Selective Catalytic Reduction of NO_x with Ammonia over Soot

Abstract

The selective catalytic reduction (SCR) of nitrogen oxides (NO x ) in the presence of NH 3 was investigated on Printex U and three diesel soot samples in the temperature range between 200 and 350°C. In the presence of NO + NO 2 , a stoichiometry similar to that of the fast SCR reaction was observed, whereas in the absence of NO, the stoichiometry shifted to values typical of the NO 2 SCR reaction. In the presence of only NO, no NO x reduction was observed. Carbon oxidation and SCR reaction minimally influenced each other. On the basis of these experimental results, a mechanism for NO x reduction over diesel soot was developed. The first step in the reaction is the disproportionation of NO 2 , which is followed by the formation of ammonium nitrates and nitrites. The nitrites decompose directly into N 2 and H 2 O, whereas the nitrates have to be reduced to nitrites either by NO or in its absence by NH 3 , which also leads to the formation of N 2 and H 2 O. The observed byproducts indicated that the acidity and the amount of surface functional groups for NH 3 are important preconditions for the occurrence of the SCR reaction over soot.

Investigation of Iron Vanadates for Simultaneous Carbon Soot Abatement and NH3-SCR

“…The four different profiles compare reaction carried out with empty reactor, and with reactor filled with soot, catalyst and soot/catalyst mixtures. Compared to the empty reactor, soot alone has a minimal effect towards ammonia oxidation, as already observed by Mehring et al [33]. FeVO 4 is the most active in the oxidation of NH 3 , followed by ceria and ceria-zirconia.…”

Diesel Soot Catalyzes the Selective Catalytic Reduction of NOx with NH3

“…For details of the involved chemistry please refer to Ref. [22,23]. The measured C-oxidation rates for different NH 3 , NO and NO 2 concentrations showed only a minor influence of the SCR conditions on the soot oxidation.…”

“…Printex U did not contain metals above the detection limits of an ICP-OES analysis. In the Euro V, Euro 0 and the mixed soot sample about 1 % Ca, 1 % Fe beside some Zn, Mg, Na and K could be found (for details refer to [22,23]). TEM measurements of Printex U and the Euro V soot showed hardly any difference in size and structure of the primary particles between the different samples.…”

An Overview on the Catalytic Materials Proposed for the Simultaneous Removal of NOx and Soot

“…In 2012, Kröcher and a coworker [130] investigated the SCR reaction in the presence of NH 3 over soot (i.e., both model and real diesel soot), in the temperature range between 200 and 350 • C. They observed SCR activity on diesel soot, which could be exploited to improve future diesel exhaust after treatment systems, where both DPF and SCR systems have to be used in series to reach the desired emission limits. The authors proposed a mechanism for NO x reduction over diesel soot where the first reaction step is the disproportionation of NO 2 , which is followed by the formation of ammonium nitrates and nitrites.…”

“…The dark points symbolize acidic carbon surface species supporting NH 3 adsorption. Reprinted with permission from [130]. Copyright © 2012, American Chemical Society.…”