R. Duś

Molecular hydrogen interactions with discontinuous and continuous thin gold films

Abstract The influence of thin gold film surface nanostructure on H2 chemisorption on unsintered gold films at 78 K was studied. Three classes of thin Au films of different thicknesses were chosen for these experiments, as follows: (i) very thin, purplish colour, transparent film formed by isolated Au islands, (ii) violet–green colour film formed by Au islands joined together and creating a kind of golden net, (iii) non-transparent, continuous Au film of golden colour. It has been found that H2 chemisorption occurs only in the case when the thin Au films are deposited at low temperatures (78 K) and unsintered. It has been suggested that this chemisorption occurs on surface Au atoms of low coordination number as a result of formation of AuH2 complexes, similarly to the compounds arising due to H2 interaction with isolated Au atoms.

Atomic hydrogen adsorption on thin gold films

Hydrogen adsorbate forms arising on thin gold films upon interaction with atomic hydrogen at 78 K have been studied by means of TDMS (thermal desorption mass spectrometry) and by determination of the adsorption rate. At low coverage (θ ≽ 0.01) hydrogen adspecies characterized by an activation energy of desorption Ed = 57 kJ/mol have been found. At higher coverages a weakly adsorbed hydrogen species is formed (Ed = 54−33 kJ/mol). Examination of the adsorption kinetics indicates that this process is accompanied by recombination via the Eley-Rideal mechanism. Initial sticking probability for atomic hydrogen on the thin gold film surface at 78 K has been estimated to be ∼ 6 × 10−3.

Atomic force microscopy of Au/Hg alloy formation on thin Au films

Abstract Atomic force microscopy was applied to the study of thin Au films deposited under ultra-high vacuum conditions on a glass support maintained at 78 K. A variety of thin gold film structures were thus determined and characterized ranging from (i) nanometric isolated gold islands, (ii) nanometric gold structures, to (iii) continuous gold films, all depending on the amount of Au deposited. The influence of the amalgamation process on the thin Au films topography was studied. The fundamental importance of surface defects on the rate of amalgamation was clearly seen. The movement of small Au islands on glass obtained as the result of thermal decomposition of Au Hg alloys was observed.

ADSORPTION OF HYDROGEN ON EVAPORATED COBALT FILMS

Hydrogen adsorption on evaporated Co films has been studied by means of measurements of the surface potential changes that occur during this process, and analysis of the desorption spectrum of hydrogen. It has been observed that hydrogen adsorbed at 78 K on Co films exists in three forms with essentially different electrical properties: atomic, electronegatively polarized β− form; atomic, electropositively polarized β+ form and reversibly adsorbed, molecular, positively polarized α form. The β− form is not homogeneous from the point of view of the bond energy with the metal surface and consists of the states βs− and β− characterized by activation energy of desorption 10.0 and 18.8 kcal/mol H2 correspondingly. The Activation energy of desorption of the β+ form is low, i.e. 2.1 kcalmol H2.

Studies on the possibility of determination of surface area of palladium films by means of adsorption of hydrogen or oxygen and reciprocal titration of these gases

Abstract The kinetics of surface potential changes in palladium films was studied in order to investigate the possibility of determining surface areas of these films by means of adsorption of oxygen or hydrogen and by titration of preadsorbed oxygen with hydrogen or preadsorbed hydrogen with oxygen at 348–370 K. It was found that the incorporation of oxygen in the bulk of palladium films and the occurrence of unreactive forms of the adsorbate in the case of titration of preadsorbed hydrogen with oxygen as well as desorption of hydrogen during the evacuation of the apparatus complicate the problem to such an extent that the volumetric method alone is inadequate. This method can be used for the determination of the surface area of palladium films only when combined with a determination of the kinetics of surface potential changes or some other method which makes it possible to follow the behaviour of the adsorbate on the surface of the metal.

Molecular hydrogen chemisorption on thin unsintered gold films deposited at low temperature

Abstract It has been found that hydrogen molecules in the ground state and possessing low kinetic energy (corresponding to 78 K) do chemisorb at 78 K on clean surfaces of thin, unsintered Au films deposited at low temperature (78 K). This deposit is characterized by a single TD peak which occurs at 125 ± 5 K. Au surface atoms of low coordination number are found to be essential in this chemisorption process. Hydrogen predissolved in the bulk of thin Au films play a minor role in H 2 chemisorption.

Atomic hydrogen solubility in thin gold films and its influence on hydrogen thermal desorption spectra from the surface

Abstract Atomic hydrogen solubility in thin gold films within the temperature interval 250–380 K has been studied, and the exothermic heat of the solution (9 kJ/mol H) has been determined. The influence of predissolved hydrogen concentration on TD spectra from the surface has been shown.

Incorporation of positively and negatively polarized hydrogen and deuterium adspecies from the surface into the bulk of thin palladium films

The incorporation of hydrogen and deuterium adspecies from the surface of a thin palladium film into the bulk has been studied measuring simultaneously the surface potential changes by means of high sensitivity (± 1 mV), short response time (0.1 s) static capacitor system, and pressure by means of the ultrasensitive Pirani type gauge. It has been found that the positively polarized β+ adspecies of hydrogen incorporate quickly at 78 K and 87 K. via a tunneling mechanism, with a diffusion coefficient of 3 × 10−13 cm2s. The β+ adspecies of deuterium incorporate with slightly increasing diffusion coefficient with the temperature increase: 1 × 10−13 cm2s at 78 K and 2 × 10−13 cm2s at 87 K. The negatively polarized adspecies β−, which strongly predominate and at 160 K, slowly incorporate in the bulk of palladium.

Reactivity of ethylene with selected forms of hydrogen deposit on evaporated palladium films

Abstract The static capacitor technique was used for measuring surface potential changes accompanying adsorption and hydrogenation of ethylene on an evaporated palladium film. It was found that ethylene is adsorbed in at least two forms, termed the σ and η forms. The σ form occurs on adsorption with decomposition. The η form is adsorbed in a second layer with no decomposition. Of the two forms, only the η form becomes hydrogenated at a temperature of 195 K with the hydrogen adsorbed in the electronegative β− form on the evaporated palladium film. The electropositive β+ hydrogen existing on the surface of palladium hydride is inactive in the hydrogenation of ethylene at 195 K.

On the influence of the way of thin gold films preparation on the character of hydrogen adsorption

Thermal desorption (TD) and surface potential (SP) measurements were applied in studying the adsorption process of molecular and atomized hydrogen on evaporated thin gold films. It has been found that H2 is adsorbed dissociatively on thin gold films deposited at 78 K and not sintered, while the H2 adsorption on the same films but sintered at T≥320 K was not detected. The course of atomized hydrogen adsorption on sintered and unsintered Au films is also reported.