In this response to the rebuttal of Krenzke and Davison, the authors of “Theoretical and Experimental Investigation of Solar Methane Reforming through the Nonstoichiometric Ceria Redox Cycle” clarify ...that the “simultaneous” and “sequential” approaches proposed by the two groups converge to become mathematically equivalent in the limit when H2 and CO constitute the only reaction products.
A kinetic relaxation study of the partial oxidation of methane over ceria in the absence of gas-phase oxygen, a promising pathway for syngas production, was performed under atmospheric pressure ...between 750 °C and 1100 °C. Using a thermogravimetric analyzer, the effect of cyclability, gas/solid diffusion, reactant/product composition, and surface contaminants on the measured rate of ceria reduction was addressed. Reaction kinetics were described by an apparent activation energy that was extracted from Arrhenius-type plots as a function of composition. Determined via two isoconversional methods, apparent activation energy was shown to vary with reaction extent between 20 kJ mol−1 and 80 kJ mol−1, increasing mostly for large reduction extents (δ > 0.15). Interestingly, this range is lower than the discrete values presented in literature and supports the idea that reaction rates are not limited by bulk diffusion. Further, although deposited carbon was not experimentally detected, evidence of surface adsorbates, such as carbonates and hydroxyls, was observed, as the reaction rate was impacted by the length of subsequent oxidation in O2. The reaction rate was also proportional to the methane partial pressure and weakly inhibited by additional hydrogen (only at large nonstoichiometries), in agreement with equilibrium thermodynamic predictions. Experimental repeatability upon isothermal cycling increased with increasing operating temperature, contrary to higher-temperature, ceria-based thermochemical cycles. Overall, this work provides new experimental insights that affect the partial oxidation of methane over ceria and help guide the immediate development of larger scale systems.
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•Reduction kinetics were described by a compositionally-dependent apparent activation energy.•Reaction rates are not limited by bulk mass transport as has been previously suggested in the literature.•Repeatable and reversible shifts between oxidized Ce4+ and reduced Ce3+ states were attainable under examined conditions.•In agreement with thermodynamics, measured reaction rates were more impacted by CH4, rather than H2, partial pressure.•Evidence of surface adsorbates and lack of carbon deposition were experimentally observed.
While ceria is the standard material for two-step water splitting, perovskites are emerging as viable alternatives. In this work, based on the orthorhombic LaMnO3 supercell, we substitute Li Na K Rb ...Mg Ca Sr Ba on the A-sites (La sites) and Al Ga In Mg Zn on the B-sites (Mn sites) at a concentration of 37.5%. The range of temperature and oxygen partial pressure at which each composition is stable is predicted. For compositions that are stable in relevant temperature and pressure ranges, the oxygen vacancy formation energies are determined for all of the oxygen vacancy site positions available in the computational supercell. Mg, Ca, Sr, and Ba A-site-substituted LaMnO3 and Al and In B-site-substituted LaMnO3 meet these two criteria for candidates in solar-thermal water splitting applications. Oxygen vacancy formation energy can also be controlled by adjusting the doping strategy.
A novel chemical-looping combustion scheme is proposed, where facile gas separation
via
steam condensation enables the production of sequestrable CO
2
from alkanes, such as CH
4
, and pure H
2
from H
...2
O. This cycle consists of two steps, namely, (1) the endothermic reduction of a ceria-based solid solution
via
the complete oxidation of CH
4
, followed by (2) the exothermic oxidation of the reduced metal oxide
via
H
2
O splitting. Relative to iron oxide-based materials and undoped ceria, ceria-zirconia solid solutions possess favorable partial molar enthalpic and entropic properties; this promotes selective production of complete combustion products, H
2
O and CO
2
, during the reforming reaction. Thermodynamic predictions suggest that the complete oxidation of CH
4
is possible by increasing the Zr content to 20 mol%, operating below 600 °C, increasing total pressure, or reducing the amount of delivered reactant. Furthermore, any H
2
, CO, or unreacted CH
4
that may persist is thermodynamically favored to oxidize if exposed to unreacted oxide downstream, as is typical for a packed-bed or downer reactor configuration. Experiments were performed to validate the thermodynamic trends using isothermal thermogravimetry coupled with residual gas analysis, which confirmed that high selectivity towards H
2
O and CO
2
is attainable for methane-driven reduction of Ce
0.9
Zr
0.1
O
2
; selectivities greater than 0.70 were observed at initial reaction extents. Importantly, metal oxide oxidation
via
H
2
O splitting and selective production of H
2
(or CO if CO
2
is the delivered oxidant) is also thermodynamically favored at the operating conditions considered for the first step. This work ultimately presents a viable avenue for the carbon-neutral conversion of CH
4
(or other alkanes) to H
2
if a renewable energy resource, such as solar energy, is leveraged to supply process heat.
