Temperature Schedules in Thermal Desorption

Heating of the adsorbent in the thermal desorption method can be performed either in a continuous or in a stepwise manner. Most usually a continuous heating is applied. 

If treatment of the experimental data used by Ehrlich (27), by Ageev, Ionov, and Ustinov (28), and by Lapujoulade (66) is employed, a mere empirical knowledge of the time-temperature dependence is sufficient (see Section I). Most authors, however, use an analytical approach to the treatment of the desorption data, where a simple analytical expression of the time-temperature function is essential. In such a case, the following two schedules are employed due to their feasibility and relative simplicity from the mathematical point of view (i) The temperature T of the adsorbent increases linearly with time t  

Carter et al. (79) have theoretically investigated the exponential increase in temperature expressed by 

Carter and Armour (80) suggested still other special temperature schedules which, however, appear to be of little practical importance. 

A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values Tx, Ts,. . ., Tn for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra s(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd is amenable as well as a better resolution of superimposed peaks on a desorption curve (see Section VI). What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. 

---