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Benben Huang, Mengzhou Yang, Xin Lei, Wen Gan, Zhixun Luo. A comparative study on the reactivity of cationic niobium clusters with nitrogen and oxygen[J]. Chin. Chem. Lett., 2022, 33(2): 1058-1064  

Benben Huang^a,b, Mengzhou Yang^a,b, Xin Lei^b, Wen Gan^a,b, Zhixun Luo^a,*     

Gas adsorption is one of the most basic interfacial phenomena and closely related to diverse applications in industrial and agricultural production and daily life. People may take it for granted that the adsorption of gas molecules on the metals is simply divided into physical adsorption and chemical adsorption; however, the underlying mechanism that determines the diverse interactions and surface reactivity at reduced sizes are illusive to be fully understood. The study of gas adsorption on solid surfaces is helpful to understand the micro-mechanism of various physicochemical processes; also, adsorption of gas molecules on nanometals is a key step in catalytic processes and important for various gas phase reactions. While physisorption is often dominated by molecular van-der Waals force and generally emits rare energies, chemisorption can be largely exothermic and may involve covalent interactions and stepwise reactions [1]. In this regard, molecular adsorption can be associated with three processes, i.e., inactivated adsorption, activated adsorption and precursor-mediated adsorptions [2]. The adsorbate may reach a chemisorption state through a barrierless process directly from physisorption state; or it needs to overcome a potential barrier simply by heating or other reaction conditions; or undergoes a precursor-mediated adsorption, where the chemisorbed state could be attained through a precursor intermediate.

On this basis, the reactivities of transition metal clusters with small molecules have been extensively studied, illustrating the distinction of physical and chemical adsorptions on surfaces [2]. Also, the interaction mechanisms and properties of metal cluster complexes have been widely analyzed by joint experimental and theoretical studies. Among them, niobium clusters have been an important research object unveiling size-dependent chemical and physical properties of transition metals at reduced sizes [3-10]. Combining mass spectrometry and gas flow reactors [11, 12], the reactivities of niobium clusters with many chemicals have been studied, such as H₂ [12-15], O₂ [16, 17], N₂ [18, 19], CO_x [20-22], NO_x [23, 24] and hydrocarbons [25-29]. The reactions of Nb clusters with N₂ and H₂ (/D₂) have similar adsorptive patterns; while the reaction with O₂ shows a difference due to the differences of atomic valence configuration and bonding nature of these diatomic molecules. In general, N₂ is inert in most reactions due to a very high dissociation energy (945 kcal/mol) of the N≡N triple bond and a high HOMO-LUMO gap (~10.82 eV); in comparison, ground-state O₂ is in triplet with two spin single electrons pertaining to its high activity [30]. However, it is not fully unveiled how the stability of such metal clusters and atomic valence configuration and bonding nature cooperatively determine the experimental observation on their reactivity.

On the other hand, previously published studies have also estimated the reaction rate constants for niobium clusters with N₂ and O₂ showing a likely difference of three orders of magnitude [18, 19]. Further studies reported the similar adsorption of N₂ on niobium clusters [18, 19, 31], and there are adsorption products of polynitrogen molecules depending on the pressure and concentration of the reaction gas at room temperature [32]. For example, Pillai et al. [33] generated Nb⁺(N₂)_n complexes (n = 3–16) with nitrogen as buffer gas and investigated the vibration in forming such complexes by infrared photodissociation spectroscopy (IRPD), showing a preferred coordination of six ligands. Meanwhile, the reactions of small Nb_n⁺ (n = 1–3) with O₂ have been studied by Loh et al. [16] who measured the cross section as a function of kinetic energy and proposed dissociation pathways and threshold values. It is anticipated that these experimental cluster physics studies of such small Nb clusters can be connected with theorical energy calculations in determining the thermodynamics and their reaction dynamics in gas phase [34].

