Theoretical analysis of reactivity on Pt(1 1 1) and Pt-Pd(1 1 1) alloys

The reactivity of Pt-Pd alloy surfaces towards the oxygen reduction reaction is studied as a function of the alloy overall composition and surface atomic distribution and compared to that on pure Pt surfaces. The systems include Pd and Pt monolayers on various substrates and Pt₃Pd, PtPd and PtPd₃ surfaces of ordered alloys. Reactivity is evaluated on the basis of the adsorption strength of oxygenated compounds which are intermediate species of the four-electron oxygen reduction reaction, separating the effect of the first electron-proton transfer from that of the three last electron-proton transfer steps. None of the alloys are found to provide better sites than those of pure Pt both for O₂ dissociation and for the reduction of O and OH to water; with the skin surfaces being the closest to pure Pt. The results are discussed in relation to those found in 10-atom clusters of similar compositions and to experiments.     

Adsorption and reactions of HN3 on Si(1 0 0)-2 × 1: A computational study

We have studied the adsorption and decomposition of HN₃ on Si(1 0 0)-2 × 1 surface using the hybrid density functional B3LYP method and effective core potential basis, LanL2DZ, with Si₁₅H₁₆ as a double dimer surface model for cluster calculations. The result shows that the barriers for the dissociative adsorption of HN₃ forming HN(a) + N₂(g) are quite low by stepwise dissociation processes occurring either on a dimer or across the dimers. The low activation energies are consistent with previous experimental observations that the molecularly adsorbed HN₃(a) can undergo decomposition producing HN(a) at low surface temperatures. On the other hand, the predicted activation energies for the N₃(a) + H(a) formation processes are all relatively higher. These results also explain the absence of the N₃(a) species in HREELS measurements following each annealing experiment. Several selected reaction paths were also confirmed with slab model calculations using an optimization approach coupling the energy and gradient calculations by the slab model with the geometrical optimization using Berny algorithm.In addition, the adsorbate effect was examined for the end-on and side-on molecular configurations. For the side-on adsorption configuration, all possible combinations with 1-4 adsorbates can exist on the four surface Si sites of the double dimers, with adsorption energies lying closely to the multiples of that of a single side-on adsorbate (LM2); i.e., adsorption energies are nearly additive. Interestingly, for the end-on adsorption, only 1 and 2 HN₃ molecules can adsorb on a dimer due to the presence of the negative charges on the remaining Si sites in the neighboring dimer. For the two end-on adsorbates on the same dimer, the total adsorption energy is close to two times that of HN₃(a) or LM1. For the mixed end-on/side-on configurations, only one of each type can co-exist on a single dimer pair (Si₁-Si₂ or Si₃-Si₄) sites with adsorption energy close to the sum of those of one end-on and one side-on adsorbate. Finally, the predicted reaction routes and vibrational frequencies showed good agreement with previous experimental results. The stabilities of many ad-species involved in these reactions with end-on and/or side-on configurations have been predicted together with the transition states connecting those species.     

Water adsorption and dissociation on BeO(0 0 1) and (1 0 0) surfaces

Plateaus in water adsorption isotherms on hydroxylated BeO surfaces suggest significant differences between the hydroxylated (1 0 0) and (0 0 1) surface structures and reactivities. Density functional theory structures and energies clarify these differences. Using relaxed surface energies, a Wulff construction yields a prism crystal shape exposing long (1 0 0) sides and much smaller (0 0 1) faces. This is consistent with the BeO prisms observed when beryllium metal is oxidized. A water oxygen atom binds to a single surface beryllium ion in the preferred adsorption geometry on either surface. The water oxygen/beryllium bonding is stronger on the surface with greater beryllium atom exposure, namely the less-stable (0 0 1) surface. Water/beryllium coordination facilitates water dissociation. On the (0 0 1) surface, the dissociation products are a hydroxide bridging two beryllium ions and a metal-coordinated hydride with some surface charge depletion. On the (1 0 0) surface, water dissociates into a hydroxide ligating a Be atom and a proton coordinated to a surface oxygen but the lowest energy water state on the (1 0 0) surface is the undissociated metal-coordinated water. The (1 0 0) fully hydroxylated surface structure has a hydrogen bonding network which facilitates rapid proton shuffling within the network. The corresponding (0 0 1) hydroxylated surface is fairly open and lacks internal hydrogen bonding. This supports previous experimental interpretations of the step in water adsorption isotherms. Further, when the (1 0 0) surface is heated to 1000 K, hydroxides and protons associate and water desorbs. The more open (0 0 1) hydroxylated surface is stable at 1000 K. This is consistent with the experimental disappearance of the isotherm step when heating to 973 K.     

Ab initio density functional study of O on the Ag(0 0 1) surface

The adsorption of oxygen on the Ag(1 0 0) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag₂O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.     

