2014
J . Phys. Chem. 1990, 94, 2014-2020
Characterization of Ru(CO), Species on Silica with 13C NMR Spectroscopy A. M. Thayer, T. M. Duncan,* and D. C. Douglass A T & T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: July 3, 1989; In Final Form: September 8, 1989)
The nature and dynamics of Ru multicarbonyl sites (Ru(CO),) are studied through a series of I3C NMR experiments. Interpretation of the NMR spectra suggests two types of multicarbonyls: motionally averaged species that produce a narrow Lorentzian line and immobile species that produce a broad powder pattern characteristic of linearly bonded CO. Evolution of CO site distributions, primarily the conversion of mobile to rigid multicarbonyls, is observed with the aging of samples. The 13C-13Cdipolar coupling of the species comprising the broad powder pattern, obtained through spin-echo spectra, support the separation into Ru(CO), species and CO adsorbed on Ru particles. Exchange between CO in motionally averaged multicarbonyl sites and CO on Ru particles is not observed through selective inversion of the Lorentzian line. Spectra observed under various CO pressures indicate that exchange with gas-phase CO is not a dominant line-narrowing mechanism. An activation energy of 0.7 kcal/mol for the motionally averaged multicarbonyls, which is consistent with hindered rotation or wagging, is obtained from a variable-temperature study.
I. Introduction because of the non-metal character of the Ru atom or strong interactions with oxygen of the alumina.6 Although it was initially The fact that the CO/Ru ratio on oxide-supported Ru usually assumed that multicarbonyl sites exist at the edges of Ru particles? exceeds the saturation coverage on Ru single crystals is attributed it is now proposed that they may exist as isolated structure^.'^^*^ to the formation of Ru(CO), species, with x L 2.1-7 The A recent work proposed ( T ~ O ) , R U ~ ~ ( C O structures ), where x is characteristics of such species were recently reviewed in detail.' 2 or 3.' The complexity of the '2CO/'3C0 IR spectra suggest The immediately relevant features are as follows. The stoithat x > 2 or (less likely) oligomeric [Ru"+(CO),], structures.' chiometry of the Ru multicarbonyl site is generally not known. The 13CN M R spectra of CO on Ru/silica can be decomposed Although the infrared spectra of R u ( C O ) ~and Ru(CO), species formed by the decomposition of Ru carbonyls (e.g., R U ~ ( C O ) ] ~ ) into three components, assigned to C O linear- and bridge-bonded to Ru particles and Ru multicarbonyl species.I6 The line shape on silica are the spectra of multicarbonyls formed by assigned to Ru(CO), species is centered at 199 ppm and is adsorbing CO onto oxide-dispersed Ru are often not interpretable, motionally averaged from a powder pattern of 250 ppm anisotropy owing to ambiguous peaks and background from the C O bonded to a Lorentzian peak of half-width 15 ppm. A subsequent to Ru particle^.'^^^^^^^'^ However, in some cases an assignment high-resolution N M R study suggested that the powder pattern is forthcoming; Ru(CO), species were proposed for silica4 and corresponding to immobile, linearly bonded C O could be resolved alumina.6 Three types of R u ( C O ) ~species, proposed to be difin two species.I7 Since the CO/Ru ratio predicted from N M R ferentiated by Ru oxidation states of 0, 1+, and 3+, were observed line-shape analysis was lower than measured by volumetric upafter the decomposition of R u ~ ( C O )on ' ~ alumina." Multitake,16one would predict the additional immobile, linearly bonded carbonyl species form on the time scale of minutes upon exposure CO was a second Ru(CO), species. This reasoning is supported of a Ru/titania substrate to -20 Torr of C0.I XPS reveals1 that by a study of