Isokinetic effect

It has been shown by several authors that a good correlation exists between activation energy (Ea) and In Aq for different reactions taking place over one catalyst [86-87]. Such a correlation is termed as compensation effect or isokinetic effect . Indeed the higher catalytic activity found with intermediate catalyst compositions on Cui.xCoxFc204 (x= 0.5) was attributed to the relatively low (Ea) for ethylation (Figure 12) on these compositions compared to the end compositions, x = 0 and 1. 

Although the term compensation effect is now in general current usage, such behavior has also been referred to as the isokinetic effect (23) and as the... 

Keywords compensation effect, isokinetic effect, Rochow synthesis, transition metal silicide phases, selective energy transfer model... 

Analyzing kinetic data of closely related reactions, the so-called compensation effect or, better, the isokinetic effect has often been found in chemistry, especially also in gas-solid reactions and heterogeneous catalysis, e.g., [1, 2]. Provided validity of the Arrhenius equation... 

That means that the phenomenon isokinetic effect is indicated by the observation that the Arrhenius lines of the closely related reactions intersect in only one point, characterized by the isokinetic pre-exponential factor kao and by the isokinetic temperature Tiso, or, in other words, there is a linear relationship between In ko and Ea with the slope IRT so-... 

Jacobsen EN, Zhang W, Giiler ML (1991) J Am Chem Soc 113 6703 Isaacs N (1995) Physical Organic Chemistry. Wiley, New York, p 296 For a discussion of related isokinetic effects, see Isaacs N (1995) Physical Organic Chemistry. Wiley, New York, p 116... 

Iron, tris(hexafluoroacetylacetone)-structure, 65 Iron, tris(oxalato)-chemical actinometer, 409 Iron, tris(l,10-phenanthroline)-absorptiometry, 549 racemization, 466 solid state, 467 structure, 64 Iron(O) complexes magnetic properties, 274 Iron(II) complexes magnetic behavior, 273 spectra, 253 Iron(III) complexes equilibrium constant solvent effect, 516 liquid-liquid extraction, 539 magnetic behavior, 272 spectra, 253 Iron(IV) complexes magnetic behavior, 272 Isocyanates metal complexes hydrolysis, 429 Isokinetic effect ligand exchange solid state, 469 Isomerism, 179-208 configurational, 180, 188 constitutional, 180,182 coordination, 183 detection, 180 history, 24... 

Isokinetic Sampling I he sample gas partial volume flow must be extracted isokinetically to avoid aerodynamic separation effects and to ensure correct particle size distribution. Isokinetics means that the velocity and direction of the sample gas partial flow at the sample nozzle are the same as at the main gas stream. ... 

AG = 0 that is, all substituent (or medium) effects on the free energy change vanish at the isokinetic temperature. At this temperature the AH and TAS terms exactly offset each other, giving rise to the term compensation effect for isokinetic behavior. 

Earlier analyses making use of AH vs. AS plots generated many p values in the experimentally accessible range, and at least some of these are probably artifacts resulting from the error correlation in this type of plot. Exner s treatment yields p values that may be positive or negative and that are often experimentally inaccessible. Some authors have associated isokinetic relationships and p values with specific chemical phenomena, particularly solvation effects and solvent structure, but skepticism seems justified in view of the treatments of Exner and Krug et al. At the present time an isokinetic relationship should not be claimed solely on the basis of a plot of AH vs. A5, but should be examined by the Exner or Krug methods. 

It is also a point of change in control of the reaction rate by the energy of activation below it to control by the entropy of activation above it. The effect of changes in structure, solvent, etc., will depend on the relation of the experimental temperature to the isokinetic temperature. A practical consequence of knowing the isokinetic temperature is the possibility of cleaning up a reaction by adjusting the experimental temperature. Reactions are cleaner at lower temperatures (as often observed) if the decrease in the experimental temperature makes it farther from the isokinetic temperature. The isokinetic relationship or Compensation Law does not seem to apply widely to the data herein, and, in any case, comparisons are realistic if made far enough from the isokinetic temperature. 

Figure 4.35. Effect of catalyst work function on the activation energy EA, preexponential factor k° and catalytic rate enhancement ratio r/r0 for C2H4 oxidation on Pt/YSZ 4 p02=4.8 kPa, Pc2H4=0-4 kPa,4,54 kg is the open-circuit preexponential factor, T is the mean temperature of the kinetic investigation, 375°C.4 T0 is the (experimentally inaccessible) isokinetic temperature, 886°C.4 25,50...

Figure 4.36. Effect of catalyst potential UWR and work function on the activation energy E (squares) and preexponential factor r° (circles) of C2H4 oxidation on Rh/YSZ. open symbols open-circuit conditions. Te is the isokinetic temperature 372°C and r is the open-circuit preexponential factor. Conditions po2=l.3 kPa, pc2n =7.4 kPa.50 Reprinted with permission from Academic Press.

Figure 4.38. NEMCA-induced compensation effect in the isokinetic point for C2H4 oxidation on Rh/YSZ. Conditions po2= 1-3 kPa, pC2H4 =7.4 kPa.50 Reprinted with permission from Academic Press.

It is worth noting that below the isokinetic point (T < T ) the reaction exhibits electrophobic behaviour, i.e. dr/dUwR > 0, while for T > T the reaction becomes electrophilic. At T = T the NEMCA effect disappears (see also the curve for T=370°C in Fig. 8.6). 

Figure 8.18. Effect of catalyst potential and work function on the apparent activation energy, E, and on the logarithm of the preexponential factor r° rfi is the open-circuit preexponential factor and T0, T are the two isokinetic points of C2H6 oxidation on Pt/YSZ for positive and negative potentials respectively.27 Reprinted with permission from Academic Press.

Figure 8.75 shows the dependence of the apparent activation energy Ea and of the apparent preexponential factor r°, here expressed as TOF°, on Uwr. Interestingly, increasing Uwr increases not only the catalytic rate, but also the apparent activation energy Ea from 0.3 eV (UWr=-2 V) to 0.9 eV (UWr-+2V). The linear variation in Ea and log (TOF°) with UWr leads to the appearance of the compensation effect where, in the present case, the isokinetic point (T =300°C) lies outside the temperature range of the investigation. 

Isokinetic point and compensation effect, 166 and electrochemical promotion, 164, 166 Isotherms... 

The idea that /3 continuously shifts with the temperature employed and thus remains experimentally inaccessible would be plausible and could remove many theoretical problems. However, there are few reaction series where the reversal of reactivity has been observed directly. Unambiguous examples are known, particularly in heterogeneous catalysis (4, 5, 189), as in Figure 5, and also from solution kinetics, even when in restricted reaction series (187, 190). There is the principal difficulty that reactions in solution cannot be followed in a sufficiently broad range of temperature, of course. It also seems that near the isokinetic temperature, even the Arrhenius law is fulfilled less accurately, making the determination of difficult. Nevertheless, we probably have to accept that reversal of reactivity is a possible, even though rare, phenomenon. The mechanism of such reaction series may be more complex than anticipated and a straightforward discussion in terms of, say, substituent effects may not be admissible. 

---