J. Phys. Chem. 1980, 84, 3211-3215
3211
EPR Study of CO and O2 Complexes of RuSt in a Y-Type Zeolite Bruce L. Gustafson, Mei-Jan Lln, and Jack H. Lunsford" Department of Chemistry, Texas A & M University, College Station, Texas 77843 (Received: December 3, 1979; In Final Form: June 19. 1980)
Ruthenium ions in zeolite Y form complexes with CO and O2which have well-resolved EPR spectra. Addition of CO at 298 K resulted in the formation of a Ru3+monocarbonyl complex or a polycarbonyl complex, depending on the partial pressure of CO. The monocarbonyl complex was characterized by g, = 2.0600 and gll = 1.9871, whlereas the polycarbonyl complex was characterized by g, = 2.0593, g2 = 2.471, and g 3 = 1.9970. Hyperfine coupling for 13Cand the two ruthenium isotopes,%u and lolRu,indicates that the unpaired electron is primarily localized (ca. 70%) on the metal ion. Upon reaction of the monocarbonyl complex with O2at 298 K, the spectrum of I I superoxide ion was observed. The Ru3+appears to be oxidized to Ru4+,and the reversible nature of the spectrum indicates that CO and O2 are simultaneously coordinated to the ruthenium. The spectrum of the complex formed with 1702 suggests that the oxgyen nuclei are nonequivalent.
Introduction Ruthenium complexes are known to be active catalysts for many types of homogeneous reactions. In addition, recent studies have shown that ruthenium zeolites are active catalysts for the water-gas shift1 and methanation2 reactions. In the case of the water-gas shift reaction, the catalytically active species was thought to be a mixture of ionic ruthenium complexes. The behavior of ionic ruthenium in zeolites is complex and not fully understood at this time. Ruthenium usually has been introduced into the zeolite through ion exchange of [RU"'(NH~)~]~' in aqueous solution, where the anion is either C1- or Br-. This complex is known to react with zeolitic water to from [Ru'~'(NH~)~(OH)]~+ (ref 1) and possibly "ru~theniuim~ e d " . The ~ , ~ [Ru("~)~]~+complex in a Y zeolite has allso been shown to undergo reduction by the ammine ligands during dehydration and decomposition at temperatures in excess of 573 K,2v6although there is disagreement regarding tho final oxidation state of the metal ion. The catalytic activity of ruthenium for the methanation reaction, as well as the potential use of ruthenium in automobile emission control, has prompted several studies of CO adsorption of supported ruthenium.M In this study, the formation of carbonyl adducts of Ru3+ has been observed in a Y-type zeolite following dehydration at 573 K. These carbonyl Complexes, in addition to a mixed ligand complex containing CO and 02,were characterized by EPR spectroscopy. Experimental Section A Ru-Y zeolite containing 2 wt % ruthenium was prepared by exchange of 4 g of Na-Y (Linde lot no. 3365-94) with the desired equivalents of [Ru(NH3)JBr3or [Ru(NH3),$1 (Strem Chemical, Inc.) in 4 L of H20. Similar results were obtained for both the Br- and the C1- salts. The [Ru(NH&]Br3 complex was obtained by oxidation of the [Ru(NH3b6]C12with Br2.' The [Ru(NH3),]CI2 complex was prepared from ItuCI3-3H20(Englehard Industries) by using the method of Lever and Powell.lo The exchange was carried out at 298 K for a period of 24 h. The sample was air dried a t 298 K and then stored over a saturated solution of NH4C1. Standard Pretreatment of the Ru--Y samples consisted of a stepwise dehydration at 100 K intervals to a final temperature of 573 K. The dehydration was conducted under a vacuum of 110" torr for 1h at each temperature. 0022-3654/80/2084-3211$01 .OO/O
Samples were clooled to 298 K under static vacuum prior to CO or O2 adsorption. Matheson Grade l2C0 (99.99%) and 13C0 (Stohler Isotopes Chemicals) were purified by brief exposure to a clean Zn film at 623 IC. The 13C0was enriched to 88% in 13C. Anhydrous 1602 (Matheson ED) and H2 (Matheson UHP) were used without further purification. Molecular oxygen enriched to 41.6% in 170and 0.98% in le0was obtained from Miles Laboratory Inc. For the l6OZexperiments a 3 vol % I6O2/Ar mixture was prepared. EPR spectra were recorded with a Varian E6-S spectrometer operating in the X-band region. All spectra were recorded at 77 IC. The g values were determined relative to DPPH (g = 2.0036). The error in the determination of g values was estimated to be f0.0005 for the CO species and fO.OO1 for the O2 species. Spin concentrations were determined by using a phosphorus-doped silicon standard as well as a 5.85% CuS04-silica standard. A numerical double integration technique was employed to calculate the spin concenitration.ll The estimated error in the absolute spin concentration was *30%. Computer simulation ~ through the use of the program of the s ectra W B achieved SIM13.
