metal–support interaction in co/sio2 and co/tio2

Applied Catalysis A: General 196 (2000) 111–123

Metal–support interaction in Co/SiO2 and Co/TiO2

Roberto Riva∗, Hans Miessner1, Roberto Vitali, Gastone Del PieroEnitecnologie, via Maritano 26, I-20097 San Donato Milanese, MI, Italy

Received 25 November 1998; received in revised form 6 October 1999; accepted 11 October 1999

Abstract

Cobalt supported on silica and titania catalysts were investigated by XPS, TPR, TPD, XRD and TEM in order to elucidatethe Co/support interactions and their effect on the dispersion and reducibility of cobalt. The reducibility of cobalt was studiedboth with TPR experiments, in which the temperature is raised at a steady rate, and with XPS after in-situ reduction treatmentsat constant temperature. Silica-supported catalysts were prepared with various Co loadings (2–27.5 wt.%) and they exhibitedsignificant structural and morphological differences. A significant Co/titania interaction was found, while no conclusive proofof any interaction was found for Co/silica. The degree of interaction between cobalt and the support affected not only theresponse of cobalt to reduction, but also its dispersion. In fact, cobalt spreads on titania during reduction and tends to sinteron silica. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:Cobalt; Silica; Titania; Support; XPS

1. Introduction

The interaction of cobalt particles with the supporthas been the subject of many studies, since supportedcobalt has important catalytic properties [1–9]. Mucheffort has been devoted to understanding the relation-ship between the dispersion of cobalt and the activityof the catalyst in Fischer–Tropsch synthesis [10,11].The evaluation of metal dispersion is a difficult taskand requires very careful interpretation of the dataobtained from different techniques (e.g. H2-TPD,XPS, TEM, XRD), which yield different pieces ofinformation on the particles of cobalt. H2-TPD givesa measure of the number of active sites available. By

∗ Corresponding author. Fax:+39-02-5203-6347.1 Present address: Gesellschaft zur Förderung der naturwissens-

chaftlich-technischen, Forschung in Berlin-Adlershof. Fax:+49-30-6392-4830.

making several strict assumptions, an estimate of theparticle size can be obtained from the H2-TPD mea-surements. H2-TPD is unable to detect any cobalt thatis chemically bonded to the support, either through theformation of a surface compound or through strongmetal–support interaction. The latter phenomenon isunderstood to entail an electron exchange between apartially reduced support and the metal and leads tothe suppression of hydrogen chemisorption [9,12–15].On the contrary XPS is sensitive to all surface cobaltwhile distinguishing different oxidation states. TEMmicrographs make it possible to measure the size ofcobalt particles directly by giving an image of them.TEM is quite different from H2-TPD in this respect.On the other hand, it must be stressed that whileTEM focuses on a single spot, H2-TPD yields anaverage value that applies to the sample as a whole.The average size of the crystallites can be obtainedfrom XRD, but without any information on the

0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.PII: S0926-860X(99)00460-3

112 R. Riva et al. / Applied Catalysis A: General 196 (2000) 111–123

degree of aggregation and on the presence of amor-phous phases.

Possible surface compounds are resistant to reduc-tion and affect negatively the activity of the catalyst[2,5,16–20]. According to the literature, the interac-tion of cobalt with titania is stronger than that withsilica. Moreover, strong metal–support interaction hasbeen found to affect the metal dispersion [19].

The present study addresses the mechanism of in-teraction between cobalt and the support, either silicaor titania. It tries to clarify at what preparation stepthe reaction between cobalt and the support occurs.

The way in which the simple reduction of the sup-ported phase affects the quantitative results of X-rayphotoelectron spectroscopy is discussed in AppendixA. It is shown that the dispersion of cobalt, as evalu-ated with XPS, can be compared after different treat-ments only if the phase of cobalt is the same, because achange of phase brings about a variation of the shield-ing power of the cobalt particles.

2. Experimental

2.1. Preparation of the samples

Johnson & Matthey Co3O4 was used as a referencecompound for XPS spectra. The quality of the samplewas checked by X-ray diffraction (XRD) before XPSanalysis.

