An-Hui Lu

Rapid synthesis of nitrogen-doped porous carbon monolith for CO2 capture.

M M U Rapid Synthesis of Nitrogen-Doped Porous Carbon Monolith for CO2 Capture N IC By Guang-Ping Hao, Wen-Cui Li, Dan Qian, and An-Hui Lu* A T IO N The development of novel materials and new technologies for CO2 capture and storage has gained great attention over the past decades. The necessity for reducing the concentration of CO2 in the atmosphere is a very important issue, since CO2 is considered as a greenhouse gas causing global warming. In addition, higher concentrations of CO2 are toxic for humans, especially in space-limited chambers like submarines and space ships. Traditional technologies for CO2 capture including absorption and adsorption-coupled membrane separation along with the corresponding sorbents such as aqueous amines, microporous coordination polymers (MCPs), zeolitic imidazolate frameworks (ZIFs), different types of nitrogen-doped zeolite, mesoporous silica, and activated carbons have been widely explored. However, MCPs and ZIFs synthesized with nitrogen-containing organic compounds as the crosslinker often suffer from structural instability and inefficiency for CO2 selectivity in the presence of water and thus are limited in their widespread use. For aminemodified solids, the main drawback is the possible loss of ammonia associated with the temperature needed for regeneration and the energy-intensive nature of the regeneration process. For instance, amine-surface-modified or amineimpregnated porous silica (e.g., MCM-41, MCM-48, SBA-15, SBA-16) and zeolite 13X materials fail to capture CO2 effectively, require high temperature and long regeneration times and lack stability over many cycles. It should be pointed out that amine-modified silica-based solids are usually prepared by a post-treatment, which is a time-consuming and costly procedure and often involves the use of toxic and corrosive reagents. Porous carbon materials have many specific features such as high surface area, thermal and chemical stability, and hydrophobic surface properties. The incorporation of basic nitrogen groups into the carbon framework ensures an improved adsorption/absorption for acidic gases. Up to date, porous carbon materials used for CO2 capture were mostly prepared by post-synthetic amine modification or ammonia treatment, which again leads to materials lacking stability and, in addition, the reagents are corrosive, which brings the same disadvantages as for modified silica and zeolites. Alternatively, nitrogencontaining porous carbons can be prepared directly from

Structurally Designed Synthesis of Mechanically Stable Poly(benzoxazine-co-resol)-Based Porous Carbon Monoliths and Their Application as High-Performance CO2 Capture Sorbents

Porous carbon monoliths with defined multilength scale pore structures, a nitrogen-containing framework, and high mechanical strength were synthesized through a self-assembly of poly(benzoxazine-co-resol) and a carbonization process. Importantly, this synthesis can be easily scaled up to prepare carbon monoliths with identical pore structures. By controlling the reaction conditions, porous carbon monoliths exhibit fully interconnected macroporosity and mesoporosity with cubic Im3m symmetry and can withstand a press pressure of up to 15.6 MPa. The use of amines in the synthesis results in a nitrogen-containing framework of the carbon monolith, as evidenced by the cross-polarization magic-angle-spinning NMR characterization. With such designed structures, the carbon monoliths show outstanding CO(2) capture and separation capacities, high selectivity, and facile regeneration at room temperature. At ~1 bar, the equilibrium capacities of the monoliths are in the range of 3.3-4.9 mmol g(-1) at 0 °C and of 2.6-3.3 mmol g(-1) at 25 °C, while the dynamic capacities are in the range of 2.7-4.1 wt % at 25 °C using 14% (v/v) CO(2) in N(2). The carbon monoliths exhibit high selectivity for the capture of CO(2) over N(2) from a CO(2)/N(2) mixture, with a separation factor ranging from 13 to 28. Meanwhile, they undergo a facile CO(2) release in an argon stream at 25 °C, indicating a good regeneration capacity.

Spatially and Size Selective Synthesis of Fe-Based Nanoparticles on Ordered Mesoporous Supports as Highly Active and Stable Catalysts for Ammonia Decomposition

Uniform and highly dispersed γ-Fe(2)O(3) nanoparticles with a diameter of ∼6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mössbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, γ-Fe(2)O(3) nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe(2)O(3)/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 °C and space velocities as high as 60,000 cm(3) g(cat)(-1) h(-1). At a space velocity of 7500 cm(3) g(cat)(-1) h(-1), complete ammonia conversion was maintained at 600 °C for 20 h. After the reaction, the immobilized γ-Fe(2)O(3) nanoparticles were found to be converted to much smaller nanoparticles (γ-Fe(2)O(3) and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe(2)O(3)/CMK-5 catalyst is much more active than the benchmark NiO/Al(2)O(3) catalyst at high space velocity, due to its highly developed mesoporosity. γ-Fe(2)O(3) nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity.

