Dengsong Zhang

Shape-controlled synthesis and catalytic application of ceria nanomaterials

Because of their excellent properties and extensive applications, ceria nanomaterials have attracted much attention in recent years. This perspective provides a comprehensive review of current research activities that focus on the shape-controlled synthesis methods of ceria nanostructures. We elaborate on the synthesis strategies in the following four sections: (i) oriented growth directed by the crystallographic structure of cerium-based materials; (ii) oriented growth directed by the use of an appropriate capping reagent; (iii) growth confined or dictated by various templates; (iv) other potential methods for generating CeO(2) nanomaterials. In this perspective, we also discuss the catalytic applications of ceria nanostructures. They are often used as active components or supports in many catalytic reactions and their catalytic activities show morphology dependence. We review the morphology dependence of their catalytic performances in carbon monoxide oxidation, water-gas shift, nitric oxide reduction, and reforming reactions. At the end of this review, we give a personal perspective on the probable challenges and developments of the controllable synthesis of CeO(2) nanomaterials and their catalytic applications.

Enhanced capacitive deionization performance of graphene/carbon nanotube composites

Graphene/carbon nanotube (GR/CNT) composites were prepared by a modified exfoliation approach and used as capacitive deionization (CDI) electrodes. SEM and TEM images demonstrate that the CNTs are successfully inserted into the GR. Nitrogen sorption analysis and electrochemical impedance spectroscopy show that the GR/CNT composites have a larger specific surface area and higher conductivity as compared with GR, which is due to the inserted CNTs inhibiting the aggregation and increasing the conductivity in the vertical direction. Through cyclic voltammetry and galvanostatic charge/discharge evaluation, we can conclude that the prepared composites have higher specific capacitance values and better stability, suggesting that the GR/CNT composite electrodes have a higher electrosorption capacity. Power and energy density analysis shows that the GR/CNT composite electrodes have higher power density and energy density and the energy density decay is relatively slow in a wide range of power as compared with GR, which indicates that the composite electrodes exhibit low energy consumption for capacitive deionization. The desalination capacity was evaluated by a batch mode electrosorptive experiment in a NaCl aqueous solution. As compared with GR and commercial activated carbon, the GR/CNT composite electrodes exhibit excellent desalination behavior, which is attributed to the improved electric conductivity and higher accessible surface area, which are quite beneficial for the electrosorption of ions onto the electrodes. The GR/CNT composites are confirmed to be promising materials for CDI electrodes.

Design of graphene-coated hollow mesoporous carbon spheres as high performance electrodes for capacitive deionization

Graphene-coated hollow mesoporous carbon spheres (GHMCSs) are rationally designed and originally used as efficient electrode materials for capacitive deionization. The GHMCSs are fabricated by a simple template-directed method using phenolic polymer coated polystyrene spheres as templates. The resulting graphene-based composites have a hierarchically porous nanostructure with hollow mesoporous carbon spheres uniformly embedded in the graphene sheets. The hierarchically porous structure of GHMCS electrodes can guarantee fast transport of salt ions, and the improved specific surface area of GHMCSs provides more adsorption sites for the formation of an electrical double layer. In addition, the graphene sheets in the GHMCSs as the interconnected conductive networks lead to fast charge transfer. The unique GHMCS structure exhibits enhanced electrochemical performance with high specific capacitance, low inner resistance and long cycling lifetime. Besides, a remarkable capacitive deionization behavior of GHMCSs with low energy consumption is obtained in a NaCl solution. The proposed carbon composite architectures are expected to lay the foundation for the design and fabrication of high-performance electrodes in the field of energy and electrochemistry.

