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.

Selective Reduction of NO into N2 Catalyzed by the RhM2O3- Clusters (M = Ta, V, and Al): Importance of the Triatomic Lewis Acid-Base-Acid Mδ+-Rhδ--Mδ+ Site.

Catalytic NO reduction by CO into N2 and CO2 is imperative owing to the increasingly rigorous emission regulation. Identifying the nature of active sites that govern the reactivity and selectivity of NO reduction is pivotal to tailor catalysts, while it is extremely challenging because of the complexity of real-life systems. Guided by our newly discovered triatomic Lewis acid-base-acid (LABA, Ceδ+-Rhδ--Ceδ+) site that accounts for the selective reduction of NO into N2 catalyzed by the RhCe2O3- cluster in gas-phase experiments, the reactivity of the RhM2O3- (M = Ta, V, and Al) clusters in catalytic NO reduction by CO was explored. We determined theoretically that the LABA site still prevails to reduce NO to N2 mediated by RhTa2O3- and RhV2O3-, and the strong M-oxygen affinity was emphasized to construct the LABA site. An overall assessment highlights that RhV2O3- functions as a more promising catalyst because the well-fitting V-O bonding strength facilitates both elementary reactions of NO reduction and CO oxidation.

Unexpected longer T1 lifetime of 6-sulfur guanine than 6-selenium guanine: the solvent effect of hydrogen bonds to brake the triplet decay.

The decay of the T1 state to the ground state is an essential property of photosensitizers because it decides the lifetime of excited states and, thus, the time window for sensitization. The sulfur/selenium substitution of carbonyl groups can red-shift absorption spectra and enhance the triplet yield because of the large spin-orbit coupling, modifying nucleobases to potential photosensitizers for various applications. However, replacing sulfur with selenium will also cause a much shorter T1 lifetime. Experimental studies found that the triplet decay rate of 6-seleno guanine (6SeGua) is 835 times faster than that of 6-thio guanine (6tGua) in aqueous solution. In this work, we reveal the mechanism of the T1 decay difference between 6SeGua and 6tGua by computing the activation energy and spin-orbit coupling for rate calculation. The solvent effect of water is treated with explicit microsolvation and implicit solvent models. We find that the hydrogen bond between the sulfur atom of 6tGua and the water molecule can brake the triplet decay, which is weaker in 6SeGua. This difference is crucial to explain the relatively long T1 lifetime of 6tGua in an aqueous solution. This insight emphasizes the role of solvents in modulating the excited state dynamics and the efficiency of photosensitizers, particularly in aqueous environments.

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.

A Paramagnetic Compass Based on Lanthanide Metal-Organic Framework.

Macroscopic compass-like magnetic alignment at low magnetic fields is natural for ferromagnetic materials but is seldomly observed in paramagnetic materials. Herein, we report a "paramagnetic compass" that magnetically aligns under ∼mT fields based on the single-crystalline framework constructed by lanthanide ions and organic ligands (Ln-MOF). The magnetic alignment is attributed to the Ln-MOF's strong macroscopic anisotropy, where the highly-ordered structure allows the Ln-ions' molecular anisotropy to be summed according to the crystal symmetry. In tetragonal Ln-MOFs, the alignment is either parallel or perpendicular to the field depending on the easiest axis of the molecular anisotropy. Reversible switching between the two alignments is realized upon the removal and re-adsorption of solvent molecules filled in the framework. When the crystal symmetry is lowered in monoclinic Ln-MOFs, the alignments become even inclined (47°-66°) to the field. These fascinating properties of Ln-MOFs would encourage further explorations of framework materials containing paramagnetic centers.

Facile CO bond cleavage on polynuclear vanadium nitride clusters V4N5.

Identifying the structural configurations of precursors for CO dissociation is fundamentally interesting and industrially important in the fields of, e.g., Fischer-Tropsch synthesis. Herein, we demonstrated that CO could be dissociated on polynuclear vanadium nitride V4N5- clusters at room temperature, and a key intermediate, with CO in a N-assisted tilted bridge coordination where the C-O bond ruptures easily, was discovered. The reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemistry calculations, and the nature of the adsorbed CO on product V4N5CO- was further characterized by a collision-induced dissociation experiment. Theoretical analysis evidences that CO dissociation is predominantly governed by the low-coordinated V and N atoms on the (V3N4)VN- cluster and the V3N4 moiety resembles a support. This finding strongly suggests that a novel mode for facile CO dissociation was identified in a gas-phase cluster study.

Water-Gas Shift Catalyzed by Iridium-Vanadium Oxide Clusters IrVO2- with Iridium in a Rare Oxidation State of -II.

