Compensation behavior Arrhenius parameters

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 following list mentions several factors that may control or influence the magnitude of one or both Arrhenius parameters and, in consequence, possibly result in the appearance of compensation behavior. Some of these parameters closely resemble, or represent alternative variations of, the reaction models described in Sections 1 -6. 

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). 

Other groups of Arrhenius parameters obtained for reactions on two or more metals and which have been shown to exhibit compensation effects include para-hydrogen conversion (on Cu, Ag and Au) (144), hydrogenation of methylacetylene (7 8) and of allene (245), and the inverse compensation behavior in decomposition of acetylene on metal films (26). 

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). 

The following example illustrates one particular quantitative application of compensation behavior for the comparison of levels of activity between different systems. The Arrhenius parameters for the steam reformation reaction over nickel alumina catalysts (290) are log A = 17.25 and E = 29.0. The position of this point on compensation diagrams would appear to be more realistically represented by the compensation relation found for oxidation and exchange processes on nickel oxide (Table V, G) than that for cracking on the metal (Table I, A). One possible mechanistic explanation for this distinction is that the active catalyst is an oxide phase [possibly including NiAl204 (290)1... 

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. 

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