J . Phys. Chem. 1985, 89, 5427-5433 that the homolysis rates of phenyl-substituted alkanes cannot be accurately estimated without taking strain energy into consideration. The relationship given by eq IV was also used to calculate enthalpy, entropy, and free energy differences from observed dissociation and recombination rate constants. These results are given in Table VI along with the values estimated for the hypothetical strain-free reactions. Also given are the differences between the two reactions for each of these quantities. On going from BD to DD the observed change at 573 K in AGc - AGCRwas -1 1.1 kcal/mol. This is 2.6 kcal/mol greater than that estimated for the hypothetical strain-free reactions. At this temperature the enthalpy contribution to this free energy discrepancy is a factor of 5 larger than the entropy contribution. Attributing this to strain implies that the central C-C bond in DD contains roughly 3 kcal/mol more strain energy than that in BD. Ruchardt et aL2 have presented evidence that an appreciable fraction (0.4) of such strain energy released in a homolysis reaction remains in the transition state. This implies a positive activation energy for recombination. The equilibrium data shown in Figure
5421
3 is consistent with a zero activation energy. However, the uncertainty in this data is large. It could be consistent with an activation energy as large as 2 kcal/mol. We are thus unable to exclude the possibility that some strain may exist in the transition state in DD. Previous work on bibenzyl diss~ciation'~ has shown that the Arrhenius parameters for the reaction are considerably higher in liquids than in the gas phase, even though actual rate constants to not differ much. Arrhenius A factors measured in the present work for BD and DD are not quite as large as that for the liquid-phase bibenzyl reaction but are nevertheless still factors of 5-9 larger than that for the bibenzyl gas-phase reactions. The precise origin of these differences is not clear, but as in our previous studies they may be at least partly due to a viscosity (temperature)-dependent cage effect. Acknowledgment. This research was supported by the Gas Research Institute. We thank Dr. M. Mautner for his critical analysis of this work. Registry No. DD, 632-50-8; BD, 1520-42-9; diphenylmethyl radical, 4471- 17-4.
Influence of the Hydrogen Uptake by the Support on Metal-Support Interactions in Catalysts. Comparison of the Rh/TiO, and Rh/SrTlO, Systems J. Sanz, J. M. Rojo, P. Malet,+ G. Munuera,+M. T. Blasco,t J. C. Conesa,*f and J. Soriat Instituto de Flsico-Qdmica Mineral. CSIC, Serrano I15 dpdo.. 28006 Madrid, Spain (Received: April 2, 1985)
Samples of Rh/Ti02 and Rh/SrTi03 catalysts subjected to thermal treatments under H2 and in vacuo have been examined by NMR, EPR, and quantitative adsorption techniques in order to ascertain the dependence of "SMSI effects" on hydrogen strongly adsorbed at high temperatures. Loss of H2 chemisorption capacity upon high-temperature reduction (monitored directly by NMR of metal-adsorbed hydrogen) occurs only in Rh/Ti02, and is accompanied by extensive incorporation of hydrogen in the support in form of hydride-type species. Both effects are reversed by high-temperature outgassing, while the amount of EPR-detected Ti3+increases. It is concluded that support-held hydrogen produces SMSI-type effects in Rh/Ti02 that cannot be explained by a conventional support reduction mechanism (generation of Ti3+ and anion vacancies) or by coverage of the metal with TiO, species migrating from the support. Electronic changes (rehybridization) induced in the small metal particles by interaction with highly reduced species generated at the support are proposed as a possible source of SMSI behavior that can be reversed by outgassing or oxidation more easily than coverage by TiO, entities.
Introduction Interest in the so-called strong metalsupport interaction (SMSI) in M / T i 0 2 systems ( M = Pt, Rh, Pd, or Ni) has been raised in recent years,',2 and though several hypotheses have been postulated3" to explain the origin of this effect, it remains still in doubt whether one or several of them will be able to account fully for the different phenomena observed. In previous works we have examined, with the aid of IR, NMR, and EPR techniques, the interaction of H2 with Rh/Ti02 and other similar systems.610 From these previous results we have concluded that at least three forms of hydrogen can be observed in Rh/Ti02 catalysts after reduction in H, a t T > 573 K. Thus, some of us6 observed in a previous work the reversible exchange of electrons between Rh (and Pt, Ru, and Pd) and the Ti02 support that generates Ti3+species, detected at 77 K by EPR. These Ti3+ species were in equilibrium with'a form of hydrogen weakly adsorbed on the metal (H,) and removable by pumping at 295 K. Further work with N M R and IR was able to show7,*
'
Departamento de Quimica General, Facultad de Quimica, Universidad de Sevilla, Sevilla, Spain. Instituto de Catilisis y Petrolecquimica, CSIC, Serrano 119, 28006 Madrid, Spain.
*
that spillover of protons to the support readily occurs under those mild conditions; at the same time 'H N M R allowed to differentiate this weakly adsorbed hydrogen from another form, H,, appearing as a 'H N M R line shifted to -120 ppm from the main line due to hydrogen on the T i 0 2 support. This second form of hydrogen, H,, bonded more tightly to the Rh particles, is retained by the (1) Tauster, S. J.; Fung, S. C.; Garten, L. R. J . Am. Chem. SOC.1978,100, 170. (2) See, e.g., papers 1-24 in: Imelik, B. et 11.
