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Langmuir 1988, 4 , 1100-1103
not readily lend themselves to such an adaptation, because the LEED optics are arranged for normal incidence. A sequential experiment, such as we describe here, would often be possible, however. Second, it is necessary to use some type of data acquisition system that is reasonably fast, i.e., a system that can gather the necessarv mot Drofiles within a minute or two. ?his is because t h e simpie is prone to contamination in the time between depositions. The data acquisition process should be relatively fast so as not to lengthen the experiment significantly. The video system which we useL4is convenient, although many other techniques (e.g., photography or resistive anode networks) could certainly be used as well. In summary, we observe diffracted intensity oscillations in the growth of Pt on Pd(100) with a conventional LEED
diffractometer. All our observations are similar to the oscillations usually observed in semiconductor MBE growth. This is a preliminary report, and more systematic results will be presented after the completion of experiments still in progress. More quantitative information will be extracted from the angular profile line shapes by taking into account the instrumental resolution.
Acknowledgment. This work is supported by a Presidential Young Investigator Award of the National Science Foundation, Grant No. CHE8451317. In addition, some equipment and all facilities are provided by the Ames Laboratory, which is operated for the U.S.Department of Energy by Iowa State University under Contract No. W-7405-ENG-82. Registry No. Pt, 7440-06-4;Pd, 7440-05-3.
Reactions of 2,2-Dimethylbutane on Pt/Re/A1203 Catalysts. Effect of Sulfur and Chlorine on the Selectivity+ A. J. den Hartog, P. J. M. Rek, M. J. P. Botman, C. de Vreugd, and V. Ponec* Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received December 29, 1987. In Final Form: April 20, 1988 Reactions of 2,2-dimethylbutane (neohexane) have been studied with two series of Pt/Re/A1203catalysts: one prepared chlorine-free, the other prepared from chloride(s) as precursor(s). The catalysts have been studied in the reduced, unsulfded state and after presulfidization by thiophene. It appears that both metallic (catalyst) components form clusters (alloys): their catalytic behavior is “nonadditive”. After sulfidization of the catalysts, the relative role of acid-catalyzed reactions increases.
Introduction The supported Pt/Re alloys (or bimetallic clusters) have attracted a lot of attention since the first report on the superior behavior of these materials in 1968.’~’ The first generation Pt catalyst (“mono metal”) has been replaced in the naptha reforming shortly afterwards by the Pt/Re catalysts that are today the most frequently used catalysts in this branch of petrochemistry. Fundamental studies devoted to the Pt/Re alloys have been already performed as well, as can be illustrated by a small selection of papers.’~~-’ The superior behavior of the Pt/Re catalysts as compared with the Pt catalysts has given rise to the formulation of several theories to explain this fact. For the benefit of the reader, various theories are summarized below into the following groups. (1) Re influences the deposition of carbon on the acidic support. The function of Re, either in the oxidic or metallic form, is to promote the fission or other ways of removal of the coke precursors, thus preventing the selfpoisoning of the cata1ysts.&l0 (2) Re modifies the availability and reactivity of hydrogen on the catalyst surface by influencing such phenomena like the “spillover” of hydrogen.”J2 *Author to whom all correspondence should be sent. t Presented at the symposium on “Bimetallic Surface Chemistry and Catalysis”, 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Campbell, Chairmen.
