Chemistry of Rh(NH3)5Cl2+ on mordenite

Chemistry of Rh(NH3)sCI 2+ on mordenite Robert A. Schoonheydt, Helene Van Brabant and Jozefien Pelgrims Katholieke Universiteit Lueven, Centrum voor Oppervlaktescheikunde en Collo~dale Scheikunde, Kard. Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium (Received 12July 1983)

At 296 K 0.225 mmole Rh(NH3)sCI 2+ is selectively exchanged into a synthetic Na-mordenite. This loading is increased by repeated exchange. The pseudo-octahedral structure of the Rh complex is retained in v a c u o and in 02 up to 448-473 K. After an oxidative pretreatment the redox chemistry of Rh o2 can be represented as Rh (lll)~zRh(O). About 50% of Rh(lll) is cationic. The other 50% is in the form of a Rh(lll)-oxide. Rh metal is present in a highly dispersed state in the channels of mordenite and as large particles on the external surface. An oxidized Rh-mordenite reacts with CO to give 2 Rh(I)(CO)2 species. The predominant species has its characteristic CO stretching bands at 2115 and 2045 cm -1, the other at 2115-2020 cm -1. Heating above 673 K is necessary for their complete destruction. On a Rh(NH3)sCI2+-mordenite three types of dicarbonyls are formed, but only one of them is formed in an equimolar CO:H20 mixture. These dicarbonyls are formed above 448K and are destroyed at 573 K. Keywords: Na-rnordenite; ion-exchange

INTRODUCTION The chemistry of Rh(III) compounds (RhCI3, Rh(NH3)5CI 2+) in faujasite-type zeolites is well understood. The reduction sequence Rh(III) --* Rh(I) ~ Rh(O) is obtained upon activation in vacuo 1"2. Activation in 02 produces small amounts of e.s.r. detectable Rh(II) species 3,4, but the majority of the Rh is present in oxide form and as Rh(III) cations 5. This chemistry is not a priori extendable to other types of zeolites. No reduction of Rh(III) was found upon activation of RhBr3-impregnated zeolite A 6. The chemistry of 3d transition metal ions is quite different in faujasite-type zeolites and in mordenite 7.w. Also, Ru(NH3)63+, activated in a watergas mixture, is an active low temperature watergas shift catalyst in faujasite-type zeolites but not in mordenite II'12. Because of these differences it was of interest to study the chemistry ofRh(NH3)5C12+ in mordenite in comparison with its chemistry in faujasites.

EXPERIMENTAL Exchange of Rh(NHs)sCI 2+ The exchange isotherm of Rh(NH3)5CI 2+ into Zeolon 100 Na (Norton) was established by immersing 200 mg Zeolon 100 Na in 20 cm 3 of an aqueous solution of constant normality (0.01 N), but varying Rh(NH3)sC12+:Na ratios. The suspensions were shaken overnight at 295 K. The equilibrium pH was in the range 7.4--8.4 for every point of the isotherm. Na + released and Rh adsorbed were determined by atomic absorption spectrometry of the supernatant solutions. In a second set of experiments 200 mg Zeolon 100 Na was exchanged 4 times successively with 20 cm 3 of a 5 mmol Rh(NH3)5CI 2+ solution. Each exchange was performed overnight at 295 K. After each exchange the supernatant was analysed for Rh and Na +. The samples for physico-chemical characterization 0144-2449184/040067-06503.00 ~) Butterworth & Co (Publishers) Ltd.

were prepared by exchange in a 0.005 mol Rh(NH3)5C12+ solution overnight at 295 K. The solid:liquid ratio was 10 g d m - . This procedure was repeated 3 times. The Rh-content was 0.275 mmol g-l. These samples were denoted RhM-0.8., where 0.8 is the number of Rh atoms per unit cell.

