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Manganese oxide (MnO2) nanomaterials represent a promising low-cost electrocatalyst material for hydrogen peroxide (H2O2) production by the oxygen reduction reaction (ORR). In this material, the introduction of oxygen vacancies is an effective strategy for boosting their electrocatalytic activities. Although the importance of oxygen defects in MnO2-based catalysts has been shown in some electrochemical systems, a facile strategy for controlling the number oxygen vacancies in MnO2-based catalysts and the systematic investigation of the effect of the vacancies on their electrocatalytic activities towards the ORR and H2O2 electrogeneration remains limited. Here, we describe the synthesis of hybrid materials composed of MnO2 nanoflowers supported on Vulcan XC-72 carbon in which the concentration of the surface oxygen vacancy sites was increased. Surprisingly, we found a volcano-type relationship between the activity and the MnO2 loading. RRDE experiments showed the greatest formation of hydroperoxide ion in alkaline media for MnO2/C 3% on carbon, displaying the highest electrocatalytic activity. This result was maintained for hydrogen peroxide during the electrolytic process using gas diffusion electrodes in acidic media. We suggest that the optimized catalytic activities observed for the MnO2/C 3% catalyst are a result of (i) the high concentrations of oxygen vacancies at the catalyst surface, which promote the generation of highly catalytically active sites towards the ORR; (ii) their strong metal oxide-carbon interactions with modification of the carbon surface; (iii) the high surface area observed for the MnO2 nanoflowers supported on Vulcan XC-72; and (iv) the use of a MnO2 nanostructure displaying well-defined shapes and uniform sizes (with petals measuring 10 ± 5 nm in width) displaying a uniform dispersion over the carbon support. We believe the results described herein shed new light on the understanding of the role of the oxygen vacancies in the electrogeneration of H2O2, which has important implications for the design of MnO2-based electrocatalysts.

Pt nanoparticles supported on carbon (Pt/C) and nitrogen-doped carbon (Pt/NC) were prepared by an alcohol-reduction process and used for ammonia electro-oxidation in alkaline media. Nitrogen-doped carbons were prepared by thermal treatment of urea and Carbon Vulcan XC72 at 800 °C under argon atmosphere. The obtained materials showed the presence of face centered cubic structure of Pt and mean particle sizes in the range of 3–3.7 nm. X-ray photoelectron spectroscopy results revealed that Pt/NC 5 (carbon prepared with 5% of urea) presents more Pt on the surface than Pt/C, and the nanoparticles are predominantly in the metallic state. The electrocatalytic activity was investigated by cyclic voltammetry and chronoamperometry experiments. Pt/NC materials showed a higher electrocatalytic activity for ammonia electro-oxidation than Pt/C, whereby the material Pt/NC 5 showed the peak current density 161% higher than Pt/C. The increase of activity might be related to the high electrochemically accessible area of Pt/NC and the improvement on the interaction with water due to the nitrogen onto the support which could contribute to the oxidation of intermediate products from ammonia electro-oxidation.

Gold nanoparticles incorporated on ceria nanowires have been employed as efficient nanocatalysts for the selective oxidation of thioanisole. The control of both physical and chemical parameters as well as metal–support interactions are important factors that determine their performances. Considering their one-dimensional morphology with large surface area, thin diameters, high concentration of oxygen vacancies, and small Au NPs uniformly on the entire CeO2 surface with a high fraction of oxidized gold species, these characteristics make them favorable nanocatalysts for oxidation transformations. The CeO2-Au nanowires displayed improved performances towards the oxidation of thioanisole when compared to the pure CeO2 nanowires and commercial CeO2-Au catalysts. The CeO2-Au nanowires catalyzed the selective synthesis of methyl phenyl sulfoxide with up to 100 % of selectivity and high conversion. The impact of solvent and temperature during the catalytic reaction was also experimentally and theoretically investigated by DFT calculations, indicating a key role in the observed activities.

The origin and influence of active sites on supported copper catalysts, and their catalytic properties for ethanol conversion, were investigated using Cu/SiO2, Cu2O/SiO2, and Cu/ZrO2. Diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO revealed that Cu1 + was more prevalent than Cu0 species at the Cu/SiO2 surface, and that Cu0 was the main species on Cu/ZrO2. The Cu1 +/Cu0 pair provided by Cu/SiO2 generates sites that are highly selective for the dehydrogenation of ethanol. Ethyl acetate can be formed from ethanol and acetaldehyde at ZrO2 or Cu surfaces. A combination of the Cu1 +/Cu0 pair and Cu0 interfaced to ZrO2 is needed to efficiently transform ethanol to ethyl acetate.