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Layer-Anion Interactions in Magnesium Aluminum Layered Double Hydroxides Intercalated with Cobalticyanide and Nitroprusside Joseph W. Boclair, Paul S. Braterman,* Brian D. Brister, and Faith Yarberry Departments of Chemistry and Materials Science, University of North Texas, Denton, Texas 76203-5070 Received March 11, 1999. Revised Manuscript Received May 13, 1999
The interactions of cobalticyanide and nitroprusside with magnesium aluminum layered double hydroxide are investigated using oriented infrared spectroscopy together with powder X-ray diffraction. Cobalticyanide is known to intercalate into layered double hydroxides in the same manner as ferrocyanide, with its 3-fold rotational axis perpendicular to the cation sheets. The intercalated cobalticyanide anion shows a small yet observable deviation from local Oh symmetry, causing small differences between its oriented and nonoriented infrared spectra. Nitroprusside is shown to intercalate into 2:1 Mg:Al LDH with decomposition to form intercalated ferrocyanide and nitrosyl groups of an unidentified nature.
Introduction Layered double hydroxides (LDHs) are a family of widely distributed anion-exchanging minerals having the general formula [MII1-xMIIIx(OH)2][Y]x‚yH2O [Y ) e.g. Cl, 1/2(CO3)], sometimes known as anion-exchanging clays.1-6 Their structural chemistry is based on that of magnesium hydroxide, brucite, in which Mg2+ ions are arranged in sheets, each magnesium ion being octahedrally surrounded by six hydroxide groups while each hydroxide spans three magnesium ion. Replacement of some fraction of the divalent ions by a trivalent ion of comparable size (e.g., Al3+, Fe3+) results in a net positive charge on the layers, which is balanced by the intercalation of anions. The pillar density (milliequivalents of exchangeable anion per gram of support) or space available for each of these anions can be controlled by varying the MII:MIII ratio. For the LDH discussed here, the pillar density is 4.68 mequiv/g, assuming an ideal layer composition [Mg2Al(OH)6‚xH2O]+n with x ∼ 2. Materials of this family have many established applications, catalysts and catalyst precursors,7-9 hosts for photoactivation and photocatalysis,10-13 and anion ex* To whom correspondence should be addressed. E-mail: psb@ unt.edu. (1) Drezdzon, M. A. A.C.S. Symp. Ser. 1990, 437, 140. (2) Lagaly, G.; Beneke, K. Colloid Polym. Sci. 1991, 269, 1198. (3) Cavini, F.; Triffiro´, F.; Vaccari, A. Catal. Today 1991, 11, 173. (4) Carrado, K. A.; Kostapapas, A.; Suib, S. L. Solid State Ionics 1988, 26, 77. (5) de Roy, A.; Forano, C.; El Malki, K.; Besse, J.-P. In Synthesis of Microporous Materials; Occelli, M. L., Robson, H. E., Eds.; Van Nostrand Reinhold: New York, 1992; Vol. 2, p 108. (6) Trifiro´, F.; Vaccari, A. In Comprehensive Supramolecular Chemistry; Atwood, J. L., Macnicol, D. D., Davies, J. E. D., Vogtle, F., Eds.; Pergamon: Oxford, 1996; Vol. 7, p 251. (7) Corma, A.; Fornes, V.; Matinaranda, R. M.; Rey, F. J. Catal. 1992, 134, 58. (8) Suzuki, E.; Ono, Y. Bull. Chem. Soc. Jpn. 1988, 61, 1008. (9) Laycock, D. E.; Collacott, R. J.; Skelton, D. A.; Tchir, M. F. J. Catal. 1991, 130, 354. (10) Takagi, K.; Shichi, T.; Usami, H.; Sawaki, Y. J. Am. Chem. Soc. 1993, 115, 4339.
