J . Phys. Chem. 1984,88. 2185-2195 the homo- and heteronuclear pentamers all lie in the range 2.03-2.08 and are significantly larger than the free spin value. This large and positive Ag has been attributed to spin-orbit interaction between the ground state and a neighboring filled level which introduces p and/or d orbital character into the singly occupied molecular ~ r b i t a l . ~ , ~ The unpaired s spin populations on the atoms bearing most of the unpaired s spin population are all similar although it does appear that silver bears a larger spin population than copper. From the measured anisotropic parameters the calculated unpaired s spin populations are reasonable whereas the p and/or d values appear to be essentially zero. It should, however, be realized that the errors involved in measuring these parameters, because of the broadness of the absorptions and the number which can be measured, are probably of the order of k 5 G. These errors are sufficiently large that while they have little effect on the calculated unpaired s spin populations they make evaluation of the p and/or d contribution less meaningful because only small changes are required in the hyperfine splittings to make substantial differences to the estimated populations. We have previously concluded that CuS2and Agj3 have a distorted trigonal-bipyramidal structure with a 2Bzin C, electronic ground state, i.e., most of the unpaired s spin population resides on the terminal equatorial atoms. The ESR parameters listed in Table I are consistent with similar structures and electronic ground states for the homo- and heteronuclear pentamers although (9) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577-82.
2185
the undistorted trigonal-bipyramidal structures for CuAg, and CuzAg3have C,, symmetry and do not need to distort to lift the degeneracy of a 2E ground state as is the case with the homo, structure indicates a *B2ground state nuclear pentamers. A C for CuzAg3whereas a ground state cannot be assigned to CuAg4 with this symmetry. We have previously noted3 that a ZB1ground state with most of the unpaired s spin population on the axial atoms of a trigonal bipyramid cannot be discounted for Ag, (and Cu,). CuAg, with the copper atom and unique silver atom occupying the axial positions, Le., a ,Al (C3,) ground state and CuzAg3with the two Cu atoms occupying the axial positions, Le., a ,A2/1 (D3h) ground state, are, therefore, possible structures for the mixed pentamers. These structures are in fact more reasonable than C, structures because the smaller Cu atom might be expected to prefer an axial position in a close-packed structure. Unfortunately, electronegativity and thermodynamic arguments are inconclusive in determining the distribution of atoms in heternuclear trigonal-bipyramidal clusters because Cu and Ag have similar electronegativities and bond strengths are not known with the required accuracy. Finally, although we cannot unambiguously assign electronic ground states to CuAg, and Cu2Ag3they have a trigonal-bipyramidal structure. Furthermore, the results presented in this paper demonstrate that bimetallic clusters of a reasonable size can be produced by simple orbital mixing processes and identified by ESR spectroscopy using our techniques. Acknowledgment. J.A.H. and B.M. thank NATO for a collaborative research grant.
FEATURE ARTICLE Structure Sensitlvlty of Hydrocarbon Synthesls from CO and H, M. Boudart* and Mark A. McDonald Department of Chemical Engineering, Stanford University, Stanford, California 94305 (Received: September 29, 1983)
Hydrocarbon synthesis from CO-HZ mixtures on group 8 metals is discussed from the viewpoint of structure sensitivity. On the most active metals which dissociate CO and produce a broad distribution of hydrocarbons under reaction conditions, the synthesis seems to proceed at the highest rate on large ensembles of surface atoms, pointing to structure sensitivity. However, for the less-activegroup 8 metals, which do not dissociate CO readily and do not produce substantial amounts of hydrocarbons heavier than methane under reaction conditions, no clear trends can be discerned. Hydrocarbon synthesis on these catalysts can be structure sensitive under some conditions, but the effects of structure are quite different from those observed for the most active catalysts. Thus, our review suggests that both the nature of the catalyst surface and the mechanism of methane synthesis vary substantially among group 8 metals, so that no general conclusion can be made about the structure sensitivity of methane synthesis.
I. Introduction The catalytic synthesis of hydrocarbons from CO and Hz has a long history. Eighty years ago Sabatier f i s t made methane from C O and Hz, a catalytic reaction now called methanation.’J Catalytic production of liquid hydrocarbons and other organic molecules from a CO-H, synthesis gas now called syngas has been carried out since the 1920’~.~3, This is the so-called Fischer~
(1) Sabatier, P.;Senderens, J. B. Compt. Rend. 1902, 134, 512. (2) Sabatier, P.; Senderens, J. B. J . SOC.Chem. Ind. 1902, 31, 504.
Tropsch (FT) synthesis, commercialized in Germany 50 years ago and currently used in South Africa to make liquid fuels from coal., The name FT synthesis will be used in this work even though we shall be concerned mostly with work at low conversion and atmospheric pressure, conditions that are remote from those of commercial operation. (3) Fischer, F.; Tropsch, H. Brennst. Chem. 1923, 4, 276. (4) Fischer, F.; Tropsch, H. Brennst. Chem. 1926, 7 , 97. (5) Dry, M. E. In ‘Catalysis, Science and Technology”, Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Heidelberg, 1981; Vol. 1, p 159.
0022-365418412088-2185$01.50/0 0 1984 American Chemical Society
2186
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
TABLE I: Effects of Metal Structure, Additives, and Nature of the Metal on Activitv for Catalvtic Reactionsa effectb of
+
H, D, = 2HD C,H, + H, = C,H, c-C,H, + H, = C,H, C,H6 + 3H, = C 6 H , , C,H,
N,
+ H , = 2CH,
structure
additives
vs vs vs vs
S S S
M
S
hl
L L
VL L
S
+ 3H, = 2NH,
M
a Adapted from ref 6. is classified as follows:
I
nature of metal
M M
The order of magnitude of the effect
10 102 104 i o 6 UP vs s M L VL
Boudart and McDonald TABLE 11: Reactions of Importance in CO-H, Synthesis Gas paraffin synthesis methanation C,+ production olefin synthesis alcohol synthesis methanol C,‘ water-gas shift carbon (“coke” C,) deposition C O disproportionation bulk carbide formation bulk oxide formation
+ CO = CH, + H,O + nCO = CnH2n+2 + rlH,O 2 n H , t nCO = C,H,, + nH,O
(1) (2)
2H, + CO = CH,OH 2nH, + nCO = C,H,,+,OH (n - l)H,O CO + H,O = CO, + H, C O H, = C t H,O nC = C, 2co= co, nC = C , xM t C=M,C yM + O=M,O
(4) (5)
3H,
( 2 n + 1)H,
+
c+
+
(3)
(6) (7)
(8) (9) (10)
11. Hydrocarbon Synthesis Processes
