4510
J. Phys. Chem. 1987, 91, 4510-4514
3p3P2transitions. These slight enhancements of J" = 2 relative to 0 and 1 may be a reflection of the population distribution within the 2p3P,, states.
Conclusions The multiphoton dissociation of CS2 in the wavelength range from 330 to 280 nm is shown to produce carbon and sulfur atoms in their lowest three electronic states and C S in its lowest triplet state. The electronic excitation of the products amounts to as much as 2.7 eV for the atoms and over 3 eV for the CS fragment. The atomic lines are all quite sensitive to power broadening and, possibly, the ac Stark effect, making accurate measurement of the line widths impossible in the present experiment. Upper limits for some Doppler widths have been measured, however, and these are only slightly greater than the room-temperature Doppler width. This leads us to conclude that little of the excess energy of photolysis is deposited as translational energy, but rather that most is taken up by internal degrees of freedom of the fragments.
Finally, we note that the resonance effects substantially change the distribution of product ions over the relatively narrow wavelength range we have investigated, and there is every reason to believe that the distribution would change even more drastically as other electronic states of CS2 are probed. In particular, the production of S+ is greatly enhanced near the 308-nm XeCl 4p 3PJ,tranexcimer band due to the two-photon 3p4 3PJt, sition in atomic sulfur, which is accidentally in resonance with the laser wavelength. We expect this resonance to play an important part in the multiphoton photolysis and ionization of many sulfur-containing compounds, and so would urge great care in the interpretation of such experiments.
--
Acknowledgment. This work was supported by the United States Army Research Office under Contract No. DAAG 2984-K-0205. Registry No. CS,, 75-15-0; CS, 2944-05-0; C, 7440-44-0; S, 770434-9.
LiganbAssisted Photochemical Reaction of Metatloporphyrins. Photodissociation of Carbon-Indium Bond in EthyJfndium(III)Tetraphenylporphyrin with the Aid of Axial Pyridine Mikio Hoshino* and Takehiro Hirai Solar Energy Research Group, The Institute of Physical and Chemical Research, Wako, Saitama 351 -01, Japan (Received: February 20, 1987)
Ethylindium(II1) tetraphenylporphyrin, C2HSIn"'TPP, undergoes photodissociation of the carbon-indium bond in the excited triplet state originating from the porphyrin ligand. The quantum yield, 0, for the photodecomposition of C2HSIn1"TPPin benzene increases with an increase in pyridine concentration: 0 = 0.046 in the absence of pyridine and 0 = 0.3 at an infinite concentration of pyridine. From the quantum yield measurements and laser photolysis studies, it is concluded that the excited triplet states, 3C2HsIn11'TPP*and 3C2HSIn11'TPP(Py)*,are in equilibrium: 3C2HsIn111TPP* + Py * 3C2HSIn11'TPP(Py)*, where Py is a pyridine molecule. The equilibrium constant is determined as 6.45 X lo2 M-' at 20 OC. The increase in the quantum yield for the photodecomposition of C2HSIn"'TPP in the presence of pyridine is interpreted in terms of the facile dissociation of the carbon-indium bond in 3C2HsIn111TPP(Py)*assisted by the axial pyridine.
Introduction The nature of metalloporphyrins in an excited state is markedly affected by the axial ligands. In previous papers,'%2we have demonstrated that the axial ligand assists the photoinduced electron-transfer reaction, resulting in an increase in the yield for the formation of ion radicals. These results suggest that the effects of axial ligands frequently play an important role in the photochemical reactions of metalloporphyrins. Recently, the photochemistry of alkylmetalloporphyrins has received increasing attention3-' These porphyrins having C O , ~ Al, In, and Rh as central metals undergo photoinduced dissociation of the carbon-metal bond upon irradiation: R-M'I'P % R'
+
(1) Hoshino, M.; Seki, H.; Shizuka, H. J . Phys. Chem. 1985,89, 470-474. (2) Hoshino, M.: Seki, H.; Yasufuku, K.; Shizuka, H. J . Phys. Chem. 1986. 90.5149-5153. (3) Hoshino, M.: Ida, H.; Yasufuku, K.; Tanaka, K. J . Phys. Chem. 1986, 90, 3984-3987. (4) Maruyama, S.; Inoue, S.; Ohkatsu, Y . Chem. Lett. 1983, 381-384. (5) Cocolios, P.; Guilard, R.; Bayed, D.; Lecomte, C. Inorg. Chem. 1985, 24, 2058-2062. (6) Tero-Kubota, S.; Hoshino, N.; Kato, M.; Goedken, V. L.; Ito, T. J . Chem. SOC.,Chem. Commun. 1985, 959-961. (7) Hoshino, M.; Yasufuku, K.; Seki, H.; Yamazaki, H. J . Phys. Chem. 1985, 89, 3080-3085. (8) In a preliminary study, we found that methylcobalt(II1) octaethylporphyrin in benzene undergoes facile photodissociation of the methyl-cobalt bond to give cobalt(I1) octaethylporphyrin.