change,14,15 and have been invoked in studies of the origins of life.16 LDHs containing transition metals via intercalated metal complex anions, such as ferrocyanide17-24 and cobalticyanide,25 have drawn interest as catalysts,22 as mixed-oxide precursors,17,18 and in modified electrode formation.21 While powder XRD is the most usual method of study for these materials, IR spectroscopy has been used to confirm the presence of such anions as sulfate, ferricyanide, and ferrocyanide (ref 3 and references therein). We have developed a simple technique for obtaining oriented IR spectra for LDH26 and now report the use of this technique to demonstrate slight deformation of the cobalticyanide force field from local Oh symmetry upon intercalation into LDH, in addition to evidence for the decomposition of nitroprusside to ferrocyanide and (11) Valim, J.; Kariuki, B. M.; King, J.; Jones, W. Molecular Crystals and Liquid Crystals 1992, 211, 271. (12) Giannelis, E. P.; Nocera, D. G.; Pinnavaia, T. J. Inorg. Chem. 1987, 26, 203. (13) Tagaya, H.; Sato, S.; Kuwahara, T.; Kadokawa, J.; Masa, K.; Chiba, K. J. Mater. Chem. 1994, 4 (12), 1907. (14) Clearfield, A.; et al. J. Inclusion Phenom. Mol. Recognit. Chem. 1991, 11, 361. (15) Ookubo, A.; Ooi, K.; Hayashi, H. Langmuir 1993, 9, 1418. (16) Kuma, K.; Paplawski, W.; Gedulin, B.; Arrhenius, G. Origins Life 1989, 19, 573. (17) Crespo, I.; Barriga, C.; Rives, V.; Ulibarri, M. A. Solid State Ionics 1997, 101-103, 729. (18) Holgado, M. J.; Rives, V.; Sanroma´n, M. S.; Malet, P. Solid State Ionics 1996, 92, 273. (19) Hansen, H. C. B.; Koch, C. B. Clays Clay Miner. 1994, 42, 170. (20) Idemura, S.; Suzuki, E.; Ono, Y. Clays Clay Miner. 1989, 37, 553. (21) Itaya, K.; Chang, H.-C.; Uchida, I. Inorg. Chem. 1987, 26, 624. (22) Cavalcanti, F. A. P.; Schutz, A.; Bilden, P. In Preparation of Catalysts IV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier Science Publishers: Amsterdam, 1987; p 165. (23) Kikkawa, S.; Koizumi, M. Mater. Res. Bull. 1982, 17, 191. (24) Mao, G.; Tsuji, M.; Tamaura, Y. Clays Clay Miner. 1993, 41, 731. (25) Suzuki, E.; Idemura, S.; Ono, Y. Clays Clay Miner. 1989, 37, 173. (26) Braterman, P. S.; Tan, C.; Zhao, J. Mater. Res. Bull. 1994, 29, 1217.
10.1021/cm990148l CCC: $18.00 © 1999 American Chemical Society Published on Web 07/17/1999
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Figure 1. Infrared spectra (nonoriented) of (a) 2:1 Mg:Al LDH chloride, (b) 2:1 Mg:Al LDH cobalticyanide, and (c) 2:1 Mg:Al LDH nitroprusside.
some sort of nitrosyl group following insertion into the LDH.
Table 1. Observed Infrared Frequencies (Nonoriented) for 2:1 Mg:Al LDHs Containing Chloride, Cobalticyanide, and Nitroprusside (cm-1)a
Experimental Section Pure water (18 MΩ/cm, purchased from Scientific Products or obtained from a Milli Q Plus water purification system), boiled and purged with nitrogen to remove carbon dioxide, was used throughout this work. Hydrated metal salts (A.C.S. reagent grade), AlCl3‚6H2O, MgCl2‚6H2O, and NaCl were used as supplied by Aldrich and Fisher. Potassium cobalticyanide [K3Co(CN)6] (reagent grade) was used as supplied by Aldrich and sodium nitroprusside [Na2Fe(CN)5(NO)] (reagent grade) was used as supplied by Mallinckrodt. Mg:Al LDH chloride [Mg2Al(OH)6Cl] was prepared by adding a stoichiometric amount of 50% NaOH solution (w/w, supplied by VWR) to a solution 0.3 M in MgCl2 and 0.1 M in AlCl3, with an overall chloride ion concentration of 1.0 M, achieved by the addition of NaCl. The excess Mg2+ is present in solution to buffer the pH of the solution during formation of the 2:1 Mg:Al product.27 The resulting suspension was then refluxed overnight in its mother liquor under a slow stream of nitrogen. Following reflux, the solid was collected and washed repeatedly via centrifuge to remove excess electrolytes. The general anion exchange reaction involves exposing the parent LDH chloride to a solution containing the anion to be intercalated. These reactions take place with magnetic stirring, under a sealed nitrogen atmosphere for between 18 and 24 h. Specifically, for preparing the [Fe(CN)5(NO)]2- and [Co(CN)6]3compounds, a sample of the Mg/Al-LDH chloride (∼0.75 g of LDH, 3.04 mequiv for the 2:1 LDH) was exposed to a solution containing excess potassium salt of the appropriate anion (∼50 mL of a solution 0.1 M in the anion in question). Following the reaction period, the resulting solid was collected and washed repeatedly via centrifuge. All exchange vessels and subsequent products were protected from light to prevent photolysis of the anions and/or the exchange products. Infrared spectra were collected using a Perkin-Elmer 1760X FTIR spectrophotometer with computerized data collection and handling. Conventional (nonoriented) spectra were obtained using KBr disks containing ∼1% sample. Oriented spectra were obtained by placing a drop of aqueous suspension of the material on the horizontal top surface of a barium fluoride disk, and allowing the water to evaporate. The disk was then mounted perpendicular to the sample beam. Thermogravimet(27) Boclair, J. W.; Braterman, P. S. Chem. Mater. 1999, 11, 298.
