Herbert Dilger

In-situ spin trap electron paramagnetic resonance study of fuel cell processes

A novel method allows the monitoring of radical formation and membrane degradation in-situ in a working fuel cell which is placed in the microwave resonator of an electron paramagnetic resonance (EPR) spectrometer. By introduction of a spin trap molecule at the cathode the formation of immobilized organic radicals on the membrane surface is observed for F-free membranes, revealing the onset of oxidative degradation. For Nafion® there is much less evidence of degradation, and the hydroxyl radical is detected instead. At the anode, free radical intermediates of the fuel oxidation process are observed. No traces of membrane degradation are detected on this side of the fuel cell.

Maximum hydrogen chemisorption on KL zeolite supported Pt clusters

Platinum clusters supported on KL zeolites were characterized by EPR, HRTEM, and EXAFS. Two kinds of hydrogen chemisorption experiments both result in a saturation value of 2.9 hydrogen atoms per platinum atom, significantly more than that reported so far. A hydrogen coverage-dependent cluster restructuring is suggested.

Towards the determination of partition coefficients of cosurfactants at surfactant bilayer interfaces by muon spin resonance spectroscopy

Avoided level crossing muon spin resonance (ALC-μSR) studies on the muonated cyclohexadienyl radical derived from the amphiphilic cosurfactant 2-phenylethanol have been used to derive cosurfactant partitioning and local environment information when dispersed in a concentrated lamellar phase dispersion. The study of partitioning at the bilayer/water interface at high surfactant concentrations is technically difficult and has consequently received very little attention. Calibration of the working range of fundamental resonance positions facilitates direct determination of cosurfactant partitioning with respect to the oil/water environment. Additional resonances yield other information about the local environment such as the degree of ordering at bilayer interfaces, thereby presenting a self-consistent picture of the local environment of the tracer molecule.

Mass and temperature effects on the hyperfine coupling of atomic hydrogen isotopes in cages

Abstract The hyperfine coupling constant A of a hydrogen isotope confined in a cage differs from its vacuum value, demonstrating that the atom acts as a probe of its environment. Room-temperature values of A for muonium in different cube-shaped Si8O12 units (T8 units or octasilsesquioxanes), in H2O, and in D2O were determined with an accuracy of about 1 MHz. The results are compared with those obtained from high-resolution ESR on H in H2O, and D in D2O, and in different silsesquioxanes in the temperature range 40–300 K. Both the strong isotope effect and the temperature dependence are well described by a single-oscillator model of Roduner et al. (J. Chem. Phys. 102 (1995) 5989). Striking differences seen between the different silsesquioxanes and between the porous Optipur and bulk Suprasil reflect varying spatial constraints and electronic interactions.

Surface diffusion of the cyclohexadienyl radical adsorbed on silica and on a silica supported Pd catalyst studied by means of ALC-μSR

Abstract The dynamic behaviour of the cyclohexadienyl radical adsorbed on plain silica and on a silica supported Pd catalyst is investigated by means of the avoided-level-crossing muon spin resonance technique. The influence of benzene coverage and of surface texture on the hyperfine coupling constants, on the signal amplitudes and on the mobility of the radical is studied. Information about surface diffusion is obtained using a theoretical model based on a stochastic Liouville formalism. The radical mobility increases with coverage of the surface with benzene. Arrhenius parameters are discussed within the framework of a hopping model based on transition-state theory for surface diffusion. It is suggested that the dynamics of the surface hydroxyl groups play a crucial role in the diffusion process.

Effect of mass on particle diffusion in liquids studied by electron spin exchange and chemical reaction of muonium with oxygen in aqueous solution

The dephasing rate constant of the muon precession signal of the light hydrogen isotope muonium (Mu) in the presence of oxygen in liquid water is (1.8 ± 0.1)× 1010 l mol–1 s–1 at 297 K. (90 ± 17)% of this is ascribed to electron spin exchange interaction, leaving no measurable amount for chemical reaction. Based on a spin statistical analysis, the diffusion-limited rate constant of Mu encounters with O2 is determined to be (5.7 ± 0.5)× 1010 l mol–1 s–1 for Mu at 297 K, which is a factor of 1.78 higher than the value deduced from literature data for H. From this we conclude that the ratio of diffusion coefficients, DMu/DH, is 2.0 ± 0.2 and DMu=(14 ± 4)× 10–9 m2 s–1. This reveals an unprecedented mass effect which by comparison with literature values appears, in clear contradiction to Stokes–Einstein behaviour, to scale approximately with the inverse cube root of the mass of the diffusing particle. The behaviour is discussed by comparison with literature values for the diffusion of noble gases.

