Universal Scaling Relationship to Screen an Efficient Metallic

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Universal Scaling Relationship to Screen an Efficient Metallic Adsorbent for Adsorptive Removal of Iodine Gas Under Humid Conditions: First-Principles Study Hoje Chun, Joonhee Kang, and Byungchan Han J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00721 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

Universal Scaling Relationship to Screen an Efficient Metallic Adsorbent for Adsorptive Removal of Iodine Gas under Humid Conditions: First-Principles Study

Hoje Chun, Joonhee Kang and Byungchan Han*

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 03722, Korea

*Corresponding author E-mail address: [email protected] Tel: +82-2-2123-5759 Fax: +82-2-312-6401

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Abstract Safe control and removal of radioactive iodine gases (I-129 and I-131) leaking from the accidents in chemical factories or nuclear industries are of importance due to its critical damage to the biosphere. We study the adsorptive removal of the off-gaseous iodine using transition metals of group ten and group eleven under humid conditions. First principles calculations enable to capture key adsorption natures of iodine and water molecules on the adsorbent surfaces. The underlying mechanism is analyzed by thermodynamic free energies, electronic structures, and surface work function changes. Our results unveil why silver metal show notably outstanding efficiency for the iodine removal. We propose an innovative and insightful map to guide sorting out the best metal adsorbents and impregnants for dramatic improvement of the adsorptive removal of the radioactive iodine gas. Our study is useful for preventing critical risks from chemical and nuclear accidents.

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1. Introduction Sustainability of our modern society considerably depends on how to supply a large-scale of energy and chemical products, in which nuclear power plants and chemical factories play the substantial role. For example, hundreds of nuclear stations are in charge of about 11 percent of the world’s electricity.1 Although beneficial aspects of the facilities are difficult to deny, it is certainly true that a catastrophic accident can be lethal to human society by leaking of radioactive or toxic gases into our biosphere.2-3 Among the off-gases, radioactive iodine (I) has especially drawn a great attention due to the difficulty in capture its organic compounds. The radioactive isotope I-129 has a very long half-life of 15.7 × 10 6 years4, while I-131 does only 8.02 days but its high volatility is a major concern in the safe removal.5-6 Particularly, radioactive iodine was well known to leak into atmosphere as off-gas by severe accidents in nuclear powerplants such as in Fukushima in 2011 at the conditions of T = 30 ℃ and high relative humidity RH = 95 %.7 In order to remove radioactive iodine, various technologies have been developed to date.8 For instance, activated carbons with a tertiary amine and various kinds of metals as impregnants are commercialized adsorbents to capture radioactive organic iodine gases. It was known that the tertiary amines undergo chemical reactions with methyl iodide to form a quaternary ammonium salt under humid conditions.6,

9-12

The activated carbons, however, have low ignition

temperatures and risks to explode. Li et al. reported a tunable crystalline porous material, a metal-organic framework (MOF), by introducing amine functional groups to overcome the 3



limitations of the activated carbon adsorbents.13-14 Moreover, functionalized solid adsorbents such as cation-exchanged mordenites or MOFs with open metal sites that have high affinity towards iodine species have been extensively studied due to their excellent performance for removing iodine gases.15-22 Most of the technologies rely on selectively stronger interaction of the impregnants, such as triethylenediamine (TEDA) or silver, with iodine than other gases (such as hydrogen or CO2).23-24 High material cost of silver impregnants, however, remains problematic in wider applications of the materials.25 One of the challenging issues in the adsorptive removal of radioactive iodine gas by metallic adsorbents is serious degradation of the efficiency under humid conditions. Thus, it is of paramount importance to identify fundamental mechanism how to decouple the competitive adsorptions of iodine and water.20-21 In our previous study, we evaluated the role of TEDA in CH3I adsorption in activated carbon. It was proposed that the impregnated TEDA substantially reduces activation energy to dissociate CH3I into atomic iodine and CH3-group, even in humid conditions.9 Degradation under humid conditions was also diminished by the introduction of metal species of high interaction energy with iodine forming halides.19, 21 Still, the reason for outstanding performance of silver in adsorbing iodine gas compared to other metals, which can also form metal-iodide, is unclear. Roman and associates studied chemical adsorption of various halogen gases in atomically close-packed metal surfaces. They reported that the behaviors considerably deviated from what was expected from conventional understanding, and were ascribed to strongly coupled chemical interaction between adsorbates and metallic surfaces.26-27 Yoo et al. investigated the surface nanomorphology of Br/Pd as a 4