The discovery of compounds containing transition metals with an unusual and well-established oxidation state is vital to enrich our horizon on formal oxidation state. Herein, benefiting from the study of the water-gas shift reaction (CO + H2O → CO2 + H2) mediated with the iridium-vanadium oxide cluster IrVO2-, the missing -II oxidation state of iridium was identified. The reactions were performed by using our newly developed double ion trap reactors that can spatially separate the addition of reactants and are characterized by mass spectrometry and quantum-chemical calculations. This finding makes an important step that all the proposed 13 oxidation states of iridium (+IX to -III) have been known. The iridium atom in the IrVO2- cluster features the Ir═V double bond and resembles chemically the coordinated oxygen atom. A reactivity study demonstrated that the flexible role switch of iridium between an oxygen-atom like (Ir-IIVO2-) and a transition-metal-atom like behavior (Ir+IIVO3-) in different species can drive the water-gas shift reaction in the gas phase under ambient conditions. This result parallels and well rationalizes the extraordinary reactivity of oxide-supported iridium single-atom catalysts in related condensed-phase reactions.

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.

Synthesis of Surface-Oxygen-Vacancy-Rich (GaN)0.5(ZnO)0.5 Particles with Enhanced Visible-Light Photodegradation Performance.

In this study, zinc-gallium oxynitrides with a Zn:Ga mole ratio of 1:1 [(GaN)0.5(ZnO)0.5] were synthesized from a Zn/Ga/CO3 layered double hydroxide (LDH) precursor. The microstructure and photoactivity of the (GaN)0.5(ZnO)0.5 particles were tuned by adjusting the nitridation conditions of the LDH. It is revealed that the quantity of the LDH, or, equivalently, the partial pressure of the water during nitridation, plays a pivotal role in the defect structure of the obtained oxynitrides. A reduction in the quantity of the LDH precursor can effectively suppress the formation of defects including Ga(Zn)-O bonding, bulk anion vacancies, and surface-deposited Ga/ON···VGa complexes, leading to a better charge-separation efficiency for the photogenerated electron-hole pairs in the oxynitride. Furthermore, a suitable introduction of methane during nitridation would not only increase the crystallinity of the bulk materials but also enhance the density of the surface oxygen vacancy (VO), which would raise the charge-injection efficiency by working as an electron trap and a reaction site to form O2•-. O2•-, as well as photogenerated holes, have been proven to be the dominant active species for the photodegradation of phenol. 25CH4-ZnGaNO, with the lowest density of bulk defects and the highest density of surface VO, exhibited the best photoactivity under visible-light irradiation for the photodegradation of Rhodamine B and phenol. The obtained surface-VO-rich (GaN)0.5(ZnO)0.5 particles can be applied as a high-performance visible-light-driven photocatalyst in the photodegradation of organic pollutants.

ZnGaNO Photocatalyst Particles Prepared from Methane-Based Nitridation Using Zn/Ga/CO3 LDH as Precursor.

Methane-based nitridation was employed to produce wurtzite zinc-gallium oxynitride (ZnGaNO) photocatalyst particles using Zn/Ga/CO3 layered double hydroxides (LDHs) as precursor. Introduction of methane to nitridation would promote the formation of Zn-O bonding and suppress shallow acceptor complexes such as V(Zn)-Ga(Zn) and Ga-Oi in ZnGaNO particles. On the other hand, high flow rate of methane would induce breaking of Ga-N bonding and enhance surface deposition of metallic Ga atoms. After loading with Rh and RuO2, ZnGaNO particles had free electron density in an order of S50 > S20 > S90 > S0, which correlated well with their photocatalytic performance upon visible-light irradiation. The best performance of the loaded S50 was ascribed to the relatively flat surface band bending of the particle. Methane-based nitridation of Zn/Ga/CO3 LDHs would provide a new route to tune the surface chemistry of ZnGaNO and enhance the photocatalytic performance to its full potential.

Noble-Metal-Free Single-Atom Catalysts CuAl4 O7-9 - for CO Oxidation by O2.

The single copper atom doped clusters CuAl4 O7-9 - can catalyze CO oxidation by O2 . The CuAl4 O7-9 - clusters are the first group of experimentally identified noble-metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl4 O9 CO- is much more reactive than CuAl4 O9 - in the reaction with CO to generate CO2 . One adsorbed CO is crucial to stabilize Cu of CuAl4 O9 - around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble-metal catalysis was discussed.

Oxidation of SO2 to SO3 by Cerium Oxide Cluster Cations Ce2O4(+) and Ce3O6(.).

Cerium oxide cationic clusters (CeO2)1-3(+) were generated through laser ablation and then reacted with sulfur dioxide (SO2) at ambient conditions in an ion trap reactor and those reactions were studied and characterized by combining the art of time-of-flight mass spectrometry (TOF-MS) with density functional theory (DFT) calculations. Molecule association and oxygen atom transfer (OAT) were observed for the CeO2(+) and (CeO2)2,3(+) reaction systems, respectively. The mechanistic analysis indicates that the weak Ce-O bond strength associated with the oxygen release capacity of cerium oxide clusters is considered as the key factor to achieve the oxidation of SO2. To our best knowledge, this research should be the first example to identify the OAT reactivity of metal oxide cluster ions toward sulfur dioxide under thermal collision conditions, and a fundamental understanding of the elementary oxidation of SO2 to SO3 is provided.

