Density Functional Theory Study on the Adsorption ... - ACS Publications

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Density Functional Theory Study on the Adsorption of H2S and Other Claus Process Tail Gas Components on Copper- and Silver-Exchanged Y Zeolites Chun-Yi Sung,† Saleh Al Hashimi,‡ Alon McCormick,† Michael Tsapatsis,† and Matteo Cococcioni*,† † ‡

Chemical Engineering and Materials Science Department, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemical Engineering, Abu Dhabi Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates

bS Supporting Information ABSTRACT: The potential use of Cu- and Ag-exchanged Y zeolites as selective adsorbents for hydrogen sulfide (H2S) from Claus process tail gas was investigated with density functional theory (DFT). The adsorption energies of H2S and other Claus tail gas components (CO, H2O, N2, and CO2) were computed for these zeolites as well as for Li
Y, Na
Y, and K
Y on a cluster model. Comparison of adsorption energies for H2S versus the other components indicated that Ag
Y has potential for selective adsorption of H2S, whereas Cu
Y is subject to strong adsorption of CO, and alkali metal-exchanged Y zeolites are subject to H2O adsorption. Comparison with alkali metalexchanged Y zeolites was performed to clarify the role of d electrons, while the influence of the zeolite framework was assessed by comparing adsorption energies on the cluster model with those on bare cations. Absolutely localized molecular orbital energy decomposition analysis (ALMO EDA) revealed that for Cu- and Ag-containing systems, transfer of electrons between the cation and the adsorbate, i.e., the donation of d electrons and the acceptance of electrons in the unoccupied orbitals of the cation, plays an important role in determining the adsorption energy. On the other hand, for alkali metals-containing systems, charge transfer is negligible and adsorption energies are dominated by interactions due to electrostatics, polarization, and structural distortions.

1. INTRODUCTION Desulfurization is an operation required for many industrial processes such as refining of transportation fuels (gasoline, diesel, and jet fuels), natural gas processing, and fuel cells applications.1
3 The removal of sulfur from transportation fuels was driven by the stringent regulations in the past decade that required cleaner fuels4 with lower emission of sulfur dioxide (SO2), one of the major elements forming acid rain. In the natural gas processing and petroleum refinery, the removal of sulfur is needed because even low levels of sulfur can poison the catalysts severely in the catalytic reforming units.5,6 Deep desulfurization is also needed to prevent sulfur poisoning of the electrode in fuel cells.7 One of the common sulfur-containing species is hydrogen sulfide (H2S). For example, the amount of H2S from the wellhead of raw natural gas ranges from parts per million up to 5%.8 The current technology for desulfurization is hydro-desulfurization (HDS), a catalytic process operated at elevated temperatures and pressures using metal oxide-based sorbents.9 In the HDS process, H2S is removed by solvent extraction using amine solutions.10 The H2S removed from the HDS process is then converted into elemental sulfur in a Claus process.11,12 The tail r 2011 American Chemical Society

gas coming out of the Claus process contains H2S ranging from a few parts per million to ∼0.1% and other gaseous components such as CO2, CO, water vapor, and N2 that are much richer in concentration than H2S, as noted in our previous paper.13 Therefore, the development of a process that can selectively remove refractory H2S at low concentrations in the presence of other gases such as CO2, CO, water vapor, and N2 is needed. Adsorptive desulfurization for hydrocarbon fuels has been extensively investigated as an alternative method over the HDS process due to its simplicity and effectiveness. Due to the high surface area, size-selective adsorption property, and the large number of active cation sites, cation-exchanged zeolites are one type of material that has been evaluated as possible adsorbents for sulfur compounds. Various cation-exchanged zeolites have been tested;1
3,14
19 among them, Cu(I)- and Ag(I)-exchanged Y zeolites have been reported as attractive sorbents for desulfurization of transport fuels and natural gas.1
3 However, these studies were conducted with a focus on sulfur compounds bulkier than H2S. Adsorption selectivity is expected to Received: October 10, 2011 Revised: December 23, 2011 Published: December 27, 2011 3561

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The Journal of Physical Chemistry C be a crucial factor in determining the superiority of a specific adsorbent for the removal of H2S and represents the focus of this work. Our previous study13 showed that Ag
Y has better selectivity for H2S adsorption than Cu
Y. Since copper is more economical, this study aims at better understanding of the mechanisms that govern the adsorption energies on the two cation-exchanged zeolites, which might provide insights to tune the selectivity of Cu
Y or to identify other cation-exchanged zeolites as selective sorbents for H2S in the future. Quantum calculations are now widely used to study adsorption on zeolites.1,19
24 The employment of these types of calculations to study zeolites is facilitated by the zeolite welldefined crystalline structure and the determination of locations of active sites from experiments.25,26 Often, computational studies are also performed24,27 to gain atomic level understanding and complement experiments. In this work, we performed density functional theory (DFT) calculations on cluster models that represent the zeolite framework to study the adsorption properties of H2S and other components such as CO, H2O, N2, and CO2 on cation-exchanged Y zeolites. The use of the cluster model is supported by the qualitative and quantitative agreements with periodic calculations reported in our previous study13 and by van Santen et al.28 We are mostly interested in Cu- and Ag-exchanged Y zeolites, but to elucidate the role of d electrons in the adsorption, alkali metal-exchanged Y zeolites were also considered as an alternative group of monovalent cationexchanged zeolites for comparison. Three alkali metal-exchanged Y zeolites, i.e., Li
Y, Na
Y, and K
Y, are included to study the effect of the cation size. To analyze the calculated adsorption energies on the zeolite cluster, several further calculations were performed. First, simplified calculations, including only the adsorbate and the bare cation, were performed to assess the effect of the zeolitic framework on the adsorption properties. Second, natural orbital analysis (NBO)29,30 was employed to reveal the changes in the charges of the adsorbate, the cation, and the zeolite cluster (when present) upon adsorption. Third, absolutely localized molecular orbital energy decomposition analysis (ALMO EDA)31,32 was performed to decompose the adsorption energy into components of different physical origin, as further described in the following section. Last, significant complementary occupied-virtual orbitals pairs (COVPs),32 with major contributions to the charge-transfer interactions for selected adsorption complexes, were identified and visualized. The simplified calculations showed that while the presence of the zeolite does not change radically the trend in the adsorption energies for alkali metal cations, in the case of transition metal cations, it enhances the CO adsorption markedly relative to all other adsorbates. On the basis of NBO analysis, no clear correlation could be found between the changes in the charges and the adsorption energies. Therefore, ALMO EDA was performed to understand the fundamental nature of the interactions between the aforementioned adsorbates and cationexchanged zeolites. It was found that, for transition metal cations, the charge transfers in both directions between the adsorbate and the cation contribute significantly to the adsorption energies.33 Such a synergic process, i.e., the donation of electrons from the filled lone electron pair orbital of the ligand into an empty orbital of the metal (forward donation) and the release of electrons from an nd orbital of the metal into the empty π*-antibonding orbital of the ligand (backward donation) is common for transition metals.34
37 The role of the zeolitic framework in stabilizing the adsorbates through charge transfer in a particular direction was

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Figure 1. Structure of faujasite zeolite showing site II (a) and top view of the cluster model including a single six ring (S6R) and a cation M at site II (b), where M could be a Li, Na, K, Cu, or Ag atom. Four topologically distinct framework oxygen atoms are labeled as O(1), O(2), O(3), and O(4). Atoms that were allowed to relax are shown in ball and stick representation, and terminal hydroxyl groups that were held fixed are shown in tube representation. The default color palette in GaussView is used: aluminum atoms are shown in pink, silicon atoms are shown in green, oxygen atoms are shown in red, and hydrogen atoms are shown in light gray.

also examined by comparing the charge-transfer terms of that direction in the simplified and the cluster calculations. The COVPs analysis showed that the types of orbitals responsible for the charge transfer between the adsorbate and the transition metal cations were invariant with the cation type.

