Molecular adsorption-desorption reactions of ammonia on alkali halide clusters and nanocrystals

The initial adsorption of ammonia molecules (NH₃) on larger alkali halide clusters and small nanocrystals (mainly NaF) has been investigated by flow-reactor methods as a function of cluster size and temperature. An analysis of the strong size-dependent reactivity of positively-charged NaF clusters, along with their computed structures, indicates that a particular type of defect in the nanocrystal structure facilitates adsorption. This defect is formed by removing an ion-pair from adjoining face and internal sites of a perfect crystallite, creating a basket-like opening. KF nanocrystals show very similar reactivity patterns, reflecting their corresponding structures, but LiF clusters follow a different pattern. It was established that NH₃ adsorption on preformed NaF nanocrystals takes place under equilibrium conditions. The equilibrium constant for initial adsorption-desorption increases with decreasing temperature (250-340 K) and allows one to derive heats of NH₃ adsorption near 0.2 eV for the more reactive (defective) nanocrystals. The much lower reactivity of negatively-charged clusters is ascribed to an additional kinetic-dynamic barrier to adsorption.