In this study, we develop a facile hydrothermal route to build a nanostructure of porous CeO2 nanobundles with hierarchical architectures by the design and growth of anisotropic CeO2 precursors in a carbonate-assisted formaldehyde hydrothermal system without templates and surfactants. The key to this synthesis is to control the formation of their anisotropic CeO2 precursors with formate and carbonate promoted by carbonate and ammonium ions during the hydrothermal treatment. The as-prepared CeO2 nanobundles have a hierarchical porous structure assembled by numerous nanorods with a small diameter, and they show much higher catalytic activities for CO oxidation compared with CeO2 nanorods, nanowires and nanoparticles prepared by formaldehyde-assisted hydrothermal treatment and traditional precipitation methods. The outstanding catalytic performance for the nanobundles is attributed to their excellent physicochemical properties such as larger lattice cell parameters, much larger surface areas and the best redox behaviour of surface oxygen on the CeO2 surface.

The present work was undertaken to investigate the influence of binary Ce–Mn species confined in carbon nanotube (CNT) channels on the catalytic properties for oxidative dehydrogenation of ethylbenzene (EB) to styrene utilizing CO2 as a mild oxidant. 7.0 wt% of Ce–Mn oxides were filled in CNTs by an incipient wetness impregnation method. The texture and physicochemical properties of the as-prepared materials were characterized by TEM, XRD, H2-TPR, Raman and XPS. The binary Ce–Mn oxides confined in CNTs exhibited much higher catalytic activities than those of single Ce or Mn. Among the catalysts tested, the sample CeMn-in-CNTs with Mn/(Ce + Mn) = 0.375 exhibited the highest conversion of EB and selectivity for styrene. The superior catalytic performance could be attributed to the fact that the doped Mn species could accelerate the oxidation of Ce3+ towards Ce4+ by the reduction of the high valence of Mn species, the occurrence of surface oxygen vacancy and activated oxygen resulting from migration of Mn, and the change of equilibrium caused by coupling a reverse water gas shift reaction.

As metal-free catalysts, the functionalization of multi-walled carbon nanotubes (MWCNTs) can usually provide the activated sites for oxidative dehydrogenation (ODH). In this paper a novel functionalization of MWCNTs has been proposed via an alkali-assisted hydrothermal treatment on the sectional defects produced by ball milling. The as-prepared hydroxylated MWCNTs exhibit twice the catalytic activity of MWCNTs without ball milling, because the ball-milled treatment leads to the production of more sectional defects located on exposed edges or terminals of MWCNTs and these defects can induce the creation of more hydroxyl groups by an alkali-assisted hydroxylation. The research on the reaction mechanism reveals that the ODH of ethylbenzene (EB) in the presence of CO2 for the hydroxylated MWCNTs takes on a possible mechanism of two-step dehydrogenation by coupling a reverse water–gas shift reaction and CO2 may oxidize hydroxyl groups into carbonyl groups to maintain the catalytic cycle.

The well-defined structure and favorable surface properties of adsorbent materials are essential for the adsorption of pollutants in water. Highly dispersed Ce–Fe/graphene hybrid materials with the size of about 1–2 nm nanoparticles (NPs) and the high specific surface area of 322 m2 g−1 are synthesized by using reduced graphene oxide (RGO). The small particle size is due to the inhibition of different species on crystal growth and the high specific surface area is due to the monolayer dispersive action of reduced graphene. The obtained Ce–Fe/RGO hybrid material exhibits the superior adsorption ability for Congo red (CR) in water. Research results suggest that the driving force for the adsorption is the electrostatic action between adsorbent materials with positive electricity and CR with negative electricity. The Fe doping modifies the electronegativity of adsorbent materials and the CR molecule was firmly adsorbed on the hybrid material by the bidirectional force. The new hybrid material displays the high adsorption capacity of 179.5 mg g−1 for the CR in water, suggesting the potential use of Ce–Fe/RGO hybrid materials in water treatment.

