Catalyst Support Sintering

This is another reason for loss of catalyst activity and it also is irreversible. This is also a result of high temperatures and particularly in connection with high water partial pressures. In this case the catalyst support material can lose surface area from a collapse of pores, or from an increase in the diameter of pores, with the pore volume remaining approximately constant. 

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Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

It is implicit in reaction 9.4 that the equilibrium yield of ammonia is favored by high pressures and low temperatures (Table 9.1). However, compromises must be made, as the capital cost of high pressure equipment is high and the rate of reaction at low temperatures is slow, even when a catalyst is used. In practice, Haber plants are usually operated at 80 to 350 bars and at 400 to 540 °C, and several passes are made through the converter. The catalyst (Section 6.2) is typically finely divided iron (supplied as magnetite, Fe304 which is reduced by the H2) with a KOH promoter on a support of refractory metallic oxide. The upper temperature limit is set by the tendency of the catalyst to sinter above 540 °C. To increase the yield, the gases may be cooled as they approach equilibrium. 

Structural promotion A highly dispersed support can provide and (or) stabilize a high surface area of the catalyst supported by it. A typical example is ammonia synthesis where the thermal sintering of the iron catalyst is inhibited by alumina (although the phase configuration is different). 

The catalyst support should provide optimal dispersion of the active component, good accessibility, and stability against sintering. In many solid catalysts, the active... 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

From previous experimental studies of sintering [2,9 11 12] it is evident that sintering and redispersion are strong functions of temperature time atmosphere and support. Sintering/redispersion rates are also significantly affected by choice of metal and/or promoter metal loading, and catalyst preparation. The discussion below of previous work will focus on how sintering rates are affected by these variables. 

The deactivation of the catalyst observed in Stage II is probably caused by the sintering of the vanadium oxide layer. The results of our previous work [13] show that reoxidation of Pd 0) is only possible with highly dispersed vanadium oxide. Catalysts on bulk vanadium pentaoxide show rapid deactivation due to the fact that reoxidation of Pd(0) hardly proceeds. Also catalysts supported on silica show rapid deactivation, as a result of the low dispersion of vanadium oxide on silica. 

The second application has a direct bearing on catalysis. Powell, Somerjai, and Montgomery (6) have used this pressure-sintering process to determine the rate of growth of platinum particles on a catalyst support. 

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