Characterization and Quantification of Reduced Sites on Supported Vanadium Oxide Catalysts by Using High‐Frequency Electron Paramagnetic Resonance

Dinse, Arne; Carrero, Carlos A.; Ożarowski, Andrzej; Schomäcker, Reinhard; Schlögl, Robert; Dinse, Klaus−Peter

doi:10.1002/cctc.201100412

Cited by 24 publications

(38 citation statements)

References 69 publications

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“…Figure 13 shows spectra of Fe co-doped samples with increasingL ia nd Fe content.S imilar to the Li/MgO samples with increasing Li content, the 7 Li resonances get significantly broader pointingt oamore disordered lattice environment. For higher Li and Fe concentrations, such as the sample containing 20 at %L ia nd 20 at %F e, the width of the 7 Li resonance is comparable to that of Li/MgO samples with al ine width of 1300 Hz. Figure 14 shows the 7 Li solid-state NMR spectra of the Fe-Li/ MgO-2 sample before and after calcination.…”

Section: Solid-state Nmr Spectroscopymentioning

confidence: 80%

“…For higher Li and Fe concentrations, such as the sample containing 20 at %L ia nd 20 at %F e, the width of the 7 Li resonance is comparable to that of Li/MgO samples with al ine width of 1300 Hz. Figure 14 shows the 7 Li solid-state NMR spectra of the Fe-Li/ MgO-2 sample before and after calcination. After calcination the spectrum exhibits an arrower 7 Li resonance line.…”

Section: Solid-state Nmr Spectroscopymentioning

confidence: 90%

“…Details of the experimental set-up used are described elsewhere. [7,8] For identification of the effective spin of the paramagnetic centers as pecific two-dimensional EPR technique (Transient Nutation EPR, TN-EPR)i su sed, mapping the Rabi nutation frequencies of the paramagnetic centers along one axis, as functiono ft he EPR absorption pattern (second axis). [9,10] The principle of the TN-EPR spectroscopy is based on the fact that the transition moment between spin sublevelso nr esonance is proportional to the nutation frequency n of spin magnetization in the rotating frame of an oscillatingM Wf ield.…”

Section: Epr Spectroscopymentioning

confidence: 99%

“…The width of the p/2 pulse was 20 ns in Xa nd Q band, the initial HTAp ulse was 32 ns, the total length T set to 512 4ns, the initial delay t was 300 or 100 ns, respectively. Li using aB ruker 4mmdouble-resonancep robe head operating at as pinning rate of 12 kHz for 7 Li. Composite pulse proton decoupling was applied during the acquisition.…”

Section: Epr Spectroscopymentioning

confidence: 99%

“…Apart from their potential direct role in catalysis, these dopants are anticipated to stabilize Li in the catalyst under the reactionc onditions, allowing further clarificationo f the role of Li. In the case of Gd/Li co-doping, changes observed in EPR and 7 Li-NMR spectra indicate the formation of correlated, next-neighborL i the local topology aroundt he paramagnetic centers formed by alio-valent Li + and Me 3 + ions. In ap reviousC ommunication, we have shown that Fe and Gd can be used as spin sensors for electron paramagnetic resonance (EPR) spectroscopy to investigate the local structure of such materials [5] by determining the site symmetry at the paramagnetic centers formed by alio-valent doping.…”

Section: Introductionmentioning

confidence: 97%

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Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

