J. Phys. Chem. 1986,90, 3381-3386
3381
Rhodium Dicarbonyl Sites on Alumina Surfaces. 1. Preparation and Characterization of a Model System John L. Robbins Exxon Corporate Research Science Laboratories, Clinton Township, Annandale, New Jersey 08801 (Received: November 27, 1985; In Final Form: March 10, 1986)
A high surface area alumina readily chemisorbs the Rh carbonyl compound Rh2(CO),C12 from alkane solvents at room temperature. Samples prepared in this manner containing ca. 1 wt % Rh have been characterized by transmission electron microscopy, transmission infrared spectroscopy,solid state I3C NMR with magic-angle spinning, and low-temperature ESR. The exchange of Rh-I2CO surface complexes with gas-phase I3COhas also been studied. These data indicate such preparations afford dispersed Al-O-Rh(C0)2 surface complexes which form dative bonds with neighboring surface hydroxyl or oxo groups, but do not vibrationally or electronically interact with each other. In this way the Rh(C0)2 species on this model system are viewed as truly 'isolated". A model is also proposed for the Rh(C0)2 sites which form when alumina-supported Rh salts are reduced in H2 and then exposed to CO. By this model those materials also contain roughly square-planar A14-Rh(CO)2 species, but here they are clustered in aggregates near a minority of paramagnetic Rh sites. The latter serve to enhance the relaxation rates of 13C0coordinated to the diamagnetic Rh" sites in NMR experiments.
Introduction is low, peaks a t 2060 f 10 and 1860 f 10 cm-l predominate. These features resemble those found by EELS for CO in terminal It has long been recognized that adsorption of C O onto H2and twofold bridging sites on the R h ( l l 1 ) surface24(2070 and reduced alumina-supported Rh generates species widely described 1870 cm-' at 300 K) and are commonly ascribed to CO bound as Rh(C0)2 or gem-dicarbonyl sites along with the anticipated to large metallic Rh particles. In the supported systems these IR CO-covered Rh crysta1lites.l Understanding the detailed elecbands show coverage-dependent frequency shifts which confirm tronic and geometric structure of the Rh(C0);s has been the goal that assignment. Additional carbonyl features near 2100 and 2030 of numerous experimental efforts utilizing chemisorption meacm-' are often observed for Rh/alumina, but no analogues for surements,2-s adsorbate infrared spectro~copy,~~"-l~ electron mic r o s ~ o p y , ~ *temperature-programmed ~~*'~ r e d u c t i ~ n EX, ~ ~ ~ ~ ~these bands are found in vibrational studies of CO on either single-crystal or polycrystalline Rh. The 2100- and 203O-cm-' AFS,2@22 and magnetic resonance method^,'^^^^ to name a few. The resuls of early experimental studies were reviewed re~ently,'~ bands are always seen as a pair and appear to increase in intensity relative to the 2060- and 1890-cm-' features as the Rh dispersion so we confine ourselves here to an outline of the principal reincreases. In fact, the former are the only resolvable carbonyl maining areas of contention and a brief overview of more recent bands on Rh/alumina samples whose H/Rh (determined from relevant data. H2 chemisorption) approaches 2.0. Most workers are now in Infrared spectra of C O chemisorbed on reduced Rh/alumina accord assigning the 2100,2030 cm-' pair to the symmetric and typically show four absorption When the Rh dispersion antisymmetric CO vibrations of a Rh gem-dicarbonyl site, but the local coordination environment of the Rh(CO)z remains controversial. ( I ) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957,61, 1504-1512. (2) Wanke, S.E.; Dougharty, N. A. J . Catal. 1972, 24, 367-384. According to several groups, the Rh(CO), groups are atomically (3) Yao, H. C.; Japar, S.;Shelef, M. J. Catal. 1977, 50 407-418. dispersed and are attached to the surface via Rh-0 support (4) Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. J . Catal. 1979, 57, b o n d ~ . ~ ~ ~ ,Such ' ~ - ' 'a structure is depicted in Figure 1 where we 41-63. also allow for the possibility of dative interactions with adjacent ( 5 ) Yates, J. T.; Cavanagh, R. R. J . Catal. 1982, 74, 97-109. (6) Yates, J. T.; Duncan, T. M.; Worley, S.D.; Vaughan, R. W. J . Chem. surface hydroxyl groups. The argument for isolated Rh(C0)2 Phys. 1979, 70, 1219-1224. sites is supported experimentally by the fact that the IR band (7) Yates, J. T.; Duncan, T. M.; Vaughan, R. W. J . Chem. Phys. 1979, positions are insensitive to the degree of CO converage. 1 6 7 9 12,15-17
71, 3908-3915. (8) Antoniewicz, P. R.; Cavanagh, R. R.; Yates, J. T. J . Chem. Phys. 1980, 73 3456-3459. (9) Cavanagh, R. R.; Yates, J. T. J . Chem. Phys. 1981, 74, 4150-4155. (10) Yates, J. T.; Kolanski, K. J . Chem. Phys. 1983, 79, 1026-1030. (11) Wang, H. P.; Yates, J. T. J . Catal. 1984, 89, 79-92. (12) Primet, M.; J . Chem. SOC.,Faraday Trans. 1 1978, 74,2570-2580. (13) Tanaka, Y.; Iizuka, T.; Tanabe, K. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 2215-2225. (14) Yao, H. C.; Rothschild, W. D. J . Chem. Phys. 1978,68,4774-4780. (15) Rice, C. A.; Worley, S.D.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1981, 74, 6487-6497. (16) Worley, S. D.; Rice, C. A.; Mattson, G. A.; Curtis, C. W.; Guin, J. A,; Tarrer, A. R. J . Chem. Phys. 1982, 76, 20-25. (17) Worley, S. D.; Rice, C. A.; Mattson, G. A,; Curtis, C. W.; Guin, J. A.; Tarrer, A. R. J . Phys. Chem. 1982.86, 2714-2717. (18).Yates, D. J. C.; Murrell, L. L.; Prestridge, E. B. In 'Growth and Properties of Metal Clusters"; Bourdon, J., Ed.; Elsevier: Amsterdam, 1980; pp 137-149. (19) Huizinga, T. Thesis, Eindhoven University of Technology, 1983. (20) Van't Blik, H. F. J.; Van Zon, J. B. A. D.; Huizinga, T.; Koningsberger, D. C.; Prins, R. J . Phys. Chem. 1983, 87, 2264-2267. (21) Van Zon, J. B. A. D.; Koningsberger, D. C.; Van't Blik, H. F. J.; Prins, R.; Sayers, D. E. J. Chem. Phys. 1984.80, 3914-3915. (22) Katzer, J. R.; Sleight, A. W.; Gajardo, P.; Michel, J. B.; Gleason, E. F.: McMillan. S. Discuss. Faradav SOC.1981. 72. 121-133. '(23) Duncan, T. M.; Yates, J.'T. Jr.; Vaughan, R. W. J . Chem. Phys. 1980, 73,975-985.
