Perovskite characterization

Two types of oxygen deficient perovskites, characterized by a dimensional character have been isolated. The first one, La3Ba3Cus0i4 (15) results from the ordered elimination of rows of oxygen atoms parallel to the <110> direction of the cubic perovskite. It results that the host lattice of the oxide (Figure... 

Most niobates and tantalates, however, are insoluble and may be regarded as mixed oxides in which the Nb or Ta is octahedrally coordinated and with no discrete anion present. Thus KMO3, known inaccurately (since they have no discrete MO3 anions) as metaniobates and metatantalates, have the perovskite (p. 963) stmcture. Several of these perovskites have been characterized and some have ferroelectric and piezoelectric properties (p. 57). Because of these properties, LiNb03 and LiTa03 have been found to be attractive alternatives to quartz as frequency filters in communications devices. 

This method of approach, when applied to the above mentioned compounds leads to a Na2Ta306F5 structure in which n = 3 (n denotes the number of layers) if the perovskite positions remain vacant, as shown in Fig. 36 (a). The Na4Ta5OioF9 phase corresponds to n = 2.5, which leads to the proposed structure consisting of two types of alternating layers, characterized by n = 2 and n = 3, as shown in Fig. 36 (b). The central positions of the perovskite layers remain vacant in this structure as well. 

In this paper we attempt a preliminary investigation on the feasibility of catalytic combustion of CO/ H2 mixtures over mixed oxide catalysts and a comparison in this respect of perovskite and hexaaluminate type catalysts The catalysts have been characterized and tested in the combustion of CO, H2 and CH4 (as reference fuel). The catalytic tests have been carried out on powder materials and the results have been scaled up by means of a mathematical model of the catalyst section of the Hybrid Combustor. 

Kaliaguine, S., Van Neste, A., Szabo, V. et al. (2001) Perovskite-type oxides synthesized by reactive grinding Part I. Preparation and characterization, Appl. Catal. A 209, 345. 

Ding X, Liu Y, Gao L, and Guo L. Synthesis and characterization of doped LaCrOs perovskite prepared by EDTA-citrate complexing method. J. Alloys Compounds 2007 458 346-350. 

Three series of LaCoi. CuxOs, LaMni.xCuxOs, LaFei x(Cu, Pd)x03 perovskites prepared by reactive grinding were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O2, NO + O2, and CsHg in the absence or presence of H2O, Fourier transform infrared (FTIR) spectroscopy as well as activity evaluations without or with 10% steam in the feed. This research was carried out with the objective to investigate the water vapor effect on the catalytic behavior of the tested perovskites. An attempt to propose a steam deactivation mechanism and to correlate the water resistance of perovskites with their properties has also been done. 

Kirchnerova J Klvana D. Preparation and characterization of high surface perovskite electrocatalysts. Int. J. Hydrogen Energy., 1994, Volume 19, Issue 6, 501-506. 

Ciambelli, P Cimino, S Lisi, L Faticanti, M Minelli, G Pettiti, 1 Porta, P. La, Ca and Fe oxide perovskites preparation, characterization and catalytic properties for methane combustion. Appl Catal, B Environmental, 2001, Volume 33, Issue 3, 193-203. 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

Octahedral tunnel structures. Metal-oxygen octahedra, BO, form host lattices characterized by large tunnels where cations are located. Thus, perovskites form a large family which are described by four-sided tunnels. There are a large number of tunnel structures which can be classified according to the size and the shape of the tunnels. Tunnel structures with angles of 90° or 60 -120° occur commonly. We shall briefiy examine the general features of tunnel structures. (Note that the bronzes we discussed earlier are tunnel structures). 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN)3 5 on supports, and LaCoC3 Perovskite. There are many others (79—85). Studies on the well-characterized Mo(II) monomer and Mo(II) dimer on silica carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkali or alkaline earth, and B is a small, octahedrally coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals are characterized by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

---