1936
J. Phys. Chem. 1980, 84, 1936-1939
Mossbauer and Raman Spectra of Carbon-Supported Iron Phthalocyanine Carlos A. Melendres Argonne National Laboratory, Chemical Engineering Division, Argonne, Illinois 60439 (Received January 17, 1980) Publication costs assisted by Argonne National Laboratory
Mossbauer spectra have been obtained for iron phthalocyanine supported on high-surface-area carbon and prepared by the dissolution-precipitation technique. A number of isomer shifts (6) and quadrupole interaction parameters (A) are observed, which, at high loadings (-45 wt % FePc), appear to be determined primarily by interaction with the solvent and atmospheric oxygen. At low loadings (C15 wt %), a quadrupole doublet (6 = 0.32, A = 0.66 mm/s) is observed which is attributed to interaction with the carbon substrate. Raman spectra taken at different exciting laser frequencies exhibit resonance enhancement. Significant differences are observed between the spectra of pure FePc and the carbon-supportedFePc which provide further evidence of catalyst-support interaction.
The physicochemical properties of iron phthalocyanine are of interest in a number of areas. In electrochemistry, the material is being extensively studied in view of its catalytic activity for the electrochemical reduction of oxygen.l It is an analogue of the biologically significant iton porphyrin and is representative of a class of compounds, i.e., the transition metal macrocyclics, which play a central role in the processes of photosynthesis and biochemical energy conversion and tora age.^^^ Our own interest in this material stems from the first application. In a program of basic research, we are attempting to understand the mechanism of oxygen electrocatalysis by macrocyclic compounds and seeking correlations between their structure, properties, interaction with the support and electrolyte, etc. A number of works that have appeared in the literature claimed a dependence of catalytic activity of the phthalocyanines on the nature of the material that they are supported 0n.435 High-surface-areacarbon is commonly used as support in fuel-cell applications. We have studied the state of iron phthalocyanine supported on carbon by using Mossbauer and Raman spectroscopy and report here evidence of such catalyst-support interaction.
Experimental Section Iron phthalocyanine (FePc) was obtained from Eastman Kodak. Elemental (CHN) and emission spectral analysis together with infrared and Mossbauer spectroscopy were used to determine sample purity. No further purification was undertaken. Vulcan XC 72 (Cabot Corp.) and Norit BRX (American Norit Co.) carbon were used as received. Manufacturer's specifications indicated surface areas of -250 m2/g and -1500 m2/g, respectively. The FePc was supported on carbon by the dissolution-precipitation technique from aqueous and organic solutions. In this method, the FePc was first dissolved in a suitable solvent, e.g., concentrated sulfuric acid, pyridine, dimethylformamide (DMF), dimethyl sulfoxide (Me2SO),etc.; the appropriate amount of carbon to give loadings of 15-45 wt % FePc was then added and mixed thoroughly. Finally, salting out of the FePc and precipitation onto the carbon were effected by addition of distilled water. The material was filtered, washed well with water, and allowed to dry in ambient air. Mossbauer absorbers with a loading of 30-40 mg FePc/cm2 were prepared by mounting on Plexiglass holders. Samples for Raman spectroscopic measurement were prepared by pressing the carbon-supported FePc with KBr to form a disk. A Ranger Engineering Co. Mossbauer spectrometer of the constant-acceleration type was used with a 100 mC
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0022-3654/80/2084-1936$01.0010
57Co-in-rhodiumsource. Velocity calibration was made by using an iron foil enriched in 57Fe. All isomer shifts (6) and quadrupole interaction parameters (A) quoted here are referred to metallic iron as standard. These were obtained by a least-squares fitting procedure developed in this laboratory.6 The Raman spectrometer used was a SPEX Model 1401 with a photomultiplier tube detector. Coherent Radiation Model 52G Ar+ and Kr+ lasers were used as excitation sources. The power level of any exciting line used was kept below 150 mW to prevent sample decomposition. Sample rotation was also employed.
