J . Phys. Chem. 1989, 93, 5846-5850
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Influence of Ceria on Alumina-Supported Rhodium: Observations of Rhodium Morphology Made Using FTIR Spectroscopy Ron Dictor* and Scott Robertst Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 (Received: May 31, 1988; In Final Form: September 21, 1988)
The adsorbed states of carbon monoxide on a 0.5% Rh/ 10% Ce/AI,O, catalyst were investigated by infrared spectroscopy in order to better understand the promotional effects ceria exerts on rhodium in automobile catalysts. It was found that the IR spectra of freshly prepared catalysts were dominated by the dicarbonyl species Rh(C0)2-indicative of a high degree of atomic dispersion. Prolonged exposures to CO at 473 K effectively agglomerated the rhodium as evidenced by the simultaneous growth of linear- and bridged-carbonyl IR bands (Rh-CO and Rh,(CO), respectively) and disappearance of the dicarbonyl features. However, the total integrated absorbance of all adsorbed C O species did not decrease significantly, suggesting that the Rh is coalescing from an atomically dispersed state to one of high dispersion. The agglomerated state would presumably be characterized by small or flat Rho particles. Brief exposures to 0, at 673 K followed by exposure to C O at 373 K showed that the catalyst could be returned to a state of 100% atomic dispersion, again suggesting that the particles are small or flat and easily oxidized. Ceria's primary role as a promoter may be the maintenance of high Rh dispersions; other reputed benefits such as increased water-gas shift activity may be secondary effects arising from small Rh particle sizes.
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Introduction It has been recognized for some time that the addition of CeO, to automotive catalyst formulations improves the conversion performance and/or thermal stability of catalytic converters. However, ceria's role as a promoter is not fully understood, and the additive has been the subject of numerous investigations. Much of the attention paid to ceria centers upon its redox chemistry. It is believed that ceria enhances oxidation activity by storing oxygen during air/fuel ratio excursions to the lean (02-rich) side of the stoichiometric point and releasing it when the exhaust gas is fuel rich, effectively switching between CeO, and Ce203.1-3 While this process is plausible, it is difficult to explain why CO oxidation activity does not always increase with increases in C e loading. In fact, it has been reported that activity may actually decrease with increases in a catalyst's C e ~ o n t e n t . ~ It has also been argued that ceria promotes water-gas shift thus catalyzing an alternate route for CO oxidation: CO H20= C 0 2 H2. It is also believed that ceria stabilizes both the BET surface area of the alumina support' and the dispersion of noble m e t a l ~ ~during . ' ~ exposures to high temperatures. This last quality, increased resistance to sintering, is of great interest to the automotive catalysis community because catalytic converters are often exposed t o both oxidizing and reducing environments a t high temperatures (>>750 K), resulting in loss of active metal surface area. The maintenance of high, active surface area in catalytic converters is critical for meeting emission standards throughout 50000 miles of vehicle service. It is interesting, however, that recent studies by Hecker et al.11j'2show that ceria may promote R h dispersion yet decrease catalytic activity for NO reduction by C O . This is consistent with earlier observations by DictorI3 that NO dissociates more readily over reduced Rh surfaces than over isolated R h atoms (or ions) which are present on very highly dispersed catalysts. Other studies have also pointed to the structure sensitivity of NO d i s ~ o c i a t i o n . ' ~ Solymosi and co-workersI5 have demonstrated that ceria can enhance rhodium dispersion on a silica support by stabilizing the atomically dispersed species, R h i ( C 0 ) 2 . In comparing a Ce0,-promoted catalyst to an unpromoted one, these authors found the isolated Rh species of the promoted catalyst to be more resistant to reductive agglomeration a t high temperatures. A recent study of the Rh/Ce/AI2O3 system by Oh and Eickeli6 has shown that the addition of cerium alters CO oxidation kinetics under moderately oxidizing or reducing conditions. Decreases in the apparent activation energy and the O2 reaction order,
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' G M R summer student. Present address: Chemical Engineering Department, University of Pennsylvania. Philadelphia, PA 19104.
