Bimodal Pore Structure Research Articles

Pt–CeO2nanoporous spheres – an excellent catalyst for partial oxidation of methane: effect of the bimodal pore structure

Bimodal pore size distribution played the most important role for the catalyst's superior activity during POM.

Effect of the desilication of H-ZSM-5 by alkali treatment on the catalytic performance in Fischer–Tropsch synthesis

Hierarchical HZSM-5 zeolites were prepared by post-synthesis treatment of HZSM-5 zeolite with different concentrations of NaOH. The physicochemical properties of these zeolites loaded by cobalt nanoparticles in Fischer–Tropsch (FT) synthesis were investigated. It was found that the activity and CH4 selectivity increased at first, then decreased closely related to the change of catalyst reducibility indicating that higher cobalt reducibility was a prerequisite for hierarchical zeolites to improve FT catalytic performances. The catalyst treated with 0.1 M NaOH was found to be optimal for C12–18 selectivity, which was mainly attributed to bimodal porous structure to improve diffusion behavior of products combining with moderate acid sites to mitigate second reactions. While the catalyst treated with 0.15 M NaOH showed the lowest activity and higher CH4 selectivity due to its lower reducibility even the largest mesopores were observed.

Hydrologic response of engineered media in living roofs and bioretention to large rainfalls: experiments and modeling

AbstractThe hydrologic response of engineered media plays an important role in determining a stormwater control measure's ability to reduce runoff volume, flow rate, timing, and pollutant loads. Five engineered media, typical of living roof and bioretention stormwater control measures, were investigated in laboratory column experiments for their hydrologic responses to steady, large inflow rates. The inflow, medium water content response, and outflow were all measured. The water flow mechanism (uniform flow vs. preferential flow) was investigated by analyzing medium water content response in terms of timing, magnitude, and sequence with depth. Modeling the hydrologic process was conducted in the HYDRUS‐1D software, applying the Richards equation for uniform flow modeling, and a mobile–immobile model for preferential flow modeling. Uniform flow existed in most cases, including all initially dry living roof media with bimodal pore size distributions and one bioretention medium with unimodal pore size distribution. The Richards equation can predict the outflow hydrograph reasonably well for uniform flow conditions when medium hydraulic properties are adequately represented by appropriate functions. Preferential flow was found in two media with bimodal pore size distributions. The occurrence of preferential flow is more likely due to the interaction between the bimodal pore structure and the initial water content rather than the large inflow rate.

Bimodal porous TiO2 structures templated by graft copolymer/homopolymer blend for dye-sensitized solar cells with polymer electrolyte

Bimodal porous TiO2 (BP-TiO2) with large surface area, high porosity, good interconnectivity, and excellent light-scattering ability are synthesized via a facile one-step method using a self-assembled blend template consisting of an amphiphilic poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer and a hydrophobic poly(vinyl chloride) (PVC) homopolymer. The hydrophilically surface-modified TiO2 nanoparticles selectively interact with the hydrophilic POEM chains, while the addition of the PVC homopolymer increases the hydrophobic domain size, resulting in the formation of dual pores (i.e., macropores and mesopores). The sizes and numbers of macropores can easily be controlled by changing the molecular weight and amount of the PVC homopolymer. The polymer electrolyte dye-sensitized solar cells (DSSCs) fabricated with BP-TiO2 photoanodes exhibited energy conversion efficiencies of up to 7.6% at 100 mW cm−2, which is much higher than those of mesoporous TiO2 (5.8%) with PVC-g-POEM only and conventional nanocrystalline TiO2 (4.9%) with commercial Dyesol paste. The enhanced energy conversion efficiencies mostly resulted from the light-scattering effects of the macropores, which increased the light-harvesting efficiencies. The improved light-harvesting and photovoltaic performances of the DSSCs were characterized by UV–vis spectroscopy, incident photon-to-current conversion efficiency analysis, electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and intensity-modulated photovoltage spectroscopy.

Proton Conduction in Sulfonated Organic-Inorganic Hybrid Monoliths with Hierarchical Pore Structure.

