2602
J . Phys. Chem. 1988, 92, 2602-2604
Effect of Molybdenum Addition to Rh/SiO, Catalysts: Appearance of a Second Rhodlum Dicarbonyl Species Michael D. Wardinsky and William C. Hecker* B W Catalysis Laboratory, Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602 (Received: September 25, 1987; In Final Form: November 13, 1987) The effects of molybdenum addition to silica-supported rhodium have been studied by using infrared spectroscopy. A new rhodium dicarbonyl species with characteristic bands at 21 10 and 2052 cm-' was observed during treatment of Rh-Mo/SiO, catalysts with CO at room temperature following reaction of nitric oxide with CO over the catalysts at 200 OC. These bands are approximately 15 cm-' higher than the bands usually reported for the rhodium dicarbonyl species. Infrared spectra show the presence of both rhodium dicarbonyl species simultaneously which suggests that the new species is not a shifted form of the lower frequency species, but rather a new entity. It is also observed that the new species is much less stable in a reducing atmosphere than the lower frequency species. It is therefore proposed that the new dicarbonyl species contains a more oxidized form of rhodium than the traditional Rh' dicarbonyl species and that the addition of molybdenum promotes this increased oxidation of the Rh surface sites.
Introduction The need to reduce nitric oxide (NO) emissions from automotive sources requires the use of rhodium (Rh) as a catalyst in three-way automotive catalytic converters.] The scarcity of rhodium supplies has led to numerous attempts2J to find catalysts capable of replacing or partially supplementing the rhodium used in three-way catalytic converters. Incorporation of base metal oxides in three-way catalyst formulations to improve the NO reduction performance is one possible alternative. Addition of molybdenum (Mo) to Pt-Rh catalysts reportedly improves the catalytic properties of Pt-Rh for the reduction of N O to N2.4 The purpose of this study is to determine the effects of molybdenum addition on the activity of silica-supported Rh-Mo catalysts for N O reduction by CO. Infrared data presented in this paper represent an attempt to further understand the interactions which occur between rhodium and molybdenum. The adsorption of CO on supported rhodium catalysts has been extensively investigated by using infrared spectroscopy.s-12 In addition to the linearly bound C O species, Rh-CO, which exhibits a characteristic infrared band between 2045 and 2075 cm-', chemisorbed CO on rhodium also exists in the form of a dicarbonyl species, Rh(CO),, which exhibits infrared bands at about 2095 cm-' (for the symmetric stretch) and 2030 cm-' (for the antisymmetric stretch). Two types of bridged rhodium carbonyl species have also been reported,', R h 2 C 0 with a characteristic band at approximately 1860 cm-I and Rh2(C0)3with a characteristic band at approximately 1920 cm-'. Both the linear and bridged carbonyl species are believed to be indicative of threedimensional rhodium crystallite surfaces while the dicarbonyl species is thought to represent atomically dispersed Rh sites or edge sites of small two-dimensional rhodium rafts.* On the basis of their infrared data, Rice and co-workersI2 have proposed the oxidation states of the Rh in the various rhodium carbonyl species. The Rh (CO), species is attributed to stable Rh' ions while the ( I ) Taylor, K. C. In Caralysis Science and Technology; Anderson, J . R., Boudart, M., Eds.; Springer-Verlag: New York, 1984; Vol. 5. (2) Shelef, M . Caral. Reu. 1975, 1 1 , I . (3) Harrison, B.; Wyatt, M.; Bough, K. G. In Caralysis; The Royal Society of Chemistry: London, 1982; Vol. 5. (4) Gandhi, H. S.; Yao, H . C.; Stepien, H. K. In Caralysis Under Transient Conditions; Bell, A. T., Hegedus, L. L., Eds.; American Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 178. (5) Arai, H.; Tominaga, H. J . Coral. 1976, 43, 131. (6) Yang. A. C.; Garland, C. W. J . Phys. Chem. 1957, 61, 1504. (7) Yao, H. C.; Rothschild, W. G. J. Chem. Phys. 1978, 68, 4774. (8) Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. J. Coral. 1979, 57, 41. (9) Yates, J. T.; Duncan, T. M.; Worley, S . D.; Vaughn, R. W. J. Chem. Phys. 1979, 70, 1219. (IO) Yates, J . T.; Duncan, T. M.; Vaughn, R. W. J . Chem. Phys. 1979, 71, 3908. ( 1 1 ) Cavanagh, R. R.; Yates, J. T. J . Chem. Phys. 1981, 74, 4150. (12) Rice, C. A.; Worley, S. D.; Curtis, C. W.; Guin, J. A,; Tarrar, A. R. J . Chem. Phys. 1981, 74, 6487.
