Catalytic Conversion of NO and CO by Noble-Metal-Free Copper-Vanadium Oxide Cluster Anions CuVO3,4.

Herein, by using state-of-the-art mass spectrometry, we demonstrated experimentally that the bimetallic copper-vanadium oxide cluster anions CuVO3,4- can catalyze the reduction of NO by CO into N2O and CO2. Note that the catalysis of NO reduction by CO has been rarely established in the gas phase and noble-metal containing clusters were commonly emphasized. Benefiting from quantum-chemical calculations, the Cu-V synergistic effect that both metal atoms work energetically to favor NO adsorption, N-N coupling, and CO oxidation by facilitating electron transfer can be understood at a strictly molecular level. Theoretical results demonstrated that the precaptured NO molecule encourages the adsorption of the second NO by electron donation. This finding deepens our understanding on NO reduction that NO functions not only as a reactant but also as a promoter during the reactions. This discovery could be helpful to permeate the nature and mechanism of active sites on related copper-vanadium heterogeneous catalyst used in real-life NO reduction.

Role of H2O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO2)nO- (n = 1-4).

Water (H2O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H2O molecules in CO oxidation poses a challenging but pivotal task in real-life catalysis. Herein, benefiting from state-of-the-art mass-spectrometric experiments and quantum chemical calculations, we identified that the dissociation of a H2O molecule on each of the cerium oxide cluster anions (CeO2)nO- (n = 1-4) at room temperature can create a new atomic oxygen radical (O•-) that then oxidizes a CO molecule. The size-dependent reactivity of H2O-mediated CO oxidation on (CeO2)nO- clusters was rationalized by the orbital compositions (O2p) and energies of the lowest unoccupied molecular orbitals of active O•- radicals modified by H2O dissociation. Our findings not only provide new insights into H2O-mediated CO oxidation but also demonstrate the importance of H2O in modulating the reactivity of the O•- radicals.

Reactivity of Atomic Oxygen Radical Anions in Metal Oxide Clusters.

Atomic oxygen radical anion (O•-) represents an important type of reactive centre that exists in both chemical and biological systems. Gas-phase atomic clusters can be studied under isolated and well controlled conditions. Studies of O•--containing clusters in the gas-phase provide a unique strategy to interpret the chemistry of O•- radicals at a strictly molecular level. This review summarizes the research progresses made since 2013 for the reactivity of O•- radicals in the atomically precise metal oxide clusters including negatively charged, nanosized, and neutral heteronuclear clusters benefitting from the development of advanced experimental techniques. New electronic and geometric factors to control the reactivity and product selectivity of O•- radicals under dark and photo-irradiation conditions have been revealed. The detailed mechanisms of O•- generation have been discussed for the reaction systems of nanosized and heteroatom-doped metal oxide clusters. The catalytic reactions mediated by the O•- radicals in metal clusters have also been successfully established and the microscopic mechanisms about the dynamic generation and depletion of O•- radicals have been clearly understood. The studies of O•- containing metal oxide clusters in the gas-phase provided new insights into the chemistry of reactive oxygen species in related condensed-phase systems.

Dry Reforming of Methane to Syngas Mediated by Rhodium-Cobalt Oxide Cluster Anions Rh2CoO.

Dry reforming of methane (DRM) to syngas is an important route to co-convert CH4 and CO2. However, the highly endothermic nature of DRM induces the thermocatalysis to commonly operate at high temperatures that inevitably causes coke deposition through pyrolysis of methane. Herein, benefiting from the mass spectrometric experiments complemented with quantum chemical calculations, we have discovered that the bimetallic oxide cluster Rh2CoO- can mediate the co-conversion of CH4 and CO2 at room temperature giving rise to two free H2 molecules and two adsorbed CO molecules (COads). The only elementary step requiring the input of external energy (e.g., high temperature) is desorption of COads from the reaction intermediate Rh2CoOC2O2-. The doping effect of Co has also been clarified that the Co could tune the charge distribution and orbital energy of the active metal Rh, enabling the enhancement of cluster reactivity toward C-H activation, which is essential to facilitating the DRM to syngas. This work not only underlines the importance of temperature control over elementary steps in practical thermocatalysis but also identifies a promising active species containing the late 3d transition metal to drive DRM to syngas. The findings could provide novel insights into design of bimetallic catalysts for co-conversion of CH4 and CO2 at low temperatures.

