Zeolite ammonia-loaded

Figure 234 (A and C) IR spectra recorded after the adsorption of increasing amounts of NH3 on zeolite TON (Si/AI = 27) at a temperature of 400 K, with the ammonia loadings given in the figure (B and D) the difference spectra (differences between the spectra of two consecutive additions of ammonia). The symbols M and D indicate the bands of monomeric ammonium ions and dimers, respectively. Reproduced from Ref. (428).

Kubinski DJ, Visser JH (2008), Sensor and method for determining the ammonia loading of a zeolite SCR catalyst. Sensor and Actuator, 130 425 29... 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

Despite the industrial importance of amines and imines, hydroamination, i.e. the direct reaction of alkenes or alkynes with primary or secondary amines, is only used in one commercial process where isobutene and ammonia are converted in the presence of a zeolite catalyst to /-butylaminc. Turnover frequencies are generally very low and consequently, high catalyst loadings are necessary, which in turn demands efficient recycling. 

Temperature-programmed desorption of ammonia from molybdenum-loaded Y-zeolites... 

The aim of this study was to measure the acidity of several dealuminated zeolitic supports and their Mo-loaded equivalents via temperature-programmed desorption (t.p.d.) of ammonia and correlate it with their ability to decompose ammonia. 

Figures 3 and 4 show the results of ammonia t.p.d. from both Mo free and Mo-loaded dealuminated zeolites. These results are standardised to a imiform sample weight, therefore the intensities of the MS signals are comparable in all cases. Moreover, blank experiments with Mo-loaded zeolites, but without ammonia adsorbed, indicated that traces of CO (the same m/e=28 as for nitrogen) do not influence the signal intensity of N2 during decomposition of NH3.

For the adsorption of two methanol molecules per bridging hydroxy groups (2 1 loading) studies on all zeolites, SOD [29], CHA [22, 30], FER [22] agree that a pro-tonated methanol dimer is formed (Fig. 22.3). The obvious reason is the high PA of the methanol dimer that exceeds even the PA of ammonia (Table 22.1). 

Selective Catalytic Reduction (SCR) using ammonia as the reductant provides NOx reduction levels of greater than 80%. Three types of catalyst systems have been deployed commercially noble metal, base metal and zeolites. Noble metals are typically washcoated on inert ceramic or metal monoliths and used for particulate-free, low sulfur exhausts. They function at the lower end of the SCR temperature range (460-520°F) and are susceptible to inhibition by SOx [14]. Base metal vanadia-titania catalysts may either be washcoated or extruded into honeycombs [11]. Typically washcoated catalysts are only used for treating particulate-free, clean gas exhausts. Extruded monoliths are used in particulate-laden coal and oil-fired applications. The temperature window for these catalysts is 600-750°F. Zeolites may also be washcoated or extruded into honeycombs. They function at relatively high temperatures of 650-940°F [15]. Zeolites may be loaded with metal cations (such as Fe, Cu) to broaden the temperature window [16]. 

Results consistent with those of ammonia adsorption were obtained in the analysis of supported vanadia and molybdena. The development of protonic acidity with the increase of the vanadia loading on dealuminated BEA zeolite can be easily traced by adsorption of pyridine (Figure 2.39). 

Another approach to managing the stored ammonia for improved low-temperature performance is described by Yasui et al. [53] and illustrated in Fig. 1.15. They use two Fe-zeolite SCR catalysts placed downstream from the DPF system. An ammonia sensor is placed between the two SCR catalysts, and ammonia is generously injected to keep the first catalyst loaded at all times, as conditions allow. This accomplishes two goals. First, the efficiency of the SCR system is improved as there is plentiful ammonia present in the system. More importantly, the strategy helps cold start management. In traditional cold start thermal management, the SCR catalyst is heated as fast as possible to get it. Here, the catalyst is always loaded with ammonia, and the catalyst is heated slowly to prevent rapid release of ammonia during this period. 

The efficiency of Cu-BEA and Cu-ZSM-5 as SCR catalyst coupled with Pt/ Ba0/Al203 was compared by De La Torre et al. in a very recent paper [68]. Both zeolites lead to very active co-catalysts in promoting the NOx reduction by the NSR catalyst alone. The optimal Cu loading is obtained for 1.4 % Cu in ZSM-5 and 2.1 % Cu in BEA (Table 19.5). Cu-ZSM-5 and Cu-BEA can increase the NOx conversion by 20-30 % in the 200-300 °C temperature range. A significant formation of ammonia is observed on the NSR catalyst alone which is used for the SCR reaction (a part of NH3 being oxidized by O2). Cu-ZSM-5 and Cu-BEA have very similar effects so that activity per Cu ions appears higher over Cu-ZSM-5. 

The two zeolite catalysts were characterized by De La Torre et al. [68]. Total acidity is higher over BEA but ZSM-5 shows a higher number of strong acid sites desorbing ammonia beyond 220 °C. In the optimized catalysts (1.4 % Cu-ZSM-5 and 2.1 % Cu-BEA), all the copper remains in the Cu " state. Increasing Cu loading leads to H2/CU < 1 in TPR experiments, which confirms the formation of Cu" and may be Cu° species. Reduced species of copper appear to be less active and less selective to N2 (higher formation of N2O). 

A calorimetric and IR study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [192]. The active sites for both N2O and CO are Cu(I) ions, which are present as a result of the pre-treatment in vacuum at 673 K. The measured amounts of chemisorbed species in the investigated systems and the values of differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mol ) and carbon monoxide (between 140 and 40 kJ mor ) demonstrate the dependence of the adsorption properties on the copper content. The samples were additionally characterized by ammonia adsorption microcalorimetry at 423 K [192]. 

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