Changming Li

Layered Double Hydroxide‐based Nanomaterials as Highly Efficient Catalysts and Adsorbents

Layered double hydroxides (LDHs) are a class of anion clays consisting of brucite-like host layers and interlayer anions, which have attracted increasing interest in the fields of catalysis/adsorption. By virtue of the versatility in composition, morphology, and architecture of LDH materials, as well as their unique structural properties (intercalation, topological transformation, and self-assembly with other functional materials), LDHs display great potential in the design and fabrication of nanomaterials applied in photocatalysis, heterogeneous catalysis, and adsorption/separation processes. Taking advantage of the structural merits and various control synthesis strategies of LDHs, the active center structure (e.g., crystal facets, defects, geometric and electronic states, etc.) and macro-nano morphology can be facilely manipulated for specific catalytic/adsorbent processes with largely enhanced performances. In this review, the latest advancements in the design and preparation of LDH-based functional nanomaterials for sustainable development in catalysis and adsorption are summarized.

Catalytic conversion of syngas to mixed alcohols over CuFe-based catalysts derived from layered double hydroxides

A uniform and highly dispersed CuFe-based catalyst was obtained via a calcination–reduction process of a CuFeMg-layered double hydroxide (LDH) precursor, which exhibits good activity and selectivity towards catalytic conversion of syngas to mixed alcohols. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that the CuFeMg-LDH precursor possesses high crystallinity with a particle size of 40–60 nm. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) demonstrate a high dispersion of copper and iron species on the catalyst surface. The CuFe-based catalyst derived from CuFeMg-LDHs shows high CO conversion (56.89%) and the total alcohol yield (0.28 g mLcat.−1 h−1), as a result of the high dispersion of active species as well as the synergistic effect between the copper and the iron species revealed by X-ray photoelectron spectra (XPS) and H2 temperature-programmed reduction (H2-TPR) techniques. Therefore, this work provides a facile and effective method for the preparation of CuFe-based catalysts with high catalytic activity, which can be potentially used in syngas conversion to mixed alcohols.

Supported nickel–iron nanocomposites as a bifunctional catalyst towards hydrogen generation from N2H4·H2O

Hydrogen represents an important alternative energy feedstock for both environmental and economic reasons. Development of highly selective, efficient and economical catalysts towards H2 generation from hydrogen storage materials (e.g., hydrous hydrazine, N2H4·H2O) has been one of the most active research areas. In this work, a bifunctional NiFe-alloy/MgO catalyst containing both an active center and a solid base center was obtained via a calcination–reduction process of NiFeMg-layered double hydroxides (LDHs) precursor, which exhibits 100% conversion of N2H4·H2O and up to 99% selectivity towards H2 generation at room temperature, comparable to the most reported noble metal catalysts (e.g., Rh, Pt). The XRD, HRTEM and HAADF-STEM results confirm that well-dispersed NiFe alloy nanoparticles (NPs) with diameters of ∼22 nm were embedded in a thermally stable MgO matrix. The EXAFS verifies the electronic interaction between nickel and iron elements in NiFe alloy NPs, accounting for the significantly enhanced low-temperature activity. The CO2-TPD results indicate that the strong basic sites on the surface of the NiFe-alloy/MgO catalyst contribute to the high H2 selectivity.

Binary Cu–Co catalysts derived from hydrotalcites with excellent activity and recyclability towards NH3BH3 dehydrogenation

The “Hydrogen economy” as an energy solution has received worldwide attention. Development of efficient, economic and recyclable catalysts for hydrogen generation from hydrogen storage materials (e.g., NH3BH3, AB) under moderate conditions has been one of the most active research areas. In the well-studied transition metals, cobalt (Co) and copper (Cu) are very efficient catalysts towards NH3BH3 dehydrogenation. In this work, we demonstrate the preparation of binary Cu–Co catalysts via the LDH precursor approach, which exhibit largely enhanced catalytic activity towards dehydrogenation of AB. The catalyst with a Cu/Co molar ratio of 1/1 yields a hydrolysis completion time less than 4.0 min at a rate of ∼1000 mL (min−1 gcat) under the ambient conditions, comparable to the most reported noble metal catalysts (e.g., Ru, Pt). XRD, H2-TPR, XPS and HRTEM measurements verify that the synergistic effect between highly dispersive metallic Cu and Co3O4 species plays a key role in the significantly enhanced activity of the Cu–Co catalyst. In addition, a monolithic Cu–Co film catalyst was fabricated by an in situ growth-reduction method, which displays further enhanced catalytic activity, recyclability and long-term reusability. This work provides an effective strategy for the fabrication of excellent Cu–Co catalysts for NH3BH3 decomposition, which can be used as promising candidates in pursuit of practical implementation of AB as a hydrogen storage material.

Photohole-oxidation-assisted anchoring of ultra-small Ru clusters onto TiO2 with excellent catalytic activity and stability

Ultra-small metal clusters with good activity and stability are of great significance for their practical applications in catalysis and materials science. Here we report a photohole-oxidation-assisted approach for anchoring ultra-small Ru clusters (∼1.5 nm) with an extremely high density (∼1017 m−2) onto TiO2 support. The resulting clusters have good thermal stability and exhibit excellent long-term catalytic activity for the hydrogenation of CO2 to methane (methanation). The anchoring process involves the oxidation of Ru3+ in solution by photogenerated holes on the TiO2 surface to give tiny RuO2 species (<0.8 nm) immobilized on the surface, followed by a H2 reduction step to produce Ru0 clusters. Aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) observations identify the Ru–Ru bond length contraction at the metal surface (relative to the interior of the particle) as well as bond length changes in the defect region at the metal–support interface. Density functional theory (DFT) calculations further demonstrate that the ultra-small Ru clusters are well stabilized and tightly anchored onto the TiO2 substrate via Ru–O covalent bonding in the defect region of the metal–support interface. The high-dispersion of ultra-small Ru clusters as well as the strong chemical bonding at the interface account for their surprisingly high catalytic reactivity and excellent thermal/reaction stability. This synthetic method may open up a new way to fabricate thermally stable ultra-small metal clusters for practical industrial applications in catalysis.