0022-3654/88/2092-2602$01.50/0
linear and bridged rhodium carbonyl species are attributed to Rho. Infrared spectra presented in this paper indicate the effects of molybdenum addition to silica-supported rhodium catalysts. A pair of absorption bands assigned to the symmetric and antisymmetric stretching modes of a dicarbonyl species, Rh(C0)2, are observed at higher wavenumbers for the Rh-Mo/SiO, catalysts than those traditionally reported for Rh/SiOz catalysts. This is interpreted to mean that the addition of Mo to Rh/silica leads to an increased oxidation state of the Rh. Infrared spectra also show that both forms of dicarbonyl species can exist on a 1% Rh/4% Mo/Si02 catalyst at the same time.
Experimental Section Catalyst Preparation. The catalysts used in this study were prepared by incipient wetness impregnation techniques. Cab-0-si1 silica (Cabot Corp.) was impregnated consecutively with aqueous solutions containing first (NH4)6M07024*4HZ0 (Johnson Matthey Chemicals) and then RhC134H20(Alfa Products) salts after being calcined at 873 K for a period of 4 h. Distilled and deionized H 2 0 was used to dissolve the catalyst precursors. A wetting ratio of 2.0 (cm3 of solution/g of catalyst) was used for the first impregnation followed by a wetting ratio of 1.3 for the second impregnation. Following each impregnation, the catalysts were dried for a period of 12 h under an infrared heat lamp at a temperature of 328 K. After each drying step the catalysts were calcined in air at 773 K for a period of 6 h. Rhodium loadings were held constant at 1 wt % while the molybdenum loadings varied from 1 to 4 wt %. Experimental Apparatus. The infrared reactor cell used in this study, which doubled as a flow reactor, was designed by Savat~ky.'~ It is composed of two stainless-steel flanges each containing a 9/ 16 in. diameter calcium fluoride (CaF,) window through which the infrared beam can pass. An aluminum gasket placed between the knife-edge of each flange was used to seal the reactor. A thin self-supporting catalyst wafer of 3/4 in. diameter (pressed from the catalyst powder) rests inside the reactor and splits the incoming feed gas with the resulting flow over both faces of the wafer. A type K thermocouple (Omega) placed in close proximity to the wafer edge was used in conjunction with a digital temperature indicator (Omega, Model 650/660) to record the catalyst temperature. Heat to the infrared reactor was supplied by two 200-W heaters which were connected in series to a closed-loop temperature controller (Omega, Series 4000). The temperature measured inside the reactor was used as the feedback temperature to the controller. Infrared spectra were recorded by averaging 100 scans of the catalyst wafer using a Fourier transform infrared spectrometer (Nicolet, 5MX) with a resolution of 4 cm-'. Absorbance plots were obtained by subtracting the appropriate background (13) Savatsky, B. J. Ph.D. Thesis, University of California, Berkeley, CA,
1980.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2603
Effect of Molybdenum Addition to Rh/Si02 Catalysts
z
s
1 2200
1 2100
1
1 2000
I
I
I
2100
2000
1
0
Wavenumber (cm " )
Figure 1. Infrared spectra of (a) 1% R h / S i 0 2 ; (b) 1% Rh/l% Mo/Si02; (c) 1% Rh/3% Mo/SiO,; (d) 1% Rh/4% M o / S i 0 2 at 298 K following treatment A.
