J. Phys. Chem. 1984,88, 2764-2771
2764
+
Ebinstead of D = De. The absolute values of ‘ / 2 h ~are 3 then found to differ considerably (for example, by a factor of 6 for ID1 (DIM)) although the trends in the isotopic ratios are still consistent with those expected from the mass ratios (20). In principle, a better procedure would be to calculate D for each isotope by the DIVAH model. It is interesting to note from eq 15 and 17 that the ratio w ~ / w I in the valence force field approximation is independent of kl and is given by w ~ / w I = (1 + 2 m y / m x ) 1 / 2 (21)
= (2my/mx)’I2
(22)
for a heavy-light-heavy molecule. The results in Table X show trends consistent with eq 22. Again, part of the reason for the discrepancies is in the choice of D discussed above. Next, we consider some of the spectroscopic consequences of the results in Table X. First, we make the obvious point that all bound-to-bound transition energies must be smaller than the zero point energies E,(XY) (see Table I). However, bound-to-resonance state transitions can result in energies larger than Eo(XY). The width of these transitions will depend on the lifetime of the resonance. Table X also shows that hwl < hw2 whereas the usual rule is h w , > ha2. Another stable molecule which has h a l < hw2 is the F X F anion31s32with X = H , and D. As already analyzed above, h w , is weakly dependent on isotopic substitution, whereas hw2shows a pronounced isotopic dependence. It is also interesting to note that ‘/2Awl is smaller than the zero point energy of the Y2 molecule. Equations 7 and 8 can also be used to estimate the number of states with v2h# 0 and v3 # 0, although the harmonic approximation is expected to become very poor for highly excited vibrational states. Comparing I / 2 h ~ 1 ( Xand ) hw2(X) for the LEPS and DIM surfaces shows that 1/2hwl(LEPS)= 1 / 2 h w l (DIM) and hw2(LEPS) = hw2(DIM) for a given isotope, although the corresponding vibrational bond energies (Table IX) exhibit a pronounced sensitivity to the potential surface. V. Conclusions In this paper we have made a systematic study of isotope and potential surface effects on vibrational bonds for 1x1and BrXBr, (31) J. J. Rush, L. W. Schroeder, and A. J. Melveger, J. Chem. Phys., 56, 2793 (1972). (32) R. A. More OFerrall, “Proton Transfer Reactions”,E. Caldin and V. Gold, Eds., Chapman and Hall, London, 1975, Chapter 8; J. Emsley, Chem. SOC.Rev., 9, 91 (1980).
with X = Mu, H, and D. For 1x1, we used extended LEPS and three-center DIM potential surfaces, and for BrXBr a three-center DIM potential surface was employed. For each of the nine systems, variational calculations of the vibrational energy levels were carried out both for the collinear configuration and in 3D (J = 0). The variational method uses the exact Watson Hamiltonian and provides a rigorous upper bound to the vibrational eigenenergies. The bond energies of the YXY molecules increase in the order D < H < Mu, which is the opposite effect to that normally encountered. Comparison of the LEPS and DIM results for 1x1 shows that the presence of van der Waals wells and lower barrier height of the DIM surface increases the number of bound states. The bond energies also show a sensitive dependence on the nature of the potential surface. In addition, we approximately partitioned our exact vibrational energies into normal-mode contributions: l/zhwl, hw2, and 1 / 2 h ~ 3For . all nine systems, we found that h a l < hw2, unlike the case for normal molecules where h w , > haz. The isotopic , and ‘ l 2 h w ,is consistent with that dependence of 1 / 2 h ~ 1hw2, expected for the valence force field model. We hope the calculations reported in this paper will help in the spectroscopic detection of vibrationally bonded species. Our calculations should also be relevant to the spectroscopic properties of the triatomic anions YXY- which have been studied in noble gas matrices.33 Finally, we note that another possible route to the experimental study of the neutral molecules YXY is by photodetachment spectroscopy of the anion YXY-.34
Acknowledgment. We thank Dr. I. Last (Rehovot) for sending
us a copy of his Fortran code for the DIM potentials and Dr. J. Manz (Munich) for helpful correspondence and for preprints. J.N.L.C. thanks NATO for a Senior Scientist Award and Professor R. A. Marcus for his hospitality at the A. A. Noyes Laboratory of Chemical Physics, California Institute of Technology, where part of this research was carried out. Support of this research by the National Science Foundation is also gratefully acknowledged. The numerical calculations were performed on the CDC 7600 computer at the University of Manchester Regional Computer Centre. Registry No. IMu, 79104-08-8; IHI, 12694-71-2; IDI, 391 17-75-4; BrMu, 12587-64-3; BrHBr, 11071-85-5; BrDBr, 391 17-76-5. (33) L. Andrews, Annu. Reu. Phys. Chem., 30, 79 (1979). (34) J. L. Beauchamp, private communication, 1983; R. R. Coderman and W. C. Lineberger, Annu. Reu. Phys. Chem., 30, 347 (1979).
