- Email: [email protected]
SiO2 catalysts prepared from Co(acac)3 by gas phase deposition
Applied Catalysis A: General 208 (2001) 223–234
Characterisation of Co/SiO2 catalysts prepared from Co(acac)3 by gas phase deposition L.B. Backman a,∗ , A. Rautiainen b , M. Lindblad b , O. Jylhä c , A.O.I. Krause a a
Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland b Fortum Oil and Gas OY, P.O. Box 110, FIN-00048 Fortum, Finland c Microchemistry Ltd, P.O. Box 132, FIN-02631 Espoo, Finland Received 27 September 1999; received in revised form 23 June 2000; accepted 26 June 2000
Abstract Silica supported cobalt catalysts prepared by a chemical vapour deposition technique, atomic layer epitaxy (ALE), were characterised in order to study the interaction between cobalt species and silica. The catalysts were prepared by chemisorption of cobalt(III)acetylacetonate from the gas phase onto silica. The metal loading on the catalysts varied from 5.7 to 19.5 wt.%. Concentration profiles obtained with SEM/EDS showed that the cobalt species were evenly distributed through the catalyst particles. The dispersion was estimated by hydrogen chemisorption. The metallic cobalt was well dispersed on samples containing less than 6 wt.% cobalt but the dispersion decreased with increasing cobalt loading. XRD showed only weak reflections even for a sample containing 19.5 wt.%, which indicated weakly ordered cobalt species. The degree of reduction estimated by XPS was less than 30% on all samples even after reduction at 550◦ C for 7 h. The low reducibility could be explained by the formation of cobalt silicate during air calcination. The presence of silicates was also indicated by XRD and XPS. The catalysts were tested for gas phase toluene hydrogenation in a microreactor system. The reaction rate per gram sample increased with cobalt loading but the turn over frequency remained essentially constant on all samples. This indicated that the surface area of metallic cobalt is the main factor in determining the overall activity of the catalysts in this reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cobalt catalysts (supported); Atomic layer epitaxy; Gas phase deposition; Cobalt silicate
1. Introduction Cobalt has been extensively used as catalyst for hydrogenation and hydrotreating reactions. One of the most common applications is the Fischer–Tropsch reaction, which has been studied in numerous publications. Cobalt based catalysts are already used in industrial scale operation for synthesis of hydrocarbons [1,2]. Studies of the kinetics of aromatics hydro∗ Corresponding author. Tel.: +358-9-451-2627; fax: +358-9-451-2622. E-mail address: [email protected] (L.B. Backman).
genation on cobalt are far less frequent [3–5]. In addition, cobalt catalysts prepared from cobalt acetylacetonate [6] have shown promising results in ethene hydroformylation [7]. Cobalt has also been used for preparation of ethylamines by reductive amination of ethanol. According to Sewell et al. [8], cobalt is considerably more active than nickel for this reaction. The wide variety of applications of supported cobalt in catalysis underlines the importance to understand the interaction between the catalytic material and the support. Silica has generally been regarded as an inert support material compared to other commonly used inorganic oxides [9,10]. However, there
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 7 0 7 - 9
224
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
have been several reports of strong interaction between cobalt species and silica [6,11–25]. Formation of silicates and hydrosilicates has been observed during precipitation of Co/SiO2 catalysts due to slightly alkaline conditions [14] and during aqueous impregnation due to a pH ≥ 5 [15,16]. The reducibility of cobalt can be strongly influenced by the drying procedure [16–18]. High temperature calcination both in oxygen (940–1000◦ C) [15–19] and in an inert atmosphere (810◦ C) [19] can form silicates. Poorly reducible CoOx –SiO2 species have also been proposed to form due to reductive decomposition of Co(NO3 )2 [20]. High reduction temperatures have been suggested to induce formation of Co–Si solid solutions and silicides [11]. Kogelbauer et al. showed that reduced cobalt produces silicates/hydrosilicates under hydrothermal conditions [21]. The same has been observed after a secondary aqueous impregnation of pre-reduced and passivated Co/SiO2 catalysts during drying at >50◦ C [22]. The precursor [20,23], the impregnation solvent [16], and support surface area [14,16,19] also affects the amount of silicate that is formed during the different stages. Formation of Co2 SiO4 has also been observed during rapid thermal annealing of cobalt thin films on SiO2 [24,25]. The trend towards ‘tailor made’ catalysts has led to the use of new preparation methods. The preparation of catalysts using chemical vapour deposition (CVD) techniques is an approach to prepare catalysts with well-defined properties. A related method, atomic layer epitaxy (ALE), differs from conventional CVD techniques mainly in the choice of process conditions [26]. In suitable conditions, defined as the process window, only reaction between functional groups at the surface and the catalyst precursor occurs. The number of available surface sites determines the amount chemisorbed on the surface at saturation density, which makes the process self-controlled. The number and nature of the reactive sites can be controlled by changing the pre-treatment temperature of the support [26]. Gas–solid reactions can thus be used for preparation of structurally well-defined catalysts. Well dispersed alumina supported nickel catalysts active for hydrogenation of toluene have been prepared by ALE [27]. In this paper, we expanded the scope of our research to cobalt catalysts. Silica supported cobalt catalysts were prepared by ALE using cobalt acetylacetonate as precursor. This work was carried
out in order to study the interaction of well-dispersed cobalt species with the silica support. The catalyst activity was tested by toluene hydrogenation. Hydrogenation of toluene is considered to be a structure insensitive reaction [28], therefore the reaction rate should be a good measure of the cobalt surface area.
2. Experimental 2.1. Catalyst preparation The catalysts were prepared by chemisorption of cobalt(III)acetylacetonate Co(acac)3 (Merck, >98%) from the gas phase onto silica (Silica Grace 432) [29]. The purity of the support has been reported earlier [30]. Thermal pre-treatment of the silica support was carried out with air in a muffle furnace at 600◦ C for 16 h. The support was further heated in a flow of dry nitrogen at 180 or 450◦ C immediately before the chemisorption of the precursor. The deposition process was carried out in an ALE reactor [31] with the silica particles (d = 0.5–1.0 mm) placed in a fixed bed. The precursor was evaporated into flowing nitrogen at 180◦ C and the surface of the support was saturated by dosing excess of precursor. The chemisorption step was followed by air treatment at 450◦ C in order to remove the acac-ligands. A metal content of about 6 wt.% was reached by saturating the surface once. To obtain higher cobalt contents additional cycles were used, each followed by air treatment at 450◦ C. The catalysts were prepared using one to five sequential chemisorption and air treatment cycles, which gave catalysts with cobalt contents ranging from 5.7 to 19.5 wt.%. 2.2. Physisorption and chemisorption measurements The surface area (BET) and pore volume, of calcined catalyst samples and of the support pre-treated at 600◦ C, were measured by nitrogen physisorption at the temperature of liquid nitrogen. Both the adsorption and desorption isotherms were measured. Prior to measurement the samples were outgassed at 90◦ C for 1 h followed by outgassing at 300◦ C down to less than 10–5 mbar for 3 h. Hydrogen and CO chemisorption measurements were performed by the static volumetric method. The hydrogen chemisorption capacity
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
was measured at 30◦ C on catalysts reduced in situ in flowing hydrogen at 500, 550 and 600◦ C for 7 h. Hydrogen chemisorption was also performed at 100◦ C on samples reduced at 550◦ C. The CO chemisorption capacity at 30◦ C was determined for samples reduced at 550◦ C. The instrument used for both the physisorption and chemisorption measurements was a Coulter OMNISORP 100CX. The samples were outgassed for 60 min at the reduction temperature before allowing them to cool down in vacuum to the measurement temperature. Both the total and reversible chemisorption isotherms were recorded and the samples were outgassed for 40 min in between at the measurement temperature. The chemisorption capacity was determined by extrapolating the flat portion of the isotherm between 8 and 27 kPa (60–200 Torr) to zero pressure. The dispersion was calculated using the total hydrogen uptake at 100◦ C [32,33]. The average crystallite size was calculated by assuming spherical geometry of the cobalt metal particles. A surface site density of 14.6 atoms/nm2 for the Co crystallites was assumed [32].
225
model 101, Surface Science Instruments, VG Fisons). The binding energy scale was fixed by using a binding energy of 103.7 eV for the Si 2p peak [34]. In the case of Co3 O4 where no silicon was present carbon (C 1s, 284.6 eV) was used instead. The degree of reduction was estimated from the ratio of the metallic peak area to the total peak area of the Co 2p2/3 region. Overlapping peaks in the XP-spectra were deconvoluted by fitting symmetrical Gauss–Lorenzian (80/20) peaks and using the Shirley background subtraction. The calculations were made by using the peak fitting routines supplied by the manufacturer. The estimation involves some uncertainties since the method used for background subtraction can influence the values obtained for the degree of reduction [35]. The particle size can affect the result due to the surface sensitivity of XPS, i.e. the core of large particles will not be probed. A difference in size between the oxidic and metallic cobalt particles could also affect the estimation of the degree of reduction [36]. When using Al K␣ radiation the Auger transition Co L3 M2,3 M4,5 can increase the intensity of the low binding energy side of the Co 2p3/2 peak [37].
2.3. X-ray diffraction 2.5. TEM and SEM/EDS The crystal structure of the cobalt species was studied by X-ray diffraction (XRD). The wide angle X-ray scattering patterns were collected in reflection mode using a Siemens D500 diffractometer equipped with a Cu-anode and a graphite monochromator in the reflected beam. Identification of the reflections was done with reference data from the JCPDS reference library. Both calcined and reduced (550◦ C/7 h) samples were studied. The samples were transferred to the measurement chamber through air. 2.4. X-ray photoelectron spectroscopy Both calcined and reduced samples were analysed by X-ray photoelectron spectroscopy (XPS). The XP spectra of cobalt oxide (Co3 O4 , Aldrich, 99.995%) and cobalt silicate (Co2 SiO4 , Pfaltz & Bauer) were also measured. Catalyst samples containing 5.7, 13.4 and 19.0 wt.% cobalt were pre-reduced in flowing hydrogen at 550◦ C for 7 h and transferred inertly into the measurement chamber. The other samples were transferred through air. The measurements were done using monochromatized Al K␣ X-rays (X-probe
The transmission electron microscope (TEM) micrographs were obtained with a JEOL JEM-1200EX II apparatus operated at an accelerating voltage of 100 keV. The samples were finely ground using an agate mortar, and were collected on a carbonised copper-grid. Two sets of every sample were ground separately. The samples were studied visually at different magnifications to confirm the homogeneity of the samples. The actual micrographs were obtained using a magnification of 100,000 times. The lower limit for measuring crystallite diameters with this apparatus was about 2 nm. The distribution of cobalt through the catalyst particle was studied by a scanning electron microscope equipped with an energy dispersive spectrometer (SEM/EDS). The samples for the SEM/EDS analyses were moulded into epoxy resin. The hardened sample blocks were sliced with a microtome equipped with a glass knife to obtain cross-section cuts of the particles. The SEM/EDS analyses were made from smooth cross-section cuts. The line profiles of silicon and cobalt were measured through five particles.