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together with a suppression of the CO inhibition effect (on oxidation rate), were explained by proposing an alternate reaction pathway involving surface reaction between adsorbed CO and lattice oxygen from the R h - C e 0 2 interface. Similar observations were made by Yu Yaol' for both CO and hydrocarbon oxidation. The author interpreted these results in the context of two parallel kinetic mechanisms, one of which was attributed to reaction over relatively large metal particles and the other attributed to oxidation involving highly dispersed noble metals. The presence of C e 0 2 was said to promote the latter kinetics. This study was done to investigate, using infrared spectroscopy, the influence of ceria on the dispersion of Rh in a Rh/CeO2/AI20, catalyst. W e have recently observed that R h supported on unpromoted alumina sinters readily in C O a t low temperatures" and that this sintering is only partially reversed through brief, high-temperature oxidative treatments. As will be shown in this report, cerium promotes the complete reversal of the sintering process, thus allowing a sintered catalyst to become fully redispersed. It is believed that cerium physically separates Rh particles, thus maintaining a highly dispersed state (small or flat particles) upon sintering.
Experimental Section Low-density O-AI2O3beads (Grace Chemical), typical of those ( I ) Yao, H. C.; Yu Yao, Y . F. J . Curd. 1984, 86, 254. (2) Su, E. C.; Montreuil, C. N.; Rothschild, W. G . Appl. Cural. 1985, 17, 75. (3) Herz, R. K. ACS Symp. Ser. 1982, No. 178, 59. (4) Summers, J. C.; Ausen, S. A. J . Curd. 1979, 58, 131. (5) Schlatter, J. C.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Deu. 1980, 19, 288. (6) Kim, G. Ind. Eng. Chem. Prod. Res. Deu. 1982, 21, 267. (7) Su, E. C.; Rothschild, W. G. J . Curd. 1986, 99, 506. (8) Hindin, H. G. Engelhard Minerals & Chemicals Corp., U S . Patent 3,870,455, 1975. (9) Sergey, F. J.; Maselli, J. M.; Ernest, M. V., W. R . Grace Co., US. Patent 3,903,020, 1974. ( I O ) Graham, J. R.; Ernest, M. V.; Maselli, J. M., W. R. Grace Co., US. Patent 3,850,847, 1974. (1 I ) Hecker, W. C.; Breneman, R. B. Curulysis and Automutiue Pollution Conirol; Elsevier Science Publishers; Amsterdam, 1987, p 257. (12) Hecker, W. C.; Wardinsky, M. D.; Clemmer, P. C.; Breneman, R. B. Presentation at the 9th International Catalysis Congress, Calgar, June, 1988. (13) Dictor, R. J . Cutul. 1988, 109, 89. (14) (a) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J . Curd. 1986, 100, 360. (b) Peden, C. H . F.; Goodman, D. W.; Blair, D. S.; Berlowitz, P. J.; Fisher, G . B.; Oh, S. H . J . Phys. Chem. 1988, 92, 1563. (15) Solymosi, F.; Pasztor, M.; Rakhely, G. J . Cutul. 1988, 110, 413. (16) Oh, S. H.: Eickel, C. C. J . Card. 1988, 112. 543. ( 1 7 ) Yu Yao, Y. F. J . Curd. 1984, 87, 152. ( 1 8) Dictor, R . J . Phys. Chem. 1989, 93, 2526
0 1989 American Chemical Society
Influence of Ceria on Alumina-Supported Rhodium used in commercial catalytic converters, were used in these experiments. Cerium was first deposited on the catalyst by aqueous impregnation. A sufficient quantity of cerium nitrate, C e ( N 0 3 ) 3 - 6 H 2 0 ,was used to make catalysts containing 10 wt % Ce (cerium and not ceria). Following the impregnation, the catalysts were allowed to dry in air overnight before calcining in air at 773 K for 2 h. Rhodium was subsequently introduced by using the incipient wetness technique. The ammine salt of rhodium dicarbonyl bromide, ( ~ - B U ~ N ) , R ~ ~ ( C was O ) ~dissolved B ~ ~ , in acetone and used to wet the ceria/alumina beads. The beads were dried at room temperature and then calcined at 773 K in air for 2 h to remove residual hydrocarbon. They were then crushed and sieved to 100/170 mesh. Approximately 45 mg of the impregnated powder was pressed into a 15-mm-diameter wafer by using a momentary pressure of 2200 kg/cm2. The wafers were mounted in the reactor, and all subsequent treatments were performed in situ. The rhodium loading, determined by analyzing the supernatant for residual rhodium, was calculated to be 0.46 wt %. This is comparable to the 0.48% loading reported earlier for the Ce-free cataIyst.18 The infrared cell and gas manifold system have previously been described in great detail.I8 Briefly, a T-shaped reactor with removable zinc sulfide windows and approximately 30-cm3 dead volume was used as an IR cell in a custom-made attachment to a Nicolet 60SX FTIR spectrometer. The gas feed to the reactor was comprised principally of helium into which pure component gases were switched as needed. Flows were controlled by means of mass flow controllers. All lines, when