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J. Phys. Chem. 1993,97, 5211-5212
Characterization of Copper Ion Catalyst Encapsulated in Si02 Matrices by the Sol-Gel Method and Their Photocatalytic Activity for Decomposition of NO into N2 and 0 2 at 275 K Nobuaki Negishi, Masaya Matsuoka, Hiromi Yamashita, and Masakazu Anpo' Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Gakuen-cho. Sakai, Osaka 593, Japan Received: February 4, 1993; In Final Form: March 30, I993
Copper(1) ions highly dispersed in Si02 matrices are prepared by a combination of sol-gel processes and the reduction treatment with H2. These Cu+ species encapsulated in Si02 exhibit a stronger photoluminescence at around 515 nm with a longer lifetime and higher photocatalytic activity for the direct decomposition of N O into NZand 0 2 at 275 K as compared to those of the Cu+/SiO2 catalyst prepared by an ion-exchange method.
Anchored copper ion catalyst, especially, the ion-exchanged copper/ZSM-5 catalyst,] has been given a great deal of attention because of its potential utilization as a catalyst for the direct decomposition of NO into N2 and 0 2 . On the other hand, we have already reported that the Cu2+ions supported onto SiOz2 or ZSM-53 prepared by an ion-exchange method are easily reduced to Cu+ when the samples are evacuated at temperatures higher than 573 K and that the Cu+ ion formed in this way decomposes NO molecules photocatalytically and stoichiometricallyinto N2 and O2 even at 275 K. Recently, a novel synthetic method for the preparation of the glasslike solid materials, the sol-gel method, has also attracted interest because the materials prepared by this method exhibit unique photophysical and photochemical properties due to their photostability and tran~parency.~ Also, the sol-gel method has two major advantages: energy conservation due to roomtemperature polymerization and the ability to prepare various kinds of materials by a suitable choice of starting monomers. Therefore, although this sol-gel method is expected to become a new and dramatic method for preparing active and selective photocatalysts, there has not yet been any report concerning the characterization and photocatalstic activity of the catalysts prepared through sol-gel processes except for the report done by Avnir et al.s In the present work, wedeal with thecharacterizationof copper(I) ion catalysts encapsulated in Si02 matrices that are prepared by a combinationof the sol-gel process and the reduction treatment with H2, and their photocatalytic activity for the direct decomposition of NO into N2 and 0 2 at 275 K by means of ESR and dynamic photoluminescence measurements, as well as by the analysis of reaction products. Cu2+/SiO2samples were prepared by the sol-gel method from a mixture of tetraethyl orthosilicate (40 mL), Cu(NO3)2*3H20 (0.25 g) in ethanol (15 mL) and water (2 mL). The Cu2+/Si02 gels were obtained by keeping the mixture at room temperature for 10 days. Thus formed Cu2+/Si02gels (0.59 Cu wt % as metal) were washed by sufficient amounts of boiled water for 9 h and calcined at 773 K for 6 h. The BET surface area of Cu2+/ Si02sample was 120 m2/g. Photoreactions were carried out at 275 K using a Toshiba mercury lamp (SHLS-1002B) (A > 280 nm) and the products were analyzed by gas chromatography. The Cu2+/SiO2 sample shows a characteristic ESR signal assigned to the Cu2+ions. This signal always has thesame nature withgll> g, > g,, indicating that the Cu2+ions exist in an axially symmetricenvironment of tetragonal symmetry.6 Figure la shows the changes in intensity of the ESR spectrum of the Cu2+/Si02 sample with increasing the temperature of the H2 reduction treatment. As shown in Figure 1, before reduction the original Cu2+/SiO2sample exhibits a high intensity in its ESR spectrum 0022-3654/93/2097-5211$04.00/0
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Figure 1. (a) Change of the intensity of the ESR signal of the Cu2+/Si02 sample with increasing the temperature of the H2 reduction treatment and (b) the change of the intensity of the photoluminescence spectrum of the Cu+/SiOZ catalyst with the temperature of the H2 reduction treatment (ESR and photoluminescence spectra were recorded at 77 K).
