J . Phys. Chem. 1984, 88, 2771-2775 or reduction was not quantified, the catalyst is in a largely dehydroxylated and mildly reduced condition. The hydrogen desorption properties of anatase and rutile are known to be quite differenteZ8Thus, it is important to limit the catalyst temperature during TPD to avoid conversion to rutile. A more detailed study of the effects of reduction, hydroxyl groups, oxide forms, and other factors affecting the spillover may be worthwhile in light of the use of these systems to catalyze hydrogenation reactions.
Conclusion Thermal desorption spectroscopy has been used to study D2 adsorption on a Pt/TiO, catalyst. Two thermally activated TPD
Spillover of Deuterium on Pt/TIO,. Oxygen
2771
states were found, one of which increased as a function of dose pressure and duration. These states do not appear on either Pt or nonmetallized Ti02 and are assigned to spillover from Pt to the oxide. The present study is qualitatively in agreement with earlier reports on similar metal/metal oxide catalysts, with the exception that an additional state is observed and is thought to arise either from hydrogen adsorbed on TiO, particles on the Pt or from sites located at the Pt/support interface.
Acknowledgment. This work was supported in part by the Office of Naval Research and the Robert A. Welch Foundation. Registry No. Pt, 7440-06-4; TiOz, 13463-67-7; hydrogen, 1333-74-0; deuterium. 7782-39-0.
2. Sequential H,-D,
Exposure and Effects of
D. D. Beck, A. 0. Bawagan, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: November 23, 1983)
The adsorption and spillover of hydrogen on Pt/TiO, have been carried out by using temperature-programmed desorption (TPD) in an ultrahigh-vacuum system. Sequential dosing of hydrogen isotopes at temperatures between 200 and 500 K results in variable isotope exchange, with the desorption for H2and D2 reversed with respect to the dose sequence. This is interpreted in terms of spillover from Pt onto TiO, sites. High-temperature oxidation confirms the presence of an intermediate state in the spillover mechanism. Oxygen titration experiments, coupled with the desorption of sequentially dosed isotopes, indicate that desorption of spillover deuterium proceeds by a mechanism involving surface migration back to Pt sites rather than recombination and desorption directly from the oxide.
Introduction The migration of hydrogen from metal centers to supporting oxide surfaces is thought to play a role in reactions over oxidesupported metal catalysts and has been studied by a variety of techniques.' Evidence for hydrogen spillover has been found in M / T i 0 2 s y ~ t e r n s . ~ -It~ is generally proposed that oxide-bound hydrogen is removed by the reverse spillover to the metal, where association and desorption of molecular hydrogen takes place.6 Evidence for spillover hydrogen removal via a reverse spillover pathway has been shown with titration experiments on other catalyst systems' and with effects on the CO infrared spectrum on Pt/TiO,.j In previous work,8 three states of D2adsorption on reduced Pt/TiOz were found. One state (designated state I) corresponds to chemisorbed deuterium on Pt, whereas the other two states are thermally activated and have been attributed to spillover states on the oxide. Evidence indicates that one of the latter two (designated state 11) is probably a precursor to the other (designated state 111). On the basis of these results the following mechanism was proposed: (1) P. A. Sermon and G . C. Bond, Caral. Reu., 8, 21 1 (1973). (2) T. M. Apple and C. Dybowski, Surf. Sci., 121, 243 (1982). (3) T. Huizinga and R.Prins, J . Phys. Chem., 85, 2156 (1981). (4) J. C. Conesa and J. Soria, J . Phys. Chem., 86, 1392 (1983). ( 5 ) G. M. Pajonk, S. J. Teichner, and J. E. Germain, Eds., "Spillover of Adsorbed Species", Elsevier, Amsterdam, 1983. (6) T. Fleisch and R. Abermann, J . Catal., 50, 268 (1977). (7) P. A. Sermon and G. C. Bond, J. Chem. Soc., Faraday Trans. 1,72, 145 (1976). ( 8 ) D. D. Beck and J. M. White, J . Phys. Chem., preceding paper in this issue.
0022-3654/84/2088-2771$01 SO10
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where direct interaction of hydrogen with Ti02is slow. The degree of spillover depended upon a variety of factors including hydroxylation and reduction of the catalyst surface. A detailed discussion of the relative amounts of hydrogen on the metal and on the oxide was also given. In this paper we provide further evidence for spillover and address whether desorption occurs by means of reverse spillover. Attention is focused on state 111. The effects of substrate oxidation at 973 K are also discussed. Isotope scrambling experiments are detailed which provide strong evidence for spillover, as noted in a preliminary a c c ~ u n t . ~
Experimental Section The two-chamber apparatus and the procedures have been described in detail.* Anatase powders with Pt loadings of 0.6 and 5 wt % were used. Typically these were reduced in flowing H2 at 773 K for 6 h before mounting =lo0 Mg on a previously described tungsten loop. After installation in the preparation chamber,8 samples were outgassed at 300 K for 3 h and then at 800 K for 1 h. For those experiments involving a Pt filament of about 2.5-cmZsurface area, a cleaning procedure described by Norton and Richards'O was adopted. Contamination of the sample (9) D. D. Beck and J. M. White, J. Phys. Chem., 88, 174 (1984).
0 1984 American Chemical Society
Beck et al.
2772 The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
200
Mx)
600
Figure 2. Peak temperature (T,) of H2,HD, and D2vs. dose temperature for sequential isotope doses: (a) H2 dosed at 9.9 X IO-* torr for 100 s and then dosed with D2at 7.7 X torr for 100 s; (b) same as in a, except dose order is reversed; (c) same as in a, except experiments carried out on Pt filament. Note that ordinate scales for a and b are different.
200
300
400
500
600
700
(K) Figure 1. TPD spectra of sequential isotope doses on Pt/Ti02 (all D2 torr for 100 s, H2doses at 9.9 X torr for 100 s, doses at 7.7 X masses 2, 3, and 4 monitored): (a) H2 dosed at 300 K, followed by D2 at 300 K; (b and c) control doses of D2 and H2, respectively (100-s exposure, followed by 30-s evacuation, and then an additional 100-s exposure to the same gas); (d) spectra of a, except the intensity of the HD peak is added to the D2peak and to the H2 peak; (e) H2dosed at 501 K, followed by D2 at 300 K. T
was minimized by flash-heating just prior to exposure to adsorbate gas. After dosing, samples were transferred under vacuum to the ultrahigh-vacuum analysis chamber for temperature-programmed desorption (TPD) by mass spectrometry. The heating rate was 4 K/s. The measured ion signals were corrected for relative sensitivities. X-ray photoelectron spectra (XPS) were acquired with a Vacuum Generators Escalab 5 system. Pretreated samples were pressed into In foil. Binding energies were referenced to the C(1s) peak. Results Sequential Adsorption of H2 and D2 on Reduced PtlTiO? In one series of sequential dosing experiments, a reduced Pt/Ti02 sample was heated to a preselected dose temperature, Td,exposed first to H2at 9.9 X torr for 100 s, evacuated for 30 s, then exposed to D2 at 7.7 X torr for 100 s, and finally cooled to 140 K after evacuation at Td. In Figure la, the subsequent TPD spectra are shown for Td = 300 K. In this example, the peak temperatures of the three isotopic hydrogen molecules are different, with D2the lowest. The H2TPD area is relatively small compared to D2. This indicates significant, but by no means complete, displacement of the first adsorbate during evacuation and exposure to the second. If the dosing order is reversed, the H2desorbs first. The results of sequential dosing experiments performed for Td ranging from 140 to 600 K appear in Figure 2. The desorption peak temperature, T,, for H,, HD, and D2 are plotted as a function of Td. For dosing temperatures between 250 and 500 K, T , for H2 and D2 differ by 70-90 K, and T , for H D lies between them. For Td below 250 K and above 500 K, the T , (10) P. R. Norton and P. J. Richards, Surf. Sci., 41, 293 (1974).