226
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
2.6. Hydrogenation runs The performance of the catalysts was tested in gas phase hydrogenation of toluene in a microreactor system. The activity runs were done with a down flow quartz glass tubular reactor at atmospheric pressure with a total molar flow of 0.8 mol/h and a molar ratio of hydrogen to toluene of 7 to 1. The reaction temperature was 170◦ C and the amount of catalyst was between 0.3 and 0.5 g. A fresh sample was loaded for every run and the catalyst was activated by reduction in situ in flowing hydrogen at 550◦ C for 7 h. The product stream was analysed on-line with a Gasmet FT-IR multi-component gas analyser (TEMET Instruments). The IR absorbance spectra were measured over a wave-number range of 950 to 4000 cm−1 with a resolution of 8 cm−1 . The analyser was calibrated by flowing pure compounds diluted in nitrogen through the measurement cell. The analysis was performed by fitting a linear combination of the reference spectra to the measured spectra [38]. The activity data reported in this publication was collected after 1 h of reaction time. The reaction rate per exposed cobalt or turn over frequency (TOF) was calculated using the amount of surface atoms obtained from the total hydrogen uptake at 100◦ C.
3. Results and discussion 3.1. Distribution of the cobalt species The catalysts were prepared by applying one to five sequential Co(acac)3 chemisorption and air calcina-
tion steps which gave catalysts with cobalt contents ranging from 5.7 to 19.5 wt.% (Table 1). The catalyst preparation is described in detail elsewhere [29]. The precursor Co(acac)3 is proposed to react with the OH-sites on the support resulting in the loss of one acac-ligand [29]. One preparation cycle on the support pre-treated at 600 and 180◦ C gave a cobalt loading of 5.9 wt.% and two cycles gave a loading of 10.6 wt.% Co. The increase in cobalt content was almost linear during the following chemisorption steps, about 3.1 wt.% per cycle. The cobalt content on the samples prepared on supports pre-treated at 600 and 450◦ C was slightly lower possibly due to a different amount or distribution of OH-groups on the surface. Line profiles of cobalt measured through the catalyst particles by SEM/EDS showed that the distribution of cobalt was even through the catalyst particles. The nitrogen physisorption measurements showed a decrease in surface area and pore volume with increasing cobalt content (Table 1). The surface area decreased from 330 m2 /gsample for the pure support to 230 m2 /gsample for the sample containing 19.5 wt.% Co. Calculated per gram of silica, assuming a stoichiometry of 1:1 (Co:O) for the cobalt species, the surface area decreased from 330 to 306 m2 /gSiO2 . This implies that the cobalt species are finely distributed on the support. The pore volume decreased from 1.19 to 0.81 cm3 /gsample . Calculated per amount of silica the pore volume decreased from 1.19 to 1.08 cm3 /gSiO2 , i.e. only slight pore blocking due to the cobalt species. In addition, a slight shift towards smaller pores was observed in the pore size distribution probably due to deposition of cobalt species on the surface of the pore
Table 1 Metal content, surface area, pore volume and average pore diameter (d ave = 4∗ A/V) as a function of preparation cycles Cycles
Silica pretreatment temperature (◦ C)
Co content (wt.%)
SABET (m2 /gsample )
SABET (m2 /gSiO2 )
Pore volume (cm3 /gsample )
Pore volume (cm3 /gSiO2 )
dave (nm)
– 1 2 3 4 5 10 30 50
600 600 + 180 600 + 180 600 + 180 600 + 180 600 + 180 600 + 450 600 + 450 600 + 450
– 5.9 10.6 13.7 16.8 19.5 5.7 13.4 19.0
330 299
330 324
1.19 1.06
1.19 1.15
14.0 14.2
266
322
0.92
1.11
13.8
230
306
0.81
1.08
14.1
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
227
Fig. 1. Hydrogen uptake as a function of reduction temperature: (䊊) 5.9; (䊐) 13.7; and (×) 19.5 wt.% Co.
walls. However, the average pore diameter remained constant at 14.0 ± 0.2 nm (Table 1). The total hydrogen uptake of the samples was determined by static volumetric measurements at 30◦ C. The irreversible uptake was also measured and the general trends were the same as for the total uptake. The catalysts were reduced in situ in flowing hydrogen for 7 h at 500, 550 and 600◦ C. A maximum hydrogen uptake at 30◦ C was observed on all samples after reduction at 550◦ C (Fig. 1). The hydrogen uptake at 30◦ C increased with cobalt content but remained modest even on the 19.5 wt.% catalyst (18.4 mol/gsample ). The uptake per gram cobalt decreased with increasing metal loading indicating a decreasing dispersion (Table 2). The chemisorption
capacity was also measured at 100◦ C on samples reduced at 550◦ C. The hydrogen uptake increased as the measurement temperature was increased from 30 to 100◦ C (Table 2) which indicated that the chemisorption of hydrogen was activated on these catalysts. The increase varied from 5 to 46% but did not show any clear trend as a function of metal loading (Table 2). Activated chemisorption of hydrogen on cobalt has been reported earlier by Bartholomew et al. [32,33]. The activation energy of hydrogen chemisorption on cobalt has been reported to increase with increasing cobalt–support interaction [33]. Therefore, the increase would be expected to be higher at low cobalt contents, i.e. at higher dispersions with stronger interaction between cobalt and support.
Table 2 Hydrogen uptake, degree of reduction (f), dispersion (D) and crystallite size (d) after reduction at 550◦ C Cycles
Co content (wt.%)
H2 uptake at 30◦ C (mol/gsample )
H2 uptake at 30◦ C (mol/gCo )
H2 uptake at 100◦ C (mol/gsample )
Activation factora
f (%)
D (%)
d (nm)
1 2 3 4 5 10 30 50
5.9 10.6 13.7 16.8 19.5 5.7 13.4 19.0
8.96 13.0 15.6 15.3 18.4 4.59 9.26 15.5
152 122 114 91 94 81 69 82
11.7 13.7 19.2 22.3 23.2 5.74 11.1 17.4
1.30 1.05 1.23 1.46 1.26 1.25 1.20 1.12
12
19
4.9
a b
The activation factor is defined as the ratio of the uptake at 100◦ C to the uptake at 30◦ C. Determined by O2 -titration at 400◦ C.
50b
3.3
64b 4 23 28
2.2 29 4.3 3.8
29 44 3.4 23 25
228
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
Fig. 2. TEM image of calcined samples (a) 13.7 and (b) 19.5 wt.% Co. Bar shown in figure equals 50 nm.
The dispersion and average crystallite size of the cobalt species on the reduced catalyst was estimated from the total hydrogen uptake at 100◦ C (Table 2). The values were corrected with the degree of reduction (obtained by XPS or O2 -titration [39]) by assuming that the metallic cobalt forms a separate phase [36]. The stoichiometry for total hydrogen chemisorption on cobalt at the temperature of maximum uptake can be taken as 1.0 (H/Co) [32]. The dispersion of cobalt on the 5.7 wt.% sample was 29%, i.e. a high value for Co/SiO2 catalysts. The corresponding size of the metallic cobalt particles was 3.4 nm. However, the dispersion decreased with increasing cobalt content, e.g. at 13.4 wt.% the dispersion was 4.3%. A further increase in cobalt content did not significantly decrease the dispersion (Table 2). Calcined samples made by three and five preparation cycles were studied by TEM (Fig. 2). The TEM images showed round shaped cobalt particles of about 10 nm in size. This corresponds to a dispersion of about 10% assuming spherical particles. This is roughly in agreement with the average particle sizes obtained by hydrogen chemisorption (27 and 28 nm),
taking into account that some sintering probably occurred during the reduction prior to the chemisorption measurements. Also some larger particles of 100 to 200 nm were observed and especially on the catalyst prepared by five cycles some crystalline-like phases were seen in the TEM images. 3.2. The nature of the cobalt species The CO chemisorption capacity was measured at 30◦ C on both calcined and reduced (550◦ C/7 h) samples (Table 3). The total CO uptake on the reduced samples was nearly independent of cobalt loading (44 ± 6 mol/g). The relative variations in the irreversible uptake were bigger and a slight increase in the uptake with metal loading was observed. Between 29 and 49% of the CO uptake was irreversible (Table 3). The total CO uptake was 55 mol/g on both the calcined samples containing 13.7 and 19.5 wt.% which was 5 and 17 mol/g higher than compared to the reduced samples, respectively. However, only 7–9% of the uptake was irreversible. The decrease in total chemisorption of CO from calcined to reduced
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
229
Table 3 CO uptake measured at 30◦ C on calcined (450◦ C) and reduced (550◦ C) samples Cycles
Co content (wt.%)
Treatment
Total CO uptake (mol/gsample )
Irreversible CO uptake (mol/gsample )
Irreversible CO uptake (%)
3 5 1 2 3 4 5
13.7 19.5 5.9 10.6 13.7 16.8 19.5
Calcined Calcined Reduced Reduced Reduced Reduced Reduced
54.9 54.5 44.8 42.8 49.7 43.3 38.0
4.0 4.7 13.0 13.7 22.4 14.2 18.6
7.4 8.7 29 32 45 33 49
samples could indicate sintering of the cobalt species or a different adsorption stoichiometry. The adsorption of CO on different cobalt species has been studied by infra-red (IR) spectroscopy [9,40]. The IR measurements have shown that CO adsorbs in a linear form on Co2+ , Coδ+ and Co0 and in a bridged form on Co0 , i.e. the metallic cobalt adsorbs somewhat less CO. The ratio of CO(tot) to H(tot, 30◦ C) was found to be bigger than or equal to unity for all metal contents, ranging from 2.5 for 5.9 wt.% to 1.0 for 19.5 wt.% Co (Fig. 3). When the hydrogen uptake was measured at 100◦ C the ratio decreased, ranging from 1.9 to 0.82, respectively. The ratio of CO(irrev) to H(tot, 30◦ C) varied between 0.72 and 0.46 but did not change monotonously with cobalt loading (Fig. 3). Adsorption of CO on non-metallic cobalt species probably distorts the amounts of total uptake. It is known that
cobalt oxide (Co3 O4 ) readily adsorbs CO [41]. However, it is not likely that Co3 O4 would be present after reduction at 550◦ C but other non-metallic species could remain on the samples, such as cobalt silicates. The changes in the CO to H ratio could also be explained by a change in stoichiometry for the chemisorption of CO on cobalt as a function of the particle size. Reuel and Bartholomew [32] have reported that the stoichiometry of irreversible adsorption (CO/Co) varied between 0.4 and 2.3 depending on metal loading, support and preparation. The crystalline species on both calcined and reduced catalysts with cobalt contents of 13.4 and 19.0 wt.% were studied by XRD. The calcined sample, that contained 19.0 wt.% Co showed weak reflections at 36, 59 and 65◦ 2θ . These reflections coincide with the strongest reflections of Co3 O4 but also with
Fig. 3. Ratio of CO and H uptakes as a function of cobalt loading. (䉬) CO(tot)/H(tot, 30◦ C); (䉱) CO(tot)/H(tot, 100◦ C); (䉫) CO(irrev)/H(tot, 30◦ C); (4) CO(irrev)/H(tot, 100◦ C).
