As revealed by ALMO EDA results, for Cu- and Ag-containing
systems, the CT terms in both directions of the charge flows
contribute significantly to the adsorption energies. However, no
single energetic term dominates the trend. The manifestation of
this is that while H2S, H2O, and CO are the three strongest
adsorbates on bare Cu and Ag cations (H2S > H2O > CO), an
order similar to that exhibited by the alkali metals would be
reproduced by simply excluding the CT terms: H2O>H2S > CO.
In other words, bare Cu and Ag cations would behave just like
bare alkali metals if the CT interactions could simply be removed.
In the presence of the zeolitic framework, though, both the CT
term and non-CT term change significantly from their values
without the zeolitic framework, and the balance of all different
energetic terms leads to a trend in adsorption energy on Cuexchanged
(or Ag-exchanged) Y cluster substantially different
from that on the bare Cu (or Ag) cation. In particular, CO
adsorption becomes significantly stronger than on bare cations,
and it competes with H2S.
On the other hand, on bare alkali metal cations and on alkali
metal-exchanged clusters, the non-CT terms constitute the dominant
contributions to the adsorption energies. All of the features of the
adsorption energies are consistent with those observed for the non-
CT terms. Comparison between the non-CT terms (Figure 8)
shows that the effect of the cation size on the adsorption energy is
mitigated by the presence of the zeolite cluster through the reduction
of the non-CT terms. The extent to which the effect of the cation
size is mitigated by the zeolite cluster varies with the cation type.
This is reflected in the following two trends: the lowering of the
charge of the alkali cations as mentioned in section 3.3 (Li >Na > K),
and the extent to which the adsorption strength of bare cation reduces
to that of the alkali metal-exchanged Y cluster: (Li > Na > K).
This is possibly due to the shielding effect of the zeolitic framework
atoms: the smaller the cation, themore the cationis recessedintothe
S6R and therefore is shielded by the zeolitic framework atoms.