values tend to converge. Different spillover states have been observed on this catalyst in these low- and high-temperature regions.s As compared to Figure 2a, the convergence in Figure 2b (reversed dosing sequence) appears incomplete at very low and very high temperatures. This is attributable in part to uncertainties in the analysis of the data. For example, at high temperatures the peaks are very broad, making peak temperatures somewhat uncertain. It is of interest to note that T , obtained from a TPD experiment in which only one adsorbate is employed coincides with T , obtained from the second adsorbate dosed in a sequential experiment. The single-dose experiments do reveal a small isotope effect; for the conditions of adsorption used, T , for H, is always 5-10 K lower than that for D2. An identical series of sequential dosing experiments was performed on a polycrystalline Pt filament for sample temperatures in the range 140-400 K. A plot of T , for Ha, HD, and D, vs. the dosing temperature is shown in Figure 2c. At most, the T , values differ by 10 K, and the difference in the H2 and D2 peak areas is no more than 30%. These data support the interpretation of the above results as occurring by a spillover process. Several control experiments were done. In one case the sample was exposed to either D2 or Hz for 100 s, followed by evacuation for 30 s, and then exposure to the same gas for an additional 100 s. The results at 300 K appear in Figure 1b for D2 and in Figure IC for H2. Figure Id shows the results of Figure l a replotted with one-half the instantaneous intensity of the HD peak added to both the H2and D2 peaks (if isotopic mixing did not occur, this result would be expected). The location and T , of the D2 peaks are nearly the same (Figure Id vs. Figure lb), but the H2 peaks, as expected, are very different (Figure Id vs. Figure IC). This occurs for all dose temperatures in the range 250-500 K. Thus, the differences in peak temperatures in Figure 2 are not simply the result of an isotope exchange effect. In another experiment, the sample was exposed to one gas at 500 K and then to the other at 300 K. An example, given in Figure le, has been previously d i s c ~ s s e d . ~Several observations are noteworthy: (1) There is no significant shift in either T , value from that measured in a single-dose experiment at the same Td (Le., 500 K for H2 and 300 K for D2). (2) The contribution from H D is very small compared to H, or D2, indicating very limited isotope scrambling. (3) There is no desorption of H2 induced by D2. The same facts pertain when the dosing sequence is reversed. However, when the sample is exposed to H 2 at 300 K and then to D2 at 140 K, there is extensive isotopic mixing. The peak
Spillover of Deuterium on Pt/Ti02 I
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The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2773 I
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Figure 4. Area of TPD spectra of D2adsorbed on Pt/TiO, at various dose temperatures, Td: (a) D2dosed on a heavily oxidized (973 K) form of Pt/TiO,; (b) area of D, TPD attributed to spillover on prereduced Pt/Ti0,.8
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TABLE I: XRD Results
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300
400
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Figure 3. TPD of D, adsorbed on Pt/TiO,: Dashed curves are for D, dosed for 100 s and 7.7 X torr on a TiO, sample prereduced in 1 atm of flowing H2 at 773 K for 6 h before impregnation (after impregnation and drying in air at 393 K for 6 h, sample was again reduced in 1 atm of flowing H,at 473 K for 2 h). This sample was oxidized at 973 K for 1 h in 1 atm of flowing O2 The solid curves are for doses of D, at 7.7 X lo-* torr for 100 s. The dose temperature is indicated on each curve.
