J. Chem. Soc., Furaduy Trans. I, 1987, 83 (8), 2605-2617

Characterization of CO-Rh Species formed on Rh-Y Zeolite by Temperature-programmed Desorption

and Infrared Techniques Nobuo Takahashi," Akihiko Mijin, Takahisa Ishikawa, Kenji Nebuka and

Hanzo Suematsu Department of Industrial Chemistry, Kitami Institute of Technology, 165 Koencho,

Kitami, Hokkaido 090, Japan

The characterization of CO-Rh species which are formed on Rh-Y zeolite during exposure to CO has been carried out using the techniques of temperature-programmed description (t.p.d.) and infrared spectroscopy (i.r.). The formation of three kinds of CO-Rh species was observed. The first is characterized by a t.p.d. peak at 473 K as well as i.r. bands at 2092 and 1834 cn-'. The second species is characterized by a t.p.d. peak at 500 K and i.r. bands at 2098 and 1763 cm-'. A similarity between the i.r. spectra of these species and those of the Rh,(CO),,/Na-Y system suggests that these two kinds of CO-Rh species are cluster-like species formed at the external surface or in the outermost supercages of the zeolite particles. The third species, characterized by a t.p.d. peak at 573 K and four i.r. bands at 21 15, 2098, 2044 and 2019 cm-', is considered to be an Rh'(CO), species formed in the zeolite cavities. Conversion of the cluster-like species into the Rh' (CO), species is accelerated by higher temperatures, lower concentrations of CO and lower rhodium contents of the catalyst. In addition, the removal of water, physically adsorbed to fill the zeolite pores, accelerated the conversion. The rate of the exchange reaction between adsorbed and gaseous CO was much faster on the Rh'(CO), species than on the cluster- like species.

Rhodium is a transition metal which exhibits particular and pronounced effects on the activation of carbon monoxide (CO). Rh-Y zeolite has been suggested as a heterogenized rhodium complex catalyst for reaction involving CO, and in fact methanol carbonyla- t i ~ n l - ~ and olefin hydr~formylation~-~ can be catalysed by Rh-Y zeolite. To understand the catalysis of Rh-Y zeolite in such reactions, it is important to clarify the interaction between the rhodium species and CO on the zeolite support. Infrared (i.r.) spectroscopy and X-ray photoelectron spectroscopy (X.P.S.) have been used for this purpose. The oxidation state of the rhodium species on fresh Rh-Y zeolite has been found to be trivalent.*-12 On exposure of Rh-Y zeolite to CO, the trivalent rhodium species are converted into monovalent rhodium dicarbonyl species, Rh'(CO),,** 13* l4 according to the following reaction by analogy with the homogenous system :15

Rh"' +3CO + H 2 0 + Rh'(CO), +CO, +2H+. However, further details of this process have not yet been clarified. In a study of the CO/ Rh-Y system using the temperature-programmed desorption (t.p.d.) technique we found that at least two different types of CO-Rh species are formed during exposure of Rh-Y zeolite to CO; the first releases CO at a maximum temperature of ca. 473-500 K and the second releases CO at a maximum of ca. 573 K.'' Our purpose in the present study is to characterize the CO-Rh species formed during exposure of the Rh-Y zeolite to CO by using t.p.d. and i.r. techniques.

2605

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2606 Characterization of CO-Rh Species on Rh-Y Zeolite

Experimental Catalyst

Rh-Y zeolites with rhodium contents of 0.7, 1 .O and 2.0 wt Oh were prepared from Na-Y zeolite (obtained from Toyo Soda Manufacturing Co. Ltd as a fine powder without binder) and an aqueous solution of rhodium trichloride trihydrate (obtained from Nippon Engelhard Ltd) by stirring the mixture at 353 K for 12 h. After the solid was thoroughly washed with distilled water, it was dried at 393 K overnight. It was then exposed to water vapour over a saturated NH,C1 aqueous solution at room temperature for 2 days in a closed container.