M"P, where R-M"'P, R', and M"P represent the alkylmetalloporphyrin, alkyl radical, and the metalloporphyrin having a divalent central metal, respectively. For ethylindium"' tetraphenylporphyrin, C2HSId1'TPP,it has been revealed that the photodissociation of the carbon-indium bond occurs from the triplet state originating from the porphyrin ligand.3 The yield for the photodissociation in benzene is as low as 0.046. By addition of 2,4,7-trinitrofluorenone,TNF, the yield is increased from 0.046 to 0.27. The formation of a triplet exciplex between the triplet C2H51n1'1TPPand T N F is considered to be responsible for the increase in the quantum yield. Presumably, the TPP ligand becomes electron deficient by forming the triplet exciplex with TNF, a strong electron acceptor, leading to the facile dissociation of the carbon-indium bond. In the present work, we have found that the carbon-indium bond in C2HSIn1"TPPis readily photodissociated with the aid of a pyridine molecule as an axial ligand in the excited triplet state. The reaction mechanisms for the photodissociation are discussed on the basis of steady light photolysis, laser flash photolysis, and ESR studies.
Experiment81 Section Ethylindium(II1) tetraphenylporphyrin, C2HSIn1"TPP, was synthesized and purified according to the method reported prev i ~ u s l y .Reagent ~ grade benzene and pyridine supplied from Wako Pure Chem. Ind. Co. Ltd. were used without further purification.
0022-3654/87/2091-4510$01.50/00 1987 American Chemical Societv
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4511
Photochemical Reaction of Metalloporphyrins I,
1
I
10,
___ O'
500
600
,J
.__..."
700
'
.._.._... .
800
,
900
WAVELENGTH( nm) WAVELENGTH( n m )
Figure 1. Absorption spectral changes upon irradiation at 580 nm for a benzene solution of C,HSIn"'TPP in the presence of 4.95 X M pyridine: (1) before irradiation; (2)after 5.0 min irradiation; (3) after 10 min irradiation; (4)after 15 min irradiation; (5) after 20 min irradiation; ( 6 ) after 25 min irradiation; (7) after 60 min irradiation.
Optical absorption and ESR spectra were recorded on a Hitachi 330 spectrophotometer and a Jeol JES-FE-3AX X-band spectrometer, respectively. Laser photolysis studies were carried out by using the second harmonic (532 nm) of a Nd:YAG laser (Model H Y 500 from JK Lasers Ltd.); the duration and the energy of a laser pulse were ca. 20 ns and 100 mJ/pulse, respectively. The detection system for the transient absorption spectra was described e l ~ e w h e r e . ~ Steady light photolysis was performed with a 250-W USH250D high-pressure mercury lamp. The quantum yields were determined with the use of monochromatic light (580 nm) from a xenon lamp incorporated in a Hitachi M P F 4 spectrofluorimeter.I0 The photon flux at 580 nm was determined from (1) light intensity distribution of the xenon lamp a t various wavelengths obtained by the measurement of the excitation spectrum of rhodamine B in ethylene glycol (8 g/L) and (2) the photon flux at 355 nm obtained with the use of an aerated methylcyclohexane solution of N-methyldiphenylamine as an actinometer; the yield for the photochemical formation of N-methylcarbazole from N-methyldiphenylamine in aerated methylcyclohexane has been established as 0.42." The concentration of C2H51nr1'TPPused in the present study was ca. 4 X lU5M. Sample solutions were degassed on a vacuum line by repeated freeze-pump-thaw cycles. Changes in the concentration of C2H51n"'TPP upon irradiation were monitored at 580 nm.
