Perovskite thermal stability

Co-, Mn-, and Fe-based perovskites, NO peaks appearing at similar temperatures as those in NO-TPD experiments (apart from a minor desorption at medium temperature and an intense one at high temperature) were also found and aseribed to the desorptions of physically adsorbed NO, nitrosyl species and nitrate species in the order of their thermal stability. The NO peaks obtained at low (80-110 °C), medium (200-210 °C), and high (310-390 °C) temperatures in the present NO-TPD analyses were thus correlated to physically adsorbed NO, nitrosyl and nitrate species. 

Qi et al. studied perovskite systems obtained by partially replacing La with Ce at the A-site of LaNi03 for ATR of gasoline and its subrogate n-octane with or without thiophene additive. They found similar activities and mechanical and thermal stabilities with both pellet and monolith Lao.8Ceo.2Ni03 [73]. 

Layered perovskites with the formula A2BO4 (e.g. the K2Nip4 structure) have a well-defined bulk structure and high thermal stability. Examples include... 

Solid oxide catalysts such as hexaaluminates and perovskites, in which an active metal catalyst is incorporated into a coke-resistant lattice, are effective for liquid hydrocarbon reforming due to their thermal stability over a broad-range of temperature. However, sulfur tolerance of those materials has yet to be demonstrated. 

Moreover, the environmental systems demonstrate unique diversity and versatility of the processes depending on the actual conditions, viz. the reaction directions and rates are sensitive to many diverse parameters, sometimes even difficult to be perceived. The example may be the dependence of the photocatalytic activity of Sr—A1—Nb—0 double perovskite on the cation ordering in the oxides (267), or the effect of the in-plane twist of the quinoline-based co-ligand on the thermal stability and yield of NO photorelease from the [Ru(NO)] nitrosyl complexes (96). 

Extensive studies have been carried out on the microwave dielectric materials with high K and thermal stability for the miniaturization of microwave passive component. In particular, CaTi03-based materials have attracted considerable interest due to their high K. These titanates can be easily combined with other perovskite compounds to form solid solutions. However, they have a large TCF for practical apphcations. Various attempts have been made to control the TCFs of CaTiOj-based materials. However, most of them are mainly empirical, such as the addition of the materials with negative TCF values. 

Ruthenium has always been considered as one of the most active catalysts for several reactions of commercial and environmental importance. In early seventies, ruthenium was studied for its possible application as a de-NOx catalyst for automobiles. Its volatile nature in oxide form was soon realized and efforts were directed to stabilize ruthenium, without much affecting its catalytic properties. Attempts were also made towards synthesis of mthenium based perovskite t3 e and other mixed oxide compositions, as the incorporation of ruthenium in perovskite structure can substantially improve its thermal stability... 

The results obtained point to the real possibility of practical application of Mn REEAEE/Ce/0-Al2O3 catalysts for utilization of CH4 lean mixtures in catalytic heat generators. As to its activity and thermal stability, the catalyst developed is not inferior to well-known analogs using perovskites, aluminates, and manganese hexaluminates. 

The catalytic activity of cation-doped hexaaluminate was not as high as the Pd catalyst or some perovskite-type oxides. But the thermal stability is superior for hexaaluminate to the Pd or perovskite-based catalysts. Pd/cordierite is the popular combustion catalyst due to its very high thermal shock resistance resulting from the low thermal expansion coefficient of the support material. However, the thermal stabilities are not high enough due to its low melting point. 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

The main drawback with the use of perovskites is the poor thermal stability of the materials compared with hexa-aluminate based catalysts. Zwinkels et al. have compared the thermal stability of two different hexa-aluminates with a SrZr03-perovskite, a pyrochlore, see Section 3.2.4, and two spinels, see Section 3.2.3. The perovskite, as well as the pyrochlore and one of the spinels decreased in their surface area significantly. One of the explanations of the much lower stability of the perovskites compared with the hexa-aluminates is that the crystal growth will take place in three dimensions and thereby yield a material with a low surface area. Lowe et al. have studied several different perovskites and their thermal stability and conclude that the surface area of the perovskites is not sufficient for use in high temperature catalytic combustion. Similar results have been shown by Cristiani et al. ... 

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