Silica-supported samples with various degrees ofcobalt loading (from 2 to 27 wt.%) were prepared fol-lowing the incipient wetness impregnation method.Amorphous Kieselgel 60 (Merk) silica was impreg-nated with an aqueous solution of Co(NO3)2·6H2O,aged overnight and dried at 100◦C in air for 2 h andfinally calcined at 400◦C in air for 4 h. The surfacearea of the Merck silica was found to be 430 m2/g,its particle size being in the range 15–45mm with anaverage pore radius of 35 Å.

Titania-supported samples containing 12 wt.% Cowere prepared with the same procedure, using De-gussa P25 titanium dioxide. Before impregnation, thesupport was calcined in air at 650◦C for 2 h for achiev-ing a high rutile to anatase ratio (75–85% Rutile).After this treatment the surface area of the support wasfound to be ca. 40 m2/g with an average particle sizearound 0.1mm.

2.2. Characterization techniques

2.2.1. X-ray diffraction (XRD)The XRD data were collected at ambient conditions

using a Philips diffractometer with monochromatic CuKa radiation (λ = 1.5418 Å). Qualitative phase analy-sis was carried out using the Siemens Diffrac AT pack-age run on a IBM PC330 P-75. For titania-supportedsamples, the quantitative phase analysis was carriedout by using the Rietveld profile fitting method [21]with the procedure proposed by Hill and Howard [22].Structural data were taken from Wyckoff [23]. Forsilica-supported samples, the conventional method re-ported by Klug and Alexander was used [24]. Crystalsize was calculated from line broadening applying theScherrer equation [24].

2.2.2. Transmission electron microscopy (TEM)TEM experiments were done on a Philips 420T mi-

croscope (120 kV, maximum resolution 5 Å) equippedwith an EDAX PV9900 EDS apparatus. The cata-lyst powder was embedded in epoxy resin and thenmicrotomed with a diamond knife to obtain sections400–700 Å thick. Images were obtained at 100 kV.

2.2.3. Temperature-programmed reduction (TPR)The TPR experiments were performed in a

U-shaped tubular quartz reactor. After loading thesample, the reactor was flushed with He at 150◦C for1 h, then cooled down to 50◦C in flowing He. Thegas flow (2% H2-He) was adjusted for each samplein such a way as to maintain a roughly constant ratiobetween the amount of cobalt contained in the sam-ple and the H2 available. The temperature was thenraised at the constant rate of 10◦C/min from 50◦Cto the desired temperature (700–900◦C). The contentof H2 and H2O in the outflowing gas was monitoredwith a VG-Fisons quadrupole mass spectrometer.

2.3. Hydrogen chemisorption

The experiments were carried out by means ofa flow technique for the measurement of hydrogenchemisorption through H2-temperature-programmeddesorption (H2-TPD), as proposed by Jones andBartholomew [25]. The samples examined were pre-viously reduced (T= 400◦C in flowing H2 for 16 h)

R. Riva et al. / Applied Catalysis A: General 196 (2000) 111–123 113

and passivated in 2% O2–N2 atmosphere at roomtemperature for 2 h. The H2-TPD procedure was asfollows: after being loaded into the reactor, the passi-vated sample was re-reduced by heating it up to 400◦Cin 2% H2-He at the rate of 10◦C/min. Then it wascooled down to 200◦C and the gas flow was switchedto pure H2 in order to allow the activated adsorptionof H2. The sample was then cooled to−80◦C (dryice/acetone) and the gas flow was switched from pureH2 to pure He. After calibrating the analyzer withseveral 50ml pulses of H2, the temperature was raisedto 400◦C at the rate of 40◦C/min in flowing He. Thecomposition of the outflowing gas was measured witha VG-Fisons quadrupole mass spectrometer.

The amount of desorbed H2 was used to evaluate thedispersion (%D) of cobalt and the mean particle size(d) with the formulas given by Reuel and Bartholomew[3]:

%D = 1.179× N/(wt.% × R) (1)

d (nm) = 6.59× SD(Co)/%D (2)

where:N is the number of mmoles of H2 desorbed pergram of catalyst; wt.% is the Co weight percentage;R is the fraction of cobalt reduced, according to TPR;SD (Co) is the site density of Co, assumed to be equalto 14.6 Co atoms/nm2.

