Open AccessCCS ChemistryRESEARCH ARTICLE28 Apr 2022Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions Chaonan Cui†, Yuhan Jia†, Hanyu Zhang, Lijun Geng and Zhixun Luo Chaonan Cui† Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 †C. Cui and Y. Jia contributed equally to this work.Google Scholar More articles by this author , Yuhan Jia† Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 †C. Cui and Y. Jia contributed equally to this work.Google Scholar More articles by this author , Hanyu Zhang Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Lijun Geng Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Zhixun Luo *Corresponding author: E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201879 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The activation and reduction of N2 to produce ammonia under mild conditions is of great interest, but challenges remain. Here, we report a breakthrough in efficient dinitrogen cleavage by employing small Ptn+ (n = 1–4) clusters and convenient plasma assistance. The reactivity of Pt3+ is found to be substantially higher than that of other clusters, and the formed Pt3N7+ shows prominent mass abundance among the odd-nitrogen products. We illustrate that a chain reaction path within dual cluster cooperation, especially via a “3+2” mode, is beneficial to N≡N triple bond dissociation, embodying efficient synergistic catalysis. A key intermediate containing a bridged N2 of binding with two Pt clusters facilitates N2 activation with significantly enhanced interactions between the d orbitals of Pt and the antibonding π*-orbitals of N2. Furthermore, by reacting the PtnNm+ clusters with H2, we observed hydrogenation products of both even- and odd-hydrogen species, indicative of ammonia release. The in situ synthesized platinum nitride clusters, typically Pt3N7+, induce a highly active N site for hydrogen anchoring, enabling a cost-effective hydrotreating process for ammonia synthesis. Download figure Download PowerPoint Introduction Dinitrogen activation and fixation under mild conditions are important but challenging subjects in view of the extremely strong N≡N triple bond (with a bond dissociation energy of ∼941 kJmol−1).1,2 To date, industrial nitrogen fixation mainly relies on the Haber–Bosch method (N2 + 3H2 ⇌ 2NH3), which utilizes iron-based catalysts and requires high temperature and high pressure (∼500 °C, >100 atm) and consumes more than 1.4% of the fossil fuel worldwide along with 1.6% of total global CO2 emissions.3,4 Considering that the rate-limiting step of dinitrogen fixation is N≡N bond activation,5–7 ongoing efforts have been devoted to exploring energy-efficient and environmentally friendly catalysts for nitrogen reduction reactions under controllable conditions.8–16 Transition metals (TMs) and related complexes have been widely studied for this purpose.17–21 The electrons in the occupied d orbitals of TMs allow π backdonation to the antibonding π* orbitals of N2, as well as electron donation from the σ bond of N2 to the empty d orbitals of TMs, giving rise to enhanced metal–nitrogen interactions that are thus beneficial to dinitrogen activation.22–27 However, identifying a central paradigm to understand the deepest chemical mechanism is an outstanding challenge, which essentially limits the rational design of ideal catalysts for dinitrogen fixation under mild conditions. Different from the Haber–Bosch process, biological nitrogen fixation is realized by utilizing nitrogenases to convert dinitrogen to ammonia under ambient conditions.28–30 Nitrogenases generally include two components (e.g., MoFe protein and Fe protein), and they have to be chelated together to generate nitrogenase activity. Thus, developing highly efficient TM catalysts and improving the synergistic effect are essential for artificial nitrogen fixation under mild conditions.31 To optimize the synergistic effect