(3) (a) Meriaudeau, P.; Dutel, J.; Dufaux, M.; Naccache, C. In ref 2, p 95. (b) Santos, J.; Phillips, J.; Dumesic, J. A. J . Catal. 1983, 81, 147. (c) Resasco, D. E.; Haller, G. L. J . Catal. 1983, 82, 279. (4) (a) Short, D. N.; Mansour, A. N.; Cook Jr., J. W.; Sayers, D. E.; Katzer, J. R. J . Catal. 1983, 82, 299. (b) Belton, D. N.; Sun, Y. M.; White, J. M. J . Phys. Chem. 1984, 88, 1690. ( 5 ) Kelley, M. J.; Short, D. R.; Swartzfager, D. G. J . Mol. Catal. 1983, 20, 235. (6) Conesa, J. C.; Soria, J. J . Phys. Chem. 1982, 86, 1392. (7) Conesa, J. C.; Munuera, G.; MuAoz, A,; Rives, V.; Sanz, J.; Soria, J. Stud. Surf. Sei. Catai. 1983, 17, 149. (8) Conesa, J. C.; Malet, P.; Mufioz, A.; Munuera, G . ;Sainz, M. T.; Sanz, J.; Soria, J. Proc. 8th. Int. Congr. Catal., West Berlin, 1984, 1984, 5, 217. (9) Conesa, J. C.; Malet, P.; Munuera, G.; Sanz, J.; Soria, J. J . Phys. Chem. 1984, 88, 2986. (10) Sanz, J.; Rojo, J. M. J . Phys. Chem. 1985, 89, 4974.
I 0 1985 American Chemical Society 0022-3654/85/2089-5427$01.50/0 I
,
al. Stud. Surf. Sei. Cutal. 1982,
5428
The Journal of Physical Chemistry, Vol. 89, No. 25, 1 9'85
metal after outgassing at 295 K and corresponds to the chemisorbed H from which metal dispersions are routinely evaluated in volumetric experiments. A detailed analysis of the behavior of this shifted line in presence of gaseous hydrogen indicates that both H, and H, readily exchange even at 295 K, while simultaneous reversible electronic changes occur in the Rh particle^.^^'^ Finally we have showns that, at T > 423 K, the H, species was able to undergo spillover to the reduced TiO, support, where it may become stabilized at higher temperatures by interaction with existing Ti3+ions, probably being incorporated to nearby anionic vacancies, to give diamagnetic species (Le., (Ti-H)3') at the surface layers of the reduced support; thus, the metal would act as a "porthole" to allow the incorporation of hydrogen to the support as a hydride-like species. The aim of this work was to examine, following those previous results, the role of this third form of hydrogen (hydride-like species a t the Ti02surface) in the SMSI-type phenomena occurring at the Rh/Ti02 system. The loss of H2 chemisorption capacity, directly observable with N M R or detectable through volumetric adsorption measurements, will be taken here as a measure of the behavior to which the term SMSI is usually applied. Of course, other experimental manifestations of it (Le., changes in adsorption capacity for other molecules or in catalytic activity for several reactions) exist, which might behave differently if they were due to different specific features of the SMSI situation. As a reference, a Rh/SrTi03 sample was used.in our study. Since T i 0 2 and SrTi03 are n-type semiconductors of rather similar band gap (3.Ck3.2 eV)," and both Ti02-and SrTi03-supported Rh samples have received great attention in connection with their potential use for water cleavage under band gap i r r a d i a t i ~ n ' ~to - ' ~generate H,, a considerable amount of information has been accumulated in the past years1"* on the characteristics of these systems, which can be useful for our purposes.
Experimental Section The catalyst precursors were prepared by incipient wetness impregnation of T i 0 2 (Degussa P-25) and an "ex-oxalate" SrTi03 with an aqueous solution of RhC13, according to the method described elsewhere.' The resulting solids were air-dried at 383 K and then treated at 773 K under H, flow for 2 h and stored in air at 295 K. The Rh/Ti02 and Rh/SrTi03 specimens thus prepared had respectively SBET = 25 f 1 and 44 f 1 mz g-I, and metal loads of 2.5% and 2% by weight. As in previous work, the samples are designated H when in the reduced state, and HR when the passivated specimens, after storage in air, have not been reconditioned under H2. Characterization by XRD indicates that the Ti02support has suffered extensive rutilization (ca. 90%), while SrTiO, has a well-defined crystalline state. IR spectroscopy shows the presence of bands due to carbonate species on both samples. XPS spectra show incomplete reduction of rhodium in both H R samples (stored in air), where part of the metal remains in a Rh' state. Outgassing, thermal treatments, and adsorption of H2 and O2 (as supplied by SEO, reagent grade) at pressures 20-60 torr (unless otherwise stated) were performed in greaseless vacuum manifolds having residual pressures better than torr. Quantitative adsorption was determined on 1 g of sample in a 120 f 1 mL quartz cell by means of a MKS Baratron capacitance (11) Memming, R. Electrochim. Acta 1980, 25, 7 7 . (12) (a) Bulatov, A. V.; Khidekel, J. L. Izu. Akad. Nauk SSSR, Ser. Khim. 1976, 1902. (b) Bard, A. J. Science 1980, 207, 139. (c) Duonghong, D.; Borgarello, E.; Grltzel, M. J. Am. Chem. Soc. 1981, 103, 4685. (d) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83. (13) (a) Lehn, J. M.; Sauvage, J. P.; Ziessel, R. N o w . J . Chim. 1980, 4 , 623. (b) Carr, R. G.; Somorjai, G. A. Nature 1981, 290, 577. (14) Connolly, J. S., Ed. "Photochemical Conversion and Storage of Solar Energy"; Academic Press: New York, 198 1. (15) Grltzel, M., Ed. "Energy Resources through Photochemistry and Catalysis"; Academic Press, New York, 1983 and references therein. (16) Munuera, G.; Soria, J.; Conesa, J. C.; Sanz, J.; GonzBlez-Elipe, A. R.; Navlo, A.; LBpez-Molina, E. J.; Mufioz, A,; Fernlndez, A,; Espinbs, J. P. Stud. Surf. Sci. Catal. 1984, 19, 335. (17) Aspnes, D. E.; Heller, H. J . Phys. Chem. 1983, 87, 4919. (18) Hcrrmann, J.-M.; Pichat, P. Stud. Surf. Sci. Cafal. 1983, 17, 77.
