Methane Activation by Gas Phase Atomic Clusters - Accounts of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Methane Activation by Gas Phase Atomic Clusters Published as part of the Accounts of Chemical Research special issue “Hydrogen Atom Transfer”. Yan-Xia Zhao,†,§ Zi-Yu Li,†,§ Yuan Yang,†,‡,§ and Sheng-Gui He*,†,‡,§ †

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State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center of Excellence in Molecular Sciences, Beijing 100190, P. R. China CONSPECTUS: The increasing supply of natural gas has created a strong demand for developing efficient catalytic processes to upgrade methane, the most stable alkane molecule, into value-added chemicals. Currently, methane conversion in laboratory and industry is mostly performed under hightemperature conditions. A lot of effort has been devoted to exploring chemical entities that are able to activate the C−H bond of methane at lower temperatures, preferably room temperature. Gas phase atomic clusters with limited numbers of atoms are ideal models of active sites on heterogeneous catalysts. The cluster systems are being actively studied to activate methane under room-temperature conditions. State-of-theart mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been combined in our laboratory to reveal the molecular-level mechanisms of methane activation by atomic clusters. In this Account, we summarize our recent progress on thermal methane activation by metal oxide clusters doped with noble-metal atoms (Au, Pt, and Rh) as well as by oxygen-free species including carbides and borides of base metals (V, Ta, Mo, and Fe). In contrast to the generations of CH3• free radicals in many of the previously reported cluster reactions with methane, the generations of stable products such as formaldehyde, acetylene, and syngas as well as closed-shell species AuCH3 and B3CH3 have been identified for the cluster reaction systems herein. Besides the well recognized mechanisms of methane activation by the O−• radicals through hydrogen atom abstraction and by metal atoms through oxidative addition, the new mechanisms of synergistic methane activation by Lewis acid−base pairs (such as Auδ+−Oδ− and Bδ+−Bδ−) and by dinuclear metal centers (such as Ta−Ta) have been recently revealed. In the reactions between methane and oxide clusters doped with noble-metal atoms, the oxide cluster “supports” can accept the H atoms and the CHx species delivered through the noble-metal atoms and then transform methane into stable oxygenated compounds. The product selectivity (such as formaldehyde versus syngas) can be controlled by different noble-metal atoms (such as Pt versus Rh). The electronic structures of base metal centers can be engineered through carburization so that the lowspin states can be accessible to reduce the C−H bond of methane. Such active base metal centers in low-spin states resemble related noble-metal atoms in methane activation. The boron clusters (such as B3 in VB3+) can be polarized by the metal cations to form the Lewis acid−base pair Bδ+−Bδ− to cleave the C−H bond of methane very easily. These molecular-level mechanisms may well be operative in related heterogeneous catalysis and can be a fundamental basis to design efficient catalysts for activation and conversion of methane under mild conditions. nonoxidative pathways.3−6 However, the reported direct methane transformations were mostly operated at high temperatures (>600 K), especially for nonoxidative methane conversion (T ∼ 1000 K). It is very important to understand the mechanisms of methane activation and conversion that involve the basic chemical process of hydrogen atom transfer (HAT), in order to transform methane directly under mild conditions.

1. INTRODUCTION The activation and conversion of methane have attracted significant attention in recent decades due to the growing availability of the unconventional natural gas (e.g., shale gas and gas hydrate). Methane is a nonpolar molecule possessing high C−H bond strength (4.55 eV), high ionization energy (12.6 eV), low proton affinity (5.72 eV), and negligible electron affinity.1 Consequently, the chemical transformation of methane is challenging and the current industrial methane conversion usually involves an indirect route via syngas generation.2 A lot of effort has been devoted to exploring an energy-efficient manner that can directly transform methane through oxidative or © XXXX American Chemical Society

