A combined ab initio and atomistic simulation study of the surface and interfacial structures and energies of hydrated scheelite: introducing a CaWO4 potential model

Density functional theory (DFT) calculations of the calcium tungstate material scheelite CaWO₄ have shown that water introduced into the bulk material remains undissociated and leads to swelling and layering of the structure, a behaviour which may resemble silicate clays more than three-dimensional poly-anionic materials, but which results in a structure that is even more similar to a rare hydrous calcium carbonate phase--a finding which suggests the existence of semi-crystalline hydrous pre-cursor phases to the dehydrated scheelite material. An interatomic potential model was derived for CaWO₄ which adequately reproduces structural and physical properties of the material and is in good agreement with the DFT calculations in respect of the structure and energy of hydration (DFT: 85 kJ mol⁻¹, atomistic: 105 kJ mol⁻¹). Atomistic simulations of a range of scheelite surfaces confirm the dominance of the experimental {1 0 1} and {0 0 1} cleavage planes in the morphology of the dry crystal and the presence of the experimentally found {1 0 3} and {1 0 1} surfaces in the hydrated morphology. Hydration of the surfaces shows non-Langmuir behaviour, where the interactions between surface calciums and oxygen atoms of the water molecules outweigh hydrogen-bonding to the surface oxygen atoms or intermolecularly within the water layer. The hydration energies indicate physisorption of water, ranging from 22 kJ mol⁻¹ on the {0 0 1} surface to 78 kJ mol⁻¹ on the more reactive {1 0 3} surface.     

Effect of Au proximity on the LSMO surface: An ab initio study

The effect of the proximity of Au on the electronic and magnetic properties of La_0.66Sr_0.33MnO₃ (LSMO) has been investigated by means of ab initio calculations within the density-functional theory. The calculations show an orbital reconstruction of the interfacial Mn, which is due more to the presence of a discontinuity rather than to a real chemical interaction with Au atoms. In fact, the same orbital reconstruction is found in the free LSMO surface. In both cases of Au/LSMO and LSMO surface, the Mn magnetic moments change very little with respect to the bulk case. In general, the calculations show a negligible influence of the Au atomic layer on LSMO at an ideal interface, with the LSMO surface magnetic and electronic properties essentially unchanged.     

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F_s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.     

CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

Adsorption of N2 and N2O on Ni(7 5 5) surface: ab initio periodic density functional study

Adsorption of N₂ and N₂O at various sites on Ni(7 5 5) has been investigated by density functional theory (DFT) method (periodic DMol³). Several possible adsorption structures (attaching the nitrogen atom to the surface, or lying parallel) are found for both molecules. There is a clear binding energy preference of N₂ and N₂O for step sites in contrast to the case of CO. It is revealed that the decomposition of N₂O occurs exclusively near the step, but not on the terrace. Two decomposition channels can be considered; dissociative adsorption and spontaneous decomposition during TPD ramp. Three possible candidates for the precursor of the spontaneous decomposition of N₂O during TPD ramp are discussed.     

First-principles study on the catalytic role of Ag in the oxygen adsorption of LaMnO3(0 0 1) surface

In the present paper, the catalytic role of Ag in the oxygen adsorption of LaMnO₃(0 0 1) surface has been theoretically investigated using first-principles calculations based on the density functional theory (DFT) and pseudopotential method. The O₂ adsorption energy is larger for the vertical adsorption and the covalent bond was formed between O₂ molecule and surface Mn. The calculation of electronic properties of interaction between Ag atom and LaMnO₃(0 0 1) surface demonstrates that the most stable position for Ag adsorption is hollow site. The O₂ adsorption energy dramatically increased from 0.298 eV to 1.108 eV due to Ag pre-adsorbed. It is Ag pre-adsorbed that facilitates O₂ adsorption on surface. The bond length and bond population of O₂ molecule indicate that Ag atom facilitates O₂ molecule dissociative adsorption. The Ag atom strengthens LaMnO₃(0 0 1) substrate activity and activity center was formed on surface, which enhances the electrocatalytic activity of LaMnO₃ as solid oxide fuel cells cathode material at low temperature.     

Cluster and band structure ab initio calculations on the adsorption of CO on acid sites of the TiO2(110) surface

The interaction of CO with the cationic sites of the TiO₂(110) surface has been investigated with cluster models and band structure calculations. The analysis of Hartree-Fock and correlated wavefunctions has shown that CO adsorbs at a distance of 4.4–4.5 bohr and a binding energy of 0.7–0.8 eV at low coverage. The bond strength is determined by the CO polarization and the CO б-donation to the surface while the electrostatic attraction is almost exactly cancelled by the Pauli repulsion. The adsorption is accompanied by two important measurable features, a considerable blue shift of the CO vibrational frequency and an increase of the CO 5б ionization potential. Both these effects have been analyzed in detail. We found that the CO ω shift is largely due to a combination of the electrostatic Stark effect and of the repulsion occurring when the CO molecule stretches in the presence of the rigid surface. The change in the 1π–5б binding energies, on the other hand, has an entirely electrostatic origin. Neither the ω shift nor the 5б binding energy shift are determined by the б-donation mechanism. Nevertheless, the occurrence of a charge transfer from CO to the empty levels of the Ti centers is well documented by (a) the energy and dipole moment change associated to this mechanism, (b) the expectation value of a projection operator which measures the charge associated with a given orbital, and (c) the CO dynamic dipole moment. The same analyses also rule out the occurrence of a Ti-to-CO back donation.     

