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

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.

Polynuclear Speciation of Trivalent Cations near the Surface of an Electrolyte Solution

Despite long-standing efforts, there is no agreed upon structural model for electrolyte solutions at air-liquid interfaces. We report the simultaneous detection of the near-surface and bulk coordination environments of a trivalent metal cation (europium) in an aqueous solution by use of X-ray absorption spectroscopy. Within the first few nanometers of the liquid surface, the cations exhibit oxygen coordination typical of inner-sphere hydration of an aquated Eu(3+) cation. Beyond that, outer-sphere ion-ion correlations are observed that are otherwise not present in the bulk electrolyte. The combination of near-surface and bulk sensitivities to probe metal ion speciation in electrolyte solutions is achieved by detecting electron-yield and X-ray fluorescence signals from an inverted pendant drop. The results provide new knowledge about the near-surface chemistry of aqueous solutions of relevance to aerosols and ion transport processes in chemical separations and biological systems.

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.

Investigations on the demetalation of metalloporphyrins under ultrasound irradiation

An efficiently facile method for the demetalation from metalloporphyrins has been developed, which uses high-intensity ultrasound to initiate the ligand dissociation in a mixed solvent of (CH(3)CO)(2)O/HCl with FeSO(4). The influences of substituents, bath temperature and reaction time on the reactions were also investigated, on the base of which the mechanism of demetalation and the effect of ultrasound irradiation on metal-ligand cleavage have been discussed in detail.

Langmuir-Schaefer films of a set of achiral amphiphilic porphyrins: aggregation and supramolecular chirality

A series of new achiral porphyrins with hydrophobic dodecyl chains and hydrophilic substituents were designed in order to clarify the relationship between molecular geometry and aggregation as well as the supramolecular chirality in Langmuir-Schaefer films. These achiral porphyrins have zero (TPPA0), one (TPPA1), two (TPPA2a, TPPA2b), three (TPPA3) and four (TPPA4) long hydrophobic chains, respectively. Most of the compounds showed good spreading behaviour on a water surface except TPPA4 and could be fabricated into LS films easily. Depending on the number of alkyl chains, the porphyrin derivatives showed different aggregation behaviour in the LS films. The transferred films were characterized by UV-vis, circular dichroism (CD) and FTIR spectroscopy, and atomic force microscopy (AFM). Interestingly, some of the porphyrin assemblies were found to be optically active in the LS films although the compounds themselves are achiral. TPPA3 showed a strong Cotton effect in the LS film and fiber-like morphology on mica surface. Weak supramolecular chirality was detected from the LS films of TPPA0, TPPA1 and TPPA4, but no Cotton effect was observed for the LS films of TPPA2a or TPPA2b. On the other hand, when the LS film of TPPA3 was dipped into hexane or exposed to HCl gas, which altered the hydrophobic effect among the dodecyl chains and destroyed the π-π interactions between the porphyrin macrocyclic rings, the CD signal decreased or disappeared. The different aggregation behaviour and supramolecular chirality in the films was suggested to be related to the molecular structure and subsequent packing in their organized molecular films.