Three-dimensional macroporous graphene architectures as high performance electrodes for capacitive deionization

In order to obtain excellent desalination behavior during the capacitive deionization (CDI) process, electrodes should provide efficient pathways for ion and electron transport. Here we open up a new opportunity to prepare high performance capacitive deionization (CDI) electrodes based on three-dimensional macroporous graphene architectures (3DMGA). The 3DMGA were fabricated by a simple template-directed method using polystyrene microspheres as sacrificial templates. The resulting 3DMGA exhibited a 3D interconnected structure with large specific surface area and high electric conductivity. The electrochemical behavior of the 3DMGA electrodes was analyzed by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. It was found that the 3DMGA showed superiority in electrosorption capacitance, low inner resistance, high reversibility and excellent stability. The power and energy density analysis further demonstrated that the 3DMGA electrode had a higher power output and lower energy consumption. According to the electrochemical measurements, the 3DMGA is quite desirable for high performance and low energy consumption capacitive deionization. The desalination capacity was evaluated by a batch mode electrosorptive experiment in a NaCl aqueous solution. An excellent desalination behavior of the 3DMGA was obtained due to the large accessible surface area, high electric conductivity and unique 3D interconnected macroporous structure. The 3DMGA was confirmed to be a promising material for CDI application.

Three-dimensional graphene-based hierarchically porous carbon composites prepared by a dual-template strategy for capacitive deionization

A three-dimensional graphene-based hierarchically porous carbon (3DGHPC) has been prepared by a dual-template strategy and explored as the electrode for capacitive deionization (CDI). The texture analyses indicate that the incorporation of porous carbon into the three-dimensional graphene (3DG) has constructed a hierarchical pore network with a bimodal pore distribution. Compared to those of the 3DG (250.3 m2 g−1, 0.49 cm3 g−1), the 3DGHPC exhibits a higher specific surface area of 384.4 m2 g−1 and an improved pore volume of 0.73 cm3 g−1. The electrosorption behavior investigated by electrochemical techniques demonstrates that the 3DGHPC electrode displays superiority in specific capacitance and cyclability, as well as electrical conductivity. The CDI performance evaluated by batch mode experiments at a direct voltage of 1.2 V in a 25 mg L−1 NaCl aqueous solution reveals that the 3DGHPC electrode presents a higher electrosorptive capacity of 6.18 mg g−1 and an increased desalination efficiency of 88.96%. The enhanced deionization properties are deduced to arise from the improved specific surface area and increased pore volume, as well as an elevated electrical conductivity.

Design of meso-TiO2@MnOx–CeOx/CNTs with a core–shell structure as DeNOx catalysts: promotion of activity, stability and SO2-tolerance

Developing low-temperature deNOx catalysts with high catalytic activity, SO2-tolerance and stability is highly desirable but remains challenging. Herein, by coating the mesoporous TiO2 layers on carbon nanotubes (CNTs)-supported MnOx and CeOx nanoparticles (NPs), we obtained a core-shell structural deNOx catalyst with high catalytic activity, good SO2-tolerance and enhanced stability. Transmission electron microscopy, X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NH3 temperature-programmed desorption have been used to elucidate the structure and surface properties of the obtained catalysts. Both the specific surface area and chemisorbed oxygen species are enhanced by the coating of meso-TiO2 sheaths. The meso-TiO2 sheaths not only enhance the acid strength but also raise acid amounts. Moreover, there is a strong interaction among the manganese oxide, cerium oxide and meso-TiO2 sheaths. Based on these favorable properties, the meso-TiO2 coated catalyst exhibits a higher activity and more extensive operating-temperature window, compared to the uncoated catalyst. In addition, the meso-TiO2 sheaths can serve as an effective barrier to prevent the aggregation of metal oxide NPs during stability testing. As a result, the meso-TiO2 overcoated catalyst exhibits a much better stability than the uncoated one. More importantly, the meso-TiO2 sheaths can not only prevent the generation of ammonium sulfate species from blocking the active sites but also inhibit the formation of manganese sulfate, resulting in a higher SO2-tolerance. These results indicate that the design of a core-shell structure is effective to promote the performance of deNOx catalysts.

