Interaction Of H2O Research Articles

Effect of U Content on the Activation of H2O on Ce1–xUxO2+δ Surfaces

The Ce–U–O system raises growing interest due to its potential importance for water splitting at low temperatures. The variable possible oxidation states of Ce (Ce3+ and Ce4+) and U (U4+, U5+, and U6+) lead to the formation of point charged defects on the surface. These point charges are active sites for the chemisorption of H2O, which is the rate-determining step for H2 production. In the present work, the interaction of H2O with the surface of Ce1–xUxO2+δ oxides in a wide range of compositions (x = 0, 0.1, 0.25, 0.5, 0.75, and 1) is studied through the measurements of adsorption and calorimetric isotherms at room temperature. The oxides’ structure is determined by X-ray diffraction, and the charge distribution between the U and Ce cations is inferred from X-ray photoelectron spectroscopy (XPS) measurements. The adsorption thermodynamics is analyzed within the electronic theory of chemisorption on semiconducting surfaces. On the basis of this analysis, correlated to the XPS results and to density functional theory calculations on a representative oxide, an atomic scale understanding of the chemisorption process is proposed. Partial charge transfer between the Ce and U cations is shown to be a key factor for creating adsorption sites for H2O activation. This charge transfer is shown to occur most efficiently in the mixed oxides with low U content. The analysis proposed explains the adsorption behavior of the different mixed oxides and provides an explanation for the improved efficiency for H2O splitting reported on reduced Ce1–xUxO2 oxides with low U content.

Calculation of the Raman Q branch of hydrogen in water and comparison with experiments

Calculations of the Raman Q-branch line shifts and line widths of H2 in water at three different pressures and at 280 K are presented and compared with available experimental results. The calculations are based on a perturbative method in which the perturbation consists of those components of the H2–H2O interaction that depend on vibrational and/or rotational coordinates. In the unperturbed system the rotovibrational motion of the hydrogen molecule is separated from the dynamics of the rest of the system. The unperturbed rotovibrational motion of H2 is treated quantum mechanically whereas the dynamics of the rest of the system is calculated by classical molecular dynamics (MD). The MD calculations are repeated with two different models for the water–hydrogen potential energy. Both are Lennard-Jones models but in one the parameters are derived from the Lorentz–Bertholet combination rules and in the other they are taken from gas phase scattering data. The numerical results for the Raman Q-branch spectra are compared with the measurements in order to determine which model is more dependable.

Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics and NBO analysis

Structural and dynamical properties of Cu+ in 18.6% aqueous ammonia solution at 298.15 K have been investigated via quantum mechanical charge field molecular dynamics (QMCF MD). The QM region was set to a radius of 6.7 Å to include the first and second solvation shell. The Hartree-Fock (HF) level was applied to calculate the ion-ligand and ligand-ligand interactions in the QM region using the LANL2DZ-ECP basis set for the ion and DZP-Dunning for the ligands. The Cu+N and Cu+O radial distribution functions showed maximum first shell probabilities at distances of 2.23 and 2.30 Å, respectively. Predominantly, four NH3 molecules were found to form a tetrahedral [Cu(NH3)4]+ complex, although the formation of a short-lived intermediate [Cu(NH3)3H2O]+ complex was also observed. The mean residence times of NH3 and H2O ligands in the first solvation shell were estimated as 14.6 ps and 1.3 ps, respectively, reflecting the strong interaction between Cu+ and ammonia as well as the occurrence of rapid water exchange. The vibrational power spectrum of the Cu+N vibration in the first solvation shell revealed a wave number of 252 cm−1 with a corresponding force constant of 43.0 Nm−1. In addition, an NBO analysis was carried out, confirming the strong electrostatic character of the Cu+NH3 and Cu+H2O interaction, and highlighting that the presence of H2O ligands may destabilize the first solvation shell.

On the Significance of Lone Pair/Lone Pair and Lone Pair/Bond Pair Repulsions in the Cation Affinity and Lewis Acid/Lewis Base Interactions.

