[Show abstract][Hide abstract] ABSTRACT: Water oxidation by Ti(OH)4 in the ground and excited states was investigated using density functional (∆SCF, TDDFT) methods gauged by the coupled cluster (CCSD, CCSD(T)) calculations. O2 and H2 are generated in a reaction sequence that starts with Ti(OH)4 reacting with H2O. This reaction can proceed by either nucleophilic attack by H2O or by H-atom abstraction from H2O. The nucleophilic attack has high energy barriers (40-120 kcal/mol) in both the ground and excited states. On the other hand H abstraction is effected by Ti(OH)4 in the excited state with a low energy barrier (4-8 kcal/mol), generating OH*. This is the rate-limiting barrier in the chain of O2 formation reactions proposed in this work. The production of free OH* radicals is not energetically feasible in the ground state. By absorbing two photons, two hydroxyl radicals are produced, which then form H2O2: By a stepwise H-abstraction from H2O2 and OOH*; O2 is generated by absorbing two more photons. In each H-abstraction reaction a Ti(OH)4 is consumed and a Ti(OH)3H2O is produced. H2 production can proceed thermally from the latter in a very exothermic (68- 105 kcal/mol) bimolecular reaction. The solvent effects, modelled by explicit water molecules, have a limited influence on the reactivity.
Physical Chemistry Chemical Physics 07/2015; DOI:10.1039/C5CP01812A · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To understand the formation of silicate oligomer in the initial stage is a key for zeolite synthesis. The use of different organic structure directing agents is known to be a key factor in the formation of different silicate species and the final zeolite structure. Tetramethylammonium (TMA+), for example, is indispensable for the formation of the LTA zeolite type. However, the role of a TMA+ template has not yet been elucidated at molecular level. In this study, ab-initio molecular dynamic simulations were combined with thermodynamic integration to arrive at an understanding of the role of TMA+ in the formation of various silicate species from dimer to 4-ring. Free energy profiles show that trimer and 3-ring silicate are less favourable than other oligomers such as linear tetramer, branched tetramer and 4-ring structures. TMA+ exhibits an important role in controlling the predominant species in solution via its close interaction with silicate structures during reaction process. This can explain that formation of D4R.8TMA crystals, as observed in experiment, is controlled by the single 4-ring formation step.
Physical Chemistry Chemical Physics 06/2015; DOI:10.1039/C5CP02068A · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The water electrolysis process requires the use of the proper catalyst in order to lower the overpotential needed to overcome the thermodynamic limitations of the process. The study presented in the article makes use of the MD simulation with explicit solvent to evaluate the relative stability of the particular intermediates in the electrocatalytic system. The cooperation of the two sites is essential in the O–O bond formation which occurs at the inter-phase of the Co oxide particle and TiO 2 support.
[Show abstract][Hide abstract] ABSTRACT: The increasing availability of computational data from quantum-chemical calculations on the reactivity and electronic structure of catalytically active oxidic systems make a revisitation of the classical questions on chemical bonding aspects of catalytically reactive systems useful.
[Show abstract][Hide abstract] ABSTRACT: A computational study is presented of the cooperative effect of a small four atom Co oxide cluster supported on the TiO2 anatase (100) surface in the electrochemical water splitting reaction. Results have been obtained including explicit solvent water molecules by means of Car-Parrinello MD simulations. Reaction steps in the catalytic cycle determined involve the formation of TiO2 surface hydroxyl groups as well as elementary reaction steps on the Co oxide cluster. Essential is the observation of O-O bond formation at the inter-phase of Co oxide particle and TiO2 support.