Spillover of Deuterium on Pt/Ti02. 1. Dependence on Temperature, Pressure, and Exposure D. D. Beck and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: February 21, 1984) The adsorption of D2on Pt/Ti02 powders has been studied by thermal desorption spectroscopy in an ultrahigh-vacuum system. In addition to chemisorption on Pt, two thermally activated states are observed and are attributed to spillover states on the oxide. Application of a surface diffusion model yields diffusion activation energies of 5.9 and 7.6 kcal/mol, respectively. One state is ascribed to spillover on a special oxide site, possibly TiO, associated with the Pt metal. The degree of spillover is dependent upon the condition of the catalyst surface with respect to a variety of factors including reduction and hydroxylation.
Introduction The use of TiOz as a support in heterogeneous catalysts has been the object of much attention in recent years. Of particular interest are the photocatalytic properties of the Pt/TiO, (1) S. Sat0 and J. M. White, J . Phys. Chem., 85, 336 (1981).
0022-3654/84/2088-2764$01.50/0
and the strong metal-support interaction (SMSI) in reduced R/Ti02!’S In addition, a mmber of recent Papers have W g e s t d (2) A. V. Bulatov and M. L. Khidakel, Tzu. Akad. Nauk SSSR, Ser. Khim. 1902 (1976). (3) T. Kawai and T. Sakata, Nature (London), 282, 283 (1979).
0 1984 American Chemical Society
Spillover of Deuterium on Pt/TiO,
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2765 transfer rod
TABLE I: Sample Designation“ sample composition 1A 0.6% Pt by weight on Degussa P25 TiO, 1B Degussa P25 TiOz 2A 0.6% Pt by weight on MCB TiOz MCB TiOz 2B
Cu block
/ 2
-
insulator
,,/’,AI,o~
///
,LU
L
oar
W support wire
,thermocouple junction
aAll TiO, starting material was reduced in flowing H,, 1 atm, for 6 h. caD. manometer
8amphg valve MS
I
liq.
N, lines
feedthroughs
Figure 2. Diagram of sample support probe.
4 r
rough P U ~ P
diffusion pump
TMP
TMP
ion pump
Figure 1. Diagram of vacuum system.
evidence for hydrogen migration, commonly referred to as spillover, in M/Ti02 catalysts. These include a series of studies by N M R of pretreated Rh/TiOz in which two types of adsorbed hydrogen were Other reports have indicated reversible formation of a Ti3+ ESR signal upon exposure of M/Ti02 to H2 at temperatures below 773 K.8,9 Furthermore, the presence of supported metal is important for the reduction of TiO, by H, and one reduction mechanism involves spillover.1° A photoconductivity study related hydrogen spillover to the formation of hydroxyl groups on the oxide. Additional evidence for hydrogen spillover on M/TiO, systems is presented elsewhere.I2 Alternative models have been proposed in which hydrogen interactions with Pt/TiO, involve either a surface reaction with Ti3+l3 or electron transfer to the bulk.14 A study of D2 adsorption on Pt/TiOz using temperature-programmed-desorption spectroscopy (TPD) in an ultrahigh-vacuum apparatus is presented here. Through the use of a variety of adsorption/desorption sequences at various temperatures, we are able to identify two states of deuterium not bound to Pt. Both states are believed to arise from spillover.
Experimental Section Catalysts were prepared by impregnation of commercially available pretreated TiO, powders (anatase, Degussa P25, 50 m2/g, and anatase, MCB, 10 m2/g) with chloroplatinic acid solution. Prior to impregnation, both supports were reduced in flowing H, (1 atm, 30 mL/min flow rate) at 773 K for 6 h. Sample designations are listed in Table I. All impregnated samples were dried in air at 393 K for 6 h, reduced in flowing H, (1 atm, 30 mL/min flow rate) at 473 K for 2 h, and stored in air. (4) S. J. Tauster, S . C. Fung, and R. L. Garten, J . Am. Chem. SOC.,100, 170 (1978). (5) B.-H. Chen and J. M. White, J . Phys. Chem., 86, 3534 (1982). (6) P. Gajardo, T. M. Apple, and C. Dybowski, Chem. Phys. Lett., 74,306 (1980). (7) T. M. Apple, P. Gajardo, and C. Dybowski, J. Catal., 68,103 (1981). (8) T. Huizinga and R. Prins, J . Phys. Chem., 85, 2156 (1981). (9) J. C. Conesa and J. Soria, J . Phys. Chem., 86, 1392 (1982). (10) S. J. DeCanlo, T. M. Apple, and C. Dybowski, J . Phys. Chem., 87, 194 . .
