Surface chemistry of ordered mesoporous carbons

Ordered mesoporous carbons (OMC) were produced by pyrolysis of hydrocarbons adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in either hydrofluoric acid or sodium hydroxide. Some carbons were subsequently heat treated at temperatures of up to 1600 °C. The chemistry of the external surface was studied by X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SIMS). Information on the graphitic order of the surface of the mesopores was obtained from low-pressure nitrogen adsorption data. The external and internal surface of the OMC has a polyaromatic, graphite-like character. This character increases considerably with increasing pyrolysis and/or post-pyrolysis temperature, as expected. According to the XPS and the nitrogen adsorption data, this increase was especially pronounced for temperatures above 1100 °C. In spite of the different pore structures, only small differences in the polyaromatic character were found for OMC synthesised either in a MCM-48 or in a SBA-15 matrix. Differences exist for the non-carbon elements. When hydrofluoric acid is used for dissolution of the silica matrix, organic fluorine compounds are formed. Their concentration is higher when a MCM-48 matrix as opposed to a SBA-15 matrix was used. Dissolution of the silica matrix in sodium hydroxide yielded a less contaminated OMC as compared to dissolution in hydrofluoric acid.     

Synthesis of ordered mesoporous silicon oxycarbide monoliths via preceramic polymer nanocasting

Highly ordered mesoporous silicon oxycarbide (SiOC) monoliths have been synthesized using liquid poly(hydridomethylsiloxane) (PHMS) as starting preceramic polymer and mesoporous carbon CMK-3 as direct template. Monolithic SiOC-carbon composites were generated via nanocasting of PHMS into CMK-3, pressing without any additive, cross-linking at 150 °C under humid air and subsequent thermolysis at 1000 or 1200 °C under argon atmosphere. The carbon template was finally removed by the thermal treatment at 1000 °C in an ammonia atmosphere, as a result of the generation of monolithic SiOC ceramics with ordered mesoporous structures. The products were characterized by scanning electron and transmission electron microscopes, X-ray diffraction, Fourier transformation infrared spectrometer, X-ray photoelectron spectroscope and nitrogen absorption-desorption analyzer. The as-prepared SiOC monoliths exhibited crack-free, ordered 2-dimentional hexagonal p6mm symmetry with high specific surface areas. With increasing the calcination temperature, the ordered mesoporous structure was still remained and the specific surface area just had a slight reduction from 616 to 602 m² g⁻¹. Moreover, the porous SiOC monoliths possessed good compression strengths and anti-oxidation properties.     

The effect of the cobalt loading on the growth of single wall carbon nanotubes by CO disproportionation on Co-MCM-41 catalysts

Highly ordered MCM-41 mesoporous molecular sieves in which silicon was isomorphously substituted with 0.5-4 wt.% cobalt were synthesized using an alkyl template with a 16 carbon atoms alkyl chain length. These materials were used as catalysts for the synthesis of uniform diameter single wall carbon nanotubes (SWNT) by CO disproportionation (Boudouard reaction). The SWNT synthesis conditions were identical for all Co-MCM-41 samples, and consisted of pre-reduction of the Co-MCM-41 catalyst in hydrogen at 500 °C for 30 min followed by reaction with pure CO at 800 °C and 6 atm for 1 h (conditions previously optimized for 1 wt.% Co-MCM-41). The SWNT grown in the Co-MCM-41 catalysts were characterized by TGA, multi-excitation energy Raman spectroscopy and TEM. The state of the catalyst and the size of the metallic cobalt clusters formed in Co-MCM-41 during the SWNT synthesis were characterized by X-ray absorption spectroscopy. The mechanism controlling the diameter distribution of the SWNT produced is related to the size uniformity of the cobalt clusters nucleated in the Co-MCM-41 catalytic template: the SWNT growth selectivity and size uniformity is influenced by the cobalt concentration in the framework. If the cobalt is not initially strongly stabilized in the MCM-41 framework during template synthesis, the catalyst produces SWNT with a wider diameter distribution. Co-MCM-41 catalysts with up to 3 wt.% cobalt can be used to grow SWNT with a diameter distribution similar to that obtained with 1 wt.% Co-MCM-41, but at yields greater by a factor of approximately 2.4.     

Textural property tuning of ordered mesoporous carbon obtained by glycerol conversion using SBA-15 silica as template

“Wet” and “dry” template methods were used to simultaneously control the pore size, morphology, and graphitization in structurally well-ordered mesoporous carbons. A novel structure has been prepared using glycerol as the carbon source. Depending on the loading amount of the glycerol, the structural characteristics of the mesoporous carbon materials can be controlled. The structurally well-ordered carbon materials have been characterized by various techniques such as nitrogen sorption, XRD, Raman spectroscopy, TGA measurements and SEM. They have graphitic character, high BET surface area (ca 1440 m²/g) and a tunable pore size. They are likely to be useful in a variety of applications including gas storage, electrode materials or catalyst supports.     

