J. Phys. Chem. C 2008, 112, 18551–18558
18551
Chemistry of Silica-Supported Cobalt Catalysts Prepared by Cation Adsorption. 2. Neoformation of Cobalt Phyllosilicate Raquel Trujillano, Jean-Franc¸ois Lambert, and Catherine Louis* Laboratoire de Re´actiVite´ de Surface, UMR 7609 CNRS, UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, F75252 Paris Cedex 05, France ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: September 9, 2008
The goal of this study was to determine whether it was possible to avoid the formation of cobalt silicate during the preparation of silica-supported Co catalysts since this too-stable cobalt phase is detrimental to catalyst activity. The strategy was to use the strong chelating ligand ethanediamine (en) to complex Co cations, then to adsorb them on silica, and to determine whether an optimal en:Co ratio could be found to avoid the formation of cobalt silicate. In a previous paper (Trujillano, R.; Villain, F.; Louis, C.; Lambert, J. F. J. Phys. Chem. C 2007, 111, 7152), which explored short contact times between Co complex solutions and silica (15 min and 2 h), it was found that cobalt ethanediamine adsorbed very quickly on silica. In the present paper, a longer contact time (7 days) was explored. XRD, UV-visible-near IR, and FT-IR characterization showed that after the fast phenomenon of cation adsorption, a second slow chemical phenomenon occurs, leading to the formation of cobalt phyllosilicate. The amount depended on the en:Co ratio, the silica surface area, and the atmosphere under which the suspension was maintained. Formation of cobalt phyllosilicate was avoided when the en:Co ratio ) 3 or when preparations were performed under air bubbling in the solution, i.e., when Co(II) was oxidized into Co(III). 1. Introduction This paper is the companion paper of a study dealing with the preparation of Co/SiO2 catalysts using cobalt(II) ethanediamine complexes as precursors (NH2-CH2-CH2-NH2, en).1 The general goal of this work was to investigate the speciation of cobalt in aqueous solutions containing cobalt nitrate and ethanediamine with three different en:Co ratios (1, 2, and 3), and then after adsorption on silica support for various times of contact with these solutions and after subsequent washing and drying stages (samples so-called Coen1, Coen2, and Coen3). The idea underlying this study was to determine whether the use of a chelating ligand such as ethanediamine could prevent the formation of cobalt silicates during the preparation of Co/ SiO2 catalysts.2-4 It is well-known that cobalt silicates form easily, and that they are detrimental especially in the case of use as catalysts for Fischer-Tropsch synthesis, because they are poorly reducible at moderate temperature, and therefore the cobalt involved in silicate does not contribute to the activity of Co/SiO2 catalysts.5-10 We believed that, by blocking some of the substitution positions in the Co coordination sphere, the chelating ligands would prevent the formation of silicates. This was observed for [Ni(en)3]2+ 11,12 and [Cu(en)2(H2O)2]2+ 13 adsorbed on silica. Therefore, we investigated three en:Co ratios (1, 2, and 3), to determine the optimum number of ethanediamine ligands, which prevented the formation of cobalt silicates. In the first paper,1 the speciation of cobalt was investigated in solution and on silica after short contact times between Co complex solutions and silica (15 min and 2 h). We showed that the speciation of cobalt in solution with three different en:Co ratios (1, 2, and 3), containing as majority species [Co(en)(H2O)4]2+, [Co(en)2(H2O)2]2+, and [Co(en)3]2+, respectively, changed when silica was introduced into the solution. The pHbuffering effect of the silica support caused speciation shifts, * Corresponding author. E-mail:
[email protected].
especially for the en:Co ) 2 ratio, where the main species became [Co(en)1(H2O)4]2+, instead of [Co(en)2(H2O)2]2+. After a contact time of either 15 min or 2 h with silica, the same amount of cobalt (2.6-2.8 wt %, i.e., ∼10% of the Co present in solution) was adsorbed on silica irrespective of the en:Co ratio, indicating fast cation adsorption. The speciation of the adsorbed Co complexes depended on the en:Co ratio. They mostly consisted in dimers in Coen1 and Coen2, with each Co grafted to silica through one bond ([(SiO)(en)CoII]2(µ-O)), and in monomeric bis-ethanediamine Co complexes in Coen3, possibly also bonded to silica. Upon drying at 100 °C, residual water molecules in the coordination sphere of adsorbed Co(II) complexes were reversibly lost, causing the establishment of an octahedral/tetrahedral coordination equilibrium. Upon calcination at 450 °C, the ethanediamine ligands were decomposed, and the monomeric complexes in Coen3 remained grafted onto silica through two bonds ((SiO)2Co(II)(OH2)4) while the dimeric complexes in Coen1 and Coen2 mostly gave rise to polymerized surface complexes (62-75%), whose X-ray absorption fine structure (XAFS) features were similar to those of a cobalt phyllosilicate. In the present work, we explored longer contact time (7 days) and observed that, after this fast phenomenon of cation adsorption, a second, slower chemical phenomenon occurred, leading to the formation of cobalt phyllosilicate. The amount depended on the en:Co ratio, the silica surface area, and the atmosphere under which the suspension was maintained. As in our first paper,1 we used an array of characterization techniques such as UV-visible-near IR and Fourier transform infrared (FT-IR) spectroscopies and X-ray diffraction (XRD). Both FTIR and XRD provided information on the possible presence of Co-phyllosilicate in the samples while UV-visible spectroscopy provided information on the Co oxidation state and symmetry of the cations.
10.1021/jp806556c CCC: $40.75 2008 American Chemical Society Published on Web 11/01/2008
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TABLE 1: Colors of the Suspensions, Washing Solutions and Co/SiO2 Samples of the Coen1, Coen2, and Coen3 Preparationsa Coen1
Coen2
preparation
static air
static air
suspension at t ) 0 suspension after 7 days solution after the first centrifugation solution after the third washing solution after the fifth washing wet sample sample after drying at RT sample after drying at 100 °C
purple-blue purple-blue-brown brown pink colorless purple-blue pink pink
purple-blue purple-blue-brown brown pink colorless purple-blue pink pink
a
static
Coen3 Arb
purple-blue purple-blue brown pink colorless purple-blue pink-purple pink
air
flowa
brown brown brown pink colorless brown pink-purple purple
static air
static Arb
air flowb
pink-brown brown brown pink colorless grayish-pink grayish-pink purple-blue
pink pink brown pink colorless grayish-pink grayish-pink purple-blue
brown brown brown pink colorless brown grayish-pink purple
The colors are roughly the same whatever the silica surface area (50, 200, and 380 m2 · g-1). b Performed only with silica 380 m2 · g-1.
2. Experimental Section 2.1. Sample Preparation. The method used for the preparation of silica-supported Co samples was the following: 2 g of silica was suspended into 50 mL of a 0.2 M cobalt nitrate solution, which corresponds to a total Co amount of 0.3 g per gram of silica. Nonporous silicas (Aerosil Degussa, Germany) of three different specific surface areas (AD380, AD200, and OX50) were used, having BET surface areas of ∼380, 200, and 50 m2 · g-1, respectively. Ethanediamine was then added (0.2, 0.4, or 0.6 M) to give three different en:Co molar ratios, i.e., 1, 2, and 3. The initial pHs of the suspensions were 6.7-6.8, 7.2-7.7, and 8.8-9.2, respectively, whatever the silica surface area. The suspensions were stirred in a closed vessel in static air at room temperature for 1 week. Two sets of samples, Coen2(380) and Coen3(380), were also prepared either under static argon or under a flow of air bubbling in the solution for 1 week. Afterward, the solids were separated by centrifugation and washed five times with distilled water (5 min of stirring in 80 mL of water and then centrifugation). After washing, nitrate anions were not detected in the supernatant by titration with ferrous ammonium sulfate.14 