6758
J . Phys. Chem. 1989, 93, 6758-6763
I R Spectroscopic Studies of Adsorbate Dlffusion in Porous Catalysts Todd H. Ballinger, Pam Basu,+ and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: February 28, 1989; In Final Form: May 3, 1989)
Transmission infrared spectroscopy has been used to study xenon diffusion through high-area porous catalysts such as alumina and silica via xenon interactions with IR-active surface species. At low temperatures, xenon condenses on the external surface of silica. As the catalyst temperature is increased and the vapor pressure of Xe increases, diffusion occurs into the pores where Xe interacts with surface hydroxyl groups, a majority of which are contained within these pores. The dynamic dipole of OH induces a dynamic dipole image in the polarizable Xe, which is opposite in sign to the OH dipole. This interaction lowers the normal frequency of OH by 28 cm-'. This effect is also observed for the chemisorbed Rh'(CO), oscillator on AI2O3. Evidence is presented that cooling Xe-filled pores below 153 K causes Xe condensation on itself in the pores, reducing the Xe-HO interaction as monitored spectroscopically.
-
Introduction Porous high-area metal oxides such as silica and alumina have been used as supports for catalytically active metals in heterogeneous catalysts for many years. Diffusion of gases from the outer surfaces of these materials to sites deep within the pores affects the reaction rates and selectivity of chemical reactions.' Thus, gas diffusion has been investigated both theoretically and experimentally1-" for both reactive and nonreactive gases through the pores of various zeolites. More recent studies5v6have used thermal desorption spectroscopy to distinguish gases adsorbed on the outer zeolite surfaces from gases that have diffused within the pores. The mechanism proposed for these processes involves the gas initially adsorbed on the outer surface and then diffusing into the pores by an activated surface migration process. Noble gases are good probe molecules to study pore structure and diffusion in porous media. Xenon is particularly suited since it is nonreactive, and it is sensitive to interactions with other surface species and to the surrounding environment. 129Xe,with nuclear spin number has been used in nuclear magnetic resonance spectroscopy (NMR) since the xenon resonance is dependent on the collisions that the atom undergoes with atoms at the solid surface. Such solid-state N M R techniques as ' H dipolar decoupling, Iz9Xe-]H cross-polarization, and magic-angle spinning have been used to study the physical shift in '29Xe N M R due to such collisions in zeolites. Ito and Fraissard first studied Iz9Xe in a wide variety of Cheung et al. used Iz9XeN M R also as a probe of the electronic environment inside zeolite cages to determine the kinds of cations in the cages and cage v01ume.~ Menorval and Fraissard showed that Xe N M R could be used to determine the mean size of platinum particles and to distinguish between bare metal and metal with chemisorbed hydrogen.1° Ripmeester used the physical shift in '29Xe N M R to distinguish between various phases of adsorbed xenon on zeolites." Shoemaker and Apple used Xe N M R along with transmission electron microscopy and redox studies to find that high-temperature reduction caused Ru migration out of the cages into the exterior of zeolite crystallites.I2 Scharpf et al. similarly found that nickel-exchanged zeolites also have metal migration out of cages upon red~ction.'~B a n d and Dybowski later discussed the factors that affect the chemical shift in the xenon resonance in nickelexchanged ze01ites.l~ Davidson et al. combined calorimetry with Xe N M R to determine composition, chemical potential, and dissociation heats of xenon hydrates.I5 Xenon surface migration at low temperatures has been applied in studies of pores in condensed silver films by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), thermal desorption (TDS), and photoemission of adsorbed xenon These techniques, along with LEED and electron energy loss spectroscopy (EELS), have been applied to study defects on single crystals, using 'Current address: Washington Research Center, W. R. Grace and Co., Columbia, MD 21044.
0022-3654/89/2093-6758$01 SO10
Xe as a probe of the surface.'8-22 Here it has been found that Xe preferentially adsorbs at defect sites.18-23 Recently, xenon has been coadsorbed with chemisorbed carbon monoxide on a Ni( 1 11) single crystaLZ4 Infrared reflection absorption spectroscopy (IRAS) of carbon monoxide shows a coverage-dependent red shift of up to 60 cm-' in the bridged CO species due to its interaction with physisorbed xenon. This interaction is much greater than the red shift experienced by CO in a xenon matrix25 and has been attributed to the presence of dipole images in the Xe and to changes in the Ni work function due to Xe polarization. Two other infrared investigations have been conducted with high surface area materials and xenon. Yates and HallerZ6found ~
~~
~~
~~~
(1) Jackson, R. Transport in Porous Catalysts; Elsevier: New York, 1977; Chapters 1,8. (2) Dullien, F. A. L. Porous Media: Fluid Transport and Pore Structure; Academic: New York, 1979; Chapters 1,4.
