J . Phys. Chem. 1990, 94, 5059-5062
J A cannot be positive. On a physical point of view that means that we have a t least two different type of interactions! The surprising fact which comes out of our analysis is that the competition between these interactions leads to this remarkable linear behavior of the effective coupling constant J which is reproduced in Figure 4. The detailed microscopic description of this last property would still require some more studies. At this stage, using a simple three-parameter model (a2,c, and d), we are nevertheless able to describe with a very good accuracy the behavior of the interfacial tensions between water and alcohols. The geometric meaning of a2 is not clear at all for the moment. It could be the cross section of a molecule of alcohol. The corresponding values of a2,calculated from Table I, show, however, a decreasing behavior with the length of the carbon chain which
5059
does not support this naive geometrical interpretation. From the value of the energetic parameter d , we deduce the order of magnitude of the energy of interaction per mole: 2 kcal/mol. This value, which corresponds to a low hydrogen bond, accounts for interactive energy between water and alcohol across the interface. As a final remark, let us point out an intriguing property revealed by our analysis. The temperature of the maxima of T ( T ) which can be determined from the experimental data (cf. ref 4) can be reduced by some energetic parameter a2 ( - C ) I / ~ (P= k T / a 2 (-c)Il2). It turns out that for the eight alcohols studied here (from C4 to C I 2 ) P of the maxima is independent of the nature of the alcohol within a few percent as can be seen in Table 1.
Thermal Behavior of a Rh/AI2O3 Model Catalyst: Disappearance of Surface Rh upon Heating Jingguang G.Chen,+ M. Luigi Colaianni, Peijun J. Chen, John T. Yates, Jr.,* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
and Galen B. Fisher Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090- 9055 (Receiced: Nouember 3, 1989)
The thermal behavior of a Rh/AI,O, model catalyst has been investigated under ultrahigh-vacuum conditions by Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (EELS), and carbon monoxide capacity using thermal desorption spectrometry (TDS). Our results indicate that, upon heating the Rh/AI,O, to 1100 K (825 "C), the capacity for CO chemisorption is completely lost as Rh is observed to move from the top surface layer, either by the diffusion of Rh into the bulk A1203or via covering by a thin A1203film, while remaining in the near surface region. The formation of large clusters of rhodium upon heating is not consistent with our data. These findings appear to relate to the behavior of high surface area supported Rh/AI20, catalysts heated to high temperature.
I. Introduction A major use of the precious noble metals platinum and rhodium is in automobile emission control cata1ysts.l Because of their high cost and limited supply, the maintenance of a high active surface area for the supported noble metals is desirable. This is particularly true for rhodium which is the principal catalytic component ~ , ~ is also effective for catalyzing C O oxifor NO r e d ~ c t i o nand d a t i ~ n especially ,~ during catalyst w a r m - ~ p . ~ The most pronounced deactivation of Rh-containing catalysts occurs a t temperatures 1875 K (>600 "C), particularly in oxidizing environments.l Despite recent progress, a detailed understanding of the deactivation mechanisms is still emerging. Mechanisms which have been suggested include the formation of a rhodium oxide which is difficult to reducesf' and the formation of Rh particles with atypical morphologies (rafts or platelets).' Several studies, performed in oxidizing environments above 875 K, have proposed that Rh interacts strongly with y-A1203,even to the extent of forming compounds with the alumina or diffusing below its surface.8-'0 Catalysts treated this way are difficult to reduce and to reactivate even when reduced near 875 More recent studies using techniques including infrared spectroscopy," temperature-programmed reaction," in situ transmission electron microscopy," and in situ dispersionI2 and reactivity"Xi3 measurements suggest that in high-temperature oxidizing environments rhodium diffuses into y-alumina or changes to some form that is very difficult to reduce. However, the original
K.89'0-'3
'Current address: Exxon Research and Engineering Co., Route 22, Annandale, NJ 08801,
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amount of rhodium can again be made accessible by a hightemperature reduction in hydrogen for an hour a t 1075 K (800 "C) followed by a moderate oxidation for an hour at 775 K (500 "C)." Hence, although rhodium supported on high surface area alumina diffuses from the surface and/or changes its chemical state to become unreactive under high-temperature oxidizing conditions, these effects are not so severe that rhodium cannot be made available again at the surface. In this paper, we report a new experimental approach for studying the thermal behavior of a model Rh/AI20, catalyst; both the fully oxidized AI2O3films and the Rh overlayer (submonolayer coverages) were prepared by in situ deposition and ultrahighvacuum (UHV) surface techniques. Most importantly, the ( 1 ) Taylor, K. C. Automobile Catalytic Converters; Springer-Verlag: New York, 1984. (2) Schlatter, J. C.; Taylor, K. C. J . Catal. 1977, 49, 42. (3) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J . Coral. 1986. 100. 360. (4) Banerle, G. L.; Service, G.R.; Nobe, K . Ind. Eng. Chem. Prod. Res.