Received: August 10, 2018

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DOI: 10.1021/acs.accounts.8b00403 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

relative energies of low-lying electronic states for some of the systems.34,35

Atomic clusters composed of limited numbers of atoms have well-defined compositions and structures and the cluster species serve as ideal models of active sites on condensed phase catalysts. The reactivity of atomic clusters with methane can be jointly characterized by state-of-the-art mass spectrometric and spectroscopic experiments and quantum chemistry calculations7−16 without being obscured by the complexity (e.g., aggregation and solvent effects) of condensed phase systems. While the cluster approach never accounts for all the details that prevail in condensed phase methane conversion, the molecular level insights into crucial issues such as elementary steps, reactive species, doping effects, and ligand effects can be revealed. Transition metal oxides (TMOs) are widely used as catalysts and catalytic support materials.17,18 Activation of methane by TMO clusters (MxOyq) has been extensively studied in the past decades. We have defined oxygen deficiency (Δ = 2y − nx + q, n is the number of valence electrons of M) to clarify the oxygenrichness or poorness for oxide clusters of early transition metals and related main group elements.19 It was found that the clusters with unit oxygen deficiency (Δ = 1) usually contain the atomic oxygen radical anion (O−•), which is highly reactive to bring about C−H activation of methane through reaction 1.20−22 The distribution of spin densities and local charges around the O−• radicals can be readily controllable by doping with heteroatom(s) of transition metals or main group elements (such as Al, Si, and P),23−25 leading to variable rate constants of the cluster reactions with methane. The O−• radicals over atomic clusters generally transform CH4 to free CH3• radicals, which prohibits direct transformation of methane to stable neutral products by the cluster species. [M−O−•] + CH4 → [M−OH−] + CH•3

2. METHANE ACTIVATION BY SINGLE NM ATOM-DOPED OXIDE CLUSTERS Heterogeneous catalysis with oxides supported single NM atoms has attracted significant attention in recent years because such catalytic processes not only maximize the effective use of the precious metals but also offer great potential for achieving high activity and product selectivity.36,37 To understand the nature of NM atoms as well as the oxide supports in the elementary reactions with methane, the Au1, Rh1, and Pt1 doped metal oxide clusters (Au1VxOy±, Au1TixOy±, Rh1AlxOy±, and Pt1AlxOy±) have been generated and reacted with methane under thermal collision conditions. It was generally found that most of the generated clusters (∼90%) were inert with methane while some of them (AuV2O6+, AuTi3O7,8−, RhAl3O4+, RhAl2O4−, and PtAl2O4− as shown in Figure 1) could indeed

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Noble metal (NM) catalysts have high reactivity in C−H activation.26 In recent years, the doping of NM atoms into metal oxide clusters to improve the product selectivity of direct methane conversion has been explored in our research group. The reactivity of carbide and boride clusters of some base metals toward methane has also been investigated to discover the reactive clusters with similar electronic structures and reactivity to the costly NM systems. Stable neutral products such as formaldehyde, water, syngas, ethylene, and acetylene have been generated in the reactions of methane with these atomic clusters. The mechanisms of these cluster reactions are the main focus of this Account. The cluster ion reactions summarized herein can be well described by the approximation of pseudo-first-order reaction mechanism on the analysis of variation of the reactant and product ion intensities with respect to the CH4 pressure in the experiments. The concepts of two-state-reactivity27,28 and relativistic effect29,30 are very useful to understand the mechanisms of these reactions. The conversion of methane to stable neutral products such as methanol and ethylene mediated by gas phase ions has also been identified by other researchers.31−33 In addition to this Account, one may read some other review articles14,15,21−25 to understand more aspects of methane activation by gas phase species. While mass spectrometric and spectroscopic techniques as well as computational methodologies can be found in our original works, it is important to point out that the metal carbide clusters often have complicated electronic structures so the multireference wave function based methods have been employed to calculate the

Figure 1. DFT calculated structures of (a) reactive metal oxide clusters doped with NM atoms and (b) reactive carbide and boride species.

