4426
J . Phys. Chem. 1988, 92, 4426-4432
in the early phase of the oxidation and the increased temperature dependence in the later phase. Acknowledgment. We acknowledge the valuable aid of Susan
J. Packer for polishing the quartz crystals used for the thin-film
deposition and her advice on the deposition techniques. This work was supported financially in part by the US.Air Force Geophysics Laboratory (OPR). Registry No. Ag, 7440-22-4; 0, 17778-80-2.
Adsorbed States of CO on Dispersed Metals: Quantitative Analysis with 13C NMR Spectroscopy T. M. Duncan* and T. W. Roott AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: August 28, 1987; In Final Form: February 9, 1988)
The ”C NMR spectra of CO adsorbed on silica-supported Ru and Rh provide quantitative analysis and descriptions of the three adsorbed states: linear- and bridgebonded CO on metal particles and multicarbonyls on isolated metal atoms. Isolated Rh atoms adsorb two CO’s, and the multicarbonyls on Ru have two or three CO’s per Ru. The species adsorbed on the metal particles are immobile at 295 K (on the NMR time scale of a few milliseconds) whereas the multicarbonylsare motionally averaged. The ratio of linear to bridging CO on Ru particles is -4:l. On Rhlsilica, this ratio is 1:1.5. The isolated Rh atoms are not paramagnetic, which indicates that migration from the metal particles to form dicarbonyls is accompanied by oxidation of the Rh atom. At least two metal particle morphologies, spheres and rafts, are consistent with the CO site distributions, the overall CO-to-metal ratio, and the local density of CO measured by I3C NMR spectroscopy.
I. Introduction Transition metals dispersed on oxides exist as multiatom particles and, in some cases, as isolated metal atoms. The catalytic properties depend on the morphology of these metal particles at reaction conditions, which defines the number and type of surface adsorption sites. We describe here the analysis of I3C N M R spectra of CO adsorbed on silica-supported Ru and Rh catalysts, which directly yields the distribution of adsorption sites and a description of the metal. C O adsorbs on metal particles in terminal on-top sites and in bridging (e.g., p2 or p 3 ) sites. For microcrystallites dispersed on oxides, the relative fraction of on-top and bridging sites is influenced by repulsive C O C O interactions and the distance between adsorption sites, which depend on the facet of the crystallite. Ru and Rh may also be present as metal atoms isolated on the silica support that adsorb two (or more) carbonyls per atom. The first, and perhaps the most distinct, evidence of a gem-dicarbonyl on isolated atoms was a study of Rh/alumina with infrared spectroscopy.’ The infrared spectra revealed a doublet at 2100 and 2030 cm-’, the position of which was independent of coverage and resembled that of [Rh(C0)2C1]2.1Further evidence for isolated Rh atoms dispersed on alumina includes selective exchange of gem-dicarbonyls exclusive of C O on particles2 and preferential oxidation of C O adsorbed on Rh particles without affecting gem-di~arbonyls.~There is also spectroscopic evidence of spatial separation between gem-dicarbonyls and CO on rafts: proximity-determined N M R relaxation times are different for the two sites4and an EXAFS study of Rh/alumina revealed that the mean number of Rh-Rh nearest neighbors was 1.5, suggesting that some Rh atoms had no Rh nearest neighbor^.^ Dispersed Ru catalysts also contain sites that adsorb more than one linearly bonded C O molecule per metal Dicarbonyl Ru sites yield infrared spectra with a doublet at -2035 and 1975 cm-’, and tricarbonyl sites show infrared spectra with features at about 2140,2070, and 2010 cm-1.6-10 The consensus of studies to date is that multicarbonyl Ru sites are a t the edges or a t steps on Ru particles, although there is circumstantial evidence to suggest these sites are separate from the particles,” as is the case for Rh. For
-
‘Present address: Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706.
