SiO2 catalysts for Fischer–Tropsch synthesis; effect of Co loading and support modification by TiO2

Catalysis Today 197 (2012) 18–23

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Co/SiO2 catalysts for Fischer–Tropsch synthesis; effect of Co loading and support modification by TiO2 Anna Maria Venezia a,∗ , Valeria La Parola a , Leonarda F. Liotta a , Giuseppe Pantaleo a , Matteo Lualdi b , Magali Boutonnet b , Sven Järås b a b

Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR, Via Ugo La Malfa 153, 90146 Palermo, Italy Department of Chemical Technology, KTH, 10044 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 6 April 2012 Received in revised form 28 May 2012 Accepted 30 May 2012 Available online 27 June 2012 Keywords: Cobalt catalyst Fischer–Tropsch TiO2 grafting SiO2 support

a b s t r a c t The influence of cobalt loading and titania addition to the silica support on Fischer–Tropsch synthesis activity is investigated over two series of catalysts with Co loading of 6 wt% and 12 wt%. The pure silica support is prepared by sol–gel procedure in acid conditions. The modification by TiO2 is performed by grafting with titanium isopropoxide. The catalysts are prepared by wet-impregnation over amorphous SiO2 and over SiO2 modified by TiO2 (5 wt%). The samples, characterized by N2 -adsorption–desorption analyses, X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), are tested in the low-temperature Fischer–Tropsch synthesis using the conditions of 483 K, 20 bar and H2 /CO = 2.1. The improved conversion rate and the increased SC5+ of the titania containing catalysts are discussed in terms of the stronger interaction between cobalt and titania affecting the cobalt oxide reducibility. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Fischer–Tropsch synthesis (FTS) is a polimerization process assisted by catalysts that, starting from the synthesis gas (a mixture of CO and H2 ), allows to produce hydrocarbons of different chain length and functionalities [1]. The increased technological interest in such process is related to the possibility of making valuable liquid fuels from the CO and H2 obtained from partial oxidation of the relatively cheap natural gas or from biomass gasification [2]. Moreover, the FT synthesis produces cleaner products, being lower in sulfur and in heavy metals, as compared to those derived from crude oil. The kind of products are related to the catalytic active species. Iron-based catalysts, generally operating at high temperatures, have high selectivity to oxygenates and branched hydrocarbons [1]. Cobalt based catalysts have high activity at low temperature (below 523 K) and high selectivity to the C5+ linear hydrocarbons. The most important features of a cobalt catalyst, relevant to the FTS reaction, are represented by its reducibility and its surface dispersion determining the amount of the active Co metal sites. To this respect the cobalt loading may be crucial for the catalytic performance in terms of activity and selectivity [3]. Another important requirement is the chemical and structural stability under the reaction conditions. As typical of heterogeneous

∗ Corresponding author. E-mail address: [email protected] (A.M. Venezia). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.05.042

catalysis, all of the above mentioned catalyst properties are influenced by the catalyst support. Therefore the design of a good cobalt FTS catalyst must take into account the textural, structural and chemical property of the carrier [3,4]. Among the oxides most commonly used as catalyst supports, SiO2 is considered as inert oxide, whereas TiO2 and Al2 O3 are considered as active oxides, being more interacting with the supported metals [5–7]. The use of a strongly interacting oxide like alumina may increase the dispersion but decrease the reducibility. On the contrary, the use of the inert oxide like silica generally leads to a good reducibility but to an easy sintering of the Co metal. Therefore a compromise between the cobalt dispersion and the percentage of metallic cobalt should be considered [8]. The influence of cobalt dispersion on the cobalt site-time yield (mole CO converted/mole Co/sec) is still a debatable matter [9]. For Co particle size above 10 nm, Iglesia concluded in favor of the structure-insensitivity for the FTS reaction [1,10]. On the contrary, for cobalt supported over carbon nanofibers, Bezemer et al. found a positive correlation between the particle size and the CO conversion in the particle size range 3–8 nm, with the smaller particles giving both lower site activity and lower C5+ selectivities [10,11]. As a way to tune the metal-support interaction, in the present study, the two types of oxides, TiO2 and SiO2 , are combined together to form the support for FTS cobalt catalysts. TiO2 is added by grafting the SiO2 surface with Ti(IV) precursor. Thereafter, cobalt species are supported by wet-impregnation. In order to distinguish a purely Co structural effect, strictly related to the particle size, from a support-metal chemical interaction effect two

