Solubility of cerium in LaCoO3-influence on catalytic activity

Cation-Deficient Ce-Substituted Perovskite Oxides with Dual-Redox Active Sites for Thermochemical Applications

Identifying thermodynamically favorable and stable non-stoichiometric metal oxides is of crucial importance for solar thermochemical (STC) fuel production via two-step redox cycles. The performance of a non-stoichiometric metal oxide depends on its thermodynamic properties, oxygen exchange capacity, and its phase stability under high-temperature redox cycling conditions. Perovskite oxides (ABO_3-δ) are being considered as attractive alternatives to the state-of-the-art ceria (CeO_2-δ) due to their high thermodynamic and structural tunability. However, perovskite oxides often exhibit low entropy change compared to ceria, as they generally have one only redox active site, leading to lower mass-specific fuel yields. Herein, we investigate cation-deficient Ce-substituted perovskite oxides as a new class of potential redox materials combining the advantages of perovskites and ceria. We newly synthesized the (Ce_xSr_1-x)_0.95Ti_0.5Mn_0.5O_3-δ (x = 0, 0.10, 0.15, and 0.20; CSTM) series, with dual-redox active sites comprising Ce (at the A-site) and Mn (at the B-site). By introducing a cation deficiency (∼5%), CSTM perovskite oxides with both phase purity (x ≤ 0.15) and high-temperature structural stability under STC redox cycling conditions are obtained. Thermodynamic properties are evaluated by measuring oxygen non-stoichiometry in the temperature range T = 700-1400 °C and the oxygen partial pressure range pO₂ = 1-10^-4 bar. The results demonstrate that CSTM perovskite oxides exhibit a composition-dependent simultaneous increase of enthalpy and entropy change with increasing Ce-substitution. (Ce_0.20Sr_0.80)_0.95Ti_0.5Mn_0.5O_3-δ (CSTM20) showed a combination of large entropy change of ∼141 J (mol-O)^-1 K^-1 and moderate enthalpy change of ∼238 kJ (mol-O)^-1, thereby creating favorable conditions for thermochemical H₂O splitting. Furthermore, the oxidation states and local coordination environment around Mn, Ce, and Ti sites in the pristine and reduced CSTM samples were extensively studied using X-ray absorption spectroscopy. The results confirmed that both Ce (at the A-site) and Mn (at the B-site) centers undergo simultaneous reduction during thermochemical redox cycling.

Role of ceria in the improvement of NO removal of lanthanum-based perovskite-type catalysts

Lanthanum-based perovskite-type oxides represented by LaBO3 (B = Co, Fe, Mn) have been thought to present strong limitations for practical application although they are active for catalytic removal of NO. Cerium (Ce) substitution has been extensively studied to modify the properties of perovskites. It is noted that a new phase of ceria (CeO2) can be separated from perovskites when the doping ratio exceeds the solution limit (x > S). This review outlines the relationship between the existence of CeO2 phase and catalytic activity. CeO2 dispersing on the lattice surface or small particles are beneficial for catalytic activity, but larger particles are adverse. Ce-doped LaBO3 perovskites exhibiting the best activity must contain additional CeO2 phases. In addition, CeO2-supported LaBO3 perovskite catalysts are discussed.

Clean Diesel Power via Microwave Susceptible Oxidation Catalysts

The problem of soot emissions from diesel engines is introduced and the possible solution of combining doped perovskites and microwave (mw) irradiation to "clean up" diesel soot filters is outlined. Eighteen doped perovskite catalysts are synthesized and tested for propane and CO oxidation, which are taken as model components for soot. The activity, selectivity, and SO2 tolerance are compared under conventional heating and mw irradiation. By combining mw irradiation and doped perovskites, one can create "hot spots" on the catalyst, resulting in efficient and selective heating of the active site, as well as less poisoning. Sr-doped and Ce-doped manganese perovskites show the highest activity. These catalysts are also the most selective, and have a high mw susceptibility. Optimal SO2 tolerance is displayed by Cr perovskites, from which the La0.8Ca0.2CrO3 combination uniquely converts propane before CO, and therefore can be used to remove >C2 hydrocarbons from a mix with CO. Possible mechanistic scenarios are presented and discussed.

A computational modelling study of oxygen vacancies at LaCoO3 perovskite surfaces

Atomistic computational modelling of the surface structure of the catalytically-active perovskite LaCoO(3) has been undertaken in order to develop better models of the processes involved during catalytic oxidation processes. In particular, the energetics of creating oxygen ion vacancies at the surface have been investigated for the three low index faces (100), (110) and (111). Two mechanisms for vacancy creation have been considered involving dopant Sr(2+) cations at the La(3+) site and reduction of Co(3+) to Co(2+). For both mechanisms, there is a general tendency that the smaller the cation defect separation, the lower the energy of the cluster, as would be expected from simple electrostatic considerations. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. The importance of individual crystal surfaces in catalysis is discussed in terms of the energetics for the creation of oxygen vacancies.