(1983). , - - - - I -
(1 1) J. Disdier, J.-M. Herrmann, and P. Pichat, J . Chem. SOC.,Faraday Trans. I , 79, 651 (1983). (12) G.M. Pajonk, S . J. Teichner, and J. E. Germain, Eds., “Spillover of Adsorbed Species”, Elsevier, Amsterdam, 1983. (13) X.-Z. Jiang, T. F. Hayden, and J. A. Dumesic, J . Coral., 83, 168 (1987) , - - - - I .
(14) T. Ichikawa, S . Fujimoto, and H. Yoshida, Chem. Phys. Lett., 103, 80 (1983).
Lecture bottles of O,, D,, and Hzwere obtained from Linde. All but H, were further purified by liquid-N, trapping. The experiments were performed in a partitionable stainless steel ultrahigh-vacuum chamber shown in Figure 1. The preparation chamber for thermal treatment and dosing is connected by a valve to a 120 L/s turbomolecular pump (TMP) and is equipped with an ionization gauge, a capacitance manometer (100-torr full scale), and a gas dosing apparatus. The analysis chamber for desorption is connected to an 80 L/s ion pump through a gate valve and is equipped with a UTI lOOC quadrupole mass spectrometer. Ambient gases in the preparation chamber can be analyzed through the connecting gate valve, or through a sampling line employing a leak valve (for pressures > torr). The sample support system shown in Figure 2 consists of a 1in.-0.d. stainless steel transfer rod that is centerless ground and polished to a surface finish of 8 RMS (root mean square for roughness, microinches). The tube is sealed at one end with an ultrahigh-vacuum feedthrough unit containing four electrical leads and two thermally isolated tubes connected to a hollow Cu block used for cooling the sample with liquid N,. Two smaller Cu bars are mounted on this reservoir through a thin (0.010 in.) polished alumina insulating plate. The powder sample is mixed with H 2 0 to form a thick slurry which is applied and air-dried on the center 0.5-cm section of a W wire loop to which a chromel-alumel thermocouple junction has been previously spot-welded. Typical sample weights ranged from 50 to 500 pg (samples were removed from the wire and weighed after the experiment). The ends of the wire loop are connected to the Cu bars on the reservoir, and the sample is heated by passing current through the support wire. By passing liquid N, through the reservoir and employing electrical heating, one obtains sample temperatures between 135 and 1300 K under vacuum. The sample probe is capable of linear and rotary motion, and sealing between the preparation chamber and atmosphere is accomplished through a series of differentially pumped Teflon seals. An additional set of seals serves to separate the two chambers when the sample probe is fully injected into the analysis chamber. The base pressure of both chambers is 2 X torr after bakeout, and typical working pressure in the preparation chamber rises to about 4 X torr after dosing. During tranfer from the preparation chamber to the analysis chamber, a momentary pressure burst of l order of magnitude occurs in the preparation chamber. Temperature-programmed-desorption (TPD) experiments were performed in the following manner. The sample was positioned under the doser, outgassed at 800 K for 1 h, and then cooled to a preselected temperature for adsorption. Dosing was accomplished by using one of two methods. In a dynamic dose, adsorbate gas was admitted for a certain time through a leak valve with the valve above the TMP left open. In a static dose, the valve above the TMP was closed, and the leak valve was closed after the desired pressure was reached. In both cases, the pressure was measured either with the ionization gauge or the capacitance manometer. The ionization gauge filament was turned off when the desired pressure was reached. At the end of the exposure time (for static doses), the valve above the TMP was opened. Dynamic doses were
2766
Beck and White
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
TABLE II: XRD Results
composition, wt samole anatase rutile 1A 71 28 1B 72 28 2A 78 21 2B 78 22
%
Pt 0.6
none 0.6
none
mean particle diam, nm TiO, Pt 2.5-3.0 18.5 19.0 none >40.0 9.0 >40.0 none
0
See text. calibrated by comparison of TPD peak areas with those obtained from static doses. Usually, the sample was cooled to 140 K after the exposure and pumpout of the chamber. Sample transfer to the analysis chamber required 100 s (300 s for dose pressures >1 torr). TPD spectra were generated by resistively heating the W wire at a linear rate of 4 K/s with a temperature programmer. During the heating program, the partial pressures of desorbing gases were measured by the mass spectrometer and recorded as a function of time. The heating rate varied by &lo% over the range of 150-800 K. Using a multiplexer, we recorded three ion signals and the temperature during a single experiment. TiOz and Pt/TiO2 samples were subjected to temperatures no greater than 800 K in order to minimize phase conversion to rutile and surface reduction. All materials listed in Table I were studied by X-ray diffraction using a General Electric XRD-5 diffractometer with Cu Ka radiation and a graphite monochromator. Relative compositions were obtained from intensities corrected for known crystal plane sensitivities, while volume mean particle diameters were estimated from diffraction peak widths by using the Sherrer method.I5
(15) H. P. Klug and L. E. Alexander, "X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials", Wiley, New York, 1974. (16) P. Pichat, M.-N. Mozzanega, J. Disdier, and J.-M. Herrmann, Nouu. J. Chirn., 6, 559 (1982). (17) P. Meriauceau, H. Ellestad, and C. Naccache, Proc. I n t . Congr. Catal. 7th, 2, 1464 (1980).