Results and Discussion Mossbauer Spectra. Some typical Mossbauer spectra obtained are shown in Figures 1 and 2. In Table I, Mossbauer parameters derived for some spectra are given. In the spectral-fitting process, the line widths were constrained to about 0.4 mm/s to allow for absorber thickness broadening. Mossbauer spectra of FePc (with and without carbon) precipitated out of concentrated H2SO4 are best fitted with three sets of quadrupole interaction parameters indicating the presence of three different iron sites. The state with a low 6 (0.22 mm/s) and high symmetry (A = 0.49 mm/s) is most probably oxidized FePc (i.e., Fe3+)and is also present in small amounts in the starting material (appearing as the inner doublet in Figure la). The two other quadrupole doublets are probably due to interaction with the solvent and/or oxygen. The outermost doublet does not appear to have a significantly different 6 than the pure FePc, but the A parameter indicates a slightly different symmetry as is observed when FePc is mixed with concentrated H2S04(6 = 0.44 and A = 2.10 mm/s). It is difficult to speculate on the coordination of iron in the state with 6 0.2 and A 1.1. It is significant to note here that unequivocal identification of any interaction of FePc with the carbon support does not appear possible because of the presence of multiple peaks. Carbon-supported FePc prepared from organic solvents shows spectra which are best fitted with two quadrupole doublets (cf. for example Figures 2 and 4.) The outer doublet with larger quadrupole splitting corresponds to either pure Fe(I1)Pc or an adduct of FePc and the solvent. The parameters obtained for the outer doublet in pyridine, for example, correspond to the sixfold coordinated FePc(bpy), adduct described by Hudson and Whitfielda7 At high loadings (-45 w t % FePc), the inner doublet is characterized by 6 0.2 and A 0.4 mm/s corresponding to oxidized FePc. This assignment is confirmed by the following
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0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 75, 1980
Spectra of Carbon-Supplotted Iron Phthalocyanine
1937
TABLE I: Mossbaueir Parameters for Carbon-Supported Iron Phthalocyanine E za
Ela
8 , b,d
E3a
8 2b,d
~j,b,d
~
~
c ~ , dc , d
A 3C'd
FePc (pure) 0.08 0.01 0.40 (2) 0.22 (4)* 2.63 (2) 0.49 (4) 45% FePc on Cf,(H,SO,)e 0.02 0.03 0.02 0.36 ( 2 ) 0.31 (5) 0.18 ( 2 ) 2.48 ( 3 ) 1.11 (2) 0.53 ( 8 ) 15%FePc on C (H,SO,) 0.0046 0.0091 0.008 0.38 0.25 0.32 2.54 1.19 0.54 100%FePc (H,SO,) 0.01 0.016 0.014 0.36 ( 2 ) 0.21 (3) 0.2 (1) 2.46 (2) 1.12 (3) 0.45 (5) 45% FePc on C (pyridine) 0.03 0.007 0.26 0.23 2.01 0.36 13%FePc on C (pyridine) 0.019 0.008 0.27 0.33 (2) 2.03 0.64(2) 0.04 0.02 0.38 0.25 2.63 0.42 47% FePc on C (DMF) 0.029 0.0061 0.40 0.30 ( 2 ) 2.63 0.66 ( 2 ) 16%FePc on C (DMF) 0.013 0.032 0.39 0.28 2.61 0.41 44% FePc on C (Me,S(D) 0.005 0.012 0.40 0.34 (2) 2.62 0.66 (2) 12%FePc on C (Me,SIO) 14%FePc on Norit BRX carbon (DMF) 0.005 0.009 0.40 0.30 2.61 0.61 A = quadrupole splitting, mm/s. Parentheses indicate probable E = fraction absorption. 8 = isomer shift, mm/s. error in last digit. e Parentheses indicate solvent in which FePc was dissolved. f Vulcan XC-72 carbon unless otherwise specified. a
I
I
I
I
I
O
-I 0
I
I
Y
L 30
2.0
0
I O
-1.0
-20
-30
2
I
I
I
20
10
I
I -20
-3 0
P
fn 2 W
,
H z
0 C
n
30
20
,_
, 0
I D
-I
I
I
0
-2 0
20
-3 0
10
O
-10
-2 0
VELOCITY, m m h
~
a
sm
Flgure 2. Mossbauer spectra of FePc on C prepared from pyridine solution: (a) 45 wt % FePc; (b) 13 wt % FePc.
