Construction of the Fischer–Tropsch regime with cobalt catalysts

Catalysis Today 71 (2002) 351–360

Construction of the Fischer–Tropsch regime with cobalt catalysts�

Hans Schulz∗, Zhiqin Nie, Farid OusmanovEngler-Bunte-Institute, University of Karlsruhe, Kaiserstraße 12, 76131 Karlsruhe, Germany

Abstract

Changes of activity and selectivity during the initial phases of Fischer–Tropsch (FT) synthesis have been measured withthree promoted cobalt catalysts. It is shown that the FT regime is formed in situ in a slow process lasting several days.A “construction” of the “true FT catalyst” is therefore assumed. Taking into account complementary investigations, thisconstruction is assigned to the segregation of the catalyst surface caused by strong CO chemisorption. This process wouldbe accompanied by an increase of the number of active sites and their disproportionation into sites of higher and lowercoordinations, which would exhibit different catalytic properties. The observed initial activity and selectivity changes are wellto be explained with this concept. © 2002 Published by Elsevier Science B.V.

Keywords:Cobalt catalyst; Fischer–Tropsch regime; CO

1. Introduction

Changes of product composition in the initialstages of conversion on the surface of a solid cata-lyst may have several causes. In a fixed bed reactorwhen the flow of reactants is established, the partialpressures will shift and the reactor temperature willbe affected by the reaction enthalpies. Adsorptionof educts, products and intermediates will provide adynamic coverage of the surface. Additionally, thecatalyst itself can undergo structural and composi-tional changes. Thus, it can be that the “true catalyst”is formed and stable only in situ.

The multicompound composition of Fischer–Tropsch (FT) products reflects a complex set of surfacereactions. It can be regarded as a source of informationabout the involved active sites. As the product

� Lecture at the conference “Fischer–Tropsch at the Eve of theXXI Century”, Krüger National Park, November 5–8, 2000; or-ganised by the Catalysis Society of South Africa.

∗ Corresponding author. Tel.:+49-721-608-4238;fax: +49-721-693019.E-mail address:[email protected] (H. Schulz).

composition changes with time, it mirrors changesof the catalytic system. With this concept in mind,initial changes of reaction rate and product composi-tion during FT synthesis with three promoted cobaltcatalysts are presented and discussed in this paper.

2. Experimental

The cobalt catalysts have been prepared by quickprecipitation with an ammonia solution being addedto the nitrates solution in which small SiO2 particles(aerosil,dp ∼ 200 Å) had been suspended [1] to obtainthe compositions (weight ratios): 100 Co:100 SiO2(aerosil):11 Zr:0.45 Pt; 104 Co:100 SiO2 (aerosil):9Re; 100 Co:100 SiO2 (aerosil):14 Ir.

Platinum was added to the zirkonia promoted cat-alyst by impregnation of the dried precipitate witha H2PtCl6 solution. The precipitates were dried at393 K, ground and sieved(dp � 0.1 mm), diluted(weight ratio, 1:10) with fused silica spheres (dp =0.25–0.4 mm) and 6 ml of this mixture, correspond-ing to 0.68 g of cobalt, filled into the fused silica

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352 H. Schulz et al. / Catalysis Today 71 (2002) 351–360

reactor tube of 9 mm inner diameter. The catalystswere calcined and reduced in situ (calcination: argonflow of 40 ml(NTP)/min, 1 bar, heating 2 K/min upto 973 K, 2 h at this temperature; reduction: gas flow40 ml(NTP, H2:Ar = 1:3)/min, heating 2 K/min upto 973 K isothermal at this temperature until H2Ofrom reduction was no longer to be detected). Theconditions of synthesis were 463 K, 5 bar, H2/CO =1.9, GHSV = 300 h−1 (GHSV referred to the catalystbed volume of 6 ml).

