1576
J . Phys. Chem. 1990, 94, 1576-1581
Adsorption of Carbon Monoxide on ManganesePromoted Rhodium/Silica Catalysts As Studied by Infrared Spectroscopy S. A. Stevenson,+ A. Lisitsyn,*and H. Knozinger* Institut fur Physikalische Chemie der Universitat Miinchen, Sophienstrasse 11, 8000 Miinchen 2, FRG (Received: May 22, 1989)
Infrared spectroscopywas used to study the adsorption of carbon monoxide on silica-supported rhodium catalysts promoted with manganese oxide. Addition of the promoter suppressed overall adsorption, weakened the adsorption of the linear CO species, and created a broad new absorption band with a maximum near 1715 cm-l; this new low-frequency band is thought to be due to a tilted Rh-C-0-Mn species. On the promoted catalyst both the new low-frequency species and the linear CO species react with hydrogen much more rapidly than does the linear species on the unpromoted catalyst. Additionally, the promoted catalyst does not chemisorb CO at 80 K; adsorption on this catalyst is an activated process, and maximum adsorption is observed only when the sample is exposed to CO at 373 K. This increase in adsorption is believed to be caused by a reconstruction of the manganese oxide overlayer.
importantly, to study the stability and reactivity of the low-freIntroduction quency band in an attempt to determine whether its formation It has long been recognized that the addition of small quantitites of a variety of metal oxides to supported group VI11 metals can significantly alter their activity and/or selectivity for the hydrogenation of carbon m o n ~ x i d e . ~ - One ' ~ often-studied example is ( I ) Sachtler, W. M. H. Proc. Int. Congr. Catal., 8rh 1984, I , 151. the promotion of supported rhodium with manganese oxide; the (2) Sachtler, W. M. H. Actas Simp. Iberoamer. Catal., Merida, Venezuela addition of as little as 0.2 wt % manganese can increase the rate 1986, 2, 1327. of synthesis gas conversion by a factor of 10 (e.g., ref 16 and 17). ( 3 ) v. d. Lee, G.; Ponec, V. Coral. Rev.-Sci. Eng. 1975, 29, 183. A significant increase in the understanding of this system was the (4) Poels, E. K.; Ponec, V. In Catalysis; The Royal Society of Chemistry: recognition that the manganese oxide species are present on the London, 1983; Vol. 6, p 196. surface of the metallic rhodium particles,I7 where a mixed surface (5) Bell, A. T. Proc. Int. Congr. Catal., 9th 1989, 5, 134. oxide was originally thought to be f ~ r m e d . ' ~ . ~The * migration (6) Kowalski, J.; v. d. Lee, G.; Ponec, V. Appl. Catal. 1985, 19, 423. of the manganese oxide and other oxide p r o m ~ t e r ' ~ ~species ~ ~ * ' ~ - * ~ (7) Mori, T.; Miyamoto, A.; Takahashi, N.; Fukagaya, M.; Hattori, T.; Murakami, Y. J. Phys. Chem. 1986, 90, 5197. is now thought to be similar to that established for support cations (8) Bond, G. C.; Richards, D. G. Appl. Coral. 1986, 28, 303. (e.g., Ti, Nb) in systems exhibiting so-called strong metalsupport (9) Kiennemann, A.; Breault, R.; Hindermann, J. P.;Laurin, M. J. Chem. interactions;21-26once on the surface of the metal particles, these SOC.,Faraday Trans. I 1987,83, 2119. SMSI support oxide species are imagined to block surface sites, (10) v. d. Lee, G.; Bastein, A. G. T. M.; v. d. Boogert, J.; Schuller, B.; alter metal ensembles, interact electronically with metal atoms, Hong Luo; Ponec, V. J. Chem. Soc., Faraday Trans. I 1987, 83, 2103. and possibly create new catalytically active sites. (11) v. d. Lee, G.; Schuller, B.; Post, H.; Favre, T. L. F.; Ponec, V. J. One possible form that such a new active site might take was Catal. 