DeNOx

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

Exxon Thermal DeNOx Similar to SCR, the Exxon Thermal DeNOx process utilizes the NO /ammonia reaction. However, this process does not use a catalyst to aid the reaction. Rather, tightly controlled temperatures are used to steer the reactions. Optimum reaction temperatures are found between IbOOT (871°C) and... 

Thus zeolite ZSM-5 can be grown (ref. 15) onto a stainless steel metal gauze as shown in Figure 6. Presumably the zeolite crystals are chemically bonded to the (chromium-) oxide surface layer of the gauze. After template removal by calcination and ion exchange with Cu(II) a structured catalyst is obtained with excellent performance (ref. 15) in DeNOx reactions using ammonia as the reductant. 

In many cases supports are shaped into simple cylinders (1-5 mm in diameter and 10-20 mm in length) in an extrusion process. The support powder is mixed with binders and water to form a paste that is forced through small holes of the desired size and shape. The paste should be sufficiently stiff such that the ribbon of extmded material maintains its shape during drying and shrinking. When dried, the material is cut or broken into pieces of the desired length. Extrusion is also applied to make ceramic monoliths such as those used in automotive exhaust catalysts and in DeNOx reactors. 

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... 

Poisoning of deNOx catalysts by SO2 could also be a problem since diesel fuels contain small amounts of sulfur compounds. Only a few studies deal with this subject [11-13]. It appears from the literature that for Cu catalysts the use of MFI as a support reduces the inhibition by SO2. Support effects also appear in the case of Co since Co/MFI is much less sensitive to SO2 than Co/ferrierite [13]. Since this support effect may be related to acidity, it becomes important, to investigate the influence of SO2 on the properties of Cu catalysts supported on Si02, AI2O3, MFI, BEA and unpromoted or sulfate promot Ti02 and Zr02- These latter have been reported active for deNOx [14]. 

In conclusion, Cu on Ti02 or Zr02 show a unique and interesting behaviour since their deNOx activity is promoted and not inhibited by the presence of sulfur in the feed. This effect can hardly be attributed to a selective inhibition of the oxidation of decane, and is better explained by the promotion of a bifunctional mechanism involving the acid sites created on the support by the reaction of SO2. 

A molecular view of reactions involved over DeNOx catalysts - Mechanisms and kinetics... 

Past and Present in DeNOx Catalysis P. Granger and VI. Parvulescu (Editors)... 

The backbone of the DeNOx process over mononuclear TMI encaged in zeolites can be epitomized in the form of three interconnected cycles associated with the formation of the N2 and 02 reaction products (Figure 2.6), inferred from the steady state and transient rate data combined with spectroscopic evidence for surface species and... 

Their decomposition into NO and 02 is apparently the most difficult step of the whole DeNOx process and requires elevated temperatures (Figure 2.24). Most likely it takes place in two following steps ... 

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