RESUMO
Catalytic conversion of NO has long been a focus of atmospheric pollution control and diesel vehicle exhaust treatment. Rhodium is one of the most effective metals for catalyzing NO reduction, and understanding the nature of the active sites and underlying mechanisms can help improve the design of Rh-based catalysts towards NO reduction. In this work, we investigated the detailed catalytic mechanisms for the direct reduction of NO to N2 by fullerene-supported rhodium clusters, C60Rh4+, with density functional theory calculations. We found that the presence of C60 facilitates the smooth reduction of NO into N2 and O2, as well as their subsequent desorption, recovering the catalyst C60Rh4+. Such a process fails to be completed by free Rh4+, emphasizing the critical importance of C60 support. We attribute the novel performance of C60Rh4+ to the electron sponge effect of C60, providing useful guidance for designing efficient catalysts for the direct reduction of NO.
RESUMO
ConspectusBuckminsterfullerene, C60, was discovered through a prominent mass peak containing 60 atoms produced from laser vaporization of graphite, driven by Kroto's interest in understanding the formation mechanisms of carbon-containing molecules in space. Inspired by the geodesic dome-shaped architecture designed by Richard Buckminster Fuller, after whom the particle was named, C60 was found to have a football-shaped structure comprising 20 hexagons and 12 pentagons. It sparked worldwide interest in understanding this new carbon allotrope, resulting in the awarding of the Noble Prize in Chemistry to Smalley, Kroto, and Curl in 1996.Intrinsically, C60 is an exceptional species because of its high stability and electron-accepting ability and its structural tunability by decorating or substituting either on its exterior surface or interior hollow cavity. For example, metal-decorated fullerene complexes have found important applications ranging from superconductivity, nanoscale electronic devices, and organic photovoltaic cells to catalysis and biomedicine. Compared to the large body of studies on atoms and molecules encapsulated by C60, studies on the exteriorly modified fullerenes, i.e., exohedral fullerenes, are scarcer. Surprisingly, to date, uncertainty exists about a fundamental question: what is the preferable exterior binding site of different kinds of single atoms on the C60 surface?In recent years, we have developed an experimental protocol to synthesize the desired fullerene-metal clusters and to record their infrared spectra via messenger-tagged infrared multiple photon dissociation spectroscopy. With complementary quantum chemical calculations and molecular dynamics simulations, we determined that the most probable binding site of a metal, specifically a vanadium cation, on C60 is above a pentagonal center in an η5 fashion. We explored the bonding nature between C60 and V+ and revealed that the high thermal stability of this cluster originates from large orbital and electrostatic interactions. Through comparing the measured infrared spectra of [C60-Metal]+ with the observational Spitzer data of several fullerene-rich planetary nebulae, we proposed that the complexes formed by fullerene and cosmically abundant metals, for example, iron, are promising carriers of astronomical unidentified spectroscopic features. This opens the door for a real consideration of Kroto's 30-year-old hypothesis that complexes involving cosmically abundant elements and C60 exhibit strong charge-transfer bands, similar to those of certain unidentified astrophysical spectroscopic features. We compiled a VibFullerene database and extracted a set of vibrational frequencies and intensities for fullerene derivatives to facilitate their potential detection by the James Webb Space Telescope. In addition, we showed that upon infrared irradiation C60V+ can efficiently catalyze water splitting to generate H2. This finding is attributed to the novel geometric-electronic effects of C60, acting as "hydrogen shuttle" and "electron sponge", which illustrates the important role of carbon-based supports in single-atom catalysts. Our work not only unveils the basic structures and bonding nature of fullerene-metal clusters but also elucidates their potential importance in astrophysics, astrochemistry, and catalysis, showing the multifaceted character of this class of clusters. More exciting and interesting aspects of the fullerene-metal clusters, such as ultrafast charge-transfer dynamics between fullerene and metal and their relevance to designing hybrid fullerene-metal junctions for electronic devices, are awaiting exploration.
RESUMO
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 (CH3â¢) 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+, â¢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 CH3â¢/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.
RESUMO
Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.
RESUMO
The gas-phase reaction of NiAl2 O4 + with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2 O4 H+ and NiAl2 O3 H2 + , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2 O4 + contains Ni in the +II oxidation state and features a terminal Al-O.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3 â ) or formaldehyde (CH2 O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3 â loss or CH3 â migration to an O2- site next to the Ni2+ center. The CH3 â attaches as CH3 + to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+ . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.