0022-3654/88/2092-4426$01.50/0
example, the hydrogenation of C O on Ru, which proceeds via the dissociation of CO and requires an adjacent vacant site,I1J2is not catalyzed by Ru sites on silica that adsorb two ~arbony1s.l~ Whereas the atoms comprising the microcrystallites have oxidation states of 0, it is proposed that the atoms responsible for multicarbonyl adsorption are Rh gem-dicarbonyl sites have an oxidation state of 1 and Ru multicarbonyl sites are +2 or +3. Among the arguments are the similarity in the I R spectra and X-ray photoelectron spectra of Rh(C0)2 and [Rh(CO)2C1]2,14and the width of the 13CNMR spectrum suggests C O is adsorbed on diamagnetic Rh in Rh(C0)2.4J5 The dispersion and topology of the metals depend on the adsorbate, as suggested by recent studies of Rh/alumina with EXAFSl63l7and infrared spectroscopy.18J9 The rhodium on fresh
+
(1) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957, 61, 1504. (2) Yates, Jr., J. T.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J. Chem. Phys. 1979, 70, 1219. (3) Cavanagh, R. R.; Yates, Jr., J. T. J . Chem. Phys. 1981, 74, 4150. (4) Yates, Jr., J. T.; Duncan, T. M.; Vaughan, R. W. J. Chem. Phys. 1980, 73, 975. (5) Via, G. H.; Meitzner, G.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 79, 1528. (6) Kuznetsov, V L.; Bell, A. T.; Yermakov, Y. I. J. Catal. 1980,65, 374. (7) Davydov, A. A.; Bell, A. T. J. Catal. 1977,49, 332. (8) Chen, H.-W.; Zhong, Z.; White, J. M. J . Catal. 1984, 90, 119. (9) Kobayashi, M.; Shiraski, T. J. Caral. 1973, 28, 289. (10) Johnson, B. F. G.; Johnston, R. D.; Josty, P. L.; Lewis, J.; Williams, I. G. Nature (London) 1967, 213, 901. (11) Cant, N. W.; Bell, A. T. J . Cafal. 1982, 73, 257. (12) Winslow, P.;Bell, A. T. J. Caral. 1984, 86, 158. (13) Kellner, C. S.; Bell, A. T. J. Coral. 1982, 75, 251. (14) Primet, M.; Vedrine, J. C.; Naccache, C. J. Mol. Catal. 1978, 4.41 1. (15) Robbins, J. L.J . Phys. Chem. 1986, 90, 3381. (16) (a) Van’t Blik, H. F.; Van Zon,J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D.; Prins, R. J . Phys. Chem. 1983,87, 2264. (b) Van? Blik, H. F.; Van Zon, J. B. A. D.; Koningsberger, D.; Prins, R. J. Mol. Caral. 1984, 25, 379. (c) Van’t Blik, H. F.; Van Zon, J. B. A. D.; Huizinga, T.; Koningsberger, D.; Prins, R. J. Am. Chem. SOC.1985, 107, 3139. (17) (a) Van Zon, J. B. A. D.; Koningsberger, D.; Van? Blik, H. F.; Prins, R.; Sayers, D. E. J . Chem. Phys. 1984,80, 3914. (b) Van Zon, J. B. A. D.; Koningsberger, D.; Van’t Blik, H. F.; Sayers, D. E. J . Chem. Phys. 1985, 82, 5742. (c) Koningsberger, D.; Van Zon, J. B. A. D.; Van’t Blik, H. F.; Visser, G. J.; Prins, R.; Mansour, A. N.; Sayers, D. E.; Short, D. R.; Katzer, J. R. J. Phys. Chem. 1985, 89,4075.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4427
N M R of CO on Dispersed Metals TABLE I: Descriptions of Silica-Supported Catalysts
sample
metal Ru Ru Rh Rh
A
B C D
loading, wt % 2.5 2.5
5 2.5
prepn method amine exchC amine exch acq impregn amine exch
Pm,Torr 23 100 9.5 8.8
CO-to-metal ratiob volumetric uptake NMR int 1.54 1.66 0.61 1.1 1
1.29 1.40 0.66 1.14
distribution of CO" (fO.02) linear bridged isolated 0.64 0.51 0.30 0.34
0.12 0.15 0.49 0.47
0.23 0.34 0.21 0.19
