Pore anisotropy and microporosity in nanostructured mesoporous solids

In this study, we carried out an investigation related to the determination of the anisotropy (b) of pores as well as the extent of microporosity (mic%) in various groups of nanostructured mesoporous materials. The mesoporous materials examined were fifteen samples belonging to the following groups of solids: MCM-48s, SBA-15s, SBA-16s, and mesoporous TiO(2) anatases. The porosities of those materials were modified either during preparation or afterward by the addition of Cu(II) species and/or 3(5)-(2-pyridinyl) pyrazole (PyPzH) into the pores. The modification of porosity in each group took place to make possible the internal comparison of the b and mic% values within each group. The estimation of both the b and mic% parameters took place from the corresponding nitrogen adsorption-desorption isotherms. The new proposed method is able to detect a percentage of microporosity as low as a few percent, which is impossible by any of the methods used currently, without the use of any reference sample or standard isotherms. A meaningful inverse relationship is apparent between the b and mic% values, indicating that large values of b correspond to small values of mic%.     

Immobilizing redox enzymes at mesoporous and nanostructured electrodes

Three key challenges are stimulating intensive research in the development of productive direct electron transfer mode enzyme electrodes: proper enzyme orientation, high enzyme loading, and full retention of enzyme activity. In this review, we summarize some significant advances that have been reported in the last years on the design of mesoporous and nanostructured electrodes as enzyme scaffolds and of innovative methodologies for wiring enzymes to electrodes. Particular attention is given to investigations on physical factors that determine a favorable enzyme immobilization, to provide rational guidelines for the design of productive enzymatic electrodes. Finally, some emerging trends focused on the spatial organization of either single enzymes or enzyme cascades are also briefly addressed.     

Incorporation of platinum nanoparticles in ordered mesoporous carbon

Platinum nanoparticles were incorporated within the pore system of ordered mesoporous carbon (OMC) by impregnating the carbon with a water-in-oil (w/o) microemulsion containing dissolved platinum salt followed by reduction of the platinum ions in situ inside the carbon pore system. The procedure provides preparation of metallic nanoparticles from hydrophilic precursors inside the hydrophobic carbon support structure with simultaneous control of the maximum metal particle size. Electron tomography was used to verify the presence of platinum nanoparticles inside the carbon material.     

Morphology,thermo-mechanical properties and surface hydrophobicity of nanostructured epoxy thermosets modified with PEO-PPO-PEO triblock copolymer

In this paper, we report on the effect of amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer (TBCP) on the miscibility, phase separation, thermomechanical properties and surface hydrophobicity of diglycidyl ether of bisphenol-A (DGEBA)/4,4'-diaminodiphenylmethane (DDM) system. The blends were nanostructured. The phase separation occurred via self-assembly of PPO blocks followed by the reaction induced phase separation of PEO blocks. The surface roughness increased with increase in concentration of TBCP due to increased phase separation of PEO blocks at higher concentration. The phase separated PEO blocks formed the crystalline phase in the amorphous crosslinked epoxy matrix. The TBCP has a strong plasticizing effect on the matrix and decreased the glass transition temperature (T_g) and modulus of the thermoset. The incorporation of TBCP improved impact strength and tensile properties and 5 phr TBCP content was found to be optimum to achieve balanced mechanical performance. Moreover, the thermal stability of the epoxy system was retained while hydrophobicity was improved in the presence of TBCP.     

Thermal stability of platinum particles embedded in mesoporous silicates

Summary The thermal stability of platinum nanoparticles immobilized in SBA-15 silicate was investigated. The Pt nanoparticles were prepared in aqueous solution using polyvinyl-pirrolidone (PVP) as shape- and size controlling agent. The appropriate pretreatment conditions of cleaning the metal surface were determined.     

Influence of solvent on platinum catalyst formation

The adsorption properties of Pt-black reduced in various solvents (water, sulfuric acid, ethanol, alkali, heptane) have been studied. The nature of solvent is shown to influence the catalyst surface formation, the heat of hydrogen adsorption and the ratio of its forms.

