2117
J . Phys. Chem. 1987, 91, 21 17-2120 k
+ 2 h + v ~20 2 + 2H+,g 2H20 + 2 h + v ~--%H202 + 2H+,,, H202
(9) (4)
(All above reactions are "overall" processes which do not imply mechanistic detail.) If the concentrations of O2and H+, are taken to be constant and a steady-state concentration of e c B and h+VB is assumed during illumination, the overall production of H202 can be described by the following kinetic equation d[H2021/dz = ki - k2[H2021
(24)
where kl is a lumped pseudo-zero-order constant reflecting the sum of eq 22 and 4 and k2 is a lumped pseudo-first-order constant reflecting the sum of eq 23 and 9. Integration of (24) yields a best fit of the kinetic data as shown in Figure 9. The initial zero-order formation of H 2 0 2under O2saturation leads to kl = 2.2 X M/s (see Figure 9a) which can be used to compute a quantum yield of do= 0.48. Values of doare listed in Table I for different catalysts and experimental conditions and have already been discussed. At steady state (d[H202]/dt = 0) the depletion rate can be calculated via
k2= = k,/[H202] = 2.4 X
s-l
Values for k2* do not represent "real" rate constants since they always include the photon flux into the sample. While some of the differences between various measurements (last column in Table I) are therefore simply due to different light intensities, the to 1.7 X IO-' s-' as the dioxygen apparent increase from 0.9 X lr3 concentration is decreased can be mechanistically explained. It has already been pointed out that a cobalt-superoxide complex is stable toward the attack of H202;however, reaction 23 requires that H 2 0 2enters the coordination sphere of the metal center to be significant. Reaction 25 is therefore envisioned to compete TiO--Co"TSP-H20
-
+ H202
TiO--Co"'TSP-OH
+ OH(25)
with the uptake of O2in the catalytic cycle (cf. left-hand part of Figure 7) and will thus be favored at lower O2content. Absorption of two photons will then lead to the release of another OH-. The absence of any complex formation with hydrogen peroxide in the dark which is strongly suggested by our results can easily be explained since the light-dependent cycle (left side in Figure 7) is a prerequisite for reaction 25. Values for k2 can also be calculated from the depletion rate s-l); of H202(the solid line in Figure 9b yields k2 = 2.8 X they are listed in the last column of Table 11. Even though these
k2dvalues agree reasonably well with k p from the steady-state calculations, a quantitative comparison is not warranted since the experimental conditions differed too much.
Conclusion It has been demonstrated that Co"TSP can be employed as an efficient electron relay on T i 0 2 provided it is chemically bound to the semiconductor surface. The formation of octahedral cobalto complexes involving 0-groups from the oxide support is strongly indicated. This hybrid complex imparts in the high degree of chemical stability and results in efficient electron-transfer p r o p erties. The reduction of dioxygen to yield hydrogen peroxide is photocatalyzed by this hybrid catalyst upon bandgap irradiation of the T i 0 2 with extremely high quantum yields. Since the superoxide adduct of Co"'TSP is very stable, two-electron-transfer steps are strongly favored and no evidence of free radical intermediates has been observed. The formation of H 2 0 2 by the oxidation of water by photogenerated could not be established unambiguously. Indirect evidence in favor of this reaction stems from the formation of steady-state concentrations of H 2 0 2 up to 25 pM in the absence of any detectable hole scavenger and from the observation that long-lived holes do not exist. Hydrogen peroxide and molecular oxygen were both measured as photoproducts, suggesting that surface adsorption of these molecules does not present a serious problem in the investigated systems. The polarographic method of the oxidase probe employed in this study allows the in situ observation of H202 formation and decay with a detection limit of lo-' M without the addition of any interfering enzymes or chemicals. The mechanism of the photocatalytic reduction of O2and the depletion of H 2 0 2 by use of the hybrid catalyst has been discussed in detail. We finally like to emphasize that Ti02-Co"TSP may have a considerable potential as an oxidation catalyst in oxic environments since the reduction of O2yields H202,which is a better oxidant, and the simultaneously formed h+VBare extremely powerful oxidants anyway. Initial experiments on the oxidation of SO3" indeed suggest that quantum yields close to unity can be achieved reproducibly with this catalyst.60 Acknowledgment. We gratefully acknowledge the financial support of the U.S. EPA (Grants CR812356-01-0 and R811612-01-0), and in particular we want to thank Drs. Donald Carey and Marcia Dodge for their support. Free samples of titanium dioxide P25 from the Degussa Corp. were highly appreciated. (60)
Hong, A. P.; Bahnemann, D. W.; Hoffmann, M. R.,unpublished
data.