Results The absorption spectra of C2H51n"'TPP in benzene were found to be identical irrespective of the absence or the presence of 10-5-6.2 M pyridine. It, therefore, seems unlikely that C2H51n"'TPP and pyridine form a molecular complex or six-coordinated species .5 Figure 1 shows the absorption spectral changes upon irradiation observed for a benzene solution of C2H51n"'TPP in the presence M pyridine. Before irradiation, the spectrum of 4.95 X exhibits major absorption peaks at 582 and 625 nm in the Q-band region. The intensities of these peaks decrease and new peaks appear at 558, 597, and 627 nm with a small and broad peak around 810 nm. When the concentration of pyridine is lower than M, the profile of the spectral changes upon irradiation 2.55 X is very similar to that observed for the benzene solution without ~yridine.~ Figure 2 shows the absorption spectra of C2H51n1'ITPP in benzene containing 5.0 M pyridine, observed before and after 5.0-s irradiation with the mercury lamp. The spectrum after photolysis shows distinct absorption peaks a t 755 and 873 nm in the nearinfrared region. These peaks of the photoproduct become obM. servable when the pyridine concentration exceeds 5 X (9)Hoshino, M.;Imamura, M.; Wabatane, S.;Hama, Y. J . Phys. Chem. 1984,88, 45-49.
(10) Yamamoto, S.;Hoshino, M.; Yasufuku, K.; Imamura, M. Inorg. Chem. 1984, 23, 195-198. (11) Forster, E. W.; Grellman, K. H.; Linschitz, H. J . A m . Chem. SOC. 1973, 95, 3108-31 15.
Figure 2. Absorption spectra observed for a benzene solution of C2HS1n"'TPP in the presence of 5 M pyridine (A) before and (B) after irradiation for 5.0 s with the mercury lamp.
\\II 3380 FIELDIG)
Figure 3. ESR spectrum observed for a benzene solution of C2H51n"'TPP in the presence of 5.0 M pyridine after irradiation for 5.0 s with the
mercury lamp. The 755- and 873-nm bands gradually decrease in intensity with irradiation time. The gradual decrease in these band intensities is also observed when the irradiated solution is kept in the dark. These results indicate that the product undergoes further photochemical and thermal reactions. As mentioned above, the absorption spectrum of the photoproducts differs markedly, particularly in the near-infrared region, depending on the pyridine concentration. The photoproducts from C2H51n1"TPPare investigated by ESR. At low concentration of pyridine, the irradiated solution exhibiting a broad band around 810 nm gives only a very weak ESR signal of unidentified species with a peak to peak width of 7.5 G at g = However, the irradiated solution in the presence of high concentration of pyridine gives a strong ESR signal. Figure 3 shows the ESR signal observed for the irradiated benzene solution of C2H5In"'TPP containing 5.0 M pyridine. The signal exhibits 10 hyperfine lines due to the indium atom (Z'" = 9/2). The ESR parameters are obtained as g = 2.009 and Ais' = 9.4 G. When the irradiated solution is exposed to air, the ESR signal as well as the absorption bands at 755 and 873 nm disappear. From this result, we consider that the species giving the ESR signal is identical with the photoproduct which has absorption peaks at 755 and 873 nm. The quantum yields for the photodecomposition of C2H5In'I'TPP were measured according to the analogous method described previously.' When the photoreaction is expressed by hu
A-B the rate for the disappearance of molecule A, -dCA/dt, is formulated as -dCA/dt = @Zo(l - 10-D)DA/(DA+ D B ) (1) where @ is the quantum yield for the photodecomposition of molecule A and Z, the incident light intensity; D = D, DB;DA and DB are, respectively, the optical densities of molecules A and B measured at an excitation wavelength. With the use of an initial and an absorption coefficient, €A, of molecule concentration, A, eq 1 is transformed to
+
e,,
-In ( C A / ~ =)
$'zo€AJ(l
- 1 0 - D ) / Ddt = @Io€AY(t)
(2)
where Y(t) is defined as Y(t) = J ( 1
- 1 0 - D ) / Ddt
The concentration, CA,is expressed as CA = ( D - D..)/(€A- tg)
(3)
4512
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
Hoshino and Hirai
3 .II
2l 1 1
O'
50
15
10
20
25
'
50
[py] X103, M
Figure 4. Quantum yields for the photodecomposition of C2H51n"'TPP in benzene solutions represented as a function of pyrdine concentration. The solid curve is the quantum yields calculated according to the reaction mechanism proposed for the photodecompositionassisted by pyridine (see text).
