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Applied Catalysis A: General 475 (2014) 82– 89

Contents lists available at ScienceDirect

Applied Catalysis A: General

j ourna l h omepa ge: www.elsev ier .com/ locate /apcata

-silicon carbide as a catalyst support in the Fischer–Tropschynthesis: Influence of the modification of the support by

pore agent and acidic treatment

osé Antonio Díaza,∗, María Calvo-Serranoa, Ana Raquel de la Osaa,lba María García-Minguillána, Amaya Romerob,. Giroir-Fendlerc, José Luis Valverdea

Facultad de Ciencias y Tecnologías Químicas Departamento de Ingeniería Química, Universidad de Castilla la Mancha, Avenida de Camilo José Cela 12,3071 Ciudad Real, SpainEscuela de Ingenieros Agrónomos Departamento de Ingeniería Química, Universidad de Castilla la Mancha, Avenida de Camilo José Cela 12, 13071 Ciudadeal, SpainInstitut de recherches sur la catalyse et l’environment de Lyon (IRCELYON), 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France

r t i c l e i n f o

rticle history:eceived 30 July 2013eceived in revised form 9 January 2014ccepted 11 January 2014vailable online 21 January 2014

eywords:-silicon carbideischer–Tropsch synthesis

a b s t r a c t

The present work studies four �-silicon carbide (�-SiC) samples supplied by SICAT. They consisted ofthe parent material (SiC-A), SiC-A modified by a pore agent (SiC-B) or purified by an acidic treatment(SiC-C), and SiC-B purified by the referred acidic treatment (SiC-D). The pore agent treatment resultedin a modification of the pore size distribution, whereas the acidic one led to materials with less metalimpurities content. The effects of these pretreatments in the resulting materials were characterized byN2 adsorption–desorption, Hg intrusion porosimetry, temperature-programmed reduction (TPR), X-raydiffraction (XRD) and NaOH titrations. They were then used as supports in cobalt-based catalysts forthe Fischer–Tropsch synthesis (FTS). The resulting catalysts were also characterized by the techniques

cidic treatmentore agent

mentioned above as well as by atomic absorption (AA) and oxygen pulses. It was observed that theacid washing, in addition to remove some metal impurities increased the number of acid sites over theSiCs, which seemed to promote the reducibility of the cobalt particles of the corresponding catalyst. Onthe other hand, the addition of pore agent strongly increased the macropore volume of the SiCs, whichfavoured the FTS products desorption, keeping the catalytically active sites available. All these facts causedthat the catalyst supported on pore- and acid-treated SiC presented the highest catalytic activity.

. Introduction

Silicon carbide (SiC) is a covalent material with a tetrahedraltructural unit. This compound has excellent thermal stability, highechanical strength, excellent resistance to oxidation and corro-

ion, a low coefficient of thermal expansion and a high thermalonductivity [1]. These properties make silicon carbide useful inarious applications, including the reinforcement of other materi-ls [2], fuel cells [3], semiconductors [4] and abrasives [5]. One of theain drawbacks of silicon carbide is related to its very low specific

urface area (0.1–1 m2 g−1). However, in the 1980s Ledoux et al.

6] designed a new method for the preparation (called shape mem-ry synthesis, SMS) of medium-specific-surface SiC (10–40 m2 g−1),hich made this material useful for catalytic applications.

∗ Corresponding author. Tel.: +34 926295300.E-mail address: joseantonio.diaz@uclm.es (J.A. Díaz).

926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2014.01.021

© 2014 Elsevier B.V. All rights reserved.

This way, SiC can be useful to prevent the formation of hotspots, which could appear in some exothermic reactions. Amongthe most recent publications related to the use of SiC as a cata-lyst support, Nguyen et al. [7] used MoVTeNbO supported on SiCfoam in the oxidative dehydrogenation of ethane and the ammox-idation of propane. García-Vargas et al. [8] prepared various Ni/SiCcatalysts for the methane tri-reforming process. Marin et al. [9]prepared SiC foam-supported Pd catalysts for the combustion ofmethane. Finally, SiC has been used as a catalyst support for theFischer–Tropsch synthesis (FTS) [1,10,11].

The FTS is a heterogeneously catalyzed polymerization processin which syngas (CO and H2) is converted into a wide variety ofhydrocarbons. This process represents an alternative way to obtainpetroleum-derived fuels, which are today affected by numerous

problems: growing demand, reduced supply of products and polit-ical instability in petroleum-rich countries [12]. There are sixoperating FTS plants in the world: three are running in South Africa(two by Sasol and one by PetroSA), one in Malaysia (by Shell), and

lysis A: General 475 (2014) 82– 89 83

tp

C

kcobma

amlaiw

saw(ma

itcpbt[vtociqtoCatbTdTTt

l(tpaca

2

S

Table 1Data of the SiC supports supplied by SICAT.

Sample Brief description Chemical composition

SiC-A 1 mm �-SiCextrudates

Al: 980 ppmCa: 500–600 ppmFe: 3000 ppmMn: 70 ppmTi: 300 ppmS < 60 ppmNa: 250 ppmK: 60 ppm

SiC-B SiC-A+pore agent Same as SiC-ASiC-C SiC-A+acidic

treatmentCa < 50 ppmFe: 3000 ppm in a silicide formMn < 50 ppmS < 60 ppmNa < 50 ppm

J.A. Díaz et al. / Applied Cata

wo more in Qatar (Oryx and Pearl). In addition, there are more FTSlants under construction.

