Calcination temperature, surface

Magnesia Calcination temperature, °C Surface area, m /g CrystaUite size, p.m Porosity, %... 

In general, the increase of preparation or calcination temperature helps to increase the crystallite size of anatase titania. However, anatase phase is thennally unstable and is easily converted to rutile phase. Moreover, reactive surface area decreases with increasing the... 

Extensive data on the charaaerization and the thermal evolution of the different catalysts have been reported elsewhere [9-14]. Phase composition, cell parameters and surface area of the final materials are summarized in Table 1. The XRD data indicate that for all the hexaaluminate-type samples the formation of the final phase begins at 1273 -1373 K and requires calcination temperatures of 1473-1573 K to be completed. 

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951 Harper, 1967 Sorrell Armstrong, 1976 Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 

Figure 3.35. Change of BET surface area of an AI2O2 as a function of calcination temperature.

Fig. 3.35 shows the decrease of the specific surface area of a certain alumina as a function of calcination temperature. Apparently, the alumina is rather stable at 1000 K still over 50 % of the original surface area is retained. For most applications in catalysis the reaction temperature is far below 1000 K, and, as a consequence, the thermal stability of alumina is often sufficient. Activated carbon, which is also often used, is even more stable. 

Calcined [MgAl] LDH was also used to adsorb penicillin G acylase [121]. The calcined LDH phases have porous structures, large specific surface areas and abundant basic sites to bind the enzymes. The effect of varying the composition of the LDH precursor and calcination temperature on the activity of the immobilized enzyme has been reported. In this case, the percentage of immobilized proteins increases up to 88 %. 

It appeared that the chemical composition was close to the theoretical one. Surface area values decreased from Mg to Sr samples, which are related to the calcination temperatures necessary to eliminate the nitrate anions and organic compounds. Moreover, the SG samples presented higher surface area than the evaporated ones, although the calcination temperatures were higher (Table 1). 

Figure 9.27 The apparent chromium coverage decreases with increasing calcination temperature. However, at low initial loadings the Cr coverage is much more stable against calcination. We assign the decrease in apparent Cr coverage to desorption of chromate, which happens most easily from highly covered surfaces (courtesy of P.C. Thiine, Eindhoven).

No determination was made as to whether differences between C0/AI2O3 and C0/K-AI2O3 were due to the presence of K, the higher support calcination temperature, or the lower surface area of K-AI2O3. It should be noted that although the total amount of bulk cobalt reducible in H2 at 480°C had increased in... 

The TEM images (Fig. 6.14a) show that Ag nanoparticles about 5 nm are dispersed uniformly in Si02 carrier before the Ag/Si02 composite prepared by the one-step method is calcined. With the increase of the calcination temperature, Ag nanoparticles grow out from the Si02 carrier. Ag nanoparticles are about 10 nm and disperse on the surface of the Si02 carrier when the composite is calcined at 450°C (Fig. 6.14b). 

Ag/Si02 composites prepared by the one-step method and two-step methods were characterized by TGA/DTA, FTIR, XRD, and TEM. The results showed that Ag or silver oxide nanoparticles in 20 to30 nm were dispersed on the surface of the carrier Si02. The calcination temperature and prepared method affect the crystal phase of the functional component. 

The internal surface area of a porous inorganic membrane is often significantly affected by the heat treatment temperature. Leenaars, Keizer and Burggraaf (1984) have shown that, even if the crystallite size of the membrane precursor particles remains essentially the same (from the X-ray line-broadening measurements), the surfaee area of a transition-phase alumina membrane decreases with increasing calcination temperature. Con-... 

There is no clear evidence to identify the active material for SO2 removal in a MgAl20 stoichiometric system. Figure 13 shows results for a 50-50 mole% magnesia-alumina material prepared from magnesium hydroxide and alumina sol and calcined at various temperatures. An attempt was made to correlate SO2 removal with compound formation, as measured by X-ray diffraction, and surface area. As indicated in the figure, SO2 removal ability decreased with Increasing calcination temperature as did surface area. X-ray diffraction analysis showed spinel formation increases as... 

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

Lithium introduced in the structure of the clay allows to control the density of the pillars and the strength of interaction between the pillar and the clay layer. At low calcination temperature, the interlayer distances and the surface area increased. The thermal stability of the clay, calcined at temperature higher than 400°C, drastically decreases. 

It has to be noted that the introduction of Li into the structure of the clay before pillaring and a calcination temperature lower than 3(X)°C increase the surface area of the solids. A calcination temperature higher than 5(X)°C gives amorphous solids. The Li clay structure collapses. In addition, these solids treated at 700°C present the same surface area as the Na montmorillonite. 

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