1986, 98, 522. suggested by Burch and Flambard27and Bracey and Burch,% who (12) Du Yu-Hua; Chen De-An; Tsai Khi-Rui Appl. Catal. 1987,35, 77. proposed that the oxygen end of adsorbed CO might interact with (13) Kip, B. J.; Smeets, P. A. T.; van Grondelle, J.; Prins, R. Appl. Catal. an oxygen vacancy in a reduced titania species, facilitating easier 1987, 33, 181. C O dissociation and hence a higher CO hydrogenation activity. (14) Underwood, R. P.; Bell, A. T. J. Coral. 1988, I l l , 325. Similar interactions have been considered for various metal-oxide (1 5) Kieffer, R.; Kiennemann, A.; Rodriguez, M.; Bernal, S.; RodriguezIzquierdo, J. M. Appl. Catal. 1988, 42, 77. combinations,2e32 and model studies on TiO, overlayers on rhodium single-crystal surfaces provided additional s ~ p p o r t . ~ ~ J ~ (16) Ellgen, P. C.; Bartley, W. J.; Bhasin, M. M.; Wilson, T. P. Adu. Chem. Ser. 1978, No. 178, 147. Sachtler',2 suggested that a similar process might operate in (17) Wilson, T. P.; Kasai, P. H.; Ellgen, P. C. J . Catal. 1981, 69, 193. manganese-promoted supported rhodium catalysts, in which the (18) v. d. Berg, F. G. A,; Glezer, J. H. E.; Sachtler, W. M. H. J. Caral. oxygen atom in an adsorbed CO molecule might be attracted to 1985, 93, 340. a manganese cation on the rhodium surface. The first spectro(19) Rieck, J. S.; Bell, A. T. J. Catal. 1986, 99, 278. scopic evidence for such a species was provided by Ichikawa and (20) Kip, B. J.; Smeets, P. A. T.; van Wolput, J. H. M. C.; Zandbergen, F ~ k u s h i m a ,who ~ ~ observed that promotion with increasing H. W.; van Grondelle, J.; Prins, R. Appl. Catal. 1987, 33, 157. amounts of manganese oxide shifted the peak for bridged CO from (21) Stevenson, S. A.; Dumesic, J. A.; Baker, R. T. K.; Ruckenstein, E. 1880 cm-l to between 1700 and 1820 cm-I and created a weak Metal-Support Interactions in Catalysis, Sintering and Redispersion; Van Nostrand Reinhold: New York, 1987. shoulder at approximately 1520 cm-' as well; similar results were (22) Engels, S.; Freitag, B.; Morke, W.;Roschke, W.; Wilde, M. Z . Anorg. obtained for rhodium promoted with titania, zirconia,35vanadia, Allg. Chem. 1981, 472, 162. and n i ~ b i a . ) ~From comparison with the infrared frequencies (23) Santos, J.; Dumesic, J. A. Stud. Surf. Sci. Catal. 1982, 11, 43. of carbonyls in known organometallic compounds containing an (24) Resasco, D. E.; Haller, G. C. J. Catal. 1983, 82, 279. M - C U M ' bond (e.g., ref 32, 37, and 38), it was suggested that (25) Rieck, J. S.; Bell, A. T. J. Catal. 1986, 99, 262. the downward shift in frequency was caused by tilting of bridged (26) Van't Blik, H. F.; Vis, J. C.; Huizinga, T.; Prins, R. Appl. Catal. CO toward promoter cations. 1985, 19, 405. The goal of this work was to prepare manganese-promoted (27) Burch, R.; Flambard, A. R. J. Catal. 1982, 78, 389. Rh/Si02 catalysts that exhibited a well-defined low-frequency (28) Bracey, J. D.; Burch, R. J. Catal. 1984, 86, 384. CO band, if possible to separate this band from the bridged band, (29) Brown, T. L. J. Mol. Catal. 1981, 12, 41. to address the question of the assignment of this band, and, most (30) Shriver, D. F. ACS Symp. Ser. 1981, No. 152, 1. 'Current address: Central Research Laboratory, Mobil Research and Development, Princeton, NJ 08540. *Institute of Catalysis, Novosibirsk, USSR.
(31) Vannice, M. A.; Sudhakar, C. J . Phys. Chem. 1984, 88, 2429. (32) Sachtler, W. M. H.; Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. J . Catal. 1985, 92, 429.