Methods and procedures Reflectance spectroscopy Reflectance spectra were recorded on a Cary 17 with diffuse reflectance attachment type I in the range 2000-210 nm. The signal was computer-processed and plotted as F(R®) against wavenumber, where F(R=) ~ (1 - R ® ) 2 / 2 R = , the Kubelka-Munk function . R= is the ratio of the light intensity reflected from the sample to that reflected from the standard. The samples were prepared in a reflectance flow cell 14. Spectra were recorded of the decomposition of RhM-0.8 in a stream of He between 273 and 573 K, after activation in 02 at 773 K and the subsequent reaction with CO. Infrared spectroscopy The redox chemistry and the interaction with CO were followed by i.r. spectroscopy (1200-3800 cm-l) on a Perkin-Elmer 580B. Self-supporting wafers (5 mg cm -2) were introduced into a home-made cell. All treatments were performed in situ. Volumetric measurements The CO, H2 and 02 consumptions were determined in a calibrated recirculation system with a linear temperature program of 0.0833 K s -1 after oxidative pretreatment at 773 K for 10.8 ks. The apparatus and the working conditions were described by Verdonck et al. 15. The initial pressure of the reagents was 16.67 + 3.33 kPa (CO) and 93.3 kPa for H 2 and 02. Blank experiments were performed on Na-mordenite after an identical pretreatment.

ZEOLITE$, 1984, Vol 4, January

67

Rh(NH3)sCI2+-mordenite: R. A. Schoonheydt et al.

For the isothermal CO adsorption, the sample was heated in 30 kPa CO to the desired temperature and kept at that temperature until the pressure remained constant. CO was then evacuated, the sample cooled to 296 K and CO2, which was condensed in a solid CO2 trap was measured and pumped away. This procedure was repeated for every temperature investigated. The H2-chemisorption on RhM-0.8, reduced in H2 at 673 K, was measured by saturation of the sample at 296 K in 100 kPa H~ and evacuation, followed by a temperature programmed desorption (t.p.d.) up to 673 K. The amount of H2 desorbed in the t.p.d, was'taken as the chemisorbed H2.

RESULTS Exchange of Rh(NHs)~CI 2+ into mordenite The exchange isotherm is shown in Figure 1. Under our experimental conditions there is a selective uptake of 0.225 mmol Rh(NH3)5CI 2+. The 4 points indicated by an arrow are those obtained with 4 successive exchanges in 0.005 M Rh(NH3)sCI 2+ solutions. These successive exchanges result in a monotonous increase of the exchange level up to 0.365 mmol Rh(NH3)sCI 2+ per gram mordenite. At the same time, however, a cation deficiency is created as more Na + is released in the solution than Rh is taken up (dotted line in Figure 1). When the exchange is performed in 0.025 M Rh(NH3)sC12+ the exchange level obtained is 0.29 mmol per gram and the reaction is almost stoichiometeric as 0.53 mmol Na + is released. Exchanges with solid:liquid ratios of 20 mg in 20 cm 3 give such small uptakes that they cannot be measured quantitatively. Reflectance spectroscopy of Rh(NH~)sCI ~+ on mordenite A freshly prepared, air-dry sample is characterized by a broad band centred at 29500 cm -I and a shoulder around 31000 cm -I (Figure 2). These bands are to be compared with the 28700 cm -I band for

J

J

i

0.5

i

n-

1 Q5 Rh/Rh + No

Figure 1 Exchange isotherm of Rh(NH3)sCI2* (0) in Namordenite, O: Na ÷ release. For Rh(NH3)sCI z+ the ordinate scale has to be divided by 2

68

ZEOLITES, 1984, Vol 4, January

2.76

5

2.1

I.~

0.70

0.0:' 5

H~

23,04 3206 -I cm x I(~ 5

41.08

50.10

Figure 2 Reflectance spectra of Rh(NH3)sCI2*-mordenite. 1, air-dry sample; 2, evacuated at 295 K; 3, evacuated at 404 K; 4, evacuated at 503 K; 5, evacuated at 588 K

1.775-

% Q835

Q03~

2

14.02

?.3.04

32.06

41.08

5QIO

¢rn-I x i0 -3

Figure 3 Reflectance spectra of Rh(NH3)sCI2+-mordenite, pretreated in 02 at 773 K (3), followed by CO-interaction at 423 K (1) and 523 K (2)

