Adsorption of hydrogen on zinc oxide. Energy of interaction and related mechanisms

Abstract

The heat of adsorption of hydrogen on zinc oxide (Kadox 25) has been measured at room temperature using a Calvet microcalorimeter.Two main forms of adsorbed hydrogen have been found, coinciding with the literature types I and 11. Both always occur simultaneously (although with different kinetics) and a clear-cut separation between the two forms is not achieved on the basis of their reversibility. Differential heats of adsorption decrease with coverage from 60 to 14 kJ mol-*. Apart from the lowest coverages, where a real heterogeneity in site energies is present, the decrease is basically due to the simultaneous adsorption of type I (ca. 40 kJ mol-I) and type I1 (ca. 14 kJ mol-l), the latter prevailing at increasing coverage. The surprising feature that the 'irreversible' type I1 has a lower adsorption heat than the reversible type I is interpreted as evidence for diffusion of the adsorbate into the bulk, in agreement with kinetic observations.

The effect of small addition of copper on the growth process, structure, surface charge and adsorption properties of ZnO films in the pyrolysis of dithiocarbamates

“…136,137 Moreover, on the surface of the ZnS or ZnO films decorated with copper, a great variety of individual centers (Zn/Cu-H and Zn/Cu-OH) and their complex structures (Cu + -O-Zn) were observed. [138][139][140] (3) Most of these materials exhibit high or even maximum (Cu-containing catalysts) activity in the range of 220-280 1C. This is typically associated with the participation and interconversion of Cu + , Cu 2+ ions and metallic copper; this changes not only the electronic structure, but also the morphology of the material.…”

ZnO Oxygen Vacancies Formation and Filling Followed by in Situ Photoluminescence and in Situ EPR

“…Furthermore, in order to get closer to the dynamic treatment involved in real catalytic applications, EPR treatments were not only carried out under gaseous flows before in situ experiments, but dynamic in situ EPR was also implemented to record the spectra under dynamic gaseous flows at various temperatures. Two powdered ZnO samples, selected for their catalytic interest, ,,− namely kadox and ex-carbonate ZnO samples, were investigated.…”

Calorimetric study of the kinetics of adsorption of hydrogen loaded titanium oxide

“…Hydrogen sorption at low pressure by 7-A120 ~ wasfound to be time dependent at 27 ~ and some surface diffusion of an entity other than hydrogen atom or ion was suggested in order to explain the chemisorption kinetics [18]. According to Fubini et al [9], a diffusion process can account for the very low heat of adsorption of H 2 on ZnO in spite of the irreversibility of the process. TirT'~, rain In the present work a process with slow kinetics was found as the surface coverage increases, while the equilibrium pressure was reached faster than the thermal equilibrium.…”