Sanz et al. Rh/SrTi 03 13.
T ..............
T'
I
Figure 1. H2 adsorption/desorption at 295 K on Rh/TiO, and Rh/ SrTiO, samples. (A) H, dosing on samples reduced at 773 K and then titrated with oxygen at 295 K (expanded initial pressure = 8 torr); (B) evacuation and closure of the cell: (C) temperature programmed desorption run (with closed cell); (D) outgassing and new H, adsorption at 295 K.
gauge; a cold trap at 77 K was always used in order to condense water during the heating runs up to 773 K. Oxygen and hydrogen adsorptions and titrations (OA,HA or OT,HT) were measured at 295 K on both samples reduced in situ at 773 K and then outgassed for 2 h at the same temperature, leading to values of H/Rh = 0.14 f 0.03 for both samples, and OT/HA and HT/OA ratios close to 1.5 and 3.0, respectively. EPR (at 77 K) and N M R (at 295 K) spectra were recorded respectively with Bruker (Model ER 200 D, X band) and Bruker (Model SXP4/100, frequency 50-90 MHz) spectrometers, using tubular sample cells provided with greaseless stopcocks where the catalyst specimens could be subjected to treatments under controlled atmosphere prior to the recording of the spectra. Results Incorporation of H2to the Supports. A few experiments were carried out to detect the possible spillover of H, species from the metallic particles to the T i 0 2 and SrTi03 supports following the same procedure described elsewhere.* Figure 1 shows adsorption experiments carried out on both Rh/Ti02 and Rh/SrTi03 which had been reduced in H2 at 773 K, outgassed at the same temperature for 2 h, and then dosed with oxygen at 295 K (OA/Rh = 0.13 f 0.03 for both samples). If, after removal of the excess of oxygen, H2is introduced in the cell at 295 K, a strong adsorption is recorded (ca.7.0 torr in both cases) corresponding to the titration of the preadsorbed oxygen (H,/Rh N 0.40 f 0.03). When the excess of H2 is evacuated and the temperature raised up to 473 K for 1 h with the sample under closed vacuum, changes in the pressure could not be detected for Rh/Ti02 in these conditions, but a small increase (ca. 1 torr) was observed for Rh/SrTi03. Once the sample was cooled down to 295 K, a new dosing with hydrogen indicated that adsorption occurs on the Rh/Ti02 as expected for a clean Rh surface (HA/Rh = 0.14 f 0.02, Le., HT/.HA 3), suggesting that after heating at 473 K the Rh particles have lost the adsorbed H, hydrogen, which however was not recovered either in the gas phase or as H 2 0 in the cold trap. A small adsorption of H, was also observed under similar conditions on the Rh/SrTi03 sample (HA/Rh = 0.05) corresponding to about 30%of its monolayer capacity, but it roughly corresponds in this case to the amount of water found in the cold trap after the experiment. Incorporation of the H adsorbed on the metal to the TiOz support, suggested by the previous experiment, was clearly observed, Figure 2, by heating at 773 K in H2 a sample which had been previously reduced at 773 K and then outgassed for 2 h. When H2 was introduced in the cell, a sustained adsorption was measured, which fits a diffusion kinetics. A similar experiment with the Rh/SrTiO, sample only produced a small change in H2 pressure, but in this case H 2 0 was found in the cold trap indicating that further reduction of the SrTiO, support is slowly taking place without a net incorporation of hydrogen similar to that for TiO,. The different behavior of TiO, and SrTiO, supports toward hydrogen incorporation on them was clearly shown by 'H N M R spectra, Figure 3. While for Rh/SrTi03 the spectra, recorded
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5429
Metal-Support Interactions in Catalysts
1.967 I
\ I
0 t(min)
F i 2. H2adsorption at 773 K on Rh/Ti02 reduced at 773 K and then outgassed at 773 K during 2 h. Inset: Fit of experimental data to a diffusion kinetics.
Figure 4. EPR spectra of Rh/Ti02 sample reduced at the indicated increasing temperatures and then outgassed at 295 K (recording temperature 77 K). - A Ippml
A
h 9 -
d
H
-
H
Figure 3. 'H NMR spectra of Rh/SrTiOp (left) and Rh/Ti02 (right) samples reduced in H2at the indicated increasing temperatures and then outgassed at 295 K.
at 295 K in vacuo after reduction in H2 (60 torr) at increasing temperatures in the range 473-773 K, do not show changes in the intensity of the main line (line A) corresponding to hydrogen species on the ~ u p p o r t for , ~ the Rh/TiO, sample a considerable growth of this line occurs at T > 573 K. Meanwhile, it is worthy of note that the intensity of the shifted line (line B), corresponding to the H, species adsorbed on the Rh,9 remains unchanged in the Rh/SrTi03, while for Rh/TiO, the intensity of this line decreases, particularly after reduction at 773 K, duly reflecting the expected loss of chemisorption capacity. In both samples, however, the value of the shift of this line remains unchanged at ca. -140 and -120 ppm for Rh/SrTi03 and Rh/TiO,, respectively. Figure 4 shows several EPR spectra recorded at 77 K after pretreatments similar to those described above for the N M R experiments. Integration of the Ti3+ signal in the spectra shows that the intensity of the signal goes through a maximum for a reduction temperature close to 573 K, and then decreases for higher reduction temperatures in spite of the continuous incorporation of H2 into the TiO, support in these conditions. This latter process seems thus to operate against the possibility of detecting the reduced Ti ions formed. Similar experiments with a Rh/SrTi03 sample only allow to detect signals due to minor = 1.995, g, = 1.884 for the main line) which amounts of Ti3+ (g,, increase with the reduction temperature. The Rh/SrTi03 catalyst was also examined for reversible metalsupport electron transfer a t room temperature. Contrary to what was observed with Rh/Ti02,6 adsorption of H2 at 295 K on the passivated Rh/SrTi03 sample, before reduction at 773
I I
,
\O
I
I
100
I
I
200
I
I
300
1
... ,
'-L
I
LOO
P (torr) H2
Figure 5. Changes in the shift of 'H NMR line B with hydrogen pressure at 295 K for Rh/Ti02 and Rh/SrTi03 samples: (A)samples reduced at 773 K, exposed to air (HR-type),and outgassed at 523 K (0)samples reduced in situ at 773 K (H-type) and outgassed at 295 K; ( 0 )sample reduced in situ at 773 K, reoxidized in 40 torr of O2 at 573 K, and outgassed at 573 K.
