Redox Kinetics Study of Fuel Reduced Ceria for Chemical-Looping Water Splitting

Chemical-looping water splitting is a novel and promising technology for hydrogen production with CO₂ separation. Its efficiency and performance depend critically on the reduction and oxidation (redox) properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface redox pathways, including the complex interactions of mobile ions and electrons between the bulk and the surface, along with the adsorbates and electrostatic fields, remain yet unresolved. This work presents a detailed redox kinetics study with emphasis on the surface ion-incorporation kinetics pathway, using time-resolved and systematic measurements in the temperature range 600–1000 °C. By using fine ceria nanopowder, we observe an order-of-magnitude higher hydrogen production rate compared to the state-of-the-art thermochemical or reactive chemical-looping water splitting studies. We show that the reduction is the rate-limiting step, and it determines the total amount of hydrogen produced in the following oxidation step. The redox kinetics is modeled using a two-step surface chemistry (an H2O adsorption/dissociation step and a charge-transfer step), coupled with the bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted with excellent agreement with measurements. The model reveals that the surface defects are abundant during redox conditions, and charge transfer is the rate-determining step for H₂ production. The results establish a baseline for developing new materials and provide guidance for the design and the practical application of water splitting technology (e.g., the design of OC characteristics, the choice of the operating temperatures, and periods for redox steps, etc.). The method, combining well-controlled experiment and detailed kinetics modeling, enables a new and thorough approach for examining the defect thermodynamics in the bulk and at the surface, as well as redox reaction kinetics for alternative materials for water splitting

Enhancing co-production of H[subscript 2] and Syngas via Water Splitting and POM on Surface-Modified Oxygen Permeable Membranes

In this article, we report a detailed study on co-production of H2 and syngas on La[subscript 0.9]Ca[subsvript 0.1]FeO[subscript 3−[delta] (LCF-91) membranes via water splitting and partial oxidation of methane, respectively. A permeation model shows that the surface reaction on the sweep side is the rate limiting step for this process on a 0.9 mm-thick dense membrane at 990°C. Hence, sweep side surface modifications such as adding a porous layer and nickel catalysts were applied; the hydrogen production rate from water thermolysis is enhanced by two orders of magnitude to 0.37 μmol/cm2•s compared with the results on the unmodified membrane. At the sweep side exit, syngas (H[subscript 2]/CO = 2) is produced and negligible solid carbon is found. Yet near the membrane surface on the sweep side, methane can decompose into solid carbon and hydrogen at the surface, or it may be oxidized into CO and CO[subscript 2], depending on the oxygen permeation flux. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4427–4435, 2016Shell Oil CompanyKing Abdullah University of Science and Technolog

Kinetic Effects of Non-Equilibrium Plasma-Assisted Methane Oxidation on Diffusion Flame Extinction Limits

The kinetic effects of plasma assisted fuel oxidization on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH 4 into the oxidizer stream. The experiments showed that the non-equilibrium plasma can dramatically accelerate the fuel oxidization at low temperature. The prompt fuel oxidization resulted in fast chemical heat release and extended the extinction limits significantly. The O production and the products of plasma assisted fuel oxidation were measured, respectively, by using twophoton absorption laser-induced fluorescence (TALIF) method, Fourier Transform Infrared (FTIR) spectrometer, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H 2 O and H 2 concentrations and under-predicted CO 2 concentration. The O concentration prediction from the kinetic model intersected with experimental results. A path flux analysis showed that O was majorly generated by the discharge and dictated the plasma assisted fuel oxidization. So the deviation between experiments and simulations was caused by the inaccurate prediction of O. This is due to missing reaction pathways, such as those involving excited species (e.g. excited O) and the validity of radical consumption reactions with hydrocarbon species at low temperature range

Toward enhanced hydrogen generation from water using oxygen permeating LCF membranes

Hydrogen production from water thermolysis can be enhanced by the use of perovskite-type mixed ionic and electronic conducting (MIEC) membranes, through which oxygen permeation is driven by a chemical potential gradient. In this work, water thermolysis experiments were performed using 0.9 mm thick La[subscript 0.9]Ca[subscript 0.1]FeO[subscript 3−δ] (LCF-91) perovskite membranes at 990 °C in a lab-scale button-cell reactor. We examined the effects of the operating conditions such as the gas species concentrations and flow rates on the feed and sweep sides on the water thermolysis rate and oxygen flux. A single step reaction mechanism is proposed for surface reactions, and three-resistance permeation models are derived. Results show that water thermolysis is facilitated by the LCF-91 membrane especially when a fuel is added to the sweep gas. Increasing the gas flow rate and water concentration on the feed side or the hydrogen concentration on the sweep side enhances the hydrogen production rate. In this work, hydrogen is used as the fuel by construction, so that a single-step surface reaction mechanism can be developed and water thermolysis rate parameters can be derived. Both surface reaction rate parameters for oxygen incorporation/dissociation and hydrogen–oxygen reactions are fitted at 990 °C. We compare the oxygen fluxes in water thermolysis and air separation experiments, and identify different limiting steps in the processes involving various oxygen sources and sweep gases for this 0.9 mm thick LCF-91 membrane. In the air feed-inert sweep case, the bulk diffusion and sweep side surface reaction are the two limiting steps. In the water feed-inert sweep case, surface reaction on the feed side dominates the oxygen permeation process. Yet in the water feed-fuel sweep case, surface reactions on both the feed and sweep sides are rate determining when hydrogen concentration in the sweep side is in the range of 1–5 vol%. Furthermore, long term studies show that the surface morphology changes and silica impurities have little impact on the oxygen flux for either water thermolysis or air separation.Shell Oil CompanyKing Abdullah University of Science and Technolog

