4806
J . Phys. Chem. 1986, 90, 4806-4810
Thermodynamics of the Hydrogenation of Oxides of Carbon Robert B. Anderson Department of Chemical Engineering and Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4L7 (Received: October 15, I985)
Thermodynamics is appropriate to a symposium honoring Paul Emmett. Paul used thermo sagaciously in his research, and when reliable values were not available, he determined the required data. The energetics for producing a vast array of organic molecules by hydrogenating oxides of carbon are favorable. However, if the possible products are assumed to be in equilibrium, the predominant and nearly only product is methane. Thus, to make higher paraffins, olefins, alcohols, etc. the catalyst must be selective. Part of this selectivity arises from selective poisoning of the catalyst by CO. Similarly, equilibrium is usually not attained in the water gas shift, except for some conditions with catalysts containing Fe, Mo, and W. Hence, the distribution of product should be of great diagnostic value for interpreting the mechanisms of the Fischer-Tropsch and higher alcohol syntheses
Emmett's Studies of Thermodynamics Paul Emmett understood the value of sound thermodynamic data for his research in catalysis. For several systems for which reliable data were not available, he determined their thermodynamics. His contributions to thermodynamic data include the following. 1. the Fe-H-0 system^^^^^^ 2. the Co-H20-COO-H2 reactionI2 3. the Co-C02-COO-CO reactioni4 4. the water gas shifti4J5 5 . the Fe-N system including y' and e nitridesi0 6. the Fe2C and Fe,C-H2-Fe-CH4 systemss 7. the Ni,C-H2-Ni-CH4 systemsg 8. the thermodynamics of bulk carbidic carbon incorporating in the Fischer-Tropsch synthesis'* The research in study 1 was related to the reduction of iron ammonia synthesis catalysts. The ratio PH+-,/PH2 required to form bulk magnetite was about 1000-fold larger than the ratio needed to deactivate an iron catalyst for the ammonia synthesis." In the poisoning by water vapor, oxygen was chemisorbed on the surface iron atoms; these atoms are very much more reactive than bulk iron. Studies 2 and 3 provide the thermodynamics of the reduction of cobalt oxide and the water gas shift, WGS. Work in topic 4 established the equilibrium constant for WGS directly and included elegant detective work to establish that divergent experimental values in the literature resulted from complications by thermal diffusion. (1) Anderson, R. B. In Caralysis, Emmett, P. H., Ed., Vol. 4, Van Nostrand-Reinhold: New York, 1956; Chapter I . (2) Anderson, R. B. Catal. Rev.-Sci. Eng. 1980, 21(1), 53. (3) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic: New York, 1984. (4) Anderson, R. B.; Lee, C. B.; Machiels, J. C. Can. J . Chem. Eng. 1976, 54, 590. (5) Anderson, R. B.; Schultz, J. F.; Hofer, L. J. E.; Storch, H. H. C.S. Bur. Mines,Bull. 1959, No. 580. (6) Barin, I.; Knacke, O., Thermochemical Properties of Inorganic Subsfances;Springer-Verlag: Berlin, 1973. (7) Barin, I.; Knacke, 0.;Kubaschewski, 0. Thermochemical Properties of Inorganic Substances, Supplement; Springer-Verlag: Berlin, 1977. (8) Browning, L. C.; DeWitt, T. W.; Emmett, P. H. J . Am. Chem. Soc. 1950, 72, 421 1. (9) Browning, L. C.; Emmett, P. H . J . Am. Chem. Soc. 1952, 74, 1680. (10) Brunauer, S.; Jefferson, M. E.; Emmett, P. H.; Hendricks, S. B. J. Am. Chem. Soc. 1931, 53, 1778. (11) Emmett, P. H.; Brunauer, S. J . Am. Chem. Soc. 1930, 5 2 , 2682. (12) Emmett, P. A,; Shultz, J. F. J . Am. Chem. SOC.1929, 51, 3249. (13) Emmett, P. H.; Shultz, J. F. J. Am. Chem. SOC.1930, 52, 4268. (14) Emmett, P. H.; Shultz, J. F. J. Am. Chem. Sor. 1930, 52, 1782. ( 1 5 ) Emmett, P. H.; Shultz, J. F. J . Am. Chem. Soc. 1933, 55, 1376. (16) Kolbel, H.; Ralek, M. The Fischer-Tropsch Synthesis, Anderson, R. B., Ed.; Academic: New York, 1984; pp 265-292. (17) Kollar, John CHEMTECH 1984, 14, 504. (18) Kummer, J. T.; Browning, L. C.; Emmett, P. H. J. Chem. Phys. 1948. 16. 139.
