Adsorption of NH3
Figure 6d and 6i show NH3 adsorbed at its most
preferential site – top. Supplementary Data Figure S5 gives the spin
density of the catalyst before and after bonding. On the (111) and (100)
surfaces, ammonia adsorbs on Ir atom 5 (a top position atom). The spin
density drops by ~0.6-1 unpaired electron at that site
(the same magnitude as the drop in density for NH2),
indicating NH3 may be covalently bonding with the top Ir
atom. However, the adsorption energy is still much lower than for
NH2.
Table 6a gives bond lengths and energies of ammonia’s adsorption. Bond
lengths are similar on the (111) surface. On the (100) surface, the bond
length with B3LYP is 0.12 Å longer than with B3LYP-D3, whose bond length
is 0.12 Å longer than with B97-D3. The trend in binding energy is
EB3LYP > EB3LYP-D3> EB97-D3.
Table 6b lists the principal vibrational modes of NH3 in
gas-phase and Ir-adsorbed phase.48–50 For all
gas-phase vibrations, all three methods overestimate the experimental
benchmarks. B97-D3 comes closest to experiment for both of the
stretching modes and the bending mode, significantly reducing the error
involved in ignoring anharmonic frequencies during
calculations.23 After adsorption the stretch
vibrations are red-shifted ~30 cm-1,
and the wag is blue-shifted ~160-200
cm-1. Other theoretical work comes closest to B97-D3
predictions for all except the weak linear stretch between
NH3 and the surface, for which B3LYP comes
closest.38 The wag is a strong IR mode. The symmetric
modes are strongly Raman active (very strong on the B3LYP (100) cluster)
and all other modes are weakly Raman active.