Structures of the Subnanometer Clusters of Cadmium Sulfide

Article pubs.acs.org/JPCC

Structures of the Subnanometer Clusters of Cadmium Sulfide Encapsulated in Zeolite Y: Cd4S6+ and Cd(SHCd)46+ Dae Jun Moon and Woo Taik Lim* Department of Applied Chemistry, Andong National University, Andong 36729, Korea

Karl Seff* Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822, United States S Supporting Information *

ABSTRACT: The structures of the subnanometer clusters of CdS that form in zeolite Y (FAU), prepared as follows, have been determined. A single crystal of Cd2+-exchanged zeolite Y was prepared by the exchange of Na75−Y (|Na75|[Si117Al75O384]−Y with aqueous Cd(NO3)2 at 294 K, followed by vacuum dehydration at 723 K (crystal 1). A second crystal, similarly prepared, was exposed to 6.7 × 104 Pa of dry H2S(g) for 6 h at 294 K and evacuated (crystal 2). Their structures were determined crystallographically using synchrotron X-rays and were refined using all data to the final error indices (calculated using only Fo > 4σ(Fo)) of R1/wR2 = 0.040/0.156 and 0.049/0.186, respectively. In crystal 1, Cd2+ ions primarily occupy sites I and II. In crystal 2, |Cd 27 (Cd 4 S 6+ ) 0.6 (Cd(SHCd)46+)0.9H12|[Si117Al75O384]−FAU, tetrahedral Cd4S6+ ions center about 7.5% of the sodalite cavities and tetrahedral Cd(SHCd)46+ ions center about 11% of the supercages. In Cd(SHCd)46+, a Cd2+ ion at the center of a supercage bonds tetrahedrally to four SH− ions, each of which bonds in turn to an 8-ring Cd2+ ion. Of the 36.5(9) Cd2+ ions per unit cell (indicating complete or nearly complete Cd2+ exchange) before treatment with H2S(g), only 6.9 participate in the above clusters. Most, 27, remain uncomplexed, and 2.6 have left the zeolite as CdS(s). No neutral clusters of formula CdnSn are present within the zeolite. the Cd2+-exchanged zeolites either by treatment with Na2S(aq)2−6 or by dehydration followed by treatment with H2S(g).7−13 The first method introduces Na+ ions into the zeolite; the second introduces H+. The experimental conditions (concentrations, pressures, times, and temperatures) for the steps of sample preparation vary from report to report.2−13 Thus, a wide variety of experimental procedures have been used and the “CdS clusters” prepared may have had a wide variety of compositions, concentrations, and structures. The style of nearly all of the above reports, which are discussed in more detail in this section, has been to use the words “CdS clusters” freely. This suggests that they are neutral and have the formula CdnSn. In contrast with that expectation is the work of Kim et al., who determined the structure of hydrated |(Cd 6 S 4 )(Cd2Na2S)0.6(H2O)≥5.8|[Si12Al12O48]−LTA prepared by treatment of Cd−LTA with 0.05 M aqueous Na2S. Cd6S44+ clusters occupied about 40% of the sodalite cavities, and Cd2Na2S4+ was

1. INTRODUCTION 1.1. Studies of CdS in Zeolites. The II−VI compounds, those between the elements of group IIB (Zn, Cd, and Hg) and group VI (O, S, Se, and Te), have characteristic semiconducting properties.1 Zeolites, because their dehydrated structures have voids, have been widely studied as hosts for quantum dots of semiconductors.2−13 CdS, like ZnS, crystallizes in two forms: the cubic zinc blende (sphalerite) structure and the hexagonal wurtzite structure.14 In both structures, all cadmium and sulfide ions are tetrahedral.2 The band gaps in the two structures are both 2.5 eV at 300 K.15 The transitions occur as a broad absorption band with wavelengths below 515 nm; thus the blue part of the visible spectrum is fully absorbed and both forms of CdS are bright yellow.1 A wide variety of experimental procedures have been used to introduce quantum dots of CdS into various zeolites.2−13 In all reports, Cd2+ is first exchanged into the zeolite from aqueous solution. When the pH is not low enough, OH− ions can accompany Cd2+ into the zeolite,16 and when CdCl2 was used, Cl− could do so;16 the resulting CdS clusters may then contain oxide or chloride ions. Sulfide ions have been introduced into © 2016 American Chemical Society

