>i<Ab Initio>/i< investigation of the adsorption of molecules, involved in carbon dioxide and glycerol utilization, on transition-metal substrates

Abstract

This study was motivated by technologies involving the utilization of glycerol and carbon dioxide to obtain higher value products, which are economically promising and can benefit the environment. Carbon dioxide is a greenhouse gas, abundant in the atmosphere, that could be converted into monomers, fuels and other moieties, while glycerol is a polyol, accumulated as a byproduct in the production of biodiesel, that can be used to obtain other three-carbon alcohols. The aforementioned technologies could be developed by the use of transition-metal substrates as nanoparticles or extended surfaces to design catalysts with great control over properties, such as particle size, morphology and composition, which can be tuned towards optimal catalytic performance. Our study focused on adsorption, a pivotal step of the catalytic process. In this thesis, we present an extensive ab initio investigation, based on density functional theory, of the adsorption of carbon dioxide, glycerol and additional small model molecules on finite-sized and extended TM substrates. Concerning size effects, we found that the adsorption properties for physisorbed and chemisorbed CO 2 depended significantly on the particle size. For CO and H 2 , there was alternation of the most stable adsorption site for the various substrate sizes, which resulted in small size-induced oscillations of properties, while for H 2 O, the adsorption was almost independent of size effects. We also studied the adsorption of CO 2 on Pt-based nanoalloys and found that for systems containing metals with completely filled d-states, alloying with Pt facilitated the charge transfer to CO 2 ; on the other hand, for metals from groups 8, 9 and 10 of the periodic table, alloying with increasing Pt content either caused small effects on the adsorption properties or impaired charge transfer to CO 2 . Moreover, we explored several stable morphologies and chemical orderings and found that CO 2 tends to bend on low-coordinated and heterogeneous adsorption sites. Concerning glycerol and other three-carbon alcohols, we systematically studied the influence of OH groups on the adsorption on model surfaces and found that increasing the number of OH groups increases the adsorption strength, while changing their relative positions leads to similar adsorption strength, but with distinctions of other properties, such as bond stretching for the molecules, such trends were discussed in terms of the reactivities of the alcohols.