Paramagnetic Surface Research Articles

Evidence of magnetic ordering of paramagnetic surface defects on partially hydroxylated MgO nanocrystals

EPR experiments carried out at various temperatures for partially hydroxylated nanocrystalline MgO exposed to atomic hydrogen reveal that the entrapped electrons arising from spontaneous dissociation of hydrogen atoms exhibit an antiferromagnetic ordering. The measured magnetic data is found to be compatible with a phenomenological model of interacting paramagnetic centres at high temperature, and with a Pauli like behaviour below a certain temperature. This is further corroborated with density functional calculations of the effective magnetic coupling parameter for Heisenberg model between entrapped electrons on suitable embedded cluster models of MgO nanocrystals.

Hyperpolarized 83Kr and 129Xe NMR Relaxation Measurements of Hydrated Surfaces: Implications for Materials Science and Pulmonary Diagnostics

In this proof of principle work, a technique is introduced to study hydrated surfaces using hyperpolarized (hp) 83Kr NMR spectroscopy. The longitudinal (T1) relaxation of hp-83Kr is shown to be extremely sensitive to the presence of adsorbed water on hydrophilic borosilicate and hydrophobic siliconized glass surfaces. The krypton surface relaxation is found to be largely independent of the total gas pressure applied to the studied materials, and the presented technique is therefore fairly robust. However, the relaxational properties of hp-83Kr can be "tuned" by adjusting the composition of the optical pumping gas mixture. This effect may be important for practical applications such as hp-83Kr MR imaging and can be achieved without sacrificing signal intensity. Complementary information to that of hp-83Kr surface relaxation data can be obtained from hp-129Xe relaxation measurements that are sensitive to the presence of paramagnetic surface sites. In contrast to the signal decay of hp-129Xe, the longitudinal relaxation of 83Kr is largely unaffected by paramagnetic impurities, and in some materials, 83Kr and 129Xe show comparable T1 times that are caused by two completely different relaxation mechanisms. Finally, the relaxation times of 83Kr in contact with bovine lung surfactant coated glass pores that are similar in size to mammalian alveoli are presented. The results suggest that in vivo MR studies may be feasible and could provide valuable information about changes in pulmonary surface chemistry.

Reduction and fragmentation of CS2 at the surface of electron-rich MgO: an EPR study

Sulfur contamination of alkaline earth oxide surfaces has important consequences in many fields of chemistry, such as surface science and catalysis. We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of CS2 molecules under different pressure and temperature conditions, with the bare and electron-enriched surface of MgO. CS2 reacts on the oxide surface through two distinct reaction paths, one leading to diamagnetic species, the other to paramagnetic surface entities. Both these reaction paths lead to the fragmentation of the CS2 molecule and, in some cases to oligomerization processes. EPR experiments allowed to follow the formation of the unstable CS 2 − radical intermediate and its evolution through formation of several surface radicals like S−, S 3 − and S n − (n ⩾ 3), depending on the reaction conditions.

Paramagnetic relaxation of spin polarized 3He at bare glass surfaces

In this first in a series of three papers on wall relaxation of spin polarized, gaseous 3He we investigate both by theory and by experiment surface-induced spin relaxation due to paramagnetic sites in the containing glass. We present experimental and theoretical evidence that — contrary to the traditional opinion — distant dipolar coupling to paramagnetic impurities in the glass, in particular iron ions, cannot be the dominant relaxation mechanism of 3He-spins, although iron dominates the bulk static permeability. Instead dangling-bond type defects in the glass matrix are found to interact much stronger via the isotropic Fermi contact interaction. A model of paramagnetic site controlled 3He relaxation including the Fermi contact interaction is presented. With reasonable semi-empirical assumptions our model allows to describe satisfactorily the measured relaxivities, both in the dissolution-dominated regime of fused silica or borosilicate glasses of the Pyrex type as well as in the surface dominated situation of aluminosilicate glasses which have only a low permeability for He atoms. In a large sample of 1.1 litre cells, built from various aluminosilicate glasses, an average relaxation time of 150 h is reached in case contaminant ferromagnetic particles have been demagnetized beforehand. From the maximum observed value of 250 h we derive after subtraction of dipolar relaxation in the gas phase a paramagnetic surface relaxivity of ρ<0.005 cm/h at room temperature.