2.3.1. X-ray photoelectron spectroscopy (XPS)The XPS spectra were collected with a VG Escalab

MKII spectrometer. A non-monochromatic Al X-raysource was used. The binding energy values given inthe literature for the following peaks were used as areference: Si 2p 103.3 eV for silica-supported sam-ples, Ti 2p 458.7 eV for titania-supported samples, O1s 530 eV for unsupported Co3O4 [26,27]. A reactionchamber was connected to the vacuum system of thespectrometer allowing the samples to be transferredinto the measurement chamber without exposure to airafter reducing and oxidizing treatments. The reducingtreatments were carried out in 3% H2–Ar at varioustemperatures and for various lengths of time. The ox-idizing treatments were done in synthetic air at 400◦Cfor 5 h at least. The Co 2p and the Si 2s or Ti 2p peakswere used for the quantitative analysis, by assumingthe composition of the sample to be uniform through-out the volume probed by XPS. Under this assump-

tion the intensityIi of the XPS peak of the elementiis [27–29]:

Ii = Ip×T (Ei) × σ(Ei) × niλ(Ei) (3)

where: Ip is the X-ray flux on the sample [time−1],Ei represents the kinetic energy of the photoemittedelectrons,T(Ei) is the transmission function of thespectrometer,σ (Ei) cross-section of photoionizationof the electron orbitali [area],ni represents the atomicdensity of the elementi [volume−1], λ(Ei) is the meanfree path of the photoelectrons of kinetic energy equalto Ei [length].

Following Briggs and Seah (26):

λ(Ei) = c(Ei)1/2 (4)

while T(Ei) was given by the manufacturer of the spec-trometer as :

T (Ei) = c′(Ei)−1/2 (5)

Using [3–5], the atomic ratio betweeni andk elementsis given by:

ni

nk

=[

(Ii/σ (Ei))

(Ik/σ (Ek))

](6)

The dispersion of cobalt over the two supports wasstudied by analysis of Co/Si and Co/Ti atomic ratios,respectively.

3. Results and discussion

3.1. Unsupported cobalt

The response of unsupported Co3O4 to reducingtreatments has been studied with XPS analysis. Fig.1 shows that Co3O4 is stable up to 200◦C and is re-duced completely at 300◦C. The reduction processoccurs in two steps: first Co3O4 is reduced to CoO(third curve from the bottom), then CoO is reduced tometallic cobalt (curve on the top). Table 1 gives thebinding energy of the Co 2p3/2 peak and the differ-ence in binding energy between the Co 2p1/2 and theCo 2p3/2 peaks (1E). Metallic cobalt is easily dis-tinguished from oxidized cobalt because of the largedifference in binding energy. On the contrary, the dif-ference in binding energy between Co3O4 and CoO isvery low; however, the concomitant difference of1E


Fig. 1. XPS Co 2p peak of unsupported cobalt after varioustreatments.

(15 eV for Co3O4, 15.7 for CoO) and lineshape makeit possible to distinguish between the two phases. Theassignments made in Table 1 agree very well with lit-erature data [1,30]. Reportedly, both the lineshape andthe1E value of CoO apply to Co2+ in general, evenwhen cobalt forms silicate or titanate through reactionwith the support.

3.2. Silica-supported cobalt

Only cubic Co3O4 is detected by XRD insilica-supported calcined samples. The amount of thisphase increases with an increase in cobalt loading, thequantity of all samples being close to that calculatedfor pure crystalline Co3O4. The oxidation of cobalt toCo3O4 in calcined samples is confirmed also by theXPS results (Table 2): both the binding energy andthe 1E values agree perfectly with those reported in

Table 1Binding energy values of the spectra plotted in Fig. 1a

Binding 1E (Co 2p1/2 −energy (eV) Co 2p3/2) (eV)

Co3O4 780.1 15CoO 780.5 15.7Metallic Cob 777.8 15

a Reference energy value O 1s 530 eV.b The oxygen peak is barely visible in this case: the Co/O

atomic ratio is equal to 22.

