NOx

About: NOx is a research topic. Over the lifetime, 26367 publications have been published within this topic receiving 496555 citations.

The smart surface modification of Fe2O3 by WOx for significantly promoting the selective catalytic reduction of NOx with NH3

Abstract: The deposition of WOx species using conventional impregnation method is a very simple and efficient way to adjust the surface acidity and redox ability of hematite Fe2O3 simultaneously, significantly improving its catalytic activity and N2 selectivity for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The deposited WOx species on Fe2O3 surface was in highly unsaturated coordination state acting as an effective surface modifier. Through the electronic inductive effect in W O Fe bonds, the oxidation ability of surface Fe(3+δ)+ species was well increased, yet the over-oxidation of NH3 on bulk Fe2O3 was significantly suppressed, which was quite beneficial to the improvement of NH3-SCR activity and N2 selectivity simultaneously. Besides, the deposited WOx species could also supply abundant surface reactive Lewis and Bronsted acid sites for NH3 adsorption and activation during the SCR reaction. The NH3-SCR process on this WOx/Fe2O3 catalyst mainly followed an Eley-Rideal (E-R) reaction pathway between gaseous NO and active NH3 adsorbed species, which was the main reason for its superior SO2 resistance. This opens a new route for the preparation of highly efficient, selective, cost effective and SO2 resistant NH3-SCR catalysts for the deNOx process on diesel engines.

Side Reactions in the Selective Catalytic Reduction of NOx with Various NO2 Fractions

Abstract: The main and side reactions of the three selective catalytic reduction (SCR) reactions with ammonia over TiO2−WO3−V2O5 catalysts have been investigated using synthetic gas mixtures matching the composition of diesel exhaust. The three SCR reactions are standard SCR with pure NO, fast SCR with an equimolar mixture of NO and NO2, and NO2 SCR with pure NO2. At high temperatures the selective catalytic oxidation (SCO) of ammonia and the formation of nitrous oxide compete with the SCR reactions. Water strongly inhibits the SCO of ammonia and the formation of nitrous oxide, thus increasing the selectivity of the SCR reactions. However, water also inhibits SCR activity, most pronounced at low temperatures. NO2 fractions exceeding 50% enhance the formation of nitrous oxide at low temperatures. If the feed of NOx consists of pure NO2, the formation of nitrous oxide may occur by two different reactions having different temperature regimes. The reaction responsible for N2O formation at low temperatures probably invo...

Low-Temperature Pd/Zeolite Passive NOx Adsorbers: Structure, Performance, and Adsorption Chemistry

Abstract: Pd/zeolite passive NOx adsorber (PNA) materials were prepared with solution ion-exchange between NH4/zeolites (Beta, ZSM-5, and SSZ-13) and PdCl2 solutions. The nature of Pd (dispersion, distribution, and oxidation states) in these materials was characterized with Na+ ion exchange, TEM imaging, CO titration with FTIR, and in situ XPS. The NOx trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites and PdO2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO2 clusters. NOx trapping and release are not simple chemisorption and desorption events but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO2 clusters are reduced by N...