T.P.D. For t.p.d. experiments the Rh-Y zeolite powder was pressed, crushed and then sieved to 20-42 mesh size. A t.p.d. run was carried out in a conventional flow system at atmospheric pressure. The catalyst (1 .O g) was placed in the reactor (a Pyrex glass tube of 10 mm id.) and heated to the desired temperature at a rate of 2 K min-l in an He-CO stream [total flow rate 50 cm3 (s.t.p.) min-'1. After exposure to the He-CO stream at a given temperature for a fixed time (usually 5 h), the catalyst was cooled to room temperature in a helium stream. It was then heated to 723 K at a constant rate of 2 K min-' in a helium stream at a flow rate of 30 cm3 (s.t.p.) min-'. The concentrations of CO and CO, were determined by gas chromatography with a thermal conductivity detect or.

I.R. For i.r. experiments the Rh-Y zeolite powder (0.7 wt YO Rh) was pressed into a thin circular disc. Exposure of the disc to CO was carried out in a glass i.r. cell equipped with KBr windows, which was connectable to both a gas flow line and a vacuum line. 1.r. measurements were carried out at room temperature using the transmission method with a Fourier-transform infrared spectrometer (JEOL Ltd JIR-40), (resolution 4 cm-l).

The exchange reaction of l2C0 adsorbed on the Rh-Y zeolite with gaseous 13C0 was carried out as follows. The disc was placed in the i.r. cell connected to a conventional static apparatus for gas adsorption. The sample was evacuated under appropriate conditions. Introduction of l2C0 (6.6 x lo3 Pa) into the system at room temperature was followed by heating the sample to a desired temperature at which CO adsorption was performed. After cooling the sample to 308 K, the system was evacuated for 5 min in order to remove CO in the gas phase. After recording the i.r. spectrum of the sample, 13C0 (99 atom %) at 6.6 x lo3 Pa was introduced into the system. The sample was then heated to the desired temperature at which the exchange reaction was performed.

Results and Discussion T.P.D. and CO adsorbed on Rh-Y Zeolite When the fresh Rh-Y zeolite was heated in an He-CO (30%) stream to 353 K and kept at this temperature a considerable amount of CO, was formed, as shown in fig. 1. The amount of CO, desorbed from the catalyst was estimated to be ca. 200 x lop6 mol g,-,',, which approximately agreed with the amount of Rh'" loaded (194 x mol g;it), suggesting that almost all the Rh"' species was reduced by CO during exposure, even at 353 K for 4 h.

T.p.d. spectra of CO and CO, obtained from CO adsorbed on the fresh Rh-Y zeolite (2.0 wt %) with an He-CO gas stream (30%) at three different temperatures, 353, 393

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N . Takahashi et al.

I---+

-

-

-

0- ~0-O-o-p~ c

2607

353

333 M 2

313

293

5*0 1373

tlh Fig. 1. Formation of CO, during exposure of the fresh Rh-Y zeolite to an He-CO (30%) stream

[50 cm3(s.t.p.) min-'1 at 353 K.

