Strong-metal–support interaction by molecular design: Fe–silicate interactions in Fischer–Tropsch catalysts

Journal of Catalysis 289 (2012) 140–150

Contents lists available at SciVerse ScienceDirect

Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat

Strong-metal–support interaction by molecular design: Fe–silicate interactions in Fischer–Tropsch catalysts Ramoshibidu P. Mogorosi, Nico Fischer, Michael Claeys, Eric van Steen ⇑ DST-NRF Centre of Excellence in Catalysis cchange, Centre for Catalysis Research, Department of Chemical Engineering, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

a r t i c l e

i n f o

Article history: Received 20 December 2011 Revised 1 February 2012 Accepted 3 February 2012 Available online 10 March 2012 Keywords: Metal–support interactions Iron Silica Fischer–Tropsch Activity Selectivity

a b s t r a c t Metal–support interactions in the form of iron–silicate were investigated by an inverse approach, that is, modification of nano-sized iron oxide with surface silicate groups. The presence of surface silicate groups in the calcined catalyst precursor was confirmed using diffuse reflectance infra-red Fourier transform analysis. The genesis of the various iron phases in the presence of surface silicate groups after H2-activation and the Fischer–Tropsch synthesis was followed. The surface silicate groups are preserved after a hydrogen treatment at 350 °C for 16 h, and these surface ligands are associated with the residual iron oxide phase, wüstite. During the Fischer–Tropsch synthesis, a-Fe is mostly converted into v-Fe5C2, whereas FeO is the main source for e-Fe2C. The activity per unit surface area of hexagonal carbide, eFe2C, is ca. 25% higher than that of v-Fe5C2. The presence of surface silicate ligands on e-Fe2C results in a further enhancement of the rate per unit surface area of e-Fe2C by a factor of ca. 3. This is being ascribed to the enhanced availability of hydrogen on the surface due to the presence of the surface silicate groups, which also results in an increase in the methane selectivity, a decrease in the olefin content and a decrease in formation of branched product compounds. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction The catchphrase ‘‘Strong-metal–support interaction’’ or SMSI was originally introduced to describe the drastic changes in chemisorption properties of group VIII noble metals when supporting these metals on reducible oxides after high-temperature treatment [1–3] and in particular the decrease in the strength of adsorption of CO and H2 [2,4]. The change in the adsorption energies was originally ascribed to electron transfer from a partially reduced support to the metal particle [5]. However, strong-metal–support interactions have also been claimed for non-reducible supports, such as alumina [6,7] and silica [8,9]. Nowadays, strongmetal–support interaction (SMSI) seems to be a collective term describing effects introduced by the intimate contact of the support and the metal, which may be caused by specific type of pre-treatment, for example, alloy formation [10,11] or electronic effects due to presence of hydrogen [12]. The support may also bind to the surface of the catalytically active phase anchoring the catalytically active phase to the support [13]. This anchor can be considered as a ligand to the surface. The modification of the (inter)metallic surface with support-like structures as a ligand may also impart a change in the adsorption properties [14]. These anchors may be created during catalyst

⇑ Corresponding author. E-mail address: [email protected] (E. van Steen). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2012.02.002

preparation, due to the coordination of the ionic metal precursors to the support [15–17] or during the thermal pre-treatment of the catalyst [18]. These anchors are not easily cleaved during standard catalyst pre-treatment steps, such as hydrogen activation. The effect of this anchor-type of structures on the resulting catalytic activity is largely unexplored. In this study, the effect of the modification of the surface of iron nano-particles with surface silicate ligands on the catalytic activity in CO hydrogenation is investigated, which can be regarded as a model for the anchors present when using silica as a binder [19] or support [20]. Silica is typically added to iron-based Fischer–Tropsch catalysts as a binder [19–28] to reduce attrition during Fischer–Tropsch synthesis [19,23] (especially attrition induced by the chemical transformation of the catalytically active phase) in turbulent environments. The interaction of silica with iron depends on the mode of addition of silica during catalyst synthesis [24] with the addition of silica prior to calcination imparting the largest effect. The interaction between iron and silica in the catalyst may result in the formation of FeAOASi bonds [28,29], which may form in a hydrolysis reaction involving surface hydroxyl groups on iron oxide/hydroxide and silica. The addition of silica to iron-based Fischer–Tropsch catalysts suppresses that adsorption of CO on a pre-carburized sample with a simultaneous decrease in the strength of adsorption [25]. This may be attributed to strong-metal–support interaction, but it should be realized that the presence of other promoters (Cu, K) may have attributed to this change as well. On a pure, pre-carburized Fe/

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

SiO2-system, the strength of adsorption of CO as evidenced by the recombination of surface carbon with surface oxygen seems to increase with increasing silica loading [29], although at high loading of silica the peak maximum temperature in CO-TPD seems to decrease. As a consequence of the change in the strength of adsorption of CO (and thus increased hydrogen availability of the surface), the product spectrum in CO hydrogenation shifts toward lighter products [30]. Furthermore, it should be realized that the addition of silica to the sample results in a reduction in the average iron crystallite size [24–26]. Small crystallites with a size less than 6–7 nm show a reduced turnover frequency in the Fischer–Tropsch synthesis with a simultaneous shift toward lighter hydrocarbons [31,32]. Metal–support interactions are typically investigated by putting the catalytically active phase on some support followed by heat treatment or in the case of iron-based Fischer–Tropsch catalyst by varying the amount of binder in the sample [19–29]. In this study, a novel approach is taken by modifying the surface of the catalyst precursor with covalently bonded ligands mimicking the metal–support interactions. The effect of these ligands anchored to the surface of the catalyst precursor may lead to the formation of surface modified nano-crystallites during activation and/or the Fischer–Tropsch synthesis. The use of surface modified nano-crystallites, a well-documented approach for biomedical [33], and material science [34–36] applications, may find its way into the field of catalysis as model systems or even as an improvement of the catalytically active phase. The resulting activity and selectivity in the Fischer–Tropsch synthesis over the surface modified nanocrystallites will be reported. 2. Experimental 2.1. Synthesis of model systems Nano-sized iron crystallites were prepared by precipitation in a water-in-oil micro-emulsion (reverse micelles) [31,37] Two microemulsions both in n-hexane with polyethylene dodecyl glycol ether (PEDGE; Berol 050) containing an 0.5 M aqueous iron nitrate solution and 2 M aqueous ammonium carbonate solution, respectively, were prepared with water-to-surfactant ratio of 0.3 g/g. In the absence of silicate modification, this should yield Fe2O3 crystallites with an average diameter of ca. 10 nm and a narrow crystallite size distribution [37]. The two micro-emulsions were mixed under stirring at 800 rpm for 3 h. Due to collision and coalescence of the droplets, the metal salt is brought into contact with the precipitation agent. The precipitate remains confined to the interior of the micro-emulsion droplets, and the emulsions are separated from the liquid phase by flocculating with acetone. The precipitate is subsequently washed with acetone to remove the surfactant. The washed precipitate is re-dispersed in n-hexane and a solution containing 1 vol.% of tetra-ethoxy silane (TEOS) in n-hexane is injected. The precipitate was separated from the slurry by centrifugation and dried at room temperature for 3 h, followed by drying at 120 °C for 24 h. The precipitate is finally calcined at 300 °C for 3 h in a fixed bed reactor in air (flow rate: 300 ml (NTP)/min; heating rate: 10 °C/min). The ex situ reduction was performed in a fluidized bed at 350 °C for 16 h in hydrogen (space velocity 400 ml (NTP)/min/g; heating rate 0.5 °C/min). For characterization purposes, the reduced sample was passivated by passing gaseous CO2 over the sample for 1 h.

