Dissociative Adsorption Of H2O Research Articles

Molecular and Dissociative Adsorption of H2O on ZrO2/YSZ Surfaces

The work involves first-principles calculations to study the mechanism of adsorption of water molecules on the surface of ZrO2 and their yttrium-stabilized structures (YSZ). Calculations of the electronic properties of ZrO2 showed that during the m-t phase transformation of ZrO2, the Fermi level first shifts by 0.125 eV towards the conduction band, and then in the t-c region goes down by 0.08 eV. In this case, the band gaps for c-ZrO2, t-ZrO2 and m-ZrO2, respectively, are 5.140 eV, 5.898 eV and 5.288 eV. Calculations to determine the surface energy showed that t-ZrO2 (101) and m-ZrO2 (111) have the most stable structure, on the basis of which it was first discovered that the surface energy is somehow inversely related to the value of the band gap, since as the band gap increases, the surface energy tends to decrease. An analysis of the mechanism of water adsorption on the surface of t-ZrO2 (101) and t-YSZ (101) showed that H2O on unstabilized t-ZrO2 (101) is adsorbed dissociatively with an energy of −1.22 eV, as well as by the method of molecular chemisorption with an energy of −0.69 eV and the formation of a hydrogen bond with a bond length of 1.01 Å. In the case of t-YSZ (101), water is molecularly adsorbed onto the surface with an energy of −1.84 eV. Dissociative adsorption of water occurs at an energy of −1.23 eV, near the yttrium atom. The obtained results complement the database of research works carried out in the field of the application of biocompatible zirconium dioxide crystals and ceramics in green energy generation, and can be used in designing humidity-to-electricity converters and in creating solid oxide fuel cells based on ZrO2.

Nanoscale-mixed ZnNiCu hydroxide composite catalyst for improved photocatalytic hydrogen evolution

The advantage of a complex catalyst in which several catalysts are mixed is that the overall reaction rate can be efficiently increased by increasing each sub-reaction rate. Each constituent catalyst in the complex catalyst must be as well-mixed as possible—while maintaining high catalytic activity—owing to the short reaction time of these sub-reactions. This study reports a facile synthetic method to mix two catalysts homogeneously at the nanoscale while keeping their crystal structure intact. ZnNiCu hydroxide nanoplates with a mixed crystal structure composed of ZnNi and ZnCu hydroxides, which could promote H2O dissociative adsorption and H2 desorption, respectively, were synthesized by a simple cation exchange of ZnO nanoparticles and a precursor mixture of Ni and Cu. ZnNi and ZnCu hydroxides in the nanoplates were mixed at the nanoscale while maintaining their respective crystal structures; density functional theory calculations show that these structures could effectively perform H2O dissociative adsorption and H2 desorption, respectively, resulting in high activity in the overall photocatalytic hydrogen evolution reaction.

The Consequences of Water Interactions with Nitrogen-Containing Carbonaceous Quantum Dots-The Mechanistic Studies.

Despite the importance of quantum dots in a wide range of biological, chemical, and physical processes, the structure of the molecular layers surrounding their surface in solution remains unknown. Thus, knowledge about the interaction mechanism of Nitrogen enriched Carbonaceous Quantum Dots' (N-CQDs) surface with water-their natural environment-is highly desirable. A diffusive and Stern layer over the N-CQDs, characterized in situ, reveals the presence of anionic water clusters [OH(H2O)n]-. Their existence explains new observations: (i) the unexpectedly low adsorption enthalpy (ΔHads) in a pressure range below 0.1 p/ps, and ΔHads being as high as 190 kJ/mol at 0.11 p/ps; (ii) the presence of a "conductive window" isolating nature-at p/ps below 0.45-connected to the formation of smaller clusters and increasing conductivity above 0.45 p/ps, (iii) Stern layer stability; and (iv) superhydrophilic properties of the tested material. These observables are the consequences of H2O dissociative adsorption on N-containing basic centers. The additional direct application of surfaces formed by N-CQDs spraying is the possibility of creating antistatic, antifogging, bio-friendly coatings.

