Ab Initio Cluster Model Study of the Chemisorption of CO on Low-Index Platinum Surfaces

Electron density topological and adsorbate orbital analyses of water and carbon monoxide co-adsorption on platinum

The electron density topology of carbon monoxide (CO) on dry and hydrated platinum is evaluated under the quantum theory of atoms in molecules (QTAIM) and by adsorbate orbital approaches. The impact of water co-adsorbate on the electronic, structural, and vibrational properties of CO on Pt are modelled by periodic density functional theory (DFT). At low CO coverage, increased hydration weakens C-O bonds and strengthens C-Pt bonds, as verified by changes in bond lengths and stretching frequencies. These results are consistent with QTAIM, the 5σ donation-2π^* backdonation model, and our extended π-attraction σ-repulsion model (extended π-σ model). This work links changes in the non-zero eigenvalues of the electron density Hessian at QTAIM bond critical points to changes in the π and σ C-O bonds with systematic variation of CO/H₂O co-adsorbate scenarios. QTAIM invariably shows bond strengths and lengths as being negatively correlated. For atop CO on hydrated Pt, QTAIM and phenomenological models are consistent with a direct correlation between C-O bond strength and CO coverage. However, DFT modelling in the absence of hydration shows that C-O bond lengths are not negatively correlated to their stretching frequencies, in contrast to the Badger rule: When QTAIM and phenomenological models do not agree, the use of the non-zero eigenvalues of the electron density Hessian as inputs to the phenomenological models, aligns them with QTAIM. The C-O and C-Pt bond strengths of bridge and three-fold bound CO on dry and hydrated platinum are also evaluated by QTAIM and adsorbate orbital analyses.

Boosting the activity of hydrogen release from liquid organic hydrogen carrier systems by sulfur-additives to Pt on alumina catalysts

Liquid organic hydrogen carriers represent an interesting alternative for hydrogen storage and transport. We demonstrate a method to simultaneously increase the activity of LOHC dehydrogenation catalysts and reduce side product formation.

Electronic structure and spectroscopy of homoleptic compounds of dimolybdenum using TDDFT

Ten compounds of dimolybdenum are studied using density functional theory and time-dependent density functional theory. The energy of the strongest symmetry-allowed bands is calculated. The results are then compared with experimental data, when available. The PW91 functional gives results for geometry and for the energy of the δ→δ* band that show good agreement with experimental data. However, the B3LYP functional gives more realistic values for the whole spectrum when the results are compared with experimental data. Finally, the different values of energy of these bands are explained analyzing the molecular orbitals involved in these transitions. Some ligands can act as an unsaturated system in conjugation with the delta bond, modifying the energies of the electronic transitions.

SIMULATION OF CO CHEMISORPTION ON Pt SUPPORTED ON FRACTAL SURFACES

CO chemisorption on Pt supported on fractal surfaces was simulated in order to compute chemisorption dimension and active sites fractal dimension. Pt deposition was simulated using different models on both fractal and planar surfaces. The potential energy surface with two adsorption positions model was used to compute Pt–CO interaction and a Lennard–Jones 6–12 potential was used to simulate CO–CO interaction. Two Pt phases on fractal surface, one at low concentration — the dispersed phase and the second at high concentration — the aggregated phase characterized by weak interactions with support are obtained. The results are in accord with experimental data of CO chemisorption on Pt supported on γ-alumina. Computed data obtained for planar support are compared with those obtained on fractal support. The effect of fractal support on chemisorption data is underlined.