0022-3654/86/2090-3381$01.50/0
9
9
2
3
Also it was shown that samples exchanged with mixtures of l2CI6O and 1 2 C i 8 0have IR spectra consistent with a mixture of independent Rh( 12C160)2, Rh( 12Ci60) ( l2CI8O), and Rh( 12C'80)2 oscillators.I0 Analogous spectra were observed as Rh+(CO),'s bound to amine or phosphine derivatized silicas underwent exchange with isotopically labeled C0.25 The structures shown in Figure 1 imply the Rh has a formal oxidation state of +1. Magnetic resonance measurements provide some support for this. The Rh(C0)2 site is thought responsible for a I3C resonance with an isotropic chemical shift of 177 ppm (6) vs. Me4Si. Although it was not noted in the original we point out that a I3C resonance for C O bonded to isolated Rh atoms may be difficult to observe. Rapidly fluctuating magnetic field gradients at paramagnetic 4d9 Rho centers would cause bound I3C to relax extremely rapidly.26 Broadening induced by such fast relaxation often precludes observation of a I3C resonance. (24) Dubois, L. H.; Somorjai, G. A. Surf. Sci. 1980, 91, 514-532. (25) Knozinger, H.; Thornton, E. W.; Wolf, M. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 1888-1899. (26) Martin, M. L.; Martin, G. J.; Delpuech, J. J. "Practical NMR Spectroscopy"; Heyden: London, 1980.
0 1986 American Chemical Society
3382 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 oc
aggregated in clusters without Rh-Rh bonds.
oc \Rh/co
\JO /
/\
0
0'
I
I
AI
OH
0'
AI
AI
1
AI
I
i OH
i,
p" AI
C
a
Robbins
b
Figure 1. Proposed structures for alumina-bound Rh+(CO)*species.
Low-temperatue ESR spectra of reduced Rh/alumina at high dispersion show paramagnetic Rh moieties are present, but these may be Rh2+species and in any case only represent 5% of the Rh present (vide supra).lg Diamagnetic carbonyl compounds containing Rh+ in a square-planar geometry are common in organometallic chemi~try.~'A surface Al-O-Rh(C0)2 complex could have such a geometry if adjacent surface hydroxyl or oxide groups act as an additional two-electron donor to the Rh centers. This is in fact the type of surface site Basset et al. claim forms when Rh carbonyl clusters oxidatively fragment on hydroxylated ceramic surfaces.28 Although the arguments for isolated A l U R h ( C 0 ) 2 structures seem persuasive, they do not readily account for two other experimental observations. TPR studies of highly dispersed Rh/ alumina catalysts show more than 90% of the metal is reduced to its metallic state in H2 at T > 473 K.3J9 In an IR/TEM/ chemisorption study of Rh/alumina at various dispersion levels Yates et aL4 found a sample with H/Rh = 1.16 and CO/Rh = 1.73 exhibited a profusion of Rh particles with a mean diameter of 15 A.4 C O adsorbed on that specimen showed an intense 2100,2030 cm-' pair, but only a weak shoulder near 2072 cm-I. They argued the gem-dicarbonyl IR bands reflect CO pairs bound to the periphery of small one-atom-thick Rh particles, and further suggested that adsorption of CO onto these "rafts" attenuates lateral bonding interactions between adjacent Rh atorns.l8 They did not speculate on the origin or nature of such an effect, but metals do form we recognize that group VI11 (groups stronger bonds with C O than with themselvesz9 and that there are many examples in the chemistry of metal-metal-bonded species where addition of CO reduces the number or order of the metal-metal bonds. An example most pertinent to these discussions is the conversion of O S ~ ( C O )a~bicapped ~, tetrahedron with 12 Os-Os bonds and an average Os-Os coordination number (CN) of 4, to OS~(CO),~, a planar Os cluster with 9 Os-Os bonds and an average C N of three.30 Recent EXAFS results, which we will discuss in more detail later, confirm the presence of low-nuclearity Rh particles on highly dispersed reduced Rh/alumina.20~2z Furthermore, they show that CO adsorption can perturb the local coordination environment of metal atoms in these clusters too.20 These data do not, however, yield information concerning rhodium's oxidation state in the resulting gem-dicarbonyl site. In the light of these issues, we felt it would be informative to prepare an alumina sample containing AI-Q-Rh(CO), units homogeneously and atomically dispersed on its surface and compare the spectroscopic properties of that system with those of the dicarbonyl sites on CO-treated Rh/alumina. This work describes the preparation of such systems via adsorption of Rh+ carbonyl compounds onto a high surface area alumina and their characterization by infrared spectroscopy with isotopic labeling, magic-angle spinning solid-state I3CNMR, electron microscopy, and ESR spectroscopy. Our results show the postulated AI-0-Rh(CO), surface complex is indeed a viable and extremely stable entity. Drawing from our own and others's spectroscopic data on the dicarbonyl site on reduced Rh/alumina catalysts, we suggest the same units are present on such surfaces, but here they are (27) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry", 4th ed.; Wiley: New York, 1980; pp 934-943. (28) 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. Cfiem. 1979, 18, 3104-3112. (29) Connor, J. A. Top. Curr. Cfiem. 1977, 71, 71-110. (30) Goudsmit, R. J.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Whitmire, K. H. Chem. Commun. 1984, 610-642.