of TM active sites for dinitrogen activation, reasonable research interest has been focused on diatomic catalysts (di-metal sites or two-atom single cluster catalysts),32–44 trinuclear catalysts,7,45–51 and diverse metal clusters, including nitrides and carbides.52–56 These catalysts take advantage of more active sites and synergistic interactions compared with single-atom catalysts,57 providing a distinctive way to identify the elementary steps on catalytic surface active sites,58–63 enabling the breakthrough of the scaling relationship limitation between reaction intermediates, which is beneficial for improving N2 dissociation activity and efficiency. However, to the best of our knowledge, the dual-cluster synergistic effect has not been explored. To learn from nature and motivate the two components to chelate together to generate nitrogenase activity, we propose a strategy of dual-cluster catalysis for N2 activation. We investigated the reactions of small Ptn+ (n ≤ 5) clusters by introducing N2 gas from a rare laser vaporization (LaVa) source along with a buffer gas to enable reactions under plasma bathing. Pt catalysis takes advantage of its chemical stability (resistance to oxidation and corrosion, etc.) and low overpotential in electrocatalytic hydrogen evolution while plasma assistance has been established as a very powerful and mild strategy for the activation of inert chemicals.64–75 As a result, a few PtnNm+ clusters with an odd number of nitrogen atoms were observed, providing robust evidence for N≡N cleavage. Furthermore, we studied the downstream reactions of the formed platinum nitrides with hydrogen and observed the replacement reaction products PtnH2x+ and hydrogenation species PtnNmHx+, including odd-hydrogen products pertaining to ammonia release. We fully demonstrated the chain reactions for “PtnN2m+ + Ptn+” (n = 1–4) channels, unveiled the dual-cluster synergistic effect for highly efficient N2 activation, and elucidated the reaction coordinates of Pt3N7+ with two H2 molecules to form ammonia. Experimental and Thoretical Methods Experimental methods This study used a customized reflection time-of-flight mass spectrometer combined with a LaVa cluster source.76 A pulsed 532 nm Nd:YAG laser (10 Hz) was employed for the ablation of the platinum disk (ɸ = 16 mm, 99.99%) with 5% N2 seeded in He (99.999%) as carrier gas (∼10 atm) which was controlled by a pulsed general valve (Parker, Serial 9). The laser power was tuned in a range of pulse energy at 5∼55 mJ to obtain proper surface plasma on the Pt disk; meanwhile, the Pt disk was kept rotating noncoaxially to ensure that fresh surfaces were constantly exposed to laser ablation. The PtnNm+ clusters were formed in the LaVa source due to supersonic expansion, showing size dependence on the multiple collisions in the nozzle (ɸ = 1.35 mm; L = 35 cm) and plasma atmosphere (controlled by laser power density). The formed PtnNm+ clusters were then skimmed (ɸ = 2 mm) into a differentially pumped chamber in which the cationic clusters were orthogonally accelerated by five electrode grids. After several microseconds of flight and reflection, the cluster ions were detected by a dual microchannel plate detector and recorded with a digital oscilloscope (Teledyne LeCroy HDO6000). To investigate the hydrogenation of platinum nitride clusters, the PtnNm+ clusters produced in the LaVa source were guided to react with hydrogen by a downstream flow tube reactor (ɸ = 2 mm; length = 60 mm). The H2 reactant (30% in He, 1 atm) was injected into the PtnNm+ cluster beam and controlled by another pulsed valve. The small, medium, and large amounts of H2 correspond to on-time of the pulsed value at 150∼225 μs per period (100 ms, i.e., 10 Hz). Similarly, the cluster beam of the reaction product was skimmed into the time-of-flight (TOF) chamber for mass spectrometry experiments. Computational methods Density