Electron paramagnetic resonance (a review)

For the purposes of this review, the termelectron paramagnetic resonance is taken to refer to the resonant absorption of electromagnetic radiation by electronic systems which possesspermanent magnetic moments (due, in general, to the orbital as well as to the spin angular momentums of electrons) and which are therefore paramagnetic. The termparamagnetic resonance, which is frequently encountered, refers strictly to the magnetic resonance of permanent magnetic dipoles of any type, including nuclear magnetic resonance. On the other hand, the termelectron resonance includes cyclotron resonance (diamagnetic resonance).In a short article of this nature, it is clearly impossible to discuss, in detail, all aspects of such a wide field. Rather, the intention is to discuss the basic principles of the electron paramagnetic resonance technique together with some results in certain fields which illustrate its range, applications, and potentialities.     

Electron paramagnetic resonance of some ABX3 compounds

Electron paramagnetic resonance studies of NaMnCl₃, KMnCl₃, KCuCl₃, NH₄CuCl₃ were made between 400 and 5 K. The peak-to-peak derivative linewidths of these substances were found to be 14, 70, 10 and 5 mT, respectively. The line splitting below 15 K in NaMnCl₃ was attributed to hyperfine interaction with the Mn²⁺ nucleus. The g values and the peak-to-peak derivative linewidths of KMnCl₃, KCuCl₃ and NH₄CuCl₃ were found to be temperature independent within the limits of experimental errors. The g values of NaMnCl₃ and KMnCl₃ are isotropic, however, of KCuCl₃ and NH₄CuCl₃ are slightly anisotropic at room temperature and their principal values were determined. Hyperfineless structure of the spectra and the broad linewidths seem to be mainly due to exchange interactions of Mn²⁺ and Cu²⁺ ions among themselves in the respective compounds.     

Rh/CeO2 catalyst preparation and characterization for hydrogen production from ethanol partial oxidation

Hydrogen production for fuel cells from ethanol partial oxidation was evaluated on 1% Rh/CeO₂ catalyst. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were used to analyze carbon formation and the possible sintering of the metallic phase. X-ray fluorescence (XRF), nitrogen adsorption, and temperature-programmed reduction (TPR) were also used for catalyst characterization. Two groups of rhodium particles of different sizes were observed. By SEM/EDS analysis, no residual chloride was identified. TEM made it possible to identify the presence of rhodium in small clusters. On the other hand, products distribution was affected by reaction temperature. At 473 K, only traces of acetaldehyde were detected for the Rh/CeO₂ catalyst. In the reaction conditions, no deactivation of the catalyst due to carbon deposition or sintering of the metal was observed. Overall, our results show that the performance of Rh/CeO₂ catalyst points to promising applications in terms of H₂ production for fuel cells technology by ethanol partial oxidation.     

Electron paramagnetic resonance: recent developments and trends

Major recent advances:High-field multi-frequency EPR improves our knowledge of magnetic materials, conductive polymers, and spin systems with large zero-field splittings. High-field electron nuclear double resonance provides better access to subtle details of the electronic structure of materials. Materials structure on length scales between 1.5 and 8 nm can now be characterized more precisely.     

One pot synthesis and characterization of ultra fine CeO2 and Cu/CeO2 nanoparticles. Application for low temperature CO oxidation

Ultra fine cerium oxide and copper doped cerium oxide nanoparticles are prepared in a one-step reaction by thermal decomposition of Ce acetate in commercial oleylamine. The products are highly crystalline and were characterized by XRD, Raman spectroscopy, XPS, TEM and BET. The TEM images show that the CeO2 particles prepared are uniformly nanosized. The size of the nanoparticles can be controlled in the sub-10 nm range by the presence of other capping agent in the reaction mixture such as tri-octylphosphine oxide and oleic acid. The copper doped cerium oxide nanoparticles show high specific surface area (up to 299 m2/gr) and high catalytic activity for the low temperature CO oxidation even at low copper loading such as 9 at.%.     

Electron paramagnetic resonance of Cu2+ and VO2+ ions in phosphate glasses

Electron spin resonance (ESR) spectra ofx(CuO · V₂O₅ (1 –x) (Na₂O · P₂O₅) andx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) glasses for 0 x 40 have been studied at the X-band and at 300 K. It is found that forx 5, both Cu²⁺ and VO²⁺ are present, mostly as isolated species. Forx 10, broad resonance lines atg = 2.1524 forx(CuO · V₂O₅) (1 –x) (Na₂O · P₂O₅) and atg = 2.1448 forx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) are observed which may be mainly due to dipole-dipole type interaction between transition metal (TM) ions. Spin Hamiltonian parameters of TM ions have been calculated. Optical spectra of the sodium phosphate glasses doped with single TM ions have also been studied. The theoretical optical basicity, A_th, of these doped glasses has been calculated. It is found that for VO²⁺ ionsg ,g and increase whileA ,P and g _|/g decrease with increase in A_th. However, no significant change is observed in the spin Hamiltonian parameters of Cu²⁺ with the change in A_th.     

