EPR Characterization and Reactivity of Surface-Localized Inorganic Radicals and Radical Ions

Chiesa, Mario; Giamello, Elio; Che, Michel

doi:10.1021/cr800366v

Cited by 183 publications

(168 citation statements)

References 192 publications

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“…4(a) and 4(b) do not resemble previous EPR spectra of MgO with isolated defects. [33][34][35] Comparison between the samples shows that the low-field EPR spectra of the AP MgO-NSs possess a large broad peak, conclusively showing that the spins in the system are strongly coupled and form spin clusters, probably at dislocations. 36 Note that the intensity distribution of this peak further shifts to lower fields with decreasing temperature as more spins become coupled and the interacting spin clusters increase in size.…”

Section: -13mentioning

confidence: 91%

“…However, upon annealing at 1323 K, where a substantial sintering process occurred, the defect density was reduced, as seen from the reduction in peak area, and the new split defect signal appeared primarily at g ∼ 1.97 and g ∼ 2.17, probably related to trapping of superoxide (O − 2 ) radicals in the air-annealed MgO. 33 The small peak apparent in the spectra at ∼1500 G indicates the presence of a low concentration of high-spin Fe 3+ . Characteristic multiplets of manganese ions were not visible in any of the EPR spectra.…”

Section: -13mentioning

confidence: 98%

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Defect-induced magnetism in chemically synthesized nanoscale sheets of MgO

Maoz¹,

Tirosh²,

Bar‐Sadan³

et al. 2011

Phys. Rev. B

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Highly defective MgO nanosheets were prepared by a colloidal synthesis and exhibited low-temperature ferromagnetism which was significantly larger that the magnetization potentially obtainable from the low transition-metal impurity concentration. Electron paramagnetic resonance experiments confirmed that the magnetization did not significantly involve impurities and that the nanosheets consisted of strongly interacting spin clusters which disappeared upon high-temperature annealing. These spins were concentrated along extended defects, possibly as unpaired electrons trapped at oxygen vacancies. DIM should be highly sensitive to the various defect or impurity characteristics, posing experimental and theoretical difficulties that demand prudence. Experimentally, Abraham et al.14 and Garcia et al. 15 showed the ease with which a ferromagnetic signal is obtained in such oxides due to impurities. The problem of paramagnetic transition-metal ion impurities as a source of magnetism was recently comprehensively studied by Khalid et al. 16 Much of the previous experimental work claiming to have detected DIM in MgO 9-13 as well as other materials ignored the need to precisely determine the paramagnetic metal impurity concentration in the analysis of results. Preparation of samples by deposition in a stainlesssteel apparatus, 12 using characterization techniques such as x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) with insufficient sensitivity to low concentrations of impurities, 9,13 or using less-than ultrapure precursors or not addressing precursor purity 10,11 may lead to confusion about the source of the observed ferromagnetism. The theory of DIM is also problematic: In the early 70s large magnetic moments were predicted on neighboring oxygen atoms due to Mg vacancies.17 Elfimov et al. 3 predicted cation vacancy-induced ferromagnetism using density functional theory (DFT) calculations. Also, further theoretical work attributed the formation of magnetic moments and the resulting FM to spin-polarized holes residing on cation p orbitals either at vacancy or impurity sites. 3,18 Most DIM studies, both experimental [10][11][12][13] and theoretical, 12,[19][20][21] suggested that the origin of the observed FM in these oxides is related to cation vacancies or even nitrogen doping in MgO.22-24 Recent theoretical work discussed interdefect coupling mechanisms, 9,25-27 such as high carrier mobility near interfaces. Very recently, Zunger and coworkers 28,29 and Droghetti et al. 2 drew attention to the Achilles' heel of all DFT calculations performed with local approximations of the exchange correlation potentials, such as the local spin density approximation and the generalized gradient approximation. These methods overestimate spin density delocalization, leading to a half-metallic ground state with a relatively strong interdefect ferromagnetic interaction.In this paper we present results obtained on highly defective MgO nanosheets (MgO-NSs) prepared by a colloidal synthesis from the ultrapure precurso...