141

ergy dispersive Fissons Kevex X-ray spectrometer) and using Xray fluorescence (Bruker S4 Explorer XRF). The average crystallite size in the calcined samples was determined using transmission electron microscopy (TEM) images taken on a LEO 912 OMEGA. The samples were crushed in methanol using a glass rod. Two drops of the methanol with the fine particles dispersed were dropped on carbon-coated copper grids. The specific surface area and pore size distribution of the calcined samples were determined using N2-physiosorption on a Tristar-Micromeritics apparatus. Ca. 0.5 g of the sample was degassed under N2-flow at 393 K overnight prior to measurements at 77 K using an equilibration time of 10 s. The phase composition of the calcined, reduced and spent catalysts was determined using a Bruker AXS D8 Advance X-ray laboratory diffractometer utilizing a Co source (kCo Ka1 = 0.178897 nm) and a VÅNTEC position-sensitive detector. The average crystallite sizes were determined by Rietveld refinement of the X-ray diffractograms in TOPAS 4.2 (Bruker AXS). Furthermore, the reduction of the calcined samples in hydrogen was investigated using in situ XRD (SV: 400 ml (NTP)/min/g; heating rate 0.5 °C/min). IR spectra were obtained using an FTIR Nicolet 5700 spectrometer (for the calcined and reduced catalyst in the diffuse reflectance infrared Fourier transform (DRIFTS) mode and for the spent catalyst in the transmission mode). Ten milligrams of each sample was diluted with 1 g of KBr. The spent catalyst was pressed into a translucent wafer for measurement in the transmission mode. A total of 1000 scans were taken in the range between 500 and 4000 cm1 for each sample at a resolution of 4 cm1. The reducibility of the calcined materials was determined by temperature programmed reduction (TPR) using Micromeritrics Autochem 2950 apparatus. About 0.03 g sample was loaded in a quartz tube. The catalyst was dried in argon (52 ml (STP)/min) for 30 min at 120 °C after which the system was allowed to cool down to 50 °C. Gas supply was then switched from Ar to a 5.6 vol.% H2AAr mixture (50 ml (STP)/min). The temperature was ramped at 10 °C/min from 50 °C to 900 °C. The degree of reduction of the samples was determined by TPR, after reduction of hydrogen at 350 °C for 16 h in pure hydrogen (400 ml (NTP)/min/g). 2.3. Catalytic activity and selectivity testing The effect of silicate–iron interaction on the activity and selectivity in the Fischer–Tropsch synthesis was explored in a fixed bed reactor under industrially relevant conditions for the lowtemperature iron-catalyzed Fischer–Tropsch synthesis. The calcined material (m = 0.1 g diluted with 6 g SiC) was in situ calcined in argon (30 ml (NTP)/min) and subsequently reduced in hydrogen (40 ml (NTP)/min). Both treatments were performed at 350 °C for 16 h. After reduction, the catalyst was cooled down to reaction temperature (250 °C). The Fischer–Tropsch synthesis was performed at 20 bar with synthesis gas (60 ml (NTP)/min; (H2:CO)inlet = 2). The conversion of CO and H2 was monitored using an on-line GC-TCD. The exit gas product stream was sampled using the ampoule technique [38] and analyzed using an off-line GC-FID. After 24 h on stream, the catalyst was cooled down in inert gas (Ar) and passivated by flowing CO2 over the catalyst for 1 h at room temperature. After the Fischer–Tropsch synthesis the catalyst was recovered and separated from the silicon carbide using a 100 lm sieve. 3. Results and discussion

2.2. Characterization of model system 3.1. Calcined samples The composition of the calcined materials was determined using energy dispersive X-ray spectroscopy (SEM-EDX, LEO S440 equipped with a four quandrant back scatter detector and an en-

The silicon-to-iron content was varied between 0 and 98 mmol/ mol. The silicon content determined using EDX correlates well

142

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

thermodynamically more stable phase under the conditions applied here [40]. However, surface energy contribution modifies the relative phase stability significantly [39–42], and maghemite becomes the more stable phase for nano-sized crystallites [39,40], which transforms into hematite upon sintering. The transformation of maghemite to hematite has to obey the thermodynamic constraint that the Gibbs free energy of the system is at a minimum. Hence, the critical diameter for maghemite (dmaghemite,c) beyond which the transformation to hematite is thermodynamically allowed is given by

with XRF-measurements. The calcined materials have a predominantly spherical morphology (see Fig. 1) with a narrow size distribution. The EDX-measurement indicated a rather even distribution of silicon over the sample. Iron in these calcined materials is mainly present as maghemite (c-Fe2O3) with a smaller amount of hematite (a-Fe2O3) present as well. The volume averaged crystallite size determined from TEMimages correlates well with the crystallite size of the dominant maghemite phase present in the samples as determined by XRD. The average crystallite size does not correlate well with the BET-

6c

6c

M

M

Fe2 O3 Fe O hematite lhematite  lmaghetite ¼ 0l0hematite  l0maghetite þ dhematite;c  d maghemiteq 2 3 ¼ 0 qhematite maghemite;c maghemite

dmaghemite;c ¼ 



6cmaghemite MFe

2 O3



l0maghetite l0hematite qmaghemite

chematite cmaghemite





 qmaghemite 2=3 qhematite

surface area determined using N2-physiosorption (as also shown by Yen et al. [39]). This might be a result of the coalescence of individual crystallite over an amorphous layer. The average crystallite size of hematite is systematic larger than the crystallite size of maghemite (see Table 1). Hematite is the



1

Here, l is the chemical potential (in J/mol; with l0 the chemical potential at standard conditions), dmaghemite is the diameter of the maghemite crystallite (in m), c is the surface energy (in J/m2), M is the molar mass of Fe2O3 (in g/mol), and q is the density of the respective phases (in mol/m3).

200 nm

200 nm

Si/Fe = 0 mmol/mol 200 nm

Si/Fe = 30 mmol/mol 200 nm

Si/Fe = 48 mmol/mol

Si/Fe = 10 mmol/mol 200 nm

Si/Fe = 39 mmol/mol 200 nm

Si/Fe = 98 mmol/mol

Fig. 1. TEM-images of the calcined samples with different silicon content.

143

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150 Table 1 Physio-chemical characteristics of the calcined samples. Si/Fe, mmol/mola 2

0

10

30

39

48

98

SBET, m /g

34.4

6.5

70.6

–

27.2

50.9

dpore, nmb

6.7

15.1

8.3

–

10.8

6.4

dTEM, nm

12.4 ± 3.4

14.9 ± 4.9

11.8 ± 3.2

9.6 ± 2.1

8,5 ± 1.7

6.9 ± 1.2

12 17

23 26

9 15

11 16

– –

– –

88 11

76 16

91 11

88 11

100 9

100 8

a-Fe2O3 wt.% dXRD, nm

c-Fe2O3 wt.% dXRD, nm a b

Silicon content relative to iron as determined using EDX. Average pore diameter determined from BJH-adsorption branch.