0022-3654/90/2094- 1576$02.50/0 0 1990 American Chemical Society
Adsorption of C O on Mn-Promoted Rh/Silica Catalysts
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1577
is indeed related to the enhanced C O hydrogenation activity of these catalysts. Since the commencement of this work, a number of other studies have been published that confirm that a lowfrequency band can be obtained for C O adsorbed on rhodium promoted with a variety of oxides. For example, IR studies of C O adsorbed on ceria-promoted Rh/Si02 catalystsg showed a series of low-frequency bands at 1696, 1600, 1525, and 1370 cm-I; the band at 1696 cm-l was proposed to be a tilted adsorbed C O species, while the other bands were thought to be due to carbonate species. A bridged species well separated from the 1696-cm-l band was also observed, so that it was not implied that the low-frequency species was a bridged species. Similarly, a band at 1680 cm-' observed on Rh/SiO, promoted with molybdenum oxide was attributed to a tilted adsorbed C O species.3g A band at 1725 cm-' on lanthana-promoted rhodium catalysts was attributed to a similar tilted species;4othis sample showed bands at 1620, 1520, and 1370 cm-' due to carbonates as well. Experimental Details
Rh/SiO, was prepared by the impregnation of Aerosil200 silica U LL z with RhCI, from methanol solution. Following evaporation of cl the solution, the sample was calcined in flowing oxygen at 570 a *a 0 K for 7 h. The resulting catalyst contained 4 wt % rhodium. m a Manganese-promoted catalysts were prepared in two steps. First, Mn(N0,),.4H20 was impregnated onto Aerosil200 from methanol solution; following drying, the manganese nitrate was decomposed by calcination for 2 h at 483 K in flowing oxygen. RhCI, was then impregnated onto this material from methanol solution; the resulting catalyst was calcined for 7 h at 573 K. This I I 2300 2000 1700 1 Lci0 catalyst contained 8 wt % manganese and 4 wt % rhodium. WAVENUMBERS / c m - ' Sample pretreatment and IR measurements were performed Figure 1. Room-temperature spectra of R h / S i 0 2 reduced a t 623 K, in a glass and metal system with a dynamic vacuum of approxexposed to CO at 295 K, and evacuated for 5 min at progressively higher imately Pa. In this system the sample wafer could be raised temperatures: (A) in 100 Torr of CO at 295 K, (B) evacuated a t 363 into an oven for in situ treatments under vacuum or flowing gas at temperatures between 300 and 800 K. The infrared ~ e 1 1 ~ ~ K, * ~(C)~ evacuated a t 401 K, (D) evacuated a t 423 K, (E) evacuated a t 450 K, (F) evacuated at 468 K, (G) evacuated a t 483 K, (H) evacuated was normally at room temperature but could be cooled to 80 K at 503 K, (I) evacuated at 523 K, (J) evacuated at 538 K, (K) evacuated when so desired. The self-supporting wafers used typically conat 553 K, (L) evacuated a t 578 K, (M) evacuated at 600 K, (N) evactained between 0.05 and 0.10 g of sample. Prior to measurements uated a t 630 K, (0)reexposed to 140 Torr of CO at 295 K. the samples were reduced in flowing hydrogen for 2 h at either 623 or 773 K; following reduction the hydrogen was evacuated a geminal dicarbonyl species with twin bands at 2092 and 2038 for 30 min at 623 K. Hydrogen (99.999% purity), carbon moncm-l (for assignments see ref 44-49). Following collection of oxide (99.997%), and deuterium (99.7%) were obtained from this spectrum, the stability of these species was studied by the Linde; the hydrogen and carbon monoxide were further purified removal of the CO under vacuum a t progressively higher temby passage through Oxisorb cartridges. I3COwas obtained from peratures (Figure 1B-N). The sample was raised into the MSD Isotopes. treatment oven and heated at the indicated temperature under Spectra were recorded in the region 2400-1300 cm-' at a vacuum for 5 min and then lowered into the sample cell where resolution of approximately 6 cm-' with a Perkin-Elmer speca room-temperature spectrum was collected. As can be seen, the trometer type 580B. Background subtraction was performed using intensity of the C O bands decreases until all CO is removed a Perkin-Elmer 3600 computer system. following treatment at 630 K. Figure 1 0 shows the sample following reexposure to C O at room temperature. The readExperimental Results sorption of a similar amount of C O suggests that the original CO was desorbed rather than dissociated. Figure 1 shows room-temperature spectra collected from the The stability of the various C O species in the presence of unpromoted rhodium catalyst following reduction at 623 K. In deuterium on the unpromoted sample can be seen in Figure 2, the presence of C O (spectrum 1A), bands due to three distinct where spectra collected following exposure to 95 Torr of CO, C O species can be observed: a linear species a t 2067 cm-I, a evacuation at room temperature, exposure to 103 Torr of D,, and bridged species a t 1901 cm-l (for assignments see ref 43), and treatment in deuterium at the same sequence of temperatures are shown. In the presence of deuterium essentially all of the C O is removed at a temperature of about 550 K. Because this tem(33) Levin, M. E.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. J . Chem. perature is significantly lower than the desorption temperature Soc., Faraday Trans. I 1987,83, 2061. under vacuum, the decrease in C O band intensity is almost surely (34) Levin, M. E.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. J . Catal.
