Aqueous ammonia abatement on Pt- and Ru-modified TiO2 : selectivity effects of the metal nanoparticles deposition method

A series of TiO2 \u2013 based photocatalysts obtained by deposition of Pt nanoparticles (NPs) has been tested in the photocatalytic decomposition of aqueous ammonia under UVA irradiation. Two main deposition routes were employed, i.e. (i) the deposition of surfactant-stabilized preformed metal NPs and (ii) a modified version of the well-known deposition-precipitation technique, employing urea as precipitating agent. The effects that the deposition route and the amount of deposited metal have on both ammonia conversion (XNH3) and selectivity (SY) towards the different N-containing products (Y = N2, NO2\u2013, NO3\u2013) have been investigated systematically, in relation to the morphological distribution of NPs on TiO2, as evidenced by HR-TEM analysis. The combination of Pt (0.8 wt.%) with a relatively low amount of Ru NPs (0.1 wt.%) as co-catalysts, both deposited on TiO2 under optimized conditions, results in an exceptional stabilization of nitrite ions (SNO2\u2013 ~ 70%), from which N2, the most desired ammonia oxidation product, might subsequently be obtained catalytically

XAFS, IR, Raman and UV-Vis Characterization of Framework Ti(IV) Species in Ti-Silicalites

The main results obtained by using IR, Raman and UV-Vis techniques concerning the structure of the Ti(IV) center in Ti-silicalite is briefly reviewed and discussed with recent XAFS results. The Ti(IV) is in tetrahedral coordination in vacuo and expands its coordination sphere upon interaction with adsorbates. Present EXAFS results indicate, for the dehydrated sample, a first shell coordination number higher then four (4.45 ± 0.3), suggesting, in a fraction of sites, the substitution of a bridged oxygen with two OH groups

Photoelectrochemical properties of nanostructrured WO3 prepared with different dispersing agents

Different chemical routes for obtaining nanostructured WO3 on transparent conductive oxides have been tried by considering the Santato-Augustynski recipe as a main guideline. The preparation started from tungstate salts as precursors and involved different organic dispersing agents. Calcination in air at 550 1C produced the final oxide. Aim of this work is to test the oxide materials as photoanodes for the photo-electrochemical water splitting. WO3 is a wide bandgap semiconductor, able to transport photogenerated carriers under irradiation, provided that it has a good crystalline structure. The morphology and the crystal size of the obtained WO3 nanoparticles have been investigated by SEM and XRD. This work shows that the particle size of the WO3 film decreases with increase in molecular weight of the organic dispersing agents. The current–voltage characteristic curves of WO3-based electrodes have been obtained by biasing a photo-electrochemical cell with an external voltage ramp, both in the dark and under simulated solar irradiation. Results showed that under incident irradiance a noticeable photocurrent density is obtained with a reasonably steep current–voltage slope and with a plateau current of the order of 1.6–1.8 mA/cm2 under incident 0.14 W/cm2 AM 1.5 irradiation

Evolution of Fe3+ from framework to extra-framework species in Fe-silicate as a function of the template burning temperature

We report a XAFS study of the Fe3+ local environment in Fe-silicalite as a function of the template burning temperature. We investigate the structural changes of the zeolitic structure, upon thermal treatments and interaction with NH3. Our XAFS results show that, before template burning, Fe3+ are in tetrahedral symmetry with 4 oxygens at 1.85 Å, while after template removal and in vacuum conditions, they exhibit a distorted tetrahedral symmetry with 3 oxygens at 1.87 Å, and one at 2.10 Å ; dosage of NH3 partially restore the tetrahedral symmetry of the site. These results are strongly supported by parallel IR, Raman, UV-Vis, TPR and EPR investigations and by recent theoretical simulations

Structure and Reactivity of Framework and ExtraframeworkIron in Fe-Silicalite as Investigated by Spectroscopic and Physicochemical Methods

The application of IR, Raman, UV-visible, EPR, XANES, EXAFS, and TPR techniques to the determination of the Fe3+local environment in Fe-silicalite is described and discussed. These methods give information concerning the structural changes of the zeolitic structure, upon thermal treatments and interaction with adsorbates. In particular the effect of template burning at 773 K or at 973 K on the local Fe3+environment has been discussed in detail. The reactivity of the zeolite towards NH3has also been considered

Searching for new redox-complexes in organic flow batteries

The study of redox couples based on Fe(III)/(II) and Co(II)/(I) organic complexes has demonstrated chemically reversible redox processes as well as good stability in organic solvents. These active complexes, obtained with polypyridine ligands, present low cost, low toxicity and good chemical stability. Moreover, they demonstrated fast redox kinetics and for that, they are candidate for active species in redox flow cells. A wide library of polypyridine complexes have been prepared and tested as acceptor ligands to reach an open circuit voltage up to 2 V, in a mixture of ethylene carbonate and propylene carbonate (EC/PC) chosen for their low volatility and electrochemical stability. Solubility data are presented after tuning ligand design to optimize metal-complex solubility. The best compounds were [Fe(bpy)3]Tf2 (Tf = CF3SO3−, bpy = 2.2′-bipyridine) and [Co(bpy)3]Tf2 which generated current densities of the order of 30 mA/cm2 in thin layer static cells. These complexes were also preliminary tested in a complete flow cell equipped with a Nafion membrane, with LiTf electrolyte, and ca. 90% coulombic efficiency was observed. The decrease of performance observed after 8 h is under investigation and assigned, for now, to membrane degradation. A change of membrane characteristics should be considered to exploit the full potentiality of these redox mediators

Toward understanding the catalytic synergy in the design of bimetallic molecular sieves for selective aerobic oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO2+ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems