Molecular chemisorption of N₂ on IrO₂(110)

We investigated adsorption of N₂ on stoichiometric and O-rich IrO₂(110) surfaces using temperature programmed desorption (TPD) experiments and density functional theory (DFT) calculations. TPD shows that N₂ desorbs predominantly from the stoichiometric-IrO₂(110) surface in a well-defined peak at 270 K for N₂ coverages below about 0.5 ML and that a shoulder centered near 235 K develops in the N₂ TPD traces as the coverage approaches saturation, indicating that adsorbed N₂ molecules destabilize at high N₂ coverages. Experiments of N₂ adsorption onto O-rich IrO₂(110) surfaces provide evidence that N₂ adsorbs exclusively on the coordinatively unsaturated Ir atoms (Ir_cus) of the surface and that pre-adsorbed O-atoms ("on-top" oxygen) stabilize adsorbed N₂ molecules, causing the main N₂ TPD peak to shift toward higher temperature with increasing oxygen coverages. Consistent with prior results, our DFT calculations predict that an N₂ molecule preferentially adsorbs into an upright configuration on an Ir_cus atom of the IrO₂(110) surface and achieves a binding energy of about 100 kJ/mol. The computed binding energy agrees well with our experimental estimate of ∼90 kJ/mol for low N₂ coverages on stoichiometric IrO₂(110). The DFT calculations also quantitatively reproduce the observed stabilization of N₂ by co-adsorption on-top O-atoms and predict the destabilization of N₂ on IrO₂(110) as the N₂ adlayer becomes crowded at high coverages.