Calcination temperature, 3"-alumina

Figure 2. The BET surface area of alumina derived from SOL-IA as a function of calcination temperature. alumina. washcoated aluminas.

Table 5, Effect of calcination temperature of Cera hydrate on the fired properties of p" -alumina...

We also see (Fig. 6) that raising the calcination temperature of rare earth modified silica-aluminas from 500 to 600°C, may slightly sinter the rare earth oxide. Although the x-ray pattern... 

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. 

M. -Trung Tran, N. S. Gnep, G. Szabo, and M. Guisnet, Influence of the calcination temperature on the acidic and catalytic properties of sulphated zirconia, Appl. Catal. A 171, 207-217 (1998). P. Canton, R. Olindo, F. Pinna, G. Strukul, P. Rieflo, M. Meneghetti, G. Cerrato, C. Morterra, and A. Benedetti, Alumina-promoted sulfated zirconia system Structure and microstructure characterization, Chem. Mater. 13, 1634-1641 (2001). 

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). 

Ermakova and co-workers manipulated the Ni particle size to achieve large CF yields from methane decomposition. The Ni-based catalysts employed for the process were synthesized by impregnation of nickel oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium dioxide, zirconium oxide and magnesia). The optimum particle size (10 0 nm) was obtained by varying the calcination temperature of NiO. The 90% Ni-10% silica catalyst was found to be the most effective catalyst with a total CF yield of 375 gcp/gcat- XRD studies by the same group on high loaded Ni-silica... 

Aluminum nitride UFPs have been synthesized by thermal decomposition from many kinds of precursor such as polyminoalanef l/ ) AIH(NR)] (50), aluminum polynuclear complexes of basic aluminum chloride (BAC) or basic aluminum lactate (BAL) (51), and (hydroxo)(succinato) aluminum(lll) complex, A1(0H)(C4H404) jfLO (52). These precursors were calcined under N2 or NH, gas flow. The calcination temperatures, which depend on the individual precursor, can be lower by 600-200°C than the 1700°C in ihe conventional carbothermal reduction method. The XRD measurements at intermediate stages of the calcination process showed the phase change from an amorphous state to a trace of y-alumina with very fine grains and finally to wurtzite-type AIN (51,52). Lowering the calcination... 

The characterization data of BaAl12Oi9 samples obtained with different preparation procedures and of a pure alumina sample are summarized in Table 2, in which the phase compositions at relevant calcination temperatures are reported, and in Figure 4, in which the plots of surface area versus calcination temperature are compared. 

Alumina will also bind Cr03 and stabilize it to 900°C, and it can polymerize ethylene when reduced to Cr(II). High surface area y alumina can be made having the porosity necesssary for good activity. Besides the electronic differences between Si—O—Cr and A1—O—Cr bonds, such alumina catalysts typically have 50-100% more hydroxyl groups than silica at normal calcining temperatures. This is clear in Fig. 21, which shows the hydroxyl populations of three different supports. The hydroxyl concentration was measured by reaction with methylmagnesium iodide. 

Silica and aluminum phosphate have much in common. They are isoelec-tronic and isostructural, the phase diagrams being nearly identical even down to the transition temperatures. Therefore, aluminum phosphate can replace silica as a support to form an active polymerization catalyst (79,80). However, their catalytic properties are quite different, because on the surface the two supports exhibit quite different chemistries. Hydroxyl groups on A1P04 are more varied (P—OH and A1—OH) and more acidic, and of course the P=0 species has no equivalent on silica. The presence of this third species seems to reduce the hydroxyl population, as can be seen in Fig. 21, so that Cr/AP04 is somewhat more active than Cr/silica at the low calcining temperatures, and it is considerably more active than Cr/alumina. 

More reactive compounds, like chromocene, are active on silica but aluminum phosphate is often better. High calcining temperatures, such as 500-800°C, are usually preferred, probably because isolated hydroxyls remain. But this depends to some extent on the particular compound. Alumina is usually the worst choice of support, probably due to its larger hydroxyl population. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

CoMo-124 Alumina was impregnated first with cobalt stepwise. The sample was dried at 120°C and calcined at 650 °C after each impregnation step. Afterwards the catalyst was impregnated with molybdenum. Final calcination temperature 650°C. The composition was the same as for MoCo-124. 

