Junya Wang

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.

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.

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.

Polypropylene/Mg3Al–tartrazine LDH nanocomposites with enhanced thermal stability, UV absorption, and rheological properties

We report the synthesis of coloured polypropylene (PP)/Mg3Al–tartrazine layered double hydroxide (LDH) nanocomposites with enhanced thermal stability, UV absorption capacity, and rheological properties using a modified solvent mixing method for the first time. SEM images indicated that the LDH nanoparticles were evenly dispersed within the PP matrix due to the favourable interactions between PP and LDHs. TGA and DSC analysis indicated that the thermal stability of PP/LDH nanocomposites was significantly increased. Decreased G′ and G′′ ascribed to the enhanced mobility (relaxation) of the confined polymer chains at the interface of the PP/LDH layers suggested that the LDH is nano-dispersed in the composites. UV-Vis spectroscopy showed that the addition of Mg3Al–tartrazine LDH significantly enhanced the UV absorption characteristics of PP. Since tartrazine is a nontoxic additive, these coloured PP/Mg3Al–tartrazine LDH nanocomposites are expected to have many promising applications such as for food packing and childrens toys, etc.

Synthesis of LiAl2-layered double hydroxides for CO2 capture over a wide temperature range

Although there are many reports on layered double hydroxide (LDH) derived CO2 adsorbents, none of them have studied the special case of LiAl2 LDHs. Here we report the first detailed investigation of the performance of LiAl2 LDHs as novel CO2 adsorbents. LiAl2 LDHs were synthesized using both traditional coprecipitation and gibbsite intercalation methods. All the materials were thoroughly characterized using XRD, SEM, TEM, FTIR, BET, and TGA. The CO2 capture performance of these LDHs were investigated as a function of charge compensating anions, Li/Al ratio in preparation solution, calcination temperature, adsorption temperature, and doping with K2CO3. The data indicated that LiAl2 LDHs derived compounds can be used as CO2 adsorbents over a wide temperature range (60–400 °C), with a CO2 capture capacity of 0.94 and 0.51 mmol g−1 at 60 and 200 °C, respectively. By doping LiAl2–CO3 LDH with 20 wt% K2CO3, the CO2 adsorption capacity was increased up to 1.27 and 0.83 mmol g−1 at 60 and 200 °C, respectively. CO2 adsorption–desorption cycling studies showed that both pure LiAl2 LDH and the K2CO3-promoted LiAl2 LDH had stable CO2 capture performance even after 22 cycles. Considering its high CO2 capture capacity and good cycling stability, LiAl2 LDH based novel CO2 adsorbents have significant potential for CO2 capture applications.

Synthesis of Pt doped Mg–Al layered double oxide/graphene oxide hybrid as novel NOx storage–reduction catalyst

We report the synthesis of Pt doped Mg–Al layered double oxide/graphene oxide (Pt–LDO/GO) hybrid as novel NOx storage and reduction (NSR) catalyst. For the preparation of layered double hydroxide/GO hybrids, LDHs and graphite oxide were first exfoliated into single-layers, followed by self-assembly. LDO/GO hybrids were obtained by thermal treatment of LDH/GO. The obtained LDH/GO and LDO/GO hybrids were thoroughly characterized using XRD, SEM, TEM, FT-IR, and BET analyses. Then the NOx storage capacity of neat LDO and LDO(10)/GO hybrids were compared by isothermal NOx adsorption tests. The influence of adsorption temperature, gas flow, calcination temperature, and LDH dispersion concentration were systematically studied. The results demonstrated that the NOx storage capacity of neat LDO was significantly improved from 0.175 to 0.314 mmol g−1 by introducing only 7 wt% of GO, which could be attributed to the enhanced particle dispersion and stabilization. Moreover, the NOx storage capacity of the hybrid could be further increased close to 0.335 mmol g−1 catalyst by doping with 2 wt% Pt. The Pt–LDO(1)/GO also exhibited excellent lean-rich cycling performance, with an overall 71.7% of NOx removal. This work provided a new scheme for the preparation of highly dispersed LDH/GO hybrid type NSR catalyst.

Unexpected highly reversible topotactic CO2 sorption/desorption capacity for potassium dititanate

Potassium dititanate (K2Ti2O5) was revealed to possess an unexpected, highly reversible CO2 sorption/desorption capacity at ca. 750 °C, which is promising as a high-temperature CO2 adsorbent for sorption enhanced hydrogen production (SEHP) processes. In contrast to numerous other adsorbents that are severely sintered during cycles at high temperatures, the CO2 sorption/desorption cycles over K2Ti2O5 exhibited a contrast particle size “break-down” process. The large K2Ti2O5 particles gradually breakdown into K2Ti2O5 nanofibers after 20 cycles, leading to a very stable CO2 sorption/desorption performance with very rapid kinetics. A reversible CO2 capture capacity as high as 7.2 wt% was achieved at 750 °C. Moreover, only 6 min is required for complete CO2 desorption at 750 °C, indicating that this adsorbent can be practically run with a simple pressure swing sorption scheme. Surprisingly, an interesting structure switching phenomenon between K2Ti2O5 and K2Ti4O9 caused by CO2 sorption and desorption was revealed. A detailed mechanism was proposed based on XRD, FTIR, SEM, HR-TEM, and SAED analyses and was further verified by density functional theory calculation. Considering its relatively high CO2 capture capacity, superior cycling stability, and excellent regeneration ability, we believe K2Ti2O5 offers significant potential as a practical, novel high-temperature CO2 adsorbent.

Heteroepitaxy of Tetragonal BiFeO3 on Hexagonal Sapphire(0001)

Highly elongated BiFeO3 is epitaxially grown on hexagonal sapphire(0001) substrate within a rather narrow synthesis window. Both X-ray reciprocal space maps and Raman characterizations reveal that it is of true tetragonal symmetry but not the commonly observed MC type monoclinic structure. The tetragonal BiFeO3 film exhibits an island growth mode, with the island edges oriented parallel to the ⟨10-10⟩ and ⟨12-30⟩ directions of the sapphire substrate. With increasing deposition time, a transition from square island to elongated island and then to a continuous film is observed. The metastable tetragonal phase can remain on the substrate without relaxation to the thermally stable rhombohedral phase up to a critical thickness of 450 nm, providing an exciting opportunity for practicable lead-free ferroelectrics. These results facilitate a better understanding of the phase stability of BiFeO3 polymorphs and enrich the knowledge about the heteroepitaxial growth mechanism of functional oxides on symmetry-mismatched substrates.