Platinum vs. Iridium (effect of catalyst)
Previous calculations using B3LYP on a 15-atom (111) cluster of platinum are directly comparable to the B3LYP results in this study.23
In the ground state of the platinum cluster, there are 12 unpaired electrons. In this study, there are 27, in addition to there being a higher cohesive energy of Ir. Preferential catalyst positions are a close match, the only difference being this study shows a small preference for adsorption of N and NH at the HCP site over the FCC site.
Binding strength increases with the same trend NH3<NH2<NH<N as on iridium. By removing the thermal corrections from this study’s binding energies, they directly compare to the electronic binding energies calculated in the platinum study. The results are as follows: N adsorption is 33 kJ/mol stronger, NH is 40 kJ/mol stronger, NH2 is 26 kJ/mol stronger, and NH3 is 4 kJ/mol weaker on iridium than on platinum. The weaker Ir-NH3 bond is interesting because platinum shows a reduction in spin density of only ~0.2, suggesting a non-covalent dipole interaction, while on iridium the spin density decreases by ~1.2, suggesting a stronger, covalent interaction. Additionally, the shorter bond length of adsorbed NH3 (2.23 Å on platinum and 2.17 Å on iridium) would seemingly indicate a stronger Ir-NH3 bond. For comparison, bond lengths for NH2 are ~0.01 Å longer, for NH are ~0.03 Å longer, and for N are ~0.04 Å longer on Ir than on Pt.