The Journal of
Physical Chemistry
0 Copyright, 1990, by the American Chemical Society
VOLUME 94, NUMBER 23 NOVEMBER 15, 1990
LETTERS 13C NMR Spectra of 13C0 Adsorbed on Silica-Supported Palladium Particles: Particle Size Dependence of the Surface Diffusion Rate and the 13C Knight Shift Kurt W. Zilm,* Laurent Bonneviot,+Gary L. Haller,f Oc Hee Han, and M. Kermarecs Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 0651 1 (Received: March 22, 1990; In Final Form: July 24, 1990)
The I3C NMR spectra of 13C0 adsorbed on a 56% dispersion silica-supported Pd catalyst are found to be quite different from those reported for I3CO on a 19% dispersion Pd catalyst. The mobility of the "CO is greatly reduced on these small Pd particles, which is interpreted as a consequence of the increase in the relative number of low-coordination,electron-deficient Pd sites such as on edges and corners and the concomitant increase in the tendency for CO to bind there in a linear fashion. This is supported by the observation that linear 13C0on this catalyst has very little Knight shift due to the decrease in the local electron density at the Fermi level for these sites, while bridging "CO on terraces is still substantially Knight shifted. Comparison of the "C NMR and FTIR spectra shows that the linear CO extinction coefficient is approximately half that for the bridging CO. These NMR results correlate well with previous work on the dispersion dependence of the magnetic susceptibility of silica-supported Pd particles and with the IR spectra for CO adsorbed on such catalysts.
Introduction 13CNMR studies have been especially informative on the nature of the bonding of CO to t'F and Pd on oxide-supported catalyst~.l-~ The observation that the 13C resonance is Knight shifted provides a measure of the involvement of the metal's conduction electrons in the surface chemical Line shapes in the I3C NMR spectra for I3CO on these metals are quite broad due to the demagnetizing field of the metal particles and a wide site-to-site variation in the isotropic shift.s Surface diffusion of the CO over entire particles is observed to lead to a narrowing of the 13C resonance line shape with increasing t e m p e r a t ~ r e . ~Ansermet4 .~ has analyzed this motional narrowing for I3CO on Pt and finds an E,,, for diffusion on the order of 14 kcal mol-I. Work in our 'Department de Chimie, Universite Laval, G I K 7P4, Quebec, Canada. *Department of Chemical Engineering, Yale University. 4 Laboratoire de Reactivite de Surface et Structure, University Paris 6, 75252 Paris Cedex 05, France.
laboratoryS has provided evidence for the diffusion being a twostage process. Surface diffusion over crystal planes is quite facile6 but leads to very little motional narrowing because significant shifts in the I3CO resonant frequency occur only when the CO moves from a crystal face at one orientation to another. The overall diffusion rate measured by NMR on small Pt particles is then determined by the slower face-to-face diffusion, which involves (1) Rudaz, S. L.; Ansermet, J. P.; Wang, P. K.; Slichter, C. P.; Sinfelt,
J. H . Phys. Reu. Lett. 1985, 54, 71-74. (2) Ansermet, J. P.; Wang, P. K.; Slichter, C. P.; Sinfelt, J. H. Phys. Reo. B 1988,37, 1417-1428.
(3) Shore, S.E.; Ansermet, J. P.; Slichter, C. P.; Sinfelt, J. H. Phys. Reu. Lett. 1987, 58, 953-956. (4) Ansermet, J. P. Ph.D. thesis, University of Illinois, Urbana-Champaign, IL, 1985. ( 5 ) Zilm, K. W.; Bonneviot, L.; Hamilton, D. M.; Webb, G. G.;Haller, G.L. J. Phys. Chem. 1990, 94, 1463-1472. (6) Reutt-Robey, J. E.; Chabal, Y.J.; Doren, D. J.; Christman, S. B. J . Vac. Sci. Technol. 1989, 7, 2227-2234.