( 3 ) Cunningham, R . E.; Williams, R. J. J. Diffusion in Gases and Porous Media; Plenum: New York, 1980; Chapter 1. (4) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd. ed.; Academic: New York, 1982; Chapter 1. (5) Kiskinova, M.; Griffin, G. L.; Yates, J. T., Jr. J . Catal. 1981, 71, 278. (6) Basu, P.; Yates, J. T., Jr. Surf. Sci. 1986, 177, 291. ( 7 ) Ito, T.; Fraissard, J. J . Chem. Phys. 1982, 76, 5225. (8) Ito, T.; De Menorval, L. C.; Guerrier, E.; Fraissard, J. P. Chem. Phys. Lett. 1984, 111, 271. (9) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J. Phys. Chem. 1988, 92, 5 170. (10) De Menorval, L. C.; Fraissard, J. P.; Ito, T. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 403. (11) Ripmeester, J. A. J . Am. Chem. SOC.1982, 104, 289. (12) Shoemaker, R.; Apple, T. J. Phys. Chem. 1987, 91, 4024. (13) Schorpf, E. W.: Crecely, R. W.; Gates, B. C.; Dybowski, C. J. Phys. Chem. 1986, 90, 9. (14) Bansal, N.; Dybowski, C. In Catalysis 1987; Ward, J. W., Ed.: Elsevier: New York, 1988; p 243. (15) Davidson, D. W.; Handa, Y. P.; Ripmeester, J. A. J . Phys. Chem. 1986, 90, 6549. (16) Wandelt, K. J . Vac. Sci. Technol., A 1984, 2, 802. (17) Albano, E. V.; Daiser, S.: Miranda, R.; Wandelt, K. Surf. Sci. 1985, 150, 367. (18) Engel, T.; Gomer, R. J . Chem. Phys. 1970,52, 5572. (19) Chiang, T. C.; Kaindl, G.; Eastman, D. E. Solid State Commun. 1982, 41, 661. (20) Demuth, J. E.; Avouris, Ph.; Schmeisser, S. Phys. Rev. Lett. 1983, 50, 600. (21) Palmberg, P. W. Surf. Sci. 1971, 25, 598. (22) Bruce, L. A,; Sheridan, M. H. J. Chem. Soc., Faraday Trans. I 1972, 68, 997. (23) Siddiqui, H. R.; Chen, P.; Guo, X.;Yates, J. T., Jr. Manuscript in
preparation. (24) Sherman, M. G.; Xu,2.;Yates, J. T., Jr.; Antoniewicz, P. J. Chem. Phys., submitted for publication. (25) Barnes, A. J. In Vibrational Spectroscopy of Trapped Species; Hallam, H. E., Ed.; Wiley: New York, 1973; Chapter 4. (26) Yates, J. T., Jr.; Haller, G. L. J. Phys. Chem. 1984, 88, 4660.
0 1989 American Chemical Society
IR Studies of Adsorbate Diffusion in Porous Catalysts t
h
-L-
1
8o
I-
O = TNO A
v
23
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6159
0.
Xa(a) vapor pnrruro
= Experlmmtal vapor prorrun
A-
with trmpwotura corractlon: T,," ,"**"") = '2 K
60
-
(Tlh.m.coupl.
v)
e n v,
40
1 0
1
~~~
80
100
120
140
Temperature (K) Figure 1. IR cell thermocouple calibration with experimental and true Xe vapor pressures. With 12 K being subtracted from the experimental values, good agreement between the two vapor pressure curves was obtained. xenon to interact with nitrogen chemisorbed on Rho sites present on a Rh/A1203porous catalyst. Here too, a red shift was observed for the N-N stretching mode. Earlier, McDonald2' studied xenon interactions with hydroxyl groups on silica. These experiments show a 19-cm-l red shift in the hydroxyl group frequency upon xenon adsorption. We have employed the physisorption of Xe on high-area porous oxides to study Xe transport processes through the pores. The specific interaction of physisorbed and/or condensed Xe with surface O H groups or with chemisorbed CO species on Rh has been used to observe the Xe transport through the porous solid. This is done by monitoring the 0-H or C-0 stretching frequency as it is perturbed by physisorbed neighbor Xe species. This use of IR spectroscopy to monitor the pore diffusion of a weakly adsorbed molecule represents the first example of this experimental method.
