Hongjun Wu

STEP wastewater treatment: a solar thermal electrochemical process for pollutant oxidation.

A solar thermal electrochemical production (STEP) pathway was established to utilize solar energy to drive useful chemical processes. In this paper, we use experimental chemistry for efficient STEP wastewater treatment, and suggest a theory based on the decreasing stability of organic pollutants (hydrocarbon oxidation potentials) with increasing temperature. Exemplified by the solar thermal electrochemical oxidation of phenol, the fundamental model and experimental system components of this process outline a general method for the oxidation of environmentally stable organic pollutants into carbon dioxide, which is easily removed. Using thermodynamic calculations we show a sharply decreasing phenol oxidation potential with increasing temperature. The experimental results demonstrate that this increased temperature can be supplied by solar thermal heating. In combination this drives electrochemical phenol removal with enhanced oxidation efficiency through (i) a thermodynamically driven decrease in the energy needed to fuel the process and (ii) improved kinetics to sustain high rates of phenol oxidation at low electrochemical overpotential. The STEP wastewater treatment process is synergistic in that it is performed with higher efficiency than either electrochemical or photovoltaic conversion process acting alone. STEP is a green, efficient, safe, and sustainable process for organic wastewater treatment driven solely by solar energy.

STEP organic synthesis: an efficient solar, electrochemical process for the synthesis of benzoic acid

This study presents the first demonstration of STEP for organic synthesis. The synthesis of benzoic acid was fully driven by solar energy without the input of any other forms of energy. STEP (the Solar Thermal Electrochemical Process) was established to drive chemical reactions by using solar heat to minimize the energy and maximize the rate of a growing portfolio of electrolysis reactions. The use of solar energy can minimize or eliminate the carbon footprint associated with the production of societal staples. To date this has included STEP chemistries for the high solar efficiency generation of hydrogen fuels, carbon dioxide splitting to generate fuels and graphite, hematite (iron ore) splitting for iron metal, of lime from limestone, and the production of bleach, magnesium and other useful chemistries. Benzoic acid is produced by the electrolysis of toluene. Solar thermal and solar electrical energy synergistically drive the electrolysis; solar thermal decreases the requisite electrolysis voltage, and improves the kinetics, selectivity and yield of the reaction, while solar electrical current drives the electrolysis. The low electrolysis potential inhibits over-oxidation of the product. In this STEP organic synthesis of benzoic acid at an applied photopotential of 3 V the reaction conversion of benzoic acid is 3.9% at a temperature of 30 °C, 12.4% at 60 °C, and 32.0% at 90 °C. The results demonstrate that the STEP is suitable for an efficient synthesis of benzoic acid from toluene.

Rational design of binder-free noble metal/metal oxide arrays with nanocauliflower structure for wide linear range nonenzymatic glucose detection

One-dimensional nanocomposites of metal-oxide and noble metal were expected to present superior performance for nonenzymatic glucose detection due to its good conductivity and high catalytic activity inherited from noble metal and metal oxide respectively. As a proof of concept, we synthesized gold and copper oxide (Au/CuO) composite with unique one-dimensional nanocauliflowers structure. Due to the nature of the synthesis method, no any foreign binder was needed in keeping either Au or CuO in place. To the best of our knowledge, this is the first attempt in combining metal oxide and noble metal in a binder-free style for fabricating nonenzymatic glucose sensor. The Au/CuO nanocauliflowers with large electrochemical active surface and high electrolyte contact area would promise a wide linear range and high sensitive detection of glucose with good stability and reproducibility due to its good electrical conductivity of Au and high electrocatalytic activity of CuO.

Effect of molten carbonate composition on the generation of carbon material

Carbon dioxide, CO2, is thought to be a main culprit leading to global climate change and a wide variety of strategies have been proposed to reduce atmospheric CO2 levels. Here, CO2 is captured and subsequently electrochemically split into carbon materials in an electrolyzer comprising a eutectic mixture of carbonates, an Fe cathode and a Ni anode, at 600 °C and current densities of 50, 100, 200 mA cm−2. SEM, EDS, XRD and BET are employed to analyze the morphology, elemental composition, crystal structure as well as the BET surface area of the synthetic cathodic products. In addition, coulomb efficiency under different electrolytic conditions is measured via the comparison between moles of formed carbon product and the Faradays of charge passed during the electrolysis reaction. This paper investigated the effect of molten carbonate compositions on carbon product generation, and confirmed the visible dependence of produced carbon on the electrolytes.

Modified hierarchical TiO2 NTs for enhanced gas phase photocatalytic activity

In this paper, three kinds of noble metal nanoparticles (NMNs) were successfully loaded on hierarchical TiO2 nanotube arrays (TiO2 NTs) to improve photocatalytic (PC) activity of gas phase pollutants. The hierarchical TiO2 NTs, with unique top-nanoporous and bottom-nanotubular structure, were prepared through a facile two-step anodization method, and then the noble metal nanoparticles were loaded on the TiO2 NTs by means of a photo-reduction method. The gas phase photocatalytic activity of TiO2 NTs and NMNs/TiO2 NTs were estimated by decomposition of gaseous methanol. The formation of Schottky junctions between TiO2 NTs and NMNs significantly improved the PC due to they could significantly accelerate the electron transfer and thus reduction of the recombination of photogenerated electrons and holes.