Table 1. The size of the Co3O4 crystallites, evaluatedby XRD, tends to increase with increasing cobaltloading (from 120 to about 160 Å), even though thevalues are rather scattered.

The same measurements have been done on reducedand passivated samples. At high temperature (900◦C)cobalt crystallizes as cubic metal, while at lower tem-perature (400◦C) a fraction of Co crystallizes also inthe hexagonal form and some residual CoO is present,probably due to the passivation process. Crystal sizetends to increase with cobalt loading, as found for cal-cined samples, with a strong dependence on the re-duction temperature (Fig. 3).

TEM analysis shows that the cobalt phases donot distribute homogeneously in the silica supportbut form spherical aggregates both inside and at thesurface of the silica particles (Fig. 2a,b). These aggre-gates are made up of crystallites (Co3O4 in calcinedsamples and a mixture of cobalt metal and cobaltoxides in the reduced and passivated samples) withdimensions comparable to those evaluated by XRD(Fig. 2c shows a close-up of the interface between anaggregate of cobalt crystallites and the support). Atlow loading, small (500–800 Å) spherical aggregatesof cobalt phase are observed, most of which are insidethe silica particles. Increasing the cobalt loading, theaggregates become larger (0.3–0.5m) and statisticallymore present on the surface of the silica particles.Above 17–18% Co, the aggregates reach micrometerdimensions and are preferentially located near thesilica surface (Fig. 2b).

The dimensions of the cobalt particles, derived fromH2-TPD as described in Section 2, are larger than thosederived from XRD data (Fig. 3). The cobalt particles,as obtained from the amount of H2 desorbed duringTPD, are in the range of 20–50 nm without a signifi-cant dependence on the loading. The 2 wt.% Co sam-ple has been omitted because the amount of hydrogendesorbed was too low for a reliable measurement tobe made.

Reducing treatments of the 18 wt.% Co–SiO2 sam-ple, studied with XPS, evidence that the surface cobaltoxide is completely reduced at 300◦C, just like un-supported cobalt (Fig. 4), but shorter treatments weresufficient to achieve complete reduction, compared tothe latter. Treatments at higher temperatures do not af-fect the Co 2p peak any more. Reduction experimentson the sample containing 9.7 wt.% cobalt confirm that


Table 2Calcined silica supported samples: XPS results

Co (wt.%) Co 2p3/2 binding 1E Co2p1/2 − Atomic Co/Si: as Atomic Co/Si:energy (eV) Co2p3/2 (eV) received samples ground samplesa

2 779.9 15.1 0.02 Not detectable5.1 779.9 15.1 0.15 0.0119.7 779.9 15.1 0.20 0.014

18.4 779.8 15 0.72 0.04722.8 779.6 15 0.70 0.05427.5 779.7 15.1 0.59 –

a The samples were ground manually with an agate mortar.

Fig. 2. (a) TEM image of a calcined silica-supported sample (13.3 wt.% Co–SiO2). (b) SEM image of a calcined silica-supported sample(17.4 wt.% Co–SiO2). (c) TEM image of a calcined silica-supported sample (13.3 wt.% Co–SiO2) at a higher magnification.


Fig. 2. (Continued).


Fig. 3. Crystal size (XRD) and particle size (H2-TPD) ofsilica-supported cobalt vs. cobalt loading.

cobalt is completely reduced at 300◦C in 2 h. Smalldifferences between the Co 2p lineshape of reducedCo/SiO2 (Fig. 4) and that of unsupported cobalt (Fig.1), such as a slightly more intense high binding energytail, may stem from the presence of a minor fraction ofunreduced cobalt or from slight differential chargingeffects.

Fig. 4. XPS Co 2p peak of silica-supported cobalt (18.4 wt.%)after various treatments.

Fig. 5. TPR-900◦C of a silica-supported sample containing5.1 wt.% cobalt.

