TiO₂ Nanotubes with Open Channels as Deactivation-Resistant Photocatalyst for the Degradation of Volatile Organic Compounds

We synthesized ordered TiO₂ nanotubes (TNT) and compared their photocatalytic activity with that of TiO₂ nanoparticles (TNP) film during the repeated cycles of photocatalytic degradation of gaseous toluene and acetaldehyde to test the durability of TNT as an air-purifying photocatalyst. The photocatalytic activity of TNT showed only moderate reduction after the five cycles of toluene degradation, whereas TNP underwent rapid deactivation as the photocatalysis cycles were repeated. Dynamic SIMS analysis showed that carbonaceous deposits were formed on the surface of TNP during the photocatalytic degradation of toluene, which implies that the photocatalyst deactivation should be ascribed to the accumulation of recalcitrant degradation intermediates (carbonaceous residues). In more oxidizing atmosphere (100% O₂ under which less carbonaceous residues should form), the photocatalytic activity of TNP still decreased with repeating cycles of toluene degradation, whereas TNT showed no sign of deactivation. Because TNT has a highly ordered open channel structure, O₂ molecules can be more easily supplied to the active sites with less mass transfer limitation, which subsequently hinders the accumulation of carbonaceous residues on TNT surface. Contrary to the case of toluene degradation, both TNT and TNP did not exhibit any significant deactivation during the photocatalytic degradation of acetaldehyde, because the generation of recalcitrant intermediates from acetaldehyde degradation is insignificant. The structural characteristics of TNT is highly advantageous in preventing the catalyst deactivation during the photocatalytic degradation of aromatic compounds

Singlet-Oxygen Generation in Alkaline Periodate Solution

A nonphotochemical generation of singlet oxygen (¹O₂) using potassium periodate (KIO₄) in alkaline condition (pH > 8) was investigated for selective oxidation of aqueous organic pollutants. The generation of ¹O₂ was initiated by the spontaneous reaction between IO₄^– and hydroxyl ions, along with a stoichiometric conversion of IO₄^– to iodate (IO₃^–). The reactivity of in-situ-generated ¹O₂ was monitored by using furfuryl alcohol (FFA) as a model substrate. The formation of ¹O₂ in the KIO₄/KOH system was experimentally confirmed using electron spin resonance (ESR) measurements in corroboration with quenching studies using azide as a selective ¹O₂ scavenger. The reaction in the KIO₄/KOH solution in both oxic and anoxic conditions initiated the generation of superoxide ion as a precursor of the singlet oxygen (confirmed by using superoxide scavengers), and the presence of molecular oxygen was not required as a precursor of ¹O₂. Although hydrogen peroxide had no direct influence on the FFA oxidation process, the presence of natural organic matter, such as humic and fulvic acids, enhanced the oxidation efficiency. Using the oxidation of simple organic diols as model compounds, the enhanced ¹O₂ formation is attributed to periodate-mediated oxidation of vicinal hydroxyl groups present in humic and fulvic constituent moieties. The efficient and simple generation of ¹O₂ using the KIO₄/KOH system without any light irradiation can be employed for the selective oxidation of aqueous organic compounds under neutral and near-alkaline conditions

Accelerated Reduction of Bromate in Frozen Solution

Bromate is a common disinfection byproduct formed during ozonation. Reducing bromate into bromide can remove this toxic pollutant, however, not many studies have been done for its environmental fate. In this work, we demonstrate a new transformation pathway that bromate can be efficiently reduced to bromide in frozen solution in the presence of organic reductants like humic substances (HS). The results showed that bromate in frozen solution could be removed by 30–40% in dark condition and 80–90% in irradiation condition (λ > 300 nm) in 24 h, while around 1% bromate was reduced in aqueous solution. The bromate reduction by HS induced a partial oxidation of HS, which was confirmed by X-ray photoelectron spectroscopic analysis of the HS sample recovered from the frozen solution. Photoluminescence analysis of HS revealed that the fluorescence quenching by bromate was observed only with very high concentration of bromate (0.1–0.2 M) in aqueous solution whereas the quenching effect in frozen solution was seen with much lower bromate concentration (5–100 μM). The highly enhanced removal of bromate in ice is ascribed to the freeze concentration effect that bromate and HS are concentrated by orders of magnitude to accelerate the bimolecular transformation in the ice grain boundary region. Freezing process in cold environments would provide a unique chemical mechanism for the removal of persistent bromate

TiO₂ Nanotube Array Photoelectrocatalyst and Ni–Sb–SnO₂ Electrocatalyst Bifacial Electrodes: A New Type of Bifunctional Hybrid Platform for Water Treatment

Bifunctional hybrid electrodes capable of generating various reactive oxygen species (ROS) over a wide range of potentials were developed by coupling electrocatalysts and photoelectrocatalysts. To achieve this, Ni-doped Sb-SnO₂ (NSS) was deposited on one side of a titanium (Ti) foil while the other side was anodized to grow a TiO₂ nanotube array (TNA) for electrochemical ozone generation and photoelectrochemical hydroxyl radical generation, respectively. Surface characterization indicated that NSS and TNA were formed and spatially separated yet electrically connected through the Ti substrate. While each catalyst possessed unique electrochemical properties, the coupling of both catalysts resulted in mixed electrochemical properties that drove electrocatalysis at high potentials and photoelectrocatalysis at low potentials. The performance of the NSS/TNA electrode for phenol decomposition was ∼3 times greater than that of single-layer catalysts and ∼1.5 times greater than the combined catalytic performances of the individual NSS and TNA catalysts. This synergistic effect was attributed partly to the simultaneous generation of hydroxyl radicals and ozone, followed by the production of other ROS. A mechanism for the generation of ROS was discussed

Concentration-Dependent Photoredox Conversion of As(III)/As(V) on Illuminated Titanium Dioxide Electrodes

The photoconversion of As­(III) (arsenite) and As­(V) (arsenate) over a mesoporous TiO₂ electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (μM–mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As­(III) can be oxidized, but also As­(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As­(III)/As­(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O₂ is far more efficient as an electron acceptor than As­(V). Although As­(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As­(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As­(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As­(III) is ascribed to the role of As­(IV) as a charge recombination center. Being an electron acceptor, the addition of As­(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed

Effect of Agglomerated State in Mesoporous TiO₂ on the Morphology of Photodeposited Pt and Photocatalytic Activity

Two mesoporous TiO₂ samples (M1-TiO₂ and M2-TiO₂) with different morphologies were synthesized, and the photocatalytic and photoelectrochemical properties of both TiO₂ and their photoplatinized counterparts (0.05, 0.1, and 1.0 wt % of Pt) were systematically investigated. Electron microscopic analysis showed that M1-TiO₂ consists of densely packed nanoparticles forming spherical secondary particles (0.5 to 1.0 μm), whereas M2-TiO₂ is made up of loosely agglomerated nanoparticles. Subsequently, this morphological difference led to the formation of different Pt clusters (photodeposited on them): large Pt nanoparticles on M1-TiO₂ versus well-dispersed smaller Pt nanoparticles on M2-TiO₂. The photocatalytic activities of platinized M1-TiO₂ and M2-TiO₂ were investigated for H₂ production and 4-chlorophenol degradation. Whereas M1-TiO₂ exhibited the highest photoactivity with 0.1 wt % Pt loading, the activity of M2-TiO₂ increased with increasing Pt loading (up to 1.0 wt %). The critical role of surface Pt morphology on the photocatalytic behavior of M1-TiO₂ and M2-TiO₂ was investigated using electrochemical impedance spectroscopy and photocurrent measurements. In the case of M1-TiO₂, an increase in Pt cluster size enhanced the charge-transfer resistance and reduced the interfacial electron transfer efficiency, whereas the same loading of Pt on M2-TiO₂ effectively enhanced the interfacial charge transfer. This dissimilar interfacial charge-transfer kinetics for M1-TiO₂ and M2-TiO₂ indicates that the TiO₂ microstructure controls the photodeposited Pt morphology, which subsequently affects the photocatalytic activity. This study reveals that the agglomerated state of TiO₂ nanoparticles can be an important parameter in determining the photocatalytic activity in both the suspension and film states

