The Thermal Dehydrogenation of Methane Enhanced by Âµ2-Oxo

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C: Physical Processes in Nanomaterials and Nanostructures

The Thermal Dehydrogenation of Methane Enhanced by µ-Oxo Ligands in Tantalum Cluster Cations [TaO], x=4,5 2

x

+

Jan F. Eckhard, Tsugunosuke Masubuchi, Martin Tschurl, Robert N. Barnett, Uzi Landman, and Ueli Heiz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07729 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

The Thermal Dehydrogenation of Methane Enhanced by µ2-Oxo Ligands in Tantalum Cluster Cations [TaxO]+, x=4,5 Jan F. Eckhard†, Tsugunosuke Masubuchi†, Martin Tschurl*†, Robert N. Barnett‡, Uzi Landman*‡ and Ueli Heiz†. †

Department of Chemistry and Catalysis Research Center, Technical University of Munich,

Lichtenbergstraße 4, 85748 Garching, Germany ‡

School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, U.S.A.

AUTHOR INFORMATION Corresponding Author *M.T.: E-mail: [email protected]. *U.L.: E-mail: [email protected].

ABSTRACT

The gas-phase reactions of [TaxOy]+ (x=4,5; y=0,1) nanoclusters with methane have been explored in a ring-electrode ion trap under multi-collision conditions, and theoretically with the use of firstprinciples quantum simulations. At room temperature Ta4+ dehydrogenates consecutively two methane molecules with the concurrent elimination of H2, whereas Ta5+ is found to be unreactive. Both of the corresponding mono-oxides, [Ta4O]+ and [Ta5O]+, demonstrate significantly increased reaction rates. Binding of the methane molecule to the tantalum clusters is found to occur through Pauli-repulsion-driven polarization of the electronic charge distribution in the metal cluster,

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induced by the closed-shell methane molecule. The subsequent dehydrogenation reaction is found to entail active participation of up to four tantalum atoms, whereas the doping oxygen atom does not form bonds to the methane molecule or the reaction intermediates, and acts merely as a clustercharge-polarizing ligand spectator. Clusters exhibiting such enhanced reactivity, influenced by oxo-ligand modification of the local electronic charge distribution, with consequent tuned local Lewis acid-base-pair balancing, may serve as potent models for active centers in small particle and surface metalorganic chemistry or heterogeneous nanocatalysis.


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INTRODUCTION Methane is the most abundant natural gas and the smallest hydrocarbon, offering prospects, coupled with significant challenges, of utilizing it in the synthesis of venerable and valuable more complex chemicals. The process of activating methane, however, is hindered by its intrinsic molecular properties, such as strong sp3-hydridized C–H bonds, a low pKa value and a small polarizability, which pose a challenge to the activating agent.1-2 On an extended surface, silicasupported tantalum hydrides catalyze the conversion of methane into ethane.3 Bulk metal oxides also facilitate C–H bond cleavage in methane, albeit at elevated reaction temperatures and accompanied by effects that obstruct their systematic investigation.2, 4 Studies of gas-phase atoms and small (nano-scale) clusters, which mimic active centers of more complex systems, may therefore provide important insights about operative molecular-level processes and mechanisms.5 Various 5d transition metal cations, including Ta+, dehydrogenate methane and form (distorted) carbene structures.6-8 The reaction of [TaO3]+ with methane leads to the formation of two valueadded products, formaldehyde and methanol.9 As observed in the comparison of [MCH2]+ and [MO2]+ (M = V, Nb, Ta)

10-11,

the increased reactivity of tantalum-containing compounds is

generally attributed to relativistic effects that enable the formation of very strong Ta–C bonds. Instead of dehydrogenation and concomitant H2 evolution, a direct hydrogen-atom transfer (HAT) mechanism prevails for many metal oxide systems that contain terminal12,

13

(and less for

bridging13) oxygen atoms with localized spin density, e.g. stoichiometric vanadium and niobium oxide clusters.13 In these systems, the local charge influences reaction barriers.14 Lewis acid base pairs found in metal oxide clusters may also abstract hydrogen atoms by means of a proton-coupled electron transfer (PCET).12,

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Whereas a number of single-collision studies focus on methane

activation (using mostly gaseous single atom metal ions),16-22 such investigations under well-


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defined multi-collision conditions are scarcer.23-26 The latter experiments enable constant thermal equilibration of the reactants and the buffer gas, and observation of consecutive reactions, e.g. metal-catalyzed C–C coupling of two reactant molecules. The catalytic reaction of Au2+ with CH4 and O2 exemplifies this benefit, as the reaction product changes from ethylene to formaldehyde with decreasing temperature.24 Due to promising properties of tantalum clusters and the corresponding mono-oxides, uncovering and elucidating their reaction mechanisms promises to aid and accelerate the rational design of catalysts mediating C–H bond activation. Herein, we report a combined experimental and theoretical investigation of the reactions of [TaxOy]+ (x=4,5; y=0,1) with methane in an ion trap. Such combined studies of C–H activation on metal clusters are complicated due to the large isomerism and fluxionality of the clusters, as well as the abundance of alternative reaction pathways. We find that binding of the methane molecule to the tantalum clusters occurs through Pauli-repulsion-driven polarization of the electronic charge distribution in the metal cluster, induced by the closed-shell methane molecule. We show that the addition of a single oxygen atom to the investigated tantalum cluster cations brings about a rearrangements of the local electron distribution in the metal cluster, resulting in a significant increase in the dehydrogenation reaction rate; notably, the Ta5+ cluster which is found to be inert toward methane, converts to a potent CH-activating cluster when doped by oxygen to form Ta5O+. First-principles Born-Oppenheimer spin-density functional theory (SDFT) simulations reveal that the bridging oxygen atom rearranges the local electronic distribution in the doped metal cluster, but otherwise no direct binding between the μ2-oxo ligand and the methane molecules, or with any dehydrogenation intermediates or products, occurs at any stage of the reaction. This is in stark contrast to essentially all other oxide systems investigated previously, for which hydroxide


