Faraday Discuss. Chem. SOC., 1989, 87, 227-237 Carbon Monoxide Hydrogenation Selectivity of Catalysts derived from Ruthenium Clusters on Acidic Pillared Clay and Basic Layered Double-hydroxide Supports Thomas J. Pinnavaia,* M. Rameswaran, Emmanuel D. Dimotakis, Emmanuel P. Giannelis and Edward G. Rightor Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824, U.S.A. Acidic pillared clays, e.g. alumina pillared montmorillonite (APM), and basic layered double hydroxides, e.g. hydrotalcite (HT), provide well defined surface environments for dispersing metal-cluster carbonyl complexes. In the present work, FTIR spectroscopic studies have been used to elucidate the surface organometallic chemistry of R U ~ ( C O ) , ~ on APM and HT.For APM as the support, cluster binding occurs initially by protonation to form HRu7(CO)T2 cations on the intracrystalline gallery surfaces of the clay. Further reaction results in the grafting of mononuclear sites of the type [Ru(CO), (OAIZE)~],, ( x = 2 , 3 ) to the pillar surfaces. The reaction of R u ? ( C O ) , ~ with HT affords chemisorbed HRu,(CO);, anions which can be transformed t o surface-bound [Ru(CO), (OM=):],, ( M = Al, Mg) com- plexes analogous to the grafted species on APM. The reduction of the grafted complex o n both supports results in active ruthenium catalysts for CO hydrogenation. Ru-APM exhibits very high selectivity for isomerized hydrocarbons (branched alkanes and internal alkenes). The isomerized products arise from the unique texture and bifunctional nature of Ru-APM; the clay-embedded ruthenium catalyses Fischer-Tropsch chain propagation, and the intracrystalline Bronsted acidity of the clay host catalyses alkene rearrangements through carbenium-ion mechanisms.In contrast, the Ru-HT system gives very different product distributions containing a high fraction of oxygenates, specifically methanol and lesser amounts of C,-C, alcohols. The high alcohol selectivity, which i s atypical for CO hydrogenation over Ru, is ascribed in part to the inhibition of CO dissociation on the metal particles by decoraments provided by the highly basic support. Catalysts derived from smectite clays were introduced in the petroleum industry when the fixed-bed Houdry process came on stream in 1939.' Following World War 11, clay-cracking catalysts were replaced by mixed-metal oxides and, eventually, by synthetic zeolites with improved steam stability and shape selectivity.However, recent develop- ments in the intercalation of smectite clays by robust polyoxometal oligomers has rekindled interest in clays as catalyst precursors.' This resurgence of interest is due partly to the possibility of developing materials known as metal-oxide pillared clays having unique two-dimensional (2D) galleries and zeolitic microporosity. For instance, montmorillonite clays interlayered with alumina or zirconia aggregates are highly selective acid catalysts for petroleum cracking,'-' alcohol dehydration7-10 and other acid-catalysed reactions."." Recently, reactive transition-metal centres have been introduced into the galleries of metal-oxide pillared clays in an effort to enhance the catalytic versatility of these materials.Initial approaches to metal immobilization on pillared clays have utilized impregnation techniques. ''*I4 Often, the latter methods lack the specificity needed to form clean metal crystallite~.'~-'' On the other hand, more specific metal immobilization 227228 C 0 Hydrogenation Selectivity methods based on specific surface organometallic reactions18 can afford well defined molecular species on the support which can be converted to well characterized catalytic materials. Initial studies of alumina pillared montmorillonite" indicate that the intercalated oxide reacts with metal cluster carbonyls to afford surface-grafted species analogous to those formed on the surfaces of bulk alumina.The layered double hydroxides (LDHs) of the type [ ( M ~ ~ ~ , M . ~ ' ( ~ H ) , I [ A ~ - I . ~ , ~ . z H ~ o are complementary to smectite clays insofar as the layers are 2D hydroxycations and the gallery species are anions.20*21 LDHs pillared by polyoxometallate anions have been reported recently,22 but most catalytic studies to date have focused on non-pillared hydrotalcite derivatives wherein M" = Mg, M"' - - Al, and An- = CO:-. Upon decompo- sition at elevated temperatures, these materials afford highly basic oxides for vapour- phase aldol reactions and alkene isomerizations. 