Langmuir 1993,9, 1284-1289
1284
Osmium Subcarbonyls on y-Alumina: Characterization of the Metal-Support Bonding by Infrared, Nuclear Magnetic Resonance, and X-ray Absorption Spectroscopies S.E.Deutsch, J.-R. Chang, and B.C . Gates* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 21,1992. In Final Form: February 1,1993 [ O S ~ ( C O )reacted ~ ~ ] with the surface of r-A1203 to give a supported osmium carbonyl cluster, which was fragmented under He at 150 OC to give osmium subcarbonyla. These surface species were characterized by infrared, l3C NMR, and extended X-ray absorption fine structure (EXAF'S) spectroscopies and by X-ray absorption near edge spectroscopy (XANES). They are formulated as [Os(CO),(HOAl},(OAl),1, where the braces represent groups at the yAlzO3 surface and IC is 2 or 3 and y + z = 6 - x . When the osmium tricarbonyl (IC= 3) was treated under vacuum at 300 O C , it lost one CO ligand and gained one surface oxygen ligand. The surface species are both coordinatively saturated and characterized by an Os-Osupport bonding distance of 2.17 A; in these respects, they are close analogues of organometallic compounds.
Introduction Supported metal catalysts consist of metal clusters or crystallites dispersed on high-area porous supports that are usually metal oxides. The metal-support interface in these materials is important in determining the resistance of the metal to sintering, and there is evidence that the interface structure affects the catalytic activity for a number of hydrocarbon reactions.' However, the structure of the interface between metals and metal oxide surfaces is still not well understood. We have investigated this interface in simple supported metal complexes, consisting of nearly uniform, well-defined mononuclear metal carbonyls bonded to metal 0xides.u Infrared and NMR spectroscopies provide information about the symmetry of the surface species and their interactions with the support.612 Extended X-ray absorption fine structure (EXAFS) Spectroscopy provides quantitative structure data, including metal-to-surface oxygen bonding distances and coordinationnumbers.' Characterization of supported mononuclear (singlemetal atom) complexes is easier than characterization of supported clusters or crystallites because in the former the greatest fraction of the EXAFS
* Towhom correspondenceshouldbe addreesed at the Department of Chemical Engineering,University of California,Davis, CA 95616. (1)Koninpberger, D. C.; Gates, B. C. Catal. Lett., 1992,14,271. (2)Kirlin, P. S.;van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1990,94, 8439. (3)Chang, J.-R.; Gron, L. U.; Honji, A.; Sanchez, K. M.; Gates, B. C. J. Phys. Chem. 1991,95,9944. (4)Papile, C. J.; Gates, B. C. Langmuir 1992,8,74. (5)Lamb, H.H.; Haelbring, L. C.; Dybowski, C.; Gates, B. C. J . Mol. Catal. lSS9,56,36. Frauenhoff, G.R.; Shapley, J. R.; Oldfield, E. Inorg. (6)Walter, T. H.; Chem. 1991,30,4732. (7)Hanson, B. E.;Wagner, G. W.; Davis, R. J.;Motell, E. Inorg. Chem. 1984,23,1636. (8) Hasselbring,L.;Puga, J.; Dybowski,C.; Gates,B. C. J.Phys. Chem. 1988,92,3934. (9)Derouane, E. G.;Nagy, J. B.; Vedrine, J. C. J. Catal. 1977,46,434. (10)Gelin, P.; Lefebvre, F.; Elleuch,B.; "cache, C.; Ben Taarit, Y. In Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 298; American Chemical Society; Washington, DC, 1983;p 455. (11)Nagy, J. B.; Van Eenw, M.; Derouane, E. G.; Vedrine, J. C. In Magnetic Resonance in Colloid and Interface Science; Fraissard, J. P., Resing, H. A,, Eds.; D. Reidel: Dordrecht, Holland, 1980;p 591. (12)Ben Taarit,Y.;Wicker, G.;Naccache, C. InMagnetic Resonance in Colloid and Interface Science; Fraissard, J. P., Resing, H. A,, Eds.; D. Reidel: Dordrecht, Holland, 1980; p 497.
signal arises from the metal-upport contributions, where-
as in the latter metal-metal contributions dominate. The supported metal complexes chosen for this investigation are osmium subcarbonyls on y-&o3, represented as [OS(CO),~HOA~J,(OA~J,~, where x is 2 or 313J4and y and z are expected to be in the range 1-3;13J4 the braces denote groups terminating the metal oxide support. In this notation, the surface -OH group, for example, is presumed to be coordinated to several A13+ ions of the support. The osmium subcarbonyls are among the few supported metal subcarbonyls that are known to be reversibly carbonylated and de~arbonylated.'~J~-'~ Thus we were motivated to investigate the supported osmium subcarbonyls to determine how a change in the number of CO ligands in the surface-bound complex affects the bonding of the complex to the ligands provided by the support surface.