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moieties are formed via PCET/HAT mechanisms12 or in the course of the subsequent reaction steps.27 EXPERIMENTAL AND THEORETICAL METHODS Experimental setup. In brief, a modified Smalley-type laser vaporization source generates metal clusters using a 100 Hz ablation laser (532 nm, Innolas Spitlight DPSS) and a pulsed Piezo valve to introduce helium carrier gas (He 5.0, Westfalen).28 In order to produce mono-oxide metal clusters, oxygen (O2 5.5, Air Liquide) is synchronously pulsed into the expansion nozzle of the source by a general valve (Pulse Valve Series 9, Parker). The resulting cluster beam undergoes a supersonic expansion into the vacuum chamber, thereby quenching the clusters’ internal degrees of freedom. Clusters with small rotational and vibrational temperatures (of 5-120 K and 50-325 K, respectively) are typically generated in similar ablation sources.29-31 The cluster beam is guided by various ion optics into a quadrupole mass filter (Model 5221, Extrel) which selects ions of a single mass, while the remainder is suppressed. Generally, a size-selected cluster beam generated by a single shot of the ablation laser is collected (within ~5 ms) and stored for a specified reaction time in a homebuilt ring-electrode ion trap.32-33 When the clusters initially enter the ion trap, their excess kinetic energy is quickly dispersed via collisions with the buffer gas (He 6.0, Westfalen) and solely ions with a sufficiently low energy are successfully stored. A constant stream of buffer gas facilitates a total pressure of 0.77 Pa inside the trap, which causes approximately 100 collisions of the clusters with the buffer gas per millisecond. Thermalization of the clusters thus occurs within a few milliseconds, while the observed reactions proceed on a significantly larger time scale. Using a closed-circuit helium cryostat (RW 2, Leybold) connected to the ion trap in combination with a resistive-heating cartridge, the ion trap and subsequently the buffer gas is kept at a constant temperature of 300 K. As the buffer gas energy is described by a Maxwell-Boltzmann distribution


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and the ions are equilibrated with the buffer gas, the reaction conditions are isothermal and energy may be provided to overcome reaction barriers in excess of endothermicity.34 When a defined amount of methane (CH4 5.5, Riessner Gase) is added to the buffer gas, reaction products are formed. Charged reaction products are transferred into a homebuilt Wiley-McLaren type time-offlight mass spectrometer with a reflector stage (m/Δm≈2000), where they are analyzed. The time-dependent reaction behavior is investigated through systematic variation of the storage time. In order to describe the reaction progression by an underlying reaction scheme (see below), kinetic simulations are used and several reaction models are compared to achieve the best fit between the calculated and measured ion abundances as a function of reaction time. From this analysis, pseudo-first order apparent rate constants k(1) are extracted as fitting parameters of the kinetic simulation, as the reactions are found to be of first order with respect to the methane partial pressure (see Fig S1 and further details in the SI). In addition, we have made measurements with deuterated methane (Linde AG). These experiments were done in order to confirm the peak assignment, and to enable the determination of the kinetic isotope effect, which provides deeper insights into the reaction mechanisms (a more detailed description is given in the Supporting Information). Spin-density functional methodology. The theoretical explorations of the atomic arrangements and electronic structures of the tantalum clusters and their derivatives were performed with the use of the Born-Oppenheimer spin density-functional theory (SDFT) molecular dynamics (BO-SDFTMD) method35 with norm-conserving soft (scalar relativistic for Ta) pseudopotentials36 and the generalized gradient approximation (GGA)37 for electronic exchange and correlations (using the so-called PBE functional37). In these calculations we have used a plane-wave basis with a kinetic energy cut-off Ec = 62 Ry, which yields convergence. This corresponds to a real-space grid spacing


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of 0.4 a0 (Bohr radius); the real-space grid spacing for the density (and potential) was 0.133 a0 corresponding to Ec = 555 Ry. In the construction of the Ta pseudopotentials the valence electrons, 5d3 and 6s2, were characterized by core radii rc(s) = 2.55 a0 and rc(d) = 2.00 a0, with the s orbital treated as local, and (unoccupied) 6p0 core radius rc(p) = 3.00 a0. Additionally for Ta there is a non-linear core correction with the pseudo-core containing 10% of the [Xe]4f14 core. For the carbon atom pseudopotential the valence 2s2 and 2p2 electrons were treated with rc(s) = rc(p) = 1.45 a0, with the p orbital treated as local. The 1s electron of the H atoms was described by local pseudopotential with rc(s) = 0.95 a0. The oxygen 2s2 and 2p4 core radii are rc(s) = rc(p) = 1.45 a0 with p local. The BO-SDFT-MD method36 is particularly suitable for investigations of charged systems since it does not employ a supercell (i.e., no periodic replication of the ionic system is used). In all the calculations the dependence on spin multiplicity has been checked, and the results that we report correspond to the spin multiplicities with the lowest energies. In particular, it is pertinent to note here that in all our calculations the spin-degree of freedom is optimized and used in the computation, unless a particular spin configuration (spin multiplicity) is prescribed. At each step of the calculation the energy levels of the SDFT up-spin and down-spin manifolds in the vicinity of the Fermi level are examined, and the occupation is adjusted such that the spin-Kohn-Sham level with the lower energy eigenvalue gets occupied. When a particular spin configuration is desired, the corresponding numbers of electrons with up and down spins are kept fixed throughout the calculation -- that is, Ns electrons with spin s =  or , so that the spin excess is given by μ = N - N; to convert to a commonly used notation we note that the spin multiplicity is given by 2s+1 = 2 (μ/2) +1.


