J . Phys. Chem. 1987, 91, 5953-5956 that the effect is general for N phases and is only enhanced in this particular case. W e would like to permit ourselves to suggest one possible solution to this puzzle: to assume, as sketched in Figure 4a, a string of cylindrical micelles, but with the cylindrical axis perpendicular to the string, that is, cylinders with lateral positional correlation. Such strings could be formed in a fluctuating mode, as already discussed26 in terms of interaction forces between micelles in N phases. Figure 4b shows a sketch of the aleatory distribution of strings in the plane perpendicular to Z, defining a uniaxial N, phase. We remark that the cross section of the cylindrical micelles need not be perfectly circular and that the strings need not necessarily have a fixed number of micelles. Such a model would also be able to account for the biaxial phase Nbx,in which there would be a definition of the string direction (induced by an order-disorder transition, by a change in the form of the micelles, or just by a change in the relative fraction of a mixturez7 of rods and disks, the last presenting a short-range order of columnar type, typical of Nd phases). Such a type of structure with strings and columns would also account for cholesteric lyotropic phases. The main difference between our proposal and the previous one1’ is that in our case the exact form of the individual micelles (26) Amaral, L. Q.; Figueiredo Neto, A.
M.Mol. Cryst. Liq. Crysr. 1983,
is left open (in particular, a cylindrical form in N, phases is allowed); the biaxial objects to which mean field theories’j could apply would correspond to the whole agglomerate, which may have much larger anisotropies than the individual micelle and variable size. This proposal is, however, tentative, and further investigation is necessary to better define the micellar structure of N phases. The XD result in the I phase shows that the same type of positional correlation is present in the isotropic micellar solution. A better integration with research being carried on micelles in isotropic solution is necessary to improve micellar models at the I-N transition. Quantitative determination of changes in Z 2 / Z , and a detailed and S(3) by fitting to the Z(3 curve are in modeling of P(?) progress and may represent a powerful tool for studying size and form of micelles as well as intermicellar interactions. Systems with SLS, by the double aspect of extensive study in I phases and R values near divergence in N phases, are particularly well suited for studying controversial aspects of the isotropicnematic transition, by changes in both concentration and temperature.
Acknowledgment. Financial support of FINEP and CNPq are acknowledged. R. Itri had a postgraduate fellowship from FAPESP.
98,285. (27) Rosenblatt, C. J . Phys.
5953
Registry No. SLS, 151-21-3;decanol, 112-30-1.
(Les Ulis, Fr.) 1986, 47, 1097.
Variation of Catalytic Activity over PtRe/y-Al,O, S. M. Augustine* and W. M. H. Sachtler Zpatieff- Laboratory, Catalysis Center, Department of Chemistry, Northwestern University, Evanston, Illinois 60201 (Received: March 2, 1987)
Hydrogen isotope exchange (at 100 “C) and hydrogenolysis (at 240 “C) were studied over y-AI2O3-supportedPt, Re, and a series of PtRe catalysts, where the composition and pretreatment conditions were varied. Sites catalyzing the rate-limiting step of the former reaction appear to be monoatomic, while those for the latter reaction are multiatomic. Alloys behave as physical mixtures for the isotope-exchangereaction, since the strength of the adsorption bond depends little on the atomic environment of the site. For hydrogenolysis, however, the PtRe catalysts behave differently than the Pt or Re monometallic catalysts. This is attributed to the formation of ensembles of atoms that contain both Pt and Re on the bimetallic catalyst and thus exhibit a heat of adsorption which is intermediate between the two. The most ensemble-specific reaction is total hydrogenolysis to methane. This rate is highest for sites on which adsorption is stronger than on Pt but weaker than on Re. This reaction, therefore, probes the extent of alloying on the supported particles.