0'
. 500
-
,
600 700 WAVELENGTH j n m )
800
Figure 5. Transient absorption spectrum observed for a benzene solution M pyridine at 50 ns after of C2H,In"'TPP in the presence of 2.5 X laser pulsing.
where D, and q, are, respectively, the optical density measured after the completion of photolysis and a molar absorption coefficient of molecule B. From eq 2 and 3, we obtain -In (D- Dm)= cPl,tAY(t) const (4)
+
The plots of -In (D- D,) vs. Y(t)gives a straight line. From the slope of the line, light intensity Z,, and molar absorption coefficient tA, the quantum yield % is determined. Figure 4 shows the quantum yields represented as a function of the pyridine concentration. The yield increases with an increase in the pyridine concentration and exhibits a leveling off at a M. Evidently, the addition concentration higher than 2.0 X of pyridine accelerates the photodecomposition of C2H,In1*ITPP in benzene. The solid curve in Figure 4 represents the quantum yield, cPobsd, calculated according to the reaction mechanism for this system as mentioned later. The laser photolysis studies were carried out in order to elucidate the reaction mechanisms for the photodecomposition of C2H51n"'TPP assisted by pyridine. Figure 5 shows the transient absorption spectrum of C2HSIn"'TPP in benzene containing 2.5 X M pyridine, observed at 50 ns after laser pulsing. The spectrum has a strong absorption band around 470 nm. Except for rather smooth absorption bands at wavelengths longer than 650 nm, the spectral profile is very similar to the triplet-triplet absorption spectrum of C2H51n"'TPP in benzene without ~ y r i d i n e . ~ It, therefore, is concluded that the transient is ascribed to the triplet state of C2HSIn"'TPP, 3C2H51n111TPP*.The addition of pyridine is considered to result in broadening of the transient absorption spectrum in the wavelength region X > 650 nm. Further examination was made for the transient absorption bands in the long wavelength region observed for the benzene solutions containing pyridine. At 45 ws after laser pulsing, the triplet state completely decays, leaving residual absorption. The residual spectrum was found to be in good agreement with the spectrum of the photoproduct measured by steady light photolysis of the solution. The decay of 3C2H51n111TPP* follows first-order kinetics irrespective of the presence or absence of pyridine. The decay rate
10
15
20
25
'
[ p v ~x lo3 M Figure 6. Rate constants for the decay of the triplet C2H,In"'TPP represented as a function of pyridine Concentration. The solid curve is the rate constants calculated according to the reaction mechanism (see text). constant, for example, was determined as 7.5 X lo4 s-l for a benzene solution of C2H5In"lTPP in the presence of 2.5 X M pyridine. Figure 6 shows the decay rate constants of 3C2H51n111TPP* represented as a function of the pyridine concentration. The rate constant increases with an increase in the pyridine concentration. However, there is a leveling off at higher concentration of pyridine. This fact implies that 3C2H51n111TPP* interacts with pyridine to form an excited molecular complex which has its own characteristic lifetime. Since pyridine is a highly eoordinating molecule, it is likely that the excited molecular complex has a structure 3C,H51n11'TPP(Py)*,in which a pyridine molecule, Py, is located in the axial position of the central indium atom. It has already been reported that a pyridine molecule attaches to an axial position of monopyridinate of nickel(I1) tetraphenylporphyrin upon irradiation.I2-l4 The leveling off of the decay rate constant at higher concentrations of pyridine indicates that there is an equilibrium between the triplet states, 3C2H51n111TPP and 3C2H,In11'TPP(Py)*: 3C2HsIn11'TPP*+ Py %z
