J. Phys. Chem. 1989, 93, 2583-2590
2583
Adsorbed States of CO on Dispersed Metals: A High-Resolution Solid-state NMR Study T. M. Duncan,* AT& T Bell Laboratories, Murray Hill, New Jersey 07974
K. W. Zilm,* Department of Chemistry, Yale University, New Haven, Connecticut 06520
D. M. Hamilton, Department of Chemistry, Yale University, New Haven, Connecticut 06520
and T. W. Root Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received: July 13, 1988)
CO adsorbs on oxide-supported transition metals in three forms: linear- and bridge-bonded states on metal clusters and multicarbonyls on isolated single metal atoms. Recent broad-line 13Cnuclear magnetic resonance (NMR) studies of adsorbed CO have demonstrated the ability to provide quantitative site distributions, descriptionsof the adsorbate bonding and geometry, models for adsorbate motions, and metal particle geometries. This study reports the high-resolution I3C NMR spectra of CO adsorbed on silica-supported Rh and Ru obtained by magic-angle sample spinning, which corroborate the broad-line measurements and provide additional information. The salient features derived from the high-resolution spectra are the isotropic chemical shifts and chemical heterogeneity of the adsorbate states. The isotropic shifts support the results for linearly bonded CO and multicarbonyls extracted from broad-line spectra and allow refinement of the principal shielding components for these two species. The low heterogeneity for these sites indicates the metal particles contribute negligible susceptibility broadening, which suggests that on these highly dispersed samples the metal aggregates are two-dimensional, raftlike structures. Finally, comparison to the broad-line spectra reveals possible caveats of magic-angle spinning spectra of adsorbates: the apparent absence of components with large site heterogeneity, the inability to resolve components clearly differentiated in broad-line spectra, and site distributions inconsistent with volumetric uptake data.
1. Introduction A recent 13C N M R study of the broad-line spectra of adsorbed C O contained sufficient spectral definition to allow the first NMR identification of the individual components of the three adsorbed states of C O on dispersed metal catalysts.’ In this present study we report high-resolution 13C N M R spectra of similar catalysts obtained by spinning the sample at the magic angle. By analyzing the high-resolution spectra concurrently with the broad-line spectra, we may corroborate the previously proposed components, refine the accuracy of the isotropic shifts of the components, and assess the relative merits of the two techniques. The three archetypal forms of C O adsorbed on oxide-supported transition metals are diagramed below.
linear
bridging
multicarbonyl
The linear- and bridge-bonded states exist on metal particles whereas the multicarbonyls form on isolated metal atoms.’ The bridge-bonded C O is shown here with 2-fold bonding. Threefold bonding and higher are also possible, depending on the nature of the metal surface; on Rh[ 11 13, a 2-fold geometry is preferred. For Rh, extensive studies with infrared spectroscopy, isotopic exchange, and quantitative uptake measurements have shown that isolated Rh adsorbs two CO’s. For Ru, it is proposed that both dicarbonyls and tricarbonyls are formed. Each C O state has a distinctive signature in a broad-line 13C NMR spectrum. Linear CO has a chemical shift anisotropy (( u ) - u33)of -250 ppm, and its two downfield parameters are degenerate, consistent with the axial symmetry of the carbon. Bridged CO has a smaller anisotropy, 125-150 ppm, and an asymmetry parameter ((all - u ~ ~ ) / ( (-uu)3 4 )of 0.15-0.40. The ( I ) Duncan, T. M.; Root, T. W. J. Phys. Chem. 1988, 92, 4426.
