Threefold Hollow Sites Research Articles

Exploring CO dissociation on Fe nanoparticles by density functional theory-based methods: Fe13 as a case study

The interactions of CO and of C+O with Fe13 have been studied by the PW91, PW91+U, B3LYP density functional theory-based methods. In each case, various possibilities have been explored for N α –N β . All methods predict an icosahedral structure for the naked particle. Moreover, all density functionals predict CO adsorption on threefold hollow site. Depending on the density functional used, the interaction of CO with the Fe13 cluster induces or not a change in the total number of unpaired electrons and, therefore, of the electronic state. The same situation can appear for the dissociation of adsorbed CO to co-adsorbed C+O. In the most stable configuration, C and O are proximate, bound to a common Fe atom that is oxidized. These results suggest the proper description of the reactivity of magnetic metallic nanoparticles toward dissociation of simple diatomic molecules is likely to involve various spin states, which has implications for the search of the corresponding transition states which cannot be restricted to a single potential energy surface.

Coverage-Dependent Adsorption at a Low Symmetry Surface: DFT and Statistical Analysis of Oxygen Chemistry on Kinked Pt(321)

We explore the influence of adsorbate interactions on the thermodynamic and spectroscopic properties of oxygen on the stepped, kinked Pt(321) surface. The ground state arrangements of atomic oxygen are identified with the aid of a cluster expansion and analyzed for coverages up to one oxygen per surface Pt (1 ML). We find that oxygen prefers to bind in bridge sites at the step edge at coverages up to 0.2 ML, but at higher coverages oxygen atoms actually experience mild, localized attractions such that both bridge and threefold hollow sites are occupied to form square planar, fourfold-coordinated PtO4-like structures. These structures progressively dominate the surface with increasing coverage up to 0.8 ML, at which point every kink Pt is saturated with four oxygens. We compute stability regions for these ground states with respect to gas-phase O2 and to NO/NO2 mixtures. The ground state structures at 0.2, 0.6, and 0.8 ML dominate over a wide range of conditions, with the 0.6 ML structure being most prominent. We also explore site preferences for molecular O2 adsorbed on key O ground state structures. Calculations of vibrational modes and core electron binding energy shifts allow us to relate both ground state and non-equilibrium structures to experimental HREELS and XPS results. We show that adsorption sites are primarily characterized by their surface coordination, such that O in atop, bridge, and threefold hollow sites possess distinct and identifiable vibrational modes and core level shifts. However, within these broad categories, we find variability due to interactions with proximal adsorbates. Adsorption energies and vibrational modes of O2 are found to be particularly sensitive to the local adsorption environment. Lastly, we develop a one-dimensional adsorption model to understand and rationalize experimentally observed non-equilibrium behavior at low coverages.

Properties of Cu films on Pt(1 1 1) revealed by AES, LEED, and DEPES

The experimental and theoretical results presented in this work concern the growth mechanism and structural investigations of the ultrathin copper layers on Pt(111) at the adsorption temperatures from 330K to 450K. The growth mode of Cu on Pt(111) and the crystalline structures up to the first four adsorbate monolayers (4ML) were studied by using Auger electron spectroscopy, low-energy electron diffraction, and (in the case of 1ML) directional elastic peak electron spectroscopy. At 330K Cu forms a misfit adsorbate layer with a supercell of Cu(13×13) on Pt(12×12). The formation of the misfit Cu overlayer characterized by the LEED pattern continues to about 4ML. At higher coverages the diffraction pattern characteristic of the Cu(111) surface was observed. At 450K the pseudomorphic growth of 1ML of adsorbate was noted. The experimental DEPES anisotropy maps compared with the theoretical data obtained by using the multiple scattering formalism confirms the misfit structure of the Cu layer at 330K and pseudomorphic overlayer with threefold hollow adsorption sites of type A at 450K.