On these bases, here we have performed a further in-depth study of the thermalized gas-phase Nb_n⁺ (n = 1–10) clusters in reacting with sufficient N₂ and O₂ gas in a customized multiple ions laminar flow tube in tandem with triple quadrupole mass spectrometer (MIFT-TQMS). It is found that the Nb_n⁺ clusters, produced by the home-made magnetron sputtering (MagS) source, readily react with both N₂ and O₂, but form different series of products respectively, seen as Nb_nN_2m⁺ (m = 1–3) and Nb_nO_x⁺ (x = 1–4). Theoretical calculations results reveal that the Nb_nN_2m⁺ clusters all adopt end-on orientation adsorption for N₂ on Nb_n⁺ with much smaller binding energy than that of correlative Nb_nO₂⁺ series which correspond to strong hollow-site adsorption allowing for occasional dissociative adsorption. Further, we conducted a comprehensive study thermodynamic energetics, HOMO-LUMO gaps, charge transfer, Wiberg bond index (WBI), density of state (DOS), natural atomic orbital (NAO) and energy decomposition analyses based on natural orbitals for chemical valence (EDA-NOCV). In addition, we carried out potential scan for N₂ and O₂ in approaching an Nb_n⁺ cluster until the formation of Nb₃N₂⁺ and Nb₃O₂⁺, illustrating how cluster-molecule interactions initiate their reaction. Finally, we explore the reaction pathways of both N₂ and O₂ with Nb₃⁺ as a representative, demonstrate the N₂-adsorption behavior and the reaction dynamics in producing Nb₃O₂⁺ from O₂-adsorption, to dissociative intermediate, and to ultimately exhaust of NbO (/NbO⁺).

The experiments in this study are conducted on our customized multi-ions laminar flow tube reactor in tandem with a triple quadrupole mass spectrometer (MIFT-TQMS). A self-designed MagS source with a DC power supply of 5 kW was used to obtain the clean mass distributions of small niobium clusters [35]. A niobium disk (99.95% purity, 50.8 mm diameter, 4 mm thickness) was used as the sputtering target. High-purity helium (> 99.999%) and high-purity argon (> 99.995%) were used as carrier gas and sputtering work gas, introduced from the rare of the MagS source and the magnetron head respectively, with the gas flow rates controlled by two mass flowmeters (Alicat, a range of 0–100 sccm and 0–100 slm, respectively). Typical parameters for the MagS source to produce the niobium clusters with n = 1–10 are: 170 V DC voltage, 4 A current, a background pressure of source chamber at ~7 Torr, He carrier gas at ~12 slm helium, while Argon is 60–120 sccm. Different concentrations of N₂ and O₂ (ca., 0.5%–30% in He) are injected into the flow tube (60 mm diameter, 1 m long) which is maintained at 0.9 Torr pressure for stable laminar flow and sufficient collisional reaction. The detailed values for these parameters may differ with dependence on the target situation and vacuum status, etc.

All the DFT calculations were performed using Gaussian 09 software package [36]. B3LYP exchange-correlation functional was used to optimize the geometric structures of all the niobium clusters and products, the basis sets of Lanl2dz was used for Nb with an extra d polarization function, while 6–311G(d) for N and O atoms [36, 37]. Vibrational frequency calculations were performed for each of the optimized structures and zero-point vibrations were corrected for all the energy calculations. Thermodynamic data, Wiberg bond index (WBI), partial density of states (PDOS), natural atomic orbital (NAO) and electron configuration are analyzed by using the Multiwfn software [38]. The geometric structures, frontier orbitals, charge distributions by natural population analysis (NPA), and the PDOS patterns of the Nb_n⁺, Nb_nN₂⁺, Nb_nO₂⁺ (n = 1–10) are plotted by visual molecular dynamics (VMD) [39]. Energy decomposition analysis was conducted based on natural orbitals for chemical valence (EDA-NOCV), calculated with Amsterdam Density Functional (ADF) program [40, 41] at the B3LYP/TZP level of theory.