0022-3654/90/2094-8495$02.50/00 1990 American Chemical Society
8496 The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
passage through edge and corner sites. CO binds more tightly at these sites, and for this reason the E,,, measured from the narrowing of the NMR line shape is much higher than that measured for diffusion on single-crystal plane^.^ For example, Reutt-Robey et al. find an E,,, of only 4.4 kcal mol-' for the diffusion of CO on Pt (1 1 1) from time-resolved IR studies? Shore and co-workers3 have also studied the Knight shift and diffusion of I3CO on supported Pd particles. In comparison to R,they find that on Pd the 13Cresonance is more strongly Knight shifted and that overall surface diffusion is much more facile, having an E,,, of only 6 kcal mol-'. These results have also been confirmed in our laboratory5 on a similar catalyst. This paper reports I3C NMR data for I3CO adsorbed on much smaller silica-supported Pd particles. In contrast to the results for I3CO on larger Pd particles, motional narrowing of the I3C resonance does not occur at room temperature. Comparison of magic angle spinning (MAS) spectra with and without total suppression of spinning sidebands (TOSS), in combination with T I relaxation time measurements and IR spectroscopy, indicates a significant fraction of linear C O is present. While the bridging CO experiences a wide site-to-site variation in the isotropic Knight shift, the linearly bound C O apparently is not Knight shifted. These observations are all interpreted as a result of having a larger fraction of the surface Pd occupying low coordination sites such as at edges or corners on these smaller Pd particles. The tighter binding of CO at such sites in comparison to terraces inhibits the surface diffusion, and the decoupling of these sites from the conduction electrons results in the quenching of the Knight shift for the CO bound there. By making the latter connection, the NMR results reported here allow us to relate the particle size dependence of the surface diffusion rate, the I3CO Knight shift, the bulk magnetic susceptibility, and the electrodeficiency of small Pd particles all to one underlying cause. Experimental Section A 5.4 wt 5% Pd/Si02 catalyst was prepared by dropwise addition of a [Pd(NH,),(OH),] solution to a vigorously stirred silica gel (Degussa Aerosil 200, 200 m2 g-l).' The [Pd(NH,),(OH),] solution was prepared by treatment of a [Pd(NH,),(CI),] solution with an OH--saturated anion-exchange resin (Amberlite IRN-78). Metal content was determined by atomic absorption of a solution obtained from acid extraction of Pd. Oxidation of the catalyst at 5 7 3 K in pure flowing O2 was followed by reduction for 3 h at the same temperature in a flow of pure H,. After pretreatment and adsorption of I3CO,the catalyst was transferred through a sidearm on the Pyrex reactor to a 5-mm NMR tube that was then flame sealed. Several short contacts at IO Torr of "CO were used followed by evacuation to 1 O-* Torr, resulting in a l3CO coverage of close to 0.5. Details of the adsorption procedure have been published el~ewhere.~ Particle sizes were determined by H 2 chemisorption at 373 K. At this temperature this is no palladium hydride formation, and the stoichiometry of adsorption is 0.84 H/surface Pd.* The H/Pd ratio was found to be 0.47, corresponding to a dispersion of 56% or an average particle diameter of 19 A. High-resolution electron microscopy using a JEOL 100 CX reveals a narrow size distribution with a majority of the particles being 15 A (57%), some at 20 (37%), and a few at IO (7%) and 25 A (4%). This distribution is consistent with the average size obtained from hydrogen chemisorption.' Details concerning the NMR spectrometers used are contained in a previous p~blication.~ MAS spectra were obtained by using a probe incorporating a spinner turbine similar in design to one reported by Gay' that spins vacuum-sealed 5-mm N M R tube sample cells at rates of up to 3 kHz. Procedures for obtaining static, MAS, and TOSS spectral0 were the same as in our previous study of I3CO on supported Pt and Pd catalysts. Digitization (7) (a) Benesi, H. A.; Curtis, R. M.; Studer, H. P. J . Catal. 1968,10, 328. (b) Candy, J . P.; Perrichon, V . J . Catal. 1984, 89, 93. (8) Boudart, M . ; Hwang, H.J . Catal. 1975, 39. 44. (9) Gay, I. D. J . Magn. Reson. 1984, 58, 413-420. (IO) Dixon, W. T. J . Chem. Phys. 1982, 77, 1800-1809.