Ordered mesoporous carbons: Implication of surface chemistry, pore structure and adsorption of methyl mercaptan

Three ordered mesoporous carbon materials (OMC) were prepared by pyrolysis of sucrose filled in the mesoporous channels of SBA-15 at three different temperatures of 600 °C (OMC-600), 850 °C (OMC-850) and 1100 °C (OMC-1100), and followed by dissolution of the silica matrix in hydrofluoric acid. The pore structures and surface characteristics of the OMC materials were evaluated using XRD, nitrogen adsorption, FTIR spectroscopy, pH of carbon surface, and the amount of acidic surface groups from Boehm titration, respectively. The increase of the pyrolysis temperature resulted in an increase in surface pH, but a decrease in the amount of acidic surface groups. The surface area and micro- and mesopore volumes increased by increasing the pyrolysis temperature from 600 °C to 850 °C, but there were no significant changes in these properties above 850 °C. In this paper, adsorption characteristics of methyl mercaptan on the OMC materials were studied using a dynamic adsorption method in a fixed bed. The results showed that the adsorption of methyl mercaptan was strongly influenced by pore structure and surface chemistry of the OMC.     

Nitrogen-containing carbon spheres with very large uniform mesopores: The superior electrode materials for EDLC in organic electrolyte

We reported the electrochemical studies on mesoporous carbon spheres (MCS) enriched in nitrogen on frameworks serving as an electrode for electric double layer capacitor (EDLC) in an organic electrolyte. The preparation of the carbon spheres is involved in a facile polymerization-induced colloid aggregation method by using melamine-formaldehyde resin (MF) as the carbon precursor, and commercial fumed silica (Aerosil-200) as a hard template. After the carbonization of as-formed resin-template composites at 1000 °C under a nitrogen atmosphere, and the removal of silica template by HF treatment, monodisperse MCS with diameter size of ∼1.2 μm, high specific surface area (up to 1460 m²/g) and uniform pore size as large as 31 nm can be obtained. The MCS product presents a high specific capacitance as 159 F/g at 0.5 A/g. The high specific capacitance of the MCS is believed to be associated with its suitable nitrogen content that can afford pseudocapacitance as well as the high specific surface area. Furthermore, the specific capacitance of MCS can remain 130 F/g at high current density of 20 A/g. Our results show that the moderate nitrogen content can enhance the surface wettability and reduce the resistance, the large pore size can accelerate the diffusion process for the ions solvated with big organic solvent molecules in the pores. In view of the facts that precursors used in this simple process are commercially available low-cost chemicals, researches on the synthesis of such MCS materials templated by silica nanoparticles may not only be theoretically important, but also provide more options for economical and large-scale productions of mesoporous carbon materials with desired nanostructures, which might find practical applications in the fields of EDLC with high power performance.     

MCM-41介孔碳材料的合成及其电化学性能研究

该文以十六烷基三甲基溴化铵（CTAB）为模板剂,可溶性淀粉(Starch)为碳源,通过软模板法一步合成了高度有序的介孔碳材料。通过热重分析（TG）、X-射线衍射（XRD）、N2吸附-脱附和透射电子显微镜（TEM）等对材料的结构进行了表征。结果表明：当模板剂与可溶性淀粉质量比为m(CTAB):m(Starch)=1.0：1.5,650 ℃焙烧碳化3 h,并用氢氟酸去除二氧化硅后,所得到的MCM-41有序介孔碳材料（OMC-650）的比表面积为985 m2/g,平均孔径为孔径分布均匀,平均孔径为2.5 nm。XRD和TEM分析表明,OMC-650具有典型的MCM-41结构特征,有序性良好。以OMC-650作为工作电极,氧化汞为参比电极,铂为辅助电极,用6 M•L-1 KOH做电解液,测其比电容为150 F/g,且经过1000次循环后,其比电容仍为138 F/g,为原电容的92%,说明所合成的材料具有良好的电容稳定性。     

Pore structure and graphitic surface nature of ordered mesoporous carbons probed by low-pressure nitrogen adsorption

Ordered mesoporous carbons (OMCs) were produced by pyrolysis of sucrose adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in hydrofluoric acid. Subsequently, some of these OMCs were heat-treated at temperatures of up to 1600 °C. The OMC pore structure was studied by low-pressure nitrogen adsorption. Information on the graphitic order of the surface of the mesopore walls was also obtained from the nitrogen adsorption data. These results were correlated to the order of the graphene layers at the outer surface, which was studied by X-ray photoelectron spectroscopy (XPS).

