Temperature-programmed desorption ammonia

Temperature Programmed Desorption. Ammonia has been used as a probe molecule in a number of studies of crystalline borosilicate molecular sieve (22,33,45). It has been shown that the ammonia is desorbed from the borosilicate samples at low temperature, 465°K, indicating the weak acidity of the hydroxyls (33). The hydroxyls have been shown to have higher acidity than silanol groups and lower acidity than those of ZSM-5 (45). The results are consistent with IR and calorimetric data for NH3 adsorption/desorption (35.). ... 

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... 

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. 

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. 

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. 

The number of acid sites on pillared clays was determined by means of temperature programmed desorption (TPD) of ammonia. In each TPD experiment, a sample weighing about 0.5 g was treated in vacuo for 1 h at a given temperature in the range 400 - 600°C. Ammonia was adsorbed at a desired temperature (100-300°C) for 30 min and evacuated for 30 min. This sample was heated to 700°C at a rate of 10°C/min and desorbed ammonia was monitored by thermal conductivity detector. As water was desorbed simultaneously with ammonia, 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. 

Ammonia TPD Measurement. The acidic properties of the catalysts were characterized using temperature programmed desorption (TPD) of ammonia. The experiments were carried out on a flow-type apparatus equipped with a fixed-bed and a thermal conductivity detector. The samples were activated in a helium flow of 5 L/h at 773 K for 1 hour. 300 mg of the H+-form of each dehydrated sample were used to perform the ammonia TPD. Pure ammonia, with a flow rate of 3 L/h, was then passed through the sample at 423 K for 30 min. The sample was subsequently purged with helium at the same temperature for 1.5 hours in order to remove the physisorbed ammonia. The TPD was performed under a helium flow of 6 L/h from 423 K to 873 K with a heating rate of 10 K/min and subsequently at the final temperature for 30 min. 

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,... 

Temperature programmed desorption (TPD) of ammonia shows weak acidic centers on the Al-MCM-41 carrier material. On the immobilised complex the amount of desorbed ammonia slightly decreases compared to the pure carrier material. This indicates an interaction of the immobilised complex with acidic sites on the Al-MCM-41. This result is also supported by IR-spectroscopy. The infrared spectra show no change of... 

The most likely reason for the high activity of zeolite BEA is the relatively high BET surface area of the catalyst (750 m2/g). Furthermore there are hints by temperature-programmed desorption (TPD) of ammonia that a large amount of acid sites are present. We assume that the alkoxylation of limonene takes place inside the pore structure of the beta zeolite. The high selectivity of zeolite BEA might originate from suitable acid sites in pores of its defined size and shape. 

The most likely reasons for the high activity of beta zeolite are the relatively high BET surface area of the catalyst (750 m2/g) and the large amount of acid sites measured by temperature-programmed desorption (TPD) of ammonia(58). 

Changes in the catalytic activity of activated alumina surfaces have also been probed by Temperature Programmed Desorption (TPD) and FTIR photoacoustic measurements [74]. Ammonia TPD allows a fast and convenient determination of the overall acidity of a solid surface. The desorption profiles provide information on the distribution, the amount and strengths of the acid surface sites, since molecules adsorbed at weaker sites desorb at lower temperatures than those adsorbed at stronger acidic sites. The activation of y-alumina with CHC1F2 resulted in a sig-... 

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... 

In this paper, we will discuss the use of temperature programmed desorption as a complementary technique for obtaining lattice concentrations for high-silica materials. While TPD of ammonia is commonly used as a measure of the number of acid sites in zeolites, this paper will discuss the use of the reactive probe molecules, 2-propanamine and 2-propanol. It has previously been demonstrated that well-defined adsorption complexes, with a stoichiometry of one molecule/Al atom could be observed in high-silica zeolites for a number of adsorbates. These complexes were found to be independent of Si/Al2 ratio for a series of H-ZSM-5 zeolites and were unaffected by... 

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). 

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. 

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