Catalytic Synergism in Physical Mixtures of Supported Iron−Cerium

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Catalytic Synergism in Physical Mixtures of Supported Iron-Cerium and Supported Noble Metal for Hydroisomerization of 1,3-Butadiene Hsuan Chang and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802 Received July 9, 1996. In Final Form: November 11, 1996X In an effort to better understand the excellent synergism of certain physical mixtures of supported catalysts for the 1-butene double bond shift described previously, the butadiene hydroisomerization selectivity/activity of the same mixtures was studied. Limited activity/selectivity synergism for the hydroisomerization of butadiene was measured for physical mixtures of FeCe/Grafoil (alloy) and Pt/Grafoil or Pd/Grafoil (noble metal). A high degree of synergism was noted for mixtures in which the ratio of alloy to noble metal was less than about 20:1. Additions of alloy to mixtures which increased the ratio to values higher than 20 did not change the activity or selectivity. For Pd/Grafoil containing mixtures both activity and selectivity synergism could be explained completely on the basis of a hydrogen spillover model. For example, the limit in activity synergism was ascribed to the depletion of hydrogen atoms away from palladium centers (radial gradient around each noble metal particle) due to the use of these atoms in hydrogenation of butadiene on FeCe particles. In contrast, to explain the nonequilibrium ratio of 2-butenes in platinum-containing mixtures, it is necessary to hypothesize two processes: both hydrogen spillover and bifunctional catalysis.

Introduction Recent articles1,2 suggest that hydrogen spillover3-8 can be “harnessed” to yield highly active and selective catalysts. For example, hydrogen atoms generated by a noble metal can activate bond-shift type isomerization on a physically separated second surface. Specifically, kinetic studies, in which FeCe/Grafoil was added in increments to a bed which initially only contained Pt/Grafoil, showed the physical mixture to be far more active (10×) than the sum of the activities of the individual materials. Indeed, it was found that each incremental addition of FeCe/ Grafoil led to an increase in activity far larger (100×) than that which could be anticipated on the basis of the measured activity of FeCe/Grafoil in the absence of platinum. It was also found that the excellent overall selectivity of the mixture was similar to that of the FeCe surface and far better than that of the Pt/Grafoil. Taken together the dramatic synergistic increase in activity, as well as the excellent selectivity, provides strong support for the hypothesis that in a physical mixture containing Pt/Grafoil and FeCe/Grafoil that hydrogen atoms generated on Pt/Grafoil “spillover” to activate 1-butene bond-shift isomerization on the FeCe/Grafoil. Alternative hypotheses did not appear to be consistent with the data. For example, the finding that palladiumand platinum-containing mixtures behaved very differently in terms of activation energy and selectivity sug* Corresponding author. E-mail: [email protected]. Fax: (814) 8657846. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Lu, W.-C.; Chang, H.; Phillips, J. J. Catal. 1994, 146, 608. (2) Chang, H.; Phillips, J. Langmuir 1996, 12, 2756. (3) Conner, Jr., W. C.; Pajonk, G. M.; Teichner, S. J. Adv. Catal. 1986, 34, 1. (4) Sermon, P. A.; Bond, G. C.; Catal. Rev., 1993, 8, 211. (5) Weng, L.-T.; Delmon, B.; Appl. Catal. A 1992, 81, 141. (6) Bianchi, D.; Gardes, G. E. E.; Pajonk, G. M.; Teichner, S. J. J. Catal. 1975, 38, 135. (7) Willey, R. J.; Teichner, S. J.; Pajonk, G. M. J. Mol. Catal. 1992, 77, 201. (8) Sermon, P. A.; Bond, G. C. J. Chem. Soc., Faraday Trans 1 1976, 72, 730.