multicarbonyl form is assigned to a motionally narrowed peak which is typically Lorentzian with a half-width of 1-2.5 kHz. The surface density of C O on silica-supported Rh at saturation, estimated from the I3C T i s , ( was consistent with that observed for R h [ l l l ] , 0.75. Three high-resolution N M R studies of C O on Ru and Rh catalysts have been reported to date. Gay observed that the broad-line spectrum of CO on Ru/silica, which resembled a single powder pattern of axial symmetry, narrowed to a single peak a 194 ppm when the sample was spun at the magic angle.2 Shoemaker and Apple measured the high-resolution spectrum of CO on Ru/Y zeolite and proposed the existence of peaks at 203, 180, and 168 ppm, interpreted as bridging, linear, and dicarbonyl species, re~pectively.~Robbins studied a Rh/alumina catalyst in which CO was present exclusively as dicarbonyls; magic-angle spinning (MAS) revealed a doublet, centered at 180.9 ppm, with a splitting of 65 Hz owing to I3C-lo3Rh J c o ~ p l i n g . ~ 11. Experimental Procedures The 2.5 wt % Rh and 2.5 wt % Ru catalyst samples were prepared by amine cation exchange using a procedure similar to that of Gay,5 as in the previous broad-line N M R study.I MC13-xH20(where M Ru, Rh) was dissolved in 10 mL of H 2 0 per gram of final product desired, and then silica was added incrementally with stirring. The mixture was chilled in an ice bath, and 0.8 mL of hydrazine hydrate per gram was added dropwise, with stirring. After N2 evolution ceased, the catalyst was covered and stirred overnight. The catalyst was then filtered, washed with cold 1 M NH,OH, and air-dried at room temperature. To accommodate study by MAS, we constructed a reactor based on a concept used in a reactor for a combined NMR-ESR-TPD (2) Gay, I. D. J. Magn. Reson. 1984, 58, 413. (3) Shoemaker, R. K.; Apple, T. M. J. Phys. Chem. 1985, 89, 3185. (4) Robbins, J. L J. Phys. Chem. 1986, 90, 3381. (5) Gay, I. D. J. Catal. 1983, 80, 231.
0022-365418912093-2583$01.50/0 0 1989 American Chemical Society
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The Journal of Physical Chemisfry, Vol. 93, No. 6 , 1989 PIREX.
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CO Adsorbed or
STAINLE55 STEEL _175 0 . 0 .
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Figure 1. Schematic diagram of the stainless steellPyrer reactor used to prepare the catalyst samples. The portion below the VCR coupling is shown to scale whereas the length of the 5-mm Pyrex tube at the top is proportionately larger than shown (approximately 5 in.). study? diagramed in Figure 1. This provides for flow reduction of the catalyst. quantitative volumetric measurement of the CO uptake, and high-vacuum sealing in an axially symmetric container, as follows. Unreduced catalyst is loaded onto the fritted glass support via the VCR port. The VCR port is sealed with a copper gasket, the reactor is connected to a gas-handling system via 0.25-in. Ultratorr connectors, and the reactor is lowered into a fluidized sand bath and is heated under flowing H,. The Ru catalyst is reduced a t 350 "C for 2 h, and the Rh catalyst is reduced a t 2 0 0 T for 6 h. The catalysts are then evacuated, purged with flowing He at reaction temperature, and cooled to room temperature. Afterward the downstream valve is closed and C O is adsorbed to the desired loading. The Rh/silica sample weighed 0.166 g, was sealed under 86.4 Torr of CO, and had a C O R h ratio of 1.25 at saturation. The Ru/silica sample weighed 0.141 g, was sealed under 0.36 Torr of CO, and had a C O R u ratio of 1.29 at saturation. The C0:metal ratios determined by volumetric uptake have 95% confidence limits at f7%. The inlet valve is then closed, and the reactor is separated from the gashandling system and inverted, spilling the catalyst into the 5mm-0.d. tip (a Wilmad Precision-Grade tube grafted onto the reactor). Finally a section of the 5-mm tube is collapsed with a flame to yield an ampule approximately 7 cm long. Care is taken to produce an axially symmetric seal, essential for achieving high spinning speeds. The apparatus for spinning the sample tubes at the magic angle is adapted from a design by Gay;, a rotor sleeve is mounted on the 5-mm-0.d. tube near the seal, and the bottom of the tube extends through the stator and into a solenoid N M R sample coil. N M R spectra are measured a t 2.35 T (25.15-MHz Larmor frequency for "C) and 7.07 T (75.67 MHz for "C) with home-built spectrometers based on the design of Gross and Zumbulyadis.' A spectrum of a stationary sample was obtained as a spin echo refocused by a 180°, pulse 25 ps after the initial 90°, pulse. Spectra of spinning samples were also derived from spin echoes, refocused one rotor period after the 90°, pulse (e.g., (uJ' = 0.4 ms for a.sample spinning at 2500 Hz). The I3C N M R spectra are plotted on the 6 scale for chemical shifts, relative to TMS, such that downfield is positive (Le., C,H, is at 128.7 ppm and CS, is at 192.8 ppm). The ubiquitous CO, impurity was used as an internal frequency reference and set to 124 ppm8 (6) (a) App1c.T. M.:Gajardo. P.; Dybawski, C. 3. Carol. 1981,68, 103. (b) Apple. T. M.: Dybowski. C. Sur5 Sci. 1982. 121. 243. (7) Grass, R.: Zumbulyadis, N. Reo. Sei. Instrum. 1982, 53. 615.
