J . Phys. Chem. 1989, 93, 2093-2100
2093
Paramagnetic Rhodium Species in Zeolites. 2. RhK-Lt A. Sayari,* J. R. Morton, and K. F. Preston Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1 A OR9 (Received: November 20, 1987)
Activation of RhK-L under flowing oxygen at different temperatures generated a large number of Rh(I1) species all of which have been characterized by electron paramagnetic resonance (EPR). The EPR parameters of these species were found to bear little resemblance to those of Rh(I1) species generated in RhNa-X and RhNa-Y. The relative stability of the Rh(I1) species has been studied under different conditions but no consistent trend was found. Evacuation at high temperature of preoxidized samples was accompanied by a significant reduction probably involving 0" from the zeolite framework. The small rhodium metallic particles thus generated are believed to disintegrate in the presence of CO without being oxidized, leading to paramagnetic Rh(0)-CO species. In the presence of 02,these metallic particles are oxidized, giving rise to two different Rh(I)-02- species.
Introduction
Under appropriate conditions of activation and reduction, supported Rh zeolites lead to a highly dispersed metallic phase with several potential catalytic applications.'-3 In addition, Rh zeolite systems furnish an excellent case study for the investigation of posssible relationships between homogeneous and heterogeneous c a t a l y ~ i s . ~In - ~order to better understand the effect of various activation procedures on the properties of the final catalyst, much ongoing research activity has focused on characterizing such catalysts at different stages of activation. The techniques that have been used include infrared6 spectroscopy, temperatureprogrammed oxidation and reduction,' hydrogen chemi~orption,'-~ TEM,' XPS,3,8X-ray d i f f r a ~ t i o n EXAFS,Io ,~ and EPR.ll-'S As far as the use of electron paramagnetic resonance (EPR) is concerned, all previous studies have dealt with rhodium in X, Y, and A ~eo!ite."-'~ Activation of RhNa-Y under flowing O2 at 500 OC and then under vacuum at the same temperature generates an axially symmetric species (g,,= 2.0, g, = 2.6) interpreted by Naccache et al.I2 as Rh(I1) located in the 0 cage or the hexagonal prisms. Similar EPR signals were also obtained by Atanasova et al.Ila and considered as indicative of the occurrence of pairs of mixed valence Rh ions, Rh(I1)-Rh(1) or Rh(1)-Rh(O), in the a cage. The same treatment carried out at about 200 OC gives rise to a rhombic species (gl= 2.09, gz = 2.06, g3 = 1.97) assumed to be Rh(I1) still attached to some of the original ligands of the rhodium-chloroammine complex used for the sample preparation.I2 Two major paramagnetic Rh species were also detected in RhNa-X13a and assigned to Rh(I1) species in the 6 cage and in the hexagonal prisms. Additional species were also formed upon exposure to various adsorbates including 02,CO, and Goldfarb and Kevan'3a found that the total spin concentration was not a monotonic function of activation temperature. As suggested earlier by Naccache et a1.,I2they assumed the Occurrence of a diamagnetic Rh(I1) dimer that split into monomers at high temperatures or upon adsorption of various molecules. Our previous EPR investigations on RhNa-Y and RhNa-XI4 did not seem to support such an interpretation. An alternative explanation based on the occurrence of two distinct mechanisms of reduction of Rh(II1) was found to account better for our findings.'4c Moreover, comparison between reports on RhNa-X,'3a,'3b RhCa-X,'3c and RhNa-Y l1,I2 seems to indicate some fundamental changes in Rh behavior in each zeolite. Part of these differences has been attributed to variations in activation procedures. Indeed, in our previous studies,'4b we found that, by closely monitoring the activation procedure of RhNa-X and RhNa-Y, not only can more paramagnetic species be detected, but a close parallel between conditions of formation, stability, and g values can be drawn. In this paper we wish to extend our investigation to Rh in K-L zeolite which is a large-pore zeolite having a different crystal NRCC No. 29786.
0022-3654/89/2093-2093$01 S O / O
structure from the faujasite-type zeolites X and Y.16 It consists of a series of columns along the c axis (Figure l a ) where t cages and D6R units alternate. These columns are cross-linked to each other as shown in Figure 1b, producing wide channels (diameter 7.5 A) parallel to the c axis. There are five types of cation sites located as indicated in Figure 1.
-
Experimental Section
K-L zeolite was kindly provided by Dr. J. G. Goodwin, Jr., of the University of Pittsburgh, and chloropentaamminerhodium chloride, [Rh(NH3)5C1]C12,was purchased from Strem Chemicals. The sample used in this study was prepared by the usual ionexchange method." A quantitative analysis was performed by (1) Khaufherr, N.; Primet, M.; Dufaux, M.; Naccache, C. C. R . Acad. Sci., Paris 1978, C286, 131. (2) Tebassi, L.; Sayari, A.; Ghorbel, A.; Dufaux, M.; Ben Taarit, Y.; Naccache, C. In Proceedings of the Sixth International Zeolite Conference;
Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 368. (3) Tebassi, L.; Sayari, A.; Ghorbel, A,; Dufaux, M.; Naccache, C. J . Mol. Catal. 1984, 25, 397. (4) Gelin, P.; Lefebvre, F.; Elleuch, B.; Naccache, C.; Ben Taarit, Y. In Intrazeolite Chemistry;Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 455. (5) Ben Taarit, Y . ;Che, M. Stud. Surf.Sci. Catal. 1980, 5, 167. (6) (a) Primet, M. J . Chem. SOC.,Faraday Trans. I 1978, 74,2570; (b) Primet, M.; Vedrine, J. C.; Naccache, C. J . Mol. Catal. 1978, 4, 411. (c) Iizuka, T.; Lunsford, J. H. J . Mol. Catal. 1980,8, 391; (d) Lefebvre, F.; Ben Taarit, Y. Nouu.J . Chimie 1984, 8, 387. (e) Lefebvre, F.; Auroux, A.; Ben Taarit, Y. Stud. Surf. Sci. Catal. 1985, 24, 411. ( f ) Rode, E. J.; Davis, M. E.; Hanson, B. E. J . Catal. 1985, 96, 574. (g) Miessner, H.; Gutschick, D.; Ewald, H.; Muller, H. J . Mol. Catal. 1986, 36, 359. (h) Burkhardt, I.; Gutschick, D.; Lohse, U.; Miessner, H. J . Chem. Soc., Chem. Commun. 1987, 291. (7) (a) van Brabant, H.; Schoonheydt, R. A.; Pelgrims, J. Stud. Surf.Sci. Catal. 1982,12,61. (b) Schoonheydt, R. A.; van Brabant, H.; Pelgrims, H. Zeolites 1984, 4, 67. (8) Shannon, R. D.; Vedrine, J. C.; Naccache, C.; Lefebvre, F. J . Catal. 1984, 88, 43 1 and references therein. (9) (a) Bergeret, G.; Gallezot, P.; Gelin, P.; Ben Taarit, Y.; Lefebvre, F.; Shannon, R. D. Zeolites 1986,6, 392. (b) Bergeret, G.; Gallezot, P.; Gelin, P.; Ben Taarit, Y . ;Lefebvre, F.; Naccache, C.; Shannon, R. D. J . Catal. 1987, 104, 279. (IO) Tzou, M. S.; Teo, B. K.; Sachtler, W. M. H. Langmuir 1986,2,773. (11) Atanasova, V. D.; Shvets, V. A,; Kazanskii, V. B. (a) Kine?. Catal. (Engl. Transl.) 1977,18,628; (b) Kine?. Catal. (Engl. Trawl.) 1979,20,427; (c) React. Kine?. Catal. Lett. 1978, 9, 349. (12) Naccache. C.: Ben Taarit. Y . :Boudart. M. In Molecular Sieues II: Katzer, J. R., Ed.; ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977; p 156. (13) Goldfarb, D.; Kevan, L. (a) J . Phys. Chem. 1986, 90, 264; (b) J . Phys. Chem. 1986,90,2137; (c) J . Phys. Chem. 1986,90,5781; (d) J . Am. Chem. SOC.1987, 109, 2303. (14) Sayari, A.; Morton, J. R.; Preston, K. F. (a) J . Phys. Chem. 1987, 91, 899; (b) J . Chem. Sac., Faraday Trans. 1 1988,84,413; (c) Sayari, A,;
Morton, J. R.; Preston, K. F.; Brown, J. R. In Proceedings 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada; Ottawa, 1988; Vol. 1, p 356. (15) Goldfarb, D.; Kevan, L.; Davis, M. E.; Saldarriaga, C.; Rossin, J. A. J . Phys. Chem. 1987, 91, 6389. (16) Breck, D. W. Zeolite Molecular Sieues; Willey: New York, 1974; Chapter 2.
Published 1989 by the American Chemical Society
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The Journal of Physical Chemistry, Vol. 93. No. 5, 1989
Sayari et al.
I'
2.273 240
Figure 1. Zcolile I. (a. upper) schematic reprscntation of the %trumure showing thc extraframeuork cation rites: (b. lower) prqcctiun parallel to the c axis. atomic ahsorption to determine K concentration and by inductively coupled plasma atomic emission spectroscopy to determine R h content. The dehydrated unit cell compusition was Kho,rrK,,,(AWdSiO~hT. Batches of RhK-L were hcated in a greaseless Pyrex or quartz microreactor under flowing oxygen. The temperaturc was slowly raised (2 OC min-l) to a preset value in the range 1 5 0 4 3 0 OC. The camplcs were kept overnight under those conditions, cooled to room tcmpcrature under oxygen, and then flushed u i t h dry nitrogen. For EPR measurements. the samplcs were transferred under a dry nitrogen atmosphere to 4 mm o.d. S u p r a 4 tubes equipped with a vacuum valvc. Subsequent evacuation for 16-18 h u a s carried uut at different temperatures in the range 25-600 OC. EPR examination was carried out after 0, trestment at various temperatures followed by evacuation at 25 "C and after each step of evacuation at higher temperatures. The purpose was, as shown ~ i l r l i c r to . ~distinguish ~ between processes taking place under 0, activation and those occurring during subsequent evacuation. Throughout this repon. a sample that has bcen treatcd under 0, at x 'C. then under vacuum at y OC for z h. w i l l be dcsignated as RhK-L x l y l z . EPR measurcments were also carried out un selected samples after exposure to various gases including H, (UHP. Linde). 0, (UIIP, Linde), and CO (Matheron). A l l gases were uscd after further purification as reported elsewhere.",'8 When necessary. the following labeled compounds were uced as received from .MSD Isotopes. Montrcsl: carbon-I3 monoxide (99.3% and oxygen-I7(70.9atom % "0). Measurements of spin concentrations were carried out on a number of selected samples. For this purpose, we used a Rrukcr ESP 300 spectrometer equipped with a ESP 1600 data system u i t h integration capabilities. Both copper sulfate (Anachemia) and 2.2.6.6-tetramcthylpiperidinyloxy free radical (Aldrich) were used as standards. I n each case 100 scans were accumulated in order to improve the quality ofthe spectra. To ascertain that the ~
~
~~
~
~~
(17) Ouhci, R.; Sayari, A.; Gwdwin, 1.0..Ir. J. Coral. 1986, 102. 126. (18) Sayari, A.; Wan& H.T.;Gwdwin. J. G., 11.J. Coral. 1985.93,368.
I I
1.641
2.326
V Figure 1. First-derivativeEPR s e r a of RhK-L x/25/18: (a) x = 150 ' C , (b) x = 200 'C, ( e ) x = 250 T, (d) x = 300 'C, (e) x = 340 O C , (fj x = 400 *C,( 9 ) x = 460 "C.(h) x = 500 OC. All spectra were recorded at -196 OC with a microwave frequency of 9.1 GHz. samples did not undergo any structural collapse or loss of crystallinity after the various trehtments, a selected set of samples were examined by X-ray diffraction. A computerized Rigaku instrument with Ni-filtered C u radiation was used under the following conditions: voltage, 40 kV; current, 30 mA; scan, 20 = 5-75O; scan speed, 0.08 deg/min.