300
400
T
500
600
700
( K )
Figure 3. TPD of D2 adsorption of 140 K on Pt/Ti02 for various dose pressures of 100-s duration: (a) 5.5 X IO-l, (b) 7.7 X IO-*, (c) 9.9 X (d) 1.5 X (e) 4.9 X and (f) 1.5 X torr.
I '
Results
XRD. The results of the X-ray diffraction studies are shown in Table 11. The anatase form predominates in every case. Pt peaks are not easily detected at 0.6% metal loading on Degussa TiO,; however, catalysts prepared in the same manner with metal loadings of 1%, 2%, and 5% on Degussa TiOz all yielded a Pt particle size of 2.5-3.0 nm. (In the present study, these were not used further.) Other studies of Pt impregnated on this type of support reveal a preference for metal crystallites with a mean diameter from 2.0 to 4.0 nm,11,16,17 independent of metal loading below 5%. On this basis, a mean particle size of 2.5-3.0 nm is assumed for the Degussa type catalyst. Although the TiO, exhibits a blue color upon reduction, no evidence of Ti9Ol7formation appears in the powder diffraction pattern. Its appearance in earlier work from this laboratorySis attributed to the use of a higher reduction temperature (875 "C). The majority of this investigation was carried out with samples 1A and 1B. A few experiments performed on catalysts 2A and 2B gave results qualitatively similar to those on catalysts 1A and 1B. We conclude that the results are insensitive to oxide surface area and to minor impurity variations from one oxide sample to another. Temperature-Programmed Desorption (TPD). About 100 pg of sample 1A was slurry-dried on the support wire and outgassed under vacuum at 300 K for 3 h and then at 800 K for 1 h. Then the sample was exposed to D, at 300 K and 0.077 torr for 10 min and heated under vacuum at 800 K for 10 min. The purpose of the latter treatment will become apparent below. A series of experiments were then carried out in which the dynamic D2 dose pressure varied between 1.5 X lo4 and 5.5 X lo-' torr; the dose time was 100 s and the dose temperature was 140 K. The resulting TPD spectra (Figure 3, for example) are broad and have peak temperatures (T,) that decrease with increasing dose, indicative
I
I
200
I
I
T
600
700
800
I
b
3 h
P
v) v)
a
I W-
5 2 I-
15W
a:
v
k 2
w
a:
50 ' 2
0
0
200
300
400
500 T ( K )
Figure 4. TPD of D2 adsorption of Pt/Ti02 at 140 and 300 K for 100-s doses at 7.7 X torr: (a) adsorption at 140 K on a new sample; (b) first D2 exposure at 300 K; (c) subsequent exposures at 140 K; (d) subsequent exposures at 300 K, (e) adsorption at 140 K, followed by heating in vacuo at 4 K / s to 300 K, then cooled to 140 K; (inset) repeatability of TPD experiments starting with a new catalyst.