0743-7463 /88/2404-1100$01.50/0
(3) Re functions as an anchor preventing the sintering of the Pt particles and possibly influences the properties of the Pt catalyst as (4) Re forms an irreversible sulfide during sulfidization as the bond strength of Re-S is larger than that of Pt-S. (1)Charcotset, H. Rev. Znst. I+. Pet. 1979,34,238.Charcosset, H. Int. Chem. Eng. 1983,23,411. (2)Kluksdahl, H. E. US.Patent 3415737, 1968. Jacobson, R. L.; Kluksdahl,H. E.; McCov, C. S.; Davis, R. W. In Proc.-Am. Pet. Inst., Diu. Refin. 1969,504. (3) Haining, I. H. B.; Kemball, C.; Whan, D. A. J. Chem. Res. Symp. 1978,364;1977,170. (4)Betizau, C.; Leclercq, G.; Maurel, R.; Bolivar, C.; Charcoeset, H.; Frety, R.; Tournayan, L. J.Catal. 1976,45,179.Bolivar, C.; Charcosset, H.; Frety, R.; Primet, M.; Tournayan, L.; Betizau, C.; Leclercq, G.; 1976,45,163. Charcosset, H.;Frety, Maurel, H. J. Catal. 1975,39,249; R.: Leclerca. G.:. Mendes., E.:. Primet., M.:. Tournavan. - . L. J. Catal. 1979. 56;468. (5) Biloen, P.; Helle, J. N.; Verbeek, H.; Dautzenberg, F. M.; Sachtler, W. M. H. J. Catal. 1980, 63, 112. Sachtler, W. M. H.; Biloen, P. Prepr.-Am. Chem. SOC.,Div. Pet. Chem. 1983,482. (6) Shum, V. K.; Butt, J. B.; Sachtler, W. M. H. J.Catal. 1985,96,371; 1986,99,126. Hasan, A.; Kawakami, K. J. Catal. 1984,88,150; (7)Coughlin, R. W.; 1984,88,163. (8)Ludlum, L. H.; Eischens, R. P. Prepr.-Am. Chem. SOC.Div. Pet. Chem. 1976,375. (9) Burch, R.; Mitchell, A. J. Appl. Catal. 1983,6,121. (10)Bertolacini, R.J.; Pellet, R. J. In Catalyst Deactivation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1980; p 73. (11)Margitfdvi, J.; GoMlos, S.;Kwayaser, E.; Hegedus, M.; Nagy, F.; Koltai, L. React. Kin. Catal. Lett. 1984,24,315. (12)Traffano, E. M.; Parera, J. M. Appl. Cotal. 1986,28,193. (13)Yermakov, Yu, I.; Kuznetaov, B. N. J. Mol. Catal. 1980,9,13. (14)Engels, S.;Wilde, M.; Tran Kim Thanh Z.Chem. 1977,17, 10. (15)Graham, A. G.; Wanke, S. E. J. Catal. 1981,68,1. _I
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2,2-Dimethylbutane on PtlRelA1203 Catalysts The Res, species separate the S-free Pt sites and influence the selectivity in this manner (ensemble size e f f e ~ t ) . ~ ( 5 ) It is known that alloying Pt with an inactive metal can suppress the deposition of carbonaceous specieslBand the conversion of these species into graphite. The function of Re or of the Res; species has been explained in a similar manner.6 (6) Pt/Re alloys should have an electronic structure very different from that of individual components, and this should be responsible for the phenomena observed, such as different selectivities in hydrocarbon reactions, a modified carbonaceous residue deposition, a different hydrogen availability, reactivity, etcS4J2 Theories 4 and 5 correctly reflect the fact that the bimetallic Pt/Re catalysts are superior to the monometallic Pt catalyst only when a sulfidization of the catalysts has been carried out (see also results presented below). Thus ideas 1-3, in spite of the fact that they most likely describe existing effects, cannot offer a complete explanation why the Pt/Re catalysts are so much better in industrial naptha reforming than the monometallic ones. Point 6 is not supported by any solid-state physics data or by chemisorption studies. In order to achieve a better understanding of the effects, bimetallic catalysts containing varying Pt/Re contents were tested with sulfidization by thiophene and without this sulfidization step. Neohexane (2,2-dimethylbutane) was used as an "arche-type molecule", providing information about the formation of some of the possible intermediates. Finally, the effect of chlorine has been checked.
Experimental Section Several series of catalysts have been prepared by a classical impregnation technique, derived from 1wt % or 2 wt % Pt/A1203 by replacing the indicated atomic ratio of Pt by Re. The catalysts were prepared either chlorine free or as chlorine-containing ones. All results reported on chlorine-free and sulfded catalysts concem 1 wt % loading catalysts. The chlorine-free catalysts (coded Pt/Re/A120g) were prepared from H2Pt(OH), and Re dissolved in HN03, while the chlorine-containing catalysts (coded as Pt/ Re/C1/Al2O3)were prepared from Pt and Re dissolved in aqua regia. All Catalysts, except the 100% Re catalyst, were calcined (450 "C) for 5 h and reduced in situ for 5 h at 400 "C. In the case of the suMded catalysts the reduction was followed by a treatment with a saturated pressure (20 "C) of thiophene/H2 for 3 h (10 mL/min) at 300 "C followed by an additional reduction step at 400 OC for 16 h to remove weakly bound sulfur species (a method similar to that described in ref 23). Prior to the reaction the catalysts were, again, reduced in situ for 5 h at 400 OC. Two commercial catalysts tested were CK 455 (0.472 wt % Pt, 0.475 wt % Re) and CK 473 (0.295 wt % Pt, 0.695 wt % Re). A comparison of all results obtained with the model and commercial catalysts revealed that in the range of loadings used the behavior is not influenced by the total loading. The catalysts were characterized by high-resolution electron microscopy. The Pt/Re/C1/A1203 catalysts contained particles of about 1.0 nm, the pure Pt and pure Re catalysts of about 2.0 nm. With Pt/Re/A1203(chlorine-free) catalyst no metal particles could be made visible. The chlorine content of the catalysts was determined after reduction in situ. Various chlorine-containing catalysts show a chlorine content of 0.7-0.9 wt %, The catalytic experiments were performed in a fured bed, plug flow reactor at varying temperatures. An increasing temperature program was followed by a decreasing temperature program, to check deactivation effects. at each temperature there was an elapse of 30 min between the temperature step and analysis. All (16) Langeveld, van A. D.; Delft, van F. C. M. J. M.; Ponec, V. Surf. Sci. 1983, 134, 665.