Ionic liquid C16mimBF4 assisted synthesis of poly(benzoxazine-co-resol)-based hierarchically porous carbons with superior performance in supercapacitors

Hierarchically porous carbons with variable pore sizes at multi-length-scale, a nitrogen and boron co-doped and local graphitized framework, and high mechanical strength were synthesized through the self-assembly of poly(benzoxazine-co-resol) with ionic liquid C16mimBF4 and a carbonization process. In this synthesis, the ionic liquid acts both as a structure directing agent and a heteroatom precursor. The obtained porous carbons have a specific surface area lower than 376 m2 g−1 and thus a high skeleton density. With such heteroatom doped skeleton structures and fully interconnected macropores, mesopores and micropores, the hierarchically porous carbon shows outstanding electrochemical performance, e.g. a superior high gravimetric capacitance (Cg) of 247 F g−1, an interfacial capacitance (CS) of 66 μF cm−2 (calculated based on the discharge curve with a constant current density of 0.5 A g−1), whilst a high volumetric capacitance (Cv) of 101 F cm−3 compared to those reported in the literature. Cycling stability tests indicate that the carbon exhibits a capacitance retention of ∼96.2% after 4000 charge–discharge cycles, strongly reflecting an excellent long-term cyclability of the electrode. Due to its unique skeleton structure and high conductivity, such hierarchically porous carbon shows promise as an electrode material for supercapacitors.

Taking Nanocasting One Step Further: Replicating CMK-3 as a Silica Material

The replication of nanoscale structures by a direct templating process has been used in recent years in several creative ways for the synthesis of carbon replicas of zeolites[1] or ordered mesoporous carbons, such as CMK-1[2] or SNU-1.[3] Such processes rely on the fact that an ordered pore system, provided by the zeolite or ordered mesoporous silica, can be filled with a carbon precursor which is pyrolyzed and the silica leached with NaOH or HF solution. However, the technique is difficult to apply to the synthesis of framework compositions other than carbon, since the leaching of the silica typically also affects the material which is filled into the silica pore system. This problem could possibly be circumvented by not using the silica as the mold, but to instead go one step further and use the mesoporous ordered carbons as templates, which could then easily be removed by combustion or other techniques, as suggested recently.[4] On the macroscale, that is, for the production of photonic crystals, similar approaches are well known, where latex spheres are used as templates which can be removed by calcinations.[5] Also carbon black has been used as a TMtemplate∫, for instance to synthesize mesoporous zeolite single crystals, in which the pores, however, are disordered.[6] In a first attempt to show the feasibility of using ordered mesoporous carbon to synthesize ordered mesoporous oxides, we decided to template mesostructured silica by using an ordered mesoporous carbon. Although this brings one only back to the starting point, that is, a mesoporous silica, it COMMUNICATIONS

Temperature-Programmed Precise Control over the Sizes of Carbon Nanospheres Based on Benzoxazine Chemistry

On the basis of benzoxazine chemistry, we have established a new way to synthesize highly uniform carbon nanospheres with precisely tailored sizes and high monodispersity. Using monomers including resorcinol, formaldehyde, and 1,6-diaminohexane, and in the presence of Pluronic F127 surfactant, polymer nanospheres are first synthesized under precisely programmed reaction temperatures. Subsequently, they are pseudomorphically and uniformly converted to carbon nanospheres in high yield, due to the excellent thermal stability of such polybenzoxazine-based polymers. The correlation between the initial reaction temperature (IRT) and the nanosphere size fits well with the quadratic function model, which can in turn predict the nanosphere size at a set IRT. The nanosphere sizes can easily go down to 200 nm while retaining excellent monodispersity, i.e., polydispersity <5%. The particle size uniformity is evidenced by the formation of large areas of periodic assembly structure. NMR, FT-IR, and elemental analyses prove the formation of a polybenzoxazine framework. As a demonstration of their versatility, nanocatalysts composed of highly dispersed Pd nanoparticles in the carbon nanospheres are fabricated, which show high conversion and selectivity, great reusability, and regeneration ability, as evidenced in a selective oxidation of benzyl alcohol to benzaldehyde under moderate conditions.

Fabrication of superior-performance SnO2@C composites for lithium-ion anodes using tubular mesoporous carbon with thin carbon walls and high pore volume

A tubular composite, including ultrafine SnO2 particles encapsulated in ordered tubular mesoporous carbon with thin walls and high pore volume, is fabricated through the in situ hydrolysis method. It is observed that up to 80 wt% of SnO2 particles with size between 4–5 nm are highly dispersed and homogeneously encapsulated in the mesopore channels and no bulky aggregates are visible. The tubular composite exhibits a considerably high reversible capacity of 978 mA h g−1 and a high initial efficiency of 71% at a current density of 200 mA g−1 between 0.005–3 V. Its reversible capacity even increases up to 1039 mA h g−1 after 100 cycles, which is much higher than the conventional theoretical capacity of SnO2 (782 mA h g−1), meanwhile, it also displays fast discharge/charge kinetics at a high current density of 1500 mA g−1. The excellent electrochemical performance is ascribed to its unique mesostructure by recruiting tubular mesoporous carbon with thin carbon walls (∼2 nm) and high pore volume (2.16 cm3 g−1). This tubular nanostructure provides confined nanospace for hosting immobilized ultrafine SnO2 with high loading, compensates volume expansion of SnO2, warrants efficient contact between nanoparticles and carbon matrix before and after Li+ insertion. We believe this special structure model might be extended for the fabrication of other cathode and anode electrode materials, to achieve high performance LIBs.