8
Results CO Adsorption. Before pretreatment of a Ru-Y sample, the infrared spectrum exhibited a band at 1355 cm-l which was assigned to NH3 coordinated to a Ru3+ Following the standard pretreatment this band was completely removed, indicating a total loss of coordinated NH3 ligands. During the standard pretreatment the Ru-Y samples changed from white to tan in color. There was no EPR signal observed following the evacuation at 573 K however, when the dehydrated sample was exposed to 100 torr of l2C0 at 298 K, the spectrum shown in Figure l a was obtained. The spectrum is characterized by gl = 2.0593, g2 = 2.0471, and g3 = 1.9970, which indicated nonaxial symmetry for the complex designated species A. The computer simulation of this species is shown in Figure lb. Evacuation of the gas-phase l2C0 at 298 K to pressures less than torr produced another spectrum with g, = 2.0600 and gll = 1.9871, as shown in Figure IC. The g values indicate that this complex, labeled species B, has axial symmetry. Ruthenium has two naturally occurring isotopes with nuclear spin I = 5 / 2 (12.7% wRu and 17.0% lolRu) which result in the hyperfine structure shown in 0 1980 American Chemical Society
3212
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
9,
Q2
Gustafson A
q3
A
b
Figure 1. EPR spectrum of '*CO adsorbed on a dehydrated Ru-Y zeolite: (a) 100 torr of I2CO added at 298 K; (b) computer simulation of (a); (c) 100 torr of "CO at 298 K followed by evacuation at 298 K to torr; (d) computer simulation of (c).
Figure IC. The simulated spectrum of Figure I d was obtained by using hyperfine values of lAll = 14.5 G and lAlrl = 17.0 G for %Ru, and lA,l = 15.5 G and lAlll = 19.0 G for 'OlRu. For an unpaired electron in a d,z orbital, as discussed in the following section, the calculated spin density was found to be -0.70. The spin density on the metal s orbital was less than 0.01. The adsorption of 13C0 on the dehydrated sample yielded the spectra shown in Figure 2. Under a pressure of 100 torr of 13C0, spectrum 2a was observed which is characterized by JAllacl = 59 G, = 50 G, and /APCl = 48 G. Accurate determination of AllSC and A23Cfor species A was difficult because of the large line widths which were observed. Estimation of these values was accomplished through computer simulation as shown in Figure 2b. It should be noted that the program used for simulation did not compensate for possible differences in line width for the x and y components. Following evacuation of the gas-phase 13C0 at 298 K, the spectrum of Figure 2c was observed. The hyperfine constants for this axially symmetric species, as determined from the computer simultation (Figure 2d), were IA113C( = 60 G and IAl;"cI = 66 G. The computer simulations for the 13C0adducts accounted for the fact that 12% of the CO present was W O . The 13C0hyperfine splitting for species B indicates that the spin density on the 13C 2s and 2p orbitals was 0.056 and 0.062, respectively. For species A the spin density on the 13C 2s orbital was 0.047. For both 13C0 species the hyperfine structures indicate that the unpaired electron was localized on only one 13C0 moiety. Species B was stable at 298 K under static vacuum torr) for several days, whereas species A reacted further with gas-phase CO at 298 K. In the presence of 100 torr of CO, the signal (Figure la) had decayed to less than half the original level within a period of 2 h. This paramagnetic species could not be re-formed by simply adding more CO.