Porous organic-inorganic hybrid monoliths with hierarchical porosity exhibiting macro- and mesopores are prepared via sol-gel process under variation of the mesopore size. Organic moieties in the pore walls are incorporated by substituting up to 10% of the silicon precursor tetramethylorthosilicate with bisilylated benzene molecules. After functionalization with sulfonic acid groups, the resulting sulfonated hybrid monoliths featuring a bimodal pore structure are investigated regarding proton conduction depending on temperature and relative humidity. The hierarchical pore system and controlled mesopore design turn out to be crucial for sulfonation and proton conduction. These sulfonated hybrid hierarchical monoliths containing only 10% organic precursor exhibit higher proton conduction at different relative humidities than sulfonated periodic mesoporous organosilica made of 100% bisilylated precursors exhibiting solely mesopores, even with a lower concentration of sulfonic acid groups.

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9MPa and 12.4MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600°C.

Facile fabrication of nanoporous gold with bimodal pore structure

A hierarchical nanoporous gold (HNPG) membrane with a bimodal pore size distribution was fabricated by a simple one-step dehydration reaction of H3BO3, which is different from the complex methods of multi-cyclic electrochemical co-alloying/dealloying. The HNPG exhibits a wide pore distribution from 10 to 120nm. The special architecture of HNPG significantly enhances the Raman signal to crystal violet and electrocatalytic activity toward the oxidation of methanol. The formation mechanism of the bimodal structure is discussed.

Bimodal Porous Iron/Nitrogen-Doped Highly Crystalline Carbon Nanostructure as a Cathode Catalyst for the Oxygen Reduction Reaction in an Acid Medium

Doped carbon nanomaterials as non-precious-metal catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells have received intense attraction. The improvement of ORR performance for the doped porous carbon nanostructures with high specific surface areas is mainly attributed to multidoped electrochemical active sites provided by the metallic (Fe, Co) and nonmetallic species (N, B, and S). Here, we prepared porous iron/nitrogen-doped carbon nanostructured materials via a simple synthesis process using silicate beads (500 and 50 nm diameter) as templates in the presence of 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin (TMPP) or (5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphyrin)iron(III) chloride (FeTMPP). The resulting samples exhibited a bimodal porous structure, homogeneous heteroatomic doping, and a fairly large specific surface area. In particular, the sample prepared using both 500 and 50 nm silicate beads with FeTMPP (FeTMPP-C-500/50) exhibited much improved ORR performance in an acid solution. The enhanced ORR properties of FeTMPP-C-500/50 could result from the fairly large specific surface area, mixed macro-/mesoporous structure, high crystallinity, and codoping of metal and nitrogen.

Pt/Co3O4/3DOM Al2O3: Highly effective catalysts for toluene combustion

Three-dimensionally ordered macro-/mesoporous alumina (3DOM Al2O3)-supported cobalt oxide and platinum nanocatalysts (xPt/yCo3O4/3DOM Al2O3, Pt mass fraction (x%) = 0−1.4%, Co3O4 mass fraction (y%) = 0−9.2%) were prepared using poly(methyl methacrylate) templating, incipient wetness impregnation and polyvinyl alcohol-protected reduction. The resulting xPt/yCo3O4/3DOM Al2O3 samples displayed a high-quality 3DOM architecture with macropores (180–200 nm in diameter) and mesopores (4–6 nm in diameter) together with surface areas in the range of 94 to 102 m2/g. Using these techniques, Co3O4 nanoparticles (NPs, 18.3 nm) were loaded on the 3DOM Al2O3 surface, after which Pt NPs (2.3–2.5 nm) were uniformly dispersed on the yCo3O4/3DOM Al2O3. The 1.3Pt/8.9Co3O4/3DOM Al2O3 exhibited the best performance for toluene oxidation, with a T90% value (the temperature required to achieve 90% toluene conversion) of 160 °C at a space velocity of 20000 mL g−1 h−1. It is concluded that the excellent catalytic performance of the 1.3Pt/8.9Co3O4/3DOM Al2O3 is owing to well-dispersed Pt NPs, the high concentration of adsorbed oxygen species, good low-temperature reducibility, and strong interaction between the Pt and Co3O4 NPs, as well as the unique bimodal porous structure of the support.