spectra from the sample spectra. Experimental Procedures. Prior to obtaining the infrared spectra presented in this paper the Mo/Si02, Rh/Si02, and Rh-Mo/SiOz catalysts were reduced in the infrared cell with flowing H2at a temperature of 473 K for a period of not less than 16 h. After reduction the catalysts were subjected to one of two types of treatments. (i) Treatment A . The reactor cell was purged with flowing He at 473 K for a 10-min period during which a background spectrum was collected and stored in the FT-IR. The H e flow was then shut off and the wafer was exposed to a flowing reaction mixture comprised of 0.9% NO and 3.3% CO in He. After a steady-state rate of NO depletion was reached, the catalyst temperature was reduced to 363 K and the NO flow stopped. Removal of adsorbed NO from Rh sites and the subsequent growth of rhodium carbonyl and Rh-NCO bands were monitored using the FT-IR. After the catalyst wafer was subjected to the 3.3% CO/He mixture for a period sufficient to remove the NO from Rh sites, the cell was cooled to 298 K and the infrared spectra collected. This process required approximately 1 h after the NO flow was stopped. (ii) Treatment B. The reactor cell was cooled to 298 K and the H2 flow was shut off and replaced with flowing He for a period of 10 min during which time a background spectrum was collected and stored in the FT-IR. The He was then replaced with a flowing 0.9% N0/3.3% C O reaction mixture and heated to 473 K. After 2 h a t 473 K the catalyst was cooled to 423 K and the N O flow shut off for 15 min. The 3.3% CO/He mixture was then replaced with a flowing 0.9% NO/He mixture for a period of 30 min. The NO/He mixture was then replaced by flowing 3.3% CO/He and the infrared spectra were collected as indicated in Figure 2.
Results and Discussion The effect of Mo addition to silica-supported rhodium catalysts is evident upon comparison of the infrared spectra presented in Figure 1. The strong band appearing at 2060 cm-' in Figure l a for a 1% Rh/Si02 catalyst is indicative of the linear species, Rh-CO, assigned to Rh in the zero oxidation state.lZ With the addition of 1% Mo the linear band is observed to decrease in strength in Figure l b and shift to a higher frequency of 2072 cm-I. The appearance of a pair of bands at 2108 and 2045 cm-' in the spectra of the 1% Rh/l% Mo/SiO2 catalyst is attributed to the symmetric and antisymmetric stretching modes, respectively, of a dicarbonyl species Rh(C0)2. The trend toward a weaker linear band (both in absolute intensity and in relation to the dicarbonyl bands) and a shift of the
2200
2100
2000
Wavenumber (cm
2100
-'
2000
1900
)
Figure 2. Infrared spectra of 1% Rh/4% Mo/Si02 at 423 K after treatment B followed by (a) 15 min in 3.3% CO/He; (b) 1 h in 3.3% CO/He; (c) 1.5 h in 3.3% CO/He; (d) 1.5 h in 3.3% CO/He and 10 min in flowing H2.
