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The highly crystalline ZnAl layered double hydroxides (ZnAl-NO3-LDHs) are utilized for the potential transformation into mixed metal oxides (MMOs) through thermal decomposition and used further for the photodegradation of phenol to assess the influence of calcination on ZnAl-LDHs with enhanced photoactivity. The structure, composition, and morphological evolution of ZnAl-LDHs to ZnO-based MMO nanocomposites, which are composed of ZnO and ZnAl2O4, after calcination at different temperatures (400–600 °C), are all thoroughly examined in this work. The final ZnO and ZnAl2O4-based nanocomposites showed enhanced photocatalytic activity. The findings demonstrated that calcining ZnAl-LDHs from 400 to 600 °C increased the specific surface area and also enhanced the interlayer spacing of d003 while the transformation of LDHs into ZnO/ZnAl2O4 nanocomposites through calcining the ZnAl-LDH precursor at 600 °C showed significant photocatalytic properties, leading to complete mineralization of phenol under UV irradiation. [ABSTRACT FROM AUTHOR]

• Melamine and s-triazines derivatives are degraded by VUV-photolysis. • Melamine can be mineralized upon VUV-photolysis. • VUV-photolysis and TiO 2 photocatalytic degradation of melamine have been compared. VUV-irradiation experiments with aqueous solutions of melamine and related triazine derivatives were carried out in the presence or absence of molecular oxygen. Substrate degradation, total organic carbon and evolution profiles of intermediates were monitored. Relevant mineralization of the substrates was observed, in particular for melamine, its extent depending on the concentration of dissolved O 2 and on the number of amino substituents at the s -triazine moiety of the substrate. In all experiments, cyanuric acid was found to be the only persistent final product. Experimental results indicate that the route of reactions leading to mineralization is independent of that leading to amino group substitution. Mechanistic hypotheses are proposed and compared with findings coming from TiO 2 -based photocatalysis. [ABSTRACT FROM AUTHOR]

The photoinduced transformation of 2,4,6-triamino-1,3,5-triazine (melamine) was studied by using different advanced oxidation technologies under a variety of experimental conditions. The systems involving homogeneous hydroxyl radicals, as generated by H 2 O 2 /hν, Fenton reagent, and sonocatalysis are ineffective. However, melamine is degraded under photocatalytic conditions or by SO 4 − (S 2 O 8 2− /hν). The time evolution of long-living intermediates, such as 2,4-diamino-6-hydroxy-1,3,5-triazine (ammeline) and 2-amino-4,6-dihydroxy-1,3,5-triazine (ammelide), has been followed, being 2,4,6-trihydroxy-1,3,5-triazine (cyanuric acid) the final stable product. During both photocatalytic and S 2 O 8 2− /hν experiments, in the early steps, a fairly stable intermediate evolving to ammelide is observed in a large extent. This intermediate was identified as 2,4-diamino-6-nitro-1,3,5-triazine. This indicates that the primary photocatalytic event is the oxidation of the amino-group to nitro-group through several consecutive fast oxidation steps, and that a hydrolytic step leads to the release of nitrite in solution. To elucidate the nature of the oxidant species hole scavengers such as methanol and bromide ions were added to the irradiated TiO 2 . They completely stop the degradation, whereas chloride and fluoride ions decrease the degradation rate. The study of the photocatalytic degradation rate of melamine at increasing concentrations using two different commercial titanium dioxides, such as P25 and Merck TiO 2 , showed an intriguing behavior. A drastic abatement of the melamine transformation rate was observed when coagulation of the P25 slurry occurs due both to the pH change and melamine concentration effect that increase melamine adsorption. In the presence of TiO 2 (Merck) the melamine initial degradation rates are significantly lower than those observed in the presence of P25 but are not depressed at larger concentrations. The experimental evidences (e.g., absence of melamine adsorption onto TiO 2 surface at low concentrations or at acidic pH or due to the catalyst surface texture, and the lack of reactivity toward OH free and bound) suggest that the effective photocatalytic mechanism is based on an outer sphere direct hole transfer to the melamine. Its formal potential lies in the range 1.9–2.3 V vs NHE. Then, the photodegradation of melamine is an efficient tool to evaluate the direct hole transfer ability of a photocatalyst. [ABSTRACT FROM AUTHOR]

