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Abstract

ABSTRACT: We report accurate time-resolved measurements of
NH3 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 NH3 translation parallel to
the surface; this approach yields accurate values for the NH3
binding energy to Pt(111) (1.13 ± 0.02 eV) and the diffusion
barrier (0.71 ± 0.04 eV). In addition, we determine NH3’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.