I n d . Eng. Chem. Res. 1994,33, 209Q-2095
2090
Geometric Factors in K and A1 Promoting of the Fe/MgO Fischer-Tropsch Catalyst Goran Boskovic2t Goran Vlajnic, and Erne Kis University o f Novi Sad, Faculty of Technology, 21000 Noui Sad, Yugoslavia
Paula Putanov Vojvodina Academy of Sciences, 21000 Novi Sad, Yugoslavia
Laszlo Guczi and Karoly Lazar Surface Science and Catalysis Laboratory, Institute of Isotopes, P.O. Box 77, H-1525, Budapest, Hungary
T h e activity of K- and Al-promoted Fe/MgO catalyst for Fischer-Tropsch synthesis was measured under atmospheric pressure, with the aim t o determine the promoter’s influence on the catalyst selectivity, particularly in terms of olefins and isomers of lower hydrocarbons. The results have been correlated to the data of a previous study on the same catalyst, made under elevated pressure and designed for the alcohol production. It has been concluded that the promoting effect of K should be attributed t o a geometric influence rather than to electronic promotion. The influence of A1 promoter manifests itself like the “skin effect”, resulting in suppression of adsorption of both CO and H2.
Introduction Compared to other metal catalysts for Fischer-Tropsch
(FT)synthesis, an iron-based catalyst is distinguished by higher conversion, selectivity to the lower olefins, and flexibility to the process parameters (Dry, 1981; King et al., 1981; Snel, 1987). However, the use of iron catalyst does not solve the problem of insufficient selectivity, which represents a general limitation of FT synthesis. Two main approaches have been pursued to overcome the limitation imposed by the Schulz-Flory product distribution. The first one is the application of the new support materials such as zeolites in the goal either to limiting the chain growth by the shape selectivity effect (Ballivet-Tkatchenko and Tkatchenko, 1981; Bein et al., 1986) or to intercept the molecular intermediates in the chain propagation sequence (Caesar et al., 1979; Egiebor et al., 1989). The second approach concerns the chemical modifications of the iron-based catalyst, by adding certain promoters. Studies made in this direction showed different effects of a number of metals applied as promoters of bulk or supported iron catalyst (Borghard and Boudart, 1983; Sachtler et al., 1985; Tau et al., 1989). So far there is no consensus on their mechanisms. Among the numerous promoters of the iron-based catalysts, K and A1 are most frequently investigated. They are the typical representatives of two different groups of metals, whose mechanisms of promotion were claimed to be different, i.e., electronic and textural. It has been shown that the promoting effect of A1 on Fe depends on the A1 portion in a catalyst. When present up to 10% in relation to Fe, A1acts by the “skin effect” causing a suppression of Fe sintering (Borghard and Boudart, 1983). However, when A1 or its oxide is a dominant component in a catalyst, as in Fe/A1203, a strong metalsupport interaction (SMSI) occurs, resulting in the suppression of the Fe reduction (Sushumna and Ruckenstein, 1985). To attain the promoting effect of K, at least two + Temporary address: University of British Columbia, Chemical Engineering Department, 2216 Main Mall, Vancouver, BC,
V6T 124, Canada.
conditions have to be met: the real basicity of the promoter has to be above the minimum strength (quality requirement) and the promoter must cover a certain part of the catalyst surface (quantity requirement) (Dry and Oosthuizen, 1968). In the case of the K-promoted Ru/SiOz catalyst, the significant decrease of catalytic activity was considered in terms of two possibilities: either the availability of Hz on the surface is reduced by alkali metal or the influence of K is related to an ensemble effect (McClory and Gonzales, 1984). There is no unique explanation of the mechanism of the K promoting effect. The long valid model of electrondonor properties of K (Dry et al., 1969) has been opposed to the model based on metal-promoter-anion interaction, where the negatively charged metal compensates the charge of electropositive promoter (Sachtler et al., 1985). Between two possible ways of promoter action, either by localized interaction with CO or by an interaction through the metal, the preference is given to the former one (Sachtler et al., 1985; Lee and Ponec, 1987). The verification of this mechanism is based on the evidence of SMSI between the metal and reducible oxide (Foger, 1984). In our previous works (Guczi et al., 1991;Putanov et al., 1992a) the influence of different promoters on the Fe/ MgO catalyst was investigated under elevated pressure, by catalytic reaction experiments designed to alcohol production. The observed catalytic behavior was connected to both changes of the state of the Fe component and the changes of textural properties of the catalyst (Boskovic et al., 1991; Putanov et al., 1992b,c). In the present work, the activity and selectivity of the Fe/MgO catalyst, promoted by K and Al, were investigated at atmospheric pressure with the accent on olefin and isomer selectivity of the lower hydrocarbons (up to six C atoms). The results were correlated to the catalytic and physicochemical properties previously obtained for the same catalysts (Guczi et al., 1991;Boskovic et al., 1991;Putanov et al., 1992a-c).