K, does not lead to the generation of Ti3+species with intensity depending reversibly at 295 K on the H2 pressure. +Znteractionbetween H,,, and H,at R h Particles. Changes in the shift of line B in the N M R spectra in presence of H2in the gas phase had been previously observed by us for Rh/Ti02,8 and ascribed to the effect of chemical exchange between this hydrogen (H, form) and the weakly adsorbed species H, together with a modulation, by the latter, of the metal-H, interaction?JO Figure 5 shows a plot of the observed changes in the shift, A, against the H2 pressure in the cell. Data for Rh/SrTi03, either oxidized (exposed to the air) or reduced in situ at 773 K, fit on the same curve, showing a moderate change in the shift with increasing H, pressures. However, as shown in the same figure, for Rh/TiO; samples reduced at 773 K the value of the shift decreases much more markedly with increasing hydrogen pressure, while if the sample is reoxidized with 0,at 573 K a curve close to that observed for Rh/SrTi03 is found. Since changes in the size of the Rh particles could be discarded from H, adsorption measurements performed after this oxidation treatment, we must conclude that the different behavior of the Rh/TiOz sample in its reduced and oxidized state reflects in some way changes occurring at the TiO, support (and not at SrTi03) which in turn affect the rhodium-hydrogen in-
5430 The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
Sanz et al.
.
A
100 ppm
1
IOOoom
A
n
T,
= 2 95 K
A=-65
31 I
\
A
Figure 6. 'H NMR spectra of Rh/Ti02 samples reduced at 773 K (H-type) and outgassed during 2 h at the indicated increasing temperature T,. The spectra were recorded after adsorption of H2 ( 3 0 torr) at 295 K followed by evacuation of the gas phase at the same temperature. 0.1 0.2 0.3 0.4 0.5 I, ( a u ) V
TV-373K
A
W
EPR spectra of reduced Rh/TiO, samples outgassed at increasing temperatures for 2 h. The spectra were recorded at 77 K after treatment in H2 at 773 K followed by evacuation of the gas phase at temperature T,. Figure 7.
teraction. As we can see in the same figure, exposure of a reduced Rh/TiOz sample to the atmosphere for a long period of time leads to a new curve that suggests a partial reoxidation of the TiO, support in these conditions. Removal of Hydrogen from the TiO, Support. Since incorporation of H2 to the T i 0 2 support seems to be accompanied by a decrease in the amount of H, species adsorbed on the Rh particles, leading to the well-known SMSI behavior, experiments were performed to examine the effect of removal of this hydrogen from the support. Figure 6 shows 'H N M R spectra of a Rh/Ti02 sample which had been reduced in H2 at 773 K and then evacuated at 295, 573, and 773 K for 2 h. The spectra were recorded once a new adsorption of H2 (30 torr) had been carried out at 295 K, followed by further evacuation at the same temperature. As is clearly seen, removal of the hydrogen incorporated into the TiO, support (nonshifted line A) occurs with increasing outgassing temperatures, while the subsequent dosing with H2 at 295 K shows that such removal is accompanied by an increase in the intensity of line B corresponding to the H, species directly bonded to the Rh particles, thus suggesting a removal of the SMSI condition. EPR experiments carried out under similar conditions, Figure 7, indicate that outgassing removes the hydrogen directly responsible for the undetectability of the reduced Ti ions created in the T i 0 2 reduction at T > 573 K, since a signal due to Ti3+ species progressively appears now with increasing outgassing temperatures. To see whether a relationship could be established between the amount of hydrogen incorporated into the T i 0 2 support and the H, species in the Rh particles, a set of N M R experiments was carried out on a R h / T i 0 2 sample reduced at 773 K and subse-
Figure 8. Top: 'H NMR spectra of Rh/Ti02 sample reduced in H, at 773 K (H-type), outgassed at the indicated temperatures for 2 h, and dosed with H,. The spectra were recorded at 295 K under an H, equilibrium pressure of 30 torr. Bottom: plot relating the intensities of lines A and B in the spectra.
quently outgassed at several temperatures in the range 295-773 K for 2 h, by adsorbing H2 at 295 K and an equilibrium pressure of 30 torr after each outgassing stage. The spectra obtained in such conditions, together with the shift A measured for line B and a plot of the relationship between the intensities of both lines, are given in Figure 8. An inverse correlation can be appreciated between the intensities of the two lines, while the value measured for the shift of line B under 30 torr of H2 increases from ca. -65 to -95 ppm with the outgassing temperature. When these shifts are examined in connection with Figure 5 , it is observed that outgassing is restoring a situation which approaches that of the passivated Rh/TiOZ sample; this would correspond to a progressive loss of the SMSI behavior. Thus, we may conclude that either outgassing at high temperature or mild reoxidation, two processes that can remove strongly held hydrogen species from the reduced sample, produces a comparable effect on the capacity of this sample to adsorb hydrogen at 295 K on the Rh particles.
