Apj Tonek Jansen

Monte Carlo simulations of a simple model for the electrocatalytic CO oxidation on platinum

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 f...

Direct versus Hydrogen-Assisted CO Dissociation

The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.

Methane activation and dehydrogenation on nickel and cobalt: a computational study

We have studied the adsorption of CH 3 and H on nickel clusters of various size and shape. As a next step we have chosen a one-layer 7-atom cluster and a spherical 13-atom cluster to model the nickel and cobalt surface and we have studied the adsorption of CH3, CH2, CH, C, and H on these clusters. Starting from gas phase CH4, the formation of adsorbed CH 3 (CH3a) and adsorbed H (H a) is endothermic on all clusters, but the endothermicity is strongly reduced on the 13-atom clusters (142 kJ/mol on Ni7, 135 kJ/mol on Co 7, 30 kJ/mol on Nil3, and 8 kJ/mol on Co13 ). The formation of adsorbed CH 2 (CHEa) and H a from Cn3a is endothermic by 25-40 kJ/mol on all clusters, except on Co 7 (3 kJ/mol exothermic), mainly because of the much stronger adsorption of CH 2 on this cluster. The formation of adsorbed CH (CH a) and H a from CH2a is exothermic on all clusters, but the exothermicity differs a factor two between the 7- and 13-atom clusters (61 kJ/mol on Ni7, 60 kJ/mol on Co7, 27 kJ/mol on Ni13, and 32 kJ/mol on Co13). Finally, the formation of adsorbed C (C a) and H a from CH a is strongly endothermic on the 7-atom clusters, but the endothermicity is again strongly reduced on the 13-atom clusters (92 kJ/mol on Ni 7, 77 kJ/mol on Co 7, 27 kJ/mol on Ni13, and 14 kJ/mol on Cola).

Monte Carlo simulations of chemical reactions on a surface with time-dependent reaction-rate constants

Abstract We present Monte Carlo methods with a correct real-time dependence for simulating chemical reactions on a surface that have reaction-rate constants that may vary in time. Explicit expressions are derived for the simulation of temperature-programmed desorption experiments, where temperature is a linear function of time. Using the first-reaction method it is shown that the computational time per reaction scales only as the logarithm of the linear dimension of the surface.

Monte Carlo simulations of surface reactions

Abstract Numerical simulations based on the Monte Carlo method offer a powerful approach for detailed studies of complex reaction sequences, such as those associated with heterogeneous catalysis. In this article, we summarize some of the recent work based on discrete models for irreversible surface reactions. Particular emphasis is placed on kinetic phase transition, bistability, and oscillatory (nonstationary) reactions. In addition to discussing some of the fundamental aspects of nonequilibrium kinetics, we show through specific examples that explicit Monte Carlo simulations can transcend traditional approaches based on rate-equation methods, in particular those invoking the mean-field approximation. This is particularly the case when local correlations and fluctuations among the reactants are important.

Electronic structure calculations and dynamics of methane activation on nickel and cobalt

The dissociative chemisorption of CH4 on nickel and cobalt has been studied using different cluster models. Density functional theory is used to determine the structure and potential energy surface in the reactant‐, transition state‐, and product region. The transition state is explicitly determined on a single atom, a one layer 7‐atom cluster and a spherical 13‐atom cluster. We find transition state barriers of 41 kJ/mol for a single nickel atom, 79 kJ/mol for a single cobalt atom, 214 kJ/mol for the Ni7‐cluster, 216 kJ/mol for the Co7‐cluster, 121 kJ/mol for the Ni13‐cluster, and 110 kJ/mol for the Co13‐cluster. The overall reaction energies are −34, 6, 142, 135, 30, and 8 kJ/mol, respectively. The higher barrier for the single cobalt atom in comparison with the nickel atom can be attributed to the difference between both atoms in the occupation of the s‐orbital in the lowest lying states. The higher and almost the same barrier for the 7‐atom clusters can be attributed to the intrinsic lower reactivity ...

MCTDH study of CH4 dissociation on Ni(111)

We present results of quantum simulations using the multi-configuration time-dependent Hartree method for the dissociation of CH., on Ni( 111) with a potential-energy surface based on density-function calculations. As varied coordinates we include the distance of the methane molecule to the surface, a C-H distance, and the orientation of the CH,. Taking into account the latter coordinate decreases the dissociation probability. For the range of translational energies we studied, the dissociation occurs via tunneling.

Monte Carlo simulations of a surface reaction model showing spatio-temporal pattern formations and oscillations

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.

Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.

Bond breaking in vibrationally excited methane on transition-metal catalysts

The dissociative adsorption of methane on transition metals is an important reaction in catalysis; it is the rate limiting step in steam reforming to produce syngas, and it is prototypical for catalytic C-H activation. Although the reaction mechanism has been studied intensively, it is not been fully understood yet. A number of molecular beam experiments in which the dissociation energy was measured as a function of translational energy have observed that vibrationally hot CH 4 dissociates more readily than cold CH 4, with the energy in the internal vibrations being about as effective as the translational energy in inducing dissociation. 1‐7 Two inde