3C2H51n11'TPP(Py)*
Since the absorbance of the transient observed at 470 nm immediately after laser pulsing is not altered by addition of pyridine, it is assumed that (1) the triplet yield is independent of the pyridine concentration and (2) the molar absorption coefficient of 3C2H51n111TPP*at 470 nm is almost identical with that of 3C2HSIn111TPP(Py)*.From (1) the assumptions described above and (2) the decay rate constants, k l and k2, for 3C2HSIn'*1TPP and 3C2H51n11'TPP(Py)*,and the equilibrium constant, K*, the triplet decay rate constant (kobsd)observed by laser photolysis is formulated as* kobsd
= (kl
+ K*k2[Pyl)/(1
-k K*[Pyl)
(5)
Here it is postulated that 3C2H51n111TPP* is not quenched by pyridine without forming 3C2HsIn1T1TPP(Py)*.Equation 5 is transformed to
= (kobsd
- k,)/[Pyl
= -kobsdK* + k2K*
(6)
The value of k l has already been reported as 1.43 X lo4 s-'. The plot of I? vs. kobsd gives a straight line. From the slope and the intercept of the line, K* and k2 are obtained as 6.45 X lo2 M-' and 7.5 X lo4 s-I, respectively. The solid curve shown in Figure 6 represents k&d calculated with the use of eq 5 and the values of K* and k2 obtained above.
Discussion The carbon-indium bond in C2H51n"'TPP is considered to be composed of a 5s orbital of an indium atom and an sp3 orbital of a carbon atom in C2H5.I5 In benzene, C2H51n"*TPPundergoes (12) Kim, D.; Holten, D. Chem. Phys. Lett. 1983, 98, 584-589 (13) Kim, D.; Kirmaier, C . ;Holten, D. Chem. Phys 1983, 75, 305-322 (14) Hoshino, M. Inorg. Chem. 1986, 25, 2476-2478 (15) Hoshino, M.; Yamaji, M.; Hama, Y . Chem Phys. Lett 1986, 125, 369-372
The Journal ofphysical Chemistry, Vol. 91, No. 17, 1987 4513
Photochemical Reaction of Metalloporphyrins
c
photoinduced homolytic cleavage:'
-
7
hu
C2H51n"*TPP
F-.
C2H5.
+ [InIITPP]
LuMO-..---';--.----.-.,.,.'..-T.
The unpaired electron in [Inl*TPP]is expected to be located in the 5s orbital of the indium atom. However, [In'ITPP] is highly unstable in solutions. W e could not detect [In"TPP] by optical and ESR spectroscopy. Since the irradiated solution gives no ESR signal ascribed to [In"TPP], it is likely that [InIITPP] is readily transformed to a diamagnetic species. Photolysis of C2HSIn111TPPin 2-methyltetrahydrofuran, MTHF, also results in the homolysis of the carbon-indium bond.I5 The photoreaction is expressed as C2H51n"'TPP
hu
C2H5'
+ (In"')+TPP*-
=
(*O
+ *-K*[Pyl)/(l
+ K*[Pyl)
(7)
where a. and aobsd are the quantum yields for the photodecomposition of C2H51n111TPP in the absence of pyridine and at a given pyridine concentration, [Py]. As shown in Figure 4, the quantum yield increases from 0.046 toward a maximum value, am,with an increase in the pyridine concentration. The solid curve in Figure 4 is the quantum yield, aobsd, calculated with the use of eq 7, = 0.3, and K* = 6.45 X lo2 M-' obtained from laser photolysis studies. The quantum yields calculated are in good agreement with those obtained experimentally, indicating that (1) 3C,H,In111TPP(Py)* undergoes facile dissociation of the carbon-indium bond 'C2HsIn"'TPP(Py)*
-
C2H5'
IITPP
$H51/TPP
Figure 7. Simple correlation diagram for the molecular orbitals of C2H,', C2H5In1"TPP,In"TPP, and (In'T')+TPP'-.