Apparatus and procedure for FT synthesis hadspecifically been developed: small catalyst particlesfor only negligible mass transfer influences, isother-mal operation through high catalyst dilution, homoge-neous gas flow and low pressure drop by applicationof relatively big fused silica dilution particles, precisemeasurement and control of flow and composition ofin- and out-going streams for accurate mass balances(also with the help of the internal standards) and timeresolution of reaction rate and selectivity by meansof quick ampoule sampling (≤0.1 s) from the hot(473 K) gaseous product stream [2–4] after passinga hot trap. GC analysis, developed for the ampoulesamples, covered the range C1 to ca. C16 with highprecision, high resolution and no discernible discrimi-nation applying a temperature programme from 353 to493 K and that also in a version with olefin precolumnhydrogenation as used for the investigation of chainbranching [2]. Importance for this work has also beenthe earlier development of a kinetic model [5–7]. Thismodel allows to transform the observed multicom-pound product composition data into a set of kineticcoefficients of the reactions of chain prolongation, ter-mination and branching, each as a function of carbonnumber. Additionally, the olefin reactions readsorp-tion for growth, hydrogenation and double bond shiftare quantified with dependence on carbon number.

3. Results and discussion

3.1. Development of activity with time

The activity of the three cobalt catalysts has been de-termined as CO conversion with dependence on time.CO2 was not present in the product stream. Thus, COconversion equalled the yield of organic compoundson a carbon basis.

Fig. 1. CO conversion as a function of time (texp.) with threepromoted cobalt catalysts at 463 K, 5 bar, H2/CO = 1.9. Forfurther conditions, see Section 2. CO conversion is mainly relatedto the reaction of chain prolongation (formation of CH2 and itsaddition to a surface species for growth).

The results are shown in Fig. 1. From the shape ofthe curves, three kinetic episodes can be distinguished.During the first ca. 10 min (episode I), the apparentCO conversion decreases drastically as reflecting fastchanges due to the delayed build up of the CO partialpressure in the product stream, because of CO reten-tion in the reactor, to fill its void volume and of COadsorption on the catalyst. In episode II, the CO con-version increases slowly with time to a much highervalue from ca. 27 to 80% with the Co–Re-, from ca.15 to 30% with the Co–Zr–Pt- and ca. 5 to 15% withthe Co–Zr-catalysts. It follows that the fresh (well re-duced) cobalt catalyst is not much active, however,then in a slow process lasting 3–4 days (episode II),the catalyst generates activity up to an about threetimes higher level. It is assumed that—as this processis so slow—it involves solid state transformations toperform the “in situ construction” of the FT catalyst.

Comparing the three catalysts, the highest activitywas obtained with the rhenium promoted one. How-ever, this result cannot be generalized as it would needfurther investigations to optimize each catalyst com-position and also the procedures and conditions of itspreparation and pre-treatment.

3.2. Chain growth and product desorption

The intrinsic feature of FT synthesis is aliphaticchain growth on the catalyst surface. The repeated ad-dition of a monomer is commonly visualized as CH2

H. Schulz et al. / Catalysis Today 71 (2002) 351–360 353

addition. Formation of the main products,�-olefinsand paraffins, is then assumed as associative desorp-tion of an alkyl species together with a H-atom or itsdissociative desorption via�-H-abstraction.

Fig. 2 showspg,3, the chain prolongation probabil-ity at carbon numberNC = 3 of the growing species,as a function of time (duration of the experiment,texp.)for the experiments with the three catalysts. Growthprobability atNC = 3 has been chosen, as at this car-bon number, it is the least affected by olefin secondaryreactions [8].

It is seen that during the period of “in situconstruction” of the catalyst, the selectivity changestowards a much higher growth probability.

Growth or desorption are alternative reaction possi-bilities of the hydrocarbon chains on the catalyst sur-face as indicated in Fig. 2 (right). The ratio of thesetwo reaction rates is the same as that of the proba-bilities of reaction of the respective surface species(rg/rd = pg/pd). The following rate ratios have beentaken from Fig. 2.

