7152
J. Phys. Chem. C 2007, 111, 7152-7164
Chemistry of Silica-Supported Cobalt Catalysts Prepared by Cation Adsorption. 1. Initial Localized Adsorption of Cobalt Precursors Raquel Trujillano,‡,† Franc¸ oise Villain,§ Catherine Louis,*,‡ and Jean-Franc¸ ois Lambert‡ Laboratoire de Re´ actiVite´ de Surface, UMR 7609 CNRS, UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, F75252 Paris Cedex 05, France, and Laboratoire de Chimie Inorganique et Mate´ riaux Mole´ culaires, UMR 7071 CNRS, UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, F75252 Paris Cedex 05, France ReceiVed: January 16, 2007; In Final Form: March 19, 2007
The speciation of cobalt ethanediamine (en) complexes [Co(en)x(H2O)6-2x]2+ (x ) 1, 2, and 3) in aqueous solution changes when a silica support is introduced into the solution ((en/Co ) 1), (en/Co ) 2), and (en/Co ) 3) preparations). The pH-buffering effect of the silica support causes speciation shifts, especially for the precursor complex [Co(en)2(H2O)2]2+ in solution, which transform into [Co(en)1(H2O)4]2+, as the main species. Once adsorbed on silica and after drying (25 and 100 °C), UV-visible and XAS characterization show that, for the (en/Co ) 1) and (en/Co ) 2) preparations, the Co(en)1 complexes form dimers bonded to silica through one silanol group per Co ([(SiO)(en)(H2O)2CoII]2(µ-O)). For the (en/Co ) 3) preparation, the adsorbed Co(II) complexes are monomeric and contain two ethanediamine ligands. Upon drying at 100 °C, residual water molecules in the coordination sphere of adsorbed Co(II) complexes may be lost reversibly, causing the establishment of an octahedral/tetrahedral coordination equilibrium. Upon calcination at 450 °C, the ethanediamine ligands are eliminated. The monomeric complex in (en/Co ) 3) probably becomes grafted onto silica through two bonds ((SiO)2CoII(OH2)4), while the dimer initially formed in (en/Co ) 1) and (en/Co ) 2) mostly gives rise to species reminiscent of Co silicate germs.
1. Introduction Systems based on cobalt supported on silica and/or mesoporous silicas are the object of persistent interest as they constitute efficient catalysts for Fischer-Tropsch syntheses1-12 or for other catalytic applications.13-20 Despite several studies of the preparation of such systems,21-29 uncertainty remains on the basic mechanisms of the metal complexes/support interaction during the initial steps of preparation. In particular, it is uncertain whether the initial adsorption is simply electrostatic or implies some kind of specific interaction, such as the formation of innersphere Co complexes with surface groups. Moreover, it is a challenge to avoid the formation of cobalt silicates since these species are known to reduce the activity of Co/SiO2 catalysts for Fischer-Tropsch synthesis because of their low reducibility.8,25,30-32 In comparison with simpler systems such as Ni(II)/SiO2,33-36 the Co/SiO2 system presents the additional complication of possible Co(II)/Co(III) redox equilibria. Moreover, redox equilibria are not independent from ligand exchange equilibria since Co(II) is readily oxidized to Co(III) when bound to strong-field ligands such as ammine or alkylamines.21 In addition, the coordination chemistry of Co(II) is dominated by the formation of both octahedral and tetrahedral complexes, with easy transformations between the two geometries. Despite these complications, the use of amine complexes as precursors for supported metals is worth studying. More * To whom correspondence should be addressed. E-mail: louisc@ ccr.jussieu.fr. ‡ Laboratoire de Re ´ activite´ de Surface. § Laboratoire de Chimie Inorganique et Mate ´ riaux Mole´culaires. † Current address: Departamento de Quı´mica Inorga ´ nica, Universidad de Salamanca, Plaza de la Merced s/n, 37800 Salamanca, Spain.
specifically, chelating amines such as ethanediamine (en: NH2-CH2-CH2-NH2) may simplify the chemistry involved in the first steps of metal deposition on the support, as previously observed in the [Ni(en)3]2+/SiO2 system.35,37,38 In particular, we were motivated by the idea that by blocking some of the substitution positions in the coordination sphere, the amine ligands could prevent the formation of cobalt silicates. Cobalt ethanediamine complexes have been used previously as Co/SiO2 precursors by Be´land et al.24 However, they chose to study the substitutionally inert Co(III) complexes as a precursor, while our work deals with the more labile Co(II) complexes. In this paper, we discuss the speciation of cobalt ethanediamine complexes in solutions containing various en/Co ratios (1, 2, and 3) and upon contact of these solutions with the silica support and subsequent drying and calcination. Phenomena with very different kinetics were observed during the deposition step. Therefore, the present paper is concerned with deposition mechanisms for short solution/support contact times (e2 h), while a companion paper will deal with longer contact times.39 We show that preparation with en/Co ) 3 is a way to avoid the formation of germs of Co phyllosilicate. This is confirmed after longer contact times.39 We used an array of characterization techniques including UV-visible and K-edge XAS spectroscopies. Both can provide information on the Co oxidation state and symmetry of the cations. In principle, XAS may provide more quantitative information, but fitting of the spectra was complicated here by the difficulty of distinguishing O from N neighbors and the frequent coexistence of species with different molecular environments.