A.M. Venezia et al. / Catalysis Today 197 (2012) 18–23

different Co loadings are considered. The correlation between catalytic performance and catalyst structural and morphological properties as obtained with a variety of techniques such as XRD, TPR and XPS are discussed. 2. Experimental 2.1. Catalyst preparation Amorphous silica is prepared according to the method described by Pecchi et al. [12] Briefly, 38 ml of TEOS is dissolved in 20 ml of ethanol. At the temperature of 318 K, 24 ml of acetic acid solution at pH 5 are added and the resulting solution is left under stirring until formation of a gel. The temperature is increased to 353 K and the gel is kept at this temperature for 1 h. After this time the solution is slowly evaporated and the resulting powder calcined at 723 K for 4 h. For the preparation of the titania modified silica, the powder SiO2 is first activated by thermal treatment at 373 K for 1 h and then suspended in 35 ml of dry toluene. To this suspension a solution of titanium isopropoxide and acetylacetonate (mlTi-isopro. /mlacetiacetonate = 2.9) dissolved in a minimum amount of toluene is added drop by drop under stirring. The resulting solution is refluxed 24 h at 383 K under vigorous stirring [13]. The powder is then filtered, washed thoroughly with ethanol, dried and calcined at 723 K for 4 h. The final support, as determined by XRF spectroscopy, contained a TiO2 loading of 5 wt% in accord with the Ti(IV) precursor amount. The catalysts with 6 wt% and 12 wt% of cobalt are prepared by wet-impregnating the pure SiO2 and the Ti(IV) doped SiO2 supports with aqueous solution of Co(NO3 )2 6H2 O [14]. The samples are left drying overnight and then calcined at 673 K for 4 h. The nominal 6 wt% and 12 wt% Co loadings were confirmed by X-ray fluorescence analyses. 2.2. Catalyst characterization X-ray diffraction (XRD) analyses are performed with a Bruker goniometer using Ni-filtered Cu K␣ radiation. A proportional counter and 0.05◦ step sizes in 2 are used. The assignment of the crystalline phases is based on the JPDS powder diffraction file cards [15]. The line broadening of the main reflection peaks is used to determine particle sizes via the Scherrer equation, with a lower detection limit of 3 nm [16]. N2 adsorption–desorption isotherms are measured with a Carlo Erba Sorptomat 1900 instrument. Before the measurements the samples are outgassed at 393 K for 4 h. The fully computerized analysis of the nitrogen adsorption isotherm at 77 K allows to estimate the specific surface areas of the samples, through the BET method in the standard pressure range 0.05–0.3 p/po . By analysis of the desorption curve, using the BJH calculation method, the pore size distribution is also obtained [17]. Temperature programmed reduction (TPR) experiments are carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD). The gas mixture with composition 5% H2 in Ar (50 ml/min) is used to reduce the samples (100 mg), heating from room temperature up to 1273 K at the rate of 10 K/min. Before starting the TPR analyses, the catalysts are pretreated at 623 K with a flowing gas mixture of 5% O2 in He (50 ml/min) in order to purge the surface, then cooling down under Ar. The X-ray photoelectron spectroscopy (XPS) analyses are performed with a VGMicrotech ESCA 3000Multilab, equipped with a dual Mg/Al anode. The spectra are excited by the unmonochromatized Al K␣ source (1486.6 eV) run at 14 kV and 15 mA. The analyzer is operated in the constant analyzer energy (CAE) mode. For the

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individual peak energy regions, a pass energy of 20 eV set across the hemispheres is used. Survey spectra are measured at 50 eV pass energy. The sample powders are analyzed as pressed in a stub holder lined with a gold foil. “In situ” reduction of selected samples is carried out inside the high pressure reaction cell attached to the preparation chamber of the spectrometer, where the sample heated at 623 K is exposed for 16 h to pure hydrogen, flowing with a flow rate of 4 ml/s, reproducing the same reduction conditions adopted prior to the catalytic reaction. The constant charging of the samples is removed by referencing all the energies to the internal reference line Si 2p set at 103.9 eV as previously calibrated for the pure SiO2 with respect to the adventitious carbon. The invariance of the peak shapes and widths at the beginning and at the end of the analyses ensures absence of differential charging. Analyses of the peaks are performed with the software provided by VG, based on non-linear least squares fitting program using a weighted sum of Lorentzian and Gaussian component curves after background subtraction according to Shirley and Sherwood [18,19]. Atomic concentrations are calculated from peak intensity using the sensitivity factors provided with the software. The binding energy values are quoted with a precision of ±0.15 eV and the atomic percentage with a precision of ±10%.

