Abstract:

The activation of methane (CH₄) is a key step in its conversion to more valuable products. The activation mechanisms of CH₄ on catalyst surfaces have been widely studied using gas-phase cluster models, which can be operated on systems with a precise number of atoms and determined structures. Herein, we have used MV₃O_y^q (M = Au/Ag, y = 6–8, q = 0 or ±1) clusters, in which a single Au or Ag atom was supported on vanadium oxide clusters, as simple models to mimic the properties of newly developed single-atom catalysts. The adsorption and activation of CH₄ on these MV₃O_y^q clusters were systematically studied via density functional theory calculations at the B3LYP/Def2-TZVP level, which provided insights into the geometric structures, adsorption energies, and charge distributions of the adsorption systems. Five Au-containing clusters, AuV₃O₆, AuV₃O₇, AuV₃O₈, AuV₃O₆⁺, and AuV₃O₇⁺, were able to activate CH₄, while other clusters, including all Ag-containing clusters, were inert. In the active clusters, all Au atoms were adsorbed on the O-atom sites of the supporting V₃O_y^q cluster and served as the active sites for CH₄ activation. The activation of CH₄ was characterized by the lengthened C―H bond (approximately 115 pm), short distances between CH₄ and Au (approximately 184 pm), relatively high adsorption energies of CH₄ (~0.590–1.145 eV), and significant electron transfer from CH₄ to the clusters (above 0.08e). In particular, AuV₃O₈, which is a neutral cluster with a close-shell electronic state, can activate CH₄ with a C―H bond length of 115 pm, Au―H bond length of 183 pm, the adsorption energy of CH₄ of 0.853 eV, and the charge on CH₄ of +0.088e. The charge state of the cluster has a significant effect on the activation ability: cationic clusters are the most active, followed by neutral clusters, while anionic clusters have the lowest activities toward CH₄. Consistently, the local charge on the M atom has a positive correction with the activation ability of MV₃O_y^q clusters with a certain M. However, as compared to Au-containing clusters, Ag-containing clusters have lower activities despite the higher local charges on Ag in each MV₃O_y^q cluster. The results indicate that the inclusion of D3 dispersion correction has a small effect on structures and energies. This study may serve as a foundation for further research on the activation of CH₄ on single-atom catalysts and provides useful information on rational designing of single-atom catalysts for CH₄ conversion at low temperatures.

Key words: Methane activation, Metal oxide clusters, Noble metal, Single-atom catalysis, Density functional theory