Langmuir 1994,10, 1461-1471
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Infrared Spectroscopy and Temperature Programmed Desorption Study of CO on Rh/A1203 Catalysts: Probing Overlayer and Support Sites Anthony L. Diaz,+Wes W. C. Quigley, Hiro D. Yamamoto, and Mark E. Bussell' Department of Chemistry, M.S. -9150,Western Washington University, Bellingham, Washington 98225 Received June 14,1993.I n Final Form: February 23,199P The adsorption of CO on reduced RNA1203catalysts has been investigatedusing infrared (IR)spectroscopy and temperature programmed desorption (TPD). At -120 K, carbon monoxide adsorbs on AF+ sites of the y-Al2O3 support and in the form of gem-dicarbonyl, linear and bridge bound CO species on the Rh overlayer as identified by IR spectroscopy. TPD of CO from a 5 5% RhIAl2O3catalyst reveals two desorption features with maximum rates of desorption at 170-185 K and 490-500 K. The former peak is assigned to CO desorbing from A13+sites while the latter is assigned to CO desorbing from the Rh overlayer. The CO adsorption capacity of the Rh overlayer has been quantified as a function of anneal temperature in ultrahigh vacuum using TPD. Sintering of the Rh overlayer has been identified as the primary mechanism by which exposed Rho is lost from the catalyst surface. Indirect evidence is provided which suggests that loss of catalyst surface area (T1 1200 K) may also be a mechanism by which surface Rh is lost, possibly due to encapsulation of Rh particles. Structural changes in the Rh overlayer have been found to occur duringCO TPD with oxidative disruption occurringat low temperaturesand reductive desorption occurring at high temperatures.
Introduction Despite their high cost, supported rhodium catalysts are used in a variety of technologicallyimportant processes. An example is the automobile catalytic converter in which alumina supported rhodium (RhIA1203) catalyzes the reduction of NO to N2 and, during warm-up, the oxidation of CO to C02. Given rhodium's expense, a considerable amount of research has focused on the processes whereby supported Rh catalysts become deactivated over time. Two deactivation processes which have been identified are the formation of catalytically inactive Rh+ species from Rho on the catalyst surface (discussed below) and the decrease in the amount of exposed Rho on the surface of the catalyst when it is heated above 875 K.' While the typical catalyst temperature in an automobile catalytic convertor is in the range from 675 to 875 K, it is not uncommon for the temperature to rise considerably higher under extreme driving conditiom2 Numerous accounts have appeared in the literature in which infrared (IR) spectroscopy has been utilized to investigate the adsorption and reactions of CO and NO on RhIA1203 catalysts.' The variety of adsorbed CO species which exist at the surface of supported Rh catalysts makes interpretation and quantitation of the IR spectra difficult and the system presents a complex characterization problem for surface scientists. Three different adsorbed CO species are typically observed at the surface of Rh/ A1203 catalysts: gem-dicarbonyl (Rh1(C0)2), terminal bound (Rh-CO), and bridge bound (Rh2-CO) species. The gem-dicarbonyl species exhibits characteristic symmetric and asymmetric CO stretching frequencies at -2100 and 2030 cm-l respectively, while vco frequenciesfor terminal
-
* Author to whom correspondence should be addressed. t Present address: Department of Chemistry, Oregon State University, Corvallis, OR 97331. Abstract published in Advance ACS Abstracts, April 1, 1994. (1) See: Ballinger, T. H.; Yates, J. T., Jr. J.Phys. Chem. 1991, 95, 1694, and references therein. (2) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill Book Co.: New York, 1980.
0743-7463/94/2410-1461$04.50/0
and bridge bound CO are found at -2060 and 1850-1935 cm-l, respectively. The gem-dicarbonyl species is produced via oxidative disruption of Rh,O particles to produce dispersed Rh+ species upon exposure of the catalyst to gas-phase C0.3 Prins and co-workers have suggested the overall reaction presented below for the formation of the gem-dicarbonyl species4
+
2Rh0 4CO
+ 2A1-OH
-
2A1-O-Rh1(CO),
+ H2
The rate of production of the gem-dicarbonyl species has been shown to depend upon the concentration of OH groups on the A1203 supports and to involve isolated OH groups instead of H-bonded hydroxyl species.6 The gemdicarbonyl species is catalytically inactive and a number of studies have investigated inhibition of its formation by removing hydroxyl groups via thermall*s and chemical dehydroxylation method^.^-^ While effective, the thermal treatments necessary to dehydroxylate the support (annealing at 1100 K) led to a decrease of CO chemisorption capacity as determined by IR spectroscopy.ltsJ0 Possible explanations for the decreased chemisorption capacity are loss of exposed Rho due to sintering of the Rh overlayer, diffusion of rhodium into the support, encapsulation of Rh particles via diffusion of A1203 on top of the metal overlayer, or formation of a Rh-Al203 compound on which CO chemisorption is s u p p r e ~ s e d . l ~Functionalization ~J~ of the y-A1203support withsilylgroups (Al-O-Si(CH3)3)718 and potassium cations (A1-OKlBwas found to be effective in suppressing formation of the gem-dicarbonyl species and, in the case of potassium, stable to hydrolysis at relatively high temperatures (770 K). (3) Primet, M. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 2570.