-
1987, 106, 401. (35) Ichikawa, M.; Fukushima, T. J. Phys. Chem. 1985, 89, 1564. (36) Sachtler, W. M. H.; Ichikawa, M. J . Phys. Chem. 1986, 90, 4752. (37) Horwitz, C. P.; Shiver, D. F.Adv. Organomet. Chem. 1984,23, 219. (38) Knbzinger, H. In Homogeneous and Heterogeneous Catalysis; Yermakov, Yu.,Likholobov, V., Eds.; VNU Science Press: Utrecht, 1986; p 789. (39) Kip, B. J.; Hermans, E. G. F.; van Wolput, J. H. M. C.; Hermans, N. M. A.; van Grondelle, J.; Prins, R. Appl. C a r d 1987, 35, 109. (40) Underwood, R. P.; Bell, A. T. J . Catal. 1988, 109, 61. (41) KnBzinger, H. Acta Cient. Venez. 1973,24 (Suppl. 2), 76. KnBzinger, H.; Stolz, H.; Biihl, H.; Clement, G.; Meye, W. Chem.-Ing.-Tech. 1970, 42, 548. (42) Kunzmann, G . Doctoral thesis, University of Munich, 1987.
(43) Sheppard, N.; Nguyen, T. T. Adv. Infrared Raman Spectrosc. 1987, 5 , 67. (44) Van't Blik, H. F. J.; Vanzon, J. 9. A. D.; Huizinga, T.; Vis, D. C.; Koningsberger, J. C.; Prins, R. J . Am. Chem. SOC.1985, 107, 3139. (45) Wang, H.; Yates, Jr., J. T. J . Catal. 1984, 89, 79. (46) Solymosi, F.;Pasztor, M. J . Phys. Chem. 1986, 90, 5312. (47) Robbins, J. L. J . Phys. Chem. 1986, 90, 3381. (48) Worley, S. D.; Rice, C. A,; Mattson, G. A.; Curtis, C. W.; Guin, G. A.; Tarrer, A. R. J . Phys. Chem. 1982, 86, 2714. (49) Zaki, M. I.; Kunzmann, G.; Gates, B. C.; KnBzinger, H. J . Phys. Chem. 1987, 91, 1486.
1578 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
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Figure 2. Room-temperature spectra of Rh/SiO, reduced at 623 K, exposed to CO at 295 K, evacuated at 295 K, and treated in deuterium for 5 min at progressively higher temperatures: (A) in 95 Torr of CO at 295 K, (B) evacuated, then 103 Torr of D, at 295 K, (C) after 363 K in D,,(D) after 398 K in D2,(E) after 423 K in Dz,(F) after 453 K in D,, (G) after 475 K in D,,(H) after 491 K in D,, (I) after 518 K in D,, (J) after 540 K in D2.(K) reexposed to 100 Torr of CO at 295 K. 2073
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2330
1400
Figure 3. Room-temperature spectra of Rh/Mn/SiO, reduced at 623 K: (A) in 110 Torr of I2CO, (B) in 31 Torr of "CO.
due to reaction with deuterium. The subsequent readsorption of CO at room temperature (Figure 2K) in a manner indistinguishable from the original adsorption suggests that the dissociated CO is removed from the surface, presumably as methane. In addition, the geminal dicarbonyl species, which were originally seen as shoulders (Figure 2A), have vanished during D2treatment. Figure 3A shows the IR spectrum of the manganese-promoted rhodium catalyst at room temperature in 110 Torr of l2C0following reduction in hydrogen at 623 K. As in the unpromoted sample, the bands a t 2073 and 1878 cm-' and the shoulders at approximately 2100 and 2038 cm-l can be assigned to the linear, bridged, and geminal dicarbonyl species, respectively. Promotion appears to decrease the total amount of adsorption and decrease the area of the bridged band relative to that of the linear band;
1
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Figure 4. Room-temperature spectra of Rh/Mn/Si02 reduced at 623 K, exposed to CO at 295 K, and evacuated for 5 min at progressively higher temperatures: (A) in 122 Torr of CO at 295 K, (B) evacuated at 368 K, (C) evacuated at 398 K, (D) evacuated at 423 K, (E) evacuated at 443 K, (F) evacuated at 466 K, (G) evacuated at 490 K, (H) evacuated at 523 K, (I) evacuated at 548 K, (J) reexposed to 119 Torr of CO at 295 K.
