Pt(100)-Catalyzed Ammonia Oxidation Studied by DFT: Mechanism and Microkinetics

Abstract

Density functional theory simulations were used to study the mechanism of ammonia oxidation over Pt(100). The conversion of NH 3 leading to NH x intermediates upon reaction with adsorbed oxygen-containing species and ultimately forming reaction products (NO, N 2 O, N 2 , and H 2 O), 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 NH 3 dehydrogenation over Pt(100) are drastically decreased with respect to the nonoxidative dehydrogenation, particularly when the number of hydrogen atoms in the NH x fragment is decreased. Ammonia activation and subsequent NH x dehydrogenation steps are greatly favored by O ads with respect to OH ads 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. Nitrogencontaining products are formed by recombination of chemisorbed N with N (N 2 ), O (NO), and NO (N 2 O). Water is formed via recombination of adsorbed OH, regenerating an active O. The O-mediated abstraction of the first proton of NH 3 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 NH 3 /O 2 ratio.

A DFT Study on the Adsorption of Formic Acid and Its Oxidized Intermediates on (100) Facets of Pt, Au, Monolayer and Decorated Pt@Au Surfaces

“…These values agree well with the previously reported results on Pt (100) surface with PW91 functional and (2 9 2) supercell (-3.09, -4.25 and 3.79 eV for atop, bridge and hollow sites, respectively) [45]. Ge et al [46] also theoretically studied the binding energies for a c(2 9 2)-O overlayer on Pt(100)-(1 9 1) surface and concluded that the bridge site possessed the strongest binding, followed by four-fold hollow site, while the atop site was the least stable.…”

Interactions between Oxygen Atoms on Pt(100): Implications for Ordering during Chemisorption and Catalysis

“…Methfessel-Paxton smearing [44] (with N = 1 and s = 0.2 eV) is used for efficiency. We perform calculations for various square supercells with areas 1, 2, 4, 5, 8, 9, and 10 a 2 , where a is the surface lattice constant, corresponding to (1 1), c (22) …”

“…One might then question the difference in stability for (2 1)(h) and c (22) ordering, where in both cases each oxygen atom is supported by two Pt atoms. However, the weak NN attractions, which are present only in the former case, can explain the enhanced stability.…”

“…[19][20][21][22] Ge et al [19] showed that at an O coverage of 0.5 ML, oxygen bonds more strongly to the bridge sites than the fourfold hollow (4fh) sites, contrary to earlier theoretical analysis. [23] This conclusion was supported by ref.…”

“…Typically, one expects strong NN repulsion between oxygen atoms on fcc (100) transition metal surfaces, and therefore c (22) ordering for oxygen coverages around 0.5 ML. As we show here from DFT calculations on oxygen on Pt(100), the NN interaction between oxygen atoms adsorbed at the preferred bridge sites is strongly anisotropic.…”