2 3 ~ 2 4 In the present work, we compare the surface organometallic chemistry of R u ~ ( C O ) ] ~ on an acidic pillared clay and a basic hydrotalcite support.Since the selectivity of a catalyst can depend greatly on the support material, we have also examined the Fischer- Tropsch selectivity properties of the ruthenium catalysts derived from these support systems. Ruthenium was especially suitable for this latter objective, because it is the most active metal known for syn-gas c o n v e r ~ i o n . ~ ~ * ~ ~ Also, the unsupported metal is known to be selective toward formation of straight-chain hydrocarbon products with little or no selectivity for oxygenate f~rmation.'~ Thus, deviations from straight-chain hydrocarbon selectivity and low oxygenate yields could be correlated directly with support effects. Experimental Alumina Pillared Montmorillonite Alumina pillared montmorillonite was prepared by the reaction of sodium montmorillo- nite (Crook County, Wyoming) with aluminum chlorohydrate solution (Chlorohydrol Reheis Chemical Company) containing A1 1304(0H)24( H20)Tl oligomers according to previously described procedures.28329 The product was air-dried on a glass plate and subsequently dehydroxylated to the alumina pillared form by heating at 623 K under vacuum for 2 h. The N2 B.E.T.surface area was 300 m2 g-I, and the X-ray basal spacing was 1.85 nm. Chemical analysis indicated the unit-cell formula of the pillared clay to be [Al(OH )2.8012.87[A13.1 1 Fe,.42Mg0.481( Si7.92AlO.08)020(OH ) 4 . Hydrotalcite Synthetic hydrotalcite was prepared by a coprecipitation method.23 A 3 : 1 molar ratio of Mg(N03)2.6H20 and A1(N03)3.9H20 in distilled water was added to an aqueous solution of NaOH and Na2C03 until the pH of the mixture was 10.0. The resulting slurry was heated overnight at 338 K.The product was then recovered by centrifugation, washed with distilled water, and dried in air at room temperature. The X-ray basal spacing was 0.776 nm. Chemical analysis indicated the unit-cell formula for this material to be [Mg,Al2(0H),,](CO3)~4H20. Surface Reactions of RU~(CO),~ The reaction of Ru,(CO) 1 2 with alumina pillared montmorillonite was carried out according to previously described methods.29 A solution of R U ~ ( C O ) ~ ~ (0.02 mmol) inT. J. Pinnavaia et al. 229 Table 1. Carbonyl stretching frequencies for ruthenium complexes on alumina pillared clay (APM) and hydrotalcite (HT) supports compound frequency/cm- ’ [HRU,(CO);~]-APM [Ru(CO),~(OA~~),],,-APM 2070s, 2000s [HRU,(CO),,I-HT 2074vs, 201054 1984s, 1947vs [Ru(CO),(OM_),],,-HT(M = Mg or Al) 2047s, 1965s 2 128s, 2099s, 2077vs, 2060s, 2030s 40cm3 CH,Cl, was added under an argon atmosphere to 0.50g alumina pillared montmorillonite which had been previously dried under vacuum at 298 K for 4 h.The reaction mixture was allowed to stir for 20 h and then transferred to a nitrogen-filled glove box. The product was filtered, washed with CH2C12 and dried under a stream of argon. The ruthenium loading was 1.0 wt ‘/o, and the X-ray basal spacing was 1.85 nm. The surface reaction of Ru,(CO),~ and hydr~talcite”~” was carried out in a manner analogous to that described above for alumina pillared montmorillonite.Hydrotalcite (1 g) was dried under vacuum at room temperature for 4 h. A solution of Ru,(CO),~ (0.084 mmol) in 100 cm3 of degassed CH2C12 was added, and the reaction mixture was stirred for 20 h at 298 K under an argon atmosphere. The red reaction product was filtered and washed with a small amount of CH2C12. Chemical analysis indicated the ruthenium loading to be 0.45 wt %. Fischer-Tropsch Synthesis The catalytic hydrogenation of carbon monoxide was carried out in a stainless-steel single-pass tube reactor. The reactor tube was fitted with a quartz glass liner and a quartz glass frit to contain the catalyst. All reacting gases were ultra-high purity grade and were purified further by passing them through a manganese/silica adsorbent to remove oxygen, Linde 4A for a molecular sieve to remove water, and an alumina adsorbent at 201 K to remove metal carbonyl contaminants.Samples