Room Temperature Methane Capture and Activation by Ni Clusters Supported on TiC(001): Effects of Metal-Carbide Interactions on the Cleavage of the C-H Bond.

Methane is an extremely stable molecule, a major component of natural gas, and also one of the most potent greenhouse gases contributing to global warming. Consequently, the capture and activation of methane is a challenging and intensively studied topic. A major research goal is to find systems that can activate methane, even at low temperatures. Here, combining ultrahigh vacuum catalytic experiments, X-ray photoemission spectra, and accurate density functional theory (DFT) based calculations, we show that small Ni clusters dispersed on the (001) surface of TiC are able to capture and dissociate methane at room temperature. Our DFT calculations reveal that two-dimensional Ni clusters are responsible for this chemical transformation, confirming that the lability of the supported clusters appears to be a critical aspect in the strong adsorption of methane. A small energy barrier of 0.18 eV is predicted for CH4 dissociation into adsorbed methyl and atomic hydrogen species. In addition, the calculated reaction free energy profile at 300 K and 1 atm of CH4 shows no effective energy barriers in the system. Comparison with other reported systems which activate methane at room temperature, including oxide and zeolite-based materials, indicates that a different chemistry takes place on our metal/carbide system. The discovery of a carbide-based surface able to activate methane at low temperatures paves the road for the design of new types of catalysts which can efficiently convert this hydrocarbon into other added-value chemicals, with implications in climate change mitigation.


INTRODUCTION
Although carbon dioxide (CO 2 ) is the main greenhouse gas, in terms of its overall contribution to global warming, methane (CH 4 ), the simplest, most abundant and stable alkane molecule, immediately follows. 1 Even though CH 4 emissions are five times smaller than CO 2 , its greenhouse effect is 23 times larger, and, therefore, its contribution to climate change comparable to that of CO 2 . The Earth's atmospheric methane content has oscillated between 350 and 800 volumetric parts per billion (ppb) during the last glacial and interglacial periods, 2 with main release sources being wetlands, living organisms, and permafrost melting processes. 3 However, since the industrial revolution, CH 4 levels have risen to the present value of 1770 ppb, 4,5 mostly from anthropogenic sources, including unsustainable landfills, livestock farming, and fossil fuels used for energy production and transportation. [6][7][8] Different combined strategies oriented towards reducing the content of greenhouse gases in the atmosphere include: reducing emissions, more efficient use of fossil fuel sources of energy, using alternative renewable, green sources of energy. A particularly appealing strategy is to use carbon capture and storage (CCS) technologies 9 where appropriate, specific, materials are used as CO 2 or CH 4 scrubbers. Even more interesting are carbon capture and usage (CCU) technologies, where the corresponding molecules, once adsorbed, are converted, ideally catalysed by the same substrate material, into other valuable chemicals. 10 In the case of captured methane this is a real challenge because of the high 4.5 eV C-H bond strength, the absence of low-energy empty orbitals, and the presence of high energy occupied orbitals. 7 An ideal transformation process would involve the production of synthesis gas (syngas for short) -a gas stream mixing carbon monoxide (CO) and hydrogen (H 2 ). This syngas can be used as feedstock for Fischer-Tropsch synthesis, after H 2 enrichment or diminution, to obtain long-chain hydrocarbons. 11 The syngas stream formation can pass through different reactions, including methane dry reforming (Eq. 1), [12][13][14] methane steam reforming (Eq. 2), 11,15,16 or methane partial oxidation (Eq. 3). [17][18][19] (1) The common limiting steps for such processes are, first, methane adsorption, and, second, its subsequent activation. These steps are intimately related to the first hydrogen abstraction from CH 4 , which is regarded as the rate determining step of the overall mechanism. [20][21][22] In addition, methane also can be transformed directly into commodity chemicals such as methanol, ethylene, or benzene. 7 An optimal catalyst for CH 4 capture and conversion would be one that simultaneously meets two criteria, namely: i) it must adsorb CH 4 ⎯ideally even at room temperature, and ii) it must feature a low enough energy barrier for C-H bond scission ⎯preferably to be overcome at room temperature as well. On a reaction free energy profile, these criteria would imply that the first dehydrogenation reaction step transition state would be lower than the molecular desorbed state.
There is an ongoing search for materials that can accomplish the activation of methane at low temperatures to enable the direct conversion of CH x fragments into commodity chemicals. 7 Most of the studies dealing with the activation of methane have focussed their attention on metals, bulk oxides, homo-and hetero-nuclear oxide clusters, zeolites, and also on biomimetic approaches. 7,21-25 On metal surfaces, methane binds weakly and the probability for dissociation is low. 26  activation under mild conditions.

