J . Phys. Chem. 1992, 96, 5488-5491
5488
Solid-State Ion Exchange in H-SAPO-34: Electron Spin Resonance and Electron Spin Echo Modulation Studies of Cu( II)Location and Adsorbate Interaction Maggie Zamadics, Xinhua Chen, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: November 26, 1991;
In Final Form: February 3, 1992) The solid-state reactions of H-SAPO-34 with various copper compounds, such as CuO,CuCI2,and CuF,, are examined in this work. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) techniques are employed to study the location of the Cu(I1) ion exchanged through a solid-state reaction and the interaction of the Cu(I1) ion with water, alcohols, ammonia, and ethylene. The migration of Cu(I1) ions from the exterior surface of the molecular sieve into interior cationic positions is observed with the three cupric compounds investigated. The hyperfine splitting of the Cu(I1) ion does not vary with the type of copper compound employed. The results of this investigation gave similar results to those found for Cu(I1) ion incorporation through an aqueous ion exchange method. The ESR spectra of dehydrated samples exposed to dimethyl sulfoxide,ethylene, ammonia, ethanol, 1-propanol,and water indicate the formation of one type of Cu(I1) complex, while the adsorption of methanol influences the formation of two distinct Cu(I1) complexes. Through analysis of the ESEM signal it was determined that Cu(I1) interacts with one dimethyl sulfoxide molecule, one ethylene molecule, three ethanol molecules, three ammonia molecules, and three water molecules.
Introduction The incorporation of Cu(I1) ions into molecular sieves has generally taken place in a liquid phase.lv2 Recently, Cu(I1) ion incorporation has been reported to occur through a solid-state reaction between a variety of zeolites and various cupric comp o u n d ~ .This ~ ~ ~technique is also effective in the dispersion of transition metals other than Cu(I1) into the zeolite lattice.+' This method is potentially useful for incorporating large transition-metal complexes into the molecular sieve which might be difficult to exchange through conventional methods. Previous electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies have examined Cu(I1) ion location and adsorbate interaction within the SAPO-34 lattice for Cu(I1) ions incorporated by aqueous ion exchange.* In this study the Cu(I1) ions are incorporated by a high-temperature solid-state reaction. This work has investigated the reaction of H-SAPO-34 with various copper compounds, such as CuO, CuCl,, and CuF2. All copper compounds readily supply the Cu(I1) cation into charge-compensating positions within the molecular sieve. The behavior of the Cu(I1) ion upon dehydration and adsorption of various molecules is found to be independent of the source of the Cu(I1) ion. The observed 27Almodulation of freshly prepared samples suggests that the location of the Cu(I1) ion, when incorporated by the solid-state method, is further away from the six-ring window than when Cu(I1) ions are incorporated by liquid-phase ion exchange. Comparison of ESR and deuterium modulation of the ESEM signals of Cu(I1) ions incorporated by liquid-phase and solid-phase ion exchange demonstrates that the Cu(I1) ion location is not dependent upon the method of incorporation.
