J . Phys. Chem. 1993, 97, 10-12
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N M R Evidence of Temperature-Dependent Structural Changes of the CO Adlayer on Supported Pd Clusters Lino R. Becerrat and Charles P. Slichter’** Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1110 West Green St.. Urbana, Illinois 61801
John H. Sinfelt Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received: September 23, 1992; i n Final Form: November 17, 1992
I3CN M R studies of 13C0adsorbed on supported Pd clusters are reported. The 13CN M R spectrum consists of two lines, each line reflecting a particular bonding site (bridge or on-top) of the CO molecules. The relative CO population of the two sites varies with temperature and cluster size, corresponding to differing arrangements (ordered arrays) of the CO’s on the surface. In going from a low-temperature arrangement to the hightemperature one, a massive rearrangement of bonding sites must occur. The structure of the low-temperature arrangement depends on cluster size: on small clusters the CO is bonded mainly to on-top sites, but on large clusters is bridge bonded. In the high-temperature arrangement, the CO is bridge bonded.
Introduction CO adsorbed on metal surfaces has been studied by many techniques,’-5 including infrared spectroscopy1 and nuclear magnetic resonance (NMR).6-8 On Pt and Pd 1 11 single-crystal surfaces, at full coverage, the CO molecules occupy the so-called ~ ( 2 x 4patterns. ) However, though the CO patterns are the same on the two metals, the bonding sites differ. On Pt, half the CO are on-top (or linearly) bonded and half are bridge b ~ n d e d , ~ whereas on Pd all C O S are bridge bonded.I0 At low temperatures (e.g. 77 K) the observed N M R line shape of adsorbed 3C0is broad. (The frequency width Aw corresponds to several hundred ppm owing to I3C chemical shift anisotropy and the large paramagnetic susceptibility of the metal.l’*l*) However, if one raises the temperature the CO begins to diffuse, averaging out the sources of the line broadening.I2 Initially, a narrow line appears on top of the broad low-temperature line. The narrow line’s relative intensity is higher, the higher the temperature, as a larger and larger fraction of the clusters are motionally narrowed until one reaches a point a t which the whole line is narrow. For narrowing to occur on a cluster, the C O must diffuse between the “poles” of the cluster and the “equator” in a time shorter than l/Aw. Originally, this led us to conclude that the smallest diameter clusters in a given sample would narrow at the lowest temperatures, whereas CO on the larger clusters would exhibit narrowing only at higher temperatures. We discovered, however, that this reasoning must be wrong by comparing samples of different mean cluster size (Figure 1). There we see that at room temperature the motionally narrowed line is a larger fraction of the intensity the larger the mean cluster size of the sample. (We also found by varying the temperature that the larger the mean cluster size, the lower the temperature at which the line narrowed!) Moreover, we had always observed another result which defied explanation: The motionally narrowed line is shifted in frequency from the average frequency of the low-temperature line. Similar results have been reported by Zilm’s group.8a.b They studied samples of low (1 6%) and high dispersion ( 5 6 % ) . They found that at room temperature the low-dispersion ‘Current address: MIT National Magnet Lab, NW14, Rm. 5122, 170 Albany St., Cambridge, MA 02139. f Also Department of Chemistry. * To whom correspondence should be addressed.
.1 0 0 0
0
1000
2000
0
1000
2000
Frequency (ppm) Figure 1. 13CNMR line shapes for I3COon Pd for different dispersions at 300 K. Part E shows for Pd46CO the variation of line shape with echo pulse separation, Tdclay. for 300 K: (a) 2Tdclay = 140, (b) 300, and (c) 1 2 0 0 ~ s Verticallinesindicate . thefrequencypositionsatwhich relaxation times were determined.
sample presents a motionally narrowed line whereas the highdispersion one maintains a broad line up to 400 X. In this paper, we report an investigation and explanation of these mysteries. All our samples have I3CO adsorbed to full (Le. saturation) coverage at room temperature. We find that the explanation for the strange results arises in the existence of a massive rearrangement of bonding sites on a given cluster as one changes the temperature. Thus at low temperatures, many of the CO’s are linearly bonded on Pd on small clusters, but at elevated temperatures they are overwhelmingly bridge bonded.