Letters
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Figure 1. (A) Static "C spectrum at 7.05 T of I3COon a silica-supported Pd catalyst. (B) MAS spectrum of 2.35 T with a four-pulse TOSS sequence to remove spinning sidebands from the spectrum. The main feature at high field comes at 192 ppm from TMS. (C) Partially relaxed inversion recovery spectrum taken at 7.05 T showing the anisotropy of the spin-lattice relaxation time T I . The I3C were first inverted by a ?r pulse and allowed to relax for 30 ms, and the magnetization was then read out by using a Hahn echo. This spectrum was taken with the same gain and number of scans as in A and is plotted on the same absolute
scale.
commenced 10-15 dwell times early in the TOSS experiments to permit observation of the formation of the echo. This was done to verify that the TOSS sequencelo was working as expected. MAS, TOSS, and static spectra were obtained at field strengths of both 2.35 and 7.05 T. Static spectra were also acquired at 7.05 T from room temperature up to 400 K and required 4000-20000 scans. TOSS and MAS spectra taken by using rotation synchronized echoes at this field required -5 times this number of scans to compensate for signal loss from T2relaxation. The spin-lattice relaxation time was also investigated a t room temperature at 7.05 T by inversion recovery techniques. Spectra on the low-field instrument were typically the result of several hundred thousand scans. Chemical shifts are referenced to tetramethylsilane (TMS) with downfield taken as positive. The FTIR spectrum for CO adsorbed on this catalyst was acquired at a resolution of 4 cm-' by using a Perkin Elmer M1700 FTIR spectrometer and a classical vacuum-tight in situ IR cell with CaF, windows. CO was initially adsorbed at I O Torr, and the sample then evacuated. Results Figure 1A depicts a representative 13C spectrum for "CO adsorbed on this Pd on silica catalyst. As in our previous study: the static and MAS "C spectra are very similar, and the line shape on a ppm scale is independent of field strength. The spectrum shown in Figure IA is a 4000-scan static spectrum acquired a t 7.05 T which has a first moment of 410 ppm. The line shape is observed to remain unchanged up to 400 K. Since the breadth of the spectrum shown in Figure 1A is greater than the sample spinning frequency, the lack of any narrowing under MAS can simply be the result of having a wide distribution in isotropic shifts as we have discussed beforeS5 To investigate this possibility TOSS was used to acquire centerband-only spectra. Both f o w l 0 and six-pulse" TOSS spectra were taken at the two field strengths, and the patterns obtained checked for independence from the spin rate. In all TOSS experiments the spectrum broke ( I I ) Raleigh, D. P.; Olejniczak, E. T.: Vega, S.; Griffin, R. G. J . Am. Chem. SOC.1984, 106, 8302-8303.
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8497
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Figure 2. FTlR spectrum of CO adsorbed on the 56% dispersion silica supported Pd catalyst.
up into three regions independent of any of the experimental parameters. Severe line-shape distortions are observed in TOSS spectra taken at 7.05 T due to insufficient spin rate" and radio frequency field amplitude as expected. These difficulties can be overcome by operating at 2.35 T, and TOSS spectra acquired at this low field are the same irrespective of the TOSS sequence used. A representative TOSS spectrum acquired at this field strength is shown in Figure 1 B. The basic structure seen in this spectrum is believed to be a fair representation of the sideband-free MAS spectrum although there may be some intensity distortion due to Tz effects. These apparently are not very large as the first moment in this TOSS spectrum is within experimental error of that for the static and MAS spectra. Such spectra are not easily obtained as the combination of small sample size, low field strength, and T2 relaxation during the TOSS sequence make this a low-sensitivity experiment. The TOSS spectrum shown in Figure 1B was acquired only after accumulating 330 000 scans. Spin-lattice relaxation time measurements at 7.05 T show that the T I is very anisotropic across the line shape. The partially relaxed inversion recovery spectrum shown in Figure 1C was acquired with a relaxation delay of 30 ms between the x pulse and the Hahn echo. While the low-field portion is almost fully relaxed, the high-field end is still inverted, and it can be seen that TImonotonically decreases from high to low field. The average TI(as determined from the inversion recovery null points) is only 26 ms at 700 ppm, while at 250 ppm T I is 1 18 ms. Further details on these NMR relaxation studies will be published at a later date. Figure 2 contains the IR spectrum for CO adsorbed on this catalyst. The principal bands observed are CO at 2088 cm-I for linear and at 1965 and 1880 cm-I for bridging CO. The ratio of the integrated intensities of the bands for bridging CO to that for linear CO is 3.4.
