Turn-over frequence

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

Figure 2. A comparison of the rate (turn-over frequency) of methane synthesis over single crystal and supported ruthenium catalysts. Total reactant pressure for the single crystal studies was 120 Torr.

The effect of tin on the catalytic activity in terms of turn over frequencies (TOP) is more complex. When Sn/Pts increases from 0 to 0.85, the catalytic activity based both on the total number of Pt atoms or even on surface Pt... 

GP 1] [R 1] A comparison of four micro reactors with different Pt loadings (Pt impregnated on anodically oxidized alumina support) and different Pt structures confirmed that cluster size has an impact on the single Pt-atom activity (6 vol.-% NHj, 88 vol.-% O2, balance He 0.51 ms 260-380 °C) [28, 98]. At low Pt loadings, isolated atoms are formed. Calculated ammonia consumption rates amount to 20 s at 300 °C. At high Pt loadings, clusters are formed. Turn over frequencies (TOP) of about 40 s are determined. 

Kinetic data turn-over frequency (TOP) and activation energies... 

A hypothesis that edge and corner sites work as active sites can explain why turn over frequency (TOF), which is defined as the reaction rate per one active site, in the case of metal catalysts, per surface exposed metal atom, increases with a decrease in the diameter of gold particles. However, it fails to explain the significant contribution of support materials and the contact structure of gold NPs. It seems to be reasonable that those edges and corners act as the sites for adsorption of one of the reactants, for example, CO in its oxidation. 

Figure 11. Turn over frequency for CO oxidation over Au/ Ti02/Mo model catalysts [42].

Figure 9.6 Activity for CO oxidation at room temperature as a function of gold coverage on an Mo(l 12)—(8 x 2)-TiOx surface. The CO 02 ratio was 2 1 and the total pressure 5 Torr. Two discrete gold structures were investigated, (lxl) and (1 x 3). The initial turn over frequencies (TOF) over the (1 x 1) gold monolayer structure were significantly lower than that for the (1 x 3) bilayer structure. (Reproduced from Ref. 20).

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

Fig. 19. Turn-over frequencies for hydrogenation of allyl alcohol and N-isopropyl acrylamide obtained in water using dendrimer-encapsulated Pd nanoparticles of constant average size. The hydroxyl-terminated PAMAM dendrimer generation varies from G4 to G8...

Table 2. Substrate, structures, and turn-over frequencies obtained for hydrogenation reactions in fluorous biphasic systems employing dendrimer-encapsulated Pd nanoparticles ...

Turn over frequency (TOP) = [Product]Moi/([catalyst]MoixThour) [Pyc] = 70.56 pMol./gram of NPyc ScHo = percentage of aldehyde selectivity by GLC. 

Similarly, a turn-over frequency (TON) of 227 of the polymerization process was distinctly low for 77d with [M]/[I] = 350, at 110 °C for 6 h, using in the melt polymerization conditions. Biocompatible calcium complex 77a used as catalyst at 110 °C produced in 30 min PLAs with high molecular weight (65,000-110,600) and narrow polydispersities (1.02-1.05) using [M]/[I] = 350-700. It is worthy of note that complex 77a displayed a notable heteroselectivity (probability of racemic linkages between monomers, = 0.73, see Sect. 4.2) in polymerization of rac-lactide in THF at 33 °C. Data on the aforementioned calcium initiators and their lactide polymerization are listed in Table 4. 

The turn over frequency (TOF) and the activity of carbonylation are defined by Equations 3 and 4, respectively. 

The data in Table IV and Figure 8 show that the dispersion of nickel decreases with increasing nickel loading from 90% (1 wt%) to 22% (10wt%). However, the turn over frequency of the carbonylation is independent of the Ni loading, indicating that this reaction is a structure insensitive reaction. 

The CO oxidation reaction occurs rapidly at room temperature and below. As an example, on a Au(lll) surface at 250 K with onear unity exposed to a constant CO pressure of Pco = 2 x 10 Torr, the reaction rate expressed as turn-over frequency (TOF [molecules CO2 (Au atom s)" )/ is approximately 2.5 x 10-3 immediately after the reaction has been initiated and then declines at a relatively constant rate reaching a value of 3 x 10" after the reaction has proceeded approximately 800 seconds. Reaction order... 

In the following we will usually calculate turn over frequencies, i.e the number of reactions per site per second. The adsorption rate, o, is the rate per m per second. 

Recently, the enantioselective hydrogenation of ethyl pyruvate catalyzed by cinchona modified Pt/Al203 (ref. 1) was shown to be a ligand accelerated reaction (ref. 2). The rate of reaction for the fully modified system is more than 10 times faster than the racemic hydrogenation using unmodified catalyst. Under certain reaction conditions, this liquid phase hydrogenation exhibits a turn-over frequency of up to 50 s 1 (3.4 mol/kg-cat s). Emphasis until now has been directed at empirically increasing optical yields (ref. 3,4). 

Such a high reaction rate strongly suggested the potential for mass transport problems. Indeed, a turn-over frequency on the order of 1 s 1 is considered appropriate for the purpose of mechanistic studies normally conducted in the gas phase (ref. 5). At higher rates, various complications including intraparticle diffusion problems, often arise. The situation is even more severe in the liquid phase where the bulk diffusivity of species is considerably reduced. A... 

Intraparticle Resistance. The Koros/Nowak (ref. 7) or Madon/Boudart (ref. 8) criterion states that, in the absence of mass transfer influences, the activity of a heterogeneous catalyst should be proportional to the number of active sites. In other words, the observed turn-over frequency (TOF) should be independent of the particle size if there is negligible intraparticle resistance since all active sites are fully effective. 

The particle size effect on the activity was measured for two supports, LTL [0.47] and SiC>2. The results for these catalysts are given in Table 3. The TOF (turn over frequency) was of the same order of magnitude, irrespective to the particle size. However, the apparent activation energy is clearly smaller for the larger particles. This lower Eapp is compensated by a smaller pre-exponential factor In Aapp. 

In Table 3, the turn-over frequency (TOF, mol CP mol Pt 1 s"1) and apparent activation energies (kJ/mol) for the conversion of CP and the roll-over mechanism are given. With the exception of the basic catalysts (Pt/LTL [K/Al > 1.0] and Pt/HT), the TOF at 75°C for the different catalysts are all of the same magnitude, approximately 5-10 mol CP mol Pt 1 s 1. The basic catalysts show activities that are up to 104 times smaller. The low activity of the basic catalysts prevented the accurate determination of selectivities, activation energies and orders in D2 and CP for these catalysts. 

The method of determination of the ASA creates some carbon-oxygen surface complexes which are nonreactive and which cause the still significant dependence of the turn over frequency (TOF) on bum-off or... 

TOF0 = initial turn over frequency (mmol of product per mmol of catalyst per minute). 

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