10.1021/jp0703612 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007
Localized Co Adsorption in Co/SiO2
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TABLE 1: Colors of the Solutions/Suspensions during the Various Steps of Preparation of the (en/Co ) 1), (en/Co ) 2), and en/Co ) 3 Samples; the Same Results Are Obtained for 15 min or 2 h of Adsorption steps of preparation solution without SiO2 suspension after adding SiO2 solution after the first centrifugation solution after the third washing solution after the fifth washing wet solid phase sample after drying at RT sample after drying at 100 °C sample after calcination at 450 °C
method
(en/Co ) 1)
(en/Co ) 2)
(en/Co ) 3)
2 2
red purple-blue
red purple-blue
orange pink
1
brown
brown
brown
1
pink
pink
pink
1
colorless
colorless
colorless
1 and 2 1 2 1 and 2
purple-blue purple-pink purple purple
purple-blue purple-pink purple purple
grayish-pink purple-pink purple-pink purple
1 and 2
pink
pink
pink
2. Experimental Section 2.1. Sample Preparation. Reference homogeneous [Co(en)x(H2O)6-2x]2+ solutions (x ) 1,2,3) were prepared in the following way; 0.2, 0.4, or 0.6 M solutions of ethanediamine were deaerated for 30 min, and the required amount of Co(NO3)2‚6H2O to obtain [Co] ) 0.2 M was added under stirring. To avoid Co(II) f Co(III) oxidation, Schlenk-type techniques were used throughout, and solution spectra were recorded under an argon atmosphere. The Co/SiO2 samples were prepared according to equilibrium adsorption procedures with a silica support AD380 (Aerosil Degussa, 380 m2‚g-1, nonporous). In method 1, 50 mL of a 0.2 M aqueous solution of cobalt nitrate and 2 g of silica were deaerated with an argon flow. Dissolution of cobalt nitrate provides [Co(H2O)6]2+ species under these conditions. In a second step, ethanediamine (also previously deaerated with an argon flow) was added to the suspension. Molar en/Co ratios of 1/1, 2/1, and 3/1 were studied using this deposition method. The resulting suspensions were stirred at room temperature for 15 min or 2 h under static argon. The solids were separated by centrifugation under ambient air and then washed five times with distilled water (80 mL of H2O for 5 min) with a centrifugation after each washing step. The absence of nitrate ions in the washing solutions was verified by the ferrous ammonium sulfate test.40 Three different partners are involved in this synthesis, the silica support, the cobalt ions, and the ethanediamine ligands. To ascertain if the order of addition had any lasting influence on the adsorption mechanism, a second preparation method was studied. In method 2, reference solutions of [CoII(en)(H2O)4]2+, [CoII(en)2(H2O)2]2+, and [CoII(en)3]2+ with [Co] ) 0.2 M were first prepared in controlled atmospheres, as explained above. In a second step, 2 g of the silica support deaerated under vacuum was contacted with 50 mL of these solutions in a Schlenk tube under argon. After 15 min of equilibration, washing was performed under air, in the same way as for method 1. The samples are hereafter referred to as Coen x/t/y, where x is the en/Co molar ratio, t is the contact time of silica with the solution (15 min or 2 h), and y corresponds to the state of the sample, W, RT, or 100 for samples wet, dried at room temperature, and dried at 100 °C for 24 h, respectively. They were also calcined under an oxygen flow (25 cm3‚min-1) from room temperature to 450 °C with a heating rate of 5 °C‚min-1 and then maintained at this temperature for 2 h. The samples were characterized after these different steps.
An ill-crystallized cobalt phyllosilicate of the talc type, synthesized by A. Decarreau (HydrASA, URA 721 CNRS, Universite´ de Poitiers, France) under hydrothermal conditions41 was used as a reference for the characterization of the Co/SiO2 samples. In this compound, Co is exclusively octahedral. 2.2. Characterization Techniques. The chemical analyses of the samples dried at 100 °C (Co, C, N, NO3-) were performed by inductive coupling plasma in the CNRS Center of Chemical Analysis (Vernaison, France). The Co loadings in the samples were expressed in wt % of Co per g of dry silica; dry silica contents were determined from weight loss at 1000 °C. Remarkable color changes were observed at the different steps of preparation and are summarized in Table 1. 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. XAS (extended X-ray absorption) measurements were performed at the Co K edge on the dried and calcined Co/SiO2 samples at the XAS 13 beam line of the DCI storage ring (operating with positrons at 1.85 eV and a mean ring current of 300 mA) of LURE synchrotron radiation facility (Orsay, France). The samples were diluted with cellulose and pressed as a pellet. The EXAFS (extended X-ray absorption fine structure) spectra were recorded at 77 K and the XANES (Xray absorption near-edge structure) spectra at RT in the transmission mode using two ionization chambers. For the EXAFS measurements, a channel-cut Si (111) monochromator was used, and the energies were scanned with 2 eV steps from 7600 to 8600 eV. For the XANES measurements, a double crystal Si (311) monochromator was used, and the energies were scanned with 0.3 eV steps from 7650 to 7830 eV. The energy was calibrated using a Co metal foil reference. For each sample, the XANES spectra were recorded twice, and the EXAFS spectra were recorded three times. After background correction, the XANES spectra were normalized in the middle of the first EXAFS oscillation. The EXAFS analyses have been made with the software package of programs “EXAFS pour le Mac”.42 The kχ(k) functions were extracted from the data following the procedure proposed by Lengeler and Eisenberger43 using a linear preedge background and a combination of polynomials and spline atomic absorption background. The Fourier transforms (FT) were calculated on w(k)k2χ(k) where w(k) is a KaiserBessel window with a smoothness parameter equal to 3. The k
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Trujillano et al.
