TRANSITION METAL MACROCYCLES SUPPORTED ON HIGH AREA CARBON: PYROLYSIS-MASS

SPECTROMETRY STUDIES

D. SCHERSON, A. A. TANAKA,* S. L. GUPTA, D. TRYK, C. FIERRO, R. HOLZE and E. B. YEAGER

Case Center for Electrochemical Sciences, The Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, U.S.A.

and

R. P. LATTIMER

The BF Goodrich Company, Research and Development Center, 9921 Brmksville Road, Brecksville, OH 44141, U.S.A.

(Received 27 January 1986)

Abstract-A number of cobalt and iron porphyrins heat treated at temperatures as low as 400°C exhibit substantial electrocatalytic activity for O2 reduction and reasonable stability in alkaline electrolytes. The effects of this heat treatment on the structure and overall properties of these materials, however, are not well understood. Differential thermogravimetry analyses have shown that in the case of the Co and Fe-y-ox0 forms of tetra methoxyphenyl porphyrin (TMPP) and the metal-free form as well, the onset for partial decomposition occurs at temperatures of about 400-500°C for the macrocycles either as crystals or dispersed on high area Vulcan XC-72. The results obtained with several pyrolysis-mass spectrometric techniques have indicated that the fraction of volatile nitrogen to non-nitrogen containing species generated during the heat treatment is much higher for the metal-free than for the iron-p-oxo or CoTMPP. Microanalysis also confirms that with the Fe-PC-oxo and CoTMPP part of the nitrogen is retained. Possible models for the nature of the active sites are discussed.

INTRODUCTION

The dispersion of certain transition metal macrocycles on high area carbons followed by a heat treatment in an inert atmosphere at temperatures as high as 850°C has been found to yield materials which exhibit high activities for the electrochemical reduction of oxygen[l-71. The overall performance of Teflon bonded &-fed cathodes of the fuel cell type contain- ing cobalt tetramethoxyphenylporphyrin (CoTMPP) prepared in such fashion, has been reported to be comparable to that of more conventional Pt-based electrodes in alkaline media[7,8]. Equally encourag- ing results have been obtained with the heat treated Fe- p-0x0 form of the same macrocycle for 0, reduction in hot concentrated phosphoric acid[9]. This rather remarkable phenomenon has prompted a number of research groups, including ours, to use an array of analytical and spectroscopic techniques to examine the physicochemical modifications induced by the thermal treatment of these materials in an effort to identify the structural and morphological factors responsible for the pronounced enhancements in the electrochemical activity and stability[2,3,7, 10-143. Although several explanations for these catalytic effects have indeed been offered in the literature by the different groups

*On leave from Instituto de Fisica e Quimica de Sao Carlos-USP, Sao Carlos (SP), Brazil.

working in this area, no consensus has yet been reached regarding such key issues as the oxidation state and binding site of the transition metal after the thermal activation. This may be attributed in part to differences in the specific conditions under which the heat treat- ment was conducted. Extraordinary care must be exercised, for instance, with the heat treatment of small specimens over extended periods of time to avoid or minimize the effects associated with the presence of trace oxygen in either the inert gas or the vacuum environment. As a means of circumventing some of these difficulties, most of the attention in this labora- tory has been focussed on techniques capable of providing information on specimens prepared and/or analyzed in an environment as similar as possible to that employed during the heat treatment and overall handling of electrochemically active materials for practical air cathodes.

This communication presents pyrolysis-mass spec- trometry Cpy-ms) and pyrolysis-gas chromato- graphy-mass spectrometry by--gc-ms) results ob- tained for both supported and unsupported cobalt, iron and metal-free tetramethoxyphenylporphyrins heat treated in situ under one atmosphere pressure of a flowing inert gas. Similar conditions were employed in differential thermal gravimetry experiments aimed at establishing the onset decomposition temperature of the porphyrins in bulk form. Particular emphasis has been placed in the purification of the macrocycles, especially the removal of traces of H,TMPP from the

1247

1248 D. SCHERSON et al.

metal containing specimens, a compound found to exhibit a behavior unlike that observed for the Co and Fe-p-oxo porphyrins. The purity of the three macro- cycles was monitored with a number of techniques including ultraviolet-visible spectroscopy (UU-uis), Fourier transform infrared (ffir), proton Fourier transform nuclear magnetic resonance (ftnmr), fast atom bombardment (fub) and field desorption (fd) mass spectrometry.