1001
i."-"-"
I
0
o
50 %Re
100
Figure 1. Selectivity in a,@-hydrogenolysis(2C fission of the C3-C4bond in 2,2-dimethylbutane) as a function of atomic % Re. The selectivities toward total hydrogenolysis and the secondary products (propane and butane) are also shown (2% Pt/Re/Cl/ A1203 catalysts). Selectivities for two commercial Ketjen Cyanamid catalysts are plotted in the same graph (+) (CK 455, CK 473), T = 275 "C: X, total hydrogenolysis; 0,a,@hydrogenolysis; - -,C3+ Ck Selectivityin isomerization (not shown) is a mirror image of the selectivity in hydrogenolysis with its maximum at the minimum of hydrogenolysis.
-
analyses were fully automated. The ratio neohexane/H2 was 18, and the total flow was 10 mL/min. The conversion was kept as low as possible to minimize consecutive reactions. Data acquisition, evaluation, and selectivitydetermination have been described previously." The various products resulting from the neohexane/H2 reactions point to certain adsorption modes of the hydrocarbon on the metal surface. From the products, neopentane was used for evaluation of the so-called a,B-hydrogenolysis (C3-C4 bond fission), isopentane for the e,y-and isobutane for the a,y'-hydrogenolysis. The isomers 2-methylpentane and 3-methylpentanewere used to calculate the extent of the a,yand a,y'-isomerization, respectively. This is shown schematically by the following:
C
Isomerization: 2,3-dimethyl butane 2- methylpentone
C
3-methyl pent one
Hydrogenolysis:
C1,2-methylbutane C2,Z-methy lpropane
C~,Z-methylbutane Cq,neopentane
Results The Pt/Re catalysts were reduced in situ and tested with 2,2-dimethylbutane (neohexane)/H, reactions a t temperatures below 400 "C. The most important results obtained with various catalysts are presented in Figure 1 for the series of S-free, chlorine-containing catalysts (Pt and Pt/Re/C1/A1203). Catalysts containing high percentages of Re manifest themselves as typical hydrogenolysis catalysts and favor fission a t the C3-C4 bond. On the other hand, Pt and Pt-rich catalysts start hydrogenolysis a t the quaternary (17) Ponec, V.; Sachtler, W. M. H. In Proceedings 5th Znt. Congr. Catal.; Palm Beach, 1972, Hightower, J. W.,Ed.;North Holland/&vier: Amsterdam, 1973; Vol. 1, p 645. Schepers, F. J.; van Broekhoven, E. H.; Ponec, V. J . Catal. 1985, 69,82.
1102 Langmuir, Vol. 4, No. 5, 1988
den Hartog et al.
the main product of isomerization on sulfided catalysts is 2,3-dimethylbutane. This is the only product that can be formed with the classical bifunctional mechanism: CH3 HsC-C-CH=CH2
I
+
H+
-
CH3
I
CH3
-
+-CH3
H3C-C-CH CH3
CH3
I
H3C-C+-CH-CH3
I
CH3
Figure 2. Conversion per gram of catalyst at standard conditions
as a function of the catalyst composition for the sulfided chlorine-free catalyst (T = 350 "C)(1%Pt/Re/S/Al2O9): X, values obtained upon an increasing temperature program; 0,upon a decreasing temperature program. Also plotted is the activity per gram of catalyst for unsulfided chlorine-containing Catalysts (+) (5" = 275 "C) (2% Pt/Re/C1/A1203).
1
100
s
( O h )
@ /o-
..
+
50-
0x
.