gested that “special” sites formed on FeCe surfaces by hydrogen atom spillover during reduction were not responsible for the observed synergism. Still, the unusually large synergism observed suggested the need for additional study to confirm the earlier mechanistic hypothesis. The following study was conducted to test a corollary to the earlier mechanistic hypothesis. To wit: In any reaction in which hydrogen atoms are both “catalytic activators”, and reactants, there should be a finite limit to the synergism observed for physical mixtures. Incremental additions of the surface which is “activated” (acceptor) by the hydrogen atoms (generated by donor) will not lead to unlimited increases in activity. At some point further addition of acceptor material will be ineffectual as the concentration of hydrogen atoms which reach this new surface will be zero. Indeed, hydrogen atoms will be completely “used” to create product molecules on those surfaces closer to the noble metal source of hydrogen atoms. The activity and selectivity for butadiene hydrogenation/ isomerization of physical mixtures of Pd/Grafoil (or Pt/ Grafoil) and FeCe/Grafoil were studied as a function of the amount of FeCe/Grafoil added to a reactor bed initially containing only noble metal/Grafoil. As the reaction of hydrogen with the diolefin to create the olefin requires hydrogen, it was hypothesized that synergistic effects would only be observed over a limited Pt/FeCe ratio. Beyond a finite limit, it was suggested, additional FeCe increments would be found ineffectual as hydrogen atoms would not “reach” the additional surface. As described below the experimental results were completely consistent with the hypothesis. However, selectivity studies indicate that the process is more complex in some cases. It appears that olefins which form on the hydrogen atom acceptor surface (FeCe) will in some instances diffuse back to the noble metal (platinum or palladium) where they react to form butane. This secondary effect is a form of “polyfunctional catalysis”. Thus, in effect both hydrogen spillover and “polyfunctional catalysis” operate simultaneously. This study illustrates limits to hydrogen spillover

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Table 1: Catalyst Composition and Preparationc atomic ratio total metal, wt % nominal measureda,b nominal measureda

catalyst no.

metal content

1 2 3 4 5 6 7

Fe:Ce Ce Fe Pd Pt Fe:Ce:Pt Fe:Ce:Pd

1:1

1:1:0.2 1:1:0.2

1:0.92

1:0.84:0.32 1:0.79:0.19

5 3.6 1.4 5 5 5 5

4.85 3.21 1.48 4.95 4.94 5.33 4.08

a Galbraith Laboratories, Inc., Knoxville, TN. b In plotting data measured values used in all cases. c The following salts were used to prepare aqueous solutions of appropriate concentration for incipient wetness: Fe(NO3)3‚9H2O, Ce(NO3)3‚6H2O, PdCl2, 5 wt % solution in 10 wt % HCl, and PtCl2‚4H2O (Aldrich Chemical Co., Inc., Milwaukee, WI).

as a means to activate selective surfaces and illustrates the impact of concomitant spillover and polyfunctional catalysis. Experimental Section Catalyst Preparation. Six catalysts were prepared on GTAgrade Grafoil (Union Carbide), a moderately high surface area (22 m2/gm), high purity, graphitic material,9-11 via the incipient wetness technique. An FeCe/Grafoil bimetallic catalyst was made, containing approximately the same atomic number of iron and cerium (always present in approximately a 1:1 atomic ratio), as shown in Table 1. Pd/Grafoil, Pt/Grafoil, Fe/Grafoil, and Ce/ Grafoil catalysts were also made. Finally, a multimetallic FeCePd/Grafoil catalyst was prepared. The catalysts were fabricated by adding ground and temperature-treated (900 °C reduction) Grafoil12 gradually to a solution composed of a mixture of one or two aqueous solutions prepared from salts obtained from Aldrich Chemical. Nitrates were used to prepare the iron and cerium solutions, and chloride was used to prepare the platinum and palladium solutions (Table 1). The total metal loadings were around 5% in all cases, except for the noble metal catalysts for which the metal weight loading was about 1%. Kinetics. The 1,3-butadiene hydrogenation/isomerization reaction was studied using a differential Pyrex microreactor operated at 1 atm pressure, as described elsewhere.13,14 The reaction gas was purchased from Matheson and mixed with rotameters to yield a reaction gas with 2% 1,3-butadiene, 18% hydrogen, and helium as balance. Analyses were done using a 5890 Series II Hewlett-Packard gas chromatograph equipped with a TC detector and a packed column containing 0.19% picric acid on Carbograph ZAP 80/100 mesh (Alltech). In all studies intended for the purpose of comparing levels of activity, total conversion was kept to less than 12% in order to justify the “differential reactor” approximation. Also, flow rates were always above 250 cm3/min STP, such that mass transfer limits on measured rates could be ignored at these conversion levels.15 In order to test the impact of mixtures on catalytic activity and selectivity, physical mixtures containing different absolute amounts and different relative amounts of each “catalyst” were required. To change the composition of the catalyst bed, 10 mg increments of catalyst were added through the differential reactor sidearm. Next, after vigorous mixing, all the catalyst material was reduced in flowing hydrogen at 400 °C for 4 h. After the catalyst was cooled, the reaction gas mixture was introduced and the system allowed to equilibrate for 2 h before any data were taken. At least three measurements of activity/selectivity were collected at any set of conditions, although only the average values are reported. The spread in activities/selectivities was never greater than 5%. It should also be noted that deactivation (9) Lin, S. C.; Phillips, J. J. Appl. Phys. 1985, 58, 1943. (10) Phillips, J.; Clausen, B.; Dumesic, J. A. J. Phys. Chem. 1980, 84, 1814. (11) Bukshpan, S.; Sonnino, T.; Dash, J. G. Surf. Sci. 1975, 52, 460. (12) Gatte, R. R.; Phillips, J. J. Phys. Chem. 1987, 91, 5961. (13) Durr, H.; Phillips, J. J. Catal. 1990, 126, 619. (14) Silva, S. M.; Phillips, J. J. Mol. Catal. 1994, 94, 97. (15) Durr, H. M.S. Thesis, The Pennsylvania State University, 1990.

