CO adsorption on Cu(1 1 1) and Cu(0 0 1) surfaces: Improving site preference in DFT calculations

CO adsorption on Cu(1 1 1) and Cu(0 0 1) surfaces has been studied within ab initio density functional theory (DFT). The structural, vibrational and thermodynamic properties of the adsorbate–substrate complex have been calculated. Calculations within the generalized gradient approximation (GGA) predict adsorption in the threefold hollow on Cu(1 1 1) and in the bridge-site on Cu(0 0 1), instead of on-top as found experimentally. It is demonstrated that the correct site preference is achieved if the underestimation of the HOMO–LUMO gap of CO characteristic for DFT is corrected by applying a molecular DFT + U approach. The DFT + U approach also produces good agreement with the experimentally measured adsorption energies, while introducing only small changes in the calculated geometrical and vibrational properties further improving agreement with experiment which is fair already at the GGA level.     

CO and hydrogen adsorption on Pd(2 1 0)

We have studied the adsorption of CO on Pd(2 1 0) by performing density functional theory (DFT) calculations within the generalized gradient approximation. We find a relatively small corrugation in the CO adsorption energies with the two bridge sites being energetically almost degenerate. CO is furthermore known as a strong poison in heterogeneous catalysis. We have therefore also addressed the coadsorption of CO with atomic hydrogen. There is a significant inhibition of the hydrogen adsorption due to the presence of CO which is analysed in terms of the electronic structure of the adsorbate system.     

First principles study of H2S adsorption and dissociation on Fe(1 1 0)

We report first principles density functional theory (DFT) results of H₂S and HS adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference of H₂S, HS, and S on Fe(1 1 0). H₂S is found to weakly adsorb on either the short bridge (SB) or long bridge (LB) site of Fe(1 1 0), with a binding energy of no more than 0.50 eV. The diffusion barrier from the LB site to the SB site is found to be small (∼0.10 eV). By contrast to H₂S, HS is predicted to be strongly chemisorbed on Fe(1 1 0), with the S atom in the LB site and the HS bond oriented perpendicular to the surface. Isolated S atoms also are predicted to bind strongly to the LB sites of Fe(1 1 0), where the SB is found to be a transition state for S surface hopping between neighboring LB sites. The minimum energy paths for H₂S and HS dehydrogenation involve rotating an H atom towards a nearby surface Fe atom, with the S-H bonds breaking on the top of one Fe atom. The barrier to break the first S-H bond in H₂S is low at 0.10 eV, and breaking the second S-H bond is barrierless, suggesting deposition of S on Fe(1 1 0) via H₂S is kinetically and thermodynamically facile.     

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).     

CO on Pd(110): determination of the optimal adsorption site

We present ab initio pseudo-potential plane-wave total-energy calculations for the geometric and electronic structure of the CO-covered Pd(110) surface. Our calculations were performed within the local-density approximation (LDA) of density functional theory (DFT). There has been some controversy as to whether CO prefers to adsorb at a bridge or on-top site when exposed to Pd(110). Total energy calculations for a CO monolayer adsorbed at the on-top and bridge adsorption sites revealed the bridge site adsorption to be favored by 0.59 eV per CO molecule. The preferential adsorption of CO to the bridge site was further corroborated by our band-structure calculations, with only the bridge site results being in good agreement with recent inverse photoemission experiments.     

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.     

SiO adsorption on a p(2 × 2) reconstructed Si(1 0 0) surface

We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O₂ adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO₂ interface and may be used for studying SiO_x growth.     

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.     