'See Discussion of Results. bConfidence limits on CO-to-metal ratios are &5%. 'Reduced in D2. catalysts is entirely reduced to the Rho state, as determined with TPR?O and exists only as microcrystallites; unpaired electrons of isolated Rho sites would yield an ESR signal, which is not observed.16 When C O contacts Rh particles, gem-dicarbonyls form slowly at 295 K, which reduces the number of Rh-Rh nearest neighbors detected by EXAFS,17 and the characteristic doublet appears in the infrared spectra.19 The formation of the gemdicarbonyl involves two steps in yet-undetermined order: oxidation of Rho to Rh' and separation from the particle. Proposed schemes for oxidation are the reaction with hydroxyl groups to yield a or by the dissociation of C0.l6 0-Rh bond and Consistent with this scheme, EXAFS measurements indicate that the Rh of the gem-dicarbonyl is coordinated to three oxygen a toms. The potential for N M R spectroscopy to differentiate between the adsorbed states of CO, and thus provide quantitative distributions, was demonstrated for the spectra of metal carbonyls in the solid state.23 The chemical shift anisotropy (6 = ( a ) - a33) of a terminal carbonyl is large; the mean anisotropy for the dozen compounds measured to date is 260 The anisotropy for a bridging carbonyl is less (6 110 ppm), owing to bonding nonparallel to the C-0 axis. The powder pattern of a gem-dicarbonyl is similar to that of a terminal carbonyl, as evidenced by an anisotropy of 267 ppm for [Rh(C0)2C1]2.4 However, a previous study has shown that gem-dicarbonyls are motionally averaged at 295 K and may be readily identified by a narrowed peak.4 The 13CN M R spectra for adsorbed C O reported to date4,24*2s lack the resolution to allow a direct identification of powder patterns. To date, the only method to decompose these overlapping features involved time-consuming spin-echo measurements which capitalized on differences in relaxation times.4 However, recent catalyst preparations have yielded well-defined adsorption sites for CO, as revealed by sharp features in the I3C N M R powder att tern,^,^^ and narrow peaks in N M R spectra measured with magic-angle spinning.26g28 1/2H21S,18921922
'
-
11. Experimental Procedures Two ultradisperse Ru/silica samples were prepared by amine cation e ~ c h a n g e . ~ ,Sufficient ,~ RuC13.xH20 to yield a final Ru loading of 2.5 wt % was dissolved in 10 mL of H 2 0 / g of final product desired. After addition of silica, the mixture was chilled in an ice bath and 0.8 mL of monohydrazine hydrate/g was added dropwise, with agitation. After N2 evolution ceased, the catalyst was covered and stirred overnight. The catalyst was then filtered, (18) Theolier, A.; Smith, A. K.; Leconte, M.; Basset, J. M.; Zanderighi, G. M.; Psaro, R.; Ugo, R. J. Organomet. Chem. 1980, 191, 415. (19) Solymosi, F.; Pisztor, M. J . Phys. Chem. 1985, 89, 4789. (20) Vis, J. C.; Van't Blik, H. F.; Huizinga, T.; van Grondelle, J.; Prins, R. J . Mol. Catal. 1984, 25, 367. (21) Smith, A. K.; Hugues, F.; Theolier, A.; Basset, J. M.; Ugo, R.; Zanderighi, G. M.; Bilhou, J. L.; Bilhou-Bougnot, V.;Graydon, W. F. Inorg. Chem. 1979, 18, 3104. (22) (a) Basu, P.; Panayotov, D.; Yates, Jr., J. T.J . Phys. Chem. 1987, 91, 3133. (b) Basu, P.; Panayotov, D.; Yates, Jr., J. T. J . Am. Chem. SOC. 1988, 110, 2074. (23) Gleeson, J. W.; Vaughan, R. W. J . Chem. Phys. 1983, 78, 5384. 124) Duncan, T. M.: Winslow, P.: Bell, A. T. J . Catal. 1985. 93. 1. (25) Rudaz, S. L.; Ansermet, J.-P.; Wang, P.-K.; Slichter, C . P.; Sinfelt, J. H. Phys. Rev. Lett. 1985, 54, 71. (26) Gay, I. D. J . Magn. Reson. 1984, 58, 413. (27) Takahashi, N.; Miura, K.; Fukui, H. J. Phys. Chem. 1986,90,2797. (28) Shoemaker, R. K.; Apple, T. M. J. Phys. Chem. 1985, 89, 3185. (29) Gay, I. D. J . Catal. 1983, 80, 231.
