J . Phys. Chem. 1985,89, 4789-4793
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An Infrared Study of the Influence of CO Chemisorption on the Topology of Supported Rhodium Frigyes Solymosi* and Monika Pdsztor Institute of Solid State and Radiochemistry, University of Szeged, and Reaction Kinetics Research Group of the Hungarian Academy of Sciences, H-6701 Szeged, Hungary (Received: April 1, 1985)
The adsorption of CO on alumina-supported Rh of different crystallite sizes produced by reduction at 573-1273 K has been investigated by following the development of the infrared bands due to Rh'(CO)2 and Rh-CO species. It is concluded that the effect of CO on the state of the dispersed Rh is a twofold one. In harmony with recent EXAFS measurements of Prins et al., CO adsorption at around 300 K leads to disruption of the Rh clusters and to the formation of isolated Rh' sites, as indicated by the slow development of the infrared bands of gem-dicarbonyl. Above 423 K, another effect of adsorbed CO comes into prominence, which leads to the formation of Rh crystallites at the expense of isolated Rh' sites. It is demonstrated that the formation of isolated Rh' sites occurs more slowly under dry conditions.
Introduction Great attention is currently being paid to the structure and surface chemistry of Rh, as a result of its wide application as a catalyst in many technologically important reactions. The use of infrared spectroscopy to study surface species produced by room temperature C O adsorption is an accepted and widely used approach in the characterization of highly dispersed Rh. As established in the pioneering work of Yang and Garland,' the predominant forms of C O chemisorbed on Rh are the linearly bonded form, Rh-CO (absorbing a t 2040-2060 cm-l depending on the coverage), the gem-dicarbonyl
(giving bands at 2098 and 2027 cm-', representing the symmetrical and antisymmetrical modes, respectively), and the bridged form
-
(absorbing at 1850 cm-I). This work was followed by a number of interesting studies which verified the results of Yang and Garland and disclosed several further details of the system. It was also demonstrated that, under certain conditions, a number of other minor surface species, mostly quite labile, are also f ~ r m e d . ~Whereas .~ the various investigators agreed that metal crystallites are involved in the formation of the linearly and bridged bonded C O species, interpretation of the gem-dicarbonyl species gave rise to Data accumulating in the past few years strongly support the view that the gem-dicarbonyl is formed on isolated Rhl+ sites, and this picture is now generally accepted. The present state of knowledge is well documented in the papers by Worley2 and Yates3 where references to previous work can also be found. However, it turned out recently that the chemisorption of C O on Rh can exert a dramatic influence on the state of Rh dispersed on a support. Prins et a1.* studied the structure of rhodium (0.57 (1) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (2) Rice, C. A.; Worley, S. D.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1981, 74, 6748. Worley, S. D.; Rice, C. A,; Curtis, C. W.; Guin, J. A. J . Phys. Chem. 1982, 86, 2714. Worley, S. D.; Rice, C. A,; Mattson, G. A.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1982, 76, 20. (3) Wang, H. P.; Yates, J. T. Jr. J. Catal. 1984, 89, 79. (4) Primet, M. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2570. (5) Primet, M.; Vedrine, J. C.; Naccache, G. J . Mol. Catal. 1978, 4, 411. (6) Yates, J. T. Jr.; Duncan, T. M.; Vaugham, R. W. J . Chem. Phys. 1979, 71, 3908. (7) Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. J . Catal. 1979, 57, 41. (8) Van't Bilk, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Phys. Chem. 1983, 87, 2264.
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TABLE I: Adsorption of H2on Rh/AI2O3 at 300 K sample reduction temp, K H2 wmol/g 1% Rh%A1203 1% Rh/A1203 1% Rh/A1,0,
5% Rh/A1203
573 673 1173 1173
10.64 9.06 14.60 0.55 4.6
H/Rh 0.22 0.19
0.30 0.01 0.024
wt % Rh/y-Alz03) by determining the coordination of Rh atoms to adjacent atoms by means of extended X-ray absorption fine structure spectroscopy (EXAFS). They found that the reduced system (reduction was performed at 593 K for 1 h under flowing H2) is highly dispersed, but not as rhodium atoms or ions. The metal crystallites consist of 15-20 rhodium atoms. Their most interesting observation was that the adsorption of CO on the above sample at 298 K significantly decreased the amplitude of oscillations due to Rh-Rh nearest-neighbors. They concluded that CO adsorption causes a significant disruption of the Rh crystallites, leading to isolated Rh' sites and formation of the gem-dicarbonyl species, Rh(C0)2. The possibility of this disrupting effect of CO was first pointed out by Yates et a1.6 in one of the most illuminating articles written on the Rh-CO system. In our present study, it will be shown that the effect of C O adsorption on the structure of rhodium is more complex. While our infrared spectroscopic measurements in situ clearly confirm that the adsorption of CO increases the number of isolated Rh' sites (very probably through the disruption of the Rh-Rh bond) on which gem-dicarbonyl can be formed and provide more data relating to the occurrence of this process, they also demonstrate that at elevated temperature, where most catalytic reactions occur, an opposite effect prevails.