Reactions of a Paramagnetic Silver Cluster with NH, and C,H,+ J. R. Morton,* K. F. Preston, A. Sayari, and J. S. Tse Division of Chemistry, National Research Council, Ottawa, Ontario, Canada Kl A OR9 (Received: November 18, 1986)
The paramagnetic silver cluster Ag6+.8Ag+in zeolite 4A has been reacted with ammonia and ethylene. The products of these reactions, which include a new hexameric cluster and a cluster thought to be Ag:+, have been studied by EPR spectroscopy. Ab initio calculations support the proposed assignments.
Introduction We have recently become interested in the properties of small silver clusters trapped in A-type zeolites. Our electron paramagnetic resonance (EPR) spectroscopic studies have enabled us Issued as NRCC No. 27336.
0022-3654/87/2091-2117$01.50/0
to identify in y-irradiated, silver-loaded 4A zeolite Ag2+,Ag$+, and as successive Products of an annealing Process.' The latter is a PflicularlY interesting S@es, being derived from Seff's 14-atom cluster Ag6*8Ag+-an octahedral Ag6 molecule Sur(1) Morton, J.
R.;Preston, K.F. J . Mogn. Reson. 1986, 68, 121.
Published 1987 by the American Chemical Society
2118
The Journal of Physical Chemistry, Vol. 91. No. 8, 1987
Morton et al. EPR spectrum (Figure 1) which we have ascribed to Ag6+.8Ag+ could readily be detectede2 In addition, samples were prepared by Hermerschmidt and Haul's m e t h ~ da, ~method which involves high-temperature dehydration followed by treatment with H2 at room temperature. The EPR spectra of the A&* cluster, before and after reaction with ammonia or ethylene, were examined at 77 K with a Varian E l 2 EPR spectrometer equipped with frequency and magnetic field measuring devices.
Figure 1. First-derivativeEPR spectrum at 77 K of Ag6+.8Ag+in zeolite Ag-A, obtained with *OSAg.
rounded by a cube of Ag+ ions.2 The paramagnetic Ag6+, also surrounded by up to eight Ag+ ions, is stable in air at 50 OC. For this reason, and because of the potential catalytic importance of silver-loaded zeolites, we decided to investigate the reactions of the paramagnetic cluster, using its EPR spectrum as a probe both for the survival or destruction of Ag6 and for the generation of any new paramagnetic species which might be formed. Two distinct EPR spectra have been assigned to the hexanuclear cluster. In addition to the spectrum referred to above,' Hermerschmidt and Haul3 published a spectrum in 1980 which they attributed to This was obtained by dehydration at 400 OC, followed by treatment with H2 a t room temperature. The difference, spectroscopically, between Hermerschmidt and Haul's spectrum and ours is that ours clearly showed a "superhyperfine" interaction with eight Ag+ ions. This "superhyperfine" interaction was easily resolved in the outside lines of the spectrum, even in samples containing silver not enriched in either Io7Agor lo9Ag. We suspect, therefore, that Hermerschmidt and Haul's (HH) cluster lacks some of the Ag+ ions which form the surrounding cube in Seffs cluster. As part of this study, we compare the reactivities toward ammonia and ethylene of Ag12-A containing each form of the cluster. It is known, of course, that the Ag6 cluster reacts with ammonia4 and e t h ~ l e n e . Seffs ~ work on the adsorption of these gases by Ag12-A proved that adsorption of ammonia led to the formation of N3H5 and N3H3 in the a- and @-cages,respectively, of the zeolite. It was our hope that we would be able to monitor the early stages of this reaction by studying the effects of added ammonia on the EPR spectrum of Ag6+. In the case of the reaction of C2H4 with the silver cluster, Seff has reported that the Ag6 moiety remains intact in the &cage and that ethylene molecules in the a-cage are coordinated to the Ag+ ions.5 Our experiments were devised to see if the paramagnetic cluster Ag6+ survived ethylene adsorption, and if not, with what results.