0022-3654/86/2090-4806$01.50/0
TABLE I: Equations for the Production of Olefins and Thermodvnamic Data for Production of 1-Hexene'
equation (1) (2)
(3) (4) (5)
+ IC02 = 2H20 + I/n(CnH2,) + I C 0 = H70 + l/n(C,H,.) H2> 2CO = CO; + l/h(C,H2,j"' H20 + 3CO = 2C02 + l/n(CnH2J H 2 0 + CO = H2 + C 0 2 3Hz 2H,
AGO
&
5.68
-25.96 -35.01 -44.06 -58.52
2.63 -0.42 -3.47 -3.04
-9.06
Data for 1-hexene at 427 OC given in kilocalories per carbon atom in the hexene. Reproduced with permission from ref 3. a
The research in study 5 defined conditions for producing iron nitrides and showed that these compounds cannot be produced in the synthesis of ammonia. Investigations on topics 6, 7, and 8 showed that carbides of iron and nickel can be produced under conditions of FTS. Bulk carbides cannot be hydrogenated to yield higher hydrocarbons; however, some of this carbon may react with H2 + C O to produce higher hydrocarbons. Thus, thermodynamics was very useful to Paul Emmett in investigating the ammonia and Fischer-Tropsch syntheses.
Introduction The present paper considers chiefly the hydrogenation of oxides of carbon typical of the Fischer-Tropsch synthesis (FTS) and the methanol and higher alcohol syntheses, and reactions that may occur in catalytic materials. Thermodynamic data were largely obtained from ref 6, 7, and 22 and have been summarized in ref 3. Many years ago, Franz Fischer postulated that carbohydrates might be formed by hydrogenating CO. Was he correct? Hydrogenation of Oxides of Carbon In synthesis reactions, for each organic molecule, a family of equations such as those for olefins, in Table I, may be obtained by adding the water gas shift, WGS, eq 5, successively to eq 1. Equation 2 is typical of many primary synthesis reactions, but with Fe, Mo, and W catalysts WGS proceeds nearly as rapidly as eq 2. Table I also presents standard-state Gibbs free energy and enthalpy changes AGO and AHo for the production of 1hexene by eq 1-4 at 427 "C. In proceeding from eq 1 to 4 the values of AGO and AH" become more negative. Equation 4 is known as the Kolbel-Engelhardt synthesis.'6 Since the moles of products are usually smaller than the moles of reactants, the equilibrium conversions may be increased enormously by increasing the operating pressure, and at moderate to high pressures sizeable conversions may be obtained even when AGO is positive (19) Machiels, J. C.; Anderson, R. B. J . Catal. 1979, 58, 268. (20) Monnier, J.; Quilliam, M. A.; Anderson, R. B. Can. J . Chem. Eng. 1986, 64, 469. (21) Smith, K. J.; Anderson, R. B. J . Catal. 1984, 85, 428. (22) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Wiley; New York, 1969.