Received: April 29, 2016 Revised: July 3, 2016 Published: July 5, 2016 16722

DOI: 10.1021/acs.jpcc.6b04369 J. Phys. Chem. C 2016, 120, 16722−16731

Article

The Journal of Physical Chemistry C

with aqueous CdCl2.16 Finding both OH− and Cl− uptake in those structures, they used a 0.05 M aqueous solution of Cd(C2H3O2)2 half-saturated with Cd(OH)2 to investigate the structure of Cd2+-exchanged zeolite A evacuated at 500 °C and that of its Cd(g) sorption complex.18,19 The latter contained both monatomic Cd+ and diatomic Cd22+ ions. The crystal structures of dehydrated |Cd6|[Si12Al12O48]−LTA and that of its ethylene sorption complex were reported by Koh et al.20 In both structures, the six Cd2+ ions per 12.3-Å unit cell occupied two distinct 3-fold axis positions. In the complex, four ethylene molecules were found per unit cell; each formed a lateral π complex with a Cd2+ ion. Jang et al. treated dehydrated |Cd6|[Si12Al12O48]−LTA with 0.1 Torr of Rb vapor at 523 K and observed crystallographically the reduction of Cd2+ ions by Rb.21 They found the cationic clusters (Rb2)+ and (Rb3)+ in the sodalite cavities of the zeolite. Jang et al. reported the single-crystal structures of |Cd6| [Si12Al12O48]−LTA evacuated at 1023 K and that of its cadmium sorption complex, |Cd11|[Si12Al12O48]−LTA.22 They observed the reduction of Cd2+ ions to Cd+ and suggested that a (Cd)54+ cluster of low symmetry had formed in the large cavity. Kwon et al. reported two anhydrous Cd2+-exchanged zeolite X structures, |Cd46|[Si100Al92O384]−FAU and |Cd24.5Tl43| [Si100Al92O384]−FAU.23 The structures of fully dehydrated and excessively Cd2+exchanged zeolites Y, |Cd27.5(Cd8O4)2|[Si121Al71O384]−FAU (Si/Al = 1.70)24 and |Cd27.5(Cd8O4)2.5|[Si117Al75O384]−FAU (Si/Al = 1.56),25 were investigated by Lee et al. and Seo et al., respectively. In the first structure the zeolite was ion exchanged using aqueous 0.05 M Cd2+ (0.025 M Cd(NO3)2 and 0.025 M Cd(C2H3O2)2) and dehydrated at 723 K. The resulting composition, |Cd27.5(Cd8O4)2|[Si121Al71O384]−FAU, indicated that eight molecules of Cd(OH)2 had been imbibed per unit cell. Eight of these nonframework oxide ions participated in the formation of two Cd8O48+ clusters per unit cell. The ability of Cd2+ ions to hydrolyze at pH 7 facilitated this process, as did the ability of the sodalite unit to host and stabilize Cd4(OH)44+ clusters.24 The crystal for the second structure had been Cd2+exchanged at a higher temperature (353 K) and had more Cd8O48+.25 Seo et al. prepared three single crystals of excessively Cd2+exchanged zeolite Y (Si/Al = 1.56). Cd2+ exchange was done using an aqueous stream 0.025 M each in Cd(NO3)2 and Cd(C2H3O2)2 at 294 K (crystals 1 and 2) and 353 K (crystal 3), followed by vacuum dehydration at 423 K (crystal 1) and 723 K (crystals 2 and 3).26 They reported that crystals 1 and 2 imbibed about six Cd(OH)2 molecules per unit cell, perhaps as CdOH+ ions. Crystal 3 imbibed 10, indicating increased hydrolysis and imbibition with increasing temperature. These decomposed upon vacuum dehydration at 723 K to give, with additional Cd2+ ions, Cd8O48+ clusters (symmetry 4̅3m (Td)) in the sodalite cavities of crystals 2 and 3. 1.3. Objective. This work was done to determine the formula, the structure, and the location of the CdS clusters that would form in fully dehydrated Cd2+-exchanged zeolite Y upon the sorption of dry H2S(g). An acidic Cd(NO3)2 exchange solution would be used to avoid the uptake of hydroxide and halide ions.

found in the remaining 60%. Neither was a neutral CdnSn cluster. Cd6S44+ consists of a central Cd4S4 group (interpenetrating tetrahedra) with two Cd2+ ions bonded to two of its sulfide ions.6 Cd2Na2S4+ is tetrahedral about its central sulfide ion. Wang et al. prepared CdS clusters in zeolites A, X, and Y and studied their photoluminescence. They characterized their samples using IR, X-ray powder diffraction, extended X-ray absorption fine structure (EXAFS) spectroscopy, and optical absorption and emission spectroscopies.7−9 They suggested that the cluster size was less than 13 Å at a low-loading level, and that a three-dimensional superlattice might form at higher loading.7 From other experiments, they reported that discrete Cd4(S or O)4 cubes formed in zeolite Y at low loading, and that these cubes begin to interconnect to form an imperfect supercluster structure at higher loadings.8 Their EXAFS analysis showed that the discrete cubes consisted of interpenetrating tetrahedra of Cd and S atoms with a Cd−S bond length of 2.47 Å in the sodalite cavities of zeolite Y, and that pairs of Cd atoms occupied double 6-rings with a Cd−Cd distance of ca. 6 Å. They suggested that the stability of the supercluster came from the interaction between the Cd4(S or O)4 clusters in adjacent sodalite cavities. Using nonlinear optical absorption and photoconductivity, Barnakov et al. reported the optical, electric, and photoelectric properties of the CdS clusters in zeolites X (Si/Al = 1.1) and A.2 Peng et al. used positron annihilation spectroscopy (PAS) to learn the locations of the CdS clusters in zeolite Y. They did this because previous studies (of the blue-shifted reflection absorption spectra with respect to that of bulk CdS) had not been able to determine whether the CdS clusters were in the sodalite cavities or the supercages.3 They concluded that they were in the sodalite cages. The photoactivity, photophysical properties, and thermoluminescence of the CdS clusters in zeolites were studied by Fox et al.,4 Chen et al.,5 and Liu et al.10 Using infrared spectroscopy, chemical analysis, ESR, and absorption spectroscopy, Liu et al. reported that the CdS clusters formed in the largest cages of the zeolites studied: the sodalite cage in sodalite, the large cavity in zeolite A, the supercage in zeolite X, and in the main channels of zeolites with channels.10 Recently, Jeong et al. and Kim et al. attempted to synthesize CdS quantum dots in zeolite Y and to investigate their photovoltaic properties.11−13 They prepared their material by treating fully dehydrated Cd2+-exchanged zeolite Y with dry H2S and characterized it by thermogravimetric analysis (TGA), solid-state 13C NMR, UV−vis spectra, transmission electron microscopy (TEM), and X-ray diffraction. Their results indicated strongly that CdS quantum dots had formed in the supercages of zeolite Y, and not the sodalite cavity.11 In some of their work, the sodalite cavities were prefilled with tetraalkylammonium ions, so that these dots could only form in the supercages. It is clear from the above that there has been much uncertainty regarding the nature and locations of the “CdS clusters” in zeolite Y for several decades. Definitive singlecrystal crystallography could not be done until suitably large single crystals of zeolite Y became available in 2001.17 1.2. Single Crystal Structures of Cd2+-Exchanged Zeolites. Many structures of Cd2+-exchanged zeolites A, X, and Y have been reported.16,18−26 McCusker et al. determined the structures of hydrated and dehydrated zeolite A exchanged

2. EXPERIMENTAL SECTION 2.1. Preparation of Cd2+-Exchanged Zeolite Y. Large colorless single crystals (mean diameter ca. 0.20 mm) of Na−Y 16723