activate and transform methane.38−42 The typical time-of-flight (TOF) mass spectra in Figure 2 indicate that the reaction of AuV2O6+ with CH4 generates diverse products including the simple addition complex AuV2O6CH4+ and many other species such as AuV2O5H2+ + CH2O, V2O6CH3+ + AuH, V2O5H2+ + CH2O + Au, and V2O5H+ + CH2O + AuH.38 The density functional theory (DFT) calculations and collision induced dissociation (CID)12 experiments characterized that the oxidation state of gold in AuV2O6+ cluster isomers can be + I or + III. As shown in Figure 3, the most stable isomer with AuIII can activate and transform methane into various products whereas the low-lying isomers with AuI only adsorb CH4 to form the simple addition product.38 In AuIIIV2O6+, the AuIII cation is a Lewis acid site and an adjacent bridging-oxygen anion (Ob2−) can be a Lewis base site. Such Lewis acid−base pair (LABP)43 AuIII−Ob2− is active enough to cleave the first C−H bond of methane (I1 → TS1 → I2, Figure 3): Au δ +−Oδ − + CH4 → CH3−Au···O−H

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After the C−H cleavage, the methyl group transfers from Au atom to the “oxide support” to make a CH3−O bond (I3) and additional energy is released. A HAT from methyl group to Au B


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molecule and the AuTi3O7H2− ion.39 It should be emphasized that most of the reported metal oxide clusters that can cleave the C−H bond of methane under thermal collision conditions have open-shell electronic structures (such as the TMO clusters with Δ = 1) while AuV2O6+ and AuTi3O7,8− clusters are closed-shell species. The C−H activation of methane by these closed-shell species should be better described as proton (H+) transfer (O2− + H+→ OH−) rather than hydrogen atom (H•) transfer (O−• + H•→ OH−), the mechanistic variations of which can be well distinguished by a diagnostic plot of the deformation energies of the reactants in the transition state versus the corresponding barrier relative to the encounter complex.44 For the oxide cluster systems doped with gold, the strong relativistic effect on gold leads to its high electronegativity and a rather weak Au−O chemical bond (2.27 eV).45 Moreover, the Au/H analogy30,46−48 results in relatively strong Au−CH3 (2.40 eV by DFT) and Au−H (3.03 eV by DFT) bonds so that evaporation of neutral Au, AuH, and AuCH3 species from the intermediates is possible in the reaction with methane. Unlike gold, platinum and rhodium have relatively strong M−O bonds (Pt−O: 4.01 eV; Rh−O: 4.15 eV).45 Thus, loss of M, MH, and MCH2,3 (M = Pt and Rh) from the oxide reaction system is unlikely to take place and the highly selective transformation of methane can be identified.40−42 Figure 4 shows that the reactions of methane with PtAl2O4− and RhAl2O4− cluster anions selectively generate PtAl 2 O 3 H 2 − + CH 2 O and RhAl2O4CH2− + H2, respectively.40,41 The DFT studies indicated that Pt and Rh atoms are the active sites to activate methane through oxidative addition:49

Figure 2. TOF mass spectra for the reactions of mass-selected AuV2O6+ (a) with CH4 (b) and CD4 (c) for 1.2 ms. The relative signal magnitudes are amplified by a factor of 3 for m/z < 400. The AuxVyOz+ and AuxVyOzX+ are labeled as x,y,z and x,y,z,X, respectively. Adapted with permission from ref 38. Copyright 2016 American Chemical Society.