Interaction of H2O, H2S, H2Se, NH3, PH3, and AsH3 with cations H+, CH3+, Cu+, Al+, Li+, Na+, and K+ was studied from the energetic and structural viewpoint using B3LYP/6-311++G(d,p) method. The charge transfer from the Lewis bases to the cations reduces lone pair/lone pair (LP/LP) repulsion in H2O, H2S, and H2Se and LP/bond pair (LP/BP) repulsion in NH3, PH3, and AsH3. In parallel, changes in the H–M–H angles (M = O, S, Se, N, P, and As) are observed. The change in the H–M–H angle during the interactions was proportional to the amount of charge transferred from the bases to the cations and electron density (ρ) at the molecule/cation bond critical point. Also, the opposite trend for proton affinities of these two families, that is, NH3 > PH3 > AsH3 and H2O < H2S < H2Se, was interpreted on the basis of LP/BP repulsion in their neutral and protonated forms. Interaction of the Lewis bases with neutral Lewis acids including BeH2, BeF2, and BH3 was studied energetically and structurally. The calculated energies for interactions of H2O and NH3 with BeH2, BeF2, and BH3 are larger than the corresponding values for H2S, H2Se, PH3, and AsH3. This difference was interpreted on the basis of the lower stability of H2O and NH3 because of large LP/LP and LP/BP repulsion in H2O and LP/BP repulsion in NH3.

A DFT study on the interaction of small molecules with alkali metal ion-exchanged ETS-10

Abstract In this paper, we present a systematic quantum-mechanical density functional theory (DFT) study of adsorption of small gas molecules in cation-exchanged Engelhard titanosilicate ETS-10 crystalline materials. Adsorbates with a range of polarities were considered, ranging from polar (H2O), quadrupolar (CO2 and N2), to apolar (CH4) atmospheric gases. Starting from the base-case of Na-ETS-10, other extra framework cations such as Li+, K+, Rb+ and Cs+ were considered. The DFT calculations were performed with the M06-L functional and were corrected for basis set superposition error with the counterpoise method in order to provide accurate and robust geometries and adsorption energies. For all adsorbates, the adsorption enthalpies decrease in the order Li+&gt;Na+&gt;K+&gt;Rb+&gt;Cs+, while adsorbate – cation interaction distances increase along the same order. For the two extreme cases, the enthalpies calculated at the M06-L/6-31++G** level of theory for CH4, N2, CO2, and H2O interaction with Li+(Cs+) exchanged materials are −21.8 (−1.7) kJ·mol−1, −19.0 (−10.7) kJ·mol−1, −34.4 (−21.3) kJ·mol−1, and −70.5 (−36.1) kJ·mol−1, respectively. Additionally, the calculated vibrational frequencies are found to be in quite good agreement with the characteristic vibrational modes of alkali metal cation-exchanged ETS-10 and also with the available experimental frequencies for CH4, N2, CO2, and H2O interactions with alkali metal cations in the 12-membered channel of ETS-10.

Anharmonic Densities of States for Vibrationally Excited I-(H2O), (H2O)2, and I-(H2O)2.

Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, Nanh( E), for I-(H2O), (H2O)2, and I-(H2O)2 versus internal energy up to their dissociation energies. The anharmonic density of states, ρanh( E), is found from the energy derivative of Nanh( E). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H2O···H2O interactions and an accurate two-body potential for the I-···H2O fit to quantum chemical calculations. The extensive Monte Carlo samplings are computationally demanding, and the use of computationally efficient potentials was essential for the calculations. Particular emphasis is directed toward I-(H2O)2, and distributions of its structures versus internal energy are consistent with experimental studies of the temperature-dependent vibrational spectra. At their dissociation thresholds, the anharmonic to harmonic density of states ratio, ρanh( E)/ρh( E), is ∼2, ∼ 3, and ∼260 for I-(H2O), (H2O)2, and I-(H2O)2, respectively. The large ratio for I-(H2O)2 results from the I-(H2O)2 → I-(H2O) + H2O dissociation energy being more than 2 times larger than the (H2O)2 → 2H2O dissociation energy, giving rise to highly mobile H2O molecules near the I-(H2O)2 dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations.

