A series of manganese-cerium oxide composites with Mn concentrations in the range of 1-20 mol % in ceria was prepared by the solution combustion technique using urea as fuel. The nature, type, and oxidation state of Mn species in ceria were investigated by X-ray diffraction (XRD), diffuse reflectance UV-visible spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy, and temperature-programmed reduction techniques. The study reveals that the method of preparation significantly influences the type of manganese species in ceria. Wet-impregnation, coprecipitation, and solid-state synthesis techniques lead to clustered MnO x -like species in the ceria matrix, while the present method of preparation (solution combustion route) yields a highly dispersed form of Mn species. In the reported series of samples, Mn is present mainly in +2 and +3 oxidation states and there is no evidence for the presence of Mn 4+ species. Powder X-ray diffraction studies at variable temperatures (298-1323 K) indicate the formation of Ce 1-x Mn x O 2-δ solid solutions. No separate MnO x -type phase was detected even at 1323 K. EPR studies reveal that the isolated Mn 2+ and Mn 3+ species are present in at least three different structural locations: species A, Mn ions in ceria-lattice defect sites; species B, Mn ions in framework Ce 4+ locations; and species C, Mn ions in interstitial locations and at the surface of ceria. The Mn 3+ ions in ceria exhibit a facile reduction and reoxidation behavior when exposed to dry hydrogen and subsequently to air at elevated temperatures. A highly dispersed state of Mn 3+ and Mn 2+ in ceria, facile redox behavior, and a synergistic Mn-ceria interaction are some of the unique properties of this material prepared by the solution combustion procedure.

The study of the defective surface sites of many oxides has received considerable interest, as these sites are coordinately unsaturated and exhibit extraordinary activity in many catalytic reactions. 1 Most of the research work in this particular area is mainly focused on theoretical simulations. 2 However, the influence of the defect sites on the chemical nature of the oxide surface and their role in the mechanism of activation of a substrate have not yet been fully explored. In this Communication, we take the example of ceria and provide direct evidence wherein the defect-site-enriched oxide promotes the surface reorganization and thereby influences the activation of ethylbenzene (EB) in the oxidative dehydrogenation in presence of N 2 O.Ceria is an interesting oxide and is one of the constituents in many catalyst formulations. 3 The method of preparation strongly influences the structural and surface properties of ceria. 4 We intend to explore its catalytic activity in the oxidative dehydrogenation (ODH) of ethylbenzene using N 2 O. A nanocrystalline ceria sample enriched with Ce 3+ -O --Ce 4+ -type defect sites was prepared by the alcoholysis method (ceria-A). For comparison, three other ceria samples were prepared, one by conventional precipitation and the other two by a solution-combustion method using urea and glycine as fuels (ceria-P,-U and -G), respectively, as reported elsewhere. 4 Interestingly, the nanocrystalline ceria prepared by the alcoholysis method exhibits Ce 3+ -O --Ce 4+ -type defect sites predominantly confined to the surface, as evidenced from diffuse reflectance UVvisible spectroscopy (a weak band at around 650 nm). 5 A rough estimate of the surface to bulk defect ratio follows the order Ceria-A > Ceria-G > Ceria-U/P based on the EPR spectroscopic analysis 6 (Supporting Information, Figure S1) and the temperature programmed reduction 7 of the samples ( Figure S2). These defect sites promote the dehydrogenation of EB using N 2 O at 598 K, which is far lower than the temperatures normally encountered in the existing conventional processes for the dehydrogenation of EB (873 K). An equilibrium conversion of 45 mol % and a styrene selectivity of 94%, comparable to that obtained in commercial practice are achieved by the ceria catalyst prepared by the alcoholysis method. However, the ceria samples prepared by conventional and combustion methods showed comparable activity and selectivity to styrene at much higher activation temperatures (Ceria-G at 648 K and Ceria-U/P at 723 K).To address the role of defect sites in governing the temperature of activation, EPR investigations were done on the ceria samples by subjecting each one of them to hydrogen treatment, as the dehydrogenation process involves interaction of hydrogen atoms with the catalyst surface. Figure 1 shows the direct correlation between the concentrations of Ce 3+ -O --Ce 4+ -type defect sites [at g ⊥ ) 1.96 and g | ) 1.933 (D signal) and 1.940 (A signal)] and the temperature of activation. In the case of defect-site-enriched ceria-A...