were analysed using a Hewlett-Packard 5890 Gas Chromatograph equipped with a flame-ionization detector, a thermal conductivity detector and automatic gas-sampling valves. The catalyst supported on alumina pillared montmorillonite was reduced in flowing hydrogen at 673 K for 2 h. Since hydrotalcite begins to decompose at a temperature near 573 K,23 the reduction of ruthenium in this support system was carried out at 548 K under flowing hydrogen for a reaction time of 16 h. The reduction temperatures were achieved at a ramp rate of 5 K min-’. Hydrogenation of carbon monoxide over the reduced catalysts was carried out under differential reaction conditions ( ( 5 % conver- sion) at gauge pressures of between 0 and 200 lbf and at 548 K.Resu 1 t s Surface Organometallic Chemistry The reaction of alumina pillared montmorillonite (APM) with Ru3(CO) 12 in CHzClz solution results in the formation of gallery-bound HRu3(CO)T2 cations on the gallery surfaces.” The infrared frequencies for the terminal carbonyl groups of the protonated cluster (cf: table 1) lie very near ( * 5 cm-’) the frequencies observed for the PF, salt of ? 1 Ibf in-’ = 6.894 76 x lo3 Pa.230 CO Hydrogenation Selectivity HRu,(CO)T2 .I9 Under the conditions of the protonation reaction, the clay retains 4.5 wt YO water. This adsorbed water undoubtedly plays a role in determining the Brgnsted acidity of the gallery regions. As the [HRu,(CO)T2]-APM is allowed to age at 298 K, the protonated cluster reacts further to form pillar-grafted mononuclear ensembles of the type [ Ru( CO),( 0Al=)Jn, where x = 2 , 3 and 0 A l ~ represents aluminate groups on the pillaring aggregates.Conversion of the protonated clusters to the pillar-grafted ensembles is complete within 24 h at 298 K. The vibrational frequencies of the terminal CO groups on the grafted ensembles are provided in table 1. These frequencies lie near those for analogous species formed on the surfaces of bulk The reactions of hydrotalcite (HT) with R U ~ ( C O ) ~ ~ in CH2Cl2 solution results in the formation of surface-bound H Ru3( CO) anions. The infrared vibrational frequencies of the cluster anion, presented in table 1, are in good agreement with those observed for authentic salts of HRu3(CO)LI .,' Upon exposure to air the HRu,(CO),-HT reacts further to form grafted [ Ru( CO),( OM-),]n (M = Mg, Al) species, analogous to the grafted complexes formed on APM. The carbonyl stretching frequencies for the HT-bound complex are provided in table 1 .Carbon Monoxide Hydrogenation The reduction of [Ru(CO),(OAI=),],-APM with hydrogen at 673 K results in the formation of ruthenium crystallites in the acidic microporous support. X-Ray diffraction measurements indicate that the 1.85 nm basal spacing of APM is not altered in the reduction reaction. Previously reported transmission electron microscopy indicate the ruthenium particle size to be <5.0nm. The reduction of [ R u ( C O ) . ( O A ~ ~ ) ~ ] ~ - H T in hydrogen at 548 K also leads to the formation of ruthenium crystallites which can be observed by electron microscopy.TEM analysis indicates the average ruthenium particle size to be 5.9 nm. The LDH support matrix remains crystalline after the reduction step, as evidenced by the presence of 001 reflections in the X-ray diffraction pattern. Also, I R spectroscopy indicated the presence of carbonate in the support matrix. The product distribution obtained for the hydrogenation of carbon monoxide over Ru-APM at 1309 kPa and 548 K follows Anderson-Schulz- Flory (ASF) statistics over the product range C,-C,. As can be seen from fig. 1, the methane yield is much higher than expected based on ASF statistics with a chain-propagation probability of (Y = 0.500*0.002.The high methane yields are presumed to result from hydrogenolysis of higher hydrocarbons. The carbon-number distribution for CO conversion in the highly basic Ru-HT catalyst system also follows ASF statistics. As shown by the data in fig. 2 for products in the C2-C8 range, the chain-propagation probability, (Y = 0.447 f 0.002, is similar to the value observed for the APM support system. Also, the methane yield is much higher than expected based on ASF statistics. Despite the similarities in reactivity and chain-propagation probabilities for ruthenium dispersed