| Abstrakt: |
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)2species grafted onto reduced graphene oxide (rGO/MoO2) are characterized by XPS, DRIFTS, powder XRD, N2physisorption, 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 MoO2moiety is not strongly acidic, while Mo 3d5/2XPS assigns the oxidation state as Mo(VI), and XRD shows little rGO periodicity change on MoO2grafting. Contact angle analysis shows that MoO2grafting consumes rGO surface polar groups, yielding a more hydrophobic surface. The rGO/MoO2DRIFTS assigns features at 959 and 927 cm–1to 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/MoO2structural data are consistent with isolated (C–O)2–Mo(═O)2species having two Mo═O bonds and two Mo–O bonds at distances of 1.69(3) and 1.90(3) Å, respectively. rGO/MoO2is also more active than the previously reported AC/MoO2catalyst, with reductive carbonyl coupling TOFs approaching 1.81 × 103h–1. rGO/MoO2is environmentally robust and multiply recyclable with 69 ± 2% of the Mo sites catalytically significant. Overall, rGO/MoO2is a structurally well-defined and versatile single-site Mo(VI) dioxo heterogeneous catalytic system. |
