J. Phys. Chem. 1993,97, 10599-10606
10599
Characterization of NaY Zeolite-Encaged Tetrairidium Clusters by Infrared and X-ray Absorption Spectroscopies S. Kawi,tgt J.-R. Chang,? and B. C. Gates'lts Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Department of Chemical Engineering, University of California, Davis, California 9561 6 Received: April 21, 1993; In Final Form: August 9, 1993"
[Ir(CO)2(acac)] in the pores of N a Y zeolite was treated in CO and converted into [Ir4(CO)12]. The zeoliteencaged [Ir4(C0)121 was characterized by infrared and extended X-ray absorption fine structure spectroscopies, with the data indicating a n average Ir-Ir coordination number of 2.6 and a n average Ir-Ir distance of 2.69 A, in agreement, within the experimental error, with the published crystallographic data for solid [Ir4(C0)12]. Structurally simple zeolite-encaged iridium clusters were made by decarbonylation of the [Ir4(CO)12] a t 325 O C in flowing He followed by Hz. The decarbonylated clusters had an average Ir-Ir coordination number of 3.4 and a bond distance of 2.70 A, consistent with the inference that the tetrahedral framework structure of [Ir4(C0)121 had been retained after decarbonylation; thus, the cluster is represented as tetrahedral Ir4. Infrared spectra showed that [Ir4(CO)12] was re-formed when the sample was treated in CO at 60 O C .
Introduction
Experimental Section
Nanostructures dispersed in the molecular-scale cages of zeolites include metals,' metal oxide^,^^^ and metal sulfide~.~,5 The encaged metals are potential shape-selective catalysts.' The encaged metal oxides2and metal sulfides4J include semiconductors that potentially offer novel electronic and optical properties. However, most of these materials are structurally nonuniform and difficult to characterize. Understanding of their chemistry and performance will be advanced by development of methods for synthesis and identification of discrete, structurally uniform nanostructures in zeolite cages.6 Metal carbonyls in zeolite cages, formed by "ship-in-a-bottle" syntheses, have been used as precursors in attempts to prepare encaged metal catalysts with uniform and nearly molecular structure^.^-^ [Ni(C0)4] in zeolite X could be reversibly decarbonylated in the cages.7 Metal carbonyl clusters, such as [Ir6(CO)ls]2- in NaX zeolitelO.l* and [h6(c0)16] in NaY z e ~ l i t e , are ~ J ~stabilized during catalytic hydrogenation of CO. However, numerous attempts to form uniform decarbonylated clusters have been unsuccessful because decarbonylation typically leads to fragmentation and/or aggregation of the metal, yielding mixtures of clusters not only inside the cages but also outside the zeolite crystallites. However, decarbonylation of robust metal carbonyl clusters has been suggested to give nearly uniform Ptls on MgO,l3 and preliminary reports suggest the formation of nearly uniform OSIO on MgOl4 and Ir4 on MgO.I5 We have presented results indicating that decarbonylated Ir6 clusters formed in NaY zeolite can be recarbonylated r e v e r ~ i b l y , ~apparently J~ without significant changes in cluster nuclearity (number of metal atoms) or morphology. Preliminary infrared results* indicate that [Ir4(CO)1~]and decarbonylated clusters, suggested to be Ir4, have been formed in NaY zeolite supercages. Here we present a report of the chemistry of tetrairidium clusters in NaY zeolite, including the characterization by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and evidence that the encaged clusters retain the nuclearity of 4 after decarbonylation.
Materials. Crystalline NaY zeolite powder (LZY-52) was supplied by Union Carbide. It had a unit cell size of 24.7 A and a silica/alumina molar ratio of 4.74. Prior to preparation of the zeolite-supported samples, the zeolite was either evacuated at Torr at room temperature for 2 h or it was evacuated at this pressure a t 500 OC for 2 h. [Ir(C0)2(acac)] (Strem) was used without purification. Reagent grade mixed hexanes were purged with N2 for several hours before use as a solvent. Sometimes this solvent was rigorously dried by redistillation from sodium/ benzophenone. H e and Hz (Matheson, 99.999%) were purified by passage through traps containing particles of Cu20 and particles of activated zeolite to remove traces of 0 2 and moisture, respectively. C O (Matheson, UHPgrade) was purified by passage through a trap containing particles of activated alumina heated to a temperature exceeding 250 OC to remove any traces of metal carbonyls from the high-pressure gas cylinder and through particles of activated zeolite 4A to remove moisture. Sample Preparation. Synthesis of the zeolite-supported organometallics was performed with samples in a Braun MB- 150M glovebox purged with Nz that recirculated through 0 2 - and moisture-scavenging traps or on a Schlenk vacuum line that was purged with N2 (99.999%). The glovebox was equipped with 0 2 and moisture detectors, and the concentrations of these contaminants were < I ppm. In the preparation of the zeolite-supported Ir samples, [Ir(CO)2(acac)] (60 mg/g of NaY zeolite) was dissolved in hexanes and brought in contact with one of the samples of preevacuated zeolite. With either of the zeolite samples, the powder became dark gray, and the initially greenish-black solution became clearer after stirring for several hours. After 2 days, the slurry was black. The mixture was filtered and the zeolite washed thoroughly with hexanes and dried under vacuum at room temperature for 12 h. The weight of the residual iridium precursor, measured after evaporation of the solvent, indicated that thezeolitecontained about 0.8 wt % Ir. The samples were stored in the drybox. Infrared Spectroscopy. Transmission infrared spectra of the zeolite samples were collected with a Nicolet 7199 spectrometer with a resolution of 4 cm-I. Samples were pressed into semitransparent wafers in the drybox and mounted in the infrared ce11.16 The experiments were performed with the samples in controlled atmospheres; purified He, N2, CO, or H2 (or any of
t
University of Delaware.
t University of California.
