Catalysts Prepared from Solutions of Coba - American Chemical Society

4780

J. Phys. Chem. C 2007, 111, 4780-4789

A Study of Cobalt Speciation in Co/Al2O3 Catalysts Prepared from Solutions of Cobalt-Ethylenediamine Complexes Franç ois Dumond, Eric Marceau,* and Michel Che† Laboratoire de Re´ actiVite´ de Surface (UMR 7609 CNRS), UniVersite´ Pierre et Marie Curie-Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France ReceiVed: NoVember 22, 2006; In Final Form: January 14, 2007

The thermal reduction of Co/Al2O3 catalysts prepared by impregnation of Co(II) nitrate generally leads to migration of cobalt ions into alumina with formation of cobalt aluminate. In order to prevent the latter, it is attempted to reduce cobalt by decomposition of supported precursor salts containing ethylenediamine (en), a procedure successfully applied earlier to nickel. Alumina is impregnated with an aqueous solution containing Co(II) nitrate and two or three equivalents of en. After drying, several cobalt compounds are detected on the support by X-ray diffraction, diffuse reflectance UV-vis spectroscopy and X-ray absorption spectroscopy: crystals of [Co(III)(en)3](NO3)3 whatever the initial en/Co stoichiometry, Co(II) and Co(III)-en complexes, grafted or not, as well as a Co(II)(OH)2-like phase. The composition of these phases reveals that dramatic evolutions in cobalt speciation have taken place in the impregnating solution, driven by the oxidation of complexed cobalt by dissolved oxygen and by the precipitation of the Co(III) salt. Decomposition of these phases in an inert gas atmosphere does not lead to metal particles exclusively, as is the case of nickel, but to a mixture of oxidic and metallic phases. A treatment in hydrogen at 500 °C completes cobalt reduction, leading to 6-20 nm metal particles. This rather broad size distribution is linked to the wide range of cobalt species on the support, which stems itself from the coordination chemistry of cobalt bonded to ethylenediamine in aqueous solutions.

Introduction Co/Al2O3 catalysts are usually prepared by impregnation of alumina with an aqueous solution of Co(II) nitrate, followed by thermal treatments in air and reduction in hydrogen. Unfortunately, thermal treatments induce the migration of a fraction of cobalt into alumina to form an aluminate-like phase which cannot be reduced below 800 °C. At that temperature, metal particles sinter, leading to a low active metal surface. Large Co3O4 particles can be reduced at temperatures below 400 °C, but the resulting metal particles are also large. Moreover, Co3O4 and γ-Al2O3 have isotype crystal structures, and this contributes to ease the migration of ions from cobalt oxide into the underlying support.1-3 These chemical phenomena limit the optimization of cobalt catalysts when metal dispersion and degree of reduction are critical factors for the application. An example can be found in the choice of the metal for the FischerTropsch process, which allows transforming synthesis gas (CO/ H2 mixture) into long-chain alkanes, alkenes or alcohols.1,4-6 Iron, the cheapest metal active toward Fischer-Tropsch reaction, has been the most used. It is, however, subject to deactivation by water produced by the reaction and for a similar degree of attrition, it is less active than cobalt.2,3,7,8 Cobalt would thus be a promising candidate on the long term, were it not for its cost, 1000 times higher than that of iron, which precludes its use unless the catalyst preparation is optimized in terms of metal content and particles size, which has been demonstrated to be a critical factor for the activity and selectivity of the reaction.9

Varying the mode of introduction of cobalt by impregnation or deposition at equilibrium,10,11 tuning the electrostatic adsorption of cobalt complexes on oxidic surfaces,12 decomposing cobalt-ammine complexes,13 and using chelating ligands in the impregnation solution, such as citrates8,14-16 or EDTA,17 have helped increase the metal final dispersion. However, calcination may still lead to some barely reducible aluminates.8 Another route can thus be investigated, in which oxidic particles are reduced as early as they are formed, and oxygen has to be avoided in thermal treatment. The decomposition in inert atmosphere of supported salts containing nitrates as counterions and organic ligands such as ethylenediamine (en) has led in the case of Ni/Al2O3 catalysts to well dispersed metal particles.18,19 This exploratory procedure takes advantage of a controlled succession of reactions between the chemical partners deposited on the support by impregnation. Decomposition of the crystallized salts (e.g., [Ni(en)2(H2O)2](NO3)2) occurs upon oxidation of en ligands by nitrate ions and nickel ions are almost immediately reduced by hydrogen produced by adsorbed organic residues, making their migration into alumina impossible. In this paper, this decomposition method is applied to the preparation of Co/Al2O3 catalysts. Macroscopic and microscopic characterization techniques, including UV-vis and X-ray absorption spectroscopies, are employed in order to check whether molecular phenomena proper to cobalt and influencing its speciation interfere or not with the final successful formation of dispersed cobalt metal particles. Experimental methods

* Corresponding author. Tel: + 33 1 44 27 60 04. Fax:+ 33 1 44 27 60 33. E-mail: [email protected]. † Institut Universitaire de France.