BACKGROUND

Early attempts aimed at acquiring detailed informa- tion concerning the nature of the gases evolved during the thermal treatment of a selected number of macro- cycles have been reported by van Veen et al. in 1981[3]. These workers employed differential thermal analysis and thermogravimetry (DTA/DTG), ESCA and a high vacuum adsorption apparatus coupled with a quad- rupole mass spectrometer for the identification of the gas phase decomposition products released by the macrocycles in question as a function of the heating temperature, T. Their studies included among others, iron and cobalt tetra p-chlorophenylporphyrms, T@- Cl)PP, dispersed on Norit BRX carbon, and unsup- ported Fe* and Co tetramethoxyphenylporphyrins, TMPP. The main reaction observed at T-c 500°C involved the loss of the p-group, a process that occurred at a much lower temperature for the chloro- than for the methoxy-substituted compounds as moni- tored by ESCA and DTA/DTG, respectively. In ad- dition, the iron porphyrins exhibited a lower onset for thermal decomposition than the Co counterparts. The information obtained from dissolution experiments in concentrated sulfuric acid provided evidence that for T > 50O”C, the amount of iron which dissolved in this media in the case of FeTMPP/Norit BRX specimens became larger as the heating temperature was in- creased, an effect that was not observed for the supported CoTMPP. Furthermore, in the case of iron and cobalt phthalocyanines, FePc and CoPc, sup- ported on a graphitized carbon, the mass spectrometric analysis indicated considerable H, and HCN release at temperatures higher than 5OO”Cr31.

In related im&tigations, Brodskii et al. [lo] and Radvushkinarl 11 have examined bv nvrolvsis mass specirometry-the thermal behavior In vacuum of unsupported Co, Fe and the metal-free forms of tetramethoxyphenylporphyrin, HsTMPP. The total ion current as a function of temperature showed peaks with maxima in the range 5&2OO”C, attributed to organic impurities, and also at - 395°C for H,TMPP, and at the surprisingly low temperature of - 28C&3OO”C for the Fe and Co counterparts. These authors proposed that three major types of decompo- sition products were associated with the peaks with maxima at 395°C and 28(r3OO”C. The first was

*The axially uncoordinated monomeric ferrous por- phyrins are readily oxidized when exposed to air even in the bulk phase yielding /wxo dimers. The compound referred to by van Veen et nL[3] as FeT(p-OCH,)PP is in fact (FeTMPP),O as their own Mossbauer spectra indicate.

reported to consist of methoxybenzene, phenol, ben- zene, methane and formaldehyde and believed to originate from the methoxyphenyl group. The second type of products was assumed to arise from the cleavage of the porphyrin ring and included aldimines, acetonitrile and acrylonitrile. The temperature range in which these two types of products were detected was rather narrow. Lastly, the third class of gas phase pyrolysis products, obtained primarily for T > 400°C involved fragments with masses between 128 and 191 amu and were attributed to polycyclic aromatic com- pounds. It was also established by these authors that the onset temperature for the release of the ben- zomethoxy group was much lower for the Co and Fe macrocycles than for the metal-free analog. At higher temperatures, however, the mass spectrometry results showed fragments for H,TMPP with m/z values larger than 200. Such species were not found in the case of the metal-containing porphyrins. On the basis of these data, it was concluded that the metal center in the macrocycle weakens the bond between the meso- substituent and the ring, and at the same time leads to an overall stabilization of the unsubstituted porphyrin ring. It should be mentioned, however, that excess transition metal not bound to the macrocycle and in the form of oxide might act as an oxidation catalyst and promote the decomposition of the macrocycle.

With the sole exception of the iron and cobalt phthalocyanine investigations referred to earlier[3], all thermal treatment-mass spectrometry experiments reported to date have involved unsupported macrocyc- les and in every case the measurements have been conducted by heating the materials under vacuum. These conditions are different from those employed in the preparation of materials found to exhibit high catalytic activity for the reduction of dioxygen. Hence, the information arising from such experiments, al- though interesting in itself, may not be relevant to the specimens of electrochemical interest.