MoCo-153 Alumina was impregnated first with molybdenum, dried and calcined at 650°C. Then the cobalt was brought hereupon. Two final calcination temperatures were applied, 480 and 650°C. The composition was 15 wt% M0O3 and 3 wt% CoO. 

NiMo-124 The catalysts were prepared according to the Dutch Patent 123195 (17). The alumina carrier was impregnated first with nickel. Two final calcination temperatures were applied, 480 and 650°C. These samples were impregnated with molybdenum. Final calcination temperatures 480 and 650°C. The composition of the four catalysts, which were obtained was 12 wt% M0O3 and 4 wt% NiO, The catalysts are indicated by the applied calcination temperatures after each impregnation e.g. NiMo-124 480/480. 

Visible Reflection Spectra. The final calcination temperature of MoCo-124 samples has been varied in order to study its influence on the coordination of the cobalt ions. The reflection spectra are shown in Figure 1. The spectra of MoCo-124, calcined at 400 and 500°C show a broad absorption band, covering the whole spectral region, with a weak superposition of the characteristic triplet of cobalt aluminate. This indicates that the cobalt ions are for the greater part still on the catalyst surface and not in the alumina lattice. The spectra of the MoCo-124 samples, calcined at 650-700 °C show a strong increase in intensity of the triplet band, while the broad absorption band has disappeared. This indicates the formation of a cobalt aluminate phase. 

These experiments indicate that at low calcination temperatures the cobalt ions are present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate surface layer. At the higher calcination temperatures, the cobalt ions move into the alumina lattice. The BrGnsted acid sites reappear, indicating that the situation on the molybdate surface is restored. 

As to calcination temperatures, it is advisable to keep them relatively low, say about 723 K. At significantly higher T, the promoter ions tend to go subsurface (incipient spinel formation) and so are lost from the surface, and eventually from the active phase. Also, in the case of a P-containing catalyst, further surface-area loss may occur due to migration of the phosphate anions into the bulk of the alumina. 

The interaction of cupric ions with alumina supports has subsequently been studied more extensively as a function of the support surface area, metal loading, and calcination temperature (93,279) by means of EXAFS and X-ray absorption-edge shifts, in conjunction with XRD, EPR, XPS, and optical reflectance spectroscopy. These techniques, each sensitive to certain structural and electronic aspects, allow a unified picture of the phases present and the cation site location. Four Cu2 + ion sites are distinguished in the catalysts. In low concentrations (typically below about 4 wt. % Cu/100 m2/g support surface area) Cu2 + ions enter the defect spinel lattice of the A1203 support. The well-dispersed surface copper aluminate has Cu2+ ions predominantly occupying tetragonally (Jahn-Teller) distorted octahedral sites, although... 

The name activated alumina is generally applied to an adsorbent alumina (usually an industrial product) prepared by the heat treatment of some form of hydrated alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has been known for many years that certain forms of activated alumina can be used as powerful desiccants or for the recovery of various vapours. It was apparent at an early stage that the adsorbent activity was dependent on the conditions of heat treatment. For example, in 1934 Bayley reported that the adsorption of H2S by a commercial sample of activated alumina was affected by prior heating of the adsorbent at different temperatures, the maximum uptake being obtained after heat treatment at SS0°C. During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930) found that a calcination temperature of c. 400°C was required to optimize the adsorption of the alcohol reactants, whereas calcination at 600°C was preferable for the adsorption of the olefine products. 

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

Figure 2 shows the effectiveness of this procedure as a function of the preoxidation temperature. At 500°C preoxidation no difference is observed between either a He or H reduction. After a 600°C calcination the He reduction was less effective, especially for higher temperature peaks, and this difference was more pronounced after a 700°C calcination temperature. It is important to note that more than oxidation occurs at high temperatures, sintering of the Pt and dehydration of alumina also occur. However, it is not surprising that hard to reduce Pt occurs at the highest temperature of preoxidation. All these factors contribute to the change in the... 

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