Experimental Section Catalyst Samples. The precursor samples of Al- and K-promoted Fe/MgO catalyst were prepared in the way
0888-5SS5/94/2633-2090~04.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2091 Table 1. Weight Percent of Iron and Promoter in the Catalyst Samples sample iron promoter Fe/MgO 7.0 K-Fe/MgO 7.5 0.8 Al-Fe/MgO 10.0 1.1 Table 2. Activity of the Catalyst Samples Measured after 500 min of Time on Stream, 1V R (pmobo/(g,,.$-S)) 294 Fe/MgO K-Fe/MgO 24 46 AI-Fe/MgO
described earlier (Guczi et al., 1991; Putanov et al., 1992a,b). The precursor of Al-promoted sample was dried in air, whereas the one with K was dried in vacuum. After the calcination at 450 "C in air, the samples were activated in the hydrogen stream of 1500 h-1, at 430 "C, over 5 h, to provide the transformation of the iron oxide phase to its reduced form (Putanov et al., 1991). The compositions of the samples, determined by spectrophotometric analysis, are presented in Table 1. Methods. The catalytic reaction measurements were performed in a small fixed-bed flow reactor, with the constant mass of the reduced catalyst sample of 0.2 g. The catalyst sample was heated in H2 stream to the working temperature and was kept in the H2 atmosphere, at this temperature, for 1 h before exposure to the premixed synthesis gas. The reaction was carried out with a HdCO = 2 mixture, at a space velocity of 1500h-l, at 250 "C, and under atmospheric pressure. The conversion was always below 10%. The products were analyzed by a gas chromatograph (GC) Hewlett Packard 5890 Series 11, supplied by both flame-ionization and thermal-conductivity detectors, using He as a carrier gas and having a 1.8m-long Porapak S column. The GC analyses were performed under a ramping temperature (Boskovic,1991). Such conditions enabled the separation of the olefins from the saturated and branched hydrocarbons up to six C atoms. The first GC analysis was taken for 10-15 min after switching to the synthesis gas. Every experiment was carried out over 10 h, during which five to six GC analyses were taken. The BET surface area (Micromeritics) of reduced and used samples was measured by the low temperature N2 adsorption using He as a carrier gas. The samples were previously degassed at 105 "C in a stream of He. Elementary microanalysis (EMA) (JEOL JXA-3A, 25 kV) was performed by X-ray line scanning for Fe and the associated promoter. Both reduced samples were previously formatted in tablets whose surfaces were polished and gold plated.
Results The catalytic activity, in terms of the CO reaction rate, was calculated after over 500 min of time on stream, when the pseudo-stationary state is supposed to be attained (Table 2). The changes of the activity with time on stream (Figure 1) show a significant decrease due to the adding of K and Al. The activity decline is particularly pronounced for the first 2 h of time on stream. The selectivity of examined samples has been presented as the number of C atoms in hydrocarbons, produced at the pseudostationary state (Table 3). The production of hydrocarbons different from methane, as a criterion of a catalyst quality, distinguishes the unpromoted Fe/MgO catalyst. However, an analysis of the production of other compounds such as C02, olefins in the c2-C~fraction,
2 1 0
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o
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Figure 1. Logarithm of activity of unpromoted and promoted samples with time on stream (activity: micromoles of CO reacted per gram of catalyst per second). 07
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+C O ~ / P I O ~ U C ~ S
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nc Figure 2. Selectivity of Fe/MgO sample with time on stream. ~=C/total
Table 3. Product Distribution by Means of the Number of C Atoms, mass 7'0 sample Fe/MgO K-Fe/MgO Al-Fe/MgO
c1
c2
c3
41.3 48.9 51.1
23.1 17.8 20.7
15.9 13.3 13.9
Cr 8.3 6.7 5.5
cs 7.2 11.1
5.8
cs 2.3 2.2 1.9
c7+
1.9 0 1.1
Table 4. Catalysts Selectivity toward Cor, Olefins, and Branched Hydrocarbons, mass % sample Fe/MgO K-Fe/MgO AI-Fe/MgO
cod
Zproducta H=C/HC= 34.0 40.4 0 71.4 0 73.1
H=C/EHC i-HC/HC@ 18.3 19.5 26.7 50.0 27.3 25.9
total olefins in hydrocarbons, and isomers in the c4-C.~ fraction (Table 4) gives different pictures of promoted samples. These results also concern the selectivity at the stationary state. The results of the product distribution, obtained for the unpromoted and promoted samples with time on stream (Figures 2-41, points at the various promoting mechanisms. The addition of promoters to Fe/MgO catalyst decreases BET surface area after reduction (Table 5). For the used catalysts, however, the positive influence of the promoters can be observed, which reflects a stabilization of the surface area. The micrographs obtained by EMA show the variation of the Fe concentration (Figures 5a and 6a), which might indicate the Fe position in the agglomerates inside the MgO support. The concentration of both promoters is independent of the active metal concentration. It is particularly characteristic for A1 (Figure 6b), which is uniformly distributed over the MgO support.