Catalyst rg/rd at texp.= 10 min

rg/rd at texp.= 104 min

Factor ofincrease

Co–Re 2.3 6.1 2.6Co–Zr 1.3 4.0 3.0Co–Ir 1.0 3.7 3.7

Fig. 2. Growth probabilitypg,3 as a function of time (texp.) withthree promoted cobalt catalysts (463 K, 5 bar, H2/CO = 1.9).

These numbers show that the ratio of growth to des-orption rates increase with time by a factor of about3. It is concluded that during catalyst construction—in addition to the FT activity increase—the particularfeature of the FT regime is established, the propensityfor chain growth, or in other words the “polymeriza-tion nature” of FT synthesis. In an earlier publication[9], the specific inhibition of desorption reactions hasbeen defined as characteristic of the FT regime. Corre-spondingly, it is concluded that a change in the natureof the active sites happens during episode II of earlycobalt catalyst life.

Ideal polymerization performance of the FT reac-tion would imply carbon number independent chaingrowth probability as representing a horizontal line inthe diagram of growth probability as a function of car-bon number of the growing speciesNC. It is seen inFig. 3 that there occur distinct deviations from ideal-ity and this particularly occurs in the early stages ofan experiment (texp. ∼= 100 min):

• low value of pg at NC = 1 indicating increasedmethane formation;

• high value ofpg atNC = 2 indicating strong ethenereadsorption for chain growth [8];

• increase ofpg with NC in the range of C6–C9, asto be explained by with carbon number increas-ing olefin-(1) readsorption for further chain growth[8,10].

At steady state (texp. = 8620 min), the ideal (poly-merization) behaviour is almost achieved, except forNC = 1, where the low value reflects the additionalmethane formation. It is concluded that only after re-structuring, the active sites of FT synthesis are clearlydeveloped (see the further discussion below).

3.3. Chain branching

The FT CO hydrogenation preferentially produceslinear chains of CH2 groups. In addition, a smallextent of methyl branching occurs. Any branchingreaction should be assumed to be more demandingin space at the active site than linear chain growth.This would explain the generally observed decreaseof branching probability with chain length [7,11],as the chain to be branched becomes more spaciouswith its length. AtNC = 4 of the growing species,branching probability commonly is the highest. This


Fig. 3. Growth probabilitypg as a function of carbon numberNC at the beginning (texp. = 100 min) and at steady state (texp. = 8620 min)of FT synthesis with the catalyst Co–Zr–SiO2 (463 K, 5 bar, H2/CO = 1.9).

probability,pgbr,4, has been chosen to discuss the timedependence of branching probability (Fig. 4). Withthe Zr- and Ir-promoted catalysts, branching probabil-ity is relatively high at the beginning (pgbr,4 = 0.075and 0.055, respectively attexp. = 15 min) but de-clines to 0.03 and 0.015, respectively, within about10 h (texp. = 600 min). This is interpreted as an in-crease of spatial constraints at the sites on whichchain growth occurs. With the Re-promoted catalyst,branching probability is very low(pgbr,4 = 0.02)already at the beginning and merely changes withincreasing time. Then, this means that the spatialconstraints at the sites of chain growth exist from thebeginning and the kind of these sites—in contrast tothose with the otherwise promoted catalysts—wereconsidered to be the same from the beginning in spiteof the high increase of activity with time. At steady

Fig. 4. Branching probabilitypgbr,4 as a function of time (texp.) for three cobalt catalysts (463 K, 5 bar, H2/CO = 1.9).

state (e.g.texp. = 104 min), branching probability islow and almost the same (ca. 2%) with all the threecatalysts.

Amazingly at the beginning of the FT experiment,when the branching probability is high (curve fortexp. = 162 min in Fig. 5), a principally differentshape of the curve of branching probability with de-pendence on carbon number, as compared with thecurve for steady state (texp. = 9600 min), has beenobtained. FromNC = 5 onwards here, the branch-ing probability increases (not decreases) with carbonnumber. Thus at the beginning, a different mecha-nism appears to contribute to chain branching. As atthe beginning, the spatial constraints at the FT sitesare to be assumed low, then�-olefin readsorption inposition 2 can be possible for further chain growth toresult in a methyl side group.