Discussion The NMR results given here are quite different than those reported by Shore et aL3 and ourselvesS for I3COadsorbed on Pd on alumina catalysts of lower dispersion (19% and 16%). In the latter studies the I3C resonance was found to be motionally narrowed at room temperature, indicating facile surface diffusion. This is in marked contrast to the spectra reported here where no evidence for motional narrowing is observed even at 400 K. This result is interpreted as evidence for a significant slowdown in the large scale diffusion of CO on these smaller Pd particles. In terms of the two-stage model5 of CO surface diffusion this means that more CO is binding at edge and corner sites and that the binding there is becoming tighter thereby slowing the face to face diffusion. The linear CO in these sites will be less mobile than the bridge-bonded CO on terraces and will also retard the mobility of the bridging CO owing to there being fewer bridging sites for the CO to diffuse among. The fact that the line shape does not motionally narrow even at 400 K is a striking result; at room temperature the rate for overall diffusion on this catalyst is IO6 times slower than on catalysts with larger Pd particles. This model is supported by the IR spectrum shown in Figure 2, which shows a significant proportion of linear CO. Previous IR s t u d i e ~ ~have ~ . ' ~found an approximate linear correlation between the Pd dispersion as measured by CO uptake and the intensity ratio of the bridge-bonded CO and linear CO peaks in
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(12) Ichikawa, S.; Poppa, H.; Boudart, M. J . Coral. 1985, 9 / , I . ( 1 3) Sheu, L.: Karpinski, 2.:Sachtler, W. M. H. J. Phys. Chem. 1989.93, 4890-4894.
the IR spectrum.13 The bridge-to-linear ratio of 3.4 reported here is in quantitative agreement with this correlation if the dispersion as defined by CO uptake is taken to be equivalent to that measured by H2chemisorption. The latter assumption has been shown to be valid for similar Pd catalystsl2 in the particular range of dispersion being considered here. The TOSS spectrum and the anisotropy of the spin lattice relaxation time TI across the static line shape allow us to show that the linear CO on this catalyst is largely coordinated at electron-deficient edge, corner, or adatom sites. Assignment of the TOSS spectral bands is made possible by comparison to the 13CN M R results reported by Bradley et for I3COadsorbed on colloidal Pd (20 f 3 A) stabilized by poly(isobutyla1uminoxane) in solution. IR spectra of their samples shows that the majority of these CO molecules are linearly bound. Most of the surface on these colloidal Pd particles is associated with the polymer and only 20% of the sites are accessible to CO. The I3CO that binds to these sites resonates at 190 ppm and has a T I close to that observed in this work at this resonance position in the static spectrum. On the basis of this result, the well-resolved peak in the TOSS spectrum at 192 ppm can also be assigned to linear CO. The intensity of this peak indicates that 35% of the CO is linear. Although the TOSS technique is prone to intensity distortions, these are apparently small in the present case for reasons discussed earlier. By comparing the N M R and IR spectral intensities,I5 one can then conclude that the extinction coefficient for the bridging CO is about twice that for the linear CO in fair agreement with previous studies.I6 Given this spectral assignment it is possible to arrive at some interesting conclusions with regard to the variation in the electronic structure from site to site on the surface of these Pd particles. The typical 13Cchemical shift range for terminal Pd and Pt carbonyls in diamagnetic compounds is 159-177 ppm." The 192 ppm shift observed here and in Bradley and Millar's work for the linearly bonded CO is close to this range, indicating that this CO experiences little if any Knight shift. On the other hand, the remaining bands for our sample are significantly shifted to lower field, indicating that the bridging CO is still Knight shifted. This interpretation is supported by the room-temperature measurement of the I3Cspin-lattice relaxation times TI. The interaction of the I3C nuclei with the conduction electrons that produces the Knight shift also provides for an efficient relaxation mechanism.lJ Therefore substantial differences in the Knight shift for the two types of CO should lead to differences in T I . The order of magnitude variation in T I across the static line shape, with the most rapidly relaxing components resonating at low field, is then accounted for by a wide variation in the Knight shift consistent with the present interpretation. The two types of CO also appear to have very different shift distributions. Linear CO appears as a relatively narrow line in the TOSS spectrum while the bridging I3CO resonance splits into two components with a shift dispersion comparable to the width of the entire static powder pattern. This behavior is also mirrored in the IR spectrum; the linear CO gives a single narrow peak while the spectrum for the bridging CO is comprised of several overlapping bands with a total width about 6 times that of the linear CO peak. The picture arrived at then is that as the Pd particles become smaller, more edge, corner, and adatom sites become available. Sites of this type are electronically decoupled from the bulk metallic core and the "CO bound to these sites experiences little Knight shift. Since these sites can more readily support linear bonding to CO, the "CO with the smallest Knight shifts are more likely to be the linearly bonded ones. If one assumes that the (14) Bradley, J . S.;Millar, J.; Hill, E. W.; Melchior, M. J . Chem. SOC. Chem. Commun. 1990, 705-706.
