Isothermal desorption kinetics of benzoic acid on a single SnO2 microhotplate sensor
The authors describe a technique that utilizes the fast heating rates 106 K/s of a microhotplate sensor along with a calibrated thermal desorption system to determine the initial coverage and kinetic parameters using isothermal desorption on a millisecond time scale. Models for isothermal desorption including both pumping and desorption rate effects are presented for zero, first, and second order kinetics. Analysis of the first order model illustrates the domain of the desorption, pumping speed, and heating rate time constants that permit the desorption parameters to be estimated from the mass spectrometer signal. The technique is demonstrated using isothermal temperature programed desorption of benzoic acid from a single SnO2 covered microhotplate at surface temperatures ranging from 296 to 347 K. The data indicate that desorption is best represented by first order kinetics. The first order preexponential factor and the desorption energy in the zero coverage limit are determined to be 11017 s−1 and 97 kJ/mol, respectively, from desorption of 108 molecules which corresponds to an initial coverage of 1012 cm−2 0.005 ML.