not used to feed a reactant or pretreatment gas, were continually purged with helium. Heat was provided by wrapping a heating tape about the cell. The highest attainable temperature for the system was 673 K-largely because of the water-cooled windows. All helium used in these experiments was purchased at 99.999% purity and further purified in Oxysorb (02-removal) traps. Hydrogen was only used for pretreatments and was used, at 5% concentration, as received (99.995%). C O was first passed through a bed of alumina beads at 573 K to decompose carbonyls before being mixed in the feedstream. O2was used as received (99.999%). Prior to each experiment, the catalyst was reduced by holding the wafer in situ a t 673 K in 5% H 2 for a prescribed period of time. H2 was then removed from the feed, and the reactor was flushed with He a t 673 K for -2-3 min to remove adsorbed H. The catalyst was then cooled to adsorption temperature and stabilized before adding CO, typically at 0.5% concentration. The total feed flow rate was nominally 60 L / h (STP). Pressure was maintained at 900 Torr (120 kPa) throughout the experiments. A Nicolet 60 SXB FTIR spectrometer with a KBr beam splitter and broad-band MCT detector was used to collect mid-IR spectra of C O adsorbed on the catalyst surface. Nicolet’s Rapidscan software was used to collect interferograms continuously during the experiments. The operating variables of the instrument were set to produce a 4-cm-I resolution spectrum approximately every 4 s, each spectrum being derived from the coaddition of 20 interferograms to improve the signal-to-noise ratio. Quantitation of the dicarbonyl bands was done using spectral substractions of (purely) dicarbonyl spectra obtained from desorption experiments. Integration of bridged and linear features was done after subtracting the dicarbonyl bands and those of gas-phase CO. The latter subtraction was necessary since all the adsorption spectra were collected in flowing CO, with the IR beam path length (of the reactor) being IO cm. Results CO Adsorption. Figure 1 shows a series of adsorption spectra taken at 373 K when C O is introduced to a freshly prepared catalyst wafer that has been reduced in H 2 at 673 K for 16 h. It had been observed earlierI8 that 373 K is a good “probe” temperature for C O since the CO-induced morphology changes of both reduced and oxidized rhodium catalysts occur very slowly with respect to the time needed to collect IR spectra. It can be seen in the figure that all three commonly observed C O adsorbates
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Frequency (cm-’) Figure 1. Adsorption of CO on a freshly prepared catalyst following reduction for 16 h in 5% H,at 673 K. Spectra were collected over 112 min; the first seven spectra were collected in the first 77 s. The inset is a difference spectrum of the spectrum collected at 112 min less a spectrum collected at 3 min (when the intensity of the linear feature was greatest). form simultaneously. The dicarbonyl bands develop at 2015 and 2087 cm-I and are associated with atomically isolated Rh atoms (or +1 ions). The linear- and bridged-CO features are associated with reduced, contiguous rhodium surfaces and develop near 2050 and 1850 cm-’, respectively. The mixture of C O adsorbates indicates that the Rh dispersion may be less than the 100% one might expect from a purely dicarbonyl spectrum. It is interesting to note that the intensity of the linear-CO feature actually passes through a maximum during the course of adsorption as depicted in Figure 1. This is evidenced by the overlapping spectral lines near 2050 cm-’ as well as the negative-going region of the difference spectrum shown in the inset (spectrum collected a t t = 112 min minus that collected a t t = 3 min). The growth of dicarbonyl features at the expense of the linear bands has been observed before for Rh/A120, a t low temperature^.'**'^ At temperatures of 1 4 7 3 K, however, dicarbonyl features diminish as the intensities of the linear- and bridged-CO bands increase. This behavior is illustrated by the spectra in Figure 2 which were collected after introducing C O at 473 K to a freshly prepared catalyst wafer. One can see that the three principal forms of adsorbed C O appear simultaneously, but the intensity of the dicarbonyl features reaches a maximum in spectrum e ( t = 4.7 min) and then declines through the course of the overnight experiment. Spectrum i ( t = 450 min) shows predominantly linear and bridged features with clear resolution of the high-frequency dicarbonyl band. The low-frequency dicarbonyl band contributes to the asymmetry of the already lopsided linear feature. It will be shown below that