Experimental Reactivity of (MoO3)NO- (N = 1-21) Cluster Anions with C1-C4 Alkanes: A Simple Model to Predict the Reactivity with Methane.

Metal oxide clusters with atomic oxygen radical anions are important model systems to study the mechanisms of activating and transforming very stable alkane molecules under ambient conditions. It is extremely challenging to characterize the activation and conversion of methane, the most stable alkane molecule, by metal oxide cluster anions due to the low reactivity of the anionic species. In this study, using a ship-lock type reactor that could be run at relatively high pressure conditions to provide a high number of collisions in ion-molecule reactions, the rate constants of the reactions between (MoO3)NO- (N = 1-21) cluster anions and the light alkanes (C1-C4) were measured under thermal collision conditions. The relationships among the reaction rates of different alkanes were obtained to establish a model to predict the low rate constants with methane from the high rate constants with C2-C4 alkanes. The model was tested by using available experimental results in literature. This study provides a new method to estimate the relatively low reactivity of atomic oxygen radical anions with methane on metal oxide clusters.

Conversion of Methane at Room Temperature Mediated by the Ta-Ta σ-Bond.

Metal-metal bonds constitute an important type of reactive centers for chemical transformation; however, the availability of active metal-metal bonds being capable of converting methane under mild conditions, the holy grail in catalysis, remains a serious challenge. Herein, benefiting from the systematic investigation of 36 metal clusters of tantalum by using mass spectrometric experiments complemented with quantum chemical calculations, the dehydrogenation of methane at room temperature was successfully achieved by 18 cluster species featuring σ-bonding electrons localized in single naked Ta-Ta centers. In sharp contrast, the other 18 remaining clusters, either without naked Ta-Ta σ-bond or with σ-bonding electrons delocalized over multiple Ta-Ta centers only exhibit molecular CH4-adsorption reactivity or inertness. Mechanistic studies revealed that changing cluster geometric configurations and tuning the number of simple inorganic ligands (e.g., oxygen) could flexibly manipulate the presence or absence of such a reactive Ta-Ta σ-bond. The discovery of Ta-Ta σ-type bond being able to exhibit outstanding activity toward methane conversion not only overturns the traditional recognition that only the metal-metal π- or δ-bonds of early transition metals could participate in bond activation but also opens up a new access to design of promising metal catalysts with dual-atom as reactive sites for chemical transformations.

Toward Designing Reactive Metal Clusters for Dinitrogen Activation: A Guideline Based on N2 Initial Adsorption.

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N2 activation mechanism. However, the effective design and search of reactive clusters in N2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N2 initial adsorption energy (ΔEads) is an important parameter to control the N2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of ΔEads, we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV2S2-, TaV2C2-, and TaV2C3-, the high N2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of ΔEads revealed in this work not only reshapes our understanding of N2 activation reactions in the gas phase but also could have implication for other N2 activation processes in the condensed phase. The more general establishment of this new perspective on N2 activation reactivity warrants future experimental and computational studies.

Size-dependent reactivity of VnO+ (n = 1-9) clusters with ethane.

Cost-effective and readily accessible 3d transition metals (TMs) have been considered as promising candidates for alkane activation while 3d TMs especially the early TMs are usually not very reactive with light alkanes. In this study, the reactivity of Vn+ and VnO+ (n = 1-9) cluster cations towards ethane under thermal collision conditions has been investigated using mass spectrometry and density functional theory calculations. Among Vn+ (n = 1-9) clusters, only V3-5+ can react with C2H6 to generate dehydrogenation products and the reaction rate constants are below 10-13 cm3 molecule-1 s-1. In contrast, the reaction rate constants for all VnO+ (n = 1-9) with C2H6 significantly increase by about 2-4 orders of magnitude. Theoretical analysis evidences that the addition of ligand O affects the charge distribution of the metal centers, resulting in a significant increase in the cluster reactivity. The analysis of frontier orbitals indicates that the agostic interaction determines the size-dependent reactivity of VnO+ cluster cations. This study provides a novel approach for improving the reactivity of early 3d TMs.

Machine Learning for Experimental Reactivity of a Set of Metal Clusters toward C-H Activation.