EXPERIMENTAL DETAILS
The reactivity of Ni/TiC(001) surfaces towards methane was studied in a system which combines an ultrahigh-vacuum (UHV) chamber (base pressure ~ 7·10 -10 mbar) and a batch reactor. [33][34][35][36] Within this system, the sample could be transferred between the reactor and UHV chamber without exposure to air. The UHV chamber was equipped with instrumentation for X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), low-energy electron diffraction (LEED), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD).
The TiC(001) single crystal was cleaned following methodologies reported in the literature. 37,37 Nickel was vapour-deposited on the TiC(001) surface at 300 K. 37 The admetal doser consisted of a resistively heated tungsten basket with a drop of ultrapure Ni inside. 37 Initially the flux of the doser was calibrated by taken thermal desorption spectra for the desorption of Ni from a Mo(100) substrate. 33,38 This information was then used to calibrate admetal coverages estimated by means of XPS. 33 In the tests of methane activation, the Ni/TiC(001) samples were transferred to the reactor at ~ 300 K, then 1.33 mbar (1 Torr) of methane was introduced for a period of five minutes. After this exposure, the methane gas was removed and each sample was transferred back to the UHV chamber for surface characterization. Maximum activity was found for small coverages, 0.2-0.3 monolayers (ML), of nickel on the carbide substrate.  Figure 1), and a Ni supported nanorod model (see Figure 2) featuring low coordinated sites were investigated. These particular Ni clusters sizes have been selected as they feature a compact packaging with high symmetry, which makes them, a priori, very stable, thus maximizing the atomic coordination. The calculations employed the Perdew-Burke-Ernzerhof (PBE) 42 exchange-correlation (xc) functional, and the contribution of dispersion (vdW) terms was included through the D3 correction as proposed by Grimme (PBE-D3). 43 For the systems containing Ni NCs, as well as for gas phase CH 3 and H (see below), spin polarization was taken explicitly into account. The transition states (TS) were located using the climbing-image NEB (CI-NEB) method. 44 Initial guesses for the five employed intermediate images were generated by means of the Atomic Simulation Environment (ASE) 45 using the Image Dependent Pair Potential (IDPP). 46 The energy reaction profiles were obtained at 0 K and in vacuum but also at working conditions of temperature and CH 4 partial pressure by using the atomistic thermodynamics approach. 47 All calculations were carried out using the Vienna Ab Initio Simulation Package (VASP) code. 48