washed with cold 1 M NH,OH, and air-dried at room temperature. From this batch, two Ru/silica samples (A and B) were prepared, as described below. Two Rh/silica catalysts were prepared: a 5 wt % Rh/silica catalyst (C) was prepared by impregnation of silica to incipient wetness with an aqueous solution of RhC13 followed by drying in air at room temperature, and a 2.5 wt % Rh/silica catalyst (D) was prepared by amine cation exchange as described above. The metal loadings on the catalysts were inferred from the amount of metal delivered via the HzO solution, which assumes that all the metal adsorbed on the substrate and negligible amounts were lost in the NH40H wash.29 After drying, catalysts were packed into glass U-tube reactors described previously" and reduced in flowing H2; the Ru catalyst at 200 "C for 2 h then 350 OC for 2 h and the Rh catalyst at 200 "C for 6 h. (One Ru/silica sample (A) was reduced in D2.)The catalysts were then purged with flowing H e and cooled to 295 K. After cooling, one port of the reactor was sealed mechanically, as described elsewhere.24 C O (99% 13Cenriched) was delivered to each catalyst in small aliquots at 295 K. Adsorption was slow near saturation coverage, approximately mol of CO/(mol of metal-h). Consequently, several days were allowed for each to reach equilibrium. Finally, the remaining arm of the U-tube was pinched off and the catalyst sealed under CO. The physical descriptions of the catalysts are summarized in Table I. The 13C N M R experiments were performed on a Bruker CXP-200 spectrometer, operating at 50.34 MHz. Spectra were measured by Fourier transforming the nuclear precession observed immediately after a 90" pulse (free-induction decay) and observed as echoes stimulated by a 180" pulse (spin-echo). Spurious signals from the excitation pulses were minimized with add-subtract routines. The free-induction decays were alternately acquired from (90",-observe) and (180",-r-90",-observe) sequences with 7 = 3 ms and successive signals were added and subtracted. Similarly, the spin-echo experiment was composed of alternating (90°x-~18OoX-r~bserve)and (90°,-~1800,-~bserve) pulse sequences. The 13C N M R spectra are plotted on the 6 scale for chemical shifts, relative to TMS, such that downfield (deshielded) is positive (Le., C6H6 is at 128.7 ppm and CS2 is a t 192.8 ppm). The silica used in this study is Degussa Aerosil 380. RuC13.xH20 and RhC13.3H20 were obtained from Alfa Products, and monohydrazine hydrate was obtained from Aldrich. The suppliers and purities of the gases are as follows: H2 (Scientific Gas Products, 99.9999%), D, (Liquid Carbonic, 99.7% enriched), and C O (Prochem, 99% I3C enriched). 111. Experimental Results
The I3C N M R spectra of CO adsorbed on silica-supported Ru and Rh are shown in Figures 1 and 2, respectively. The integrated spectral intensities of the Rh/silica catalysts C and D correspond to coverages of 0.66 and 1.14 CO/Rh, respectively, in agreement with the volumetric uptake (0.61 and 1.11). The confidence limits on these coverages are &5%, estimated by a propagation-of-error calculation. The integrated areas of the spectra of CO on Ru/silica samples A and B cprrespond to 1.29 and 1.40, which are 15% lower than the amounts measured by volumetric uptake. The complex shape and the presence of more than three inflection points suggest that the spectra are composed of multiple overlapping components. Motivated by the results of previous 13CN M R studies of metal carbonyls and adsorbed CO, we simulate the spectra with the sum of three components corresponding to C O adsorbed in terminal, bridging, and motionally averaged sites. We assumed that spectra for these sites will be (1) a powder pattern with an antisotropy
4428 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Duncan and Root
TABLE II: "C NMR Wilding Parameters of Carbonyls" linear
bridging u11
9 2
a33
(a)
-97 -84
239 244
325 329
294 300
98 103
198 198
310 319
-89 -63
203
275 295
-56 -84
232 242
611
u22
Ru/silica (A) Ru/silica (B) Ru/silica Ru/Y zeolite RU3(C0)12
182 187 194 180 201
335 334
306 311
320 348
Rh/silica (C) Rh/silica (D) Rh/alumina Rh/alumina Rh/Y zeolite RhdC0)16 [Rh(CO)2CI],
176 175
308 314
180 169
315 306
multicarbonyl
(0)
(a)
033
305 299
-80 -99
231
622
all
ref
u33
this study this study 25 27 22
168 314 318
296
268 276
116 130
296
this study
175 172 177 181 180
this study 296
296
4 15 26 22 4
-51
102
"In ppm, relative to tetramethylsilane (TMS). Estimated confidence limits for the parameters obtained in this study are f 5 ppm for linear sites, *lo ppm for ( 6 )of the bridging sites, 12.0 ppm for ( a ) of the multicarbonyl sites, and f10 ppm for u l l , 622, and ajj.