Introduction Of all the parameters that have been used to characterize metals, such as position and filling of the d band, density of states, or Fermi level, the one which correlates best with the activity of heterogeneous metal catalysts is the ability to form chemisorption bonds. This has been effectively illustrated in graphs where the catalytic activity of a metal for a certain reaction is plotted against the heat of formation of the chemisorption bond acrass a transition series. Such graphs show a peak in activity a t an intermediate heat of adsorption. This was first shown by Balandin, who called these plots “volcanc-shaped curves”.l A detailed thermodynamic explanation for this phenomenon has been presented by Schuit et a1.,2 who also considered the entropy of forming the chemisorption complex in their calculations. They further showed that the enthalpy of chemisorption of a metal could be correlated to the heat of formation of its lowest oxide. (1) Balandin, A. A. Ivz. Akad. Nauk SSSR 1955, No. 4,624; Dokl. Akad. Nauk SSSR 1954, 97(4), 667. ( 2 ) Schuit, G . C. A.; Van Reijen, L. L.; Sachtler, W. M. H.; Proc. Second Congr. Catal. 1960, Paris 1960, 893.
For the purpose of the present paper it suffices to state that chemisorption is always accompanied by a loss in entropy. Therefore, for metals which form weak chemisorption bonds, the coverage of the active surface by the reaction intermediate during the steady state will be low. The consequence is a low activity. Where the metal forms very strong bonds with the adsorbate the surface intermediate is formed rather easily, but the breaking of these bonds to form the product is slow. For an intermediate heat of chemisorption, both adsorption and desorption are fairly rapid; Le., such a catalyst is near the peak of the volcano curve. While this behavior has been well established for pure metals, the question arises as to whether this principle can be applied to catalysts where two active metals are combined. If we accept that an active site on a catalyst may consist of not just one but several zerovalent metal atoms, then the overall character of the site would be intrinsically related to the nature of each of the atoms. A special case of interest combines two metals of markedly different heats of adsorption such that the pure metals are located on opposite sides of the volcano curve. A mixed metal site would then display a collective heat of adsorption that would be intermediate between the two and display a higher activity than mo-
0022-3654/87/2091-5953$01.50/0 0 1987 American Chemical Society
5954 The Journal of Physical Chemistry, Vol. 91, No, 23, 1987
nometallic sites of each of the constituent metals. In other words, as the metal composition of the catalyst is varied from 100% to one metal to 100% of the other, the catalytic activity for certain reactions should pass through a maximum. This simple speculation is based on a number of assumptions. The first is that for supported catalysts there is sufficient interaction between the two metals on the surface so that there is a high concentration of bimetallic clusters. The second condition is that the reaction being studied is catalyzed by a site of several metal atoms, Le., an active ensemble. Platinum and rhenium are two metals which do have a very different heat of bond formation, and a volcano-type behavior for a series RRe/A1203 catalysts has been demonstrated in some very interesting work with n-butane and 2,2-dimethylbutane by Haining et al.3 They showed that the activity for these reactions passes through a maximum as the catalyst composition is changed from pure Pt to pure Re in six regular steps. What has to be clarified is whether this behavior does correlate with both the extent of metal interaction and whether or not the reaction being studied is sensitive to the size of the catalytic site. Then it should be possible to find a correlation of catalytic activity with the composition of bimetallic catalysts on the basis of the heats of adsorption. To do this we have studied several PtRely-Al203 catalysts with similar metal loadings but different Pt/Re ratios. The extent of metal interaction was varied by using different pretreatment conditions. Two different reactions were studied, one that required a large multiatom site and one that did not. These were the hydrogenolysis of cyclopentane and the deuterium exchange of the same molecule, respectively. Besides the overall hydrogenolysis, the “deep” hydrogenolysis to methane was monitored. Methane formation is, of course, highly undesirable in industrial naphtha reforming with supported PtRe catalysts. In previous papers we have shown that methane production can be suppressed by adding carefully dosed amounts of sulfur to the PtRe/y-A1203 catalyst. In the present work, however, sulfur was excluded, because we wish to exploit the high selectivity for methane production of mixed PtRe ensembles, The activities were compared in terms of a turnover frequency with the amount of exposed metal on the surface being measured by H / D titration.
