1190
J . Phys. Chem. 1990, 94, 1190-1196
Bimetallic Catalysts from Alumina-Supported Molybdenum-Iridium Clusters John R. Shapley,* Winston S. Uchiyama, and Robert A. Scott7 School of Chemical Sciences and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 (Received: July 5, 1989; In Final Form: November 17, 1989)
The heterometallicclusters C ~ M O I ~ , ( C O and ) ~Cp2MozIrz(CO),o ~ (Cp = $-CsHs) were deposited onto fumed alumina (Degussa A120,-C, 0.2 wt % Ir) as catalyst precursors. Comparable materials were prepared from Ir4(CO)12and C~,MO,(CO)~ as well as from a stoichiometricmixture (Ir, + 2Mq) of these homometallic compounds. The supported compounds were activated by heating to 500 O C in flowing Hz. Methane evolution profiles observed during activation were characteristic for each heterometallic cluster precursor and were distinct from profiles associated with the homometallic compounds. H2 and CO 1). The IR-containing chemisorption on the activated Ir-containing materials indicated high dispersion (H/Ir 2 1, CO/Ir materials, [Mo21r2],[MoIr,], [Ir,], and [Ir, + 2M02], were active catalysts for the hydrogenolysis of n-butane at 215 OC, whereas [Mo,] was inactive. The [MoIr,] catalyst exhibited enhanced activity (5-10 times) over the [Ir4] and [Ir4 2MoJ catalysts, but the selectivity toward ethane production, 70-75%, was the same for all these samples. In contrast, the [Mo,I!,] catalyst exhibited a change in ethane selectivity to ca. 50% but had catalytic activity comparable to the [Ir4 + 2M02]material. The differences in catalytic properties displayed by the heterometallic-cluster-derived catalysts are attributed to bimetallic interactions maintained in the activated materials. Mo K edge X-ray absorption spectra were collected in order to characterize the metal-metal interactions. The near-edge spectra (XANES) showed unique features for [MoIr3]and [MqIr,] in comparison with [IT, + 2Moz];the spectrum of the latter was identical with that of [MoJ alone. A XANES feature at 20010 eV, indicative of Mo=O moieties, was apparent in oxygen-contaminated samples, but it could be removed by in situ activation. Fourier transforms of the EXAFS data collected on in situ activated samples of [MoIr,], [MoJr,], and [Ir4 2M02] showed that each sample had structurally unique molybdenum sites. In particular, both heterometallic-cluster-derived samples, [MoIr,] and [Mo21rz],displayed evidence for Mo-M interactions at R ' = 2.5-2.6 A, but the sample derived from the stoichiometric mixture, [Ir, + 2Moz], did not.
-
+
+
I. Introduction
11. Experimental Section
Bimetallic catalysts are of interest for strong practical as well as intellectual One route to bimetallic catalysts is the use of heterometallic molecular clusters as precursor^.^-^ This approach offers the potential of controlling the composition of the derived catalyst particles,6 allowing one to probe the role of the individual components in a bimetallic catalyst. A constant concern with the bimetallic cluster precursor approach, however, is whether the different metals will segregate either immediately upon contact with the support or under the conditions necessary to form an active ~ a t a l y s t . ~ - ~ Previous work from these laboratories dealt with the compounds CpxWxlr4-x(CO)lz-x(Cp = qS-CsHs,x = 1,2) as precursors to alumina-supported tungsten-iridium catalysts.I0 Evidence for unique catalytic properties dependent on the specific cluster precursor was developed from chemisorption and catalysis data. This paper deals with the corresponding molybdenum-iridium clusters CpMolr,(CO),, and Cp2MoZIr2(CO),,,which are prepared by analogous procedures and have analogous structures, consisting of a pseudotetrahedral core of metal atoms.l0."
A. Catalytic Studies. The apparatus used to activate and test supported samples included a gas handling system, chemisorption line, catalysis line, and gas chromatographic analytical system. The gas handling system provided helium, hydrogen, oxygen, carbon monoxide, and argon to both lines, with appropriate drying and deoxygenation pretreatment. The catalysis line was based on the system originally created by HardwickI2 from his association with Brennera73l3 It was a grease-free system capable of maintaining a vacuum of less than 1 X IO4 Torr and pressures greater than 15 psig. Reactant gas streams were exposed to brass valves with kel-F seats, copper and glass tubing, Teflon filters, and Teflon and Viton seals. The high-vacuum manifold was centered around a three-stage glass oil diffusion pump, backed by a two-stage mechanical pump, which could achieve a maximum vacuum of 1 X Torr with a pumping speed of 25 L/s. The U-tube catalysis reactor was designed to accommodate sample preparation in an inert atmosphere box, with anaerobic transfers to and from the catalysis line. ( 1 ) Sinfelt, J. H. Bimetallic Cafalysts;Academic Press: New York, 1983. (2) Strohl. J. K.; King, T. S. J . Catal. 1989, 116, 540, and references
CP
The results of the current work demonstrate that when these heterometallic clusters are deposited on A1203,the active catalysts derived from them exhibit unique properties that are attributed to residual bimetallic interactions in the active state. The unique characteristics of the heterometallic-cluster-derived catalysts are also apparent under examination by X-ray absorption spectroscopy (XAS). 'Current address: Departments of Chemistry and Biochemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, GA 30602.