Fig. 1 presents the mass spectra of the Nb_n⁺ (n = 1–21) clusters in the absence and presence of N₂ and O₂ gas reactants respectively, where the concentrations and gas flowrates are controlled by a mass flowmeter (ALICAT-LD12). As is shown in Fig. 1a, most of the Nb_n⁺ clusters react with N₂ just to form molecular adsorption products Nb_nN_2m⁺ (m = 1–3) with size dependence, which is in contrast to the sequence reactions between Nb_n⁺ and O₂ in forming Nb_nO_x⁺ (x = 1–4). From mass spectra in Fig. 1a and the variation of mass abundances of the nascent Nb_n⁺ clusters and Nb_nN_2m⁺ products (Fig. S1 in Supporting information), it is seen that Nb₃₋₆⁺ clusters react faster than the other clusters and convert to Nb_nN₂⁺products. In comparison, the Nb₇⁺ is seen to prefer an adsorption of several nitrogen molecules to form Nb₇N_{4, 6}⁺ (also seen for Nb₄⁺); The cation Nb⁺, also Nb₈⁺ and Nb₉⁺ slowly react with N₂, but Nb₂⁺and Nb₁₀⁺display rare reaction products with N₂ in the experimental condition of this study. The variation of Nb_n⁺ clusters reacting with N₂ reflects their size-dependent stability and reactivity, assuming they undergo identical collision annihilation with the wall of the flow tube. We have plotted the changes in relative intensity of Nb_n⁺ (n = 3–9) and –ln(I/I₀) of Nb_n⁺ as a function of the gas flow rate (Fig. S3 in Supporting information). The experimental results display that the N₂ addition on Nb_n⁺ (n = 1–10) closely follow the quasi first-order reactions for which the rate constants have been estimated simply by using the equation ln(I_A/I₀) = -k₁(P_eΔt/k_BT) [42], where k_B is the Boltzmann constant, T is the temperature, P_e and Δt correspond to the effective pressure and reaction time. As results, the size-dependent reaction rates of Nb_n⁺ (n = 1–10) exhibit a wide diversity, and Nb₃⁺ is found to display a maximal value of ~10⁻⁹ cm³ molecule⁻¹ s⁻¹.

Figure 1

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Fig. 1. The mass spectra of Nbn+ clusters in the absence and presence of (a) 30% N2/He and (b) 0.5% O2/He, respectively. The weak mass peaks marked with stars (*) in the nascent Nbn+ cluster distributions correspond to trace amount of oxygen contamination.

There is different case. As shown in Fig. 1b, the Nb_n⁺ (n = 1–10) clusters are highly reactive with O₂ even at a very low concentration (e.g., 0.8 sccm of 0.5% O₂ in He gas). Even in the presence of a small amount of oxygen, the dissociative products Nb_nO⁺ are observed. This is reasonable considering the large Nb-O bond energy up to ~770 kJ/mol which is even larger than the O-O bond strength (~400 kJ/mol); and electron transfer readily occur between niobium and oxygen [43, 44], with a variety of exothermic reaction channels as given in Table 1. Among the thermodynamically favorable channels, the O₂-dissociaed equation to produce a NbO neutral suggests the maximal energy release for all the Nb_n⁺ (n = 2–10) clusters. This could well explain the experimental observation of dissociative products Nb_nO⁺ which was also found in previous experimental study [17]. It is worth noting that, such a favorable reaction channel "Nb_n⁺ + O₂ → Nb_n-1O⁺ + NbO" largely differs from the previously determined reactions of "Al_n⁺ + O₂ → Al_m⁺+ Al_n-mO₂" [45], "Al_n⁻ + O₂ → Al_n−4⁻ + 2Al₂O" [46] and "Co_n⁻ + O₂ → Co_n−1⁻ + CoO₁⁻" [47]. Due to the large Nb-O bond energy, here the reaction channels with Nb^0/+or Nb₂^0/+removal are much less favorable from the thermodynamics, which is well consistent with the experimental observation that Nb⁺ and Nb₂⁺ do not display increased mass abundance in the varied reaction conditions. Also, this is consistent with the previous study by Radi et al. [17], where the dominant reaction pathway of mass-selected small niobium clusters with O₂ in a drift reactor was proposed to be the one in producing primary products of NbO and NbO⁺.

Table 1

Table 1 DFT-calculated energies (eV) for the likely diverse reaction pathways of Nbn+ (n = 1–10) with O2. All the data are calculated at the B3LYP/Lanl2dz level for Nbn+ and B3LYP/6-311G(d) level for N/O.

Table 1 DFT-calculated energies (eV) for the likely diverse reaction pathways of Nbn+ (n = 1–10) with O2. All the data are calculated at the B3LYP/Lanl2dz level for Nbn+ and B3LYP/6-311G(d) level for N/O.

Utilizing DFT calculations we have provided further insights into the diverse reactivities of niobium clusters (for details of the lowest-energy structures, spin multiplicity, bond lengths, and point group see Fig. S6 in Supporting information). For the adsorption complexes, N₂ adopts an end-on adsorption orientation on the Nb_n⁺ clusters of which the nascent structures and spin multiplicities almost do not change, except Nb₂N₂⁺ which shows a V-shape structure with a high spin multiplicity. In contract, Nb_nO₂⁺ (n > 2) clusters all adopt hollow-site adsorption mode with elongated O-O bond length ranging from 1.2 Å to 1.5 Å, although the structures of Nb_n⁺ in Nb_nO₂⁺do not show much changes. Moreover, the spin multiplicities of the odd-number small niobium clusters (i.e., Nb_2n+1O₂⁺) reduced in the formation of Nb_nO₂⁺ (except Nb₉O₂⁺); whereas, for the even-number niobium clusters (i.e., Nb_2n⁺), the spin of the cluster can be aligned opposite to that of the ³O₂ molecule thus spin conservation in the Nb_2nO₂⁺products [48, 49].

