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J . Phys. Chem. 1990, 94, 7250-7255
The application of a linearized PB model to this type of system appears to reproduce well the experimental observations and, in addition, provides some physical insight into the origin of the behavior of charged polymers in size-exclusion chromatography. Some of the discrepancies between experiment and theory that remain may be due to assumptions made in applying the PB model. First and foremost among these is, of course, the assumption that the PB equation can be linearized, an assumption which has been considered by other^.^' Other assumptions that could prove to (31) McQuarrie, D. A. Statistical Mechanics; Harper & Row: New York, 1976; p 336.
be invalid upon closer inspection are the neglect of any chemically specific adsorption effects, the assumption that the polymer can be treated as a porous sphere with homogeneously distributed charge, and lack of polarization and shape-change effects due to strong interactions which may be important at close range.
Acknowledgment. Assistance was received from the donors of the Petroleum Research Fund, administered by the American Chemical Society. Support in the form of graduate fellowship (to C.J.W.) from the Johnson's Wax Fund is gratefully acknowledged. We thank Prof. G. S. Manning for pointing out ref 18 to us and Prof. W. M. Deen for helpful communications.
Infrared Spectroscopic Study on CO-Induced Structural Changes of Iridium on an Alumina Support F. Solymosi,* E. Novaik, and A. Molnir Reaction Kinetics Research Group of the Hungarian Academy of Sciences, Institute of Solid State and Radiochemistry, University of Szeged, P.O.Box 105, H-6701 Szeged, Hungary (Received: January 3, 1990; In Final Form: March 22, 1990)
The interaction of CO with alumina-supported iridium, reduced at different temperatures, was investigated by infrared spectroscopy. The dominant spectral features for the low-temperature reduced samples ( TR = 473-673 K) at 300 K are the bands at 2090-2107 and 2010-2037 cm-I. No spectral changes were observed, even after an extended adsorption time. I n the case of high-temperature reduced samples ( T R= 773-1 173 K), the adsorption of CO at 300 K initially produces a broad absorption band peaking at around 2060 cm-I, but after an extended adsorption time, bands at 2090-2107 and 2010-2037 cm-' gradually appear and grow in intensity. This process is attributed to the oxidative fragmentation of Ir, crystallites and to the formation of isolated iridium sites (Iro and Ir'), which involves the participation of OH groups on alumina. When the sample exhibiting strong adsorption bands at 2090-2107 and 2010-2037 cm-l is heated to 423-573 K in the presence of CO, both bands are gradually reduced in intensity and a strong band is formed at 2050-2080 cm-'. This process is explained by the reductive agglomeration of Ir' sites.
Introduction Of primary interest to us here is the evaluation of the possible structural changes of Ir crystallites due to the adsorption of CO. Recently it has been demonstrated by EXAFS and infrared spectroscopy that C O not only is adsorbed strongly on supported Rh crystallites at 300 K but also results in its morphological changes, i.e., in the disruption of Rh crystallites to isolated Rh atoms and in subsequent oxidation of the latter to Rh'.'s2 The addition of H 2 0 facilitated this process,2 whereas that of H2 hindered it.34 Changes in infrared spectra following the C O adsorption on supported Ru indicated the occurrence of a similar process. As the driving force of the oxidative disruption of metal crystallites (M) is probably the high bond strength between C O and M, which does not vary significantly for the Pt metals, similar CO-induced structural changes are expected for alumina-supported Ir, as for the other metals. This could obviously be an activated process: its evaluation requires the study of the CO-Ir interaction as a function of adsorption temperature and time. As one expects that the vibration of CO bonded to Ir depends on whether it is '38