EXPERIMENTAL

Synthesis

The tetramethoxyphenylporphyrin H,TMPP was prepared following the procedure described by Adler et al.[lS]. This consists in adding freshly distilled pyrrole (28 ml, 0.4 mole) and p-anisaldehyde (49 ml, 0.4 mole) to refluxing propionic acid (1.5 1). The reac- tion is allowed to proceed for 30 min under constant magnetic stirring, at which point the heating is inter- rupted. After the solution reaches room temperature it is filtered through a fine frit glass funnel. The crystals are subsequently washed with methanol and hot water, dried in an oven at 110°C for three days, and sub- sequently dissolved in a minimum amount of chloro- form. The solution is then chromatographed in a silica column employing a benzene-chloroform mix- ture (1 : 1 v/v) as the eluant. The fraction containing the H,TMPP is filtered in a sintered glass funnel (fine size) to remove silica particles and evaporated to dryness in a roto-evaporator. The resulting crystals are finally dried under vacuum at 80°C for 4 h and then stored in plastic cap glass vials. For H,TMPP (C,,H,,N,O,):

Pyrolysis-mass spectrometry studies 1249

Anal. Cal& C, 78.47: H, 5.18; N, 7.63; 0, 8.72 Found:* C, 78.32: H, 5.56: N, 6.85; 0, 8.45%. The insertion of the cobalt center into the porphyrin

was performed according to the method reported by Adler et al.[16]. The H,TMPP (0.67 g, 9 x 10v4 mole) is dissolved in refluxing N, N’-dimethylformamide (56 ml). Cobalt acetate (0.22 g, 9 x 10e4 mole) is then added to this solution and the reaction progress is monitored bv the decrease in the long-wavelength fluorescence -associated with the H,TMPP. Small amounts (ca lo”/,) of extra Co(CH,COO), are often needed to achie;e a complete n&alla& of the porphyrin. At that point the reaction vessel is cooled in an ice-bath, and a portion of an ice-water mixture is added to the solution which is then filtered through a Buchner funnel. The solid material is washed with water until the filtrate is clear and later dried in an oven at 110°C for 24 h. The purification procedure consists in dissolving the porphyrin crystals in a minimum amount of chloroform followed by chromatography in a silica column using a benzene-chloroform mix- ture (1: 1 v/v) as the eluant. The first collected fraction is CdTMPP’and the second H,TMPP. The solution containing the metalloporphyrin is filtered in a sin- tered glass funnel and evaporated to drynesss in a roto-evaporator. The resulting crystals are dried under vacuum at 80°C for 4 hand stored in a plastic cap glass vial. For CoTMPP (C48Hs6N404C~):

Anal. Cal& C, 72.82; H, 4.55; N, 7.08; 0, 8.09; CO, 7.46

Found: C, 72.53; H, 4.85; N, 6.92; 0, 7.93; CO, 7.81 %.

The p-0x0 form of the iron tetramethoxyphenylpor- nhvrin. tFeTMPP),O. was synthesized by the route he&i&d by Torrens et al.[17]. A fresh ferrous acetate solution, prepared by dissolving 0.04 g of metallic iron powder in acetic acid, is added to refluxing glacial acetic acid (150 ml) containing the H,TMPP (0.520 g, 7 x lo-& mole). The completion of the reaction is determined using fluorescence as a monitor as de- scribed above. The solution is then diluted with water and the porphyrin extracted with benzene in a sep- aratory funnel. An aqueous potassium hydroxide solution (100 ml, 25 oA in water) is added to the benzene fraction and the mixture stirred overnight. The ben- zene solution is separated and washed with water several times and the porphyrin is then obtained by evaporating the solvent. The unreacted H,TMPP is removed by column chromatography employing a benzene-chloroform mixture (1: 1 v/v). The (FeTMPP),O is then eluted using a benzene- methanol mixture (20: 2 v/v) and subsequently filtered in a sintered glass funnel and evaporated to dryness. The crystals are finally dried under vacuum at 80°C for

*The elemental analyses of the three porphyrins as well as those of specimens involving the same compounds dispersed on carbon before and after heat treatment was performed by Galbraith Laboratories (Knoxville, TN).

4h. For (FeTMPP),O (C96H72N809FeZ):

Anal. Calcd: C, 72.36; H, 4.52; N, 7.04; 0. 9.04: Fe, 7.04.

Found: C, 71.58; H, 4.90; N, 6.62; 0, 9.54; Fe, 6.58 %.

Spectroscopic characterization

Fourier transform infrared (stir), uv-visible, proton jitnmr and field desorption ($4 and fast atom bombard- ment mass spectrometry ~ahm) were used to assess the purity of the porphyrins. To the level of sensitivity of each of these methods no contaminants were detected other than those associated with the solvents involved in the spectroscopic measurements themselves.