temperatures for H2 and D, were still well separated, but 36% of all the hydrogen desorbed was HD. Activated Adsorption of D2 on Oxidized P t / T i 0 2 . Another series of experiments was conducted on a Pt/TiO, sample which was oxidized in flowing 0,(1 atm, 30 mL/min flow rate) at 973 K for 1 h, and then reduced at 473 K in flowing H2 (1 atm, 30 mL/min flow rate). This reduction removes chemisorbed oxygen from Pt but does not reduce the titania.3 This sample was exposed to D2 at 7.7 X lo-, torr for 100 s at various Td,ranging from 140 to 600 K. The resulting TPD spectra appear in Figure 3. For Td = 140 K, the spectrum (solid curve) has a different peak shape and a greatly reduced area (10-fold) relative to that obtained with a reduced sample (dashed curve) at 140 K. As Td increases, the resulting TPD area increases, and the desorption from the oxidized samples (solid curves) takes on a shape more like that observed in similar experiments with the reduced samples (dashed curves). However, unlike reduced Pt/TiO,,* for Td > 300 K, there is no distinct high-temperature state located at or near 650 K. The TPD area is plotted in Figure 4a as a function of the dosing temperature. Figure 4b shows the TPD area attributed to spillover on a reduced catalyst in which the oxidation step was omitted.* When a Pt filament was used, there was no evidence for activated chemisorption and, as expected, no artifacts that might be mistakenly attributed to spillover in the Pt/Ti02 experiments. As discussed below, the similar trends of Figure 4, a and b, below 400 K are significant. In Table I, X-ray diffraction measurements are compared. As a result of high-temperature oxidation some sintering has occurred, and there is partial conversion of the oxide to rutile. In Pt/Ti02 samples oxidized above 823 K, the D2 TPD spectral areas illustrated in Figure 4a were not altered by repeated dosing in D2 or
sample Pt/Ti02 Pt/TiO,, oxidized
composition, wt % anatase rutile Pt 71 28 0.6 17 82 0.6
mean particle diameter, nm Ti02 Pt 18.5 2.5-3.0 29.0 26.0
TABLE 11: XPS Binding Energies
BE: eV sample Ti ( 2 ~ Pt ~ (4f712) ~ ~ ) 0 (1s) Ti026 458.5 529.7 0.6% Pt/TiO? 458.5 70.1 529.7 0.6% Pt/Ti02C 458.5 d 529.7 5.0% Pt/TiO,b 458.3 70.6 529.8 5.0% Pt/TiOZc 458.5 71.6, 529.8, 73.8 530.7
C (1s) 284.6 284.6 284.6 284.6 284.6
"Mg anode, 20-eV pass energy. bPrereduced. COxidizedin 1 atm of flowing 0, at 973 K for 1 h. dPeaks not resolved.
H2 at temperatures 5600 K. Oxidation treatments at temperatures below 823 K produce similar results, but repeated treatment with D, or H, above 300 K gradually restores the hydrogen uptake capability to that of the reduced sample (see Figure 3 and ref 8). Both the oxidized and reduced 0.6 wt % Pt catalysts were examined with XPS (Table 11). The major difference between the two is observed in the Pt(4f) peaks which are relatively well resolved for the reduced but not for the oxidized sample, where only a sloping base line is evident. The two forms with a 5 wt % metal loading were also examined. In this case, the oxidized sample exhibits two major Pt(4f) peaks separated by about 2.2 eV (Table 11),while the O(1s) peak has a shoulder located at about 1 eV higher binding energy. High-temperature (973 K) treatments with He instead of 0, did not produce these additional peaks. Titration of O2 with Spillover D2. To determine whether desorption takes place by means of a reverse spillover mechanism, or simply by recombination and molecular desorption from the oxide, deuterium, initially present only on the oxide, was used to titrate oxygen chemisorbed only on the Pt to form water. In a series of experiments, the sample was exposed to D2 at 7.7 X torr for 100 s at dose temperatures ranging from 300 to 500 K, and then to 0, at 2.2 X lo-' for 100 s at temperatures ranging from 140 to 500 K. TPD spectra for the low- and high-temperature extremes appear in Figure 5a-d. Unlike Pt or TiO, alone (see below), a high-temperature D 2 0 peak is present with T , ranging between 550 and 700 K. The area of the high-temperature D 2 0 peak increases strongly with O2 dosing
2114
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 I
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Beck et al.
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