The supported catalysts will be hereafter referred to as CoenxNy, with x ) molar ratio en/Co and y ) molar ratio NO3-/

10.1021/jp067781w CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007

Cobalt Speciation in Co/Al2O3 Catalysts Co in impregnating solutions. Three catalysts will be discussed: Coen2N2, Coen3N2, and Coen3N3. 1. Preparation of the Solutions. Aqueous impregnation solutions were prepared, for samples CoenxN2, by successive additions in water of hexaaquacobalt(II) nitrate Co(NO3)2·6H2O (Aldrich) and x equivalents of ethylenediamine (NH2CH2CH2NH2, noted en, Aldrich). Liquid ethylenediamine is volatile with a density 0.9. Its exothermic addition to the Co(II) solution should be carried out dropwise with care. Solutions turned from dark pink to brown-dark red upon ligands addition. They were stirred for 10 min in ambient atmosphere before their use for impregnation. For sample Coen3N3, taken as reference, the solution was prepared in several steps. One equivalent of HNO3 from concentrated nitric acid (69.5 wt %, Prolabo) was first added dropwise to the cobalt solution prior to en addition, as to a reach a molar ratio NO3-/Co ) 3. After addition of 3 equiv of en and stirring for 2 h, the solvent was evaporated. A crude solid formed which was dissolved in 10 mL of water. After filtration to remove impurities, the solution was stored at 5 °C for 5 days. Orange crystals of [Co(III)(en)3](NO3)3 were then separated by filtration, washed with small fractions of ethanol and dried at room temperature in a desiccator. Elemental analyses were found to be in agreement with the formula mentioned above (found: C, 16.97; H, 5.61; N, 29.48. Calcd for [Co(C2N2H8)3](NO3)3: C, 16.94; H, 5.65; N, 29.65%). Impregnating solution was finally prepared by dissolution of this salt in water. 2. Preparation of Supported Catalysts. A 3 g sample of γ-Al2O3 (IFP 2520, 250-400 µm grains, specific surface area 200 m2 g-1, pore volume 0.6 cm3 g-1) was added at room temperature under stirring to 35 mL of each 0.2 mol Co L-1 impregnating solution. The solution concentration was chosen so as to eventually lead to a 12 Co wt% reduced catalyst. After 1 h, water was evaporated under reduced pressure (0.1 bar), and the humid light brown solids were dried in static air at 100 °C for 16 h. The dried samples were then pretreated for 1 h in air or argon, with a heating rate of 7.5 °C min-1 up to 500 °C. For Ar-treated materials, a posttreatment in pure H2 was performed for 2 h at 500 °C, (heating rate of 7.5 °C min-1). Gas flows of 100 mL min-1 were used in all cases. 3. Characterization Techniques. Chemical analyses were performed by inductively coupled plasma (Co) or catharometry after fast calcination (C, H, N) at the Vernaison Center of Chemical Analysis of the CNRS. X-ray diffraction (XRD) analyses were carried out on a Siemens D500 diffractometer, using Cu KR radiation (1.5418 Å). [Co(en)3](NO3)3 powder diffraction pattern was simulated by use of Crystal Diffract software (Crystalmaker Software Ltd). Combined differential thermal and thermogravimetric analyses (DT-TGA) were obtained on a Seiko DT-TGA 320 module operated by a Seiko SSC5200 disk station, with a heating rate of 7.5 °C.min-1 in an air or nitrogen atmosphere. Temperature-programmed reduction (TPR) was performed under 5% H2 in Ar (25 mL min-1). The hydrogen consumption was measured by catharometry, from room temperature to 1000 °C with a heating rate of 7.5 °C min-1. A HPR20/DSMS mass spectrometer (MS) was used to analyze the gases produced upon thermal treatment in air, in argon, or during TPR. A 2 m capillary line connected to the reactor outlet was used to collect the gases at atmospheric pressure. Gases were analyzed after fragmentation in an ionic source.

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4781 TABLE 1: UV-Vis Absorption Band Positions and Crystal Field Parameter for Some Co(III) (d6, Low Spin) Reference Complexesa complex

λ1 (nm)

λ2 (nm)

∆o (cm-1)

ref

1. [Co(H2O)6]3+ 2. cis-[Co(en)2(H2O)2]3+ 3. cis-[Co(en)2(OH)2]+ 4. [Co(en)2(µ-OH)]24+ 5. [Co(en)3]3+

405 361 371 375 340

606 495 517 532 467

18300 21800 21000 20600 23200

22 22 23 23 24

complex

λ1, λ2 (nm)

6. trans-[Co(en)2(H2O)2]3+ 343, 444 7. trans-[Co(en)2(OH)2]+ 380 (single band)

λ3 (nm) ∆ (cm-1) 549 526

18800 15500

25 25

a Complexes 1-5: λ1:1A1g f 1T2g, λ2:1A1g f 1T1g (interpreted on the basis of Oh symmetry, as an approximation for symmetries C2V (complexes 2-4) and D3 (complex 5))25 Complexes 6 and 7: λ1:1A1g f 1Eg, λ2:1A1g f 1A2g, λ3:1A1g f (1B2g, 1Eg) (interpreted on the basis of D4h symmetry)25

UV-vis-near infrared (UV-vis-NIR) spectra of the solutions were recorded in the transmission mode on a Cary 5 spectrometer (Varian; resolution, 1 nm). UV-vis-NIR spectra of the solids were recorded in the diffuse reflectance mode on the same spectrometer equipped with an integration sphere, using Teflon as reference. The spectra were subsequently submitted to the Kubelka-Munk transform. XANES and EXAFS spectra were recorded in transmission at the Co K edge at Sincrotrone Elettra Trieste (Trieste, Italy) on beamline XAFS (Coen2N2 and Coen3N2 dried and washed, measurement at 77 K) and at LURE (Orsay, France) on beamline XAS 13 of the DCI storage ring (Coen2N2 treated in argon, measurement at room temperature). For XANES measurements, a double-crystal Si(311) monochromator was used and the energies were scanned in 0.3 eV steps from 7680 to 7830 eV. For EXAFS measurements, a channel-cut Si(111) monochromator was used, and the energies were scanned in 2 eV steps from 7600 to 8600 eV. The energy was calibrated using a Co metal foil reference. After background correction, XANES spectra were normalized in the middle of the first EXAFS oscillation. EXAFS analyses were performed in the framework of single-scattering treatments with the package of programmes “EXAFS pour le Mac.” 20 Fourier transforms (FT) were calculated on w(k)k3χ(k), where w(k) is a Kaiser-Bessel window with a smoothness parameter equal to 2.5. The k limits were 2 and 11.7 Å-1. FT are presented without phase correction. Single-scattering fits of experimental curves were performed with the Round Midnight program.21 It was checked that multiple scattering is negligible in the 0-3.5 Å distance range for phases containing ionic cobalt, but this domain can be extended to 4.4 Å for metallic cobalt. Transmission electron micrographs (TEM) were collected on a JEOL 100 CXII UHR microscope. Results 1. Characterization of the Solutions. The solution used for Coen3N3 impregnation and prepared from [Co(en)3](NO3)3 exhibits the UV-vis absorption bands of Co3+ in [Co(en)3]3+, with two maxima at 337 and 465 nm and a minimum at 385 nm, in agreement with literature data (Table 1).24 In contrast, after 1 h of stirring under air atmosphere, the solutions used to prepare Coen2N2 and Coen3N2 are characterized by a continuously increasing absorption in the visible and UV domains (750 down to 250 nm). No shoulder can be located precisely. Very weak bands are detected in the near-infrared domain (at 1050 nm for Coen2N2 and 1000 nm for Coen3N2).