( a ) of the
CO Adsorbed on Rh/Silica
CO Adsorbed on Ru/Si 1 i ca
Sample ( C ) CO/Rh = 0.61
Sample ( A ) CO/Ru = 1.54
-
Sample ( D ) CO/Rh 1.11
&
I
I
I
I
I
I
I
I
600 500 400 300 200 100 0 -100 -200 -300 Frequency, in ppm, Relative to TMS Figure 1. I3C NMR spectra of CO adsorbed on silica-supported Ru catalysts A and B. The spectra were derived from free-induction decays measured at room temperature and represent the accumulation of 15 000 scans for sample A and 6000 scans for sample B. The solid lines through
the experimental data points are the superposition of the four component spectra shown by broken lines. of 260 f 40 ppm, (2) a powder pattern with an anisotropy of 110 f 40 ppm, and (3) a Lorentzian resonance. line, respectively. The powder patterns are convoluted with Lorentzian functions with constant width across the peak. The composite spectra determined from least-squares fits are given by the solid lines drawn with the data points in Figures 1 and 2. The component spectra are indicated by dashed lines, and the fitted parameters are summarized in Table 11. For all four spectra, the line shapes require three-component peaks for an acceptable fit; omitting any component increases the mean-squared deviation by a t least a factor of 2.5. Qualitatively, the Rh/silica spectra are better fit by a three-component model than are the Ru/silica spectra. The
b
I
I
I
I
I
I
I
I
600 500 400 300 200 100 0 -100 -200 -300 Frequency, in ppm, Relative to TMS Figure 2. ')C NMR spectra of CO adsorbed on silica-supported Rh catalysts C and D. The spectrum of sample C was derived from an echo at 0.2 ms and is the accumulation of 15 000 scans. The spectrum of sample D was derived from a free-induction decay and is the accumu-
lation of 50000 scans. Both spectra were measured at r c " temperature. The solid lines through the experimental data points are the superposition of the four component spectra shown by broken lines. Ru/silica spectra show discrepancies near uI (320 ppm) and on either side of the motionally narrowed peak. The spectra also contain a sharp peak at 124 ppm, which corresponds to C 0 2 . This is most evident in the spectrum of CO on %/silica (C) in which C02accounts for 3.2%of the integrated area (again, the error limits are estimated to be f5%). Smaller amounts of C 0 2 were detected on the other samples: Ru/silica (A), 0.5%; Ru/silica (B), 0.4%; and Rh/silica (D), 0.4%. The amount of gas-phase CO, estimated from the void fraction and the pressure, ranges from 1.8% of the CO on Ru/silica (A) to 0.14% of the CO adsorbed on the Rh/silica samples. We presume
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4429
N M R of CO on Dispersed Metals CO Adsorbed on Ru/Silica ( B )
+. - --__ ‘.I
9.-
J -
I
CO Adsorbed on Rh/Silica (C)
Spin Echo Study
- - - \
Spin Echo Study
Echo Delay = 0 ms (FID) .-.- .- -
_.
