53
Ind. Eng. Chem. Fundam. 1086, 25, 53-58
The Remarkable Active Site: AI in SiOl Paul 9. Welsz Department of Chemical Engineering, Un/versity of Pennsylvania, Philadelphia, Pennsylvania 19 104
Since the demonstration of intracrystalline catalysis, with alumlnosillcate zeolites, a symbiotic development of technology and of fundamental scientlfic Insights has taken place. The nature and quantity of the acidic catalytic sites can be controlled. They can be obtained as substantially identical entitles, analogous to species in ideal solutions. For many reactants, conversion pathways proceed through similar olefinic species as intermediates in a characteristic “box” of catalyst sites. In the silica structure, there is a remarkable coexistence of structural stability with chemical interchangeability of atomic constituents. Elemental substitutions between silicon, aluminum, and other elements can be induced. Tetrahedral aluminum sites can be introduced in several ways, including by migration from a solid alumina binder. Aluminum sites possess uniquely high catalytic turnover numbers. The homopolar nature of the tetrahedral 0-Si-0 bond structure also stands confirmed experimentally.
Water Sorption by Siliceous Solid High surface area silica, SiOz (silica gel), has been a traditional sorbent for water. Also, zeolites, fvst identified in the mineral world, were actually named in recognition of their strong power to sorb water. This has lead to a conscious association of water affinity with Si02 This is, in fact, quite contrary to expectations for the symmetrical tetrahedral structure and nature of the 0-Si-0 bonds (Pauling, 1945), which would ascribe high homopolarity to the bond structure. This matter has been clarified by zeolite research. Figure 2 shows, on the same plot, the molar amounts of water observed by different authors to strongly sorb on zeolites of varying, but low, Al contents. This includes data on various mordenites achieved by various degrees of dealumination (Chen, 1967; Mikunov et al., 1973) and on ZSM-5 zeolites synthesized with varying low aluminum contents (Olson et al., 1980; Olson, 1980). The water molecules are seen to be associated with framework aluminum atoms in a stoichiometric ratio of four H 2 0 atoms per A1 site; this is independent of the particular crystallographic structure of the silica. Thus, the intracrystalline, Le., stoichiometrically correct, SiOz environment is hydrophobic as predicted by theory (Pauling, 1945). Previous experience with classical zeolite structures could not reveal this because, as seen in Figure 2, the amounts of aluminum in the traditional zeolites caused water sorption to exceed the volumetric sorption capacity in all cases. In the zeolites, it is the polar, ionic A1 site that binds water. On the surface of silica gel, we do not have an SiOz environment but Si-OH terminations offering ample hydrogen-bonding interaction for water sorption.
Since the demonstrations of intracrystalline catalysis (Weisz and Frilette, 1960; Frilette et al., 1962; Weisz et al., 1962), with aluminosilicate zeolites, progress in catalytic science and development of technology have occurred simultaneously and in a tightly symbiotic relationship. There are now at least seven new industrial processes based on shape-selective, intracrystalline catalysis operating around the world. In a few months another process will go on stream in New Zealand which is to supply one-third of the country’s gasoline fuels. These processes rely on the A1 sites of zeolite ZSM-5. I will enumerate a few of the fundamental insights that have resulted from the closely knit search for science and technology. Evolving a Definable and Controllable Site The objective of designing practically useful molecular shape-selective catalysts created the incentive to produce siliceous zeolites with structural stability, thermally and particularly toward hydrolytic attack of the structure, in the environments of chemical processing. Laboratory research, using the only synthetic zeolites available (A, X, and Y) and several mineral zeolites (chabazite, erionite, and mordenite), indicated that hydrolytic attack on the framework aluminum atom would cause loss of structural integrity. Two logical pathways were examined: One was the dealuminization of existing zeolites, to remove structural Al and hopefully to “heal” the vacancies; the other was to synthesize new zeolites with fewer A1 atoms in the framework. With the discovery of zeolite ZSM-5 (Argauer and Landolt, 1972; Olson et al., 1981), we were able to move from aluminosilicate frameworks in which both silicon and aluminum were major and structure-determiningbuilding blocks to basically SiO, frameworks, with occasional, near randomly positioned aluminum substitutions. This is well illustrated visually by a comparison of the framework models of zeolite Y (left in Figure 1) and zeolite ZSM-5 (right in Figure 1). The A1 sites are now available in a dilute and isolated state. In physical-chemical terms we now have a situation analogous to that of an ideal solution. Furthermore, these sites, being intracrystalline, are accurately definable, chemically and stoichiometrically, in contrast to surface sites traditionally terminating a solid. As a consequence, we are now in a position to rigorously examine many basic questions concerning the reactivities of sites.
