Guandao Gao

Directed Synthesis of Mesoporous TiO2 Microspheres: Catalysts and Their Photocatalysis for Bisphenol A Degradation

This paper describes the fabrication of two different 3D mesoporous TiO2 microspheres via one-step solvothermal process without templates using different titanium sources. The resulting materials were characterized by XRD, FESEM, TEM, and nitrogen adsorption techniques. Their photodegradation of bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] in aqueous suspension was investigated under UV irradiation. The experimental results revealed that the photocatalytic effect of the two 3D mesoporous TiO2 microspheres was superior to the commercial P25 TiO2, and as-prepared samples as catalysts demonstrated that the smaller pore size it is, the higher the effective degradation for BPA is. Particular attention was paid to the identification of intermediates and analysis of photocatalytic degradation mechanism of BPA by HPLC-MS and HPLC-MS-MS. Five main intermediates were formed during photocatalytic degradation, and their evolution was discussed. On the basis of the evidence of oxidative intermediate formation, a detailed degradation pathway of BPA degradation by two mesoporous TiO2 microspheres photocatalysts are proposed.

Electrochemical Carbon Nanotube Filter Oxidative Performance as a Function of Surface Chemistry

An electrochemical carbon nanotube filter has been reported to be effective for the removal and electrooxidation of aqueous chemicals and microorganisms. Here, we investigate how carbon nanotube (CNT) chemical surface treatments including calcination to remove amorphous carbon, acid treatment to remove internal residual metal oxide, formation of surficial oxy-functional groups, and addition of Sb-doped SnO(2) particles affect the electrooxidative filter performance. The various CNT samples are characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) and electrochemically evaluated by cyclic voltammetry, open circuit potential versus time analysis, and electrochemical impedance spectroscopy. Voltammetry results indicate that the near CNT surface pH is at least two units lower than the bulk pH. The electrooxidative performance of the various CNT samples is evaluated with 1 mM of methyl orange (MO) in 100 mM sodium sulfate at a flow rate of 1.5 mL min(-1). At both 2 and 3 V, the efficacy of electrochemical filtration is observed to be function of CNT surface chemistry. The samples with the greatest electrooxidation were the calcinated then HCl-treated CNTs, i.e., the CNTs with the most surficial sp(2)-bonded carbon, and the Sb-SnO(2)-coated CNTs, i.e., the CNTs with the most electrocatalytic surface area. At 3 V applied voltage, these CNT samples are able to oxidize 95% of the influent MO within the liquid residence time of <1.2 s. The broader applicability of electrochemical filtration is evaluated by challenging the C-CNT-HCl and C-CNT-HNO(3) networks with various organics including methylene blue, phenol, methanol, and formaldehyde. At 3 V applied voltage, both CNTs are able to degrade a fraction of all the organics with the extent organic degradation dependent on both CNT and organic properties. The C-CNT-HCl network generally had the better oxidative performance than the C-CNT-HNO(3) network with an exception being the positively charged methylene blue. The extent of MO degradation, steady-state current, anode potential, effluent pH, and back pressure are also measured as a function of applied voltage (1-3 V) and CNT surface chemistry. Mass spectrometry of electrochemical CNT filter effluent at 2 and 3 V is utilized to evaluate plausible electrooxidation products. Energy consumption as compared to state-of-the-art electrodes and strategies to tailor the CNT surface for a specific target molecule are discussed.

Facet-dependent catalytic activity of nanosheet-assembled bismuth oxyiodide microspheres in degradation of bisphenol A.

Photocatalysts with different exposed facets often exhibit different photochemical performances, but the underlying mechanisms are not fully understood. In this study, we synthesized two nanosheet-assembled bismuth oxyiodide (BiOI) microspheres with exposed (110) and (001) facets, respectively, to further investigate facet-dependent photocatalytic activity. Our experimental results showed that the BiOI microspheres with exposed (110) facets exhibited much greater catalytic activity than the BiOI microspheres with exposed (001) facets in the degradation of bisphenol A under visible light irradiation. Density functional theory calculation revealed that the (110) facets can adsorb a greater amount of O2 and, thus, form more O2(•-) and (•)OH radicals than the (001) facets. The electron spin resonance spectroscopy and radical scavenging experiments verified that the BiOI microspheres with exposed (110) facets could produce a greater amount of O2(•-) radicals than the BiOI microspheres with exposed (001) facets, and more importantly, between the two BiOI products, only the BiOI microspheres with exposed (110) facets could generate (•)OH radicals directly. The facet-dependent radical formation mechanisms were previously unidentified. The findings of this study may have important implications for the understanding of the facet-dependent photochemical performance of photocatalysts and the design of novel catalytic materials with inorganic nanostructures.