this decrease in total adsorption is consistent with volumetric measurements for manganese-supported catalysts.% In addition, a broad band specific to the promoted samples is present with a maximum at 1716 cm-I. Figure 3B shows the IR spectrum collected from the same sample a t room temperature in 31 Torr of 13C0. Shifts in frequency calculated from the reduced masses of the labeled and unlabeled CO assuming a pure carbon-oxygen vibration agree with the observed shifts (linear: 2027 cm-' predicted, 2022 cm-I observed; bridged: 1836 cm-I predicted, 1846 cm-' observed; new low-frequency species: 1677 cm-' predicted, 1675 cm-I observed) given the constraints of experimental accuracy, a lower pressure of labeled CO (resulting in a slightly lower CO coverage and a slight downward shift in wavenumber for the linear species), and difficulties in determining accurately the maximum of the broad but weak bridged band. Figure 4 shows spectra collected from the promoted catalyst following reduction at 623 K, exposure to CO at room temperature, and subsequent evacuation for 5 min at progressively higher temperatures. As can be seen from the spectra, evacuation at 548 K was sufficient to remove virtually all adsorbed CO, indicating that all adsorbed species were more weakly bound on the promoted catalyst than on the unpromoted material. Additionally, the new low-frequency species is removed more easily than the linear species. Figure 5 shows the results of exposing the same catalyst to CO, evacuating the CO at room temperature, and treating the sample in deuterium at elevated temperatures. No CO could be observed after treatment above 480 K, although CO could be readsorbed following evacuation and cooling to room temperatiire. The temperature of the reaction between CO and D2was significantly below the reaction temperature on the unpromoted catalyst, as shown in Figure 2. The effects of reduction at 773 K on the promoted catalyst are shown in Figures 6 and 7, which present spectra collected following treatment under vacuum and in deuterium, respectively. The primary effect of reduction at a higher temperature was to suppress (50) Tauster,
S.A.; Fung, S.C. US.Patent 4402869, 1983.
Adsorption of C O on Mn-Promoted Rh/Silica Catalysts
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1579
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Figure 5. Room-temperature spectra of Rh/Mn/Si02 reduced at 623 K, exposed to CO at 295 K, evacuated at 295 K, and treated in deu-
terium for 5 min at progressively higher temperatures: (A) in 124 Torr of CO at 295 K, (B) evacuated, then 73 Torr of D, at 295 K, (C) after 363 K in D2, (D) after 393 K in D2, (E) after 422 K in D2,(F) after 448 K in Dl,(G) after 475 K in D,. I
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Figure 7. Room-temperature spectra of Rh/Mn/SiO, reduced at 773 K, exposed to CO at 295 K, evacuated at 295 K, and treated in deu-
terium for 5 min at progressively higher temperatures: (A) in 110 Torr of CO at 295 K, (B) evacuated, then 90 Torr of D2 at 295 K, (C) after 366 K in Dz,(D) after 403 K in D,,(E) after 423 K in D2, (F) after 448 K in D2,(G) after 473 K in D,, (H) after 494 K in D2, (K) reexposed to 91 Torr of CO at 295 K.
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Figure 8. Low-temperature spectra of Rh/Si02 reduced at 623 K: (A) in 140 Torr of CO at 80 K, (B) in 140 Torr of CO at 80 K following treatment in 140 Torr of CO at 373 K for 10 min.
Figure 6. Room-temperature spectra of Rh/Mn/SiO, reduced at 773 K, exposed to CO at 295 K, and evacuated for 5 min at progressively higher temperatures: (A) in 109 Torr of CO at 295 K, (B) evacuated at 362 K, (C) evacuated at 403 K, (D) evacuated at 422 K, (E) evacuated at 449 K, (F) evacuated at 473 K, (G) evacuated at 508 K, (H) evacuated at 522 K, (I) reexposed to 107 Torr of CO at 295 K.
overall adsorption; additionally, the area of the low-frequency peak relative to the area of the linear and twin species increased by approximately 3 0 4 0 % . To further study the adsorptive properties of the catalysts, the following experiment was conducted: following reduction and evacuation, the sample was cooled under vacuum to 80 K and then exposed to approximately 100 Torr of CO. After a spectrum was collected at this temperature, the sample was raised into the treatment oven and heated for 10 min at 373 K in CO and then lowered into the sample cell and recooled to 80 K so that another spectrum could be collected. The results of this experiment for the unpromoted catalyst are shown in Figure 8, while the results for the promoted catalyst are shown in Figure 9. In addition to the bands at 2066 and 1870 cm-l due to the linear and bridged species, two new bands at 2153 and 2136 cm-' can
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Figure 9. Low-temperature spectra of Rh/Mn/SiO, reduced at 623 K: (A) in 131 Torr of CO at 80 K, (B) in 131 Torr of CO at 80 K following
treatment in 131 Torr of CO at 373 K for 10 min.