D 1986 American Chemical Societv
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4807
Hydrogenation of Oxides of Carbon
6 0
I
Q
/
C3H4
-30
r/ !I
I
200
I
I
400
I
I
600
I
I
I
I
I
CH20
800
TEMPERATURE, "C Figure 1. Standard heats of reaction per carbon atom in the organic product for equations of type 2 in Table I, where C3H4is methylacetylene, C6H6is benzene, and C, is graphite. WGS, water gas shift. Reproduced with permission from ref 3. Copyright 1984, Academic.
or the equilibrium constant, K , is less than one. Figures 1 and 2 present changes of enthalpy, AH",and Gibbs free energy, AGO, for the formation of a variety of molecules by reactions typical of eq 2, i.e., water and not C 0 2 as the principal product, and including formaldehyde and methanol for which neither H 2 0 nor C02is produced. To plot these data conveniently, the values are divided by the number of carbons, n, in the product, as was also done in Table I. All of these reactions are exothermic and in designing practical reactors the removal of this heat from the catalyst is the major engineering problem. The heat of reaction for producing CHI is about 80% of the heat of combustion of CO. For paraffins the values of A H " / n decrease to a constant value as the number of carbons increases. Introducing oxygen as alcohols or aldehydes or olefinic bonds makes A H " / n less exothermic by more or less the same amount compared with the corresponding paraffin and an acetylene bond by roughly twice this amount. Let us call these "special groups", SG, with an acetylenic bond being worth 2SG. In contrast to the relatively constant enthalpies, the Gibbs free energies of reaction increase rapidly with increasing temperature, Figure 2. Here, the equilibrium constant K is related to the Gibbs free energy by (AG"/n) n = -RT In K. At temperatures below 300 "C the values of AGO are negative for all molecules shown except formaldehyde, acetylene, methanol, and methylacetylene, and at moderate pressure large equilibrium conversions are possible even for methanol and methylacetylene. At this point we digress to state that it is usually thermodynamically possible for products from the synthesis to react further with H2 CO; this building-in process is called "incorporation". Alcohols, aldehydes, ketones, olefins, and acetylenes can react with syngas in any ratio; however, paraffins, particularly methane, can be incorporated in only limited amounts. These incorporations are the principal reactions in hydroformylation reactions that occur in homogeneous systems. As mentioned earlier, Kummer, Browning, and Emmett1*showed that higher hydrocarbons could not be produced by hydrogenating bulk carbides of iron and nickel;
+
I
Figure 2. Standard free energies of reaction per carbon atom of the organic product for equations of type 2 in Table I, where C3H4is methylacetylene, C is graphite, and the top of the C2,, band represents the 1-olefinand the bottom of the n-araffin. -AGO = RT In K. WGS, water gas shift. Reproduced from ref I . Copyright 1956, Van NostrandReinhold. Note that the ordinate is Gibbs free energy and would be written as AGO now. TABLE 11: Changes of Gibbs Free Energy for Syngas Reactions Producing C2 and C3Compounds" AGO, kcal/mol
compd ethane ethylene acetylene acetaldehyde ethanol ethylene glycol methyl ether acetic acid propane propylene cyclopropane methylacetylene allene I-propanol allyl alcohol acetone
227 O C -29.18 -1 1.09 +16.85 -4.05 -6.78 +15.77 7.21 -5.78 -37.28 -23.06 -1 1.60 +2.71 +4.83 -15.94 C0.28 -18.63
427
OC
-5.88 +5.90 27.52 13.36 16.41 38.94 3 1.20 13.18 -2.24 +5.43 18.38 25.19 27.5 1 18.96 29.27 1 1.06
+
"From data of Stull et a1.22for reactions forming water, e.g. 4H2 2CO = C2H4+ 2H20, except for acetic and ethylene glycol where the molecule is the only product.