(!) Van't Blik, 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; J . Am. Chem. Sac. 1985, 87, 2264. (2) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1985, 89, 4789. (3) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1986, 90, 5312. (4) Zaki, M. 1.; Kunzmann, G.;Gates, B. C.; Knozinger, H . J . Phys. Chem. 1987, 91, 2486. ( 5 ) Basu, P.;.Panaytov. D.; Yates, J . T., Jr. J. Phys. Chem. 1987.91, 3133. (6) Solymosl, F.; Knozinger, H. J . Chem. Soc., Faraday Trans. I 1990, 86, 389. (7) Solymosi, F.; Rasko, J . J . Catal. 1989, 115, 107. (8) Robbins, J. J . Caral. 1989. 115. 120. 0022-3654/90/2094-7250$02.50/0
bonded to Ir, crystallite, to isolated Iro atoms (formed in the disruption of Ir, crystallites), or to Ir' (produced by surface oxidation), registering changes in the infrared spectrum of adsorbed C O may be indicative of the occurrence of these structural changes of Ir. Although the adsorption of CO on supported Pt metals has been the subject of several detailed studies, the CO-Ir interaction has received relatively less a t t e n t i ~ n . ~ -Lynds9 '~ observed only one band at 2070 cm-' following C O adsorption on Ir/AI20,. Guerra and Schulmanlo found strong bands at 2030 and 2080 cm-' and weaker ones at 1993 and 1890-19 10 cm-I for Ir/Si02. Howell and Solymosi and RaskoI2 identified two C O bands at 2060-2080 and 2020 cm-' for reduced Ir/A1203 and Ir/Si02 and attributed them to linearly bonded CO to larger and smaller Ir clusters, respectively. The variation in frequencies was explained by Irsupport interaction of different extents. The effect of crystallite size on the adsorption of CO was investigated in detail by McVicker et aI.l3 They stated that the highly dispersed catalysts display a major carbonyl band at 2060 cm-I, while the larger (9) Lynds, L. Spectrochim. Acta 1964, 20, 1369. (IO) Guerra, C. R.; Schulman, J. H. Surj. Sci. 1967, 7, 229. ( I I) Howe, R. F. J . Catal. 1977, 50, 196. (12) Solymosi, F.;Rasko, J . J . Catal. 1980, 62, 253. (13) McVicker, G.B.; Baker, R. L.; Garten, R. L.; Kugler, E. L. J . Coral. 1980, 65, 207. (14) Toolenaar, F. J. C. M.; Bastein, A. G . T. M.; Ponec, V. J . Caral. 1983, 82, 35. ( I 5 ) Gelin, P.; Coudurier. G.; Ben Taarit, Y.; Naccache, C. J . Catal. 1981, 70, 32. (16) Tanaka, K.; Watters, K. L.; Howe, R. F. J . Catal. 1982, 75, 23. (17) Krishnamurthy, S.;Landolt. G. R.; Schoennagel, H. J. J . Catal. 1982, 319, 78. (18) Erdohelyi, A.; Solymosi, F., to be published.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7251
Ir on an Alumina Support TABLE I: Adsorption of
sample
0.2% Ir/AI20, 1% Ir/AI2O3
5% 1r/A1203 1% Ir/Si02
t T%
H2and CO on Ir/A1203 at 300 K reduction temp, K 513 573 873 1 I73 573 573
H/Ir
CO/Ir
2.1 1.85 1.67 1.19 1.21 0.69
1.76 1.41 1.13 0.78 0.62 0.42
crystallites exhibit a band at 2020-2025 cm-I. On the basis of TEM and chemisorption data they concluded that the highly dispersed system contains isolated Ir atoms that can adsorb up to two CO molecules. Toolenaar et aI.l4 suggested that the high-frequency band at 2075 cm-' is due to CO adsorbed on high-coordinate sites (planes), while the low-frequency band at 2050 cm-l was associated with CO bonded to low-coordinate sites (edges, corners, etc.). Different assignments were proposed by Gelin et al.I5 for absorption bands produced by interaction of CO with iridium-exchanged Na-Y zeolite, where the oxidation state of Ir was 3. An intense pair of bands was observed at 2086 and 2001 cm-I, when the sample was treated with CO at 443 K. As the relative intensities of these bands remained constant with the reaction temperature and the iridium loading, they were attributed to iridium tricarbonyl Ir1(CO)3species. The formation of Ir' was described by reduction of Ir"' by CO, and the positive charge was assumed to be balanced by halide ion. Tanaka et a1.I6 used alumina and silica as supports, and the iridium was deposited by the decomposition of Ir carbonyl, Ir4(CO)12.The pair of bands observed at 2080 and 2008 cm-' for SiOzand 2070 and 1995 cm-' for A1203was assigned to dicarbonyl adsorbed on edges of twodimensional rafts, while the middle band observed at 2054-2020 cm-I was rendered to CO bonded to Ir clusters, Ir,-CO. Experimental Section Materials. Ir/A1203samples were prepared by incipient wetting of A1203(Degussa, BET area 100 m*/g) with an