and 433 K, are shown in fig. 2(a), (b) and (c) , respectively. The observed desorption of CO and CO, was attributed to CO adsorbed on the rhodium species on the catalyst, since no appreciable desorption of CO and CO, was observed from the Na-Y zeolite when exposed to the He-CO stream at 393 K for 5 h. The peak positions of CO, correspond to those of CO, except in the temperature region above 593 K, where the desorption of CO, continued after CO desorption was complete. The colour of the catalyst following the adsorption of CO was white-yellow. After t.p.d. runs the colour changed to black. When CO had been adsorbed at 353 K, the t.p.d. spectrum of CO showed a maximum at ca. 473 K, and most of the CO was desorbed from the catalyst before the temperature reached 5 13 K [fig. 2 (a)]. The CO-Rh species for this desorption peak is denoted by A,. The Rh-Y zeolite subjected to adsorption at 393 K clearly exhibited two peaks, as shown in fig. 2(b); the maxima for these two peaks were found at ca. 500 K for the first peak and ca. 573 K for the second peak. Here we denote the first peak as A, and the second as B. The species corresponding to A, cannot be identified as the species corresponding to A,, since the temperature difference between 473 K for A, and 500 K for A, is too large. On all t.p.d. runs except for run 1, the lower t.p.d. peak was observed at ca. 500 K. On the Rh-Y zeolite which was exposed to the stream at 433 K, the first peak diminished and the second peak grew significantly [fig. 2(c)]. The amounts of desorbed CO and CO, estimated from the t.p.d. spectra are shown in table 1 as runs 1, 2 and 4, where both A, and A, are incorporated into A.

The effects of other factors in the CO adsorption conditions on the t.p.d. results are also summarized in table 1. A longer exposure (run 5 ) and a lower concentration of CO (run 6) brought about a decrease in the first peak and an increase in the second peak. Under the same adsorption conditions the Rh-Y zeolite with a lower rhodium content showed a higher ratio of B/A (see runs 2, 7 and 8). Molar ratios of CO +CO, desorbed from the catalyst us. rhodium loaded are found in the range 2.0-2.4 for all t.p.d. runs. This suggests that the rhodium species responsible for B are formed by conversion of the rhodium species responsible for A.

The Rh-Y zeolites used in t.p.d. runs 1-8 were almost fully saturated with water. By exposing the Rh-Y zeolite to a helium stream at moderate temperatures it is possible to remove partly physisorbed water without changing the oxidation states of the rhodium species. The t.p.d. spectrum of an Rh-Y zeolite pretreated with a helium stream at 433 K for 5 h [fig. 3(b)] is compared with that on the non-pretreated Rh-Y zeolite [fig. 3(a)], where the CO adsorption was carried out at 413 K. The amounts of CO and

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2608 Characterization of CO-Rh Species on Rh-Y Zeolite

300 400 500 600 700

Fig. 2. T.p.d. spectra of CO adsorbed on Rh-Y at (a) 353, (b) 393 and (c) 433 K. 0, CO; 0, T/K

co,.

CO, desorbed are also listed in table 1 as run 3 for the former and as run 9 for the latter. Following the pretreatment with helium, peak A, at 500 K was reduced and a single peak was observed at the position of B. This result indicates that physically adsorbed water on the Rh-Y zeolite, which fills the pores, retards the formation of the rhodium species responsible for B.

With respect to the route for the formation of CO, during t.p.d. runs, the following three reactions may be proposed : (1) the reaction of CO adsorbed on Rh-Y with residual 0, in the catalyst, (2) the Boudouard reaction (2CO + CO, +C) and (3) the reaction of CO with H,O or zeolite framework oxygen.

Another reaction may be possible for the formation of CO, at peak A, i.e. reduction of Rh"' species into RhI(CO), species according to the equation given in the introduction. However, this possibility may be excluded since the reduction was almost

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N. Takahashi et al. 2609

Table 1. Effects of adsorption conditions of CO on the t.p.d.

amount desorbedl mol g-I adsorption

Rh conditions co run content no. (wt %) CO (YO) T/K t/h A B total B/A CO, CO+CO, (CO+CO,)/Rh

1 2 3 4 5 6 7 8 9"

2.0 2.0 2.0 2.0 2.0 2.0 0.7 1 .o 2.0

30 353 5 242 37 279 0.15 150 429 30 393 5 268 70 338 0.26 106 444 30 413 5 158 149 307 0.94 127 434 30 433 5 44 277 321 6.30 112 433 30 393 10 230 119 349 0.52 120 469 10 393 5 109 157 266 1.44 126 392 30 393 5 11 83 94 7.55 66 160 30 393 5 54 81 135 1.50 92 227 30 413 5 0 247 247 - 133 380

2.2 2.3 2.2 2.2 2.4 2.0 2.4 2.3 2.0

a Pretreated with a helium stream [50 cm3 (s.t.p.) min-l] at 433 K for 5 h.

