10898
J. Phys. Chem. 1992, 96, 10898-10905
the kinetic coupling between the moieties, is quite accurate. This is especially convenient, since the latter is readily implemented (e.g., ref 11) and has (with appropriate extensions) been shownI0 to give a good quantitative prediction of rate coefficients for reactions of small species. While the present method enables RRKM calculations with these Hamiltonians to take precise account of the complexity of the kinetic energy in the quantification of these Gorin-style models, a major unsolved problem is in the potential term between the moieties when one or both are bulky. The variational calculation then gives a frequency factor that is highly sensitive to the assumed hard-sphere radii and the model cannot be employed meaningfully for a priori prediction of experiment: more sophisticated treatment of the transitional modes needs to be employed in such circumstances, clearly a fruitful area for future study. However, the present treatment could be used to give a physically meaningful fit (by scaling the van der Waals radii) of experimental data for reactions involving large moieties over a limited pressure and/or temperature range, and the k(E,J) so obtained could be used to predict the entire pressure and temperature dependence by using standard' methods.
Acknowledgment. The financial support of the Australian Research Grants Committee is gratefully acknowledged.
References and Notes (1) Gilbert, R. G.; Smith, S.C. Theory of Unimolecular and Recombinarion Reacrions; Blackwell Scientific: Oxford and Cambridge, MA, 1990.
(2) Wardlaw, D. M.; Marcus, R. A. J. Chem. Phys. 1985, 83, 3462. (3) Klippenstein, S. J.; Marcus, R. A. J . Phys. Chem. 1988, 92, 5412. (4) Wardlaw, D. M.; Marcus, R. A. Adu. Phys. Chem. 1988, 70, 231. (5) Gorin, E. Acra Physiochim. URSS 1938, 9, 691. (6) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York, 1976. (7) Klippenstein, S. J. J . Chem. Phys. 1991, 94, 6469. (8) Pitt, I. G.; Gilbert, R. G.; Ryan, K. R. Ausr. J . Chem. 1990,43, 169. (9) Greenhill, P. G.; Gilbert, R. G. J . Phys. Chem. 1986, 90, 3104. (10) Jordan, M. J. T.; Smith, S. C.; Gilbert, R. G. J . Phys. Chem. 1991, 95, 8685. (11) Gilbert, R. G.; Smith, S.C.; Jordan, M. J. T. UNIMOL program suite (calculation of fall-off curves for unimolecular and recombination reactions); 1992; available directly from the authors: School of Chemistry, Sydney University, NSW 2006, Australia. (12) Smith, S. C. J . Chem. Phys. 1991, 95, 3404. (13) Forst, W. Theory of Unimolecular Reactions; Academic Press: New York, 1973. (14) Beyer, T.; Swinehart, D. F. Commun. Assoc. Comput. Machin. 1973, 16, 379. (15 ) Goldstein, H. Classical Mechanics; Addison-Wesley: Reading, MA, 1980. (16) Eidinoff, M. J.; Aston, J. G. J. Chem. Phys. 1935, 3, 379. (17) Pitzer, K. S. J . Chem. Phys. 1937, 5, 469. (18) CGme, G. M. In Pyrolysis: Theory and Indusrrial Practice; Albright, L. F., Crynes, B. L., Cocoran, W. H.,Eds.; Academic: New York, 1983. (19) Baldwin, A. C.; Lewis, K. E.; Golden, D. M. Inf. J. Chem. Kinei. 1979, 11, 529. (20) Klippenstein, S. J.; Marcus, R. A. J. Phys. Chem. 1988, 92, 3105. (21) Wardlaw, D. M.; Marcus, R. A. J. Phys. Chem. 1986, 90, 5383. (22) Troe, J. Z . Phys. Chem. N.F. 1989, 161, 209. (23) Troe, J. J . Chem. Phys. 1987,87, 2773. (24) Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 242. (25) Quack, M.; Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 240.