linear band to higher frequencies continues with the addition of Mo. For a 1% Rh/3% Mo/Si02 catalyst which appears in Figure IC, the linear band is now observed at a frequency of 2078 cm-I, a shift of 6 cm-' from the linear band present in Figure 1b and a shift of 18 cm-' from the band appearing in Figure l a for the 1% Rh/SiOz catalyst. A pair of bands assigned to the dicarbonyl species are observed at 2108 and 2051 cm-' in Figure I C for the 1% Rh/3% Mo/SiOz catalyst with the antisymmetric band having also been shifted to a slightly higher frequency compared to that for the catalyst containing 1% Mo. For a 1% Rh/4% Mo/Si02 catalyst the linear band now appears at 2087 cm-I in Figure Id, a shift of 27 cm-I from the linear band observed for the 1% Rh/Si02 catalyst. The dicarbonyl bands at 21 10 and 2051 cm-' continue to grow and are much stronger than the linear band in the spectra for the 1% Rh/4% Mo/Si02 catalyst. It should be noted that the bands appearing at approximately 2180 cm-I in Figure 1 (and Figure 2) are attributed to the isocyanate species, Rh-NCO, whose assignment and behavior are discussed elsewhere. A similar decrease in intensity and shift to higher frequencies of the linear carbonyl band has previously been observed by Yang and Garlande6 In their study, sintered 8% Rh on alumina was subjected to a partial C O coverage and then exposed to O2 at increasing temperatures. Upon addition of O2 at room temperature, a band appearing at 2062 cm-' assigned to the linear Rh-CO species was observed to decrease in intensity while shifting in frequency to 2087 cm-I. A dicarbonyl species was also observed to appear during this oxidation treatment. Based on the similarities between this behavior and that observed in the present study, it appears that the effect of molybdenum addition to R h / S i 0 2 is to facilitate the oxidation of rhodium during the NO-CO reaction. Further infrared spectra of C O adsorbed on the 1% Rh/4% Mo/Si02 catalyst (see Figure 2) suggest that the dicarbonyl species observed in Figure 1 at about 2108 and 2050 cm-I is different from the dicarbonyl species traditionally observed on supported Rh catalysts. After treatment B and upon exposure to a 3.3% CO/He reducing atmosphere at 423 K, the formation of bands centered at 2093 and 2037 cm-' is noted in Figure 2b and seems to increase in Figure 2c with increased exposure to the reducing atmosphere. These bands are assigned to the type of dicarbonyl species usually observed on Rh/Si02 catalysts while (14) Hecker, W. C.; Bell, A. T. J . Coral. 1984, 85, 389.
J . Phys. Chem. 1988, 92. 2604-2613
2604
the pair of bands which appear a t higher frequencies (approximately 2106 and 2047 cm-') are presumed to be from the same dicarbonyl species observed in Figure 1. Thus, two different dicarbonyl species are observed at the same time. It is possible, although not certain at this point, that the appearance and growth of the lower frequency dicarbonyl bands (in Figure 2) with exposure to the 3.3% CO/He mixture is a reduction effect accompanied by a breakup of the rhodium ensemble as proposed by Van't Bilk and c o - ~ o r k e r s . ~ ~ ~ ~ ~ The possibility of a Mo(CO)? species leading to the observation of the higher frequency pair of infrared bands was dismissed after examination of the infrared spectra (not shown) of a 4% Mo/Si02 catalyst subjected to treatment B. No bands were observed in the infrared region corresponding to CO adsorbates on molybdenum. This is in good agreement with the l i t e r a t ~ r e ~ 'which .'~ suggests only very weak adsorption of CO in a linear form on Mo sites. Miessner et al.I9 have recently observed a rhodium dicarbonyl species appearing at 21 18 and 2053 cm-I in the infrared spectrum of a calcined dealuminated zeolite Y catalyst containing 1% Rh. It is interesting to note that upon reduction with H2and treatment with CO the calcined zeolite catalyst exhibits both a high-frequency pair of rhodium dicarbonyl bands (at 21 18 and 2053 cm-I) and a lower frequency pair appearing at 2093 and 2036 an-'.The appearance of the lower frequency bands upon reduction of the zeolite catalyst is not inconsistent with the events occurring in (15) Van? 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. (16) Van't Blik, H. F. J.; Van Zon,J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. A m . Chem. SOC.1985, 107, 3139. (17) Howe, R. F.; Kemball, C. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1153. (18) Peri, J. B. J . Phys. Chem. 1982, 86, 1615. (19) Burkhardt, I.; Gutschick, D.; Lohse, U.; Miessner, H. J . Chem. SOC., Chem. Commun. 1987, 291.
