The Structures and Compositions Design of the Hollow Micro-Nano-Structured Metal Oxides for Environmental Catalysis

In recent decades, with the rapid development of the inorganic synthesis and the increasing discharge of pollutants in the process of industrialization, hollow-structured metal oxides (HSMOs) have taken on a striking role in the field of environmental catalysis. This is all due to their unique structural characteristics compared to solid nanoparticles, such as high loading capacity, superior pore permeability, high specific surface area, abundant inner void space, and low density. Although the HSMOs with different morphologies have been reviewed and prospected in the aspect of synthesis strategies and potential applications, there has been no systematic review focusing on the structures and compositions design of HSMOs in the field of environmental catalysis so far. Therefore, this review will mainly focus on the component dependence and controllable structure of HSMOs in the catalytic elimination of different environmental pollutants, including the automobile and stationary source emissions, volatile organic compounds, greenhouse gases, ozone-depleting substances, and other potential pollutants. Moreover, we comprehensively reviewed the applications of the catalysts with hollow structure that are mainly composed of metal oxides such as CeO2, MnOx, CuOx, Co3O4, ZrO2, ZnO, Al3O4, In2O3, NiO, and Fe3O4 in automobile and stationary source emission control, volatile organic compounds emission control, and the conversion of greenhouse gases and ozone-depleting substances. The structure-activity relationship is also briefly discussed. Finally, further challenges and development trends of HSMO catalysts in environmental catalysis are also prospected.

Solvothermal Synthesis Routes to Substituted Cerium Dioxide Materials

We review the solution-based synthesis routes to cerium oxide materials where one or more elements are included in place of a proportion of the cerium, i.e., substitution of cerium is performed. The focus is on the solvothermal method, where reagents are heated above the boiling point of the solvent to induce crystallisation directly from the solution. This yields unusual compositions with crystal morphology often on the nanoscale. Chemical elements from all parts of the periodic table are considered, from transition metals to main group elements and the rare earths, including isovalent and aliovalent cations, and surveyed using the literature published in the past ten years. We illustrate the versatility of this synthesis method to allow the formation of functional materials with applications in contemporary applications such as heterogeneous catalysis, electrodes for solid oxide fuel cells, photocatalysis, luminescence and biomedicine. We pick out emerging trends towards control of crystal habit by use of non-aqueous solvents and solution additives and identify challenges still remaining, including in detailed structural characterisation, the understanding of crystallisation mechanisms and the scale-up of synthesis.

Exploiting oxidative coupling of methane performed over La2(Ce1−xMgx)2O7−δ catalysts with disordered defective cubic fluorite structure

The oxidative coupling of methane reaction to produce C₂ compounds was studied using La₂(Ce_1−xMg_x)₂O_7−δ catalysts with disordered defective cubic fluorite structures, varying the Mg content (0.0 ≤ x ≤ 1.0), CH₄/O₂ ratio, temperature, and WHSV.

Intercalation of laminar Cu-Al LDHs with molecular TCPP(M) (M = Zn, Co, Ni, and Fe) towards high-performance CO2 hydrogenation catalysts

A confined space is broadly applied to enhance the dispersion and limit the aggregation of catalytically active sites, especially at high temperatures. In this work, we provided an efficient approach to immobilize transition metal ions (e.g., Zn2+, Co2+, Ni2+, and Fe2+) into the confined space of laminar Cu-Al layered double hydroxides (LDHs) using a range of molecular metalloporphyrins (viz., TCPP(M)) as shuttles. The deprotonated TCPP(M) not only provides nitrogen-based coordination sites to anchor a series of transition metal ions, but also intercalates and diffuses facilely into the interlayer gallery of LDHs by ion exchange. The obtained TCPP(M)@Cu-Al LDHs were then used as solid precursors for the fabrication of a series of heterogeneous catalysts for CO2 hydrogenation via high-temperature calcination. Two restriction forces contributed to the enhanced dispersion of the active species over the catalyst surface structures. Remarkably, the transition metals positioned within the confined space of LDHs significantly affected the catalytic performance of CO2 hydrogenation. Mainly CO, methanol, and methane were found as the C1 products, and their selectivities are highly dependent on the reaction intermediates, as suggested by the in situ DRIFTS study. Moreover, the designed catalysts fabricated via molecular TCPP(M) intercalation exhibited much better performance than the conventional catalysts derived from surface-supported CA-LDHs, due to their better metal dispersion and smaller particle size.