Au-doped Ni/GDC as an Improved Cathode Electrocatalyst for H2O Electrolysis in SOECs

The present work deals with the comparison between single electrolyte supported Solid Oxide Electrolysis Cells that comprise either a 3 wt.% Au-Ni/GDC or a Ni/GDC steam/hydrogen electrode. Specifically, their electrochemical performance was investigated for the H2O electrolysis reaction by varying the pH2O/pH2 ratio (from 1 to 9), the operating temperature (800-900 °C) and the applied current. Physicochemical characterization was also performed both in the form of powders and as half cells with ex-situ and in-situ techniques, such as SEM, BET, XPS and TGA, including specific measurements in the presence of H2O. Different structural and activity properties were observed for each cermet, where the cell comprising the 3Au-Ni/GDC electrode exhibited better electrochemical performance, especially at 850 °C and above, having lower polarization resistance. This improvement is ascribed to the formation of a surface Ni-Au solid solution, which causes weaker interaction of H2O and of the resulting adsorbed Oads species with the modified cermet. The outcome is an electrode with a lower degree of surface oxidation and increased “three phase boundaries” length, where the charge transfer and electrode processes are enhanced for the H2O electrolysis reaction.

Force Field for Water over Pt(111): Development, Assessment, and Comparison.

Metal/water interfaces are key in many natural and industrial processes, such as corrosion, atmospheric, or environmental chemistry. Even today, the only practical approach to simulate large interfaces between a metal and water is to perform force-field simulations. In this work, we propose a novel force field, GAL17, to describe the interaction of water and a Pt(111) surface. GAL17 builds on three terms: (i) a standard Lennard-Jones potential for the bonding interaction between the surface and water, (ii) a Gaussian term to improve the surface corrugation, and (iii) two terms describing the angular dependence of the interaction energy. The 12 parameters of this force field are fitted against a set of 210 adsorption geometries of water on Pt(111). The performance of GAL17 is compared to several other approaches that have not been validated against extensive first-principles computations yet. Their respective accuracy is evaluated on an extended set of 802 adsorption geometries of H2O on Pt(111), 52 geometries derived from icelike layers, and an MD simulation of an interface between a c(4 × 6) Pt(111) surface and a water layer of 14 Å thickness. The newly developed GAL17 force field provides a significant improvement over previously existing force fields for Pt(111)/H2O interactions. Its well-balanced performance suggests that it is an ideal candidate to generate relevant geometries for the metal/water interface, paving the way to a representative sampling of the equilibrium distribution at the interface and to predict solvation free energies at the solid/liquid interface.

Structures and Spectroscopic Properties of F-(H2O) n with n = 1-10 Clusters from a Global Search Based On Density Functional Theory.

Using a genetic algorithm incorporated in density functional theory, we explore the ground state structures of fluoride anion-water clusters F-(H2O) n with n = 1-10. The F-(H2O) n clusters prefer structures in which the F- anion remains at the surface of the structure and coordinates with four water molecules, as the F-(H2O) n clusters have strong F--H2O interactions as well as strong hydrogen bonds between H2O molecules. The strong interaction between the F- anion and adjacent H2O molecule leads to a longer O-H distance in the adjacent molecule than in an individual water molecule. The simulated infrared (IR) spectra of the F-(H2O)1-5 clusters obtained via second-order vibrational perturbation theory (VPT2) and including anharmonic effects reproduce the experimental results quite well. The strong interaction between the F- anion and water molecules results in a large redshift (600-2300 cm-1) of the adjacent O-H stretching mode. Natural bond orbital (NBO) analysis of the lowest-energy structures of the F-(H2O)1-10 clusters illustrates that charge transfer from the lone pair electron orbital of F- to the antibonding orbital of the adjacent O-H is mainly responsible for the strong interaction between the F- anion and water molecules, which leads to distinctly different geometric and vibrational properties compared with neutral water clusters.

