ft2

Journal of Catalysis 248 (2007) 143–157

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Cobalt dispersion, reducibility, and surface sites in promotedsilica-supported Fischer–Tropsch catalysts

J.-S. Girardon a,1, E. Quinet a, A. Griboval-Constant a, P.A. Chernavskii b,L. Gengembre a, A.Y. Khodakov a,∗

a Unité de Catalyse et de Chimie du Solide, UMR 8181 CNRS, Université des Sciences et Technologies de Lille, Bat C3, Cité scientifique,59655 Villeneuve d’Ascq, France

b Department of Chemistry, Moscow State University, 119992 Moscow, Russia

Received 12 October 2006; revised 3 March 2007; accepted 5 March 2007

Available online 18 April 2007

Abstract

Cobalt particle size, cobalt reducibility, and metal surface sites in a series of ruthenium- and rhenium-promoted cobalt silica-supported Fischer–Tropsch catalysts were studied by X-ray diffraction, UV–vis spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectronspectroscopy, DSC-TGA thermal analysis, and propene chemisorption. The catalysts were prepared by co-impregnation; in several catalyst synthe-ses, sucrose was added to the impregnating solutions. Mononuclear octahedral cobalt complexes were observed in the catalysts after impregnationand drying. Cobalt repartition on silica in the impregnated and dried catalysts depended primarily on the pH of the impregnating solution. Cobaltrepartition was uniform on the silica surface if the pH of the impregnating solution was higher than the point of zero charge (PZC) of silica, butwas less uniform at pH below that of the PZC of silica. Cobalt dispersion proceeded during catalyst calcination in air. Decomposition of cobaltnitrate and crystallization of cobalt oxide seemed to be the crucial steps in the preparation of highly dispersed cobalt catalysts. Promotion withnoble metals resulted in greater cobalt dispersion, probably due to higher concentrations of cobalt oxide crystallization sites. Addition of sucrosemodified the structure of supported cobalt complexes and led to higher temperatures of crystallization of cobalt oxide and to catalysts with ex-tremely high cobalt dispersion. In situ magnetization measurements show that promotion with Ru moderated the temperature of reduction of cobaltoxide to metal phases, whereas the effect was less significant for Re-promoted catalysts. The addition of sucrose during impregnation, althoughsignificantly enhancing cobalt dispersion, did not diminish cobalt reducibility. Due to a combination of high cobalt dispersion and reducibility,the ruthenium- and rhenium-promoted catalysts prepared using sucrose had the highest number of cobalt metal surface sites. Fischer–Tropschreaction rates were determined principally by the number of cobalt surface sites, with high cobalt dispersion and easy reducibility resulting inmore active Fischer–Tropsch catalysts.© 2007 Elsevier Inc. All rights reserved.

Keywords: Clean fuels; Fischer–Tropsch synthesis; Nanoparticles; Catalyst preparation; Point of zero charge (PZC); EXAFS; Cobalt catalysts; Promotion;Dispersion; Reducibility

1. Introduction

Fischer–Tropsch (FT) synthesis produces clean fuels fromsyngas, which can be generated from natural gas, coal, andbiomass. Most VIII group metals have measurable activity in

* Corresponding author. Fax: +33 3 20 43 65 61.E-mail address: [email protected] (A.Y. Khodakov).

1 Current address: Max-Planck-Institut for Coal Research, Kaiser-Wihelm-platz 1, 45470 Mülheim an der Rhur, Germany.

0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcat.2007.03.002

carbon monoxide hydrogenation but yield different products,including hydrocarbons, alcohols, acids, and esters. Cobalt-supported catalysts are particularly suited for the production oflong-chain hydrocarbons [1–3]. Preparation of cobalt FT cata-lysts involves several essential steps. The catalysts for FT syn-thesis are commonly synthesized using aqueous impregnationor co-impregnation with cobalt nitrate and promoters, followedby oxidative and reductive pretreatments. The catalysts are thenexposed to syngas in an appropriate reactor to conduct FT reac-tion.

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144 J.-S. Girardon et al. / Journal of Catalysis 248 (2007) 143–157

It has been shown [3–6] that for cobalt particles larger than7–8 nm, FT activity is a function of the number of surfacemetal sites. Thus, cobalt dispersion and reducibility seem tobe important parameters affecting the number of cobalt sur-face sites and thus the overall catalytic performance. Our pre-vious reports [7–12] have shown that the support texture andcatalyst calcination could provide efficient tools for control-ling cobalt particle size, reducibility, and catalytic propertiesof monometallic supported cobalt catalysts. FT catalysts areoften promoted with noble metals (e.g., Ru, Pt, Re). Previouswork suggests that introduction of noble metals could lead toseveral phenomena, including much easier reduction of cobaltoxide particles [12–15], formation of bimetallic particles andalloys [16,17], a lower fraction of barely reducible mixed ox-ides, enhanced cobalt dispersion [18], inhibition of catalystdeactivation [19,20], appearance of additional hydrogen acti-vation sites [21], and increased site intrinsic reactivity [22].Culross and Mauldin showed [23–25] that the addition of dif-ferent chelating molecules during catalyst impregnation couldaffect deposition of cobalt phases, cobalt dispersion, and thenumber of cobalt active sites in the reduced catalysts.

This paper addresses the impact of different preparationsteps, the use of metallic promoters and a chelating agent oncobalt dispersion, reducibility, and number of cobalt surfacesites in silica-supported FT catalysts. The catalysts have beenstudied using a comprehensive set of characterization tech-niques. The catalytic performance in FT synthesis was mea-sured in a fixed-bed microreactor. The catalytic results, as wellas the characterization data, are discussed.