Understanding the mechanisms of C-H activation of alkanes is a very important research topic. The reactions of metal clusters with alkanes have been extensively studied to reveal the electronic features governing C-H activation, while the experimental cluster reactivity was qualitatively interpreted case by case in the literature. Herein, we prepared and mass-selected over 100 rhodium-based clusters (RhxVyOz- and RhxCoyOz-) to react with light alkanes, enabling the determination of reaction rate constants spanning six orders of magnitude. A satisfactory model being able to quantitatively describe the rate data in terms of multiple cluster electronic features (average electron occupancy of valence s orbitals, the minimum natural charge on the metal atom, cluster polarizability, and energy gap involved in the agostic interaction) has been constructed through a machine learning approach. This study demonstrates that the general mechanisms governing the very important process of C-H activation by diverse metal centers can be discovered by interpreting experimental data with artificial intelligence.

Reverse water-gas shift catalyzed by RhnVO3,4- (n = 3-7) cluster anions under variable temperatures.

A fundamental understanding of the exact structural characteristics and reaction mechanisms of interface active sites is vital to engineering an energetic metal-support boundary in heterogeneous catalysis. Herein, benefiting from a newly developed high-temperature ion trap reactor, the reverse water-gas shift (RWGS) (CO2 + H2 → CO + H2O) catalyzed by a series of compositionally and structurally well-defined RhnVO3,4- (n = 3-7) clusters were identified under variable temperatures (298-773 K). It is discovered that the Rh5-7VO3,4- clusters can function more effectively to drive RWGS at relatively low temperatures. The experimentally observed size-dependent catalytic behavior was rationalized by quantum-chemical calculations; the framework of RhnVO3,4- is constructed by depositing the Rhn clusters on the VO3,4 "support", and a sandwiched base-acid-base [Rhout--Rhin+-VO3,4-; Rhout and Rhin represent the outer and inner Rh atoms, respectively] feature in Rh5-7VO3,4- governs the adsorption and activation of reactants as well as the facile desorption of the products. In contrast, isolated Rh5-7- clusters without the electronic modification of the VO3,4 "support" can only catalyze RWGS under relatively high-temperature conditions.

Spectroscopic Characterization of Thermal Methane Activation by Lewis-Acid-Base Pair in a Gas-Phase Metal Nitride Anion Ta2N3.

Activation and transformation of methane is one of the "holy grails" in catalysis. Understanding the nature of active sites and mechanistic details via spectroscopic characterization of the reactive sites and key intermediates is of great challenge but crucial for the development of novel strategies for methane transformation. Herein, by employing photoelectron velocity-map imaging (PEVMI) spectroscopy in conjunction with quantum chemistry calculations, the Lewis acid-base pair (LABP) of [Taδ+-Nδ-] unit in Ta2N3 - acting as an active center to accomplish the heterolytic cleavage of C-H bond in CH4 has been confirmed by direct characterization of the reactant ion Ta2N3 - and the CH4-adduct intermediate Ta2N3CH4 -. Two active vibrational modes for the reactant (Ta2N3 -) and four active vibrational modes for the intermediate (Ta2N3CH4 -) were observed from the vibrationally resolved PEVMI spectra, which unequivocally determined the structure of Ta2N3 - and Ta2N3CH4 -. Upon heating, the LABP intermediate (Ta2N3CH4 -) containing the NH and Ta-CH3 unit can undergo the processes of C-N coupling and dehydrogenation to form the product with an adsorbed HCN molecule.

CO Oxidation Promoted by NO Adsorption on RhMn2O3- Cluster Anions.

CO oxidation represents an important model reaction in the gas phase to provide a clear structure-reactivity relationship in related heterogeneous catalysis. Herein, in combination with mass spectrometry experiments and quantum-chemical calculations, we identified that the RhMn2O3- cluster cannot oxidize CO into gas-phase CO2 at room temperature, while the NO preadsorbed products RhMn2O3-[(NO)1,2] are highly reactive in CO oxidation. This discovery is helpful to get a fundamental understanding on the reaction behavior in real-world three-way catalytic conditions where different kinds of reactants coexist. Theoretical calculations were performed to rationalize the crucial roles of preadsorbed NO where the strongly attached NO on the Rh atom can greatly stabilize the products RhMn2O2-[(NO)1,2] during CO oxidation and at the same time works together with the Rh atom to store electrons that stay originally in the attached CO2- unit. The leading result is that the desorption of CO2, which is the rate-determining step of CO oxidation by RhMn2O3-, can be greatly facilitated on the reactions of RhMn2O3-[(NO)1,2] with CO.