atom occurs (I3 → I4) and the reaction system then relaxes (I4 → I5) to form AuH and CH2O moieties in I5 that has enough energy to evaporate AuH and CH2O successively to produce the experimentally identified V2O6CH3+ and V2O5H+ ions (P1 and P3). Alternatively, the H atom bonded with Au in I5 can further transfer to one terminal O atom and a more stable intermediate I6 with an Au−V chemical bond is formed. The CH2O molecule and the Au atom can then be desorbed successively to produce AuV2O5H2+ and V2O5H2+ ions (P2 and P4). It is noteworthy that the intermediates I3−I6 have large internal energies (>3 eV) while the rates (>109 s−1) of internal conversions into products (I3 → P1 and I6 → P2) are much higher than the rates (104 − 106 s−1) of collisions with CH4 and the bath gas (He) in the reactor. As a result, the energetic intermediates I3−I6 have no chance to react with a second CH4 molecule. In addition, CH2O and CO + H2 are almost thermal neutral (CH2O → H2 + CO, ΔH = +0.059 eV) while generation of CO and H2 requires the cleavage of all of the four C−H bonds of CH4. Test calculations indicated that the cleavage of the C−H bond of [CH2O] from I4 and I5 is subject to high reaction barriers (>2.3 eV), so it is very unlikely that the loss of [CH2O] is due to the successive loss of H2 and CO. In contrast to the high reactivity of AuIII−O2− in AuIIIV2O6+ species, methane activation by the AuI−O2− LABP in AuIV2O6+ encounters overall positive energy barriers (>0.30 eV) owing to the weak Lewis acid property of AuI than that of AuIII.38 The reactivity of the AuI−O2− with CH4 can be enhanced by changing the cluster support from cationic vanadium oxide to anionic titanium oxide.39 The AuTi3O7− cluster (Figure 1a) has a one-fold coordinated AuI and two terminally bonded oxygen anions (Ot2−). The CH4 molecule can be trapped by the AuI site and then delivered to an Ot2− ion so that the C−H cleavage is mediated by the AuI···Ot2− LABP, resulting in the formation of Au−CH3 bond and O−H bond. The AuCH3 moiety can be further desorbed to generate Ti3O7H− ions identified in experiments.39 More interestingly, when one of the Ot2− ions in AuTi3O7− is replaced by a peroxide unit (O22−), the O22− anion in AuTi3O8− cluster (Figure 1a) is able to activate the C− H bond of AuCH3 moiety to produce the neutral CH2O

M + CH4 → CH3−M−H

(M = Pt and Rh)

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The different product selectivity of the PtAl2O4− and RhAl2O4− reaction systems is due to the fact that Rh (4d85s1) has one less valence electron than Pt (5d96s1), which leads to much lower barrier for activation of the C−H bond of the formed CH2O unit in the Rh reaction system.41 As a result, all of the four C−H bonds in CH4 can be cleaved by Rh to form H2 and adsorbed CO while PtAl2O4− cluster selectively transforms CH4 into free CH2O. In addition to RhAl2O4−, the Rh atom in RhAl3O4+ can also cleave the four C−H bonds of CH4 to form syngas (CO + H2) as identified by the experiments.42 In both of the RhAl2O4− + CH4 and RhAl3O4+ + CH4 reactions, the cluster supports (AlxOyq) supply oxygen atoms to oxidize the activated CH4 to form CO, which reveals the molecular-level origin for a puzzling experimental observation that trace amounts of Rh can promote the direct participation of lattice oxygen of chemically very inert alumina (Al2O3) to oxidize methane to carbon monoxide.50 It is noteworthy that NM atoms and ions typically activate methane to form H2 and metal carbene species (M = CH2).51,52 For a NM atom doped metal oxide cluster (NM)1MxOyq, the cluster support (MxOy) can accept H atom(s) delivered from NM atom (such as in reaction 3) and supply an O atom to oxidize the activated C atom of CH4.

3. METHANE ACTIVATION BY TRANSITION METAL CARBIDE CLUSTERS While NMs have superior activity in CH4 activation, a lot of effort has been devoted to exploring NM-free systems with comparable performance to the NMs. Transition metal carbides (TMCs) were reported as promising alternatives of NMs.3 The reactivity studies of TMC clusters (MoC3−, Mo2C3O−, Ta2C4−, and FeC3−, as shown in Figure 1b) have indicated that C


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Figure 3. DFT calculated potential energy profile for AuV2O6+ + CH4. Relative energies are given in eV and bond lengths are in pm. Adapted with permission from ref 38. Copyright 2016 American Chemical Society.