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Section: -13mentioning

confidence: 91%

Section: -13mentioning

confidence: 98%

Defect-induced magnetism in chemically synthesized nanoscale sheets of MgO

Maoz¹,

Tirosh²,

Bar‐Sadan³

et al. 2011

Phys. Rev. B

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“…It can thus be used for studying surface acid-base centers using paramagnetic nitroxide probes [93][94][95][96]. Nitroxide group [N Á À O, acting as a Lewis base, binds to a surface electron acceptor site (A) through oxygen lone pair according to…”

Section: Electron Spin Resonance Techniquementioning

confidence: 99%

Acid–base characterization of heterogeneous catalysts: an up-to-date overview

Védrine

2015

Res Chem Intermed

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In this overview, we focus on the different and current methods of acidbase characterization of heterogeneous catalysts and on their potential relation with catalytic properties in gas and liquid phases. Special emphasis is drawn on the main techniques currently employed such as the use of Hammett's indicators, the use of basic or acidic probes of different strength for adsorption measurements in microcalorimetry and of their thermal programmed desorption, the use of other techniques such as FTIR, ESR, NMR, photoluminescence, Raman, UV-Vis, and XPS, to identify and describe acid-base sites, the use of model catalytic reactions, and of theoretical/modeling approaches. Relationships between such a characterization and catalytic properties in different acid or base reactions are discussed, and implemented for different catalysts and different reactions. The concept of thermodynamic acidity/basicity (determined via chemical physical characterization) versus reactivity (determined via catalytic properties) of acid-base sites is presented, and explains why relationships between acid-base properties and catalytic properties are very often met but are difficult to be established unambiguously and cannot be generalized. The case of acid-base properties in liquid phase ''polar'' (water, alcohol) and ''apolar'' (cyclohexane) as ''effective'' and ''intrinsic'' properties, respectively, and their relation with catalytic properties in liquid phase are also discussed.Keywords Acid-base properties of solid catalysts Á Related catalytic properties Á Intrinsic versus effective acidity/basicity Á Acidity/basicity in liquid polar or apolar media

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“…While NMR experiment sensitivity remains a limiting parameter, EPR spectroscopy is the reference technique for the study of trace amounts of paramagnetic centers in metal oxide powders [128]. It therefore constitutes a valuable analytical tool in heterogeneous catalysis research [102][103][104]. A comprehensive review of this technique and its importance for the elucidation of defects on oxide surfaces is given in Chap.…”

Section: Magnetic Resonance Techniquesmentioning

confidence: 99%

Defects in Metal Oxide Nanoparticle Powders

Berger

Diwald

2015

Defects at Oxide Surfaces

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Metal oxides are mainly used in powder or particulate form for their application as functional materials. Insights into functional as well as unwanted properties of different defect types ranging from oxygen vacancies to grain boundaries and pores require an integrated characterization approach and are extremely difficult to establish. After a brief introduction into the complexity of particle systems and related heterogeneities, we discuss examples where for defects and other distinct structural features of MgO or TiO 2 particle systems firm structureproperty relationships have been established. Moreover, we want to point out that processing of particle matters. Related microstructural changes can induce the formation of solid-solid interfaces upon transformation of nanoparticle powders into mesoporous nanoparticle networks. This change in aggregation level and microstructure can substantially modify the electronic, optical and spectroscopic properties of metal oxide nanoparticle ensembles and, for this reason, plays a critical role for the generation of structural and-at the same time-functional defects inside particle ensembles. Introduction Particle Systems and the Hierarchy of DefectsLarge quantities of metal oxide particle powders are widely used in materials science and employed in very diverse areas such as engineering [1] catalysis [2] electronics [3][4][5] and energy technology [6]. The behavior of a metal oxide powder

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EPR Characterization and Reactivity of Surface-Localized Inorganic Radicals and Radical Ions

Cited by 183 publications

References 192 publications

Defect-induced magnetism in chemically synthesized nanoscale sheets of MgO

Defect-induced magnetism in chemically synthesized nanoscale sheets of MgO

Acid–base characterization of heterogeneous catalysts: an up-to-date overview

Defects in Metal Oxide Nanoparticle Powders

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