Compensation behavior surface reactions

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). 

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... 

None of the mechanistic explanations of compensation behavior have enabled the values of Arrhenius parameters for untested systems to be predicted. Thus, every compensation plot consists of a number of individual points (log Ai,E1 log A2, E2 log A3, E3 ... log Ah f ...) each point is defined by a single reaction, and the line through these yields the characteristic values of B and e for that series of related reactions. In the absence of control over the magnitudes of A and of E, Eq. (2) is not a realizable continuous function. In principle, this might be achieved by appropriate variations in conditions if a meaningful mechanistic explanation of the surface behavior were available. 

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. 

No single theoretical explanation of compensation behavior has been recognized as having general application. It is appropriate, therefore, to consider in this context the conditions obtaining on a catalyst surface during reaction, with particular reference to the factors that control the rate of product evolution and to the interpretation of kinetic measurements. This discussion of surface behavior precedes a critical assessment of the significance of measured values of A and E. 

It is reasonable to suppose that between the members of a group of related reactions there will be modifications, but not drastic changes, in the positions of surface equilibria and in the temperature dependences of c1 c2, and As. Such variations, when subject to appropriate constraints, are capable of providing an explanation of compensation behavior (Appendix I and Section II, A, 5). From this it follows that the compensation effect appears as a general or at least a widely occurring characteristic feature of surface processes, rather than an exceptional phenomenon that requires an exceptional explanation. 

Compensation behavior occurs in the decomposition of hydrogen peroxide on Ag-Au alloys (25) and, unlike most other alloy systems, there is a systematic change in the Arrhenius parameters with proportions of metals present. This behavior is ascribed to the progressive transformation, with alloy composition, of the reaction mechanism from that characteristic of one metal to that which occurs on the other. In contrast, decomposition of hydrogen peroxide on Pd-Au alloys (27) does not correlate with ratios of metals present in the catalyst, and kinetic parameters are sensitive to surface pretreatment. 

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... 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

Common features in the various theoretical explanations of compensation behavior referred to in Section II, A, 1-7 are the occurrence of parallel reactions that are characterized by different values of the kinetic parameters (A, E) and/or a systematic change in the effective concentrations of reactants across the temperature interval used in the measurements of the Arrhenius parameters. Both influences are based on reaction models for which the kinetic behavior cannot be represented as a single desorption step and, indeed, the overall surface interactions could be much more complicated. 

The above representation of compensation behavior is consistent with reactions involving the surface equilibrium model. In this model the con-... 

While exploring the kinetic consequences of variations in surface occupancy upon reaction rate, a further mechanistic explanation of compensation behavior, of particular relevance in the consideration of adsorption kinetics, became apparent. If the total quantity of gas adsorbed by a surface df varies with temperature and the rate of adsorption dd/dt is proportional to the... 

In studying interfacial electrochemical behavior, especially in aqueous electrolytes, a variation of the temperature is not a common means of experimentation. When a temperature dependence is investigated, the temperature range is usually limited to 0-80°C. This corresponds to a temperature variation on the absolute temperature scale of less than 30%, a value that compares poorly with other areas of interfacial studies such as surface science where the temperature can easily be changed by several hundred K. This "deficiency" in electrochemical studies is commonly believed to be compensated by the unique ability of electrochemistry to vary the electrode potential and thus, in case of a charge transfer controlled reaction, to vary the energy barrier at the interface. There exist, however, a number of examples where this situation is obviously not so. 

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. 

Since reactions at comparable concentrations of surface species and involving an almost identical bond redistribution energy requirement Es must be expected to exhibit at least approximate isokinetic behavior, it follows that the observed values of A must compensate for changes in E — Es — t — 2). 

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)]. 

The redox behavior of nucleic acids, especially at mercury and carbon electrodes, has been studied for decades, and a variety of quantitative methods exist [117-120, and references therein]. Recently, under-ivatized nucleic acids in solution have been selectively quantitated by ac voltammetry at copper electrodes [121], while DNA bound to gold electrode surfaces via gold-thiol chemisorption has been quantitated by chronocoulometry using charge-compensating redox markers [122]. Derivative square wave voltammetry has been used to examine chemically induced DNA damage following reaction with styrene oxide [123]. 

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