Fei-Yang Yu,Hao Sun,Hua-Qiao Tan,Yang-Guang Li,Shuit-Tong Lee,Zhen-Hui Kang
The dynamic evolution and structure-performance relationships of single-atom catalysts (SACs) under operating conditions are one of the most concerned hotspots at present. Here, we employed Anderson-type polyoxometalates as a precursor to prepare the SAC Rh-MoS 2 , which shows excellent hydrogen evolution reaction (HER) performance with a mass activity of 4.26 A mg −1 at 100 mV. Such a result is much better than that of 20% Pt/C. Extensive experimental characterization combined with density functional theory (DFT) calculations suggest that the active site undergoes a dynamic evolution from Rh-S to Rh-S/O, while at the same time, the support is transformed from 1T MoS 2 to 1T-2H MoS 2 . It is this dynamic evolution of the catalyst structure that causes a series of changes in the activity, and eventually, a balance between catalytic activity and stability is achieved. This work offers a good research demonstration of the dynamic evolution of SACs under operating conditions and provides a new perspective on the design of SACs.
The heterogenization of homogeneous catalysts is of critical importance for both fundamental research and practical applications. The key obstacle to industrial applications of heterogenized homogeneous catalysts is the long-term stability of the immobilized molecular complexes. We unambiguously demonstrate in this report that the vinyl-functionalized PPh 3 polymers supported single Rh sites (Rh/POL–PPh 3 ) catalyst provided not only excellent catalytic activity but also ultrahigh stability for fixed-bed olefins hydroformylation , which is one of the most important industrial homogeneous catalytic processes for the synthesis of aldehydes and alcohols. HAADF-STEM and EXAFS results indicated that the Rh atoms were present in single Rh sites which were strongly coordinated with the exposed phosphorous atoms of the POL–PPh 3 support. The Rh/POL-PPh 3 catalyst provided high activity and selectivity because it maintained the catalytic functionalities similar to the homogeneous HRh(CO)(PPh 3 ) 3 complex, as demonstrated both by 31 P MAS NMR and in situ FT-IR experiments. Thus a reaction mechanism, similar to that over the homogeneous HRh(CO)(PPh 3 ) 3 complex, was proposed for olefin hydroformylation over the novel Rh/POL–PPh 3 catalyst.
The local coordination environment of central active sites plays a critical role in tuning the catalytic properties of heterogeneous single-metal-site catalysts (HSMSCs). However, carbon supports with inert surface properties provide weak interaction with the active sites, which hamper the effective modulation of the coordination structure of HSMSCs. Herein, this work shows that introducing N and S atoms into carbon support can modify the coordination environment of single-Rh-site and effectively improves the catalytic activity for the methanol carbonylation reaction. The synthesized Rh/AC-NS with single-Rh-site exhibits a high methanol conversion of 88% and a stable acetyl production rate of 2812 h −1 , in contrast with 1285 h −1 of Rh/AC without N- and S- functional groups, and meanwhile remarkably higher than those of Rh/AC-N and Rh/AC-S with only N- or S- functional group under the same mild reaction conditions. As revealed by the experimental result and density functional theory (DFT) calculations, the special combination of N- and S- functional groups offers favorable coordination configuration and also beneficial electronic structure for facilitated CH 3 I oxidative addition and CO insertion steps of single-Rh-site. This work offers new opportunities for rationally tuning the coordination structure of HSMSCs for heterogeneous catalysis.
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