Thin Catalyst Layer Research Articles

A theoretical model for phase transfer catalyzed reaction with the third‐liquid phase

The behaviors of phase transfer catalysts under the formation of a third‐liquid phase were analyzed in the present work. A mathematical model concerning mass transfer with reaction in an organic droplet was developed. The third‐liquid phase was considered to form between the aqueous/organic interface. The diffusion resistance of active intermediate (QY) in the catalyst phase and mass transfer resistance between the droplet and the bulk aqueous phase were all considered. The surface conversion factor (?) and mass Biot number (Bi) were defined to interpret their relative importance on the overall reaction rate. The system equations were solved by finite difference and fourth‐order Runge‐Kutta methods. From the simulation results, the conversion increases with increasing Bi and decreases with increasing ?. The overall reaction rate is influenced not only by the diffusivity of organic reactant but also by its surface reaction rate. In addition, the diffusion rates of QY and QX in the third liquid phase are not important to the overall reaction rate because QY and QX are highly concentrated within the thin catalyst layer. These results exhibit the phenomena of phase transfer reaction catalyzed by the third‐liquid phase.

Performance of a polymer electrolyte membrane fuel cell with thin film catalyst electrodes

In order to develop a kW-class polymer electrolyte membrane fuel cell (PEMFC), several electrodes have been fabricated by different catalyst layer preparation procedures and evaluated based on the cell performance. Conventional carbon paper and carbon cloth electrodes were fabricated using a ptfe-bonded Pt/C electrocatalyst by coating and rolling methods. Thin-film catalyst/ionomer composite layers were also formed on the membrane by direct coating and transfer printing techniques. The performance evaluation with catalyst layer preparation methods was carried out using a large or small electrode single cell. Conventional and thin film membrane and electrode assemblies (MEAs) with small electrode area showed a performance of 350 and 650 mA/cm2 at 0.6 V, respectively. The performance of direct coated thin film catalyst layer with 300 cm2 MEAs was higher than those of the conventional and transfer printing technique MEAs. The influence of some characteristic parameters of the thin film electrode on electrochemical performance was examined. Various other aspects of overall operation of PEMFC stacks were also discussed.

Screening Binary and Ternary Catalyst Formulations for Direct-Methanol-PEM Fuel Cells

In an exploratory approach binary and ternary nanodisperse alloy catalysts were synthesized containing the elements Pt, Ru and W, Mo or Sn, respectively. The catalysts were prepared applying a colloid method developed by Bonnemann, which was found to be highly suitable for synthesizing this kind of supported catalyst, as determined by TEM, EDX and electrochemical measurements. The catalysts were tested for their activity towards anodic oxidation of H 2 /CO-mixtures and methanol in PEM single cells. It turned out that for H 2 /CO oxidation, the systems Pt/Ru/W and - at higher overpotentials -Pt/Ru/Sn were the most active systems, for methanol oxidation again Pt/Ru/W was the most active system. By varying the molar composition of the system Pt/Ru/W it was found that both for oxidation of H 2 /CO and methanol the most active system tested was a ternary catalyst with a molar ratio Pt/Ru/W of 1/1/1,5. In another series of experiments the cocatalytic activity of some transition metal phtalocyanine complexes was investigated. It turned out that none of the systems tested had a beneficial effect on oxidation of H 2 /CO, for methanol oxidation the systems Pt/Cobaltphtalocyanine and Pt/Nickelphtalocyaninetetrasulfonic acid showed a cocatalytic activtiy. For Pt/Nickelphtalocyaninetetrasulfonic acid, the effect can still be enhanced by pyrolyzing the complex. Furthermore, a pre-screening method for methanol oxidation is introduced which makes it possible to test a number of electrocatalyst systems at the same time using graphite model electrodes which are covered with a thin layer of catalyst and ionomer.

