Synthesis and Structure-Activity Characterization of a Single-Site MoO₂ Catalytic Center Anchored on Reduced Graphene...

Molecularly derived single-site heterogeneous catalysts can bridge the understanding and performance gaps between conventional homogeneous and heterogeneous catalysis, guiding the rational design of next-generation catalysts. While impressive advances have been made with well-defined oxide supports, the structural complexity of other supports and the nature of the grafted surface species present an intriguing challenge. In this study, single-site Mo(═O)₂ species grafted onto reduced graphene oxide (rGO/MoO₂) are characterized by XPS, DRIFTS, powder XRD, N₂ physisorption, NH₃-TPD, aqueous contact angle, active site poisoning assay, Mo EXAFS, model compound single-crystal XRD, DFT, and catalytic performance. NH₃-TPD reveals that the anchored MoO₂ moiety is not strongly acidic, while Mo 3d_5/2 XPS assigns the oxidation state as Mo(VI), and XRD shows little rGO periodicity change on MoO₂ grafting. Contact angle analysis shows that MoO₂ grafting consumes rGO surface polar groups, yielding a more hydrophobic surface. The rGO/MoO₂ DRIFTS assigns features at 959 and 927 cm^-1 to the symmetric and antisymmetric Mo═O stretching modes, respectively, of an isolated cis-(O═Mo═O) moiety, in agreement with DFT computation. Moreover, the Mo EXAFS rGO/MoO₂ structural data are consistent with isolated (C-O)₂-Mo(═O)₂ species having two Mo═O bonds and two Mo-O bonds at distances of 1.69(3) and 1.90(3) Å, respectively. rGO/MoO₂ is also more active than the previously reported AC/MoO₂ catalyst, with reductive carbonyl coupling TOFs approaching 1.81 × 10³ h^-1. rGO/MoO₂ is environmentally robust and multiply recyclable with 69 ± 2% of the Mo sites catalytically significant. Overall, rGO/MoO₂ is a structurally well-defined and versatile single-site Mo(VI) dioxo heterogeneous catalytic system.