Guoqing Guan

Nanostructured catalysts for electrochemical water splitting: current state and prospects

Hydrogen is an ideal candidate for the replacement of fossil fuels in the future due to zero emission of carbonaceous species during its utilization. Water electrolysis is a dependable link of primary renewable energy and stable hydrogen energy. In this work, the fundamentals of water electrolysis, current popular electrocatalysts developed for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in liquid electrolyte water electrolysis are reviewed. The main HER catalysts include noble metals, non-noble metals and composites, noble metal-free alloys, metal carbides, chalcogenides, phosphides and metal-free materials while the OER catalysts are focused on efficient Co-based, Ni-based materials and layered double hydroxide (LDH) materials. The strategies to improve catalytic activity, long-term durability and endurance to electrochemical erosion are introduced. The main challenges and future prospects for the further development of electrodes for water electrolysis are discussed. It is expected to give guidance for the development of novel low-cost nanostructured electrocatalysts for electrochemical water splitting.

LaMnO3/CdS nanocomposite: a new photocatalyst for hydrogen production from water under visible light irradiation

Abstract A LaMnO 3 /CdS nanocomposite was developed as a new type of visible light sensitive photocatalyst that can produce hydrogen from water containing electron donors. The nanocomposite catalyst, prepared by a reverse micelle method, was found to show high photocatalytic activity, as compared to that of CdS; on the other hand, LaMnO 3 showed no photocatalytic activity. The proposed energy diagrams for the LaMnO 3 /CdS composite, based on the results obtained from photoemission yield measurements, suggest that photogenerated holes in the valence band of CdS could move to that of LaMnO 3 , thus separating photogenerated charge-carriers and improving the activity.

Separation of low concentration of cesium ion from wastewater by electrochemically switched ion exchange method: Experimental adsorption kinetics analysis

A series of experiments were performed to evaluate the continuous separation of cesium based on an electrochemically switched ion exchange (ESIX) process using a diaphragm-isolated reactor with two identical nickel hexacyanoferrate/porous three-dimensional carbon felt (NiHCF/PTCF) electrodes as working electrodes. The effects of applied potential, initial concentrations and pH values of the simulation solutions on the adsorption of cesium ion were investigated. The adsorption rate of cesium ion in the ESIX process was fitted by a pseudo-first-order reaction model. The experiments revealed that the introduction of applied potential on the electrodes greatly enhanced the adsorption/desorption rate of Cs(+) and increased the separation efficiency. H(3)O(+) was found to play a dual role of electrolyte and competitor, and the adsorption rate constant showed a curve diversification with an increase in pH value. Also, it was found that the electrochemically switched adsorption process of Cs(+) by NiHCF/PTCF electrodes proceeded in two main steps, i.e., an ESIX step with a fast adsorption rate and an ion diffusion step with a slow diffusion rate. Meanwhile, the NiHCF/PTCF film electrode showed adsorption selectivity for Cs(+) in preference to Na(+).

Ultrastable coaxial cable-like superhydrophobic mesh with self-adaption effect: facile synthesis and oil/water separation application

A coaxial cable-like stainless steel wire@conducting polymer (polyaniline or polypyrrole) with special hierarchical micro–nanoscale roughness was successfully synthesized using a facile, inexpensive and controlled pulse electro-polymerization method (PPM). Conjugated with stearoyl chloride (SC) through an acylation reaction, the obtained meshes exhibit both superhydrophobic and superoleophilic properties with a water contact angle of 154° and an oil contact angle of 0°, which was successfully applied in separating a series of oil/water mixtures with a separation efficiency of over 98%. The separation efficiency also remains high even after 25 reuse cycles (99.90% for a hexane/water mixture). More importantly, owing to their self-adaption aging protection and antioxidant properties, the obtained meshes retain ultrastable superhydrophobicity and high separation efficiency in long-term storage, ultrasonic treatment, and even under extreme environment conditions of strong acidic, alkaline, hyperhaline and oxidizing solutions. In addition, electrochemical measurements indicate that the corrosion dynamic rate of the superhydrophobic mesh is 1/60 lower than that of bare stainless steel mesh. Therefore, this work provides a novel method to fabricate a high quality superhydrophobic and superoleophilic mesh for effective oil/water separation.

Photocatalytic hydrogen production from water over a LaMnO3/CdS nanocomposite prepared by the reverse micelle method

A LaMnO3/CdS nanocomposite was prepared by a reverse micelle method and examined for its photocatalytic activity for H2 production from water containing electron donors (Na2S and Na2SO3) under visible light irradiation (λ > 420 nm). The prepared LaMnO3/CdS nanocomposite of 10–20 nm particle size showed 6 times higher photocatalytic activity than that of CdS; on the other hand, LaMnO3 showed no photocatalytic activity. It was also found that LaMnO3/CdS can continuously produce hydrogen from water in the presence of electron donors for more than 200 h. In order to account for the improvement in the activity, the valence edge potentials of CdS and LaMnO3 were investigated by photoemission yield measurements. The proposed energy band scheme for the LaMnO3/CdS composite suggests that photogenerated holes in the valence band of CdS can move to that of LaMnO3 and react with Na2S while photogenerated electrons remain in the conduction band of CdS and react with water to produce H2; this charge-carrier separation is responsible for the improved activity.