Counting the Sites The proportionality between the number of water molecules adsorbed and the aluminum contents of the zeolite already suggested the uniformity of sites in the silica framework. The measurement of the amount of water sorbed can be considered to be a titration method for the counting of these sites. High-resolution NMR techniques for nAl that have been developed (Fyfe et al., 1982; Oldfield and Kirkpatrick, 1985) now provide a unique and sensitive tool for quantification, specifically, of the number of A1 atoms in the tetrahedral sllicon oxide framework down to levels of parts per million of A1 atoms. The evolution of this technique 0
1986 American Chemical
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54
I d . Eng. Chem. Fundam.. Vol. 25. No. 1. 1986
!
Figure I. Models of zeolites. (left) Zeolite Y. AI stoma (black spheres) are integral parts of the structure. (right) ZSM-5. AI atoms are in arbitrarily small "dilution".
Enzyme Competition We have stated (Haag et al., 1984) that the turnover numbers of the AI sites compare with enzymatic performance, except that we employ higher temperatures. Actually, the question whether the latter qualification is needed presents quite a challenge to research. Some of the test reactions used require the higher temperatures for thermodynamic reasons. On the other hand, reactions like double-bond isomerization are subject to experimental difficulties because of polymerization side reactions prevalent a t lower temperature. Thus,we have yet to test the reaction rates of the zeolite sites a t experimentally meaningful conditions, for suitable molecules and reactions a t temperatures approaching, say,
.I5
.IO
.os
C
40 "C. A
OWXOENITES 0 ZSM-5's
MDRDENlTE
Figure 2. Water aorption w. AI content (T= 25 OC, PIP,, = 0.042): ( 0 ) Chen, 1967; (A)Mikunov et al.. 1973; ( 0 )Olson et al., 1980; Olson, 1980. Water molecules are sorbed in 4 1 ratio at AI sites. In traditional zeolites, AI concentration is high but water sorption is limited hy available sorption volume.
was influenced by the interest in the ZSM-5 zeolite as a prominent catalytic material. This development and ultraelectron microscopy may well be the fmt technique to (a) d e w and count catalytic sites in a solid and to (b) observe their molecular environment, respectively (Weisz, 1982). Absolute Reaction Rates a n d Turnover Numbers These developments now allow us to determine per-site turnover rates and minimum turnover numbers for a few hydrocarbon reactions (Haag et al., 1984). They range up to magnitudes of at least 105-106per minute. Rates of some reactions turn out to be so fast that on siliceous zeolites they can cause high conversions under typical catalytic operating conditions a t AI concentrations frequently unrecognized or dismissed as "trace" quantities. For some reactions, like olefin double-bond isomerization, appreciable conversion can obviously take place for AI concentrations well below the detection limit of many analytical procedures. In 1962 we reported (Mikovsky and Weisz, 1962) the generation of double-bond shift activity in a "pure" silica by neutron irradiation. We concluded then that the nuclear cross sections for nuclear Si-to-AI transformations were surely too small to explain the result by AI generation; that conclusion may require reexamination.
Individual Molecule-Site Interactions The strong sorption of ammonia as ammonium ion has been adopted as another measure for the number of acid aluminum sites (Ken and Chester, 1971; Topsoe et al., 1981) characterized by a desorption peak at about 400 "C in temperature-programmed desorption (TPD). A TPD procedure which follows preparation of the ammoniumzeolite by ion exchange yields particularly "clean" results. Excess ammonia can produce lower temperature desorption peaks that sometimes are interpreted (Anderson et al., 1979; Babu et al., 1981) as catalytically relevant sites of lower acidity. Such a "lower acidity" peak can be associated with ammonia adsorption a t a residual sodium cation site. It contributes no activity, nor does a still lower temperature signal associated with surface or impurity Si-OH groups.. We are enterine an era of observing. auantitativelv and in real-time circukstances, the fate Gf a variety of molecules on the active sites. We can move from sorbents, like H 2 0 and NH3, to potential or real reactants. For example, Negishi et al. (1984) observed irreversible sorption of methanol on ZSM-5, at 25 OC. Using their data, we find two methanol molecules chemisorbing per AI site. Grady and Gorte (1985) have shown by craftsmanship in technique that in the case of 2-propano1, too, two molecules will associate with ZSM-5 sites at 20 "C. They follow the sequence of reaction events upon raising the temperature. Figure 3 sketches their observations for the fate of 2-propanol on the sites. Near 100 O C one alcohol molecule is left and converted to a propylcarbenium ion and H20. The former will desorb at somewhat higher temperature. Later, a little Cs olefin desorbs, probably as a result of occasional contact of
Ind. Eng. Chem. Fundam.. Vol. 25, No. 1, 1986
Oc
w'%l
I
Methanol
-- 1 c,.
