Molecular chemisorption of N-2 on IrO2(110)

We investigated adsorption of N-2 on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed desorption (TPD) experiments and density functional theory (DFT) calculations. TPD shows that N-2 desorbs predominantly from the stoichiometric-IrO2(110) surface in a well-defined peak at 270 K for N-2 coverages below about 0.5 ML and that a shoulder centered near 235 K develops in the N-2 TPD traces as the coverage approaches saturation, indicating that adsorbed N-2 molecules destabilize at high N-2 coverages. Experiments of N-2 adsorption onto O-rich IrO2(110) surfaces provide evidence that N-2 adsorbs exclusively on the coordinatively unsaturated Ir atoms (Ir-cus) of the surface and that pre-adsorbed O-atoms ("on-top" oxygen) stabilize adsorbed N-2 molecules, causing the main N-2 TPD peak to shift toward higher temperature with increasing oxygen coverages. Consistent with prior results, our DFT calculations predict that an N-2 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 similar to 90 kJ/mol for low N-2 coverages on stoichiometric IrO2(110). The DFT calculations also quantitatively reproduce the observed stabilization of N-2 by co-adsorption on-top O-atoms and predict the destabilization of N-2 on IrO2(110) as the N-2 adlayer becomes crowded at high coverages.