Fundamental understanding of hydrocarbon synthesis from syngas has lagged behind these commercial applications, despite extensive research during the past decade. The structure sensitivity of hydrocarbon synthesis is among the unresolved questions. Structure is defined as the atomic arrangement at the surface. Turnover frequencies (Le., number of molecules of reactants consumed or products yielded per titrated site per second) of structure-sensitive reactions should vary appreciably with varying catalyst structure, e.g., on different crystallographic planes or on small particles of 1-10 nm size. The reverse is true for structure-insensitive reactions. As noted by one of us, this is because a structure-sensitive reaction seems to require a multiple-atom site or ensemble on the catalyst surface.6 Thus, recognition of structure sensitivity not only indicates the existence of an optimum catalyst with the most active structure but may also suggest the nature of the optimum sites involved in the rate-determining step. One of us has described three classes of experiments that can be used to infer structure sensitivity.6 Variation in structure has already been mentioned. Rates of structure-sensitive reactions also show a strong decrease upon introduction of additives, as in alloying. Alloying is defined here as the addition of an inactive metal or poison, e.g., sulfur, to the surface of a catalytically active metal. Changes in rate upon introduction of additives can be ascribed to an ensemble effect or a ligand effect. Finally, rates of structure-sensitive reactions should also depend strongly on the choice of catalytic metal used (specificity). Table I shows the size of these effects on structure-insensitive and structure-sensitive reactions, respectively. This article reviews studies pertinent to structure sensitivity of hydrocarbon synthesis, both methanation and FT reactions. Following a discussion of CO hydrogenation processes in section I1 and of reaction mechanism as it pertains to structure sensitivity in section 111, the relevant experimental work will be reviewed in sections IV-VI. No attempt will be made to review the extensive methanation and FT literature or to discuss in detail proposed reaction mechanisms, since this has been done e l ~ e w h e r e . ~ - ’ ~
(6) Boudart, M. Proc. Inr. Congr. Catal., 6rh 1977, 1, 1. (7) Mills, G. A.; Steffgen, F. W. Caral. Reu. 1974, 8, 159. (8) Anderson, R. B. In ‘Catalysis”; Emmett, P. H., Ed.; Reinhold: New York, 1956; Vol. 4, p 29. (9) Greyson, M. In ‘Catalysis”; Emmett, P.H.,Ed.; Reinhold: New York, 1956; Vol. 4, p 473. (10) Cohn, E. M. In ‘Catalysis”; Emmett, P. H., Ed.; Reinhold: New York, 1956; Vol. 4, p 443. (11) Pichler, H. Adu. Cafal.1952, 4,271. (12) Vannice, M. A. Cafal. Rev.-Sci. Bng. 1976, 14, 153. (13) Vannice, M. A. In “Catalysis, Science and Technology”; Anderson, J. R.; Boudart, M., Eds.; Springer-Verlag: Heidelberg, 1982; Vol. 3, p 139. (14) Somorjai, G. A. Cutal. Reu.-Sci. Eng. 1981, 23, 189. (15) Bell, A. T. Card. Reu.-Sci. Eng. 1981, 23, 203. (16) King, D. L., Cusumano, J. A., Garten, R. L. Caral. Rev.-Sci. Eng. 1981, 23, 233. (17) Biloen, P.; Sachtler, W. M. H. Adu. Coral. 1981, 30, 165. (18) Storch, H. H.; Golumbic, N.; Anderson, R. B. “The Fischer-Tropsch and Related Syntheses”; Wiley: New York, 1951. (19) Ponec, V. Catalysis 1982, 5,48.
The hydrogenation of C O produces a wide variety of products depending on temperature, reactant partial pressures, and nature of the catalyst. Except for production of the C1 products (CH4 and CH30H), selectivity of CO-H2 reactions to a single organic species has not been accomplished thus far. Conversion of syngas results in a mixture of different products requiring additional separation. These organic products are linear paraffins, olefins, and alcohols, with little chain branching.8 The main reactions involved in hydrocarbon synthesis are listed in Table I1 together with the side reactions affecting activity and selectivity. Methanation converts a syngas of low heating value (5-6 M J m-3) to a synthetic natural gas (mainly CH4) with heating values of about 35-40 M J m-3.7 Nickel catalysts are preferred for methanation because of their low cost and good activity and selectivity. Typical conditions are 1-7 MPa at 550-675 K. A ratio of H 2 / C 0 = 3.0 is desirable, as it favors methane production while minimizing carbon deposition. Promoted Co supported on kieselguhr or reduced Fe with oxide promoters are the traditional FT catalysts. The H 2 / C 0 ratio is less than 2.0, the limiting stoichiometric ratio for production of heavy hydrocarbons. Typical conditions are 0.5-5.0 MPa at 450-550 K. The product yield differs on the two catalysts.18 Unlike other FT catalysts, Fe produces a large amount of alcohols and olefins at these pressures. Furthermore, metallic iron is converted to a complex mixture of metal, carbide, and oxide phases18,20and is completely carbided at lower conversions. The Co catalysts make mostly paraffins, some olefins, essentially no alcohols, and remain in the metallic state. Ruthenium catalysts are more active than Fe or Co and produce even heavier hydrocarbons at 5-15 MPaZ1but have not been used commercially because of cost. Other group 8 metals (Rh, Pd, Ir, Pt, Os) are less active in methanation and even worse for FT synthesis.8 Among nongroup 8 elements, Mo and W have shown activity for hydrocarbon s y n t h e s i ~ . ~ ~ * ~ ~ Bench-scale C O hydrogenation studies are typically done a t atmospheric pressure and differential C O conversions. For simplicity, the most studied catalysts are Ru or Ni, since they produce only hydrocarbons, although carbon deposition at the surface often deactivates the catalysts. With Fe, bulk carbides are also formed, which are accompanied by increased catalyst a ~ t i v i t y . ~Less ~,~~ active group 8 metals show little carbon deposition.26 111. Mechanistic Arguments for Structure Sensitivity
The large amount of work during the past decade has prompted a correspondingly large number of proposed reaction mechanisms (20) Anderson, R. B., Hofer, L. J. E.; Cohn, E. M.; Seligman, B. J . Am. Chem. SOC.1951, 73, 944. (21) Pichler, H. Brennsr. Chem. 1938, 19, 226. (22) Leclerq, L.; Imura, K.; Yoshida, S.; Barbee, T.; Boudart, M. “Preparation of Catalysts”; Delmon, B.; Grange, P.; Jacobs, P.; Poncelet, G., Eds.; Elsevier: Amsterdam, 1979; Vol. 11, p 627. (23) Kelley, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Cutul. 1977, 50, 301. (24) Amelse, J. A,; Butt, J. B.; Schwartz, L. H. J. Phys. Chem. 1978,82, 558. (25) Raupp, G. B.; Delgass, W. N. J . Catal. 1979, 58, 361. Vannice, M. A. J . Cutal. 1981, 71, 167. (26) Wang, S.-Y.;Moon, S. H.;
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2187
Feature Article for methanation and FT synthesis. The methanation mechanisms tend to fall into two categories, both consistent with kinetic data: hydrogenation of molecularly adsorbed C O followed at some point by subsequent d e h y d r a t i ~ n , ~ or J ~dissociation * ~ ~ * ~ ~ of C O (possibly assisted by adsorbed hydrogen) followed by hydrogenation of carbon and oxygen fragments.14*15*17,29 While the hydrogenation mechanism was favored during most of the past 30 years, the dissociation mechanism has become more popular recently. With the latter mechanism, one out of two kinetically significant steps must be retained: dissociation of CO and hydrogenation of carbon species on surface sites *: co 2* e c * (1)
+
CH,*
+ o* + yH* & CH,,* + y*
(2)
The above steps may or may not be reversible and the number of sites or values of x and y are debatable. Methanation of CO-H2 is suspected a priori to be structure sensitive because of its analogy with NH3 synthesis from N2-H2.30 The structure sensitivity of N H 3 synthesis” results from the structure-sensitive ratedetermining step, N2 dissociation, analogous to eq 1: Nz 2* + 2N* (3) The C O molecule is isoelectronic with Nz:z so that it is plausible that the same types of sites are required for dissociation of both molecules. Therefore, methanation can be structure sensitive unless CO dissociation is an equilibrated step in the reaction sequence. Less is known about the structure sensitivity of the proposed hydrogenation step, eq 2. But if that reaction is structure sensitive from right to left as has been s ~ g g e s t e d ?it~is also likely to be structure sensitive from left to right, unless conditions are too far removed from equilibrium. Ideas about FT synthesis parallel those about methanation. For many years, researchers favored mechanisms involving CO or -CHOH addition to a growing alkyl chain, followed by hydrogenation and d e h y d r a t i ~ n . ~ *More ~ ~ * recent ~ ~ * ~ work, ~ however, supports the formation of CH, intermediates (formed from C O dissociation and subsequent hydrogenation of the carbon species) which add onto the growing hydrocarbon chains C,Hm:15917,29 C,Hm* + CH,* 3 Cn+lHm+,* * (4)
