Standard SCR reaction

The fast SCR reaction , which involves both NO and N02, exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO ... 

The simplest variant of the selective catalytic reduction of NO with NH3 is the standard-SCR reaction, in which NH3 and NO comproportionate in a 1 1 stoichiometry to nitrogen. This reaction is efficiently catalyzed with high activity and selectivity between 300 and 400°C by V205/W03-Ti02 catalysts, which are wide-spread in stationary SCR systems [1],... 

The decline of the DeNO. curve for N02 fractions above 50% is much stronger than the incline below 50% due to the different reaction rates of standard- and N02-SCR. The latter is much slower than the fast-SCR reaction and even slower than the standard-SCR reaction. The promoting effect of N02 levels off above 350°C, because the rate constants of standard-, fast- and N02-SCR reactions are converging at higher temperatures. 

Different proposals have been advanced for the mechanism of the standard SCR reaction, which have been reviewed by Busca et al. [32]. Inomata et al. proposed that ammonia is first adsorbed as at a V-0 H Bronstcd site adjacent to a V =0 site and then reacts with gas-phase NO to form nitrogen and water while V" -O H groups are reoxidized to =0 by gaseous oxygen [33]. Janssen etui, demonstrated thatone N... 

Also, the MR kinetics provided a much better description than the modified ER kinetics of fast SCR transients originated by high-frequency NHj feed pulses in a stream of 1000 ppm NO, 2% v/v O2 and 1% v/v H2O and similar to those associated with the operation of SCR after treatment devices for vehicles [56]. Indeed, the MR model is definitively more chemically consistent than the modified ER model in view of the redox character of the standard SCR reaction. 

It is worth mentioning that the same chemistry is involved when surface nitrite and nitrate are considered instead of gas-phase nitrous and nitric add. Indeed, surface nitrate has been suggested to take part in the reoxidation of the reduced catalyst sites, thus accounting for the higher rate of the fast SCR reaction compared with the standard SCR reaction [74]. 

To describe the NH3 + NO/NO2 reaction system over a wide range of temperatures and NO2 NOxfeed ratios in addition to ammonia adsorption-desorption, ammonia oxidation and standard SCR reaction with the associated kinetics already discussed in Section 2.3.2, the following reactions and kinetics have been considered by Chatterjee and co-workers [79] ... 

The reactivity of the NH3-N0/02 system was studied first by TPR experiments in order to explore the effects of some operative conditions, namely temperature, water and oxygen feed contents and space velocity, on the standard SCR reaction. In this case NH3 (1,000 ppm) + NO (1,000 ppm) with 02 (2 or 6% v/v), H20 (1%) and balance He were initially fed at 50°C and then the catalyst temperature was continuously increased at 2 °C/min up to 450°C. 

No significant influence of the water feed content on the standard SCR reaction between 1% and 10% H20 was found, while a moderate promoting effect of oxygen on the SCR activity was clearly apparent (Chatterjee et al., 2005 Ciardelli et al., 2004a Nova et al., 2006a Tronconi et al., 2005). Afterwards the reactivity in dynamic conditions was systematically studied by means of TRM experiments, i.e. by performing step changes of the NH3 feed... 

The following reactions were included in the kinetic model NH3 adsorption (R3 in Table V), NH3 desorption (R4 in Table IV), NH3 oxidation (R5 in Table IV) and standard SCR (R6 in Table V). Mass balances for adsorbed ammonia and nitrogen now include the standard SCR reaction. Moreover, the mass balance of gaseous NO was introduced, too... 

For the standard SCR reaction, the following rate expression was considered,... 

For Eqs. (48), (52) and (68)—(69), the same rate parameter values were herein adopted as used in the previous work addressing the standard SCR reaction only (Chatterjee et al., 2005 Ciardelli et al., 2004a). A set of 32 experimental runs was used for the estimation of the additional rate parameters for the reactions involving N02 according to the procedures described in (Chatterjee et al., 2005, 2006 Ciardelli et al., 2004a). 

A temperature ramp (from 100°C to 450°C) was performed. Equimolar conversion of NO and NH3 was observed, in agreement with the stoichiometry of the standard SCR reaction (R6, Table V). The observed reactivity was again well predicted by the model simulation in the whole T-range. 

Reaction 1 is the most important and desirable reaction and is quoted as the standard SCR reaction it proceeds rapidly on the catalyst at temperatures between 250°C and 450°C in excess oxygen and accounts for the overall stoichiometry of the SCR process. NO2 accounts for only 5% of the NO in combustion gases and accordingly reactions 2 and 3, although very fast, play a minor role in the process, particularly for stationary applications. Reactions 2 and 3 are quoted as NO2 SCR and fast SCR reaction, respectively. 

