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Molecular dynamics (MD) and molecular statics (MS) simulations of crack propagation in the presence of defects in brittle crystalline materials under mode I loading are carried out on the (0 0 1)[1 0 0] crack system using the embedded atom method (EAM) interatomic potential. Substitutional impurity point defects are introduced into a 3D thin-strip slab of 160,000 atoms at various distances from the crack tip. The critical load required for the initiation of crack propagation is obtained, along with the atomic level stress distribution near the crack tip. The results indicate that the critical load is dependent on the defect species, geometry, and position. When located directly at the crack tip, the defects reduce the peak internal stress, increasing the critical load relative to the defect-free system. As the defects are moved away from the crack tip the critical load goes through a minimum and approaches the value of the pure material asymptotically. In addition, the critical loads calculated for the defect-free systems exceed the Griffith value, given by 2 γ s , for a pure brittle material. This difference is investigated by analyzing the lattice trapping phenomenon using a series of constrained energy minimizations (drag method) and the nudged elastic band (NEB) method in a defect free crack system.

A hydrogen atom can either physisorb or chemisorb onto a graphene surface. To describe the interaction of H with graphene, we trained the C-C, H-H, and C-H interactions of the ReaxFF CHO bond order potential to reproduce Density Functional Theory (DFT) generated values of graphene cohesive energy and lattice constant, H2 dissociation energy, H on graphene adsorption potentials, and H2 formation on graphene using the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) processes. The results, generated from the trained H-graphene potentials, are in close agreement with the corresponding results from DFT. The advantage of using optimized CH potentials is, for example, the inclusion of physisorption interactions and quantum mechanical features of chemical bonding in the functional forms of the potentials. The trained CH potentials are utilized to study the energetics of formation of an H2 molecule on graphene using the Eley-Rideal and Langmuir-Hinshelwood processes. Potential energy surfaces for the formation of H2 through ER are generated for the collinear and oblique approach of the second hydrogen atom. Energetics of the formation of H2 through LH is studied for a variety of cases such as when hydrogen atoms are chemisorbed or physisorbed and when hydrogen occupies ortho, meta, or para chemisorption sites. The likelihood of H2 formation through LH for various configurations is discussed. Furthermore, the tunneling probability of an atom through a continuous symmetric/asymmetric barrier is calculated and applied to an adsorbed hydrogen atom on graphene.

Palladium supported on carbon (Pd/C) has been shown to be an effective catalyst for the cross-coupling of potassium p-tolyltrifluoroborate with a variety of aryl bromides and iodides. Yields ranging from moderate to good were obtained using Pd/C in ethanol/water mixtures with potassium carbonate as base at 50 °C under an air atmosphere.

The Suzuki–Miyaura cross-coupling reaction using heterogeneous Pd/C has a homogeneous component. The soluble palladium concentration increases during the reaction reaching a maximum at ca. 90% conversion before falling to

An optimal chiral surface in the cinchona-modified Pt/Al2O3 catalytic system is established for fast enantioselective hydrogenation of ethyl pyruvate. A makeup protocol is used to compensate for the destructive hydrogenation of the chiral modifier, thus maintaining the optimal chiral surface over the course of the hydrogenation reaction. Hydrogenation over the optimal surface (Ptsurface/modifier = 5–12) results in high enantioselectivity (94% ee) under mild conditions (5.8 bar and 17°C) with high turnover frequency (4 s-1) and turnover numbers (pyruvate/modifier>28,000, pyruvate/Ptsurface>5,500).

Several in situ probes for continuously monitoring rate of catalytic reactions under reaction conditions are described. They are reaction calorimetry, measurements of hydrogen uptake in the case of hydrogenation, and infrared spectroscopy. In studying catalytic hydrogenation reactions, for example, these in situ probes provide kinetic details of the reactions from different perspectives over the entire course of the reaction. The reaction calorimetry and the hydrogen uptake measure directly, continuously, and in a non-invasive manner the rate of reaction, while the in situ infrared spectroscopy provides time-resolved compositional information in the liquid phase. A combination of the information thus obtained leads to a clear and coherent kinetic picture of the reaction under study which can greatly facilitate pathway analysis and mechanistic description of the catalytic reaction. In this report, the usefulness of the combination of these in situ probes is illustrated with two examples of heterogeneously-catalyzed hydrogenation reactions.