The photocatalytic activity of semiconductor oxides, in particular TiO2 powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti–OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O2 reduction (by conduction band electrons) is strongly influenced by the presence of Zn2+ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of ≡Ti–OH surface sites blocked by inner sphere complexation of Zn2+ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of ≡Ti–OH surface sites by Zn2+ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn2+ is accompanied by an increasing production of H2O2 during formate degradation in the presence of O2. Zn(II) cations are not complexed by peroxide/superoxide species derived from O2 reduction. When ≡Ti–OH sites are blocked by Zn2+, the complexation on the TiO2 surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO2. [ABSTRACT FROM AUTHOR]

The photocatalytic activity of semiconductor oxides, in particular TiO2 powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti–OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O2 reduction (by conduction band electrons) is strongly influenced by the presence of Zn2+ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of ≡Ti–OH surface sites blocked by inner sphere complexation of Zn2+ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of ≡Ti–OH surface sites by Zn2+ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn2+ is accompanied by an increasing production of H2O2 during formate degradation in the presence of O2. Zn(II) cations are not complexed by peroxide/superoxide species derived from O2 reduction. When ≡Ti–OH sites are blocked by Zn2+, the complexation on the TiO2 surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO2. [ABSTRACT FROM AUTHOR]

An interesting aspect of the chemistry of nitrite is the possibility for this compound to interact with other environmental factors and many oxidising species, which results in the oxidation of nitrite to nitrogen dioxide. This is a potentially interesting process that can lead to the formation of nitroaromatic compounds in the environment. In previous papers we have shown that nitrite can interact with dissolved Fe(III) and nitrate under irradiation, Fenton and heterogeneous photo-Fenton reagents, and semiconductor oxides such as TiO2, α-Fe2O3, and β-FeOOH under irradiation. This paper reports on the interaction between nitrite/nitrous acid and the Mn(III,IV) (hydr)oxides β-MnO2 and γ-MnOOH, both in neutral solution under irradiation and in acidic conditions in the dark. β-MnO2 and γ-MnOOH originate from the oxidation of Mn(II) and play a key role in the redox cycling of manganese in the environment. These Mn(III,IV) (hydr)oxides show some photocatalytic activity, and they can act as thermal oxidants at acidic pH. The photoinduced oxidation of nitrite and the thermal oxidation of nitrous acid by Mn(III,IV) (hydr)oxides yield nitrogen dioxide and lead to the formation of nitrophenols in the presence of phenol. These processes can take place at the water–sediment or water–colloid interface in natural waters and on the surface of atmospheric particulate. Furthermore, the phenol/γ-MnOOH/HNO2 system in dark acidic solution is an interesting model due to the formation of phenoxyl radical upon phenol monoelectronic oxidation by γ-MnOOH. The kinetics of nitrophenol generation under such conditions indicates that phenol nitration is unlikely to take place upon reaction between phenoxyl and ⋅NO2 and suggests a solution to a literature debate on the subject. [Copyright &y& Elsevier]

The study of the photocatalytic activity of TiO2inverse opals showed that these structures behave as metamaterials: their properties arise principally from the 3D periodic structure of the material and marginally from porosity, reflectivity and scattering. [ABSTRACT FROM AUTHOR]