Catalytic behavior of supported Ru nanoparticles on the (101) and (001) facets of anatase TiO2

Ru/TiO2 heterogeneous catalysts were prepared by immobilizing Ru nanoparticles onto the (101) and (001) facets of anatase TiO2 substrate, and the influence of metal–support interactions on the catalytic behavior of Ru/TiO2 towards CO2 methanation was studied from the viewpoint of electronic structure. Structural investigations based on temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) indicate that a stronger metal–support interaction occurs between Ru and (101) facet in contrast to the Ru and (001) one. This gives rise to an enhancement in CO2 adsorption as well as spill-over hydrogen at the interface of Ru/TiO2(101), accounting for its largely enhanced catalytic activity towards CO2 methanation. In addition, a theoretical study based on density functional theory (DFT) calculations reveals that the Ru nanoparticles supported on the (101) plane have a relatively lower activation energy for CO dissociation (the rate-determining step), which results in their high activity toward CO2 methanation reaction.

Patterned fluorescence films with reversible thermal response based on the host–guest superarchitecture

This paper reports patterned films with thermal colorimetric and fluorescent response fabricated by a combined approach based on electrophoretic deposition (EPD)–photolithography. A composite film of diacetylene (DA)/layered double hydroxide (LDH) was prepared by the method of EPD, and the photolithography technique was subsequently employed to further obtain a polydiacetylene (PDA)/LDH patterned fluorescence film via UV-induced polymerization of DA in the two-dimensional (2D) gallery of LDH matrix. The PDA/LDH film shows a well c-orientation of LDH platelets (the ab plane of the LDH platelets parallel to the substrate) confirmed by XRD and SEM. Both the in situUV-vis absorption and fluorescence emission spectroscopy indicate that the composite film exhibits marked thermal colorimetric and fluorescent behavior in the temperature range 20–130 °C, which is reversible over a number of heating/cooling cycles. It should be noted that the pristine PDA shows no reversible thermal colorimetric and fluorescent performance at all. The transformation of an organic chromophore from irreversible to reversible thermal response material upon incorporation into a 2D layered matrix is the most distinct feature in this work. It was demonstrated that the thermally response behavior resulted from the strong hydrogen bond interaction between the PDA and LDH matrix, which was confirmed by in situ Raman and in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. Therefore, this work provides new opportunities for the fabrication of thermally responsive patterned films with high stability and reversibility, which can be used in intelligent response and display devices.

Confined synthesis of ultrafine Ru–B amorphous alloy and its catalytic behavior toward selective hydrogenation of benzene

How to control the size and morphology of metal nanocatalysts is of vital importance in enhancing their catalytic performance. In this work, uniform and ultrafine Ru–B amorphous alloy nanoparticles (NPs) supported on titanate nanosheets were fabricated via a confined synthesis in titanate nanotubes (TNTs) followed by unwrapping the tube to sheetlike titanate (TNS) (denoted as Ru–B/TNS), which exhibit excellent catalytic performance toward the selective hydrogenation of benzene to cyclohexene (yieldcyclohexene: 50.7%) without any additives. HRTEM images show the resulting Ru–B NPs are highly dispersed on the titanate nanosheets (particle size: 2.5 nm), with a low Ru–Ru coordination number revealed by EXAFS. Moreover, XPS demonstrates the surface-enriched B element and a strong electron transfer from B to Ru, which facilitates the formation and desorption of cyclohexene on the Ru active-sites, accounting for the significantly enhanced catalytic behavior. The surfactant-free confined synthesis and additive-free catalytic system make the Ru–B/TNS catalyst a promising candidate for the selective hydrogenation of benzene.

Preparation of a ternary Pd–Rh–P amorphous alloy and its catalytic performance in selective hydrogenation of alkynes

Palladium–rhodium–phosphorus amorphous alloy nanoparticles (~5.2 nm) were prepared via a facile one-pot synthesis method, exhibiting excellent catalytic behaviour in selective hydrogenation of alkynes under mild conditions.

Cu‐Decorated Ru Catalysts Supported on Layered Double Hydroxides for Selective Benzene Hydrogenation to Cyclohexene

The selective hydrogenation of benzene to cyclohexene is of high value for the chemical industry owing to its inexpensive feedstock, atom economy, and operational simplicity. A tunable catalytic behavior towards the selective hydrogenation of benzene was obtained over Cu‐decorated Ru catalysts supported on a layered double hydroxide (denoted as RuxCuy/MgAl‐LDH), reaching a maximum cyclohexene yield of 44.0 % over Ru1.0Cu0.5/MgAl‐LDH at 150 °C and 5.0 MPa without employment of any additives. CO‐TPD (TPD=temperature‐programmed desorption) and in situ CO‐FTIR techniques demonstrated that Cu atoms preferentially deposit on the surface of low‐coordinated Ru atoms in RuxCuy/MgAl‐LDH catalysts, resulting in a low adsorption energy of cyclohexene on the modified sites as revealed by DFT calculations. This work not only gives an understanding of the correlation between the surface exposure of Ru active sites and the resulting selectivity, but also provides a green and additive‐free catalytic process for the selective hydrogenation of benzene.