0022-3654/90/2094- 1 190$02.50/0
therein. (3) Gates, B. C.; Guczi, L.; Knozinger, H., Eds. Metal Clusters in Cafalysis; Elsevier: Amsterdam, 1986. (4) Iwasawa, Y., Ed. Tailored Metal Catalysts; Reidel: Boston, 1986. (5) Braunstein, P.; ROSE,J . In Stereochemistry of Organometallic and Inorganic Compounds; Bernal, I., Ed.; Elsevier: Amsterdam, 1988; Vol. 3. (6) Choplin, A,; Huang, L.; Theolier, A,; Gallezot, P.; Basset, J. M.; Siriwardane, U.; Shore, S. G.; Mathieu, R. J. Am. Chem. SOC.1986, 108, 4224. (7) Brenner, A. In Metal Clusrers; Moskovits, M., Ed.; Wiley: New York, 1986.
(8) Fung, A. S.;Tooley, P. A.; McDevitt, M. R.; Gates, B. C.; Kelley, M. J. Polyhedron 1988, 7, 2421. (9) Bergmeister, J. J., 111; Hanson, B. E. Organometallics 1989, 8, 283. ( I O ) Shapley, J. R.; Hardwick, S. J.; Foose, D. S.;Stucky, G. D.; Churchill, M. R.; Bueno, C.; Hutchinson, J. P. J . Am. Chem. SOC.1981, 103, 7383. ( 1 I ) Churchill, M. R.; Li, Y.-J.; Shapley, J. R.; Foose, D. S.: Uchiyama, W. S. J . Organomet. Chem. 1986, 312, 121. (12) Hardwick, S. J. Ph.D. Thesis, University of Illinois at UrbanaChampaign, 1981. (13) Brenner, A,; Hucul, D. A.; Hardwick, S. J. Inorg. Chem. 1979, 18, 1478
0 1990 American Chemical Society
Catalysts from Alumina-Supported Mo-Ir Clusters
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1191 TABLE I: Quantitative Determination of Evolved Carbon Monoxide and Methane
CO/comprecursor complex(es) plex CP2M%(C0)6 1.6 I~~(CO)I~ 2.6 CpMolrdCO) I I 1.7 CP2MozIr2(CO),, 1.3 1/211r,(C0)12 + 1.7 2CpWWOM (2.9)'
b
CH,/complex 3.5 9.0 9.0 4.2 1.1 (8.0)'
total C,/complex 5.1 11.6 10.7
5.5 9.4
(10.9)"
'These numbers are the weighted average of the entries due to pure Cp2MO2(C0)6.
h4(co)l2and
TABLE 11: Chemisorption Data for Freshly Activated Samples' sample H/Ir CO/Ir O/M cm3 0 2 / g [Mot]b c 0.44 0.055 0.67 0.07 1 1.3 1.2 [Ird 1.2 1.o 0.64 0.133 [Ir4 + 2M021b 0.108 1.o 0.8 0.69 [MoIr,lb 1.16 0.255 [Mo21r21b 1 .o 1.2 '0.2 wt % iridium or equivalent. bThe expression in brackets is an abbreviation that only indicates the precursor(s) to the supported metal species, with no implications regarding actual particle size, shape, or composition. eAdsorptionof CO in this case was ca. 1% of that by the iridium-containingsamples.
e
I 100
200
300
400
TEMPERATURE ('C)
Figure 1. Temperature-programmed reaction (TPR) in flowing hydrogen of (a) CPMOZI~~(CO)IO, (b) CpMoIr3(CO)11, (4 Ir4(CO)ll, (4 Cp&fo2(CO)6,and (e) (Ir4(CO)12 + 2Cp2M02(CO)6]deposited on alu-
mina-C.
The support was Degussa Alumina-C. In order to improve handling of this fumed material, it was slurried with water, dried at 115 OC, and then calcined at 500 OC for 2 h. The resulting chunks were crushed and sieved to 60-80 mesh. The BET surface area of this material was measured as 44 m2/g. Prior to use the support was calcined with dry oxygen at 500 OC for 1-2 h, evacuated at 500 OC for 1 h, cooled to room temperature in vacuo, and then taken into the drybox. The organometallic compounds were adsorbed onto this material from cyclohexane solutions. The standard conditions used for temperature-programmed reaction (TPR) of the supported clusters were 40 cm3/min of flowing H2and a heating rate of 4 OC/min. Chemisorption measurements were determined volumetrically by using conventional procedures (double isotherm^).'^ Catalytic activity for n-butane hydrogenolysis was monitored by gas chromatography. Conversions were generally kept to less than 5%. Activities at different temperatures were bracketed by runs at the standard temperature (21 5 "C) to minimize possible catalyst deactivation.lS Both activities and product distributions showed good reproducibility at any given temperature. The flow was generally 60 cm3/min of 5% butane in hydrogen. (14)
McVicker, G. B.; Baker, R. T. K.; Garten, R. L.; Kugler, E. L. J .