The process can be considered as a polymerization in which theCH2 molecular fragment is the monomer:

O + 2H2 → –CH2– + H2O �H0FT = –165KJmol−1

A large number of paraffins, olefins, alcohols, aldehydes andetones can be obtained by FTS. Iron-, nickel-, ruthenium- andobalt-based materials are the classic FTS-active metals. However,nly cobalt- and iron- based catalysts attract particular interest inench-scale testings. Cobalt catalysts are less expensive than nobleetal catalysts, and have a very low water–gas-shift (WGS) activity,

nd a high selectivity to long-chain hydrocarbons.Typical cobalt and iron catalysts usually contain between 10

nd 30 wt.% of active phase and, often, small amounts of a secondetal (typically noble metals) or a metal oxide (typically zirconia,

anthanum oxide or cerium oxide) acting as promoters [13]. Thective phase is usually supported on a high surface area oxide (sil-ca, alumina or titania), and is mainly incorporated by the incipient

etness or the slurry impregnation techniques [14].The use of SiC as a catalyst support in FTS is of interest because

ome of the properties of this material positively influence the cat-lytic performance. In addition of its high thermal conductivity,hich could prevent the formation of hot spots, their pore network

mainly constituted by meso- and macropores) would favour theass transfer during FTS whereas their chemical inertness would

llow to recover the active phase and the support [15].After its synthesis, the obtained SiC presented some metal

mpurities into its structure. The presence of impurities such as lan-hanum, titanium, or some alkali and alkaline earth metals [16,17]ould significantly alter the catalytic performance by changing thehysicochemical properties of the support. Very few studies haveeen carried out on the influence of the presence of impurities inhe support on the performance of the final catalyst. Park et al.16] reported that typical FTS supports could be modified witharious metals, such as La, Ti or Mg, with the aim of improvinghe dispersion of the metal particles. They studied the influencef the presence of titanium in a Co/Ti–Al2O3 system. They con-luded that the presence of low levels of Ti (0.2 wt.%) in the supportncreased the reduction degree of the final catalyst and, conse-uently, improved the catalytic activity. Balonek et al. [17] studiedhe effect of alkali metal impurities from biomass-derived syngasn Co–Re catalysts. It was concluded that the metals studied (K,a, Li, Na) decreased both the reduction degree and the catalyticctivity. This finding was attributed to geometrical effects that ledo the blockage of active sites. However, the catalytic activity coulde improved when the presence of some of these components, e.g.iO2, is increased. In this sense, Liu et al. [15] prepared titania-ecorated SiC (17% TiO2) to be used as a catalyst support in FTS.hey found that the interaction between the cobalt particles andiO2 led to the formation of small cobalt particles which enhancedhe FTS activity.

The aim of this work was to study the FTS performance of cata-ysts consisting of Co incorporated to four supports based on �-SiCin the pellet form) supplied by SICAT. Before the metal incorpora-ion, and taking as a reference a parent SiC, the other supports wererepared by modification of the former by a pore agent and/or ancidic treatment. The pore agent treatment resulted in a modifi-ation of the pore distribution of the parent material whereas thecidic one led to materials with less metal impurities content.

. Experimental

Four samples of �-SiC (in the pellet form) were supplied byICAT. They consisted of the parent material (SiC-A), SiC-A modified

K < 50 ppmSiC-D SiC-B+acidic

treatmentSame as SiC-C

by a pore agent (SiC-B) or purified by an acidic treatment (SiC-C),and SiC-B purified by the referred acidic treatment (SiC-D). The poreagent treatment resulted in a modification of the pore distributionwhereas the acidic one led to materials with less metal impuritiescontent. Their compositions are listed in Table 1. The pore-agentSiC compositions were the same as those of non-treated ones. SiC-supported cobalt catalysts were prepared by the incipient wetnessimpregnation technique, using Co(NO3)2·H2O as the salt precursor.Firstly, the support was outgassed at ambient temperature for 2 hin order to remove water and other surface impurities. An aqueoussolution of the salt precursor was then poured over the supportand the sample was outgassed at 363 K for 2 h. The final catalystwas dried overnight at 403 K and then calcined in a static air atmo-sphere at 823 K for 5 h. The final samples were denoted as Co/SiC-A,Co/SiC-B, Co/SiC-C and Co/SiC-D.

Surface area/porosity measurements were carried out usinga Quantachrome Quadrasorb SI apparatus, with N2 as the sor-bate at 77 K. All samples were outgassed prior to analysis at453 K under vacuum (1 × 10−2 Torr) for 12 h. The total specific sur-face areas were determined by the multipoint BET method andthe mesoporosity by the Barrett–Joyner–Halenda (BJH) method.Mercury intrusion porosimetry was used to determine the mac-roporous distribution of the supports. These analyses were carriedout in a Micromeritics Autopore IV 9420 porosimeter. XRD analyseswere conducted with a Philips XıPert instrument using nickel-filtered Cu–K� radiation; the samples were scanned at a rate of0.02◦ step−1 over the range 5◦ ≤ 2� ≤ 90◦ (scan time = 2 s step−1)and the diffractograms were compared with the PDF-ICDD ref-erences. Temperature-programmed reduction (TPR) experimentswere conducted on a commercial Micromeritics AutoChem 2950HP unit with TCD detection. Samples (ca. 0.15 g) were loaded in aU-shaped quartz reactor and ramped from room temperature to1173 K (5 K min−1) under a reducing atmosphere (17.5% v/v H2/Ar,60 cm3 min−1). Thermogravimetric analysis (TGA) experimentswere conducted using a Mettler Toledo TGA/DSC 1 apparatus.Approximately 10 mg of the sample was placed in an Al2O3 ceramicpan. The sample was heated from room temperature to 1173 K atprefixed heating rate of 10 K min−1. The flow rate of pure oxygenwas kept constant at 40 cm3 min−1. O2 titration experiments wereconducted in the same apparatus as used for the TPR experiments.After reduction, the sample (ca. 0.1 g) was kept at 673 K in He andheld for 1 h to desorb any chemisorbed H2. Calibrated pulses of O2were then added into the continuous He flow until further con-

sumption of O2 was not detected by the TCD located downstreamfrom the reactor. The extent of reduction was calculated assum-ing stoichiometric re-oxidation of Co0–Co3O4 [18]. OH titrationswere performed on a Metrohm 686 titrator with a Dosimat 665