anion Cl[Co(CN)6]3[Fe(CN)5(NO)]2a
ν(OH) δ(OH)
ν,(CN and/or NO)
lattice vibrations
3546, 1631 672, 447 3467 3428 1619 2128 677, 449 3465 1636 2143, 2037, 1936 660, 448 20 (2143, 2040, 1940)
Literature values are given in parentheses.
ric analysis was performed using a Perkin-Elmer TGA-7 instrument to a maximum temperature of 1000 °C at a rate of 20.0 °C/min under a nitrogen atmosphere. Powder X-ray diffraction (XRD) data were collected on a Scintag XDS 2000 using Cu Ka radiation. All powder XRD samples contained ∼5% CaF2 as an internal standard. Carbon, hydrogen, and nitrogen content were analyzed by Atlantic Microlab Inc. Metal content was determined using a Perkin-Elmer 5500 ICP/AES. The ICP samples were digested in a 1:1 mixture of 5% nitric and 5% hydrochloric acids. Standard metal solutions obtained from Aldrich and Alfa Æsar were used as calibrants for the ICP/AES. Mixed standards were used for magnesium and aluminum calibration. Magnesium and aluminum contents were determined simultaneously, while the particular anion metal content was determined in separate runs. We performed an average of nine determinations for each metal in each material.
Results and Discussion The insertion of cyanide-containing anions into LDH is very easily demonstrated by the appearance of the ν(CN) vibration in the 2200-2000 cm-1 region of the infrared spectrum of the material. Figure 1 shows the infrared spectra (nonoriented) of chloride-, cobalticyanide-, and nitroprusside-containing 2:1 Mg:Al LDHs. The associated data for the LDHs are listed in Table 1. Table 2 compares infrared data for LDH ν(CN) and ν(NO) with data for the aqueous anions and the potassium (or sodium for nitroprusside) salts. Hansen and Koch reported that the infrared spectra of intercalated and aqueous ferrocyanide were very similar.19 Likewise the IR data of cobalticyanide reported here are indeed
Mg Al Layered Double Hydroxides
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Figure 2. Expanded infrared spectra of 2:1 Mg:Al LDH cobalticyanide: (a) nonoriented and (b) oriented.
Figure 3. Expanded infrared spectra of product from attempted preparation of 2:1 Mg:Al LDH nitroprusside: (a) nonoriented and (b) oriented.
virtually identical in aqueous and intercalated environments, showing ν(CN) values of 2128 and 2127 cm-1 for the intercalated and aqueous species, respectively. However, close inspection of the intercalated sample shows a broadening of the ν(CN) peak (Figure 2), associated with a slight splitting clearly detectable in the full-size spectral printout.31 The oriented infrared spectrum of intercalated cobalticyanide is also shown in Figure 2. This ν(CN) of the oriented sample is broadly similar to that of the nonoriented sample, showing an absorbance maximum at 2133 cm-1, but with loss of the lower frequency component. The IR spectrum of nitroprusside (Figure 3) also shows some features that are highly similar in aqueous (28) Jones, L. H.; Memering, M. N.; Swanson, B. I. J. Chem. Phys. 1971, 54, 4666. (29) Bates, J. B.; Khanna, R. K. Inorg. Chem. 1970, 9, 1376. (30) Khanna, R. K.; Brown, C. W.; Jones, L. H. Inorg. Chem. 1969, 8, 2195. (31) Available as Supporting Information.