Discussion As mentioned above, a previous analysis9J0of the intensity and position dependence of N M R line B vs. H2 pressure in the Rh/TiO2 system indicated that two forms of hydrogen (H, and H,) directly adsorbed on the metal contribute to this shifted line. The more strongly adsorbed form, H,, that remains on the Rh particles after pumping at 295 K and shows (in evacuated conditions) an intrinsic shift of -120 ppm, and the weaker reversible H, form, with an intrinsic shift near zero, undergo chemical exchange. Upon adsorption of the H, species, changes in the interaction between the metal and the strongly adsorbed hydrogen H, were also detected that contribute to the observed decrease in the shift A of line B with the hydrogen pressure. A similar model can be applied (at least qualitatively) to the Rh/SrTi03 sample studied here as a reference, since changes with H2 pressures of similar characteristics are found in the N M R spectra; these are probably characteristic of the interaction between hydrogen and small Rh particles. These phenomena associated to the H, form could be related, in the case of the Rh/TiO, system, to the simultaneous reversible generation of Ti3+ centers via electron injection from the metal to the support observed by EPR6 and to the reversible electrical conductivity changes observed by Herrmann and Pichat.'* The different behavior found for Rh/SrTi03 in the EPR spectra, and in particular the lack of reversibly generated Ti3+species, suggests
The Journal of Physical Chemistry,Vol. 89,No.25, 1985 ,5431
Metal-Support Interactions in Catalysts that the electron-transfer phenomenon should be strongly dependent on the electronic structure of the support. In fact, this difference can be related with the development of an ohmic contact between Rh and T i 0 2 in Rh/TiOz systems, occurring according to Aspnes and Heller17 under a H, atmosphere, allowing thus a nonactivated transfer of electrons across the Rh/Ti02 interface. It was also shown by the same authors that a small Schottky barrier (ca. 0.2 mV) still remains in Rh/SrTi03 samples under similar conditions, probably due to the somewhat higher energy of the conduction band edge in SrTi03 as compared to rutile;" this may prevent hydrogen-induced electron transfer from the metal to the support in our Rh/SrTi03 system. Another important difference between the two reduced supports lies in the way in which H, reduction affects both the intensity of line B and the dependence of its shift on H2 pressure. The smaller intensity of this line found in highly reduced Rh/TiOz samples, after removal of the gas phase at 295 K, indicates a decrease in the amount of the hydrogen form H, chemisorbed on the metallic particles, and therefore the presence of SMSI whatever its origin. This decrease in the amount of H, species is accompanied by a more marked decrease of the value of the B-line shift with increasing Hz pressure as shown in Figure 5. This can be explained by taking into account the (because of SMSI) necessarily lower contribution of these species to line B in comparison with the number of H, species. On the basis of our previous results:JO an additional change in the influence of this latter H, species on the electronic characteristics of the Rh-H, bond (weakening of this bond) might need to be considered in order to explain the observed differences between the reduced and oxidized catalysts. None of these changes with the oxidation state of the sample were observed for Rh/SrTi03, so we must conclude that SMSI does not occur in this sample even after reduction at 773 K, though the number of Ti3+centers irreversibly formed at the S r T i 0 3 support does grow moderately in intensity with increasing reduction temperature, indicating that also in this oxide the Fermi level is being slowly but progressively raised during these reduction ~retreatment5.l~ On the contrary, in the case of Rh/Ti02, H2 reduction in the range 295-573 K first generates Ti3+centers in a mostly irreversible way (associated to oxygen vacancies), and this is followed at higher reduction temperatures by an important increase in the intensity of line A in the N M R spectra and a simultaneous decrease in the Ti3+ signal. Both facts are associated to the onset of an important reduction of the TiOz support. In its first stages (up to 573 K), reduction takes place according to '/2H2 Ti4+...02-
+ Rh,
-
+ Rh, ...H, -+
20H-
-
Rh,*-H,
Ti3+...OH-
02-+ V,
(1)
+ Rh,
+ H,O(g)
(2) (3)
so that progressive reduction originates an increase in the number of Ti3+species close to surface vacancies, Vo. This can be then followed by an additional and sustained incorporation of hydrogen to the support, detected by a growth of line A in N M R spectra; this hydrogen would become stabilized at the previously generated anionic vacancies, giving diamagnetic hydride-like species Ti3+.-V0
+ Rh, - H,
-
(Ti-H)3+
+ Rh,
(4)
and decreasing thus the number of Ti3+ centers detected by EPR at the highest temperatures. Preliminary calculations22using a model similar to that used by H o r ~ l e ybut , ~ ~considering one He (19) Contrarily to our results, SMSI effects have been reported in the patent literature for SrTi0,-supported catalysts.20 This may be due to the unintentional presence of TiO, in the system. It is known that the coverage of an inert support with near-monolayer amounts of TiOl induces SMSI behavior?' We found in fact that, when preparing the mixed oxide, incomplete reaction of the starting material led to the presence of small amounts of T i 0 2 (and SrC03) detectable with XRD. (20) Tauster, S. J.; Murrell, L. L.; Fung, S. C. U.S. Patent 4 149998, 1979. (21) KO, E. I.; Wagner, N. J. J . Chem. SOC., Chem. Commun. 1984, 52, 61.
