The effects of exposure time and pressure on the temperature-programmed desorption spectra of systems with bulk states

A mean-field model describing temperature programmed desorption (TPD) spectra for coupled reaction-diffusion (RD) systems is introduced. This finite medium model includes two states; a surface state and a state in the bulk which supports concentration gradients. A unified model which treats both exposure and desorption during a typical TPD experiment is presented. The substantial concentration gradients developed in the bulk during exposure depend sensitively on the rate of exposure, and profoundly influence the spatiotemporal evolution of concentration profiles during TPD. Our approach follows the development of the experiment accurately and can be used to probe the detailed characteristics of coupled RD systems. The nominal exposure in Langmuir units (L) is not adequate for coupled RD systems, since diffusion into the bulk during adsorption depends strongly on time. The detailed exposure schedule used, including exposure pressure and exposure time, must be reported for the correct interpretation of the TPD spectra of RD systems.