Each sample was separated into three batches for characterization after washing (wet), drying at room temperature (RT), and drying at 100 °C for 24 h. The samples are thereafter referred to as Coenx(y), where x is the en:Co molar ratio in solution and y is the specific surface area of silica (50, 200, or 380 m2 · g-1). For the samples prepared under static Ar and under air flow, the letters “Ar” and “AF” are added. Cobalt phyllosilicate with a talc-like structure, synthesized by A. Decarreau (Poitiers, France) according to a hydrothermal process,15 was used as a reference for the characterization of the Co/SiO2 samples. Such a cobalt phyllosilicate is a layered compound of structural formula Si4Co3O10(OH)2 when it is wellcrystallized, and the layers consist of two sheets of linked SiO4 tetrahedral units, which sandwich a sheet of linked Co(II)O6 octahedra16 (T-O-T phyllosilicate). 2.2. Characterization Techniques. The chemical analyses of the samples dried at 100 °C (Co, C, N, NO3-, and fire loss at 1000 °C) were performed by inductive coupling plasma in the CNRS Center of Chemical Analysis (Vernaison, France). The Co loadings in the samples are expressed in weight percent Co per gram of silica. The UV-visible-near IR spectra were recorded on a Cary 5E (Varian) spectrometer equipped with a Cary4/5 diffuse reflectance sphere in the 190-2500 nm range. The baseline was recorded using a polytetrafluoroethylene reference. The XRD patterns were recorded on a Siemens diffractometer (D500) using Cu KR radiation. The FT-IR spectra were recorded in transmission mode at room temperature in the 4000-400 cm-1 range on a Bruker
Vector 22 spectrometer equipped with a DGTS detector. The spectral resolution was 4 cm-1, and 20 scans were recorded for each spectrum. The samples were finely ground and dispersed into KBr pellets. 3. Results 3.1. Sample Colors and Elemental Analysis. The suspensions, washing solutions, and Co/SiO2 samples exhibit various colors, which also depends on the Co speciation (Table 1). The blue color indicates the presence of tetrahedral Co(II), pink indicates octahedral Co(II), and brown indicates octahedral Co(III). More details can be found in our earlier paper.1 According to this paper, the main Co species present in the suspension of the Coen1 and Coen2 preparations (en:Co ) 1 and 2, purple-blue in static air) is [Co(en)1(H2O)4]2+, while the main species for Coen3 (pink-brown) is [Co(en)3]2+. When the suspensions are maintained under argon (Table 1), the colors are purple-blue (Coen1 and Coen2) and pink (Coen3), and when the preparations are performed in air, the colors are still different because of the oxidation of Co(II). The solutions collected after the first centrifugation of the three preparations are brown, indicating the presence of Co(III) species. Further washing leads to pink solutions, indicating that the main species present is octahedral Co(II). After the fifth washing, the solutions are colorless, obviously indicating that all the Co complexes noninteracting with silica have been eliminated. The wet samples obtained after washings roughly keep the same colors as those of the suspension, except maybe sample Coen3, where cobalt is more or less oxidized in air. Table 2 shows that the higher the silica surface area, the higher the Co loading. The Co loadings and the variations in the Co loadings are lower for Coen3 (1.6-3.0 wt %) than for Coen1 and Coen2 (1.7-18.1 wt %). For each type of silica, the Co loadings vary as follows with the en:Co ratio: Coen2 > Coen1 > Coen3. The Co loadings do not reach the maximum loading of 30 wt % Co corresponding to total adsorption, but for the Coen1 and Coen2 samples they are considerably higher than those reported for short equilibration periods. The atmosphere of preparation (static air, static Ar, or air flow) has no influence on the Co loading for the Coen3(380) samples (Table 2). In the case of Coen2(380), the preparation under air flow leads to a much lower Co loading (1.7 wt %) than in static air (18.1 wt %). The preparation under argon also leads to a lower Co loading than in static air, but the effect is less pronounced. The amount of NO3- detected by chemical analysis is very low (100-500 ppm) in all the samples. We conclude that the N present in the samples arises from ethanediamine only, allowing calculation of the en:Co ratios in the solid samples.
