Josep M. Ricart

Critical size for O(2) dissociation by au nanoparticles.

Density functional theory calculations predict that the presence of low-coordination Au atoms is not enough to dissociate O(2), that there is a common pathway for O(2) dissociation on Au nanoparticles and that there is critical size for Au nanoparticles to dissociate O(2) (see figure).Density functional theory calculations carried out for a series of Au nanoparticles as well as for extended systems containing low-coordinated sites show that the presence of low-coordinate Au atoms is not enough to dissociate O(2). Strong adsorption of molecular oxygen on Au nanoparticles is a necessary but not sufficient condition for O(2) dissociation, there is a common pathway for O(2) dissociation on these nanoparticles and there is critical size for Au nanoparticles to dissociate O(2).

A theoretical study of the adsorption and reaction of SO2 at surface and step sites of the MgO(100) surface

Abstract The interaction of SO 2 with acid and basic sites of a clean, fully dehydroxilated, MgO(100) surface has been investigated by means of ab initio cluster model calculations. Full geometry optimization of the surface complexes has been carried out at the SCF level. The nature of the bonding has been analyzed in detail by performing a decomposition of the interaction energy as the sum of electrostatics, polarization and charge transfer contributions. SO 2 adsorbs molecularly at five-coordinated Mg 2+ 5c surface cations forming an electrostatically bound complex where SO 2 is bridge-bonded to two adjacent Mg 2+ centres. The formation of a strongly chemisorbed sulphite, SO 2− 3 , species occurs when SO 2 interacts with the basic O 2− ions. However, a very different reactivity is observed for five-coordinated, surface, and four-coordinated, step, O 2− sites. In fact, the O 2− 5c ions of the surface are rather unreactive while the formation of the very stable SO 2− 3 species at O 2− 4c step sites is a non-activated process. The different reactivity of the regular and defect basic sites of the MgO surface is explained in terms of the different Madelung potential at these sites hence of the different energetic cost for the MgOSO 2 charge transfer. No evidence is found for the formation of sulphate, SO 2− 4 , species. The computed vibrational and photoemission features of the two forms of adsorbed SO 2 , physisorbed and chemisorbed, are consistent with the corresponding experimental spectra.

On modelling the interaction of CO on the MgO(100) surface

Abstract In this work we discuss the interaction of an adsorbate on an ionic surface taking as representative example CO on a perfect MgO(100) surface. The main goal is to investigate the different contributions to the interaction and how to model them by using a finite cluster model. To this end we use three different ab initio Hartree-Fock approaches. First, we discuss the convergence properties of the array of point charges used to simulate the Madelung potential. Next, we use large cluster models to show that there is an oscillatory behavior of the interaction energy. We show that rather large clusters are needed to avoid such oscillations. Also, we discuss how to represent these large clusters by using model potentials and compare the results with those obtained from periodic Hartree-Fock calculations. The interaction of CO on MgO is found to be weak and of electrostatic origin, with no noticeable chemical contributions.

Selected versus complete configuration interaction expansions

Full CI (FCI) and selected multireference CI (MRCI) calculations have been carried out for the first row atoms (B to F) and hydrides (BH to FH). The multireference selected CI calculations were carried using the CIPSI algorithm (CIPSI stands for configuration interaction by perturbative iterative selection). An analysis of the relationship between variational and perturbational correlation energy contributions is also reported. By diagonalizing a matrix corresponding to a CI expansion selected from the full MRCI space, 99% of the total FCI correlation energy is obtained. The contribution of a large number of generated determinants to the correlation energy can be accounted for by second order perturbation theory. This contribution to the correlation energy is one order of magnitude smaller than the error with respect to the FCI value. The calculations were carried out using two different basis sets. The first one is based on the atomic natural orbital (ANO) contraction, and the second follows the segmente...

Dynamical and nondynamical correlation effects in ab initio chemisorption cluster model calculations. Ground and low lying states of H on Cu(100) and Ag(100)

The nondynamical correlation effects for the interaction between an adsorbate and a surface cluster model can be recovered by means of a simple and accurate method. A projection localization procedure permits to distinguish between cluster and adsorbate MOs and between lone pairs or bonds. Valence virtual MOs are obtained by an inexpensive method. Once the valence space is defined CASCI calculations account for these correlation effects, the results being in good agreement with CASSCF calculations. The dynamical correlation effects are taken into account by using a perturbative approach in which the CAS wave function is taken as the zeroth order one. Results, thus obtained, are also in agreement with accurate CIPSI calculations. The overall procedure is illustrated by means of calculations on the ground and low lying electronic states of the on top chemisorption of atomic hydrogen on the Cu5 and Ag5 cluster models simulating the (100) metal surface. The main effects of nondynamical correlation in the elec...