CO adsorption on the Cu(1 1 1) surface: A density functional study

The adsorption of CO on the Cu(1 1 1) surface has been studied with ab initio density functional theory. The adsorbate-metal system was analyzed with the local density approximation, the gradient corrected functional of Perdew and Wang and the B3LYP hybrid functional, for comparison. A slab model was used for the pattern at a coverage of 1/3. The local density approximation and the gradient corrected functional give the fcc site as the favorable adsorption site. In contrast, the B3LYP functional results in the preference of the top site, in agreement with the experiment. These results confirm the suggested explanation for the failure of standard functionals, based on the position of the highest occupied and lowest unoccupied molecular orbital. The results of total energy calculations are presented, together with projected densities of states and Mulliken populations. In addition, the basis set superposition error is discussed for CO/Cu(1 1 1) and for CO/Pt(1 1 1).     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

Molecular origins of surface poisoning during CO oxidation over RuO2(1 1 0)

Metal oxides are of interest as environmental oxidation catalysts, but practical applications are often limited by poorly understood surface poisoning processes. RuO₂ is active for CO oxidation under UHV conditions but is deactivated by some surface poisoning processes at ambient pressures. In this work, we report kinetic models of surface poisoning during CO oxidation over RuO₂(1 1 0), based on data obtained from plane-wave, supercell DFT calculations. While a surface carbonate is stable at low O₂ pressures and high CO₂ exposures, it is not stable under catalytic conditions. A surface bicarbonate is more stable and deactivates the RuO₂ surface over a wide range of CO and oxygen pressures in the presence of trace amounts of water.     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

NO dissociation pathways on Rh(1 0 0), (1 1 0), and (1 1 1) surfaces: A comparative density functional theory study

The chemisorption and dissociation pathways of NO on the Rh(1 0 0), (1 1 0), and (1 1 1) surfaces are studied by the plane-wave density functional theory (DFT) with CASTEP program. In addition, the electronic and geometrical effects that affect the NO dissociation reactions have been investigated in detail. The calculation results are presented as following: The effective activation energies of the best NO dissociation pathways on the Rh(1 0 0), the Rh(1 1 0), and the Rh(1 1 1) are 0.63, 0.66 and 1.77 eV, respectively. The activity of the Rh planes for NO dissociation is in the order of Rh(1 0 0) ≈ Rh(1 1 0) > Rh(1 1 1). The low dissociation barrier for Rh(1 0 0) and Rh(1 1 0) is associated with the existence of a lying-down NO structure which acts as a precursor for dissociation. By Mulliken population analysis and structure analysis, both electronic and geometrical effects are found to affect the NO dissociation reactions, but the geometrical effect exceed the electronic. The energy decomposition scheme has been used to provide further insight into the NO dissociation reactions. Based on the calculations, the interaction energy between N and O in the transition state on the Rh(1 1 1) is found much larger than that on the Rh(1 0 0) and the Rh(1 1 0). The major differences of should originate from the variation of the bonding competition effect.     

Conformation-selective adsorption of 2,3-butanediol on Si(0 0 1)

The adsorption of 2,3-butanediol on the Si(0 0 1) surface is studied by means of first-principles pseudopotential calculations. Molecular adsorption on top of the Si dimers resulting in a 6-membered ring of the O-C-C-O segment with the dimer atoms is energetically favored, in agreement with the interpretation of recent experiments. The adsorption energy difference for butanediol adsorbed in either gauche or anti conformation is nearly one order of magnitude larger than the energy difference between the respective conformers in gas phase, pointing to a conformation-selective adsorption.     

Reaction of chlorinated benzenes with Si(1 0 0)2 × 1: a theoretical study

Theoretical (HF + DFT) investigations of the adsorption of chlorobenzene (ClPh), 1,2- and 1,4-dichlorobenzene (1,2-diClPh and 1,4-diClPh) on a silicon (1 0 0) surface are reported for the first time, and are compared with one another and with benzene. Binding energies for various structures with the molecules attached on-top and in-between the surface dimer rows are correlated with the STM experimental data. Novel structures with the molecules linking two dimer rows, stabilised by detachment of Cl (or H)-atoms forming Cl-Si (or H-Si) bonds, are described. For 1,4 and 1,2 binding, these linking structures are predicted to attach the phenyl ring parallel or perpendicular to the Si surface, respectively, while preserving its aromaticity. The potential-energy barriers between several different structures are evaluated, and compared with available experimental evidence. For 1,4-diClPh it is shown that the potential-energy barrier for the second Cl transfer is significantly lower than for the first one in contrast to the gas-phase, and comparable to the barrier for lifting the Bz-ring into a vertical position and forming a singly bonded ‘displaced’ structure. The predicted barrier-heights are consistent with the experimentally observed relative occurrence of the on-top, linking, and displaced structures.     

Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(1 1 1)

The adsorption of 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene and cyclohexane on Pt(1 1 1) was studied using ab initio density functional theory. For 1,3-cyclohexadiene three adsorption modes were distinguished: bridge 1,2-di-σ/3,4-π, hollow 1,4-di-σ/2,3-π and bridge 1,4-di-σ/2,3-π with adsorption energies of −155, −147 and −75 kJ/mol, respectively. Three stable adsorption modes were also identified for 1,4-cyclohexadiene: bridge quadra-σ, hollow di-σ/π and bridge di-π with adsorption energies of −146 kJ/mol, −142 kJ/mol and −88 kJ/mol, respectively. Cyclohexene was found to adsorb in six modes: 4 di-σ and 2 π-adsorption modes. The preferred configuration was found to be boat di-σ with an adsorption energy of −81 kJ/mol. The three other di-σ adsorption modes have comparable adsorption energies, ranging from −64 to −69 kJ/mol. Molecular strain and CPt bonding energies are used to elucidate stability trends. Cyclohexane is found to adsorb only at the hollow site whereby the axial hydrogen atoms are positioned over surface Pt-atoms with an adsorption energy of −37 kJ/mol. The calculations correctly predict the weakening of the axial CH bonds and provide a possible explanation for the large shift in the vibrational frequencies.     

First principle calculations of benzotriazole adsorption onto clean Cu(1 1 1)

The adsorption of benzotriazole (BTAH or C₆N₃H₅) on a Cu(1 1 1) surface is investigated by using first principle density functional theory calculations (VASP). It is found that BTAH can be physisorbed (<0.1 eV) or weakly chemisorbed (∼0.43 eV) onto Cu(1 1 1), and the chemical bond is formed through nitrogen sp² lone pairs. The weak chemisorption can be stabilized by reaction with neighboring protonphilic radicals, like OH⁻. Furthermore, the geometries and associated energies of intermolecular hydrogen bonds between adsorbates on Cu(1 1 1) are also calculated. A model of the first layer of BTAH/BTA⁻ on Cu(1 1 1) surface is developed based on a hydrogen bond network structure.     

Nitrogen adsorption and thin surface nitrides on Cu(1 1 1) from first-principles

Experimental studies of nitrogen adsorbed on a Cu(1 1 1) surface show that the surface layer undergoes a reconstruction to form a pseudo-(1 0 0) structure. We use ab initio techniques to demonstrate the theoretical stability of this reconstructed surface phase over a range of conditions. We systematically investigate the chemisorption of N on the Cu(1 1 1) surface, from 0.06 to 1 ML coverage. A peculiar atomic relaxation of N atoms for 0.75 ML is identified, which results in the formation of a (metastable) “N-trimer cluster” on the surface. We have also investigated surface nitride formation, as suggested from experiments. A surface nitride-like structure similar to the reported pseudo-(1 0 0) reconstruction is found to be highly energetically favored. Using concepts from “ab initio atomistic thermodynamics”, we predict that this surface nitride exists for a narrow range of nitrogen chemical potential before the formation of bulk Cu₃N.     

Kinetic Monte Carlo simulations of the partial oxidation of methanol on oxygen-covered Cu(1 1 0)

The partial oxidation of methanol to formaldehyde on oxygen-precovered Cu(1 1 0) has been studied using kinetic Monte Carlo simulations. The rates entering the simulation have been derived from density functional theory calculations within the generalized gradient approximation using transition state theory. We demonstrate that kinetic Monte Carlo simulations are a powerful tool to elucidate the microscopic details of the reaction kinetics on surfaces. Furthermore, the comparison of calculated and measured temperature programmed desorption rates allows a genuine assessment of the calculated barrier heights. We find that some of the calculated barriers and adsorption energies have to be adjusted by up to 0.5 eV in order to reproduce the measured desorption spectra. Possible reasons for the discrepancies between experiment and theory are discussed.     

Theoretical simulation of butane isomers adsorption on a Pt(1 0 0) surface

The adsorption of the two butane isomers on Pt(1 0 0) has been characterised with use of density functional simulations. The adsorption energies corresponding to various adsorption configurations were evaluated in good agreement with experimental values. Limited changes of the molecular structure were evidenced. The C-H bond length increases at a degree depending on the surface-hydrogen distance, while the C-C bond length remains similar to that of the free molecule. The surface on-top Pt sites exert a preferential attraction on the molecule, probably through the interaction with the H atoms. The local density of states curves around H as well as C of the adsorbed molecules show dispersed states below the metal Fermi level indicating a molecule-Pt mixing demonstrating a chemical interaction.     

First principles calculations of oxygen adsorption on the UN(0 0 1) surface

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (0 0 1) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(0 0 1) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.