Density-functional study of the CO adsorption on ferromagnetic Co(0 0 0 1) and Co(1 1 1) surfaces

The regular CO overlayers at coverage θ = 1/3 adsorbed on the (0 0 0 1) surface of hcp Co and (1 1 1) surface of fcc Co are studied by first-principles density-functional theory with the exchange-correlation component in the PBE form. Adsorption in atop, bridge, and three-fold hcp or fcc position are considered. The adsorption energies, CO stretching frequencies, geometry, work function, and local magnetic moments are studied, and, when possible, compared with experimental or theoretical data. Particularly, we show that the recently proposed correction to adsorption energy of CO prefers correctly the atop adsorption site, whereas the remaining sites are almost degenerate in energy. The CO molecule lowers magnetization on neighbouring Co atoms, and the effect decreases with the adsorption site coordination. We show, however, that this trend is not the result of the different C-Co separation at different adsorption sites. A very small magnetic moment appears on CO that couples antiferromagnetically to Co. Most results are very similar for the Co(0 0 0 1) and Co(1 1 1) surfaces.     

Microscopic model of CO oxidation on Pt(1 1 1)

A new microscopic model, based on DFT/LDA modeling, is suggested for the Langmuir-Hinshelwood reaction of catalytic CO oxidation in coadsorbed O-CO layers on Pt(1 1 1). It has been found that only the oxygen atoms occupying threefold hollow sites of hcp type are chemically active. The potential barrier for the oxidation reaction significantly decreases due to changes in the adlayer oxygen states in the proximity to CO. The oxygen electronic density distribution is affected by approaching CO molecule which alters the oxygen position. Height of the barrier is estimated as 1.15 eV, which may be attributed to the upper limit of activation energy for the net reaction process.     

Adsorption of N@C60 on Si(1 0 0)

The interactions between endohedrally doped N@C₆₀ molecules and the Si(1 0 0) surface have been explored via ab initio total energy calculations. Configurations which have the cage located upon the dimer row bonded to two dimers (r2) and within the dimer trench bonded to four dimers (t4) have been investigated, as these have previously been found to be the most stable for the C₆₀ molecule. We have investigated the differences between the adsorption of the C₆₀ and N@C₆₀ molecules upon the Si(1 0 0) surface and found that there are only minimal differences. Two interesting cases are the r2g and t4d configurations, as they both exhibit differences that are not present in the other configurations. These subtle differences have been explored in-depth. It is shown that the effects on the endohedral nitrogen atom, due to its placement within the fullerene cage, are small. Bader analysis has been used to explore differences between the C₆₀ and N@C₆₀ molecules.     

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom-restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH₃OH dissociation favouring a basic cleavage (adsorption of OH and CH₃ fragments) rather than an acidic one (adsorption of H and OCH₃ fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.     

Coadsorption of methyl radicals and oxygen on Rh(1 1 1)

The chemisorption of CH₃ on Rh(1 1 1) is studied to understand the origin of the weakened symmetric stretch mode. A few different explanations for this weakened mode have been suggested in previous studies. These include C-H bond depletion and donation into C-H anti-bond orbitals either in an upright or tilted geometry. We investigate these possibilities by performing first-principles density functional calculations. Our results show strong adsorption at all high-symmetry sites with methyl in two possible orientations. A thorough analysis of the adsorption geometry shows that C_3v symmetry is preferred over a tilted species, ruling out tilting as a mechanism for C-H mode softening. Evidence of a multi-center bond between methyl and the surface rhodium atoms (similar to the kind shown recently by Michaelides and Hu for methyl on Ni(1 1 1)) is presented, showing that C-H bond depletion is the cause of mode-softening for methyl on Rh(1 1 1). Experimental results have shown that mode-softening diminishes when an electronegative species is coadsorbed, suggesting that donation into C-H anti-bonding orbitals is the mechanism for mode-softening. We therefore examine the coadsorption of oxygen and methyl on Rh(1 1 1). Our results suggest a new model for the effect of O on CH₃. Analysis of charge density differences shows that the dominant initial effects of O coadsorption are the removal of charge from the C-surface bond and the transfer of charge to the C-H bond. Subsequent increase of the H-Rh distance further reduces mode softening.     

