Ammonia adsorption temperature

For decationated mordenites, an increase in the ammonia adsorption temperature caused a significant increase in the differential heat of adsorption at low coverages, the largest increase of the initial heat being from 130 to 170 kJ mol" , corresponding to Lewis acid sites on a 90% decationated mordenite that was dehydroxylated at 923 K (88-90). The effect of increasing the... 

Dehydroxylation at high temperatures produces a dramatic change in the acidity spectrum of HY zeolite. As the activation temperature of HY zeolites with a Si/Al ratio of about 2.4 is increased from 573-673 K to 873-923 K, the differential heat of ammonia adsorption at 423 K indicates that stronger acid sites are created in the range of 150 to 180 kJ mol" 91, 147, 149, 153, 154). Equivalent results are obtained for samples with the same Si/Al ratio and dehydroxylation temperatures but an ammonia adsorption temperature... 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

Most of the work with alumina was done, however, attempting to elucidate the nature of the catalytically active sites in dehydrated alumina. The catalytic activity of alumina is enhanced by treatment with hydrofluoric acid. Oblad et al. (319) measured a higher activity in the isomerization of 1- and 2-pentene. Webb (339) studied the effect of HF treatment on ammonia adsorption by alumina. There was no difference in the capacity. However, the ammonia was more easily desorbed at a given temperature from the untreated sample. Apparently, the adsorption sites grew more strongly acidic by the treatment. No NH4+ ions, only NHj molecules were detected by their infrared spectra, indicating that the ammonia was bound by Lewis acids rather than Bronsted acids. 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

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 pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The effect of temperature on ammonia adsorption by ZSM5 samples has been investigated by microcalorimetry, varying the adsorption temperature from 150 to 400°C [235]. The initial heats of adsorption were independent of temperature up to 300°C. When the adsorption temperature increased, there was a competition between the formation of ammonium ions on Brpnsted sites and their decomposition. The total number of titrated sites decreased with increasing adsorption temperature. It appeared that an adsorption temperature between 150 and 300°C is appropriate for these calorimetric experiments. 

In this study, we analyze this situation using Si-MAS-NMR spectroscopy and high-temperature ammonia-adsorption calorimetry. The acid strength will be determined from the heat of adsorption of ammonia. On adsorption of ammonia, the reaction. 

As expected from the TPD results, Al-sapo was more active for the cracking of cumene on a per weight of catalyst basis than Al-mont. In order to compare the catalytic activity on a basis of active sites, we evaluated the number of active sites on these catalysts. TPD spectra were measured with varying the temperature of ammonia adsorption. Typical results on Al-mont are shown in Fig. 2. By integrating these spectra, the concentration of acid sites corresponding to different strength of acidity can be determined. 

Because of the high molecular weight materials at the upper end of the boiling range for medium lube oil stocks, the desorption technique of choice should probably employ a displacement chemical with a high heat of adsorption, in order to overcome the high heat of adsorption of the in-paraffins. Ammonia at temperatures near 660 K and near atmospheric pressure appears to have good potential. 

In these terms, the ammonia adsorption is not entirely reversible. Figure 12.1 shows the total and irreversible ammonia adsorption (at room temperature) on Kieselgel 60, thermally pretreated at 473, 673 and 973 K. The adsorption capacity is exceptionally expressed as /nm2. The figure clearly demonstrates that the total ammonia adsorption on silica is determined by the silanol number, in a 1 1 relationship. This means that ammonia adsorption on silica involves an attachment (by a hydrogen bond) of 1 NH3 molecule to 1 silanol group. Such species produce an infrared band at 3419 cm1, assigned by Peri1 to the v3 valence vibration of ammonia. 