Carol. 1980, 65, 207.
(15) Yates, D. J. C.; Taylor, W. F.;Sinfelt, J. H. J. Am. Chem. SOC.1964, 86, 2996.
B. XAS Instrumentation and Analysis. All XAS data were obtained on wiggler beam line VII-3 under dedicated conditions at the Stanford Synchrotron Radiation Laboratory (SSRL). The monochromator utilized two Si[220] flat crystals in a conventional double-crystal arrangement. Sample cells for model compounds were made from aluminum with Mylar tape windows. These cells proved inadequate for activated samples, which were measured in situ in a furnace/cryostat designed by Dr. F. Lytle and borrowed from Dr. G.Via and Dr. J. Sinfelt of Exxon. This unit accommodated in situ activation at 500 OC in hydrogen and XAS data collection at liquid nitrogen temperature. The sample holder was a beryllium boat, which was incorporated in an insert placed within the sample chamber for activation and data collection. The addition of a valve system allowed the samples to be loaded and maintained in an air-free environment. Three types of detectors were used: gas ionization chambers for transmission work and an array of scintillation counters16 or an ionization chamber with Soller slits for fluorescence detection of samples in the furnace/cryostat. XAS data on pure samples were collected by standard transmission detection. Supported samples, due to their low metal loading (0.2 wt a), required fluoresence detection. The Kr-filled furnace/cryostat utilized Soller slits between the 25-pm zirconium filter (for Mo XAS) and the ion chamber to reduce the amount of filter fluorescence. The data reduction and analysis were performed as described elsehere.'^,'^ C S ~ [ M O ~ O ~ ( C ~ O ~ )was ~ ( provided H ~ O ) ~by ] Dr. S. P. Cramer; the crystal structure of the compound is reported in the 1iterat~re.I~
111. Results and Discussion
A . Activation (Temperature-Programmed Reaction) of Supported Clusters. The supported clusters were activated by passing hydrogen gas over a freshly impregnated sample while increasing the temperature to 500 OC. This temperature-programmed reaction (TPR) resulted in the desorption of small molecules, with carbon monoxide and methane comprising the bulk of gas desorbed. The carbon monoxide evolution profiles were very similar for all samples; detection occurred initially near ambient temperature and ended at approximately 150 OC. Methane evolution Rev.Sci. Instrum. 1981, 52, 395. ( I 7) Scott, R. A. Methods Enzymol. 1985, 117, 414. (1 8) Scott, R. A. In Physical Techniques in Biological Research; Roseau, D., Ed.;Academic Press: New York, 1985; Vol. 2. (19) Bino, A.; Cotton, F. A,; Dori, 2.J . Am. Chem. SOC.1978, 100, 5252. (16) Cramer, S. P.; Scott, R. A.
Shapley et al.
1192 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 TABLE 111: Hydrogenolysis of n-Butane'
selectivity at 215 O C turnover a t 215 "C, catalyst % C, % C2 % C3 molecule/s Ir 1.4 X m41 15.4 74.3 10.3 1.8 X 16.0 72.0 12.0 9.6 2.9 X 10-l [Ir, 2M02] 15.6 74.8 9.6 3.5 x 10-2 16.2 74.2 10.0 X [MoIrJ 18.1 71.3 10.6 8.0 X 18.8 70.5 10.7 2.0 x 10-2 [Mo21r,] 31.0 53.9 15.1 3.0 X 30.2 54.6 15.2
+
L
'Total sample size 500 mg,0.2 wt 76 Ir, on alumina-C. Flow rate 60 cm'/min of 5% n-butane in hydrogen mixture.
ic
100
200
300
E,, kcal/mol 41 42 41 39 35 36 36 36
o i
400
TEMPERATURE ('C)
-5
Figure 2. TPR methane profile of chemisorbed carbon monoxide on (a) [Mo21r2I, (b) [MoIrJ, and ( c ) [ I d .
