Reaction of H2 + D2⇆2HD Catalyzed by Molybdenum and Observed by Mass Spectrometer

Abstract

Papers I, II, and III of this series emphasize chemical reactions of the same designation. I describes H2–D2 exchange, II describes 28N2–30N2 exchange, and III reports some measurements on ammonia synthesis. All reactions were catalyzed by the same Mo ribbon. I and II are simpler than III and their study facilitates understanding III. Publications for I show a trend toward the expected equilibrium, but evaluation of catalytic effectiveness usually involved many sorptions of each reactant molecule with the heated catalyst. Our technique permits evaluation resulting from a single sorption interaction and shows for I that sorbed molecules leave our catalyst S at relative abundances agreeing with the equilibrium constant K of the exchange reaction. This equilibration during a single interaction occurs throughout the experimentally accessible temperature range 335°K to above 1000°K and for covers between 0.05 and approximately a complete monolayer. Contamination by nitrogen and by CO does not affect equilibration. Results imply that the catalyst splits the reactant molecules at least intermittently and may suggest atomic, rather than molecular sorption. However, published experiments warn against complete acceptance of atomic sorption. If exchange reactions involve only the catalyst surface, as generally thought, relative departure rates for H2, D2 and HD molecules would satisfy the usual conservation relation calculated from incidence rates from the gas phase onto the catalyst. This relation is independent of K but, with K, would completely specify relative desorption rates of the three molecular forms of hydrogen. Experimentally, only K is satisfied. Desorption rates are not directly related to incidence rates and are only understood if much of the hydrogen and deuterium participating in exchange comes from a phase dissolved in the catalyst, rather than from an adsorbed layer. This and other results suggest that H2–D2 exchange is related more critically to the volume of the catalyst than to its surface. If such results occur for other catalysts and other reactions, widely accepted models for catalysis must be reconsidered.