aqueous solution of IrC13-H20. The solvent was allowed to evaporate in air with continuous stirring. After impregnation, the samples were dried in air at 330 K. For 1R studies, the dried powder was pressed into thin selfsupporting wafers (30 X 10 mm, 20 mg/cm2). The pretreatment of the Ir samples was performed in the vacuum cell: the samples were (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 30 min at 573 K, (c) evacuated for 15 min, and (d) reduced with 100 Torr of H2 for 60 min at selected temperatures (473-1073 K). This was followed by degassing at 623 K for 45 min, which is required to achieve a complete removal of adsorbed hydrogen from Ir sample^.'^ When the reduction temperature was lower than 623 K, the sample was degassed first at the reduction temperature for 15 min, and the final evacuation was carried out at 623 K. The gases were circulated during the oxidation and reduction processes by using a trap cooled by liquid nitrogen. We note here that according to XPS results a fraction of the supported Ir has a slight positive charge even after reduction at 573 K.'9-22 The gases used were of commercial purity. CO (99.9%)was purified by bubbling through a Mn(OH), suspension. Water vapor was frozen out by a trap cooled with a dry ice-acetone mixture. Methods. 1R spectra were recorded with a Specord 75 IR double-beam spectrometer (Zeiss, Jena) with a wavenumber accuracy of f5 cm-I. In this case difference spectra were produced and magnified with the help of a Data system (Tracor Northern, TN 1710). More sensitive measurements were performed with another spectrometer (Specord M80) that contained a Data ( I 9) Escard, J.; Pontvianne, B.;Contour, J. P. J . ElecfronSpecrrosc. 1975, 6, 17. (20) Escard, J.; Leclere, C.; Contour, J. P. C. R. Acad. Sci., Ser. C 1972, 274, 1645. (21) Leclere, C.; Contour, J. P.; Pannetier, G.Ann. Chim. 1974, 9, 221. (22) Tanaka, K.; Watters, K. L.;Howe, R. F.; Anderson, S.L. T. J . Cufol. 1983, 79, 251.
,473 K
0.2% lr/A$03 1% Ir/A$O,
5% Ir/AhO,
2b60 2100
2100
2000 cm-'
2000
cm"
-
Figure 1. (A) Infrared spectra of 0.2% 1r/Al2O3, 1% Ir/AI2O3, 5% Ir/A120,, and 1% Ir/Si02 (TR= 573 K) following IO Torr of CO adsorption at 300 K. Adsorption time was 5 min. (B) Effects of reduction temperature of 1% 1r/Al2O3on the infrared spectrum of adsorbed CO (50Torr) at 300 K. Adsorption time was 5 min.
10%
2101 L
-D
2100
2000
cm-l
Figure 2. Infrared spectra of 1% Ir/AI2O3 ( T R= 473 K) as a function of CO pressure at 300 K. Adsorption time was 5 min at each pressure.
system. The IR cell used made it possible to register the spectra at the temperature of CO adsorption at both low and high temperatures. The dispersion of supported Ir reduced at different temperatures has been determined by H2 and CO adsorption at 300 K.I2 Data obtained are shown in Table I. Results Infrared Spectroscopic Measurements. Low- Temperature Adsorption of CO, T I 300 K . Figure 1 displays IR spectra of adsorbed CO on alumina- and silica-supported Ir reduced at 573 K (denoted T R = 573 K). In the case of 5% Ir/AI2O3 three absorption bands were produced at 2102, 2081, and 2040 cm-' even at the lowest CO exposure. For convenience, the bands are referred to as the high-, medium-, and low-frequency bands and denoted HF, MF, and LF. At lower Ir content, 1% Ir/AI2O3, only two absorption bands, H F and LF bands, appeared at 2107 and 2035 cm-l. For Ir/Si02 we observed only one band at 2070 cm-l. The effect of the temperature of reduction on the IR spectrum of adsorbed CO was investigated for 1% Ir/AI2O3. IR spectra obtained after adsorption for 5 min are displayed in Figure I . There is significant variation in the IR spectrum of adsorbed CO. Whereas two well-resolved bands can be distinguished for samples reduced at low temperature, only a very broad band is obtained for catalysts reduced at high temperature. The effect of the CO partial pressure on the development of the CO bands for a sample reduced at 473 K is shown in Figure
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Solymosi et al.
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
Figure 3. Difference spectra of OH region for 1% Ir/AI2O3( T R= 573 K ) as a function of CO pressure at 300 K.