Results Oxygen Activation. EPR spectra of several RhK-L x/25/18 (x = 150-500 "C) samples are shown in Figure 2. Several paramagnetic species with substantially different gvalues can be distinguished. In assigning these species we made use of the
Paramagnetic Rh Species in Zeolites
F
gI1 =1.958, g1 =2.101
g,, =2.067, g
4 E
D,D'
m g,=2.094, I
I
2.102
1 =2.197
g ,=2.326, g 2 =2.247 g3 =1.907, 9; = 1.880
A
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2095
I
I
g 2 =2.045, 9, =1.975
I
200 300 400 500 TEMPERATURE OF OXYGEN ACTIVATION ( " 0
L E
Figure 3. Schematic representation of the conditions of formation and stability of the various Rh(I1) species under 02.
following methods: (i) comparison of the EPR spectra with each other, (ii) selective power saturation, and (iii) EPR examination at rmm temperature. As seen in Figure 2a, upon oxygen activation at 150 OC, a first species designated A(L) was generated; its orthorhombic EPR spectrum is characterized by the following g values: g, = 2.094, g2= 2.045, and g3= 1.975. Species A(L) can also be detected by EPR a t room temperature. Oxygen activation a t 200 O C gave rise, in addition to species A(L), to a new species referred to as B(L) which had an axially symmetric g matrix with gll = 2.008 and g, = 2.1 15 (Figure 2b). Increasing the temperature of oxygen activation to 250 OC led to the disappearance of A(L) and the development of a third species, C(L), having rhombic symmetry with g, = 2.273, g2 = 2.240, and g, = 1.907. At room temperature, species B(L) exhibits the same spectrum as the one obtained at -196 OC, while C(L) becomes an axial species with gl, = 1.907 and g, = 2.274 instead of a slightly rhombic species. The EPR spectrum of RhK-L 300/25/18 is shown in Figure 2d. In the middle region of the spectrum, new features began to develop; their assignment can be made by comparison to EPR spectra of samples heated in O2at higher temperatures (see below). In the low-field region, two features of a rhombic species are seen at g, = 2.326 and g2= 2.247. The third component at g3of about 1.85 seems to be a doublet with a splitting of ca. 50 G; however, under other conditions (see below) one of two species D(L) and D'(L) having the same g, = 2.326 and g2= 2.247 components can be selectively stabilized. The third component is g, = 1.907 for D(L) and g', = 1.880 for D'(L). Therefore we assume that both species are present in RhK-L x/25/18 with x = 300-400 OC (Figure 2d-f). This assignment is also in agreement with the fact that a 50-G splitting would be unusually high for lo3Rh. As x increased further, two additional species developed. An axially symmetric species, E(L), began to show up a t x = 340 "C (Figure 2e); its g values are gll = 2.067, g, = 2.197. This species remains stable under O2at temperatures up to about 500 OC. At room temperature species D(L), D'(L), and E(L) are hardly detectable as their signals become very broad. An additional species, F(L), which can be observed either at -196 OC or at room temperature developed, under oxygen at x = 400 OC (Figure 2 0 , and vanished at slightly higher temperature. It had an axially symmetric g matrix with gll = 1.958 and g, = 2.101. Almost no EPR signals were left for x higher than 550 OC. In order to summarize these results, Figure 3 is a quick way to visualize the conditions of formation of the various paramagnetic species under O2activation along with their domains of stability and their g values. It is understood that, in this diagram, the temperatures of formation and disappearance of all species are only approximate. It is worthwhile to note that RhK-L treated under O2at low temperature (1 50-250 "C) or at high temperature (630 "C) are
t
1.961
Figure 4. First-derivative EPR spectrum of species G(L) obtained by exposure of RhK-L treated in O2at 340 "C to ambient air for 5 days. Spectral conditions as in Figure 2.
not very sensitive to air. By contrast, samples pretreated in oxygen at intermediate temperature (300-460 "C), when left in air (ambient conditions), develop the same EPR signal as displayed in Figure 4 while all the original signals (Figure Id-f) disappear. The shape of this signal does not change at room temperature or when contacted with up to 350 Torr of O2 or with microwave power. Therefore, we suggest that the signals shown in Figure 4 correspond to only one species, G(L), with g, = 2.102, g2 = 2.037, and g, = 1.961 and the 39-G splitting in the g2component is due to hyperfine interaction with a single Io3Rhnucleus (natural abundance loo%, S = The paramagnetic species were found to consist of about 5% of the rhodium in RhK-L 150/25/18 and 10 to 15% in samples heated under O2 at 250-500 OC. The same percentage was also found in samples containing species G(L) shown in Figure 4. The proportion of paramagnetic Rh decreased dramatically in samples with x > 500 OC. Vacuum Activation. As in the case of RhNa-X and RhNa-Y14 prepared from [Rh(NH3)5Cl]C12,thermal activation under vacuum of nonpreoxidized RhK-L did not generate any paramagnetic species. This is in line with the results of Schoonheydt et al.' who reported that such activation leads, via autoreduction, to a poorly dispersed metallic phase. The color change from white to black is also an indication in favor of such a proposal. Subsequent evacuation of RhK-L x/25/ 18 at increasing temperatures led to different results depending on the temperature x of O2activation. For x = 150-200 OC, the paramagnetic species (A(L), B(L)) disappeared under vacuum at about 250 OC without further changes. For x = 250-500 OC,the overall intensity of the EPR spectra decreased as the temperature of evacuation increased. For example, when RhK-L 300/25/18 was further evacuated at 530 OC for 18 h, the spin concentration decreased from 10-15% to 3-5%. In these experiments, small quantities of D'(L) and E(L) could be detected at temperatures as high as 500-550 OC; however, unlike RhNa-X and RhNa-Y,'4b no new species were generated by vacuum treatment. For x = 630 OC, no changes were noticed upon further evacuation at 25-600 OC. Finally, for samples that have been stored in air after O2treatment (Figure 4), species G(L) was destroyed under vacuum at about 150 "C; at higher temperatures the samples behave as if they had never been stored in air. As a summary, Figure 5 provides a graphical representation of the conditions under which the various paramagnetic species are generated as a result of oxygen activation and subsequent evacuation. It is understood that Figure 5 does not indicate the relative intensities of the various signals; however, as a general rule the most powerful spectra are those obtained after outgassing at room temperature.
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The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Sayari et al.
