Possible reasons for the superior performance of zeolite-based catalysts in the reduction of nitrogen oxides

Literature data and results obtained in this laboratory are used to compare catalysts supported on either zeolites or amorphous oxides and used in NO _x reduction to N₂. The data show that the catalytic activity of the zeolite-based materials is higher if alkanes, alkenes, or organic oxygenates are used as reductants, but with ammonia the performance of both groups of catalysts is comparable. These findings are rationalized in the context of the present knowledge of the mechanism of NO_x reduction. Transition metals form oxo-ions on zeolites, but solid solutions on alumina. On zeolites, optimal Coulomb stabilization can be achieved if heterolytic dissociation of molecules transforms multipositive cations to monopositive cations. N₂O₄ dissociates to form NO++NO3- on BaNa/Y. In contrast, no NO⁺ ions are detected on BaO/γ-Al ₂O₃. Another important parameter is the high heat of adsorption of small molecules in the narrow zeolite pores. Because oxygenates are superior to alkanes in competing against water for active sites, NO _x reduction with alkanes is favored by zeolites with low Al/Si ratios, but NO_x reduction with acetaldehyde is more efficient on a faujasite with a high Al/Si ratio. Over these catalysts, water vapor actually enhances NO_x reduction by preventing formation of crotonaldehyde, which would poison catalyst sites.