Search

The immobilization of nanostructured photocatalysts is of great importance in the purification of both liquids (e.g. industrial wastewaters) and polluted air. Titanium dioxide films with nanotubular morphology and good photocatalytic efficiency in both environments can be produced by anodic oxidation, which avoids release of nanoscale materials in the environment. Here we evaluate different oxidation processes in order to identify the most efficient and robust technology for the production of TiO2 layers with high specific surface area. The role of the electrolyte in particular is crucial: nanoporous or nanotubular structures show different activities in liquid or gas phase degradation reactions, and within nanotubular structures a large variable is represented by the electrolyte, which influences both overall activity and treatment robustness.

Titania nanotube arrays (TNTAs) are very interesting photocatalysts. Particularly, TNTAs obtained by anodic oxidation of metallic titanium foils allow the direct synthesis of high efficiency supported photocatalytic layers without the drawbacks related to the high temperature sintering of anatase nanopowders. The synthesis of TNTAs is however a critical process with several parameters to be optimized in order to obtain high photocatalytic activities. This work investigates the optimization of titanium supported TNTAs for the photocatalytic degradation of toluene in air at ambient concentration. Metal-supported TNTAs were produced by titanium foils anodization using a solution of 0.5 wt.% NaF/1M Na2SO4. The anodization process (3-6 h) was performed at 20 V. The samples were then thermally annealed in air at different temperatures and duration times in order to achieve the crystallization of the obtained amorphous TiO2. The photocatalytic activity was assessed by measuring the toluene degradation in air using a continuous-flow stirred photoreactor operated at constant toluene concentration (0.75 µmol m-3). The TNTAs photocatalytic activity demonstrates to be strictly dependent on several process parameters (e.g. the anodic oxidation solution, the oxidation time and the post-synthesis thermal annealing). The oxidation time was studied in the 3-6 h range demonstrating the best photocatalytic performance for the longest time. The post-synthesis annealing was studied in the 400-600 °C temperature range for 60 and 120 min duration. For both annealing duration times the highest activities were found at 600 °C process temperature, with >6.5 factor improvement from 400 to 600 °C for 120 min process and >4 factor improvement from 500 to 600 °C for 60 min process. At the highest temperature the curing time is however critical and the best activity was found for 60 min curing, with ?30% activity decrease shown by the 120 min annealed sample. The results demonstrate the severe dependence of the TNTAs gas-phase photocatalytic activity on the synthesis process parameters. Particularly, the final TNTAs crystalline structure is critically dependent on post-synthesis annealing with a very pronounced dependence of the obtained photocatalytic activity on the process temperature and duration. The criticality of the annealing process is due both to the amorphous/crystalline and then the anatase/rutile phase transitions and to the morphological transformations triggered at high temperatures. TNTAs obtained by anodization of titanium foils can thus be very interesting natively-supported photocatalysts after a fine tuning of the anodization and annealing process parameters.

In the last years titania nanotubes arrays (TNTAs) have attracted increasing attention in photocatalysis due to their unique properties, not least the possibility of a direct production of supported TNTAs by anodic oxidation of metallic titanium foils. Besides, loading of TiO2-based photocatalysts with Au nanoparticles (NPs) has been extensively investigated so as to improve the photocatalytic efficiency. In order to combine the advantages of TNTAs with Au NPs decoration and to overcome the problems derived from current deposition methods this work proposes a simple and scalable synthetic protocol able to load size-controlled (10 µg cm-2) a pronounced deactivation was observed. Using the described method the Au loading was finely tuned ranging from 1.5 to 11.6 ?g cm-2 by controlling the support/Au dipping time without significant change of Au NPs mean size. No further thermal or chemical post-treatments were necessary. The Au loadings strongly influenced the photocatalytic activities and the best performance results in a three times enhancement of the bare TNTAs activity obtained with a sample containing 3.3 ?g cm-2 of Au. The proposed protocol demonstrated a remarkable enhancement of the photocatalytic activity of metal-supported TNTAs at room temperature without any post-deposition process.