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The energy minimization to find the optimal cluster geometry was done with a steepest-descent method. The convergence criteria were that the maximum force magnitude on any particle is less than 0.0005 Hartree/Bohr and that the average over all particles is less than 0.00025 Hartree/Bohr. In some cases BO-SDF-MD simulations of typically a few picosecond duration at 300 K (that is, canonical, constant temperature, simulations, with stochastic thermalization) were used to ensure that the resulting optimal configurations were stable; a time-step of 0.25 fs was used in these simulations. Spin Isomers. We found for the Ta4+ and Ta4O+ clusters, both in the bare clusters and with adsorbed CH4, that the ground state always has the lowest possible spin, i.e. =1 ( is the difference between numbers of majority and minority spin electrons). The calculated energy differences of the two higher-spin isomers with respect to the ground state isomer are: E[Ta4+ (=3)] - E[Ta4+ (=1)] = 0.92 eV; E[Ta4+CH4 (=3)] - E[Ta4+CH4 (=1)] = 0.86 eV; E[(HTa4CH3)+ (=3)] - E[(HTa4CH3)+ (=1)] = 0.60 eV. The energy differences between spin isomers for the Ta5+ and Ta5O+ clusters are smaller and for Ta5O+CH4 the ground state changes from =2 to =0 after the first deprotonation as shown in the first-principles theoretically simulated reaction path profiles shown below. Reaction pathway calculations. In the first-principles SDFT calculations of the reaction profiles (shown below) the total energy of the system is calculated using the SDFT method as a function of a sequence of values of a judiciously chosen reaction coordinate; i.e., steered SDFT simulations. The reaction coordinate can be the distance between two atoms or a function involving several interatomic distances, angles, etc. For example, for the case of activation and dissociation of a C–H bond of a CH4 molecule adsorbed on TaxOy+ cluster (x=4,5 and y=0,1), that is TaxOyCH4+ →


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TaxOy(HdCH3)+, the reaction coordinate can be the distance between the activated bond length r(C–Hd), where Hd is the dissociating hydrogen atom, or the ratio between that bond length and the distance between Hd and chosen Ta* atom of the metal cluster, i.e. r(C–Hd) / r(Ta*–Hd). In our calculations we generally used the C–Hd bond for the first deprotonation and the above-noted distance-ratio choice for subsequent deprotonation. The H–H bond length was used for the barrier to the adsorbed H2 molecule. For each value of the reaction coordinate, the total energy of the system was optimized through unconstrained relaxation of all of the other degrees of freedom of the system; as aforementioned, the relaxation process includes optimization with respect to the spin degrees of freedom. The reaction profile (reaction path) was obtained via incrementing the reaction coordinate by small steps to find the local minima and barrier configurations. These calculations yield results that are the same as, or close to, those obtained by other methods, e.g., the nudged elastic band and variants thereof; see the discussion on pp. 89 and 90 in Reference 3838, were we also quote accuracy estimates. Bader charge analysis. The wave-functions resulting from the SDFT calculations were used in evaluation of the partial charges on the atoms of the cluster systems, using the Bader-charge analysis39-41 This analysis gives the partial charge associated with each of the atoms in the cluster complex, and allows one to estimate the gain or loss of charge on each atom in the cluster environment. RESULTS AND DISCUSSION Mass-selected beams of two bare tantalum clusters (Ta4+ and Ta5+) and their mono-oxides (Ta4O+ and Ta5O+) were stored and exposed under isothermal conditions to methane in the a ring-electrode ion trap32 at a constant total pressure of 0.77 Pa. At this pressure, collisions with the gas inside the


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trap lead to rapid thermalization of the stored ions. Time-of-flight mass spectra originating from the reaction with methane and kinetic simulations of [TaxOy]+ clusters exposed to a defined fraction of methane in the helium buffer gas are shown in Fig. 1.

Figure 1. Intensity distribution of reactants and products in the reaction of [TaxOy]+ (x=4,5; y=0,1) with CH4 at 300 K. The mass spectra (a,c,e) display: (a) Ta4+ species after 0.15 s, 0.02 Pa CH4, 0.75 Pa He ; (c) [Ta4O]+ species, after 0.07 s, 0.008 Pa CH4, 0.76 Pa He, and (e) [Ta5O]+ species after 0.3 s, 0.008 Pa CH4, 0.76 Pa He. The respective kinetic abundances as a function of time are shown to the right in (b), (d) and (f); experimental data given by the dots, and kinetic simulations by the solid lines. ■: [TaxOy]+, ●: [TaxOy(C,2H)]+, ▲: [TaxOy(C,2H)2]+, ♦: [TaxOy(C,2H)2(C,4H)]+, ◄: [TaxOy(C,2H)2(C,4H)2]+, □: [Ta5O(2C,2H)]+, ○: [Ta5O(2C,2H)(C,4H)]+. The simplest reaction scheme (as e.g. shown for Ta4+ in scheme 1), reproducing the experimental data best, is used to extract the corresponding rate constants. Note that this reaction model does not necessarily represent elementary steps (as described by microkinetics).


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Scheme 1. The reaction scheme of Ta4+ with methane obtained from the kinetic analysis. First, methane is hydrogenated in two consecutive, irreversible reaction steps, in which molecular hydrogen is released. After these, dehydrogenation stops and methane is formally adsorbed as a whole in two subsequent reaction steps, which are reversible. After the reaction with four methane molecules no further products are observed. Note that analogous reaction schemes apply for the reaction of Ta4O+ and the main reaction channel of Ta5O+ with methane (see Scheme S1). The apparent rate constant k1 associated with the first dehydrogenation step (yielding [Ta4CH2]+) and of k2 (corresponding to the second dehydrogenation step producing [Ta4(CH2)2]+), are given in connection with Table 1. A summary of all apparent rate constants is given in Table S1.

