Low Activation Energy Pathway for the Catalyzed Reduction of Nitrogen Oxides to N2 by Ammonia

Abstract

A low activation energy pathway for the catalytic reduction of nitrogen oxides to N 2 , with reductants other than ammonia, consists of two sets of reaction steps. In the first set, part of the NO x is reduced to NH 3 ; in the second set ammonium nitrite, NH 4 NO 2 is formed from this NH 3 and NO + NO 2 . The NH 4 NO 2 thus formed decomposes at $100 C to N 2 + H 2 O, even on an inert support, whereas ammonium nitrate, NH 4 NO 3 , which is also formed from NH 3 and NO 2 + O 2 , (or HNO 3 ), decomposes only at 312°C yielding mainly N 2 O. Upon applying Redhead's equations for a first order desorption to the decomposition of ammonium nitrite, an activation energiy of 22.4 is calculated which is consistent with literature data. For the reaction path via ammonium nitrite a consumption ratio of 1/1 for NO and NO 2 is predicted and confirmed experimentally by injecting NO into a mixture of NH 3 + NO 2 flowing over a BaNa/Y catalyst. This leads to a yield increase of one N 2 molecule per added molecule of NO. Little N 2 is produced from NH 3 + NO in the absence of NO 2 .

An acid catalyzed step in the catalytic reduction of NO x to N2

“…After 20 min the TPD program was started. The peak at $100°C is found at the same position as with genuine NH 4 NO 2 /BaNa/Y that had never been exposed to HCl, as shown in our previous paper [11].…”

“…Ammonium nitrite supported on Ba,Na/Y or quartz, was prepared by impregnating the support with an aqueous solution of 3.5 M NH 4 NO 2 , followed by drying at room temperature, as described in [11]. In the absence of acid sites, the rate of decomposition is known to be negligible at room temperature for both *To whom correspondence should be addressed.…”

“…However, ammonium nitrate does not decompose at the temperatures where ammonium nitrite does; the decomposition of the nitrate requires a higher temperature than that of the tailpipe of many diesel-driven vehicles. The nitrate ions thus have the potential to block the active sites of the catalyst [11]. At higher temperature when ammonium nitrate does decompose, the major N-containing product is nitrous oxide, N 2 O, a greenhouse gas.…”

Mechanistic Study of Lean NO2 Reduction by Propane Over HZSM-5 in the Presence of Water

“…Isotope labelling shows that one N atom in the formed N 2 origins from the NO and the other from the deposit. Formation of a heteronuclear N 2 molecule has also been reported by, for instance, Sun et al [9,10] and Li et al [11] for ammonia-SCR. The reaction pathway is suggested to include formation of ammonium nitrite as a reaction intermediate, Yeom et al [12] put a similar discussion forward for the HC-SCR reaction, The nitrogen oxides and the organic molecules are proposed to react and form nitromethane, from which isocyanic acid (HNCO) may be formed via the aci-form of the molecule.…”

“…For instance, Liu et al [15] have studied the reaction of nitroethane over Cu-MFI under HC-SCR conditions and propose a reaction mechanism starting from the nitroparaffin via isocyanate species, which are likely formed through dehydrogenation of the aciform over Brønsted acid sites, and then further hydrolysed to amine species. Alternatively, ammonia can be formed from isocyanic acid (HNCO) which may react with NO 2 forming ammonium nitrite and eventually nitrogen [9][10][11][12]. However, this reaction route involves formation of N 2 O 3 on the sample surface and even though we find a peak in the N=O region (1930-1880 cm )1 ) [10] the symmetric and asymmetric stretching vibrations of NO 2 , at 1590-1550 and 1305-1290 cm )1 , respectively [10], are not detected.…”