spin ground electronic states are inert toward methane under thermal collision conditions55−57 because of the high C−H activation barriers. The low-spin excited states of the metal center could have very small or negligible barriers for C−H activation while such low-spin states have relatively high energies and are thus inaccessible (Figure 5, left), so the atomic systems are inert even if the two-state reactivity27,28 is considered. The photoelectron imaging spectroscopy (PEIS) and quantum chemistry studies on MoC3− + C2H6/CH4 systems indicated that the active low-spin metal center for C− H oxidative addition can be accessible (Figure 5, right) by forming metal carbide species.34 In addition to carburization, adsorption of alien molecules can also tune the spin state of the metal center, which then promotes methane activation through oxidative addition.35 The mass spectrometry experiments indicated that the Mo2C2− cluster is inert with CH4. In contrast, with the presence of CO in the reactant gas, the CO addition product Mo2C3O− (Mo2C2− + CO → Mo 2 C 3 O − ) can react with CH 4 to generate Mo2C3OCH2− (Mo2C3O− + CH4 → Mo2C3OCH2− + H2). The PEIS and DFT studies demonstrated that CO can be molecularly or dissociatively adsorbed in Mo2C3O− and the dissociative CO-adsorption tunes down the spin density on one

Figure 4. TOF mass spectra for the reactions of RhAl2O4− (a) and PtAl2O4− (d) with CH4 (b, e) and CD4 (c, f). The reaction times are 2.2 ms (b, c) and 1.6 ms (e, f). Adapted with permission from refs 40 and 41. Copyright 2014 Wiley-VCH and 2018 American Chemical Society.

carburization is an effective way to engineer the electronic structures of base metal centers to activate methane.34,35,53,54 The atomic species 6 Mo+, 6 Mo−, and 7 Mo that all have highD


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experiment. The reaction mechanism study demonstrated that methane activation by a single Ta atom of Ta2C4− (isomer 1) is subject to a positive barrier of 0.62 eV. In contrast, methane activation by the cooperation of the two Ta atoms to generate an intermediate is kinetically favorable: M−M + CH4 → CH3 − M−M−H

(M = Ta)

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To explore the ligand effect on the cooperative methane activation, the reactivity of one- and two-carbon-less clusters Ta2C3− and Ta2C2− has also been experimentally studied and both of them are inert with methane. The topological analysis of the electron localization functions (ELFs) shows that the symmetrical C2 ligands in Ta2C4− (isomer 1) afford an electronrich dinuclear metal center (Figure 6d) over which C−H cleavage of methane is kinetically favorable.53 However, when one or both of the two C2 ligands are replaced with the C1 ligands to form Ta2C3− or Ta2C2−, the electron density stored by the two Ta atoms decreases progressively, resulting in the increase of the C−H activation barrier (+0.27 and +0.65 eV for Ta2C3− and Ta2C2−, respectively). The presence of symmetrical C2 ligands with π-donating ability in Ta2C4− (isomer 1) is thus important to facilitate the cooperative methane activation. The reported reactions between gas phase clusters and methane were usually performed under room-temperature conditions. The elevation of reaction temperature is required to better mimic the condensed phase methane activation that was typically performed under high-temperature conditions.3 With an upgraded ion trap reactor,60 we have recently investigated the reaction between FeC3− and methane at variable temperatures (300−610 K)54 and the results are shown in Figure 7. At 300 K, the association complex FeC4H4− (FeC3− + CH4 → FeC4H4−) was identified while the other two products FeC4H2− and FeC2H2− were barely observable. With the elevation of temperature from 300 to 610 K, the branching ratios (BRs) of FeC4H2− and FeC2H2− increased significantly (Figure 7g), indicating the consecutive reactions of FeC3− + CH4 → FeC4H4− → FeC4H2− + H2 (or FeC2H2− + C2H2). The reaction mechanism study revealed that the doublet FeC3− is the reactive component at low temperatures (575 K) the quartet FeC3− can also contribute to the observed reactivity, leading to a sharp increase of k1 value from 575 to 610 K in the experiments (Figure 7h). The production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization,61 was

Figure 5. Simplified potential energy profiles and energetics for the processes of C−H oxidative addition by naked metal atom (left) and metal carbide (right) in low-spin (LS) and high-spin (HS) states. Adapted with permission from ref 34. Copyright 2015 Wiley-VCH.

