NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γphases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure MM centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

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Noncontact atomic force microscopy (nc-AFM) with a CO-functionalized tip yields high resolution images under many situations. However, nc-AFM images are sometimes difficult to interpret when visualizing polycyclic aromatic hydrocarbons. The authors employ real-space pseudopotentials constructed using density functional theory to simulate nc-AFM images of benzene and dibenzo(cd,n)naphtho(3,2,1,8-pqra)perylene molecules with selected probe tips (such as CO, H2, N2, Br, and CH2O). The selected tips provide accurate simulations, save for the tip functionalized with a Br atom. The authors find contrast inversion with CO and N2 tips at small tip heights and image distortion with the CH2O tip.

Bond breaking and forming are essential components of chemical reactions. Recently, the structure and formation of covalent bonds in single molecules have been studied by non-contact atomic force microscopy (AFM). Here, we report the details of a single dative bond breaking process using non-contact AFM. The dative bond between carbon monoxide and ferrous phthalocyanine was ruptured via mechanical forces applied by atomic force microscope tips; the process was quantitatively measured and characterized both experimentally and via quantum-based simulations. Our results show that the bond can be ruptured either by applying an attractive force of ~150 pN or by a repulsive force of ~220 pN with a significant contribution of shear forces, accompanied by changes of the spin state of the system. Our combined experimental and computational studies provide a deeper understanding of the chemical bond breaking process.