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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Platinum Surface Oxide and Oxygen Reduction Reaction Kinetics during Transient Fuel Cell Operation

Pt surface oxide coverage (POC) and Oxygen Reduction Reaction (ORR) were measured under a range of operating conditions in a Proton Exchange Membrane Fuel Cell (PEMFC). Sensitivity analysis suggests that adsorption of sulfonate groups of the ionomer on the Pt surface plays an important role in POC and ORR kinetics. Although both Pt oxide growth and ORR activity decay follow a logarithm of time behavior, it is found that ORR kinetics are affected by at least two different types of Pt oxide. A semi-empirical ORR kinetic model is proposed taking into account the effect of ionomer and Pt oxide types. The model is capable of providing a quantitative prediction of POC and ORR activity over a range of potential, temperature, relative humidity, and time that is relevant to normal PEMFC operation.

Electrochemistry↗
DOE OSTI2023

A Comparative Study of Electrical Double Layer Effects for CO Reduction Reaction Kinetics

Solvation models describe how the interactions between the solutes and solvents affect the reactivity and selectivity in electrochemical processes. In this study, we developed a framework for evaluating the effects of applied potential and electrical double layer on CO reduction (COR), comparing fully explicit, implicit, and hybrid solvation models at the standard hydrogen electrode (SHE) scale. We analyzed all crucial intermediates leading to the production of C 1 and C 2 products and found good agreement across these models. Some notable differences were observed in the implicit description of *C and *CO adsorption at higher overpotentials and overall trends in the adsorption energies within the hybrid model. Using this unified SHE framework, we built comparable microkinetic models for COR kinetics and rates. Despite small differences in thermodynamics, solvation-model-based microkinetic simulations showed good agreement for onset potentials against the benchmark experiment. Only qualitative difference was observed for C 1+ versus hydrogen evolution at high overpotentials for the implicit model. Finally, we constructed a generalized C 2 selectivity map in descriptor space (U SHE , ΔG electrolyte CO ), which highlights the limitations of a copper-based COR catalyst and guides the search for optimal descriptor parameters to maximize C 2 selectivity. In conclusion, these findings demonstrate the importance of considering electrical double layer effects in reduction reactions and offer a useful framework for comparing solvation models and predicting optimal electrochemical conditions for specific applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Reduction of Iron‐Ore Pellets Using Different Gas Mixtures and Temperatures

Direct reduction of iron ore (DRI) is gaining an increased attention due to the growing need to decarbonize industrial processes. The current industrial DRI processes are performed using reformed natural gas, which results in CO 2 emission, although it is less than carbothermic reduction in the blast furnace. Carbon‐free reduction may be realized through the utilization of green H 2 as a reducing agent, in place of natural gas. Herein, the effects of various gas mixtures and temperature on the reduction kinetics of the hematite iron‐ore pellets are focused on in this work. Pellets are reduced at 700, 800, 850, and 900 °C in hydrogen and using various gas mixes at 850 °C. Morphology of the pellets is investigated with the help of scanning electron microscopy and mercury intrusion porosimetry. The effects of temperature and gas composition on the reduction kinetics and porosity of the pellets are discussed. A notable effect of reduction rate on the internal structure of the pellets is detected, slower reduction rate yielded bigger pores offsetting the gas composition. Higher temperature results in coarser pores and higher porosity. Finally, increase of CO content in the gas mix also leads to bigger pore size.

36 MATERIALS SCIENCE↗
NASA NTRS2010

Modeling of Alkane Oxidation Using Constituents and Species

It is currently not possible to perform simulations of turbulent reactive flows due in particular to complex chemistry, which may contain thousands of reactions and hundreds of species. This complex chemistry results in additional differential equations, making the numerical solution of the equation set computationally prohibitive. Reducing the chemical kinetics mathematical description is one of several important goals in turbulent reactive flow modeling. A chemical kinetics reduction model is proposed for alkane oxidation in air that is based on a parallel methodology to that used in turbulence modeling in the context of the Large Eddy Simulation. The objective of kinetic modeling is to predict the heat release and temperature evolution. This kinetic mechanism is valid over a pressure range from atmospheric to 60 bar, temperatures from 600 K to 2,500 K, and equivalence ratios from 0.125 to 8. This range encompasses diesel, HCCI, and gas-turbine engines, including cold ignition. A computationally efficient kinetic reduction has been proposed for alkanes that has been illustrated for n-heptane using the LLNL heptane mechanism. This model is consistent with turbulence modeling in that scales were first categorized into either those modeled or those computed as progress variables. Species were identified as being either light or heavy. The heavy species were decomposed into defined 13 constituents, and their total molar density was shown to evolve in a quasi-steady manner. The light species behave either in a quasi-steady or unsteady manner. The modeled scales are the total constituent molar density, Nc, and the molar density of the quasi-steady light species. The progress variables are the total constituent molar density rate evolution and the molar densities of the unsteady light species. The unsteady equations for the light species contain contributions of the type gain/loss rates from the heavy species that are modeled consistent with the developed mathematical forms for the total constituent molar density rate evolution; indeed, examination of these gain/loss rates shows that they also have a good quasi-steady behavior with a functional form resembling that of the constituent rate. This finding highlights the fact that the fitting technique provides a methodology that can be repeatedly used to obtain an accurate representation of full or skeletal kinetic models. Assuming success with the modified reduced model, the advantage of the modeling approach is clear. Because this model is based on the Nc rate rather than on that of individual heavy species, even if the number of species increases with increased carbon number in the alkane group, providing that the quasi-steady rate aspect persists, then extension of this model to higher alkanes should be conceptually straightforward, although it remains to be seen if the functional fits would remain valid or would require reconstruction.