Experimental Methods Sample Preparation. The supported osmium subcarbonyls were prepared in the near absence of air on a Schlenk vacuum line, and the sample handling and transfer were carried out in a nitrogen-purged Braun MB15OM glovebox. Y&03 (Degussa aluminum oxide C, 104 mZ/g)was slurried with deionized water and dried overnight. It was then ground and calcined at 400 O C for 5 h under flowing 02 (Matheson,99.999 %) and for 2 h under Nz (Matheson, 99.999%) and cooled overnight under vacuum. n-Octane (Aldrich) was refluxed under NPin the presence of Ndbenzophenoneto remove tracesof water. [Os&O)12] (Strem) was used without further purification. The supported osmium carbonyl cluster was prepared as follows:i6 A mixture of 1.06 g of yA1209 and 0.015 g of [Osh (CO)l~Jwas refluxed in 50 mL of n-octane for 4 h, filtered, then washed twice with 25 mL of n-octane to remove weakly adsorbed species,filtered,and dried under vacuum overnight. A pale yellow powder containingapproximately 1 wt % Os was obtained,which is formulated as [HOs3(CO)l~(HOAlJy(OA1J,], where y + z = l.14 The supported osmium subcarbonyl~Os(CO),(HOAl]y(OAl),l was formed by oxidative fragmentation of the supported trios(13)Psaro, R.; Ugo, R.; Zanderighi, G. M.; Besson, B.; Smith, A. K.; Basset, J.-M. J . Organomet. Chem. 1981,213,215. (14)Duivenvwrden, F. B. M.; Koningsberger, D. C.;Uh, Y. S.; Gates, B. C. J . Am. Chem. SOC. 1986,108,6254. (15)Kndzinger, H.; Zhao, Y. J . Catal. 1981, 71,337. (16)Deeba, M.; Gates, B. C. J . Catal. 1981,67,303. (17)DOgSi, L.;Fuai, A.;Psaro,R.; Zanderighi, G.M. Appl. Catal. 1989, 46,145.
0743-7463/93/2409-1284$04.00/0 1993 American Chemical Society
Langmuir, Vol. 9, No. 5, 1993 1285
Osmium Subcarbonyls on y-Alumina mium cluster as the sample was treated in flowing He at 150 "C and 1 atm for 3 h. The supported osmium subcarbonyls were light tan. The W-enriched supported triosmium carbonyl cluster was prepared from [Os&O)l2] that had been enriched in 13C by stirring in decalin at 150 "C for a week under 13C0 (99.97 % , supplied by Isotec). The degree of enrichment of [Os3(CO)121 was 61 & 5 % , as determined by mass spectrometry. The supported triosmium cluster that was labeled with 13C was prepared as stated above for the unlabeled supported cluster. Labeled osmium subcarbonyls were prepared by oxidative fragmentation of the enriched supported cluster at 200 "C for 3 h in flowing He. This sample was converted into [os(co)3{HOA1~y{OAIJ,I by heating for 3 h a t 200 OC under 1 atm of 13C0. The enriched osmium dicarbonyl was made by treatment of the enriched osmium tricarbonyl for 4 h at 300 "C under vacuum. Infrared Spectroscopy. Infrared spectraweremeasured with a Nicolet 510-M FTIR spectrometer. A wafer of the supported species was pressed inside the drybox and loaded into a gas-tight cell. Treatments of the sample were done in situ. The wafer was heated for 3 h in He at 150 O C to partially decarbonylate the supported cluster and convert it into osmium subcarbonyls.l"18 It was then heated for 3 h at 200 OC in flowing CO at 1 atm to convert it largely to an osmium tricarbonyl,lSl8 followed by heating in vacuo at 300 "C for 3 h to convert it largely to an osmium dicarbonyl.13-18 1 3 c NMR Spectroscopy. In preparation for a cross-polarization magic angle spinning NMR spectroscopy experiment, a powder sample was loaded into a Pyrex NMR rotor insert and sealed under vacuum as a thin neck in the insert was melted in a flame. The insert was placed in the 25-MHz spectrometer (ChemagneticsM100S) and the spectrum (approximately 50 OOO scans) was recorded over several days with a spinning rate of 2.53 kHz and a spin-lock field of 36.8 kHz. The spectral width was 12.5 kHz; 1024 data points were acquired with a 5-8 relaxation delay and 2-ms contact time for cross polarization. The data were Fourier transformed with 2048 data points. EXAFS Spectroscopy. EXAFS measurements were performed at Beamline X-11A at the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, Long Island, NY. The ring energy was 2.528 GeV, and the ring current was at least 110 mA for all EXAFS measurements. The powder sample incorporating the supported osmium cluster ww pressed into a wafer, with the amount of sample chosen to give an absorbance of 2.5. The sample in a sealed EXAFS cell was then partially decarbonylated and converted into osmium subcarbonyls by heating under He for 3 h at 200 OC, followed by 3 h in flowing CO at 200 OC and 1 atm to give largely the osmium tricarbonyl, whereupon the sample in the presence of CO was cooled to nearly liquid nitrogen temperature ahd the EXAFS spectrum scanned twice. The sample was removed from the beam line and treated under vacuum (10-5 Torr) for 3 h at 300 "C to give largely the osmium dicarbonyl,13-18and the EXAFS spectrum was again measured twice.