230
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
Fig. 4. XRD spectra of reduced samples containing (a) 19.0, (b) 13.4 wt.% Co and of (c) the pure support.
the strongest reflections of Co2 SiO4 . Following hydrogen reduction at 550◦ C broad reflections at 36, 44, 61 and 65◦ 2θ were obtained (Fig. 4). Co2 SiO4 and Co3 O4 have their strongest reflections at 36.6 and 36.8◦ , respectively. The strongest reflection of metallic cobalt is at 44.2◦ 2θ, but the peak observed at 44◦ 2θ probably has a contribution of Co2 SiO4 at 44.5◦ and Co3 O4 at 44.8◦ 2θ. The features at 61 and 65◦ 2θ are probably the sums of the peaks of Co2 SiO4 at 58.9 and 64.7◦ and of Co3 O4 at 59.3 and 65.2◦ 2θ. The XRD measurements indicate the presence of Co3 O4 and/or Co2 SiO4 . However, the crystallite sizes of the different phases were not determined due to broad, weak and partly overlapping peaks. The XRD analysis of samples with a cobalt loading of 13.4 wt.% showed reflections at similar 2θ values as the 19.0 wt.% Co, but the intensities were even weaker (Fig. 4). The particle sizes obtained from the chemisorption measurements, ranging from about 4.2 to 28 nm (Table 2), are considerably larger than indicated by the weak XRD pattern. This result suggests that the cobalt particles consist of weakly ordered species. It is also possible that the cobalt species are present as a two-dimensional surface silicate [14], which would be difficult to detect by XRD. Three reduced samples (one, three and five cycles) and one calcined sample (three cycles) were studied by XPS to determine the chemical state of the surface cobalt species. These samples were prepared on
silica pretreated at 600 and 450◦ C. Reference spectra of Co3 O4 and Co2 SiO4 were also measured. The reduced samples were pre-reduced in flowing hydrogen at 550◦ C for 7 h and transferred inertly into the XPS measurement cell. The other samples were transferred through air. The Co 2p3/2 XP spectra of samples prepared by one, three and five cycles are shown in Fig. 5. The peak at about 778.3 eV that is present in all
Fig. 5. XP spectra of catalysts prepared by 1 (a), 3 (b) and 5 (c) preparation cycles. The curves are scaled to the same maximum intensity.
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
231
Table 4 Binding energies (Co 2p3/2 ), intensity ratios of Co 2p3/2 /Si 2p and degree of reduction Sample
Co (wt.%)
Co2 SiO4 3, calcined 1, reduced 3, reduced 5, reduced Co3 O4
13.4 5.7 13.4 19.0
a
BE Co0 (eV)
778.3 778.3 778.4
BE Co2+ (eV)
Co/Si×100
Co0 (%)
781.9 781.8 781.7 781.0 780.4 779.5a
48.3 18.2 4.5 6.4 8.2
4 23 28
Binding energy reference C 1 s, 284.6 eV.
three spectra of the reduced samples can be ascribed to metallic cobalt, although in the spectrum of the one-cycle sample it can be seen only as a very weak shoulder. The shake-up satellite features at about 6 eV higher binding energy from the peaks on the high binding energy side of the metallic peak suggests that Co2+ is present on the surface (Fig. 5) [42]. The ratios of the Co 2p3/2 and the Si 2p peak areas of the reduced samples increased with metal content (Table 4). The Co 2p3/2 and Si 2p peak ratio of the calcined three-cycle sample was 2.9 times higher than on the reduced sample. According to the formula of Kerkhof and Moulijn [43] the ratio increases with increasing dispersion at constant cobalt loading. A significant decrease in the ratio during the reduction at 550◦ C could indicate that some sintering had occurred. Other possible reasons would be encapsulation of cobalt by silica due to migration of silica [44,45], or migration of cobalt into the support. The degree of reduction was estimated from the ratio of the metallic peak area to the total area of the Co 2p2/3 region. The degree of reduction increased with cobalt content (Table 4). The sample prepared with one deposition cycle (5.7 wt.%) gave a very weak signal for metallic cobalt and the degree of reduction was estimated to be only about 4%. On the three-cycle sample (13.4 wt.%) the degree of reduction was considerably higher (23%), but even on the five-cycle sample (19.0 wt.%) only about 28% of the cobalt had reduced to metallic cobalt. This shows that a large amount of the cobalt species on the catalyst is in a poorly reducible state. Temperature programmed reduction studies of these catalysts [6] showed that reduction mainly occur at temperatures above 700◦ C which indicated that the samples contained cobalt species that are hard to reduce, e.g. cobalt silicates. The degrees of reduction of the samples prepared on silica calcined at 600 and
180◦ C are given in Table 2. The reduction extents are higher on these samples. This is probably partly due to the different measurement method, i.e. XPS versus oxygen titration [6]. XPS seem to underestimate the amount of metallic cobalt [35]. The extents of reduction were all below 60%, independent of the measurement method, which can be considered low taking into account the high reduction temperature (550◦ C/7 h). The position of the metallic peak remained constant for all the samples at 778.3 ± 0.1 eV while the binding energies of the non-metallic peaks decreased with metal loading (Table 4). The peak shift could be a chemical shift due to an increased amount of silicate [46] on the surface, i.e. at low metal loading the cobalt is present almost completely as silicates. A similar peak shift has been reported for calcined Co/Al2 O3 samples by Chin and Hercules [36] and by Okamoto et al. [47]. Furthermore, the Co 2p3/2 region in the XP spectra of the one-cycle sample, the calcined three-cycle sample and the Co2 SiO4 were of similar shape. These findings suggest that the samples contain high concentrations of cobalt silicate, which agrees with the indications obtained from the XRD measurements. 3.3. Cobalt silica interaction Strong interaction between cobalt species and silica can cause formation of silicates and/or hydrosilicates [6,12–25]. Alkaline conditions during the precipitation/impregnation have increased the formation of silicates [14–16]. The drying procedure has also been proven important for the reducibility of the catalyst [16–18]. In our case, where the preparation does not involve any solvent, these are not important. However, the cobalt acetylacetonates have been proposed to interact with the OH groups on the silica [29]
232
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
forming bonds between silicon, oxygen and cobalt. These highly dispersed cobalt species could then convert into a surface silicate during the calcination in air at 450◦ C, or possibly even partly during the nitrogen purge at 450◦ C. Silicates have been formed during high temperature calcination (>900◦ C) in oxygen [15–19] or in an inert [19]. The formation of silicates during high temperature (>500◦ C) oxygen treatment is thermodynamically favourable based on calculations for bulk compounds [48,49]. The conditions during the calcination are probably slightly hydrothermal. However, the hydrothermal conditions have the greatest effect on metallic cobalt [21,23]. Introduction of oxygen has been reported to decrease the formation of silicates during hydrothermal treatment [21]. The thermodynamic calculations showed that when water and excess of oxygen are present the cobalt silicate formation is not probable below 400◦ C. When hydrogen and water is present the main cobalt species are CoO, Co and Co2 SiO4 . Water vapour gives mainly Co2 SiO4 and CoO. Reductive decomposition of the Co(NO3 )2 precursor has also led to poorly reducible species [20]. Indications of a similar behaviour have also been reported for the Co(acac)3 precursor [6]. However, the samples studied here are calcined before exposure to a reductive gas. Formation of silicides [11] is not thermodynamically favourable in the conditions that were used [48,49]. It should however be pointed out that the thermodynamic considerations presented here are valid only for bulk compounds. The conditions at the interfaces between the dispersed cobalt particles and the support may differ from bulk conditions. 3.4. Toluene hydrogenation The catalysts were tested in gas phase hydrogenation of toluene. The samples were reduced at 550◦ C, which was the temperature that gave a maximum hydrogen uptake. Previous hydrogenation runs [50] showed that reduction at 400◦ C was not sufficient to reduce the catalysts while reduction at 500◦ C gave active catalysts. This behaviour was completely different compared to catalysts prepared from Co-nitrate where the maximum in activity was reached after reduction at 300◦ C. Furthermore, reduction at 500◦ C of the Co-nitrate based samples resulted in almost inactive catalysts probably due to agglomeration [35].