O2 adsorption and dissociation on neutral, positively and negatively charged Aun (n = 5–79) clusters

The adsorption and dissociation of an O(2) molecule on gas-phase gold clusters of size varying from 5 to 79 atoms have been investigated by means of first principles density functional theory calculations. The adsorption energies and dissociation barriers have been determined for neutral, positively and negatively charged gold clusters in order to analyze in a systematic way the role of the charge on the cluster reactivity. While there is beneficial effect on O(2) activation of an extra electron on the small gold clusters (Au(5) and Au(13)), the effect is absent for positively charged clusters. The effect of the charge vanishes rapidly by increasing the cluster size and is not visible for clusters containing about 40 atoms or more. Au(38) appears to be the most reactive among the clusters considered and strong oscillations in adsorption energies and dissociation barriers are found even for clusters containing several tens of atoms like Au(38), Au(55), and Au(79).

Approximate natural orbitals and the convergence of a second order multireference many‐body perturbation theory (CIPSI) algorithm

In this paper we discuss a method to obtain approximate natural orbitals (ANOs) by means of the zero‐order and first‐order one‐particle reduced density matrices. Convergence of the multireference second‐order perturbation CI by means of CIPSI method, in its two‐class and three‐class versions, appears to be much improved when using these ANOs. The method requires only a single additional integral transformation. A comparison with canonical and valence virtual orbitals defined through the PHO procedure is also reported. Finally, the use of ANOs together with the multireference analog of the Davidson’s correction, as well as a suitable extrapolation procedure in the CIPSI method is discussed. Results are compared with available full CI calculations for H2O in a double‐ξ and a double‐ξ plus polarization basis set.

Density functional studies on the adsorption and decomposition of SO2 on Cu(100)

Density functional calculations in combination with cluster and slab models (periodic super-cell approach) were used to investigate the bonding and chemistry SO2 on Cu(100). For small coverages of SO2, the stability of the different bonding modes increases according to the η1-S<η2-S,O<η2-O,O<η3-S,O,O sequence. Large coverages make impossible η3-S,O,O bonding, and under such conditions the most stable conformations involve η2-O,O or η2-S,O bonding. These adsorption geometries can be expected when SO2 is coadsorbed with substantial amounts of O. The bonding mechanism of SO2 on copper involves a Cu(3d,4s)→SO2(LUMO) electron transfer that leads to a weakening and elongation of the S–O bonds. The η3-S,O,O conformations exhibit the biggest adsorption energies, the largest charge transfers, and the weakest S–O bonds. These conformations are ideal precursors for the dissociation of the SO2 molecule. From a thermochemical viewpoint, it is much easier to generate SO3(3SO2→2SO3+S,ΔE=∼+1 kcal/mol) than to form SO(SO2...

Charge Polarization at a Au–TiC Interface and the Generation of Highly Active and Selective Catalysts for the Low‐Temperature Water–Gas Shift Reaction

Au atoms in contact with TiC(001) undergo significant charge polarization. Strong metal-support interactions make Au/TiC(001) an excellent catalyst for the low-temperature water-gas shift (WGS), with turnover frequencies orders of magnitude larger than those observed for conventional metal/oxide catalysts. DFT calculations indicate that the WGS reaction follows an associative mechanism with HOCO as a key intermediate.

Electronic structure of Rh, RhH, and Rh2 as derived from ab initio (configuration interaction) calculations

The electronic structure of Rh, RhH, and Rh2 in their ground and low‐lying electronic states has been studied using nonempirical relativistic pseudopotentials and moderately large basis sets including f‐type functions. The energy splitting 4F–2D, ionization potential and electron affinity of Rh have been obtained and compared with experimental results. A rather good agreement being found in all cases. For RhH the 3Δ, 3Π, 3Φ, 3Σ− and 1Σ+ electronic states were studied. The ground state being 3Δ in agreement with previous theoretical studies. The calculated De for RhH was found to be in a nice agreement with the experimental value. For Rh2 eight electronic states symmetrically dissociating into the ground state of the isolated atoms have been selected. Moreover a search for lowest electronic states has also been performed among those electronic states dominated by electronic configurations which (at re ) do not correlate with the ground state of the isolated atoms. The lowest electronic state was a 5Δu, the...