Experimental Section The samples were prepared with a spray deposition technique that has been described elsewhere.28 Briefly, it involves dispersing the powered material in a slurry of water and acetone. The slurry is uniformly sprayed by an atomizer onto a calcium fluoride disk (5.07 cmz in geometrical area on one face), which is heated to 50-60 "C to flash evaporate the solvents. The disk containing the sample for study was then placed into the IR cell29and was supported there by a copper ring. The ring was soldered to a tube that conducts a mixture of N2(l) and N2(g) for temperature control ( f 2 K). The stainless steel cell is connected to a stainless steel gas line that may be rough-pumped by a zeolite sorption pump and further pumped by a 20 L/s ion pump. The base pressure typically obtained was
0
,I
i
b.-t?l'
,
80
110
,
140
,
,
,
170
,
,
,
200
,
,
,
230
,
,
,
260
,
in
a
a
290
Temperature (K) Fipre 2. Experimental Xe vapor pressure vs corrected cell temperature. The curve flattens out at 153 K since all the Xe(s) present has vaporized. +Resolution 3 . 2 cm-'
A
A bs
I
3800
1
I
3700 Wavenumber
I
1
36C
(cm-'
)
Figure 4. Infrared spectra of unaffected and Xe-affected SiOH groups
at several temperatures, showing the isosbestic point. surface of the silica, spectrum b was then taken. No change in isolated OH peak position or intensity is observed. As the cell was slowly warmed, a new Si-OH feature begins to appear at 3705 cm-I. This feature maximizes in absorbance at 148 K, as seen in spectrum f. As the sample is warmed further, the peak at 3705 cm-l decreases in intensity [spectra g-i]. By 288 K, spectrum j, obtained in the presence of Xe(g), is identical with spectrum a, taken under vacuum. These changes can be better understood by looking at the difference spectra shown in Figure 3B. Because spectrum a and b are similar, spectrum b was used as a "base-line" spectrum to subtract from the other spectra obtained at higher temperatures. Initially, the difference spectra resulting from b - b is a horizontal line. At higher temperatures, as the 3733-cm-' feature loses intensity, it appears as a negative difference feature at -3734 cm-l in the difference spectra. Opposite this effect, as the 3705-cm-' feature increases in intensity, a positive feature appears at -3708 cm-I in the difference spectra. The maximum effect of xenon on the silica hydroxyl groups occurs near 148 K, spectrum f - b. As the temperature is further increased, the trend reverses itself up to 288 K, where almost no effect of Xe is observed on the Si-OH groups. It should be noted that the observed trend involves the growth of a new IR feature and not a shift of the 3733-cm-' feature. This can be detected by the presence of an isosbestic point in the spectra,36which is indicative of the conversion process involving unaffected O H groups changing to xenon-affected O H groups. Figure 4 shows three overlaid infrared spectra taken at 128, 133, and 148 K and the resulting isosbestic point at 3726 cm-' as the new 3705-cm-' infrared feature becomes more prominent at the expense of the 3733-cm-' feature. Another way of measuring the process of Xe interaction with Si-OH groups is to study the integrated area of the negative feature at 3734 cm-' in the difference spectra. The integrated area is plotted in Figure 5 as a function of temperature. The circular points represent the integrated area of the negative difference spectra peaks as the cell is warmed from 88 to 288 K. The most negative area represents the maximum effect of xenon, which occurs near 153 K. This can be correlated to the temperature point near 153 K in Figure 2 at which there is no longer any Xe(s) remaining. As the cell is warmed above 153 K, the trend reverses itself as the temperature increases to 288 K. Following IR measurements at 288 K with xenon gas still present in the cell and gas line, the sample is once again slowly cooled while IR spectra are measured. The integrated area of the 3734-cm-' difference feature for this part of the experiment
-
( h ) 168
( i ) 188 01
u c
:6 0
u)
I Difference
n
a
Spectra
Resolution 3~2cm-'
1 3708
3708
(b-b) J
100
s
l
~
'
I
.