Table 5 Reaction rate (catalysts reduced at 550◦ C) and TOF for toluene hydrogenation Cycles
Co content (wt.%)
Reaction rate (10−5 mol/gsample ×s)
TOF (s−1 )
1 3 5 10 30 50
5.9 13.7 19.5 5.7 13.4 19.0 4.6
0.51 0.57 0.95 0.30a 0.36a 0.73a 0.86a
0.29 0.18 0.26 0.26 0.17 0.21 0.22
b a
Reaction rate data from Backman et al. [6]. 4.6 wt.% Co/SiO2 catalyst prepared from Co(NO3 )2 by aqueous impregnation [35]. Catalyst reduced at 300◦ C. b
The hydrogenation runs were performed under atmospheric pressure at 170◦ C (H2 :toluene = 7:1), reaction rates and turn over frequencies (TOF) are shown in Table 5. The rate data shown in Table 5 was recorded after 1 h of reaction. The deactivation observed during that time was less than 10%. The reaction rate increased with cobalt content but no obvious trend was observed in the TOF values. The samples prepared on the support calcined at 600 and 450◦ C showed lower reaction rates than the samples prepared on the support calcined at 600 and 180◦ C. On the other hand, the hydrogen uptakes were also lower and therefore the TOF values were in the same range on all samples. Despite that the one-cycle ALE catalyst was well-dispersed (Table 2) it did not show a high reaction rate. The reaction rate on a 4.6 wt.% cobalt nitrate based catalyst prepared by aqueous impregnation [6,35] was clearly higher (Table 5). This was mainly due to the low extent of reduction of the ALE catalyst, which was 4% (Table 2), while the extent of reduction on the nitrate based catalyst was 84% [6]. The reaction rate per exposed cobalt (TOF) was, however, essentially equal (Table 5). This indicates that the surface area of reduced cobalt is the most important factor for determining the activity of these catalysts in this reaction, i.e. toluene hydrogenation on cobalt would not be a structure sensitive reaction. The hydrogenation of aromatics is often considered to be a structure insensitive reaction [28]. The reaction rate per gram cobalt shows a decrease with increasing metal loading. This suggests that a smaller part of the metal is available for the reaction.
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
233
This is in agreement with the dispersion estimations obtained from the chemisorption measurements.
cal Analysis of the Helsinki University of Technology are thanked for the cobalt determinations.
4. Conclusions
References
Silica supported cobalt catalysts with cobalt contents of 5.7 to 19.5 wt.% were prepared by ALE using Co(acac)3 as precursor. The catalysts were prepared by 1 to 5 sequential precursor addition and air calcination steps. The distribution of the cobalt species through the catalyst particles was found to be even on all samples. The surface area and pore volume decreased only slightly with increasing metal loading. The cobalt species were well dispersed on the samples prepared in one cycle. The dispersion decreased with increasing cobalt loading. The XRD reflections were very broad and weak even for a sample containing 19.5 wt.% cobalt. Considering the XRD, TEM and chemisorption measurements the cobalt species are dispersed on the support either as poorly ordered particles or as a two-dimensional surface silicate layer. The low degree of reduction even at high reduction temperatures (550◦ C) suggested a strong interaction between the cobalt species and the support. The formation of silicates was indicated by the XRD and XPS measurements. Cobalt silicates were probably formed during the calcination in air. The chemisorption of hydrogen on these catalysts was shown to be activated. The XPS and CO chemisorption measurements indicated that some sintering occurred during the reduction at 550◦ C. However, the catalysts were thermally stable compared to nitrate based cobalt catalysts. The reaction rate in toluene hydrogenation increased slightly with cobalt loading but the reaction rate per exposed cobalt (TOF) remained nearly constant on all samples indicating that hydrogenation of toluene on cobalt is a structure insensitive reaction.
Acknowledgements The Academy of Finland is gratefully acknowledged for the financial support. Fortum Oil and Gas Oy, Analytical Research is thanked for XPS, XRD, TEM and SEM/EDS measurements. The Technical Research Centre of Finland and the Centre for Chemi-
[1] J.H. Gregor, Catal. Lett. 7 (1990) 317. [2] A. Chauvel, B. Delmon, W.F. Hölderich, Appl. Catal. A 115 (1994) 173. [3] W.F. Taylor, H.K. Staffin, J. Phys. Chem. 71 (1967) 3314. [4] J. Völter, M. Hermann, K. Heise, J. Catal. 12 (1968) 307. [5] L. Babernics, P. Tétényi, L. Kertész, Z. Phys. Chem. Neue Folge 89 (1974) 237. [6] L.B. Backman, A. Rautiainen, A.O.I. Krause, M. Lindblad, Catal. Today 43 (1998) 11. [7] T.A. Kainulainen, M.K. Niemelä, A.O.I. Krause, Catal. Lett. 53 (1998) 97. [8] G. Sewell, C. O’Connor, E. van Steen, Appl. Catal. A 125 (1995) 99. [9] A. Lapidus, A. Krylova, V. Kazanskii, V. Borokov, A. Zaitsev, J. Rathousky, A. Zukal, M. Jancálková, Appl. Catal. 73 (1991) 65. [10] D.G. Castner, D.S. Santilli, ACS Symp. Ser. 248 (1984) 39. [11] J.M. Jablonski, D. Potoczna-Petru, J. Okal, L. Krajczyk, React. Kinet. Catal. Lett. 54 (1995) 15. [12] D. Potoczna-Petru, J.M. Jablonski, J. Okal, L. Krajczyk, Appl. Catal. A 175 (1998) 113. [13] H.F.J. van’t Blik, D.C. Koningsberger, R. Prins, J. Catal. 97 (1986) 210. [14] I. Puskas, T.H. Fleisch, J.B. Hall, B.L. Meyers, R.T. Roginski, J. Catal. 134 (1992) 615. [15] H. Ming, B.G. Baker, Appl. Catal. A 123 (1995) 23. [16] E. van Steen, G.S. Sewell, R.A. Makhothe, C. Micklethwaite, H. Manstein, M. de Lange, C.T. O’Connor, J. Catal. 162 (1996) 220. [17] K.E. Coulter, A.G. Sault, J. Catal. 154 (1995) 56. [18] K.E. Coulter, A.G. Sault, Mat. Res. Soc. Symp. Proc. 368 (1995) 115. [19] J.M. Jablonski, M. Wolcyrz, L. Krajczyk, J. Catal. 173 (1998) 530. [20] M.P. Rosynek, C.A. Polansky, Appl. Catal. 73 (1991) 97. [21] A. Kogelbauer, J.C. Weber, J.C. Goodwin Jr., Catal. Lett. 34 (1995) 259. [22] G.J. Haddad, J.G. Goodwin Jr., J. Catal. 157 (1995) 25. [23] Y. Okamoto, K. Nagata, T. Adachi, T. Imanaka, K. Inamura, T. Takyu, J. Phys. Chem. 95 (1991) 310. [24] T. Nguyen, H.L. Ho, D.E. Kotecki, T.D. Nguyen, J. Appl. Phys. 79 (1996) 1123. [25] E. Kondoh, R.A. Donaton, S. Jin, H. Bender, W. Vandervorst, K. Maex, Appl. Surf. Sci. 136 (1998) 87. [26] S. Haukka, E.-L. Lakomaa, A. Root, J. Phys. Chem. 97 (1993) 5085. [27] S. Smeds, T. Salmi, L.P. Lindfors, O. Krause, Appl. Catal. A 144 (1996) 177. [28] M. Che, C.O. Bennett, Adv. Catal. 36 (1989) 55. [29] A. Rautiainen, M. Lindblad, L.B. Backman, J. Chem. Soc., Faraday Trans. 1 (2000), submitted.
234
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
[30] M.K. Niemelä, A.O.I. Krause, T. Vaara, J. Lahtinen, Top. Catal. 2 (1995) 45. [31] S. Haukka, E.-L. Lakomaa, T. Suntola, in: A. Dabrowski (Ed.), Adsorption and its Applications in Industry and Environmental Protection, Elsevier, Amsterdam, Stud. Surf. Sci. Catal. 120 (1999) 715. [32] R.C. Reuel, C.H. Bartholomew, J. Catal. 85 (1984) 63. [33] J.M. Zowtiak, C.H. Bartholomew, J. Catal. 83 (1983) 107. [34] E. Gorlich, J. Haber, A. Stoch, J. Stoch, J. Solid State Chem. 33 (1980) 121. [35] M.K. Niemelä, L. Backman, A.O.I. Krause, T. Vaara, Appl. Catal. A 156 (1997) 319. [36] R. Chin, D.M. Hercules, J. Phys. Chem. 86 (1982) 360. [37] N.G. Farr, H.J. Griesser, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 293. [38] A. Hakuli, A. Kytökivi, E.-L. Lakomaa, A.O.I. Krause, Anal. Chem. 67 (1995) 1881. [39] L.B. Backman, A. Rautiainen, M. Lindblad, A.O.I. Krause, Appl. Catal. A 191 (2000) 55. [40] K. Sato. Y. Inoue, I. Kojima, E. Miyazaki, I. Yasumori, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 841.
[41] D. Pope, D.S. Walker, L. Whalley, R.L. Moss, J. Catal. 31 (1973) 335. [42] C.R. Brundle, T.J. Chuang, D.W. Rice, Surf. Sci. 60 (1976) 286. [43] F.P.J.M. Kerkhof, J.A. Moulijn, J. Phys. Chem. 83 (1979) 1612. [44] D.C. Castner, P.R. Watson, I.Y. Chan, J. Phys. Chem. 94 (1990) 819. [45] C.R.F. Lund, J.A. Dumesic, J. Catal. 72 (1981) 21. [46] C.D. Wagner, L.H. Gale, R.H. Raymond, Anal. Chem. 51 (1979) 466. [47] Y. Okamoto, T. Adachi, K. Nagata, M. Odawara, T. Imanaka, Appl. Catal. 73 (1991) 249. [48] I. Barin, Thermochemical data of pure substances, VCH Verlags Gesellschaft, Weinheim, 1989. [49] O. Knacke, O. Kubaschewski, K. Hesselman, Thermochemical Properties of Inorganic Substances, 2nd Edition, Springer, Berlin, 1991. [50] L.B. Backman, M.Sc. Thesis, Helsinki University of Technology, Finland, 1995, p. 70.