Experimental Section Materials. Air-sensitive materials were prepared, manipulated and stored inside a Vacuum Atmospheres drybox with a recirculating atmosphere. Spectroscopic grade heptane was distilled under N, from CaHz and then stored inside the drybox. Hydrated RhC13 (42% Rh) was used as received from Engelhard. The Rh2(C0)4C12was prepared according to a literature procedure3' and was resublimed prior to use. The alumina used in this study is a fumed 7-Alz03obtained from Degussa (Aluminum Oxide-C) with a BET surface area of 100 M2 g-I. It was calcined in 0, at 723 K for 11 h and then stored in air prior to use. Analysis of the calcined material by ICPAES revealed Ca (0.13 wt %) and Si (0.01 wt %) as the principal metallic impurities. Natural abundance CO (Matheson, purity > 99.99%) and 10 vol % H2 in H e (>99.99% H2 He) were passed through Matheson gas purifiers to reduce H 2 0 and 0, contaminants to manufacturer-stated levels of less than 1 ppm. I3C-enriched CO, 90 and 99.8 atom %, was used as received from Pro-Chem and Mound Research Labs, respectively. The 99.8% I3CO also contained 13.5 atom % lSO. Surface area measurements, ICPAES analyses, and Rh determinations (X-ray fluorescence) were performed in the laboratories of our Analytical and Information Division. Sample Preparation. Rh2(C0).,C12/A1203 (I). Calcined alumina was evacuated to Torr for 16 h at 473 K and then transferred to the drybox under vacuum. The alumina (9.3 g) was stirred in dry heptane (300 mL) and a solution of Rh2(CO)4C12(0.175 g) in heptane (30 mL) was added rapidly. The mixture was stirred for 30 min and then filtered to yield a colorless solution and a pale yellow solid. The solid was dried under vacuum Torr) at room temperature for 20 h. This material was transferred under vacuum to the drybox for storage. Rh Analysis: 0.98 wt. %. Sampling Methods and Instrumentation. Thin, translucent wafers of alumina and Rh-treated alumina were prepared by pressing the dry powder to 20 MPa inside a 16-mm metallurgical die. The typical wafer density was 37 mg ern-,. The circular wafers were cut to 8 X 14 mm rectangles and loaded into the IR cell described below. For I these operations were carried out inside the drybox. When pressed, the alumina used in this study adhered tenaciously to the polished steel die faces. To circumvent this problem, the powder was sandwiched between disks of glassene weighing paper to prevent contact with the metal dies. Infrared-detectable quantities of organic contaminants were not transferred to the support by this method. Our infrared cell is based upon a previous design.3z It consits of a glass U-tube reaction zone 20 cm in length and 12 mm in diameter coupled via a standard taper 14/20 joint to a circular glass transition cell fitted with CaF, windows. Triple O-ring Teflon stopcocks terminating in 10/30 standard taper joints are mounted on the ends of the U-tube. One joint attaches to a stainless steel manifold which has access to a greaseless, liquid nitrogen trapped oil diffusion pumped vacuum system, three dosing gases, and flowing hydrogen. The other joint provides a vent for wafer reductions in flowing H,. Standard taper joint and glass-to-CaF, window connections were sealed with a high-melting, hard wax (Crystalbond 501, Aremco Products). Fully assembled, the entire system including the I R cell and steel manifold was evacuable to less than 10" Torr. Pressue inside the manifold was measured with a Baratron capacitance manometer or a BayardAlpert gauge. The leg of the U-tube not joined to the IR cell was normally capped, but could also be fitted with a quartz ESR tube that terminates in a 14/20 glass joint. The U-tube was lowered into a thermostated furnace for high-temperature wafer treatments. Rotation of the assembly allowed the wafer to drop into the IR cell and loose powders to fall into the ESR tube. The EPR tube was flame sealed and removed from the assembly under vacuum.
+
(31) McCleverty, J. A.; Wilkinson, G. Inorg. Synrh. 1966, 8, 211-214. (32) Yates, D. J. C. J. Colloid Interface Sci. 1969, 29, 195.