2. We obtained only the H F and L F bands for sample with TR = 473 K. The intensities of the two bands grew as the pressure of CO was increased. The positions of the H F and L F bands remained the same throughout the measurements. It is important to point out that after saturation no spectral changes were experienced, even after an extended adsorption time of 24 h. When the sample was reduced at 1 173 K the adsorption of C O at 300 K produced a broad adsorption band peaking at 2060 cm-’ (Figure 4C). With the increase of the pressure of C O the band became stronger and shifted to higher wavenumbers. There was only slight indication of the formation of H F and LF bands at a CO pressure of 10 Torr and adsorption time of 5 min. Attention was also paid to changes in other frequency regions. The adsorption of CO produced no detectable spectral features in the lower frequency range, 1200-1 900 cm-I. I n the high-frequency region, a broad absorption feature centered at 3550 cm-’ and two small shoulders at 3725 and 3640 cm-’ were registered before C O adsorption. These features correspond well to the vibrations of different O H groups on the alumina s ~ r f a c e . ~The ~ , perturbations ~~ of these bands following CO adsorption cannot be seen in the ordinary spectra, but they can be clearly observed in magnified difference spectra when the background spectrum is subtracted from each subsequent spectrum (Figure 3). A negative feature appears at 3679 cm-’, which becomes larger at higher CO exposure. This change suggests that the intensity of the band of the corresponding O H group decreases following CO adsorption on 1% Ir/A1203. Evacuation at room temperature caused only a negligible attenuation of the CO absorption bands. A more significant decrease in the intensities occurred above 473 K, and elimination of all the HF, MF, and L F bands was achieved at 623 K. It is important that no other CO band developed during the evacuation, and at most a shift of 5-7 cm-’ to lower wavenumber occurred in the positions of the two CO bands. As the IR spectrum of adsorbed C O after a short adsorption time differed basically for samples reduced at low and high temperatures (Figure I ) , the effects of adsorption time were investigated for the catalysts reduced at different temperatures. Some selected results are plotted in Figure 4. When the sample was reduced at lower temperature, TR = 473 K, the adsorption of CO at 300 K produced two intense bands, at 2104 and 2024 cm-’; these underwent only a slight change in position even after several hours. I n the case of catalyst reduced at higher temperature, the spectra initially exhibited only one broad absorption band peaking at 2060 cm-I. However, after an extended adsorption time, two additional bands can be clearly resolved at 2080-2085 and 2000-2005 cm-I. These new absorption bands developed more slowly on increase of the reduction temperature. The above spectral changes are displayed for the sample reduced at 1173 K in Figure 4, which also shows the difference spectra as a function of adsorption time. These spectra clearly demonstrate that the (23) Peri, J . B.; Hannan. R. B. J . Phys. Chem. 1960, 64, 1526. (24) Knozinger, H.; Ratnasamy, P. Catal. Rev. Sei. Eng. 1960, 1 7 , 31
C
2052
2082
DiffereKe spectm
B
193 K 283 X
n3
K
2*3 % 2’3 K I83 K is3
x
Figure 5. Infrared spectra of 1% Ir/A120, (TR= 573 K) in the presence of 10 Torr of CO as a function of temperature: ( A ) CO frequency region; (B) difference spectra in OH frequency region.
intensities of all the three bands increase with an increase of the adsorption time (Figure 4C,D). To see whether any change occurred in the composition of the gas phase during the interaction of C O with supported Ir, the amount of 1% Ir/AI2O3 ( T R= 573 K) was increased to 1 g, and a mass spectrometer was connected to the IR cell. In this case the sample was reduced with D2 at 573 K. After the introduction of 5 Torr of CO, a well-detectable increase was observed at mass number 4, and a further increase was registered when the temperature was raised to 373-573 K. In subsequent measurements, the adsorption of CO was performed at 93 K, and spectral changes were registered in the course
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7253
Ir on an Alumina Support A
b
Oofference spectra
lh
2100
L 2000 1900~6'
2MM
Figure 7. Infrared spectra for CO adsorbed on 1% Ir/AI2O3(TR= 573
Figure 6. Infrared spectra of 1% Ir/AI2O3(TR= 873 K) in the presence of IO Torr of CO as a function of temperature. The spectrum taken at 98 K after CO introduction was subtracted from each subsequent spectrum.
