Nanostructured Perovskites for Catalytic Combustion

Abstract

Until recently, for reducing the environment pollution, the combustion catalysts based on noble metals was considered the most active, practically irreplaceable. Their high cost, high thermal instability, high sensitivity to deactivation and to the attack of some harmful elements or compounds determine the intensification of studies to replace them with new cheaper and stable catalysts. Numerous experimental data from literature indicate that the semiconductive oxidic compounds can compete with combustion catalysts based on noble metals from catalytic activity standpoint. Recent studies led to the realization of remarkable catalytic activity at moderate (350-600°C) and high (over 600°C) temperatures at some oxidic perovskite compounds, which contain transition metals. In this study are presented a series of nanostructured oxidic compounds with perovskite structure, based on transition metals and synthesized by the precursor method of self-combustion, using polyvinyl alcohol as colloidal medium, for catalytic combustion of some volatile organic compounds at low (50-350°C) and moderate temperatures. The catalytic activity of the perovskite compounds in the total oxidation reactions of the gases is largely determined by the amount of weakly bound surface oxygen species which in turn depends on the presence of oxygen vacancies.

The Structural, Impedance and Dielectric a Ferrite Core of Iron Manganite and Its Composite

“…The crystal structure of each sample is MnFeO3 (hexagonal, space group R -3 c), ZnFe2O4 (cubic, space group F m -3 m), and LaMnO3 (monoclinic, space group I 1 2 / a 1). The same results were obtained by other researchers, namely MnFeO3 with a hexagonal structure [11], ZnFe2O4 with a cubic crystal structure [5], and LaMnO3 being monoclinic [12]. The complete results of XRD characterization can be seen in Table 1.…”

Nanocrystalline FeMnO3 Powder as Catalyst for Combustion of Volatile Organic Compounds

“…Although the mechanism regarding the catalytic activity of oxide compounds with a perovskite structure is still under debate, according to some widely accepted opinions, at low temperatures (<400 °C), the catalytic activity of oxide compounds with a perovskite structure in total oxidation reactions of gases is, to a large extent, determined by the amount of weakly bound surface oxygen species [ 15 , 36 ], which in the present case are assumed to be much more available for the oxidation of alcohols (ethanol and methanol) compared to those for the oxidation of hydrocarbons (toluene and xylene). The weaker the oxygen bond on the catalyst surface, the more active the catalyst is for the complete oxidation of gases [ 15 , 36 , 37 ]. Of course, the roles of specific surface area and particle size are very important.…”

“…The apparent activation energies E a of the catalyst in the combustion reactions of ethanol, methanol, toluene and xylene were determined using Arrhenius curves (the natural logarithm of the reaction rate constant k at low conversion, below 15%, as a function of the inverse of the absolute temperature, 1/T) [ 15 ]. This graph is a straight line, and from its slope, the apparent activation energy was calculated [ 15 , 37 ].…”