Abstract published in Aduance ACS Absrracrs, September 15, 1993.
0022-3654/93/2097-10599$04.00/0
0 1993 American Chemical Society
Kawi et al.
10600 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
these gases containing some water) could be delivered to the cell, which was part of a flow system. A typical gas flow rate was 20-30 mL (NTP)/min. Samples were scanned 32 or more times and the data averaged. X-ray AbsorptionSpectroscopy. The EXAFS experiments were performed on X-ray beamline X- 11A at the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, Long Island, NY. The ring energy was 2.5 GeV and the ring current 80-220 mA. The spectra were recorded with the sample in a cell that allowed treatment in flowing gases prior to the measurements. The powder samples were pressed into wafers with a C-clamp insidea glovebag purged with N 2boiloff gas from a liquid nitrogen cylinder. The sample mass (approximately 150 mg) was calculated to give an absorbance of 2.5 at the Ir LIIIedge. After the sample had been pressed, it was unloaded from the pressing die and loaded into the EXAFS cell. The cell was then closed with a positive pressure of N2, removed from the glovebag, aligned at the beamline, and cooled with liquid nitrogen. The EXAFS data were recorded in the transmission mode after cooling the cells with liquid nitrogen. The data were collected with a Si( 11 1) double crystal monochromator, which was detuned about 30%to minimize the effects of higher harmonics that were present in the X-ray beam. Each sample was scanned twice near the Ir L l l ~ edge (11215 eV). Two samples were characterized by EXAFS spectroscopy. The first was prepared by adsorption of [ Ir(C0)2(acac)] in NaY zeolite followed by treatment in a Schlenk flask with flowing CO at 125 OC for 12 h. After treatment in CO, the sample was exposed to air at room temperature for 5 min and then treated at room temperature for 1 h under vacuum (0.01-0.001 Torr). The sample was removed from the Schlenk flask inside the glovebox. It was stored in three layers of glass vials that were sealed with parafilm. The sample was transported to the synchrotron, where it was pressed and loaded into the EXAFS cell. The cell was then cooled to approximately liquid nitrogen temperature, and the EXAFS data were collected at the beamline. The second set of EXAFS measurements was made with this sample after further treatment. After collection of the first set of data, the cell was allowed to warm to room temperature, and flow of He [50-100 mL(NTP)/min] at 1 atm was started. The sample was then decarbonylated in flowing He by heating the cell at a rate of 3 OC/min from 25 to 325 OC and holding at 325 OC for 2 h. After cooling to room temperature under He flow, the sample was then treated under H2 while being heated to 325 OC at 3 OC/min. After 2 h at 325 "C,the sample was cooled to room temperature under flowing Hz. The cell was closed under a slightly positive pressure of Hz, the sample was cooled with liquid nitrogen, and the EXAFS data were collected. EXAFS Reference Data. The EXAFS data were analyzed with experimentally determined reference files obtained from EXAFS data for materials of known structure. The Ir-Ir and Ir-Osupprt interactions (where Osupprtrefers to oxygen of the zeolite lattice) were analyzed with phase shifts and backscattering amplitudes obtained from EXAFS data for Pt f0il17 and NazPt(OH)6,18 respectively. The transferability of the phase shift and backscattering amplitudes for Pt and Ir, which are near neighbors in the periodic table, is justified by both experimental19 and theoretical results.z0 The Ir-C and I r a * interactions (where O*denotes the oxygen of a carbonyl ligand) were analyzed with phase shifts and backscattering amplitudesobtained from EXAFS data for crystalline [Ir4(CO)12] (which has only terminal CO ligands)21 that was mixed with Si02. [Ir4(CO)12] is a good reference material because the multiple scattering effect in the Ir-O* shell is significant (because of the linearity of the Ir-C-0 moiety), and it was necessary to fit the data for the iridium carbonyl in the zeolite with a reference that exhibits multiple
TABLE I: Crystallographic Data Characterizing the Reference Compounds and Fourier Transform Ranges Used in the EXAFS Analysis' crystallographic data Pt foil Na2Pt(OH)6 [Ir4(CO)l2]
Fourier transform
shell
N
R,A
Ak,A-'
&,A
n
Pt-Ptb Pt-Oe
12
2.77 2.05 2.69 1.87 3.01
1.9-19.8 1.4-17.7
1.9-3.0 0.5-2.0
3 3
2.8-16.5 2.8-16.5
1.1-2.0 2.0-3.3
3 3
sample
6
Ir-I# Ir-Cd Ir-O*d
3
3 3
Notation: N, coordination number for absorber-backscatterer pair; R, distance; Ak, limits used for forward Fourier transformation (kis the wave vector); Ar, limits used for shell isolation ( r is distance); n, power of k used for Fourier transformation. Crystal structure data from ref 17. Crystal structure data from ref 18. Crystal structure data from ref 21.
2
1
t
I
I
2100
2000
1900
1 D
WAVENUMBER, cm.1
Figure 1. Infrared spectra taken during treatment of the initially prepared sample made from [Ir(C0)2(acac)] and NaY zeolite: (A) sample after evacuation for 12 h at room temperature; the other spectra indicate the change resulting from contact with flowing CO at 1 atm and 40 OC; (9) after 2 h; (C) after 6 h; (D) [Ir4(CO)l2] in T H F solution. scattering.22 The details of the preparation of the reference files are described elsewhere,23 and the parameters used to extract these files from the EXAFS data are summarized in Table I.
