Infrared Study of CO Chemisorption on Zeolite and Alumina Supported Rhodium BY MICHEL PRIMET* Institut de Recherches sur la Catalyse, 79 Boulevard du 11 November 1918, 69626 Villeurbanne Cedex, France Received 5th January, 1978 The chemisorption properties of supported rhodium with respect to carbon monoxide are investi- gated by infrared speclrometry. Zeolite and alumina supported rhodium is obtained following hydrogen reduction. Highly dispersed rhodiuin is prepared on the zeolite support whatever the temperatme of reduction over the range 280-600°C. For rhodium supported on alumina, the dispersion is less good and decreases when the conditions of reduction become more drastic. For particles of larger diameter than 20& chemisorption of CO leads mainly to gem dicarbonyl species, the site of adsorption being one rhodium atom in the oxidized state.These gem dicarbonyl complexes are not formed at low temperature, but develop on warning up to room temperature. Dissociation of carbon monoxide is then assumed, CQ in excess adsorbs on the surface Rh-0 species to give gem dicarbonyl complexes, probably of Rhr, which interact with thc lattice in the case of zeolite support. Infrared spectroscopy of adsorbed species was extensively used to characterize various catalysts. In tbie case of srtpported metals, the specific adsorption, on the metal, of compounds like CO or NO has given useful information concerning factors frequently involved in catalytic process : effect of the support,l particle size effect,2 modifications of surface properties by alloying, etc.Unsupported metals, in the form of ribbons, foils, f i l m or monocrystals, were used in order to examine a wider spectral range or to remove the expected support effect. In the case of metals like Ni, Pt, Pd or Fe, the results obtained for supported or Unsupported systems are in good agreement both for bands positions and the number of bands. This analogy in behaviour is not observed for CO adsorption on rhodium :4-7 bands attributed to linear and bridged species are detected on unsup- ported rhodium,4* whereas gem dicarbonyl species are mainly observed for alumina or silica supported rhodium.6* This discrepancy between supported and unsupported rhodium catalysts prompted us to investigate CO adsorption on supported rhodium upon varying the nature of the support, the metal particle size and the temperature of the adsorption experiment.The use of zeolite as a support enables us to examine the spectral range below 1200 cm-l from which information concerning the zeolite structure and the effect of the reduction on the lattice are expected. EXPERIMENTAL The rhodium form of Y zeolite was prepared from a sample of NaY zeolite with a Si/AI ratio of 2.4 (Linde, S.K. 40). Cation exchange was carried out by stirring the NaY zeolite with an aqueous solution of [Rh(NH3)5C1]2+ complex. The sample was carefully washed with distilled water and dried at 323 K. Rhodium chemical analysis indicated that the sample contained 3.1 % wt of rhodium. Ammonia and chlorine coordinates were removed 2570M. PRIMET 2571 by a thermal treatment at 623 K in a flow of oxygen in accordance with previous E.S.C.A measurements. * Reduction was performed under hydrogen over the temperature range 473-573 K.Bare surfaces of rhodium were obtained by removal of adsorbed hydrogen under vacuum at 623 K. Electron microscope determinations, carried out on the samples, showed the presence of highly dispersed metal with a particle size between 4 and 8 w . Rh/A1203 catalysts were prepared by stirring the alumina (Aluminum Oxid P from Degussa) with an aqueous solution of rhodium trichloride. Water was eliminated by evaporation under vacuum, then the solid was dried at 393 K. Chemical analysis gives a Rh content of 3.68 % wt. Reduction was performed under a flow of hydrogen either at 773 K, or at 1273 I<.In both cases adsorbed hydrogen was removed at 773 K under vacuum. For 500°C hydrogen reduction (Rh/A1203 773, electron microscope measurements showed the presence of rhodium particles ; 85 % of them had a diameter between 5 and 20 A with some crystallites of 50A. For 1273 K hydrogen reduction (Rh/A1203 1273), 70 % of the metal was found in particles of 20-40A diameter, some crystallites in the range 100-140A were also detected, but no particles of diameter lower than lOA were observed. Samples