A periodic density functional theory analysis of CO chemisorption on Pt(111) in the presence of uniform electric fields

Periodic DFT calculations are used to study the effect of a homogeneous electric field applied perpendicular to a Pt(111) surface on the bond distances, binding energies, and vibrational frequencies of atop- and fcc-adsorbed CO at various coverages. The observed structural and energetic modifications can be understood in terms of modest field-induced charge transfer between charged metal surface and adsorbate and are well-described by classical first and second-order Stark models. Electronic differences between atop and fcc adsorption cause CO in these sites to respond differently to applied fields. After correcting for the GGA site preference error, CO adsorption is predicted to shift from atop to fcc at potentials <-0.19 V A(-1). The results are in qualitative agreement with previously reported cluster-based DFT models but differ quantitatively due to difference in modeled coverage, surface relaxation, and finite size effects. The calculated 44.4 cm(-1) V(-1) A shift in C-O stretch frequency with electric field (Stark tuning rate) compares favorably with UHV experiments but is significantly lower than the value obtained in electrochemical measurements, highlighting the importance of adsorbate environment on the magnitude of the tuning rate. The calculated coverage dependence of the tuning rate is in good agreement with previous UHV experiments.

Interface between platinum(111) and liquid isopropanol (2-propanol): A model for molecular dynamics studies

A molecular dynamics model and its parametrization procedure are devised and used to study adsorption of isopropanol on platinum(111) (Pt(111)) surface in unsaturated and oversaturated coverages regimes. Static and dynamic properties of the interface between Pt(111) and liquid isopropanol are also investigated. The magnitude of the adsorption energy at unsaturated level increases at higher coverages. At the oversaturated coverage (multilayer adsorption) the adsorption energy reduces, which coincides with findings by Panja et al. in their temperature-programed desorption experiment [Surf. Sci. 395, 248 (1998)]. The density analysis showed a strong packing of molecules at the interface followed by a depletion layer and then by an oscillating density profile up to 3 nm. The distribution of individual atom types showed that the first adsorbed layer forms a hydrophobic methyl "brush." This brush then determines the distributions further from the surface. In the second layer methyl and methine groups are closer to the surface and followed by the hydroxyl groups; the third layer has exactly the inverted distribution. The alternating pattern extends up to about 2 nm from the surface. The orientational structure of molecules as a function of distance of molecules is determined by the atom distribution and surprisingly does not depend on the electrostatic or chemical interactions of isopropanol with the metal surface. However, possible formation of hydrogen bonds in the first layer is notably influenced by these interactions. The surface-adsorbate interactions influence the mobility of isopropanol molecules only in the first layer. Mobility in the higher layers is independent of these interactions.

Vibrational lifetimes of molecular adsorbates on metal surfaces

We report density functional theory calculations of electron-hole pair induced vibrational lifetimes of diatomic molecules adsorbed on metal surfaces. For CO on Cu(100), Ni(100), Ni(111), Pt(100), and Pt(111), we find that the C-O internal stretch and the bending modes have lifetimes in the 1-6 ps range, and that the CO-surface stretch and the frustrated translational modes relax more slowly, with lifetimes >10 ps for all cases except CO on Ni(111). This strong mode selectivity confirms earlier calculations for CO on Cu(100) and demonstrates that the trends carry over to other metal substrates. In contrast, for NO adsorbed on Pt(111), whereas we still find that the bending mode has the shortest lifetime, about 1.3 ps, we predict the other three modes to have almost equal lifetimes of 8-10 ps. Similarly, for CN adsorbed on Pt(111), we calculate that the internal stretching and molecule-surface stretching modes have approximately equal lifetimes of about 15 ps. Our results are in reasonable agreement with experiment, where available. We discuss some of the underlying factors that may contribute to the observed mode selectivity with adsorbed CO and the altered selectivity with NO and CN.

Fermi Level Alignment in Self-Assembled Molecular Layers: The Effect of Coupling Chemistry

Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.

Comparative study of cluster- and supercell-approaches for investigating heterogeneous catalysis by electronic structure methods: Tunneling in the reaction N + H → NH on Ru(0001)

Different ruthenium clusters of various sizes are constructed with the aim to model the Ru(0001) surface with a sufficient accuracy for predicting catalysis by hybrid density functional methods (B3LYP). As an example reaction the hydrogenation step N(ads) + H(ads) --> NH(ads) from the catalytic production cycle of ammonia is chosen. A cluster of 12 ruthenium atoms is found to reproduce experimental geometries and frequencies of the various reactants on the surface satisfyingly. To get the geometries of adsorbed hydrogen qualitatively correct it is shown that second layer atoms have to be included in the model cluster. Boundary effects are believed to have minor effects on optimized geometries, whereas the effects on reaction barriers are significant. A comparison of model cluster calculations to a periodic supercell approach employing plane waves and density functional methods (RPBE) reveals similar barriers for reaction. The influence of tunneling in this reaction is determined by the small curvature tunneling approach on the electronic surfaces.

