Compensation behavior active surface

Combustion, 27 189, 190 reaction, sites for, 33 161-166 reaction scheme, 27 190, 196 Commercial isomerization, 6 197 CoMo catalysts, 40 181 See also Cobalt (nickel)-molybdenum-sulfide catalysts Compact-diffuse layer model, 30 224 Compensation behavior, 26 247-315 active surface, 26 253, 254 Arrhenius parameters, see Arrhenius parameters... 

The theoretical and mechanistic explanations of compensation behavior mentioned above contain common features. The factors to which references are made most frequently in this context are surface heterogeneity, in one form or another, and the occurrence of two or more concurrent reactions. The theoretical implications of these interpretations and the application of such models to particular reaction systems has been discussed fairly fully in the literature. The kinetic consequence of the alternative general model, that there are variations in the temperature dependence of reactant availability (reactant surface concentrations, mobilities, and active areas Section 5) has, however, been much less thoroughly explored. 

Arrhenius parameters for nickel carbide hydrogenation 162) is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that of the metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a period of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). 

Compensation behavior in the decomposition of formic acid on silver has been ascribed to the participation of (at least) two active areas of surface... 

While kinetic measurements are available for reactions on a number of alloy systems, detailed mechanisms of the surface steps involved have not always been established and in some system have only been partially characterized. The identification of Arrhenius parameters with specific processes is not always practicable since several factors may be involved these include the possible influences of electronic, elemental, and crystallographic structures of the active catalyst surfaces. Compensation behavior could arise... 

The compensation relationships mentioned here for the decomposition of formic acid on metals (Table III, K-R and Figs. 6 and 7) cannot be regarded as established, meaningful kinetic descriptions of the reactions concerned, since the magnitudes of the calculated values of B and e depend on the selection of data to be included in the calculation. While there is evidence of several sympathetic interrelationships between log A and E, the data currently available do not accurately locate a specific line and do not define values of B and e characteristic of each system, or for all such systems taken as a group. The pattern of observations is, however, qualitatively attributable to the existence of a common temperature range within which the adsorbed formate ion becomes unstable. The formation of this active intermediate, metal salt, or surface formate, provides a mechanistic explanation of the observed kinetic behavior, since the temperature dependence of concentration of such a participant may vary with the prevailing reaction conditions. 

A number of correlations involving linear relationships between pre-exponential factors and activation energy are widely quoted for reactions on surfaces, as we shall see in the next chapter. This behavior, commonly termed the compensation effect implies a linear relationship between A7/ and A5. An exact linear relationship between A77 and TAS means that there is no variation in AG for the series. An example of this for a liquid-phase reaction is given by Fairclough and Hinshelwood for ethyl benzoate hydrolysis [R.A. Fairclough and C.N. Hinshelwood, J. Chem. Soc., 1573 (1937)]. 

Despite the failure of the activation of TCE on clusters observed in TPD, EES experiments of deposited selected clusters of similar sizes were performed. The exclusion of additional molecules and the chemisorption behavior for selected clusters, reduces the origin of possible shifts in peak energy positions in EE spectra to the effect of physisorption induced relaxation shifts. Thus, it allows for uncomplicated comparison of the differences in physisorption on the surfaces and supported clusters (if omitting compensating effects of potential energy and electron relaxation, as discussed in Sect. 2.2.4). 

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