José L. C. Fajín

Effect of the exchange-correlation potential and of surface relaxation on the description of the H2O dissociation on Cu(111)

The role of the exchange-correlation density functional (PBE, PW91, RevPBE) and of surface relaxation in the determination of the adsorption energies, reaction energy barriers, and reaction rate constants has been analyzed taking water dissociation on Cu(111) surface as a test case. The PBE and PW91 functionals yield similar adsorption geometries and, adsorption and activation energies, but differ significantly from RevPBE results. For each of the functionals tested, surface relaxation was found to have only a minor effect on the calculated (co)adsorption geometries and (co)adsorption energies. The calculated energy barriers for water dissociation are more affected by the functional used, especially in the case of the RevPBE, with obvious implications on the calculated energy barriers and derived reaction rate constants.

Density functional theory study of the water dissociation on platinum surfaces: general trends.

We report a comparative periodic density functional theory study of the reaction of water dissociation on five platinum surfaces, e.g., Pt(111) Pt(100), Pt(110), Pt(211), and Pt(321). These surfaces were chosen to study the surface structural effects in the reaction of water dissociation. It was found that water molecules adsorb stronger on surfaces presenting low coordinated atoms in the surface. In the cases of the stepped Pt(110) and kinked Pt(321) surfaces, the activation energy barriers are smaller than the adsorption energies for the water molecule on the corresponding surfaces. Therefore, the calculations suggest that the dissociation reaction will take place preferentially at corner or edge sites on platinum particles with the (110) orientation. The inclusion of the results obtained in this work in previous derived BEP relationships confirms that the adsorption energy of the reaction products arises as the most appropriate descriptor for water dissociation on transition metal surfaces.

Density functional theory model study of size and structure effects on water dissociation by platinum nanoparticles

Size and structure effects on the homolytic water dissociation reaction mediated by Pt nanoparticles have been investigated through density functional theory calculations carried out on a series of cubooctahedral Pt(n) nanoparticles of increasing sizes (n = 13, 19, 38, 55, 79, and 140). Water adsorption energy is not significantly influenced by the nanoparticle size. However, activation energy barrier strongly depends on the particle size. In general, the activation energy barrier increases with nanoparticles size, varying from 0.30 eV for Pt(19) to 0.70 eV for Pt(140). For the largest particle the calculated barrier is very close to that predicted for water dissociation on Pt(111) (0.78 eV) even though the reaction mediated by the Pt nanoparticles involves adsorption sites not present on the extended surface.

DFT Study of the Adsorption of d-(l-)Cysteine on Flat and Chiral Stepped Gold Surfaces

The adsorption of cysteine onto the intrinsically chiral gold surface, Au(321)(R,S), was investigated by means of a periodic supercell density functional theory approach. The results are compared to those obtained at the same level of theory with a nonchiral surface having the same terrace orientation, the Au(111) surface. Neutral and zwitterionic cysteine forms of the L and D enantiomers are considered, as are surface coverage effects. It was found that at high coverage the zwitterionic forms of L- and D-cysteine are more stable on the Au(321)(R,S) faces of the stepped surface and also on the flat Au(111) surface, leading to highly organized cysteine monolayers. However, at low coverage the adsorption of cysteine dimers, with the pairs interacting through their carbonyl groups, is more favorable than or at least equally favorable to the adsorption of single cysteine molecules on both surfaces. A comparison between the cysteine adsorption on the two different surface structures shows that the adsorption on the stepped surface is clearly more favorable than on the flat surface, revealing the importance of the low-coordinated gold atoms in the adsorption of these species. Furthermore, non-negligible differences between the adsorption energy of the enantiomers of cysteine were found both at high and low coverage, thus showing the enantiospecificity of this intrinsically chiral surface regarding cysteine adsorption. This adsorption occurs with the cysteine binding the surface through only one contact point (by its sulfur atom), in contrast to previous work where the enantiospecific adsorption of cysteine has been related to two nonequivalent binding sites of the cysteine enantiomers with the surface.

Fluorobenzene–argon ground-state intermolecular potential energy surface

The ground-state intermolecular potential energy surface for the fluorobenzene-argon van der Waals complex is evaluated using the coupled-cluster singles and doubles including connected triple excitations model, with the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. In the surface minima the Ar atom is located above and below the fluorobenzene plane at a distance of 3.562 A from the fluorobenzene center of mass and at an angle of 6.33 degrees with respect to the axis perpendicular to the fluorobenzene plane. The corresponding binding energy is 391.1 cm(-1). Both these results and the eigenvalues obtained from the potential compare well with the experimental data available.

On the Need for Spin Polarization in Heterogeneously Catalyzed Reactions on Nonmagnetic Metallic Surfaces.