Theftir spectra were recorded with a Digilab FTS- 14 spectrophotometer using KBr pellets. The H,TMPP and CoTMPP prepared in this laboratory yielded results which were essentially identical to those reoorted earlier bv Thomas and Martellf181~. with all the peaks at the &me frequencies. The ‘we& peak at about 3320 cm-’ in H,TMPP due to the N-H stretch was not observed in the case of the metallated derivat- ives. The corresponding spectrum for (FeTMPPhO shown in Fig. 1 exhibited two weak bands at 874 and 893 cm-‘. These can be assigned to the F-Fe stretching modes based upon-the assignment made previously for the (FeTPP),O[19-211.

Excellent agreement with the uo-ois spectra reported in the literature was obtained for theH,TMPP and CoTMPPfZZ-251. All these measurements were con- ducted in-benze;e solutions (1.5 x lo-’ M) with a Cary Model 15 spectrophotometer using 1Omm quartz cuvettes. One mm quartz cells were employed to record the intense Soret band in the near uv range. The spectrum obtained for the (FeTMPP),O is shown in Fig. 2. The adsorption bands (411,572 and 614 nm) are veiy similar to those reported‘for the (FeTPP),O (408, 571, and 612 nm)[26]. The small energy shift towards longer wavelengths induced by the methoxy groups (bathochromic shift) has also been observed for H,TMPP and CoTMPP.

The proton ftnmr spectra were obtained with a Varian XL-200 instrument, using CDCl, as a solvent and TMS as a standard. The spectrum for H,TMPP [N-H, 6 - - 2.8 ppm; p-OCH,, 6 - -4.i5 ppm: m-H(nhenv1). 6 - 7.25 nnm: 0-H(ohenv1). 6 - 8.12 ppm: ‘pyrrdlt&H, S - 8.85 ppm] is tfl exceilknt agree- ment with that reported by Walker et aI.[27], whereas that of CoTMPP (see curve a, Fig. 3) rm-H(phenv1). 6 - 9.50 ppm; 0-H(phenyl), 6 i 13.15ppm; pyrroi&H, 6 - 16.05 ppm; p-OCH,: 6 - 5.25 ppm] when ac- count is made for the different p-phenyl substituents, is very similar to that obtained by La Mar and Walker for Co(p-CH,)TPPr28]. In the case of (FeTMPP),O rp- OCH,, &‘- 4.15 ppm; m-H and ‘O-H@henyl),--6 - 7.25 pnm: pvrrole-H, 6 - 13.35 nnml, as shown in curve 6,-F& -3, the spectrum exhibit& an unsplit phenyl proton resonance in the expected aromatic region. The same phenomenon was noted earlier by La Mar et aI.[29] in their nmr studies of (FeTPPbO.

1250 D. SCHERSON et 01

-0.24

,0.16

0 Srcm-‘) 450

Fig. 1. Ftir spectrum of (FIzTMPP)~O in a KBr pellet.

t

Absorbance Units 0.6

Fig. 2. Uv-visible spectrum of a 1.5 x lo-’ M (FeTMPP)*O benzene solution.

The mass spectrometric analyses were performed using a Finn&an MAT3 11A double focussing instru- ment with a Finnigan Incas 2400 data system. Field desorption-mass spectra ua_mS) were acquired using a combination electron impact (ei), field desorption ($4 and fast atom bombardment (jiab) ion source. The accelerating voltage was either + 3 kV or + 2.7 kV, and the extraction plate voltage was - 6 kV. Samples were dissolved in dichloromethane for deposition on the field emitter using the conventional dipping tech- nique. Standard high-temperature activated carbon emitters were used. The instrument resolution (M/AM) was - 500, and the ion source temperature was - 80°C. Fd spectra for all three compounds are given Fig. 4; these are spectra summed as the emitter was heated with 0 up to 30 mA heating current. The fd technique produced molecular ions almost exclusively, as is common for this method.

Fast atom bombardment Ua&ms) was performed using the same ion source at a temperature of - 50°C and a resolution (M/AM) of - 1000. Thioglycerol was

used as thefab solvent. Fast xenon atoms (7 keV) were supplied by an Ion Tech FAB-ll-GG gun. The fab spectrum of (FeTMPP),O is given in Fig. 5. The major higher mass ions observed are m/z 789 and 805, which correspond to protonated forms of FeTMPP and FeTMPPO, respectively. The spectrum contains many low mass fragments ions, as is common infib analysis. Some of the low mass ions are due to the thioglycerol solvent (eg m/z 109 and 217). Qualitatively similar fub-ms results have been recently reported by Zhang et aI.[30] for a variety of p-substituted H2 and CuTPP compounds.

