Oxidation of NOX

Fig. 8.21 A reaction scheme presumed for the catalytic oxidation of NOx (NO and N02) to HNC, (N03 ) by the Ti02-AC-Fe203 mixture under pnotoillumination.

Plasma-Stimulated Combined Oxidation of NOx and SO2 in Air Simultaneous Industrial Exhaust Gas Cleaning of Nitrogen and Sulfur Oxides... 

TiOa is one of the most commonly used photocatalysts. In Hong Kong, a study titled Ambient Air Treatment by Titanium Dioxide Based Photocatalyst was conducted in 2002 (116). The purpose of the study was to determine the durability of TiOa catalyst. Extensively used Ti02-coated paving blocks were tested for then-use in the photocatalytic oxidation of NOx. 

Concerning the second part of your question, I suppose with nitrogen you mean nitrogen oxides. I don t know if the NOx emitted will not be solidified by certain components in precipitation. Anyway, I did not contend the opposite. I think that under certain circumstances, NOx in contact with rain water or cloud water will be transformed into NO3, for example at high ozone concentrations. It is a fact that under normal conditions the oxidation of NOx higher oxides needs much more time than the oxidation of SO2 into SO3. 

The plate reactors are often used for photocatalytic oxidation of NOx in the gas phase [81-86]. These types of reactors were described in the previous subsection. Ao et al. studied photocatalytic oxidation of NO2 using plate reactor with inner source of irradiation (6 W UV lamp) [81]. The reactor s surface was coated by a Teflon film. Ti02 powder was coated on the glass fiber filter. Moreover, the plate reactor made of stainless steel with inner source of irradiation was used by Chen et al. for photocatalytic oxidation of NO [60, 82]. Yu et al. used plate reactor with outer source of irradiation for removal of NO [83]. This reactor was made of non-adsorbing plastic material. Top of the reactor was covered with a borosilicate plate. The photocatalyst was illuminated by 25 W cool daylight lamps. Other types of the reactors used for photocatalytic oxidation of inorganic pollutants were described elsewhere [87, 88]. 

Is it fair that Congress has focused on reducing SO2 emissions, even though NOx emissions also contribute to acid rain There are several reasons for this selective attention. In the eastern U.S., more than two thirds of the acidity in rain comes from SO2 emissions, while only one-third comes from NOx emissions. Furthermore, the oxidation of SO2 produces twice as much acidity as the oxidation of NOx (see Chapter 3). Therefore, molecule for molecule, it makes more sense to reduce SO2 emissions. Acid sulfate appears to play a somewhat greater role than acid nitrate in acidifying lakes and streams, because acid sulfate runs off more easily into lakes and streams while acid nitrate is retained... 

America. They do so directly in the form of nitrates generated from oxidation of NOx emitted mostly from combustion of fossil fuels but also from nitrogen fertilizers. Their indirect contribution to acidification begins with ammonia volatilized from animal wastes and nitrogen fertilizers and released during the combustion of fossil fuels NH4 deposited from the atmosphere is either taken up by plants or, more likely, nitrified, and both of these processes produce hydrogen ions. 

A high pressure level results in a higher energy requirement with possibly higher utility eosts. On the other hand, today s striet environmental pollution laws with regard to NOx diseharged into the atmosphere are easier to meet at higher pressures, and oxidation of NO to NO2 is favored by inereased pressure and low temperature. 

Basis for NOx Prevention. Emissions from turbines are a funetion of temperature and thus a funetion of the F/A ratio. Figure 10-20 shows that as the temperature is inereased the amount of NOx emissions are inereased and the CO, and the unburnt hydroearbons are deereased. The prineipal meehanism for NOx formation is the oxidation of nitrogen in air when exposed to high temperatures in the eombustion proeess, the amount of NOx is thus dependent on the temperature of the eombustion gases and also, to a lesser amount on the time the nitrogen is exposed to these high temperatures. 

The challenge in these designs is to lower the NO without degradation in unit stability. In the combustion of fuels that do not contain nitrogen compounds, NOx compounds (primarily NO) are formed by two main mechanisms, thermal mechanism and the prompt mechanism. In the thermal mechanism, NO is formed by the oxidation of molecular nitrogen through the following reactions ... 

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... 

In addition to carbon monoxide (CO) and unbumed hydrocarbons (UHC), the most significant products of combustion are the oxides of nitrogen (NOx). At high temperatures, free oxygen not consumed during combustion reacts with nitrogen to form NO and NO2 (about 90% and lO /i of total NOx, respectively). 