Analysis of EXAFS Data EXAFS data analysis was done with the Koningsberger difference file technique.'3Jg The analysis was based on experimentallydetermined phase shiftsand backscattering amplitudes characterizing materials with known crystal structures. The Os-Osupport interactions (where Osupportis oxygen of the yA1203 surface) were analyzed with phase shifts and backscattering amplitudes obtained from EXAFS data for crystalline [NH4Re041.20 The 0s-C and Os-O* interactions (where O*denotes carbonyl oxygen) were analyzed with phase shifts and backscattering amplitudes obtained from EXAFSdata for crystalline [oS3(18) Barth,R.;Gates,B.C.;Zhao,Y.;Kn6zinger,H.;Hulse,J. J.Catal. 1983,82, 141. (19) van Zon, J. B. A. D.; Koningsberger, D.C.; van't Blik, H. F. J.; Sayers, D. E. J. Chem. Phys. 1985,82,5142. (20) Brown, R. J. C.; Segel, S. L.; Dolling, G. Acta Crystallogr. 1980, B36,2195.
Table I. Structural Parameters Characterizing the Reference Compounds Used in the EXAFS Analysisa crvstallomaDhic " dah * Fourier t r d o r m reference comDound shell N R . A Ak.A-l &.A n ref NH&04 Re-0 4 1.74 2.2-13.2 0.5-2.06 3 20 Os4 OS-O*
Os3(CO)12
4 4
1.95
3.09
1.9-12.4 2.9-12.4
0.9-2.0 2.0-3.3
3 3
21 21
Notation: Shell, absorber-backscatterer pair, N,coordination number for absorber-backscatterer pair; R, radial distance from crystal structure data; Ak, limits used for forward transform (kis the wave vector); Ar, limits used for shell isolation (r is distance); n, power of k used for Fourier transformation.
1'
2200
'
"
I
'
2150
"
"
"
2100
2845 ' ' ' 1 1' 2041 " "
2050
"
2000
'
'
I
"
1950
''4
1900
wavenumber (cm") Figure 1. Infrared spectra of osmium subcarbonyle: A, spectrum of sDecies formed after decomDositionof [0edCO)1,1on r-&Ol andtreatment with CO at 200-OC for 3 h;B, s&tr& of spe6es formed after additional treatment under vacuum at 300 OC for 3 h.
(CO)12] mixed with SiOza2lThe approximation that Os behaves equivalently to Re as an absorber atom is justified by experimental and theoretical Deteile of the preparation of the reference film are given ebewhere.l4 EXAFS parameters for the reference materiala are summarized in Table I. Rssults Infrared Spectroscopy. After the treatment of the supported cluster in the infrared cell to oxidatively fragment it, bands appeared in the vco region at 2124, 2041, and 1970cm-l (Figure 1).During further treatment for 3 h under CO at 200 "C,the band at 2124 cm-l grew concomitantlywith a decrease of the 1970-cm-1band. After treatment under vacuum at 300 OC, the band at 2124 cm-l had shrunk, the one at 1970 cm-l had grown, and the one at 2041 cm-l had shifted to 2045 cm-l. The infrared bands characterizing the osmium subcarbonyls supported on y-A1203 have been assigned by Knazinger and Zhao.16 The bands at 2124 and 2041 cm-l are representative of osmium tricarbonyls bonded to y-Al2O3, and the bands at 2124,2045, and 1970 cm-I are representative of osmium dicarbonyls bonded to 7 4 2 0 3 . The infrared spectra indicate mixtures of these two species in each sample. r n e spectrum of Figure 1A hae IJCO bands at 2124 m, 2040 s, and 1970 w cm-1. T his spectrum shows that the surface osmium carbonyl species were predominantly (roughly 80%)osmium tricarbonyls. The spectrum of Figure 1B has peaks at 2124 m, 2045 8, and 1970 m cm-'. This spectrum shows that the surface species were principally (roughly 60%) osmium dicarbonyls. These estimates were made by comparing integrated (21) Corey, E. R.; Dahl,L.F. Znorg. Chem. 1962, 1, 521. (22) Teo, B.-K.; Lee, P. A. J. Am. Chem. SOC.1979,101,2815.