(4) Van't Blik, H. F. J.; Van Zonn, J. B. A. D.; Huizinga, T.;Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Phys. Chem. 1988,87, 2264. (6)Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789. (6) Baeu, P.; Panayotov,D.; Yaks, J. T., Jr. J. Phys. Chem. 1987,91, 3133.
(7) Zaki, M. I.; Kunzmann, G.; Bates, B. C.; Kndzinger, G . J. Phys. Chem. 1987,91, 1486. (8) Paul, D. K.; Yates, J. T., Jr. J. Phys. Chem. 1991,95,1699. (9) Zaki, M. 1.;Ballinger, T. H.; Yates, J. T., Jr. J. Phys. Chem. 1991, 95,4028. (IO)Wong, C.; McCabe, R. W. J. Catal. 1989, 119, 47.
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Recent work in our laboratory has shown that temperature programmed desorption (TPD) can be used in conjunction with IR spectroscopy to quantify the amount of CO adsorbed on catalyst surfaces.ll This capability is particularly important for studies of CO adsorption on supported Rh catalysts for which overlap of IR absorption features for the gem-dicarbonyl and terminal-bound CO speciesoccurs, making quantitation of the IR data difficult. Investigation of thermal desorption of CO from Rh/A1203 catalysts has been reported by ~ t h e r s . ~ JTo ~J~ our knowledge, however, no studies have appeared in the literature in which TPD has been used to quantify the amount of CO adsorbed on the surface of a supported Rh catalyst in ultrahigh vacuum. Described here are the results of experiments in which the combined techniques of IR spectroscopy and TPD have been used to investigate how annealing Rh/A1203 catalysts under ultrahigh vacuum (UHV) conditions affects their CO adsorption properties. In addition to identifying which Rh bound CO species are present on the surface, the quantity of CO adsorbed on the catalysts has been measured for progressively higher anneal temperatures. Furthermore, the capability of cooling the catalyst samples to low temperatures (- 120 K) has permitted sites on the 7-A1203support to be probed by CO adsorption in addition to sites on the Rh overlayer. Our results provide new insight with regard to the mechanism of high temperature deactivation of Rh/A1203 catalysts. Experimental Section Catalyst Preparation and Mounting. Rh/A1203 catalysts were prepared by impregnation Of -y-AlzOa(Engelhard AL-3945) with aqueous solutions of rhodium(II1) trichloride (JohnsonMatthey). The 7-A1203had a BET surface area of 255 m2/gand a pore volume of 0.60 mL/g. The alumina support (l/l&. extrusions) was ground to a fine powder prior to use. Following impregnation, the catalysts were dried for 24 hat 393 K and then calcinedfor 3 h in air at a temperature of 773 K. Catalyst samples (-10.0 mg) were pressed at 10 000 psi onto a tantalum metal mesh (50 X 50 mesh size, 0.003 in. wire diameter). The area of the pressed samples was 0.90 cm2 and sample thickness were normalized using their masses. The temperature of the sample was monitored by means of a chromel-alumel thermocouplespotwelded to the tantalum mesh. Ultrahigh Vacuum/High-Pressure System. This research was carried out in a bakeable,stainless steel UHV systempumped by a 110 L/s ion pump and equipped with a high-pressure cell that can be isolated from the vacuum chamber; the system has been described in detail elsewhere." In brief, the high-pressure cell consists of a 23/4in. cube cross equipped with flangemounted CaF2windows. The sampleholder is comprised of a 23/4-in.conflat flange outfitted with feedthroughs for resistive heating, temperature measurement, and liquid nitrogen cooling. The sample holder is mounted to the cell via the top face of the cube cross, perpendicular to the CaF2 windows, permitting infrared measurements to be conducted in the transmission mode. Samples supported on the tantalum metal mesh are clamped onto copperberyllium sample supports and can be cooled to -120 K and heated to -1200 K. Sample heating is accomplished using a home-built temperature controller which allows linear sample heating at rates of 0.1-10 K/s. Gases are introduced into the high-pressurecellvia a welded,stainless steel gashandling system connected to the cell by l/4 in. tubing. Infrared measurements are accomplished usinga MattsonRS-1FTIRspectrometerwhich has a water-cooledsource and a narrow-band MCT detector and is interfaced to a personal computer for data acquisition and treatment. For TPD experiments, the UHV system is outfitted with a Lyebold-InficonQuadrex 200 quadrupole mass spectrometer which is also interfaced to a personal computer. The mass (11) Diaz, A. L.;Bussell, M. E. J. Phys. Chem. 1993,97, 470. (12) Aria, H.; Tominaga, H. J . Catal. 1976,43, 131. (13) Yao, H.C.;Rothschild, W. G . J . Chem. Phys. 1978,68, 4774.