An example of the TPR curves measured onsilica-supported samples is given in Fig. 5. The TPRspectra show two major peaks at 340◦C and 430◦C,and a broad peak at higher temperatures. The twomajor peaks are similar to those obtained with pureCo3O4. Only the high temperature peak is new. Theratio between the H2 consumed at 430◦C and thatconsumed at 340◦C is 3 : 1. A similar behaviour wasobserved by other authors [8,17,31,32]. It is generallyagreed that this picture represents the reduction ofCo3O4 particles to metallic cobalt through the CoOstep, as already pointed out in a previous paragraphfor unsupported Co3O4.

As regards the dependence of the reduction be-haviour of cobalt on its content in the catalyst, thepeaks of the first reduction step show a small trend to-wards higher reduction temperatures with lower cobaltloading, not so evident for the second step (Fig. 6).Moreover, the samples differ significantly from oneanother with respect to the third reduction step (athigher temperature). This reduction step occurs in arather broad interval between 500 and 700◦C, depend-ing on the degree of cobalt loading. Only the sam-ples containing 2, 5 and 10 wt.% cobalt are reduced


Fig. 6. Temperatures of the two peaks of H2 consumption of TPRprofiles of the silica-supported samples vs. cobalt loading.

completely in experiments with 700◦C as the maxi-mum temperature. The others require treatments up to900◦C. The fraction of cobalt reducible only at a tem-perature higher than 500◦C is around 30% for all thesamples except for 2% Co–SiO2, for which the figureincreases to 70%. The assignment of this high temper-ature reduction peak (500–700◦C) is not straightfor-ward and two hypotheses can be made: (1) the peakmay be due to the presence of cobalt silicate, which isless reducible than Co3O4; (2) the peak may be due tothe reduction of the fraction of cobalt that is containedin the inner cavities of the silica support (Fig. 2a),whose reducibility is limited by the diffusion throughthe pores of the H2O formed during the reduction.

The XPS data (Fig. 4) indicate the absence of asignificant amount of Co2+ on the surface. This wouldstrengthen hypothesis (2) leading to the reasonableconclusion that the cobalt located in the cavities of thesupport is less reducible because of the limitation ofH2O transport through the pores, as already suggestedby Castner et al. [31]. However, the presence of cobaltsilicate at the interface between the cobalt particles andthe support cannot be ruled out and may contribute tothe high temperature reduction peak.

As regards the cobalt distribution in the support,XPS data indicate that the abundance of Co on theouter shell of the silica particles is higher than inthe core. This is proved by the decrease of the Co/Siatomic ratio observed after grinding the calcined sam-ples (Table 2). Grinding has already been used by otherauthors for probing the bulk composition by XPS anal-ysis [8,33]. The presence of an outer Co-rich shell thatis more easily reducible than the fraction of cobaltcontained in the bulk is consistent with TEM obser-vations.

In order to get more information about the coverageof SiO2 by cobalt, the samples were exposed to air af-ter 700◦C-TPR, reduced again for 2 h at 400◦C in thereaction chamber connected to the XPS spectrometerand then analyzed. Cobalt proved to be completely re-duced, irrespective of the degree of loading. The sam-ples were then reoxidized and their spectra were col-lected again. The measured Co/Si ratios, listed in Table3, need some comments. Let us consider the samplecontaining 5.1 wt.% cobalt: the Co/Si ratio decreasesafter reduction and reaches practically the initial valueafter reoxidation. The coverage of SiO2 by cobalt turnsout not to have changed in this case. The lower Co/Siratio measured on the reduced sample can be attributedto the fact that the Co3O4 particles shield the substratemore strongly than the corresponding metallic parti-cles, which were obtained after reduction. This pointis discussed with a simple model in the Appendix A.If reoxidation restores the original value, one can rea-sonably conclude that the dimensions and the disper-sion of the cobalt particles were not affected by reduc-tion and reoxidation. If the Co/Si ratio obtained afterreoxidation is lower than the starting value, as foundfor the samples containing 18.4 and 22.8 wt.% cobalt(Table 3), sintering probably occurred.