--LITTLE
---1I
loo{
55
C;
t
Cg DESORBS---------
I
20
%"
DESORBS
10
0
and 1 H20 SORBED
CJOH+E~
--I
T
---1.4
370%
DESORBS
t
n20
t ---2.4
C$ SORB
Carbon number
Figure 5. Similarity of product distributions (carbon number, aliphatics, aromatics) obtained from a large variety of charge molecules, on ZSM-5 catalyst. 2-PROPANOL
Figure 3. Stoichiometric events, sorption, conversion, and desorption observed (Grady and Gorte, 1985) on AI sites of ZSM-5. The symbol C indicates that a earhenium ion species is presumed. CATALYST
zsu-5
MONTMORILLONITE (AI
T I M ON STREAM rims
HYDROCARBON PRODUCTS. C,+
x a MTHIML io(.)
0 TO 5000
80
0.1
C70
TROPSCH
BIOMASS
LIOUIO
(El 0.5 TO 1.5
C8
SCUDS ONLY DISTRIBUTION
Figure 6. The 'olefin boa" concept of conversion steps proceeding in a bed of acidic conversion catalyst. Figure 4. Comparison of reactivity of acidic clay and H-ZSM-5 catalyst for methanol-to-hydrocarbonreaction (370 "C, 1 atm).
propylene with carbenium ion. The high reactivity at 20 "C of the propylcarhenium ion entities toward gas-phase propene is demonstrated on the right. Contacting sites with propene at 20 "C immediately generate C-6 and C-9 products; only these products and no propene can henceforth be desorbed. Collisions between gas-phase propene and reactive site-sorbed propene molecules cannot be avoided during loading. This is a preview of potentials for the study of reactivities of a great variety of individual molecules on carbenium species on sites. Reactivity of the Site Assembly The discovery by Chang and Silvestri (1977) of selective transformation of methanol molecules t o a hydrocarbon mixture which has the composition of an aromatic gasoline led to the development of the methanol-to-gasolineprocess (Meisel et al., 1976) and also spawned important insights into basic characteristics of the reactions of hydrocarbon entities on the acidic catalyst sites. An early reaction to the discovery was the assumption that ZSM-5 was performing a new "C-l^ chemistry; how-
ever, further research in the Mobil laboratories convinced us that other acidic solids were also able to perform that chemistry, except for an impracticable short period of activity. As seen in the data of Figure 4, a clay catalyst will also generate hydrocarbons, but only for a short period of time (Weisz, 1983). ZSM-5, by virtue of its intracrystalline constraint on molecular shape and size, does not generate appreciable aromatic species larger than C9to C,o, which precludes the generation of "coke" precursors and the otherwise inevitable deactivation of catalyst. By now we have accumulated many observations of conversions of many other organic structures. When we view these in their entirety, we observe that, at temperatures above about 700 K, a similar product spectrum is approached when the starting materials are as diverse as methanol, butanol, heptanol (Chang and Silvestri, 1977), olefins (Haag et al., 1982), paraffins, the entire FischerTropsch synthesis product (Weisz, 1982), isoprene, limonene, squalene, dipentene, triglycerides, and triglyceride esters (Weisz et al., 1979; Haag et al., 1982). This is well illustrated by the comparisons in Figure 5. These observations lead to the conclusion that there is a common feature to the mechanisms of transformation of these diverse reactants, that, in fact, at some point of
56
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 .5
I
I
I
I
0
.05
.I
.I5
.2
MOLES OF AROMATICS
Figure 7. Computer collection of diverse data from development of the methanol-to-gasoline process. Paraffin/aromatics are in 3:l molar ratio.
