A comparative study on the reactivity of cationic niobium clusters with nitrogen and oxygen

Unusual Inertness of a Ta8+ Cluster in Dinitrogen Reactions

Clusters serve as the optimal model to elucidate the structure-property relationship of materials, bridging condensed matter and individual atoms. The pursuit of exceptionally stable clusters has garnered significant interest. The distinctive electronic configuration and symmetrical geometry generally provide a consistent rationale for their stability. However, this principle does not quite correspond to the behavior of all transition metal clusters. Utilizing our customized apparatus, we successfully produced pure tantalum clusters Tan+ (n = 1-16) and examined their reactions with dinitrogen under sufficient gas-collision conditions. Significantly, with the introduction of N2 gas reactants, the Ta8+ cluster became the predominant species. Comprehensive theoretical analyses indicate that the inertness of Ta8+ is due to not only its unique electronic configuration and superatomic feature but also its unfavorable N2 adsorption dynamics and N≡N activation kinetics on the cluster. We demonstrate the contributions of frontier orbitals, the natural population of charges, and their interactions with lone-pair electrons of N2, together with the rate coefficients derived from Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This study provides comprehensive insights into the cluster stability and activity, which can be used as a reference for the development of gas separation materials that are resistant to N2.

Weak Interactions Initiate C-H and C-C Bond Dissociation of Ethane on Nbn + Clusters

The conversion of ethane into value-added chemicals under ambient conditions has attracted much attention but the mechanisms remain not fully understood. Here we report a study on the reaction of ethane with thermalized Nb_n ⁺ clusters based on a multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). It is found that ethane reacts with Nb_n ⁺ clusters to form both products of dehydrogenation and methane-removal (odd-carbon products). Combined with density functional theory (DFT) calculations, we studied the reaction mechanisms of the C-C bond activation and C-H bond cleavage on the Nb_n ⁺ clusters. It is unveiled that hydrogen atom transfer (HAT) initiates the reaction process, giving rise to the formation of Nb-C bonds and an elongated C-C distance in the HNb_n ⁺ CH₂ CH₃ motif. Subsequent reactions allow for C-C bond activation and a competitive HAT process which is associated with CH₄ removal or H₂ release, resulting in the production of the observed carbides.

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.