Characterisation of Co/SiO2 catalysts prepared from Co(acac)3 by gas phase deposition L.B. Backman a,∗ , A. Rautiainen b , M. Lindblad b , O. Jylhä c , A.O.I. Krause a a
Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland b Fortum Oil and Gas OY, P.O. Box 110, FIN-00048 Fortum, Finland c Microchemistry Ltd, P.O. Box 132, FIN-02631 Espoo, Finland Received 27 September 1999; received in revised form 23 June 2000; accepted 26 June 2000
Abstract Silica supported cobalt catalysts prepared by a chemical vapour deposition technique, atomic layer epitaxy (ALE), were characterised in order to study the interaction between cobalt species and silica. The catalysts were prepared by chemisorption of cobalt(III)acetylacetonate from the gas phase onto silica. The metal loading on the catalysts varied from 5.7 to 19.5 wt.%. Concentration profiles obtained with SEM/EDS showed that the cobalt species were evenly distributed through the catalyst particles. The dispersion was estimated by hydrogen chemisorption. The metallic cobalt was well dispersed on samples containing less than 6 wt.% cobalt but the dispersion decreased with increasing cobalt loading. XRD showed only weak reflections even for a sample containing 19.5 wt.%, which indicated weakly ordered cobalt species. The degree of reduction estimated by XPS was less than 30% on all samples even after reduction at 550◦ C for 7 h. The low reducibility could be explained by the formation of cobalt silicate during air calcination. The presence of silicates was also indicated by XRD and XPS. The catalysts were tested for gas phase toluene hydrogenation in a microreactor system. The reaction rate per gram sample increased with cobalt loading but the turn over frequency remained essentially constant on all samples. This indicated that the surface area of metallic cobalt is the main factor in determining the overall activity of the catalysts in this reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cobalt catalysts (supported); Atomic layer epitaxy; Gas phase deposition; Cobalt silicate
1. Introduction Cobalt has been extensively used as catalyst for hydrogenation and hydrotreating reactions. One of the most common applications is the Fischer–Tropsch reaction, which has been studied in numerous publications. Cobalt based catalysts are already used in industrial scale operation for synthesis of hydrocarbons [1,2]. Studies of the kinetics of aromatics hydro∗ Corresponding author. Tel.: +358-9-451-2627; fax: +358-9-451-2622. E-mail address: [email protected] (L.B. Backman).
genation on cobalt are far less frequent [3–5]. In addition, cobalt catalysts prepared from cobalt acetylacetonate [6] have shown promising results in ethene hydroformylation [7]. Cobalt has also been used for preparation of ethylamines by reductive amination of ethanol. According to Sewell et al. [8], cobalt is considerably more active than nickel for this reaction. The wide variety of applications of supported cobalt in catalysis underlines the importance to understand the interaction between the catalytic material and the support. Silica has generally been regarded as an inert support material compared to other commonly used inorganic oxides [9,10]. However, there
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 7 0 7 - 9
224
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
have been several reports of strong interaction between cobalt species and silica [6,11–25]. Formation of silicates and hydrosilicates has been observed during precipitation of Co/SiO2 catalysts due to slightly alkaline conditions [14] and during aqueous impregnation due to a pH ≥ 5 [15,16]. The reducibility of cobalt can be strongly influenced by the drying procedure [16–18]. High temperature calcination both in oxygen (940–1000◦ C) [15–19] and in an inert atmosphere (810◦ C) [19] can form silicates. Poorly reducible CoOx –SiO2 species have also been proposed to form due to reductive decomposition of Co(NO3 )2 [20]. High reduction temperatures have been suggested to induce formation of Co–Si solid solutions and silicides [11]. Kogelbauer et al. showed that reduced cobalt produces silicates/hydrosilicates under hydrothermal conditions [21]. The same has been observed after a secondary aqueous impregnation of pre-reduced and passivated Co/SiO2 catalysts during drying at >50◦ C [22]. The precursor [20,23], the impregnation solvent [16], and support surface area [14,16,19] also affects the amount of silicate that is formed during the different stages. Formation of Co2 SiO4 has also been observed during rapid thermal annealing of cobalt thin films on SiO2 [24,25]. The trend towards ‘tailor made’ catalysts has led to the use of new preparation methods. The preparation of catalysts using chemical vapour deposition (CVD) techniques is an approach to prepare catalysts with well-defined properties. A related method, atomic layer epitaxy (ALE), differs from conventional CVD techniques mainly in the choice of process conditions [26]. In suitable conditions, defined as the process window, only reaction between functional groups at the surface and the catalyst precursor occurs. The number of available surface sites determines the amount chemisorbed on the surface at saturation density, which makes the process self-controlled. The number and nature of the reactive sites can be controlled by changing the pre-treatment temperature of the support [26]. Gas–solid reactions can thus be used for preparation of structurally well-defined catalysts. Well dispersed alumina supported nickel catalysts active for hydrogenation of toluene have been prepared by ALE [27]. In this paper, we expanded the scope of our research to cobalt catalysts. Silica supported cobalt catalysts were prepared by ALE using cobalt acetylacetonate as precursor. This work was carried
out in order to study the interaction of well-dispersed cobalt species with the silica support. The catalyst activity was tested by toluene hydrogenation. Hydrogenation of toluene is considered to be a structure insensitive reaction [28], therefore the reaction rate should be a good measure of the cobalt surface area.
2. Experimental 2.1. Catalyst preparation The catalysts were prepared by chemisorption of cobalt(III)acetylacetonate Co(acac)3 (Merck, >98%) from the gas phase onto silica (Silica Grace 432) [29]. The purity of the support has been reported earlier [30]. Thermal pre-treatment of the silica support was carried out with air in a muffle furnace at 600◦ C for 16 h. The support was further heated in a flow of dry nitrogen at 180 or 450◦ C immediately before the chemisorption of the precursor. The deposition process was carried out in an ALE reactor [31] with the silica particles (d = 0.5–1.0 mm) placed in a fixed bed. The precursor was evaporated into flowing nitrogen at 180◦ C and the surface of the support was saturated by dosing excess of precursor. The chemisorption step was followed by air treatment at 450◦ C in order to remove the acac-ligands. A metal content of about 6 wt.% was reached by saturating the surface once. To obtain higher cobalt contents additional cycles were used, each followed by air treatment at 450◦ C. The catalysts were prepared using one to five sequential chemisorption and air treatment cycles, which gave catalysts with cobalt contents ranging from 5.7 to 19.5 wt.%. 2.2. Physisorption and chemisorption measurements The surface area (BET) and pore volume, of calcined catalyst samples and of the support pre-treated at 600◦ C, were measured by nitrogen physisorption at the temperature of liquid nitrogen. Both the adsorption and desorption isotherms were measured. Prior to measurement the samples were outgassed at 90◦ C for 1 h followed by outgassing at 300◦ C down to less than 10–5 mbar for 3 h. Hydrogen and CO chemisorption measurements were performed by the static volumetric method. The hydrogen chemisorption capacity
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
was measured at 30◦ C on catalysts reduced in situ in flowing hydrogen at 500, 550 and 600◦ C for 7 h. Hydrogen chemisorption was also performed at 100◦ C on samples reduced at 550◦ C. The CO chemisorption capacity at 30◦ C was determined for samples reduced at 550◦ C. The instrument used for both the physisorption and chemisorption measurements was a Coulter OMNISORP 100CX. The samples were outgassed for 60 min at the reduction temperature before allowing them to cool down in vacuum to the measurement temperature. Both the total and reversible chemisorption isotherms were recorded and the samples were outgassed for 40 min in between at the measurement temperature. The chemisorption capacity was determined by extrapolating the flat portion of the isotherm between 8 and 27 kPa (60–200 Torr) to zero pressure. The dispersion was calculated using the total hydrogen uptake at 100◦ C [32,33]. The average crystallite size was calculated by assuming spherical geometry of the cobalt metal particles. A surface site density of 14.6 atoms/nm2 for the Co crystallites was assumed [32].
225
model 101, Surface Science Instruments, VG Fisons). The binding energy scale was fixed by using a binding energy of 103.7 eV for the Si 2p peak [34]. In the case of Co3 O4 where no silicon was present carbon (C 1s, 284.6 eV) was used instead. The degree of reduction was estimated from the ratio of the metallic peak area to the total peak area of the Co 2p2/3 region. Overlapping peaks in the XP-spectra were deconvoluted by fitting symmetrical Gauss–Lorenzian (80/20) peaks and using the Shirley background subtraction. The calculations were made by using the peak fitting routines supplied by the manufacturer. The estimation involves some uncertainties since the method used for background subtraction can influence the values obtained for the degree of reduction [35]. The particle size can affect the result due to the surface sensitivity of XPS, i.e. the core of large particles will not be probed. A difference in size between the oxidic and metallic cobalt particles could also affect the estimation of the degree of reduction [36]. When using Al K␣ radiation the Auger transition Co L3 M2,3 M4,5 can increase the intensity of the low binding energy side of the Co 2p3/2 peak [37].
2.3. X-ray diffraction 2.5. TEM and SEM/EDS The crystal structure of the cobalt species was studied by X-ray diffraction (XRD). The wide angle X-ray scattering patterns were collected in reflection mode using a Siemens D500 diffractometer equipped with a Cu-anode and a graphite monochromator in the reflected beam. Identification of the reflections was done with reference data from the JCPDS reference library. Both calcined and reduced (550◦ C/7 h) samples were studied. The samples were transferred to the measurement chamber through air. 2.4. X-ray photoelectron spectroscopy Both calcined and reduced samples were analysed by X-ray photoelectron spectroscopy (XPS). The XP spectra of cobalt oxide (Co3 O4 , Aldrich, 99.995%) and cobalt silicate (Co2 SiO4 , Pfaltz & Bauer) were also measured. Catalyst samples containing 5.7, 13.4 and 19.0 wt.% cobalt were pre-reduced in flowing hydrogen at 550◦ C for 7 h and transferred inertly into the measurement chamber. The other samples were transferred through air. The measurements were done using monochromatized Al K␣ X-rays (X-probe
The transmission electron microscope (TEM) micrographs were obtained with a JEOL JEM-1200EX II apparatus operated at an accelerating voltage of 100 keV. The samples were finely ground using an agate mortar, and were collected on a carbonised copper-grid. Two sets of every sample were ground separately. The samples were studied visually at different magnifications to confirm the homogeneity of the samples. The actual micrographs were obtained using a magnification of 100,000 times. The lower limit for measuring crystallite diameters with this apparatus was about 2 nm. The distribution of cobalt through the catalyst particle was studied by a scanning electron microscope equipped with an energy dispersive spectrometer (SEM/EDS). The samples for the SEM/EDS analyses were moulded into epoxy resin. The hardened sample blocks were sliced with a microtome equipped with a glass knife to obtain cross-section cuts of the particles. The SEM/EDS analyses were made from smooth cross-section cuts. The line profiles of silicon and cobalt were measured through five particles.
