Nanoscale Catalysts Research Articles

Carbon nanotubes as electronic interconnects in solid acid fuel cell electrodes

Carbon nanotubes have been explored as interconnects in solid acid fuel cells to improve the link between nanoscale Pt catalyst particles and macroscale current collectors. The nanotubes were grown by chemical vapor deposition on carbon paper substrates, using nickel nanoparticles as the catalyst, and were characterized using scanning electron microscopy and Raman spectroscopy. The composite electrode material, consisting of CsH2PO4, platinum nanoparticles, and platinum on carbon-black nanoparticles, was deposited onto the nanotube-overgrown carbon paper by electrospraying, forming a highly porous, fractal structure. AC impedance spectroscopy in a symmetric cell configuration revealed a significant reduction of the electrode impedance as compared to similarly prepared electrodes without carbon nanotubes.

Ethylene Formation by Catalytic Dehydration of Ethanol with Industrial Considerations.

Ethylene is the primary component in most plastics, making it economically valuable. It is produced primarily by steam-cracking of hydrocarbons, but can alternatively be produced by the dehydration of ethanol, which can be produced from fermentation processes using renewable substrates such as glucose, starch and others. Due to rising oil prices, researchers now look at alternative reactions to produce green ethylene, but the process is far from being as economically competitive as using fossil fuels. Many studies have investigated catalysts and new reaction engineering technologies to increase ethylene yield and to lower reaction temperature, in an effort to make the reaction applicable in industry and most cost-efficient. This paper presents various lab synthesized catalysts, reaction conditions, and reactor technologies that achieved high ethylene yield at reasonable reaction temperatures, and evaluates their practicality in industrial application in comparison with steam-cracking plants. The most promising were found to be a nanoscale catalyst HZSM-5 with 99.7% ethylene selectivity at 240 °C and 630 h lifespan, using a microreactor technology with mechanical vapor recompression, and algae-produced ethanol to make ethylene.

Mesostructured thin films as electrocatalysts with tunable composition and surface morphology

Among the most challenging issues in technologies for electrochemical energy conversion are the insufficient activity of the catalysts for the oxygen reduction reaction, catalyst degradation and carbon-support corrosion. In an effort to address these barriers, we aimed towards carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored physical properties. We present here a new class of metallic materials with tunable near-surface composition, morphology and structure that have led to greatly improved affinity for the electrochemical reduction of oxygen. The level of activity for the oxygen reduction reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycrystalline Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.

Photodegradation of methyl orange catalyzed by nanoscale zerovalent iron particles supported on natural zeolite

A nanoscale catalyst Fe0(FeNPs) supported on the natrolite zeolite nanoparticles (NANPs) is successfully synthesized and characterized by FT-IR, X-ray diffraction (XRD) and scanning electron microscopy (SEM) and thermogravimetric-differential thermal analysis (TG-DTA). The photodegradation of methyl orange (MO) is studied in aqueous suspension containing the catalyst under UV irradiation and H2O2. The effect of various reaction parameters such as initial dye concentration, irradiation time, pH, H2O2 concentration and catalyst dosage on the decolorization of methyl orange is investigated. The degradation study reveals that the reactivity of the catalysts is in order of: photo-NANPs–FeNPs–H2O2 > photo-NANPs–H2O2 > photo-NANPs–FeNPs > photo-H2O2 > NANPs–FeNPs–H2O2. The results show that methyl orange can be effectively decolorized by NANPs–FeNPs via the pseudo-first-order kinetic model.

Carbon Nanotube Pattern Formation—Precise Routing on Silicon Oxide Using Nanoscale Catalyst Breadboards

A method for directly routing single-walled carbon nanotubes on silicon dioxide is presented. The lack of a preferred direction on the amorphous oxide film was overcome by placing nanoscale catalyst particles on the surface in the form of a grid or breadboard. Colloidal lithography was used to create an ordered nanoscale array of uniformly distributed Fe/Mo catalyst particles to serve as the breadboard. Following chemical vapor deposition at 900 °C with a methane/hydrogen gas mixture, intricate nanotube patterns that corresponded to the underlying breadboard were formed. Most patterns were found to consist of individual tubes or few-tube bundles. A “lift-and-drop” model was used to describe the nanotube pattern formation process whereby a combination of gas flow and van der Waals interactions allows tube growth to be precisely guided onto the breadboards without requiring an external impetus. These results indicate a potential path to carbon nanotube integration on standard oxide films for future deep nanoscale electronics.