Adsorption of hydrogen on the surface and sub-surface of Cu(111)

The interaction of atomic hydrogen with the Cu(111) surface was studied by a combined experimental-theoretical approach, using infrared reflection absorption spectroscopy, temperature programmed desorption, and density functional theory (DFT). Adsorption of atomic hydrogen at 160 K is characterized by an anti-absorption mode at 754 cm(-1) and a broadband absorption in the IRRA spectra, related to adsorption of hydrogen on three-fold hollow surface sites and sub-surface sites, and the appearance of a sharp vibrational band at 1151 cm(-1) at high coverage, which is also associated with hydrogen adsorption on the surface. Annealing the hydrogen covered surface up to 200 K results in the disappearance of this vibrational band. Thermal desorption is characterized by a single feature at ∼295 K, with the leading edge at ∼250 K. The disappearance of the sharp Cu-H vibrational band suggests that with increasing temperature the surface hydrogen migrates to sub-surface sites prior to desorption from the surface. The presence of sub-surface hydrogen after annealing to 200 K is further demonstrated by using CO as a surface probe. Changes in the Cu-H vibration intensity are observed when cooling the adsorbed hydrogen at 180 K to 110 K, implying the migration of hydrogen. DFT calculations show that the most stable position for hydrogen adsorption on Cu(111) is on hollow surface sites, but that hydrogen can be trapped in the second sub-surface layer.

First-principles study on the mechanism of coking inhibition by the Ni(111) surface doped with IB-group metals at the anode of solid oxide fuel cells

Focusing on the mechanisms of coking inhibition properties of the nickel-based alloy catalysts, the adsorption and diffusion of single C atoms and C dimer on the (111) surfaces of pure metals (Ni, Cu, Ag and Au), as well as the bimetallic systems (Ni/M) with 1/4 ML of M atoms in the surface layer of Ni(111) are studied based on spin-polarized density functional theory calculations, where M represents the IB metals (Cu, Ag and Au). It is confirmed that C atoms are energetically favorable to be adsorbed at the three-fold hollow sites on the pure Ni and M surfaces. Introducing M into Ni surface can weaken the adsorption of C due to the 3d-bands of the dopant atoms are farther from the Fermi level than those of Ni, which makes the three-fold hollow sites with IB dopant neighbor(s) unstable for carbon adsorption. The diffusion barriers for the process of C-dimer formation (C + C → C dimer) on the bimetallic surface are all higher than that on pure nickel. The results provide a proper explanation on the suppression effects of carbon deposition on the nickel-based alloy catalysts.

Controlling surface crowding on a Pd catalyst with thiolate self-assembled monolayers

The relationship between surface crowding and catalytic activity was investigated using thiolate self-assembled monolayers (SAMs) on Pd/Al2O3 catalysts. The surface density of the thiolate modifier was controlled by varying the steric bulk of the organic substituent. A straight-chain alkanethiol 1-octadecanethiol (C18), with a nearest-neighbor spacing of ∼4.7Å on Pd(111) surfaces, created a denser SAM coating than 1-adamantanethiol (AT) with a nearest-neighbor spacing of ∼6.4Å. Diffuse reflectance infrared spectroscopy revealed that CO adsorbate molecules adsorbed only on threefold hollow and atop sites on C18-modified surfaces. On AT-modified surfaces, however, access to bridging and additional linear sites was also observed. Analysis of adsorption isotherms suggested that CO adsorption energies were comparable on AT-modified and C18-modified catalysts. Acetylene hydrogenation, which results in uncontrolled crowding due to carbonaceous “coke” formation on the catalyst, was found to be insensitive to modification by the thiols. For hydrogenation reactions less associated with uncontrolled coking, crowding – and therefore reactivity – could be controlled systematically using SAMs. In particular, ethylene hydrogenation was 17 times faster on AT-coated surfaces than on C18-coated surfaces, consistent with the additional accessibility to specific sites unavailable on C18-modified surfaces. The effect of modifier density on reactivity was found to be dramatically different for several mono- and bi-functional reactants in a manner consistent with previous literature reports, suggesting that controlled crowding with SAMs can be used to understand reaction structure sensitivity and active site requirements in catalysis.

Simulation of scanning tunneling microscope image of benzene chemisorbed on a Pd(111) electrode surface by density functional theory

A computational method based on density functional theory was used to simulate the scanning tunneling microscopy (STM) images of benzene chemisorbed on a Pd(111) electrode in order to confirm the adsorption site of the aromatic molecule on the metal surface held at a certain applied potential. The simulated STM images on various adsorption sites were obtained and compared with the experimental electrochemical STM images. The simulation results indicate that when the potential of the Pd electrode is held at 0.3 V, benzene is chemisorbed on a threefold hollow site; at 0.55 V, the molecule is adsorbed on a position between a threefold and a twofold bridge site. These findings corroborate previously published experimental elec - trochemical STM results.