0
2
sb
"_
Re
/*
I
100
Figure 3. Group selectivities of neohexane (as a function of the relative atomic % Re) for the sulfided catalysts (1%Pt/Re/ S/A1203): X, hydrogenolysis; 0,isomerization. Also indicated are the values for a 1:l physical mixture consistingof Pt/S/Al,O, and Re/S/A1203 (+). Treact= 350 "C.
carbon (3C, a,?-and a,?'-adsorption, leading to hydrogenolysis). The activity and selectivity pattern change dramatically when the catalysts are presulfided. The deposition of sulfur suppresses the overall activity of the catalysts, as can be seen in Figure 2. Although the activity is much more suppressed a t the Re side compared to the Pt-rich catalysts (the maximum disappears), the activity always remains measurable also with a 100% Re catalyst (Cl-free catalyst at 350 "C). The selectivity is influenced in a way, as indicated in Figure 3. I t is important to realize that
With this mechanism dehydrogenation to olefin on the metal site is followed by the formation of a carbenium ion by proton addition on the acid site of the support. Carbenium ions induce a methyl shift, resulting in isomerization from 2,2-dimethyl- into 2,3-dimethylbutane. For this reason, it was necessary to learn whether the activity of the sulfided catalysts is indeed related to the acid function of the catalyst. To this end, pyridine has been injected into the stream of neohexane/H2 in doses of 5 mL of gas consisting of saturated vapor pressure of pyridine at room temperature (ca. 18 Torr). Dosing by pyridine started at the steady-state performance of the catalyst. In Figure 4,the effect of accumulated doses of pyridine is shown on the rate of hydrogenolysisand isomerization for a sulfided catalyst (a catalyst of the series containing chlorine). The second part of the figure shows the effect when the addition of pyridine is stopped, the third part shows the rates after a re-reduction a t 400 "C for 5 h. Isomerization is suppressed to a larger extent than the hydrogenolysis reactions, as is to be expected for a reaction that makes use of the acidic centers of the catalyst. It is known from other experiments that poisoning of metallic (nonacidic) catalysts is to a great extent reversible, unlike the case shown in Figure 4.
Discussion There has been already a discussion in the literature (see the Introduction) whether some Re is present at all in the working catalysts as a metal. To determine the ionic Re content, the Pt/Re catalysts have been analyzed by chemical extraction methods. Neither of these methods (extraction by diethylamine/acetic acid (4%) or by acetylacetone) was found really satisfactory for Re; the first one tends to underestimate the Re ionic content, the other one to overestimate it. The first method shows that after the standard reduction only a few percent of Re might be ionic,18 according to the second method as much as 25% 40.0 Y (g-l 1
20.0
3
Figure 4. Rate per unit weight of catalyst (1 wt % total loading) of isomerization and hydrogenolysis (75% Pt, 25% Re/S/A1,03, chlorine containing), T,,,& = 350 "C: X, hydrogenolysis; 0,isomerization. First part of the ordinate, mL of pyridine, injected into the hexane/H2 flow; 2nd part, time in hours following the pyridine injections; 3rd part, cf. 2nd part, after a reduction at 400 "C for 5 h.
2,2-Dimethylbutane on PtlRelA1203Catalysts of Re might remain ionic.lg Recently, it has been shown by ESR that the content of Re(IV), the most probable form of “ionic” rhenium, is less than 10%,mand this is probably the most reliable of the available figures. In any case, these are good reasons to discuss below the results obtained here with Pt/Re catalysts, as obtained with “alloy” or “bimetallic’’ cluster catalysts. Although Re itself is a strong hydrogenolytic catalyst, an addition of Re to Pt causes no increase (rather a small decrease) in hydrogenolysis (and in parallel a small increase in isomerization) (see Figure 1). Such nonadditive behavior indicates that alloys are formed. Re, which binds hydrocarbon fragments more strongly than Pt and, hence, slows down the multiple reactions in the adsorbed state,3 can, by suppression of hydrogenolysis of isomerized adsorbed species, enhance the isomerization selectivity. The selectivity Sa, shows that, up to about 50% Re in the catalysts, the behavior is ’Pt-like”, whereas above the mentioned value it is “Re-like”. Both components keep their identity, a t least as seen by evaluating the parameter Sap The two industrial catalysts of a much lower loading of Pt fit very well in the overall picture obtained with model catalysts. The form of the curves of Sa,,and of the overall activity is not unique with 2,2-dimethylbutane (neohexane). Also, some other hydrocarbons show a similar form of curves.3” The apparent decrease of Sa,, a t higher Re content is an effect of multiple reactions. The product used for the determination of the CY,@ adsorption mode (dimethylpropane, neopentane) is further hydrogenolyzed. The fact that multiple reactions are important at high Re content is also demonstrated by the increase in the formation of propane and butane. These products cannot be formed in one step, and thus they