A
n 1.06 ms
A 2.1 ms
5.1 ms
;7--;73“--2 . 0 ms
600
600 500 400 300 200 100 0 -100 -200 -300 Frequency, in ppm, Relative to TMS Figure 3. 13Cspin-echo spectra of CO adsorbed on silica-supportedRu (sample B).
that the absence of a sharp peak at 181 ppm, where CO, ) is expected, in the four spectra is due to low concentration and/or exchange broadening. W e estimate that the C02 detected by N M R spectroscopy represents -20% of the C 0 2 produced by CO adsorption, presumably by oxidation or disproportionation, as follows. The ratio of CO,,,/CO2 in sample D is -3 in the voids of the catalyst, a volume of -0.5 mL, whereas it was -30 in the 0.15-L volume above the sample, as measured by mass spectrometry. The component line shapes arbitrarily derived by multicomponent analysis are also distinguished by different relaxation times. Most notable are the differences in spectra acquired with various delays in the spin-echo experiment, shown in Figures 3 and 4. For example, after 2 ms, only the Lorentzian function at 198 ppm remains in the spectra of C O on Ru/silica (B). By decomposing each of the spin-echo spectra as was done for the spectra in Figures 1 and 2, the component intensities as a function of echo time yield the individual spin-spin relaxation times T2. In each case, the intensities decreased exponentially, yielding a T2 of 0.6 f 0.1 ms for the terminal and bridging species and 2.5 f 0.3 ms for the motionally averaged species for C O on Ru/silica (B). For CO on Rh/silica, the T2’s were less disparate; for sample (C) the terminal and bridging CO had T2’s of 0.45 f 0.05 ms and the motionally averaged peak had a T2 of 0.65 f 0.05 ms, and on sample (D) these T i s were 0.55 f 0.10 and 0.90 f 0.15 ms. The T2’s for sample (A) were not measured. The spin-lattice relaxation times for the component line shapes also differ. However, in each case it was not possible to determine a precise T 1 for each component for two reasons. First, the decomposition of the data was complicated by inhomogeneous saturation of the peaks; typically the upfield intensities (corresponding to carbonyls aligned nearly parallel to the external field) saturated before the downfield intensity (perpendicular orientations). Second, the spectral intensities are not simple exponential
500
400 300 200 100
0 -100 -200 -300
(30) Mann, B. E.; Taylor, B. F. ” C NMR Data for Organometallic Compounds: Academic: New York, 198 1.
4430 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
-
Rh/silica, the motionally averaged peak is centered at 174 ppm, slightly below C O on monorhodium compounds (mean = 183 ppm, standard deviation = 6 ppm) and C O on multirhodium compounds (mean = 186, standard deviation = 5 ppm). Thus, the line shape parameters alone do not indicate which spectrum corresponds to gem-dicarbonyl and which corresponds to C O linearly bonded on metal particles. To assign the peaks it is necessary to consider other information. In a study of C O on R h / a l ~ m i n a the , ~ narrowed Lorentzian peak was assigned to gem-dicarbonyls on the basis of the preferential exchange of gem-dicarbonyls species at low temperatures; for a catalyst in which the Lorentzian peak represented 34% of the N M R intensity, 33% of the adsorbed CO exchanged with the gas phase at 195 K. In the present study, for both Ru and Rh catalysts the Tl's and T i s of the broad powder pattern are similar to those of the bridged component, suggesting that the broad component derives from linear species on metal particles rather than on isolated particles. Since the bridged carbonyl is not motionally averaged on a time scale less than 1 ms, one would expect that its nearest neighbors, the terminal carbonyls, would also be orientationally fixed. The I3C N M R parameters are compared to those of previous studies in Table 11. Qualitatively, the line shapes are similar. However, there are differences which may be due to sample preparation, the type of oxide support, or interpretation of the data. The isotropic shifts of CO on Ru/Y zeoliteB are consistently 10-30 ppm upfield of those obtained for Ru/silica in this study. For a highly dispersed Ru/silica sample,26prepared with the same procedure29as the Ru/silica catalyst in the present study and containing "one monolayer (based on metal area) of CO", the I3C N M R spectrum revealed only the broad powder pattern (6 -270 ppm). Although there was slight evidence of a narrowed Lorentzian peak, there was no evidence of bridging carbonyls. Motionally narrowed line shapes for the multicarbonyl sites are consistent with previous report^.