5.0

4.0

3.0

2.0 " I

5 ," 1.0 & - 'p 8 0

< 2 6.0 \

8 5.0

4 .O

3.0

2.0

1.0

0 300 400 500 600 700

TIK Fig. 3. Effects of pretreatment of Rh-Y with a helium stream on the t.p.d. of CO (CO adsorption at 413 K for 5 h): (a) without pretreatment, (b) with pretreatment. 0, CO; 0, CO, at 433 K

for 5 H. 86 F A R 1

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2610 Characterization of CO-Rh Species on Rh-Y Zeolite

8.0 1 -

M" & 6.0 "

4 z 'p 4.0 2 \

2.0

0 300 400 500 600 700

T/K Fig. 4. T.p.r. spectra of CO adsorbed on Rh-Y into an He*, (30 %) stream. (1) 0, CO adsorption

at 353 K for 5 h. (2) 0, at 433 K for 5 h.

complete during the adsorption of CO at 353 K, as shown in fig. 1. In addition, if CO, formation corresponded to the reduction, other desorption peaks of CO and/or CO, attributed to the'Rh'(CO), species should be observed in the t.p.d. spectrum of fig. 1 (a) at higher temperatures than peak A. However, no substantial peaks were observed there. In order to clarify the origins of the CO, formed, a temperature-programmed reaction (t.p.r.) run using a He-0, (30%) stream instead of a helium stream was performed on the Rh-Y zeolite exposed to CO at the same conditions as with run 1 (at 353 K) or with run 4 (at 433 K). As shown in fig. 4, CO, was predominantly formed during t.p.r. runs. The amount of CO, formed was estimated to be ca. 500 x mol g;it in both cases, although their accuracy was not good. The CO-Rh species responsible for peak B can react with 0, at lower temperatures than 500 K, suggesting that CO, for peak B was formed by a reaction other than (1). Comparing the spectrum for run (1) in fig. 4 with spectrum (a) in fig. 1, the pattern of CO, formation for the CO-Rh species responsible for peak A is seen to be influenced in the presence of 0,. However, the possibility of reaction (1) still remains for CO, at peak A, since the changes would not be great. Reaction (2) presumably took place in part, since ca. 50 x mol g;:t of CO, was recovered during a t.p.r. run into an He-0, (30%) stream which was carried out consecutively on the catalysts used for t.p.d. runs 1 and 4. However, the amount of CO, formed in the second run (ca. one third of the amount of CO, evolved in the first run) seems too small in order to attribute all the CO, formation to the Boudouard reaction. Although the origins of CO, for peak A are still uncertain, reaction (3) seems to be important as a route for CO, formation at peak B during t.p.d. runs into a helium stream.

Infrared Study of CO adsorbed on Rh-Y Zeolite

A thin disc of fresh Rh-Y zeolite was exposed to an H e 4 0 (30%) stream at 353 K for 3 h in a flow system. The i.r. spectrum of this sample in the CO stretching region is shown in fig. 5 as spectrum (a) where i.r. bands at 2092 and 1834 cm-l are pronounced. 1.r. spectra of CO adsorbed at 393 K are shown in fig. 5[(b) for CO adsorption for 0.5 h and (c) for 5 h]. At this temperature a decrease in the intensity of the band at 1834 cm-' and an increase in that at 1763 cm-l is observed with increasing adsorption time. These changes were accompanied by the propagation of four bands at 21 15,2098,

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N . Takahashi et al. 261 1

2200 2080 1960 6840 1720 wavenumber/cm-'

Fig. 5. 1.r. spectra of CO adsorbed on Rh-Y in a flow system: (a) adsorbed at 353 K for 3 h; (b) adsorbed at 393 K for 0.5 h; (c) adsorbed at 393 K for 5 h; (d ) sample (c) followed by heating in a helium stream at 473 K for 0.5 h; (e ) CO adsorbed on the Rh-Y pretreated with a helium stream

at 433 K for 5 h.