Origin of site preference of CO and NO adsorption on Pd(1 1 1) at different coverages: A density functional theory study

First-principles calculations based on density functional theory have been performed to investigate the adsorption of CO and NO on Pd(111) surface. The average and differential adsorption energies of CO and NO at the high-symmetry adsorption sites on Pd(111) at different coverages, namely 0.11, 0.25, 0.50, and 0.75ML are calculated. The results show that at low coverages of 0.11 and 0.25ML, both CO and NO are preferentially adsorbed at the threefold hollow sites. At the medium coverage of 0.50ML, CO molecules occupy both hollow and bridge sites with almost degenerate adsorption energies, while NO molecules prefer the hollow sites to the bridge sites, which can be attributed to the more corrugated potential energy surface of NO on Pd(111) surface. At high coverage of 0.75ML, both CO and NO form the p(3×3)- 3CO structures with the adsorbates at the hollow and atop sites. The calculated stretching vibrational frequencies are in good agreement with the experimental values. The electronic structure analysis shows that the adsorption of CO and NO on Pd(111) surface follows a donation and back-donation mechanism, and the high-coordinated sites promote the donation and back-donation process. The energy decomposition scheme indicates that the site preference at higher coverage is a compromise between the adsorbate–substrate interaction and adsorbate–adsorbate interaction. The relationship between the adsorption energies and the surface coverage has been well established, and linear relations are identified for both CO and NO. We suggest that the coverage-dependent adsorption energies of CO and NO on Pd(111) are useful parameters for the kinetic studies on the relevant catalytic reaction systems.

The Adsorption of Oxygen and Coadsorption of CO and Oxygen on Structurally Well‐Defined PdAg Surface Alloys

The dissociative interaction of oxygen with structurally well-defined monolayer Pd(x)Ag(1-x)/Pd(111) surface alloys of different compositions, with well-known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012, 14, 10754-10761), and the coadsorption of/reaction with CO on oxygen pre-covered surfaces were studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd(67)Ag(33)(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold-hollow sites on Pd(3) sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.

Origin of synergistic effect over Ni-based bimetallic surfaces: A density functional theory study

Density functional theory calculations have been conducted to explore the physical origin of the synergistic effect over Ni-based surface alloys using methane dissociation as a probe reaction. Some late transition metal atoms (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) are substituted for surface Ni atoms to examine the variation in electronic structure and adsorption property of Ni(111). Two types of threefold hollow sites, namely, the Ni(2)M and Ni(3) sites, are taken into account. The calculated results indicate that the variation in the CH(x) adsorption energy at the Ni(2)M and Ni(3) sites is dominated by the ensemble and ligand effect, respectively, and the other factors such as surface and adsorbate distortion and electrostatic interaction affect the catalytic properties of the bimetallic surfaces to a smaller extent. Both the Brønsted-Evans-Polanyi relationship and the scaling correlation hold true on the Ni-based bimetallic surfaces. With the combination of these two linear energy relations, the corrected binding energy of atomic C is found to be a good descriptor for representing the catalytic activity of the alloyed surfaces. Considering the compromise between the catalytic activity and catalyst stability, we suggest that the Rh/Ni catalyst is a good candidate for methane dissociation.

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces through first-principles calculations

First-principles calculations were performed to study the properties of O adsorption on Ni3Al (001), (011), and (111) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption sites are identified. The atomic and electronic structures and adsorption energies are predicted. The adsorption sites for O on the Ni3Al (001) surface are at the 2Ni–2Al fourfold hollow site, whereas O prefers to adsorb at the Ni–Al bridge site on (011) surface and 2Ni–Al threefold hollow site on (111) surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized when O adsorbs on (001) surface, while the affinity of O for Al on (011) surface is weaker than (001) surface, and (111) surface is the weakest. The stronger O and Al affinity indicates more stable Al2O3 when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in different orientations improves in the order of (111), (011), and (001) surface, suggesting that the oxidation in different crystallographic orientations may be related to the affinity of O for Al in the surface.