Experimental Section Catalysts. In this work we compared catalysts made by two preparation techniques, viz., coimpregnation of both metals or impregnation of Re onto prereduced Pt/y-A1203, which enabled us to vary the extent of metal interaction. All supported catalysts were prepared by the incipient wetness impregnation method. Cyanamid PHF y-A1203 (surface area 180 m2/g, pore volume 0.5 cm3/g; mesh size 60-80) was used as the catalyst support. The procedure for crushing the alumina extrudate and subsequent treatment followed that of Kobayashi et al! The impregnating solutions were composed of Pt(NH3)4(N03)2(Alfa no. 88960) or NH4Re04(Aldrich no. 20416-1) dissolved in doubly distilled water. Coimpregnation made use of a solution containing both the Pt and the Re compounds. After impregnation, the catalysts were dried at 120 O C overnight, calcined a t 500 OC in flowing dry air for 3 h, and then stored in bottles before use. These catalysts were subsequently dried for 1 h in Ar and reduced for 2 h in H2 (or D2) at 500 O C prior to the reaction. The flow rates were 60 cm3/min. Catalysts prepared as such included 0.33 wt % Pt-0.22 wt % Re/y-A1203, 0.35 wt % Pt/y-A1203, and 0.2 wt % Re/y-A1203. The second preparation method started with impregnation, calcination, and reduction of Pt onto 7-A1203 as above. The catalyst was then impregnated with Re, dried overnight at 120 “C,and allowed to pick up moisture from the atmosphere for a week. The purpose of this is to facilitate later surface migration (3) Haining, I. H. B.;Kemball, C.; Whan, D. A. J. Chem. Res. 1977,2056; J . Chem. Res. (91978, 364. (4) Kobayashi, M.:Inoue, Y.; Takahashi, N.; Burwell, R. L., Jr.; Butt, J. B.; Cohen, J. B. J. Cutul. 1980, 50, 464.
Augustine and Sachtler
u uuu Figure 1. High vacuum recirculation reactor interfaced with mass spectrometer, which was used for deuterium-exchange reactions. B, bakeable valve; BT, bakeable table; BV, butterfly valve; CP, circulation pump; CP‘, cyclopentane; CV, calibrated volume; I, ionization gauge; LN, liquid nitrogen trap; M, molecular sieves; MP, mechanical pump;
MS, mass spectrometer;MnO, oxygen trap; PT, pressure transducer; T, Teflon valves; TP, turbomolecular pump; VL, variable leak valve. of Re oxide. The effect of two different pretreatments was also studied. The first was identical with the one mentioned previously with a drying step of 1 h and a reduction step of 2 h; this will be termed the “dry” pretreatment. The second bypassed the drying step and reduction was done by heating from room temperature to 500 O C in 1 h and then keeping the temperature constant for 2 h; this will be called the “wet” pretreatment. The catalysts prepared in this way were a 0.43 wt % Pt-0.20 wt % Re/y-A1,03 (66/34 PtRe), and a 0.21 wt % Pt-0.43 wt % Re/y-A1203 (34/66 PtRe). Reagents. Cyclopentane was purchased from Fluka and exceeded 99% purity. It was dried prior to use by exposure to a 4A molecular sieve in a static vacuum for 3 days. All gases were purchased from Matheson and were classified as ultrahigh purity. They were further purified by passing them through reduced, Si02-supported manganese oxide traps and molecular sieve traps at -78 OC. In addition to this, the hydrogen and deuterium were also passed through Deoxo catalyst traps prior to the other traps. The D content of the deuterium gas was determined by mass spectrometry to be 97%. HID Isotopic Exchange of Qclopentane. These reactions were done a t low pressure in a batch reactor which is schematically represented in Figure 1. It consists of a reaction loop (156 mL total volume) in which the gaseous components are mixed by a magnetically driven circulation pump. The gas mixture is continuously bled through a Granville-Phillips variable leak valve at a rate such that the pressure inside the reaction vessel