The influence of reaction conditions on enantioselectivity in the Ru II -(binap)-catalyzed asymmetric hydrogenation of allylic alcohols is discussed. This work highlights the importance of considering kinetic influences in addition to the stereochemical aspects of the chiral catalytic environment in interpreting catalytic behavior in asymmetric hydrogenation reactions.

Examples from both homogeneous and heterogeneous catalytic systems are presented which demonstrate the role that reaction dynamics may play in dictating the ultimate enantioselectivity observed in asymmetric hydrogenation reactions. The hydrogenation of allylic alcohols using Ru(S)-binap and the hydrogenation of α-keto esters using cinchona-modified supported Pt are discussed. This work highlights the importance of considering reaction kinetics in addition to the stereochemical aspects of the chiral catalytic environment in interpreting catalytic behavior in asymmetric hydrogenation reactions.

Due to its ability to provide directly reaction rate data and its in-situ nature, reaction calorimetry has become one of the most powerful probes of reaction pathways and mechanisms of chemical reactions by virtue of providing high-quality kinetic data. In this paper, examples of enantioselective hydrogenation and selective consecutive hydrogenation reactions are presented to demonstrate the high quality of kinetic data obtainable from the reaction calorimetry. They are also used to illustrate the use of reaction calorimetry for elucidating reaction pathways and mechanisms from detailed kinetic and thermodynamic information about individual step involved in multi-step reactions that is otherwise difficult to obtain without calorimetry.

Marked shifts in enantioselectivity from ca. 20 to 60 ee% as a function of reactor agitation speed were observed in the hydrogenation of ethyl pyruvate over cinchona-alkaloid-modified Pt. Concomitant with this shift in enantioselectivity was a change in the observed kinetics from the zero-order substrate dependence typical of gas–liquid diffusion control to a positive dependence on substrate concentration, even for a series of reactions carried out at constant pressure. A strong relationship between the solution hydrogen concentration and this rate and enantioselectivity behavior was confirmed. For systems where enantioselectivity exhibits a positive dependence on hydrogen concentration, the intrinsic ability of a catalyst to effect asymmetric hydrogenation may be masked in a reaction carried out under conditions where gas–liquid diffusion is the rate-limiting step.

The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient behavior at the beginning of the reaction, and a period of rising rate and enantioselectivity was linked to conversion of substrate for a wide range of reaction temperatures and initial substrate concentrations. A “reaction-driven equilibration” of the surface environment for optimal enantioselective catalysis was proposed for these reactions carried out under conditions where moderate enantioselectivities are observed.

Marked shifts in enantioselectivity in the asymmetric hydrogenation of several prochiral substrates were observed as a function of the availability of hydrogen to the catalyst in both heterogeneous and homogeneous catalytic reactions. The key kinetic parameter affecting enantioselectivity was found to be concentration of molecular hydrogen in the liquid phase, [H2], rather than hydrogen pressure in the gas phase, and it was observed that under typical reaction conditions, [H2] could differ widely from its equilibrium saturation value. It was demonstrated that the reported pressure dependence on enantioselectivity may in fact be reproduced at constant pressure for several systems by varying the rate of gas−liquid mass transfer. The general significance of the conclusions suggest that considerations of hydrogen diffusion limitations might be important in other asymmetric hydrogenation studies reported in the literature. For systems where enantioselectivity depends positively on hydrogen pressure, the intrin...

Several unexpected observations were made in studying the hydrogenation of ethyl pyruvate over dihydrocinchonidine-modified Pt. A marked trend of increasing optical yield with increasing conversion was observed over a wide range of reaction temperatures, ultimately reaching a plateau between 54-58% enantiomeric excess at high conversion. In addition, reactions carried out under bulk hydrogen diffusion limitations exhibited a strong suppression in optical yield. These results reveal the dynamic nature of the catalyst surface as the reaction progresses and demonstrate the need to develop a complete kinetic picture of the reaction.