Performance of PEM Fuel Cells Operating on Synthetic Reformate: Experimental and Modeling Studies

Experimental results and modeling simulating PEM Fuel Cell operation on reformed gasoline are reported. These studies cover observed and calculated effects of hydrogen dilution at the level expected from gasoline reforming and the extent of catalyst poisoning by CO under such anode operation conditions. At flow rates corresponding to less than 80% hydrogen utilization, modest losses in cell power density are observed experimentally from hydrogen dilution alone. Modeling suggests that the performance loss caused by hydrogen dilution stems primarily from combined effects of limited protonic conductivity and gas permeability within the thin film anode catalyst layer. Effects due to limited transport through the gas diffusion backing are expected to be relatively minor. Under the experimental conditions employed, little evidence was found for deleterious effects caused by intemal generation of CO from CO 2 . Finally, we demonstrate complete tolerance to 100 ppm CO in a 40% H 2 inlet stream at stoichiometric flow of 1.3, using air injection into the anode.

Laser-assisted selective metallisation of diamonds by electroless Ni and Cu plating

Results are presented on area-selective metallisation of diamond single crystals and CVD diamond films realised by means of electroless metal deposition onto laser-activated diamond surfaces. Two techniques of laser activation of the diamond surface are applied: (1) prenucleation of the surface with a thin layer of Pd catalyst via laser-induced decomposition of a corresponding acetyl-acetonate solid film, and (2) laser-induced damage of the diamond surface. The activated surface is characterised with Auger spectra and X-ray microanalysis techniques. The possible mechanisms of surface activation are discussed along with alternative methods of diamond activation for electroless plating. The adherence strength of the Cu deposit is 1.5–2 N/mm 2.

Laser activation of diamond surface for electroless metal plating

Selective area electroless nickel and copper deposition onto the surface of diamond single crystals and polycrystalline diamond films has been realized. Three methods of laser-assisted activation of diamond surface were applied: (i) prenucleation of diamond surface with a thin layer of palladium catalyst via laser-induced decomposition of a palladium acetyl-acetonate [Pd(acac)2] solid film; (ii) deposition of palladium by means of the decomposition of Pd(acac)2 dissolved in dimethylformamide; (iii) laser-induced damage of diamond surface.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer‐electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects the majority of the platinum to conditions that sustain particle growth. X‐ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m2/g Pt eventually stabilize to about 40 to 50 m2/g in the cathode and 60 to 70 m2/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributable to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

Gas Sensitivities of Silicon MIS Capacitors Incorporated with Catalyst and Adsorptive Oxide Layers

A new family of gas sensors based on catalyst‐adsorptive MIS capacitor has been developed. The structure integrates the catalytic properties of Pd, Pt, or Ag as a thin catalyst layer, and the gas sensing properties of or as a gas adsorptive oxide with a surface sensitive silicon MIS capacitor. The choice of catalyst layer and adsorptive oxide employed in the structure modifies the gas sensing characteristics: gas selectivity, sensitivity, activation energy, and reaction rate constant. Moreover, the sensors can detect, , and gases at a much lower temperature than conventional solid state gas detectors.

Biotechnological applications of fiber-optic sensing: multiple uses of a fiber-optic fluorimeter

The development of a simple but very effective modular fiber-optic fluorimeter is described. Using this instrumentation, different fiber-optic chemo- and biosensors are investigated to demonstrate the application of optical sensors in biotechnology. To obtain information about vital biomass concentrations and the metabolic state of organisms during cultivation, a non-invasive fast-responding on-line sensor based on the combination of turbidometric and spectrofluorometric methods (culture fluorescence monitoring) is presented. In another system, permeabilized cells are confined in front of the fiber tip and changes in the NAD(P)H pool of membrane-bound enzyme complexes inside the cells are monitored during enzymatic reactions with different analytes to study the metabolic activity of the system and to use the system as a biosensor. Another fiber-optic sensor uses the effect of luminescence quenching of a special Ru(II) complex by oxygen to form a sterilizable oxygen optode for measuring dissolved and exhaust-gas oxygen levels. The oxygen optode can also sere as a transducer; various chemo- and biosensors can be constructed by placing a thin layer of a specific catalyst directly onto the oxygen-sensitive membrane. The development of a glucose sensor is used to illustrate this concept.