2044 and 2019 cm-', while the band at 2094 cm-' in (b) shifted slightly to 2098 cm-l in (c). As shown in spectrum ( d ) other changes were also observed when the sample for spectrum (c) was heated in a helium stream up to 473 K and kept at the temperature for 0.5 h; i.e. the intensity of the band at 2098 cm-l decreased and the band at 1763 cm-' almost disappeared.

The effect of the pretreatment of the fresh Rh-Y zeolite with a helium stream at 433 K for 5 h on CO adsorption was examined, since the pretreatment considerably affected the conversion of rhodium species from A to B in t.p.d., as described in the previous section. The i.r. spectrum of CO adsorbed at 393 K for 3 h on the pretreated Rh-Y is shown in fig. 5 as spectrum (e). The CO-Rh species formed agreed with the species for spectrum (d) , which are characterized by four sharp bands at 21 15,2098,2044 and 2019 cm-'. No bands of adsorbed CO are observed in the region 1700-1900 cm-' on this sample. The four bands observed correspond to the bands for Rh'(CO), species formed in Y

13- l4 The Rhl(CO), species were characterized by i.r. bands at 21 16, 2101, 2048 and 2022 cm-'. The doublets at 2101-2022 and 21 16-2048 cm-' are attributed to Rh'

86-2

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2612 Characterization of CO-Rh Species on Rh-Y Zeolite

2280 2120 1960 1800 1640 wavenumber/cm -l

Fig. 6. Exchange reaction of CO adsorbed on Rh-Y with gaseous CO. (The Rh-Y was evacuated at 433 K for 3 h prior to CO adsorption.) (a) l2C0 adsorbed at 433 K for 3 h; (b) after exposure of sample (a) to 13C0 at room temperathre for 0.5 h; (c) 13C0 adsorbed at 433 K for 3 h ; (d) after

exposure of sample (c) to ' T O at room temperature for 0.5 h.

(CO),(O,), and Rh'(CO),(O,)(H,O), respectively, by Shannon et al. ,14 where 0, represents a zeolite framework oxygen atom.

Comparing spectrum (c) with spectrum (d) , the intensity ratio of 2098 to 21 15 cm-' on (c) is much greater than that on (d), while the intensity ratio of 2044 to 2019 cm-l is almost the same for these two spectra. Therefore it may be possible to deconvolute spectrum (c) into the CO-Rh species responsible for the 2098 and 1763 cm-' peaks and the Rh'(CO), species.

In order to elaborate this point the exchange reaction of l2C0 adsorbed on the Rh- Y zeolite with gaseous 13C0 or vice versa was studied. Four sharp bands at 21 15, 2098, 2044 and 2019 cm-' are predominantly observed in spectrum (a) in fig. 6 , where l2C0 was adsorbed on Rh-Y (evacuated at 433 K for 3 h) at 433 K for 3 h. This suggests that the predominant CO-Rh species on this sample are Rh'(CO), species. Andersson and Scurrell previously found that the exchange between CO in Rh'(CO), on Rh-Y and gaseous CO was fast at 323 K,12 but did not report further details of the exchange reaction. On the exposure of this sample to 13C0 at room temperature for 0.5 h, spectrum (a) was changed to (b), where the four bands shifted to 2065, 2050, 2002 and 1975 cm-'. Spectrum (b) is identical to spectrum (c), which was obtained on the Rh-Y zeolite exposed directly to 13C0 at 433 K for 3 h. Replacement of adsorbed 13C0 by gaseous l2C0 at room temperature also took place easily, as shown by spectrum (d). On