The pyrolysis-mass spectrometry was conducted using the same instrument described above with a CDS Model 100 Pyroprobe unit. A schematic diagram of the experimental arrangement is shown in Fig. 6. The sample ( - l-2 mg) is placed in the center of a quartz pyrolysis tube (15 mm long x 1 mm diameter) that is sealed at one end, and the tube then placed in the platinum coil probe for pyrolysis. The pyrolysis is conducted in a flowing helium environment (flow rate

Pyrolysis-mass spectrometry studies 1251

L- - I6

b

Fig. 3. Proton ftnmr spectra of (a) CoTMPP and (b) (FeTMPP),O in CDC13. The features labelled with an

asterisk correspond to impurities present in the solvent.

- 25 cm3min-’ ). The pyroprobe is interfaced to the mass spectrometer via an all-glass transfer line using a jet separator to remove most of the carrier gas (see Fig. 6). The ionization was accomplished by electron impact at energies specified in the figures. The instru- ment resolution (M/AM) was 1000.

The pyrolysis experiments were conducted in two different mode: “slow” pyrolysis mass spectrometry (sp-ms) and “flash” pyrolysis mass spectrometry (fp-ms). In sp-ms, the pyrolysis unit was stepped at 50°C intervals every 2 min from 150°C to 600°C as shown in Fig. 7. The total time for a run was 20 min. An ionizing voltage of 34-38 eV was used. The lowered voltage reduces mass spectral fragmentation so that molecular ions are more prominent; ie the mass spectra are thus less complex than at the more conventional setting of 70 eV. In_&-ms, the sample is heated quickly (in a few ms with the ramp in the ‘off position) to an equilibrium temperature of 550°C. This gives an

overall picture of the pyrolyzates in a short period of time, but it does not reveal the details regarding the temperature profiles at which the various pyrolyz- ates are released. For the pyrolysis-gas chro- matography-mass spectrometry (py-gc-ms) ex- periments, the pyrolyzates were formed by fpms and flowed directly onto a 2 m x 2 mm i.d. Ultrabond column, programmed from 40°C to 240°C at 8°C min-’ with a 2 min hold at 40°C after pyrolysis. The eluted components were flowed through a jet separator interface into the ei ion source.

The differential thermal gravimetry analyses (&a) were performed with a Perkin-Elmer TGS-2 instru- ment in a nitrogen flowing atmosphere (rate 40 cm3 min-I). The heating rate was 20°C min-’ and the sample weights in the range l-l.2 mg.

The dispersion of the macrocycles on the Vulcan XC-72 carbon was accomplished by dissolving the compounds [4.64mg for H,TMPP, 4.98 mg for CoTMPP and 5.01 mg for (FeTMPP),O] in acetone (125 ml) and then adding 100 mg of XC-72 to the solution under ultrasonic agitation. This procedure has been described in more detail in a previous communication[il].

The heat treatment of the macrocycles dispersed on the high area carbon was performed by placing the dry specimens in porcelain boats, which had been first cleaned and pre-heated at 800°C for 2 h in a flowing Ar atmosphere. These were then introduced into a quartz tube (2.5 cm diameter) which fitted into a horizontal furnace (Lindberg, Wisconsin). The temperature in the immediate neighborhood of the sample was automati- cally regulated by means of a control unit (Omega Engng Inc.) coupled to a chromel-alumel thermo- couple junction. The pyrolysis was conducted under a flowing Ar atmosphere (100 cm3 min-I) at 800°C for 2 h. The Ar (Matheson, 99.995 %) was further purified by interposing a multistage train between the cylinder and the quartz tube inlet, consisting of silica gel to remove water, Alfa De-Ox catalyst (Alfa Ventron Corp.) to eliminate O2 and molecular sieves 4.& and 8A to remove water and organic contaminants, re- spectively. A water trap was installed at the outlet of the tube to prevent the back diffusion of gases from the atmosphere. O-ring joints and copper tubing were used to assemble the manifold. Prior to the heat treatment, the samples were flushed with room tem-

R.?lOtiW Intensity

H’TMPP CoTMPP IFeTMWk 0

Fig. 4. Field desorption mass spectra of H2-, Co- and Fe-/r-ox0 tetramethoxyphenylporphyrins in crystal form (isotope peaks are not included).

1252 D. SCHERSON er al.

RdOtiW Intensity

-50 789

60:

1 ..a I. .A

1 I

I50 200 250 750 ml2 800

Fig. 5. Fast atom bombardment-mass spectrum of (FeTMPP), in crystal form. The peaks at m/z 109 and 217 are due to the thioglycerol solvent (see Experimental section).

