40 research outputs found

    Cooperative Effect between Cation and Anion of Copper Phosphate on the Photocatalytic Activity of TiO<sub>2</sub> for Phenol Degradation in Aqueous Suspension

    No full text
    Surface modification of TiO<sub>2</sub> with CuO or calcium phosphate (CaP) can result in enhancement in the photocatalytic activity for organic degradation. In this work, we report on a synergism between the cation and anion of copper phosphate (CuP) on the photocatalytic activity of TiO<sub>2</sub>, for phenol degradation in aerated aqueous suspension under UV light at wavelengths longer than 320 nm. Photocatalysts were prepared by mixing TiO<sub>2</sub> and CuP powders in isopropyl alcohol, followed by drying at 90 °C. As CuP loading increased, the activity of the modified TiO<sub>2</sub> first increased and then decreased. The maximum activity was observed with the catalyst containing 0.1 wt % CuP, which was about 1.9–3.4 times that of bare TiO<sub>2</sub> (anatase, rutile, and their mixture) and also exceeded that of the modified TiO<sub>2</sub> with CuO or CaP. During five repeated tests, the catalyst activity was stable, without detectable leaching of cupric and phosphate ions into aqueous solution. Solid characterization with several techniques including electron paramagnetic resonance (EPR) spectroscopy revealed that CuP particles at low loading were highly dispersed onto TiO<sub>2</sub> as a kind of clusters, whereas the TiO<sub>2</sub> phase in different samples remained nearly unchanged in terms of the crystal structure, surface area, and crystallinity. Upon exposure to UV light, the EPR signal of Cu­(II) in CuP or CuO-modifed TiO<sub>2</sub> was unchanged in air but slightly decreased in N<sub>2</sub>. Moreover, CuP-modified TiO<sub>2</sub> showed a higher capacity than bare TiO<sub>2</sub> and CuO- or CaP-modified TiO<sub>2</sub> for the uptake of 2,4-dichlorophenol from water. It is proposed that cupric and phosphate ions act as an electron scavenger and organic sorbent, which facilitate electron and hole transfer, respectively. Their co-operation would significantly improve the efficiency of charge separation, and thus increase the rate of phenol degradation

    Incorporative Effect of Pt and Na<sub>2</sub>CO<sub>3</sub> on TiO<sub>2</sub>‑Photocatalyzed Degradation of Phenol in Water

    No full text
    Carbonate anions are often present in aqueous solution, but their effect on the semiconductor-photocatalyzed reaction has been rarely studied. In this work, we report a positive effect of Na<sub>2</sub>CO<sub>3</sub> on the TiO<sub>2</sub>-photocatalyzed degradation of phenol, 2,4-dichlorophenol, and H<sub>2</sub>O<sub>2</sub> in an aerated aqueous suspension at initial pH 8.0. The rate of phenol degradation, upon the addition of 2.0 mM Na<sub>2</sub>CO<sub>3</sub>, 0.52 wt% Pt, and 2.0 mM Na<sub>2</sub>CO<sub>3</sub> plus 0.52 wt % Pt, was increased by 1.78, 3.38, and 6.63 times, respectively. Such positive effect of carbonate was also observed from a TiO<sub>2</sub> and Pt/TiO<sub>2</sub> film electrode for the photoelectrochemical oxidation of phenol, but not water. However, the rates of phenol degradation over TiO<sub>2</sub> and Pt/TiO<sub>2</sub> became decreased as carbonate concentration exceeded 5.0 and 2.0 mM, respectively. It is proposed that CO<sub>3</sub><sup>•–</sup> radicals are formed mainly from the hole oxidation of dicarbonate adsorbed on TiO<sub>2</sub>, followed by phenol degradation. At a high concentration, the CO<sub>3</sub><sup>•–</sup> radicals would recombine to a peroxocarbonate that easily decomposes into CO<sub>2</sub> and O<sub>2</sub>. The carbonate-mediated hole transfer from TiO<sub>2</sub> to phenol would incorporate with the Pt-mediated electron transfer from TiO<sub>2</sub> to O<sub>2</sub>, consequently resulting in a great improvement in the efficiency of charge separation for the reactions at interface

    Enhanced Visible-Light Photoactivity of CuWO<sub>4</sub> through a Surface-Deposited CuO