Co/SiO2 Catalysts
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18553
TABLE 2: Characteristics of the Co/SiO2 Samples Dried at 100 °C chemical analysis sample
en:Co
silica surface area (m2 · g-1)
initial pH of suspensions
preparation conditions
Co (wt %)
en:Co (mol/mol)
FT-IR I670 cm-1/I800 cm-1
XRD I2.6 Å/I4 Å
Coen1(50) Coen1(200) Coen1(380) Coen2(50) Coen2(200) Coen2(380) Coen2(380)Ar Coen2(380)AF Coen3(50) Coen3(200) Coen3(380) Coen3(380)Ar Coen3(380)AF
1 1 1 2 2 2 2 2 3 3 3 3 3
50 200 380 50 200 380 380 380 50 200 380 380 380
6.7 to 6.8 7.2 to 7.7 8.8 to 9.2 -
static air static air static air static air static air static air static Ar air flow static air static air static air static Ar air flow
2.3 8.7 10.3 5.7 14.7 18.1 13.6 1.7 1.9 1.6 3.0 3.1 2.9
0.27 0.22 0.27 0.24 0.16 0.17 0.60 1.69 1.08 -
0 0.04 0.08 0.05 0.22 0.47 0.20 0 0 0 0 0 0
0 0.515 0.550 0.464 0.624 0.870 0.750 0 0 0 0 0 0
Table 2 shows that, for all samples, the en:Co ratio is much lower than that initially present in solution. This is especially true for the Coen1 and Coen2 samples for which en:Co < 0.3. This indicates that most of the Co ions in these samples are not coordinated by ethanediamine. 3.2. Infrared Spectroscopy. There is no difference between the FT-IR spectra of the samples dried at RT and those dried at 100 °C. The spectrum of Coen3(380) (Figure 1c) is roughly the same as that of silica AD380 alone, except for the presence of weak bands at ∼3340 and 3280 cm-1 attributable to νNH2, at 2965 and 2925 cm-1 attributable to νCH2, and at 1460 cm-1 attributable to δCH2 vibrations of ethanediamine.17 The IR spectra of Coen1(380) and Coen2(380) are similar (Figure 1a,b). They show additional bands at 3630 and 670 cm-1 as well as a shoulder at ∼1010 cm-1. The same bands are observed in the spectrum of bulk Co-phyllosilicate (Figure 1d). The band attribution is probably the same as for Ni-phyllosilicate: (i) the band at ∼3630 cm-1 is characteristic of the νOH vibration mode of the OH-3Co group, which is an isolated OH surrounded by three Co atoms18 belonging to the octahedral layer of the phyllosilicate; (ii) the band at 1010 cm-1 is attributable to the νSiO vibrations19 in the phyllosilicate tetrahedral layer; (iii) the band at 670 cm-1 is the resultant of two vibration modes, the δOH vibration of the OH-3Co group and the νSiO vibration of a SiO4 group.20,21 No vibration band of NO3- is visible at
1385 cm-1,22 which is in agreement with chemical analysis. The band at 1630 cm-1 is attributable to δH2O vibration.23 The same trends are observed for the samples prepared with silica of lower surface areas (50 and 200 m2 · g-1) (spectra not shown). The IR spectra of Coen3(50) and Coen3(200) are similar to that of Coen3(380). In the case of Coen1 and Coen2 (50 and 200 m2 · g-1), the vibration bands corresponding to Cophyllosilicate are weaker than in Coen1 and Coen2 (380 m2 · g-1), which is consistent with the lower Co loading (Table 2). In line with that, the intensity ratio of the band at 670 cm-1 characteristic of Co-phyllosilicate to that at 800 cm-1 belonging to silica (I670 cm-1/I800 cm-1) (Table 2) decreases when the Co loading decreases. It may be noted that the IR spectra of the samples with the lowest Co loadings, Coen1(50) (2.3 wt %), Coen2(380)AF (1.7 wt %), and the Coen3 series, do not show any bands of Co-phyllosilicate. 3.3. X-Ray Diffraction. There is no difference in the XRD patterns for the samples dried at RT and at 100 °C, as was also observed for the IR spectra. However, two types of patterns can be distinguished: (1) The first type is those with no diffraction lines except the broad band of amorphous silica at 2θ ∼ 20° (∼4 Å) (Figure 2a); this is the case for the samples with Co loadings lower than 5 wt %, i.e., Coen1(50), Coen2(380)AF, and the Coen3 series. (2) The second type of pattern is those showing broad diffraction bands at 2θ ∼ 35 and 60° (2.6 and 1.5 Å) (Figure 2b,c); this is the case for all other samples. The intensity ratio
Figure 1. FT-IR spectra of Co/SiO2 samples (static air, silica 380 m2 · g-1) dried at 100 °C: (a) Coen3(380); (b) Coen2(380); (c) Coen1(380). (d) FT-IR spectrum of bulk cobalt phyllosilicate.