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The Journal of Physical Chemistry C zeolite, |Na75|[Si117Al75O384]−FAU (Na75−Y, Si/Al = 1.56), were prepared by Lim et al.27 using the method of Ferchiche et al.17 They have a lower Si/Al ratio and therefore a higher ion exchange capacity than the zeolite Y of industry (Si/Al ca. 2.5). That benefits this work by allowing higher Cd2+ contents to be achieved. This increases both the reliability and the precision of the results. Crystals of hydrated Cd−Y (Cd37.5−Y or Cd36.5H2−Y) were prepared by the static ion-exchange method using aqueous 0.05 M Cd(NO3)2·4H2O (Aldrich, 99.999%, K 0.9 ppm, Na 0.8 ppm, Zn 0.3 ppm, Ag 0.1 ppm, Cr 0.1 ppm), pH 3.65. Hydrated Na75−Y (10 mg) was mixed with 20 mL of 0.05 M Cd(NO3)2 in a 25 mL conical tube. The mixture was stirred on a shaking incubator at 294 K for 36 h. This was repeated two times with fresh Cd(NO3)2 solution. The product was washed each time with distilled water followed by filtration and drying at 323 K for 24 h. 2.2. Preparation of Crystals for Structure Determination. 2.2.1. Fully Dehydrated Cd−Y. A single crystal, a clear pale brown octahedron about 0.20 mm in cross section, was selected and lodged in a fine Pyrex capillary. This capillary was attached to a vacuum system, and the crystal was cautiously dehydrated by gradually increasing its temperature (ca. 25 K/h) to 723 K. The crystal remained at this temperature and a dynamic vacuum of 1 × 10−4 Pa for 48 h. Protected from the sorption of water that might diffuse to it from the more distant parts of the vacuum system that had not been baked out, the crystal was allowed to cool to room temperature and was sealed off by torch under vacuum in its capillary. The crystal had become dark brown. 2.2.2. Cd−Y with Sorbed H2S. A second crystal of similar size was prepared as above and was treated with 6.7 × 104 Pa of zeolitically dry H2S(g) (Rigas 99.9%) at 294 K for 6 h, after which the excess H2S was removed by evacuation for 0.5 h. The capillary containing the crystal under vacuum was then sealed off and removed from the vacuum system by torch. This crystal had also become dark brown. 2.3. X-ray Diffraction. Diffraction data were collected for the two crystals using synchrotron X-radiation. Their temperatures were maintained at 100(1) K by a flow of cold nitrogen gas. Preliminary cell constants and an orientation matrix were determined from 72 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic scale file was prepared using the HKL3000 program.28 The reflections were successfully indexed by the automated indexing routine of the DENZO program.28 The diffraction data were harvested by collecting 72 sets of frames with 5° scans with an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects, and a very small correction for crystal decay was applied. The space group Fd3̅m, standard for zeolite Y, was determined by the program XPREP.29 A summary of the experimental and crystallographic data with some additional details is presented in Table 1.

Table 1. Summary of Experimental and Crystallographic Data crystal 1 crystal cross section (mm) ion exchange with Cd2+ T (K), t (h) dehydration T (K), P (Pa) sorption T (K), t (h), P (Pa) evacuation T (K), t (h) crystal color data collection T (K) space group, Z X-ray source detector detector to crystal distance, mm wavelength (Å) unit cell constant, a (Å) 2θ range in data collection (deg) total reflections harvested no. of unique reflections, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters, a/b Rinta Rsigmab final error indices R1/wR2 (Fo > 4σ(Fo))c R1/wR2 (all intensities)d goodness of fite

0.20 294, 72

crystal 2 0.20 294, 72

723, 1 × 10−4 723, 1 × 10−4 − 294, 6, 6.7 × 104 − 294, 0.5 dark brown dark brown 100(1) 100(1) Fd3̅m, 1 Fd3̅m, 1 Pohang Light Source (PLS) (Beamline 2D SMC) ADSC Quantum ADSC Quantum 210 210 63 63 0.70000 0.70000 24.780(1) 24.708(1) 59.02 66.95 79882 89825 1092 1390 1081 1281 53 20.6 0.065/172.3 0.0137 0.0117

66 21.1 0.081/192.9 0.0065 0.0097

0.040/0.156 0.044/0.170 1.46

0.049/0.186 0.067/0.256 1.42

Rint = ∑|Fo2 − Fo2(mean)|/∑[Fo2]; Rint is calculated from the merging of equivalent data for internal agreement for all reflections. b Rsigma = ∑[σ(Fo2)]/∑[Fo2]. cR1 = ∑|Fo − |Fc||/∑Fo and wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2; R1 and wR2 are calculated using only the reflections for which Fo > 4σ(Fo). dR1 and wR2 are calculated using all unique reflections measured. eGoodness of fit = [∑w(Fo2 − Fc2)2/ (m − s)]1/2, where m is the number of unique reflections and s is the number of variables. a

structure determination and refinement as new atomic positions were found on successive difference Fourier electron density functions are shown in Table 2. The most difficult part of completing this structure involved the S2 position. What was its symmetry, and how should four of them be placed in the 24 equivalent positions (see Figure 1) about each Cd5 ion? (S2 is at Wyckoff position 192(i), and there are 8 supercages per unit cell: hence 192/8 = 24 positions per supercage.) They were placed so that the resulting S2− Cd5−S2 angles would be closest to tetrahedral (see Table 4). This is an example of the chemical judgment that must be added if sense is to be made of structures with many partially occupied positions. Before that could be done, the symmetry at the S2 position needed to be established. Because pairs of the 24 positions are close together (Figure 1), it might have been possible to select fewer positions of higher symmetry. However, the error indices and the less reasonable thermal parameters argued against these alternatives. Still, in the actual structure, there may be some fluxional motion at the S2 position as that atom moves from one of its positions to an equivalent one. Such motions may be correlated among the four S2 atoms.