High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells

Polymer electrolyte membranes are catalyzed by the direct application of thin film catalyst layers cast from solutions of suspended Pt/C catalyst and solubilized Nafion® ionomer. Both the ionomeric membrane and the solubilized ionomer are in the Na+form during casting to enable higher curing temperatures, which results in more robust catalyst layers. In addition to simplifying the fabrication process, the direct application apparently provides enhanced bonding at the interface between the membrane and the catalyst layer. Consequently, the performances of fuel cells utilizing these catalyzed membranes with ultra‐low platinum loadings are superior to those achieved with other approaches to polymer electrolyte membranes of low Pt loading.

Thin-film catalyst layers for polymer electrolyte fuel cell electrodes

New structures for the Pt/C catalyst layer of polymer electrolyte fuel cell electrodes have been developed that substantially increase the utilization efficiency of the catalyst. Fabricating the catalyst layers and gas diffusion backings separately makes it possible to formulate each structure with the properties that are most suitable for its function. In the case of the catalyst layer, the optimal properties are hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. The catalyst layers are cast from solution as thin films that utilize the ionomer itself as a binder. The thin films are hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings are inserted when the cells are assembled. The performances of fuel cells based on the thin film catalyst layers are comparable with those of gas diffusion electrode designs that utilize several times as much platinum, thus the specific activities of the Pt catalysts in the new structures are significantly higher.

Dependence of performance of solid polymer electrolyte fuel cells with low platinum loading on morphologic characteristics of the electrodes

This paper describes the results of transmission electron microscopic, scanning electron microscopic and/or Rutherford backscattering spectroscopic analyses of platinum electrocatalysts supported on carbon, and of low catalyst loading gas-diffusion electrodes used in proton-exchange-membrane (PEM) fuel cells. We looked for correlations between the performance of PEM fuel cells and the morphology of low-catalyst-loading electrodes, taking into account the thickness of the catalyst layers, the crystallite sizes of the platinum electrocatalyst supported on carbon and the increased Pt catalyst content near the front of the electrodes. We reached the conclusion that the use of electrodes with thin catalyst layers (made by using platinum on carbon electrocatalysts with a high Pt/C weight ratio) and with more platinum localized near the front surface had the effect of diminishing the overpotentials in PEM fuel cells.

Linear sweep and cyclic voltammetry for electrocatalysis at modified electrodes with very thin films

The expression for linear sweep and cyclic voltammograms is derived for the case where electrocatalysis for oxidation of a dissolved substrate occurs at an electrode modified with a very thin layer of catalyst. The voltammogram consists of a simple sum of the catalytic component and the surface wave for the catalyst couple. The voltammogram involving only the catalytic component varies with both the kinetic parameter, Λ = k fΛ T( RT/ FvD R2 ) 1 2 , and the thermodynamic parameter, Δζ= F[( E 2) 1 2 − E° 1′]/ RT, where k f is the forward rate constant of the catalytic reaction. Λ T is the total amount of catalyst on the electrode surface, v is the potential sweep rate. E° 1′ is the formal potential of the catalyst and ( E 2) 1 2 is the half-wave potential of the substrate in solution. The dependence of the peak current and of the peak potential on these two parameters is discussed. From this dependence, one can assess the catalytic efficiency and, further, evaluate the reaction rate constant.

A selective absorber formed by catalytic deposition of pyrolytic carbon on a silver infrared reflecting layer

A carbonaceous selective absorber of rather high solar absorptance and quite low thermal emittance has been studied to determine the optimum conditions for its formation. A large number of variables are involved in the formation of this selective absorber, making the determination of these conditions rather time consuming. This absorber is formed by chemically depositing silver on glass, electroplating a thin nickel catalyst layer on the silver, and exposing this surface to hot acetylene at temperatures usually in the range of 475 to 535°C. Under proper conditions a selective absorber is formed which has an air-mass-two normal solar absorptance near or above 0.90, a 100°C hemispherical thermal emittance near or below 0.03, and which appears to be stable in vacuum at temperatures near 500°C. The carbonaceous coating formed is rather hard and adhesive, such that it can withstand the “scotch tape test”. This absorber has the advantage that its formation is entirely at atmospheric pressure using inexpensive materials, and thus can be produced more cheaply than many other high temperature absorbers.