2092 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 0.9
0.8
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CH jlolal HC i-HC/HC,,
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Figure 3. Selectivity of K-Fe/MgO sample with time on stream. ,
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Figure 4. Selectivity of Al-Fe/MgO sample with time on stream. Table 5. BET Surface Area, m2/g
sample Fe/MgO K-Fe/MgO Al-Fe/MgO
reduced 167 84 104
used 53 63 93
Discussion There is much controversy concerning the mechanism of the FT synthesis. In the case of a regular stepwise chain growth, i.e., a polymerization process, the product distribution should followthe Schulz-Flory (S-F) equation M p = (ln2 a)PaP (Dwyer and Somorjai, 1979), where M , is the weight fraction of hydrocarbon with P carbons and a is the chain growth probability factor defined by the rates of the propagation and the termination. From the logarithmic form of the S-F equation, and by using the product distribution data from Table 3, the plot of lg(Mp/ P)vs P was obtained (Figure 7), with the linearity which implies an agreement to the S-F type kinetics (King et al., 1981). The a values, calculated from both the slope and the ordinate intercept of the straight line, are presented in Table 6. The calculation procedure assumes the sum of hydrocarbons up to six carbons as 100% ,since the exact number of carbons in the Cy+ fractions is not known. The agreement between two values of a,calculated for every particular sample, was taken as a criterion of the "goodness" of the S-F fit (Dwyer and Somorjai, 1979). The excellent agreement of a values, obtained for all the samples, indicates that the products distribution follows the S-F equation (Table 6, Figure 7). The behavior of K-promoted Fe/MgO catalyst is peculiar, since the yield of hydrocarbons with five C atoms is almost doubled in comparison with that one predicted from S-F kinetics (Figure 7b, Table 3) (11.1mass % C obtained vs 5.0 mass% C predicted).
A slightly higher a value for the K-promoted catalyst indicates the formation of longer hydrocarbon chains. A similar influence of K was reported recently (Donnely and Satterfield, 1989). However, there is no evidence of a double S-F distribution in our case, since the second a value is typical for the CIO+ fractions (Donnely and Satterfield, 1989;Huff and Satterfield, 1984). The higher selectivity to methane on the K-promoted in comparison to unpromoted sample, at the same time when a grows, is curious. It is opposite to our earlier results (Putanov et al., 1992a) and is a good example of the influence of experimental conditions on reaction mechanism. A higher HdCO ratio and lower total pressure, applied in the present experiments, seem to facilitate methane formation, while the expected favorable influence of K toward higher hydrocarbons is still present through enhanced selectivity toward C5hydrocarbons (Table 3). This is in accordance with earlier observation of electron donor properties of K which result in higher chain growth probability, Le., enhanced higher hydrocarbon selectivity (Dry et al., 1969; Holmen at al., 1989). Besides, the lowest BET surface area in the whole series (Table 51, and the more than doubled Fe particle size (Putanov et al., 1992a) and MgO particle size (Boskovic, 1991) in comparison with the unpromoted samples, could be further facts of interest attempting to explain different behavior of K-containing sample. The references give examples of Fe crystal growth in the presence of K (Dry and Oosthuizen, 1968; Moral et al., 1987). The fact that K may preferentially affix to a support (Snel, 1987; McVicker and Vannice, 1980) could explain the MgO crystal growth, although their interaction is not clear due to similar acid-base properties. Further, it was shown elsewhere that K does not react with MgO, but rather migrates and reacts with the reactor walls (Perrichon and Durupty, 1988). The results of EMA indicate the independent location of both Fe (Figure 5a) and K (Figure 5b) on the surface of the catalysts. Since the Tamman melting temperature of K is lower than the calcination temperature applied in this work, K might be located in "islands" formed from the melted K. According to Vannice (19821,the possibility of promoting action of K in clusters is connected with a critical number of promoter atoms. The quantity of promoter of 2 atom % has been mentioned as usual in the case of K-promoted iron (Dry, 1981). In