Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Fusion of Multiple Spectra for Investigating Chemical Bonding Properties via Machine Learning

Chemical bonding properties are crucial to understanding the chemical behavior of molecules. Spectroscopy is a versatile technical tool to study various microscopic properties, but its interpretation suffers from human biases and the loss of high-dimensional information. Here, we present a machine learning approach to predict diverse bonding properties, including the bond dissociation energy, bond length, and α-C connectivity of hydroxyls in organic molecules, by fusing multiple spectra with different physical mechanisms. Combining nuclear magnetic resonance and vibrational spectroscopy exhibits higher prediction accuracy than what they did separately. On the hold-out test data set, the models achieve a mean absolute error of 1.243 kcal/mol and 1.041 × 10-4 Å for BDE and bond length and an accuracy of 95.09% for hydroxyl α-C connectivity. Our models demonstrate strong extrapolation capabilities when they are transferred to different molecules, external electric fields, and solvation environments. These end-to-end models pave the way to investigating chemical bonding properties by using spectroscopic observables.

Plasma-Assisted Dinitrogen Activation on Small Cobalt Clusters: Co4 N9 + with Enhanced Stability

We have performed a study on the accommodation of nitrogen doping toward superatomic states of transition metal clusters. By reacting cobalt clusters with N₂ in the presence of plasma radiation, a large number of odd-nitrogen clusters were observed, typically Co₃ N_2m-1 ⁺ (m=1-5) and Co₄ N_2m-1 ⁺ (m=1-6) series, showing N≡N bond cleavage in the mild plasma atmosphere. Interestingly, the Co₃ N₇ ⁺ , Co₄ N₉ ⁺ , and Co₅ N₉ ⁺ clusters exhibit prominent mass abundances. First-principles calculation results elucidate the stability of the diverse cobalt nitride clusters and find unique stability of Co₄ N₉ ⁺ with a swallow-kite structure of which four coordinated N₂ molecules causes a significantly enlarged HOMO-LUMO gap, while the single N atom doping gives rise to superatomic states of 1S² 1P³ ||1D⁰ . We reveal an efficient dinitrogen activation strategy by reacting multiple N₂ molecules with cobalt clusters under a plasma atmosphere.

Gas-Phase Synthesis of Metal Olefins: Plasma-Assisted Methane Dehydrogenation and C═C Bond Formation

Methane dehydrogenation and C-C coupling under mild conditions are very important but challenging in chemistry. Utilizing a customized time of flight mass spectrometer combined with a magnetron sputtering (MagS) cluster source, here, we have conducted a study on the reactions of methane with small silver and copper clusters simply by introducing methane in argon as the working gas for sputtering. Interestingly, a series of [M(C_nH_2n)]⁺ (M = Cu and Ag; n = 2-12) clusters were observed, indicating high-efficiency methane dehydrogenation in such a plasma-assisted chamber system. Density functional theory calculations find the lowest energy structures of the [M(C_nH_2n)]⁺ series pertaining to olefins indicative of both C-H bond activation of methane and C-C bond coupling. We analyzed the interactions involved in the [Cu(C_nH_2n)]⁺ and [Ag(C_nH_2n)]⁺ (n = 1-6) clusters and demonstrated the reaction coordinates for the "Cu⁺ + CH₄" and "Ag⁺ + CH₄." It is illustrated that the presence of a second methane molecule enables us to reduce the necessary energy of dehydrogenation, which concurs with the experimental observation of an absence of the metal carbine products Cu⁺CH₂ and Ag⁺CH₂, which are short-lived. Also, it is elucidated that the higher-lying excitation states of Cu⁺ and Ag⁺ ions enable more favorable dehydrogenation process and C═C bond formation, shedding light on the plasma assistance of the essence.

Ladder Oxygenation of Group VIII Metal Clusters and the Formation of Metalloxocubes M13O8. J Phys Chem Lett 2022;13:733-739. [PMID: 35025527 DOI: 10.1021/acs.jpclett.1c04098]

