Oxides, compensation behavior

Fig. 5. Compensation behavior in reactions on palladium metal. The lines (Table 11) were calculated (Appendix II) for cracking ( ) and for exchange (x) and oxidation (o) processes (data references given in text).

Deren et al. (271) have reported compensation behavior in the oxidation of carbon monoxide on nickel oxide containing various amounts of chromium oxide the effect is attributed to the modification of the Fermi level at... 

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). 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

Compensation behavior found for the decomposition of hydrogen peroxide on preparations of chromium (III) oxide, which had previously been annealed to various temperatures, was attributed to variations in the energy states of the active centers (here e 0.165). Compensation behavior has also been observed (284) in the decomposition of hydrogen peroxide on cobalt-iron spinels the kinetic characteristics of reactions on these catalysts were ascribed to the electronic structures of the solids concerned. 

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... 

Five-membered heterocycles with two heteroatoms have the jr-electron deficiency of Y-type heteroatoms compensated by the jr-electron excessive character of the X-type atoms therefore, this category includes some of the most stable heterocycles. For example, NMR spectral data and chemical behavior (e.g., resistance to oxidation by potassium permanganate) suggest that pyrazole and imidazole have delocaliza-... 

The compensation trend present in data reported by Shannon et al. (285) for the isomerization of n-butenes over a number of different oxide catalysts is given in Table V, P (omitting from the calculation the point for MnO, which shows a marked deviation). Dehydrogenation of cyclohexane over oxides (286) exhibited similar behavior the calculated line is given in Table V, Q. Hydrocarbon exchange over alumina (287) also gave a slight compensation trend, for which e = 0.132 and ae = 0.024. 

The features of behavior described above for the reactions on oxides bear a close resemblance to the kinetic characteristics of reactions on metals these include, for example, ranges of obedience to Eq. (2), the magnitudes of the calculated values of B, e, and a, and other features. Again compensation trends were found within groups of rate processes that involved either a common chemical transformation catalyzed by several oxides or related reactions on a single oxide. Representative instances of such observed obedience to Eq. (2) are included in Table V. The quantity of kinetic information available for reactions on particular oxides was, however, often insufficient to enable values of B and e to be estimated meaningfully. Also in the... 

The closed loop process control permits the process to be stabilized in the region of the unstable transition mode between B and C, allowing compensation for the dynamic behavior of the target oxidation by corresponding... 

Figure 11 Transport- and IR-compensated 02 reduction currents as a function of potential recorded on nuclear fuel (U02) in 0.1 mol dm3 NaC104 (pH = 9.5). The electrode was cathodically reduced before the experiment. (1) Data recorded from the most negative to the most positive potential, showing the behavior on a reduced U02 surface. (2) Data recorded from the most positive to the most negative potential after corrosion in aerated solution, illustrating the behavior on an oxidized U02+x surface.

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