Results Reactivity of [Ir(CO)z(acac)] in NaY Zeolite in the Presence of CO. [Ir(C0)2(acac)] was adsorbed by NaY zeolite from hexane solution. Uptake of the organometallic species by the zeolite required some hours and was not complete even after 2 days of contacting. The infrared spectrum of the supernatant solution showed that some [Ir(CO)l(acac)] was still present, and there was no evidence of other metal carbonyls. Upon removal of the solvent from the black [Ir(CO)2(acac)]containing zeolite by evacuation, the solid became pale yellow. Reintroduction of the fresh hexane solvent gave a brown material, which became pale yellow again upon evacuation. The infrared spectrum of the pale yellow solid (Figure 1A) has two strong carbonyl bands, at 2070 and 2000 cm-I, consistent with the presence of an iridium dicarbonyl species.
NaY Zeolite-Encaged Tetrairidium Clusters The carbonylation of the iridium dicarbonyl species formed from [Ir(CO)2(acac)] adsorbed in NaY zeolite was monitored by infrared spectroscopy with the solid in the presence of CO at 1 atm. When the sample had been prepared with thoroughly dried reagents and with zeolite that had been evacuated at 500 OC, there was no discernible change in the vco infrared spectrum when the sample was treated in C O at temperatures as high as 100 OC. In contrast, when the treatment in CO was done with samples that had been prepared with reagents that had not been dried and with zeolite that had been evacuated only at room temperature, there were significant changes in the vco infrared spectrum. After exposure for 2 h a t 40 OC, the carbonyl band at 2000 cm-l decreased in intensity and a new band appeared a t 2030 cm-1 (Figure 1B). Continuation of the C O treatment for 6 h led to a spectrum with bands at 21 17 w, 2070 s, 2060 sh, and 2032 m cm-1 (Figure 1C). The resultant spectrumclosely resembles that of [Ir4(CO)12] in tetrahydrofuran (THF) solution (Figure 1D). The wafer sample was removed from the cell in the glovebox; it was light yellow, the color of [Ir4(C0)12] in T H F solution. Attempts were made to extract the iridium carbonyl species from the zeolite to allow a comparison of the infrared spectrum with that of [Ir4(CO)12] in T H F solution. The extractions were attempted under N2 in the drybox with THF and with a solution of excess of [PPN] [Cl] in THF. The wafer remained light yellow after the extraction attempt, and the supernatant solutions remained colorless. No iridium carbonyls were observed in the solution by infrared spectroscopy, indicating that no such species were extracted from the zeolite. When the C O in the carbonylation experiment was replaced by a stream of COcontaining saturated water or with an equimolar CO H2 mixture, the same results were observed. Heating the initially prepared sample to 50 OC and holding for 4 h in flowing C O at 1 atm in a copper-lined tubular flow reactor also gave the same results. Reactivity of Zeolite-Supported Iridium Carbonyl Clusters in Air. Another sample prepared from [Ir(C0)2(acac)] and NaY zeolite was treated in the infrared cell in flowing C O at 125 OC for 12 h to form an iridium carbonyl that has been suggested12 to be the isomer of [Ir6(co)16] with edge-bridging ligands (Figure 2A). The sample was cooled to room temperature in flowing CO. The cell was then purged with N2 for 15 min to remove CO gas. The spectrum of [Ir6(co)16] was maintained. The sample was thenexposedtoair. Thebridging bandat 1816cm-Ideaeased in intensity, and the terminal bands shifted to lower frequencies after exposure to air for 5 min (Figure 2B). After 15 min of exposure, the infrared spectrum was similar to that of [Ir4(CO) (Figure 2C). This method of preparing the sample with the spectrum resembling that of [Ir4(CO)12]is more efficient than that described above. EXAFS Analysis: Iridium Carbonyl Clusters in NaY Zeolite. The sample of NaY zeolite with the infrared spectrum indicating the presence of [Ir4(CO)I~],prepared as described in the immediately preceding paragraphs, was characterized by EXAFS spectroscopy. The normalized EXAFS function was obtained from the averaged X-ray absorption spectra by a cubic spline background subtraction and normalized to the height of the absorption edge. The raw EXAFS data (Figure 3A) show oscillations up to a value of k, the wave vector, of about 14 A-1, indicating the presence of near-neighbor high-atomic-weight scatterers. Since carbonyl ligands were present in the sample, thedata were first analyzed for Ir-Ir, Ir-C, and Ir-O* interactions (where O* refers to carbonyl oxygen). The EXAFS analysis was done with the experimentally determined reference files described above. The data were analyzed by Fourier transforming the raw EXAFS data with k2-weighting over the k range 3.52-14.19 A-1 with no phase correction. There was no evidence in the Fourier transform of
+
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10601
I
I
2100
zoo0
WAVENUMBER,
I
1900 cm-1
11 I
Figure 2. Infrared spectra characterizing the reactivity of Nay-supported [Ir6(co)16] with edge-bridging ligands in air at 25 OC: (A) NaYsupported [Ir6(CO)16] with edge-bridging ligands; (B) after exposure to air at 25 OC for 5 min; (C) after exposure to air at 25 OC for 15 min.
