Selective Conversion of Methane by Rh₁‑Doped Aluminum Oxide Cluster Anions RhAl₂O₄^–: A Comparison with the Reactivity of PtAl₂O₄^–

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

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

Reactivity of Stoichiometric Lanthanum Oxide Cluster Cations in C–H Bond Activation

Lanthanum oxide cluster cations are prepared by laser ablation and reacted with alkane molecules (<i>n</i>-butane and methane) in a fast flow reactor under thermal collision conditions. A reflectron time-of-flight mass spectrometer is used to detect the cluster distributions before and after the reactions. Hydrogen atom abstraction (HAA) from <i>n</i>-butane by (La₂O₃)_<i>N</i>⁺ (<i>N</i> = 1–8, <i>N</i> ≠ 6) is observed, while the HAA from methane is only observed for (La₂O₃)₅⁺. The experimentally determined rate constants for HAA vary significantly with the cluster sizes. Density functional theory (DFT) calculations are performed to study the structures and reactivity of (La₂O₃)_<i>N</i>⁺ (<i>N</i> = 1–6) clusters. The DFT results suggest that the experimentally observed C–H bond activation by (La₂O₃)_<i>N</i>⁺ is facilitated by oxygen-centered radicals. The position of oxygen-centered radicals binding onto the clusters can heavily influence the reactivity in C–H bond activation. This gas-phase study improves our understanding about the chemistry of oxygen-centered radicals

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

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

C–H Bond Activation by Oxygen-Centered Radicals over Atomic Clusters

Saturated hydrocarbons, or alkanes, are major constituents of natural gas and oil. Directly transforming alkanes into more complex organic compounds is a value-adding process, but the task is very difficult to achieve, especially at low temperature. Alkanes can react at high temperature, but these reactions (with oxygen, for example) are difficult to control and usually proceed to carbon dioxide and water, the thermodynamically stable byproducts. Consequently, a great deal of research effort has been focused on generating and studying chemical entities that are able to react with alkanes or efficiently activate C–H bonds at lower temperatures, preferably room temperature.To identify low-temperature methods of C–H bond activation, researchers have investigated free radicals, that is, species with open-shell electronic structures. Oxygen-centered radicals are typical of the open-shell species that naturally occur in atmospheric, chemical, and biological systems. In this Account, we survey atomic clusters that contain oxygen-centered radicals (O^–•), with an emphasis on radical generation and reaction with alkanes near room temperature. Atomic clusters are an intermediate state of matter, situated between isolated atoms and condensed-phase materials. Atomic clusters containing the O^–• moiety have generated promising results for low-temperature C–H bond activation.After a brief introduction to the experimental methods and the compositions of atomic clusters that contain O^–• radicals, we focus on two important factors that can dramatically influence C–H bond activation. The first factor is spin. The O^–•-containing clusters have unpaired spin density distributions over the oxygen atoms. We show that the nature of the unpaired spin density distribution, such as localization and delocalization within the clusters, heavily influences the reactivity of O^–• radicals in C–H bond activation.The second factor is charge. The O^–•-containing clusters can be negatively charged, positively charged, or neutral overall. We discuss how the charge state may influence C–H bond activation. Moreover, for a given charge state, such as the cationic state, it can be demonstrated that local charge distribution around the O^–• centers can also significantly change the reactivity in C–H bond activation. Through judicious synthetic choices, spin and charge can be readily controllable physical quantities in atomic clusters. The adjustment of these two properties can impact C–H bond activation, thus constituting an important consideration in the rational design of catalysts for practical alkane transformations