2.3. Fischer–Tropsch syntheses The Fischer–Tropsch synthesis (FTS) is carried out in a downflow stainless steel fixed-bed reactor (i.d. 9 mm) with a catalyst loading of 0.7–1 g (pellet size:53–90 ␮m) diluted with 5 g SiC. The process conditions are: 483 K, 20 bar, H2 /CO = 2.1. The experimental rig and analysis system are described in details elsewhere [20]. The reactor tube is heated by means of an oven, regulated by cascade temperature control with one sliding thermocouple in the catalyst bed and another one placed in the oven. This system, together with an aluminum jacket placed outside the reactor, allows for an even temperature profile along the catalyst bed (483 ± 1 K). Prior to reaction, the catalyst is reduced in situ in pure H2 (250 N cm3 /gcat min) at 623 K at atmospheric pressure for 16 h (heating rate: 1 K/min). After reduction the catalysts is cooled at 443 K and then flushed with He before increasing the pressure to the desired level. Then the feed is switched to syn gas (containing 3% N2 as internal standard). Subsequently the temperature is slowly increased to 483 K (rate: 0.15 K/min). The initial reactant flow (period A) is 200–250 N cm/min in order to reach a constant weight hourly spaced velocity (WHSV) of 17,000 N cm3 /(gcat h) and held for about 24 h in order to reach a pseudo steady state condition. Then the space velocity is lowered to achieve about 30% CO conversion (period B) and held for another 24 h. The heavy HCs and most of the water are condensed in two consecutive traps kept at 383 K and room temperature respectively. The product gases leaving the traps are depressurized and analyzed on-line by means of a gas chromatograph Agilent 6890 equipped with a TCD and a flame ionization detector (FID). H2 , N2 , CO, CH4 and CO2 are separated by a Carbosieve II packed column and analysed by the TCD. C1 –C6 products are separated by an alumina-plot column and quantified on the FID allowing for SC5+ determination. The SC5+ is defined as follows: SC5+ = 100 − (SC1 + SC2 + SC3 + SC4 ) C5 and C6 are present both in gas and liquid phase. By assuming gas/liquid equilibrium in the cold trap (calculated by using Aspen HYSYS 2006, equation of state: Lee Kesler Plocker 298 K, 20 bar) the total amount of C5 and C6 hydrocarbons are estimated from the gas analysis. Formation of oxygenates over these Co-based catalysts has been assumed negligible. All reported selectivities are

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A.M. Venezia et al. / Catalysis Today 197 (2012) 18–23

Table 1 Specific surface area (SBET ), average pore diameters (dpav ) and pore volume (Vp ) of the supports and calcined catalysts and XRD derived Co3 O4 crystallite sizes (dCo3O4 ) of the Co catalysts. Samples

SBET (m2 /g)

dpav (nm)

Vp (cm3 /g)

dCo3O4 (nm)

SiO2 6Co/SiO2 12Co/SiO2 TiSiO2 6Co/TiSiO2 12Co/TiSiO2

457 395 336 389 350 322

6.5 6.5 6.5 7.0 7.3 7.7

0.61 0.52 0.52 0.63 0.50 0.51

– 15 15 – 11 12

C-atom based and CO2 -free (i.e. SC5+ if excluding CO2 from the carbon balance [20]). 3. Results and discussion 3.1. Catalyst characterization In Table 1 the textural properties of the catalysts and related supports as determined by N2 physisorption measurements are listed. The sol–gel prepared silica is characterized by high surface area which, after the sequential deposition of the titania by grafting and then cobalt by wet-impregnation with calcinations steps in between, undergoes a significant decrease. Such variation is likely determined by the added species partially filling the silica pores. The catalysts with the higher cobalt loading exhibit lower surface areas. The average pore diameter of the silica does not change upon cobalt deposition, on the contrary it increases slightly in the titania modified silica. In this case also an increase of the pore size is observed in the corresponding cobalt catalysts. In Fig. 1 the XRD patterns of the calcined catalysts with 6 wt% and 12 wt% of cobalt over the promoted and unpromoted silica are displayed. Only peaks attributable to the spinel Co3 O4 are present. From the linewidth of the most intense reflection peaks, using the Scherrer equation, the average Co3 O4 crystallite diameters are calculated and their values are listed in Table 1. In both series, quite large sizes are obtained which do not relate with the amount of cobalt but rather with the presence or not of the TiO2 in the supports. Indeed slightly smaller sizes are obtained over the titania promoted silica as compared to the cobalt over the pure silica. In agreement with a recent study on similar samples, the smaller sizes of the cobalt species could be related to a chemical interaction between titania and cobalt [21]. The formation of these cobalt oxide crystallites may locally modify the pore structure causing the increase of the pore diameter observed for the titania doped