Atomic scale insight into the mechanisms of chloride induced steel corrosion in concrete

Chloride (Cl) induced steel corrosion is a major cause of durability issues of reinforced concrete (RC) structures and causes a large economic loss every year. Understanding the mechanisms of Cl induced steel corrosion in concrete is of great importance to develop anti-corrosion methods and further lowers down the repair and strengthening frequency and cost. Hence, the nature of steel depassivation and corrosion induced by Cl at atomic scale was revealed through the density functional theory (DFT) calculations to better understand the Cl induced corrosion mechanism. The results indicated that Cl species weakened the bonding of oxygen (O) with outer iron (Fe) atoms while enhanced the hybridization of O with inner Fe atoms, thus induced the breakdown of passive films. Besides, Cl facilitated the charge loss of outer Fe atoms and improved the interactions of water molecular (H2O) with these Fe atoms. When co-adsorption with O atom, hydroxyl (OH) generated by dissociative adsorption of H2O enhanced the charge loss of outer Fe atoms. Then OH strongly bonded with the neighboring Fe atoms to form the initial corrosion products, iron hydroxides (Fe-OH). The above DFT calculations are consistent with the experimental results about the steel depassivation and corrosion processes in concrete and hopefully these results can guide the anti-corrosion design of RC structures.

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition.

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H2O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

Local Ordering of Molten Salts at NiO Crystal Interfaces Promotes High‐Index Faceting

AbstractGiven the strong influence of surface structure on the reactivity of heterogeneous catalysts, understanding the mechanisms that control crystal morphology is an important component of designing catalytic materials with targeted shape and functionality. Herein, we employ density functional theory to examine the impact of growth media on NiO crystal faceting in line with experimental findings, showing that molten‐salt synthesis in alkali chlorides (KCl, LiCl, and NaCl) imposes shape selectivity on NiO particles. We find that the production of NiO octahedra is attributed to the dissociative adsorption of H2O, whereas the formation of trapezohedral particles is associated with the control of the growth kinetics exerted by ordered salt structures on high‐index facets. To our knowledge, this is the first observation that growth inhibition of metal‐oxide facets occurs by a localized ordering of molten salts at the crystal–solvent interface. These findings provide new molecular‐level insight on kinetics and thermodynamics of molten‐salt synthesis as a predictive route to shape‐engineer metal‐oxide crystals.

Dissociative adsorption of H2O and CO2 on the clean and O-pre-covered high index Ru surfaces: Corrugated Ru(11−21) and stepped Ru(20−21) surfaces

Periodic density functional theory (RPBE) computation has been used to assess dissociative adsorption of H2O and CO2 on the clean and O-pre-covered Ru(11−21) and Ru(20−21) surfaces. It is found that surface H2O prefers desorption on the clean Ru(11−21) and Ru(20−21) surfaces, while dissociative adsorption on O-pre-covered Ru(11−21) and Ru(20−21) surfaces, in contrary to that on O-pre-covered Ru(0001) surface. The H2O dissociation capability on these clean and O pre-covered Ru surfaces follows a sequence of Ru(11−21) > (20−21) > (0001). In addition, CO2 desorption energy is comparable with the barrier of CO2 dissociation on Ru(11−21) and Ru(20−21) (0.81 vs. 0.77 eV, 0.14 vs. 0.20 eV, respectively), indicating that CO2 would not desorb in quantity before it occurs dissociation, which is contrary to the case on Ru(0001) (-0.32 vs. 0.23 eV). For surface O removed via H2O or CO2 formation, H2O formation has lower effective barrier than CO2 formation on these three Ru surfaces, in line with the experiments, and the effective barrier of H2O and CO2 formation follows the increasing order of Ru(11−21) < (20−21) < (0001).

Role of chromium in Cr–Fe oxide catalysts for high temperature water-gas shift reaction – A DFT study

Carcinogenic hexavalent Cr in the current iron oxide-based catalysts of high temperature water-gas shift (HT-WGS) reaction is great environmental concern. Interpreting the role of the Cr in this important industrial catalyst system is required. In the present study, we investigated substitution of Cr atoms into the most stable termination of Fe3O4 (111) slab surface by spin-polarized periodic DFT approach to comprehend the role of Cr. We applied the projector augmented-wave (PAW) method within the Perdew-Wang 1991 (PW91) form of the generalized gradient approximation (GGA) on the Vienna Ab-initio Simulation Package (VASP). The calculations point out that Cr atoms choose being below the surface FeO6 sites. Cr slightly affects the dissociative H2O adsorption. There is no effect on the CO adsorption. Oxygen vacancy is favored to form on the topmost layer with less vacancy formation energy. Substitution of Cr into the structure increases the oxygen vacancy formation energy. This indicates that Cr does not act as a chemical promoter and does not affect the catalytic activity positively, which is experimentally confirmed by the previous studies.

