EPR as a Tool To Investigate the Transition Metal Chemistry on Oxide Surfaces

Dyrek, K.; Che, Michel

doi:10.1021/cr950259d

Cited by 192 publications

(147 citation statements)

References 96 publications

(224 reference statements)

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“…16 Simulations of the spectra in Figure 4 verify that the multiplet is due to a well-defined magic number Pt 13 cluster and to a significant decrease of the g anisotropy (see below). Furthermore, exchange of deuterium for hydrogen leads to slightly narrower lines of the multiplet, resulting in a better resolution.…”

Section: Resultsmentioning

confidence: 74%

Support Effects on Hydrogen Desorption, Isotope Exchange, Chemical Reactivity, and Magnetism of Platinum Nanoclusters in KL Zeolite

et al. 2013

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Platinum clusters were prepared by ion exchange of a KL zeolite, followed by oxygen calcination and hydrogen reduction, and characterized by electron paramagnetic resonance (EPR) and hyperfine sublevel correlation experiment (HYSCORE) spectroscopy. Simulations indicate that the cluster contains 12 equivalent platinum atoms. Therefore, the most likely structure is an icosahedral or cuboctahedral magic number Pt 13 cluster with 12 platinum atoms at the surface.One atom in the center possesses only a small spin density. H/D desorption and readsorption experiments monitored via EPR and HYSCORE measurements provide information about the structure of the clusters and about the reversibility of the adsorption/desorption and isotope exchange process. Deuterium desorption experiments result in a value of 2.1±0.2 eV for the D 2 desorption energy of the Pt 13 clusters dispersed in KL zeolite. This is more than double of the 2 value for a (111) Pt single-crystal surface, revealing a finite size and/or support effect. Oxygen adsorption on deuterium covered Pt 13 clusters did not show a vigorous oxyhydrogen reaction. It appears that the reaction is inhibited by the deuterium coverage.

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Section: Resultsmentioning

confidence: 74%

Support Effects on Hydrogen Desorption, Isotope Exchange, Chemical Reactivity, and Magnetism of Platinum Nanoclusters in KL Zeolite

et al. 2013

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“…Although there are to the best of our knowledge no studies about the position of framework oxygen atoms of microporous aluminophosphates in the spectrochemical series, we believe that the ligand field strength of a framework oxygen is lower than that of a water molecule. Taking into account the rule of the average environment 68 (the decrease of the crystal field strength upon substitution of water by framework oxygen atoms is a linear function of the number of oxygen ligands), one can conclude that V 4+ ions in VAPO-16 are surrounded by more framework oxygen atoms than in the AEL and AFI structures. All the Co-containing AST aluminophosphate molecular sieves, regardless of the form, possess two overlapping triplet structures ( Figure 9B), which can be resolved in two sets of three Gaussian bands by spectrum deconvolution.…”

Section: Methodsmentioning

confidence: 99%

Incorporation of Transition Metal Ions in Aluminophosphate Molecular Sieves with AST Structure

et al. 2001

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Transition metal ion containing aluminophosphate molecular sieves with AlPO 4 -16 (AST) structure have been synthesized using the classic template, quinuclidine, and the corresponding salts of V 4+ , Co 2+ , Cr 3+ , and Mn 2+ . The cubic AST form was the main product, but in the presence of fluoride ions, the tetragonal AST form could also be obtained. The AST materials were characterized with X-ray diffraction, thermal analysis, scanning electron microscopy, electron spin resonance, infrared spectroscopy, and diffuse reflectance spectroscopy in the UV-vis-NIR region. Spectroscopic measurements showed that small amounts of Co 2+ and Mn 2+ can be incorporated in the framework. The diffuse reflectance spectra are indicative for the presence of two types of Co 2+ , most probably taking the two framework positions T 1 and T 2 with a T 1 :T 2 occupation ratio of 3-8. In contrast, V 4+ is present in defect sites by coordination to two (or three) framework oxygen atoms. The incorporation of Cr 3+ in the AST framework is also uncertain and two surface Cr 3+ species were observed: clusters similar to the bulk Cr 2 O 3 oxide and isolated Cr 3+ ions in strongly distorted octahedral coordination. These spectroscopic observations were confirmed by thermal analysis results, which showed that the degree of metal ion incorporation is inversely proportional with the phase transition temperature of AST to berlinite. Furthermore, Co 2+ and V 4+ are very stable in the AST structure and difficult to oxidize upon heating.

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“…This with the aim to gather information on the electron hopping between Mo 5+ and Mo 6+ centres [24], as well as the local structure of these materials and possible binding modes of the oxygen to the MB nanoring (SS-EPR, section 3.1.2) [25].…”

Section: Dr-uv Spectroscopy Of Molybdenum Bluementioning

confidence: 99%

Insights into the Reaction Mechanism of Cyclohexane Oxidation Catalysed by Molybdenum Blue Nanorings

et al. 2015

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Molybdenum blue (MB), is a polyoxometalate with a nanoring structure comprising Mo 5+ -OMo 6+ bridges, which is active for the catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone. However, little is known about the mechanistic features responsible of this catalytic activity. In the present work, the Mo 5+ -O-Mo 6+ moieties embedded in the MB nanoring structure were characterized using diffuse reflectance-UV-Visible spectroscopy and solid state EPR spectroscopy. The amount of Mo 5+ centres was then varied by thermal treatment of the polyoxometalate in the absence of oxygen, and the resultant effect on the catalytic activity was investigated. It was observed that, an increased amount of Mo 5+ centres preserved the conversion of cyclohexane (ca. 6%) but led to a loss of selectivity to cyclohexanol giving cyclohexanone as the major product, and the simultaneous formation of adipic acid. To rationalise these results the catalysts were studied using EPR spin trapping to investigate the decomposition of cyclohexyl hydroperoxide (CHHP), a key intermediate in the oxidation process of cyclohexane. This analysis showed that CHHP has to be bound to the MB surface in order to explain its catalytic activity and product distribution.

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EPR as a Tool To Investigate the Transition Metal Chemistry on Oxide Surfaces

Cited by 192 publications

References 96 publications

Support Effects on Hydrogen Desorption, Isotope Exchange, Chemical Reactivity, and Magnetism of Platinum Nanoclusters in KL Zeolite

Support Effects on Hydrogen Desorption, Isotope Exchange, Chemical Reactivity, and Magnetism of Platinum Nanoclusters in KL Zeolite

Incorporation of Transition Metal Ions in Aluminophosphate Molecular Sieves with AST Structure

Insights into the Reaction Mechanism of Cyclohexane Oxidation Catalysed by Molybdenum Blue Nanorings

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