Rh(CO)2 species in which three N M R components their formation is attendant to the oxidation of a portion of the immobile dicarbonyls, dicarbonyls with mutually Ruo to Ru", probably by reaction with OH g r o ~ p s , ~similar - ~ * ~ ~ ~ were - ~ ~identified: ~~ exchanging CO's, and isotropically averaged dicarbonyls.18 to the mechanism demonstrated for Rh dicarbonyls on silica and The goal of this study is two-fold. First, we explore the posalumina.13 At 300 K, C O in Ru multicarbonyl species on titania sibility of more than one Ru(CO), species as detected by I3C does not exchange with CO,, whereas C O on the Ru particles NMR. Second, we investigate the motional averaging of the does;' this is contrary to the case of Rh(CO)2/silica species, which previously identified Ru(CO), species by measuring the rate of readily exchange at temperatures >200 K.I4 Ru multicarbonyl exchange with the CO in the gas phase and C O adsorbed in other sites on titania are inert to O2at room temperature,' similar to sites and by measuring the activation energy of the motion. Rh(C0)2.'5 although Ru multicarbonyls surface-grafted from R u ~ ( C O )were , ~ not.I2 Contrary to Ru in metal particles, Ru11. Experimental Procedures (CO)2/alumina species do not catalyze CO hydrogenation, possibly A . Catalyst Samples. This study involves two samples of 2.5 wt % Ru on silica prepared by amine exchange. Most experiments ( I ) Robbins, J . L. J . Catal. 1989, 115, 120. (low temperature, spin-labeling, and variable field) were performed (2) Dalla Betta, R. A. J . Phys. Chem. 1975, 79, 2519. on a sample with a CO/Ru ratio of 1.66 under a C O pressure (3) Kobayashi, M.; Shirasaki, T. J . Catul. 1973, 28, 289. of 100 Torr. Previous studies of this sample, in which it was (4) Davydov. A . A,: Bell, A . T. J . Catal. 1977, 49, 332. referred to as sample B, described the site distribution of adsorbed ( 5 ) Goodwin, J. G., Jr.; Naccache, C. J . Catal. 1980, 64, 482. (6) Kellner, C. S.; Bell, A. T. J. Catal. 1982. 7 5 , 2 5 1 . C016317 and the surface diffusion of CO on the Ru particle^.'^ A ( 7 ) Chen, H. W.; Zhong, Z.; White, J. M. J . Catal. 1984, 90, 119. second sample (E) was made from 0.297 g of the same catalyst (8) Kuznetsov, V. L.; Bell, A. T.; Yermakov, Y . I. J . Catal. 1980,65, 374. preparation used for sample B by methods described previously.I6 (9) Zecchina. A.; Guglielminotti, E.; Bossi, A.; Camia. M . J . Catal. 1982, However, the u-tube reactor for sample E was fitted with a Teflon 74, 225. (10) Schwank, J.; Parravano, G.; Gruber, H. L. J . Catal. 1980, 61, 19. vacuum stopcock on one arm to allow reconnection to the vacuum ( I 1 ) Guglielminotti, E.; Zecchina, A.; Bossi, A,; Camia, M. J . Catal. 1982, system for studies as a function of pressure. 74, 240. The silica used in this study is Degussa Aerosil 380. Ru(12) Zanderighi, G . M.; Dossi, C.; Ugo, R.; Psaro, R.; Theolier, A.; CI,.xH,O was obtained from Alfa Products, and monohydrazine Choplin, A.: D'Ornelas, L.; Basset, J. M. J . Organomet. Chem. 1985, 296, 127. (13) (a) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J . Phys. Chem. 1987, 91, 3133. (b) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J . Am. Chem. SOC. 1988. 110, 2074. (14) (a) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J . Chem. Phys. 1979, 71, 3908. (b) Wang, H. P.; Yates, J. T., Jr. J . Catal. 1984, 89, 79. ( 1 5 ) Cavanagh, R. R.; Yates, J. T., Jr. J . Chem. Phys. 1981, 7 4 , 4150.
0022-3654/90/2094-2014$02.50/0
(16) Duncan, T. M.; Root, T. W. J . Phys. Chem. 1988, 92, 4426. (17) Duncan, T. M.; Zilm, K. W.; Hamilton, D. M.; Root, T. W. J . Phys. Chem. 1989, 93, 2583. (18) Thayer, A. M.; Duncan, T. M. J . Phys. Chem. 1989, 93, 6763. (19) Duncan, T. M.; Thayer, A. M.; Root, T. W. J . Chem. Phys , in press.