however, it is thermodynamically possible to form higher hydrocarbons by incorporating carbidic carbon with H 2 CO. Table I1 gives changes of free energy for C2 and C, molecules by reactions typical of eq 2. Here the free energy change becomes more positive compared with paraffins when an olefin bond or oxygen as aldehyde, ketone, or alcohol are added, and an acetylene bond is worth about two of these special groups. Oxygen in ethers or organic acids does not follow the same pattern. Although data are not available on carbohydrates, a guess may be made on Fischer's postulate: the formation of carbohydrates would not be expected to be favorable thermodynamically. Glucose for example with six carbons and six special groups should have
+
4808
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
TABLE 111: Equilibrium Constants for Reactions Producing Ethylene Glycol
Anderson FEED 3Hz+ICOz 2H2+IC0 IH,t IC0 IH,+2CO
3H2 + 2CO = C2H.502 CH30H + HCHO = C2H602 CH30H + Hz + CO = C2H602 OHCCHO + 2H2 = C2H.502 CHjCOOH + H2 = C2H602 C2H2 + 2H2O = CzH.502
1.3 X lo-' a t ~ n - ~ 4.2 atm-'
2.1
h /'i
IOOI-
K p at 500 K
IO
X
0.031 atm-2 3.8 X 10"O atm-I
.I
2.95
IHz0+3C0
,
262°C
I-
]p-li IO
11
.01
L
8
350°C
2100b0
+
IO
+
'14
-l
IO
:
.I
5
.01 W
P 8
500°C
0'
I
I
I
1
I
0.4 0.6 0.8 FRACTIONAL CONVERSION OF n-HEXANE
0.2
t
I
+
/ / I
Figure 3. Product distribution of hydrogenolysis of n-hexane over Ru on alumina at 149 OC. Reproduced with permission from ref 19. Copyright
1979, Academic thermodynamics similar to ethylene glycol with two carbons and two special groups. This molecule can be produced only at very high pressures. Some molecules with unfavorable thermodynamics may be produced at alternate routesi7 as shown in Table 111; however, such options will probably require the development of new catalysts.
Equilibria in Multireaction Systems It is instructive here to consider the hydrogenolysis of hydrocarbons on the FTS metals. These reactions, which are moreor-less the reverse of FTS, proceed rapidly at 100 to 200 O C , cf. the selectivity plot for the hydrogenolysis of n-hexane on supported Ru in Figure 3.19 Before unravelling this paradox, the thermodynamics of syngas reactions should be considered for a situation in which a number of organic molecules can be produced and equilibrium is obtained for all possible reactions of reactants and products. In our example, synthesis gas of several initial feed compositions are in equilibrium with methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, ethylene, propylene, acetone, formaldehyde, acetaldehyde, formic acid, methanol, ethanol, and acetylene at several typical temperatures and press u r e ~ These .~ calculations involve minimization of the Gibbs free energy for all components using a sophisticated trial-and-error procedure. The results in Figure 4 show the conversion of H2 and CO by dotted curves and as solid curves the mole percent of product molecules exclusive of H 2 0 and C 0 2 . At all temperatures the product is virtually all CH,, and other molecules are found in significant amounts only at the right side of the graph where the synthesis gas does not contain enough hydrogen to produce all CH4. In the FTS higher hydrocarbons and olefins are produced because the hydrogenolysis activity of the metal catalysts are selectively poisoned by C0.3920 In the higher alcohol synthesis the oxidic catalysts seem to lack hydrogenolysis and methanation capabilities. Because the products from these syntheses are generally far from equilibrium, the selectivity data often have large diagnostic value in interpreting reaction mechanism^.^^^^ For example, the author's simple chain growth scheme, in which carbons are added one-at-a-time to the end or penultimate carbons at one end of the growing chain, has been useful in predicting
PRESSURE, ATM. Figure 4. Equilibrium compositions and conversions for forming the group of molecules given in the text: ---, percentage conversion of H2 + CO; 1 , methane; 2, ethane; 5 , n-pentane, 3, propylene. Reprodwed with permission from ref 4. Copyright 1976, Can. J . Chem. Eng. TABLE I V Complicating Factors in the Fischer-Tropsch Synthesis"
factor carbon deposition carbide formation volatile carbonyl formation oxide formation water gas shift forms nitrides low activity
metal other Pt Ru Rh metals Co Ni Fe Mo, W +b
+
+
0 I'
0 0
0 0
+ + + + + + I + I-
0 0 0
0 0 0
0 0 0
o O o
O
o o
+
0
0
0
O
O
?