of second-order desorption kinetics. A similar trend is observed for a series in which the dosing time is varied for a specific pressure at 140 K. A second-order Redhead kinetic analysis1*is linear at low dose pressures torr) and short dose times, yielding an activation energy of 17.9 kcal/mol. This state will be designated I. A control experiment was performed on catalyst 1B. The Dz uptake was alwasy 400 K). A more subtle effect is the filling of a thermally activated state that overlaps the low-temperature state observed for adsorption at 140 K. This is more apparent in the following analysis. The relative amount of D2 adsorbed in the activated process is calculated from the data of Figure 6. Figure 6a is a plot of the total TPD area as a function of adsorption temperature. Figure 6b is the total TPD area obtained after the sample was (1) exposed to D2 at the same pressure and duration as in Figure 6a, but at 140 K, (2) heated at a rate of 4 K/s in vacuo up to the adsorption temperature of Figure 6a, (3) immediately cooled to 140 K, and (4) heated at 4 K/s to give the TPD spectrum. The difference between these two curves is a reproducible function of the dose temperature and is plotted in Figure 7a. Similar experiments with the dose time extended to 500 s are summarized in Figure 7b. These differences go to zero when a polycrystalline Pt foil is used. Similar experiments carried out on an equivalent amount of prereduced TiOz containing no Pt are summarized in Figure 6, c and d. The very small differences between these curves indicates that Pt greatly facilitates the observed process. A series of blank experiments were carried out on the W support wire only (Figure 6e); it is clear that contributions from the background are negligible. Two prominent features are observed in Figure 7: (1) There is an increase with adsorption temperature above 400 K, which will be denoted state 111; and (2) there is a peak in the TPD area below 300 K with a maximum at about 215 K which will be denoted state 11. State 111is attributed to activated spillover of D atoms from the Pt particles onto the TiO,. It should be pointed out that some population of state I11 occurs even upon adsorption of D, at 140 K. Therefore, Figure 7 should be considered a lower limit of detectable spillover in our system. The population of state I1 at 140 K is not directly detectable but is probably a small
2768
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 I
I
I
I
I
Beck and White
1
I
200
300 400 DOSE TEMPERATURE ( I< )
500
I
I
,
I
I
5.0
5.5
-43
I I1 I11 With the following test, it is possible to determine qualitatively the temperature regions where k2 > k-l and where k3 > k+ Experiments like those of Figure 5 (shown again in Figure Sa) were done but the dosed sample was heated under vacuum for 100 s at the adsorption temperature just prior to obtaining a spectrum. The results are shown in Figure 8b for a variety of adsorption temperatures. For a given dose temperature, if the difference in the two TPD areas (Figure 8a-b) is small, then the forward process is favored. There are two temperature regions where the difference is minimal (Figure 8c): (1) below 200 K, where kz > k+ and (2) around 525 K, where k3 > k..2. (The latter region probably extends to 550 K, since the difference in Figure 8, a and b, at this point is primarily due to the inability to measure areas beyong 800 K.) For experiments carried out at 500-s exposures the results (not shown) are qualitatively the same. This result is important in the calculation of kinetic parameters to be discussed below. The population of state I11 is dependent on the duration of the dose. The growth of this state was measured as a function of exposure time up to 2000 s at various temperatures ranging from 140 to 600 K. The relatively slow uptake may be related to the slow uptake of H2 on reduced Pt/Ti02 observed by Dumesic et al.l3 The increase in state I11 with temperature exhibited behavior similar to that reported by Kramer and Andre19 for a Pt/AIzO3 system and, for comparison, their analysis was applied. In their model, it is assumed that diffusion of deuterium atoms between oxide sites is the rate-determining step in adsorption, that diffusion proceeds according to a surface model consisting of circular sources of radius r and constant surface deuterium concentration c, within the sources, that spillover products from one source do not interfere with those migrating from a neighboring source, and that the reverse process (recombination and desorption) is negligible. (Temperature regions where this assumption is valid may be chosen on the basis of the data of Figure 8.) Within this framework the surface diffusion coefficient is approximately
(2)
assuming only D is temperature dependent. In eq 2, D = diffusion coefficient of D atoms, c = spillover concentration in atoms/cm2,
30
'
I
1-37
L,'
I
\
(19) R. Kramer and M. Andre, J. Cuful.,58, 287 (1979). (20) K. Foger and J. R. Anderson, Appl. Surf. Sci., 2, 335 (1979).
I
30 0
fraction of the maximum amount detected at about 215 K. Whereas the major spillover state (111) is filled at relatively high adsorption temperatures (>400 K), the low-temperature feature (11) is interpreted as an intermediate state (Da,,,,Jwhose population is governed by the following equilibrium:
- In ( 4 ~ ' / ~ N r c , ) )
I
600
Figure 8. Effect of heating in vacuo after dose: (a) TPD areas of D2 torr for 100 s at various dose temadsorption on Pt/TiO, at 7.7 X peratures; (b) same as for part a, followed by heating in vacuo at the same temperature for an additional 100 s; (c) difference in a and b.