TABLE 2: Evolution of Suspension pH during Deposition (Method 1) pH of the adsorption pH of the fifth solution/suspensiona washing solution sample
pH of the solution
15 min
2h
15 min
2h
(en/Co ) 1) (en/Co ) 2) (en/Co ) 3)
7.9 9.4 10.9
6.2 6.5 9.2
6.3 6.6 n. d.
8.4 8.4 9.2
7.7 8.3 n.d.
a
Gathered after the first centrifugation.
limits are 2.6 and 14 Å-1. It may be noted that the FT’s are presented without phase correction in the figures. Using the FEFF7 code,44,45 the inelastic reduction factor S02, the constant of the mean free path Γi and the functions |fi(k,Ri)| (amplitude) and |φi(k,Ri)| (phase shift) were calculated from the structures of Co(en)2Cl2 for Co-N and Co-C and Co phyllosilicate of the talc-type for Co-O. Moreover, we have checked from models calculated with the FEFF7 code from crystallographic structures of different compounds (Co(en)3Cl3‚3H2O,46 Co(en)2Cl2‚Cl,47 and Co phyllosilicate of the talc-type48 that multiple scattering is negligible until 4 Å; therefore, single scattering fits of experimental curves were performed with the Round Midnight program.49 The simulated spectra were compared to the experimental EXAFS signals and the FT’s (both imaginary and real part) of our compounds. Thermal analyses were performed on a Seiko SSC 5200H thermal analyzer. The samples were placed in a platinum crucible and heated at 5 °C‚min-1 under a 100 mL‚min-1 flow of air, with a 60 min plateau at 120 °C to ensure elimination of physisorbed water prior to other transformations. The reference was an empty platinum crucible. Emitted gases were analyzed using an on-line HPR 20 mass spectrometer (Hiden Company). The speciation of the various Co(II) complexes (ethanediamine, aqua, ethanediamine-aqua complexes, and hydroxo complexes) for solutions containing cobalt nitrate (0.2 M) and ethanediamine (0.2, 0.4, and 0.6 M) was calculated using the ChemEQL software.50 3. Results 3.1. Supernatant and Washing Solutions. The characteristic colors of the Co/en solutions, compatible with the expected speciations (Table 1), were altered upon contact with the silica particles; the resulting suspensions were bright purple-blue for (en/Co ) 1) and (en/Co ) 2) and pink to grayish-pink for (en/ Co ) 3). The order of addition of the three components appeared to have little influence since the observed colors were almost identical for methods 1 and 2 (with a possible exception for room-temperature-dried samples; see Table 1). Washing led to wet solids with the same color as the solutions, with only slight
changes upon drying at RT. Drying at 100 °C caused all samples to turn purple and calcination to turn pink. After adsorption, the pH of the supernatant solutions was slightly acidic for (en/Co ) 1) (6.2) and (en/Co ) 2) (6.5) and basic for (en/Co ) 3) (9.2) (Table 2). The pH of the washing solutions became basic in all cases (Table 2). These values will be discussed in section 4.2. The color of the supernatant solutions after centrifugation in air was brown, while that of the washing solutions became pink at the third washing and colorless after the fifth one (Table 1). 3.2. Elemental Analysis. Table 3 shows that the amount of Co adsorbed is almost invariant between 15 min and 2 h of contact. This suggests that an initial adsorption equilibrium is reached within 15 min, at most (however, a very slow additional uptake is probably occurring for longer times, as will be discussed in a further paper).39 The maximum cobalt loading is almost the same for all three precursors at ≈2.7 wt %, that is, ≈0.72 Co per nm2, assuming that the 380 m2‚g-1 of silica are accessible since silica is nonporous; this Co loading does not depend on the order of addition of the three components (compare methods 1 and 2). This loading corresponds to the adsorption of only about 9% of all Co complexes present in the solution. Such a low value could be explained either by the saturation of a limited number of adsorption sites or of a limited adsorption capacity in the case of electrostatic adsorption. Alternatively, it could be due to a low adsorption affinity on a higher number of adsorption sites. In the latter case, the adsorption would probably be reversible; thus, the washing steps would induce desorption, which is not observed. The amount of NO3- anions adsorbed in the (en/Co ) 1) and (en/Co ) 2) samples is very low, and the en/Co ratio is close to 1. In (en/Co ) 3) samples, the amount of NO3- is slightly higher, and the en/Co ratio is ≈2. The C/N ratio is generally 1.2-1.3; the difference with C/N ) 1 in the original (en) molecules probably lies within experimental error. These conclusions hold for both methods 1 and 2. Experiments were also carried out under ambient atmosphere. The amounts of adsorbed cobalt fell down to 1.9 wt % for (en/ Co ) 1) and (en/Co ) 2) samples and to less than 0.5 wt % for (en/Co ) 3) (where Co speciation should make it particularly prone to oxidation). Aging the cobalt solutions for one week prior to deposition caused a further decrease of the adsorbed Co amount. Obviously, the presence of an oxidant in the atmosphere is an important experimental parameter. 3.3. UV-Visible Near-IR Spectroscopy. The UV-visible near-IR spectra were recorded for all of the samples (wet, dried at room temperature and at 100 °C, and calcined) in order to study the possible changes in the oxidation state and in the symmetry of the cobalt cations. Almost indistinguishable spectra are obtained for methods 1 and 2. Thus, it seems that the same
TABLE 3: Elemental Analyses for Silica-Supported Cobalt Catalysts after Drying at 100 °C samples
method
Co (wt %)
NO3-/Co (mol./mol.)