Experimental Section Rh/A1203samples were prepared by incipient wetting of Alz03 (Degussa, BET area 100 m2/g) with an aqueous solution of RhC13. After impregnation, the samples were dried in air at 373 K. For infrared studies, the dried Rh/Alz03powder was pressed into thin self-supporting wafers (30 X 10 mm, 20 mg/cm2). The pretreatment of the samples was performed in the vacuum IR cell: the sample was (a) heated (20 K min-I) to 573 K under constant evacuation, (b) oxidized with 100 torr of O2 (1 torr = 133.3 Pa) for 60 min at 573 K, (c) evacuated for 30 min, and (d) reduced with 100 torr of H2for 60 min at 573-1273 K. This was followed by degassing at the same temperature for 30 min and by cooling the sample to the temperature of the experiment. The gases were circulated during oxidation and reduction processes. The gases used were of commercial purity. C O (99.9%) was purified by bubbling through M ~ I ( O Hsuspension. )~ Water vapor was frozen out by a trap cooled with a dry ice-acetone mixture. The hydrogen was purified by passage at the temperature of liquid air through a trap filled with molecular sieve. 0 1985 American Chemical Societv
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Solymosi and Pasztor
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 1 min
15 min 90 min 120 min
180 min 240 min _In_
, 473 K.I'J2 We may also consider the possibility that an electronic interaction occurs between small Rh clusters and the alumina support, which can basically influence the surface properties of Rh. The first experimental evidence for this type of electronic interaction ~ this was presented for Ni/TiO2 more than 20 years a g 0 ' ~ 3 'and idea gained ground recently.17-19 It is very likely that this type of electronic interaction is involved in the exceptionally high
-
(14) Yates, J. T. Jr.; Williams, E. D.; Weinberg, W. H. Surf. Sci. 1981,
-
. 91., . 5 6 -2..
(1 5 ) SzaM, Z . G.; Solymosi, F. 'Proceedings of 2nd International Congress on Catalysis"; Editions Technip: Paris, 1961; p 1627. (16) Solymosi, F. Catal. Rev. 1967, I , 233. (17) Schwab, G. M. Adu. Catal. 1978, 27, 1. ( 1 8) Bond, G. C. Stud. Surf. Sei. Caral. 1982, 11, 1. (19) Solymosi, F.; Tomblcz, I.; Koszta, J. J . Catal., in press.
Solymosi and Pasztor
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
4192
50 Torr CO 1 min
90 min 300min
evacuation and admission of ‘Torr H20, 10 min
1
2ioo
ctn-
I
2doo
2100 cm-1
50min admision of 50Torr CO l 0 m i n
?&
Figure 6. Changes in the infrared spectrum of adsorbed C O on 1% Rh/AI2O3 (reduced at 573 K) in the presence of 50 torr of C O a t 473 K (A), and 523 K (B). In the latter case, after C O treatment at 523 K
60 min
l8hwrs
for 10 min the sample was cooled in the presence of 50 torr of C O to 300 K, where further spectral changes were followed in time.
catalytic activity of Rh/Ti02 in the dissociation of CO and in the hydrogenation of C O and C 0 2 over Rh/Ti02.20,21As the work function of Rh is 4.98 eVZ2and that of alumina is 5.3 eV, we envisage that electrons flow from Rh to the alumina, and as a result the Rh is partially positively charged. This model was also used by Hyde et aLZ3in the explanation of the formation of cationic character of Rh on oxides, as some electron transfer from the rhodium to the supporting oxides had been detected by ESCA measurement^.^^^^^ Hyde et al.23rightly argue that this interaction is more significant for monatomically dispersed rhodium on which gem-dicarbonyl is formed. Although we would welcome the application and the further development of the electronic interaction between metal and oxidic support, we feel that it cannot be applied as such in the present case. The formation of Rh1(C0)2 (which may mean the oxidative disruption of the Rh, clusters) also occurred on magnesia-, titania-, and silica-supported Rh,2,’0 when the electron flow is presumed to proceed in the opposite direction (Rh TiOz)e (the work function of reduced TiO, is 4.6 eVI9) or, very probably, not at all (Rh-Si02). This makes the charge-transfer model very doubtful, unless we assume another type of an electronic interaction between Rh and the oxidizing sites of the supports modifying the electronic state of the RhSZ5 At the moment, the most probable mode of production of Rh’ is the oxidation of Rh, by an OH group of the support during the carbonylation, as suggested by Prins et al.?