Carbon nanotube membrane stack for flow-through sequential regenerative electro-Fenton.

Electro-Fenton is a promising advanced oxidation process for water treatment consisting a series redox reactions. Here, we design and examine an electrochemical filter for sequential electro-Fenton reactions to optimize the treatment process. The carbon nanotube (CNT) membrane stack (thickness ∼ 200 μm) used here consisted of 1) a CNT network cathode for O2 reduction to H2O2, 2) a CNT-COOFe(2+) cathode to chemical reduction H2O2 to (•)OH and HO(-) and to regenerate Fe(2+) in situ, 3) a porous PVDF or PTFE insulating separator, and 4) a CNT filter anode for remaining intermediate oxidation intermediates. The sequential electro-Fenton was compared to individual electrochemical and Fenton process using oxalate, a persistent organic, as a target molecule. Synergism is observed during the sequential electro-Fenton process. For example, when [DO]in = 38 ± 1 mg L(-1), J = 1.6 mL min(-1), neutral pH, and Ecell = 2.89 V, the sequential electro-Fenton oxidation rate was 206.8 ± 6.3 mgC m(-2) h(-1), which is 4-fold greater than the sum of the individual electrochemistry (16.4 ± 3.2 mgC m(-2) h(-1)) and Fenton (33.3 ± 1.3 mgC m(-2) h(-1)) reaction fluxes, and the energy consumption was 45.8 kWh kgTOC(-1). The sequential electro-Fenton was also challenged with the refractory trifluoroacetic acid (TFA) and trichloroacetic acid (TCA), and they can be transferred at a removal rate of 11.3 ± 1.2 and 21.8 ± 1.9 mmol m(-2) h(-1), respectively, with different transformation mechanisms.

Facile preparation of hierarchical Nb2O5 microspheres with photocatalytic activities and electrochemical properties

Hierarchical flower-like Nb2O5 microspheres have been prepared via a facile hydrothermal approach without any additives. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to clarify the structure and morphology of the Nb2O5 microspheres. Structure and morphology evolution mechanisms have been proposed for the hierarchical structure in detail. During the symmetric Ostwald ripening, the resultants formed aggregates composed of two-dimensional nanoflakes as building blocks. Photocatalytic activity of the as-prepared Nb2O5 microspheres was evaluated by the photodegradation of Rhodamine B (RhB), and over 90% of RhB was degraded within 30 min under the irradiation of UV light. The as-prepared Nb2O5 exhibits higher photocatalytic activity than commercial Degussa P25. Moreover, Nb2O5 was tested as an anode material of lithium-ion batteries, which displayed high reversibility and excellent rate stability at a current density of 50 mA g−1.

Doped Carbon Nanotube Networks for Electrochemical Filtration of Aqueous Phenol: Electrolyte Precipitation and Phenol Polymerization

Electrochemical filtration with anodic carbon nanotube (CNT) networks is reported to be effective for chemical and microbiological water treatment. Here, we investigate how CNT doping affects the electrochemical filtration performance toward the remediation of aromatic wastewaters. Purified and well-characterized undoped (C-CNT), boron-doped (B-CNT), and nitrogen-doped (N-CNT) anodic carbon nanotube networks are challenged with aqueous phenol in a sodium sulfate electrolyte. Steady-state current and effluent total organic carbon (TOC) measurements are utilized to evaluate the oxidative performance as a function of voltage and electrolysis time. In terms of steady-state TOC removal, at an applied voltage of 3 V all three anodic CNT networks are able to remove approximately 7 to 8 mgC L(-1) of the influent TOC within the ~1 s liquid residence time of the electrochemical filter. The anodic CNT networks are partially passivated over the 5 h electrolysis time with the B-CNT network displaying the least passivation. The extent of passivation was observed to be inversely correlated to the CNT work function. SEM, XPS, and TGA of the electrolyzed CNT networks are used to identify the two primary passivation mechanisms of electrochemical phenols polymerization and electrochemical electrolyte precipitation. In agreement with chronoamperometry results, the B-CNT network has the lowest extent of passivating polymer and precipitate formation. The precipitant is determined to likely be sodium persulfate or carbonate and is removed with a simple acidic water wash. The polymer is determined to likely be polyphenylene oxide and is partially removed with the wash. All three anodic CNT networks display potential for energy efficient electrochemical filtration of aromatic wastewaters and the B-CNT are determined to be the most resistant to passivation.