226
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
2.6. Hydrogenation runs The performance of the catalysts was tested in gas phase hydrogenation of toluene in a microreactor system. The activity runs were done with a down flow quartz glass tubular reactor at atmospheric pressure with a total molar flow of 0.8 mol/h and a molar ratio of hydrogen to toluene of 7 to 1. The reaction temperature was 170◦ C and the amount of catalyst was between 0.3 and 0.5 g. A fresh sample was loaded for every run and the catalyst was activated by reduction in situ in flowing hydrogen at 550◦ C for 7 h. The product stream was analysed on-line with a Gasmet FT-IR multi-component gas analyser (TEMET Instruments). The IR absorbance spectra were measured over a wave-number range of 950 to 4000 cm−1 with a resolution of 8 cm−1 . The analyser was calibrated by flowing pure compounds diluted in nitrogen through the measurement cell. The analysis was performed by fitting a linear combination of the reference spectra to the measured spectra [38]. The activity data reported in this publication was collected after 1 h of reaction time. The reaction rate per exposed cobalt or turn over frequency (TOF) was calculated using the amount of surface atoms obtained from the total hydrogen uptake at 100◦ C.
3. Results and discussion 3.1. Distribution of the cobalt species The catalysts were prepared by applying one to five sequential Co(acac)3 chemisorption and air calcina-
tion steps which gave catalysts with cobalt contents ranging from 5.7 to 19.5 wt.% (Table 1). The catalyst preparation is described in detail elsewhere [29]. The precursor Co(acac)3 is proposed to react with the OH-sites on the support resulting in the loss of one acac-ligand [29]. One preparation cycle on the support pre-treated at 600 and 180◦ C gave a cobalt loading of 5.9 wt.% and two cycles gave a loading of 10.6 wt.% Co. The increase in cobalt content was almost linear during the following chemisorption steps, about 3.1 wt.% per cycle. The cobalt content on the samples prepared on supports pre-treated at 600 and 450◦ C was slightly lower possibly due to a different amount or distribution of OH-groups on the surface. Line profiles of cobalt measured through the catalyst particles by SEM/EDS showed that the distribution of cobalt was even through the catalyst particles. The nitrogen physisorption measurements showed a decrease in surface area and pore volume with increasing cobalt content (Table 1). The surface area decreased from 330 m2 /gsample for the pure support to 230 m2 /gsample for the sample containing 19.5 wt.% Co. Calculated per gram of silica, assuming a stoichiometry of 1:1 (Co:O) for the cobalt species, the surface area decreased from 330 to 306 m2 /gSiO2 . This implies that the cobalt species are finely distributed on the support. The pore volume decreased from 1.19 to 0.81 cm3 /gsample . Calculated per amount of silica the pore volume decreased from 1.19 to 1.08 cm3 /gSiO2 , i.e. only slight pore blocking due to the cobalt species. In addition, a slight shift towards smaller pores was observed in the pore size distribution probably due to deposition of cobalt species on the surface of the pore
Table 1 Metal content, surface area, pore volume and average pore diameter (d ave = 4∗ A/V) as a function of preparation cycles Cycles
Silica pretreatment temperature (◦ C)
Co content (wt.%)
SABET (m2 /gsample )
SABET (m2 /gSiO2 )
Pore volume (cm3 /gsample )
Pore volume (cm3 /gSiO2 )
dave (nm)
– 1 2 3 4 5 10 30 50
600 600 + 180 600 + 180 600 + 180 600 + 180 600 + 180 600 + 450 600 + 450 600 + 450
– 5.9 10.6 13.7 16.8 19.5 5.7 13.4 19.0
330 299
330 324
1.19 1.06
1.19 1.15
14.0 14.2
266
322
0.92
1.11
13.8
230
306
0.81
1.08
14.1
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
227
Fig. 1. Hydrogen uptake as a function of reduction temperature: (䊊) 5.9; (䊐) 13.7; and (×) 19.5 wt.% Co.
walls. However, the average pore diameter remained constant at 14.0 ± 0.2 nm (Table 1). The total hydrogen uptake of the samples was determined by static volumetric measurements at 30◦ C. The irreversible uptake was also measured and the general trends were the same as for the total uptake. The catalysts were reduced in situ in flowing hydrogen for 7 h at 500, 550 and 600◦ C. A maximum hydrogen uptake at 30◦ C was observed on all samples after reduction at 550◦ C (Fig. 1). The hydrogen uptake at 30◦ C increased with cobalt content but remained modest even on the 19.5 wt.% catalyst (18.4 mol/gsample ). The uptake per gram cobalt decreased with increasing metal loading indicating a decreasing dispersion (Table 2). The chemisorption
capacity was also measured at 100◦ C on samples reduced at 550◦ C. The hydrogen uptake increased as the measurement temperature was increased from 30 to 100◦ C (Table 2) which indicated that the chemisorption of hydrogen was activated on these catalysts. The increase varied from 5 to 46% but did not show any clear trend as a function of metal loading (Table 2). Activated chemisorption of hydrogen on cobalt has been reported earlier by Bartholomew et al. [32,33]. The activation energy of hydrogen chemisorption on cobalt has been reported to increase with increasing cobalt–support interaction [33]. Therefore, the increase would be expected to be higher at low cobalt contents, i.e. at higher dispersions with stronger interaction between cobalt and support.
Table 2 Hydrogen uptake, degree of reduction (f), dispersion (D) and crystallite size (d) after reduction at 550◦ C Cycles
Co content (wt.%)
H2 uptake at 30◦ C (mol/gsample )
H2 uptake at 30◦ C (mol/gCo )
H2 uptake at 100◦ C (mol/gsample )
Activation factora
f (%)
D (%)
d (nm)
1 2 3 4 5 10 30 50
5.9 10.6 13.7 16.8 19.5 5.7 13.4 19.0
8.96 13.0 15.6 15.3 18.4 4.59 9.26 15.5
152 122 114 91 94 81 69 82
11.7 13.7 19.2 22.3 23.2 5.74 11.1 17.4
1.30 1.05 1.23 1.46 1.26 1.25 1.20 1.12
12
19
4.9
a b
The activation factor is defined as the ratio of the uptake at 100◦ C to the uptake at 30◦ C. Determined by O2 -titration at 400◦ C.
50b
3.3
64b 4 23 28
2.2 29 4.3 3.8
29 44 3.4 23 25
228
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
Fig. 2. TEM image of calcined samples (a) 13.7 and (b) 19.5 wt.% Co. Bar shown in figure equals 50 nm.
The dispersion and average crystallite size of the cobalt species on the reduced catalyst was estimated from the total hydrogen uptake at 100◦ C (Table 2). The values were corrected with the degree of reduction (obtained by XPS or O2 -titration [39]) by assuming that the metallic cobalt forms a separate phase [36]. The stoichiometry for total hydrogen chemisorption on cobalt at the temperature of maximum uptake can be taken as 1.0 (H/Co) [32]. The dispersion of cobalt on the 5.7 wt.% sample was 29%, i.e. a high value for Co/SiO2 catalysts. The corresponding size of the metallic cobalt particles was 3.4 nm. However, the dispersion decreased with increasing cobalt content, e.g. at 13.4 wt.% the dispersion was 4.3%. A further increase in cobalt content did not significantly decrease the dispersion (Table 2). Calcined samples made by three and five preparation cycles were studied by TEM (Fig. 2). The TEM images showed round shaped cobalt particles of about 10 nm in size. This corresponds to a dispersion of about 10% assuming spherical particles. This is roughly in agreement with the average particle sizes obtained by hydrogen chemisorption (27 and 28 nm),
taking into account that some sintering probably occurred during the reduction prior to the chemisorption measurements. Also some larger particles of 100 to 200 nm were observed and especially on the catalyst prepared by five cycles some crystalline-like phases were seen in the TEM images. 3.2. The nature of the cobalt species The CO chemisorption capacity was measured at 30◦ C on both calcined and reduced (550◦ C/7 h) samples (Table 3). The total CO uptake on the reduced samples was nearly independent of cobalt loading (44 ± 6 mol/g). The relative variations in the irreversible uptake were bigger and a slight increase in the uptake with metal loading was observed. Between 29 and 49% of the CO uptake was irreversible (Table 3). The total CO uptake was 55 mol/g on both the calcined samples containing 13.7 and 19.5 wt.% which was 5 and 17 mol/g higher than compared to the reduced samples, respectively. However, only 7–9% of the uptake was irreversible. The decrease in total chemisorption of CO from calcined to reduced
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
229
Table 3 CO uptake measured at 30◦ C on calcined (450◦ C) and reduced (550◦ C) samples Cycles
Co content (wt.%)
Treatment
Total CO uptake (mol/gsample )
Irreversible CO uptake (mol/gsample )
Irreversible CO uptake (%)
3 5 1 2 3 4 5
13.7 19.5 5.9 10.6 13.7 16.8 19.5
Calcined Calcined Reduced Reduced Reduced Reduced Reduced
54.9 54.5 44.8 42.8 49.7 43.3 38.0
4.0 4.7 13.0 13.7 22.4 14.2 18.6
7.4 8.7 29 32 45 33 49
samples could indicate sintering of the cobalt species or a different adsorption stoichiometry. The adsorption of CO on different cobalt species has been studied by infra-red (IR) spectroscopy [9,40]. The IR measurements have shown that CO adsorbs in a linear form on Co2+ , Coδ+ and Co0 and in a bridged form on Co0 , i.e. the metallic cobalt adsorbs somewhat less CO. The ratio of CO(tot) to H(tot, 30◦ C) was found to be bigger than or equal to unity for all metal contents, ranging from 2.5 for 5.9 wt.% to 1.0 for 19.5 wt.% Co (Fig. 3). When the hydrogen uptake was measured at 100◦ C the ratio decreased, ranging from 1.9 to 0.82, respectively. The ratio of CO(irrev) to H(tot, 30◦ C) varied between 0.72 and 0.46 but did not change monotonously with cobalt loading (Fig. 3). Adsorption of CO on non-metallic cobalt species probably distorts the amounts of total uptake. It is known that
cobalt oxide (Co3 O4 ) readily adsorbs CO [41]. However, it is not likely that Co3 O4 would be present after reduction at 550◦ C but other non-metallic species could remain on the samples, such as cobalt silicates. The changes in the CO to H ratio could also be explained by a change in stoichiometry for the chemisorption of CO on cobalt as a function of the particle size. Reuel and Bartholomew [32] have reported that the stoichiometry of irreversible adsorption (CO/Co) varied between 0.4 and 2.3 depending on metal loading, support and preparation. The crystalline species on both calcined and reduced catalysts with cobalt contents of 13.4 and 19.0 wt.% were studied by XRD. The calcined sample, that contained 19.0 wt.% Co showed weak reflections at 36, 59 and 65◦ 2θ . These reflections coincide with the strongest reflections of Co3 O4 but also with
Fig. 3. Ratio of CO and H uptakes as a function of cobalt loading. (䉬) CO(tot)/H(tot, 30◦ C); (䉱) CO(tot)/H(tot, 100◦ C); (䉫) CO(irrev)/H(tot, 30◦ C); (4) CO(irrev)/H(tot, 100◦ C).