Oxidation and reduction of substances in aqueous solution in presence of water-repel lent catalyst

The oxidation of ethylene to acetaldehyde over noble metal (Pt and Pd) hydrophobic thin layer catalysts, in a three phase system, at mild conditions was investigated. Fe+3/Fe+2 redox couple in sulfuric acid solution (pH=1) has been used as oxygen carrier. The reaction was studied in the temperature range 353–393 K. Kinetic measurements by operating in a batch mode have been performed. A novel approach to attempt selective oxidation reactions of hydrocarbons is pointed out.

Kinetics of temperature-programmed reactivation of a hydrodesulphurization catalyst

The kinetic parameters of reactivation of a carbonized hydrodesulphurization (HDS) catalyst by air were evaluated from combined thermogravimetric (TG) and differential thermal analysis (DTA) data. In addition, the gaseous products leaving a temperature-programmed reactor with a thin layer of catalyst were analyzed chromatographically. Two exothermic processes were found to take part in the reactivation, and their kinetics were described by 1st order equations. In the first process (180-400 °C), sulphur in Co and Mo sulphides is oxidized to sulphur dioxide; in the second process (300-540 °C), in which the essential portion of heat is produced, the deposited carbon is oxidized to give predominantly carbon dioxide. If the reaction heat is not removed efficiently enough, ignition of the catalyst takes place, which is associated with a transition to the diffusion region. The application of the obtained kinetic parameters to modelling a temperature-programmed reactivation is illustrated on the case of a single particle.

Tin-Palladium Catalysts for Electroless Plating

The application of electroless plating as a surface coating technique for producing both functional and decorative finishes has increased substantially with the growth in the requirement for printed circuit boards, and with the widespread use of plastics for so many purposes. During the plating process a thin layer of catalyst is applied to the dielectric surface and a layer of metal, generally copper or nickel, is then electroless plated on top. The versatility and cost-effectiveness of tin-palladium catalysts have resulted in their almost universal use.

Dynamic measurements of the electrical conductivity of a non-metallic catalyst during adsorption and catalysis

Describes a novel device, the 'conductivity probe', consisting of a thin layer of catalyst deposited between two platinum wires wound in a bifilar configuration, developed for observing changes in the electrical conductivity of the catalyst under operating conditions. Using a catalyst of uranium-antimony oxide, it is shown that, when the probe is placed in a reactor also containing the catalyst in its conventional form, measured changes in conductance accompanying adsorption and reaction reliably reflect those taking place in the main bulk of the catalyst.

Apparatus for electrochemical determinations of hydrogen in inert gas mixtures

A potentiometric method for determinations of hydrogen in inert gas mixtures is proposed which makes use of a cell involving porous graphite electrodes activated with a thin layer of platinum-black catalyst. These electrodes are similar to the high-performance ones used for fuel cells, and the equilibrium is rapidly reached and maintained. This method results in an accuracy to better than ±0.2% and is applicable for continuous determinations of hydrogen varying from 100 to 0.1% in the inert gas mixture.

Improved Porous Electrode for Studying Electrocatalytic Reactions of Gases and Vapors

An improved porous electrode is described which is a convenient basis for a comparative study of electrocatalytic activity of noble metals and/or for investigations of electrocatalytic processes on gases. The suggested technique makes use of porous graphite disks having a part of their surface covered with a thin layer of catalyst kept in position by means of a thin network of sintered Teflon drops. Such an electrode is to be disposed horizontally with its upper catalyst side kept in the atmosphere of the concerned gas, the remaining part being immerged in the solution. Solution by capillarity moves upward to moisten catalyst. Teflon prevents solution from reaching a too high level which would submerge the catalyst. This way the triple contact electrode‐solution‐gas is complete and efficient, and both solution level and graphite porosity are not critical factors. Polarization curves and charge‐discharge curves have been determined in order to evaluate performances of that porous electrode in various electrochemical systems and processes.