1694
J. Phys. Chem. 1991,95, 1694-1698
High-Temperature Behavior of Rh/A1,03 Catalysts Todd H. Ballinger and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: April 9, 1990; In Final Form: August 13, 1990)
The behavior of highly dispersed Rh/AI2O3catalysts (0.15% Rh) has been investigated at extreme temperatures up to 1400 K under vacuum and under CO(g) pressure. Infrared spectroscopicstudies have shown that the removal by heating of isolated AI-OH groups on the support, beginning near 400-600 K, decreases the tendency of the Rh; species to form Rh' species (as Rh1(CO)2)following CO adsorption. This is in agreement with the work of others where isolated AI-OH groups have been shown to be necessary for oxidation of supported Rh; species in the presence of CO. Heating under vacuum or under CO(g) yields similar behavior, although additional degradation of Rh," sites for CO chemisorption occurs under CO(g), possibly due to carbon deposition. Heating to 1400 K causes the conversion of y-Al,O, to a-A1203and results in the complete loss of CO chemisorption capability for the Rh," species as judged by IR spectroscopy.
1. Introduction The behavior of supported metal catalysts at extreme temperatures is of technological importance for understanding catalyst deactivation. Because of the importance of rhodium in automotive catalytic converters,' a number of high-temperature studies of Rh/A1203have been carried out.2-" Rhodium is well-known to and CO o~idation.'~ have an important role in NO A major problem in catalytic converters is metal interaction with the support material at high temperatures, which results in a loss in catalytic activity of the metal. Metal sintering",*' occurs at high temperatures in a hydrogen atmosphere, causing a loss of surface area of the metal. An oxygen environment disperses the metal at elevated temperatures; here it is believed that the formation of metal oxides reduces the surface tension below that which exists for the metal particles relative to the s ~ p p o r t , ~ allowing spreading or redispersion of the metal oxides to occur. Mixtures of H2, O,,and CO have been used in chemisorption studies13to investigate catalysts treated at high temperatures. Yao et aL4" proposed that treating Rh/AI2O3 catalysts at high temperatures under oxidizing conditions caused Rh203to diffuse into the alumina. More recently, Beck and Carr6v' used temperature-programmed desorption to examine thermal aging of Rh/ AI2O3;these studies found that although oxidized Rh dissolves into the support at 973 K in oxidative environments, most of the metal can be recovered again by reducing in H2at 973 K. This was confirmed in a very thorough study by Wong and McCabe* using temperature-programmed reduction, H, chemisorption, infrared spectroscopy, transmission electron microscopy, and CO oxidation kinetic analysis. CO adsorption as monitored by infrared spectroscopy is a good probe of the Rh site distribution, since oxidized Rh' atoms can be identified by the production of Rhl(CO), (gem-dicarbonyl) species; Rh? sites on rhodium crystallites are identified by both terminal and bridging CO specie^.^^*^^ Solymosi and Pasztor9 used the observation of such species to determine the effects of high-temperature reductions up to 1273 K on highly dispersed Rh/AI2O3. They found that catalysts reduced up to 673 K in H2 are dispersed upon CO chemisorption as judged by Rh'(CO), production at 300 K, but prolonged treatment in high-pressure C O (50 Torr) at 448 K and above re-formed Rh," sites from Rhl(CO), by the reducing action of CO. Catalysts reduced in Hz at higher temperatures underwent a metal sintering process, producing larger RhXoparticles. Prolonged CO adsorption on high-temperature reduced and sintered catalysts at 300 K showed a partial production of isolated Rhl sites. Variation in the amount of Rh'(CO), formation with reduction temperature is certainly related to variations in the concentration of isolated AI-OH surface species which have been shown to be necessary for oxidation, producing Rh' sites from Rho sites in the presence of C0.26*27Zaki et al." found similar results for re* Author to whom correspondence should be addressed.