high-shell Ir-Ir contributions. The Fourier-transformed data were then inverse transformed from 0.87 to 3.25 A to isolate the major contributions from low-frequency noise. With the Koningsberger difference file t e c h n i q ~ e ? ~the . ~ ~first-shell Ir-Ir contribution, the largest in the EXAFS spectrum, was then estimated. However, since the Ir-O* contribution was strongly coupled with the Ir-Ir contribution, these two contributions had to be analyzed simultaneously. The structural parameters were estimated initially by fitting the data in the high-k range (7.50 k < 13.0 A-I). The multiple scattering associated with IrC-O* groups was found to be significant in this range, with Ir-C and Ir-O,,,,,contributions being insignificant. Further analysis following the subtraction of the calculated Ir-Ir and Ir-O* contributions from the raw data led to characterization of the Ir-C and Ir-Osupprt contributions. The structural parameters characterizing these contributions were determined by fitting the residual spectrum. The initial guesses for parameter estimation were obtained by adjusting the coordination parameters to give the best fit of the residual spectrum in r (distance) space. The calculated Ir-C and Ir-Osupportcontributions were then subtracted from the raw data. Better estimates of the parameters characterizing the Ir-Ir and Ir-O* contributions were then obtained by fitting the residual spectrum. The refinement through this iteration was continued until good overall agreement was obtained. The final results of the fitting are summarized in Table 11, and the comparisons of the data and the fit, both in k space and in r space, are shown in Figure 3B-D. The residual spectrum determined by subtracting the Ir-Ir + Ir-OsupPrtcontributions from the EXAFS data (which gives evidence of the carbonyl ligands) is shown in Figure 3E. The number of parameters used to fit the data in this mainshell analysis, n, is 16; the statistically justified number is approximately 17, estimated from the Nyquist theorem,26n = ( 2 A k A r l ~ )+ 1, where Ak and Ar respectively are the k and r ranges used in the forward and inverse Fourier transforms (Ak = 10.67 A-I, Ar = 2.38 A).
Kawi et al.
10602 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 0.8
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Figure 3. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing zeolite-supported Ir carbonyl after Ir-C exposure to CO at 1 atm and 125 OC for 1 day: (A) raw EXAFS data; (B) experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-Os + Ir-Osuppn contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (kl-weighted, Ak = 3.80-13.80 8,-I) of Ir-C + Ir-O* + Ir-Osuppn contributions (dashed line); (D) imaginary part and experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-C magnitude of Fourier transform (&)-weighted, Ak = 3.80-13.80 A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir Ir-OS Ir-Osuppn contributions (dashed line); (E) residual spectrum illustrating the contributions of carbonyl groups: imaginary part and magnitude of Fourier transform (&)-weighted, Ak = 3.80-13.80 A-l) of raw data minus calculated Ir-Ir Ir-O,,pponEXAFS (solid line) and calculated Ir-C + Ir-OS EXAFS (dashed line).
+
+
+
+
Ir-Ir Ir-CO Ir-C Ir-O*
+
+
TABLE 11: EXAFS Results Characterizing the NaY Zeolite-Supported Iridium Carbonyl Species Formed after Treatment in CO at 125 OC and 1 atm for 8 h Followed by Exposure to Air at Room Temperature for 5 mine6 shell
+
N
R, 8,
A&,
2.6
2.68 1.87 3.02 2.13
2.3 2.2 Ir-OsuppR 0.76
Az
AEo, eV
EXAFS reference
0.0004
4.88
Pt-Pt
0.0017 0.0028 0.0075
6.03 -4.63 -3.78
Ir-C Ir-O* Pt-0
a Notation as in Table I. Estimated precision: N, k20% (Ir-Osupprt, &30%); R, &2% (Ir-Ir, & l % ) ; AaZ, *30%; A E o , &lo%.
Infrared Data Characterizing Cluster Decarbonylation in NaY Zeolite. The zeolite incorporating iridium carbonyl clusters with infrared spectra resembling that of [Ir4(CO)1~]was treated in flowing H2 at 1 atm in the infrared cell (Figure 4). Exposure for 1 h at 50 OC led to no change in the spectrum, but as the temperature increased beyond 75 "C, the peaks in the carbonyl region decreased in intensity, broadened, and shifted to lower frequencies. After the sample had been held for 1 h at 325 "C in Hz, the vco bands disappeared, indicating that the sample had been decarbonylated. The sample was then evacuated for 15 min at 325 OC and cooled under vacuum to room temperature. The wafer removed from the cell was beige.