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-
0'
E
2'015-il
1
D'
D D1
W
I
1 30
Figure 5. Schematic representation of the conditions of formation and stability of the various paramagnetic species in RhK-L with regard to the temperature of O2 activation and the temperature of subsequent
evacuation.
100
G
H
Figure 7. First-derivative EPR spectra of RhK-L 460/500/18 after exposure to O2at room temperature: (a) 5 Torr, (b) 0.1 Torr, (c) 0.01 Torr. Spectral conditions as in Figure 2. The numbers on the right-hand side indicate the relative gain.
jgo7
I
rl
Figure 6. First-derivative EPR spectra of RhK-L 300/25/18 in the presence of 100 Torr of H2(a) at room temperature for 18 h (gain = 6.3 X lo2) and (b) at 160 "C for 2 h (gain = 1.6 X 10)). Spectral conditions as in Figure 2.
At this point, it is worthwhile to note that O2 treatment had no detectable effect on the crystallinity of the samples as revealed by X-ray diffraction studies. However, under vacuum treatment at >550 OC, a slight decrease in crystallinity occurred. Hydrogen Treatment. Several RhK-L x/25/18 were exposed to 150 Torr of H2 for 24 h at room temperature and then for 2 h at 20 OC increments from 60 to 160 OC. The main results are as follows: (a) for x = 150 and 500 "C all signals practically disappear at 80-100 OC; (b) for x = 630 OC, no signals are generated; (c) for x = 300-400 OC, species D(L) increases at the expense of other species; it reaches a maximum after a few hours at room temperature. At 160 OC, the only signal left is a broad and weak signal at g = 2.16. Typical results are shown in Figure 6. It is interesting to note that no major species are generated by H 2 as was the case for RhNa-X and RhNa-Y.13r'4C,19 Adsorption of Oxygen. As may be anticipated, samples activated under flowing oxygen and evacuated at room temperature (19) Sayari, A,, to be submitted for publication.
do not exhibit any extra paramagnetic species upon exposure to oxygen at room temperature. Four samples, RhK-L x/25/18 ( x = 250, 300, 400, and 500 "C), were contacted with O2 at pressures between 5 and 150 Torr and no changes occurred in their EPR spectra except for some line broadening which affected species D(L) and D'(L) more than the others. On the other hand, adsorption of O2 on samples previously treated under flowing oxygen at temperatures above 200 OC and then under vacuum at high temperature resulted in very strong EPR signals. Spin concentration measurements on a RhK-L 300/500/18 in the presence of 4 Torr of O2 show that the new signals correspond to about 35% of the total rhodium content. Typical EPR spectra recorded at different O2 pressures are disTorr, only played in Figure 7. At low O2 pressure, ca. 1 X one axial species referred to as 0-1(L) is generated (Figure 7c). Its EPR parameters are g! = 2.042 and g, = 1.991. At higher pressures at least one additional species, 0-2(L), develops (Figure 7a). We tentatively assign the following parameters for this species: gl = 2.015, g2 = 1.960, and g3 = 1.912. Use of ''0: at pressures in the range 0.1-1 Torr gave only a broad signal having the same shape as the 0-2(L) signal with barely resolved hyperfine structure corresponding to a hyperfine splitting constant of about 70 G. Lower pressures of 1702 led to weak signals that could not be analyzed with sufficient accuracy. Adsorption of Carbon Monoxide. Three RhK-L x/25/ 18 samples with x = 250, 400, and 500 OC were exposed at room temperature to 150 Torr of C O and examined by EPR over a period of 5 days. The main results are (i) species C(L), D(L), and D'(L) disappear immediately, (ii) species B(L) disappears after about 2 h, (iii) species E(L) and F(L) decrease steadily without complete disappearance, and (iv) no new species were formed in any case. Exposure of samples pretreated under O2at temperatures exceeding 200 OC and then under vacuum at 400-600 "C generated two new species even at -196 "C (Figure 8b) with a tremendous increase in spin concentration. For example, upon exposure of RhK-L 300/530/18 to 350 Torr of CO at 25 OC, the spin concentration increases from 3-5% to ca. 35-40% of total Rh. About 60% of this increase is due to species CO-1(L), the remaining being attributable to C0-2(L). Note that the proportion of paramagnetic Rh involved in CO-1(L) and C0-2(L) is comparable to that involved in the EPR signals originating from O2adsorption. The first species generated by CO adsorption, CO-1(L), had an axial
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2091
Paramagnetic Rh Species in Zeolites
25
a
x 20 x 10
TIME lh)
Figure 10. Relative intensities of species CO-l(L) and C0-2(L) generated in RhK-L 400/500/18 in the presence of 200 Torr of CO vs temperature: (I) 25 OC, (11) 100 OC, (111) 200 OC, (IV) 250 "C,(V) evacuated at 300 OC, (VI) reexposed to 200 Torr of CO at 25 OC. I
I
Figure 8. First-derivative EPR spectra of RhK-L 250/600/18 in the presence of I2C0(a) before adsorption, (b) exposed to CO at -196 OC (equivalent pressure at 25 OC of 150 Torr), (c) same as (b) allowed to warm to room temperature, (d) exposed to 400 Torr of CO at 25 O C , (e) evacuated for 5 min at 25 OC. Spectral conditions as in Figure 2. The numbers on the right-hand side indicate the relative gain.
