86,818 research outputs found
F-doped TiO2 prepared by flame spray pyrolysis: effect on the photoelectrochemical performance
No full textPhotoelectrochemical water splitting performed with semiconductor materials has attracted attention due to potential clean energy production, achievable utilizing light. In this context, TiO2 is the most investigated semiconductor, though characterized by i) low rate of photopromoted e- transfer and ii) high recombination rate of photoproduced charge carriers (e-/h+ pairs). Doping with non-metal elements is often suggested as a valuable remediation to limit the occurrence of these two drawbacks.
In line with this, a series of fluorine-doped TiO2 samples (with F/O ratio varying in the 1–10 at.% range) were synthesized, in single step, by flame spray pyrolysis starting from organic solutions containing titanium tetra-isopropoxide (Ti(OC3H7)4) and hexafluorobenzene (C6F6) as titanium and fluorine precursors, respectively.
By means of the selected synthesis technique, doping can be conveniently carried out by co-dissolving the dopant source, along with the TiO2 precursor, directly in the employed solvent (i.e., xylene), with a precise engineering of dopant concentration during synthesis.
Quantitative crystalline phase analysis, performed by applying the Rietveld refinement method to XRPD data, highlighted the presence of both anatase and rutile phases in all the prepared samples, with the former being always predominant. Accordingly, all samples exhibited a UV-vis absorption edge at λ = ca. 400 nm.
The prepared powders were then deposed on fluorine-doped tin oxide (FTO) glass by means of the doctor-blade technique, to investigate the effect of F-doping on the photoelectrochemical activity of TiO2. Incident Photon-to-Current Efficiency (IPCE), reflecting the semiconductor’s ability of generating and transferring photopromoted e-/h+ under irradiation and suitable applied voltage, was found to increase with increasing the F-dopant concentration, up to a F/O ratio of 5 at.%.
This finding pointed to an improved separation of photopromoted electron-hole pairs, ascribed to the formation of intra-band gap states, induced by fluorine doping and able to efficiently trap photoproduced charge carriers. Conversely, when the amount of F-doping exceeded the optimum value, structural defects acting as e-/h+ recombination centers were formed, reasonably yielding lower photocurrent
Effects of the W precursor and amount on the photocatalytic activity of WO3-TiO2 mixed oxides
No full textIn order to overwhelm one of the limits affecting TiO2 as photocatalyst, i.e. the high recombination rate of photogenerated charge carriers, one of the most widely adopted strategy consists in producing TiO2-based mixed oxide photocatalysts. For that purpose, WO3-TiO2 mixed oxides were synthesized by a sol-gel method employing titanium tetra-isopropoxide (Ti(OC3H7)4) as titanium source and either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti molar ratios. Formic acid, undergoing direct photomineralisation without forming any stable intermediate species, was used as model substrate for the abatement of organic pollutants in water under UV-visible irradiation. The photocatalytic degradation of acetaldehyde, one of the main odour-causing gases in indoor air, was employed as photocatalytic test reaction in the gas phase. Furthermore, the so-obtained materials were characterized by XRPD, BET, UV-vis reflectance and XPS analyses, and also by photocurrent (IPCE) measurements.
The synthesis of mixed WO3-TiO2 photocatalysts was successfully achieved starting from the organic precursors of the two metals. Materials with an intimate contact between the two oxides ensured an efficient photoproduced charge separation and a consequent enhanced photoactivity paralleled by increased IPCE values, with respect to both pure TiO2 prepared by the same route, and commercial benchmark P25 TiO2 from Degussa (Evonik). The particle sizes decrease observed with increasing the W content, the parallel beneficial increase of surface area and the persistence of the highly active crystalline anatase phase, even after calcination at 700 °C, also contribute to the higher photocatalytic performance of these materials. By contrast, a less performing photocatalyst was obtained when the inorganic precursor was employed in the sol-gel synthesis, due to its persistence in separate domains within the TiO2 structure. The optimal tungsten content was confirmed to correspond to a 3% W/Ti molar ratio, as in recent studies, as both the photocatalytic activity and the IPCE curves were negatively affected by the presence of larger tungsten amounts. Since WO3 is not a photocatalyst as effective as TiO2, the photocatalytic activity of the mixed oxides thus decreased as a consequence of overdoping, further proving that the photoactivity increase observed upon tungsten addition is to be mainly related to an improved electron-hole separation in TiO2 and it is not originated by the action of WO3 as photocatalyst
PHOTOCATALYTIC REACTIONS FOR ENERGY CONVERSION
No full textGeneral upward trends in fossil fuel consumption and CO2 emissions, along with the accepted belief that global chemistry substantially influences climate, require that scientific research provides efficient remedies and/or alternatives to the present scenario. Photocatalysis is often proposed as one of the most promising technique to achieve these purposes.