Adsorption of thiophene on Ni(1 0 0), Cu(1 0 0), and Pd(1 0 0) surfaces: ab initio periodic density functional study

Adsorption of thiophene on the (1 0 0) surfaces of Ni, Cu, and Pd has been investigated by the ab initio density functional theory method (periodic DMol³). Several parallel and perpendicular adsorption geometries are examined in detail. For Ni(1 0 0), both dissociative and molecular adsorption structures are found with small difference in energy. Thiophene adsorbs only molecularly on Cu(1 0 0) and Pd(1 0 0). The most stable molecular adsorption structures on all the surfaces are quite similar, where thiophene adsorbs on top of a 4-fold hollow with the symmetry axis rotated 45° from the metal rows. These stable structures arise from a good matching of the thiophene molecule to the metal surfaces. The calculated adsorption geometries are in reasonable agreement with XAFS experiments.     

Adsorption and diffusion of H2O molecule on the Be(0001) surface: A density-functional theory study

Using first-principles calculations, we systematically study the adsorption behavior of a single molecular H₂O on the Be(0001) surface. We find that the favored molecular adsorption site is the top site with an adsorption energy of about 0.3 eV, together with the detailed electronic structure analysis, suggesting a weak binding strength of the H₂O/Be(0001) surface. The adsorption interaction is mainly contributed by the overlapping between the s and pz states of the top-layer Be atom and the molecular orbitals 1b₁ and 3a₁ of H₂O. The activation energy for H₂O diffusion on the surface is about 0.3 eV. Meanwhile, our study indicates that no dissociation state exists for the H₂O/Be(0001) surface.     

Acrolein coupling on reduced TiO2(1 1 0): The effect of surface oxidation and the role of subsurface defects

Reactions of acrolein, water, and oxygen with the vacuum-reduced surface of TiO₂(1 1 0) are reported in a temperature programmed reaction study of the interaction of an aldehydic pollutant with a reducible metal oxide. A total of 25% of the acrolein that binds to the surface is converted to products. Notably, carbon-carbon coupling occurs with 86% selectivity for formation of C₆ products: C₆H₈, identified as 1,3-cyclohexadiene, in a peak at 500 K and benzene immediately thereafter at 530 K. Acrolein is evolved from the surface in three peaks: a peak independent of coverage at 495 K, attributed to decomposition of an intermediate that is partly converted to C₆H₈; a coverage-dependent peak that shifts from 370 K (low coverage) to 260 K (high coverage), which is attributed to adsorption at 5-fold coordinated Ti sites; and a multilayer state at 160 K. Water and acrolein compete for 5-fold coordinated titanium sites when dosed sequentially. The addition of water also opens a new reaction pathway, leading to the hydrogenation of acrolein to form propanal. Water has no effect on the yield of 1,3-cyclohexadiene. Exposure of the surface to oxygen prior to acrolein dosing quenches the evolution of acrolein at 495 K and concurrently eliminates the coupling. From these results, we propose that reduced subsurface defects such as titanium ion interstitials play a role in the reactions observed here. The notion that subsurface defects may contribute to the reactivity of organic molecules over reducible oxide substrates may prove to be general.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

Effect of Y2O3 addition on the conductivity and elastic modulus of (CeO2)1 − x(YO1.5)x

The conductivity and elastic modulus of (CeO₂)_1 − x(YO_1.5)_x for x values of 0.10, 0.15, 0.20, 0.30, and 0.40 were investigated by experiments and molecular dynamics simulations. The calculated conductivity exhibited a maximum value at approximately 15 mol% Y₂O₃; this trend agreed with that of the experimental results. In order to clarify the reason for the occurrence of the maximum conductivity, the paths for the transfer of oxygen vacancies were counted. The numerical result revealed that as the content of Y₂O₃ dopant increases, the number of paths for the transfer of oxygen vacancies decreases, whereas the number of oxygen vacancies for conductivity increases. Thus, the trade-off between the increase in the number of vacancy sites and the decrease in the vacancy transfer was considered to be the reason for the maximum conductivity occurring at the Y₂O₃ dopant content of approximately 15 mol%. The calculated elastic modulus also exhibited a minimum value at approximately 20 mol% Y₂O₃, which also agreed with the experimental results. It was shown that the Y–O–Y bonding energy increased with the increasing content of Y₂O₃ dopant. Thus, the trade-off between the increase in the number of vacancy sites and that in the Y–O–Y bonding energy was considered to be the reason for the minimum elastic modulus occurring at the Y₂O₃ dopant content of approximately 20 mol%.