Chapter 4. Incorporation of Inorganic Interaction Sites in Rationally Designed Porous Materials
4.1 Enhanced CO Selectivity and Adsorption Stability in Cu(Ⅰ)Zn@MIL-100(Fe)
4.1.3 Results and Discussion
The increase in the binding strength of Cu(I) and CO molecules upon incorporation of Zn(II) was theoretically investigated via DFT calculations. As shown in Figures 4.1.3 and 4.1.4, for the pristine CuCl (111) surface, the binding energy (ΔB.E.) of CO was significantly higher than that of CO2, indicating chemisorption-like behavior for CO and physisorption-like behavior for CO2. Interestingly, when the CuCl surface was doped with Zn(II), ΔB.E. for CO increased compared to the case of the pristine CuCl surface. Furthermore, the affinity for CO increased as the amount of the Zn(II) dopant increased.
(a) (b)
Front view
Top view
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Figure 4.1.3 Optimized configurations of (a) CO2 and (b) CO on pristine CuCl. Optimized configurations of CO on (c) singly Zn(Ⅱ)Cl-doped and (d) doubly Zn(Ⅱ)Cl-doped surface. Color scheme is same as Figure 4.1.2. Reproduced from ref. 1 with permission from Elsevier B.V., copyright 2020.
Figure 4.1.4 Binding energies (ΔB.E.) for CO and CO2 on pristine and Zn(Ⅱ)-doped CuCl surfaces.
Reproduced from ref. 1 with permission from Elsevier B.V., copyright 2020.
(a) (b) (c) (d)
Front view
Top view
CO2, pristine CO pristine CO 1Zn2+ CO 2Zn2+
0 5 10 15 20 25 30 35
-B.E. (kcal/mol)
32.02
4.00
32.68 33.67
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To elucidate this phenomenon, the change in the electronic structure induced by incorporation of Zn(II) was further studied by density of states (DOS) and orbital analyses. When a CO molecule is adsorbed on the CuCl surface, three orbitals of CO, i.e., 1π,5σ, and 2π*, participate in bonding with Cu near the Fermi level (Figure 4.1.5). As the amount of Zn(II) dopant in-creases, the intensity of the partial DOS corresponding to the 2π* or-bital is slightly reduced, while that corresponding to 5σ increases (Figure 4.1.6a). Furthermore, a new peak emerges around E – Ef = −8 eV, where the corresponding orbital is localized on the Zn(II) dopant and Cu–CO bond that is dominated by the characteristics of the 5σ orbital (Figure 4.1.6b and c). Thus, these results suggest that the improved adsorption of CO in Cu(I)Zn@MIL-100(Fe)-x is attributed to enhanced σ-bonding between Cu and CO.
Figure 4.1.5 (a) Partial density of states (DOS) of Cu d orbital (red line) and CO molecule (black line) on CuCl surface; (b) molecular orbital distribution of 1π, 5σ, and 2π* orbitals of CO molecule on CuCl surface. Color scheme is same as Figure 4.1.2. Reproduced from ref. 1 with permission from Elsevier B.V., copyright 2020.
-10 -8 -6 -4 -2 0 2 4
Partial density of states (a.u.)
E-Ef (eV) Cu (I), adsorbed
CO, adsorbed 1π
5σ
2π*
5σ
2π*
1π 5σ 2π*
(a)
(b)
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Figure 4.1.6 (a) Partial density of states (DOS) of CO molecule on Zn-doped CuCl surface. Molecular orbital distribution of 5σ orbital of CO molecule on (b) singly Zn-doped and (c) doubly Zn-doped surface. Reproduced from ref. 1 with permission from Elsevier B.V., copyright 2020.
In addition to the enhancing effect on CO adsorptivity, incorporation of Zn(Ⅱ) was found to improve the stability of adsorbent under atmospheric air conditions. As described in the following equation, it is known that Cu(Ⅱ) is reduced to Cu(Ⅰ), which is an active form for CO adsorption, while Fe(Ⅱ) site is oxidized to Fe (Ⅲ) with Cl– adsorption.16,27
Fe(Ⅱ) + Cu(Ⅱ)Cl2 → Fe(Ⅲ)-Cl– + Cu(Ⅰ)Cl
Interestingly, when ZnCl2 was added to Cu(Ⅰ)Zn@MIL-100(Fe), the experimental results showed that the vibrational mode of ʋas(Fe(Ⅲ)–O) became considerably more intense than that of normal Cu(Ⅰ)@MIL-100(Fe), suggesting that a larger amount of Fe(Ⅲ) was regenerated during the activation process. This indicates that the reduction of Cu(Ⅱ) to Cu(Ⅰ) inside the MOF may have been enhanced
-10 -8 -6 -4 -2 0 2 4
Partial density of states (a.u.)