The catalytically important Mn/CeO2−TiO2 (MCT) solid solutions were synthesized by solution combustion technique using glycine, urea, or PEG as fuel. These samples along with those prepared by coprecipitation and wet impregnation methods were characterized for its structural, textural and redox properties using various physical and spectroscopic techniques, viz., powder X-ray diffraction, high-resolution transmission electron microscopy, temperature-programmed reduction, diffuse reflectance UV−visible, electron paramagnetic resonance, and X-ray photoelectron spectroscopy. The samples prepared by the solution combustion method showed high thermal stability, even at temperatures of 1323 K, whereas the samples synthesized by conventional routes lost its structural integrity at higher temperatures due to fast sintering. The combustion and coprecipitation methods stabilize Mn species both in +2 and +3 oxidation states, while in the MCT sample prepared by wet impregnation, Mn ions are present in +3 state exclusively. The latter possesses highly reversible redox properties, whereas the samples prepared by combustion and coprecipitation techniques show quasireversible redox behavior. On comparing the physicochemical properties of MCT solid solutions with that of Mn/CeO2, we elucidate that Ti incorporation into the ceria matrix alters the redox properties of both cerium and manganese ions. Though the nature of the fuel did not have significant effect on the oxidation states of Mn species stabilized in the solid solutions, the relative concentration of Mn2+ and Mn3+ ions and their existence in the bulk or on the surface of the material did vary with the nature of the fuel used in the combustion process. These variations could be attributed to the intrinsic properties of combustion process, viz., the adiabatic/real flame temperature and mode of combustion and its duration. Employing PEG as fuel in the combustion process, one can obtain thermally stable nanocrystalline MCT samples with high phase purity.

Rosmarinic acid (RA) was assessed for its quorum sensing inhibitory (QSI) potential against Aeromonas hydrophila strains AH 1, AH 12 and MTCC 1739. The pathogenic strains of A. hydrophila were isolated from infected zebrafish and identified through biochemical analysis and amplification of a species-specific gene (rpsL). The biofilm inhibitory concentration (BIC) of RA against A. hydrophila strains was found to be 750 μg ml. At this concentration, RA reduced the QS mediated hemolysin, lipase and elastase production in A. hydrophila. In FT-IR analysis, RA treated A. hydrophila cells showed a reduction in cellular components. Gene expression analysis confirmed the down-regulation of virulence genes such as ahh1, aerA, lip and ahyB. A. hydrophila infected zebrafish upon treatment with RA showed increased survival rates. Thus, the present study demonstrates the use of RA as a plausible phytotherapeutic compound to control QS mediated biofilm formation and virulence factor production in A. hydrophila.

Metal oxide nanoparticles (NPs) have found a variety of applications in numerous industrial, medical, and environmental fields s, attributable to recent advances in the nanotechnology field. Titanium dioxide nanoparticles (TiO2-NPs) have gained importance as metal oxide NPs due to their potential in various fields, particularly nanomedicine and other biomedicine fields. Several studies have confirmed that NPs produced via the biosynthesis route using natural resources have significant advantages such as fewer toxic contaminants, less subsequent complex chemical synthesis, environmental friendliness, cost-effectiveness, and stability when compared to NPs produced by conventional methods, and its production with controlled shapes and sizes. Therefore, considerable effort is being expended to implement biological synthesis methods with these proven advantages. TiO2-NPs can be made using a variety of biological, chemical, and physical methods. Physicochemical methods are costly, emit high levels of toxic chemicals into the atmosphere, and consume a lot of energy. On the other hand, the biological approach is an environmentally safe, cost-effective, dependable, convenient, and easy way to synthesize TiO2-NPs. In this review, the bio-mediated synthesis, as well as various biomedical applications of TiO2-NPs, were discussed.