on acidic APM and basic HT supports, two dramatic differences in product selectivity are observed for these support systems: ( i ) Ru-AMP exhibits very high yields of isomerized products (branched alkanes and internal alkenes) relative to normal products (n-alkanes and terminal alkenes), whereas Ru-HT affords low ratios of isomerized to normal product ratios, and (ii) Ru-APM yields only trace amounts of oxygenated products (< 1 "/o), whereas Ru-HT affords substantial yields of alcohols in the C,-C4 range.Table 2 provides the ratio of isomerized to normal-chain hydrocarbons obtained for Ru-APM and Ru-HT. For C4-C9 products produced over the acidic APM support, 64-90% of the hydrocarbon chains have been isomerized. In contrast, the C,-C, productsT. J. Pinnavaia et al. 23 1 7 0 60 50 h 8 40 E f 30 '0 x lz 20 10 0 1 2 3 4 5 6 7 8 carbon no. 0 - 1 - 2 - 3 --. 3= v - 4 2 -5 -6 - 7 Fig. I . CO hydrogenation product distribution and Anderson-Shulz- Flory plot for Ru- APM catalyst at 548 K and 1309 kPa.formed over the basic HT support are only 30-38% isomerized. The extent of isomeriz- ation in the Ru-APM system is highly dependent on the H2: CO ratio. As shown in fig. 3, the isomerized to normal hydrocarbon products in the C,-C, range increase substantially with decreasing H., : CO ratio. It is particularly significant that Ru-HT produces substantial yields of methanol, while no methanol was observed with Ru-APM. The selectivity towards alcohol forma- tion depends on the reaction pressure and temperature. The pressure dependence is indicated by the data in table 3. Note that the total alcohol yield increases with increasing pressure. Although the methane yield decreases with increasing pressure, the fraction of total alcohol present as methanol (ca.7 5 % ) changes little over the pressure range investigated. Also, decreasing the temperature of CO hydrogenation over Ru-HT from 548 to 533 K, at 826 kPa increases the alcohol yield from 13.2 to 30.8%. The same temperature change causes the fraction of total alcohol present as methanol to increase slightly from 75 to 8 1'7'0. Discussion Surface Organometallic Chemistry R U ~ ( C O ) , ~ binds initially to partially hydrated APM as the protonated species HRu3(CO)r2. The protonation of the ruthenium cluster carbonyl attests to the strong Bronsted acidity of the pillared clay support. Normally, acid strengths equivalent to 98% H2S04 or trifluoroacetic acid are required for protonation of the cluster in homogeneous s o l ~ t i o n .~ ~ ~ ~ ~ The acidity of the pillared clay is believed to arise from the partial thermal dehydration and dehydroxylation of the intercalated Al 1304(OH)24+x( H20)\7,_",'+ ion to form alumina aggregates and ionizable p r o t o n ~ . ~ ~ - ~ ~232 CO Hydrogenation Selectivity 60 50 40 h 8 3 U v 30 -f! E: V T3 c" 20 10 0 -1 - 2 -3 ; 1 3- - 4 f v - 5 -6 -7 1 2 3 4 5 6 7 carbon no. Fig. 2. CO hydrogenation product distribution and Anderson-Shulz-Flory plot for Ru-HT catalyst at 548 K and 1309 kPa. Table 2. Isomerized/normal hydrocarbon product ratios" for CO hydrogenation over Ru supported on APM and HT (1309 kPa, 548 K) no. Ru-APM Ru-HT 4 1.79 0.45 5 5.32 0.5 1 6 6.8 1 0.63 7 9.75 - 8 6.94 - 9 2.98 - " This ratio is defined as sum of branched hydro- carbons and internal alkenes divided by the sum of n-alkanes and terminal alkenes.Upon ageing, or upon exposure to air, the chemisorbed HRu3(CO)T2 ion is converted to the pillar-grafted complex [ RU(CO).(OA~E)~]~. Grafting to the intercalated alumina pillars rather than the external surfaces of the clay layers is supported in part by the fact that unpillared montmorillonite is capable of binding only trace amounts of the ruthenium complex. Conversion of the pillar grafted ruthenium ensembles to metallic ruthenium crystallites is readily accomplished by reduction with hydrogen at 673 K. Earlier electron microscopy studies have demonstrated that the ruthenium crystallites are embedded within the microporous pillared clay particles with very little rutheniumT.J. Pinnavaia et al. 233 1 ’ I I I I 0.0 0.5 1 .o 1.5 2.0 H*/CO Fig. 3. Dependence of isomerized/normal hydrocarbon ratios for C4-C6 products on the H,/CO reactant ratio for conversion over Ru-APM catalyst at 548 K and 1309 kPa. 0, C,; 0, C,; 0, C,. Table 