difference spectra gathered during desorption help resolve the “linear” feature into a t least two bands. The agglomeration of rhodium may proceed further than that shown in Figure 2i by exposing the catalyst to H2 a t 673 K or CO at 573 K. This is shown in Figure 3, spectra a-c. Spectrum a is the result of an overnight exposure to C O a t 473 K. An additional 17.5-h reduction in H 2 a t 673 K results in spectrum b, and a subsequent 13.5-h exposure to C O at 573 K yielded spectrum c. With the possible exception of a slight broadening of the base of the linear feature, the spectra are qualitatively quite similar. However, the integrated absorbance of all CO, features (19) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1985, 89, 4789.
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Figure 4. Example of 02-induced redispersion. IR spectra collected following ( I ) CO-induced coalescence, (2) exposure to 1% O2at 673 K for I O s, and (3) cooling to 373 K and introducing CO. Spectra were collected over 21 h, the first 12 of 17 collected within the first 2.5 h. Presence of only dicarbonyl bands suggests atomic dispersion of Rh. 1
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Frequency (cm-’) Effects of increased severity of the reducing (sintering) treatment: (a) following overnight exposure to 0.5% CO at 473 K, (b) following additional 17.5 h in 5% H2at 673 K, (c) after additional 13.5 h in CO at 573 K. Spectra d and e were collected following IO-s exposures of 1% O2 at 673 K to catalysts characterized in spectra b and c, respectively. Integrated absorbances for entire carbonyl region of spectra a through e are 18.2. 14.7, 12.8, 29.2, and 25.1 cm-I, respectively. Figure 3.
(21 10-1750 cm-l) decreased from 18.2 cm-’ in spectrum a to 14.7 and 12.8 cm-l in spectra b and c, respectively, indicating further decreases in Rh dispersion upon additional H2 or CO treatment. Following the H2 and C O treatments cited immediately above, the catalysts were exposed to a IO-s pulse of 1% 0, (flow rate = 60 L/h) at 673 K before cooling to 423 K and introducing CO. As had been observed earlier for the Rh/A1203 cataIysts,l8 and 0, pulse and subsequent exposure to CO induced a partial re-
dispersion of the rhodium as evidenced by the diminution of the linear-CO band and the reappearance of large dicarbonyl features. The resulting spectra are presented in Figure 3d,e. The two spectra are quite similar except for the higher background interference of H 2 0 in spectrum d (inadequate purge of the spectrometer). The integrated absorbances are 29.2 and 25.1 cm-I, each nearly 2-fold greater than their agglomerated precursors, spectra b and C. A very dramatic illustration of the ability to redisperse the rhodium through a brief, high-temperature oxidation is shown in Figure 4. A catalyst that had been treated overnight in C O a t 473 K (Rh more fully agglomerated than the sample depicted in Figure 2i) was heated to 673 K, exposed to 1% 0, (900 Torr, 60 L/h) for I O s, cooled to 373 K, and exposed to CO. The spectra presented in Figure 4 were gathered over the course of 21 h, though it was evident that the intensity of the bands was still increasing a t the time data collection was discontinued. There was no evidence for bridge-bonded C O and very little evidence for linear CO in these spectra. Procedurally, the only difference between the spectra of Figure 4 and those of Figure 3d-e is that the latter were collected a t a catalyst temperature of 423 K as compared to the 373 K for those of Figure 4. This comparison points to a thermal instability of the dispersion in the temperature region of 373-423 K. Additionally, it is interesting to note that the atomic dispersion of the redispersed catalyst appears to be greater than that of the fresh catalyst (Figure I ) . At this time it should also be noted that the loss of integrated absorbance for Rh/Ce/Al,O, catalysts had been much less than the IO- to 20-fold loss typically observed earlier for the unpromoted catalyst.I8 Over the course of 2-4 weeks of repeated experiments, intensities of IR bands on the promoted catalysts would decrease 25-50%-much less dramatic than the case of the unpromoted catalyst. In fact, in the case of Rh/AI2O3 the loss of intensity was so great that the dominant IR features would ultimately be those of gas-phase CO which had intensities well under 0.1 absorbance unit. When the highly redispersed catalyst of Figure 4 was heated in pure He to 673 K, cooled back to 373 K, and reexposed to CO, the spectra in Figure 5 resulted. The supported rhodium, heated with only dicarbonyl CO present, was clearly agglomerated as evidenced by the appearance of linear and bridged features.