1580 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 TABLE I: Area of Linear and Low-Frequency Bands at 80 K Relative to Area followinn Treatment at 373 K T..A. K
Rh/Si02 Rh/Mn/SiO, Rh/Mn/Si02
623 623 773
linear 0.68
low frea
0.1 1
0.20
0.07
0.00
TABLE 11: Temperature at Which Band Area Is Reduced to 10%of Band Area in CO at 295 K TIM,OC under vacuum in deuterium Trd, K linear low freq linear low freq
Stevenson et al. TABLE 111: Absorption Maxima and Standard Deviations (in Brackets)" T-..,. K
Rh/SiO, Mn/Rh/Si02 Mn/Rh/Si02
623 623 773
"Spectra were collected in
wavenumber [SD], cm-' linear bridged low frea 2067 [1.0] 1901 [21.3] 2073 [1.4] 1882 [9.1] 1718 [9.1] 2069 [0.6] 1865 [6.8] 1709 [4.2]
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100 Torr of CO at 295 K.
Table 111, where the average peak locations of the linear, bridged, and low-frequency species are tabulated. For the manganesepromoted sample reduced at 623 K, the frequency of the linear species is shifted up approximately six wavenumbers, in spite of the fact that the lower coverage of C O suggested by weaker IR 270 Rh/Si02 623 305 bands would decrease the frequency due to reduced dipole-dipole 623 240 190 200 200 Rh/Mn/Si02 interactions. When the promoted catalyst is reduced at 773 K, Rh/Mn/SiO, 773 230 195 225 215 the maximum of the linear species shifts down in frequency, presumably due to the much lower coverage on this sample. be seen on the unpromoted catalyst in Figure 8A; these bands can Table I1 indicates that the low-frequency species present in the be assigned to C O adsorbed on silanol groups and C O physically promoted sample is indeed more reactive toward deuterium than adsorbed on the support, r e ~ p e c t i v e l y . ~Following ~ * ~ ~ treatment is the linear species on the unpromoted material. However, the at 373 K in CO and recooling to 80 K (Figure 8B), the same bands linear species on the promoted catalyst is also much more reactive are present, although they have increased somewhat in area, as toward deuterium, being removed 45-70 K lower in temperature indicated quantitatively in Table I where the initial areas of the than on the unpromoted sample. Indeed, the low-frequency species various bands relative to their area after treatment at 373 K are is not significantly more reactive on the promoted catalyst than tabulated. is the linear species. Thus, it seems that the manganese oxide Figure 9A shows the spectrum of CO adsorbed at 80 K onto promoter has a 2-fold effect: it creates a new low-frequency species the promoted catalyst. Again strong bands at 2164, 2154,and which is more weakly bound and more reactive toward hydrogen 2135 cm-' assigned to adsorption on the support can be seen (the than is adsorbed C O on unpromoted rhodium, and at the same band at 2164 cm-' is due to new acid sites created by manganese time it weakens the adsorption of linearly adsorbed C O and makes cations on the surface of the silica, as shown by the adsorption it more reactive toward deuterium. The increased reactivity of of C O on Mn/Si02), but only weak linear and bridged bands can the linear CO may occur through one of a variety of mechanisms: be observed. However, following treatment in CO at 373 K,strong (1) alteration of the Rh-CO bond via an electronic interaction linear, bridged, and low-frequency bands are present, as shown between neighboring Rh and Mn atoms, (2)conversion of linear in Figure 9B. The relative areas of the bands are presented in CO to tilted C O in the presence of hydrogen, probably with Table I . reduction of manganese oxide as the slow step preceding a rapid interconversion, since this conversion is not seen in the absence Discussion of hydrogen, or (3) an increase in the hydrogen surface coverage Although the identity of the low-frequency species present in due to the weakening of CO adsorption on the promoted catalyst. the manganese-promoted catalyst cannot be definitively assigned, A number of possible explanations must be considered to explain the evidence is in accord with a tilted C O species of the form why no C O is chemisorbed on the promoted catalyst at 80 K while proposed by Ichikawa and S a ~ h t l e r . The ~ , ~experiment ~ ~ ~ ~ with warming in CO produces strong linear, bridged, and low-frequency labeled C O establishes that a C-0 vibration is involved, the bands. First, it is possible that some contamination by water occurs absence of bands in the region 1600-1300 cm-l eliminates the during evacuation and cooling of the sample and that more C O possibility of surface carbonates, and the location of the peak at can be adsorbed after this water is removed by heating at 373 approximately 1715 cm-I is compatible with the frequencies of K. However, no significant amount of adsorbed water was seen M-C-0-M' analogues in the organometallic l i t e r a t ~ r e . ~ ~ with ~ ~ ~IR,~ and ~ ~ contamination cannot explain the large and reproUnlike the spectra of Ichikawa and F ~ k u s h i m a the , ~ ~low-freducible difference