3. STRUCTURE DETERMINATION Full-matrix least-squares refinement (SHELXL2014)30 was done on F2 using all data for both crystals. Each refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O1, O2, O3, and O4] in dehydrated |Tl75| [Si117Al75O384]−FAU.31 Each initial refinement used anisotropic thermal parameters and converged to the high error indices (given in step 1 of Table 2) R1/wR2 = 0.52/0.86 and 0.53/0.90 for crystals 1 and 2, respectively. The further steps of 16724


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The Journal of Physical Chemistry C Table 2. Steps of Structure Determination and Refinement occupancya step

Cd1

Cd1′

Cd2′

Cd2a

Cd2b

Cd3a

Cd3b

Cd5

S1

S2

R1

wR2

0.52 0.40 0.103 0.088 0.071 0.0554 0.0554 0.0403

0.86 0.80 0.28 0.25 0.21 0.1761 0.1762 0.1561

0.53 0.093 0.081 0.069 0.063 0.0589 0.0587 0.0561 0.0551 0.0551 0.0551 0.0567 0.0485

0.90 0.29 0.26 0.24 0.22 0.2083 0.2075 0.2041 0.1988 0.1981 0.1972 0.2007 0.1857

Crystal 1, |Cd36.5H2|[Si117Al75O384]−FAU 1b 2 3 4 5 6 7c 8d

12.0(5) 15.0(2) 15.0(2) 14.7(1) 14.7(1) 14.7(1) 14.6(1)

1b 2 3 4 5 6 7 8 9 10c 11e 12f 13g

13.5(1) 13.8(1) 13.9(1) 13.8(1) 13.8(1) 13.8(1) 13.7(1) 13.7(1) 13.7(1) 13.7(1) 13.7(1) 13.7(1)

2.7(2) 2.6(1) 2.8(1)

2.2(2) 2.5(1) 2.9(1) 2.7(1) 2.7(1) 2.6(1) 2.5(1) 2.3(1) 2.3(1) 2.3(1) 2.3(1)

2.5(2) 2.3(2) 2.8(2) 2.8(2) 2.7(2)

16.1(2) 15.3(2) 13.2(3) 2.7(3) 13.4(2) 2.7(2) 13.3(2) 2.7(2) 13.7(7) 2.6(6) Crystal 2, |Cd27(Cd4S6+)0.6(Cd5(SH)46+)0.9H12|[Si117Al75O384]−FAU

1.7(1) 1.8(1) 1.9(1) 1.9(1) 1.9(1) 2.0(1) 2.0(1) 2.0(1) 2.1(1) 2.3(1)

6.3(6) 6.1(5) 5.8(5) 5.7(5) 6.5(5) 6.8(5) 7.2(5) 8.6(6) 11.0(5)

4.0(4) 4.1(4) 3.9(4) 4.4(4) 4.7(4) 5.1(4) 3.6(2) 3.6(2)

0.4(1) 0.4(1) 0.4(1) 0.9(1) 0.9(1)

0.7(3) 1.2(4) 1.5(5) 1.4(5) 0.6(1) 0.58(2) 0.58(2)

6.1(8) 6.2(8) 6.4(8) 6.7(9) 3.6(2) 3.6(2)

a The occupancy is given as the number of ions or molecules per unit cell at each position. bOnly the zeolite framework positions were included in the initial structure model; all were refined anisotropically. cThe occupancies at Cd1 and Cd1′ were constrained (D6Rs; site I + site I′/2 = 16). dAll Cd2+ positions were refined anisotropically. eThe occupancies of Cd1′ and S1 were constrained to the ratio 4:1. fThe occupancies of Cd3b, S2, and Cd5 were constrained to the ratio 4:4:1. gCd1, Cd1′, Cd2, and Cd3a were refined anisotropically. Cd3b, Cd5, S1, and S2 were refined isotropically.

All scattering factors were modified to account for anomalous dispersion.34,35 The final error indices are given in Table 1. The structural parameters are given in Table 3, and selected interatomic distances and angles can be found in Table 4.

4. DESCRIPTION OF THE STRUCTURES 4.1. Zeolite Y Framework and Exchangeable Cation Sites. The framework structure of zeolite Y (FAU) is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cuboctahedron), and the supercage (see Figure 2). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations, which balance the negative charge of the zeolite Y framework, usually occupy some or all of the sites shown with Roman numerals in Figure 2. The orders (maximum occupancies, ions per unit cell) of the cation sites I, I′, II′, II, and III are 16, 32, 32, 32, and 48, respectively. Site III′ in zeolite Y studied using space group Fd3̅m is a 192-fold position. Site V, order = 8, is at the center of the supercage, too far from the zeolite framework to bond to it. Further description is available.37,38 4.2. Crystal 1, |Cd36.5H 2|[Si117Al75O384]−FAU. The 36.5(9) Cd2+ ions per unit cell are distributed over five equipoints, sites I, I′, II′, and two sites II. The 14.6(1) Cd2+ ions per unit cell at Cd1 nearly fill site I (at the centers of the D6Rs, see Figure 3a). Each coordinates to the six O3 framework oxygens of its D6R at 2.370(3) Å, a little longer than the sum of the corresponding conventional ionic radii, 0.97 + 1.32 = 2.29 Å.39 (Shannon’s crystal radii for 6-coordinate Cd2+ and 2-coordinate O2− give 0.95 + 1.21 = 2.16 Å; a radius for 3-coordinate O2− is not given.40) The O3−Cd1−O3 bond

Figure 1. The 24 equivalent positions about Cd5 into which four S2 atoms must be placed.