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N. Takahashi et al. 2613

1 1 I 1 2280 2120 1960 1800 1640

wavenumberlcm Fig. 7. Exchange reaction of CO adsorbed on Rh-Y with gaseous CO. (Rh-Y was evacuated at room temperature for 5 min prior to CO adsorption.) (a) 'TO adsorbed at 393 K for 3 h; (b) after exposure of sample (a) to Y O at room temperature for 0.5 h; (c) after exposure of sample (b) to 13C0 at 373 K for 0.5 h; (d) 13C0 adsorbed at 393 K for 3 h; (e) after exposure of sample (d) to

l2C0 at 353 K for 3 h; cf) after additional exposure of sample (e) to l2C0 at 353 K for 2 h.

spectra (b) and (c) another i.r. band is observed at 1951 cm-l in addition to the four bands mentioned above. This suggests the presence of another kind of adsorbed CO species, while the band for the l2C0 species was masked by the band at 2019 cm-'. We will not refer to this type of CO species further in the present study, since its concentration was much lower than that of the Rh'(CO), species.

On the Rh-Y sample evacuated at room temperature for 5 min and then exposed to l2C0 at 393 K for 3 h, i.r. bands at 2098 and 1763 cm-l are mainly observed, together with weak bands at 2038 cm-' as shown by spectrum (a) in fig. 7. Spectrum (d) was obtained on the sample which was first exposed to 13C0 instead of l2C0 under the same

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2614 Characterization of CO-Rh Species on Rh-Y Zeolite

conditions as spectrum (a). In this case the bands at 2098, 2038 and 1763 cm-' shifted to 2048, 1990 and 1728 cm-l, respectively. Spectrum (b) was obtained from the sample which was subsequently exposed to 13C0 at 333 K for 1 h after spectrum (a) was recorded. The band at 2038 cm-' was shifted to ca. 1990 cm-l, together with the appearance of bands at ca. 2050 cm-' which correspond to the two higher of the four bands given by the Rhr(l3CO), species. However, the bands at 2098 and 1763 cm-l remained at the same positions. As shown in spectrum (c), the rate of the exchange reaction with respect to the 12CO-Rh species for the bands at 2098 and 1763 cm-' was fast at 373 K. As shown by spectra (e) and 0, the 13C0 responsible for the i.r. bands at 2048 and 1728 cm-' was gradually replaced by l2C0 when the sample for spectrum ( d ) was subsequently exposed to l2C0 at 353 K. Thus the rate of exchange of adsorbed CO by gaseous CO on the rhodium species responsible for the i.r. bands at 2098 and 1763 cm-' is much slower than that on the Rh'(CO), species.

On the Rh-Y zeolite sample evacuated at room temperature for 1 h followed by exposure to l2C0 at 393 K for 3 h, spectrum (a) in fig. 8 was obtained. Subsequent exposure of this sample to 13C0 at room temperature for 0.5 h changes the spectrum into (b), suggesting that this sample contains a comparable amount of the 12CO-Rh species responsible for the bands at 2098 and 1763 cm-l with that of the Rh1(l3CO), species responsible for the four bands at 2065,2050,2002 and 1975 cm-l. On heating this sample in vacuo up to 433 K followed by cooling in vacuo, spectrum ( c ) was obtained. The intensities of the Rh'(13CO), species were almost unchanged, while the splitting into four bands progressed. However, the band at 1763 cm-' was almost removed by the evacuation. In addition, the intensity of the band at 2098cm-l was considerably weakened. This result indicates that CO molecules in the CO-Rh species responsible for the bands at 2098 and 1763 cm-' are more easily removed by evacuation than CO molecules in the Rh'(CO), species. However, the appearance of low-intensity bands due to the Rh1(12CO), species suggests that some of the 12CO-Rh species at 2098 and 1763 cm-' was converted into the Rh1(l2CO), species on heating the sample under evacuation.