Bellan, Jasette↗
DOE OSTI2021

Autoignition of Premixed Liquefied Petroleum Gas in a Rapid Compression Machine: Experimental Results and Chemical Kinetic Mechanism Reduction

Liquefied petroleum gas (LPG) has many properties that make it an attractive alternative fuel such as lower cost than conventional fuels and an established distribution infrastructure. The development of high efficiency, spark ignited LPG engines is currently limited by engine knock and misfire. The knock and misfire limits are further complicated by the wide range of chemical reactivity in LPG, particularly in international markets. In this study, a rapid compression machine (RCM) was used to characterize the effects of variation in LPG fuel reactivity, equivalence ratio, and exhaust gas recirculation (EGR) on autoignition of LPG/oxidizer/inert/EGR blends. Experiments were conducted with 100% propane (C3H8) and blends of propane with propene, ethane, isobutane, and n-butane. EGR was simulated with mixtures of Ar, CO2, CO, and NO at substitution percentages from 0 to 30 mass percent. Equivalence ratio was varied from 0.75 to 1.5. Ignition delay period under homogeneous autoignition conditions was measured at compressed pressures and temperatures of 23 to 25 bar and 701 to 921 K, respectively. Zero-dimensional simulations of the RCM experiments were performed using Chemkin with several detailed chemical kinetic mechanisms to determine their suitability at predicting ignition delay periods. Multiple reduced chemical kinetic mechanisms were created from the NUIGMech1.1 mechanism to determine the optimal balance between accuracy and computational efficiency for future three-dimensional, time-dependent spark-ignited engine computations.

42 ENGINEERING↗
DOE OSTI2025

Spatially Patterned Architectures to Modulate CO 2 Reduction Cascade Catalysis Kinetics

Electrochemical CO 2 reduction using renewable sources of electrical energy holds promise for converting CO 2 into fuels and chemicals. The complex interactions among chemical/electrochemical reactions and mass transport make it difficult to analyze the effect of an individual process on electrode performance based only on experimental methods. Here, we developed a generalized steady-state simulation to describe an electrode surface in which sequential cascade catalysts are patterned in a periodic trench design. If appropriately constructed, this trench geometry is hypothesized to be able to yield a higher net current density for a CO 2 reduction (CO 2 R) cascade reaction. We have used realistic experimental reaction kinetics to investigate the role of trench geometry in mass transport, local microenvironments, and selectivity for a model CO 2 R cascade reaction. The model considers local concentration gradients of bicarbonate species at quasi-equilibrium and catalytic surface reactions based on concentration-dependent Butler–Volmer kinetics. Our results suggest that varying the spatial distribution of active sites plays a significant role in facilitating effective mass transport between active sites, modulating selectivity for the cascade reaction, and enhancing the yield of desirable cascade products. Moreover, we observe that this trench geometry significantly alters the cascade reaction rate by affecting the local pH, which can cause inadvertent depletion of available aqueous CO 2 to limit the CO 2 R cascade kinetics and modest suppression of the hydrogen evolution reaction (HER). The results highlight the trade-offs between mass transport, pH, and reaction kinetics that become apparent only when considering the coupled physics of all processes at the electrode surface. Here, this model can thus serve as a primary tool to build more selective and efficient patterned architectures for the CO 2 R cascade catalysis.

CO2 reduction↗
NASA NTRS1972

Kinetics of carbothermal reduction of quartz under vacuum.

Study of the kinetics of carbothermal reduction of quartz in a batch reactor between 0.25 torr and 1 microtorr. The effects of temperature, CO partial pressure, SiO2 phase transformation, and alkali and alkaline-earth oxide additives on the rate of reaction are reported. The results are consistent with a model based on a transitory phase intermediate.