Magnetization studies of the nuclear spin ordered phases of solid 3He in silver sinters

Solid 3 He, in the bcc lattice between 34 and 100 bar, exhibits two nuclear magnetic ordered phases in the sub-mK temperature range, the so called U2D2 low (magnetic) field phase and the "high field phase" above 0.4 T. To determine the exact spin structure of these phases we started a project of neutron scattering from the ordered solid in collaboration with the Hahn-Meitner Institute, Berlin, and other European and US groups. For this experiment it is crucial to grow a single crystal within the sinter needed for cooling the solid to temperatures of the order of 500 µK (or even twenty times lower in the case of the hcp lattice which is formed above 100 bar) and to keep it there long enough to measure a magnetic neutron reflection. We studied the growth of crystals in Ag sinters of different pore size and with different growth speeds to find an optimal way to obtain single crystalline samples. As a first diagnostic step we performed pulsed NMR measurements in the ordered phases of solid 3 He in a sinter of 2700 ˚ article size down to temperatures of 450 µK at various molar volumes. We could keep the samples in the ordered state for as long as 140 h. The second method we used was SQUID magnetometry. For the low field phase TN was indicated by a drop of the intensity, both in the NMR signal and in the dc magnetization, whereas in the high field phase an increase of about 30% was observed below the ordering temperature. For the fabrication of the sinters a packing fraction of 50% and subsequent annealing proved to be very favorable to obtain cold ordered solid. Furthermore, we find that a paramagnetic surface contribution from a few monolayers of 3 He exists down to 500 µK in addition to the bulk magnetization.

Promoting effects of H2 on photooxidation of volatile organic pollutants over Pt/TiO2

Superior photocatalytic activity and durability of Pt/TiO2 for decomposing volatile organic pollutants (typically aromatic compounds) have been obtained by adding trace H2 into an O2-rich photooxidation system. The order of photodegradation efficiencies for volatile organic compounds is cyclohexane < acetone < benzene < toluene < ethylbenzene. Exceptional stability of Pt/TiO2 for benzene photodegradation is still maintained after 22 h of testing, by the repeated use of the catalyst four times. The promoting effects of H2 in the photocatalysis were studied by infrared spectroscopy (IR), spin-trapping electron paramagnetic resonance (EPR) and surface photovoltage spectroscopy (SPS). Results demonstrated that the introduced H2 has several beneficial effects on heterogeneous photocatalysis, namely, a comparatively clean surface of Pt/TiO2 with no persistent aromatic intermediates, an increased number of surface hydroxyl radicals in the photocatalytic process, and an enhanced separation efficiency of photogenerated electron–hole pairs. A mechanism is proposed to elucidate the promoting effect of H2 on the photooxidation of volatile organic pollutants over Pt/TiO2.

Alkali Metal Doping of MgO: Mechanisms of Formation of Paramagnetic Surface Centers

AbstractFor Abstract see ChemInform Abstract in Full Text.

The interaction of positronium with paramagnetic surface defect at low temperatures in fine Al 2O 3 powders

We used the age-momentum correlation technique at the high energy position beam in Stuttgart for the study of ortho-positronium in fine alumina powders. At low temperatures, the irradiation of the positron beam creates stable paramagnetic defects on the surfaces of the powder grains. The age-momentum correlation spectra are analyzed in terms of the age-dependent line-shape and peak-to-valley parameter. A model considering a surface positronium state and a slow thermalizing inter-particle positronium state is proposed to explain the spectra.

Alkali Metal Doping of MgO: Mechanisms of Formation of Paramagnetic Surface Centers

The adsorption of alkali-metal atoms (M = Li, Na, and K) on the surface of MgO has been studied by means of embedded-cluster DFT calculations. Alkali-metal atoms bind preferentially to the oxide anions with energies of the order of I eV. On these sites the ns valence electron remains localized on the alkali atom, but the substantial polarization (ns-np mixing) leads to major changes in the isotropic hyperfine coupling constants. In the presence of specific defect sites, like a bare oxygen vacancy, F S 2+ center, a net charge transfer occurs, with formation of F S + color centers. At higher coverages, once the F S 2+ centers have been saturated, a different process takes place. At specific neutral morphological defects, like a cationic reverse corner, the alkali atom valence electron is transferred to the surface with formation of M + (e - ) t r a p p e d pairs. The computed properties of the unprecedented M + (e - ) t r a p p e d pairs (hyperfine constants and optical transitions) are consistent with the experimental measures and show that the trapped electron and the adsorbed alkali-metal cation are separated by short distances.