I7
CO Adsorbed o n
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 / , 1 1 , , , 1 1 , , , 1 1 , , ,
600 500 400 300 200 1 0 0
0 -100-200-300
F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Elgum 2. Summary of "C N M R spectra of adsorbed C O measured for this study. The spectra of CO on silica-supported Rh were measured at room temperature with (A) a stationary sample at 7.07 T and spinning samples at (B) 2.35 T and (C) 7.07 T. Analogous spectra for CO on silica-supportedRu are shown in (D), (E), and (F). The silica used in this study is Degussa Aerosil 380. RuCI,.xH,O and RhCI,.3H20 were obtained from Alfa Products, and hydrazine monohydrate was obtained from Aldrich. The suppliers and purities of the gases are as follows: H2(Scientific Gas Products, 99.9999%) and CO (Prochem, 99% "C enriched).
Ill. Experimental Results Three l3CN M R spectra were measured for each of the catalyst samples: a broad-line spectrum at 7.07 T and two high-resolution scectra at 2.35 and 7.07 T. A " m i t e of the six mectra is shown in Figure 2. A . CO Adsorbed on RhlSilica. Figure 3 shows the spectrum of C o o n K h , silica measured at 2.35 ? w i t h the sample spinning at 2900 H7 at the magic angle. The spectrum is the accumulation of 56000 scans acquired n t 3 - 5 intervals. An intewe peak of half-width 6.2 ppm .ippexs at 177 ppm. flanked by tun sidebands on either side 3t multiplcc of 2900 HI ( I I5 ppm). A minor peak observed at 124 ppm of half-uidth 2.9 ppm is interpreted as CO,. (8) Stothers, J. B. Carbon-13 NMRSpc~roscopy:Academic: New York,
1972.
Adsorbed States of C O on Dispersed Metals
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2585
1
Rh/S i 1i c a
CO A d s o r b e d o n
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2L D i c a r b o ny l
C a r b o n Dioxide
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(xO.25)
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Carbon D i o x i d e
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F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Figure 3. I3C NMR spectrum of CO adsorbed on silica-supportedRh measured at 2.35 T with the sample spinning at 2900 Hz at the magic angle. The solid line through the data is the sum of the three components plotted separately: C02, dicarbonyl, and linearly bonded CO.
F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Figure 4. "C NMR spectrum of CO adsorbed on silica-supportedRh measured at 7.07 T with the sample spinning at 2950 Hz at the magic angle. The solid line through the data is the sum of the three components plotted separately: C02, dicarbonyl, and linearly bonded CO.