It can be estimated using a non-size-dependent surface energy that for non-hydrated surfaces, this transition at the calcination temperature applied here occurs for maghemite crystallites of ca. 19.4 nm (the corresponding crystallite size of the hematite crystals is 18.9 nm). For hydrated surfaces, this transformation may occur for much smaller maghemite crystallites, but the accurate transition size requires the incorporation of size-dependent surface energies. In this study, the average size of the hematite crystallites after calcination in the sample not containing any silicon was ca. 17 nm with maghemite crystallites being ca. 11 nm. This is well in line with the predicted thermodynamic transition point. It can be noticed that the average crystallite size decreases with increasing silicon content with exception for the material with Si/ Fe = 10 mmol/mol, and as a consequence the amount of iron present as maghemite increases with increasing silicon loading. The iron oxide phases, maghemite and hematite, are formed during the calcination process, since the dried sample contains solely small crystallites of two-line ferrihydrite. Hence, the variables in the calcination process control the crystallite size distribution and thus the phases present. The controllable narrow crystallite size distribution [31,37] is presumably induced by residual surfactant in the sample. Silicon added to the sample in the form of tetraethoxy silane apparently inhibits the sintering process resulting in an on average smaller crystallite size. It might be argued that termination of nano-crystallites with silicate groups protects the nano-crystallites against further growth [28,29].

The calculated surface loading of silicon is estimated to be between 0 and 4.5 atoms of Si per nm2, if it can be assumed that all silicon is attached to the iron oxide surface and not agglomerated as (amorphous) silica within the samples. Fig. 2 shows the DRIFTSspectra of the calcined samples. The main adsorption bands are in the ranges 3000–3400 cm1 (OH stretching frequency mainly from adsorbed water [43]), 1300–1550 cm1 (possibly due to carbonaceous material left from the surfactant used [44], although also observed in superparamagnetic magnetite [43]), and bands below 1050 cm1. Upon the addition of tetra-ethoxy silane to the sample preparation, a shoulder at the band of 755 cm1 (previously observed for superparamagnetic iron oxide [43] and possibly due to FeAOAH stretching [45]) starts developing at ca. 870 cm1. With increasing silicon loading, the adsorption at this frequency becomes more dominant and shows a blue shift. Adsorption bands in this region are typically ascribed to the FeAOASi stretching frequency [28,29,43,46,47], with the adsorption at 940–952 cm1 being ascribed to a bidentate ((„FeO)2ASi(OH)2) [46,47]. The blue shift observed in this band might be attributed to a change from tridentate, bidentate to monodentate bonding of the surface silanol group. It should, however, be noted that isolated silanol groups will also show an adsorption band at 960 cm1 [48]. The typical adsorption band for the asymmetric SiAOASi vibration at ca. 1090 cm1 [49] is absent implying that at most a minimal amount of tetraethoxy silane is converted into amorphous silica, although the formation of oligomeric silicate species, which yield an adsorption

monomeric, bidentate (≡FeO) 2-Si(OH)OH [41,42]

Si/Fe (mmol/mol):

Si/Fe (mmol/mol):

48 98 39

Abundance, a.u.

Absorbance. a.u.

98

30

10

48 39 30 10 0

0 3400

2400

1400

wavenumber, cm-1

400

1250

1000

750

wavenumber, cm-1

Fig. 2. DRIFTS-spectra of the calcined samples modified with tetra-ethoxy silane.

144

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

band at ca. 1020 cm1 [46], cannot be excluded in the samples with a high Si/Fe-content. It can thus be concluded that the surface modification of the iron oxide nano-particles was successful. 3.2. Hydrogen activation The activation of iron-based Fischer–Tropsch catalysts can be performed in hydrogen, carbon monoxide, or synthesis gas. In this study, a hydrogen activation step at 350 °C for 16 h was chosen. The effect of the modification of the samples by addition of tetra-ethoxy silane on the hydrogen activation was investigated using temperature programmed reduction (see Fig. 3). The temperature programmed reduction profile of the unmodified sample shows the typical two-step reduction observed for the reduction of hematite [50,51] and maghemite, which is typically ascribed to the consecutive process Fe2O3 ? Fe3O4 ? Fe with the first reduction step consuming 11.1% of the total hydrogen consumption. Our study indicates that the reduction of maghemite to magnetite proceeds typically at a lower temperature than that of hematite, whereas the reduction of magnetite to metallic iron is seemingly unaffected by the origin of the magnetite phase. An intermediate step in the reduction of magnetite to metallic iron via FeO can be observed above 570 °C (the temperature above which wüstite is stable against its disproportionation into magnetite and metallic iron) [27,51], if the reduction process is kinetically limited (e.g., by applying a high heating rate). With increasing silicon loading, the reduction peak typically ascribed to the reduction of Fe2O3 to magnetite first shifts toward higher temperatures, then to lower temperatures and at higher silicon loadings back toward higher temperatures. This peculiar kinetic behavior during the H2-activation can be understood in terms of the phase composition of the calcined sample and their modification with silicon. The initial shift toward a higher reduction temperature observed with the sample containing 10 mmol Si/mol Fe might be ascribed mainly to a change in the phase composition accompanied by the increase in the average crystallite size

(see Table 1). The silica surface modification seems to inhibit the first reduction step as can be seen in samples with Si/Fe > 39 mmol/mol. The main difference in the TPR-profiles of the samples modified with tetra-ethoxy silane is in the range between 450 and 800 °C. The unmodified sample shows a main peak with a maximum at ca. 570 °C with a shoulder at ca. 710 °C. With increasing silicon loading, this main peak diminishes in intensity and the shoulder becomes the dominant feature in the TPR-profile. The shift toward higher reduction temperatures will be accompanied by a change in the reduction pathway, with the pathway via FeO becoming dominant [51]. This indicates that the silicon modification retards the transformation of magnetite; the high reduction temperatures required to transform magnetite into metallic iron then proceeds via wüstite. At high silicon loading (Si/Fe > 48 mmol/mol), a new high-temperature peak starts appearing at ca. 840 °C. In the case of the sample containing 48 mmol Si/mol Fe, the amount of hydrogen consumed corresponding to this peak was 26 mmol/mol Fe, and in the case of the sample containing 98 mmol Si/mol Fe, the amount of hydrogen consumed amounted to 100 mmol/mol of Fe. This may indicate that the hydrogen consumed at temperatures larger than 800 °C is linked to the presence of silicon in the sample. The effect of hydrogen activation was investigated using in situ XRD using pure hydrogen (SV: 400 ml (NTP)/min/g; heating rate 0.5 °C/min). Fig. 4 shows the XRD-patterns recorded during the reduction for the unmodified sample and the sample containing 98 mmol Si/mol Fe. The XRD-patterns during the reduction of the unmodified sample (0 mmol Si/mol Fe) confirm the consecutive nature of the H2-activation process, viz. Fe2O3 ? Fe3O4 ? Fe with the metallic iron phase starting to appear at ca. 300 °C. The first reduction step of the silica modified sample in the reduction of

Si/Fe = 0 mmol/mol

γ-Fe2O3 Fe3O4

340 oC 570 oC 700 oC

Si/Fe mmol/mol

840 oC

98

Hydrogen consumption, a.u.

α-Fe

350 350 250 150 50 20

48

40

60

80

Diffraction angle (Co-K α ), 2Θ , o

Si/Fe = 98 mmol/mol

39

FeO (wüstite) α-Fe γ-Fe2O3

30

Fe3O4

Fe3O4

350 350 250 150 50

10

0 20 0

200

400

600

800

1000

Temperature, oC Fig. 3. Temperature programmed reduction profiles of the tetra-ethoxy silane modified samples after calcination in air (50 ml (NTP)/min 5% H2/Ar; msample  0.03 g; heating rate 10 °C/min).

40

60

80

Diffraction angle (Co-K α ), 2Θ , o Fig. 4. In situ XRD of the hydrogen reduction of the unmodified sample (top) and the sample containing 98 mmol Si/mol Fe (bottom). Reduction: pure hydrogen, 400 ml (NTP)/min/g; heating rate: 0.5 °C/min – only main diffraction peaks indicated.