RESUMO
Isolated nickel-doped aluminum oxide cations (NiOm)(Al2O3)n(AlO)+ with m = 1-2 and n = 1-3 are investigated by infrared photodissociation (IRPD) spectroscopy in combination with density functional theory and the single-component artificial force-induced reaction method. IRPD spectra of the corresponding He-tagged cations are reported in the 400-1200 cm-1 spectral range and assigned based on a comparison to calculated harmonic IR spectra of low-energy isomers. Simulated spectra of the lowest energy structures generally match the experimental spectra, but multiple isomers may contribute to the spectra of the m = 2 series. The identified structures of the oxides (m = 1) correspond to inserting a Ni-O moiety into an Al-O bond of the corresponding (Al2O3)1-3(AlO)+ cluster, yielding either a doubly or triply coordinated Ni2+ center. The m = 2 clusters prefer similar structures in which the additional O atom either is incorporated into a peroxide unit, leaving the oxidation state of the Ni2+ atom unchanged, or forms a biradical comprising a terminal oxygen radical anion Al-Oâ¢- and a Ni3+ species. These clusters represent model systems for under-coordinated Ni sites in alumina-supported Ni catalysts and should prove helpful in disentangling the mechanism of selective oxidative dehydrogenation of alkanes by Ni-doped catalysts.
RESUMO
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.
RESUMO
The effects of NaCl and its partial substitutes (KCl, MgCl2 and CaCl2) on solubility, structural characteristics and aggregation behaviors of myofibrillar protein (MP) from pearl mussel muscle were investigated and compared. MP at 0.6 M NaCl was beneficial to protein unfolding and showed excellent potential functional properties. When NaCl was substituted in low level, MPs also showed good solubility and ordered microstructure as well as NaCl, especially MgCl2 and CaCl2, due to the unfolding of α-helical structures and subsequently exposed tyrosine residues and hydrophobic groups. However, the obviously increased disulfide bonds and hydrophobic interactions in high substitution level indicated the excessive non-sodium salts had negative effects on molecular rearrangement, leading to irregular and overly tight of microstructure. Thus, NaCl partially substituted by KCl, MgCl2 and CaCl2 in low substitution level is promising to improve functional properties of MP in low-sodium meat products.
Assuntos
Bivalves , Produtos da Carne/análise , Proteínas Musculares/química , Agregados Proteicos , Cloreto de Sódio/química , Animais , Interações Hidrofóbicas e Hidrofílicas , Desdobramento de Proteína , Alimentos Marinhos , SolubilidadeRESUMO
The activation and hydrogenation of nitrogen are central in industry and in nature. Through a combination of mass spectrometry and quantum chemical calculations, this work reports an interesting result that scandium nitride cations Sc3N+ can activate sequentially H2 and N2, and an amido unit (NH2) is formed based on density functional theory calculations, which is one of the inevitable intermediates in the N2 reduction reactions. If the activation step is reversed, i.e., sequential activation of first N2 and then H2, the reactivity decreases dramatically. An association mechanism, prevalent in some homogeneous catalysis and enzymatic mechanisms, is adopted in these gas-phase H2 and N2 activation reactions mediated by Sc3N+ cations. The mechanistic insights are important to understand the mechanism of the conversion of H2 and N2 to NH3 synthesis under ambient conditions.
RESUMO
The gas-phase reaction of the heteronuclear oxide cluster [VPO4 ].+ with C2 H4 is studied under multiple collision conditions at 150â K using cryogenic ion-trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier-free thermal C2 H4 âCH3 CHO conversion. Interestingly, in the course of the reaction, the emerging CH3 CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2 H4 O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.
RESUMO
The strategy of synthesizing diazeniumdiolates (X-N(O)=NO-) through the coexistence of nitric oxide and alkoxides (RO-) was introduced by Wilhelm Traube 120 years ago. Today, despite the wide use of diazeniumdiolate derivatives to release nitric oxide in the treatment of cancer, the first step of the reaction mechanism for diazeniumdiolate synthesis remains a mystery and is thought to be complex. We have studied the gas-phase reactions of nitric oxide with alkoxides at room temperature. An electron-coupled hydrogen transfer is observed, and the radical anion HNO- is the only ionic product in these reactions. HNO- can further react with nitric oxide to form N2O and HO-.
