Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping.

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

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.

C-H Activation by Iron-Vanadium Bimetallic Oxide Cluster Anions FeV3 O10 - and FeV5 O15 - : A Comparison with Scandium-Vanadium Oxide Clusters.

Late transition metal-bonded atomic oxygen radicals (LTM-O⋅- ) have been frequently proposed as important active sites to selectively activate and transform inert alkane molecules. However, it is extremely challenging to characterize the LTM-O⋅- -mediated elementary reactions for clarifying the underlying mechanisms limited by the low activity of LTM-O⋅- radicals that is inaccessible by the traditional experimental methods. Herein, benefiting from our newly-designed ship-lock type reactor, the reactivity of iron-vanadium bimetallic oxide cluster anions FeV3 O10 - and FeV5 O15 - featuring with Fe-O⋅- radicals to abstract a hydrogen atom from C2 -C4 alkanes has been experimentally characterized at 298 K, and the rate constants are determined in the orders of magnitude of 10-14 to 10-16  cm3 molecule-1 s-1 , which are four orders of magnitude slower than the values of counterpart ScV3 O10 - and ScV5 O15 - clusters bearing Sc-O⋅- radicals. Theoretical results reveal that the rearrangements of the electronic and geometric structures during the reaction process function to modulate the activity of Fe-O⋅- . This study not only quantitatively characterizes the elementary reactions of LTM-O⋅- radicals with alkanes, but also provides new insights into structure-activity relationship of M-O⋅- radicals.

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.

Thermal Methane Conversion to Formaldehyde Mediated by NiAlO3+ in the Gas Phase.

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al2O3, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO3+ cations toward CH4 is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl2O4+, we find evidence for the formation of both the methyl radical (CH3•) and formaldehyde (CH2O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)2AlO• from methane forming a [Ni(O)2AlOH+, •CH3] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O2--Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to Al2O3+/CH4 and YAlO3+/CH4 systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH3•/CH2O branching ratio of 0.62/0.38. The present study provides molecular-level insights into the highly efficient gas-phase reaction mechanism contributing to an improved understanding of methane conversion by Ni/Al2O3 catalysts.

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.

Methane activation by vanadium oxide cluster anions (V2O5)NO- (N = 1-18).

The reactivity of vanadium oxide cluster anions (V2O5)NO- (N = 1-18) that feature with vanadium oxyl radicals (V-O⋅-) toward the most stable alkane, methane, at 273 K has been characterized by employing a newly home-made ship-lock type reactor coupled with a time-of-flight mass spectrometer. The rate constants were determined in the orders of magnitude of 10-16-10-18 cm3 molecule-1 s-1, which significantly breaks the detection limit of predecessors that the reactivity of metal-oxyl radicals (Mn+-O⋅-) with rate constants higher than 10-14 cm3 molecule-1 s-1 could usually be measured. The dynamic structural rearrangement of the cluster skeleton has been proposed to account for the size-dependent reactivity of (V2O5)1-5O- clusters, which may also function in tuning the reactivity of large-sized (V2O5)6-18O- clusters. This work provides new insights into the mechanism of Mn+-O⋅--mediated C-H activation of methane at a strictly molecular level and expands the activity landscape of Mn+-O⋅- radicals.

Size-dependent reactivity of rhodium deuteride cluster anions Rh3Dn - (n = 0-3) toward dinitrogen: The prominent role of σ donation.

Nitrogen (N2) fixation is a challenging task for chemists. Adsorption of N2 on transition metal (TM) sites has been identified as a prerequisite for activating the very stable N≡N triple bond in both industrial and biological processes. The importance of π back-donation (filled orbitals of TM → π* orbitals of N2) between metal sites and N2 has been well elucidated while the role of another classic orbital interaction, namely σ donation (σ orbitals of N2 → empty orbitals of TM), remains ambiguous. Herein, the size-dependent reactivity of trinuclear rhodium deuteride cluster anions Rh3Dn - (n = 0-3) toward N2 adsorption in the gas phase was investigated experimentally and theoretically. A reverse relationship that higher electron-donating ability of clusters corresponds to lower N2 adsorption reactivity was experimentally observed, which is uncommon in N2 activation by gas-phase species. Theoretical analysis revealed that the σ donation rather than the π back-donation plays a predominant role in the adsorption complexes Rh3DnN2 - and the enhanced reactivity upon D addition is ascribed to the lowered energy levels of active orbitals in Rh3Dn - as n increases. This study provides the first experimental evidence to declare the important role of σ donation and new clues for the design of reactive metal species in nitrogen fixation.