145

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

Table 2 Physio-chemical characteristics of the hydrogen reduced samples (reduction at 350 °C for 16 h in H2 (400 ml (NTP)/g/min). Si/Fe, mmol/mola

10

30

39

48

98

92

77

62

61

58

57

100 39

94 33

71 30

69 31

61 29

52 27

Fe3O4 wt.% dXRD, nm

– –

6 10

– –

– –

– –

– –

FeO wt.% dXRD, nm

– –

– –

29 12

31 10

39 11

48 7

% Reductionb

0

a-Fe wt.% dXRD, nm

a

Silicon content relative to iron as determined using EDX. Degree of reduction determined by the hydrogen consumption using TPR (50– 900 °C) of a previously reduced sample. b

Si/Fe (mmol/mol): 900 cm-1 98

48

Absorbance, a.u.

Fe2O3 to Fe3O4. Subsequently, at a temperature of ca. 325 °C both FeO and a-Fe are formed in an apparently parallel process. It should be noted that the transformation of magnetite is not severely hindered by the modification of the sample with tetra-ethoxy silane. However, silicon in the sample stabilizes the FeO-phase at temperatures substantial below its critical temperature for disproportionation of wüstite into magnetite and metallic iron (570 °C). The disproportionation of FeO (4FeO ? Fe3O4 + Fe) is kinetically feasible at temperatures higher than 150 °C in hydrogen containing atmospheres, and at temperatures higher than 200 °C in inert atmospheres [51]. The silicon-containing samples were left at 350 °C for 16 h in hydrogen. It must thus be concluded that the disproportionation is either severely kinetically hindered by the silicon modification or that the ligands introduced by the silicon modification lower the surface energy of nano-sized FeO and thus disallowing the disproportionation into magnetite and a-Fe thermodynamically. Table 2 shows the phase composition of the reduced, modified samples (reduction in pure H2, 400 ml (NTP)/min/g, 350 °C, 16 h). With increasing amount of tetra-ethoxy silane added to the sample, the amount of metallic iron and the average crystallite size decreases. The average crystallite size of a-Fe is much larger than the original hematite/maghemite crystallite sizes implying severe sintering during the H2-activation step. However, the sintering process is somewhat impeded for the samples containing silicon. This might be ascribed to the physical separation of the metallic iron crystallites due to the presence of the oxide phase. The relative amount of iron oxide in the sample increases with increasing silicon loading showing that the silicon modification leads to a retardation of the reduction process. The resulting crystallite sizes of the iron oxide phases decrease with increasing silicon content and correlate reasonably with the average crystallite size of maghemite in the original sample, taking the size reduction due to oxygen removal into account. This implies that the iron oxide phases are residuals of individual maghemite crystallites in the original sample. If all silicon is associated with the iron oxide phases, the ratio of silicon to surface iron for the samples containing FeO as the iron oxide phase is ca. 1.20 ± 0.06 (assuming an surface iron density of 10.7 Fe/nm2 for FeO(1 0 0)), which can be taken as an indication that the wüstite crystallites can be fully covered with surface silicate structures (in addition to possibly some amorphous silica). The presence of FeAOASi bonds in the reduced sample was confirmed using DRIFTS (see Fig. 5). With increasing silicon loading, an adsorption band at ca. 900 cm1 starts to become more intense. A weak adsorption band in the sample containing 10 mmol Si/mol Fe appears at ca. 949 cm1, whereas the stronger adsorption band ap-

30

10

0

3400

2400

1400

400

wavenumber, cm-1 Fig. 5. DRIFTS-spectra of the reduced, and subsequent passivated samples (reduction: pure hydrogen, 350 °C, 16 h, 400 ml (NTP)/min/g; heating rate: 1 °C/min; passivation in flowing CO2 at room temperature for 1 h).

pears in samples with a higher Si/Fe ratio at ca. 900 cm1. This might be ascribed to the FeAOASi stretching frequency, with iron incorporated in different iron oxide phases. The FeAOASi stretching with Fe incorporated into Fe2O3 yields an adsorption band at 940–952 cm1 [28,29,43,46,47]. A lowering in the oxidation state of iron will result in a shift in the FeAOASi stretch toward lower frequency. The adsorption band becomes wider with increasing silicon loading possible due to SiAOAH stretch in isolated silanol groups [48]. The typical adsorption band for the asymmetric SiAOASi vibration at ca. 1090 cm1 [49] is absent implying that at even after reduction at most, a minimal amount of cross-linked silica is present. Hence, it can be concluded that silicon is still (mostly) present as surface ligands possibly attached to the remaining iron oxide phase in the sample. 3.3. Characterization of the spent catalyst samples The catalyst samples were subjected for a period of 24 h to industrial conditions of iron-based Fischer–Tropsch synthesis (viz. 250 °C, 20 bar, (H2/CO)inlet = 2). After the Fischer–Tropsch synthesis, the catalyst samples were passivated and separated from the diluent, silicon carbide. The recovered spent catalyst was weighed and subsequently analyzed. Fig. 6 shows the XRD-pattern of the spent catalysts with the presence of a residue of long chain hydrocarbons (wax), which was found on the spent catalyst after Fischer–Tropsch synthesis as evidenced by the diffraction peaks at 2H = 25° and 28°. The relative intensity of these peaks decreases with increasing Si loading indicating qualitatively a lower amount of wax attached to the catalyst when the sample was modified with tetra-ethoxy silane. The sample with Si/Fe = 0 mmol/mol contains some amorphous carbon as evidenced by the diffraction peak at ca. 31°. Furthermore, the separation of the diluent from the catalyst sample was not always perfect, and diffraction peaks attributable to SiC are present indicating the presence of a low amount of the diluent SiC. Table 3 shows the abundance of the various iron phases present in the catalyst samples and their average crystallite sizes. Magnetite is present as the iron oxide phase in the spent catalyst samples

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150 Si/Fe, mmol/mol 98 48 39 30 10 0

Fig. 6. XRD patterns of the spent, and subsequent passivated samples after exposure for 24 h to industrially relevant Fischer–Tropsch conditions (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g).

Table 3 Iron phases in the spent catalyst and their average crystallite sizes as determined using Rietveld refinement of the X-ray diffractogram (catalyst exposed for 24 h to synthesis gas (SV = 600 ml (NTP)/min/g; (H2:CO)inlet = 2) at 20 bar and 250 °C). Si/Fe, mmol/mola