Figure 4 shows the same sample measured at 7.07 T and spinning at 2950 Hz. The spectrum is the accumulation of 54000 scans acquired at 0.8-s intervals. In comparison to Figure 3, measuring at a higher magnetic field provides better resolution at the expense of a more complicated spectrum. Except for the C02peak at 124 ppm (half-width 1.2 ppm), all discernible peaks are separated by the rotor frequency, 39.0 ppm. Considering only the peak positions, only peaks that coincide in Figures 3 and 4 are candidates for isotropic shifts. These are 177, 293, 61, and 124 ppm (CO,). Figure 5 contains the broad-line spectrum of C O on Rh/silica which is the summation of 7736 scans measured at 1 scan per 5 s. The integrated area corresponds to a C O R h ratio of 1.28, which agrees with 1.23 measured by volumetric uptake. The center of mass of the line shape is 193 ppm, and the shape is qualitatively similar to broad-line spectra observed previously with C0:Rh ratios of 0.61 and 1.11.' B. CO Adsorbed on RulSilica. Figure 6 shows the spectrum of CO on Ru/silica measured at 2.35 T with the sample spinning at 2575 Hz at the magic angle. The spectrum is the accumulation of 149 000 scans acquired at 1 -s intervals. The component peaks of the Ru sample are broader than those of the Rh sample and have qualitatively different shapes; whereas the peaks in the Rh
spectrum are essentially Lorentzian, the Ru peaks are asymmetric. Figure 7 shows the spectrum of CO on Ru/silica measured at 7.07 T with the sample spinning at 3010 Hz at the magic angle. The spectrum is the accumulation of 58 500 scans acquired at 0.8-s intervals. The enhanced resolution and change in the intensities of the center peaks relative to their sidebands, relative to 2.35 T, reveal the presence of multiple species. Because the ratio of the rotor speeds at the two different magnetic fields was not an integral multiple of the field strengths, the coincident peaks in Figures 6 and 7 must be isotropic shifts. A sharp peak occurs in both spectra at 199 ppm, and other unresolved species lie in the range 200-175 ppm. The C 0 2 peak is less intense in the Ru sample and is indistinguishable from the noise in both Figures 6 and 7 . The broad-line spectrum of C O on Ru/silica, shown in Figure 8, is the summation of 11 000 scans measured at 1 scan per 8 s. The integrated area corresponds to a CO-Ru ratio of 1.11, which is 14% lower than the coverage measured by volumetric uptake. In a previous study' the C O coverages derived from N M R integrations were also consistently lower than the results of volumetric uptake. The center of mass of the line shape is 21 3 ppm, and the shape is qualitatively similar to broad-line spectra observed previously for samples with CO:Ru ratios of 1.54 and 1.66, al-
2586 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Duncan et al.
CO Adsorbed o n
CO A d s o r b e d o n
7.07 T
Ru/Sil ica 2.35 T
Rh/S i l i c a
~2 Dicarbonyl
I!
Multicarbonyl
b
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/1/////111l~LllI L l ~ l l IL I l I 3 0 0 200 1 0 0
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0
-100-200-300
F r e q u e n c y , i n ppm, R e l a t i v e t o TMS
I
Figure 5. I3C NMR spectrum of CO adsorbed on silica-supportedRh measured at 7.07 T with a stationary sample. The solid line through the data is the sum of the four components that are plotted separately.
though the sharp feature at -200 ppm is considerably smaller.'
IV. Discussion of Results A. Analytical Decomposition of NMR Spectra. A method to decompose N M R spectra into subspectra for each chemical component must yield results consistent with both high-resolution and broad-line spectra, with proper emphasis on the fortes of each method. Specifically, high-resolution spectra obtained by magic-angle spinning (MAS) yield accurate isotropic shifts but are less sensitive to the orientational shielding anisotropy and asymmetry. Conversely, the shoulders in the broad-line spectra reveal the positions of the principal shielding components, but multiple overlapping line shapes obfuscate the correct pairing of components and thus the isotropic shifts are uncertain. We employ here an iterative procedure to decompose the spectra, as follows. First the broad-line spectrum is fitted by least squares to the sum of four components (linear, bridging, multicarbonyl, and C 0 2 )with the algorithm described previously.' The parameters derived from the fit, the rotor frequency, and magnetic field strength are used to calculate the sideband intensities for the corresponding MAS spectra, using the formalism of Herzfeld and Berger.g The high-resolution spectra are then decomposed into a sum of sets of Lorentzian peaks; the peaks in each set have the same width and are separated by integral multiples of v , , ~ . The integrated intensity of each set of peaks is unrestricted while the relative intensities within a set are maintained. The fit to the high-resolution spectrum yields accurate isotropic shifts for the three (9) Herzfeld, J.; Berger, A. E. J . Chem. Phys. 1980, 73, 6021.