our work the quantity of K was 0.4 atom % (calculated on the basis of Fe/MgO), which appears to be sufficient for promoting effect. However, the depth profile of K in the related sample after F-T reaction, obtained by Auger electron spectroscopy (not shown), gives some 4 atom % K at the depth of 10 nm (Boskovic, 1991). This is, of course, the consequence of the applied preparation method for this sample (wetness impregnation) rather than the result of K diffusion. The high initial production of C02, followed by its rapid decline, indicates the change of the reaction mechanism with time on stream in the case of K-promoted Fe/MgO catalyst (Figure 3). C02 can be formed either in the water gas shift reaction, CO + H2O COz + H2, or in the Boudouard reaction, 2CO COS+ C. Since the intensity of the former is proportional to a degree of oxidation state of Fe (Madon and Taylor, 19811, and no iron oxide was detected by the Mossbauer spectroscopy of K-Fe/MgO reduced sample (Putanov et al., 1992a), we can assume the Boudouard reaction as the predominant reaction in the C02 production. The dissociative adsorption of CO, which precedes this reaction, can be performed on a-Fe (Reymond et al., 1982). Oxygen atom, obtained by the
- -
Ind. Eng. Chem. Res., Vol. 33. No. 9, 1994 2093
j I.
a
j
b
I
i
.
*.
Y
Figure 6. X-ray line scanning micrographs of Fe (a) and AI (b) in AI-Fe/MgO sample (magnification 1500X) (reproduced at 95% of the original)
dissociation on FeO, reacts with CO from the gas phase forming COz, while C is partly hydrogenated to methane but partly reactswith FeO, forming carbides. Thereaction of the iron carburization was found to be very fast (Reymondetal., 1982). Assoonasa-Fe has beenconsumed and transformed to carbides, the formation of COz is stopped. Then the FT reaction proceeds over the iron carbides. This is in accordance with the Moasbauer results obtained earlier on the K-promoted sample, indicating that all quantity of the a-iron was transformed to Fe&z during the FT reaction (Putanov et al., 1992a). The decrease of the catalytic activity in the case of K-promoted Ru/SiOz was explained by the modification of the rate at which Hz adds to C atom on the surface; the role of an ensemble effect was supposed rather than an electronic effect (McCloryand Gonzales, 1984). It is likely that a similar geometric factor plays essential role in our catalyst as well. However, instead of limiting the activity of metal clusters by acting as a “spacer” between the iron surfaces (Sushumna and Ruckenstein, 1985). in our case Kprobably acts by “sticking”Fe particlestogether, causing the decrease of the surface area and the effective area for bothCOand Hzadsorption. Suchaphenomenon is known in the literature as a “cement effect” (Perrichon and
Durupty, 1988). Indeed, K-promoted samplecharacterizes a 2 times lower BET surface area (Table 5) and a more than doubled crystal size ofthe iron (Putanovetal., 1992a), in comparison with the unpromoted sample. Similar iron crystal growth in the presence of K was mentioned earlier (Dry and Oosthuizen, 1968; Moral et al., 1987). The domination of the sticking effect of K, over its electron donor properties, might be the reason for overall activity decline in the present experiments. However, a certain amount of K stillaets asan electron promoter and provokes the decrease of the hydrogenation rate by increasing the strength of CO adsorption. The same conclusion was made before by Dwyer and Hardenbergh (1984). rather than the idea of promoting influence through the facilitate dissociation of adsorbed CO. The increasing strength of CO adsorption put the hydrogen in the position similar to coadsorption onto a previously covered surface, which is known asdisadvantageous for hydrogen (Vannice,1982). In such a competitive chemisorption between hydrogen and CO, concentration of atomic hydrogen on the surface is suppressed and methanation is restricted (Dwyer and Hardenbergh, 1984). This results in the higher portion of the gasoline fraction (Cs fraction) and the higher olefin to alkaneratio(Tables3and4). Theappearanceofthehigher
2094 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
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Figure 7. Schulz-Flory plot of Fe/MgO (a), K-Fe/MgO (b), and Al-Fe/MgO (c). Table 6. a Chain Growth Probability Factor. sample ff’ ff” Fe/MgO 0.416 0.407 K-FelMgO 0.429 0.429 Al-Fe/MgO 0.387 0.386 a a’,calculated from the slope; a”, calculated from the intercept.