Origin of the Electrocatalytic Properties for O2 Reduction of Some Heat-Treated Polyacrylonitrile and Phthalocyanine Cobalt Compounds Adsorbed on Carbon Black As Probed by Eiectrochemlstry and X-ray Absorption Spectroscopy M. C.Martins Alves, J. P. M e l e t 2 D. Guay, M. Ladouceur, and G. Tourillon* LURE, Batiment 209 D, 91405 Orsay, France (Received: February 27, 1992)
Electrochemicaland X-ray absorption techniques have been used to determine the influenceof heat treatment in electrocatalytic activity for O2reduction for two cobalt catalysts. The catalysts are cobalt phthalocyanine (catalyst 1) and polyacrylonitrile cobalt acetate (catalyst 2) adsorbed on carbon black and heat treated at several temperatures. A maximum far the catalytic activity was obtained for PcCo at 850 "C and for the PAN + Co catalyst at 950 "Cwith subsequent decrease. The results obtained by XANES and EXAFS data clearly show that metallic cobalt aggregates with different size are synthetized in the range of increased activity. In the region of highest activity were observed the smallest cobalt clusters (20 A). For higher temperatures these cobalt aggregates became bigger (100-200 A), which corresponds to the decrease in the catalytic activity. TEM was utilized as a complementary technique and it confirms the influence of the annealing temperature in the size of the cobalt aggregates obtained. XANES measurements at the Co and N K edges confirm that CON, centers and nitrogen atoms are no longer detected after heat treatment in the region of increased activity.
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Introduction In fuel cell technology the search for efficient and cost-effective electrocatalysts for cathodic oxygen reduction is a crucial prob1em.I With the knowledge that platinum is one of the best electrocatalysts for oxygen reduction, the investigation of compounds that do not use precious metal is very important from practical and theoretical points of view. In particular, N4-metal chelates appeared to be good candidates as catalysts for the oxygen reduction r e a c t i ~ n . ~These , ~ chelates became more interesting when it was demonstrated that the heat treatment of these materials adsorbed on high-area carbons improves their stabilities and activities for O2r e d u ~ t i o n . ~ ~ Several authors have attempted to explain the origin of the high activity and ~ t a b i l i t y ~and - ~ especially the nature of the active 'On sabbatical. Permanent address: INRS, Energie, C.P. 1020, Varenes,
F Q J3X 1S2, Canada 11.
0022-3654/92/2096-10898$03.00/0
species for the oxygen reduction after the thermal treatment: Wiesenerg proposed initially that during the heat treatment, a special "kind of carbon" with fupional chemical surface groups is synthetized on the substrate. The role of the metal ion is to influence the thermal formation of the active catalyst. According to others,'*I2 the annealing treatment does not lead to the complete destruction of the chelate macrocycle, but rather to a ligand modification which preserves the central N4-metal part. The electrocatalytic activity would come from this N,-metal group. Gupta et al.I3 demonstrated that the N4-metal centers are not essential to the electrocatalysis. They studied a system composed of a mixture of cobalt or iron salts and polyacrylonitrile adsorbed on carbon black (Vulcan XC-72) and annealed up to 1000 OC. The catalytic activities of such compounds are identical to those of the corresponding transition metal-N4 macrocycles. They proposed a model where the active species is a modified carbon 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10899
Heat Treatment and Electrocatalytic Activity surface on which transition metal ions are adsorbed through interactions with residual nitrogens derived from the heat-treated macrocycles. The idea that nitrogen is necessary in the electrocatalytic reaction is sustained by other authors,I4 but the exact chemical nature of the active sites is unknown. The formation of a mixture of oxides'5 and elementary during the heat treatment is also reported as a possible origin of the catalytic activity for the O2 reduction. Recently X-ray absorption spectroscopy was used to characterize the annealed products. Joyner et a1.16camed out an EXAFS (extended X-ray absorption fine structure) study on cobalt tetraphenylporphyrin (CoTMPP). From the data obtained on two samples of CoTPP adsorbed on Norit BRX, the