TEMPERATURE-PROGRAMMED DESORPTION - A STATISTICAL RATE THEORY APPROACH

The equation traditionally used to interpret temperature programmed de sorption (TPD) spectra, the Polanyi-Wigner equation, does not contain explicitly the coverage and temperature dependence necessary to predic t TPD spectra in several important systems including CO-Ni(lll). Herei n, the statistical rate theory (SRT) approach is used to formulate equ ations for temperature programmed desorption which are then used to ex amine TPD spectra reported in the literature for CO-Ni(lll). The molec ular and material properties for the CO-N1(111) system have been previ ously established. One experimental spectrum has been chosen to determ ine the apparatus constants. The material properties and the apparatus constants are then used in the SRT equations to predict the eight add itional TPD spectra for different initial coverages. A critical compar ison can then be made between the theory and these eight experimental spectra, since no fitting constants were used in these eight cases. Th e results show that there is clearly qualitative agreement. The SRT eq uations are then used along with the heat of adsorption to derive an e quation for the pre-exponential factor appearing in the Polanyi-Wigner equation. A prediction is made for the pre-exponential factor that is in agreement with that found empirically. The agreement found between the SRT predictions and the measured spectra indicates that all of th e coverage and temperature dependence necessary to predict TPD spectra is given explicitly by the SRT approach. Hence, the experimental supp ort for the SRT approach is enhanced. The SRT equations are then used to predict CO-Ni(lll) spectra that would occur if the heating rate wer e varied. (C) 1997 American Institute of Physics.