0

10

30

39

48

98

Fe3O4 wt.% dXRD, nm

– –

– –

–b –

3 9

7 9

22 7

Fe5C2 wt.% dXRD, nm

100 13

62 17

61 15

60 14

62 13

53 14

Fe2C wt.% dXRD, nm

– –

38 22

39 17

37 22

31 23

25 22

Scarbide, m2/gsamplec

46.4

33.8

38.7

36.6

36.3

24.7

a

Silicon content relative to iron as determined using EDX. Trace of magnetite. Surface area of carbide phases based on average crystallite size obtained from XRD per unit mass of the calcined sample. b

c

with a high Si/Fe ratio, whereas the reduced catalyst showed the presence of FeO implying oxidation of wüstite to magnetite during the Fischer–Tropsch synthesis, which is thermodynamically feasible for the bulk compounds at pH2/pH2O < 890 (or in the absence of water–gas shift activity at XCO > 0.2%). It was deduced that the original FeO crystallites were fully covered with silicate ligands (vide supra). The average crystallite size of the resulting magnetite crystallites are in accordance with the average crystallite size of the original wüstite crystallites suggesting that the oxidation of wüstite to magnetite occurs without sintering. The silicon cannot be solely associated with the magnetite phase, since the calculated surface coverage, if all silicon was to be associated with the magnetite phase, would result in more than one silicate group being associated with a surface iron in magnetite (assuming a surface density of 9.8 Fe/nm2 for Fe3O4(1 1 1)). The main iron phases in the spent catalyst samples are the iron carbide phases (see Table 3). It has been argued that the active Fischer–Tropsch catalyst consists of a core of Fe3O4 surrounded by a layer of iron carbide [52]. The absence of (XRD-visible) magnetite in the samples with Si/Fe < 30 mmol/mol does not support this model. This may originate from the method of catalyst activation (H2-activation in this study vs. CO activation [52]) or the differences in the average crystallite size in the catalyst, since the stability of the various iron phases is dependent on the applied reaction conditions [52] and the crystallite size [41,42]. The unmodified sample only contains Hägg carbide (v-Fe5C2), whereas the silicon containing catalyst samples also contain

hexagonal iron carbide (e-Fe2C). Hägg carbide is commonly observed as one of the carbide phases in iron-based Fischer–Tropsch catalysts. The molar fraction of iron present as Hägg carbide in the spent catalysts corresponds to the molar fraction of iron present as metallic iron in the reduced catalyst for the catalyst samples with a high silicon content (Si/Fe = 48 and 98 mmol/mol) (see Fig. 7) suggesting that in these samples as well Hägg carbide is formed upon exposing a-Fe to synthesis gas. The molar Fe-content in Hägg carbide is slightly lower than the Fe-content in a-Fe in the corresponding reduced sample for spent samples with a lower Si/Fe ratio implying that some of a-Fe is transformed into either eFe2C or Fe3O4. The average crystallite size of Hägg carbide is in all cases substantially smaller than the average crystallite size of the original a-Fe crystallites implying crystallite break-up during carburization due to strain within the crystal introduced by diffusion of carbon into the structure. Carbides with carbon in trigonal bipyramide interstices (TP-carbides), such as Hägg carbide, are enthalpically less stable than carbides octahedral interstices (O-carbides), such as e-Fe2C [53] (with the exception of g-Fe2C [53,54]). The formation of e-Fe2C, or e0 Fe2.2C, two phases which are difficult to distinguish by XRD [54], can be found in silica-supported systems [25,27,55,56]. It seems from our study that a surface modification of the starting iron oxide results in the formation of the enthalpically favored e-F2C. It might be argued that the surface ligands stabilize the hexagonal carbide further. However, the relatively large size of these carbide crystallites (17–23 nm) would result in a relative small effect of surface modification on the bulk crystal stability. Hence, it is deduced that the formation of e-Fe2C is thermodynamically favored and becomes kinetically favored due to a retardation of the carbon diffusing into the iron matrix allowing for sufficient time for lattice expansion leading to formation of O-carbides. e-Fe2C originates, at least for a significant part, from the silicon stabilized FeO-phase. The average crystallite size of e-Fe2C is larger than the size of the original wüstite phase implying that the transformation is associated with sintering. Hence, the protection of the crystallite by surface silicate ligands must have been lost during the Fischer–Tropsch synthesis. It can be postulated that the product water of the Fischer–Tropsch synthesis may result in hydrolysis of some of the silicate ligands resulting in the formation of amorphous silica. The (partially) naked oxide surface may sinter via condensation between hydroxyl groups [42], which upon carburization yields e-Fe2C (see Scheme 1). It might further be

1

(nFe, phase/nFe,total)spent catalyst

146

0.8

α-Fe

χ-Fe5C2

FeO

Fe3O4

FeO

ε-Fe2C

0.6

0.4

0.2

0

0

0.2

0.4

0.6

0.8

1

(nFe, phase/nFe,total)reduced catalyst Fig. 7. Correlating the phases present in the reduced catalyst (350 °C, pure hydrogen, 1 atm, SV = 400 ml (NTP)/min/g) with the phases present in the catalysts exposed for 24 h to industrially relevant Fischer–Tropsch conditions (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g).

147

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

Reduced catalyst

with increasing silicon loading of the sample, possibly associated with the formation of oligomeric silicate species [46]. The presence of amorphous silica cannot be confirmed beyond doubt based on the typical adsorption band for the asymmetric SiAOASi vibration (at ca. 1090 cm1 [49]). Numerically, all silicon modified samples with exception of Si/Fe = 10 mmol/mol should have some amorphous silica, since the calculated surface coverage of e-Fe2C with silicate ligands would exceed full coverage (assuming a surface iron density of 9.2 Fe/nm2 taking only the top surface Fe-atoms on the ridge of the corrugated e-Fe2C(1 1 1) surface due to geometric constraints imposed by the silicate ligand and assuming that all Fe3O4 crystallites are fully covered with silicate ligands). The sample with Si/Fe = 10 mmol/mol would have a coverage of the Fe-atoms on the ridge of the e-Fe2C(1 1 1) surface of 0.55, if no amorphous silica was formed during the formation of the hexagonal iron carbide phase or the Fischer–Tropsch synthesis.

Fischer-Tropsch conditions Fe5C2

α

+ (H2)/CO Fe5C2

Fe5C2

ε-Fe2C

Fe3O4

+ (H2)/CO + H2O

FeO

+ H2O

silica

3.4. Catalytic performance

Fe3 O4

Scheme 1. Transformation of a-Fe and FeO stabilized by silicate surface ligands during the Fischer–Tropsch synthesis.

argued that not all silicate groups are removed from the surface, since rapid diffusion of carbon into the oxide structure will lead to Hägg carbide and cementite [57]. These surface silicate ligands may, however, become progressively hydrolyzed during the Fischer–Tropsch synthesis. Fig. 8 shows the IR-spectra of the spent catalyst. The spectra show adsorption bands due to the presence of wax, and the spectra are further complicated due to the presence of amorphous carbon. The spectra for the samples containing silicon do show a feature at ca. 890 cm1, which might be ascribed to silicate groups attached to the carbide phase. The IR-spectrum for the sample with Si/ Fe = 98 mmol/mol shows a shoulder at ca. 950 cm1 typically associated with silicate adsorbed on magnetite [28,29,43,46,47]. Furthermore, an adsorption band at ca. 1025 cm1 starts appearing

The catalysts were tested under typical industrially relevant conditions for iron-based low-temperature Fischer–Tropsch synthesis. Fig. 9 shows the CO-conversion as a function of time on line. The observed trends are very comparable to the results obtained during the FT-synthesis over Fe/SiO2 [29], with a strong, initial decline in CO-conversion for the catalysts with a low silicon content (Si/Fe < 10 mmol/mol). It might be argued that the CO-conversion after 24 h on line is also affected by the high initial CO-conversion. However, similar levels of CO-conversion after 24 h on line were obtained with the catalyst containing no silicon after slowly increasing the reaction temperature from 220 °C to 250 °C to avoid a possible, high initial exotherm, that is, the CO-conversion after 24 h on line is independent of the start-up conditions for this catalyst. The CO-conversion increases with increasing time on line for the catalysts containing silicon. It can be further noted that the increase in the CO-conversion level seems to shift toward longer time on line with decreasing levels of CO-conversion. This might be attributed to a slow three-step transformation process of silicate covered wüstite to e-Fe2C under the applied Fischer–Tropsch

Si/Fe (mmol/mol):

1025 cm -1

890 cm -1

Si/Fe (mmol/mol): 98

absorbance, a.u.

absorbance, a.u.

