Nitrogen oxides catalytic removal

Most measures to counteract acid deposition involve lowering the amount of acidic substances released into the atmosphere. Nitrogen oxides are removed from vehicle emissions using catalytic converters, and sulfur dioxide emissions from coal-fired power stations can be decreased in several ways. 

Removal of NOx from stack gas presents some formidable problems. Possible approaches to NOx removal are catalytic decomposition of nitrogen oxides, catalytic reduction of nitrogen oxides, and sorption of NOx by liquids or solids. 

Catalytic combustion is a process in which a combustible compound and oxygen react on the surface of a catalyst, leading to complete oxidation of the compound. This process takes place without a flame and at much lower temperatures than those associated with conventional flame combustion. Due partly to the lower operating temperature, catalytic combustion produces lower emissions of nitrogen oxides (NOx) than conventional combustion. Catalytic combustion is now widely used to remove pollutants from... 

Reactions involving the catalytic reduction of nitrogen oxides are of major environmental importance for the removal of toxic emissions from both stationary and automotive sources. As shown in this section electrochemical promotion can affect dramatically the performance of Rh, Pd and Pt catalysts (commonly used as exhaust catalysts) interfaced with YSZ, an O2 ion conductor. The main feature is strong electrophilic behaviour, i.e. enhanced rate and N2 selectivity behaviour with decreasing Uwr and , due to enhanced NO dissociation. 

Bioprocesses for the removal of nitrogen oxides from polluted air are an interesting alternative [58], but current reaction rates are still too low for large-scale applications. Advanced biological processes for the removal of NO from flue gases are based on the catalytic activity of either eukaryotes or prokaryotes, e.g. nitrification, denitrification, the use of microalgae and a combined physicochemical and biological process (BioDeNO ). 

Peng, X., Lin, H., Huang, Z. et al. (2006) Effect of catalysis on plasma assisted catalytic removal of nitrogen oxides and soot, Chem. Eng. Technol. 29, 1262-6. 

The postcombustion systems used at power generating plants and factories are somewhat different. These systems remove nitrogen oxides from the waste gases jlue gases) processes using classified as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). Oxides of nitrogen are also removed by some systems... 

The potential for catalytic removal of nitrate was introduced in the late eighties and bears some similarity to the electrochemical process in as much as both involve stepwise reduction of the nitrogen oxidation state via formation of nitrites, ammonia and N2. As with the electrochemical process, the major drawback is the potential for formation of undesired products such nitrite and ammonium which must be depleted at more stringent levels than the nitrate itself... 

Catalysts help customers comply cost-effectively with clean-air regulations. Hydrocarbons, carbon monoxide, and nitrogen oxides can be removed using supported precious metal catalysts. Organic sulfur compounds are converted to H2S using nickel/molybdenum or cobalt/molyb-denum on alumina catalysts. Sulfur can be recovered in a Claus process unit. The Claus catalytic converter is the heart of a sulfur recovery plant. 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

Most of the existing processes for nitrogen oxide removal are chemically based requiring high temperature or expensive catalysts. The main techniques involve either selective noncatalytic reduction (SNCR) or selective catalytic reduction (SCR). SNCR uses ammonia for conversion of NO to N2 and H20 at elevated temperatures (550-850 K). SCR can use catalysts such as Ti02 with active coatings of V2Os and WO, . 

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