Experimental Section Linde 4A molecular sieve was stirred in the dark for 16 h with 1.0 M AgNO, solution, in sufficient quantity to provide for a twofold excess of Ag+ for the exchange with Na+. The Ag12-A produced was washed with distilled water and dehydrated at 105 O C over a stream of helium. Some samples were made with silver enriched to 98% in one of the isotopes Io7Ag or lo9Ag. These isotopes were obtained from Oak Ridge National Laboratory, Oak Ridge, TN. After y-irradiation for ca. 30 min at 77 K in a 700 TBq %o y-cell, followed by annealing to room temperature, the (2) Kim, Y.; Seff, K. J. Am. Chem. SOC.1977, 99, 7055. (3) Hermerschmidt, D.; Haul, R. Ber. Bunsenges. Phys. Chem. 1980,84,
902. (4) Kim, Y.; Gifje, J. W.; Seff, K. J. Am. Chem. SOC.1977, 99, 7057. ( 5 ) Kim, Y.; Seff, K. J. Am. Chem. SOC.1978, 100, 175.
Theoretical Section Molecular orbital calculations were performed with the unrestricted Hartree-Fock ab initio pseudopotential method. For the first-row atoms, we employed the valence Gaussian basis sets and effective core potentials (ECP) derived recently by Stevens et a1.6 The basis set and ECP for the silver atom were taken from the tabulation of Wadt and Hay.' In the Ag, cluster calculations, the Gaussian basis sets were contracted to double-{ quality. For the larger Ag:+, A&+, and Ag6+.8Ag+ clusters, a minimal valence basis set was used. In the Ag6+.8Ag+ cluster calculations, the effect of Ag+ cations was mimicked by eight point charges. The spin contamination in all the calculations was found to be very small. Results The spectrum which we have assigned to the Ag6+.8Ag+ cluster,I was obtained as described above and had the following EPR parameters a t 77 K: g = 2.0233 f 0.0005 ~109(6)= 208 f 2 MHz ~log(8)= 15.3 f 0.5 MHz The numbers in parentheses are the number of silver nuclei having the given hyperfine interaction. These parameters were obtained with silver enriched in the isotope Io9Ag. Our measurements on Hermerschmidt and Haul's spectrum differ somewhat from theirs. This may have been the result of a typographical error. In any event, we obtained g = 2.0250 f 0.0010 alo9(6) = 202 f 2 MHz Thus, except for the absence of a small hyperfine interaction with eight Ag nuclei, the spectral parameters of the two species are quite similar. As we shall see, however, the properties of the two clusters toward reaction with ammonia and ethylene are quite different. Reaction with Ammonia. When Ag12-A molecular sieve containing the Ag6+.8Ag+ cluster was placed in contact with 500 Torr of ammonia at -80 "C and periodically examined at 77 K with an EPR spectrometer, certain changes in the EPR spectrum occurred. These changes resulted in the development over a 16-h period of a new spectrum which also exhibited quite large hyperfine interactions with six equivalent silver nuclei. The spectral changes occurred in two stages. The first stage merely involved the loss of the 15-MHz (IWAg)superhyperfine structure and took place in less than 1 h at -80 OC. The second stage, which required approximately 16 h at -80 O C for completion, resulted in a spectrum having a quite different g factor and hyperfine interactions than those of the initially present Ag6+.8Ag+. The isotropic parameters describing the new spectrum obtained from samples made with Io9Ag-enrichedsilver were as follows:
* 0.0005 aiog(6) = 247 * 10 MHz g = 1.9882
The new spectrum described by these parameters appeared without (6) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984,81,6026. (7) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
Paramagnetic Silver Cluster in Zeolite 4A
Figure 2. First-derivative EPR spectrum at 77 K obtained by treating sample of Figure 1 with CzD4.
