Surface area alumina

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Data from Bartholomew et al. [23,27] for Ni/alumina catalysts (Table 2 and Figure 8) provide perspective regarding the role of alumina surface area, structure, and pretreatment on the thermal stability of nickel. From Figure 8 it is evident that at any given temperature the rate of sintering is greater for Ni supported on 5-alumina (SA = 98 m2/g) than for Ni on y-... 

As with alumina, ceria has several roles to play within the catalyst formulation. It has some effect on stabilizing alumina surface area at high temperatures, and it is also capable of stabilizing the dispersion of platinum in these systems, important because the effect is particularly marked in the 600-800°C region, where many present-day catalysts operate. In addition, ceria allows two other more directly performance-related phenomena to take place oxygen storage and the water gas shift reaction shown in Eq. (9) ... 

The presence of small amounts of impurities can either accelerate or decelerate phase transformations In alumina. The presence of platinum at concentrations from 0.1 to 3 v% has been reported to accelerate sintering, with the production of the S phase, in particular, being favoured (Figure 3) [27]. On the other hand, 3% Mi was found to lead to the Initial stabilisation of alumina surface area, followed by rapid collapse of the structure (Figure... 

The effects of metals are, however, less significant than the effect of m tal oxides, which are used widely to stabilise alumina [28], Lanchana, in particular, is widely used to maintain alumina surface area In autooiobile Catalysis [29]. 

More detailed studies demonstrate that crystalline Al2(W04)3 is produced from the direct solid-state reaction between crystalline WO3 and the alumina support and occurs by the migration of alumina to the WO3 particles at these elevated temperatures. The initial transformation of the surface tungsten-oxide phase to crystalline WO3 particles at elevated temperatures, when the alumina surface area decreases, reveals that the tungsten-oxide species are not miscible in the alumina support and are actually diffusing away from the alumina support. Thus, it appears that the surface tungsten oxide-alumina mixed oxide forms an "immiscible oxide system" because of the inability of the alumina support to accommodate the large charge and size of the cation. More recent Raman studies... 

Alumina from Degussa (Aluminum Oxid C 8-alumina surface area of 100 m. g ) was used in powder form made of microspheres of about 100 A. This powder was first wetted with water to prepare a slurry. Then it was dried overnight at 393 K in an oven, ground and sieved to collect the fraction between... 

FTIR spectroscopy (BRUKER IFS 110) study of the adsorption of carbon dioxide on the solids was used to determine the free alumina surface area. More precisely, the adsorption of CO2 onto hydroxyl groups of the alumina produced characteristic bands of hydrogenocarbonate species [6,7], The optical density of the band at 1235 cm l, (5 C-O-H bending mode), after taking into accoimt the bands displayed by ceria, was measured and used to determine the free alumina surface. From this method and by comparison with total BET surface area, a ceria surface has been estimated. Ceria-aluminas, alumina and ceria were used as self supported wafers (0.01 to 0.03 g/cm ) they were thermally treated up to 673 K, under oxygen and then under high vacuum, in situ, before adsorption of CO2 at room temperature. Then, the catalysts were evacuated at 295, 373, 473 and 573 K, for 1 h. IR spectra were recorded at room temperature, after CO2 adsorption and after each desorption temperature. 

Triphenylarsine (AsPhs) and benzene were purchased from Aldrich. The n-heptane used as solvent was freshly distilled and was kept in a glass flask, under argon. Alumina supported nickel were prepared by the incipient wetness procedure. The alumina surface area was measured by N2 adsorption and the metal loading was measured by elemental analysis. 

The catalyst particles were 0.3 x 0.3-cm cylinders, 10% Ni on alumina. Surface area was 30m /g, and the diffusivity of hydrogen within the catalyst at 25 °C was 0.07cm /s. Is there evidence that diffusion was important Assume Knudsen diffusion in the catalyst pore structure. The rate of addition of deuterium to 2-butyne at 25 °C over a 0.03% Pd/ AI2O3 catalyst was reported to be 0.11 gmol/h-cm3 (catalyst). Experimental conditions were feed, D2 saturated with 2-butyne nt 25 °C catalyst dimension, 40 mesh D ff 0.15 of the bulk diffusivity of 2-butyne in hydrogen at 25 °C. Was intraparticle diffusion important in this experiment Note Persevere in obtaining all the exact numbers you need to do this calculation. They come from various sources, and it takes some time. After all, this is real life). 

The first category we will cover will be oxides that are sintered in an oxidizing atmosphere and that have a relatively low surface area. The model formulation selected is a polyvinyl butyral-based binder system using a 94 wt% aluminum oxide powder with an alumina surface area of 3.3 mVg. 

Reports in the literature show that the addition of La, Zr and Si modify the alumina surface areas and its thermal stability (1.2). AIjOj-TiOj supports have been reported to offer higher acidity than alumina, and improved performance in hydrotreating reactions (3). On the other hand the combination ZrO -AljOj provides greater mechanical strength which results in improved resistance to attrition (4,5), however the interaction does not modify the acidity of alumina, but decreases the Bronsted basicity of zirconia (6). 

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