Palladium hydrogenation activation energy

Table III lists the kinetic equations for the reactions studied by Scholten and Konvalinka when the hydride was the catalyst involved. Uncracked samples of the hydride exhibit far greater activation energy than does the a-phase, i.e. 12.5 kcal/mole, in good accord with 11 kcal/mole obtained by Couper and Eley for a wire preexposed to the atomic hydrogen. The exponent of the power at p amounts to 0.64 no matter which one of the reactions was studied and under what conditions of p and T the kinetic experiments were carried out. According to Scholten and Konvalinka this is a unique quantitative factor common to the reactions studied on palladium hydride as catalyst. It constitutes a point of departure for the authors proposal for the mechanism of the para-hydrogen conversion reaction catalyzed by the hydride phase.

Fig. 8. Arrhenius plots for the formic acid decomposition on palladium foil (1) and small pieces of this foil (2) at a higher temperature range, when hydrogen evolving as a product of the reaction was absorbed by Pd and transformed into the /3-Pd-H hydride phase. At the lower temperature range the reaction proceeds on the a-Pd-H phase, with a higher activation energy when the foil was hydrogen pretreated (2a), and a lower activation energy for a degassed Pd foil (3a). After Brill and Watson (57).

The high MIBK selectivity over the palladium catalysts suggested that the hydrogenation of MO was facile and that the overall rate-determining step may lie in the aldol part of the sequence rather than with the hydrogenation. If this was the case the case then we may have expected to have similar activation energies for the formation of MO and MIBK. In a previous study (II) an activation energy of 23+4 kj.mof was calculated for the formation of MO from... 

The solubility of hydrogen atoms in gold is very small at 273 K the H/Au ratio was 4.4 x 10-3, and the activation energy for desorption between 250 and 380 K was 57 5kJ mol-1 the heat of absorption was1 - 9kJ mol-1. Hydrogen atoms are also able to diffuse easily through a gold layer when supplied via a palladium hydride phase.10... 

Since the E-alkene is the quickest to form under these conditions, cis addition of HCl must have a smaller activation energy barrier than trans addition. This suggests that reaction occurs on the surface of the alumina with both the H and the Cl added to the triple bond simultaneously from the same side rather like cis-hydrogenation of triple bonds on a palladium catalyst (chapter 24). 

A palladium-hydrogen-mordenite catalyst with a 10.8/1 silica/alumina mole ratio was evaluated for the hydroisomerization of cyclohexane. The rate of reaction followed a first-order, reversible reaction between cyclohexane and methylcyclopentane. The energy of activation for this reaction between 400° and 500°F was 35.5 it 2.4 kcal/mole. Cyclohexane isomerization rates decreased with increasing hydrogen and cyclohexane-plus-methylcyclopentane partial pressure. These effects are compatible with a dual-site adsorption model. The change of the model constants with temperature was qualitatively in agreement with the expected physical behavior for the constants. 

Catalysts are usually needed in the hydrogenation of alkenes because the reaction s activation energy is too large without them. Catalysts such as powdered platinum or palladium provide a surface that adsorbs the reactants and makes their electrons more available to bond to other atoms. 

---