Temperature-Programmed Desorption TPD Methods

University of Belgrade, Nemanjina 6, 11080 Zemun, Serbia e-mail vesna.rakic tfh.bg.ac.rs 

University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia e-mail ljiljana ffh.bg.ac.rs 

Springer Series in Materials Science 154, DOI 10.1007/978-3-642-11954-5 4, Springer-Verlag Berlin Heidelberg 2013 

In general, temperature-programmed methods are applicable for the investigation of both porous materials (such as real catalysts) and well-defined surfaces of single-crystalline samples. In addition, their application is experimentally simple and inexpensive, what explains their wide application in several scientific domains. 

Since that time, the method is widely developed experimental setups are improved and adjusted to many different purposes (e.g. for the investigations of oxidation and reduction reactions). Today, two main types of equipment are available those operating under ultrahigh vacuum and so-called flow systems. WeU-defined surfaces of single-crystalline samples are investigated in a continuously pumped ultra-high vacuum (UHV) chamber (this technique is often referred to as thermal desorption spectroscopy—TDS [5]). The equipment that is constructed to allow adsorption-desorption in the gas flow are most often used for the investigation of porous materials (catalysts, for example). Vacuum setups are customarily used for surface science studies, but they can be also useful for the characterization of porous materials. 

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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 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). 

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. 

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. 

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