0 1990 American Chemical Society
Characterization of Ru(CO), Species on Silica with I3C N M R
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2015
TABLE I: Chemical Shift Parameters of CO Species on Ru'
species linearly bonded I linearly bonded I1 bridge bonded
195 180 250
260 155
multicarbonyl
199
-
().
6 254
CO Adsorbed on Ru/S i l i c a
7
0.02 0.07 0.20 -
' ( u ) , the isotropic shift, and 6, the anisotropy, are in parts per mil( u ) is relative to tetramethylsilane (TMS). 7 is the asymmetry. The chemical shielding parameters are defined as follows: 6 = ( 0 ) -
lion; 03)
and 7 = ( u I I -
hydrate was obtained from Aldrich. The suppliers and purities of the gases are as follows: HZ,Scientific Gas Products, 99.9999%, CO, Prochem, 99% 13C-enriched. B. N M R . N M R spectra were obtained with a Bruker Model CXP-200 console at a Larmor frequency of 75.4 MHz for I3C. Spectra were also obtained with the same console at 50.3 MHz. The spectra are Fourier transformed accumulations of free induction decays obtained after 90" pulses. A 180" pulse was applied prior to the 90" pulse on alternate accumulations followed by subtraction of the signal in order to reduce the effects of receiver recovery. Spectra taken with spin-labeling were accumulations of 90°,-1 8O0, echo sequences alternately subtracted from a 90D,-1 80", sequence; details of the spin-labeling experiments are given e1~ewhere.l~ Variable temperature spectra were obtained between 80 and 375 K with a Bruker Model VTlOOO temperature control unit with an accuracy of f3 K. The N M R spectra are plotted on the 6 scale for chemical shifts, relative to TMS, with downfield field signals being positive (i.e., C6H6 is at 128.7 ppm and CS, is at 192.8 ppm). The method used to extract individual components for CO from the I3C N M R spectra is described elsewhere.16 In brief, the experimental spectra are fit with a sum of three components: CO linearly bonded to Ru, C O bridge-bonded to Ru, and motionally averaged multicarbonyls. The I3C N M R line shapes for these three components are a broad powder pattern with negligible asymmetry, an asymmetric powder pattern approximately half as broad, and a relatively narrow Lorentzian peak, respectively. In addition, a magic-angle-spinning study revealed there are two distinct linearly bonded C O species on Ru/silica.l' The chemical shift parameters for these components used to obtain uniform analyses of the spectra are listed in Table I. Motivated by a I3C NMR study of Rh dicarbonyls,l*we also explored the possibility of the existence of a fifth C O species; a powder pattern with an anisotropy -0.5 times that of linearly bonded CO caused by mutual exchange of the CO's. However, including this fifth component did not yield statistically significant improvements to the simulations of the experimental spectra. Thus, the "flipping" dicarbonyl species proposed in studies of Rh on silica and aluminaIs is not detected in the Ru/silica samples studied here. Finally, a sharp peak at 124 ppm is attributed to c02. 111. Experimental Results A . Evolution of CO Site Distributions. Spectra of sample B taken 17 months apart are shown in Figure 1. Aging has decreased the area of the narrow Lorentzian peak and has increased the broad component that spans 300 to -100 ppm, as shown by the difference spectrum. The integrated areas of these changes are approximately compensating, as given in Table 11. In addition, the amount of C 0 2 has increased by a factor of 3. The effects seen with aging are accelerated at higher temperatures. Shown in Figure 2 are spectra taken at 298 K of sample B at 38 months and after an additional 4 days at 375 K. The area of the broad component is further increased, at the expense of the narrow Lorentzian peak. The height of the Lorentzian peak in Figure 2 is greater than that of the 17-month spectrum because the line is narrower at higher field; the integrated intensities are given in Table 11. B. Spin-Echo Studies. The aging experiments suggest that one of the powder patterns corresponding to immobile, linearly bonded C O is created at the expense of mobile multicarbonyl
600 500 400 300 200 100
0 -100-200-300
Frequency, i n ppm, R e l a t i v e t o TMS
Figure 1. I3C NMR spectra at 298 K of CO on 2.5 wt % Ru/silica (sample B) measured at (A) 3 months and (B) 17 months after preparation. The sample aged under 100 Torr of CO at 298 K.
