Temperature of calcination

Ib/ft ). Again, dolomitic limes average about 4% denser than their high calcium counterparts. The severity of the calcination process largely determines the porosity of a quicklime the higher the temperature of calcination and the longer its duration, the more the porosity declines. 

Fig. 8. The polymerization kinetics of Cr/silica depends on the temperature of calcining, which controls the surface hydroxyl population.

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

The surface area is a function of the temperature of preparation of the oxides [169, 486]. Thus, the apparent electrocatalytic activity decreases with increasing temperature of calcination (which is usually in the range 350°-500 °C) [227, 475]. However, if the calcination is carried out at too high temperature, the electrode surface is deactivated probably as a consequence of some dehydration, and the observed Tafel slope can be very high [487], The important relationship between the acid-base properties of an oxide surface in solution and its electrocatalytic properties has been pointed out by the present author [488]. 

Calcination. Catalyst samples were calcined in a tube immersed in a bath of molten salts equipped with electric heating, at a constant flow of air. The rate of the temperature increase was 10 deg/h. The final temperature of calcination was kept constant for three days. Next, the active mass was dried in air at 110 °C for two days, and then calcined in a muffle furnace at a temperature increased at a rate of 20 deg/h. 

The bulk density of the catalyst is 1360 kg/m3. The kinetic constants found well describe the operation of a fresh catalyst over less than twenty days. At present, the results are being extended onto a period of a couple of months. In the catalysts studied a drop in activity by 40 % was found when the temperature of calcination was increased from 420 to 440 °C over three days, and by as much as 79 % following the calcination at 460 4C. 

The temperature of calcination of a support before impregnation understandably modifies the interaction between the precursor and the support, as shown in the above case and that of alumina-supported nickel catalysts [68]. 

Figure 10.16. BET area of calcined hydrated aluminas versus the temperature of calcination for 5 hours (Lippens, 1961 Sing, 1972).

Temperature of calcination (°C) given in brackets. Each sample heated for S hours at recorded temperature (Sing, 1972). 

Figure 10.29. BET nitrogen area of chromia gel versus temperature of calcination (in air, in vacuo or in stream of dry nitrogen) (Carruthers and Sing, 1967).

The materials thus prepared were examined in the acid-catalyzed conversion of methanol into dimethyl ether. The conversions are shown as a function of calcination temperature of the catalysts in Figure 17.9. The highest activity for 40%- and 80%-WO3/SnO2 was observed at a calcination temperature of800°C, but materials with lower quanhties of W required higher temperatures of calcination for optimum achvity. This indicates the main tendency that the lower the quantity, the higher the temperature showing the optimum activity namely, 900, 1000, 1100, and 1150°C for the materials with 20, 10, 5, and 2% W, respectively. 

Among the various intrinsic defects at the MgO surface, the oxygen vacancy has received the strongest attention. O vacancies can be created by thermal treatments of hydroxylated MgO surfaces. The treatment conditions of the sample, such as the temperature of calcination and the extent of surface dehydration, significantly affect the morphology and defectivity of the oxide... 

The X-ray diffraction patterns of 30NA-coppt. and 50NA-coppt. calcined at different elevated temperatures gave the information that the sample calcined below 300° gave unknown broad peaks at 26 = 14, 28, 38, 49.5, and 65° and that the elevation of the calcination temperature over 500° made peaks at 26 = 14, 28, and 49.5° disappear and new peaks which coincided with those of NiAl204 spinel appear at 26 = 37.5, 45, and 66°. These new peaks became sharper as the temperature of calcination was elevated. 

Fio. 19. Schematic diagram showing the possibility and impossibility of formation of a polydentate chelate complex on the surface of aluminosilicate, depending on temperature of calcination of the aluminosilicate. 

As the temperature of calcination in air was raised, Dx increased at first and then became constant above 500° on the contrary, A changed in just the adverse way ij, however, stayed constant throughout. These results indicate synthetically that calcination in air causes the growth of palladium particles, but no diminution of the imperfection of crystals. 

Fig. 13. Cyclohexane adsorption vs temperature of calcination for modified faujasite catalysts.

Calcination of RUO2 XH2O from Ventron at Different Temperatures Properties Originally amorphous, crystallinity increases with temperature of calcination, TGA, DTA results available [1745],... 

It was found in previous studies [13,14] that when the temperature of calcination of freshly prepared iron phiosphate is raised, the surface area decreases markedly and the structure changes clearly, that is, amorphous phase FeP04 is transformed into tridymite type FeP04 at 400 to 500°C, and the tridymite type FeP04 is then transformed into quartz type FeP04 at a temperature above 500 to 550°C. 

Fig. 14. Effect of the temperature of calcination during 16 hours (left) and of the time of calcination at 823 K under air (right) on the CH, conversion, selectivity and Mo loading of the 23 HPMo Catalyst. Reaction Conditions CHU (67%) N fi (33%) Tr = 843, U = 0.5 q, F = 30 mLmi n-1 Symbols ...

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

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