2.319
A
/ \
2.160
i A
I
1.985
2.194
1.994
Figure 9. First-derivative EPR spectra of RhK-L 460/600/18 in the presence of 13CO: (a) 1 Torr at 25 OC,(b) 300 Torr at 25 OC. Spectral conditions as in Figure 2. g matrix with gll = 1.985 and g, = 2.319 (Figure 8e). The parallel
feature was a doublet with a splitting of 33.6 G probably due to the interaction of the unpaired electron with one Io3Rh nucleus. The second species, referred to as C0-2(L), had slightly rhombic symmetry with g, = 2.194, g2 = 2.160, and g3 = 1.994 (Figure 9b). The g3feature was split into two hyperfine lines 20.7 G apart. Using I3CO instead of I2CO did not affect the CO-1(L) signals while all three EPR features of species C0-2(L) were split into doublets. The "CO hyperfine splitting constants were A , N A2 = 102.2 G and A3 = 119.5 G (Figure 9b). This demonstrates that species C0-2(L) involves one Rh and one CO. There are other differences between these two species: (i) species CO-1(L) can be detected by EPR at room temperature while C0-2(L) cannot, (ii) species CO-1(L) reaches its maximum concentration upon exposure to 1 Torr of CO for 1 min at room temperature
(Figure 9a) while species C0-2(L) begins to form at a slightly higher C O pressure and its concentration is pressure dependent up to 400-500 Torr of CO, (iii) species C0-2(L) disappears immediately upon evacuation at room temperature while, under similar conditions, CO-1 (L) is stable even though it also vanishes at 300 OC. The stability of both species CO-1(L) and C0-2(L) under C O pressure was unexpectedly high. In one experiment a RhK-L 400/500/18 sample was exposed to 200 Torr of C O at room temperature. Then, as shown in Figure 10, the relative amounts of CO-1(L) and C0-2(L) species were monitored as a function of time at different temperatures. After treatment at 250 OC (area IV of Figure lo), the sample was evacuated at room temperature and exposed to 200 Torr of CO. N o increase in the EPR signal was noticed. However, after evacuation at 300 OC for 3 h, a new exposure to 200 Torr of CO at room temperature restores the EPR signals of CO-1(L) and C0-2(L) to a significant extent (area VI of Figure 10). When the sample is outgassed again at 300 O C for 3 h then exposed to 50 Torr of O2at room temperature, the same strong signal as shown in Figure 7a readily develops.
Discussion RhK-L ~125118.As discussed in our earlier papers,14 we assign all signals generated in RhK-L x/25/18 to Rh(I1) species having different symmetries. For some as yet unknown reasons, the EPR parameters of Rh(I1) species are extremely sensitive to their immediate environment. The large number of signals with substantially different g values reported here and elsewhere14J9 together with the lack of any significant relationship between the EPR spectra of species generated in quite similar supports such as K-L and faujasite-type zeolites are relevant examples. The frequent lack of hyperfine structure makes it even more difficult to propose detailed assignments. As far as the paramagnetic species generated by O2activation are concerned, the only species common to RhK-L and Rh fauj a ~ i t eis' ~A(L). ~ The reason for this disparity may be, in principle, attributed to the differences between the exchangeable cation (K+ vs Na') and/or to structural differences among the host zeolites (K-L vs N a faujasite). The structure of the zeolite is certainly an important factor that governs the EPR parameters of Rh(I1) species simply because these species are necessarily located in cation sites, the symmetries of which depend on the zeolite structure. The nature of the neutralizing cations in the zeolite seems, at least at low levels of exchange, to play only a limited role. This is illustrated by the work of Goldfarb and Kevan13who did not find major differences between paramagnetic Rh species in Na(Ca)-X and Na(Ca)-Y. As discussed we suggest that A(L) corresponds to [Rh"(NH,),Cl]+ generated according to
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The Journal of Physical Chemistry, Vol. 93, No. 5, I989 02
[Rh(NH3)5ClI2+
15&2,,,
oc*
[Rh(NH3)4Cl]+ + %NH3 + y6N2
+ H+
Owing to its size, species A(L) can only be located in the main channel of the zeolite probably in site D. The absence of line broadening in the presence of oxygen is indicative of a coordinatively saturated species where a direct interaction between gaseous oxygen and the paramagnetic center is prevented by the ligands. There are several reports dealing with the formation of 02in rhodium organic complexes.2s22 In all cases the EPR parameters of Rh(III)-O< or [Rh(II1)l2-pO2- were found to be quite similar to those of our species A(L). However, upon exposure of a RhK-L sample containing species A(L) to 10 Torr of I7O2,no hyperfine structure appeared suggesting that A(L) is indeed a Rh(I1) species. As the O2 activation temperature increases, several Rh(I1) species appear. It is believed that these species differ from each other by at least one of the following parameters: (a) number of ligands, (b) nature of ligands, (c) location inside the zeolite framework. However, it is not yet possible to identify the precise environment of each species and its location. The lack of line broadening in the presence of gaseous O2 indicates that most of these species, if not all of them, are located inside the smaller cavities of the zeolite. During O2 treatment between 250 and 500 "C,the spin concentration remains practically constant, suggesting that, as x increases, the same Rh(I1) centers are gradually losing their ligands and moving toward more stabilizing sites. At x > 500 OC, we believe that the spin concentration drops because of reoxidation of the Rh(I1) centers. Under different conditions, these Rh(I1) paramagnetic species exhibit different stability but with no consistent trends. For example, D(L) is the most stable species under a H2atmosphere, whereas in the presence of CO it rapidly vanishes. On the other hand, under vacuum at increasing temperatures, species D'(L) and E(L) are by far the most stable. These differences may originate from both an intrinsic stability for each species and differences in their mobility depending on the experimental conditions. The relatively high reactivity of Rh(I1) species in the presence of H2 or CO is in fact expected. Indeed, temperature-programmed reduction of preoxidized Rh supported on a variety of oxides , ~ ~Na-Y eol lite^^^* including A1203:3-26 Si02,27Ti02:627 Z I - O ~and indicates that the reduction of rhodium to the metallic state occurs at a rather low temperature, ca. 80-150 OC. Recent XPS measurements confirmed these finding^.^^^*^ There is also evidence that Rh(II1) may be reduced at least partly to Rh(0) by hydrogen at room t e m p e r a t ~ r e . ' ~ J ~ , ~ ~ ~ ~ As for the interaction of CO with oxidized rhodium, particularly in zeolites, several investigations mostly using infrared techniques have been published.6a-f~7~9*~33-35 At moderate temperature, ca. (20) Wayland, B. B.; Newman, A. R. (a) J . Am. Chem. SOC.1979,101, 6472; (b) Inorg. Chem. 1981, 20, 3093. (21) Caldararu, H.; DeArmond, K.; Hanck, K. Inorg. Chem. 1978, 17, 2030. (22) Raynor, J. B.; Gillard, R. D.; Pedrosa de Jesus, J. D. J . Chem. SOC., Dalton Trans. 1982, 1165 and related references therein. (23) van? Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Am. Chem. SOC.1985, 107, 3139. (24) Yao, H . C.; Japar, S.; Shelef, M. J . Catal. 1977, 50, 407. (25) Dall'Agnol, C.; Gervasini, A.; Morazzoni, F.; Pinna, F.; Strukul, G.; Zanderighi, L. J . Catal. 1985, 96, 106. (26) Vis, J. C.; van't Blik, H. F. J.; Huizinga T.; van Grondelle, J.; Prins, R. (a) J . Mol. Catal. 1984, 25, 367; (b) J . Catal. 1985, 95, 333. (27) Panda, N. K.; Bell, A. T. J . Cutal. 1986, 97, 137. (28) Niwa, M.; Lunsford, J. H. J . Catal. 1982, 75, 302. (29) Lee, C.; Schmidt, L. D. J . Carol. 1986, 101, 123. (30) DeCanio, S. J.; Miller, J. B.; Michel, J. B.; Dybowski, C. J . Phys. Chem. 1983.87, 4619. (31) Miller, J. B.; DeCanio, S. J.; Michel, J. B.; Dybowski, C. J . Phys. Chem. 1985.89, 2592. (32) Foley, H. C.; DeCanio, S.J.; Tau, K. D.; Chao, K. J.; Onuferko, J. H.; Dybowski, C.; Gates, B. C. J . Am. Chem. SOC.1983, 105, 3074. (33) Lefebvre, F.; Gelin, P.; Naccache, C.; Ben Taarit, Y. In Proceedings of the 6th International Zeolite Conference;Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 435.