Mesoporous CeO2 nanobelts have been synthesized by a facile hydrothermal route via controlling cationic type and concentration of alkali without any surfactant or template. The key synthesis of CeO2 nanobelts is forming the beltlike precursors in the presence of enough NaOH (Na/Ce molar ratio ⩾16.3 at 120 °C) during hydrothermal process. The enough OH− ions induce the two steps of Cannizzaro disproportionation reaction to result in conversion of formate partly into carbonate, and Na+ ions with small ionic radius allow coexistence of carbonate and formate in structure and accordingly are favorable to anisotropic growth of beltlike precursors. The increased hydrothermal temperature can promote the formation of carbonates and the minimal required Na/Ce molar ratio is decreased from 16.3 at 120 °C to 10.8 at 140 °C. When Na+ is substituted by the same concentration of K+ or NH4+, the obtained precursor products are finally nanowires or irregular morphology. XRD measurement proves that the sodium ions do not enter the crystalline frame of CeO2 nanobelts and can be easily removed by simple washing with deionized water. After washing, CeO2 nanobelts with enlarged mesoporous pores show superior catalytic performance for CO oxidation compared with CeO2 nanoparticles prepared with traditional methods.Graphical abstractMesoporous CeO2 nanobelts are synthesized via a facile hydrothermal route including formaldehyde solution. The synthetic key factor of CeO2 nanobelts is forming the beltlike precursors with the coexistence of carbonate and formate by controlling Na/Ce molar ratio and hydrothermal temperature. After simple washing with deionized water, the CeO2 nanobelts show better catalytic performance for CO oxidation than the CeO2 nanoparticles prepared by traditional methods.Highlights► Mesoporous CeO2 nanobelts are prepared by a facile hydrothermal method. ► The synthetic key is forming beltlike precursors at enough high Na/Ce molar ratio. ► Synergy of OH− ions and Na+ ions induces production of CeO2 nanobelt precursors. ► CeO2 nanobelts show better catalytic performance than CeO2 nanoparticles.

CeO2 nanobelts, nanorods and nanowires have been synthesized by a formaldehyde-assisted hydrothermal system. It is found that the morphologies of these one-dimensional (1D) CeO2 nanomaterials are dependent on the components of the corresponding precursors, which is achieved to control the reaction degree of Cannizzaro disproportionation tuned by Na/Ce molar ratio, hydrothermal temperature and type of strong alkali. The characterization results indicate that the morphologies of 1D CeO2 nanomaterials are closely correlated to their structural features, such as lattice cell parameters, specific surface area and pore volume. These 1D nanomaterials show excellent morphology-dependent optical absorption and catalytic performance. From nanowires, nanorods to nanobelts, the visible absorption gradually decreases along with the increasing of band gap values, whereas the catalytic activity of CO oxidation increases along with lattice cell expansion. This synthetic route using formaldehyde may open a new insight into fabricating one-dimentional rare earth oxides with different morphologies and extending their potential applications.

Carbon nanotubes (CNTs) filled with CeO2 particles are prepared by wet impregnation assisted by capillary force. Compared to CeO2 outside CNTs, these composites show superior catalytic performance of oxidative dehydrogenation (ODH) of ethylbenzene (EB) to styrene in the presence of CO2. Transmission electron microscopy, temperature-programmed reduction, Raman and X-ray photoelectron spectroscopy are used to investigate the effect of CNT confinement on the catalytic performance of CeO2 inside CNTs. The results indicate that CNT tubular structure results in strengthened interaction between CeO2 and inner wall, which induces distortion and reducibility of CeO2 lattices to promote the activation of surface lattice oxygen and the formation of oxygen vacancy. The activated surface oxygen and oxygen vacancy from CeO2–CNT composites play an important role in two-step ODH reaction by promoting reverse water–gas shift reaction. In addition, CeO2 filled into shorter CNTs exhibits higher catalytic activities due to decreasing the diffusion resistance of reactants and products in CNT channels. The fact that CeO2–CNT composites exhibit excellent thermostability in the atmosphere of CO2 provides a positive choice for enhancing catalytic efficiency at elevated temperature using CNTs as supports.Graphical abstractHighlights► CeO2 inside CNTs shows higher dehydrogenation activity than CeO2 outside CNTs. ► Enhanced catalytic performance is attributed to the confinement of CNT channel. ► CeO2 particles inside shorter CNTs display higher dehydrogenation activity. ► Good thermostability of composites in CO2 can enhance their catalytic efficiency.

ZnO particles about 200 nm were prepared through a facile hydrothermal method. Compared with single ozonation, the degradation efficiency of phenol increased about 23.7% and the degradation efficiency of intermediates improved about four times in the presence of ZnO at 298 K. In addition, the catalyst had good stability in the ozonation process. The influence of temperature was investigated and it was found that the better catalysis efficiency could be obtained at lower temperature.

To elucidate the growth mechanism of Cu2O/MWCNTs synthesized by spontaneous redox reaction, the evolution of copper oxides was investigated during the synthesis process. It was found that the pH of the reaction system and defects in MWCNTs play an important role for Cu2O/MWCNTs formation. According to the calculation of oxidation and reduction potentials at different conditions, only pH 9 satisfies conditions for the formation of Cu2O/MWCNTs thermodynamically in the presence of NH3·H2O which is used for adjusting the pH of reaction system. The high defect density of MWCNTs is favorable for Cu2O/MWCNTs synthesis dynamically. For spontaneous redox, Cu2O NPs are located on the defect sites of CNTs, and a strong interaction is formed between defects and Cu2O, which could promote electron transfer in catalysis. Thus, the as-prepared Cu2O/MWCNTs show catalytic performance superior to that of the catalysts prepared by a hydrothermal method or other methods in the hydrogenation of benzene to cyclohexane.