4782 J. Phys. Chem. C, Vol. 111, No. 12, 2007

Dumond et al.

TABLE 2: UV-Vis-NIR Absorption Band Positions and Crystal Field Parameter for Some Co(II) (d7, High Spin) Reference Compoundsa compound 1. [Co(H2O)6]2+ 2. β-Co(OH)2 3. cis-[Co(en)2(H2O)2]2+ 4. [Co(en)3]2+

λ1 (nm) λ2 (nm) λ3 (nm) ∆o (cm-1) ref 510 532 493 483

621 645 562 535

1250 1300 1100 1010

8600 8300 9500 9900

24 25 24 27

a

λ1:4T1g(F) f 4T1g(P), λ2:4T1g(F) f 4A2g(F), λ3:4T1g(F) f 4T2g(F) (interpreted on the basis of Oh symmetry, as an approximation for symmetries C2V (complex 3) and D3 (complex 4)).33

Because no absorption is expected in this region for Co(III) complexes (Table 1), these bands are attributed to complexes containing Co2+ and present in small quantities, a mixture of [Co(en)2(H2O)2]2+ and [Co(en)3]2+ for Coen2N2, and [Co(en)3]2+ for Coen3N2, respectively (Table 2). Therefore, the strong absorption in the UV-vis region is ascribed mainly to Co(III) complexes: not to a single species, for which two well-separate bands are expected, but rather to a mixture of species of various en/Co stoichiometries whose spectra overlap. It can be noticed that, for example, the maximum of absorbance expected for cis or trans-[Co(en)2(OH)2]+ around 370-380 nm coincides with the minimum of absorbance of [Co(en)3]3+ (Table 1). The addition of ethylenediamine into the solution of Co(II) nitrate has thus led to the formation of Co(II)-en complexes containing two or three equivalents of en (hereafter named “bis” or “tris” complexes for the sake of brevity), subsequently transformed into Co(III) complexes upon oxidation by dissolved oxygen. A gross calculation from Co(II)-en stability constants (log βi ) 5.4, 10.2 and 13.8)28 leads to the relative concentrations in solution as [Co(en)(H2O)4]2+/[Co(en)2(H2O)2]2+/ [Co(en)3]2+ ) 17%/66%/17% and 0%/4%/96%, in the case of 2 or 3 equiv of en added, respectively. This calculation rests on the hypothesis that equilibrium has been reached, but this may not be the case because the parallel reaction of oxidation consumes Co(II) complexes, with a rate higher for the bis species than that for the tris species.29 Once formed, it is likely that Co(III) complexes do not undergo ligands substitution due to the kinetic inertness of low spin Co3+, as evidenced by the fact that [Co(en)3]3+ is the only species present in solution prepared from [Co(en)3](NO3)3. [Co(en)3]3+, [Co(en)2(H2O)2]3+, and its deprotonated forms are probably present in Coen2N2 and Coen3N2 impregnating solutions. The existence of the Co(III)-containing dimer [Co(en)2(µ-OH)]24+ is also conceivable, but mixed-valence Co(II)/Co(III) entities can be ruled out because no intervalence band is observed between 700 and 800 nm.30 The pH of the solutions after 1 h is found to be close to 8 and thus to the zero-point of charge of alumina.31 Electrostatic interactions between the charged surface and complexes are consequently not expected to influence much the adsorption of complexes at the solid-liquid interface during initial contact with the solution. 2. Characterization of the Supported Systems after Drying. After drying, elemental analyses give molar ratios N/Co and C/Co close to the values expected from the stoichiometries inside impregnating solutions. Beside lines given by γ-alumina, X-ray diffractograms of all the dried samples present a pattern at lower angles typical of [Co(en)3](NO3)3, by comparison with the diffractogram of the pristine salt and that simulated from its crystal structure32 (Figure 1). In order to assign the least resolved diffraction lines on Coen2N2, one fraction of the sample was not dried after water evaporation but kept wet in a humidificator for 1 month. Peaks

Figure 1. X-ray diffractograms of reference salt [Co(en)3](NO3)3 (with a simulation based on the salt structure) and of supported samples.