Detection and characterization of singly deuterated silylene, SiHD, via optical spectroscopy.

Singly deuterated silylene has been detected and characterized in the gas-phase using high-resolution, two-dimensional, optical spectroscopy. Rotationally resolved lines in the 00 (0)X̃(1)A(')→Ã(1)A(″) band are assigned to both c-type perpendicular transition and additional parallel, axis-switching induced bands. The extracted rotational constants were combined with those for SiH2 and SiD2 to determine an improved equilibrium bond length, rSiH, and bond angle, θ, of 1.5137 ± 0.0003 Å and 92.04° ± 0.05°, and 1.4853 ± 0.0005 Å and 122.48° ± 0.08° for the X̃(1)A(')0,0,0 and Ã(1)A(″)(0,0,0) state respectively. The dispersed fluorescence consists of a long progression in the Ã(1)A(″)(0,0,0)→X̃(1)A(')(0,ν2,0) emission which was analyzed to produce vibrational parameters. A strong quantum level dependence of the rotationally resolved radiative decay curves is analyzed.

Thermal conversion of methane to formaldehyde promoted by gold in AuNbO3(+) cluster cations.

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO3(+) heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass-selected to react with CH4 in an ion-trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au-O bond into an Au-Nb bond during the reaction.

Metal-catalyzed formation of 1,3-cyclohexadienes: a catalyst-dependent reaction.

A metal-dependent and complementary catalytic method to synthesize the cyclohexadienes has been developed. When gold or indium salts were used as catalysts, 1,3-cyclohexadiene (1,3-CHD) could be obtained; when Cu(OTf)2 was used as the catalyst, however, another isomer 2,4-cyclohexadiene (2,4-CHD) was furnished instead.

Detection of the Thorium Dimer via Two-Dimensional Fluorescence Spectroscopy.

Thorium dimer, Th2, has been detected in the gas phase via two-dimensional laser-induced fluorescence electronic spectroscopy. The visible excitation spectra are broad, unstructured features with an approximate line width of 10 cm(-1). The spectrum consists of vibrational progressions associated with excitation from the ground electronic state to two different excited electronic states. The dispersed fluorescence was analyzed to give ground state vibrational constants ωe = 134.86 ± 0.66 cm(-1) and ωexe = 0.50 ± 0.04 cm(-1). Vibrational constants ωe = 169 ± 3 and 212 ± 2 cm(-1) were determined for the two excited electronic states. The radiative lifetimes were measured. A comparison with theoretical predictions is given.

CO Oxidation Promoted by the Gold Dimer in Au2 VO3 (-) and Au2 VO4 (-) Clusters.

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O(.-) and O(2-) bonded to early-transition-metal oxide clusters have been shown to oxidize CO. This study reports that when an Au2 dimer is incorporated within the cluster, the molecular oxygen species O2 (2-) bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au2 VO4 (-) cluster ions which then reacted with CO in an ion-trap reactor to produce Au2 VO3 (-) and then Au2 VO2 (-) . The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O-O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.

Reactivity of oxygen radical anions bound to scandia nanoparticles in the gas phase: C-H bond activation.

The activation of C-H bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O(-·)) is an important species in C-H activation. The mechanistic details of C-H activation by O(-·) radicals can be well understood by studying the reactions between O(-·) containing transition metal oxide clusters and alkanes. Here the reactivity of scandium oxide cluster anions toward n-butane was studied by using a high-resolution time-of-flight mass spectrometer coupled with a fast flow reactor. Hydrogen atom abstraction (HAA) from n-butane by (Sc2O3)(N)O(-) (N=1-18) clusters was observed. The reactivity of (Sc2O3)(N)O(-) (N=1-18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc8O13(-)) and 12 (Sc24O37(-)). Larger (Sc2O3)(N)O(-) clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc2O3)(N)O(-) (N=1-5) clusters, which were found to contain the O(-·) radicals as the active sites. The local charge environment around the O(-·) radicals was demonstrated to control the experimentally observed size-dependent reactivity. This work is among the first to report HAA reactivity of cluster anions with dimensions up to nanosize toward alkane molecules. The anionic O(-·) containing scandium oxide clusters are found to be more reactive than the corresponding cationic ones in the C-H bond activation.

Mechanistic insight into transition metal-catalyzed reaction of enynal/enynone with alkenes: metal-dependent reaction pathway.

A systematic study of the transition metal-catalyzed reaction of enynal/enynone with alkenes has been reported. It was found that the reaction has two metal-dependent reaction pathways. One led to the formation of 1,2-DHN, while another led to cyclic-o-QDM.

Cu(OAc)2-promoted cascade carboamination/oxidative cyclization of C-acylimines with alkenes.

A Cu(OAc)2-promoted cascade carboamination/oxidative cyclization of alkenes with α-imino esters has been explored. This transformation provides a concise approach to rapid assembly of 2-oxo-3-iminopyrrole derivatives in moderate to good yields.