further irradiation, and there was no indication of "superhyperfine" structure. The same experiment was carried out on ABlZ-A containing the HH cluster, that is, on samples which had been dehydrated at 400 O C and exposed to 30 Torr of H2at room temperature for a few minutes. The consequences of exposing such samples to ammonia a t -80 OC were dramatically different from the preceding results: the signal attributed by Hermerschmidt and Haul to Ag6X+was destroyed by a 15-s exposure to ammonia at -80 OC. After such treatment only a broad signal attributable to the conduction EPR of silver crystallites remained. Reaction with Ethylene. The reaction of the Ag6+.8Ag+cluster with ethylene was more rapid than with ammonia; in a typical experiment a sample containing the Ags+43Ag+ signal was exposed to 350 Torr of ethylene at -80 OC. The sample was then allowed to warm to room temperature for 4 min and then recooled to 77 K for examination of the spectrum. After five such cycles (Le., a total time at room temperature of ca. 20 min) the spectrum of Ag6+*8Ag+ had been completely replaced by a new spectrum having quite large, almost isotropic, hyperfine interactions of ca. 300 M H z (lo9Ag) with three silver nuclei (Figure 2). Some anisotropy is apparent, with g,, N 2.025 and g, = 2.0384. As can be seen from Figure 2, the individual lines of the spectrum are quite broad (AHN 25 G, peak to peak), even in a sample prepared from isotopically enriched '"Ag and C2D4. Extra structure is evident, although barely resolved. This is presumably due to superhyperfine structure from other Io9Ag nuclei. In the case of the HH cluster, exposure to 350 Torr of ethylene a t -80 O C resulted in immediate loss of the Ag6X+spectrum and the emergence of a single, broad resonance which may be associated with large silver particles.
Discussion The remarkable feature of our results is the difference in chemical behavior between Ag12-A containing the Ag6+.8Ag+ cluster and that containing the HH cluster. Elsewhere, we have shown evidence that in the HH cluster the cube of eight Ag+ ions in Ag6+.8Ag+ is incomplete, in that one or more of them are missing? This conclusion, based on the superhyperfine structure of the surrounding Ag+ ions, suggests that the reactivity of the HH cluster is due to the accessibility of the Ag6+ moiety to the reagent, via the 6-window from which an Ag+ is missing. Reaction with Ammonia. It would appear from our data that the action of ammonia on Ag,,-A containing the Ag6+.8Ag+ cluster is to leave the Ag6+ moiety intact and to react with the Ag+ ions forming the surrounding cube. We suspected at first that the new spectrum was that of the naked A&+ cluster, stripped by the ammonia of the surrounding cube of Ag+ ions. However, the experiments with the HH cluster convinced us that this was not the case and that a naked Ag6+ cluster would itself be destroyed by ammonia.