Release of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures

The release characteristics of Ca during a Chinese bituminous coal pyrolysis and char gasification in different gasifying agents was studied. Residual chars with different carbon conversion levels were obtained by gasifying in a quartz fixed-bed reactor under H2O, CO2, and H2O/CO2 atmospheres at 800–1000 °C. Sequential chemical extraction and microwave digestion were used to determine the modes of occurrence of Ca in chars. X-ray diffraction and scanning electron microscopy were used to analyze the ash composition and the morphology of minerals. The contents of Ca in chars were measured by atomic absorption spectroscopy. The results indicate that water-soluble form and HCl-soluble form Ca transforms mainly into gas phase during pyrolysis. The release ratio of Ca at 800 °C under different atmospheres is H2O/CO2 >H2O > CO2. The main reason is that minerals are more prone to sintering in CO2 atmosphere. The interaction of H2O and CO2 can expand pores, which precipitate the minerals to the char surface and release into the gas phase. Increasing temperature induces the collapse of the pore structure which changed the release rule of Ca. The composition of the ash obtained from gasification at 1000 °C in CO2, H2O and H2O/CO2 are nearly same.

Propane oxidation in a Jet Stirred Flow Reactor. The effect of H2O as diluent species

Advanced combustion technologies, such as MILD, LTC (Low Temperature Combustion) reduce emission of pollutants by controlling system working temperatures to values not critical to promote the formation of several classes of pollutants. To access this temperature range, a significant dilution of reactants is required. At the same time, reactants have to be preheated to sustain the oxidation process. Such conditions are achieved by a strong recirculation of exhaust gases. Such a strategy implies that high contents of CO2 and H2O interact with reactants oxidation chemistry. In order to characterize this aspect of the combustion processes under diluted conditions, experimental tests were carried out for propane/oxygen/nitrogen mixtures in presence of variable amounts of H2O in a quartz Jet Stirred Flow Reactor (JSFR). Experiments were carried out at atmospheric pressure, over the temperature range 720–1100 K, from fuel lean to rich conditions and at a residence time of 0.5 s. Temperature and species concentration measurements suggest that the oxidation of propane is significantly altered by H2O in dependence of mixture inlet temperatures and equivalence ratios.Numerical analyses were performed to explore the interaction of H2O with the oxidation chemistry of propane. Results suggested that such a species alters the main radical branching mechanisms, i.e. in termolecular reactions as a third body species with high collisional efficiency or directly participating in bimolecular reactions.

Formation of 45° Silicon (110) Surface Using Triton X-n Surfactants in Potassium Hydroxide for Infrared Applications

Silicon (Si) micromirrors are an integral feature for many micro-optomechanical systems (MOEMS). Such mirrors are generally wet etched in alkaline solution at elevated temperature. For 90° beam steering applications, 45° slanted Si (110) plane is the prime choice fabricated with the incorporation of tensioactive surfactants. Here, Triton-Si and Triton-hydroxide (OH−)/H2O interaction using varying hydrophilic chain length Triton (X-45, X-100 and X-405) were investigated. The surfactant concentration was varied from 0 to 1000 ppm in potassium hydroxide (KOH). Triton molecules were shown to adsorb preferentially on (110) than on (100) surface. Longer chain length Triton hampered OH− access to Si surface resulting in slower etch rate. In contrast, contact angle measurement suggested that shorter Triton interfaced better with Si surface. Later, Si wafers etched in Triton 10 ppm – KOH were examined. The measured output for (110)X-45, (110)X-100, (110)X-405 and polished Si wafer reference (Rq < 5Å) mirrors were 0.58, 0.76, 0.72 and 1.25 mW, respectively. Subsequently, Si-SiO2 thin film in [HLHL]2-substrate configuration was fabricated. Broadband micromirror for use in 3.0–5.5 μm spectrum range was experimentally realized with reflected efficiency of 73%.

