Carolina Pistonesi

Density functional study of H–Fe vacancy interaction in bcc iron

The Fe–H interaction in the vicinity of a vacancy in bcc iron was studied within the framework of the density functional theory and the findings compared with previous results obtained by semiempirical molecular orbital methods. Calculations were performed using an iron cluster containing 12 atoms and a vacancy. Geometry optimizations were performed to find the most stable positions for one and two hydrogen atoms. Changes in the electronic structure of Fe atoms near the vacancy were analysed when one and two H atoms are added to the iron cluster. Fe atoms surrounding the vacancy weaken their bond when hydrogen is present. This is interpreted in terms of the formation of Fe–H bonds.

The electronic structure and bonding of a H-H pair in the vicinity of a BCC Fe bulk vacancy

Abstract The H–Fe interaction near a bcc Fe vacancy is analysed using a semi-empirical theoretical method. Calculations were performed using a Fe 86 cluster with a vacancy. Hydrogen atoms are positioned in their local energy minima configurations. Changes in the electronic structure of Fe atoms near a vacancy were analysed for the system without H, with one H and with two H atoms. Fe atoms surrounding the vacancy weaken their bond when hydrogen is present. This is due to the formation of H–Fe bonds. Hydrogen influences only its nearest-neighbour Fe atoms. The H–H interaction was also analysed. For H–H distance of 0.82 A an H–H association is formed, while H–Fe interaction and Fe–Fe weakening is markedly reduced, when compared with other H–H interactions.

A computational study of H-Fe vacancy interaction

The semi-empirical atom superposition and electron delocalization molecular orbital (ASED-MO) theory was used to study the H-Fe interaction near a Fe vacancy. Calculations were carried out using clusters of the type and simulating the absorption of a hydrogen atom near a vacancy located at the centre of the cluster. The formation of H-H pairs was also considered. Our calculations involve a system in which experiments are difficult to perform and could contribute to a better understanding or insight based on microscopic exploration. The results indicate that in general the H-Fe interaction gets stronger when the H atom is close to the vacancy but not at its centre due to a strong indirect interaction mediated by the Fe matrix. Minima regions are obtained at eccentric positions. A H-H pair is suggested to be formed near the vacancy region. The relaxation of the first neighbour Fe atoms towards the vacancy is also addressed in the presence and absence of H. For all situations studied, the most stable configuration corresponds to a cluster contraction.

Study of zirconocene and MAO interaction with SiO2 surfaces

The atom superposition and electron delocalisation method was used to study a model of metallocene catalyst based on zirconocene supported on silica with methylaluminoxane (MAO) as a co-catalyst. The system was analysed for completely and partially hydrated silica. Our results suggest that the production of a cationic zirconocene as a final step for the active site formation may be energetically favourable, suggesting that the process may occur preferentially on partially hydrated silica.

CO on Pd(100)/SiO2 : a computational study of the effect of CaO on the energetics of adsorption

Abstract The semiempirical atom superposition and electron delocalisation molecular orbital (ASED-MO) theory was used to study the CO/Pd–Ca–SiO 2 interaction. Addition of a single CaO molecule to simulate a different interaction with both the metal and the CO molecule was also considered. Ca was added as a sublayer between the SiO 2 and the metallic cluster. A simple diatomic CaO molecule was positioned on the Pd 21 surface. The energetics of CO adsorption was studied in both cases. In our adsorption scheme, CO was considered to be perpendicular to the surface. The results indicate that in general adsorption on twofold sites seems to be more favourable. The Ca sublayer stabilises CO adsorption by less than 0.1 eV for twofold short bridge adsorption site. Fourfold site is de-stabilised by 0.6 eV and is 2 eV less favourable than the twofold sites. The effect of Ca and CaO on the adsorption is of small magnitude.

A computational simulation for H-dislocated BCC Fe interaction

Abstract A model was performed to simulate hydrogen diffusion and trapping on an Fe35 cluster with a (100) dislocation over three different paths, to obtain the relative amount of hydrogen atoms in different sites along each path. The results indicate that hydrogen locates preferentially on the surface zone and on the void produced by the dislocation. For hydrogen energies greater than certain values that depend on the path, the hydrogen concentration predominates on the void region.

The electronic structure and bonding of MAO on the SiO2 (111) hydrated surface

This work examines a polymerisation precatalyst based on methylaluminoxane (MAO) on the silica (111) surface. The electronic structure calculations are computed considering the β-cristobalite structure for SiO 2 and within the Extended Huckel Tight-Binding theory. The density of states of Si, O and surface OH are reported before and after MAO adsorption on the silica surface. MAO is also computed freely in space as an infinite linear chain. Our results suggest that MAO is stabilised when bonded to the silica (111) surface as a result of reinforcement of all the Al-O bonds present on our mode Infinite MAO model bonded to the silica surface.

Zirconocene interaction with MAO on (111) and ( 100) silica surfaces

This work examines a polymerisation catalyst based on zirconocene with methylaluminoxane (MAO) as a cocatalyst on silica surfaces, Calculations were carried out using the Atom Superposition and Electron Delocalisation method (ASED-MO) considering the (111) and (100) silica planes, both completely and partially hydrated. Our results suggest the production of a cationic zirconocene as a final step for the active site formation for (111) silica plane, occurring preferentially on partially hydrated silica. On the contrary this may not be possible for the (100) plane, resulting in this case in the formation of a MAO-zirconocene complex as a final and most stable state.

Tungsten oxide nanowire on copper surfaces: a DFT model

We studied a model of a one dimensional tungsten oxide nanowire on a reconstructed Cu(110) surface by density functional theory calculations. The effect of oxygen vacancies on the electronic structure was analyzed (DOS). The nanowire equilibrium geometric configuration corresponds to tungsten oxide strips oriented along the Cu[10] direction, in which W–W distances are similar to the bulk data, in agreement with electron diffraction experiments. Bader charge analysis shows that the electron charge of the W atoms corresponds to a tungsten atom in the 6+ oxidation state. The presence of oxygen vacancies increases the electronic charge of the tungsten atoms. Formation of the oxygen vacancies by removal of terminal oxygen atoms is energetically favorable. Calculated PDOS shows stabilization of the tungsten states to lower energies due to the presence of oxygen vacancies.