spectrometerwas calibratedfor TPD measurementsby simulating a CO TPD spectrum with a known amount of gas measured volumetrically in the high-pressure cell. This calibration was readily reproducible and is believed to be accurate within 10 % . IR/TPD Measurements. Following mounting of a catalyst sample in the vacuum system, it was outgassed for 16 h at 475 K. Catalyst samples were then reduced in pure hydrogen (Matheson Gas Products, 99.999% minimum purity) by heating to 475 K in 100 Torr Hzfor 30 min followed by outgassingat 475 K in UHV for 30 min. This process was repeated 4 times with the final outgassingcarried out for 16 h. The typical system base pressure following such a procedure was -2 x 10-8 Torr. After the sample was cooled to 120 K, an IR spectrum was recorded; IR spectral acquisitionconsistedof eight scansof the region 40001000 cm-l at a resolution of 4 cm-l and took less than 10 s to complete. For spectra acquired in vacuum,the backgroundwith which the sample spectrum was ratioed was that of a blank tantalum mesh mounted in the sample holder. Following acquisition of the predose IR spectrum, the high-pressure cell was isolated from the UHV system and pressurized to an equilibrium CO pressure of 5.0 Torr. The CO (Matheson Gas Products, 99.99% minimum purity) was passed through a l/B-in. stainless steel coil submerged in a pentane slush prior to dosing in order to remove metal carbonyl impurities. Following acquisition of an IR spectrum at a CO pressure of 5.0 Torr (using a similar background as above but with PCO= 5.0 Torr), the carbon monoxide was evacuated using a mechanical pump prior to opening the valve linking the high-pressure cell to the UHV system. When the system pressure had lowered to 6.0 X 10-8 Torr ( - 5 min),IR and TPD spectra were acquired. Temperature programmed desorption experiments were carried out using a heating rate of 1 K/s while acquiring data for mass 28 (CO) at a sampling frequency of 2 points/K. Experiments performed using a blank Ta mesh revealed no contribution to the TPD spectrum from CO desorbing from the sample holder. Initially, TPD data were also acquired for masses 2 (Hz)and 44 (COz),but no reproducible signals above the background were obtained. Followingcollectionof the TPD spectrum, the sample was either heated to the next annealing temperature for 30 min and then cooled or cooled immediately to -120 K depending upon the experiment. Three kinds of experiments were carried out. The first type investigated the effect of annealing 5% Rh/A1203 catalysts in vacuum on the COadsorptionproperties of the catalysts. Infrared and CO TPD spectra were acquired after annealing the catalysts at progressively higher temperatures in UHV. Followinga given anneal (e.g. 475 K), the sample was cooled and dosed with CO, and IR and TPD spectra were acquired. TPD experiments were carried out from the starting temperature of -120 K up to the next highest anneal temperature (e.g. 600 K) at which the sample was maintained for 30 min. The subsequent TPD experiment was conducted in a similar manner except that the final temperature was increased to the next anneal temperature (e.g. 800 K). In the second type of experiment, IR spectra were acquired during TPD experiments for a 1 % Rh/A1203catalyst which had been previously annealed at 475 K for 30 min. IR spectra were acquired at 10-5 intervals during the TPD experiment which was carried out from 140 to 800 K. The third type of experiment investigated the effect of repeated temperature cycling (i.e. TPD experiments) upon the CO adsorption properties of aluminasupported Rh catalysts. For these experiments, a 5% Rh/AlzOa catalyst was annealed at 900 K for a period of 1 hand then cooled to 120 K and exposedto CO followingthe procedure described above. Infrared and TPD spectra were then acquired in the normal fashion with the exception that the TPD experiment was stopped at 850 K and the sample immediately cooled to 120 K. The catalyst was then dosed again with CO, IR and TPD spectra were acquired, and the process was repeated. The anneal temperature of 900 K was chosen for this experiment because it is just above that needed to completely desorb CO from the Rh overlayer (-800 K) during a TPD experiment. All IR and TPD spectra are reproduced without any further background or smoothing treatment. Infrared peak areas were calculated in the uco region from 2250 to 2150 cm-l for CO adsorbed in AP+sites and from -2150-1965 cm-1 and -1965-
-
-
Study of CO on RhIA1203 Catalyst
Langmuir, Vol. 10, No. 5, 1994 1463
1690 cm-1 for CO bound in Rh sites using the Mattson FTIR software. The integrated absorbancesplotted in Figure 9 for the gem-dicarbonylspecies were determined using the curve fitting capabilities of the Mattaon FTIR software. The peak parameters used for the V. and var absorbances of thegem-dicarbonylspecies were taken from the 320 K spectrum (seeFigure 7) of CO adsorbed on a 1%Rh/A1203catalyst. Curve fitting of the absorbance of the linear CO speciesproved to be more difficult and ita integrated absorbance was instead determined by subtraction of the synthesized spectrum for the gem-dicarbonyl species from the experimentalspectrum. The integrated absorbancefor the bridge CO specieswas determined by directintegration of its absorbance peak. TPD peak areas were calculated using points on either side of the peak for which the mass spectrometer signal was at its base-linelevel,or where necessary,reached ita minimum value between two peaks.