A special but important case where the SCR reacting system includes NO2 in signiflcant amounts is represented by the new generation of urea-SCR converters for diesel vehicles integrated with an upstream preoxidation catalyst, which partially oxidizes NO to NO2 (10,11). In such devices, the so-called fast SCR reaction 3, which may be over ten times faster than the standard SCR reaction 1 at low temperatures, plays a critical role in boosting the DeNOx activity at 180 -300°C (12). However, large NO2 feed contents result also in the occurrence of two additional unselective reactions, not observed in the presence of NO-NH3 only, namely the formation of ammonium nitrate, which is critical below 180°C,... 

Mechanism of the SCR Reactions. Several proposals have been advanced in the literature concerning the mechanism of the standard SCR reaction (reaction 1) over vanadia-based catalysts, some of which have been reviewed by Busca and co-workers (71). 

Steady-State Kinetics of the SCR Reaction. The kinetics of the standard SCR reaction over vanadia-based catalysts has been investigated by several authors (64,66,67,86,87,92,93), and different kinetic rate expressions have been proposed. Most of them refer to steady-state conditions, but kinetic studies performed under transient conditions have been reported as well (94-102). 

As previously reported, an Eley-Rideal mechanism is generally accepted for the standard SCR reaction, which implies the reaction between adsorbed NH3 and gas-phase NO accordingly, a kinetic expression, which is in agreement with the observed dependencies of ammonia, NO, oxygen and water on the rate of reaction, is given by... 

However, in transient experiments performed at low temperature (T < 250 C) when the ammonia feed is shut off, the NO concentration trace decreases, goes through a minimum and then begins to increase and eventually recovers the inlet value. The minimum in the NO concentration and the symmetrical maximum in N2 evolution, which eventually confirms the occurrence of the standard SCR reaction, have been attributed to the inhibiting effect of excess NH3, possibly caused by a competition between NO and ammonia in adsorbing onto the catalyst. 

The transient kinetic model of the standard SCR reaction over a commercial V-based catalyst for vehicles reported in Reference (101) is the only treatment available so far accounting both for the redox nature of the SCR catalytic mechanism and for the ammonia inhibition effect. It relies on a dual-site redox scheme, whereby ammonia is first adsorbed onto acidic sites, but reacts with NO on different redox sites associated with the vanadium component. The redox sites can, however, be blocked by excess ammonia. Adopting a Mars-Van Krevelen formal approach, the following modified redox (MR) rate expression was derived (27) ... 

It is worth noticing that equation 27 formally reduces to equation 16 in the limit of low ammonia coverages (0nh3 0). In other words, the dual-site modified-redox SCR rate law predicts ammonia kinetic order less than one and o gen kinetic effects at low temperatures, but becomes equivalent to the well-established Eley-Rideal rate law at T > 250°C, where such effects are indeed negligible. It is also worth stressing that a modified red-ox rate law is more consistent with the mechanism of the standard SCR reaction 1. 

As reported in the section SCR Chemistry, other reactions may occur in the SCR reactor besides the standard SCR reaction 1, namely the reduction of NO2 (reactions 2 and 3), the oxidation of ammonia (reactions 4-6) and the oxidation of SO2 (reaction 7). 

According to a more simplified approach. Reference (131) presents an extension of the dual-site redox rate law for the standard SCR reaction discussed in the section Unsteady-State Kinetics of the SCR Reaction, equation 27, to NO2 - containing feeds It is assumed in this case that the rate-limiting reoxidation of V-sites is carried out in parallel by gaseous oxygen and by surface nitrates, formed by NO2. Of course, the latter route prevails by far as long as NO2 is available. This provides a unifying approach to the kinetics of standard and fast SCR reactions. 

Reaction (1.1) is generally the standard SCR reaction . As NO2 is always present in the exhaust to some extent (maybe 10 % of NOx), Reaction (1.2) is also pertinent, and is in fact the fastest and preferred NOx reduction reaction. To promote this fast SCR reaction a DOC is commonly used to form more NO2 over platinum by the following reaction ... 

In this chapter, the reaction mechanisms of the standard SCR reaction are discussed from various perspectives including steady-state kinetics, the relations with NH3/NO oxidation ability and acid site amount, the effect of coexisting gases, and transient reaction behavior. Through these comprehensive analyses, some similarities and differences of the reaction mechanism among the conventional SCR catalysts could be extracted. Also, new perspectives on standard SCR mechanism could be suggested. 

Fig. 8.1 a NOx conversion under standard SCR reaction, b totai NH3 conversion and c NH3 -> NOx conversion under NH3 oxidation reaction, d NO -> NO2 conversion under NO... 

As mentioned in Sect. 8.2.3, the oxidation of NO would be a crucial step in the standard SCR reaction. In fact, there are many reports in the literature which suggest that the SCR activity is correlated with the NO oxidation conversion when the same type of active metal is compared [12-17]. On the other hand, some literature indicates that there is no correlation between them especially when comparing different active species, such as Fe and Cu [11, 18-20]. In this section, the reason for this inconsistency has been discussed. 

Regarding standard SCR reaction, on the other hand, the NO2 desorption does not seem to be the rate-determining step because produced NO2 should react with NH3 immediately, which is elucidated and discussed in more detail in the other sections. Therefore, it is difficult to make a correlation between SCR activity and... 

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