Synthesis and electrochemical study of PtPd nanodendrites

Here we report on an advanced nanoscale catalyst system of PtPd nanodendrites and their electrochemical catalytic activity toward the oxidation of methanol. The Pt and Pd based nanostructured materials with different compositions, Pt, PtPd(25%), PtPd(50%), PtPd(75%) and Pd, were prepared using a facile hydrothermal method. These catalysts were tested for efficacy in the oxidation of methanol using cyclic voltammetry and chronoamperometry, as well as in situ electrochemical infrared (IR) spectroscopy. In terms of methanol oxidation, the PtPd(50%) nanodendrite catalyst was found to have the highest electrocatalytic activity over its counterparts as well as pure Pt and Pd. With the increase of the Pd component, the intensity of the CO peaks decreased and the CO band shifted to a lower wavenumber. The final product of methanol oxidation, CO2, was observed at the lowest potential (100mV) for the PtPd(50%) among the nanoporous Pt and PtPd nanodentrites investigated in this study, whereas no CO and CO2 peaks appeared in the IR spectra recorded for the nanoporous Pd. The enhanced Pt activity in the oxidation of methanol via the incorporation of Pd may be attributed to a synergistic catalytic effect. With a balance of 50:50 of Pt/Pd, not only are the maximum number of sites made available for the formation of the COads and the OHads species, but also those sites that are in closest proximity between the formed Pt–COads and Pd–OHads. These synergistic effects translate to a significant improvement in the electrochemical oxidation of methanol through the formation of PtPd(50%) nanodentrites.

Electron microscopic studies of growth of nanoscale catalysts and soot particles in a candle flame

Scanning electron microscopy and high-resolution transmission electron microscopy have been used to investigate non-classic crystal growth of catalytic nanoparticles, such as zeolites, perovskites, metal and alloy particles. Growth mechanisms of some crystals with novel morphologies, for example, BiOBr flower-like particles and ZnO twin-crystals, have also been studied. A development of sampling method for soot particles inside a candle flame allows us to reveal all four well-known carbon forms, amorphous, graphitic, fullerenic and nanodiamond particles. This article demonstrates that electron microscopy is a powerful tool to study the microstructures of small particles, giving us more freedom to develop new materials.

Dendritic Catalysis in Asymmetric Synthesis

In this review, we have focused our attention on the application of dendrimers as asymmetric catalyst in organic synthesis, which appeared in the literature. Dendrimers have been identified as a promising candidate for catalysis application. The combination of high surface area and high solubility makes dendrimers as useful nanoscale catalysts. The main advantage of the dendritic catalysts is they can be separated from the reaction mixture by simple solvent precipitation and reused for several times with similar catalytic activity. Importance of catalytically active substances like metal salts or organic molecule like pyrrolidinylmethanol thatare attached either to the core or to the periphery of the dendrimers have been highlighted and their applications in important organic reactions such as Baylishillman, Mannich-type, Diels-Alder, Wittig reaction, Michael addition, addition of dialkylzinc to aldehyde, asymmetric epoxide ring opening, asymmetric hydrogenation and asymmetric epoxidation are also presented. Use of dendrimer based salts and chiral compounds in asymmetric syntheses are discussed and the advantages and disadvantages of those catalysts are brought out in this review. Keywords: Dendritic catalyses, Asymmetric syntheses, Organic transformations, Chiral catalysts, residues, boundchiral ligands, Dendrimers, catalysts, pyrrolidinylmethanol, ethylvinyl ketone

New surfaces stabilized by adsorbate-induced faceting

Faceting is a form of self-assembly of single-crystal surfaces at the nanometer-scale in which an initially planar surface converts to a ‘hill-and-valley’ structure, exposing new faces of low-index surfaces. Our recent studies revealed that, upon annealing in O2, three-sided nanoscale pyramids form on Ir(210) exposing smooth {311} and partially restructured (110) faces. Through a combination of scanning tunneling microscopy and density functional theory, we identify this structure to be a stepped double-missing-row reconstruction, which is only stable on nanopyramidal facets, not on a planar Ir(110) surface. This faceting-enabled stabilization of a hitherto unstable surface points to a new approach to prepare nanoscale model catalysts for structure-sensitivity studies in heterogeneous (electro-)catalysis with high selectivity and reactivity.

ChemInform Abstract: Transition Metal Nanoparticle Catalysis in Ionic Liquids

AbstractReview: 193 refs.