Results IR Spectroscopy and TPD of CO on yAl2O3 and a Rh/A1203 Catalyst. Samples of pure r-Al2O3 and of a 5% Rh/A1203 catalyst were mounted in the UHV system, outgassed as described in the Experimental Section, and then annealed at a temperature of 800 K for 30 min. Following cooling to 120 K and exposure of the samples to an equilibrium CO pressure of 5.0 Torr (-1 min exposure, followed by evacuation to UHV pressures), IR spectra were acquired in the uco region (see Figure la). The IR spectrum for the -pA1203sample exhibits a single absorption peak at 2199 cm-l. Consistent with previous work in our laboratory'' and the work of ~ t h e r s , this ~~J~ absorbance is assigned to CO weakly chemisorbed on A13+ sites created by dehydroxylation of the yA1203 support. This peak is also observed for the 5% Rh/A1203 catalyst treated similarly, but the IR spectrum for this sample exhibits additional vco absorbances associated with CO bound to the Rh overlayer. Focusing on these latter CO species, the absorption features at 2104 and 2038 cm-l are assigned to the symmetric and asymmetric stretches of the Rh bound gem-dicarbonylspecies, respectively.' The two remaining features, located at 2069 and 1923cm-' are assigned to Rh bound CO species adsorbed on linear and bridge sites, respectively.' Following acquisition of the IR spectra, temperature programmed desorption spectra were acquired for the pure 7-Al203 and 5% Rh/A1203 samples dosed with CO (see Figure lb). The TPD spectrum for the pure yA1203 sample exhibits a single desorption feature with a maximum rate of desorption at -170 K; this peak has been assigned previously to desorption of CO from AP+ sites.ll For the 5 % Rh/A1203 catalyst, two desorption features are apparent with the peak at 170 K being assigned to CO desorption from AP+ sites while the broad peak with maximum rate of desorption at -500 K is assigned to CO desorption from the Rh overlayer. This latter assignment is consistent with those for CO desorption from the Rh(111)and Rh(ll0) single crystal surfaces for which the maximum rate of desorption occurs at 500 K with a shoulder at -425 K.'6J7 The peak at 500 K has been identified to be due to desorption of linear bound CO while the shoulder at -425 K is due to desorption of the more weakly bound bridge CO species. The adsorption of CO has also been investigated on model Rh/A1203 catalysts prepared by vapor deposition of Rh either onto an A1203 film grown on a Mo(ll0) single crystal or directly on an
-
(14) Della Gatta, G.; Fubini, B.; Ghiotti, G.; Morterra, C. J. Catal. 1976, 43, 90. (15) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041. (16) Root, T. W.; Schmidt, L. D.;Fisher, G. B. Surf. Sci. 1985,150,173. (17) Baird, R. J.; Ku,R. C.; Wynblatt, P. Surf. Sci. 1980, 97, 346.
2199
2400
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1700
Wavenumbers (cm-1) I
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i
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,
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/
l
~
,
~
17YK
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200
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,
l
400
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i
500
.
l
600
.
l
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,
l
800
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i
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Temperature ( K )
Figure 1. (a) IR spectrum of the vco region of a 5% Rh/A1203 catalyst in UHV following annealing at 800 K and dosing with an equilibrium pressure of 5.0 Torr of CO at 120 K. (b) TPD spectrum for CO on a 5% Rh/A1203. The TPD experiment was
carried out in UHV following acquisition of the IR spectrum shown in Figure la.
a-A1203(0001)single crysta1.l8J9 For both of these model catalysts, the CO desorption spectrum closely resembles those obtained from the Rh single crystal surfaces, albeit with the -425 K shoulder associated with bridge bound CO less well resolved than is observed for the single-crystal substrates. Excludingthe desorption feature for CO bound to coordinately unsaturated AP+ sites, the only significant differences apparent between the CO TPD spectrum acquired in this study for a 5% Rh/A1203 catalyst and those for the Rh single crystals and thin film catalysts is the lack of resolution of the shoulder associated with bridge site CO species and the presence of a high-temperature tail in our spectrum. For the Rh single crystals and the model Rh/A1203catalysts, the CO TPD signal has returned to its baseline value by a temperature of 600 K while in the case of the 5% Rh/A1203 catalyst this does not occur until -850 K. There are at least three possible explanations for this difference. In contrast to the Rh singlecrystal surfaces and the model supported catalysts, there is a significant concentration ofgem-dicarbonyl CO species on the surface of the 5 % Rh/A1203 catalyst in addition to (18) Chen, J. G.; Colaianni, M. L.; Chen, P. J.; Yates, J. T., Jr.; Fisher, G. B. J. Phys. Chem. 1990, 94, 5059. (19) Altman, E. I.; Gorte, R. J. Surf. Sci. 1988, 195, 392.
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1464 Langmuir, Vol. 10, No. 5, 1994
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600 K
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4000
3800
3600
3400
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475 K
Wavenumbers (cm-1) Figure 2. IR spectra of the Y ~ region H for a 5 % Rh/A1209catalyst annealed in UHV at the indicated temperatures for 30 min.