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Ionic strength-controlled virtual area of mesoporous platinum electrode

Mesoporous electrodes provide an unusual opportunity to observe the dramatic transition of the electrochemical potential distribution in vicinity to mesoporous surfaces as the ionic strength varies. The experimental results were in accordance with what the classical Gouy-Chapman theory predicts on the basis of the correlation between Debye length (kappa-1) and the diameter of mesopores. Using the phenomenon that the electrochemically effective area of mesoporous electrode depends on the ionic strength, the faradaic current density of dioxygen reduction could be controlled by the electrolyte concentration.     

Triarylphosphine-stabilized platinum nanoparticles in three-dimensional nanostructured films as active electrocatalysts

Ligand-stabilized platinum nanoparticles (Pt NPs) can be used to build well-defined three-dimensional (3-D) nanostructured electrodes for better control of the catalyst architecture in proton exchange membrane fuel cells (PEMFCs). Platinum NPs of 1.7 +/- 0.5 nm diameter stabilized by the water-soluble phosphine ligand, tris(4-phosphonatophenyl)phosphine (TPPTP, P(4-C6H4PO3H2)3), were prepared by ethylene glycol reduction of chloroplatinic acid and subsequent treatment of the isolated nanoparticles with TPPTP. The isolated TPPTP-stabilized Pt NPs were characterized by multinuclear magnetic resonance spectroscopy (31P and 195Pt NMR), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). The negatively charged TPPTP-Pt NPs were electrostatically deposited onto a glassy carbon electrode (GCE) modified with protonated 4-aminophenyl functional groups (APh). Multilayers were assembled via electrostatic layer-by-layer deposition with cationic poly(allylamine HCl) (PAH). These multilayer films are active for the key hydrogen fuel cell reactions, hydrogen oxidation (anode) and oxygen reduction (cathode). Using a rotating disk electrode configuration, fully mass-transport limited kinetics for hydrogen oxidation was obtained after 3 layers of TPPTP-Pt NPs with a total Pt loading of 4.2 microg/cm2. Complete reduction of oxygen by four electrons was achieved with 4 layers of TPPTP-Pt NPs and a total Pt loading of 5.6 microg/cm2. A maximum current density for oxygen reduction was reached with these films after 5 layers resulting in a mass-specific activity, i(m), of 0.11 A/mg(Pt) at 0.9 V. These films feature a high electrocatalytic activity and can be used to create systematic changes in the catalyst chemistry and architecture to provide insight for building better electrocatalysts.     

Morphology and gas-sensing behavior of in situ polymerized nanostructured polyaniline films

Nanostructured polyaniline films could be easily in situ polymerized on microslides via rapidly mixing aniline with ammonium persulfate in acid solution. The influence of different conditions including aniline, acid concentration, reaction time and acid kinds on the morphology and thickness of the resulting films was studied. The gas-sensing behavior of the nanostructured films to 100 ppm of NH₃ was investigated through monitoring the change of the resistance.     

Morphology effects on the biofunctionalization of nanostructured ZnO

A stepwise surface functionalization methodology was applied to nanostructured ZnO films grown by metal organic chemical vapor deposition (MOCVD) having three different surface morphologies (i.e., nanorod layers (ZnO films-N), rough surface films (ZnO films-R), and planar surface films (ZnO films-P). The films were grown on glass substrates and on the sensing area of a quartz crystal microbalance (nano-QCM). 16-(2-Pyridyldithiol)-hexadecanoic acid (PDHA) was bound to ZnO films-N, -R, and -P through the carboxylic acid unit, followed by a nucleophilic displacement of the 2-pyridyldithiol moiety by single-stranded DNA capped with a thiol group (SH-ssDNA). The resulting ssDNA-functionalized films were hybridized with complementary ssDNA tagged with fluorescein (ssDNA-Fl). In a selectivity control experiment, no hybridization occurred upon treatment with non complementary DNA. The ZnO films' surface functionalization, characterized by FT-IR-ATR and fluorescence spectroscopy and detected on the nano-QCM, was successful on films-N and -R but was barely detectable on the planar surface of films-P.     

