Chromium adsorption effects

In addition to film preparation by adsorption from the melt, Compound D was also adsorbed on pure platinum surfaces by adsorption from solution in nitrobenzene, ethyl alcohol, and ethyl alcohol-water solutions. As was true of films adsorbed on chromium, adsorption on platinum from solution resulted in surface coatings exhibiting lower contact angles than those formed from the molten acid. The same postadsorption treatments were also effective in increasing the contact angles of films of Compound D on platinum. 

The mechanism of complex formation of metals with chitosan is manifold and is probably dominated by different processes such as adsorption, ion exchange, and chelation under different conditions. In Fig. 15.6, it can be seen that the sorption capacity of chromium and cadmium is similar, while zinc has approximately double the affinity of these two metal ions for chitosan. Chitosan has almost three times the removal capacity for copper sorption than that for cadmium or chromium. These effects have proved complex to interpret but are a function of a number of parameters ionic radii ionic charge electron structure and possibly some hydration capacity of the metal ions solution pH and nature and availability of sites for chitosan. 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

Poisoning experiments usually show very selective adsorption. As the first increments of poison are added, activity declines rapidly. Then with later increments the effect of the poison is less severe, probably because the adsorption is less discriminating between active and inactive chromium. This suggests that the active site population is that portion of the reduced chromium which, due to surface heterogeneity, is most coordinatively insaturated. Presumably this would correspond to some of the CrA population identified by the Turin group, which under favorable conditions comprised almost half the total chromium (76, 40). 

The metal surface properties also change with the bath constituents and thereby affect the particle-electrode interaction. Metal deposition constitutes a multi-step reaction mechanism that depends on the bath composition. In quite a number of reaction mechanism adsorbed intermediates, e.g. the presence of chromium and catalyst polyoxides on the metal surface during chromium plating, are involved. Not the metal surface, but the adsorbed intermediates will determine the particle-electrode interaction and might even compete for adsorption sites on the electrode surface with the particle. Although the reverse, i.e., the change in metal deposition mechanism due to the presence of particles has been investigated (see Section 3.U), no studies on the effect of the deposition mechanisms on particle codeposition have been reported. 

Griffin, R.A., A.K. Au, and R.R. Frost. 1977. Effect of pH on adsorption of chromium from landfill leachate by clay minerals. J. Environ. Quai. Health A12 431-449. 

S. W. Weller Houdry Process Corporation) Pretreatment is frequently important not only in determining catalytic activity but also in changing the actual chemical composition of the surface and, therefore, the electronic configuration attributed to the metal ions. With Cr203, for example, pretreatment with O2 at elevated temperature results in substantial coverage of the surface with adsorbed O2, and, in effect, the surface chromium ions have a valence number greater than three. Pretreatment with Ha similarly results in high adsorption of H2 and an effective decrease in the valence number of chromium, probably to two. 

In a study of the effect of gas pressure on the chemisorption of hydrogen on chromium oxide gel, Taylor and Burwell (6f) found it necessary to resort to an arbitrary process of subtraction from experimentally observed amounts of adsorbed gas in order to display a relatively uniform area of gel for hydrogen chemisorption. Within close limits, at temperatures of 457 and 491° K. and at pressures of 1, 0.5 and 0.25 atmospheres, the energy of activation for the adsorption process was nearly independent of the amounts of adsorption (0-35 cc.) and the activation energy was about 21 kcal. In the temperature range 383 to 457° K. no such uniformity was observed. For example, if the amounts adsorbed with time at 405 and 427° K. be used to calculate activation energies the values received increase from 0 to 18.5 kcal. as the amount of gas adsorbed on the surface increases from 1.6 to 8.5 cc. Similar results can be drawn from the earlier measurements of Kohlschutter (17). 

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