the reaction paths, substantially the same set of reaction intermediates is generated. It appears, as illustrated in Figure 6, that olefin species are generated (unlessthe reactants are olefins already), and these undergo transformations among themselves in the ”box” which represents the assembly of catalyst sites. The transformations (oligomerization, polymerization, cracking) tend toward a distribution in size determined by thermodynamics (Haag et al., 1982; Weisz et al., 1979; Haag, 1984). In the case of methanol, the rapid alkylation of olefin by methanol adds to the carbon chain growth process among these olefins, as long as methanol reactant is available (Chen and Reagan, 1979; Kaeding and Butter, 1980; Anderson et al., 1980). Then, for those olefinic entities having six or more carbon atoms, there follow hydrogen-transfer reactions which form aromatics and paraffins (Haag et al., 1984). The latter reaction has a stoichiometric requirement of producing three moles of paraffins per mole of aromatics. In the development of the methanol-to-gasoline conversion process, a computer was called upon to read out the stored data of total molar quantities of paraffins and of aromatics produced from a variety of laboratory and pilot operations, catalysts (ZSM-5), and operating conditions. The data are presented in the plot, Figure 7, confirming the 3/ 1 ratio. The relative amount of aromatics is determined by the stoichiometric constraint of the H/C ratio of the starting material. When heteroatoms are present, an effective H/C can be defined (Haag et al., 1980) which takes into account the loss of hydrogen by dehydration, deamination, etc. For example, in oxygen-containingcompounds, we have (H/C),ff = (H - 2 ( 0 X w))/(C - 0 X cm - 0
X
cd
X
0.5)
where w, cm, and cd are the fractions of oxygen rejected as water, carbon monoxide, and carbon dioxide, respectively. The selectivity, i.e., the relative amounts of product species, is determined by the point in the above reaction path sequence at which the process is ended. Selectivity: Quantity and Quality of Sites A multitude of experiences with hydrocarbon reactions suggest that, as regards selectivity toward reactants or toward product species, the quality of Bransted A1 sites is the same-at least for the “dilute”, siliceous zeolitesregardless of “acidity” in terms of A1 site density. That is, that performance is solely determined by the number
100
10,boo
__t
1
bo
1,do0
Si%/ALzO,
1
RATIO
Figure 8. Propylene conversion (20%). Catalytic conversion is the same for any silica/alumina ratio when feed rate is adjusted proportionately, Le., activity per A1 site is the same.
of sites in the reactor and the residence time of the charge therein. Figure 8 illustrates this for H-ZSM-5 catalyst. The same performance is obtained (Haag, 1984) in the cracking of propylene for a thousandfold variation in the A1 contents (Le., activity) of the catalyst, provided, of course, the residence time is adjusted in inverse proportion. In this case the A1 contents were varied from 1.3% down to 160 ppm which corresponds to a silica/alumina ratio of 5600. Frequently, apparent “selectivity” changes (such as different olefin/aromatics product ratios) have been attributed to qualitative catalyst variants. Often they result from employing fewer active A1 sies, that is, a less active catalyst. Different methods of eliminating sites may also give the same result, as exemplified by using partial alkali metal ion poisoning as compared to higher silica/alumina ratios (i.e., much lower Al concentration), as shown by Dwyer and Garwood (1984) for the conversion of Fischer-Tropsch synthesis product. Chemical Integrity of the Silica Framework The utility of zeolites as industrial catalyasts naturally induced studies concerning their stability. Also, there has been a logical interest in possibly substituting the Al sites by other elements. Such inquiries are generating new information and discoveries concerning the chemical integrity and exchangeability of framework elements. For example, the tetrahedral framework of silica, once presumed to be one of the stablest configurations of chemical bonds, will exchange oxygen atoms with water molecules with great facility, as shown by von Ballmoos and Meier (1982). A t 95 “C, four oxygens are exchanged per A1 in H-ZSM-6, in a matter of 2 days. This, we observe, is consistent with the finding (see above) of the intimate association of four water molecules per acid A1 site. In addition, the framework Si-0-Si oxygens are also exchanged at a slower rate but are still countable in units of days; this process itself appears to be catalyzed by the number of A1 sites present. The tetrahedral atoms of the central framework have proven to be exchangeable for other elements.
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 57 ZEOLITE
ZEOLITE
ZEOLITE
40
c; c3 c4 c, c5
+
c-, c, c, c5
c, +
c5
+
Figure 9. Creation of aluminosilicate site activity by solid-to-solid transfer of A1 atoms from alumina to siliceous zeolite.