1
M
CO Adsorbed o n
I
t 4 days a t 375 K
L'vvsirrJ?rLv
I
l , , , , l , / / , 1 ,
, , l l l , , l l
600 500 400 300 200 100
, / I
l l l l l / 1 I l 1 ~ I 1
0 -100-200-300
Frequency, i n ppm, R e l a t i v e to TMS
Figure 2. ')C NMR spectra at 298 K of CO on 2.5 wt % Ru/silica (sample B) measured at (A) 38 months and (B) 38 months plus 4 days at 315 K.
2016
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
Thayer et al.
TABLE II: CO Site Distributions after Aeine"
linear
I
I1
brideed
0.33 0.31 0.32 0.30
0.17 0.30 0.28 0.33
0.15 0.18
+ 4 days a t 375 K
CO Ru/Silica Adsorbed on
F
linear aee 3 months at 298 K 20 months at 298 K 38 months a t 298 K
0.21 0.22
,
multicarbonvl 0.34 0.21 0.19 0.13
Estimated confidence is f0.02. All spectra are of sample B measured at 298 K; 3- and 20-month spectra measured at 4.7 T, 38-month spectra measured at 7.05 T. (I
Ru/Silica
I
I
Delay=
I
' I
i/'
- -, j
600
500
400
300
1
,
200
8
1
8
1
100
1A
-
L
-
2
0 -100-200-300
Frequency, in ppm, Relative to T M S
Figure 4. I3C NMR spectra at 298 K of CO on 2.5 wt 7' 6 Ru/silica (sample B) measured at (A) 4.7 and (B) 7.05 T. TABLE 111: Temperature Dependence of Line Shape
fractional areas" line width of temp, K linear I linear I1 bridged multicarbonyl Lorentzian 80 0.44 0.27 0.17 0.12 55 120 0.45 0.22 0.16 0.16 53 160 045 0.26 0.13 0.16 44 185 0.42 0.18 0.22 0.17 26 225 0.37 0.28 0.14 0.20 22 0.16 0.33 0.32 0.19 16.2 260 295 0.32 0.31 0.23 0 21 13.5 345' 0.32 0.23 0 31 0.14 13.0 37jC 0.29 0.29 0.24 0.13 12.3 - - . . I ,
600
j
500
,
,
,
400
,
, 1 !
I . I . . . 1 1 1
300
200
,
, 1
100
I
! , I , I , , . L L I 0 -100-200-300
Frequency, i n ppm, Relative to TMS
Figure 3. I3C N M R spectra at 298 K and 4.7 T of CO on 2.5 wt % Ru/silica (sample B) measured with a spin-echo sequence at various delays. The age of the sample was 17 months.