+
+ +
+ + O?
+ + + +
"Reproduced with permission from ref 3. 6 + , denotes a phenomenon that occurs readily; 0, does not occur; I, may be a problem in some cases. 'Ru may also produce polynuclear carbonyls of low volatility. carbon-number and isomer distributions from FTS3 A growth scheme has also been developed for the higher alcohol synthesis.2' In this process growth occurs most rapidly on the adjacent-to-end carbon. For this reason the principal product is isobutyl alcohol, and the alcohols larger than C, or C7 are not found in significant amounts.
Reactions of Catalytic Materials Thermodynamic data can also be used to predict adverse reactions that may occur to catalytic materials. In homogeneous catalysis metal carbonyls decompose and precipitate metals if the partial pressure of CO is too low or the temperature too high. For polynuclear carbonyls, cluster compounds, etc., an excessively high partial pressure of CO may cause these large complicated molecules to decompose to simple carbonyls. The heterogeneous catalysts in the FTS may undergo a larger variety of secondary reactions, usually undesirable, as listed in Table IV. High partial pressures of CO and/or temperatures
Hydrogenation of Oxides of Carbon
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4809
Figure 5. Effect of pretreatment of a precipitated ferric oxide catalyst on selectivity. Tests with 1H2 + 1CO gas at 7.8 atm. Reproduced with permission from ref 2. Copyright 1980, Dekker.
that are too low may lead to the formation of carbonyls. Volatile or soluble carbonyls will be removed from the catalysts. Even if the carbonyls stay in the reaction system, the net result is usually unfavorable as carbonyls generally have low activity in many syngas reactions and may have a different selectivity. All of these metals deposit elemental carbon particularly at high temperatures. Carbon deposits may cause catalyst particles to disintegrate, clog fixed beds and pores, and make the active surface inaccessible. Metallic iron, molybdenum, and tungsten may be converted to oxides by the water and C 0 2produced in synthesis reactions, as shown by equilibrium constants for these oxidation reactions in Table V. These materials are also active catalysts for the water gas shift; that is, this reaction proceeds as rapidly as the primary reaction, and the overall synthesis reaction often follows eq 3. Under virtually all synthesis conditions, it may be expected that the surfaces of Fe, Mo, and W will be covered with oxygen or some other chemisorbed species. Iron group metals, Mo, and W can be converted to carbides and nitrides; however, nitrides of Ni and Co are relatively unstable and apparently cannot be produced in catalysts by treatment with NH3. Chlorides of iron group metals are relatively stable, and in some instances if chlorides are use in catalyst preparation, residual chlorine is difficult to remove by washing. Chlorine can usually be removed from the Pt metals by treatment with hydrogen. We should note here that chlorine as well as oxygen and sulfur, discussed in the next paragraph, are not necessarily undesirable components of catalysts, but they usually are for FTS. Table V gives the equilibrium constants for producing oxides and sulfides in terms of H20/H2and HzS/Hzratios. The ratio H2S/H2is the minimum required to produce the bulk sulfide. Chemisorption of H2Sand poisoning of the catalyst can occur at very much lower ratios.