Thus, adsorption of NH3 on alumina resembles that of water in many respects. Both molecules are adsorbed molecularly at low temperatures but are chemisorbed dissociatively at higher temperatures. Ammonia is held strongly on A1203 surfaces and cannot be removed completely even on desorption at 500°C. Various species occur simultaneously, their relative importance being determined by the OH content of the surface. Furthermore, displacement adsorptions may take place. Thus, NH2" ions readily replaced chloride ions on surfaces of chlo-rided aluminas (166). One has, therefore, to conclude that ammonia retention on aluminas cannot be an acceptable measure of surface acidity and can hardly be related to catalytic activity. Ammonia adsorption on aluminas as studied by infrared spectroscopy, perhaps combined with TPD experiments (173), gives ample information on surface properties but ammonia cannot be used as a specific poison on alumina. 

Catalytic properties Phosphorus is known to have deactivation effects for some automotive catalysts and the formation of CeP04 has been identified in phosphorus contaminated catalysts (Uy et al., 2003). Nanocrystalline LaP04 would act as Lewis acid in a catalytic process, which could be determined by a temperature-programmed ammonia adsorption/desorption process (Onoda et al., 2002 Rajesh et al., 2004, 2007). In addition, the rare earth phosphate NCs could act as supports for example, Pd, Pt, or Rh supported on RPO4 show excellent catalytic reduction of NO into N2 and O2 (Tamai et al., 2000), and gold supported on RPO4 shows catalytic activity and stability for CO oxidation. 

Increasing the pre-treatment temperature modifies the surface acidity of the solids. For y-alumina, there are numerous surface models, and various acid sites having different strengths are formed on the surface during dehydration. The influence of the pre-treatment temperature, between 573 and 1073 K, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry. The number and strength of the strong sites, which should be... 

Ammonia adsorption was studied on several activated carbons with different textural and chemical characteristics by flow adsorption microcalorimeter. The textural and chemical nature of the samples was measured by N2 and CO2 adsorption and temperature programmed desorption (TPD-MS) respectively. The ammonia adsorption consists in reversible (related to physisorption) and irreversible (related to chemisorption on chemical groups) components. From the molar heats of adsorption it can be concluded that the samples have a wide distribution of acidic sites some of which are very strong. However, they are not always easily accessible to ammonia because constrictions in the pore-network hinder the access, forcing the adsorbed molecules to re-arrange. 

Recently, there have been various studies of the effect of the adsorption temperature on the acidic properties of metal oxide catalysts. Tsutsumi and co-workers (84,85) studied calorimetrically the adsorption of ammonia and pyridine on H Y and NaY zeolites, silica-alumina, and silica between 313 and... 

The small decrease of the heat of adsorption of ammonia with temperature can be explained by the following thermodynamic equation representing the temperature dependence of the heat evolved from a certain site at a given coverage (85) [derived from Eq. (32) in Section II,A,3] ... 

Kapustin et al. (87) studied the adsorption of ammonia on sodium mor-denites between 303 and 673 K (see Table V). As the temperature was increased on wide-pore mordenite (see Section V,B), the initial heat of adsorption decreased from 109 to 80 kJ moU. Using Eq. (96) it was calculated that for localized adsorption of a diatomic gas an increase in temperature of 100 K would produce a decrease of 1.3 kJ mol"whereas experimental differences observed for ammonia were between 4 and 13 kJ mol" . On a narrow-pore mordenite, the heats of adsorption at 303 and 573 K were identical and coincided with the heats at 573 K on the wide-pore mordenite. Apparently, in the wide-pore mordenite, the sodium cations are more weakly bound to the zeolite than in the narrow-pore mordenite, and as the adsorption temperature is increased, they change their localization which in turn changes the heat of adsorption. 

The results at 303 K indicate that NaY zeolite is only weakly acidic, displaying heats of adsorption between 94 kJ mol for ammonia (17, 57, 81) and 124 kJ mor for pyridine (81). This material also contains weak basic sites as determined by the heat of COj adsorption at 298 K (747). Increasing the temperature from 298 to 573 K confirms the effect of adsorption temperature on weakly acidic samples previously discussed in Section IH,A. As the adsorption temperature increases, the initial differential heat of ammonia adsorption increases slightly from 84 (35) to 94 kJ mol" (17,57,81) at 303 K before decreasing continuously to 65 kJ mol" at 573 K (55). Increasing... 

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