Estudo da adsorção de álcoois sobre clusters e superfícies de metais de transição utilizando a Teoria do Funcional da Densidade

Abstract

This work was motivated by the difficulty in breaking alcoholic molecules of small carbon chains in fuel cells of direct alcohol, on surfaces of platinum-based catalysts. Although the extensive number of studies is based on the nature of the catalyst, the interaction between alcohol and catalyst is not yet fully understood on atomic scale. Thus, this work presents a theoretical investigation based on the Density Functional Theory using the PBE exchange and correlation functional proposed by Perdew–Burke–Ernzerhoff, added by the D3 correction proposed by Gimme to consider the van der Waals interactions, in order to obtain a better understanding about the adsorption process of methanol, ethanol and ethylene glycol on transition metals (MT). This investigation involved two approaches: (i) the study of the adsorption of methanol, ethanol andethylene glycol on clusters of MT15and (ii) the study of ethylene glycol adsorption on surfaces based on Pt(100). From the first, it was observed that the alcohol molecule approximates of the transition metal, preferentially, through the oxygen atom, maintaining a distance of 2 Å and with the C – C bond tilted in relation to the position of the metal atoms that interact with the molecule. In the case of ethylene glycol, the adsorption occurs by one and by the two atoms of O and, in the latter mode of adsorption, the C – C bond is almost parallel to the interacting metals. Adsorption does not affect the structure of clusters and for the molecule the main variation is the elongation of the C – O and O – H bonds by up to about 3 %. Variations in charges indicate charge transfer from the molecule to cluster and from cluster to the molecule, in some of the cases of ethylene glycol/cluster systems. Depending on the transition metal, adsorption causes local or extended disturbance in the electronic density. From the study of the adsorption of ethylene glycol on Pt(100), the adsorption of the molecule by one or two atoms of O is observed, and the interaction with two O atoms is more intense, according to the energies of interaction. In the adsorption of ethylene glycol to surfaces, there was an elongation of the C1 – O1 bond (with O1 attached to the metal) and a shortening of the C2 – O2 bond for adsorption via a O atom and for both adsorbed O atoms, the two CO bonds undergo extension. There was also a great variation in the dihedrals OCCO due to the rotation of the molecules. The introduction of dopant (MT) in platinum, even without causing significant changes in the clean surface properties, it changed the intensity of ethylene glycol adsorption, especially when the dopant occupied a (cationic) site on the surface. Change in the effective charge of the molecule indicated the charge transfer of the molecule to the surface. Thus, understanding the form and intensity with which the adsorption of alcohols occurs helps to clarify the understanding of this process.