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

摘要

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, N-2 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) angstrom, respectively. rGO/MoO2 is also more active than the previously reported AC/MoO2 catalyst, with reductive carbonyl coupling TOFs approaching 1.81 x 10(3) 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.