J. Phys. Chem. 1991, 95, 269-214 mechanism of chain growth at the interface of the transition metal and of intrinsic fluorite-type oxide for the dissociation of CO on palladium and the formation of high alcohols in syngas conversion. Some points however remain unclear. Thus a high alcohol selectivity is found on the intrinsic fluorite-type oxide. The catalytic site has been assigned to a set formed by transition-metal atom (or adjacent atoms) and a couple of adjacent vacant anionic sites and anionic sites on the support (Scheme VII). Now we may expect from the intrinsic nature of these oxides an ordered distribution of surface anionic vacancies. Thus the specific nature
269
of the catalytic site may induce some special alcohol selectivity rather than a Schulz-Flory distribution. To test this hypothesis, further work must be undertaken in order to prepare catalysts supported on fluorite-type oxides with a controlled and homogeneous distribution of anionic vacancies. Acknowledgment. Many thanks are due to Mr. Lavergne (Universitt de Paris VI), Mrs. Bacri, and Mr. Vennegues (Universitt de Strasbourg) for TEM, BET, and XRD experiments, respectively.
Photoionization on Insulator Surfaces. Diffuse Reflectance Laser Flash Photolysis of Distyrylbenzenes Adsorbed on Silica and Alumina Dieter Oelkrug,* Steffen Reich, Institut fur Physikalische und Theoretische Chemie der Universitat, D- 7400 Tubingen, Federal Republic of Germany
Francis Wilkinson, and Philip A. Leicester Department of Chemistry, University of Technology, Loughborough, Leicestershire, LEI 1 3 TU, United Kingdom (Received: March 28, 1990; In Final Form: July 19, 1990)
The absorption and fluorescence of 0-,m-,and p-distyrylbenzenes on surfaces of polycrystalline silica and alumina were investigated in the adsorbed state. Time-resolved diffuse reflectance transient absorption spectra were recorded following pulsed nanosecond laser excitation at 354 nm. In each case, a long-lived transient was detected and assigned to radical-cation absorption, and in two cases a short-lived transient was observed and assigned to triplet-triplet absorption. The radicals were also detected by ESR spectroscopy after laser excitation. Experimental observations of the laser fluence dependence of the radical-ion formation efficiency are consistent with a mechanism involving the sequential absorption of two photons during the same laser pulse. It is shown that model calculations for one- and two-photon excitation processes give predictions that are in good agreement with the measurements of the triplet-state and radical-ion production, respectively. The radicals decay by electron-ion recombination. The experimentally observed decay curves are well described by a temperature-activated diffusion model. Decay kinetics are discussed and tested in relation to this theoretical treatment.
Introduction Adsorbed organic molecules can be photoionized with photon energies far below their ionization potentials, IP, as long as the adsorbent consists of a semiconductor. After photoexcitation of either the adsorbent or the adsorbate, an electron or hole is transferred between the adsorbate and an energy band of the semiconductor, and the excess charge diffuses into the bulk of the semiconductor, leaving the radical ion on the surface. The corresponding process on insulators is very unfavorable since lowenergy conduction bands are not available. Nevertheless, radical cations of conjugated hydrocarbons with IP > 7.5 eV can be produced on insulators with laser pulses that have photon energies of only 3-4 eV.'J Beck and Thomas propose that photoionization on insulator surfaces is the result of two-photon absorption and that the adsorbed radical decays via recombination involving either electron tunneling or a diffusion process.' The adsorbed radical ions have been detected by the method of diffuse reflectance laser flash phot~lysis~.~ using polycrystalline microporous metal oxides such as silica or alumina as adsorbents. Until now, neither the mechanism of the creation nor the deactivation of the adsorbed radicals was fully understood. One of the aims of this paper is to quantify these reaction steps on the basis of new experimental data. Since a laser pulse produces in a strongly light-scattering sample a very high photon flux density within a very small volume, it is reasonable to assume that nonlinear optical effects are of importance in the excitation process. An obvious starting point is to consider two consecutive absorption