C/Na (at./at.)
en/Coa (mol./mol.)
(en/Co ) 1)/15min/100 (en/Co ) 1)/2h/100 (en/Co ) 1)/15min/100 (en/Co ) 2)/15min/100 (en/Co ) 2)/2h/100 (en/Co ) 2)/15min/100 (en/Co ) 3)/15min/100 (en/Co ) 3)/2h/100 (en/Co ) 3)/15min/100
1 1 2 1 1 2 1 1 2
2.6 2.6 2.7 2.7 2.7 2.7 2.7 2.6 2.8
0.008 0.008 0.004 0.020 0.011 0.004 0.052 0.049 0.004
1.2 1.2 1.3 1.3 1.3 1.3 1.4 1.3 0.7
1.09 0.95 1.10 1.10 0.83 1.15 1.93 1.84 1.85
a The N amount has been corrected by subtracting the amount of NO3- present in the samples; this corrected value has been used to determine the number of (en) ligands.
Localized Co Adsorption in Co/SiO2
Figure 1. UV-visible near-IR spectra of (en/Co ) 2)/2h (a) wet, (b) RT-dried, and (c) 100 °C-dried. The inset presents the near-IR spectrum of the 100 °C-dried sample at a different scale to better distinguish the vibrational bands of water and -NH2 from the d-d electronic bands of cobalt (arrows).
state of equilibrium is reached in both cases; the system does not remember if cobalt ions were adsorbed first and then modified with ethanediamine or if Co/en complexes were formed prior to adsorption. 3.3.a. Wet and Dried (en/Co ) 1) and (en/Co ) 2) Samples. The spectra of samples (en/Co ) 1) and (en/Co ) 2) are very similar and are also the same after 15 min or 2 h of adsorption. Therefore, only the spectra of (en/Co ) 2)/2h are reported and discussed (Figure 1). A first group of bands can be immediately assigned in the near-IR region, namely, the overtone and combination bands of H2O vibrations (labeled “W” in Figure 1). The (ν+δ)HOH and (2ν)OH of molecular water are apparent at 1925 and 1450 nm,51 respectively, for both wet samples and samples dried at room temperature (inset 1b). In wet samples, they are intense enough to preclude the observation of any other bands above 1400 nm; in addition, other bands of water are apparent, (2ν+δ)HOH at 1170-1200 and (3ν)OH at 970 nm51 (Figure 1a). In the samples dried at 100 °C (Figure 1c and inset 1c), the bands of molecular water are much smaller and shifted to a lower wavelength, at 1900 nm for (ν+δ)HOH and at 1410 nm for (2ν)OH, due to a different state of hydrogen bonding. Silanol bands are observed at ≈2250 nm assigned to (ν+δ)OH of H-bridged silanols and at ≈2210 and 1380 nm assigned to (ν+δ)OH and (2ν)OH of free silanols, respectively.52 Other bands appear at ≈2040 and 1540 nm, corresponding to (ν+δ)NH and (2ν)NH of NH2 in ethanediamine;35 the corresponding CH bands ((2ν)CH) are sometimes observable around 1700 nm, although with smaller intensity. These findings are consistent with the existence of intact ethanediamine species at least until drying at 100 °C. The UV-visible region contains the d-d transitions of cobalt ions that should be diagnostic of the oxidation state and molecular environment of Co-containing species. The UVvisible spectra of the wet and dry samples are quite similar, with a strong band at 220-240 nm, a shoulder at ≈320 nm, and a broader feature showing three maxima at 520, 580, and 640 nm. In addition, in the samples dried at 100 °C, the disappearance of water vibration bands reveals a weak and broad d-d band between 1000 and 1800 nm (sometimes exhibiting separate maxima at 1240-1260 and 1720-1740 nm; see inset of Figure 1).
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7155
Figure 2. UV-visible near-IR spectra of (en/Co ) 3)/2h (a) wet, (b) RT-dried, and (c) 100 °C-dried. Inset: near-IR spectrum of the 100 °C-dried sample, as in Figure 1.
Figure 3. UV-visible near-IR spectra of (a) calcined (en/Co ) 1)/2h and (b) calcined (en/Co ) 3)/2h. Inset: near-IR spectrum of the calcined samples.
