Oxide Cluster Anions Research Articles

Factors Determining the Selectivity of NO Reduction Catalyzed by Copper-Vanadium Oxide Cluster Anions Cu2VO3‒5.

Catalytic NO reduction by CO is imperative to satisfy the increasingly rigorous emission regulations. Identifying the structural characteristic of crucial intermediate that governs the selectivity of NO reduction is pivotal to having a fundamental understanding on real-life catalysis. Herein, benefiting from the state-of-the-art mass spectrometry, we demonstrated experimentally that the Cu2VO3-5- clusters can mediate the catalysis of NO reduction by CO, and two competitive channels to generate N2O and N2 can co-exist. Quantum-chemical calculations were performed to rationalize this selectivity. The formation of the ONNO unit on the Cu2 dimer was demonstrated to be a precursor from which two pathways of NO reduction start to emerge. In the pathway of N2O generation, only the Cu2 dimer was oxidized and the VO3 moiety functions as a "support", while both moieties have to contribute to anchor oxygen atoms from the ONNO unit and then N2 can be generated. This finding displays a clear picture to elucidate how and why the involvement of VO3 "support" can regulate the selectivity of NO reduction.

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.

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.

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.

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.

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.

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.

Size-dependent reactivity of chromium oxide cluster anions (CrO3)1−4O− towards alkanes

The reactivity of chromium oxide cluster anions (CrO3)1−4O− towards low carbon (C1−C4) alkanes has been experimentally investigated at 298 K by employing a homemade ship-lock-type reactor coupled with a time-of-flight mass spectrometer. The results demonstrate that while CrO4− and Cr2O7− clusters could abstract a hydrogen atom from C2H6 and CH4, respectively, Cr3O10− and Cr4O13− clusters were found to be inert towards n-C4H10 within the detection limit. Theoretical results reveal that CrO4− and Cr2O7− clusters possess delocalized chromium-bonded oxygen radicals (Cr−O−·), which rationalizes the hydrogen atom abstraction reactions between (CrO3)1,2O− clusters and alkanes. However, the active sites of (CrO3)3,4O− clusters evolve to peroxide species (O22−), which exhibit inferior activity compared to O−· radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrOy unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O22− and selective generation of reactive O−· radicals in small-sized (CrO3)1,2O− clusters. This study not only enriches the chemistry of metal-bonded O−· radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O22− and O−· radicals over metal oxides.

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 273K 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 10-16-10-18cm3molecule-1s-1, which significantly breaks the detection limit of predecessors that the reactivity of metal-oxyl radicals (Mn+-O⋅-) with rate constants higher than 10-14cm3molecule-1s-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.

Synthesis of aspirin catalyzed by supramolecular compound based on Keggin-Type phosphomolybates with flexible 1,3-bis(4-pyridyl)propane

With an aim to catalyze the synthesis of aspirin in a green approach, an unreported supramolecular compound [H3(C13H14N2)2][PMo12O40] (PMo12O40-Bpp) based on Keggin-type has been regarded as catalyst in synthesis of aspirin, which is prepared with flexible 1,3-bis(4-pyridyl)propane (C13H14N2, Bpp) and characterized structurally. Structural analysis of X-ray single crystal diffraction reveals that charge-assisted hydrogen bonds (CAHBs) N+-H…O- is formed between N+ of protonated Bpp molecule in PMo12O40-Bpp as hydrogen bond donor and molybdenum phosphate oxide cluster anion [PMo12O40]3-, and 3D supramolecular framework with porous is connected by intermolecular hydrogen bonds C9-H9…O3, C11-H11…O3 and C6-H6…O7 alternatively. When reactants of pharmaceutical esterification are salicylic acid and acetic anhydride, by screening dose of catalyst PMo12O40-Bpp, temperature and reaction time, the optimal conditions are determined as follows: 0.15 g PMo12O40-Bpp (mass fraction 5%), 90 °C and 55 min. Under these conditions, the yield of aspirin, the conversion rate of salycilic acid and the selectivity of catalyst can reach up to 69.24%, 98.02% and 70.64%, which indicating that catalytic activity is similar to that of concentrated sulfuric acid (85%) under the same reaction conditions. The kinetic results of pharmaceutical esterification of aspirin catalyzed by PMo12O40-Bpp show that reaction order is 1.37, rate constant is 1.13 × 10-3 k/L·(mol·s)-1, and apparent activation energy is 64.39 kJ·mol−1. After PMo12O40-Bpp catalyzes pharmaceutical esterification of aspirin repeatedly, PXRD, IR and lifetime of catalyst show that structure of PMo12O40-Bpp is unchanged and catalytic activity does not decrease significantly, indicating that PMo12O40-Bpp can be recycled for multiple times. Above experimental results may be due to specific “pseudo-liquid phase” phenomenon of POM-based supramolecular compounds to make full contact with reactants in the catalytic process, further stabilizing carbocation intermediates effectively through [PMo12O40]3- with greater surface electronegativity, which is more conducive to attack of hydroxyl oxygen atom on salicylic acid to promote pharmaceutical esterification reaction of aspirin catalyzed by PMo12O40-Bpp. This successful work implies that POM-based supramolecular compounds as catalysts in pharmaceutical synthetic process have potential to reduce environmental pollution and provide newfangled inspirations for green pharmaceutical chemistry.

Methane Activation by (MoO3 )5 O- Cluster Anions: The Importance of Orbital Orientation.

