NO conversion

Performance criteria for SCR are analogous to those for other catalytic oxidation systems NO conversion, pressure drop, catalyst/system life, cost, and minimum SO2 oxidations to SO. An optimum SCR catalyst is one that meets both the pressure drop and NO conversion targets with the minimum catalyst volume. Because of the interrelationship between cell density, pressure drop, and catalyst volume, a wide range of optional catalyst cell densities are needed for optimizing SCR system performance. 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

The most popular SCR catalyst formulations are those that were developed in Japan in the late 1970s comprised of base metal oxides such as vanadium pentoxide [1314-62-1J, V20, supported on titanium dioxide [13463-67-7] Ti02 (1). As for low temperature catalysts, NO conversion rises with increasing temperatures to a plateau and then falls as ammonia oxidation begins to dominate the SCR reaction. However, peak conversion occurs in the temperature range between 300 and 450°C, and the fah-off in NO conversion is more gradual than for low temperature catalysis (44). 

Typically NO conversion ranges from 80 to 95% and there are corresponding decreases in CO and hydrocarbon concentrations. Potential problems associated with NSCR appHcations include catalyst poisoning by oil additives, such as phosphoms and 2inc, and inadequate control systems (37). 

Figure 4.51. Transient effect of a constant applied current on the rates of C02, N2 and N20 production, on NO conversion (XN0) and on catalyst potential (Uwr) during NO reduction by propene in presence of gaseous 02 on Rh/YSZ.70 Reprinted with permission from Elsevier Science.

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

In the context of this study, the extent of reaction refers to the conversion of sites from active to inactive, and is given by Equation 6 (i.e., x = 1 for no conversion, x = 0 for total conversion). For a single site mechanism it can be shown easily that F(x) reduces to 1.0. Solution of Equation 7 and substitution into Equation 3 yields the expected result ... 

The use of the enolsilyl ether of 1-menthone [16, 19, 21-23] and of some free triflic acid favors the formation of the thermodynamically controlled products as with free 2,2 -dihydroxydiphenyl [22] and only subsequently added HMDS 2 [22]. On reacting silylated alcohols and carbonyl compounds with pure trimethylsilyl triflate 20 under strictly anhydrous conditions no conversion to acetals is observed [24]. Apparently, only addition of minor amounts of humidity to hydrolyze TMSOTf 20 to the much stronger free triflic acid and hexamethyldisiloxane 7 or addition of traces of free triflic acid [18-21, 24, 26] or HCIO4 [25] leads to formation of acetals. 

The NO and NO2 conversion data (Fig. la, Ic and le) indicate that the presence of NO improves the NO2 conversion while NO2 improves the NO conversion, with some minor exceptions in the case of V20s/Ti02 catalyst. It is particularly noteworthy in Fig. le that a small amotmt of NO (i.e., NO2/NOx=0.75) can make a big improvement in the NO2... 

When a 1 1 mixture of NO and NO2 (i.e., NO2/NOx=0,5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6] Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction... 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

Figure 10.16. NO conversion and ammonia slip as a function of the NH3/NO ratio in the presence of O2 and H2O over a V203/Ti02 catalyst at 623 K. The lines represent the model based on reactions (9)-(14) and the parameters in Tab. 10.7. [Adapted from).A. Dumesic, N.-Y. Topsoe, H. Topsoe, Y. Chen, and T. Slabiak, J. Catal. 163 (1996) 409.]...

At higher temperatures the NO conversion is enhanced for the Pd/Al-Zr-Ba and Pd/Al-Zr solids but the Pt-Rh/Al203 solid remains the best one At the maximum of conversion, the activities obey the order ... 

Figure 2 NO conversion with the CO-NO-O2-C3H6-H2O (10 vol %) (s = 1.03) mixture on Pd/AljOj, A Pd/Zr02, Pd/AljOj-ZrOj, O PtRh/AljOj and Pd/AljOj-ZrOj-BaO...

The catalytic properties were characterized in a simplified manner by two parameters the maximal conversion Cm and the corresponding temperature Tm- The selectivity of NO conversion to N2 is always very high (> 98%). The formation of NO2 is marginal on these Cu catalysts. 