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2119 Bearing in mind our belief that the Ag6+.8Ag+ cluster occupies the &cage, that the central hexamer is intact, and that the Ag+ ions occupy the centers of the 6-windows, we concluded that the new spectrum was that of Ag6+-8(Ag(NH3),)+. By this formulation we mean that an unknown number (x) of NH3 molecules have reacted with the Ag+ ions in the centers of the 6-windows, modifying the environment of the Ag6+ cluster. The ground-state symmetry of a bare Ag6+ cluster in the ideal octahedral geometry is 4Al,,. Under this geometrical restriction, all the silver atoms in the cluster are equivalent. The character of the singly occupied molecular orbital (SOMO) is almost 80% Ag 5s and 20% Ag 5p. We have performed a series of calculations varying the Ag-Ag bond distance from 2.66 to 3.10 A and find no dramatic changes in the character of the SOMO. In contrast, the experimental parameters signify a fundamental change in the electronic structure of the radical upon reaction with ammonia: the g factor, formerly greater than the free-spin value, falls well below that value in the ammoniated cluster, and the spin populationg in each Ag 5s orbital increases from ca. 0.10 to ca. 0.12. Our calculations suggest that diffusion of the positive charge of the outer eight Ag+ ions during ammoniation would indeed lead to minor increases in the 5s spin populations of the cluster. However, we are at a loss for an explanation of the dramatic change in g factor. Reaction with Ethylene. We have difficulty offering a definitive identification of the carrier of the spectrum shown in Figure 2. Three silver atoms are clearly indicated by their large hyperfine interactions, although the line shapes strongly suggest the possibility of a more complex cluster. The obvious candidates, Ag, and Ag?', both have g factors less than free spin and can be eliminated on this ground alone. Moreover, Ag,, being a JahnTeller molecule, would be expected to distort away from the equilateral triangular configuration. In fact, both acute- and obtuse-angled forms of Ag, have been The alternative, Ag32+,is an equally well established molecule, which has been reported not only in zeolite matrices but in various frozen solvents.l2 It has hyperfine interactions with three equivalent Io9Ag nuclei of ca. 640 MHz and a g factor of ca. 1.98. Thus, assignment of the spectrum of Figure 2 to either Ag, or Ag32+ would appear to be untenable, and we are obliged to contemplate species of higher nuclearity. We assume that the carrier is derived from Ag3 or Ag?+ with the addition of one or two Ag+ ions and the retention of a threefold axis. These possibilities can be formulated Ag3X+.-yAg+,where x = 0 or 2 and y = 1 or 2. The carrier can thus be chosen from Ag4+, Ag4,+, A g t + , or Ag54+in which there are one or two Ag+ ions lying on the threefold axis. We have carried out ab initio calculations on all four of these molecules, using as a variable r, the distance of the apical atom(s) from the plane of the other three. In the case of Ag4+, the calculations predict that the hyperfine interaction of the unique silver nucleus increases monotonically with r , and at large r the species resembles Agf-.Ag0. There is no r for which the hyperfine interaction of the unique nucleus is less than those of the other three. The opposite is true for Ag?+: the calculations show that the isotropic hyperfine interaction decreases monotonically with r. Tetrahedral Ag4,+ has a 'Al ground-state whose EPR spectrum in A-type zeolites has been claimed by Narayana and KevanI3 and in various frozen matrices by Symons and Alesbury.l2 The Io9Aghyperfine interactions are ca. 350 MHz. At sufficiently large distances r the cluster is best described as Ag,'+-Ag+, and the hyperfine interaction of the unique nucleus is zero, while those of the three equivalent silver nuclei have almost doubled (alogin Ag3'+ = 640 MHz). Throughout this range, however, the g factor (9) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577. (10) Kernisant, K.; Thompson, G. A,; Lindsay, D. M. J. Chem. Phys. 1985, 82, 4739. (11) Howard, J. A,; Preston, K. F.; Mile, B. J . Am. Chem. SOC.1981,103, 6226. .__.
(12) Symons, M. C. R.; Alesbury, C. K. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 3629.
(8) Morton, J. R.; Preston, K. F. Zeolites, 1987, 7 , 2.
(13) Narayana, N.; Kevan, L. J . Chem. Phys. 1982, 76, 3999.
