Standard-SCR

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],... 

Standard-SCR is also the basic reaction over SCR catalysts in diesel vehicles, as more than 90% of the NO in diesel exhaust is composed of NO under usual operation conditions. 

Figure 9.6. Compliance of the SCR stoichiometry in dependency of the vanadia concentration under standard-SCR conditions. Extruded V205AV03—Ti02 catalysts with ( ) 3%, (A) 1.9% and ( ) 1.7% V205. (a) Thermal treatment at 550°C for 50 h. (b) Thermal treatment at 550°C for 50 h and at 600°C for 30 h.

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. 

Comparison of metal-exchanged zeolites and vanadia catalysts under standard-SCR conditions... 

Figure 9.24. DeNO at 10 ppm NH3 slip for the ( ) fresh, ( ) dry aged, and (A) wet aged Cu-ZSM5 monolith catalyst under standard-SCR conditions.

The main reactions, which have to be considered on SCR catalysts, are the standard-SCR, fast-SCR, and the N02-SCR reactions, beside the ammonia oxidation and the formation of N20. The fast-SCR reaction is promoted by N02 in the feed that can be generated from NO in a pre-oxidation catalyst. However, the right dimensioning of the oxidation catalyst is critical in order to prevent the production of an excess of hazardous N02. This problem is further aggravated if a continuous regenerating DPF is installed in front of the SCR system, as part of the N02 produced by the oxidation catalyst is always consumed in the filter for soot oxidation. 

The standard SCR process is based on the reduction of NO with ammonia to water and nitrogen according to the following main reaction ... 

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. 

To describe the NH3 + NO + O2 (standard SCR) reacting system, NH3 adsorption-desorption, ammonia oxidation to nitrogen and standard SCR have been considered with the kinetics already presented in the previous section. 

This reaction has been known since the 1980s to be faster by one order of magnitude than the standard SCR (Equation 13.1) at low temperatures [64]. 

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

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] ... 

This represents the typical feed mixture to SCR converters when no oxidation precatalyst is applied, as for instance in the case of NOx abatement from stationary sources. With such a feed mixture the main deNOx reaction occurring over V-based catalysts is the so-called standard SCR (R6 in Table V). Transient experiments in a wide range of temperatures (50-550°Q were performed in order to develop a suitable kinetic model of the NH3-N0/02 reacting system. The study of the standard SCR kinetics was particularly focused on the characteristics that are critical for mobile applications, namely the behavior during transient operation and the reactivity in the low temperature region. 

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... 

Reactions R13, Table VI and R14, Table VI describe the redox cycle in the NO + NH3 standard SCR, where reaction (R14, Table VI) is the rate limiting reoxidation step involving gaseous oxygen. 

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. 

After the initial transient, NO and ammonia signals pointed to a steady-state conversion of about 40%. Again the standard SCR stoichiometry was respected. At time of about 300 s, the NF13 feed was shutoff. Consequently its outlet signal slowly decreased, while NO, after a short transient, reached its feed value of 1,000 ppm. Again, a good correlation between experiment and predictive simulation on the integral monolith reactor level was obtained the model was able to correctly predict both the steady-state levels of the reactants and their temporal evolution. 

As discussed, the low temperature deNOx efficiency of SCR converters for automotive exhaust aftertreatment can be significantly enhanced by converting part of the nitric oxide to N02, e.g. by means of a DOC located upstream of the SCR. In fact, the so-called fast SCR reaction, involving the reaction between NH3 and equimolar amounts of NO and N02, can be faster by one order of magnitude than the standard SCR in the low-T region (Ciardelli et al., 2007a Koebel et al., 2001). Effective exploitation of fast SCR reactivity is certainly important... 

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