Ion * Sowce

Jet Separator

Fig. 6. Schematic diagram of the pyroprobe and jet separator used in the pyrolys&mass spectrometry experiments

I I I I I 6 I 0 2 4 I I 10 I I 14 16 18 20 I

t lmin)

Fig. 7. Temperature program employed in the slow pyrolysis-mass spectrometry experiments

perature AT for - 10 min inside the quartz tube. After RESULTS AND DISCUSSION the pyrolysis was completed, the heating was inter- rupted and the specimens allowed to cool down to room temperature without discontinuing the flow of

The slow pyrolysis-mass spectrometry (sp-ms)

gas, before they were removed from the quartz tube. curves for the three porphyrins both in the form of bulk crystals and dispersed in Vulcan XC-72 carbon

Pyrolysis-mass spectrometry studies 1253

are shown in Figs 8 and 9. These relative ion current us temperature diagrams include only those species found to be present in the highest concentrations. Water, benzene, chloroform, methylene chloride and other solvent related impurities were detected at T 4 400°C. In the case of (FeTMPP),O, diethylphthalate and other minor contaminants were also observed. All these compounds are removed at temperatures much higher than those employed during the purification procedure and may be responsible for the slight discrepancies between the theoretical and actual ele- mental composition analyses. The fact that the tem- peratures required to eliminate such impurities are much lower than those at which the decomposition was found to mur provides strong evidence that their presence in the original materials may not be. expected to interfere with the overall nature of the thermal fragmentation patterns, nor with that of the pyrolysis products.

Based upon the results obtained from sp-ms and the differential thermogravimetry (dtg) experiments shown in Fig. 10, all specimens investigated exhibited fairly well-defined onsets for thermal decomposition (T,,) in the range between 400°C and 450°C. In particular, the value of T,, for CoTMPP (bulk) was

b

150,200,250dCO,350 J 400,450 ~500~55Q_~6~ V-C>

I I I I I 0 5 lo IS 20 trmin)

Fig. 8. Slow pyrolysis-mass spectra of (a) H*TMPP, (b) CoTMPP and (c) (FeTMPP)IO in crystal form. The tem- perature program is shown in Fig. 7. The zero for the time axis has been set arbitrarily at a temperature of 150°C. The label in each of the curves corresponds to the molecular weight of the species being monitored. The structure of these. compounds is

given in Table 1.

Fig. 9. Slow pyrolysis-mass spectra of (a) HITMPP (4.4 % w/w), (b) CoTMPP (4.7 % w/w) and (c) (FeTMPP)*O (4.8 % w/w) dispersed on high area Vulcan XC-72 carbon. (See

caption Fig. 8.)

slightly higher than that of the (FeTMPP),O (bulk) in agreement with the observations made by van Veen et aI.[3]. Unfortunately, the sensitivity of the dtg instru- ment employed in this study was found to be not high enough to yield reliable data in the case of the supported macrocycles.

The integrated pyrolysis-mass spectra for the ther- mal decomposition products for all the samples ex- amined are shown in Figs 11 and 12. Phenol (94), methoxybenzene and/or methylphenol (108) and et- hylphenol and/or methyl-methoxybenzene (122) (orig- inating from the meso-substitutents of the macro- cycle), were detected for all specimens in varying amounts (see Table 1). It should be stressed that the magnitude of the peak associated with the methoxy group (m/z 31) was negligible as compared to that of benzene or its derivatives. Hence, the major thermal decomposition pathway involves the cleavage of the complete meso-substituent and not just the outer fringe of the chelate as reported by van Veen et af[3]. Furthermore, in contrast to the claims made by Brodskii and Radyushkina et aL[lO, 111 referred to in the Background section, the value of T,, associated with the release of these spec& gases was slightly higher

1254 D. SCHERSON et al.

Fig. 10. Thermogravimetry and differential thermogravi- metry analysis curves for (a) HZTMPP, (b) CoTMPP and

(c) (FeTMPP)ZO in crystal form.

for the Fe-p-oxo than for the metal-free and co- balt porphyrins. The pyrolysis fragments with masses between - 130and - 200 (with the exception of 134 and 136 which correspond to isomeric mixtures of Me- &COMe and C-3 phenols) consist mostly of a pyrrole group linked by an aliphatic chain to a substituted benzene moiety (see Table 1). These relatively high

molecular weight species are present in much higher proportion in the case of H,TMPP than (FeTMPP),O and could not be detected for CoTMPP. These results are at variance with those obtained by the Soviet workers[ 10, 111. In particular, none of the polycyclic aromatic compounds reported by these authors were found in this study. This may be due, however, to the high vacuum conditions under which the measurements reported by these authors were performed.