2. COMPUTATIONAL METHODS Extra framework cations in faujasite zeolites are found in different crystallographic sites. The definitions of the different conventional cation sites can be found elsewhere.38 Among them, site II has been reported to be a site occupied by all extra framework cations considered in this work (Li, Na, K, Cu, and Ag),38 and is considered a site of interest to study adsorption due to its accessibility by adsorbates. For this reason, although other sites may contribute to adsorption, we focus here on site II. Quantum chemical calculations are usually performed using cluster models around the site of interest to study adsorption or reactions on zeolites that have large unit cells.1,19
24,27,39,40 In our previous study,13 we performed DFT calculations of the adsorption on periodic Cu
Y, Ag
Y, and Na
Y zeolites and on cluster models of the local environment around site II. The comparison between results obtained from the two types of calculations show that the cluster model contains enough structural freedom to accurately represent the adsorption geometry and the adsorption energy trends. Therefore, the cluster model of the local environment of site II of the Y zeolite was employed in this work to reduce computational cost. The cluster we used contains a single six ring (S6R), composed of five SiO4 and one AlO
4 tetrahedra, next to site II of the crystal structure of Y zeolite41 (Figure 1). The dangling bonds were capped by hydrogen atoms directed along the bond vector of what would have been the next zeolite framework atom in the crystal structure. The bond lengths of the terminal OH groups were set to be 0.98 Å, which is within the range of possible OH bond lengths.42 The positions of the terminal OH groups were fixed, while the coordinates of all other atoms were allowed to vary during geometry optimizations. When a Si atom is replaced by an Al atom in the S6R, a cation is needed to counterbalance the negative charge of AlO
4. 3562

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Figure 2. Side views of the optimized structures of (a) Cu
Y, (b) Ag
Y, (c) Li
Y, (d) Na
Y, and (e) K
Y cluster models at site II. Atomic representations can be referred to in the caption for Figure 1. The Al atom is in the far left and not visible.

Figure 3. Top (upper) and side (bottom) views of the structures of adsorbed (a) CO, (b) H2O, (c) H2S, (d) N2, and (e) CO2 on the Cu
Y cluster model. Atomic representations can be referred to in the caption for Figure 1. The optimized geometries of CO, H2O, H2S, N2, and CO2 on the Cu
Y cluster were also presented in our previous work.13

In this study, the charge balancing cations are Cu, Ag, Li, Na, and K to represent CuI
Y, AgI
Y, Li
Y, Na
Y, and K
Y zeolites, respectively, as shown in Figure 2. Geometry optimizations were performed with Gaussian 0943 with B3LYP functional.44 Stuttgart RSC 1997 basis set containing effective core potentials (ECP)45,46 was used for transition

metals. The 6-311+G(2d,p) and the 6-31+G(d) basis sets were used for alkali metals and all other atoms, respectively. The longrange van der Waals (vdW) interactions were assumed to have minimal effect on the adsorption energies on the basis of the consistent trend in the adsorption energies on the Cu
Y cluster obtained with B3LYP and ωB97X-D47 functionals as shown 3563

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Figure 4. Optimized structures of adsorbed (a) CO, (b) H2O, (c) H2S, (d) N2, and (e) CO2 on bare Cu cation. Atomic representations can be referred to in the caption for Figure 1.

Figure 6. Adsorption energies of small molecules adsorbed on (a) bare alkali cations, (b) bare alkali cations with the adsorbates and the cations constrained at their atomic positions from the cluster calculations, and (c) alkali metal exchanged Y zeolite clusters.

geometry optimization calculations: ΔE ¼ EðZÞ þ EðXÞ
EðZ 3 XÞ

Figure 5. Adsorption energies of small molecules adsorbed on (a) bare Cu and Ag cations, (b) bare Cu and Ag cations with the adsorbates and the cations constrained at their atomic positions from the cluster calculations, and (c) Cu
Y and Ag
Y zeolite clusters.

in Figure S1 of the Supporting Information. Geometry optimizations of the isolated adsorbate molecules, of the bare cationexchanged zeolite clusters, and of the adsorption complexes were carried out separately. The adsorption energy of each adsorbate was calculated from the electronic energies of three independent

ð1Þ

where E(Z 3 X) is the energy of the adsorption complex, E(Z) is the energy of the bare zeolite cluster, and E(X) is the energy of the isolated adsorbate molecule. Besides the cluster calculations, we also performed two types of simplified calculations to study, by comparison, the effect of the zeolitic framework on the adsorption properties. In both types of simplified calculations, only an adsorbate and a cation were included. In the first kind of these calculations, the zeolitic framework atoms were removed from the optimized structures of the adsorption complexes obtained from simulations on the zeolite cluster, and single-point calculations were performed for the adsorbate and the bare cation with a total charge of +1. The differences in adsorption energies and electronic structure between these calculations and those obtained with the cluster model is indicative of the effect of the zeolite framework. In the second type of simplified calculations, the adsorbate and the cation 3564

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Table 1. Natural Bond Orbital Analysis for Total Charges of the Adsorbate, Copper and Silver, and the Zeolitic Framework Atoms If Presenta cluster calculations adsorbate

bare cation calculations with atomic positions from cluster calculations

cation

zeolite

adsorbate

cation

none

0.77 ([Ar]4s0.193d9.904p0.13)


0.77

CO H2O

0.09 0.07

0.35

0.64 ([Ar]4s 3d 4p ) 0.75 ([Ar]4s0.293d9.854p0.10)


0.73
0.82

0.11 0.08

0.89 ([Ar]4s0.253d9.844p0.02) 0.92 ([Ar]4s0.143d9.934p0.01)

H2S

0.17

0.64 ([Ar]4s0.323d9.884p0.16)


0.81

0.24

0.76 ([Ar]4s0.283d9.934p0.02)

0.01

0.25

Cu

N2 CO2

0.03

9.73

0.27

1.0 ([Ar]3d10)

0.22

)


0.76

0.02

0.98 ([Ar]4s0.133d9.884p0.01)

0.16

)


0.77

0.02

0.98 ([Ar]4s0.023d10)

0.83 ([Kr]5s0.084d9.985p0.13)


0.83

0.31

5p

0.27


0.77

0.09

0.91 ([Kr]5s0.194d9.895p0.02)

