Yanshan Gao

Recent advances in solid sorbents for CO2 capture and new development trends

Carbon dioxide (CO2) capture using solid sorbents has been recognized as a very promising technology that has attracted intense attention from both academic and industrial fields in the last decade. It is astonishing that around 2000 papers have been published from 2011 to 2014 alone, which is less than three years after our first review paper in this journal on solid CO2 sorbents was published. In this short period, much progress has been made and the major research focus has more or less changed. Therefore, we feel that it is necessary to give a timely update on solid CO2 capture materials, although we still have to keep some important literature results published in the past years so as to keep the good continuity. We believe this work will benefit researchers working in both academic and industrial areas. In this paper, we still organize the CO2 sorbents according to their working temperatures by classifying them as such: (1) low-temperature ( 400 °C). Since the sorption capacity, kinetics, recycling stability and cost are important parameters when evaluating a sorbent, these features will be carefully considered and discussed. In addition, due to the huge amounts of cost-effective CO2 sorbents demanded and the importance of waste resources, solid CO2 sorbents prepared from waste resources and their performance are reviewed. Finally, the techno-economic assessments of various CO2 sorbents and technologies in real applications are briefly discussed.

Flame retardant polymer/layered double hydroxide nanocomposites

Recently, there has been rapid growth in research related to the synthesis and application of flame retardant polymer–layered double hydroxide (LDH) nanocomposites. In order to outline the potential and to promote further developments in the field we have prepared a critical review of the most recent progress in the area. We discuss the techniques and indices (e.g. micro calorimetery, limiting oxygen index, cone calorimetry and UL-94) for evaluating the flame retardant properties. The flame retardant mechanism of LDHs, the types of polymers studied, the effect of LDH chemical composition and the synergistic effect with other fire retardants are reviewed. It is hoped that this review will not only introduce the synthesis, characterisation and application of polymer–LDH nanocomposites for flame retardancy, but also prompt new discussion on the use of LDH dispersions in polymer-based materials.

A critical review on the heterogeneous catalytic oxidation of elemental mercury in flue gases.

Nowadays, an increasing attention has been paid to the technologies for removing mercury from flue gases. Up to date, no optimal technology that can be broadly applied exists, but the heterogeneous catalytic oxidation of mercury is considered as a promising approach. Based on a brief introduction of the pros and cons of traditional existing technologies, a critical review on the recent advances in heterogeneous catalytic oxidation of elemental mercury is provided. In this contribution, four types of Hg oxidation catalysts including noble metals, selective catalytic reduction (SCR) catalysts, transition metals, and fly ash have been summarized. Both the advantages and disadvantages of these catalysts are described in detail. The influence of various acidic gases including SO2, SO3, NH3, NOx, HCl, Cl2, etc. have been discussed as well. We expect this work will shed light on the development of heterogeneous catalytic oxidation of elemental mercury technology in flue gases, particularly the synthesis of novel and highly efficient Hg(0) oxidation catalysts.

Synthesis of nano-sized spherical Mg3Al–CO3 layered double hydroxide as a high-temperature CO2 adsorbent

Due to its layered structure, layered double hydroxides (LDHs) generally prefer to form either “sand rose” or platelet-like morphologies. To the best of our knowledge, nano-sized spherical LDHs have not been previously reported. In this work, we present the first successful synthesis of nano-sized spherical Mg3Al–CO3 LDH using a facile isoelectric point (IEP) method. SEM and TEM analyses confirmed that the size of the nanospheres is very uniform, with an average value of ca. 20 nm. Furthermore, a mesoporous LDH sample composed of the above synthesized uniform nano-spheres can be prepared, and this material showed a H1 type hysteresis loop in the N2 BET analysis. Such mesoporous LDH possesses large mesopores (18 nm) and a high surface area (103 m2 g−1), which we believe make it a promising adsorbent, catalyst, or support material. We demonstrated that its CO2 capture capacity is 0.83 mmol g−1 at 200 °C and 1 atm and it can be further increased up to 1.21 mmol g−1 by doping with 20 wt% K2CO3.

Synthesis of layered double hydroxides/graphene oxide nanocomposite as a novel high-temperature CO2 adsorbent

Abstract In this contribution, a novel high-temperature CO2 adsorbent consisting of Mg-Al layered double hydroxide (LDH) and graphene oxide (GO) nanosheets was prepared and evaluated. The nanocomposite-type adsorbent was synthesized based on the electrostatically driven self-assembly between positively charged Mg-Al LDH single sheet and negatively charged GO monolayer. The characteristics of this novel adsorbent were investigated using XRD, FE-SEM, HRTEM, FT-IR, BET and TGA. The results showed that both the CO2 adsorption capacity and the multicycle stability of LDH were increased with the addition of GO owing to the enhanced particle dispersion and stabilization. In particular, the absolute CO2 capture capacity of LDH was increased by more than twice by adding 6.54 wt% GO as support. GO appeared to be especially effective for supporting LDH sheets. Moreover, the CO2 capture capacity of the adsorbent could be further increased by doping with 15 wt% K2CO3. This work demonstrated a new approach for the preparation of LDH-based hybrid-type adsorbents for CO2 capture.