duction by monitoring CO adsorption from 77 to 773 K. Water and oxygen were found to increase dispersion on highly reduced catalysts, but cyclic oxidative and reductive treatments suppressed metal dispersion. Recently, electron energy loss spectroscopy experiments on a model Rh/AI2O3catalyst were conducted to examine the metallic rhodium interaction with a1~mina.I~ The catalyst was prepared under ultrahigh vacuum by creating a thin film of A1203on a Mo(110) substrate. Afterward, rhodium was evaporated onto this A1203 layer. The thermal behavior of the model catalyst was studied by electron energy loss spectroscopy (EELS) using CO as a probe of surface rhodium. A lack of the CO adsorption feature after annealing the Rh/Alz03 interface to 1 100 K revealed that rhodium had disappeared from the surface. Auger spectroscopic ~ ~ ~ ~ studies indicated that the rhodium was located in the near surface region of the model catalyst film and that the loss of CO chemisorption capacity could not be explained by Rh (1) Taylor, K. C. Automobile Catalytic Cowerters; Springer-Verlag: New York, 1984. (2) Fiedorow, R. M. J.; Chahar, B. S.; Wanke, S . E. J . Catal. 1978, 51, 193. (3) (4) (5) (6) (7)
Wanke, S. E.; Dougharty, N. A. J . Catal. 1972, 24, 367. Yao, H. C.; Japar, S.; Shelef, M. J. Carol. 1977, 50, 407. Yao, H. C.; Stepien, H. K.; Gandhi, H. S.J . Catal. 1980, 61, 547. Beck, D. D.; Carr, C. J. J . Phys. Chem., submitted for publication. Beck, D. D.; Carr, C. J. J . Catal., submitted for publication. (8) Wong, C.; McCabe, R. W. J. Catal. 1989, 119, 47. (9) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1985, 89, 4789. (10) Solvmosi. F.: Pasztor. M. J . Phvs. Chem. 1986. 90. 5312. (11) Zaii, M.'I.;'Kunzmann, G.; Gates, B. C.; Kn&inger, H. J . Phys. Chem. 1987, 91, 1486. (12) Zaki, M. I.; Tesche, B.; Kraus, L.; Knozinger, H. Surf Interface Anal. 1988, 12, 239. (13) Chen, J. G.; Colaianni, M. L.;Chen, P.; Yates, Jr., J. T.; Fisher, G. B. J . Phys. Chem. 1990, 94, 5059. (14) Burkhardt, J.; Schmidt, L. D. J. Catal. 1989, 116, 240. (15) Duprez, D.; Barrault, J.; Geron, C. Appl. Catal. 1988. 37, 105. (16) Duprez, D.; Delahy, G.; Abderrahim, H.; Grimblot, J. J. Chim. Phys. 1986, 83, 465. (17) Kiss, J. T.; Gonzalez, R. D. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 216. (18) Schlatter, J. C.; Taylor, K. C. J . Catal. 1977, 49, 42. (19) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J . Catal. 1986, 100, 360. (20) Hughes, R. Deactioation of Catalysts; Academic: New York, 1984; Chapter 4. (21) Butt, J. B.; Petersen, E. E. Activation, Deactivation and Poisoning of Catalysts; Academic: New York, 1988; Chapter 5. (22) Ruckenstein, E. In Sintering and Heterogeneous Catalysis; Kuczynski, G. C., Miller, A. E., Sargent, G. A., Eds.; Plenum: New York, 1984; Chapter 15. (23) Ruckenstein,E. In Metal-Support Interactions in Catalysis, Sintering, and Redispersion; Stevenson, S. A., Dumesic, J. A., Baker, R. T. K., Ruckenstein, E., Eds.; Van Nostrand Reinhold: New York, 1987; Chapter 11. (24) Cavanagh, R. R.; Yates, Jr, J. T. J. Chem. Phys. 1981, 74, 4150. (25) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957, 61, 1504. (26) Basu, P.; Panayotov, D.; Yates, Jr., J. T. J . Am. Chem. SOC.1988, 1IO, 2074. (27) Basu, P.; Panayotov, D.; Yates, Jr., J. T. J. Phys. Chem. 1987, 91, 3 133.
