Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI)_(initial)] = 2.00 × 10^?4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10^?5 mol/L (no ZSM-5 zeolite, 4.0 × 10^?3 mol/L oxalate) and 1.0 × 10^?5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10^?4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10^?3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.     

Size controlled ZSM-5 on the structure and performance of Fe catalyst in the selective catalytic reduction of NOx with NH3

Fe/ZSM-5 catalysts with various morphologies and sizes were prepared and the catalytic properties in NH₃-SCR were also investigated. The different ZSM-5 morphologies and sizes indeed influence the dispersion of Fe species. The Fe/ZSM-5 catalyst, which was cauliflower-like morphology of ZSM-5 support aggregated by small nano-crystal zeolite with crystallite size of about 50 nm, exhibited the best NH₃-SCR activity (T_≥ 90% = 280–650 °C). This specific morphology and size of ZSM-5 support were considered to benefit the distribution of isolated Fe^3 + species, which was proved to be the main active sites in SCR reaction.     

Role of acidity of catalysts on methane combustion over Pd/ZSM-5

Palladium-based catalysts were prepared by the impregnation (I) and ion-exchange method (E) with ZSM-5 and γ-Al₂O₃ as support respectively. The high activity of Pd/ZSM-5(I) and Pd-ZSM-5(E) catalysts for methane combustion was observed. The order of activity is consistent with Brønsted acidity of catalysts: Pd/ZSM-5(I) > Pd-ZSM-5(E) > Pd/Al₂O₃. It is shown by FT-IR that methane adsorbs on acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. Symmetric v₁ C–H stretching vibrations of methane shift to low frequency due to the interaction between methane molecules and Brønsted acid sites or Pd²⁺, indicating that methane molecules can be activated.     

Thermal-catalytic degradation kinetics of polypropylene over BEA,ZSM-5 and MOR zeolites

In this study, thermal degradation of additive-free polypropylene powder over different type of zeolite catalysts was investigated. BEA, ZSM-5 and MOR with different surface areas, pore structures, acidities and Si/Al molar ratios were used as solid catalysts for degradation of polypropylene (PP). Degradation rate of the PP over zeolites was studied by thermogravimetric analysis (TGA) employing four different heating rates and apparent activation energies of the processes were determined by the Kissinger equation. The catalytic activity of zeolites decreases as BEA > ZSM-5a (Si/Al = 12.5) > ZSM-5b (Si/Al = 25) > MOR depending on pore size and acidity of the catalysts. On the other hand, initial degradation is relatively faster over MOR and BEA than that over both ZSM-5 catalysts depending on the apparent activation energy. It can be concluded that acidity of the catalyst is the most important parameter in determining the activity for polymer degradation process as well as other structural parameters, such as pore structure and size.     

Nitrogen-doped ordered mesoporous carbons based on cyanamide as the dopant for supercapacitor

Nitrogen-doped ordered mesoporous carbons (N-doped OMCs) with a high surface area of 1741 m²/g and nitrogen content up to 15 wt.% have been synthesized by nanocasting approach by using SBA-15 as a hard template, phenolic resin (resol) as a carbon source and high nitrogen-containing cyanamide as the nitrogen dopant. The introduction of cyanamide not only incorporates high-content nitrogen into the carbon matrix in the primary forms of pyridinic and quaternary species, but also greatly increases the surface area of materials. The obtained N-doped OMCs have large surface area with mesoporosity up to 92%, uniform and appropriate pore size (3.6–4.1 nm), large pore volume (1.2–1.81 cm³/g). These merits together with high nitrogen enrichment lead to a specific capacitance (230 F/g at 0.5 A/g) and good rate capability (175 F/g at 20 A/g with capacitance retention of 77.4%) in 6 M KOH aqueous electrolytes.     