Adsorption, Diffusion, and Dissociation of H2O on Kaolinite (001): a Density Functional Study

Density functional theory is used to investigate the adsorption, diffusion, and dissociation of H2O on kaolinite(001) surface. It is found that the preferred adsorption sites on the kaolinite(001) surface for H2O are the threefold hollow sites with the adsorption energies ranging from 1.06 to 1.15 eV. H2O does not adsorb on the six-fold hollow site of the aluminium(001) face of the third layer of kaolinite, implying that it is difficult for water molecules to penetrate the ideal kaolinite(001) surface. In addition, we calculate the energetic barriers for the diffusion of H2O between the most stable and next most stable adsorption sites, which range from 0.073 to 0.129eV. The results also show that H2O molecules are easy to diffuse on kaolinite(001) surface. Finally, our study indicates that no dissociation state exists for the H2O on kaolinite(001) surface.

Adsorbate-induced segregation in a PdAg membrane model system: Pd3Ag(1 1 1)

Thin PdAg alloy membranes with 20–25% Ag are being developed for hydrogen separation technology. Despite many investigations on such membranes as well as representative experimental and theoretical model systems, unresolved issues remain concerning the effect of the alloy surface structure and composition on adsorption and vice versa. Therefore, the interaction between hydrogen, carbon monoxide or oxygen with the surface of a PdAg model alloy was studied using periodic self-consistent density functional theory (DFT-GGA) calculations. In particular, the adsorption structure, coverage dependence and possible adsorption-induced segregation phenomena were addressed using Pd3Ag(111) model surfaces with varying degrees of surface segregation. In agreement with previous experimental and theoretical investigations, we predict Ag surface termination to be energetically favorable in vacuum. The segregation of Ag is then reversed upon adsorption of H, CO or O. For these adsorbates, the binding is strongest on Pd three-fold hollow sites, and hence complete Pd termination is favored at high coverage of H or CO, while 25% Ag may remain under oxygen because of the lower O-saturation coverage. CO adsorption provides a somewhat stronger driving force for Pd segregation when compared to H, and this may have implications with respect to permeation properties of PdAg alloy surfaces. Our predictions for high coverage are particularly relevant in underlining the importance of segregation phenomena to the hydrogen transport properties of thin PdAg alloy membranes.

THE DIRECT INVERSION OF HELIUM ATOM SCATTERING (HAS) DIFFRACTION SPECTRA

An algorithm that provides direct, efficient ND polynomial factorization is presented to solve the numerical issues that arise during the direct inversion of helium atom scattering (HAS) diffraction spectra. For an n-variate polynomial the algorithm directly deflates the polynomial to n-single variable equations by evaluating the ratio of pairs of polynomial coefficients. Error estimation of the coefficients of the 1D polynomials is then performed automatically using standard 1D search techniques. The effectiveness of the technique is demonstrated against bi- and trivariate polynomials and an approximate range of validity for error prone polynomials is demonstrated. To demonstrate the effectiveness of the technique, HAS diffraction spectra for the low coverage (2 × 1)-H/Pd(311) system have been analyzed using direct inversion and have revealed that H binds in a three-fold hollow site.

Surface structure and phase transition of K adsorption on Au(111): By ab initio atomistic thermodynamics

We studied the interactions between atomic potassium (K) and Au(111) at a range of coverage (i.e., Θ(K) = 0.11-0.5 monolayer (ML)) by ab initio atomic thermodynamics. For K on-surface adsorption, we found that K energetically favors the three-fold hollow sites (fcc or hcp), while the most significant surface rumpling was obtained at the atop sites. The incorporation of gold atoms in the adsorbate layer gradually becomes energetically favorable with increasing K coverage. We proposed a possible model with a stoichiometry of K(2)Au for the (2 × 2)-0.5 ML phase observed in lower energy electron diffraction (LEED): one K at atop site and the other K as well as one Au adatom at the second-nearest fcc/hcp and hcp/fcc, respectively. Clear theoretical evidences were given for the ionic interaction of K on Au surface. Additionally, phase transitions were predicted based on chemical potential equilibrium of K, largely in line with the earlier reported LEED observations: the clean surface → (√3 × √3)R30° → (2 × 2), and (2 × 2) → (√3 × √3)R30° reversely at an elevated temperature.