1280 Langmuir, Vol. 9, No. 6,1993
Deutsch et al.
l"""'""'""''"'"'~ 1001
I1
1996
-0.2 2~ ~ " 4" " " "6 " " " 8" ' " 10 " " ~ '1 2
k 2150
2100
2050
2000
1950
(k')
14
16
1900
wavenumber ( c m-') Figure 2. Infrared spectra of 13C-labeledosmium subcarbonyls: A, spectrum of species formed after decomposition of 10%-
ll
(l3C0)12]on 7-4203 and treatment with 13CO at 200 "C for 3 h; B, spectrum of species after additional treatment under vacuum at 300 "C for 4 h. -0.1 -0.2
t 2
I 4
4
6
8
k
10
(A3
12
14
16
Figure 4. Raw EXAFS data characterizing osmium subcarbo-
nyls: A, spectrum of species formed after decomposition of [Os3(CO)l~lon y-Alz03 and treatment with CO at 200 "C for 3 h; B, spectrum of species formed after additional treatment under vacuum at 300 OC for 3 h.
PPM (relitive to TMS)
Figure 3. NMR spectra of W-labeled osmium subcarbonyls:
A, spectrum of species formed after decomposition of [Oss(l3C0)12]on y A l 2 0 3 and treatment with W O at 200 "C for 3 h; B, spectrum of species after additional treatment under vacuum at 300 "C for 4 h.
intensities characterizing the tricarbonyl and dicarbonyl with those characterizingthe enriched samples, assuming that all the extinction coefficients were the same. Infrared spectra of the labeled osmium subcarbonyls (Figure 2) indicate virtually complete conversion of the initial mixture of osmium di- and tricarbonyls to the osmium tricarbonyl upon treatment with W O at 200 OC and virtually complete replacement of the W O ligands with WO. Figure 2A shows uco bands at 2080 and 1996 cm-l. Upon treatment of the osmium tricarbonyl under vacuum at 300 OC, there was nearly complete conversion to the W-enriched osmium dicarbonyl, as shown by the shift in the uco bands to 2001 and 1935 cm-l (Figure 2B). W NMR Spectra. The spectrum characterizing each sample is shown in Figure 3. The chemical shifta are centered at 168 and 171 ppm (both relative to TMS) for the supported osmium tricarbonyl and osmium dicarbonyl, respectively. Spinning sidebands are evident in both the spectra; they are very small for the former sample. EXAFS Spectra of the Osmium Subcarbonyls. EXAFS data from two scans were averaged and analyzed by the difference f i e technique of Koningsbergeret al.,14J9 as before.3J3 The EXAFS spectra are shown in Figure 4. The noise amplitude at k = 10 A-' is 0.00s;the value of the EXAFS function at k = 4 A-1 is 0.27 (k is the wave ~~
~~
(23)Honji, A.; Gron, L.U.;Chang, J.-R.; Gates, B.C. Langmuir 1992,
8, 2715.