Experimental study on the interactions of supercritical CO2 and H2O with anthracite

ABSTRACTTo better understand the mechanism of interactions of supercritical CO2 (SCCO2) and H2O, untreated, SCCO2 treated and SCCO2-H2O treated anthracites were adopted to analyze changes in pore structure, adsorption capacity. Observations from experimental data reveal that mineral and other substances in the fractures and matrix of coal body are dissolved and mobilized by the carbonic acid formed from the mixture of SCCO2 and H2O, which may contribute to smaller pore development and enhance the adsorption capacity. These findings may provide new insights into effective and safe storage of CO2 in coal reservoir.

Adsorption of ammonia molecules and humidity on germanane nanosheet—A density functional study

The structural stability and electronic properties of pristine and Ga substituted germanane nanosheet were investigated using the density functional theory technique. The stability of bare and Ga substituted germanane nanosheet is substantiated with formation energy. The energy band gap opens upon hydrogenation on germanene sheet, which is utilized as a sensor material for the detection of NH3 and H2O molecules. The interaction of ammonia and humidity on germanane nanosheet is explored using the projected density of states, Bader charge transfer, adsorption energy, average energy gap variation, energy gap and electron density. The interaction of H2O and NH3 gas molecules on germanane material is studied in atomistic level. The interaction of humidity on pristine germanane nanosheet and NH3 on Ga substituted germanane nanosheet is found to be more favorable. The findings recommend that germanane nanosheet can be utilized as a chemi-resistor for the detection of humidity and trace levels of ammonia gas in the environment.

σ-Holes on Transition Metal Nanoclusters and Their Influence on the Local Lewis Acidity

Understanding the molecular interaction behavior of transition metal nanoclusters lies at the heart of their efficient use in, e.g., heterogeneous catalysis, medical therapy and solar energy harvesting. For this purpose, we have evaluated the applicability of the surface electrostatic potential [VS(r)] and the local surface electron attachment energy [ES(r)] properties for characterizing the local Lewis acidity of a series of low-energy TM13 transition metal nanoclusters (TM = Au, Cu, Ru, Rh, Pd, Ir, Pt, Co), including also Pt7Cu6. The clusters have been studied using hybrid Kohn–Sham density functional theory (DFT) calculations. The VS(r) and ES(r), evaluated at 0.001 a.u. isodensity contours, are used to analyze the interactions with H2O. We find that the maxima of VS(r), σ-holes, are either localized or diffuse. This is rationalized in terms of the nanocluster geometry and occupation of the clusters’s, p and d valence orbitals. Our findings motivate a new scheme for characterizing σ-holes as σs (diffuse), σp (localized) or σd (localized) depending on their electronic origin. The positions of the maxima in VS(r) (and minima in ES(r)) are found to coincide with O-down adsorption sites of H2O, whereas minima in VS(r) leads to H-down adsorption. Linear relationships between VS,max (and ES,min) and H2O interaction energies are further discussed.

From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O and CO2 with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. The experimental data are summarized in a Temperature-PO2 diagram to visualize the dominant reactions at each temperature and PO2 for the two cathode materials. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. We will present our studies on the effect of cathodic polarization on LSM and LSCF. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

Catalytic Oxidation of Chlorobenzene over MnxCe1-xO2/HZSM-5 Catalysts: A Study with Practical Implications.