of the mixtures was relatively slow, such that no more than a 15% decline in activity was noted over 5 h. Still, in order to minimize the impact of deactivation on activity measures, all readings were taken between one and 1 and 11/2 h after reduction.

Results In order to determine the presence/absence of synergism two types of experiments were carried out in the present work: control studies and true physical mixture studies. The control studies were designed to separately determine the activities and selectivities of each of the components present in the physical mixtures. The physical mixture studies were intended to determine the activity and selectivity of physical mixtures with a range of compositions. Specifically, the amount of inactive alloy (ironcerium) was gradually increased while keeping all other parameters, including the amount of noble metal, constant. Taken together the control studies and the mixture studies permitted a semiquantitative evaluation of synergism. Control Studies. The first control studies were designed to determine the activity and selectivity of FeCe/ Grafoil. Initially 18 mg of FeCe/Grafoil and 2 mg of Grafoil were present in the microreactor and activity and selectivity measured. Thereafter 10 mg increments of FeCe/ Grafoil were added and thoroughly mixed. After each mixing the material was reduced at 400 °C in flowing hydrogen for 4 h and then cooled. The activity and selectivity were measured again. This procedure was repeated until 90 mg of material was present in the microreactor. For all mixtures studied (Figure 1) the activity of FeCe/Grafoil alone for 1,3-butadiene hydrogenation was found to be very low. However, the selectivity for butenes, rather than butane, was high. The butenes:butane ratio in the product was about 4:1. The control studies for Pd/Grafoil were carried out in a different fashion than those for FeCe/Grafoil. This was done to better mimic the situation prevailing in the physical mixture studies described below. For the Pd/ Grafoil control studies the reactor was initially loaded with 2 mg of Pd/Grafoil and 18 mg of unloaded Grafoil. After the activity and selectivity of this mixture were determined, 10 mg of treated, but “unloaded” Grafoil was added and all of the catalyst thoroughly mixed. The entire mixture was then reduced at 400 °C for 4 h in flowing hydrogen and cooled. The activity and selectivity were again measured as described (Figure 1). This procedure was repeated until there were 2 mg of Pd/Grafoil in the reactor and 88 mg of Grafoil. A virtually identical control procedure was carried out using Pt/Grafoil (Figure 2) instead of Pd/Grafoil, except it was necessary to conduct experiments at higher temperatures. The rates over Pt/ Grafoil were too low to operate the reactor in a differential mode at sufficient flow rates below 353 K. In contrast to FeCe/Grafoil the activity of Pt/Grafoil for 1,3-butadiene conversion was very high and the selectivity lower. In fact, over Pt/Grafoil the butenes:butane ratio was never higher than 1.5:1. It is also important to note that little changed in terms of activity and selectivity with each addition of unloaded Grafoil. Pt/Grafoil showed trends similar to Pd/Grafoil in that neither activity nor selectivity changed much with the addition of Grafoil. It should be noted that the selectivity of Pt/Grafoil was lower than that of Pd/Grafoil, with butane rather than butenes as the dominant product. The ratio of butenes to butane was around 1:1.5. The lower selectivity of platinum relative to palladium has frequently been reported.16-19 (16) Phillipson, J. J.; Wells, P. B.; Wilson, G. R. J. Chem. Soc. A 1969, 1351.