* To whom correspondence should be addressed. 0022-3654/9 1/2095-0269$02.50/0
steps. This process is well understood from jet-beam experiments in the gas p h a ~ e ,where ~ , ~ the radicals are stabilized in molecular donor-acceptor clusters. Similar processes are assumed to operate also in the liquid in micelles,I0 and in polymers,'I and the proof is mainly given by showing that the initial rate of the radical formation increases quadratically with the intensity of the exciting light. It is especially difficult to evaluate corresponding experiments in light-scattering media because the photometric laws are much more complex than in transparent media and because even a very small self-absorption by the adsorbent is crucial for the initial rate of transient formation. However, we have been able to develop numerical methods, based on the Kubelka-Munk model,'2 which allow us to quantify diffuse reflectance laser flash experiments with respect to decay kinetics, ( I ) Beck, G.; Thomas, J. K. Chem. Phys. Len. 1983, 94, 553. (2) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J . Phys. Chem. 1988, 92, 3589. (3) Wilkinson, F. J . Chem. SOC.,Faraday Trans. 2 1986,82, 2073. (4) Wilkinson, F.; Willsher, C. J. Chem. Phys. Lert. 1984, 104, 272. (5) Brutschy, B.; Jane, C.; Eggert, J. Ber. Bunsen-Ges.Phys. Chem. 1988, 92. 74. ( 6 ) Riihl, E.; Brutschy, B.; Biding, P.; Baumgartel, H. Ber. Bunsen-Ges. Phvs. Chem. 1988. 92. 194. 17) Gratzel, M.: Thomas, J. K. J . Phys. Chem. 1974, 78, 2248. (8) Thomas, J . K.; Piciulo, P. Adu. Chem. Ser. 1980, 184, 97. (9) Bauer, H.; Reske, G. J . Photochem. 1978, 9, 43. (IO) Almgren, M.; Thomas, J. K. Photochem. Phorobiol. 1980, 31, 329. ( I I ) Tsuchida, A.; Nakano, M.; Yoshida, M.; Yamamoto, M.; Wada, Y. Polym. Bull. (Berlin) 1988, 20, 297. (12) Kubelka, P. J . Opt. SOC.Am. 1948, 38, 448.
0 1991 American Chemical Society
.
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400
800
6oo Atnm Figure 2. Diffuse reflectance transient absorption spectra of 0 - , m-, and p-DSB adsorbed on silica ( c = 7 X mol g-I, T A = 120 "C), recorded with a 5 0 - p ~delay after laser pulse excitation at 354 nm.
radicat
T TA so0
300
Figure 1. Fluorescence and fluorescence excitation spectra of p-distyrylbenzene @-DSB): ( I ) sample in ethanol (c = 3 X IOd M); (2) sample adsorbed on silica-60 (c = 7 X mol g-', T A = 120 "C); (3) sample adsorbed on alumina-90 (c = 7 X mol g-l, T A = 120 "C).
quantum yields of product formation, and local distribution of reactants,I3-l5 and these methods are used below. The adsorbates investigated in this study are the three isomeric trans,trans-distyrylbenzenes(DSB) which can form a variety of
o-,m-,p-distyrylbenzene
0
(OSB'
rotational conformers and which can undergo a series of photoreactions involving trans-cis isomerization, ring closure, dimerization, and polymerization,'6-20 not only in solution but also in the adsorbed state.2' Thus these molecules are good candidates for the study of (a) the preferred molecular geometry of adsorbates on solids, especially on microporous ones, (b) internal and translational mobility, and (c) because of their extended *-electron systems, the interaction with donor and acceptor states in the bulk or on the surface of the adsorbent.
Experimental Section The three isomeric trans,trans-distyrylbenzeneswere adsorbed onto alumina-90 (active, neutral: Fa. Merck No. 1077) with a specific surface area of about 100 m2 g-I, silica-60 (Fa. Merck No. 7754) with a specific surface area of about 500 m2 g-I, and Fractosil 1000 (Fa. Merck No. 16420), a special silica with a specific pore size of 100 nm. Adsorption took place either under high vacuum or from liquid n-hexane. Prior to adsorption, the ( I 3) Kessler, R. W.; Krabichler, G.; Oelkrug, D.; Hagan, W. P.; Hyslop, J.; Wilkinson, F. Opt. Acta 1983, 30, 1099. (14) Oelkrug, D.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J . Chem. Soc., Faraday Trans. 2 1987, 83, 208 1. (IS) Honnen, W. Ph.D. Thesis, University of Tubingen, FRG, 1986. (16) Misumi, S.; Kuwana, M. Bull. Chem. SOC.Jpn. 1960, 33, 711. (17) Dietz, F.; Scholz, M. Tetrahedron 1968, 24, 6845. (18) Zertani, R.: Meier, H. Chem. Ber. 1986, 119, 1704. (19) Tol, A. J. W.; Laarhoven, W. H. J . Org. Chem. 1986, 51, 1663. (20) Oelkrug, D.; Rempfer, K.;Prass, E.; Meier, H. 2.Naturforsch. 1988, 43A. . , 583. ... (21) Laarhoven, W. 1970, 26, 1069.
