Zinc oxide adsorbates

Hydrous zinc oxide adsorbs undissociated sodium alizarate to form a purple lake. There is no way at present to tell whether there is some alizarin ion present or not. 

A sample of zinc oxide adsorbs hydrogen only after an activating pretreatment. This activation may be carried out either by heating the sample in vacuo around 400°C. or by treating it in hydrogen at a slightly lower temperature. Water vapor and carbon dioxide are evolved during the pretreatment (3). 

A suitably activated and evacuated zinc oxide adsorbent takes up considerable amounts of hydrogen. Below room temperature hydrogen is rapidly adsorbed in a reversible manner. At 77°K., a temperature that is some 2.3 times the critical point of hydrogen, and at 1 atm, pressure hydrogen covers 80% of the B.E.T. surface (4). 

Sulphur compounds in the fuel (0.05-0.01 %) must be removed because they would poison the reformer catalyst. Sulphur compounds are removed by using a hydrodesulphurization catalyst (HDS), typically noble metals on alumina, to convert the organic sulphur to hydrogen sulphide. Then a zinc oxide adsorbent bed is used to trap the hydrogen sulphide ... 

At temperatures below 300 °C, hydrogen sulfide can be reduced to below 100 ppb by zinc oxide in the absence of carbon monoxide and steam [294]. At higher temperatures, a mixture of zinc oxide and titania avoids the partial reduction of zinc oxide by the hydrogen contained in the reformate [293]. While zinc oxide has a high adsorption capacity of around 10 g S per g zinc oxide in dry gas, this value decreases considerably in the presence of steam to 0.1 g S per g zinc oxide [107]. According to Li an King, the addition of 1 vol.% carbon monoxide lowered the sulfur removal capability of their zinc oxide adsorbent [294]. The formation of carbon oxide sulfide by the reaction ... 

The reformate left the reformer with a temperature of 814 °C and entered a zinc oxide trap. However, this would he not feasible in a practical system, because zinc oxide adsorbent materials cannot tolerate temperatures exceeding 450 °C. The reformate, which was cooled to 440 °C in heat-exchanger E-2 was then passed to the water-gas shift reactor. This reactor was cooled by steam generation at 15-bar pressure and a temperature of200 °C in a counternoble metal based rhenium/alumina catalyst at the inlet section followed by a copper/zinc oxide catalyst at the outlet section. Despite the fact that a water-gas shift catalyst of fairly low activity had been chosen for the... 

Desulphurization is a typical gas-solid absorbing reaction on zinc oxide. Sulfur adsorption capacity with the mass fraction of sulfur absorbed per gram catalyst is about one percent if only surface zinc oxide is reactive. Therefore, the desulphmiza-tion performance of zinc oxide adsorbent not only depends on the content of zinc oxide, but also on the utilization ratio of zinc oxide (related to porous structme and surface area), and thereby preparation conditions. It is commonly proposed that the zinc oxide prepared from zinc carbonate possesses small crystal size, high smface areas and therefore good desulphurization performances. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

The experiment was carried out in a reaction cell shown in Fig. 3.3 with inner walls covered by a zinc oxide film having thickness 10 pm [20]. The surface area of the measuring film on the quartz plate was about 1/445 of the total film area on the wall of the vessel. The results of direct experimental measurements obtained when the adsorbent temperature was -196 C and temperature of pyrolysis filament (emitter of H-atoms) 1000°C and 1100°C, are shown on Fig. 3.4. One can see a satisfactory linear dependence between parameters A r (the change in film conductivity) and APh2 (reduction of hydrogen pressure due to adsorption of H-atoms), i.e. relations... 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

Besides the experimental data mentioned above, the kinetic dependencies of oxide adsorption of various metals are also of great interest. These dependencies have been evaluated on the basis of the variation of sensitive element (film of zinc oxide) conductivity using tiie sensor method. The deduced dependencies and their experimental verification proved that for small occupation of the film surface by metal atoms the Boltzman statistics can be used to perform calculations concerning conductivity electrons of semiconductors, disregarding the surface charge effect as well as the effect of aggregation of adsorbed atoms in theoretical description of adsorption and ionization of adsorbed metal atoms. Considering the equilibrium vapour method, the study [32] shows that... 

Stationary concentration of adsorbed acceptor particles of O- and N-atoms on a film of zinc oxide is attained for the most part due to the competition between the chemisorbtion of particles and their interaction, i. e. mutual recombination on the adsorbent surface, and with free atoms attacking the adsorbed layer of the adsorbent from outside. 

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

The above results put forward the question, whether or not one can directly compare in experiment the influence produced by adsorbed atomic and molecular silver particles on electric conductivity of a zinc oxide film. 

The results obtained in above experiments confirm the removal of chemisorbed particles in the process of immersion of the film with preliminary chemisorbed radicals in a liquid acetone. Note that at low pressures of acetone, the CHa-radicals absorbed on ZnO film could be removed only by heating the film to the temperature of 200 - 250°C. Moreover, if the film with adsorbed radicals is immersed in a nonpolar liquid (hexane, benzene, dioxane), or vapours of such a liquid are condensed on the surface of the film, then the effect of removal of chemisorbed radicals does not take place, as is seen from the absence of variation of electric conductivity of the ZnO film after it is immersed in liquid and methyl radicals are adsorbed anew onto its surface. We explain the null effect in this case by suggesting that the radicals adsorbed on the surface of the ZnO film in the first experiment remained intact after immersion in a nonpolar liquid and blocked all surface activity of the adsorbent (zinc oxide). 

From the above consideration we suppose that the increase of electric conductivity a of the zinc oxide film is proportional to [Hj], as is the case when hydrogen atoms produced by pyrolysis or discharge in the gas phase are adsorbed on the zinc oxide film. Reverse (competitive) changes of electric conductivity are proportional to the a value of the semiconductor film and to [ ]. Thus, taking into account both chemi-... 

For this purpose, the authors used a special vacuum cell with a controlled focused electron beam incident on a zinc oxide film target. In these experiments, the role of the film was twofold. It served as an adsorbent and as a high-sensitivity detector of hydrogen atoms (10 at/cm ). Hydrogein atoms were produced due to surface dissociation of adsorbed molecular hydrogen. This process was induced by heating or bombardment of the adsorbed layer by an electron beam. 

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