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Figure 1. Synergism of physical mixtures of Pd/G and FeCe/G for 1,3-butadiene hydroisomerization: (A) activity of the physical mixtures; (B) selectivity of the physical mixtures measured at approximately 12% conversion in all cases, selectivity defined as this product ratio, n-butenes/(n-butenes + n-butane); (C) bar graph illustrating synergism. Note that the activity of 88 mg of FeCe/Grafoil is so low that it barely registers on the scale used.

The fact that the activity/selectivity changes little upon the addition of Grafoil is consistent with the general understanding of catalysis. Grafoil is not a catalyst. The addition of noncatalytic materials to a packed bed is expected to have minimal impact. In fact more change was detected than might be expected. That is, there was (17) Bond, G. C.; Wells, P. B. Adv. Catal. 1964, 15, 91. (18) Bond, G. C.; Winterbottom, J. M. Trans. Faraday Soc. 1969, 65, 2779. (19) Bates, A. J.; Leszczynski, Z. K.; Phillipson, J. J.; Wells, P. B.; Wilson, G. R. J. Chem. Soc. A 1970, 2435.

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Figure 2. Synergism of physical mixtures of Pt/G and FeCe/G for 1,3-butadiene hydroisomerization: (A) activity of the physical mixtures; (B) selectivity of the physical mixtures measured at approximately 12% conversion in all cases; (C) bar graph illustrating synergism. Note that the activity of 88 mg of FeCe/Grafoil is so low it barely registers on the scale.

a trend toward slightly lower activities with each addition of Grafoil. This probably resulted from a slow sintering of the platinum/palladium particles and a concomitant loss of surface area. Indeed, it is well-known that metal particles sinter rapidly on Grafoil.20 The relative concentration of the various butenes is also worth noting (Table 2). Over Pd/Grafoil the cis: trans ratio is about 3:1. This is the same ratio found for FeCe/Grafoil. In contrast, over Pt/Grafoil it is only about 1.2:1. Earlier isomerization experiments2 conducted using 1-butene in place of 1,3-butadiene, but otherwise con(20) Wu, N.-L.; Phillips, J. J. Surf. Sci. 1987, 184, 463.

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Chang and Phillips Table 2. Measured Specific Selectivity,a %

1-butene (weight of mixture, mg)

cis-2-butene (weight of mixture, mg)

trans-2-butene (weight of mixture, mg)

catalyst

30

60

90

30

60

90

30

60

90

Pt/G Pd/G FeCe/G FeCe/G + Pt/G FeCe/G + Pd/G

46.3 58.4 25.5 26.0 35.7

38.4 55.5 25.8 29.1 36.5

37.4 55.1 26.1 30.3 36.0

0 0 14.4 6.3 10.1

0 0 14.4 6.4 10.4

0 0 13.9 6.4 10.0

0 3.1 43.5 9.0 26.2

0 3.0 42.9 9.6 26.7

0 2.9 42.5 10.0 26.9

a Specific selectivity ) concentration of listed product/concentration of all product species. Products are 1-butene, cis-2-butene, trans2-butene, and butane. The “listed product” in the first three columns is 1-butene, in the second three cis-2-butene, and trans-2-butene in the last three.

