MOLECULAR ADSORPTION-DESORPTION REACTIONS OF AMMONIA ON ALKALI-HALIDECLUSTERS AND NANOCRYSTALS

The initial adsorption of ammonia molecules (NH3) on larger alkali hal ide 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-ch arged NaF clusters, along with their computed structures, indicates th at a particular type of defect in the nanocrystal structure facilitate s adsorption. This defect is formed by removing an ion-pair from adjoi ning face and internal sites of a perfect crystallite, creating a bask et-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 preforme d NaF nanocrystals takes place under equilibrium conditions. The equil ibrium constant for initial adsorption-desorption increases with decre asing temperature (250-340 K) and allows one to derive heats of NH3 ad sorption near 0.2 eV for the more reactive (defective) nanocrystals. T he much lower reactivity of negatively-charged clusters is ascribed to an additional kinetic-dynamic barrier to adsorption.