The tendency of the supported particles towardssintering is further evidence of the lack of interac-

Table 3Silica-supported samples: XPS Co/Si atomic ratios after differenttreatments

Co Calcined After 700◦C-TPR+ Reoxidized(wt.%) reduction at 400◦C

5.1 0.15 0.10 0.149.7 0.20 0.13 0.19

18.4 0.72 0.42 0.5622.8 0.70 0.32 0.52


Table 4Titania supported samples (12 wt.% Co–TiO2): XRD results

Fraction of Co Fraction of Co Fraction of Crystalcrystallized as Co3O4 crystallized as cubic Co amorphous Co size (Å)

12% Co/TiO2 calc. (85% Rutile) 100% – 0% 300 (Co3O4)Reduced and passivated – 17% 83% 220 (Co cubic)

12% Co/TiO2 calc. (76% Rutile) 100% – 0% 400 (Co3O4)Reduced and passivated – 17% 83% 190 (Co cubic)

tion between cobalt and silica, since sintering causesthe area of the interface between the two phases todecrease. Table 3 also shows that the Co/Si ratioincreases steadily with the content of cobalt up to18 wt.% and then levels off at a constant value. Thisbehaviour can be explained with the onset of particlegrowth at high levels of cobalt content.

3.3. Titania-supported cobalt

The XRD spectra of several titania-supported sam-ples (with ca 12% Co and Rutile/Anatase ratio rangingfrom 76/24 to 85/15) indicate that all the cobalt con-tained in the calcined samples is in the form of crys-talline Co3O4. After reduction and passivation mostof the cobalt is amorphous and only a small fractioncrystallizes as cubic Co (Table 4), whereas both therutile to anatase ratio and the morphology of the sup-port do not change. Therefore, the reduction treatmentaffects the phase composition of cobalt quite strongly,turning the oxidized crystalline phase into a mainlyamorphous phase after reduction.

Based on H2-TPD experiments, the particle size isestimated to be around 600 Å, larger than the crystalsize of cubic Co determined via XRD (about 200 Å).This can be explained by moderate aggregation of thecrystallites and/or cluster formation by the amorphousfraction.

The response of cobalt to reducing treatments hasbeen studied with XPS. The results are shown in Fig.7. Co3O4 is readily reduced to Co2+ with a 2 h treat-ment at 300◦C, but the complete reduction of Co2+ tometallic cobalt is not accomplished even after 66 h at300◦C. In fact, a high binding energy shoulder indi-cates that a fraction of the cobalt is not reduced and isprobably in the Co2+ oxidation state. This behaviouris markedly different from that of unsupported Co3O4

and is probably due to the partial formation of cobalttitanate, which is less reducible than Co3O4, accord-ing to the literature. Treatments at higher temperaturesimprove the degree of reduction. This behaviour isconfirmed by tests on samples that have been preparedin different batches and it stands as XPS evidence ofthe well known metal–support interaction.

The comparison between the Co/Ti atomic ratiosobtained before reduction and after reduction and re-oxidation provides further evidence of the formation ofsurface compounds between cobalt and titania. In fact,the Co/Ti atomic ratio increases appreciably after re-duction and reoxidation. This proves that cobalt tendsto spread on the support, probably to form a compoundwith it. Table 5 gives the atomic ratio and the bindingenergy values obtained after two consecutive reducingand oxidizing treatments. The increase in the Co/Tiratio is very high after the first reduction–reoxidationstep, since it increases from 0.53 to 0.94, and does

Fig. 7. XPS Co 2p peak of titania-supported cobalt after varioustreatments.


Table 5Response of 12 wt.% Co–TiO2 to various treatments

Treatment Atomic Co/Ti Co 2p3/2 binding energy (eV) 1E (Co 2p1/2 − Co2p3/2) (eV)

Calcined 0.53 779.7 153% H2 400◦C 15 h 0.67 777.1 15Reoxidized 0.94 779.7 153% H2 400◦C 15 h 0.78 776.9 15Reoxidized 0.92 779.8 14.9

not vary appreciably after the second redox treatment(final Co/Ti ratio 0.92), which means that the secondredox step does not affect the coverage of TiO2 bycobalt any more. The Co/Ti atomic ratio measured af-ter reduction increases from 0.67 to 0.78. Once morethe coverage of TiO2 by cobalt could be comparedwith the original value of the calcined sample only af-ter the sample had been reoxidized (see the appendix).It must be remarked that the reduction step is neces-sary in order to obtain an increase in the coverage ofTiO2 by cobalt. In fact, treating the calcined samples inair at 400◦C for 10 h does not affect the Co/Ti atomicratio. This behaviour is consistent with the model pro-posed by Horseley, which depicts the metal–supportinteraction as an electron exchange between a partiallyreduced support and the metal [9].