first one heated at 700 OC and the other one untreated, they concluded that the C0-N4 group is retained in the active heat-treated catalyst. Van Wingerden et al." reported EXAFS analysis of heat treated 5,10,15,20-tetrakis(p-chlorophenyl)porphyrinatombalt(III)adsorbed on Norit BRX in the range of temperatures between 550 and 850 OC. For this system, the maximum catalytic activity was at 550 OC. From the EXAFS spectra, they concluded that at this temperature the CON, part is retained in the catalyst. For higher temperatures they observed the destruction of the porphyrin structure with subsequent formation of metallic cobalt. As Joyner et a1.,16 these authors also proposed that the increased activity derives from the C0-N4 part. McBreen et a1.I0 reported a study on iron tetramethoxyporphyrin (FeTMPP) and CoTMPP adsorbed on Vulcan XC-72 annealed at several temperatures in the range 600-1OOO OC range. EXAFS analysis identified several products including metal oxides and metallic cobalt particles which makes the interpretation difficult. For this reason, the samples were chemically treated to remove the oxides and the metal particles. They concluded that the metal-N, species are still present at temperatures yielding the maximum catalytic activity. However no X-ray absorption near edge structure experiments (XANES) were performed to confirm these results. Finally, a recent review on the electrocatalytic activities for the oxygen reduction was presented by Vasudevan et a1.,I8 but no clear conclusion was presented. Thus it appears that the question related to the nature of the catalytic site obtained after the thermal treatment of the precursors is still controversial. In order to understand the processes occurring during the thermal treatment, we studied two systems by electrochemical and X-ray absorption techniques. These two systems are cobalt phthalocyanine (PcCo) and polyacrylonitrile (PAN) Co acetate adsorbed on carbon black annealed at different temperatures. X-ray absorption spectroscopy (X-ray absorption near edge structure, XANES;and extended X-ray absorption f i e structure, EXAFS) is a well-adapted technique to get information about the local arrangement of the atoms in a matrix. The nature of the neighbors, their distances from the probed atom, and its coordination number are obtained from the EXAFS domain, while the symmetry of the site and the oxidation state of the selected atom are obtained from the XANES characteristics. Thus it has been possible to correlate electrochemical activity and intrinsic structures of the catalysts. We used the Van der Putted9 method for obtaining the electrochemical characteristics since it gives very reproducible results and avoids geometrical and wetting problems. Transmission electron microscopy (TEM) was used as a complementary technique to get information about the morphology and the particle sizes of the catalysts.
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Experimental Section Catalyst Preparation. Catalyst I. This catalyst was prepared by dissolving 30 mg of cobalt phthalocyanine (Fluka Chemie) in 10 mL of 96% H2S04(Merck). A 250-mg portion of Vulcan XC-72 (Cabot, 254 m2/g) was added and the solution was stirred during 1 h to yield PcCo/support 1/8 (w/w). The mixture was filtered, washed with distilled water, and then dried at 100 OC in air. Catalyst 2. This catalyst was prepared by dissolving 58.9 mg of PAN (Aldrich) with 14.7 mg of cobalt(I1) acetate (Aldrich)
in 2 mL of warm dimethylformamide (Merck). A 500-mg portion of Vulcan XC-72 was added to this solution. The dimethylformamide was removed by evaporation at 160 OC under flowing argon. The final concentration was 10% PAN 0.59% Co Vulcan XC-72. They represent the optimum concentrations as reported by Gupta et al.13 The carbon support (Vulcan XC-72) had a very low content of metallic impurities, in particular the concentration of cobalt metal was less than 1 ppm. Heat Treatment. The catalyst was placed in a quartz boat and heated during 2 h at various temperatures in a horizontal quartz furnace under continuous flow of argon and then allowed to cool down to room temperature under flowing argon. Electrode Preparation. The working electrode was prepared by following the method proposed by Van der Putten et al.I9 A thii porous layer of the catalyst was applied onto a vitreous carbon disk (area = 0.08 cm2) which made part of a rotating disk electrode. This layer was