The Journal of Physical Chemistry, Vol. 90, No. IS, 1986 3383
Rh Dicarbonyl Sites on Alumina Surfaces
t
t
0;l
O r
3500
3000
2500 2000 Frequency (Cm
12,lZ 12,13 1313
&
1500 I)
Figure 2. IR spectrum of a pressed wafer of I under vacuum.
For the N M R measurements, wafer and powder samples of I were loaded into adjacent legs of the U-tube. The samples were repeatedly evacuated and exposed to I3CO (20 Torr) until no further changes were noted in the wafer's IR spectrum. The cell was then closed and passed back into the drybox. Here, ca. 0.25 g of the powder was transferred to a Kel-F rotor. The rotor was sealed with a Teflon-taped threaded cap and then taken to the spectrometer under N2 inside a sealed vial. As a qualitative test for sample integrity, I R spectra of the powders were measured both before and after the N M R experiment and compared with the wafer's original IR spectrum. For this, 30-50 mg of powder was spread between 25-mm KBr plates which were then placed inside an O-ring sealed metal housing and removed from the drybox. N o spectral shifts or new IR features were observed at the conclusion of the N M R studies. Infrared spectra were recorded on a Perkin Elmer Model 624 spectrophotometer interfaced to a PE Model 3600 Data Station. Spectra depicted here were averaged (4-9 scans) to enhance signal to noise, but smoothing functions were not employed. Spectral slit widths of 3.54.5 cm-' were employed in the 2300-1600-~m-~ region. Frequencies were calibrated by using the rotation-vibration spectrum of gas-phase CO. Magic-angle spinning solid-state 13C N M R spectra were recorded at 300 K on a JEOL FX60Q spectrometer equipped with a Chemagnetics probe operating a t 14.5 MHz. A 3-s pulse delay was employed in these experiments. X-band ESR spectra were measured on a Varian E109 instrument with a He-cooled cavity.
Results and Discussion Transmission IR Studies. The dimeric Rh' complex, Rh2(CO)4C12,adsorbs rapidly from heptane solution onto the surface of a partially dehydrated y-alumina to afford, after filtration and drying, a pale yellow powder, I. The Rh loading levels employed here correspond to 0.6 Rh atoms/100 A2. At this level the dimer adsorption is judged irreversible in that it cannot be extracted from the surface into dry heptane. Others have adsorbed this dimer onto ceramic oxide surfaces and concluded that chemisorption proceeds as in eq 1, where the labile Rh-CI bonds are replaced Rh2(C0)4C12+ 2surface-O-H = 2~urface-O-Rh(CO)~+ 2HC1 (1) by Rh-O-support bonds.28*33 While the spectroscopic data presented below pertain to materials prepared from the chlorocarbonyl dimer, we have also examined a material derived from adsorption of a monomeric and C1-free complex, [Rh(CO),(33) Bowser, W. M.; Weinberg, W. H. J . Am. Chem. Soc. 1981, 103, 1453-1458. (34) Yates, D. J. C.; Prestridge, E. B. Nature (London) 1971, 234, 345-341.
Frequency (cm
1)
Figure 3. IR spectrum of I under vacuum following successive exposures to 99 atom II3CO: (a) no "CO;(b) a after 2 Torr of "CO for 30 s; (c) b after three 30-s exposures to 0.5 Torr of I3CO;(d) c after 1.5 Torr of I3CO for 30 s; (e) d after 30 Torr of "CO for 10 min. Note the abscissa scale expands below 2000 cm-I.
(acetone),]BF4, onto the alumina. IR spectra of this solid show bands associated with adsorbed acetone (1690 cm-I) and fluoroborate (1000 cm-I), but they are otherwise indistinguishable from I. This supports our contention that little if any of the Rh in I contains C1 in its first coordination sphere. Electron micrographs of I show no discernible images other than the alumina particles a t a final magnification of 680 000. From our own and others' studies on the same instrument4J8we are assured that Rh aggregates >5 A in diameter are not present. The IR spectrum of a pressed wafer of I is shown in Figure 2. The broad absorption near 3600 cm-' is associated with physisorbed water and surface hydroxyl groups. The peaks near 2900 cm-I reflect hydrocarbons remaining on the support from the impregnation. Of interest to us here are the strong and sharp carbonyl absorptions at 2098 cm-' (fwhm = 22 cm-') and 2026 cm-' (fwhm = 26 cm-I). Prolonged evacuation at 310 K does not affect either the band positions or intensities. The carbonyl bands are similarly insensitive to brief exposure to dry O2 (20 Torr, 310 K, 10 min), but they do disappear entirely when I is exposed to air for the 24 h. The carbonyl bands shift to 2090 and 2020 cm-l when I is exposed to 20 Torr of H 2 0 for 1 min. The original band positions are restored by overnight evacuation a t 300 K. In all these respects the behavior of the Rh-CO species in I closely mimics that reported for the dicarbonyl site on reduced Rh/ alumina. 1,4,9,14J5 When I is repeatedly evacuated and treated with I3CO (0.3-1.0 Torr, 30 s) a t 310 K, the 2098- and 2026-cm-' bands rapidly decline in magnitude and new features appear at 2081,2049, 1993, and 1977 cm-l (Figure 3). The 2081- and 1993-cm-' bands dominate when the wafer is equilibrated with a 50:50 mixture of natural abundance and 99% I3C-enriched CO, but decline with further growth of the 2049- and 1977-cm-I bands as the I3C enrichment is increased beyond that level. These spectral changes are reversed upon evacuation and addition of '*CO. If we assume the 2098- and 2026-cm-I absorptions are due to Rh(12C160)2 species, the 2049- and 1977-cm-I bands are readily
