the contact. Notably, the presence of trapped particles was associated with higher values of wear coefficients of both disks and pins. In addition, a lower roughness was observed along the direction of sliding in the case of water-containing ionic liquid. The observed trends in wear and the combined results of the various techniques, as well as the comparison with tests carried out in the presence of pure water, all point to the characteristic tribochemical reactions of water with silicon-based materials, namely, the formation of a sacrificial layer of hydrated oxide and the dissociative adsorption of water at crack tips of SiO2. In the absence of water, the lack of a tribochemical mechanism forming a sacrificial layer leads to a microfracture-dominated wear mechanism over the entire duration of the test, thus leading to more severe wear. The possible occurrence of stress-induced phase transformation of silicon during sliding is also discussed.

Adsorption of Water on Yttria-Stabilized Zirconia

Water adsorption on the cubic (111) surface of yttria-stabilized zirconia (YSZ) was investigated using density functional theory calculations. Relaxation of atomic positions away from ideal cubic lattice sites, in particular of the oxygen anion sublattice, is observed on including both phase-stabilizing dopants and water adsorption. A large slab model has been used to explore the effects of extended relaxation throughout the anionic sublattice and the role of vacancy–vacancy interactions on water adsorption. Dissociative adsorption of water to fill a surface vacancy site, accompanied by concerted oxygen movement in the vacancy cluster region of the slab, leads to a very strong adsorption of −2.20 eV, blocking surface sites for oxygen activation. We show that the use of larger slab models leads to a more detailed representation of the YSZ surface system.

Coverage Dependent Water Dissociative Adsorption on the Clean and O-Precovered Fe(111) Surfaces

Water dissociative adsorption on the clean and O-precovered Fe(111) surfaces at different coverage have been studied using the density functional theory method (GGA-PBE) and ab initio atomistic thermodynamics. On the clean p(3 × 3) Fe(111) surface, surface H, O, OH, and H2O species can migrate easily. Considering adsorption and H-bonding, the adsorbed H2O molecules can be dispersed or aggregated in close energies at low coverage, while in different aggregations at high coverage, indicating that the adsorbed H2O molecules might not have defined structures, as observed experimentally. On the O-precovered surface (nO = 1–8), the first dissociation step, nO + H2O = (n – 1)O + 2OH, has a very low barrier and is reversible; and the barriers of the sequential OH dissociation steps, (n – 1)O + 2OH = nO + H + OH and nO + H + OH = (n + 1)O + 2H, are close (0.9–1.2 eV). All of these barriers are coverage independent. For OH and H adsorption at 1/3 ML coverage, surface OH forms a trimer (OH)3 unit, and surface O forms a regular linear pattern. At one ML coverage, there are three dispersed (OH)3 units for OH adsorption and three well-ordered parallel lines for O adsorption. The average adsorption energies for OH and O adsorption are coverage independent. Desorption temperatures of H2O and H2 under ultrahigh vacuum conditions are computed. Systematic comparison among the Fe(110), Fe(100), and Fe(111) surfaces reveal their intrinsic differences in water dissociative adsorption and provide a basic understanding of water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces.

Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations

The molecular and dissociative adsorption of water on a Ag-supported 1 ML, 2 ML and 3 ML-a six atomic layer-thick MgO films with a single Au adatom is investigated using density functional theory calculations. The obtained results are compared to a bulk MgO(001) surface with an Au atom. On thin films the negatively charged Au strengthens the binding of the polar water molecule due to the attractive Au–H interaction. The adsorption energy trends of OH and H with respect to the film thickness depend on an adsorption site. In the case OH or H binds atop Au on MgO/Ag(001), the adsorption becomes more exothermic with the increasing film thickness, while the reverse trend is seen when the adsorption takes place on bare MgO/Ag(001). This behavior can be explained by different bonding mechanisms identified with the Bader analysis. Interestingly, we find that the rumpling of the MgO film and the MgO–Ag interface distance correlate with the charge transfer over the thin film and the interface charge, respectively. Moreover, we employ a modified Born–Haber-cycle to analyze the effect of film thickness to the adsorption energy of isolated Au and OH species on MgO/Ag(001). The analysis shows that the attractive Coulomb interaction between the negatively charged adsorbate and the positive MgO–Ag-interface does not completely account for the weaker binding with increasing film thickness. The redox energy associated with the charge transfer from the interface to the adsorbate is more exothermic with the increasing film thickness and partly compensates the decrease in the attractive Coulomb interaction.

Coverage dependent water dissociative adsorption on Fe(110) from DFT computation.