obtained by the incorporation of the catalyst particles in a polypyrrole f h . In order to prepare the polypyrrole film, 2.5 mg of catalyst was added to a 5 mL solution composed of 0.1 M LiC1044.5 M pyrrole and acetonitrile (Merck). The pyrrole was distillated just before use. The suspension was then put in contact with the working electrode and a potential of 0.6 V vs saturated calomel electrode (SCE) was applied. The potential was switched off when a charge of 40 mC was obtained. The deposited layer was flushed with ethanol and then dried. Electrochemical Measurements. Oxygen reduction measurements were performed in H2S04 solution (0.3 N, pH = OS), saturated with oxygen. A platinum grid was used as counter electrode and a saturated calomel electrode (Tacussel) as reference. The rotating disk was the working electrode. The net oxygen reduction currents reported are the differences at 4.150 V/SCE between currents at 25 rotations per second (rps) and current responses of the stagnant electrode in 02-saturated conditions. The sweeping rate was always 10 mV/s. The voltammograms were obtained with a Princeton Applied Research Model 273 potentiostat. X-ray Absorption Experiments. The experiments at the Co K edge were performed at the DCI storage ring, LURE Orsay, running at an energy of 1.85 GeV and a current of 300 mA. The X-rays were monochromatized with a Si 331 channel cut single crystal with an energy resolution of 1 eV at the Co K edge. The incident beam was collimated by slits and its intensity was measured by an ionization chamber. The XANES and EXAFS spectra of the catalysts and of the model compounds (Co metal foil, cobalt phthalocyanine and cobalt acetate) were recorded at room temperature in fluorescence and transmission mode, respectively. The thickness of the samples in transmission mode was adjusted such that px ( p is the absorption coefficient and x the thickness) on the high-energy side of the absorption edge was 1. The calibration of the energy scale was determined by recording the X-ray absorption spectra of a metallic cobalt foil. In the transmission mode the transmitted (r) beam intensity was determined by using an ionization chamber. The spectra recorded in the fluorescence mode were obtained using a detector especially designed at LURE which is based on a plastic scintillator and a photomultiplier.20 The XANES data were collected in scans with an energy step of 0.5 eV and two scans were added to obtain a good signal-to-noise ratio. The experiments at the N K edge were conducted in an ultrahigh vacuum (UHV) system (base pressure of about 1O-IoTorr). They were carried out at the VUV Super-ACO storage ring, LURE, on the SACEMOR beam line using a high-energy TGM monochromator (resolution of 0.2 eV at the N K edge). For XANES, the incident photon beam (Io)was monitored by collecting the total electron yield from a 85% transmission coppcr metal grid freshly coated with gold. The total electron yield from the sample ( r ) was then normalized with respect to (Io). The analysis of the EXAFS data involves a background subtraction by means of a cubic spline function.21 The various neighboring shells were obtained by a Fourier transformation of the EXAFS signal. The various peaks were sorted out by a
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Martins Alves et al.
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F i p w 1. I-E curves of the electrode in H2SO4 !p,H = 0.5, 0.3 N )
saturated with oxygen: (A) polypyrrole film containing PAN + Co + Vulcan XC-72 treated at 800 OC on vitreous carbon (-, stagnant electrode; ---, electrode at 25 r p ) ; (B) polyprrole film on vitreous stagnant electrode; ---, electrode at 25 rps). Scan rate carbon (-, = 10 mV/s, disk area = 0.08 cm2.
200
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ENERGY (ev)
Figme 3. Co K edge XANES for PcO on Vulcan XC-72: (a) pure PCCO; (b) PcCo on Vulcan XC-72 untreated sample; (c) PcCo on Vulcan XC-72 at 700 OC; (d) at 800 OC; (e) at lo00 OC; (f) Co metal. The zero energy reference corresponds to 7709 eV. TABLE I: Energies d Proporea Adgmu~tsfor Featurea obscned in the Co K XANES Spectrum of Cobalt Phthrlocyraiae
LOO
600
800
TEMPERATURE
1000
(OCI
Figure 2. Electrocatalytic current variations for 0,reduction in H2S04 (pH = 0.5) at -150 mV vs SCE, as a function of the annealing temperature: (A)PcCo on Vulcan XC-72 1/8 (w/w); (0)PAN 1096, Co 0.59% Vulcan XC-72. Catalyst loading = 6.4 mg/cm2.
