The thermodynamics of ceria-based metal oxides M x Ce1–x O2, where M = Gd, Y, Sm, Ca, Sr, have been studied in relation to their applicability as reactive intermediates in solar thermochemical redox ...cycles for splitting H2O and CO2. Oxygen nonstoichiometry was modeled and extrapolated to high temperatures and reduction extents by applying an ideal solution model in conjunction with a defect interaction model. Subsequently, relevant thermodynamic parameters were computed and equilibrium H2 and CO concentrations determined as a function of reduction conditions (T, P O2 ) and ensuing oxidation temperature. At 1 atm and above 1673 K, the degree of reduction is negatively correlated to dopant concentration, regardless of the type of dopant considered. Consequently, at a given reduction temperature, more H2 and CO is generated at equilibrium for pure ceria compared to any of the other doped ceria materials considered. Although the reduction enthalpy decreases as the dopant concentration increases, the overall solar-to-fuel energy conversion efficiency is greater for pure ceria (20.2% at δ = 0.1, P O2 = 10 ppm). Only when considering heat recovery of nearly 100% are theoretical efficiencies higher for the dopants.
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 H2O and CO2 to H2 ...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 pO2. 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.
We present results on the thermochemical redox performance and analytical characterization of Hf4+, Zr4+, and Sc3+ doped ceria solutions synthesized via a sol–gel technique, all of which have ...recently been shown to be promising for splitting CO2. Dopant concentrations ranging from 5 to 15 mol % have been investigated and thermally cycled at reduction temperatures of 1773 K and oxidation temperatures ranging from 873 to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr doped materials is substantially higher than those of pure ceria and Sc doped ceria and increases with dopant concentration. Overall, 10 mol % Hf doped ceria results in the largest CO yields per mole of oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based on measured mass changes during oxidation. However, these yields were largely influenced by their rate of reoxidation, not necessarily thermodynamic limitations, as equilibrium was not achieved for either Hf or Zr doped samples after 45 min exposure to CO2 at all oxidation temperatures. Additionally, sample preparation and grain size strongly affected the oxidation rates and subsequent yields, resulting in slightly decreasing yields as the samples were cycled up to 10 times. X-ray diffraction, Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX have been applied to characterize the elemental, crystalline, and morphological attributes before and after redox reactions.
The kinetics of CO2 reduction over nonstoichimetric ceria, CeO2−δ, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of ...nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ T ≤ 1273 K), and CO2 concentrations (0.005 ≤ p CO2 ≤ 0.4 atm). Samples were reduced thermally at 1773 K to probe low nonstoichiometries (δ < 0.05) and chemically at lower temperatures in a H2 atmosphere to prevent particle sintering and probe the effect of higher nonstoichiometries (δ < 0.25). For extents greater than δ = 0.2, oxidation rates at a given nonstoichiometry are hindered for the duration of the reaction, presumably because of near-order changes, such as lattice compression, as confirmed via Raman Spectroscopy. Importantly, this behavior is reversible and oxidation rates are not affected at lower δ. Following thermal reduction at very low δ, however, oxidation rates are an order of magnitude slower than those of chemically reduced samples, and rates monotonically increase with the initial nonstoichiometry (up to δ = 0.05). This dependence may be attributed to the formation of stable defect complexes formed between oxygen vacancies and polarons. When the same experiments are performed with 10 mol % Gd3+ doped ceria, in which defect complexes are less prevalent than in pure ceria, this dependence is not observed.
Formaldehyde (HCHO) is an important air pollutant from both an atmospheric chemistry and human health standpoint. This study uses an instrumented photochemical Air Quality Model, CMAQ-DDM, to ...identify the sensitivity of HCHO concentrations across the United States (U.S.) to major source types and hydrocarbon speciation. In July, biogenic sources of hydrocarbons contribute the most (92% of total hydrocarbon sensitivity), split between isoprene and other alkenes. Among anthropogenic sources, mobile sources of hydrocarbons and nitrogen oxides (NO x ) dominate. In January, HCHO is more sensitive to anthropogenic hydrocarbons than biogenic sources, especially mobile sources and residential wood combustion (36% of national hydrocarbon sensitivity). While ozone (O3) is three times more sensitive to NO x than hydrocarbons across most areas of the U.S., HCHO is six times more sensitive to hydrocarbons than NO x , largely due to sensitivity to biogenic precursors and the importance of low-NO x chemistry. In winter, both HCHO and O3 show negative sensitivity to NO x (increases with the removal of NO x ), although O3 increases are larger. Relative sensitivities do not change substantially across different regions of the country.