of pyridine. This result suggests that (In"')+TPP'-(Py) equilibrium with [In"TPP]
where (In1")+TPP*- is a zwitterionic indium porphyrin in which an unpaired electron is located in the porphyrin ligand. The zwitterionic porphyrin, In(III)+TPP'-, is detected as a stable product by optical and ESR measurements. The present study has shown that the photoproduct of C2H5InIf1TPPin a benzene solution containing 5 M pyridine exhibits (1) the absorption spectrum having peak maxima at 755 and 873 nm in the near-infrared region and (2) the ESR spectrum with hyperfine structures due to an indium atom. Both the absorption and ESR spectra are similar to those observed for the zwitterionic porphyrin in an M T H F solution of C2H51n"'TPP produced by irradiation. Based on these results, we consider that the photoproduct is (In"')+TPP*-(Py) in which a pyridine molecule, Py, is located in the axial position. The absorption and ESR spectra of (In"')+TPP'-(Py) are slightly different from those of (Inlll)+TPP'-: (1) the absorption peaks of (In"')+TPP'-(Py) in the near-infrared region are redshifted by ca. 3-7 nm in comparison with those of (In"')+TPP'in M T H F and (2) the hyperfine coupling constant, A!:o, of (In"')+TPP'-(Py) is slightly smaller than that (10.5 G) of (In"')+TPP'- in MTHF.15 These differences are probably ascribed to the effects of the axial pyridine in (Inlll)+TPP'-(Py). It has been demonstrated that the photocleavage of the carbon-indium bond in C , H J ~ I ~ ~ ~ Toccurs P P from the excited triplet state originating from the porphyrin ligand. Addition of pyridine in the benzene solution of C2H51n"'TPP was found to give marked effects on the decay rate constant of 3C2H51n111TPP*as well as the quantum yield for the photodecomposition of C,H5In"'TPP: both the rate constant and the quantum yield increase with an increase in the pyridine concentration. The laser photolysis studies revealed that an excited-state equilibrium between 3C2H51n111TPP* and 3C2H51n111TPP(Py)* is responsible for the changes in the triplet decay rate constant by addition of pyridine. The assumption that C2H51n111TPP decomposes with a quantum yield, am,at an infinite concentration of pyridine leads to aobsd
5+p"
+ (Inlll)+TPP'-(Py)
and (2) 3C,H51n111TPP*is not quenched by pyridine without forming 3C2H51n111TPP(Py)*as postulated previously. Steady light photolysis of C2HSIn1"TPPin benzene has revealed that the photoproduct changes from a diamagnetic species having a characteristic absorption band around 8 10 nm to a paramagnetic species, (Inlll)+TPP'-(Py), on going from low to high concentration
(In"*)+TPP'-(Py) followed by [InIITPP]
-
F?
[In"TPP]
is in
+ Py
diamagnetic species
At higher concentration of pyridine, the equilibrium is shifted principally toward left. We, therefore, can detect (In"')+TPP'-(Py) as the product by optical absorption and ESR techniques. The quantum yield, a0,for the photodecomposition of C2H5InlITTPPin benzene is expressed by a0
=
@STkr/kl
(8)
Here aST, k,, and k , are the yield of 3C2H51n111TPP*,the rate constant for the dissociation of the carbon-indium bond in 'C2H51n"'TPP*, and the decay rate constant of 3C2H51n111TPP*, respectively. Analogously, the quantum yield, a,, for the photodecomposition of C2H51n"'TPP at an infinite concentration of pyridine is formulated with the use of (1) the yield, aST(Py),of 'C2H51n"'(Py)*, (2) the rate constant, k,(Py), for the dissociation of the carbon-indium bond in 'C2H5In1''TPP(Py)*, and (3) the decay rate constant, k,, of 3C2HSIn111TPP(Py)*:
a- = aST(pY)kr(pY) / k2
(9)
As mentioned previously, the addition of pyridine into the solution does not affect the triplet yield. This fact leads to @ST
= aST(pY)
(10)
From eq 8-10, we obtain @ O / k
= krk,/kr(PY)kl
(1 1)