0022-3654/58/2097-0010$04.00/00 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993 11 1.o 0.8 0.6 0.4
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Figure 2. Variation of the I3C NMR line shape with temperature for Pd52CO.
Since the samples have full coverage of CO, a change of a substantial fraction of CO's from linear to bridge bonding almost surely also requires that the bridge bonded CO's change the bridge sites at which they are bonded, much as would be necessary if the Pt ~ ( 2 x 4 arrangement ) were to shift to the Pd ~ ( 2 x 4 ) .We find that the temperature at which this rearrangement occurs depends on cluster size-the smaller the cluster, the higher the temperature of the transition. We also find that C O diffuses much more rapidly in the high-temperature arrangements; hence, motional narrowing only arises once the C O arrangement has transformed to its high-temperature arrangement. In this paper, we present the evidence for this picture. It is based on the fact that the I3CN M R resonance frequency of I3CO on a metal differs for the linearly and bridge bonded species. As the sample is heated progressively above 77 K, the position and shape of the resonance line change, requiring therefore a change in the type of bonding. Experimental Method Our samples are small Pd clusters (diameters 10-200 A) supported on silica. They are 5 wt % Pd. The fraction of Pd atoms at the surface of the clusters is called the dispersion and is measured by C O chemisorption. We have used samples with 12,22, 33,46, and 52% dispersion (labeled Pd12C0, Pd22C0, Pd33C0, Pd46C0, and Pd52C0, respectively). The clean metal surface is covered with CO enriched in I3C at room temperature. Our sample preparation procedure has been discussed elsewhere.11 Data are collected by spin echoes in a static field of 8.2 T with absorption line shapes obtained by taking the Fourier transform of the second half of the echo. Results Direct evidence for the two types of adsorbed species and the temperature variation of their populations is given in Figure 2, which shows the N M R absorption line of Pd52CO as a function of temperature. At 77 K, the line is centered at about 400 ppm and is strongly asymmetric with its peak at low frequency, but if the temperature is raised to 300 K, the resonance becomes roughly Gaussian and the peak shifts to 650 ppm, where it remains as the temperature is raised to 500 K. The position of the resonance for a particular I3COmolecule is determined by the chemical shift and the Knight shift.
0.0
7
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1
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6
Figure 3. Fractions of the low ( *)and high ( 0 )frequency lines as a function of 1/T for Pd52C0, Pd33C0, and PdlZCO.
Therefore, a change of average line position from 400 to 650 ppm requires a change in bonding, hence a structural rearrangement of the CO adlayer. As we explain below, we interpret the lower frequency absorption near 400 ppm as arising from linearly (or 'on-top") bonded C O and the higher frequency absorption near 650 ppm as arising from bridge bonded CO (superposed on the tail of the linear bonded absorption). Therefore, we conclude that much of the CO on Pd52 is linearly bonded at 77 K but that as one warms the sample to around 250 K a significant amount of the CO changes from the linearly bonded form to the bridge bonded form. We can decompose the spectrum into contributions from the low-frequency (linearly bonded) and high-frequency (bridge bonded) species by varying the time Tdelaybetween the 90' and 180' pulses used to form the echo (this is a "T2 experiment"). In Figure 1E we show the changes in the spectrum for Pd46CO a t 300 K for different Tdelay.The high-frequency contribution has a faster decay than the low-frequency one. Following the intensities at the two frequencies marked by the lines in Figure 1 (300 and 700 ppm), we can obtain the relaxation times TZ's for the two lines. Assuming that the high-frequency line does not overlap the low-frequency one at 300 ppm, the intensity of the low-frequency line is fitted to a single exponential. The intensity at 700 ppm is fitted to the sum of two exponentials because at that frequency there is overlap of both lines. One of the latter exponentials has the time constant determined from the lowfrequency measurements. In this way we can determine the relaxation time constants for the two CO species. By fitting the total area of the line to the sum of these exponentials, we can deduce the relative fraction of each species versus temperature. Fits were better when the temperature or the sample dispersion provided significant amounts of each phase. Figure 3 depicts the variation of the fractions of the two species with temperature for three samples. In general, the ratio of the low-frequency to the high-frequency species increases with decreasing cluster size, and the high-frequency species becomes the dominant one at sufficiently high temperatures (500 K). For both Pdl2CO and Pd33CO at high temperatures (400-500 K),
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The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993
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Figure 4. Effect of cluster diameter on temperature of transition from the low-temperature to the high-temperature arrangement for CO on Pd clusters. HTA and LTA refer to high-temperature and low-temperature arrangements, respectively.