were pressed to form thin pellets 18 m in diameter and weighing 30-40 mg for Rh/A12Q3 catalysts, or about 15 mg for RhNaY samples for experiments in the range 4000-1300cin-'. Very thin pellets of about 6-8 mg were necessary to investigate the 1000-500 tin-l range of the zeolite framework vibrations. The pellet was then introduced in a cell where the reduction treatments were performed in situ.Depending on the type of cell used, infrared spectra were recorded either at 300 K or at a lower temperature using a Fourier Transform spectrometer (Digilab F.T.S. 14) with a resolution of 4 crn-l. Every spectrum was the sum of 200 scans and was stored in the disk memory. The initial spectrum of the sample was used as a reference for the following adsorptions in order to remove the bands due to the background of the support (especially in the case of zeolite). RESULTS AND DISCUSSION ADSORPTIONS OF co ON BARE SURFACES Carbon monoxide was admitted under a pressure of 0.3 kN m-2 onto RhNaY samples. Whatever the temperature of hydrogen reduction in the range 473-873 K, the infrared spectra showed the same features after CO adsorption.A typical spectrum is presented in fig. 1. Two main bands around 2100 and 2040 cm-I developed during the contact time under CO. The intensities of both bands increased with time, but the intensity ratios remained constant. These two absorptions were split into two components 21 15-2100 and 2050-2025 cm-l. In addition, a broad band was observed between 1920 and 1750 cm-l; it is possible to resolve it into bands located at 1870-1830 and 1760 cm-'. In the range 1000-500 cm-l, a band at 910 cm-I was detected and its intensity grew with the time of contact under CO. After oxygen treatment at room temperature, the bands between 1920 and 1750 cm-1 completely disappeared whereas the main bands near 2100 and 2040 cm-' and that at 910 cm-I were unaffected.Nevertheless, the 2100 and 2040 cm-l bands were now better resolved, which means that the species vibrating between 2100 and 2040 cm-1 was removed by room temperature oxygen treatment. Indeed, the comparison of spectra recorded before and after reaction with oxygen revealed the disappearance of a band close to 2070 crn-I. Carbon monoxide adsorbed onto Rh/A1,03 1273 gave a single strong band at 2065 cm-I with a small shoulder at 2100 cm-I and an absorption at 1935 cm-I (fig. 1). By oxygen treatment at 300 K, the two low frequency bands disappeared and the spectrum showed new bands at 2128-2100-2038 cm-I in addition to absorption due to carbonated species bound to the support.Adsorption of CO onto Rh/Al,O3 773 gave an intermediate state between that on the two preceding samples, i.e., appearance of the 2110-2062-2038 cm-' bands2572 co ON SUPPORTED RHODIUM with comparable intensities and of a broad band at 1875 cm-l (fig. 1). As with the previous solids, the 21 10 and 2038 cm-l bands did not disappear by oxygen treatment at 300 K in contrast to the 2062 and 1875 cm-l bands. 2110 - 2200 m 1800 (a wavenumber/cm- 2200 2000 (a FIG. 1.--Infrared spectra of CO adsorbed at 300 K on supported rhodium (A) RhNaY sample treated under oxygen, then under hydrogen and finally under vacuum at 623 K. (B) Rh/A1203 sample treated under flowing hydrogen at 773 K and desorbed at the same temperature. (C) Rh/A1203 sample treated under flowing hydrogen at 1273 K and desorbed at 773 K.Spectra (a) correspond to CO irreversibly adsorbed at 300 K. Spectra (b) were obtained after treating the previous samples with 15 kN m-2 oxygen at 300 K. In comparison with the results obtained for CO adsorption on rhodium we attribute the 2060-2070 cm-l band to a linear Rh-CO species. The bands situated in the range 1920-1760 cm-1 are assigned to multicentred species. The band at 2128 cm-l which developed after oxygen treatment at 300 K is due to Rh / O ‘co surface complexes. According to the pioneer work of Garland et aZ.,6 the two bands near 2100 and 2040 cm-l are attributed respectively to the asymmetric and symmetric modes of a gem dicarbonyl complex of metallic rhodium Rh co <,, From these assignments, it appears that particles of diameter smaller than lOA are highly suitable for the formation of Rh(CO)z species, since this surface complex is observed mainly on RhNaY samples.When the mean particle size is >20 A (Rh/Alz03 1273), this complex is no longer observed : only linear and multicentred forms are detected.M. PRIMET 2573 Without further experiments, it is difficult to find an explanation for the splitting of the bands due to the gem dicarbonyl species in RhNaY samples. It is probable that the specific properties of rhodium in zeolites must be considered since this phenomenon is not encountered for Rh/Al,03 catalysts. I I I I I 2200 21 00 2000 1900 1800 wavenumber /cm-' FIG. 2.