+
+
This implies one-carbon fragments add on to growing hydrocarbon chains until the chain growth is terminated (desorption as an olefin or paraffin). If conditions favor chain growth, then a hydrocarbon distribution similar to that observed in addition polymerization is usually referred to as the Schulz-Flory distribution. The chain growth step shown in eq 4 suggests structure sensitivity in the FT reactions. Indeed, it would require a relatively large site, larger than that required for C O dissociation. Furthermore, the reverse reaction is very similar to a hydrogenolysis reaction,38 which is known to be structure sensitive: If it is kept in mind that C O dissociation is also expected to be structure sensitive, it is reasonable that FT reactions would be structure sensitive. This discussion is meant only to suggest that structure sensitivity of CO-H2 reactions can be expected. The steps for hydrocarbon synthesis shown in eq 1, 2, and 4 are derived mainly from work on Ru or Ni. Iron? Rh,39and Pd@ can synthesize hydrocarbons ~~
~~
~~
~~
(27) Kozub, G. M.; Rusov, M. T.; Vlasenko, V. M. Kiner. Karol. 1965,6, 244. (28) Pichler, H.; Schulz, H. Chem. Ing. Tech. 1970, 42, 1162. (29) Ponec, V. Catal. Reus-Sci.Eng. 1978, 28, 151. (30) Jones, A.; McNicol, B. D. J. Card. 1977, 47, 384. (31) Boudart, M. Caral. Rev.-Sci. Eng. 1981, 23, 1. (32) Madon, R. J., Shaw, H. Catal. Reo.-Sci. Eng. 1977, 15, 69. (33) Frennet, A.; Lienard, G.; Degols, L.; Cruq, A. Bull. Soc. Chim. Belg. 1979, 88, 621. (34) Olive, H.; Olive, S. Angew. Chem., Inr. Ed. Engl. 1976, 15, 136. (35) Friedel, R. A.; Anderson, R. B. J . Am. Chem. SOC.1950, 72, 1212. (36) Friedel, R. A.; Anderson, R. B. J . Am. Chem. SOC.1950,72, 2307. (37) Herrington, E. F. G. Chem. Ind. 1946, 65, 347. (38) Dalmon, J. A.; Martin, G. A. Proc. Inr. Congr. Catal. 7rh 1981,402. (39) Castner, D. G.; Blackadar, R. L.; Somorjai, G. A. J. Catul. 1980,66, 257.
0
20
40 OIo
60
80
cu
Figure 1. Rate of hydrocarbon synthesis (arbitrary units) on Ni-Cu alloys, as a function of atomic percent copper: (1) formation of C2 and C3 hydrocarbons; (2) formation of methane (Ni-Cu powders, atmospheric pressure, 593 K, H 2 / C 0 = 1, ref 43); (3) formation of methane (Ni-Cu foils, 77 Pa, 573 K, H 2 / C 0 = 5, ref 41). Adapted from van Barneveld and Ponec, ref 43.
and/or oxygenates, depending on experimental conditions and the chemical state of the catalyst. Such a change in products almost certainly reflects a change in reaction mechanism. It seems unlikely, then, that a reaction mechanism for Ni or Ru should explain results for Fe or Pd equally well. IV. Effects of Additives A. Nickel. Araki and Ponec studied methanation on Ni and Ni-Cu films in a batch system at 77 Pa, HJCO = 5.0, and T = 573 Ke4’ The rate of methanation based on total metal area dropped an order of magnitude upon addition of 10% Cu to Ni. The authors attributed this substantial effect of alloying to depletion of ensembles of surface Ni atoms required for C O dissociation, since apparent activation energies showed no trend with respect to percent Cu. Although Cu surface enrichment in Ni-Cu alloys has been observed in the case of Ni-Cu powders42and could explain the magnitude of the alloying effect, bulk and surface compositions were found to be the same for the films of Araki and Ponec. van Barneveld and pone^^^ obtained similar results on unsupported Ni-Cu powders in a differential flow reactor at atmospheric pressure, with H 2 / C 0 = 1.0. These conditions yielded C2 and C3 hydrocarbons in addition to CH4. The methanation rate decreased two orders of magnitude upon addition of 10% Cu. Production of higher hydrocarbons decreased by a factor of almost 100 over the same range. Thus, selectivity to C2+hydrocarbons increased slightly with Cu addition. These results are shown in Figure 1, along with the previous results.41 The alloying effect was again attributed to a depletion of Ni ensembles on the catalyst surface required for both CH4 and Cz+production. Work involving supported Ni-Cu catalysts may show particle size effects in addition to alloying effects, but the latter appears to be dominant and of the same magnitude as on unsupported Luyten et al. prepared supported Ni-Cu/Si02 catalysts of 5% Ni and Cu/Ni atomic ratios between 0.0 and 1.0.44 Hydrogen adsorption used to titrate surface Ni showed no Cu surface enrichment and no important particle size effect. The turnover frequency of C O conversion to hydrocarbons, Nco, (40) Fajula, F.; Anthony, R. G.; Lunsford, J. H. J . C a r d 1982, 73, 237. (41) Araki, M.; Ponec, V. J . Cural. 1976, 44, 439. (42) Iglesia, E. I.; Boudart, M. J . Caral. 1983, 82, 204. (43) van Barneveld, W. A. A.; Ponec, V. J . Cara2. 1978, 51, 426. (44) Luyten, L. J. M.; v. Eck, M.; v. Grondelle, J.; v. Hooff, J. H. C. J . Phys. Chem. 1978,82, 2000.
2188 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
Boudart and McDonald
10
0
20 OIo
30
40
cu
Figure 3. Total turnover rate, N , for synthesis of CH4 (0),C2H6( O ) , and CoHa (A) on Ni-Cu/Si02 as a function of atomic percent copper,
at atmospheric pressure and 523 K from H2/C0 = 4. Adapted from Dalmon and Martin, ref 38.
1.6
1.8
-/ 1o3
2.0 K-'
T Figure 2. Arrhenius plot for the turnover rate for CO hydrogenation to
-7
-i
0.1
Y, \
hydrocarbons, Nco, on silica-supported Ni-Cu and Ru-Cu at atmospheric pressure from H2/C0 = 3. Adapted from Luyten et al., ref 44. measured in a flow reactor at atmospheric pressure decreased two orders of magnitude upon Cu addition (top half of Figure 2). This is a strong alloying effect that suggests structure sensitivity, since Nco based on active metal surface should show no decrease upon alloying for a structure-insensitive reaction. In contrast to results decreased slightly of Ponec and c o - ~ o r k e r s , Cz+ ~ ~ . selectivity ~~ with Cu addition. However, apparent activation energies for CO conversion remained constant, in agreement with the trend for unsupported Ni-Cu. The alloying effect was also ascribed to the geometric effect of dilution of Ni ensembles, and ligand effects were unimportant. Dalmon and Martin found similar results with Ni-Cu/Si02 with metal particles ca. 6 nm in diameter.38 They measured CHI, C2H6, and C3H8production in a differential flow reactor at atmospheric pressure (H,/CO = 4.0, T = 523 K). Figure 3 shows that alloying Cu to Ni decreased both activity and selectivity to Cz+ products by factors similar to those in ref 44. Similarly, activation energies for CH4, C2H6,and C3H8production did not depend on Cu mole fraction. Researchers at the National Bureau of Standards have studied the nickel (100) plane with varying amounts of sulfur adsorbed on the otherwise clean surface.45 Deposition of uniform sulfur layers on Ni( 100) should have an effect on Ni ensembles analogous to that of Cu addition. At 16.0 kPa, H2/C0 = 4.0, and 600 K, the turnover rate for methanation, NCH, decreased by a factor of 50 as the sulfur coverage, Os, increased to 0.25, corresponding to thep(2X2) sulfur layer (Figure 4). Further deposition of sulfur up to saturation coverage [Os = 0.5, c(2X2)] did not change the rate. The apparent activation energy for methanation did not change with addition of sulfur, implying that sulfur blocks sites on the N i surface without exerting substantial ligand effects. B. Ruthenium. Both NCHIand selectivity to Cz+ products on ruthenium are higher than on nickel.* Also, unlike Ni, Ru is (45) Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981, 105, L265.
