Photocatalytic degradation of organophosphorus flame retardants in aqueous solutions: a review and future prospects

The widespread use of organophosphorus flame retardants (OPFRs) in industrial and household products increases the risk of their environmental exposure, posing a serious threat to ecosystems and human health. Photocatalytic technology has been widely used in wastewater treatment due to its high efficiency, mild reaction conditions, and robustness. This review summarizes the current status of research on photocatalytic degradation of OPFRs, focusing on the effect of different types of catalysts on the degradation efficiency, the effects of pH, and co-existing inorganic and organic ions. And pH and co-existing inorganic mainly affect the active oxygen and the active surface sites of the catalyst. In addition, toxicological calculations of the intermediates of the degradation pathway using T.E.S.T. and ECOSAR showed that photocatalysis could effectively reduce the toxicity of OPFRs. Development of new photocatalytic materials, in-depth study of the degradation mechanism of different catalysts and flame retardants, and attention to practical applications and toxicity issues can be the development direction of future research.

Reactive Radicals on Reactive Surfaces: Heterogeneous Processes in Catalysis and Environmental Pollution Control

Many reactions that occur on solid surfaces are mediated by free radicals. A review is presented of both mechanistic and practical investigations in relation to catalysis and environmental applications. The review begins with actual imaging of surface adsorbed reactive radicals using scanning tunnelling microscopy (STM), and then discusses a range of examples, mainly as underpinned by electron spin resonance (ESR) measurements. Included are surface defects and their reactions, studies of the redox behaviour of zeolites, and the use of radicals adsorbed in zeolites as molecular surface probes of diffusion and reactivity within these important materials. Photocatalysis, mainly using TiO2-based materials, is reviewed both from the fundamental perspective and in terms of some practical examples relating to pollution control. Other reactive oxide surfaces are considered, including silica, and the nature of paramagnetic centres that may be induced thereon by a variety of activation procedures. Evidence is presented for the formation of radical species during heterogeneous reactions on metal surfaces. Finally, the role of free radical generation in creating and modifying polymer and nanomolecular systems is discussed, and the health implications of the ability of some solids such as quartz to generate reactive oxygen radicals in contact with biological media.

Designing an ultrathin silica layer for highly durable carbon nanofibers as the carbon support in polymer electrolyte fuel cells

A critical issue for maintaining long-term applications of polymer electrolyte fuel cells (PEFCs) is the development of an innovative technique for the functionalization of a carbon support that preserves their exceptional electrical conductivity and robustly enriches their durability. Here, we report for the first time how the formation of a partially coated, ultrathin, hydrophobic silica layer around the surfaces of the carbon nanofiber (CNF) helps improve the durability of the CNF without decreasing the significant electrical conductivity of the virgin CNF. The synthesis involved the adsorption of polycarbomethylsilane (PS) on the CNF's sidewalls, followed by high temperature pyrolysis of PS, resulting in a highly durable, conductive carbon support in PEFCs. The Pt nanoparticles are in direct contact with the surface of the carbon in the empty spaces between unevenly coated silica layers, which are not deposited directly onto the silica layer. The presence of a Pt nanoparticle layer that was thicker than the silica layer would be a quite advantageous circumstance that provides contact with other neighboring CNFs without having a significant adverse effect that deeply damages the electrical conductivity of the neighboring CNF composites with the silica layer. Furthermore, the ultrathin, hydrophobic silica layer around the surfaces of the CNF provides great potential to reduce the presence of water molecules in the vicinity of the carbon supports and the ˙OH radicals formed on the surface of the Pt catalyst. As a result, the CNF with a 5 wt% silica layer that we prepared has had extremely high initial performance and durability under severe carbon corrosion conditions, starting up with 974 mA cm(-2) at 0.6 V and ending up with more than 58% of the initial performance (i.e., 569 mA cm(-2) at 0.6 V) after a 1.6 V holding test for 6 h. The beginning-of-life and end-of-life performances based on the virgin CNF without the silica layer were 981 and 340 mA cm(-2) at 0.6 V, respectively. The CNF having a silica layer had long-term durability which was superior to that of the virgin CNF.