linear and bridge bound CO species. Neither of these other surfaces have the A1-OH groups needed for formation of the gem-dicarbonyl species and it is therefore not surprising to expect differences in the CO TPD spectra. As will be discussed later, the gem-dicarbonyl CO is the most strongly bound of the three CO species and therefore desorbs at a higher temperature. A second possible explanation for the high-temperature tail in the CO TPD spectrum relates to the porous nature of the 5 % Rh/A1203 catalyst. Herz and co-workers carried out a TPD study of CO on a Pt/A1203 catalyst in vacuum and found that adsorption equilibrium is approached under conditions similar to ours.20 Calculations modeling the CO TPD experiment showed that if CO readsorption is accounted for, then broadened desorption peaks and shifts of peak maxima to higher temperatures are predicted.20 Thus while the peak temperature in our spectrum is similar to that of the single crystal and thin film catalysts, the broadness of the peak may be related to CO readsorption phenomena. Lastly, temperature gradients within the sample cannot be ruled out as a possible explanation of the broad CO desorption peak from Rh sites. To our knowledge,the CO TPD spectra presented here for a 5% Rh/A1203 catalyst are the first to appear in the literature in which a single adsorbate has been used to probe sites in both the metal overlayer and the catalyst support simultaneously. As mentioned above, both the Rh/A1203/Mo(110) and Rh/a-A1203(0001) catalysts lack A1-OH groups and the concomitant coordinately unsaturated AP+sites created by dehydroxylation. It will be shown shortly that the ability to probe AF+ and Rh sites simultaneously is useful for investigating possible interactions between the alumina support and the Rh overlayer. Effect of AnnealingontheIRSpectraofaReduced 5 5% Rh/AlzOsCatalyst. Shown in Figure 2 are IR spectra in the VOH region for a reduced 5 % Rh/A1203catalyst which was outgassed in UHV at progressively higher temperatures and then cooled to -120 K. The IR spectrum (20) Herz, R. K.; Kiela, J. B.; Marin, S. P. J. Catal. 1982, 73, 66. (21) Zaki, M. I.; Knljzinger, H. Spectrochim. Acta 1987,43A, 1455.
I
... ZlW
2300 2200 2100 2000 1900 1800 1700
Wavenumber (cm-1)
Figure 3. IR spectra of the YCO region for a 5 % Rh/A1208catalyst in UHV following annealing at the indicated temperature and dosing with an equilibrium pressure of 5.0 Torr CO at 120 K.
following the 475 K anneal, while differing in relative peak intensities, is similar to that observed in our laboratory previously for pure 7-A120311and exhibits peak maxima a t 3438 and 3689 cm-l as well as shoulders at 3733 and 3764 cm-I. This spectrum and the spectra corresponding to the higher anneal temperatures are also similar to those observed by Yates and co-workers for a 0.15% Rh/A1203 catalyst.* Similar to what is observed for pure 7-A1203, heating the Rh/Al203 catalyst in UHV results first in removal of associated hydroxyl groups (3438 cm-l) at the lower anneal temperatures followed by removal of isolated hydroxyl groups (3689, 3733, 3764 cm-l) at the higher anneal temperatures. Excluding the hydroxyl region, no other changes were observed in the IR spectra of the Rh/ A1203 catalysts as a function of heating in UHV conditions. Effect of Annealing on the IR Spectra of a 6 % Rh/ A1203 Catalyst Dosed with CO. After annealing a 5% Rh/A1203 catalyst in UHV at temperatures of 475, 600, 800, 1000, and 1200 K, the catalyst was cooled to -120 K and exposed to CO as described in the Experimental Section. Following evacuation to a pressure of 6.0 X 10-8 Torr, IR spectra were acquired and the CO spectral region is shown in Figure 3 for the different anneal temperatures. IR absorption features are observed for CO adsorbed on both the r-Al203 support and the Rh overlayer as discussed above. As the anneal temperature is increased from its lowest value of 475 K, a number of significant changes are apparent in the vco region of the IR spectra. As expected, the intensity of the vco feature associated with CO bound to A13+ sites increases as the anneal temperature is increased due to the creation of these sites via dehydroxylation of the alumina support. For anneal temperatures above 800 K, two vco absorbances are apparent with the peak at 2199 cm-1 assigned to CO adsorbed in
Study of CO on RhlA1209 Catalyst
octahedral AP+ sites and the peak at 2223 cm-l assigned to CO adsorbed in tetrahedral A13+ sites.20 It has been shown previously, that a linear correlation exists between the loss of hydroxyl groups and the increase in the amount of CO chemisorbed to A13+sites of pure y-A1203.11*15This correlation is also observed for the 5% Rh/Al203 catalyst for all but the highest anneal temperature (1200 K) for which the integrated area of the uco absorbance associated with AP+ sites decreases despite the fact that dehydroxylation of the alumina support was not complete until this anneal temperature. Focusingon the vco region associated with Rh bound CO, a number of prominent changes are apparent in the spectra as the anneal temperature is increased. The intensity of each of the Rh-bound CO species decreases with the most pronounced decrease associated with the gem-dicarbonylspecies. By an anneal temperature of 1200 K, the symmetric (2100 cm-1) and asymmetric (2038 cm-') stretches of the gem-dicarbonyl species are essentially gone and the uco stretches of the linear (2068 cm-l) and bridge bound (1932 cm-l) species are greatly diminished. The spectra are similar in general details to those observed by Ballinger and Yates for a more highly dispersed Rh/A1203 catalyst (0.15% Rh, 104 m2/g)except that there appears to be a relatively smaller loss of IR intensity for Rh-bound CO species for their more highly dispersed catalyst.' Comparison of the IR spectra from the 475 and 1200 K anneals reveals a significant shift from 1880 to 1932 cm-l for the bridge bound CO species. Others have associated such shifts with an increase in the size of Rh particles to which the bridge CO species is bound.' Effect of Annealing on the TPD Spectra of a 5% Rh/A1203Catalyst Dosed with CO. Following acquisition of the IR spectra described immediately above, temperature programmed desorption experiments were carried out and the corresponding spectra are shown in Figure 4. In each case, the TPD experiment was carried out until the next highest anneal temperature was reached which