Theoretical and experimental studies of hydrogen adsorption and desorption on Ir surfaces

Abstract

We report adsorption and desorption of hydrogen on planar Ir(210) and faceted Ir(210), consisting of nanoscale {311} and (110) facets, by means of temperature programmed desorption (TPD) and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. TPD spectra show that only one H₂ peak is seen from planar Ir(210) at all coverages whereas a single H₂ peak is observed at around 440 K (F1) at fractional monolayer (ML) coverage and an additional H₂ peak appears at around 360 K (F2) at 1 ML coverage on faceted Ir(210), implying structure sensitivity in recombination and desorption of hydrogen on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in recombination and desorption of hydrogen on faceted Ir(210) for average facet sizes of 5–14 nm. Calculations indicate that H prefers to bind at the two-fold short-bridge sites of the Ir surfaces. In addition, we studied the stability of the Ir surfaces in the presence of hydrogen at different H coverages through surface free energy plots as a function of the chemical potential, which is also converted to a temperature scale. Moreover, the calculations revealed the origin of the two TPD peaks of H₂ from faceted Ir(210): F1 from desorption of H₂ on {311} facets while F2 from desorption of H₂ on (110) facets.