decreases less than 3% per hour. The products are analyzed by a Dycor MlOO mass spectrometer which is interfaced to a Zenith 158 personal computer for data retrieval and processing. The system is differentially pumped by two Balzers turbomolecular pumps. Due to the temperature limitations of the variable leak valve and the mass spectrometer the system cannot be baked at high temperature, but it is periodically pumped at a temperature of 150 O C overnight to remove adsorbed molecules from the walls and achieve a vacuum of lod Torr. Prior to each experiment cyclopentane and deuterium were mixed in a control volume, where their pressures were monitored by a Datametrics diaphragm manometer, and allowed into the reaction volume. The catalyst was preheated to 100 ‘C, the hydrogen-cyclopentane ratio was 25, and the total pressure was 26 Torr. Hydrogenolysis of Cyclopentane. The series of hydrogenolysis reactions was carried out at atmospheric pressure. A flow type reactor was used for this work. The reaction temperature was 240 O C , and the saturator temperature was held at -7 O C to obtain
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5955
Catalytic Activity over PtRe/A1203
50
-
8
>
L Pt
__-50%
Re
C O M P O S I T I O N C O M P O S I T I O N
Figure 2. Hydrogenolysis of cyclopentane turnover frequency vs. PtRe catalyst composition. Circles represent activity of catalysts dried at 500 OC for 1 h before reduction. Squares represent activities of undried
catalysts. a hydrogen-cyclopentane ratio of 9. The products from this reaction were analyzed by a Hewlett-Packard 5790 gas chromatograph with a cross-linked methylsilicone capillary column and a 3390A integrator. The conversions for these reactions were generally kept well within the differential kinetic regime. Dispersion Measurements. It has been reported that supported Re adsorbs very little hydrogen at room temperature." Presumably this adsorption is a kinetically limited process. Therefore, we used a different method to measure metal surface area, viz., the room temperature exchange of preadsorbed deuterium with gaseous protium after saturating the catalyst with adsorbed deuterium. This was done by reducing the catalyst at 500 OC in deuterium and then cooling it. The reactor was then purged at 20 OC for 0.5 h in Ar and evacuated for at least 3 h at Torr. A quantity of hydrogen which was at least 20 times greater than the expected amount of adsorbed deuterium was then introduced into the reactor system. Metal-catalyzed exchange of hydrogen with adsorbed deuterium atoms reaches equilibrium almost instantaneously at room temperature, while exchange of the surface hydroxyl groups is a much slower process. Therefore, the percentage of hydrogen exchanged as a function of time was extrapolated to zero time to determine the amount of hydrogen adsorbed on the metal.
Results The results of these experiments are summarized in Figures 2-4, where the activity, expressed as molecules of cyclopentane ( 5 ) Baumgarten, E.; Denecke, E. J. Caral. 1986, ZOO, 377. (6) Ponec, V. Advances in Catalysis; Academic: New York, 1983;Vol. 72 n --7 r 149. -
( 7 ) Sachtler, W. M. H.; van Santen, R. A. Advances in Catalysis; Academic: New York, 1977;Vol. 26,p 69. (8) Roberti, A.; Ponec, V.;Sachtler, W. M. H. J. Caral. 1973, 28, 381. (9) Biloen, P.; Dautzenberg, F. M.; Sachtler, W. M. H. J. Catal. 1977,50,
,.
77 ,
(10)Sachtler, W. M. H.; van der Plank, P. Surf. Sci. 1969, 18, 62. (1 1) Soven, P. Phys. Rev. 1967, Z56(3), 809. (12) Hedge, R. I.; Sinha, A. P. B. Appl. Spectrosc. Rev. 1983, Z9(1), 1. (13) Isaacs, B. H.; Petersen, E. E. J. Caral. 1982, 77, 43. (14) Wagstaff, N.;Prins, R. J. Catal. 1979, 59, 434. (15) Bolivar, C.;Charcosset, H.; Frety, R.; Primet, M.; Tournayan, L.; Betizeau, C.; Leclerq, G.; Maurel, R. J . Caral. 1975, 39, 249.