High performance ordered mesoporous carbon/carbon nanotube composite electrodes for capacitive deionization

In this study, a novel ordered mesoporous carbon/carbon nanotube (OMC/CNT) composite electrode was proposed and its capacitive deionization properties were investigated in aqueous solution. The OMC/CNT composites were obtained by an organic–inorganic self-assembly route. SEM and TEM observations show the 2D hexagonally ordered mesoporous channels and nanotubular morphology of the composites. The OMC/CNT composites with high specific surface area and excellent conductivity are favorable for capacitance deionization due to the easily available electrochemical double layer behaviors. The electrochemical properties are measured by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements. Through all the measurements above, it was found that the OMC/CNT electrodes with 10 wt% of CNTs exhibit superiority in electrosorption capacitance, low resistance, high reversibility and electrochemical stability during the charge/discharge process. The comparative electrosorption properties of the OMC/CNT electrodes were also investigated in various aqueous solutions, and it is found that the electrosorption capability of the OMC/CNT electrodes depends on the hydration radius and valence of the ions in the aqueous solutions. The power and energy density were evaluated by a pair of OMC/CNT electrodes with various current loads, and this showed that the OMC/CNT composite electrodes have higher power density and lower energy density characteristics as compared with the pristine OMC electrode, which indicates that the composite electrodes exhibit low energy consumption for capacitive deionization. In desalination measurements, the OMC/CNT composite electrodes exhibit excellent desalination behavior in a flow-through apparatus, which is attributed to the low resistance and high specific surface area.

Three-dimensional hierarchical porous carbon with a bimodal pore arrangement for capacitive deionization

In this work, three-dimensional hierarchical porous carbon (3DHPC) has been prepared via a double-template strategy with a colloidal crystal SiO2 as the hard template as well as a triblock copolymer as the soft template and its capacitive deionization (CDI) behavior in a NaCl aqueous solution was investigated for the first time. The resultant 3DHPC exhibits a bimodal porous structure with numerous mesopores defined in the well-interconnected macroporous walls. The unique pore architecture, resulting from an effective combination of the mesopores and macropores, presents a higher surface area as well as an improved electronic conductivity, which are quite beneficial for a high CDI performance. The electrochemical behaviors of the prepared electrodes were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements and 3DHPC shows great superiority when used as the CDI electrode. Moreover, the dependence of the CDI performance on the ion size and valence were also investigated in salt solutions with various cations and anions. Briefly, ions with a smaller hydrated radius and higher valence ensure a stronger electrostatic force onto the 3DHPC electrode, associated with an improved electrosorption behavior. Also, the desalination performance and regeneration behavior of the electrodes were evaluated by batch mode CDI apparatus. The high desalination capacity and excellent regeneration performance render the 3DHPC electrode a great potential in the CDI process.

Graphene prepared via a novel pyridine–thermal strategy for capacitive deionization

A novel pyridine–thermal strategy for successive exfoliation and reduction of graphite oxide with the use of pyridine as the intercalating agent and dispersant is reported, and the obtained graphene exhibits a good performance in capacitive deionization.

Highly dispersed CeO2 on carbon nanotubes for selective catalytic reduction of NO with NH3

Highly dispersed CeO2 on carbon nanotubes (CNTs) is successfully prepared by a pyridine-thermal route for selective catalytic reduction (SCR) of NO with NH3. This catalyst is mainly characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction by hydrogen (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD) and X-ray photoelectron spectroscopy (XPS). The results of the XRD, TEM and TPR analysis show that the CeO2 particles on the CNTs are highly dispersed with a strong interaction between the particles and the CNTs. The NH3-TPD profiles indicate that this catalyst exhibits abundant strong acid sites. Furthermore, the O 1s XPS spectra show that the Oα/(Oα + Oβ) ratio of this catalyst is very high, which can result in more surface oxygen vacancies and therefore favor the NH3-SCR reaction. Compared with the catalysts prepared by impregnation or physical mixture methods, the catalyst prepared by the pyridine-thermal route presents the best NH3-SCR activity in the temperature range of 150–380 °C as well as favourable stability and good SO2 or H2O resistance. More than 90% of NO can be removed in the range of 250–370 °C with a desirable N2 selectivity. Moreover, the NO conversion can be kept at about 97% with the presence of SO2 or H2O at 300 °C. In addition, this catalyst shows a high catalytic activity with a NO conversion remaining constant at ca. 98% during a 16 h continuous run duration at 300 °C. Highly dispersed CeO2 on the CNTs as well as the strong interaction between the particles and the CNTs, the large amounts of strong acid sites and the high Oα/(Oα + Oβ) ratio could be ascribed to the excellent NH3-SCR performance of the catalyst prepared by the pyridine-thermal route.