and intercalated environments (2142, 1936 and 2143, 1935 cm-1, respectively). There are, however, some significant differences: the appearance of a split band with maximum absorbance at 2037 cm-1 in the Mg:Al LDH nitroprusside, and a noticeable splitting of the ν(NO) band. We attribute the 2037 cm-1 band, completely absent in both the aqueous and solid sodium salt spectra, to intercalated ferrocyanide, generated by decomposition of the nitroprusside following insertion into the layers. We attribute the splitting of the ν(NO) peak to the generation of nitrosyl groups of unknown structure, a byproduct of the nitroprusside to ferrocyanide decomposition. The oriented IR spectrum of Mg: Al LDH nitroprusside, shown in Figure 3, exhibits significant differences from the nonoriented spectrum. Specifically, the oriented spectrum in the ν(CN) region shows a suppression of the 2037 cm-1 maximum with a symmetric peak centered at 2045 cm-1 remaining as well as a complete resolution of the two ν(NO) peaks,
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Table 2. Observed IR Frequencies of Octahedral Cyanide-Containing Anions in Differing Environmentsa IR frequencies (cm-1) intercalated [Co(CN)6]3-
solid salt [cation]
aqueous
2129 [K] (2128)28
2128
[Fe(CN)5(NO)]2- 2143, 2037, 2174, 2159, 2144, 1935 1940 [Na] (2173, 2162, 2143, 1945)29 a
2127 (2127)28 2142, 1936 (2142)30
Literature values given in parentheses.
Table 3. Observed Powder XRD Spacings for Nitrate- and Nickelocyanide-Containing Mg:Al LDHsa anion
d003
d006
d012
Cl[Co(CN)6]3[Fe(CN)5(NO)]2-
7.87 (7.86)32 10.75 (10.8)25 11.09 (11.0)4
3.96 5.44 5.57 (5.47)4
2.65 2.61 2.59 (2.65)4
a Literature values listed in parentheses. (hkl assignment is based on ref 34.)
with enhancement of the higher frequency absorption relative to the one at lower frequency. Reported interlayer spacings show that ferrocyanide,23 ferricyanide,22 nitroprusside,4 and cobalticyanide25 all orient themselves in a LDH interlayer with their 3-fold rotational axis perpendicular to the hydroxyl sheets. Table 3 lists our observed XRD data. Figure 4 shows the powder XRD traces of 2:1 Mg:Al LDH cobalticyanide and 2:1 Mg:Al nitroprusside. The observed basal spacings of 10.75 Å for the 2:1 Mg:Al LDH cobalticyanide and 11.09 Å for the 2:1 Mg:Al LDH nitroprusside are consistent with the values previously reported.4,25 The XRD trace for the 2:1 Mg:Al LDH nitroprusside seems unusual in that the observed 006 reflection is of greater intensity than the 003 reflection. We have, however, observed similar intensity ratios in other similar systems, e.g., LDH ferrocyanide. Table 4 lists the results of the elemental analysis of these samples. Since this table combines data obtained by completely different techniques (ICP; microanalytical GC) in different laboratories, the agreement between observed and predicted M:C and M:N ratios is satisfactory. Perhaps the most interesting result is the absence of any extensive loss of N (and hence NO) from the nitroprusside. The Al:Fe ratio in the LDH nitroprusside is found to be 3.57. This value falls between the values expected for complete intercalation of nitroprusside (2.00) and ferrocyanide (4.00). The LDH cobalticyanide exhibits a Al:Co ratio of 3.44, close to the theoretical value of 3.00, indicating nearly complete replacement of chloride by cobalticyanide. In each case, the Mg:Al ratio falls below the expected value of 2.00, implying an enrichment in Al relative to Mg during intercalation. Such results have been reported by other workers for LDHs intercalated with similar anions.17,22,24 We performed TGA/DTA analysis on the Mg:Al LDH cobalticyanide, nitroprusside and ferricyanide for comparison. All three of the samples showed the expected weight loss between 100 and 200 °C normally associated with loss of interlayer water. The rest of the thermograms are more complicated and are under further investigation.31 Deformation of Intercalated Cobalticyanide. For cobalticyanide, ferrocyanide, and the 2142 and 1936 cm-1 bands of nitroprusside, the LDH intercalate fre-
Figure 4. Powder XRD patterns for (a) 2:1 Mg:Al LDH chloride, (b) 2:1 Mg:Al LDH cobalticyanide, and (c) product from attempted preparation of 2:1 Mg:Al LDH nitroprusside. (Asterisks (*) indicate standard; the cross (+) indicates contaminant Mg:Al LDH carbonate phase.)