5p

0.10


0.83

0.06

0.94 ([Kr]5s0.064d9.985p0.01)

0.16

0.75 ([Ar]4s

0.16

0.74 ([Ar]4s

9.77

3d

9.93

3d

4p 4p

Ag none CO H2O

0.10 0.05

0.67 ([Kr]5s

0.15

0.78 ([Kr]5s

9.80

4d

9.95

4d

) )

1.0 ([Kr]4d10)

H2S N2

0.16 0.06

0.64 ([Kr]5s 4d 5p ) 0.74 ([Kr]5s0.174d9.905p0.18)


0.80
0.80

0.20 0.03

0.80 ([Kr]5s0.214d9.975p0.02) 0.97 ([Kr]5s0.054d9.965p0.01)

CO2

0.04

0.78 ([Kr]5s0.094d9.985p0.16)


0.82

0.02

0.98 ([Kr]5s0.024d10)

0.25

9.93

a

The electronic configuration of the cation is also shown in parentheses. In bare cation calculations, the cations and the adsorbates were held at their positions from the optimization cluster calculations (see text).

were allowed to relax around each other with a total charge of +1. The difference in the energetics between these two types of calculations for a given adsorbate
cation complex gives the energy penalty that is due to the geometric constraints imposed by the zeolite framework. The adsorption energies of these two types of simplified calculations were computed with respect to isolated adsorbate molecules and bare cations. Natural bond orbital analysis (NBO)29,30 was carried out to obtain the total charges of the adsorbate, the cation, the zeolite cluster, and also the electron configurations of the cations. In ALMO EDA, a system is divided into two or more molecular fragments. The occupied molecular orbitals (MOs) on a fragment are constructed from only the atomic orbitals of the same fragment,31,32 as opposed to conventional MOs that are delocalized over the entire system. The constructed MOs that are localized on fragments are called absolutely localized molecular orbitals (ALMOs). Such a localization constraint enables decomposition of the overall binding energy between fragments into the following terms: ΔE ¼ ΔEGD þ ΔEFRZ þ ΔEPOL þ ΔECT þ ΔEHO

ð2Þ The geometry distortion (GD) term is the energy penalty required to distort the isolated fragments from their equilibrium geometries into the geometries they have in the complex. The frozen density (FRZ) term is the energy change resulting from bringing the infinitely separated distorted fragments into the complex geometry without any relaxation of the MOs on the fragments (apart from readjustments to satisfy the Pauli principle). The FRZ term constitutes a combination of Coulomb and exchange-correlation interactions. The polarization (POL) term accounts for the energetic stabilization due to the intramolecular relaxation of each fragment’s absolutely localized MOs in the presence of the other fragment(s). The charge-transfer (CT) term is the difference between the energy obtained with the relaxed

ALMOs (i.e., after polarization has been allowed) and that from the self-consistent (SCF) calculation with fully delocalized MOs. The CT term includes the energy lowering due to electron transfer from occupied orbitals on one fragment to virtual orbitals of another fragment and further energy change induced by the occupied-virtual mixing. The occupied-virtual mixing term can be decomposed further into forward and backward components through a perturbative single Roothaan step correction.48 Induction effects accompanying occupied-virtual charge transfer are included in the higher order (HO) term and are generally small.31 Lastly, the bondings between fragments are represented in terms of just a few localized orbitals called significant COVPs.32 The visualization of significant COVPs for adsorbed CO and H2S is presented in the following section. ALMO EDA calculations were performed with QChem 3.249 with B3LYP functional on the Gaussian-optimized geometries of the adsorption complexes. The same ECP basis set was used for transition metals, and the 6-311++G(2d,p) basis set was used for all other atoms. Basis set superposition error (BSSE) corrections evaluated by the counterpoise method50 were taken into account.

3. RESULTS 3.1. Structure. 3.1.1. Zeolite Y Clusters with Different Cations. The side views of the optimized structures of Cu-, Ag-, Li-, Na-, and K-exchanged Y zeolite cluster models are shown in Figure 2. The cation positions are described here with respect to the O(2) framework atoms, one of the four topologically distinct framework oxygen atoms as shown in Figure 1b. Two geometric parameters associated with the cation and O(2) have been given in Table S1 of the Supporting Information: the distances between the cation and three O(2) atoms in the S6R, and the height of the cation from the plane formed by three O(2) atoms in the S6R as shown in Figure 2. For all five cations, the three interatomic distances between the cation and O(2) atoms show that the cation resides more closely to the framework O atom next to the Al atom and thus slightly off-center 3565

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Table 2. Natural Bond Orbital Analysis for Total Charges of the Adsorbate, Alkali Cation, and the Zeolitic Framework Atoms If Presenta cluster calculations adsorbate

bare cation calculations with atomic positions from cluster calculations

cation

zeolite

adsorbate

cation

Li none

0.80 ([He]2s0.052p0.15)


0.80

1.0 ([He])

CO H2O

0.17 0.05

0.56 ([He]2s0.132p0.30) 0.69 ([He]2s0.072p0.22)


0.73
0.74

0.05 0.02

0.95 ([He]2s0.032p0.02) 0.98 ([He]2s0.012p0.01)

H2S

0.12

0.62 ([He]2s0.102p0.273p0.01)


0.74

0.06

0.94 ([He]2s0.042p0.02)

0.10

0.10

N2 CO2

0.06

0.26

)


0.73

0.02

0.98 ([He]2s0.012p0.01)

0.22

)


0.74

0.02

0.98 ([He]2s0.01)

0.86 ([Ne]3s0.043p0.10)


0.86

0.69 ([Ne]3s0.093p0.21)


0.80

0.03

0.97 ([Ne]3s0.023p0.01)

0.14


0.82

0.01

0.99 ([Ne]3s0.01)

0.16

0.63 ([He]2s

0.08

0.68 ([He]2s

2p 2p

Na none CO H2O

0.11 0.02

0.05

0.80 ([Ne]3s

3p

)

1.0 ([Ne])

H2S N2

0.04 0.07

0.76 ([Ne]3s 3p ) 0.74 ([Ne]3s0.073p0.18)


0.80
0.81

0.04 0.01

0.96 ([Ne]3s0.033p0.01) 0.99 ([Ne]3s0.013p0.01)

CO2

0.04

0.79 ([Ne]3s0.053p0.15)


0.83

0.01

0.99 ([Ne])

0.88 ([Ar]4s0.023d0.054p0.05)


0.88

0.77 ([Ar]4s0.053d0.074p0.11)


0.84

0.02

0.98 ([Ar]4s0.014p0.01)

0.02

0.07

K none CO

0.07

1.0 ([Ar])

H2O


0.01

)


0.86

0.01

0.99 ([Ar])

H2S

0.00

0.80 ([Ar]4s0.043d0.064p0.09)


0.80

0.03

0.97 ([Ar]4s0.013d0.014p0.01)

0.05 0.02

0.04


0.85
0.86

0.01 0.01

0.99 ([Ar]) 0.99 ([Ar])