ducted in a manner identical to the experiments described in the present report, also yielded cis:trans ratios of 3:1 over FeCe/Grafoil and Pd/Grafoil. In contrast over Pt/ Grafoil the ratio was again only about 1.2:1. Additional control studies were conducted to determine if the high selectivity of FeCe is related to the alloy structure or is a property of just one of the components. The selectivities of each of these is less than that found for an FeCe alloy. Mixtures. Initially 2 mg of Pd/Grafoil and 18 mg of FeCe/Grafoil were placed in the reactor and reduced at 400 °C in hydrogen for 4 h. The activity of this mixture was significantly higher than that for Pd/Grafoil and Grafoil (Figure 1). Clearly, the net activity was higher than that expected for a simple sum of the activities of the two components as the activity of the 18 mg of FeCe was found in the control experiments to be very small. Next, an additional 10 mg of FeCe/Grafoil was added and mixed and the entire mix reduced. The activity and selectivity of the mixture were measured. This procedure was repeated until the reactor contained 88 mg of FeCe/ Grafoil and 2 mg of Pd/Grafoil. It is clear from Figure 1 that the first four additions of FeCe/Grafoil led to increases in activity and selectivity which are greater than those anticipated from a simple addition of the separately determined activities/selectivities of the components. This clearly establishes the existence of synergism in the physical mixtures. The existence of a synergism is illustrated in a more dramatic fashion in Figure 1c. The addition of more than 40 mg of FeCe/Grafoil had little impact (Figure 1). The activity and selectivity of the mixture were virtually unchanged for the fourth through seventh addition of FeCe/Grafoil. In regard to selectivity, it is important to note that the ratio of cis: trans 2-butenes was around 3:1 in all mixtures containing Pd/Grafoil and FeCe/Grafoil. It is also interesting to note that 1-butene was the predominant product in all cases. Mixture experiments using Pt/Grafoil were also carried out. These experiments were virtually identical to those described above with two changes: (i) 2 mg of Pt/Grafoil was used instead of 2 mg of Pd/Grafoil; (ii) higher temperatures were required to obtain sufficient activity at required flow rates. There were several important differences between the results of the mixture experiments employing Pt/Grafoil and those employing Pd/Grafoil. First, only the first addition of FeCe to the Pt/Grafoil led to a significant increase in activity. Only very small increases in activity were detected for additions of FeCe subsequent to the first (Figure 2). Second, the synergism in selectivity appears smaller than that seen for the Pd/Grafoil studies. Third, the distribution of butenes is significantly different. In the platinum-containing mixtures 1-butene is the dominant product, and the ratio of cis:trans is far lower than that observed for the palladium-containing mixtures. Fourth, the rates were lower with platinum in the mixture

(required higher temperature). This suggests that hydrogen production and subsequent spillover occur at a lower temperature with palladium. Finally, some “control” style mixing studies were carried out. No significant synergism was found for mixtures of Pt/Grafoil and (i) Ce/Grafoil, (ii) Fe/Grafoil, or (iii) Ce/ Grafoil and Fe/Grafoil. Discussion It was found that continuous additions of FeCe/Grafoil did not lead to continuous increases in the activity and selectivity of the mixtures (Figures 1 and 2). For the mixtures containing Pd/Grafoil, increases in activity and selectivity ceased after the fourth addition of FeCe/Grafoil. For mixtures containing Pt/Grafoil significant increases stopped after the first addition of FeCe/Grafoil (Figure 2). These findings are consistent with the hypothesis advanced in the Introduction. That is, since butadiene hydrogenation/isomerization requires that some of the hydrogen is taken-up by the product species, it is anticipated that a hydrogen atom “shortage” will develop if more than an empirically determined amount of FeCe/G is added to a bed of noble metal. The following model of the reactor bed is offered as a means to better understand the above conclusion. The catalyst bed is pictured as an ideal mixture in which each platinum (or palladium) particle is surrounded by a uniform bed of FeCe particles. The concentration of butadiene and hydrogen molecules is constant everywhere (differential reactor approximation). Hydrogen atoms generated on the platinum particle surfaces diffuse evenly in all directions. They react with butadiene to form butene and butane on the FeCe particles. This leads to a radial (relative to the noble metal particle) gradient of hydrogen atoms. The concentration decreases outwardly from the noble metal particle because the atoms are “used” to form products. Beyond a certain point the hydrogen atom concentration drops to zero. FeCe particles beyond this point have very little activity as no hydrogen atoms reach them. Thus, beyond a finite limit further addition of FeCe/ Grafoil is ineffectual. This model is consistent with observations made in the present study. Alternatives to the hydrogen spillover model are sometimes invoked to explain synergism in physical mixtures. Indeed, the “polyfunctional catalyst” model21 appears to explain synergism which leads to ring opening and enlargement of some cyclic hydrocarbons.22,23 The polyfunctional catalyst model requires that a stable identifiable molecular species (intermediate) form on one type of catalytic site and then diffuse via the gas phase to a second type of catalytic site on a second surface. At the second site a catalytic process takes place, leading to (21) Weisz, P. B. Adv. Catal. 1962, 13, 137. (22) Chow, M.; Park, S. H.; Sachtler, W. M. H. Appl. Catal. 1985, 19, 349. (23) Bai, X.; Sachtler, W. M. H. J. Catal. 1991, 129, 121.