Fig. 2. H2 -TPR patterns of different Co catalysts. (a) 12Co/SiO2 , (b) 6Co/SiO2 , (c) 12Co/TiSiO2 , (d) 6Co/TiSiO2 .

catalysts and reported in Table 1. It is worth noticing that in all cases the average Co3 O4 crystallite sizes exceed the pore sizes of the supports, suggesting that the particles are mainly on the external surface of the pores. The reduction behavior of the catalysts as a function of the cobalt loading and titania promotion is investigated by temperatureprogrammed reduction (TPR). The TPR profiles of the various catalysts are given in Fig. 2. As widely reported in literature, the reduction profiles of cobalt catalysts containing Co3 O4 species are characterized by either one or two peaks due to the two step reduction of Co3 O4 to Co metal through the CoO oxide [3,14,22]. In the case of cobalt over pure silica two well resolved peaks are generally observed [22]. Indeed as shown in Fig. 2 for the two differently loaded Co/SiO2 catalysts, the low temperature peak at ∼550 K attributable to the reduction of Co3 O4 to CoO and the peak at ∼590 K attributable to the reduction of CoO to Co(0) are both present [22]. The profiles of the 6Co/SiO2 and 12Co/SiO2 contain also a low intensity high temperature peak at 948 K and 1027 K respectively, attributed to cobalt oxide stabilized by the support, like a cobalt silicate [23]. In the presence of titania, the two peaks in the range 550–590 K merge into one, centered at lower temperature as compared to the average position of the two peaks observed for the cobalt catalysts over pure silica. In addition, stronger and broad peaks are observed at high temperature, at about 1100 K, due to the reduction of Co species strongly interacting with the titania containing support [21]. The H2 -TPR peak maxima and H2 uptake of the calcined Co catalysts are summarized in Table 2. Within the experimental error, mainly related to the integration procedure, the total amount of consumed hydrogen is in accordance with the Co3 O4 amount. With respect to the pure silica supported catalysts it is worth noticing that the relative amount of the hydrogen uptake of the second peak to the hydrogen uptake of the first peak is close to the stoichiometric value of 3 corresponding to the reduction of Co3 O4 to CoO and CoO to Co(0). In accord with the profiles of Fig. 2, the titania promoted catalysts present most of the hydrogen uptake Table 2 H2 -TPR peak maxima and H2 uptake (±15%) of calcined Co catalysts. Sample

Fig. 1. XRD patterns of Co catalysts.

6Co/SiO2 12Co/SiO2 6Co/TiSiO2 12Co/TiSiO2

Tmax (K)

V(ml/gcat )

I peak

II peak

III peak

I peak

II peak

III peak

553 553 556 577

586 593 693 727

948 1027 1095 1100

6 10 6 13

16 30 6 12

1 2 10 17

A.M. Venezia et al. / Catalysis Today 197 (2012) 18–23

Fig. 3. Experimental and fitted Co 2p photoelectron spectra of sample 12Co/SiO2 (a) calcined and (b) after being reduced in situ at 623 K for 16 h.

in correspondence of the high temperature peaks, attributed to the reduction of cobalt interacting with the support. Moreover, as for the cobalt over the pure silica, no changes in the degree of the cobalt reduction as function of the cobalt loading is observed in the titania doped catalysts. These results reflect similar Co3 O4 particle sizes in accord with the results obtained from the XRD analyses within the two series of differently loaded samples. Investigation of the catalyst surface, in the calcined state and after reduction, is performed by XPS spectroscopy. The binding energies of the main peaks, Ti 2p3/2 , O 1s and Co 2p3/2 are listed in Table 3 along with the Co/(Si + Ti) atomic ratio obtained from the intensity of the XP peaks, normalized by the atomic sensitivity factors. The Co 2p XP spectra with the two spin orbit components Co 2p3/2 and Co 2p1/2 are shown in Figs. 3 and 4 for the 12Co/SiO2 and for the 12Co/TiSiO2 catalysts respectively. The experimental spectra of the calcined samples are curve fitted with the two chemical components Co3+ and Co2+ , present in the Co3 O4 particles. In the pure silica cobalt samples, as shown in Fig. 3a for the 12Co/SiO2 , the low energy Co 2p3/2 component at 779.8 eV (±0.1 eV) is attributed

