[Show abstract][Hide abstract] ABSTRACT: The water-splitting process on electrochemical cells is studied with focus on the energetics of the oxygen evolution reaction at the TiO2-based anodes. New reaction mechanisms are proposed that lead to the decomposition of water molecules on TiO2 clusters. The oxygen evolution reaction at the anode is investigated using electronic structure calculations based on density functional theory (DFT). Simulations are carried out for different cluster sizes (monomers and dimers). For each reaction path, the free energy profile is computed, at different biases, from the DFT energies as well as the entropic and the zero-point energy contributions. The mechanisms of the oxygen evolution reaction explored in the present work are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on cluster approximations of the surface of the photocatalyst. Finally, the representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface.
Full-text · Article · Dec 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: In the literature it is proposed that acrolein formation from propylene occurs via a two-step mechanism. In the first step, propylene is dehydrogenated to form an allyl radical after which a second dehydrogenation step including oxygen insertion occurs to form acrolein. Our previous study showed that direct hydrogen stripping from propylene is not possible on Ag2O (001) surface due to direct propylene oxide (PO) or surface oxametallocycle (OMC) formation. In this study allyl formation from PO and OMC are studied. Preliminary results show that radical formation from PO and OMC are more favorable than desorption of those products.
[Show abstract][Hide abstract] ABSTRACT: The kinetics of synthesis gas conversion on the stepped Rh(211) surface were investigated by computational methods. DFT calculations were performed to determine the reaction energetics for all elementary reaction steps relevant to the conversion of CO into methane, ethylene, ethane, formaldehyde, methanol, acetaldehyde, and ethanol. Microkinetics simulations were carried out on the basis of these first-principles data to predict the CO consumption rate and the product distribution as a function of temperature. The elementary reaction steps that control the CO consumption rate and the selectivity were analyzed in detail. Ethanol formation can only occur on the stepped surface, because the barrier for CO dissociation on Rh terraces is too high; step-edges are also required for the coupling reactions. The model predicts that formaldehyde is the dominant product at low temperature, ethanol at intermediate temperature, and methane at high temperature. The preference for ethanol over long hydrocarbon formation is due to the lower barrier for C(H) + CO coupling as compared with the barriers for CHx + CHy coupling reactions. The C(H)CO surface intermediate is hydrogenated to ethanol via a sequence of hydrogenation and dehydrogenation reactions. The simulations show that ethanol formation competes with methane formation at intermediate temperatures. The rate-controlling steps are CO removal as CO2 to create empty sites for the dehydrogenation steps in the reaction sequence leading to ethanol, CHxCHyO hydrogenation for ethanol formation, and CH2 and CH3 hydrogenation for methane formation. CO dissociation does not control the overall reaction rate on Rh. The most important reaction steps that control the selectivity of ethanol over methane are CH2 and CH3 hydrogenation as well as CHCH3 dehydrogenation.
[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.
[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.
Full-text · Article · Jun 2015 · Physical Chemistry Chemical Physics
[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.
Full-text · Article · Feb 2015 · Physical Chemistry Chemical Physics
[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.
Full-text · Article · Nov 2014 · Physica A: Statistical Mechanics and its Applications
[Show abstract][Hide abstract] ABSTRACT: Mikrokinetik-Simulationen wurden für die Analyse der komplexen Fischer-Tropsch (FT)-Reaktion eingesetzt. In der Zuschrift auf S. 12960 ff. unterscheiden E. J. M. Hensen et al. zwischen drei kinetischen Domänen, in denen die katalytische Leistung der modernen FT-Katalysatoren durch CO-Aktivierung, Produktdesorption bzw. O2-Entfernung bestimmt ist. Ein besseres Verständnis der FT-Reaktion wird für die gezielte Entwicklung verbesserter Katalysatoren für die Syngas-Umwandlung benötigt.
No preview · Article · Nov 2014 · Angewandte Chemie
[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.
Preview · Article · Nov 2014 · Angewandte Chemie International Edition
[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.
No preview · Article · Aug 2014 · Angewandte Chemie International Edition
[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.