explains why the TPD spectrum following the 475 K anneal was interrupted prior to ita completion (i.e. at 600 K). As discussed earlier, the CO TPD peak with a maximum rate of desorption a t 170-185 K is associated with CO desorption from AP+ sites while the peak with a maximum at -490-500 K is due to desorption of CO from the Rh overlayer. Because quantitative analysis of shifts in TPD peak maxima is complicated for porous catalysts, TPD is used in this work solely to quantify the amount of CO adsorbed on the y-Al2Os support and Rh overlayer. I t is interesting to note, however,that the TPD spectrum following the 475 K anneal rises more rapidly and peaks at a slightly lower temperature (-490 K)than for the higher anneals for which the TPD peak is broader and has a maximum rate of desorption at -500 K. This observation is contrary to what one might expect in that it can be inferred from the IR spectra that the catalyst has the highest concentration of Rh-bound CO following the 475 K anneal, and readsorption equilibrium would therefore be most closely approached during the TPD experiment for this anneal. The narrowness of the peak and the slightly lower peak temperature suggest otherwise. Comparing the TPD spectra associated with the different anneal temperatures, two trends are clearly evident. Excluding the 1200 K anneal, the quantity of CO adsorbed on the y-A1203support increases with increasing anneal temperature, reflecting the fact that the concentration of AP+ sites increases as the support becomes progressively more dehydroxylated. The quantity of CO adsorbed on the Rh overlayer, on the other hand, decreases with
Langmuir, Vol. 10, No. 5, 1994 1465 I
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~
100
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Figure 4. TPD spectra for CO on a 5% Rh/A1203 catalyst following annealing at the indicated temperature and dosingwith an equilibrium pressure of 5.0 Torr of CO at 120 K. The TPD spectra were acquired in UHV.
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0.0 1 400
, 600
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Anneal Temperature ( K )
Figure 5. A plot of the integrated CO TPD peak areas versus anneal temperature for a 5% Rh/A1203 catalyst.
increasing anneal temperature indicating a decrease in the amount of exposed rhodium. These trends are shown graphically in Figure 5 in which the CO TPD peak areas, converted to CO moleculesper unit surface area of catalyst, are plotted as a function of the anneal temperature for anneals of 600 K and above. The 475 K data were not included in this plot as the TPD experiment for this anneal was interrupted at 600 K. When converted to units of CO. molecules per Rh atom, the CO adsorption capacity of the Rh overlayer following the 600 K anneal has a value of 0.45 CO molecule/Rh atom which decreases to 0.13 CO molecule/Rh atom following the 1200 K anneal. Based upon the data presented in Figure 5, two important observations can be made. The most precipi-
Diaz et al.
1466 Langmuir, Vol. 10, No.5, 1994
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70.0
Table 1. Integrated IR Extinction Coefficients for CO Adsorbed on Supported Rh Catalysts zco (cm/molecule) catalyst gem-dicarbonyl linear bridge ref Rh/SiOz 49 X 10-1' 4.7 X l0-l' 16 x 10-1' 23 Rh/&03 34 X 10-" 4.2 X i t i 7 14 X 10-1' 24 Rh/A1203 2.3 X 10-1' 25 Rh/A1203 4.2X 10-'7 this work
0 60.0 -
50.0
-
40.0
-
30.0 20.0
-
10.0
c
-I
, 875 K) has been identified as one of the mechanisms by which threeway catalysts used in automobile catalytic converters become deactivated. The TPD results of the current study indicate that the loss of exposed Rho sites on a 5% Rh/ A1203catalyst begins at temperatures as low as 800 K with a 71% decrease in the adsorption capacity of the Rh overlayer occurring from an anneal temperature of 600 K (0.45 CO/Rh) to 1200 K (0.13 CO/Rh). The results presented here suggest at least two mechanisms are responsible for the loss of Rho from the surface of the catalyst. Reference to Figure 3 shows that the infrared absorbance associated with bridge CO species shifts from 1880 cm-1 for the catalyst annealed at 475 K to 1932 cm-l following the 1200 K anneal. The stretching frequency of this vco absorbance is thought to be sensitive to the size of the Rh particle to which the CO is bound with blue shifts associated with an increase in the size of the metal p a r t i ~ l e .The ~ IR spectra presented in Figure 3 suggest, therefore, that sintering of the Rh overlayer begins at temperatures as low as 600 K and leads to a significant decrease in the CO adsorption capacity of the rhodium. In addition to sintering of the Rh overlayer, the results of the current study suggest that there is a second deactivation mechanism for Rh/A1203 catalysts at high temperatures. As shown in Figure 2, annealing a 5% Rh/ A1203catalyst in UHV results in dehydroxylation of the 7-A1203support with complete removal of hydroxyl groups accomplished by heating to 1200 K. Reference to Figure 3 shows that dehydroxylation of the support brings about two important changes in the chemisorptive properties of the 5% Rh/Al203 catalyst: (1)suppression of the production of gem-dicarbonyl species via the oxidative disruption reaction; (2) creation of AP+ sites on which CO weakly chemisorbs. Using IR spectroscopy, Ballinger and Yates have shown that a linear correlation exists between the integrated absorbances of the vco band associated with CO bound to AP+ sites and the VOH band of pure y-Al~03;'~ this inverse relationship between hydroxyl groups and CO bound to AP+ sites has recently been confirmed in our laboratory and quantified via CO TPD.ll Interestingly, in the work presented here, this correlation was not observed when the 5 % Rh/A1203 catalyst was annealed above 1000 K. As shown in Figure 2, dehydroxylation of the alumina support (and therefore creation of AP+ sites) was not complete until 1200 K, yet the quantity of CO adsorbed on Al3+ sites decreases by 17 ?4 from the 1000 K anneal to the 1200 K anneal. The loss of CO adsorption capacity associated with AP+ sites suggests a loss of surface area of the 7-A1203support when the catalyst is annealed at 1200 K. Ballinger and Y ates have also investigated the effect of high temperature annealing on the CO adsorption properties of Rh/A1203 catalysts; using IR spectroscopy they found that an anneal temperature of 1200 K was necessary
Diaz et al.