H.;Cuppin, T. J. H. M.; Nivard, R. J. F. Tetrahedron
0.6
0.4
0.2 0 4 00
500
6oo
Atnm
Figure 3. Time-resolved diffuse reflectance transient absorption spectra of p-DSB adsorbed on silica, recorded with different time delays after laser pulse excitation at 354 nm.
adsorbents were preheated a t a temperature of TA= 120 OC to remove oxygen and physisorbed water. The resulting adsorbate concentrations were 3 x IO-9 to 7 x mol g-I for silica, 2 x mol g-' for alumina, and 7 X mol g-' for to 7 x Fractosil. Fluorescence and excitation spectra were recorded on a Spex Fluorolog 222 spectrometer with two double monochromators. The radicals were identified by ESR spectroscopy. All ESR measurements were recorded at T = 77 K after laser pulse excitation at 308 nm. The flash photolysis apparatus has been described, and full details have been given e l ~ e w h e r e . ~Excitation ,~~ was achieved with the 354-nm harmonic of a pulsed Nd-YAG laser with a pulse width of about 8 ns and a pulse intensity of 20 mJ. A pulsed 250-W xenon arc lamp was used as the monitoring light. In addition, we used an XeCl excimer laser with an excitation wavelength of 308 nm in some experiments.
Results and Discussion 1. Fluorescence and Fluorescence Excitation Spectra. Fluorescence was excited with steady-state illumination of very low intensity in order to avoid photochemical reactions. The spectra of the three isomers as adsorbates are similar to their absorption and fluorescence spectra in indicating that no strong electronic interactions between the solids and the adsorbates have to be considered. In Figure 1, the spectra of p-DSB are presented as an example. The vibrational structure of p-DSB adsorbed on alumina is more pronounced than in ethanol, whereas on silica it is less pronounced. This is a consequence of changes in the torsional mobility of the phenyl-ethenyl groups that are strongly suppressed on alumina, stabilizing one definite conformation. However, in the case of silica torsional mobility is sufficient to allow equilibration between different rotamers. This suggests that repulsive forces between (22) Leicester, P. Ph.D. Thesis, University of Loughborough, U.K., 1990. (23) Dale, J. Acta Chem. Scand. 1957, 1 1 , 971. (24) Bush, T. E.; Scott, G. W. J . Phys. Chem. 1981, 85, 144. (25) Meier, H.; Zertani, R.; Noller, K.; Oelkrug, D.; Krabichler, G. Chem. Ber. 1986, 119, 1716.
Photoionization on insulator Surfaces
, 4OG ,
fl
I
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 271
gr2.0028
0.6
- 0.4 I
,/
7
0.2 n
5 10 1s pulse energy I m J ,cm-z
Figure 5. Decrease of the diffuse reflectance R of p-DSB adsorbed on silica (c = 7 X mol g-I, T A = 120 " C ) by transient absorption at (1) 410 nm and (2) 600 nm vs laser pulse energy density (At = 20 ns, X = 354 nm).
R+
Figure 4. ESR spectrum of o-DSB adsorbed on silica (c = 7 X mol g-l, TA = 120 "C), recorded at T = 77 K after laser pulse excitation at 308 nm. Dashed line: standard EPR sample (PITCH).