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Figure 6. Rates of CO uptake as evidenced by changes in the integrated absorbance of the entire carbonyl region (2120-1710 cm-I) as a function of time: (a) freshly loaded catalyst reduced 16 h in 5% H2 at 673, (b) pretreatment as described in Figure 5 , (c) reduced as in (a) and then exposed to I % O2at 673 K for 20 s, (d, e) reduced as in (a) and then exposed to 5% O2 at 673 K for 2 and 15 h, respectively.
The rates of C O adsorption on differently pretreated catalysts are of interest since they reflect the influence CO exerts on Rh morphology and oxidation state. Figure 6a is a plot of integrated absorbance versus time for a fresh catalyst that has been reduced 16 h (Figure I ) , Figure 6b is from a catalyst that has been reheated after a C O exposure and cooled back to 373 K for readsorption (Figure 5), and Figure 6c-e is derived from catalysts that had undergone three different oxidative treatments. Curves a and b of Figure 6 are largely coincident, the difference between them likely arising from the partial coalescing of the rhodium as seen in Figure 5. The continued growth of the spectrum in curve a most likely arises from continued reduction of oxidized rhodium and/or CO-induced redispersion (as was noted for Figure 1). It is interesting that a small dose of 02,20 s of 1 % O2 at 673 K, dramatically slows the rate of C O uptake (curve c). Longer exposures of 2 and 15 h to 5% O2 appear to have little additional impact (beyond that of the 20-s treatment) on the rate of CO uptake (curves d and e). None of the experiments were conducted for a sufficient length of time to observe a final absorbance value.
Figure 7. Desorption of CO from sintered catalyst at 473 K. Spectra collected over 0.60-h period: (a) All spectra, (b) difference of spectra collected at t = 0 and t = 54 s, (c) difference of spectra collected at t = 0.33 h and t = 0.60 h.
Even the data in curve c, which appear to be asymptotically approaching an absorbance value of -30 cm-l, are in very good agreement with comparable data derived from the experiment of Figure 4 which show absorbance still on the rise a t t = 21 and A = 40 cm-I. An earlier study'j pointed to the need to first reduce oxidized supported Rh before being able to adsorb C O on it. Desorption. Desorption studies were performed by simply removing C O from the feed and collecting spectra while the reactor temperature was maintained at constant temperature-always the temperature of the pretreatment. All of these studies were performed a t T I 473 K. For catalysts containing all three principal C O adsorbates, the linear and bridged species desorbed with near-parallel behavior a t rates greatly exceeding that of the dicarbonyl species. This can be observed in Figure 7a where the small 2088-cm-' feature of the dicarbonyl species remains largely intact while the bands associated with linear and bridged C O diminish. Figure 7b,c (arbitrarily scaled) are difference spectra derived from the spectra of Figure 7a. Spectrum b is the difference between the first and third spectra ( t = 0 less t = 54 s), and curve c is the difference between the sixth and eight spectra (spectrum from t = 0.33 h less the one collected at t = 0.60 h). Time t , of course, represents the length of desorption or, in other words, the length of time in the absence of gaseous CO. Individual bands a t 2054, 2036, 2024, and 2002 cm-' are resolved in curves b and c of Figure 7. Similar features, though not always as clearly resolved, have consistently been observed throughout these experiments and even in the absence of ceria.l* The band at 2054 cm-' disappears most rapidly as evidenced by the absence of this band in the difference spectra at later times (curve c), The band at 2036 cm-l red shifts with increasing length of desorption and eventually evolves into the 2024-cm-l feature shown in Figure 7c. The small broad feature near 2002 cm-I appears to be enveloped by this band as the latter shifts to lower frequencies. The bridged-CO band at 1845 cm-' is well-behaved during desorption. Extinction Coefficient. An estimate of the extinction coefficient, t , for the dicarbonyl species R h ( C 0 ) 2 may be obtained by assuming that the rhodium can be (completely) atomically dispersed as indicated by the spectra of Figure 4. From the weight loading of Rh in the sample we can calculate an exact content of 2.43
