Helen Dannetun

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.

Kinetic modeling of hydrogen adsorption/absorption in thin films on hydrogen‐sensitive field‐effect devices: Observation of large hydrogen‐induced dipoles at the Pd‐SiO2 interface

A kinetic modeling of the hydrogen interaction with a Pd‐SiO2‐Si (Pd‐MOS) device is reported. The model is fitted to a number of experimental results, mainly from mass spectrometric desorption meas ...

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.

Hydrogen adsorption states at the external and internal palladium surfaces of a palladium‐silicon dioxide‐silicon structure

The hydrogen adsorption states at the external and internal Pd surfaces of a hydrogen sensitive Pd‐SiO2‐Si (Pd‐MOS) structure have been studied in high and ultrahigh vacuum. The steady‐state response of the Pd‐MOS structure (due to the hydrogen present at the Pd‐SiO2 interface) shows a logarithmic dependence on hydrogen pressure, independent of external surface condition. The heat of adsorption of the internal surface is coverage dependent and described by ΔHi =ΔH0(1‐aθi) giving ΔH0≊1.4 eV/molecule with a=1. Thus, there are always hydrogen adsorption states available at the internal surface independent of the applied pressure. This is why the Pd‐MOS structure works as a hydrogen sensor over a large pressure range. The importance of a surface cleaning procedure in order to obtain an atomically clean Pd surface after Ar sputtering is pointed out. It is observed that a, probably carbon, contaminated Pd surface has hydrogen adsorption properties similar to those of the palladium‐oxide interface. A clean Pd surface has adsorption sites with smaller heat of adsorptions than those at the interface. Furthermore, these adsorption sites give an increase in work function, whereas a decrease in work function is observed at the interface. It is pointed out that the difference in the heat of adsorption between surface and interface hydrogen adsorption sites means that hydrogen atoms are detected at the interface even when the hydrogen coverage at the clean surface is very small.

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 ...

The water-forming reaction on palladium

The water-forming reaction on Pd has been studied on a Pd---SiO2---Si (Pd-MOS) structure in the temperature range 323–473 K. The reaction is found to be of the Langmuir-Hinshelwood type with the formation of OH beeing rate limiting. Since the Pd-MOS structure works as a sensitive hydrogen detector unique information on the behaviour of hydrogen during this catalytic reaction has been obtained. The reaction can be described in a model where the hydrogen atoms on the Pd surface have a large temperature activated lateral mobility and with no evidence of beeing in hot precursor states. At T = 473 K this means that for oxygen coverages 0.01 monolayers all hydrogen adsorbed will also react with oxygen. For smaller oxygen coverages unreacted hydrogen will not initially desorb towards the vacuum but towards the internal Pd surface of the Pd-MOS structure. Futhermore, hydrogen adsorption is blocked by adsorbed oxygen. The sticking coefficient for hydrogen on the bare Pd surface is, however, close to one and only weakly temperature dependent. An effect giving rise to a hysteresis in the work function versus oxygen coverage curve during oxygen adsorption - desorption is also discussed.

A hydrogen-sensitive Pd-MOS structure working over a wide pressure range

A Pd-MOS structure has been developed which has proven to be sensitive to hydrogen both in air at atmospheric pressures and in UHV. The ability to work over a wide pressure range makes it an inte ...

Water production on palladium in hydrogen−oxygen atmospheres

Abstract We have studied the water production on Pd in various oxygen-hydrogen atmospheres in combination with work function measurements, photoelectron spectroscopy and hydrogen sensitive measurements on a PdSiO 2 Si structure. The existence of a critical oxygen to hydrogen pressure ratio is confirmed. Above the critical ratio, the water production rate decreases with increasing oxygen pressure and increases with increasing hydrogen pressure. Below the critical ratio, the water production rate is proportional to the oxygen pressure and independent of the hydrogen pressure. These features, together with the variations in oxygen and hydrogen coverage, are described by a simple Langmuir-Hinshelwood model, assuming that hydrogen has a large lateral mobility and that both hydrogen and oxygen adsorption-dissociation is blocked by adsorbed oxygen.

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 ...

The H2-O2 reaction on palladium studied over a large pressure range : Independence of the microscopic sticking coefficients on surface condition

We have studied the H2---O2 reaction on Pd over a large pressure range by letting the reaction take place on the Pd gate of a Pd-MOS structure. By measuring shifts in the capacitance versus volt ...