0
0.1
0.2
0.3
0.4
0.5
9s Figure 4. Turnover rate for methanation, NCH,, on Ni(100) at 16.0 kPa and 600 K from H2/C0 = 4, as a function of Os, the fraction of surface covered with sulfur. Adapted from Goodman and Kiskinova, ref 45.
immiscible with Cu, although supported Ru-Cu bimetallic clusters can be prepared.46 Support interactions and particle size effects may make an alloying effect more difficult to interpret for small bimetallic Ru-Cu clusters, yet results with Ru-Cu are consistent with those obtained with Ni-Cu. Luyten et al." measured Nm for Ru-Cu/SiOz catalysts under essentially the same conditions as for Ni-Cu/SiOz catalysts: H2/C0 = 3.0 or 1.O, atmospheric pressure, differential conversion, Cu/Ru from 0.0 to 1.0 with constant 2% Ru loading. Results shown in the lower half of Figure 2 are quite similar to those reported for Ni-Cu, as previously discussed, and were similarly explained as the result of dilution of active Ru ensembles. With addition of Cu, apparent activation energies remained constant, and selectivity to C2+products decreased. Catalyst characterization is less certain than for Ni-Cu systems, but the H/Ru ratios determined by hydroen titration decreased linearly with percent Cu, indicating that Cu was replacing Ru atoms at the catalyst surface. A complication may arise as it is unclear how addition of Cu to Ru influences the particle size and, as will be seen later, areal rates have been found to increase with particle size.17 Furthermore, the catalysts with atomic ratio Cu/Ru = 0.5 and 1.O showed evidence of increased Ru dispersion during reaction. (46)Sinfelt, J. H.J . Coral. 1973, 29, 308. (47) King, D.L.J . Coral. 1978, SI, 386.
Feature Article Therefore, although results for Ru-Cu cannot be considered as clear-cut, they indicate the same trends as those for Ni-Cu. The work of Bond and T ~ r n h a m ,on ~ Ru-Cu alloys shows a smaller effect of Cu addition and a greater uncertainty in catalyst characterization. Their samples contained 1% Ru on S i 0 2with Cu/Ru atomic ratios varying from 0.0 to 1.O. Rates were measured in a recirculation reactor, with total pressure typically equal to 10.7 Wa, H 2 / C 0 = 3.0, and T = 600 K. Rates were calculated from the change in total pressure, with the assumption that CH, and water were the only significant products. Values of NCH4 decreased by a factor of 50 as the atomic ratio Cu/Ru changed from 0.0 to 1.O. The reaction yielded relatively small amounts of C2+products, but amounts of C2 and C3 products decreased by a factor of 100 over the same range, of Cu/Ru ratios, Le., C2+ selectivity decreased from the addition of Cu. Apparent activation energies for methanation were constant within experimental error. These results are consistent with those of Luyten et al.,, But it must be kept in mind that it is uncertain what degree Ru and Cu were incorporated in the same particles and to what degree Cu segregated to the surface. Furthermore, the use of pressure change in a recirculation system is not the best way to measure rates of C O hydrogenation. The results of Elliott and L ~ n s f o r dmust ~ ~ also be considered qualitative. They prepared Ru, Ru-Ni, Ru-Cu, Ni, and Cu clusters in Y zeolites. Catalysts were made with a constant 0.5% Ru loading while the Cu loading varied, giving samples with Cu/Ru from 0.0 to 6.5. Initial NcH4values (80 kPa, H 2 / C 0 = 3, T = 553 K) decreased by an order of magnitude as Cu/Ru increased to 1.0. The C2+ selectivity increased over the same Cu/Ru range, in agreement with the findings of van Barneveld and P0nec.4~ Contrary to previous results, the apparent activation energy increased upon Cu addition. From the viewpoint of structure sensitivity, these results must also be regarded as qualitative because of the same factors clouding interpretation of Ku-Cu studies and because zeolites may interact more strongly with the metallic particles than SO,, thus influencing selectivity in a manner which is unclear. Miura and G o n ~ a l e have z ~ ~ recently found evidence of what appears to be a ligand effect in methanation on Ru-Pt clusters supported on Si02. In this case, addition of Pt to Ru resulted in the same trend as for Cu addition. Rates of methanation of an H 2 / C 0 = 3.0 synthesis gas at 500 K showed a greater-thanh e a r decrease with percent surface Pt, indicating that Ru atom ensembles are necessary for reaction. Apparent activation energies decrease monotonously, indicating a ligand effect. This decrease in rate correlated with their finding that the temperature needed to dissociate C O (via disproportionation) increased with percent Pt. C. Effect of Additives: A Summary. As can be seen from Table I, rates of structure-sensitive reactions are affected by several orders of magnitude as a result of alloying. Sinfelt et ala5'found that addition of 6.2% Cu to Ni decreased ethane hydrogenolysis activity three orders of magnitude, while further addition of Cu up to 74% decreased activity by an additional factor of 100. Addition of Cu (or S) to Ni or Ru does not decrease rates of CH4 or C,+ production as dramatically, but the effect is certainly much larger than for the structure-insensitive reactions in Table I. Besides, in assessing alloying effects in CO-HZ reactions, the relatively small differences in activity between pure Ni or Ru catalysts and pure Cu (or S-poisoned Ni) catalysts must be kept in perspective. Vannice reports NCH4on Ru/SiOz andNi/Si02 to be three and two orders of magnitude greater than on Cu/Si02, r e ~ p e c t i v e l y . ~Similarly, ~ NCH4 on Ni is 50 times greater than on a thoroughly S-poisoned catalysts.53 Consequently, one would not expect alloying effects of a larger magnitude than have been found. More important is the rapid decrease in activity with Bond, G. C.; Turnham, B. J . Cutul. 1976, 45, 128. Elliott, D. J.; Lunsford, J. H. J . Cutul. 1979, 57, 11. Miura, H.; Gonzalez, R. D. J . Cutul. 1982, 74, 216. Sinfelt, J. H.; Carter, J. L.; Yates, D. J. C. J . Curd 1972, 24, 283. (52) Vannice, M. A. J . Curd. 1977, 50, 228. (53) Dalla Betta, R.A.; Piken, A. G.; Shelef, M. J . Cutul. 1975, 40, 173. (48) (49) (50) (51)
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2189 TABLE 111: Effects of Additives activity decrease'
catalyst
CH,
Ni-Cu foil Ni-Cu powders Ni-Cu/ SiO,
10 100 5 5 10 5 2
Ni-Cu/Si02 Ni(100)-S Ru-Cu/SiO, Ru-Cu/SiO,
Ru-CU/Y
ref
C2+
41 43 44 38 45 44
50 10 10
10
48
2 2
49 a The factors denote the decrease in activity upon addition of 10% Cu (or S) for synthesis of CII, and C 2 + . 2
I
I
I
0 0
I 10
I 20 o/o
I
I
'
I
30
40
50
60
Ni EXPOSED
Figure 5. Turnover rate for methanation, NCH4, on Ni/SiOz at 525 K (0,0) and Ni/AI,O, at 500 K (A), as a function of percent Ni exposed. Adapted from Bartholomew et al., ref 58. addition of a small amount of Cu or S, characteristic of a structure-sensitive reaction requiring a sizable ensemble. The Ni-Cu work in particular supports such an interpretation, in a more decisive way than the work with Ru-Cu. But taken together, the evidence from both types of alloys studied indicates that an ensemble effect is important, and ligand effects are minor. N o cogent reason exists to believe that, in FT synthesis, C2+ production is more structure sensitive than CH4 production. The change in the C2+/CH4ratio with alloying was a factor of two or less in each of these studies. Such changes are small compared to the magnitude of alloying effects on activity and may be caused by alloying effects on secondary reactions occurring under FT conditions (such as hydrogen~lysis,~~ h y d r ~ g e n a t i o nisomeri,~~ ~ a t i o n , , and ~ , ~ incorporation ~ of C2 and C3 olefins into growing hydrocarbon chainss7). The low molecular weight fractions (Ci-C,) typically deviate from Schulz-Flory distributions and are therefore apparently sensitive to such secondary reactions. Small changes in selectivity with alloying are thus unimportant and indicate that CH4 and C2+production in FT synthesis have the same degree of structure sensitivity, possibly as a result of a common reaction step. A summary of alloying effects can be found in Table 111.