Activation of Methane by Rhodium Clusters on a Model Support C20H10.

Supported metals represent an important family of catalysts for the transformation of the most stable alkane, methane, under mild conditions. Here, using state-of-the-art mass spectrometry coupled with a newly designed double ion trap reactor that can run at high temperatures, we successfully immobilize a series of Rhn- (n = 4-8) cluster anions on a model support C20H10. Reactivity measurements at room temperature identify a significantly enhanced performance of large-sized Rh7,8C20H10- toward methane activation compared to that of free Rh7,8-. The "support" acting as an "electron sponge" is emphasized as the key factor to improve the reactivity of large-sized clusters, for which the high electron-withdrawing capability of C20H10 dramatically shifts the active Rh atom from the apex position in free Rh7- to the edge position in "supported" Rh7- to enhance CH4 adsorption, while the flexibility of C20H10 to release electrons further promotes subsequent C-H activation. The Rh atoms in direct contact with the support serve as electron-relay stations for electron transfer between C20H10 and the active Rh atom. This work not only establishes a novel approach to prepare and measure the reactivity of "supported" metal clusters in isolated gas phase but provides useful atomic-scale insights for understanding the chemical behavior of carbon (e.g., graphene)-supported metals in heterogeneous catalysis.

Selective Reduction of NO into N2 Catalyzed by Rh1-Doped Cluster Anions RhCe2O3-5.

Catalytic conversion of toxic nitrogen oxide (NO) and carbon monoxide (CO) into nitrogen (N2) and carbon dioxide (CO2) is imperative under the weight of the increasingly stringent emission regulations, while a fundamental understanding of the nature of the active site to selectively drive N2 generation is elusive. Herein, in combination with state-of-the-art mass-spectrometric experiments and quantum-chemical calculations, we demonstrated that the rhodium-cerium oxide clusters RhCe2O3-5- can catalytically drive NO reduction by CO and give rise to N2 and CO2. This finding represents a sharp improvement in cluster science where N2O is commonly produced in the rarely established examples of catalytic NO reduction mediated with gas-phase clusters. We demonstrated the importance of the unique chemical environment in the RhCe2O3- cluster to guide the substantially improved N2 selectivity: a triatomic Lewis "acid-base-acid" Ceδ+-Rhδ--Ceδ+ site is proposed to strongly adsorb two NO molecules as well as the N2O intermediate that is attached on the Rh atom and can facilely dissociate to form N2 assisted by both Ce atoms.

Size-Dependent Reactivity of Con- (n = 5-25) Cluster Anions toward Carbon Dioxide.

A fundamental understanding of the reactivity evolution of nanosized clusters at an atomically precise level is pivotal to assemble desired materials with promising candidates. Benefiting from the tandem mass spectrometer coupled with a high-temperature ion-trap reactor, the reactions of mass-selected Con- (n = 5-25) clusters with CO2 were investigated and the increased reactivity of Co20-25- was newly discovered herein. This finding marks an important step to understand property evolution of subnanometer metal clusters (Co25-, ∼0.8 nm) atom-by-atom. The reasons behind the increased reactivity of Co20-25- were proposed by analyzing the reactions of smaller Co6-8- clusters that exhibit significantly different reactivity toward CO2, in which a lower electron affinity of Con contributes to the capture of CO2 while the flexibility of Con- could play vital roles to stabilize reaction intermediates and suppress the barriers of O-CO rupture and CO desorption.

Superior Reactivity of Molybdenum-Sulfur Cluster Anions Mo5S2- and Mo5S3- toward Dinitrogen.

Inspired by the fact that Mo is a key element in biological nitrogenase, a series of gas-phase MoxSy- cluster anions are prepared and their reactivity toward N2 is investigated by the combination of mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The Mo5S2- and Mo5S3- cluster anions show remarkable reactivity compared with the anionic species reported previously. The spectroscopic results in conjunction with theoretical analysis reveal that a facile cleavage of N≡N bonds takes place on Mo5S2- and Mo5S3-. The large dissociative adsorption energy of N2 and the favorable entrance channel for initial N2 approaching are proposed as two decisive factors for the superior reactivity of Mo5S2- and Mo5S3-. Besides, the modulation of S ligands on the reactivity of metal centers with N2 is proposed. The highly reactive metal-sulfur species may be obtained by the coordination of two to three sulfur atoms to bare metal clusters so that an appropriate combination of electronic structures and charge distributions can be achieved.