began typically above 100 OC, but by 500 OC it had decreased to below the limit of our flame-ionization detector. Therefore, this was deemed a suitable temperature of activation. Our column system did not allow the determination of evolved cyclopentadiene or its possible hydrogenation products; however, the standard activation treatment was expected to remove the cyclopentadienyl ligand as well. Methane evolution profiles depended in a characteristic way on the precursor complexes. The profiles for all of the systems examined are compared in Figure 1. Both the Cp2M021r2(CO)loand CpM~Ir~(CO)~~-derived samples displayed a low-temperature shoulder at approximately 250 OC followed by a maximum at approximately 320 OC. These profiles are virtually identical with those observed previously for the Cp2WzIr2(CO)lo- and CpWIr3(CO)I,-derived materials.I0 The supported Ir4(C0)1z sample showed a low-temperature maximum (approximately 260 "C) with a high-temperature shoulder (approximately 330 "C), whereas the profile for supported C ~ , M O ~ ( C O was ) ~ a broad structureless peak. The profile for the stoichiometric mixture appeared to be the superposition of the methane evolution profiles of the separate components and was distinct from those displayed by the heterometallic-cluster-derived samples. Variations in the metal concentration (0.1-0.4 wt 5% iridium) and the amount of sample (0.3-0.6 g) undergoing TPR did not alter the profile. Ethane and propane evolutions were detected in all samples. All iridium-containing samples exhibited ethane maxima at both 225 and 325 OC. The quantities of ethane evolved reached as much as 25% of the methane evolution maximum, whereas much less propane was detected. The CpzMozIr2(CO)lo-derivedsample displayed maxima at 225 and 325 OC, paralleling the ethane maxima. All other samples showed only one propane maximum at 310 "C. Quantitative measurement of evolved methane and carbon monoxide was attempted by trapping the effluent on silica gel cooled to liquid nitrogen temperature, followed by desorption and G C analysis. The resultant values are listed in Table I, and they show generally good agreement between the total C I detected (CO + CH4) and the number of C O S initially present in the adsorbed precursor. In the particular case of Ir,(CO)lz the quantities of C O and CH4 evolved are closely comparable with values reported by Hucul and Brenner.20 The only sample that was clearly (20) Hucul, D. A.; Brenner, A. J . Am. Chem. SOC.1981, 103. 217.
-6
, 1.96
, 1.98
2.00
I
2.02
1000/T
2.04
2.06
2.08
(K-')
Figure 3. Arrhenius plots for n-butane hydrogenolysis: (e) [MoIr3]; (*) [Mo21r21;( 0 ) [Ir4 + 2M021; (A) [Ir41.
anomalous was C ~ , M O ~ I ~ , ( C Ofor ) , ~which , the quantity of methane evolved was low. As noted before, this sample had enhanced ethane and propane production during TPR. However, the quantities of ethane and propane did not appear to be substantial enough to account for the C1discrepancy. Perhaps higher chain hydrocarbons, not detected, accounted for the remaining amounts. B . Chemisorption Measurements. Chemisorption data were collected as partial physical characterization of the activated cluster-derived materials. (These will be denoted as [Mol], [Ir4], [Ir4 2M02], [Mo21r2],and [MoIrJ as an indication of the specific precursor(s) .) The quantities of irreversibly adsorbed hydrogen, carbon monoxide, and oxygen are summarized in Table 11. The H/lr ratios for [MoJrz], [MoIr,], [Ir4], and [Ir4+ 2M02] were all 1.O or greater. The CO/Ir ratios observed were generally lower than the H/Ir ratios, falling between 0.8 and 1.2 CO/Ir for supported [MoIr3], [Ir4], and [Ir4 2M02]. In contrast, [Mo21r2]had a slightly greater CO/Ir than H/Ir ratio (1.2 vs 1,O), Previous studies of highly dispersed iridium particles on alumina have observed H/Ir and CO/Ir ratios between one and two, although the specific value in this range may depend on the morphology of the metal atom-support aggregate.14*2' The hydrogen and carbon monoxide chemisorption values observed in the current work, although consistent with high dispersion of the iridium atoms, are slightly lower than those determined in the earlier work with the analogous W/Ir clusters.I0 This may have resulted from the lower surface area of the alumina used in the current study. Oxygen differed from the other absorbate in that it apparently chemisorbed to both iridium and molybdenum. The sum of the oxygen quantities adsorbed by the homometallic samples [Ir4] and [ Mo,] was 0.126 cm3/g, which is in good agreement with the value of 0.1 33 cm3/g measured for the stoichiometric mixture [Ir4 + ~ M o , ] . I n contrast, [MoIr,] adsorbed 0.108 cm3/g, which was
+
+
(21) Tanaka, K.; Watters, K. L.; Howe, R. F. J. Carol. 1982, 75, 23. (22) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J . Caral. 1987, 105, 26.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1193
Catalysts from Alumina-Supported Mo-Ir Clusters TABLE IV: Effects of Sintering on n-Butane Hydrogenolysis Properties 76 c,
catalvst [Ir4 + 2M02]
treatment conditions 10 min, 02.500 O C 30 min, 02, 500 "C 10 min, 0,. 500 "C IO min, 02, 500 OC
sintered
unsintered
re1 activity' (sintered/ unsintered)
71 56
77
0.4
76 0.2 [1r41 57 71 0.2 [Molr,l 48 54 0.1 [ M021r21 'Samples were reactivated by treatment with H2up to 500 "C.
i e
0.42
t
z
i
0.22
0.02 19.95
19.98
20.01
I
i 19.95
19.98
20.01 ENERGY
20.04
20.07
20.10
(hV)
Figure 5. Mo K edge fluorescence excitation XANES spectra of activated [Mo21r2](a) after a 15-minexposure to air, (b) during reactivation, and (c) after reactivation was complete.