a
30
20
10
-I
0
-2 0
-3 0
VELOCITY, mm/s Flgure 1. Typical Mossbauer spectra: (a)FePc starting material; (b) FePc on C (45 wt % ) prepared from concentrated H2S04; (c) FePc on C (15 wt % ) precipitated from concentrated H,SO,; (d) FePc precipitated from concentrated H2S04.
simple experiments. Pure FePc is sealed in a Pyrex tube and equilibrated with oxygen for 5 days with occasional heating to -150 "C. At the end of this period, the Mossbauer spectrum is obtained. A significant increase in intensity of the inner doublet is observed (Figure 3a). Similarly, a solution of FePc in DMF is prepared, and oxygen is bubbled through for 0.5 h. The FePc is subse-
quently reprecipitated by addition of distilled water. The Mossbauer spectrum of the material shows a considerably enhanced inner doublet compared with the starting material (Figure 3b). For low loadings (-15 wt %) of FePc on carbon, a species characterized by 6 = 0.33(3) and A = 0.65(5) mm/s is observed (Figures 2b, 4a, and 4b). This parameter appears to be independent of the solvent used in the dissolution process and could be attributed to an interaction of the FePc with the carbon support. Some variation of the intensity of the inner doublet with the solvents used can be correlated with differences in solubility of the FePc in them. The intensity of this doublet also increases with an increase in the surface area of the carbon used, i.e., Norit BRX carbon (Figure 44. It is interesting to note that the shift in 6 is toward lower values as is observed when basic ligands are coordinated to FePc.' The basicity of the surface (or whatever functional group may be on it) is apparently somewhat less than that of pyridine if one applies the correlation found by Hudson7 that the more basic the ligand the lower 6 is. The lower value of 6 compared to that in pure FePc indicates an increase in the density of s electrons at the nuclear position, presumably due to a decrease in d-electron density (and hence less shielding of the s electrons) of the iron brought about by delocalization or back-bonding to the ligands. The high symmetry, A = 0.65 mm/s, is also suggestive of probable coordination to the sixth position, presumably by oxygen.
1938
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980
Melendres
t 2
zYc
I
1
2000
-3 IO
-0055
to055
0
I , 1500
I ,
,
,
,
,
I000
500
0
R I M A N SHIFT, cm-'
t 3 10
Figure 5. Laser Raman spectra of FePc in KBr pellet (20 wt %) at various exciting frequencies: (a) X = 647.1 nm, P = 70 mW; (b) h = 514.5 nm, P = 75 mW; (c) X 487.9 nm, P, = 50 mW.
I
20
0
10
-I 0
I
-20
VELOCITY, mm/s
Figure 3. MGssbauer spectra of oxygenated FePc: (a) FePc equilibrated with 0,;(b) FePc reprecipitatedform DMF solution after bubbling 02. a
I 20
0
10
I
-I 0
-2.0
b
by the FePc is considerably enhanced when the material is dissolved in the organic solvents employed here. Presumably, coordination of basic ligands to the iron in FePc results also in activation of the FePc toward oxygen uptake. This is probably why, at high loadings, a significant fraction of the FePc is oxidized and masks any evidence of catalyst-support interaction. At low loadings, the fraction bonded to the support appears to predominate. It is also worthwhile to note here that the interaction of FePc with the carbon support appears more pronounced and presumably stronger with the monomeric FePc than the polymeric materials. The value of 6 allegedly found my Meier et a1.9 for polymeric FePc on carbon did not differ significantly from that of the pure material. Although the quadrupole interaction parameter of the carbon-supported material is significantly lower, it cannot be ruled out that this may be due to a slightly different stacking arrangement of the FePc layers rather than an unequivocal evidence of interaction with the carbon substrate. It may also be due to interaction with the