Development in Process of Direct Decomposition of Nitrogen Monoxide over Copper Ion-exchanged Zeolite Catalysts

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Direct decomposition and selective reduction of nitrogen monoxide over copper ion-exchanged zeolite catalysts are proposed as new methods for removal of NO. The copper ion-exchanged ZSM-5 zeolite (Cu-Z) was the most active catalyst for decomposition of NO. The activity of Cu-Z zeolites increased with increase in the exchange level. The zeolites with copper ion-exchange levels of 100% or more, which could be prepared by repeating ion exchange of the ZSM-5 zeolite using aqueous copper (II) acetate solution or addition of ammonia into the aqueous copper (II) nitrate solution, showed significantly high activity even in the presence of oxygen and at high GHSV region. It was clarified concerning Cu-Z, by using IR, ESR, phosphorescence, TPD, and CO adsorption measurements that (1) the Cu^<2+> ions exchanged into zeolite were reduced to Cu^+ and/or Cu^+-Cu^+ through evacuation at elevated temperature, (2) after exposure to oxygen at 773 K and subsequent evacuation, about 40% copper ions in zeo … More lite existed as Cu^+ ions, (3) the NO^- species formed by adsorption of NO on Cu^+ would be an intermediate in the NO decomposition, and (4) redox cycle of Cu^+<@2<@>D2 <@D1>@>D1Cu<@D12+@>D1 in the zeolite is probably a key step to achieve the decomposition reaction. Selective reduction of NO by hydrocarbon in the presence of oxygen was first found by the authors and Cu-Z was remarkably effective for NO removal at temperatures as low as 473-573 K. The activity for this selective reduction in NO+C<@D23@>D2H<@D26@>D2+O<@D22@>D2 system was not poisoned very much by addition of SO<@D22@>D2. The conversion into N<@D22@>D2 was changed to 85% (773 K) in the presence of SO<@D22@>D2, from 100% in the absence of SO<@D22@>D2, which is in contrast with the fact that the catalytic activity for direct decomposition of NO was completely lost on adding the same amount of SO<@D22@>D2. Furthermore, the reduction rate over Cu-Z at 525 K was higher than those over H-zeolite and alumina catalysts at 673 and 773 K, respectively, which have been reported to be active, after findings by the authors. Less