20.04
20.07
20.10
ENERGY (k*V)
Figure 4. Mo K edge fluorescence excitation XANES spectra of activated [Mo21r2]after extended exposure to air. The spectra of [Mo,] and [Ir4 + 2M02] after similar treatment are identical.
clearly more than that for iridium only (0.071 cm3/g) or that calculated for segregated molybdenum and iridium (0.090 cm3 g). The [Mo21r2]material adsorbed much more oxygen (0.255 cm /g) than any other sample, almost twice as much as its corresponding stoichiometric mixture. Oxygen chemisorption has been widely used to characterize catalytic surface areas in Mo-containing catalysts, e.g., for HDS,23 although the relationship between the specific O/Mo ratio observed and the structure and oxidation state of the Mo site is not well-established. In a previous study of M O ~ ( ~ , - C , Hon~ )y-~ alumina, 0.6% Mo samples reduced at 585 O C took up oxygen at 0 O C to give a O / M o ratio of 0.97; the oxidation state was claimed to be predominantly M O ( I I ) . ~It~ is likely that the Mo sites in our activated [Mo2] sample are more highly oxidized, which is consistent with the lower oxygen uptake. However, in order to account for the enhanced oxygen uptake in the [Mo21r2] sample, the corresponding Mo sites must be either less highly oxidized or positioned adjacent to other metal atoms. Note that XANES data for the [Mo21r2]material (see below) suggest the formation of M e 0 moieties upon exposure to oxygen, which tends to support the former explanation. As long as the activated samples were exposed only to mild conditions, e.g., air at room temperature, reactivation in H2 at 500 OC followed by chemisorption at 25 OC showed no change in the amount of gas adsorbed. Alumina-supported iridium samples are known to sinter upon exposure to air at 500 OC;I4 the large I r 0 2 particles produced can be reduced again to metal, but the loss of surface area is reflected in much lower H/Ir ratios. The [ Ir4] sample had a H/Ir ratio of 1.O after 30-min exposure to air at 500 OC, compared to 1.3 H/Ir before. Sintering apparently occurred much more rapidly in the [Mo21r2]and [MoIr,] samples with this treatment, since only a few minutes exposure was required to reduce the hydrogen chemisorption values from 1.0 to 0.3 H / h . Extending the reduction time (at 500 "C in hydrogen) for the sintered samples did not increase the hydrogen chemisorption values. It is interesting that the presence of Mo apparently promotes the sintering of iridium under these conditions, whereas the presence of Pt in bimetallic Pt/Ir/A1203 catalysts has been shown to inhibit the agglomeration of iridium.'*25
i
/-------I
I
*
(23) Tauster, S. J.; Pecoraro, T. A.; Chianelli, R. R. J . Calal. 1980, 63, 515. (24) Iwasawa, Y.; Sato, Y . ;Kuroda, H. J . Carol. 1983, 82, 289.
J 19.95
19S8
20x)l ENERGY
20.04
20.07
20.10
(bV)
Figure 6. Mo K edge fluorescence excitation XANES spectra of activated samples of (a) [MoIr,], (b) [Mo21rZ],(c) [Ir, + 2M02],and (d) [Mo2](during parasitic time at SSRL).
C . Temperature-Programmed Reaction of Chemisorbed Carbon Monoxide. The TPR of chemisorbed carbon monoxide showed profiles that were characteristic of active iridium sites in the specific sample. Methane evolution profiles of [MoIr,], [Mo21r2],and [1r4] are shown in Figure 2. The profile obtained for [Ir4 + 2M02] was identical with that for [Ir4], with a single methane evolution maximum at 240 OC. The [Mo21r2] and [Mo1r3]samples evolved methane won after heating began. They showed a low-temperature shoulder (100 "C), followed by the methane evolution maximum at 170 and 195 OC for [Mo21r2]and [ Molr,], respectively. All of these profiles were reproducible, and all samples showed a weak ethane evolution maximum at 300 O C as well. The profiles of the sintered samples (after exposure to air at 500 "C) had a single methane evolution peak similar to that observed initially for [Ir4]. Since these modified samples are likely to have relatively large iridium particles, the dispersion of iridium is apparently not distinguished by this technique. D. Hydrogenolysis of n-Butane. One of the simplest hydrocarbon conversion reactions for which selectivity as well as activity can be determined is the hydrogenolysis of n-butane. Furthermore, a previous study of supported iridium catalysts showed that product selectivity was a strong function of particle size, with smaller particles showing high selectivity for ethane and larger particles ( 2 5 ) Huang, Y.-J.; Fung, S . C.; Gates, W. E.; McVicker, 1989, 118, 192.