^*^* However, the I3C N M R spectrum of a gem-dicarbonyl has also been observed as a static powder pattern; the I3C N M R spectrum of a sample prepared by adsorbing C O on RhC1, (unreduced)/Y zeolite at 393 K was dominated by a powder pattern with an anisotropy of 230 ~ p m . ~ ' However, on the basis of the infrared spectra, this peak was assigned to gem-dicarbonyl species.27 B. Analysis of Line Shapes. In principle, the geometry of the bridge-bonded C O can be determined from three features of the powder pattern; the center of mass, the anisotropy (6) and the asymmetry (7 = (uI1- U ~ ~ ) / S ) . The I3CN M R data of organometallic compounds indicate a weak correlation between the isotropic shift and the coordination of the bridging carbonyl. The range of isotropic shifts for 20 edge-bridging ( p 2 ) carbonyls on Rh is centered at 228 ppm (standard deviation = 12 ppm), which is slightly upfield from the range calculated for 6 face-bridging b3)carbonyls on Rh (mean = 238, standard deviation = 5 ppm).M On this basis, the bridging carbonyl on Rh/silica (( u) = 237 ppm) is 3-fold coordinated. For Ru compounds, only data on p 2 carbonyls have been tabulated; six carbonyls lie in a range centered at 255 with a standard deviation of 5 ppm. The bridging carbonyl on the Ru/silica catalyst is centered at 242 ppm, which is upfield of the p 2 range and presumably even more upfield of the yetunknown p 3 range. This suggests that the bridging carbonyl on Ru/silica is 2-fold coordinated. Interpretation of the anisotropy of the bridging carbonyls is frustrated by a lack of reference data. The anisotropies for only two bridging carbonyls have been reported:23a fi3-carbonylin Rh6(C0),6(129 ppm) and a p2-carbonyl in ($-C5H5)2Fe2(C0)4(93 ppm). It is interesting to compare to the anisotropy of the 2-fold-coordinated C=O moiety in aldehydes and ketones, which have a mean of l 13 ppm (standard deviation = 14 ppm) for seven compounds reported in the litera t ~ r e .The ~ ~ anisotropies of the bridged carbonyls reported here, -141 ppm for Ru/silica and -114 ppm for Rh/silica, are consistent with bridged carbonyls, but because of a lack of data on reference compounds, the anisotropies do not indicate the
-
(3,l) Duncan,T. M. J . Phys. Chem. ReJ Dora 1987,16, 125 and references therein.
Duncan and Root symmetry of the bridged bonding. In principle, the asymmetry parameter (7) is the best indicator of site geometry: for three or more equivalent bonds to the surface, 7 = 0, whereas a 2-fold site has finite asymmetry. The asymmetry parameter of seven aldehydes and ketones3] averages 0.37 with a standard deviation of 0.16. The bridged carbonyls on Ru/silica and Rh/silica have asymmetries of 0.21 and 0.38, respectively, suggesting 2-fold coordination. However, this suggestion is softened by an inability to guarantee that uI1and u22are indeed split (i.e,, setting u l l= u22minimally degrades the fit) and the possibility that the apparent splitting may be due to multiple overlapping peaks of different bridged carbonyls. The NMR data of this study lack the resolution to indicate the geometry of the bridging site on Ru/silica or Rh/silica. Improved resolution of the principal components and the isotropic shift as well as data on model compounds is needed. The linearly bonded CO on Ru and Rh have spectra comparable to previous reports, as shown in Table 11. A recent study of I3C N M R spectra of compounds in which carbon is bonded exclusively along one axis proposed that uilapproaches a fiducial value of -90 Off-axis bonding at the carbon of interest shifts the upfield component by over 150 For example, the upfield component of ketone carbons is at about 85 ~ p m . ~Furthermore, ' deviation from axial or 3-fold (and higher) symmetry at the atom adjacent to the carbon also shifts the component upfield. For example, it is reported that the 13CN M R spectra of O=C=O and O=C=C=C=O have uiIat -90 ppm whereas for CH2= C*=O the upfield component is at 77 ppm (32). The effect of low symmetry at an adjacent metal atom is unknown but is expected to be less. The uIIcomponent of C O on Ru/silica is -86 ppm, which suggests exclusively axial bonding at the carbon and 3-fold or higher bonding of the Ru atom. The upfield shift of the ullcomponent of CO on Rh/silica to -54 ppm may imply lower symmetry of the Rh bonding. In both cases, the u,,component indicates that the meta1-C-O moiety is colinear and the C O axis is perpendicular to the metal surface. Reorientation of the multicarbonyl species narrows the I3C N M R spectrum from one that presumably is comparable to the linearly bonded CO to Lorentzian peaks with half-widths of 16 ppm (0.80 kHz). Similar averaging has been reported for the gem-dicarbonyl on Rhlalumina, which displayed a Lorentzian peak with half-width of 0.93 kHz (18 ppm in this study). The data of the present study do not give a detailed description of the reorientation but do allow some qualitative restrictions, as follows. The residual line width is due to chemical shielding. That is, the T i s of the line shapes suggest that homonuclear dipolar couplings contribute approximately 1.3 ppm for a motionally narrowed line shape on Ru/silica and 5 ppm for Rh/silica. (The observed broadening is the square root of the sum of the squares of the individual contributions.) Furthermore, the line shape can be further narrowed by magic-angle spinning26,28 which suggests that the line broadening is due to orientational anisotropy of the chemical shift, not chemical shift dispersion. The chemical shielding of an axially symmetric species is given by ( I ) , where O ( t ) is the time-dependent angle between the symmetry axis and the external magnetic field Ho.