IAPCJ
Flgure 2. EPR spectrum of I3CO adsorbed on a dehydrated Ru-Y zeollte: (a) 100 torr of "CO added at 298 K; (b) computer simulation torr; (d) computer slmulation of (a); (c) evacuation at 298 K to of (c).
The spectrum of Figure l a could be reproduced by adsorption of 100 torr of l2C0 at 298 K onto the sample which contained species B. When the W O was evacuated at 473 K, no paramagnetic species were observed, but the spectra of Figure 1could be reproduced with no apparent loss in spin concentration by once again adding "CO. Species B could also be obtained by the addition of a small amount of l2C0 (5 torr) to a dehydrated Ru-Y sample. Spin concentration calculations indicate that the observable paramagnetic Ru-CO species accounted for only 1-2% of the total ruthenium present in the zeolite. Final dehydration temperatures of 423 and 673 K resulted in the same EPR spectra, but with a loss of -30% of the signal intensity. Hydrogen reduction at 573 K for 19 h resulted in complete loss of observable paramagnetic ruthenium species. Kobayashi and Shirasaki have reported the EPR spectra of CO adsorbed on ruthenium at elevated temperatures.l* However, in the present study adsorption of CO at 373 K did not result in an observable EPR signal. Subsequent adsorption at 298 K did not produce either species A or B, indicating a loss of the ruthenium in the particular oxidation state which formed the carbonyl complexes. O2Adsorption, Exposure of a dehydrated Ru-Y sample to 90 torr of the 1602/Armixture resulted in a broad signal was adsorbed onto as shown in Figure 3a. When 1602 species B, the spectrum in Figure 3b was obtained. Evacuation of the ls02/Ar mixture at 298 K to 1 X torr did not immediately alter the observed spectrum, but, when the evacuated sample was left in static vacuum at 298 K, the signal due to species B slowly reappeared. After 19 h the spectrum shown in Figure 3c was obtained. This cycle was reversible to some extent, although, after several oxygen exposures, a small amount of the oxygen species could not be removed by evacuation at 298 K. Substitution of 13C0for W O did not alter the observed spectra for the oxygen adducts, There appear to be at least two forms of dioxygen present in the observed spectrum. The one present in greater concentration is characterized by g1 = 2.001, g2 = 2.006, and g, = 2.083, whereas the other has gi = 2.056.
The Journal of Physical Cliemistty, Vol. 84, No. 24, 1980 3213
CO and 0, Complexes of Ru3+ in a Y-Type Zeolite
170100-
H100 G
_ 1
170170-
x 25
1 Figure 4. EPR spectrum of '70-labeled O2(3 torr) adsorbed on specks B at 298 K. For the r70170species, only 12 of the possible 72 lines are shown for clarity.
c
-
x2
li
,.J I
Figure 3. EPR spectrum of % adsorbed , on a dehydrated Ru-Y zeoliie: (a) 80 torr of O,/Ar mixture added at 298 K (b) 80 torr of 02/Armlxture added to species B at 298 K; (c) evacuation of O,/Ar mixture to 1 X torr at 298 K, static vacuum at 298 K for 19 h.