-Influence of preadsorbed oxygen or hydrogen on the spectrum of CO irreversibly adsorbed at 300K on Rh/A1203 773 sample.(a) Bare sample, (b) sample precovered by oxygen at room temperature, (c) sample precovered by oxygen at 473 K, (d) sample precovered by hydrogen at 300 K.2574 co ON SUPPORTED RHODIUM According to the previous results, the i.r. spectra of CO adsorbed on rhodium should give a useful clue concerning the dispersion of the metal : the appearance of the doublet 2100-2040 cm-l for highly dispersed rhodium and formation of the 2070-2060 cm-1 band for poor dispersions, i.e., high temperatures of reduction or rhodium films. The assignment of the 2100-2040 cm-I doublet to two CO molecules adsorbed on the same site is in agreement with the experimental data, except in the zerovalent state where the site is identified with one metal atom. We bear in mind the following facts: (i) the doublet is not affected by treating the sample under oxygen at 300 K, which is unusual for carbon monoxide adsorbed on a metal, (ii) hydrogen treatment at room temperature led to a strong decrease in the high frequency band (2100 cm-I), the low frequency one (2040 cm-l) being overlapped by the shift in the 2070-2060 cm-1 band towards lower wavenumbers ; (iii) the frequen- cies observed for this gem dicarbonyl species are very close to the frequencies of [Rhr (@O),Cl], complex in the solid state 2089-2033 cm-1 lo or adsorbed on a NaY zeolite 21 12-2095 and 2050-2030 cm-l." Considering these observations, it seems unlikely that a metallic rhodium atom could be the adsorption site involved in the formation of the gem dicarbonyl species.ADSORPTION OF co O N HYDROGEN OR OXYGEN PRECOVERED SURFACES Rh/A1,03 773 sample was chosen as reference in the investigation of the effect of preadsorbed oxygen or hydrogen on the metal. If the surface was precovered by oxygen at room temperature, CO adsorption gave intense bands due to the gem dicarbonyl species, whereas the bands due to linear and multicentred forms decreased (fig. 2). If the rhodium surface was oxidized by heating under O2 at 473 K, the subsequent adsorption of CO at 300K produced only the bands due to the gem dicarbonyl species, i.e., the surface of the catalyst was fully oxidized after oxygen treatment at 473 M. On the other hand, if CO was adsorbed on Rh/A1203 which had been first exposed to hydrogen, then evacuated at 300 M, CO adsorption gave a spectrum which showed weaker bands due to gem dicarbonyl species (fig. 2).In addition, a cycle of 0,-CO treatments was performed at room temperature on the Rh/Al,03 1273 sample. Every adsorption of oxygen led to a spectrum of adsorbed CO with an increase of the intensity of the doublet close to 2100-2040 cm-l. ft appears that the formation of the doublet at around 2100 and 2040 cm-l by CO adsorption must be correlated with the presence of oxygenated species bound to the metal. Indeed, the higher the amount of chemisorbed oxygen, the more dominant the doublet in the infrared spectrum. In contrast, the presence of preadsorbed hydrogen reduces the extent of rhodium oxidation. ADSORPTION OF CARBON DIOXIDE The chemisorption of carbon monoxide on supported rhodium was then accom- panied by an oxidation process for highly dispersed metal.Two types of reaction can be taken into account to explain this result : (i) Disproportionation of CO followed by dissociation of CO, : 2 c o 3 co, + C(S) co, -+ C(s) +20(s) co + C(s)+O(s). (iif dissociation of CO 111 bcpth cases, carbon monoxide in excess was adsorbed on the oxygenated surfaceM. PRIMET 2575 species to give the gem dicarbonyl complex. In the hypothesis of C 0 2 formation as an intermediate reactant, adsorption of CO, must be favoured in the case where the widest doublet was observed, i.e., that with the zeolite support. At 300 K, C 0 2 adsorption on RhNaY sample did not give v CO bands in the i.r. spectrum. Under the same conditions with Rh/A1203 