230
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
Fig. 4. XRD spectra of reduced samples containing (a) 19.0, (b) 13.4 wt.% Co and of (c) the pure support.
the strongest reflections of Co2 SiO4 . Following hydrogen reduction at 550◦ C broad reflections at 36, 44, 61 and 65◦ 2θ were obtained (Fig. 4). Co2 SiO4 and Co3 O4 have their strongest reflections at 36.6 and 36.8◦ , respectively. The strongest reflection of metallic cobalt is at 44.2◦ 2θ, but the peak observed at 44◦ 2θ probably has a contribution of Co2 SiO4 at 44.5◦ and Co3 O4 at 44.8◦ 2θ. The features at 61 and 65◦ 2θ are probably the sums of the peaks of Co2 SiO4 at 58.9 and 64.7◦ and of Co3 O4 at 59.3 and 65.2◦ 2θ. The XRD measurements indicate the presence of Co3 O4 and/or Co2 SiO4 . However, the crystallite sizes of the different phases were not determined due to broad, weak and partly overlapping peaks. The XRD analysis of samples with a cobalt loading of 13.4 wt.% showed reflections at similar 2θ values as the 19.0 wt.% Co, but the intensities were even weaker (Fig. 4). The particle sizes obtained from the chemisorption measurements, ranging from about 4.2 to 28 nm (Table 2), are considerably larger than indicated by the weak XRD pattern. This result suggests that the cobalt particles consist of weakly ordered species. It is also possible that the cobalt species are present as a two-dimensional surface silicate [14], which would be difficult to detect by XRD. Three reduced samples (one, three and five cycles) and one calcined sample (three cycles) were studied by XPS to determine the chemical state of the surface cobalt species. These samples were prepared on
silica pretreated at 600 and 450◦ C. Reference spectra of Co3 O4 and Co2 SiO4 were also measured. The reduced samples were pre-reduced in flowing hydrogen at 550◦ C for 7 h and transferred inertly into the XPS measurement cell. The other samples were transferred through air. The Co 2p3/2 XP spectra of samples prepared by one, three and five cycles are shown in Fig. 5. The peak at about 778.3 eV that is present in all
Fig. 5. XP spectra of catalysts prepared by 1 (a), 3 (b) and 5 (c) preparation cycles. The curves are scaled to the same maximum intensity.
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
231
Table 4 Binding energies (Co 2p3/2 ), intensity ratios of Co 2p3/2 /Si 2p and degree of reduction Sample
Co (wt.%)
Co2 SiO4 3, calcined 1, reduced 3, reduced 5, reduced Co3 O4
13.4 5.7 13.4 19.0
a
BE Co0 (eV)
778.3 778.3 778.4
BE Co2+ (eV)
Co/Si×100
Co0 (%)
781.9 781.8 781.7 781.0 780.4 779.5a
48.3 18.2 4.5 6.4 8.2
4 23 28
Binding energy reference C 1 s, 284.6 eV.
three spectra of the reduced samples can be ascribed to metallic cobalt, although in the spectrum of the one-cycle sample it can be seen only as a very weak shoulder. The shake-up satellite features at about 6 eV higher binding energy from the peaks on the high binding energy side of the metallic peak suggests that Co2+ is present on the surface (Fig. 5) [42]. The ratios of the Co 2p3/2 and the Si 2p peak areas of the reduced samples increased with metal content (Table 4). The Co 2p3/2 and Si 2p peak ratio of the calcined three-cycle sample was 2.9 times higher than on the reduced sample. According to the formula of Kerkhof and Moulijn [43] the ratio increases with increasing dispersion at constant cobalt loading. A significant decrease in the ratio during the reduction at 550◦ C could indicate that some sintering had occurred. Other possible reasons would be encapsulation of cobalt by silica due to migration of silica [44,45], or migration of cobalt into the support. The degree of reduction was estimated from the ratio of the metallic peak area to the total area of the Co 2p2/3 region. The degree of reduction increased with cobalt content (Table 4). The sample prepared with one deposition cycle (5.7 wt.%) gave a very weak signal for metallic cobalt and the degree of reduction was estimated to be only about 4%. On the three-cycle sample (13.4 wt.%) the degree of reduction was considerably higher (23%), but even on the five-cycle sample (19.0 wt.%) only about 28% of the cobalt had reduced to metallic cobalt. This shows that a large amount of the cobalt species on the catalyst is in a poorly reducible state. Temperature programmed reduction studies of these catalysts [6] showed that reduction mainly occur at temperatures above 700◦ C which indicated that the samples contained cobalt species that are hard to reduce, e.g. cobalt silicates. The degrees of reduction of the samples prepared on silica calcined at 600 and
180◦ C are given in Table 2. The reduction extents are higher on these samples. This is probably partly due to the different measurement method, i.e. XPS versus oxygen titration [6]. XPS seem to underestimate the amount of metallic cobalt [35]. The extents of reduction were all below 60%, independent of the measurement method, which can be considered low taking into account the high reduction temperature (550◦ C/7 h). The position of the metallic peak remained constant for all the samples at 778.3 ± 0.1 eV while the binding energies of the non-metallic peaks decreased with metal loading (Table 4). The peak shift could be a chemical shift due to an increased amount of silicate [46] on the surface, i.e. at low metal loading the cobalt is present almost completely as silicates. A similar peak shift has been reported for calcined Co/Al2 O3 samples by Chin and Hercules [36] and by Okamoto et al. [47]. Furthermore, the Co 2p3/2 region in the XP spectra of the one-cycle sample, the calcined three-cycle sample and the Co2 SiO4 were of similar shape. These findings suggest that the samples contain high concentrations of cobalt silicate, which agrees with the indications obtained from the XRD measurements. 3.3. Cobalt silica interaction Strong interaction between cobalt species and silica can cause formation of silicates and/or hydrosilicates [6,12–25]. Alkaline conditions during the precipitation/impregnation have increased the formation of silicates [14–16]. The drying procedure has also been proven important for the reducibility of the catalyst [16–18]. In our case, where the preparation does not involve any solvent, these are not important. However, the cobalt acetylacetonates have been proposed to interact with the OH groups on the silica [29]
232
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
forming bonds between silicon, oxygen and cobalt. These highly dispersed cobalt species could then convert into a surface silicate during the calcination in air at 450◦ C, or possibly even partly during the nitrogen purge at 450◦ C. Silicates have been formed during high temperature calcination (>900◦ C) in oxygen [15–19] or in an inert [19]. The formation of silicates during high temperature (>500◦ C) oxygen treatment is thermodynamically favourable based on calculations for bulk compounds [48,49]. The conditions during the calcination are probably slightly hydrothermal. However, the hydrothermal conditions have the greatest effect on metallic cobalt [21,23]. Introduction of oxygen has been reported to decrease the formation of silicates during hydrothermal treatment [21]. The thermodynamic calculations showed that when water and excess of oxygen are present the cobalt silicate formation is not probable below 400◦ C. When hydrogen and water is present the main cobalt species are CoO, Co and Co2 SiO4 . Water vapour gives mainly Co2 SiO4 and CoO. Reductive decomposition of the Co(NO3 )2 precursor has also led to poorly reducible species [20]. Indications of a similar behaviour have also been reported for the Co(acac)3 precursor [6]. However, the samples studied here are calcined before exposure to a reductive gas. Formation of silicides [11] is not thermodynamically favourable in the conditions that were used [48,49]. It should however be pointed out that the thermodynamic considerations presented here are valid only for bulk compounds. The conditions at the interfaces between the dispersed cobalt particles and the support may differ from bulk conditions. 3.4. Toluene hydrogenation The catalysts were tested in gas phase hydrogenation of toluene. The samples were reduced at 550◦ C, which was the temperature that gave a maximum hydrogen uptake. Previous hydrogenation runs [50] showed that reduction at 400◦ C was not sufficient to reduce the catalysts while reduction at 500◦ C gave active catalysts. This behaviour was completely different compared to catalysts prepared from Co-nitrate where the maximum in activity was reached after reduction at 300◦ C. Furthermore, reduction at 500◦ C of the Co-nitrate based samples resulted in almost inactive catalysts probably due to agglomeration [35].