Stable Hydrogen Storage Cycling in Magnesium Hydride, in the Range of Room Temperature to 300 °C, Achieved Using a New Bimetallic Cr-V Nanoscale Catalyst

We created a bimetallic chromium vanadium hydrogen sorption catalyst for magnesium hydride (MgH2). The catalyst allows for significant room-temperature hydrogen uptake, over 10 cycles, at absorption pressures as low as 2 bar. This is something that has never been previously achieved. The catalyst also allowed for ultrarapid and kinetically stable hydrogenation cycling (over 225 cycles) at 200 and at 300 °C. Transmission electron microscopy analysis of the postcycled samples revealed a nanoscale dispersion of Cr-V nanocrystallites within the Mg or MgH2 matrix. TEM analysis of the partially absorbed specimens revealed that even at a high absorption pressure, that is, a high driving force, relatively few hydride nuclei are formed at the surface of the pre-existing magnesium, ruling out the presence of any contracting volume (also termed contracting envelope or core shell) type growth. HRTEM of the cycled and desorbed powder sample demonstrated that the bcc Cr-V phase is crystalline and nanoscale. We experimentally demonstrated that the activation energy for hydrogen absorption is not constant but rather evolves with the driving force. This finding sheds new insight regarding the origins of the wide discrepancy in the literature – reported values of the hydrogenation activation energy in magnesium hydride and in related metal hydride systems.

Facile synthesis of a platinum–lead oxide nanocomposite catalyst with high activity and durability for ethanol electrooxidation

Aimed at searching for highly active and stable nano-scale Pt-based catalysts that can improve significantly the energy conversion efficiency of direct ethanol fuel cells (DEFCs), a novel Pt-PbO(x) nanocomposite (Pt-PbO(x) NC) catalyst with a mean size of 3.23 nm was synthesized through a simple wet chemistry method without using a surfactant, organometallic precursors and high temperature. Electrocatalytic tests demonstrated that the as-prepared Pt-PbO(x) NC catalyst possesses a much higher catalytic activity and a longer durability than Pt nanoparticles (nm-Pt) and commercial Pt black catalysts for ethanol electrooxidation. For instance, Pt-PbO(x) NC showed an onset potential that was 30 mV and 44 mV less positive, together with a peak current density 1.7 and 2.6 times higher than those observed for nm-Pt and Pt black catalysts in the cyclic voltammogram tests. The ratio of current densities per unit Pt mass on Pt-PbO(x) NC, nm-Pt and Pt black catalysts is 27.3 : 3.4 : 1 for the long-term (2 hours) chronoamperometric experiments measured at -0.4 V (vs. SCE). In situ FTIR spectroscopic studies revealed that the activity of breaking C-C bonds of ethanol of the Pt-PbO(x) NC is as high as 5.17 times that of the nm-Pt, which illustrates a high efficiency of ethanol oxidation to CO(2) on the as-prepared Pt-PbO(x) NC catalyst.

Enhanced response characteristics of SnO2-ZnO hetrostructures loaded with nanoscale catalyst clusters for methane gas detection

ABSTRACTMethane is an important explosive gas, used extensively at the domestic and industrial sites. It is the main constituent of natural gas, which is the main fuel supplied to homes and industries including automobiles. Detection of trace level of methane gas is very important to avoid any accidental explosion due to its leakage and may cause loss of valuable human life and property. The present paper is focused on the development of new sensing material in the form of composites to improve sensitivity, selectivity and stability. Present work shows the enhanced response of SnO2-ZnO composite structures for methane sensing and further increases its sensing response by loading appropriate catalyst on the sensor surface keeping in view of Fermi energy control mechanism and spillover mechanism. A stable sensor response of 77-85 % was obtained for SnO2-ZnO-Pd sensor structure over a wider range of temperature (160-260oC).

Catalytic decomposition of nitrogen-containing heterocyclic compounds with highly dispersed iron nanoparticles on carbons

Decomposition of pyrrole or pyridine with iron catalysts supported on carbons has been studied with a cylindrical quartz-made pulse reactor, into which liquid pyrrole or pyridine is injected as a pulse, in order to develop a novel hot gas cleanup method of removing the nitrogen present in tar as N2. The catalyst is mainly prepared by heating FeOOH precipitated onto powdery cellulose from FeCl3 solution. Nanoscale iron catalysts with the average particle sizes of 25–30nm can promote decomposition reactions of the N-containing heterocyclic compounds in inert gas at >500°C and provide N2 yields of 40–45% after the almost complete decomposition of pyrrole at 600°C or pyridine at 650–700°C. Iron catalyst with the mean size of 100–500nm, prepared from Fe(NO3)3 impregnated with a commercial activated carbon, is also active for the decomposition of the heterocyclic N-compounds, but it is almost inactive for N2 formation, the yields being as low as less than 2% in all cases. The increase in the number of pulses lowers the catalytic activity of iron nanoparticles for N2 formation, whereas in situ H2 treatment at 500°C after reaction can restore it to the almost original state. On the basis of the above-mentioned results, it is likely that the activity of iron catalyst for N2 formation from pyrrole or pyridine is very sensitive to the iron particle size, and the in situ H2 treatment is effective for recovery of the decreased catalytic performance.