Storage of molecular hydrogen into ZSM-5 zeolite in the ambient atmosphere by the sealing of the micropore outlet

The storage of molecular hydrogen into ZSM-5 zeolite in the ambient atmosphere was examined by hydrogen filling into the micropore and the following sealing of the micropore outlet to prevent the release of hydrogen to the outside. The surface grafting of 1,4-bis(hydroxydimethylsilyl)benzene onto ZSM-5 zeolite was applied to the sealing of the micropore outlet. Pressurized hydrogen (10 MPa) was filled into the micropore of ZSM-5 at liquid nitrogen temperature (−196 °C), and then the sample was heated at 150 °C for forming strong binding between the zeolite surface and the disilane compound under the hydrogen pressure. The hydrogen sorption isotherm at −196 °C showed that the adsorption of hydrogen onto the disilane-grafted ZSM-5 thus obtained was reduced to less than 20% from the original ZSM-5. The remarkable hysteresis between the adsorption and desorption branches of the isotherm indicated that the kinetic trap of hydrogen occurred by the narrowed outlets of the micropores of ZSM-5 with the disilane compound. Even after exposing the disilane-grafted ZSM-5 to the atmosphere over a few months, hydrogen could be discharged by heating over 150 °C. This result demonstrated that molecular hydrogen was successfully stored into ZSM-5 zeolite in the ambient atmosphere for a long time.     

Internal versus external surface active sites in ZSM-5 zeolite: Part 2: Toluene alkylation with methanol and 2-propanol catalyzed by modified and unmodified H3PO4/ZSM-5

Vapor-phase methylation of toluene with methanol and isopropylation of toluene with 2-propanol has been investigated in a down flow reactor under atmospheric conditions using N₂ gas carrier over a series of surface modified and unmodified ZSM-5 (Si/Al = 60–170) loaded with H₃PO₄, differing in the external surface treatment of the zeolites. The feed molar ratios of toluene/methanol and toluene/2-propanol were varied over a wide range (8–0.125), and the optimum feed ratio of toluene/alcohol was less than 0.5 in both cases. Space velocity employed in toluene methylation reported as WHSV (toluene) = 1.2 h⁻¹, and the space velocity employed in toluene isopropylation reported as WHSV (toluene) = 0.8 h⁻¹. The methylation reactions were carried out in the temperature range of 623–773 K, and the isopropylation reactions were carried out in the temperature range of 483–583 K. Atmospheric pressures was maintained in all runs. Catalysts containing 0–4.9 wt.% P were prepared using modified and unmodified ZSM-5 zeolites, and their catalytic performance for vapor-phase alkylation of toluene with methanol and 2-propanol were investigated. The optimum phosphorous content for methylation was 2.1 wt.% P which was greater than the optimum phosphorous loading for isopropylation (0.7 wt.% P).     

Photocatalytic reduction of CO2 to energy products using Cu–TiO2/ZSM-5 and Co–TiO2/ZSM-5 under low energy irradiation

Copper or cobalt incorporated TiO₂ supported ZSM-5 catalysts were prepared by a sol–gel method, and then were characterized by XRD, BET, XPS and UV–vis diffuse reflectance spectroscopy. Ti^3 + was the main titanium specie in TiO₂/ZSM-5 and Cu–TiO₂/ZSM-5, which will be oxide to Ti^4 + after Co was doped. With the deposition of Cu or Co, the efficiency of the CO₂ conversion to CH₃OH was increased under low energy irradiation. The peak production rate of CH₃OH reached 50.05 and 35.12 μmol g^− 1 h^− 1, respectively. High photo energy efficiency (PEE) and quantum yield (φ) were also reached. The mechanism was discussed in our study.     

One-pot hydrothermal synthesis and supercapacitive performance of nitrogen and MnO co-doped hierarchical porous carbon monoliths

Nitrogen and MnO co-doped hierarchical porous carbon monolith (N-MnO-HPCM) materials were synthesized through a facile one-pot hydrothermal method. The resulting N-MnO-HPCM materials had hierarchical porous structure, high BET surface area (606 m²/g), large pore volume (0.33 cm³/g), and contained evenly dispersed MnO nanoparticles of about 6 nm in the carbon matrix. Their electrochemical performances as electrodes for supercapacitors were investigated. N-MnO-HPCM material exhibited an excellent electrochemical performance with a specific capacitance of 261.7 F/g at a current density of 1 A/g. It also showed a good rate capability with 74% capacity retention at high current density (5 A/g), indicating its potential applications in supercapacitors.     