EXAFS Analysis: Decarbonylated Iridium Clusters in NaY Zeolite. The EXAFS data characterizing the decarbonylated sample formed from iridium carbonyl clusters as described in the preceding paragraph (Figure 5A) were analyzed by a method that is nearly the same as that stated above for the sample before decarbonylation. The EXAFS data were Fourier transformed with k2 weighting and no correction over the useful range (3.61 < k < 14.66 A-l). The major contributions were isolated by inverse Fourier transformation in the range 0.65 < r < 3.25 A. The Ir-Ir contribution was estimated by calculating an EXAFS function that agreed as closely as possible with the experimental results in t h e high-k range (7.50< k < 14.0 A-l); the metalsupport contributions in this rangearesmall. An EXAFS function calculated with the first-guess parameters was then subtracted from the raw data, with the residual spectrum being expected to represent the Ir-Osupportinteractions. The difference file was estimated with two Ir-0 contributions, as both shortZ7s2*and long29330 metal-support oxygen distances have been frequently observed for metal clusters on metal oxide supports. As a first approximation, only four free parameters were estimated (A$, the Debye-Waller factor, and A&, the inner potential correction, were set equal to 0) to shorten the time for parameter estimation. The first-guess Ir-Ir and Ir-Osupportcontributions were then added and compared with the raw data in r space, and the fit was not satisfactory. Then the Ir-Osupponcontribution was subtracted
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10603
NaY Zeolite-Encaged Tetrairidium Clusters
yellow. These infrared spectra show that the carbonylationdecarbonylation process was reversible, as reported previously.*
I
Discussion
I
I
I
2100 2000 1900 WAVENUMBER, cm-1
18
Figure 4. Decarbonylation of the iridium carbonyl cluster in NaY zeolite: (A) sample following treatment in CO (the spectrum is virtually the same as that of [Ir4(C0)1~]in THF solution). The other spectra show the effects of treatment of this sample in H2 for various times at various temperatures: (B) 125 ' C for 10 min; (C) 200 ' C for 30 min; (D) 275 'C for 30 min; (E) 300 'C for 2 h. from the data, and better estimates of the parameters characterizing the Ir-Ir contribution were determined. The improved fit for the Ir-Ir contribution was subtracted from the data, and more accurate parameters for the contributions of the metalsupport interface were determined by fitting the Ir-Osupprt contributions to the residual spectrum, both in k space and in r space. This process was repeated, but even after many iterations, the fit was not good in the low-r region. It was thus inferred that another small contribution, attributed to a low-Z (atomic-weight) backscatterer, had to be accounted for. A difference file was calculated by subtracting the best estimated Ir-Ir + Ir-Osupprt contribution from the experimental EXAFS function. The additional contribution was calculated by fitting the difference file with four adjustable parameters. The additional low-Z backscatterer is not identified; it may be carbon remaining from the carbonyl ligands, and we tentatively refer to the contribution as Ir-C. The Ir-Ir, Ir-C, and two Ir-Osuppn contributions were then added, representing the overall fit of the data. To show the goodness of the fit for both the high- (Ir) and low-atomic-weight (C, 0) contributions, the raw data are compared with the fit, both in k space (with k2 weighting) and in r space (with both kl and k3 weighting) (Figure 5B-D). The agreement is good. The structural parameters are shown in Table 111, and the Ir-O,uppn contributions are shown in Figure 5E. The number of parameters used to fit the data in this first-shell analysis is 16; the statistically justified number, calculated as above, is approximately 19. Infrared Data Characterizing the Recarbonylation of the Decarbonylated Iridium Clusters in NaY Zeolite. The decarbonylated sample (Figure 6A) was exposed to C O in the infrared cell as the temperature was raised. When the temperature reached 40 OC, infrared bands in the carbonyl region grew in at 2035 and 207Ocm-I. Thegrowth inintensitycontinuedfor about 4 h (Figure 6B-D), with the spectrum finally becoming virtually the same as that prior to decarbonylation. Again, the sample was light
Characterizationof Zeolite-Entrapped[Ir@O) 121 by EXAFS Spectroscopy. Both the infrared and EXAFS results are consistent with the inference that the iridium carbonyl species formed in high yield from [Ir(CO)z(acac)] in undried NaY zeolite after treatment in C O a t 1 atm and 125 OC for 12 h followed by cooling to room temperature and treatment in air at 25 OC were [Ir4(CO)12]. The evidence supporting this conclusion is the following: (1) the color of the zeolite containing the supported iridium carbonyl matches that of the THF solution of [Ir4(CO)12], (2) there is close agreement between the infrared spectrum of the zeolite-supported iridium carbonyl and that of the THF solution of [Ir4(CO)12](Figure l), and (3) the EXAFS spectrum of the zeolite-supported iridium carbonyl is consistent with the presence of [Ir4(CO)12],but not exclusively [Ir4(CO)12]. The latter point is developed in the following paragraphs. The data indicate that there was an EXAFS contribution in addition to that characteristic of [Ir4(CO)12]; the EXAFS data suggest the presence of a small amount of an Ir-containing species having a lower nuclearity than [Ir4(CO)12]. It may arise from unconverted [Ir(C0)2(acac)] or other mononuclear species.12Jl-33 The Ir-0 distances (Table 11) are indistinguishable from those that have been observed quite generally for metals supported on metal oxides. The shorter distance suggests bonding of positively charged Ir atoms to oxygen of the support, and the longer distance is still a matter of debate.34 All the Ir atoms in [Ir4(CO)12] in the crystalline state (Table I) are stereochemically equivalent, with each being bonded to three Ir atoms (at an average distance of 2.69 A) and to three terminal carbonyl ligands (with an average Ir-C distance of 1.87 8, and an Ir-O* distance of 3.01 A).21 EXAFS data for solid [Ir4(CO)12] mixed with inert boron nitride are consistent with the crystallographic structure parameterse35 Consistent with these results, the EXAFS data characterizing the supported iridium carbonyl (Table 11) indicate an Ir-Ir coordination number of 2.6 with an average Ir-Ir distance of 2.69 A. This coordination number, although it is not distinguishable within the estimated experimental error of f20% from the value of 3 for crystalline [Ir4(CO)12],suggests the presence of an Ir species with a lower nuclearity than 4. The Ir-C coordination number of 2.3 and the Ir-O* coordination number of 2.2 are also less than the crystallographic values of 3 for [Ir4(CO)12] and also suggest lower-nuclearity species. The Ir-C and Ir-O* distances (Table 11) match the crystallographic values for [ Ir4(CO) 121, consistent with the presence of this cluster as the predominant Ir-containing species. The EXAFS results indicate that about 75% of the Ir atoms were present in the [Ir4(CO)12] clusters. The remaining Ir-containing species are unknown. In summary, there is good agreement between the EXAFS results characterizing the sample formed from [ Ir(CO)2(acac)] in NaY zeolite after treatment in CO at 125 OC followed by treatment inair (TableII) and thecrystalstructureof [1r4(co)12] (Table I). We conclude, therefore, that the predominant entrapped iridium carbonyl species in the zeolite was [Ir4(CO)12]. Gelin et al.,36 on the basis of infrared spectra, also reported the synthesis of [Ir4(CO)l2] in NaY zeolite from a mononuclear iridium carbonyl in the presence of CO. Chemistry of the Formationof [Ir4(CO)1~]inNaYZeoliteCages. Infrared results reported elsewhere12 indicate the formation of [Ir6(co)l6] from [Ir(C0)2(acac)] in undried NaY zeolite. The chemistry of synthesis of [Ir6(co)l6] in the zeolite is consistent with that of the formation of iridium carbonyl clusters in solution: They are synthesized by carbonylation of iridium salts37-39 or by carbonylation of [Ir(CO)z(acac)] .40 The infrared results of the present work indicate that [Ir4(CO)12] was an
Kawi et al.