Khalafalla, S. E.↗
NASA NTRS1973

The kinetics of ulvoespinel reduction - Synthetic study and applications to lunar rocks.

The kinetics of Fe2TiO4 reduction to FeTiO3 + Fe were studied using CO-CO2 gas mixtures with fO2 measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. Isothermal rate studies at 900 C suggest that the mechanism of Fe2TiO4 reduction is one of nucleation and growth, where the growth stage may be controlled by the diffusion of the reactant through the product layer or volume diffusion. The activation energy for the growth stage of the process was determined to be 46 plus or minus 4 kcal/mole.

Mccallister, R. H.↗
NASA NTRS1994

Experimental space weathering of regolith material

Significant advances in the understanding of space weathering processes were recently reported. Submicroscopic iron blebs were produced in lunar simulant glass and natural terrestrial minerals by high-temperature reduction in controlled atmosphere furnaces. These experiments altered the samples' optical properties and microtextures so that they resembled those of extremely mature lunar soil. The results contributed to a revised model for natural reduction in the regolith. Subsequently, supporting results were obtained by reduction of lunar samples. Research to date has focused on reduction of three lunar surface components: basalt, pyroclastic glass, and mare soil. An extensive set of H reduction experiments with simulants has led to a detailed understanding of reaction mechanisms and kinetics. Reduction experiments using lunar basalt were recently conducted by Carbotek. Reduced samples from these test were analyzed. Reduction experiments on lunar glass 74220 were run at temperatures of 900-1100 C. Reduction efficiency of volcanic glass proved to be a function of the sample's FeO abundance and reaction temperature. We also reduced mare soil 75061 at temperatures of 900-1050 C. Partial reduction of FeO in olivine and pyroxene occurred, but was slower and less complete than reduction of ilmenite. Our experiments on simulants and lunar samples have indicated that the most readily reduced phases in the regolith are ilmenite and glass. Based on initial tests with simulants we proposed refinements to the accepted model for space weathering of the regolith. The impact of a micrometeoroid flash heats and melts and ejects from the impact point a small volume of soil that contains trapped solar wind H and C. Reduction occurs rapidly, while the melt volume is still in motion. When a droplet encounters unmelted soil, it envelopes cold mineral grains. The melt is chilled rapidly. Our analyses of experimentally reduced lunar basalt, glass, and mare soil support the proposed model for space weathering.

Mckay, D. S.↗
DOE OSTI2025

Spatial Composition Influenced by Solvent in High-Entropy Alloy Nanoparticle Synthesis via Polyol Reduction

The performance of nanoparticles in catalytic reactions depends critically on their surface composition. In a multielemental system, this issue becomes even more important. Here, we report that the choice of solvent in polyol reduction synthesis of high entropy alloy (HEA) nanoparticles can have a subtle but significant influence on the elemental distribution near the surface of individual nanoparticles. Our study reveals that longchain polyethylene glycol typically produces a more uniform multielement distribution within a nanoparticle than short-chain triethylene glycol under identical experimental conditions. We performed electrochemical reduction of metal salts in both solvents to understand the reduction kinetics of metal salts, which shows that a solvent capable of improving the co-reduction of metal salts leads to the synthesis of more homogenized nanoparticles, whereas a solvent with a varying degree of reduction potency will lead to inhomogeneous elemental distribution in an HEA nanoparticle.

Catalyst↗
DOE OSTI2022

Modeling Isothermal Reduction of Iron Ore Pellet Using Finite Element Analysis Method: Experiments & Validation

Iron ore pellet reduction experiments were performed with pure hydrogen (H2) and mixtures with carbon monoxide (CO) at different ratios. For direct reduction processes that switch dynamically between reformed natural gas and hydrogen as the reductant, it is important to understand the effects of the transition on the oxide reduction kinetics to optimize the residence time of iron ore pellets in a shaft reactor. Hence, the reduction rates were studied by varying experimental parameters such as the temperature (800, 850 & 900 °C), reactant gas flow rate (100, 150 & 200 cm3/min), pellet size and composition of the reactant gas mixture. The rate of reduction was observed to increase with an increase in temperature and reactant gas flow rate, but it decreased with an increase in pellet size. SEM greyscale analysis was performed to analyze the porosity and phase composition of partially reduced pellets. The porosity of the pellets was observed to increase from 0.3 for unreacted pellet to 0.42 for a completely reduced pellet. Energy-dispersive X-ray spectroscopy (EDAX) analysis was performed to identify the phases observed in the SEM images. The fraction of iron phase was observed to increase from the shell region of the pellet to the core region with an increase in the degree of reduction. A 2D-axisymmetric numerical model was developed on COMSOL Multiphysics, and it was validated using the conversion (X) vs. time curves obtained from each experiment. The model was able to accurately predict the total time needed for the complete conversion of a single iron ore pellet for multiple experiments. Effects of changes in the porosity and tortuosity of the pellet on the model were also studied and the rate of reduction was observed to be sensitive to changes in both porosity and tortuosity. The SEM analysis and the model results show that tortuosity is higher for pellets reduced with H2 than for pellets reduced with H2-CO gas mixtures.