NO reduction over nanostructure M-Cu/ZSM-5 (M: Cr,Mn, Co and Fe) bimetallic catalysts and optimization of catalyst preparation by RSM

The selective catalytic reduction (SCR) of NO with NH₃ in the presence of oxygen over a series of H-ZSM-5 supported transition metal oxides (Co, Mn, Cr, Cu and Fe) was investigated. Among them, Cu/ZSM-5 nanocatalyst was found to be the most promising catalyst based on activity. The modification of Cu/ZSM-5 by adding different transition metals (Co, Mn, Cr and Fe) to improve the efficiency of NO conversion was studied. The results indicated that the Fe–Cu/ZSM-5 bimetallic nanocatalyst was the highest active catalyst for NO conversion (67% at 250 °C and 93% at 300 °C). Response surface methodology (RSM) involving central composite design (CCD) was employed to evaluate and optimize Fe–Cu/ZSM-5 preparation parameters (Fe loading, calcinations temperature, and impregnation temperature) in SCR of NO at 250 °C. The optimum condition for maximum NO conversion was estimated at 4.2 wt.% Fe loading, calcinations temperature of 577 °C and impregnation temperature of 43.5 °C. Under these condition, experimental NO conversion efficiency was 78.8%, which was close with the predicted value (79.4%).     

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp³ average surface hybridization, sp³ / (sp² + sp³) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10¹⁴ cm^? 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.     

Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions

Activated carbons were prepared from sodium lignosulfonate by phosphoric acid activation at carbonization temperatures of 400–1000 °C. The resulting materials were characterized with regard to their surface area, pore volume, pore size distribution, distribution of surface groups and ability to adsorb copper ions. Activated carbons were characterized by nitrogen adsorption, scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analyses. The results indicate that with increasing carbonization temperature, the surface area decreased from 770 m²/g at 400 °C to 180 m²/g at 700 °C and increased at higher temperatures to 1370 m²/g at 1000 °C. The phosphorus content peaked at 11% for carbon obtained by carbonization at 800 °C. Potentiometric titration revealed the acidic character of all the phosphoric acid-activated carbons, which were found to have total concentrations of surface groups of up to 3.3 mmol/g. The carbons showed a high adsorption capacity for copper ions even at pH values as low as 2.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Synthesis of nanostructured carbons by the microwave plasma cracking of methane

One of the attractive methods of producing hydrogen and high value-added carbon is plasma-reforming of hydrocarbons. Here, nanostructured carbon was produced by methane cracking in a relatively low-energy cold plasma reactor designed in-house specifically for such purpose. Carbon samples collected at different positions in the reactor show similar structural morphologies, indicating extensive structural uniformity of the carbon during processing. Surface area and microstructure of the materials were characterized by BET surface area analysis, X-ray diffraction and transmission electron microscopy (TEM). The effects of flow rate, temperature and power were evaluated for the formation of the carbon structures. The results show that the BET surface area and pore volume of the carbon materials vary from 74 to 125 m²/g and from 0.12 to 0.20 cm³/g, respectively. Such variations are closely associated with the magnitude of temperature drop at the sample collection position in the cold-plasma chamber before and after methane loading. The highest BET surface area of 125 m²/g is obtained at a power of 2000 W. TEM shows that the carbon consists of spherical particles of 40.8 ± 8.7 nm in diameter and graphene sheets.     

Preparation of sulfur-doped microporous carbons for the storage of hydrogen and carbon dioxide

Structurally well ordered, sulfur-doped microporous carbon materials have been successfully prepared by a nanocasting method using zeolite EMC-2 as a hard template. The carbon materials exhibited well-resolved diffraction peaks in powder XRD patterns and ordered micropore channels in TEM images. Adjusting the synthesis conditions, carbons possess a tunable sulfur content in the range of 1.3–6.6 wt.%, a surface area of 729–1627 m² g^?1 and a pore volume of 0.60–0.90 cm³ g^?1. A significant proportion of the porosity in the carbons (up to 82% and 63% for surface area and pore volume, respectively) is contributed by micropores. The sulfur-doped microporous carbons exhibit isosteric heat of hydrogen adsorption up to 9.2 kJ mol^?1 and a high hydrogen uptake density of 14.3 × 10^?3 mmol m^?2 at ?196 °C and 20 bar, one of the highest ever observed for nanoporous carbons. They also show a high CO₂ adsorption energy up to 59 kJ mol^?1 at lower coverages (with 22 kJ mol^?1 at higher CO₂ coverages), the highest ever reported for any porous carbon materials and one of the highest amongst all the porous materials. These findings suggest that S-doped microporous carbons are potential promising adsorbents for hydrogen and CO₂.     