[Show abstract][Hide abstract] ABSTRACT: A reactive force field has been developed that is used in molecular dynamics (MD) studies of the surface transformation of the cobalt (0001) surface induced by an overlayer of adsorbed carbon atoms. Significant surface reconstruction is observed with movement of the Co atoms upward and part of the C atoms to positions below the surface. In a particular C ad atom coverage regime step edge type surface sites are formed, which can dissociate adsorbed CO with a low activation energy barrier. A driving force for the surface transformation is the preference of C adatoms to adsorb in 5- or 6-fold coordinated sites and the increasing strain in the surface because of the changes in surface metal atom-metal atom bond distances with the increasing surface overlayer concentration. The process is found to depend on the nanosize dimension of the surface covered with carbon. When this surface is an overlayer on top of a vacant Co surface, it can reduce stress by displacement of the Co atoms to unoccupied surface positions and the popping up process of Co atom does not occur. This explains why small nanoparticles will not reconstruct by popping up of Co atoms and do not create CO dissociation active sites even when covered with a substantial overlayer of C atoms.Keywords: catalysis; surface reconstruction; reactive force field; molecular dynamics; size dependence; step-edge sites
[Show abstract][Hide abstract] ABSTRACT: The target of our study is to approximate numerically and, in some particular
physically relevant cases, also analytically, the residence time of particles
undergoing an asymmetric simple exclusion dynamics on a stripe. The source of
asymmetry is twofold: (i) the choice of boundary conditions (different
reservoir levels) and (ii) the strong anisotropy from a nonlinear drift with
prescribed directionality. We focus on the effect of the choice of anisotropy
in the flux on the asymptotic behavior of the residence time with respect to
the length of the stripe. The topic is relevant for situations occurring in
pedestrian flows or biological transport in crowded environments, where lateral
displacements of the particles occur predominantly affecting therefore in an
essentially way the efficiency of the overall transport mechanism.
[Show abstract][Hide abstract] ABSTRACT: Microkinetics simulations were employed to analyze the complex Fischer–Tropsch (FT) reaction. In their Communication on page 12746 ff., E. J. M. Hensen et al. distinguish between three kinetic regimes, in which the performance of current FT catalysts is limited by CO activation, product desorption, or oxygen removal. A better understanding of the FT reaction is required for the design of improved catalysts for syngas conversion.
[Show abstract][Hide abstract] ABSTRACT: Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.
[Show abstract][Hide abstract] ABSTRACT: Low-temperature Fischer–Tropsch reaction data are reported for Ru nanoparticles suspended in the water phase. Their activity and selectivity strongly depends on particle size, when varied between 1 to 5 nm. Small particles display high oxygenates selectivity. The Anderson–Schulz–Flory (ASF) chain-growth probability for oxygenates is significantly lower than that observed for hydrocarbons. The chain growth parameter for hydrocarbon formation is independent of particle size. For oxygenates it is constant only for particles larger than 3 nm. Oxygenate and hydrocarbon formation occur on different sites. The ASF chain-growth probability for oxygenate formation increases with temperature. For very small 1.2 nm particles it shows a maximum as a function of temperature. This unusual temperature dependence is due to relatively slow CO dissociation compared to the rate of C–C bond formation.
[Show abstract][Hide abstract] ABSTRACT: A comprehensive density functional theory study of the Fischer–Tropsch mechanism on the corrugated Ru(111) surface has been carried out. Elementary reaction steps relevant to the carbide mechanism and the CO insertion mechanism are considered. Activation barriers and reaction energies were determined for CO dissociation, C hydrogenation, CHx + CHy and CHx + CO coupling, CHxCHy–O bond scission and hydrogenation reactions, which lead to formation of methane and higher hydrocarbons. Water formation that removes O from the surface was studied as well. The overall barrier for chain growth in the carbide mechanism (preferred path CH + CH coupling) is lower than that for chain growth in the CO insertion mechanism (preferred path C + CO coupling). Kinetic analysis predicts that the chain-growth probability for the carbide mechanism is close to unity, whereas within the CO insertion mechanism methane will be the main hydrocarbon product. The main chain propagating surface intermediate is CH via CH + CH and CH + CR coupling (R = alkyl). A more detailed electronic analysis shows that CH + CH coupling is more difficult than coupling reactions of the type CH + CR because of the σ-donating effect of the alkyl substituent. These chain growth reaction steps are more facile on step-edge sites than on terrace sites. The carbide mechanism explains the formation of long hydrocarbon chains for stepped Ru surfaces in the Fischer–Tropsch reaction.