Benzotriazoles are a new class of organic emerging pollutants ubiquitously found in the environment. The increase of their concentration to detectable values is the consequence of the inability of the Conventional Waste Water Plants (CWWPs) to abate these products. We subjected 1H-benzotriazole (BTz), tolyltriazole (TTz), and Tinuvin P (TP, a common UV plastic stabilizer) to photocatalytic degradation under UV-irradiated TiO2 in different conditions. The principal photoformed intermediates, the relationship between the degradation rate and the pH, the degree of mineralization, and the fate of the organic nitrogen were investigated. Under the adopted experimental conditions, all the studied substrates were rapidly photocatalytically transformed (the maximum degradation rates for BTz and TTz were (3.88 ± 0.05) × 10−2 and (2.11 ± 0.09) × 10−2 mM min−1, respectively) and mineralized (the mineralization rate for BTz and TTz was 4.0 × 10−3 mM C min−1 for both substrates). Different from the 1,2,4-triazole rings that are not completely mineralized under photocatalytic conditions, 1H-benzotriazole and tolyltriazole were completely mineralized with a mechanism that involved a partial conversion of organic nitrogen to N2. The photocatalytic process activated by UV-irradiated TiO2 is an efficient tool to abate 1H-benzotriazole and its derivatives, avoiding their release in the environment. [ABSTRACT FROM AUTHOR]

Anatase nanoparticles in suspension have demonstrated high photoactivity that can be exploited for pollutant removal in water phases. The main drawback of this system is the difficulty of recovering (and eventually reusing) the nanoparticles after their use, and the possible interference of inorganic salts (e.g., sulfates) that can reduce the performance of the photocatalyst. The present work describes the development of a cordierite-honeycomb-supported TiO2 film to eliminate the problems of catalyst recovery. The catalyst was then tested against phenol in the presence of increasing concentrations of sulfates in a specially developed recirculating modular photoreactor, able to accommodate the supported catalyst and scalable for application at industrial level. The effect of SO42− was evaluated at different concentrations, showing a slight deactivation only at very high sulfate concentration (≥3 g L−1). Lastly, in the framework of the EU project Project Ô, the catalyst was tested in the treatment of real wastewater from a textile company containing a relevant concentration of sulfates, highlighting the stability of the photocatalyst. [ABSTRACT FROM AUTHOR]

Photonic efficiency measurements of the photoeffects at semiconductor surfaces for pollutant abatement and value‐added product generation (e. g. H2 in the hydrogen evolution reaction, HER) are usually carried out under continuous irradiation of the catalyst. In the last decades, Controlled Periodic Illumination (CPI) was proposed as a way to increase the photonic efficiency of irradiated semiconductor systems. CPI consists in the modulation of the incident radiant power, alternating light and dark periods with given duty cycle (γ), intensity (I) and frequency (f). The rationale underlying the use of CPI to enhance photonic efficiency is the exploitation of dark periods to allow slow reactions proceed, to induce new reactions or to free up space on the surface of the catalyst. In this short review, we will highlight the strengths and weaknesses of some of the works published so far on the abatement of pollutants and the production of hydrogen using CPI. [ABSTRACT FROM AUTHOR]

It has been often reported that an efficient and green photocatalytic dissociation of water under irradiated semiconductors likely represents the most important goal for modern chemistry. Despite decades of intensive work on this topic, the efficiency of the water photolytic process under irradiated semiconductors is far from reaching significant photocatalytic efficiency. The use of a sacrificial agent as hole scavenger dramatically increases the hydrogen production rate and might represent the classic "kill two birds with one stone": on the one hand, the production of hydrogen, then usable as energy carrier, on the other, the treatment of water for the abatement of pollutants used as sacrificial agents. Among metal oxides, TiO2 has a central role due to its versatility and inexpensiveness that allows an extended applicability in several scientific and technological fields. In this review we focus on the hydrogen production on irradiated TiO2 and its fundamental and environmental implications. [ABSTRACT FROM AUTHOR]

