555
COMMUNICATIONS TO THE EDITOR
Intermediate of Oxygen Exchange Reaction over Illuminated Titanium Dioxide
Sir: In the past several years, three forms of oxygen, 0 - , Oz-, and 0 3 - ,have been observed on oxides by the esr technique,l but the reactivity or selectivity of these species have not been firmly characterized. The author has shown that 0- is the reactive intermediate in the catalytic oxidation of carbon monoxide over Zn02*3and 0 2 is less reactive in the oxidation as well as in the isotopic exchange of oxygen between 0 2 - and CO, and 0 2 - and C02.4 Naccache has also shown the high reactivity of 0 on MgO for 0 2 , CO, and C2HdU5 This communication suggests an 0 3 - intermediate in the photocatalytic exchange reaction of l6Oz and l8O2 over illuminated TiOz. The isotopic analysis of the gasphase oxygen as well as the desorbed oxygen were carried out simultaneously by connecting the reactor directly to the mass spectrometer. Rutile type Ti02 (1.50 g of Titanox RA-10 from Titanium Pigment Corp.) was mounted in a Pyrex glass reactor, evacuated a t ca. 410" for more than 10 hr, and cooled to room temperature in uacuo, after which l s 0 z of 0.6 cm pressure was first adsorbed at room temperature for 6 hr and then removed by 30 min of evacuation. l6O2 of 1.0 cm pressure was added to the above l 8 0 2 preadsorbed Ti02 for 1.5 hr a t room temperature and then removed by 30 min of evacuation. The reactor was connected to the mass spectrometer for isotopic analysis of the desorbed oxygen, which has a maximum around 180" and has been identified as 0 2 - by esr. The isotopic composition of the desorbed oxygen from Ti02 changes with increasing temperature and approaches to a uniform composition at higher temperatures as shown in Figure 1. This is the same as has been observed on Zn0,4 suggesting the heterogeneity of the adsorption strength of 0 2 - ; adsorbed oxygen which will desorb at the lower temperature takes much perturbation from the gasphase oxygen. A mixture of l s O ~and l6O2, each of 1 mm pressure, was admitted to the Ti02 on which l8O2- and l602- had been coadsorbed, and was subjected to illumination with a medium pressure mercury lamp (Toshiba H-400-P) at room temperature. The reactor was shaken during illumination so that the Ti02 powder may be exposed to light homogeneously. Figure 2 gives a typical result of the isotopic analysis of gas-phase oxygen and of desorbed oxygen. The isotopic exchange reaction of gas-phase oxygen is so enhanced by illumination that equilibrium has been established within 3 min of illumination. The exchange reaction over Ti02 in the dark a t room temperature is slow compared with that under illumination and illumination without Ti02 gives no exchange. Accordingly, the rapid exchange observed in gas phase under illumination is undoubtedly a photocatalytic reaction taking place over TiOz. After the 3 min illumination, the gas-phase oxygen was removed by evacuation and the thermal desorption was carried out. The iso-
100
0
200
Temperature ("0 Figure 1. Isotopic composition of desorbed oxygen from T i 0 2 which has been exposed to I*O2 and then to 1602. The dotted line is the approximate amount of desorption.
1
Photocatalysis
Thermal Desorption
I
0
2
3
Illurninorion Time irnin)
50'
70'
-70°C -llO"C
110"
-140T
140' -180°C
Figure 2. The change of isotopic composition of gas-phase oxygen under illumination and that of desorbed oxygen from T i 0 2 after 3 min of illumination. The dotted lines show the equilibrium composition.
topic composition of the desorbed oxygen is unambigously far from the gas-phase equilibrium composition. The fraction of l8O in the desorbed oxygen increases with desorption temperatures as observed in the desorption experiment shown in Figure 1. From the results of the thermal desorption, it is obvious that the oxygen with maximum desorbtion around 180", Oz-, has not been the intermediate species of the homomolecular oxygen exchange reaction over the illuminated TiOz. Accordingly, either the dissociative or the associative mechanisms, such as (i) 0 2 - e 2 0 - or (ii) 0 2 + 0 2 - e 0 4 - , including 0 2 - should be ruled out. The 0 3 - species have been directly detected by Tench and Lawson6 and Lunsford and Wong7 over uv irradiated MgO, and 0 3 - over uv irradiated MgO appears inactive for the exchange reaction. However, Kazansky, et a1.,8 have suggested the 0 3 - intermediate in the homomolecular oxygen exchange reaction on vanadium oxide supported on silica by reason of unstability of the 0 3 - species. Accordingly, we may conclude that the homomolecular oxygen exchange reaction taking place over illuminated Ti02 proceeds via weakly held 0 3 - intermediates formed from 0 2 and 0-, because the desorption of 0 - is implausible at room temperature. This result perhaps reveals the important role of 0 3 - in catalytic oxidation particularly in photocatalytic oxidation over TiO2. The Journal of Physical Chemistry, Vol. 78, No. 5 , 1974
556
Communications to the Editor