species. This correlation, taken with the volumetric uptake, suggests that this species is a rigid multicarbonyl. Spin-echo experiments were performed to attempt to separate the two powder patterns by differences in l3C--l3C nuclear dipolar couplings. Qualitatively, CO linearly bonded to a Ru particle has stronger dipolar couplings than CO in isolated multicarbonyls because there are more nearest-neighbors on a saturated surface and, for a flat surface, the coupling is homonuclear. The couplings are calculated in the Discussion. The changes in line shape as a function of spin-echo time are slight, as shown in Figure 3. The difference spectrum reveals that the major part of the change resembles a powder pattern with a perpendicular component at 310 i 15 ppm. The difference spectrum is distorted from that of an ideal powder pattern, owing to orientational variation in the dipolar coupling. C. Magnetic Field Dependence. The relative contributions from chemical shift and nuclear magnetic dipolar couplings can be revealed by changes in the spectra as a function of magnetic field; chemical shift is proportional to the applied field whereas dipolar couplings are independent of field. Room-temperature I3CNMR spectra measured at 4.7 and 7.05 T, shown in Figure 4, contain only subtle differences. The broadening convoluted into the spectra of the linearly bonded CO species decreases from 8.7 to 7.5 ppm, which affords a slightly better resolution of the principal components. However, the fits are insensitive to this convoluted broadening, so an extraction of the dipolar contribution is not
Estimated confidence is f0.02. All spectra are of sample B measured at 7.05 T. *Half-width at half-height, in parts per million. 'From spectra reported in ref 19. meaningful. The line width of the Lorentzian peak changes from 15.8 to 13.2 ppm with increasing field. This indicates the chemical shift contributes 11 ppm and the dipolar coupling contributes 0.6 kHz to the line width. D. Spin-Labeling Experiments. The remaining experiments explore the causes of the motional narrowing of the Lorentzian peak. The spin labeling experiments in this section probe for exchange with rigidly bonded CO on Ru particles. These experiments would also detect interconversion with rigid multicarbonyls, such as a mobile intermediate between rigidly held sites. The study of the pressure dependence is intended to measure the effects of exchange with the gas phase. Finally, the temperature dependence is measured to determine the activation energy for the motion. These three series of experiments were performed at 7.05 T. To probe the chemical exchange between the Lorentzian line and the CO comprising the powder patterns, a selected portion of the I3Cnuclear magnetic dipoles were inverted. The maximum proportion of Lorentzian species are labeled by irradiating at 199 ppm, which also labels some CO species bonded to Ru particles. The spectra in Figure 5, measured on sample B at 38 months (after 4 days at 375 K) show the evolution of spectra labeled with a Gaussian hole of width 21 ppm, which inverts most of the 16 ppm wide Lorentzian line. The width of the hole remains approximately constant as it recovers, and no new holes appear elsewhere in the spectrum. E . Temperature Dependence. The spectra taken at temperatures between 80 and 295 K are shown in Figure 6. With decreasing temperature, the most obvious change is the broadening
Characterization of Ru(CO), Species on Silica with 13C N M R
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2017
TABLE IV: Pressure Dependence of Line Shape
pressure change, Torr 0
--
6.5 690
6.5 690 0.46
-
CO/Ru
linear I
1.26 1.4 1.4
0.24 0.23 0.23
fractional areas" linear I1 bridged 0.42 0.26 0.34
0.08 0.18 0.06
multicarbonyl
line width of Lorentziad
0.25 0.30 0.36
18.0 13.2 16.7
'Estimated confidence is f0.02. All spectra are of sample E measured at 7.05 T. bHalf-width at half-height, in parts per million.
600 500 400 300 200 100
0 -100-200-300
600 500 400 300 200 100
0 -100-200-300
Frequency, i n ppm, R e l a t i v e to TMS
Frequency, in ppm, R e l a t i v e t o TMS
Figure 5. I3C NMR spectra of CO on 2.5 wt 7% Ru/silica (sample B) with the narrow peak selectively inverted, shown as a function of the delay between the inverting pulse and the detection of the signal.
Figure 6. "C NMR spectra of CO on 2.5 wt 7% Ru/silica (sample 9) as a function of temperature between 80 and 295 K.