Figure 2. XRD patterns of Co/SiO2 samples (static air, silica 380 m2 · g-1) dried at 100 °C: (a) Coen3(380); (b) Coen2(380); (c) Coen1(380). (d) XRD pattern of bulk cobalt phyllosilicate.
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Figure 3. UV-visible-near IR spectra of Coen2(380) prepared in static air: (a) wet; (b) RT dried; (c) 100 °C dried. Figure 4. UV-visible-near IR spectra of Coen3(380) prepared in static air: (a) wet; (b) RT dried; (c) 100 °C dried.
of the band at 2.6 Å over that of silica at 4 Å (I2.6 Å/I4 Å) increases with the Co loading (Table 2). As a reference, the XRD pattern of the bulk Co-phyllosilicate is shown in Figure 2d with the line assignment established by Decarreau.24 The diffractograms of the Coen1 and Coen2 samples show two band characteristics of phyllosilicate: the (13,20) band at 2θ ) 35° (2.6 Å) and the (06,33) band at 2θ ) 60° (1.5 Å) (Figure 2b,c). However, they are much broader than those of the bulk phyllosilicate because of the smaller domains of coherence in the (a,b) plane due to the poor crystallinity of phyllosilicates formed in the presence of silica. Moreover, the (02,11) band at 2θ ) 19.6° (4.5 Å) overlaps the broad band of silica, and the (001) and (002) lines corresponding to the basal spacing are not visible, probably because the crystalline coherence along the c axis is very weak, as supported Co-phyllosilicate contains two sheets at most. This is typical for phyllosilicates synthesized at room temperature.25 In agreement with the IR results, XRD shows that Coen1 and Coen2 samples contain Co-phyllosilicate, but Coen3 does not. 3.4. UV-Visible-Near IR Spectroscopy. a. Influence of the en:Co ratio in the Coenx(380) Samples. The UV-visible-near IR spectra of the Coenx(380) samples prepared in static air were recorded after washings (wet), after drying at RT, and after drying at 100 °C. As for IR and XRD, the UV-visible-near IR spectra of the Coen1 and Coen2 samples are similar, so only the spectrum of Coen2 is reported in the following figures. In the near IR range, the spectra of the wet Coen2 and Coen3 samples (Figures 3a and 4a) show a strong absorption in the 1300-2500 nm range due to water adsorbed on silica. Other bands of water are also visible at ∼1190 and ∼970 nm. They are attributed to (2ν + δ)HOH and (3ν)OH, respectively.26 When Coen2(380) is dried at RT, other strong bands of molecular water become visible at 1925 and 1450 nm (Figure 3b) due to (ν + δ)HOH and (2ν)OH vibrations of water, respectively.26 When it is dried at 100 °C, these bands decrease in intensity and shift to 1900 and 1410 nm, respectively (Figure 3c). New bands are visible at ∼2050 and 1540 nm, and can be assigned to (ν + δ)NH2 and (2ν)NH of NH2 in ethanediamine,27 respectively. The corresponding CH bands ((2ν)CH) around 1700 nm are sometimes observable, although with smaller intensity. Other bands observed at ∼2250 nm are assigned to (ν + δ)OH of H-bridged silanols, and those at 2210 and 1380 nm are assigned to (ν + δ)OH and (2ν)OH of free silanols, respectively.28,29 Note that a band at 2330 nm is also observed. The spectra of Coen3(380)
in the near IR range are the same as those of Coen1 and Coen2, except that the bands of water at 1410 and 1900 nm are less intense, and the band at 2330 nm is not visible (Figure 4b,c). In the UV-visible range, the wet Coen2(380) sample exhibits charge transfer bands at 225-250 nm with a shoulder at 360 nm and a strong band at ∼530 nm with a shoulder at ∼650 nm (Figure 3a). When the sample is dried at RT and 100 °C, the same bands are observed, and in addition a broad band at 1200-1260 nm (Figure 3b,c), which was probably present in the spectrum of the wet sample but hindered by the strong absorption of water. The UV-visible spectra of the