Kinetics of NH3Desorption and Diffusion on Pt: Implications for the Ostwald Process

Dmitriy Borodin, Igor Rahinov, Oihana Galparsoro, Jan Fingerhut, Michael Schwarzer, Kai Golibrzuch, Georgios Skoulatakis, Daniel J. Auerbach, Alexander Kandratsenka, Dirk Schwarzer, Theofanis N. Kitsopoulos, Alec M. Wodtke

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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.

Original languageEnglish
Pages (from-to)18305-18316
Number of pages12
JournalJournal of the American Chemical Society
Issue number43
StatePublished - 3 Nov 2021

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© 2021 The Authors. Published by American Chemical Society.


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