Rh(NH3)sCI 2+ and the 31200 cm -I band for Rh~NH3)5(OH)2+ 16. They are the characteristic IAl~ --'* "Tig transitions of pseudo-octahedral R h ( I I I ) l ~ The second d-d transition, IAlg ----> Iy2g, is expected around 36000 cm -I. I t is not resolved from the zeolitic background spectrum and the CF- ~ Rh(III) charge transfer band with maximum around 48000 cm-I Upon degassing the 29500 cm-I band broadens at its low frequency side and shifts to 28900 cm -I. A weak band in the range 35000-36000 cm -I becomes visible. This is probably t h e IAig ~ IT2g transition. This spectrum remains almost unchanged upon degassing at increasing temperatures except that the 48000 cm-l charge transfer band shifts to 45500 cm-l and that a weak band around 24000 cm -] becomes apparent. After evacuation at 503 K and higher Rh(NH3)~C12+ is completely destroyed and metallic Rh is the main species on the surface. When RhM-0.8 is pretreated in an O2-flow, the Rh(NH3)sC12+ spectrum remains essentially intact up to 473 K except for a shift of the 29500 cm-I band to 28000 cm-l and the appearance of a shoulder around 23000 cm -l. Above 473 K the Rh-complexes are destroyed. A spectrum is produced which is characteristic for Rh-oxides (Figure 3). This oxide is destroyed by CO: the 2 characteristic bands of the Rh(I)(CO)2

Rh(NH3)sCI~+-mordenite: R. A. Schoonheydt et al.

moiety 5 at 31900 c m - ' and 39200 cm -I grow at the expense of the oxide spectrum above 423 K. Infrared

spectroscopy

When RhM-0.8 is heated in an atmosphere of CO, typical carbonyi bands start to a p p e a r at 448 K (Figure 4): a triplet with m a x i m a at 1995, 2010 and 2035 c m - ' and a band at 2110 cm -I. T h e CO2 band is found at 2350 c m - ' ; bands in the region 2100-2200 cm - l are due to gaseous C O and a weak band at 2280 c m - l is the absorption of a cyanato group ( - N C O ) 17. T h e carbonyl spectrum simplifies upon evacuation of C O at increasingly higher temperatures. After evacuation

J 2200

cm -I

1900

Figure 5 I.r. spectra of the interaction of a 1:1 ; CO/H20 mixture with Rh(NH3)sCI2+-mordenite. 1, heated at 483 K and 2, at 503 K; 3, evacuated at 295 K; 4, evacuated at 448 K, 5, evacuated at 498 K; 6, evacuated at 563 K. Spectra are taken at the indicated temperatures

2500

2200

1900 cm -I

Figure 4 I.r. spectra of the interaction of CO (37.5 kPa) with Rh(NH3)sCIZ+-mordenite. 1, heated at 448 K in 36.14 kPa CO during 1800 s; 2, idem at 498 K; 3, evacuated at 328 K during 3600 s; 4, evacuated at 423 K for 1800 s; 5, evacuated at 473 K for 3600 s; 6, evacuated at 553 K for 1800 s. Spectra are taken at the indicated temperatures

at 473 K the remaining carbonyl bands are at 2110 cm - l , 2035 cm - l and 2010 cm -I with, at 553 K, a weak broad band around 1975 cm - l . When this procedure is repeated on a freshly prepared sample in the presence of a 1:1 C O : H 2 0 mixture the same bands are produced (Figure 5) with other relative intensities. Thus, the pair of bands 2100-2030 c m - I is predominant. Only by evacuation at 448 K and higher is it possible to observe significant band intensities at 2010 and 1995 cm -I and, at 498 K and at 565 K, at 1975 cm - l . T h e latter band grows gradually as the other carbonyl bands decrease in intensity. These changes are accompanied by the shift of t h e 2 1 0 0 c m - I band t o 2 1 1 0 c m - I .