1470 Langmuir, Vol. 10, No. 5, 1994 to completely dehydroxylate the support and that the CO adsorption capacity of the Rh overlayer was completely lost following heating to 1400 K.' X-ray diffraction analysis of 0.15% Rh/A1203 catalysts annealed at 1300 and 1400 K showed the former to have the crystalline structure of 7-A1203 while the latter the structure of a-A1203, such a structural transformation had been identified previously for pure ?-A1203 in the temperature range 1375-1450 K.26 a-A1203 is the thermodynamically stable phase of alumina at high temperatures, and its formation from yA1203 involves a collapse of the pore structure to produce a material with much lower surface area.2 Morterra et al. have recently investigated the adsorption of CO on a-Al203 and observeda YCO absorbance at 2165 cm-l which they assigned to CO adsorbed on octahedral AP+ sites.27 As shown in Figure 3, the IR spectrum for the 1200 K anneal shows vco absorbances at 2199 and 2223 cm-l which are assigned to octahedral and tetrahedral AP+ sites of 7-A1203;there is no evidence in the spectrum of an absorption feature at 2165 cm-'. Thus, while annealing the Rh/A1203 catalyst to 1200 K in UHV has apparently not converted the support to a-AlzO&the decrease in AP+ sites of the yAl2O3 support provides indirect evidence that structural rearrangement of the catalyst has begun which results in a loss of surface area and, possibly, encapsulation of Rho particles. One final point which needs to be addressed is the fact that structural changes occur in the Rh overlayer during CO TPD from Rh/A1203 catalysts. The results presented in Figures 7-10 indicate that oxidative disruption of the Rh overlayer occurs during heating to -320 K and leads to a net increase in CO adsorption sites for a freshly annealed catalyst. As a 1%Rh/A1203 catalyst dosed with CO is heated above -190 K, the difference spectra reproduced in Figure 8 indicate that gem-dicarbonyl species are formed on the catalyst surface, presumably due to oxidative disruption of the Rh overlayer to produce isolated Rh+ species. This observation is supported by IR spectroscopy studies carried out by others which indicate that this conversion is an activated process which occurs at temperatures above 150K,7,X*28gBconsistent with the IR spectra presented in Figures 7-9. Coincident with the increase in integrated absorbance of the gem-dicarbony1species as the 1% Rh/A1203 catalyst is heated above 190K is adecrease in integrated absorbance of the linear and bridge CO species. This observation is noteworthy because no CO desorption was observed from the Rh overlayer of a 5 % Rh/A1203 catalyst previously annealed at 475 K until -320 K (see Figure 4), suggesting that the loss of CO from linear and bridge sites at lower temperatures is due to another process. Given that the TPD experiments are carried out in the absence of CO gas (i.e. in vacuum), the two possible sources of CO for formation of the gem-dicarbonyl species are CO desorbing from AP+ sites of the support (140-260 K) and/or CO adsorbed on the Rh overlayer. The first possibility can be eliminated because the integrated extinction coefficient calculated for CO bound on AP+ sites is in close agreement with the value determined for pure 7-Al2O3, indicating that quantitative desorption occurs for CO adsorbed in A13+sites. Thus, the two CO molecules needed to form a gemdicarbonyl species apparently originate from linear and/
-
(26) Stumpf, H.C.; Russell, A. S.; Newsome, J. W.; Tucker, C. M. Ind. Eng. Chem. 1950,42,1938. (27) Morterra, C.; Magnacca, G.; Del Favero, N. Langmuir 1993, 9, 642. (28) Solymosi,F.; Kndzinger,H. J. Chem. Soc., Faraday Trans. 1990, 86, 389. (29) Dictor, R.; Roberta, S. J. Phys. Chem. 1989,93, 2526.