3.3.b. Wet and Dried (en/Co ) 3) Samples. The UV-visible spectra of sample (en/Co ) 3) are reported in Figure 2. In the near-IR range, vibrational features similar to samples (en/ Co ) 1) and (en/Co ) 2) are observed and will not be discussed further. In the UV-visible region, however, different electronic transitions are observed for the wet sample (Figure 2a), with three bands at 360, 530, and 820 nm. For the RT-dried sample (Figure 2b), in addition to these bands, a feature similar to the triplet of bands observed in the corresponding (en/Co ) 1) and (en/Co ) 2) samples becomes visible in the 520-644 nm region (Figure 1b). Drying at 100 °C produces an UV-visible spectrum (Figure 2c) essentially identical to those of (en/Co ) 1) and (en/Co ) 2) (Figure 1c). 3.3.c. Calcined Samples. The UV-visible spectra of samples (en/Co ) 1), (en/Co ) 2), and (en/Co ) 3) are very similar; only those of (en/Co ) 1) and (en/Co ) 3) are reported in Figure 3. In the near-IR range, they exhibit bands readily assigned to molecular water and SiOH vibrations (cf. supra). The only difference with the 100 °C-dried samples is that the NH and CH bands have disappeared. In the UV-visible range, all three samples exhibit three bands, at 515, 580-585, and 640-645 nm, respectively, that are reminiscent of those already observed in the 100 °C-dried samples (Figures 1c and 2c) but that are less well resolved, as well as the band at about
7156 J. Phys. Chem. C, Vol. 111, No. 19, 2007
Trujillano et al. TABLE 4: Best Parameters for the Simulation of the EXAFS Signals of the (en/Co ) 2)/15min Samples samples
Figure 4. XANES spectra of (en/Co ) 2)/15 min (a) RT-dried, (b) 100 °C-dried, (c) calcined, (d) ill-crystallized Co phyllosilicate, and (e) Co0.5O-Zn0.5O mixed oxide.
Figure 5. EXAFS signal (A) and Fourier transform (B) of (en/Co ) 2)/15 min (a) RT-dried, (b) 100 °C-dried, (c) calcined, and (d) illcrystallized Co phyllosilicate.
220 nm. Instead of the shoulder at ≈320 nm, there is now a rather-well-resolved band with a maximum at 350 nm for (en/ Co ) 3) and 370 nm for (en/Co ) 1) and (en/Co ) 2). A broad d-d band is observed between 1000 and 1800 nm for all samples (not shown). 3.4. XAS at the Co K edge. XAS data have been obtained for samples (en/Co ) 2)/15 min and (en/Co ) 3)/15 min. 3.4.a. The (en/Co ) 2) Samples. The XANES spectra of the 25 and 100 °C-dried (en/Co ) 2) samples are close to each other (Figure 4a,b) but different from that of the calcined sample (Figure 4 c), indicating a change in the Co coordination during calcination. The XANES spectrum of ill-crystallized Co phyllosilicate as a reference for octahedral Co(II) is reported in Figure 4d, and that of the Co0.5O-Zn0.5O wurtzite as a reference for tetrahedral Co(II) is reported in Figure 4e. The EXAFS signals (Figure 5A) and the moduli of the corresponding Fourier transforms (Figure 5B) of the 25 and 100 °C-dried (en/Co ) 2) samples are also very similar (compare spectra a and b in each case), while those of the
backscatterers Na σ (Å)b R (Å)c ∆E0 (eV)d F (%)e
RT-dried
O N C Co Si
3 2 2 1 1
0.06 0.06 0.06 0.06 0.08
1.94 2.16 2.80 3.13 3.22
0 -2 -3 -5 -5
4.2
100 °C-dried
O N C Co Si
3 2 2 1 0.6
0.07 0.10 0.06 0.07 0.08
1.93 2.14 2.85 3.10 3.21
-2.5 -6 -1 -5 -5
2.6
calcined
O O Co Si
1.5 4.5 4.5 3
0.08 0.09 0.09 0.08
1.88 2.05 3.13 3.20
-4 -4 -1 -1.1
4.0
a N: number of neighbors. b σ (Å): Debye-Waller factor. c R (Å): distance between Zn and a backscatterer. d ∆E0 (eV): energy shift. e F (%): agreement factors.