773, bands were observed at 2025 and 1860 cm-l ; they disappeared on oxygen treatment at 300K and C02 then dissociated on the Rh/A1,03 catalyst.The frequencies observed were lower than those found for GO adsorption on the same solid; weak CO coverage can be explained by the position of these bands. If the Rh/AI2O3 sample was precovered with O2 at 300K, the dissociation of C02 did not occur. Thus, the dismutation of CO must be ruled out, since C 0 2 is not dissociated on ihe catalysts which produced the highest content of oxidized form after CO adsorption. LOW TEMPERATURE EXPERIMENTS Fischer-Tropsch reactions involved the dissociation of carbon monoxide on metals and a carbide intermediate was postulated. This dissociation of CO occurred during the methanation of CO on nickel catalysts ;12* l3 it was found that alloying nickel with copper strongly reduced the CO dissociation and in the same way the rate of methanation.14 On nickel, we have found that the CO dissociation occurred via a Ni4C0 intermediate which led to Ni3C and NiO surface species.12 For rhodium films, CO adsorption is considered as non-dissociative. 1.r.measurements by transmission 4 9 or by reflection absorption confirm this statement. When the conditions of absorption became more severe, CO dissociation was observed. Marbrow and Lambert l6 have shown that CO was adsorbed in a single state at 300 K on Rh (1 lo), but under electron impact carbon monoxide was dissociated, i.e., surface carbon was shown to inhibit CO chemisorption, whereas surface oxygen fed to the formation of a new more tightly bound form of CO.Hydrogenation of CO and C 0 2 between 523 and 623 K over polycrystalline foils of rhodium confirmed the decomposition process, since under these reaction conditions, the surface is covered with a catalytically active carbonaceous deposit while some oxygen is located below the surface. Dissociation of CO on highly dispersed Rh seems to be the only way to account for our experimental data. This reaction must be temperature dependent and the nature of adsorbed species is expected to change drastically with the adsorption temperature. In order to investigate modification of the surface species with the temperature, CO adsorption was performed below 300K on Rh/Al,03 773 and RhNaY samples. Adsorption of CO at 77 K under a pressure of 0.27 kN rn-, on Rh/A1203 gave only the bands at 2075 and 1920-1870 cm-l (fig.3) assigned to linear and multicentred forms, respectively. In addition, small bands at 2190 and 2160 cm-l were detected and attributed to CO interacting with the support. When the temperature of adsorption was increased, bands at 2095 and 2028 cm-l developed and reached their maximum intensities after one hour contact at 300 K. The resulting spectrum was identical to that obtained by direct adsorption of CO at room temperature. If the temperature of adsorption was raised to 350K, the 2095 and 2028cm-l bands increased. The doublet attributed to the gem dicarbonyl species is not observed at tempera- tures below 193 K. Its progressive formation on increasing the temperature of adsorption must be correlated with dissociation of CO on rhodium followctl by adsorption of CO on the oxidized surface groups.2576 co ON SUPPORTED RHODIUM -1 I 2190 2160 --la--- > 10% LILlq4q 2095 I I I I I 2200 ZOO0 1800 .wavenurnber/cm-l sample. Carbon FIG. 3.-1.r. spectra of CO adsorbed at low temperature on Rh/A1203 773 monoxide was introduced under a pressure of 0.27 kN m-2 at 77 K and then the temperature was raised to (a) = 103 K, (b) = 203 K, (c) = 300 K, (d) = 353 K. A similar experiment was carried out on very thin pellets of RhNaY catalyst. CO adsorption under a pressure of 0.27 kN m-2 gave, between 77 and 143 K, a spectrum which had two main bands at 2040 and 1910 cm-1 ; CO adsorption on Naf ions l 8 was also detected by a peak at 2170 cm-' (fig.4). Thus, the spectrum observed under CO at 143 K is quite different from the spectrum obtained at 300 KM . PRIMET 2577 and particularly the doublet which is split for the RhNaY sample is not detected. Nevertheless this spectrum is difficult to explain. Indeed, the frequencies are slightly different from those mentioned for U/Al2O3 at 143 K (2075-1920-1870 cm-l). The differences noticed can tentatively be explained by the presence of peculiar sites in the small particles of rhodium in zeolites or by the formation of rhodium carbonyl complexes, as already evidenced by Ozin et aZ.19 By increasing the temperature above 193 K, the 21 15-2098 and 2048-2020 cm-1 bands developed similarly. At the same time, a band appeared at 910 cm-l and grew as the temperature increased.1898 1830 I 2041 I I I I 2200 2100 2ooo 1900 1800 I 910 wavenumber /cm-' FIG. 4.