(22) GonzBlez-Elipe, A. R., unpublished results.
atom at an oxygen vacancy around a five-coordinated Ti3+ ion, confirm the hydride character of such species. This hydrogen incorporation on TiO, is accompanied by a decrease in the capacity for H2 adsorpion on the metal (directly detected by N M R ) corresponding to the SMSI effect. It is worthy of note that the reduction temperature at which the decrease in hydrogen adsorption is observed does not coincide with the temperature giving the maximum intensity of Ti3+ signal in EPR (ca. 573 K), but with the higher temperatures at which diamagnetic (Ti-H)3+ species are being generated. It seems therefore that the mere presence of Ti3+species (Le., reduction of the T i 0 2 support), even associated with anion vacancies, is not enough to generate the observed SMSI-type effects and that species with a deeper degree of reduction (such as (Ti-H)3+) might be important to give rise to the SMSI behavior. This correspondbnce between the SMSI effects and the presence of hydrogen incorporated into the reduced Ti02 support is confirmed by the fact that removal of this hydrogen (by evacuation at increasing temperatures up to 773 K) is accompanied by a parallel increase in the capacity for H, adsorption at 295 K on the metal (as detected by NMR, figures 6 and 8), in spite of the increase in the number of Ti3+ centers detected in the outgassed samples by EPR. This increase in the number of detected Ti3+ ions would not be expected if the reason for the decrease of the EPR signal upon reduction in Hz at T > 573 K was the coupling of Ti3+ ions in pairs rather than reaction 4. The strong H2 uptake at T > 573 K on the reduced TiOz support, clearly detected by the increase in the intensity of line A in the N M R spectra, was absent in Rh/SrTi03, where even a slight decrease of this line could be observed, which is probably due to progressive reduction and dehydroxylation of the SrTi03 support (as detected by EPR, and preliminary IR results) according to reactions similar to eq 1-3. The lack of SMSI in this sample again suggests that Ti3+ and V, vacancies generated on the SrTi03 do not induce by themselves the SMSI behaviori at least in the deeper form that, in the case of Ti02, develops fully only when more reduced hydride-like species are incorporated to the support during reduction in the range 573-773 K. These species are strongly stabilized, so that exposure to the air at 295 K, even for long periods of time, does not produce their complete elimination, though they can react readily with water vapor at T > 373 K giving H2.24 The stability of such species is confirmed also by the work of Hongli et showing that a TPD run up to 773 K is unable to eliminate them completely. The mechanism by which the reduced T i 0 2 support can induce the SMSI behavior in groups 8-1040 metals is still a matter of great controversy. The extent of a hypothetical electron transfer from the reduced TiOz support to the metal, one of the hypotheses considered initially to explain the phenomena, has proved to be difficult to establish from XPS binding energies,26-28since this parameter is influenced by final-state (relaxation) as well as initial-state (electronic configuration) effects, both of them depending on particle size and shape as stressed by Huizinga et al.27 and Mason,28respectively. More recently, several a ~ t h o r s ~ ~ - ~ l have suggested, on the basis of data obtained with surface analysis techniques on singlecrystal or thin-film models and following some initial proposal^,^ that migration of reduced TiO, species to the surface of the metal overlayer occurs upon heating Rh and Pt deposited on TiO,, this "poisoning" effect decreasing the chem(23) Horsley, J. A. J . Am. Chem. SOC.1979, 101, 2870. (24) Duprez, D.; Miloudi, A. In ref 2, p 179. (25) Hongli, W.; Sheng, T.; Maosong, X.; Guoxing, X.; Xiexian, G. In ref 2, p 19. (26) Fung, S. C. J . Cutul. 1982, 76, 225. (27) Huizinga, T.; van't Blik, H. F. J.; Vis, J. C.; Prins, R. SurJ Sci. 1983, 135, 580. (28) Mason, M. G. Phys. Reo. E 1983, 827, 748. (29) (a) Chung, Y. W.; Xiong, G.;Kao, C. C. J. Cutul. 1984,85, 237. (b) Sadeghi, H. R.;Henrich, V. E. J. Curd. 1984, 87, 279. (c) Takatani, S.; Chung, Y. W. J . Curd. 1984, 90, 75. (d) Belton, D. N.; Sun, Y. M.; White, J. M. J . Am. Chem. SOC.1984, 106, 3059. (30) KO, C. S.; Gorte, R. J. J . Cutul. 1984, 90, 59. (31) Belton, D. N.; Sun, Y. M.; White, J. M. J . Phys. Chem. 1984, 88, 5 172.