Both signals showed a decrease in intensity as the signal due to species B reappeared. Intensity correlations between the oxygen adducts and species B were not possible after a few hours because of the overlap of the two EPR signals. It appeared, however, that the EPR signal due to species B was fully restored only after complete removal of the O2 species. Spin concentration measurements confirmed tlhat, within experimental error, the signal due to species B contained the same number of spins as were present in the O2 adducts. Two other small signals were observed at g = 2.023 and 2.033 in several of the samples. EPR spectra of the oxygen complex weire recorded at various temperatures between 77 and 298 K in an effort to determine the origin of these two signals. At all temperatures these two signals remained constant iin both position and relative intensity, which indicates that these signals are not related to motion of the radical speeies.15 To determine the nature of the O2 coordination on ruA small amount thenium, we substituted I6Ol7Ofor 1602. (25%) of 1702was also present in this mixture. If both oxygen nuclei were equivalent, a singly labeled dioxygen adduct would yield 21 + 1, or six, lines of equal intensity for each principal direction. The I7O2species would have 2(20 + 1, or 11, lines with relative intensity ratios of 1:2:3:4:5:6:5:4:3:2:1for each principal direction. If there were two different 160170 species, each having equivalent oxygen atoms but different A values, there would be two sets of six equally intense lines with H less intense hyperfiie at the midpoint between pairs of 160170 line due to 1702 hyperfine lines.16 When 170-labeledO2 (3 torr) was exposed to species B, the spectrurn shown in Figure 4 was obtained. The observed 170hyperfine splitting pattern does not appear to
correspond to either of the splitting patterns discussed above. Rather the hyperfine splitting seems to indicate that there are two separate sets of six lines, each of approximately equal intensity, centered on gl. This type of splitting is characteristic of a peroxy-type radical which has two nonequivalent oxygen n~c1ei.l~ The two splittings were found to be (ATo(l)l= 80 G and IAF0(2)1 = 67 G, where the labels 1 and 2 are used to indicate the two different oxygen nuclei. There was no resolved splitting of either the g2or g3 components.
Discussion CO Adsorption. Following the dehydration and deammination of the Ru(NH3)$+-Y sample, a significant portion of the Ru3+ions have been reduced to some extent. Elliott and Lunsford2 have concluded that the ruthenium has undergone reduction to an average oxidation state of 1.7 on the basis of N2 evolution. Pearce et ala5proposed that the ruthenium has been reduced to the metal on the basis of their X-ray diffraction study. It is probable that some ruthenium metal is formed during decomposition of the ammine complex although the system may best be described as a mixture of several oxidation states following evacuation at 573 K. The existence of higher oxidation states of ruthenium following dehydration at 573 K is evident from the observation of EPR spectra in this study. Although the amount of ruthienium observed in this study is small, a larger fraction of the paramagnetic ion may reside in the small cages following dehydration. Since a CO molecule cannot enter the small cages at 298 K,I8ruthenium in these cages would not be available for the formation of carbonyl complexes, assuming that ruthenium does not migrate out of the hidden sites. An EPR signal could arise from several oxidation states of ruthenium, namely, low-spin Ru+(4d7)and Ru3+(4d5) and high-spin Ru2+(4d6)ions. The EPR spectra of a high-spin Ru2+ion would be difficult to observe and is not consistent with the observed EPR spectra in this study.19 A Ru0(4da)atom, as well as low-spin Ru2+(4d6)ion, would not give rise to an EPR spectrum.lg Recently Naccache et aL20 reported the EPR spectrum of a low-spin Rh2+(4d7)adduct with CO in a Y -type zeolite. The EPR spectrum was assigned to a monocarbonyl Rh2+ species which was experiencing a tetragonally distorted octahedral field. The unpaired electron was thought to be localized in the d,z orbital on the rhodium ion. The spectrum reported for the Rh2+-CO complex is qualitatively similar to the observed spectrum of species B in this study, and it is tempting to assign this species to a complex