The role of oxygen vacancies on SnO2 surface in reducing cross-sensitivity between ambient humidity and CO: A first principles investigation

The sensing properties of SnO2 based CO gas sensors can be affected by ambient humidity and oxygen pressure. In this study the effect of oxygen vacancy along with the adsorption mechanism of H2O and CO on SnO2 (110) surface has been investigated by performing density functional theory (DFT) calculations. The influence of ambient humidity has been taken into consideration in order to better understand the CO sensing mechanism in SnO2 based gas sensors. Molecular and dissociative adsorption of H2O on both stoichiometric and oxygen deficient surface were simulated. Adsorption energies, Bader charge analysis and the projected density of states were investigated by employing electronic structure calculations. The findings show that CO adsorption in most cases is less favorable in the presence of H2O and on the reduced SnO2 surface. However, oxygen vacancies provide more sites to adsorb H2O molecules to compensate the humidity cross-interference. Furthermore, CO to CO2 conversion along with dissociative H2O adsorption on the vacancy of oxygen site presents the highest charge transfer and very strong adsorption energy among other adsorption configurations. These findings provide a guideline for fabricating more efficient CO sensors.

Competitive Adsorption of H2O and SO2 on Catalytic Platinum Surfaces: a Density Functional Theory Study

ABSTRACT Platinum has been widely used as the catalyst of choice for the production of hydrogen in the hybrid sulphur (HyS) cycle. In this cycle, water (H2O) and sulphur dioxide (SO2) react to form sulphuric acid and hydrogen. However, the surface reactivity of platinum towards H2O and SO2 is not yet fully understood, especially considering the competitive adsorption that may occur on the surface. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the competitive effect of both H2O and SO2 on the Pt (001), (011) and (111) surfaces. Comparing the adsorption of a single H2O molecule on the various Pt surfaces, it was found that the lowest adsorption energy (Eads = -1.758 eV) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the molecular adsorption on the (011) surface (Eads = -0.699 eV) and (111) surface (Eads = -0.464 eV). For the molecular SO2 adsorption, the trend was similar, with the lowest adsorption energy (Eads = -2.471 eV) obtained on the (001) surface, followed by the (011) surface (Eads = -2.390 eV) and (111) surface (Eads = -1.852 eV). During competitive adsorption by H2O and SO2, the SO2 molecule will therefore preferentially adsorb onto the Pt surface. If the concentration of SO2 increases, self-reaction between two neighbouring SO2 molecules may occur, leading to the formation of sulphur monoxide (SO) and -trioxide (SO3) on the surface, which could lead to sulphur poisoning of the Pt catalytic surface. Keywords: Platinum, water, sulphur dioxide, hydrogen, adsorption, density functional theory.

Coverage-Dependent Water Dissociative Adsorption Properties on Nickel Surfaces

Periodic density functional theory calculations were employed to study the mechanisms of H2O dissociative adsorption on the Ni(211) surface at different coverages. Due to stronger H-bonding, H2O cl...

A kinetic model for evolution of H2 and CO over Zr-doped ceria

Among notable catalytic assisted processes by ceria-abased materials are the production reactions of H2 and CO from solar thermochemical splitting of water and carbon dioxide, respectively, for applications pertinent to hydrogen fuel cell and synthesis of syngas. Doping ceria with isovalent cations, most notably Zr, was shown to enhance the yield of O2, H2, and CO. Via sophisticated measurements techniques, literature provided several global-like rates for the overall evolutions of H2 and CO. However, these values remain without a mechanistic verification by means of first-principles calculations. Herein, we map out mechanisms and construct a detail chemistry kinetic model for the dissociative adsorption of H2O and CO2 over pure and Zr-decorated ceria surfaces. The overarching objective of the kinetic model is to deduce temperature-dependent profiles that account for the experimentally observed evolution of O2, H2, and CO during the redox cycles of ceria and to reveal the effect of Zr dopants. Ce surface atoms in the latter incur higher positive charges, and hence more affinity to capture gas phase molecules. Splitting of H2O into a gas phase H2 over the Zr-substituted surface ensues in two consecutive steps through activation energies of 77.7 kJ/mol and 191.2 kJ/mol. Our computed rate constant for evolution of H2 over ceria amounts to 2.67 mL/g. min, within the narrow range of the experimental values scattered between 1.5–5.4 mL/g. min. Formation of H2 and CO arises at a ∼ 100 K lower temperature over the Zr-doped surface in reference to pure ceria. The developed kinetic model is expected to describe - on a precise atomic basis - the H2O/CO2 redox cycles of ceria-based materials and the promoting effects of dopants.