The OMCs were predominantly mesoporous, but they also contained micropores. For OMCs produced in an SBA-15 matrix, the micropore volume decreased upon heating. After heating to 1600 °C, nearly all micropores had disappeared. Furthermore, upon heating the width of the mesopores increased from 35 to 50 Å. All these changes can be explained by a shrinking of the OMC framework upon heating. A different behavior was found for OMCs derived from MCM-48. Upon heating these materials at increasingly high temperatures, the width of the mesopores first decreased, and for temperatures above 1100 °C it increased again. For all OMCs studied the graphitic order of the mesopores and the order of the graphene layers at the outer surface increased upon heating. For a given temperature, the graphitic surface order of OMCs derived from SBA-15 and MCM-48 was similar.     

PtCo supported on ordered mesoporous carbon as an electrode catalyst for methanol oxidation

A catalyst for methanol oxidation, PtCo supported on graphitized mesoporous carbon, has been synthesized and its electrochemical activity for methanol oxidation has been investigated. The graphitized mesoporous carbon support with ordered pore structure and high surface area of 585 m² g⁻¹ was prepared by one-step melt casting method using Al doped hexagonal mesoporous silica as hard templates and mineral pitches as carbon precursors followed by carbonization at 800 °C. The materials were characterized by X-ray diffraction, Raman spectra, field emission scanning electron microscopy, transmission electron microscopy and nitrogen sorption techniques. Cyclic voltammetry and amperometric i-t tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that the graphitized mesoporous carbon exhibits large electrochemical capacitance and good electric property. After supported with 20 wt%Pt or 20 wt%PtCo nanoparticles, the resultant mesostructured composites show 26-97% higher electrochemical catalytic activity for methanol oxidation than commercial catalyst 20 wt%Pt/C in mass activity (mA mg Pt⁻¹).     

Synthesis of thermally stable, high surface area, nanocrystalline mesoporous tetragonal zirconium dioxide (ZrO2): Effects of different process parameters

Nanocrystalline mesoporous zirconium dioxide powder with high surface area and remarkable thermal stability was synthesized using ethylene diamine and zirconyl chloride octahydrate. Ethylene diamine used as precipitating agent also acted as a colloidal protecting agent. The material retained high surface area (193.1 m²/g) mesoporous structure even after calcination at 900 °C with the surface area of the as-prepared material exceeding 440 m²/g depending upon the preparation conditions. Effects of different process parameters such as digestion time, pH, precursor concentration and calcination temperature on structural properties of the material were studied. These preparation conditions significantly affected the structural stability, crystal size and the crystal phase of the final material. The material was characterized by nitrogen adsorption-desorption, X-ray diffraction, scanning electron microscopy, thermogravimetric and differential scanning calorimetry, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The final material has very high tetragonal phase stability even after calcination at 1000 °C temperature.     

Synthesis of ultra-large mesoporous carbons from triblock copolymers and phloroglucinol/formaldehyde polymer

Ultra-large mesoporous carbon materials were synthesized with the use of triblock copolymers and phloroglucinol/formaldehyde polymer as filler under acid conditions. The carbons obtained by employing F127 as template and carbonizing at 600 °C exhibit large mesopores with a narrow pore-size distribution centered at 19.2 nm. With the assistance of decane as a swelling agent, the pore size of the P123-templated 600 °C-carbonized carbons could be enlarged from 11.5 to 14.7 nm. It is demonstrated that the low synthesis temperature and high reactivity of phloroglucinol are two key factors for the formation of large mesopores.     

Kinetic behavior of sunflower oil pyrolysis over mesoporous materials

Mesoporous materials type MCM-41 and AlMCM-41 were synthesized by the hydrothermal method and characterized by X-ray diffraction and nitrogen adsorption. Analyses have showed that the hexagonal structures of the materials were preserved and the template completely removed. In this work, the degradation of pure sunflower oil and mixed with mesoporous materials were evaluated. A kinetic study of the oil degradation was performed by thermal analysis under heating rates of 5, 10 and 20 °C min⁻¹ using the model-free kinetic in order to determine the kinetic parameters for this process. From this method, the obtained values of apparent activation energy for sunflower oil/AlMCM-41 was of 167 kJ mol^− 1.     