Clean and reactive nanostructured cellulose surface

A simple, solvent-free and low cost method to activate the surface of nanofibrillated cellulose films for further functionalization is presented. The method is based on the oxidative properties of UV radiation and ozone, to effectively remove contaminants from nanocellulosic surface, which remains clean and reactive for at least a week. The efficiency of the method is demonstrated by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. In clear contrast to previous results on nanoscaled cellulose the relative atomic concentration of non-cellulosic carbon atoms was only 4 %, and water completely wetted the surface within seconds. After activation, neither chemical degradation nor morphological changes on cellulose were observed. This surface activation is essential for further functionalization of the film in dry state or nonpolar media. The surface activation was confirmed by silylation and a four times higher degree of substitution was achieved on the activated sample compared to non-activated reference film, as monitored with XPS.     

Controlled synthesis of highly dispersed platinum nanoparticles in ordered mesoporous carbons

A novel route has been developed to fabricate ordered carbon mesoporous materials with well-dispersed, highly stable Pt nanoparticles of ca. 2-3 nm on the pore walls using platinum acetylacetonate as the co-feeding carbon and Pt precursor.     

Microwave-induced formation of platinum nanostructured networks with superior electrochemical activity and stability

Platinum nanostructured networks (PNNs) can be synthesized through the chemical reduction of H(2)PtCl(6) by benzyl alcohol under microwave irradiation without the introduction of any surfactants, templates, or seeds. The synthesis route utilizes benzyl alcohol as both the reductant and the structure-directing agent, and thus, the process is particularly simple and highly repeatable. The formation of the PNN structure was ascribed to the collision-induced fusion of Pt nanocrystals owing to the cooperative functions of microwave irradiation and benzyl alcohol. Compared with a commercial Pt/C catalyst, the as-prepared PNNs possessed superior electrochemical activity and stability on the oxidation of methanol because of the unique 3D nanostructured networks and abundant defects formed during the assembly process. This study may provide a facile microwave-induced approach for the synthesis of other 3D nanostructured noble metals or their alloys.     

Interface chemistry of nanostructured materials: ion adsorption on mesoporous alumina

This paper presents a part of our work on understanding the effect of nanoscale pore space confinement on ion sorption by mesoporous materials. Acid-base titration experiments were performed on both mesoporous alumina and alumina particles under various ionic strengths. The point of zero charge (PZC) for mesoporous alumina was measured to be approximately 9.1, similar to that for nonmesoporous alumina materials, indicating that nanoscale pore space confinement does not have a significant effect on the PZC of pore surfaces. However, for a given pH deviation from the PZC, (pH-PZC), the surface charge per mass on mesoporous alumina was as much as 45 times higher than that on alumina particles. This difference cannot be fully explained by the surface area difference between the two materials. Our titration data have demonstrated that nanoscale confinement has a significant effect, most likely via the overlap of the electric double layer (EDL), on ion sorption onto mesopore surfaces. This effect cannot be adequately modeled by existing surface complexation models, which were developed mostly for an unconfined solid-water interface. Our titration data have also indicated that the rate of ion uptake by mesoporous alumina is relatively slow, probably due to diffusion into mesopores, and complete equilibration for sorption could take 4-5 min. A molecular simulation using a density functional theory was performed to calculate ion adsorption coefficients as a function of pore size. The calculation has shown that as pore size is reduced to nanoscales (<10 nm), the adsorption coefficients of ions can vary by more than two orders of magnitude relative to those for unconfined interfaces. The prediction is supported by our experimental data on Zn sorption onto mesoporous alumina. Owing to their unique surface chemistry, mesoporous materials can potentially be used as effective ion adsorbents for separation processes and environmental cleanup.     

Role of the solvent in the adsorption-desorption equilibrium of cinchona alkaloids between solution and a platinum surface: correlations among solvent polarity, cinchona solubility, and catalytic performance

The role that the nature of the solvent plays in defining the extent of cinchona alkaloid adsorption-desorption equilibrium on platinum surfaces has been studied both by testing their solubility in 54 different solvents and by probing the stability of adsorbed cinchona in the presence of those solvents. The solubilities vary by as much as 5-6 orders of magnitude, display volcano-type correlations with solvent polarity and dielectric constant, and follow a cinchonine < cinchonidine < quinine, quinidine sequence. The adsorption-desorption equilibrium shifts toward the solution with increasing dissolving power of the solvent. The relevance of these results to the behavior of cinchona as chiral modifiers in hydrogenation catalysis is discussed.     