Fig. 5. Branching probability as a function of carbon num-ber NC at the beginning (texp. = 162 min) and at steady state(texp. = 9600 min) of FT synthesis with the catalyst Co–Zr–SiO2

(463 K, 5 bar, H2/CO = 1.9).

This adsorption should be favoured at increasingsize of the olefin molecule. (Under steady state con-ditions, olefins adsorb on FT sites rather exclusivelywith the terminal carbon atom, as this readsorptionfor further chain growth has been observed to be notassociated with chain branching [12].)

Remarkably, branching probability is always lowfor NC = 3, the smallest species where branching ispossible. To explain this behaviour, it is recalled thatin the kinetic model for calculation of branching prob-ability, the desorption probability for branched andnonbranched species is taken as equal; however, thereaction of desorption as an olefin (the main FT des-orption reaction) can be assumed to be strongly spa-tially demanding at the site in case of a neighbouredtertiary C-atom and therefore specifically slow.

3.4. Olefinicity

Among the product olefins of different chain length,those with carbon number 4 appear to be the leastaffected by secondary reactions [8]. Therefore, thecomposition of the C4 hydrocarbon product fraction is

Fig. 6. Olefin content of the C4 fraction of FT products as afunction of time (texp.) for three cobalt catalysts (463 K, 5 bar,H2/CO = 1.9).

used for the discussion of olefinicity with dependenceon time,texp. (Fig. 6).

As the olefin content increases with time, it is con-cluded that during the catalyst formation period thosesites on which secondary hydrogenation occurs disap-pear. This conclusion will be discussed more deeplybelow. It is also seen that the differently promotedcatalysts show a different time dependence in thisaspect. With the Co–Re catalyst from the beginning,the olefin content in the C4 fraction remains almostthe same indicating again that with this catalyst—from the beginning—the kind of active sites remains(almost) the same.

Similar conclusions are drawn for the time depen-dence of secondary double bond shift from Fig. 7showing the olefin-(1) content among the C4 olefinsas a function of timetexp.

3.5. In situ FT catalyst construction

From the above described development of activityand selectivity with time of cobalt catalysts for FT

Fig. 7. Olefin-(1) content in the C4 fraction as a function of time(texp.) for three cobalt catalysts (463 K, 5 bar, H2/CO = 1.9).


synthesis, it is concluded that the kinetic regime ofFT synthesis is generated slowly under reaction condi-tions. An in situ construction of the “true FT catalyst”is therefore envisaged.

This “catalyst construction” shall now be consid-ered more profoundly. CO chemisorbs strongly oncobalt (as well as on Ni and Ru) and it has beenpointed out by Pichler [13] that FT synthesis performsunder conditions not so far from those which allow(thermodynamically) carbonyl formation from thesemetals. Then the reaction of CO with the metal sur-face can be assumed to induce surface restructuringin direction to surface enlargement through segrega-tion. Fig. 8 gives a schematic thermodynamic pictureof segregation in competition with sintering. Segrega-tion of the surface will be favoured against sinteringif the free energy of CO adsorption overcompensatesthat of reducing the surface area through sintering.Surface segregation can explain the observed activityincrease with time, as segregation means an increaseof the specific surface area and, respectively, of thenumber of active sites. Nickel surface segregation byCO adsorption has been reported by Ponec et al. [14].Images of a cobalt metal surface which had been usedfor FT synthesis have been obtained by scanning tun-nelling electron microscopy by Wilson and de Groot[15]. It is deduced from these pictures that segregationproduces an ordered surface structure. The depth ofsegregation is generally just one atomic layer and the

Fig. 8. Thermodynamic view of segregation against sintering of cobalt particles. Specific enthalpy of cobalt as for big crystals: smallcrystals and segregated small crystals with and without chemisorbed CO.

rather uniform distance of the peaks is ca. 30 Å. It fol-lows that by segregation through CO chemisorption,a disproportionation of plane sites happens in sites oflow coordination on top positions and sites of highcoordination in hole positions.