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Figure 3. The working hypothesis of physical mixtures for 1,3-butadiene hydroisomerization. Table 3. Measured Total Selectivity,a % measured corrected (weight of mixture, mg) (weight of mixture, mg) catalyst

30

60

90

Pt/G Pd/G FeCe/G FeCe/G + Pt/G FeCe/G + Pd/G

40.3 61.5 83.4 42.0 72.0

38.4 58.5 83.1 44.9 73.6

37.4 58.0 82.5 46.6 73.7

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60

90

69.3b,c 69.8b,c 70.2b,c 87.9b 78.5b 78.2b

a Total selectivity ) sum of product olefin concentrations/sum of product olefin and butane concentrations. b Determined after subtracting activity of noble metal component. c Corrected for conversion of trans-2-butene on platinum surface, see text. These values were not measured, but rather computed.

the formation of the desired product. This model does appear to partially explain some of the observations made for the platinum-containing mixtures. Palladium-Containing Mixtures. It is clear that the hydrogen spillover model is consistent with all the results obtained for these mixtures (Figure 3a). For example, the net selectivity of the mixtures is very close to that which would be expected if both the palladium surfaces and the FeCe surfaces retain their separately determined values (Table 3). That is, in this model it is assumed that the activity is the sum of the activities of the two surfaces. Once the activity of the palladium surface for each product species is subtracted based on values obtained in the control studies, the activity and selectivity of the FeCe surfaces can be computed. The selectivity of the FeCe surface thus determined can be used as a check on the validity of the hydrogen spillover model. Indeed, the hydrogen spillover model predicts the activity of the FeCe surfaces is greatly increased, but the selectivity is expected to remain relatively constant. As shown in Table 3 the selectivity of FeCe in the mixtures is close to that measured for FeCe in control studies. In addition to the fact that the hydrogen spillover model explains all data, there are two objections which can be raised against using the polyfunctional model to explain the palladium mixture results. The first objection is based on the identity of the intermediate. The only stable identifiable intermediate between a paraffin and a diolefin

is an olefin. Yet, it has already been shown that FeCe/ Grafoil has very low activity for olefin conversion. Thus, previous studies demonstrate that in the event palladium creates an olefinic intermediate (1-butene or 2-butene), from butadiene, it would not lead to enhanced activity at FeCe sites. It also unlikely that palladium converts intermediates. That is, control studies in the present work clearly show FeCe/Grafoil does not make olefins from butadiene at any appreciable rate; thus FeCe is not capable of providing intermediates to palladium at a significant rate. The second objection is based on more detailed investigations of mechanism. It is widely agreed that both diolefin and olefin hydrogenation/isomerization proceed via hydrogen atom addition on metal surfaces.24-26 In fact, the high activity of butene reactions is limited to metals (e.g., Pt, Pd, Rh, Ni) which dissociate hydrogen molecules at low temperatures. The hydrogen spillover model includes a postulated source of hydrogen atoms for accelerating reaction at sites (i.e., FeCe) which cannot independently dissociate hydrogen. In contrast, the polyfunctional model requires high activity for diolefin/ olefin hydrogenation/isomerization in the absence of atomic hydrogen. Platinum-Containing Mixtures. Different arguments are required to explain the results obtained for platinum-containing mixtures. The hydrogen spillover model is not consistent with all the data obtained. The selectivity for butenes of the FeCe surfaces in the mixtures, obtained as described above, is only about 50%. This is significantly less than that of FeCe as tested in the control studies and, hence, indicates the spillover hypothesis alone, although it is consistent with the large activity synergism, is not a sufficient explanation for all observations. A proposed solution to the dilemma is outlined in Figure 3b. It is postulated that both hydrogen spillover and polyfunctional behavior are simultaneously at work in the platinum-containing mixtures. Hydrogen atoms activate the reaction on the FeCe surfaces, creating butenes with a high degree of selectivity, as in the palladium mixture case. In the platinum mixtures; however, an additional process is postulated to take place. To wit: Some of the products of reactions on FeCe surfaces diffuse back to platinum surfaces where they are converted to butane at a very high rate. Specifically, it is postulated that trans-2-butenes created at the FeCe surface are converted very rapidly on platinum surfaces. It is postulated that platinum, relative to palladium, has very high activity for the conversion of trans-2-butene to butane. Evidence for this can be seen in the fact that the ratio of cis to trans 2-butenes over Pt/Grafoil, or any mix containing Pt/Grafoil (Table 2), is far less than that found over all the other catalysts studied. It must also be noted that differences between beds containing platinum and palladium cannot be attributed to dispersion. Dispersions of Pt/Grafoil and Pd/Grafoil catalysts were measured after both 4 and 20 h of hydrogen reduction using oxygen isotherms. Employing standard O/metal ratios for both Pd27,28 and Pt29 revealed dispersions for both metals of between 28 and 31% in all cases. The suggestion that different noble metals will display different catalytic behavior in these reactions is consistent (24) Garra, S.; Ragaini, V. J. Catal. 1968, 10, 230. (25) Macnab, J. I.; Webb, G. J. Catal. 1968, 10, 19. (26) Ragaini, V. J. Catal, 1974, 34, 1. (27) Guo, X.; Hoffman, A.; and Yates, J. T. J. Chem. Phys. 1989, 90, 5787. (28) Peterson, L. G.; Danetun, H. M.; Lundstrom, I. Surf. Sci. 1985, 161, 72. (29) Nishiyama, Y.; Wise, H. J. Catal. 1974, 32, 50.