DFT study on interaction of hydrogen with Pd(1 1 1)

The interaction of hydrogen atoms with Pd(1 1 1) surface is studied with density functional theory (DFT) method. Various coverages ranging from 0.25 to 2.00 monolayer (ML) are considered. Particular attention is paid to the thermodynamics and kinetics of the adsorption or absorption processes and to the structural and electronic properties. The results show that the threefold hollow sites, FCC and HCP are most energetically favorable. The diffusion processed in the same layer is more preferential than the penetration processed in two different layers. For the coverages more than 1.00 ML, it is predicted that the additional hydrogen easily penetrates into subsurface. PDOS research shows that the interaction of hydrogen atoms with Pd leads to a splitting of d electron density of the outmost Pd(1 1 1) and to a gradual decrease of d band center of surface Pd atoms with coverages increasing.

Adsorption of CO and O2 on W2C(0001)

CO, O(2), and H(2) adsorption on a clean W(2)C(0001)√13×√13 R ± 13.9° reconstructed surface at room temperature (RT) were investigated using high-resolution electron energy loss spectroscopy (HREELS). The W(2)C(0001) adsorbs CO molecularly and adsorbs O(2) dissociatively, but does not adsorb H(2) at RT. In the CO adsorption system, two C-O stretching (antisymmetric CCO stretching) modes were found at 242.3 meV (1954 cm(-1)) and at 253.0 meV (2041 cm(-1)). The low-frequency site is occupied at first with subsequent conversion to the high-frequency site with increasing coverage. Additionally, a small peak was apparent at 104.5 meV (843 cm(-1)), and a middle peak at 50-51 meV (400-410 cm(-1)), which are assignable to a symmetric stretching mode and a hindered translational mode, respectively, of a CCO (ketenylidene) species. These observations are consistent with the CO adsorption model on top of the surface carbon. For oxygen adsorption, two adsorption states were found at 65.2-68.1 meV (526-549 cm(-1)) and 73.6 meV (594 cm(-1)): typical frequencies to oxygen adsorption on metal surfaces. Results suggest that atomic oxygen adsorption occurred on a threefold hollow site of the second W layer.

C-N coupling on transition metal surfaces: A density functional theory study

We have investigated the formation of C-N bonds from individual atoms and single hydrogenated moieties on a series of transition metals. These reactions play a role in HCN formation at high oxygen coverage, also known as Andrussow oxidation, and they are fundamental to understand the ability of other materials to form part of alloys where Pt is the major component. Dehydrogenations take place quite easily under these high oxygen conditions and thus, the C+N, HC+N, and N+CH recombinations to form HCN or its isomer CNH might represent the rate-limiting steps for the reaction. For all the metals in the present study we have found that the activation energy for the reactions between H(x)C and NH(y) (x,y = 0,1) involved in C-N formation follow a linear relationship with the adsorption energy of the N atom. This is due to the common nature of all these transition states, where N-containing fragments get activated from three-fold hollow sites to bridge positions. The slopes of the linear dependence, though, depend on the valence of the N fragment, i.e., smaller slopes are found for NH moieties with respect to N ones.

Dynamic Double Lattice of 1-Adamantaneselenolate Self-Assembled Monolayers on Au{111}

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.

Computational Studies of Experimentally Observed Structures of Sulfur on Metal Surfaces

First-principles electronic structure calculations were carried out to examine the experimentally observed structures of sulfur on close packed surfaces of a number of important metals - Ag(111), Cu(111), Ni(111), Pt(111), Rh(111), Re(0001) and Ru(0001). At low coverages ({le} 1/3 ML), the prediction is consistent with the typical pattern of preferred sulfur occupancy of threefold hollow sites, notably the fcc site on the (111) surfaces and the hcp site on the (0001) surfaces. Theoretical confirmation for the existence of pure sulfur overlayer phases on Pt(111), Rh(111), Re(0001) and Ru(0001) at higher coverages (> 1/3 ML) was provided. For the ({radical}7 x {radical}7) phase seen on Ag(111), the most preferred structure identified for adsorbed S trimer consists of an S atom on the top site bonded to two S atoms situated on the nearest neighbor off-bridge site positions. Among the different densely packed mixed sulfur-metal overlayer models suggested for the ({radical}7 x {radical}7) phase on Cu(111), the structure which consists of metal and S atoms in a hexagonal-like arrangement on the top substrate was found to be the most energetically favorable. For the (5{radical}3 x 2) phase on Ni(111), the calculations confirm the existence of clock-reconstructed top layer metal atoms onto whichmore » sulfur atoms are adsorbed.« less