Fig. 2 plots the DFT-calculated HOMO-LUMO gaps, binding energies (BE), and natural population analysis (NPA) of charge distributions of the Nb_n⁺, Nb_nN₂⁺ and NbO₂⁺ (n = 1–10) clusters. More energetics data such as the VSE, ASE, binding energy per atom (BE) and Δ₂E of the Nb_n⁺ are given in Fig. S9 (Supporting information). While no surprise that the Nb_nN₂⁺clusters show much smaller cluster-molecule binding energies and charge transfer comparing with that of the Nb_nO₂⁺products, the HOMO-LUMO gaps of the nascent Nb_n⁺clusters and the subsequent Nb_nN₂⁺ and Nb_nO₂⁺ are comparable with each other except a jump at n = 2, 7, 8 pertaining to altered spin multiplicity (Nb₇O₂⁺), symmetry (Nb₂O₂⁺) or structure changes (Nb₈O₂⁺). Note that, O₂ always serves as an electron acceptor in interacting with the Nb_n⁺ clusters, with a total electron transfer of 0.6–0.9 |e|, which is in sharp contrast to that of the Nb_nN₂⁺ complexes of which the σ donations from N≡N to the Nb_n⁺clusters are enabled especially for the larger ones at n ≥ 6. Detailed information about NPA charge and natural electron configuration of Nb_3/6⁺, N₂, O₂, Nb_3/6N₂⁺ and Nb_3/6O₂⁺ is shown in Tables S3 and S4 (Supporting information).

Figure 2

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Fig. 2. (a) DFT-calculated HOMO-LUMO gaps of Nbn+, NbnN2+ and NbO2+ (n = 1–10). (b) Binding energy of NbnN2+ and NbnO2+. (c) NPA charge transfer of Nbn+ to N2 in NbnN2+ compared with Nbn+ to O2 in NbnO2+. All the data are calculated at the B3LYP/Lanl2dz level for Nbn+ and B3LYP/6-311G(d) level for N/O.

Table 2 lists the Wiberg bond indexes (WBI) between the atoms in Nb_nN₂⁺ and Nb_nO₂⁺ (n = 1–10). As is shown, every WBI of the N(1)-N(2) bond in Nb_nN₂⁺ changes to 2.69-2.90 from the primary N≡N triple bond in dinitrogen. Meanwhile, the WBI of the linked N(1)-Nb bond is about 0.3–0.5, while the WBI of N(2)-Nb is as small as 0.05–0.15 indicating weak interaction between the Nb_n⁺ clusters and N₂ corresponding to physical adsorption. By contrast, every WBI of O-O bond changes to about 1 from O=O double bond in the ground state oxygen molecule, and the bond length elongated from 1.2 Å to 1.4~1.5 Å), which is similar to O-O bond length and bonding mode in H₂O₂, as named peroxy bond (or peroxide state) after a chemical adsorption. Moreover, every O atom links to 2 Nb atoms in the 3Nb-2O elementary units when n ≥ 3, showing two types of O-Nb bonds with a minor different WBI, that is, O(1)-Nb(1, 2) and O(2)-Nb(1, 3) bond. It is notable that Nb₂O₂⁺ possesses 4 equal Nb-O bonds (WBI = 0.48) and the WBI of O-O bond is 1.02. In all, the WBI of every Nb-N and Nb-O provides detailed information of their bonding strengths.

Table 2

Table 2 The WBI analysis between the atoms in NbnN2+ and NbnO2+ (n = 1–10).

Table 2 The WBI analysis between the atoms in NbnN2+ and NbnO2+ (n = 1–10).