Nano-structured gold catalysts supported on CeO2 and CeO2-Al2O3 for NOx reduction by CO: effect of catalyst pretreatment and feed composition

Gold catalysts supported on ceria and ceria-alumina were studied in NOx reduction by CO. Gold was loaded using deposition-precipitation method. The ceria-alumina (20 wt% alumina) support was synthesized by co-precipitation. The average size of gold and ceria nano-particles was bellow 10 nm. It was established that the type of pretreatment do not have a substantial effect on the catalytic activity. The presence of O2 in the feed leads to a high conversion of CO to CO2 but no NO conversion was registered. Both NO and CO conversion was increased adding H2 to the feed. The catalytic activity became higher upon adding higher amount of H2. Supplementary to the main reaction parallel reactions took place. Bellow 200 degrees C N2O formation and at 250 degrees C and above the NH3 formation was detected. At around 200 degrees C it was established 100% selectivity to N2. The addition of water to the feed influenced positively the CO conversion and did not influence negatively the conversion of NO. The selectivity to N2 at around 200 degrees C remained 100% independent of the presence of moisture. Alumina in the mixed support prevents the sintering of both gold and ceria nano-particles. The results obtained make the catalysts containing gold supported on ceria-alumina promising for practical application.     

Electron paramagnetic resonance-based pH mapping using spectral-spatial imaging of sequentially scanned spectra

The development of electron paramagnetic resonance (EPR)-based mapping of pH is an important advancement for the field of diagnostic imaging. The ability to accurately quantify pH change in vivo and monitor spatial distribution is desirable for the assessment of a number of pathological conditions in the human body as well as the monitoring of treatment response. In this work we introduce a method for EPR-based pH mapping utilizing a method of spectral-spatial imaging of sequentially scanned spectra to decrease the missing gradient rotation angle, without increasing the spatial field of view. Repeated in vitro measurements of pH phantom tubes demonstrated higher precision measurements of the hyperfine coupling constant (HFC) compared to previous EPR-based methods, resulting in mean pH values accurate to less than 0.1 pH across a range of physiologically observed values.     

Enhanced activity and stability of Cu-Mn and Cu-Ag catalysts supported on nanostructured mesoporous CeO2 for CO oxidation

Cu, Mn and Ag nanoparticles are loaded on nanostructured mesoporous CeO2 as catalysts for CO oxidation. The Cu/CeO2 catalyst exhibits an obvious deactivation after the stability test at 95 degrees C for 60 h. This is caused by carbon deposition as confirmed by FTIR spectroscopy, Raman spectroscopy and thermogravimetry-differential scanning calorimetry-mass spectroscopy (TG-DSC-MS) analysis. It is found that the Cu-Mn or Cu-Ag binary metal catalysts supported on the nanostructured CeO2 exhibit much improved activity and stability in CO oxidation. In ease case, carbon deposition is absent in the similar stability test, due to enhanced oxygen adsorption property.     

Electron paramagnetic resonance and optical absorption studies of Cu2+ ion doped polyvinyl alcohol films

Electron paramagnetic resonance (EPR) and optical absorption studies have been carried out on Cu²⁺ ions doped in polyvinyl alcohol films (PVA). EPR spectrum at room temperature exhibits hyperfine structure characteristic of Cu²⁺ ions in tetragonal symmetry. The EPR spectra have also been recorded at various temperatures. The number of spins participating in the resonance is measured as a function of temperature and the activation energy is calculated. The paramagnetic susceptibility (χ) is calculated from the EPR data at various temperatures and the Curie constant is evaluated from 1/χ versus T graph. The optical absorption spectrum exhibits a broad band which has been assigned to the transition ²B_1g → ²B_2g.     

Electron paramagnetic resonance characterization of aluminum ion implantation-induced defects in 4H-SiC

Deep-level defects in silicon carbide(SiC) are critical to the control of the performance of SiC electron devices. In this paper, deep-level defects in aluminum ion-implanted 4 H-SiC after high-temperature annealing were studied using electron paramagnetic resonance(EPR) spectroscopy at temperatures of 77 K and 123 K under different illumination conditions. Results showed that the main defect in aluminum ion-implanted 4 H-SiC was the positively charged carbon vacancy( VC~+), and the higher the doping concentration was, the higher was the concentration of VC~+. It was found that the type of material defect was independent of the doping concentration,although more VC~+ defects were detected during photoexcitation and at lower temperatures. These results should be helpful in the fundamental research of p-type 4 H-SiC fabrication in accordance with functional device development.