Two hydrothermal techniques under microwave irradiation were used to synthesize γ-MnO2 from 90°C to 150°C in 10−30 min. The first technique is based on reducing KMnO4 with MnSO4, and the second one involves liquid-phase oxidation between MnSO4 and (NH4)2S2O8. The structures and morphologies of the samples were analyzed using X-ray diffraction, scanning electron microscopy, and N2 physisorption measurements. The electrochemical properties were evaluated through cyclic voltammetry and electrochemical impedance spectroscopy. The γ-MnO2 materials obtained by the first technique mainly exhibited nanorods with diameters of 40–60 nm, and the samples obtained by the second technique showed flower-like microspheres with diameters of 1−2 µm; each flower was composed of nanosheets with a thickness of 10−20 nm. The processing time directly depends on the size of the nanorods. The sample synthesized by the first technique at 150°C and 10 min has the highest specific surface area of up to 59.08 m2 g−1 and mean pore diameter of 34.11 nm. Furthermore, this sample exhibits a near-rectangular cyclic voltammetry curves and high specific capacitance of 331.3 F g−1 in 0.1 M Na2SO4 solution at 5 mV s−1 scan rate. Graphic abstract

The fluorite/perovskite composite with general formula Ce 0.9 Pr 0.1 O 2−δ /Pr 0.6 Sr 0.4 Fe 0.5 Co 0.5 O 3−δ was chemically synthesized by the citrate-EDTA method and subsequent calcination. First, the microstructural and textural properties of the ceramic powder were determined by several techniques such as XRD, N 2 physisorption, SEM, and TEM. Then the catalyst was loaded with Co as a promoter phase. Next, the catalytic evaluations were performed using thermogravimetric analysis to determine the combustion temperature of soot. Moreover, a series of cyclic analyses were carried out to evaluate the thermal stability of the catalyst. Based on the thermogravimetry data, the kinetic parameters of the catalytic soot combustion process were obtained.