In the segregated catalyst state, thus three sorts ofsites can be visualized:

1. Peak sites and mountain sites of low coordina-tion with several free valences for binding surfacespecies and “ligands”. On these site reactions alike,those known for carbonyl complexes can be as-sumed and specifically the reaction step of chaingrowth should be possible.

2. Hole and valley sites of high coordination. Theseshould be suited for CO dissociation.

3. Remaining plane sites with the common activ-ity/specificity of hydrogenation catalysts, however,being much “poisoned” by strong CO chemisorp-tion in the FT regime.

The observed initial changes of selectivity andactivity during formation of the FT regime are wellexplained in consequence of surface site dispropor-tionation as for high and low coordinations.

It has been observed [16] that high performanceof cobalt catalysts in FT synthesis was only obtainedwhen during the initial stages of a run the reactiontemperature was approached very gradually. This sup-ports the concept of in situ FT catalyst construction


through segregation which would need sufficient timeat favourable conditions: relatively low temperature atrelatively high CO partial pressure.

3.6. The FT alkyl grove

The major (primary) products of FT synthesis oncobalt are linear olefins with terminal double bonds(70–80 mol%) and linear paraffins [12]. These com-pounds are thought to be formed by desorption of alkylgroups: associative desorption for a paraffin moleculeand dissociative desorption for an olefin molecule:

As the desorption probability is—in a first appro-ximation—carbon number independent (except forNC = 1), it follows a similar distribution on carbonnumbers of the chemisorbed alkyls as that of the hy-drocarbon products. Thus, alkyl species of differentchain length bonded to the catalyst surface can beimagined to form a kind of an “alkyl grove”. Theindividual trees of which are soaring into the fluidphase. As the growth probability increases duringcatalyst formation, the mean height of this grove willincrease accordingly. As the number of FT sites in-creases, simultaneously, the density of the alkyl grovewill increase also. Alkyl species should be favourablylocated on sites of low coordination (on top sites)

Fig. 9. Principal view of the “living alkyl grove” during FT synthesis on cobalt: alkyl chains are the very stable intermediate speciesin FT synthesis; their desorption reaction is strongly suppressed; the alkyl chains are preferentially located on cobalt peak sites of lowcoordination; further, free coordinations may be occupied by “ligands” as CO, CH2 and others; the “alkyl grove” reaches its steady stateafter completion of surface restructuring; at steady state, growth and decay of “alkyl trees” are in balance.

with distinct complex character, similar to the wellknown alkyl–carbonyl-complexes in homogeneouscatalysis, as pertinent, e.g. in olefin hydroformylation.“Carbonyl surface complexes” have been proposedby Pichler [13] to be of mechanistic importance in FTsynthesis.

Remarkably, the olefin desorption probability ishigher than that for paraffin [9]. This means that inthe FT regime the alkyl species are very stable againstdesorption as paraffin molecules. Even desorption asolefin is slow as compared with chain prolongation(this is a necessary conclusion for any chain growth

to occur) proving again the high stability of alkylspecies in the FT regime.

The view that chemisorbed alkyls are particularlystable surface species (intermediates) in FT synthesisis supported by several observations:

1. The reaction of double bond shift has been pro-posed to perform according to the following kineticscheme [9]:


Relatively high degrees of double bond shift havebeen noticed at moderate degrees of hydrogena-tion [8]. It is concluded that addition of the first Hto the olefin is fast and reversible, whereas addi-tion of the second H (associative desorption of theparaffin molecule) is slow and irreversible. Againthe alkyl species is seen to be very stable againsthydrogenation.