08 HYDROGEN↗
DOE OSTI2023

Numerical modeling of chemical looping oxidative dehydrogenation of ethane in parallel packed beds

Chemical looping oxidative dehydrogenation (CL-ODH) of ethane has the potential to be a highly efficient alternative to steam cracking for ethylene production. Accurate reactor modeling is of critical importance to efficiently scale up and optimize this new technology. This study reports a one-dimensional, heterogeneous packed bed model to simulate the CL-ODH of ethane to ethylene with a Na 2 MoO 4 -promoted CaTi 0.1 Mn 0.9 O 3 redox catalyst. Here, the overall reaction kinetics was well-described by coupling the gas-phase steam cracking of ethane with the reduction kinetics of the redox catalyst by H 2 and C 2 H 4 . The impact of H 2 on the formation rate of CO 2 byproduct from C 2 H 4 conversion was also thoroughly investigated to validate the applicability of the kinetic model under operational environments. The temperature variation within the different CL-ODH steps and the temperature distribution along the bed were also carefully considered. The accuracy of the model was validated by experiments conducted in a large lab-scale packed bed reactor (200 g catalyst loading), with an average deviation of 2.8% in terms of ethane conversion and ethylene yield. The model was subsequently used to optimize the operating parameters of the CL-ODH reactor, indicating that up to 63.7% single-pass C 2 +olefin yield can be achieved with the current redox catalyst bed whereas further optimization of the redox catalyst to inhibit C 2 H 4 activation can result in 69.4% single-pass C 2 +yield while maintaining low CO 2 selectivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Elucidating key reducing species beyond ions in hydrogen plasma smelting reduction of iron ore

Hydrogen plasma smelting reduction (HPSR) of iron ore has attracted significant attention over the past decade due to its high-temperature operation, rapid plasma mediated reduction kinetics, and simpler density-based separation of molten iron product, compared to H2-based solid-state reduction. All of these attributes enable processing of low-grade ores for downstream use in electric-arc furnaces, as virgin iron with low gangue content is required for high quality steel and improved furnace operation. While positive ions exist within the plasma arc, this work demonstrates that near the anodic ore surface, hydrogen radicals and vibrationally excited hydrogen species dominate and their densities correlate well with observed reduction rates. Species concentrations in the transferred plasma arc and at the plasma-ore interface are evaluated using coupled thermal plasma and near-wall non-equilibrium plasma models. The thermal plasma model is validated against experimental voltage data and spectroscopic measurements of plasma temperature and density for varying current inputs. Modeling of the near surface thermochemical non-equilibrium and micrometer scale anode sheath layer reveals, in addition to the expected H + , significant concentrations of ArH + and H$^+_3$ ions, typically not observed in thermal plasmas under thermodynamic equilibrium. Our results show that the inverted sheath structure at the anodic ore surface strongly suppresses reactive positive ion fluxes, while non-equilibrium electron-impact processes generate abundant hydrogen radicals and vibrationally excited species. These findings highlight the critical role of non-equilibrium effects in hydrogen arc-driven iron ore reduction and advance understanding beyond prevailing hypotheses centered on hydrogen ion-driven mechanisms.

08 HYDROGEN↗
DOE OSTI2023

Kinetic Effects of Anion Clusters on the Interfacial Stability between Solid-State Electrolyte and Metal Anode

The success of all-solid-state batteries (ASSBs) depends on the solid-state electrolyte (SSE) exhibiting high interfacial stability and room-temperature ionic conductivity. However, the current SSEs, especially those with practical ionic conductivities (≥10 –3 S/cm) at room temperature, often develop unstable interfaces at the metal anode, in some cases with even greater severity than with liquid organic electrolytes. Despite persistent efforts, achieving interfacial stability and sufficient ionic conductivity simultaneously represents one of the greatest challenges in ASSBs. The current approaches focus on stabilizing the interface by incorporating secondary interlayers or introducing coatings by surface engineering. The method is often material-specific, and the added interlayers often deteriorate during cycling. In this work, using phase analysis and explicit interface modeling, we demonstrate a strategy to kinetically stabilize the interface between the SSE and metal anode by incorporating selected monoanion clusters in the SSE; they can effectively lower or even halt the reduction kinetics at the interface by promoting on-site formation of interphases that are highly electron insulating. The study provides insight into the kinetic effects to achieve SSEs with superior properties in bulk and at the interface.

25 ENERGY STORAGE↗