Synthesis of Mesoporous Wall‐Structured TiO2 on Reduced Graphene Oxide Nanosheets with High Rate Performance for Lithium‐Ion Batteries

Mesoporous wall-structured TiO2 on reduced graphene oxide (RGO) nanosheets were successfully fabricated through a simple hydrothermal process without any surfactants and annealed at 400 °C for 2 h under argon. The obtained mesoporous structured TiO2 -RGO composites had a high surface area (99 0307 m(2)  g(-1)) and exhibited excellent electrochemical cycling (a reversible capacity of 260 mAh g(-1) at 1.2 C and 180 mAh g(-1) at 5 C after 400 cycles), demonstrating it to be a promising method for the development of high-performance Li-ion batteries.

A novel integrated active capping technique for the remediation of nitrobenzene-contaminated sediment.

The objective of this study was to develop a novel integrated active capping system and to investigate its efficiency in the remediation of nitrobenzene-contaminated sediment. An integrated Fe(0)-sorbent-microorganism remediation system was proposed as an in situ active capping technique to remediate nitrobenzene-contaminated sediment. In this system, nitrobenzene was reduced to aniline by Fe(0), which has a much better biodegradability. The sorption capacity and structural properties of cinder was measured to examine its applicability as the sorbent and matrix for this integrated capping system. Indigenous microorganisms from Songhuajiang River sediment, which was contaminated by nitrobenzene and aniline in Chinese Petrochemical Explosion in Jilin, were acquired one month after the explosion and used in this active capping system to degrade nitrobenzene and its reduced product, aniline. A bench-scale remediation experiment was conducted on a mimicked nitrobenzene-contaminated sediment to investigate the efficiency of the integrated capping system and the synergistic effects of the combined components in the active capping system. The results show that this integrated active capping system can effectively block the release of target pollutants into the upper-layer water and remove the compounds from the environment.

CNT–PVDF composite flow-through electrode for single-pass sequential reduction–oxidation

In this study, a five-layer electrochemical CNT–PVDF filter that is thin, flexible, and stable is demonstrated to be effective and efficient for single-pass nitrobenzene mineralization by sequential reduction–oxidation. Key to the technology is development of a CNT–PVDF membrane that is mechanically stable, electrically conductive, and non-Faradaic, which is used to prevent CNT release from and electrochemical degradation of the Faradaic CNT electrodes. A five-layer electrochemical filter: (1) conductive & protective CNT–PVDF, (2) Faradaic CNT electrode, (3) insulating PVDF separator, (4) Faradaic CNT electrode, and (5) conductive & protective CNT–PVDF is formed by mechanical press. At a total cell potential of 4 V, the sequential electrochemical reduction–oxidation process is able to reduce >99% of the influent nitrobenzene to aniline then oxidize >80% of the aniline to non-aromatic products (mostly carbon dioxide) in a single-pass (∼5 s hydraulic residence time). The mineralization current efficiency is ∼20%, total cell potential is completely (>99%) distributed to the two electrodes, and the energy requirement is ∼36000 kJ per mole of nitrobenzene degraded.

Transformation of graphene oxide by ferrous iron: Environmental implications.

Abiotic transformation of graphene oxide (GO) in aquatic environments can markedly affect the fate, transport, and effects of GO. The authors observed that ferrous iron (Fe[II])-an environmentally abundant, mild reductant-can significantly affect the physicochemical properties of GO (examined by treating aqueous GO suspensions with Fe(2+) at room temperature, with doses of 0.032 mM Fe(2+)  per mg/L, 0.08 mM Fe(2+)  per mg/L, and 0.32 mM Fe(2+)  per mg/L GO). Microscopy data showed stacking of GO nanosheets on Fe(2+) treatment. Spectroscopy evidence (X-ray diffraction, Fourier transform infrared transmission, Raman and X-ray photoelectron spectroscopy) showed significant changes in GO surface O-functionalities, in terms of loss of epoxy and carbonyl groups but increase of carboxyl group. The reduction mechanisms were verified by treating model organic molecules (styrene oxide, p-benzoquinone, and benzoic acid) resembling O-containing fragments of GO macromolecules with Fe(2+). With sedimentation and adsorption experiments (using bisphenol A as a model contaminant), the authors demonstrated that Fe(2+) reduced GOs still maintained relatively high colloidal stability, whereas their adsorption affinities were significantly enhanced. Thus, reduction of GO by mild reductants might be of greater environmental concerns than by stronger reducing agents (e.g., N2H4 and S(2-)), because the latter can result in too significant losses of surface O-functionalities and colloidal stability of GO. This interesting aspect should be given consideration in the risk assessment of GO.