sulfuric acid which the authors appear to have overlooked. Finally, from the relative intensities of the spectra of FePc on carbon at low loadings, it is apparent that a significant fraction of the FePc is not supported on the carbon, This may stem from the relatively low solubility of FePc in the solvents used and/or may reflect an inherent inefficiency in the process of preparing carbonsupported electrocatalysts by the dissolution-precipitation process. Not to be ruled out is also the possibility that most of the FePc is not oriented parallel to the surface. This would be the case if, for example, the peripheral groups of the FePc are the ones bonded to the surface such that there is little perturbation of the electronic structure of the iron. Raman Spectra. Figure 5 shows the Raman spectra of solid iron phthalocyanine (as 20 wt % in KBr pellet) at different exciting laser frequencies. Figure 6 shows the spectra of FePc supported on Norit BRX carbon ( 14 wt 3'% FePc) prepared by the dissolution-precipitation technique from DMF, then formed into a pellet with KBr (-95 w t %). It is evident from Figure 5 that resonance enhancement of certain bands (notably those at 1508,1133, 952, and 743 cm-l) occurs as the exciting laser line is changed from the blue (A = 488 nm) to the red (A = 647.1 nm). The bands at 1332 and 1448 cm-' appear to be enhanced also in going from blue to green. This resonance enhancement is not unexpected since excitation at 647.1 nm is well into the electronic absorption band of FePc.loJ1 It is worthwhile to compare our spectrum with the only N
1
I
20
I
10
I
0
1
-10
-2 0
VELOCITY, m m l l
Flgure 4. Mossbauer spectra of FePc on C prepared from organic solutions: (a) 12 wt % FePc on C with Me,SO in solvent; (b) 16 wt % FePc on C with DMF as solvent: (c) 14 wt % FePc on Norit BRX carbon with DMF as solvent.
It is known that, for iron porphyrin, addition of a 7rbonding ligand greatly increases the affinity of the iron to coordinate to a second one so that low spin octahedral complexes result.* We have observed that oxygen uptake
The Journal of Physical Chemistry, Vol. 84,
Spectra of Carbon-Supported Iron Phthalocyanine %
No. 15, 1980 1939
exciting frequency also differ for the C-supported and pure FePc. For example, with X = 647.1 nm excitation, the ratio of intensity of the 1503-cm-' band to that at 743 cm-l for the pure FePc is -2, while for the supported material it is obviously less. This is significant inasmuch as the 1503-cm-' band is reported12to be sensitive to the type of central metal atom (among the metal phthalocyanines) and indicates a perturbation of the metal environment when the FePc is supported on carbon. Other metal-sensitive bands at 1450 and -950 cm-l while relatively strong for the pure FePc have become rather weak for the carbonsupported sample. We find new bands and the disappearance of others in going from pure FePc to carbonsupported FePc. For example, with blue laser excitation, new bands are present at 350, 316, and 218 cm-' for the FePc on C and the bands at 276,259,232, and 183 cm-l found in pure FePc are no longer evident. Since these bands are presumably associated with vibrational modes involving the Fe atom, it is evident that the environment (symmetry) and bonding toward the Fe are significantly different for the two systems. This is what one might expect if additional coordination to iron has changed the symmetry of the FePc to other than the original D4h. The difference in excitation profiles for the two types of samples certainly is indicative of a change in molecular electronic structure in going from the pure FePc to carbonsupported FePc.
-
2000
I500
1000
!IO0
I 0
R A M A N SHIFT, Cm-'
Figure 6. Laser Raman slpectra of FePc supported on Norit BRX carbon: (a)
h = 647.1 nm, P = 150 mW; (b) h = 568.2 nm, P = 110 mW;
(c) h = 487.9 nm, P = mW. (* grating ghost).