2. At NC = 1, the methyl species cannot form anolefin molecule, and only desorbs associatively

Fig. 10. Schematic picture of surface species on the segregated cobalt surface and main FT reactions.

with hydrogen to yield methane. Paraffin desorp-tion has been concluded to be particularly slow.Then, this explains the suppression of methaneformation (which would be much favoured thermo-dynamically), a prerequisite for any chain growthreaction to proceed.

3. High stability of methyl species on a palla-dium catalyst has been reported by Albers et al.[17]. A used catalyst was found to be coveredwith methyl groups which had even poisoned its


hydrogenation activity. The methyl species werethought to be located on top positions of the cat-alyst surface which nicely supports the aboveconclusion of alkyl chains to be preferentiallybonded to peak sites of the segregated catalyst.The method of investigation had been neutronscattering.

4. Characterization of the FT regime with cobalt asthe catalyst has also been performed by means of13CO steady state isotopic transient studies [18] in-dicating a high relative surface coverage with CO ofca. 70%. The C1-coverage (e.g. methyl) amountedto 27% and that of C2+ to about 1%. Replacementof C1Hx species was specifically slow as consistentwith a high stability of the methyl species.

Generalizing from the several observations and con-clusions, a simplified schematic picture of the alkylgrove on the catalyst surface during FT synthesis withcobalt is presented in Fig. 9.

A picture of FT synthesis with the cooperating dif-ferent cobalt sites is now schematically presented inFig. 10. Chain growth is related to top sites wherereactions, similar to those in homogeneous complexcatalysis, are possible. CO dissociation is assumed toperform on sites of high coordination as pertinent forholes and valleys of the surface. Hydrogen is availableall around; however, the reaction of H for associativeparaffin desorption is seriously suppressed. Collabora-tion of the different sorts of sites is achieved by migra-tion of particularly CH2 in direction to top positions.

Remaining plane sites of the Co catalyst are widelypoisoned through CO chemisorption, but to someextent still available for minor reactions, as olefin iso-merization, olefin hydrogenation and methane forma-tion. Several further reactions are minorily possible.Their relative participation varies during constructionof the “true catalyst” and also with changing reactionconditions. Thus these minor reactions are interest-ing indicators for tracing the kinetic regime on thecatalyst surface (as will be objective of a separatecommunication).

4. Summary

Changes of reaction rate and selectivity in the ini-tial stages of FT synthesis have been investigated with

three promoted catalysts. Reaction rate increased byabout a factor of 3 and selectivity changed towardshigher chain growth probability and lower branchingprobability. As these changes are only slow, they ap-pear to indicate a catalyst restructuring process. Thedetails of selectivity changes are used to characterizethe changes of the nature of the active sites.

This FT cobalt catalyst “in situ construction” isaddressed as surface segregation through strong COchemisorption as supported from complementary lit-erature. By this process, the number of sites increaseslargely and the sites disproportionate into sites ofhigher and lower coordinations on the expense ofplane sites. The on top sites of low coordinationallow for reactions similar to those known from ho-mogeneous complex catalysis and the sites of lowcoordination (in holes and valleys) preferentially giveCO dissociation.

It follows from the selectivity studies, but also neu-tron scattering on used hydrogenation catalysts, thatalkyl species are particularly stable intermediates. Dueto the observation/conclusion that the reaction of as-sociative paraffin desorption, the combination of analkyl with a H is particularly slow in the FT regime,a kind of “alkyl grove” is imagined to exist on thesurface of the operating FT catalyst.

Then the steady state kinetic regime of FT synthesiswith cobalt (and Ni and Ru) is understood as estab-lished during an initial process of self-organization inwhich the catalyst surface is segregated to produce thetrue collaborating FT sites of high and low coordina-tions and a steady state “alkyl grove” on the catalystsurface, whereas the reactions on common hydrogena-tion sites (plane sites) are seriously poisoned (throughstrong CO chemisorption).

References

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Construction of the Fischer–Tropsch regime with cobalt catalysts