NOX selectivity

Synthesis and properties of new catalytic systems based on zirconium dioxide and pentasils for process of NOx selective catalytic reduction by hydrocarbons... 

The reaction pathway of the NOx selective catalytic reduction with ammonia (NH3-SCR) is described by the following reactions (reactions 19.21-19.24) ... 

The surface area of the support greatly affected the performances (VOC conversion and NOx selectivity) of the catalyst. The catalyst in which the precious metal was dispersed onto a high surface area carrier (150 m /g) showed complete conversion at around 225°C, while the catalyst with alow surface area (25 m /g) showed complete conversion at 300°C (Fig. 7.15). 

Figure 7.16 shows the effect of surface area on the NOx selectivity for the platinum catalyst (% NOx formed = 100 ppm NOx/ppm acrylonitrile converted). It can be seen that the low surface material makes a slightly higher amount of NOx than the high surface area catalyst. 

Selective Catalytic Reduction (SCR) SCE is a process to reduce NO, to nitrogen and water with ammonia in the presence of a catalyst between 540-840 F (282-449 C). Ammonia is usually injected at a 1 1 molar ratio with the NOx contaminants. Ammonia is used due to its tendency to react only with the contaminants and not with the oxygen in the gas stream. Ammonia is injected by means of compressed gas or steam carriers. Efficiencies near 90% have been reported with SCR. See Exxon Thermal DeNO. ... 

Selective catalytic reduction (SCR) and selective noncatalytic reduction processes (SNCR) are widely employed in large industrial and utility boiler plants, as well as in municipal waste incineration plants and other combustion processes. They are used to complement mechanical improvements (such as low NOx burners and furnace design modifications) as an aid to reducing the emission levels of NOx, S02, and other noxious gases into the atmosphere. 

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. 

G., and Forzatti, P. (1998) Monolithic catalysts for selective reduction of NOx with NH3 from stationary sources, in Structured Catalysts and Reactors, 1st edn. Chapter 5 (eds A. Cybulski and J.A. Moulijn), Marcel Dekker, New York, p. 121 (b) Nova, 1., Beretta,... 

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. 

A mathematical model for the design of extruded honeycomb reactor for selective catalytic reduction of NOx... 

Ammonia Coulometry (e.g. Messier method) Ion selective electrode Oxidation to NOx and chemiluminescence... 

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... 

Nitrous oxide has received increasing attention the last decade, due to the growing awareness of its impact on the environment, as it has been identified as an ozone depletion agent and as a Greenhouse gas [1]. Identified major sources include adipic acid production, nitric acid and fertilizer plants, fossil fuel and biomass combustion and de-NOx treatment techniques, like three-way catalysis and selective catalytic reduction [2,3]. 

NOx levels are increased in plasma and urine of septic animals. Many nonse-lective NO synthase inhibitors (e.g., L-NMMA) are used in several models with experimental induced sepsis (S40). In most studies it was shown that the cardiovascular abnormalities associated with sepsis were reversed, increasing blood pressure and systemic vascular resistance (F7, K9, M26, N5), together with a improvement in renal function (B42, H24). Also, selective inhibition of iNOS prolonged survival in septic rats (A7). 

Example 25.5 A gas turbine exhaust is currently operating with a flowrate of 41.6 kg s-1 and a temperature of 180°C after a heat recovery steam generator. The exhaust contains 200 ppmv NOx to be reduced to 60 rng rn 3 (expressed as N02) at 0°C and 1 atm. The NOx is to be treated in the exhaust using low temperature selective catalytic reduction. Ammonia slippage must be restricted to be less than 10 mgm 3, but a design basis of 5 mg-rn 3 will be taken. Aqueous ammonia is to be used at a cost of 300 -1 1 (dry NH3 basis). Estimate the cost of ammonia if the plant operates... 

Figure 25.31 Removal of NOx using selective catalytic reduction.

Wood SC (1994) Select the Right NOx Control Technology, Chem Eng Prog, Jan 32. 

Development of Catalytic Processes for the Selective Catalytic Reduction of NOx into N2... 

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 ... 

A recent area of development is to use these NRS catalysts for the combined treatment of NOx and particulates. The combination of the NOx trap and of a particulate filter depends on various factors (1) the selection of the best suited filter technology... 

From a general point of view, in heterogeneous catalysis, acidity has a very important influence on both activity and selectivity. More specifically, in the context of NOx... 

Busca, G., Lietti, L., Ramis, G. et al. (1998) Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts A review, Appl. Catal. B Environ., 18, 1. 

Brosius, R., Bazin, P., Thibault-Starzyk, F. et al., (2005) Operando FTIR study of reaction pathways of selective catalytic reduction of NOx with decane in the presence of water on iron-exchanged MFI-type zeolite, J. Catal., 234, 191. 

When the Ba loading increased up to 10% (w/w) (Pt—Ba/ y-AljC 1/11.5/100 w/w sample, Figure 6.18c), the reduction step was longer than in the previous case, in line with the higher amounts of NOx species that the catalyst was able to store during the previous adsorption phase (see Figure 6.6). Also in this case, low amounts of NO were found at the reactor outlet so that the process presented a selectivity rate close to 100% (Figure 6.19). 

Burch, R., Breen, J. and Meunier, F. (2002) A Review of the Selective Reduction of NOx with Hydrocarbons under Lean-Burn Conditions with Non-Zeolitic Oxide and Platinum Group Metal Catalysts, Appl. Catal. B Environ., 39, 283. 

The actual issues of EuroV standards aim at optimizing engine s design to decrease the engine-out N(), emissions in order to avoid the need for expensive after-treatments in the exhaust line. Only some heavily loaded applications would need such NOx after-treatment. Today, two major technological ways of NOx treatment are identified the NOxTrap and the selective catalytic reduction with ammonia (SCR-NH3). 

Finally, unconventional OSC supports have also been investigated. Machida et al., for instance, studied a Pd/MnOx-Ce02 as NOx-storage materials [118]. This catalyst was tested in lean/rich conditions using H2 as reducer with, however, an unusual period (9 min. lean/3min. rich). The catalyst showed both excellent activity and selectivity (almost 100% to N2). 

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