A customized reflection
time-of-flight (Re-TOF) mass spectrometer
combined with a 177 nm deep-ultraviolet laser has enabled us to observe
well-resolved cobalt clusters Co
n
±/0 and perform a comprehensive study of their reactivity
with ammonia (NH3). The anions Co
n
– are found to be inert, the neutrals allow the
adsorption of multiple NH3 molecules, while the cationic
Co
n
+ clusters readily react
with NH3 giving rise to dehydrogenation. However, incidental
dehydrogenation of NH3 on Co
n
+ is only observed for n ≥ 3.
The dramatic charge- and size-dependent reactivities of Co
n
±/0 clusters with NH3 are studied by the density functional theory (DFT)-calculation results
of energetics, density of states, orbital interactions, and reaction
dynamics. We illustrate the dehydrogenation from two NH3 molecules, where a significantly reduced transition-state energy
barrier is found pertaining to the dimolecular co-catalysis effect.
The reactivity of Co3
+ with NH3 is
illustrated showing effective catalysis for N–H dissociation
to produce hydrogen applicable for designing ammonia fuel cells.