The TPR spectra of titania-supported samples arequite different from those of silica-supported samples:only two peaks are detected and their maxima occurat higher temperatures, 380–400◦C and 500–600◦C,respectively (Fig. 8), the latter being very broad. Theshape of the peak at 500–600◦C is very different fromthat of the second reduction peak (430◦C approx-imately) of both unsupported and silica supportedcobalt. This difference suggests that the interactionof cobalt with titania takes place during that particu-lar step in the reduction. The above TPR evidence isexplained by the presence of cobalt titanate, which ismore resistant to reduction than Co3O4.

To summarize, there is experimental evidence thata reaction occurs between the cobalt particles and thesupport during reduction of cobalt/titania catalysts. Inparticular:• XRD data indicate that cobalt is prevailingly amor-

phous in the reduced and passivated samples, whilecrystallizing completely as Co3O4 in the calcinedsamples.

• XPS reduction tests show that a fraction of cobaltis not reducible at 300◦C in 3% H2. This does not

Fig. 8. TPR-900◦C of a titania-supported sample.

happen in both unsupported and silica-supportedCo3O4. Moreover, the coverage of TiO2 by cobalt(Co/Ti atomic ratio) increases appreciably after re-duction and reoxidation, compared to the startingcalcined samples.

• The TPR peaks fall at higher temperatures for ti-tania supported samples than for silica supportedsamples.

4. Conclusions

This study has addressed the interaction of cobaltwith two different kinds of support: silica and titania.The interaction is much stronger in the case of tita-nia, as indicated by the formation of a surface com-


pound between cobalt and titania that is more resis-tant to reduction than Co3O4. On the contrary the be-haviour of silica supported samples is very similar tothat of unsupported Co3O4 under reducing treatments.The different reactivity of cobalt with silica and tita-nia explains why reducing and reoxidizing treatmentshave opposite effects on the dispersion of cobalt de-pending on whether it is supported on SiO2 or TiO2.The low reactivity of cobalt with silica favours sinter-ing effects, after reduction and reoxidation treatments.Conversely, due to the high reactivity of cobalt with ti-tania, the coverage of TiO2 by cobalt tends to increaseafter the same treatments.

Such changes in the morphology of the supportedcatalysts may affect their activity and selectivity. Infact the activity and selectivity of Fischer–Tropsch cat-alysts has been shown to depend strongly on intrapel-let diffusion phenomena of the reactants and reactionproducts [37]. As a result, changes in the density of ac-tive sites, brought about by sintering or by the spread-ing of the metallic particles on the support, can affectthe performance of the catalysts. The knowledge ofthe response of supported cobalt to reducing and ox-idizing treatments may also help devise regenerationprocedures that affect the supported phase the least.

Acknowledgements

The authors wish to thank Mr. Otello Farias, Ms.Monica Catrullo, Dr. Andrea Gusso and Dr. RobertoZennaro for their helpful cooperation. This work wassupported by ENI spa.

Appendix A. Change of the XPS atomic ratio as aconsequence of the reduction of the supportedphase

A simple calculation shows that the atomic ratioof cobalt to support is bound to vary after reduction,even if the cobalt particles do not change their shapein the process, but just shrink as a consequence of thechange of phase from Co3O4 to metallic cobalt. Twophenomena having opposite effects must be taken intoaccount: the decrease in the volume of the particlesand the decrease in the mean free path of the photo-electrons.

