Activated chemisorption

Effect of temperature on amount adsorbed for simultaneous physical adsorption and activated chemisorption. (Adapted from Chemical Engineering Kinetics by J. M. Smith. Copyright 1970. Used with permission of McGraw-Hill Book Company.)... 

We ivill discuss the reaction of hydrogen and oxygen on transition metals first. This reaction has been extensively studied in our laboratory 18-32) using evaporated metal films as a catalyst. From our previous considerations it follows that as a consequence of the choice of this particular system we must restrict ourselves to certain problems only. We cannot identify the surface species (we can indirectly indicate only some of them) nor understand completely their role in the reaction. Because of the polycrystalline character of the film, all the experimental results are averaged over all the surface. Several new problems thus arise, such as grain boundaries, and, consequently, the exact physical interpretation of these results is almost impossible it is more or less a speculative one. However, we can still get some valuable information concerning the chemical nature of the active chemisorption complex. The experimental method and the considerations will be shown in full detail for nickel only. For other metals studied in our laboratory, only the general conclusions will be presented here. 

Aharoni, C, and Ungarish, M. (1976). Kinetics of activated chemisorption. I. The non-Elovichian part of the isotherm. J. Chem. Soc., Faraday Trans. 72, 400-408. 

In general, for both facile and activated chemisorption systems, the chemisorption probability is calculated using the following equation ... 

A fair amount of research has been conducted on so called facile precursor-mediated chemisorption systems. The section that follows is a review of selected existing studies and will serve as a useful introduction to these gas-surface dynamics and will provide a foundation for later discussions of activated chemisorption systems. 

ABC physical adsorption DFGH activated chemisorption DFH rton activated chemisorption... 

The essential parts of a volumetric apparatus are a closed system of known volume, a source of adsorbate and a pressure measuring device. This type of apparatus is more commonly used for the determination of adsorption isotherms, but kinetics can be measured provided that the adsorbent surface is large and the response of the pressure measuring device is much faster than the rate of adsorption. These criteria are usually only satisfied by powders or films which show activated adsorptions. Bond has discussed the phenomenon of activated adsorption. Although the results may often be ascribed to contamination or incorporation into the bulk of the material, in other instances a genuine activated chemisorption is found. 

It seems very probable that the chemisorption of oxygen and carbon monoxide diagnoses the number of surface sites, (Cr3+) (cus), but the degree to which n is 1 or 2 is uncertain. At lower temperatures of activation, chemisorption of carbon dioxide probably diagnoses the number of strongly basic sites, 02-(cus) or OH (cus). The reliability of the second conclusion is less than that of the first. We suggest that the catalytic reactions which we have studied use various eombinations of these sites. 

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

This suggested mechanism is consistent with a number of observations made here and with previous work reported in the literature. For example it was found that recarbonation rates were relatively insensitive to temperature. This would indicate non-activated chemisorption and, as Fischbeck and Snaidt report (10), mineral recarbonation is often independent of temperature when the temperature dependencies of the decarbonation rate conr-stant and the equilibrium constant are similar. This is exactly what is observed in western oil shales (Ref (9), Equations (6) and (7)). Previous work has also pointed to the role of chemisorption phenomena in mineral decomposition reactions. Spencer and Topley (11) have suggested that finely grained oxides can chemisorb HoO as well as H2 and Soni and Thomson (12) observed higher recarbonation rates when CO2 was produced on the surface during oil shale char combustion. 

Reduction and oxidation took place in flowing hydrogen and oxygen, respectively. After evacuation (10-2 Pa) at the same temperature the cell was cooled down to room temperature and the chemisorption of H2 was started. The H2 used was purified by passing through a Pd diffusion cell. To check for activated chemisorption, in some cases the chemisorption of H2 was already started at the same temperature as that of evacuation and the catalyst was then cooled down to room temperature under H2. Similar procedures were used for oxygen chemisorption. 

The above conclusion applies as well to non-activated chemisorption, fn chemisorption, however, the chemical specificity of the reaction is such that factors like the crossing of electronic curves, the existence of steric constraints, and the coupling with the heat reservoir are expected to play a fundamental role, so that no general rule can be specified. 

The last point deserves a special discussion. In fact, due to the short-range nature of the forces involved in chemisorption, E Q) is expected to depend on 0 only for 0 1/c, where c is the coordination number i.e., the number of nearest neighbours) of each adsorption site. Since the Elovich behaviour is associated with the final stages of the process, this is not a serious difficulty in activated chemisorption, where the appearance of the logarithmic kinetics is associated with high coverage. 

The barrier to chemisorption, E, with 0 for kinetically nonreactive systems, >0 for activated chemisorption, and E <0 for nonactivated chemisorption ... 

The dependence of Sq on T follows from Eq. (2.3). For activated chemisorption (i.e., E > 0), Sq increases with increasing T this behavior is not unique since it can also occur for activated reactions in both direct and indirect mechanisms. By contrast, for nonactivated chemisorption (i.e., Ej < 0), So decreases with increasing T this is a unique signature for such a process that is often looked for experimentally. 

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