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X mol of Rh in the 50-mg, 15-mm circular wafer. Using the Lambert-Beer law together with a n estimate of -70 cm-I for the maximum integrated absorbance of the dicarbonyl species (based on the data of Figure 6), we find that t N 51 % IO6 mol-Ian. This value is roughly 4-fold greater than that reported by Cavanagh and Yates for 0.2% Rh/A120320and one-fourth that reported by Duncan and co-workers.2’ This calculation is based on several assumptions, the most important being that all the rhodium in the sample adsorbs CO. Since the dicarbonyl species is isolated, we expect the extinction coefficient to be independent of coverage. The calculation of the extinction coefficient for R h ( C 0 ) 2 is more straightforward than those for the Rh-CO and Rh2(CO) species because of the ability to fully disperse the rhodium in the catalyst. The assumption given above, full participation of R h in the adsorption process, is also more valid for an atomically dispersed species than for adsorbates crowded on metal particle surfaces and subjected to steric, electronic, and dipole-dipole interactions.
Discussion Perhaps the most significant result reported here is the observation that the total integrated absorbance of the carbonyl absorbates on Rh/Ce/A1203 does not decline significantly during the course of a single sintering experiment or over the period of several investigations. Further, light oxidative treatments coupled with exposure to C O a t 373 K cause agglomerated rhodium particles to redisperse to the extent that the only observable C O adsorbate is Rh(CO), (Figure 4). The retention of high absorbance coupled with the effectiveness of light oxidative treatments suggests that the exposure of R h atoms does not decline appreciably while rhodium coalesces. In other words, a highly dispersed Rh/Ce/A1203 catalyst can remain highly dispersed as coalescence proceeds at 473 K in CO, presumably because atomically dispersed rhodium atoms (or ions) are coalescing to form 2-dimensional rhodium surfaces and/or small particles. This is clearly consistent with claims in the patent literature and elsewhere that ceria helps stabilize noble-metal dispersion^.^-'^ Solymosi and co-workers, however, showed that the addition of ceria to a Rh/SiOz stabilized the isolated dicarbonyl species and inhibited reductive agglomeration of rhcdium.I5 Though these authors also observe enhanced noble-metal dispersion owing to CeO,, it is unclear why they observed inhibited agglomeration while we observed agglomeration to small or flat particles. Some of the differences may be attributed to differences in support and/or the lower temperatures used in their investigation (573 K). Despite arguments by others that R h particles on alumina substrates are there is no direct evidence in this study that this may be true. Spectroscopic differences between small and flat particles would be difficult to discern. W e propose that the CO-induced coalescence results in a loss of atomic dispersion and the creation of reduced, continguous rhodium surfaces associated with flat and/or small particles. Dictor and Robertsi8 previously suggested that the mobile precursor to coalescence is the dicarbonyl species Rh(C0)2. That proposal is consistent with the data shown here for the ceriapromoted catalyst-particularly the results given in Figure 5 where agglomeration occurs after a catalyst exhibiting exclusively dicarbonyl CO, is heated beyond 473 K. One can thus envision the process as
(20) Cavanagh, R. R.; Yates, Jr., J . T.J . Chem. Phys. 1981, 7 4 , 4150. (21) Duncan, T. M.; Yates, Jr., J. T.; Vaughan, R. W. J . Chem. Phys. 1980, 7 3 , 975. (22) Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. J . Card. 1979, 57. 41. (23) Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. Growth and Properties of Meral Clusters; Elsevier Scientific Publishing: Amsterdam, 1980. (24) Fuentes, S.; Vazquez, A,; Perez, J. G.:Yacaman, M. J, J . Catal. 1986, 99,492.
