Showing papers by "Gaetano Granozzi published in 2019"

Carbon Dots from Citric Acid and its Intermediates Formed by Thermal Decomposition

Abstract: Thermal decomposition of citric acid is one of the most common synthesis methods for fluorescent carbon dots; the reaction pathway is, however, quite complex and the details are still far from being understood. For instance, several intermediates form during the process and they also give rise to fluorescent species. In the present work, the formation of fluorescent C-dots from citric acid has been studied as a function of reaction time by coupling infrared analysis, X-ray photoelectron spectroscopy, liquid chromatography/mass spectroscopy (LC/MS) with the change of the optical properties, absorption and emission. The reaction intermediates, which have been identified at different stages, produce two main emissive species, in the green and blue, as also indicated by the decay time analysis. C-dots formed from the intermediates have also been synthesised by thermal decomposition, which gave an emission maximum around 450 nm. The citric acid C-dots in water show short temporal stability, but their functionalisation with 3-aminopropyltriethoxysilane reduces the quenching. The understanding of the citric acid thermal decomposition reaction is expected to improve the control and reproducibility of C-dots synthesis.

Effect of Ni Doping on the MoS2 Structure and Its Hydrogen Evolution Activity in Acid and Alkaline Electrolytes

Abstract: We have investigated three-dimensional (3D) MoS2 nanoarchitectures doped with different amount of Ni to boost the hydrogen evolution reaction (HER) in alkaline environment, where this reaction is normally hindered. As a comparison, the activity in acidic media was also investigated to determine and compare the role of the Ni sites in both media. The doping of MoS2, especially at high loadings, can modify its structural and/or electronic properties, which can also affect the HER activity. The structural and electronic properties of the Ni doped 3D-MoS2 nanoarchitecture were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning and transmission electronic microscopy (SEM; TEM), and X-ray photoemission Spectroscopy (XPS). XPS also allowed us to determine the Ni-based species formed as a function of the dopant loading. The HER activity of the materials was investigated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH. By combining the physicochemical and electrochemical results, we concluded that the Ni sites have a different role in the HER mechanism and kinetics in acidic and in alkaline media. Thus, NiSx species are essential to promote HER in alkaline medium, whereas the Ni-Mo-S ones enhance the HER in acid medium.

Combined high degree of carboxylation and electronic conduction in graphene acid sets new limits for metal free catalysis in alcohol oxidation

Abstract: Graphene oxide, the most prominent carbocatalyst for several oxidation reactions, has severe limitations due to the overstoichiometric amounts required to achieve practical conversions. Graphene acid, a well-defined graphene derivative selectively and homogeneously covered by carboxylic groups but maintaining the high electronic conductivity of pristine graphene, sets new activity limits in the selective and general oxidation of a large gamut of alcohols, even working at 5 wt% loading for at least 10 reaction cycles without any influence from metal impurities. According to experimental data and first principles calculations, the selective and dense functionalization with carboxyl groups, combined with excellent electron transfer properties, accounts for the unprecedented catalytic activity of this graphene derivative. Moreover, the controlled structure of graphene acid allows shedding light upon the critical steps of the reaction and regulating precisely its selectivity toward different oxidation products.

Ethanol aerobic and anaerobic oxidation with FeVO4 and V2O5 catalysts

Abstract: This study compares the aerobic and anaerobic transformation of ethanol using FeVO4 and V2O5 catalysts. Despite their different structure, the two oxides showed very similar catalytic performances and their main product was acetaldehyde. However, in the absence of oxygen, the catalysts produced an equimolar amount of ethane and acetaldehyde, and this aspect has been little studied in the literature. In-situ XPS and DRIFT spectroscopy studies showed that the active species for the disproportionation of the alcohol into ethane and aldehyde was the reduced V3+ ion; nevertheless, the Fe in the FeVO4 catalysts was responsible for directing the reduction of metals toward the formation of a Fe-V-O spinel phase which was homogeneous and more stable than V2O5. Moreover, an in-situ DRIFT spectroscopy study showed that ethanol adsorbs in different ways on the surface of the catalysts during the reduction of samples (anaerobic reaction), forming H-bonded and dissociated ethoxy species, and giving rise to new surface OH groups that participate in the aldehyde/alkane formation. To conclude, a new mechanism of hydrogen transfer for the anaerobic ethanol disproportionation into ethane and acetaldehyde is proposed. This research completes the picture about ethanol oxidation to acetaldehyde on V-based catalysts, demonstrating that the catalytic behavior is mainly affected by the oxidation degree of the vanadium species which, in turn, depends on the reaction environment, and not on the structure itself.

CeOx/TiO2 (Rutile) Nanocomposites for the Low-Temperature Dehydrogenation of Ethanol to Acetaldehyde: A Diffuse Reflectance Infrared Fourier Transform Spectroscopy–Mass Spectrometry Study

Abstract: Two nanocomposites of different layer thicknesses were synthesized by impregnation of a rutile titania support with a reduced ceria solution. An in situ diffuse reflectance infrared fourier-transfo...

Reversible adsorption of oxygen as superoxide ion on cerium doped zirconium titanate

Abstract: Zirconium titanate (ZrTiO4) modified via cerium ions insertion in the oxide lattice has been successfully prepared via Sol-Gel synthesis forming solid solutions in the molar range between 0 and 10%. Cerium ions are hosted both as Ce4+ and Ce3+ ions whose ratio depends on the treatment undergone by the solid, with trivalent cerium always abundantly present even in strong oxidative conditions. Surface exposed Ce3+ ions are capable to adsorb O2 in the range of temperature between 273 K and 77 K, as superoxide anion (O2 −), having side-on structure and characterized by complete surface-to-molecule electron transfer (about 98% of spin density on O2 molecule). Surprisingly such abundant adsorption is pressure dependent and fully reversible opening the perspective of applications of this system in the field of oxygen separation from gas mixtures.

Electrochemical Surface Science: Basics and Applications

Abstract: The great success of the Surfaces Special Issue entitled “Electrochemical Surface Science (EC-SS): Basics and Applications” reflects the great vitality and relevance of the addressed topic [...]