Titanium dioxide as a catalyst support in heterogeneous catalysis

The lack of stability is a challenge for most heterogeneous catalysts. During operations, the agglomeration of particles may block the active sites of the catalyst, which is believed to contribute to its instability. Recently, titanium oxide (TiO₂) was introduced as an alternative support material for heterogeneous catalyst due to the effect of its high surface area stabilizing the catalysts in its mesoporous structure. TiO₂ supported metal catalysts have attracted interest due to TiO₂ nanoparticles high activity for various reduction and oxidation reactions at low pressures and temperatures. Furthermore, TiO₂ was found to be a good metal oxide catalyst support due to the strong metal support interaction, chemical stability, and acid-base property. The aforementioned properties make heterogeneous TiO₂ supported catalysts show a high potential in photocatalyst-related applications, electrodes for wet solar cells, synthesis of fine chemicals, and others. This review focuses on TiO₂ as a support material for heterogeneous catalysts and its potential applications.

Unpaired electrons as probes of catalytic systems

An overview is provided of the importance of molecular species containing unpaired electrons in catalytic systems, as revealed using ESR spectroscopy. The review aims to demonstrate the considerable extent of scientific progress that has been made in this broad topic during the past few decades. Studies of catalytically active surfaces, including zeolites, are surveyed, and the detection of radical species, formed as intermediates in their reactions, using matrix isolation and spin-trapping techniques. Radical cation formation in zeolites is discussed, and the employment of muon spin rotation and relaxation techniques to study the mobility of labelled radicals in various porous and catalytic media. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, degradation of environmental pollutants, hydrocarbon conversions, fuel cells and sensor devices employing graphene. The review concludes with recent developments in the study of enzymes and their reactions, using ESR-based methods.

Adsorption and UV/Visible photocatalytic performance of BiOI for methyl orange, Rhodamine B and methylene blue: Ag and Ti-loading effects

Adsorption and UV/visible photocatalytic activity of echinoid-like Ag and Ti-loaded BiOI were tested for methyl orange, Rhodamine B and methylene blue.

Degradation of Toluene Using Modified TiO2 as Photocatalysts

Volatile organic compounds (VOCs), especially toluene as the typical indoor air pollutants, are toxic and environmentally persistent whose removal is undoubtedly becoming increasingly urgent matter over these years. Titania is one of the most promising photocatalysts for the degradation of organic compounds, whereas the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic effciency. These defects can be tackled by modifying the electronic band structure of titania including various strategies like metal deposition, non-metal atoms substitution, transition metal ions doping, and coupling with a narrow band gap semiconductor, etc. This review encompasses several advancements made in these aspects, and also the influence factors such as physical morphologies changing, humidity, as well as the presence of O2 etc, are involved. To be practically considering, TiO2 photocatalysts require being fixed on the bulky supports like silica, alumina, clays and activated carbons. Moreover, photocatalytic coatings deposited on external building materials, like roofing tiles and corrugated sheets, is becoming the attractive application potentials to remove toluene from air.

Nanostructuring cadmium germanate catalysts for photocatalytic oxidation of benzene at ambient conditions

A nanostructured Cd(2)Ge(2)O(6) photocatalyst was successfully prepared by a hydrothermal process. The photocatalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV/vis, N(2) adsorption-desorption, and Fourier transform infrared (FTIR) techniques. The photocatalytic property of the material was evaluated via the decomposition of benzene in the gas phase with light illumination and was compared with that of commercial titania (Degussa P25) and Pt/TiO(2). The electronic band structure of Cd(2)Ge(2)O(6) was analyzed by density functional theory (DFT) calculation. Results reveal that the prepared Cd(2)Ge(2)O(6) has unique geometric and electronic properties, which in combination with its superior textural properties makes it a new semiconductor photocatalyst for environmental purification of benzene in air with molecular oxygen under ambient conditions. It was also found that the Cd(2)Ge(2)O(6) was more active and stable than TiO(2)-based catalysts in the photocatalytic decomposition of other volatile aromatic pollutants including toluene and ethylbenzene. The enhanced photocatalytic performance of Cd(2)Ge(2)O(6) can be explained by the special band structure, and geometric and electronic feature, in unison with the high surface area nanoporous framework.