The quantum yield measurements and laser photolysis studies have confirmed a0= 0.046, am= 0.30, k , = 1.43 X lo4 SKI,and k , = 7.5 X lo4 s-l. By substitution of these values into eq 11, k,(Py)/k, is determined as 34.2. This result implies that the rate for the dissociation of the carbon-indium bond in 'C2H51n"'TPP(Py)* is ca. 34 times faster than that in 'C2H51n"'TPP*: the axial pyridine in 'C2H5In'''TPP(Py)* accelerates the photodissociation of the carbon-indium bond. The dissociation of the carbon-indium bond in 3C2H51n111TPP* is assisted by axial coordination of a pyridine molecule. Molecular orbital consideration is necessary for understanding the bond cleavage assisted by axial pyridine. Figure 7 shows a simple correlation diagram for molecular orbitals of C2H51n1"TPP, InIITPP, C2H5, and (In"')+TPP'- . In the case of C2H51n1"TPP, we assume that a bonding (r orbital composed of an sp3 orbital of C2H5 and a 5s orbital of a central indium atom is located between the HOMO and LUMO orbitals of the porphyrin ligand. This assumption leads to the consideration that the lowest excited triplet state of C2H51n"'TPP is possibly a charge-transfer triplet state: an electron is transferred from the u orbital to the porphyrin LUMO orbital. The strength of the carbon-indium bond is predicted to become weaker in the charge-transfer triplet state, resulting in the facile dissociation to form a triplet radical pair, C2H5' and In'ITPP. Since an unpaired electron in 1n"TPP is located in the 5s orbital of the central indium atom, the triplet
J. Phys. Chem. 1987, 91, 4514-4519
4514
radical pair readily reproduces C,H,In"'TPP by recombination after spin inversion to a singlet radical pair. Therefore, the yield for the photodecomposition of C2H51n"'TPP in benzene is considered to become as small as 0.046. We could not detect the charge-transfer state in the present study, presumably, owing to its short lifetime. The optical absorption and ESR studies have shown that the photoproduct of C2H51n"'TPP in benzene containing high concentration of pyridine is ascribed to the zwitterionic porphyrin, (In"')+TPP'-(Py). This finding indicates that the 5s orbital of the central indium atom becomes higher in energy than the porphyrin LUMO orbital owing to the effects of the axial pyridine
as depicted in Figure 7: the porphyrin moiety in a radical pair produced from 3C2H51n111TPP(Py)*is expressed as (In"')+TPP-(Py) in which an unpaired electron is located in the LUMO orbital of the porphyrin. As far as the unpaired electron is located in the porphyrin ligand, regeneration of C2H,In1"TPP by recombination reaction in the radical pair is not expected. Accordingly, the rate constant for the dissociation of the carbon-indium bond in 3C2H51n111TPP(Py)*becomes much faster than that in 3C2H51n111TPP*. Acknowledgment. We thank Ms. M. Kogure for her assistance during the course of this study.
Reactions of Cobalt Surface Carbides with Water and Their Implications for the Mechanism of the Flscher-Tropsch Reaction A. Ekstrom* and J. A. Lapszewicz CSIRO Division of Energy Chemistry, Lucas Heights Research Laboratories, Sutherland, N S W 2232, Australia (Received: June 10, 1986; In Final Form: January 9, 1987)
The reaction of steam with carbides deposited onto the surface of a cobalt Fischer-Tropsch catalyst by the disproportionation of carbon monoxide results in the formation of carbon dioxide, methane, ethane, and hydrogen. At constant temperature the carbon dioxide yield is virtually independent of space velocity in the range 15-1950 h-I, whereas the CH4 yield was strongly dependent on the space velocity. The C 0 2 yield was found not to be less than 50% of the carbide reacted and did not exceed 50% of the total carbide present. These data are interpreted in terms of a primary reaction between the carbide and steam to give an intermediate which undergoes a bimolecular disproportionation to give C 0 2 and methane. The mechanism of this reaction and its role in the Fischer-Tropsch synthesis are discussed.
