Alumina specific surface area

The general trend observed for specific surface areas of the CH-CQ catalysts in Table 2 is that with increase of ruthenium content on the surface of alumina, specific surface area reduces rather linearly. This can be understood in terms of modifications of original carrier porosity by the metal and its related surface species [12]. 

Aluminum oxides have several crystal forms. Active alumina (porous alumina) used as an adsorbent is mainly y alumina. Specific surface area is in the range of 150 and 500 m g with pore radius of 15 to 60 A (1.5 to 6 nm), depending on how they are prepared. Porosity ranges from 0.4 to 0.76 which gives particle density of 1.8 to 0.8 g/cm ... 

Alumina Specific surface area (m. g ) Pore volume (crn g- ) Mean pore diameter (nm) FWMH (nm) Ni wt%... 

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. 

Metal Oxide - Since metals are less electrophilic than silicon, metal oxide adsorbents show even stronger selectivity for polar molecules than do siliceous materials. The most commonly used metal oxide adsorbent is activated alumina, used primarily for gas drying. Occasionally, metal oxides find applications in specific chemisorption systems. For example, several processes are under development utilizing lime or limestone for removal of sulfur oxides from flue gases. Activated aluminas have surface areas in the range of 200 to 1,000 ftVft Average pore diameters range from about 30 to 80 A. 

Catalyst material Pt/Sn on alumina/silica filament Specific surface area lOSm m ... 

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. 

Retention on silica or alumina adsorbents is a function of the specific surface area, the degree of surface continination (particularly that of water), the prior thermal conditioning of... 

Activated carbon has high specific surface area with respect to its volume, and thus has high adsorption capacity. Activated carbon adsorption is considered to be one of the most versatile treatment technologies and can remove classical pollutants such as COD, TOC, BOD, and nitrogen, as well as toxic pollutants such as phenol, refractory organic compounds, VOCs, and soluble heavy metals.38 Activated alumina and peat have also demonstrated similar abilities. 

The lipophilicity and specific surface area of a similar set of synthetic dyes was also determined on an alumina-based RP-TLC stationary phase and the linear relationship between the two hydrophobicity parameters was calculated. The result of the calculation is depicted in Fig. 3.6. The good correlation between these physicochemical parameters indicated that from the chromatographic point of view these compounds behave as a homologous series of analytes, however, their chemical structures are markedly different [87],... 

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]. 

In principle all solids insoluble in common liquids can be used as stationary phases provided they have sufliciently high specific surface area. Only silica and alumina will be discussed here as examples of polar stationary phases inasmuch as the former is used almost exclusively, and the latter to a lesser extent, in current practices of HPLC. 

Aluminium hydroxide has a Moh hardness of about 3 and a specific gravity of 2.4. It decomposes endothermically with the release of water at about 200 °C and this makes it a very useful flame retardant filler, this being the principal reason for its use in polymers. The decomposition temperature is in fact too low for many thermoplastics applications, but it is widely used in low smoke P VC applications and finds some use in polyolefins. For these applications low aspect ratio particles with a size of about 1 micron and a specific surface area of 4-10 m g are preferred. The decomposition pathway can be diverted through the mono-hydrate by the application of pressure, and this may reduce the flame retardant effect [97]. This effect can be observed with the larger sized particles. Although it is chemically the hydroxide, it has for many years been known as alumina trihydrate and by the acronym ATH. 

These results indicate that the alumina has about 7 times more positively charged sites at pH 5.2 as compared to pH 7.2. Since the specific surface area of the alumina is 1.5 x 104 m2 kg-1, these results imply AEC values ... 

---