An equilibrium ab initio atomistic thermodynamics investigation of hydrogen adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

Abstract

The ab initio atomistic thermodynamics technique was used to calculate the surface free energy diagrams. These diagrams were obtained for hydrogen over the unreconstructed low-index iridium surfaces. Results indicated that the Ir(110) surface acquired the highest hydrogen intake among the low-index surfaces with a coverage of up to 2.33 ML. However, high-pressure hydrogen conditions were essential to maintain the stability of this phase. The Ir(100) surface required −0.81 eV of hydrogen chemical potential to produce a stable H/Ir phase with a 1.0 ML coverage. This potential represented the lowest value among the low-index surfaces. Phase diagram calculations of H/Ir(111) illustrated that the 0.75 ML phase has the highest stability at low to moderate hydrogen pressures. On the other hand, the 1.25 ML-H/Ir(111) phase became the most stable under high-pressure conditions. Finally, Wulff construction was implemented to predict the shapes of iridium nanoparticles in a hydrogen environment. Predictions indicated a truncated-octahedron shape for the nanoparticles irrespective of the value of the hydrogen chemical potential.