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function of Br chemical potential to figure out thermodynamic equilibrium crystal structure of Pd nanoparticles.28 Likewise, most of the previous studies was focused on analyzing the bonding natures of various halogen gases in prescribed metals. Yet, understanding on its adsorption mechanism in different metal substrates and the effect of the humidity are rather incomplete. It is certainly an essential step toward improving the removal efficiency of radioactive iodine gas by innovative design of adsorbents or screen of an optimum metal impregnant. In this paper we utilized first principles density functional theory (DFT) calculations to accomplish the goals. This methodology has been successfully applied to such adsorption problems. For example, DFT calculations in heterogeneous catalysis, in which adsorption, diffusion and desorption process play key roles, have identified thermodynamic reaction path, kinetic rate, and often provided design concept for catalysts to substantially improve the activity.26, 29-32 We exploited DFT calculations to unveil the adsorption mechanism of iodine in face-centered cubic (fcc) metals (Ni, Pd, Pt Cu, Ag and Au) under humid conditions. Removal efficiency for iodine was characterized by the Gibbs free energy of the adsorption and its underlying mechanism was analyzed with electronic structure theory and surface work function. Our results clearly proposed why silver is so outstanding in the adsorption efficiency for iodine under humid conditions, and identified a design concept to further improve the removal efficiency

2. Computational methodology and models All DFT calculations were performed using the Vienna ab-initio simulation package (VASP).335



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Electron-core interactions were accounted for by the projector augmented wave (PAW)35 basis

sets and cutoff energy of the plane wave was 520 eV. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)36 exchange-correlation functional was used to substitute core electrons. All calculations were allowed to relax until the total energy and force converged within 10-5 eV and 0.02 eV/ Å, respectively with spin polarization and dipole corrections. The van der Waals (vdW) interaction was considered with DFT-D3 method by Grimme37-38 in all of the calculations. As shown in Figure 1 metal surface was modeled by a slab of five atomic layers stacked into direction and used a periodic boundary condition with 15 Å of vacuum space in the perpendicular direction of the surface to preclude its image interaction. To generate dilute surface coverage, we used a model system up to 3×3×1 size of a unit cell. In geometry optimization and total energy calculation the gamma-centered 5×5×1 kpoint scheme was used to integrate the Brillouin zone and the gamma-point only for a gas molecule such as I2 or H2O. Electronic properties were obtained with the gamma-centered 9×9×1 k-point and PBE exchange-correlation functional after the geometry optimization. Work functions ( Φ ) of the metal surfaces with and without adsorbate were calculated by the energy level difference between vacuum level ( Evac ) and fermi level ( ε F ) as described in equation 1. Φ = Evac − ε F (1)

3. Results and discussion To characterize the adsorption behaviors of iodine and water molecules in the metal surface we 6


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calculated not only their adsorption Gibbs free energies but also density of states (DOS) of electrons, changes of work function change ( ∆Φ ) and d-band center energy of metals induced by the adsorbates. Thermodynamic stability of the adsorbate/adsorbent system was estimated by the energy convex hull diagram obtained by DFT computing. Inspecting the electronic structures enabled us to capture a key descriptor for the adsorptive removal efficiency of the prescribed metal surface. These results were, then, combined to consistently elucidate the outstanding performance of Ag(111) surface toward iodine adsorption that exhibits the scaled-off relation between the iodine and water adsorption, unlike other metals.