Dissociative adsorption of H2O onto a Pt thin film in direct contact with GaN (0001): Effect of electronic communications between catalyst and a semiconducting support

Heterogeneous catalysts that contain metal/semiconductor heterojunctions have been applied in numerous chemical transformations. While the involvement of these heterojunctions in catalytic processes has been recognized widely, limited information about their local chemical, geometrical, and electronic structures is available, particularly under catalytic operating conditions. Here, using Pt/GaN (0001) and Pt (111) as model systems, we investigated the interfacial chemistry of H2O/Pt under operando conditions using ambient pressure X-ray photoelectron spectroscopy (APXPS). We found that H2O remains intact in its molecular form on pristine Pt (111) at room temperature; however, when a thin Pt film was brought into direct contact with a semiconductor, i.e., GaN (0001), electronic communications at the heterojunction were established and were observed spectroscopically. The build-up of hot carriers under steady-state X-ray photoexcitation drives the dissociative adsorption of H2O onto Pt at room temperature that otherwise does not proceed on bare Pt (111). H2O and its dissociation products, hydroxyls (OH) and surface oxides, were observed binding to surface Pt atoms under elevated pressures and/or temperatures, which in return altered the hot carrier transfer and therefore, the band alignment at the Pt/GaN interface, leading to the observed evolutions in Pt, O, and Ga core-level photoemission spectra. Our findings demonstrate that the selection of semiconducting supports can govern the outcome of a chemical transformation event directly; the catalytic performance and reaction pathway of metal (or oxides)-based catalysts can be modulated by electronic modifications at the catalyst/support interface and drive reactions that are kinetically sluggish or non-spontaneous.

Heteroatom doping effects on interaction of H2O and CeO2 (111) surfaces studied using density functional theory: Key roles of ionic radius and dispersion.

Understanding heteroatom doping effects on the interaction between H2O and cerium oxide (ceria, CeO2) surfaces is crucially important for elucidating heterogeneous catalytic reactions of CeO2-based oxides. Surfaces of CeO2 (111) doped with quadrivalent (Ti, Zr), trivalent (Al, Ga, Sc, Y, La), or divalent (Ca, Sr, Ba) cations are investigated using density functional theory (DFT) calculations modified for onsite Coulomb interactions (DFT + U). Trivalent (except for Al) and divalent cation doping induces the formation of intrinsic oxygen vacancy (Ovac), which is backfilled easily by H2O. Partially OH-terminated surfaces are formed. Furthermore, dissociative adsorption of H2O is simulated on the OH terminated surfaces (for trivalent or divalent cation doped models) and pure surfaces (for Al and quadrivalent cation doped surfaces). The ionic radius is crucially important. In fact, H2O dissociates spontaneously on the small cations. Although a slight change is induced by doping as for the H2O adsorption energy at Ce sites, the H2O dissociative adsorption at Ce sites is well-assisted by dopants with a smaller ionic radius. In terms of the amount of promoted Ce sites, the arrangement of dopant sites is also fundamentally important.

Water‐Based Paintable LiCoO2 Microelectrodes: A High‐Rate Li‐Ion Battery Free of Conductive and Binder Additives

AbstractA simple, low‐cost, and environmentally friendly water‐based colloidal spray‐painting process is developed to obtain mechanically stable microelectrodes of pure LiCoO2 on stainless steel disks, which display reversible high‐rate discharge capacity and good cyclability. By using a simple nanostructuring process with commercially available LiCoO2, the use of dispersant, conductive, and binder additives during the electrode preparation is completely avoided. The nanostructuring approach is based on: i) planetary ball milling and annealing of LiCoO2 microparticles and ii) LiCoO2 surface modification through dissociative H2O adsorption during contact with air‐ and water‐based colloid preparation. The coating technique involves: i) water‐based colloid preparation, ii) spray‐coating, and iii) post‐treatment (drying and calcination) processes. The effects of particle size distribution, shape, crystallinity, specific surface area, and LiCoO2 surface modification on the electrode architecture (roughness, thickness, packing density, and porosity) are studied, together with their impact on the final electrochemical performances of batteries assembled with such a LiCoO2 electrode.

Interaction of H2O with the Platinum Pt (001), (011), and (111) Surfaces: A Density Functional Theory Study with Long-Range Dispersion Corrections.