Mixing at micrometric and nanometric scale in mechanically alloyed Fe 60Cr 40

Mechanical alloying (MA) of iron and chromium powder mixtures was performed from 0 to 190 h. SEM, transmission electron microscopy with energy-dispersive X-ray analyses and Mössbauer spectrometry were used to study the mixing at micrometric and nanometric scale. At the stationary mixing state (≈20 h of milling), MA of Fe 60Cr 40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of crystalline Fe 60Cr 40 alloy surrounded by a disordered paramagnetic grain surface (at 300 K), which increases during milling. At 85 h, the nanograins are totally disordered. For a longer time of milling, a de-mixing occurs by partial recrystallization of this disordered structure.

Characterization of a model Ziegler–Natta catalyst for ethylene polymerization

Based on the work of the Somorjai group [Magni and Somorjai, Catal. Lett. 35, 205 (1995)] we have prepared a thin well ordered MgCl2(001) film by MgCl2 evaporation from a Knudsen cell. This film does not absorb TiCl4 at room temperature if it is not activated by increasing the defect density via electron or ion bombardment. The nature of some of the defects created is characterized by in situ ESR measurements and Auger spectroscopy. Paramagnetic surface defects are altered by the bonding of TiCl4 to the surface as observed by ESR spectroscopy. Ti3+ centers are detected if particularly severely defected MgCl2 layers are prepared. Reactivity studies show however, that these species are not correlated with polymerization activity. Interaction with aluminum alkyl leads to the formation of the active catalyst and we observe for the first time directly ethyl radicals formed from trimethyl-aluminum in an abstraction process which may be formulated as TiCl4/surface+AlMe3→Me−TiCl3/surface+AlMe2Cl, Me−TiCl3/surface→TiCl3/surface+Me⋅, and Me⋅+Me3Al→C2H5⋅+AlH(Me)2. The presence of the aluminum alkyl is observed via in situ IRAS in the same apparatus.

Site selective hydroxylation of the MgO surface

On the surface of MgO nanoparticles 1- and multiple (n = 3 and 4) coordinated hydroxyl groups are generated site-selectively and studied by transmission infrared spectroscopy. Their formation is based either on the stepwise dehydration of previously hydrated samples or on simple surface reactions involving molecular hydrogen on the one hand, and molecular oxygen, nitrous oxide or properly selected UV quanta on the other as reactants. 4-Coordinated OH groups may interact (a) with adjacent surface adsorbants such as hydrides (Mg2+H−) or superoxide anions (Mg2+⋯O−2) and (b) with surface defects like oxygen anions on steps (O2−) or paramagnetic surface colour centers. In each case the frequency of the respective IR-active OH stretching vibration is indicative of a specific local surface environment to which the OH probe is exposed. On the other hand, only perfectly free and isolated 3-coordinated OH groups have so far been observed. For sterical reasons they are exempt from any interaction with adjacent surface groups or surface defects.

Positronium in Metal-Oxide Powders Studied with Age. The Age-Momentum Correlation Technique

For the first time positronium is investigated in fine powders of MgO and Al 2 O 3 using age-momentum correlation technique based on a relativistic positron beam. The application of this technique for investigating the interaction of positronium with the grain surfaces is discussed and compared with other techniques. The previously reported interaction of the positronium with paramagnetic centres is further studied. A qualitative interpretation of the spectra is given. In the Al 2 O 3 samples we found some unexpected behaviour of the conversion quenching of ortho-positronium at irradiation induced paramagnetic surface defect.

Electron paramagnetic resonance and surface resistance measurements in MgB 2 and Mg 1− xLi xB 2

The temperature dependence of the electron paramagnetic resonance and surface resistance of MgB 2 and MgB 2 doped with lithium is studied. A single narrow symmetric line having an isotropic g value of 2.0036 is observed which is attributed to mobile spin 1/2 holes in the hexagonal boron planes. Both the g value and line width are independent of temperature in the normal state but there is a marked decrease in the intensity of the spectrum as the temperature is lowered to T c. The surface resistance is observed to decrease linearly with temperature in the normal state of the undoped material. In the doped material there is some deviation of the surface resistance from the linear temperature dependence starting at 150 K suggesting the possibility of the existence of a spin gap in the less than optimally doped material. Large changes in surface resistance as a function of dc magnetic field are observed in the superconducting state resulting from dissipation due to vortex movement.