84 J.A. Díaz et al. / Applied Catalysis A: General 475 (2014) 82– 89

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

N2 a

dsor

bed

(cm

3 g-1)

SiC-A SiC-B SiC-C SiC-D

1001010.00

0.05

0.10

0.15

0.20

0.25

dV/d

(log

r)

Pore radius (nm)

SiC-A SiC-B SiC-C SiC-D

a(rts

43t8TwaWpogci(h

3

3

s

80604020

SiC-D

SiC-C

SiC-B

Inte

nsity

(a.u

)

2θ

SiC-A

TP

Relative press ure (P/P0)

Fig. 1. Nitrogen adsorption–desorption isotherms of the SiC supports.

utomatic dosager. Sample (25 mg) was immersed in 0.1 M NaCl50 cm3), acidified to pH ca. 3 with HCl (0.1 M) with constant stir-ing under a N2 atmosphere. A 0.1 M NaOH solution was used ashe titrant and was added dropwise (3.6 cm3 h−1). The starting NaClolution served as a blank.

FTS tests were performed under the following conditions:93 K, 20 bar, H2/CO ratio 2, gas space hour velocity (GSHV) of000 NmL g−1 h−1. The catalyst (2 g) was placed in a tubular reac-or (i.d. = 9 mm) with a porous plate. The catalyst was reduced at23 K for 5 h with an ultrapure hydrogen flow of 100 Ncm3 min−1.he reactor was conditioned until the desired reaction conditionsere achieved. A flow of syngas (H2:CO:N2 of 60:30:10; N2 used

s internal standard) was then passed through the reactor for 60 h.axes were condensed in a wax trap at 393 K and the rest of the

roducts passed to a liquid–liquid–gas separator, which consistedf a Peltier cell. The liquid products were condensed (278 K) and theaseous products and unreacted gases were analyzed in a micro-hromatograph with a TCD analyzer. Liquid samples were analyzedn a chromatograph equipped with a flame ionization detectorFID). The CO reaction rate (as rate of CO converted pf long-chainydrocarbons (C5+) were calculated using the following equations:

rate =NCO,converted

(mol × min−1)

(nCobalt) (mol)

yieldC5+ =NCO,converted

(mol × min−1)

NCO,o

(mol × min−1) × SC5+

. Results

.1. Support characterization

The main physicochemical properties of the silicon carbidesupplied by SICAT are listed in Table 2. Regarding the textural

able 2hysicochemical properties of SiC supports.

Sample SiC-A

BET surface area (m2 g−1) 28

N2 adsorption–desorption mesopore volume (cm3 g−1) 0.109

Hg intrusion porosimetry mesopore volume (cm3 g−1) 0.156

Macropore volume (cm3 g−1) 0.308

VNaOH consumed (cm3) 0.58

Fig. 2. X-ray diffractograms of the SiC supports.

properties, the differences between the samples were less markedin the mesopore range and more remarkable in the macroporeone. The four samples presented a type II–IV isotherm (IUPACclassification, Fig. 1) that is characteristic of macroporous andmesoporous materials, without nitrogen adsorption in the micro-pore range (P/P0 ≤ 0.03) and a small increase in the mesopore range.The hysteresis loops observed (Type H3, IUPAC classification) [19]are characteristic of samples with slit-type pore and were due tocapillary condensation in the mesopores. No meaningful differ-ences in the mesopore distribution were observed when comparedthe different samples (Fig. 1). The four SiC samples mainly pre-sented pores of ca. 10 nm in radius. However, the pores size, asan average, in samples SiC-B and SiC-C were slightly higher thanthose in samples Co/SiC-A and Co/SiC-D. On the other hand, Hgintrusion porosimetry experiments (Table 2) showed the high con-tribution of macropores (>50 nm, IUPAC classification) to the totalporosity of these solids. It can be observed that the addition ofpore agent directly affected the macropore volume. For example,sample SiC-B experienced a significant increase in the macroporevolume after treatment with the pore agent whereas the meso-pore volume remained constant. On the other hand, the acidictreatment caused an increase of the pore volume, probably dueto the removal of the metal impurities which could block someSiC pores. Note that the strong increase in the mesopore volumein sample SiC-C and in the macropore volume in sample SiC-D,would suggest the exact location of these impurities. The meso-pore volume (corresponding to a range of pore sizes between 2 and50 nm, according to IUPAC classification) measured by Hg intru-sion porosimetry (Table 2) was slightly higher than that obtainedby nitrogen adsorption–desorption.

Four diffraction peaks were observed in the X-ray diffrac-tograms at 2� = 36◦, 41◦, 60◦ and 72◦ for all the supports hereconsidered (Fig. 2), which corresponded to the (1 1 1), (2 0 0), (2 2 0)

and (3 1 1) diffraction peaks of �-SiC, respectively (PDF-00-29-1129). These results are consistent with those reported elsewhere[20]. Regarding the samples characterized, each of these peaks

SiC-B SiC-C SiC-D

28 30 300.121 0.142 0.1150.162 0.148 0.1770.512 0.325 0.5900.59 0.58 0.60

J.A. Díaz et al. / Applied Catalysis A: General 475 (2014) 82– 89 85

300 60 0 90 0 120 096

97

98

99

100

101

% W

eigh

t

SiC- A SiC- C

(a)

300 60 0 90 0 120 0

Temperature (K)

SiC-B SiC-D

(b)

D (analysis conditions: up to 1173 K, heating ramp 5 K min−1, 10 Nml min−1 pure O2).

pap

tmtfmppirft

3

ogsie(spieB

80604020

Co/SiC-D

Co/SiC-C

Co/SiC-BInte

nsity

(a.u

.)