We have conducted orbital analysis to reveal the essential interaction of Nb_n⁺ with N₂ and O₂. Fig. 3 presents a comparison for the partial density of states (PDOS) and corresponding orbitals of 2 N(/O) atoms and linked 3 Nb atoms typically in Nb₃N₂⁺ and Nb₃O₂⁺ respectively. Firstly, the overlap area in PDOS of Nb₃O₂⁺ is much more than Nb₃N₂⁺. For Nb₃N₂⁺, this is displayed by the reconstituted Nb₃N₂⁺ orbital which possesses a pattern involving both the cluster and nitrogen molecule. The PDOS of non-dissociative adsorption of N₂ gives rise to seen as σ_2s, σ_2s^*, σ_2pz which are associated with the dominant N₂→Nb₃⁺ σ donation interactions, and two equivalent orbitals π_2px and π_2py involving very few interactions, vacant π_2px^* and π_2py^* of nitrogen are associated with the Nb₃⁺→N₂ π backdonation interactions (detailed orbital information including the natural atomic orbital (NAO) contributions is seen in Table S5 in Supporting information) [50]. While in Nb₃O₂⁺, the interacting orbitals also possess character of free O₂ molecule, where corresponding to σ_2s, σ_2s^*, σ_2pz of oxygen refer to O₂→Nb₃⁺ σ donation, and two equivalent orbital π_2px and π_2py both contribute to the O₂→Nb₃⁺ π donation interaction; meanwhile, the strong DOS correlative to π_2px^* and π_2py^* of oxygen accounts for the Nb₃⁺→O₂ dominant π backdonation (Table S5) [51]. Similarly, the NAO analyses for Nb₆N₂⁺ and Nb₆O₂⁺ are given in Table S6 (Supporting information).

Figure 3

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Fig. 3. Energy level of Nb3N2+ and Nb3O2+ together with PDOS of 2 N atoms and 3 Nb atoms in Nb3N2+ and PDOS of 2 O atoms and 3 Nb atoms in Nb3O2+. Insets show the corresponding orbitals.

Further, we conducted energy decomposition analysis based on natural orbitals for chemical valence (EDA-NOCV) [52-55]. The typical results of Nb₃N₂⁺ and Nb₃O₂⁺ are shown in Table 3, where the strength of the intrinsic interaction energies (ΔE_int) are divided into three components (ΔE_int = ΔE_pauli + ΔE_elstat + ΔE_orb) corresponding to pauli exclusion (ΔE_pauli), electrostatic interaction (ΔE_elstat) and orbital interaction (ΔE_orb) which can be further divided into σ donation, π donation, π backdonation, polarization and the rest interactions. For Nb₃N₂⁺, ΔE_elstat and ΔE_orb each contributes a half of the attractive interactions; while in Nb₃O₂⁺, the ΔE_orb contributes about 60%. In total orbital interactions, the N₂→Nb₃⁺ σ donation interaction and polarization interaction contribute ~48%; Nb₃⁺→N₂ π backdonation contributes ~40%. In contrast for Nb₃O₂⁺, ΔE_orb is much larger than ΔE_elstat in total interactions, among which and the two O₂→Nb₃⁺π donation interactions contribute only ~13%, reversely, Nb₃⁺→O₂ π backdonation contributes ~83% of the interactions. It is worth mentioning that the interacting fragments ³Nb₃⁺ and ³O₂ are both in spin triplet, but ¹Nb₃O₂⁺ in singlet state, which results in a larger value of ΔE_int than the BE in forming Nb₃O₂⁺ because of the differences between the unactivated fragments and the activated complex (which involves an additional energy to activate the precursor).

Table 3

Table 3 EDA results for 3Nb3N2+ and 1Nb3O2+ at the B3LYP/TZP level of theory using ADF, taking Nb3+ and N2 (/O2) as interacting fragments. ΔEint is the intrinsic interaction energies. ΔEpauli is the repulsion energy caused by the Pauli exclusion principle. ΔEelstst and ΔEorb are the attraction energies due to electrostatic and orbital interactions, respectively. The values in parentheses show the contribution to ΔEint and the total orbital interaction ΔEorb.

Table 3 EDA results for 3Nb3N2+ and 1Nb3O2+ at the B3LYP/TZP level of theory using ADF, taking Nb3+ and N2 (/O2) as interacting fragments. ΔEint is the intrinsic interaction energies. ΔEpauli is the repulsion energy caused by the Pauli exclusion principle. ΔEelstst and ΔEorb are the attraction energies due to electrostatic and orbital interactions, respectively. The values in parentheses show the contribution to ΔEint and the total orbital interaction ΔEorb.

