Abstract
Raw exhaust gases may contain notable levels of formaldehyde that can negatively impact the efficiency of after-treatment systems. In the selective catalytic reduction (SCR) of NOx over V2O5/WO3-TiO2, formaldehyde was found to react with NH3 to produce HCN at concentrations above the threshold limit value set by environmental/safety organizations. Due to this side reaction, NH3 is consumed parasitically and the NOx conversion decreases by up to 15 %, even after compensating for the fraction of lost NH3. Under similar conditions, the non-reducible TiO2 support also produced HCN moderately, thereby showing that redox sites promote the reaction but are not a necessary condition. To understand the chemistry responsible for HCN formation, the roles of reaction temperature, water, and oxygen were investigated. Our results suggest a new pathway for HCN production through the direct reaction of formaldehyde and NH3, which is active at high temperature and does not proceed through the formate route previously proposed.
Highlights
The rise in anthropogenic nitrogen oxide (NOx) emissions has im pelled the development of exhaust after-treatment technologies
We show that formaldehyde reacts with NH3 to form hydrogen cyanide (HCN) with high yields under selective catalytic reduction (SCR) conditions, and that the reaction does not require the formation of ammonium formate
The performance of V2O5/WO3-TiO2 in the absence of formaldehyde (Fig. 2; SCR feed) was comparable with those reported in the literature for the same V loading and GHSV [28,30]
Summary
The rise in anthropogenic nitrogen oxide (NOx) emissions has im pelled the development of exhaust after-treatment technologies. Because of its unparalleled efficiency, wide operating window, and moderate cost, SCR is the technology of choice for NOx reduction in Diesel-powered vehicles and thermal power plants [1,2,3]. The presence of nitrogen dioxide (NO2) in the exhaust gas invokes the occurrence of the so-called fast SCR reaction (Eq 2), wherein the reoxidation step of the metal center occurs much faster because of the stronger oxidizing power of NO2 than oxygen (O2) [4,5]. The inherent complexity of the exhaust gas composition and gascatalyst interactions can promote side reactions during the SCR process. The most significant is the direct oxidation of NH3 with O2 (Eq 3) [7,8]
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