Layered Double Hydroxide/Nanocarbon Composites as Heterogeneous Catalysts: A Review

The synthesis and applications of composites based on layered double hydroxides (LDHs) and nanocarbons have recently seen great development. On the one hand, LDHs are versatile 2D compounds that present a plethora of applications, from medicine to energy conversion, environmental remediation, and heterogeneous catalysis. On the other, nanocarbons present unique physical and chemical properties owing to their low-dimensional structure and sp2 hybridization of carbon atoms, which endows them with excellent charge carrier mobility, outstanding mechanical strength, and high thermal conductivity. Many reviews described the applications of LDH/nanocarbon composites in the areas of energy and photo- and electro-catalysis, but there is still scarce literature on their latest applications as heterogeneous catalysts in chemical synthesis and conversion, which is the object of this review. First, the properties of the LDHs and of the different types of carbon materials involved as building blocks of the composites are summarized. Then, the synthesis methods of the composites are described, emphasizing the parameters allowing their properties to be controlled. This highlights their great adaptability and easier implementation. Afterwards, the application of LDH/carbon composites as catalysts for C–C bond formation, higher alcohol synthesis (HAS), oxidation, and hydrogenation reactions is reported and discussed in depth.

Universal Strategy for HF-Free Facile and Rapid Synthesis of Two-dimensional MXenes as Multifunctional Energy Materials

Two-dimensional MXenes are promising for various energy-related applications such as energy storage devices and electrocatalysis of water-splitting. MXenes prepared from hydrofluoric (HF) acid etching have been widely reported. Nonetheless, the acute toxicity of HF acid impedes the large-scale fabrication of MXenes and their wide utilization in energy-related applications. It is thus greatly encouraging to explore a more innocuous protocol for MXenes synthesis. Thereby, a universal strategy based on thermal-assisted electrochemical etching route is developed to synthesize MXenes (e.g., Ti₂CT _x, Cr₂CT _x, and V₂CT _x). Furthermore, the cobalt ion doped MXenes show an exceptionally enhanced capability of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity, demonstrating their multifunctionalities, which is comparable to the commercialized catalysts. Moreover, we successfully exploited our MXenes as cathodes for the novel aqueous rechargeable battery, with proficient retention and excellent electrical output performance. This work paves a nontoxic and HF-free route to prepare various MXenes and demonstrates practical applications of the materials.

A robust core–shell nanostructured nickel–iron alloy@nitrogen-containing carbon catalyst for the highly efficient hydrogenation of nitroarenes

A core–shell nanostructured NiFe alloy@ nitrogen-containing carbon catalyst exhibited robust catalytic performance in the hydrogenation of nitroarenes.

Fabrication of Highly Stable Metal Oxide Hollow Nanospheres and Their Catalytic Activity toward 4-Nitrophenol Reduction

In this paper, hollow nanospheres (HNSs) of metal oxides (NiO, CuO, and NiO/CuO) coated with a porous carbon shell (HNSs@C) with good structural stability were successfully prepared on the basis of the nanoscale Kirkendall effect. The formation process was based on a template-free method, and the as-prepared HNSs@C are very clean compared with products of the template process. In addition, the results of N₂ adsorption-desorption noted that both the metal oxide HNSs and the coated carbon were mesoporous structures. Therefore, small molecules can access the inner space of the whole HNSs@C, which was expected to increase the active site area and to show better performances in applied fields, such as catalysts and sensors. As an example of the functional properties, the obtained HNSs@C were investigated as the catalyst for the hydrogenation of 4-nitrophenol (4-NP) and manifested highly catalytic activity and excellent stability. This work has opened up a novel route for the development of metal oxide HNSs nanocatalysts. This straightforward method is of significance for development of clean metal oxide HNSs with high stability and multiplied applications.

MOF-derived Ni-based nanocomposites as robust catalysts for chemoselective hydrogenation of functionalized nitro compounds

Highly dispersed Ni nanoparticles within graphitic carbon layers were prepared by facile thermolysis of a Ni-MOF, which exhibited outstanding catalytic performance in the chemoselective hydrogenation of diverse functionalized nitro compounds.

Highly active electron-deficient Pd clusters on N-doped active carbon for aromatic ring hydrogenation

Pyridinic nitrogen species in N-doped active carbon (xN-AC) are responsible for high activity of ring hydrogenation via the formation of a high percentage of electron-deficient Pd clusters.

The promotional effect of surface defects on the catalytic performance of supported nickel-based catalysts

Supported nickel-based catalysts derived from layered double hydroxides exhibited superior catalytic hydrogenation performance due to the promotional effect of surface defects.

Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects

Oxidation and hydrogenation catalysis plays a crucial role in the current chemical industry for the production of key chemicals and intermediates. Because of their easy separation and recyclability, supported catalysts are widely used in these two processes. Layered double hydroxides (LDHs) with the advantages of unique structure, composition diversity, high stability, ease of preparation and low cost have shown great potential in the design and synthesis of novel supported catalysts. This review summarizes the recent progress in supported catalysts by using LDHs as supports/precursors for catalytic oxidation and hydrogenation. Particularly, partial hydrogenation of acetylene, hydrogenation of dimethyl terephthalate, methanation, epoxidation of olefins, elimination of NOx and SOx emissions, and selective oxidation of biomass have been chosen as representative reactions in the petrochemical, fine chemicals, environmental protection and clean energy fields to highlight the potential application and the general functionality of LDH-based catalysts in catalytic oxidation and hydrogenation. Finally, we concisely discuss some of the scientific challenges and opportunities of supported catalysts based on LDH materials.

Magnetically recoverable Ni/C catalysts with hierarchical structure and high-stability for selective hydrogenation of nitroarenes

A magnetic Ni/C catalyst with hierarchical structure has been synthesized and exhibits a high stability for selective hydrogenation of nitroarenes.

Direct synthesis of hybrid layered double hydroxide–carbon composites supported Pd nanocatalysts efficient in selective hydrogenation of citral

A facile route for the synthesis of hybrid layered double hydroxide–carbon composites supported Pd nanocatalysts efficient in the selective hydrogenation of citral.