Sayari et al. 25-100 OC, the most important species detected is the diamagnetic Rhl(CO)> In preoxidized RhNa-Y, reduction of Rh(II1) to Rh(1) by CO, particularly in the presence of moisture takes place at temperatures as low as -80 oC;6a,bat room temperature, Rh(1) undergoes a further reduction accompanied by clustering to Rh,(CO) 16.'~ It is clear from this brief review that Rh(II1) is easily reduced in the presence of H2 or CO. We therefore infer that Rh(II), which is a much less stable species than Rh(III), will undergo the same type of reactions at least as easily. However, it should be pointed out that the stabilizing effect of the zeolite cannot be overlooked as some of these Rh(I1) species are far more stable than the extremely labile monomeric Rh(I1) complexes in solution.21.36,37 As compared to RhNa-X and RhNa-Y, the striking fact is the absence of any new paramagnetic species when a series of RhK-L x/25/18 samples are exposed to H2 or C O or when they are outgassed at increasing temperatures. In particular, we reportedI4 that evacuation of RhNa-X x/25/18 or RhNa-Y x/ 25/ 18 (x > 200 O C ) at high temperature generated a new species designated E(X or Y) (g,,= 2.02-2.07, g, = 2.6-2.7) that had never been observed after O2 activation alone. This species, which we assigned to Rh(0) atoms,I4hccan also be generated by exposure of RhNa-X 500/25/18 to CO, H2, and other reducing molecules at room t e m p e r a t ~ r e ' or ~ , by ~ ~evacuation of RhNa-Y ex-RhC1, at high temperature with or without previous activation in flowing 02.14b Therefore, it seems that such a species cannot be stabilized in K-L zeolite. In order to further substantiate this conclusion, a RhK-L (1.75% w/w Rh) sample was prepared from RhC13 following the same recipe used earlier.14b As anticipated, evacuation of this sample at 25-500 O C did not give rise to any paramagnetic species. A last point concerns species G(L) which is generated by exposing RhK-L 300-460/25/18 to air for a few days (Figure 4). This species has very similar g values to species A(L) which probably implies that both species have the same symmetry and location. We suggest that, in the presence of moist air, the existing Rh(I1) species in RhK-L 300-460/25/18 migrate toward the main channel of the zeolite where they become coordinatively saturated by contacting OH or H 2 0 ligands. This new species would be G(L) with the tentative formula Rh1*(OH),(H20)5-,. This interpretation is compatible with all experimental observations, in particular with the fact that the spin concentration corresponding to G(L) is the same as that found in the original samples (RhK-L 300-460/25/18). The difficulty in generating species G(L) in samples activated in O2 at low temperatures (150-200 "C) is probably due to the fact that the existing Rh(I1) species are already coordinatively saturated. Under vacuum at increasing temperatures, species G(L) is expected to dissociate progressively and the resulting species migrate back to the inner sites as if the samples were never exposed to moist air. The absence of O2 line broadening is explained as for species A(L). RhK-L x / y / 1 8 (y > 25 " C ) . As already mentioned, the interaction of O2 and C O with RhK-L x/25/18 does not yield any new paramagnetic species. However, exposure of samples treated in O2 at 250-630 "C and then under vacuum at 400-600 OC to O2 or CO, even at -196 "C, immediately generates the species whose EPR spectra are displayed in Figures 7 and 8. Therefore, the appearance of such signals must be related to some processes taking place during the evacuation of preoxidized samples at high temperature. Several paramagnetic adducts of Rh species with either 0 2 or C O have been reported in the l i t e r a t ~ r e . ' ~ - ~ ~Let ? ~ *us- ~first ~ (34) Takahashi, N.; Mijin, A.; Miyauchi, M.; Sato, A. Chem. Letr. 1985, 1911. (35) Takahashi, N.; Miura, K.; Fukui, H. J . Phys. Chem. 1986, 90,2797. (36) Kadish, K. M.; Yao, C. L.; Anderson, J. E.; Cocolios, P. Inorg. Chem. 1985, 24, 4515. (37) Anderson, J. E.; Yao, C. L.; Kadish, K. M. Inorg. Chem. 1986, 25, 718. ( 3 8 ) Yao, H. C.; Shelef, M. Stud. Surf. Sci. Catal. 1981, 7A, 329. (39) Beringuelli, T.; Gervasini, A,; Morazzoni, F.; Strumolo, D.; Martinengo, S.; Zanderighi, L. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 1479.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2099
Paramagnetic Rh Species in Zeolites
TABLE I: Conditions for the Occurrence of Species C0-2(L) samples and pretreatments
C0-2(L)'
RhNa-Y, RhNa-X, RhK-L, RhNa-M, and Rh-A1203 preoxidized at 300-500 OC, then evacuated at 500-600 O C RhNa-Y, RhNa-X, and RhK-L prepared from RhC13 and evacuated at 500-600 "C without oxygen pretreatment RhNa-Y, RhNa-X, and RhK-L prepared from [Rh(NH3)5CI]C12 and evacuated at 400-600 OC without oxygen pretreatment RhK-L preoxidized at 400 OC, cooled in 02,reduced in flowing H2 at 25 OC, and evacuated at 540 OC
yesb
Rh-A1203 oxidized at 400 OC (or not), reduced in H 2 at 450 OC and evacuated at 580 OC
yesc
yesb noc yesc
comments significant reduction took place during the evacuation step accompanied by the Occurrence of metallic Rh'" metallic Rh was found in similar samples by XPS5'p52and by catalytic activityIg Rh metallic particles are too large' reduction at 25 OC of proxidized samples generated some metallic Rh as shown by benzene hydr~genation.'~ N o C0-2(L) signal was observed in sample reduced at high temperature: probably because of the inherent generation of O H groups along with the reduction of Rh(II1)
'Yes: species C0-2(L) detected, no: species C0-2(L) not detected. bPrior to CO adsorption almost no EPR signals were present in RhK-L. Very weak signals were present in Rh-A1203 and in RhNa-M.I9 In RhNa-Y and RhNa-X species E(Y) or E(X)'4b was present but it is definitely not a precursor of C0-2(L). C N o paramagnetic species were detectable before exposure to CO. d o t h e r EPR signals were obtained; they are currently investigated.