Additive and pyrolysis atmosphere effects on polysiloxane-derived porous SiOC ceramics

Porous silicon oxycarbide (SiOC) is emerging as a much superior ultrahigh surface area material that can be stable up to high temperatures with great tailorability through composition and additive modifications. In this study, bulk SiOCs were fabricated from a base polysiloxane (PSO) system by using different organic additives and pyrolysis atmospheres followed by hydrofluoric acid (HF) etching. The additives modify the microstructural evolution by influencing the SiO₂ nanodomain formation. The SiOC ceramics contain significantly less SiC and more SiO₂ with Ar + H₂O atmosphere pyrolysis compared to Ar atmosphere pyrolysis. Water vapor injection during pyrolysis also causes a drastic increase in specific surface areas. The addition of 10 wt% tetraethyl orthosilicate (TEOS) with Ar + H₂O pyrolysis produces a specific surface area of 1953.94 m²/g, compared to 880.09 m²/g for the base PSO pyrolyzed in Ar. The fundamental processes for the composition and phase evolutions are discussed as a novel pathway to creating ultrahigh surface area materials. The ability to drastically increase the specific surface area through the use of pyrolysis atmosphere and organic additives presents a promising processing route for highly porous SiOC ceramics.     

Microstructure,mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Boron carbide (B₄C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m² g^?1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m^1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W₂B₅ platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B₄C–W₂B₅ composites could be potential candidate materials for structural applications.     

Superhigh-rate capacitive performance of heteroatoms-doped double shell hollow carbon spheres

The salient practical application feature of an ideal supercapacitor is its ability to deliver high energy density stably even at ultrahigh power density. Therefore, a rational design of electrode materials is essentially required for achieving high current, energy and power densities. In this work, a special “in situ replicating” strategy is employed to fabricate double shell hollow carbon spheres with homogeneously doped heteroatoms. The KOH activation introduces micropores to the thin shells of the hollow carbon spheres. Materials characterizations show that these carbon spheres have such merits as large surface area, easy-accessible micropore surface with faradaic reaction sites, and high conductivity. All these result in ultrafast ion transport from electrolyte to the micropores in the carbon spheres and endow the carbon with outstanding capacitive performance, e.g., an unprecedentedly high specific capacitance of 270 F g⁻¹ at a very high current density of 90 A g⁻¹. Moreover, a high energy density of 11.9 Wh kg⁻¹ at a respectable power density of 30,000 W kg⁻¹ is achieved in 6 M KOH electrolyte.     

Enhanced production of high octane gasoline blending stock from methanol with improved catalyst life on nano-crystalline ZSM-5 catalyst

Methanol value addition reaction has been studied on lab-synthesized nano-crystalline ZSM-5, Si/Al = 13 (NZ) possessing particle size of ∼29–51 nm and a micro-crystalline ZSM-5 (MZ) of similar atomic ratio is also taken as standard for comparison studies. The NZ sample exhibited excellent catalytic activity to produce 50.7 wt.% of high octane (Research Octane Number = 137) gasoline blending stock rich in desired toluene and xylene components, while the undesired benzene is very low, suitable for fuel applications. The superior performance of NZ to MZ catalyst reflected in three fold increase in gasoline yield and considerably high time-on-stream performance.     

High-performance charge storage by N-containing nanostructured carbon derived from polyaniline

Nitrogen-containing nanostructured carbon materials, C-nanoPANI, C-nanoPANI-DNSA and C-nanoPANI-SSA, were prepared by the carbonization of nanostructured polyaniline (PANI) doped with sulfuric acid, 3,5-dinitrosalicylic acid (DNSA), and 5-sulfosalicylic acid (SSA), respectively. The charge storage ability of these materials was investigated in alkaline solution. It was found that the specific capacitance increased in the order: C-nanoPANI-DNSA < C-nanoPANI < C-nanoPANI-SSA. The highest capacitance, amounting to 410 F g^?1 at a scan rate of 5 mV s^?1, was found for C-nanoPANI-SSA. At a large rate of 10 A g^?1, its capacitance displayed a stable value close to 200 F g^?1. To explain the observed differences in charge storage properties, the materials were characterized by different techniques able to ascertain their morphology, elemental composition, nitrogen surface concentration, chemical state of nitrogen, pore structure and electrical conductivity. All materials were essentially microporous with relatively small fraction of mesopores and displayed conductivities in the range 0.32–0.83 S cm^?1. The best charge-storage performance of C-nanoPANI-SSA was attributed to its highest surface fraction of nitrogen, the highest surface content of pyridinic nitrogen groups, and the highest electrical conductivity, as well as to its well-balanced micro- and mesoporosity and highest content of mesopores.