2. Experimental

2.1. Catalyst preparation

Cobalt catalysts were synthesized via incipient-wetnessimpregnation or co-impregnation using aqueous solutions ofcobalt nitrate and promoters. Cab-o-sil M-5 fumed silica(SBET = 214 m2/g, Cabot) was used as a catalytic support in allcatalyst preparations. Before impregnation, Cab-o-sil M5 wasagglomerated by wetting and dried at 373 K. The precursorsof noble metal promoters were either a commercial solution ofruthenium nitrosyl nitrate in HNO3 (Ru = 1.5 wt%, Sigma–Aldrich) or an aqueous solution of perrhenic acid (HReO4,Sigma–Aldrich). In several preparations, the impregnating so-lutions also contained sucrose with the Co/sucrose molar ratioof 10. The pH of the relevant impregnating solutions is pre-sented in Table 1. The pH of the solutions containing rutheniumwas lower than that of the solutions containing rhenium becauseof the presence of nitric acid in the ruthenium precursor. Thecontents of cobalt and promoting noble metal (Ru or Re) inthe catalysts were respectively 5.5–8.2 wt% and 0.1–0.2 wt%(Table 1). The impregnated catalysts were dried in an oven, cal-cined in a flow of air at different temperatures (373–673 K),and then reduced in hydrogen at 673 K for 5 h. The temper-ature ramp rates during calcination and reduction were 1 and3.3 K/min, respectively. The cobalt content in the catalystswas measured by atomic absorption at the Service Central

Table 1Chemical composition and surface area of promoted cobalt silica supportedcatalysts

Catalysts Cobaltcontent(wt%)

pH ofimpreg-natingsolution

Co/Ru orCo/Re molarratio

Relativecarboncontent(nC/nCo)

BETsurfacearea(m2/g)

CoRu(S) 5.5 0.3 76 1.1 112CoRu673(S) 7.7 75 <0.2 191CoRe(S) 6.3 1.3 73 1.8 113CoRe673(S) 7.7 72 <0.2 174CoRu 5.5 36 – 118CoRu373 6.5 0.1 102 – 174CoRu673 8.2 354 – 160CoRe 5.6 58 – 124CoRe373 8.1 0.9 111 – 154CoRe673 7.0 160 – 162

d’Analyse du CNRS, (Vernaison, France). Specific surface ar-eas and porosity of the calcined catalysts were measured bylow-temperature nitrogen adsorption using the BET method(Table 1).

The catalysts are designated CoMT(S), where M denotes thepromoting metal (Ru or Re) and T and (S) denote the temper-atures of catalyst calcination and eventual addition of sucroseduring impregnation, respectively. The impregnated and driedcobalt catalysts (before calcination at T � 373 K) are desig-nated CoM(S).

2.2. X-ray diffraction

Powder X-ray diffraction patterns were recorded by aSiemens D5000 diffractometer using Cu(Kα) radiation. Theaverage crystallite size of Co3O4 was calculated according toScherrer’s equation [26] using a (440) peak at 2θ = 65.344.

2.3. UV–vis spectroscopy

Diffuse reflectance UV–vis spectra of catalysts and catalystprecursors were obtained at ambient conditions with a Varian-Cary 4 spectrophotometer using BaSO4 as a reference.

2.4. DSC-TGA

Simultaneous differential scanning calorimetry and thermo-gravimetric analysis were carried out in a flow of air at heatingrate of 1 K/min using a DSC-TGA SDT 2960 thermal analyzer.The sample loading was typically 10–15 mg.

2.5. X-ray absorption

The X-ray absorption spectra at the Co K-edge weremeasured at the European Synchrotron Radiation Facility(DUBBLE-CRG, Grenoble, France) and the Elettra SynchrotronLight Laboratory (beamline 1.1, Trieste, Italy). In situ de-composition of catalyst precursors and reduction in hydrogenwere performed at ESRF using our in situ X-ray absorptioncell described elsewhere [27], while ex situ characterization of

J.-S. Girardon et al. / Journal of Catalysis 248 (2007) 143–157 145

Fig. 1. XANES spectra of the reference compounds: Co(NO3)2 6H2O (1),Co3O4 (2), CoO (3), α-cobalt silicate (4), cobalt foil (5).

calcined catalysts using XANES and EXAFS was done at Elet-tra. The measurements were performed in transmission mode,with two ionization chambers used for X-ray detection. TheSi (111) double-crystal monochromator was calibrated by set-ting the first inflection point of the K-edge spectrum of Co foilat 7709 eV. Measuring an X-ray absorption spectrum (7600–8400 eV) took about 30–40 min. The X-ray absorption datawere analyzed using the conventional procedure. The XANESspectra after background correction were normalized by theedge height. After subtracting metal atomic absorption, theextracted EXAFS signal was transformed without phase cor-rection from k space to r space to obtain the radial distributionfunction (RDF).

Crystalline Co3O4, CoO, Co foil, and α- and β-cobalt sili-cate were used as reference compounds for XANES and EX-AFS data analysis. The XANES spectra of the reference com-pounds are shown in Fig. 1. Additional information about co-ordination of cobalt in the reference compounds is availableelsewhere [9,10,28,29].

2.6. XPS

Surface analyses were performed using the VG ESCALAB220XL spectrometer. The Alkα nonmonochromatized line(1486.6 eV) was used for excitation with a 300 W appliedpower. The analyzer was operated in a constant pass energymode (Epass = 40 eV). Binding energies were referenced to theSi2p core level (103.6 eV) of SiO2 support. The reproducibilitywas ±0.2 eV for Co2p binding energy. The vacuum level dur-ing the experiments was >10−7 Pa. The powdered catalyst waspressed as a thin pellet onto a steel block. In situ reduction wascarried out in pure hydrogen at 673 K for 5 h in the reactor cellof the preparation chamber attached to the analysis chamber ofthe spectrometer. The temperature ramp rate was 3.3 K/min,with a hydrogen flow rate of 20 mL/min. The reduced sam-ple was then transferred from the preparation to the analysischamber of the spectrometer under vacuum without exposure

to air. The pressure in the XPS preparation chamber during thetransfer was about 10−6 Pa.

The XPS of cobalt catalysts were systematically comparedwith the XPS spectra of the reference CoO, Co3O4, and amor-phous cobalt silicate using relevant literature references. The2p1/2 2p3/2 spin–orbital splittings and intensities of satellitestructures were used in addition to the absolute Co2p bindingenergies for the analysis of XPS data.

2.7. In situ magnetic measurements

In situ magnetic measurements were performed using aFoner vibrating-sample magnetometer as described previously[30,31]. Design of magnetometer allows recording curves ofmagnetization during temperature-programmed heating or un-der isothermal conditions at 280–973 K. Temperature-program-med reduction experiments were carried out in pure hydrogen.The sample amount was about 30 mg in all experiments. Thegas flow velocity was 60 mL/min. The appearance of cobaltmetallic species in the catalysts was monitored in situ by a con-tinuous increase in sample magnetization during the reduction.

