Hydrogen-sensitivePd–SiO₂–Si and Pt–SiO₂–Si metal–insulator–semiconductor (MIS) devices have been studied inultrahigh vacuum in the temperature range of 223–523  K. Adsorption/absorption ofhydrogen occurs at the metal surface, in the metal bulk,and at the metal–insulator interface. The sensor signal, caused byhydrogen adsorption at the interface, shows a logarithmic dependence onthe applied hydrogen pressure. The Pt-MIS device, which is fullyfunctional at atmospheric pressures, is sensitive to changes in hydrogenpressure down to the 10^–12-Torr scale. We propose that theinterface adsorption follows a so-called Temkin isotherm with an interfaceheat of adsorption that varies with hydrogen coverage as H_i0(1–a).The initial heat of adsorption H_i0 is determined to 0.78  eV/hydrogenatom. The adsorption potential at the external Pt surface isfound to be 0.45  eV/hydrogen atom. These values were obtained bymodeling the hydrogen interaction with the MIS devices and fittingthe model to a number of experimental results. Also studiesof Pd-based devices were performed and compared with Pt. Thehydrogen adsorption on the metal surface, previously treated as afirst-order process on Pd, is shown to follow a second-orderprocess. Qualitatively the results from the Pd- and Pt-MIS devicesagree. Quantitatively there are differences. The hydrogen sensitivity of thePt-MIS device is only approximately one-third compared to that ofthe Pd-MIS structure. This agrees with the result that theconcentration of available hydrogen adsorption sites at the Pt–SiO₂ interfaceis approximately 7×10¹⁷ m^–2 whereas the concentrations of sites at thePd–SiO₂ interface is roughly three times larger (2×10¹⁸ m^–2). An estimateof the size of the dipole moments (0.6–0.7  D) implies thatthe interface hydrogen atoms are strongly polarized. Differences are alsoobserved in the microstructure of the metal films. Atomic forcemicroscopy results show that the Pd surface reconstructs during H₂–O₂exposures, while the Pt surface shows no such change atthese temperatures.

Hydrogen-sensitive Pd–SiO₂–Si and Pt–SiO₂–Si metal–insulator–semiconductor (MIS) devices have been studied in ultrahigh vacuum in the temperature range of 223–523 K. Adsorption/absorption of hydrogen occurs at the metal surface, in the metal bulk, and at the metal–insulator interface. The sensor signal, caused by hydrogen adsorption at the interface, shows a logarithmic dependence on the applied hydrogen pressure. The Pt-MIS device, which is fully functional at atmospheric pressures, is sensitive to changes in hydrogen pressure down to the 10⁻¹²-Torr scale. We propose that the interface adsorption follows a so-called Temkin isotherm with an interface heat of adsorption that varies with hydrogen coverage as ΔH_i0(1−aθ). The initial heat of adsorption ΔH_i0 is determined to 0.78 eV/hydrogen atom. The adsorption potential at the external Pt surface is found to be 0.45 eV/hydrogen atom. These values were obtained by modeling the hydrogen interaction with the MIS devices and fitting the model to a number of experimental results. Also studies of Pd-based devices were performed and compared with Pt. The hydrogen adsorption on the metal surface, previously treated as a first-order process on Pd, is shown to follow a second-order process. Qualitatively the results from the Pd- and Pt-MIS devices agree. Quantitatively there are differences. The hydrogen sensitivity of the Pt-MIS device is only approximately one-third compared to that of the Pd-MIS structure. This agrees with the result that the concentration of available hydrogen adsorption sites at the Pt–SiO₂ interface is approximately 7×10¹⁷ m⁻² whereas the concentrations of sites at the Pd–SiO₂ interface is roughly three times larger (2×10¹⁸ m⁻²). An estimate of the size of the dipole moments (0.6–0.7 D) implies that the interface hydrogen atoms are strongly polarized. Differences are also observed in the microstructure of the metal films. Atomic force microscopy results show that the Pd surface reconstructs during H₂–O₂ exposures, while the Pt surface shows no such change at these temperatures.

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 permeation rate, oxygen needs to be added to the alcohol supply. Without oxygen, the decomposition products will form a contaminating layer on the upstream membrane surface. The extraction of hydrogen from ethanol is six times more effective through a Pd70Ag30 membrane than through a pure Pd membrane (at optimum conditions). For methanol, the hydrogen permeation is 30% larger through a Pd70Ag30 membrane than through a membrane of pure Pd. The increased hydrogen permeation yield through Pd70Ag30 compared to Pd can be attributed mainly to a lower upstream consumption of hydrogen due to water formation, but also to an increased conversion of the alcohol in the presence of oxygen. © 2001 Elsevier Science B.V.

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 20Å 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. © 2001 Elsevier Science B.V. All rights reserved.

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 was continuously sparged with nitrogen (30 ml/min) fed in from the bottom. The gas stream from the biorector passes through a sulphide trap (ZnO) and then through a heated palladium-silver membrane reactor to separate hydrogen from the gas stream. In this way, waste remediation and biological hydrogen production is combined in a process where a large proportion of the hydrogen produced can be collected, free of other gaseous species from the fermentation. © 2001 International Association for Hydrogen Energy.

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 O2, 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 O2 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+O2+H2, the hydrogen permeation can become either larger or smaller than that due to only O2+H2 or CO+H2 depending on the CO/O2 ratio. It is thus possible to find a situation where carbon monoxide and oxygen react to form CO2 leaving adsorbed hydrogen free to permeate the membrane.