Ammonia Dehydrogenation over Platinum-Group Metal Surfaces. Structure, Stability, and Reactivity of Adsorbed NH_x Species

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 NH 3 and its dehydrogenation intermediates (NH x , x ) 0, 1, 2). On the six surfaces investigated, NH 3 adsorbs preferentially on top sites, NH 2 on bridge, and NH and N on hollow sites. However, the adsorption energies of the NH x 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 (NH x f 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 NH 3 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.

Mechanism of ammonia decomposition on clean and oxygen-covered Cu (1 1 1) surface: A DFT study

“…1, which combines with Cu (1 1 1) surface on the top site through N atom. Any attempt to find a minimum of energy in the other symmetric sites leads to the top site after complete optimization, which is in agreement with previous theoretical studies of NH 3 adsorption on transition metals [13][14][15][16][17][18][19][20][21][22][25][26][27]. In the preferred adsorption geometry, NH 3 adsorbs on the surface with atomic N downwardly and the C 3 axis of NH 3 is perpendicular to the surface, which is similar to the previous calculation results on Pd (1 1 1) surface [27].…”

“…The most stable co-adsorption configurations of relevant species on Cu (1 1 1) surface. energies of NH on the Pt (1 1 1) [26], Pd (111) [26,27] and Ir (1 1 1) [5] surfaces, the smaller value indicates a weaker interaction between NH and Cu (1 1 1) surface.…”

“…Especially, periodic DFT calculations with the slab model have been a powerful approach to study the adsorption and dissociation of small molecules on the metal surfaces. Much attention has been paid to ascertaining the adsorption and dehydrogenation mechanism of NH 3 on numerous metal surfaces, such as Fe [11,12], Co [12], Ni [13][14][15][16], Ir [5,[17][18][19][20][21][22], Cu [23,24], Pd [25][26][27], Pt [28][29][30][31][32], Au [33]. Most of these works were based on density functional theory (DFT) http calculations.…”

Theoretical study of adsorption and dissociation of NH3 on the Ir{110}(1×2) surface

“…The Ir-Ir distance increases by 0.013 nm in order to better accommodate the NH 2 species with respect to the clean relaxed surface. These features are similar to NH 2 adsorption on Pd{100}, Pt{100}, and Rh{100} [10] .…”

NH3 adsorption on PtM (Fe, Co, Ni) surfaces: Cooperating effects of charge transfer, magnetic ordering and lattice strain

“…The (111) surface was constructed from this relaxed bulk structure. In the Table II, we report the adsorption energies of NH 3 molecule on elemental metal surfaces as well as on MPt surfaces for the atop positions, which is the most preferred adsorption site for NH 3 on the transition metal surfaces [9,10,11]. For the elemental metal surfaces, NH 3 has highest adsorption energy on Pt (111) surface followed by Ni (111), Fe (110) and Co (0001).…”