-1.r. spectra of CO adsorbed at low temperature on RhNaY sample reduced at 773 K. Carbon monoxide was introduced under a pressure of 0.27 k N m-2 at 77 K, and then the temperature was raised to (a) = 143, (b) = 153, (c) = 228, (d) = 300 K. As for the Rh/A1,03 sample, the appearance of the gem dicarbonyl complex must be associated with the dissociation of carbon monoxide. Hindered at low temperature, this process is achieved to a large extent at 300 K. In a previous study * of the properties of RhrJr ions in NaY zeolite, it was shown that the reduction of Rh"' ions is performed at 300 K in presence of carbon monoxide and water. E.S.C.A. measurements indicate that rhodium is in the state Rh' after reaction.The doublet due to Rhr species was found at the same position as it was for CO adsorption on supported rhodium zeolite. This analogy strongly co <co2578 co ON SUPPORTED RHODIUM suggests that dissociation of CO on RhNaY leads to rhodium in the plus one oxidation state which is able to chemisorb two CO molecules to form a gem dicarbonyl complex. Indeed, recent work performed here by U.V. reflectance spectrometry has shown that chemisorption of CO at room temperature on reduced RhNaY samples leads to the formation of a large number of planar Rh' complexes.20 The dissociation of CO on supported rhodium is enhanced for the highly dispersed metallic phase, whereas this process is strongly limited for poor dispersions. Metal surface areas were often measured by chemisorption of H2, O2 or CO. In the latter case, the stoichiometry of the adsorption must depend strongly on the particle size of rhodium.Actually, Wanke et found an increase of the CO/Rh ratio with decrease in the particle size. INTERACTION WITH ZEOLITE LATTICE In the case of the RhNaY sample, a band at 910 cm-I appears by adsorption of CO. The evolution of its intensity follows that of the doublet near 2100 and 2040 cm-l. Such a band has been already observed at 900 cm-l after removal of the NH3 ligands present in a NaY zeolite exchanged with palladium or rhodium ammine complexes.22 This band disappeared during hydrogen reduction at 150°C.22 For Cu" exchanged zeolite, a band at 900 cm-I was also created by removal of water.22 TABLE 1 .-ZEOLITE FRAMEWORK INFRARED ASSIGNMENTS 2J* 26 internal tetrahedra (structure insensitive) external linkages (structure sensitive) asym.stretch 1250-950 cm-l asym. stretch 1050-1150 cm-l sym. stretch 720-650 cm-l sym. stretch 750-820 cm-1 T-0 bend 420-500 cm-' double ring 650-500 cm-' pore opening 300-420 cm-l The band at around 900cm-' is not very sensitive to the nature of the cation Mn+ (Rh, Pd, Cu) bound to the zeolite lattice. Thus, it is difficult to attribute it to a Mn+-oxygen vibration. It appears more realistic to consider a local deformation of the T (T = Si or Al) oxygen bonds of the zeolite due to interaction of the oxygen atom with the cation. Indeed, X-ray measurements have shown that interaction of cations such as Pd" 23 or CoI1 24 with the lattice of a NaY zeolite lead to an important lengthening of the T-0 bond which distorts the tetrahedron.This increase causes a weakening of the force constant of the T-0 bond which displaces the T-0 vibration towards lower frequencies. The bands in the vibrational spectra of the zeolite framework have been assigned by Flanigen et aZ.25* 26 Some of these assign- ments are summarized in the table 1. In Y-zeolites, internal tetrahedron TO4 vibrations include the two most intense bands : the strongest at around 1000 cm-1 (T-0-T asymmetric stretching mode) and the other of medium intensity at around 469 cm-' (0-T-0 bending mode). From infrared data and unit cell parameters of synthetic alumino silicate sodalites, Taylor et al.27 observed a linear relationship between the T-0 distance and the wavenumber of the v,, mode : d(T-O)(A) = 2.195- (5.263 x 10-4)(~,, T-0-T).By applying this relationship to a NaY zeolite for v,, (T-0-T) = 1035 cm-', we find a T-0 distance of 1.65 A which is close to the mean value observed by X-ray diffra~tion.