Chemical Oxidation of SrCoO3−δ

Abstract

Various chemical oxidants were investigated for their capability to oxidize brownmillerite SrCoO 2:5 to perovskite SrCoO 3À . Among the oxidants tested, KMnO 4 /HCl was found to be the strongest to yield SrCoO 3À with % 0:06.The perovskite SrCoO 3À system is known for its wide range of oxygen nonstoichiometry. Hence, it serves as a kind of prototype for the recently highlighted functional oxides of mixed-valent (III/IV) cobalt. With the proper choice of process parameters it can be obtained as different types of metastable single phases with orthorhombic (0:5 < < 0:6), 1,2 tetragonal (0:1 < < 0:25) 2,3 or cubic ( % 0) 3-5 symmetry. Synthesis of the oxidized phases usually goes through the orthorhombic brownmillerite phase SrCoO 2:5 , 1 which has ordered oxygen vacancies such that the structure is composed of alternately stacked layers of corner-sharing CoO 4 tetrahedra and CoO 6 octahedra. For the oxidation of SrCoO 2:5 , electrochemical [3][4][5] and high-pressure 2,6 techniques have been employed. Possibility to utilize chemical ways for the control of oxygen content was brought up by Nemudry et al.3 who oxidized SrCoO 2:5 by immersing the sample into an aqueous solution of Br 2 and NaOH. Compared to the other methods chemical oxidation requires only a very simple experimental set up. In the present work, several other chemical oxidizing agents are utilized.The SrCoO 2:5 precursor was prepared from Co and SrCO 3 powders, dissolved in stoichiometric amounts in 3 M HNO 3 solution and then chelated by EDTA/NH 3 solution containing EDTA in 50% excess. The solution was dried, burned and then calcined at 900 C. The calcination product was ground and pressed into pellets for the final heat treatment in air at 1000 C for 40 h, followed by quenching to room temperature. The X-ray diffraction pattern for the product revealed a singlephase brownmillerite with an oxygen content of 2.50 per formula unit, as determined by iodometric titration.For the oxidation experiments, the SrCoO 2:5 pellets were crushed and ground. Four different oxidizing media were employed: (1) aqueous solution of 0.2 M KMnO 4 and HCl in various concentrations, (2) aqueous solution of Na 2 S 2 O 8 saturated at room temperature, (3) 1:1 dispersion of Br 2 and H 2 O, and (4) aqueous solution of 0.3 M Br 2 and 5 M NaOH. The experiments were carried out for 50-mg samples in 2-mL plastic vials except for experiment (4), which was carried out for a 200-mg sample in a 20-mL glass tube. Immersion time varied from 1 to 24 h. Temperature was controlled using a water or an ice bath and a thermometer. After the oxidation treatment, the sample powder was washed with deionized water plus ethanol [(1) and (2)] or CH 3 CN plus ethanol [(3) and (4)] and subsequently dried in air at room temperature.X-ray diffraction (XRD) patterns were collected for the samples with a Rigaku RINT2550 diffractometer equipped with a rotating Cu anode. Software JANA 2000 was used in the profile-fitting mode for the lattice-parameter refinements [in space groups, I4=mmm (SrCoO 3À ) and P...

Oxygen Absorption Capability of YBaCo4O7+δ

“…From previous works on various functional cobalt oxide materials, chemical oxidation methods have been found promising as well. 5,6 Therefore, in addition to the HP techniques we tested the capability of a Br 2 /H 2 O dispersion to oxidize YBaCo 4 O 7þ . Moreover, for the sake of comparison normalpressure annealing experiments in oxygen, air, and nitrogen atmospheres were performed.…”

“…From previous works on various functional cobalt oxide materials, chemical oxidation methods have been found promising as well. 5,6 After evaporating the solvent and burning the residue, the remaining ash was ground, pressed into pellets, and sintered in an N 2 gas flow at 1050 C for 20 h. From iodometric titration, oxygen content of the thus synthesized XRD-pure YBaCo 4 O 7þ sample (Sample B) was determined at ¼ 0:13. A nearly oxygen-stoichiometric sample of ¼ 0:03 (Sample A) was obtained by annealing a specimen of Sample B powder in a perfectly air-tight thermobalance (MAC Science TG-DTA 2000S) in flowing N 2 gas at 500 C for 2 h. All oxidation experiments were carried out for specimens of Sample B powder.…”

Syntheses and Magnetic-Properties of Zn-Diluted Sr-Based Perovskite Cobalt Oxides, Sr1−x Zn x CoO3; 0.05≤x≤0.3

“…Due to huge diversity in composition, stoichiometry and structure, perovskite materials exhibit number of interesting properties on structural, magnetic and transport properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. These features promise for a wide range of applications in the solid oxide fuel cells, magnetic recording heads, permanent magnets, and suitable catalysts or support for combustion processes, CO oxidation, methane reforming, SO 2 reduction, N 2 O to N 2 and O 2 , etc.…”

“…These features promise for a wide range of applications in the solid oxide fuel cells, magnetic recording heads, permanent magnets, and suitable catalysts or support for combustion processes, CO oxidation, methane reforming, SO 2 reduction, N 2 O to N 2 and O 2 , etc. [10][11][12][15][16][17][18][19][20][21][22]. SrCoO 3 is the well studied perovskite in the Sr homologous cobalt oxide series.…”

“…Amongst the different types of perovskite oxides investigated, the rare earth cobaltites have been extensively studied for their fascinating structural, magnetic and transport properties. So far these perovskite materials have been obtained by different methods viz., solid-state classical ceramic reaction route, high-pressure and high temperature (HPHT) route, solid state synthesis by different precursor sources, electrochemical oxidation and soft-chemistry routes [5,11,12,15,25]. Our previous investigations on the perovskite cobalt oxide materials have brought many interesting and exciting innovative results [25][26][27][28][29][30][31][32][33].…”