Figure 2. UV-vis-NIR spectra of samples Coen2N2 and Coen3N2 after drying at 100 °C overnight, and after subsequent washing with water (absorbance in the NIR region × 4 on the graph).

from [Co(en)3](NO3)3 are clearly visible after this aging treatment. Coen2N2 thus has the smallest or the least organized crystals, and despite minor shifts, we can assume that the phase detected by XRD on the dried system corresponded to [Co(en)3](NO3)3. No other known crystalline phase can fit the position of the peaks. Except for Coen3N3, the stoichiometry of the supported phase detected by XRD differs from those of the impregnating solutions. UV-vis spectra of dried samples Coen2N2 and Coen3N2, both recorded after dilution in BaSO4, exhibit a broad band in the UV-vis region (Figure 2) with two maxima close to the positions expected for [Co(en)3]3+, in line with our interpretation of XRD results. The width of the band suggests the presence of other supported complexes. An interesting feature though not easily reproducible is seen around 1000 nm on some samples (see spectrum of Coen2N2 in Figure 2). As mentioned in the section devoted to the solutions, it corresponds to a fraction of Co(II)-en complexes which has not been oxidized. Finally, the NIR region reveals overtone and combination bands from the hydrated surface of alumina (1385 and 1930 nm) as well as from the en ligands: overtones of N-H and C-H vibrations around 1560 and 1690 nm, respectively; combinations of N-H and C-H stretching and deformation

Cobalt Speciation in Co/Al2O3 Catalysts

Figure 3. Comparison between the UV-visible spectra of a [Co(en)3](NO3)3 solution and of the filtrate obtained by washing of Coen3N2 with water.

bands around 2040-2060 and 2260 nm, respectively.19 The shift of the N-H combination band from 2040 to 2064 nm between Coen2N2 and Coen3N2 can be linked to the higher quantity of [Co(en)3](NO3)3 on Coen3N2. This band is actually seen at 2074 nm on Coen3N3 whose spectrum (not given) shows that [Co(en)3](NO3)3 is the only species present on the support. This shift to lower energy is attributed to the probable presence of hydrogen bonds between N-H and nitrate ions in the crystallized compound.32 3. Characterization of the Supported Systems after Drying and Washing. Because UV-vis spectra from dried Coen2N2 and Coen3N2 exhibit overlapping bands difficult to analyze, the samples were washed with water; washing would keep the coordination sphere of inert Co3+ complexes untouched and reveal the other phases remaining on the alumina surface. Washing was performed with a vacuum flask by slow addition of 100 mL of water onto the solid placed on a Bu¨chner, till the supernatant liquid above the solid appeared colorless upon stirring. UV-vis spectra of both red-orange filtrates are quite similar and show two separate bands which correspond to Co(III) complexes (Figure 3). The position of the band associated to the 1A1g f 1T2g transition (362 nm) is closer to that expected for a bis Co(III)-en complex, while the other band (469 nm) falls closer to the position expected for the tris complex. The difference in intensities compared with a solution containing [Co(en)3]3+ only, and the asymmetry of the band at higher wavelengths suggest that along with [Co(en)3]3+ originating from [Co(en)3](NO3)3, bis complexes exist in the filtrate and thus were initially present on the support in ill-crystallized phases. It should be noted that if present on the solids, Co(II)en complexes formerly identified by UV-vis were probably oxidized into Co(III) upon contact with air-containing water. The study of the washed solids gives insight into the insoluble species present on the samples. Washed solids are pink, with a darkish taint for Coen2N2. No supported phase can be detected by XRD. The quantity of cobalt amounts to 2.6 and 1.6 wt % for Coen2N2 and Coen3N2, respectively: more cobalt has resisted washing on Coen2N2. An en/Co ratio close to 1 is found by elemental analysis. UV-vis spectra of both samples (recorded without dilution in BaSO4) are globally similar (Figure 2). They exhibit a band

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4783 at 512-517 nm, with at least one shoulder at 615-624 nm (a second one may be seen on Coen2N2 at 568 nm), as well as a broad band in the near-infrared region (1300 nm), more intense on Coen2N2 and which was difficult to identify on the unwashed solids. This band shows that at least part of the phases resisting washing contains Co2+. The bands at 512-517 nm are indeed those expected for octahedral Co2+ ions (4T1g(F) f 4T1g(P) transition). This is supported by the shoulder at higher wavelengths assigned to the 4T1g(F) f 4A2g(P) transition of a d7 high spin configuration.33 Figure 4a presents the XANES spectra of washed samples Coen2N2 and Coen3N2, and of two reference compounds, a phyllosilicate containing Co2+ ions only34 and the Co(III)containing salt [Co(en)3](NO3)3. The position of the preedge and edge for the supported samples is intermediate between those of the two references, indicating the coexistence of Co(II) and Co(III) ions. They are expected to be present in octahedral symmetry, given the low intensity of the preedge (1s f 3d transition), similar to that observed for the two reference compounds with octahedral cobalt ions (Figure 4c). EXAFS best fit parameters of cobalt first and second shells are listed in Table 3, while the comparisons between experimental and calculated oscillations are shown on Figure 5a,b. k3-weighted Fourier transforms corresponding to the total signals and to the calculations limited to the first two shells are given on Figure 5c,d. No shell can be fitted using parameters from one element only and several distances were used in order to improve the fit. However, adding parameters can be questioned because it increases uncertainties (the value of ∆E0 appears to be quite high for N, as is the distance of 2.17 Å between Co and O) and the significance of these results must be examined with those concerning the second shell. For the second shell, two components are extracted quite easily, one corresponding to the carbons of en around 2.85 Å (which supports the need for N atoms in the first shell) and the other corresponding to Co atoms. The value R ) 3.16 Å would not be consistent with the presence of Co-en dimers such as [Co(en)2(µ-OH)]24+, in which the Co-Co distance is always shorter than 3 Å,36-38 neither with CoOOH (Co-Co ) 2.85 Å, obtained from CoOOH structure).39 This first insoluble phase detected could rather derive from Co(OH)2, characterized by a Co-Co distance of 3.18 Å and a Co-O distance of 2.11 Å,26,40 which can account for the upper value found for the Co-O pair during the simulation of the first shell. The shorter Co-N and Co-O distances can then be related to the other fraction of cobalt ions, those bonded to en. Given the XANES results and the fact that Co(OH)2 involves Co(II), these complexes contain Co(III); distances shorter than 2 Å in the first coordination sphere are indeed characteristic of Co(III) octahedral complexes.23,32,36-38 On the basis of the octahedral symmetry of cobalt complexes (XANES, UVvisible), on an average en/Co ratio of 1, and on former results showing that similar cobalt complexes which are not eliminated by washing are grafted on the support,41 we can suppose that about one-half of cobalt is grafted, with [Co(III)(en)2(AlO)2]+ as a possible formula, as found for similar nickel complexes,19 the other half being present as Co(OH)2 or as a related structure such as hydrotalcite. Though results on the adsorption of bis Co(III)-en complexes on alumina would rather support the stability of grafted complexes bearing one en ligand only,41 this possible speciation is in agreement with UV-vis spectroscopy (Tables 1 and 2): maxima at 517 nm for [Co(III) (en)2(OH)2]+ (with the high-


Dumond et al.