G.B. J . Calal.
1194 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
-41 0
'
1
'
I '
"
'
"
8
4
'
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I
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-20' 0
Shapley et al.
'
"
12
16
20
12
16
20
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20
-7L 0
'
'
4
'
'
8
1
o
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Figure 7. Mo K edge EXAFS of (a) CS~[MO~O~(C~O~)~(H~O)~], (b) CpMoIr3(CO)11,(c) activated [Mo21rz], (d) activated [MoIr,], (e) activated [Ir, + 2M02], and ( f ) air-exposed [MoIr,]. All data were collected at 80 K. Note the scale differences on the ordinates.
a more nearly random cracking pattern.26 Results of catalytic studies with our cluster-derived materials are listed in Table 111, with representative Arrhenius plots shown in Figure 3. Both the AI20,-C support and the [Mo2] sample were inactive under reaction conditions (190-235 "C), although A1203-Calone showed some activity above 280 O C . The [Ir4] and [Ir4 2Mo,] samples demonstrated similar catalytic properties in terms of both activity and selectivity; that is, the presence of the molybdenum in the stoichiometric mixture had no significant effect on the catalytic properties of the iridium sites. The results for these catalysts were in close agreement with the values reported% for highly dispersed indium, specifically, 76% ethane, 2 X molecule/s Ir(surface), and E, = 42 kcal/mol. The [MoIr3] catalyst maintained a high selectivity toward ethane production, but it was much more active at 21 5 OC and had a lower activation energy. The [Mo21r2]catalyst exhibited a significantly lower selectivity toward ethane (54%), with similar activity to the [Ir4] catalyst but a lower activation energy. Since the differences in catalytic properties displayed by the heterometallic-clusterderived catalysts cannot be ascribed to major differences in iridium particle size, on the basis of the chemisorption data described above, the modified activity and selectivity patterns provide strong evidence for residual molybdenum-iridium interactions in the activated materials that modify the character of the catalytic sites. A closely similar pattern of activity and selectivity differences was observed in the previous study of W/Ir-cluster-derived catalysts,1° which suggests that the effect of the group VI metal on the catalytically active platinum metal is essentially the same in both cases.
+
It is premature to offer a detailed mechanistic explanation for these selectivity differences; however, some connections with other observations can be made. In an extensive study of hydrocarbon hydrogenolysis on two single-crystal faces of iridium?' it was found that the hydrogenolysis of n-butane showed a high selectivity for ethane on the more open, corrugated (1 IO)-( 1X2) face but a high selectivity for methane on the close-packed (1 11) face. It was suggested that the intermediate leading to ethane was a metallocyclopentane (Le., 1,4-coordination at one iridium atom) and that this intermediate was sterically inhibited on the smoother surface. This result correlates with the structure sensitivity displayed by supported iridium particles for this reaction,26since the smallest particles have the greatest fraction of rough faces. A similar explanation may obtain for the heterobimetallic-cluster-derived catalysts, i.e., that the [MIr,] catalysts contain a sufficient number of sterically unhindered Ir sites to allow high ethane selectivity, the same as that displayed by the highly dispersed [Ir4] catalyst, whereas the [M21r2]catalysts have sterically hindered Ir sites and therefore cannot form a metallocyclic intermediate. However, since supported molybdenum itself displays a demethylation selectivity pattern:* the specific influence of the adjacent Mo centers must be considered in regard to both selectivity and activity differences in the bimetallic catalysts. Exposure of the samples to dry oxygen at 25 O C (chemisorption) and subsequent reactivation did not alter the catalytic properties. Sintering at 500 OC in oxygen did cause a marked change in catalytic properties, consistent with a large decrease in active metal (27) Engstrom, J. R.; Goodman,D. W.; Weinberg, W . H. J . Am. Chem.
SOC.1988, 110, 8305.
(26) Foger, K.; Anderson, J. R. J . Catul. 1979, 59, 325.
(28) Nakamura, R.; Burwell, R. L., Jr. J . Curd. 1985, 93, 399.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1195
Catalysts from Alumina-Supported Mo-Ir Clusters 40
1
a
0
30,
IC
15
,
I
3.0
45
6.0
0
7.5
1
4.0
40
Id'
15
30
4.5
6.0
75
1
'
f
Figure 8. Fourier transform ( E = 20005 eV, k = 5-15 A-1, k3-weighted) of Mo K edge EXAFS collected at 80 K: (a) C~,[MO~O~(C~O,)~(H~O),~, (b) CpMoIr,(CO)II,(c) activated [Mo21r,], (d) activated [MoIr,], (e) activated [Ir, + 2M02], and (f) air-exposed [MoIr,]. Note the scale differences on the ordinates.