2θ

Co/SiC-A

TP

Temperature (K)

Fig. 3. Thermogravimetric analysis of (a) SiC-A and SiC-C and (b) SiC-B and SiC-

resented the same intensity, demonstrating that neither the poregent nor the acidic treatment affected the crystalline structure ofarent silicon carbide.

The oxidation behaviour at high temperature (up to 1173 K) ofhe four supports was studied under an oxygen atmosphere by

eans of TGA (Fig. 3). All of them were stable up to 900 K. At thisemperature, a slight weight loss associated with the removal ofree carbon compounds was noted [21]. This decrease was more

arked in samples modified by a pore agent (SiC-B and SiC-D),robably due to the stronger presence of these free carbon com-ounds after their preparation. Anyway, the weight loss differences

n all samples were lower than 4%. As the temperature continued toise, an increase in weight was observed, which was related to theormation of SiO2 [22]. No plateau was reached, therefore meaninghat SiC oxidation could continue at higher temperatures.

.2. Catalyst characterization

After metal incorporation, catalysts were also characterized inrder to evaluate their physical and chemical properties. Nitro-en adsorption–desorption experiments showed that the isothermhapes (not shown) were similar to those of the supports. Metalncorporation did not affect the silicon carbide structure. How-ver, the metal incorporation produced a decrease in pore volumeTable 3). This decrease was caused by the pore blockage by metalpecies. These blockages were different in each sample. Thus, the

ore volume decrease in sample Co/SiC-A was low and located

n the macropores (note that mesopore volume of the catalyst isven higher than that of the support SiC-A). In turn, sample Co/SiC-, with the higher values of pore volume experienced a higher

able 3hysicochemical properties of the catalysts prepared.

Co/SiC-A

Cobalt content (wt.%) 14.7

BET surface area (m2 g−1) 23

N2 adsorption–desorption mesopore volume (cm3 g−1) 0.117

Hg intrusión porosimetry mesopore volume (cm3 g−1) 0.124

Macropore volume (cm3 g−1) 0.263

TPR-Tred (K) 596

TPO-degree of reduction (%)a 60.5

XRD-cobalt particle size (nm)b 39

VNaOH consumed (cm3) 0.60

a Assuming reoxidation of Co0–Co3O4.b Calculated from the XRD peak at 44◦ using the Debye–Scherrer equation.

Fig. 4. X-ray diffractograms of the reduced catalysts (823 K, 5 h).

blockage. Finally, samples Co/SiC-C and Co/SiC-D presented dif-ferent pore blockages. It seemed that the pores affected by acidictreatment (mesopores in SiC-C and macropores in SiC-D) weremore susceptible to be occupied by metal species.

X-ray diffractograms of the reduced catalysts (Fig. 4) showed thediffraction planes of silicon carbide, as mentioned above, along with

two additional peaks at 2� = 44◦ and 52◦, which were attributed tothe (1 1 1) and (2 0 0) planes of the cobalt crystal lattice, respec-tively (PDF 15-0806). From the main peak (44◦), the average cobalt

Co/SiC-B Co/SiC-C Co/SiC-D

16.0 13.1 13.628 23 28

0.093 0.078 0.1090.145 0.138 0.1550.445 0.292 0.428

602 588 60164.4 83.2 69.542 47 52

0.62 0.72 0.64

86 J.A. Díaz et al. / Applied Catalysis A

1200900600300

Co/SiC-B

Co/SiC-A

Co/SiC-C

TCD

Sig

nal (

a.u.

)

Co/SiC-D

p(fBtgdttessbchas

rscop(itassCfic

TC

Temperature (K)

Fig. 5. TPR profiles of the prepared catalysts.

article size could be estimated with Debye–Scherrer equationTable 3). The following increasing order can be established as aunction of the estimated metal particles size: Co/SiC-A < Co/SiC-

< Co/SiC-C < Co/SiC-D. This order would clearly indicate that thereatment of the parent sample (SiC-A) with the pore agent, whichenerated a slight increase in the macropore volume, led to theeposition of bigger cobalt particles. On the other hand, the acidicreatment of the parent sample (leading to SiC-C and SiC-D) led tohe deposition of bigger particles if compared to that of the par-nt one (SiC-A and SiC-B, respectively). It has been reported thatome metal impurities could have improved the cobalt dispersion,pecifically Ca and K [23]. Thus, the removal of them would lead toigger cobalt particles, as seen when samples SiC-C and SiC-D areompared to samples SiC-A and SiC-B, respectively. On the otherand, previous reports [16,17] have shown that the effect of cat-lyst acidity on metal particle size can be low, as observed in thistudy.