1
I
I
3800 3600 Wavenum ber (cm-' 1 Figure 3, Infrared spectra of Xe/SiOH in the 0-H stretching region recorded as a function of temperature. Part A shows the actual spectra, and part B shows the difference spectra where spectrum b was used as the base line. 3600
B. Xenon Interaction with SiOH Groups. The infrared spectra in the hydroxyl region of silica are shown in Figure 3A. Spectrum a was taken in vacuum at 88 K. The isolated hydroxyl groups on silica exhibit a characteristic, sharp OH stretching band at 3733 cm-I; the tail in the 3700-3500- and 3500-3400-cm-' regions is due to weakly and strongly H-bonded O H groups, respectively.34,35 After the condensation of 2.5 mmol of xenon onto the (34) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967; Chapter 4. ( 3 5 ) Peri, J. B. J . Phys. Chem. 1966, 70,2937.
(36) Ewing, G . W. Instrumental Methods of Chemical Analysis, 4th ed.; McGraw-Hill: New York, 1975; Chapter 3.
The Journal of Physical Chemistry, VO~. 93, No. 18, 1989 6761
IR Studies of Adsorbate Diffusion in Porous Catalysts
I
'
I
~
I
I
I
Experimental Frequency Shift
L
U
p = 127 Torr Xe
L 3
under
c
T=
0 Q
-3
v)
Resolution 3 . 2 cm-'
P)
0
c
2Q
-5
r
'c
&
-7
-
0
A
= Heating = Cooling
10.02Abs
I . . , . , , . , , , . , ,, ,., .I , , 80
110
140
200
170
230
260
290
E 0
Figure 5. Xe transport phenomena to and from Si-OH groups as shown by the integrated area of the negative feature in the OH difference spectra.
,
2103.51..
7
E
J
2103.0
-
n
2101.5
-
3
2101.0 -
$ 0 3
2100.5
Q
E c
-
2100.0
. .
'.'.
2102.5 2102.0
-
I
\
'
2
. .
'
8
Background, no Xe
Asymmetric C - 0 stretch
?
2 o
I1 I1
Modeled Frequency Shift model under X e
in vacuum
a
I
I
I I
Modeled S h i f t e d Bond Components I
C
4
,, I:x
model--+ under X e I
I!\
shifted band 4 cm- ) ( 6 4 % )
(A=
-
80
110
140
170
200
230
260
290
Temperature (K) Figure 6. Effect of Xe adsorption on the symmetric stretching frequency of the gem-dicarbonyl species as a function of wavenumber.
-
is represented by triangles in Figure 5. As the sample is cooled to 153 K, reversible spectral behavior (within experimental error) is observed. However, when the sample is cooled below 153 K, the warming and cooling curves diverge somewhat and a hysteresis effect is observed. Even at 88 K, a difference exists between the heating and cooling experiments. C. Interaction with Chemisorbed Rh'(CO),. A similar effect of Xe adsorption and desorption on a different surface oscillator has also been studied. When carbon monoxide is chemisorbed on rhodium-supported catalysts, the gem-dicarbonyl species, Rhl(CO),, is produced. This species is characterized by the symmetric C-O stretching mode at 2104 cm-l and the asymmetric C-0 stretching mode at 2030 cm-1.37,38 The gem-dicarbonyl species is the dominant carbonyl species for highly dispersed Rh catalysts on a l ~ m i n a . ~ 'For * ~ ~these experiments involving the Rh1(C0)2.-Xe interaction, a 0.30% rhodium/A1203catalyst was used. The xenon condensing and warming experiment was done as before. The results obtained are shown graphically in Figure 6 for the symmetric stretching mode of the gem-dicarbonyl species; similar results were obtained for the asymmetric stretching mode. The dashed line represents the experimental change in frequency of this mode with temperature in the absence of Xe(a), and this shift with temperature has been thoroughly studied p r e v i o ~ s l y . ~ ~ The data points represent the Rhl(CO), frequency shift during a heating experiment in the presence of Xe(g). The actual spectra of the unaffected and Xe-affected carbonyl species are shown in Figure 7A. Because the frequency change between the two spectra is small (-2 cm-I), we have modeled
-
Symmetric
C-0 stretch a, 0
Temperature (K)
'?