N2 CO2

0.87 ([Ar]4s

0.05

3d

0.07

0.06

4p

0.10

0.80 ([Ar]4s 3d 4p ) 0.84 ([Ar]4s0.033d0.064p0.07)

a The electronic configuration of the cation is also shown in parentheses. In bare cation calculations, the cations and the adsorbates were held at their positions from the optimization cluster calculations (see text).

of the S6R. The heights of the Cu and Ag cations from the O(2) plane increase in the same order as the ionic radius: Cu+ (0.77 Å) < Ag+ (1.34 Å).51 The alkali metals also show a growing trend in their heights from the O(2) plane with the ionic radius: Li+ (0.76 Å) < Na+ (1.02 Å) < K+ (1.38 Å). 3.1.2. Adsorption of H2S and Other Small Molecules on M
Y Zeolite Clusters (M = Cu, Ag, Li, Na, or K). The optimized structures of CO, H2O, H2S, N2, and CO2 adsorbed on the Cu
Y cluster are shown in Figure 3. Linear molecules undertake the end-on configuration with respect to Cu cation. CO, N2, and CO2 are coordinated to Cu at 1.81, 1.85, and 2.66 Å with C, N, and O atoms, respectively. H2O is coordinated to Cu with O atom at 1.97 Å, and H2S is coordinated to Cu with S atom at 2.22 Å. For each adsorbate, similar configurations were also found on other MY clusters. Note that in H2O and H2S, one of the two H atoms is coordinated to a framework oxygen atom, denoted as Oz in Table S2 of the Supporting Information. Selected geometric parameters of the adsorbate molecules in the gas phase and adsorbed on the M
Y clusters are tabulated in Table S2. The geometric parameters within the adsorbate do not change markedly from their gas-phase values, and the distance between the cation and the adsorbate atom coordinated to the cation varies with the cation type for a given adsorbate. 3.1.3. Adsorption of H2S and Other Small Molecules on Bare Cations M+ (M = Cu, Ag, Li, Na, or K). The optimized structures of CO, H2O, H2S, N2, and CO2 adsorbed on bare Cu cation are shown in Figure 4. All of the adsorbates studied undertake the same configurations with respect to the cations as when adsorbed

on the Y clusters. The configuration of each adsorbate does not change with the cation type (not shown), but the distance between the cation and the adsorbate atom coordinated to the cation does. Adsorption on the cations causes almost no change from their gasphase configurations, as shown by selected geometric parameters tabulated in Table S3 of the Supporting Information. 3.2. Energetics. In Figure 5 we compare three sets of adsorption energies involving Cu and Ag cations: adsorption on bare Cu and Ag cations with full structure relaxation (Figure 5a), adsorption on bare Cu and Ag cations with the adsorbate and the cation constrained at their atomic positions from the cluster calculations (Figure 5b), and adsorption on Cu- and Agexchanged Y zeolite clusters (Figure 5c). The same order in the adsorption energy was found on bare Cu and Ag cations as shown in Figure 5a. H2S is the strongest adsorbate, followed by the other four species: H2S > H2O > CO > N2 ≈ CO2. A similar trend was found upon the imposition of the geometric constraints: H2S > H2O > CO > N2 > CO2, as shown in Figure 5b. It is expected that all of the adsorption energies in Figure 5b decrease from the corresponding values in Figure 5a due to the constraints imposed on the atomic positions. With or without the geometric constraints, the adsorption strengths of bare Cu and Ag cations for all of the adsorbates decrease with increasing cation radius: Cu > Ag. On the zeolite clusters, as shown in Figure 5c, adsorption energies show somewhat different trends and overall decreases in their values. On the Cu
Y cluster, the main change consists of the fact that adsorption energy of CO exceeds those of the other adsorbates markedly. On the Ag
Y 3566

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Figure 7. ALMO EDA results for the adsorption of CO, H2O, H2S, N2, and CO2 on bare (a) Cu and (b) Ag cations, on bare (c) Cu and (d) Ag cations with the adsorbates and the cations constrained at their atomic positions from the cluster calculations, and on (e) Cu
Y and (f) Ag
Y zeolite clusters.

cluster, the adsorption energy of CO becomes comparable to that of H2S. The same order in adsorption energy holds for all other adsorbates: H2S > H2O > N2 > CO2. The adsorption energies on systems involving alkali metal cations are shown in Figure 6. At variance with Cu and Ag cations, H2O is the strongest adsorbate on bare alkali metal cations. The same order in the adsorption energy was found: H2O > H2S > CO2 > CO > N2 (Figure 6a). Adsorption on isolated cations with atomic positions constrained to the configurations from the cluster calculations result in a similar trend in the adsorption energy: H2O > H2S > CO2 ≈ CO > N2 (Figure 6b). For each adsorbate, the adsorption strength decreases with increasing cation radius in spite of the geometric constraints: Li > Na > K. The presence of the zeolite clusters (Figure 6c) does not change the overall trend in the adsorption

energies, but it lowers the adsorption energies of all the adsorbates from the corresponding values in Figure 6b. The decreases for Li
Y and Na
Y with respect to the isolated cations are larger than those for K
Y; this leads to comparable adsorption strengths of Li
Y and Na
Y to that of K
Y. Overall, the transition metal-containing systems exhibit very different trends from the alkali metal-containing systems in the adsorption energies. Upon adsorption on a given cation, the geometric constraints imposed by the zeolitic framework result in the reduction of the adsorption energies for all adsorbates, but no drastic change appears in the trend of the adsorption energies among different adsorbates. For bare transition metals, the trend in the adsorption energy is H2S > H2O > CO > N2 J CO2, while on transition metal-containing Y clusters, the adsorption of CO 3567

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Figure 8. ALMO EDA results for the adsorption of CO, H2O, H2S, N2, and CO2 on bare (a) Li, (b) Na, and (c) K cations, on bare (d) Li, (e) Na, and (f) K cations with the adsorbates and the cations constrained at their atomic positions from the cluster calculations, and on (d) Li
Y, (e) Na
Y, and (f) K
Y zeolite clusters.

is stronger or comparable to that of H2S: CO J H2S > H2O > N2 > CO2. For systems based on alkali metals, a general trend is observed: H2O > H2S > CO2 J CO > N2. While adsorption energy decreases with the radius of bare cation for both groups of metals, in the presence of zeolite this trend is not maintained. The electronic properties of the cations, i.e., the partial charge and the electronic configuration of its valence orbitals, are expected to be one of the factors relevant to the different adsorption trends observed for the two groups of metals. Thus, our first attempt to understand these results was made through the natural bond orbital (NBO) analysis that was performed for cluster and for bare cations with the geometric constraints imposed. 3.3. Natural Bond Orbital Charge Analysis. The NBO analysis results involving transition metals and alkali metals are tabulated in Tables 1 and 2, respectively. To gain more insight

into the role of the zeolitic framework atoms upon adsorption, the total charges of the adsorbate, the cation, and the zeolite cluster are listed separately for cluster calculations. The total charges of the adsorbate and the cation in the bare cation calculations with the geometric constraints imposed are also listed for comparison. The electron configurations of the cations are shown in parentheses. Overall we can observe that there is a significant exchange of electrons between the adsorbate, the zeolite, and the cation. Transition metals lose d electrons, but electrons received on s and p orbitals overcompensate for this loss. Alkali metals can only receive electrons on s and p orbitals. The retrieval of electrons in the valence s and p orbitals decreases the net charges of alkali metal cations in this order: Li > Na > K. These exchanges are more significant in the presence of the zeolite framework; the zeolite enhances, in particular, the recovery of electrons in p orbitals. 3568

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Figure 9. Side views of significant COVPs between CO and Cu, denoted as (a) Cu_CO(i) and (b) Cu_CO(ii). Top and side views of significant COVPs between CO and Cu
Y cluster, denoted as (c) CuY_CO(i), (d) CuY_CO(ii), and (e) CuY_CO(iii). The contribution of each of these COVPs to ΔECT is listed in Table 3. Occupied orbitals are represented with glossy surfaces, and virtual orbitals are represented with transparent surfaces. Isovalue surface is 0.1 au. Atomic representations can be referred to in the caption for Figure 1.