surface area (Table IV). These results provide further evidence that [Mo21r2]and [MoIr3] agglomerated more rapidly than [Ir,], since a large drop in activity and selectivity occurred after 10-min exposure. The properties of [Ir,] decreased to the same extent only after 30-min treatment. E . X-ray Absorption Spectroscopy ( X A S ) . XAS studies of bimetallic catalysts have been undertaken with increasing frequency in an attempt to determine a structural basis for their unique catalytic properties.',2e32 We present preliminary Mo K edge data herein for our Mo-Ir-cluster-derived catalysts. Initial supported sample data, collected under ambient conditions, indicated that air contamination was occurring. The Mo K edge spectrum of activated [ Mo21rz] collected under these conditions is shown in Figure 4. The distinct shoulder at ca. 20010 eV is known to occur when Mo=O bonds exist.', A very similar Mo XANES spectrum has been reported for a material derived from supporting the heterobimetallic cluster CpzMo2FezS2(CO)8on AI2O3followed by heating in hydrogen.29 Figure 5 shows that the features caused by air contamination in our system can be eliminated by in situ activation, and therefore this was adopted as standard procedure. Figure 6 displays the Mo edges of in situ activated [MoIr3], [Mo21r2],[Ir,], and [Ir, + 2M02] collected at liquid nitrogen (29) Curtis, M. D.; Penner-Hahn, J. E.; Schwank, J.; Baralt, 0.;McCabe, D. J.; Thompson, L.; Waldo, G. Polyhedron 1988, 7 , 2411. (30) Ichikawa, M.; Fukushima, T.; Yokoyama, T.; Kosugi, N.; Kuroda, H.J . Phys. Chem. 1986, 90, 1222. (31) Asakura, K.; Iwasawa, Y. J . Chem. Soc.,Furuduy Trans. 1 1988.84, 2445. (32) Tzou, M. S.; Teo, E. K.; Sachtler, W. M .H. Langmuir 1986, 2, 773. (33) Kutzler, F. W.; Scott, R. A.; Berg, J. M.; Hodgson, K. 0.; Doniach, S.; Cramer, S. P.; Chang, C. H. J . Am. Chem. SOC.1981, 103, 6083.
temperature. The Mo edges of the various catalyst samples corroborated the catalytic results that each heterometallic-cluster-derived sample was unique. [Ir, 2M02] and [Mo,] exhibit the same characteristic edge features (Figure 6c,d), indicating that the Mo remains segregated from the Ir in the activated stoichiometric mixture. Mo EXAFS data were collected on the model compounds Cs2[Mo,04(C204)3(H20)3] and C p M 0 1 r ~ ( C 0 ) in ~ , order ~ ~ to define Mo-Mo and Mc-Ir interactions, respectively. EXAFS data also were collected on in situ activated samples of [MoIr,], 2M02] as well as on [MoIr,] following [Mo21r2],and [Ir, exposure to moist air. The raw EXAFS data are shown in Figure 7 . The two pure compounds and the air-exposed sample of [ MoIr,] have significant EXAFS intensities, whereas the anaerobic activated samples display relatively low intensities (normalized per Mo). This suggests significant heterogeneity of Mo sites in the activated samples. For this reason and because of significant low-frequency, background contributions to the EXAFS of the activated samples (see Fourier transforms in Figure 8), we have not attempted quantitative curve-fitting analyses of these data. The Fourier transforms (FTs) of the Mo EXAFS data for the model compounds Csz[M~304(C204)3(HZ0)3] (Figure 8a) and C p M 0 l r ~ ( C 0 )(Figure ,~ 8) are dominated by Mc-M peaks at R' = 2.1 A (RMo-Mo = 2.486 A) and 2.4 A (RMeIr = 2.824 A), respectively. Figure 8d shows that the activated [MoIr3] sample retains a Mo-Ir interaction as does the activated [Mo,Ir,] sample (Figure 8c). In contrast, the activated mixture [IT, + 2MoJ sample provides evidence of a fragmented molybdenum dimer, since no Mo-M interaction is observed. This structural difference is presumably related to the inability of the mixture of supported homometallic clusters to mimic the catalytic behavior of the supported heterometallic clusters.
+
+
J . Phys. Chem. 1990, 94, 1196-1203
1196
Exposure of the activated [Mob3] sample to air resulted in a return of FT intensity to the Mo-M peak (Figure 80, signaling a return to a more nearly homogeneous Mo site structure. This implies that the clusters were not highly fragmented during activation and that air exposure resulted in recondensation of cluster species. It should be noted, however, that the position of the new FT peak is nearer to that expected for Mo-Mo than for Mo-Ir as seen in the EXAFS of the precursor cluster (Figure 8b). In addition, the activated [MozIrz] sample did not show the same behavior. Examination of the Ir EXAFS should improve our understanding of these supported cluster transformations.