J. Phys. Chem. 1987, 91,2120-2122
2120
remains below free spin, being ca. 1.98 in Ag?+ and ca. 1.97 in Ag43+. These facts enable us to discard distorted Ag4+ and A&3+ from the list of possibilities. We turn to the pentanuclear clusters Ag52+and Ag54+. Our calculations predict for Ag52+very similar behavior to that for A&+: the unpaired spin resides predominantly on the apical nuclei. This conclusion confirms that of Ozin, Mattar, and McIntosh,14 who predicted for Ag52+apical lWAg hyperfine interactions of ca. 600 MHz. Clearly, hyperfine interactions of this magnitude are inconsistent with the observed spectrum, and we can discard the possibility that our spectrum is due to Ag52+. We are left with Ag54+. Our calculations predict, for an equilateral triangular biprism, 2.85 A on a side, spin populations in Ag 5s atomic orbitals of 0.204 for the three equatorial atoms, but only 0.086 for the two apical atoms. Silver atoms ("Ag, s2s1/2)in the gas-phase have a hyperfine interaction of 1977 MHz,15 so that the above spin populations correspond to ca. 400 MHz for the three equatorial nuclei and ca. 170 MHz for the two apical nuclei. These calculations on A g t + are similar in some respects to the predictions of Ozin, Mattar, and McIntosh,14 who obtained (for IWAg)isotropic hyperfine interactions of 350 and 200 MHz respectively for the equatorial and apical nuclei. They also predicted with remarkable accuracy the g factors of Ag54+: gll = 2.015, g, = 2.047. Thus, we see that an assignment of the observed spectrum to A p t + cannot be lightly dismissed: both the predicted g tensor and the hyperfine interactions of the equatorial silvers agree quite well with the measurements. There is, however, a considerable overestimate of the participation of apical Ag 5s atomic orbitals in the SOMO. We estimate that the apical lWAg hyperfine interactions are close to 30 MHz rather than 200 MHz. Spectral simulations suggest g = (2.0384, 2.0384, 2.02), alW(3)
= (317, 305, 305 MHz), alw(2) N (30, 30, 30 MHz), where tensor components are in the order x, y , z with z the direction of the threefold axis. There is, of course, considerable uncertainty in the hyperfine anisotropy extracted from an almost solution-like spectrum (Figure 2). Even slight anisotropy in silver hyperfine interactions translates15 into considerable spin density in atomic 4d or 5p orbitals, however. This must account for the appreciable (50%) deficit in unpaired spin density indicated by the isotropic lo9Ag hyperfine interactions. Since the displacements of the g tensor components from the free-spin value are positive, one may conclude that Ag 4d make more important contributions than Ag 5p to the SOMO. In contrast to the reaction with ammonia, which yielded a hexanuclear adduct, the reaction of the Ag6+.8Ag+ cluster with ethylene appears to result in the formation of a "naked" silver cluster. This conclusion was confirmed by the recent observations of Michalik and Kevan,I6 who obtained the same spectrum directly by H2 reduction or y-irradiation of AgNa-A sieves. Their suggestion that the carrier of the spectrum is Ag3 appears to be untenable however, as we have seen. A comparison of our conclusions with those of Seff and his co-workers indicates that much remains to be understood in the area of silver-zeolite chemistry. Seff s single-crystal X-ray data show conclusively that reaction of the Ag6.8Ag+ cluster with ammonia results in the destruction of the cluster and the formation of N3H3and N3H5. Our results perhaps indicate that the initial step in this process is ammonia attack on the Ag ions in the cube surrounding the Ag6 cluster. With ethylene, on the other hand, our results suggest the formation of a new cluster, probably Ag54+. Seffs data indicate that the Ag6 cluster is intact but that two Ag+ ions have been lost from the surrounding cube of Ag+ ions. Registry No. Ag6+-8Ag+,106928-96-5; Ag?, 106905-98-0; ammonia, 7664-41-7; ethylene, 74-85-1.
(14) Ozin,G. A.; Mattar, S.M.; McIntosh, D. F. J. Am. Chem. Soc. 1984, 106, 7765. (15) Dahmen, H.; Penselin, S.2.Phys. 1967, 200, 456.
(16) Michalik, J.; Kevan, L. J . Am. Chem. SOC.1986, 108, 4247.
The Redox Potential of the Azide/Azidyl Couple Zeev B. AIfassi,+Anthony Hamiman,$ Robert E. Huie, S. Mosseri, and P. Neta* Chemical Kinetics Division, National Bureau of Standards, Gaithersburg, Maryland 20899, and The Royal Institution, London WlX 4BS, England (Received: October 21, 1986)
Pulse radiolysis experiments were carried out with neutral aqueous solutions containing azide with iodide, bromide, or thiocyanate to examine possible one-electron transfer rates and equilibria involving the N3'/N3- couple. The N3 radical was found to oxidize I- with a rate constant of 4.5 X lo8 M-' s-l. No reaction was observed between 12*- and N