A more detailed discussion can be found in the Supporting Information. While these trends are clear, a more precise interpretation of adsorption energy in terms of charge transfer is still needed. Therefore, absolutely localized molecular orbital energy decomposition analysis was carried out to quantify the contributions of different energy components to the adsorption energies that vary with the adsorbate and the cation. 3.4. Absolutely Localized Molecular Orbital Energy Decomposition Analysis. The ALMO EDA was performed for adsorption energies on bare cations, on bare cations with geometric constraints, and on cation-exchanged Y zeolite clusters. The results associated with transition metals (Cu and Ag) and alkali cations (Li, Na, and K) are presented in Figure 7 and Figure 8, respectively. The two fragments are the adsorbate X and the cation-exchanged zeolite M
Y (or bare cation M). The energetic contributions associated with the charge transfer from and to the adsorbate are denoted as CT(XfM
Y) (CT(XfM) for bare cation) and CT(M
YfX) (CT(MfX) for bare cation), respectively. The FRZ, POL, and GD terms are consolidated into the non-chargetransfer (non-CT) term in Figures 7 and 8 for simplification, whereas all of the energy components in eq 2 are tabulated in Tables S4
S6 in the Supporting Information. Upon adsorption on bare Cu and Ag cations, the charge transfer between the adsorbate and the cation has significant

contributions to the adsorption energy as shown in Figure 7a,b. For all of the adsorbates studied, contributions from charge transfers of both directions are present, and the contribution from the charge transfer from adsorbate to cation is bigger than that in the opposite direction. For the adsorption energy of a given adsorbate, the CT contributions on the two cations are comparable. The non-CT term is larger on Cu than on Ag and is responsible for the lower adsorption energies on Ag. Comparison between Figure 7a,b and Figure 7c,d shows that the distortion of the adsorbate
cation complex into the geometry obtained from the cluster calculations generally results in slight reduction of the non-CT contributions to the adsorption energies for all of the adsorbates. An extreme case is the non-CT term of CO on Ag that becomes negative. Parts e and f of Figure 7 show that the zeolite cluster greatly reduces the contributions from the non-CT terms, and the CT terms exceed them in magnitudes. In the cases of CO, H2S, and N2 adsorbed on Cu
Y and Ag
Y clusters, the non-CT terms contribute negatively to the adsorption energies. These negative contributions to the adsorption energies could be due to the repulsion between the d electrons of Cu or Ag and p electrons of the adsorbate atom in close vicinity. However, these adsorbates remain stable energetically due to the fact that the positive contributions from the CT terms of both directions exceed the negative contributions from the non-CT terms. 3569

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Figure 10. Side views of significant COVPs between CO and Ag, denoted as (a) Ag_CO(i) and (b) Ag_CO(ii). Top and side views of significant COVPs between CO and Ag
Y cluster, denoted as (c) AgY_CO(i), (d) AgY_CO(ii), and (e) AgY_CO(iii). The contribution of each of these COVPs to ΔECT is listed in Table 3. Occupied orbitals are represented with glossy surfaces, and virtual orbitals are represented with transparent surfaces. Isovalue surface is 0.1 au. Atomic representations can be referred to in the caption of Figure 1.

For example, the CT term associated with the charge inflow into CO is as high as ∼100 kJ/mol. Together with the energetic contributions from the CT term in the opposite direction and the negative non-CT term, CO still has the largest adsorption energy on Cu
Y cluster among all adsorbates. It is also worth noting that the CT term associated with the outflow of the charge from a given adsorbate when adsorbed on the clusters is smaller than the corresponding one pertaining to the bare cation, whereas the opposite is observed regarding the CT term associated with the inflow of the charge into a given adsorbate. This suggests that the zeolite framework reduces the electron-accepting tendencies of Cu and Ag cations and increases their electron-donating capabilities. As a result, the zeolite enhances the stabilization of the adsorbate through charge transfer into the adsorbate, but it weakens adsorption based on the donation of electrons from the adsorbate. This effect benefits adsorption complexes with strong back-donation components. For example, CO is an excellent electron acceptor, its complexes have significant CT(MfCO) terms (Figure 7), and thus the strength of its adsorption is increased relative to H2S when cations are embedded in the zeolite. In contrast to Cu- and Ag-containing systems, the adsorption energies on bare Li, Na, and K cations are dominated by the nonCT terms as shown in Figure 8a
c. The charge transfer from the

adsorbate to the cation has minor contribution to the adsorption energy, and the contribution from the charge transfer in the opposite direction is absent. The non-CT terms of the adsorption energies of the adsorbates studied decrease in the same order for the three alkali cations: H2O > H2S > CO2 > CO > N2. Even though in polar molecules the dipole moment could contribute to strong electrostatic interactions,25 the above trend does not show correlation with the dipole moments of these adsorbates: H2O (1.84 D) > H2S (0.98 D) > CO (0.12 D) > CO2 = N2 (0 D).52 For a given adsorbate, the non-CT term contribution to its adsorption energy decreases with increasing cation radius: Li > Na > K. Comparison between Figure 8a
c and Figure 8d
f shows that when the adsorbate and the cation are distorted to their atomic positions in the cluster calculations, the aforementioned features are qualitatively preserved except for the non-CT terms in the adsorption of CO2 and CO that become comparable. As shown in Figure 8g
i, there is noticeable contribution from the charge transfer from the zeolite to the adsorbate in the presence of the zeolitic framework. Compared to Figures 8d
f, the main difference is that the non-CT contributions to the adsorption energies are comparable on Li
Y, Na
Y, and K
Y clusters instead of varying with the size of the cation. As a results, the adsorption strength of Li
Y, Na
Y, and K
Y to the adsorbates studied are indistinguishable. 3570

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Table 3. Contributions of Significant COVPs to ΔECT in kilojoules per mole and Their Percentages of ΔECT for CO and H2S Adsorbed on Cation M and M
Y Zeolite Cluster (M = Cu or Ag) M (or M
Y) to CO

CO to M (or M
Y)

COVP

ΔECT (kJ/mol)

percentage

Cu_CO(i)a

21.4

44.9

Cu_CO(ii)

