Desorption of hydrogen from iron—Influence of the activity coefficient of adsorbed hydrogen

Abstract

Within the framework of absolute reaction rate theory as applied to surface reactions, the chemical activities of the adsorbed species can be formulated in terms of thermodynamic activity coefficients. This leads directly to the prediction that the Arrhenius parameters of activation energy and preexponential factor also depend on the thermodynamic activity coefficients of the adsorbed species. This development also produces a simple algebraic relationship between activation energy and preexponential factor which not only provides a thermodynamic rationale for the compensation effect but also permits calculation of thermodynamic activity coefficients from experimentally observed compensatory behavior of the Arrhenius kinetic parameters. The development is carried out for the case of desorption of hydrogen from iron, which involves the chemical combination of adsorbed hydrogen atoms to hydrogen molecules as the rate-limiting step. Activity coefficients and entropies of activation are also thereby computed from measured rates of desorption of hydrogen from assiduously cleaned iron wire which provides a homogeneous, smooth surface. In this case, the compensation effect involves variation of activation energy and preexponential factor depending on surface coverage, and this information is used to compute the thermodynamic activity coefficient of adsorbed hydrogen atoms as a function of surface coverage. The results can be interpreted as a departure from two-dimensional, adsorbed-phase, ideal-gas behavior as surface coverage increases. The results also demonstrate that the intrinsic relationship between activation energy and preexponential factor can be explained in terms of variation of thermodynamic activity coefficients of adsorbed reactants.