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where the asterisks represent individual R h atoms. Additional sintering of flat particles 3-D particles or small particles large particles will occur if enough thermal energy is provided. The spectra in Figure 3 show that rhodium that coalesced in CO at 473 K will undergo additional sintering in C O at 573 K, though a t these conditions the sintering is slow and adsorbed C O likely plays no role. It can also be seen in Figure 3 that reduction in H2 a t 673 K also induces more sintering. Since H2does little to agglomerate rhodium on a fresh catalyst a t the same temperature, it may be suggested that the CO, is the “coalescing agent” for Rh. Once rhodium is coalesced into small or flat particles, thermal energy in excess of the (coalescing) pretreatment, 473 K, will lead to additional sintering. This is very similar to observations made by Schmidt and ~ o - w o r k e r swho , ~ ~used ~ ~ ~oxidative treatments to flatten Rh particles and subsequent reductions in H, at different temperatures to “ball up” flattened particles. The convoluted linear I R band shown in Figure 7 might be comprised of linear C O adsorbed onto different crystalline faces, the populations of the crystal planes being dependent on particle size aiid immediate past history of the catalyst. Schmidt and c o - w o r k e r ~used ~ ~ a similar rationalization to account for 1000-fold increases in reactivity they observed for ethane hydrogenolysis over R h / S i 0 2 catalysts that received oxidation-reduction treatments. The structural response of R h to a brief, high-temperature oxidation may also account for the dramatic increase in water-gas shift activity observed by Dictor for otherwise inactive Rh/A1203 catalysts.27 It should be emphasized that the coalescence/dispersion behavior of ceria-promoted Rh/AI2O3 is qualitatively very similar to that of the unpromoted catalyst, the notable exception being that the promoted catalyst maintains a high dispersion during CO-induced sintering a t 473 K. Because the agglomerated R h particles are small and/or flat and the fraction of R h atoms exposed to the gas phase is high, brief oxidations are more effective for dispersing R h in this system than for the unpromoted catalyst. The sintered Rh particles of the non-ceria catalyst are 3-dimensional and much bigger than those of the promoted catalyst, as evidenced by the significant loss of absorbance in the former as compared to the latter.18 It is believed that small particles are easier to oxidize than large particles.28 Ceria may promote Rh dispersion by covering the alumina and interacting with R h or simply by providing a physical barrier between R h deposits on the alumina surface. An understanding of the promotional effects of ceria is currently being sought through combined infrared and chemisorption studies. Finally, we note that the stabilization of the rhodium dispersion may be the primary effect of adding ceria to the catalyst whereas some of the reputed benefits of ceria may actually be secondary effects derived from the maintenance of high dispersion. In particular, Kim6 reported that ceria increased C O conversion over three-way catalytic converters by increasing water-gas shift activity. Concurrent with this result was the observation that NO, reduction activity was reduced by promotion with ceria. D i ~ t o r ~ ~ recently noted that brief oxidative treatments similar to those used in this study would stimulate significant W G S activity over Rh/AI,O3. Since the oxidation is more effective for the small (or flat) particles of the ceria-promoted catalyst, it is reasonable to expect WGS activity to be greater for this system. Similarly, NO reduction activity may be reduced since N O dissocation preferentially occurs on reduced rhodium surfaces.I3 This is very consistent with the work of Hecker and co-workers.”.l2 Additional studies in this area would be worthwhile. Acknowledgment. The authors thank Michael G. Zammit for preparing the catalysts. Registry No. Rh, 7440-16-6; Ce, 7440-45-1; CO, 630-08-0. (25) Lee, C.; Schmidt, L. D. J . Catal. 1986, 101, 123. (26) Wang, T.; Schmidt, L. D. J . Card. 1981, 70, 187; 1981. 7 1 , 411. (27) Dictor, R. J . Catal. 1987, 106, 458. (28) Vis, J . C.; Van’t Blik, H . F.; Huizinga, T.; van Grondelle, J.; Prins, R. J . Catal. 1985, 95, 3 3 3 .
