Highly Hydroxylated Alumina

Most y-alumina powders placed under high vacuum and heated at moderate temperature (373-423 K) undergo complete water removal, and nearly all of the surface aluminum atoms are hydroxylated. Additionally, for hydroxylated alumina thin films prepared via aluminum evaporation/water oxidation cycles, although the surface density of OH groups would be the same as for powders, the specific surface area differs significantly [9]. 

Equation 9.3 summarizes the evolution of the surface molybdenum species  

Equation 9.4-9.S summarize the results obtained under CVD conditions  

On a y-alumina powder in a fluidized bed at 493 K it has been shown by XPS that [Mo(CO)6] decomposes mainly to molybdenum carbide [13]. In the early stages 

Importantly, under CVD conditions the temperature can be sufficiently high to induce dehydroxylation of the alumina surface, so that the mechanism could follow reaction routes other than the one observed when GPID is carried out. This phenomenon can be even more pronounced when experiments are performed under dynamic UHV. 

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On an alumina support, independently of the cobalt carbonyl precursor used, complex cobalt sub-carbonyls compounds, [Co(CO)4] and hydrogencarbonate species formed [143, 149]. However, the reactivity of the alumina surface depends on the degree of hydroxylation highly hydroxylated alumina is more reactive against Co2(CO)g and facilitates decarbonylation, whereas dehydroxylated alumina favors the formation of high nuclearity species like [Co6(CO),5] , which would need higher temperatures than the initial Co2(CO)8 to be decarbonylated [149]. 

Cr/alumina can be modified like Cr/silica. Adding titania is not particularly useful, but replacing the hydroxyls with fluoride does boost the activity by as much as 10-fold (62). An example is shown in Fig. 22, where activity is plotted versus the amount of fluoride impregnated onto a highly porous alumina. Too much fluoride tends to sinter the alumina and destroy the activity. Other modifications which improve the activity of Cr/alumina include adding chloride, sulfate, boria, phosphate, or 1-5% silica (62, 78). 

Mo(CO)3 ads> which is 100% molecularly dispersed. Above 200° C remaining CO groups of Mo(CO)3 ads ar liberated, but detectable amounts of Mo(CO)2,ads Slid Mo(CO)ads do not accumulate. The likely dominant species after activation at 270° C is (a-0")2Mo . At higher temperatures, e.g 500° C, the zero-valent decarbonylated molybdenum is oxidized by the surface hydroxyl groups of alumina to an average oxidation number of about 5.6. Molybdenum hexacarbonyl on highly dehydroxylated alumina, le., pretreated at 950° C, behaves differently Mo(CO)3 a[Pg.114]

The typical activated aluminas used in water treatment are 28 X 48 mesh (0.6-0.3 mm diam) mixtures of amorphous and gamma aluminum oxide (y-AI2O3) prepared by low-temperature (300-600°C) dehydration of precipitated A1(0H)3. These highly porous materials have surface areas of 50-300 m /g. Using the model of hydroxylated alumina surface subject to protonation and deprotonation, the following ligand exchange reaction, Eq. (1), can be written for arsenate adsorption in acid solution (alumina exhaustion) in which =A1 represents the alumina surface and an overbar denotes the solid phase ... 

In view of the widespread use of nitrogen and argon in surface area and porosity studies, data for the construction of the standard a,-curves for these adsorbates on hydroxylated silica, are given in Table 2.14 (p. 93) for nitrogen and in Table 2.15 for argon. From the arguments of Section 2.12, these should be adequate for other oxides such as alumina, if high accuracy is not called for. 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Eigure 3.56 depicts LEIS spectra for two completely different types of AI2O3 sample, i. e. a-alumina (sapphire) and y-alumina (a powder with high specific surface area) which show very similar results in both cases after thermal treatment at 400 °C [3.142]. Reduction of the A1 signal in y-alumina was ascribed to shielding by hydroxyl groups formed by water molecules, which are typical adsorbates on y-alu-mina. 

Of all the aluminas, y-alumina is the most used alumina, due to its high surface area and the high concentration of surface hydroxyl groups. 1 1-alumina is u.sed in i.somerization becau.se it has a higher density of acid sites at the surface. 

The use of highly dehydroxylated AI2O3 and zerovalent M(CO)6 (M = Cr, Mo, W) led to highly dispersed low-valent, surface-subcarbonyl catalysts, in which the oxidation of M by the hydroxyl groups of alumina was prevented [19, 20] ... 

Silica-alumina is the most popular mixed oxide support, combining characteristic features of silica and alumina, including (i) high surface area, (ii) persistent OH population at high temperature and (iii) strong Lewis acidic sites. The predominant surface hydroxyl groups on silica-alumina are [=Si-OH], while [Als-OH] surface species have not been observed [79, 89, 90]. Note that the silica-alumina bulk is mainly composed of [=Si-0-Si=] along with [=Si-0-Als] moieties (Lewis... 

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