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window separation. By an inverse Fourier transformation into
k space, the EXAFS oscillations corresponding to only one neighbor shell were obtained.22 Comparison of the experimental phase and amplitude functions deduced from a model compound (cobalt metal) with those of the sample yields the structural parameters.
R d Q md Discussion Ekctrocbemlcrl Characteristics. Figure 1A shows the current-potential (i-E) curves of the electrode in H$04 (0.3 N,pH = 0.5) at 10 mV/s. The solid curve is the response in a saturated O2solution of the polypyrrole layer containing the catalyst (PAN Co XC-72) heated at 800 OC. The dotted line corresponds to a rotation frequency of 25 rps of the same electrode. Figure 1B shows the i-E curve obtained for polypyrrole layer alone. It demonstrates that the carbon disk covered by the polypyrrole fdm is only responsible for a very low activity for O2reduction compared with the activity obtained for the catalyst loaded film. Moreover with this electrode preparation technique, very reproducible values are obtained. Figure 2 presents the performances at a potential of -0.150 V/SCE for both catalyst materials for a wide range of temperatures. For the PAN Co catalyst, the activity increases from room temperature up to 950 OC where a maximum is obtained. The catalyst activity decreases for higher temperatures. For PcCo the activity is first quite constant until 700 OC. A maximum is obtained around 850 OC. For temperatures higher than 950 O C , the current for both catalysts is very similar. It is well known2’ that cobalt phthalocyanine is active for O2reduction and for this
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features 1 2 3 4 5
energy/eV 7709 7716 7724 7728 7735
1s 1s 1s 1s 1s
---
assignment 3d (p-d hybridization) 4p, + ligand hole 4p, 4pxy+ ligand hole 4P,
reason the appearance of a maximum is less pronounced than for the catalyst made of cobalt acetate which is inactive without heat-treatment. X-ray Absorption Spectroscopy at the Co K Edge. XANES Chamcteristica of Catalyst 1. Figure 3 shows the Co K edge XANES data recorded for the catalyst 1 (PcCo/Vulcan XC-72) as a function of the annealing temperatures. The XANES data of the pure compounds (cobalt phthalocyanine (a), cobalt metal (0)and PcCo on Vulcan XC-72 untreated sample (b) are given for comparison. The reference energy value (0 eV) corresponds to the first inflection point of the metallic cobalt edge (7709 eV). Cobalt phthalocyanine has a DG symmetry where the metallic center atom is in a square-planar environment. The cobalt phthalocyanine spectrum (Figure 3a) exhibits several transitions labeled 1, 2, 3,4, and 5; their energy values are listed in Table I. The fmt transition is assigned to a dipole forbidden 1s 3d transition. In square-planar complexes p d mixing is forbidden by the inversion symmetry and the 1s 3d transition is predominantly quadrupole cou led with only a small contribution from d-p vibronic coup1ing.k The second transition is assigned to a shake down satellite involving the 1s 4p, transition with simultaneous ligand to metal charge transfer. The third transition is a pure 1s 4p,. The fourth and fifth features are due to 1s 4px, transitions. The relative energy positions of the px, py, and pz orbitals depend on the coordination of the metallic center. In a D4*symmetry, the absence of axial ligands stabilizes the pz orbital in comparison with the px, py ones and the 1s 4p, transition appears at a lower energy compared to the 1s 4px ones. The transition labeled 2, which is observed for all compound in a square-planar environment is a fingerprint of the Co-N, structurez and any modification of the coordination greatly affects this transition. These assignments were recently confirmed by
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The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10901
Heat Treatment and Electrocatalytic Activity
3
nn
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4 6 DISTANCE (A)
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1
Figure 4. Fourier transform of the k3 weighted EXAFS data for PcCo on Vulcan XC-72: (a) pure PcCo; (b) PcCo on Vulcan XC-72 untreated sample; (c) PcCo on Vulcan XC-72 treated at 700 OC; (d) at 800 OC; (e) at lo00 OC; (f) Co metal. (A& = 1.65-6.48 A-'.) Arrows show the limits of the inverse transform chosen to best fit the data.
