In this paper we present a study on stable radicals and short-lived species generated in anion-exchange membrane (AEM) fuel cells (AEMFCs) during operation. The in situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals is monitored by the EPR spin trapping technique. For the first time, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation. The main detected adducts during the operation of the micro-AEMFC are DMPO-OOH and DMPO-OH on the cathode side, and DMPO-H on the anode side. These results indicate that oxidative degradation involving radical reactions has to be taken into account when stability of AEMFCs is investigated.

Photocatalytic carboxylation of acetylacetone with carbon dioxide has been performed by using ZnS‐based photocatalysts. The formation of two isomeric carboxylic acids, as proven by IR and 13C NMR spectroscopy, was observed. The reaction yield was enhanced after deposition of ruthenium nanoparticles on ZnS. The reaction encompasses ruthenium‐mediated one‐electron reduction of CO2 to CO2.− with electrons from the conduction band of ZnS and one‐hole oxidation of acetylacetone to the relevant radical. Coupling of photogenerated radicals leads to the formation of carboxylic acids. Generation of CO2.− has been confirmed by spin‐trapping EPR measurements. The process described herein may find applications for the solar‐light‐driven green synthesis of Cn+1 carboxylic compounds from Cn substrates by utilising carbon dioxide.

Highly active photocatalysts were obtained by impregnation of nanocrystalline rutile TiO 2 powders with small amounts of Cu(II) and Fe(III) ions, resulting in the enhancement of initial rates of photocatalytic degradation of 4-chlorophenol in water by the factor of 7 and 4, compared to pristine rutile, respectively. Detailed structural analysis by EPR and X-ray absorption spectroscopy (EXAFS) revealed that Cu(II) and Fe(III) are present as single species at the rutile surface. The mechanism of the photoactivity enhancement was elucidated by a combination of DFT calculations and detailed experimental mechanistic studies including photoluminescence measurements, photocatalytic experiments using scavengers, OH radical detection, and photopotential transient measurements. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing "dark" redox reactions at the interface, improving thus the normally very low quantum yields of UV light-activated TiO 2 photocatalysts. The exact mechanism of the photoactivity enhancement differs depending on the nature of the cocatalyst. Cu(II)decorated samples exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. At Fe(III)-modified rutile the rate of dioxygen reduction is not improved and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, it was demonstrated that excessive heat treatment (at 450 °C) of photocatalysts leads to loss of activity due to migration of Cu(II) and Fe(III) ions from TiO 2 surface to the bulk, accompanied by formation of oxygen vacancies. The demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modified photocatalysts holds promise for developing further tailored single-site-modified photocatalysts for various applications. 19calculations that have the surface exposed to vacuum. Furthermore, the oxygen atom of the cluster forms a short bond of 1.79 Å with a fivefold coordinated titanium atom of the surface, closing a TiO 6 octahedra. Figure 10. (a) DOS of the TiO 2 (R)-Cu system, spin-up channel only, aligned to the DOS of the ideal TiO 2 (R) surface. (b) The spin up channel DOS of the final relaxed electron polaron system for the TiO 2 (R)-Cu system. Cu contribution shown. The zero of the x-axis is fitted to the VBM for both plots.The electronic structure of TiO 2 (R)-Cu is shown in Figure 10a. The Cu atom has a significant presence on the CBE. This is primarily due to the Cu d-states. Further, we compare the position of the decorated rutile TiO 2 surface to the bare rutile surface, by comparison and alignment of the electrostatic potential in the vacuum region. 61 When the alignment is taken into action, the Cu-state is margi...

The unique physicochemical properties and biocompatibility of zinc oxide nanocrystals (ZnO NCs) are strongly dependent on the nanocrystal/ligand interface, which is largely determined by synthetic procedures. Stable ZnO NCs coated with a densely packed shell of 2-(2-methoxyethoxy)acetate ligands, which act as miniPEG prototypes, with average core size and hydrodynamic diameter of 4-5 and about 12 nm, respectively, were prepared by an organometallic self-supporting approach, fully characterized, and used as a model system for biological studies. The ZnO NCs from the one-pot, self-supporting organometallic procedure exhibit unique physicochemical properties such as relatively high quantum yield (up to 28 %), ultralong photoluminescence decay (up to 2.1 μs), and EPR silence under standard conditions. The cytotoxicity of the resulting ZnO NCs toward normal (MRC-5) and cancer (A549) human lung cell lines was tested by MTT assay, which demonstrated that these brightly luminescent, quantum-sized ZnO NCs have a low negative impact on mammalian cell lines. These results substantiate that the self-supporting organometallic approach is a highly promising method to obtain high-quality, nontoxic, ligand-coated ZnO NCs with prospective biomedical applications.

The photocatalytic activity of materials synthesized by titanium dioxide impregnation with chromates(VI) was studied in the processes of 4-chlorophenol oxidation and photocurrent generation. The materials show measurable activity when excited with visible light. Electron paramagnetic resonance (EPR) studies revealed the presence of chromium(V) species even without irradiation. Detection of photogenerated reactive oxygen species, together with elucidation of electrochemical properties of the materials, enabled assumption of a very unique mechanism of TiO 2 photosensitization, involving a photoinduced hole injection from the excited photosensitizer species to the valence band. Photoelectrochemical studies revealed that visible light induced both hole injection to the valence band and electron injection to the conduction band, depending on the electrode potential. The former process is responsible for anodic, whereas the latter is responsible for cathodic photocurrent generation. This counterintuitive behavior results from a peculiar arrangement of electronic levels in the studied systems. Although the (photo)stability of studied materials, as well as the efficiency of the photosensitization process are moderate, the system represents a very unique and therefore interesting mode of titania photosensitization.

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Heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25 and 10% ethylene (E), respectively, as ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals and FTIR of films prepared by compression molding. The spatial distribution of the HAS-derived nitroxide radicals was obtained by 1D ESRI. These experiments enabled the visualization of an outer region of thickness ≈100 μm that contained a lower amount of nitroxides and believed to arise from the loss of the stabilizer by diffusion (“blooming”) and in chemical reactions during aging. Nondestructive (“virtual”) slicing of the 2D spectral spatial ESR images resulted in a series of ESR spectra that indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. 1D and 2D ESRI allowed the detection of faster degradation of EPR in the amorphous phase represented by the fast spectral component. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.

We present the application of 1D spatial and 2D spatial-spectral electron spin resonance imaging (ESRI) in order to deduce the intensity profile and the spatial variation of line shapes for nitroxides formed during accelerated UV irradiation and thermal degradation of poly(acrylonitrile−butadiene−styrene) (ABS) containing a hindered amine stabilizer (HAS). The detection of two dynamically different sites for the HAS-derived nitroxide radicals (“fast” and “slow” components) and the decrease of the relative intensity of the fast component with UV irradiation or heating time point to the butadiene-rich domains as the vulnerable site for degradation. Spatial variation of the nitroxide intensity and of the line shapes was detected in the UV-irradiated samples. The nitroxide signal is strong on the irradiated side, increases with time on the opposite side, and is very weak in the sample interior; these results were taken as evidence for diffusion-limited oxidation. By contrast, the radical concentration and the line shapes are spatially homogeneous in the polymer undergoing thermal degradation at 333 K. This study has demonstrated the potential of imaging methods based on ESR to provide details on polymer degradation and stabilization that cannot be deduced from properties averaged over the entire sample.