Enhanced intermediate-temperature CO[subscript 2] splitting using nonstoichiometric ceria and ceria–zirconia

CO[subscript 2] splitting via thermo-chemical or reactive redox has emerged as a novel and promising carbon-neutral energy solution. Its performance depends critically on the properties of the oxygen carriers (OC). Ceria is recognized as one of the most promising OC candidates, because of its fast chemistry, high ionic diffusivity, and large oxygen storage capacity. The fundamental surface ion-incorporation pathways, along with the role of surface defects and the adsorbates remain largely unknown. This study presents a detailed kinetics study of CO[subscript 2] splitting using CeO[subscript 2] and Ce[subscript 0.5]Zr[subscript 0.5]O[subscript 2] (CZO) in the temperature range 600-900 °C. Given our interest in fuel-assisted reduction, we limit our study to relatively lower temperatures to avoid excessive sintering and the need for high temperature heat. Compared to what has been reported previously, we observe higher splitting kinetics, resulting from the utilization of fine particles and well-controlled experiments which ensure a surface-limited-process. The peak rates with CZO are 85.9 μmole g[superscript -1]s[superscript -1] at 900 °C and 61.2 μmole g[superscript -1]s[superscript -1] at 700 °C, and those of CeO[subscript 2] are 70.6 μmole g[superscript -1]s[superscript -1] and 28.9 μmole g[superscript -1]s[superscript -1]. Kinetic models are developed to describe the ion incorporation dynamics, with consideration of CO[subscript 2] activation and the charge transfer reactions. CO[subscript 2] activation energy is found to be -120 kJ mole[superscript -1] for CZO, half of that for CeO[subscript 2], while CO desorption energetics is analogous between the two samples with a value of ∼160 kJ mole[superscript -1]. The charge-transfer process is found to be the rate-limiting step for CO[subscript 2] splitting. The evolution of CO[subscript 3][superscript 2-] with surface Ce[superscript 3+] is examined based on the modeled kinetics. We show that the concentration of CO[subscript 3][superscript 2-] varies with Ce[superscript 3+] in a linear-flattened-decay pattern, resulting from a mismatch between the kinetics of the two reactions. Our study provides new insights into the significant role of surface defects and adsorbates in determining the splitting kinetics.King Abdullah University of Science and TechnologyBritish Petroleum Compan

Kinetic Effects of Non-Equilibrium Plasma-Assisted Methane Oxidation on Diffusion Flame Extinction Limits

A new plasma assisted combustion system was developed by integrating a counterflow burner with nano-second pulsed non-equilibrium discharge. The kinetic effects of plasma assisted fuel oxidization on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH 4 into the oxidizer stream. The non-equilibrium discharge accelerated dramatically the fuel oxidation. The O production and the products of plasma assisted fuel oxidation were measured, respectively, by using two-photon absorption laserinduced fluorescence (TALIF) method, Fourier Transform Infrared (FTIR) spectrometer , and Gas Chromatography (GC). The product concentrations were used to validate an existing plasma assisted combustion kinetic model. The comparisons showed the kinetic model prediction was poor due to missing reaction pathways, such as those involving carbon formation, H 2 excitation and dissociation, and interactions of excited species with hydrocarbon species. The path flux analysis determined that O was the critical species for kinetic modeling because it was generated by the discharge and dictated the oxidization process. The extinction strain rate measurements showed the non-equilibrium plasma discharge extended the extinction limit significantly. Strong emission from Ar* was observed at high plasma repetition rates and numerical modeling showed that Ar* contributed significantly to the enhancement of extinction limit. Nomenclature a = extinction strain rate

Redox Kinetics and Nonstoichiometry of Ce_0.5Zr_0.5O_2−δ for Water Splitting and Hydrogen Production

Water splitting and chemical fuel production as a promising carbon-neutral energy solution relies critically on an efficient electrochemical process over catalyst surfaces. The fundamentals within the surface redox pathways, including the complex interactions of mobile ions and electrons between the bulk and the surface, along with the role of adsorbates and electrostatic fields remain yet to be understood quantitatively. This work presents a detailed kinetics study and nonstoichiometry characterization of Ce_0.5Zr_0.5O_2−δ (CZO), one of the most recognized catalysts for water splitting. The use of CZO leads to >60% improvement in the kinetic rates as compared with undoped ceria with twice the total yield at 700 °C, resulting from the improved reducibility. The peak H₂ production rate is 95 μmol g^–1 s^–1 at 700 °C, and the total production is 750 μmol g^–1. A threshold temperature of 650 °C is required to achieve significant H₂ production at fast rates. The redox kinetics is modeled using two-step surface chemistry with bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted, and the model predictions show good agreement with the measurements. The enthalpy of bulk defect formation for CZO is found to be 262 kJ/mol, >40% lower than that of undoped ceria. As oxygen vacancy is gradually filled up, the surface H₂O splitting chemistry undergoes a transition from exothermic to endothermic, with the crossover around δ = 0.04 to 0.05, which constrains the further ion incorporation process. Our kinetics study reveals that the H₂O splitting process with CZO is kinetics limited at low temperature and transitions to partial-equilibrium with significantly enhanced backward reaction at high temperature. The charge-transfer step is found to be the rate-limiting step for H₂O splitting. The detailed kinetics and nonstoichiometric equilibria should be helpful in guiding the design and optimization of CZO as a catalyst, oxygen storage material, as well as oxygen carrier for water-splitting applications