In D = 2(ln (dc/dt'12)
I C
a
10
\ 6.0
,
6.5
I
1
1.5
I
2.0
1
2.5
1/T (X 10-4
Figure 9. Population of deuterium spillover states on Pt/Ti02 with dose duration: (a) TPD area of state I1 vs. to for various adsorption temperatures between 140 and 190 K, (b) plot of In D vs. I / T for state 11; (c) TPD area of state I11 vs. for various adsorption temperatures between 410 and 551 K, (d) plot of In D vs. 1/T for state 111.
t = exposure time at specified temperature, N = number of Pt crystallites/cm2, r = radius of source, and c, = surface concentration within source area. In agreement with this model, the amount of spillover deuterium (represented by the area of state 111) increases as the t l I 2 at low coverages (Figure 9c). A plot of In D vs. 1 / T is linear in the region 410-551 K and yields an activation energy of 7.6 kcal/mol (Figure 9d). Nonlinearity of this relationship below about 300 K implies that the activation energy is dependent on the coverage of state 11. Likewise, In D determined from the population of state I1 in the region 140-190 K increases as the t 1 I z (Figure sa). This state is characterized by a diffusion activation energy of 5.9 kcal/mol (Figure 9b). Preexponential factors were estimated by assigning the following values for the constants: c, = 8.2 X l O I 4 atoms/cm2 on Pt,Z' r = (2.5-3.0) X cm, N = (7.9-13.7) X 1OloPt particles/cm'. Different Pt particle geometries (rafts, hemispheres, etc.), average particle diameters up to 6.0 nm, and values for c, up to a factor of 3 smaller were used in the calculation of kinetic parameters to account for uncertainties in the above constants. These changes do not alter activation energies but cause changes of up to 2 orders of magnitude in the preexponential factors. The calculated kinetic parameters are listed in Table I11 along with those reported in the literature for other catalyst systems. The population in state I11 was also measured in higher pressure (1 10 torr) experiments with no significant difference in the results. Reliable data for state I1 could not be obtained for these conditions because for dose pressures higher than 1 torr the sample temperature could not be reduced below 200 K. Figure 10 shows the effect of dose pressure on TPD peak area. In these experiments, the sample was exposed to D2 at 140 and 300 K for 100 s at various pressures ranging from 1.5 X to 5.5 X lo-' torr. (The right-hand panel extends the data of the left-hand panel to higher pressures.) At low pressures adsorption at 140 K has the shape of a Langmuir isotherm (Figure loa); (21) R. Herz, J. Kiela, and S. Marin, J . Curd., 73, 66 (1982).
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2769
Spillover of Deuterium on Pt/Ti02 TABLE I11 Comparison of Kinetic Parameters
diffusion state I1 state I11 E,, kcal/m preexp, cm2/s Ea, kcal/m preexp, cm2/s 5.9 f 2.0 5.0 X lo-" 7.6 2.5 1.9 x 10-13
system D2/Pt/Ti0," H2/TiOt H2/Pt/Al,O,' H2/Pt/Ag2Sd H,/Pt/AgzS'
*
28-29 15.5 21
2.3
X
desorption (state 111) E,, kcal/m 39 i 7 41.4 f 10.1 45
7 x 10-5
3.2 X lo-*
"This work. bReference 28 cReference 19. dReference 39. eReference 40.
0
0
2
0
4
0
6
0
8
~ 1
2
3
4
5
DOSE PRESSURE/(IO-' TORR)
Figure 10. Uptake curves (TPD area vs. pressure) for D2 adsorption on Pt/TiO,: (a) total area, 140 K doses: (b) total area, 300 K doses; (c) total spillover at 300 K (d) area of state I11 only, 300 K doses. Left half is an expanded version of the right half.