~~.
Deo. 1972. 11. 54. ~
~~
~i
~
( 5 ) Fiedorow, R. M. J., Chahar, B. S.; Wanke, S. E. J . Card. 1978, 51, 193.
(6) Kiss, J. T.; Gonzalez, R. D. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 216. (7) Yates, D. J. C.; Prestridge, E. B. J . Coral. 1987, 106, 549. (8) Yao, H. C.; Japar, S.; Shelef, M. J . Coral. 1977, 50, 407. (9) Yao, H. C.; Stepien, H. K.; Gandhi, H. S. J . Carol. 1980, 61, 547. (IO) Duprez, D.; Delahay, G.; Abderrahim, H.; Grimblot, J. J . Chim. Phys. 1986.83, 465. (11) Wong, C . ; McCabe, R. W. J . Catal. 1989, 119, 47. (12) Beck, D. D.; Carr, C. J. Submitted to J . Phys. Chem. (13) Beck, D. D.; Carr, C. J. Submitted to J . Catal.
0 1990 American Chemical Society
5060 The Journal of Physical Chemistry, Voi. 94, No. 12, 1990
Rh/AI2O3 films were prepared on a refractory metal substrate, Mo( 1 lo), allowing the film to be heated over a wide temperature range. Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (EELS), and thermal desorption spectroscopy (TDS) for investigation of CO chemisorption were applied in these studies. This approach allows us to investigate the fundamental properties of this Rh/AI2O3model catalyst under well-controlled experimental conditions. However, it is not presently clear how model AI2O3 layers made by the oxidation of metallic aluminum compare to A1203conventionally used for the support of metal catalyst particles. Early EELS studies of Rh/AI2O3 which did not involve the investigation of the thermal stability of Rh films on A1203were originally carried out by Dubois et In our study, we have observed that Rh deposited onto A1203 at 120 K loses its CO chemisorption capacity upon heating the Rh/AI2O3 layer under vacuum to 1000 K (725 “C). This can be attributed to either the diffusion of R h into the near surface region of the Alz03 bulk, reaction of Rh with Alz03, or the covering of the Rh by a thin film of the A1203 substrate. The possibility of aggregation of Rh to form large Rh clusters upon heating to 1100 K is excluded on the basis of the combined AES, EELS, and TDS results. 11. Experimental Methods The experiments reported here were performed in a stainless mbar) equipped steel UHV chamber (base pressure
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.-E
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.-ee
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(a) 120 K (2000 2500
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li
100 300 500 700 900 110(
Energy L o s s (cm-l) Temperature M) Figure 3. EELS studies of CO adsorption on Rh/AI20, after heating to increasing temperatures. A saturation exposure of CO was introduced to the surface between 120 and 150 K after each heating.
c
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Thermol Desorption Studies o f C O / R h / A 1 ~ 0 ~ / M o ( 1 1 0 ) T.al=l20K
' dRh- 1 2 b,
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-
(c) CO on CRh/Al~O~/Ma(I10)
Figure 2. EELS studies using chemisorbed CO as a sensor of the thermal behavior of Rh/AI,O,.
is a bulk AI2O3 film, it differs from high surface area 7-A1203 in its lack of hydroxyl groups, and its crystal structure is not definitively known. The main observation regarding the thermal behavior of a Rh film on the Al2O3surface is shown in Figure 2b,c: A new feature at 2065 cm-' (cross hatched), readily assigned as the u(C0) mode of CO on Rh, is observed when the Rh/AI2O3 layer is exposed to a saturation exposure of C O molecules (Figure 2b). This corresponds with the frequency of terminally bound C O on Rh(1 1 However, the EELS spectrum recorded after momentarily heating this Rh/AI2O3 layer to I100 K followed by a similar saturation exposure to C O at 120 K reveals no v ( C 0 ) vibrational feature. This indicates that Rh following this heating treatment is no longer able to chemisorb C O (Figure 2c). In fact, Figure 2c is almost identical with the EELS spectrum of A1203/Mo(l IO) recorded prior to the Rh deposition (Figure 2a). A more detailed EELS study of the CO adsorption on Rh/ AI2O3layers heated to various temperatures is presented in Figure 3. The Rh/AI2O3layer was heated (dT/dt = 4 K/s) momentarily to the indicated temperature. Following heating, the layer was exposed to a saturation CO exposure at a surface temperature between 120 and 150 K. The intensity of the v ( C 0 ) feature at saturation C O coverage decreases almost linearly in the temperature range of 120-900 K; no u(C0) feature is observed after the Rh/AI2O3 layer is heated to 11000 K. The loss of C O adsorption capability by Rh upon heating is further supported by the thermal desorption studies, as shown in Figure 4. The C O desorption feature observed at T 150 K is related to the desorption of C O from the W heating leads which support the sample and may be disregarded. As expected, C O does not adsorb on A1203at 120 K, as indicated by the absence of any C O desorption feature in Figure 4a. When a C O / R h / AI2O3/Mo(110) layer, prepared at 120 K, is heated (Figure 4b), a very broad C O desorption feature is observed in the temperature range of -260-550 K ; this C O desorption state is very similar to that observed for a CO/Rh( 1 1 1) layer.24s25 This agrees with l).22323
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-
(22) Crowell, J. E.; Somorjai, G. A. Appl. Surf. Sci. 1984, 19, 73. ( 2 3 ) Root, T. W.; Fisher, G. B.; Schmidt, L. D. J . Chem. Phys. 1986.85, 4687.
