Hydrogenolysis energies

Figure 13. Dependence of ethane hydrogenolysis TOF and apparent activation energy on Pt particle size. TOFs decrease by two orders of magnitude over the size range, while the apparent activation energy increases. Coordinatively unsaturated surface atoms in small particles have a higher reactivity and subsequently a smaller barrier for hydrogenolysis than highly coordinated surface atoms of larger particles. TOFs were measured at 20 Torr C2H6, 200 Torr H2, and 658 K [16]. (Reprinted from Ref [16], 2006, with permission from American Chemical Society.)...

Table 5 Hydrogenolysis of AcOBu on lRelPt(e,red) catalyst in SPR16 reactor. Reaction order for AcOBu and apparent activation energy.

Schultz and Linden Ind. Eng. Chem. Process Design and Development, 1 (111), 1962] have studied the hydrogenolysis of low molecular weight paraffins in a tubular flow reactor. The kinetics of the propane reaction may be assumed to be first-order in propane in the regime of interest. From the data below determine the reaction rate constants at the indicated temperatures and the activation energy of the reaction. 

Fig. 5. Apparent activation energies of the ethane hydrogenolysis and cyclopropane hydrogenation reactions on the group VIII noble metals. The activation energies were determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

Recently, Vayner and coworkers [239] have revisited the model proposed by Augustine et al. [34] which is based on the assumption that the QN can make a nucleophilic attack to an activated carbonyl. According to this model the two possible zwitterionic intermediates that can thus be formed have different energies, which leads to the selective formation of one of the two intermediates, and, therefore, to e.s. after hydrogenolysis by surface hydrogen. This model nevertheless does not explain the e.d. of nonbasic modifiers, such as the one reported by Marinas and coworkers [240], which have no quinuclidine moiety and no nitrogen atom, and thus no possibility to form zwitterionic intermediates. Furthermore, in situ spectroscopic evidence for hydrogen bond formation between the quinuclidine moiety of cinchonidine and the ketopantolactone has been provided recently [241], which supports the hypothesis of the role of weak bond formation rather than the formation of intermediates such as those proposed by Vayner and coworkers. 

The catalytic activity of strained-layer Ni on W(llO) for methanation and ethane hydrogenolysis has been studied as a function of Ni coverage. The activity per Ni atom site for methanation, a structure-insensitive reaction, is independent of the Ni coverage and similar to the activity found for bulk Ni. The activation energy for this reaction is lower on the strained-metal overlayer, however, very likely reflecting the lower binding strength of CO on the bimetallic system. 

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