or bridge sites in the Rh overlayer as suggested by the decrease in the integrated absorbance of these species before the onset of CO desorption from Rh sites. As shown in Figure 9, the integrated absorbance of the gem-dicarbonyl species reaches its maximum a t -320 K and continued heating of the 1% Rh/Al203catalyst above this temperature results in a decrease in integrated absorbance of all three Rh-bound CO species, in agreement with the onset of CO desorption from the Rh overlayer. The data presented in Figure 9 indicate that desorption of linear and bridge CO species is complete by -600 K, while desorption of the gem-dicarbonyl is not complete until -800 K. The results presented here are in agreement with studies by others of CO desorption from Rh single crystal and thin film surfaces which show CO desorption from linear and bridge sites is finished by 600 K.16-19 It can be concluded that the high temperature tail observed in the CO TPD spectra of the current study is due to desorption of gem-dicarbonyl species which are more strongly bound to the Rh overlayer than either the linear or bridge CO species. It should be noted that in the CO TPD studies from Rh single crystal and thin film catalysts that the bridge CO species was found to desorb at a lower temperature than the linear CO specie^.^^^^ In the current study, IR spectra acquired during CO TPD suggest no difference in the range of temperatures over which the two species desorb from the Rh overlayer of a 1%Rh/ A1203 catalyst. This observation is supported by the fact that no shoulder is observed in the TPD peak associated with CO desorption from Rh sites on a 5% Rh/Al203 catalyst (see Figure 4). As mentioned earlier, however, a red shift of the absorbance peak of the linear CO species occurs above -260 K which complicates curve fitting of the absorbance features associated with this species and the asymmetric stretch (v-) of thegem-dicarbonyl species because of overlap of the two bands. Close examination of Figure 9 reveals that the decrease in the integrated absorbance of the linear CO species accelerates above 260 K, suggesting that the red shift of its absorbance peak is in fact causing an underestimation of its integrated absorbance. The red shift of the linear CO absorbance feature has been studied in detail by Yates et aLm In their work, a downward shift of the linear CO absorbance from 2082 cm-l to -2030 cm-l was observed to occur at temperatures 1 265 K. The authors suggested that this shift is associated with a structural rearrangement of the Rh site to which the linear CO species is bound, resulting in a strengthened Rh-CO bond.30 Focusing again on the current study, the IR spectra presented in Figures 7 and 8 provide no evidence for a reductive desorption process in which isolated Rh+ species agglomerate to produce RhXoparticles.
-
2A1-O-Rh1(CO)2
-
XRhO-CO
Al-O-A1+ 2RhO-CO
-
Rh,-CO
+ CO + CO,
+ (X - 1)CO
Of course, it is possible that because of the high catalyst temperature (2' > -450 K) at which the surface becomes depleted of gem-dicarbonyl species, bridge CO species formed would desorb immediately and its concentration would therefore simply be too small to detect via IR spectroscopy. Infrared spectra acquired for a 5% Rh/ A 1 2 0 3 catalyst subjected to repeated CO TPD experiments (Figure 11) do provide indirect evidence that reductive desorption occurs during CO TPD. The first difference (30) Yates, J. T., Jr.; Vaughan,R. W. J. Chem. Phys. 1980, 73, 975.
Study of CO on RhIA1203 Catalyst spectrum, prepared by subtracting the IR spectrum prior to the first TPD run from the IR spectrum acquired before the second TPD run, shows a significant increase in the absorbance peaks for both the linear and bridge CO species and a decrease in the absorbance bands associated with the gem-dicarbonyl species. As discussed earlier, the concentration of gem-dicarbonyl species was observed to increase in the temperature range 190-320 K of a TPD experiment. However, following completion of the TPD experiment and reexposure of the catalyst to CO at -140 K, the concentration of gem-dicarbonyl species has decreased below its level prior to the first TPD run. It can be inferred, therefore, that Rh+ sites created via oxidative disruption between 190and 320K are converted to small Rh, particles via reductive desorption at higher temperatures. As shown in Figure 10,this process leads to a net increase (17%) in CO adsorption capacity of the Rh overlayer for a freshly annealed 5 % Rh/A1203 catalyst. Subsequent TPD experimentsproduce only minor changes in the IR spectra acquired prior to the TPD run and no measurable change in the catalyst’s adsorption capacity. Conclusion Temperature programmed desorption has been used in conjunction with infrared spectroscopy to identify and quantify adsorbed CO species on a 5% Rh/A1203 catalyst. FollowingCO adsorption at -120 K, CO TPD reveals two
Langmuir, Vol. 10, No. 5, 1994 1471 desorption features, a peak with maximum rate of desorption a t 170-185 K associated with CO desorption from AF+ sites of the ?A1203 support and a broad peak with maximum rate of desorption at -500 K due to CO desorption from the Rh overlayer. Sintering of the Rh overlayer has been identified as the primary mechanism by which exposed Rho is lost from the catalyst surface. The decrease in Al3+ sites upon annealing to 1200 K provides indirect evidence for loss of catalyst surface area as a second mechanism by which Rh adsorption sites are lost, possibly due to encapsulation of Rh particles. The decrease in CO adsorption capacity of the Rh overlayer has been quantified as a function of anneal temperature using TPD. In the course of a CO TPD experiment, structural changes occur in the Rh overlayer of Rh/A1203 catalysts with oxidative disruption occurring at low temperatures and reductive desorption occurring at high temperatures. Acknowledgment. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This research was also supported by an award from the Research Corporation and a precious metals loan from the Johnson-Matthey Corporation. The authors acknowledge Patrick Sofarelli for his assistance in acquiring some of the data reported in this study.
