Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis.

Some fundamental concepts of catalysis are not fully explained but are of paramount importance for the development of improved catalysts. An example is the concept of structure insensitive reactions, where surface-normalized activity does not change with catalyst metal particle size. Here we explore this concept and its relation to surface reconstruction on a set of silica-supported Ni metal nanoparticles (mean particle sizes 1-6 nm) by spectroscopically discerning a structure sensitive (CO hydrogenation) from a structure insensitive (ethene hydrogenation) reaction.

Deactivation Kinetics of Solid Acid Catalyst with Laterally Interacting Protons.

Differences in catalyst deactivation kinetics in solid acid catalysis are studied with catalyst models that allow for lateral interaction between protons. Deactivation of a solid acid catalyst with laterally interacting protons induces inhomogeneity of proton reactivity that develops with time. As a consequence, product selectivity changes and deactivation will accelerate.

The challenge of catalyst prediction.

New insights and successful use of computational catalysis are highlighted. This is within the context of remaining issues that prevent theoretical catalysis to be fully predictive of catalyst performance. A major challenge is to include in modelling studies the transient initiation as well as deactivation processes of the catalyst.

Hydride Transfer versus Deprotonation Kinetics in the Isobutane-Propene Alkylation Reaction: A Computational Study.

The alkylation of isobutane with light alkenes plays an essential role in modern petrochemical processes for the production of high-octane gasoline. In this study we have employed periodic DFT calculations combined with microkinetic simulations to investigate the complex reaction mechanism of isobutane-propene alkylation catalyzed by zeolitic solid acids. Particular emphasis was given to addressing the selectivity of the alkylate formation versus alkene formation, which requires a high rate of hydride transfer in comparison to the competitive oligomerization and deprotonation reactions resulting in catalyst deactivation.

Scaling Relations for Acidity and Reactivity of Zeolites.

Zeolites are widely applied as solid acid catalysts in various technological processes. In this work we have computationally investigated how catalytic reactivity scales with acidity for a range of zeolites with different topologies and chemical compositions. We found that straightforward correlations are limited to zeolites with the same topology.

Hierarchical control of enzymatic actuators using DNA-based switchable memories.

Inspired by signaling networks in living cells, DNA-based programming aims for the engineering of biochemical networks capable of advanced regulatory and computational functions under controlled cell-free conditions. While regulatory circuits in cells control downstream processes through hierarchical layers of signal processing, coupling of enzymatically driven DNA-based networks to downstream processes has rarely been reported. Here, we expand the scope of molecular programming by engineering hierarchical control of enzymatic actuators using feedback-controlled DNA-circuits capable of advanced regulatory dynamics.

Particle-based modeling of heterogeneous chemical kinetics including mass transfer.

Connecting the macroscopic world of continuous fields to the microscopic world of discrete molecular events is important for understanding several phenomena occurring at physical boundaries of systems. An important example is heterogeneous catalysis, where reactions take place at active surfaces, but the effective reaction rates are determined by transport limitations in the bulk fluid and reaction limitations on the catalyst surface. In this work we study the macro-micro connection in a model heterogeneous catalytic reactor by means of stochastic rotation dynamics.

Constrained-Orbital Density Functional Theory. Computational Method and Applications to Surface Chemical Processes.

We present a method for performing density-functional theory (DFT) calculations in which one or more Kohn-Sham orbitals are constrained to be localized on individual atoms. This constrained-orbital DFT (CO-DFT) approach can be used to tackle two prevalent shortcomings of DFT: the lack of transparency with regard to the governing electronic structure in large (planewave based) DFT calculations and the limitations of semilocal DFT in describing systems with localized electrons or a large degree of static correlation. CO-DFT helps to address the first of these issues by decomposing complex orbital transformations occurring during elementary chemical processes into simpler and more intuitive transformations.

Simple diffusion hopping model with convection.

