Ethylene and propylene adsorption and the formation of hydrocarbon complexes with NO 2 upon interaction of adsorbed olef
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Ethylene and Propylene Adsorption and the Formation of Hydrocarbon Complexes with NO2 upon Interaction of Adsorbed Olefins with an NO + 02 Mixture over HZSM-5 and CuZSM-5 Zeolites Studied by TPD and ESR A. N. Irichev*, V. A. Matyshak*, V. N. Korchak*, and Yu. B. Yan** *Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia **Institute of Marine Geology and Geophysics, Far East Division, Russian Academy of Sciences, Yuzhno-Sakhalinsk, 693002 Russia ReceivedJune 21, 1999 Abstract--Adsorption of C2H4 and C3H6 on copper in oxidized samples of CuZSM-5 is found to increase with the copper concentration; simultaneously, olefin adsorption on the Br/~nstedacid sites decreases. The Cu2+cations in the square-planar coordination exhibit higher reactivity in olefin adsorption than copper cations in the square-pyramidal coordination. Thermal treatment of CuZSM-5 with hydrogen results in regeneration of the Br6nsted acid sites for olefin adsorption and the disappearance of CuE+cations, the active sites of adsorption, due to the reduction of Cu2+ to Cu+ and Cu0 . Desorption peaks appear in the TPD spectra upon the interaction between the adsorbed hydrocarbons and NO2. These peaks are not observed upon separate adsorption of the reactants, and they are likely due to decomposition of NO2-hydrocarbon complexes over both the Br6nsted and copper-containing sites of the zeolite. INTRODUCTION Nitroxyl-hydrocarbon complexes are possible intermediates in the selective catalytic reduction of NO x by hydrocarbons in excess oxygen over HZSM-5 and CuZSM-5 zeolites [1, 2]. The mechanism of their formation and the role of hydrocarbons in this process are currently under discussion. Shelef [ 1] discussed the activation of hydrocarbons o n the Br6nsted acid sites of CuZSM-5 zeolite, leading to oligomeric radical cations. Nitroxyl organic compounds are formed upon the subsequent interaction of these oligomeric species with N O 2 molecules. On the other hand, according to Takeda and Iwamoto [2], nitromethane-type complexes can be formed directly on the copper cations. Hayes et al. discussed nitromethane formation by the reaction of n-allyl complexes with NO2 on copper ions. The formation of nitromethane is also possible upon the interaction of NO 2 with the methyl group on the metal cation, as was suggested for the CoZSM-5 catalyst [4, 5]. The conversion of nitromethane as the nitrogencontaining organic intermediate in the selective catalytic reduction of NO x by hydrocarbons over CuZSM-5 and CoZSM-5 zeolites was studied in [6, 7]. However, nitrogen-containing compounds were not observed under the conditions of NO x reduction by propane over CuZSM-5 [8]. Rather, only ammonia [6, 8] and HNCO [9] were found, which were formed by the conversion of nitrogen-containing compounds. Note that in [10],
nitrogen-containing organic compounds were detected only under precatalytic conditions. Earlier [ll, 12], we showed that the interaction between adsorbed olefins and NO or NO 2 over HZSM-5 and A1203 leads to nitroxyl complexes. The
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