Table 5 Reaction rate (catalysts reduced at 550◦ C) and TOF for toluene hydrogenation Cycles
Co content (wt.%)
Reaction rate (10−5 mol/gsample ×s)
TOF (s−1 )
1 3 5 10 30 50
5.9 13.7 19.5 5.7 13.4 19.0 4.6
0.51 0.57 0.95 0.30a 0.36a 0.73a 0.86a
0.29 0.18 0.26 0.26 0.17 0.21 0.22
b a
Reaction rate data from Backman et al. [6]. 4.6 wt.% Co/SiO2 catalyst prepared from Co(NO3 )2 by aqueous impregnation [35]. Catalyst reduced at 300◦ C. b
The hydrogenation runs were performed under atmospheric pressure at 170◦ C (H2 :toluene = 7:1), reaction rates and turn over frequencies (TOF) are shown in Table 5. The rate data shown in Table 5 was recorded after 1 h of reaction. The deactivation observed during that time was less than 10%. The reaction rate increased with cobalt content but no obvious trend was observed in the TOF values. The samples prepared on the support calcined at 600 and 450◦ C showed lower reaction rates than the samples prepared on the support calcined at 600 and 180◦ C. On the other hand, the hydrogen uptakes were also lower and therefore the TOF values were in the same range on all samples. Despite that the one-cycle ALE catalyst was well-dispersed (Table 2) it did not show a high reaction rate. The reaction rate on a 4.6 wt.% cobalt nitrate based catalyst prepared by aqueous impregnation [6,35] was clearly higher (Table 5). This was mainly due to the low extent of reduction of the ALE catalyst, which was 4% (Table 2), while the extent of reduction on the nitrate based catalyst was 84% [6]. The reaction rate per exposed cobalt (TOF) was, however, essentially equal (Table 5). This indicates that the surface area of reduced cobalt is the most important factor for determining the activity of these catalysts in this reaction, i.e. toluene hydrogenation on cobalt would not be a structure sensitive reaction. The hydrogenation of aromatics is often considered to be a structure insensitive reaction [28]. The reaction rate per gram cobalt shows a decrease with increasing metal loading. This suggests that a smaller part of the metal is available for the reaction.
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
233
This is in agreement with the dispersion estimations obtained from the chemisorption measurements.
cal Analysis of the Helsinki University of Technology are thanked for the cobalt determinations.
4. Conclusions
References
Silica supported cobalt catalysts with cobalt contents of 5.7 to 19.5 wt.% were prepared by ALE using Co(acac)3 as precursor. The catalysts were prepared by 1 to 5 sequential precursor addition and air calcination steps. The distribution of the cobalt species through the catalyst particles was found to be even on all samples. The surface area and pore volume decreased only slightly with increasing metal loading. The cobalt species were well dispersed on the samples prepared in one cycle. The dispersion decreased with increasing cobalt loading. The XRD reflections were very broad and weak even for a sample containing 19.5 wt.% cobalt. Considering the XRD, TEM and chemisorption measurements the cobalt species are dispersed on the support either as poorly ordered particles or as a two-dimensional surface silicate layer. The low degree of reduction even at high reduction temperatures (550◦ C) suggested a strong interaction between the cobalt species and the support. The formation of silicates was indicated by the XRD and XPS measurements. Cobalt silicates were probably formed during the calcination in air. The chemisorption of hydrogen on these catalysts was shown to be activated. The XPS and CO chemisorption measurements indicated that some sintering occurred during the reduction at 550◦ C. However, the catalysts were thermally stable compared to nitrate based cobalt catalysts. The reaction rate in toluene hydrogenation increased slightly with cobalt loading but the reaction rate per exposed cobalt (TOF) remained nearly constant on all samples indicating that hydrogenation of toluene on cobalt is a structure insensitive reaction.
Acknowledgements The Academy of Finland is gratefully acknowledged for the financial support. Fortum Oil and Gas Oy, Analytical Research is thanked for XPS, XRD, TEM and SEM/EDS measurements. The Technical Research Centre of Finland and the Centre for Chemi-
[1] J.H. Gregor, Catal. Lett. 7 (1990) 317. [2] A. Chauvel, B. Delmon, W.F. Hölderich, Appl. Catal. A 115 (1994) 173. [3] W.F. Taylor, H.K. Staffin, J. Phys. Chem. 71 (1967) 3314. [4] J. Völter, M. Hermann, K. Heise, J. Catal. 12 (1968) 307. [5] L. Babernics, P. Tétényi, L. Kertész, Z. Phys. Chem. Neue Folge 89 (1974) 237. [6] L.B. Backman, A. Rautiainen, A.O.I. Krause, M. Lindblad, Catal. Today 43 (1998) 11. [7] T.A. Kainulainen, M.K. Niemelä, A.O.I. Krause, Catal. Lett. 53 (1998) 97. [8] G. Sewell, C. O’Connor, E. van Steen, Appl. Catal. A 125 (1995) 99. [9] A. Lapidus, A. Krylova, V. Kazanskii, V. Borokov, A. Zaitsev, J. Rathousky, A. Zukal, M. Jancálková, Appl. Catal. 73 (1991) 65. [10] D.G. Castner, D.S. Santilli, ACS Symp. Ser. 248 (1984) 39. [11] J.M. Jablonski, D. Potoczna-Petru, J. Okal, L. Krajczyk, React. Kinet. Catal. Lett. 54 (1995) 15. [12] D. Potoczna-Petru, J.M. Jablonski, J. Okal, L. Krajczyk, Appl. Catal. A 175 (1998) 113. [13] H.F.J. van’t Blik, D.C. Koningsberger, R. Prins, J. Catal. 97 (1986) 210. [14] I. Puskas, T.H. Fleisch, J.B. Hall, B.L. Meyers, R.T. Roginski, J. Catal. 134 (1992) 615. [15] H. Ming, B.G. Baker, Appl. Catal. A 123 (1995) 23. [16] E. van Steen, G.S. Sewell, R.A. Makhothe, C. Micklethwaite, H. Manstein, M. de Lange, C.T. O’Connor, J. Catal. 162 (1996) 220. [17] K.E. Coulter, A.G. Sault, J. Catal. 154 (1995) 56. [18] K.E. Coulter, A.G. Sault, Mat. Res. Soc. Symp. Proc. 368 (1995) 115. [19] J.M. Jablonski, M. Wolcyrz, L. Krajczyk, J. Catal. 173 (1998) 530. [20] M.P. Rosynek, C.A. Polansky, Appl. Catal. 73 (1991) 97. [21] A. Kogelbauer, J.C. Weber, J.C. Goodwin Jr., Catal. Lett. 34 (1995) 259. [22] G.J. Haddad, J.G. Goodwin Jr., J. Catal. 157 (1995) 25. [23] Y. Okamoto, K. Nagata, T. Adachi, T. Imanaka, K. Inamura, T. Takyu, J. Phys. Chem. 95 (1991) 310. [24] T. Nguyen, H.L. Ho, D.E. Kotecki, T.D. Nguyen, J. Appl. Phys. 79 (1996) 1123. [25] E. Kondoh, R.A. Donaton, S. Jin, H. Bender, W. Vandervorst, K. Maex, Appl. Surf. Sci. 136 (1998) 87. [26] S. Haukka, E.-L. Lakomaa, A. Root, J. Phys. Chem. 97 (1993) 5085. [27] S. Smeds, T. Salmi, L.P. Lindfors, O. Krause, Appl. Catal. A 144 (1996) 177. [28] M. Che, C.O. Bennett, Adv. Catal. 36 (1989) 55. [29] A. Rautiainen, M. Lindblad, L.B. Backman, J. Chem. Soc., Faraday Trans. 1 (2000), submitted.
234
L.B. Backman et al. / Applied Catalysis A: General 208 (2001) 223–234
[30] M.K. Niemelä, A.O.I. Krause, T. Vaara, J. Lahtinen, Top. Catal. 2 (1995) 45. [31] S. Haukka, E.-L. Lakomaa, T. Suntola, in: A. Dabrowski (Ed.), Adsorption and its Applications in Industry and Environmental Protection, Elsevier, Amsterdam, Stud. Surf. Sci. Catal. 120 (1999) 715. [32] R.C. Reuel, C.H. Bartholomew, J. Catal. 85 (1984) 63. [33] J.M. Zowtiak, C.H. Bartholomew, J. Catal. 83 (1983) 107. [34] E. Gorlich, J. Haber, A. Stoch, J. Stoch, J. Solid State Chem. 33 (1980) 121. [35] M.K. Niemelä, L. Backman, A.O.I. Krause, T. Vaara, Appl. Catal. A 156 (1997) 319. [36] R. Chin, D.M. Hercules, J. Phys. Chem. 86 (1982) 360. [37] N.G. Farr, H.J. Griesser, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 293. [38] A. Hakuli, A. Kytökivi, E.-L. Lakomaa, A.O.I. Krause, Anal. Chem. 67 (1995) 1881. [39] L.B. Backman, A. Rautiainen, M. Lindblad, A.O.I. Krause, Appl. Catal. A 191 (2000) 55. [40] K. Sato. Y. Inoue, I. Kojima, E. Miyazaki, I. Yasumori, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 841.
[41] D. Pope, D.S. Walker, L. Whalley, R.L. Moss, J. Catal. 31 (1973) 335. [42] C.R. Brundle, T.J. Chuang, D.W. Rice, Surf. Sci. 60 (1976) 286. [43] F.P.J.M. Kerkhof, J.A. Moulijn, J. Phys. Chem. 83 (1979) 1612. [44] D.C. Castner, P.R. Watson, I.Y. Chan, J. Phys. Chem. 94 (1990) 819. [45] C.R.F. Lund, J.A. Dumesic, J. Catal. 72 (1981) 21. [46] C.D. Wagner, L.H. Gale, R.H. Raymond, Anal. Chem. 51 (1979) 466. [47] Y. Okamoto, T. Adachi, K. Nagata, M. Odawara, T. Imanaka, Appl. Catal. 73 (1991) 249. [48] I. Barin, Thermochemical data of pure substances, VCH Verlags Gesellschaft, Weinheim, 1989. [49] O. Knacke, O. Kubaschewski, K. Hesselman, Thermochemical Properties of Inorganic Substances, 2nd Edition, Springer, Berlin, 1991. [50] L.B. Backman, M.Sc. Thesis, Helsinki University of Technology, Finland, 1995, p. 70.