of heating the sample in the presence of CO. Relevant spectra for the sample reduced at 573 K are displayed in Figure 5. The adsorption of CO at 100 K produced two new intense bands at 2 155 and 2 180 cm-' (not shown in the spectrum), which can be eliminated either on degassing at 100 K or on increasing the adsorption temperature to about 180 K. The positions of these bands agree well with those observed following CO adsorption on alumina at this temperature: the 2155-cm-I band is attributed to H-bonded CO, and the band at 2180 cm-I to CO coordinated A more stable and broad band was formed between to AI3+ 2000 and 2100 cm-I, which was not seen in the spectrum of the Ir-free sample, and it is therefore associated with CO bonded to Ir. This broad absorption consists of three separate bands at 2095, 2060, and 2020 cm-'. With an increase in temperature, the spectrum shows clear evidence of higher and lower frequency bands at 2082 and 2015 cm-I, which increased in intensity as the temperature was raised. The spectrum obtained at room temperature agreed well with those seen in Figure 1; the intensities of the H F and LF bands clearly exceed that of the less-resolved M F band. Similar measurements were also performed with sample reduced at 873 K. In this case the CO chemisorbed on iridium gave a broad band at 2060 cm'. On warming the sample, spectral changes occurred only above 200 K, when the H F and L F bands appeared as shoulders. This is more clearly seen in the difference spectra (Figure 6). Spectral changes were also registered in the OH frequency region. As can be seen in Figure 5B, the intensity of the negative feature at 3669 cm-l in the difference spectra is enhanced on increase of the adsorption temperature. The spectral changes that occurred in the O H frequency range indicated the possible involvement of the O H groups in the COinduced surface processes. To obtain more information concerning the role of OH groups, an Ir/AI2O3sample was prepared under the driest possible conditions: the alumina support was dehydroxylated before impregnation at 1073 K, and the IrCI3 was dissolved in methanol. The IrCI3-AI2O3suspension was dried in vacuum, and all further manipulations were performed in dry nitrogen. The spectral changes observed in the presence of CO for this sample are depicted in Figure 7. In another experiment, 2 Torr of H20was mixed with the CO, and the IR spectra were again registered. From a comparison of the two series of measurements, it appears clearly that the development of the HF and LF bands is accelerated by water moieties. This is more clearly seen in difference spectra. High-Temperature Adsorption of CO, T L 423 K . A completely different picture was obtained in the presence of CO at higher temperature (423-573 K); both the H F and the L F bands gradually attenuated, while the M F band gained in intensity. This can be seen clearly for the sample reduced at 573 K, where the adsorption of CO at 300 K produced strong H F and L F bands at 2104 and 2024 cm-I. When the temperature was suddenly
K) prepared under dry conditions (A) and the effect of H 2 0 (2 Torr)(B) at 300 K. The pressure of CO was 5 Torr in both cases.
I
no0
m
I
2100
zdw
I
21m
Moo cm-f-
Figure 8. Changes in the infrared spectrum of 1% Ir/AI2O3(TR= 573 K) in the presence of 50 Torr of CO at 423 (A), 448 (B),and 473 K (C) as a function of adsorption time. Samples have been kept in CO at 300 K for 60 min before the temperature was raised, when similar spectra as shown in Figures lB, 2, and 5A were obtained.
Figure 9. Changes in the infrared spectrum of 1% Ir/A1203(TR= 573 K) in the presence of 50 Torr of CO at 523 (A) and 573 K (B)and after cooling back the sample (B) to 300 K in the presence of CO (C).
raised to 423-473 K in the presence of CO, a new intense band appeared at 2068-2080 cm-', and its absolute and relative intensities increased in time. Degassing of the sample at 423 K attenuated all bands, particularly the M F one, which was accompanied by a shift in its position to lower frequencies. Relevant spectra are displayed in Figure 8. A similar procedure at higher temperatures caused a more basic change: after a certain time, in the presence of CO, the M F band at 2063-2052 cm-' became the dominant band, and the H F and L F bands appeared only as shoulders (Figure 9). As regards the thermal stability of the
7254
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
adsorbed species during degassing, the M F band exhibited the lowest thermal stability: it disappeared even after degassing at 373 K. The other two bands were much more stable: their complete elimination required evacuation at 573-623 K. Similar results were obtained for the sample containing S% Ir. The reversibility of the above transformation was investigated by cooling the sample to room temperature in CO. The reactivity of supported Ir toward C O was found to be decreased after high-temperature C O treatment as both the H F and the LF bands developed much slower than in the case of the untreated sample. When the sample was treated in C O at 573 K for 300 min, only the 2065-cm-l band was seen even after 18 h of CO adsorption at 300 K (Figure 9C).