Transition Metal Nanoparticle Catalysis in Ionic Liquids

The main “molecular” physicochemical aspects involved in the catalytic applications of “soluble” transition metal nanoparticles in ionic liquids are used to explain their properties. In particular, a special view on the concepts of coordination catalysis is used to describe the properties of these nanoscale catalysts in solution as well as their scope and limitations in multiphase catalysis.

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO2, TiO2, Fe2O3, Co3O4, and SiO2, were added to the LiBH4/2LiNH2/MgH2 system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe2O3>Co3O4>CeO2>TiO2>SiO2. The onset dehydrogenation temperature was below 70°C for the samples doped with Fe2O3 and Co3O4 with 10wt.%. More than 5.4wt.% hydrogen was released at 140°C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH2)2 during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH2. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.

Roles of Pb and MnOx in PtPb/MnOx-CNTs catalyst for methanol electro-oxidation

Design of novel nano-scale catalysts with high activity and low cost for methanol oxidation reaction is crucial for the development of direct methanol fuel cell. In this study, MnOx, Pt and Pb were forced to precipitate successively on the surface of carbon nanotubes for fabricating a PtPb/MnOx-CNTs catalyst. Physical characterizations indicated that there existed a mass of Mn (IV, Ⅴ), Pb (Ⅱ) and Pt (0) species, and partial alloying between Pt and Pb in this catalyst. Methanol oxidation reaction with this novel composite exhibited over 3 times higher specific activity (140.9 mA cm−2) and somewhat lower onset potential (−0.1 V vs. Hg/Hg2SO4) than the values on Pt/CNTs (44.2 mA cm−2 and 0 V, respectively). Fundamental understanding in reaction mechanisms enabled us to reveal the distinguishing functions between Pb and MnOx in methanol oxidation processes. The addition of Pb resulted in the enhanced intrinsic activity towards electro-oxidation of residual intermediate species, while dehydrogenation in methanol oxidation processes was obviously improved by using MnOx-CNTs as a support.

Contrast on COD photo-degradation in coking wastewater catalyzed by TiO 2 and TiO 2–TiO 2 nanorod arrays

This paper deals with photo-degradation of chemical oxygen demand (COD) in coking wastewater catalyzed by TiO 2 nanorod arrays (TNAs) and special secondary structural TiO 2–TiO 2 nanorod arrays (TTNAs). The photo-catalysts, TNAs and TTNAs, were prepared by hydrothermal process and characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). Well-aligned TNAs and TTNAs were prepared on the pretreated quartz substrates. It is proved that Rutile TiO 2 crystal seeds induce the nucleation and growth of TNAs preferably and the controllable synthesis of TTNAs with different secondary morphologies via hydrothermal approach could be achieved. Chemical oxygen demand analyses, photochemical kinetic analysis and Fourier Transform infrared spectroscopy (FTIR) analysis have been carried out to obtain the details of the photo-catalytic degradation and mineralization of contaminants. The results show that these nanoscale catalysts exhibit high photo-catalytic activities in degradation of COD in coking wastewater and TTNAs have significant advantages in COD removal. The optimum degradation efficiencies of COD reach 77.3% and 89.8% respectively using TNAs and TTNAs with catalyst addition of 0.04 g L −1 in 60 min reaction period. The effects of operating parameters on photo-degradation efficiency and the kinetics of COD photocatalytic degradation were investigated and discussed.

Synergy of elemental Fe and Ti promoting low temperature hydrogen sorption cycling of magnesium

We studied the catalytic effects of Titanium, Iron and FeTi intermetallic on the desorption kinetics of magnesium hydride. In order to separate the catalytic effects of each element from additional synergistic and alloying effects, Mg–Ti and Mg–Fe mixtures were studied as a baseline for Mg–Fe–Ti elemental and Mg–(FeTi) intermetallic composites. Sub-micron dimensions for MgH 2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy ball-milling as confirmed by analytical electron microscopy techniques. The composites containing Fe shows desorption temperature of 170 K lower than as-received MgH 2 powder, which makes it suitable to be cycled at relatively low temperature of 523 K. Furthermore, the low cycling temperature prevents the formation of Mg 2FeH 6. In sorption cycling tests, Mg-10% Ti and Mg-10% (FeTi), after about 5 activation cycles, show fast desorption kinetics initially, but the kinetics also degrade faster than for all other composites, eventually slowing down by a factor of 7 and 4, respectively. The ternary Mg–Fe–Ti composite shows best performance. With the highest BET surface area of 40 m 2/g, it also shows much less degradation during cycling. This is attributed to titanium hydride acting as a size control agent preventing agglomeration of particles; while Fe works as a very strong catalyst with uniform and nanoscale dispersion on the surface of MgH 2 particles.

Shape Selective Nanoscale Catalysts with Energy Applications

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