Preferential oxidation of carbon monoxide catalyzed by platinum nanoparticles in mesoporous silica

Preferential oxidation (PROX) of CO is an important practical process to purify H2 for use in polymer electrolyte fuel cells. Although many supported noble metal catalysts have been reported so far, their catalytic performances remain insufficient for operation at low temperature. We found that Pt nanoparticles in mesoporous silica give unprecedented activity, selectivity, and durability in the PROX reaction below 353 K. We also studied the promotional effect of mesoporous silica in the Pt-catalyzed PROX reaction by infrared spectroscopy using the isotopic tracer technique. Gas-phase O2 is not directly used for CO oxidation, but the oxygen of mesoporous silica is incorporated into CO2. These results suggest that CO oxidation is promoted by the attack of the surface OH groups to CO on Pt without forming water.     

Anion-exchange behaviour of the platinum metals and gold in hydrochloric acid-organic solvent media

The anion-exchange characteristics of the platinum metals and gold in hydrochloric acid media are described. The distribution coefficients of these elements were measured on the strongly basic anion-exchange resin Dowex 1 x 8 in mixtures of hydrochloric acid with water and several organic solvents, i.e., dimethylformamide, acetone, tetrahydrofuran, dioxan, methanol, acetic acid and pyndine. Based on these data the conditions most suitable for quantitative separation are indicated and discussed.     

Electrocatalytic activity and simultaneous determination of catechol and hydroquinone at mesoporous platinum electrode

The electrochemical oxidation of catechol and hydroquinone was investigated using cyclic and differential pulse voltammetries at nanostructured mesoporous platinum film electrochemically deposited from the hexagonal liquid crystalline template of C₁₆EO₈ surfactant. The mesoporous platinum electrode has shown an excellent electrocatalytic activity and reversibility towards the oxidation of catechol and hydroquinone redox isomers in 1.0 M HClO₄. The oxidation and reduction peak separation (ΔE) has been decreased from 485 to 55 mV for hydroquinone and from 430 to 75 mV vs. SCE for catechol at polished polycrystalline and mesoporous platinum electrodes, respectively. The differential pulse voltammograms in a mixture solution of catechol and hydroquinone have shown that the oxidation peaks became well resolved and are separated by about 100 mV, although the bare electrode gave a single broad oxidation peak. Moreover, the oxidation current of hydroquinone and catechol has been enhanced by a factor of two and four times, respectively, at mesoporous platinum electrode. Using differential pulse voltammetry, a highly selective and simultaneous determination of hydroquinone and catechol has been explored at mesoporous platinum electrode.     

Synthesis of highly active nanostructured PtRu electrocatalyst with three-dimensional mesoporous silica template

Nanostructured PtRu material has been successively synthesized via chemical co-reduction of hexachloroplatinic acid and ruthenium trichloride using three-dimensional (3D) hexagonal mesoporous SBA-12 silica as a solid template, and has been studied as an electrocatalyst toward methanol electro-oxidation. The ordered nanostructure of the PtRu particles has been disclosed by transmission electron micrographs and is characterized by regular pores of ca. 3.0 ± 0.3 nm in diameter separated by walls of ca. 3.0 ± 0.3 nm thick. X-ray diffraction and energy dispersive X-ray spectroscope studies indicate that the PtRu material comprises of complicated phases rather than a single alloy phase of Pt and Ru. The specific electrochemical surface area of the nanostructured powder measured using both CO and underpotential deposited Cu stripping techniques is 74–78 m² g^–1, higher than that of unsupported precious metal catalysts prepared using standard techniques. The combination of high surface area and periodic nanostructure of the templated PtRu makes it an interesting promising fuel cell electrocatalyst. This has been demonstrated by the high activity of the templated PtRu towards the methanol electrooxidation. Therefore the solid template route based on 3D mesoporous silica with controlled pore size and high pore interconnectivity provides an interesting alternative to produce promising high-surface-area electrode materials.