Hydrogenolysis catalyst particle size

C complexes, 32 185-186 CFjHCFjH, 39 340 chemisorption complexes, 32 170-172 CjH, enthalpies, 37 141, 143 "C-labeling studies, 25 166-172 commercial, 6 197 complex molecules, 30 58-72 medium-sized rings, 30 68-72 polymethylcycloalkanes, 30 59-65 substituted aromatics, 30 65-68 cyclic-acyclic product ratio, 30 8-9 cycloalkanes, 30 68-69 function, hydrogen pressure, 30 12, 15-16 hydrocarbon reaction models, 32 202-205 hydrogenolysis and, 23 93, 103 interconversion, 30 81-82 isopentane, 30 17 label scrambling, 30 7, 12-13 mechanism, 30 5-16 bifunctional, 30 4 catalyst particle size and, 30 72-85 concerted, 30 20... 

Figure 12. TEM images of the Pt(X)/SBA-15 catalysts X = (a) 1.7 nm, (b) 2.6 nm, (c) 2.9 nm, (d) 3.6 nm, and (e) 7.1 nm (the scale bars represent 20 nm) (1) structure sensitivity of ethane hydrogenolysis on 1% Pt(X)/SBA-15 with Pt particle sizes ranging from X= 1.7 to 7.1 nm. Rates corrected to 20Torr C2H6, 200Torr H2, and 643 K. (Reprinted from Reference [143], 2005, with permission from American Chemical Society).

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

These advances in catalyst preparation techniques have certainly stimulated the already growing interest in the relations between the catalytic and sorptive properties of catalysts and their mode of preparation. Many authors have studied the dependence of specific reaction rate upon particle size, mainly in hydrogenation, dehydrogenation, and hydrogenolysis reactions. The results of this work have recently been compiled by Schlosser (6). 

By using ratio the number of edge (C7) atoms to the number of (111) face (Cg) atoms (Fig. 12) to define an effective particle size , the selectivity of n-butane hydrogenolysis as a function of particle size for the two surfaces could be plotted and compared to selectivities measured on supported Ir catalysts This comparison is shown in Fig. 13. Clearly, the results on single... 

Fig. ii. Selectivity for ethane production from n-butane hydrogenolysis on iridium as a function of effective particle size. (From R s. HI. 112.) Also shown are data for n-butane hydrogenolysis on supported Ir catalysts The temperature is 47S K in all cases. 

The catalytic behavior of small metal particles in heterogeneous catalysts varies with metallic particle size and shape a phenomenon referred as a structure-sensitivity. Simple alkanes such as ethane, propane, n-butane and isobutane can be used as archetype molecules for studying hydrogenolysis reactions as they... 

The ratio of rates of formation and removal (by H2) of firmly bound species ( carbon ) is different with different metals. Evidently, Pt and Pd keep more carbon on their surfaces than do the good methanation catalysts such as Ni, Ru, or Rh. The surface of, say, Pt is better blocked and thus protected against hydrogenolysis than are surfaces of other metals. The often-found particle size sensitivity of hydrocarbon reactions on Pt (less on other metals) might be related to this. 

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

With linear alkanes having five or more carbon atoms, cyclization becomes possible as well as isomerization and hydrogenolysis. With n-pentane, cyclization is minimal and with n-hexane it does not exceed 25% in the range 470-570 K [6] with the latter molecule, isomerization predominates above 520 K. Product selectivities are particle-size sensitive, and Pt/SiC>2 catalysts having lower dispersion give more hydrogenolysis and cyclization. 

The activity of platinum clusters in zeolites for the hydrogenolysis of neopentane, ethane, and butane exceeds that of conventional platinum catalysts, and in two cases the results were interpreted in terms of an intrinsic change of electronic properties with particle size. In butane hydrogenolysis the sticking probability of alkane on the electrophilic platinum cluster was high compared with that on conventional platinum crystallites. ... 

A similar explanation may well be valid for a study by Yates and Sinfelt of the specific activity of rhodium catalysts supported on silica for the hydrogenolysis of ethane to methane 49). As in the example just discussed, there appears to be a sharp contrast in order of magnitude between the specific activity of catalysts with particle size below 40 A (the sensitive range of Poltorak) and above that size (130-2500 A) where bulk behavior is expected. In this case, I speculate that, with very small particles, hydrocarbon surface residues which appear to play an important role in the hydrogenolysis of ethane may well perturb the metallic character of the small rhodium particles, just like adsorbed oxygen in the case of Poltorak and co-workers. 

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