Abstract: Glycerol is a good probe molecule to understand what type of photocatalytic mechanism could occur at the catalyst surface, as the analysis of the produced intermediates is able to discriminate between a direct electron transfer and a radical mediated oxidation mechanism. The mechanism of transformation of glycerol is explored by an extensive analysis of formed intermediates as a function of glycerol concentration on two different commercial TiO2 powders. Over TiO2 Degussa P25 the transformation rate of glycerol shows a sharp maximum as a function of the substrate concentration. The type of product detected at low glycerol concentration (glyceraldehyde and dihydroxyacetone) changes after the sharp maximum giving mainly formaldehyde and glycolaldehyde. On Merck TiO2, characterized by a lower density and more uniform population of hydroxyls at surface sites, mainly glyceraldehyde and dihydroxyacetone are observed. Because these products are formed through an echanism, the products observed on P25 derive from a direct electron transfer. The results are rationalized invoking the surface complex of glycerol where an inner sphere electron transfer occurs, which leads to glycerol fragmentation, and where the substrate-mediated recombination plays the major role. A simplified kinetic approach confirms the observed rate dependence on concentration. The working hypothesis is also confirmed by addition of fluoride anions that compete for surface sites and impede glycerol complexation. Without the possibility of inner sphere electron transfer, on P25 only glyceraldehyde and dihydroxyacetone are observed. Thus different catalysts differ for the possibility of surface complexation. [Copyright &y& Elsevier]

Abstract: This paper reports the results of a study on the titanium dioxide Wackherr''s ¿Oxyde de titane standard¿, which shows very interesting photocatalytic activity. Produced for cosmetic purposes as a white pigment, its features make it very interesting in the field of heterogeneous photocatalysis. The results obtained with this photocatalyst are compared with the behaviour of the well-studied and widely used TiO2 Degussa P25 under the same conditions. In particular, the TiO2 Wackherr induces relevantly faster phenol degradation than P25 when high photocatalyst loading is used (up to 2.00gl-1), as phenol degradation rate in the presence of TiO2 Wackherr continues to increase with increasing photocatalyst loading. This is most likely due to the lower radiation scattering in the UV region of TiO2 Wackherr when compared with Degussa P25, which is linked with the higher particle size of TiO2 Wackherr. The initial phenol degradation rate by TiO2 Wackherr as a function of phenol concentration has a maximum for [phenol]¿3×10-4M, and decreases for higher concentration values. The addition of fluoride ions to TiO2 Wackherr at pH 3.7 increases phenol degradation rate, as already found for Degussa P25. The increase is more relevant for higher phenol concentration values and makes the maximum as a function of phenol concentration to disappear. Comparable results are also obtained when benzoic acid is used as a substrate, with some differences from phenol that can be accounted for by benzoic acid more strongly interacting with the surface. The use of TiO2 Wackherr in heterogeneous photocatalysis can be desirable when high photocatalyst loading is required. [Copyright &y& Elsevier]

Particle aggregation and agglomeration influence the optical properties of materials and therefore their ability to absorb and scatter the incoming radiation, also affecting their photocatalytic activity [1–7] . We have studied the correlation between aggregation and photocatalytic activity for titanium dioxide by means of experimental measurements of extinction and photocatalytic activity and calculations of their optical properties (extinction, absorption and scattering cross-sections). This approach can be adopted to quantitatively assess the quantum yields of the heterogeneous photocatalytic systems. The study was performed on TiO 2 PC105 Cristal ACTiV™, made of aggregated (and agglomerated) primary particles of anatase. The size of the aggregates has been reduced with ultra-sonication. Aqueous suspensions of the obtained materials were characterized by measuring the optical properties (UV–vis extinction), the sizing properties (DLS) and the photocatalytic activity (degradation of phenol under standard conditions). The extinction and absorption spectra of the suspensions were derived from the calculated coefficients, considering also the size distributions measured with DLS, and revealed that light absorption is maximized when particle aggregation and agglomeration are avoided, while diffusion of the incoming radiation dominates when large aggregates and agglomerates are present. The present paper represents a valuable approach to the accurate and reproducible measurement of the photocatalytic activity of TiO 2 nanoparticles suspensions, thus allowing a more reliable comparison of the properties of different materials. [ABSTRACT FROM AUTHOR]