unsaturated hydrocarbons and oxide surfaces are of more importance on silica than on alumina, as does the fact that the quantum yield (ptC of trans-cis isomerization on silica is almost as high as in low-viscosity solutions. However, cptc is reduced by a factor of 10-100 on 2. Transient Absorption Spectra. The samples were excited by a laser pulse with a typical energy density of about 20 mJ which in the most concentrated samples (7 X lo-' mol g-l) produces a deep color. Figure 2 shows the corresponding transient absorption spectra recorded 50 ps after the laser pulse. Negative absorption in the region X < 390 nm is a consequence of ground-state depletion. After 50 ps the spectral shapes are time independent and the color persists for ca.100 s. Immediately after the laser pulse the o-DSB and p-DSB samples exhibit additional transients which absorb between 400 and 500 nm and decay in the microsecond time domain. Time-resolved spectra for p-DSB adsorbed on silica in this time range are reproduced in Figure 3. Similar results are found for p-DSB adsorbed on alumina except that absorption by the short-lived transient (60% absorption) is stronger than that by the long-lived one (35% absorption). In addition, the absorption maximum of the long-lived transient is red-shifted by ca. 300 cm-I. In a series of experiments, the surface loading was systematically lowered in order to determine whether bimolecular reactions were responsible for the transient production or decay and in order to test the sensitivity of the method. The observed transient absorption decreased in intensity for lower loadings, but the shape of the spectra remained constant. The lowest concentrations that gave detectable transients were 3 X IO4 mol g-' for silica-60 and 3 X 10%mol g-' for alumina-90. No significant differences were observed in the spectra of samples prepared by gas-phase adsorption under high vacuum or by adsorption from liquid n-hexane. 3. Assignment of the Transients. Transients in the spectra of adsorbed DSB are assigned, by analogy to the transient spectra of adsorbed diphenylpolyenes,2to short-lived triplets and long-lived radical cations. The triplet-triplet absorption (TTA) spectra are well-known from solution data for many polyenes and stilbene^.^' In the physisorbed state, the band positions changed very little and the decay times increased slightly from < I to 1-5 ps. The only transient absorption band that we were able to detect in ethanolic solution of p-DSB was at 480 nm and decayed with a lifetime of 0.8 1s. In solution no radical absorption was observed. It is reasonable to compare the peak at 480 nm directly with the corresponding peak at 485 nm of the short-lived transient in the adsorbed state (Figure 2) and to assign both to TTA. Since the So S, transition energies increase in the series p- < 0- < m-DSB, it is also reasonable to assume the TTA energies will increase in the same way. In keeping with this expectation, we were able to detect the onset of TTA in adsorbed o-DSB at 400-450 nm
-
(26) Brun, M. Diploma Thesis, University of Tubingen, FRG, 1989. (27) Gorner, H . J . fhorochem. 1982, 19, 343.
one photon process
soiid
two photon process
Figure 6. Reaction scheme of the one- and two-photon excitation processes (for parameters see text).
but not in m-DSB, presumably because the TTA was then outside the accessible range of our laser photolysis apparatus. The assignment of the long-lived transients is based mainly on their close similarity to features of the absorption spectra of stable radical cation^.^^,^^ In the case of DSB, the samples were flashed at 77 K, producing transients that were stable over a period of days and allowing the recording of ESR spectra at this temperature. Very intense ESR signals were obtained for all three isomers of adsorbed DSB, one of which is shown in Figure 4. The fine structure of the resonance is not resolved in the adsorbed state so that no detailed structural information is given, but the position of the main peak at g = 2.0046 is undoubtedly that of a radical. 4. Mechanism of Radical-Ion Formation. The energy of 3.51 eV of the laser photons is too low to dissociate or ionize the adsorbates directly, and since no low-energy excited states that could support ionization exist in the adsorbents, certainly not in silica, the most probable mechanism of ionization is by a twophoton or multiphoton absorption process. Qualitatively, the one-photon process can be excluded at once. The samples give a very high radical-ion absorption signal when they are irradiated with HL = 5 X einstein cm-2 within an interval of 10 ns. However, not a trace of radical is formed if the same photon flux HL,or even a much higher one, is used from a continuous Hg or Xe arc lamp with a monochromator irradiating over an interval of 1 s. Since the deactivation of the radicals is almost negligible during this time interval, this proves that their formation yield cannot be linearly proportional to the photon flux. A semiquantitative result in favor of the two-photon process was obtained in the following way. The intensities of both the TTA at X = 410 nm and the radical absorption at X = 600 nm were monitored as a function of energy of the laser pulses, always with the same pulse width of 10 ns. The results are shown in Figure 5. The TTA increases very steeply at low pulse energy Contrary to this, and saturates at pulse energies >IO mJ the radical absorption increases very slowly at low pulse energy but comes up to the TTA intensity at higher pulse energies and When we use the does not seem to saturate even at 20 mJ (28) Wallace, S. C.; Gratzel, M.; Thomas, J. K. Chem. fhys. Leu. 1973, 23, 359. (29) Alkaitis, S. A.; Beck, G.; GrPtzel, M. J . Am. Chem. SOC.1975, 97, 5123.
