To
further improve the intrinsic reactivity of single-atom catalysts
(SACs), the controllable modification of a single site by coordinating
with a second neighboring metal atom, developing double-atom catalysts
(DACs), affords new opportunities. Here we report a catalyst that
features two bonded Fe–Co double atoms, which is well represented
by an FeCoN6(OH) ensemble with 100% metal dispersion, that
work together to switch the reaction mechanism in alcohol dehydrogenation
under oxidant-free conditions. Compared with Fe-SAC and Co-SAC, FeCo-DAC
displays higher activity performance, yielding the desired products
in up to 98% yields. Moreover, a broad diversity of benzyl alcohols
and aliphatic alcohols convert into the corresponding dehydrogenated
products with excellent yields and high selectivity. The kinetic reaction
results show that lower activation energy is obtained by FeCo-DAC
than that by Fe-SAC and Co-SAC. Moreover, computational studies demonstrate
that the reaction path by DACs is different from that by SACs, providing
a rationale for the observed enhancements.
Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO 2 reduction reaction (CO 2 RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO 2 RR. The selectivity and activity of CO 2 reduction to formic acid on Snsingle-atom sites are shown to be correlated with Sn(IV)-N 4 moieties axially coordinated with oxygen (O−Sn−N 4 ), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (j HCOOH ) of 74.8 mA•cm −2 at −1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and 119 Sn Mossbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO 2 RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn−O−CO 2 species over the O−Sn−N 4 sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn−N 4 sites, thereby greatly facilitating CO 2 -to-HCOOH conversion.
The electronic structure of the newly discovered superconductor KNi(2)Se(2) is studied by first-principles calculations. Our results show that its ground state is a quasi-two-dimensional metal and the Fermi surface is contributed by the Ni 3d and Se 4p states. These states determine the significant physical properties of the heavy-fermion superconductor KNi(2)Se(2), such as the heavy-fermion behavior and low superconducting transition temperature. Based on the multi-orbital character of our calculation, we suggest that KNi(2)Se(2) may have a complicated superconducting multi-gap structure. The influence of the Coulomb interaction on the electronic structure and the 3d orbital character in this material is also investigated.
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