Synthesis Tests on an Iron Catalyst A set of experiments from Bureau of Mines ~ o r killustrates ~ , ~ a number of pertinent points on FTS. Here a ferric oxide gel containing copper and K2C03was tested in lHz and 1CO gas at an hourly space velocity of 100 after a variety of pretreatments indicated in Figure 5. For each test a histogram depicts the
TABLE V Thermodynamics of Forming Oxides and Sulfides at 327 OC
metal Fe co Ni
cu Zn
Mo
Ru W
Re Ir
H@/H, 5.5 x 10-2 7.6 X 10' 3.4 x 102 5.6 X 10' 3.6 x 10-7 6.5 x 10-3 7.1 x 109 6.5 x 10-3 2.1 x 104 8.5 X 10l2
HS/H, 7.0 X 10" 9.6 X 1.4 x 10-4
2.5 x 1.4 x 4.8 x 8.3 x 2.6 X 1.4 x 1.0 x
10-5 1043
IO-* 10-5 lo-' 10-3 10-2
production distribution, the top part showing gaseous fractions and the bottom the results of a one-plate distillation. The two lower boiling fractions were analyzed by infrared spectroscopy to give the weight percent hydroxyl group denoted by OH, weight percent carbonyl group by CO, and the olefin content by a bromine number Br. The olefins in the C2 and C3 and C4 fractions, in mol percent are shown by the numbers following the equal sign. Parenthetically, this procedure was how products were characterized before the advent of temperature-programmed capillary gas chromatography coupled with mass spectrometry. The data in Figure 5 demonstrates the versatility of iron catalysts in FTS. After 1 or 2 days of synthesis the selectivity was constant to the end of the tests after 6 to 8 weeks, and activity changes were small. Thus, many of the pretreatments produced surfaces with unique selectivities. Catalysts that were initially reduced in H2 to a-Fe at 300 O C had essentially the same activity (tests X245, -220, -273, and -324), but samples pretreated initially with C O or synthesis gas were about twice as active. Catalysts pretreated with l H z + 1CO at 230 "C (X149) or C O at increasing temperatures from 200 to 250 OC (X341), both at atmosphere pressure, were largely converted to magnetite plus some FezC carbides and elemental carbon. These samples produced the largest wax yields and lowest amount of gaseous hydrocarbons. The catalyst in X245 reduced to a-Fe and a similar preparation subsequently treated with C O at 180-300 OC to convert the iron
4810
J . Phys. Chem. 1986, 90, 4810-4819
to x-Fe,C (X324) had similar selectivities. This result was not unexpected, because the reduced catalyst is partly converted to Hagg carbide in the early part of the synthesis. Although reduction of the used carbided catalyst (X324C) removed some carbon and wax, the activity and selectivity were not changed. All three of these reduced and carbided samples produced a lower molecular weight product than obtained in tests X149 and X341. Reduced samples were also treated with N H , to product iron nitrides, Fe4N in X220 and Fe2N in X273A. We note that iron nitrides are hydrogenated to N H 3 in pure hydrogen at 230 OC in I or 2 h. In the presence of CO the nitrogen persists for many months, and the nitrogen removed is replaced by carbon to form an t-carbonitride. The Fe,N in X220 gained carbon rapidly early in the FTS to become t-carbonitride. The similarity of activity and selectivity may be expected. After 5 weeks of FTS the sample in X273A was treated with pure hydrogen at 300 "C, and the nitrogen was completely removed, but the sample showed the presence of t and x phases. The presence of nitrogen in the catalyst, probably as an t-nitride or carbonitride, caused a marked shift in selectivity toward lower molecular weight products and increased yields of oxygenates, particularly alcohols. The products reverted to those typical of reduced and/or carburized catalysts when the nitrogen was removed. These data suggest that the iron catalyst can exist in metastable states that persist for 6 or more months. Catalysts for FTS seem to be selectively poisoned by CO, particularly for reactions of hydrogen. For iron this includes the hydrogenation of olefins, hydrogenolysis of hydrocarbons, and hydrogenation of nitrides and carbides. Other reactions including deposition of carbidic
and elemental carbon, the water gas shift, and magnetite formation proceed at measurable rates. Concluding Remarks Thermodynamics is useful in many ways for studying syngas reactions. The energetics of producing a large variety of organic molecules are favorable under conditions of FTS or the alcohol syntheses. Introducing one or more hydroxyl or aldehyde groups and/or double or triple bonds decreases this reaction tendency. Thermodynamically the production of acetylene seems unlikely, and methanol and ethylene glycol require moderate and high operating pressure, respectively. The thermodynamics of producing carbohydrates as postulated by Fischer may be similar to those of ethylene glycol. The production of methane is so favorable that CH4 is virtually the only product possible if equilibria is permitted between all molecules considered. The products of many syngas reactions are often far from the equilibrium composition. Thus, the distribution of products has great diagnostic value in interpreting the mechanisms of the reactions. Thermodynamics also provides useful information on catalytic materials; will they reduce, carbide or nitride, or form sulfides, oxides, and volatile carbonyls? Synthesis tests on an iron catalyst pretreated in several ways suggest that many metastable steady states are possible. In addition, the catalyst is severely inhibited, presumably by carbon monoxide, for reactions with hydrogen. However, reactions forming carbides, carbonitrides, elemental carbon, and iron oxide proceed at a modest rate. Registry No. CO, 630-08-0; C 0 2 , 124-38-9.