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The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
TABLE I : EPR Data for l 701 60Hyperfine Interaction 00)
O(2)
A,,G A,, G A,, G Aiso, G 2B, G
-82 -0 -0 - 21
- 69 -0 -0
- 55
- 23 - 46
ps
0.016 0.54
0.013 0.45
p ZPng*
of Rut(4d7). Such an assignment is, however, inconsistent with the EPR results. In order for a ruthenium ion to interact with a CO molecule, it must be accessible to the supercage. The most probable location for this ion would be site 1121where the ruthenium can interact with three lattice oxygens. In conjunction with this study, molecular orbital calculations22 have been carried out for a linear monocarbonyl complex of a Ru+(4d7)ion at site 11. A linear monocarbonyl complex at this site would result in C3?symmetry with the C-0 bond directed along the C3 axis. The unpaired electron for this type of species was found to occupy a degenerate orbital. This result is similar to that of Klier et for complexes of Co2+in A-type zeolite under CSUsymmetry. Thus a low-spin Rut ion at site I1 would not give rise to the EPR spectra observed in this study. The molecular orbital calculations were also carried out for a linear monocarbonyl complex of a low-spin Ru3+ion at site 11. In this case the relative ordering of the energy levels is such that the unpaired electron occupies an a, orbital which is over 90% d,z in character. There are two degenerate states, one at higher energy and the other at lower energy than the a, orbital. The value of g, is largely determined by a hole transition between the a, orbital and the lower e state which is predominately d,, and dyz.in character. This result is consistent with the observation of g, > gll = g, for species B in this study. Species B may therefore be assigned to a linear monocarbonyl complex of Ru3+at site I1 in which the C-0 bond is oriented along the C3 axis. The 13C0hyperfine structure indicates that only one 13C atom interacted with the metal orbital which contained the unpaired electron for both species A and B. Similar results have been obtained for Co(CH3Nl3C):+ complexes in a Y-type zeolite where the sixth coordination site of the complex is interacting with the zeolite surface.24 The unpaired electron in this complex is mainly localized in the d,z orbital in the cobalt, and only one of the ligands gives rise to superhyperfine structure in the EPR spectra. It is conceivable that excess CO may result in the formation of a polycarbonyl complex in which only one CO interacts with the d,z orbital on the ruthenium. The reduction in symmetry would result in a nonaxial g tensor as observed for species A. If the ruthenium is coordinated to the lattice oxygens at site 11, the most probable assignment for species A is a dicarbonyl or tricarbonyl adduct. Attempts to distinguish between the different Ru3+-(CO), species by infrared spectroscopy were unsuccessful principally because of the small amount of Ru3+ present or accessible to CO following deammination. O2 Adsorption. It has been shown that the addition of O2 at 298 K to a reduced ruthenium zeolite results in an immediate oxidation to form RuOP6 When O2was added to the ruthenium zeolite prepared in this study, only a broad signal was observed while the sample turned dark gray in color. This color is characteristic of RuO2 formation, and the observed EPR signal is probably due to some form of Ru-02- species. When CO was first adsorbed to
Gustafson
form species B, addition of O2 did not result in a change in color from tan to gray. Furthermore, the observed EPR spectrum is characteristic of 02-coordinated to metal ions.26,26 The formation of this 0, species results in a complete, reversible loss of the Ru3+-C0 signal. Since the spin concentrations were found to be the same for the Of species and species B, it is resonable to postulate the formation of a complex in which the Ru3+ion is oxidized to a Ru4+ion as follows: 0
I
r
0
,./o-