Ta4+ clusters are found to facilitate H2 elimination upon reacting with the first methane molecule and generate [Ta4(C,2H)]+ as an intermediate (Fig. 1a). This species is also capable of releasing H2 from a second (sequentially) adsorbed CH4 molecule, thereby forming [Ta4(C,2H)2]+. Subsequently, the reaction progresses via a consecutive slow adsorption of two additional CH4 molecules (see Fig. 1a-b). As H2 is not released in these (latter) steps, the reactions become reversible (see scheme 1). In contrast, Ta5+ clusters are found to be inert at room temperature (300K) towards dehydrogenation of methane (see Fig. S2). At a lower temperature (100K) we observed the adsorption of methane molecules to the cluster; compare Fig. S3a and Fig. S3b for time-of-flight data recorded at 300K and 100K, respectively). The comparison between the low and high temperature data reflects weak-binding of methane to the Ta5+ cluster (confirmed by the first-principles theoretical calculations discussed below, see also the theory part of the Supplementary Information). This weak molecular adsorption to the Ta5+ clusters leads at 300K to an adsorption/desorption equilibrium which is reflected in no adsorbed methane signal at the


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higher temperature. On the other hand, as discussed above for Ta4+, subsequent to molecular adsorption at 300K, activated dehydrogenation takes place, resulting in observation of a [Ta4 (C,2H)]+ (see Fig. 1a); with the same found for the oxygen-doped tetramer and pentamer tantalum cluster cations (see below). Indeed for the reactive clusters, molecularly adsorbed CH4 is also not detected at low temperatures, with dehydrogenation occurring faster than the experimental time scale. In conclusion, these observations suggest inhibition of the dehydrogenation of methane on Ta5+ due to an excessively high activation barrier for the dehydrogenation process (past the weak (molecular) adsorption step); this expectation is indeed confirmed by the microscopic simulations of the reaction path discussed below. Next we explore the reactivity of oxygen-doped Tax (x=4, 5) clusters. The reaction of [Ta4O]+ proceeds analogously to Ta4+, i.e. the corresponding reaction intermediates and products are formed, with the reaction progressing at a significantly faster rate compared to the undoped cluster case (see Fig. 1 c-d), and the apparent rate constant of the first reaction step is more than an order of magnitude larger (see Table 1). Oxygen-doping is found to activate the Ta pentamer cluster, with [Ta5O]+ showing the maximum rate constant of all reactions that we measured in the process of formation of the first intermediate, [Ta5O(2C,2H)]+. As displayed in Fig. 1e-f, this species is generated very rapidly and also mediates activation of a second CH4 via H2 evolution. In a parallel reaction channel, [Ta5O(C,2H)]+ reacts with CH4 by eliminating two H2 molecules, thus forming [Ta5O(2C,2H)]+. For all clusters, side reactions with possible residual gases in the vacuum chamber are ruled out as no products are formed when the clusters are stored for prolonged times without mixing methane into the buffer gas (see Fig. S2b-d). Furthermore, isotopic labeling with CD4 confirms the product assignment (Fig. 2a,c,e), and Table 1 displays experimentally-


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determined apparent rate constants for the corresponding reactions as well as the kinetic isotope effect (KIE).

Figure 2. Mass spectra (left) and time-dependent abundances (right) of reactants and products in the reaction of [TaxOy]+ (x=4,5; y=0,1) with CD4 at 300 K. Mass spectra display Ta4+ species (a, after 0.075 s, 0.05 Pa CD4, 0.72 Pa He), [Ta4O]+ species (c, after 0.075 s, 0.008 Pa CD4, 0.762 Pa He) and [Ta5O]+ species (e, after 0.5 s, 0.008 Pa CH4, 0.762 Pa He). The change of the corresponding ion abundances as a function of reaction time is shown on the right (b,d,f), with species of interest depicted as symbols and the results of the kinetic modeling displayed as solid lines. ■: [TaxOy]+, ●: [TaxOy(C,2D)]+, ▲: [TaxOy(C,2D)2]+, ♦: [TaxOy(C,2D)2(C,4D)]+, □: [TaxOyN2]+/[Ta5O(2C,2D)]+, ○: [TaxOyN2(C,2D)]+, Δ: [TaxOyN2(C,4D)]+, ♢ : [TaxOyN2(C,2D)(C,4D)]+.


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Table 1. Apparent (experimentally obtained) termolecular rate constants (see SI) in units of 10-26 cm6/s in the reaction of [TaxOy]+ (x=4,5; y=0,1) with CH4 and CD4 at 300 K as well as kinetic isotope effects (KIE) of the respective reaction steps. Uncertainties do not take deviations of the total pressure into account. The definition of the rate constants k1 and k2 are given in [a] and [b] at the bottom. CH4 Cluster Ta4+

k1[a]

CD4 k2[b]

k1[a]

KIE k2 [b]

k1[a]

k2 [b]

1.20 ± 0.24

1.02 ± 0.21

0.17 ± 0.03

0.62 ± 0.12

6.6 ± 2.0

1.5 ± 0.5

[Ta4O]+

17.92 ± 3.66

6.19 ± 1.32

1.72 ± 0.35

0.71 ± 0.15

9.4 ± 3.0

7.9 ± 2.6

[Ta5O]+

24.28 ± 4.99

1.08 ± 0.23

2.68 ± 0.55

0.16 ± 0.03

8.2 ± 2.6

6.2 ± 2.0

[a] k1([TaxOy]+→[TaxOy(C,2H)]+) or k1([TaxOy]+→[TaxOy(C,2D)]+), [b] k2([TaxOy(C,2H)]+→[TaxOy(C,2H)2]+) or k2([TaxOy(C,2D)]+→[TaxOy(C,2D)2]+).

The molecular structures of the reactants, and of the transitional intermediates, products, and microscopic reaction pathways, including transitional activation barriers, obtained from BornOppenheimer SDFT simulations (including scalar-relativistic effects for Ta),35, 42 are displayed in Figures 3 and 4; the ground state configuration of the Ta atom is [[Xe] 4f14 5d10 6s2 6p1] (where we list here the Xe-core and the outer angular-momentum levels for principal quantum numbers 4,5, and 6), resulting in excess spin in the ground state-configurations of the Tax+ (x=4,5) clusters. We found for the Ta4+ and [Ta4O]+ clusters, both in the bare clusters and with adsorbed CH4, the ground state always has the lowest possible spin, i.e. =1 (where μ = N - N is the difference between numbers of majority and minority spin electrons). The ground state structure of Ta4+ is a slightly Jahn-Teller-distorted tetrahedron (Fig. 3a) and that of Ta5+ is a square-base pyramid (see


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Fig. 3c, and Fig. S3). The equi-excess-spin-density isosurfaces displayed in Fig. 3a-d for Ta4+, [Ta4O]+, [Ta4(CH4)]+, and [Ta4O(CH4)]+, respectively, show a nearly uniform distribution of the excess spin spread over all the metal atoms (for a similar result for the non-vanishing excess spin Ta-pentamer cluster ions, see Fig. S5a-d), corresponding to a delocalized, itinerant, character of the excess spin in these clusters.