We demonstrate that Pd/C without added ligands catalyzes the Suzuki cross-coupling reaction with aryl chlorides. The ability of heterogeneous Pd to activate the C−Cl bond is explained in terms of a synergistic anchimeric and electronic effect that occurs between the Pd surface and the aryl chloride. Furthermore, the importance of selectivity control following C−Cl bond activation is illustrated by the striking role that solvents play in determining homo- vs cross-coupling pathways of the aryl chlorides.

Palladium supported on carbon (Pd/C) has been shown to be an effective catalyst for the cross-coupling of potassium p-tolyltrifluoroborate with a variety of aryl bromides and iodides. Yields ranging from moderate to good were obtained using Pd/C in ethanol/water mixtures with potassium carbonate as base at 50 °C under an air atmosphere.

The nitro group in 1-(4-nitrobenzyl)-1,2,4-triazole (4NBT) was selectively hydrogenated to the amine using a Pd/C catalyst. Kinetics and reaction pathways were studied using in-situ kinetic tools, i.e. , measurements of heat flow by reaction calorimetry and rate of hydrogen uptake, in addition to analysis of samples taken from the reactor. The hydrogenation reaction follows predominantly a two-step monomeric pathway from 4NBT to the hydroxylamine intermediate and then to the aniline. The first step, hydrogenation of 4NBT to the hydroxylamine, follows nearly zero-order kinetics. Activation energies and heats of hydrogenation associated with each step in the consecutive hydrogenation were determined. The heat of hydrogenation of the second step, hydrogenation of the hydroxylamine (−58 kcal/mol), is slightly less than that of the first step (−65 kcal/mol). However, the activation energy associated with the second step is higher than that associated with the first step. Consequcently, the rate of hydrogenation of the hydroxylamine increases with increasing temperature at a faster pace than that of hydrogenation of the 4NBT does. Hydrogenation via the dimeric pathway, as evidenced by presence of a trace amount of the azoxy intermediate (formed from a coupling reaction of the nitroso and the hydroxylamine intermediates) contributes only ∼0.1% to the overall hydrogenation reaction. The low probability of the reaction following the dimeric pathway may be attributed to the high rate of hydrogenation of the nitroso on the catalyst.

A two-site, two-step kinetic model is proposed to rationalize the observed effects of solution hydrogen concentration on enantioselectivity in the asymmetric hydrogenation of ethyl pyruvate using a dihydrocinchonidine-modified heterogeneous Pt catalyst. The model successfully predicted enantioselectivity at a hydrogen concentration outside the range used in the kinetic fit. This work demonstrates how the perturbation from equilibrium adsorption of the organic substrate on a heterogeneous catalyst may account for the observed effects of pressure on enant ioselectivity. Both positive and negative hydrogen dependences on enantioselectivity may be rationalized using the same model.

The diffusion of molecular hydrogen (H2) on a layer of graphene and in the interlayer space between the layers of graphite is studied using molecular dynamics computer simulations. The interatomic interactions were modeled by an Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. Molecular statics calculations of H2 on graphene indicate binding energies ranging from 41 meV to 54 meV and migration barriers ranging from 3 meV to 12 meV. The potential energy surface of an H2 molecule on graphene, with the full relaxations of molecular hydrogen and carbon atoms is calculated. Barriers for the formation of H2 through the Langmuir-Hinshelwood mechanism are calculated. Molecular dynamics calculations of mean square displacements and average surface lifetimes of H2 on graphene at various temperatures indicate a diffusion barrier of 9.8 meV and a desorption barrier of 28.7 meV. Similar calculations for the diffusion of H2 in the interlayer space between the graphite sheets indicate high and low temperature regimes for the diffusion with barriers of 51.2 meV and 11.5 meV. Our results are compared with those of first principles.