The EPR spectrum of coordinated Of would be observed only if the resulting Ru4+ion were in the low spin state. Furthermore, if the electron is largely localized on the oxygen nuclei, substitution of 13C0for l2C0would not alter the spectrum, which is in agreement with our results. Ben Taarit et al.27have reported the formation of a C02-02-radical on MgO through the interaction of O2with an adsorbed COf species. The formation of a similar type of radical on the Ru-CO species seems unlikely since there are no electrons on the CO ligand available for transfer to the O2 moiety. Rather, it appears that the electron is derived from Ru3+ and the resulting dioxygen anion is coordinated to the metal. Substitution of C02for CO in this study did not result in any observable EPR signal either before or after O2 adsorption. As mentioned above, the 170hyperfine splitting indicates that the two oxygen nuclei are not equivalent. The experimental hyperfine tensor for each oxygen can be resolved into an isotropic and anisotropic component. The electron densities on each oxygen can be calculated from the theoretical values of Ai, = -1659 G and 2B = -102 G.19 The results are shown in Table I. The results indicate that the unpaired electron is localized (-99%) on the oxygen 2p,, orbitals. The remaining hyperfine lines evident in the spectrum are due to further splitting by the second 170nucleus which is molecules. present in the doubly labeled 1702
Conclusion The results presented here indicate that Ru3+ ions, presumably at site I1 in a Y-type zeolite, reversibly complex with CO to form both a monocarbonyl species as well as some form of a polycarbonyl complex. The monocarbonyl complex was capable of reversibly bonding O2 with oxidation of the Ru3+to a Ru4+superoxide complex. The two oxygens are not equivalent in the superoxide complex. Acknowledgment. We are indebted to Dr. John Pearce for carrying out the molecular orbital calculations on the ruthenium complexes. We acknowledge the support of this work by the National Science Foundation under Grant No. CHE77-06792. References and Notes (1) Verdonck, J. J.; Jacobs, P. A.; Uytterhoeven, J. B. J . Chem. Soc., Chem. Common. 1979, 181. (2) Elliott, D. J.; Lunsford, J. H. J . Catal. 1979, 57, 11. (3) Madhusudhan, C. P.; Patll, M. D.: Good, M. L. Inorg. Chem. 1979, 18, 2384. (4) Schoonheydt, R. A., personal communication. (5) Pearce, J. R.; Mortier, W. J.; Uytterhoeven, J. B. J. Chem. SOC., Faraday Trans. 1. 1979, 75, 1395. (6) Davydov, A. A,; Bell, A. T. J. Catal. 1977, 32, 254. (7) Brown, M. F.; Gonzalez, R. D. J. Phys. Chem. 1976, 80, 1731. (8) Dalla Betta, R. A. J . Phys. Chem. 1975, 79, 2519. (9) Fergusson, J. E.; Love, J. L. Inofg. Synth. 1972, 73,208.
J. Phys. Chem. 1980, 84, 3215-3223 Lever, F. M.; Powell, A. R. J . Chem. Soc. A 1969, 1477. Lin, M. J. R.D. Disertation,Texas A & M Unhrerslty,Cdlege Station, TX, 1976. Computer program SIM 13 written by Lozos, G., Hoffman, B., and Franz, C., Northwestern Universlty. Nakamoto, K. “Infrared Spectra of Inorganic and Coordination Compounds”, 2nd ed.;Wlley: New Ywk, 1970. Kobayashi, M.; Shirasakl, T. J. Cafal. 1974, 32, 254. Schlick, S.; Kevaln, L. J . Phys. Chem., In press. Lunsford, J. H. C:atal. Rev. 1973, 8 , 135. Ben Taarit, Y.; Lunsford, J. H. J . Phys. Chem. 1973, 77, 780. Breck D. W. “Zeolite Molecular Sleves”; Wiley: New York, 1974. Goodman, B. A,; Flaynor, J. B. Adv. Inorg. Chem. Radiochem. 1970, 13, 135.
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(20) Naccache,C.; Ben Taarit, Y.; Boudart, M. ACS Symp. Ser. 1977, No. 40, 156. (21) The nomenclature for site locations follows the conventlon of Smith, J. V. Adv. Chem. Ser. 1971, No. 101, 171. (22) Pearce, J. R.; Sherwood, D.; Lunsford,J. H.; Hal, M. B.,to be submitted for publication. (23) Kellerman, R.; Kller, K. Surf. Defect Prop. SolMs 1975, 4 , 1. (24) Vansant, E. F.; Lunsord, J. H. J. Chem. Scc.,Faraday Trans. 2 1973, 69, 1028. (25) Vansant, E. I:.; Lunsford, J. H. Adv. Chem. Ser. 1973, No. 121, 441. (26) Wang, K. M.; Lunsford, J. H. J. Phys. Chem. 1970, 74, 1512. (27) Ben Taarit, Y.; Vedrine, J. C.; Naccache, C.; de Montgolfier, Ph.; Meriaudeau, P. J . Chem. Phys. 1977, 6 7 , 2880.