portion of the branched hydrocarbons (Table IV), which might be connected to the absence of the C,+ fraction observed only in the case of the K-promoted catalysts (Table 3), is staying for the moment without explanation. The lowest a values in whole series, in the case of AlFe/MgO catalyst (Table 6, Figure 7c), are reflected in the highest CHI and lowest C4+ selectivities (Table 3). The overall activity is, as in the case of the K-promoted sample,
significantly lower than the activity of the unpromoted catalyst (Table 2). The selectivity of the Al-Fe/MgO catalyst with time on stream (Figure 4) indicates also the change in the mechanism over the first 2 h. Here again the sharp decline of the CO2 amount occurs, after high initial production. The state of the iron oxides, already known as excellent catalysts for the water gas shift reaction, deserves special attention. The Mossbauer analysis of the reduced AlFe/MgO catalyst (Putanov et al., 1992b) showed that the major part of Fe was in the different oxide phases, whereas a very small part was reduced to the metallic iron. We concluded previously that the hindered reduction was caused by a certain mechanism imposed by Al, which starts from y-Fe2O3, and by topotactic reaction transforms to Fe304 (Putanov et al., 199213). In all other cases, i.e., unpromoted and both Mo- and Ca-promoted catalysts, the reduction of iron was found to start from a-FezO3 and through the mixed oxide (Fe,Mg)O phase goes to metallic Fe, as proposed earlier by Boudart et al. (1975). Under the conditions of FT reaction applied in the present work, nondissociative adsorption of CO can proceed on Fe304, taking an oxygen atom from it and forming COz. In that way, the active center is reduced, but it can be reactivated by reaction with water, which is formed as the primary product of the FT syntheses (Madon and Taylor, 1981). However, this mechanism cannot explain the sharp decline of COZ (Figure 41, since the Mossbauer analysis showed no decrease in the oxide phases; instead, all the iron was in the Fe304 phase after the FT reaction (Putanov et al., 1992~).Therefore, it is more likely that the CO adsorption, which precedes the Boudouard reaction, for this particular catalyst is dissociative in its nature. This kind of adsorption may occur on either a-Fe or Fe304; the resulting C was claimed to be more reactive from the oxide (Reymond et al., 1982; Blanchard et al., 1982). Therefore, parallel CO dissociation may proceed on FeO and Fe304. Oxygen atom obtained by the dissociation on FeO reacts with CO from the gas phase forming C02, while C reacts with FeO giving carbides, as it was proposed earlier by Blanchard et al. (1982). The absence of the carbides in the Mossbauer spectra of the used catalyst (Putanov et al., 1992c) might be understood as a consequence of the low quantity of a-Fe in the reduced sample (4% of relative intensity). In the afterward reaction sequence, iron did transform to the different carbide phases, but in particular quantities which were below the sensitivity of the applied method. When all the metallic iron was transformed to the carbides, the C02 formation stopped. As was mentioned before, A1 influences the iron in the early stage of the catalyst preparation, leaving the major part of Fe (96% ) after reduction in the oxide phase (Guczi et al., 1991; Putanov et al., 1992b). During the reaction, the FeaO4 phase was found to be unchanged for a certain stabilization effect of A1 (Putanov et al., 1992~).This effect was attributed to the “skin” effect of Al, as was supposed earlier for the same catalyst (Boskovic et al., 1991). A uniform distribution of A1 (Figure 6b) and the BET surface area decrease (Table 5) speak in favor of the promoter model, where A1 at the same time acts as a “spacer”between the iron surfaces, and as a “cover”of part of the iron oxide. As a consequence, all the iron retains the same oxidation state during reduction and FT reaction. Both CO and H2 adsorption are suppressed for the fraction of Fe which is hidden under the A1203 ‘shell”. This results in the significant decrease of activity (Table 2). In such competitive CO and H2 adsorption, the adsorption of H2 might
Ind. Eng. Chem. Res.,Vol. 33, No. 9, 1994 2095 be suppressed to a higher extent, resulting in lower hydrogenation activity and more olefins in the products (Table 4).