Synthesis of Highly Efficient Flame Retardant High-Density Polyethylene Nanocomposites with Inorgano-Layered Double Hydroxides As Nanofiller Using Solvent Mixing Method

High-density polyethylene (HDPE) polymer nanocomposites containing Zn2Al-X (X= CO3(2-), NO3(-), Cl(-), SO4(2-)) layered double hydroxide (LDH) nanoparticles with different loadings from 10 to 40 wt % were synthesized using a modified solvent mixing method. Synthesized LDH nanofillers and the corresponding nanocomposites were carefully characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, etc. The thermal stability and flame retardancy behavior were investigated using a thermo gravimetric analyzer and microscale combustion calorimeter. Comparing to neat HDPE, the thermal stability of nanocomposites was significantly enhanced. With the addition of 15 wt % Zn2Al-Cl LDH, the 50% weight loss temperature was increased by 67 °C. After adding LDHs, the flame retardant performance was significantly improved as well. With 40 wt % of LDH loading, the peak heat release rate was reduced by 24%, 41%, 48%, and 54% for HDPE/Zn2Al-Cl, HDPE/Zn2Al-CO3, HDPE/Zn2Al-NO3, and HDPE/Zn2Al-SO4, respectively. We also noticed that different interlayer anions could result in different rheological properties and the influence on storage and loss moduli follows the order of SO4(2-) > NO3(-) > CO3(2-) > Cl(-). Another important finding of this work is that the influence of anions on flame retardancy follows the exact same order on rheological properties.

Comprehensive investigation of CO2 adsorption on Mg-Al-CO3 LDH-derived mixed metal oxides

Layered double hydroxides (LDHs) have been intensively studied for high-temperature CO2 capture. However, big differences in the CO2 capture capacity, ranging from 0.28 to 0.6 mmol g−1, have often been reported for the same Mg–Al–CO3 LDH. Furthermore, how the active Mg–O species that are responsible for CO2 adsorption are formed is still unclear. In this work, we have performed a comprehensive investigation on the CO2 adsorption characteristics of Mg–Al–CO3 LDH-derived mixed metal oxides. Based on these results we proposed the possible adsorption sites and the mechanisms for CO2 adsorption. Initially, the effects of synthesis method, Mg : Al ratio, pretreatment conditions, adsorption conditions, and thermal stability on the CO2 adsorption capacity were systematically studied. By carefully examining the structural changes during thermal treatment using X-ray diffraction and solid state NMR, we suggest that the active Mg–O species could be induced either by the substitution of Mg2+ by Al3+ in the periclase MgO lattice, or by the diffusion of Al3+ out of the octahedral brucite layers. This work not only suggests the optimal testing conditions for LDH-derived CO2 adsorbents, but also provides a clearer understanding of the CO2 adsorption sites and mechanisms on LDH-derived mixed oxides and sheds light on the synthesis and utilization of LDH-derived high-temperature CO2 adsorption materials.

Synthesis of polypropylene/Mg3Al–X (X = CO32−, NO3−, Cl−, SO42−) LDH nanocomposites using a solvent mixing method: thermal and melt rheological properties

In this contribution, polypropylene (PP) nanocomposites with four different inorganic anion (CO32−, NO3−, Cl−, and SO42−) intercalated Mg3Al layered double hydroxides (LDHs) as nanofillers were prepared using a solvent mixing method for the first time. The influence of interlayer inorganic anions on the thermal stability, melting and recrystallisation behavior and rheological property of PP was then systematically compared. The thermal stability was significantly improved, with Mg3Al–CO3 and Mg3Al–Cl LDHs, and T0.5 increased by about 44 °C compared to pure PP. The incorporation of LDHs increased the melting temperature (Tm) and the recrystallization temperature (Tc) of PP by about 3–4 °C and 10–13 °C, respectively. The influence on the storage modulus (G′) and loss modulus (G′′) follows the order of Mg3Al–SO4 > Mg3Al–NO3 > Mg3Al–CO3 > Mg3Al–Cl. Although the incorporation of all LDHs led to an improvement in the thermal stability, Tm, Tc, G′, G′′, and complex viscosity of PP/Mg3Al–X LDH nanocomposites, the extent of changes varied with the type of inter-layer anions. The study finally demonstrated that even very similar inorganic anions such as CO32−, NO3−, Cl−, and SO42− could lead to a significant difference in the property of the synthesized PP/LDH nanocomposites, and it must be taken into account for the future design of polymer/LDH nanocomposites.

Highly sensitive p-nitrophenol chemical sensor based on crystalline α-MnO2 nanotubes

This paper reports the successful fabrication and characterization of a highly sensitive p-nitrophenol (p-NP) amperometric chemical sensor based on crystalline α-MnO2 nanotubes. The α-MnO2 nanotubes were successfully synthesized using a simple hydrothermal treatment of potassium permanganate (KMnO4) and concentrated hydrochloric acid (HCl). The prepared nanotubes were examined in detail by using various analytical methods which revealed that the synthesized nanotubes are grown in very high density, possessing good crystallinity and high purity. The as-synthesized α-MnO2 nanotubes were used for the fabrication of the p-NP chemical sensor which exhibited high sensitivity of 19.18 mA mM−1 cm−2 and a low detection limit of 0.1 mM. To the best of our knowledge, this is the first report that used α-MnO2 nanotubes to fabricate a p-NP chemical sensor with such high sensitivity and low detection limit.

Layered double hydroxide (LDH) derived catalysts for simultaneous catalytic removal of soot and NOx

Nitrogen oxides (NO(x)) and soot which come from vehicle engine exhausts cause serious environmental pollution and human health problems. Recently, the catalytic purification technology, particularly the simultaneous catalytic removal of soot and NO(x), has received more and more attention. For this technology, the key is to develop highly efficient and robust catalysts. Due to the unique chemical and structural properties of layered double hydroxides (LDHs), LDH-derived catalysts have shown great potential, and much effort has been devoted to this type of catalyst. In this manuscript, we reviewed the latest progress in the LDH derived catalysts by classifying the LDH precursors according to the number of metals into binary, ternary, and quaternary, and discussed their advantages and disadvantages in detail. We hope that this review paper could provide a clearer picture of this topic and theoretical support for its better development.