ZEOLITES, 7984, Vol 4, January 69

Rh(NH3)sCl2+-mordenite: R. A. Schoonheydtet al. The H2- and O2-consumptions of an oxidized RhM-0.8 are given in Table 1. The reduction rate is maximal in the temperature region 373-393 K. The reduction is completed at 400--420 K. During oxidation 0.11 mmol 02 g-l is taken up instantaneously at 300 K. The maximal rate of the second oxidation step is around 673 K. Except for the first reduction, the H2- and O2-consumptions are indicative for the conversion R h ( O ) ~ Rh(III). Namordenite, also pretreated in O9 at 773 K does not adsorb detectable amounts of H2 or O2 at 296 K. The amount of chemisorbed H2 on RhM-0.8, reduced at 673 K with H2, is 0.0403 mmol g-I. This gives a H:Rh ratio of 0.29. In the X-ray pattern a broad band of Rh-metal particles at 41 ° is visible, superposed on lines of the mordenite lattice. A crude estimate of the average particle size from the width of the X-ray line gives particle sizes in the range 3.0-7.5 nm. Oxidized RhM-0.8 adsorbs 0.31 mmol CO g-t at 300 K. In the temperature programmed experiment 0.26 mmol is desorbed at 400 K. Above 400 K a new CO adsorption process becomes apparent with its maximum CO consumption at 600 K. Na-mordenite adsorbs 0.37 mmol CO g-t at 300 K and this amount is gradually desorbed upon heating until at 450 K no CO is left on the zeolite. When the CO chemistry is studied in isothermal steps the results of Table 2 are obtained. Again, there is an instantaneous adsorption of 0.30 mmol CO g-] at 300 K, almost equal to the amount adsorbed by Na-mordenite. Upon heating, a partial desorption of CO occurs until, from 383 K on, CO2 formation starts and CO consumption increases again. The maximum amount of CO consumption is attained at 500 K. If the amount of CO adsorbed at 300 K is subtracted, because it is due to physisorption, the total amount of adsorbed CO in the range 300-500 K is 0.83 mmol g-I, corresponding to a C O / R h ratio of 2.95. The total amount of CO2 produced between 383 2300

1900 cm-I

Figure 6 I.r. spectra of the interaction of CO (20 kPa) with Rh(NHa)sCI2+-mordenite pretreated in 02 at 723 K: 1,295 K; 2, 323 K; 3, 373 K; 4, 523 K. The spectra were taken at 295 K after evacuation of CO at 295 K

Finally, in Figure 6 the carbonyl bands are shown of an oxidized RhM-0.8. Here, the carbonyl formation starts at room temperature. The 2 main bands are at 2115 cm -] and at 2045 cm -l. A low frequency shoulder at 2020 cm-l is present from the beginning but it is a 'significant band only at 473 K and higher. The weak band at 2140 cm -I disappears at 473 K. The 3-band spectrum 2115-2045-2020 cm -] attains its maximum intensity at 523 K but temperatures above 673 K are necess~iry to destroy it completely.

Quantitative redox chemistry A RhM-O.8 wafer, pretreated in 02 at 623 K in the i.r. cell has a weak OH band at 3615 cm -j. This intensity is doubled by H2-reduction at the same temperature and reduced to its original value in 02. These OH band intensity changes are reversible.

70

ZEOLITES, 1984, Vol 4, January

Table 1 H2- and 02- uptakes of RhM-0.8 pretreated in 02 at 773 K Cycle

1 2

H2 HzO 02 H20 H2 (mmol g-1)(mmol g-1)(mmol g-1)(mmol g -1) Rh 0.30 0.43

0.19 0.19

0.21 0.20

0.11 0.11

02 Rh

1.09 1.56

0.76 0.72

Table 2 CO-consumption and CO2-production of oxidized RhM-0.8

T(K) 300 332 383 422 500

CO(mmolg-1)CO2(mmolg-1) CO

C02

Rh

Rh

0.30 0.14 0.11 0.15 0.41

-0.05 0.11 0.23

1.07 0.51 0.40 0.55 1.49

--0.18 0.40 0.84

SUM 1.1 CO(sum)-CO(300 K) 0.81

0.39 --

4.04 2.95

1.43 --

565 629

0.14 0.09

-0.011 -0.05

-

-

Rh(NHa)~CI2+-mordenite: R. A. Schoonheydt et al.

and 500 K is 0.39 mmol or CO2:Rh equals 1.43. Above 500 K a small amount of CO is desorbed with a simultaneous production CO2.

