L.-G. Petersson

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 ...

Clean and Oxygen Exposed Potassium Studied by Photoelectron Spectroscopy

Clean evaporated potassium films have been studied in a combined UV-X-ray photoelectron instrument. The valence band and the K 3p level were studied using 21.2 and 40.8 eV radiation. In the 40.8 eV excited spectrum a structure attributed to surface and volume plasma losses was resolved. Using AlKα excitation the binding energies of the core levels in potassium were determined. Potassium films at a temperature of 77 K were exposed to oxygen at a pressure of about 10-5 Pa for increasing time periods. The intensity and shape of the core levels as well as of the valence band were studied after each exposure. The shapes of the O 1s line and the valence band excited by 40.8 eV radiation were followed as a function of increasing temperature after an initial exposure of 45 L O2. For comparison spectra from a sample exposed to about 45 L H2O were recorded.

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.

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.

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 ...

Reactions between hydrocarbons and an oxygen covered palladium surface

Isothermic desorption studies where an oxygen covered polycrystalline Pd surface (T = 325–575 K) is exposed to small unsaturated hydrocarbons have been performed. The studied gases were acetylene, propene, propadiene, propyne, butene, butadiene, butyne, and also for comparison hydrogen. Electron energy loss spectroscopy (EELS) was available for adsorbate studies and a hydrogen sensitive Pd-MOS structure was used as a sample. The temperature limit for total dissociation is observed to decrease slightly for an oxygen covered surface compared to the clean surface. On an oxygen covered surface the dissociation is complete above 400 K and the reaction products are H2O, CO2, and adsorbed C. Varying the adsorption temperature, the carbon dioxide production is not detectable at room temperature, where all carbon is adsorbed and only water is formed, but increases to reach a constant level above 475 K. For temperatures above 400 K the licking coefficient is close to unity and there is no blocking due to preadsorbed oxygen (as there is for H2). Both the water and carbon dioxide formations are of Langmuir-Hinshelwood type but while the H2O desorption rate is found to depend only on impinging hydrocarbon flux the CO2 desorption rate depends also on the oxygen coverage.

Local gas sampling and surface hydrogen detection in catalysis on planar surfaces

Abstract A new experimental set-up for in situ studies of heterogeneous catalytic reactions on metal films in the pressure range 10 −8 –2000 mbar is described. The reactions take place on the outermost layer of a metal-insulator-semiconductor structure, whose capacitance-voltage characteristics are extremely sensitive to variations of the hydrogen coverage on the metal surface. It is shown how this device can be used in combination with mass spectrometry and work function measurements. Mass spectrometry is performed with high sensitivity and fast response even at atmospheric pressures, owing to local gas sampling with a capillary leak. Using this technique reactant pressure gradients close to a catalyst surface can be quantified, allowing true rates of mass transfer limited reactions to be determined. The performance of the system is illustrated by the H 2 + O 2 reaction on Pt in the mbar region. The maximum efficiency of water production is found for a hydrogen/oxygen pressure ratio of 0.1, and the reaction probabilities are S H 2 = 4 × 10 −3 and S O 2 = 8 × 10 −4 at this optimum point. The behaviour of the experimental set-up is further demonstrated by the CO + O 2 oscillatory reaction on Pd and Pt.

Dehydrogenation of acetylene and ethylene studied on clean and oxygen covered palladium surfaces

The interaction of acetylene and ethylene with a clean and oxygen covered Pd surface has been studied at a temperature of 473 K. The measurements were performed on a hydrogen sensitive Pd-MOS str ...