Active site density

As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts at ambient temperatures and mild pH, and have significantly higher reaction selectivity than precious metals. The main disadvantage in applying redox enzymes for electrocatalysis arises from their large size, which means that the catalytic active site density is low. Enzymes also have a relatively short hfetime (usually not more than a few months), making them more suited to disposable applications. 

In agreement with the TPR results, the hydrogen chemisorption/pulse reoxidation data provided in Table 8.3 indicate that, indeed, the extents of reduction for the air calcined samples are -20% higher upon standard reduction at 350°C (compare 02 uptake values). Yet in spite of the higher extent of reduction, the H2 desorption amounts, which probe the active site densities (assume H Co = 1 1), indicate that the activated nitric oxide calcined samples have higher site densities on a per gram of catalyst basis. This is due to the much smaller crystallite that is formed. The estimated diameters of the activated air calcined samples are between 27 and 40 nm, while the H2-reduced nitric oxide calcined catalysts result in clusters between 10 and 20 nm, as measured by chemisorption/pulse reoxidation. 

Table 1. Composition, Ammonia and Oxygen Uptakes, Surface Coverage, Active Site Density, Methanol Conversion and Product Selectivities of Catalysts...

Zakharov et al. have used a radio tagging technique to measure the active site density in which polymerization is killed with labeled methanol (72, 73). They found only about 1 % or less of the chromium to be active, or about one tenth of Hogan s number. But because they calcined Cr/silica at only 400-500°C, their catalyst was probably only one-tenth as active. So the two studies are not necessarily in conflict. As expected, the active site density found by tagging increases with time during a polymerization run. 

Both the properties of the active sites (density, strength, etc.) and of the pore systems can be tailored by well-known methods.[3,9]... 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

It is also evident that the collision frequency (or pre-exponential factor) differs between different metal oxides for comparable surface areas. Hence, the active site density depends on the type of metal oxide and most likely also on pre-treatment and manufacturing of the oxide particles. In practice, determining the active site density is very difficult or even impossible since it is based on BET measurements of the specific surface area. The BET surface area can give a measure of the sorption site density which deviates significantly from the actual specific surface area. Again, this demonstrates the difficulties encountered when trying to compare the reactivity of different solid materials at a given temperature on the basis of the specific surface area. 

We also examine the structure sensitivity of CO hydrogeneration on Co and Ru crystallites supported on various metal oxide supports at conditions that favor high selectivity (>80%) to C5+ products. We describe procedures for the synthesis of catalytic materials with high active site densities and controlled intrapellet distributions of sites. Finally, we review the extensive literature that has previously described metal dispersion, support, and transport effects on FT synthesis rate and selectivity. 

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... 

Lutzerrkirchen. J. et al. Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces, Geochim. Cosmochim. Acta. 66. 3389. 2002. 

It is emphasized that each method of measuring active-site density has limitations, and some common techniques are actually invalid. For example, counting the number of chains formed provides no useful information, because Cr/silica is not a living system under industrial conditions and therefore each site produces many chains each second. Nor can kinetics be used in a stand-alone way, because the measured activity is always the product of the number of sites times the rate per site, both of which are unknown. Poisoning experiments give an upper bound. Below is a discussion in more detail of those techniques that the author believes provide most insight into the commercial catalyst. 

In some experiments, the selectivity of the poison decreased with increasing reaction temperature. That is, the catalyst became more tolerant of some of the poisons, and more had to be added to achieve an equivalent loss in activity. Most likely, the poison blocks a site by adsorption onto it, and at higher temperatures it is not held as tightly. In general, these plots show the difficulty of using titration by poisons to quantify the active-site density. In every case, even the most severe one, the poisoning is probably not 100% selective, so again the value calculated can serve only as an upper limit. This indicates once more that only a small fraction of the total chromium accounts for the observed polymerization activity. 

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