functional theory calculations using the Gaussian 09 program77 were conducted to determine the ground state structure and multiplicity of platinum nitride clusters as well as the reaction mechanisms for N2 adsorption and dissociation on the Ptn+ clusters. The Lee–Yang–Parr correlation functional (B3LYP)78,79 combining the basis set of Stuttgart/Dresden (SDD) and corresponding effective core potential were employed for the Pt atoms, and 6-311+G(2d) was used for N atoms, namely, B3LYP/6-311+G(2d), SDD. To validate the B3LYP functional, the optimized Pt3Nm+ clusters were calculated by various functionals (see Supporting Information Table S2). Generalized gradient approximation methods with the B3LYP functional have been proven to provide accurate energetic and geometric results for TMs and nitrides. Vibrational frequency calculations were performed to ensure that the reaction intermediates and transition states have zero and one imaginary frequency, respectively. The corrections of zero-point energy were included for the energy analysis. Natural population analysis (NPA)80 was performed to check out the atomic point charge distribution on the PtnNm+ clusters. The molecular orbital patterns were plotted via visual molecular dynamics (VMD) and Multiwfn software.81,82 Results and Discussion The formation of odd-nitrogen PtnNm+ clusters The TOF mass spectra of small Ptn+ (n = 1–5) cationic clusters reacting with N2 molecules (5% N2 in He) under plasma atmosphere were obtained at different laser power densities. As shown in Figures 1a–1d, only PtnN2m+ clusters with an even number of N atoms (such as Pt1N4+, Pt2N8+, Pt3N6+, Pt4N8+, and Pt5N10+) were observed under a lower laser power density (c.a. 0.5 mJ). By increasing the laser power density, several PtnN2m+1+ clusters with odd numbers of nitrogen atoms appear in the mass spectra corresponding to complete cleavage of the N≡N triple bond in the reactions of N2 with Ptn+ clusters. With a further increase in the laser power, more cationic platinum nitride clusters with odd numbers of N atoms emerge in the spectra, seen as Pt1N5+, Pt2N5+, Pt3N7+, Pt3N9+, Pt4N9+, and Pt5N11+, indicative of more N2 molecules being dissociated. Under even higher laser power conditions (≥24.9 mJ), no more PtnNm+ products appear, indicative of saturation of N2 adsorption on the small Ptn+ clusters (for details see Supporting Information Figure S1). Figure 1 | Typical mass spectra of Ptn+ clusters reacting with N2 under plasma atmosphere. The plasma is provided by different laser powers focused on a Pt disk target. (a–d) The laser pulse energies are 0.5, 1.4, 12.1, and 24.9 mJ, respectively. Download figure Download PowerPoint Interestingly, more N2 dissociation products with stronger intensity are observed for the Pt3N2m+1+ products, indicating a higher reactivity of Pt3+ toward dinitrogen cleavage than the other Ptn+ clusters. To illustrate the superiority of Pt3+ for N2 dissociation, we plotted the mass abundances of the platinum nitride clusters with odd/even numbers of N atoms on each small Ptn+ cluster (n = 1–5), as shown in Figure 2a. It can be clearly seen that the Pt3N2m+1+ series possesses a maximal intensity and the highest conversion efficiency among all the product clusters of this study. In comparison, Pt1+ shows weak efficiency in forming Pt1N2m+1+ products; instead, Pt1+(N2)2 dominates its products, pertaining to reasonable N2 coordination on the Pt cation. Figure 2b summarizes the nitrogen adsorption limits (written as PtnNm+_max and PtnNm+_min) and the most intense N-absorbed complex (i.e., PtnNm+_int) in the “Ptn+ + xN2” reaction processes. Among the studied Ptn+ clusters (n = 1, 2, 4, and 5), the most intense cluster corresponds to the maximum number of N atoms (i.e., PtnNm+_int = PtnNm+_max), indicating a tendency of N saturation in forming stable PtnNm+ clusters. However, for Pt3+, the most intense peak “Pt3Nm+_int” corresponds to m = 7 or 6 (instead of PtnNm+_max) for odd or even N atoms, respectively, revealing the unique reactivity of Pt3+ for N2 dissociation. Figure 2 | The product analysis of small Ptn+ clusters reacting with N2. (a) Mass abundances of the formed PtnNm+ clusters on each small Ptn+ cluster (n = 1–5) at a laser power of 24.9 mJ. (b) Recorded N-atom adsorption limits of “PtnNm+_max” (hollow circle) and “PtnNm+_min” (solid circle), and the most intense N-absorbed complex “PtnNm+_int” (star), at a laser energy of 24.9 mJ. The red symbols represent the products with an odd number of nitrogen atoms while blue symbols refer to the even ones. The black line represents the 1:1 stoichiometry of N2 number and Pt atoms (m = 2n). The hatched area highlights the difference between “PtnNm+_max” and “PtnNm+_min” of the products with an odd number of N. (c) The ratio of the total mass abundances of the formed odd-N products PtnN2m+1+ relative to that of PtnNm+ clusters, with regard to the increasing n value of the small Ptn+ clusters (n = 1–5) at different laser energies. Download figure Download PowerPoint To demonstrate the critical contribution of plasma activation, we conducted a comparative experiment by preparing pure cationic Ptn+ clusters (n = 1–5) via the same LaVa cluster source and studied their reactions with N2 gases in a downstream flow tube reactor ( Supporting Information Figure S2). As a result, only PtnN2m+ cluster species with an even number of nitrogen atoms were observed, seen as Pt1N4+, Pt2N6+, Pt3N6+, Pt4N8+, and Pt5N10+, with an increasing coordination number of N2 on the larger Ptn+ clusters. The absence of PtnN2m+1+ products suggests that local plasma assistance is indispensable for dinitrogen dissociation on Ptn+ clusters under ambient conditions. This is also consistent with previous studies in terms of the dissociation of N≡N triple bonds on Rhn+ clusters.83 Considering that plasma creation is also an energy-consuming process, we have provided a simple estimation to evaluate the performance of plama assistance. Figure 2c plots the calculated ratios of the formed odd-N product PtnN2m+1+ relative to the PtnNm+ products at different laser energies. The ratio of PtnN2m+1+/PtnNm+ increases with rising laser energy, indicating enhanced efficiency for N2 dissociation. With laser energy at about 12.1 mJ/pulse, the relative reaction efficiency to produce odd-N products reaches a maximum. Downstream reactions of Pt3Nm+ with H2 Having obtained odd-nitrogen PtnNm+ clusters, we then studied the in situ hydrogenation reactions. Different amounts of H2 were added into the downstream flow tube reactor where H2 gas encounters and reacts with the prepared PtnNm+ clusters ( Supporting Information Figure S3). Diverse hydrogenated products were observed, but Pt1N4+ and Pt2N8+ were found to be inert in reacting with hydrogen ( Supporting Information Figures S3 and S4), likely due to their high coordination, which impedes the bonding of hydrogen onto the metal. Figures 3b and 3c present the enlarged area of the mass spectra for Pt3Nm+ reacting with H2, where the nascent Pt nitrides Pt3N6-9+ were observed to fully react with hydrogen, giving rise to a variety of combination products involving both N2-removal and H2-addition reactions, such as Pt3N7H2+, Pt3N4H4,6+, and Pt3N2H6,8+ (for more details, see Supporting Information Figures S3 and S5). In the presence of a large quantity of the H2 reactant gas, full substitution products (e.g., Pt3H12+) and more hydrogen components are involved in the Pt3NmHx+ species, among which a broad peak corresponding to a combination of Pt3N6+, Pt3N6H6+, and Pt3N6H+ indicates H2 dissociation in the reaction process (details for peak assignment can be found in Supporting Information Figures S6 and S7 and Table S1). Figure 3 | Mass spectra of the Pt3Nm+ clusters reacting with H2. (a) Mass spectra of Pt3+ clusters