    No full text
    Several papers have shown that CuWO<sub>4</sub> is active under visible light for water oxidation at an applied potential bias and for organic degradation in an aerated aqueous suspension. In this work, we report that the observed reduction of O<sub>2</sub> on the irradiated CuWO<sub>4</sub> is a multielectron transfer process with the formation of H<sub>2</sub>O<sub>2</sub>. More importantly, the surface modification of CuWO<sub>4</sub> with 1.8 wt % of CuO can increase the activity by approximately 9 times under UV light and by 5 times under visible light, for phenol degradation in aerated aqueous suspension. The catalyst was prepared by a hydrothermal reaction between Cu­(NO<sub>3</sub>)<sub>2</sub> and Na<sub>2</sub>WO<sub>4</sub>, followed by thermal treatment at 773 K. High-resolution transmission electron microscopy revealed that triclinic CuWO<sub>4</sub> (40 nm) was covered by monoclinic CuO (4 nm). Through a combination of photo- and electrochemical measurement, a plausible mechanism responsible for the activity enhancement is proposed, involving an interfacial electron transfer from CuO to CuWO<sub>4</sub> and an interfacial hole transfer from CuWO<sub>4</sub> to CuO

    Generation of H<sub>2</sub>O<sub>2</sub> and OH Radicals on Bi<sub>2</sub>WO<sub>6</sub> for Phenol Degradation under Visible Light

    No full text
    In thermodynamics, the one-electron reduction of O<sub>2</sub> by the conduction band electrons of Bi<sub>2</sub>WO<sub>6</sub> or WO<sub>3</sub> is not allowed. However, many studies have reported that Bi<sub>2</sub>WO<sub>6</sub> is photocatalytically active for organic degradation in aerated aqueous suspension. In this work, the photocatalytic activities of Bi<sub>2</sub>WO<sub>6</sub> and WO<sub>3</sub> under visible light have been compared by using phenol degradation as a model reaction. In aerated aqueous solution, Bi<sub>2</sub>WO<sub>6</sub> and WO<sub>3</sub> were indeed active and inactive, respectively, as reported. However, by using Pt as a catalyst for O<sub>2</sub> reduction, or by using H<sub>2</sub>O<sub>2</sub> as an electron scavenger, Bi<sub>2</sub>WO<sub>6</sub> became much less active than WO<sub>3</sub>. Similar results were also obtained in the production of H<sub>2</sub>O<sub>2</sub> under visible light, and in the generation of <sup>•</sup>OH radicals under UV light, measured by a spin-trapping electron paramagnetic resonance (EPR) spectroscopy. Moreover, in the presence of catalase to completely remove H<sub>2</sub>O<sub>2</sub>, the EPR signal due to <sup>•</sup>OH radical was reduced, but not disappeared. These observations indicate that the irradiated Bi<sub>2</sub>WO<sub>6</sub> is not only active for water oxidation to <sup>•</sup>OH but also active for the two-electron reduction of O<sub>2</sub> to H<sub>2</sub>O<sub>2</sub>, the latter of which hardly occurs with the irradiated WO<sub>3</sub>

    Larger Effect of Sintering Temperature Than Particle Size on the Photocatalytic Activity of Anatase TiO<sub>2</sub>

    No full text
    Photocatalytic activity of anatase TiO<sub>2</sub> that increases with the increase of its synthesis temperature has been widely reported, but the reason for that remains incompletely understood. In this work, the positive effect of synthesis temperature, presumably due to the growth of particle size, has been examined. Three series of anatase samples with various particle sizes were prepared from the hydrolysis of TiOSO<sub>4</sub> in water at 150 °C, followed by calcinations in air. The particle size of TiO<sub>2</sub>, estimated by X-ray diffraction and N<sub>2</sub> adsorption, increased with the increase of the hydrothermal time, calcination time, and calcination temperature. For phenol photodegradation in aerated aqueous suspension, three series of the catalysts showed different correlation between the activity and particle size of TiO<sub>2</sub>. However, with the same amount of Ag<sup>+</sup> adsorbed on the oxide surface for phenol photodegradation in a N<sub>2</sub>-purged aqueous suspension, those catalysts showed activities all increasing with the particle size of TiO<sub>2</sub>, whereas at given particle size, the thermally treated TiO<sub>2</sub> was much more active than the hydrothermally treated one. These observations are discussed in terms of the solid crystallinity, surface area, exposed facets, surface hydroxyl groups, and light absorption. But, there only appears a positive correlation between the particle size, calcination temperature, and the number of surface defects, as revealed by photoluminescence spectroscopy. The increase of surface defects may facilitate separation of the photogenerated charges, consequently improving the efficiency for phenol degradation at the solid–liquid interface