consider species C0-2(L) that has already been reported in RhA1203,38RhNa-Y,'1c,'2-'3d and RhCa-X.13c In most cases this species was ascribed to a Rh(I1)-CO adduct. The precursor of this adduct was assumed to be a diamagnetic Rh(I1) dimer that forms at high temperature and splits into Rh(I1)-CO upon exposure to C O at room temperature.12J3 The occurrence of a Rh(I1) dimer does not seem to be consistent with our re~u1ts.l~ The fact that during exposure of RhK-L x/25/18 to CO all Rh(I1) species tend to disappear without the formation of any new paramagnetic species is a strong indication that Rh(I1) species are unlikely to form stable adducts with CO. We believe that they reduce to diamagnetic Rhl(CO)z. The fairly good stability of C0-2(L) species in the presence of gaseous CO (Figure 10) together with its immediate disappearance upon evacuation of C O indicates that a rather unreactive Rh species would better explain the formation of such a weak adduct. Because the rhodium species involved in C0-2(L) must be paramagnetic, the only possibility left is Rh(0) atoms. The low reactivity of isolated "metalic" atoms has already been suggested at least for Pt atoms in Y zeolite42which did not strongly chemisorb Hz or Oz. Figure 10 is also consistent with such a proposal. After lengthy exposure to C O at temperatures up to 250 OC, it is likely that all rhodium has been converted into Rh1(C0)2. However, the possibility that some of the rhodium has been clustered into Rh6(CO)169or even into small metallic particles43 cannot be ruled out. As Rhl(CO)z is stable under vacuum at room temperature,6d-z8~44 evacuation of such a sample at room temperature and reexposure to C O did not generate the adducts. After evacuation at 300 OC no EPR signals were found, suggesting that all the rhodium is now in the metallic state as a result of the reductive desorption of C0.7v23Readmission of C O at room temperature restores the EPR signals of C O adducts to a significant extent (Figure 10). Up to this point, assignment of C0-2(L) species to a R h ( 0 ) X O adduct located in the main channel of the zeolite appears to be quite reasonable. However, as we have never detected any Rh(0) atoms by EPR, it is necessary to delineate the origin of the Rh(0) precursor. We suggest the following: (1) Evacuation at high temperature of preoxidized samples generates very small and EPR-silent metallic particles in an accessible location.'" This may occur via reduction with 02-from the zeolite framework as in the case of C U N ~ - Yand ~ ~Ag-A46 xRh"+
+ (nx/2)02-
-
Rh,(O)
+ (nx/4)o2
(40) Gervasini, A.; Morazzoni, F.; Strumolo, D.; Pinna, F.; Strukul, G.; Zanderighi, L. J . Chem. SOC.,Faraday Trans. 1 1986,82, 1795. (41) Shubin, V. E.; Shvets, V. A.; Kazanskii, V. B. Kinet. Catal. (Engl. Traml.) 1978, 19, 1026. (42) Gallezot, P.; Alarcon-Diaz, A.; Dalmon, J. A.; Renouprez, A. J.; Imelik, B . J . Catal. 1975, 39, 334. (43) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1985, 89, 4789. (44) Takahashi, N.; Mijin, A.; Ishikawa, T.; Nebuka, K.; Suematsu, H. J . Chem. SOC.,Faraday Trans. 1 1987,83, 2605.
