Lowering the C-H Bond Activation Barrier of Methane Using SAC@Cu(111): A Periodic DFT Investigations

Methane has long captured the world's spotlight for being the simplest and yet one of the most notorious hydrocarbon. Exploring its potential to be converted into valueadded products has raised a compelling interest. In the present work, we have studied the e ciency of Single-Atom Catalysts (SACs) for methane activation employing Density Functional Theory (DFT). The Climbing Image-Nudged Elastic Bond (CI-NEB) method is used in tandem with the Improved Dimer (ID) method to determine the minimum energy pathway for the rst C-H bond dissociation of methane. Our study reported that the transition-metal doped Cu(111) surfaces enhance adsorption, activate C-H bond, and reduce activation barrier for rst C-H bond cleavage of methane. The results suggest Ru/Co/Rh doped Cu(111) as promising candidates for methane activation with minimal activation barrier and less endothermic reaction. For these SACs, the calculated activation barriers for rst C-H bond cleavage are 0.17 eV, 0.24 eV, and


Introduction
Earth is home to rich reserves of methane, making it an attractive feedstock and a foremost competitor for the production of green fuel. 1 The myriad occurrence of methane can be attributed to its stable nature. Higher symmetry (Td) along with close shell electronic conguration, wide HOMO-LUMO gap (≈ 8.9 eV), and four stable C-H bonds (E bd ≈ 4.5 eV), makes it thermodynamically and kinetically stable at room temperature. Given the signicant mass percentage of hydrogen (25.13%) in methane, it is widely used for hydrogen production. 2 Steam Methane Reforming (SMR) and Fischer-Tropsch (FT) synthesis are widely used processes for conversion of methane to hydrogen as well as to value-added products like methanol, formic acid, formaldehyde, higher hydrocarbons, and FT fuels. However, stability of methane makes it resistant to electrophilic and nucleophilic attacks. For most methane conversion reactions, the rst C-H bond cleavage is the primary and the rate-limiting step.
This motivates the rational design of a catalyst aiming at reducing the activation barrier for C-H bond dissociation.
Planar and stepped nickel surfaces are go-to catalysts used industrially in the aforementioned processes, primarily because Ni is procured at cheap rates and portrays excellent reactivity towards methane. The drawback of Ni catalysts is that they can completely dehydrogenate methane into carbon and hydrogen, which causes coking. Nullifying or preventing coking in methane conversion reactions is challenging area of current research. 3,4 The literature cites multiple studies in both theoretical and experimental verticals across various catalyst classes discussing methane activation. For example, it has been investigated on supported metal clusters, 5 noble metal surfaces, 69 mixed-metal alloys, 10 zeolites, 11 metalorganic frameworks(MOFs), 12 metal oxides, 1315 perovskite, 16 and supported single-atom catalysts (SACs) 17 to name a few. Lately, SACs have emerged as a focal point in active research surrounding methane activation. 13,16,18 Doping the base metal with a single atom increases the number of active sites on the surface, thus reducing the use of precious metals as traditional catalysts. SACs, as an interesting class of catalyst subtly modify the electronic structure of the base metal and the dopant itself. This unique alteration of properties is a consequence of changing the atomic environment and dopant-doped interaction. Pt doped rutile TiO 2 (110) catalyst ( E ads = -0.62 eV, E act = 0.15 eV) 13 and Ag-doped CeO 2 (100)(E ads = -1.01 eV, E act = 0.21 eV) 14 have shown considerable activity in terms of bondlength activation, adsorption energy, and activation barrier. IrO 2 (110) activates methane at low temperature, 19 and IrO 2 nanoparticles activate it at temperatures as low as 110 0 C. 20

Computational Details
Kohn-Sham Formalism of Density Functional Theory (DFT) is employed to carry out all the calculations. Projector Augmented Wave potential is used, 21,22 with Perdew-Burke-Ernzerhof (PBE) 23 approximation for the exchange-correlation and generalized gradient approximation 24 as implemented in plane-wave, pseudopotential based code, Vienna Ab-initio Simulation Package (VASP). 2527 Within our framework, the calculated value of the lattice constant for Cu is found to be 3.62 Å which is in agreement with the experimental value of 3.61 Å. 28 Atomic Simulation Environment (ASE) 29 is used to cleave Cu(111) surface. We substituted one of the Cu surface atom with the dopant to model the SACs under investigation. We used a 3x3x4 supercell with a 5x5x1 Monkhorst Pack grid resulting into 13 K-points in the IBZ for primary screening of twenty-two SACs towards methane adsorption.
The k-points convergence exercise was carried out by increasing the Monkhorst-Pack grid for each system. It was observed that the dierence in energies was less than 4 meV/atom for every system. Ten potential candidates were then investigated for rst C-H bond cleavage, in a 4x4x4 supercell with a 3x3x1 Monkhorst Pack grid. Our calculations reported that a change in system setup is not associated with any observable changes in adsorption energies and bondlength activation. 24 Å of vacuum is found to be sucient to avoid interaction between adjacent images of planes along the z-direction. The criteria of a force cuto of 0.01 eV/Å on the unxed atoms and the total energy convergence below 10 −5 eV for each SCF cycle are employed for geometry optimization. The Van der Waals corrections are applied to all the calculations. The adsorption energy (E ads ) is calculated as, E ads = E slab+methane -(E slab + E methane ); where E slab+methane is the energy of the system when methane is placed on the slab, E slab is the energy of the bare slab, E methane is the energy of the methane molecule.     Table 1. Considering Metal-C bondlength and accompanied adsorption energies, we dene physisorption at the energy range of -0.10 eV to -0.35 eV and chemisorption at an energy range from -0.35 eV and below (as shown in SI Fig. 1 (A and   B)).
Adsorption energy decreases across the period with increasing metal-carbon bondlength as depicted in SI Fig. 1  Interaction between methane and SACs describing the charge transfer from the surface to methane molecule as reported in SI Tab. 2, could be understood from pDOS plotted with respect to vacuum and shown in Fig. 4. For isolated methane, C(2p) peak is sharp with the highest intensity. C(2p) peak of physisorbed methane is same in nature but with reduced intensity. C(2p) peak of chemisorbed methane is shown for two cases, the most activated one (Ru/Cu(111)) and the most stable one(Mo/Cu(111)). Not only the peak intensity decreases but also the broadening of the peak along with a secondary peak is observed in both the cases as evident from Fig.4. In addition, our study revealed that the mid-transition doped SACs show more C-H bond activation. tion of about 3% -5% were selected to study dissociative adsorption (DA) of methane.
The dissociative adsorption of methane (CH 3 + H) on Cu(111) surface is found to be thermodynamically unfavorable, whereas it is favorable for the SACs. For all the cases studies, dissociation of methane is an endothermic phenomenon, and the values are reported in Table   1. We observed that the lowest activation barrier is accompanied with the most elongated C-H bond. When methane adsorbed strongly, the activation barrier is higher as a result of the formation of a stable conguration (C-M-H sigma complex) as shown in Fig.4

Conicts of Interests
There are no conicts to declare.