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.