Atomic and Molecular Adsorption on Ir(111)

Abstract

The chemisorption of atomic (H, O, N, S, C), molecular (N2, CO, NO, NH3), and radical (CH3, OH, NOH) species on Ir(111) has been systematically studied. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using PW91 and RPBE functionals, have been used to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage. The following order in binding energies from least to most strongly bound was determined: N2 < NH3 < NO < CH3 < CO < OH < H < NOH < O < N < S < C. A preference for 3-fold sites for the atomic adsorbates was observed, with the exception of atomic H, which prefers top sites. Molecular species showed a preference for top sites with the exception of NOH; this species preferred fcc sites. Surface relaxation had only a small effect on energetics in most cases. Calculated vibrational frequencies, in general, were in good agreement with experimental frequencies and support experimentally proposed site preferences for all adsorbates where data are available. Finally, the thermochemistry of CO, NO, NOH, N2, NH3, and CH3 decomposition on Ir(111) was examined, leading to predictions for each species of the preference for either desorption or decomposition.

Theoretical Study of the Dissociative Adsorption of Methane on Ir(111): The Role of Steps and Surface Distortions at High Temperatures

“…The fact that on Ir(111), CH 3 adsorption is most stable on top sites is in agreement with DFT results reported previously 36 . The stability ordering of adsorption of CH 3 on the high-symmetry sites we have obtained, is the same as the one reported in ref.…”

“…Nevertheless, keeping fixed the lateral coordinates (x H , y H ) of the H atom on bridge, we obtained E ads = -2.66 eV, a value similar to the ones obtained for the other high-symmetry sites indicating a very low energetic corrugation of the H/Ir(111) PES. The highest stability of top sites for adsorption of H on Ir(111) is in good agreement with experiments [33][34][35] , and the relative stability of the four high symmetry sites considered here is also consistent with previous DFT calculations 5,8,36 . The E ads values reported here are slightly smaller (adsorption more stable) than those reported in ref.…”

“…The E ads values reported here are slightly smaller (adsorption more stable) than those reported in ref. 36 (by between 0.02 eV and 0.17 eV), and larger (adsorption less stable) than those of ref. 5 (by between 0.13 eV and 0.17 eV).…”

Analysis of reaction schemes using maximum rates of constituent steps

“…The next step is to estimate values of binding energies for adsorption of all species on the surface, for example using experimental results from the literature, results from DFT calculations, or scaling relations (20)(21)(22)(23)(24). In addition, it is necessary to estimate values for the entropies of the adsorbed species, for example using results from experimental studies (25), correlations in the literature (26), results from DFT calculations of vibrational frequencies (27)(28)(29)(30)(31)(32)(33)(34), combined with hindered translator and hindered rotor models for the three modes associated with motion parallel to the surface (35,36), or by assuming models for the extent of mobility on the surface. It is now possible to estimate values for lumped equilibrium constants, K ads,i , to form the adsorbed species from the reactants and/or products, followed by calculation of θ p .…”

Energetics of hydrogen coverage on group VIII transition metal surfaces and a kinetic model for adsorption/desorption

“…Mavrikakis and co-workers 30 report D e for H on Ir͑111͒ at 1/4 ML coverage in top position of 63.0 or 60.4 kcal/mol, depending on the density functional used. For the fcc site, the BE becomes 60.0 or 56.5 kcal/mol.…”