0022-365419 1/2095-1694%02.50/0 0 1991 American Chemical Society
High-Temperature Behavior of Rh/A1203 Catalysts
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
sintering. In an effort to compare these results to the behavior of an actual Rh/AI2O3catalyst, we have undertaken a study of the thermal effects on a very highly dispersed supported Rh catalyst. The behavior of the surface rhodium was observed by using infrared spectroscopy to monitor CO adsorption at 200 K after each heating cycle. By use of a high-temperature infrared cell,28 0.15% Rh/A1203catalysts, reduced at various temperatures, were studied both under vacuum and in CO(g) following various treatments in the temperature range 200-1400 K. 11. Experimental Section The low temperature-high temperature infrared cell used in these studies has been described previously.28 The unreduced catalyst is deposited by a spraying technique onto a tungsten grid which is held rigidly by nickel clamps. Electrical heating power may be used to control the grid temperature using an electronic controller.29 In addition, the grid and supported catalyst can be cooled by using liquid N,. The temperature of the catalyst is measured by a chromel/alumel thermocouple (0.077." diameter) spot-welded to the top central region of the grid. The grid support is held in the center of a stainless steel cell containing ports for gas delivery and for admission of the IR beam. Calcium fluoride optical windows, mounted in standard 2.75-in.-diameter stainless steel flanges, allow IR measurements in the 40001OOO-cm-' spectral range. The stainless steel ultrahigh-vacuum system used for this work is equipped with a liquid N 2 cooled zeolite sorption pump and a 30 L/s ion pump. In addition, a baratron capacitance manometer and a quadrupole mass spectrometer are used for gas measurements. Degussa aluminum oxide C (104 m2/g) and RhCI33H2Owere used to prepare the Rh/A1203 catalysts. The rhodium content of the catalysts was 0.15 wt %. The amounts required of these materials to obtain 0.15% Rh/A1203 were added simultaneously into an appropriate volume (1 10 mL/g support) of a liquid mixture of water and acetone (1 :9 volume ratio), and the resulting suspension was agitated ultrasonically for 30 min. The slurry thus obtained was uniformly sprayed, by a N,-pressurized atomizer, onto the entire exposed grid area (5.2 cm2). The grid was electrically heated during spraying to 323-333 K to flash evaporate the liquid phase.28 The net weight of the catalysts sprayed onto the grid was 37.4-45.7 mg (7.2-8.8 mg/cm2). The sample was reduced in situ by heating under vacuum at the designated temperature for 12 h. Reduction was achieved at the desired temperature with four successive exposures of 400 Torr of H2 (99.9995% pure, Matheson) for 15-60 min (after each exposure, the cell was evacuated for 30 min), followed by outgassing at the reduction temperature for 12 h. The chloride content of Rh/AI2O3catalysts prepared in the same manner has been reported p r e v i o u ~ l y . ~ ~ The carbon monoxide used with adsorption experiments was 99.9% pure obtained from Matheson Gas Products in a break-seal glass storage bulb. IR spectra were taken from the catalyst and adsorbed species in a purged double beam Perkin-Elmer Model 580 B infrared grating spectrometer coupled with a Model 3500 data station for data storage and manipulation. Spectra were signal-averaged, for data acquisition times of 2.2 s/cm-l for the uOH region (3900-3200 cm-I) and 3.7 s/cm-l for the uco region (2150-1700 cm-'), acquired at 1 point/cm-'. The spectra showing the adsorbed species were then obtained by subtracting out a background of CO(g) held in the cell under identical measurement conditions. The spectral resolution was 5.3 cm-I. X-ray powder diffractograms were obtained with a Model XRD 700 Diano diffractometer. A Model CA-8L Diano generator operated at 50 kV and 32 mA provided a source of Ni-filtered (28)Basu, P.; Ballinger, T. H.; Yates, Jr., J. T. Reu. Sci. Instrum. 1988, 59, 1321. (29)Muha, R. J.; Gates, S. M.; Yates, Jr., J. T.; Basu, P. Reu. Sci. Instrum. 1985,56,61 3. (30)Duncan, T.M.; Yates, Jr., J. T.; Vaughan, R. W. J . Chem. Phys. 1980,73,975.
\
\(a)
1695
200K
I
4TaK* 600 K I
f
800 K .( 1000 K
\ J
IO0
~
3700
I
~
(f) I
1200K ~
I
I
3500 3300 Wavenumber ( c m - ' )
Figure 1. Infrared spectra in the uOH region for a 0.15% Rh/AI2O3 catalyst following heating under vacuum at the temperatures indicated. The Rh catalyst was reduced at 475 K. Measurement temperature = 200 K.