66.1

96.3

Ag_CO(i)a

12.3

40.0

Ag_CO(ii)

72.0

97.1

CuY_CO(i) CuY_CO(ii)

50.3 38.2

50.4 38.3

CuY_CO(iii)b

28.8

53.3

AgY_CO(i)

25.2

42.2

AgY_CO(iii)

56.0

94.6

AgY_CO(ii)

19.6

32.8

COVP

M (or M
Y) to H2S COVP

a

ΔECT (kJ/mol)

percentage

H2S to M (or M
Y)

ΔECT (kJ/mol)

percentage

COVP

ΔECT (kJ/mol)

percentage

Cu_H2S(iv)

80.4

92.6

Ag_H2S(iv)

82.8

92.9

CuY_H2S(iv)

50.0

90.1

Ag_H2S(iv)

53.1

90.3

Cu_H2S(i)

4.6

29.3

Cu_H2S(ii)

5.9

37.4

Cu_H2S(iii)

4.4

28.4

Ag_H2S(i) Ag_H2S(ii)

2.7 1.6

46.3 27.3

Ag_H2S(iii)

1.3

22.6

CuY_H2S(i)

8.0

28.8

CuY_H2S(ii)

7.5

27.0

CuY_H2S(iii)

3.1

11.1

AgY_H2S(i)

3.5

20.0

AgY_H2S(ii)

4.0

22.8

AgY_H2S(iii)

2.3

13.4

This COVP is doubly degenerate. b Two orbitals contribute to the remaining ΔECT can be found in Figure S2 of the Supporting Information.

3.5. Significant Complementary Occupied-Virtual Orbitals Pairs Analysis. Up to this point, we have presented the adsorp-

tion energies of CO, H2O, H2S, N2, and CO2 on the bare cations and cation-exchanged zeolite clusters and discussed their compositions in terms of different energetic terms based on the ALMO EDA. For systems that have a similar adsorption complex but differ significantly in the ALMO EDA, further attention should be paid in order to understand the origin of these changes. For example, when CO is adsorbed on Cu
Y and Ag
Y zeolite clusters, the contributions to the adsorption energies from transfer of electrons to CO is significantly larger than those when CO is adsorbed on bare Cu and Ag cations. In addition, the contribution to the adsorption energy from the intake of charge by CO on the Cu
Y cluster is larger than that on the Ag
Y cluster. Accordingly, two issues are addressed here: (1) Does the presence of the zeolitic framework provide additional orbitals for the charge transfer into CO? (2) Are there more orbitals involved in the charge transfer into CO on the Cu
Y cluster than on the Ag
Y cluster? To resolve the preceding questions, COVPs analysis32 was invoked to provide a simple description of intermolecular electron-transfer effects in terms of just a few localized orbitals. The visualizations of the significant COVPs53 that contain major energetic contributions to the charge-transfer effects for the adsorption of CO on Cu cation/Cu
Y cluster and Ag cation/Ag
Y cluster are shown in Figures 9 and 10, respectively. The contributions of significant COVPs to ΔECT and their percentages of ΔECT in both directions of charge transfer are tabulated in Table 3. As evident from Figures 9a,b and 10a,b, the bondings between CO and the two bare transition metals come predominantly from electron backward

donation from the occupied d orbitals of Cu and Ag into unoccupied π*-antibonding orbitals of CO, and electron forward donation from CO to Cu and Ag unoccupied orbitals through carbon. The two doubly degenerate d orbitals together account for 90 and 80% of ΔECT of the backward donation for CO adsorbed on Cu and Ag cations, respectively, as shown in Table 3. These results agree well with the work of Khaliullin et al.32 In the presence of the zeolitic framework atoms, the same types of orbitals are responsible for the charge transfer between CO and the Cu
Y (or Ag
Y) cluster as shown in Figures 9c
e and 10c
e. However, the contributions from COVPs of the back-donation to the adsorption energies increase significantly in the presence of zeolite, resulting in competitive adsorption between CO and H2S. It is interesting to note that even though significant COVPs of the back-donation for Cu and Ag cations seem to be the same, the energetic contribution is approximately doubled when CO adsorbs on Cu cation (21.4 kJ/mol) than on Ag cation (12.3 kJ/mol). The same is found for CO adsorbed on Cu
Y (50.3 and 38.2 kJ/mol) and Ag
Y clusters (25.2 and 19.6 kJ/mol). While the backward donations take place only between CO and the cation upon adsorption on Cu
Y and Ag
Y clusters, the participation of additional orbitals were found upon the adsorption of H2S on Cu
Y and Ag
Y clusters with respect to the case of isolated Cu and Ag cations. As shown in Figure 11e
g and Figure 12e
g, in the presence of the zeolite framework, the additional orbitals are associated with charge transfers from the framework O atoms to the H atoms of H2S coordinated to those framework O atoms that do not appear in Figure 11a
c and Figure 12a
c. Again, with or without the zeolite cluster, the types of orbitals involved in the charge transfer between H2S and the 3571

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Figure 11. Side views of significant COVPs between H2S and Cu cation, denoted as (a) Cu_H2S(i), (b) Cu_H2S(ii), (c) Cu_H2S(iii), and (d) Cu_H2S(iv). Top and side views of significant COVPs between H2S and the Cu
Y cluster, denoted as (e) CuY_H2S(i), (f) CuY_H2S(ii), (g) CuY_H2S(iii), and (h) CuY_H2S(iv). The contribution of each of these COVPs to ΔECT is listed in Table 3. Occupied orbitals are represented with glossy surfaces, and virtual orbitals are represented with transparent surfaces. Isovalue surface is 0.1, Au atomic representations can be referred to in the caption of Figure 1.

cation do not change with the cation type. However, the energetic contribution of the same type of COVP differs in magnitude with the cation type as shown in Table 3.

4. DISCUSSION 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 nonCT 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, the more the cation is recessed into the S6R and therefore is shielded by the zeolitic framework atoms. The electronic configurations of bare Cu and Ag cations (Table 1) and the visualizations of the significant COVPs (Figures 9
12) show that both valence s and d orbitals are involved in the charge transfer with the adsorbate. Such flexibility for charge transfer is missing for alkali cations. Therefore, it is reasonable to expect that with the zeolite cluster acting like an electron reservoir, there is also charge transfer between Cu (or Ag) cation and the zeolite that leads to charge re-distribution in Cu
Y (or Ag
Y) cluster. Consequently, the zeolite has a more pronounced effect on the adsorption energies on transition metals than on alkali metals. This is supported by the drastic 3572

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Figure 12. Side views of significant COVPs between H2S and the Ag cation, denoted as (a) Ag_H2S(i), (b) Ag_H2S(ii), (c) Ag_H2S(iii), and (d) Ag_H2S(iv). Top and side views of significant COVPs between H2S and the Ag
Y cluster, denoted as (e) AgY_H2S(i), (f) AgY_H2S(ii), (g) AgY_H2S(iii), and (h) AgY_H2S(iv). The contribution of each of these COVPs to ΔECT is listed in Table 3. Occupied orbitals are represented with glossy surfaces, and virtual orbitals are represented with transparent surfaces. Isovalue surface is 0.1 au. Atomic representations can be referred to in the caption for Figure 1.