Fig. 4. Experimental and fitted Co 2p photoelectron spectra of sample 12Co/TiSiO2 (a) calcined and (b) after being reduced in situ at 623 K for 16 h.

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to Co3+ , the high energy component at 782.1 eV, with the shake up satellite peak 5 eV higher than its main peak, is attributed to CoO [24]. It should be pointed out that the value of this component is rather higher (0.5 eV) as compared to what is reported in literature for the CoO. One possible explanation could be the presence of the precursor nitrate not completely decomposed at the calcination temperature of 673 K [23]. Moreover, the relative percentages of the Co3+ and Co2+ chemical species is not equal to the stoichiometric value of 2 typical of the Co3 O4 (Co2 O3 + CoO) but is close to 1. Such discrepancy suggests that part of the cobalt, during the calcinations step does not entirely crystallize as spinel Co3 O4 but exists as amorphous CoO and/or some precursor nitrate, not detected by the XRD. After the “in situ” reduction at 623 K for 16 h, metallic cobalt Co(0) is formed, with the typical Co 2p3/2 binding energy at ∼777.5 eV (Fig. 3b). The other Co 2p3/2 component has a binding energy of ∼781 eV, an intermediate value between those for Co2+ and Co3+ species. On the bases of the cobalt species reducibility, according to TPR measurements, such small peak is reasonably attributed to Co2+ species. The presence of CoO and Co metal is also in agreement with findings for a reduced Co/SiO2 catalyst [25]. Based on the atomic concentration calculation, as given in Table 3, about 70% of cobalt reduction has occurred after the reduction treatment performed in situ using the same conditions as those adopted prior to the catalytic reaction. The extent of this reduction is in close accord with what measured by Storsæter et al. by oxygen titration on silica supported cobalt catalysts upon reduction at 623 K for 16 h [23]. Moreover, the Co/Si atomic ratio stays constant after the reduction treatment. The O 1s binding energy at 532.9 eV listed in Table 3 is typical of the oxygen in SiO2 . Co 2p spectra of the 12Co/TiSiO2 catalyst are displayed in Fig. 4. Within the error of the curve fitting and the charging compensation procedure, the binding energy values reported in Table 3 agree with the presence in the calcined state of the two cobalt species, Co2+ and Co3+ . In terms of binding energies and atomic species percentages, the spectra of the calcined Co catalysts over the titania promoted silica do not differ significantly from the spectra of the pure silica supported cobalt catalysts. Given the similarity of the Co 2p spectra of CoO and CoTiO3 and considering the broadness of the experimental spectra it is not possible to ascertain the presence of a cobalt titanate [24]. As seen in the table, the atomic ratio between the cobalt and the support species is double in the titania containing catalysts as compared with the ratio in the pure silica supported catalysts. The Ti 2p3/2 component at 459.1 eV(±0.2 eV) and the O 1s component at 530.4 eV are both typical of TiO2 . After reduction, as clearly seen in Fig. 4b, the metallic cobalt and the Co2+ species are present. At variance with the 12Co/SiO2 sample, the presence of the Co2+ species after reduction is unequivocally established by the large shake up satellites. By comparing the spectra of the reduced 12Co/SiO2 (Fig. 3b) and reduced 12Co/TiSiO2 (Fig. 4b) and comparing the corresponding atomic percentages in Table 3, one important difference between the two types of catalysts reside in the significantly lower percentage of reduction achieved with the titania promoted catalyst, 47% against 76%. Reduction of about 50% of the total amount of cobalt has been reported in literature for cobalt supported over another interacting support like alumina [22]. Moreover, as observed in the case of the 12Co/SiO2 catalyst, the invariance of the atomic ratio between Co and the support species (Ti and Si) after the reduction indicates that no sintering of the surface cobalt species is occurring. In agreement with recent investigation on the mechanism of cobalt particle regeneration, during the reduction treatment at 623 K the cobalt mobility over the support (sintering), regardless the type of support is nil or very low [26]. In Fig. 5 the space-time yields to hydrocarbons, corresponding to the amount of hydrocarbon per grams of catalysts per hour, as a function of time on stream (TOS) are shown for the cobalt