3.1. Adsorption energy calculations Alkyl iodides have been experimentally studied that they readily dissociate in most of the metal surfaces yielding chemisorbed iodine atoms and various organic chains.39-40 Thus, we only focused on the atomic iodine adsorption in this study. We defined the adsorption energy of an iodine atom on metal (111) surface as equation 2,

I Eads =

N 1 ( EI / surface − Esurface − I EI2 ) (2) NI 2

where N I , EI / surface , Esurface and EI2 are the number of iodine atoms adsorbed, and the total energies of metal (111) surface with the iodine adsorbate, a pure metal (111) surface and I2 gas molecule, respectively. More negative value of the adsorption energy means a stronger bond strength. We calculated the adsorption energy at a dilute coverage of 1/9 monolayer (ML) to

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decouple the effect of adsorbate-adsorbate interactions. As shown in Table 1, the adsorption energies considerably depend on the kinds of metal surfaces and adsorption sites (atop, fcc, hcp and bridge in Figure 1). The fcc (hcp) sites in the group 10 (11) metals adsorb iodine the most strongly. Iodine atom prefers the group 10 metals to the group 11 for the adsorption. More interestingly, over all of the metals we considered the adsorption energies between the fcc and hcp sites are not significantly different (about 0.11 eV). This signifies that the metal surfaces may adsorb iodine atoms at both of the two sites, which will be conspicuous as the iodine coverage increases. It is noteworthy that the adsorption energy in Ag(111) surface is very weak, even weaker than that in Cu(111) surface. It is not the adsorption energy that makes silver as a good metallic impregnant for the adsorptive removal of iodine gas. There may be other underlying mechanisms letting it be widely used. In order to take the humidity conditions into consideration in the adsorption of iodine, we calculated adsorption energy of water as monomer and hexagonal ring cluster on the metal surfaces. Since water adsorption on various metal surfaces was already extensively studied,41-44 we excerpted some of the results to reduce the computational redundancy such as the H O monomer

identification of adsorption sites. Adsorption energies of a H2O monomer ( Eads2 H 2O hexagonal ring cluster ( Eads

cluster

) were described

H 2O Eads

H 2O Eads

monomer

cluster

as equation 3 and 4, respectively,

= Emonomer / surface − Esurface − EH2O (3)

1 = ( Ecluster / surface − Esurface − 6 EH 2O ) (4) 6 8


) and a

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where Emonomer / surface is the total energy of water monomer adsorbed metal (111) surface,

Esurface represents the total energy of the clean metal (111) surface, EH 2O is the energy of an isolated water molecule and Ecluster / surface is the total energy of the metal (111) surface with the adsorbed water cluster. The adsorption energy was normalized by the number of adsorbed water molecules. Our calculations showed that a H2O monomer is adsorbed on atop with a parallel orientation. As shown in Figure 1 (c), a hexagonal ring cluster H2O forms two distinct kinds of structures: parallel or vertical orientation that H points into the metal surface. Compared to monomer the hexagonal cluster is adsorbed more strongly (Table 2). This is because of an extra stabilization by the hydrogen bonding networks in the cluster. Iodine atoms are more strongly adsorbed (~ -2. 27 eV) than water (~ -0.79 eV) in the metal (111) surface. As discussed, Ag(111) surface does not have the highest adsorption energy for the iodine as well as water among the metals we considered. It implies there should be a certain correlation between the adsorption energies of iodine and water molecules. In addition, water molecules (humidity) may keep the metal surface from the adsorption of iodine atom via surface poisoning effect. This scenario proposes that optimum metal surface should have moderate adsorption energies for iodine and water molecules.

3.2. Electronic properties and bonding characteristics Using an electronic structure theory, we pursued to figure out fundamental origin for the 9