Separation of nitrogen from oxygen using a titanosilicate membrane prepared on a porous α-alumina support tube

A zeolite membrane was formed on a porous α-alumina support tube by hydrothermal reaction using a solution which was appropriate for the synthesis of ETS-4 zeolite. The morphology and x-ray diffraction pattern of the top surface of the support tube were consistent with those of the ETS-4 zeolite. The permeances to single-component H2, CO2, O2, N2, and CH4 were in the range (0.8–2.5)×10− 8 mol m− 2 sec− 1 Pa− 1, while those to n-C4H10 and iso-C4H10 were less than 10− 11 mol m− 2 sec− 1 Pa− 1. This suggests that the membrane functioned as a molecular sieve with pores smaller than 0.4 nm. At a permeation temperature of 310K, the ideal N2/O2 separation factor was 2.8, and the N2/O2 separation factor was 2.3 and 3.2 for a mixture of and 4, respectively. Thus, the membrane is nitrogen-selective and can be used to enrich the oxygen level in air, which is fed on the pressurized side, without much loss of compression energy.

GAS PERMEATION PROPERTIES OF ION-EXCHANGED LTA-TYPE ZEOLITE MEMBRANES

NaA-type zeolite membranes were prepared on a 1.7-mm i.d. porous α-alumina tube by repeating a hydrothermal reaction. The membranes, which were prepared by four repetitions, were ion-exchanged with Ca2+ and K+, and the permeation for single-component and mixed gases were investigated at 35°C. The NaA-type membrane showed a hydrogen permeance of (0.8–2.6) × 10−8 mol · m−2 s−1 Pa−1 and a hydrogen/n-butane ideal separation factor of 12–15. The permeability properties of the membrane for binary mixtures were dependent on the size of the permeating molecules and their adsorptivity to the zeolite. The KA-type zeolite membrane showed a higher ideal separation factor for a hydrogen/nitrogen system than did the NaA- and CaA-type zeolite membranes. The pores of the CaA-type zeolite membrane were larger than those in the NaA- and KA-type zeolite membranes, and showed higher permeances to n-butane and i-butane. All the zeolite membranes contained non-zeolitic pores, which decreased the selectivity of permeation.

Facile preparation of ion-imprinted composite film for selective electrochemical removal of nickel(II) ions.

A facile unipolar pulse electropolymerization (UPEP) technique is successfully applied for the preparation of ion-imprinted composite film composed of ferricyanide-embedded conductive polypyrrole (FCN/PPy) for the selective electrochemical removal of heavy metal ions from wastewater. The imprinted heavy metal ions are found to be easily removed in situ from the growing film only by tactfully applying potential oscillation due to the unstable coordination of FCN to the imprinted ions. The obtained Ni(2+) ion-imprinted FCN/PPy composite film shows fast uptake/release ability for the removal of Ni(2+) ions from aqueous solution, and the adsorption equilibrium time is less than 50 s. The ion exchange capacity reaches 1.298 mmol g(-1) and retains 93.5% of its initial value even after 1000 uptake/release cycles. Separation factors of 6.3, 5.6, and 6.2 for Ni(2+)/Ca(2+), Ni(2+)/K(+), and Ni(2+)/Na(+), respectively, are obtained. These characteristics are attributed to the high identification capability of the ion-imprinted composite film for the target ions and the dual driving forces resulting from both PPy and FCN during the redox process. It is expected that the present method can be used for simple preparation of other ion-imprinted composite films for the separation and recovery of target heavy metal ions as well.

Selective catalytic conversion of bio-oil over high-silica zeolites.

Four high silica zeolites, i.e., HSZ-385, 890, 960, and 990 were utilized for the selective catalytic conversion of bio-oil from Fallopia japonica to certain chemicals in a fixed-bed reactor. The Beta-type HSZ-960 zeolite showed the highest selectivity to hydrocarbons, especially to aromatics as well as PAH compounds with the lowest unwanted chemicals while HSZ-890 showed high selectivity to aromatics. NH3-Temperature Programmed Desorption (TPD) analysis indicated that different amounts of acid sites in different zeolites determined the catalytic activity for the oxygen removal from bio-oil, in which the acid sites at low temperature (LT) region gave more contribution within the utilized temperature region. The reusability test of HSZ-960 showed the stability of hydrocarbons yield at higher temperature due to the significant contribution of coke gasification which assisted further deoxygenation of bio-oil. These results provide a guidance to select suitable zeolite catalysts for the upgrading of bio-oil in a practical process.

Facile preparation of electroactive amorphous α-ZrP/PANI hybrid film for potential-triggered adsorption of Pb2+ ions

An electroactive hybrid film composed of amorphous α-zirconium phosphate and polyaniline (α-ZrP/PANI) is controllably synthesized on carbon nanotubes (CNTs) modified Au electrodes in aqueous solution by cyclic voltammetry method. Electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM) and X-ray power diffraction (XRD) analysis are applied for the evaluation of the synthesis process. It is found that the exfoliated amorphous α-ZrP nanosheets are well dispersed in PANI and the hydrolysis of α-ZrP is successfully suppressed by controlling the exfoliation temperature and adding appropriate supporting electrolyte. The insertion/release of heavy metals into/from the film is reversibly controlled by a potential-triggered mechanism. Herein, α-ZrP, a weak solid acid, can provide an acidic micro-environment for PANI to promote the electroactivity in neutral aqueous solutions. Especially, the hybrid film shows excellent potential-triggered adsorption of Pb(2+) ion due to the selective complexation of Pb(2+) ion with oxygen derived from P-O-H of α-ZrP. Also, it shows long-term cycle stability and rapid potential-responsive adsorption/desorption rate. This kind of novel hybrid film is expected to be a promising potential-triggered ESIX material for separation and recovery of heavy metal ions from wastewater.