EPR spectroscopic investigation of radical-induced degradation of partially fluorinated aromatic model compounds for fuel cell membranes

EPR spectroscopic investigations of reactions between monomeric model compounds representing typical structural moieties of poly(aryl) ionomers and photochemically generated hydroxyl radicals are reported. Deoxygenated solutions of the model compounds (in a water/methanol mixture) containing hydrogen peroxide at defined pH values were exposed to UV light in the flow cell within the cavity of an EPR spectrometer. Spectra were analyzed by computer simulation and the formed radicals were assigned by comparing their g-factors and hyperfine coupling constants (hfccs) with those from the literature and from density functional theory (DFT) calculations. The relevance for polymer electrolyte membrane fuel cells (PEMFCs) and alkaline-anion exchange membrane fuel cells (AAEMFCs) is discussed.

Why ALC μSR is superior for gas‐phase radical spectroscopy

ALC μSR spectra of the muonated ethyl and cyclohexadienyl radicals in the gas phase are reported. They have surprisingly narrow lines for a magnetic resonance type technique under conditions near ambient temperature and near 1 atmosphere pressure. The main reason for this behaviour is the dramatic reduction of electron spin relaxation in high magnetic fields.

Chiral Induction in Lyotropic Liquid Crystals: Insights into the Role of Dopant Location and Dopant Dynamics†

The pitch P of the helix can be directly observed in the polarizing optical microscope as the periodic pattern of the “fingerprint texture”. Chiral induction in liquid crystals (LCs) is one of the most sensitive methods for the detection of chirality. The unique chirality effects in LCs have been studied widely, including the molecular induction mechanism in thermotropic LCs and in a self-assembled twodimensional model system. For LLCs, however, the molecular induction mechanism in the N* phase has been a matter of discussion for more than 20 years. There are two proposed mechanisms: a) a dispersive chiral interaction between dopants in adjacent micelles (the dopant should preferentially be located at the micellar surface), and b) a steric dopant–amphiphile interaction yielding distorted micelles (in this case the solubilization of the chiral dopant within the micelle should be favorable). 10, 11] The temperature dependence of the pitch P(T) is expected to differ for the two mechanisms: in (a), P increases linearly with T, whilst in (b), P may decrease hyperbolically (with T ; see Supporting Information). Experimental studies on the chiral induction mechanism include pitch measurements of varied guest–host systems 7] and X-ray diffraction, however, the latter has not provided clear evidence of distorted micelles. The pitch was found to depend on the chemical composition of the LLC host phase, the temperature, the dopant concentration and, in particular, the chemical nature of the dopant. A general correlation between properties of the chiral dopant and its chiral induction power in a host phase has not yet been established for LLCs, in contrast to the molecular concepts developed for thermotropic LCs (see, for example, Ref. [1] and Ref. [12]), which are also important for LLCs, as we will discuss later. A crucial point of discussion regarding LLCs, especially in view of the suggested mechanisms, is the actual location of the chiral dopants in the N* phase: within the apolar core of the micelle or at the micellar surface. The dopant location is proposed to play an important role for the chiral induction power, 13] but locating the dopants experimentally has not yet been successful. Recently, a magnetic resonance method suitable for studying dopants present at low concentrations, avoidedlevel-crossing muon spin resonance (ALC-mSR), was used to reveal the dopant location in lamellar LLC phases. The ALCmSR technique involves the formation of a radical by the addition of muonium, Mu, a light hydrogen isotope (mH = 9mMu) with a positive muon m + as the nucleus, to an unsaturated bond (Scheme 1). The time-integrated muon spin polarization, which is proportional to the muon decay asymmetry A, is measured as a function of an external magnetic field. Resonances occur when there is coupling between eigenstates of the three-spin-=2 system composed of the radical electron, the muon, and the proton bound to the same carbon as the muon. The resonance field Bres is determined by the hyperfine coupling constants of the radical; these coupling constants are, among other factors, sensitive to the polarity of the surroundings, and Bres is shifted to higher values with increasing polarity. The polarity of the local environment and thus the location in the LLC is determined by comparing Bres in the LLC with the values in a Figure 1. Schlieren texture and model of the nematic (N) LLC host phase with disk-like micelles (left). Fingerprint texture and model of the chiral nematic (N*) phase; micelles represent the helical modulation of the director n with pitch P induced by doping the host phase with 4.37% R-MA (right).

Reorientation dynamics of cyclohexadienyl radicals in zeolites

The dynamics of the muonium substituted cyclohexadienyl radical adsorbed on silicalite and NaZSM‐5 is investigated by means of avoided level crossing muon spin resonance. The influence of benzene loading on the mobility of the radical is studied. At low loadings the radicals were found to be located on a single adsorption site where they undergo a wobbling type of motion. With increasing loading an additional species adsorbed on a different site is observed.