the low-frequency species has disappeared. For Pd52CO (the smallest clusters), there is still 20% of the low-frequency species at 500K,but thedatasuggest that theamount ofthelow-frequency species is still decreasing as the temperature increases. At lower temperatures (below 300 K), the low-frequency species is much more evident and becomes the predominant species for the smallest Pd clusters. We assign the lower frequency line to linearly bonded C O and the higher frequency line to bridge bonded C O for several reasons: (a) In metal carbonyl molecules, the average shift is more paramagnetic for bridge bonded than for linear bonded CO;I3 (b) IR studies show the C O tends to be more frequently linearly bonded on small clusters than on large clusters;] (c) our CO bond length measurements using 13C-170 doubleresonanceI2 gave a bond length for the high-frequency line of 1.20 f 0.03 A typical of bridge bonding. As indicated earlier, we visualize two different arrangements of adsorbed C O molecules, one consisting of bridge bonded species and the other of a mixture of linearly and bridge bonded species, the exact mixture being a function of cluster size. Figure 3 tells us that the temperature of transition from one arrangement to the other depends on cluster size. The transition temperature is higher for smaller clusters. It is reasonable to assume that for an individual cluster the temperature of transition from one arrangement to the other depends on the size of the cluster regardless of the distribution of clusters in the sample. We can expect that in a particular sample at a certain temperature there is a cluster diameter such that the CO species on all clusters smaller than that size are in the low-temperature arrangement, whereas those on clusters with larger sizes are in the hightemperature arrangement. From our T2 experiments we know the fraction of low-temperature arrangement for a given temperature. We also know the probability distribution of cluster diameter, d, from previous studies in our group.I4 Therefore, we can find for a given sample what diameter corresponds to the fraction of low-temperature arrangement at that temperature. This is equivalent t o determining the temperature of transition from the low-temperature arrangement to the high-temperature arrangement for a cluster of given diameter. We plot in Figure 4 the temperature of transition vs cluster diameter for all the samples. The figure is analogous to a phase diagram for the low- and high-temperature arrangements. We do not have a way to determine the nature of the transition by NMR, i.e. first order, continuous, etc. Our analysis, however, assumes that there is a transition temperature from one arrangement to the other. That temperature is more clearly defined in a discontinuous transition than in a continuous one.
