Polarized total-reflection x-ray absorption fine structure for self-assembled monolayer of zinc porphyrin at air–water interface

Symmetry Evolution Induced 2D Pt Single Atom Catalyst with High Density for Alkaline Hydrogen Oxidation

The performance of single-atom catalysts is greatly influenced by the chemical environment surrounding the central atom. Here, a salt-assisted method is employed to transform the tetrahedral coordination structure of zeolitic imidazolate frameworks - 8 (ZIF-8) into a planar square coordination structure without altering the ligands. During the subsequent carbonization process, concurrent with the evaporation of zinc atoms, the structure of the nitrogen and carbon carriers (NC carriers) undergoes a transition from five-membered rings to six-membered rings to preserve the 2D structure. This transition results in the generation of additional defect sites on the 2D-NC substrates. Hence, the Pt single-atom catalysts with planar square coordination symmetries can be precisely prepared via electrodeposition (denoted as 2D-Pt SAC). The Pt loading of 2D-Pt SAC is 0.49 ± 0.03 µg cm-2, higher than that of 3D-Pt SAC (0.37 ± 0.04 µg cm-2). In the context of the hydrogen oxidation reaction electrocatalysis, under an overpotential of 50 mV, these single-atom catalysts with 2D coordination exhibit mass activities of 2396 A gPt -1 (32 times higher than commercial Pt/C catalyst, 2 times higher than 3D-PtNC).

Axial Oxygen Ligands Regulating Electronic and Geometric Structure of Zn-N-C Sites to Boost Oxygen Reduction Reaction

Zn-N-C possesses the intrinsic inertia for Fenton-like reaction and can retain robust durability in harsh circumstance, but it is often neglected in oxygen reduction reaction (ORR) because of its poor catalytic activity. Zn is of fully filled 3d10 4s2 configuration and is prone to evaporation, making it difficult to regulate the electronic and geometric structure of Zn center. Here, guided by theoretical calculations, five-fold coordinated single-atom Zn sites with four in-plane N ligands is constructed and one axial O ligand (Zn-N4 -O) by ionic liquid-assisted molten salt template method. Additional axial O not only triggers a geometry transformation from the planar structure of Zn-N4 to the non-planar structure of Zn-N4 -O, but also induces the electron transfer from Zn center to neighboring atoms and lower the d-band center of Zn atom, which weakens the adsorption strength of *OH and decreases the energy barrier of rate determining step of ORR. Consequently, the Zn-N4 -O sites exhibit improved ORR activity and excellent methanol tolerance with long-term durability. The Zn-air battery assembled by Zn-N4 -O presents a maximum power density of 182 mW cm-2 and can operate continuously for over 160 h. This work provides new insights into the design of Zn-based single atom catalysts through axial coordination engineering.

Communication: Coordination structure of bromide ions associated with hexyltrimethylammonium cations at liquid/liquid interfaces under potentiostatic control as studied by total-reflection X-ray absorption fine structure

Total-reflection X-ray absorption fine structure (TR-XAFS) technique was applied for the first time to an interface between two immiscible electrolyte solutions under potentiostatic control. The hydration structure of bromide ions was investigated at polarized 2-octanone/water interfaces. TR-XAFS spectra at Br K-edge measured in the presence of hexyltrimethylammonium bromide (C6TAB) were slightly modified depending on the Galvani potential difference (Δ(o)(w)φ). The extended X-ray absorption fine structure analysis exposed hydration structure changes of bromide ions at the polarized interface. The coordination structure of bromide ions at the interface could be analyzed as compared with bromide ions dissolved in aqueous solution and Br(-)-exchanged resin having quaternary ammonium groups. The results indicated that bromide ions were associated with C6TA(+) at the polarized interface. The relative contribution of ion association form of bromide ions with quaternary ammonium groups was enhanced at a potential close to the ion transfer of C6TA(+), where the interfacial concentration of C6TA(+) is increased as a function of Δ(o)(w)φ.

Simulation of X-ray Absorption Near Edge Spectra of Organometallic Compounds in the Ground and Optically Excited States

The Mg K-edge and Zn K- and L3-edge X-ray absorption near edge spectra of Mg and Zn porphyrins in the ground state and low-lying optically excited states are calculated. Also computed are X-ray absorption near edge spectra of Fe(II) spin crossover compound in its ground and low-lying optically excited states, motivated by a recent experiment (J. Phys. Chem. A 2006, 110, 38). The calculated absorption spectra of optically excited states can be used to simulate ultrafast optical pump/X-ray probe experiments.

Interfacial behavior of tetrapyridylporphyrin monolayer arrays

The behavior of monolayer films of free base 5,10,15,20-tetrapyridylporphinato (TPyP) and 5,10,15,20-tetrapyridylporphinato zinc(II) (ZnTPyP) on pure water, 0.1 M CdCl2, and 0.1 M CuCl2 subphases was investigated by surface pressure-area isotherms, specular X-ray reflectometry, and polarized total reflection X-ray absorption spectroscopy (PTRXAS). Surface pressure-area isotherms showed significant differences in the area per molecule on pure water compared to that on salt subphases, with a marked increase in the area observed on the salt solutions. This behavior was noted for both forms of the porphyrin and both salts investigated. Modeling of specular X-ray reflectometry data indicated that thinner and more electron dense layers on salt subphases best fit the observed profiles. These data suggest that the porphyrin macrocycle is oriented parallel to the interface on salt subphases and takes on a tilted conformation on pure water. In the case of ZnTPyP, PTRXAS was used to determine the orientation of the porphyrin moiety relative to the surface and to probe the coordination of the central Zn ion. In agreement with the pressure-area isotherms and reflectometry, the PTRXAS data indicate a change in orientation on the salt subphases.

Zinc(II) porphyrins at the air-water interface as studied by polarized total-reflection X-ray absorption fine structure

The polarized total-reflection X-ray absorption fine structure method was applied to characterize zinc porphyrins at the air-water interface. The X-ray absorption near edge structure exhibited a significant difference depending on the polarization of the X-ray. A shoulder peak of the Zn K-edge corresponding to the 1s-4p(z) transition for a square planar metal complex without axial coordination(s) was observed at 9662 eV, which indicates that the axial coordination sites of zinc porphyrin molecules examined are not fully hydrated at the air-water interface. The molecular orientation of zinc porphyrins was determined by analyzing the polarization dependence of the transition peak intensity. The meso-substituted porphyrin derivative 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) orients rather parallel to the solution surface. In contrast to ZnTPP, the zinc(II) protoporphyrin IX (ZnPP) with hydrophilic carboxyl groups at one side of the molecule stands up with respect to the solution surface, and the average tilting angle of the porphyrin plane to the surface was evaluated to be between 57 degrees and 43 degrees. In addition, the axial coordination of ZnPP is modified depending on the surface concentration, in which the axial hydration to the zinc center is effectively inhibited in the compressed surface layer.