Platinum is a noble metal that is widely used for the electrocatalytic production of hydrogen, but the surface reactivity of platinum toward water is not yet fully understood, even though the effect of water adsorption on the surface free energy of Pt is important in the interpretation of the morphology and catalytic properties of this metal. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the interaction of H2O with the Pt (001), (011), and (111) surfaces. During the adsorption of a single H2O molecule on various Pt surfaces, it was found that the lowest adsorption energy (Eads) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the (011) and (111) surfaces. When the surface coverage was increased up to a monolayer, we noted an increase in Eads/H2O with increasing coverage for the (001) surface, while for the (011) and (111) surfaces, Eads/H2O decreased. Considering experimental conditions, we observed that the highest coverage was obtained on the (011) surface, followed by the (111) and (001) surfaces. However, with an increase in temperature, the surface coverage decreased on all the surfaces. Total desorption occurred at temperatures higher than 400 K for the (011) and (111) surfaces, but above 850 K for the (001) surface. From the morphology analysis of the Pt nanoparticle, we noted that, when the temperature increased, only the electrocatalytically active (111) surface remained.

DFT + U Study of the Adsorption and Dissociation of Water on Clean, Defective, and Oxygen-Covered U3Si2{001}, {110}, and {111} Surfaces.

The interfacial interaction of U3Si2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U3Si2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H2 formation mechanisms. The adsorption strength decreases in the order U3Si2{001} > U3Si2{110} > U3Si2{111} for both molecular and dissociative H2O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H2O on the oxygen-covered U3Si2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H2O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces.

How Do Structurally Distinct Au/α‐Fe2O3 Interfaces Determine Surface OH/H2O reactivity, Intermediate Evolution, and Product Formation in Low‐temperature Water‐gas Shift Reaction?

AbstractThree structurally distinct interfaces, i. e. Au/α‐Fe2O3‐THB‐Air, Au/α‐Fe2O3‐HS‐Air, and Au/α‐Fe2O3‐QC‐Air, with the major substrate facet being {113}, {001}, and {012} respectively, were controllably prepared. The low temperature water gas shift (WGS) reaction (100–260 °C) was applied to these unique systems to reveal how these structurally distinct interfaces can impact on the reaction behavior. With the detailed performance evaluation plus the characterizations of CO‐TPSR, XPS, and in situ‐FTIR in particular, the correlations between the evolution of different intermediates and the distinct catalytic behaviors have been established. The substrate facet structure and the Au entity largely determine the form and reactivity of surface OH/H2O species which in turn determine the formation of different intermediates and eventually the product formation. The Au/α‐Fe2O3‐THB‐Air is enriched with oxygen vacant sites, favorable for dissociative adsorption of H2O, carboxyl intermediate formation and H2 production. The Au/α‐Fe2O3‐HS‐Air is enriched with surface lattice oxygen, favorable for molecular adsorption of H2O, formate intermediate formation, and CO2 production. The Au/α‐Fe2O3‐QC‐Air interface is enriched with surface Fe sites, favorable for dissociative adsorption of H2O and evolution of stable carboxyl intermediate as well as isolated OH species at low reaction temperatures. The findings are informative to control the interfacial structure for the WGS and other reactions.

Robust nanocomposites of α-Fe2O3 and N-doped graphene oxide: Interfacial bonding and chemisorption of H2O

Nanocomposites of α-Fe2O3 (hematite) and (N-doped) graphene oxide (GO) were investigated using first-principles calculations with focus on structure, chemical bonding, electronic structure and H2O adsorption. The nanocomposites were modeled as the interface between the α-Fe2O3 (0 0 0 1) surface and the basal plane of reduced graphene oxide, comprising epoxy groups (C:O ratio of 8) as well as graphitic and pyridinic nitrogen doping. The composite structures exhibited strong chemical bonding by the formation of a bridging Fe–O–C bond. The calculated binding energy between the materials was −0.56 eV per Fe–O–C bond for GO and up to −1.14 eV for N-doped GO, and the binding energies were found to correlate with the charge of the bridging oxide ion. The composites exhibited partly occupied carbon states close to or above the α-Fe2O3 valence band maximum. Dissociative adsorption of H2O was found to be more exothermic for the composites compared to the individual materials, ranging from about −0.9 to −1.7 eV for the most stable configurations with hydroxide species adsorbed to GO and protons forming NH groups or adsorbed to the α-Fe2O3 surface.

Coverage dependent structure and energy of water dissociative adsorption on clean and O-pre-covered Ni (100) and Ni(110)

H2O dissociative adsorption on clean and O pre-covered Ni(100) and Ni(110) surfaces has been computed systematically on the basis of periodic density functional theory and ab initio atomistic thermodynamics.