Coen3(380) sample (Figure 4) are different from those of Coen2(380). For the wet sample (Figure 4a), there is a well-defined band instead of a shoulder at 360 nm, and a very broad absorption at ∼530 nm, hiding any other features that might be present in this range. In addition, a new band is visible at 820 nm. After drying at RT, some modifications are already apparent: the broad shoulder at ∼530 nm has transformed into a band at 510 nm with a shoulder at 640 nm (Figure 4b), the band at 360 nm is much weaker, and the band at 820 nm is still visible. After drying at 100 °C, the spectrum has become completely different from that of the initial wet sample. It exhibits a shoulder at 320 nm and a triplet of bands at 520, 580, and 640 nm, while the band at 820 nm has almost disappeared (Figure 4c). A very broad band between 1200 and 1700 nm is also observed. Figure 5 shows the UV-visible-near IR spectrum of the reference Co-phyllosilicate. The main bands are at 510 and 1200 nm, and they are attributable to the 4T1g(F) f 4T1g(P) and 4T1g(F) f 4T2g electronic transitions of octahedral Co(II) (CoO6), respectively.30,31 The 4T1g(F) f 4A2g transition expected at about 630 nm is not resolved, which is sometimes the case for octahedral Co(II) ions.31 The spectrum also shows a band at 2335 nm attributable to the (ν + δ)OH vibration modes of isolated OH-3Co groups of the Co-phyllosilicate structure (νOH ) 3630 cm-1 and δOH ) 670 cm-1; see section 3.2). The (2ν)OH vibration mode at 1410 nm is at the same frequency as the (2ν)OH vibration mode of water. Other small bands at 1900 and 2210 cm-1 can be attributed to (ν + δ)HOX vibrations of water and of free silanols, respectively. The band at 2420 nm could not be assigned. We can now attempt to assign the bands of the silicasupported Co samples. The band at 530 nm with the shoulder at 650 nm and the broad band at ∼1200 nm in the spectra of
Co/SiO2 Catalysts
Figure 5. UV-visible-near IR spectra of bulk cobalt phyllosilicate.
Coen2(380), either wet or dried (Figure 3), can be attributed to the three transitions of octahedral Co(II).30,31 The presence of a band at 2330 nm in the spectrum of the sample dried at 100 °C (Figure 3c) may indicate that octahedral Co(II) belongs to a Co-phyllosilicate structure, which is consistent with the IR and XRD results, and with the fact that the en:Co ratio is very low, 300 m2 · g-1), but not with small ones ( static Ar > air flow. However, the oxidizing power of the solution varies as follows: static Ar (almost only Co(II)) < static air (Co(II) and Co(III)) < air flow (mostly Co(III)). It seems that to some extent the presence of a small amount of Co(III) together with Co(II) favors the formation of Co-phyllosilicate. This point deserves further investigation. Finally, it is interesting to recall that, after calcination at 450 °C, the XAFS features at the Co threshold of the Coen1(380) and Coen2(380) samples prepared during short times1 were very close to those of ill-crystallized Co-phyllosilicate. Here, we show in addition that, after long time of contact with silica in solution, Co-phyllosilicate forms in the same samples, as if the grafted Co dimers formed at the initial stage of contact acted as precursors for Co-phyllosilicate formation during calcination, or as germs for the slow growth of Co-phyllosilicate when the contact time is extended. In the case of Coen3, fast adsorption of Co monomers with nonbridging ethanediamine ligands leads to grafted monomeric Co species on silica after calcination, and inhibits the formation of phyllosilicate after long time of contact. It must be noted that cobalt silicates, lamellar or otherwise, such as different polymorphs of Co2SiO4, can also form during calcination, i.e., during precursor decomposition, especially when the decomposition is exothermic. This was observed for instance with samples prepared with cobalt nitrate precursors47 or acetate34,48 or under hydrothermal conditions simulating for instance the conditions of the Fischer-Tropsch reaction.49,50 It