Cobalt reducibility was studied by temperature-programmededuction analysis. The TPR profiles of the prepared catalysts arehown in Fig. 5. Two reduction peaks can be observed in allases. The first peak (≈600 K) can be attributed to the reductionf Co3O4, which appeared during the calcination step, to metallichase Co0, as reported in previous works [24,25]. The second peak≈900 K) was ascribed to the reduction of Co species that stronglynteract with the SiC support to form Co silicates [26,27]. In addi-ion, samples Co/SiC-A and Co/SiC-C (not treated with the poregent) were reduced at slightly lower temperatures. This findinguggests the existence of stronger metal-support interactions in

amples Co/SiC-B and Co/SiC-D, if compared to those in sampleso/SiC-A and Co/SiC-C. It is worth mentioning that the TPR pro-les of the supports (not shown) did not result in any hydrogenonsumption peak.

able 4atalytic results. Experimental conditions: 493 K, 20 bar, H2/CO 2, 3000 cm3 g−1 h−1, TOS

Catalyst Co/SiC-A

Conversion (%) CO 13.9

H2 16.0

Reaction rate 102 ((molCO min−1)molCobalt−1) 4.0

Selectivity (%) CO2 1.31

CH4 15.9

C2–C3 4.9

C5+ 77.9

Yield (%) C5+ 10.8

Olefin/paraffin ratio (C7–C20) 0.34

˛ASF (chain growth probability) 0.82

: General 475 (2014) 82– 89

The acid-base character of the supports and the correspond-ing catalysts were evaluated by NaOH titrations (Fig. 6). Prior tothe metal incorporation, all samples presented a basic character,i.e., they consumed the same amount of NaOH in the titrationprocess (Fig. 6a), which was lower than that consumed in theblank experiment. This fact would indicate that SiC samples pre-sented basic centres that neutralized part of the HCl used. However,after metal incorporation, the basic character of the resulting cata-lysts changed (Fig. 6b). Sample Co/SiC-C clearly consumed moreNaOH to reach the equilibria, since it was more acid than theother samples. Taking into account the results, it seemed that theacidic treatment led to acid sites in the samples SiC-C and SiC-D which later interacted with cobalt. Furthermore, the acid sitesconcentration seemed to be higher in the less-porosity sampleCo/SiC-C.

Finally, the degrees of reduction of the different samples werecalculated by pulses of oxygen. Percentages of reduced cobalt arelisted in Table 3. It can be observed that the degrees of reduc-tion obtained for the catalysts supported on acid-treated SiCs werehigher than those obtained for the non-treated ones. It seemed thatthe presence of impurities hindered the reduction of the cobalt par-ticles. Consequently, the reduction of cobalt particles resulted to beeasier when the impurities were removed.

Regarding the pore agent treatment, sample Co/SiC-B wasslightly better reduced than Co/SiC-A, whereas the degree of reduc-tion of sample Co/SiC-D was lower than that obtained for sampleCo/SiC-C. However, the particle sizes in samples Co/SiC-B andCo/SiC-D were higher than in samples Co/SiC-A and Co/SiC-C,respectively. According to the literature, bigger particles interactmore weakly with the support than smaller ones, and are moresusceptible to be reduced. Consequently they would be reducedto a higher extent [27,28]. However, no relationship between thecobalt particle size and the degree of reduction could be establishedin this work.

This fact could be a consequence of the strong metal-supportinteraction (SMSI) effect between the active phase and the sup-port and the acid sites mentioned above. The SMSI effect is notonly related to the particle size, i.e., it depends on many factors:metal loading, metal incorporation method, and support properties[29,30]. A stronger SMSI effect hinders the reduction of the metalparticles. The results obtained with pulses of oxygen and TPR pro-files suggested a stronger SMSI effect in the catalysts supported onpore-treated SiC supports, i.e., samples Co/SiC-B and Co/SiC-D. As aconsequence, the degree of reduction of the cobalt particles did notmatch with the cobalt particle size. In addition, the reduction tem-peratures of the cobalt particles were higher in the case of samplesCo/SiC-B and Co/SiC-D. On the other hand, there was a markedlyrelation between the degree of reduction and the acid character ofthe catalyst: the higher the acid sites concentration of the catalyst,

the higher the degree of reduction was. Cobalt particles that inter-acted with acid sites were easier to reduce. This effect was reportedelsewhere in zeolite-based catalysts [31].

60 h.

Co/SiC-B Co/SiC-C Co/SiC-D

16.1 20.2 26.321.6 27.8 28.9

4.6 5.8 7.61.52 1.48 1.12

12.0 14.2 14.05.6 5.1 4.6

80.8 79.2 80.213.0 16.0 21.1

0.21 0.14 0.220.71 0.73 0.66

J.A. Díaz et al. / Applied Catalysis A: General 475 (2014) 82– 89 87

0.0 0.2 0.4 0. 6 0.8 1.0 1.2 1.4 1. 62

4

6

8

10

12pH

Blank (neutral pH) Si C-A Si C-B Si C-C SiC-D

(a)

SUPPORTS

Surface acidity

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1. 6

Blank (neutral pH) Co/SiC-A Co/SiC-B Co/SiC-C Co/SiC-D

(b)

CATALYSTS

Surface acidity

) supp

3

oct1a

rCap

atcovseaw

Sntittr

(tuiitrw

Fpm

methane selectivity was not affected by the acidity of the catalysts(14 ± 2 mol%), thus showing the methanation process is not an acidsite-sensitive reaction.

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Rea

ctio

n ra

te ((

mol

COm

in-1)m

olC

obal

t-1)

Co/Si C-A Co/Si C-C Co/Si C-B Co/Si C-D

VNaOH (ml)

Fig. 6. NaOH titrations of the (a

.3. Catalytic tests

The results of catalytic tests revealed that the main productsbtained in the FTS process were CH4, CO2, C2H6, C3H8 and long-hain hydrocarbons (C5+). All the catalytic tests were performedwice, and the relative errors obtained in all cases were less than0%. Experiments without cobalt-impregnated SiC did not showny relevant catalytic activity.

Table 4 shows the catalytic activity values obtained. The reactionate increased in the following order: Co/SiC-A < Co/SiC-B < Co/SiC-

< Co/SiC-D. It can be seen that the addition of pore agent and thecid treatment carried out over the SiCs, improved the catalyticroperties of the catalysts supported on them.