Hydrocracking of petroleum gas oil over NiW/MCM-48-USY composite catalyst

MCM-48-USY composite materials were prepared by coating USY zeolite by a layer of MCM-48 mesoporous material at different meso/microporous ratios (SiO₂/USY ratios of 0.1, 0.2, 0.3, 0.4, 0.5) and used as support for nickel and tungsten. The NiW/MCM-48-USY catalysts were prepared using the incipient wetness method. The prepared catalysts were characterized by TPD-TGA acidity, TGA thermal stability, BET surface area, pore volume, pore size, XRD, SEM and TEM and then tested for hydrocracking of petroleum gas oil at reaction temperature of 450 °C, contact time of 90 min and catalyst to gas oil ratio of 0.04. In all prepared samples, the catalyst activity and properties were improved with increasing SiO₂/USY ratio and found that maximum values of a total conversion and liquid product (total distillate fuels) were obtained at SiO₂/USY ratio of 0.5. Finally, the obtained results from hydrocracking of gas oil over composite MCM-48-USY catalysts were compared with those obtained over physically mixed USY and MCM-48 catalysts.     

Adjusting the texture and nitrogen content of ordered mesoporous nitrogen-doped carbon materials prepared using SBA-15 silica as a template

Mesoporous nitrogen-doped carbon (N-MC) with highly ordered two dimensional hexagonal structures has been synthesized using diaminobenzene (DAB) as carbon and nitrogen sources, ammonium peroxydisulfate (APDS) as an oxidant, and SBA-15 as a hard template. The effect of synthesis temperatures and precursors on the textures and nitrogen content of the N-MC is investigated. By adjusting the synthesis temperatures in a range of 70-100 °C, the pore diameter of the N-MC materials can be tuned from 3.43 to 4.15 nm, while the specific surface area of the N-MC with a nitrogen content of 26.5 wt.%, can be tuned from 281.8 to 535.2 m²/g. The C/N molar ratio of the N-MC can be tuned in a range of 3.25-3.65 by adjusting the mole ratio of DAB/APDS precursors at a synthesis temperature of 80 °C, while the pore diameter of the N-MC be tuned in a range of 4.12-3.66 nm. With the nitrogen doping content increasing, the lattice constant of N-MC materials decreases. Investigation indicates that well ordered N-MC materials with large specific surface area, high total pore volume and high nitrogen content can be fabricated under a synthesis temperature of 80 °C and a molar ratio of 1.5 for DAB/APDS precursors.     

Easy synthesis of highly nitrogen-enriched graphitic carbon with a high hydrogen storage capacity at room temperature

An easy method for synthesizing highly nitrogen-enriched graphitic carbon was developed and its hydrogen storage capacity was explored. The synthesis method uses a solution-based, stepwise condensation reaction between cyanuric chloride and melamine at low temperature (e.g., 0, 25, and 120 °C) and ambient pressure using conventional glassware without the need for an autoclave vessel. The physical and chemical structure of the synthesized highly nitrogen-enriched graphitic carbon was investigated by powder X-ray diffraction, scanning and transmission electron microscopy, selected area electron diffraction, energy dispersive spectroscopy, elemental analysis, Fourier transform infrared spectroscopy, X-ray photoemission spectroscopy, and electron energy loss spectroscopy. The analyzes confirmed that the product has a highly crystalline nitrogen-enriched graphitic structure (d₀₀₂ = 0.324 nm) with a carbon-to-nitrogen ratio of 1:1.12 (>50 atomic% nitrogen content). The material was determined to have an excellent hydrogen storage capacity of 0.34 wt% at room temperature under 100 bar in spite of its low BET surface area of only ∼10 m²/g.     

MCM-48 supported chromium catalyst for trichloroethylene oxidation

MCM-48 supported chromium (Cr/MCM-48) catalyst was prepared by introduction of chromium chloride during gel-preparation for the hydrothermal synthesis of MCM-48. Based on the N₂ adsorption/desorption isotherm, BET, pore size distribution, XRD, FTIR, TGA, and DTA data, Cr/MCM-48 was found to have high surface area (832 m²/g), uniform pore size distribution (24 Å), mesoporous structures similar to MCM-48 itself, and incorporation of about 3 wt% of Cr component in the mesoporous framework. Cr/MCM-48 was very active for the oxidative destruction of trichloroethylene (TCE), which is a typical chlorinated volatile organic compound (CVOC); 100% conversion of TCE was achieved at 350°C. Based on the TGA of trichloroethylene/water adsorption study, it was found that MCM-48 had high adsorption capacity (>0.25 g TCE/g catalyst). In addition, the hydrophobicity of the adsorptive properties of Cr/MCM-48 materials could be modified. The high adsorption capacity and catalytic activity of Cr/MCM-48 material make it suitable for adsorption/catalysis bifunctional systems for energy-saving treatment of low concentration of VOC or CVOC.     