however, the amount adsorbed continues to increase slowly with pressure above 0.10 torr. Figure 1Oc represents the total amount in states I1 and I11 obtained by the same method used to obtain Figure 7. Several observations deserve mention. First, activated states are detected only above a threshold pressure of about 3 X torr. Second, Figure 1Oc plateaus between 8.0 X and 2.0 X lo-' torr but increases again above 2.0 X lo-' torr. This increase coincides with the increase in the area of state I11 (Figure lOd), indicating that there is a correlation between the filling of state I1 and the onset of population into state 111. In the pressure range 0.3-1.0 torr, the increase in state 111 is first order in D, pressure. The apparent desorption activation energy of state I11 can be estimated from the slope of a plot of In (Tm2//3) vs. 1/ T m ,where /3 = heating rate in K/s, yielding a value of 39 7 kcal/mol. Effect of Reduction. Reduction treatments of catalyst 1A modified the uptake of D2. After reduction at 700 K for 1 h, heating under vacuum at 800 K for 1 h, and exposure to D2 at 0.077 torr for 100 s at various sample temperatures between 140 and 550 K, there was a significant reduction in the population of both states I1 and I11 with respect to the data discussed above, while the population of state I was only slightly reduced. Exposure to O2at 2.2 X lo-' torr for 100 s at 300 K restores the catalyst to its original condition. (Several D2 doses at 300 K followed by TPD are necessary before repeatability is achieved, but TPD areas are within 10%of those for the catalyst before reduction treatments confirm restoration.) Exposure to H 2 0vapor at 300 K, followed by heating under vacuum to 800 K for 10 min, also restores the activity. This procedure was adopted as a means of restoring the catalyst whenever D2 experiments carried out at relatively high temperatures led to the loss of uptake capacity in either state I1 or 111.
*
Discussion Experimental Method. Most conventional methods of performing thermal desorption experiments with powdered samples do not employ a high-vacuum environment, but rather are conducted in a carrier medium.20 Recently, however, high- and
ultrahigh-vacuum conditions have been successfully ~ t i l i z e d . ~ ~ - ~ ~ The primary advantage of this method is the minimization of interparticle diffusion and readsorption effects.23Since the sample bed geometry determines the contribution of these phenomena,21 we employed as thin a bed as possible in this investigation (lo4 torr), a sharp peak with T , = 200 K superimposed on the state I peak grows in and is interpreted as a molecular state of D2 on Pt.,' The D2 uptake on Pt-free titania is at most one-tenth that of the Pt/Ti02 system and exhibits TPD spectra typical of weakly held molecular D2, consistent with earlier report^.^ Although a hydrogen desorption peak in the region 623-673 K has been observed on Ti02, it does not appear under our conditions probably because a significant number of surface OD groups were not formed.28 The strongest evidence of spillover is the appearance of a relatively high temperature TPD peak, state 111, on the supported Pt catalyst but on neither Pt nor the TiOz alone. This conclusion is remarkably similar to that found for Pt on Al2O3l9and for Rh on Ti02.29 Evidently, supported Pt particles function as a catalyst for the formation of chemisorbed D(a) on the oxide, and this is largely due to the ability of Pt to dissociatively chemisorb D2. Physisorption of D2 on titania can also lead to D(a) but this is significant only at relatively high pressures (>100 torr).5 States ZZ and ZZI. The observation of the thermally activated state (11), which overlaps that known to represent chemisorbed D(a) on Pt (state I), has no known analogue on alumina-supported catalyst^.^^,^^ Several adsorption models can be proposed to explain the existence of states I1 and 111. One model assigns states I1 and I11 to distinct spillover sites on anatase and rutile. The oxides used in this study are mixtures of the two and the TPD of hydrogen chemisorbed on pure anatase differs from that on rutile.28 This model does not fit our results for the following reason. Less hydrogen chemisorbs on rutile than on anatase but it desorbs with Under our conditions, for a a higher peak temperature (Tm).28 given dose pressure and dose time, state I11 always accommodates more deuterium than state 11, yet T , for state I1 is much lower than T , for state 111. Another model involves the migration of dissociated hydrogen from the metal into the bulk as well as onto the surface of the (22) M. Kiskinova, G. L. Griffin, and J. T. Yates, Jr., J. Catal., 71, 278
(1981).
(23) B. Halpern and J. E. Germain, J . Cutal., 37, 44 (1975). (24) K. Kawasaki, T. Kcdama, H. Miki, and T. Kioka, SurJ Sci., 64,349 (1977). \ - -
(2i) J. H. Craig, Jr., Appl. Surf. Sci., 103, 315 (1982). (26) J. J. Stephan, V. Ponec, and W. W. Sachtler, Surf. Sci., 47, 403 11975) ,--
. - I .
(27) M. Procop and J. Volter, Surf. Sci., 33, 69 (1972). (28) T. Iwaki, J. Chern. SOC.,Faraday Trans. 1, 79, 137 (1983). (29) K. W. Jozwiak and T. Paryjcak, J . Catal., 79, 196 (1983).
