DeNOx mechanism

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... 

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... 

Kinetics and Mechanism of the Thermal DeNOx Reaction The discovery of the Thermal DeNOx reaction was followed by studies of its mechanism by the author and his coworkers and by other research groups. The former efforts culminated in the development of a kinetic data base and of a computer model2. The data base consisted of 742 data points distributed over a range of temperatures, reaction times, and initial concentrations of NO, NH3, 02, H2 and H2 O. The computer model used a set of 31 elementary reaction rates. Of these 31 reactions 27 had rate constants which were accurately known or could reasonably be estimated because they had little effect on the model s predictions. By using the remaining 4 reaction rate constants as adjustable parameters it was possible to fit the data base with its 7% experimental uncertainty. 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

Technical constraints are often imposed on the design of the monolith geometry by the extrusion process, as well as by the mechanical properties of the extrudate the specific SCR application (e g., high-dust vs. low-dust) is also crucial for the definition of the catalyst geometrical features. Here, attention is paid to the influence that the monolith parameters (wall thickness, channel size, channel shape) have on both DeNOx reaction and SO2 oxidation in order to advance guidelines for optimization of the catalyst geometry. 

In a related set of catalysts a loading of 1% vanadium(v) oxide on titanium dioxide is used. These eatalysts are applied in selective catalytic reduction (SCR) reactions, i.e. the removal of NOx through selective reaction with ammonia from streams eontaining large volumes of o g gen. These eatalysts require the redox aetivity as diseussed above, but also an acidic functionality (to activate the seleetive reduetant (ammonia)). It transpires that the produet of reduetion of the surface V=0 groups (V-OH) is sufficiently acidie for this purpose. This means these V=0 species oriented into the reaction medium beeause of their interaction with Ti02 can act as both redox and acidic surface sites. The mechanism of the deNOx reaction has been extensively studied over these materials. ... 

The mechanism of the deNOx reaction has been extensively studied [33,34] over these materials. 

There has been significant work on the mechanism of the deNOx [42,43] and the related fast deNOx reaction [44-46] over Fe-containing zeolites and it is considered that the presence of NO within the reaction mixture is essential for the fast deNOx process. Subsequent to this there has been much work on adding NO oxidation catalysts to the base Fe zeolite catalyst in order to promote NO formation in order to allow the fast deNOx process. Fe zeoHtes play a major role in the nanocomposite materials discussed in Section 1.3. 

By the example of various catalytic reactions (ethylene epoxidation over Ag, deNOx with methane on Co-ZSM-5, Fischer-Tropsch synthesis over Co-based systems), we show how the reaction mechanism was revealed and the concentrations of key intermediates and reaction rate coefficients were estimated using different isotope labels ( 0, etc.). A single instance... 

In this work the merits of the use of a natural fibrous mineral, sepiolite, as a binder to produce titania based monoliths of high mechanical strength and abrasion resistance is discussed. The monoliths of square channels were conformed with an initial 7.5 channels cm and 1 mm wall thickness. TTie textural characterization was made by mercury intrusion porosimetry (MIP), nitrogen adsorption/desorption (BET), and X-ray diffraction (XRD). The mechanical resistance, dimensional changes and weight losses al each stage of heat treatment were also determined. The thermal expansion coefficients (TEC) of the monoliths were determined between 200 and 400 C, since in practice the usual working temperature of DENOX catalysts lies between 250°-350 C. 

So far we have proven that not only nitrates are stored onto Fe- (and Cu-) zeolite catalysts in the presence of NO2, but also that they do participate effectively in the NH3-SCR catalytic chemistry, being indeed responsible for the very high DeNOx activity associated with the Fast SCR reaction. In the next paragraph we make use of transient reaction analysis to elucidate in more detail the reactivity of surface nitrates with NO and NH3, i.e., the SCR reactants in so doing, we will also explore the individual steps of the Fast SCR mechanism. 

As part of his investigation. Silver (1981) also modeled laboratory data of Muzio et al (1976) on Thermal DeNox using a 54-reaction mechanism, including the two effective channels (1) and (2), and found that a = 0.4 provided a best overall fit at T = 1250 K. In work of a similar nature, Salimian... 

---