(15) Duncan, T. M.; Yates, J. T., Jr.; Vaughan, R. W. J . Chem. Phys. 1980, 73, 975-985.
(16) Vannice, M . A.; Wang, S.-Y. J . Phys. Chem. 1981.85, 2543-2546. (17) (a) Chisholm, M. H.; Clark, H. C.; Manzer, L. E.; Stothers, J. B.; Wand, J. E. H. J . Am. Chem. Soc. 1973, 95, 8574. (b) Mann, B. E.; Taylor, B. F. In "C N M R Data for Organometallic Compounds: Academic Press: New York, 1981.
8498 The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
average for the bridging I3CO Knight shift on Pd in Shore's 540 ppm shift (regardless of dispersion) and that all linear C O have an average shift of 190 ppm as in Bradley and Millar's sample, one can calculate the average shift for a given sample as a weighted average of these two CO components. From the average shift of 410 ppm in our sample, the fraction of linear C O can be estimated in this fashion as 37%, in agreement with the TOSS spectral intensity. Therefore the data taken as a whole support the hypothesis that the decrease in the average Knight shift with particle size and the accompanying increase in the amount of linearly bound C O are intimately related. With this model in mind, it is instructive to look at the dependence of the 13C0 resonance center of gravity on dispersion and to compare this to the dependence of the magnetic susceptibility on dispersion. Following the work of Laddas et a1.,I8one expects the magnetic susceptibility of the Pd particles to be a linear function of the dispersion. Since the Knight shift and the magnetic susceptibility depend on similar electronic factors,I9 it could be reasoned that they would have a similar dependence on the dispersion. From the average shifts noted above for the 19% and 56% catalysts, extrapolation to 100%dispersion gives an average I3C shift of -255 ppm. This is close to where bridging carbonyls on diamagnetic Pt cluster compounds resonateZo(228 ppm). As 13Cshifts for Pd and Pt carbonyl compounds are ~ i m i l a r , "this ~ shift is about what would be expected for a bridging "CO on Pd when the Knight shift disappears at 100% dispersion. This is an interesting correlation, as Laddas et al. notedi8that the linear decrease in the susceptibility of silica-supported Pd particles with dispersion argued for this being a surface effect as opposed to a particle size effect.I9 The interpretation of the I3C NMR results here is consistent with this idea and permits some refinement. Since the Knight shift decrease parallels the susceptibility quenching, it can be argued that this is largely a result of a greater fraction of the Pd being incorporated into the low coordination surface sites, which are electron deficient and thus do not significantly contribute to the bulk susceptibility. Therefore the majority of the susceptibility quenching can be associated with the increased percentage of these types of Pd surface sites at low dispersion rather than a direct result of having smaller particles. This interpretation is further supported by calculations21that show the electron density at the Fermi level is reduced at low coordination sites in comparison to terrace sites on small particles and by IR studies.I3 The results here are also in line with other observations of dispersion-dependent phenomena on supported Pd catalysts attributed to the electrodeficiency22 of small Pd particles with a greater concentration of low coordination sites. The latter can be inferred from the decrease in selectivity and activity in the hydrogenation of I-butyne to I-butene over Pd as dispersion increase^.^) When a Lewis base such as piperidine is adsorbed on a highly dispersed Pd catalyst, the electrodeficiency is ame(18) Laddas, S.;Dalla Betta, R. A.; Boudart, M. J . Curd. 1978, 53, 356-365. (19) Halperin, W. P. Reo. Mod. Phys. 1986, 58, 533-606. (20) Washecheck, D. M.; Wucherer, E. J.; Dahl, L. F.; Ceriotti, A.; Longoni, G . ; Manassero, M.; Sansoni, M.; Chini, P. J . Am. Chem. Soc. 1979, 101. 61 10-61 12. (21) van Santen, R. A. 1.Chem. Soc., Faraday Trans. I 1987.83, 1915. (22) Gallezot, P.; Webb, R.; Della Betta, R. A.; Boudart, M. J . Carol.
