Ammonia temperature-programmed

Lok, B.M., Marcus, K.K., and AngeU, C.L (1986) Characterization of zeolite addity. 11. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTIR spectroscopy techniques. Zeolites, 6, 185-194. 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

Evidence of poisoning of acidic sites was also obtained by IR spectroscopy. The number of protons on the ferrierite surface was estimated by the intensity of the OH band at 3600 cm". After a few hours on stream, the absorbance of this OH vibration was reduced by a factor of 2.4 in contrast, the acidity of the sohd probed by ammonia temperature-programmed desorption (TPD) decreased by a larger factor (53,58). These results were considered as evidence of the poisoning of a large number of acidic sites and a partial blocking of pores, rendering some acidic sites inaccessible to the reactant. 

Reactivity and acidity Ammonia temperature-programmed desorption (NH3TPD) Calorimetry liquid-liquid, liquid-solid and gas-solid Probe molecule microreactivity testing... 

Ammonia temperature programmed desorption (NH3-TPD) curves were recorded by placing 1.5 g of zeolite particles (60x140 mesh) into a tube furnace and heated to 125°C. Aimnonia was supplied to the zeolite in dry air at a concentration of 1,000 mg/m (1,450 ppm) and a flow rate of 2 L/min. The aimnonia flow was terminated once the effluent concentration equaled the feed concentration, and clean air was subsequently purged through the zeohte imtil ammonia was no longer detected in the effluent. The temperature was then... 

Alternatively, can be used to calculate the activation energy of adsorption for the chemisorbed base, which is proportional to the heat of adsorption. However, determination of heats of adsorption from ammonia temperature-programmed desorption (TPD), although popular for its simplicity, may lead to inconsistent data because the experimental protocols may greatly influence the results [21]. 

Fig. 3.51 Ammonia temperature-programmed desorption from the iron (111) face with and without co-adsorbed potassium ...

Figure 4.10. Ammonia temperature-programmed desorption from clean Fe(lll) and K/Fe(lll) surfaces/ The desorption temperature of ammonia from Fe(lll) is lowered in the presence of potassium. Thus potassium lowers the adsorption energy of ammonia on the iron surface.

The studies on reaction rates and ammonia temperature-programmed desorption suggest that planes containing C7 sites, such as the Fe(211) and Fe(lll) surfaces, are being created during the water-vapor pretreatments whether aluminum oxide is present or not. However, only when aluminum oxide is present will these active surfaces remain stable and not reconvert to less active surfaces [i.e., Fe(llO) and Fe(lOO) planes]. 

Figure 5. Ammonia temperature-programmed desorption curves of various titanium oxide samples.

The number of acid sites on pillared montmorillonite was determined by means of ammonia temperature-programmed desorption(TPD). In each TPD experiment, a sample of 0.5 g was treated in vacuo at 673 K for 1 h and then cooled to 373 K. Ammonia was adsorbed at 373 K for 30 min and evacuated for 30 min. This sample was heated from 373 to 973 K at a rate of 10 K/min and the desorbed ammonia was monitored by thermal conductivity detector. As water was simultaneously desorbed with ammonia above 673 K, the ammonia TPD spectrum was obtained by point-by point subtraction of the water desorption spectrum obtained with the sample which had not adsorbed ammonia. 

Temperature Programmed Desorption (TPD). Chemisorbed molecules are bonded to the surface by forces dependent on the nature of the sites. For instance, ammonia will be strongly adsorbed on acid sites, whereas it is only weakly adsorbed on basic sites. Consequently, the adsorbate complex formed with the basic sites will decompose at lower temperatures than that formed with the acid sites. The following example regarding the NH.i-zeolite H-ZSM-5 system will illustrate this. 

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.

It has been known for some time that the spectroscopic signature of Ob-vacs can be healed by exposure to 02 [42-46], In addition, Epling etal. [47] show that temperature-programmed desorption (TPD) spectra of water and ammonia are perturbed when the surface is predosed with 02. This implies that oxygen is left on the surface in some form when Ob-vacs are healed by 02, As such, Epling et al. proposed that one Ob-vac is healed per 02 molecule with the other O atom being adsorbed at a Ti5c site (Oad), a dissociation mechanism supported by theoretical calculations [48, 49]. 

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. 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

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

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. 

The surface nethoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabilities were studied by Temperature-Programmed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed ammonia was... 

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