Activation energy for catalysis

Table IV. Activation Energies for Catalysis of Hemoglobin Hydrolysis...

Smith and Smith230 compared the activation energies for catalysis of the mutarotations by the water molecule, and by each of three acids and four bases, by use of the equation k = PZe EIHT. The values of E found for the various catalysts differed only slightly, and the observed... 

Proton coupled electron transfer. Titration of 1 with acetic acid results in the reduction of Mn(III) to Mn(II) as shown in Figure 8. Thus, the transfer of an electron will facilitate the addition of a proton and vice versa. Consequently, the activation energy for catalysis by both above mechanisms is lowered. 

Catalysis by hydrogen chloride or iodine monochloride in chlorination in carbon tetrachloride has also been examined. For the chlorination of pentamethylbenzene, the reaction was first-order in both aromatic and chlorine and either three-halves, or mixed first- and second-order in hydrogen chloride, but iodine monochloride was more effective as a catalyst and the chlorination of mesitylene was first-order in iodine monochloride the activation energy for this latter reaction (determined from data at 1.2 and 25.0 °C) was only 0.4 273. 

Thomas and Long488 also measured the rate coefficients for detritiation of [l-3H]-cycl[3,2,2]azine in acetic acid and in water and since the rates relative to detritiation of azulene were similar in each case, a Bronsted correlation must similarly hold. The activation energy for the reaction with hydronium ion (dilute aqueous hydrochloric acid, = 0.1) was determined as 16.5 with AS = —11.3 (from second-order rate coefficients (102At2) of 0.66, 1.81, 4.80, and 11.8 at 5.02, 14.98, 24.97, and 34.76 °C, respectively). This is very close to the values of 16.0 and —10.1 obtained for detritiation of azulene under the same condition499 (below) and suggests the same reaction mechanism, general acid catalysis, for each. 

First, the rate of heat production is again related to the sum of the rates of depositional and burning processes, and if the predominant factor affecting the overall rate is temperature, then it does not seem likely that the specific effect of water vapor on the oxidation reported here is chemical catalysis, since a lowering of activation energy for either process would result in an increase in the overall rate relative to dry oxidation. 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

CO oxidation catalysis showed that, for all the supports, the bimetallic catalyst was more active at low temperatures than the corresponing monometallic and cometallic catalysts. Apparent activation energies for monometallic Pt and Au catalysts were very consistent, near 32 and 80 kJ/mole, respectively. The s)uiergism for PtieAuie catalysts also shows up in the apparent activation energies for these catalysts, which were consistently around 23 kJ/mole. 

The decrease in activation energy for the chemisorption process as a result of 77 complex adsorption is readily explained by Lennard-Jones general theory of catalysis (19). Curve I in Fig. 1 represents the van der... 

Activation Energies for Carbon Monoxide Oxidation on Nickel Oxide Catalysis... 

The mechanism of the cycloaddition process was partly clarified on the basis of experiments with acetylenic dithio derivatives 102, which were found to be thermally transformable to allenic isomers 103.149 The latter gave expected 2/7-thiopyrans 104 in 41 to 78% yields under triethylamine catalysis, whereas isomeric thiophenes 105 were formed in the presence of protic acids (Scheme 5). The activation energies for some of the processes were also measured.149... 

The fact that each of the reaction steps outlined above has an inverse that is energetically accessible leads readily to the formulation of catalytic cycles. This is not simply a restatement of microscopic reversibility. The distinction is that the activation energies for all of these reactions are not prohibitively high so that catalysis can occur. 

---