Regarding the effect of pore agent, it can be observed that pore-gent-treated samples (Co/SiC-B and Co/SiC-D) were more activehan the respective non-treated ones (Co/SiC-A and Co/SiC-C). Asommented above, the pore agent led to SiCs with higher macrop-re volume. Then, it can be asserted that the increase of macroporeolume favoured the catalytic activity. The pore-agent-treatedamples presented a higher macropore volume that facilitated thevacuation of intermediate products, keeping the active sites morevailable for further reactants. This effect has been reported else-here [15].

On the other hand, the catalysts supported over acid-treatediCs (Co/SiC-C and Co/SiC-D) were more active than their respectiveon-treated ones (Co/SiC-A and Co/SiC-B, respectively). This acidicreatment removed the metal impurities of the SiCs. These metalmpurities hindered the catalytic reaction of CO and H2. Moreover,he acidic treatment led to SiCs with acid sites which promotedhe cobalt reduction, as pointed above. Then, a higher degree ofeduction led to more active sites available for FTS.

The variation in reaction rate as a function of time-on-streamTOS) for the four catalysts studied is represented in Fig. 7. Afterhe replacement of the reactor dead volume, the activity increasedntil a plateau was reached, at approximately 20 h of TOS. This

ncrease could be related to the further reduction of cobalt particlesn the reaction media. This fact could explain the faster increase inhe catalytic activity of catalyst Co/SiC-C, the one with the highesteduction degree at the beginning of the reaction. Once a plateauas reached, the four catalysts were stable for at least 60 h.

Selectivities towards the different products obtained duringTS for the four catalysts are listed in Table 4. All samplesresented similar values of selectivities. The low rates of CO2 for-ation observed were an indicative of a low extension of the

VNaOH

(ml)

orts and (b) prepared catalysts.

water–gas-shift (WGS) reaction. Since a low selectivity to CH4 wasobserved, the selectivity to long-chain hydrocarbons (C5+) was con-sequently high in all cases.

It is worth noting that C5+ selectivity was slightly higher forthe pore-agent-treated samples. Furthermore, the acidic and poreagent treatments did not have a marked effect on the selectivity ofthe final catalysts.

Liquid hydrocarbon distributions (C7–C20) obtained for the fourcatalysts are represented in Fig. 8. In all cases, the distributions weresimilar. The presence of gasoline (C7–C10) and kerosene (C11–C14)fractions in the products is remarkable.

It can be observed that the catalysts supported over acid-treatedSiCs yielded lighter hydrocarbons. This fact can be attributed to theoccurrence of acid sites during the acidic treatment. It has beenreported [32,33] that catalytic supports containing acid sites, likezeolites, converted long-chain hydrocarbons to gasoline and othershort-chain fractions by cracking. Thus, the acid sites presentedover SiC in samples Co/SiC-C and Co/SiC-D promoted the crackingof long-chain hydrocarbons, specially the sample Co/SiC-C, whichpresented higher acid sites concentration. On the contrary, the

60402000.00

Time (h)

Fig. 7. Reaction rate versus time-on-stream (TOS).

88 J.A. Díaz et al. / Applied Catalysis A

20191817161514131211109870

2

4

6

8

10

12

14

16

18

% m

ol

Co/SiC-A Co/SiC-B Co/SiC-C Co/SiC-D

A

wnWtaFtart

latwCtci[tC

TC

TP

Carbon number

Fig. 8. C7–C20 hydrocarbon distribution.

The liquid products distributions were also described by thenderson–Schulz–Flory (ASF) equation:

Wn

n= (1 − ˛)2˛n−1

here Wn is the mass fraction of a hydrocarbon with chain length, and is the chain growth probability. A plot of logarithm ofn/n vs n would produce a straight line plot whose slope is related

o ˛. Obtained values are shown in Table 4. It can be appreci-ted that these values matched with the distributions observed inig. 8. Catalysts supported over pore-agent-treated SiCs favouredhe evacuation of products, whereas the ones supported overcid-treated SiCs presented acid sites that promoted the crackingeactions, as commented above. As a consequence, the values ofhese catalysts were lower than that obtained for sample Co/SiC-A.

Bearing in mind the above results, sample Co/SiC-D, the cata-yst supported on acid- and pore agent-treated SiC, was the mostctive catalyst for FTS at 20 bar and 493 K. In order to evaluatehe activity of this sample, its catalytic parameters were comparedith the values obtained with typical FTS catalysts (Co/SiO2 ando/Al2O3) at similar conditions. Table 5 lists the activity and selec-ivities towards methane and C5+ results for Co/SiC-D typical FTSatalysts, by using the most cited previous works [34,35]. Herein,

t can be observed that Co/SiC-D converted more CO than Co/SiO234]. However, the former was more selective towards the forma-ion of CH4. On the other hand, the CO conversion reached witho/Al2O3 [35] was slightly higher than that obtained by Co/SiC-D.

able 5omparison between catalytic activity reached with Co/SiC-D and those obtained with cl

Catalyst Cobalt content Temperature Pressure

(wt.%) (K) (bar)

Co/SiC 13.6 493 20

Co/SiO2 25 493 20

Co/Al2O3 15 493 19.7

able 6hysicochemical properties of the catalysts after reaction.

Co/SiC-A

BET surface area (m2 g−1) 3

Mesopore volume (cm3 g−1) 0.029

TGA-weight loss percentage (%) 17

XRD-cobalt particle size (nm)a 37

a Calculated from the XRD peak at 44◦ using the Debye–Scherrer equation.

: General 475 (2014) 82– 89

However, the selectivity towards CH4 obtained with the former waslower.