DISCUSSION The exchange of 0.225 mmol Rh(NH3)sCI 2+ is very selective. This corresponds to 17% of the cation exchange capacity, based on the theoretical framework formula AlsSi40096, or 0.69 Rh(NH3)sC12+ molecules per unit cell. The exchange level can be increased to 1.15 Rh(NH3)sCI ~+ per unit cell by successive exchanges but simultaneously a cation deficiency of 0.22 mmol Na + g-l is created• We assume that these Na + ions are replaced by H +. They originate from the partial hydrolysis of Rh(NHa)sC12+: Rh(NH3)sCI 2+ + H 2 0 Rh(NH3)s(OH) 2+ + H + + C1-

(1)

Na-mordenite + H + + C1 H-mordenite + Na + + C1-

(2)

g

Rh203 + 3H2

~

2Rh(O) + 3H20

2Rh(III) + 3H2 4Rh(O) + 302

--> ~

2Rh(O) + 6H + 2Rh~Oa

4Rh(O) + 302 + 12 Z O H

~

(3) (4) (5)

Rh(III) + 6 H 2 0 + 12ZO(6)

Z represents the lattice.

Evidence for reactions (1) and (2) comes from the reflectance spectra. The argument runs as follows. The symmetry ofRh(NH3)sC12+ is not octahedral but C4v. The Tl~ level is then split in 2 components and the IAlg---) "Tl transition is composed of 2 bands Because C1- is a weaker hgand than NH3 one band (IAl ~ IA2) is expected at the position of the IAlg ITlg transition, the second band (IA ~ IE) is expected at lower energies 18. We find a band at 29500 cm- , almost the position of the band of Rh(NH3)sC12+ in solution (28700 cm-I) 16 and a band at 31000 cm -I, i.e. above the original band of 29500 cm -l. In solution Rh(NH3)sOH 2+ has a band at 31200 cm -l. We conclude that both Rh(NH3)sC12+ and Rh(NHa)sOH 2+ are present on the surface according to reactions (1) and (2). Upon evacuation of Rh(NH3)sC12+-mordenite below 448-473 K the band broadening around 28000 cm-~ and the shift to lower frequency of the charge transfer band from 48000 cm -I to 45500 cm -l are indicative for ligand replacement reactions without a major change in the effective symmetry of the coordination sphere. At the same time NH~'formation is visible in the i.r. spectra. Therefore, some NH3 is released and replaced by lattice oxygens, H 2 0 or O H - ligands. Above 448-473 K the octahedral Rh(III) structure collapses and Rh(III) is reduced to the metallic state in vacuo. The thermal stability of the octahedral coordination sphere of Rh(III) is also apparent from the i.r. spectra of the CO interaction: bands of coordinated CO are only visible after pretreatment in the range 448-473 K. In 02, qualitatively the same scheme applies below 448-473 K, but above 473 K, Rh-oxide is formed. Reflectance spectra, typical for the oxide and metallic Rh are seen in Figures 2 and 3. We are working at the detailed explanation of these spectra. •

first reduction after the oxidative pretreatment the H2-consumption is less. This was also observed for Rh-Y zeolites 5. It means that after the pretreatment not all the Rh is in the +3 state. The formation of H 2 0 and the reversible O H band intensity changes are indicative for the following reactions:

.

Redox chemistry The H 2- and O2-consumptions of Table 1 can be 02 explained by the scheme Rh(III) Rh(O). In the 1-12