metal center, which then promotes methane activation through oxidative addition. This work35 again emphasized the importance of the low-spin metal center in oxidative C−H addition (Figure 5). Metals typically activate CH4 through oxidative addition with a single metal atom center (reaction 3). Dinuclear metal centers58 are ubiquitous on catalytic metal surfaces as well as in many transition-metal complexes. It has been reported in organometallic chemistry that the cooperation of two metal atoms can induce the cleavage of C−H bonds of alkenes, alkynes, and aromatics.59 Our mass spectrometric experiments indicated that under thermal collision conditions, the tantalum carbide cluster Ta2C4− can dehydrogenate methane53 (Ta2C4− + CH4 → Ta2C5H2− + H2) with the pseudo-first-order rate constant (k1) of 1.5 × 10−11 cm3 molecule−1 s−1 that is the largest among the reported NM-free cluster anions [for example, k1(Mo2C3O− + CH4) = 8 × 10−12 cm3 molecule−1 s−1].16,35 This implies that different mechanism is operative in methane activation by the dinuclear Ta2C4− species. The DFT calculations determined two low-lying isomers of Ta2C4− with very close energies (Figure 6). The cryogenic PEIS of Ta2C4− at 11 K characterized that the isomer with two C2 ligands (isomer 1) has a simulated spectrum matching the experimental one, which assigned the cluster structure (isomer 1) in the

Figure 6. (a) Photoelectron image and spectrum of Ta2C4− cluster at 636 nm laser excitation. (b, c) DFT calculated structures of Ta2C4− isomers (1 and 2) and the simulated photoelectron spectra. (d) Two-dimensional ELFs of the reactive Ta2C4− (isomer 1) and unreactive Ta2C2,3− clusters. The ELF distributions are shown on the σxz and σyz planes and a larger ELF value means that electrons are more localized. Adapted with permission from ref 53. Copyright 2017 American Chemical Society. E


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Figure 7. TOF mass spectra for the reactions of mass-selected FeC3− cluster with (a) He at 610 K, (b) CH4 at 300 K, (c) CH4 at 410 K, (d) CH4 at 610 K, (e) CD4 at 610 K, and (f) 13CH4 at 644 K for around 14.5 ms. Branching ratios (g) and rate constants (h) at different temperatures are also shown. Adapted with permission from ref 54. Copyright 2018 Wiley-VCH.

successfully characterized in FeC3− + CH4 under hightemperature conditions while the formation of C2H2 was rarely reported for cluster reactions with methane under roomtermperature conditions.62 The 13CH4 experiment (Figure 7f) indicated that the carbon atom of C2H2 product can come solely from Fe12C3− (Fe12C3− + 13CH4 → Fe12C13CH2− + 12C2H2), which is supported by the reaction mechanisms.54

4. METHANE ACTIVATION BY METAL BORIDE CLUSTERS In addition to TMC clusters, transition metal boride (TMB) clusters have also been reported to activate methane. Our recent experiments indicated that VBn+ (n = 3−6) clusters could dehydrogenate methane (VBn+ + CH4 → VBnCH2+ + H2) under thermal collision conditions.63 For the reactions of VB3+ and VB4+ with CH4, there were competing reaction channels to generate B3CH3 and B4CH4, respectively. The mechanism of methane activation by the TMB clusters can be very different from that by the TMC clusters that usually activate methane through oxidative addition of a C−H bond onto a metal center. Figure 8 indicates that methane activation by VB3+ is much more favorable through the nonmetal (B) site (I7 → I8) than through the metal(V) site (I7′ → I8′). The charge analysis indicated that the B3 moiety in VB3+ can be polarized by the atomic V+ cation (+0.96 e). The B atom (denoted as Bα) far away from the V atom has the positive charge of +0.32 e while each of the other two B atoms (Bβ) carries a negative charge of −0.14 e. The Bαδ+−Bβδ− can be considered as LABP induced by the V+ cation. Note that the LABP in metal oxide systems (AuV2O6+ and AuTi3O7,8−) is based on the analysis of oxidation states of metal (AuIII or AuI) and oxygen (O2−) while it is hard to determine the oxidation states of B atoms in VB3+. As a result, the charge analysis is carried out for the metal boride system. It turns out that methane activation by the LABP Bδ−−Bδ+ is facile: Bδ +−Bδ − + CH4 → CH3−B−B−H