and decrease in intensity of the Lorentzian peak centered at 199 ppm; at 80 K, very little of the narrow Lorentzian component is observed. The decrease of the Lorentzian peak coincides with a monotonic increase in the linear species I. The distribution of CO and the Lorentzian line widths are given in Table 111. On average, the area of the bridging species is constant, although this conclusion is imprecise; variations in its area cause only small fitting errors. Overall relaxation times change only slightly with temperature, being on average -2-3 s. The disappearance of the peak at 124 ppm at low temperatures is due to condensation of the gaseous COz. All changes observed in the spectra are reversible. F. Pressure Dependence. The rate of adsorption of C O was recorded at each C O pressure. During the initial adsorption from 0.0 to 1.26 CO/Ru, the pressure above the sample decreased from 9.2 to 6.6 Torr over a 24-h period, behaving as a sum of two exponentials; the first was weighted 0.38 and had a rate constant of 0.18 m-I, and the second was weighted 0.62 with a rate constant of 0.0027 m-I. The sample was held at 6.6 Torr for an additional 24 h while the N M R spectrum was measured. The sample was then exposed to 690 Torr, where the pressure decreased as a single
-
exponential with a rate constant of 0.12 m-l, which increased the CO/Ru ratio to 1.4. The 13C N M R spectra of this sample, E, as a function of the C O pressure appear in Figure 7 . The results of analysis of these three spectra are given in Table IV. At 690 Torr, the increased area of the narrow peak at 199 ppm relative to the earlier spectrum at 6.6 Torr correlates with an increased adsorption of C O at higher pressure. In contrast, as can be seen in the spectrum taken at 0.46 Torr, decreasing the pressure does not cause a decrease in this narrow component. At 690 Torr, the Lorentzian peak is narrower and is shifted upfield (toward CO, at 180 ppm) by 1.2 f 0.4 ppm. There is no difference in the overall relaxation times as measured a t the highest and lowest pressure; the spin-lattice relaxation time, TI,was -0.8 s in both cases. IV. Discussion of Results A . Conversionfrom Mobile to Rigid Multicarbonyl Sites. As a Ru/silica catalyst ages, the intensity of the motionally averaged Lorentzian peak decreases commensurate with the increase in a powder pattern corresponding to rigid, linearly bonded CO. From the model proposed originally, this would be interpreted as the condensation of an isolated multicarbonyl site into a Ru particle,
2018
Thayer et al.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 20
1
/-
CO A d s o r b e d o n Ru/Sllrca I
\
C 0 on R u / S I L I C A ( E )
6 . 6 Torr
=
0 W
ctwc/
SELECTIVE INVERSION AT i 9 9 p p m
4
0
IC1 01
1 1 1 l l l I ~ l
1
1 1 / l l l l l 1
10
1
I11!/11l
io0
I
L l , l I l
I
11
1000
RECOVERY TIME ( m s )
Figure 8. Percentage of unrecovered magnetization as a function of recovery time for 13C NMR spectra of CO on Ru/silica (sample B) at 298 K. The solid line is the upper limit imposed by spin-lattice relaxation at 298 K. The dashed line includes the effects of CO diffusion on the surface of Ru particles.
species is reorienting nearly isotropically and rapidly ( 7 , C ( ( A W ) ~ ) -= ~ /20 ~ ps). For example, if the species is a tricarbonyl with nearly mutually orthogonal Ru-C-0 bonds, the line nar0 4 6 Torr rowing may be caused by intrasite exchange of the C O groups. ".--,-%u-However, the line has some residual width either because the time constant of the motion is not short enough relative to the , , , , ~,, , , / , , , , , , , , , , , , . , ,u broadening interactions or the motion is not completely isotropic. 6 0 0 5 0 0 400 300 200 100 0 -100-200-300 Selective population inversion experiments probe the possibility F r e q u e n c y , i n ppm, R e l a t i v e t o TMS of exchange of the multicarbonyl with C O that resonates at Figure 7. I3C N M R spectra of CO on 2.5 wt % Ru/silica (sample E) different positions of the powder pattern. A cursory examination as a function of CO pressure, which was varied from 0.0 to (A) 6.6 Torr, of the spectra in Figure 5 reveals that the hole persists for much to (B) 690 Torr, to ( C ) 0.46 Torr. longer than 20 pus, and thus, exchange is not the dominant mechanism. However, the hole does recover faster than predicted with the desorption of one or more CO's. However, this is by spin-lattice relaxation alone, as shown in Figure 8. We now thermodynamically unfavorable under 100 Torr of C O as in demonstrate that the recovery can be completely accounted for sample B here. Rather, it is more likely that the new powder by the behavior of that portion of the inverted