Catalytic activity and surface area

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. 

Fig.l Surface area and catalytic activity for methane combustion of AMnAlii-Oi9methane conversion level is 10%. Reaction condition CH4,1 vol% air, 99 vol% space velocity, 48 OOOh ... 

Generally, as we have discussed earlier, catalytic reactions at elevated temperatures lead to a loss of metal surface area and catalytic activity. Under certain gas treatments it is shown that re-dispersion of sintered or deactivated metal particles is posssible. Both published and patent literature cite examples of particle regeneration (e.g. Nakayama et al 1984). For example, in Pt/alumina... 

Supported M0S2 and WS2 No Promoter. Supports include high surface area oxides especially 7-AI2O3 and Si02, and carbon. The role of the support is to disperse the active components so increasing their effective surface area and catalytic activity. Oxide supports may also participate in isomerization and cracking. Interaction of an active component with a support during... 

Surface areas and catalytic activities of the freshly-prepared Mo nitride and CoMo nitrides for thiophene HDS are summarized in Table 1. The HDS conversion activity of the CoMo nitride (even with a factor of 10 lower surface ar. a) is comparable to that of M02N. However the specific activity (based on surface area) is four times higher for the CoMo nitride phase. 

Recent studies have indicated direct correlations between surface area and catalytic activity of metal oxide sensors (Li et al. 1999). Therefore, it is evident that incorporating catalytic particles by coating techniques may affect surface area and catalysis. Studies by Lee and Bhat (2003) have demonstrated that by incorporating small amounts of electrospun nanoflbers in spun-bond and melt-blown nonwovens, barrier properties such as flitration efficiency and air permeability can be improved. 

With supported catalysts in particular the catalytically active surface is much smaller than the total surface area, because the surface area of the support is usually much larger than the active surface area. With non-supported catalysts, such as Raney metals, it is, however, also highly important to compare the total surface area with the active surface area to assess whether residual alumina covers a significant fraction of the metal surface. Therefore separate measurements of total surface area and catalytically active surface area are required. 

The phases, surface areas, and catalytic activities of Mn-substituted hexaaluminates with various cation compositions in the mirror plane are summarized in Table 3. The crystal structures of both BaMnAlijOig. and SrMnAluOj9.(jj are the hexaaluminate type. The surface area was quite large for hexaaluminate samples even after heating at 1,300 °C. Although the hexaaluminate phase was contained in CaMnAljjOi9. , the coexisting... 

Table 3 Phases, surface areas, and catalytic activities of Mn-substituted hexaaluminates with various cation composition in the mirror plane ...

Table 5 Surface areas and catalytic activity over Pd/Al203-M0 calcined at 800 °C...

In spite of the above-mentioned factors that might interfere with correlations between surface areas and catalytic activity, it has often been found that a good correlation does exist. For example it has been pointed out in a paper by Owen (38) that the catalysts used in dehydrogenating butene to form butadiene have activities that are directly proportional to the total surface areas. For such materials the surface area measurements may serve as a good control for predicting the activity that will be possessed by a given catalyst. 

The catalytic performances of LaxMOy oxides could be considerably improved if they were incorporated in a support allowing to enhance selectivity and dispersion. Indeed, previous attempts showed a noticeable increase of the surface area and catalytic activity of perovskite pillared montmorillonite in comparison with pure perovskite [3]. Therefore, we were interested in elaborating nanocomposites made of LaxMOy oxides dispersed in a layered silicate matrix. For that, we used a process based on the Cationic Exchange Capacity (CEC) of Na-Montmorillonite the sodium cations are exchanged with heterobinuclear complex cations and subsequent heat treatment leads to the nanocomposite [4]. 

As described previously, the cations in LDHs are evenly distributed in thebrucite-like layers. Thus, in principle, the catalytic activity of LDHs can be well controlled by varying the cation ratio and incorporating different cations. Catalytically active constituents of LDH include the hydroxide groups and the metal ions themselves, especially if these are redox active. The introduction of catalytically active anions, such as polyoxometalates (POMs), can further modify the properties of LDHs. Thermal decomposition (calcination) of LDH gives mixed basic oxides of high surface area and catalytic activity. Finally, the reduction of LDH can give rise to finely divided catalytically active metal and to the prospect of metal/base bifunctional catalyst. 

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