Another difficult step in solving this structure involved recognizing that the Cd3 position should be resolved into two positions, Cd3a and Cd3b, and that only one could be refined anisotropically. Fixed weights were used initially; the final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [max(Fo2,0) + 2Fc2]/3; the refined values of a and b are given in Table 1. Atomic scattering factors for Cd2+, O−, S2−, and (Si,Al)1.80+ were used.32,33 The function describing (Si,Al)1.80+ is the weighted (for composition) mean of the Si4+, Si0, Al3+, and Al0 functions (half formal charges were assumed). 16725


16726

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 192(i) 192(i) 8(b) 8(a) 192(i)

Si, Al O1 O2 O3 O4 Cd1 Cd1′e Cd2 Cd3a Cd3bf Cd5f S1e S2f

I I′ II III′ III′ V

I I′ II′ II II

cation site

y 12261(3) 0 −305(10) 6361(10) 8092(11) 0 6373(42) 19929(46) 22025(12) 23485(238)

12345(2) 0 −426(8) 6344(7) 8124(8) 0 6128(22) 23708(33) 29517(86) 26326(83) 37500 12500 31000(97)

x −5412(3) −10811(10) −305(10) −3251(14) 8092(11) 0 6373(42) 19929(46) 22025(12) 23485(238)

−5343(2) −10959(8) −426(8) −3504(11) 8124(8) 0 6128(22) 23708(33) 17992(67) 20743(83) 37500 12500 29676(99)

U11b or Uiso U22

U33

Crystal 2, |Cd27(Cd4S6+)0.6(Cd5(SH)46+)0.9H12|[Si117Al75O384]−FAU 3571(2) 159(3) 117(3) 129(3) 10959(8) 231(7) 228(12) 231(7) 14322(11) 230(8) 230(8) 194(11) 6344(7) 246(12) 192(7) 192(7) 31840(11) 184(7) 184(7) 243(12) 0 115(2) 115(2) 115(2) 6128(22) 363(20) 363(20) 363(20) 23708(33) 452(35) 452(35) 452(35) 41436(65) 1283(119) 2478(187) 1435(127) 39184(82) 534(51) 37500 1046(108) 12500 764(278) 35571(100) 149(55)

Crystal 1, |Cd36.5H2|[Si117Al75O384]−FAUd 3559(3) 148(5) 115(4) 123(4) 10811(10) 213(10) 219(16) 213(10) 14415(14) 211(10) 211(10) 176(16) 6361(10) 190(15) 180(9) 180(9) 31465(16) 205(11) 205(11) 295(19) 0 151(3) 151(3) 151(3) 6373(42) 601(46) 601(46) 601(46) 19929(46) 367(49) 367(49) 367(49) 22025(12) 154(12) 154(12) 154(12) 23485(238) 470(142) 470(142) 470(142)

z

−21(2) −65(6) −4(7) 19(9) −5(6) 8(1) 36(19) 277(33) −650(121)

−20(3) −49(9) 3(9) 16(12) 31(10) 23(2) 189(44) 152(47) 26(9) 242(152)

U23

10(2) −17(9) −4(7) 43(6) −5(6) 8(1) 36(19) 277(33) −778(93)

−2(3) 15(13) 3(9) 21(8) 31(10) 23(2) 189(44) 152(47) 26(9) 242(152)

U13

−5(2) −65(6) 68(9) 43(6) 56(9) 8(1) 36(19) 277(33) −275(113)

−3(3) −49(9) 54(12) 21(8) 72(13) 23(2) 189(44) 152(47) 26(9) 242(152)

U12

192 96 96 96 96

192 96 96 96 96

fixed

13.67(6) 2.31(6) 2.33(11) 11.0(5) 3.59(22) 0.90(6) 0.58(2) 3.59(22)

14.59(6) 2.81(12) 2.75(18) 13.7(7) 2.6(6) 36.5(9)

varied

occupancyc

a Positional ×105 and thermal parameters ×104 are given. Numbers in parentheses are the esd’s in the units of the least significant figure given for the corresponding parameter. bThe anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. cThe occupancy is given as the number of atoms or ions per unit cell. dThis is the composition determined crystallographically by counting Cd2+ ions. The composition may be (is only about one esd from being) Cd37.5−Y. H+ ions from the Cd2+ exchange solution (pH 3.65) or the final washing step (section 2.1) may have exchanged into the zeolite. eMembers of the Cd4S6+ cluster. fMembers of the Cd(SHCd)46+ cluster.

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e)

Wyckoff position

Si, Al O1 O2 O3 O4 Cd1 Cd1′ Cd2′ Cd2a Cd2b ∑Cd2+

atom

Table 3. Positional, Thermal, and Occupancy Parametersa

The Journal of Physical Chemistry C Article


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The Journal of Physical Chemistry C Table 4. Selected Interatomic Distances (Å) and Angles (deg)a (Si,Al)−O1 (Si,Al)−O2 (Si,Al)−O3 (Si,Al)−O4 mean (Si,Al)−O Cd1−O3 Cd1′−O3 Cd1′−S1 Cd2′−O2 Cd2−O2 Cd2a−O2 Cd2b−O2 Cd3a−O1 Cd3a−O4 Cd3b−O1 Cd3b−O4 Cd3b−S2 Cd5−S2 O1−(Si,Al)−O2 O1−(Si,Al)−O3 O1−(Si,Al)−O4 O2−(Si,Al)−O3 O2−(Si,Al)−O4 O3−(Si,Al)−O4 (Si,Al)−O1−(Si,Al) (Si,Al)−O2−(Si,Al) (Si,Al)−O3−(Si,Al) (Si,Al)−O4−(Si,Al) O3−Cd1−O3 O3−Cd1′−O3 O3−Cd1′−S1 S1−Cd1′−S1 O2−Cd2′−O2 O2−Cd2−O2 O2−Cd2a−O2 O2−Cd2b−O2 O1−Cd3a−O4 O1−Cd3b−S2 Cd3b−S2−Cd5 S2−Cd5−S2

crystal 1

crystal 2

1.6420(18) 1.6742(15) 1.7048(17) 1.6297(10) 1.663 2.370(3) 2.385(11) − 2.327(7) − 2.209(4) 2.34(3) − − − − − − 112.23(18) 106.85(19) 112.06(18) 105.60(16) 106.85(19) 112.06(18) 131.16(24) 139.72(24) 126.87(20) 161.5(3) 89.43(12) 90.57(12) 89.9(5) − − 110.2(5) − 119.54(4) 109.5(23) − − − −

