Effects of interaction and mobility on selectivity of a simple reaction scheme

Abstract

Using the random patch model, the effects of interaction between and mobility of adsorbed molecules have been accounted for in a straightforward manner. For considering surface heterogeneity four widely accepted site energy distributions are used. Expressions for selectivity and rates are derived as a function of pressure, with finite limits of heats of adsorption. For the case of localized adsorption with interactions the Fowler-Guggenheim model and the quasi-chemical approximation model are considered, both for the square and the hexagonal lattice. Mobile adsorption with interaction is analyzed using an isotherm similar to the Hill-deBoer model. An approximation method called the condensation approximation is employed to estimate mean surface and mean squared surface coverages and compared with the exact numerical method. The condensation approximation is good (1% deviation) for calculating the mean surface coverage, but is only partially successful for determining the mean squared surface coverage (5% deviation). Methods are given for determining the characteristic pressure at which a desired selectivity is obtained. The Fowler-Guggenheim model and the quasi-chemical model predict nearly identical characteristic pressures. For the case of mobile adsorption, values of characteristic pressure are much lower when compared with the localized model. We investigate the relationship arising between the activation energy of surface reaction and heat of adsorption as a consequence of the Polanyi-Bronsted relationship assumed to exist between activation energy and enthalpy change for an elementary step. The pressure-dependent parts of the rates show appreciable fall compared to the case when statistical independence is assumed between the activation energy and the heat of adsorption. Besides, the characteristic pressure gets shifted to higher values. The analysis presented can be applied to a simple reaction scheme (alcohol dehydration) as well as a more complex scheme such as that involved in catalytic cracking and disproportionation reactions.