-
=
6 + -[I 2
- 3 COS^ e(t)l
(1)
Analytical expressions for the line shape can be derived from the specifics of the motion. For rotation about an axis a t an angle Orot relative to the chemical shielding symmetry axis, (1) becomes
where Oaxis is the angle between the rotation axis and Ho. For gem-dicarbonyl species on Rh/alumina, the relative intensities of the symmetric and antisymmetric vibrational modes suggested that the OC-Rh-CO angle is ~ 9 0 O Rapid . ~ rotation of the Rh (32) Beeler, A. J.; Orendt, A. M.; Grant, D.M.; Cutts, P. W.; Michl, J.; Zilm, K. W.; Downing, J. W.; Facelli, J. C.; Schindler, M. S.:Kutzelnigg, W. J . Am. Chem. SOC.1984, 106, 7672.
N M R of C O on Dispersed Metals
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4431
TABLE III: Calculated 13C T2's as a Function of Coverage
surface R h [ l l l ] or Ru[0001] (hexagonal)
Rh[100] (square)
structure (1 x 1) (2 x 2) (43x 43) (2 x 2) c(2 x 2)
CO overlayer 0 distribn, linear; bridged 1.o 0.75 0.33 0.25 0.5
l.O:O.O or 0.0: 1.O 0.6; 0.4 1.0; 0.0 or 0.0; 1.0 1.0; 0.0 or 0.0; 1.0 1.0; 0.0 or 0.0; 1.0
I'C T20 homonucl, ms
heteronucl, ms
0.25 0.38 1.2 2.0 0.78
0.38 0.58 1.9 3.0 1.2
'Powder average for T2. - T2's - calculated for CO on Rh assuming an metal interatomic distance of 2.69 A; T i s for CO on Rh for which do = 2.65
A are
-5% shorter.
gem-dicarbonyls about an axis perpendicular to the surface (e,,, = 45O) would yield a powder pattern with the same shape as the unaveraged spectrum, but with one-fourth the anisotropy. That is, this model predicts a spectrum with ul at 225 ppm for Ru/silica and at 210 ppm for Rh/silica. Thus, pure rotation is insufficient to cause the observed narrow peaks; additional motion is required. Other possible motions include rapid, large librations of the rotation axis (>45O) and/or exchange with other sites or the gas phase. For example, Wang and Yates have shown that the rhodium dicarbonyl exchanges with gas-phase C O by adsorbing a C O to form Rh(C0)3 from which one C O desorbs.33 A description of the motion is likely to be obtained from a study of the motionally averaged line shape as a function of temperature and C O pressure. Low-temperature I3C N M R spectra of the samples described here suggest that the motion is quenched at 150 K, although the static line shape of this component could not be unambiguously extracted. However, it is preferable to perform such experiments on catalysts containing C O adsorbed exclusively as gem-dicarbonyls, prepared by deposition of [Rh(C0)2X]234or R u ( C O ) ~ X prepared ~, by impregnation of metal salts at low loadings (