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The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
o ~ i d e . 2 ~Diffusion 3~~ into the bulk can be facilitated by bulk Ti3+.13 It is doubtful that states I1 and I11 can be accounted for in this way. The reversible formation of state I1 (by exposure to D, at 1300 K followed by TPD) correlates with the behavior of a Ti3+ ESR signal reported by Conesa and Soria9 when they exposed a reduced Rh/Ti02 catalyst to H2at room temperature and then evacuated the sample at room temperature. Within this framework, state I1 would be associated with the surface reduction of Ti4+to Ti3+ possibly through the formation of surface hydroxyl groups and state 111would be assigned to migration into the bulk. On the other oxides, however, the formation of hydroxyl groups by a spillover mechanism is accompanied by a hydrogen TPD state with a relatively high Tm19,29 in the general range observed for state 111. The direct interaction between deuterium and Ti3+31,32 is also unimportant since the amount of spillover in both states decreases after high-temperature reduction (after the sample is installed in the vacuum system). It is unlikely that state I1 involves a surface reaction" because it would have to be completely reversible in an adsorption/desorption cycle to account for the repeatability of the spectra at dose temperatures below 700 K. For example, a reaction between D(a) and OH to form water would deplete OH over repeated experiments and reduce the state I1 intensity. If, on the other hand, the reaction was simply isotope exchange, H2 and/or HD would appear in state 11, which was not observed. We believe that state I1 can be identified as a spillover site involving TiO, species associated with Pt.33,34 These sites can be located either on or at the edges of Pt parti~1es.I~ Pt adsorption sites are not completely blocked since state I is always observed and correlates well with desorption from polycrystalline Pt. When the chemisorption sites were blocked by preadsorbing CO, TPD after exposure to Dz gave a much smaller desorption peak with a lower T,. The data shown in Figure 10 indicate that the population of state I1 saturates before a significant population builds up in state 111. This is not inconsistent with either of the Pt/TiO, species. If state I1 functions as a precursor to state 111, it is probable that most of the TiO, species are located at the Pt/bulk TiO, interface. If state 11 is not a precursor to state 111, then isolated TiO, islands on Pt are attractive since the observed lack of isotope exchange between states I1 and I11 is readily e ~ p l a i n e d . ~At~ present, we favor the precursor model. The high-temperature TPD peak (state 111) has been observed by other investigator^'^*^^^^^-^^ and, as indicated above, we favor a mechanism involving spillover deuterium distributed among oxide sites at various distances from Pt c e n t e r ~ . ' At ~ high exposure temperatures, a slow falloff with exposure time in the rate of population of state I11 is observed for t > 2000 s, indicating saturation. The maximum amount of this spillover deuterium is about equal to that required to saturate the Pt centers. This corresponds to about 1 X 1014spillover sites/cm2 of catalyst, based on the above assumptions of Pt crystallite size. This represents about 3% of the exposed surface Ti02 units, or 0.08% of the total bulk TiO, units. This is not surprising since reduced TiO, is expected to have relatively few oxide sites for adsorption of spillover species. The measured diffusion activation energy for state I11 is much lower than that reported for Pt/A120319and Pt/Ag2S39*40 systems (30) M. Che, B. Canosa, and A. R. Gonzalez-Elipe, J . Chem. Soc., Faraday Trans. I , 78, 1043 (1982). (31) M. L. Knotek, Surf. Sci., 91, L17 (1980). (32) D. Behar and A. Samuni, Chem. Phys. Lett., 22, 105 (1973). (33) Y.-W. Chung, G. Xiong, and C.-C. Kao, J . Catal., 85, 237 (1984). (34) P. Meriaudeau, J. F. Dutel, M. Dufaux, and C. Naccache, Stud. Surf. Sci. Cutal., 11, 95 (1983). (35) D. D. Beck, A. Bawagan, and J. M. White, J . Phys. Chem., following paper in this issue. (36) P. C. Aben, H. van der Eijk, and J. M. Oelderik, Proc. Int. Dongr. Cutal. 5th, 1972, 1, 717 (1973). (37) W. Hongli, T. Sheng, X. Maosong, X. Guoxing, and G. Xiexian, "Metal-Support and Metal-Additive Effects in Catalysis", Vol. 1, B. Imelik et al., Eds., Elsevier, Amsterdam, 1982, p 19. (38) J. P. Candy, P. Foulloux, and M. Primet, Surf. Sci., 72, 167 (1978). (39) T. Fleish and R. Abermann, J . Cutal., 50, 268, (1977).
Beck and White (Table 111). In the present study, the activation energy for D diffusion, not H diffusion, in state I11 was measured. The desorption energy for H2 from state 111was 37 kcal/mol, lower than for D2 by only 2 kcal/mol. From an isotope study of H2/D2 adsorption on MgO, Ito et al.41found that molecular desorption energies for a dissociatively adsorbed state of H2 and D2 differed by
