Studies on perovskite type rare earth transition metal oxides as oxidation catalysts

Abstract

The introductory chapter begins with a classification of the thermal decomposition of solids, followed by a brief account of the decomposition of simple and complex oxalates as well as complex nitrites. The salient features of the perovskite structure are discussed. Since rare-earth cobaltites have been reported as promising catalysts for automobile exhaust treatment, the literature on the catalytic applications of these oxides is briefly reviewed. Previous work on the decomposition of trioxalato-cobaltates of rare-earth and bivalent metals is summarized, and the scope of the present investigation is outlined. Chapter II This chapter deals with the preparation of various trioxalato-cobaltates and rare-earth hexanitrocobaltates. Procedures for chemical analysis and the physical techniques employed are described in detail. A variety of physical techniques were utilized during this investigation, including thermoanalytical methods such as TG (thermogravimetry), DTA (differential thermal analysis), infrared and electronic spectroscopy, X-ray diffraction, surface area measurements, and magnetic susceptibility studies. Chapter III Detailed studies on the thermal decomposition of solid ammonium trioxalato-cobaltate(III) trihydrate, [(NH?)?[Co(C?O?)?]·3H?O], are reported in this chapter. Although this compound has been known for a long time, its thermal decomposition in the solid state has not been previously documented. Our results indicate that decomposition occurs mainly in two steps: dehydration and oxalate decomposition. Dehydration is accompanied by partial oxalate breakdown, and the oxalate decomposition is found to be a multistep process, ultimately yielding Co?O? as the final product in air. A decomposition scheme in air is proposed. The surrounding atmosphere significantly influences the nature of the final product: in non-oxidizing atmospheres, the product is CoO, whereas in vacuum or reducing atmospheres, a mixture of Co?O? and CoO is obtained. Chapter IV This chapter focuses on the thermal decomposition of rare-earth trioxalato-cobaltate(III) hydrates, [Ln[Co(C?O?)?]·xH?O], where Ln = La, Pr, Nd. The results of the present investigation reveal that the previously proposed decomposition scheme is inadequate. Based on thermal analysis, isolation, identification, and characterization of residues at various stages using both chemical and physical methods, a new decomposition scheme is proposed. This consists of three steps: 1. Dehydration of the hydrate, accompanied by partial oxalate decomposition; 2. Oxalate decomposition to form an oxycarbonate, LnCo(CO?)(C?O?); 3. Carbonate decomposition to yield LnCoO?. While dehydration occurs in a single step for lanthanum and praseodymium compounds, the neodymium compound undergoes dehydration in two or more steps. The first dehydration step, accompanied by partial oxalate decomposition, results in the formation of Nd[Co(C?O?)?]·1.5H?O. The remaining water is lost during subsequent oxalate decomposition, leading to the formation of an oxycarbonate, which then decomposes to give the cobaltite. Rare-earth trioxalato-cobaltates are photosensitive and, upon storage at room temperature for a month or more, decompose to form a pink-colored residue of composition Ln[Co(C?O?)?]. While the green trioxalato-cobaltate complexes contain cobalt in the low-spin trivalent state, the photodecomposed salts contain cobalt in the divalent state. This involves the reduction of Co(III) to Co(II) with the evolution of one mole of carbon dioxide by oxidation of 0.5 mole of oxalate per mole of the complex. This auto-redox reaction invariably accompanies the dehydration step for all trioxalato-cobaltates, as observed for other compounds as well. Since the composition of the photoreduced compound corresponds to that of a mechanical mixture of rare-earth and cobalt oxalates in a 1:2 molar ratio, the thermal behavior of the two has been studied for the lanthanum salt. The results show that while dehydration occurs in two steps for both the photoreduced compound and the mechanical mixture, oxalate decomposition proceeds differently. The photoreduced oxalate decomposes in a single step to form an oxycarbonate similar to that formed by the original complex oxalate, which further decomposes to form lanthanum cobaltite. In contrast, oxalate decomposition in the mechanical mixture occurs in two steps, and the final products are La?O? and Co?O? with traces of LaCoO?. This indicates that the residue obtained during photoreduction is a complex containing both lanthanum and cobalt in a chemically bound state. Chapter V The thermal decomposition of potassium–alkaline earth/lead trioxalato-cobaltate(III) hydrates, [K?M[Co(C?O?)?]·xH?O], where M = Ba, Sr, Ca, or Pb, is reported in this chapter. There is no prior literature on these compounds, making their decomposition behavior particularly interesting. Thermal analysis, complemented by other physical and chemical techniques for the isolation and identification of intermediates at various stages, reveals that decomposition occurs in two steps: 1. Partial dehydration, accompanied by partial oxalate decomposition; 2. Loss of remaining water, followed by the main oxalate decomposition. Chapter VI The thermal decomposition of these compounds results in the formation of potassium carbonate, alkaline earth carbonates, and cobalt oxide. For lead salts, lead oxide is formed along with Co?O?. The calcium carbonate produced from calcium salts undergoes further decomposition at higher temperatures to yield calcium oxide. Decomposition schemes have been proposed for these compounds in air, and the influence of the surrounding atmosphere on the decomposition pathway has also been examined. Chapter VI comprises a study of the thermal decomposition of ammonium–alkaline earth trioxalato-cobaltate(III) hydrates, [(NH?)???M?[Co(C?O?)?]·xH?O], where M²? = Ba, Sr, and Ca. While ammonium trioxalato-cobaltate(III) trihydrate is well known, the partial substitution of ammonium groups by bivalent metals is reported here for the first time. The present investigation shows that these compounds decompose in a manner analogous to the corresponding potassium salts. Thermal decomposition was studied under different atmospheric conditions, including air, carbon dioxide, and vacuum. The results indicate that decomposition proceeds in a stepwise fashion. Dehydration, accompanied by partial oxalate decomposition, occurs in two stages. The main oxalate decomposition leads to the formation of alkaline earth carbonates and cobalt oxide. Calcium carbonate further decomposes at higher temperatures in air and vacuum to form calcium oxide. No alkaline earth cobaltite was identified in any of the residues. In vacuum and carbon dioxide atmospheres, further reduction of Co?O? occurs, yielding either CoO or metallic cobalt. Chapter VII Studies on the thermal stability of rare-earth hexanitrocobaltates, [Ln[Co(NO?)?]], where Ln = La, Pr, Nd, and Sm, form the subject matter of this chapter. Decomposition was carried out mainly in air and vacuum. All compounds exhibit a similar decomposition scheme comprising two steps: 1. Decomposition of the complex to form stable rare-earth nitrite and cobalt oxide; 2. A two-stage process involving a rapid reaction followed by a slower reaction over a wide temperature range. The second step involves decomposition of the rare-earth nitrite and its reaction with cobalt oxide, leading to the formation of rare-earth cobaltite. These two steps overlap and are not significantly influenced by the surrounding atmosphere. The final product of decomposition is primarily the rare-earth cobaltite, with small amounts of undecomposed nitrite. Results based solely on TG and DTA experiments are inconclusive, as overlapping reactions can lead to variable weight losses. Therefore, dynamic thermoanalytical techniques must be supplemented by other physicochemical methods to establish reliable decomposition schemes for complex compounds. From the present study, rare-earth trioxalato-cobaltates have been identified as excellent precursors for the low-temperature preparation of cobaltites. Although hexanitrocobaltates are not as effective as oxalato complexes, they are reasonably good starting materials for the synthesis of rare-earth cobaltites.