(2) In the presence of CO, it is suggested that such particles react according to Rh,(O) + xCO * xRh(0)-CO This equilibrated reaction is pressure dependent. In agreement with the experimental results, at increasing C O pressures the equilibrium shifts toward the formation of Rh(0)-CO adduct, while evacuation of C O regenerates the original Rh,(O) particles. The next problem to be addressed is how this mechanism relates to the numerous papers on C O interaction with either reduced or oxidized rhodium. Let us first summarize the present state of knowledge. As discussed earlier, in the presence of CO, oxidized supported Rh reduces very easily to Rhl(CO)z. As for the interaction of C O with reduced ample^,'^^^^^^^^,^^^^^^^ two major conclusions have been reported: (i) small metallic particles undergo an easy oxidation into Rh(I)(CO), even at subambient temperature^^@^^^ particularly in the presence of moisture or OH groups, and (ii) larger particles simply adsorb C O in linear or bridged forms. Several mechanisms for the oxidative adsorption of C O on metallic Rh have been proposed.23 A very interesting scheme was recently published by Yates et al.47 According to this proposal, upon exposure to C O the sma!l metallic particles first dissociate into mobile Rh(O)-CO intermediates without being oxidized. The oxidation step takes place only afterwards as a result of the migration of the Rh(0)-CO intermediate and its oxidative interaction with isolated O H groups. The Rh(I)-CO thus generated accepts another C O ligand to yield the well known Rh1(C0)2 species. The question to be answered now is the following: could the Rh(0)-CO intermediate be stabilized in a supported Rh sample reduced at fairly low temperature and then evacuated at high temperature so that most of the O H groups are depleted without extensive sintering? We propose that such intermediates can indeed be stabilized under appropriate conditions including the presence of tiny metallic particles and the extensive depletion of O H groups. We further suggest that these intermediates correspond to our C0-2(L) species. Additional experimental observations gathered in Table I lend further support to our proposal. However, even though our in~
~~~~~~
(45) Jacobs, P. A.; de Wilde, W.; Shoonheydt, R. A.; Uytterhoeven, J. B.; Beyer, H. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 1221. (46) Texter, J.; Kellerman, R.; Gonsiorowski, T. J . Phys. Chem. 1986,90, 21 18 and references therein. (47) Basu, P.; Panayotov, D.; Yates, J. T., Jr. (a) J . Phys. Chem. 1987, 91, 3133, (b) J . Am. Chem. SOC.1988, 110, 2074. (48) Zaki, M. I.; Kunzmann, G.; Gates, B. C.; Knozinger, H. J . Phys. Chem. 1987, 91, 1486. (49) Hamadeh, I. M.; Griffiths, P. R. Appl. Spectrosc. 1987,41,682 and ~~
~
references therein. (50) Wang, H. P.; Yates, Jr., J. T. J . Catal. 1984, 89, 79. (51) Okamoto, Y.; Ishida, N.; Imanaka, T.; Teranishi, S . J . Catal. 1979, 58, 82. (52) Andersson, S . L. T.; Scurrell,
M.S. J . Catal. 1979, 59, 340.
2100
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
terpretation seems to be consistent with all the experimental facts, more work is still needed to better undstand such behavior; in particular, a very carefully designed infrared study would be of much help. We have already mentioned that species C0-2(L) can be generated in RhNa(Ca)-X, RhNa(Ca)-Y, RhK-L, RhNa-M, and Rh-Al,O,; however, all attempts to stabilize such a species in Rh-Si02, Rh-TiO,, and Rh-MgO have so far failed. Investigation of this support, effect is currently being tackled. As for species CO-1(L) with gll = 1.985 (All = 33.6 G) and g, = 2.319, the EPR technique does not provide much information regarding its nature. Goldfarb and KevanllCalso obtained a similar species (gll 2.0, g , = 2.312) upon exposure of RhCa-X to CO under similar conditions, but no comment was given. The same species was later found in RhCa-A and assigned to a Rh( 11)-CO adduct. Under identical conditions, a species with comparable g values can also be generated in RhNa-M.53 We believe that species CO-I(L) is a cluster probably involving more than one CO and more than one Rh even though the EPR spectrum seems to show only one Rh. Exposure of RhK-L x/25/18 to 0, a t room temperature did not change any of the EPR spectra except for some occasional line broadening. This result together with the fact that Rh(I1) species are actually formed under 0, at high temperature is strong evidence that Rh(I1) stabilized in zeolites, unlike Rh(I1) dimers in solution, cannot be oxidized at subambient or room temperature into paramagnetic Rh(III)-O; species or form paramagnetic Rh(I1)-0, adducts. Therefore, it seems unlikely that the EPR signals displayed in Figure 7 correspond to such species. As in the case of C O adsorption, species 0-1(L) and 0-2(L) are generated only in samples heated under vacuum at high temperature prior to exposure to 0,. The vacuum treatment must be preceded by 0, activation for samples prepared from [Rh(NH3)5Cl]C12but not for samples prepared from RhCI,. Before addition of 02,neither type of sample exhibits EPR signals. Consequently, the precursors of O-I(L) and 0-2(L) species must be diamagnetic or EPR-silent species generated during the evacuation step. We strongly suspect that the precursors of 0-1(L) and 0-2(L) are again small metallic particles. This is consistent with the fact that comparable amounts of Rh are involved in the
-
(53) Sayari, A., to be submitted for publication.
Sayari et al. EPR signals generated by either CO or 0, adsorption. This leads us to consider that 0-1(L) and 0-2(L) are simply two 0,- species adsorbed on Rh(1) in two different sites. In order to check the likelihood of our assignment, a RhK-L sample was treated in flowing 0, at 350 "C (3 h) and then under flowing H, at 200 "C (2 h) and finally evacuated at 350 "C for 18 h and exposed to 0, at room temperature. A very strong EPR signal identical in shape to the one displayed in Figure 7 was obtained. Before 0, adsorption, only a small amount of D(L) was detected but it was not affected by 0,. Moreover, the only Rh(I)-O,species we found in the literature38had g values (g,,= 2.043, g, = 2.004) surprisingly close to those of our 0-1(L) species (ga= 2.042, g, = 1.991). ConcIusions 1. Activation of RhK-L under flowing 0, at different temperatures generates several Rh(I1) species. It is believed that they differ from each other by at least one of the following parameters: (a) number of ligands, (b) nature of ligands, (c) location inside the zeolite framework. The relative stability of these species has been studied under different conditions and no clear trend was found. 2. Except for species A(L), none of these Rh(I1) species bears a resemblance to the many Rh(1I) species detected earlier in RhNa-X and RhNa-Y.14 3. Subsequent evacuation of RhK-L at high temperature is accompanied by a significant reduction probably via 0,- pertaining to the zeolite framework. The tiny rhodium metallic particles thus generated seem to have very peculiar properties. It is believed that, in the presence of CO, they disintegrate without being oxidized, leading to paramagnetic Rh(0)-CO species. In the presence of 02,the small particles are oxidized with the concomitant formation of two different Rh(I)-02- species. 4. In contrast to RhNa-X and RhNa-Y, evacuation at high temperatures of preoxidized RhK-L does not generate new Rh paramagnetic species. In particular, isolated Rh(0) atoms could not be stabilized in K-L zeolite. Acknowledgment. We gratefully acknowledge the help of Dr. J. Tse in X-ray diffraction and Dr. H. Joly in spin concentration measurements. Registry No. CO, 630-08-0; Rh, 7440-16-6.