-
2Rh
+ 4CO + 2A10H
-+
2Al-O-Rh(CO)2
+ H2
This reaction route was proposed on the basis of results obtained for the transformation of Rh6(C0),6on Al2O3. In studies of the decomposition of Rh6(C0)16it was postulated that surface hydroxyl on A1203is effective in converting Rh6(C0)16to a species in which the Rho in the carbonyl is oxidized to Rh’
co
co
‘RJ
d
I AI
‘0
1
AI
with the evolution of H2.26,27 (20) Solymosi, F.; ErdBhelyi, A,; Binslgi, T. J . Catal. 1981, 68, 371. (21) Solymosi, F.; Tomblcz, I.; Kocsis, M. J . Catal. 1982, 75, 78. (22) ‘Handbook of Chemistry and Physics”, 63rd ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1982-1983. (23) Hyde, E. A.; Rudhow, R.; Rochester, C. H. J . Chem. SOC.,Faraday Trans. I 1983, 79, 2405. (24) Katzer, J. R.; Sleight, A. W.; Gajardo, P.; Michel, J. B.; Gleason, E. F . ; McMillan, S. Faraday Discuss. Chem. SOC.1982, 72, 121. (25) Figureas, F.; Gomez, R.; Primet, M. Ado. Chem. Ser. 1973, 121, 266. (26),Smith, A. K.; Hugues, F.; Theolier, A,; Basset, J. M.; Ugo, R.; Zanderighi, G. M.; Bilhou, J. L.; Bilhou-Bougnol, V.;Graydon, W. F. Inorg. Chem. 1979, 18, 3104.
o f t e r evocuotion 15 min I
2200
21cc cm-‘
h o
Figure 7. Infrared spectra for C O adsorbed on 1% Rh/AI2O3 (reduced at 1273 K) in the presence of 50 torr of CO, as a function of adsorption time at 300 K. The sample was prepared from RhCI, dissolved in methanol. Care was taken to perform all manipulations under dry conditions.
As the number of O H groups gradually decreases with elevation of the temperature of A1203pretreatment,28this may contribute to the slower development of gem-dicarbonyl following hightemperature reduction, provided that the Rh particle size is affected only to a minor extent by the reduction temperature. However, the gem-dicarbonyl likewise developed (rather slowly) on Rh/A1203 reduced and evacuated at 1173 K, even though this sample contained no O H groups detectable by IR spectroscopy. This indicates either that a very small amount of isolated OH groups (even undetectable by IR spectroscopy) is sufficient for the slow oxidation, or that during the very slow surface reaction (3-24 h) enough H 2 0vapor can diffuse into the vacuum cell to produce O H groups and oxidize the Rh, clusters. In order to examine these effects more clearly, a Rh/Al2O3 sample was prepared under dry conditions. In this case, alumina dehydroxylated at 1273 K was impregnated with RhC1, dissolved in methanol. The suspension was dried at 373 K under vacuum. The further treatment of the sample in the IR cell was the same as before. Although great care was taken to perform all manipulations in dry nitrogen, we cannot exclude completely the possibility that moisture adsorbed on the sample during compression of the powder into a disk and/or when it was placed in the ir cell. Following CO adsorption at 300 K on 1% Rh/A120, reduced at 1273 K, we obtained an intense Rh-CO band, with only a very slight indication the presence of Rh(CO)2. The bands due to this latter species developed more slowly than in the previous cases (Figure 7). However, the admission of 1 torr of H 2 0 into the cell, which produced a broad band due to OH groups, clearly accelerated this process, as shown in Figure 7. These results may be considered as confirming that the oxidation of Rho can occur through surface O H groups. How can the opposite effect of CO on the state of Rh observed at higher temperatures be described? Accepting that the oxidation state of Rh in the Rh(C0)2 species is + 1 , a reductive process should occur in the action of C O before the formation of a Rh cluster. Due to the reducing properties of CO, this process can be more easily envisaged than the oxidative adsorption of CO. (27) Theolier, A,; Smith, A. K.; Leconte, M.; Basset, J. M.; Zanderighi, G . M.; Psaro, R.; Ugo, R. J . Organomet. Chem. 1980, 1919 415. (28) For a review of properties of the OH group of alumina see, for example: Knozinger, H. Adu. Caral. 1976, 25, 184 and references therein.
