Heavy metal removal from water/wastewater by nanosized metal oxides: a review.

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications

Single-crystalline and uniform nanopolyhedra, nanorods, and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at temperatures in the range of 100-180 degrees C under different NaOH concentrations, using Ce(NO3)3 as the cerium source. According to high-resolution transmission electron microscopy, they have different exposed crystal planes: {111} and {100} for polyhedra, {110} and {100} for rods, and {100} for cubes. During the synthesis, the formation of hexagonal Ce(OH)3 intermediate species and their transformation into CeO2 at elevated temperature, together with the base concentration, have been demonstrated as the key factors responsible for the shape evolution. Oxygen storage capacity (OSC) measurements at 400 degrees C revealed that the oxygen storage takes place both at the surface and in the bulk for the as-obtained CeO2 nanorods and nanocubes, but is restricted at the surface for the nanopolyhedra just like the bulk one, because the {100}/{110}-dominated surface structures are more reactive for CO oxidation than the {111}-dominated one. This result suggests that high OSC materials might be designed and obtained by shape-selective synthetic strategy.

Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes.

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Frustrated Lewis pair chemistry: development and perspectives.

CeO2 is a catalytic material of exceptional technological importance, and the precise role of oxygen vacancies is crucial to the greater understanding of these oxide materials. In this work, two ceria nanorod samples with different types and distributions of oxygen vacancies were synthesized. A direct relationship between the concentration of the larger size oxygen vacancy clusters and the reducibility/reactivity of nanosized ceria was revealed. These results may be an important step in understanding and designing active sites at the surface of metal oxide catalytic materials.

Oxygen Vacancy Clusters Promoting Reducibility and Activity of Ceria Nanorods

Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes

Using bottom-up chemistry techniques, the composition, size, and shape in particular can now be controlled uniformly for each and every nanocrystal (NC). Research into shape-controlled NCs have shown that the catalytic properties of a material are sensitive not only to the size but also to the shape of the NCs as a consequence of well-defined facets. These findings are of great importance for modern heterogeneous catalysis research. First, a rational synthesis of catalysts might be achieved, since desired activity and selectivity would be acquired by simply tuning the shape, that is, the exposed crystal facets, of a NC catalyst. Second, shape-controlled NCs are relatively simple systems, in contrast to traditional complex solids, suggesting that they may serve as novel model catalysts to bridge the gap between model surfaces and real catalysts.

Catalysis Based on Nanocrystals with Well-Defined Facets

Ceria nanotubes with extended cavities can be obtained by a facile oxidation−coordination-assisted dissolution process of the partially oxidized one-dimensional Ce(OH)3. Moreover, the ceria nanotubes with large cavities and thin walls exhibit excellent reducibility and high oxygen storage capacity, indicating they are potential novel catalytic materials.

Highly reducible CeO2 nanotubes

Controllable synthesis of ultrasmall atomically ordered intermetallic nanoparticles is a challenging task, owing to the high temperature commonly required for the formation of intermetallic phases. Here, a metal-organic framework (MOF)-confined co-reduction strategy is developed for the preparation of sub-2 nm intermetallic PdZn nanoparticles, by employing the well-defined porous structures of calcinated ZIF-8 (ZIF-8C) and an in situ co-reduction therein. HAADF-STEM, HRTEM, and EDS characterizations reveal the homogeneous dispersion of these sub-2 nm intermetallic PdZn nanoparticles within the ZIF-8C frameworks. XRD, XPS, and EXAFS measurements further confirm the atomically ordered intermetallic phase nature of these sub-2 nm PdZn nanoparticles. Selective hydrogenation of acetylene evaluation results show the excellent catalytic properties of the sub-2 nm intermetallic PdZn, which result from the energetically more favorable path for acetylene hydrogenation and ethylene desorption over the ultrasmall particles than over larger-sized intermetallic PdZn as revealed by density functional theory (DFT) calculations. Moreover, this protocol is also extendable for the preparation of sub-2 nm intermetallic PtZn nanoparticles and is expected to provide a novel methodology in synthesizing ultrasmall atomically ordered intermetallic nanomaterials by rationally functionalizing MOFs.

Kebin Zhou

Papers

Oxygen Vacancy Clusters Promoting Reducibility and Activity of Ceria Nanorods

Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes

Catalysis Based on Nanocrystals with Well-Defined Facets

Highly reducible CeO2 nanotubes

MOF-Confined Sub-2 nm Atomically Ordered Intermetallic PdZn Nanoparticles as High-Performance Catalysts for Selective Hydrogenation of Acetylene