specific excellency of Ag(111) surface for the relatively high chemical inertness toward humidity. A projected density of states (PDOS) of electrons have been widely used to understand covalency of chemical bonding between adsorbates and metal substrates.45 In Figure 2, black dashed line represents total DOS of each metal, and blue and red solid lines do that of the adsorbates (5p orbital of an iodine atom and 2p orbital of oxygen in a water molecule) at the most stable adsorption sites. In iodine adsorption we found strong overlaps between the DOS of iodine and metal surfaces (Ni, Pd, Pt, Cu and Au) resulting in the hybridized bands. For example, in I/Cu(111) surface, the 5p orbital of I splits into an anti-bonding (-2 ~ -1 eV) and a bonding (-5 ~ -4 eV) levels through the hybridization with a d-band of Cu. These electronic structures imply that the iodine forms covalent bondings with the metals (Ni, Pd, Pt, Cu and Au). Contrastingly, in Ag(111) surface, the iodine 5p orbital is clearly outlying from the d-band of Ag, which was also found in previous study.46 Andryushechkin et al. compared the DOS projected on iodine and Ag atoms for a Ag(100)-c(2 × 2)-I structure with that of a sandwich-like silver-iodide (AgI) model. The DOS for the Ag(100)-c(2 × 2)-I structure was to our calculated one for I/Ag(111) system, in which the iodine orbital lies outside the Ag band. Unlikely, DOS for sandwich-like AgI structure was similar to that of our calculated DOS for I/Cu(111) surface, where iodine orbital splits into the hybridized anti-bonding and the bonding levels. These two contrastingly different characteristics in the electronic structures clearly identify completely disparate types of chemical bondings in the iodine adsorption. The DOS for the adsorption of iodine in Ag(111) surface indicates that electron in Ag(111) surface should be completely transferred to the iodine adsorbate, leading to an ionic bonding nature, while there are substantial covalency in other metals. 10


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For the water monomer adsorption, we found two molecular orbitals, 1b1 and 3a1, as shown in Figure 2, mentioned in previous literatures.42-43 It was known that the energy difference between 1b1 and 3a1 is a quantitative descriptor for the bonding strength between metal surface and water molecule: the larger the difference the weaker the adsorption energy. Our results provided in Table 2 agree well with the fact. In order to further rationalize the bonding characteristics of iodine and water with the metal substrates, we applied DFT calculations to obtain a d-band center energy ( ε d ) of each metal. It is well-known that the d-band center energy ( ε d ) is an important indicator for adsorption energy of gas molecules on metal substrate. The key idea of the theory is that the higher the ε d lies with respect to fermi energy the stronger is the adsorbate adsorbed on the metal substrate for the fixed d-band filling.45 We figured out a certain correlation between the d-band center energy and adsorption energies of iodine and water molecules, as plotted in Figure 3. DFT calculations dictate that the adsorption energy of each adsorbate (atomic iodine and water monomer) is overall in linear correlation with the d-band center energy of adsorbent metals, except Ag(111) surface. This provides another clear evidence that iodine adsorption mechanism in Ag(111) surface is different from other metals. Since the theory of d-band center energy is essentially based on the band overlapping with adsorbate orbital, Ag(111) surface should adsorb iodine not by sharing electrons as the covalent bonding but rather by transferring electrons completely like ionic bonding. On the contrary, the adsorption behavior of the water monomer in all metal surfaces including Ag(111) surface lies within the linear correlation predicted by the d-band center energy. The results signify that the bonding characteristics of water molecule in metal 11



surfaces are well captured by the d-band center theory, whereas that of iodine cannot be explained solely by it. We thought that the chemical bonding natures of iodine in the metal substrate should leave its trace the work function change of the metal. Quantum mechanically, electronic wavefunctions in a metal surface extend into vacuum space (here to the z-direction), leading to the formation of a net dipole moment. With adsorption of specific atoms/molecules the wavefunctions of electrons in metal surfaces are redistributed accordingly. This property can be utilized to analyze the bonding characteristics.26, 47-49 According to the spill out mechanism47-48 proposed by a simple jellium model, positively charged adsorbate reduces the surface dipole and hence, the surface work function decreases. Likewise, the induced surface work function change should be positive for the negatively charged adsorbate. On the other hand, adsorbates of noble gas, large organic molecules, or large polarizable atoms are controlled by the pillow effect47-48 that the redistribution of the surface electron density compresses the electronic dipole tail and reduces the work function. Roman et al. studied halide adsorption on metal surfaces and claimed that the bonding types change from ionic to covalent natures as the size of halide increase, due to the enhanced polarization.26 We computed induced surface work function change as shown in Table 1. Our results are in good accordance with the previous finding