Physical and Chemical Characterlzation of Cr-Y and Cr-X Zeolites J. IR. Pearce, D. E. Sherwood, M. 8. Hall, and Jack H. Lunsford“ Department of Chemlsfry, Texas A & M Universky, College Sfaflon, Texas 77843 (RecelveO: March 13, 1980; In Final Form: July 3, 1980)
The physical and chemical properties of trivalent chromium supported in zeolites X and Y have been investigated by infrared, EPR, and diffuse reflectance spectroscopies. Nitric oxide was also used as a chemical probe and several chromium-nitrosyl complexes were characterized on the zeolite surface. Earlier investigatorshave reported that the dehydration of Y-type zeolites containing Cr(II1) resultri in the reduction of a major fraction of the Cr to the divalent state. In contrast, we find no evidence for this reduction. The infrared and EPR parameters of chromium-nitrosyl complexes formed upon the addition of NO, to dehydrated Cr-Y samples are explained on the basis of a Cr(II1) precursor. In addition, Fenske-Hall molecular orbital calculations were performed for several models representing Cr-nitrosyl complexes in zeolitic coordination sites. These calculations support tho assertion that Cr(II1) is the predominant cation in Cr-Y samples.
Introductilon I t is well known that silica and aluminosilicate catalysts containing supported chromium are active in the polymerization of ethylene; however, attempts to determine the oxidation state of Cr which gives rise to this activity have led to different conclusions. Indeed, every oxidation state from I1 to V has been Much of this uncertainty arises from the complexity of systems containing ionic chromium at ,a solid interface since in addition to the multiplicity of available oxidation states, a number of chromium complexes may be stabilized on solid surfaces.&l0 The present study is an attempt to resolve some of the ambiguity surrounding the nature of supported chromium. Recently, several authors have investigated Cr ions supported in ~eolites.~l-~* These studies have been conducted primarily on Y-type lattices in order to take advantage of their regular surfaces and well-defined coordination sites. Preparation of these samples generally involves introduction of Cr via an aqueous ion exchange with the [Cr(H20)6]3’ complex cation. This complex is then decomposed by evacuation at 300-500 OC. Several research g r o u ~ p s have ~ ~ proposed ~ ~ ~ ~ ~that ~ -a ~major ~ fraction of the Cr(II1) is reduced to the divalent state during this dehydration by either of the following simplified mechanisms:laJ7 2Cr(III) + nH20 -* 2Cr(II) + (n - 1)H20+ 2H+ + 1/202 (1) or
-
2Cr(III) iOlatticeP- 2Cr(II) + 1/202
(2) Evidence in support of this model has been provided by 0022-3654/80/2084-3215$01 .OO/O
diffuse reflectance spectroscopy, analysis of gases over the samples, and the EPR and infrared spectra of the dehydrated zeolites after adsorption of probe molecules such as CO or NO. ]However,as the present results will demonstrate, reduction of Cr(II1) does not occur to any great extent upon dehydration of Cr-Y zeolites. Rather, the EPR and infrared parameters of nitrosyl complexes formed upon addition of nitric oxide to dehydrated Cr-Y are better explained on the basis of a Cr(II1) precursor. In addition, a theoretical model will be described whereby the EFR parameters of model chromium-nitrosyl complexes were derived from molecular orbital calculations. This technique proved to be quite successful, and the results provide further evidence that trivalent chromium predominates in dehydrated Cr-Y zeolites. The situation is somewhat different in the less stable Cr-X zeolites. Experimental evidence suggests that some Cr(I1) may be produced upon high-temperature evacuation of these samples.
Experimental Section Materials. The CO, NO, H2, and O2gases used in this study were supplied by Matheson. All of them except NO were ultrahigh purity and were used as received. The NO was purified by repeated vacuum distillation at the freezing point of n-pentane (-129 “C). Nitrogen-15-labeled NO was obtained from Stohler Isotope Chemicals and was similarly purified. Chromium-53-labeled Cr203 (96% 53Cr) was purchased from Oak Ridge. The sodium forms of zeolites Y and X were supplied by Linde. Sample Preparation. Samples of Cr-Y and Cr-X were prepared from the Na zeolites by aqueous ion exchange in solutions of CrC13. The exchange cation in such solutions was [Cr(H20)6]3+.After filtering, repeated rinsing, 0 1980 Amerlcan Chemical Soclety