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Table 3 XPS binding energies and XPS derived Co/Si atomic ratios of calcined and in situ reduced catalysts. The binding energies are referred to the internal standard Si 2p = 103.9 eV. Samples

Co 2p3/2

6Co/SiO2 12Co/SiO2 6Co/TiSiO2 12Co/TiSiO2

O 1s

Ti 2p

Calc.

Red.

Calc.

Red.

779.9 (50%) 782.3 (50%) 779.7 (47%) 782.0 (53%) 779.6 (56%) 781.9 (44%) 779.4(40%) 781.7(60%)

777.6(70%) 781.0(30%) 777.4(76%) 780.9(34%) n.d

532.9

532.9

532.9

532.9

533.0(93%) 530.4 (7%) 532.9(91%)530.4(9%)

777.6(47%) 780.9(53%)

Calc.

Co/(Si + Ti) Red.

Calc.

Red.

–

–

0.01

0.01

–

–

0.02

0.02

n.d

459.3

–

0.03

n.d.

532.9(92%) 530.1(8%)

459.0

458.9

0.04

0.04

n.d.; not determined.

period B. The conversion values from period A refer to the steady state conditions. The corresponding turn over frequencies (TOF) are calculated from the Co crystallite sizes, estimated from the XRD derived Co3 O4 sizes [22,28]. Indeed, according to the observation reported earlier by Lee et al. on Co/Al2 O3 systems and corroborated in the present study by the invariance of the XPS atomic ratio of Co/support species, before and after reduction, the dispersion of the reduced catalysts is largely fixed after the calcinations step [29]. Therefore, assuming that no sintering is occurring during the reduction process, taking into account the molar volumes of metallic cobalt and of Co3 O4 , the resulting conversion factor for the diameter d of a given Co3 O4 particle being reduced to metallic cobalt is then d(Co◦ ) = 0.75 · d(Co3 O4 ).

Fig. 5. Space time yields of hydrocarbon for the two differently loaded cobalt catalysts supported on SiO2 and on TiSiO2 , during 50 h. The data refer to the first period A at the constant space velocity and to period B at approximately constant conversion and variable space velocity.

The reliability of this approach to determine the Co metal particle sizes has been recently confirmed by the good agreement between Co particle sizes determined by XRD, TEM and H2 -chemisorption measurements [30]. In order to have the total amount of the surface active cobalt metal atoms, the cobalt dispersion (D) is calculated from the metal particle sizes, assuming spherical, uniform particles with site density 14.6 atoms/nm2 , by use of the following formula [31] D=

catalysts with the different loadings and different supports. The data are reported for the period A at the higher space velocity of 17000 N ml/(gcat h) and for period B at lower space velocity and constant conversion to allow for selectivity comparison [27]. In both type of experimental conditions the beneficial effect of the titania over the cobalt catalyst activity is observed. Going from period A to period B, all the catalysts show an increase of the space time yield to hydrocarbons. The catalytic performance in terms of CO conversion (XCO ), catalyst time yield (CTY) (mole CO converted/(gcat h)), turn over frequency (TOF) and selectivity to different hydrocarbons, SCn , for the pure silica and for the Ti promoted silica cobalt catalysts are summarized in Table 4 for the two different period A and

96 [D = %, d = nm]. d

The obtained dispersions are multiplied by the degree of reduction obtained by XPS analyses. Considering the data in period A, the TOF values increases with the cobalt loading and with the presence of the titania in the support, the latter feature playing the larger effect. Selectivities are not much influenced by the cobalt loading in the pure silica catalysts. On the contrary, by comparing the two differently loaded catalysts over the TiSiO2 support a substantial increase of the SC5+ from ∼76% to ∼80% takes place. At the low space velocity, in period B, with an increase of the catalytic conversion to about 30%, a decrease of the CH4 selectivity and an increase of the C5+ selectivity is observed for all the catalysts. The

Table 4 Catalytic performances in period A and B for all catalysts. Experimental conditions: 483 K, 20 bar, H2 /CO = 2.1. Sample

