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.