272 The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991
Oelkrug et al.
---_---_----__ ' ---__ 0 '
0.5-
W 0
05
10
15 tls
p-mI M V / q nm
0.5
z
'I 1. D - DSB/ Fractosil
0'
0-
5 1
15
10 2
3
4 1Osa
ImJcm-' HilElnstMtW'
Figure 7. Calculated absorptivities (1 - R ) of transients generated by one-photon (T, upper) and two-photon (R', lower) absorption as a function of the photon flux HLof rectangular laser pulses with a width of At = IO ns. Adsorbate concentration c,, = 5 X lo4 M; extinction coefficients (base e) t , = f2 = q = fRt = IO' M-l cm-'; kF = 5 X IO8 s-l; scattering coefficient of the adsorbent S = 80 cm-l. Formation efficiencies of the transients (see eq 1) are variable parameters.
TTA intensity versus pulse energy trace as an internal standard for one-photon absorption, it becomes evident from this experiment that the radical ions must be formed by a two-photon or multiphoton absorption process. 5. Model Calculations. The curves of Figure 5 can be simulated for various transient formation processes by the numerical methods described p r e v i ~ u s l y .Figure ~ ~ ~ ~6~shows the minimum reaction scheme. The first photon hul induces the transition So SI,t,with an extinction coefficient e l . The vibronic state Si,+ deactivates very quickly to SI:o.From here the transient species T is formed, or fluorescence IS emitted, or a second photon hvl is absorbed with extinction coefficient e2, populating a higher excited singlet state, S,; i.e., S I, + hu2 Sn,o. From this state the radical cation (transient R4) is formed by electron ejection or by electron transfer to some highly energetic states, e.g. to the conduction band of the adsorbent. This last reaction step must be very fast in order to compete with the very high deactivation rate from S , to S I . Since the laser pulse is much longer than the lifetime of S i (in the case of p-DSBlsilica we measured a fluorescence lifetime rF= 2.2 x s), activation-deactivation processes within the singlet manifold take place repeatedly and result in substantial formation of T and R+. The parameters of the scheme ( t i , k F , kT) are experimentally known for the onephoton process and in some cases also the extinction coefficient cT of the transient T. In our model, all extinction coefficients were set equal, i.e. el = t2 = 9 = CRt, and the expected initial absorptions were calculated by using the efficiencies of the one-photon and , as two-photon transient production, yT and Y ~ respectively, variables, where
-
-
YT = k ~ ( + k b)-' ~ YR = k ~ ( k+~kid-' (1) The scattering coefficients of the adsorbents were determined from the reflectances and transmittances of thin slices.M Some results of our simulations are presented in Figure 7. The main features of the calculated curves are in good agreement with those of the
(30) Kortiim, G.;Oelkrug, D. Z.Naturforsch. 1964, / 9 A , 28.
I 9
0
12.5
25
37.5 t/ms
Figure 8. Normalized radical absorption decay curves of p-DSB on various adsorbents recorded at X = 600 nm: (1A) curve for sample adsorbed on silica from high vacuum, recorded at T = 295 K (1 B) curve for same sample in an hour time scale; ( 2 ) curve for sample adsorbed on silica from liquid n-hexane (c = 3 X IO4 mol g-l), recorded at T = 295 K; (3) curve analogous to ( 2 ) but with c = 7 X IO-' mol g-I; (4A) curve for sample adsorbed on Fractosil from liquid n-hexane (c = 7 X IO-' mol g');(4B) curve for fast component of (4A) on a millisecond time scale.