calcined sample (spectra c) are drastically different; in particular, the FT of the calcined sample shows two contributions in the first peak, and the second peak is of considerably higher intensity than in the uncalcined samples. Actually, the XANES, EXAFS signal, and FT of the calcined sample look rather similar to those of a reference ill-crystallized Co phyllosilicate (compare spectra c and d in Figures 4, 5A, and 5B). To go further in the determination of the environment of the Co species, it would be necessary to simulate the EXAFS signal. Because of the complexity of the systems studied in which a mixture of Co species is present, the EXAFS simulations were very difficult to perform, and often, several different models were found to fit the experimental data equally well. While two different models gave fits of equal quality, we have chosen to present here the simulations which are most consistent with the results obtained by the other techniques and with the known chemistry of cobalt complexes. The EXAFS signal of the first shell of the FT of (en/Co ) 2) dried at RT and 100 °C (Figure 5Bb) can be simulated with five O/N atoms as first neighbors and two distances at 1.94 and 2.16 Å (cf Table 4), consistent with the length of Co-O bonds of tetrahedral and octahedral Co(II) species, respectively (cf Table 5). In the second shell, two carbon atoms are present. Interestingly, a good fit between the simulated and experimental spectra could only be obtained, if in addition to the C atoms, one Co backscatterer was introduced. There is more uncertainty regarding the presence of Si second neighbors. The addition of about one Si atom in the second shell was compatible with (F ) 2.6%) but generally not indispensable for a good fit (F )2.8%). Therefore, EXAFS does not allow us to determine unequivocally if bonds of the CoO-Si type (grafted complexes) are present or not. The simulation of the EXAFS signal of the calcined samples requires the presence of two types of oxygen as first neighbors and of both cobalt and silicium atoms as second neighbors (Table 4). 3.4.b. The (en:Co)3) Samples. The XANES spectra (Figure 6), the EXAFS signals (Figure 7A), and the moduli of their Fourier transforms (Figure 7B) of the 25 and 100 °C-dried (en/Co ) 3) samples are very similar, while important changes are observed after calcination. The XANES spectra of the 25 and 100 °C-dried (en/Co ) 3) samples (Figure 6a,b) are different from those of the (en/Co ) 2) samples (Figure 4a,b), while that of the calcined sample (Figure 6c) is rather similar to that of calcined (en/Co ) 2)
Localized Co Adsorption in Co/SiO2
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7157
TABLE 5: Co-O and Co-N Distances in Various Compounds According to the Cobalt Oxidation Number and the Symmetry of Its Environment symmetry
dCo-O (Å)
ref
dCo-N (Å)
ref
Co(II) Oh
2.13 in CoO 2.08 in Co phyllosilicate (talc) 1.99 in Co0.5O-Zn0.5O (wurtzite) 1.94 for Co(II) Td in Co3O4 1.96 in Co2Al2O4 1.96 for Co(III) Oh in Co3O4
68 49 70 71 72 71
2.17 in Co(propanediamine)2(SCN) 2
69
Co(II) Td Co(III) Oh a
-a
2.00 in Co(en)3Cl3 1.99 in [Co(en)2Cl2]Cl
46 47
Data not available.
(Figure 4c). In contrast, the EXAFS signals and the Fourier transforms are rather different (compare spectrum c in Figures 5A and 7A and in Figures 5B and 7B). Simulation of the 25 °C-dried sample (Table 6) gave six N/O neighbors at 1.94 Å and four carbon second neighbors consistent with two en/Co, that is, with the chemical analysis (Table 3). The simulation of the 100 °C-dried sample is not very different from that of the 25 °C-dried sample. In the case of the calcined sample, the best fit necessitated both Si and Co in the second sphere, but the simulation converged to a smaller number of Co neighbors than in calcined (en/Co ) 2). 3.5. XRD. The diffractograms of all of the samples which have been prepared during 15 min or 2 h are similar, showing only the typical pattern of amorphous silica with a broad band between 10 and 40° (i.e., centered at ≈4 Å). In other words, no new crystalline phase is formed in detectable amounts during cobalt deposition, at least if the contact time is kept lower than 2 h. 3.6. Thermal Analysis. Thermal analysis of the samples was carried out in oxidizing atmospheres (dry air). The TG, DTG (first derivative of TG), and DTA curves are very similar for all of the preparations. Phenomena that might occur under 150 °C are obscured by the strong water loss typical of silica and silica-supported catalysts. Two well-individualized weight losses are observed on the DTG curve at ≈220 and ≈330 °C, the second one being exothermal (DTA). The gases evolved during the decomposition of one of these solids were studied by mass spectroscopy. The profile of the m/e ) 30 amu signal (corresponding to the NH2CH2+ fragment) shows two peaks at 250 and 320 °C. The temperature maxima of these peaks correspond to those of the DTG curve, which confirm that they arise from the decomposition of ethanediamine. Furthermore, it was checked that ethanediamine directly adsorbed on the silica surface desorbs before 200 °C; thus, the desorption maxima at higher temperatures in cobalt-containing samples corresponds to a different species, most probably ethanediamine ligands coordinated to Co ions, whose elimination is complete at 400 °C. Therefore, all samples calcined at 450 °C may be considered as free of ethanediamine. 4. Discussion 4.1. UV-Visible Transitions and Color of the Co(II) and Co(III) Complexes. Co(II) and Co(III) complexes exhibit different chemical (notably redox) behavior and different spectroscopic properties, depending on the symmetry of the coordination sphere and the nature of the ligands. Tables 7-9 summarize the positions and assignments of the d-d transition bands reported in the literature for cobalt compounds with oxidation numbers of +II or +III, tetrahedrally or octahedrally coordinated, in order to help interpret the UV spectroscopic data in further discussion.