Removal of A1 and “healing”, that is, their exchange by Si atoms, is accomplished by Sic& (Chang, 1981; Fyfe et al., 1983). Highly siliceous structures are thus made. The inverse can also be accomplished: A1 can be introduced (Chang et al., 1984) from gaseous AlC13 and also from aqueous aluminates. Also, one substituent, like Al, can be replaced by another by exposure to the appropriate trichloride, as illustrated by the reaction A1(Si02) + BC13 =B(Si02) i + AlC13 This was recently demonstrated by E. G. Derouane et al. (1985). In the case of the pair A1 and B, only the A1 site was shown to contribute catalytic activity.
Solid-to-Solid Transfer of Aluminum Atoms It is important to note that even solid-to-solid transfer and migration of Al atoms will take place from an external alumina to the intracrystalline framework of zeolite crystals (Shihabi et al., 1985; Chang et al., 1985; Chu et al., 1985). This generates new or additional catalytically active aluminosilicate sites in the zeolite. This occurs with steam in hydrothermal environments, both at low and high temperature. Since such conditions accompany even the routine operations of extruding a zeolite catalyst with an alumina binder, the catalytic consequences of this phenomenon can be startling (Shihabi et al., 1985), as demonstrated in Figure 9. Here, a fivefold increase in catalytic activity is obtained by the “mere” binding of the catalyst by extrusion. The product distribution also changes due to the progress of reaction further down the olefin reaction path. Thus, it is difficult to avoid the creation of active aluminosilicate zeolite entities, when alumina-bound zeolite composites are made from relatively “pure” siliceous zeolite. The A1 atom transfer has been demonstrated (Chu et al., 1985)to be further facilitated by the presence initially of framework substitutions other than Al, such as by the presence of framework boron sites in ZSM-5. Again, catalytic activity is generated by the new aluminosilicate sites. This is discussed by C. T.-W. Chu and C. D. Chang (1985) at this meeting and is consistent with the results reported by E. G. Derouane et al. (1985); no measurable activity could be attributed to boron sites. The revelations of the chemical variability of the siliceous framework have important implications, not only to catalytic science and developments but to the chemistry
of siliceous oxides, to the mineralogy of zeolites and other siliceous oxides, and to geochemistry. Conclusions The evolution of zeolite catalysis provides an outstanding example for symbiosis between basic research and applied industrial development. Not only did basic findings open doors to new technology, but the observations and inquiries undertaken during development provided new insights to basic catalytic science. The following are some of the key elements of new knowledge and capabilities that emerged: The substantial homopolarity of Si-0 bonds in the tetrahedral silica structure (hydrophobicity), predicted by Pauling, was demonstrated early in the process of zeolite catalyst research. Zeolites have made it possible to control and quantify the numbers of active sites for acid catalysis and to have them available in substantial uniformity. As a result, well-defined, individual sorption, desorption, and reaction steps can be studied. Some characteristic common reaction steps in acid-catalyzed hydrocarbon conversion have been identified. The protonated, tetrahedral aluminum site in the silica structure has been clearly demonstrated to be the “active site”. High catalytic activities per site (turnover numbers) can be created and determined quantitatively. Parts per million of aluminum can evoke useful, high conversion rates. Contrary to conventionalthought that an Si02structure must be very stable and very inert to chemical change, some surprising structural flexibilities at relatively mild conditions have been revealed: Nearly all structural oxygen can be exchangeable in a matter of hours; aluminum atoms can replace silicon and, often unintentionally, generate catalytic activity. Registry No. Al, 7429-90-5. Literature Cited Anderson, J. R.; Foger, K.; Mole, T.; Rajadhyaksha, R. A.; Sanders, J. V. J . . . Catal. 