Fig. 4 displays the principal orbitals correlation diagrams of Nb₃N₂⁺ and Nb₃O₂⁺ based on the corresponding fragments, with only α-orbitals being shown as the correlation of β-orbitals is similar. In Nb₃N₂⁺ (Fig. 4a), N₂→Nb₃⁺ σ donation interaction dominantly forms compound orbital HOMO-8, which is mainly contributed (about 83%) by HOMO of N₂ (σ_2pz) and vacant LUMO+1 orbital of Nb₃⁺; and Nb₃⁺→N₂ π backdonation forms new compound orbitals (HOMO, HOMO-1, HOMO-6) which are composed by the occupied orbitals of Nb₃⁺ and LUMO of N₂ (π_2px/y^*). In Nb₃O₂⁺ as a comparison (Fig. 4b), two O₂→Nb₃⁺ π donation interactions form HOMO-8, HOMO-9 orbitals of the complex, which are mainly from the occupied antibonding HOMO (π_2px^*) and one of the degenerate orbitals (π_2py) of O₂, coupled with vacant orbitals (LUMO, LUMO+1) of Nb₃⁺. Here π backdonation interactions sourcing from bonding orbitals of Nb₃⁺ and LUMO (π_2py^*) of O₂ (Nb₃⁺→O₂) play a dominant contribution and form HOMO-7 orbital of the Nb₃O₂⁺ complex. The electrons on O₂ fragment π_2py^* orbital (Fig. S14 in Supporting information) stem from charge transfer and orbital interactions [56, 57]. In addition, NOCV analysis including detailed values of interaction energies and the shapes of electron deformation densities (Δρ) are shown in Figs. S17 and S18 (Supporting information), which display the weak interaction/bonding between Nb₃⁺ and N₂, while much larger donor-acceptor interaction energies for Nb₃O₂⁺ in the single state.

Figure 4

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Fig. 4. Kohn–Sham orbital correlation diagrams of (a) Nb3N2+ in the triplet state. (b) Nb3O2+ in the stable triplet state. Blue lines are for orbitals composited by one part, and red lines are for orbitals interaction of two fragments with percentage which solid lines are for occupied orbitals and dotted line are for vacant orbitals. All the data are calculated at the B3LYP/TZP level of theory using ADF.

Figs. 5a and b present the potential scan curves of Nb₃⁺ in reacting with N₂ and O₂ respectively to form the products (more details of varying directions are given in Figs. S20 and S21 in Supporting information), where the NPA charge distribution is displayed in color with dependence on the varying distances of the reactants. For "Nb₃⁺···N₂" (Fig. 5a), the two overlap curves display minor vibration of the N-N bond distance ranging from 1.09 Å to 1.11 Å. Considering that the product ¹Nb₃O₂⁺has an altered spin multiplicity and oxygen state comparing with the nascent ³Nb₃⁺ and ³O₂, we have performed the potential scan from two directions, allowing the geometrical configuration and spin multiplicity to be equal to the reactants and the products respectively. As is shown in Fig. 5b, the two scanning curves for "Nb₃⁺···O₂" display a crossing at ~2.4 Å, which indicates a critical value for the interaction distance of chemical reaction, and the energy minima point at a Nb₃⁺···O₂ distance of 1.74 Å corresponds to the oxygen-activated state (i.e., ¹Δ_g state, with O-O bond distance at 1.44 Å). Furthermore, Kohn–Sham orbital correlation diagrams, charge transfer diagrams along with orbital interactions and detailed NOCV analysis of Nb₃O₂⁺ in imaginary quintuplet state at a Nb₃⁺···O₂ distance of 1.80 Å and O-O bond distance of 1.20 Å are shown in Figs. S13, S15 and S19 in Supporting information).

Figure 5

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Fig. 5. Potential scan curves of the Nb3+ cluster in approaching (a) N2 and (b) O2 from two directions with the geometrical configuration and spin multiplicity equal to the reactants and the products respectively. Red lines refer to real approaching process due to energy crossing point, while light blue/green lines refer to imaginary scanning processes. The insets show the NPA charge of every atom as a function of different interaction distance.