For evaluation of cobalt metal particle sizes from magneticdata, the catalyst was first reduced in a flow of hydrogen at773 K. Once a constant value of magnetization was attained atthat temperature, the catalyst sample was cooled to 473 K. Thehydrogen flow was replaced by argon flow at 473 K to desorbhydrogen species from the catalyst, after which the sample wascooled in argon to 280 K. The dependency of magnetization onthe intensity of the magnetic field (field dependency) was mea-sured at 280 K by scanning the intensity of the magnetic fieldto 6.3 kOe.

2.8. Propene chemisorption

Recently, [32] we proposed a new method for evaluatingthe number of cobalt active sites using propene chemisorp-tion. We showed that propene chemisorption was irreversibleon cobalt catalysts. The method has been tested for a numberof cobalt-supported catalysts [10,12], and good agreement hasbeen found between propene chemisorption and results of othercharacterization techniques. The measurements could be donein the pulse mode in the same fixed-bed reactor used for mea-suring catalytic performance.

In brief, in the experimental procedure used for propenechemisorption, after reduction in pure hydrogen at 673 K for5 h, the catalyst sample (0.1 g) was cooled and purged withHe at 323 K. Then 0.25-mL pulses of propene were introducedto the flow of He. The relative number of metal surface siteswas estimated from the amount of chemisorbed propene. Nopropene chemisorption was observed on pure silica support.Analysis of the reaction products was done by gas chromatog-raphy with a packed column containing XOA 400 silica. Thepulse experiments were completed when the detector showedno propene chemisorption. This method provides only relativeinformation about cobalt metal sites. Note that no assumption ismade about the stoichiometry of propene chemisorption or thus


about the absolute number of cobalt metal sites in the reducedcobalt catalysts.

2.9. Catalytic measurements

Carbon monoxide hydrogenation was carried out in a fixed-bed stainless steel tubular microreactor (dint = 9 mm) operatingat 463 K under atmospheric pressure. The thermocouple wasin direct contact with the catalyst. The reactor design allowedmeasurement of temperature along the catalyst bed; no hot spotwas detected in the catalyst bed during the FT reaction. Thecarbon monoxide conversion was <10% in all experiments.

The catalyst was crushed and sieved to obtain catalyst grainsof 0.05–0.2 mm diameter. The catalyst loading was typically0.5 g. The samples were reduced in hydrogen flow at 673 Kfor 5 h. After the reduction, a flow of premixed synthesis gaswith molar ratio H2/CO = 2 was introduced to the catalysts.The carbon monoxide contained 5% nitrogen, which was usedas an internal standard for calculating carbon monoxide conver-sion. To avoid possible condensation of the reaction products,the gas transfer lines were constantly heated at 423 K. Gaseousreaction products were analyzed online by gas chromatogra-phy. Analysis of H2, CO, CO2, and CH4 was performed using a13X molecular sieve column and a thermal conductivity detec-tor. Hydrocarbons (C1–C18) were separated in a 10% CP-Sil5on Chromosorb WHP packed column and analyzed by a flame-ionization detector. The hydrocarbon selectivities were calcu-lated on a carbon basis. The Anderson–Schulz–Flory (ASF)chain growth probabilities in the C4–C16 hydrocarbon rangewere calculated from the slope of the curve ln(Sn/n) versus n,where n is the carbon number and Sn is the selectivity to the Cn

hydrocarbon. The FT reaction rate is expressed as cobalt-timeyield (in mol of converted CO/s divided by the total amountof cobalt [in mol] loaded into the reactor). The total amount ofcobalt in the catalysts was determined by elemental analysis.

3. Results

3.1. Impregnation and drying

The UV–vis spectra of the cobalt nitrate solutions containingRu and Re promoters with and without the addition of sucrose(not shown) exhibited two broad bands at 470 and 510 nm char-acteristic of high-spin Co2+ ions in octahedral coordination[33–35]. Cobalt octahedral coordination in the impregnatingsolutions prepared without a promoter (Re or Ru) was pre-viously observed by UV–vis spectroscopy [10]. This suggeststhat adding Ru, Re, or sucrose to the impregnating solutions ofcobalt nitrate did not significantly modify cobalt local coordi-nation.

The UV–vis spectra of the impregnated and dried samplesare shown in Fig. 2. The maxima of major bands in the spec-tra attributed to cobalt species are located at 520 and 470 nm.These bands indicate that cobalt is predominately in the octa-hedral environment [33–35] in the dried catalysts. The sharpband seen at 295–300 nm in the UV–vis spectra is related to ni-trate ions. More specifically, this band is due to the NO− ions
3
Fig. 2. UV–visible spectra of impregnated and dried cobalt catalysts.

present in cobalt nitrate complexes and to the NO−3 ions from

the commercial solution of ruthenium nitrosyl nitrate diluted innitric acid used for promotion with ruthenium. For the catalystsprepared with the addition of sucrose, a shoulder also could beseen at 360 nm. This feature cannot be attributed unambigu-ously using our UV–vis data. For the dry CoRu catalyst, theUV–vis data suggest some slight decomposition of cobalt ni-trate. Cobalt nitrate decomposition results in a broad UV–visband at 730 nm. This band is characteristic of Co3O4 [34,36].Slight decomposition of cobalt nitrate after impregnation anddrying of the unpromoted silica-supported catalysts was alsoobserved in our previous study [10].

The XANES and EXAFS results for impregnated and driedcatalysts (Figs. 3 and 4) are consistent with UV–vis data. TheXANES of both impregnated and dried CoRu and CoRu(S) cat-alysts are almost identical to that of cobalt nitrate (Fig. 3, curvesTroom, air). The EXAFS Fourier transform moduli (Fig. 4,curves Troom, air) show an intense peak at 1.6 Å, which is re-lated to a Co–O coordination shell in cobalt nitrate. No cobaltatoms can be seen in the second cobalt coordination shell.