1979, 5 8 ; 114. Anderson, J. R.; Mole, T.; Christov, V. J . Catal. 1980, 6 1 , 477. Argauer, R. J.; Landott, G. R. US. Patent 3 702 886, 1972. Babu, G. P.; Hegde, S. G.; Kulkarnl. S. B.; Ratnasamy, P. J . Catal. 1981, 8 1 , 471. Cha.ng,C. D.; Sllvestrl, A. J. J . Catal. 1977, 4 7 , 249. Chang, C. D. U. S. Patent 4273753, 1981. Chang, C. D.; Chu, C. T.-W.; Miale, J. N.; Bridger, R. F.; Calvert, R. B. J . Am. Chem. SOC. 1984, 106, 8143. Chang, C. D.; Hellring, S. D.; Miale, J. N.; Schmltt, K. D. J . Chem. SOC., Faraday Trans. 1 , In press. Chang, C. D.; Chu, C. T.-W.; Kuehl, G. H.; Lago, R. M. Prep.-Dlv. Pet. Chem., Am. Chem. SOC. 1985. Chen, N. Y. J . Phys. Chem. 1976, 8 0 , 60. Chen, N. Y.; Reagan, W. J. J . Catal. 1979, 5 9 , 123. Chu, C. T.-W.; Kuehl, G. H.; Lago, R. M.; Chang, C. D. J . Catal., in press. Derouane, E. G.; Baitusis, L.; Dessau, R. M.; Schmltt, K. D. “Catalysis by Acids and Bases”; Imelik, B., Ed.; Elsevier: Amsterdam, 1985; p 135 ff. Dwyer, F. G.; Garwood, W. E. “Catalytic Conversions from Synthesis Gas and Alcohols to Chemicals”; Herman, R. G., Ed., Plenum Press: New York, 1964. Frilette, V. J.; Welsz, P. 8.; Golden, R. L. J . Catal. 1962, 1 , 301. Fyfe, C. A.; Gobbi, G. C.; Kllnowski, J.; Thomas, J. M.; Ramdas, S. Nature (London) 1982, 2 9 6 , 530. Fyfe, C. A.; Thomas, J. M.; Klinowski, J.; Gobi, G. C. Angew. Chem. 1983, 22, 259. Grady, M. C.; Gorte, R. J. J . Catal., in press. Haag, W. 0.:Lago, R. M.; Welsz, P. 8. Nature (London) 1984, 309, 589-591. Haag, W. 0.; Lago, R. M.; Rodewaid, P. G. J . Mol. Catal. 1982, 17, 161. Haag. W. 0.;Rodewald, P. G.; Weisz, P. B. Prepr.-Div. Pet. Chem., Am. Chem. SOC. 1980. Haag, W. 0. In “Proceedings of the 6th International Conference on Zeolites”; Bisio, A., Olson, D. H., Olson, Eds.; Butterworth Scientific Ltd.: Surrey, England, 1984. Haag. W. 0.;Olson, D. H.; Welsz, P. 8. Chem. Future, R o c . IUPAC Congr., 29th, 1983 1984. Haag, W. O., Mobil Research and Development Corp., personal communication, 1984. Kaedlng, W. A.; Butter, S. A. J . Catal. 1980, 6 1 , 115. Kerr, G. T.; Chester, A. W. Thermochim. Acta 1971, 3 , 113. Meisel, S.L.; Lechthaler, C. H.; McCuiiough. J. P.; Weisz, P. B. CHEMECH 1978, 6 , 86-89.
58
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Chem. Fundam. 1986, 25, 58-62
Mlkovsky, R. J.; Welsz, P. B. J. Catal. 1962, 7 , 345. Mlkunov, B. I.; Yakerson, V. I.; Lafer. L. I.; Rublnshteln, A. M. Izv. Akad. Nauk SSSR, Ser. Khim. 1973, 449. Ngishl. H.; Sasakl, M.; Iwaki, T.; Hayes, K. F.; Yasunaga, T. J. Phys. Chem. 1984, 88, 5584. Oldfield. E.; Kirkpatrick, R. J. Nature (London) 1985, 227, 1537. Olson, D. H.; Kokotallo, G. T.; Lawton, S. L. J . Phys. Chem. 1981. 85, 2238. Olson, D. H.; Haag, W. 0.; Lago, R. M. J . Catal. 1980, 67,390. Olson, D. H., Mobll Research and Development Corp., private communication, 1980. Pauling, L. “The Nature of the Chemical Bond”; Cornel1 University Press: Ithaca, NY, 1945. Shihabi, D. S.;Garwood, W. E.; Chu, P.; Miale, J. N.; Lago, R . M.; Chu, C. T.-W.; Chang. C. D. J. Catai., In press. Topsoe, N. Y.; Pederson, K.; Derouane, E. G. J. Catal. 1981, 7 0 , 41.
von Ballmoos, R.; Meler, W. M. J. Phys. Cbem. 1962, 86,2698. Welsz, P 6.Faraday Discuss. Chem. SOC. 1982, 72, 378. Welsz, P. E.,paper presented at the Catalysis Meeting of the Japanese Petroleum Institute, Tokyo, May 9, 1983. Weisz, P. B., paper presented at the International Coal Conversion Conference of the CSIR, Pretoria, S.A., 1982. Welsz, P. 6.;Frilette, V. J. J. Pbys. Chem. 1960, 6 4 , 382. Weisz, P. 6.; Frilette. V. J.; Maatman, R . W.; Mower, E. 6.J. Catal. 1962, 7 . 307. Welsz, P. 6.; Haag, W. 0.; Rodewald, P. G. Science 1979, 206, 57.