Different lengths of short multi-walled carbon nanotubes (MWNTs) were prepared by mechanical ball milling over 12 h. TEM observation indicated that the extent of shortening and opening of MWNTs increased on prolonging the milling time. The short nanotubes with length of 100–200 nm were obtained after ball milling for 30 h. The adsorptive performance for aniline in aqueous solution indicated that the adsorptive capacity of short open-ended MWNTs increased obviously from 15 to 36% compared to the unmilled MWNTs. The measurements of pore size distribution proved that the inner pore diameter of 3 nm remains constant and the aggregated pore diameter decreased obviously after ball milling. The increasing and expanding of hystersis loop in N2 adsorption–desorption isotherms confirmed that the capillarity plays an important role in adsorption on the milled short MWNTs.

In this work, magnesia from natural brucite mineral has been used firstly for catalytic degradation of nitrobenzene and aniline in presence of ozone. Compared with single ozonation, the catalytic ozonation accelerated markedly the degradation of nitrobenzene and aniline. The influences of hydroxyl radical scavengers, pH values, and reaction temperatures on degradation were investigated. It was found that the essential of catalysis was the homogeneous catalysis of hydroxyl ions in water, which accelerated the generation of hydroxyl radicals. As a catalyst, magnesia from natural brucite has supplied an economical and feasible choice for catalytic ozonation of nitrobenzene and aniline in industrial wastewater.

•Ni/CNTs catalyst with high reducibility is facilely synthesized.•The synergy of CNTs and ethylene glycol promoted the reduction of highly dispersed NiO.•Inherent metallic Ni can induce the reduction of more NiO during catalytic hydrogenation.•As-prepared Ni/CNTs exhibits excellent catalytic performance of benzene hydrogenation even without pretreatment by H2.A facile synthesis of Ni/CNTs catalyst with high reducibility has been performed. The as-prepared Ni/CNTs sample exhibited excellent catalytic performance for benzene hydrogenation even without pretreatment by H2. The results indicated that using carbon nanotubes (CNTs) as support in presence of ethylene glycol (EG) may result in the formation of a small amount of metallic Ni. These metallic Ni would induce to produce more metallic Ni and maintain the high activity during catalytic process. The synergy between CNTs and EG has been investigated in detail. The proposed preparation method and the promotion mechanism for catalytic hydrogenation provided a new convenient approach for the research and practical application of CNTs.Download high-res image (187KB)Download full-size image

As metal-free catalysts, the functionalization of multi-walled carbon nanotubes (MWCNTs) can usually provide the activated sites for oxidative dehydrogenation (ODH). In this paper a novel functionalization of MWCNTs has been proposed via an alkali-assisted hydrothermal treatment on the sectional defects produced by ball milling. The as-prepared hydroxylated MWCNTs exhibit twice the catalytic activity of MWCNTs without ball milling, because the ball-milled treatment leads to the production of more sectional defects located on exposed edges or terminals of MWCNTs and these defects can induce the creation of more hydroxyl groups by an alkali-assisted hydroxylation. The research on the reaction mechanism reveals that the ODH of ethylbenzene (EB) in the presence of CO2 for the hydroxylated MWCNTs takes on a possible mechanism of two-step dehydrogenation by coupling a reverse water–gas shift reaction and CO2 may oxidize hydroxyl groups into carbonyl groups to maintain the catalytic cycle.

In this study, we develop a facile hydrothermal route to build a nanostructure of porous CeO2 nanobundles with hierarchical architectures by the design and growth of anisotropic CeO2 precursors in a carbonate-assisted formaldehyde hydrothermal system without templates and surfactants. The key to this synthesis is to control the formation of their anisotropic CeO2 precursors with formate and carbonate promoted by carbonate and ammonium ions during the hydrothermal treatment. The as-prepared CeO2 nanobundles have a hierarchical porous structure assembled by numerous nanorods with a small diameter, and they show much higher catalytic activities for CO oxidation compared with CeO2 nanorods, nanowires and nanoparticles prepared by formaldehyde-assisted hydrothermal treatment and traditional precipitation methods. The outstanding catalytic performance for the nanobundles is attributed to their excellent physicochemical properties such as larger lattice cell parameters, much larger surface areas and the best redox behaviour of surface oxygen on the CeO2 surface.