Nature of the Metal−Support Interaction in Bifunctional Catalytic Pt/H-ZSM-5 Zeolite

The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Brønsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Brønsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Brønsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.

Electric field induced electron transfer at the adsorbate–surface interface. Effect of the type of metal surface

The ab initio two-state model for electron transfer induced by an external electric field has been applied to the chloride oxidation on Cu, Rh, Pd, Ag, Pt and Au (001) surface models. The two electronic states involved in the model represent physical situations where the electron transferred from the chloride anion to the metal surface lies either on the halide or on the metal substrate. The model assumes that electron transfer takes place when these two states become degenerate and this is achieved by applying an external electric field. Two different situations representing either ultrahigh vacuum or electrochemical conditions have been considered. For the former the present study shows that electric field necessary to achieve degeneracy of the two electronic states is directly related to the metal surface work function whereas for the latter, it is found to be rather insensitive to the metal surface.

Adsorbate-Geometry Specific Subsurface Relaxation in the CO/Pt(111) System

A dramatic multilayer substrate relaxation is observed for the (square root 19 x square root 19)-13CO adlayer phase on a Pt(111) electrode by surface X-ray scattering. Within the (square root 19 x square root 19) unit cell, a vertical expansion of 0.28 A was determined for the Pt atoms under near-top-site CO molecules, whereas only 0.04 A was found under near-bridge-site CO molecules. The lateral displacements involve small rotations toward more symmetric bonding. Both the expansions and rotations extend into the bulk with a decay length of 1.8 Pt layers. This nonuniform layer expansion, hitherto unseen, appears to be a manifestation of the differential stress induced by CO adsorption at different sites.

What can we learn about electrode-chemisorbate bonding energetics from vibrational spectroscopy? An assessment from density functional theory

An analysis is presented of the manner and extent to which the metal surface-chemisorbate bond energetics and geometries as functions of the metal and the applied field can be correlated with vibrational frequencies, with specific reference to electrochemical systems. Emphasis is placed on metal-adsorbate stretching frequencies, vM-A, using oxygen and carbon monoxide chemisorption as illustrative examples; the intramolecular stretch (vCO) of the latter adsorbate is also examined in view of the extensive experimental utilization of this vibrational mode. Results based on Density Functional Theory (DFT) are presented for finite-cluster models of Pt-group and coinage-metal (111) surfaces. The DFT calculations enable a separation between steric repulsion and orbital contributions to the potential-energy surface (PES), and additionally, in the case of CO chemisorption, between the 5sigma and 27pi* orbital components. While rough metal-dependent correlations between vM-A and the surface binding energy, -Eb, are observed, such a relationship is not expected in general. Thus for CO chemisorption, the variations in -Eb are affected more by changes in the 5sigma rather than 2pi* orbital energies, whereas these components influence the M-CO stretching frequency, vM-CO, to a comparable extent. Moreover, the metal-dependent vCO frequencies do not correlate even qualitatively with -Eb; this is because the former are dominated by 2pi*, rather than 5sigma, interactions. The factors influencing the field (F) (and hence electrode potential) dependence of Eb versus VM-CO and vCO mirror somewhat this pattern. While the field-dependent influence of the 5sigma and 2pi* interactions are offsetting, the latter affects the vM CO-F, and especially the vCO-F, behavior to a greater extent than the -Eb-F dependence. Generally, then, the lack of broad-based correlations between chemisorbate vibrational frequencies and binding energetics can be understood in terms of the differing influence of the individual interaction components on the PES well shape and depth. The description of such bonding contributions in terms of dipole-moment parameters is illustrated. Also considered are relations between vibrational frequencies and bond lengths.