The XPS spectra of all impregnated and dried cobalt cat-alysts (Fig. 5) exhibit an intense satellite structure [37,38].High-energy XPS Co2p3/2 peaks (782.3 eV) indicate the pres-ence of Co2+ ions in the impregnated and dried catalysts. TheICo/ISi ratio of the XPS intensities provides information aboutthe repartition of cobalt ions on silica surface after drying. Fig. 6represents the relationship between the ICo/ISi ratio and pH ofthe impregnating solutions. Higher ICo/ISi ratios are character-istic of higher cobalt dispersion in the dried cobalt catalysts;they are observed when the catalyst is impregnated by solu-tions with high pH. This effect could be interpreted in terms of


Fig. 3. In situ XANES spectra measured for CoRu catalysts prepared without (a)and with (b) sucrose addition under flow of air and hydrogen at different tem-peratures. The spectra are offset for clarity. The XANES of reference Co3O4 isdisplayed in (a).

electrostatic attraction or repulsion between Co2+ ions and thesilica surface. At pH higher than that of the PZC (pH 3 [39]), thesilica surface is negatively charged and there is a certain attrac-tion between cobalt ions and the surface. XPS shows uniformrepartition of cobalt ions on the surface at these conditions. AtpH below that of the silica PZC, the surface is charged posi-tively and Co2+ ions are repulsed from the silica surface. Thisresults in nonuniform cobalt repartition and lower cobalt dis-persion. Thus, cobalt dispersion in the impregnated and driedcobalt catalysts is significantly affected by the pH of the im-pregnating solution.

Table 1 shows that catalyst impregnation results in somedecrease in BET surface areas of the samples (from 214 to110–125 m2/g in Cab-o-sil). BET surface areas increase (from110–125 to 150–190 m2/g) after calcination. Catalyst calcina-tion results in decomposition of cobalt precursor and sucrose.The increase in the apparent surface area after calcination sug-gests [40] that cobalt nitrate and eventually sucrose introducedon impregnation were preferentially located inside the silicapores.

Fig. 4. In situ EXAFS Fourier transform moduli measured for CoRu catalystsprepared without (a) and with (b) sucrose addition under flow of air and hy-drogen at different temperatures. The moduli are offset for clarity. The EXAFSFourier transform modulus of reference Co3O4 is displayed in (a).

Fig. 5. Co2p XPS spectra of impregnated and dried catalysts.


Fig. 6. ICo/ISi ratio measured by XPS in the impregnated and dried cobalt cat-alysts versus pH of the impregnating solutions. Co1.6, Co2.6, and Co sampleswere obtained using impregnation of silica with solutions of cobalt nitrate; pHof these solutions was adjusted respectively to 1.6, 2.6, and 4.3 using nitricacid.

3.2. Decomposition of cobalt precursors in the presence ofnoble metals and sucrose

The DSC-TGA curves of the catalysts prepared from cobaltnitrate promoted with ruthenium and rhenium and with or with-out the addition of sucrose are shown in Fig. 7. All of theDSC-TGA curves were measured in air flow. The decomposi-tion profiles for the CoRu and CoRe catalysts are similar andresemble the DSC-TGA curves of unpromoted cobalt silica-supported catalysts [10]. Two endothermic weight losses (6.7–7.6%) are observed at 323–328 and 363–364 K. These lossescan be attributed to the dehydration of cobalt nitrate and sil-ica. An additional weight loss (10.4–11.7%) was detected at431–432 K, attributed to the decomposition of NO−

3 groups. Aswith monometallic cobalt catalysts [10], for all promoted cat-alysts, the total experimental weight loss is much smaller thatthe theoretical one calculated from decomposition of supportedbulk cobalt nitrate to Co3O4 (∼29%). This difference can beexplained in terms of lower extent of hydration and partial de-composition of cobalt nitrate before calcination (during catalyst

Fig. 7. DSC-TGA curves of cobalt silica-supported catalysts promoted with Re and Ru and prepared without and with addition of sucrose. Temperatureramp 1 K/min.


Fig. 7. (continued)

impregnation and drying). UV–vis spectroscopy (Fig. 2) con-firms partial decomposition of cobalt nitrate in the impregnatedand dried CoRu catalysts.

The addition of sucrose to the impregnating solution modi-fies the DSC-TGA curves significantly. The decomposition ofcobalt precursors in CoRu(S) and CoRe(S) samples turns fromendothermic into exothermic (Figs. 7b and 7d). The relevantexothermic peaks are situated at slightly higher temperaturesthan the endothermic peak of decomposition of cobalt nitratein the catalysts prepared without sucrose. This suggests that inCoRu(S) and CoRe(S) decomposition of supported cobalt com-plexes proceeds at higher temperatures than decomposition ofcobalt nitrate in both promoted and unpromoted catalysts pre-pared without sucrose.

The in situ X-ray absorption data confirm the DSC-TGA re-sults. Figs. 3 and 4 show that calcination of CoRu catalyst at423 K results in decomposition of cobalt nitrate. The XANESand EXAFS Fourier transform modulus of CoRu catalysts afterheating in air at 423 K are almost identical to those of Co3O4,whereas calcination of the CoRu(S) sample even at 473 K doesnot result in any noticeable modification of the Co adsorptionedge and EXAFS Fourier transform modulus. This supposes

higher temperatures of decomposition of cobalt complexes andcrystallization of Co3O4 phase in the catalysts prepared with su-crose addition than in the catalysts prepared without sucrose. Inother words, sucrose addition to cobalt silica-supported FT cat-alysts seems to alter the mechanism of decomposition of cobaltnitrate and to affect nucleation and crystallization of the result-ing cobalt oxide species.

3.3. Calcined catalysts

The chemical compositions of the catalysts calcined at 373 K(100 h) and 673 K (5 h) under air are presented in Table 1.Calcination of the catalysts prepared without sucrose leads toa significantly decreased ruthenium content. The Co/Ru molarratio is multiplied by 3 after calcination at 373 K and by 10 aftercalcination at 673 K. These data seem to confirm earlier resultson the sublimation of ruthenium under oxidizing atmosphere insupported catalysts [41]. The rhenium content varies less sig-nificantly after calcinations at 373 and 673 K (Table 1).