On Rh/Al,O, catalyst systems the presence of three types of CO species adsorbed on Rh atom(s) has been reported; these are the twin, linear and bridged tyes.17-" Yates et al. reported that twin-type CO can exchange with gaseous CO even at 200 K."*18 They also reported that bridged CO, which is observed at 1870 cm-l, is replaced by gaseous CO at temperatures > 250 K. Besides the difference found in the position of the i.r. band for bridged CO ongoing from Rh/Al,O, to Rh-Y, the difference in the exchangeability of bridged CO with gaseous CO found in these two systems suggests that the CO-Rh species responsible for bridged CO (1763 cm-') on the Rh-Y zeolite are different from those on metallic rhodium particles.

Accordingly, at least three different types of CO-Rh species could be detected by i.r. methods on the Rh-Y zeolite which was exposed to CO; for l2C0 the first species (denoted by I,) is characterized by bands at 2092 and 1834cm-l, the second species (denoted by I,) by bands at 2098 and 1763 cm-l and the third species (denoted by 11) by bands at 21 15, 2098, 2044 and 2019 cm-l, respectively.

Correspondence between T.P.D. and I.R. The correspondence betweeen t.p.d. and i.r. results is summarized in table 2 on the basis of the following observations. (1) On increasing the adsorption temperature, the conversion of I, to I, and then to I1 was observed by i.r., as was the conversion of A, to A, and then to B by t.p.d. (2) CO found in the CO-Rh species for I, was easily desorbed from the catalyst by evacuation of the sample at 433 K, while CO found in the CO-Rh species for I1 was still present after evacuation. (3) On the Rh-Y zeolite pretreated in a helium stream at 433 K, the predominant CO-Rh species found by t.p.d.

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N. Takahashi et al. 261 5

1 I I I 1 22 80 21 20 1960 1800 1640

wavenumber/ cm

Fig. 8.1.r. spectra of CO adsorbed on Rh-Y (Rh-Y was evacuated at room temperature for 1 h). (a) l2C0 adsorbed at 393 K for 3 h; (b) after exposure of sample (a) to 13C0 at room temperature

for 0.5 h; (c) sample (6) evacuated at 433 K for 0.5 h.

Table 2. Correspondence between t.p.d. and i.r. results

t.p.d. 1.r.

species T,,/K species absorption bands/cm-'

A, 473 I, 2092, 1834 A2 500 -+ I, 2098, 1763 B 573 - I1 2116, 2098, 2044, 2019

were CO-Rh species for B; these corresponded to I1 by i.r. found by spectroscopy. The CO-Rh species for B/II correspond to the Rh'(CO), species formed on Y zeolite. Our results indicate that Rh*(CO), species are formed via the first CO-Rh species (Al/Il) and the second CO-Rh species (A2/12).

The i.r. spectrum of the first CO-Rh species (Al/Il) is very similar to the spectrum of Rh,(CO),, weakly adsorbed on the external surface of Na-Y zeolite particles which was reported by Gelin et uL20 They also reported that the first CO-Rh species is converted into the second CO-Rh species (A2/12) when the sample is exposed to air at 298 K followed by exposure to CO at 373 K. Mantovani et ul.' also reported that the second

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2616 Characterization of CO-Rh Species on Rh-Y Zeolite species was formed when the Rh-Y zeolite was treated with a CO-H, (1 : 1) mixture at 403 K and 8 x 10, Pa. Gelin et al. speculated the species to be rhodium carbonyl clusters, Rh,(CO),, m = 6-13.,' However, the information available on the CO-Rh species with bridging CO is insufficient to discuss their structures in detail. Nevertheless, it is obvious that these CO-Rh species are composed of several rhodium atoms, which enable the CO species with a bridging structure to be formed. On the Rh-Y zeolite prepared by our technique, the concentration of rhodium species at the external surface of the zeolite particles is much higher than that in the pore^.^-^^ Therefore, it is feasible that the formation of cluster-like CO-Rh species, which are responsible for the t.p.d. peaks for A, and A,, occurs at the external surface of the zeolite particles. The higher ratio of B/A on the Rh-Y sample with a lower rhodium content may reflect a decrease in the rhodium concentration at the external surface.