Experimental Section SAPO-34 was synthesized by the method described by Xu et ale9using triethylamine as the templating agent as described previously.* A hydrated sample of H-SAPO-34 and a copper compound were ground with a mortar and pestle for about 30 min to achieve a homogeneous mixture. The mixture was compressed with a pressure of 5000 lb into pellets of 12-mm diameter and 3-mm thickness. The pellets were broken into small pieces, placed in a ceramic boat, and heated in a muffle furnace. Initially, the samples were heated a t 550 OC, the temperature being raised slowly over a 5-h period and heated at 550 OC for 1 h. This heating period and temperature were sufficient to complete the reaction for H-SAPO-34 and CuC1,. For the reaction of HSAPO-34 with CuO and CuF2 it was necessary to heat to 800 OC to complete the reaction. With these samples the mixtures which were previously heated to 550 OC were returned to the
furnace and slowly heated over an 8-h period to 800 OC and held at 800 OC for 1 h or more. The X-ray powder diffraction pattern of all samples following the high-temperature treatment was recorded. No detectable loss of crystallinity was observed. For all samples 0.06 wt % Cu(I1) ions are incorporated into the molecular sieve. The change in the coordination environment of the Cu(I1) as a function of the dehydration process was monitored with ESR spectroscopy. All samples were dehydrated under vacuum to a residual pressure of about lo4 Torr. Samples dehydrated to temperatures higher than 300 OC were heated in 760 Torr of oxygen for 2 h and then evacuated a t that temperature. The interaction of Cu(I1) ion with various adsorbates was also examined. Before exposure to the adsorbate, the samples were dehydrated at 400 OC in flowing oxygen for 4 h and then evacuated to remove the oxygen. Samples treated in this manner are referred to as activated. Liquid adsorbates were exposed to the activated samples overnight at their room temperature vapor pressure, and gaseous adsorbates were exposed at 200 Torr. The types of adsorbates studied included DzO, ND3, ISNH3,CzD4,CH30D, CH3CH20D,CH3CH2CH20D(Cambridge Isotope Laboratories), and (CD3),S0 (Aldrich). ESR spectra were recorded at 77 K on a modified Varian E-4 spectrometer previously described.'O ESEM signals were recorded at 4 K on a home-built ESE spectrometer which is described elsewhere.]I~'*In a three-pulse experiment a pulse sequence of 9O0-~-9Oo-T-9O0 is employed.and T is swept. To suppress modulation from zeolitic aluminum, T was fixed at 0.28 c(s. The ESEM data were transferred to a HCP386 IBM PC compatible computer for data analysis. Simulation of the deuterium modulation observed in the ESEM signal was performed using the analytical expression derived by Dikanov et al.I3 Data were extracted from the modulation pattern by comparing the experimental ESEM signal with the calculated signal. The best fit was found by varying the following parameters: the number of equivalent interacting nuclei N , the interaction distance R,and the isotropic hyperfine interaction Ab, until the sum of the squared residuals was minimized. Results Solid-stateReaction. Prior to heating SAPO-34 and the copper compound no ESR signal is observed due to isolated Cu(I1) ions. Heating to 550 OC is sufficient to generate the diffusion of Cu(I1) ions into the interior of the molecular sieve from CuCl,, CuO, and CuF2 compounds. An increase in the reaction temperature to 800 OC generates an increase in the ESR signal intensity for the samples doped with CuO and CuF,. No change of the hyperfine splittings in the ESR signal is observed with this tem0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96,No.
Solid-state Ion Exchange in H-SAPO-34
CUH-SAPO-34t CZD,
TABLE I: ESR Parameters at 77 K for Cu(I1) Doped via a Solid-State Reaction with CuCll in H-SAPO-34 Molecular Sieve AI,
sample metreatment fresh evac 115 O C evac 420 OC +methanol +ethanol +propanol +ammonia +ethylene +dimethyl sulfoxide
glib 2.382 2.380 2.338 2.341 2.379 2.381 2.414 2.373 2.375 2.376 2.357 2.381
(104
143 135 162 147 129 139 122 146 146 171 150 144
gLb
2.074 d d
-
d d d d
x A
2.063 2.068 2.046 2.055 2.074
a evac = evacuation. Estimated uncertainty is h0.005. Estimated uncertainty is *0.005. dNot resolved.
