Air calcination

Cl y Conversion. The starting material for this process is kaolin, which usually must be dehydroxylated to y /i7-kaolin by air calcination. At 500—600°C, yW /t -kaohn forms, followed by a mulliti2ed kaolin at 1000—1050°C. 

The added value, variety of use, and methods to apply zeohte coatings or films in sensor apphcations has been convincingly demonstrated. Although current trends focus on miniaturization of sensors and creating smaUer zeohte crystals and thinner films, to decrease the response time of the sensor [79], often thick-film technology is sufficient to apply zeohte films for this type of application. Some sensor materials cannot withstand the high temperatures necessary for template removal by air calcination. Recent work demonstrated that ozonication yields... 

The kinetic methodology used by The University of Kentucky Center for Applied Energy Research (CAER) for analyzing the kinetic data has been reported elsewhere [10,12,17], Using the same approach, we obtained the kinetic parameters for the reduced air calcined 15% Co/Si02 and 25% Co/Si02 catalysts (see Table 3.1). 

A comparison of catalyst activity and selectivities to hydrocarbon and C02 over the reduced air calcined and nitric oxide calcined 15% Co/Si02 catalysts is shown in Figures 3.1 through 3.4 and Table 3.3. Initial CO conversion at an SV of 10 Nl/g-cat/h over the reduced air calcined sample was 33%, but was significantly... 

Figure 3.4 and Table 3.3 show that C02 selectivity over the 15% Co/Si02 catalyst is quite low, less than 0.4% regardless of the calcination procedure used, indicating a small extent of the water-gas shift (WGS) reaction over the catalysts. However, as shown in Table 3.3, a slightly lower average C02 selectivity was observed over the NO calcined 15% Co/Si02 catalyst compared to the air calcined one (0.19 vs. 0.29%), another indication that the NO calcination benefited FTS performance. 

FIGURE 8.2 Comparison of pore size distributions for the (solid) air calcined and... 

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. 

Turning to the XANES results (Figure 8.4), upon reduction at 350°C, the extent of reduction is found to be higher for the H2-activated air calcined catalysts. This is evident in the shoulder at the edge (-7,709 eV), which is a measure of metallic content, as well as the lower white line intensity for the activated air calcined catalyst at -7,725 eV. The catalysts appear to contain a combination of mainly Co metal and CoO, in agreement with the interpretation of TPR profiles previously discussed. 

FIGURE 8.7 CO conversion vs. time on stream in the CSTR (220°C, 280 psig, H2/CO = 2.5) at (squares) 10 NL/g.at/h and (circles) 20 NL/g.at/h for H2-activated (filled circles) air calcined and (unfilled circles) NO calcined catalysts, including (top) 15% Co/Si02 and (bottom) 25% Co/Si02. The NO calcined catalysts clearly exhibit higher CO conversion rates on a per gram of catalyst basis. 

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