The paper is oriented to the study of chlorides corrosion of carbon steel in simulating alkaline concrete pore solutions.Cyclic voltammetries (CV) were performed in order to evaluate the influence of pH, in the range of 12.6-13.8, on theinhibiting properties of nitrite and anions of two organic acid, lactate and aspartate, on localized corrosion initiation.The results confirm that the corrosion resistance of steel in solutions with the same pH of concrete is strictly dependentupon the chlorides-alkalinity ratio (Figures 6, 7). The inhibitors showed an additive contribute to alkalinity withdifferent weight for the three substances considered during testing.The CV tests were performed in aerated solutions at room temperature on disk specimens machined from a carbonsteel bar, with an exposed surface of 0.67 cm2. Disk electrode (RDE) rotating at 2500 rpm was employed for reducingthe effect of diffusion. The test was performed after conditioning of the working electrode at -1 V vs SCE for 60 s, reequilibrationfor 15 s and two polarization cycles between -1.7 and + 1 V vs SCE at 50 mV / s scan rate.Before test, the specimen was passivated in the test solution for 72 hours to grant the formation of a stable protectivelayer on the steel surface without chloride addition. The inhibitors were added into the passivation solution beforethe immersion of specimen, at different concentrations, 0.01, 0.1 and 1 M for sodium nitrite, 1 M for sodium lactateand 0.1 M for sodium aspartate. Two characteristic potentials related to pitting initiation susceptibility, E1,1st and E1,2nd,were derived by the first and the second CV cycle, respectively (Figures 1, 3, 4, 5). The results confirm that the criticalthreshold for localized corrosion initiation can be expressed as a function of a critical [Cl-] / [OH-] ratio; the probabilityof pitting initiation is low in the case of [Cl-] / [OH-] ratio lower than 0.6 (eqs. 1-4). The nitrite addition causes anincrement in the critical [Cl-] / [OH-] ratio (Figure 8-10) that can be quantified by introducing an efficiency parameterf* (eq. 9; Figure 13). This parameter defines the effectiveness of the inhibitor compared to alkalinity. It is about onefor nitrite ions. Thus, nitrite ions show an inhibition of corrosion initiation similar to that of hydroxyl ions. For aspartateions (Figure 11), f* depends upon pH (Figures 14, 15); above 12.6, the higher the pH, the higher the f* was noticed. AtpH equal to 13.8, the aspartate showed a f* value higher than nitrites at the same concentration.Sodium lactate showed an almost nil f* parameter, denoting a negligible inhibition on pit initiation (Figure 12). Thus,the positive effect observed during previous long-term tests on concrete specimens should be ascribed to an effect onthe propagation phase rather than initiation of the localized corrosion.

As mentioned in previous chapters, the benefits provided by innovation in the paper and board business are connected with the possible chemical modification of fibres, with coupling with other materials (composites), as well as with special surface treatments. Today paper and board can substitute polymers by virtue of their enhanced ability to act as barriers to gases, while displaying an active, antimicrobial, photocatalytic, etc. behaviour. The desired barrier effect is primarily against oxygen. Considerable interest was aroused in the past by substances like artificial or natural antioxidant additives, capable to act as a barrier to oxygen by preventing its action, as well as by solutions that prevent oxygen contact with food, such as vacuum and modified atmosphere. One interesting option today is nanocellulose, a strong barrier to oxygen and other gases that can truly compete with resins currently in use to produce composite materials. A water barrier is also required for paper and board. Surface treatments are therefore available for paper, enhancing its resistance to water contact without affecting its recyclability and biodegradability properties. Also important is the grease barrier. Greaseproof paper can be obtained through the enhanced disaggregation of the fibre mesh, which can be achieved either chemically or mechanically. Alongside mass treatment, surface coatings are also available. These act as a physical barrier to grease by means of paraffin, fluorinated emulsions, or silicone-based composites. Coatings based on polymer solutions have a severe drawback – they prevent paper recyclability. Therefore latest research in the paper and board packaging pipeline is aimed at producing new grease-barrier coating options, to be used without jeopardizing the medium’s recyclability. In these natural coatings, the “active” part, i.e. the fraction responsible for the desired barrier properties, is made up of biodegradable substances, including polysaccharides, proteins, and fatty acids. Microbial proliferation is another important cause for food decay. Bacterial contamination occurs, in particular, on the product’s surface, and incorporating antimicrobial agents in packaging allows to reduce the amount of preservatives added to food and to modulate their release. At present, research is carried out based on a variety of approaches: these range from a release mechanism, where the antimicrobial agent is incorporated in the material and migration to food occurs through diffusion and distribution with a decreasing effect in time, to a contact-based mechanism, where the antimicrobial agent is immobilized on the material and the antimicrobial action is displayed at the interface, thus ensuring a long-lasting effect. Abundant research is carried out on chitosan, another natural substance and a structural component of the shell of certain crustaceans. It is a biodegradable material with excellent antimicrobial properties. Lastly, several studies focus on titanium dioxide, which displays both an antibacterial and a photocatalytic action and is therefore applied in the paper and board sector on food packaging, as well as on other products, such as air purification devices.