3384 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 assignable to Rh(13C160)2sites (predicted v ( I ~ C ~ ~= O0.977; ) V ( ~ ~ C '= ~O 2050 ) and 1979 cm-I). Using the frequencies measured in I2Cl60and the secular equations for the CO stretching modes of a ~ i s - M ( C 0 oscillator,35 )~ we calculate values for the CO force constant, k = 1718 N m-I, and the CO-CO interaction constant, ki = 60 N m-l. Using those parameters we then calculate stretching frequencies of 2081 and 1996 cm-' for the asymmetrically labeled Rh(1%?60)(13C160). In accord with that prediction we observe that the 2081- and 1993-cm-I features are the most intense IR bands when the surface is equilibrated with a 5050 mixture of l2CO and "CO. Here the Rh(CO)z site distribution should be 1:2:1in Rh(12C0)2,Rh('2C0)(13CO),and Rh('3C0)z, respectively. Additional shoulders appear near 2030 and 1950 cm-I at high levels of I3C enrichment when the 99 atom % 13C0 is employed in the exchange experiments. These are not apparent when the 90 atom % 13C is used. This can be understood when we recall that the former gas is also enriched to 13.5 atom % in I 8 0 , while the latter contains less than 1 atom % l 8 0 . With 13.5 atom % 13C180we can anticipate 27%of the surface species will be Rh(13C160)('3C180) as their isotopic composition approaches that of the enriched gas. Another calculation predicts CO stretching frequencies of 2032 and 1946 cm-l for that site. The foregoing exercise illustrates two points. First, it demonstrates that I contains vibrationally uncoupled monomeric Rh(C0)2 units. A C,, Rh(12C0), structure would generate four isotopic isomers as it equilibrates with "CO and a total of eight carbonyl IR bands should be observed.36 For a simple Rh dimer, such as the X-bridged [Rh(CO),X], (X = C1, Br, I, etc.), seven isotopic isomers can form during exchange with labeled CO. Seventeen C O stretching bands have been identified as such complexes exchange with 12C180.37 We have measured the IR spectrum of I at over 20 different degrees of I3COenrichment, but no features other than those already described proved discernible. We also call attention to the striking similarities in the CO stretching frequencies and derived parameters for I and the gem-dicarbonyl site on reduced Rh/alumina as reported by Yates and Kolanski.Io They found v(l2Cl60) = 2098 and 2027 crn-'. Six IR bands were seen as the site exchanged with 12C180and their average values of k (1719 cm-I) and ki (60 cm-I) were indistinguishable from those reported here. Despite these close similarities, the Rh(CO)* units on I and on reduced, CO-treated specimens can be distinguished by their behavior in ESR and solid-state I3C N M R experiments. Those measurements further allow us to make more definitive statements regarding rhodium's oxidation state and dispersion in the two systems. Magnetic Resonance Studies. At 20 K, the X-band ESR spectrum of I shows no evidence for paramagnetic Rh species. A resonance is found at g = 4.3, but this also appears in spectra of the native support. Others have observed such a feature and attributed it to low levels of Fe3+j8 impurities in the alumina.19 I also exhibits a weak, but sharp resonance at g = 2.001 which shows no added structure in samples exchanged with l3C0. That suggests the resonance is not associated with Rh(C0)2 centers. An analogous feature appears in the spectra of an alumina sample (35) The force constant k and interaction constant ki were calculated for Rh('ZC'60), by using the relationships k = (X, + Aas)/2p; k, = (X, - A )/2p; and = 5.889 X 10-2v>(as), where p is the reduced mass of 1 2 C ' 6 and Y , and uar are the experimentally determined symmetric and asymmetric CO stretching frequencies in cm-I. Using these constants, we calculated the stretching frequencies for multiply labeled Rh(CO)*(CO) species by expanding the determinant
Here
p
and
p*
are the reduced masses of the respective CO's and
Y+
=
(A+/5.8915 X 10-J)'/2. These equations are taken from ref 36. (36) Braterman, P. S. Metal Carbonyl Spectra; Academic: London, 1975; pp 24-75. (37) Johnson, B. F. G.; Lewis, J.; Robinson, P. W. J . Chem. SOC.A 1969, 2693-2695. (38) Dowsing, R. D.; Gibson, J. A. J . Chem. Phys. 1969, 50, 294-303.
Robbins
I
200
180 PPM (6)
160
I
Figure 4. Magic-angle spinning solid-state I3C N M R spectrum of I enriched to >90% in ')CO.76000 FIDS were madded and averaged for this spectrum.