Solar-driven thermo- and electrochemical degradation of nitrobenzene in wastewater: Adaptation and adoption of solar STEP concept.

The STEP concept has successfully been demonstrated for driving chemical reaction by utilization of solar heat and electricity to minimize the fossil energy, meanwhile, maximize the rate of thermo- and electrochemical reactions in thermodynamics and kinetics. This pioneering investigation experimentally exhibit that the STEP concept is adapted and adopted efficiently for degradation of nitrobenzene. By employing the theoretical calculation and thermo-dependent cyclic voltammetry, the degradation potential of nitrobenzene was found to be decreased obviously, at the same time, with greatly lifting the current, while the temperature was increased. Compared with the conventional electrochemical methods, high efficiency and fast degradation rate were markedly displayed due to the co-action of thermo- and electrochemical effects and the switch of the indirect electrochemical oxidation to the direct one for oxidation of nitrobenzene. A clear conclusion on the mechanism of nitrobenzene degradation by the STEP can be schematically proposed and discussed by the combination of thermo- and electrochemistry based the analysis of the HPLC, UV-vis and degradation data. This theory and experiment provide a pilot for the treatment of nitrobenzene wastewater with high efficiency, clean operation and low carbon footprint, without any other input of energy and chemicals from solar energy.

UV-light aided photoelectrochemical synthesis of Au/TiO2 NTs for photoelectrocatalytic degradation of HPAM

TiO2 nanotube arrays (TiO2 NTs) loaded with Au nanoparticles were fabricated as an electrode for enhanced photoelectrocatalytic activity toward partially hydrolyzed polyacrylamide (HPAM) degradation. The honeycombed TiO2 NTs were prepared by a two-step anodization method and modified by Au nanoparticles via an UV-light aided photoelectrochemical process. The photoelectrocatalytic (PEC) activities of TiO2 NTs and Au/TiO2 NTs were characterized by decomposition of HPAM. The results showed that Au/TiO2 NTs exhibit much higher PEC activity than that of pristine TiO2 NTs. The size and amount of Au nanoparticles can be well controlled by adjusting the concentrations of metal ion precursor in the photoelectrochemical process. Structures, element components and morphologies of TiO2 NTs and Au/TiO2 NTs were measured by XRD, XPS, EDS and FESEM. Photoresponse of the as prepared samples were evaluated by UV-vis DRS. The UV-light aided photoelectrochemical synthesis of Au/TiO2 NTs contributes to the rational design of the plasmonic photocatalytic composite material based on wide band gap metal oxides for photoelectrochemical applications on degradation of polymers. As a consequence, an optimum loading amount of Au were obtained with weight percentage of 1.24 wt%.

Platinum decorated hierarchical top-porous/bottom-tubular TiO2 arrays for enhanced gas phase photocatalytic activity

In this communication, Pt nanoparticles (NPs) were successfully loaded on hierarchical TiO2 nanotube arrays (TiO2 NTs) for efficient decomposition of gas phase pollutants. The loading of Pt NPs on TiO2 NTs significantly enhanced the photocatalytic activity due to reduction of the recombination of photogenerated electrons and holes.

A novel route to synthesize carbon spheres and carbon nanotubes from carbon dioxide in a molten carbonate electrolyzer

The process of molten salt CO2 capture and electrochemical conversion provides us with a new way to close the present carbon cycle and mitigate global climate change by transforming the greenhouse gas CO2 into carbonaceous fuels or chemicals. In this paper, carbon spheres and carbon nanotubes that can be used as a societal resource to serve mankind are synthesized from CO2 in diverse electrolyte composites with inexpensive metallic electrodes. Carbon products, subsequent to electrolysis, are characterized by EDS, SEM, TEM, Raman, TGA, FTIR, BET and XRD to reveal the elemental composition and morphological and structural features. The results demonstrate that Li–Ca–Na and Li–Ca–K carbonate electrolytes favor carbon sphere formation rather than carbon nanotube formation, and in particular, K2CO3 shows enhanced interference with carbon nanotube growth. In contrast, Li–Ca–Ba and Li–Ba carbonate composites present an increase in the carbon nanotube fraction. Additionally, CNTs generated from Li–K, Li–Ba and Li–Ca–Ba present a different diameter. In this way, the CO2-derived carbon products of carbon spheres and carbon nanotubes could be alternatively synthesized through the appropriate regulation of the electrolyte composition.

Effect of BaCO3 addition on the CO2-derived carbon deposition in molten carbonates electrolyzer

Electrolysis of molten carbonates provides an effective means to achieve the direct conversion of the greenhouse gas carbon dioxide to readily available carbonaceous fuel or chemicals. In this study, CO2 was electrolyzed in a Li–Na–K ternary carbonates mixture using galvanized Fe as the cathode and nickel as the anode. In particular, the effect of BaCO3 on the generation of carbon was studied. The results demonstrate that BaCO3 addition facilitates the deposition of compact carbon materials. Moreover, decreasing the temperature and increasing the current density lead to the formation of loosened carbon products. Though Li–Na–K–Ba electrolysis exhibits a higher cell voltage than that using Li–Na–K carbonates, BaCO3 addition can significantly ease anode corrosion and the corresponding corrosion inhibiting action is discussed. In summary, a facile CO2 absorption and a mitigatory nickel anode corrosion were observed when BaCO3 was dissolved in Li–Na–K carbonates and further investigation is expected.