Catalytic performances Period A a

X 6Co/SiO2 12Co/SiO2 6Co/TiSiO2 12Co/TiSiO2 a b c d

CO

3.4 8.5 8.5 18.7

(%)

Period B b

c

−1

CTY (molCO/(gcat , h))

TOF (s

0.005 0.013 0.013 0.030

0.017 0.021 0.032 0.038

)

SC1 (%)

SC2 −C4 (%)

SC5+ (%)

GHSV

CTY (molCO/(gcat , h))

SC1 (%)

SC2 −C4 (%)

SC5+ (%)

14.3 13.7 12.3 10.7

12.3 13.2 11.4 10.6

73.4 73.1 76.3 79.7

3100d 4200 4100 7200

0.006 0.017 0.016 0.031

12.0 10.4 10.0 9.8

10.1 9.2 9.2 8.7

77.9 80.4 80.8 81.5

The conversions refer to the steady state conditions. Catalyst time yield. TOF after 10 h on stream. The conversion XCO in period B was around 30 for all the catalysts except for this catalysts exhibiting the low value of 14.

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observed increase of the SC5+ as the conversion is increased (upon lowering the space velocity) is usually ascribed to a higher extent of the ␣-olefin readsorption due to the longer bed residence time and/or to the increased partial pressure of the indigenous water [20,27,32–34]. The best performing catalyst with the highest selectivity and the largest FT activity is the 12Co/TiSiO2 . Moreover, it is worth noticing the similar catalytic performances of the 6Co/TiSiO2 and the 12Co/SiO2 catalysts exhibiting the same C5+ selectivity and similar catalyst time yield in spite of the different cobalt loading. As reported in literature, the FT activity is affected by both the size of the cobalt particles and the extent of the reduction. With respect to the first factor, the Co3 O4 particle size of the 6Co/TiSiO2 (11 nm) and consequently of the metallic particles is slightly smaller as compared to the Co3 O4 size (15 nm) of the 12Co/SiO2 sample. However both particle sizes are well above the value of 10 nm, above which the cobalt site-time yields have been traditionally considered independent of both cobalt dispersion and support identity [1]. Therefore, neglecting the effect of the cobalt size, the extent of reduction before the catalytic reaction must play the most important role. As clearly indicated by the TPR profiles and quantified by the XPS analyses, the cobalt, following the reduction treatment at 623 K, is reduced by 70% over pure silica, and less than 50% over the titania doped silica. Oxidation of the cobalt metal active phase has been for long time considered one of the main deactivation processes for these catalysts. It was recently shown that cobalt catalysts supported on alumina and with particle size exceeding 6 nm do not undergo surface or bulk oxidation during FTS, but given the strongly reducing FT environment, further reduction of the unreduced CoO takes place [35]. The mechanism of sintering is generally present during the FTS and contribute to up 30% of deactivation. Such mechanism, related to the cobalt mobility over the support, dominates during the early stage of the reaction, thereafter being replaced by the polymeric carbon formation, responsible for the long-term deactivation [26,35]. As said before, during the hydrogen treatment at 623 K, obviously the presence of titania due to its stronger interaction with the supported cobalt, hinders the reducibility of the cobalt oxide. The presence of the unreduced cobalt oxide, strongly bound to the oxide carrier, by hindering the cobalt mobility on the support, may then contribute to the increased activity observed for the titania containing cobalt catalysts as compared to the cobalt over the silica support. 4. Conclusion Adding a small amount of titania to a sol–gel prepared silica, has a beneficial effect on the FTS activity of the supported cobalt catalysts. According to the XRD investigation of the calcined samples, only Co3 O4 crystallites are present in all the catalysts. The XRD derived crystallite sizes are slightly smaller in the presence of titania and are not influenced by the cobalt loading. XPS analyses of the catalysts, in situ reduced at the same FT pretreatment conditions, confirm a harder reducibility of the cobalt when supported over the titania modified silica. Such property is also assessed by the H2 -TPR profiles exhibiting strong hydrogen consumption peaks at elevated temperature (∼1100 K) which are not present in the catalysts of cobalt over pure silica. Comparing the catalytic performance of the catalysts in the FT reaction at high space velocity, increased FT conversion rates and larger C5+ selectivities are observed for the titania doped catalysts. When running the tests at low space velocity and constant conversion, still slightly higher C5+ selectivity is obtained for the titania doped catalysts. The improvement of the

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