Conclusions (i) Under the applied experimental conditions, the influence of K is mainly expressed through a geometric factor. The “cement effect” of K causes Fe agglomeration and the decrease of active surface for CO adsorption. Electron promotion, if any, occurs probably through an interaction with the electron-rich oxygen of CO, since K distribution has been found to be independent of Fe agglomerates. (ii) A1 acts by the “skin effect”, covering the major part of iron oxide and protecting it from the reduction. For low catalytic activity the absence of a-Fe and iron carbides is not decisive, but it can be explained with the reduced number of free (uncovered) active sites, such as FeaOr. Acknowledgment This work was supported by Ministry of Science of Serbia. Literature Cited Ballivet-Tkachenko, D.; Tkatchenko, I. Small particles in zeolites as selective catalysts for the hydrocondensation of carbon monoxide. J. Mol. Catal. 1981,13,1-11. Bein, Th.; Schmiester, G.; Jacobs, P. A. Characterization of a new iron-on-zeolite Y Fischer-Tropsch Catalyst. J. Phys. Chem. 1986, 90,4851-4856. Blanchard, P.; Reymond, J. P.; Pommier, B.; Teichner, S. J. On the mechanism of the Fischer-Tropsch synthesis involvingunreduced iron catalyst. J. Mol. Catal. 1982,17,171-181. Borghard, W. S.;Boudart, M. The textural promotion of metallic iron by alumina. J. Catal. 1983,80,194-206. Boskovic, G. Uticaj nosaca i promotora na mehanizam delovanja katalizatora na bazi gvozdja u reakcijama hidrogenovanja ugljenmonoksida. Ph.D. Dissertation, The University of Novi Sad, 1991. Boskovic, G.; Vlajnic, G.; Putanov, P. HT-XRD, DSC and X-ray microprobe analysis of Cu and Al promoted Fe/MgO. React. Kinet. CUtUl. Lett. 1991,45 (2),313-318. Boudart, M.; Delbouille,A,; Dumesic,J. A.; Khammouma, S.;Topsoe, H. Surface catalytic and magnetic properties of small iron particles. I. Preparation and characterization of samples. J.Catal. 1975,37, 486-502. Caesar, P. D.; Brennan, J. A.; Garwood, W. E.; Ciric, J. Advances in Fischer-Tropsch chemistry. J. Catal. 1979,56,274-278. Donnelly, T. J.; Satterfield, C. N. Product distributions of the FischerTropsch synthesis on precipitated iron catalysts. Appl. Catal. 1989, 52,93-114. Dry, M. E. The Fischer- Tropschsynthesis; Anderson, J. R., Boudart, M., Eds.; Catalysis: Science and Technology 1; AkademieVerlag: Berlin, 1981;pp 159-255. Dry, M. E.; Oosthuizen, G. J. The correlation between catalyst surface basicity and hydrocarbon selectivity in the Fischer-Tropsch synthesis. J. Catal. 1968,11, 18-24. Dry, M. E.; Shingles, T.; Boshoff, L. J.; Oosthuizen, G. J. Heats of chemisorption on promoted iron surfaces and the role of alkali in Fischer-Tropsch synthesis. J. Catal. 1969,15,190-199. Dwyer, D. J.; Somorjai, G. A. The role of readsorption in determining the product distribution during CO hydrogenation over Fe single crystals. J. Catal. 1979,56,249-257. Dwyer, D. J. ; Hardenbergh, J. H. Catalytic reduction of carbon monoxide over potassium modified iron surface. Appl. Surf. Sci. 1984,19,14-27. Egiebor, N. 0.; Cooper, W. C.; Wojciechowki, B. W. Synthesis of motor fuels from HY-Zeolite supported Fischer-Tropsch iron catalysts. Appl. Catal. 1989,55,47-64.