Conversion of CO-Rh species from A, to B by t.p.d. as well as from I, to I1 by i.r. spectroscopy corresponds to the conversion of the cluster-like CO-Rh species into monovalent rhodium dicarbonyl species, Rh'(CO),. As shown in table 1, this conversion was retarded by an increase in the concentration of CO in the atmosphere, suggesting that the cluster-like species are stabilized at a higher concentration of CO. This conversion is then accelerated by the removal of weakly physisorbed water from the pores of zeolite, as can be seen from fig. 3 and 5. The appreciable difference among the spectra of fig. 6(a), fig. 7(a) and fig. 8(a), where CO adsorption was carried out at 393 K for 3 h in every case, reflects the difference in water contents of these samples. However, the formation of Rh'(CO), species was not significant on the Rh,(CO),,/Na-Y system,l* where Na-Y dehydrated under severe conditions (evacuation at 773 K) was used as support. This suggests an important role for adsorbed water in the conversion of cluster- like species into the Rh'(CO), species, whereas an excess amount of adsorbed water retards the conversion. On the Rh,(CO),,/Al,O, system, Smith et al. reported that the molecular nature of Rh,(CO),, is maintained when the content of water on alumina is low, whereas the conversion of the cluster into Rh'(CO), species takes place in the presence of OH groups at an appropriate concentration.2' Previously, on an Rh-Y zeolite sample which had been evacuated at room temperature, we found that the majority of rhodium species present at the external surface of the zeolite particles migrates into the pores during exposure of the Rh-Y zeolite to CO.,, The fragmentation of the cluster-like species into Rh'(CO), species enables the migration through the narrow windows of the zeolite cavities as a result of the reduction in size of the CO-Rh species. Bilk et a1. studied the CO-Rh/Al,O, system by using EXAFS, and they found that extra-fine rhodium metal particles dispersed on Al,O, were converted into atomically dispersed rhodium dicarbonyl species, Al-0-Rh'(CO),, during exposure of the catalyst to CO at room t empera t~ re .~~ Recently, Solymosi et al. reported the important role of water in the fragmentation of metal particles of rhodium into Rh' (CO), species on an Al,O,

Consequently, the following conclusions are obtained. (1) At the first stage of exposure of the fresh Rh-Y zeolite to CO, the formation of

CO-Rh species characterized by a t.p.d. peak at 473 K and by i.r. bands at 2092 and 1834 cm-l takes place, which is followed by the formation of a second type CO-Rh species, characterized by a t.p.d. peak at 500 K and i.r. bands at 2098 and 1763 cm-l. The presence of bridged CO in these two types of CO-Rh species suggests that they contain several rhodium atoms. Judging from the similarity of the i.r. spectra for these species to those of the Rh,(CO),,/Na-Y system, these CO-Rh species are considered to be of a cluster-like type formed at the external surface of the zeolite particles or in the supercages which are the nearest to the external surface.

(2) The cluster-like CO-Rh species are subsequently converted into monovalent rhodium dicarbonyl species which are characterized by a t.p.d. peak at 573 K and i.r. bands at 2115, 2098, 2044 and 2019 cm-l. The formation of Rh'(CO), species is

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N. Takahashi et al. 2617

accompanied by the migration of the rhodium species from the external surface into the pores.

(3) CO molecules on the cluster-like CO-Rh species are more easily removed from the catalyst by evacuation at 433 K than CO molecules on the Rh'(CO), species.

(4) The rate of the exchange reaction between adsorbed CO and gaseous CO on the Rh'(CO), species is much faster than that on the cluster-like CO-Rh species.

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Paper 611958; Received 6th October, 1986 Publ

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