This review summarizes state of the art metal oxide materials used in two-step thermochemical redox cycles for the production of H2 and CO from H2O and CO2 using concentrated solar energy. Advantages ...and disadvantages of both stoichiometric (e.g. iron oxide based cycles) and nonstoichiometric (e.g. ceria based cycles) materials are discussed in the context of thermodynamics, chemical kinetics, and material stability. Finally, a perspective aimed at future materials development and requirements necessary for advances of process efficiencies is discussed.
Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing ...total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.
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•Deposition budgets for inorganic nitrogen have improved over the last decade in the U.S.•Important data and knowledge gaps in monitoring and modeling of total nitrogen deposition remain.•Expanded monitoring of deposition in agricultural and urban areas is needed.•Monitoring of organic N deposition and improvement of organic N in atmospheric models is needed.•Land use specific modeled deposition estimates are needed for critical load assessments.
Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO₂ and H₂O. They feature dual-scale porosity with mm-size pores for ...effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier's law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10
to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09-0.9 in both the strut's µm-scale and bulk's mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors.
•Advancements in redox materials for two-step solar thermochemical fuel production.•Effects of varying enthalpy and entropy changes on redox performance.•Perovskites suffer from reduced oxidation ...favorability compared to ceria.•The use of excess oxidant may lead to unrealistic expectations of performance.
Solar thermochemical (STC) redox cycles have made substantial advances in recent years, in large part due to the utilization of nonstoichiometric ceria over other iron oxide and zinc oxide based materials that undergo crystallographic or solid-to-gas phase changes. These changes render their utilization in a cyclic nature to be challenging because of the ever-changing physical properties over time and difficulty in preventing reverse reactions when cooling, for example Zn(g)+0.5O2(g)→ZnO(s). However, such phase changes also have the distinct benefit of being accompanied by large changes in entropy which is typically favorable from a thermodynamic perspective. As a result, the theoretical solar-to-fuel energy conversion efficiencies of ceria-based cycles are usually predicted to be lower than their volatile and nonvolatile stoichiometric counterparts; however, in actuality their measured performance is superior. For this technology to become commercially viable, there is a need to develop new materials that can outperform ceria in terms of solar-to-fuel energy conversion efficiency and operate at more benign conditions. This has been a large focus in the thermochemical community over the last 5–6years and, to date, most of the effort has been centered on reducing the relatively high operating temperatures that are required while maintaining ceria’s desirable characteristics such as favorable oxidation thermodynamics, rapid reaction kinetics, and crystallographic stability. This effort resulted in many perovskite related materials that operate several hundred degrees lower (e.g. 1473K). Unfortunately, however, their entropy change is usually lower than that of ceria and the results are consistently a compromise in the thermodynamic driving force for oxidation that results in less efficient materials overall. This work focuses on the thermodynamic, experimental, and computational aspects related to the discovery and characterization of new and better performing redox materials and the attributes necessary of them in order to drive the next generation of efficient STC redox materials.
A thermodynamic and experimental investigation of a new class of solar thermochemical redox intermediates, namely, lanthanum–strontium–manganese perovskites, is presented. A defect model based on ...low-temperature oxygen non-stoichiometry data is formulated and extrapolated to higher temperatures more relevant to thermochemical redox cycles. Strontium contents of x = 0.3 (LSM30) and x = 0.4 (LSM40) in La1–x Sr x MnO3−δ result in favorable reduction extents compared to ceria in the temperature range of 1523–1923 K. Oxidation with CO2 and H2O is not as thermodynamically favorable and largely dependent upon the oxidant concentration. The model is experimentally validated by O2 non-stoichiometry measurements at high temperatures (>1623 K) and CO2 reduction cycles with commercially available LSM35. Theoretical solar–fuel energy conversion efficiencies for LSM40 and ceria redox cycles are 16 and 22% at 1800 K and 13 and 7% at 1600 K, respectively.
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