Ammonia dehydrogenation over platinum-group metal surfaces. Structure, stability, and reactivity of adsorbed NHx species
authors Novell-Leruth, G; Valcarcel, A; Perez-Ramirez, J; Ricart, JM
nationality International
journal JOURNAL OF PHYSICAL CHEMISTRY C
keywords DENSITY-FUNCTIONAL THEORY; SELECTIVITY-DIRECTING FACTORS; SINGLE-CRYSTAL SURFACE; HYDROGEN-ADSORPTION; AB-INITIO; MOLECULAR ADSORPTION; TEMPORAL ANALYSIS; PRODUCTS REACTOR; PGM GAUZES; OXIDATION
abstract Periodic DFT calculations using plane waves have been applied to comparatively study the adsorption and decomposition of ammonia on the (111) and (100) surfaces of platinum-group metals (Pd, Rh, Pt). Different adsorption geometries and positions have been studied for NH3 and its dehydrogenation intermediates (NHx, x = 0, 1, 2). On the six surfaces investigated, NH3 adsorbs preferentially on top sites, NH2 on bridge, and NH and N on hollow sites. However, the adsorption energies of the NHx moieties differ considerably from one surface to another. All of the species adsorb more strongly on the (100) than on the (111) planes. Rh(100) provides the maximum stability for the various intermediates. The reaction energies, the structure of the transition states, and the activation barriers of the successive dehydrogenation steps (NKx -> NH(x-)1 + H) have been determined, making it possible to compute rate coefficients at different temperatures. Our calculations have confirmed that ammonia decomposition over noble metal catalysts is structure sensitive. As a general trend, the first dehydrogenation step is rate determining, especially for Pd. In agreement with experiments, Rh is a better catalyst for NH3 decomposition than are Pt and Pd. The former strongly stabilizes the highly dehydrogenated NH and N species and also leads to the lowest activation barriers. For the set of dehydrogenation reactions, a linear relationship between the transition state potential energy and the adsorption energy of the final state has been obtained.
publisher AMER CHEMICAL SOC
issn 1932-7447
year published 2007
volume 111
issue 2
beginning page 860
ending page 868
digital object identifier (doi) 10.1021/jp064742b
web of science category Chemistry, Physical; Nanoscience & Nanotechnology; Materials Science, Multidisciplinary
subject category Chemistry; Science & Technology - Other Topics; Materials Science
unique article identifier WOS:000245005300051
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journal impact factor 4.484
5 year journal impact factor 4.691
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