Discussion Characterization of Reduced Ir Samples. Previous studies indicated that the reduction of Ir/A1203 produces highly dispersed Ir clusters, the size of which depends on the reduction temperature and the preparation From the TPR results described elsewhere we obtained that the reduction of lr3+ on an alumina support is complete at around 580 K.l8 The hydrogen uptake of reduced samples and the H / l r ratio depended primarily on the Ir loading. In harmony with previous studies, up to two atoms of hydrogen were found to be chemisorbed per iridium atom at metal loadings of 0.2-1% ( TR = 573 K, Table I ) . The CO/lr ratio for 0.2% Ir/Alz03 was almost 2 at 300 K, but a value higher than 1 was obtained for 1% 1r/Al2O3also. This feature suggests that the highly dispersed Ir is capable of binding two CO molecules. With the increase of reduction temperature the H / l r and CO/Ir ratios gradually decreased. The uptake of hydrogen and CO was always less when silica support was used. This result is reflected in the IR spectrum of adsorbed C O where only one band appeared at 2070 cm-l (Figure 1A). Spectral Features Induced by CO Adsorption. One of the most important findings of this work is that significant spectral changes occur during C O adsorption on supported Ir, the nature of these depending sensitively on the temperature of adsorption of CO. At low temperature, 100-373 K, the H F and L F bands at 2090-2107 and 2010-2037 cm-I gradually developed at the expense of the M F band at 2050-2060 cm-l. At higher temperatures, above 423 K, the opposite process was observed: attenuation of the H F and LF bands and intensification of the M F band. If it is assumed that these bands represent different adsorbed species, these spectral features suggest that the adsorption of CO induces a structural change in the supported Ir and creates adsorption sites that were not present before C O adsorption. As the direction of these structural changes is primarily determined by the temperature, for convenience we discuss the results separately. Oxidative Disruption of Ir, Crystallites. Although the characteristics of the IR spectrum of C O adsorbed on Ir exhibited a significant variation, a firm basis for the interpretation of the spectral changes is that the band at 2050-2080 cm-l (the M F band in the present case) is most probably due to vibration of the CO linearly bonded to an iridium cluster, Ir,-CO. This is supported by the results obtained for an Ir single crystal by using HREELS and for I r film by using reflection IR spectroscopy, when the formation of isolated Ir sites can be excluded.28 C O adsorption produced a band at 2050 cm-I, which exhibited a significant shift (50 cm-I) with variation of the surface concentration of adsorbed CO, due to dipoledipole coupling. This feature was also observed In the present case, this M F band was clearly for Ir/A1z03.14*16 resolved for the low-temperature adsorption of CO on highly dispersed Ir samples (Figure 6) and also at room-temperature adsorption for high Ir loading (Figure I ) . In all other cases, even in the case of lr/AIzO3 reduced at high temperature, a very broad band was produced that contains several absorption bands not resolved in the presence of gaseous CO at and below room temL. D. J . Carol. 1980, 66, 301. (26) Foger. K.:Hay, D.; Jaeger, H. J . Carol. 1985, 96, 170. (27) Foger. K.; Hay, D.;Jaeger, H. J . Coral. 1985, 96, 170. (28) Reinalda. D.;Ponec, V. Sur5 Sci. 1979, 91, 113; Reinalda, D.; Ponec, V . Appl. Surf Sci. 1980, 5. 98. ( 2 5 ) Wang. T.; Schmidt,
Solymosi et al. perature. Its peak position shifted to higher wavenumbers on increase of the CO exposure. It is to be pointed out that there was no spectral indication of the formation of bridged-bonded CO, which makes a distinction between the Ir/Al2O3and the Rh/A120, systems. The most important result of this study is that with the increase of the adsorption temperature, the band at 2050-2060 cm-I slightly decreased in intensity above 150 K, and two new bands were produced at 2098 and 2030 cm-'. These bands gained in intensity with further increase of temperature to 300 K. In the case of highly dispersed Ir (0.2 and 1% Ir/Al2O3 reduced at 473 and 573 K, respectively), these two bands are the dominant spectral features at room temperature. When C O was adsorbed on these samples at 300 K, the M F band was not seen (or only at the beginning of C O adsorption), but the two other bands, at 2107 and 2037 cm-I, appeared at once in the spectrum. For a less dispersed system, they formed only after extended contact with CO (Figure 5). These results clearly indicate that both the H F and the L F bands are produced in an activated process by prolonged contact with CO. In other words, the corresponding adsorption sites are absent before CO adsorption, and their formation is induced by CO adsorption. Analogously as for supported Rhl" and R u ~ ~ ~ we assume that the adsorption of CO leads to the disruption of the Ir crystallites, producing smaller clusters or even isolated Ir atoms, on which the coordination and adsorption energy of C O are different: Ir,
