Catalytically Active Surface Area

The acceleration of a chemical reaction by solid catalysts proceeds at the surface of the catalyst. The catalytic activity of solids is therefore generally proportional to the surface area of the catalytically active component of the catalyst per unit weight or per unit volume. The surface area per unit volume depends on the size of the solid particles and the bulk density. Assuming a bulk density of 

When it is possible to fill a reactor with catalyst up to the bulk density, these surface areas per unit volume can be achieved. We now will calculate the rate of production per unit volume of catalyst that can be obtained provided transport limitations do not interfere. We assume a product having a molecular weight of 80. Usually the number of catalytically active atoms at the surface of a solid catalyst is smaller than the total number of surface atoms, which in metal surfaces is of the order of lO m . Here we will assume that 2 X lO atoms m are catalytically active. The turnover number indicates the number of molecules reacting s per active site. The turnover number usually ranges between 10 and 10 s [1]. For this calculation we will take a turnover number of 1 s. Finally we use a working day of 8 h-the fine-chemical industry does not usually work continuously. 

For 1-cm catalyst particles with an active surface area of 225 m m production is 1.8 kg day, whereas 10-pm catalyst particles with a surface area of 

A reactor completely filled with catalyst to the level of the bulk density is not, however, often employed in the fine-chemical industry. Usually a weight fraction of catalyst varying from 0.07 % (w/w) to about 2.5 % (w/w) is utilized. Assuming a catalyst loading of 1 % (w/vv), a density of reactants of 1 g cm , and the above density of the catalyst of 4 g cm , the volume fraction of the catalyst is 

5 X 10 m m . With 1-cm catalyst particles the surface area is now only 0.56 m m . The resulting production is a mere 4.3 g day , which is evidently not acceptable. With 10- and 1-pm catalyst particles the production is 4.3 and 

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As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area AG but also the length, t, of the three-phase-boundaries between the solid electrolyte, the catalyst film and the gas phase (Fig. 4.7). 

Uses of adsorption studies Determination of catalytically active surface area and elucidation of reaction kinetics Determination of specific surface areas and pore size distributions... 

The most common techniques utilize the chemisorption of hydrogen, oxygen, or carbon monoxide on the surface of the sample. The volume of a gas produced in a catalytic reaction may also be used to calculate catalytically active surface area. 

A kinetic expression for the above reaction can be obtained by assuming a pseudosteady state for the adsorbed species A(ad]) and P(ad3). The measure of the concentrations of these species is the degree of occupation of the surface, 0A and 0P, respectively 0A and Qp are defined such that they range from zero (no active surface area covered at all) to unity (all active surface area covered). We further assume a pseudosteady state for the degrees of occupation of the catalytically active surface area, so that 0A and 0P do not change in time, so d6Jdt = d6,Jdt = 0. If the concentration of active sites in the catalyst pellet is C, then the free sites, on which A can be adsorbed, are given by C ( 1 -0A- 0B) and the rate of adsorption by... 

In the previous section we dealt with the surface area per unit weight or unit volume required to achieve technically acceptable conversions. The conclusion was that porous catalyst particles must usually meet the demands both of pressure drop (fixed bed catalysts) or of viability of separation (suspended catalyst particles) and of the extent of the catalytically active surface area. When the catalyst must be thermally pretreated, the active surface area should not severely drop as a result of sintering of the active particles. Although solid catalysts in fine-chemistry operations are usually employed in liquid phases, i. e. not at highly elevated temperatures, sintering of the active component(s) should not occur during the reaction. 

Other data required to classify industrial catalysts are total surface area per unit volume or per unit weight pore volume and pore-size distribution and catalytically active surface area per unit volume or per unit weight. 

With supported catalysts in particular the catalytically active surface is much smaller than the total surface area, because the surface area of the support is usually much larger than the active surface area. With non-supported catalysts, such as Raney metals, it is, however, also highly important to compare the total surface area with the active surface area to assess whether residual alumina covers a significant fraction of the metal surface. Therefore separate measurements of total surface area and catalytically active surface area are required. 

Catalytically Active Surface Area Per Unit Weight of Catalyst... 

A wider variety of alcohols [18] and alkenes [17] were investigated under CPO conditions on platinum- and rhodium-coated monoliths with 5 wt.% of catalytically active species. The results obtained with monoliths having the same dimensions as for methanol CPO revealed that a small surface area of active species can lead to higher alkene selectivity by homogeneous reactions. Platinum was also judged to be unable to dissociate higher alkenes completely. Thus, an increase in catalytically active surface area by means of a washcoat or a reduction in the channel size can lead to a higher H2 yield. 

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