Stability of adsorbed states and site-conversion kinetics: CO on Ni(100) (original) (raw)
Related papers
Surface diffusion of H on Ni(100): Interpretation of the transition temperature
Physical Review B, 1995
We calculate surface-diffusion coefficients for hydrogen on the (100) face of nickel over a temperature range from 40 to 1000 K. The calculations include tunneling contributions from discrete-energy states. The results are in very good agreement with experiment. We find a dramatic leveling off of the Arrhenius plot at approximately 66 K, below which temperature the diffusion coefficient is virtually independent of temperature. The existence and magnitude of such a transition temperature agrees well with experimental findings and also with previous theoretical work based on path-integral transitionstate theory. The present treatment provides insight into the origin of the effect. We evaluate the transition temperature analytically in terms of local quadratic approximations to the potential and find it to correspond approximately to the temperature at which the various low-energy bound-reactant states contribute equally to the diffusion coefficient. The nearly temperature-independent diffusion rate below the transition temperature corresponds to tunneling primarily from the ground state. The analytical expression for the transition temperature depends strongly on the magnitude of the frequency at the top of the potential barrier. We also demonstrate that this transition temperature does not correspond to a transition from over-barrier activated diffusion to tunneling diffusion, which has been previously proposed, and that the surface-diffusion process proceeds largely by a tunneling mechanism even well above the transition temperature. 'Christmann, Schober, Ertl, and Neumann, Ref. 42.
Diffusion on Metal Surfaces: Formalism and Application to CO Diffusion
The Journal of Physical Chemistry C, 2008
An analytical approach searching for the saddle point on a semiempirical adiabatic potential surface (SP-SE-APES) is proposed in order to study diffusion of adsorbed molecules on a metal surface. Three reaction coordinates are used to construct this potential surface: the perpendicular adsorption height z, the distance x between adsorption sites (top, bridge, or hollow), and coordinate θ describing phonon vibrations of the metal atoms. Harmonic approximation is used to describe the potential energy along the θ coordinate, and anharmonic Morse functions are used to describe the potential energies along z and x coordinates. The theory is applied to study CO diffusion on five transition metals (rhodium, ruthenium, palladium, iridium, and platinum), and the activation energies and the prefactors are calculated for various hopping mechanisms. The one-step top-bridge-top path with the bridge state as a transition state, which was generally assumed to be the dominant mechanism of CO diffusion on these metals, is shown not to be the most likely path. Results of calculation of the diffusion coefficients are compared with experimental data.
Effect of anharmonicity on diffusion jump rates
Physical Review B, 1984
Rate theory is modified to take explicit account of anharmonicity. In general, the "watershed" saddle surface is curved rather than. planar. Certain almost tangential trajectories then cut the saddle surface two or more times without intervening randomization. We calculate explicit saddle surfaces for model fcc crystals using potential-energy functions derived from Morse and Lennard-Jones pair potentials. The theory is developed to determine how the fraction of dynamical return jumps which occur in thermal equilibrium depends on the shape of the saddle surface. For models of Ag, Al, Ar, and Cu we determine that an upper limit of only about 5% of return jumps take place, even at temperatures near the melting temperatures, and still less occur at lower temperatures. The validity of the rate-theory approach is therefore established. The shape of the saddle surface also determines the isotope effect in diffusion. The anharmonic contribution to the isotope effect factor a. does not exceed 2% of the harmonic part for any of the four substances, and the values calculated from the model results appear reasonable when compared with experimentally detern:ined values for metals. The results presented here prepare the way for explicit Monte Carlo calculations of the jump frequency and of its precise dependence on state variables in model crystals.