between the promoted and unpromoted samples. quency band is well-separated from the bridged band, indicating Secondly, the rhodium surface may reconstruct slightly in the that the new band is not simply a downshifting of the bridged band. presence of CO;53 such a reconstruction would be kinetically Whatever the exact identity of the low-frequency species, the limited and could not occur at 80 K. We would suggest that metal spectra collected following heating under vacuum show that this reconstruction is probably responsible for most if not all of the species is less strongly adsorbed on the rhodium surface than is small increase in integrated C O area for the unpromoted sample the linear species, which in turn is more weakly adsorbed than following treatment in CO at 373 K. Finally, the manganese oxide the linear species on the unpromoted catalyst. To more quanoverlayer may reconstruct in the presence of CO. It can be titatively understand the results presented in Figures 1, 2, and imagined that, following reduction in hydrogen and evacuation, 4-7, the areas under the various peaks have been integrated as the rhodium surface is covered by manganese oxide moieties, with a function of treatment temperature. Table I1 presents the temmany Mn-0-Rh bonds present. Many of the 0-Rh bonds will perature at which the area of the various bands was reduced to be weaker than Rh-CO bonds and could be displaced by the only 10% of the original area. Because the low-frequency band adsorption of CO, leaving terminal Mn-0 bonds, but this process can be removed at a temperature 35-50 K lower than the linear would be thermally activated and could not occur at 80 K. species on the promoted catalyst, it seems clear that this species Therefore, we suggest that the fraction of the surface covered by is less strongly bound. In turn, however, the linear species can manganese oxide in promoted rhodium catalysts can change be evacuated 65-75 K sooner on the promoted catalyst than on dramatically with changes in temperature and gas-phase comthe unpromoted catalyst, indicating that the promoter not only position, vacuum favoring high coverages and carbon monoxide creates a new adsorbed species but also weakens the chemisorptive favoring lower coverages, with the manganese forming islands and bond between the linearly adsorbed C O and rhodium surface. more rhodium atoms available to the gas phase in the latter case. Further evidence that this bond is indeed weaker is presented in Similar phenomena have been observed for Ti0,- and Nb0,(51) Beebe, T. E.;Gelin, P.; Yates, Jr., J. T. Surf. Sci. 1984, 148, 526. (52) Zaki, M. 1.; Knozinger, H.Mater. Chem. Phys. 1987, 17, 201.
(53) Zaki, M. I.: Tesche, B.; Kraus, L.; Knozinger, H. SurJ Interface Anal. 1988, 12, 239.
J . Phys. Chem. 1990, 94, 1581-1586 promoted Rh/Si02 catalyst^.^^ Conclusions and Summary
The infrared studies reported here suggest that manganese oxide plays a complicated role in the promotion of supported rhodium catalysts. The addition of manganese oxide to Rh/Si02 creates a new adsorbed C O species at about 1715 cm-l. Although this band cannot be definitivelyassigned, comparison to organometallic analogue^^'*^^ suggests that it may be the Rh-C-0-Mn species postulated earlier by other a ~ t h o r s . ~Promotion - ~ ~ ~ ~weakens - ~ ~ the strength of adsorption of the linear C O species; the new lowfrequency species is also weakly bound. On the promoted catalyst adsorbed C O will react with deuterium at a much lower temperature than on the unpromoted catalyst; however, both the low-frequency and linear species react at about the same temperature. Thus, it is possible that the enhanced activity of manganese-promoted rhodium is not due simply to the presence of the new low-frequency CO species, but to an enhancement of the reactivity of the linear species as well. This enhancement may be due to promoter-metal electronic interactions, a rapid conversion of linear to tilted CO initiated by the reduction of manganese oxide, or higher concentration of surface hydrogen. The (54)Kraus, L.;Tesche, B.; Zaki, M. I.; Knozinger, H. J . Mol. Catal., in
1581
low-frequency species may, however, have special properties for controlling ~electivity,’3~~* a proposition which still remains to be experimentally proven. The unpromoted catalyst adsorbed only slightly more C O at room temperature or above than it did at 80 K. In contrast, the promoted catalysts would not adsorb significant amounts of CO at low temperature. Warming these catalysts in the presence of CO allowed adsorption to occur. It is suggested that under vacuum following reduction the manganese covers virtually all of the rhodium surface but that in the presence of C O this oxide overlayer partially reconstructs through the displacement of weak M n a R h bonds by adsorbed CO, a process that is presumed to be thermally activated. These results suggest that the surface coverage of rhodium by manganese oxide is dynamic and depends on gas-phase composition and temperature.