26

. The positive change of the work function in Ag

(111) surface as iodine is adsorbed indicates that the bonding nature is ionic, while all the other metals show negative changes of surface work functions meaning the covalent bondings caused by the pillow effect from the large polarizable iodine atom. It is of interest that only the silver 12


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exhibits the completely different type of bonding with iodine unlike other metals (Ni, Pd, Pt, Cu and Au). This may allow the silver to have a moderate binding energy of iodine but a weak interaction with water, which leads to a good candidate for a metal impregnant in the adsorptive removal of iodine in highly humid condition. To verify the conjecture we calculated thermodynamically plausible surface coverages of iodine atoms in Ag(111) surface. It is also important to know that since the amount of the impregnation with silver in an adsorbent is practically limited.

3.3. Surface structures of iodine adsorbed Ag(111) surface We calculated the surface coverages iodine using different sizes of supercell models (Figure S1). Interestingly, iodine adsorption sites of fcc and hcp were degenerated as the iodine coverage increases. At 0.11 ML at the supercell size of 3 × 3 ×1 iodine atoms are adsorbed on hcp site, but as the coverage increases the adsorption energy is almost the same as that of fcc site. At 0.33 ML the iodine atoms prefer the fcc sites to hcp sites with a slight difference. This feature opens up the possibility of appearing various I/Ag(111) surface structures. Accordingly, we estimated the stability of each model system using an area-normalized Gibbs free energy of adsorption,50-53 ∆Gads , in Ag(111) surface defined as

∆Gads =

1 (GI / M − N M µ M − N I µ I ) (5) A

13



where GI / M is the Gibbs free energy of iodine adsorbed metal system and A is the surface area of the corresponding Ag(111) surface. Here, µ M and µ I are the chemical potentials of the metal atom and iodine, respectively. The chemical potential of the metal atom was referenced to a bulk reservoir and that of the iodine was with respect to the dissociation energy of an isolated I2

1 molecule, ∆µ I = µ I − EI2 . 2 As shown in Figure 4, the stability of Ag(111) surface significantly depends on the surface coverage of iodine. The iodine adsorption starts at the value of ∆µ I larger than -1.79 eV. Before the AgI formation starts, p( 3 × 3) configuration is thermodynamically the most stable, and the difference in thermodynamic stability between hcp and fcc sites is only -0.67 meV. This signifies that the p( 3 × 3) structure with iodine at either hcp or fcc site is energetically indistinguishable. Moreover, the adsorption energies at 0.25 and 0.4 ML are similar to

p( 3 × 3) structure, implying that these structures are also formed in the Ag(111) surface but as metastable states, driven by any kinetic or a statistical preference. It is noteworthy that the iodine adsorption is thermodynamically possible only up to 0.5 ML, beyond which bi- or trilayers of iodine adsorbates form AgI surface film structures with Ag atoms from the subsurfaces through surface segregation. Thus, the 0.5 ML should be the thermodynamic limit for adsorptive removal of iodine using Ag(111) surface. To estimate thermodynamic driving force for an initial stage of the surface segregation of Ag atom from the subsurfaces, we calculated the energy cost for exchanging an iodine adsorbate with Ag atom just below at 0.25 and 0.5 ML as shown in Table S1. The energy tremendously 14


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decreases as the iodine coverage reaches 0.5 from 0.25 ML. This is consistent with our result of ∆Gads that I/Ag(111) surface starts to form AgI solid film at beyond a certain coverage of 0.5 ML, and that a full monolayer adsorption of iodine in the Ag(111) surface is thermodynamically not plausible. The thermodynamic limit to the surface coverage of iodine atoms in Ag(111) surface can be also understood by the concept of the energy convex hull obtained from the formation energy