Having unveiled the different interactions involved in "Nb₃⁺ + N₂" and "Nb₃⁺ + O₂", we then plotted the reaction coordinates for the reaction dynamics of the niobium clusters with oxygen and nitrogen, as shown in Fig. 6. Two reaction pathways are considered for Nb₃⁺ with N₂ in view of the likely varying molecular orientation of dinitrogen in approaching the cluster. As is shown, when the dinitrogen takes an end-on adsorption on a vertex atom of the Nb₃⁺ cluster (black line), it gets a 0.62 eV energy gain but subsequent path toward the final dissociative Nb₃N₂⁺ needs to overcome a few endothermic steps and the rate-determine transition state TS3 is 0.27 eV higher than the relative energy of the reactants. Alternatively, if the dinitrogen molecule could approach the cluster via an orientation orthogonal to one of the triangular edges of the Nb₃⁺, the reaction path (green line) needs to overcome a relatively smaller energy barrier but the rate-determine TS1 is still 0.24 eV higher in energy than the reactants. This energy barrier is small especially when considering the N≡N bond dissociation energy of ~9.79 eV, but it could not proceed spontaneously in the thermalized flow tube reactors (where previous studies suggest a threshold value could be ~0.1 eV by taking into consideration of vibrational, rotational and translational kinetic energy) [56, 58].

Figure 6

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Fig. 6. The calculated reaction pathways for (a) "Nb3+ + N2" and (b) "Nb3+ + O2". All the thermodynamic data are calculated at the B3LYP/Lanl2dz for Nb and B3LYP/6-311G (d) for O and N.

In sharp contrast, the reaction of "Nb₃⁺ + O₂" is largely exothermic (8.10 eV in our calculation) to form an oxygen-dissociative products with two bridge oxygen atoms bonding to two edges of the triangular Nb₃⁺ cluster, and the spin multiplicity turns to be singlet indicative of spin accommodation in the reaction process of ³Nb₃⁺ with ³O₂ [48, 59]. Note that, this reaction to form oxygen-dissociated product is highly thermodynamic and also kinetics-favorable, with the TS1 being 2.33 eV (or 3.2 eV for the yellow line) lower than the total energy of the reactants. Therefore, the mass spectrometry observation of Nb₃O₂⁺ (and the other Nb_nO₂⁺) could be completely dissociated products. This is largely due to the large Nb-O bond energy (~770 kJ/mol) in contrast to the bond strengths of O-O (~400 kJ/mol) and Nb-Nb (~513 kJ/mol). The large amount of energy release by dissociative adsorption makes it feasible to produce odd-oxygen produts (e.g., Nb₂O^+/0, Nb_nO_{1, 3}⁺) along with the NbO^0/+ removal.

In summary, utilizing our customized instrument MIFT-TQMS we report here an in-depth study to compare the gas-phase reactivity of Nb_n⁺ clusters with N₂ and O₂. Under sufficient backing gas pressure (up to 1 Torr), the Nb_n⁺ clusters readily react with N₂ and form a series of adsorption products Nb_nN_2m⁺. In comparison, the Nb_n⁺ clusters react with O₂ in a comparable rate but produce Nb_nO₁₋₄⁺along with a rapidly growing product NbO⁺, showing the O-O bond dissociation and metal cluster etching reactions akin to the previous findings on anionic aluminum and cobalt clusters. Energetics calculations show that the adsorption energies of Nb_nO₂⁺ (2.5~4 eV) are significantly larger than that of Nb_nN₂⁺ (0.5~0.9 eV). While the dinitrogen all follow an end-on orientation adsorption on the Nb_n⁺ clusters, the chemisorptive Nb_nO₂⁺ species conform to hollow-site adsorption along with spin-flip and perfect accommodation to be singlet. PDOS and frontier orbital analyses reveal strong π backdonation from Nb_n⁺ to O₂; which is in contrast to the Nb_nN₂⁺ system which involves both σ donation from N₂ to Nb_n⁺ and simultaneously weak π backdonation from Nb_n⁺ to N₂. Potential scan plots a van der Waals curve for the N₂ molecule in approaching the Nb₃⁺ cluster, whereas the potential scan for "Nb₃⁺···O₂" displays a crossing from two directions with the spin multiplicity equal to the reactants and the products respectively, shedding light on the spin crossing effect. Further, reaction dynamics calculation unveils feasible O-O bond dissociation and finally NbO (/NbO⁺) removal from such Nb_nO_2m⁺ clusters.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was financially supported by the CAS Instrument Development Project (No. Y5294512C1), the National Natural Science Foundation of China (No. 21722308) and Key Research Program of Frontier Sciences (CAS No. QYZDBSSWSLH024).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.04.020.