    Borate-Mediated Hole Transfer from Irradiated Anatase TiO<sub>2</sub> to Phenol in Aqueous Solution

    No full text
    The development of a highly active TiO<sub>2</sub> photocatalyst for energy and environmental use is a great challenge. In this work, we report that the addition of sodium borate to an aqueous suspension of anatase TiO<sub>2</sub> at neutral pH can result in a significant enhancement in the rate of phenol degradation. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H<sub>2</sub>O<sub>2</sub> decomposition, and chromate reduction in the presence of phenol. This borate-induced rate increase for phenol degradation was determined not only by the amount of borate adsorption but also by the structure of borate species (pH effect). A (photo)­electrochemical measurement with the TiO<sub>2</sub> film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O<sub>2</sub> were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO<sub>2</sub> was negatively shifted by 81 mV. Since the hole oxidation of water to O<sub>2</sub> remained unchanged, it is proposed that a borate radical is produced, followed by regeneration through phenol oxidation. This borate-mediated hole transfer would promote the electron transfer to O<sub>2</sub> and consequently improve the efficiency of the charge separation for phenol degradation at interfaces

    Brookite vs Anatase TiO<sub>2</sub> in the Photocatalytic Activity for Organic Degradation in Water

    No full text
    Brookite is the least studied TiO<sub>2</sub>, and its photocatalytic activity higher or lower than that of anatase still remains unclear. In this work, three different model reactions have been used for the activity assessment. Phase-pure brookite and anatase were homemade at different temperatures (<i>T</i><sub>s</sub> = 200–500 °C), as confirmed by X-ray diffraction and Raman spectroscopy. For phenol oxidation in aerated aqueous solution, brookite showed an apparent activity higher and lower than that of anatase at low and high <i>T</i><sub>s</sub>, respectively. For chromate reduction in aerated aqueous suspension, the apparent activity of brookite was always lower than that of anatase. However, with the same amount of Cr­(VI) or Ag­(I) adsorbed on the oxide in water for Cr­(VI) reduction or for phenol degradation under N<sub>2</sub>, the intrinsic activities of brookite and anatase not only became similar at given <i>T</i><sub>s</sub> but also increased with the increase of <i>T</i><sub>s</sub>. Moreover, for O<sub>2</sub> reduction to H<sub>2</sub>O<sub>2</sub> in the presence of excess phenol, the BET surface area normalized activity of brookite was always higher than that of anatase, the trend of which was similar to that observed from phenol degradation in aerated aqueous solution. It is proposed that brookite has a stronger affinity to O<sub>2</sub> in water than anatase. Then, the observed difference between brookite and anatase in the apparent photocatalytic activity for phenol degradation is ascribed to the combined effect of <i>T</i><sub>s</sub>, surface area, and sorption capacity toward the dissolved O<sub>2</sub> in water

    Self-Assembled Nano-FeO(OH)/Reduced Graphene Oxide Aerogel as a Reusable Catalyst for Photo-Fenton Degradation of Phenolic Organics

    No full text
    Fabrication of visible-light-responsive, macroscopic photo-Fenton catalysts is crucial for wastewater treatment. Here, we report a facile fabrication method for nano-FeO­(OH)/reduced graphene oxide aerogels (FeO­(OH)-rGA) equipped with a stable macrostructure and a high efficiency for catalytic degradation of phenolic organics. The structure of FeO­(OH)/rGA was characterized by SEM, TEM, XPS, Raman analysis. The FeO­(OH) is the main constituent of ferrihydrite, which dispersed in the graphene aerogel with a particle size of ∼3 nm can efficiently activate H<sub>2</sub>O<sub>2</sub> to generate abundant •OH. The excellent performance of the FeO­(OH)/rGO aerogel was specifically exhibited by the outstanding catalyst activity, sustained mineralization and eminent reaction rate for phenolic organics. A synergy effect between FeO­(OH) and graphene aerogel was observed, which came from the extensive electron transfer channels and active sites of the 3D graphene aerogel and the visible-light-activated FeO­(OH) and H<sub>2</sub>O<sub>2</sub> consistently producing •OH. The FeO­(OH)/rGA could be reused for 10 cycles without a reduction in the catalytic activity and had less iron leaching, which guarantees that the active ingredient remains in the gel. Moreover, the FeO­(OH)/rGA induced photo-Fenton degradation of 4-chlorophenol under near neutral pH conditions because the tight connection of FeO­(OH) with the rGO aerogel results in less iron leaching and prevents the generation of Fe­(OH)<sub>3</sub>. The 4-chlorophenol was completely removed in 80 min with a 0.074 min<sup>–1</sup> rate constant in the FeO­(OH)-rGA/H<sub>2</sub>O<sub>2</sub> photo-Fenton system under visible-light irradiation, and mineralization rate was up to 80% after 6 h. Oxidative •OH can continuously attack 4-chlorophenol, 2,4,6-trichlorophenol and bisphenol A without selectivity. These results lay a foundation for highly effective and durable photo-Fenton degradation of phenolic organics at near neutral pH and sufficient activation of H<sub>2</sub>O<sub>2</sub> for future applications