Industrial-use catalysts usually encounter severe deactivation after long-term operation for catalytic oxidation of chlorinate volatile organic compounds (CVOCs), which becomes a "bottleneck" for large-scale application of catalytic combustion technology. In this work, typical acidic solid-supported catalysts of MnxCe1-xO2/HZSM-5 were investigated for the catalytic oxidation of chlorobenzene (CB). The activation energy (Ea), Brønsted and Lewis acidities, CB adsorption and activation behaviors, long-term stabilities, and surficial accumulation compounds (after aging) were studied using a range of analytical techniques, including XPS, H2-TPR, pyridine-IR, DRIFT, and O2-TP-Ms. Experimental results revealed that the Brønsted/Lewis (B/L) ratio of MnxCe1-xO2/HZSM-5 catalysts could be adjusted by ion exchange of H• (in HZSM-5) with Mnn+ (where the exchange with Ce4+ did not distinctly affect the acidity); the long-term aged catalysts could accumulate ca. 14 organic compounds at surface, including highly toxic tetrachloromethane, trichloroethylene, tetrachloroethylene, o-dichlorobenzene, etc.; high humid operational environment could ensure a stable performance for MnxCe1-xO2/HZSM-5 catalysts; this was due to the effective removal of Cl• and coke accumulations by H2O washing, and the distinct increase of Lewis acidity by the interaction of H2O with HZSM-5. This work gives an in-depth view into the CB oxidation over acidic solid-supported catalysts and could provide practical guidelines for the rational design of reliable catalysts for industrial applications.

DFT study of the interactions of H2O, O2 and H2O + O2 with TiO2 (1 0 1) surface

The interactions of H2O, O2 and H2O+O2 with TiO2 (101) surface was studied using density functional theory (DFT). The adsorption energy of H2O+O2 on the TiO2 surface is the largest, followed by O2, and H2O. H2O molecule and O2 molecule adsorbed respectively on TiO2 surface are not dissociated, while the coadsorption of H2O and O2 results in the dissociation of H2O molecule and O2 molecule. H2O and O2 molecules not only interact respectively with TiO2 surface, but also interact with each other when H2O and O2 molecules adsorb simultaneously on the TiO2 surface. The interaction of H2O+O2 with TiO2 surface is more complex than those of single H2O or O2, including the formations of five new bonds and the cleavages of four bonds, which suggests the strong interaction between H2O+O2 and TiO2 surface. Two hydrogen-oxygen bonds of H2O molecule are all broken, and two hydrogen atoms interact with two oxygen atoms of TiO2 surface, respectively, while oxygen atom of H2O interacts simultaneously with a titanium atom of TiO2 surface and an oxygen atom of O2 molecule. Another oxygen atom of O2 molecule is bonded to another titanium atom of TiO2 surface.

From in-Situ to in-Operando Evaluation of SOFC Cathodes for Enhanced ORR Activity and Durability

To study the SOFC cathode oxygen reduction reaction (ORR) mechanism and the effects of ambient gas composition on ORR, we used in-situ 18O isotope exchange techniques to probe the exchange of H2O and CO2 with two of the most common SOFC cathode materials, (La0.8Sr0.2)0.95MnO3-x (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF). A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of H2O with the cathode surface as a function of temperature, oxygen partial pressure, and contaminant gas concentration. We further report on the development of a novel in-operando 18O isotope exchange technique by modifying our heterogeneous catalysis system for operation and measurement under electrochemical polarization. The system allows ORR investigations under real time, real life, operating conditions. The studies show the effects of operating voltage, temperature, and oxygen partial pressure on the oxygen exchange coefficient. We will also present our model for the understanding and analysis of the isotope oxygen exchange profiles under in-operando conditions.

Effect of external strain on the charge transfer: Adsorption of gas molecules on monolayer GaSe

First-principles calculations were performed to investigate the interaction of small gas molecules, including H2, O2, H2O, NH3, NO, NO2, and CO, with monolayer GaSe. The energetics, charge transfer and band structures were evaluated by considering the Grimme-D2 correction. Due to the low adsorption energies and the moderate charge transfer, monolayer GaSe could be a promising candidate as a sensor for O2 and NO2. The theoretical results for adsorption of O2 on monolayer GaSe are consistent well with the most recent experimental observation. Diverse projected band structures of these gas molecule-adsorbed systems demonstrate that there exist two kinds of charge transfer mechanisms: traditional and orbital mixing theories. Based on the proposed charge transfer mechanisms, external strain exerted different influences on the charge transfer for gas molecules adsorbed on the GaSe monolayer. The present study provides theoretical insight leading to a better understanding of the novel two-dimensional materials, such as graphene, phosphorene, and transition metal dichalcogenides.