Acknowledgment. This material is based upon work supported by the North Atlantic Treaty Organization under a grant awarded to S.A.S. in 1987; the authors are thankful for the financial support of the National Science Foundation in the form of this award. The research work was financially supported by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie. A loan of RhCI, from Johnson-Matthey is gratefully acknowledged. Registry No. Rh, 7440-16-6;CO, 630-08-0;manganese oxide, 1 1129-60-5.
press.
Assoclatlon of Chlorophyll with Inverted Micelles of Dodecylpyridinium Iodide in Toluene G.R.Seely,* X. C. Ma, R. A. Nieman, and D. Gust Department of Chemistry and Center for the Study of Early Events in Photosynthesis,? Arizona State University, Tempe, Arizona 85287- 1604 (Received: May 26, 1989)
Dodecylpyridinium iodide forms inverted micelles in water-containing toluene at concentrations higher than lo4 M, as it reportedly does in other nonpolar solvents. Micelle formation is characterized by changes in the charge-transfer absorption band, and in the chemical shifts of protons, especially those on or near the pyridinium group. The micelles associate with chlorophyll a, also dissolved in the toluene, as evidenced by large changes in the chemical shift of some of the surfactant and the chlorophyll resonances. Temperature effects are small and are consistent with a loosening of the association at higher M temperatures (to 345 K). The fluorescence quantum yield of chlorophyll is little reduced by the presence of 2,2’-dithiobis( 5-nitropyridine), a quencher which is soluble in toluene and probably associates weakly with the micelles, but is strongly reduced by the presence of the bis(tetramethy1ammonium)salt of 5,5’-dithiobis(2-nitrobenzoicacid), which is solubilized only in the presence of the inverted micelles, and furthermore forms a complex with chlorophyll. These cationic inverted micelles constitute a new environment for the pursuit of chlorophyll model system investigations.
Introduction
A model system of interest to us for photochemical energy conversion studies consists of chlorophyll a (Chl) adsorbed to the interface between particles of polyethylene swollen with tetradecane, and an aqueous suspending medium. Normally, an amphiphile with Lewis basic properties is included in the preparation to ligate the Mg of Chl and stabilize its monomeric state.’V2 Cationic surfactants do not ligate Mg but are sometimes included to confer a positive charge on the particles which assists their dispersion. However, recent preparations in which dodecylpyridinium iodide (DPI) was used as cationic surfactant possessed Drouerties that suaaested that chl was not entirelv at the interface but‘was trapped Liide the particles, perhaps in inverted micelles of DPI. Other surfactants, specifically cetylpyridinium chloride and dodecyltrimethylammonium chloride, had not shown such indications. The situation in tetradecane systems has been in-
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vestigated further and will be reported in due course; it was considered worthwhile to investigate also the possibility of interaction with inverted micelles in toluene, a hydrocarbon solvent in which DPI is considerably more soluble than it is in tetradecane. The present report is an account of that investigation. There are a few previous references to inverted micelles of dodecylpyridinium salts. The existence of micelles of DPI in benzene, with an aggregation number of about 7, was concluded from vapor pressure ~ s m o m e t r y . ~They solubilized tetracyanoquinodimethane as the anion. Smaller micelles of the C1- salt exist in chloroform above 4.5 X lo4 M, which solubilize transitionmetal salts with various amounts of water, and organic^.^-^ E. R. (1) Pho,ochem. Seely, G. R.;Photobiol. Rutkoski, A. 1982, M.; Kusumoto, 36, 633, Y.; Senthilathipan, V.;Shaw, (2) Kusumoto, Y.; Seely, G. R.; Senthilathipan, V. Bull. Chem. SOC.Jpn. 1983, 56, 1598. (3)Harada, S.; Schelly, 2.A. J. Phys. Chem. 1982, 86, 2098. (4) Masui, T.; Watanabe, F.;Yamagishi, A. J. Phys. Chem. 1977,81,494. ( 5 ) Yamagishi, A,; Watanabe, F. J. Colloid InterfaceSci. 1977, 59, 181.
0 1990 American Chemical Society