IV. Effects of Structure A . Nickel. Bartholomew et al.58measured effects of particle size and support on hydrocarbon synthesis on Ni supported on A1203,SOz, and TiO,. This was done in a differential conversion flow reactor at 140 kPa and 500-550 K with H 2 / C 0 = 4.0 in 95% N2. These catalysts were well characterized by C O and H2 adsorption, TEM, and XRD.59 For Ni/A1203, NCH4 increased fourfold with increasing Ni particle diameter d , inversely proportional to percent metal exposed. For Ni/SiO, a small, though similar, increase of NCHa with d was also found (Figure 5). (54) Kellner, C. S.; Bell, A. T. J . Cutul. 1981, 70, 418. (55) Ott, G. L.; Fleisch, T.; Delgass, W. N. J . Cutuf. 1980, 65, 253. (56) Novak, S.; Madon, R. J.; Suhl, H. J . Cutul. 1982, 77, 141. (57) Dwyer, D. J.; Somorjai, G. A. J . Cutul. 1979, 56, 249. (58) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J . Cutuf.1980, 65,
335. (59) Bartholomew, C . H.; Pannell, R. B. J . Cutul. 1980, 65, 390.
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
2190
Boudart and McDonald I
s
. -z
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Figure 6. Ratio of (total Cz+hydrocarbons)/CH4on Ni/Si02 at 525 K (0)and Ni/A120, at 500 K ( 0 ) . Adapted from Bartholomew et al., ref
58.
Selectivity to C2+decreased with d on both Ni/Al2O3 and Ni/Si02 (Figure 6 ) . The effect of d on C2+ selectivity is stronger for Ni/A1203 catalysts even when the different temperatures used are accounted for, 500 K for Ni/A1203vs. 525 K for Ni/Si02. Apparent activation energies for methanation varied but showed no trend with respect to d or support. Decreased H2 adsorption on N i / T i 0 2 and small particles of Ni on A1203 was considered indicative of a so-called strong metal-support interaction (SMSI), the nature of which remains unclear. The main effect on activity, then, appears to be the nature of the support used, as NCH4decreased in the order Ni/Ti02 > Ni/Al2O3 > Ni/Si02, which is also the order of decreasing support interactions. The increase in C2+ selectivity with decreasing d for Ni/A1203 correlated with the increase in ratio of CO/H2 uptake on small Ni particles. Thus, the hydrogen deficiency on small particles of Ni on A1203favored chain growth over methane production. Since the magnitude of particle size effect increased with magnitude of support interaction, the former was considered to be of secondary importance. Bhatia and othersm studied methanation on Ni/A1203 catalysts in a nondifferential flow reactor operating at atmospheric pressure. At T = 548 K and H2/C0 = 3.0, NCH4 increased by a factor of approximately six as d increased from 2.1 to 14.4 nm. Apparent activation energies varied somewhat, although no general trend was observed. However, these results are somewhat questionable, primarily because the measurements of percent metal exposed appear anomalous. Indeed, the CO adsorption on the catalyst of lowest Ni loading appears abnormally low, making it the catalyst with the largest calculated N i particle size. This is a departure from the usual observation of increasing particle size with increasing metal loading, as observed on all other samples. In the work of Coenen et NCH4 increased tenfold with d (Figure 7) over the 0.5-5-nm range for Ni/Si02 at 479 K, H2/C0 = 10, and 100 kPa. Apparent activation energies for methanation increased slightly with increasing particle size. Ethane was the only other hydrocarbon observed, with selectivity (ethane/CO converted) dropping abruptly from 2% for smallest particles to 0.8% for d > 1.2 nm. Dalmon and Martin3* also studied particle size effects on hydrocarbon synthesis. They used Ni/Si02 with metal particle sizes between 2.5 and 25 nm as determined by H2uptake and magnetic methods. Particle size did not affect rate of methanation at the conditions used ( H 2 / C 0 = 4.0, T = 488 K), while the rate of ethane production increased more than one order of magnitude with increasing particle size (Figure 8). Activation energies of CH4, C2H6, and C3Hs production were the same as those for Ni-Cu/Si02 catalysts. These particle size effects on activity and C2+selectivity clearly contradict those reported by Bartholomew and ~ o - w o r k e r s . ~Considering ~,~~ their companion Ni-Cu/Si02 work, they concluded that ensemble effects were important and proposed that FT reactions be regarded as reverse hydrogenolysis. (60) Bhatia, S.; Bakhshi, N. N.; Mathews, J. F. Can. J. Chem. Eng. 1978, 56, 575. (61) Coenen, J. W. E.; Schats, W. M. T. M.; van Meertetl, R. Z. C. Bull. SOC.Chim. Belg. 1979, 88, 435.
dlnrn Figure 7. Turnover rate for methanation, N,,,
on Ni/Si02 as a function of Ni particle size d, at atmospheric pressure and 479 K from H 2 / C 0 = 10. Adapted from Coenen et al., ref 61.
0
10
20
30
d/nm Figure 8. Turnover rate, N , for synthesis of CH4 (0)and CzH6(0) on Ni/Si02 as a function of Ni particle size d at atmosphericpressure and 488 K from H2/C0 = 4. Adapted from Dalmon and Martin, ref 38.