E-Ef (eV) Pristine, CO
Zn 1, CO Zn 2, CO
(a)
(b) (c)
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by the presence of Zn(Ⅱ), i.e., the Zn(Ⅱ) species could weaken the interaction between the Cu(Ⅱ) species and MOF framework, facilitating the reduction of Cu(Ⅱ) by Fe(Ⅱ). Therefore, it is necessary to understand the role of ZnCl2 in improving these features of the Cu(I)Zn@MIL-100(Fe) adsorbent.
Based on XPS analyses of the Fe 2p and Zn 2p core-level,1 we speculate that Zn(II) might adsorb Cl– that was expelled from the newly formed Fe(III)-Cl− during the activation process, leading to the regeneration of Fe(II). A possible reduction mechanism is proposed as follows:
(1) Formation of Fe(II) sites:
Fe(Ⅲ)-X– → Fe(Ⅱ)–□ + 1/2 X2 (X = F)
or 2 Fe(Ⅲ)-X– → 2 Fe(Ⅱ)–□ + H2O + 1/2 O2 (X = OH) (2) Reduction of Cu(Ⅱ) to Cu(Ⅰ):
Fe(Ⅱ)–□ + Cu(Ⅱ)Cl2 → Cu(Ⅰ)Cl + Fe(Ⅲ)-Cl− (3) Regeneration of Fe(II) sites:
Fe(Ⅲ)-Cl− + Zn(Ⅱ)Cl2 → Fe(Ⅱ)–□ + Zn(Ⅱ)Cl3·(radical) → Fe(Ⅱ)–□ + Zn(Ⅱ)Cl2 + 1/2Cl2
where □ indicates the ligand vacancy at Fe(Ⅱ) sites. The regeneration of the Fe(Ⅱ) sites after reducing Cu(Ⅱ) to Cu(Ⅰ) may be one reason for the increased Cu(Ⅰ) concentration in the Cu(Ⅰ)Zn@MIL-100(Fe) adsorbents. Additionally, the higher Cu(Ⅰ)/Cu(Ⅱ) and Fe(Ⅱ)/Fe(Ⅲ) ratios in Cu(Ⅰ)Zn@MIL-100(Fe) compared to those in Cu(Ⅰ)@MIL-100(Fe) may account for the superior oxygen resistance of the former upon exposure to atmospheric air as Fe(Ⅱ) might protect the formed Cu(Ⅰ), which could be easily oxidized to Cu(Ⅱ).37
Furthermore, the present DFT calculations show that Zn(Ⅱ) species could form more stable intermediate states when Fe(Ⅲ) is reduced to Fe(Ⅱ). For the DFT calculations, it was assumed that a radical intermediate (i.e., Cl0) could be generated in the process of the redox reaction. As shown in Figure 4.1.7, detachment of the Cl0 radical from the Fe(Ⅲ) site in the cluster model requires a reaction energy of 52.67 kcal/mol (Path 0).
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Figure 4.1.7 (a) Reaction energy profiles for Fe(Ⅱ) generation without (Path 0) and with (Path a and b) Zn(Ⅱ). Note that the production of Cl2 gas was only assumed to finalize the reaction. Optimized configurations of reaction intermediates corresponding to (b) Int 0-a, (c) Int 1-a, and (d) Int 1-b states in (a). For (b)–(d), orange solid lines represent the distances between atoms in Å. Copper, chlorine, carbon, oxygen, hydrogen, iron, and zinc atoms are colored in reddish brown, yellow-green, gray, red, white, blue, and violet, respectively. Reproduced from ref. 1 with permission from Elsevier B.V., copyright 2020.
With the ZnCl2 molecule, two possible adsorption sites were considered in the cluster model, i.e., near the metal node (Path a) and the Cl− ligand (Path b). In Path a, when ZnCl2 is adsorbed near the metal node, −4.32 kcal/mol of energy is released (Figure 4.1.7b), and the subsequent reaction, where the Cl0 radical is detached from Fe(Ⅲ) to produce the neutral ZnCl3 radical, only requires 6.88 kcal/mol (Figure 4.1.7c). For ZnCl2 attached to Cl− (Path b), the binding energy of −13.45 kcal/mol is required to form Fe(Ⅲ)-Cl-ZnCl2 (Figure 4.1.7d). In addition, even though the ZnCl3 radical molecule was isolated from the cluster model, it is still in the lower energy state compared to the Cl0 radical, and a lower heat of reaction from each intermediate state is required (i.e., 29.74 kcal/mol (Path a) and 45.75 kcal/mol (Path b), respectively). These calculations support our hypothesis that the recovery of Fe(Ⅱ) could be facilitated by the addition of Zn(Ⅱ).
Int 0-a Int 1-a Int 1-b
-15 0 15 30 45 60
Relative energy (kcal/mol)
Reaction coordinate Path 0
Path a Path b
0.0
Fe(II) + Cl0
Fe(II) + Cl2 + ZnCl2 52.67
2.56
32.30
25.53
-4.32 Fe(III)-Cl-
-13.45 Int 0-a
Int 1-a
Int 1-b
Fe(II) + ZnCl3•
(a)
(b) (c) (d)
3.65 Å
3.85 Å 2.23 Å
3.55 Å 3.47 Å
2.35 Å
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