+ 2CO = Tr,-,-CO + IrO-CO
or IrO-CO
+ CO = Iro(CO)z
However, there is additional experimental evidence suggesting that isolated Ir atoms are oxidized, very likely to Ir', during C O adsorption at 150-300 K: (i) Recent XPS data revealed an additional shoulder following the adsorption of CO on 1% Ir/Al2O3 ( TR = 673 K) a t 300 K, which can be attributed to the partial oxidation of Ir, clusters.29 This signal can be eliminated by treating the sample with hydrogen or C O at higher temperatures. (ii) Analysis of the spectral changes in the O H frequency region clearly indicates an attenuation of the band at 3670 cm-' (manifested as a negative peak in the difference spectra) as the HF and L F bands of C O develop with increasing CO pressure at 300 K. The same feature was observed when the sample was heated from 100 to 300 K in the presence of CO. This result indicates the involvement of isolated OH groups on the support in the CO-induced structural changes of the Ir, crystallites. The most probable route of involvement of the O H group is reaction with the isolated Ir atoms produced by the CO-induced disruption process: Iro
+ OH- = Ir' + Y2H2+ 02-
The detection of substituted deuterium during the C O adsorption provides strong evidence in favor of the above reaction. (iii) The development of the H F and LF bands was slower under dry conditions, but addition of a small amount of water to the C O accelerated their formation. This further confirms the occurrence of a surface oxidation and the involvement of OH groups on the alumina. The oxidation of iridium was also observed by Hucol and Brenner30 during the decomposition of Ir4(CO)12deposited on alumina. As the consumption of OH groups (Le., the oxidation of Iro) and the development of the H F and L F bands occur simultaneously, it is rational to assume that these bands are due to CO bonded to Ir' formed in the CO-induced oxidative disruption process. Analogous with the CO-Rh system, this species could be a dicarbonyl species, Ir'(CO)z. The H F and L F bands would correspond to asymmetric and symmetric stretches of this surface (29) Bertbti, J.; Solymosi, F., unpublished results. (30) Hucol, D. A.; Brenner, A . J , Am. Cbem. SOC.1981, 103, 217.
J. Phys. Chem. 1990, 94, 7255-7259 complex. This species could form in the reaction Ir0(C0)2
+ OH- = Ir'(CO)2 + 02+ y2H2
or without the transitory species Iro(C0),: Ir'
+ 2CO = Ir'(CO)2
The final state can be described by the reaction 21r 4CO 2Al-OH = 2AI-O-Ir(CO),
+
+
+ H,
An alternative route for the formation of Ir' from Iro is the dissociation of C O and the subsequent oxidation of Iro by adsorbed oxygen. This route was favored by several workers in the case of the CO-Rh i n t e r a c t i ~ n . ~ . ~However, ' ? ~ ~ it has been pointed out24*33*34 that this process could not be responsible for the oxidation of Rho as the oxidative disruption of the Rh, crystallites proceeds at as low as 150 K, while the dissociation of C O on supported Rh was detected only well above 473 K . j 5 3 In a similar investigation of the dissociation of C O on the Ir samples used in the present study, the dissociation of C O was not observed below 473 K on either small or large particles of Ir supported by alumina.Is It was detected only above 523 K, when surface carbon and carbon dioxide were formed. While we cannot exclude the possibility of the dissociation of C O to a small extent (which escaped detection) below 523 K, we are convinced that we need not consider the occurrence of this process in the temperature range 150-300 K, where the oxidative disruption process of Ir, crystallites was assumed to proceed. The existence of gem-dicarbonyl in this system is strongly suggested by earlierI3-l6and present chemisorption data (Table I), which clearly show that Ir can adsorb more than one C O molecule. This led several authors to assume the formation of (31) Primet. M. J . Chem. Soc., Faraday Trans. 1 1978,89, 79. (32) Bergeret, G.; Gallezot, P.; Gelin, P.; Ben Taarit, Y.; Lefebre, F.; Naccache, C.; Shannon, R . D. J . Catal. 1987, 104, 279. (33) Solymosi, F.; Pasztor, M. J . Catal. 1987, 104, 312. (34) Solymosi, F.; Bansagi, T.; Novak, E. J . Catal. 1988, 112, 183. (35) Solymosi, F.; Erdohelyi, A. Surf. Sci. 1981, 110. L630. (36) Erdohelyi, A,; Solymosi, F. J . Catal. 1983, 84, 446.