CO Hydrogenation over Carbon-Supported Iron-Cobalt and Potassium-Iron-Cobalt Carbonyl Cluster-Derived Catalysts A. A. Chen,? M. Kaminsky,t G . L. Geoffroy,t and M. A. Vannice*+ Department of Chemical Engineering and Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: October 15, 1985: In Final Form: March 24, 1986)
FeCo ana K-promoted Fe-Co CO hydrogenation catalysts were prepared by dispersing iron, cobalt, and iron-cobalt carbonyl clusters on a clean. high-surface-area carbon support. High dispersions were achieved as indicated by chemisorption and XRD measurements. Because of their zero-valent state and the absence of oxygen functional groups on the carbon surface, these catalysts required no high-temperature reduction for activation. The CO,(CO)~/Ccatalyst was particularly active, produced only paraffins, and exhibited very good activity maintenance, whereas the C-supported Fe3(CO),, clusters had the low turnover frequencies and higher olefin/paraffin ratios associated with small Fe crystallites. The mixed-metal cluster catalysts showed intermediate behavior. Addition of one K atom to the clusters markedly decreased specific activity but greatly enhanced selectivity to olefins along with increasing the rate of CO disproportionation. Co, which is known to produce essentially all paraffins, was modified by the K SO that it produced only olefins and CH4. The K significantly increased activation energies for CO hydrogenation, increased the pressure dependency on H2,and made the CO pressure dependency more negative. The variation observed in specific activities indicated that some surface enrichment in Fe may have occurred in the mixed-metal catalysts.
Introduction The high activity and relatively low cost of traditional iron and cobalt Fischer-Tropsch catalysts have given them a preeminent position in catalysis research pertaining to CO hydrogenation. However, one limitation with these catalysts is that they are typically poorly dispersed. The preparation of well-dispersed metal and mixed-metal particles is a desirable goal since they may have improved activity, selectivity, and on-stream durability. Two approaches to this end are the employment of metal carbonyl clusters (MCCs) as catalyst precursors's2 and the use of very 'Department of Chemical Engineering. *Department of Chemistry.
0022-3654/86/2090-4810$01.50/0
high-surface-area supports, preferably with no reactive surface oxygen, that can entrap and stabilize small metal clusters or particles. In previous work we have successfully used an amorphous carbon (1400 m2/g) to prepare Fe and Fe-Ru catal y s t ~ ,and ~ , ~in this study we report the synthesis and characterization of a series of MCC-derived, carbon-supported Fe-Co catalysts along with a series of MCC-derived, K-promoted Fe-Co (1) Anderson, J. R.; Mainwaring, D. E. J. Catal. 1974, 35, 162. (2) Zwart, J.; Snel, R. J. Mol. Catal. 1985, 30, 305. (3) Jung, H. J.; Vannice, M. A.; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N. J . Catal. 1982, 76, 208. (4) Kaminsky, M.; Yoon, K. J.; Geoffroy, G. L.; Vannice, M. A. J. Catal. 1985, 91, 338.
0 1986 American Chemical Society