98

48

30

48

30

10

10 0

0 3400

2400

1400

wavenumber, cm -1

400

1250

1000

750

wavenumber, cm-1

Fig. 8. IR-spectra (transmission mode) after exposing the catalysts for 24 h to industrially relevant Fischer–Tropsch conditions (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g).

148

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

CO-conversion, XCO, %

50 Si/Fe (mmol/mol):

40

0 10 30 39 48 98

30 20 10 0 10

100

1000

Time on line, min Fig. 9. CO-conversion as a function of time on line in the Fischer–Tropsch synthesis under industrially relevant conditions (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g).

conditions (see Scheme 1), with the first step being the oxidation of wüstite to magnetite by the product water. The change in the CO-conversion over FeASiO2 systems was previously ascribed to an effect of the average crystallite size of the catalytic activity [29]. The average crystallite sizes of the various phases present in the spent catalysts are very similar and can thus not be the origin in the variation of the CO-conversion. The integral rate of the Fischer–Tropsch synthesis per unit surface area of carbide (v-Fe5C2 or e-Fe2C) passes a maximum as a function of the silicon content in the sample (see Fig. 10). The specific rate of e-Fe2C can be deduced by assuming that v-Fe5C2 in all samples has a similar specific rate per unit surface area. The assumption that the rate of the Fischer–Tropsch synthesis over Hägg carbide is unaffected by the silicon modification in the sample is reasonable, since it was made plausible that metallic iron in the reduced catalyst was the source of Hägg carbide, whose surface was not modified with silicate (vide supra). The specific integral rate of e-F2C per unit surface area for the sample containing Si/ Fe = 10 mmol/mol is ca. 25% higher than that of v-Fe5C2, whereas the activity for specific integral rate of e-Fe2C per unit surface area for the samples containing more silicon (Si/Fe > 10 mmol/mol) is almost four times higher than the specific rate of v-Fe5C2. It might be argued that the specific activity of e-Fe2C obtained with catalyst with Si/Fe = 10 mmol/mol represents that of an (almost) bare eFe2C surface, since the CO-conversion over the catalyst with Si/ Fe = 10 mmol/mol was initially high resulting in an initial high

water partial pressure. This may have led to removal of almost all silicate surface ligand by hydrolysis. The site density on eFe2C(1 1 1), defined as the number of surface iron atoms per unit area, is ca. 5% higher than on v-Fe5C2(1 0 0), which may explain some of the observed enhanced activity. However, the observed rate of the Fischer–Tropsch synthesis is a complex variable depending amongst others on the strength of CO and H2 adsorption, surface coverage of the adsorbed species and the activation barrier for surface CHx-species hydrogenation [58–60]. The samples with high silicon content show a much higher rate of the Fischer–Tropsch synthesis over e-Fe2C per unit surface area (ca. 2.9–3.5 times higher than the rate per unit surface area of eFe2C obtained with Si/Fe = 10 mmol/mol). This might be ascribed to a promotional effect of the surface silicate groups as ligands. The addition of silica to Fe-based catalyst results in a change in the CO adsorption on a pre-carburized catalyst [25,29]. A weakening of the strength of CO adsorption may result in an increase in the rate of Fischer–Tropsch synthesis [60,61]. The Fischer–Tropsch synthesis requires the removal of oxygen from the surface active for the formation of organic product compounds, leading to the formation of the co-products water or carbon dioxide [60,61]. The product water may in a secondary reaction react further with CO in the water–gas shift reaction yielding CO2. The latter is typically linked with the presence of magnetite in the catalyst sample [62,63]. Fig. 11 shows the CO2 selectivity as a function of the CO-conversion in order to distinguish between primary and secondary effects. The Fischer–Tropsch synthesis over the unmodified catalyst resulted in a very low CO2 selectivity (ca. 3 C-%), whereas the reaction over modified catalysts yielded a CO2 selectivity in the range between 6.5 and 8.5 C-%. The higher CO2 selectivity for the silicon containing sample containing Si/Fe = 10 mmol/mol compared to the unmodified sample despite the lower level of CO-conversion may reflect a higher probability of oxygen removal via CO2 formation than via H2O formation. It is estimated that the probability for oxygen removal as CO2 is ca. 3% for Hägg carbide and ca. 14% for e-Fe2C. The CO2 selectivity for the samples with Si/Fe = 30 mmol/mol is slightly lower than the selectivity obtained over the catalysts with Si/Fe = 10 mmol/ mol despite the higher CO-conversion. This implies a lowering of the probability of O-removal as CO2 neglecting secondary formation of CO2. The CO2 selectivity increases further with increasing silicon content with a concomitant lower CO-conversion. The integral rate of CO2 formation over the samples with high silicon content cannot be linked to the increase in the surface area of Fe3O4, which increases strongly with increasing silicon content, implying

20

3

per m2 of ε-Fe2C

2

2

per m of carbide

1

0

per m of χ-Fe5C2 2

CO2 -selectivity, SCO2, C-%

-rFT, integral , μmol/m2/s

4

15

10

(98) (48)

(10)

(39)

5

(30)

(0) 0

20

40

60

80

100

Si/Fe, mmol/mol

0

0

10

20

30

40

50

CO-conversion, XCO, mol-% Fig. 10. Integral rate of the Fischer–Tropsch synthesis (CO consumption for the formation of organic product compounds) as a function of the silicon content in the catalyst samples (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g – curves drawn as a guide).

Fig. 11. CO2 selectivity as a function of the level of CO-conversion after 24 h on line in the Fischer–Tropsch synthesis (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/ min/g – numbers in brackets indicate Si/Fe in mmol/mol).

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

a higher probability of O-removal as CO2 possibly due to the interaction of surface OH-groups or product water with surface silicate groups. Differences in the selectivity of the surface polymerization reaction may give insight in subtle differences in catalyst behavior [30,64,65] through changes in its complex, though regular product distribution. The product spectrum is not only determined by the primary processes taking place on the catalyst surface, but also by secondary reactions. Hence, the selectivity of the Fischer–Tropsch synthesis is discussed as a function of the conversion of CO (see Fig. 12). 10

Methane selectivity, SCH4, C-%

A 8 (98) 6

4 (48) (10) (0)

2

(30) (39)

0

0

10

20

30

40

50

CO-conversion, X CO , mol-% 100

Olefin content in n-C5 hydrocarbons, mol-%

B 80 (0) 60

(39)

(10) (98)

(30)

(48)

40

20

0

0

10

20

30

40

50

CO-conversion, X CO , mol-% 0.6

iso-C5 /n-C5, mol/mol

C (10) (0)

0.4

(30) (39) 0.2

(48) (98)

0

0

10

20

30

40

50

CO-conversion, X CO , mol-% Fig. 12. Selectivity characteristics as a function of the level of CO-conversion after 24 h on line in the Fischer–Tropsch synthesis (250 °C, 20 bar, (H2/CO)inlet = 2, SV = 600 ml (NTP)/min/g – numbers in brackets indicate Si/Fe in mmol/mol). (A) Methane selectivity. (B) Olefin content in the fraction of linear C5-hydrocarbons. (C) Ratio of branched to linear hydrocarbons in C5.

