DFT Characterization of Adsorbed NH_x Species on Pt(100) and Pt(111) Surfaces

Abstract

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111).

Shape-controlled synthesis of Pt-Ir nanocubes with preferential (100) orientation and their unusual enhanced electrocatalytic activities

“…Density functional theory (DFT) calculations also support that the adsorption of the NH 2 fragment is more favorable on Pt (100) than on Pt (111) [78]. Besides, DFT calculations indicate a much lower adsorption energy of the atomic nitrogen on Pt (100) than that on Pt (111) and Pt (110), slowing down the deactivation process of Pt (100) [78]. This could explain the high activity of the Pt (100) surface.…”

“…However, Pt (111) surface gives a deeper dehydrogenation of NH 3,ads to strongly adsorbed NH and N fragments, which fails to form N 2 at an appreciable rate [74]. Density functional theory (DFT) calculations also support that the adsorption of the NH 2 fragment is more favorable on Pt (100) than on Pt (111) [78]. Besides, DFT calculations indicate a much lower adsorption energy of the atomic nitrogen on Pt (100) than that on Pt (111) and Pt (110), slowing down the deactivation process of Pt (100) [78].…”

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

“…Despite outstanding theoretical studies in the topic of ammonia synthesis over a number of surfaces, 26-28 only few theoretical works have systematically examined the fragmentation of adsorbed NH x species on Pt(111), [29][30][31] Pd(111), 32 and Rh(111). 33 In fact, ammonia dehydrogenation studies over (100) surfaces are to the best of our knowledge not reported in the literature.…”

“…Our recent work 30 assessed the adsorption and relative stability of ammonia and the resulting dehydrogenated NH x species (x ) 0, 1, 2) on Pt(111) and Pt(100) surfaces using periodic slab DFT calculations. NH 3 and NH 2 were found to be more stable on Pt(100) than on Pt(111), while NH and N exhibited similar stabilities.…”

“…Following our previous study, 30 this work systematically investigates the non-oxidative dehydrogenation of ammonia over PGM surfaces. The structure, stability, and reactivity of adsorbed NH 3 , NH 2 , NH, N, and H species have been comparatively analyzed over the (100) and (111) surfaces of the Pt, Rh, and Pd.…”