changes in ALMO EDA between bare transition metals and transition metal-exchanged Y cluster; such changes were not observed for alkali metal systems. To further demonstrate the importance of the charge transfer between Cu (or Ag) and the zeolite, the binding energies of Cu, Ag, Li, Na, and K cations on the zeolitic framework were calculated in the absence of adsorbate, and the ALMO EDA was carried out to quantify the energetic effects associated with the charge transfer. As shown in Table 4, the CT interactions do not dominate the binding energies, but there are noticeable differences in the magnitudes of the CT contributions between the transition metal- and alkali metal-exchanged Y clusters. The energetic effects associated with charge transfer from the zeolitic framework to the cation are greater for Cu and Ag cations (59.6 and 47.7 kJ/mol, respectively) than for alkali metals (2.7
16.0 kJ/mol). While the energetic effects associated with charge transfer in the opposite direction are close to zero for alkali metals, those for Cu and Ag cations are 15.9 and 5.8 kJ/mol, respectively. Due to the ability of transition metal cations to exchange electrons with both the zeolite cluster and the adsorbate, the zeolite cluster serves as a bigger perturbation to adsorption energy when the exchanged cations are transition metals rather than alkali metals. As a result, the adsorption energies on Cu
Y and Ag
Y clusters depend on the balance of different energy

components that change greatly from adsorption on bare Cu and Ag cations, whereas those on alkali metal-exchanged Y clusters are governed by the non-CT terms, as is the case for adsorptions on bare alkali metals. To summarize, several trends are observed from our calculations. H2S is the strongest adsorbate on bare Cu and Ag cations, whereas the adsorption energy of H2S is smaller or comparable compared to the strongest adsorbate, CO, on Cu
Y and Ag
Y zeolites where CT terms are substantial. The geometric constraints imposed by the zeolitic framework only lead to reduction of adsorption energies on bare cations and cannot reproduce the trend in adsorption energies on Cu
Y and Ag
Y zeolite clusters. As shown by COVPs results, the different trends in adsorption energy on bare transition metal cations and on transition metal-exchanged Y zeolites are not due to different types of orbitals involved in charge transfer, nor is the different selectivity for H2S between Cu
Y and Ag
Y. This suggests that the effect of the zeolite is not purely geometric, and its participation in the electronic process is significant. On alkali metal-based systems, the non-CT term is the dominant factor to the adsorption energy, and H2O is the strongest adsorbate. While alkali metal-exchanged Y zeolites are found to be selective adsorbents for H2O due to strong non-CT interaction, a selective adsorbent for H2S should have the ability for significant charge transfer with 3573

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Table 4. Binding Energies (kJ/mol) of Cu, Ag, Li, Na, and K Cations on Zeolitic Frameworka binding energy

CT(MfZ)

CT(ZfM)

Cu
Y

708.7

15.9

59.6

Ag
Y Li
Y

643.0 712.9

5.8 0.2

47.7 16.0

Na
Y

612.5


0.3

2.7

K
Y

529.0

0.7

15.3

a

The CT contributions (kJ/mol) are also listed for charge transfer from the cation to the framework (MfZ) and the charge transfer in the opposite direction (ZfM).

the adsorbate. The presence of zeolite, however, complicates the balance between different terms and the adsorption of other species as CO may become dominant. This could be related to the observation that the zeolite framework saturates the cations with electrons on s and p orbitals that are also responsible for accepting electrons from the adsorbate, and it also increases the amount of d electrons being donated. In fact, the adsorption of CO is strongly favored with respect to H2S by charge transfer on Cu
Y. The ideal situation for selective H2S adsorption is represented by isolated Cu or Ag cations where H2S adsorbs most strongly due to pronounced charge-transfer effect. While moderating the electron-donating tendency and encouraging the electron-accepting tendency of the cation seem to be advantageous for the selective adsorption of H2S, the overall adsorption energy depends also on interactions due to electrostatics, polarization, and structural distortions (non-charge-transfer terms). Therefore, whether these interactions can be tuned and controlled independently when modifying the electron-donating and -accepting tendencies of the cation is an important issue and worth further investigation in the future.

5. CONCLUSION The capabilities of Cu
Y and Ag
Y zeolites for selective hydrogen sulfide (H2S) removal from Claus process tail gas were investigated with DFT calculations and were compared to those of Li
Y, Na
Y, and K
Y zeolites. Cluster models that include a six-membered ring were used to represent the faujasite zeolite structure upon the adsorption of H2S and other common impurities (CO, H2O, N2, and CO2). On the basis of the comparison of the adsorption energies, Ag
Y has the best predicted selectivity for the adsorption of H2S. Cu
Y and alkali metal-exchanged Y zeolites are subject to strong adsorption of CO and H2O, respectively. Comparison between the simplified and the cluster calculations associated with alkali metal cations shows that the presence of the zeolitic framework reduces the differences in the adsorption energies among among different alkali metals but does not change the trend among different adsorbates. For Cu and Ag cations, the zeolitic framework also enhances the adsorption of CO with respect to H2S. Natural bond orbital analysis shows that electrons re-distribute upon adsorption: all of the cations studied gain electrons in their valence s and p orbitals; in addition, Cu and Ag cations donate their d electrons to the adsorbates. The retrieval of the electrons in the presence of the zeolite cluster is most pronounced in p orbitals, resulting in the lowering of the charges of different cations to different extents. ALMO EDA reveals that the CT terms of both directions have significant contributions to the adsorption energies for the adsorption on

bare Cu and Ag cations and Cu
Y and Ag
Y clusters. The visualization of the significant COVPs confirms that the charge transfers with the adsorbed CO and H2S mainly involve the valence s and d orbitals of Cu and Ag cations. This points out that the availability of the d electrons is important for CT-based adsorption mechanisms that favor H2S and CO over H2O. In fact, for alkali metals, where d electrons are missing, the adsorption is dominated by non-CT terms that favor H2O over all other species. On the basis of our calculations, an ideal adsorbent for H2S should behave as much as possible like the bare Cu cation. While Cu
Y has worse performance than Ag
Y due to the poisoning from CO, we suspect that changing Al distributions and zeolite framework type might reproduce the adsorption energies on bare Cu cations. Further calculations will be performed in the future to clarify these points.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures showing adsorption energies of CO, H2O, H2S, N2, and CO adsorbed on Cu
Y zeolite and significant COPVs of CO and the Cu
Y cluster, text describing NBO charge analysis, and tables listing distances between cation M and three O(2) atoms, selected geometric parameters, and ALMO EDA results. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was funded in part by the Nanostructural Materials and Processes Program at the University of Minnesota, by ADGAS and GASCO, and by the Abu Dhabi
Minnesota Institute for Research Excellence (ADMIRE), a partnership between the Petroleum Institute of Abu Dhabi and the Department of Chemical Engineering and Materials Science of the University of Minnesota. We thank Professor Donald G. Truhlar for helpful discussions. We are also grateful to the Minnesota Supercomputing Institute for providing resources necessary to perform this study. ’ REFERENCES (1) Yang, F. H.; Hernandez-Maldonado, A. J.; Yang, R. T. Sep. Sci. Technol. 2004, 39 (8), 1717–1732. (2) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301 (5629), 79–81. (3) Hernandez-Maldonado, A. J.; Yang, R. T. Catal. Rev.: Sci. Eng. 2004, 46 (2), 111–150. (4) Avidan, A.; Klein, B.; Ragsdale, R. Hydrocarbon Process. 2001, 80 (2), 47–53. (5) Oudar, J. Catal. Rev.: Sci. Eng. 1980, 22 (2), 171–195. (6) Cheekatamarla, P. K.; Lane, A. M. J. Power Sources 2005, 152 (1), 256–263. (7) Matsuzaki, Y.; Yasuda, I. Solid State Ionics 2000, 132 (3
4), 261–269. (8) Crespo, D.; Qi, G.; Wang, Y.; Yang, F. H.; Yang, R. T. Ind. Eng. Chem. Res. 2008, 47, 1238–1244. (9) Speight, J. G. The Desulfurization of Heavy Oils and Residua; Dekker: New York, 1999. 3574