Figure 4. XANES spectra of reference compounds and (a) Coen2N2 and Coen3N2 after drying at 100 °C overnight and washing with water; (b) Coen2N2 after thermal treatment in argon. (c) Zoom on the preedge features.

TABLE 3: Best Fit Parametersa for First and Second Shells of Samples Coen2N2 and Coen3N2 after Washing and Dryinga sample

atom

N

σ (Å)

R (Å)

∆E0 (eV)

F(%)

Coen2N2

N O O C Co N O O C Co

2.2 2.2 1.9 1.7 0.7 2.0 2.0 1.7 2.0 0.3

0.07 0.07 0.07 0.08 0.09 0.07 0.07 0.07 0.06 0.09

1.94 2.02 2.17 2.84 3.18 1.93 1.96 2.15 2.80 3.16

-5 0 0 2 2 -6 0 0 0 -1

0.2

Coen3N2

0.2

N ) number of neighbors, σ (Å) ) Debye-Waller factor, R (Å) ) distance Co-backscatterer, ∆E0 (eV) ) energy shift, F (%) ) agreement factor (Σ[kχth(k) - kχexp(k)]2/ Σ[kχexp(k)]2). Ab initio amplitudes and phases functions were calculated using FEFF7 code.35 a

energy transition hidden in the intense charge-transfer band) and at 532, 645 and 1300 nm for Co(OH)2,. 4. Consequences of a Thermal Treatment in Air on Cobalt Speciation. The outcome of a thermal treatment in air will be now studied, in order to compare the differences in behavior between the reference sample Coen3N3 containing [Co(en)3](NO3)3 only and the samples in which several phases coexist (Coen2N2, richer in ill-crystallized or insoluble phases, and Coen3N2, richer in crystallized [Co(en)3](NO3)3). The decomposition of [Co(en)3](NO3)3 starts at 220 °C as shown by mass spectrometry (CH4 (m/z 13 and 15), NH3 (m/z 15 and 17), HCN (m/z 27), en (m/z 29 and 30), CO2 (m /z 28 ans 44), and NO2 (m/z 46)), in agreement with literature data.42 The major consumption in oxygen, noticed for the three samples at 280 °C, is accompanied by a brutal release of CO2 (m/z 44) and NO2 (m/z 46), revealing the combustion of organic matter contained in the system. These events followed by DTA appear to be highly exothermic, especially for Coen3N3 with a temperature rise of 40 °C as measured under the sample holder. XRD carried out on the three calcined samples detects intense and narrow Co3O4 peaks superimposed to those from γ-Al2O3 (Figure 6). The TPR thermogram of calcined Coen3N3 (Figure 7) exhibits two peaks of hydrogen consumption only: the first one at 367 °C is narrow, while the other at 457 °C is asymmetric with a shoulder at 522 °C accompanied by a production of methane (m/z 13 and 15). The first peak

corresponds to the reduction of large Co3O4 particles to metallic Co, at the temperature of reduction of the bulk oxide,43 and the second includes the reduction of smaller oxidic particles and the hydrogenation of carbon traces which have not been burnt during calcination (around 0.1 wt % according to H2 consumption).18 It is interesting to note that in spite of the heat released during [Co(en)3](NO3)3 oxidation into Co3O4, no aluminate phase which would have been reduced at higher temperature has been formed. In contrast, not only the peaks mentioned above are present on Coen2N2 and Coen3N2 thermograms (one can note on Coen3N2 a higher hydrogen consumption in the narrow peak corresponding to large Co3O4 particles, in line with the higher quantity of crystallized [Co(en)3](NO3)3), but also reduction peaks at 660 °C and above 800 °C. Only the first peak at 660 °C is associated to water production. It is attributed to the reduction to Co0 of oxide particles in strong chemical interaction with the support, while the hydrogen consumption above 800 °C is typical of the reduction to Co0 of cobalt ions from an aluminate phase.44,45 The phases specific to Coen2N2 and Coen3N2 (poorly crystallized salts containing bis or tris Coen complexes, grafted complexes and Co(OH)2) are likely to be transformed into these species more resistant to reduction because of an initial interaction with the support stronger than [Co(en)3](NO3)3 crystals. 5. Consequences of a Thermal Treatment in Argon on Cobalt Speciation. When dried catalysts are pretreated in argon instead of air, the supported phases also start decomposing at 220 °C. Water is detected at 280 °C (m/z 17 and 18). The temperature rise is only of a few degrees, compared with 40 °C during calcination in air. H2 and CO (m/z 28) released at 400 °C come from adsorbed organic species. Processes are thus parallel to those described for the decomposition of supported Ni-en complexes, including the release of hydrogen.18,19 Ar-treated samples mainly exhibit XRD lines from γ-Al2O3, but a shoulder at 44.2° characteristic of face-centered cubic Co0 is visible for Coen3N2 and Coen3N3, which initially contain better crystallized [Co(en)3](NO3)3 (Figure 8). In order to check the presence of Co0 on Ar-treated Coen2N2, X-ray absorption spectra were recorded at the Co K edge. The Fourier transform of the EXAFS signal shows that the first shell around cobalt contains two components, one at a distance characteristic of Co-O bonds (R ) 2.02 Å) and one at 2.51 Å, the shortest Co-


J. Phys. Chem. C, Vol. 111, No. 12, 2007 4785

Figure 5. Experimental (.....) and calculated (___) EXAFS oscillations relative to the first and second shells around cobalt in Coen2N2 (a) and Coen3N2 (b) after drying at 100 °C overnight and washing with water; k3-weighted Fourier transforms of the experimental total EXAFS signals (.....), and corresponding fits for the first and second shells (__), for Coen2N2 (c) and Coen3N2 (d) after drying at 100 °C overnight and washing with water.