experimental curves of absorption saturation, as far as the height of the absorption plateau and the difference between one- and two-photon absorption traces are concerned. Of course, the agreement cannot be expected to be quantitative, since approximate extinction coefficients for the excited-state transitions were used. According to the calculations, a IO-ns pulse of 20 mJ will deplete the ground state of p-DSB almost completely within a layer 100 to 200 prn thick, beneath the illuminated surface of the sample. The rest of the sample remains almost unconverted; Le., we have a plug situation (see ref 14). 6. Kinetics of the Transient Decays. We have found by repetitive excitation at room temperature of the same sample area ofpDSBlsilica at intervals of 2 min that the intensities and decays of the transient absorption signals are completely reproducible. Thus the radicals decay mainly by electron-ion recombination to the original neutral molecule. The overall decay of the radical cations is very complex. Figure 8 shows various decays of p-DSB adsorbed on silica gel and Fractosil. In all cases, a fast initial decay rate is followed by a slow long-time component. The shapes of all decay curves are qualitatively comparable, but the absolute time scales vary by several orders of magnitude. These large variations can be correlated mainly with the nature of the adsorbent and its pretreatment. It is obvious that there is a clear difference between adsorption from the gas phase under high vacuum with outgassing for days at T A = 120 OC (curve 1A) and adsorption from the liquid phase with preheating for only 2 h at T A = 120 "C in a dry nitrogen atmosphere (curves 2 and 3). It is also obvious that p-DSB decays much faster on Fractosil (curve 4A) with a specific surface area of only 15 mz g-' (pore size 100 nm) than on silica with a specific surface area of 500 m2 g-' (average pore size 6 nm). However, only very small differences are observed in the kinetics of deactivation due to variation of the surface coverage with p-DSB. This is shown in curves 2 and 3, which are very similar but which are obtained with very different surface loadings of cDSB = 7 X lo-' mol g-I and cDsB= 3 X mol gl.Thus one observes that the process of ionization leaves the radical and the electron separated by small distances, smaller
Photoionization on Insulator Surfaces
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 273
than the mean distances between the adsorbates, and that the reactants recombine in a geminate process of pseudo first order. The kinetics are controlled by surface diffusion of both species and not by electron tunneling. This can be shown by the recombination rates, which are strongly temperature dependent; the radicals are stable for weeks at 77 K and recombine almost 1 order of magnitude faster at 325 K than a t room temperature. In addition, the recombination rate is strongly dependent on the surface pretreatment and on the magnitude of the specific surface area. This excludes bulk diffusion of electrons from being an important deactivation channel. The rate of geminate pair recombination is quantified in three-dimensional systems very often by the simple -dc/dt = k P
TABLE I: Radical-Electron Recombination Constants of Adsorbed p-DSB Evaluated by Eq 5 with the Two-LayerApproximation adsorbent surface loading, mol I-' pretreatment a, silica 3 x 10-9 liquid phase 1.25" 7 x 10-7 silica liquid phase 1.82' 7 x 10-7 19.1b silica liquid phase 7 x 10-7 0.04" silica high vacuum 3 x 10-8 1.18' alumina liquid phase 7 x 10-7 alumina 1.31" liquid phase 7 x 10-7 0.309' alumina high vacuum 7 x 10-7 4.60' Fractosil liquid phase 7 x 10-7 42.g6 Fractosil liquid phase "Measured at 295 K. bMeasured at 325 K.