Received f o r review June 24, 1985 Revised manuscript received October 24, 1985 Accepted November 8, 1985
Model Studies of Cu/Ru Bimetallic Catalysts Charles H. F. Peden and D. Wayne Goodman’ Surface Science Division, Sandla Natlonal Laboratories, Albuquerque, New Mexico 87 785
The activity of a model Ru(0001) catalyst for the methanation and ethane hydrogenolysis reactions has been measured in a high pressureultrahigh vacuum surface analysis apparatus as a function of impurity (sulfur or copper) coverage. Unlike sulfur, which was found to poison the activity at quite low coverages, the role of copper was to simply block active ruthenium sites on a one-to-one basis. This latter result is in contrast to results obtained on supported Cu/Ru catalysts which show a marked reduction in activity upon addition of Cu. The discrepancy is attributed to an error in counting the active Ru surface area on the supported catalysts by selective hydrogen chemisorption techniques. I t is demonstrated here that hydrogen, once dissociated on Ru, can spill over onto Cu, making an overcount of Ru surface atoms possible.
Introduction The modification of catalytic behavior by surface impurities is of crucial importance to most catalytic processing. At present, the mechanisms responsible for surface chemical changes induced by surface additives are poorly understood. However, the current interest and activity in this area of research promise an emerging understanding of the fundamentals by which impurities alter surface chemistry. A pivotal question concerns the relative importance of steric (local) vs. electronic (extended or “long-range”) effects. A general answer to this question will critically influence the degree to which we will ultimately be able to tailor-make exceptionally efficient catalysts by fine-tuning material electronic structure. If, indeed, low surface impurity concentrations can profoundly alter the surface electronic structure and thus catalytic activity, then the possibilities for the systematic manipulation of these properties via additive selection would appear limitless. On the other hand, if steric effects dominate the mode by which surface additives alter the catalytic chemistry, a different set of considerations for catalyst alteration come into play, a set which will most certainly be more constraining than the former. In the final analysis, an understanding will include components of “electronic” and “steric” effects, the relative importance to be assessed for each reaction and its conditions. A major emphasis of our research has been in the area of addressing and partitioning the importance of these two effects in the role of surface additives in catalysis. The modification of catalytic performance by surface additives is an extremely difficult question to address experimentally (Imelik et al., 1982). For example, the interpretation of related data on dispersed catalysts is severely limited by the uncertainty concerning the structural and compositional characteristics of the active sur0196-43131861 1025-0058$01 .SO10
face. Specific surface areas cannot always be determined with adequate precision. In addition, a knowledge of the crystal orientation, the concentration and the distribution of impurity atoms, and their electronic states is generally poor. The surface concentration of the impurity may vary considerably along the catalytic bed, and the impurity may very well reside on the suport as well as the metal. Moreover, the active surface may be altered in an uncontrolled manner as a result of sintering or faceting during the reaction itself. The use of metal single crystals in catalytic reaction studies essentially eliminates the difficulties mentioned above and, to a large extent, allows the utilization of a homogeneous surface amenable to study using modern surface analytical techniques. These techniques allow detailed surface characterization regarding surface structure and composition. Carefully controlled, single-crystal catalytic surfaces are particularly suited to the study of impurity effects on catalytic behavior because of the ease with which impurity atoms can be uniformly introduced onto the surface, in many cases, with the knowledge of the impurity atom position. To date, relatively few studies have incorporated both surface science techniques with kinetics at elevated pressures (1atm). Kinetics, however, are an essential link between these kinds of model catalytic studies and the more relevant catalytic systems, establishing the crucial connection between the reaction rate parameters. Although the studies to date are few, the results appear quite promising in addressing the fundamental aspects of catalytic modification by surface additives. Impurities whose electronegativities are greater than those for transition metals generally poison a variety of catalytic reactions, particularly those involving Hzand CO. Considerable work has been invested in defining the 0 1986 American Chemical Society