References and Notes (1) J. H. Lunsford,Catal. Rev., 8, 135 (1973). (2) K. Tanaka and G. Blyholder. Chem. Cornmun., 736 (1971); J. Phys. Chem., 76, 1807 (1972). (3) K. Tanaka and G . Blyholder, Chem. Comrnun., 1343 (1971): J. Phys. Chem., 76, 3184 (1972). (4) K . Tanakaand K. Miyahara. Chem. Commun., 877 (1973). (5) C. Naccache, Chem. Phys. Lett., 11, 1323 (1971). (6) A. J. Tench and T. Lawson, Chem. Phys. Lett., 7,459 (1970). (7) N. B. Wong and J. H. Lunsford. J. Chem. Phys., 56, 2664 (1972). (8) V. B. Kazansky. V. A. Skvets. M. Ya Kon, U. U. Nikisha, and B. N. Shelimov, Proc. 5th Int. Congr. Catal., 104 (1972)
Research Institute for Catalysis Hokkaido University Sapporo, Japan 060
Ken-ichi Tanaka
Received October 5, 7973
Solvation Numbers in Nonaqueous Solvents
Sir: In a recent paper,l Della Monica and Senatore state that in methanol monovalent cations are more solvated than divalent cations of comparable crystallographic radii. This statement was based on the Stokes radii, as calculated from conductance data2 by means of eq l.3 This conr , = 0.821ZI/(X+")q (1) clusion is contrary to that expected on the basis of Coulombic theory4 as well as the results found by this method for mono- and divalent cations in aqueous solution^,^ and is, in fact, so unexpected that it casts doubt on the estimation of solvation numbers from conductance data. Because of these discrepancies and the possibility that nonaqueous solvents, or a t least methanol, behave anomalously in their interactions with cations it seemed of value to reexamine this method of calculating solvation numbers for divalent cations in as many nonaqueous solvents as data were available. In Table I are given the X o values and the calculated Stokes radii for a number of mono- and dipositive ions in methanol, acetonitrile, propanol, and acetone. Also given
in this table are the corrected radii obtained from the Stokes radii by assuming that the crystallographic radii of the larger tetraalkylammonium ions represent their true radii in solution ( i e . , that they are unsolvated).6 From these data, it can be seen that both the Stokes and corrected radii for the divalent ions studied are greater than those for monovalent ions in all solvents. This is in agreement with expectations of a greater degree of solvation for more highly charged species and indicates that these solvents behave no differently, in this regard a t least, than water. Because the results obtained here for methanol are based on the same conductance data as those of Della Monica and Senatore, it would seem that their unusual conclusion was due to the omission of a factor of 2 (for the charge of the alkaline earth ions) in eq 1. The apparent correlation between the extent of solvation and the charge density is further substantiated by the decrease in corrected radii (and therefore in solvation) in a given solvent which is found with increasing crystallographic radii for ions of like charge. In an attempt to compare the solvation numbers found by this method with those obtained by other methods, eq 2 was applied to the data for Mg2+, using values of 0.65 A
for rcryst for Mg2+ and 50, 68, 170, and 145 A3 for the volumes of the methanol, acetonitrile, propanol, and acetone molecules, respectively. The values of h so obtained (15,14, 14, and 16, respectively) are remarkably similar although they are very dependent on the molecular volumes assumed. The value of 15 obtained for methanol is much greater than the value of 6 measured by means of nmr.8 A comparable difference occurs for the hydration and numbers of Mg2+ found from conductance ( h = nmr ( h = 6)1° data and is most likely due to the fact that the nmr results reflect the number of solvent molecules in the first solvation sphere alone,ll while conductance measurements lead to the inclusion of a t least one additional layer of the solvent sheath. In conclusion, this study gives no indication that the solvation numbers obtained from conductance data for cations in nonaqueous solvents are in any way anomalous, increasing as they do with increasing charge density of the
TABLE I: Conductivities, Stokes Radii, and Corrected R a d i i for Ions in Different Solvents
Li
3.786 3.280 2.786 2.406 2 .248 2.576 3.415 4.063 5.205
Sr2
39.6a 45.7" 53.8~ 62.38 66 . 7 b 58.2b 43.9b 36.9b 57.6' 6O.Oa 59 .Ou
5.082
5.62 5.50 5.54
Ba2+ Zn2+
59.60
5.030
5.52
+
Na K+
+
cs
+
Me4N
+
EkN+ PrlN+ BurN+ Mg2+ Ca2 +
5.000
4.73 4.425 4.122 3.89
79.gC 76.9~ 83.4. 97.6~ 94.zd 83.7d 69.6d 61.3d 94.8~
2.966 3.082 2.842 2.428 2.516 2.831 3.405 3.866 5.00
4.137 4.238 4.00 3.625
6.01
10.32' 12.45e
4.071 3.374
14.40, 2.917 15.05, 2.791 12.19' 3.446 10.17/ 4.131 9.400 8.938
4.918 4.418
8.280
72.8" 3.717 77.4gh 3.492 80.6h 3.358 96.63& 89.49' 75 .09$ 66.40' 70.20 83.60
2.801 3.024 3 ,604 4.076 7.710 8.188 6.474 7.05
8 5 . 0 ~ 6.368 94.8~ 5.00
4.625 4.42 4.294
6.950
6.01
W. Libus and H. Strzelecki, Electrochim. Acta, 17, 1749 (1969). a Reference 2.0 * E . C. Evers and A. G . Knox, J . Amer. Chem. Soc., 73, 1739 (1951). A. H. Harkness and H. M . Daggett, Can. J . Chem., 43, 1215 (1965). e T.A. Gover and P. G . Sears, J . Phys. Chem., 60, 330 (1956). f D. F. Evans and
P. Gardam, J . Phys. Chem., 7 2 , 3281 (1968). Kraua, J . Amer. Chem. Soc., 73, 3293 (1951).
0
*
P. Van Rysselberghe and R. M. Fristrom, J . Amer. Chem. SOC.,67,680 (1945). M. B. Reynolds and C. A . D. F. Evans, J. Thomas, J. Nadas, and M . A. Matesich, J . Phys. Chem., 75, 1714 (1971).
The Journal of Physical Chemistry, Vol. 78, No. 5. 1974