Six-coordinated octahedral or pseudooctahedral Co(II) species are normally pink and exhibit three spin-allowed transitions, 4T 4 4 4 4 4 1g f T2g, T1g f A2g, and T1g f T1g(P) (Table 7). The 4T 4 1g f A2g transition either appears as a shoulder or is not observed at all. Table 7 summarizes published data for some octahedral Co(II) complexes, including those synthesized in the present study (solution spectra). Four-coordinated tetrahedral and pseudotetrahedral Co(II) complexes are intense blue or green. The 4A2 f 4T1(F) and 4A f 4T (P) transitions appear as multiple absorptions (triplets) 2 1 in the near-infrared and visible regions, respectively (Table 8; the multiple maxima are due to overlap with spin-forbidden transitions).53 The molar extinction coefficients of these bands are much higher than those of octahedral species. Six-coordinated octahedral Co(III) complexes have two spinallowed transitions (1A1g f 1T1g and 1A1g f 1T2g); besides, two spin-forbidden transitions are usually observed with relatively high intensities (1A1g f 3T1g and 1A1g f 3T2g) (Table 9). Variable colors may be observed for these complexes depending on the composition of the coordination sphere. The appearance of a brown color is generally interpreted as incipient polymerization of Co(III) species. Finally, four-coordinated tetrahedral Co(III) complexes are rare and have only been characterized in special environments, such as the central site of Keggin ions.54 4.2. Cobalt Speciation in the Precursor Solutions and in the Silica Suspensions. We have evaluated the speciation of cobalt that would be expected in the precursor solutions in the absence of the silica support (Table 10).55 The species considered were Co(II) ethanediamine complexes, [Co(en)(H2O)4]2+, [Co(en)2(H2O)2]2+, and [Co(en)3]2+;40 protonated ethanediamine, (enH)+, and (enH2)2+; ethanediamine-free Co(II) complexes, [Co(H2O)6-x(OH)x](2-x)+ (x ) 0-4); and the polymeric species “Co2(OH)3+” and “Co4(OH)44+” (the latter were always minority species and generally present in vanishingly small concentrations). Speciations were calculated for the “natural pH” (that corresponding to a solution containing only cobalt nitrate and ethanediamine) and for the pH actually measured in the deposition slurry, that is, in the presence of silica. At natural pH, quite expectedly, the predominant species are the mono-ethanediamine complex, [Co(en)(H2O)4]2+, for an en/ Co ratio ) 1, the di-ethanediamine complex, [Co(en)2(H2O)2]2+, for an en/Co ratio ) 2, and the tris-ethanediamine complex, [Co(en)3]2+, for an en/Co ratio ) 3 (Table 10). The band positions of the UV-visible spectra of the precursor solutions reported in Table 7 were compatible with these theoretical speciations. Each solution exhibited three spin-allowed transitions, in agreement with pseudooctahedral Co(II) complexes; furthermore, a bathochromic effect was found when the amount of ethanediamine was decreased, as expected if strong-field ethanediamine ligands are substituted with weak-field water ligands. It is therefore likely that each spectrum corresponds to the cobalt complex predominating in the corresponding solution.
7158 J. Phys. Chem. C, Vol. 111, No. 19, 2007
Trujillano et al. TABLE 6: Best Parameters for the Simulation of the EXAFS Signals of the (en/Co ) 3)/15min Samples samples
backscatterers Na σ (Å)b R (Å)c ∆E0 (eV)d F (%)e
RT-dried
N, O C
6 4
0.06 0.08
1.94 2.85
1.5 4
3.3
100 °C-dried
N O C
3 2 3
0.06 0.08 0.08
1.95 1.93 2.84
2 1 4
5.5
calcined
O O Co Si
1.6 3.3 1.7 2.7
0.10 0.10 0.10 0.09
1.95 2.06 3.12 3.21
-2.4 -2.4 -4.1 -2.65
3.7
a N: number of neighbors. b σ (Å): Debye-Waller factor. c R (Å): distance between Zn and a backscatterer. d ∆E0 (eV): energy shift. e F (%): agreement factors.
Figure 6. XANES spectra of (en/Co ) 3)/15 min (a) RT-dried, (b) 100 °C-dried, and (c) calcined.
Figure 7. EXAFS signal (A) and Fourier transform (B) of (en/Co ) 3)/15 min (a) RT-dried, (b) 100 °C-dried, and (c) calcined.