Freezing-Enhanced Dissolution of Iron Oxides: Effects of Inorganic Acid Anions

Dissolution of iron from mineral dust particles greatly depends upon the type and amount of copresent inorganic anions. In this study, we investigated the roles of sulfate, chloride, nitrate, and perchlorate on the dissolution of maghemite and lepidocrocite in ice under both dark and UV irradiation and compared the results with those of their aqueous counterparts. After 96 h of reaction, the total dissolved iron in ice (pH 3 before freezing) was higher than that in the aqueous phase (pH 3) by 6–28 times and 10–20 times under dark and UV irradiation, respectively. Sulfuric acid was the most efficient in producing labile iron under dark condition, whereas hydrochloric acid induced the most dissolution of the total and ferrous iron in the presence of light. This ice-induced dissolution result was also confirmed with Arizona Test Dust (AZTD). In the freeze–thaw cycling test, the iron oxide samples containing chloride, nitrate, or perchlorate showed a similar extent of total dissolved iron after each cycling while the sulfate-containing sample rapidly lost its dissolution activity with repeating the cycle. This unique phenomenon observed in ice might be related to the freeze concentration of protons, iron oxides, and inorganic anions in the liquid-like ice grain boundary region. These results suggest that the ice-enhanced dissolution of iron oxides can be a potential source of bioavailable iron, and the acid anions critically influence this process

Heterogeneous Catalytic Oxidation of As(III) on Nonferrous Metal Oxides in the Presence of H₂O₂

The oxidation of As­(III) (arsenite) to As­(V) (arsenate), a critical pretreatment process for total arsenic removal, is easily achieved using chemical oxidation methods. Hydrogen peroxide (H₂O₂) is widely used as an environmentally benign oxidant but its practical use for the arsenite oxidation is limited by the strong pH dependence and slow oxidation kinetics. This study demonstrated that H₂O₂-induced oxidation of As­(III) can be markedly enhanced in the presence of nonferrous metal oxides (e.g., WO₃, TiO₂, ZrO₂) as a heterogeneous catalyst working over a wide pH range in ambient reaction conditions. In particular, TiO₂ is an ideal catalyst because it is not only active and stable but also easily available and inexpensive. Although the photocatalytic oxidation of As­(III) on TiO₂ was intensively studied, the thermal catalytic activities of TiO₂ and other nonferrous metal oxides for the arsenic oxidation have been little investigated. The heterogeneous oxidation rate increased with increasing the TiO₂ surface area and [H₂O₂] and weakly depended on pH whereas the homogeneous oxidation by H₂O₂ alone was favored only at alkaline condition. The oxidation rate in the TiO₂/H₂O₂ system was not reduced at all in the absence of dioxygen. It was not retarded at all by OH radical scavengers but markedly inhibited by hydroperoxyl radical scavengers. It is proposed that the surface complexation of H₂O₂ on TiO₂ induces the generation of the surface hydroperoxyl radical through an inner-sphere electron transfer, which subsequently reacts with As­(III). The catalytic activity of TiO₂ was maintained without showing any sign of deactivation. The heterogeneous catalytic oxidation is proposed as a viable method for the preoxidation treatment of As­(III)-contaminated water under ambient conditions

Boosting up the Low Catalytic Activity of Silver for H₂ Production on Ag/TiO₂ Photocatalyst: Thiocyanate as a Selective Modifier

Noble metal cocatalysts like Pt have been widely employed as an essential ingredient in many kinds of photocatalytic materials for solar hydrogen production. The high material cost of Pt is the biggest limitation. Silver is far less expensive but much less active than Pt and Au as a hydrogen evolving catalyst. Here we demonstrate a new strategy to boost up the activity of silver in Ag/TiO₂ for photocatalytic H₂ production via forming a simple surface complexation of thiocyanate (SCN^–) on silver. The addition of thiocyanate in the suspension of Ag/TiO₂ markedly enhanced the photocatalytic production of H₂ by about 4 times. Thiocyanate was not consumed at all during the photoreaction, which ruled out the role of thiocyanate as an electron donor. Such a positive role of thiocyanate was not observed with bare TiO₂, Pt/TiO₂, and Au/TiO₂. The selective chemisorption of thiocyanate on silver was confirmed by the analyses of Raman spectroscopy and spot-profile energy-dispersive spectroscopy. In the presence of thiocyanate, the overpotential for water reduction on Ag/TiO₂ electrode was slightly reduced, and the interfacial charge transfer resistance on Ag/TiO₂ (measured by electrochemical impedance spectroscopy) was significantly decreased, whereas other electrode systems (bare TiO₂, Au/TiO₂, and Pt/TiO₂) showed the opposite effect of thiocyanate. These results indicate that the adsorption of thiocyanate on Ag facilitates the transfer of photogenerated electrons on the Ag/TiO₂ electrode. It is proposed that the formation of Ag-SCN surface complex enhances the interfacial electron transfer rate and facilitates the reduction of protons on Ag/TiO₂