vector). These results lead to an estimated signal to noise ratio of about 33:l. An EXAFS analysis for a mixture of osmium di- and tricarbonyls on y-Al203has been reported,14and the results are summarized in Table 11. The method of data analysis used in this work is essentiallythe same as that re~0rted.l~ The method is also virtually identical to that used for analysis of data characterizing MgO-supported rhenium subcarb~nyls.~~~~ The EXAFS function (Figure 4A) shows no significant oscillations for values of k > 11 A-l, which demonstrates that there were no significant Os-Os interactions and that the fragmentation of the triosmiumcluster on the yA1203 surface was complete. The EXAFS data characterizing the sample that was primarily the osmium tricarbonyl were Fourier transformed over the useful range (3.51 < k < 10.60A-I) with k3 weighting and no phase correction. The major contributions were isolated by inverse Fourier transformation in the range 0.57 < r < 3.77 A. 0 8 4 , Os-0*, and OsOauPp~ contributionswere all needed to fit the data (Figure 5). Evidence of multiple scattering resulting from the nearly linear arrangement of the atoms in the Os-C-O* moieties is provided by a negative peak at approximately 3 A in the imaginary part of the phase-corrected Fourier transform of the spectrum (Figure 6). The fit of the data was not satisfactory until a contribution was included to account for a small peak at 3.6 A, which is tentatively assigned as an interaction between Os and a low-Zbackscatterer, which might be 0 of the support, Al of the support, or C remaining from the solvent used in the samplepreparation. Fitting of this peak is difficult because of the interference from the side lobe of the strong Os-O* multiple scattering peak located at 3 A. The side lobe arises from truncation of the Fourier transform data range. Although the overall contribution to the EXAFS signal from this contribution is small and affected by noise, it was necessary to fit this peak to get an acceptable
Langmuir, Vol. 9, No.5,1993 1287
Osmium Subcarbonyls on y-Alumina
Table 11. EXAFS Results Characterizing the Surface Species Formed after Decomposition of [O~a(CO)izlon 7-Al203"* sample osmium tricarbonyl supported on r-Al~03~
R,A
2.9 2.4 3.1
2.17 1.93 3.04
0.0018 0.0098 0.0022
-3.18 13.55 6.21
os-osupport 05%
3.9 2.0 2.4
2.17 1.85 3.04
0.0036 0.0074 0.0014
-2.84 14.85 4.16
os-osupport
3.0 2.8 2.8
2.17 1.91 3.05
-0.0025
08-oS"Pport
Os4 05-0.
osmium dicarbonyl supported on r-Al~03~
mo,eV
N
shell
A$,
A2
ref this work
this work
os-o*
mixture of osmium di- and tricarbonyls on y-Alz03
14
os4
os-o*
0 0
a Notation: Shell, absorber-backscatterer pair; N,coordination number of absorber-backscatterer pair; R,average distance from absorber to backscatterer; A$, Debye-Waller factor;AEo,inner potential correction. Estimated precision: N,*20%; R,*1%; A$, *30% ;A&, *lo%. Best-fit parameter values for a second Os-Osupportshell were estimated N = 8.5,R = 3.57, A d = O.OOO6, = 8.22 (units aa above); they are not believed to be reliable because their contribution to the total EXAFS was so small. Best-fit parameter values for a second 04,upport shell were estimated N = 9.8,R = 3.58,A$ = 0.0038,AEo = 6.62 (units as above); again, these values are not believed to be reliable.
*
4
3
E
s
L . 2 E
E -
1
0
L.
-1
0)
. I
L.
g
-2
k
-3 -4
0
1
2
r (ii13
5
4
Figure 5. Results of EXAFS analysis obtained with the best calculated Coordination parameters for the sample prepared by decomposition of [Os3(CO)121on yA1203 and treatment with C O Wweighted Fourier transform of experimental EXAFS (solid Os-O* line) and s u m of the calculated Os-Osupport+ Os-C contributions (dashed line).
+
1
1
3
E
s
L . 2 E
E
-
L.
1
0 -1
. Q,I
L.
g
k
-2
-3
k 1 2 4 5
-4 0
r
Figure 6. k3-Weighted Fourier transform of experimental EXAFS ( R e 4 phase corrected for Os-Osupportcontributions), showing evidence of multiple scattering at r = 3 A.
determination of the EXAFS parameters for the principal contributions. The parameter values determined in the fit are shown in Table 11. The number of parameters is 16; the tati is tic ally^^ justified number is approximately 16. The EXAFS data characterizing the sample containing principally the osmium dicarbonyl on y-Al203 were (24) Crozier, E. D.;
Rehr, J. J.; Ingalls, R. In X-Ray Absorption:
Principles,Applications, Techniquesof EXAFS,SEXAFS,and XAh'ES; Koningsberger, D. C., Prins, R., Eda.;Wiley: New York, 1988; Chapter 9.
-4
~
0
"
"
'
"
"
1
"
"
"
"
'
2
'
'
'
'
~
r (A13
4
5
Figure 7. Results of EXAFS analysis obtained with the best calculated coordination parameters for the sample prepared by decomposition of [Os3(CO)12] on -y-Al203 and after treatment with CO and then under vacuum: !&weighted Fourier transform of experimental EXAFS (solid line) and sum of calculated OsOsupport Os-C Os** contributions (dashed line).
+
+
analyzed in the sameway. The EXAFS data were Fourier transformed over the useful range (3.52< k < 10.79 A-1) with k3 weighting and no phase correction. The major contributions were isolated by inverse Fourier transformation in the range 0.57 C r C 4.02 A. Again, Os