10604 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 2
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E
-3
4 1 2 3 4 5
0
r, A Figure 5. Results of EXAFS analysis obtained with the best calculated coordination parameters characterizing zeolite-supported Ir clusters prepared by decarbonylationof the Ir carbonyl clusters in NaY zeolite at 325 O C in He followed by H2: (A) raw EXAFS data; (B) experimentalEXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-O,umn contributions (dashed line); (C) imaginary part and magnitude of Fourier transform (kl-weighted, Ak = 3.66-13.80 A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-O,um contributions (dashed line); (D) imaginary part and magnitude of Fourier transform (k3-weighted,Ak = 3.66-13.80A-I) of experimental EXAFS (solid line) and sum of the calculated Ir-Ir + Ir-C + Ir-O,,mn contributions (dashed line); (E) residual spectrum illustrating the EXAFS contributions characterizing the metal-support interaction: imaginary part and magnitude of Fourier transform (k3-weighted,Ak = 3.66-10.00A-I) of raw data minus calculated Ir-Ir + Ir-C EXAFS (solid line) and calculated Ir-01 + b o aEXAFS (dashed line).
TABLE 111: EXAFS Results Characterizing the Iridium Clusters Formed by Decarbonylation of [Irq(CO)12)in NaY Zeolite at 325 OC in He Followed by H+* shell N R.A Pa2. A2 AEo,eV EXAFS reference Ir-Ir 3.4 2.70 0.0030 0.06 Pt-Pt ~~
Ir-Osuppn
Ir-01
h-Oa
1.5 1.7 0.7
2.68 2.19 1.94
0.0036 0.0012 0.0028
-6.11 -3.56 -9.50
Pt-O Pt-O
Ir-C Ir-C 0 Notation as in Table I; the subscripts s and I refer to short and long, respectively. Estimated precision: N, *20% (Ir-OSumn, *30%); R, *2% (Ir-Ir, *l%); A&, f30%; A E o , *lo%.
intermediate in the formation of [Ir6(co)l6]. This pattern is borne out by the solution chemistry.41~42Water is required both in the syntheses in solution and in the zeolite. Similarly, [Ir4(CO)12]is synthesized by reductive carbonylation of adsorbed [Ir(C0)2(acac)] on a hydroxylated yA1203 surface.40 The chemistry of formation of rhodium carbonyl clusters on surfaces is apparently similar to these syntheses of iridium carbonyl clusters and also requires water. The formation of [Rh6(CO)16] on the surface of alumina has been suggested to be initiated by attack of adsorbed H2O on rhodium subcarbonyls to give mobile, nucleophilic [Rh(CO)&, which was envisaged to react with the rhodium subcarbonyl in a reductive condensation to give the c l u ~ t e r . ~We ~ *suggest ~ that similar chemistry occurred in the
formation of the iridium carbonyl cluster in the NaY zeolite45 and also in the formation of [Rb(Co)16] in NaY This hypothesis about the formation of the rhodium carbonyl cluster is supported by the fact that Rao et al.” used C O containing a small amount of water in their synthesis of the cluster from a mononuclear rhodium precursor. The results reported here indicate that the hexanuclear cluster was converted efficiently into [Ir4(CO)I2] as a result of air exposure at room temperature. This cage-mediated synthesis of thecluster is facile and straightforward. Analogous chemistry has been observed for rhodium carbonyl clusters in solution.46 To repeat, the synthesis was successful only when it was done with undried liquid reagents and the zeolite that had been evacuated only at room temperature. We conclude that water was needed for a successful synthesis, but the amount required was not determined. When the synthesis was done successfully, addition of small amounts of water to the C O did not change the chemistry. The support basicity is inferred to affect the chemistry of formation of iridium carbonyls by reductive carbonylation of adsorbed [Ir(CO)2(acac)] in zeolite cages and on metal oxide surfaces. In the relatively weakly basic NaY zeolite, formation of neutral iridium carbonyl clusters, such as [Ir4(CO)12] and [Ir6(C0)16]>-9J2isobserved.However, in the morestrongly basic NaX zeolite, various iridium carbonyl anions (such as [HIrd-
NaY Zeolite-Encaged Tetrairidium Clusters I
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10605
the inference that the nuclearity and the tetrahedral frame of the precursor were largely retained in the decarbonylated cluster. Furthermore, there are no Ir-Ir contributions at distances greater than the first Ir-Ir coordination shell. Higher-shell Ir-Ir contributions, had they been present, would have suggested the presence of larger metal clusters or crystallites.54 For example, Fourier transforms of the EXAFS function characterizing Pt foil include peaks corresponding to the second, third, fourth, and fifth shells of the pt atoms in the fcc structure.55 The lack of higher-shell Ir-Ir neighbors of the encaged decarbonylated iridium clusters shows that there was no significant sintering of theiridium to form crystallites on the outer zeolite surface. The Ir-Ir distance characterizing the decarbonylated Ir4cluster in NaY zeolite is 2.70 A, which is, within experimental error, the same as that of [Ir4(CO)12] (2.69 A).2l For comparison, the first-neighbor Ir-Ir distances in bulk Ir metal and in relatively large Ir crystallites are 2.7 15 A?5 Thus, the average Ir-Ir distance alone is not sufficient to distinguish tetrairidium clusters from larger clusters or crystallites. The comparison is, however, complicated by the fact that the decarbonylated sample was characterized by EXAFS spectroscopy in the presence of H2, and the presence of adsorbed hydrogen has been shown to increase the metal-metal distance in y-Al203-supported RhS6 and Y zeolite-supported Pt ~lusters.5~Further work is needed to determine the effect of adsorbed hydrogen on the Ir-Ir distance. 