Figure 6. X-ray diffractograms of alumina and of supported sample Coen2N2 after calcination in air (*, Co3O4).

Figure 7. TPR thermograms of the supported samples after calcination in air.

Co bond length found in metallic cobalt46 (Figure 9a,c, Table 4). It is not possible to reach a satisfactory fit by assuming the presence of Al atoms near cobalt. The next two shells (Figure 9b,d) may be fitted, though not totally satisfactorily, by assuming the presence of Co atoms at 3.55 and 4.42 Å, i. e., cobalt second and third neighbors in metallic Co0.46 Low values of N and high values of Debye-Waller parameters show that the environments around Co are diverse and

Figure 8. X-ray diffractograms of alumina and of the supported samples after thermal treatment in argon.

the average disorder high (it can be stressed that the comparison of σ values obtained on washed samples is not straightforward, because signals from washed solids were recorded at 77 K to enhance sensitivity). XANES (Figure 4b) confirms the coexistence of metallic and ionic cobalt, because of the edge position intermediate between Co0 and oxidized cobalt. The intensity of the preedge (Figure 4c) may originate from tetrahedral cobalt ions; the hypothesis of Co3O4 is favored because, as shown below, no aluminate is detected by TPR. Though the FT does not exhibit the features characteristic of Co3O4,46 its presence cannot be excluded, because of the problems of peaks identification due to the uncertainties in the fit at R > 2.5 Å. TPR thermograms of Ar-treated samples further exposed to air are different from those recorded after pretreatment in air (Figure 10). H2 consumption below 300 °C can be linked to the surface reduction of metal particles passivated by adsorbed oxygen, as noticed earlier for nickel catalysts.18 A broad peak due to methane (identified by MS) is visible in the 300600 °C region. The shoulder around 430-450 °C is associated to the production of water. Water is also produced on Coen2N2 in the course of the main H2 consumption. The production of methane is characteristic of hydrogenation of carbonaceous deposits, while water originates from the reduction of the oxidic phase. Finally, and in contrast with the result of calcinations in air, there is no peak at high temperature attributed to the reduction of cobalt aluminate for any of the samples. 6. Characterization of the Ar-Treated Samples after Posttreatment in Hydrogen. The goal of posttreatment in H2 is both to reduce the oxidic phase not fully reduced during pretreatment in argon and to eliminate carbonaceous species.


Dumond et al.

Figure 9. (a) Experimental (.....) and calculated (____) EXAFS oscillations relative to the first shell around cobalt in Coen2N2 after thermal treatment in argon; (b) comparison between the experimental total EXAFS signal (.....) and calculated EXAFS oscillations relative to the first three shells (__); (c, d) corresponding k3-weighted Fourier transforms (experimental ....., calculated ___).

TABLE 4: Best Fit Parametersa for Sample Coen2N2 after Thermal Treatment in Inert Gas (Agreement Factor G ) 1%)a atom

N

σ(Å)

R (Å)

∆E0 (eV)

O Co Co Co

3.8 1.9 1.5 2.6

0.11 0.11 0.11 0.11

2.02 2.51 3.55 4.42

-1 1 1 1

a N ) number of neighbors, σ (Å) ) Debye-Waller factor, R (Å) ) distance Co-backscatterer, ∆E0 (eV) ) energy shift, F (%) ) agreement factor (Σ[kχth(k) - kχexp(k)]2/ Σ[kχexp(k)]2). Ab initio amplitudes and phases functions were calculated using FEFF7 code.35

Figure 11. TEM micrographs of Coen2N2 (top) and Coen3N2 (bottom) after thermal treatment in argon and posttreatment in hydrogen.

decomposition in inert gas of similar Ni(II) complexes only led to 2-5 nm metal particles.18 The route using the decomposition of Co-en complexes has led to metal particles, but less dispersed than in the case of nickel. Discussion

Figure 10. TPR thermograms of the supported samples after thermal treatment in argon. Production of water and methane is evidenced by mass spectrograms (m/e 18 and 15, respectively; arbitrary units).

TPR/ MS experiments performed after posttreatment confirm that the shoulder at 450 °C is no longer observed and peaks due to carbon hydrogenation have decreased by 40%. TEM micrographs show assemblies of small grains corresponding to the ill-crystallized alumina support. Co0 particles appear as darker zones, but smaller particles are not easily detected because of the lack of contrast. On Coen2N2, some small isolated particles (arrows) coexist with well-contrasting agglomerates consisting of particles whose size varies between 6 and 50 nm (see the white circles, especially the one in upper left corner) (Figure 11, top). These large agglomerates are dominant on catalysts prepared with 3 equiv of en (Figure 11, bottom); there are also dark opaque zones attributed to unsupported cobalt particles of several 100 nm. In comparison, the

For catalysts prepared by impregnation, all species present in the initial solution remain on the support after drying. For short contact times between solution and support, and at pH for which the support surface is essentially neutral, two types of phenomena can occur: direct chemical interaction between the support and the precursor complex, or crystallization when the limit of solubility of the compound is reached during solvent evaporation.47,48 Thus, the salt [Ni(en)2(H2O)2](NO3)2 used as precursor to prepare supported nickel catalysts is recovered after drying, as evidenced by XRD.18 For low Ni contents, grafted complexes deriving from [Ni(en)2(H2O)2]2+ are detected by UV-vis; minor species in solution such as [Ni(en)(H2O)4]2+ can also become grafted onto the support but still remain in minority.19 The results presented here show that the situation with cobalt is more complex. Figure 12 summarizes the network of reactions leading to the various species identified, with emphasis on the most important phenomena occurring during Coen2N2 and Coen3N2 preparation. Compared with nickel, the successive stability constants of Co(II)-en complexes are lower by a factor of 2 to 100.19 The