Figure 3. Turnover frequency for methane production from the hydrogenolysis of cyclopentane vs. PtRe catalyst composition. Symbols have same meaning as in previous figure.
*@
t
1
0
"1
5 .
I
Pt
56%
be
C O M P O S I T I O N
Figure 4. Disappearance of light cyclopentane turnover frequency vs. PtRe catalyst composition. Symbols have same meaning as in Figure 2.
converted or methane produced per second per exposed surface metal atom, is plotted as a function of the composition of the catalysts. Data points of catalysts which were directly reduced are represented by squares, while circles represent the catalysts which were dried before reduction. Figures 2 and 3 are plots of the turnover frequencies of cyclopentane conversion and methane production, respectively. Figure 3 was included for two reasons. First, methane production at these temperatures is exclusively a metal function, so effects of the acid function will not interfere to a great extent. The second reason is that it represents the most extensive form of hydrogenolysis. Therefore, evidence of this reaction represents a stronger
5956
The Journal of Physical Chemistry, Vol. 91, No. 23, I987
interaction of cyclopentane with the surface than does simple cyclopentane conversion and thus should display a greater sensitivity toward the nature of the active site. From both these figures it can be easily seen the catalytic activity for the hydrogenolysis reactions displays a maximum at intermediate compositions. In most cases, this maximum lies in the vicinity of the catalyst with 34% Pt and 66% Re, which correlates well with results found by Haining et al.3 For the deuterium exchange, it is apparent, from Figure 4, that no maximum exists for the turnover frequency with light cyclopentane. Rather, the points scatter about a straight line. The highest rate of exchange is obtained with the pure Re/y-A1203, and as Pt is added the rate decreases. The intermediate values do appear to be low. This may be a consequence of our method of measuring surface areas. Baumgarten and Denecke have shown that, when H / D exchange occurs over a Pt/y-A1203 catalyst, spillover of the deuterium to hydroxyl groups does occur. They identify two types of hydroxyl groups, one which exchanges quickly and one which exchanges much more slowly.5 It is this rapidly exchanged hydroxyl group which is difficult to account for and could lead to an overestimation of the actual surface area of the metal. The hydrogenolysis reactions, especially methane production, appear to be most sensitive to the pretreatment conditions of the catalyst. When the catalyst is not dried, but directly reduced, the turnover frequency is enhanced by a factor of 9 for overall hydrogenolysis and a factor of over 20 for methane production in the case of the 34/66 PtRe/y-A1203 catalyst. This is markedly different from the deuterium-exchange reaction, for which no difference, beyond experimental error, is observed.
Discussion It is now quite well accepted that hydrogenolysis occurs on a large ensemble, Le., a site composed of a large number of contiguous active metal atoms. Much of the work with alloy catalysts of group VI11 (groups 8-10)16 and group IB (group 11)16metals show that when the active noble metal is diluted by a catalytically inert metal, the activity for hydrogenolysis is reduced dramatially.^,' Deuterium exchange, on the other hand, is much less sensitive to the size of the active sites on a surface. Roberti et al. analyzed both the rate of methane formation and production of the pexdeutero product from methylcyclopentane over Ni and a NiCu alloy. They found that adding 5% Cu to Ni greatly reduced the rate of hydrogenolysis relative to the rate of deuterium exchange. This proves that C-C bond scission is much more sensitive to alloying than is the rupture and re-formation of C-H bonds8 The results of the present work correlate well with these findings. When Re is added to a Pt/y-A1203 catalyst the rates of hydrogenolysis are greatly affected, due to the fact that in this reaction the molecule samples a large multiatomic site. If this site contains atoms of two different elements, then the heat of adsorption on this site will be intermediate to that on sites of the same size but containing only Pt or Re atoms. By consequence, the principle of the volcano-shaped curves predicts that the rate of hydrogenolysis on these mixed sites should be higher than on the pure sites. In the case of deuterium exchange of cyclopentane, the molecule probes only a very small ensemble. The rate-limiting step for this reaction involves possibly only one surface metal atom, as is suggested by the catalytic dehydrogenation results on PtAu al10ys.