TPD method

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

After the chemisorption step the oxygen was swept by flowing Ar, and temperature-programmed desorption -TPD method- was performed in the TGA by heating the sample at 15°C min from room temperature to 1000 °C. The desorbed gases (CO and CO2) were followed by means of a mass spectrometer (MS). The optimisation of the coupling system and the parameters used have been described elsewhere [6]. 

The lower ASA values obtained by the TPD method are probably due to the formation of C-0 complexes which are stable even at 1000 °C [11], and thus not included in the quantification of the CO and CO2 desorbed during TPD. 

The adsorption property was measured by a static method at 30 °C with a conventional volumetric apparatus as well as by the temperature programmed desorption (TPD) method. The details of the pretreatment and adsorption procedures were shown in Results and Discussion section. Metal-loaded zeolite samples were characterized by XRD, diffuse reflectance UV-Vis spectroscopy (DRS) and electron spin resonance (ESR). 

A systematic series of synthetic, characterization and butene isomerization catalysis studies of ferrierite and ferrierite-like materials such as ZSM-22,59 ZSM-23,60 and ZSM-35,6 was undertaken to study optimization of isobutylene product. Coke deposits in the pores of these materials play a key role in isobutylene formation as does the overall acidity and structure of the pore system. Such shape selective effects have been probed with TPD methods. 

Our purpose in this text is principally to present temperature scanning methods. These generally involve multiple rampings as one seeks to delineate the kinetics of a system over a wide range of conditions. However, there is a well known and established technique for the semi-quantitative study of desorption phenomena, the Temperature Programmed Desorption (TPD) method. The equations developed below are also applicable to results that can be obtained using some of the versions of the traditional TPD apparata. In such cases they can be used to quantify the TPD results to yield the kinetics of the process and/or to check for extraneous influences that can result in anomalous results, effects such as mass diffusion, heat diffusion, or purely kinetic effects. 

From the viewpoint of solid acid catalysis, the modified PTSM by Group 1 and 5 cations, is noteworthy. The cracking activities of modified catalysts fairly correlate with their acid amounts measured by the TPD method of adsorbed ammonia. The schematic structures of M" -PTSM catalysts can be depicted as shown in Figure 14-2, in connection with the fixation mode of Group 1-5 cations (Sakurai, Urabe, and Izumi 1990). However, this classification did not to be applicable to all cations, provide interesting exceptions. 

The basis of temperature programmed desorption (TPD) method is chemisorption of base vapor, ammonia being most often used, on the surface of an acidic catalyst at a given temperature, followed by its desorption as a result of a temperature rise. The area of the obtained desorption peak gives the total acid site density and the maximum of the peak characterizes the activation energy of desorption and may be considered as a measure of the acid strength. 

Characterization of Acidity/Basicity. The acid-base properties of a metal oxide play a crucial part in catalysis and are generally a function of pretreat-ment/preparation variables and catalyst morphology. It is important to be able to fully characterize the surface in terms of the strength and number of acid and base sites. Frequently, TPD methods involving probe molecules have been applied. However, caution must be exercised to simple TDP measurements, as it is often not clear whether the probe molecule is behaving in the acid-base manner assumed. An illustrative example of this is provided in a study of desorption of NH3 from CaO, in which the high temperature of desorption may lead one to believe that CaO is a strong acid (280). The explanation for this... 

Temperature-programmed desorption (TPD) (Cvetanovic and Amenomiya 1972) yields not only the heat of adsorption but also information on groups of different active sites, which are useful in characterizing reactions. While the flash filament method in which an adsorbate is desorbed from a rapidly heated filament in an ultra-high vacuum environment has been used for a long time, the usefulness of the TPD method lies in the fact that the adsorbed gas is desorbed in a programmed... 

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