In reactions (3) and (5) the Rh-oxide is formally represented as Rh203. It only means that a Rh(III)oxide is formed upon oxidation. The overall stoichiometry of (3)-(6) leads to: H J R h = 1.5 and O2/Rh = 0.75, which is confirmed by the data of Table 1. From the amount of water produced after reduction and oxidation we conclude that 0.13 mmol Rh or 47% of the total Rh-content is involved in reaction (3) and 0.073 mmol Rh or 27% of the total Rh-content is involved in reaction (6). If the 0.11 mmol 02 g-l adsorbed at 300 K on a reduced sample is taken as a measure of the amount of the monoatomically or highly dispersed Rh, as was done for Ru 19, then 0.147 mmol Rh or 53% of the total Rh content is in that state. This is exactly the amount of Rh involved in reaction (4) but, double the amount of Rh calculated on the basis of the water evolved from reaction (6). However, at these low H 2 0 pressures the readings of the pressure transducer are highly inaccurate and there is a large uncertainty on the 0.11 mmol H 2 0 g-l of Table 1. We conclude from all our evidences discussed above that about 50% of the Rh is involved in reactions (3) and (5) and 50% in reactions (4) and (6). The H2-chemisorption on Rh metal particles in mordenite is indicative for a highly dispersed metal state, as, on the average, 1 out of 3.4 Rh atoms chemisorbs a H atom. Relatively large Rh metal particles are made visible by X-ray diffraction. These 2 informations together point to a bidisperse metal particles systems: highly dispersed metal probably in the channel system and large particles on the external surface. The 2 step oxidation process (see above) confirms these conclusions. On a Rh-Y with 8 Rh atoms per unit cell, the amount of Rh reduced according to reaction (4) was estimated to be 25% of the total Rh content 6. Another more fundamental difference between Rh-Y and Rh-mordenite is that on R h - Y the redox cycle was O~ Rh(IV) -~ Rh(O) 5. Presently we cannot explain this difference".' CO-chemistry Oxidized RhM-0.8 reacts with CO, starting at 295 K, to form thermally very stable dicarbonyls. There

ZEOLITES, 1984, Vol 4, January 71

Rh(NHa)sCI~+-mordenite: R A. Schoonheydt et al.

are 2 kinds of dicarbonyls differing in the position of

the low frequency band. The dicarbonyl absorbing at 2115-2040 cm -I is formed first and the dicarbonyl absorbing at 2115-2020 cm -I grows in upon heating in CO. The weak 2140 cm -I band, only visible below 473 K is thought to be due to Rh(III)-CO. The dicarbonyl formation is confirmed by the reflectance spectra: the bands at 31900 and 39200 cm -I are similar to the 4d(Rh) --> g*(CO) transitions as they occur in [RhCI(CO)2]219. Thus, the presence of Rh(I)(CO)2 units is confirmed but the data do not allow one to draw conclusions about monomeric, dimeric or polNneric Rh(I)(CO)2 entities. The fact that there are 2 different species absorbing in the i.r. seems to suggest 2 sites for the Rh(I)(CO)2 units. The reactions can formally be represented as: Rh20 ~ + 7CO---~

Rh(I)(CO)2 + CO2

(7)

Rh(III) + 3CO + 2ZOH----> Rh(I)(CO)2 + CO2 + H 2 0 + 2Z-

(8)

Rh(III) + 3CO + Z-O-Z ---> Rh(I)(CO)2 + CO2 + Z[:]Z-

(9)

Rh(III) + 3CO + H 2 0 + 2 Z O ----> Rh(I)(CO)2 + CO2 + 2 Z O H

(10)

Reactions (8)-(10) are 3 possible variants for the participation of the lattice, Z, in the reaction. In any case, the expected CO:Rh and CO2:Rh ratios are 3.5 and 1.5 for (7) and 3 and 1 for (8)-(10). The isothermal CO-consumption and CO2-production data of Table 2, after subtraction of the physiosorbed CO at 300 K, give CO:Rh and CO2:Rh ratios of 2.95 and 1.43 respectively. The i.r. spectra indicate that reactions (7)-(10) start at 295 K. Thus, ascribing the CO adsorption at 300 K only to physisorption is an oversimplification. The real CO:Rh ratios will be somewhat higher than 2.95. All this indicates that the experimental CO:Rh and CO2:Rh ratios are within the range expected for the simultaneous occurrence of reactions (7)-(10). Temperatures of at least 448 K are necessary to initiate the interaction of CO with Rh(NH3)5CI 2+mordenite. Thus, the octahedrally coordinated Rh(III) atoms in mordenite are thermally very stable and much more stable than in faujastic-type zeolites 5. This is also the case for their reaction with (CO + H20) mixtures. The reaction with CO gives 3 types of Rh(I)(CO)2 complexes, but only one of these is formed in the presence of H20. Thus CO probes 3 complexes differing in the nature of ligands (NH3, CO, CI-, H~O and structural oxygens) and/or in the nature of the sites in the main channel system of mordenite. In any case, these complexes are destroyed by evacuation up to 573 K. The result is the formation of metallic Rh. CONCLUSIONS