1.6795(14) 1.6403(11) 1.6953(12) 1.6296(8) 1.661 2.380(3) 2.381(6) 2.727(10) − 2.395(6) − − 2.469(11) 2.898(19) 2.566(21) 3.108(21) 2.65(3) 2.56(3) 110.89(10) 104.80(12) 108.37(13) 107.78(13) 112.28(14) 112.48(13) 127.78(18) 143.44(18) 129.44(16) 153.76(19) 87.39(9) 92.61(9) 92.6(3) 123.44(20) 109.47b − 108.2(3) − − 59.4(3) 137.5(10) 147.3(11) 102.2(10) 113.2(6)

Figure 2. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1−4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein’s rule (ref 36) would be obeyed. Extraframework cation positions are labeled with Roman numerals or the letters “U” and “V”.

a

The numbers in parentheses are the esd’s in the units of the least significant digit given for the corresponding parameter. bThe tetrahedral angle by symmetry.

angles are 89.43(12) and 90.57(12)° (Table 4), nearly perfectly octahedral. The distance between sites I and I′ is only 2.74(2) Å. Intercationic electrostatic repulsion should be severe for Cd2+ ions at this distance. However, the observed occupancies indicate that this approach is being avoided. If a site I is occupied, the two I′ positions of that same D6R should not be. Accordingly, only (16 − 14.6) × 2 = 2.8 site I′ positions should be available for Cd2+ ions per unit cell. That, 2.8(1), is exactly the number of Cd2+ ions that were found at site I′ (Figure 3b), before that constraint (Table 2, footnote c) was applied. Accordingly, it is concluded that all D6Rs are occupied, either by a Cd2+ ion at site I or by two ions at two I′ sites.

Figure 3. Stereoviews of the double 6-rings (D6Rs) in crystals 1 and 2. Of the 16 D6Rs per unit cell, 14.6 and 13.7, respectively, are occupied by Cd1 ions as shown in (a). The remaining D6Rs hold 2.8 and 2.3 Cd1′ ions, respectively, as shown in (b). The zeolite Y framework is drawn with heavy bonds. The coordination of Cd2+ ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.

Each ion at Cd1′ is recessed 1.38 Å into a sodalite cavity from its three O3 plane, so the O3−Cd1′−O3 angle, 89.9(5)°, 16727


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occupy six different equipoints in this structure. At site I 13.7(1) Cd2+ ions are found, and 2.31(6) occupy site I′, so the D6Rs are not quite fully occupied; 2.3 D6Rs may host a Cd2+ ion at just one of their two I′ positions. The site II position, Cd2, has 2.33(11) Cd2+ ions per unit cell. Two different site III′ positions are occupied by 11.0(5) and 3.6(2) Cd2+ ions at Cd3a and Cd3b, respectively, and the remaining 0.9 Cd2+ ion, Cd5, is at the center of supercage (site V). The 13.7(1) Cd2+ ions at Cd1 nearly fill the 16-fold site I (Figure 3a). Each is octahedrally coordinated by the six O3 oxygen atoms of its D6R as in crystal 1. Each of the 2.3 ions at Cd1′ coordinates to three O3 framework oxygens at 2.381(6) Å as in crystal 1. In this structure, however, each also bonds at 2.727(10) Å to a sulfide ion at S1 (Figure 6). These 2.31(6) Cd2+ ions, together with

is far from trigonal planar (120°) (see Figures 3b and 4). It coordinates to three O3 framework oxygens at 2.385(11) Å. This is the same as the Cd1−O3 distance, 2.370(3) Å.

Figure 4. Stereoview of a representive sodalite cavity in crystal 1. See the caption to Figure 3 for other details.

The 2.7(2) Cd2+ ions per unit cell at Cd2′ are located at site II′ (opposite S6Rs in the sodalite cavity, Figure 4). Each coordinates near trigonally to the three O2 oxygens of its S6R at 2.327(7) Å. At site II (opposite S6Rs in the supercages), 13.7(5) and 2.3(4) Cd2+ ions are found at Cd2a and Cd2b, respectively (Figures 4 and 5). Both of these positions are in the supercage,

Figure 6. Stereoview of a sodalite cavity in crystal 2 containing a Cd4S6+ ion. Cd−S bonds are indicated by lines of intermediate thickness. See the caption to Figure 3 for other details.

0.58(2) S2− ions at S1, form about 0.6 tetrahedral Cd4S6+ clusters with symmetry 4̅3m (Td) per unit cell. They occupy only 7.5% of the sodalite cages. At site II, 2.3(1) Cd2 ions are found (Figure 7). Their positions are similar to those in crystal 1. In this structure Cd2+ Figure 5. Stereoview of a representive supercage in crystal 1. See the caption to Figure 3 for other details.

0.15 and 0.77 Å, respectively, from the plane of their three O2 framework oxygen atoms (Table 5). The corresponding Cd2+− O2 bond lengths are 2.209(4) and 2.34(3) Å, respectively. Cd2b may occupy 6-rings with fewer Al3+ ions than Cd2a does. 4.3. Crystal 2, |Cd27(Cd4S6+)0.6(Cd(SHCd)46+)0.9H12| [Si117Al75O384]−FAU. About 33.8 Cd2+ ions per unit cell Table 5. Displacements of Atoms (Å) from Six-Ring Planes

Figure 7. Stereoview of a supercage in crystal 2 containing a Cd(SHCd)46+ ion. See the captions to Figures 6 and 3 for other details.