    Improved Photocatalytic Activity of TiO<sub>2</sub> on the Addition of CuWO<sub>4</sub>

    No full text
    Various methods that aim to improve the photocatalytic activity of TiO<sub>2</sub> have been reported in the literature. Herein, we report that addition of CuWO<sub>4</sub> into the aqueous suspension of TiO<sub>2</sub> can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO<sub>4</sub> increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO<sub>4</sub> was about 2.83 times that in the absence of CuWO<sub>4</sub>. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO<sub>4</sub>/TiO<sub>2</sub> was much more stable than CuO/TiO<sub>2</sub>, due to the very high stability of CuWO<sub>4</sub> against photocorrosion. The improved activity of TiO<sub>2</sub> is not due to CuWO<sub>4</sub> and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H<sub>2</sub>O<sub>2</sub> in aqueous solution, CuWO<sub>4</sub>/TiO<sub>2</sub> was also more active than TiO<sub>2</sub>. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO<sub>2</sub> to CuWO<sub>4</sub> that facilitates the charge separation of TiO<sub>2</sub> and consequently accelerates reactions at interfaces

    Effect of a Co-Based Oxygen-Evolving Catalyst on TiO<sub>2</sub>‑Photocatalyzed Organic Oxidation

    No full text
    Cobalt phosphate (CoPi) is a promising cocatalyst for the (photo)­electrochemical oxidation of water over semiconductor electrodes in phosphate solution, but the effect of CoPi on organic oxidation reactions has been little studied. Herein, we report a compound-sensitive effect of CoPi on the TiO<sub>2</sub>-photocatalyzed oxidation of phenol, 4-chlorophenol (CP), and 2,4-dichlorophenol (DCP) in a phosphate-containing suspension at pH 7.0. A photochemical method was used to deposit Pt onto TiO<sub>2</sub> and then CoPi onto both Pt/TiO<sub>2</sub> and TiO<sub>2</sub>. In all reactions, Pt/TiO<sub>2</sub> and CoPi/TiO<sub>2</sub> were always more active and less active, respectively, than TiO<sub>2</sub>. In comparison with Pt/TiO<sub>2</sub>, CoPi/Pt/TiO<sub>2</sub> was less active for phenol oxidation but more active for CP and DCP oxidation. CoPi/Pt/TiO<sub>2</sub> was also more active than Pt/TiO<sub>2</sub> for the photocatalytic reduction of O<sub>2</sub> into H<sub>2</sub>O<sub>2</sub>. For DCP oxidation in a phosphate-free suspension at pH 7, however, CoPi/Pt/TiO<sub>2</sub> was much less active than either Pt/TiO<sub>2</sub> or TiO<sub>2</sub>, which is ascribed to the dissolution of Co<sup>2+</sup> ions that act as recombination centers. It is proposed that the Co<sup>IV</sup> species, formed by the hole oxidation of Co<sup>II/III</sup> in CoPi, are surface-bound and short-lived. They can react with a nearby adsorbed substrate (CP, DCP, and H<sub>2</sub>O<sub>2</sub>) but deactivate in the absence of either Pt (O<sub>2</sub> reduction catalyst) or phosphate (CoPi repairer). Moreover, there is a synergism between the CoPi-mediated hole transfer and the Pt-mediated electron transfer, that improves the efficiency of the charge separation and, consequently, increases the rates of O<sub>2</sub> reduction and organic oxidation
    corecore