2200 2100 2800 1900 11 In summary, the EXAFS data provide the basis for the WAVENUMBER, em-1 suggestionof a simplified structural model of theencaged clusters Figwe6. Recarbonylation of thedecarbonylated iridiumclustersinNaY in the sample derived from [Ir4(CO)12]. The clusters are zeolite. The sample was treated in flowing CO at 1 atm and 40 "C,with represented as tetrahedral Ir4 inside the zeolite supercages. To spectra measured at various times: (A) sample in the decarbonylated account for the average Ir-0 coordination number of 3.2, we form. The remaining spectra were measured with the sample in CO for the indicated times: (B) l / 2 h, (C) 1 h, (D) 2 h, (E) 4 h. speculate that three of the four Ir atoms of each cluster are positioned at the interior zeolite surface, with each Ir atom (CO)ll]- and [Ira(CO)l5]2-) have been synthesized by carboninteracting with four oxygen atoms. ylationof sorbed [Ir(CO)2(a~ac)].10Jl,~~ Comparable results have This model is simplified; others cannot be ruled out. The metal been observed for reaction on metal oxide surfaces; [Ir4(CO)12] is formed on weakly basic y-A1203,40whereas [ H I ~ ~ ( C O ) I I ] - , is depicted here as bare atoms in the cages, but since the sample was investigated in the presence of hydrogen, it is likely that the [Ir8(CO)22]2-, and [Ir6(C0)15]2-are formed on strongly basic clusters were present as iridium hydrides;58 the available data Mgo.15.48-50 provide no evidence of the presence or locations of hydrogen atoms The results reported here are consistent with a ship-in-a-bottle in the structure. Some clusters with nuclearities greater than 4 synthesis in the zeolite supercages. It is evident that the precursor may also have been present; however, since the higher Ir-Ir shells [Ir(CO)2(acac)] is small enough to fit into the interior of the are almost absent, these must have been rare. Furthermore, there zeolite, since larger metal carbonyls have been shown to fit: [CpMis a significant EXAFS contribution that is not identified. It was ( W 2 1 , [Cp*M(C0)21, and [CpM(C2H4)21 [M = Rh, Ir; CP = suggested to be an Ir-C contribution because the average C5H5; Cp* = (CH3)&,].51 Furthermore, [Ir4(CO)12] (with a absorber-backscatterer distance was found to be 1.94 A; this diameter of about 9 A) is easily small enough to fit in the distance is too small to be an Ir-O distance, which would be supercages of zeolite Y (which have diameters of about 12.5 A) about 2.1 8, in an iridium subcarbonyl.59 but too large to diffuse rapidly through the apertures (which In summary, the EXAFS results suggest that the structure of have diameters of about 7.4 A). Thus, it is plausible that the the iridium cluster frame after decarbonylation under mild clusters, once formed, would be trapped in the supercages and conditions closely resembles that of the tetrahedral frame of [Ir4not removed when the sample came in contact with THF. (CO)l2]. The decarbonylation and stabilization of the iridium Decarbonylation of NaY Zeolite-Entrapped [Ir4(CO)12]. The carbonyl clusters are similar to what has been observed for [Ir6decarbonylation of [Ir4(CO)12] in NaY zeolite was carried out (co)l6] encaged in NaY zeolite.12 We suggest that the clusters in flowing H2 by gradually increasing the temperature. The are stabilized by the rigid environment of the zeolite cages and terminal absorption bands in the infrared spectrum decreased in migrate and sinter less rapidly than they would on a support with intensity, broadened, and shifted to lower frequency as the larger pores. temperature was increased. This result is similar to those of Recarbonylation of the Supported Iridium Clusters. The Handy et al.,52 who observed that both the terminal and the infrared spectra show that the decarbonylated iridium clusters bridging bands shifted to lower energy as the decarbonylation of formed from [Ir4(CO)12]in NaY zeolite, which are suggested to [Pt15(CO)30]2- (present as the PPN salt) on y-Al203 proceeded be Ir4 clusters, can be reversibly recarbonylated. Similarly, a with the sample in flowing He. The decrease of C O coverage reversible carbonylation-decarbonylation cycle has been observed leads to diminished