The exchange at 295 K of Rh(NHs)5CI 2+ into mordenite is a selective, stoichiometric reaction up to

72

Z E O L I T E S , 1984, Vol

4, January

a loading of 0.225 mmol g-I. Higher loadings are obtained by repeated exchanges but protons are incorporated simultaneously due to partial hydrolysis of Rh(NH3)5C12+ to Rh(NH3)5(OH) 2+. These pseudo-octahedral complexes are thermally stable up to 448-473 K. At the latter temperatures interaction with CO gives 3 kinds of carbonyls but only one type in the presence of gas phase H20. After oxidation of the Rh(NH3)sC12+-mordenite at 773 K a 1:1 mixture of Rh(III)-oxides, formally represented as Rh203, and Rh(III) cations is present on the surface. They are reduced by H2 to the bidisperse metallic state and the redox reaction is reversible. The oxidized Rhmordenites react with CO to form Rh(I)(CO)2 species. The latter are thermally stable to at least 673 K but their maximum concentration is at 523 K. ACKNOWLEDGMENT

Acknowledgment is made to the Donors of the Petroleum Research Foundation, administered by the American Chemical Society, for partial support of this work. The authors acknowledge the support of the Belgian Government (Geconcerteerde Onderzoekakties). R.A.S. is indebted to the National Fund of Scientific Research (Belgium) for a position as Senior Research Associate.

REFERENCES 1 Okamoto, Y., Ishida, N., Imanaka, T. and Teranishi, S. J. Catal. 1979, 58, 82 2 Anderson, S. L. T. and Scurrel, M. S. J. Catal. 1981, 71,233 3 Naccache, C., Ben Taarit, Y. and Boudart, M. A.C.S. Syrup. Ser. 1977, 40, 156 4 Atanasova, V. D., Shvets, V. A. and Kazanskii, V. B. Kinet. Katal. 1977, 18, 753 5 Van Brabant, H., Schoonheydt, R. A. and Pelgims, J. in 'Metal Microstructures in Zeolites' (Eds. P. A. Jacobs et al). Elsevier, Amsterdam, 1982, 12, 61 6 Kuznicki, M. and Eyring, E. M. J. Cata/. 1980, 65, 227 7 Kasai, P. H. and Bishop, R. J., Jr. J. Phys. Chem. 1977, 81, 1527 8 Kasai, P. H., Bishop, R. J., Jr. and McLeod, D., Jr J Phys. Chem. 1978, 82, 279 9 Garbowski, E. D., Mirodatos, C., and Primet, M. in 'Metal Microstructures in Zeolites' (Eds. P. A. Jacobs, et al.) Elsevier, Amsterdam, 1982, 12, 235 10 Garbowski, E. D., Mirodatos, C., Primet, M, and Mathieu, M. V. J. Phys. Chem. 1983, 87, 303 11 Verdonck, J. J., Schoonheydt, R. A. and Jacobs, P. A. 'Proc. 7th Int. Congress Catalysis', (Eds. T. Seiyama and K. Tanabe) Elsevier, Amsterdam, 1981, B16, 911 12 Jacobs, P. A., Chantillon, R., De Laet, P., Verdonck, J. and Tielen, M. A.C.S. Symp. Ser. 1983, 218, 439 13 Kort(Jm, G. 'Reflectance Spectroscopy', Springer Verlag, Berlin, 1969, p 106 14 Verdonck, J. J., Schoonheydt, R. A. and Jacobs, P. A. J. Phys. Chem. 1981, 85, 2393 15 Verdonck, J. J., Jacobs, P. A., Genet, M. and Poncelet, G. J. Chem. Soc. Faraday Trans. 11980, 76, 403 16 JCrgensen, Chr. K. Acta Chem. Scandinavica 1956, 10, 500 17 Nakamoto, K. 'Infrared and Raman spectra of inorganic and coordination compounds', 3rd Edn, Wiley, New York, Part III, 1978 18 Lever, A. B. P. 'Inorganic Electronic Spectroscopy', Elsevier, Amsterdam, 1968, p. 309 19 Nixon, J. F., Suffolk, R. J., Taylor, M. J., Norman, J. G., Jr., Hoskin, D. E. and Gmur, D. J. Inorg. Chem. 1980, 19, 810