Molecular Dissociation and Reconstitution on Solids

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Abstract
The dependence of heterogeneous reactions on the nature of the solid surface is explored by analyzing the elementary rate processes involved. The relations between the rates of adsorption,surface diffusion, and evaporation, and the depth and distribution of binding sites on the surface are presented. Empirical generalizations concerning the binding energy of adatoms and the cohesive energy of solids are examined; Pauling's additivity rule is found to fail for most metal hydrides and thus does not provide a reliable base for estimating the covalent contribution to the heat of adsorption. Rates of association of atoms and dissociation of molecules on solids are estimated from a knowledge of the experimental heat of adsorption — ΔH and the postulate that the activated complex for dissociative chemisorption of a diatom has a configuration resembling two noninteracting atoms, one in its equilibrium binding position on the surface, the other in the saddle‐point position for surface migration. For chemisorption on clean metals, it is found that an activation energy can be expected only for systems in which the heat of adsorption is small (— ΔH <10 kcal/mole), in agreement with experiment. Conversely, desorption of diatoms by combination of atoms deposited in excess of the equilibrium amounts on metals on which — ΔH <0 is limited only by surface diffusion and therefore should occur rapidly, as is observed. The mechanism of heterogeneous recombination of atoms is outlined; it is shown that for hydrogen atoms only small variations in the efficiency of this process on different metals are expected at pressures of the monatomic gas less than 10—3 mm, and that these are primarily dictated by the rate of direct recombination between gas phase and adsorbed atoms. For metals on which chemisorption from the molecular gas is endothermic, the recombination coefficient should approach unity above room temperature. In the converse process, the dissociation of diatomic molecules into atoms at a hot metal surface, variations in the rate of atomization may occur for different metals; for those on which adsorption is endothermic the rate should be small since it is the adsorption step which results in specificity. The rate of atomization is also shown to depend upon the first power of the pressure of the molecular gas p 2 in the region of low pressures, changing to a dependence on p 2 ½ at higher pressures where encounters between adsorbed atoms become limiting. The same picture of the activated complex for chemisorption is applied to the interaction of gases with nonmetals. From the appreciable activation energy observed for adsorption of diatoms on nonmetals, it is concluded that binding sites on these surfaces are widely separated. This suggests that dissociation or excitation of the gas‐phase molecule to a higher vibrational state should increase the rate of adsorption, and conversely that the recombination efficiency of nonmetals for atoms should be small. These expectations are found to be in agreement with experiment.

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