149

Methane is an undesired, but inevitable product of the Fischer– Tropsch synthesis. Hence, the methane selectivity is of great industrial interest. Methane selectivity is a complex variable, which is determined by the probability that a surface carbon is transformed into a surface methyl group and the probability that the surface methyl group is hydrogenated yielding methane. Both probabilities increase with increasing hydrogen availability on the surface [30,64] and hence methane selectivity increases with increasing hydrogen availability on the catalyst surface. It has previously been shown that the incorporation of silica in iron-based Fischer–Tropsch synthesis results in an increase in the methane selectivity [25– 27], which can be related to a weakening in the strength of adsorption of CO [27]. The modification of iron oxide nano-crystallites with a small amount of silicon (Si/Fe < 40 mmol/mol) does not seem to affect the methane selectivity to a large extent. A significant increase in methane selectivity is observed for the samples with a relative high silicon loading (Si/Fe > 40 mmol/mol). This might be ascribed to a higher hydrogen availability on the surface when surface silicate groups are present (vide verde). Olefins, and in particular a-olefins, are the dominant primary products of iron-based Fischer–Tropsch synthesis, which under Fischer–Tropsch conditions can be hydrogenated, undergo double bond isomerization or act as chain starter [66] The olefin content in the fraction of n-C5-hydrocarbons obtained with the unmodified catalyst sample is ca. 78 mol%, which is thought to represent primary olefin selectivity. A lower olefin content in the fraction of n-C5-hydrocarbons (67 mol%) was obtained with the sample with Si/Fe = 10 mmol/mol, despite a lower conversion level. This may originate from a lower primary olefin selectivity or enhanced olefin re-adsorption over e-Fe2C in comparison with v-Fe5C2, with both effects enhanced by increased hydrogen availability on the surface [30]. The Fischer–Tropsch synthesis over the catalysts with a higher silicon loading yielded a decrease in the olefin content with increasing silicon loading despite a lowering of the CO-conversion with increasing silicon loading. This might be interpreted in terms of enhanced hydrogen availability on the surface leading to a decrease in the olefin content. Branched product compounds can be formed during the Fischer–Tropsch synthesis via olefin re-adsorption or chain growth involving hydrogen-poor surface species, for example, alkylidene species [65,66]. Iron-based Fischer–Tropsch synthesis results in a relative high rate of formation of branched products relative to the rate of formation of linear compounds [66], which has been attributed to a relative low hydrogen availability on the catalytically active surface. The Fischer–Tropsch synthesis over the unmodified sample, which contains only v-Fe5C2, yielded a rather high ratio of branched products in C5 relative to linear products. A similar ratio was obtained in the Fischer–Tropsch synthesis over the modified sample with Si/Fe = 10 mmol/mol. It has been argued that this sample contains e-Fe2C (in addition to Hägg iron carbide) whose surface contains almost no surface silicate groups (vide supra) implying a similar hydrogen availability on both carbide surfaces. The Fischer–Tropsch synthesis over catalyst with higher silicon loadings resulted in a significant decrease in the ratio of branched to linear C5 hydrocarbons. This can be attributed to an increased hydrogen availability on the surface due to the presence of the silicate groups. The hydrogen availability on the surface is a crucial parameter in determining the activity and selectivity of Fischer–Tropsch catalysts [30]. The catalytic activity increases with increasing hydrogen availability due to increased rate of hydrogenation of surface carbon [66]. This is associated with an increase in the rate of methane formation, which may lead to an increase in the methane selectivity. Furthermore, an increase in the hydrogen availability will lead to a decrease in the olefin content, due to an increase in the primary rate of paraffin formation and an increase in olefin

150

R.P. Mogorosi et al. / Journal of Catalysis 289 (2012) 140–150

re-adsorption [66]. The primary formation of branched product compounds is thought to proceed via hydrogen-poor surface species [64,65], and hence increasing the hydrogen availability will result in a decrease in the primary formation of branched product compounds. The obtained results for the catalyst with a high silicon loading can thus be explained in terms of the influence of surface silicate groups on the hydrogen availability on the catalyst surface. The presence of surface silicate groups may weaken the CO adsorption resulting in a higher hydrogen availability on the surface. This in turn will lead to an enhanced activity per unit surface area, increased methane selectivity, lowered olefin content, and lower ratio of branched to linear product compounds. 4. Conclusions The metal–support interaction between iron and silica in ironbased Fischer–Tropsch catalysts was investigated by surface modification of nano-sized iron oxide crystallites using tetra-ethoxy silane. The dominant phase of the iron oxide in the calcined phase is maghemite, with some larger crystallites present as hematite. The presence of FeAOASi bonds in the calcined catalysts was evidenced by additional absorption bands between 870 and 950 cm1, previously ascribed to („FeO)2ASi(OH)2. The presence of FeAOASi bonds in the calcined catalyst could also be inferred from the high-temperature peak at ca. 840 °C in the temperature programmed reduction profile of the samples with a high silicon content. The FeAOASi bonds remain mostly in tact after reduction in hydrogen at 350 °C for 16 h. The hydrogen activation of the silicon modified samples resulted in the formation of wüstite, whose disproportionation is either kinetically or thermodynamically limited by the modification with surface silicate groups. It is deduced that the FeO surface is almost fully covered with the silicate surface groups. The average size of the wüstite phase is in agreement with the size of the maghemite phase implying an inhibition of the sintering process possibly due to the presence of surface silicate groups. Sintering can take place under the conditions applied for the hydrogen activation, as evidenced by the large increase in the average crystallite size of a-Fe. Exposure of the catalysts to industrially relevant Fischer–Tropsch conditions results in the formation of v-Fe5C2, and for the silicon modified catalysts also e-Fe2C (and Fe3O4 for samples with a high silicon content). It is deduced that a-Fe is the origin of Fe5C2, whereas FeO is transformed into e-Fe2C, possibly via Fe3O4. The activity per unit surface area of e-Fe2C is ca. 25% higher than that of v-Fe5C2, which might be partially explained in terms of the difference in site density on these carbides. The catalysts with high silicon loading show an enhanced integral rate per unit surface area of e-Fe2C, which might be ascribed to the presence of surface silicate groups. It is deduced that surface silicate groups result in an increase in the hydrogen availability on the surface by reducing the strength of CO adsorption. An increased hydrogen availability on the surface will result in an increase in activity and methane selectivity, a decrease in the olefin content of the product and a decrease in the primary rate of formation of branched product compounds relative to the formation of linear product compounds. References [1] [2] [3] [4] [5]

S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. S.J. Tauster, Acc. Chem. Res. 20 (1987) 11. S.J. Tauster, S.C. Fung, J. Catal. 55 (1978) 29. M.A. Vannice, L.C. Hasselbring, B. Sen, J. Phys. Chem. 89 (1985) 2972. S.J. Tauster, S.C. Fung, R.T.K. Baker, J.A. Horsley, Science 211 (1981) 1121.