~ An abundance of experimental data on alloys of d-metals published in recent years unequivocally shows that the “ligand effect” is very small on these alloys,’“-’* and thus the strength of the adsorption bond of any adsorbate with one single surface metal atom will be almost independent of the surroundings of this atom. Therefore, to the first approximation one expects for a catalytic reaction of this type that the rate will behave in an additive manner; Le., there should be little sensitivity to the extent of alloy formation, if the comparison is made on the basis of equal numbers of surface atoms of either element. It appears that our experimental results on the
Augustine and Sachtler deuterium exchange of cyclopentane are in good agreement with this simple model. Our motivation to study the effect of different pretreatment conditions is to use catalytic hydrogenolysis as a probe for testing the different degrees of bimetallic interaction. It is well-known and understood that in a PtRe/y-A1203 catalyst Pt catalyzes the reduction of Re. It has also been shown from temperature-programmed reduction studies by Isaacs and Petersen that the degree of hydration of the 7-A1203surface plays an important role in this phen0men0n.l~ These authors assume that an enhanced interaction during reduction will result in a larger number of mixed metal clusters on the surface. This conclusion is supported by results of Wagstaff and Prins14and Bolivar et al.15 TPR studies done in this laboratory with both y-A1203and zeolite-supported PtRe show similar results and will be published at a later date. These reaction studies substantiate this view. The samples which had not been severely dried prior to reduction not only show an enhanced reduction at lower temperature, but apparently also contain a higher concentration of bimetallic particles. The resulting increase in the number of bimetallic adsorption sites, when compared to the samples which have been reduced in the conventional way, should give rise to a higher hydrogenolysis activity, while no effect is expected on the TOF for the deuterium-exchange reaction. Indeed, the present results are in excellent agreement with these expectations.
Conclusions Alloying two metals, for which the heat of adsorption of a given molecule is significantly different, leads to predictable changes in adsorption and catalysis: 1. For adsorption complexes in which a chemical bond is formed with only one surface atom, the alloy surface differs little from that of a physical mixture of the unalloyed metals. If there are differences they are due to the electronic “ligand effect” which is known to be very small for alloys of d-metals. 2. For molecules which are able to form chemisorption complexes with multiatomic sites, the alloy surface exhibits sites which are qualitatively different from those on the pure metals. The heat of adsorption on the “mixed ensembles” will be intermediate between those on the pure ensembles of the pure metals. If the heat of adsorption on the mixed ensembles is too low to establish a steady-state concentration, the “ensemble effect” will favor the adsorptiop mode on the smallest ensembles. This case is widely documented for alloys such as PtAu, CuNi, or PtSn. 3. The catalytic performance of alloys depends on the type of chemisorption complex which is a reaction intermediate to the catalytic process. For complexes involving one metal atom only, changes in catalysis are limited to the ligand effect. For reaction intermediates requiring larger ensembles, the relative activity of metal and alloys is given by the dependence of the reaction rate on the heat of adsorption, Le., the volcano-shaped curves. In such cases the effect of alloying on catalysis can be very large, as it is basically an ensemble effect. For hydrocarbon hydrogenolysis on PtRe, e.g., the alloy is intrinsically more active than either pure metal. 4. The success of preparation conditions aimed toward favoring the formation of mixed metal clusters on a support surface can be assessed by studying the turnover frequency of a catalytic reaction with an intrinsically higher rate on mixed ensembles. Acknowledgment. Financial support for this study by the Amoco Oil Co. is gratefully acknowledged. Registry No. Pt, 7440-06-4; Re, 7440-15-5; D,, 7782-39-0; cyclopentane, 287-92-3. (16) In this paper the periodic group notation (in parentheses) is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.) (17) Freel, J. Prep., Am. Chem. SOC.,Diu.Pet. Chem. April, 1973, 10.
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