The case of Co3O4 islands on a support of infinitethickness will be considered. Under this assumptionthe ratio between the intensity of the Co 2p peak andthat of the Si 2s (Ti 2p) peak of the support is givenby [27,28,34]:

ICo

ISi= f TCoσConCoλCo[1 − exp(−d/λCo)]

TSiσSinSiλss[(1 − f ) + f exp(−d/λSCo)](A.1)

where:ICo is the computed area of the XPS peak of Co;ISi represents the computed area of the XPS peak ofSi; f is the fractional coverage of the support surface;d is the thickness of cobalt phase islands;TCo, TSiare instrumental factors (transmission function);σCo,σSi are the cross-sections of photoionization of Co 2pand Si 2s orbitals;nCo represents the density of Coatoms in the cobalt phase [atoms/volume];λCo is themean free path of Co 2p photoelectrons in the cobaltphase;λSSis the mean free path of Si 2s photoelectronsin the support;λSCo is the mean free path of Si 2sphotoelectrons in the cobalt phase.

Once the phase of cobalt is specified, eitherCo3O4 or metallic cobalt, all the coefficients of(A1) are automatically determined except forf andd, related to the dimension of the islands. (A1)becomes:(

ICo

ISi

)ox

= g(dox, fox) (A.2)

for Co3O4 and:(ICo

ISi

)met

= h(dmet, fmet) (A.3)

for metallic cobalt.Moreover, by assuming that no variation of particle

shape occurs during shrinkage, the following simpleequations can be considered:

dmet =(

nox

nmet

)1/3

dox (A.4)

fmet =(

nox

nmet

)2/3

fox (A.5)

where nox is the density of cobalt atoms in Co3O4[number of Co atoms/volume];nmet is the densityof cobalt atoms in metallic cobalt [number of Co


Table 6Comparison between experimental (Table 3) and computed atomic ratios

Experimental Co/Si ratio Computed Co/Si ratio dox (Å) fox

Reduced sample Oxidized sample Reduced sample Oxidized sample

5.1 wt.% Co 0.101 0.143 0.108 0.142 >100 0.0829.7 wt.% Co 0.132 0.191 0.139 0.187 >100 0.105

atoms/volume]. Combining (A3), (A4) and (A5) weobtain:(

ICo

ISi

)met

= h

[(nox

nmet

)1/3

dox,

(nox

nmet

)2/3

fox

]

(A.6)

It must be remarked that both (A2) and (A6) dependondox andfox, that is on the dimensions of the Co3O4islands.

The expressions (A2) and (A6) can be easily appliedand proved that theICo/ISi ratio decreases after catalystreduction. This is verified for a reasonable range ofdox and fox values, both for silica and titania. Fig. 9

Fig. 9. Computed Co/Si ratios of Co3O4 particles on pure silica andof the corresponding metallic particles, obtained after reduction,vs. particle thickness (dox). Two values of fractional coverage (fox)have been considered.

gives a computed example of (A2) and (A6), where thefollowing parameters were used:Ti = 1/(Ei)1/2 whereEi is the kinetic energy of the photoelectrons;σCo,σSi given by Scofield [35];λi values given by Seahand Dench [36].

This result provides an explanation of the be-haviour of silica supported samples containing 5.1and 9.7 wt.% cobalt (Table 4). The Co/Si ratio de-creases after reduction, but rises to the initial valueafter reoxidation. The decrease observed on the re-duced samples is in qualitative agreement with thepredictions of the expressions (A2) and (A6).

Bearing in mind the discrepancies between the strictassumptions that have led to (A2), (A6) and the mor-phology of the real samples, a quantitative comparisoncan be made between the predictions of the model andthe experimental results. The atomic ratio can be com-puted from the XPS intensity ratio using the expres-sion (6), given in Section 2. The values ofdox andfoxleading to a good fit of the experimental data can befound with a trial and error procedure. Table 6 gives thevalues ofdox andfox which reproduce the experimen-tal results of Table 3. The calculation has been donefor the two samples whose dispersion was unaffectedby reduction and reoxidation. The computed thicknessof the Co3O4 islands (>100 Å) roughly agrees withthe XRD and H2-TPD results, but it must be stressedthat the model gives only a coarse description of themorphology of the samples. Therefore, the calculatedfractional coverage (fox) must be regarded as a roughestimate.

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