The reactivity of the molybdenum oxide cluster anion (MoO3 )5 O- , bearing an unpaired electron at a bridging oxygen atom (Ob .- ), towards methane under thermal collision conditions has been studied by mass spectrometry and density functional theory calculations. This reaction follows the mechanism of hydrogen atom transfer (HAT) and is facilitated by the Ob .- radical center. The reactivity of (MoO3 )5 O- can be traced back to the appropriate orientation of the lowest unoccupied molecular orbitals (LUMO) that is essentially the 2p orbital of the Ob .- atom. This study not only makes up the blank of thermal methane activation by the Ob .- radical on negatively charged clusters but also yields new insights into methane activation by the atomic oxygen radical anions.

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.

Nitrogen Activation and Transformation on Monometallic Niobium Boron Oxide Cluster Anions at Room Temperature: A Dual-Site Mechanism.

Dinitrogen activation and transformation at room temperature is a goal that has been long sought after. Despite that, it remains underdeveloped due to being a challenging research area and the need for a better mechanistic understanding. Herein, we report that well-defined NbB3O2- gas-phase clusters can activate one N2 molecule and generate the products B3N2O- and B3N2O2-, as applying mass spectrometry and theoretical calculations. This unusual N2 activation reaction results from the different functions of the Nb and B3O2 moieties in NbB3O2-. Theoretical calculations suggest that a catalytic cycle can be completed by the recovery of NbB3O2-, which is achieved through the reactions of Nb and NbO with B3O2- and B3O-, respectively. This is the first example of N2 efficient transformation at a monometallic cluster, and this method for generating dual active sites by designing proper ligands may open the way toward the development of more effective N2 fixation and functionalization methodologies.

Photooxidation of Isoprene by Titanium Oxide Cluster Anions with Dimensions up to a Nanosize.

Titania (TiO2) nanoparticles are active photocatalysts, and isoprene (C5H8) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O•-) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the π orbital of C5H8 to the Ti3d orbital in the photooxidation process (C5H8 + Ti4+-O•- → C5H8O + Ti3+) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O•- radicals by UV excitation on the TiO2 particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO2 nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO2 nanoparticles.

Study on the Reaction of Nanosized Yttrium Oxide Cluster Anions with n-Butane in the Gas Phase

Study on the Reaction of Nanosized Yttrium Oxide Cluster Anions with <i>n</i>-Butane in the Gas Phase

Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals.

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear CrIII carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb3O13] or [Ta3O13] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

Catalytic CO Oxidation by O2 Mediated with Single Gold Atom Doped Titanium Oxide Cluster Anions AuTi2 O4-6.

Gas-phase studies on catalytic CO oxidation by O2 mediated with gold-containing heteronuclear metal oxide clusters are vital to obtain the structure-reactivity relationship of supported gold catalysts, while it is challenging to trigger the reactivity of clusters with closed-shell electronic structure in O2 activation. Herein, we identified that CO oxidation by O2 can be catalyzed by the AuTi2 O4-6- clusters, among which AuTi2 O4- with closed-shell electronic structure can effectively activate O2 . The reactions were characterized by mass spectrometry and quantum chemical calculations. Theoretical calculations showed that in the initial stage of O2 activation, the Ti2 O4 moiety in AuTi2 O4- contributes dominantly to activate O2 into superoxide (O2- ⋅) without participation of the Au atom. In subsequent steps, the intimate charge transfer interaction between Au and the Ti2 O4 moiety drives the direct dissociation of the O2- ⋅ unit.

Photoassisted Selective Steam and Dry Reforming of Methane to Syngas Catalyzed by Rhodium-Vanadium Bimetallic Oxide Cluster Anions at Room Temperature.

Photoassisted steam reforming and dry (CO2 ) reforming of methane (SRM and DRM) at room temperature with high syngas selectivity have been achieved in the gas-phase catalysis for the first time. The catalysts used are bimetallic rhodium-vanadium oxide cluster anions of Rh2 VO1-3 - . Both the oxidation of methane and reduction of H2 O/CO2 can take place efficiently in the dark while the pivotal step to govern syngas selectivity is photo-excitation of the reaction intermediates Rh2 VO2,3 CH2 - to specific electronically excited states that can selectively produce CO and H2 . Electronic excitation over Rh2 VO2,3 CH2 - to control the syngas selectivity is further confirmed from the comparison with the thermal excitation of Rh2 VO2,3 CH2 - , which leads to diversity of products. The atomic-level mechanism obtained from the well-controlled cluster reactions provides insight into the process of selective syngas production from the photocatalytic SRM and DRM reactions over supported metal oxide catalysts.

Photoassisted Selective Steam and Dry Reforming of Methane to Syngas Catalyzed by Rhodium–Vanadium Bimetallic Oxide Cluster Anions at Room Temperature

AbstractPhotoassisted steam reforming and dry (CO2) reforming of methane (SRM and DRM) at room temperature with high syngas selectivity have been achieved in the gas‐phase catalysis for the first time. The catalysts used are bimetallic rhodium–vanadium oxide cluster anions of Rh2VO1–3−. Both the oxidation of methane and reduction of H2O/CO2 can take place efficiently in the dark while the pivotal step to govern syngas selectivity is photo‐excitation of the reaction intermediates Rh2VO2,3CH2− to specific electronically excited states that can selectively produce CO and H2. Electronic excitation over Rh2VO2,3CH2− to control the syngas selectivity is further confirmed from the comparison with the thermal excitation of Rh2VO2,3CH2−, which leads to diversity of products. The atomic‐level mechanism obtained from the well‐controlled cluster reactions provides insight into the process of selective syngas production from the photocatalytic SRM and DRM reactions over supported metal oxide catalysts.

Structure Assignment and Separation of Isomers of Palladium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry

Geometric structures of palladium oxide cluster anions, PdnOm– (n = 2–7, n ≥ m), were investigated by ion mobility mass spectrometry and quantum chemical calculations. Collision cross sections of t...