Physical Adsorption of Oxygen

In Fig. 6.4, experimental mass-transfer data for the adsorption of oxygen are compared for turbulent contactors and monoliths. The data for stirred tanks and bubble column follow the trend predicted by Eq. (6.3), and the different lines correspond to different Uu/V). 

The monolith data were taken from Kreutzer et al. [15]. In exactly the same setup, we have also measured the pressure drop, and data in Fig. 6.4 were obtained by calculating the power input from the experimental pressure drop data. The monolith data are consistently higher than the data for the turbulent systems at equal power input. There is significant scatter in the monolith data. The mass transfer was measured at steady state for two lengths of monohth columns. The measured outlet concentration of the short column was used as the inlet concentration for the rest of the longer column, and with the measured outlet concentra-hon the mass-transfer group was determined from 

In Fig. 6.4, we also plotted Eq. (10), and the order of magnitude, predicted in the section above, is confirmed by the experimental data. Heiszwolf et al. [6] used a correlation by Bercii and Pintar [32] and experimental data in a loop reactor where the pressure drop over the column was kept at zero to obtain 

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Physical adsorption of oxygen resulting in the formation of one or more monolayers of oxide and requiring no activation energy. 

Fig. 1. Energy scheme of chemisorption and physical adsorption of oxygen vs. distance from the surface according to Lennard-Jones. E tt is the electron affinity of atomic oxygen, Eo the dissociation energy of oxygen molecules, Ecu the chemisorption energy, and Exot the activation energy. Position A is that of physically adsorbed O2, and position B is that of chemisorbed O".

Figure 8.23 Oxide formation on a metal surface (a) physical adsorption of oxygen (O2) molecules from the air (b) chemical adsorption of separated oxygen atoms (O) strongly hound to the surface (c) penetration of some oxygen atoms into the metal to form a subsurface layer, as more oxygen arrives at the surface (d) saturation of the surface and subsurface with oxygen, leading to formation of oxide nuclei on the surface (e) surface layer of oxide grains...

Further chemisorption of atomic oxygen into a second layer and/or further physical adsorption of Oj. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

Dissociative adsorption occurring on four adjacent silver atoms (AgadJ) is responsible for nonselective oxidation to yield CO2. An optimum chlorine coverage of about 25% of surface silver atoms effectively blocks dissociative adsorption of oxygen by physical blocking ensuring increased selectivity. 

The interesting point is that only the ratio of the partial pressures of the reactants enters this equation, but not the total pressure. The physical reason is obvious. Both reactants are competing for free adsorption sites on the surface and since CO inhibits the dissociative adsorption of oxygen the partial pressure of the latter compound enters into the denominator of the rate expression. This result indicates that the actual surface composition and, in addition, the reaction mechanism will not be affected by the total pressure. The only effect of increasing the total pressure (at a fixed p0l pco ratio) will be that rmax will be shifted towards higher temperatures since reduction of... 

Physical adsorption of the impinging oxygen molecules onto the metal surface. 

Fig. 1. Initial stages of metal oxidation. (1) Oxygen impingement (2) Physical adsorption (3) Oxygen dissociation and chemisorption (4) Place exchange.

The kinetic model comprises the following elementary steps irreversible adsorption of oxygen and reversible adsorption of ethene on the noble metal surface, followed by a surface reaction between adsorbed ethene and oxygen. The values of the kinetic parameters, i.e. preexponential factors and activation energies, were estimated by non-linear regression of the ethene conversion and found to be physically meaningful. 

Physical adsorption usually involves a smaller energy change than does chemisorption. The adsorption of nitrogen on carbon evolves about 5000 calories per mole, this heat being somewhat greater than the heat of liquefaction, whereas the initial adsorption of oxygen on some carbons at 0° C liberates over 100,000 calories per mole—an amount that is greater than the heat of formation of carbon dioxide. 

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