Pyrolysis of mass-selected (V2O5)NO- (N = 1-6) clusters in a high-temperature linear ion trap reactor.

A high-temperature linear ion trap that can stably run up to 873 K was newly designed and installed into a homemade reflectron time-of-flight mass spectrometer coupled with a laser ablation cluster source and a quadrupole mass filter. The instrument was used to study the pyrolysis behavior of mass-selected (V2O5)NO- (N = 1-6) cluster anions and the dissociation channels were clarified with atomistic precision. Similar to the dissociation behavior of the heated metal oxide cluster cations reported in literature, the desorption of either atomic oxygen atom or molecular O2 prevailed for the (V2O5)NO- clusters with N = 2-5 at 873 K. However, novel dissociation channels involving fragmentation of (V2O5)NO- to small-sized VxOy - anions concurrent with the release of neutral vanadium oxide species were identified for the clusters with N = 3-6. Significant variations in branching ratios for different dissociation channels were observed as a function of cluster size. Kinetic studies indicated that the dissociation rates of (V2O5)NO- monotonically increased with the increase in cluster size. The internal energies carried by the (V2O5)NO- clusters at 873 K as well as the energetics data for dissociation channels have been theoretically calculated to rationalize the experimental observations. The decomposition behavior of vanadium oxide clusters from this study can provide new insights into the pyrolysis mechanism of metal oxide nanoparticles that are widely used in high temperature catalysis.

Catalytic Co-Conversion of CH4 and CO2 Mediated by Rhodium-Titanium Oxide Anions RhTiO2.

Catalytic co-conversion of methane with carbon dioxide to produce syngas (2 H2 +2 CO) involves complicated elementary steps and almost all the elementary reactions are performed at the same high temperature conditions in practical thermocatalysis. Here, we demonstrate by mass spectrometric experiments that RhTiO2 - promotes the co-conversion of CH4 and CO2 to free 2 H2 +CO and an adsorbed CO (COads ) at room temperature; the only elementary step that requires the input of external energy is desorption of COads from the RhTiO2 CO- to reform RhTiO2 - . This study not only identifies a promising active species for dry (CO2 ) reforming of methane to syngas, but also emphasizes the importance of temperature control over elementary steps in practical catalysis, which may significantly alleviate the carbon deposition originating from the pyrolysis of methane.

Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects.

Substituent effects play critical roles in both modulating reaction chemistry and supramolecular self-assembly processes. Using substituted terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position of the substituents was explored in terms of their ability to regulate the inherent anion complexation features of a tetracationic macrocycle, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (referred to as the Texas-sized molecular box; 14+), in the form of its tetrakis-PF6- salt in DMSO. Several of the tested substituents, including 2-OH, 2,5-di(OH), 2,5-di(NH2), 2,5-di(Me), 2,5-di(Cl), 2,5-di(Br), and 2,5-di(I), were found to promote pseudorotaxane formation in contrast to what was seen for the parent PTADA system. Other derivatives of PTADA, including those with 2,3-di(OH), 2,6-di(OH), 2,5-di(OMe), 2,3,5,6-tetra(Cl), and 2,3,5,6-tetra(F) substituents, led only to so-called outside binding, where the anion interacts with 14+ on the outside of the macrocyclic cavity. The differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly when the reaction components included additional metal cations (M). Depending on the specific choice of PTADA derivatives and metal cations (M = Co2+, Ni2+, Zn2+, Cd2+, Gd3+, Nd3+, Eu3+, Sm3+, Tb3+), constructs involving one-dimensional polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks (RSOFs), or two-dimensional metal-organic rotaxane frameworks (MORFs) could be stabilized. The presence and nature of the substituent were found to dictate which specific higher order self-assembled structure was obtained using a given cation. In the specific case of the 2,5-di(OH), 2,5-di(Cl), and 2,5-di(Br) PTADA derivatives and Eu3+, so-called MORFs with distinct fluorescence emission properties could be produced. The present work serves to illustrate how small changes in guest substitution patterns may be used to control structure well beyond the first interaction sphere.

C-N Coupling in N2 Fixation by the Ditantalum Carbide Cluster Anions Ta2C4.