These differences could be explained referring to the porousstructures of these supports: Al2O3 and SiO2 are mainly meso-porous materials, which in general better promoted the dispersionof the cobalt particles onto the support. Moreover, the evacuationof the products in mesoporous materials seems to be more diffi-cult. As a consequence, the selectivities towards CH4 and C5+ withCo/Al2O3 and Co/SiO2 were lower and higher, respectively, thanthose obtained with Co/SiC-D. Finally, this macroporous characterof SiC, which favoured the evacuation of products, would explainthe reactants conversion to be similar despite the lower dispersionof the cobalt particles.

3.4. Post-reaction catalyst characterization

In order to better understand the catalytic performance of thecatalysts studied, they were characterized once the reaction hadfinished, by N2 adsorption–desorption, XRD, and TGA.

The physicochemical properties of the catalysts after reac-tion are listed in Table 6. N2 adsorption–desorption experimentsshowed that all samples experienced a strong decrease of their sur-face area and pore volume. This decrease can be attributed to a poreblockage, caused by the waxes formed during the FTS tests. Thesephenomena were predicted before [29], and did not affect to thecatalytic performance.

The TGA profiles of the samples after reaction (Fig. 9) confirmedthe results predicted by N2 adsorption–desorption analysis. It canbe appreciated a strong weight decrease for all samples, whichappeared at the same temperature interval. This weight loss can beattributed to the waxes desorption, which were retained in the cat-alyst and partially blocked the pores. It is also worth noting that thesamples with higher pore volume (Co/SiC-B and Co/SiC-D) expe-rienced a stronger weight loss, which could be associated to thehigher porosity of these supports, having more vacancies to retainthe waxes.

Finally, the metal particle sizes of the catalysts after reactionwere evaluated by XRD diffractograms (Fig. 10), by applying theDebye–Scherrer equation to the peak corresponding to metalliccobalt at 44◦. It is remarkable that the diffraction peaks of theused samples were practically the same as those observed in thecorresponding reduced ones, but with the presence of two newdiffraction peaks in the range 20◦ < 2� < 25◦. These peaks can beattributed to some waxes with high molecular weight, which can

be crystalline, as reported elsewhere [36]. On the other hand, theaverage particle sizes of cobalt in samples Co/SiC-A and Co/SiC-Bafter the FTS test were practically the same than those obtainedfor the corresponding fresh catalysts. However, the catalysts

assic FTS catalysts.

GSHV CO conversion Selectivity (%) Ref.

(Nml g−1 h−1) (%) CH4 C5+

3000 26.3 14.0 80.2 This work2000 15.2 10.5 88.1 [34]2000 29.2 10.9 85.6 [35]

Co/SiC-B Co/SiC-C Co/SiC-D

3 4 30.029 0.032 0.027

25 18 2443 38 32

J.A. Díaz et al. / Applied Catalysis A

120090060030070

80

90

100

% W

eigh

t

Temperature (K)

Co/SiC-A Co/SiC-B Co/SiC-C Co/SiC-D

Fig. 9. Thermogravimetric analysis of catalysts after FTS tests (analysis conditions:up to 1173 K, heating ramp 5 K min−1, 10 Nml min−1 pure O2).

80604020

Co/SiC-D

Co/SiC-C

Co/SiC-B

Co/SiC-A

Inte

nsity

(a.u

.)

sssmoat

4

ctFwpmdccwvtt

[

[

[

[[[

[

[

[

[

[

[

[[

[

[[

[

[[

[

[

[[

[145–168.

2θ

Fig. 10. X-ray diffractograms of the catalysts after FTS tests.

upported over acid-treated SiCs (Co/SiC-C and Co/SiC-D) pre-ented smaller particles after the FTS test. These samples could haveuffered a cobalt re-dispersion during the reaction and further-ore; the higher the support acidity the larger was the promotion

f this re-dispersion. This effect could better explain the higher cat-lytic activity of the catalysts supported on acid-treated SiCs andhe highest activity of sample Co/SiC-D.

. Conclusions

It has been demonstrated that the pre-treatment of �-siliconarbide (�-SiC) with pore agent and acidic treatment improvedhe catalytic activity of the resulting cobalt catalysts in theischer–Tropsch synthesis (FTS). The acidic treatment led to SiCith less metal impurities and more surface acidity, whereas theore agent led to changes in the SiCs pore structures. These treat-ents allowed to increase both the macropore volume and the

egree of reduction of the cobalt particles supported on them. Theatalytic tests showed that these pre-treatments provided a finalatalyst yielding higher CO conversion in FTS at 493 K and 20 bar,

ith smaller changes in product selectivity. The higher macropore

olume led to a better evacuation of the products, which favouredhe subsequent reactants adsorption and conversion, whereashe higher reduction degree increased the catalyst active sites.

[

[

: General 475 (2014) 82– 89 89

Moreover, the removal of metal impurities favoured the increasingon the catalytic activity. After FTS tests, it was observed that somelong-chain hydrocarbons were retained in the catalyst structure.It was also detected a strong cobalt re-dispersion in the cata-lysts supported over acid-treated SiCs. It has been demonstratedthat pore-agent- and acid-treated �-SiC supported cobalt catalyst(Co/SiC-D) is a promising candidate for further FTS studies.

Acknowledgements

The authors gratefully acknowledge financial support fromthe Regional Government of Castilla-La Mancha (Projects PCI08-0020-1239 and PEII1 1-0152-2229) and the collaboration of theIRCELYON and SICAT catalyst.

References

[1] M. Lacroix, L. Dreibine, B. De Tymowski, F. Vigneron, D. Edouard, D. Bégin, P.Nguyen, C. Pham, S. Savin-Poncet, F. Luck, M.J. Ledoux, C. Pham-Huu, Appl.Catal. A: Gen. 397 (2011) 62–72.

