Re-Evaluating CeO₂ Expansion Upon Reduction: Noncounterpoised Forces, Not Ionic Radius Effects, Are the Cause

Ceria (CeO₂) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce³⁺ as compared with Ce⁴⁺. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce³⁺ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O–Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce³⁺–Ce⁴⁺ distances of 3.90 Å rather than a 3.89 Å Ce⁴⁺–Ce⁴⁺ distance in oxidized ceria). The severing of Ce–O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce⁴⁺_vac–Ce⁴⁺_vac distance of 4.14 Å). The relaxation of Ce⁴⁺ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance

Thermodynamics of paired charge-compensating doped ceria with superior redox performance for solar thermochemical splitting of H2O and CO2

Paired charge-compensating doped ceria (PCCD) using trivalent and pentavalent cations are evaluated as redox materials for the thermochemical splitting of H2O and CO2. The oxygen nonstoichiometries of PCCD materials with formulas of Ce0.9A0.05Nb0.05O2 (A = Y, La, Sc) and CexLa(1−x)/2Nb(1−x)/2O2 (x = 0.75, 0.95) were measured in a thermogravimetric analyzer over a range of temperatures (T = 1173–1773 K) and oxygen partial pressures (pO2 = 10−15–10−1 atm). Undoped and single element doped ceria (Ce0.9B0.1O2 where B = Y, La, Nb, Hf) served as a reference. At any given set of T and pO2, the relative reduction extent follows Ce0.9Hf0.1O2 > Ce0.9Sc0.05Nb0.05O2 > Ce0.9Y0.05Nb0.05O2 > CexLa(1−x)/2Nb(1−x)/2O2 > CeO2 > solely trivalent or pentavalent doped ceria. The partial molar reduction enthalpies were determined using Van't Hoff analysis coupled to defect modeling and range from 360 to 410 kJ mol−1. A system efficiency model predicts that these PCCD materials have the potential of achieving high solar-to-fuel energy conversion efficiencies because of their balanced reduction and oxidation properties. Ce0.9Y0.05Nb0.05O2 in particular can outperform undoped ceria and reach efficiency values of 31% and 28% for H2 and CO production, respectively

Comparing the solar-to-fuel energy conversion efficiency of ceria and perovskite based thermochemical redox cycles for splitting H2O and CO2

A thermodynamic analysis was conducted on a solar thermochemical plant for syngas generation via H2O/CO2-splitting redox cycles in order to determine the performance of six candidate redox materials under an array of operation conditions. The values obtained for the solar-to-fuel energy conversion efficiency are higher in relative order Zr-doped CeO2 > undoped CeO2 > La0.6Ca0.4MnO3 > La0.6Ca0.4Mn0.6Al0.4O3 > La0.6Sr0.4MnO3 > La0.6Sr0.4Mn0.6Al0.4O3. This ordering is attributed to their relative reducibility and re-oxidizability, where the doped and undoped ceria, that favor oxidation, outperform perovskites, that favor reduction and therefore require high flowrates of excess H2O and CO2 during re-oxidation. Solids-solid heat recuperation during the temperature swing between the redox steps is crucial, particularly for ceria because of its low specific oxygen exchange capacity per mole and cycle. Conversely, the efficiencies of the perovskites are more dependent on gas-gas heat recuperation due to the massive excess of H2O/CO2. Redox material thermodynamics and plant/reactor performance are closely coupled, and must be considered together to maximize efficiency. Overall, we find that Zr-CeO2 is the most promising redox material, while perovskites which seem promising due to high H2/CO production capacities under large H2O/CO2 flow rates, perform poorly from an efficiency perspective due to the high heating duties, especially for steam.ISSN:1879-3487ISSN:0360-319

Thermodynamics of paired charge-compensating doped ceria with superior redox performance for solar thermochemical splitting of H2O and CO2

ISSN:2050-7488ISSN:2050-749

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH₃) synthesis applications where AlN would be intentionally hydrolyzed to produce NH₃ and aluminum oxide (Al₂O₃), which could be subsequently reduced in nitrogen (N₂) to reform AlN and reinitiate the NH₃ synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH₃ yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH₃, and NH₃ desorption. We found activation barriers (<i>E</i>_a) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al–N bonds required to accumulate protons on surface N atoms to form NH₃. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (<i>E</i>_a = 331 kJ/mol) and mediated proton diffusion (<i>E</i>_a = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH₃ generation from AlN (<i>E</i>_a = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal–nitrogen bonds and, thus, more-facile NH₃ liberation

Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly cooccurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six “target” oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five “competing” co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents

Growth of Pt Particles on the Anatase TiO₂ (101) Surface

Growth of Pt_<i>n</i> (<i>n</i> ≤ 37) clusters on the defect-free TiO₂ anatase (101) surface has been studied using ab initio pseudopotential calculations based on density functional theory. Several initial configurations for clusters of 1, 2, 7, 10, and 37 atoms were relaxed to determine the most stable structures. All final optimized structures are three dimensional, suggesting that formation of island-like particles is favored over planar monolayers, as verified experimentally using Pt atomic layer deposition and high-resolution transmission electron microscopy. Diffusion barriers of a single Pt adatom on TiO₂ were calculated to understand the mobility of Pt atoms on the TiO₂ surface. Activation barriers of 0.86 and 1.41 eV were calculated for diffusion along the [010] and [101̅] directions, respectively, indicating that Pt atoms are relatively mobile along the [010] direction at moderate temperatures. The energy barriers for a Pt atom to escape from an 11- and a 37-atom Pt cluster on (101) anatase are predicted to be 1.38 and 2.12 eV, suggesting that particle coarsening occurs by Ostwald ripening and that Ostwald ripening of deposited Pt particles is limited by atom detachment from particles as small as several tens of atoms

Preferential adsorption of selenium oxyanions onto {1 1 0} and {0 1 2} nano-hematite facets

As the commercial use of nano metal oxides, including iron oxides, becomes more prevalent, there is a need to understand functionality as it relates to the inherent properties of the nanomaterial. Many applications of nanomaterials rely on adsorption, ranging from catalysis to aqueous remediation. In this paper, adsorption of selenium (Se), an aqueous contaminant, is used as a model sorbate to elucidate the relationships of structure, property, and (adsorptive) function of nano-hematite (nα-Fe 2O 3). As such, six nα-Fe 2O 3 particles were synthesized controlling for size, shape and surface area without capping agents. Sorbent characteristics of the six particles were then assessed for their impact on selenite (HSeO 3 −) and selenate (SeO 4 2−) adsorption capacity and mechanism. Mechanism was assessed using in-situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine edge spectroscopy (EXAFS). Regression analyses were then performed to determine which characteristics best describe adsorption capacity and binding mechanisms of Se on nα-Fe 2O 3. The results demonstrate that crystal surface structure, specifically presence of the {0 1 2} facet promotes adsorption of Se and the presence of {0 1 2} facets promotes SeO 4 2− sorption to a greater extent than HSeO 3 −. The data further indicates that {1 1 0} facets bind HSeO 3 − with binuclear complexes while {0 1 2} facets bind HSeO 3 − via mononuclear inner-sphere complexes. Specific nα-Fe 2O 3 facets also likely direct the ratio of inner to outer-sphere complexes in SeO 4 2− adsorption

First-Principles Analysis of Cation Diffusion in Mixed Metal Ferrite Spinels

Ferrite spinels are metal oxides used in a wide variety of applications, many of which are controlled by the diffusion of metal cations through the metal oxide lattice. In this work, we used density functional theory (DFT) to examine the diffusion of Fe, Co, and Ni cations through the Fe₃O₄, CoFe₂O₄, and NiFe₂O₄ ferrite spinels. We apply DFT and crystal field theory to uncover the principles that govern cation diffusion in ferrite spinels. We found that a migrating cation hops from its initial octahedral site to a neighboring octahedral vacancy via a tetrahedral metastable intermediate separated from octahedral sites by a trigonal planar transition state (TS). The cations hop with relative activation energies of Co ≈< Fe < Ni; the ordering of the diffusion barriers is controlled by the crystal field splitting of the diffusing cation. Specifically, the barriers depend on the orbital splitting and number of electrons which must be promoted into the higher energy t_2g orbitals of the tetrahedral metastable intermediate as the cations move along the minimum energy pathway of hopping. Additionally, for each diffusing cation, the barriers are inversely proportional to the spinel lattice parameter, leading to relative barriers for cation diffusion of Fe₃O₄ < CoFe₂O₄ < NiFe₂O₄. This results from the shorter cation-O bonds at the TS for spinels with smaller lattices, which inherently possess shorter bond lengths, and consequently higher system energies at their more constricted TS geometries

Extracting Kinetic Information from Complex Gas–Solid Reaction Data

We develop an approach for extracting gas–solid kinetic information from convoluted experimental data and demonstrate it on isothermal carbon dioxide splitting at high-temperature using CoFe₂O₄/Al₂O₃ (i.e., a “hercynite” cycle based on Co-doped FeAl₂O₄) active material. The reaction kinetics equations we derive account for competing side reactions, namely catalytic CO₂ splitting on and O₂ oxidation of doped hercynite, in addition to CO₂ splitting driven by the oxidation of oxygen-deficient doped hercynite. The model also accounts for experimental effects, such as detector dead time and gas mixing downstream of the reaction chamber, which obscure the intrinsic chemical processes in the raw signal. A second-order surface reaction model in relation to the extent of unreacted material and a 2.4th-order model in relation to CO₂ concentration were found to best describe the CO generation of the doped hercynite. Overall, the CO production capacity was found to increase with increasing reduction temperature and CO₂ partial pressure, in accordance with previously predicted behavior. The method outlined in this paper is generally applicable to the analysis of other convoluted gas–solid kinetics experiments