In April of 1998, the EPA published a final rule for emission of oxides of nitrogen (NOx), hydrocarbons (EIC), carbon monoxide (CO), particulate matter (PM), and smoke opacity for newly manufactured and rcmanufacturcd locomotives. The rulemaking took effect in 2000 and is estimated by the EPA to cost the railroads 80 million per year—about 163 per ton of NOx reduced, according to EPA figures. The emissions standards for the several pollutants will be implemented in three tiers—for locomotives... 

Ethylene is currently converted to ethylene oxide with a selectivity of more than 80% under commercial conditions. Typical operating conditions are temperatures in the range 470 to 600 K with total pressures of 1 to 3 Mpa. In order to attain high selectivity to ethylene oxide (>80%), alkali promoters (e.g Rb or Cs) are added to the silver catalyst and ppm levels of chlorinated hydrocarbons (moderators) are added to the gas phase. Recently the addition of Re to the metal and of ppm levels of NOx to the gas phase has been found to further enhance the selectivity to ethylene oxide. 

NOx An oxide, or mixture of oxides, of nitrogen, typically in atmospheric chemistry, noble gas A member of Group 18/VIlI of the periodic table (the helium family). 

Nitrogen Dioxide (NO2) Is a major pollutant originating from natural and man-made sources. It has been estimated that a total of about 150 million tons of NOx are emitted to the atmosphere each year, of which about 50% results from man-made sources (21). In urban areas, man-made emissions dominate, producing elevated ambient levels. Worldwide, fossil-fuel combustion accounts for about 75% of man-made NOx emissions, which Is divided equally between stationary sources, such as power plants, and mobile sources. These high temperature combustion processes emit the primary pollutant nitric oxide (NO), which Is subsequently transformed to the secondary pollutant NO2 through photochemical oxidation. 

Closed symbols in Fig. 1 show the effect of reaction temperature on ammonia oxidation over CuO by heating with a conventional electric furnace. The reaction started at about 400 K and the conversion of NH3 became 1 at temperatures higher than 500 K. Fig. 1 also indicates that selectivity to N2 was high at low temperatures but it decreased as the temperature increased. Both N2O and NO increased instead of N2, except at 623 K, at which N2O decreases. NO was detected above 583 K, and it monotonously increased by the temperature. High reaction temperature seems to tend deeper oxidation to NOx. Considering that oxidation of N2 to N2O and NO is difficult in the tested temperature range. 

Figure 10.10. Principle of operation of NOx storage catalyst. During lean combustion, NO is oxidized to NO2 and stored by BaO as barium nitrates. Once the getter is saturated, a short rich excursion of the air-fuel mixture...

It is well known also that higher alkanes suffer radical gas phase oxidation above 723 K. Therefore, their use requires catalysts active and selective for deNOx at lower temperatures. The mechanism of NOx elimination is still debated a redox mechanism involving Cu ions is probable, and isolated Cu cations exchanged into MFI [4,5] or mordenite [6] have been found to be more active than CuO clusters. It must be emphasized, however, that acid zeolites exhibit good activity at high temperature, and acid mechanisms have been proposed [7-10]. In presence of Cu this acid mechanism disappears probably due to the decrease of the acidity of mordenite upon Cu exchange [6]. According to... 

The effects of precious metals on ln/H-ZSM-5 was found not only to simply catalyze NO oxidation but also to enhance NOx chemisorption. It is noted that NO conversion on the lr/ln/H-ZSM-5 exceeded NO2 conversion in NO2-CH4-O2 reaction on in/H-ZSM-5, when the concentration of NOx was decreased [14]. This study shows the catalytic activities of ln/H-ZSM-5 promoted by precious metals for the removal of low concentration NOx and the promotive effects of the precious metal will be discussed. 

Figure 2 shows the effect of NOx concentration on the conversion of NOx reduced by CH4 in the presence of 5% H2O. In the NO-CH4-O2 system, In/H-ZSM-5 showed low catalytic activity in the whole range of NO concentration. On the other hand, this catalyst was active for the NO2-CH4-O2 reaction, while the conversion of NO2 decreased with decreasing concentration of NO2. The catalytic activity of ln/H-ZSM-5 for the reduction of 1000 ppm NO was enhanced by the addition of Ir and R almost to the level of NO2 reduction on ln/H-ZSM-5, indicating that these precious metals worked as the catalytic sites for NO oxidation, which is a necessary step for NO reduction with CH4. With decreasing NO concentration to 100 ppm, however, the increase in NO conversion was observed on lr/ln/H-ZSM-5 and the conversion of NO exceeded that of NO2 on ln/H-ZSM-5. This can not simply be explained by the catalytic activity of Ir for NO oxidation. 

NOx - Oxides of nitrogen (principally NO and NO2) are formed in combustion processes and as a byproduct of chemicals production. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

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