dipole-dipole coupling between adjacent with [Ir6(co)16] in NaY zeolite.gJ2 C O ligands and a reduced shift in the energy of the CO absorption bands.53 However, CO is not always an innocent adsorbate; it may induce Formation of Decarbonylated Ir4 Cluster in NaY Zeolite. significant morphological changes in supported metal clusters. For example, the adsorption of C O on highly dispersed Rh56.60,61 EXAFS spectroscopy was used to investigate the structure of the entrapped clusters resulting from the decarbonylation. The Irand Ru62 clusters supported on y-Al203 leads to the oxidative Ircoordination number (3.4, Table 111) is, within theexperimental fragmentation of the clusters; the resulting surface species are error, equal to the value of the crystallographically determined metal subcarbonyls. When clusters of more noble (less oxophilic) Ir-Ir coordination number of [Ir4(CO)12] (3.0), consistent with metals (Pd63 or Ptl3) in zeolites or on metal oxide supports are
10606 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
brought in contact with CO, they aggregate to form larger clusters. Iridium is intermediate in character between the relatively oxophilic Rh and Ru and the more noble Pd and Pt, and it may be optimal in its resistance to fragmentation and agglomeration when the decarbonylated clusters are brought in contact with CO. The zeolite cages may help to stabilize the tetrairidium clusters and prevent agglomeration. Thus, we are led to suggest that the combination of Ir and the stabilizing confines of the zeolite supercage may be nearly unique in allowing the reversible decarbonylation and recarbonylation with retention of cluster nuclearity .
Conclusions [Ir(CO)z(acac)] sorbed in the supercages of NaY zeolite was converted to [Ir4(CO)12]in the presence of CO at 1 atm and 125 “C, followed by air exposure at room temperature. EXAFS spectra characterizing the encaged iridium carbonyl cluster gave an average Ir-Ir coordination number of 2.6 with a bond distance of 2.69 A, consistent with the formation of [Ir4(CO)12] as the predominant species in the zeolite supercages. The decarbonylation of this sample in He and H2 at 325 OC led to the formation of extremely small and uniform iridium clusters that were found by EXAFS spectroscopy to have an average Ir-Ir coordination number of 3.4 with a bond distance of 2.70 A. The data are consistent with the inference that the tetrahedral metal frame of [Ir4(CO)12] was retained in the decarbonylated sample; thus, thecluster is represented as Ir4with a tetrahedral structure. When this sample was recarbonylated, infrared results show that [ Ir4(CO)12] was reformed.
Acknowledgment. We thank Professor D. C. Koningsberger of the University of Utrecht for many helpful discussions about EXAFS analysis. The EXAFS data were analyzed with the Eindhoven University EXAFS Data Analysis Program, developed by M. Vaarkamp and D. C. Koningsberger. We thank Dr. Edith Flanigen of Union Carbide Corp. for providing the sample of NaY zeolite. This research was supported by the National Science Foundation (CTS-9012910 and CTS-9300754). We also acknowledge thesupport of the U S . Department of Energy, Division of Materials Sciences, under contract number DE-FGO589ER45384, for its role in the operation and development of beam line X-1 1A at theNationa1 Synchrotron Light Source. The NSLS is supported by the Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-76CH00016. We are grateful to the staff of beam line X-1 1A for their assistance. References and Notes (1) Jacobs, P. A. In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., KnBzinger, H., Eds.; Elsevier: Amsterdam, 1986; p 357. (2) Ozin, G. A.; Ozkar, S.; Moller, K.; Bein, T. J. Am. Chem. SOC.1990, 112,9575. (3) Ozin, G. A.; Malek, A.; Prokopowicz, R.; Macdonald, P. M.; Ozkar, S.; Moller, K.; Bein, T. Mater. Res. SOC.Symp. Proc. 1991, 233, 109. (4) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. SOC.1989, 1 1 1 , 530. ( 5 ) Moller, K.; Eddy, M. M.; Stucky, G. D.; Herron, N.; Bein, T. J. Am. Chem. SOC.1989. I I I. 2564. (6) Gates, B.C. In’CaralysrDesign: Progress and Perspectives;Hegedus, L. L., Ed.; Wiley: New York, 1987; p 71. (7) Herron, N.; Stucky, G. D.; Tolman, C. A. Inorg. Chim. Acto 1985, inn. . - - , 1- -3-5. (8) Kawi, S.; Gates, B. C. Caral. Lett. 1991, 10, 263. (9) Kawi, S.; Gates, B. C. J. Chem. SOC.,Chem. Commun. 1991, 994. (10) Kawi, S.; Gates, B. C. J. Chem. SOC.,Chem. Commun. 1992, 702. (11) Kawi, S.; Chang, J.-R.; Gates, B. C. J. Catal. 1993, 142, 585. (12) Kawi, S.; Chang, J.-R.; Gates, B. C. J. Am. Chem. SOC.1993, 115, 4830. (13) Chang, J.-R.; Koningsberger, D. C.; Gates, B. C. J. Am. Chem. SOC. 1992, 114,6460. (14) Lamb, H. H.; Wolfer, M.;Gates, B. C. J. Chem.Soc.,Chem. Commun. 1990, 1296. (15) Maloney, S. D.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1990, 5, 161.
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