Ceria-zirconia solid solutions can facilitate the carbon-neutral conversion of CH
4
into separate streams of sequestrable CO
2
and pure H
2
.
This work encompasses the thermodynamic characterization and critical evaluation of Zr
4+
doped ceria, a promising redox material for the two-step solar thermochemical splitting of H
2
O and CO
2
to ...H
2
and CO. As a case study, we experimentally examine 5 mol% Zr
4+
doped ceria and present oxygen nonstoichiometry measurements at elevated temperatures ranging from 1573 K to 1773 K and oxygen partial pressures ranging from 4.50 × 10
−3
atm to 2.3 × 10
−4
atm, yielding higher reduction extents compared to those of pure ceria under all conditions investigated, especially at the lower temperature range and at higher
p
O
2
. In contrast to pure ceria, a simple ideal solution model accounting for the formation of isolated oxygen vacancies and localized electrons accurately describes the defect chemistry. Thermodynamic properties are determined, namely: partial molar enthalpy, entropy, and Gibbs free energy. In general, partial molar enthalpy and entropy values of Zr
4+
doped ceria are lower. The equilibrium hydrogen yields are subsequently extracted as a function of the redox conditions for dopant concentrations as high as 20%. Although reduction extents increase greatly with dopant concentration, the oxidation of Zr
4+
doped ceria is thermodynamically less favorable compared to pure ceria. This leads to substantially larger temperature swings between reduction and oxidation steps, ultimately resulting in lower theoretical solar energy conversion efficiencies compared to ceria under most conditions. In effect, these results point to the importance of considering oxidation thermodynamics in addition to reduction when screening potential redox materials.
The thermodynamics and defect chemistry of Zr
4+
-doped ceria is investigated and discussed in regards to the efficiency of solar thermochemical redox cycles.
The thermodynamics and defect chemistry of Zr
4+
-doped ceria is investigated and discussed in regards to the efficiency of solar thermochemical redox cycles.
This work encompasses the thermodynamic ...characterization and critical evaluation of Zr
4+
doped ceria, a promising redox material for the two-step solar thermochemical splitting of H
2
O and CO
2
to H
2
and CO. As a case study, we experimentally examine 5 mol% Zr
4+
doped ceria and present oxygen nonstoichiometry measurements at elevated temperatures ranging from 1573 K to 1773 K and oxygen partial pressures ranging from 4.50 × 10
–3
atm to 2.3 × 10
–4
atm, yielding higher reduction extents compared to those of pure ceria under all conditions investigated, especially at the lower temperature range and at higher
p
O
2
. In contrast to pure ceria, a simple ideal solution model accounting for the formation of isolated oxygen vacancies and localized electrons accurately describes the defect chemistry. Thermodynamic properties are determined, namely: partial molar enthalpy, entropy, and Gibbs free energy. In general, partial molar enthalpy and entropy values of Zr
4+
doped ceria are lower. The equilibrium hydrogen yields are subsequently extracted as a function of the redox conditions for dopant concentrations as high as 20%. Although reduction extents increase greatly with dopant concentration, the oxidation of Zr
4+
doped ceria is thermodynamically less favorable compared to pure ceria. This leads to substantially larger temperature swings between reduction and oxidation steps, ultimately resulting in lower theoretical solar energy conversion efficiencies compared to ceria under most conditions. In effect, these results point to the importance of considering oxidation thermodynamics in addition to reduction when screening potential redox materials.
▪Professor Aldo Steinfeld’s contributions to the fields of solar thermochemistry and energy conversion are extensive and impressive. His work has greatly contributed to the ongoing transition from ...fossil to renewable fuels. We, his former doctoral students and postdoctoral researchers, take a look back at his life and honor his contributions. His work has redefined the field and created a legacy that reverberates throughout the world. His impact is being realized through his tireless efforts towards developing cutting edge solar technologies, writing seminal papers, and his undying commitment to and leadership in the solar energy and renewable energy technology communities and beyond. This legacy has been recognized by numerous accolades and will continue after his retirement through his mentorship and guidance of the next generations of researchers dedicated to continuing his solar research.
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