Figure 3. Ground-state configurations of bare (Ta4+) and oxygen-ligated ([Ta4O]+) clusters without (a,b), and with (c,d), an adsorbed CH4 molecule. Superimposed we display equi-excessspin-density isosurfaces illustrating the itinerant nature of the excess spin direction. The superimposed numbers next to the atoms are the calculated Bader electronic charges. Ta atoms are represented by blue spheres, O by a red sphere, carbon is depicted in green and hydrogen atoms are shown as small white spheres.

In the oxygen-doped clusters (see [Ta4O]+ and [Ta4O(CH4)]+, in Fig. 3b,d, respectively and Fig. S5b,d for the corresponding Ta pentamer cluster) we find some localized excess spin density on the doping oxygen atom. However, whereas terminal oxygen-centered radicals are capable of activating methane, for the bridging configuration of the μ2-oxo ligand in the Ta clusters discussed here, the HAT mechanism13 is found to be ineffective in the methane dehydrogenation reaction. The Bader charge distributions (see Fig.3 and SI) exhibit unequal values on the metal atoms, and


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the effect of an added oxygen atom (in [TaxO]+, x=4,5) is to concentrate negative charge on the oxygen atom, resulting in depletion of the (negative) electronic charge on the metal atoms. This results in redistribution of the Bader charges on the Ta atoms, with those directly bonded to the bridging O atom acquiring a larger positive charge. Methane molecular adsorption occurs on one of the Ta atoms and interestingly, the adsorbed CH4 molecule remains neutral with the charge on the Ta atoms directly bonded to it becoming positively polarized (see Fig. 3c-d). This reflects the nature of the induced polarization bonding mode, where the 8-electron closed-shell CH4 molecule repels charge from the anchoring metal atom site (Pauli repulsion); similar results are found for the Ta pentamer cluster ion (see Fig. S5a-d).

Figure 4. Calculated reaction pathways for adsorption and dehydrogenation of a methane molecule on bare [Tax+, x=4,5; panels (a) and (c), respectively] and oxygen-ligated ([TaxO]+, x=4,5; panels


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(b) and (d), respectively) clusters. The “n-ordering” of the reaction steps, given below the bottom axis of each panel, corresponds to the reaction scheme given at the top left of each panel. The transitional activation barriers (numbered, in green) are denoted by dotted horizontal lines, and local minima are denoted by solid horizontal lines. For the bare, and oxygen-doped, tantalum tetramer (panels (a) and (b)) μ = 1 throughout. For the tantalum pentamer the color of the horizontal line indicates the spin state (corresponding to the number of unpaired electrons (μ = N N) excess-spin, electrons; note the spin transition occurring in the first activation process in (c)). The relaxed atomic arrangement corresponding to each state of the reaction pathway is shown, with Ta atoms depicted as blue spheres, O as red spheres, C in grey, and H atoms depicted as small white spheres. ΔE and ΔE’ are the calculated energy and zpe-corrected thermochemical total energies for the reaction. The depicted energy levels correspond to non-zpe-corrected values. Negative values indicate exothermicity (that is lowering of the total energy upon completion of the reaction), and a positive value indicates an endothermic reaction. The values of the activation barriers and local minima in (a-d) are given for the bare and oxygen-doped tetramer Ta cluster in Figs S6 and S7, respectively; the corresponding information for the Ta-pentamer cluster is given in Figs S8 and S9.

The cluster-mediated methane dehydrogenation reaction pathways, calculated via steered firstprinciples simulations, show that CH4 readily adsorbs on both the bare and oxo-ligated clusters. The adsorption energies are: ΔEad (Ta4+; μ=1) = 0.24 eV (0.22 eV); ΔEad (Ta5+; μ=2) = 0.40 eV (0.36 eV); ΔEad (Ta4O+; μ=1) = 0.58 eV (0.54 eV); ΔEad (Ta5O+; μ=2) = 0.28 eV (0.27 eV), with the values in parenthesis giving the zero-point-energy (zpe)-corrected adsorption energies, and μ is the number of unpaired electrons (excess spin-up over spin-down electrons). Note that the pentamer cluster possesses several isomers with the energy of the cluster having μ=0 and 2 being nearly-degenerate, and the spin isomer with μ=4 having a significantly higher energy (0.80 eV). Upon room temperature (300K) molecular adsorption (illustrated for the case of Ta5+ at 100 K in Fig. S3b), the reaction proceeds through transition states corresponding to the sequential dissociation of the first and second C–H bond via “metal atom insertion” (transition states marked as 1 and 2, green in Fig. 4a-d). Scission of the first C–H bond leads to intermediates with the H and CH3 fragments bridging two adjacent Ta–Ta bonds (n=1). The electronic charge distribution


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in the cluster, which underwent significant change upon adsorption of the methane molecule, is found to undergo further notable rearrangement in the course of the above C-H bond activation (see Fig. S10). At the top of the transition state barrier (see dotted line marked underneath it as “1” in Fig. 4a), we observe that the electronic charge depletion on the Ta atom serving as the adsorption site increases from +0.72 to +1.04, whereas the electronic (negative) charge on the CH3 fragment and the dissociating H atom is found to be -0.32e and -0.33e, respectively. At the end of the first metal insertion process, the positive charge on the Ta atom binding the CH3 fragment decreases to +0.83, and the negative charge (excess electronic charge) on the CH3 intermediate and the bridging H atom (between the anchoring Ta atom and its nearest neighbor Ta atom) increases to -0.72e and -0.70e. The above illustrates that along with the cluster geometrical dynamical fluxionality (GDF, that is variation in the relative interatomic positions along the reaction pathways,43-44 the dehydrogenation process entails dynamic electronic fluxionality (GEF) freedom, which is expressed by the marked propensity of the local electronic charge distribution to undergo sizeable variations in conjunction with the nuclear displacements along the reaction pathway, enabling crossing of the C-H bond activation (dissociation) barrier in the cluster-methane complex. The same metal insertion mechanism is found for both the Ta4+ and [Ta4O]+ clusters (see Fig 4a,b). It is pertinent to remark here that for the case of the oxo-doped cluster, we find that the local minimum (see n=1 in Fig, 3b) is more stable (lower in energy) by 0.68 eV compared to the local minimum obtained through H atom transfer to the doping oxygen (see Fig. S11); for the clustermediated dehydrogenation of methane using [Ta5O]+, the first C-H dissociation step, with the H atom transferring to a bridging metal site (see n=1 in Fig. 4d), is favored even more (by 1.28eV) compared to the hydroxylation of the doping oxygen (see Fig. S11).


