Desorption of ammonia

The nature (Bronsted or Lewis centers), the number, and the strength of the acidic sites of the Pd/Al203 and Pd/Zr02 solids have been checked using infixed spectroscopy of adsorbed pyridine and thermoprogrammed desorption of ammonia. 

Danielson LR, Dresser MJ, Donaldson EE, Dickinson JT. 1978. Adsorption and desorption of ammonia, hydrogen and nitrogen on Ru(OOOl). Surf Sci 71 599. 

N2-BET analysis and porosity measurements were done on a Micrometries ASAP 2000 apparatus at liquid nitrogen temperature. Temperature-programmed desorption of ammonia (NH3-TPD) and temperature-programmed reduction by H2 (H2-TPR) were performed with a Micromeritics AutoChem 2910 apparatus. 

The acidic properties were studied by temperature programmed desorption of ammonia (TPD-NH3). TPD experiments were carried out in the temperature range of 20 to 780°C in a flow of dry He (30 ml/min.). The rate of heating was 8 °C/min. 

TEM image, 64 temperature, 224 temperature effect, 162 temperature-programmed desorption of ammonia (TDP), 77... 

Figure 4.29 IR spectra of H-MOR sample after pretreatment at 500°C and after adsorption/ desorption of ammonia at 150°C. All spectra recorded at 25 °C.

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

Figure 5.60 Thermal desorption of ammonia from Mg(NH3)6Cl2.

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. 

Figure 13.5 Adsorption-desorption of ammonia at 280 " C on a model V2O5—WO3/TiO2 catalyst Dashed lines, inlet NH3 concentration triangles, outlet NH3 concentration solid lines, model fit with Temkin-type coverage dependence. Adapted from ref. [3].

This empirical rate expression considers the active sites of the catalyst as only a fraction of the total adsorption sites for ammonia and is consistent vfith the presence of a reservoir of ammonia adsorbed species which can take part in the reaction. The ammonia reservoir is likely associated vfith poorly active but abundant W and Ti surface sites, which can strongly adsorb ammonia in fact, nhs roughly corresponds to the surface coverage of V. Once the ammonia gas-phase concentration is decreased, the desorption of ammonia species originally adsorbed at the W and Ti sites can occur followed by fast readsorption. When readsorption occurs at the reactive V sites, ammonia takes part in the reaction. Also, the analysis of the rate parameter estimates indicates that at steady state the rate of ammonia adsorption is comparable to the rate of its surface reaction with NO, whereas NH3 desorption is much slower. Accordingly, the assumption of equilibrated ammonia adsorption, which is customarily assumed in steady-state kinetics, may be incorrect, as also suggested by other authors [55]. 

In addition to adsorption and desorption of ammonia (r, and rj, respectively, with... 

Acidity and acid strength distribution were evaluated by temperature-programmed desorption of ammonia. The acid sites were classified as weak < 300°C, medium (M) 300 < T < 450° 0 and strong (S) 450 < T < 550° 0. Ultra strong acid sites (S in a. u ) are defined as a ratio of peak height for catalyst to alumina (Ni cat./AfjOj) at 550° C,... 

Fig. 8. Mossbauer spectra of adsorption and desorption of ammonia on an iron-on-silica gel catalyst at 25°C. [From M. C. Hobson, Nature (London) 214, 79 (1967).]...

Catalysts Characterization. Following pretreatment of the SAPO molecular sieves, the catalysts were characterized by temperature programmed desorption (TPD) of ammonia and infrared spectroscopy. To assess the acidity of the samples, the desorption of ammonia from the catalysts was performed in a manner similar to that described by van Hooff et. al. [11]. For the ammonia TPD experiments, typically 0.1 gram of the molecular sieve sample was supported on quartz wool inside a 9 mm O.D. quartz reactor equipped with axial thermowell which contacted the top of the... 

H-[Fe]-ZSM-5 in air at high temperatures, the coverage of 2-propanamine corresponding to desorption of ammonia and propene is decreased by more than 80% based on the relative areas under the propene peak. 

Acid Strength Distribution After Stabilization Treatment. The acid strength distributions of the zeolites were determined by temperature-programmed desorption of ammonia equipped with a thermal conductivity detector. The detailed procedure has been given in the early paper (6). 

The temperature programmed desorption of ammonia was performed at normal pressure in a flow reactor with He as carrier gas. The flow rate was 1 cm s, the NH, concentration 3 Vol. % and the heating rate amounted to 10 K min. Sample weights of 200 mg were used. 

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

Temperature-programmed desorption was used to monitor water desorption (TPE-temperature programmed evolution) during the solid-state exchange in vacuum as well as for studies of acidity of the samples via adsorption and desorption of ammonia. TPD of ammonia (TPDA) was carried out after NH3 adsorption at 395K for 0.5h on 10 mg discs of samples followed by desorption at the same temperature for Ih. The heating rate was lOK/min. A mass spectrometer (Balzers QMG 311) was applied to determine the desorbed ammonia and water. 

Figure 3. Temperature-programmed desorption of ammonia for Zn-H-ZSM-5 catalyst.

The LT (Low Temperature) peaks observed on TPD curves are commonly related to desorption of ammonia physisorbed at 353K and chemisorbed on weak... 

Figure 4 Activation of energy of desorption of ammonia measured by TPD as a function of coverage for a series of H-Y zeoiites with different iattice aiuminum content O HjoNasAise-Y H3,tte2Ai4o-Y A Hj NaAias-Y."...

Aboul-Gheit has developed a method for using differential scanning calorimetry (DSC) to measure acid strength distributions. The desorption of ammonia gives three DSC signals which Aboul-Gheit attributed to desorption from weak sites, Lewis sites, and Brpnsted sites. 

The reaction energy diagram (Fig. 4.36) illustrates competition between dissociative adsorption of the nitrogen molecule and desorption of ammonia. This confirms the conceptual basis of the Sabatier effect (3.7) that predicts that the rate is maximum at optimum interaction with the catalyst surface the interaction energy where the rate of dissociative adsorption or another surface reaction and desorption rate are in balance. 

NH3TPD Thermal desorption of ammonia was used to characterize the acid strength of the support and catalysts. A McBain microbalance was employed using 1 mg of sample. Ammonia was adsorbed at room temperature and the total mol of NH3 10 /m adsorbed were measured. Then the sample was heated using a ramp of 6 °C/min and the remaining amount of NH3W calculated at three temperatures (200/300/400 °C). See Table 3. 

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