which contained no Rh, but had been subjected to the normal sample preparation conditions (high-temperature evacuation, exposure to alkane, etc.) We are confident the resonance reflects traces of hydrocarbon radicals which form during the impregnation sequence.39 With magieangle spinning, the 14.4-MHz solid-state I3CNMR spectrum of >90% I3C enriched I (Figure 4) exhibits a doublet centered at 180.9 ppm (6). The 13Crelaxation time is 900 ms at 300 K as established by an inversion/recovery experiment. We assign the 180.9 ppm resonance to I3CO's terminally bonded to Rh. The 60-Hz splitting reflects scalar coupling of 13Cand lo3Rh (I = 100% abundant) nuclei. From solution N M R studies of structurally characterized rhodium carbonyl clusters we expect terminal C O resonances in the 180-190 ppm range with 60-90 Hz Rh-C coupling constants. Edge- and face-bridging COSshow higher field chemical shifts (210-232 ppm) and smaller J values (25-45 Hz).~' The magnetic resonance data establish the Rh(C0)2 sites in I are diamagnetic, so the Rh+ oxidation state implied by eq 1 and Figure 1 is plausible. Monomeric Rho and Rh2+centers have 4d9 and 4d7ground-state electronic configurations, respectively. In strong crystal fields of trigonal, tetrahedral, square-planar, or pyramidal symmetry, such complexes must have one unpaired in a metal localized HOMO of a, b, or e symmetry. ESR signatures are anticipated for all such ground states and in no case would we expect to see an N M R transition for 13C0attached to these paramagnetic metal centers.'" Electron/nuclear spin interactions would lead to very short I3C Tl's and thus broaden the I3C resonance beyond recognition. In contrast, diamagnetic square-planar complexes of d8 ions such as Rh+, Pd2+, and Pt2+ are routinely encountered in organometallic chemistry. The gem-dicarbonyl site on reduced Rh/alumina is described as inert toward O2 at 300 K. The fact is frequently cited as evidence such sites contain Rh+ bonded to the oxide surface. However, stability to molecular oxygen is not a universal criterion for establishing the Rh oxidation state since the zerovalent Rh4(C0)12and Rh6(C0),642are 02-stableand a number of Rh+ compounds irreversibly bind dioxygen at 300 K.43 The dicarbonyl IR bands also appear when supported RhCI, is reduced in CO?8-" In this case a complete description of the Rh oxidation state(s) on such surfaces is complicated by the possible formation of Rh2(C0)4C1:5 and the conversion of Rh(CO)z sites to metallic (39) A similar signal was observed on alumina and reduced Pt/A1203 catalysts: Huizinga, T.; Prins, R. J . Phys. Chem. 1983, 87, 173-176. (40) Chini, P.; Martinengo, S.; McCaffrey, J. A.; Heaton, B. T. Chem. Commun. 1974, 3 10-3 11. (41) Abragam, A.; Bleany, B. Electron Paramagnetic Resonance of Transition Metal Ions, Clarendon: Oxford, U.K., 1970. (42) Chini, P.; Martinengo, S. Inorg. Chim. Acta 1969, 3, 315-318. (43) Vaska, L. Aec. Chem. Res. 1976, 9, 175-183. (44) Primet, M.; Garbowski, E. Chem. Phys. Lett. 1980, 72, 472-476. (45) Brenner, K. S.; Fischer, E. 0.;Fritz, H. P.; Kreiter, C. G. Chem. Ber. 1963, 96, 2632-2635.
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3385
Rh Dicarbonyl Sites on Alumina Surfaces Rh particles and/or saturated carbonyl clusters.z8 Each of these processes can occur when supported Rh salts are treated with CO in the presence of small quantities of water. In considering the valence of Rh in gem-dicarbonyls on Hz-reduced Rh/alumina we prefer to rely here on an analysis of the available magnetic resonance data. These should exhibit a more profound sensitivity to integral changes in the metals' formal oxidation state. In a T,-resolved solid-state N M R study of 13C0on Hz-reduced Rh/alumina, Duncan et al. identified two types of chemisorbed C0.z3 One (6 = 177 ppm; TI = 6.4 ms) was assigned to the Rh(C0)2 sites and the other (6 = 190 ppm; TI = 60 ms) to terminal and bridging COSon Rh crystallites. The chemical shift of the former is very close to that we find for isolated Rh+(CO)z sites in I, but the sites in I have a T, that is 100 times longer than their analogues in the reduced catalyst. In the original NMR study it was acknowledged that the 6.4-ms TI value seemed abnormally short for C O on an isolated metal center. Several possible relaxation enhancement mechanisms were considered including spin-spin interactions with adjacent surface hydroxyl groups or paramagnetic Fe centers on the surface. None were deemed acceptable at the time. Huizinga recorded the low-temperature ESR spectrum of a highly dispersed (H/Rh = 1.9) 0.57 wt % Rh/alumina catalyst following its reduction, evacuation, and treatment with CO.I9 At 20 K, the reduced and outgassed sample exhibited a broad (AH = 260 G) resonance at g = 2.14. Chemisorption of C O shifted this resonance to g = 2.09 and generated a new, narrow (AH = 24 G) line with gll= 2.043 and g, = 2.011. The ESR signal intensities declined rapidly with increasing temperature above 60 K, indicating that the extant paramagnetic species have very fast relaxation rates. The number of spins corresponded to 4.8% of the total number of Rh atoms in this sample and 2.5% of the Rh in a more poorly dispersed sample containing 2.0 wt % Rh. The broad resonances a t g = 2.09-2.14 were assigned to a small number of Rhz+ centers interacting with larger metallic Rh clusters. In support of this he noted that 4d7 Rh*+ ions should exhibit fast relaxation behavior and that spinspin interactions of the RhZ+with Rh atoms in the cluster would serve to broaden the Rhz+ resonance. It was postulated that such centers serve to anchor Rh clusters to the oxide surface. In an EXAFS study of the same 0.57% Rh/alumina, Van't Blik and co-workers found the reduced and evacuated sample had an average Rh-Rh distance of 2.65 f 0.01 A, a value somewhat shorter than that in bulk R h (2.69 A).z0 They computed an average Rh-Rh coordination number of 5.0 f 0.5 and concluded the system contained metallic Rh clusters with 15-20 atoms per particle. Significantly, oscillations in the EXAFS spectrum associated with Rh-Rh bonds were severely attenuated after exposure to 100 Torr of CO at room temperature. IR spectra of CO treated materials showed only the 2095- and 2023-cm-' bands associated with Rh(C0)2. They proposed that C O adsorption disrupts the metal-metal bonds in these very small Rh particles and that distances between the resulting Rh(CO)z sites are too large or too disordered to yield coherent scattering in the EXAFS experiment. These workers were the first to suggest that adsorption of CO onto small, low coordination number metal crystallites might convert such metal particles to oxide-bound Rh(CO)zs via an oxidative fragmentation reaction involving surface hydroxyl groups (eq 2). An analogous reaction is known Rh,