-20
0
20 40 ENERGY (ev)
80
60
100
Figure 5. Co K edge XANES spectra of PAN + Co + Vulcan XC-72 samples heated at various temperatures: (a) pure Co acetate; (b) PAN Co Vulcan XC-72 untreated sample; (c) PAN Co Vulcan XC-72 treated at 800 OC; (d) 900 OC; (e) 1160 OC; (f) Co metal. The zero energy reference corresponds to 7709 eV.
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polarizationdependent experiments performed on oriented PcCo thin films.26 The XANES spectra for cobalt metal in a hcp structure is given in the curve 3f. The features observed in the spectra arise from multiple scattering effects of the photoelectron by the different cobalt shells.27 When the phthalocyanine is deposited onto the carbon support and upon heating up to 700 OC (Figure 3b,c), the XANES data clearly reveal that the Co-N4 structure is retained. For temperatures above 700 "C(curves d and e), great changes are observed, especially in the pre-edge region. In particular, transition 2 is no longer observed which means that the squareplanar codiguration is destroyed. Moreover the XANES spectra become very similar to the one of cobalt metal. Thus the XANES characteristics clearly reveal the appearance of metallic cobalt particles. EXAFS Characteristics of Catalyst 1. Figure 4 shows the k3 weighted Fourier transform of the Co K edge EXAFS for catalyst 1 in the range of temperatures studied. The standard compounds are given for comparison (PcCo (a), Co metal (0). Arrows in the FT figures indicate the limits used to extract the inverse transforms. The Fourier transform of cobalt hthalocyanine (Figure 4, curve a) exhibits a peak located at 1.6 (uncorrected from the phase shift) and is relative to the Co-N distance in the phthalocyanine structure. The Fourier transform of Co metal (Figure 4, curve f) exhibits three peaks located at 2.2,4, and 4.7 A. The first peak is related to the Co-Co distance in the first neighbor shell and the others are related to more distant neighbors. For the catalysts annealed up to 700 OC (Figure 4 b and c), the peak located at 1.6 A due to the Co-N distance in the CON, structure is still observed. For temperatures between 800 and loo0 "C, the intensity of this peak continuously decreases and three new peaks appear at approximately 2.2, 4, and 4.7 A. The comparison of these distances with those found in cobalt metal demonstrates the formation of metallic cobalt in good agreement with the XANES data. XANES Characteristics of Catalyst 2. Figures 5 and 6 show the XANES spectra and the Fourier transform obtained at the Co K edge for catalyst 2 (PAN + Co Vulcan XC-72) treated
A)
+
0.6
0.5 I
.-Y C
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5
0.4;
c LL U ul
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0.3:
t d a
2
0.2
B
00
0
2
4 6 DISTANCE (A)
8
Figure 6. Fourier transform of the k3 weighted EXAFS data for PAN + Co + Vulcan XC-72 samples as a function of their annealing temperatures: (a) pure Co acetate; (b) PAN + Co + Vulcan XC-72 untreated sample; (c) PAN + Co + Vulcan XC-72 treated at 800 OC; (d) 900 OC; (e) 1160 OC; (f) Co metal. (Ak = 1.77-7.05 A-'.) Arrows show the limits of the inverse transform chosen to best fit the data.