spins corresponding pattern corresponds to multicarbonyl sites that are rigid on the to CO on Ru particles. N M R time scale, 10-100 ms. Similar conversion from motTo model the hole, we consider contribution from three different ionally averaged to rigid has been proposed for Rh dicarbonyl sites CO types: C O on Ru particles, C O in rigid multicarbonyls, and on siIica.l8 motionally averaged CO's. The behavior of C O on Ru particles, Analysis of the NMR spectra as a function of aging suggests which represents 30% of the inverted spins, is known from a that the smaller anisotropy powder pattern (linear I1 in Table I) previous study of this ~ a m p 1 e . lThis ~ intensity as a function of corresponds to the rigid multicarbonyl sites. This conclusion is recovery time is designated hpa*l(Trec).The time evolution of the somewhat arbitrary because a minor variation in rf phase, which total hole intensity, H(7rec),is thus given by is inevitable with spectra measured many months apart, can change ~
1 ,
-
the shape of the downfield feature and thus the proportion of linear I and I1 species. However, the spin-echo study also suggests that linear I is C O on Ru particles, by the following analysis. The change in shape as a function of echo delay is largest at oL. The T2 at uI for a saturation coverage of C O on a Ru surface is estimated to be 0.24-0.37 ms.I6 The decay of the magnetization due to dipolar coupling within a rigid Ru(CO), group, modeled in the Appendix, is calculated assuming a Ru-C bond length of 1.80 A and a C-Ru-C bond angle of 90°. The calculated decay is not a single exponential, but in the first 0.4 ms it has an apparent T2of 3.8 ms. From the calculated decay for Ru(CO)~,we estimate the decay for Ru(CO), to be 2.5-3.0 ms, a factor of (3/2)1/2 because of the third I3Cand the increased portion of "like" versus "unlike" couplings. The portion of the spectrum that decreases more rapidly has a T2at ul of 0.45 f 0.15 ms, which best matches that of C O on Ru particles, and its downfield extreme is 325 ppm, which best matches the linear I component. Thus we interpret the linear I component as C O on Ru particles. The definitive experiment would be to monitor the effects of aging with MAS spectra to observe which peak ( 1 95 or 180 ppm) grows with age. 6. Characterization of Motionally Averaged Multicarbonyl Sites. As discussed previously,16 that the line shape of the multicarbonyl species is Lorentzian indicates that the C O of this
The fit to the data in Figure 8 is obtained with hh(7,) = kw(7,) = 1. That is, neither multicarbonyl component is exchanging with other species, on a time scale faster than spin-lattice relaxation. The exchange rate with gas-phase CO, we, can be estimated from the line width as a function of pressure. Qualitatively, since the line width did not narrow by the ratio of the pressures between 0.5 and 690 Torr, either the gas-phase exchange is a negligible contributor to the narrowing at 0.5 Torr or the exchange mechanism is less than first order in C O pressure. One would expect that the exchange would proceed by the mechanism proposed for Rh dicarbonyl sites: adsorption of a CO to form an activated-state tricarbonyl followed by desorption of a C0.14 Thus, we assume the mechanism for exchange is first order. The rate of random modulation of the chemical shielding caused by exchange, we, is approximately M2/Aw where M2 is the second moment of the rigid line shape and Au is the width of the Lorenztian function.20 The approximation assumes a Gaussian correlation function. The second moment of a chemical shielding (20) Abragam, A. The Principles of Nuclear Magnetism; Oxford Press: London, 1961; Chapter X.
Characterization of Ru(CO), Species on Silica with I3C NMR 18
,
> A (heteronuclear), M(27) = cos ( A T ) ,as expected. The coupling between two spins with correlated chemical shift and dipolar principal axis systems may be either A2, AB, or AX, depending on the molecular orientation, as described by Zilm.2s Consider two spins in chemically identical sites with axially symmetric chemical shielding (ql = q2 = 0) with internuclear vector or length rI2that is at an angle er relative to the symmetry axis of spin 1. This geometry requires that the chemical shielding (241 Reference 20. DD 497-501. (2Sj Zilm, K.W.; W'ebb, G. G.; Cowley, A. H.; Pakulski, M.; Orendt, A J . Am. Chem. SOC.1988, 110, 2032.
3Vo6 2
ul - u2 = - -(cos2
8, - sin2 8 , cos2 +2)
Finally, to calculate the decay of the intensity at uL, we set 8, = 90°, which yields
1
Substituting into the expression for M ( 2 r ) and integrating numerically over +*, the dipolar decay is obtained. Registry No. Ru, 7440-18-8; C O , 630-08-0.