The presence of carbon atoms is detected in several cat-alysts prepared with sucrose addition. The presence of car-bon atoms in the dried and calcined catalysts indicates some


Fig. 8. UV–visible spectra of calcined cobalt silica supported catalysts.

concentration of sucrose and other organic compounds, whichcan originate from sucrose. The nC/nCo atomic ratios de-tected in dried CoRu(S) and CoRe(S) (Table 1) are relativelyclose to those expected from the chemical composition of im-pregnating solutions. Indeed, the molar ratio of nCo/nSucrose

(C12H22O11) = 10 in the impregnating solutions gives a the-oretic atomic ratio of nC/nCo = 1.2 in the dried catalysts. Cal-cination at 673 K results in a significantly decreased concentra-tion of carbon (nC/nCo < 0.2), suggesting that the sucrose andsucrose-originated organic compounds disappear during calci-nation at 673 K.

In contrast to the catalysts prepared without sucrose, insucrose-promoted samples, the concentrations of Ru and Re donot drop after calcination. The Co/Ru and Co/Re ratios remainalmost unchanged (Table 1). This suggests that sucrose helpsmaintain a high ruthenium content and prevents sublimation ofRu from the surface of silica-supported catalysts.

The structure of cobalt species in the calcined catalysts wascharacterized by UV–vis spectroscopy, XAS, XPS, and XRD.The relevant UV–vis spectra are shown in Fig. 8. All of the cat-alysts except CoRu373(S) and CoRe373(S) exhibit two broadabsorption bands at 410 nm and 710–730 nm, which are relatedto Co3O4 species [34,36]. This means that the decompositionof cobalt nitrate under air leads primarily to Co3O4.

CoRu373(S) and CoRe373(S) maintain a sharp pink color.Note that these catalysts were calcined in air at 373 K for 100 hunder exactly the same conditions as CoRu373 and CoRe373.The relevant UV–vis spectra also remain unchanged for thesesamples. This suggests that in contrast with the samples pre-pared without sucrose, cobalt oxides (Co3O4 or CoO) are notgenerated after calcination of CoRu(S) and CoRe(S) catalysts

Fig. 9. XANES spectra of calcined cobalt silica-supported catalysts.

at 373 K. Thus, local coordination of cobalt in CoRu373(S)and CoRe373(S) remains similar to that in the impregnated anddried catalysts.

The calcined catalysts were analyzed by X-ray absorptionspectroscopy at cobalt K absorption edge. The XANES spec-tra of the catalysts are presented Fig. 9. Whereas the XANESspectra of CoRu373(S) and CoRe373(S) (not shown) are al-most identical to the XANES spectra of cobalt nitrate, theXANES spectra for the CoRu673, CoRe673, CoRu673(S), andCoRe673(S) catalysts suggest the presence of Co3O4. With theCoRe373 and CoRu373 catalysts, XANES is more complex andprobably suggests the presence of several cobalt species. Fittingthe XANES of CoRe373 and CoRu373 using linear combina-tion of XANES spectra of reference compounds is indicativeof both cobalt nitrate and Co3O4. The contributions of Co3O4and cobalt nitrate to the XANES spectrum of CoRe373 are es-timated as 27% and 73%, respectively. These contributions areequal to 44 and 56% for the XANES spectrum of CoRu373.Thus, the XANES data suggest that decomposition of cobaltnitrate in both CoRu373 and CoRe373 is not complete after cal-cination at 373 K. The extent of decomposition of cobalt nitrateis higher in CoRu373 than in CoRe373, however.

The surface analysis of the calcined catalysts was carriedout by XPS (Fig. 10). For CoRu673, CoRe673, CoRu673(S),and CoRe673(S), the spectra show a Co2p3/2 binding energyat 780.1–780.8 eV and a Co2p1/2 binding energy at 795.1–795.8 eV. Very weak shake-up satellites are observed at ap-proximately 789.5 and 804.5 eV. The binding energies and lowintensity of satellites suggest the presence of Co3O4 on thecatalyst surface [38,42,43]. In CoRe373 and CoRu373, whichwere precalcined at low temperatures, the Co2p3/2 and Co2p1/2binding energies are 781.8–782.3 eV and 797.4–798.1 eV, re-


Table 2Characterization of cobalt silica-supported catalysts and their catalytic performance in FT synthesis

Catalysts ICo2p/ISi2pratio incalcinedcatalysts(XPS)

Size ofCo3O4cryst.,XRD(nm)

Fraction of metalcobalt in reducedcatalysts, XPS(%)

Propenechemisorption(µmol/g)

Cobalttime yield(10−4 s−1)

SCH4(%)

SC5+(%)

Alpha

CoRu673(S) 1.24 6 58 93 10.1 12 67 0.76CoRe673(S) 1.11 5 43 76 9.9 13 67 0.74CoRu373 1.60 9 31 60.0 8.6 15 62 0.75CoRu673 0.69 12 54 50.1 6.1 15 72 0.82CoRe373 1.70 9 24 37.6 6.8 11 71 0.80CoRe673 0.76 13 45 35 4.1 12 69 0.77

The FT reaction rate is expressed as cobalt-time yield (in moles of converted CO per second divided by the total amount of cobalt (in moles) loaded into the reactor).

Fig. 10. Co2p XPS spectra of calcined cobalt silica-supported catalysts.

spectively. An intense satellite structure is also observed at 787and 804 eV. All of these XPS features are characteristic of Co2+ions [37,38,44]. These results are consistent with XANES andEXAFS data, which show almost complete decomposition ofcobalt nitrate to Co3O4 in CoRu673, CoRe673, CoRu673(S),and CoRe673(S) and partial decomposition and the presence ofresidual cobalt nitrate species in CoRu373 and CoRe373.

Co3O4 crystalline phase is detected in all calcined catalystsby XRD. The sizes of Co3O4 particles, calculated according toScherrer’s equation, are shown in Table 2. Co3O4 crystallitesof CoRe673 and CoRu673 have an average size of 12–13 nm.The Co3O4 particles were slightly smaller (∼9 nm) in CoRe373and CoRu373. The addition of sucrose during impregnationresults in a further decrease in the size of Co3O4 crystal-

Fig. 11. Magnetization of cobalt catalysts promoted with Ru (a) and Re (b) mea-sured during in situ reduction in pure hydrogen. Temperature ramp 28.2 K/min.

lites. The Co3O4 average particle size of the CoRu673(S) andCoRe673(S) calculated from XRD patterns drops to 5–6 nm.The XRD results are consistent with ICo/ISi ratios in the cal-cined catalysts measured by XPS (Table 2). Higher ICo/ISiratios and correspondingly high cobalt dispersion are observedin the catalysts prepared using sucrose addition.