148K
(37) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J . Chem. Phys. 1979, 71, 3908. (38) Yates, J. T., Jr.; Duncan, T. M.; Worley, S.D.; Vaughan, R. W. J . Chem. Phys. 1979, 70, 1219. (39) Antoniewicz, P. R.; Cavanagh, R. R.; Yates, J. T., Jr. J . Chem. Phys. 1980, 73, 3456.
- -/= ,
1
I
I
I
'
-
I
I
2150 2100 2050 2000
Wavenumber ( c m - ' ) Figure 7. Comparison of experiment and model of Xe-induced frequency shifts in Rh'(CO)* (A) gem-Dicarbonyl at 148 K without Xe and with Xe; (B) gem-dicarbonyl at 148 K with Xe; and (C) theoretical spectrum with component parts.
the observed shift due to Rh1(CO)2--Xe interaction as follows: we assume that the observed shifting of the symmetric Rh'(C0)2 carbonyl mode is caused by a superposition of a shifted spectral feature on the remaining unshifted spectral feature. The modeled shifted band (Figure 7B) is very similar in shape to the experimental shifted band (Figure 7A) when the band contours are carefully compared on an expanded wavenumber scale (not shown). To construct the modeled shifted band, a shift of 4 cm-' was selected, and 64% of the species were assumed to be affected, leaving 36% unaffected. It was found in a series of trials that the model works for Xe-shifted features from 3 to 5 cm-' with 7540% of the carbonyl species being affected, respectively. Figure 7C shows the two-component peaks before they are added together to form the model feature for the shifted band. The general similarity of the observed behavior of Figures 5 and 6 indicates that a similar Xe-transport phenomenon through porous solid is being observed in both cases even though different spectral properties (Si-OH, absorbance; Rh1(C0)2,frequency) are being plotted. For the Si-OH groups on S O 2 , a distinct red shift of voHis observed due to interaction with Xe, and affected and unaffected O H groups may be spectroscopically distinguished allowing absorbance difference measurements to be made. In contrast, for the Rh'(CO), species on Alz03, resolution of a Xe-affected species is not possible and instead of the direct observation of xenon-affected and -unaffected carbonyl bands, we observe only a small shift to the red when the Xe-Rh'(CO), interactions occur.
Discussion A . Xe Influence on Surface Oscillators. A . I . Surface Hydroxyl Groups. The growth of the 3705-cm-' feature in the
6762 The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
infrared spectra (Figure 3) is caused by xenon interacting with the initially isolated surface hydroxyl groups of silica. This interaction may be explained qualitatively in the following way. Initially, before xenon is added, the dipole moments of the oscillating surface hydroxyl groups are oriented with the positive end of the dipole pointing away from the surface. When the xenon adsorbed on the surface interacts with these surface O H oscillators, an image dipole is formed in the highly polarizable xenon, which is oppositely oriented to the O H dipole. This induced dipole in xenon will oscillate in phase with the OH oscillator, causing a reduction in the effective force constant of the OH oscillator. This "softening" of the force constant will dampen, or lower, the O H vibrational frequency. McDonald2' attributed weak hydrogen bonding to the 19-cm-' red shift for silica surface hydroxyl groups upon Xe adsorption. Further attemptsz7to relate the polarizability of other rare gases to the magnitude of the shift were unsuccessful. Our observation of a 28-cm-l Si-OH red shift is in general agreement with McDonald.27 It should be noted that the Xe interaction with surface hydroxyl groups on A1203was also studied in our laboratory. However, because of a variety of hydroxyl groups on alumina,35this region is spectroscopically very complicated and measuring quantitative changes in the O H spectra is extremely difficult. A.2. Surface Rhodium gem-Dicarbonyl Groups. As seen in Figures 6 and 7 , the effect of xenon on Rh/AlZO3gem-dicarbonyl groups is much less than on the OH groups. However, the same physical effect (red shift) of an induced dipole in Xe interacting with the dynamic dipole of the carbonyl is still occurring. The reasons for the different magnitude of the red shift effect for uco and vOH are not well understood at present. One possibility could be due to the size of the rhodium gem-dicarbonyl species. Since the gem-dicarbonyl is larger than a hydroxyl group, it is possible that the adsorbed neighbor xenon