a
IIOOKI
(b) CO on Rh/AI2O3/Mo(110)
(a) C O on clean A I ~ O ~ / M o ( I l O )
100 200 300
400
500
600
700
800
Temperature (K) Figure 4. Thermal desorption measurements monitoring CO desorption from (a) a thin film of Al2O3.(b) a Rh/AI20, layer, and (c) the layer in (b) after heating to 1100 K, cooling below 150 K, and resaturating with CO. Spectrum b shows the CO/Rh( 11 1) TPD spectrum from ref 24 (dashed line). studies by Altman and Gorte26which have shown that similar TPD spectra are obtained from C O desorbing from rhodium single crystals, foils, films and supported rhodium clusters ranging in size from 26 to 47 A. Again, upon momentarily heating the Rh/AI20, layer to 1100 K and then adsorbing CO, Rh is seen to lose its ability to chemisorb C O as indicated by the absence of a CO desorption feature in Figure 4c. B. Models for Loss of Rh Chemisorption Capacity. From the results presented above, it is suggested from the EELS and TDS measurements that the deposited Rh disappears from the surface after heating the Rh/AI2O3 layer to 1100 K, whereas the Auger results suggest that about 70-85% of Rh still remains within the Auger sampling depth for the Rh (302 eV) transition. One can speculate that there are three possible Rh deactivation processes involving heating the Rh film to high temperatures: (A) aggregation of Rh on the A1203film to form large clusters, (B) reaction of Rh with AI2O3,and (C) diffusion of Rh into the AI2O3surface or covering of Rh by a thin AI2O3 film. 1 . Model A: Aggregation o f R h . On the basis of the combined AES, EELS, and TDS results, one can exclude the possibility of aggregation of Rh at 1100 K. We have made a rough estimation of the consequences of a process in which a uniform single monolayer of Rh a t 120 K converts by surface migration and sintering to large hemispheres upon heating under vacuum to 1100 (24) Thiel, P. A.; Williams, E. D.; Yates, Jr., J. T.; Weinberg, W. H. SurJ Sci. 1979, 84, 54.
(25) Root, T. W.; Schmidt, L. D.; Fisher, G. B. SurJ Sci. 1985. 150. 173. (26) Altman, E. 1.; Gorte, R. J. Surf. Sci. 1988, 195, 392
5062
J . Phys. Chem. 1990, 94, 5062-5068
K. Both EELS and TDS results show the absence of C O adsorption after the Rh/AI2O3 layer is heated to 1100 K. Assuming the detection limit of both EELS and TDS techniques for CO adsorption to be approximately 1% of a monolayer and that the CO is adsorbed on identical Rh hemispheres' surfaces exposing a total surface area equivalent to 1% of the original Rh monolayer, the average radius of the Rh hemispheres would be -SO0 A. However, if this Rh aggregation process were to occur on the surface of the A1203 substrate, we estimate that the intensity of the Rh (302 eV) Auger feature would be at most 6% of that measured on the original dispersed Rh layer deposited at 120 K. This predicted loss of Rh Auger intensity is due to the selfscreening of the Rh Auger signal from inside of the hemispheres by the outer layers of the hemispheres. This is clearly not the case since 10 separate Auger measurements on Rh/AI,O, layers of 6-55 A indicate that the intensity of the Rh (302 with dAJZo3 eV) Auger feature upon heating to 1100 K is about 70-85% of that measured initially at 120 K. While we can clearly rule out the aggregation of Rh into large particles, the increase in the Mo Auger signal above -700 K suggests that the Mo is less well screened after heating. This may be due to some tendency of the Rh or the Al,O, to coalesce on heating. The tendency for R h to coalesce has been observed in transmission electron microscopy work on model Rh/AI,O, catalysts.' 2. Model B: Reaction of Rh with A1203. The physical and chemical measurements employed in this work are not especially sensitive to a chemical reaction between Rh and A1203at elevated temperatures. The major experimental observation was that no significant change in the energy or line shape of the Rh Auger line occurred during this study. Studies made on AIo-rich A120316 indicate that effects identical with those reported here are also observed in the nonstoichiometric aluminum oxide. Thus, excess AIo in A1203 is not likely to be the cause of the deactivation of Rh. Conversely, since excess oxygen is unlikely to be present in AlO-rich A1203, the involvement of unreacted oxygen in stoichiometric A1203may also be ruled out as an explanation for the Rh deactivation upon heating. The reaction of Rh with A1,0, to form rhodium aluminate cannot be ruled out, except that extensive discussions and searches of the literature have not revealed any evidence that a rhodium aluminate exists.,' 3. Model C: Diffusion of Rh into A1203. Finally, the extensive loss of CO chemisorption combined with the small fractional
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(27) Beck, D. D.; McCabe, R. W.; Wong, C . Private communication.