The construction of C-N bonds by the direct incorporation of dinitrogen (N2) instead of ammonia (NH3) into active species is particularly desirable but has been rarely reported. Herein, a ditantalum carbide cluster anion (Ta2C4-) capable of cleaving the N≡N bond and constructing a C-N bond under mild conditions has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum-chemical calculations. The photoelectron spectrum of Ta2C4N2- is remarkably different from that of Ta2C4- and matches the simulated spectrum of the Ta2C4N2- species with an end-on-bonded CN unit. The formation of the C-N bond has also been supported by the CN- fragment observed in the collision-induced dissociation of Ta2C4N2-. The exceptional reactivity of Ta2C4- is ascribed to the low-valent metal center serving as an electron reservoir. This study provides a non-NH3 route to construct C-N bonds by incorporating N2 into carbide compounds to produce nitrogenous species.

Electron Attachment Reaction Ionization of Gas-Phase Methylglyoxal.

Methylglyoxal (MGLY) plays a significant role in atmospheric chemistry by serving as a key contributor to the formation of active free radicals, ozone, and secondary organic aerosol. Detection of MGLY by traditional chemical ionization such as proton-transfer reaction has several shortcomings such as parent molecule fragmentation. In this study, an electron attachment reaction (EAR) ionization method has been developed for the effective detection of MGLY. Almost no fragmentation was observed during the EAR. The generation of MGLY- anion in the EAR was further confirmed by cryogenic photoelectron imaging spectroscopy. The concentration of MGLY can be calibrated by using dibromomethane (CH2Br2) as reference gas. The detection sensitivity of MGLY was estimated to be (100 ± 2) mV/ppbv (parts per billion by volume). The O2, H2O, CO2, and trace gases in ambient air have no obvious effects on the detection of MGLY- anion by the EAR ionization method.

Mutual functionalization of dinitrogen and methane mediated by heteronuclear metal cluster anions CoTaC2.

The direct coupling of dinitrogen (N2) and methane (CH4) to construct the N-C bond is a fascinating but challenging approach for the energy-saving synthesis of N-containing organic compounds. Herein we identified a likely reaction pathway for N-C coupling from N2 and CH4 mediated by heteronuclear metal cluster anions CoTaC2 -, which starts with the dissociative adsorption of N2 on CoTaC2 - to generate a Ta δ+-Nt δ- (terminal-nitrogen) Lewis acid-base pair (LABP), followed by the further activation of CH4 by CoTaC2N2 - to construct the N-C bond. The N[triple bond, length as m-dash]N cleavage by CoTaC2 - affording two N atoms with strong charge buffering ability plays a key part, which facilitates the H3C-H cleavage via the LABP mechanism and the N-C formation via a CH3 migration mechanism. A novel Nt triggering strategy to couple N2 and CH4 molecules using metal clusters was accordingly proposed, which provides a new idea for the direct synthesis of N-containing compounds.

A ship-lock-type reactor for ion-molecule reactions of mass-selected ions under high-pressure conditions.

A ship-lock-type reactor has been developed to study ion-molecule reactions of mass-selected ions under high-pressure conditions. Neutral gas molecules can be confined in the reactor by controlling two electromagnet valves to close both the inlet and the outlet of the reactor. Gas-phase ions can be trapped in an ion funnel trap installed in the reactor and interacted with a high-pressure (up to 1000 Pa) reactant gas for a period of time (up to 1 s). The reactions of mass-selected V2O6 - with CH4 and n-C4H10 and mass-selected Au+ with n-C7H16 were investigated to evaluate the performance of the reactor. The hydrogen atom abstraction product V2O6H- was observed for the reaction of V2O6 - with CH4, the rate constant was measured to be (1.9 ± 0.4) × 10-16 cm3 molecule-1 s-1, and the kinetic isotope effect value was determined to be 5.4 ± 1.1. Furthermore, the detection limit of n-C7H16 with 1-min measurements was determined to be (19 ± 2) pptv, which is significantly lower than those in previous studies. These results indicate that the current apparatus is a prospective for the study of slow ion-molecule reactions and the detection of trace amounts of gas species, such as volatile organic compounds.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions.

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H2O into CO2 and H2 is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh2MnO1,2- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H2O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh2MnO1,2- is paramount for the generation of an electron-rich Mn+-Rh- bond that is critical to capture and reduce H2O and giving rise to a polarized Rh+-Rh- bond that functions as the real redox center to drive the WGS reaction.