Several other workers have studied particle size effects on Ni over severely limited ranges. Vannice was the first to do a systematic study of turnover frequencies for unsupported Ni and Ni on various supports, mainly SO,, 01-A1203,and ~ - A 1 ~ 0 ,A. ~ ~ small crystallite size effect was inferred, as the best-dispersed catalysts had the largest turnover frequencies, although the differences at 478 K were less than an order of magnitude. However, as particle diameters were >8 nm, this study is outside the range where particle size effects are important. F ~ n t a i n found e ~ ~ that NCHI on a Ni/Si02 catalyst for 7-nm particle diameter was twice that for a catalyst of d = 5 nm. This is also outside of the optimum range for study of particle size effect, although the results are qualitatively similar to those of Coenen et a1.61 Among the most thorough studies in recent years are those of Goodman and co-workers at the National Bureau of Standa r d ~ . ~ ~These , ~ @authors acquired an impressive amount of data on methanation on clean Ni single-crystal surfaces, mainly the (100) plane, over wide ranges of pressure (0.1-200 kPa), tem(62) Vannice, M. A. J. Catal. 1976, 44, 152. (63) Fontaine, R. Ph.D. Dissertation, Cornell University, 1973. (64) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; and Yates, J. Y.J. Card 1980, 63, 226. (65) Kelley, R. D.; Goodman, D. W. Prep., Diu.Fuel Chem., Am. Chem. SOC.1980, 25 (2), 43.
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2191
Feature Article 800K 700K 1 . 1
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' d0 ' 40 ' 60 ' 810 ' 1 j O 'lo Ru EXPOSED Figure 10. Turnover rate for methanation, NeH4,on alumina-supported (0,0, A) and unsupported (A)Ru as a function of percent Ru exposed. Adapted from Kellner and Bell, ref 66. ld6k
1.4
1.6
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Figure 9. Arrhenius plot of turnover rate for methanation, NCH4, on Ni( 100) and Ni( 111) at 16.0 kPa from H2/C0 = 4. Rates reported in ref 12 for Ni/AI20, are also shown. Adapted from Kelley and Goodman, ref 65.
TABLE IV: Activity of Supported Ruthenium Catalysts (250 "C)' STY/10-, s - '
% C O %metal convrn exposed for CO for CH, 0.5%/A1,0,a 2.8 60 28 10.4 1.25%/A1,0,a 11.0 44 60 19.5 1.8%/A10 a 18.0 44 68 20.1 2.5%/A1:0: 26.0 34 89 37.1 2.5%/A1,0,C 22.0 23 117 43.3 6.3 1.5%/SiO, 14 92 32.2 2.5 S/SiO, 12.9 8 183 73.2 2%/Si0,-Al,0,d 11.5 11 154 67.0 2%/Si0,-Al,0,e 12.0 10 148 49.3 2%/Cr,03-A1, 0, 20.0 24 114 33.5 2%/Cr ,0, -Al, O,g 9.8 49 31 7.7 2.8%/Th0, 19.0 29 69 19.6 2%/Th02-A1,03h 16.0 27 89 31.8 2.5%/Na-X 18.0 22 91 33.4 2.5%/ultrastable zeolite 21.0 35 75 26.9 unsupported Ru 14.5 0.3 193 143.0 a -y-Al,O,, pore diameter (pd) = 12 nm. y-Al,O , p d = 14 nm. y-Al,O,,, p d = 4 nm. 10% SiO,. e 85% SO,. ?15% Cr,O,, commercial preparation. 15% Cr,O,, laboratory preparation. 10% Tho,. Adapted from King, ref 47. catalyst
perature (450-800 K), and H 2 / C 0 ratio (0.1-1000). At 16 P a was the same on both the Ni(100) and and H,/CO = 4, NCH4 Ni( 111) planes65 (Figure 9). Furthermore, these measurements compare well with those of Vannice for Ni/A1203,12shown by dashed lines in the figure. Thus, no structure effects can be inferred from studies on unpoisoned Ni surfaces, whereas sulfur deposition indicated a possible structure s e n s i t i ~ i t y .The ~ ~ authors found a strong correlation between surface carbidic carbon built up under reaction conditions and the measured reaction rate. As the concentration of surface carbidic carbon increased, the rate of methanation decreased very markedly. B. Ruthenium. Several studies show a clearer particle size effect for Ru than for Ni. King47studied the methanation activity and Cz+selectivity for Ru catalysts of varying dispersion on several supports in a flow reactor, a t 400 kPa and 523 K, with Hz/CO = 2.0. Table IV shows the rate data as a function of percent metal exposed. Even thouch C O conversions are too high for precise coinparison of rates (>20% in a few cases), the site-time yield
for methanation (STYcH,, the reactor-averaged value of N C H 4 ) on supported catalysts increased by a factor of seven with increasing d . The trend is the same even if one limits the comparison to catalysts on A1203. A 2.5% Ru/A1203 catalyst with 23% metal exposed had STY values for methanation and C O conversion approximately four times those of a 0.5% Ru/Al2O3catalyst with 60% metal exposed. A fourfold increase over this small particle size range (approximately 1.5-4 nm) confirms a particle size effect on methanation, although the support had a slight effect on activity for methanation and a stronger effect on C2+selectivity. Unsupported Ru of low percent metal exposed had a STYcH, fourteen times that of 0.5% Ru/A1203but produced a much smaller C2+ fraction. Kellner and Bell also found FT activity increased with increasing d for Ru/A1203catalysts.66 They measured Cl-Cl4 production for differential CO conversions at 0.1 and 1 MPa. Figure 10 shows a particle size on NcH4 of the same magnitude as that found by King47for percent metal exposed between 30 and 70 (King's results are also shown in the figure). The value of NCH4 dropped even more dramatically when Ru exposed exceeded 70%. The C2+ selectivity was constant for Ru exposed 3 nm, and STYcH, values in both studies for Pd/Si02 were about the same for d 3 nm. Structure sensitivity for methanation on supported Pd may therefore be seen only on very small particles which can disproportionate C O much more readily than bulk Pd. Increased selectivity to methanol on larger particles and higher STYcH, on smaller particles are easily explained by disproportionation of C O on the smaller particles but not the larger ones if it is admitted that dissociative adsorption of C O leads to methane while associative adsorption of C O yields methanol. Vannice studied Pt catalysts of varying dispersion and on several supports, originally inferring a particle size effect. Indeed, on Pt black NCHl was more than two orders of magnitude less than on a well-dispersed Pt/A1203catalyst.78 However, in retrospect, this appears more likely to be a support effect, as was inferred for Pd catalyst^.^^^^^^^^ Recent work showed no particle size effect at atmospheric pressure on Pt, and that activity was determined mainly by the nature of the support.81 Values of NCH4 decreased by two orders of magnitude in the order Pt/TiO, > Pt/A1,03 > N
-
(75) Niemantsverdriet, J. W.; van der Kraan, A. M.; van Dijk, W. L.; van der Baan, H. S. J . Phys. Chem. 1980, 84, 3363. (76) Topsm, H.; Topsoe, N.; Bohlbro, H.; Dumesic, J. A. Proc. Inr. Congr. Catal. 7rh 1981. 247. (77) Dwyer, D. J.; Somorjai, G. A. J . Caral. 1978, 52, 291. (78) Vannice. M. A. J. Catal. 1975. 40. 129. (79j Vannice,’M. A.; Garten, R. L. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18, 186. (80) Ichikawa, S.; Poppa, H.; Boudart, M., to appear in ACS Symp. Ser. (81) Vannice, M. A,; Twu, C. C. J . Catal. 1983, 82, 213.
