Quasiclassical molecular dynamics simulations are performed to investigate energy dissipation to the (100) and (110) tungsten surfaces upon Eley−Rideal (ER) recombination of H 2 and N 2 . Calculations are carried out within the single adsorbate limit under normal incidence. A generalized Langevin surface oscillator (GLO) scheme is used to simulate the coupling to phonons, whereas electron−hole (e-h) pair excitations are implemented using the local density friction approximation (LDFA). Phonon excitations are found to reduce the ER reactivity for N 2 recombination, but do not affect H abstraction. In contrast, the effect of e-h pair excitations on the ER recombination cross section is small for N 2 , but can be important for H 2 . The analysis of the energy lost by the recombined species shows that most of the energy is dissipated into phonon excitations in the N 2 recombination and into electronic excitations in the H 2 recombination. In all cases, the energy dissipated into e-h pairs is taken away from the translational kinetic energy of the formed molecules, whereas dissipation to phonons, only significant for N 2 , also affects vibration. Interestingly, the electron mediated energy losses are found to be smaller in the case of N 2 when surface motion is allowed.
Using molecular dynamics simulations, we predict that the inclusion of nonadiabatic electronic excitations influences the dynamics of preadsorbed hydrogen abstraction from the W(110) surface by hydrogen scattering. The hot-atom recombination, which involves hyperthermal diffusion of the impinging atom on the surface, is significantly affected by the dissipation of energy mediated by electron-hole pair excitations at low coverage and low incidence energy. This issue is of importance as this abstraction mechanism is thought to largely contribute to molecular hydrogen formation from metal surfaces.
We report accurate
time-resolved measurements of NH
3
desorption from Pt(111)
and Pt(332) and use these results to determine
elementary rate constants for desorption from steps, from (111) terrace
sites and for diffusion on (111) terraces. Modeling the extracted
rate constants with transition state theory, we find that conventional
models for partition functions, which rely on uncoupled degrees of
freedom (DOFs), are not able to reproduce the experimental observations.
The results can be reproduced using a more sophisticated partition
function, which couples DOFs that are most sensitive to NH
3
translation parallel to the surface; this approach yields accurate
values for the NH
3
binding energy to Pt(111) (1.13 ±
0.02 eV) and the diffusion barrier (0.71 ± 0.04 eV). In addition,
we determine NH
3
’s binding energy preference for
steps over terraces on Pt (0.23 ± 0.03 eV). The ratio of the
diffusion barrier to desorption energy is ∼0.65, in violation
of the so-called 12% rule. Using our derived diffusion/desorption
rates, we explain why established rate models of the Ostwald process
incorrectly predict low selectivity and yields of NO under typical
reactor operating conditions. Our results suggest that mean-field
kinetics models have limited applicability for modeling the Ostwald
process.
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