Photocatalytic oxidation of CO on TiO(2): chemisorption of O(2), CO, and H(2)

On the surface: Adsorption of O(2) at the surface oxygen vacancy (SOV) sites of TiO(2) reconstructs the lattice oxygen (healing SOVs), resulting in a decrease of the photocatalytic activity of oxidizing CO over vacuum-pretreated TiO(2) with increasing temperature (see scheme). Adsorption of H(2) produces new SOVs at the TiO(2) surface and stabilizes the photocatalytic activity. Photocatalytic oxidation of CO over vacuum-pretreated TiO(2) is performed in a series of systems with the introduction of O(2), CO, and H(2) in different orders. The photocatalytic oxidation of CO is dependent on the order of introduction of O(2), CO, or H(2), and introducing O(2) prior to CO promotes the oxidation of CO. Moreover, an increase of reaction temperature suppresses the oxidation of CO, but the preintroduction of H(2) reduces this suppression effect. The results of the chemisorption of O(2), CO, and H(2) at the TiO(2) surface reveal that the adsorbed O(2) heals the surface oxygen vacancy (SOV) sites of TiO(2), while the adsorbed CO and H(2) promote the formation of new SOVs. It is proposed that changes in the amounts of adsorbed O(2) and SOVs are mainly responsible for the differences of CO conversion in different systems.

Degradation of benzene over a zinc germanate photocatalyst under ambient conditions

A rod-shaped Zn2GeO4 photocatalyst has been successfully prepared by a surfactant-assisted hydrothermal method. The photocatalyst was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, UV/vis, N2 adsorption-desorption, and FTIR techniques. The photocatalytic activity of the sample was evaluated by the decomposition of benzene in the gas phase under UV light illumination and was compared with that of bulk Zn2GeO4, commercial titania (Degussa P25), and Pt/P25. The results revealed that the Zn2GeO4 nanorods had the best photocatalytic activity for mineralizing benzene to CO2 among the catalysts examined. No obvious deactivation of Zn2GeO4 nanorods was observed during the prolonged operation of 140 h. It was found that the Zn2GeO4 was also more active and stable than TiO2-based catalysts toward photocatalytic decomposition of other volatile aromatic pollutants (e.g., toluene and ethylbenzene).

A new route for degradation of volatile organic compounds under visible light: using the bifunctional photocatalyst Pt/TiO2-xNx in H2-O2 atmosphere

The bifunctional photocatalyst Pt/TiO2-xNx has been successfully prepared by wet impregnation. The properties of Pt/ TiO2-xNx have been investigated by diffuse reflectance spectra, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, a photoluminescence technique with terephthalic acid, and electric field induced surface photovoltage spectra. The photocatalytic activity of the sample was evaluated by the decomposition of volatile organic pollutants (VOCs) in a H2-O2 atmosphere under visible light irradiation. The results demonstrated that nitrogen-doped and platinum-modified TiO2 in a H2-O2 atmosphere could enormously increase the quantum efficiency of the photocatalytic system with excellent photocatalytic activity and high catalytic stability. The increased quantum efficiency can be explained by enhanced separation efficiency of photogenerated electron-hole pairs, higher interface electron transfer rate, and an increased number of surface hydroxyl radicals in the photocatalytic process. A mechanism was proposed to elucidate the degradation of VOCs over PtTiO(2-x)Nx in a H2-O2 atmosphere under visible light irradiation.