Adsorption of atomic and molecular oxygen on Cu(1 0 0)

We have studied the initial stages of the oxidation of the Cu(1 0 0) surface using ab initio calculations. Both atomic and molecular oxygen are addressed. We show that subsurface oxygen is not energetically favourable, but gets stabilized by on-surface O. We discuss the adsorption of molecular oxygen using elbow plots, which can be used in order to qualitatively understand the measured sticking coefficient at different oxygen coverages.     

Studies on fluorinated polyurethanes by X-ray diffraction and density functional theory calculations with periodic boundary conditions

Thin-film wide-angle X-ray diffraction, small-angle X-ray scattering, and density functional theory calculations using B3LYP hybrid functional with the two-dimensional periodic boundary conditions (2D-PBC) have been applied to study the crystal structures of parent and fluorinated polyurethanes. The crystal structures from 2D-PBC-B3LYP calculation and experiments showed the hard-segment chains within crystallites adopted an extended-chain conformation for polyurethanes. Energetically, the parent polyurethane preferred an alternating hydrogen-bonded sheet structure while the fluorinated one adopted a progressive hydrogen-bonded sheet structure.     

Molecularly imprinted catalytic polymers with biomimetic chloroperoxidase activity

A method for the rational design of molecularly imprinted polymers with catalytic activity (MICs) that mimic hemeproteins is described. Density Functional Theory calculations applied to the pre-polymerization adduct and to a possible reaction intermediate allow to choose the best functional monomer that builds up the active sites. This functional monomer should stabilize both relevant stages, the first one in an adequate porogenic solvent and the second one in the aqueous reaction medium. In this work, a MIC was synthesized to catalyze the oxidation of 2,4,6-trichlorophenol (TCP), the substrate and template molecule. A hemin molecule was used as the catalytic centre and four functional monomers and six solvents were tested. According to the theoretical predictions, methacrylamide and 4-vinylpyridine MIPs successfully mimic the recently discovered ability of Chloroperoxidase (CPO) to catalyze the oxidative dechlorination of TCP in batch analysis. They exhibit good catalytic properties and substrate selectivity.     

Formation of Mo-carbene active sites in Mo/Beta zeolite catalysts with different olefins: Theoretical exploration of possible reaction pathways and substituent effects

Metal-carbenes act as the active sites in varieties of important reactions, and in this work, the possible pathways of Mo-carbene formation in Mo/Beta zeolite were explored by density functional calculations. Four chiral oxametallacyclobutane intermediates of different energies were determined for either propylene or 2-butylene, corresponding to the four independent reaction pathways. The main products were Mo-methylidene for propylene and syn-Mo-ethylidene for 2-butylene, respectively. On the basis of energy profiles, the activities of producing the Mo-carbene species were found to decrease in the order of propylene > 2-butylene > ethylene, which were the compromising results of electrophilic and steric effects.     

Transition metals to sulfur binding energies relationship to catalytic activities in HDS: back to Sabatier with first principle calculations

We have undertaken systematic calculations of transition metal sulfides bulk crystal structures, electronic and energetic properties at the first principles level (DFT, GGA, PW-USPP, PBC, implemented in the Vienna Ab initio Simulation Package, VASP). Relaxed cell parameters and ionic positions showed an excellent agreement with the experimental values. Computed and experimental cohesive energies agreed within 3%. We re-defined the metal–sulfur (M–S) bond strength as the cohesive energy per metal–sulfur bond: we show that all experimental HDS activities (Pecoraro and Chianelli, 1981) fit nicely on a single volcano master curve when plotted against this simple energetic parameter. Metallic (i.e. zero gap) ionic sulfides consistently exhibit the weakest M–S bonds and semi-conductor iono-covalent sulfides the strongest. However, the Sabatier principle suggests a simple kinetic interpretation of this master curve. This new interpretation also accounts for the well known synergetic effects in mixed sulfides and therefore opens new prospects for exploratory applied research.     

Tuning catalytic properties of bimetallic surfaces: Oxygen adsorption on pseudomorphic Pt/Ru overlayers

The adsorption of atomic and molecular oxygen on bimetallic Pt/Ru overlayer systems has been studied by periodic density functional theory calculations. A strong interaction between the Pt and the Ru layers is found resulting in a hcp stacking for the first Pt layer on Ru. The chemical properties of the Pt/Ru overlayers are beyond those of the single constituents Pt and Ru. Both the compression of the Pt overlayers as well as the strong direct interaction between Pt and Ru, which reaches up to the second Pt layer, reduce the atomic and molecular oxygen adsorption energies compared to pure Pt and Ru surfaces. Although the influence of the electrolyte and any external electric field are neglected, the results should still be relevant in the electrochemical context. The consequences of the theoretical findings for the oxygen reduction reaction in electrocatalysis are discussed.     

The origin of the catalysis of hydrogen peroxide reduction by functionalized graphene surfaces: A density functional theory study

The adsorption of hydrogen peroxide on pristine and modified graphene sheets is studied by means of density functional theory calculations. The system considered involves perfect and defective layers, which can be pristine or functionalized with carboxyl groups.It is found that functionalization increases (in absolute value) the adsorption energy of hydrogen peroxide and improves the reduction reaction due to a favourable change in the properties of the defective layer. These changes are analyzed and mechanisms for the reduction reaction of hydrogen peroxide on the different surfaces are proposed. The associated energy barriers are calculated by means of state-of-the-art calculation methods.     

Cluster model studies on oxygen sites at the (010) surfaces of V2O5 and MoO3

Cluster model studies have been performed to examine the electronic structure and adsorption properties near structurally different oxygen sites at the (010) surfaces of vanadium pentoxide, V₂O₅, and molybdenum trioxide, MoO₃. In addition, adsorption of hydrogen at the oxygen sites and desorption of OH groups has been studied in order to find site specific differences. The electronic properties and total energies of the clusters have been obtained from ab initio density functional theory (DFT) calculations. The surface oxygen sites are found to be ionic where bridging oxygens become more negative than terminal ones. Further, hydrogen adsorbs at all oxygen sites where binding is strongest at the bridge sites on the V₂O₅(010) surface whereas on MoO₃(010) the terminal sites are preferred. The latter difference can be understood by simple geometric arguments. Surface OH groups formed by H adsorption and involving terminal oxygens are strongly bound to the surface whereas those involving bridging oxygens are mobile and become available for subsequent reactions.