MAGNITUDE AND EFFECTS OF ADSORBENT TEMPERATURE NONUNIFORMITIES IN TEMPERATURE-PROGRAMMED DESORPTION

In a typical apparatus for temperature-programmed desorption (TPD) measurements, heat is not added uniformly to the entire adsorbent volume, but rather is generated in support wires and conducts in at the contact points, forming a temperature gradient in the adsorbent during heating. Based on a simplified, one-dimensional treatment, we determine the approximate magnitude of the temperature profile and investigate its deleterious effects in TPD simulations for first-order kinetics. Analysis of the coupled equations governing surface temperature and coverage leads to a dimensionless group of parameters involving the heating rate, the desorption rate parameters, and the size and physical properties of the adsorbent that determines the magnitudes of systematic errors introduced by the temperature nonuniformity. The simulations allow a general expression to be developed for the maximum heating rate in TPD that will give estimates of E and ln(nu) reliable to within a few percent. The predictions of the model are verified by comparison to TPD data for CO/Ni(110).