Metal-organic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor

Five nanoporous carbons (NPCs) were prepared by polymerizing and then carbonizing carbon precursor of furfuryl alcohol accommodated in a porous metal-organic framework (MOF-5, [Zn₄O(bdc)₃], bdc = 1,4-benzenedicarboxylate) template. The Brunauer-Emmett-Teller (BET) surface areas for five NPC samples obtained by carbonizing at the temperatures from 530 to 1000 °C fall into the range from 1140 to 3040 m² g⁻¹ and the dependence of BET surface areas on carbonization temperatures shows a “V” shape. All the five NPC samples have a pore size distribution centered at about 3.9 nm. As electrode materials for supercapacitor, the NPC samples obtained at the temperatures higher than 600 °C display the ideal capacitor behaviors and give rise to almost constant specific capacitance (above 100 F g⁻¹ at 5 mV s⁻¹) at various sweep rates, which is associated with their mesoporous characteristics. However, the NPC sample with the highest BET surface area (3040 m² g⁻¹) obtained by carbonizing at 530 °C gives a unusually low capacitance (12 F g⁻¹ at 5 mV s⁻¹), which may be attributed to the poor conductivity of the carbon material due to the low carbonization temperature.     

Modified supercritical CO2 extraction of amine template from hexagonal mesoporous silica (HMS) materials: Effects of template identity and matrix Al/Si molar ratio

In this work, the primary amine template has been extracted from freshly synthesized hexagonal mesoporous silica (HMS) materials by means of modified supercritical carbon dioxide at 60–85 °C under 10.0–20.0 MPa. The influences of amine identity and matrix Al/Si ratio on the extraction efficiencies and structural properties of HMS thus obtained are investigated in detail. The results show that the extraction efficiency is strongly dependent on the pore size of the HMS materials produced by five different templates. For aluminium-incorporated samples, the extraction efficiency is observed to decrease with the Al/Si molar ratio since as the Al/Si molar ratio increases, more amine will get protonated and the matrix/template interactions become stronger, subsequently rendering the extraction more difficult and the efficiency decrease. The formic acid modifier has resulted in better extraction performance than methanol, yielding higher extraction efficiencies. The SFE-treated materials exhibit better structural properties like higher pore volume and specific surface area as compared to those prepared by conventional calcinations. Besides, results of pyridine adsorption and conversion of 2-propanol to propylene suggest that the SFE-treated HMS materials may have higher acidity than the directly calcined samples.     

Electrochemical properties of an ordered mesoporous carbon prepared by direct tri-constituent co-assembly

An ordered mesoporous carbon with a high surface area of 2390 m²/g and a large pore size of 6.7 nm was synthesized through an organic-inorganic-surfactant tri-constituent co-assembly method which used resols as the carbon precursor, silicate oligomers as the inorganic precursor and triblock copolymer as the soft template. The electrochemical properties of this carbon were evaluated as an electrode material for electrochemical double layer capacitor and lithium-ion battery. It shows rectangular-shaped cyclic voltammetry curves over a wide range of scan rates even up to 200 mV/s between 0 and 3 V, with a large capacitance of 112 F/g in nonaqueous electrolyte. As a negative electrode material for lithium-ion battery, it delivers a reversible specific capacity as high as 1048 mAh/g and a good cycling ability with capacity retention of 500 mAh/g over 50 cycles.     

Surface-modified carbons as platinum catalyst support for PEM fuel cells

Carbon forms, such as activated carbon, carbon black, carbon nanofibers and nanotubes, can be used as support materials for precious metal catalysts used in fuel cell electrodes. This work first compares the ability of functionalized high surface area graphitic (carbon nanofibers) and amorphous (activated carbon) carbons to homogeneously support finely divided platinum catalyst particles, then contrasts the performance of platinum/carbon composite electrodes within a hydrogen fuel cell. Functionalization by concentrated acid treatment results in the creation of various oxygen carrying functionalities on the otherwise inert carbon surfaces. The degree of surface functionalization is found to be a function of the functionalization treatment strength. Chemical reduction of the platinum precursor complex using milder reducing agents in the temperature range of 75-85 °C, and using ethylene glycol at 140 °C yields the smallest platinum particle sizes observed in this study, a result confirmed by X-ray diffraction and transmission electron microscopy measurements. X-ray photoelectron spectroscopy measurements confirm the existence of platinum in primarily its metallic state on the functionalized carbon surfaces.