Figure 2 in which the Rh-Mo/Si02 catalyst was exposed to a 3.3% CO/He reducing environment resulting in the simultaneous appearance of both rhodium dicarbonyl species. Miessner et al. have assigned their high-frequency pair of infrared bands to a Rh+ dicarbonyl species located inside the zeolite cages while they have assigned the low-frequency pair of bands to a Rh' dicarbonyl species located outside the zeolite cages. The ease with which the pair of higher frequency dicarbonyl bands disappear in Figure 2d (except for the small peak at 2105 cm-') upon exposure to H, (at 423 K) suggests that the type of rhodium site may differ for the two dicarbonyl species observed. The fact that the bands associated with the new high-frequency species appear at frequencies higher than those usually reported for silica-supported rhodium and the fact that they disappear readily upon exposure to H2 suggests that the new species is associated with a different, more oxidized rhodium state than the traditional dicarbonyl species which is associated with Rhf.6 The infrared data also suggest that it is the addition of molybdenum that facilitates the oxidation of rhodium and leads to this new rhodium dicarbonyl species upon treatment with NO-CO mixtures. Conclusions
The results presented in this paper lead to the following important conclusions: (1) The addition of molybdenum to silica-supported rhodium catalysts followed by the treatments described previously leads to the formation of a pair of new higher frequency dicarbonyl bands that can be distinguished from the dicarbonyl bands usually seen on Rh/Si02 catalysts. (2) The higher frequency dicarbonyl bands suggest that the addition of molybdenum results in a more oxidized form of rhodium and may lead to a different type of active Rh site. Registry No. Rh, 7440-16-6; Mo, 7439-98-7; NO, 10102-43-9; CO, 630-08-0.
Interfacial Properties of a Novel Group of Solvatochromic Acid-Base Indicators in Self-Assembled Surfactant Aggregates Calum J. Drummond,* Franz Grieser, and Thomas W. Healy Department of Physical Chemistry, The University of Melbourne, Parkville, Victoria, 3052, Australia (Received: November 4, 1987)
The spectral and acid-base properties of l-hexadecyl-4-[(oxocyclohexadienylidene)ethylidene]- 1,4-dihydropyridine(HOED), 1-hexadecyl-5-hydroxyquinoline(HSHQ), and 1-hexadecyl-6-hydroxyquinoline(H6HQ) have been investigated to ascertain their suitability as probes of both the mean interfacial solvent properties and the electrostatic surface potential of self-assembled surfactant aggregates in aqueous solution. The results obtained indicate that the solvatochromic visible absorption band of the conjugate base form of HOED, HSHQ, and H6HQ can be utilized to obtain a reasonable estimate of the mean interfacial solvent properties of self-assembled surfactant aggregates. The factors that are primarily responsible for the apparent pK, values of HOED, HSHQ, and H6HQ in most types of self-assembled surfactant aggregates have been ascertained. It is concluded that the acid-base properties of HOED, HSHQ, and H6HQ can be utilized to obtain a quantitative measure of the electrostatic surface potential of self-assembled surfactant aggregates in aqueous solution.
Introduction It has long been recognized' that the magnitude of the apparent pK,, pKaobsd,of a prototropic moiety residing at or close to a charged interface depends on the electrostatic potential of the charged interface. Indeed2-23 pKaQM= pK2
F\k
-2.303RT
(1) Hartley, G. S.; Roe, J. W. Trans. Faraday SOC.1940, 36, 101. (2) Mukerjee, P.; Banerjee, K. J. Phys. Chem. 1964, 68, 3567.
0022-3654/88/2092-2604$01 S O / O
where pK2 is the intrinsic interfacial pK, of the prototropic moiety, 9 is the mean field potential at the average site of residence for (3) Tokiwa, F. Adu. Colloid Interface Sci. 1972, 3, 389. (4) Yalkowsky, S. H.; Zografi, G. J. Colloid Interface Sci. 1970, 34, 525. (5) Fromherz, P.; Masters, B. Biochim. Biophys. Acta 1974, 356, 270. (6) Bakker, E. P.; Arents, J. C.; Hoebe, J. P. M.; Terada, H. Biochim. Biophys. Acta 1975, 387, 491. (7) Funasaki, N. J. Colloid Interface Sci. 1977, 60, 54. (8) Fernandez, M. S.; Fromherz, P. J . Phys. Chem. 1977, 81, 1755. (9) Vaz, W. L. C.; Nicksch, A.; Jahnig, F. Eur. J . Biochem. 1978,83,299. (10) Mashimo, T.; Ueda, I.; Shieh, D. D.; Kamaya, H.; Eyring, H. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 5114.
0 1988 American Chemical Society
