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u)
100 200 Reaction Temperature / 'C
Figure 13. Effect of oxygen on the catalytic NO-NH3 reaction over Cu(I1)NaY. Feed: NO (0.4%)-"3 (0.4%)-02 (variable)-He (balance); concentration of 02:(1)470, (2) 2%, (3) 0.3%, (4) 0%; contact time: 1.0 g-s/cm3.
NO + NH3 + 114 02
-
N2 t 312 H20
F i g u r e 14. Reaction mechanism of the NO-NH3 reaction over Cu(I1)NaY in the presence of oxygen.
enables the NOz coordination again to form a nitrito type. Based on these results, we propose the mechanism in Scheme I for each step of the NO-NH3 reaction. We consider that n and m are probably equal to 4 and 2, respectively. In the working state, step I and 11, i.e., reduction and oxidation of Cu ions, are combined in a stationary state. The rate of step I1 becomes smaller with increasing temperature as described before. We presume that the rate decrease in caused by an increasing difficulty of NH, coordination to Cu(1) ions. Although this phenomenon is disadvantageous for NO removal, the drawback
would be remedied if the reoxidation step of Cu ions could be promoted effectively. Fortunately, this could be achieved by adding oxygen to the system as shown in Figure 13. Clearly oxygen addition had a drastic effect particularly at the higher temperature region above 120 "C, totally erasing the fd-down of the reaction rate which was inevitable in the absence of oxygen. A remarkable fact in this case was that NzO was not formed a t all. The and prostoichiometry of consumed NO, consumed 02, duced Nz was found to be nearly equal to 4:1:4. It was confirmed that NO and Oz did not react significantly without catalyst under similar conditions. We consider that the promoting effect resulb because oxygen takes over the reoxidation of Cu ions as shown in Figure 14. The scheme gives Nz only in agreement with the result. As we have seen so far, the NO-NH, reaction can be achieved effectively over Cu(I1)NaY. There, Cu ions are the active species dispersed on zeolite. The mechanism, though rather complex, has interesting features, which are probably applicable to other catalyst systems containing Cu ions. L i t e r a t u r e Cited Anderson, H. C., Gteen, W. J., Steeie, D. R., Ind. Eng. Chem., 53(3), 199 (1961). Arakawa, T., Mizumoto, M., Takita, Y., Yamazoe, N., Sejama, T., Bull. Chem. SOC. Jpn., 50, 1431 (1977). Flentge, D. R., Lunsford, J. H., Jacobs, P. A., Uytterhoeven, J. B., J . phys. Chem., 79, 354 (1975). Markvart, M., Pour, VI., J . Catal., 7, 279 (1967). Mlzumoto, M., Yamazoe, N., Seiyama, T., J . Catal., 5 5 , 119 (1978). Mizumoto, M., Yamazoe, N., Seiyama, T., J . Catal., in press, 1979. Naccache, C., ,Che, M., Ben Taarit, Y., Chem. fhys. Lett., 13, 109 (1972). Nakamoto, K., Infrared Spectra of Inorganic and Coordination Compounds", 2nd ed, Sec. 111-2, pp 160-166, Wiley, New York, N.Y., 1970. Otto, K., Shelef, M., Kummer, J. T., J . fhys. Chem., 74, 2690 (1970). Otto, K., Shelef, M., J . fhys. Cbem., 76, 37 (1972). Seiyama, T., Arakawa, T., Matstida, T., Yamazoe, N., Takita, Y., Chem. Left., 781 (1975). Sejama, T., Arakawa, T., Mats&, T.. Takita, Y., Yamazoe, N., J. Catal., 48, 1 (1977). Vansant, E. F., Lunsford, J. H., J . fhys. Chem., 76, 2860 (1972). Williamson, W. B., Lunsford, J. H., J . fhys. Chem., 8 0 , 2664 (1976).
Received for review May 17, 1979 Accepted August 28, 1979 Presented a t the 177th National Meeting of the American Chemical Society, Division of Colloid Chemistry, Honolulu, Ha., April 1979.
Surface Oxygen Species as Active Species in the Reduction of Nitrogen Oxide by Ammonia over Copper Vanadate Satohiro Yoshida,' Akio Ueda, and Kimio Tarama Department of Hydrocarbon Chemistry, Kyoto University, Sakyo-ku, Kyoto 606, Japan
Copper orthovanadate was easily reduced at 150 OC by a mixture of NO and NH3or at 200 OC by CO. A treatment of the partially reduced copper vanadate by oxygen at 200 OC made the vanadates very active catalysts at 150 O C for the reduction of NO by NH,. By temperature programmed desorption techniques, the interaction between adsorbed NO or NH3and the adsorbed oxygen species was confirmed. From the results of the experiments using isotopes, it is concluded that the reduction would proceed by a mechanism involving adsorbed NO, adsorbed NH, and the adsorbed oxygen.
Introduction The abatement of NO, in exhaust gases is one of the currently important subjects of environmental chemistry and many processes have been proposed (Klimisch and
Larson, 1975). As the process for the exhaust gases from stationary combustion facilities, the catalytic reduction of NO, by ammonia is regarded as the most favorable one and vanadium oxide supported on titania is recommended
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as a promising catalyst (Kasaoka et al., 1977). Although the catalyst system has activity at rather low temperatures, e.g., 250 "C, it is desirable to develop catalyst systems active at much lower temperatures. So far, the enhancement in the activity by oxygen gas has been noticed at many laboratories for both catalysis by metals (e.g., Anderson et al., 1961; Markvart and Pour, 1967) and by metal oxides (e.g., Bauerle et al., 1975; Todo et al., 1975). We suppose that the favorable effect of the oxygen suggests important keys to develop the active catalysts at low temperatures. Several different mechanisms have been proposed to the effect of oxygen in the catalysis a t low temperatures over vanadium oxides (Takagi et al., 1977; Miyamoto et al., 1977; Kasaoka et al., 1978) and the subject is still open to arguments. We expect that there would be active oxygen species on the surface of vanadium oxide and these play an important role in the activation of NO and ammonia. If this is correct, it is possible to develop more active catalysts than vanadium oxide by a modification that makes oxygen ions on the surface more labile or makes the surface possible to keep active oxygen species. In a previous work, we noticed that silver vanadates have peculiar active oxygen (Tarama et al., 1965). However, the silver vanadates are not thermally stable (mp 350-450 "C) so that copper vanadate seems to be more suitable as a catalyst (mp 780 "C). The present work is concerned with the surface-active oxygen species on copper vanadate in the NO + NH3 reactions.
100 bj
-* 3 z
0 m
z
+ 50 F i i
