Plasma-Assisted Dinitrogen Activation via Dual Platinum Cluster Catalysis: A Strategy for Ammonia Synthesis under Mild Conditions

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 , 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 , 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 , 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 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 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 kJžmol−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 for dinitrogen activation has been by combining multiple N2 in the presence of dual Pt clusters. we investigated the formation of Figure the chain reaction for N2 dissociation on a Pt3+ cluster in the presence of a are The adsorption of one or two N2 molecules results in a small energy and a high reaction energy for N2 dissociation. N2 molecules are a large energy can be and the energy is to to form which facilitates the dissociation process, the formation of Pt3N7+ to be = the N2 onto Pt3N7+ to form is In comparison, to the weak stability and N2 dissociation on or is and the energy with more N2 The charge on Pt3N6+ and (see in Figure show that two of the nitrogen atoms are more than the other nitrogen atoms, indicating their stronger interactions with the occupied orbitals of another Pt cluster Figure | reaction and for N2 dissociation. (a) chain reaction for + xN2" by reactions with another The on the show the charge distribution of Pt3N6+ and (b) for with different Ptn+ (n = 1–4) cationic clusters to form (c) The reaction for dissociation from with and states are All the energies are shown with to the + Natural charge of the and on in the reaction process shown in Download figure Download PowerPoint A comparison of the chain reaction for N2 dissociation on the other Ptn+ (n = 1, 2, and clusters was performed ( Supporting Information Figures It is found that bond cleavage on as energy is for the elementary and the total reaction energy is even the adsorption of N2 For + the intermediate reacts with another enabling N2 dissociation and thus and with a formation energy of In comparison, the dissociation energy of N2 + to eV, and the formation energy of is very to that of Pt2N5+, indicating that could not be The orbital analysis for and ( Supporting Information Figure also shows that the N2 on has a with the indicative of its to Pt2N5+, which the intensity of in the mass spectra. The reaction of + xN2" have a as that on Pt3+, but the corresponding nitrides higher formation energies. on the number of nitrogen atoms, the charge of the Ptn+ (n = 1–4) core ( Supporting Information Figure represents a larger on Pt3Nm+ and than on the and clusters, indicating that Pt3+ and Pt4+ enable effective electron backdonation in the interactions with nitrogen atoms. Considering the of gas collisions under we conducted calculations to unveil the chain reaction mechanism for N2 dissociation by dual clusters. A of forming Pt3N7+ by reacting with another small Ptn+ (n = 1–4) cluster is given in Figure Pt2+ in the chain reaction dinitrogen activation can be with a significantly dissociation energy = 1.35 and the formation of Pt3N7+ is thermodynamically as the reaction is highly = For reacting with is a higher activation energy, which is to the Pt3N7+ clusters reactivity than their cationic involved in the reaction ( Supporting Information Figure This is likely the cationic cluster stronger interactions with the electrons of the other reaction are calculated for N2 dissociation from Pt1N4+, Pt2N6+, and and with different (n = 1–4) clusters (details in Supporting Information Figures Similarly, the clusters enable the reaction energies to be in the formation of odd-nitrogen the formation of from Pt1N4+ is all the pathways. The most is that for different Ptn+ (n = 1–4) clusters, the N2 dissociation step can be in the presence of a Pt2+ in significantly total reaction Among all the studied chain reaction channels, N2 dissociation from + is the most thermodynamically on a reasonable "3+2" reaction on the small metal Figure further into the of + Pt2+ → Pt3N7+ + for N2 activation and dissociation. Pt2+ to nitrogen in adsorption on the ( Supporting Information Figure The two intermediates and and two transition states and but the energy for and are in view of the energy in the nitrogen adsorption After transition state bridged N2 is with both and clusters, showing a dual in which the The of an important in N2, as the N≡N triple bond has been (with an enlarged bond length to transition state which is a the bond is dissociated to form Pt3N7+ and by an energy of of the total reaction energy of to the original is with the of plasma assistance. charge analysis of the dual cluster + shows that the charge distribution on the Pt2+ and the N2 molecules have no along the the Pt3+ has a charge from to and the charge on the bridged N2 from to The Pt3+ cluster efficient coordination and electron backdonation to the antibonding π*-orbitals of N2, which is to N2 activation. In we also calculated the for a series of dual-cluster reaction The energy are plotted in Supporting Information Figures and in Supporting Information Table It is shown that the + path (i.e., a not N2 binding in the dual and has an extremely high activation = and a large total reaction energy This is consistent with the experimental observation of rare odd-nitrogen products in the mass spectra, indicative of low activity of Pt1+ for N2 dissociation. In comparison, all the other reaction channels, including and can significantly lower the activation the dual adsorption of N2, the important synergistic effect of multiple metal sites for nitrogen dissociation. Among and can be produced the + and + respectively. However, the cluster to another N2 to form more stable Pt3N7+, which for the low abundance of in the experimental It is that the reaction energy + is than that along all the other which the "3+2" reaction as the most effective for N2 dissociation by the Pt clusters. for For hydrogenation of in situ synthesized platinum