3.4. Reduction of cobalt catalysts

The results of in situ temperature-programmed magnetiza-tion measurements in pure hydrogen for promoted cobalt silica-supported catalysts are shown in Fig. 11. A magnetization curvefor unpromoted cobalt silica-supported catalyst precalcined at


673 K (Co673 [10]) is shown in Fig. 11a for comparison. Notethat only cobalt metallic particles have noticeable magnetiza-tion under these conditions. Thus, the increase in magnetizationdirectly indicates the growth of cobalt metallic phases in thecatalysts.

Fig. 11 shows that promotion with Re and Ru results in a sig-nificant decrease in the temperature of emergence of the cobaltmetal phase. This is consistent with previous reports indicatingthat promotion with noble metals results in significant decreasein the catalyst reduction temperature [13–15]. The presence ofRu seems to affect catalyst reduction to a much greater ex-tent than promotion with Re. The metallic particles emerge at451 K for CoRu373, at 473 K for CoRu673 and CoRe373, andat 523 K for CoRe673. Interestingly, sucrose addition duringcatalyst impregnation affects cobalt reducibility only slightly.

The magnetic data on cobalt reducibility in the presenceof sucrose are consistent with XANES and EXAFS findings.CoRu(S) and CoRu catalysts precalcined and then reduced inhydrogen at 673 K have almost identical XANES spectra andEXAFS Fourier transform moduli (Figs. 3 and 4); both are char-acteristic of metallic cobalt. Thus, X-ray absorption data forthese catalysts indicate a high extent of cobalt reduction. Animportant observation is that addition of sucrose during im-pregnation, although dramatically enhancing cobalt dispersion,does not diminish cobalt reducibility (Table 2). The magneticfield dependence curves (magnetization vs intensity of the mag-netic field) for CoRe673 and CoRe673(S) catalysts are shownin Fig. 12. This curve for CoRe673 exhibits a hysteresis loop,indicating the presence of cobalt metallic ferromagnetic par-ticles (>7 nm diameter), whereas the field dependence curvefor CoRe673(S) shows no hysteresis. The absence of a hys-teresis loop is characteristic of cobalt superparamagnetic par-ticles, the diameter of which is <7 nm at ambient temperatures[30,45,46]. Similar to CoRe673(S), only small cobalt super-paramagnetic particles are detected in the CoRu673(S) sample.Fitting the field dependence curves using the Langevin functionyields cobalt metal particle sizes of 4 nm in CoRe673(S) and6 nm in CoRu673(S) [31]. Note that the cobalt metal particlesizes calculated from the magnetic measurements are consistentwith XRD, taking into account some decrease in molar volumeafter reduction of Co3O4 to metallic cobalt.

The reduced catalysts were also studied by XPS (Fig. 13).For XPS measurements, the catalysts were reduced for 5 h at673 K (ramp rate, 3.3 K/min; hydrogen flow rate, 20 mL/min)and then transferred to the XPS vacuum chamber for measure-ments without exposure to air. Both cobalt metallic-phase andoxidized Co2+ species can be observed in the XPS spectra. TheCo2p3/2 peak at 778–779 eV and the Co2p1/2 peak at 793–794 eV are characteristic of cobalt metal species [47]. The pres-ence of oxidized Co2+ species can be identified by XPS peaksat 782 and 797 eV and an intense satellite structure [37,38]. Thefractions of Co2+ ions and cobalt metal species in the reducedcatalysts were evaluated using decomposition of XPS spectraas described previously [10]. The corresponding extents of re-duction are presented in Table 2. The extent of reduction isslightly higher in the catalysts promoted with Ru prepared with-

out and with sucrose addition compared with the Re-promotedcatalysts.

Propene chemisorption data, presented in Table 2, are con-sistent with the results on cobalt dispersion and reducibilityobtained by other characterization techniques. Higher num-bers of cobalt surface sites were detected in CoRu673(S) andCoRe673(S), which were prepared with the addition of su-crose. These catalysts have high cobalt dispersion along withgood cobalt reducibility. Lower concentrations of cobalt surfacesites are seen in CoRe673 and CoRe373. The decreased num-ber of cobalt surface sites is probably due principally to lowercobalt dispersion in CoRe673 and to poor cobalt reducibility inCoRe373.

3.5. Catalytic performance

The catalytic performance of cobalt silica-supported cata-lysts was evaluated in a differential catalytic reactor at 463 Kunder atmospheric pressure. To discount any transient behaviorof the fixed-bed reactor, the conversion and selectivities weremeasured after 24 h on stream. The results of catalyst eval-uation, summarized in Table 2, show that FT reaction rates(cobalt-time yields) follow the same trend as the number ofcobalt surface sites measured by propene chemisorption. TheFT rates were much higher with CoRu673(S) and CoRe673(S).CH4 selectivity was 11–15%, whereas C5+ selectivity was 61–72%. The apparent Anderson–Schulz–Flory parameter (α) cal-culated for C4–C16 hydrocarbons was 0.74–0.82.

4. Discussion

4.1. Repartition and structure of cobalt species in theimpregnated and dried catalysts

Preparation of cobalt-supported FT catalysts involves sev-eral important stages. Impregnation is one of the initial im-portant stages in catalyst preparation. Our results suggest thatimpregnation of silica with cobalt nitrate and promoters leadsto the formation of mostly cobalt mononuclear octahedrallycoordinated cobalt complexes. Fourier transform modulus ofEXAFS of impregnated and dried catalysts (Fig. 4) reveals nocobalt atoms in the second cobalt coordination sphere. UV–visspectra of impregnated and dried catalysts also confirm octahe-dral coordination of cobalt ions (Fig. 2).

The repartitioning of Co2+ ions after impregnation and dry-ing depends primarily on the pH of the impregnating solution.Contact between the silica and impregnating solution results information of an electric double layer. For silica, the PZC oc-curs at pH 2–3 [39]. When impregnation occurs at a pH abovethe PZC, the surface of silica is negatively charged. The neg-atively charged silica surface attracts the positively chargedcobalt ions. Below this PZC value, the surface of silica is posi-tively charged, which involves an electrostatic repulsion of thepositively cobalt-charged ions. Consequently, below the PZC,the silica support attracts negatively charged nitrate and hy-droxyl groups and repulses the cobalt complexes. This results


Fig. 12. Field dependences measured for reduced CoRe673 (a) and CoRe673(S) (b) cobalt catalysts.

in nonuniform Co2+ repartition on the silica surface in the im-pregnated and dried catalysts. XPS data (Fig. 6) show higherICo/ISi ratios at higher pH of the impregnating solutions. Thissuggests more uniform distribution of cobalt precursor on thecatalyst surface at pH above the PZC of silica. The addition ofsucrose does not affect cobalt repartition on the silica surface.Thus, the pH of the impregnating solution seems to be a majorparameter controlling cobalt dispersion in the impregnated anddried catalysts.