This PhD thesis is mainly focused on the investigation of TiO2-based systems for the photocatalytic oxidation of formic acid in aqueous suspension, as well as for H2 production by methanol photo steam reforming in the gas phase. Two different approaches were adopted to minimize the drawbacks usually characterizing TiO2 photocatalysts: i) TiO2-WO3 mixed oxide photocatalysts were prepared with the aim of reducing the recombination rate of photopromoted electron/hole pairs. The superior photocatalytic performance of the mixed oxide system was mainly attributed to the positive effect induced by W in efficiently trapping the photopromoted electron from the conduction band of TiO2, ensuring extended charge carriers separation. Even better results were obtained upon the surface modification with Pt nanoparticles which, by virtue of the metal high work function, further enhanced e-/h+ separation. ii) surface modification of TiO2 with Au nanoparticles, possessing a Localized Surface Plasmon Resonance (LSPR) at λ = 530 nm was proved to be an efficient way to promote TiO2 photoactivity under visible light irradiation. By selecting three titania samples (i.e., a stoichiometric, nearly non defective TiO2, a N-doped TiO2 and a oxygen vacancy-rich TiO2), evidence of two different plasmonic photoactivity mechanisms was provided, with the so-called hot electron transfer promoting plasmonic photoactivity in the stoichiometric TiO2 and Plasmon Resonance Energy Transfer accounting for the observed plasmonic visible light photoactivity of doped samples.
Being the abatement of CO2 through (photo)electrochemical reduction very challenging (E0(CO2/CO2-* = -2.14 V)), an alternative way has been studied: pyridinyl radicals (1-PyH*), photogenerated by irradiating a pyridine (Py) solution, were found to efficiently react with CO2 yielding a carbamic species (HPy-1-COOH), triggered by a stepwise mechanism where electron transfer from 1-PyH* precedes proton transfer. Formate (HCOO-) was also obtained, demonstrating that photoexcited pyridine does catalyze the 2e—reduction of CO2.
Finally, Fenton oxidation of gaseous isoprene on the surface of aqueous Fe2+ droplets, yielding carboxylic acids, polyols and carbonyl compounds, detected in situ through ElectroSpray Ionization Mass Spectrometry, accounted for alternative routes for the conversion of organic gases into secondary organic aerosol, occurring under tropospheric conditions, and may be incorporated into present atmospheric chemistry models
WO3-TiO2: effect of the W precursor on the photocatalytic activity of mixed oxides
No full textWO3-TiO2 mixed oxides were synthesized by a sol-gel method
employing an inorganic salt (Na2WO4 ∙ 2H2O) and an organic
alkoxide (W(OC2H5)6), as tungsten precursors. Different results
in terms of photoactivity in both formic acid mineralization in
aqueous suspension and acetaldehyde oxidation in the gas phase
were obtained, proving that the organic precursor may lead to the
formation of an intimately mixed oxide with enhanced
photoactivity in both reactions, also higher than that of pure TiO2
and of P25 TiO2
Role of ceramide and sphingosine-1-phosphate in the regulation of glioma cell death and survival
No full textLegal Institutions, Innovation and Growth
No full textWe analyze the relationship between legal institutions, innovation and growth. We compare a rigid (law set ex-ante) legal system and a flexible one (law set after observing current technology). The flexible system dominates in terms of welfare, amount of innovation and output growth at intermediate stages of technological development - periods when legal change is needed. The rigid system is preferable at early stages of technological development, when (lack of) commitment problems are severe. For mature technologies the two legal systems are equivalent. We find that rigid legal systems may induce excessive (greater than first-best) R&D investment and output growth.legal system, commitment, flexibility, innovation, growth
A simple and efficient Solid-Phase Microextraction – Gas Chromatography – Mass Spectrometry method for the determination of fragrance materials at ultra-trace levels in water samples using multi-walled carbon nanotubes as innovative coating
No full textThe occurrence of emerging contaminants is becoming of increasing importance to assess the impact of anthropogenic activities onto the environment. The present study reports for the first time the development and validation of an efficient method for the simultaneous determination of fragrance materials in water samples based on the use of a novel multiwalled carbon nanotubes (MWCNTs)-based solid-phase microextraction coating. Helical MWCNTs were selected as adsorbent material due to their outstanding extraction performance. The multicriteria method of desirability functions allowed the optimization of the experimental conditions in terms of extraction time and extraction temperature. Validation proved the reliability of the method for the determination of the analytes at ultra-trace levels, obtaining detection limits in the 0.2–13 ng/L range, good precision, with relative standard deviations lower than 20% and recovery rates in the 80 ± 12%–111 ± 11%. Superior enrichment factors compared to commercial fibers were also calculated. Finally, applicability to real sample analysis was demonstrated
Homogeneous reduction of CO2 by photogenerated pyridinyl radicals
No full textWe report that 1-hydropyridinyl radicals (1-PyH•) photogenerated in solution react with dissolved CO2 en route to its 2e− reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H2O mixtures saturated with 1 atm of CO2 yields a suite of products, among which we identified Na(HCOO)2−(m/z− = 113), C5H6NCOO− (m/z− = 124), and C5H10O2NCOO− (m/z− = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO2 neutrals via collisionally induced dissociation. We infer that
1-PyH• [from (1) 3Py* + 2-PrOH → 1-PyH• + •PrOH] adds to CO2, in competition with radical−radical reactions, leading to intermediates that are in turn reduced by •PrOH into the observed species. The formation of carboxylates in this system, which is shown to require CO2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e− reduction of CO2 by 2-PrOH initiated by Py*. We evaluate a rate constant (2) k2(1-PyH• + CO2 → •Py-1-COOH) ≈ O (10) M−1 s−1 and an activation energy E2 ≥ 9 kcal mol−1 that are compatible with thermochemical estimates for this reaction
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