The chemistry of DeNOx reactions over Pt/Al2O3: the oxime route to N2 or N2O.
Résumé
The reactivity of 12 nitrogen compounds with different organic functions (nitro, nitrite, nitrate, oxime, isocyanate, amide, amine, nitrile) was investigated over a 1% Pt/δ-Al2O3 catalyst (mean metal particle size, 20 nm) in oxygen excess conditions (5% O2 + 5% H2O). Two series of experiments, at temperatures ranging from 150 to 500•C, were carried out: (i) decomposition of the N compounds without any addition of NO and (ii) reduction of NO by these compounds. The results were compared with the catalytic behavior of the Pt catalyst in NO reduction by propene. Some of the N compounds as well as oxygenated products were identified by GC-MS in the NO + C3H6 reaction. The nitrogen organic intermediates could be classified into two groups: (i) class 1 compounds, which react with NO at relatively high temperatures (300°C) with a high selectivity to N2 (nitro, nitrite, nitrate, nitrile), and (ii) class 2 compounds, which reduce NO in the same temperature range as propene (200-225•C) with a selectivity of ≈50% close to that of NO reduction by propene (oximes, amides, amines, and, to a lesser extent, isocyanate). A mechanism is proposed in which oxime species play a key role in NO reduction. N2O would be produced mainly by a Nef reaction (leading to ketones and N2O), whereas N2 would be formed by successive steps starting with a Beckmann rearrangement of oxime species, with the byproducts of the N2 formation being acids, alcohols, and nitrato and nitrito species.
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