Abstract
The electrical characterization of catalysts composed of layered manganese oxide in the form of birnessite supported on γ-Al2O3, which have been successfully used in the combustion of soot, is presented. The results indicate that the electrical conduction and ion conduction processes are influenced by the amount of the active phase. There was also evidence of Grotthuss-type proton conductivity favored by the presence of surface water on the exposed alumina surface. The above is supported by the porous nature of the catalyst in which the surface area varied between 125.2 ± 1.2 and 159.0 ± 1.1 m2/g, evidencing changes in the alumina surface. The conductivity, determined from measurements of impedance spectroscopy, at low frequency showed changes associated with the amount of the active phase. The values ranged from 2.61 × 10−8 ± 2.1 × 10−9 Ω−1·cm−1 (pure alumina) to 7.33 × 10−8 ± 5.9 × 10−9 Ω−1·cm−1, 7.21 × 10−8 ± 5.8 × 10−9 Ω−1·cm−1 and 4.51 × 10−7 ± 3.6 × 10−8 Ω−1·cm−1 at room temperature for catalysts with nominal active phase contents of 5.0, 10.0 and 20.0%, respectively. Such results indicate that it is possible to modulate the electrical properties with variations in the synthesis parameters.
Highlights
Heterogeneous metal oxide catalysts are essential in many industrial processes due to their role as mediators in oxidation reactions [1,2], improving the efficiency of those processes
Among the transition metal oxides, manganese oxides supported in alumina have been used for soot combustion [8], the ozonization of volatile organic compounds [9,10] and the selective catalytic reduction (SCR) of nitric oxide [11], among other reactions, in which a charge transfer is presented between the active center and the reactants
The chemical composition, obtained by atomic absorption results (AA) (Table 1), shows a variation of the potassium and manganese content of the materials, suggesting that the manganese oxide was successfully supported and is related to the nominal concentration of the precursor used in the synthesis
Summary
Heterogeneous metal oxide catalysts are essential in many industrial processes due to their role as mediators in oxidation reactions [1,2], improving the efficiency of those processes. To supply the industrial demand for efficient, selective and highly active catalysts, it is necessary to design synthetic routes allowing the creation of high-performance catalysts. In this sense, the structure, crystal size, morphology and surface area are some of the parameters which influence the selectivity and activity of the catalyst [3]. The structure, crystal size, morphology and surface area are some of the parameters which influence the selectivity and activity of the catalyst [3] The properties of these solids can be modulated by changes in the synthesis parameters, among others, supporting the active phase in metal oxides such as Al2 O3 , SiO2 , TiO2 or MgO [4]. The most common method for supporting manganese oxides is wet impregnation, mainly with acetate salts or manganese nitrate to produce Mn3 O4 or Mn2 O3 as the active phase [9]
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