J , Phys, Chem. 1992,96, 5491-5494 (14) (15) (16)
Anderson, M.W.;Kevan, L. J . Phys. Chem. 1987, 91, 4174. Lee, C. W.; Chen, X.; Kevan, L. J . Phys. Chem. 1992, 96, 357. Hathaway, B. J.; Tomlinson, A. A. G. Coord. Chem. Reo. 1970,5,
1.
5491
(17) Hathaway, E. J.; Billing, D.E. Coord. Chem. Rev. 1970, 5, 143. (18) Ichikawa, T.; Kevan, L. J . Am. Chem. Soc. 1981, 103, 5355. (19) Sass, C. E.; Kevan, L. J . Phys. Chem. 1989, 93, 7856. (20) Sass, C. E.; Kevan, L. J . Phys. Chem. 1988, 92, 14.
Kinetlc Models and Estimation of the Constants of Photoinduced Oxygen Isotope Exchange over Semiconductor Oxides. Cases of Two Potasslum Nlobates Girard Mitrat,* URA au CNRS 'Laboratoire d'Automatique et de Glnie des ProcZdZs", UniversitC Lyon I , 69622 Villeurbanne CZdex. France
Henri Courbon, and Pierre Pichat* URA au CNRS "Photocatalyse, Catalyse et Environnement",Ecole Centrale de Lyon, BP 163, 691 31 Ecully CZdex, France (Received: December 17, 1991; In Final Form: February 25, 1992) Kinetic measurements of oxygen isotope exchange (OIE)have been carried out over two UV-illuminated powdered semiconducting potassium niobates, KNb03 and K4Nb6Ol7. For the former material, the kinetic variations of the fraction of I8O atoms in the gas phase showed that as,the content of I8O in the solid surface, remained close to zero because of fast OIE with the bulk; the mathematical treatment under that condition allows one to determine the constants K1,K2, and K,, referring to the three types of OIE mechanisms and accordingly the relative role of each of them. For &Nb6Ol7, the condition a, = 0 was not fulfilled and the mathematical model shows that, in the absence of knowledge of the number of exchangeable oxygen atoms in the solid, the kinetic data can be accounted for by an infinite number of values of K1,K2,and 4. OIE over other semiconductor oxides can be analyzed identically.
Introduction When a semiconductor oxide is illuminated at room temperature with photons of sufficient energy to excite electrons from the valence band to the conduction band, an isotopic exchange can occur between gaseous dioxygen and oxygen atoms of the semiconductor or adsorbed oxygen This photoinduced lability of oxygen, shown by the isotopic exchange, arises from both the weakening of the metal-oxygen bonds due to the removal of valence electrons from the 2p orbitals of oxygen ions of the solid and the change in the adsorption equilibrium of negatively charged adsorbed oxygen species under the action of photon^.^^-^^ In the absence of optical excitation, oxygen isotope exchange (OIE) takes place with a measurable rate only a t temperatures higher than about 0.4 times the Tamman temperature (in kelvin) of the oxide.I5-I8 The extent and mechanism of OIE produced by illumination is an important characteristic of semiconductor oxides, especially because it has been correlated with the photocatalytic properties in oxidation r e a c t i o n ~ . ~ ~ ~ ~ J ~ - ~ ~ OIE over a solid oxide can occur via three mechanisms.I6-I8 In one case, all the isotopic species can come from the gas phase and the solid catalyzes the OIE without participation of its oxygen atoms. This homoexchange corresponds to '80180(g)
+ 160160(g) F! 2160180(g)
(1)
If OIE involves isotopic species from both phases, two mechanisms of heteroexchange are distinguished, depending on whether one or two surface oxygen atoms participate in each act of exchange
+ 180(s) '802(g) + 2160(s) F! 1602(g) + 2180(s)
1802(g) + ' 6 0 ( s ) is 160180(g)
(2) (3)
These equations represent the overall exchanges without any assumption on the elementary steps. These three mechanisms result in different kinetics of OIE. An analysis of the kinetic variations in the various isotopic species of gaseous oxygen has been proposed in the case of thermally activated OIEI6 in order to determine the role of each OIE mechanism for a given system. This analysis was used for simple 0022-3654/92/2096-5491$03.00/0
semiconductor oxides submitted to UV excitation in the presence of gaseous 1802,3-5 In these systems, the mechanism represented by eq 2 was found to be the only one that intervenes. This paper presents kinetic results of photoinduced OIE over more complex oxides. Because these results cannot be interpreted by use of only one of the above OIE mechanisms, the mathematical model has been reconsidered in an attempt to determine the respective role of each of these mechanisms. This mathematical treatment shows that the information that can be drawn from the kinetic measurements depends on whether the exchanged oxygen atoms equilibrate rapidly or not with the ensemble of the oxygen atoms of the solid. The solids chosen are K N b 0 3 and K4Nb6ol7. K N b 0 3 is a semiconductor which absorbs light in the UV region (Figure 1). It is generally nonstoichiometric by lack of oxygen. Its ease of reduction by carbon monoxide shows that its oxygen atoms are not strongly bound. Interest in it stems from its ferroelectric properties. Its crystal structure is almost cubic with the oxygen atoms located near the center of the face^.^^.^^ By contrast, K4Nb6Ol7is a layer compound; the layers are formed by N b 0 6 units with the K+ cations located between them.21 We thought that this difference in structure between KNb03 and K,,Nb6OI7 might give rise to different behaviors in photoinduced OIE. This material is also a semiconductor which can be excited by UV light.22 It has been used as a photocatalyst,22-26either pure or with NiO/Ni or pt deposits, and even as a support for CdS particles, which substantiates the interest of our study.
Experimental Section Apparatus. The static cell had the shape of a cylindrical box 1 cm high closed by two 6 cm diameter parallel optical windows made of Pyrex transmitting wavelengths > 290 nm. A thin layer of sample was deposited on the horizontal lower optical window. This cell was glass-blown to the vacuum system (residual pressure 10-s-lO" Pa) equipped with an oil diffusion pump (Edwards), a Datametric Dresser barocel pressure sensor, and a Riber QMM 17 quadrupole analyzer. The sample was illuminated by a Philips HPK 125-Whigh-pressure mercury lamp, coupled with a water circulating cuvette. The radiant flux received by the sample (-3.7 0 1992 American Chemical Society
5492 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
MBtrat et al.
0
100
200
300
400
500
600
TOO
900
800
'000
!:me T i n
l
.
-. . 1 1 ,.I
Figure 2. Variations in the fractions of gaseous I8O2, 160180, or 160as 2 a function of illumination time in the course of OIE over KNbO,. The solid lines represent the best fitting through the experimental points.
i
'
i
