TOF Based on Chemisorption

Figure 1 is a typical activity/time plot for the unpromoted catalysts at low reaction temperatures. Total activity (CO TOF, based on chemisorption data from fresh, reduced catalysts) decreases slightly over the first 2-3 hours and then increases until a steady-ctate is reached after about 18 hours. The initial drop in activity can be explained by the significant drop in CO2 production as the hydrocarbon (HC) production increases. 

The results from the H -D2 experiments are shown in Figures 2 and 3. In Figures 4 and 5 the propane dehydrogenation conversion just before an H2-D2 experiment have been related to the HD formation rate. Experiments from all the runs were used. The TOF, based on the number of hydrogen chemisorption sites on a fresh catalyst, were calculated from the rate... 

In a similar vein, Aben et al. (97) have found that a TOF based on reversible (weak) hydrogen chemisorption is invariant with particle size for the hydrogenation of benzene over Pt/Al203. This system appears in Table XVI with TOF based as usual on strong hydrogen chemisorption. 

The results showed that K promotion resulted in almost 50 times the increase in the reaction rate. TOF based on the amount of hydrogen chemisorption increased by about two orders of magnitude with K-promotion. However, the intrinsic TOF based on SSITKA increased only by a factor of 16. The increase in activity with K promotion was actually due to both a significant increase by a factor of 3 in the surface concentration of intermediates, and an increase by a factor of 16 in the average intrinsic site activity. 

As summarized in Table 1, the support also has an important role on the Ru content and particle size. The steady-state CO conversion is about 8.9 % over Ru/SiOa synthesized under fluidized-bed conditions at 383 K in the second cycle, while the conversion is 5.7 % over Ru/Si02 synthesized under static conditions. Under the same conditions, the steady-state CO conversions are 17.1 % and 12.2 % over Ru/MgO and Ru/CNTs, respectively. Due to the different Ru loading and Ru dispersion, it is reasonable to calculate turnover frequencies (TOFs number of CO2 molecules formed per second per surface site) which were based on Ha chemisorption data and the steady-state CO conversion at 383... 

Little attempt seems to have been made to estimate the number of free surface metal atoms in coked catalysts, and hence to find TOFs, assuming these to be the seat of the residual activity. While the use of hydrogen chemisorption might be considered risky, that of carbon monoxide ought to be suitable. Based on the loss of its IR intensity, the active metal area of Pt/Al203 used for n-heptane reforming was only 8% of its initial value, but its extent of adsorption slowly increased as it displaced some of the carbon . ... 

Ammonia synthesis reaction rate about three kinds of catalysts with different loading amount of ruthenium and promoters were studied. Based on O2 chemisorption data, the relationship between the turnover of frequency (TOF) and the ruthenium particle size is shown in Fig. 6.51. A monotonic increase in TOF02 vs do2 is characteristic for each system. Extrapolation of the results to small crystallite diameters suggests (Fig. 6.51) that extra fine particles smaller than 0.7nm 0.8 nm (critical size) might be totally inactive. Analogous trends in the surface activities were found (not shown) when the amount of adsorbed CO were used instead of O2 uptake for the particle diameter and TOF calculation. 

Many catalytic studies, perhaps even a majority, have involved metallic systems, either unsupported or supported on a high surface area substrate which is frequently inert in the reaction of interest. Thus the reaction rate is dependent on the specific surface area (m g ) of the metal, not only because the total number of active sites can vary, but also because the average metal crystallite size is dependent on this value and some reactions, now termed structure-sensitive [14], have areal rates (and TOFs) that are dependent on crystallite size [14,15]. Consequently, it is of utmost importance to measure the metal surface areas in these catalysts and calculate metal dispersions and crystallite sizes based on this information. The three most general approaches to accomplish this involve TEM (SEM), XRD, and chemisorption methods. 

The particular reactivity of bare Si02 for the production of HCHO is a matter of debate and has not yet been completely rationalized. Parmaliana et al. [113] pointed out that the performance of the silica surface in CH4 partial oxidation is controlled by the preparation method. For several commercial Si02 samples, the following reactivity trend has been established, based on the preparation method precipitation > sol-gel > pyrolysis. The activity of such silicas has been correlated with the density of surface sites stabilized under steady-state conditions acting as O2 activation centers [114], and the reaction rate was the same for all the silicas when expressed as TOF (turnover frequency). Klier and coworkers [115] reported the activity data for the partial oxidation of CH4 by O2 to form HCHO and C2 hydrocarbons over fumed Cabosil and silica gel at temperatures ranging from 903 to 1953 K under ambient pressure. They observed that short residence times enhanced HCHO (and C2 hydrocarbon) selectivity, suggesting that HCHO did not originate from methyl radicals, but rather from methoxy complexes formed upon direct chemisorption. 

The rate per site or turnover frequency was calculated based on the rate of benzene formation at 373 K and the number of surface sites after CO chemisorption, as presented in Table 3.4. Results show that the turnover frequency (TOF) changed with the different carbon supports. 

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