The addition of silica exerts a definite buffering effect, lowering the pH with respect to the value for a pure precursor solution. As can be seen in Table 10, this is expected to result in important speciation changes; in particular, for the suspension used for the preparation of the (en/Co ) 2) sample, the predominant species should no longer be [Co(en)2(H2O)2]2+ but rather [Co(en)(H2O)4]2+, as in the (en/Co ) 1) sample. For that of the (en/Co ) 3) sample, [Co(en)3]2+ should remain predominant when the solution is contacted with silica, but up to 10% of the diaqua complex ([Co(en)2(H2O)2]2+) should be formed. No spectroscopic data are available at this stage of preparation (i.e., initial contact between precursor solutions and silica support). However, the colors of the suspensions (Table 1) may give some cues on the cobalt speciation. In the (en/Co ) 1) and (en/Co ) 2) series, the deposition suspensions were purple-
blue, which is typical of a mixture of tetrahedral and octahedral Co(II) complexes (blue and pink, respectively). In the (en/ Co ) 3) procedure, the pink color of the suspension indicates the presence of octahedral Co(II) complexes. When the solid phase was separated from the solution by centrifugation, the colors of the wet solids were the same as those of the suspensions, indicating that we were mainly observing the speciation of adsorbed cobalt complexes (rather than those remaining in solution). The color of supernatant solutions could not be observed separately, as the dissolved cobalt complexes quickly oxidized during the steps of centrifugation-washings performed in ambient air, as indicated by the appearance of a brown color (cf. supra, section 4.1). The first washing solutions were also brown (elimination of unadsorbed, easily oxidizable Co(II)/en complexes), but starting from the third washing, solutions were pink, probably due to the removal of a small amount of Co cations as [Co(H2O)6]2+. As regards to the high pH of the washing solutions in the (en/Co ) 1) and (en/Co ) 2) series compared to that of the adsorption solutions (Table 2), it could be due to the progressive release of ethanediamine molecules, giving a basic character to the solutions. Indeed the en/Co ratio is lower in the adsorbed solids (Table 3) than that in the original suspensions, and the ethanediamine ligands lost by the adsorbed complexes must therefore be eliminated during the washing step. After five washings, colorless solutions were obtained, indicating that the Co species remaining in the solid phase were irreversibly adsorbed. The nitrate test on the solution was negative; according to elemental analysis, a small amount of nitrate could remain irreversibly in the solid phase (Table 3), but it was not observed by FTIR spectroscopy (no band at 1385 cm-1).56 4.3. Cobalt Speciation in Solid Samples (en/Co ) 1) and (en/Co ) 2) after Drying and after Calcination. We will successively discuss the results of UV-visible and EXAFS spectroscopies and then try to reconcile both in a single model of Co speciation. The bands at 220-240 nm are most probably ligand-to-metal charge transfers (CT), which say little about Co speciation. There is more ambiguity regarding the bands between 315 and 370 nm; some high-energy d-d bands may be found in this region (especially for Co(III) complexes, vide infra), but interference with CT transitions is very likely, preventing the use of this spectral region as a diagnostic of Co speciation. The characteristic triplet of bands at about 520, 580, and 640 nm in (en/Co ) 1) and (en/Co ) 2) wet, RT-dried, and 100 °C-dried may be assigned to the 4A2 f 4T1(P) transition in a tetrahedral Co(II) complex (cf. literature data in Table 8). When water vibration bands do not interfere, such as in the
Localized Co Adsorption in Co/SiO2
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7159
TABLE 7: UV-Visible Absorption Maxima of Six-Coordinated Co(II) Complexes
a
complexes
first coordination sphere
ref
[Co(en)3]2+ [Co(en)3]2+ [Co(H2O)6]2+ [Co(H2O)6]2+ Co(NO3)2/SiO2 [Co(en)3]2+ [Co(en)2(H2O)2]2+ [Co(en)(H2O)4]2+
N6 N6 O6 O6 O6 N6 N4O2 N2O4
73 74 73 74 58 this work this work this work
λ1 (nm) T1g f 4T2g
4
4
λ2 (nm) T1g f 4A2g
λ3 (nm) T1g f 4T1g(P)
4
n.r.a 534 625 625 n.r. 534 n.r. n.r.
1000 1029 1235 1388 1250 1013 1051 1112
476 460 515 495 510 484 483 484
n.r.: nonresolved.
TABLE 8: UV-Visible Absorption Maxima of Four-Coordinated Co(II) Complexes
complexes
first coordination sphere
ref
N2Cl2 O4 -c O′3O′′d
73 53 58 23 21
[Co(Me4pn)Cl2a Co(II) in spinel Co(II) acetate/SiO2 CoO/SiO2 Co(II) nitrate/silica, “blue gel”
λ1 (nm) A2 f 4T1 (F)
λ2 (nm) A2 f 4T1(P)
4
4
1666,1369,1000 1663,1470,1320 n.r.b n.r. n.r.
653, 578, 555 645, 600, 560 615, 570, 520 637, 585, 525 641, 588, 520 * 660, 608, 524**
a N,N,N′,N′-tetramethylpropanediamine. b Nonresolved. c Molecular environment not identified; see comment in section 4.3. *As tabulated in article. **From apparent maxima in published spectrum.
TABLE 9: UV-Visible Absorption Maxima of Low-Spin Six-Coordinated Co(III) Complexes complex
coordination sphere
ref
[Co(en)3]3+ [Co(en)3]3+ [Co(NH3)6]3+ [Co(en)2Cl2]+ cis-[Co(en)2Cl2]+ [Co(H2O)6]3+ [Co(H2O)6]3+ [Co(NO3)6]3-
N6 N6 N6 N4Cl2 N4Cl2 O6 O6 O6
73 75 73 73 76 73 75 73
λsf.1 A1g f 3T1g
1
1
λsf.2 A1g f 3T2g
729 n.r. n.r. n.r. n.r. 1250 n.r. n.r.
571 n.r. 769 n.r. n.r. 800 n.r. n.r.
1
λ1 A1gf 1T1g
1
464 464 470 529 548 (broad) 606 606 657
λ2 A1g f 1T2g 338 338 338 379 393 405 402 444
TABLE 10: Theoretical Speciations in Aqueous Solutions Containing 0.2 M Cobalt Nitrate and 0.2, 0.4, and 0.6 M Ethanediamine in the Absence of Oxidizing Agentsa speciation at natural pH (%) en/Co ratio
natural pH
Cob
Co(en)1c
Co(en)2d
1 2 3
7.9 9.4 10.9
16.7 0.2