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with the literature. The fact that they have different selectivities for hydrogenation/isomerization of C4 hydrocarbons is clearly demonstrated in earlier work. Specifically, it is known that the mechanisms for these reactions are very different on platinum and palladium surfaces.16 In Table 3 the selectivity of the FeCe surfaces was “corrected” for the high conversion of trans-butene. It was assumed that the cis-2-butene was correct and that initially the ratio of cis:trans was 3:1 as it is over most surfaces. The corrected selectivity of the FeCe surface is reasonably close to that measured for FeCe in control studies. Summary In sum, it was shown that physical mixtures containing a graphite-supported “alloy” (FeCe) and a graphitesupported noble metal (palladium or platinum) display synergism for butadiene hydrogenation/isomerization. That is, the activity of the mixtures is far higher than that anticipated from the addition of activities measured for each material alone. Previously, synergism was observed for the same physical mixtures employed in the current study but used as catalysts for 1-butene isomerization. In that study it was found that each addition of FeCe resulted in a significant increase in activity of the bed as a whole. In contrast, in the present work it was found that there was a limit to the synergism. Beyond a certain FeCe/noble metal ratio further additions of FeCe were ineffectual. The overall activity and selectivity of the catalyst bed stabilized. This supports a corollary to the hydrogen spillover model: In reactions for which there is a net consumption of hydrogen, the degree of synergism for physical mixtures will be limited for a given value of dispersion of the noble model. It must be noted that both the original hydrogen spillover work and the present study of the “corollary” were only tested with graphitic-carbon

Chang and Phillips

supported catalysts. Additional study is required before either is extended to catalysts supported on refractory oxides, or even turbostratic carbons. Selectivity results provided further insight into the mechanism of the observed synergism. They suggested that the mechanism of synergism is complex and that it depends on the identity of the noble metal in the mixture. It was shown for mixtures containing palladium that hydrogen spillover, resulting in the activation of nearby FeCe surfaces, could account for all observations. To wit: hydrogen atoms generated on palladium particles “spill over” onto the FeCe particles and activate the isomerization and hydrogenation of butadiene on the FeCe surfaces. In contrast, to explain all the observations made for mixtures containing platinum, it was necessary to postulate that both hydrogen spillover and a form of “polyfunctional surface” catalysis were taking place simultaneously. Specifically, some of the 2-butenes formed on the FeCe surfaces, as a result of hydrogen atom activation, diffused back to the platinum where they were subsequently converted to butane. Finally, the present results add further evidence to support the hydrogen spillover hypothesis. For example, the present results are not consistent with the postulate that synergism in the mixtures results from the formation of ‘special sites’ which form on FeCe during high temperature reduction in the presence of a noble metal. Such a model does not explain the ‘limit’ to synergism observed in the present study. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for support of this research (ACS-PRF# 29695AC5). We also thank John Weigle for assistance with dispersion measurements. LA960674G