[2] A. Devaraju, A. Kumar, B. Kotiveerachari, Mater. Design 45 (2013) 576–585.[3] L. Fang, X.P. Huang, F.J. Vidal-Iglesias, Y.P. Liu, X.L. Wang, Electrochem. Com-

mun. 13 (2011) 1309–1312.[4] J.B. Casady, R.W. Johnson, Solid-State Electron. 39 (1996) 1409–1422.[5] H. Tanaka, Nippon Seram. Kyo. Gak. Ronbunshi 119 (2011) 218–233.[6] M.J. Ledoux, S. Hantzer, C.P. Huu, J. Guille, M.P. Desaneaux, J. Catal. 114 (1988)

176–185.[7] T.T. Nguyen, L. Burel, D.L. Nguyen, C. Pham-Huu, J.M.M. Millet, Appl. Catal. A:

Gen. 433–434 (2012) 41–48.[8] J.M. García-Vargas, J.L. Valverde, A. De Lucas-Consuegra, B. Gómez-Monedero,

P. Sánchez, F. Dorado, Appl. Catal. A: Gen. 431–432 (2012) 49–56.[9] P. Marín, S. Ordónez, F.V. Díez, J. Chem. Technol. Biotechnol. 87 (2012) 360–367.10] B. De Tymowski, Y. Liu, C. Meny, C. Lefèvre, D. Begin, P. Nguyen, C. Pham, D.

Edouard, F. Luck, C. Pham-Huu, Appl. Catal. A: Gen. 419–420 (2012) 31–40.11] R. Philippe, M. Lacroix, L. Dreibine, C. Pham-Huu, D. Edouard, S. Savin, F. Luck,

D. Schweich, Catal. Today 147 (2009) S305–S312.12] E. Corporan, T. Edwards, L. Shafer, M.J. Dewitt, C. Klingshirn, S. Zabarnick, Z.

West, R. Striebich, J. Graham, J. Klein, Energy Fuels 25 (2011) 955–966.13] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (2007) 1692–1744.14] A.Y. Khodakov, Catal.Today 144 (2009) 251–257.15] Y. Liu, B. De Tymowski, F. Vigneron, I. Florea, O. Ersen, C. Meny, P. Nguyen, C.

Pham, F. Luck, C. Pham-Huu, ACS Catal. 3 (2013) 393–404.16] S.J. Park, S.M. Kim, M.H. Woo, J.W. Bae, K.W. Jun, K.S. Ha, Appl. Catal. A: Gen.

419–420 (2012) 148–155.17] C.M. Balonek, A.H. Lillebø, S. Rane, E. Rytter, L.D. Schmidt, A. Holmen, Catal. Lett.

138 (2010) 8–13.18] A.R. De La Osa, A. De Lucas, J. Díaz-Maroto, A. Romero, J.L. Valverde, P. Sánchez,

Catal. Today 187 (2012) 173–182.19] J. Rouquerol, D. Avnir, D.H. Everett, C. Fairbridge, M. Haynes, N. Pernicone, J.D.F.

Ramsay, K.S.W. Sing, K.K. Unger, Stud. Surf. Sci. Catal. 87 (1994) 1–9.20] S.H. Lee, S.M. Yun, S.J. Kim, S.J. Park, Y.S. Lee, Res. Chem. Intermediat. 36 (2010)

731–742.21] X. Yuan, J. Lü, X. Yan, L. Hu, Q. Xue, Micropor. Mesopor. Mater. 142 (2011)

754–758.22] N.S. Jacobson, D.L. Myers, Oxid. Met. 75 (2011) 1–25.23] A.R. De La Osa, A. De Lucas, A. Romero, J.L. Valverde, P. Sánchez, Fuel 90 (2011)

1935–1945.24] R. Bechara, D. Balloy, J.Y. Dauphin, J. Grimblot, Chem. Mater. 11 (1999)

1703–1711.25] P. Arnoldy, J.A. Moulijn, J. Catal. 93 (1985) 38–54.26] M.S. Batista, R.K.S. Santos, E.M. Assaf, J.M. Assaf, E.A. Ticianelli, J. Power Sources

134 (2004) 27–32.27] G. Jacobs, T.K. Das, Y. Zhang, J. Li, G. Racoillet, B.H. Davis, Appl. Catal. A: Gen.

233 (2002) 263–281.28] H. Xiong, Y. Zhang, K. Liew, J. Li, J. Mol. Catal. A: Chem. 295 (2008) 68–76.29] J.A. Díaz, M. Martínez-Fernández, A. Romero, J.L. Valverde, Fuel 111 (2013)

422–429.30] J.A. Díaz, R. Díaz-Moreno, L.S. Silva, F. Dorado, A. Romero, J.L. Valverde, J. Colloid

Interf. Sci. 380 (2012) 173–181.31] S.H. Kang, J.H. Ryu, J.H. Kim, P.S. Sai Prasad, J.W. Bae, J.Y. Cheon, K.W. Jun, Catal.

Lett. 141 (2011) 1464–1471.32] V. Udaya, S. Rao, R.J. Gormley, Catal. Today 6 (1990) 207–234.33] K.M. Cho, S. Park, J.G. Seo, M.H. Youn, S.H. Baeck, K.W. Jun, J.S. Chung, I.K. Song,

Appl. Catal. B: Environ. 83 (2008) 195–201.34] B. Ernst, S. Libs, P. Chaumette, A. Kiennemann, Appl. Catal. A: Gen. 186 (1999)

35] T.K. Das, G. Jacobs, P.M. Patterson, W.A. Conner, J. Li, B.H. Davis, Fuel 82 (2003)805–815.

36] L.D. Mansker, Y. Jin, D.B. Bukur, A.K. Datye, Appl. Catal. A: Gen. 186 (1999)277–296.