Cu K, radiation (A = 1.54051 A). The diffractometer was operated with 3.0° diverging and 0.2O receiving slits at a scan rate of 2O/min and produced a continuous trace of diffracted X-ray intensity as a function of 20. The test powder samples were obtained by scraping the catalyst sample off the tungsten grid and pressing the powder onto a glass substrate. XRD patterns ( I / I o vs d spacings) were derived from the diffractograms and matched subsequently with those filed as ASTM standard^.^^
111. Results CO Interaction with Rh/A1203. Three catalyst samples were used in the experiments reported here. The first sample, Rh(475 K), was reduced at 475 K the second (Rh(600 K)) and the third (Rh(900 K)) were reduced at 600 and 900 K, respectively. CO adsorption on all three catalysts was done in the following manner: CO(g) was introduced into the cell with the catalyst at 300 K and maintained with an equilibrium pressure of 5 Torr for 30 min. Then, the catalyst was cooled to 200 K and IR spectra were taken. The catalyst was then heated to a desired temperature either in the presence of the C O gas phase or under vacuum ( P = 1 X lo-' Torr) for 1 h. The catalyst was subsequently cooled back to 200 K, CO(g) was admitted (if necessary), and IR spectra were taken after 30 min. Therefore, all CO(a) spectra shown (Figures 2 and 3) were recorded at 200 K in the presence of 5 Torr of CO(g). No production of C02(g) was observed with IR spectroscopy in these measurements. Effect of Heating under Vacuum on Hydroxyl Groups. Figure 1 shows the effect of increasing temperature under vacuum conditions on the Rh(475 K) catalyst. Spectrum a was recorded at 200 K and exhibits four OH stretching bands. The 3734- and 3680-cm-l bands are due to the dominant isolated hydroxyl groups on A1203,32while the broad bands at 3577 and -3500 cm-' are due to H-bonded (associated) hydroxyl groups. As the temperature was increased, dehydroxylation to produce H20(g)32-38 (31) ASTM Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Philadelphia, 1967. (32)Peri, J. B. J . Phys. Chem. 1965,69,220. (33)Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967;Chapter 5. (34) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960, 64,1526. (35) Knozinger, H.; Ratnasamy, P. Catal. Reu.-Sci. Eng. 1978,17, 31. (36)Knozinger, H. Adu. Catal. 1976,25, 184. (37) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975.
1696 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Ballinger and Yates I
l
l
1
1
6.R h ( 9 O O K ) Maximum heating temperature
2080
( j ) 1400K
Maximum heating temperature: (f)
Resolution
1200K
(i) 1200K
( h ) 1OOOK (9) BOOK
5.3 cm-1
Io.oc A Tscan= 200 K
Pscan= 5 Torr CO(g) (f)
600 K
(e)
400K
(d)
200K
2499 2029
Wavenumber ( c m - ' )
Figure 2. Infrared spectra in the
uco region for a 0.15% Rh/A1203 catalyst. Spectra were recorded at 200 K in 5 Torr of CO(g) after heating under vacuum to the designated temperature. The Rh catalyst was reduced at 415 K.