at different temperatures. The XANES data of pure compounds (CoAc (a), Co metal ( f ) ) and untreated sample (b) are given for comparison. The Co acetate (CO~(CH$H~COO)~.~H~O) is characterized by a C d o distance of 2.65 A. Each Co is situated in a distorted octahedral environment. Four of the six apexes of the octahedron centered on the first Co atom are oxygens from two acetate groups. The Co-0 distance is 1.97 A. Another oxygen atom from H 2 0
10902 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 TABLE II: Energies and Proposed Assignments for Features Observed in the Co K XANES Spectrum of Cobalt Acetate features energy/eV assignment 1 1109 1s 3d (p-d hybridization) 2 1124 1s 4p 3 1165 first EXAFS oscillation
--.
with a C d distance of 2.2 A is on the fifth apex while the other Co atom is on the sixth one.28 The XANES spectrum of cobalt acetate (Figure 5 , curve a) exhibits mainly three transitions at 7709, 7724, and 7765 eV respectively (Table 11). The pre-edge feature 1 is due to a dipole forbidden transition to 3d states. For octahedral complexes as for square-planar compounds, d-p mixing is forbidden and the 1s 3d transition is quadrupole allowed with only a small contribution from d-p vibronic coupling.24 The intense white line (feature 2) corresponds to the transition of the 1s electron from the core level to 4p states (feature 2). Transition 3 is the first EXAFS Co-0 oscillation. The XANES characteristicsof the catalyst annealed up to 900 OC reveal great modifications. The intensities of the white line (feature 2) and the first EXAFS oscillation (feature 3) decrease, indicating modifications in the local order of the organic matrix. For temperatures above 900 "C, the transformations are similar to those already observed with catalyst 1, the spectra evolving to that of metallic cobalt. EXAFS Chwacteristics of Catalyst 2 The k3weighted Fourier transform of cobalt acetate (Figure 6, curve a) exhibits a peak located at 1.62 A (uncorrected from the phase shift) and is relative to the Co-0 distance. For annealing temperatures up to 900 OC the Co-0 distance at 1.62 A is still present. Considering that the cobalt acetate inserted in the polymeric matrix is not stable at high temperatures, it could be transformed to an amorphous oxide compound. The fitting of the EXAFS curves with backscattering amplitude and phase shift values deduced from standard COO and Co304compounds was impossible because several (20-0 distances are present in the first shell of these compounds. For catalyst 2 at temperatures below 900 "C an amorphous oxide could be formed. For the temperatures between 900 and 1160 "C, the distances observed at 2.2,4, and 4.7 A are relevant to the COCO distances in metallic cobalt as it was demonstrated by the XANES analysis. We verified the influence of the chemical leaching with a 9 M solution of KOH at 90 "C during 0.5 h as proposed by McBreen et al.IO Identical XANES data were found for the unleached and leached samples. In order to obtain quantitative estimations of the bond lengths and the coordination numbers around the cobalt atoms, the inverse Fourier transforms of the first shell has been fitted (Figure 7). For catalyst 1 and 2 the backscattering amplitude and the phase shift values were deduced from the EXAFS spectra of the metallic cobalt foil. The fit parameters (bond lengths, coordination numbers, and Debye Waller factor) are shown in Table 111. The results clearly show a continuous increase of the coordination number with increasing temperature while the Co-Co distances remaining constant at 2.46 A. The evolution of the coordination number values from 8 to 12 suggests that in the lower temperature domain (900 OC for PcO and 950 "Cfor PAN Co) very small metallic clusters are synthesized (size in the order of 20 AZ9v3O).When the temperature increases, these aggregates become bigger and for temperatures as high as 1000 "C for PcCo and 1160 "C for PAN Co, 12 neighbors are obtained, which correspond to the metallic cobalt foil. For the PccO/XC-72 sample treated at 800 "C and Co PAN XC-72 sample treated at 900 "c,good fits cannot be obtained. At these temperatures, a mixture of oxide, PcCo, and cobalt metal is present which makes the fitting of the curves difficult. An examination by TEM of the PAN + Co catalyst was performed in the range of temperatures between 900 and 1090 "C.The pictures obtained for 900 and 1090 OC are shown in parts a and b of Figure 8, respectively. Different sizes of metallic Co
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