Determination of the thermal desorption kinetic parameters for samples with a temperature gradient

An application of the thermal desorption technique to the study of desorption from the samples with a temperature gradient is discussed. The kinetics of first- and second-order desorption from linearly and exponentially heated samples with a parabolic temperature profile is considered. It is shown that the low-temperature part of the thermal desorption curve is described by the same equations as those for the desorption from the nongradient surface with the less effective area and with the temperature equal to that at the center of the nonuniformly heated sample. The approximate analytical expressions for the amount of adsorbed surface species as a function of time are derived. These expressions enable to find the kinetics order, the activation energy E and the preexponential factor k₀ for the desorption process from thermal desorption spectra. In a first approximation the corrections for the nonuniformity of the sample temperature do not substantially change the value of E but slightly increase the value of k₀. The correction procedure for k₀ is described in detail. The possible application of the proposed method to various experimental conditions is discussed.