resumo
Density functional theory simulations were used to study the mechanism of ammonia oxidation over Pt(100). The conversion of NH3 leading to NHx intermediates upon reaction with adsorbed oxygen-containing species and ultimately forming reaction products (NO, N2O, N-2, and H2O), have been systematically computed. The reaction proceeds via an imide mechanism, while classical mechanisms postulating nitroxyl and hydroxylamine as reaction intermediates may be excluded. The barriers of oxidative NH3 dehydrogenation over Pt(100) are drastically decreased with respect to the nonoxidative dehydrogenation, particularly when the number of hydrogen atoms in the NHx fragment is decreased. Ammonia activation and subsequent NHx dehydrogenation steps are greatly favored by O-ads with respect to OHads on Pt(100). This differs from calculations on Pt(111) due to the metal sharing effect and to the lower stability of adsorbed hydroxyl in the latter facet. Nitrogen-containing products are formed by recombination of chemisorbed N with N (N-2) O (NO), and NO (N2O)Water is formed via recombination of adsorbed OH, regenerating an active O. The O-mediated abstraction of the first proton of NH3 is the slowest dehydrogenation step, whereas NO desorption determines the rate of the overall process. Rate coefficients of the elementary steps involved in the mechanism have been calculated, enabling a microkinetic analysis of the reaction. Our simplified model predicts reasonably well the product distribution obtained experimentally at different temperature, time, and NH3/O-2 ratio.
palavras-chave
DENSITY-FUNCTIONAL THEORY; SELECTIVITY-DIRECTING FACTORS; NITRIC-OXIDE DECOMPOSITION; STEPPED PLATINUM SURFACES; MONTE-CARLO-SIMULATION; NH3 OXIDATION; METHANOL DECOMPOSITION; CATALYTIC-REACTIONS; TEMPORAL ANALYSIS; PRODUCTS REACTOR
categoria
Chemistry; Science & Technology - Other Topics; Materials Science
autores
Novell-Leruth, G; Ricart, JM; Perez-Ramirez, J
nossos autores
Gerard Novell Leruth
Investigadoragradecimentos
Financial support from the Spanish Ministerio de Educacion y Ciencia (Projects CTQU2005-08459-C02-02, CTQ2006-01562PPQ, and Consolider Ingenio 2010, grant CSD2006-003), the Catalan Government (2005SGR-00104), and the ICIQ Foundation is acknowledged. G.N.-L. is indebted to the Universitat Rovira i Virgili for a predoctoral grant. Part of the computer time was provided by the CESCA/CEPBA supercomputer centers.