decrease in Auger intensity of the deposited Rh following heating suggests that most of the Rh remains in the sampling region of Auger spectroscopy, i.e., within a depth of 10-20 A in the AI2O3 surface. Penetration or diffusion of Rh into the near surface region of the A1,03 (or alternatively migration of A1203onto the surface of Rh particles) would be expected to poison the Rh for CO chemisorption, while permitting the Rh to remain near the A1203 surface. The fact that there is no correlation of the fractional decrease in Rh Auger intensity (upon heating to 1100 K) with the thickness of the A1203 film strongly supports the idea that Rh remains in the near surface region of the A1203. If Rh diffusion throughout the Al,O, were occurring, a larger fractional decrease in intensity would be expected on the thick A1203 film, and this is not observed over a 10-fold thickness range for the A1203. This model is also consistent with observations on high surface area Rh/AI,O,, where Rh reactivity is lost a t high temperature^."-'^ C. Hydrogen Activation Experiments. In the Introduction the partial reactivation of rhodium via high-temperature reduction in hydrogen is d e s ~ r i b e d . In ~ .an ~ ~attempt ~ ~ to restore the CO adsorption capability of a preheated Rh/AI20, model catalyst, hydrogen from the doser was directed onto the Rh/AI2O3interface s-] while it was held at 1100 K. A flux of 1.1 X I O i 5 H2 was maintained for 600 s. After terminating the H,flow and then cooling, it was found by EELS that CO could not be readsorbed at 1 SO K. This observation, along with the absence of a change in the Auger line shape, supports our view that rhodium is not undergoing oxidation from the support or from residual oxygen. This behavior is also consistent with the difficulty in reactivating supported Rh/AI,O, catalysts.8,10-13 IV. Conclusions I n summary, our results indicate that, upon heating a Rh/AI,O, model catalyst to 1100 K, Rh loses its capability to chemisorb CO. We conclude that diffusion of rhodium into the A1203near surface region or diffusion of an A1203 film onto the R h film surface under vacuum conditions is likely to be the cause of this effect. This observation provides useful information for the understanding of the structural properties and catalytic reactivity of rhodium-alumina catalysts.
Acknowledgment. W e gratefully acknowledge the financial support of this work by the General Motors Corp. We also thank Chor Wong and Don Beck of General Motors Research Laboratories for their careful reading of the manuscript. Registry No. Rh, 7440-16-6; CO, 630-08-0.
Micetlar Effects upon Rates of S,2 Reactions of Chloride Ion. 1. Effects of Variations in the Hydrophobic Tails Radu Bacaloglu,*~l"Andrei Blaskb, Clifford A. Bunton,* Giorgio Cerichelli,lband Francisco Ortegalc Department of Chemistry, Uniuersity of California, Santa Barbara, California 931 06 (Received: November 3, 19891
Interactions of 35CI-with trimethylammonium surfactants increase with increasing length of the hydrophobic alkyl group on the basis of NMR line widths. The dependence of line width upon [surfactant] and [NaCI] can be fitted by a Langmuir isotherm, and for alkyl = CBH17-C18H37 the binding parameters, K b l , agree reasonably well with those estimated kinetically from rate constants of the reaction of CI- with methyl naphthalene-2-sulfonate. The second-order rate constants for reaction in the micellar pseudophase decrease modestly with decreasing length of the alkyl group and are similar to those in water. For C8H17NMe3CIthere appear to be interactions of CI- with both micelles and monomeric surfactant, or small clusters of it. This nonmicellar interaction is also kinetically significant with C,H,,NMe3CI and Me4NCI, but interactions with substrate and CI- are weak.
Aqueous micelles speed bimolecular reactions by bringing reactants together or inhibit them by keeping reactants a p a r t 2
The rate enhancements can be treated quantitatively by estimating reactant concentrations at the micellar ~ u r f a c e and , ~ calculated
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