Foger, K. Dispersed metal catalysts;Anderson, R. J., Boudart, M., Eds.; Catal.Sci.Eng., 6,Springer-Verlag: Berlin, 1984;pp 227-251 Guczi, L.; Lazar, K.; Boskovic, G.; Kis, E.: Putanov, P. Promoter effectsof Ca, Mo, Al, on FelMgO catalyst on CO hydrogenation; Holmen, A., Jens, K.-J., Kalboe, S.,Eds.; Studies in Surface Science and Catalysis 61;Elsevier: Amsterdam, 1991;pp 251-256. Holmen, A.; Schanke, D.; Sundmark, G. CO hydrogenation over Fe catalyst on different support. Appl. Catal. 1989,50,211-221. Huff G. A., Jr.; Satterfield, C. N. Evidence for two chain growth probabilities on iron catalysts in the Fischer-Tropsch synthesis. J. Catal. 1984,85,370-379. King, D. L.; Cusumano,J. A.; Garten, R. L. Atechnologicalperspective for catalytic processes based on synthesis gas. CUtUl. Rev.-Sci. Eng. 1981,23(1&2), 233-263. Lee, G.; Ponec, V. On some problems of selectivity in syngas reactions on the group VI11 metals. Catal. Rev.-Sci. Eng. 1987,29(2&3), 183-218. Madon, R. J.; Taylor, W. F. Fischer-Tropsch synthesis on a precipitated iron catalyst. J. Catal. 1981,69,32-43. McClory,M. M.; Gonzales,R. D. The role of alkali metals as promoters in the methanation and Fischer-Tropsch reaction: An in situstudy. J. CUtUl. 1984,89,392-403. McVicker, G. B.; Vannice, M. A. The preparation, characterization, and use of supported potassium-group VI11 metal complexes as catalysts for CO hydrogenation. J. Catal. 1980,63,25-34. Moral, P.; Pralioud, H.; Martin, G. A. Potassium addition to Fe/ MgO and Ni/SiO reducibility, dispersity and stabilization of the promoter by the support. React. Kinet. Catal. Lett. 1987,34(l), 1-7. Perrichon, V.; Durupty, M. C. Thermal stability of alkali metals deposited on oxide supports and their influence on the surface area of the support. Appl. Catal. 1988,42,217-227. Putanov, P.; Kis, E.; Boskovic, G.; Lazar, K. Effects of the method of preparation of MgCzOI as a support precursor on the properties of iron/magnesium oxide catalysts. Appl. Catal. 1991,73,17-26. Putanov, P.; Boskovic, G.; Vlajnic, G.; Kis, E.; Lazar, K.; Guczi, L. Promoter effect of Cu, Co and K on Fe/MgO in CO hydrogenation. J. Mol. Catal. 1992a,71,81-92. Putanov, P.; Boskovic, G.; Kis, E.; Lazar, K.; Guczi, L. Effects of preparation on the properties of Mo, Al and Ca promoted Fe/ MgO catalysts. J. Solid State Chern. 1992b,97,41-47. Putanov, P.; Boskovic, G.; Vlajnic, G.; Kis, E.; Lazar, K.; Guczi, L. Iron-support-promoter interactions in the sequence of preparation of Fe/MgO catalysts or CO hydrogenation. Indian J. Technol. 1992,30,156-160. Reymond,J. P.; Meriaudeau, P.;Teichner, S.J. Changes in thesurface structure and composition of an iron catalyst of reduced or unreduced Fez09 during the reaction of carbon monoxide and hydrogen. J. Catal. 1982, 75, 39-48. Sachtler, W. M. H.: Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. Promoter action in Ficher-Tropsch catalysis. J. Catal. 1985,92, 429-431. Snel, R. Olefins from syngas. Catal. Rev.-Sci. Eng. 1987,29 (4), 361-445. Sushumna, I.; Ruckenstein, E. Role of physical and chemical interactions in the behavior of supported metal catalysts: iron on alumina -a case study. J. Catal. 1985,94,239-288. Tau, L.; Dabbagh, H. A.; Wilson, T. P.; Davis, B. H. Fischer-Tropsch synthesis with iron catalysts. Impact of alkali or added alcohol upon catalytic activity and product selectivity. Appl. Catal. 1989, 56,95106. Vannice, M. A. Catalytic activation of carbon monoxide on metal surfaces; Anderson, J. R., Boudart, M., Eds.; Catalysis: Science and Technology 3,Akademie-Verlag: Berlin, 1982; pp 139-198. Received for review May 24, 1993 Revised manuscript received November 18,1993 Accepted May 25, 1994.
* Abstract published in Advance A C S Abstracts, July 1,1994.