''
The ESR and ESEM signals of the samples doped by different copper compounds give rise to similar signals. To verify the diffusion of Cu(I1)ions into the interior surface of the molecular sieve a sample was dehydrated and then allowed to adsorb benzene which is too large to diffuse into the cavities of SAPO-34. Cu(I1)has been reported to interact with benzene;I4 thus, if Cu(I1)is located on the exterior surface, we should observe a change in the dehydrated ESR signal. The absence of this interaction through analysis of the ESR spectrum indicates that Cu(I1)is not located on the exterior surface. ESR. All ESR spectra and ESEM signals illustrated in this work were obtained from a sample of H-SAPO-34 and CuCl, heated to 550 OC. These signals are typical of those recorded from samples where the source of Cu(I1) ions was CuO and CuF,. Table I summarizes the Cu(11)magnetic parameters acquired from the samples after various treatments. Many of these ESR parameters are similar to those of liquid-phase ion-exchanged samples. Dehydration of a sample generates two Cu(I1)complexes whose ESR parameters differ from those found in the hydrated sample. The Cu(I1)species observed in a hydrated sample has ESR parameters of g,,= 2.382, A,,= 0.0143 cm-', and g, = 2.07, while the parameters for the Cu(I1)species generated by dehydration at 415 "C are g,,= 2.341, A,, = 0.0147 cm-' and gll = 2.37, A,, = 0.0129 cm-l. The ESR signal of the fresh sample is completely regenerated by exposing the dehydrated sample to the vapor pressure of water at room temperature. The adsorption of I5NH3onto the activated sample produces a single Cu(I1)complex with g,,= 2.239 and A,, = 0.0181 cm-l. The second-derivative ESR signal reveals four hyperfine splittings with relative intensities of 1:3:3:1. The observation of the hyperfine lines is indicative of direct interaction of the cupric ion with the nitrogens. The number and intensity ratio of the hyperfine lines suggest the coordination of three ammonia molecules to the Cu(I1). The adsorption of methanol generates two distinct Cu(I1) complexes while ethanol and propanol adsorption produces only one Cu(I1)species. The ESR parameters of the Cu(I1)species formed by adsorption of methanol are g,,= 2.381, A,, = 0.0139 cm-'and g,,= 2.414, All = 0.0122 cm-l. The adsorption of ethanol and propanol gives rise to similar ESR signals which have gll = 2.373 and A,, = 0.0146 cm-I. When dimethyl sulfoxide is adsorbed by a dehydrated sample, one Cu(I1)complex is formed. The gl,value of this Cu(I1)species is 2.381, and the All coupling constant is 0.0144 cm-l. The exposure of a dehydrated sample to ethylene for 48 h produces one Cu(I1)species as illustrated in Figure 1. The ESR parameters for this sample are gll = 2.360 and A,, = 0.0148 cm-I. These parameters differ significantly from those found for Cu(I1) ion exchanged by the liquid-phase method. H E M . The simulation parameters of the deuterium modulation observed in the ESEM signals from samples with adsorbed water, ethanol, dimethyl sulfoxide, ammonia, and ethylene are
gIl = 2.357
200 G
Figure 1. ESR spectra of CuH-SAPO-34 recorded at 77 K after the adsorption of C2D4.
perature increase. The integrity of the structure is maintained
upon heating to 800 OC as verified from X-ray powder diffraction.
M
'bt
1.0r I
CUH-SAPO-34+ C2D4 N=4 R = 0.33 nm ' Ais0 = 0.1 1 MHz
L r
I
g 0.2 Y
I
0
2
1
4
3 T, ~.ls
5
-
Figure 2. Experimental (-) and simulated (- -) three-pulse ESEM spectra of CuH-SAPO-34 with adsorbed C2D4. Spectra recorded at 4 K with T = 0.28 ps. TABLE II: ESEM Simulation Parameters for Cu(I1) Doped via a Solid-state Reaction with CuCI2 in H-SAPO-34 Molecular Sieve
adsorbate D20 ND3 0
4
(CD3)$O CH3CH20Dd
no. of adsorbate (MHZ)~ molecules Ai,
Na
R (nm)b
6 9 4 6 2 1
0.28 0.30 0.33 0.43 0.28 0.32
0.21 0.20 0.11 0.14 0.04 0.10
3 3 1 1 2 1
"Number of nuclei. bCu2t-D distance. Isotropic hyperfine mupling constant. dTwo-shellmodel required.