6 0 0 500 4 0 0
3 0 0 200
100
Carbon D i o x i d e
0 -100-200-300
F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Figure 6. I3C NMR spectrum of CO adsorbed on silica-supportedRu measured at 2.35 T with the sample spinning at 2575 Hz at the magic angle. The solid line through the data is the sum of four components: COz, multicarbonyl, and two linearly bonded CO's. components, which are then preset in a second fit to the broad-line spectrum. The results of the second broad-line analysis are used to calculate better estimates for the sideband intensities, and a second fit to the high-resolution spectrum is obtained. The fits are iterated until consistent results are obtained, which is usually two or three iterations. The results of the applying this method are discussed below. A discussion of the treatment of the Lorentzian species in the magic-angle spinning spectra is necessary. Specifically, there is some uncertainty in the prediction of the sideband intensities for spinning rates less than the line width, which is the case for spectra measured at 7.07 T. First, since previous measurements1 have shown that the T2for the multicarbonyls is 1.O ms or longer, the contribution from lifetime broadening is less than 150 Hz and a sharp central peak is expected. The relative intensities of the sidebands depend on the details of the motion, which are unknown. For example, if the motion is isotropic, but with a time constant comparable to the reciprocal anisotropy, spinning could effectively narrow the line shape to a single, central peak. If the motion is fast relative to the reciprocal anisotropy (250 ppm), but anisotropic, one would expect sidebands; the sideband intensities would be less than or equal to the height of the superimposed broad-line
Adsorbed States of CO on Dispersed Metals
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2587
CO Adsorbed o n Ru/S i 1i ca 7.07 T
CO A d s o r b e d o n
Ru/Silica 7.07 T
I
’ i
I
Multicarbonyl
600 500 400 300 200 100
1 1 1 1
I l l 1
/ I l l
1 1 1 1
1 1 1 1
I
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l
l
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F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Figure 8. I3C NMR spectrum of CO adsorbed on silica-supportedRu measured at 7.07 T with a stationary sample. The solid line through the data is the sum of four components: carbon dioxide, multicarbonyl, bridged, and linearly bonded COS.
0 -100-200-300
F r e q u e n c y , i n ppm, R e l a t i v e t o TMS Figure 7. I3C NMR spectrum of CO adsorbed on silica-supportedRu measured at 7.07 T with the sample spinning at 3010 Hz at the magic angle. The solid line through the data is the sum of four components: C02, multicarbonyl, and two linearly bonded CO’s. Lorentzian. In this study, we have arbitrarily chosen to include sidebands in the MAS spectra at 7.07 T. First, the former model for the motion would not necessarily yield a Lorentzian but rather a collapsed powder pattern, whereas the MAS data (vide infra) confirm a line shape whose maximum is at the isotropic shift. Second, including sidebands provides better fits and accounts for more of the multicarbonyl fraction, which is generally lower in the MAS spectra than predicted from the stationary spectra. High-resolution spectra have the propensity to reveal subtle differences not apparent in broad-line spectra. Specifically, fine structure in the central peaks of magic-angle spinning spectra suggests additional isotropic shifts. Incorporating additional components inevitably improves the fit to the data. The question is whether the marginal improvement is significant, given the level of noise in the data. We have applied the significance tests of Hamiltonlo to address this question. The tests are straightforward and convenient using the tables of significance points. We based our tests on R factors computed from normalized standard de(IO) (a) Hamilton, w. C. Acta Crystallogr. 1965, 18, 502. (b) Hamilton, W. C. Statistics in Physical Science: Estimation, Hypothesis Testing and Leos! Squares; Ronald Press: New York, 1964.
viations (R”)with uniform weighting. The NMR spectra typically contain -500 data points whereas the significance tables are most sensitive to data sets of