et al. 2017

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The role of Li in Li/MgO as a catalyst for oxidative coupling of methane (OCM) is to promote MgO surface morphology change rather than serve as a constituent of catalytically active sites. While Li/MgO is unstable at realistic conditions with respect to loss of Li, the resulting samples show enhanced selectivity towards C2 hydrocarbons versus CO2, although activity is low and close to pristine MgO. The way (co‐)doping with alio‐valent metal ions affects the catalytic performance of Li/MgO has now been explored. To analyze the structure and the stability of the samples, catalysts with well‐defined stoichiometry were prepared using a co‐precipitation method with freeze‐drying and subsequent annealing. Gd and Fe were used as dopants. Apart from their potential direct role in catalysis, these dopants are anticipated to stabilize Li in the catalyst under the reaction conditions, allowing further clarification of the role of Li. In the case of Gd/Li co‐doping, changes observed in EPR and 7Li‐NMR spectra indicate the formation of correlated, next‐neighbor Li−Mg⋅⋅⋅Gd+Mg pairs co‐existing with “isolated” Gd3+ ions at octahedral Mg lattice sites. For Li/Fe co‐doping, no significant change in the EPR pattern is observed in the presence of Li+ ions, indicating a larger distance between the Li+ and Fe3+ cations in the MgO lattice. Hybrid DFT calculations explain the difference between Fe and Gd co‐doping by a less efficient screening of the Coulomb repulsion between Gd3+ and neighboring cations in Gd doped samples, leading to the stabilization of LiMg near GdMg.

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Section: Solid-state Nmr Spectroscopymentioning

confidence: 80%

Section: Solid-state Nmr Spectroscopymentioning

confidence: 90%

Section: Epr Spectroscopymentioning

confidence: 99%

Section: Epr Spectroscopymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

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Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

et al. 2017

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New Insights into Defect‐Mediated Heterostructures for Photoelectrochemical Water Splitting

Wen

et al. 2016

Advanced Energy Materials

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Understanding the interfacial electronic structures of heterojunctions, a challenging undertaking, is extremely important to the design of photoelectrodes for efficient water splitting. The heterostructured interfaces in terms of crystal defects at the atomic‐level exemplified by TiO2/BiVO4 are studied. Results from both experimental observations and theoretical calculations clearly confirm the spontaneous formation of defective interfaces in the heterostructures. TiO2/BiVO4 junction with engineered interfacial defects can efficiently increase the carrier density and extend the lifetime of electrons. The inherent phenomenon of defective electronic structures in different heterostructures creates a significant impact on their photoelectrochemical performance. The synergetic effect between defect‐mediated mechanism and organic quantum dots sensitization yields significantly increased photoconversion efficiency, which is even superior to that of common metal sulfide sensitized ones. This result demonstrates an approach worthy for the design and fabrication of defect‐mediated heterostructures for water splitting, without utilizing harmful metal sulfides. Moreover, new insights into the influence of intrinsic defects on the interfacial charge transfer process between two different semiconductors for energy‐related applications have also been provided.

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Supported Vanadium Oxide Clusters in Partial Oxidation Processes: Catalytic Consequences of Size and Electronic Structure

2017

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The catalytic activity for ethanol partial oxidation of vanadium oxide (VOx) anchored on titanium oxide was correlated to their electronic structure. In situ Raman spectroscopy and temperature‐programmed desorption (TPD) experiments indicate that the presence of catalytically active VOx moieties is very sensitive to vanadia loading: highly dispersed VOx predominantly exists at low VOx contents whereas larger vanadia clusters coexist at higher VOx loadings. In situ UV/Vis spectroscopy revealed that a significant fraction of these larger clusters remain reduced during catalysis, and thus do not fully participate in catalytic turnovers. The electronic structures of model VOx nanoclusters of different sizes (monomer, dimer, trimer, and one‐dimensional polymers) were investigated by using periodic density functional theoretical calculations. Results indicate that their electronic structures are significantly affected by their size. Our analysis also revealed that the formation of reduced VOx species (V4+) during catalysis is concomitant to the reduction of adjacent Ti cations (Ti3+). Theoretically calculated optical absorption spectra matched the experimental spectroscopic results obtained under in situ reaction conditions. Furthermore, the determination of defect formation enthalpies reported previously as the main descriptor for catalytic activity of vanadia nanoclusters, predicted that isolated monomeric VOx clusters predominantly take part in catalytic turnovers.

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Characterization and Quantification of Reduced Sites on Supported Vanadium Oxide Catalysts by Using High‐Frequency Electron Paramagnetic Resonance

Cited by 24 publications

References 69 publications

Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

New Insights into Defect‐Mediated Heterostructures for Photoelectrochemical Water Splitting

Supported Vanadium Oxide Clusters in Partial Oxidation Processes: Catalytic Consequences of Size and Electronic Structure

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