occurs, resulting first in the disappearance of the associated hydroxyl groups. After the 600 K heating (spectrum c), most of the associated hydroxyl groups have been eliminated. Upon a further increase in temperature, the isolated hydroxyl groups commence to disappear. After the IO00 K heating, the bands at 3734 and 3680 cm-'are considerably weakened, and two additional bands can be resolved at 3705 and 3797 cm-', as seen in spectrum e. After the 1200 K heating (spectrum f), no hydroxyl groups can be detected by infrared spectroscopy. This is in agreement with Solymosi and Pasztor9 who observed no hydroxyl stretching bands after an 1 173 K evacuation of a Rh/AI,O? catalyst. The IR spectrum in the hydroxyl region, obtained after the reduction of Rh(600 K), is similar to spectrum c, while the hydroxyl spectrum obtained after the reduction of Rh(900 K) is similar to spectrum e. Thus, it is important to recognize that Rh(475 K) represents a highly hydroxylated catalyst, Rh(600 K) is a partially hydroxylated catalyst, and Rh(900 K) is a highly dehydroxylated catalyst. Spectroscopic Studies of CO Chemisorptionfollowing Heating of Rh/A1203under Vacuum. The thermal behavior under vacuum of a Rh(475 K) catalyst as monitored by infrared spectroscopy of the C-O stretching region is seen in Figure 2. This experiment is characteristic of an initially highly hydroxylated AI2O3support. Spectrum a indicates that, upon initial CO adsorption at 300 K, the gem-dicarbonyl species, Rh'(CO)*, identified by the symmetric C-O stretch at 2097 cm-' and the antisymmetric C-O stretch at 2027 cm-I, is the predominant species formed on the highly dispersed Rh/A1203.24 This is expected since it is known that Rhl(CO), is produced from the Rh,O species during CO adsorption in the presence of isolated surface hydroxyl groups which disappear as Rh'(C0)2 As this catalyst initially containing a monolayer of CO is heated to 600 K under vacuum (spectrum c), the Rhi(CO)2 features caused by C O adsorption (again to saturation CO coverage) become less intense, and terminal CO species begin to appear around 2065 cm-I. However, the Rhi(CO), species is still the spectroscopically dominant CO species which develops even after heating under vacuum to 600 K. At 800 K (spectrum d) the developing terminal-CO feature overlaps with the symmetric stretch band of the remaining RhI(CO),, resulting in a broad band with peak maximum near 2100 cm-I and with a low-frequency (38) Cornelius, E. B.;Milliken, T.H.; Mills, G.A.; Oblad, A. G . J. Phys. Chem. 1955, 59, 809.
600 K
J
I
I
I
I
(b)
400K
(a)
200K
I
! I O 0 1900 1700
Wavenumber (cm'' ) Figure 3. Infrared C-O stretching spectra of (A) 600 K reduced and (B) 900 K reduced 0.15% Rh/AI2O3catalysts. Spectra were recorded at 200 K in 5 Torr of CO(g) after heating to the designated temperature in Wg).
shoulder. Bridging CO is also produced at 1850 cm-' by CO adsorption following 800 K vacuum heating of the catalyst. The Rh'(CO), species, produced upon CO adsorption on the catalyst which has been heated, continues to decrease after heating under vacuum to 1000 K (spectrum e) and is no longer the dominant CO species, based on changes in IR intensity. Spectrum f was obtained after heating to 1200 K and shows that the terminal species can be resolved at 2069 cm-'. It should be noted that the spectra shown in Figure 2 are a result of CO adsorption at 200 K on the catalyst which has been heated under vacuum to the indicated temperature. Since the IR spectra following heating under vacuum show that all adsorbed CO has desorbed by 700 K, the significant changes above 700 K are solely due to heating the catalyst in the absence of any gas phase or chemisorbed CO. Hence, the major changes observed in spectra d-f are due to the reduction of Rh' (originally formed as Rhl(CO),) to Rho by a reduction process where the reducing agent is unknown. Spectroscopic Studies following Heating Rh/AI,O under CO(g). The previous experiments involving heating Rh/AI2O3 under vacuum have shown that as surface hydroxyl groups are thermally destroyed, the tendency of the RhXosites to form Rh' sites decreases (upon CO adsorption). In the experiments shown in this section, and in contrast to those of Figure 2, Rh: was heated under CO(g) to determine whether the same trend is present when heating occurs under the mild reducing conditions associated with CO(g). Figure 3A shows the results for Rh(600
High-Temperature Behavior of Rh/A1203 Catalysts
