Reactivity of Cobalt Clusters Co_n^±/0 with Ammonia: Co₃⁺ Cluster Catalysis for NH₃ Dehydrogenation

Abstract

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.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6+

“…The experimental setup is a home-built reflection time-of-flight mass spectrometer (Re-TOFMS); it consists of a customized vacuum system, a mini flow tube reactor (Φ = 6 mm, L = 60 mm), and a home-built laser vaporization (LaVa) source. More detailed descriptions about our Re-TOFMS can be found elsewhere. ,, In this experiment, the Co n ±/0 clusters were prepared by ablating a clean and rotating cobalt disk (99.95%) with a pulsed 532 nm laser (Nd:YAG) at a repetition rate of 10 Hz in the LaVa source. In a nozzle (Φ = 2 mm, L = 35 mm), the clusters were cooled down during a supersonic expansion process by the pulsed He carrier gas (99.999%, 10.0 atm) which was controlled by a pulsed valve (series 9, general valve).…”

“…In addition to the main group metals and coinage metals, earth-abundant light transition metals (e.g., Fe, Co, and Ni) have versatile catalytic activities for a variety of reactions due to their active 3d electrons, , such as water/ammonia dissociation, , CO/CO 2 capture, − and nitrogen fixation/activation. − The adsorption behavior of N 2 on nickel and cobalt clusters have been studied by cryo spectroscopy and infrared spectroscopy. , Earlier studies have already found relatively high reactivity of Co 4 + and Co 5 + in comparison with Co 6 + . , Also, in the reactions of small cobalt clusters with CO, it was found that the anions Co n – react faster than cations Co n + ; meanwhile, the cationic Co 6 + and Co 8 + clusters exhibit a lower reactivity than Co 5 + and Co 7 + . Inspite of these interesting results, since the transition metals (IIIB–XB) readily form coordination and oxidation products, it remains a big challenge to prepare pure metal clusters and to determine magic numbers directly by observing their mass abundances in gas-phase reactions.…”

“…Recently, we have successfully produced “metalloxocubes” by the reaction of cobalt clusters with oxygen, which is identified as a stable oxygen-passivated metal cluster, and can be classified into the general category of the superatom family. In another work, we studied the reactions of Co n ±/0 clusters with ammonia; the dehydrogenation from ammonia with both dramatic charge- and size-dependent reactivities has been observed . However, a pending issue remains: whether a magic number exists in the naked cobalt clusters which differ from the main group metals (s- and p-blocks of the periodic table of elements) and coinage metals?…”

“…Luo et al 11−13 explored the magic numbers of aluminum−magnesium cluster anions and coinage metals and found enhanced stability of superatomic Ag 13 − , Ag 17 − , and Cu 18 − , endeavoring to connect the superatoms in the gas phase with those synthesized in wet chemistry, 14−22 by proposing a concept of special and general superatoms. 15,16,23 In addition to the main group metals and coinage metals, earth-abundant light transition metals (e.g., Fe, Co, and Ni) have versatile catalytic activities for a variety of reactions due to their active 3d electrons, 24,25 such as water/ammonia dissociation, 26,27 CO/CO 2 capture, 28−31 infrared spectroscopy. 34,38 Earlier studies have already found relatively high reactivity of Co 4 + and Co 5 + in comparison with Co 6 + .…”

“…In another work, we studied the reactions of Co n ±/0 clusters with ammonia; the dehydrogenation from ammonia with both dramatic charge-and size-dependent reactivities has been observed. 26 However, a pending issue remains: whether a magic number exists in the naked cobalt clusters which differ from the main group metals (s-and p-blocks of the periodic table of elements) and coinage metals? To address this question and get a clear picture for reactions between cobalt clusters with nitrogen molecules, we performed this study.…”