cannot get as close to the gem-dicarbonyl species as it can to the hydroxyl species. Thus, since the dipole field is proportional to r-3, a xenonlgem-dicarbonyl interaction will tend to be less than the xenon/hydroxyl interaction. A second, interesting effect observed in Figure 7A is the change of relative intensities of the two C - 0 stretching modes. Before Xe is added to the sample, the peak intensity of the high-frequency symmetric C-0 stretch is greater than that of the asymmetric C-0 stretching mode. After xenon adsorption, the intensity of the asymmetric mode is greater than the symmetric mode. This effect can be qualitatively explained with the assumption that the C-O bond axis is collinear with the Rh-C bond axis. In this case, the angle between the carbonyl groups, 0, is related to the intensities40 by Isym/Iasym
= cot2 (0/2)
(1)
From the intensities of the two modes before Xe addition, Is,,,,/Ia,,,, = 1.034, yielding a calculated C-Rh-C angle of 89.03O. After = 0.8955, correXe addition, the intensity ratio is Isym/Iasym sponding to a C-Rh-C angle of 93.16'. Thus, it is possible that Xe coadsorbed with the Rh(*)(CO),species causes an opening of the C-Rh-C angle by about 4O. Possibly other explanations of this effect will be discovered in the future. B. Xe Transport through Porous Catalyst Media. The structure of Aerosil 200, a fumed silica, is accepted to be small spheres connected together to form chains.41 These spheres do not contain micropores; thus, the pores discussed here result from the spacing between the spheres as they are packed together. Initially at 88 K, Xe condenses as a solid from the gas phase only on the outer geometrical surface of the catalyst. As seen in Figures 3 and 5, Xe at this temperature does not interact with measurable quantities of the hydroxyl groups. This is because the condensed Xe cannot diffuse into the pores where a majority of the hydroxyl groups exist. When the sample is warmed and (40) Chemier, J. H. B.; Hampson, C. A,; Howard, J. A,; Mile, B. J . Chem. Soc., Chem. Commun. 1986, 730. (41) Parfitt, G . D.;Sing, K. S. W. Characterization of Powder Surfaces; Academic: New York, 1976; Chapter 8.
Ballinger et al. the Xe vapor pressure rises, diffusion into the pores can occur. Once Xe does gain enough thermal energy to desorb from the surface, the gaseous diffusion through pores can be classified by the following mechanisms: (1) molecular diffusion; (2) Knudsen diffusion; and (3) surface diffusion.' Molecular diffusion occurs when momentum is transferred from one gas-phase molecule to a n ~ t h e r .This ~ type of diffusion occurs when the mean free path of a molecule is small compared to the average pore diameter. Knudsen diffusion is present when momentum transfer occurs only between a molecule and a pore wall.3 Here, the mean free path of a molecule is large compared to the pore diameter, and the molecule-pore wall collision probability is greater than the molecule-molecule gas-phase collision probability. In surface diffusion, a gas molecule adsorbed at a site can acquire enough energy to overcome the energy barrier between two adsorption sites (but not enough energy to desorb). The molecule will "hop" to the next adsorption site as surface diffusion occurs. There is only a certain temperature region where surface diffusion will occur. If the temperature is too low, the molecule will not obtain enough thermal energy to overcome the energy barrier for migration between sites. At high temperatures, gas molecules will not adsorb to appreciable coverages and surface diffusion will not occur. In order to calculate which type of diffusion is occurring in these experiments, the mean free path, A, of Xe can be calculated from h = (f/z'/2~&)(RT/PNo)
(2)
where d is the diameter of Xe, R is the gas constant, 7'is the temperature, P is the pressure, and No is Avogadro's number. With the van der Waal's diameter of Xe as 4.4 X lo-* cm, at 153 K with a partial pressure of 152 Torr of Xe, the Xe mean free path is 1200 A. Since the average pore diameter is less than 1000 A in both our S i 0 2 and A120342samples, Knudsen diffusion will be the dominant diffusion mechanism, with some surface diffusion possibly occurring between 120-1 50 K when the vapor pressure of Xe is low. As diffusion continues in intergranular pores with increasing sample temperature, Xe interaction with O H groups becomes evident. At 153 K, when no more Xe(s) remains, the