RESUMO
Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl2O4- with the most stable alkane molecule CH4 under thermal conditions has been studied. The methane tends to be converted into syngas (free H2 and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl2O4- with CH4. Density functional theory calculations show that the difference in reactivity between RhAl2O4- and PtAl2O4- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al2O4-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.
RESUMO
For many decades, astronomers have searched for biological molecules, including amino acids, in the interstellar medium; this endeavor is important for investigating the hypothesis of the origin of life from space. The space environment is complex and atomic species, such as nitrogen and oxygen atoms, are widely distributed. In this work, the reactions of eight typical deprotonated amino acids (glycine, alanine, cysteine, proline, aspartic acid, histidine, tyrosine, and tryptophan) with ground state nitrogen and oxygen atoms are studied by experiment and theory. These amino acid anions do not react with nitrogen atoms. However, the reactions of these ions with oxygen atoms show an intriguing variety of ionic products and the reaction rate constants are of the order of 10-10 cm3 s-1. Density functional calculations provide detailed mechanisms of the reactions, and demonstrate that spin conversion is essential for some processes. Our study provides important data and insights for understanding the kinetic and dynamic behavior of amino acids in space environments.
RESUMO
Reactions of hydrogen atoms with small sulfur-containing anions, SCN-, CH3COS-, C6H5COS-, -SCH2COOH, C6H5S-, 2-HOOCC6H4S-, and related oxygen-containing anions, OCN-, CH3COO-, C6H5COO-, HOCH2COO-, C6H5O-, 2-HOOCC6H4O-, have been studied both experimentally and computationally. The experimental results show that associative electron detachment (AED) is the only channel for the reactions. The rate constants for reactions between sulfur-containing anions and H atoms are generally higher than for the related oxygen-containing anions with the exception of the reaction of SCN-. The generally higher reactivity of the sulfur anions contrasts with previous results where AED reactivity was found to correlate with reaction exothermicity. Density functional theory calculations indicate that the reaction enthalpies, the characteristics of the reaction potential energy surfaces, and other structural and electronic factors can influence the reaction rate constants. This study indicates that organic sulfur anions can be more reactive than related oxygen anions in the interstellar medium where hydrogen atoms are abundant.
RESUMO
Laser ablation generated RhAl3O4+ heteronuclear metal oxide cluster cations have been mass-selected using a quadrupole mass filter and reacted with CH4 or CD4 in a linear ion trap reactor under thermal collision conditions. The reactions have been characterized by state-of-the-art mass spectrometry and quantum chemistry calculations. The RhAl3O4+ cluster can activate four C-H bonds of a methane molecule and convert methane to syngas, an important intermediate product in methane conversion to value-added chemicals. The Rh atom is the active site for activation of the C-H bonds of methane. The high electron-withdrawing capability of Rh atom is the driving force to promote the conversion of methane to syngas. The polarity of Rh oxidation state is changed from positive to negative after the reaction. This study has provided the first example of methane conversion to syngas by heteronuclear metal oxide clusters under thermal collision conditions. Furthermore, the molecular level origin has been revealed for the condensed-phase experimental observation that trace amounts of Rh can promote the participation of lattice oxygen of chemically very inert support (Al2O3) to oxidize methane to carbon monoxide.
RESUMO
The study of chemical reactions between gold-containing heteronuclear oxide clusters and small molecules can provide molecular level mechanisms to understand the excellent activity of gold supported by metal oxides. While the promotion role of gold in alkane transformation was identified in the clusters with atomic oxygen radicals (O(-.)), the role of gold in the systems without O(-.) is not clear. By employing mass spectrometry and quantum chemistry calculations, the reactivity of Au2 VO3(+) clusters with closed-shell electronic structures toward ethane was explored. Both the dehydrogenation and ethene elimination channels were identified. It is gold rather than oxygen species initiating the C-H activation. The Au-Au dimer formed during the reactions plays important roles in ethane transformation. The reactivity comparison between Au2 VO3(+) and bare Au2(+) demonstrates that Au2 VO3(+) not only retains the property of bare Au2(+) that transforming ethane to dihydrogen, but also exhibits new functions in converting ethane to ethene, which reveals the importance of the composite system. This study provides a further understanding of the reactivity of metal oxide supported gold in alkane activation and transformation.