3. Pressure dependence of CO hydrogenation selectivity over Ru-HT‘ hydrocarbon and total alcohol yields (wt %) alcohol distribution pressure total /kPa C, C? C3 C4 C5 C,, alcohols MeOH EtOH PrOH BuOH 101 85.5 8.8 5.7 tr tr tr tr tr tr tr tr 482 65.2 8.3 7.0 4.5 2.8 1.9 10.0 78 22 tr tr 826 63.5 7.5 7.4 4.5 2.4 1.5 13.2 75 20 5 tr 1171 54.1 7.6 9.1 6.0 3.3 2.5 17.6 78 18 4 tr 1309 52.2 7.3 8.8 5.7 3.2 2.8 20.1 74 17 4 4 “ Temperature = 548 K; H,/CO = 2.0; CO Conversion<S0/~; GHSV = 1000-3000 h-’; Time on stream>24 h.immobilized at external surfaces.29336 The embedding of ruthenium crystallites within the clay particles has important catalytic consequences, as will be discussed below. The organometallic chemistry of R u ~ ( C O ) , ~ on a basic hydrotalcite support is complementary to the acid-mediated chemistry observed on alumina pillared montmorillonite. On the basic support the ruthenium cluster carbonyl undergoes reduc- tive decarbonylation to the H Ru3( CO) 1, anion. Equivalent reductive decarbonylations in homogeneous solution require the presence of very strong bases such as potassium hydr~xide.”~ The basicity of the layered double hydroxide arises from the hydrolysis of the surface carbonate anion,44 not by reaction of the lattice hydroxyls.234 CO Hydrogenation Selectivity R H+ Oxidation of HRu,(CO),,-HT in air results in the formation of grafted [ R~(CO),(OMEZ)~],, analogous to the grafted complexes formed on APM.However, since HT is non-microporous, the ruthenium grafted complexes are restricted to occupy- ing external surfaces only. This latter result is verified by the retention of the 0.77 nm X-ray basal spacing characteristic of the carbonate intercalate of the layered-double hydroxide. Upon reduction in hydrogen at 548 K, ruthenium crystallites with an average particle size of 5.9 nm are observed at the external surfaces of the support by electron microscopy. CO Hydrogenation Selectivity Ruthenium dispersed on conventional silica and alumina supports is highly selective for the catalytic hydrogenation of carbon monoxide to linear terminal alkenes and alkane^.*'^^^ In contrast, ruthenium supported on alumina pillared clay exhibits a very high selectivity towards isomerized products (branched hydrocarbons and internal alkenes) relative to normal products (n-alkanes and terminal alkenes).Between 64 and 90% of the products formed in the C,-C, range are isomerized derivatives (c$ table 2). The high yields of isomerized products obtained for the Ru-APM catalysts system most likely arises from the high Brmsted acidity of the pillared clay support. At Fischer-Tropsch reaction temperatures, the adsorption of carbon monoxide on clean ruthenium surfaces is known to involve a dissociative mechani~rn.~~ When the metal is dispersed on a conventional support, the ruthenium alkyl intermediates are converted to terminal straight-chain alkenes or normal alkanes. However, if the metal is dispersed on an acidic support, the terminal alkenes are capable of forming carbenium ions.The carbenium ions may rearrange via protonated cyclopropane intermediates to branched- chain h y d r o ~ a r b o n s . ~ ~ - ~ ~ The following scheme summarizes the elementary steps involved in converting metal alkyl intermediates to internal alkenes (paths B-D) and to branched-chain hydrocarbons (paths E-G). We propose that the pathways represented in the above scheme are responsible for the high isomerization selectivity of Ru-APM. The importance of terminal alkenes as intermediates is supported by an increase in selectivity towards isomerized productsT.J. Pinnavaia et a1 23 5 Fig. 4. Schematic representation of defect sites occupied by ruthenium in a Ru-APM catalyst. The shaded particles represent Ru crystallites, the filled circles represent alumina pillars and the slabs represent the silicate layers of the clay. with decreasing hydrogen-to-CO ratio (cf: fig. 3 ) . As indicated by pathways A and B in the scheme, the formation of terminal alkenes should be facilitated by a decrease in the hydrogen-to-carbon monoxide ratio. The selectivities towards isomerized products observed in the present work are unusual but not unprecedented. Ruthenium as a syn-gas catalyst supported on the hydrogen-exchanged form of dealuminated zeolite Y affords substantial yields of isomerized hydrocarbons. 