Estimation of the activation energies for heterogeneous catalytic processes from thermodynamic and structural considerations

Abstract

Abstract The activation energies for three different heterogeneous catalytic processes is evaluated from phenomenogical thermodynamic considerations in conjunction with structural aspects inferred from various mechanistic sequences. The surface coverages of adsorbed species, lattice coordination number of single crystals and polycrystalline metals as well as the corresponding work functions are explicitly incorporated. The reactions considered are the synthesis of ammonia, decomposition of formic acid and hydrogenation of ethylene. The estimated activation energies for the synthesis of ammonia on Fe(1 1 1) and Ru(0 0 0 1) surfaces show satisfactory agreement. In the case of formic acid decomposition, the volcano plot between the activation energy and reaction temperature is rationalized and the subtle role of work functions is indicated. The activation energy for the hydrogenation of ethylene on Pd(1 1 1) and Pt(1 1 1) is estimated and shown to be consistent with the experimental data. A simple phenomenological expression for the activation energy is shown to be valid for three different heterogeneous processes, viz. formation, decomposition and addition reactions.