Single-atom Pd in ZSM-5 for selective oxidation of methane to methanol: a DFT-based ONIOM approach

Single-atom catalysts are emerging as promising candidates for the selective oxidation of methane, providing distinctive opportunities to improve activity, selectivity, and atomic efficiency. In this study, we have applied the DFT-D3 based ONIOM approach to examine the potential energy surfaces of Pd single atoms, with oxidation states of 0, +1, and +2, enclosed within the ZSM-5 framework for the partial oxidation of methane to methanol employing N₂O as the oxidant. We have incorporated dispersion correction using Grimme's GD3 dispersion correction to account for the van der Waals interaction within the zeolite's framework. The isomorphous substitution of the framework Si atoms with the Al atoms caused the zeolites to have negative charges, which are balanced by metal cations or protons. Depending on the oxidation states of the Pd atom, we have substituted Si atoms with an equivalent number of Al atoms to maintain the overall charge neutrality of the ZSM-5 framework. The mechanism of the reaction mainly consists of three key steps: formation of the active site by N₂O, activation of the C-H bond of methane and recombination of the CH₃ and OH moieties to form methanol. Two-state reactivity has also been considered for the catalyst with close lying spin states. Furthermore, the inclusion of water during methanol desorption significantly reduces its desorption energy, thereby preventing the risk of over-oxidation and increasing the selectivity. In terms of activation barrier, kinetic analysis and methanol desorption energies, siliceous ZSM-5 with Pd⁰ is found to be the most favorable choice for partial oxidation of methane to methanol. Our findings offer significant insights into the oxidation state dependent reactivity of single atom Pd/ZSM-5 for direct conversion of methane to methanol.