Pt/SiO,, the same trend as observed for Pd catalysts. E . Effect of Structure: A Summary. The effect of structural variations on catalytic activity are generally smaller than effects of additives (Table I). Ammonia synthesis on iron is strongly structure sensitive,31and its STY increased by over an order of magnitude as particle size of Fe on MgO increased from 1.5 to 30 nm.82 Spencer et al. measured rates of N H 3 synthesis at 2 MPa and 798 K on different crystal faces of iron and found relative rates of 428:25:1for the ( 1 1 l):(lOO):(110) planes, respectively, perhaps the largest structure effect ever measured, especially at such a high t e m p e r a t ~ r e . ~By ~ contrast, structure-insensitive reactions show essentially no change in rate with particle size or crystal plane variations. The C O hydrogenation reactions, however, do not show a clear-cut behavior similar to either extreme. Results on Ni are the most contradictory. Structure variations have been reported to produce no ~ h a n g e ,a~moderate ~,~~ or a strong change61in NCH4. Similarly, C,’ selectivity has been reported to increase38or decreases8 with increasing particle size. Several factors may account for this. One possible factor is surface contamination by sulfur to which Ni is known to be extremely sensitive. Conflicting results of sulfur on methanation activity of Ni have been r e p ~ r t e d , ’but ~ it is clear that any sulfur present in the catalyst would affect the results as discussed above in the section on effects of additives. The amount of sulfur, the type of catalyst, the origin of the sulfur (Le., as sulfate during catalyst preparation, or as H2S or organic sulfur in reactant streams), and the details of catalyst treatment could influence the effect of sulfur on Ni catalysts, and thus the particle size effect expected. Let us note in this connection that the most anomalous result on the structure insensitivity of Ni in methanation is also the only one for which the absence of sulfur from the surface was ensured by the UHV methodology and checked by Auger electron spectrosc~py.~~ Another important factor is the degree of carbon coverage of the catalyst surface. Most workers have observed an accumulation of carbidic carbon on Ni during hydrocarbon synthesis. It is suggested here that, under conditions of high surface coverage by carbon, the situation is very similar to that found for hydrogenation of alkenes and aromatics on transition metals, reactions that are structure insensitive in an exemplary manners6 During the course of these reactions, the metal surface seems largely covered with hydrocarbon intermediates, the rate of reaction of which with hydrogen being what is kinetically significant. But methanation on Ni may become structure sensitive under conditions where carbon deposition is not excessive, Le., high H,/CO ratios or in the presence of sulfur. In this connection, it is interesting that the only strong particle size effect on methanation with Ni was recorded for an unusually high value of the reactant ratio H 2 / C 0 = 10.0.61Addition of S to a Ni surface is well-known to cut down surface carbon in the case of catalytic re-forming of hydrocarbons, which is stoichiometrically the reverse of hydrocarbon synthesis from syngaseg4 And indeed, as discussed in the section on alloying effects, addition of S to Ni affects methanation in a way typical of structure-sensitive reactions. Effects of structure variation are typically smaller than alloying effects and may not produce a noticeable change in rates under conditions favoring carbon deposition. Little-understood metal-support interactions in the case of Ni/Ti02 also have an effect on activity and thus, indirectly, on particle size effect. They clearly influence Cz+ selectivity, since unsupported Ni produces CH4 almost exclusively.62 Structure and alloying studies for Ru lead to the same conclusions. Kin&7 and Kellner and found a particle size effect to be compared with the alloying effects of Luyten et al.44 For Ru, the magnitude of particle size effect is similar for C2+ and CHI production, so that the degree of structure sensitivity for CHI (82) Dumesic, J. A.; Topscae, H.; Khammouma, S.; Boudart, M. J . Catal. 1975. - >
37., 503. - -
(83) Spencer, N. D.; Schoonmaker, R. C.; Somorjai, G. A. Nature (Loondon) 1981, 294, 643. (84) Rostrup-Nielsen, J. “Catalysis: Science and Technology”; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Heidelberg, 1984; Vol. 5, p 1 .
2194
The Journal of Physical Chemistry, Vol. 88, No. 1 1 , 1' 984
TABLE VI: Effects of Nature of Grouo 8 Metal activity variationa
TABLE V: Effects of Structure activity variationa system
CH,
C,'
ref
Ni/AI,O,, Ni/SiO, Ni/ A1 0
10 >10 73 Fe/carbon a These factors denote the variation of activity due to change in surface structure for synthesis of CH4 and C2+. resembles that for C2+ hydrocarbon synthesis. The variation in carbide structure with particle size for Fe/MgO and Fe/carbon complicates the interpretation of particle size effects. With that reservation clearly in mind, we may conclude that C G H 2reactions on Fe appear structure sensitive. Here again seems to be little difference in the magnitudes of the particle size effect on CH4and C2+production. The ambiguity of the particle size effect in the case of Fe catalysts is due to the fact that the carbide structure formed, the degree of carbiding, the induction time for carbiding to be completed, and the degree of carbon coverage of the surface all vary with Fe particle size. Less active Pt group metals behave differently from FT catalysts. The values of STYcH, are an order of magnitude larger for 1-nm Pd particles than for 18-nm particles, which also exhibit a greater selectivity to methanol at higher pressures.s0 This implies structure sensitivity for CH4 production on Pd, since larger particles do not disproportionate C O as easily as small particles. Larger particles therefore show smaller methanation rates and selectivity shifts toward C H 3 0 H production, since the latter does not require C-0 bond cleavage. This trend for CHI production contrasts with the one for Ru, Fe, and Ni, where rates of hydrocarbon synthesis increase with particle size. This difference may be due to the difficulty of breaking the C-0 bond on Pd and probably most Pt group metals other than Ru and the ease of dissociation of C O on FT catalysts Ru, Fe, Co, and Ni. In fact, it appears that dissociation of C O on Pd is by disproportionation on low coordination sites to be found on very small particles while straight dissociation of CO on Fe requires larger ensembles to be found on larger particles. A summary of structure effects can be found in Table V. VI. Effect of the Nature of the Metal Vannice has measured Nco and NCH4 on supported group 8 metals (excpet osmium) and found a relatively small variation of a ~ t i v i t y . The ~ ~ , first ~ ~ study compared A1203-supported catalysts68at 548 K, atmospheric pressure, and Hz/CO = 3.0. Only a two-orders-of-magnitude variation in NCb was observed between the most active Ru/A1203and the least active Ir/A1203. The Cz+ production apparently decreased by more than a factor of 100 between Ru/A1203 and Ir/A1203, since the latter is a poor FT catalyst. A similar study of group 8 metals on S i 0 2 showed N C Hfor ~ the most active Co/Si02 catalysts was three orders of magnitude greater than the smallest value observed for Pd/Si02.52 The Cz+ production apparently differed by well over three orders of magnitude. Unsupported catalysts show the largest range of activity variations. A comparison of the data of King47and V a n n i ~ yields e~~ an estimate of the size of these variations. King found NcH4 = 143 X s-l on unsupported Ru (H,/CO = 2.0, 523 K, 4 atm), while Vannice's values of NCH:8are 0.1 5 X s-' for Pd black s-l for Pt black (Hz/CO = 3.0, 548 K, 1 atm). and C0.02 X
system
CH 4
c,+
ref
supported
103
104
51,67
unsupported 104 io5 41, I1 a These factors denote the variation in activity with nature of the metal for synthesis of CH, and C,+. If we assume the reaction is zero order in C O pressure, is first order in H2 pressure, and has an apparent activation energy for methanation of 109 kJ mol-', the rates of Pd black and Pt black are essentially unchanged when corrected to the conditions of King.47 The rates on Pd and Pt would be somewhat lower if the surface site density were based on chemisorption on fresh catalyst. Vannice calculated NCH4on the basis of chemisorption on used catalysts, while King used fresh catalyst for his chemisorption measurements. The major uncertainty in this comparison is the purity of Pd and Pt black. Both N a and K remain following preparation of Pt and Pd black and may affect these results, particularly the low rate measured for Pt black, for which the importance of surface K impurities has been shown by Auger electron s p e c t r o s ~ o p y . ~ ~ For this reason, the intrinsic activity of Pt may be comparable s-' at the to Pt/Si02, which shows NCH,= (0.15-0.20) X same conditions as above.81 Thus, there remains a variation of lo3 for rates of methanation and lo4 for rates of FT reactions over unsupported group 8 metals. A summary of specificity effects is shown in Table VI. The range of methanation activity is moderate when compared to that of known structure-sensitive reactions (Table I). Turnover frequencies for ethane hydrogenolysis show a range of nine orders of magnitude over group 8 metals, and the rate of ammonia synthesis on metals that exhibit measurable activity shows a variation of at least five orders of magnitude,6 but the activity of Pt is so small that it has never been reported. The two-orders-of-magnitude difference for A1203-supported catalysts and three-orders-of-magnitude difference on Si02-supported catalysts are too small to be considered evidence of structure sensitivity. Unsupported metals show the largest range of activity; however, the actual range may be somewhat less than has been reported. The range in NCH, observed is borderline, probably more characteristic of structure-insensitive reactions such as hydrogenation of alkenes. The variations in rate of Cz+ production as a function of metal are more substantial and may be large enough to warrant a classification of Cz+ synthesis among the structure-sensitive reactions.