The different catalysts are compared in Table 6. The addition of sulfur dioxide has no effect on activity in the case of zeolites and slightly inhibits C11/AI2O3, but promotes NO conversion on Cu(l)/Ti02 and Cu(4)/Zr02. The activity is practically doubled for Cu(4)/Zr02 with a small shift of the optimal temperature. 

Reasonable NO conversion can be achieved using n-decane as reductant. In the absence of sulfur dioxide, the catalytic activity is roughly related to the r ucibility of the Cu phase of Cu ions in zeolites the reaction temperature needed to reach 20% NO conversion parallels that of the TPR peak (Table 7). This relation also practically holds for Cu on simple oxides, therefore a redox mechanism in which reduction of Cu + cations is the slow step could account for the results. 

Comparison of the temperatures needed to obtain 20% NO conversion and of the reducibility expressed by the temperature of the first peak in TPR by hydrogen. 

Figure 5. NO conversion on Cu/Ti02 catalysts prepared according different procedures. The feed is the standard one containing 20 ppm of sulfur dioxide.

In order to explore the effect of acid sites on Ihnetallic solidsa sample wasprqparedfromthe ammonic form ofthe mordenite. In this case, a decrease of about 30% in the NO conversion was observed when it was evaluated under the same conditions (Table 2). 

Figure 4 Effect of NO on the N2O Figure 5 Product composition for a conversion at 0.1 kPa N2O and space NO/N2O feed mixture over Co-ZSM-5 at time W/Fn2o= 1.52 10 g.s/mol. Included 0.1 kPa N2O and space time W/Fn2o= is the NO conversion (open symbols). 1.52 10 g.s/mol.

The effects of precious metals on ln/H-ZSM-5 was found not only to simply catalyze NO oxidation but also to enhance NOx chemisorption. It is noted that NO conversion on the lr/ln/H-ZSM-5 exceeded NO2 conversion in NO2-CH4-O2 reaction on in/H-ZSM-5, when the concentration of NOx was decreased [14]. This study shows the catalytic activities of ln/H-ZSM-5 promoted by precious metals for the removal of low concentration NOx and the promotive effects of the precious metal will be discussed. 

Figure 2 shows the effect of NOx concentration on the conversion of NOx reduced by CH4 in the presence of 5% H2O. In the NO-CH4-O2 system, In/H-ZSM-5 showed low catalytic activity in the whole range of NO concentration. On the other hand, this catalyst was active for the NO2-CH4-O2 reaction, while the conversion of NO2 decreased with decreasing concentration of NO2. The catalytic activity of ln/H-ZSM-5 for the reduction of 1000 ppm NO was enhanced by the addition of Ir and R almost to the level of NO2 reduction on ln/H-ZSM-5, indicating that these precious metals worked as the catalytic sites for NO oxidation, which is a necessary step for NO reduction with CH4. With decreasing NO concentration to 100 ppm, however, the increase in NO conversion was observed on lr/ln/H-ZSM-5 and the conversion of NO exceeded that of NO2 on ln/H-ZSM-5. This can not simply be explained by the catalytic activity of Ir for NO oxidation. 

In samples ZV(a) (Fig. 6, a), ZV(i) and ZV(acac) (Fig. 6, b), NO conversion increased with V-loading at all temperatures. In the whole temperature range, the selectivity to N2 was very high. A small amounts of N2O (< 3%) were detected only above 573 K. Pure zirconia showed some SCR activity at T > 573 K, comparable with... 

In the absence of O2, NO reduction continued, however at a rate about ten times lower than that in the presence of O2. During 20 h experiments NO conversion remained constant. On O2 addition, the catalytic activity increased with O2 content in the mixture up to about 1000 ppm, and changed little thereafter. We noticed that increasing the O2 concentration caused NO conversion to become lower than that of NH3, probably due to changes in the stoichiometry of the overall reaction (the NO/NH3 ratio passed from 1.5 to 1). Catalytic tests of NH3 oxidation with O2 yielded high selectivity to N2 (66-90%), which decreased with the higher loading catalysts. 

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