Letters liorated and the selectivity and activity are restored.24 It is worthwhile to briefly consider alternative interpretations of the data. One model to consider involves Pd particles which are flat plates. Diffusion of the I3COthen will have little or no effect on the resonance frequency and motional narrowing will not O C C U ~ . However ~ there is no reason to expect that small Pd particles on silica will form raftlike structures, and the MAS and TOSS results are inconsistent with such a picture. One might also consider the bands in the TOSS spectrum to be due to I3CO on different sizes of particles. If this was the case, motional narrowing by diffusion on the separate particles would also not lead to a collapse of the entire spectrum into a single line. However on the 19% dispersion catalysts where the particle size distribution is broader, I3CO diffusion leads to a very narrow line ~ h a p e . ~ . ~ Therefore in the absence of further data, the spectra observed here are taken to be largely indicative of the site-to-site distribution in shifts and as the result of having chemically distinct types of
co.
Conclusions
The mobility of CO on a 56% dispersion-supported Pd catalyst has been found to be greatly reduced in comparison to that on catalysts with 16%-19% dispersion. A large fraction of the 13C0 adsorbed on this catalyst is found to be bound in a linear fashion. While the linear I3CO experiences very little Knight shift, the bridging "CO is strongly Knight shifted. By combining a variety of N M R methods and IR spectroscopy, it has been possible to show that all of these effects can be understood as the result of having a larger fraction of low-coordination, electron-deficient Pd surface sites as the particle sizes decrease. CO binds more tightly and in a linear fashion to these sites leading to the reduction in the overall mobility. Since there is much less local electron density at the Fermi level at these sites, the linear I3COexperiences very little Knight shift. Bridging I3CO on these small particles is still apparently Knight shifted, and these shifts have a wide site to site distribution, This observation carries over to the IR spectrum as well; the structural heterogeneity giving rise to a wide NMR shift distribution may also account for the breadth of the bridging C O IR band. Further work correlating dispersion and surface roughness to the magnetic susceptibility, NMR line shape and shift, and IR spectra are needed to verify the interpretation put forth here. Measurements of the 13C T I versus both temperature and dispersion to obtain the dependence of the Korringa constant on particle size and for the different types of CO are important in this regard. It is expected that additional study of the NMR spectra of "CO on Pd catalysts will be helpful in furthering our understanding of the N M R of adsorbates on catalytic surfaces and how the electronic and magnetic properties of these small Pd particles varies with particle size and surface morphology. Acknowledgment. This work was supported in part by a grant from the U S . Department of Energy, DE-FG22-80791PC, and a grant from the Exxon Education Foundation. M. Bouchemoua is acknowledged for preparation of the catalyst, and M. Lavergne is thanked for his technical assistance in the electron microscopy. T. M. Duncan is acknowledged for helpful discussions and a critical reading of the manuscript.
1981, 69, 511.
(23) Boitiaux, J. P.;Cosyns, J.; Vasudevan, S . Appl. Carol. 1983, 6 , 41.
(24) Boitiaux, J . P.; Cosyns, J.; Vausdevan, S. Appl. Caral. 1985, 15, 317.
