Nitrogen oxides catalytic effect

These harmful effects of nitrogen oxides being known from several years, regulations in their emissions have been progressively introduced in most of the countries worldwide. Therefore, new technologies have been introduced to either limit their formation or convert them to N2. Among these technologies, the selective catalytic reduction (SCR) was the one which was most successfully developed. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Wichterlova, B., Sazama, P., Breen, J.P. et al. (2005) An in situ UV-vis and FTIR spectroscopy study of the effect of H2 and CO during the selective catalytic reduction of nitrogen oxides over a silver alumina catalyst, J. Catal. 235, 195. 

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. 

Nitric acid synthesis, platinum-group metal catalysts in, 19 621 Nitric acid wet spinning process, 11 189 Nitric oxide (NO), 13 791-792. See also Nitrogen oxides (NOJ affinity for ruthenium, 19 638—639 air pollutant, 1 789, 796 cardioprotection role, 5 188 catalyst poison, 5 257t chemistry of, 13 443—444 control of, 26 691—692 effect on ozone depletion, 17 785 mechanism of action in muscle cells, 5 109, 112-113 oxidation of, 17 181 in photochemical smog, 1 789, 790 reduction with catalytic aerogels, l 763t, 764... 

Bis(fluoroalkyl) and tris(lluoroalkyl) phosphites arc oxidized to the corresponding phosphates using a mixture of oxygen with nitrogen dioxide.277,278 In the oxidation of tris(2,2,3.3-tetrafluoropropyl) phosphite (5) by oxygen, bromal exhibits a strong catalytic effect, while chloral is less effective.278... 

The following method, which is an adaptation of that of Duval,6 makes use of the catalytic effect of charcoal in forming cobalt-nitrogen bonds and avoids the unnecessary use of ammonium chloride. This procedure is much shorter than air-oxidation methods and yields a relatively pure product in acceptable yield. 

The operability and reliability of processes using ammonia must also be studied. With the potential for increased ammonia use in these systems (in the selective catalytic reduction of nitrogen oxides and as an absorbent), research documenting ammonia emissions and the effects of ammonia on process equipment should be conducted. Furthermore, additional investigations should be performed to determine whether ammonium salts are formed and to document their effects on both the environment and the flue gas treatment system. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

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. 

The typical composition of the reaction products for char are shown in Table III (b). The amount of nitric oxide decomposed coincided well with the consumed hydrogen. A negligibly small amount of carbon dioxide was observed in the reaction products. This indicates that the reaction was also carried out catalyti-cally over char surface since the catalytic effect of quarz sand used for diluting char particles was not observed. The addition of hydrogen reduced the consumption of carbon to almost zero. The products were nitrogen and ammonia. 

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

C. Jung-Min Sung, L.A. Kennedy, and E. Ruckenstein, The effect of nitrogen content on the oxidation of fuel bound nitrogen in a transition metal oxide catalytic combustor, Comb. Sci. Tech. 47 315 (1984). 

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