Catalytic turn-over-frequency

Catalytic Turn-Over-Frequency. From the kinetic fit, it is possible to simulate the CO2 formation rate and thus to obtain the turn-over-frequency (TOF) of the catalytic reaction. The simulated C02-yield under the reaction conditions of Fig. 1.64a is displayed in Fig. 1.68. The corresponding TOF amounts to 0.4 CO2 molecules per gold cluster per second. For the conditions of Fig. 1.64c, a TOF of 0.3 CO2 molecules per gold cluster per second was estimated. These values are in the same order of magnitude as the catalytic activity of oxide supported gold cluster particles with a size of a few nanometers, which ranges between 0.2 s per Au atom ( 2 nm diameter particles at 273 K) and 4s per Au atom (3.5 nm particles at 350 K) [238,366,367]. 

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. 

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... 

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. 

Turn-Over Frequencies of Catalytic Reactions on Supported Clusters... 

The examples presented above showed that size-selected clusters on surfaces reveal remarkable size-effects, however, they have been studied so far only by one-cycle experiments. Thus an experimental proof that these systems are active for catalytic processes, e.g. several cycles of a catalytic reaction are promoted without destruction of the catalyst, is still missing. In the following we present an experimental scheme to study catalytic processes of clusters on surfaces with high sensitivity and we report turn-over frequencies TOFs for the oxidation of CO on size-distributed supported Pd clusters. 

To study catalytic processes of clusters on surfaces with high sensitivity, a newly designed pulsed valve with excellent pulse-to-pulse stability, in combination with absolutely calibrated mass spectrometry, was used to determine turn-over frequencies [75]. 

The most significant results were obtained by the group of Breit who demonstrated that phosphinine rhodium(I) complexes behave as very efficient catalysts. In the first two reports, most efforts focused on the hydroformylation of styrene and important conversion yields and interesting selectivities in favour of the branched aldehyde were obtained by using derivatives of 2,4,6-triphenylphosphin-ine 1 [51]. Importantly, reactions could be carried out under mild conditions in toluene at 25 °C using [Rh(acac)(CO)2] complex as catalytic precursor with a Rh phosphinine substrate ratio of 1 5 280 and a CO/H2 (1 1) pressure of 20 bars. With the triphenyl derivative, a Turn Over Frequency (TOF) of 28.7 mol sub-strate/mol catalyst/h was obtained, the conversion yield reaching 30.8%. Theses... 

The results given in Table 1 are expressed in T50 namely the temperature at which 50% conversion is reached. Table 1 also presents the number of exposed noble metal on each catalyst expressed in mmol/g of catalyst. As shown by this table, very important differences of activity are evidenced. It was thus not possible to compare all the catalysts at a common reaction temperature. Moreover, the important differences of activity do not allow any reliable comparison of turn over frequencies in this simple set of experiments. Nevertheless, it is still possible to interpret the activity as a function of the amount of metal exposed. The differences of activity between catalysts cannot be attributed to the combination of loading and dispersion. Indeed, Pd/C DP is more active with 3.3 exposed Pd/g of catalyst than Pd/C El with 3.8 exposed Pd/g of catalyst. In contrast. Figure 1 shows that an obvious relationship exists between the catalyst loading and activity expressed in T50. This correlation indicates that the catalytic activity is directly proportional to the amount of Pd deposited on the support for loadings superior to 1 wt%. It also shows that the... 

Not only does the performance depend on the nature of the foreign metal but also on the preferential surface exposed. Thus, the Pt3Ni(lll)-skin surface exhibits the highest catalytic activity recorded for the ORR, i.e. a total 90-fold increase with respect to that of Pt/C. This value corresponds to a TOF (turn-over frequency) of 2800 s a value well above the figures reported for Pt/C and PtM/C of 25 s and = 60 s respectively. Wu et have reported a facile method for synthesizing PtgNi nanoparticles with a dominant exposure of 111 facets. 

Over et al. performed in situ surface X-ray diftiaction experiments (SXRD, see Box 8.1) [19, 20]. The combination of online reaction product analysis with SXRD allowed them to correlate the turn over frequency (TOF—number of reaction products per site per second) for CO oxidation with the structure of the catalytic surface. Figure 8.3b shows that, in the mbar regime, two distinct phases can be present the RUO2 phase and a non-oxidic phase. At tanperatures below 550 K, both phases have a nearly identical activity for temperatures above 550 K, the oxide phase has the higher activity, showing that the oxide is indeed the active phase under these conditions. 

The intrinsic rate at which a catalytic cycle turns over on an active site is called the turnover frequency, r of a catalytic reaction and is defined as in Chapter 1 [Equation (1.3.9)] as ... 

Ifrrnover Refers to the ability of a catalytic species to be used many times over in a catalytic cycle. In enzymology, the catalytic constant cat, obtained from a Michaelis-Menten kinetic analysis, is often called the turnover number because it represents the number of times the enzyme turns over per unit time, although other uses of the term have arisen. The more precise term is turnover frequency. 

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