Using density functional theory calculations and ab initio atomistic thermodynamics, H2O adsorption and dissociation on the Fe(110) p(4 × 4) surface at different coverages have been computed. At the lowest coverage, the adsorbed H2O, OH, O and H species can migrate easily on the surface. For (H2O)n adsorption, H2O molecules donating H atoms for H-bonding adsorb more strongly than those accepting H atoms for H-bonding. Monomeric H2O dissociation is favored both thermodynamically and kinetically. On nO pre-covered Fe(110) surfaces (n = 1-8), H2O dissociation is accessible for nO + H2O (n = 1-7) both kinetically and thermodynamically, while H2O desorption instead of dissociation occurs at n = 8. With the increased number of surface O atoms, H2 dissociative adsorption energies vary in a narrow range for n = 1-4 and decrease for n = 5-7, while at n = 8, the surface does not adsorb H2. At low OH coverage (n = 2, 4), OH groups are perpendicularly adsorbed without H-bonding, while for n≥ 6, adsorbed OH groups are linearly arranged and stabilized by H-bonding. The maximal OH coverage (n = 12) is 0.75 ML and the reasonable O coverage (n = 7) is 0.44 ML, in line with the experiment. The calculated desorption temperatures of H2O and H2 agree well with the available experimental data. These results provide fundamental insights into water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces.

Water on titanium dioxide surface: a revisiting by reactive molecular dynamics simulations.

The behavior of surface water, especially the adsorption and dissociation characteristics, is a key to understanding and promoting photocatalytic and biomedical applications of titanium dioxide materials. Using molecular dynamics simulations with the ReaxFF force field, we study the interactions between water and five different TiO2 surfaces that are of interest to both experiments and theoretical calculations. The results show that TiO2 surfaces demonstrate different reactivities for water dissociation [rutile (011) > TiO2-B (100) > anatase (001) > rutile (110)], and there is no water dissociation observed on the TiO2-B (001) surface. The simulations also reveal that the water dissociation and the TiO2 surface chemistry change, and the new surface Ti-OH and O-H functional groups affect the orientation of other near-surface water molecules. On the reactive surface, such as the rutile (110) surface, water dissociated and formed new Ti-OH and O-H bonds on the surface. Those functional groups enhanced the hydrogen bond networking with the near-surface water molecules and their configurations. On the nonreactive TiO2-B (001) surface where no molecular or dissociative water adsorption is observed, near-surface water can also form hydrogen bonds with surface oxygen atoms of TiO2, but their distance to the surface is longer than that on the rutile (011) surface.

Zeolitic Imidazolate Framework-8 with High Efficiency in Trace Arsenate Adsorption and Removal from Water

The development of highly efficient adsorbents, especially those aimed at the capture of trace (ppb, 10–9) arsenate, is one of the principal challenges in the water treatment field. In this article, zeolitic imidazolate framework-8 (ZIF-8) was explored for the removal of trace arsenate from water. Results showed that ZIF-8 outperformed some other adsorbents and owned the first and highest reported adsorption capacity (76.5 mg g–1) at a low equilibrium concentration (9.8 μg L–1). Satisfactory adsorption properties (adsorption capacity, adsorption rate, adaptability to water environment, regeneration capacity) demonstrated the feasibility of using ZIF-8 as an efficient adsorbent for the removal of aquatic trace arsenate. In addition, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra revealed the proposed mechanism of As(V) adsorption onto ZIF-8: producing large amounts of external active sites (Zn–OH) through the dissociative adsorption of water and subsequently forming an...

Role of proton hopping in surface charge transport on tin dioxide as revealed by the thermal dependence of conductance.

The presence of water on an oxide surface can dramatically alter its electrical properties with important consequences for electrical measurements by scanning probe microscopy, and for the use of semiconducting oxides in sensing applications. Here, the thermal dependence of the conductance of tin dioxide is interpreted by combining semiconductor equilibrium carrier statistics with a proton hopping mechanism. First, the functional form of this charge transport model is fit to experimental conductance data for tin dioxide. Next, the important energy parameters in the model are computed with density functional theory. Comparing the values of the energy parameters obtained by fitting, to the values for the same parameters obtained from electronic structure calculations, yields new insight into the surface charge transport in tin dioxide. In particular, it is found that mobile protons, freed from the dissociative adsorption of water on the [110] surface, are an essential component of the observed thermal dependence of conductance in tin dioxide.

How Surface Hydroxyls Enhance MgO Reactivity in Basic Catalysis: The Case of Methylbutynol Conversion

This combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was experimentally determined on partially hydroxylated MgO: active sites were saturated with the reactant and catalyzed its conversion with an activation energy of 85 kJ·mol–1. Reaction pathways were calculated over fully hydroxylated, partially hydroxylated, and dehydroxylated MgO surfaces by means of first-principles simulations. To highlight the effect of reactant coverage, we also calculated the reaction pathway on MgO precovered with MBOH molecules. The results show that the surface OH groups generated by dissociative adsorption of water induce a lowering in the activation energy barriers when they keep a bare Mg2+-O2– pair available in the vicinity for MBOH to adsorb and react. Interestingly, OH groups do not directly interact with MBOH converting on the surface, but they modify the basic properties of the vicinal bare Mg2+-O2– pair on which MBOH adsorbs and converts. A similar effect is predicted when MBOH converts on a bare Mg2+-O2– pair in the vicinity of a second adsorbed MBOH molecule. The beneficial effect of coadsorbates on the reactivity of active sites is analyzed by means of a thermodynamic model and an electronic structure analysis.