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For Ta4+ and [Ta4O]+, the subsequent cleavage of the second C–H bond generates a second intermediate including a Ta triangle with three Ta–Ta bonds respectively bridged by a methylene unit, the first hydrogen and the second hydrogen atom (n=2). Lastly, the hydrogen atoms combine on the Ta atom opposite to the CH2-bridged Ta–Ta bond to form H2 (transition state marked as 3 in Fig. 4) and molecular hydrogen is released from the cluster. In addition, the third and fourth C–H bonds may be cleaved (n=3 and 4), which requires participation of the fourth Ta atom and causes the generation of dihydrido carbides of the Ta4+ and [Ta4O]+ clusters. For Ta5+ and [Ta5O]+, methylene formation and H2 elimination require four Ta atoms to take part and an additional step of hydrogen atom migration (transition state marked as 3, whereas H2 formation is marked as 4). In the n=2 intermediate of the pentamer clusters, the Ta atom that initially acts as the CH4 adsorption site becomes bonded to all of the three (H,H,CH2) fragments, which bridge three individual Ta–Ta bonds. For [Ta5O]+, the system crosses from the triplet spin state of the adsorption complex (n=0) into the singlet state of the first transition state to circumvent the elevated activation barrier of the triplet state. In addition, cleavage of the third C–H bond potentially generates a [Ta5O]+ carbyne cluster. The calculated thermochemistry of the reactions uncovers for Ta4+; μ=1, [Ta4O]+; μ=1, and [Ta5O]+; μ=2, our DFT calculations yield an overall exothermicity of the zpe-corrected total energy difference ΔE’ between the total energy of the product cluster with a gaseous H2 molecule and the energy of the [TaxOy]+ + CH4 (gas) reactants (see Fig. 4a,b,d). The strongly exothermic formation of dihydrido-carbide (Ta4+) and carbyne structures ([Ta5O]+) is remarkable as a carbene structure is more common and usually assumed for most metals, with the exception of the Ir+ carbyne.7 In contrast with the results for the other tantalum cluster cations discussed here, for the methane reaction mediated by Ta5+ (with μ=2) the calculations predict that the reaction is inhibited


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(in agreement with the experimental findings) by an overall endothermic energy balance (see ΔE’> 0 in Fig. 3c). Furthermore, the transitional activation energy barrier for the first C–H dissociation process (marked as 1, green in Fig. 3) is larger (about a factor of two for the zpe-corrected activation-barrier values) for the Ta5+ cluster (ΔEb(1) (Ta5+) = 0.91 eV (0.73 eV)) compared to the other cluster systems; that is: ΔEb(1) (Ta4+) = 0.50 eV (0.38 eV), ΔEb(1) ([Ta4O]+) = 0.46 eV (0.32 eV), ΔEb(1) ([Ta5O]+) = 0.40 eV (0.35 eV), with the values in parenthesis giving the zpe-corrected values. Due to isothermal, multi-collision conditions, the buffer gas may supply external energy to overcome activation barriers in excess of endothermicity.34 This process enables the dehydrogenation e.g. mediated by Ta4+, but is prevented by the exceedingly high activation energy required in the reaction of Ta5+. As noted above, in addition to the initial electronic charge polarization of the clusters, cleaving the C–H bond involves a shift of electron density from the CH4 adsorption site to the adjacent Ta atoms and subsequently to the methyl and hydrogen fragments that bind to those atoms and become negatively charged (see Figs. 3a-d and S10). The magnitude of this reaction-promoting charge-density shift is smaller for the mono-oxide clusters. This suggests that the addition of a µ2-O ligand renders the clusters in a state of higher proclivity toward methane activation, with a consequent decrease in the C–H dissociation barriers of the adsorbed CH4 molecule. Interestingly, in the case of small cationic Pd clusters with methane, the reverse is found, with the addition of an O atom likely to increase the C-H bond activation barrier.25 Furthermore, the dissociated H atom is found to favor a cluster-site with excess electron charge, but contrary to other oxidic systems does not form a hydroxide on the mono-oxide clusters. So far, a similar role of oxygen atoms has only been observed during σ-bond metathesis in the reaction of [HMO]+ (M = Ti, Mo, Ta) with methane, in which scission of a single C–H bond leads to the formation of a M-CH3 bond and simultaneous loss of H2.45-47 Rather than bringing about a state of


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higher reactive propensity, however, the oxo ligand in those systems was found to affect the MCH3 and M-H bond strengths. For Ta4+ and [Ta5O+], the first C–H dissociation is the ratedetermining step, while the scission of the second C–H bond is associated with the highest barrier in the reaction of [Ta4O]+. These findings are in excellent agreement with the observed KIE in Table 1. KIEs of >2 are characteristic for a metal-mediated C–H bond scission in the ratedetermining step.12 Accordingly, similar and high kinetic isotopic effects are observed for all reactive clusters (see also Table 1), with values of ~ 8. The three reactive clusters discussed above (Ta4+; μ=1, [Ta4O]+; μ=1, and [Ta5O]+; μ=2) demonstrate similar reaction pathways in the subsequent consecutive reaction (with an additional side reaction for [Ta5O]+ attributed to its high reactivity, see Fig. 1e). After a second CH4 is adsorbed and dehydrogenated (eliminating H2), two additional methane molecules can (formally, that is, mass-wise) molecularly adsorb on the cluster. One may expect that this (exothermic) adsorption could provide energy that may enable the release of a neutral product molecular species, e.g., as observed for ethylene generation by Au2+ and Pd2+.26, 48 As this does not occur here, a strong binding of potential C2-3 hydrocarbons or separated methylene units to the tantalum clusters may be conjectured. To explore these scenarios, we investigated the dehydrogenation of a second methane molecule using first-principle DFT-based simulations. We illustrate our results in Fig. 5 for the case of Ta4+. Our simulations show that adsorption of methane to the Ta4+-methylene cluster is exothermic (adsorption energy of ΔE’ad([Ta4(C,2H)(CH4)]+) = 0.84 eV, see Fig. 5a(1), where ΔE’ denotes the zpe-corrected energy difference involved in this process), and the formation of the most stable dehydrogenation product, a dihydrido-carbide carbene cluster, [H2Ta4C(CH2)]+, is endothermic (thermochemically, entailing an energy of 0.43 eV, see Fig. 5b(3), but could become accessible