quencies are very close to those in aqueous solution, implying similar hydrogen bonding to the terminal N lone pair. We have previously shown that the splitting of the ν(CN) peak of intercalated ferrocyanide is attributable to the T1u mode in Oh symmetry splitting into separate A2u and Eu components in D3d symmetry, indicating distortion of the anion force-field in the LDH interlayer. Both of these components are present in the conventional spectrum, but in the oriented spectrum the A2u component, being polarized in the direction of polarization of the beam, is suppressed.26 Close inspection of the nonoriented ν(CN) peak of Mg:Al LDH cobalticyanide (Figure 2) reveals very minor, yet noticeable and reproducible splitting. The corresponding peak in the oriented IR spectrum shows suppression of the higher frequency component, resulting in a symmetric peak. This small splitting, seen in the nonoriented spectrum and the suppression of the c-polarized component upon orientation, indicates slight distortion of the force field, much less than that seen for intercalated ferrocyanide, from local Oh symmetry of the cobalticyanide. Some such distortion is of necessity imposed by H-bonding to layer OH, and may or may not be accompanied by geometrical distortion. We suggest that this difference in behavior is due to the 3- cobalticyanide forming weaker hydrogen bonds with the layer than the 4- ferrocyanide.
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Table 4. Elemental Analysis Results of Mg:Al LDH Compounds [Co(CN)6]3[Fe(CN)5(NO)]2-
%Mg
%Al
%M
%C
%N
%H
Mg:Al
Al:M
C:M
N:M
15.25 14.56
10.13 9.36
6.22 5.50
6.86 7.57
7.37 9.04
2.94 3.22
1.67 1.72
3.44 3.57
5.55 6.25
5.00 6.67
Figure 5. Schematic representation of cobalticyanide-intercalated Mg:Al LDH. Bond lengths are given in angstroms.
Kikkawa was the first to suggest that the ferrocyanide anion intercalated into Mg:Al LDH with its 3-fold symmetry axis perpendicular to the cation sheets.23 Using Miyata’s reported sheet thickness of 4.8 Å,32 Kikkawa calculated the gallery height of Mg:Al LDH ferrocyanide to be ∼6 Å. Similar treatment of the Mg: Al LDH cobalticyanide was reported by Suzuki and Ono.24 We have refined this method by utilizing the scheme shown in Figure 5. We have defined an axis terminated by hydrogen atoms in the cation sheets and containing one of the anion’s NCMCN axes. On the basis of the crystal structure of potassium cobalticyanide reported by Curry and Runciman,33 the average Co-C distance is 1.89 Å and the average C-N distance is 1.17 Å, yielding a cobalticyanide 4-fold axis length of 6.12 Å. On the basis of the crystal structure of potassium ferrocyanide reported by Taylor et al.34 we assume N‚ ‚‚H hydrogen bond lengths of 2.84 Å. We use an idealized H-N-C bond angle of 180°, because of sp hybridization at N. These values yield an overall hydrogen atom to hydrogen atom axis distance of ∼11.80 Å and a corresponding gallery height between the hydrogen atom planes of 11.80/x3 or 6.84 Å. The vertical distance between H atoms, on either side of the sheet, based on Allmann’s results from XRD studies of crystalline mineral samples,35 is calculated to be ∼4.77 Å. Thus the sum of our newly defined gallery height, comprising the space between the atomic centers of opposing hydrogen layers, and the layer thickness, is 11.60 Å, moderately close to the observed powder XRD value of 10.75 Å. The difference between the calculated and observed d003 spacing could well be caused by the presence of shorter or slightly skewed hydrogen bonds between the layer and the anion. Intercalation and Decomposition of Nitroprusside. Nitroprusside shows several significant differences between aqueous and intercalated environments. Our aqueous spectrum of nitroprusside shows two peaks, one (32) Miyata, S. Clays Clay Miner. 1983, 31, 305. (33) Curry, N. A.; Runciman, W. A. Acta Crystallogr. 1959, 12, 674. (34) Taylor, J. C.; Mueller, M. H.; Hitterman, R. L. Acta Crystallogr. 1970, A26, 559. (35) Allmann, R.; Jepsen, H. P. Neues Jahrb. Mineral. Monatsh. 1969, 544.