Recent Applications of Electron Magnetic Resonance (EMR) Techniques in Heterogeneous Catalysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Recent applications of electron magnetic resonance (EMR) techniques in heterogeneous catalysis

EMR techniques have been extensively used in the past year to explore problems relevant to heterogeneous catalysis, including surface defects and radicals, redox processes with supported transition metal ions and in situ studies at elevated temperatures. The combination of these techniques with computational methods, can now be used to provide detailed structural information of active sites even in polycrystalline materials. Developments in high field EMR and the utilisation of pulsed EMR methods are providing marked improvements in sensitivity and spatial resolution of paramagnetic surface states.

Microscopic and Macroscopic Approaches of Cu(II) Removal by FSM-16

The removal of Cu(II) by a mesoporous material, FSM-16, was studied using electron paramagnetic resonance (EPR) spectroscopy and surface complexation modeling (SCM). Free copper ions, adsorbed and precipitated Cu(II) species were qualitatively identified by in situ EPR spectroscopy of Cu-FSM-16 suspensions at room temperature and at 77 K. In addition, the adsorbed species was identified as a Cu(II) species with an axial symmetry from an analysis of the EPR spectra of “dry” Cu-FSM-16 at 77 K. On the basis of the EPR results, the removal of Cu(II) as a function of pH under various experimental conditions was successfully simulated by assuming two removal mechanisms such as surface complexation and surface precipitation. In the acidic pH range (<pH 6), free copper ions were predominant, and surface complexed then surface precipitated species became dominant as the pH increased.

Detecting Sorbed Hydrocarbons in a Porous Medium Using Proton Nuclear Magnetic Resonance

An ability to detect residual (sorbed) concentrations of hydrocarbons in a porous medium using nonintrusive geophysical methods is of interest in the remediation of hydrocarbon-contaminated sites. Proton NMR is an noninvasive analytical technique that can be applied to characterize several rock properties in the lab and the field. In this study, the authors perform a series of laboratory experiments to quantify the effect of sorbed crude oil on the NMR relaxation time (T{sub 1}) of water-saturated silica gels, both in the presence and absence of iron oxyhydroxide. The experimental data indicate that in the absence of paramagnetic iron impurities, T{sub 1} remains constant in the presence of sorbed hydrocarbons. T{sub 1} increases with increasing hydrocarbon concentration only if the silica surface is coated with a paramagnetic substance such as iron oxyhydroxide. These results suggest that increases in T{sub 1} previously observed for water in natural materials coated with oil are caused by the shielding of paramagnetic surface sites by the sorbed hydrocarbons. The major implication of this study is that proton NMR is a potential means of detecting sorbed or residual hydrocarbons in natural environments.

Particle size effect on transverse NMR relaxation in aqueous char suspensions

A strong particle size influence on NMR transverse proton relaxation in aqueous suspensions of several newly synthesized carbon based chars is reported. The results are quantitatively interpreted in terms of a two-stage molecular exchange model. A porous cage effect leads to slow exchange between molecules inside and outside pores, in contrast to fast molecular exchange at the solid–liquid interface, where a familiar two-site formalism can be applied. The dependence of surface properties on the relaxation data was detected. The transverse NMR relaxation time of water protons on the paramagnetic surface of chars is estimated to be T 2s≈ 0.5×10 −5 s, which is comparable with the EPR relaxation time of the free radical centers. Dynamic nuclear polarization (DNP) is used to estimate the paramagnetic shift Δ v of water protons at the surface. The value Δ v so obtained is much less than the transverse NMR relaxation rate of water protons on the paramagnetic surface Δv ⪡ T 2 s −1 . This inequality can account for the independence of the NMR transverse relaxation times on the interval τ observed in CPMG measurements.

Spectroscopic and surface potential variations study of a CuCe oxide catalyst using H 2S as a probe molecule

X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and surface potential variations were used to study the reactivity of H 2S on CuCe oxide catalyst. In the fresh catalyst, Cu 2+ ions are present as monomers, dimers and clusters. After the H 2S treatment at 293 K, all the Cu 2+ species of catalyst become reduced. Such a reduction is irreversible even in the presence of oxygen. In parallel, S 2− and SH − groups associated with copper in CeO 2 are formed. Sulphide groups are stable and cannot be removed by further O 2 treatment. The valence of Ce(IV) is not affected with this H 2S treatment.