other published one, Le., that obtained by Aleksandrov et d.l2We find some differences in both band positions and intensity when comparing their spectrum taken at X = 632.8 nm with ours taken at X = 647.1 nm exciting line. It is difficult to say whether this is just due to the slight difference in exciting frequency or real differences in the sample. Mossbauer, IR, and emission spectral, as well as elemental, analyses have been used to check our sample purity. Our band positions are believed to be accurate within zt3 cm-l. We also observed more bands than what the Russian investigators have found. The low frequency vibrational modes (P < 500 cm-l) have not been reported before. These modes presumably involve the central metal atom (i,e., Fe). In some infrared spectral studies of metal phthalocyanine~,l~J'~ Kobayashi et al. assigned bands in the region of 1.00-200 cm-l to metal-ligand vibrations and out-of-plane deformation of the macrocyclic ring. Bands a t about 230-240 cni-l are thought to be in-plane deformation of the macrocyclic ring, while lattice vibrations are expected a t less than 100 cm-l. A complete assignment of the observed frequencies to the various normal modes of vibration of FePc is not possible at this time but is being attempted in this laboratory in connection with resonance Raman spectral studies of FePc solutions in various solvents. On the basis of observations and deductions made on similar molecules (i.e., the metal porphyrins), we may tentatively assign the bands at about 1500 cm-l to C-C and C-N stretching modes; those at around 1300 cm-l are presumably due to CH in-plane bending.15 Examination of the spectra of FePc supported on Norit BRX carbon shows plrominently two broad bands at about 1350 and 1600 crn-l. The spectra in these regions appear to be superpositions of band due to the carbon substrate and the band due to :FePc. The former occur at 1330-1350 and 1580-1615 cm-l for various carbons.16J7 Resonance enhancement of bands in the region around 1340 cm-I is also evident with change in the frequency of the exciting laser light from blue to red. The intermediate frequency bands from -500 to -1000 cm-l, however, appear to be rather insensitive to excitation wavelength. This is in contrast with that olbserved for the unsupported FePc. Relative intensities of bands in spectra taken at the same
Acknowledgment. I am grateful to Dr. F. A. Cafasso for his continued interest and support of this work. The kindness of Drs. John Ferraro and John Unik of the Chemistry Division for allowing the use of their Raman spectrometer is gratefully acknowledged. Thanks are due to A1 Smirvickas for assistance in solving occasional problems associated with operation of the Raman spectrometer and also to Condeocita Sy for help in gathering the Mossbauer data. This work was undertaken under sponsorship by the Materials Sciences Office, Division of Basic Energy Sciences of the Department of Energy.
References and Notes (1) H. Jahnke, M. Schonborn, and G. Zimmerman, Top. Curr. Chem., 61, 133 (1976). (2) K. E. Smith, Ed., "Porphyrins and Metailoporphyrins",Elsevier, Amsterdam, 1975. (3) A. B. P. Levbr, Adv. Inorg. Chem. Radiochem., 7 (1965). (4) H. Alt, H. Binder, and G. Sandstede, J. Catal., 28, 8 (1973). (5) A. Kozawa, V. E. Zitionis, and R. J. Brood, J. Nectrochem. Soc., 117, 1470 (1970). (6) J. R. Gabriel and S. L. Ruby, Nucl. Insfrum. Methods, 36, 23 (1965). (7) A. Hudson and H. J. Whitfield, Inorg. Chem., 6, 1120 (1967). (8) J. E. Falk, "Porphyrins and Metalioporphrlns",Elsevier, Amsterdam, 1964, p 63. (9) H. Meier, U. Tschirwitz, et al., J . Phys. Chem., 81, 712 (1977). (10) B. W. Dale, J. Chem. Soc., Faraday Trans., 64, 331 (1968). (11) M. J. Stillman and A. J. Thomson, J. Chem. Soc., Faraday Trans., 70. 790 (1974). (12) I. V. Aleksandkov, Y. S. Bobovich, et al., Opt. Spectrosc. (Engl. Trans/.), 37, 265 (1974). (13) T. Kobayashi, Spectrochim. Acta, Part A , 26, 1313 (1970). (14) T. Kobavashl. F. Kurokawa, N. Uveda. and E. Suito. Soectrochim. Acta, Part A , 26, 1305 (1970). . (15) H. Ogoshi, Y. Saito, and K. Nakamoto, J . Chem. Phys., 57, 4194 (1972). (16) N. M. D. Brown, W. J. Nelson, 8. Cook, and J. Louden, J. Raman Spectrosc., 8, 229 (1979). (17) M. Nakamizo, R. Kammereck, and P. L. Walker, Jr., Carbon, 12, 259 (1974).