Tuning the metal-support interaction in the thermal-resistant Au-CeO2 catalysts for CO oxidation: influence of a mild N2 pretreatment

Pretreatment is very important for altering the catalytic properties of the supported noble metal catalysts in many heterogeneous reactions. In this study, a simple and mild pretreatment with N₂ has been reported to re-activate the Au–CeO₂ catalysts that were prepared by a deposition–precipitation method followed by calcination at 600 °C. Upon N₂ pretreatment at 200 °C, the metal-support interaction between Au nanoparticles (NPs) and CeO₂ was observed with the evidence of particular coverage of Au nanoparticles by CeO₂, electronic interactions and changes in CO adsorption ability. As a result, the CO oxidation activity of the pretreated Au–CeO₂ catalysts largely improved compared with those without any pretreatment and even with those subjected to H₂ and O₂ pretreatments. N₂ pretreatment also makes the Au NPs more resistant to sintering at high temperature. Furthermore, this mild pretreatment strategy can provide a potential approach to improve the thermal stability of other supported noble metal catalysts.

The degree of encapsulation for Au–CeO₂ catalysts was identical to the catalysts exhibiting metal-support interaction, which improved the CO oxidation activity.

Catalytic oxidation of CO over mesoporous copper-doped ceria catalysts via a facile CTAB-assisted synthesis

Nanosized copper-doped ceria CuCe catalysts with a large surface area and well-developed mesoporosity were synthesized by a surfactant-assisted co-precipitation method. The prepared catalysts with different Cu doping concentrations were characterized by XRD, DLS analysis, TEM, BET, Raman, H2-TPR and in situ DRIFTS techniques. The influence of Cu content on their catalytic performance for CO oxidation was also studied. The XRD results indicate that at a lower content, the Cu partially incorporates into the CeO2 lattice to form a CuCe solid solution, whereas a higher Cu doping causes the formation of bulk CuO. Copper doping favors an increase in the surface area of the CuCe catalysts and the formation of oxygen vacancies, thereby improving the redox properties. The CuCe samples exhibit higher catalytic performance compared to bare CeO2 and CuO catalysts. This is ascribed to the synergistic interaction between copper oxide and ceria. In particular, the Cu0.1Ce catalyst shows the highest catalytic performance (T 50 = 59 °C), as well as excellent stability. The in situ DRIFTS results show that CO adsorbed on surface Cu+ (Cu+-CO species) can easily react with the active oxygen, while stronger adsorption of carbonate-like species causes catalyst deactivation during the reaction.

Morphology-dependent oxygen vacancies and synergistic effects of Ni/CeO2 catalysts for N2O decomposition

Strong morphology-dependent oxygen vacancies and synergistic effects of Ni/CeO₂ catalysts and their vital effects on N₂O decomposition.

Catalytic decomposition of undiluted methane into hydrogen and carbon nanotubes over Pt promoted Ni/CeO2 catalysts

A series of Pt promoted Ni/CeO₂ catalysts were prepared and characterized, and their catalytic activity for methane decomposition was reported.

Fe-doped CeO2 nanorods for enhanced peroxidase-like activity and their application towards glucose detection

Surface defects of Fe-doped CeO₂ nanorods were found to be active sites for increasing peroxidase mimetic activity.

Low-temperature CO oxidation over manganese, cobalt, and nickel doped CeO2 nanorods

Transition metal doped ceria nanorods exhibit a better CO oxidation activity at lower temperatures.