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The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
isorption capacity of the metal for both H2and CO. Our results indicate that this hypothesis alone cannot explain all of the SMSI characteristics. KO and Gorte30 have recently shown using AES that the reversible diffusion of partially oxidized titanium species occurs through the bulk of clean platinum at high temperature, and that these species can be removed by flashing the sample in oxygen, but not by heating it under vacuum or hydrogen; this result is in contrast with our finding that outgassing at temperatures in the range 573-773 K removes (at least partially) the SMSI behavior, restoring the chemisorption capacity for H2 in our Rh/TiO, sample. On the other hand, in recent studies using AES, SSIMS, and TPD techniques, Belton et aL3’ have concluded that, in addition to TiO, migration to the metallic surface, an electronic interaction between metal and support must contribute to the SMSI effect on Pt/TiO, and other similar thin-film model systems. In fact, according to Mason,,* small supported metal particles may suffer changes in electronic structure if the support has electron levels that overlap the valence levels of the metal, owing to the mixing of the respective valence orbitals to produce chemical bonds and substantial changes in the hybridization of the atomic orbitals involved in the metal valence bonds. Hydride-like species formed at the reduced TiO, surface might be involved in this type of electronic interactions, since the work function of the TiO, support must be relatively close to that of the Rh in the reduced catalyst; this can be deduced from the existence of electronic exchange equilibrium (ohmic contact) in our Rh/TiOz sample in presence of H,, as would occur according to Aspnes and Heller” with all groups 8-10 metals on TiO,. Therefore, orbital mixing between the metal particle and the reduced support can take place; however, we can expect that the bonding orbital thus formed becomes located at the hydrogen atom in a substantial amount because of the stabilization of the electronic orbitals supported by the hydride-like species (due to its bonding with Ti), so that the net charge transfer to the Rh metal would be small and therefore undetectable by XPS. So, we should speak rather of a rehybridization within the metallic electronic structure, induced by its interaction with this support-stabilized hydrogen. Recent band structure calculations of metal hydrides3, and of adsorbed or subsurface hydrogen33have shown that the bonding interaction between the hydrogen orbitals and the electronic states of platinum-group metals leads to two new peaks in the density of states, one of them below the metal d-band (and therefore full) and the other one with part or all of its density of states above the Fermi level (thus empty), while at the same time a fraction of density of states is lost from the metal d-band, the net result being an electronic redistribution leading to a decreased number of holes at the top of the d-band, Le., a smaller density of states at the Fermi level. Since these are the states responsible for H, chemisorption on Pt-group metals, interaction of the small metal particle with these hydride centers could contribute, according to this model, to a decrease in the number of surface states available for chemisorption, Le., to a lower adsorption capacity, although these states would not be much different in nature from those existing in the normal metal. Also, with these considerations in mind, we cannot exclude that a certain amount of this support-held hydrogen could diffuse from the hydride-rich metal-support interface into the metallic particles. contributing to the electronic changes described above. The observed SMSI characteristics are in good agreement with these hypotheses. Thus, as pointed out previously,* our N M R results indicate that the nature of the Rh-H, interaction remains unchanged after reduction at 773 K (a constant shift at -120 ppm is found in the absence of gaseous H,), while the number of these species decreases; this is in agreement with the calorimetric results reported by Vannice and C h o for ~ Pd/TiO,. ~ ~ On the other hand, Baker et aL3’ have found that morphological changes of the metal (32) Switendick, A. C. 2. Phys. Chem. (Wiesbaden) 1979, 117, 89. (33) (a) Louie, S. G. Phys. Rev. Lett. 1979, 41, 476. (b) Muscat, J . P.; Newns, D. M. Surf. Sci. 1980, 99, 609. (34) Vannice, A.; Chou, P. J . Chem. Soc., Chem. Commun. 1984, 1590. ( 3 5 ) Baker, R. T. K.; Prestridge, E. B.; Murrell, L . L. J . Catal. 1983, 79. 348.
Sanz et al. particles, a phenomenon that usually accompanies SMSI,36only occur in the Ag/TiO, system after addition of Pt, a metal that, contrarily to Ag, is able to dissociate H 2 and thus to promote its incorporation to the support. Kelly et aLs observe a lower visibility by TEM of the metal (Pt) in the SMSI state, and suggest that in these conditions the system has some of the characteristics of Pt-Ti intermetallics, while the results of their surface composition measurements using ISS are not consistent with coverage of the metal with titanium oxide species, the changes upon sputtering being typical of experiences with adsorbed gases, a result that could be expected if hydrogen is present in those samples as observed in our case. Finally, we should mention that measurements made by Short et on highly dispersed Pt/TiO, catalysts using XANES and EXAFS techniques revealed after high-temperature reduction little or no structural changes in the coordination between Pt and Ti or 0 atoms, but a different shape in the platinum LIIand LIIl“white lines” suggesting an electronic rearrangement that should be described not as an electron transfer but as a rehybridization at the metal. Bearing in mind these ideas, as well as the known influence that the detailed electronic structure at the metal surface has on the characteristics of the metal-hydrogen bond, it may be suggested that the observed effects of the H, form on the N M R parameters of the shifted line and on the reversible generation of Ti3+might be explained also in terms of a rehybridization of the metal electronic states induced by the H, form. In such case, this rehybridization should be of a different kind from that responsible for the SMSI effects, since these latter are not accompanied by a change in the shift of the H, species (only in the intensity), and since furthermore the influence of H, on A is present also in the Rh/SrTi03 system, where no SMSI behavior has been found. It should be stressed, in any case, that the recent works using several surface techniques and thin-film models, which support the hypothesis of the formation of TiO, species on the metal surface, do not give information either on the primary reduction process (eq 1-3) or on the subsequent hydrogen incorporation to the support reported in this paper. In our view, once the strong interaction between the metal and the hydrogen-saturated TiO, surface described above is established (which may be accompanied by changes in the shape of the metallic particles), diffusion of TiO, and/or (not less likely) of H atoms through the (perhaps thin and “pillbox-shaped”) metal particles may occur to reach their surface; hydrogen would be easily desorbed by treatments in vacuo at 773 K, while TiO, (or Ti) would remain decorating the metal surface under these conditions, a fact that could lead to complete encapsulation of the metal and irreversible loss of chemisorption properties upon repeated reduction-oxydation cycles or by treatments under hydrogen above 850 K as reported by other authors. In order to avoid the need of invoking long-range interactions between reduced centers at the support and surface atoms of large metal particles (as must exist in our low dispersion catalysts), it could be suggested that, under SMSI conditions, Ti-H bonds appear not only on the titania support, but also within the highly reduced “TiO,” species (that should be described in such a case as “HTiO,”) decorating the metal particles, and that these are the hydride species playing a role in SMSI. The partial reversal of the SMSI effect upon outgassing would then have to be explained assuming that, because of the nature of the electronic interactions involved, the influence of such species on the metal is stronger than that of the non-hydrided TiO, species remaining after outgassing. Such possibility is not excluded by our results. These indicate, in any case, that Ti-H centers are indeed generated at the TiOz support; this is deduced especially from the EPR spectra (where any Ti3+belonging to the TiO, centers would be probably undetectable because of interactions with the metal conduction electrons) and from the extent of the hydrogen uptake shown in Figure 2. It should be noted, however, that the distance between the support and the surface atoms of the metal particles (36) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J . Catal. 1979, 59, 293.