reacting with N2 under plasma atmosphere with a laser pulse energy at ∼9.5 mJ. (Full spectra are given in Supporting Information Figure S3.) The inset shows a simulated isotope distribution of Pt3N6+, which matches well with the experimental results. (b) Mass spectra of Pt3Nm+ reacting with a small amount of H2 (pulsed reaction time at 150 μs, 10 Hz). (c) Mass spectra of Pt3Nm+ reacting with a large amount of H2 (pulsed reaction time at 224 μs, 10 Hz). The inset shows simulated isotope distribution of the mass range covering Pt3N6Hx+ (red curve), assigned to a combination of Pt3N6+ (54%), Pt3N6H+ (21%) and Pt3N6H6+ (25%), in comparison with the experimental distribution (black curve). For details, see Supporting Information Figure S7. Download figure Download PowerPoint Interestingly, in the presence of a small amount of hydrogen gas, the products assigned to Pt3N7H2+ show higher intensity than Pt3N7+ and Pt3N9+, suggesting the transfer of both original clusters to Pt3N7H2+. Meanwhile, nitrogen reduction to hydro-nitrogen species could also occur as competitive channels. In brief, the channels for Pt3Nm+ reacting with H2 can be written as: Pt 3 N m + + H 2 → Pt 3 N m H 2 + (1) Pt 3 N m + + H 2 → Pt 3 N m − 2 H 2 + + N 2 (2) Pt 3 N m + + 2 H 2 → Pt 3 N m − 1 H + + NH 3 (3) Pt 3 N m + + 3 H 2 → Pt 3 N m − 2 + + 2 NH 3 (4)Notably, the nitrogen atoms decrease sequentially with the rising partial pressure of H2 gas until a complete replacement of the platinum nitrides with platinum hydrides (e.g., Pt3H12+) takes place. The observation of nitrogen to be replaced by hydrogen eventually also suggests that the Pt3+ clusters can be reproduced simply by degassing hydrogen at high temperature,84 indicative of reutilization and a complete reaction cycle. Reaction mechanism for N≡N bond dissociation Theoretical calculations were performed to determine the ground-state structures of the PtnNm+ (n = 1–4, m = 1–12) clusters ( Supporting Information Figures S8–S10) and to unveil the reaction mechanism. All dinitrogen molecules are inclined to bind with Ptn+ clusters in an end-on orientation while the dissociated isolated nitrogen atoms prefer to anchor at the edge site of Pt2+ and at the hollow sites of Pt3+ and Pt4+ clusters. The relative stability of these PtnNm+ clusters was evaluated by calculating the formation enthalpies of the ground-state structures ( Supporting Information Figure S11). It is shown that the PtnNm+ clusters with an even number of nitrogen atoms have relatively higher stability than those with an odd number of nitrogen atoms. Their locations at the minimum of the convex hulls (Pt1N4+, Pt2N6+, Pt2N8+, Pt3N6+, and Pt4N8+) are consistent with their dominant intensities in the mass spectra. It should be noted that several PtnN2m+1+ clusters with highly symmetric structures, such as Pt2N5+, Pt3N7+, and Pt3N9+, possess lower formation energies than the others, suggesting their preferential formation in the flow tube reaction with plasma assistance. To understand the reaction mechanism for N2 dissociation on the small Pt clusters, we examined the thermodynamically favorable reaction pathways. Considering that the reaction energy for direct N2 dissociation into two radicals N* (N2→2N*) can reach 15.21 eV, it is impracticable for direct N≡N bond splitting under ambient conditions.11,83 Alternatively, a chain reaction route for dinitrogen activation has been proposed by combining multiple N2 adsorptions in the presence of dual Pt clusters. First, we investigated the formation of Pt3N7+. Figure 4a presents the chain reaction pathways for N2 dissociation on a Pt3+ cluster in the presence of a neutral Pt3 counterpart. Five successive pathways are considered. The adsorption of one or two N2 molecules results in a small energy gain (ca. <2 eV) and hence a high reaction energy for N2 dissociation. When four N2 molecules are adsorbed, a large energy can