E f ( x) of I/Ag(111) surface as a function of the coverage ( x ) as equation 6, E f ( x) = EDFT ( x) − E0 ( x) (6) where EDFT ( x) is total energy of I/Ag(111) surface at iodine coverage x obtained by DFT calculations and E0 ( x) is a reference energy calculated with respect to the reference systems of a clean (x = 0) and a half monolayer (x = 0.5) covered Ag(111) surfaces. E0 ( x) was defined as E0 ( x) = [ E ( x = 1/ 2) − E ( x = 0)]x + E ( x = 1 / 2) (7) As shown in Figure 5, the energy convex hull diagram shows that I/Ag(111) surfaces with x = 0.33 and 0.4 have substantial stabilities against the decomposition into the reference systems, which is in an accordance with the area-normalized Gibbs free energy diagram of the iodine adsorption (Figure 4). Although not stable as much as the structure with x = 0.33 I/Ag(111) surfaces at x = 0.17, 0.2 and 0.25 are also thermodynamically plausible to form at the corresponding coverages. Interestingly, the adsorption sites of the fcc and hcp become degenerated as x increases. This dual-site adsorption mechanism can lead to more or less 15



disordered configurations of iodine adsorbates in Ag(111) surface. Our calculated formation energy of I/Ag(111) system indicates that the order-disorder transition occurs at x = 0.33 - 0.5, from the p( 3 × 3) to hcp and fcc adsorbed structures as shown in Figure 5. In summary, iodine atom is thermodynamically adsorbed in the Ag(111) surface up to coverage of 0.5 ML at hcp, fcc or both sites, and beyond it Ag atom is driven to segregate to the surface likely leading to the formation of AgI solid thin film. Also, the AgI formation of Ag(111) are in good agreement with the previous experiment observation that Ag atoms form AgI with iodine dosing.16

3.4. Evaluation of metal impregnants for iodine removal Our results indicate that benign properties of the Ag(111) surface for adsorptive removal of iodine even under humid condition are attributed to the decoupled bonding nature of iodine from water molecules. There is, however, a thermodynamic limit to the maximal surface coverage of iodine about a half monolayer. In order to screen optimum metal impregnants, we plotted adsorption energies of iodine versus water monomer/cluster for the different metal surfaces, as shown in Figure 6. As expected, adsorption energies of the two adsorbates in all but Ag metal surface show a highly linear correlation, as predicted by means of the d-band center energy theory. The outlying of the Ag(111) surface from the relationship is very contrasting, showing relatively high chemical inertness to water adsorption but higher selectivity to iodine. We propose that a proper control of the Ag particle size can even further tune adsorptive 16


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removal efficiency toward iodine gas.54-55 We setup two different sized model icosahedron nanoparticles of Ag (1.5 nm size of 147 Ag atoms and 1.0 nm size of 55 Ag atoms) to maximize the exposure of (111) surfaces. Then, we calculated adsorption energies of iodine and water monomer to see any improvement from the bulk Ag(111) surface (Table S1, S2 and Figure S2). Surprisingly, iodine adsorption energy dramatically increases as the particle size decreases, while the adsorption energy of water monomer only marginally changes. Furthermore, the adsorption sites for iodine and water were completely decoupled in that iodine is more likely to be adsorbed in hcp or fcc sites of the terraces, while water is adsorbed in edges or vertices. The results provide a smart design concept to optimize the materials properties for specific aim, for instance, how to maximize the removal efficiency of radioactive iodine gas.

4. Conclusion Using first principles DFT calculations and thermodynamic approaches, we studied the adsorption mechanism of iodine in six close-packed metal surfaces (Ni, Pd, Pt Cu, Ag and Au), and studied the efficiency of adsorptive removal under humid condition. It was found that Ag(111) surface shows outstanding performance in iodine removal by relatively high chemical inertness toward water. We unveiled that the underlying mechanism originates from different types of chemical bondings between iodine and water molecules, ionic and covalent, respectively. The thermodynamic stability of I/Ag(111) surface enabled us to evaluate the maximal surface coverage of iodine as 0.5 ML. This adsorptive removal limit of iodine, however, can be overcome by trapping iodine with AgI formation at higher concentration of iodine. Most 17



interestingly, the scaling relationship between iodine and water adsorption energies allowed to find a design concept to make better materials for the iodine removal, for example, by controlling of particle size and morphology to well decouple the competitive adsorption of iodine and water molecules.