Synthesis of Discrete and Dispersible Hollow Carbon Nanospheres with High Uniformity by Using Confined Nanospace Pyrolysis

In recent years, hollow carbon nanospheres (HCSs) have attracted a great deal of attention because of their unique properties such as high surface-to-volume ratios, and excellent chemical and thermal stabilities. In this regard, HCSs are superior to polymerand metal-based hollow nanospheres. 3] HCSs are promising materials in a variety of applications such as adsorption, lithium ion batteries, fuel cells, and catalysis, and can also serve as building blocks for complex structures. The success of HCSs in these applications relies strongly on the availability of HCSs with carefully controlled diameter and shell thickness, surface properties, crystallinity of the carbon shell, and dispersibility in media. Much effort has been devoted to the synthesis of HCSs by a nanocasting approach. However, most previous reports on HCSs have the sole aim of obtaining hollow structural units, and little attention has been paid to addressing the issue of particle conglutination. In fact, an inevitable tendency of all carbon nanostructures during high-temperature annealing is the incidental condensing and sintering. Consequently, the end results are often nondispersible and conglutinated bulky materials. This challenge becomes even greater when the carbon particles are smaller, for example, below 200 nm. Discrete and dispersible HCSs are of critical importance for both the fundamental study of carbon colloids and for many practical applications such as colloidal catalysts, drug carriers, nanodevices, and inks. However, to the best of our knowledge, no feasible solution has been reported to date that can satisfactorily resolve the issue of conglutination for carbon nanoparticles. Thus, it remains a great challenge to develop a simple and effective strategy to overcome the barrier and to produce discrete and dispersible HCSs with high uniformity. Herein, we describe a new method referred to as “confined nanospace pyrolysis” for the synthesis of discrete and highly dispersible HCSs that have both tailorable shell thickness and cavity size. This idea is inspired from nature, in particular the structure of an egg, which consists of an inorganic outer shell (eggshell), organic inner shell (egg white), and core (yolk). The presence of the inorganic outer shell is an essential element that ensures that eggs retain their original shape, and remain discrete. A designed synthesis of HCSs that was inspired by the egg structure is illustrated in Figure 1a. Firstly, monodisperse polystyrene nanospheres (PS) with a specific particle size are prepared as the seed;

Engineering of Hollow Core–Shell Interlinked Carbon Spheres for Highly Stable Lithium–Sulfur Batteries

We report engineered hollow core-shell interlinked carbon spheres that consist of a mesoporous shell, a hollow void, and an anchored carbon core and are expected to be ideal sulfur hosts for overcoming the shortage of Li-S batteries. The hollow core-shell interlinked carbon spheres were obtained through solution synthesis of polymer spheres followed by a pyrolysis process that occurred in the hermetical silica shell. During the pyrolysis, the polymer sphere was transformed into the carbon core and the carbonaceous volatiles were self-deposited on the silica shell due to the blocking effect of the hermetical silica shell. The gravitational force and the natural driving force of lowering the surface energy tend to interlink the carbon core and carbon/silica shell, resulting in a core-shell interlinked structure. After the SiO2 shell was etched, the mesoporous carbon shell was generated. When used as the sulfur host for Li-S batteries, such a hierarchical structure provides access to Li(+) ingress/egress for reactivity with the sulfur and, meanwhile, can overcome the limitations of low sulfur loading and a severe shuttle effect in solid carbon-supported sulfur cathodes. Transmission electron microscopy and scanning transmission electron microscopy images provide visible evidence that sulfur is well-encapsulated in the hollow void. Importantly, such anchored-core carbon nanostructures can simultaneously serve as a physical buffer and an electronically connecting matrix, which helps to realize the full potential of the active materials. Based on the many merits, carbon-sulfur cathodes show a high utilization of sulfur with a sulfur loading of 70 wt % and exhibit excellent cycling stability (i.e., 960 mA h g(-1) after 200 cycles at a current density of 0.5 C).

Novel porous solids for carbon dioxide capture

The development of novel materials for CO2 capture has received much attention during the past decade. Herein, we focus on the latest advances in novel porous solids as highly effective adsorbents for CO2 capture. The advantages and existing barriers of each porous material and their future perspectives will be discussed.