The most striking difference between the thermal fragmentation patterns of the materials examined is the much larger relative fraction of nitrogen contain- ing species released by the metal-free porphyrin sam- ples than by the metallated counterparts, an effect that is especially pronounced for the dispersed macrocycles. In addition, the ratio of nitrogen to non-nitrogen containing fragments appears to be much higher for the bulk than for the supported catalysts. The same trends were observed in the flash pyrolysis-mass spectrometry &ms) experiments at 550°C as shown in Figs 13 and 14, although the relative amounts of each of the fragments were slightly different than those obtained in the sp-ms measurements. It should be noted that a number of organic impurities were detected in the fp (55O”C~ms spectra for the Vulcan XC-72 carbon (see Fig. 15). The fact that the most prominent peak (92) could not be seen in most of the macrocycle samples provides a strong indication that the contribution of the carbon contaminants to the results obtained with the supported macrocycles is negligible. It is interesting to note that the microanal- yses of the same macrocycle/XC-72 specimens, shown in Table 2, indicated that after heat treatment at 800°C the amount of nitrogen retained by the metallated porphyrins dispersed on carbon was much higher than in the case of the metal-free counterpart.

The pyrolysis-gas chromatography-mass spectrom- etry @y-gc-ms) results not only confirmed the findings referred to above but also revealed the presence of several isomeric forms of the components with masses 108, 122, 136, 187 and 201. As a means of illustration, the py-gc-ms diagrams obtained for H,TMPP and CoTMPP both unsupported and dispersed on XC-72

50 I00 IS0 ml2 200 20 60 IGG ml2 140 SO W 150 m/z 200

Fig. 11. Integrated pyrolysis-mass spectra (sp-ms) for (a) H,TMPP, (b) CoTMPP and (c) (FeTMPP),O in crystal form. Electron impact ionization energy, ie = 34-38 eV. The label of each of the peaks corresponds to

the molecular weight of the species detected. The structure of these species is given in Table 1.

Pyrolysis-mass spectrometry studies 1255

(Arbitrary Units ) b

‘I

10

e

Fig. 12. Integrated pyrolysis-mass spectra for (a) H,TMP (4.4% w/w), (b) CoTMPP (4.7 % w/w) and (c) (FeTMPP),O (4.8 % w/w) dispersed on high area Vulcan XC-72 carbon. Ie = 34-38 eV. (See caption Fig. 11.)

Table 1. Molecular weights and most probable structures of species found in the mass spectrometry experiments involving

Hz-, Co- and Fe-p-oxo tetramethoxyphenylporphyrins

Molecular weight Compound

58

67

78

81

92

94

108

118

I20

122

134

136

& H3C-C-CH3

H3C -0 0 OCH3

H2 ‘>aOH “‘,‘>CaOCH3

H3C

HO

Table 1. (Con@

Molecular weight Compound

I57

171

I87

201

&?2a

&a kH3

using an Ultrabond column are shown in Figs 16 and 17, respectively, in which the peaks associated with column bleed have been omitted for clarity.

It is interesting to note that preliminary slow pyrolysis-mass spectrometry experiments involving iron phthalocyanine, FePc, dispersed on Vulcan XC- 72 using the same instrument described in the Experimental section, failed to detect any significant gas-phase decomposition products at temperatures below 550°C. At and above this temperature, ben- zonitrile and dicyanobenzene were found to be present

12.56 D. SCHERSON et al.

tntensity I Arbitrary Unitss)

20 60 I

b

I.0

c

108

i 7, 148 187

1361 157 , a, I ’ I

/SO ml2 200

Fig. 13. Flash pyrolysis (550”Ckmass spectra (fp-ms) for (a) HZTMPP, (b) CoTMPP and (c) (FeTMPP),O in crystal form. le = 34-38 eV. (See caption Fig. 11.)

Intensity 0

(Arbitrary Units I

60 K

b E

-1.0 94

Fig. 14. Flash pyrolysis (550”Ckmass spectra (jktns) for (a) H,TMPP (4.4 % w/w), (b) CoTMPP (4.7 % w/w) and (c)(FeTMPP),O (4.8 % w/w) dispersed on high area Vulcan XC-72 carbon. Ie = 34-35 eV. (See

caption Fig. 11.)