due to the Cu2+ions. However, with increasing the reduction temperature the intensity of the ESR signal decreases without anychangesin theshapeofthesignal, suggesting that thereduction of Cu2+ions has occurred. After the reduction treatment with Hz at temperatures higher than 473 K, a photoluminescencespectrumof thesample becomes observable at around 5 15nm when it is excited by 300-nm beams. Only the reduced Cu+/Si02sample exhibits the photoluminescence, being attributed to the radiative decay process from the electronically excited state of the (Cu+...Cu+) dimer species, i.e., (4sa) state.2.7 Figure l b shows the changes in intensity of the photoluminescence due to the (Cu+--Cu+) dimer species with increasing the temperatures of the H2 reduction. The intensity of the photoluminescence increases with increasing the reduction temperature. From these results obtained by ESR and photoluminescence measurements it is concluded that the Cu2+ions in the Cuz+/SiO2 sample are reduced into the Cu+ ions by the H2 reduction treatment. The Cu+/SiO2 sample prepared through the sol-gel process was found to exhibit a higher photoluminescence yield with a luminescent lifetime 10 times longer as compared to those of the catalyst prepared by the ion-exchange method. Figure 2 shows the effect of the addition of NO onto the photoluminescencespectrum of the Cu+/SiO2sample. As shown in Figure 2, the photoluminescence is quenched in its intensity and lifetime by added NO molecules, the extent depending on the pressure of added NO. These results clearly indicate that the Cu+ sites are accessible to added NO and NO molecules interact with the (Cu+--Cu+) dimer species not only in the ground state but also in the excited state. Just after the complete quenching of the photoluminescenceby the added NO, the evacuation of the 0 1993 American Chemical Society
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The products were N2 and 0 2 , and the formation of N20 and NO2 were minor and negligible. These results clearly indicate that the Cu+/Si02 catalysts decompose NO molecules photocatalytically and stoichiometrically into N2 and 0 2 even at 275 K. The Cu+/SiO2 catalyst exhibits a somewhat higher photocatalytic activity than that of the ion-exchanged Cu+/Si02 catalysts: which may be attributed to the fact that the Cu+species are stably encapsulated in Si02 with a high dispersion and that these Cu+ species located in diversified small fractal pores have a very weak interaction between these Cu+ species. From these results, the following is proposedfor the mechanism of the photocatalyticdecompositionof NO at 275 K on the Cu+/ Si02 catalyst prepared through the sol-gel process and H2 reduction treatment. During the H2 reduction, Cu2+ ions are reduced to Cu+ ions with migration in Si02 matrices, resulting in the formation of the (Cu+-Cu+) dimer species. When NO molecules are introduced onto the (Cu+.-Cu+) dimer species, a decoupling of the Cu+-.Cu+ interaction occurs to form nitrosylic adduct species (Cu+-NO). UV irradiation of the catalyst leads to an electron transfer from the (3d'O) electronic state to the (3d94s1)electronic state of the Cu+ ion which constitutes the Cu+--NO adduct. A local charge separation then occurs, Le., an electron transfer from (3d94s1)state of the electronically excited Cu+ ion to the anti-*-bonding orbital of NO molecule occurs. This electron transfer plays a significant role in the weakening of the N-O bond and initiates the decomposition of NO molecule. The study of the coordination natures of the Cu+/Si02catalyst and the detailed mechanism of the photocatalytic decomposition of NO will be the object of future work.
A,=153 G Figure 3. ESR spectrum obtained after the addition of N O onto the Cu+/SiO* catalyst (ESR spectrum was recorded at 77 K).
system was found to lead to a complete recovery of the photoluminescence in its intensity and lifetime, suggesting that the Cu+ ions in the Cu+/SiO2 sample exist stably in the Si02 matrices with high transparency and the interaction with NO is weak and does not accompany any oxidation of the Cu+ ion to the Cu2+. Figure 3 shows the ESR signal due to the NO species adsorbed on the Cu+/Si02 catalyst at 77 K. The signal consists of four equally spaced components indicating that the NO molecules are adsorbed on the Cu+ ( I = 3/2) to form a nitrosylic adduct on Cu+ ($can be written as CU+-*NO).~*~,~ Although at present it is not clear why only the (Cu+-.Cu+) dimer species are formed in Si02 matrices during the preparation through the sol-gel process and the H2 reduction, the addition of NO onto the Cu+/Si02catalyst leads to a decoupling of the Cu+--Cu+ interaction to form a nitrosylic adduct on Cu+. UV irradiation of the Cu+/Si02 catalyst in the presence of NO at 275 K was found to lead to the decomposition of NO molecules with a good linearity against the UV irradiation time.
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