RESULTS AND DISCUSSION
Experiments. In our experiments, surfaces of Ni/TiC(100), TiC(001) and Ni(111) were exposed to methane in a batch reactor. Figure 3 shows C 1s XPS spectra collected after dosing to methane a TiC(001) surface pre-covered with 0. produces a peak at ~284 eV that can be attributed to adsorbed CH 3 or CH 2 species. 32 .
We estimate that 0.2-0.3 ML of CH x are deposited on the surface as a consequence of dissociation of methane on the Ni particles or the nickel-carbide interface. The ratio of CH x species to Ni adatoms was in the range of 1-1.5. Figure  We also found that the amount of Ni present on the carbide surface had a strong effect on the reactivity of the surface towards methane. Figure 5 displays the change of the CH x intensity (peak at ~284 eV) as a function of Ni coverage in Ni/TiC(001). In these experiments, all the surfaces were exposed to 1 Torr of methane for 5 minutes at 300 K. A maximum of activity is seen at Ni coverages of 0.2-0.3 ML. Under these conditions, small particles of nickel interact strongly with the TiC substrate. 33 An increase in the Ni coverage increases the size of the Ni particles, favouring Ni-Ni interactions over Ni-TiC interactions, and the admetal nanoparticles slowly acquire the chemical properties of bulk nickel losing their reactivity towards methane. At large coverages of nickel, the dissociation of methane was negligible as seen on plain Ni(111). 21 Temperature had a strong effect on the stability of the adsorbed CH x groups. In the XPS experiments, we observed the gradual disappearance of the CH x species when the temperature was raised from 320 to 420 K, see Figure 6. Using a mass spectrometer we detected mainly the evolution of CH 4 from the CH x /Ni/TiC(001) surface between 340 and 410 K, with weaker signals for C 2 H 6 and C 2 H 4 (see Figure 7) . Thus, it is clear    Both, TiC(001) and Ni(111) are able to adsorb methane but both feature a high C-H bond breaking energy barrier well above that of the desorption limit, thus inhibiting any further reaction as inferred from the energy (E) and Gibbs free energy (G) profiles in Figure 8.  on the vertices of the clusters, see Figure S4), rather than on the TiC substrate near the Ni cluster boundary region. Furthermore, CH 4 adsorption on the 3D supported Ni 13 cluster is noticeably weaker than for the planar 2D Ni 4 and Ni 9 cases. As the size of the Ni cluster increases, its reactivity towards CH 4 and CH 3 decreases, in agreement with the experimental trend seen in Figure 5. The highest reactivity is found on the transfer. This is in agreement with experimental and computational evidence confirming that the enhanced activity/reactivity is larger for small flat metal clusters, and tends to vanish in larger, three-dimensional metal clusters and nanoparticles. 53 The CH 4 dissociation on Ni 4 /TiC appears to be facilitated when starting from a vertex site with CH 3 +H going to bridge sites with a calculated E b of 0.18 eV only (see TS1 in Figure 9); a drastic reduction when compared to the case of Ni(111) implying an   Figure S11). On this Ni 9 /TiC system, E b is just 0.25 eV, only 0.07 eV higher than corresponding value for Ni 4 /TiC (0.18 eV). Interestingly, this energy barrier is still much lower than the -0.64 eV CH 4 adsorption energy on Ni 9 /TiC (Table 1).
Finally, on the largest Ni 13 supported cluster, CH 4 and the CH 3 preferentially adsorb on a vertex of the cluster, and the released H adatom adsorbs on a three-fold hollow site of small (111) facets of the supported cluster, as shown in Figure S12. Here  Table 2. Clearly, only 2D Ni clusters supported on TiC exhibit E ads values larger than E b energy barriers. This conclusion is further supported from total energy and Gibbs free energy profiles in Figure 9; the latter corresponding to 300 K and 1 atm (1.01325 bar) of CH 4 partial pressure. The Gibbs free energy profiles indicate that on all the studied Ni n /TiC clusters both adsorption and dissociation processes are exergonic; contrary to the situation corresponding to Ni(111). Moreover, the smaller Ni 4 /TiC and Ni 9 /TiC clusters present no effective free energy barrier for CH 4 dissociation at normal conditions, confirming that small Ni particles are particularly active to trigger CH 4 dissociation at room temperature, in agreement with the experimental findings. Note also that room temperature is actually the temperature where the highest conversion is achievable, given that a rise in temperature would only place the effective energy barrier above the reactant energy limit, in accordance with experiment.   (Table 1), yet highlighting the importance of low coordinated Ni atoms on methane adsorption as well as the effect of the support. Note, however, that the E ads value for the supported nanorod is larger than that computed for the Ni 13 /TiC cluster (-0.36 eV), which is attributed to the above-stated lability of the Ni rod compared to the rigidity of the Ni 13 /TiC cluster.
However, the important aspect is that, on the supported nanorod, the CH 4 dissociation energy barrier is 0.64 eV, much higher than the corresponding values for the finite supported clusters, and also higher than the desorption energy. The latter kinetically inhibits CH 4 dissociation, with an even more unfavourable free energy profile at working conditions of temperature and pressure (see Figure 11).   Table S2 and, for convenience, do not include the ZPE term.
At this point one may wonder whether the chemistry governing C-H scission in these systems bears some similarity to that exhibited by oxides and zeolites, some of which are able to dissociate CH 4 . In a recent paper, Latimer et al. 21 where !!!"#$ is the raw DFT energy of the slab with adsorbate , !"#$ is the raw DFT energy of the slab, ! is the number of atomic species in , and ! is the reference energy of that atomic species with the reference energies ! for each atomic species defined as where again !(!) is a "raw" energy directly taken from the DFT based calculation. We note that one interesting feature of the formation energy approach (as opposed to the use of pure DFT energies) is that it does not distinguish between thermodynamic minima (adsorbed states) and saddle points (transition-states). Thus, it is possible to define the formation energy of the CH 3 -H dissociation transition state, !" ! , as !" which is related to the DFT calculated energy barrier for CH 3 -H dissociation, ! , as The analysis of the !" ! versus ! ! plots ( Figure S15) shows that while the main trend in the formation energy of the TS is well captured by hydrogen affinity ! ! descriptor, there is a considerable deviation from linearity implying that the chemical mechanism for CH 4 activation by Ni/TiC is different and does not involve the same type of radical like TS. Interestingly, plotting the natural logarithm of the raw DFT energy barrier with respect to the adsorbed hydrogen formation energy results in an almost perfect straight line as shown in Figure 13, indicating a somehow different chemistry. In the case of low-temperature methane dissociation on IrO 2 (110) 31   Our results for a range of Ni/TiC systems, including extended Ni(111) surface as a limiting case, show that a BEP relationship holds for the C-H scission. This indicates that the interaction between the metal and the underlying carbide support stabilizes the dissociation products with a concomitant decrease of the energy barrier involved in this elementary step. Moreover, it is found that the adsorbed hydrogen formation energy ! ! constitutes a very good descriptor for methane activation on these Ni-based catalysts. However, ! ! appears to be directly related to the logarithm of the raw energy barrier E b for CH 4 dissociation and not to the TS formation energy ( !" ! ) as in the case of oxides and zeolites 21 implying that a different chemistry is involved.
The present results open the way for the preparation of a new family of active materials for methane activation and conversion under mild conditions, thus widening the applications of existing natural gas resources.

CONFLICTS OF INTEREST
The authors declare no conflict of interest.

Supporting Information
The following Supporting Information is available free of charge on the ACS