and Zhan Gao

Bio: and Zhan Gao is an academic researcher from University of Detroit Mercy. The author has contributed to research in topics: Diffusion. The author has an hindex of 1, co-authored 1 publications receiving 9 citations.

Two-Dimensional Spatial−Spectral ESR Imaging of Diffusion Based on Molybdenum(V)

Abstract: The diffusion coefficients of MoV (as MoCl5) at 300 K in solutions of poly(acrylic acid) in dimethylformamide (DMF) have been measured by two-dimensional spatial−spectral electron spin resonance imaging (2D ESRI), at polymer contents of 5%, 10%, and 15% w/w in the solvent. The concentration dependence of the diffusion coefficients is consistent with the free volume theory. The central position of molybdenum complexes in redox and acid−base catalysis, together with the possibilities opened by this study, suggest that the method of ESRI based on MoV could become important for measuring transport properties and spatial characteristics of reactions in heterogeneous catalysis. Further developments of the approach taken in this study are discussed.

Hydration, dynamics, and transport across the phase diagram of aqueous poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic L64) by spin probe electron spin resonance and electron spin resonance imaging

Abstract: The aggregation of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (EO13PO30EO13; commercial name Pluronic L64) in aqueous solutions and the hydration of the PEO blocks were deduced from ESR spectra of the nitroxide spin probe perdeuterio-2,2‘,6,6‘-tetramethylpiperidone N-oxide in the temperature range 293−330 K. The isotropic 14N hyperfine splitting of the nitroxide, aN, is a key parameter that reflects the local polarity, and its variation with temperature and polymer concentration is an indicator of aggregation. The effective hydration of the polymer chains was determined by comparison of aN values of the probe across the phase diagram with “calibration” curves of aN in pure PEO, pure PPO, and mixtures of the homopolymers, measured as a function of the water content Zw, where Zw is the number of water molecules per monomer unit. The calibration curves indicated that the spin probe is preferentially located in the vicinity of the EO segments and reports on the ...

Diffusion Coefficients of Small Molecules as Guests in Various Phases of Pluronic L64 Measured by One-Dimensional Electron Spin Resonance Imaging

Abstract: The diffusion coefficients of small nitroxide probes as guests in aqueous solutions of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) were measured at 300 K by one-dimensional electron spin resonance imaging (1D ESRI). The method is based on encoding the spatial distribution of the probes as a function of time in ESR spectra recorded in the presence of magnetic field gradients and simulation of these spectra in order to extract the diffusion coefficient, D. The rate of transport of each probe as a function of polymer content in the various phases (micellar, hexagonal, lamellar, and reverse micellar) of aqueous L64 depends on the probe location in the self-assembled system. The probe site was deduced from the analysis of the ESR spectra; the isotropic hyperfine splitting, aN, from the 14N nucleus of the >NO fragment in the probes was the polarity sensitive parameter. D values for the cationic probe 4-(N,N,N-trimethyl)ammonium-2,2,6,6-t...

EPR techniques applied to the study of chemistry and catalysis on oxide surfaces

Abstract: The use of electron paramagnetic resonance (EPR) spectroscopy to study the chemistry of oxide surfaces and catalysis is discussed. Emphasis is placed on those aspects of the techniques that are directly related to the specificity of heterogeneous systems, investigations of surfaces and catalytic reactions. The characteristic features of EPR spectra of high-surface oxide materials are analyzed, and the typical means employed to increase the amount of extractable information are presented. A brief survey of new potentialities of continuous-wave EPR and typical applications is also provided.

Binding and transport of mov in perfluorinated ionomers by epr and epr imaging

Abstract: EPR spectra of MoV(as MoCl5) in ethanol solutions and in perfluorinated ionomers (PFI) with long pendant chains (Nafion) have been measured as a function of MoCl5 concentration in the solutions (in the range 3.60 × 10–6 to 1.15 × 10–4M) and degree of neutralization of the sulfonic groups in the ionomers. Two MoV sites were characterized in the ethanol solutions, and their relative intensity was found to depend on MoCl5 concentration. Both sites, however, show evidence for chlorine ligands around the paramagnetic cation. The MoV site in PFI swollen by ethanol is identical to the site detected in neat ethanol solutions at higher MoCl5 content, and was assigned to a molybdenum site with five chlorine ligands and an oxygen ligand from the solvent. The diffusion coefficients of this species were determined by 2D spatial–spectral electron paramagnetic resonance imaging (2D EPRI) in Nafion–ethanol, by simulating the concentration profiles along the spatial coordinate measured during the diffusion process. The diffusion coefficients in Nafion–ethanol are 5.0 × 10–7, 7.1 × 10–7, 10.4 × 10–7 and 13.3 × 10–7 cm2 s–1 at 280, 300, 315 and 330 K, respectively. The activation energy for diffusion is 15.4 kJ mol–1.