~~ If we consider the frequency detected for Pd"Y zeolite, we find A(T-0) = 1.72A. PdI1 ions in Y-zeolite are mainly located on the SIP site, which means that they are interacting with Oc3) oxide ions ; X-ray diffraction measurementsM. PRIMET 2579 performed on such systems have given the value 1.720 A for the T-O,,, distance, which is identical to that deduced from infrared data. Thus, the cation lattice interaction with lengthening of the T-0 bond from 1.65 to 1.72 A can explain the shift of the 1035cm-l band towards 900cm-l.The appearance of a band near 900cm-’ must be connected with the presence of charged species interacting with the zeolite framework. Since the increase in the (21 12-2098)-(2048-2020) cm-1 bands is associated with the increase in the 910 cm-l band, we can conclude that CO adsorption on RhNaY sample leads to the formation of oxidized species of rhodium interacting with the lattice. CONCLUSION It appears that adsorption of CO on rhodium is more complex than on supported platinum or nickel. For Rh, the nature of the adsorbed CO is strongly dependent on the metal particle size. For large particles, CO is mainly adsorbed in a linear Rh-CO complex and the CO/Rh ratio is close to 1. When the particle size decreases, gem dicarbonyl complexes are formed and the smaller the diameter, the greater the amount of gem dicarbonyl species ; so the CO/Rh ratio is > 1.Not only the number of CO molecules linked to one rhodium atom, but also the state of rhodium after CO adsorption is particle size dependent. On large particles (films or high reduction temperatures), adsorption of CO is not dissociative. When the particle size decreases below lOA, carbon monoxide is dissociated. This process is strongly hindered below 173 K and is mainly observed at room temperature for zeolite supported rhodium. Subsequent adsorption of CO leads to the formation of gem dicarbonyl species of oxidized rhodium probably in the Rh’ state. In such a case, the RhNaY samples would be very efficient catalysts for the hydrogenation of CO and CO, since on these solids the dissociation of CO is already achieved to a great extent at room temperature.Thanks are due to Mrs. I. Mutin and C. Leclerq for taking electron micrographs and to Mr. Marc Dufaux for his help with the infrared experiments. The author is grateful to Dr. G. Coudurier, Dr. G. Naccache, Dr. M. V. Mathieu and Dr. B. Imelik for helpful discussions. F. Figueras, R. Gomez and M. Primet, Adv. Chem. Ser. Molecular Sieves, 1973, 121, 480. ’ M. Primet, J. M. Basset, E. Garbowski and M. V. Mathieu, J. Amer. Chem. Soc., 1975,97,3655. J. A. Dalmon, M. Primet, G. A. Martin and B. Imelik, Surface Sci., 1975,50,95 ; M. Primet, M. V. Mathieu and W. M. H. Sachtler, J. Catalysis, 1976, 44, 324. C. W. Garland, R. C. Lord and P. F. Troiano, J. Phys. Chem., 1965,69, 1188. R. Queau and R. Poilblanc, J. Catalysis, 1972, 27, 200. A. C. Yang and C. W. Garland, J. Phys. Chem., 1957,61, 1504. ’ H. Arai and H. Tominaga, J. Catalysis, 1976,43, 131. * M. Primet, J. C. Vedrine and C. Naccache, J. Mol. Catalysis, in press. M. Primet, to be published. C. W. Garland and R. J. Wilt, J. Chem. Phys., 1962,36, 1094. l 1 M. Primet, unpublished results. l 2 G. A. Martin, M. Primet and J. A. Dalmon, J. Catalysis, in press. l 3 M. Araki and V. Bonec, J. Catalysis, 1976,44,439. l4 W. L. Van Dijk, J. A. Groenewegen and V. Ponec, J. Catalysis, 1976, 45,277. M. G. Wells, N. W. Cant and R. G . Greenler, Surface Sci., 1977, 67, 541. l6 R. A. Marbrow and R. M. Lambert, Surface Sci., 1977, 67,489. l7 B. A. Sexton and G. A. Somorjai, J. Catalysis, 1977, 46, 167. ’* C. L. Angel1 and P. C. Schaffer, J. Phys. Chem., 1966,70, 1413. l9 L. A. Hanlan and G. A. Ozin, J. Amer. Chenz. SOC., 1974,96,6324.2580 co ON SUPPORTED RHODIUM 2o H. Praliaud, personal communication. 21 S. E. Wanke and N. A. Dougharty, J. Catalysis, 1972, 24, 367. 22 M. Primet, unpublished results. 23 P. Gallezot and B. Imelik, Adv. Chem. Ser. Molecular Sieves, 1973, 121, 66. 24 P. Gallezot and B. Imelik, J. Chim. phys., 1974, 71, 155. 2 5 E. M. Flanigen, H. Khatami and H. A. Szymanski, A h . Chem. Ser., 1971, 101, 201. 26 E. M. Flanigen, in Zeolite Chemistry and Catalysis, ed. J . A. Rabo, A.C.S. Monograph, 1976, 27 C. M. B. Henderson and D. Taylor, Spectrochim. Actu A, 1977, 33, 283. 171, 80. (PAPER 8/018)