are an indication of the extent of multiple reactions occurring on the catalyst. Suppression of the activity by sulfur is, of course, to be expected. Since the affinity of Pt for sulfur is lower than that of Re: the higher degree of suppression of the activity of the Re-containing catalysts (as compared with Pt) does not surprise one either. It has already been r e p ~ r t e d “that ~ ~sulfidization ~ ~ ~ ~ ~ of the catalysts selectively suppresses hydrogenolysis. How. ever, it has not yet been mentioned explicitly that even at the rather low temperature (7-alumine is not that strong an acid) of 350 OC (and lower) the surviving reaction of isomerization is to a great extent catalyzed by the acid centers present in the sulfided catalysts. Yet, the experiments with pyridine point clearly in that direction. When pyridine poisons the metallic function (like with the Si02-supported catalysts, not shown here), the poisoning is to a considerable extent reversible, while the poisoning of the acid centers (see Figure 4) is irreversible.22 Chlorine-containing and chlorine-free catalysts did not differ qualitatively from each other. In both cases isomerization selectivity after sulfidization was over 95%. However, the activity was increased by the presence of (18) Botman, M. J. P.; Ph.D. Thesis, 1987, Leiden University, The Netherlands. (19) Zhang, J. Y.;Botman, M. J. P.; Ponec, V. React. Kinet. Catal. Lett. 1984,26, 189. (20) Nacheff, M. S.;Kraus, L. S.; Ichikawa, M.; Hoffmann, B. M.; Butt, J. B.; Sachtler, W. M. H. J. Catal. 1987,106, 263. (21) Menon, P. G.;Praaad, J. In Proc. 6th Int. Congr. Catal., London, 1976, Chem. SOC.London 1977, Vol. 2, p 1061. (22) Robschlager, K. W.H.; Emeis, C. A.; van Santen. R. A. J. Catal. 1984,86, 1.
Langmuir, Vol. 4, No. 5, 1988 1103 chlorine ions during the impregnation step. This effect is expected when the isomerization is an acid site reaction, since chlorine is known to have a positive effect on the acidity of the support. Reactions of hexane are no subject of this paper, but let us mention a t least in passing that all features of the changes induced by alloying Pt with Re and by the subsequent sulfur and pyridine poisoning are also observed with hexane reactions. Also, reactions of hexane were more sensitive to the acidity of the support as compared with the 2,2-dimethylbutane (neohexane) reactions. Let us return to the introduction and inspect whether something can be said with regard to the theories expressed under points 1-6, starting with the last point. Although the ideas of an essential modification by alloying of the electronic structure of the individual alloy components enjoy a quite general popularity and have been repeatedly used to explain the behavior of Pt/Re catalysts,4,12*23 there are no convincing pieces of evidence that such modification occurs, either with this system or with other relevant similar systems (for general discussion on this subject, see ref 24). On the other hand, the use of CO adsorption as a probe of the electronic structure of surface alloy atoms, whereby IR spectra supply the required information, did not reveal any substantial change in the Pt properties when Pt had been alloyed with Reaz5 Therefore, we do not discuss this point any further. The results with unsulfided catalysts show effects of alloying, but these effects are not related to the carbon deposition, hydrogen spillover, or sintering, points 1-3, although the latter effects occur. The same holds for sulfided alloys, and the results with these catalysts show also that when the effects of points 4 and 5 are operative one has to consider in addition the bifunctional character of the catalyst. Sulfur deposition results in a strong decrease in activity. The shown bifunctional character of the catalysts implies that even though sulfur changes the group selectivities by suppressing hydrogenolysis and dehydrocyclization by affecting the metal one cannot conclude that it is just this effect that results in the strong increase in the selectivity toward isomerization: the main part of the isomerization products stems from the acidic function. When sulfur prevents, graphitization, as has been suggested6 (we have no information on that by the results presented), it also-simultaneously-enhances the relative importance of the acid-catalyzed reactions, as this paper shows. The last points have been up to now neglected in the literature, which is certainly unjustified.
Acknowledgment. The investigations were supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advmcement of pure Research (ZWO). The supports by the Scientific Committees of NATO and EEC, which made a collaboration with laboratories abroad, have been highly appreciated. The AKZO company kindly supplied the commercial catalysts used in this study. Registry No. Pt, 7440-06-4;Re, 7440-15-5;C1, 22537-15-1;S, 7704-34-9; pyridine, 110-86-1. (23) Parera, J. M.;Bertramini, J. N.; Querini, C. A.; Martinelli, E. E.; Churin, E. J.; Aloe, P. E.; Figoli, N. S. J. Catal. 1986, 99,39. (24) Ponec, V. Adu. Catal. 1983, 32, 149. (25) Bastein, A. G.T. M.; Toolenaar, F. J. C. M.; Ponec, V. J. Catal. 1984,90, 88.