+ 2 n C 0 + nAlOH = nAl-O-Rh(CO)z + (n/2)H2
(2)
to occur when the C O saturated, zerovalent clusters Rh4(CO)1z and Rh6(C0)16are deposited onto hydroxylated alumina surfaces.28 In a similar vein, several groups have monitored C O adsorption onto reduced and outgassed Rh/alumina by IR in the (46) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new 3 and 13.) numbering: e.&, I11
-
90-300 K regime.7J2J1 It was concluded that the Rh(C0)2 sites form via an activated process. Drawing upon these and our own data we now propose a model for the Rh(CO),, sites on reduced and CO exposed Rh/alumina. This model, which considers their molecular geometry, electronic structure, and morphology, incorporates the following paradigms. The first and portions of the second paradigm have been suggested previously. 1 0 ~ 1 z ~ 1 5 ~ z 8 1. The Rh(CO)z sites are not directly bonded to one another. This view is supported by fractional CO coverage and mixed isotope infrared studies which show the units are not vibrationally coupled.'JO Our companion IR studies on a homogeneously dispersed model system validate that opinion. 2. The sites contain diamagnetic 4 8 Rh' centers, probably in a square-planar coordination environment. Magnetic resonance studies show that most, if not all, of the Rh(C0);s are diamagneti~.'~.*~ Monomeric Rho or RhZ+complexes must contain at least one unpaired electron. Rh+ can adopt a diamagnetic ground state in square-planar complexes with strong field ligands, where all metal levels save the 4d,2+ are filled. Paramagnetic ground states are expected for d8 ions in trigonal, tetrahedral (ez:; tZ:) or octahedral (tz{; e z t ) crystal fields. The five coordinate geometries, trigonal bipyramidal and square pyramidal, can also generate diamagnetic ground states, so we cannot yet rule out a site geometry such as that shown in Figure IC. However, the experimental observation of a 90-deg OC-Rh-CO bond angle6 leads us to prefer the roughly square-planar geometry suggested by Figure lb. 3. The Rh+(CO), sites on reduced catalysts are clustered near paramagnetic Rh centers which are minority species in the system. Huizinga showed paramagnetic Rh is present on reduced materials, probably in the vicinity of metallic Rh clusters. Their low-temperature ESR signals are also perturbed by chemisorption of C0.19 EXAFS results on the same samples showed CO adsorption strongly modifies Rh-Rh bonding in the clusters.20 Rh+(CO)is on I and on reduced materialsz3have nearly identical I3Cchemical shifts, but those on reduced Rh/alumina relax more than 100 times faster. Proximity of those sites to the rapidly relaxing paramagnetic Rh centers could introduce an additional spin relaxation mechanism (electron/nuclear dipolar interactions) and thus account for the short 13Crelaxation times. In considering their TI data, Duncan et al. considered the possibility that the isolated Rh(CO)z sites were located near paramagnetic Fe impurities in the alumina support which could similarly enhance the 13C relaxation rates. Models demonstrated such relaxation enhancement would be viable if the Rh(C0)2's were all within 35 8,of a paramagnetic center. This was considered unlikely as the 0.2 wt % Fe was homogeneously distributed throughout the alumina. In our model, CO adsorption disrupts Rh-Rh bonding in lownuclearity Rh crystallites and induces the formation of A l U R h bonds, possibly with concomitant elimination of Hz. The resulting Rh+(CO)is are no longer close enough to maintain direct Rh-Rh interactions but remain aggregated near a minority of paramagnetic Rh species. These serve to enhance the 13Crelaxation rates of nearby gem-dicarbonyls. The clustering aspect of our model is attractive in that it also rationalizes the 5-30-A images observed in TEM studies of COtreated Rh/alumina at high dispersion.lz>l8Those images might denote locally high concentrations of nonbonded A l U R h ( C 0 ) i s as well as a number of metallic Rh particles. There are several implications of this clustering model that are testable by chemical and spectroscopic methods. For example, if the Rh+(C0)2units are clustered near paramagnetic centers we would expect to find a distribution of 13Crelaxation times since the theoretical relaxation equations include a 1/r6 term ( r = distance from paramagnetic to diamagnetic n u c l e u ~ ) .In~ this ~~~~ regard it would be helpful to measure the magnetization vs. time behavior for l 3 C 0 on reduced Rh/alumina containing the fewest possible number of Rh metal-CO sites. That study might also quantify the number of truly isolated Rh(C0)2's with Tl's close to those found for our model system. Companion TEM studies
J . Phys. Chem. 1986, 90, 3386-3393
3386
would establish a viable range for the Rh2+/Rh(CO)2distances and thus aid in modeling the magnetization data. “Isolated” and “clustered” Al-O-Rh(CO),’s may also be differentiable by their kinetic behavior in certain chemical reactions. Basset et al. demonstrated that CO/H20 mixtures reduce Al0-Rh(CO),’s and cause them to aggregate to CO-covered supported Rh metal particles or saturated carbonyl clusters.28 That reaction might be more facile when the dicarbonyls are already clustered. There is also some controversy regarding the thermal stability of Rh(C0)2 on reduced ~ata1ysts.l~Rates of CO desorption from such sites might also be influenced by their proximity
to each other, especially if the desorption is accompanied by formation of Rh-Rh bonded species. Our current experiments involve a careful kinetic analysis of the aggregation and C O desorption reactions of A1-O-Rh(C0)2 sites both on our model system and on H2-reduced Rh/alumina. Acknowledgment. I thank Dr. M. Melchior, Dr.E. Prestridge, and Dr. K. Rose for their help in the N M R and electron microscopy studies. Registry No. A1203, 1344-28-1; Rh2(C0)4C12,14523-22-9; CO, 630-08-0.