VII. Summary Alloying, particle size, and specificity effects all point in the direction of structure sensitivity in FT synthesis of hydrocarbons, though the effects are less dramatic than for ammonia synthesis or hydrogenolysis of alkanes. This conclusion is derived from studies involving Ru and, to a lesser degree, Ni and Fe. Structure sensitivity seems due to ensemble effects as FT synthesis seems to proceed more rapidly on large ensembles of Ru, Ni, or Fe atoms. Data on methane synthesis are more difficult to wrestle with. On typical FT catalysts (Ru and Fe), CH4 production shows alloying and particle size effects of the same magnitude as for Cz+production. On Ni, CHI production shows moderate particle size effects58and substantial alloying effects when the Cz+fraction is not negligible. Under selective methanation conditions, however, CH4 production generally shows little dependence on crystal plane or particle although alloying effects are still s ~ b s t a n t i a l . ~ ~ It is likely that C O dissociation is a structure-sensitive step in selective methanation on Ni, but it may frequently not be a kinetically significant step in the reaction. Methane production may show structure sensitivity on Ni when Cz+ is favored and Ni becomes an FT catalyst like Fe or Ru. Similarly, high Hz/CO ( 8 5 ) O'Rear, D. Ph.D. Dissertation, Stanford University, 1980.
J. Phys. Chem. 1984,88, 2195-2200 ratios or addition of an alloying element could favor structure sensitivity in seletive methanation on Ni by switching the kinetically significant step from carbon hydrogenation to C O dissociation. Since bulk Pt group metals other than Ru and perhaps Rh do not dissociate C O readily, C O dissociation could well be the kinetically significant step for methanation on Pd, Pt, etc. Thus, structure sensitivity for methanation on Pd or Pt would be more likely than on Ni. Although Vannice and co-workers found no particle size effect for Pt or Pd, recent work shows that very small Pd particles (53 nm) disproportionate C O much better than moderate-to-large particles and thus have higher methanation rates and lower selectivity to methanol. The trend of this structure sensitivity is much different from that on Ni or Ru. Clearly, the
2195
nature of structure effects on synthesis of methane depends strongly on the catalyst, and sweeping generalizations would be foolhardy. Indeed, for Ni alone, the reaction may be structure sensitive or insensitive depending on the amount of carbon on the surface, the latter being a function of sulfur poisoning which is probably ubiquitous but rarely revealed. At any rate, examination of rate data on FT and methanation catalysts from the standpoint of structure sensitivity is a profitable exercise which can yield new insight into the mechanisms of these interesting reactions.
Acknowledgment. This work was supported by National Science Foundation Grant N S F CPE 8219066. Registry No. Methane, 74-82-8; carbon monoxide, 630-08-0.
ARTICLES Adsorption of Carbon Monoxide on a Smooth Palladium Electrode: An in-Situ Infrared Spectroscopic Study Keiji Kunimatsu Research Institute for Catalysis, Hokkaido University, Sapporo, Japan (Received: March 7, 1983; I n Final Form: November IS, 1983)
Adsorption of carbon monoxide on a smooth palladium electrode in 1 M HC104saturated with CO was studied by two in-situ IR reflectance spectroscopic methods: EMIRS (electrochemically modulated infrared reflectance spectroscopy) and LPSIRS (linear potential sweep infrared reflectance spectroscopy). Two types of adsorbed CO, linear and bridged, were identified from the observed IR spectra, the latter being the predominant surface species. The C - 8 stretching frequency of the linear CO shifts to higher frequencies at more positive potentials with a slope of 48 cm-'/V. The frequency of the bridged CO increases by 63 cm-' between -0.5 and 0.9 V(NHE) and its integrated band intensity decreases continuously in the same potential region while the intensity of the linear CO is almost constant up to 0.1 V and then decreases gradually with increasing positive potential. The surface selection rule of the IR reflection absorption spectroscopy was tested for the present system by using the p- and s-polarized light. It was found that only p-polarized light gave the IR spectra of CO adsorbed on the palladium electrode thus proving the selection rule at the electrode/solution interface.
Introduction The adsorption of carbon monoxide on palladium has been the subject of studies in electrochemical systems as well as in gas-phase systems. The nature of C O adsorbed on single-crystal surfaces of palladium'** and on supported palladium catalysts3v4has been studied directly by infrared spectroscopy, and it is now well-known, from the observed vibrational spectra, that there are two types of adsorbed CO, linear and bridged, on palladium at high surface coverages, the latter being the predominant surface species. An electrochemical study of the adsorption of C O on a palladium electrode reported recently by BreiterS showed that a large number of C O molecules on the Pd electrode occupied more than one adsorption site. It was suggested, accordingly, that they were present as bridge-bonded species (two-site adsorption). This (1) A. M Bradshaw and F. M. Hoffman, Surf. Sci., 72, 513 (1978). (2) A. Ortega, F. M. Hoffman, and A. M. Bradshaw, Surf. Sci., 119, 79 (1982). (3) R. P.Eischens, S.A. Francis, and W. A. Pliskin, J . Phys. Chem., 60, 194 (1956). (4) R. P. Baddour, M. Modell, and R. L. Goldsmith, J . Phys. Chem., 74, 1787 (1970). (5) M. Breiter, J . Elecfroanal. Chem., 109, 243 (1980).
0022-3654/84/2088-2195$01.50/0
conclusion is consistent with the data obtained in gas The agreement between the gas-phase data and that obtained on electrodes is encouraging; electrochemists can refer to the gas-phase data when discussing the nature of the adsorbed species on electrodes. It is desirable, however, to carry out in-situ IR spectroscopic studies for the actual electrochemical systems whenever possible. In fact, it has become feasible now to conduct such IR spectroscopic studies of electrochemical systems due to the recent development of the in-situ IR reflectance spectroscopy at the electrode/electrolyte solution An in-situ IR spectroscopic study of the adsorption of CO on a palladium electrode is especially interesting for two reasons. Firstly, we can study the nature of the bonding between CO and the metal surface directly by observing the vibrational spectra of the adsorbed C O as a function of electrode potential. Secondly, the Pd/CO system may offer the possibility of studying the nature (6) A. Bewick, K. Kunimatstu, and B. S.Pons, Electrochimi. Acta, 25,465 (1 980). -( 7 j A . Bewick, K. Kunimatstu, J. Robinson, and J. W. Russell, J . Elecfroanal. Chem., 119, 175 (1981). (8) B. Beden, A. Bewick, C. Lamy, and K. Kunimatsu, J . Electroanal. Chem., 121, 343 (1981). \ -
0 1984 American Chemical Society