Acknowledgment. This research was supported by grants from NSF to the Materials Research Laboratory at the University of Illinois (DMR 8316981 and 8612860). The authors appreciate the equipment graciously made available by Drs. J. H. Sinfelt and G. Via. The XAS data were collected at the Stanford Synchrotron Radiation Laboratory (SSRL), which is operated by the Department of Energy, Division of Chemical Sciences. R.A.S. is a National Science Foundation Presidential Young Investigator. Registry No. CpMoIr3(CO)II, 124442-18-8; Cp2M021r2(CO)lo, 124442-19-9; Ir4(CO)12,18827-81-1; Cp2M02(CO),, 12091-64-4; Ir, 7439-88-5; Mo, 7439-98-7; butane, 106-97-8.
Nature of Bonding in Transition-Metal Aluminldes Leo Brewer Department of Chemistry, University of California, and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, California 94720 (Received: April 3, 1989)
The binary systems of aluminum with transition metals are very complicated with many intermediate phases. Conventional chemical bonding models with consideration of availability of different electronic configurations and the role of generalized Lewis acid-base interactions provide an understanding of these systems. The stable phases of 77 binary systems of aluminum are characterized.
Introduction It is a pleasure to participate in honoring Professor Harry Drickamer. He has developed a wide range of techniques for study of materials under pressure. His measurements of the effect of pressure upon properties of materials have had a broad application to the understanding of materials under all conditions. I have found his results stimulating. The present paper follows Drickamer's lead in illustrating the manner in which understanding of the role of different electronic configurations and the role of generalized Lewis acid-base interactions can improve our understanding of the properties of materials. This discussion of bonding in aluminides is in part a response to questions raised in my graduate solid-state chemistry course last year. It was noted that A1 with nontransition metals such as Zn to Hg, Ga, to T1, and Si to Pb forms simple binary phase diagrams with no intermediate phases. The question was posed: Why are the phase diagrams for A1 with transition metals so complicated with as many as fourteen stable intermetallic phases and many more metastable phases? A consideration of the character of the chemical bonding in these systems provides a clear understanding. There are a number of factors that control the strength of chemical bonding. The contribution of each factor usually varies in a smooth way from element to element, allowing simple interpolation for an unstudied system. As one moves across the periodic table, some of these factors increase bond strength and some decrease bond strength. It is necessary to separate the contributions of these factors to gain an understanding of the variations of stability of different structures, as they may respond in different ways to yield an irregular variation of the net bonding. The discussion will be divided into three parts. The first will present a brief review of the chemical bonding factors with references to more complete discussions. The second part will deal generally with the applications to bonding in aluminides. The third part will present a detailed review of the stable intermediate phases of binary systems of AI with a large fraction of the elements 0022-3654/9O/2094- 1 196$02.50/0
(including transition metals, lanthanides, and actinides) and a discussion of the role of chemical bonding factors in determining the structures of these phases.
Role of Electrons in Chemical Bonding A. Promotion to Valence States. Almost all elements with two or more valence electrons will have a nonbonding pair in the outer s orbital.' As indicated in Table I, promotion of an s electron to a vacant p orbital is necessary to utilize all of the valence electrons of Mg, AI, and Si. For the transition metals, promotion involving d orbitals is also important. The promotion energies can be quite large and must be offset by the additional bonding contribution of the electrons that become unpaired in the atomic valence state. Tabulations of promotion energies determined from spectroscopic data are Where spectroscopic data are incomplete, particularly for lanthanides and - ~ variation of actinides, missing values can be p r e d i ~ t e d . ~ The bonding strengths of the various orbitals with atomic numbers have been discussed in B. Bond Orders. Pairs of magnesium atoms with lsz2sz2p63s2 'Sground states cannot form chemical bonds and they experience only weak van der Waals interactions. The promotion energies to a 3s3p 3P state are larger than the resulting double bond energy, ( 1 ) Moore, C. E. Atomic Energy Leuels; US.Government Printing Office: Washington, DC, 1949; Vol. I; 1952, Vol. 11; 1953, Vol. 111. (2) Martin, W. C.; Zalubas, R.; Hagan, L. Atomic Energy Leuels-The Rare Earth Elements; US.Government Printing Office: Washington: DC, 1978; NSRDS-NBS 60. (3) Brewer, L. J . Opt. Soc. Am. 1971, 61, 1101-11, 1666-86. (4) Brewer, L. In Systematics and the Properties of the Lanthanides; Sinha, S . P., Ed.; D. Reidel: Boston, 1983; pp 17-69. (5) Brewer, L. High Temp. Sci. 1984, 16, 1-30. (6) Brewer, L. In Phase Stability in Metals and Alloys; Rudman, P., Stringer, J., Jaffee, R. I., Eds.; McGraw-Hill: New York, 1967; pp 39-61,
242-5, 344-5, 560-8. (7) Brewer, L. In Alloying, Walter, J. L., Jackson, M. R., Sims, C. T., Eds.; ASM International, 1988; pp 1-28.
0 1990 American Chemical Society