CO Oxidation Catalyzed by Single Gold Atoms Supported on Aluminum Oxide Clusters

The single gold atom doped aluminum oxide clusters AuAl₃O₃⁺, AuAl₃O₄⁺, and AuAl₃O₅⁺ have been prepared and mass-selected to react with CO, O₂, and mixtures of CO and O₂ in an ion trap reactor under thermal collision conditions. The reactions have been characterized by mass spectrometry with isotopic substitution (¹⁶O₂ → ¹⁸O₂) and density functional theory calculations. The AuAl₃O₅⁺ cluster can oxidize two CO molecules consecutively to form AuAl₃O₄⁺ and then AuAl₃O₃⁺, the latter of which can react with one O₂ molecule to regenerate AuAl₃O₅⁺. The AuAl₃¹⁶O₃⁺ ions interact with a mixture of C¹⁶O and ¹⁸O₂ to produce the fully substituted ¹⁸O species AuAl₃¹⁸O_3–5⁺, which firmly identifies a catalytic cycle for CO oxidation by O₂. The oxidation catalysis is driven by electron cycling primarily through making and breaking a gold–aluminum chemical bond. To the best of our knowledge, this is the first identification of catalytic CO oxidation by O₂ mediated with gas-phase cluster catalysts with single-noble-metal atoms, which serves as an important step to understand single-atom catalysis at strictly a molecular level

Hydrogen Atom Abstraction from CH₄ by Nanosized Vanadium Oxide Cluster Cations

Reactions of vanadium oxide cluster cations with methane in a fast-flow reactor were investigated with a time-of-flight mass spectrometer. Hydrogen atom abstraction (HAA) reactions were identified over stoichiometric cluster cations (V₂O₅)_<i>N</i>^<i>+</i> for <i>N</i> as large as 11, and the relative reactivity decreases as the cluster size increases. Density functional calculations were performed to study the structural, bonding, and electronic properties of the stoichiometric oxide clusters with the size <i>N</i> = 2–6. The geometric structures were obtained by means of topological and structural unit analyses together with global optimizations. Two types of oxygen-centered radicals were found in these clusters, which are active sites of the clusters in reactions with CH₄. The size-dependent reactivity is rationalized by the charge, spin, and structural effects. This work is among the first reports that HAA from CH₄ can take place on <i>nanosized</i> oxide clusters, which makes a bridge between the small reactive species and inert condensed phase materials for CH₄ activation under low temperature

Does Each Atom Count in the Reactivity of Vanadia Nanoclusters?

Vanadium oxide cluster anions (V₂O₅)_<i>n</i>V_<i>x</i>O_<i>y</i>^– (<i>n</i> = 1–31; <i>x</i> = 0, 1; and <i>x</i> + <i>y</i> ≤ 5) with different oxygen deficiencies (Δ = 2<i>y</i>–1–5<i>x</i> = 0, ± 1, and ±2) have been prepared by laser ablation and reacted to abstract hydrogen atoms from alkane molecules (<i>n</i>-butane) in a fast flow reactor. When the cluster size <i>n</i> is less than 25, the Δ = 1 series [(V₂O₅)_<i>n</i>O^– clusters] that can contain atomic oxygen radical anions (O^•–) generally have much higher reactivity than the other four cluster series (Δ = −2, −1, 0, and 2), indicating that <i>each atom counts</i> in the hydrogen-atom abstraction (HAA) reactivity. Unexpectedly, all of the five cluster series have similar HAA reactivity when the cluster size is greater than 25. The critical dimension of vanadia particles separating the cluster behavior (<i>each atom counts</i>) from the bulk behavior (<i>each atom contributes a little part</i>) is thus about 1.6 nm (∼V₅₀O₁₂₅). The strong electron–phonon coupling of the vanadia particles has been proposed to create the O^•– radicals (V⁵⁺ = O^2–+ heat → V⁴⁺–O^•–) for the <i>n</i> > 25 clusters with Δ = −2, −1, 0, and 2. Such a mechanism is supported by a comparative study with the scandium system [(Sc₂O₃)_<i>n</i>Sc_<i>x</i>O_<i>y</i>^– (<i>n</i> = 1–29; <i>x</i> = 0, 1; and <i>x</i> + <i>y</i> ≤ 4)] for which the Δ = 1 series [(Sc₂O₃)_<i>n</i>O^– clusters] always have much higher HAA reactivity than the other cluster series

Thermal Methane Conversion to Syngas Mediated by Rh₁‑Doped Aluminum Oxide Cluster Cations RhAl₃O₄⁺

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