Figure 8. DFT calculated simplified potential energy profiles for VB3+ + CH4. Relative energies are given in eV. Adapted with permission from ref 63. Copyright 2018 PCCP Owner Societies.

5. CONCLUDING REMARKS In sharp contrast to the oxide clusters of early transition metals and related main group elements that generally activate one C− H bond of CH4 through the O−• radical to produce the CH3• radical (reaction 1), the oxide clusters doped with noble metal atoms can activate two to four C−H bonds of CH4 to transform methane into stable products including formaldehyde and syngas. The noble metal atoms function as the active sites to cleave the C−H bonds following Lewis acid−base pair mechanism (reaction 2) or oxidative addition mechanism (reaction 3). The oxide cluster supports can accept the hydrogen atom delivered through the noble metal atoms and provide oxygen atom to oxidize the carbon atom of CH4. The electronic structure of the base-metal center can be properly engineered through carburization so that the low-spin state can be accessible to activate methane through oxidative addition. Dinuclear metal center and polarized boron−boron unit can cooperatively activate methane (reactions 4 and 5). These mechanisms of

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methane activation such as reactions 1−5 revealed from gas phase systems may well be operative in related condensed phase systems. The investigations on the reactions of mixed metal oxide clusters with methane are far from systematic. Future studies will at least include (i) doping all of the eight noble-metal elements (Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au) into one type of base-metal oxide clusters (such as VxOyq) and (ii) doping one noble-metal element (such as Au) into all of the 3d-5d base-metal oxide clusters. The previous studies of gas phase cluster systems have discovered many elementary reactions involved with methane activation and conversion. In contrast, only a few atomic and diatomic species have been reported to react with methane to form catalytic cycles.64−66 Catalytic methane activation by gas phase atomic clusters is thus an important topic in the future. The studies can include (i) structure and reactivity characterization of the intermediates and products generated from the elementary reactions with methane, (ii) coupling of methane with other molecules (H2O, CO2, N2, etc.) over atomic clusters, and (iii) cluster reactions under room- to high-temperature conditions or under photo irradiation conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sheng-Gui He: 0000-0002-9919-6909 Notes

The authors declare no competing financial interest. Biographies Yan-Xia Zhao received her B.S. degree in chemistry from Shanxi Normal University in 2005, M.S. degree in chemistry from Beijing Normal University in 2008, and Ph.D. degree in chemistry from Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2011. Her current research interests are methane activation by atomic clusters. Zi-Yu Li received her B.S. degree in chemistry from Minzu University of China in 2010 and Ph.D. degree in chemistry from ICCAS in 2015. Her current research interests are bonding and reactivity of atomic clusters. Yuan Yang received her B.S. degree in chemistry from Zhengzhou University in 2015 and is currently a Ph.D. candidate with Sheng-Gui He. Sheng-Gui He received his B.S. degree in physics in 1997 and Ph.D. degree in chemistry in 2002 from University of Science and Technology of China. After postdoctoral stays with Prof. Dennis J. Clouthier at University of Kentucky and Prof. Elliot R. Bernstein at Colorado State University, he joined ICCAS in 2007. His research interests are experimental and theoretical studies of reactive intermediates including free radicals and atomic clusters.

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ACKNOWLEDGMENTS This work was financially supported by the Chinese Academy of Sciences (No. XDA09030101) and the National Natural Science Foundation of China (Nos. 91645203, 21573247, 21627803, and 21773253). Y.-X.Z. is thankful for the grant from the Youth Innovation Promotion Association, Chinese Academy of Sciences (2018041). G


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