Active sites specific site TOFs

In this equation the Rate is the molar TOF of the reaction, moles of product formed/mole of metal catalyst/unit time. The terms in [ ] are the STO measured site densities given in moles of site/mole of metal. The specific site TOFs, A, B and C, have units of moles of product/mole of site/unit time. Of these factors, the site densities are available from an STO characterization of the catalyst and the Rate is determined for the specific reaction nm over the STO characterized catalyst. When a series of at least three STO characterized catalysts is used for the same reaction, run under the same conditions, the specific site TOFs can be calculated from the simultaneous equations expressed as in Eqn. 3.6. When this approach was used in the hydrogenation of cyclohexene over a series of seven Pt/CPG catalysts specific site TOF values for the Mr and MH sites were found to be 2.1, 18.2 and 5.2 moles of product/mole of site/second, respectively.21 Not surprisingly, that site with the weakly held hydrogen was the most active and that on which the hydrogen was strongly held was the least active. 

FT-activity may be calculated per catalyst volume or weight. Calculating a site-specific activity (TOF) appears not possible and theoretically not justified in Fischer-Tropsch synthesis, tiie TOF-concept being not applicable. The active sites of FT-synthesis develop (are built) during the episodes of self-organization and can neither in number nor in nature be the same as those determined by chemisorption with the reduced fresh catalyst. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Webb, M. E., Stephens, E., Smith, A. G and Abell, C. (2003). Rapid screening by MALDI-TOF mass spectrometry to probe binding specificity at enzyme active sites. Chem. Commun., 2416-2417. 

Competition between reactant, solvent and product molecules for adsorption within the zeolite micropores is demonstrated directly (adsorption experiments) and indirectly (effect of the framework Si/Al ratio on the activity, kinetic studies) to occur during Fine Chemical synthesis over molecular sieve catalysts. This competition, which is specific for molecular sieves (because of confinement effects within their micropores), adds up to the competition which exists over any catalyst for the chemisorption of reactant, solvent and product molecules on the active sites. Both types of competition could affect significantly the activity, stability and selectivity of the zeolite catalysts. Although the relative contributions of these two types of competition cannot be estimated, the large change in the activity of the acidic sites (TOF) with the zeolite polarity seems to indicate that the competition for adsorption within the zeolite micropores often plays the major role. 

Table 10.2 presents the kinetic information for the main reactions, in which the frequency factors have been calculated from turnover-frequency (TOF) data [8, 9]. This term, borrowed from enzymatic catalysis, quantifies the specific activity of a catalytic center. By definition, TOF gives the number of molecular reactions or catalytic cycles occurring at a center per unit of time. For a heterogeneous catalyst the number of active centers can be found by means of sorption methods. Let us consider that the active sites are due to a metal atom. By definition [15] we have ... 

Rates of catalytic reactions are obtained by measurement of the conversion of a key component, often the rate limiting reactant, in laboratory reactors and relating this to the amount of catalyst used and the amount or flow rate of reactants used, to obtain an intrinsic quantity, mols-1 amount-1. For practical application the mass or volume of a catalyst is most relevant as the amount but, for comparitive studies the amount of active phase on a supported catalyst, its specific surface area or the number of active sites may be preferred. In the latter case this yields the turnover frequency (TOF) [3], which is quite relevant for fundamental studies. The number of active sites is, however, usually hard to determine and the mass of the catalyst W will be used, resulting in a rate dimensions of mol s 1 kg-1. Other quantities are easily derived from this. 

It was mentioned previously that the rate of a heterogeneously catalyzed reaction is expressed as a turnover frequency (TOF) which is the number of times an active site reacts per unit time. Since active site concentrations have not been available, in most cases the TOF is expressed as the number of molecules formed per unit time per surface atom or unit surface area. The ability to use the STO procedure to measure active site densities also provides a means of determining specific site TOFs. It is apparent that the total number of molecules formed in a catalytic reaction per unit time is the sum of the production from each active site. Thus, the reaction TOF can be expressed as the sum of the products of the specific site TOF and the specific site densities as shown in Eqn. 3.6.21... 

The value for V, ax 27" (mole site) , is the maximum rate for this reaction when it is run under saturation conditions for both substrates. In contrast to the single type of active site found in most enzymes, there are a number of different types of sites present on the surface of the platinum catalyst used for these oxidations. It was shown in a parallel study that 2-propanol oxidation takes place over the coordinately unsaturated corner atoms, that is, the single turnover (STO) characterized M, and MH sites (see Chapter 2). It was also shown that the specific site turnover frequencies (TOF) for these sites are 5.5,7.9 and 5.0 moles O2 uptake/mole site/minute respectively. 

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 specific activity of monolayer supported oxide catalysts considering the actual density of surface active sites is a factor of 3 higher than the TOFs obtained with the amount of dispersed metal atoms. This observation is expected since less than half of the exposed metal oxide atoms can simultaneously participate as active surface sites during steady-state reaction due to the steric hindrance in methanol adsorption. 

The catalytic activity per surface active site (TOP) toward a specific reaction is the right parameter to obtain reliable surface structure-activity correlations. The knowledge of the TOFs values dismissed the believes that the catalytic activity is influenced by bulk properties and that monolayer supported oxide catalysts are more active than bulk oxide catalysts. There is no doubt that the specific activity would contribute to design more active and selective catalytic materials at a molecular level. 

In order to distinguish the catalytic activity of different acid sites, a specific rate per acid site (TOF (h )) has also been calculated. The results are presented in Figure 6.33. A comparison between the results presented in Figures 6.32 and 6.33 shows that the highest intrinsic activity was displayed by the MgFi-Tl sample. The differences between the activities expressed in terms of conversion of glycerol and TOF are due to the different densities of the acid sites on the catalyst surface. 

The single crystal catalysts, -1 cm in diameter and 1 mm thick, are typically aligned within 0.5 of the desired orientation. Thermocouples are generally spot-welded to the edge of the crystal for temperature measurement. Details of sample mounting, cleaning procedures, reactant purification, and product detection techniques are given in the related references. The catalytic rate normalized to the number of exposed metal sites is the specific activity, which can be expressed as a turnover frequency (TOF), or number of molecules of product produced per metal atom site per second. 

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