The diversity of valence and bonding of transition metals makes their oxidation processes perplexing at reduced sizes. Here we report a comprehensive study on the oxidation reactions of rhodium clusters Rh_n^± (n = 3-30) and find that Rh_3,4O₄⁺, Rh_5-7O₆⁺, and Rh_8-13O₈⁺ always dominate the mass distributions showing size-dependent ladder oxygenation which is closely associated with the O-binding modes. While the Rh_8-13O₈⁺ clusters display a μ₃-O binding mode (hollow site adsorption), Rh_3-4O₄⁺ and Rh_5-7O₆⁺ favor the μ₂-O binding mode (edge-site adsorption) or a mixture of the two modes. The μ₃-O binding mode is inclined to yield a cubic Rh₁₃O₈, while the μ₂-O binding mode gives rise to oxygen-bridge protection for the metal clusters. Such ladder oxidation was also observed for Pt_n⁺, Fe_n⁺, Co_n⁺, and Ni_n⁺ clusters. We propose a three-dimensional diagram for the oxidation states and O-binding modes of metals, and highlight the metalloxocubes M₁₃O₈⁺ for cluster-genetic materials.

In situ formed Co from a Co-Mg-O solid solution synergizing with LiH for efficient ammonia synthesis

A cobalt magnesium oxide solid solution (Co-Mg-O) supported LiH catalyst has been synthesized, in which LiH functions both as a strong reductant for the in situ formation of Co metal nanoparticles and a key active component for ammonia synthesis catalysis. Dispersion of the Co-LiH composite on the Co-Mg-O support results in a significantly higher ammonia synthesis rate under mild reaction conditions (19 mmol g^-1 h^-1 at 300 °C, 10 bar).

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H^- could supply. Polarized, electron-rich N_xH_y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N₂, NH₃, and H₂, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH₃. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N₂ is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H^- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

First-principles screening of single transition metal atoms anchored on two-dimensional C9N4 for the nitrogen reduction reaction

Compared to the Haber-Bosch process, the electrochemical nitrogen reduction reaction (NRR) can convert N₂ into NH₃ under ambient conditions, and thus has attracted considerable attention in recent years. However, it remains a challenge to fabricate NRR catalysts with high faradaic efficiency and yield rate. In this work, by systematic first-principles calculations, we investigate the structure, stability and catalytic performance of single metal atoms anchored on porous monolayer C₉N₄ (M@C₉N₄) for the electrochemical NRR. A total of 25 transition metals (Sc-Zn, Zr-Mo, Ru-Ag, Hf-Au) were explored, and we screened out four promising systems, i.e., Nb, Ta, Re and W@C₉N₄, which not only exhibit high catalytic activity with low limiting potentials of -0.3, -0.42, -0.49 and -0.25 V, respectively, but also have superior selectivity that suppresses the competitive hydrogen evolution reaction. The physical origin lies in the coupling between the d orbitals of the transition metals and the 2π* orbital of N₂, which activates the N₂ molecule and facilitates the reduction process. Our proposed systems are kinetically and thermodynamically stable, which may shed light on future design and fabrication of high-efficiency single atom catalysts for various technologically important chemical reactions.

Gas-phase preparation and the stability of superatomic Nb11O15. Phys Chem Chem Phys 2021;23:15766-15773. [PMID: 34286767 DOI: 10.1039/d1cp02128a]

We report a study of the reactions of pure metal clusters Nbn- with dioxygen in the gas phase. It is found that the presence of low-concentration dioxygen reactants results in oxygen-addition products, whereas sufficient high-concentration dioxygen enables oxygen-etching reactions giving rise to molecular niobium oxides. Interestingly, in the presence of a suitable gas flow rate of an intermediate dioxygen concentration, a highly selective product Nb11O15- shows up in the mass spectra. Utilizing density functional theory (DFT) calculations, we have discussed the reactivities of Nbn- (3 ≤ n ≤ 14) clusters with oxygen, and unveiled the reasonable stability of Nb11O15- pertaining to its unique geometric structure with a D5h Nb@Nb10 core fully protected by 15 bridge-oxygen atoms. The oxygen-passivated Nb@Nb10O15- cluster exhibits a large HOMO-LUMO gap (1.46 eV) and effective multicenter bonds with remarkable superatom orbitals for all the 26 valence electrons of the Nb@Nb10 core corresponding to well-staggered energy levels. We illustrate the superatomic features in the Nb@Nb10 metallic core for which the adaptive natural density partitioning (AdNDP) analysis unveils thirteen 11c-2e bonds. Among them, one of the 11c-2e bonds accounts for the superatomic S orbital, three bonds correspond to superatomic P orbitals, another five display vivid D orbital characteristics, and the remaining four 11c-2e bonds are assigned to F orbital features. In addition, the net atomic charge of the center Nb atom is as high as -0.804 |e| rendering core-shell electrostatic interactions and the shielding effect of the Nb10O15 shell.