be gained, and the system energy is lowered to −2.90 eV to form Pt3N8+, which facilitates the subsequent N–N dissociation process, allowing the formation of Pt3N7+ to be exothermic (∆E = −0.81 eV). Moreover, the subsequent N2 addition onto Pt3N7+ to form Pt3N9+ is energetically favorable. In comparison, owing to the weak stability and steric hindrance, N2 dissociation on Pt3N10+ or Pt3N12+ is infeasible, and the energy gain diminishes with more N2 adsorption. The NPA charge distributions on Pt3N6+ and Pt3N8+ (see insets in Figure 4a) show that two of the terminal nitrogen atoms are more positively charged than the other nitrogen atoms, indicating their stronger interactions with the occupied orbitals of another Pt cluster counterpart. Figure 4 | Chain reaction pathways and kinetics for N2 dissociation. (a) Possible chain reaction pathways for “Pt3+ + xN2” followed by reactions with another Pt3 neutral cluster. The insets on the bottom left show the NPA charge distribution of Pt3N6+ and Pt3N8+. (b) Energy profiles for Pt3N8+ interacting with different Ptn+ (n = 1–4) cationic clusters to form Pt3N7+. (c) The proposed reaction pathway for N–N dissociation from Pt3N8+ interacting with Pt2+. Both singlet and triplet spin states are considered. All the energies are shown with respect to the initial reactants “Pt3+ + 4N2”. (d) Natural charge variation of the four fragments, Pt3, Pt2, N≡N, and 3N2 (based on IM2) in the reaction process shown in (c). Download figure Download PowerPoint A comparison of the chain reaction pathways for N2 dissociation on the other Ptn+ (n = 1, 2, and 4) clusters was performed ( Supporting Information Figures S12–S14). It is found that bond cleavage hardly proceeds on Pt1+, as significant energy is required for the elementary step, and the total reaction energy is largely endothermic (>1.4 eV) even after the adsorption of four N2 molecules. For “Pt2+ + xN2”, the intermediate Pt2N6+ reacts with another Pt2 cluster, enabling N2 dissociation and thus generating Pt2N5+ and Pt2N1 with a formation energy of −0.56 eV. In comparison, the dissociation energy of N2 through “Pt2N8+ + Pt2” jumps to 3.46 eV, and the formation energy of Pt2N7+ is very close to that of Pt2N5+, indicating that Pt2N7+ could not be easily produced. The localized orbital locater analysis for Pt2N5+ and Pt2N7+ ( Supporting Information Figure S15) also shows that the outer N2 on Pt2N7+ has a weaker interaction with the Pt2 cluster, indicative of its ready desorption back to Pt2N5+, which explains the reduced intensity of Pt2N7+ in the mass spectra. The reaction pathways of “Pt4+ + xN2” have a similar trend as that on Pt3+, but the corresponding nitrides display higher formation energies. Depending on the number of adsorbed nitrogen atoms, the NPA charge of the Ptn+ (n = 1–4) core ( Supporting Information Figure S16) represents a larger fluctuation on Pt3Nm+ and Pt4Nm+ than on the Pt1Nm+ and Pt2Nm+ clusters, indicating that Pt3+ and Pt4+ enable effective electron backdonation in the interactions with nitrogen atoms. Considering the existence of multibody gas collisions under rich-pressure conditions, we conducted comprehensive calculations to unveil the chain reaction mechanism for N2 dissociation mediated by dual clusters. A typical example of forming Pt3N7+ by reacting Pt3N8+ with another small Ptn+ (n = 1–4) cluster is given in Figure 4b. When Pt2+ participates in the chain reaction path, dinitrogen activation can be facilitated with a significantly lowered N–N dissociation energy (∆E = 1.35 eV), and the formation of Pt3N7+ is thermodynamically favourable, as the whole reaction is highly exothermic (∆E = −1.56 eV). For Pt3N8+ reacting with Pt1+, there is a higher activation energy, which is averse to the subsequent Pt3N7+ formation. Comparably, neutral Pt2,4 clusters exhibit less reactivity than their cationic counterparts involved in the reaction ( Supporting