Adsorption of cobalt species on the surface of silica andother oxides has been recently reviewed by Bourikas et al. [48],who reported that an increase in the impregnation pH (up toa critical value above which cobalt phases precipitate) bringsabout an increased extent of deposition and thus the Co2+ sur-face concentration. In fact, cobalt adsorption on the silica sur-face is maximal at pH 7.3–8.0.

These results are consistent with earlier data of Minget al. [49], who predicted that Co2+ deposited on the surface asa cation at pH < 5. At pH < 2, cobalt dispersion could be lowdue to repulsion between the positively charged silica surface

and Co2+ ions. Ming et al. [49] also suggested that cobalt repar-tition on silica immediately after impregnation could affect thecatalytic performance of the final FT catalysts. However, ourresults demonstrate that repartition of cobalt in the impregnatedand dried catalysts is very volatile and evolves significantly dur-ing further catalyst preparation steps.

4.2. Cobalt species in the catalysts after decomposition ofcobalt precursor and calcination

After impregnation and drying, cobalt ions seem to be an-chored only weakly to the silica surface. Cobalt repartitionchanges dramatically after decomposition of cobalt precursors.Indeed, in the present work, co-impregnation involves a solu-tion of ruthenium nitrosyl nitrate and perrhenic acid. Addingpromoters to the impregnating solution reduces the pH to val-ues below the pH of silica PZC. Due to the repulsion betweenCo2+ ions and positively charged silica surface, cobalt reparti-tion is nonuniform in the impregnated and dried catalysts. Thus,


Fig. 13. Co2p XPS spectra of cobalt catalysts reduced in situ in hydrogen at673 K for 5 h.

silica impregnation with acidic solutions of cobalt and promot-ers is expected to yield less dispersed cobalt species.

However, XRD of calcined catalysts shows that Co3O4 crys-tallites are smaller after calcination of cobalt catalysts preparedby co-impregnation with more acid solutions containing Ru,Re, and sucrose (Table 2). This suggests that the genesis ofcobalt oxide crystallites essentially occurs at the stage of de-composition of cobalt precursor and catalyst calcination, but notat the stage of impregnation and drying.

Our results also show that cobalt dispersion in silica-supported catalysts is affected by both the temperature of de-composition of cobalt precursors and catalyst promotion. Thetemperature of cobalt decomposition affects the size of cobaltoxide crystallites. In agreement with previous results for un-promoted cobalt catalysts [10], smaller Co3O4 crystallites areobserved when the catalysts are pretreated at 373 K instead of673 K (Table 2).

Decomposition of cobalt precursor and cobalt dispersion arealso strongly affected by promotion. Decomposition of cobaltnitrate in the presence of noble metals leads to smaller crystal-lites of cobalt oxide than those obtained in unpromoted silica-supported catalysts ([10], Table 2). Similar results were ob-tained earlier by Schanke et al. [18] with Pt-promoted silica-supported catalysts. One reason for the higher cobalt dispersionin the catalysts promoted with noble metals could be related to ahigher concentration of cobalt oxide crystallization sites, whichat similar cobalt content would result in an increased number ofcobalt particles and, consequently, higher cobalt dispersion.

The addition of sucrose during co-impregnation enhancescobalt dispersion considerably. This finding is consistent withearlier patents by Mauldin et al. [24,25], who claimed thatfor titania-supported cobalt catalysts, impregnation with cobaltnitrate solutions containing monosaccharide or disaccharidecould improve cobalt dispersion without any negative effecton cobalt reducibility. DSC-TGA (Fig. 7) and X-ray absorp-tion measurements (Figs. 3 and 4) show that decomposition ofcobalt precursor proceeds at much higher temperatures in thecatalysts prepared using sucrose. A high temperature of decom-position probably suggests the presence of cobalt complexes,which differ from those in the catalysts prepared without theaddition of sucrose.

It is known that in the presence of nitric ions, sucrose is oxi-dized to saccharic acid, which can polymerize on silica surfaceinto polysaccharic acid, an excellent chelating agent. The syn-thesis of supported catalysts using metal chelate complexes hasbeen reviewed by van Dillen et al. [50]. A gel-like phase formedafter decomposition of organic precursor generally favors highmetal dispersion. Highly dispersed Co3O4 crystallites are prob-ably produced after decomposition of complexes of polysaccha-ric acid. Thus, as with other promotors, the addition of sucroseaffects the mechanism of cobalt precursor decomposition andthus the genesis of Co3O4 in silica-supported catalysts.

The presence of sucrose also reduces sublimation of ruthe-nium suboxides. Table 2 shows that calcination of silica Ru-promoted catalysts prepared without sucrose results in a signif-icant loss of ruthenium. The presence of sucrose prevents theformation of volatile ruthenium suboxide and maintains a highRu content in the calcined catalysts.

Previously [10], we found that high exothermicity of cobaltacetate decomposition results in significant concentrations ofamorphous cobalt silicate. Note that there are some importantdifferences in the decomposition of cobalt acetate and cobaltcomplexes with sucrose. TGA-DSC curves [10] show that de-composition of cobalt acetate is an autocatalytic combustionand proceeds very fast in a narrow temperature range (473–510 K). Decomposition of cobalt complexes with sucrose in-volves several steps (e.g., saccharic, polysaccharic acids, resin)and proceeds in a much wider temperature range (403–495 K;Fig. 7). Multistep decomposition of cobalt-sucrose complexesleads to much lower heat flow and thus less significant forma-tion of amorphous cobalt silicate species.