Fabrication and Property of Mullite Whiskers Frameworks with an Ultrahigh Porosity by Expandable Mesocarbon Microbeads

Mullite whiskers frameworks with an ultrahigh porosity were fabricated by the vapor‐phase reaction of AlF3, Al2O3, and SiO2 and adding expandable mesocarbon microbeads (MCMB) as a pore‐forming agent. A large volume expansion of 122% for MCMB due to its layered structure occurred during the formation of mullite whiskers, resulting in the expansion of samples and high porosities of 87.7%–98.2% at 50–90 wt% MCMB contents. Perfect whiskers and a lap‐joint structure formed due to the formation of mullites through the vapor‐phase reaction. A bimodal pore structure was achieved from the spaces of the whiskers framework and burning of the expanded MCMB. High compressive strengths of 1.7 to 5.4 MPa were obtained for the porous mullite at porosities of 94.2%–87.7%, which suggested a rigid structure; these strengths at the ultrahigh porosities are attributed to the merit of the framework with high strength whiskers and their strong bonding.

Mesoporous TiO2 microspheres synthesized via a facile hydrothermal method for dye sensitized solar cell applications

Mesoporous TiO2 microspheres were successfully synthesized by a facile hydrothermal process and the obtained product was sintered at 450 °C. The sintered TiO2 powder was characterised by powder X-ray diffraction pattern and the result shows pure anatase phase with good crystalline nature. The morphological image of field emission scanning electron microscopy and high resolution transmission electron microscopy shows spherical shape and size of the particles is around 100 to 300 nm. The Brunauer–Emmett–Teller surface area of synthesized TiO2 material was 56.32 m2 g−1 and average pore width of synthesized materials was 7.1 and 9.3 nm. Bimodal pore structure of TiO2 microspheres has been very effective for electrolyte diffusion into photoanode in dye sensitized solar cells. The synthesized anatase TiO2 microsphere based dye sensitized solar cells have high surface area with light scattering effect to enhance the photocurrent and conversion efficiency than the commercial P25 photoanode material. The power conversion efficiency of synthesized mesoporous TiO2 microspheres and commercial P25 material is 4.2 and 2.7 % respectively. Therefore bimodal mesoporous anatase TiO2 microsphere appears to be a promising and potential candidate for dye sensitized solar cells (DSSC) application.

Metal-organic framework derived nitrogen-doped porous carbon@graphene sandwich-like structured composites as bifunctional electrocatalysts for oxygen reduction and evolution reactions

In this work, we have successfully prepared sandwich-like structured N-doped porous carbon@graphene composites (N-PC@G) derived from sandwich-like structured zeolitic imidazolate framework@graphene oxide (ZIF-8@GO). ZIF-8@GO was obtained by in situ controllable growth of ZIF-8 nanocrystals on both surfaces of graphene oxide (GO) sheets with different contents. Experimental results demonstrate that N-PC@G-0.02 (representing GO amount of 0.02 g in reaction precursors) obtained at 900 °C possesses high surface area (1094.3 m2 g−1), bimodal-pore structure (micropores and mesopores) and high graphitization degree, exhibiting great potential as a bifunctional electrocatalyst for both ORR and OER. Compared to commercial Pt/C catalyst, the N-PC@G-0.02 shows superior electrocatalytic activity with onset and half-wave potentials of 1.01 V and 0.80 V (vs. RHE), respectively, better durability and high resistance to methanol crossover effect toward ORR in alkaline media. Also, the metal-free N-PC@G-0.02 also exhibits high electrocatalytic activity of OER, comparable to commercial RuO2 catalyst. The superior ORR and OER performance could be due to a synergistic effect between ZIF-8 derived porous carbon and graphene with regard to structure and composition of N-PC@G-0.02 with high surface area, porous structure, and suitable N doping level and type, boosting the catalytic active sites, mass transport and electron transfer.

Preparation of MnO2–Al2O3 adsorbent with large specific surface area for fluoride removal

MnO2–Al2O3 (MOAO) binary nanocomposite with a 1:3 MnO2 to Al2O3 molar ratio was synthesized by impregnation technique using mesoporous alumina (MA) precursor. The MOAO product consisted of MA and amorphous MnO2. The manganese valence in MOAO was +4, indicative of MnO2 being coated on the surface of MA during the impregnation process. MOAO had a large specific surface area (385.266m2/g) and wormhole-like mesoporous structure. The average pore size, which could be precisely controlled over the range of 3.4–4.1nm. The optimum removal of fluoride was obtained when the initial pH was in the range of 4–10. The defluorination efficiency of MOAO was far superior to that of MA when the initial fluoride concentration exceeded 40mg/L. The large surface area and bimodal porous structure of MOAO after coating MnO2 may be responsible for the high removal efficiency in the defluorination process.

