6.2 Reaction with H 2 S and NH 3
85
Recently, utilizing the customized Re-TOF mass spectrometer combined with a
177-nm deep-ultraviolet laser, Luo et al. have been able to observe well-resolved
cobalt clusters Co n
±/0 on which they performed a comprehensive study on cobalt
cluster reactions with ammonia (NH 3 ). As results, the anions Co n
– were found to
be inert, but the neutrals were able to adsorb multiple ammonia molecules. The
neutral Co n clusters, especially those of relatively larger sizes, were observed to
can adsorbe multiple NH 3 molecules, i.e., Co n + mNH 3 → Co n (NH 3 ) m [66]. This
is in consistence with the previously established coordination chemistry theory on
cobalt-ammonia complexes. Besides the adsorptive reactions, it was proposed that
an etching-like fragmentation channel (Eq. 6.2) could also exist in the cobalt cluster
reaction with NH 3 , which is enabled in the presence of multiple NH 3 molecules. What
is interesting is that, the cationic Co n
+ clusters readily reacted with NH 3 resulting
in a series of dehydrogenation products; whereas, the dehydrogenation of NH 3 on
Co n
+ clusters was only observed for those of n ≥ 3, as shown in Fig. 6.4. It is also
notable that the dehydrogenation of NH 3 was observed only when more than two
NH 3 molecules were present indicative of a co-operative mechanism. In all, these
reactions were summarized as [66],
Co
+
n + NH 3 → Co n NH 3
+
(6.1)
Co
+
n + mNH 3 → Co x (NH 3 ) y
+
+ Co n−x (NH 3 ) m−y (n > x; m ≥ y ≥ 0) (6.2)
Co
+
n + mNH 3 → [Co n (NH 3 ) m ]
+
→ [Co n (NH 3 ) m−2 (NH 2 ) 2 ]
+ + H 2 (m ≥ 2) (6.3)
The diverse reactivities of Co n
±/0 with ammonia brought forth comprehensive
insights into the charge-dependence and size-dependence and cooperativion effect
in such metal cluster reactions [66]. The charge-dependence with Co n
+ > Co n >
Co n
– was well explained by their altered electrostatic potential for an NH 3 molecule
in approaching the Co n
±/0 clusters, also associated with altered charge distributions,
binding energies, and PDOS of dominant orbitals. Among the cationic Co n
+ clusters,
the Co
+ ion and Co 2
+ clusters lack active sites for the second hydrogen transfer, and
suffer from insurmountable rate-limiting barrier for dehydrogenation from NH 3 , as
shown in Fig. 6.5. In contrast, transition states for Co 3,6
+ are surmountable, which
concurs with the experimental observation of dehydrogenation products for Co n≥3
+
clusters. Besides, the DFT calculations indicate that two co-adsorbed NH 3 molecules
benefit to the H 2 evolution, evidencing the importance of cooperative active sites for
NH 3 decomposition.
85
Recently, utilizing the customized Re-TOF mass spectrometer combined with a
177-nm deep-ultraviolet laser, Luo et al. have been able to observe well-resolved
cobalt clusters Co n
±/0 on which they performed a comprehensive study on cobalt
cluster reactions with ammonia (NH 3 ). As results, the anions Co n
– were found to
be inert, but the neutrals were able to adsorb multiple ammonia molecules. The
neutral Co n clusters, especially those of relatively larger sizes, were observed to
can adsorbe multiple NH 3 molecules, i.e., Co n + mNH 3 → Co n (NH 3 ) m [66]. This
is in consistence with the previously established coordination chemistry theory on
cobalt-ammonia complexes. Besides the adsorptive reactions, it was proposed that
an etching-like fragmentation channel (Eq. 6.2) could also exist in the cobalt cluster
reaction with NH 3 , which is enabled in the presence of multiple NH 3 molecules. What
is interesting is that, the cationic Co n
+ clusters readily reacted with NH 3 resulting
in a series of dehydrogenation products; whereas, the dehydrogenation of NH 3 on
Co n
+ clusters was only observed for those of n ≥ 3, as shown in Fig. 6.4. It is also
notable that the dehydrogenation of NH 3 was observed only when more than two
NH 3 molecules were present indicative of a co-operative mechanism. In all, these
reactions were summarized as [66],
Co
+
n + NH 3 → Co n NH 3
+
(6.1)
Co
+
n + mNH 3 → Co x (NH 3 ) y
+
+ Co n−x (NH 3 ) m−y (n > x; m ≥ y ≥ 0) (6.2)
Co
+
n + mNH 3 → [Co n (NH 3 ) m ]
+
→ [Co n (NH 3 ) m−2 (NH 2 ) 2 ]
+ + H 2 (m ≥ 2) (6.3)
The diverse reactivities of Co n
±/0 with ammonia brought forth comprehensive
insights into the charge-dependence and size-dependence and cooperativion effect
in such metal cluster reactions [66]. The charge-dependence with Co n
+ > Co n >
Co n
– was well explained by their altered electrostatic potential for an NH 3 molecule
in approaching the Co n
±/0 clusters, also associated with altered charge distributions,
binding energies, and PDOS of dominant orbitals. Among the cationic Co n
+ clusters,
the Co
+ ion and Co 2
+ clusters lack active sites for the second hydrogen transfer, and
suffer from insurmountable rate-limiting barrier for dehydrogenation from NH 3 , as
shown in Fig. 6.5. In contrast, transition states for Co 3,6
+ are surmountable, which
concurs with the experimental observation of dehydrogenation products for Co n≥3
+
clusters. Besides, the DFT calculations indicate that two co-adsorbed NH 3 molecules
benefit to the H 2 evolution, evidencing the importance of cooperative active sites for
NH 3 decomposition.