A series of reactions including water, oxygen, hydrogen and nitric oxide dissociation and carbon monoxide or nitric oxide oxidations catalyzed by metallic surfaces have been investigated by means of periodic density functional calculations with the main aim of establishing the importance of spin polarization when the substrate is nonmagnetic. Numerical differences in the calculated total energies and bond lengths of the breaking/forming bonds corresponding to spin restricted or spin unrestricted formalisms are usually smaller than the inherent error of density functional theory based methods. Nevertheless, it is important to insist on the fact that the spin polarized solution exists and is lower in energy than the one corresponding to the spin restricted formalism, as one would expect, and from a practical point of view, results obtained without taking spin polarization into account lead to the same description of the potential energy surface.

Effect of the Exchange-Correlation Potential on the Transferability of Brønsted−Evans−Polanyi Relationships in Heterogeneous Catalysis

As more and more accurate density functional methods emerge, the transferability of Brønsted-Evans-Polanyi (BEP) relationships obtained with previous models is an open question. In this work, BEP relationships derived from different density functional theory based calculations are analyzed to answer this question. In particular, BEP relationships linking the activation energy of O-H bond breaking reactions taking place on metallic surfaces with the adsorption energy of the reaction products are chosen as a case study. These relationships are obtained with the widely used Perdew-Wang (PW91) generalized gradient approximation (GGA) exchange-correlation functional and with the more accurate meta-GGA Tao-Perdew-Staroverov-Scuseria (TPSS) one. We provide compelling evidence that BEP relationships derived from PW91 and TPSS functionals are essentially coincidental. This finding validates previously published BEP relationships and indicates that the reaction activation energy barrier can be obtained by the determination of the energy reaction descriptor value at the less computationally demanding GGA level; an important aspect to consider in future studies aimed at the computational design of catalysts with improved characteristics.

A DFT study of the NO dissociation on gold surfaces doped with transition metals

The NO dissociation on a series of doped gold surfaces (type TM(n)@Au(111) or TM(n)@Au(110), with TM(n) = Ni, Ir, Rh, or Ag and referring n to the number of dopant atoms per unit cell) was investigated through periodic density functional theory calculations. Generally, doping of Au(111) and Au(110) matrices was found to strengthen the interaction with NO species, with the exception of Ag, and was found to increase the energy barrier for dissociation, with the exception of Ni on Au(111). The calculations suggest that the NO dissociation is only possible in the case of the Ir@Au(110) bimetallic surface but only at high temperatures. The increase of the contents of Ir on Au(110) was found to improve significantly the catalytic activity of gold towards the NO dissociation (E(act) = ∼1 eV). Nevertheless, this energy barrier is almost the double of that calculated for NO dissociation on pure Ir(110). However, mixing the two most interesting dopant atoms resulted in a catalyst model of the type Ir@Ni(110) that was found to decrease the energy barrier to values close to those calculated for pure Ir surfaces, i.e., ∼0.4 eV, and at the same time the dissociation reaction became mildly exothermic.

The p-Difluorobenzene−Argon S1 Excited State Intermolecular Potential Energy Surface

The first excited state (S1) intermolecular potential energy surface for the p-difluorobenzene-Ar van der Waals complex is evaluated using the coupled-cluster method and the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. In order to calculate the S1 interaction energies we use the ground state surface evaluated with the same basis set and the coupled-cluster singles and doubles [CCSD] including connected triple excitations [CCSD(T)] model, and interaction and excitation energies evaluated at the CCSD level. The surface minima are characterized by the Ar atom located above and below the p-difluorobenzene center of mass at a distance of 3.4736 A. The corresponding interaction energy is -435.233 cm-1. The surface is used in the evaluation of the intermolecular level structure of the complex.

Accurate computations of the rovibrational spectrum of the He–HF van der Waals complex

The rovibrational spectrum of the He–HF van der Waals complex is calculated from an accurate intermolecular potential energy surface. This is obtained by fitting a considerable number of interaction energies evaluated at the Coupled Cluster Singles and Doubles level including connected triple corrections and with an augmented correlation consistent polarized valence quintuple zeta basis set extended with a set of 3s3p2d1f1g mid-bond functions. The basis set was selected after a systematic study carried out at four intermolecular geometries. The potential is characterized by two linear minima, i.e., He–HF and He–FH, with distances from the He atom to the HF centre of mass of 3.1662 Å and 2.9989 Å and binding energies of −43.844 and −26.169 cm−1, respectively. These results are compared to data available in the literature. An analytic fit to this potential energy surface is presented and used to compute several low-lying rovibrational energy states using coupled channel methods. These results are compared with fixed-frame diffusion Monte-Carlo calculations using a method developed specifically for linear molecules.