Information Figure S17). This is likely because the cationic cluster undergoes stronger interactions with the lone pair electrons of nitrogen. Additionally, the other reaction pathways are calculated for N2 dissociation starting from Pt1N4+, Pt2N6+, and Pt4N8+ and cooperating with different Ptn+/0 (n = 1–4) clusters (details in Supporting Information Figures S18–S20). Similarly, the specific counterpart clusters enable the reaction energies to be lowered in the formation of odd-nitrogen species; nevertheless, the formation of Pt1N3+ from Pt1N4+ is energetically unfavourable through all the pathways. The most noteworthy finding is that for different Ptn+ (n = 1–4) clusters, the N2 dissociation step can be largely improved in the presence of a Pt2+ cluster, resulting in significantly reduced total reaction energy. Among all the studied chain reaction channels, N2 dissociation from “Pt3N8+ + Pt2+” is the most thermodynamically favorable, shedding light on a reasonable “3+2” reaction mode on the small metal clusters.85 Figure 4c provides further insight into the detailed kinetic pathway of “Pt3N8+ + Pt2+ → Pt3N7+ + Pt2N+” for N2 activation and dissociation. Pt2+ tends to approach nitrogen in vertical adsorption on the Pt3 triangular plane ( Supporting Information Figure S21). The two main intermediates (IM1 and IM2) and two transition states (TS1 and TS2) display ascending energies, but the energy barriers for TS1 and TS2 are surmountable in view of the energy gain in the primary nitrogen adsorption process. After overcoming transition state TS1, bridged N2 is linked with both Pt3 and Pt2 clusters, showing a dual side-on mode (μ4-η2:η2-N2) in IM2 (namely, [Pt3(N6)N-NPt2]2+), which mimics the “Feynman diagram” shape. The specific configuration of IM2 plays an important role in activating N2, as the N≡N triple bond has been progressively weakened (with an enlarged bond length up to 1.21 Å). Following transition state TS2, which is a rate-determining step, the N–N bond is completely dissociated to form Pt3N7+ and Pt2N+ by overcoming an energy barrier of 2.55 eV. Because of the total reaction energy of 0.61 eV (relative to the original reactants), TS2 is surmountable with the help of plasma assistance. NPA charge analysis (Figure 4d) of the dual cluster system “Pt3N8+ + Pt2+” shows that the charge distribution on the Pt2+ fragment and the three adsorbed N2 molecules have no significant changes along the pathway, whereas the Pt3+ fragment has a charge variation from 0.43 |e| to 1.21 |e|, and the charge on the bridged N2 unit changes from 0.37 |e| to −0.75 |e|. The Pt3+ cluster allows efficient coordination and electron backdonation to the antibonding π*-orbitals of N2, which is vital to N2 activation. In addition, we also calculated the kinetic pathways for a series of dual-cluster reaction paths. The energy diagrams are plotted in Supporting Information Figures S22–S27 and summarized in Supporting Information Table S3. It is shown that the “Pt1N4+ + Pt2+” path (i.e., a “1+2” mode) does not support N2 binding in the dual side-on mode and has an extremely high activation barrier (3TS2 = 4.29 eV) and a large total reaction energy (1.65 eV). This is consistent with the experimental observation of rare odd-nitrogen products (Pt1N2m+1+) in the mass spectra, indicative of low activity of Pt1+ for N2 dissociation. In comparison, all the other reaction channels, including “3+2,” “3+3,” and “2+2”, can significantly lower the activation barriers (<2.75 eV) through the dual side-on adsorption of N2, validating the important synergistic effect of multiple metal sites for nitrogen dissociation. Among them, Pt2N5+ and Pt3N5+ can be produced through the “Pt2N6+ + Pt2+” and “Pt3N6+ + Pt2+” paths, respectively. However, the Pt3N5+ cluster prefers to adsorb another N2 molecule to form more stable Pt3N7+, which accounts for the low abundance of Pt3N5+ in the experimental