4.3. Cobalt species in the reduced cobalt catalysts and theircatalytic performance

Reduction of cobalt species in silica-supported catalysts isan important step in catalyst preparation, because catalyst re-duction generates active cobalt metal sites for FT synthesis.Reducibility of silica-supported FT catalysts is a function of thefraction of Co3O4 crystalline phase, sizes of Co3O4 crystallites,and promotion with noble metals.

In agreement with several previous reports [13–15], the re-sults of in situ magnetic measurements (Fig. 11) show mucheasier reducibility of cobalt oxides in the presence of noble met-als. Cobalt particle size is also an essential parameter affecting


cobalt reducibility. Cobalt catalysts with larger cobalt particles(CoRu673 and CoRe673) have a higher extent of cobalt re-duction (Table 2). Promotion with Ru has a greater impact oncobalt reducibility than promotion with Re. Re can be reducedto metallic state at higher temperature than Ru. It is known thatreduction of Co3O4 to metallic cobalt proceeds via intermediateformation of CoO. Thus, the promoting effect of Re on cobaltreducibility can be observed only at higher temperatures thanthe promoting effect of Ru [51–53].

The addition of sucrose during impregnation leads to ex-tremely high dispersion of supported cobalt oxide. Reductionis usually more difficult for smaller cobalt particles than largerones [29]; thus, the small cobalt particles in the samples pre-pared with sucrose might be expected to have decreased re-ducibility in hydrogen. However, as shown in Table 2, the ad-dition of sucrose does not lead to poor reducibility. Thus, it canbe suggested that the genesis of Co3O4 particles in the cata-lysts prepared with the addition of sucrose likely proceeds viaintermediate formation of chelate complexes of cobalt and a no-ble metal. Strong interaction between Co and noble metal pro-moters in such complexes and possible formation of bimetal-lic metal oxide particles could be among the reasons for therelatively high cobalt reducibility in the sucrose-promoted cat-alysts. The reduction of silica-supported cobalt catalysts pro-moted with sucrose leads to superparamagnetic cobalt particles,which are identified by the magnetic method. These superpara-magnetic cobalt particles were smaller than 7 nm.

Propene chemisorption shows that the number of cobaltsurface sites in the reduced catalysts depends on both cobaltdispersion and reducibility (see Table 2). As expected, highernumbers of cobalt metal sites are detected in ruthenium- andrhenium-promoted samples prepared with the addition of su-crose. Note that these catalysts have both highly dispersed andeasily reducible cobalt species.

There is controversy in the literature regarding the effectof catalyst preparation parameters on cobalt dispersion. Sev-eral authors have suggested that cobalt dispersion is affected

principally by the method of cobalt deposition on the catalyticsupport. Ming et al. [49] observed that smaller cobalt oxideparticles supported on silica could be obtained at higher pHof impregnating solutions. A correlation between the particlesize of cobalt oxide and the pH of the impregnating solutionof cobalt nitrate also was observed in titania-supported cata-lysts [54]. Lok et al. [55–57] obtained highly dispersed cobaltcatalysts by optimizing the deposition of cobalt precursors onthe support surface. In all of these works, the pH of the im-pregnating solutions was adjusted by adding various organicand inorganic compounds. However, the effect of the addedcompounds on decomposition of cobalt precursors and catalystcalcination was often underestimated in these reports. Dryingalso can affect cobalt repartition in the catalyst grain. It has beenshown that solvent with higher viscosity can prevent migrationof active phase during catalyst drying and enhance cobalt dis-persion [50,58]. Several reports have emphasized the role ofoxidative pretreatments in the design of metal catalysts. Joyneret al. [59–62] showed that the temperature of calcination hasa significant affect on the Pt metal particle size in zeolites. DeJong et al. [63] found that a very low heating rate during cal-cination was essential in the preparation of highly dispersedPt particles in Y zeolite. Our earlier report [10] and currentresults extend this approach to the preparation of cobalt silica-supported catalysts for FT synthesis. We find that cobalt dis-persion in silica-supported FT catalysts depends principally onthe conditions of cobalt precursor decomposition and catalystcalcination.

The catalysts with high number of cobalt surface sites dis-play higher FT reaction rates (see Table 2). Fig. 14 shows the re-lationship between the number of cobalt metal sites and cobalt-time yield for the series of unpromoted and promoted cobaltsilica-supported catalysts. A higher concentration of cobalt sur-face sites leads to higher FT reaction rates, while affecting hy-drocarbon and methane selectivities only slightly. Note that thecatalytic results were obtained in a differential reactor under rel-

Fig. 14. Cobalt time yield versus propene chemisorption on different cobalt catalysts. Note: Co373 and Co673 are monometallic cobalt silica-supported catalystsprecalcined respectively at 373 and 673 K. Additional information about the monometallic cobalt catalysts is available in Ref. [10].


atively mild conditions, when catalyst deactivation is not verysignificant.

5. Conclusion

Cobalt repartition in silica-supported impregnated and driedcatalysts is a function primarily of the pH of the impregnatingsolution; a higher pH results in more uniform cobalt repartitionon the silica surface. Although all preparation steps (i.e., im-pregnation, drying, calcination, and reduction) are significantin the design of efficient cobalt FT catalysts, cobalt dispersionis affected primarily by cobalt precursor decomposition andcatalyst calcination. Both a lower temperature of catalyst cal-cination and promotion with ruthenium, rhenium, and sucroselead to smaller cobalt particles. Promotion with Re and partic-ularly with Ru enhances cobalt reducibility. Extremely small,easily reducible cobalt particles are seen in the cobalt catalystsprepared using sucrose addition. The overall number of cobaltsurface sites in the reduced catalysts is a function of both cobaltdispersion and reducibility. The FT reaction rates are stronglyaffected by the number of cobalt surface sites. In differentialreactor and at atmospheric pressure, FT reaction rates vary as afunction of cobalt surface sites, with hydrocarbon selectivitiesaffected by cobalt dispersion only slightly.

Acknowledgments

The authors thank L. Burylo, M. Frère, O. Gardoll, andG. Cambien for the X-ray diffraction, XPS, DSC-TGA, andBET surface measurements, respectively. The help of V. Briois,S. Nikitenko, and G.V. Pankina during X-ray absorption andmagnetic measurements is particularly appreciated. ESRF andElettra are acknowledged for the use of synchrotron beamtime.P.A.C. acknowledges financial support from the Russian Foun-dation for Fundamental Research (Grant 06-03-32500-a).

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