Letters Our experiments show that the transition is reversible; we can achieve the low-temperature arrangement by cooling down a sample in the high-temperature arrangement. On a cluster, the number of edge sites is proportional to d, whereas the number of sites in a surface plane is proportional to d2. Therefore, the ratio of planar sites to edgesites is proportional to d. The fact that the transition temperature goes roughly like 1 / d (see Figure 4) suggests that it is determined by a competition between edge and planar sites, the former evidently favoring linear bonded C O and the latter bridge bonded CO. Figures 1 and 2 also show us that CO is relatively rigid in the low-temperature arrangement since that line is not motionally narrowed. However, a t sufficiently high temperatures in the hightemperature arrangement CO diffuses fast enough to narrow the line. Estimation of the diffusion constants D of both arrangem e n t ~at~ 330 ~ K gives a ratio of diffusion constants (high temperature to low temperature) of about 5000. The high-temperature arrangement seems to have an activation energy for diffusion that varies when the cluster size is changed. An analysis of the dependence of T2 on temperature for this arrangementIs yields an activation energy for diffusion of 6 f 2 kcal/mol for the small clusters and 4 1 kcal/mol for the large ones. We found no indication that motion affects T2 in the lowtemperature phase at any temperature. In conclusion, we have found that the 13Cspectrum of CO on Pd clusters can be decomposed into two components that represent two arrangements of the CO monolayer. There is a temperature of transition from one arrangement to the other that depends on cluster size. We find that each arrangement has different characteristics, especially for diffusion of CO. CO is much more mobile in the high-temperature arrangement. We have evidence that CO bonds linearly on small Pd clusters where linear sites correspond to surface atoms on edges. Since the phenomena take place on samples for which the C O coverage is full, the rearrangements require that a very large fraction of the CO molecules change bonding sites and suggest therefore that the change in arrangement is some form of collective phenomenon.
*
Ackwwledgment. We thank Dr. C. Klug for numerous discussions. We are grateful to the IBM Corporation for providing a Fellowship (held by L.R.B.) to assist our work. This research was supported by the Department of Energy, Division of Materials Science, under Grant DEFG02-9 1ER45439. References and Notes (1) Sheppard, N.; Nguyen, T.T. In Advances in Infrared and Raman Spectroscopy: Clark, R. J. H., Hester, R. E., Eds.; Wiley: London, 1978. (2) Lewis, R.; Gomer, R. Nu000 Cimento Suppl. 1967, 5, 506. (3) Poelsema, B.; Verheij, L. K.;Comsa, G. Phys. Reu. Lett. 1982,49, 1732. (4) Seebauer, E. G.; Kong, A. C. F.;Schmidt, L. D. J . Chem. Phys. 1988, 88, 6597. ( 5 ) Zhu,X. D.; Rasing, Th.; Shen, U. R. Phys. Rev. Lett. 1988,61,2883. (6) Wang, P. K.; Ansermet, J. P.; Rudaz, S. L.; Wang, Z.; Shore, S.; Slichter, C. P.; Sinfelt, J. H. Science 1986, 35. (7) Duncan, T.M.; Root, T. W. J . Phys. Chem. 1988, 92, 4426.
(8) (a) Zilm, K. W.; Bonneviot, L.; Hamilton, D. M.; Webb,0. G.; Hallcr, G . L. J . Phys. Chem. 1990, 94, 1463. (b) Zilm, K. W.; Bonneviot, L.; Haller, G. L.; Han, 0. H.; Kermarec, M. J . Phys. Chem. 1990,94,8495. (9) McCabe, R. W.; Schmidt, L. D. Surf. Sci. 1977, 65, 189. (10) Miranda. R.; Wandelt, K.; Reiger, D.; Schnell, R. D. Surf. Sci. 1984, 139. . - ., 430. .- -. ( 1 I ) Rudaz, S.L.; Ansermet, J. P.; Wang, P. K.; Slichter, C. P.; Sinfelt, J. H.Phys. Rev. Lett. 1985, 54, 71. ( 1 2) Shore, S. E.; Ansermet, J. P.; Slichter, C. P.; Sinfelt, J. H. Phys. Rev. Lett. 1987, 58, 953. (13) Basu, P.;Panayotov, D.; Yates, J. T . J . Phys. Chem. 1987,91,3133. (14) Rhodes, H. E.; Wang, P. K.; Stokes, H. T.; Slichter, C. P.; Sinfelt, J. H.Phys. Rev. B 1982. 26. 3559. ( 1 5 ) Becerra,L. R. Ph.D.Thais, Universityof IllinoisUrbana-Champaign, 199 I (unpublished).