displacement at O3a at O2b

atom

site

crystal 1

crystal 2

Cd1 Cd1′ Cd2′ Cd2 or Cd2a Cd2b

I I′ II′ II II

−1.38 1.38 −0.74 0.15 0.77

−1.43 1.31 − 0.84 −

ions occupy two different III′ sites. About 11.0(5) at Cd3a coordinate to just two framework oxygens, O1 at 2.469(11) Å and O4 at 2.898(19) Å (Figure 7). The O1−Cd3a−O4 angle is 59.4(3)°. The 3.6(2) Cd2+ ions at Cd3b each bond to just one framework oxygen, O1, at 2.566(21) Å and to a hydrosulfide ion, S2, at 2.65(3) Å. The O1−Cd3b−S2 angle is 137.5(10)° (Figure 7). The presence of a hydrogen atom bonded to S2 is inferred; it was not found crystallographically. The remaining 0.90(6) Cd2+ ions per unit cell are found at the center of the supercage at site V. Each Cd5 ion bonds to four hydrosulfide ions, S2, at 2.56(3) Å. Cd5 is thus distorted

a

Site I′ is near the plane of one 6-ring of a D6R; displacements into the sodalite unit are given as positive. A negative deviation indicates that the atom lies within a D6R. bSite II is in the supercage; displacements from its 6-rings are given as positive. A negative deviation indicates that the atom is at site II′ and lies within a sodalite cavity. 16728


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The Journal of Physical Chemistry C Table 6. Distribution of Chemical Species and Charge Balance crystal 1 atom

site

Cd1 Cd1′ Cd2′ Cd2 or Cd2a Cd2b Cd3a Cd3b Cd5 S1 S2 Hb

I I′ II′ II II III′ III′ V

chem species 2+

Cd Cd2+ Cd2+ Cd2+ Cd2+

occupancya

crystal 2 occupancya

ionic charge

Cd Cd2+

13.7(1) 2.3(1)

+2 +2

Cd2+

2.3(1)

+2

Cd2+ Cd2+ Cd2+ S2− SH− H+

+2 +2 +2 −2 −1 +1

crystal 1

11.0(5) 3.6(2) 0.9(1) 0.6(2) 3.6(2) 12 crystal 2

36.5(9) 36.5(9) − − 2 75

33.8(6) 27.0(5) 0.6 0.9 12 75

ionic charge

14.6(1) 2.8(1) 2.8(2) 13.7(7) 2.6(6)

a

2+

+2 +2 +2 +2 +2

2 total Cd2+ Cd2+ not bound to S Cd4S6+ Cd(SHCd)46+ H+ not bound to S total charge

chem species

Number of ions and atoms per unit cell. bNumber of H+ ions per unit cell required to balance the negative charge of the zeolite framework, 75−.

perhaps all, 10−12 per unit cell, originate from the sorbed H2S molecules. The following net reaction occurred when H2S was sorbed onto fully dehydrated, nearly fully Cd2+-exchanged zeolite Y at 294 K.

tetrahedral; of the six S2−Cd5−S2 angles, two are 102.2(10)° and four are 113.2(6)°. The Cd5−S2−Cd3b angle is 147.3(11)°. Four −S2−Cd3b groups extend from each ion at Cd5 to give 0.9 Cd(SHCd)46+ clusters in the eight supercages (see Figure 7), filling only 11% of them.

Cd36.5H 2−Y + 6.8H 2S(g) → Cd2 +27(Cd4S6 +)0.6

5. DISCUSSION

(Cd(SHCd)4 6 + )0.9 H+12−Y + 2.6CdS(s)

Crystal 1 is the empty Cd2+-exchanged zeolite before the sorption of H2S(g). Crystal 2 shows the structure afterward. Two distinctly different subnanometer clusters of cadmium sulfide have formed in the zeolite, one in the sodalite cavity (section 5.2.1) and one in the supercage (section 5.2.2), both cationic and both unlike small particles of CdS(s). 5.1. Crystal 1, |Cd36.5H2|[Si117Al75O384]−FAU. In crystal 1, 36.5(9) Cd2+ ions were found per unit cell. This indicates a unit cell composition of Cd36.5H2−Y, but it is only about one estimated standard deviation (esd) from being Cd37.5−Y, complete Cd2+ exchange. Although Cd2+ exchange may have been complete, the formula Cd36.5H2−Y is used in this discussion. Cd2+ ions occupy five sites in fully dehydrated Cd2+exchanged zeolite Y (Table 3). When the Cd2+ exchange was done at a higher pH, Cd(OH)2 was imbibed, presumably as CdOH+, and Cd2+ ions were found at six sites.25 Using that same higher pH solution, Cd46−X (Si/Al = 1.09) with 16 Cd2+ ions at site I and 30 at site II was prepared.23 Zeolite Y, then, because it is less polar and has a greater diversity of cation sites (e.g., 6-rings with different numbers of Al3+ ions) than zeolite X, is more prone to anion imbibition and shows a more complex distribution of Cd2+ ions over the available sites. 5.2. Crystal 2, |Cd27(Cd4S6+)0.6(Cd(SHCd)46+)0.9H12| [Si117Al75O384]−FAU. The sum of the cationic charges of the Cd2+, Cd4S6+, and Cd(SHCd)46+ ions, 63+ per unit cell, is insufficient to balance the negative charge of the product zeolite framework, 75− (Table 6). The difference is attributed to 12 additional H+ ions. Two may have exchanged into the crystal from the Cd2+ exchange solution, but the majority of them,

It might have been expected that a zeolite with 36.5 Cd2+ ions per unit cell would have sorbed many more than 6.8 H2S molecules, and retained more than 4.2 sulfide and hydrosulfide ions (0.6 S2− and 3.6 HS−). All of the hydrogen ions in the 6.8 H2S molecules that entered the zeolite per unit cell remained in the zeolite. Of the 6.8 incoming sulfide ions, 2.6 extracted Cd2+ ions to from CdS(s) on the crystal’s surface. Of the remaining 33.9 Cd2+ ions, most (27) did not coordinate to a sulfide ion; they had not reacted with H2S. Cd2+ ions occupy six different crystallographic sites in crystal 2: I, I′, II, two different III′ sites, and V (center of supercage). Cd1, Cd2, and Cd3a coordinate only to framework oxygen atoms. The Cd1′ ions bond to S2− ions at S1 at the centers of sodalite cavities (Figure 6). The Cd2+ ions at the second III′ site, Cd3b, bridge between O1 and S2 (a SH− ion) (Figure 7). The S2 atoms in turn bond to Cd5 at the center of supercage. 5.2.1. Cd4S6+ Cluster. In 7.5% of the sodalite cavities, four Cd2+ ions at Cd1′ are arranged tetrahedrally about a sulfide ion, S1, at its center (Figure 6). Its Cd−S bond length, 2.727(10) Å, is noticeably longer than the Cd−S bond lengths in α-CdS (hexagonal, wurtzite structure, 2.527 Å42) and β-CdS (cubic, zinc blende (sphalerite) structure, 2.525 Å41); both bond lengths have been converted from kX units to angstroms. The lengthening may be attributed to the strong bonds that Cd1′ makes to framework oxygen atoms. The S1−Cd1′−S1 bond angles are tetrahedral by symmetry as in both phases of CdS(s).15,41,42 This Cd4S6+ cluster is held in place by the coordination of each Cd2+ ion to three 6-ring framework oxygen atoms. 16729