J. Phys. Chem. C, Vol. 111, No. 12, 2007 4787

Figure 12. Relationships between the cobalt species identified during the successive steps of catalysts preparation. Minor species are indicated in italics.

addition of en to the solution of Co(II) nitrate thus leads to a distribution of species broader for cobalt: for instance, if no oxidation took place, [Co(en)2(H2O)2]2+ would amount to 66% of the species in the solution used to prepare Coen2N2, while the proportion of [Ni(en)2(H2O)2]2+ is over 80% in a similar solution prepared with nickel.49 Calculation shows that [Co(en)2(H2O)2]2+ and [Co(en)3]2+ would nevertheless be in majority in the solutions prepared with 2 and 3 equiv of en respectively (Results section, part 1.). Two groups of events can change these proportions: (i) Co(II)-en complexes react before equilibrium is reached; (ii) equilibria between bis and tris Co(II)-en complexes are shifted upon consumption of one of them. 1. Role of Oxygen in the Solutions. The first type of event corresponds to the oxidation of Co(II)-en complexes by oxygen dissolved in solution,50,51 in line with the influence of Co environment on Co(III)/Co(II) redox potential.41 The oxidation of [Co(en)2(H2O)2]2+

2[Co(en)2(H2O)2]2+ + 1/2 O2 + H2O f 2 [Co(en)2 (H2O)2]3+ +2 OH- (1) leads to [Co(en)2(H2O)2]3+ or to one of its deprotonated or dimeric forms, which our data do not allow to identify. We will thus use [Co(en)2(H2O)2]3+ as representing this class of species. [Co(en)3]2+ can also be oxidized into [Co(en)3]3+, but the reaction mechanism is less documented. For mixtures of bis and tris complexes and a time scale comparable to ours (1-2 h), the reaction is reported to be faster for bis Co(II) complex than for tris Co(II) complex.29 As a consequence, when the consumption of [Co(en)2(H2O)2]2+ is kinetically favored, complexation equilibria are likely to be shifted toward the formation of [Co(en)2(H2O)2]2+, even if [Co(en)3]2+ is initially the major species. These arguments explain why (i) [Co(en)3]2+ remains as such on some dried samples, even for the nominal ratio en/Co ) 2, and (ii) bis Co(III)-en complexes are detected by UV-vis on Coen3N2, for the initial nominal ratio en/Co ) 3. Finally, the kinetics of oxidation is diffusion-controlled by transfer of oxygen from air to the solution29 and, we can suppose, further to the solvent

contained in the porosity. Thus, the reaction rate depends on solution stirring, which was not monitored precisely, making the amount of [Co(en)3]2+ observed not reproducible. In conclusion, oxygen dissolved in the impregnating solution is likely to influence precursor speciation, as chemical partner. 2. Influence of Grafting onto the Support and Precipitation on Cobalt Speciation. The second type of event contributes to remove from solution the species formed by cobalt oxidation, probably during drying. Precipitation of [Co(en)3](NO3)3 (revealed by XRD) which consumes [Co(en)3]3+ seems to be the major phenomenon occurring for Coen3N2, due to the higher proportion of [Co(en)3]3+ in solution compared with Coen2N2. [Co(en)3](NO3)3 appears to be the only crystallized supported phase although other less organized phases are possible, such as those containing bis Co(III)-en and tris Co(II)-en complexes evidenced by UV-vis on the solids and in the filtrates of the washed solids. The narrow diffraction peaks observed for Coen3N2 and Coen3N3 after drying allow one to assume that the largest crystals of [Co(en)3](NO3)3 have formed from the solution and have precipitated outside the porosity of the support during the elimination of water. [Co(en)3]3+ needs 3 equiv NO3- to precipitate and, except for Coen3N3 prepared as reference with this stoichiometry, only 2 equiv NO3-/Co are initially present in solution. The other fraction of Co complexes thus exhibits a deficit of NO3- to compensate the positive charges. Two other sources of anions can lead to consume Co complexes out of the liquid phase: - grafting to surface hydroxyl groups undergoing deprotonation, leading to [Co(en)2(AlO)2]+ complexes; - reaction with OH- ions produced by reaction (1) explaining the formation and precipitation of Co(OH)2, from a fraction of complexes having lost their en ligands (in absence of en, Co3+ is no longer stable in water). This species may be related to the surface precipitates mentioned by Ataloglou et al.10,11 In presence of water and oxygen, Co(OH)2 should be oxidized into CoOOH,46 which has not been identified. It may be present as traces, for the transformation of Co(OH)2 into CoOOH can be topotactic.52,53 The two routes leading to these species resisting washing are nevertheless minor compared with the precipitation of nitrate