[6] S. Taniguchi, T. Mori, Y. Mori, T. Hattori, Y. Murakami, J. Chem. Soc. Chem. Commun. (1988) 630. [7] A.A. Khassin, T.M. Yurieva, V.V. Kaichev, V.I. Bukhtiyarov, A.A. Budneva, E.A. Paukshtis, V.N. Parmon, J. Mol. Catal. A: Chem. 175 (2001) 189. [8] R. Lamber, W. Romanowski, J. Catal. 105 (1987) 213. [9] I. Puskas, T.H. Fleisch, J.B. Hall, B.L. Meyers, R.T. Roginski, J. Catal. 134 (1992) 615. [10] R. Lamber, N. Jaeger, G. Schulz-Ekloff, Surf. Sci 227 (1990) 268. [11] B.K. Min, A.K. Santra, D.W. Goodman, Catal. Today 85 (2003) 113. [12] E.V. Benvenutti, L. Franken, C.C. Moro, Langmuir 15 (1999) 8140. [13] Z.X. Cheng, C. Louis, M. Che, Z. Phys. D 20 (1991) 445. [14] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173. [15] M. Che, Stud. Surf. Sci. Catal. 75 (1993) 31. [16] E. van Steen, G.S. Sewell, R.A. Makhote, C. Micklethwaite, H. Manstein, M. de Lange, C.T. O’Connor, J. Catal. 162 (1996) 220. [17] W.A. Spieker, J.R. Regalbuto, Chem. Eng. Sci. 56 (2001) 3491. [18] I. Puskas, T.H. Fleisch, P.R. Full, J.A. Kaduk, C.L. Marshall, B.L. Meyers, Appl. Catal. A: Gen. 311 (2006) 146. [19] H.M. Pham, A.K. Datye, Catal. Today 58 (2000) 233. [20] N.O. Egiebor, W.C. Cooper, Can. J. Chem. Eng. 63 (1985) 81. [21] D.B. Bukur, X. Lang, D. Mukesh, W.H. Zimmerman, M.P. Rosynek, C. Li, Ind. Eng. Chem. Res. 29 (1990) 1588. [22] K. Jothimurugesan, J.J. Spivey, S.K. Gangwal, J.G. Goodwin Jr., Stud. Surf. Sci. Catal. 119 (1998) 215. [23] K. Jothimurugesan, J.G. Goodwin Jr., S.K. Gangwal, J.J. Spivey, Catal. Today 58 (2000) 335. [24] H. Dlamini, T. Motjope, G. Joorst, G. ter Stege, M. Mdleleni, Catal. Lett. 78 (2002) 201. [25] H.-J. Wan, B.-S. Wu, Z.-C. Tao, T.-Z. Li, X. Ana, H.-W. Xiang, Y.-W. Li, J. Mol. Catal. A: Chem. 260 (2006) 255. [26] C.H. Zhang, H.J. Wan, H.W. Xiang, Y.W. Li, Catal. Commun. 7 (2006) 733. [27] W. Hou, B. Wu, Y. Yang, L. Tian, H. Xiang, Y. Li, Fuel Process. Technol. 89 (2008) 284. [28] M. Qing, Y. Yang, B. Wu, J. Xu, C. Zhang, P. Gao, Y. Li, J. Catal. 279 (2011) 111. [29] H. Suo, S. Wang, C. Zhang, J. Xu, B. Wu, Y. Yang, H. Xiang, Y.-W. Li, J. Catal. 286 (2012) 111. [30] H. Schulz, E. van Steen, M. Claeys, Stud. Surf. Sci. Catal. 81 (1994) 455. [31] D. Barkhuizen, I. Mabaso, E. Viljoen, C. Welker, M. Claeys, E. van Steen, J.C.Q. Fletcher, Pure Appl. Chem. 78 (2006) 1759. [32] J.-Y. Park, Y.-J. Lee, P.K. Khanna, K.-W. Jun, J.W. Bae, Y.H. Kim, J. Mol. Catal. A: Chem. 323 (2010) 84. [33] M.I. Shukoor, F. Natalio, N. Metz, N. Glube, M.N. Tahir, H.A. Therese, V. Ksenofontov, P. Theato, P. Langguth, J.-P. Boissel, H.C. Schröder, W.E.G. Müller, W. Tremelm, Angew. Chem. Int. Ed. 47 (2008) 4748. [34] T. Rajh, L.X. Chen, K. Lukas, T. Liu, M.C. Thurnauer, D.M. Tiede, J. Phys. Chem. B 106 (2002) 10543. [35] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Nano Lett. 2 (2002) 183. [36] G. Marcelo, E. Pérez, T. Corrales, C. Peinado, J. Phys. Chem. C 115 (2011) 25247. [37] E.I. Mabaso, PhD Thesis, University of Cape Town, 2005. [38] H. Schulz, S. Nehren, Erdöl Kohle – Erdgas – Petrochem. 39 (1986) 93. [39] F.S. Yen, W.C. Chen, J.M. Yang, C.T. Hong, Nano Lett. 2 (2002) 245. [40] A. Navrotsky, L. Mazeina, J. Majzlan, Science 319 (2008) 1635. [41] E. van Steen, M. Claeys, M.E. Dry, J. van de Loosdrecht, E.L. Viljoen, J.L. Visagie, J. Phys. Chem. B 109 (2005) 3575. [42] E. van Steen, M. Claeys, Chem. Eng. Technol. 31 (2008) 655. [43] C.W. Jung, Magn. Res. Imag. 13 (1995) 675. [44] S.M.I. Morsy, S.A. Shaban, A.M. Ibrahim, M.M. Selim, J. Alloys Compd. 486 (2009) 83. [45] M. Gotic, S. Music’, J. Mol. Struct. 834–836 (2007) 445. [46] X. Yang, P. Roonasi, A. Holmgren, J. Colloid Interface Sci. 328 (2008) 41. [47] P.J. Swedlund, G.M. Miskelly, A.J. McQuillan, Geochim. Cosmochim. Acta 73 (2009) 4199. [48] B.A. Morrow, A.J. McFarlan, J. Phys. Chem. 96 (1992) 1395. [49] E. Vinogradova, M. Estrada, A. Moreno, J. Colloid Interface Sci. 298 (2006) 209. [50] O.J. Wimmers, P. Arnoldy, J.A. Moulijn, J. Phys. Chem. 90 (1986) 1331. [51] W.K. Jozwiak, E. Kaczmarek, T.P. Maniecki, W. Ignaczak, W. Maniukiewicz, Appl. Catal. A: Gen. 326 (2007) 17. [52] B.H. Davis, Catal. Today 141 (2009) 25. [53] E. de Smit, F. Cinquini, A.M. Beale, O.V. Safonova, W. van Beek, P. Sautet, B.M. Weckhuysen, J. Am. Chem. Soc. 132 (2010) 14928. [54] L.-L. Bao, C.-F. Hua, C.-M. Deng, Y.-W. Li, J. Fuel Chem. Technol. 37 (2009) 104. [55] J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk, H.S. van der Baan, J. Phys. Chem. 84 (1980) 3363. [56] J.A. Amelse, J.B. Butt, L.H. Schwartz, J. Phys. Chem. 82 (1978) 558. [57] S. Li, W. Ding, G.D. Meitzner, E. Iglesia, J. Phys. Chem. B 106 (2002) 85. [58] E. van Steen, H. Schulz, Appl. Catal. A: Gen. 186 (1999) 309. [59] C.-F. Huo, Y.-W. Li, J. Wang, H. Jiao, J. Am. Chem. Soc. 131 (2009) 14713. [60] R.A. van Santen, I.M. Ciobîca˘, E. van Steen, M.M. Ghouri, Adv. Catal. 54 (2011) 127. [61] S. Krishnamoorthy, A. Li, E. Iglesia, Catal. Lett. 80 (2002) 77. [62] D.G. Rethwisch, J.A. Dumesic, J. Catal. 101 (1986) 35. [63] K.R.P.M. Rao, F.E. Huggins, V. Mahajan, G.P. Huffman, V.U.S. Rao, B.L. Bhatt, D.B. Bukur, B.H. Davis, R.J. O’Brien, Top. Catal. 2 (1995) 71. [64] H. Schulz, E. van Steen, M. Claeys, Top. Catal. 2 (1995) 223. [65] H. Schulz, H. Gorre, E. Erich, E. van Steen, Catal. Lett. 7 (1990) 157. [66] M. Claeys, E. van Steen, Stud. Surf. Sci. and Catal 152 (2004) 601.