dx.doi.org/10.1021/jp2097313 |J. Phys. Chem. C 2012, 116, 3561–3575

The Journal of Physical Chemistry C (10) Speight, J. G. Fuel Science and Technology Handbook; Dekker: New York, 1990. (11) Chung, J. S.; Paik, S. C.; Kim, H. S.; Lee, D. S.; Nam, I. S. Catal. Today 1997, 35 (1
2), 37–43. (12) Svoronos, P. D. N.; Bruno, T. J. Ind. Eng. Chem. Res. 2002, 41, 5321–5336. (13) Kumar, P.; Sung, C. Y.; Muraza, O.; Cococcioni, M.; Hashimi, S. A.; McCormick, A.; Tsapatsis, M. Microporous Mesoporous Mater. 2011, 146 (1
3), 127–133. (14) Hernandez-Maldonado, A. J.; Yang, R. T. J. Am. Chem. Soc. 2004, 126, 992–993. (15) Satokawa, S.; Kobayashi, Y.; Fujiki, H. Appl. Catal., B 2005, 56 (1
2), 51–56. (16) Lee, D.; Ko, E. Y.; Lee, H. C.; Kim, S.; Park, E. D. Appl. Catal., A 2008, 334 (1
2), 129–136. (17) Velu, S.; Ma, X.; Song, C. Ind. Eng. Chem. Res. 2003, 42, 5293–5304. (18) Wakita, H.; Tachibana, Y.; Hosaka, M. Microporous Mesoporous Mater. 2001, 46 (2
3), 237–247. (19) Lee, D.; Kim, J.; Lee, H. C.; Lee, K. H.; Park, E. D.; Woo, H. C. J. Phys. Chem. C 2008, 112, 18955–18962. (20) Chen, N.; Yang, R. T. Ind. Eng. Chem. Res. 1996, 35, 4020–4027. (21) Rejmak, P.; Sierka, M.; Sauer, J. Phys. Chem. Chem. Phys. 2007, 9 (40), 5446–5456. (22) Hass, K. C.; Schneider, W. F. Phys. Chem. Chem. Phys. 1999, 1 (4), 639–648. (23) McMillan, S. A.; Broadbelt, L. J.; Snurr, R. Q. J. Catal. 2003, 219 (1), 117–125. (24) Boekfa, B.; Choomwattana, S.; Khongpracha, P.; Limtrakul, J. Langmuir 2009, 25, 12990–12999. (25) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; Wiley: New York, 1974. (26) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. (27) Schneider, W. F.; Hass, K. C.; Ramprasad, R.; Adams, J. B. J. Phys. Chem. B 1997, 101, 4353–4357. (28) Pidko, E. A.; Mignon, P.; Geerlings, P.; Schoonheydt, R. A.; van Santen, R. A. J. Phys. Chem. C 2008, 112, 5510–5519. (29) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926. (30) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor
Acceptor Perspective; Cambridge University Press: New York, 2005. (31) Khaliullin, R. Z.; Cobar, E. A.; Lochan, R. C.; Bell, A. T.; Head-Gordon, M. J. Phys. Chem. A 2007, 111, 8753–8765. (32) Khaliullin, R. Z.; Bell, A. T.; Head-Gordon, M. J. Chem. Phys. 2008, 128 (18), No. 184112. (33) Frenking, G.; Fr€ohlich, N. Chem. Rev. 2000, 100, 717–774. (34) Jardillier, N.; Villagomez, E. A.; Delahay, D.; Coq, B.; Berthomieu, D. J. Phys. Chem. B 2006, 110, 16413–16421. (35) Datka, J.; Kozyra, P.; Kukulska-Zajac, E. Catal. Today 2004, 90 (1
2), 109–114. (36) Brand, H. V.; Redondo, A.; Hay, P. J. J. Phys. Chem. B 1997, 101, 7691–7701. (37) McNaught, A. D.; Wilkinson, A. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”); Blackwell Scientific: Oxford, U.K., 1997. XML on-line corrected version created by: Nic, M.; Jirat, J.; Kosata, B. http://goldbook.iupac.org, 2006 (updates compiled by: Jenkins, A.). (38) Frising, T.; Leflaive, P. Microporous Mesoporous Mater. 2008, 114 (1
3), 27–63. (39) Li, H. Y.; Pu, M.; Liu, K.; Zhang, B. F.; Chen, B. H. Chem. Phys. Lett. 2005, 404 (4
6), 384–388. (40) Rattanasumrit, A.; Ruangpornvisuti, V. J. Mol. Catal. A: Chem. 2005, 239 (1
2), 68–75. (41) Hriljac, J. A.; Eddy, M. M.; Cheetham, A. K.; Donohue, J. A.; Ray, G. J. J. Solid State Chem. 1993, 106 (1), 66–72.

ARTICLE

(42) Demaison, J.; Herman, M.; Lievin, J. Int. Rev. Phys. Chem. 2007, 26 (3), 391–420. (43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.01; Gaussian: Wallingford, CT, USA, 2009. (44) Becke, A. D. J. Chem. Phys. 1993, 98 (7), 5648–5652. (45) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86 (2), 866–872. (46) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta. 1990, 77 (2), 123–141. (47) Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615–6620. (48) Khaliullin, R. Z.; Head-Gordon, M.; Bell, A. T. J. Chem. Phys. 2006, 124 (20), 204105. (49) Shao, Y.; Molnar, L.; Jung, Y.; Kussmann, J.; Ochsenfeld, C.; Brown, S. T.; Gilbert, A. T. B.; Slipchenko, L. V.; Levchenko, S. V.; O’Neill, D. P. Phys. Chem. Chem. Phys. 2006, 8 (27), 3172–3191. (50) Vanduijneveldt, F. B.; vande Rijdt, J. G. C. M. V.; Lenthe, J. H. Chem. Rev. 1994, 94, 1873–1885. (51) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (52) Hillier, I. H.; Saunders, V. R. Chem. Phys. Lett. 1969, 4 (4), 163–164. (53) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33–38.

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