K) heated under CO(g) in sequential experiments up to the 600 K reduction temperature. Spectra a-c show that Rh'(C0)2 is the dominant species, with adsorption bands occurring at 2099 and 2029 cm-I, from 200 to 600 K. Only minor features due to terminal or bridging CO are observable. IR spectra of CO adsorbed on the Rh(900 K) catalyst are shown in Figure 39. Spectrum d, recorded initially after reduction at 900 K and cooling to 200 K in CO(g), clearly shows that some sintering has occurred during reduction and that the tendency to form Rhl(CO), is strongly diminished. This is seen by the production of an 1877-cm-' band of a bridging carbonyl species and the dominant 2080-cm-I band of terminal CO. A weak Rh'(CO), band can be seen at 2037 cm-I, and its weak companion is unresolved near 2100 cm-I. Heating to 400 and 600 K (spectra e and f) has little effect on these features, except for a slight decrease in the Rh'(CO), and bridging C O features. Spectra g-h in the temperature range 800-IO00 K show a further depletion of the gem-dicarbonyl species (as judged by the 2037-cm-' band) along with a decrease to low intensity of the bridging CO species. Spectrum i indicates that heating to 1200 K under CO(g) causes a significant loss in the amount of bridged or terminal CO which may be adsorbed on the catalyst. Upon heating under CO(g) to 1400 K (spectrum j), no CO adsorption features were observed upon cooling to 200 K. In summary then, the heating experiments under vacuum or under CO(g) have given the following results: (1) The formation of Rh'(C0)2 is related to the presence of isolated hydroxyl groups. For Rh/AI2O3 catalysts heated under vacuum or under CO(g), Rh1(C0)2formation is diminished as OH groups are removed from the surface. (2) The sites normally forming Rh'(CO), upon treatment with CO remain as Rh,O sites when Rh'(CO), formation is suppressed by thermal removal of OH groups. (3) Rh; sites may be detected by IR spectroscopy by the chemisorption of terminal CO and bridged CO under saturation C O coverage conditions. (4) At temperatures above -1000 K, under CO(g), the number of available terminal CO and bridged CO sites (Rh: sites) begins to decrease as measured by the infrared intensity of the chemisorbed C O at saturation. (5) By 1400 K, all ability for C O chemisorption has disappeared, as observed by IR spectrosCOPY. X-ray Diffraction Studies following High- Temperature Treatment of Rh/A1203. Inspection of the Rh(900 K) catalyst after the 1400 K treatment indicated that physical changes had occurred. The catalyst now appeared darker gray in color and was granular, instead of the usual light gray color and powdery character. It is known that y-A1203undergoes a structural reordering from a spinel-like structure to the corundum structure between 1375 and 1450 The corundum structure, known as a-A1203, is the only high-temperature thermodynamically stable oxide of aluminum. In order to determine whether such a reordering had occurred, an X-ray diffraction analysis (XRD) was made. The XRD powder diffractograms of Rh(900 K) are shown in Figure 4. Only A1203diffraction features were observed since the low loading of Rh is below the sensitivity of the XRD technique as confirmed by Wanke and D ~ u g h a r t y .The ~ expected location of Rh metal crystal diffraction features is shown by arrows in Figure 4. The bottom diffractogram shows the XRD pattern obtained after heat-treating Rh(900 K) to 1300 K for 30 min. This pattern is mainly that of the y-A1203 form (ASTM PDF card 10-425), although a small amount of the 6-A1203form also exists (d values 1.78, 2.58, 2.70 A; ASTM PDF card 4-877), as has been demonstrated by others for Degussa aluminum oxide C.4' The top pattern was obtained for Rh(900 K) following annealing to 1400 K and is entirely different in the 20 positions of the diffraction peaks. Comparison with the ASTM powder diffraction file3' K.39340
~
~~
(39) Wefers, K.;Misra, C. Technical Paper No. 19, A h a Laboratories, 1987. (40) Stumpf, H.C.; Russell, A. S.;Newsome, J. W.; Tucker, C. M. Ind. Eng. Chem. 1950, 42, 1398. (41) Lavalley, J. C.; Benaissa, M.; Busca, G.; Lorenzelli, V.Appl. Catal. 1986, 24, 249.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1697 2e '0 I
50
80 I
[m countr
I
45
40
35
I
I
I Rh (900K)heated to 1400 K I I
I
.-bm C
s-
Counts
C
Rh (900K)heated t o 1300 K
b
YA~&',
A
1 I
1.50
1.70
1.90
2.10
2.50
2.50
2.7(
d (A) Fipre 4. X-ray powder diffractograms obtained after heating a 0.15% Rh/AI2O3catalyst to 1300 K (bottom) and 1400 K (top). Comparison of major diffraction lines: observed d (A) [ASTM d (A)]. a-AI2O3: 1.395 [1.395], 1.974 [1.977], 2.274 [2.28], 2.418 [2.39]. y-A1203: 1.374 [1.374], 1.405 [1.404], 1.605 [1.601], 1.734 [1.740], 2.082 [2.085], 2.374 I2.3791, 2.553 [2.552].
confirmed that the pattern was due to a-A1203 (ASTM PDF card 10- 173). It is clear that the loss, following 1400 K annealing, of C O chemisorption capacity on Rh(900 K) (Figure 3j) is closely correlated with the structural reordering of the A1203support.
