Activation of Methane by Rhodium Clusters on a Model Support C₂₀H₁₀

Supported metals represent an important family of catalysts for the transformation of the most stable alkane, methane, under mild conditions. Here, using state-of-the-art mass spectrometry coupled with a newly designed double ion trap reactor that can run at high temperatures, we successfully immobilize a series of Rh^- (n = 4-8) cluster anions on a model support C₂₀H₁₀. Reactivity measurements at room temperature identify a significantly enhanced performance of large-sized Rh_7,8C₂₀H₁₀^- toward methane activation compared to that of free Rh_7,8^-. The "support" acting as an "electron sponge" is emphasized as the key factor to improve the reactivity of large-sized clusters, for which the high electron-withdrawing capability of C₂₀H₁₀ dramatically shifts the active Rh atom from the apex position in free Rh₇^- to the edge position in "supported" Rh₇^- to enhance CH₄ adsorption, while the flexibility of C₂₀H₁₀ to release electrons further promotes subsequent C-H activation. The Rh atoms in direct contact with the support serve as electron-relay stations for electron transfer between C₂₀H₁₀ and the active Rh atom. This work not only establishes a novel approach to prepare and measure the reactivity of "supported" metal clusters in isolated gas phase but provides useful atomic-scale insights for understanding the chemical behavior of carbon (e.g., graphene)-supported metals in heterogeneous catalysis.