7255
di- and even tricarbonyl species without considering the Occurrence of CO-induced oxidative disruption of the Ir, c r y ~ t a l l i t e s . l ~ . ' ~ , ' ~ Reductive Agglomeration of Ir'. One of the interesting features of the CO-lr/AI2O3 interaction is that the H F and L F bands are gradually reduced in intensity above 373 K, and an intense band is re-formed at 2050-2080 cm-I. This process occurs very quickly at 423-573 K. The gradual elimination of Ir1(C0)2bands cannot be attributed to the desorption of C O from Ir' sites, because the species Ir'(CO), is thermally more stable than the species Ir,-CO. The destruction of Ir' sites is also indicated when the sample is cooled to room temperature in a C O atmosphere: the spectrum shows only an intense band at 2068 cm-I, without reappearance of the H F and L F bands. (This state can also be attained by treating the sample containing the species Ir'(CO)2 with hydrogen at 373 K.) All these results suggest that above 400 K the adsorption of C O induces the reductive agglomeration of Ir' sites to an Ir cluster, which can be described by following reactions:
+ 02= Ir,-CO + C 0 2 + 2CO nIr,-CO = Ir,-CO + ( n - 1)CO
21r1(C0), where x = 2.
Conclusions The adsorption of C O on reduced Ir/AI2O3 produces three absorption bands, 2090-2107,2050-2080, and 2010-2037 cm-I (denoted by HF, MF, and LF, respectively). Their relative intensities strongly depend on the Ir loading, on the nature of the support, on the reduction temperature, and on the time and temperature of C O adsorption. The adsorption of CO at 200-300 K leads to the oxidative disintegration of Ir, crystallites and to the formation of Ir' species. This process is indicated by the enhancement of H F and L F bands of adsorbed C O on iridium and by an attenuation of the band of isolated O H groups on alumina. At higher temperature, 423-573 K, C O induces the reductive agglomeration of Ir' and the re-formation of Ir, crystallite, as suggested by the attenuation of H F and L F bands and by the growth of the M F band.
Light Scattering from Semidilute Solutions of Nonionic Surfactants (C12E5and C12E8) and the Scaling Law Tadashi Kato,* Shin-ichi Anzai, and Tsutomu Seimiya Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Fukasawa, Setagaya- ku, Tokyo 158. Japan (Received: January II. 1990; In Final Form: April 26, 1990)
Static and dynamic light scattering have been measured on D 2 0 solutions of nonionic surfactant C12ESin the concentration range 3-300 g dm-3 at different temperatures. The results for the semidilute solutions at higher temperatures can be explained by the scaling theory for entangled solutions of flexible polymers which exhibit phase separations. Temperature dependences of the light scattering data suggest that the ''0 temperature" exists at 27-30 OC (the lower critical consolution point is 30.5 "C), which is in approximate agreement with results of our previous studies for dilute solutions. Relation to the existence of liquid crystal phases is also discussed. Measurements of dynamic light scattering have been made on D 2 0 solutions of C!2E8also. The data do not follow the scaling law even at higher temperatures, which may come from the fact that C12Es micelles are much smaller than CI2ESmicelles.
Introduction In the past few years, the scaling theory for entangled solutions of flexible polymers have been applied to semidilute aqueous solutions of surfactants. Such an application was made for the first time by Candau et al.14 They have shown that concentration
dependences of scattered intensities and mutual diffusion coefficients for aqueous KBr solutions of cetyltrimethylammonium bromide (CTAB) follow the power laws derived for semidilute polymer solutions in a good solvent. They have also shown that (3) Candau, S. J.; Hirsh, E.; Zana, R.; Adam, M. J . Colloid Interface Sci.
( I ) Candau, S. J.; Hirsh, E.; Zana, R. J . Phys. (Paris) 1984, 45, 1263. (2) Candau, S. J.; Hirsh, E.; Zana, R . J . Colloid Interface Sci. 1985, 105,
521.
0022-3654/90/2094-7255$02.50/0
1988, 122, 430.
(4) Hirsh, E.; Candau, S. J.; Zana, R. In Surfactants in Solution; Mittal, K . L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 4, p 155.
0 1990 American Chemical Society