IV. Discussion Dispersion of Rh/A1203. The extremely low loading (0.15%) for rhodium on the catalyst samples was selected in order to study more clearly the interactions occurring between the rhodium and the A1203support. These results represent, to the best of our knowledge, the first studies up to 1400 K of such a highly dispersed Rh/A1203 catalyst using IR spectroscopy. Although other inv e s t i g a t o r ~ ~ have - ~ , ~ studied ~ lower loadings of Rh/A1203 by chemisorption techniques, no IR data were reported. Furthermore, no Rh/AI2O3heating experiments to 1400 K have been reported in the literature.42 The high dispersion of the catalyst is indicated by the infrared spectra obtained following CO chemisorption. The initial spectra obtained after CO absorption on Rh(475 K) show little evidence of any bridging CO species present. Bridged CO would be expected if large rhodium crystallites were present after CO chemi s o r p t i ~ n .Small ~ ~ quantities of bridged CO and terminal CO are observed on Rh(600 K). On Rh(900 K), terminal CO and bridged CO are the spectroscopically dominant surface species. Isotherm studies of CO or H2 adsorption were not carried out due to the small volumetric uptake expected on these dilute Rh/A1203preparations. In addition, difficulties in determining the stoichiometry of the saturated CO/Rh or H/Rh layers4v8make these measurements suspect for the determination of average Rh particle size. Thermal Effects under Vacuum. Figure 2 shows that the adsorption of CO on Rh/A1203 produces a very different distribution of chemisorbed species depending upon the annealing temperature under vacuum. For Rh/A1203annealing temperatures below -500 K, the dominant surface species produced, judging from the saturated CO IR spectra, is the Rh'(C0)2 species (42) A search of the Society of Automotive Engineers literature from 1965 through 1988 was conducted, and no heating experiments to 1400 K on Rh/A1203 were found.
1698 The Journal of Physical Chemistry, Vol. 95, No. 4, 19
(Figure 2a,b). Above a 600 K vacuum annealing temperature, the tendency to generate Rh'(C0)2 species is observed to decrease, and both terminal CO and bridged CO on Rh," sites are formed. We associate this tendency with the extensive loss of isolated Al-OH groups by vacuum heating as seen in the spectra of Figure I . Previously, we have shown that Rh'(CO), formation is directly proportional to isolated AI-OH consumption, both species being measured by IR s p e c t r o ~ c o p y . ~The ~ ~ ~oxidative ' process ( 1 /X)Rh>
+ AI-OH + 2CO(g)
-
(A1-O-)Rh1(CO)2 + (1/2)H2(g) (1)
Ballinger and Yates nealing temperature in terminal CO and bridged CO IR intensity a t saturation. This effect may be due to one or more of the processes listed below: (1) sintering under CO(g) of Rh; to form (2) particles of larger size, and smaller exposed surface area;9*'1*48 partial diffusion of Rho into y-A1203; (3) carbon deposition from CO on Rh? particle^.^^^^^ Following annealing of the Rh/A1203 catalyst under CO(g) at 1400 K, the complete loss of the Rh ability to adsorb spectroscopically detectable chemisorbed CO (see Figure 3j) correlates closely with the structural reordering from y-A1203to a-A1203, as observed with XRD (Figure 4). Studies by have shown that the y-A1203to a-A1203transition occurs between 1375 and 1450 K. It is likely that Rh is encapsulated into the support as the y-A1203to a-A1203structural transformation occurs. Since the pore size distribution of Degussa aluminum oxide C has been measured with a diameter range of 15-1000 A:' it is possible that the Rh residing in the micropores can become encapsulated as the pore structure collapses during this transformation. The reaction to form rhodium aluminate can be ruled out, since literature searches by others52have found no evidence for such a reaction.