4788 J. Phys. Chem. C, Vol. 111, No. 12, 2007 salt and the formation of ill-crystallized species. At this stage, at least five different coordination spheres around Co coexist in these samples, making the quantitative analysis of speciation extremely difficult, especially because some spectroscopic responses (UV-vis, EXAFS) are blurred by the system disorder and because some species are subjected to transformations upon analysis, e.g., oxidation of Co(II) complexes during washing with water. 3. Effect of Thermal Treatment in Argon on Cobalt Speciation. As in the case of nickel, the thermal decomposition of Co-en complexes in argon leads to metal particles without formation of the aluminate but, at variance with nickel, also to oxidic phases. This can be understood by investigating the first decomposition step detected by MS, i.e., the events taking place at 220 °C whatever the atmosphere. Unsupported [Co(en)3](NO3)3 decomposes not via a single-step internal reaction between ligands and NO3-, as is the case of Ni(II) salts,54 but according to Collins et al.,42 by an initial release of en, a fraction of which is thermally decomposed leading to N- and C-containing Co(NO3)2-like phases. The latter are partly transformed into Co3O4 upon decomposition of NO3- ions, and partly reduced to metallic cobalt, probably by reaction with C-containing species. Although the nature of the organic matter is not known, this proposed mechanism fits quite well our experimental data on supported samples. The situation above 250 °C depends on the gas used for thermal treatment. In the case of air, all Co species formed by [Co(en)3](NO3)3 decomposition are oxidized into Co3O4 (TPR, Figure 7) while N- and C-containing species are partly burnt (MS data). Species interacting with the support (grafted complexes, Co(OH)2) and poorly crystallized salts containing Co-en complexes have also been oxidized. In air, the large release of heat, also reported on zirconia-supported systems,55 favors their migration into alumina. A similar influence of exothermicity on ions migration has been evidenced earlier with Ni.19 This does not occur during Ar-treatment. Upon Ar-treatment of the supported systems, the Co3O4 phase produced by the decomposition of [Co(en)3](NO3)3 remains as such. It is poorly crystallized or in the form of smaller particles escaping identification by XRD and no longer behaves as bulk oxide, hence a reduction temperature higher than for treatment in air. The persistence of this oxidic phase shows that the reduction by hydrogen released at 400 °C is less efficient than in the case of nickel catalysts. Besides, the decomposition of [Co(en)3](NO3)3 and probably the in situ reduction of the illcrystallized and insoluble phases lead to metallic Co. But the better [Co(en)3](NO3)3 is crystallized, the larger the size of Co0 particles resulting from its decomposition, as proven by the well resolved diffraction peak observed for Coen3N3 and by TEM. This is also in contrast with the case of nickel catalysts, where large precursor salt crystals lead to small metal particles,18 possibly because the reaction of decomposition between en and nitrates which breaks the salt particles apart takes place inside the crystals in this case. The consequence of the peculiar speciation of Co during impregnation and drying is thus the existence of species (crystallized [Co(en)3](NO3)3) leading to larger metal particles upon thermal treatment, while other species lead to smaller particles. The presence of agglomerates is also detrimental to the obtaining of a high dispersion. The formation of [Co(en)3](NO3)3 is due to three factors: (i) modification of cobalt redox potential upon complexation with en,

Dumond et al. (ii) presence of oxygen dissolved in the impregnating solution and behaving as reactant, and (iii) precipitation of [Co(en)3]3+ with NO3- ions so as to give a salt with low solubility. Already at the first preparation step, all chemical partners in solution are able to influence the subsequent state of the catalyst, in a way that can be either anticipated (i and ii), or understood a posteriori only (iii). This study evidences the challenge of rationalizing catalysts preparation, when a new partner (e.g., chelating en) is involved and another metal is used. Speciation can follow unexpected routes, specific to the transition element studied. Conclusions Impregnation of alumina with solutions containing Co-en complexes was investigated as a route allowing the preparation of Co/Al2O3 catalysts without subjecting them to calcination treatments, which induce the migration of cobalt ions into the support. After drying, several compounds are identified on the support surface: [Co(en)3](NO3)3 crystals, whose quantity and crystallinity increase with the ratio en/Co of solution; poorly cristallized soluble compounds containing bis and tris-ethylenediamine complexes of Co(II) and (III); grafted complexes of supposed formula [Co(III)(en)2(AlO)2]+; Co(OH)2. The formation of Co(III) from Co(II) nitrate precursor is explained by the redox reaction between dissolved oxygen and the various Co(II)-en complexes in equilibrium. However, unlike what occurs in solution, some Co(II)-en complexes can escape oxidation, probably due to a too low diffusion of oxygen in the wet porosity of the support during contact with the solution. Cobalt oxidation perturbs the equilibrium between Co(II)-en complexes, especially since secondary phenomena contribute to the removal from solution of [Co(en)3]3+ (precipitation) and [Co(en)2(H2O)2]3+ (grafting), which results in surface speciation that could not be anticipated from the nominal initial en/Co stoichiometries. As a consequence, thermal treatment in argon results in a mixture of supported oxidic and metallic cobalt phases, which lead to metal particles after posttreatment in hydrogen and allow avoiding the formation of cobalt aluminate. But the size distribution is broader than in the case of a similar synthesis using nickel, for which the crystallized precursor is the only species that decomposes during thermal treatment. Acknowledgment. The authors thank Dr. Patricia Beaunier (Laboratoire de Reáctivite´ de Surface) and Dr. Franç oise Villain (Laboratoire de Chimie Inorganique et Mate´riaux Molećulaires, Universite´ Pierre et Marie Curie-Paris 6) for their help in carrying out TEM and XAS experiments respectively. Prof. Franç ois Bozon-Verduraz (Universite´ Paris VII, Denis Diderot) is gratefully acknowledged for providing access to the UVvis-NIR spectrometer. References and Notes (1) Dry, M. E. Catal. Today 2002, 71, 227. (2) Bolt, P. H.; Habraken, F. H. P. M.; Geus, J. W. J. Solid State Chem. 1998, 135, 59. (3) Wang, W. J.; Chen, Y. W. Appl. Catal. 1991, 77, 223. (4) Iglesia, E. Appl. Catal. A 1997, 161, 59. (5) Schulz, H. Appl. Catal. A 1999, 186, 3. (6) Davis, B. H. Catal. Today 2002, 71, 249. (7) Fu, L.; Bartholomew, C. H. J. Catal. 1985, 92, 376. (8) Van de Loosdrecht, J.; van der Haar, M.; van der Kraan, A. M.; van Dillen, A. J.; Geus, J. W. Appl. Catal. A 1997, 150, 365. (9) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956.

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Catalysts Prepared from Solutions of Coba - American Chemical Society

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