18558 J. Phys. Chem. C, Vol. 112, No. 47, 2008 is possible that to avoid such an undesirable phenomenon it is suitable to prepare samples containing Co monomer species, only, i.e., using Co precursors containing nonbridging ligands such as [Co(en)3]2+. 5. Conclusion The goal of this work was to determine whether by blocking some of the substitution positions in the coordination sphere the ethanediamine ligands could prevent the formation of cobalt silicates. We now have a clearer view than after the stage of short deposition time.1 We showed that much longer times of Co precursor deposition resulted in a different chemistry, with a slow process of formation of cobalt phyllosilicate within the course of several days provided that the Co coordination sphere contains enough bridging water ligands, which favor heterocondensation reactions with dissolved silicic species. The amount of cobalt phyllosilicate formed depends on a number of factors: (1) The en:Co ratio in solution: Co-phyllosilicate forms in the Coen1 and Coen2 preparations and not in the Coen3 preparations. The number of ethanediamine ligands or rather the number of water ligands in the Co(II) coordination sphere is an important factor for the formation of phyllosilicates. When the Co complex is chelated by two or three nonbridging ethanediamine ligands, Co-phyllosilicate cannot form, as complexation inhibits heterocondensation reactions. It seems that complexes with two water ligands are not sufficient to induce the formation of phyllosilicate since the initial solutions for Coen3 preparation contain a small proportion of this species. In Coen1 and Coen2 preparations, the presence of four to six water ligands makes the formation of Co-phyllosilicate possible. (2) The solution pH: More basic solutions lead to a higher amount of Co-phyllosilicate. (3) The silica surface area: The higher it is, the larger the liquid-solid interface and the higher the amount of Cophyllosilicate. (4) The atmosphere of preparation (argon, airsstatic or dynamic conditions): That is, the oxidation state of cobalt, Co(III), in the preparation of Coen2 under air flow prevents the formation of Co-phyllosilicate (Co(II)). Considering all of these parameters, one can explain that, for the samples prepared in static air, the Co loading varies as follows: Coen3 (no Co-phyllosilicate) < Coen1 < Coen2 (higher pH than Coen1) whatever the silica surface area. However, the effect of preparation conditions (argon, airsstatic or dynamic conditions) is complex, since the effect of the en:Co ratio is not monotonic. Therefore, further in-depth studies of the catalyst preparation chemistry are still necessary. Acknowledgment. Prof. A. Decarreau (Poitiers, France) is warmly thanked for helpful discussion and for providing us with well-crystallized Co-phyllosilicate. Two students, Julie Grimoult and Elodie Batmale, are acknowledged for additional experiments during short stays in the laboratory. References and Notes (1) Trujillano, R.; Villain, F.; Louis, C.; Lambert, J. F. J. Phys. Chem. C 2007, 111, 7152. (2) Puskas, I.; Fleisch, T. H.; Hall, J. B.; Meyers, B. L.; Roginski, R. T. J. Catal. 1992, 134, 615. (3) Ming, H.; Baker, B. G. Appl. Catal., A 1995, 123, 23. (4) van Steen, E.; Sewell, G. S.; Makhothe, R. A.; Micklethwaite, C.; Manstein, H.; de Lange, M.; O’Connor, C. T. J. Catal. 1996, 162, 220. (5) Ernst, B.; Libs, P.; Chaumette, P.; Kienemann, A. Appl. Catal., A 1999, 186, 145.
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