summarized in Table 11. Through deuterium modulation analysis Cu(I1)ion was determined to coordinate directly to three water molecules and three molecules of ammonia and indirectly to one dimethyl sulfoxide adsorbate. The best fit simulation of a ESEM signal for a sample after ethanol adsorption required a two-shell model. The simulation parameters suggest that two ethanol molecules are directly coordinated to the Cu(I1)ion at a distance of 0.28 nm while a third molecule is indirectly coordinated to Cu(I1) at a distance of 0.32 nm. The ESEM signal acquired for a sample with adsorbed ethylene is shown in Figure 2. The calculated parameters indicate the interaction of one ethylene molecule with a Cu(I1)-D distance of 0.33 nm. 27Almodulation from zeolitic aluminum was monitored in a three-pulse experiment from the hydrated sample by setting T = 0.40 ps, which is the ideal value for recording the deepest 27Al modulation." Pulse widths were increased to 78 ns to suppress deuterium modulation. Figure 3a illustrates the 27Almodulation from a fresh sample where the Cu(I1)ion doping was achieved
5490 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
P 0.6
Y
Zamadics et al. which is significantly shorter than that observed in ion-exchanged samples. The orientation of the ethylene molecules is similar to that found in ion-exchanged samples. The Cu(I1) ion interacts with four equivalent deuteriums which suggests that the ethylene molecule interacts with the Cu(I1) cation with its molecular plane perpendicular to a line toward the Cu(I1) ion. Cu(11) ions doped in zeolite A have a similar ethylene orientation.l* A comparison of the 27Almodulation from freshly prepared samples of liquid- and solid-phase Cu(I1) ion incorporation suggests that the location of the Cu(I1) ion relative to the framework differs slightly in the two samples. With the solid-phase method of incorporation the 27Almodulation is shallower than with the liquid-phase incorporation method, and the modulation extends to longer values of T. This type of modulation is common when the magnetic nucleus is not in close proximity to the paramagnetic species. The 27Almodulation observed in an ion-exchanged sample gives rise to a deeper modulation depth which dies out rapidly. This behavior is indicative of paramagnetic species located close to magnetic n~c1ei.I~ The observance of 27Almodulation indicates that in both cases the Cu(I1) ion is within 0.5 nm of framework aluminum.20 The different features of the ESEM signals suggest that in the solid-phase incorporation the Cu(I1) ion migrates to a location further away from the six-ring window of the SAPO-34 compared to the liquid-phase method.
-
0
1
3
2
4
5
T, PS
Figure 3. Three-pulse ESEM spectra of CuH-SAPO-34 at 4 K showing modulation: (a) Cu(I1) ion incorporated by a solid-state reaction; (b) Cu(I1) ion incorporated by aqueous ion exchange. Spectra recorded at 4 K with T = 0.40 ps.
through a solid-state reaction. For comparison, 27Almodulation from a freshly prepared, ion-exchanged sample is shown in Figure 3b. 27Almodulation is apparent in both samples.
Discussion This work illustrates that the introduction of Cu(I1) ions into the cationic positions of H-SAPO-34 is accomplished through a high-temperature solid-state reaction. The Cu(I1) ion ESR signal observed is independent of the type of copper compound employed. It is also determined that for complete migration of cupric ions from the exterior surface it is necessary to increase the reaction temperature to 800 OC when the source of the Cu(I1) ion is CuO or CuF2 which was also observed by Kucherov et aL3 and Lee et al.ls Dehydration Process. The Cu(I1) ion ESR parameters determined at various stages of the dehydration process in this study do not vary significantly from those parameters found for Cu(I1) ion incorporated into H-SAPO-34 by liquid-phase ion exchange. This suggests that the method of incorporation does not influence the location of the Cu(I1) ion upon dehydration. In a hydrated sample the ESR signal at room temperature or 77 K shows one species with octahedral coordination. Evacuation at room temperature is sufficient to remove some of the water ligands as indicated by the formation of a second Cu(I1) species in the ESR signal. Dehydration to a final temperature of 400 OC produces two distinct Cu(I1) species. Adsorbate Interaction. The ESR parameters of D20, CH$H,OD, CH3CH2CH20D,(CD,),SO, and ND3 are suggestive of a Cu(I1) species with axial symmetry.I6J7 These