J . Phys. Chem. 1985,89, 4193-4199 In addition, it can be assumed that the migration of Rh on the support at elevated temperature is promoted by adsorbed CO, which is also required for the formation of Rh crystallites. Data accumulating in the past decade strongly suggest that the surface diffusion of metal particles is influenced by adsorbed gases. In the case of supported Rh, Yates (D.J.C.)et aL7 observed that
4793
the adsorbed CO enhanced the surface mobility of Rh in hydrogen at 473 K. The results of the present study showed that the chemisorbed C O can cause the agglomeration of Rh from 448 K upward producing Rh, crystallites. Registry No. Rh, 7440-16-6; CO, 630-08-0.
Conformationally Dependent Fermi Resonances and Long-Range Interactions between c Bonds in Polymethylene Systems Derived from Their Raman Spectrat Laure Ricard, Universiti de Bordeaux I , Laboratoire de Spectroscopie Infrarouge, 351 Cours de la Liberation, 33405 Talence, France
Sergio Abbate, CNR, Istituto di Chimica- Fisica Applicata dei Materiali, Genoua, Nucleo di Spettroscopa c/o Dipartimento di Chimica Industriale, Politecnico di Milano, Italy
and Giuseppe Zerbi* Dipartimento di Chimica Industriale del Politecnico- Piazza Leonard0 da Vinci, 32-201 33 Milano. Italy (Received: February 13, 1985; In Final Form: May 8, 1985)
Further attempts in the interpretation of the Raman scattering in the C-H stretching range are presented. Such scattering consistently and characteristically occurs in all molecules containing a long alkyl residue. Attempts are presented to account for the 2900-cm-' Raman band, not yet explained. It is found that the explanation of the whole spectral pattern brings up a message of physical relevance, namely that an electronic coupling through u bonds may occur at least as far as the second-nearest neighbor.
Introduction The strong Raman scattering observed in the C H stretching region near 3000 cm-' from polymethylene chains has been and is being used as a tool for structural studies in complex organic and biological materials.' Indeed all the materials containing a long polymethylene residue show a very characteristic scattering so strong as to overcome the other Raman scattering of most of the structurally complex biological materials. In spite of its characteristic occurrence, the detailed interpretation of this scattering is far from being understood and seems to get more and more complicated, if one attempts to consider various structural and dynamical aspects. The Raman spectral features which are generally observed for polymethylene compounds are different depending on whether the molecule (or the alkyl residue in a larger system) is in the all-trans conformation or in a distorted conformational state. The Raman spectra in this frequency range are almost identical from short chains up to polyethylene. In Figure 1, A and B, the Raman spectra of crystalline (trans = T) and molten (containing gauche (G) structures) polyethylene are shown both for comparison and for use in the discussion which follows. From previous works in this field it is at present well accepted that the observed Raman scattering is strongly determined by the fact that several Fermi resonances take place, to a different extent, depending on whether the alkyl residue has an all-trans or a distorted While several of the observed features seem to be satisfactorily understood, at present the behavior of the very strong Raman line observed near 2900 cm-' seems rather puzzling since it changes frequency, intensity, and shape in a way not yet rationalized. Its 'Work presented at the IXth International Conference on Raman Spectroscopy, Tokyo, Sept 1984.
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understanding may provide a structural probe unique for these organic systems. For this reason we wish to discuss in this paper some dynamical and spectroscopic aspects of this yet unsettled problem. So far the authors working in this field have attempted to evaluate the various Fermi resonance processes which certainly affect the whole pattern in this frequency range and which may affect the band near 2900 cm-' as a consequence of the conformational distortion of the chain. On the other hand, the conclusions reachedsv6were that the 2900-cm-' Raman line is not greatly affected by Fermi resonances, even in the distorted conformation. We present in this paper an attempt to understand the whole Raman spectrum in the C H stretching region in terms of dynamics, Fermi resonances, and intensity calculations, which take into account the existence of interactions between CH2 groups up to the second-nearest neighbor. Essentially the idea which stems from this work is that CH, do not behave as isolated groups but feel the influence of the neighboring CH2 groups, in a characteristic way which depends on their distance and conformation. This idea is actually not new since it has already been presented very clearly by M ~ K e a n who , ~ has carried out very (1) I. W. Levin in "Advances in Infrared and Raman Spectroscopy", Vol. 11, R. J. H. Clark and R. E. Hester, Eds., Wiley, New York, 1984, Chapter
G. Snyder and J. R. Sherer, J. Chem. Phys., 71, 5221 (1979). G. Snyder, S. L. Hsu, and S. Krimm, Spectrochim. Acta Part A, (1978). Zerbi and S. Abbate, Chem. Phys. Lett., 80, 455 (1981). Abbate, G. Zerbi, and S. L. Wunder, J. Phys. Chem., 86, 3140 (1982). ( 6 ) S. Abbate, S. L. Wunder, and G. Zerbi, J. Phys. Chem., 88, 593 (1984).
0 1985 American Chemical Society