[Show abstract][Hide abstract] ABSTRACT: Der perfekte Katalysator: Es wird untersucht, welche Vorzüge der gezielte Entwurf von Katalysatoren aus theoretischen Grundprinzipien bietet. Aspekte der Computerchemie sowie der Kinetik und des physikalischen Zustands des reaktiven Katalysators werden diskutiert.
[Show abstract][Hide abstract] ABSTRACT: The perfect catalyst: The advances towards the ability to design a catalyst from first principles are explored. Aspects of computational chemistry as well as the kinetics and physical state of the reactive catalyst are discussed.
Angewandte Chemie International Edition in English 06/2014; 53(33). DOI:10.1002/anie.201310965 · 13.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic basis to the surface chemical bond of molecules and atoms chemisorbed to transition-metal surfaces is introduced. The chemical bonding features that determine preference of an adsorbate for different coordination sites are identified. This is related to a discussion of chemisorption as a function of particle size. Lateral effects relevant at high surface coverage are discussed with chemisorption-induced surface reconstruction.The analysis focuses on the relation of the surface chemical bond energy with degree of delocalization of surface transition-metal d-valence electrons and distribution of electrons over bonding and antibonding adsorbate complex fragment orbitals. The model of chemisorption as surface molecule complex formation embedded into the surface of a metal is shown to be a good approximation.This chapter concludes with an analysis of transition states of elementary surface reactions of small adsorbed molecular species. The structure and energy of such transition states are shown to relate in an interesting way to the nature of the chemical bond to be activated (π-bond vs σ bond) and the topology of the reaction center.
The Chemical Bond, 05/2014: pages 269-336; , ISBN: 9783527333158
[Show abstract][Hide abstract] ABSTRACT: Synchrotron X-ray diffraction coupled to atomic pair distribution function analysis and Reverse Monte Carlo simulations is used to determine the atomic-scale structure of Ru nanoparticle catalysts for the Fischer-Tropsch reaction. The rate of CO hydrogenation strongly correlates with the abundance of surface atoms with coordination numbers of 10 and 11. DFT calculations confirm that CO dissociation proceeds with a low barrier on these Ru surface atom ensembles.
Chemical Communications 04/2014; 50(45). DOI:10.1039/c4cc01687d · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Density functional theory (DFT) calculations are used to explore water adsorption and activation on different α-Ga2O3 surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low-index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is significant. Calculations of electron local density of states indicate that the electron-energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on α-Ga2O3 surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.
Chemistry - A European Journal 04/2014; 20(23). DOI:10.1002/chem.201400006 · 5.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fischer–Tropsch synthesis is an attractive process to convert alternative carbon sources, such as biomass, natural gas, or coal, to fuels and chemicals. Deactivation of the catalyst is obviously undesirable, and for a commercial plant it is of high importance to keep the catalyst active as long as possible during operating conditions. In this study, the reactivity of CO on carbon-covered cobalt surfaces has been investigated by means of density functional theory (DFT). An attempt is made to provide insight into the role of carbon deposition on the deactivation of two cobalt surfaces: the closed-packed Co(0001) surface and the corrugated Co(112̅1) surface. We also analyzed the adsorption and diffusion of carbon atoms on both surfaces and compared the mobility. Finally, the results for Co(0001) and Co(112̅1) are compared, and the influence of the surface topology is assessed.
The Journal of Physical Chemistry C 03/2014; 118(10-10):5317-5327. DOI:10.1021/jp4109706 · 4.77 Impact Factor