We present results from a new variant of a diffusion hopping model, the convective diffusive lattice model, to describe the behavior of a particulate flux around bluff obstacles. Particle interactions are constrained to an underlying square lattice where particles are subject to excluded volume conditions. In an extension to previous models, we impose a real continuous velocity field upon the lattice such that particles have an associated velocity vector.

Lattice model of reduced jamming by a barrier.

We study an asymmetric simple exclusion process in a strip in the presence of a solid impenetrable barrier. We focus on the effect of the barrier on the residence time of the particles, namely, the typical time needed by the particles to cross the whole strip. We explore the conditions for reduced jamming when varying the environment (different drifts, reservoir densities, horizontal diffusion walks, etc.

Quantum chemistry of the oxygen evolution reaction on cobalt(ii,iii) oxide - implications for designing the optimal catalyst.

Density functional theory is used to examine the changes in electronic structure that occur during the oxygen evolution reaction (OER) catalyzed by active sites on three different surface terminations of Co3O4. These three active sites have reactive oxo species with differing degrees of coordination by Co cations - a μ(3)-oxo on the (311) surface, a μ(2)-oxo on the (110)-A surface, and an η-oxo on the (110)-B surface. The kinetically relevant step on all surfaces over a wide range of applied potentials is the nucleophilic addition of water to the oxo, which is responsible for formation of the O-O bond.

Structure Sensitivity of the Oxygen Evolution Reaction Catalyzed by Cobalt(II,III) Oxide.

Quantum chemical calculations and simulated kinetics were used to examine the structure sensitivity of the oxygen evolution reaction on several surface terminations of Co3O4. Active sites consisting of two adjacent Co(IV) cations connected by bridging oxos were identified on both the (001) and (311) surfaces. Formation of the O-O bond proceeds on these sites by nucleophilic attack of water on a bridging oxo.

Light-induced water splitting by titanium-tetrahydroxide: a computational study.

Water oxidation by Ti(OH)4 in the ground and excited states was investigated using density functional (ΔSCF, 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 role of a structure directing agent tetramethylammonium template in the initial steps of silicate oligomerization in aqueous solution.

The understanding of the formation of silicate oligomers in the initial stage of zeolite synthesis is of fundamental scientific and technological importance. 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.

Synergy between TiO2 and Co(x)O(y) sites in electrocatalytic water decomposition.

A computational study 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 is presented. The 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.

The optimally performing Fischer-Tropsch catalyst.

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.

Correlating Fischer-Tropsch activity to Ru nanoparticle surface structure as probed by high-energy X-ray diffraction.

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.

DFT simulations of water adsorption and activation on low-index α-Ga2O3 surfaces.

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.

Molecular simulation of protein encapsulation in vesicle formation.

Liposomes composed of fatty acids and phospholipids are frequently used as model systems for biological cell membranes. In many applications, the encapsulation of proteins and other biomacromolecules in these liposomes is essential. Intriguingly, the concentration of entrapped material often deviates from that in the solution where the liposomes were formed.

Microkinetics of oxygenate formation in the Fischer-Tropsch reaction.

Microkinetics simulations are presented on the intrinsic activity and selectivity of the Fischer-Tropsch reaction with respect to the formation of long chain oxygenated hydrocarbons. Two different chain growth mechanisms are compared: the carbide chain growth mechanism and the CO insertion chain growth mechanism. The microkinetics simulations are based on quantum-chemical data on reaction rate parameters of the elementary reaction steps of the Fischer-Tropsch reaction available in the literature.

Catalyst nano-particle size dependence of the Fischer-Tropsch reaction.

Computational catalytic studies indicate that the elementary reactions that constitute the Fischer-Tropsch reaction strongly dependent on the structure of the catalyst reaction center. Recent experimental evidence is available that, for metallic Fischer-Tropsch catalysts such as Co or Ru, the very small metallic particles show altered catalytic performance. To distinguish between changes in the relative concentration of reaction centres, changes in chemical reactivity, or rate controlling steps, transient SSITKA data are extremely useful.