maximum SiOH-Xe effect is observed. When less than 2.5 mmol of Xe(g) was condensed on the sample and warmed, the fully developed Xe-affected Si-OH feature at 3705 cm-' in Figure 3 was less intense. Similarly, the maximum red shift (Figure 5) was reduced when smaller amounts of Xe(g) were used. Once no Xe(s) is left on the surface, and the temperature rises above 153 K, Xe begins to diffuse back out of the pores and to desorb. At 288 K, all Xe has desorbed from the silica and the infrared spectrum j in Figure 3A is similar to spectrum b taken at 88 K, where in both cases S i 4 H - X e interactions are minimal. As seen in Figure 5, when the sample is cooled under Xe(g), the trend is reversible in the temperature range of 288-153 K. However, below 153 K, one would expect that once Xe diffuses into the silica pores and is interacting with O H groups, it would freeze inside the pores, possibly maintaining the Si-OH-Xe interaction. Thus, it would be expected that the lower frequency IR feature would still be present and the triangles in Figure 5 would remain at the most negative value of the integrated area scale below 153 K. This is not what is observed in Figure 5. Although the curve is almost (but not exactly) identical with the heating curve in this temperature region, it is difficult to believe that Xe would migrate out of the pores onto the outer surface as the temperature is reduced. Furthermore, the slight hysteresis in the cooling curve below 153 K indicates that slightly more Xe-HO interactions are occurring along the cooling curve than along the heating curve. Thus, Xe rediffusion onto the outer silica surface can be ruled out during cooling. Xenon must be freezing inside the pores but not near the O H groups. Thus, the site accepting the freezing Xe atoms is a site other than an OH site that is not observed by infrared spectroscopy. A likely possibility could be that Xe is condensing on
-
-
-
(42) Basu, P.; Ballinger, T.H.; Yates, J. T.,Jr.; Langmuir 1989, 5, 502.
J. Phys. Chem. 1989, 93,6763-6769 itself in clusters away from most of the OH groups. Implicit in this assumption is that a Xe atom would rather interact with itself than an O H group. The two-body Xe-Xe interaction energy may be approximated from the heat of sublimation, which is 3.8 k c a l / m 0 1 . ~ ~ Assuming .~ a close-packed structure, a surface Xe will have nine nearest neighbors. Therefore, an individual Xe-Xe interaction will have an energy of -0.42 kcal/mol. The Xe-HO interaction will be of the van der Waals type. The energy of interaction in van der Waals complexes is typically studied by molecular beam experiments; however, to the best of our knowledge, no molecular beam studies have been conducted for Xe-HO-type clusters, so a direct measurement is not available. However, the Xe effect on O H can be estimated from the Xe-HF interaction energy that has been found by Hutson and Howard45 to be 0.478 kcal/mol. Since the dipole moments of HF and O H and 1.5 D,4' respectively, the ratio of the are similar, 1.8 two can be used to find the Xe-HO interaction energy. The induced dipole in Xe is proportional to D4s946
(3) where a is the polarizability of Xe, p is the dipole moment of either H O or HF, and r is the distance between the point dipole and its image in the Xe neighbor atom. The electric field of the HO or H F dipole is proportional to
P/r3
(4)
Thus, the interaction energy between the permanent and induced dipole will be proportional to ap2/r6
(5)
In scaling the Xe-HF interaction energy, a ratio of eq 5 for the two dipole moments (pHFand poH) will be taken. Since the Xe-HO distance is not accurately known, we will assume it to be approximately equal to the Xe-HF distance since in both cases interaction with hydrogen atoms is involved. Thus, r6(XeHF)/#(Xe-HO) N 1. Furthermore, axeis constant and will cancel out of the ratio. This leaves the scaling ratio simply as the ratio of the square of the dipole moments. On this basis, the Xe-HO interaction energy is -0.33 kcal/mol. (43) Crawford, R. K. In Rare Gas Solids;Klein, M. L., Venables, J. A,, Eds.;Academic: New York, 1977; Vol. 2, Chapter 11. (44) Tessier, C.; Terlain, A.; Carher, Y.Physica 1982, 113A, 286. (45) Hutson, J. M.; Howard, B. J. Mol. Phys. 1982, 45, 791. (46) Baiocchi, F. A.; Dixon,T. A.; Joyner, C. H.; Klemperer, W . J. Chem. Phys. 1981, 75, 2041. (47) McClellan, A. L. Tables ofExperimental Dipole Moments; Freeman: San Francisco, 1963.