earlier postulated by BurwelP and Brenner is confirmed by our ~ o r k . ~ In ~ .addition, ~' recent studies of the removal of isolated surface AI-OH groups by functionalization to produce A1-0Si(CH3)3groups has also clearly demonstrated that the isolated AI-OH groups are necessary for Rh'(C0)2 generati~n.~' In contrast to recent EELS results obtained on model Rh/A1203 catalyst^,'^ heating Rh(475 K) to 1200 K under vacuum does not destroy the low-temperature CO adsorption capabilities (Figure 2, spectrum 9. Under these conditions, both the model Rh/AI2O3 catalyst (EELS) and the catalysts employed here (IR) are completely free of O H groups as determined by vibrational spectroscopy in each experiment. V. Conclusions Two possible explanations for this difference in behavior of the The effects observed on 0.15% Rh/AI2O3 catalysts after various model (EELS) and the real (IR) catalysts below 1200 K are as high-temperature treatments are as follows: follows: (1) The structure of the A1203film substrate used in the 1. A correlation between the presence of isolated surface EELS studies differs from that of the y-A1203employed here, hydroxyl groups, AI-OH, on the alumina support, and the and this structural difference may be responsible for difference tendency of supported Rh," surface species to convert to Rh' in the thermal behavior of the two Rh/AI2O3interfaces. (2) Rho surface species in the presence of CO(g) has been confirmed. The species present on the real Rh/A1203 catalyst may in fact diffuse removal of Al-OH by heating under vacuum above -600 K has into the near surface region of the A1203support under vacuum been shown to reduce the tendency of Rh," to form Rh', as deat temperatures near 1100 K, as is found by Auger and EELS tected by Rh'(C0)2 formation. studies on the model ~ata1yst.l~ Subsequent C O chemisorption 2. Similar effects of heating Rh,0/A1203 under CO(g) are at 200 K and 5 Torr pressure may then restore Rho to the surface observed. Under these conditions, above -900 K,some additional by reduction of the free energy of the Rho through chemisorptive loss of Rh," sites capable of binding terminal CO atid bridged CO binding to CO. Such chemisorption effects on the surface segregation of alloys are well-known, as in the Ni-Cu s y ~ t e m . ~ ~ . ~ 'is observed compared to similar heating experiments under vacuum. This may be due to carbon deposition or other thermal The model Rh/AI2O3 catalyst (EELS) studies were carried out effects on the Rh," sites which partially reduce the ability of the at CO pressures of only -IO-' Torr and at 120 K, which may catalyst to chemisorb CO, as judged by IR intensities. be insufficient to cause CO-induced Rho segregation back to the 3. Heating to 1400 K results in y-Al,03 conversion to a-A1203. surface. This structural reordering for the support causes a complete loss Esfect of Heating Rh/A1203under C q g ) . Figure 3 shows that of Rh," capacity for CO chemisorption as judged by IR intensities. under CO(g) the conversion between Rh' sites and Rh," sites occurs on heating Rh/AI2O3 to temperatures where substantial Acknowledgment. We thank the General Motors Research loss of isolated AI-OH has occurred. Above -600 K, a slow loss Laboratory for support of this work and also Dr. Galen Fisher of Rh> sites is observed on heating under CO(g) at 5 Torr. This for helpful comments. We also thank Dr. Mohamed Zaki for conclusion is based on the gradual decrease with increased anhelpful discussions. Registry NO. Rh, 7440-16-6; CO, 630-08-0. (43) Brenner, A,; Burwell. Jr., R. L. J . Caral. 1978, 52, 3 5 3 . (44) Hucul, D. A.; Brenner, A. J . Phys. Chem. 1981,85,496. (45) Paul, D. K.: Ballinger, T.H.; Yates, Jr.. J. T. J . Phys. Chem. 1990, 94, 4617. (46) Kuijers, F. J.; Ponec, V. Surf. Sci. 1977, 68, 294. (47) Harberts, J. C. M.; Bourgonje, A. F.; Stephan, J. J.; Ponec, V. J . Catal. 1977, 47, 92.
(48) Dictor, R.; Roberts, S. J. Phys. Chem. 1989, 93, 2526. Erdohelyi, A.; Solymosi, F. J . Caral. 1983, 84, 446. Solymosi, F.; ErdBhelyi, A. Surj. Sci. 1981, 110, L630.
(49) (50) (51) (52)
Basu, P. Ph.D. Dissertation, University of Pittsburgh, 1988. Beck, D. D. Private communication.