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under our isothermal conditions at 300°K . Additionally, forming an ethylene unit on Ta4+ is thermodynamically hindered at 300 K (ΔE’([Ta4(C2H4)]+ = 0.78 eV, see Fig. 5b(5)) and the adsorption of two additional methane molecules, albeit exothermic, cannot therefore induce the desorption of ethylene. These results suggest the formation of a Ta4+ dihydrido-carbide carbene cluster and the absence of C-C coupling (see Fig. 5b(4)).

Figure 5. Methane adsorption to the Ta4(C,2H)+ cluster and dehydrogenation channels. The energy difference ΔE= E([Ta4 nC (4n- 2m)H]+) + mE[H2] – (E[Ta4+] + nE[CH4]), and ΔE’ includes the zero-point energy correction. EB(CH4) is the binding energy to the cluster (adsorption energy) of the last adsorbed methane molecule, and EB’(CH4) is the zpe-corrected binding energy. (a1) the optimal (highest molecular CH4 adsorption energy to the Ta4(C,2H)+ cluster). m=1 in the above expression for ΔE. (a2) A higher-energy isomer of the cluster shown in (a1). (b3) the same as (a1), but with m=2, that is further elimination of an H2 molecule resulting in a dihydrido-carbide carbene cluster, [H2Ta4C(CH2)]+. (b4) A higher energy dicarbene isomer of the cluster shown in (b3). (b5) ethylene complex with the Ta4+ cluster; note the large endothermic (positive) value of ΔE’ corresponding to this channel, indicating that multiple metane adsorption is not likely to result in C-C coupling. (b6) an even higher carbide-methane isomer. Ta atoms are depicted by blue balls, C atoms are in black, and H atoms are represented by white balls. Some interatomic distances are marked in red.


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Further adsorption of methane to the tantalum tetramere cation (past the first two molecules) does not result in hydrogen elimination from the cluster complex (see Fig. 1a,c,e and Scheme 1). For the thermochemistry of the adsorption of a methane molecule to the dihydrido-carbide carbene tantalum tetramer (Figure 6a-b), we find first a complex with a bonded CH4 molecule (adsorption energy of 0.28 eV, Fig. 6a, top). A dehydrogenated tricarbene isomer (with two bridging H atoms) having a lower energy of 0.94 eV, is shown at the bottom of Fig. 6a. Formation of the latter complex is energetically favored over the elimination of H2. (compare Fig. 6b), in agreement with the experimental observations (Fig. 1a-f).


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Figure 6. Further adsorption of a methane molecule onto the Ta4+ dihydro-carbide carbene cluster (see figure 4 in the text). The displayed optimized configurations were obtained through DFT calculations and steepest-descend energy-structural minimization. As in Fig. 5, the energy difference ΔE= E([Ta4 nC (4n- 2m)H]+) + mE[H2] – (E[Ta4+] + nE[CH4]), and ΔE’ includes the zero-point energy correction. (a) Two isomers of the (Ta4C3H8)+ cluster, that result from the adsorption of a third CH4 molecule with no H2 elimination from the cluster. m=2 in the above expression for ΔE. (b) Two isomers of the (Ta4C3H8)+ cluster following H2 elimination. Here m=3 in the above expression for ΔE. Note that the energy difference with respect to the configurations given in (a) are endothermic, in particular starting from the lowest energy isomer in (a) (that is, the bottom one with ΔE’ = -1.63 eV) we observe that H2 elimination transition in a process involving any of the two configurations displayed in (b), requires a fairly sizeable energy cost, making such process unlikely. These results are in agreement with the experimental findings (see discussion in the text in connection with Figure 1), leading us to conclude that past the sequential adsorption of the first two methane molecules with sequential elimination of two H2 molecules, further adsorption of methane to the cluster would not lead to hydrogen elimination. Ta atoms in blue, C in black, and the H atoms are depicted as white spheres.

CONCLUSION To summarize, we highlight our main finding pertaining to the enhanced methane C–H bond activation by µ2-O ligated [Ta4O+] and [Ta5O+] compared to that of (active) bare Ta4+ cluster and the (inert) bare Ta5+ one, with the lack of reactivity of the latter caused by an endothermic thermochemical energy balance and prohibitively large activation energies. C–H activated scission is the rate-determining step of the cluster-catalyzed methane dehydrogenation reaction, as evidenced by a high kinetic isotope effect, and the corresponding barriers are decreased in the mono-oxide clusters. Bonding of the CH4 molecule is found to involve Pauli-repulsion induced polarization of the electronic charge of the metal cluster, with C–H activation accompanied by (negative) charging of the -CH3 intermediate, enabled by the dynamic electronic fluxionality of the cluster, in addition to the geometric one. Up to four Ta atoms are involved in the dehydrogenation reaction, whereas the additional oxygen atom does not take direct part in the reaction (that is, no hydroxyl specie is formed). The addition of the µ2-O ligand induces an