Fabrication of bimodal porous PLGA scaffolds by supercritical CO2 foaming/particle leaching technique

ABSTRACTSpecific pore structure is a vital essential for scaffolds applied in tissue engineering. In this article, poly(lactide‐co‐glycolide) (PLGA) scaffolds with a bimodal pore structure including macropores and micropores to facilitate nutrient transfer and cell adhesion were fabricated by combining supercritical CO2 (scCO2) foaming with particle leaching technique. Three kinds of NaCl particles with different scales (i.e., 100–250, <75, <10 μm) were used as porogens, respectively. In particular, heterogeneous nucleation occurred to modify scCO2 foaming/particle leaching process when NaCl submicron particles (<10 μm) were used as porogens. The observation of PLGA scaffolds gave a formation of micropores (pore size <10 μm) in the cellular walls of macropores (pore size around 100–300 μm) to present a bimodal pore structure. With different mass fractions of NaCl introduced, the porosity of PLGA scaffolds ranged from 68.4 ± 1.4 to 88.7 ± 0.4% for three NaCl porogens. The results of SEM, EDS, and in vitro cytotoxicity test of PLGA scaffolds showed that they had uniform structures and were compatible for cell proliferation with no toxicity. This novel scCO2 foaming/particle leaching method was promising in tissue engineering due to its ability to fabricate scaffolds with precise pore structure and high porosity. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43644.

Scalable and Environmentally Friendly Synthesis of Hierarchical Magnetic Carbon Nanosheet Assemblies and Their Application in Water Treatment

Large-scale assembling of graphitic carbon nanosheets to a three-dimensional hierarchical structure is a great challenge. Herein we report a facile synthesis of hierarchical magnetic carbon nanosheet assemblies (MCNSAs) via an ambient-pressure chemical vapor deposition method. To explore the formation mechanism, the as-prepared MCNSAs as well as the intermediates of synthesis were extensively characterized. It was revealed that two different carbon deposition processes, i.e., the dissolution–precipitation process and graphitic defects triggered catalytic decomposition of methane, were involved in the formation of MCNSAs. The disclosed method is simple and environmentally friendly, which is favorable for large-scale production. The resulting MCNSAs possess large surface areas, bimodal pore structures, abundant defective sites, excellent chemical stability, and sufficient magnetism. Such features afford significant advantages for application in water cleaning. As a proof of concept, the sorption performance...

Fabrication of bimodal-pore SrTiO3 microspheres with excellent photocatalytic performance for Cr(VI) reduction under simulated sunlight

Solving the increasing contamination from toxic heavy metal ions in wastewater is an imperative issue in photocatalysis research area. In this study, three-dimensional (3D) porous SrTiO3 microspheres have been fabricated by a sol–gel-templating method using the agarose gel bead containing SrCO3 granules as the template. The resultant SrTiO3 microspheres, several tens of micrometers in diameter, exhibit a bimodal pore structure, in which the macropore about 70–150nm in size stems from SrCO3 granules and the mesopore about 3nm is formed via removing the agarose fiber embedded in the composite microspheres. The porous framework of SrTiO3 microspheres is assembled by regular single-crystalline SrTiO3 nanocubes with an edge length of 100±10nm. The highly interconnected porous network renders numerous pathways for the rapid mass transport, strong adsorption of reactants and multi-reflection of incident light. Moreover, the as-prepared SrTiO3 microspheres exhibit excellent photocatalytic performance for the Cr(VI) reduction under simulated sunlight, which can reduce nearly 100% Cr(VI) at pH 2 within 2h and retain a relatively high reduction ability after six recycles.

Wollastonite ceramics with bimodal porous structures prepared by sol–gel and SPS techniques

In this work, we suggest a novel synthetic route combining sol–gel and SPS for the fabrication of porous ceramics based on wollastonite with bimodal pore size distributions (100–500 nm and 1–500 μm) and high mechanical strength (Young's modulus from 72.5 to 172 MPa). We studied peculiarities in the formation of the biporous silicate framework using two types of pore forming additives (templates) of various natures, shapes and sizes, introduced during different stages of sol–gel and SPS synthesis: organo-nonorganic (polymer latex of “core–shell” type) and nonorganic (carbonaceous filler). The influence of carbonaceous template content (5% or 25%) on the structural and mechanical properties of ceramics synthesized by spark plasma current was described. The original approach presented here provides formation of wollastonite ceramics with high mechanical strength and a biporous structure that is similar to the texture of bone tissue and is capable of executing its main functions. The obtained ceramics meet all the requirements of the bioceramics class demanded by modern medicine.

