Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

Al‐Shankiti, Ibraheam; Alotaibi, Faisal M.; Al-Salik, Y.; Idriss, Hicham

doi:10.1007/s11244-013-0079-1

Cited by 41 publications

(37 citation statements)

References 45 publications

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“…However, U was found to increase STWS, either due to its ability to enable over‐oxidation, (i.e. Ce 1‐x U x O 2+y ) or its ability to take on multiple oxidation states, which then aid Ce 4+ reduction . Although U is unlikely to be used in commercial processes due to the radioactivity of its various isotopes, it may serve as a useful tool for identifying other dopants.…”

Section: Two‐step Stws Active Materialsmentioning

confidence: 99%

A review and perspective of efficient hydrogen generation via solar thermal water splitting

Muhich

Ehrhart

Al‐Shankiti

et al. 2015

WIREs Energy & Environment

189

160

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Solar thermal water splitting (STWS) produces renewable hydrogen from water using concentrated sunlight. Because it utilizes energy from the entire solar spectrum to directly drive the redox reactions that split water, it can achieve high theoretical solar-to-hydrogen efficiencies. In two-step STWS, a metal oxide is first heated by concentrated sunlight to high temperatures to reduce it and produce O 2 . In the second step, the reduced material is exposed to H 2 O to reoxidize it to its original oxidation state and produce H 2 . Various aspects of this process are reviewed in this work, including the reduction and oxidation chemistries of the active redox materials, the effects of operating conditions, and the solar thermal reactors in which the STWS reactions occur, and a perspective is given on the future optimization of STWS.

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Section: Two‐step Stws Active Materialsmentioning

confidence: 99%

A review and perspective of efficient hydrogen generation via solar thermal water splitting

Muhich

Ehrhart

Al‐Shankiti

et al. 2015

WIREs Energy & Environment

189

160

View full text Add to dashboard Cite

show abstract

“…33,37,[39][40][41][42] The presence of the smaller dopant cations, i.e. Zr and Hf, are believed to alleviate the expansive strain caused by the larger Ce 3+ cations via the overlap of tensile and compressive strain elds; simultaneously decreasing the extent of expansion and decreasing the reduction energy.…”

Section: Oxidationmentioning

confidence: 99%

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Muhich

Steinfeld

2017

J. Mater. Chem. A

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Ceria-based metal oxides are promising redox materials for solar H 2 O/CO 2 -splitting thermochemical cycles. Density functional theory (DFT) computations are applied to elucidate the underlying mechanism of the role of dopants in facilitating ceria based redox cycles; specifically, we explain why some dopants increase performance, while others do not. Firstly, we find that Zr and Hf dopants increase the oxygen exchange capacity of ceria because they store energy in tensilely strained Zr-or Hf-O bonds which is released upon O-vacancy formation. This finding corrects a long held assumption that Zr and Hf decrease the O-vacancy formation energy by compensating for ceria expansion upon reduction.Although the released strain energy decreases the O-vacancy formation energy, O-vacancy formation remains sufficiently endothermic to split H 2 O and CO 2 . Secondly, we show that two electrons must be promoted into the high energy Ce f-band during reduction if the O-vacancies are to store sufficient energy to drive the oxidative gas splitting step. This means that di-and trivalent dopants are not suitable for this process. Lastly, we show that dopants which break O bonds due to their small size or strongly covalent character, such as Ti and the pentavalent dopants, substantially decrease the O-vacancy formation energy because only three O bonds must break during reduction. These vacancies, therefore, are too low in energy to drive gas splitting. Based on these findings, we develop guidelines for new ceria doping strategies to facilitate solar thermochemical gas splitting cycles.

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“…Progress on the latter issue is essential to make much needed advances in all aspects of treating waste from the nuclear fuel cycle. Furthermore, uranium oxides have been studied as catalysts 14–16 and in thermal and photolytic hydrogen production 17 .…”

Section: Introductionmentioning

confidence: 99%

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Gouder

Eloirdi

Caciuffo

2018

Sci Rep

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Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

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Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

Cited by 41 publications

References 45 publications

A review and perspective of efficient hydrogen generation via solar thermal water splitting

A review and perspective of efficient hydrogen generation via solar thermal water splitting

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

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Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

Cited by 41 publications

References 45 publications

A review and perspective of efficient hydrogen generation via solar thermal water splitting

A review and perspective of efficient hydrogen generation via solar thermal water splitting

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

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Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