Table 2. Relative nitrogen content of Co-, Fe-p-oxo- and H,TMPP dispersed on Vulcan XC-72carbon before and after heat treatment at

800°C obtained by microanalysis

Found (%) Heat treated

Specimen Theoretical (%) Untreated (SCWC)

4.4’;/, H,TMPP 0.34 0.32 0.20 4.7 % CoTMPP 0.34 0.38 0.3 1 4.8 % (FeTMPP),O 0.34 0.34 0.28 5 % CoTMPP* 0.37 0.36 5 % (FeTMPP),O* 0.37 0.31

*These results were obtained independently by .I. A. S. Bett, H. R. Kunz and S. W. Smith, for specimens heat treated at S50°C[32].

in much higher amounts than hydrogen cyanide. materials before and after heat treatment, and will be Actually, no HCN was observed in the Jpms runs at presented in a forthcoming communication. These 550°C for the same specimen. additional measurements indicate that a significant

Further information has been obtained from ex fraction of the iron and cobalt centers in CoTMPP and situ[7] and in situ MBssbauer spectroscopy[31] and (FeTMPP),O dispersed in Vulcan XC-72 does not cyclic voltammetry experiments involving the same seem to remain coordinated in the porphyrin complex.

Pyrolysis-mass spectrometry studies 1257

Fig. 15. Flash pyrolysis (55O”C)-mass spectra (&-ms) for high area Vulcan XC-72 carbon. le = 3438eV. (See caption Fig. 11.)

b

Fig. 16. Pyrolysis-gas chromatography-mass spectrometry trace of H,TMPP (a) in crystal form and (b) dispersed on high area Vulcan XC-72 carbon (4.4% w/w). Molecular weights are given for each peak. (See Table 1 for the most probable

structures of these compounds.)

The difference in the pyrolysis of the metal-free and metal-containing porphyrins can be explained on the basis that the metal stabilizes the macrocyclic ring, or that the metal catalyzes pyrolytic processes which

result in the binding of the nitrogen to the substrate. Upon contact with an electrolyte solution the metal species may undergo partial or total dissolution and subsequently adsorb or coordinate to thermally

b

(22 I

108

g4jy I I I

10 6 tfmim 20

Fig. 17. Pyrolysis-gas chromatography-mass spectrometry trace of CoTMPP (a)in crystal form and (b) dispersed on high area Vulcan XC-72 carbon (4.7 o/O w/w). Ze = 3&38 eV. (See

caption Fig. 16.)

1258 D. SCHERSON er al.

formed sites on the carbon surface, most likely involv- ing nitrogen. Preliminary evidence in support of this view has been provided by the substantial increase in the electrocatalytic activity for 0, reduction in alkaline media of H,TMPP dispersed and heat treated at 450-500°C after the addition ofcobalt by precipitation of the corresponding hydroxide in the carbon matrix, or adsorption of the transition metal species from solution on the substrate. Recent experiments con- ducted in this laboratory have indicated that in the case of CoTMPP dispersed on XC-72 carbon and heat treated at 8OO”C, only about one half of the total amount of metal could be extracted with glacial acetic acid using a Soxhlet apparatus even after several hours. The activity of such specimens for 0, reduction in alkaline solutions is reduced somewhat but still very much higher than either the corresponding pyrolyzed metal-free porphyrin or the carbon support itself. The results of these experiments will be reported in due course.

SUMMARY

The slow and flash pyrolysis-mass spectrometry and pyrolysis-gas chromatography-mass spectrometry ex- periments presented in this work have indicated that:

(i) the fraction of volatile nitrogen to non-nitrogen containing species generated during thermal treatment is much higher for the metal-free than for the cobalt and iron-p-oxo tetramethoxyphenylporphyrin. This effect is especially pronounced when the macrocycles are supported on Vulcan XC-72 high area carbon. These results are consistent with the microgravimetry analyses of the same supported macrocycles before and after heat treatment at 800°C for which the amount of nitrogen was higher for the metallated compounds than for the metal-free analog, and

(ii) high molecular weight volatile fragments can be detected only in the case of H,TMPP and to a much less extent for (FeTMPP),O. These consist of a pyrrole attached to a substituted benzene group and thus do not correspond to the compounds reported earlier by other workers.

Acknowledaements-Financial sumxxt for this work was provided b; the U.S. Departmeni-of Energy through sub- contracts with Lawrence Berkeley Laboratory and Eltech Systems. The authors would like to expresss their appreci- ation to IBM for a Faculty Development Award (D.S.), to CNPq-Brasil (A.A.T.) and to the Heinrich-Hertz-Foundation for a fellowship to one of the authors (R.H.).

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