Preparation and catalytic performance of Ag, Au, Pd or Pt nanoparticles supported on 3DOM CeO2–Al2O3 for toluene oxidation

Three-dimensionally ordered macro-/mesoporous 26.9wt% CeO2–Al2O3 (denoted as 3DOM 26.9CeO2–Al2O3)-supported noble metal nanocatalysts (xM/3DOM 26.9CeO2–Al2O3, x=0.27–0.81wt%; M=Au, Ag, Pd, and Pt) were prepared using the polymethyl methacrylate-templating and polyvinyl pyrrolidone- or polyvinyl alcohol-protected reduction methods, respectively. It is shown that the xM/3DOM 26.9CeO2–Al2O3 samples displayed a high-quality 3DOM architecture with a bimodal pore (macropore size=180–200nm and mesopore size=4–6nm) structure and a surface area of 102–108m2/g, with the noble metal nanoparticles (3–4nm in size) being highly dispersed on the 3DOM 26.9CeO2–Al2O3 surface. The 0.27Pt/3DOM 26.9CeO2–Al2O3 sample performed the best (T90%=198°C at space velocity=20,000mL/(gh)) for toluene oxidation. The addition of moisture to the feedstock induced a positive effect on catalytic activity. The apparent activation energies obtained over the xM/3DOM 26.9CeO2–Al2O3 samples were in the range of 46–100kJ/mol, with the 0.27Pt/3DOM 26.9CeO2–Al2O3 sample possessing the lowest apparent activation energy. It is concluded that the good catalytic performance of 0.27Pt/3DOM 26.9CeO2–Al2O3 was associated with its higher adsorbed oxygen species concentration, better low-temperature reducibility, and stronger interaction between Pt and 3DOM 26.9CeO2–Al2O3 as well as the unique bimodal porous structure.

Jet-Fuel Range Hydrocarbons from Biomass-Derived Sorbitol over Ni-HZSM-5/SBA-15 Catalyst

Aromatics and cyclic-hydrocarbons are the significant components of jet fuel with high energy-density. However, conventional technologies for bio-fuel production cannot produce these products without further aromatization and isomerization. In this work, renewable liquid fuel with high content of aromatics and cyclic-hydrocarbons was obtained through aqueous catalytic conversion of biomass sorbitol over Ni-HZSM-5/SBA-15 catalyst. Texture characteristics of the catalyst were determined by physisorption of N2, which indicated its bimodal pore structures were microporous (HZSM-5, pore width: 0.56 nm) and mesoporous (SBA-15, pore width: 8 nm). The surface acidity included weak and strong acid sites, predominantly Lewis type, and was further confirmed by the NH3-TPD and Py-IR analysis. The catalytic performances were tested in a fixed-bed reactor under the conditions of 593 K, WHSV of 0.75 h−1, GHSV of 2500 h−1 and 4.0 MPa of hydrogen pressure, whereby oil yield of 40.4 wt. % with aromatics and cyclic-hydrocarbons content of 80.0% was obtained.

Experimental evaluation of catalyst layers with bimodal pore structure for Fischer–Tropsch synthesis

As catalyst pores are typically filled with liquid, long-chained hydrocarbons during low-temperature Fischer–Tropsch synthesis internal mass transport limitations can impede conversion rate and C5+ selectivity. Utilization of transport pores may improve reactant diffusion and thereby reduce these negative effects. In this work preparation, characterization and experimental testing of CoRe/γ-Al2O3 catalyst layers, with and without transport pores, of different thickness is presented. All prepared layers with thicknesses in the range from 50 to 600μm exhibit similar cobalt crystallite sizes of 8.7nm irrespective of the presence of transport pores. Experimental results showed an increase in methane selectivity from 10 to 40% and a drop of C5+ selectivity from 80 to 42% with increasing diffusion length. These negative effects could be retarded by use of transport pores. The highest CO conversion was achieved for layers of about 140μm thickness exhibiting 50–70% higher values than obtained with the thinnest layers (50–60μm). This trend holds also for layers with transport pores but use of additional pores did not result in a significant improvement of conversion and space time yield. Nonetheless, on a catalyst mass basis transport pores lead to a benefit in productivity of 25–50%.