adsorbates facilitate the migration of the Cu(1I) cation into a single location. From the fitting of the deuterium modulation of the ESEM signals it is determined that the Cu(I1) ion interacts directly with three water molecules and three ammonia molecules and indirectly with one dimethyl sulfoxide molecule and with three ethanol molecules, two of which interact a t a closer distance than the third. These features are also common in samples where the Cu(I1) ion is incorporated by liquid-phase ion exchange. The location of the Cu(I1) ion after exposure to these adsorbates is most probably site I. Site I will allow for the maximum coordination number and be the most accessible site to the adsorbates. Methanol adsorption produces two distinct Cu(I1) complexes, which is also observed for a sample where the Cu(I1) ion is incorporated by a liquid-phase technique. This adsorbate is smaller than the other two alcohols, and thus it may act to draw the cupric ion into a location which is not feasible for these adsorbates. The rate at which ethylene equilibrates with an activated sample is slow relative to the rate for water and alcohol adsorbates. This is also observed in liquid-phase, ion-exchanged samples. The analysis of deuterium modulation from the ESEM signal indicates that the Cu(1I) ion interacts with four deuteriums at a distance
Conclusions This work illustrates that Cu(I1) ions migrate from the exterior surface of the molecular sieve into the interior cationic positions by a high-temperature treatment. It was also found, with the exception of slight differences, that the location of the Cu(I1) ion is independent of the method of incorporation. The adsorbate interactions, with the exception of ethylene, are also unaffected by the method of Cu(I1) ion incorporation. In both the liquid and solid ion-exchanged samples the Cu(I1) ion is found to interact directly with three molecules of water and three ammonia molecules and indirectly with one dimethyl sulfoxide molecule. In a sample with adsorbed ethanol it is determined that the Cu(I1) ion interacts directly with two ethanol molecules and indirectly with one molecule of ethanol. The adsorption of ethylene gives rise to different ESR parameters and ESEM simulation parameters. The ethylene molecule maintains the same orientation as that found in the aqueous ion exchanged sample but interacts at a significantly shorter distance. From the *’A1 modulation it is suggested that the Cu(I1) ion incorporated through a solid-phase method is located further away from a six-ring window into the ellipsoidal cavity relative to the Cu(I1) ions in the aqueous ion exchanged sample. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program. Registry No. DMSO, 67-68:5;’ Cu, 7440-50-8; CuO, 1317-38-0; CuCI,, 7447-39-4; CuF2, 7789-19-7; water, 7732-18-5; ammonia, 766441-7; ethanol, 64-17-5; methanol, 67-56-1; ethylene, 74-85-1.
References and Notes (1) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 3206. (2) Lee, H.; Narayana, M.; Kevan, L. J . Phys. Chem. 1985, 89, 2419. (3) Kucherov, A. V.; Slinkin, A. A. Zeolites 1986, 6, 175. (4) Kucherov, A. V.; Slinkin, A. A,; Kondrat’ev, D. A.; Bondarenko, T. N.; Rubinstein, A. M.; Minachev, Kh. M. Zeolites 1985, 5, 320. (5) Sass, C. E.; Chen, X.; Kevan, L. J . Chem. Soc., Faraday Tram. 1990, 86, 1989. (6) Kucherov, A. V.; Slinkin, A. A. Zeolites 1988, 8, 110. (7) Kucherov, A. V.; Slinkin, A. A. Zeolites 1987, 7, 38. (8) Zamadics, M.; Chen, X.; Kevan, L. J . Phys. Chem., submitted for publication. (9) Xu, Y.; Maddox, P. J.; Couves, J. W. J . Chem. SOC.,Faraday Trans. 1990, 86, 425. (10) Chen, X . ; Kevan, L. J . Am. Chem. Soc. 1991, 113, 2861. (11) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley Interscience: New York, 1979; pp 279-341. (12) Kevan, L. In Modern Pulsed and Continuous-Wave Electron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990; pp 231-266. (13) Dikanov, S.A,; Shubin, A. A,; Parmon, V. N . J . M a p . Reson. 1981, 42, 474.
J , Phys, Chem. 1992,96, 5491-5494 (14) Anderson, M.W.;Kevan, L. J . Phys. Chem. 1987, 91, 4174. (15) Lee, C. W.; Chen, X.; Kevan, L. J . Phys. Chem. 1992, 96, 357. (16) Hathaway, B. J.; Tomlinson, A. A. G. Coord. Chem. Reo. 1970,5, 1.
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(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, 69131 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) + 2160(s) F! 1602(g) + 2180(s)
1802(g) + '802(g)
'60(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
