Solid-State Redox Kinetics of CeO2 in Two-Step Solar CH4 Partial Oxidation and Thermochemical CO2 Conversion

The CeO2/CeO2−δ redox system occupies a unique position as an oxygen carrier in chemical looping processes for producing solar fuels, using concentrated solar energy. The two-step thermochemical ceria-based cycle for the production of synthesis gas from methane and solar energy, followed by CO2 splitting, was considered in this work. This topic concerns one of the emerging and most promising processes for the recycling and valorization of anthropogenic greenhouse gas emissions. The development of redox-active catalysts with enhanced efficiency for solar thermochemical fuel production and CO2 conversion is a highly demanding and challenging topic. The determination of redox reaction kinetics is crucial for process design and optimization. In this study, the solid-state redox kinetics of CeO2 in the two-step process with CH4 as the reducing agent and CO2 as the oxidizing agent was investigated in an original prototype solar thermogravimetric reactor equipped with a parabolic dish solar concentrator. In particular, the ceria reduction and re-oxidation reactions were carried out under isothermal conditions. Several solid-state kinetic models based on reaction order, nucleation, shrinking core, and diffusion were utilized for deducing the reaction mechanisms. It was observed that both ceria reduction with CH4 and re-oxidation with CO2 were best represented by a 2D nucleation and nuclei growth model under the applied conditions. The kinetic models exhibiting the best agreement with the experimental reaction data were used to estimate the kinetic parameters. The values of apparent activation energies (~80 kJ·mol−1 for reduction and ~10 kJ·mol−1 for re-oxidation) and pre-exponential factors (~2–9 s−1 for reduction and ~123–253 s−1 for re-oxidation) were obtained from the Arrhenius plots.

Modeling of Ceria Reduction in a Solar Thermochemical Reactor via DEM Method

Solar thermochemical reactor provides an attractive approach that utilizes the most common solar radiation as the thermal driving force to motivate the reaction between CO2 and metal oxides, which is also called metal oxide redox pair-based thermochemical cycles. The CeO2/CeO2-δ is widely used in the two-step redox process due to its advantages including fast-redox kinetics, high crystallographic stability of a wide range of reacting oxygen non-stoichiometry, and relatively high oxygen solid-state conductivity. In this work, a three-dimensional transient numerical analysis has been completed to study the performance of a CeO2 reduction reaction in a 1/8th segment region of a novel partition cavity-receiver reactor. The porous CeO2 catalyst was analyzed using the discrete element method (DEM) to capture the heat transfer and reactive performances. The catalyst textural properties (particle size and void fraction) and reaction conditions (gas flow rate and radiative power input) were investigated in the CeO2 reduction reaction. The results indicated that increasing the catalyst specific surface area and the temperature are beneficial to the O2 production and further CO2 conversion.

Comparison of Au–Ce and Au–Cu interaction over Au/CeO2–CuO catalysts for preferential CO oxidation

Au decelerates reduction of copper species, while it improves ceria reduction.

Activation and deactivation of Ag/CeO2 during soot oxidation: influences of interfacial ceria reduction

Both the existence of CeO2 surface oxygen vacancies and their dynamic change during reaction may cause crucial influence on the soot oxidation behavior of Ag/CeO2.

Numerical Simulation of Heat Transfer in a 3D Cavity-Receiver

The unsteady 3D fluid flow coupled to radiative, convective, and conductive heat transfers are computed within a cavity-receiver that was successfully tested experimentally. A Monte-Carlo radiation model is used in the fluid regions of the reactor with source terms outside the cavity’s window to account for the concentrated radiative power input. Darcy’s law for the viscous regime and the Forchheimer’s term for the inertial regime are used in the momentum equation to account for the pressure drop within the porous region (RPC). Two separate energy equations for the solid and for the fluid regions of the porous domain are solved in order to capture the non-equilibrium effects in that region. Rosseland diffusion approximation is used in the solid regions of the RPC domain. The material properties and boundary conditions were taken from published experimental measurements. The simulation results are compared to the measurement data collected during the pre-heating and the ceria reduction phases, which sum up to four different radiative power inputs. Results of the comparison are very good and constitute the verification that the numerical methods, physical sub-process models and material properties are adequately selected and implemented. An analysis regarding the heat balance, the recirculating flow and, the effect of dual-scale porosity is also presented.

Experimental Study on the Deactivating Effect of KNO3, KCl, and K2SO4on Nanosized Ceria/Titania SCR Catalyst

Nanosized Ce/TiO2is effective in selective catalytic reduction of NO with NH3. The NO conversion of Ce/TiO2is 93% at 370°C. However, addition of potassium using KNO3, KCl, or K2SO4as precursors effectively deactivates Ce/TiO2. NO conversion at 370°C is reduced to 45%, 24%, and 16% after addition of KNO3, KCl, and K2SO4, respectively, with a controlled K/Ce molar ration at 0.25. The deactivation may be attributed to the changes in the structural and chemical state of ceria and the degradation of surface acidity. The transformation of amorphous ceria into ceria crystals after potassium addition, together with the decrease of surface defects, is also determined. Oxygen diffusion in the process of ceria reduction is slow, and the redox cycle is slowed down. Moreover, the surface acid sites are markedly destroyed, leading to the reduced capacity of ammonia adsorption. These results may provide useful information for the application and life management of CeO2/TiO2in potassium-rich environments such as biofuel-fired boilers.

Atomic layer deposition of epitaxial CeO2 thin layers for faster surface hydrogen oxidation and faster bulk ceria reduction/reoxidation

Thin ceria layers of 120 nm were processed by atomic layer deposition on both YSZ(100) single crystal substrates and polycrystalline YSZ ones.