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electronic charge rearrangement in the metal cluster that appears to be favorable for C–H scission through a metal insertion mechanism, rather than the HAT process (found for various other delectron metals), which is rendered ineffective for the bridging μ2-oxo configuration of the doping oxygen. Such a fluxional doping-induced change in the electronic structure may also become possible by tuning the Lewis acidity of the support material in heterogeneous catalysis. Indeed such catalytic control has been demonstrated by us recently in the context of hydrogenation reactions of ethylene by small platinum clusters on amorphous silica (a-SiO2) supports of varying stoichiometries,49 as well as through work-function changes induced by the choice of a metal substrate underlying a molecularly thin a-SiO2 film that serves as a support for the catalyzing Pt clusters. The present study thus demonstrates that small metal clusters (here bare and μ2-oxo ligated small Ta clusters) may be used as building blocks for surface metalorganic chemistry. ASSOCIATED CONTENT Supporting Information. Description of the determination of rate constants and the measurements with CD4 for the confirmation of the peak assignment and the determination of the KIE. Further details about structures and reaction pathways obtained from theory. ACKNOWLEDGMENT The research at TUM has been financed through the DFG (project TS 232/2-1). The stay of TM at TUM has been supported by the Erasmus Mundus EASED programme (Grant 2012-5538/004001) coordinated by CentraleSupélec. The work at Georgia Tech (UL and RNB) was supported by the Air Force Office of Scientific Research under Award No. FA9550-15-1-0519. Calculations were carried out at the GATECH Center for Computational Materials Science.


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16. Albert, G.; Berg, C.; Beyer, M.; Achatz, U.; Joos, S.; Niedner-Schatteburg, G.; Bondybey, V. E., Methane Activation by Rhodium Cluster Argon Complexes. Chem. Phys. Lett. 1997, 268 (3-4), 235-241. 17. Achatz, U.; Berg, C.; Joos, S.; Fox, B. S.; Beyer, M. K.; Niedner-Schatteburg, G.; Bondybey, V. E., Methane Activation by Platinum Cluster Ions in the Gas Phase: Effects of Cluster Charge on the Pt-4 Tetramer. Chem. Phys. Lett. 2000, 320 (1-2), 53-58. 18. Liyanage, R.; Zhang, X. G.; Armentrout, P. B., Activation of Methane by Size-Selected Iron Cluster Cations, Fen+ (n=2-15): Cluster-CHx (x=0-3) Bond Energies and Reaction Mechanisms. J. Chem. Phys. 2001, 115 (21), 9747-9763. 19. Liu, F. Y.; Zhang, X. G.; Liyanage, R.; Armentrout, P. B., Methane Activation by Nickel Cluster Cations, Nin+ (n=2-16): Reaction Mechanisms and Thermochemistry of Cluster-CHx (x=0-3) Complexes. J. Chem. Phys. 2004, 121 (22), 10976-10990. 20. Adlhart, C.; Uggerud, E., Reactions of Platinum Clusters Ptn+/-, n=1-21, with CH4: to React or not to React. Chem. Commun. 2006, (24), 2581-2582. 21. Citir, M.; Liu, F.; Armentrout, P. B., Methane Activation by Cobalt Cluster Cations, Con+ (n=2-16): Reaction Mechanisms and Thermochemistry of Cluster-CHx (x=0-3) Complexes. J. Chem. Phys. 2009, 130 (5). 22. Harding, D. J.; Fielicke, A., Platinum Group Metal Clusters: From Gas-Phase Structures and Reactivities towards Model Catalysts. Chem. Eur. J. 2014, 20 (12), 3258-3267. 23. Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Landman, U., Methane Activation and Catalytic Ethylene Formation on Free Au2+. Angew. Chem. Int. Ed. 2010, 49 (5), 980-983. 24. Lang, S. M.; Bernhardt, T. M., Methane Activation and Partial Oxidation on Free Gold and Palladium Clusters: Mechanistic Insights into Cooperative and Highly Selective Cluster Catalysis. Faraday Discuss 2011, 152, 337-351. 25. Lang, S. M.; Frank, A.; Bernhardt, T. M., Activation and Catalytic Dehydrogenation of Methane on Small Pdx+ and PdxO+ Clusters. J. Phys. Chem. C 2013, 117 (19), 9791-9800. 26. Lang, S. M.; Frank, A.; Bernhardt, T. M., Composition and Size Dependent Methane Dehydrogenation on Binary Gold-Palladium Clusters. Int J Mass Spectrom 2013, 354, 365-371. 27. Zhao, Y. X.; Li, Z. Y.; Yuan, Z.; Li, X. N.; He, S. G., Thermal Methane Conversion to Formaldehyde Promoted by Single Platinum Atoms in PtAl2O4- Cluster Anions. Angew. Chem. Int. Ed. 2014, 53 (36), 9482-9486. 28. Heiz, U.; Vanolli, F.; Trento, L.; Schneider, W. D., Chemical Reactivity of Size-Selected Supported Clusters: An Experimental Setup. Rev. Sci. Instr. 1997, 68 (5), 1986-1994. 29. Gole, J. L.; English, J. H.; Bondybey, V. E., Laser Spectroscopy of Cooled MetalClusters - Copper Dimer. J. Phys. Chem. 1982, 86 (14), 2560-2563. 30. Powers, D. E.; Hansen, S. G.; Geusic, M. E.; Puiu, A. C.; Hopkins, J. B.; Dietz, T. G.; Duncan, M. A.; Langridgesmith, P. R. R.; Smalley, R. E., Supersonic Metal Cluster Beams Laser Photo-Ionization Studies of Cu2. J. Phys. Chem. 1982, 86 (14), 2556-2560. 31. Hopkins, J. B.; Langridgesmith, P. R. R.; Morse, M. D.; Smalley, R. E., Supersonic Metal Cluster Beams of Refractory-Metals - Spectral Investigations of Ultracold Mo2. J. Chem. Phys. 1983, 78 (4), 1627-1637. 32. Neuwirth, D.; Eckhard, J. F.; Lange, K.; Visser, B.; Wiedemann, M.; Schröter, R.; Tschurl, M.; Heiz, U., Using Controlled Ion Extraction to Combine a Ring Electrode Trap with a Reflectron Time-of-Flight Mass Spectrometer. Int. J. Mass. Spectrom. 2015, 387, 8-15.


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TOC GRAPHICS


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The Thermal Dehydrogenation of Methane Enhanced by Âµ2-Oxo

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