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The Journal of Physical Chemistry C 5.2.2. Cd(SHCd)46+ Cluster. In 11% of the supercages, four −SH−Cd groups are arranged somewhat tetrahedrally about a Cd2+ ion at its center to give Cd(SHCd)46+ (Figure 7). The four terminal Cd2+ ions each coordinate to just one framework oxygen atom, Cd3b−O1 = 2.566(21) Å; a second framework oxygen atom is much further away, Cd3b−O4 = 3.108(21) Å. The Cd−S bond lengths in this cluster, S2−Cd5 = 2.56(3) Å and S2−Cd3b = 2.65(3) Å, are similar to those in the two phases of CdS(s), ca. 2.53 Å (section 5.2.1). It was possible to select S2−Cd5−S2 bond angles, 102.2(10) and 113.2(6)°, that were close to tetrahedral (section 3, paragraph 2). 5.3. Charge, Structure, and Composition of Cadmium Sulfide Clusters in Zeolites. To date, the structures of four “cadmium sulfide clusters” have been determined crystallographically. They were all prepared by introducing sulfide ions into Cd2+-exchanged zeolites. They are all cations; none are neutral clusters of formula CdnSn. The two CdS-containing zeolites whose structures have been determined, A and Y, each had two kinds of clusters. Each of these zeolites has two kinds of cavities, a sodalite cavity and a larger cavity, and each cavity hosted a cluster that fit it. Cd6S44+ and Cd2Na2S4+ formed when zeolite LTA was treated with Na2S(aq),6 and Cd4S6+ and Cd(SHCd)46+ formed when zeolite Y was treated with H2S(g) (this work). Only one of the these four cationic clusters, the smallest one, the tetrahedral Cd4S6+ cluster (Figure 8), has a structure that is

Zeolites may be expected to contain more than one kind of cluster, especially if they have more than one kind of cavity of sufficient size. Ions other than those intended for cluster formation, some of which are necessarily present during sample preparation, may be members of the clusters. They are primarily M+ when sulfide is introduced as M2S(aq) (M = Na in all reports to date), and H+ when sulfide is introduced as H2S(g). They may, however, include oxide and other anions that may have accompanied the cations into the zeolite, M′OH+ or M′Cl+ for example, during the cation exchange procedure. For simple electrostatic reasons, especially for high-Al zeolites, the clusters should be cationic. They would be held in the zeolite structure for the same reasons that monatomic exchangeable cations remain there: they coordinate to oxygen atoms and balance the framework charge. Indeed, most clusters found in zeolites are cationic; a lengthy list is available.42 It would be expected that neutral atoms, molecules, or clusters, unless they are retained by bonding to exchangeable cations (thereby becoming cationic clusters), would readily exit the zeolite to form their own bulk phases whose free energy would be lower. Still, some neutral ionic clusters have been found; examples are Ag4X4, X = Cl, Br, and I.43 There have been no reports of anionic clusters in zeolites.

6. SUMMARY The structures of the CdS clusters that form when dehydrated Cd2+-exchanged zeolite Y is exposed to anhydrous H2S(g) at 294 K have been determined. There are two kinds of CdS clusters, both cations; no neutral CdnSn clusters are seen. Tetrahedral Cd4S6+ clusters center 7.5% of the sodalite cavities. Tetrahedral Cd(SHCd) 4 6+ clusters center 11% of the supercages. The zeolite also contains H+ ions, in addition to those in the SH− ions. All or most are from the sorbed H2S(g) molecules; a few may have entered during the Cd2+ exchange procedure. Some CdS(s) exited the crystal and deposited on its surface.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Treating fully dehydrated Cd−Y with H2S(g) results in the formation of these two cationic cadmium sulfide clusters within the zeolite.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04369.

a subunit of a CdS(s) structure (zinc blende or wurtzite). Some atoms in the other three are tetrahedral like all of the atoms in both CdS(s) structures. Perhaps the best way to introduce a high concentration of cationic cadmium sulfide clusters into zeolites is to treat a zeolite with Cd(NO3)2(aq) at a moderately acidic pH, followed by Na2S(aq) or (NH4)2S(aq), followed by Cd(NO3)2(aq) again, and so on for several cycles.2 Alternatively, exposure to H2S(g) at elevated temperatures might be tried. 5.4. Emerging Principles Regarding the Structures of Clusters of Ionic Compounds in Zeolites. From the discussion in section 5.3, some tentative principles governing the structures of ionic compounds occluded as nanometer or subnanometer clusters in zeolites have emerged. The structures of small clusters of ionic compounds occluded within zeolites will not be subunits of their bulk solids unless these clusters are trivially small, e.g., an atom and its immediate coordination sphere.

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Table of calculated structure factors squared, and observed structure factors squared with esd’s (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82 54 820 5454. Fax: +82 54 822 5452. E-mail: wtlim@ andong.ac.kr (W.T.L.). *Tel.: 808-226-7917. Fax: 808-956-5908. E-mail: seff@hawaii. edu (K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors wish to thank the staff at Beamline 2D SMC of the Pohang Light Source, Korea, for assistance during data collection. This work was supported by a grant from 2015 Research Funds of Andong National University. 16730


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Structures of the Subnanometer Clusters of Cadmium Sulfide

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