Rate of n-butane hydrogenolysis

Figure 6 Arrhenius plot of the rate of n-butane hydrogenolysis over Sample 1 (ion-exchanged Ni catalyst) and Sample 3 (precipitated Ni catalyst).

Namizek (109) has studied the rate of n-butane hydrogenolysis over Ni/ A1203 for Ni crystallites of d varying between 1.0 and 10 nm. A maximum rate is found at about 2.5 nm, so that those results resemble closely those for C3H8 and for C2H6. The maximum in the rate curve occurs at a d that corresponds roughly to the maximum surface concentration of Bs sites, as measured by the method of van Hardeveld and van Montfoort (106). 

Somewhat different conclusions have been reached in a study of n-butane hydrogenolysis on a series of PtRe/a-AhOs catalyst covering the whole composition range. There was an astronomic factor (x 10 ) between the rates shown by platinum and the Pt25Re7s catalyst (Figure 13.25), and as little as 12.5% rhenium was sufficient to increase methane selectivity and to move the Arrhenius... 

Catalytic Activity of Nickel-Loaded Titanates. A good test of the dispersion of the active metal is the activity and selectivity for the hydrogenolysis of n-butane. For example, it is well known (6) that the hydrogenolysis of saturated hydrocarbons, or the rupture of carbon-carbon bonds by hydrogen, are structure sensitive that is, their rates per surface metal atom (TOF s) vary with the percentage of metal exposed on the catalyst (i.e.. the dispersion). Typically,... 

In later work, Namizek and Ryczkowski (110) measured rates of propane and n-butane hydrogenolysis over Ni/Al203 catalysts of various FE. They found maximum values of TOF for propane at d = 3-4 nm and for butane at 2-3.5 nm. Masson et al. (Ill) have studied n-butane hydrogenolysis on well-characterized Ni vapodeposited on silica, prepared as already discussed (78, 79). The TOF rises as d is decreased, reaches a maximum at about 2 nm, and then tends toward zero as d goes toward zero (FE = 1.0). 

Figure 5.7. Hydrogenolysis of n-butane on Rh/Ti02 at 429 K comparison of direct (O) and semi-logarithmic ( ) plots of rate versus hydrogen pressure. In this and the next figure, the curves are calculated by the rate expression ES5B to be derived in Chapter 13.

Figure 5.9. Dependence of rate of hydrogenolysis of n-butane on hydrogen pressure at 609 K using PtRe/Al203 experimental points fitted to three rate expressions. ES5B, — ES2, - - ES3, . The formulation of these rate expressions is explained in Chapter 13.

Figure 13.19. Hydrogenolysis of n-butane on 1% RU/AI2O3 variation of rate at 398 K with hydrogen pressure after HTRi and O/LTR pre-treatments. ...

Figure 13.26. Activity of PtRe/a-Al203 catalysts for n-butane hydrogenolysis as a function of rhenium content at 513 K (In (rate/molecules cm s )). ° ...

To provide adequate background for the work to be described next, some further findings by Anderson and Avery may be mentioned. The selectivity for isomerization versus hydrogenolysis (St = 77/77 ) of isobutane on evaporated films of platinum claimed to expose (111) faces predominantly was found to be enhanced by a factor of 5 relative to unoriented films this enhancement was not observed for n-butane (Table VI). Anderson and Avery (47) proposed that a symmetrical triadsorbed species (Diagram 1) is the preferred reaction intermediate for isobutane, such an intermediate not being possible for n-butane. This intermediate fits the triplets of metal atoms on the (111) plane of platinum, suggesting, they believed, a basis for the enhanced efficiency of the (111) plane for the isomerization of isobutane. We note that inspection of rates of isomerization given in the paper of Anderson and Avery shows a factor of only... 

The kinetics of alkane hydrogenolysis, that is, the dependence of rate on reactant concentration, have been the subject of numerous studies, and much effort has been devoted to devising rate expressions based on the Langmuir-Hinshelwood formalism to interpret them. Reactions of ethane, propane, and n-butane with H2 on EUROPT-3 and -4 have been carefully studied, with the surfaces in either as clean a state as possible, or deliberately carbided [21, 22] kinetic measurements at different temperature permitted adsorption heats and true activation energies to be obtained. There were two surprises (but like all surprises they were obvious afterwards) ... 

Davis et al. (23) also studied the isomerization and hydrogenolysis of isobutane, n-butane, and neopentane over flat, stepped, and kinked Pt single-crystal surfaces. The rates and selectivities of butane isomerization and consecutive rearrangements were maximized on (100) portions of the surfaces. Competing hydrogenolysis reactions were most rapid on surfaces containing the greatest step- and kink-site densities. 

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