at 2142 cm-1 attributable to ν(CN) and another at 1936 cm-1 caused by ν(NO), in close agreement with the solution values reported by Jones;30 the other bands predicted by group theory are, presumably, either very weak or not separately resolved. After exchange into Mg:Al LDH, several changes are noted in the nitroprusside IR spectrum: the ν(NO) peak loses intensity relative to the ν(CN) peaks, the ν(NO) peak shows a distinct splitting, and a peak, not previously observed, appears at 2035 cm-1. Idemura and co-workers reported a decrease in the relative intensity in the ν(NO) peak of nitroprusside upon intercalation, a finding which they attributed to the decomposition of the anion into ferrocyanide and NO.20 Closer inspection of our 2034 cm-1 peak reveals the same splitting as seen in intercalated ferrocyanide.36 In the oriented IR spectrum, the two different ν(NO) peaks are resolved, the higher frequency component gaining in relative intensity indicating a,bpolarization, while the asymmetric 2034 cm-1 peak becomes a broad, symmetric peak with a maximum at 2045 cm-1, as shown previously36 for ferrocyanide in this environment. We interpret these results as follows: in ferrocyanide, the degenerate T1u (Oh) mode shows a first-order splitting into A2u (c-polarized) and Eu (a,b-polarized) components in D3d; the c-polarized component is suppressed in the oriented spectrum. This splitting is characteristic of interlayer ferrocyanide, and quite distinct from that of ferrocyanide externally adsorbed on LDH carbonate.26 In nitroprusside (C4v), ν(CN) spans 2A1 + E (IR active) + B2 (IR inactive), while ν(NO) at lower frequency spans A1. Since the charge on the anion is only -2, interaction with the layer will be slight, and intensity in all these modes will be distributed parallel and perpendicular to the a,b-plane with intensity ratio 2:1. Thus ν(CN) (2142 cm-1) and ν(NO) (1936 cm-1) show the same relative intensity in both conventional and oriented spectra, while losing intensity in the latter relative to the a,bpolarized 2045 cm-1 band of ferrocyanide. An important corollary is that the 1954 cm-1 band in the ν(NO) region, which gains relative intensity on orientation, must correspond to a largely a,b-polarized absorption. This is not consistent with its assignment to NO coordinated to an octahedral metal ion. These results show that nitroprusside decomposes to ferrocyanide and nitrosyl groups, following insertion into the LDH, and that the nitrosyl-containing compounds so formed stay in the interlayer, parallel, or nearly parallel, to the hydroxyl sheets. Suib and co-workers also demonstrated the decomposition of nitroprusside intercalated in LDH prepared via direct precipitation. Their results from EPR and Mo¨ssbauer spectroscopy indicate NO radicals associated with the LDH.4 The presence of these nitrosyl compounds in the interlayer suggests that the decomposition of nitroprusside takes place after intercalation. This work shows that spectroscopic methods may be used to help elucidate the nature of interaction between the intercalated anion and cation sheets in certain LDH (36) Wang, Z. Masters Thesis, University of North Texas, 1997.
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systems. With the use of oriented IR, we have demonstrated that nitroprusside decomposes after intercalation into the interlayer to yield intercalated ferrocyanide and some sort of free nitrosyl species trapped in the interlayer. We have also demonstrated that like ferrocyanide, cobalticyanide experiences some deformation from Oh symmetry upon insertion into LDH. The extent of this deformation is much less for cobalticyanide than for ferrocyanide, presumably due to the lower anion charge on cobalticyanide. Acknowledgment. We thank the Welch Foundation (Grant B-1154), the University of North Texas Faculty
Boclair et al.
Research Fund, and NASA’s Exobiology (grants NAGW4620 and NAG5-4891) program for support; Kevin Menard and Brian Billeau for obtaining TGA data; and Dr. K. Balkus of the University of Texas at Dallas for obtaining XRD data and for helpful discussions. Supporting Information Available: Infrared spectra, both full-scale and cyanide region expansion, for Mg:Al LDHcobalticyanide and TGA data for Mg:Al LDH cobalticyanide, nitroprusside, and ferricyanide. This material is available free of charge via the Internet at http://pubs.acs.org. CM990148L