Theoretical study of the chemisorption of CO on bimetallic RhCu surfaces and nanoparticles

The CO interaction with bimetallic RhCu surface models representing several compositions has been studied by first principles density functional theory calculations. The analysis of the bare bimetallic clusters Rh(4s) and Cu(3s) core-level binding energies indicates that is not possible to extract information about the oxidation state of the alloy components. The present calculations predict that CO does always sit on top sites, the influence of the alloy composition on the equilibrium geometry and vibrational frequency of CO chemisorbed at a given Rh or Cu site being very small. However, there is a large difference in the structural properties corresponding to CO chemisorption above either Rh or Cu. Therefore, the absolute value of the vibrational frequency of chemisorbed CO does not permit to extract any information about the alloy composition but afford to assign the chemisorption site. Finally, the CO adsorption energy does not follow a monotonic trend with composition. The use of the Constrained Space Orbital Variation analysis permits one to firmly establish that the difference in adsorption energy for different compositions cannot be explained through differences in the σ-donation and π-backdonation mechanisms.     

CO adsorption on the Pt(1 1 1) surface: a comparison of a gradient corrected functional and a hybrid functional

The adsorption of CO on the Pt(1 1 1) surface in a pattern has been studied with the gradient corrected functional of Perdew and Wang and the B3LYP hybrid functional. A slab which is periodic in two dimensions is used to model the system. The Perdew-Wang functional incorrectly gives the fcc site as the most favorable adsorption site, in accord with a set of previous studies. The B3LYP functional gives the top site as the preferred site. This confirms results from cluster studies where it was suggested that the different splitting, dependent on the functional, between highest occupied and lowest unoccupied molecular orbital, could be the reason for this change of the adsorption site. This is supported by an analysis based on the projected density of states and the Mulliken population.     

Intermediates in the hydrogenation of benzene to cyclohexene on Pt(1 1 1) and Pd(1 1 1): A comparison from DFT calculations

Catalytic hydrogenation of aromatic compounds is an important process in petroleum industry. Understanding it through experimental or theoretical research can help to improve its efficiency. This work presents a first principles density functional theory study of the intermediates for the first four hydrogenations steps of the smallest aromatic compound, benzene, into C₆H₁₀ species, on two popular catalytic metals, palladium and platinum, described by periodic models. Different structures have been studied for the intermediate C₆H_6+n species, with a various degree of conservation of the conjugation. Some intermediates would present in gas phase a closed-shell conjugated structure, while other would correspond to multiple radicals with a massive destruction of the benzene π system. The Pd and Pt(1 1 1) surfaces strongly differ in terms of most stable structure for the intermediates. On Pd the most conjugated intermediates, i.e. the most stable in gas phase, is clearly preferred. In contrast, on Pt, multiple radical species, highly unstable in gas phase, are strongly stabilized by coupling with the surface. This thermodynamic study indicates a different trend for the hydrogenation mechanism: a clear successive 1-2-3… hydrogenation of neighboring carbon atoms on Pd keeping the largest conjugated fragment, and a nonconsecutive attack, with a maximum breaking of benzene conjugation on Pt.     

A DFT study of the transition metal promotion effect on ethylene chemisorption on Co(0 0 0 1)

Transition metals are often introduced to a catalyst as promoters to improve catalytic performance. In this work, we study the promotion effect of transition metals on Co, the preferred catalytic metal for Fischer–Tropsch synthesis because of its good compromise of activity, selectivity and stability, for ethylene chemisorption using density functional theory (DFT) calculations, aiming to provide some insight into improving the α-olefin selectivity. In order to obtain the general trend of influence on ethylene chemisorption, twelve transition metals (Zr, Mn, Re, Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au) are calculated. We find that the late transition metals (e.g. Pd and Cu) can decrease ethylene chemisorption energy. These results suggest that the addition of the late transition metals may improve α-olefin selectivity. Electronic structure analyses (both charge density distributions and density of states) are also performed and the understanding of calculated results is presented.     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

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.