27,4950 Nevertheless, the high selectivity of Ru-APM towards isomerized Fischer-Tropsch products even at low conversions, where the fugacity of alkene products is low, remains exceptional.We attribute the selectivity of Ru-APM, in part, to the special textural features of the microporous support. Pillared clays contain numerous defects which arise from distortions of the host layers and mis-matching of layer edges, as illustrated schematically in fig. 4. In the case of Ru-APM, the size of the embedded metal crystallites (1.0-5.0 nm) is larger than the size of the pillared galleries (ca. 0.85 nm). Thus, the ruthenium aggregates appear to be stabilized by the clay defect sites. The encapsulated ruthenium particles are readily accessible to hydrogen and carbon monoxide. When chain propaga- tion is terminated and a terminal alkene is released from the metal site, the alkene is obligated to migrate through the acidic microporous structure of the pillared clay where carbenium-ion formation and chain isomerization can occur.Consequently, Ru- APM is a bifunctional catalyst which serves as a support for the intracrystalline dispersion of ruthenium crystallites for Fischer-Tropsch chain propagation and, at the same time, as an acidic microporous medium for carbenium-ion formation and isomerization. The selectivity of ruthenium supported on basic hydrotalcite also is highly unusual owing to the substantial oxygenate (alcohol) yields obtained at relatively low reaction pressures. Although ruthenium normally adsorbs CO by a dissociative mechanism under syn-gas reaction conditions, basic supports such as MgO and alkali-metal-promoted alumina have been observed to form alcohols.51~5' I t is especially noteworthy, however, that Ru-HT is a much more effective support matrix for improving oxygenate yields over dispersed ruthenium than are MgO or alkali-metal-promoted aluminas.The latter supports require reaction pressures of 8000 kPa to achieve alcohol yields of 20 wt %. In contrast, analogous alcohol yields are obtained with Ru-HT at substantially lower reaction pressures (1300 kPa). The oxygenate selectivities of syn-gas catalysts are known to depend on the support matrix. For instance, in the case of rhodium, basic supports promote methanol formation, whereas non-basic supports provide high yields of hydrocarbons.The methanol selec- tivity of rhodium on basic supports is related directly to the surface coverage of236 CO Hydrogenation Selectivity non-dissociatively adsorbed CO on the metal surface.53 In general, mediation of catalytic selectivity by a support matrix may involve an electronic effect on the dispersed metal ~ r y s t a l l i t e ~ ~ or, alternatively, a support could provide decorants for the surface modification of the metal c r y ~ t a l l i t e s . ~ ~ - ~ ~ Since electronic effects of the support extend over only two to three atomic layers of clean metal, it is unlikely that such effects are operative for the Ru-HT catalyst system where the average crystallite size is 5.9 nm. We propose that the mechanism for oxygenate selectivity in the Ru-HT catalyst arises from the decoration of the metal crystallites by the basic support material.A similar process is responsible for oxygenate formation over rhodium. In this case, the function of the oxide decorament is ( i ) to block metallic Rho surface sites and suppress the production of CH, units through CO dissociation, and (ii) to stabilize non-reduced Rh"+ sites, including the possibility of Rh203 on the surface of the The unreduced Rh"' sites are responsible for methanol formation, whereas the metallic centres on the decorated metal are responsible for C l oxygenate formation.57 The latter species are most likely formed through a metal acyl reaction pathway.58 The increase in oxygenate selectivity for Ru-HT with increasing pressure ( c j table 3) and decreasing temperature is consistent with the proposed inhibition of CO dissociation by support decoraments.This research was supported in part by the National Foundation, Division of Materials Research, through grant DMR-85 14154, and the Michigan State University Center for Fundamental Materials Research. 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