Analysis of thermal desorption curve for heterogeneous surfaces: II. Nonlinear variations of activation energy of desorption

Abstract

We reported analyses of the desorption curve for the simple heterogeneous surface. This paper deals with the more general heterogeneous surface with nonlinear variation of energy of desorption as a function of surface coverage. Computer simulation shows that the characteristic changes of the curve shapes are caused by variations of the distribution pattern of the activation energy of desorption. Thus, the determination of the profile of E d ( θ) is possible by the analysis of the peak shape with a few assumptions; the rate parameters, E d ( θ) and A, of desorption can be deduced as a function of surface coverage, θ, by solving a number of simultaneous equations E d(θ) − RT ln A = RT ln ( θ n βC ) , where n is the order of desorption, β is the heating rate, C is the normalized concentration, A is the frequency factor, and E d ( θ) is the activation energy of desorption, assuming the relation E d(θ)= ∑ k=0 N α k(1−θ) k. The above analysis was verified by its application to the various simulated desorption curves. The thermal desorption curves of oxygen on ZnO and silver catalyst were analyzed by this method and found to give the reliable values of rate parameters of desorption as a function of the surface coverage; E d ( θ) varying from 21.5 ± 1.5 kcal/mol at the initial stage of desorption to 32.0 ± 3.0 kcal/mol at the end for ZnO and from 28.5 ± 0.5 to 35.0 ± 0.5 kcal/mol for silver catalyst. The result was much more improved by the simultaneous analysis of the plural desorption curves obtained at different heating rates.