The chemistry of DeNOx reactions over Pt/Al2O3: The oxime route to N2 or N2O

Abstract

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.

The selective reduction of NO with propene on Pt-beta catalyst: A transient study

“…When oxygen is eliminated from the system, CO 2 evolution ceases immediately, indicating that O 2 is essential for the reaction to occur and that O ads coverage at this temperature is very low, as the reaction does not continue in the absence of this gas. This agrees with recent results of Duprez and co-workers [25].…”

“…When oxygen is eliminated from the system, CO 2 evolution ceases immediately, indicating that O 2 is essential for the reaction to occur and that O ads coverage at this temperature is very low, as the reaction does not continue in the absence of this gas. This agrees with recent results of Duprez and co-workers [25].The fact that propene and NO profiles are identical indicates that the catalyst surface accepts both species proportionally up to 150 seconds after the switch, when saturation of the surface occurs. After the switch, both molecules are adsorbing on the catalyst to the same extent as they did under usual reaction conditions, although slower kinetics of adsorption are observed.…”

“…Recently, Duprez et al [25] investigate the reactivity of several compounds with different organic functions with the aim to explore their potential role as intermediates in the NO reduction. They come to the conclusion that oxime species should play a key role in the NOx reduction by alkenes at temperatures below 250 ºC.…”

Fourier Transforms Infrared Spectroscopy Applied in Selective Catalytic Reduction of NO by Acetylene

“…There are also some other suggestions about the reaction intermediate of HC-SCR in literature, including nitrile (Poignant et al, 2001;Delahay et al, 2007), isocyanate (Mihaylov et al, 2004;MosquedaJiménez et al, 2004b;He & KÖhler, 2006), R-NO x (Mosqueda-Jiménez et al,2003;Cowan et al, 1998), amine (Poignant et al, 2001), acetonoxime Resini et al, 2003) and ammonia (Lónyi et al, 2007). Although most of the "reduction" mechanisms were supported by Fourier transform infrared (FTIR) identification of reaction intermediates (Joubert et al, 2006), none of them has been widely accepted because of the complexity of the process (Mosqueda-Jiménez et al, 2003b) being concerned with different catalysts, reductants and reaction conditions. More investigation on the reaction mechanism is required for understanding the real reaction route of HC-SCR over different catalysts and using different reductants.…”

“…Hence, we propose that formamide species, which gives the band at 1691 cm -1 , was formed during the reaction of C 2 H 2 -SCR over 2%Zr/FER catalyst by nitric species reacting with acetylene. Correspondingly, a possible formation route of the formamide species can be proposed as follows: In the reaction of C 2 H 2 -SCR over 2%Zr/FER, formate species may be produced by the formamide species further reaction with nitric species: In literature, it is widely accepted that amine species are curtail intermediates of HC-SCR (Poignant et al, 2001;Joubert et al, 2006;Larrubia et al, 2001;Arve et al, 2007). Fig.…”