Supporting Information Energy cost to exchange an iodine adsorbate with Ag atom, adsorption energies of iodine as a function of the slab supercell size, and the adsorption energies of iodine and water monomer in different size of nanoparticles.

Acknowledgement This work was supported by the Defense Industry Technology Center (DITC) of Korea (contract grant number UC15000ID) and the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Project No. 2013M3A6B1078882).

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22


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Table 1. Adsorption energies of iodine and work function change as iodine adsorbed on the

Adsorption Energy (eV)

∆઴ (eV) top

fcc

hcp

bridge

Cu

-1.52

-1.89

-2.00

-1.85

-0.13

Ag

-1.45

-1.69

-1.72

-1.66

0.56

Au

-1.09

-1.30

-1.30

-1.26

-0.09

Ni

-1.61

-2.15

-2.10

-2.05

-0.46

Pd

-1.61

-2.27

-2.19

-2.09

-0.38

Pt

-1.57

-2.05

-1.95

-1.89

-0.84

adsorption site for different metal (111) surfaces.

23



Page 24 of 38

Table 2. Adsorption energies of water monomer and hexagonal ring cluster and the energy difference in two molecular orbitals (1b1, 3a1) of H2O. Adsorption Energy (eV)

૚࢈૚ − ૜ࢇ૚ (eV) monomer

cluster

Cu

-0.39

-0.75

1.58

Ag

-0.28

-0.63

1.78

Au

-0.29

-0.65

1.62

Ni

-0.51

-0.79

1.48

Pd

-0.46

-0.75

1.50

24


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Pt

-0.49

-0.75

25


1.37


Figure 1. Model systems for (a) fcc metal (111) surface of 3 × 3 ×1 supercell, (b) different adsorption sites of iodine adsorption and (c) hexagonal ring H2O cluster adsorption geometry.

26


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Figure 2. The density of one-electron states (DOS) for iodine atomically chemisorbed and water monomer adsorbed on the fcc metal (111) surfaces.

27



Figure 3. Relationship between d-band center energy and adsorption energy of iodine and water monomer.

28


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Figure 4. DFT calculated Gibbs free energies of iodine adsorption on Ag (111) surface as a function of the change in iodine chemical potential, ∆µ I . ∆µ I = 0 corresponds to µ I =

1 EI , 2 2

determined from the DFT calculated total energy of I2 molecule. Between two vertical dashed lines, left one represents the initialization of the iodine adsorption on Ag(111) surface and the other represents the start of the AgI formation.

29



Figure 5. DFT calculated formation energies and energy convex hull for the I/Ag(111) surface. Inset figures are ground state structures at corresponding coverage.

30


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Figure 6. Scaling relationship between water and iodine adsorption energies. Star point represents the optimal material for an iodine removal.

31



Figure 1. Model systems for (a) fcc metal (111) surface of 3×3×1 supercell, (b) different adsorption sites of iodine adsorption and (c) hexagonal ring H2O cluster adsorption geometry. 719x719mm (96 x 96 DPI)


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Figure 3. Relationship between d-band center energy and adsorption energy of iodine and water monomer. 400x640mm (96 x 96 DPI)



Figure 4. DFT calculated Gibbs free energies of iodine adsorption on Ag (111) surface as a function of the change in iodine chemical potential, ∆µI. ∆µI=0 corresponds to ∆µI=1/2EI2, determined from the DFT calculated total energy of I2 molecule. Between two vertical dashed lines, left one represents the initialization of the iodine adsorption on Ag(111) surface and the other represents the start of the AgI formation. 508x381mm (96 x 96 DPI)


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Figure 5. DFT calculated formation energies and energy convex hull for the I/Ag(111) surface. Inset figures are ground state structures at corresponding coverage. 500x381mm (96 x 96 DPI)



Figure 6. Scaling relationship between water and iodine adsorption energies. Star point represents the optimal material for an iodine removal. 550x381mm (96 x 96 DPI)


Page 36 of 38

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3x2mm (600 x 600 DPI)


Universal Scaling Relationship to Screen an Efficient Metallic

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