Introduction Surface carbides formed by the disproportionation of carbon monoxide on the surfaces of iron, nickel, cobalt, and ruthenium catalysts are believed to be important intermediates in the Fischer-Tropsch reaction, particularly when present in a partially hydrogenated form such as CH,. Repeated studies have shown that these carbides react rapidly with hydrogen to form methane and traces of higher hydrocarbons, and such observations have been cited in support of the carbide mechanism for the synthesis reaction. If carbides are indeed significant intermediates in the Fischer-Tropsch reaction, then their reaction with water, which is the major product of the synthesis, must be of considerable importance. Only two studies of this reaction have been reported. Rabo et aL2 found that nickel or cobalt carbides reacted with steam above 250 OC to form methane and carbon dioxide (the CO-thane process) according to the stoichiometry represented by eq 1, where
2.C
+ 2H2O
+
CHI
+ C02
(1)
0 represents a catalyst site. This reaction could in principle be
divided into the component reactions 2 and 3. More r e ~ e n t l y , ~ .C + 2HzO .C
-+
+ 2H2
C02 -+
+ 2H2
CH4
(2) (3)
traces of hydrocarbons extending to C I 4were obtained when cobalt (1) (a) Wise, H.; McCarthy, J. G. Surf. Sci. 1983,133, 311. (b) Araki. M.; Ponec, V. J. Coral. 1976,44,439. (c) Wentreck, P. R.; Wood, B. J.; Wise, H. J . Catal. 1976, 43, 363. (d) Biloen, P.; Helle, J. M.; Sachtler, W. M. H. J . Catal. 1979, 58, 95. (e) Brady, 111, R. C.; Pettit, R. J . Am. Chem. SOC. 1981, 103, 1287. (2) (a) Rabo, J. A,; Elek, L. F.; Francis, J. N. In Proc. 7th Inr. Congr. Catal. Tokyo 1980, 490. (b) Frost, A. C.; Elek, L. F.; Yang, C. L.; Rabo, J. A. Energ. Prog. 1982, 2, 247. (c) Rabo, J. A,; Elek, L. F.; Francis, J. N. US Patent 4242 103, 1980. (3) Ekstrom, A,; Lapszewicz, J. A. J . Phys. Chem. 1984, 88. 4577
0022-3654/87/2091-4514$01.50/0
carbides were reacted at 200 "C with steam or steam/hydrogen mixtures, but the mechanism by which this occurred was not studied in any detail. Accordingly, reaction 1 was investigated under conditions similar to those used for the atmospheric pressure synthesis reaction over a cobalt catalyst. Since previous work had shown that the presence of a support could complicate such ~ t u d i e sparticularly ,~ when isotopic tracers are used, an unsupported cobalt/thoria/magnesia catalyst was used for this work. Experimental Section The cobalt catalyst was prepared by evaporating a solution of cobalt, magnesium, and thorium nitrates to dryness followed by calcination at 450 "C in air. Analysis showed the catalyst to have the composition Co/MgO/Th02 = 100/7/4. The experiments were carried out in a fixed bed reactor (1 20 mm X 10 mm) containing 12 g catalyst of -105 to +250 pm particle size.3 Facilities were provided for temperature and gas flow controls. Accurate amounts of steam were passed over the catalyst by passing helium carrier gas through a water reservoir held at a precisely controlled t e m p e r a t ~ r e . ~The reactor effluent could be collected in a gas bag for subsequent analysis by gas chromatography (Shimadzu GC 8A and Varian Vista 6000) or high-resolution mass spectrometry (JEOL 300 DX). The effluent composition could also be monitored by an on-line quadrupole mass spectrometer (VG Masstorr FX) used with a Hewlett Packard HP85A computer. For each experiment, the catalyst was reduced in flowing H 2 (SV = 500 h-I) for 24 h at 400 "C and purged with helium at that temperature for 1 h. The reactor temperature was then (4) Not unexpectedly, it has been found that the support absorbs large amounts of water which can be removed only at elevated temperatures. This effect would complicate any work with D 2 0 . ( 5 ) Separate experiments showed that this procedure gave very accurate control of the water partial pressure within the range of helium carrier gas space velocities used in this study.
0 1987 American Chemical Society