(2)
where c is the pair concentration and n i= I . The concentration of pairs as a function of time will then be c = const - k In t
(3)
This relation has been used for example by Beck and Thomas1 in order to quantify the recombination of pyrene radicals on alumina. However, eq 3 cannot be valid for r 0 and t m, and Abel and M o z ~ m d e rhave ~ ~ ,shown ~ ~ that eq 2 must change for long times to
- -
-dc/dt = kY3l2
-
I
0
0.5
i
it&
O l 0
0.5
1
itlis
0
0.5
1
it/&
(4)
-
independent of the original distribution of pair distances. Of course, eq 4 cannot be valid a t t 0 because of co a. The pole can only be avoided with an exponent n < 1 in eq 2. In order to develop a closed analytical description of the problem, we propose the following approximation: c = co(l
-
+ a(t)-l
This equation satisfies eq 4 for long times ( a v't gives n < I as t 0
(5)
>> 1) and it also
The square-root relation of eq 6 is well-known from many nonstationary one-dimensional diffusion models. The same relation also makes sense in the initial stages of geminate pair recombination on surfaces, since the motions of oppositely charged particles with small separation are directed preferentially toward one another in. one dimension. The transition into the three-dimensional behavior of eq 4 for long times is in agreement with the approximately three-dimensional fractal surface of silica. Our results are evaluated according to eq 5 as follows: First the experimental R,? curves are converted into c,t curves within the limits of the Kubelka-Munk model. This is done (a) by the numerical methods given e l ~ e w h e r e , ' (b) ~ * ~in~a two-layer approximation (one layer saturated with transient, the other completely unconverted) that allows an analytical description of the problem,22 and (c) in a one-layer approximation using the well-known Kubelka-Munk equation (1 - R)2(2R)-' c for optically thick samples. The results arc then linearized according to
I
-
co/c = 1
+afi
(7)
In all cases, the evaluations of (a) and (b) give reasonable straight lines with almost equal slopes. In many cases also, the simple approximation (c) is sufficient to quantify the slope of eq 7. In Table I are presented the results of the approximation (b), which is much more convenient to handle than the very expanded method (a). Four examples plotted according to eq 7 are shown (31) Debye, P.; Edwards, J. 0. J . Chem. Phys. 1952, 20 (2), 236. (32) Era, K.: Shinonoya, S.;Washizawa, Y.; Ohmatsu, H. J . Phys. Chem. Solids 1968, 29, 1843. (33) Tachiya, M.: Mozumder, A. Chem. Phys. Lett. 1975, 34 ( I ) , 77. (34) Abell, G. C.: Mozumdcr, A. J . Chem. Phys. 1972, 56 ( 8 ) . 4079.
0 0.5 1 hi& Figure 9. Linearization of the experimentally observed decay curves of p D S B according to eq 7 with the two-layer approximation: (A) sample adsorbed on silica from the liquid phase (c = 3 X mol gl);(B) sample analogous to that in (A) but with c = 7 X mol g-I; (C) sample adsorbed on alumina from the liquid phase (c = 7 X IO-' mol g'); (D)sample analogous to that in (C) but with c = 3 X mol g-l.
in Figure 9. Although the linearization of the experiments is not perfect, especially in the initial time range where the simplifications of the model or the inhomogeneity of surface diffusion coefficients becomes most noticeable, the slopes of the straight lines help to quantify the radical-electron recombination kinetics in quite different environments. The radical-electron recombination constants given in Table I emphasize the temperature dependence
J . Phys. Chem. 1991, 95, 274-282
274
of the recombination process. The constants also depend strongly on the pretreatment of the samples but less on the surface loading. Conclusions
Laser cxcitation of distyrylbenzenes adsorbed on high-surface-area silica and alumina surfaces is shown to give rise to short-lived triplet states and long-lived radical cations that can be studied by time-resolved diffuse reflectance spectroscopy. The decrease in reflectivity due to triplet-triplet absorption by the adsorbate increases with increasing laser fluence until the laser dose is sufficient to create a conversion layer that is thick enough to shield the rest of the sample from spectroscopic detection. The observed variation, which rises quickly to a saturation value, follows expectations for a "one-photon" process involving absorption by ground-state adsorbate to give SI followed by intersystem crossing to give T,. In contrast, the absorption by the radical cation shows a much slower initial rise with laser fluence compatible with a sequential "two-photon" absorption process, i.e. So + hu -,SI hu S, R+ + e-.
-
+
-+
The radical-cation decay is extremely nonexponential with a proportion decaying over a few microseconds but with complete recovery taking several minutes. However, the decay is reversible; Le., all the radical cations decay to give the original adsorbate. Thus decay is due to radical ion-electron recombination. The amount of transient produced is independent of the number of times the sample has been exposed, but the decay shows a large temperature dependence, so large that cooling to 77 K virtually eliminates decay and ESR spectra of the stabilized radical ions can be measured. The decay is independent of the loading of the adsorbate and of laser fluence, which proves that the decay involves the recombination of the originally produced radical ion-electron pair. At long times, pair recombination theory predicts that pair decay will be a diffusional process and the amount of radical ion remaining will have a dependence, as is found experimentally. Acknowledgment. This work was supported by the European Community (Contract No. ST2-0069) and by the Deutsche Forschungsgemeinschaft (Oe 57/12) to whom we are grateful.