Atomic jumps during surface diffusion
Physical Review B, 2009
The characteristics of atomic displacements during surface diffusion of Cu on Cu͑111͒ are studied by means of molecular dynamics simulations. It is found that even at very low substrate temperatures, the majority of the jumps are correlated, i.e., the displacement directions are not randomly chosen but rather keep some sort of memory from the previous moves and are influenced by them. Long jumps, spanning several surface unit cells, are observed at all temperatures. From an analysis of their length probability distribution information can be obtained about the mechanisms of friction and energy transfer between the diffusing adatom and the substrate. Both long jumps and recrossings ͑displacements in which the adatom moves back and forth between two adjacent adsorption sites͒ appear with a higher activation energy than normal diffusion. Finally, the influence of the instantaneous atomic configuration of the substrate on the adatom's trajectory is also highlighted.
Coverage and nearest-neighbor dependence of adsorbate diffusion
Chemical Physics, 2005
We present data on the coverage and nearest-neighbor dependences of the diffusion of CO on Cu͑111͒ by time-lapsed scanning tunneling microscope ͑STM͒ imaging. Most notable is a maximum in diffusivity of CO at a local coverage of one molecule per 20 substrate atoms and a repulsion between CO molecules upon approach closer than three adsites, which in combination with a less pronounced increase in potential energy at the diffusion transition state, leads to rapid diffusion of CO molecules around one another. We propose a new method of evaluating STM-based diffusion data that provides all parameters necessary for the modeling of the dynamics of an adsorbate population.
A first-principles analysis of trends in metal-on-metal hopping diffusion for 64 admetal/substrate systems is presented. Focusing on the (100) facets of various transition metal substrates, we demonstrate that the calculated hopping diffusion barriers may be interpreted in terms of the cohesive energies of the admetals and substrates, as well as the lattice constants of the substrates. We further show that general linear relationships exist between the diffusion barriers and the corresponding adsorption energies on each transition metal substrate. The slopes in these Brønsted−Evans− Polanyi relationships are related to the degree of resemblance between the initial states and the transition states for hopping diffusion, and the slopes are found to depend sensitively on the nature of the transition metal substrate. Substrates with higher cohesive energies and smaller lattice constants generally exhibit smaller slopes and, therefore, a closer correspondence between the transition states and the initial states. These relationships, in addition to providing fundamental insights into trends in diffusion across different transition metal surfaces, give a powerful and convenient means of predicting diffusional kinetics from purely thermodynamic quantities. The results may ultimately provide a useful input to kinetic Monte Carlo (kMC)-type simulations, enabling efficient and accurate studies of heteroepitaxial metal-on-metal growth.
Dynamics of two-dimensional diffusional barrier crossing
1987
The time-dependent diffusion equation is solved directly, by a new numerical algorithm, for a two-dimensional potential energy surface. The dynamics gives a qualitative description of isomerization reactions in solution. Along the reaction coordinate, the potential has a double-well shape, and the diffusion coefficient is inversely proportional to solvent viscosity. Along the perpendicular coordinate there are also two wells, the barrier height varies and the diffusion coefficient is assumed independent of viscosity. Under these conditions, in the high-viscosity regime, the diffusional flux can bypass the reaction-coordinate barrier, achieving a higher reaction rate. The calculated rate coefficient shows a non-Kramers, fractional viscosity dependence, in agreement with the isomerization experiments.
Journal of Nano Research, 2009
In a set of recent papers we have shown that the diffusion asymmetry in diffusion couples (the diffusion coefficient is orders of magnitude larger in one of the parent materials) leads to interesting phenomena: i) sharp interface remains sharp and shifts with non Fickian (anomalous) kinetics [1][2][3][4][5], ii) originally diffuse interface sharpens even in ideal (completely miscible) systems [6,7], iii) an initially existing thin AB phase in A/AB/B diffusion couple can be dissolved [8], iv) there exists a crossover thickness (typically between few nanometers and 1m) above which the interface shift turns back to the Fickian behaviour [9], v) the growth rate of a product of solid state reaction can be linear even if there is no any extra potential barrier present (which is the classical interpretation of the "interface reaction control" for linear kinetics) [10]. These latter results will be summarized and reformulated according to the usual expression for linear-parabolic law containing the interdiffusion coefficient, D, and interface transfer coefficient, K. Relation between the activation energies of D and K will be analyzed and compared with available experimental data.