Activation of Dinitrogen Promoted by Adsorption of C6H6 on Fe2VC- Cluster Anions.

The introduction of organic ligands is one of the effective strategies to improve the stability and reactivity of metal clusters. Herein, the enhanced reactivity of benzene-ligated cluster anions Fe2VC(C6H6)- with respect to naked Fe2VC- is identified. Structural characterization suggests that C6H6 is molecularly bound to the dual metal site in Fe2VC(C6H6)-. Mechanistic details reveal that the cleavage of N≡N is feasible in Fe2VC(C6H6)-/N2 but hindered by an overall positive barrier in the Fe2VC-/N2 system. Further analysis discloses that the ligated C6H6 regulates the compositions and energy levels of the active orbitals of the metal clusters. More importantly, C6H6 serves as an electron reservoir for the reduction of N2 to lower the crucial energy barrier of N≡N splitting. This work demonstrates that the flexibility of C6H6 in terms of withdrawing and donating electrons is crucial to regulating the electronic structures of the metal cluster and enhancing the reactivity.

High-temperature reactivity of vanadium oxide clusters in methane activation: Vibrational degrees of freedom matter.

Understanding the properties of small particles working under high-temperature conditions at the atomistic scale is imperative for exact control of related processes, but it is quite challenging to achieve experimentally. Herein, benefitting from state-of-the-art mass spectrometry and by using our newly designed high-temperature reactor, the activity of atomically precise particles of negatively charged vanadium oxide clusters toward hydrogen atom abstraction (HAA) from methane, the most stable alkane molecule, has been measured at elevated temperatures up to 873 K. We discovered the positive correlation between the reaction rate and cluster size that larger clusters possessing greater vibrational degrees of freedom can carry more vibrational energies to enhance the HAA reactivity at high temperature, in contrast with the electronic and geometric issues that control the activity at room temperature. This finding opens up a new dimension, vibrational degrees of freedom, for the simulation or design of particle reactions under high-temperature conditions.

Catalytic NO Reduction by Noble-Metal-Free Vanadium-Aluminum Oxide Cluster Anions.

By using state-of-the-art mass spectrometry and guided by the newly discovered single-electron mechanism (SEM; e.g., Ti3+ + 2NO → Ti4+-O•- + N2O), we determined experimentally that the vanadium-aluminum oxide clusters V4-xAlxO10-x- (x = 1-3) can catalyze the reduction of NO by CO and substantiated theoretically that the SEM still prevails in driving the catalysis. This finding marks an important step in cluster science in which a noble metal had been demonstrated to be indispensable in NO activation mediated by heteronuclear metal clusters. The results provide new insights into the SEM in which active V-Al cooperative communication favors the transfer of an unpaired electron from the V atom to NO attached to the Al atom on which the reduction reaction actually takes place. This study provides a clear picture for improving our understanding of related heterogeneous catalysis, and the electron hopping behavior induced by NO adsorption could be a fundamental chemistry for driving NO reduction.

Filtration of the preferred catalyst for reverse water-gas shift among Rhn- (n = 3-11) clusters by mass spectrometry under variable temperatures.

The key to optimizing energy-consuming catalytic conversions lies in acquiring a fundamental understanding of the nature of the active sites and the mechanisms of elementary steps at an atomically precise level, while it is challenging to capture the crucial step that determines the overall temperature of a real-life catalytic reaction. Herein, benefiting from a newly-developed high-temperature ion trap reactor, the reverse water-gas shift (CO2 + H2 → CO + H2O) reaction catalyzed by the Rhn- (n = 3-11) clusters was investigated under variable temperatures (298-783 K) and the critical temperature that each elementary step (Rhn- + CO2 and RhnO- + H2) requires to take place was identified. The Rh4- cluster strikingly surpasses other Rhn- clusters to drive the catalysis at a mild starting temperature (∼440 K). This finding represents the first example that a specifically sized cluster catalyst that works under an optimum condition can be accurately filtered by using state-of-the-art mass spectrometric experiments and rationalized by quantum-chemical calculations.