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.
|Number of pages||12|
|Journal||Journal of the American Chemical Society|
|State||Published - 3 Nov 2021|
Bibliographical noteFunding Information:
D.B. and M.S. thank the BENCh graduate school, funded by the DFG (389479699/GRK2455). I.R. gratefully acknowledges the support by Israel Science Foundation, ISF (Grant No. 2187/19), and by the Open University of Israel Research Authority (Grant No. 31044). O.G. acknowledges financial support by the Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-107396GB-I00/AEI/10.13039/501100011033). T.N.K., G.S., M.S., and J.F. acknowledge support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 833404).
Open access funded by Max Planck Society.
© 2021 The Authors. Published by American Chemical Society.