Lars-Gunnar Ekedahl

Hydrogen permeation through surface modified Pd and PdAg membranes

The hydrogen permeation through surface modified Pd and Pd70Ag30 membranes has been studied at temperatures between 100 and 350°C. Silver has been evaporated on Pd and Pd70Ag30 foils with a thickness of 25µm in order to study the role of the surface composition in comparison with the membrane bulk composition. The Pd70Ag30-based membranes display the largest permeation rates at temperatures below 200°C, while Pd membranes with 20A silver evaporated on the upstream side show the largest permeation rates above 200°C. There are, consequently, different rate limiting processes above and below 200°C: at temperatures below 200°C, the bulk diffusion through the membrane is rate limiting, while at temperatures above 200°C, the influence of the surface composition starts to become significant. It has further been concluded that a sharp silver concentration gradient from the surface to the bulk is important for the hydrogen permeation rate at temperatures above 200°C. Adding oxygen to the hydrogen supply will almost totally inhibit the hydrogen permeation rate when a pure Pd membrane surface is facing the upstream side, while for silver-containing surfaces the presence of oxygen has almost no effect. On a clean Pd surface, oxygen effectively consumes adsorbed hydrogen in a water forming reaction. With Ag on the surface, no water formation is detected. Co-supplied CO inhibits the permeation of hydrogen in a similar manner on all studied membrane surfaces, independent of surface silver content.

The effect of CO and O2 on hydrogen permeation through a palladium membrane

Abstract Hydrogen permeation through a 25-μm thick palladium membrane during continuous exposures of hydrogen together with different combinations of oxygen and carbon monoxide has been studied at membrane temperatures of 100°C–250°C (total pressures of 40–150 Torr). Both CO and O 2 , individually, inhibit hydrogen permeation through the membrane. The cause of the inhibition is, however, somewhat different. CO blocks available hydrogen dissociation sites, while oxygen both blocks dissociation sites and also consumes adsorbed hydrogen through the production of water. When a combination of CO and O 2 is supplied together with hydrogen, new reaction pathways will emerge. The carbon dioxide formation will dominate the water forming reaction, and consequently, the blocking effect caused by the formation of water will be suppressed. In a mixture of CO+O 2 +H 2 , the hydrogen permeation can become either larger or smaller than that due to only O 2 +H 2 or CO+H 2 depending on the CO/O 2 ratio. It is thus possible to find a situation where carbon monoxide and oxygen react to form CO 2 leaving adsorbed hydrogen free to permeate the membrane.

Bridging the pressure gap for palladium metal-insulator-semiconductor hydrogen sensors in oxygen containing environments

A comparison has been made between the steady state response obtained from palladium metal-insulator-semiconductor (Pd-MIS) structures exposed to hydrogen in presence of oxygen under atmospheric conditions and the response calculated from a model valid under ultrahigh vacuum conditions. It is shown that the model gives a good description of the steady state response as a function of hydrogen and oxygen pressure. This is of interest not only for the understanding of the sensing mechanism of Pd-MIS hydrogen sensors but would also imply that the used model for the water forming reaction on Pd gives realistic estimates for the hydrogen coverage on the Pd surface over a vast pressure range.

Hydrogen production from organic waste

The extraction of pure hydrogen from the fermentation of household waste by a mixed anaerobic bacterial flora is demonstrated. Simulated household waste (600 g) was fermented in a bioreactor, which ...

High temperature catalytic metal field effect transistors for industrial applications

Field effect chemical sensors, utilising silicon carbide as semiconductor, can be operated at high temperature and in rough environments. Gas sensitive field effect transistors, MISiCFET, are now d ...

A model of the Temkin isotherm behavior for hydrogen adsorption at Pd–SiO2 interfaces

A simple electrostatic model of the adsorbate–adsorbateinteraction of hydrogen atoms at a Pd–SiO 2 interface is presented. The model predicts a hydrogen adsorption isotherm of the Temkin type. It i ...

SiC Based Field Effect Gas Sensors for Industrial Applications

The development and field-testing of high-temperature sensors based on silicon carbide devices have shown promising results in several application areas. Silicon carbide based field-effect sensors can be operated over a large temperature range, 100-600 °C, and since silicon carbide is a chemically very inert material these sensors can be used in environments like exhaust gases and flue gases from boilers. The sensors respond to reducing gases like hydrogen, hydrocarbons and carbon monoxide. The use of different temperatures, different catalytic metals and different structures of the gate metal gives selectivity to different gases and arrays of sensors can be used to identify and monitor several components in gas mixtures. MOSFET sensors based on SiC combine the advantage of simple circuitry with a thicker insulator, which increases the long term stability of the devices. In this paper we describe silicon carbide MOSFET sensors and their performance and give examples of industrial applications such as monitoring of car exhausts and flue gases. Chemometric methods have been used for the evaluation of the data.

Hydrogen adsorption states at the Pd/SiO2 interface and simulation of the response of a Pd metal–oxide–semiconductor hydrogen sensor

The response of a Pd–SiO 2 –Si hydrogen sensor depends on the reaction kinetics of hydrogen on the Pd surface and on the hydrogen adsorption states at the Pd/SiO 2 interface. In this work we show t ...

Real time measurements of hydrogen desorption and absorption during CO exposures of Pd: Hydrogen sticking and dissolution

Combined real time measurements of hydrogen desorption and absorption show that both processes may be induced simultaneously by CO adsorption on a hydrogen-covered Pd surface. The induced absorptio ...

Alcohol dehydrogenation over Pd versus PdAg membranes

The dehydrogenation of methanol and ethanol and the subsequent permeation of hydrogen through Pd and Pd70Ag30 membranes, respectively, have been studied. In order to keep a continuous hydrogen perm ...