J . Phys. Chem. 1985,89, 5433-5439 will be substantially reduced if these become flattened (the pillbox shape), as has been shown by TEM in several besides, quantum calculations suggest that, even when adsorption or poisoning modify the overall electronic density of the metal only at points very close to the sorbed species, the changes induced in the local density of states near the Fermi level (which are the main chemically active states) may extend across several atomic distance~.~* In may cases it will be difficult to decide experimentally whether the SMSI behavior is induced mainly by these TiO, species at the surface, as assumed by several a ~ t h o r s , ~ ~or ~ ’by~ ’hydrogen ~~~ at the metal-Ti02 interface or even in the bulk of the metallic particles. These different phenomena may play a different role in the different observable chemical properties associated usually with the SMSI behavior, i.e., changes in H2 and CO chemisorption, catalytic activity for hydrogenolysis or other reactions, etc. Because of this, it could happen that those several properties are changed to widely differing extents upon different catalyst pretreatments or modifications, if these lead to different degrees of occurrence of the several structural or electronic phenomena that are or may be currently associated with the SMSI state. We may recall, for example, that while lack of adsorption of both H2 and C O is characteristic of this situation, it has been shown that, in the case of platinum, alloying with titanium to give the Pt,Ti compound (a modification that might have features in common with high-temperature reduction of Pt/Ti02) affects very differently the capacity of chemisorption for both gases;39it is strongly (37) (a) Cairns, J. A,; Baglin, J. E. E.; Clark, G. J.; Ziegler, J. F. J. Caral. 1983, 83, 2301. (b) Kramer, R.; Zuegg, H. J . Card 1984, 85, 530. (c) Fleisch, T. H.: Hicks. R. F.; Bell, A. T. J . Cural. 1984.87, 398. (d) Tau, L. M.; Bennett, C. 0. J . Cutul. 1984, 89, 285. (38) Feibelman, P. J.; Hamann, D. R. Phys. Reu. Lett. 1984, 52, 61. (39) Bardi, U.; Somorjai, G. A.; Ross, P. N. J . Cafal. 1984, 85, 272.
5433
suppressed for H2 and only moderately weakened for CO, both effects being attributed to changes in the electronic structure of the metal. Let us point out also, on the other hand, that TiO, species diffusing to the metal surface might act in a similar way as proposed here for strongly held hydrogen, inducing rehybridization in the electronic structure of the metal particles (in addition to any geometrical poisoning effect), since such highly reduced entities (x is proposed to be close to 1,29cimplying for titanium a +2 formal redox state) may have filled orbitals energetically similar to those of hydride species and able to overlap the metal orbitals. A contribution to the SMSI effects might then arise from the electronic interaction of the metal with different types of reduced centers generated at the support, the common feature of them being the possibility of inducing in the metal orbitals a rehybridization leading to a change in the electronic states at the Fermi level that determine the sorptive and catalytic properties. In any case, the results reported in this paper, which are probably complementary to those obtained by other techniques, clearly suggest that incorporation of hydrogen to the Ti02support might induce by itself the SMSI behavior in our Rh/Ti02 sample. Acknowledgment. We thank the CAICYT, the Fundaci6n RAMON ARECES, and the Spain-US. Cooperation Treaty for financial support. Registry No. H1,1333-74-0;Rh, 7440-16-6;TiO,, 13463-67-7;SrEO3, 12060-59-2. (40) In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclaturecommittees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e&, 111 3 and 13.)
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Microcalorimetric and Fourier Transform Infrared Spectroscopic Studies of Methanol Adsorption on AI,O, Guido Busca,* Pier Francesco Rossi, Vincenzo Lorenzelli, Istituto di Chimica, Facolth di Ingegneria, Uniuersitd di Genoua, Fiera del Mare, Pad. D. 161 29 Genoua, Italy
Mohammed Benaissa, Josette Travert, and Jean-Claude Lavalley Laboratoire de Spectrochimie, Groupe “Structure et RPactiuitP d’Esp2ces AdsorbCes”, U.A. 41 4, I.S.M.Ra., UniuersitP, 14032 CAEN Cedex, France (Received: April 22, 1985)
Adsorption microcalorimetry and infrared measurements using CH30H, CD30H, and CHDzOH show the existence of at least three steps during the adsorption of methanol on alumina activated at 773 K. At very low coverages, molecules coordinated on the strongest Lewis sites are evident, which easily transform into bridged methoxide species. With increasing coverage, another form, irreversibly adsorbed at room temperature but desorbed by evacuation at 373-473 K, becomes predominant. It is identified as undissociated methanol strongly hydrogen bonded on a cation-anion couple having a strong basic character. Finally, at high coverages a reversible form hydrogen bonded to basic sites is detected.
Introduction The interaction of al,-&ols with oxide surfaces is relevant with respect to a number of heterogeneously catalyzed reactions such as dehydrogenation and dehydration. M ~ alcohols~ may also be regarded as suitable probe molecules in surface studies. ‘On leave from Istituto Chimico, Facoltl di Ingegneria, Universitl di Bologna. To whom all correspondence should be addressed at Genova University.
0022-3654/85/2089-5433$01.50/0
In the case of alumina, the interaction of alcohols produces several different adsorbed species; some of them have been clearly identified such as alkoxide group, which are P r o d u d bY alcohol molecules, ~ acidic~alcohols,’S2 ~ and chemisorbed ~ ~undissociated , (1) R. G. Greenler, J . Chem. Phys., 37, 2094 (1962). (2) J. Travert, 0. Saur, M. Benaissa, J. Lamotte, and J. C . Lavalley In ”Vibration at Surfaces”, R. Caudano, J. M. Gilles, and A. A. Lucas, Eds., Plenum Press, New York, 1982, p 333.
0 1985 American Chemical Society