6763
Ignoring all other molecular interactions, this approximation shows that the Xe-Xe interaction energy (-0.42 kcal/mol) is slightly greater than the Xe-HO interaction energy (0.33 kcal/mol) and that Xe-Xe interactions are more likely to occur than Xe-HO interactions, especially at low temperatures. This explanation supplies a rationale for the disappearance of the Xe-HO interaction during the low-temperature region of the cooling experiment where the red shift disappears on cooling.
Conclusions A new method of studying xenon diffusion through high-area porous catalysts has been demonstrated by monitoring xenoninduced changes in the infrared spectra of hydroxyl and carbonyl surface species. The phenomena observed for Xe are expected to be present for any diffusing molecule in a porous solid. These effects are summarized as follows: (1) Xe initially condenses at 88 K on the outermost surfaces of silica and alumina and causes no observable change in the IR spectra. (2) As the catalyst is warmed above 88 K, the Xe vapor pressure rises and Xe becomes mobile and begins to diffuse into the pores of the catalyst. (3) Once Xe diffuses inside the pores of silica, there are van der Waals attractions between the Xe and OH groups. For Rh/A1203, both hydroxyl and gem-dicarbonyl species interact with Xe, also through van der Waals forces. (4) The dynamic dipole image induced in a physically adsorbed Xe is opposite in sign to the dynamic dipole of OH, causing a softening of the force constant of the O H oscillator, which lowers the 1R frequency of this oscillator by 28 cm-l at 153 K in silica. The same effect is observed with a smaller vco red shift for Rh'(CO)*. Here, the carbonyl IR frequency is lowered about 3-5 cm-' upon interaction with Xe. (5) Above 153 K, Xe desorbs from the catalyst pores, and the effect of Xe on the surface oscillators is observed to disappear as the temperature increases to 288 K. When the catalyst is recooled from 288 K, Xe diffusion and adsorption occurs once again into the pores as observed by the spectroscopic reversibility. (6) Cooling the catalyst below 153 K causes Xe to condense inside the pores, but Xe does not condense near Si-OH groups. This segregation effect occurs because Xe-Xe interactions are slightly stronger than Xe-HO interactions.
-
Acknowledgment. We acknowledge, with thanks, the support of this work by the General Motors Corp. We also thank Craig Bieler and Dr. Ken Janda for helpful discussions concerning van der Waals complexes. P.B. acknowledges a fully supported educational leave from Alcoa. Registry No. Xe, 7440-63-3; SOz, 7631-86-9; Rh, 7440-16-6; CO, 630-08-0.
Rhodium Dicarbonyl Species on Silica and Alumina: Characterization with 13C NMR A. M. Thayer and T. M. Duncan* AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: January 12, 1989)
We have obtained I3C NMR spectra of several highly dispersed, low-loading Rh samples which are chiefly comprised of dicarbonyl species. In addition to the motionally averaged dicarbonyl species initially identified, the spectra reveal the presence of two other dicarbonyl species. One dicarbonyl type is characterized by rapid mutual exchange of the C O Swhereas in the other type the CO's are constrained to appear rigid on the NMR time scale. These three dicarbonyl species are inherently unresolvable with vibrational and electron spectroscopies but yield three disparate NMR line shapes. It is shown that these new species resolve the inconsistencies in a previous model, and a modified method to obtain site distributions is described.
I. Introduction CO adsorbs on oxide-supported Rh in three general forms: linear, bridging, and dicarbonyl. A recent study proposed assignments of I3CN M R spectra for these three forms, derived from
a separation of overlapping components based on similarities to spectra of metal carbonyl clusters;' the Rh dicarbonyl species was (1) Duncan, T. M.; Root, T.W. J . Phys. Chem. 1988, 92, 4426.
0022-3654/89/2093-6763$01.50/00 1989 American Chemical Society