Dynamic Surface Properties of Anionic-Cationic Mixtures P. Joos,*t J. Van Humel,$and C.Bleyst Departments of Biochemistry and Chemistry, Uniuersitaire Ins telling Antwerpen, 8-261 0 Wilrijk, Belgium (Received: December 3, 1985; In Final Form: February 19, 1986)
A thorough theoretical investigation is made on the dynamic surface properties of mixed anionic/cationic surfactant solutions containing, e.g., P+Y- and R+X- of which R+ and Y- are surface-active ions. When the system in equilibrium is subjected to small periodic area variations, both ions R+ and Y- diffuse to and from the surface. Lucassen et al. (Lucassen, J.; Holloway, F.; Buckingham, J. J . Colloid Interface Sci. 1978, 67, 432) considered both ionic species to diffuse as an electroneutral combination, and this process is ruled by the simple diffusion equation. This simplified theory results in
with Go the thermodynamic dilational modulus, wo the diffusion relaxation frequency, FRYthe adsorption of the electrolyte RY, DRY the diffusion coefficient of the electrolyteRY, and CRand Cythe bulk concentrationsof the ions R+ and Y-, respectively. However, if both ions R+ and Y- diffuse to and from the surface, a diffusion potentia) is built up if the mobilities or diffusion coefficients of both ionic species are different. This diffusion potential also affects the motion of the other ions P+ and X-. As a result the motion of each ionic species must be described by an electrodiffusion equation. This set of four differential equations is solved in order to find the concentration fluctuations of all ionic species. An essential step here is the use of the Poisson equation rather than the Laplace equation in order to relate the potential with the concentrations of the ions. This theory finally results in
For the situation where both diffusion coefficients are equal, this equation reduces to the Lucassen equation. As a general conclusion it is stated that the equation of Lucassen is a fair approximation in most practical cases.
Introduction When an aqueous solution of an anionic surfactant, say sodium dodecyl sulfate, is mixed with a cationic surfactant, say cetyltrimethylammonium bromide, a very surface-active electroneutral ionic combination can be formed: cetyltrimethylammonium dodecyl sulfate. Over a broad range of mixing ratios only this component is adsorbed at the air/water interface, resulting in a strong synergistic effect. The theory on the equilibrium properties of mixed anionic/cationic surfactant solutions was elaborated by Lucassen-Reynders et aI.’J and checked by various author^.^,^ For what concerns the dynamic surface properties of these solutions treated the theory seems not so well estabbhed. Lucassen et both equilibrium and dynamic surface properties of mixed solutions of polylysine with sodium dodecyl sulfate. Polylysine can be seen as a polyvalent cationic surfactant. These authors assumed that only the electroneutral combination polylysine/dodecyl sulfate diffuses to the changing interface, and this diffusion process is ruled by the simple diffusion equation. This is not quite correct since one has a mixture of electrolytes, hence in principle elecDepartment of Biochemistry. *Department of Chemistry.
t
0022-3654/86/2090-3386.$01SO/O
trodiffusion must be accounted for. A priori, we do not know how serious this simplification is, and to get evidence about this is the purpose of this paper. It will be shown that the equation of Lucassen must be corrected. This correction is, however, not important. Theory We confine our attention to a mixture of a cationic surfactant R+X-, say cetyltrimethylammonium bromide, with an anionic surfactant P’Y-, say sodium dodecyl sulfate. To be clear R+ stands for the surface-active cation, cetyltrimethylammonium, Xfor the bromide anion, P+ for the sodium cation, and Y - for the (1) Lucassen-Reynders, E . H. Kolloid Z . Z . Polym. 1972, 250, 356. (2) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 81, 150. ( 3 ) Lucassen, J.; Holloway, F.; Buckingham, J. H. J . Colloid Interface Sci. 1978, 67, 432. (4) Buckingham, J. H.; Lucassen, J.; Holloway, F. J . Colloid Interface Sci. 1978, 67, 423. ( 5 ) Lange, H.; Schwuger, M. J. Kolloid 2.Z . Polym. 1971, 243, 120, 129. (6) Joos, P.; Van Hunsel, J. J. Colloid Interface Sci.1985, 106, 161. (7) Bleys, G.; Joos, P. J . Phys. Chern. 1985, 89, 1027.
0 1986 American Chemical Society