Dinitrogen Photoreduction to Ammonia over Titanium Dioxide Powders Doped with Ferric Ions J. Soria, J. C. Conesa, Instituto de Catcilisis y Petroleoquimica, CSIC, 28006 Madrid, Spain
V. Augugliaro,* L. Palmisano, M. Schiavello, and A. Sclafani Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, University of Palermo, 901 28 Palermo, Italy (Received: April 2, 1990)
The production of ammonia from dinitrogen and water vapor at mild conditions of temperature and pressure on Fe3+-doped titanium dioxide powders under UV radiation has been studied in a continuous photoreactor working in the gassolid regime. A net activity decline was observed after a few hours of irradiation; this decline did not depend on the reactor temperature, the powder composition, or the amounts of produced ammonia. The highest activity is found when no excess iron is segregated at the surface; overall turnover numbers for dinitrogen reduction as high as six electrons per iron atom can then be reached before powder deactivation, showing the catalytic character of the participation of Fe in this process. An IR investigation of the active and spent specimens revealed that the irradiation determines the almost complete disappearance of the OH groups from the powder surface. Furthermore, an ESR study of all the powders showed that bulk Fe3+ions are better electron traps than Ti4+ ions so that, when a UV photon generates a hole-electron pair, the electron can be stabilized on the iron ions. The charge separation, thus favored by the latter, makes possible the adsorption of dinitrogen by its reaction with surface species activated by holes. From these data, the deactivation process is ascribed to the decrease in the amount of the reactive hydroxyls which make possible the regeneration of the bulk Fe3+traps necessary for the ammonia photoproduction; the activity, as well as the Fe3+ and OH- groups, is recovered by treatment of the powders in air at 823 K followed by slow cooling.
Introduction
The dinitrogen reduction to ammonia in the presence of water over irradiated semiconductor powders in mild conditions of temperature and pressure constitutes a new route for the solar energy conversion into chemical energy. This process is even more difficult than that of H 2 0 photodecomposition. Several factors contributing to this are the following: (a) the reduction of one molecule of N2 involves transfer of six electrons compared to two electrons in the H20reduction; (b) although the energy requirements of H20decomposition and N2 reduction are quite similar, the activation energy for N2 reduction is very high; (c) N, adsorption is a necessary requirement for catalytic activity, but most semiconductor oxides do not adsorb N2 easily; (d) in addition, when batch systems are used, any photogenerated ammonia accumulates in the reacting system and can be involved in back-reactions which ultimately block reaction progress. Schrauzer and Guth' were the first authors to report on the dinitrogen photoreduction; by testing various iron-doped and ( I ) Schrauzer, N.; Guth, T. D. J. Am. Chem. SOC.1977, 99,7189-7193.
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iron-free Ti02 (rutile) specimens, they found that Fe ions improve the activity, which however declines for Fe contents above 0.2 wt %. After that, many studies have been performed on this photoreaction, which have definitively proved its feasibility in heterogeneous conditions, and exhaustive reviews have been reported in the Among the several semiconductor oxides t e ~ t e d ,titanium ~,~ dioxide doped with various cations is found to be one of the most effective.'V6-l5 A very recent paperI6 reports (2) Schiavello, M.; Sclafani, A. Photoelectrochemistry, Photocatalysis and Photoreactors. Fundamentals and Developments; D. Reidel Publishing Co.: Dordrecht, 1985; pp 503-519. (3) Augugliaro, V.; Palmisano, L. Photocatalysis and Environment Trends and Applications; Kluwer Academic Publishers: Dordrecht, 1988; pp 425-444. (4) Endoh, E.; Leland, J. K.; Bard, A. J. J. Phys. Chem. 1986, 90, 6223-6226. (5) Khader, M. M.; Lichtin, N. N.; Vurens, G.H.; Salmeron, M.; Somorjai, G. A. Langmuir 1987, 3, 303-304. (6)Augugliaro, V.; Lauricella, A.; Rizzuti, L.;Schiavello, M.; Sclafani, A. Int. J. Hydrogen Energy 1982, 7, 845-850. (7) Augugliaro, V.; DAlba, F.; Rizzuti, L.; Schiavello, M.; Sclafani, A. Int. J . Hydrogen Energy 1982, 7, 851-855.
0 1991 American Chemical Society
