Library

Notes: Results of dynamic Monte Carlo simulations of a model for CO oxidation on a reconstructing Pt(100) surface are presented. A comparison is made between simulations that explicitly include surface diffusion of adsorbed CO and simulations without diffusion. Oscillatory behavior as well as spatio-temporal pattern formation are studied as a function of system size. In the absence of diffusion the amplitude of kinetic oscillations decreases with grid size and oscillations are not stable. Spatio-temporal patterns appear, as expected for an excitable medium. Such patterns become stabilized by structural substrate defects. The length scale of the patterns is in the order of 10–100 nm, the temporal period of the oscillations is around 200 seconds. Inclusion of diffusion stabilizes and synchronizes oscillations. Spatio-temporal features now appear with larger spatial dimensions. © 1998 American Institute of Physics.

Notes: The adsorption of ammonia on the two low index (111) and (100) surfaces of rhodium has been studied by periodic calculations with density functional theory and compared to experimental results. The geometries of the adsorbates and the surfaces are completely optimized. For both surfaces the top site is found to be the most stable while the adsorption energy of ammonia is 8–10 kJ⋅mol−1 larger on the (100) surface. The presence of steps on the (100) surface has a minor effect on the heat of adsorption. The theoretical predictions of the adsorption energies and the changes in work function by NH3 are in good agreement with experimental data. Moreover the prediction of the ontop adsorption as well as the weak interactions between the adsorbates is confirmed. The broadening of the temperature programmed desorption spectra and the two desorption peaks for the first adlayer are mainly due to an entropy effect which affects the preexponential factor of the desorption rate constant. © 1999 American Institute of Physics.

Notes: A simple lattice-gas model for the electrocatalytic carbon monoxide oxidation on a platinum electrode is studied by dynamic Monte Carlo simulations. The CO oxidation takes place through a Langmuir–Hinshelwood reaction between adsorbed CO and an adsorbed OH radical resulting from the dissociative adsorption of water. The model enables the investigation of the role of CO surface mobility on the macroscopic electrochemical response such as linear sweep voltammetry and potential step chronoamperometry. Our results show that the mean-field approximation, the traditional but often tacitly made assumption in electrochemistry, breaks down severely in the limit of vanishing CO surface mobility. Comparison of the simulated and experimental voltammetry suggests that on platinum CO oxidation is the intrinsically fastest reaction on the surface and that CO has a high surface mobility. However, under the same conditions, the model predicts some interesting deviations from the potential step current transients derived from the classical nucleation and growth theories. Such deviations have not been reported experimentally. Furthermore, it is shown that our simple model predicts different Tafel slopes at low and high potential, the qualitative features of which are not strongly influenced by the CO mobility. The comparison of our simulation results to the experimental literature is discussed in some detail. © 1998 American Institute of Physics.

Notes: Off-lattice diffusion limited cluster aggregation simulations in two dimensions have been performed in a wide volume fraction range between 0.001 and 0.60. Starting from a system of 10 000 monomers with radius 0.5, that follow Brownian trajectories, larger aggregates are generated by bond formation between overlapping aggregates. No rings are present in the nonaged structures. The influence of the initial monomer volume fraction on the fractal properties of the gels is studied and interpreted by calculation of small angle scattering structure factor patterns to find the fractal dimension. It is found that an increase of the volume fraction leads to the development of two distinct fractal regions. The fractal dimension at short length scale shows the diffusion limited cluster aggregation value of 1.45 up to the correlation length, while the long range fractal dimension gradually increases from 1.45 to 2.00, the Euclidean dimension of the simulation space. It is shown that high volume fractions lead to changes in the local morphology of the monomers as a function of the volume fraction with corresponding changes in the high-q region of the structure factor pattern. A new method to simulate aging of aggregates at modest (low pH and temperature) reaction conditions by an off-lattice nonbreaking reorganization process, called shaking, is presented. The shaking process induces small changes in the local mass distribution, causing ring formation, but does not affect the radius of gyration of the aggregates.It is found that shaking leads to the formation of a polydisperse collection of new primary scatterers with a volume fraction dependent radius between 6 and 8 initial monomer radii. For low volume fractions the fractal dimension decreases approximately by 0.2, due to the formation of the new primary scatterers. The shaking process induces changes in the fractal properties at length scales up to ∼40. A simple theory that relates the volume fraction with the correlation length and describes the influence of aging on fractal properties such as monomer radius, fractal dimension, and correlation length is developed. The diffusion limited cluster aggregation simulation results of nonaged and aged gels are compared with this theory. Satisfactory quantitative agreement between simulated and theoretical results is found. © 1995 American Institute of Physics.

Notes: The calculation of adsorption energies in zeolites in the cluster approximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximation, the long-range electrostatic effects of the infinite crystal are ignored and there are boundary effects. In order to remove a part of the disadvantages of the cluster approach, we developed a method to embed a cluster in a zeolite crystal by imposing an electrostatic potential on it. This potential adds the long-range electrostatic effects of the crystal and subtracts the electrostatic potential of the boundary of the cluster. It is calculated from the charge distribution of the crystal obtained with an ab initio calculation using the crystal program. We calculated the adsorption energies of NH3 and NH+4 on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH+4 in the crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorption energies (relative to the crystal calculations) from ≈20 to ≈2–3 kJ/mol.

Notes: The carbon–carbon coupling of C (carbide) and CH (methylidyne), C and CH2 (methylene), and C and CH3 (methyl) on nickel has been studied with density functional theory using 7- and 13-atom cluster models. Formation of CCH3 (ethylidyne) turned out to be the most exothermic reaction on both clusters. Experimentally, CCH3 has been identified unambiguously with a structure perpendicular to the metal surface. Thus for the C/CH3 coupling forming CCH3, we have determined the structure and potential energy surface in the coadsorbed state, transition state, and CC-formed state. The transition state is explicitly determined on the 7-atom cluster and the 13-atom cluster of both nickel and cobalt. We find transition state barriers of 57 kJ/mol for the Ni7 cluster, 55 kJ/mol for the Co7 cluster, 84 kJ/mol for the Ni13 cluster, and 47 kJ/mol for the Co13 cluster. The overall reaction energies are −215, −184, −66, and −89 kJ/mol, respectively. Analysis of the TS shows a dominant contribution of the CC bond to the reaction coordinate. Cluster size effects play a role in all studied CC coupling reactions. © 1995 American Institute of Physics.