Thermal Processes and Sintering

Thermal influences can often affect the catalyst compositian. In many cases one or more metastable phases are formed from the active components or the support materials. Phase changes can limit the catalyst activity or lead to catalyst- bstrate interactions. We have already dealt with the transformation of y-Al203 into a-Al203 with its lower surface area. Another example is the phase transformation of Ti02 from anatase to rutile in V2O5/ Ti02/corundum catalysts for the oxidation of o-xylene to phthalic anhydride. 

Sintering is a well-known phenomenon in metallurgy and ceramics technology. Sintering processes are also of importance in catalysis, even at low temperatures. Reasons for this are the extremely small crystalhtes, porous supports, and reactive gases. Catalyst atoms already become mobile at temperatures between one-third and one-half of the melting point. 

The rate of sintering increases with increasing temperature, decreasing crystallite size, and increasing contact between the crystallite particles. Other factors are the amount and type of impurities on the crystallite surface and the support composition in supported catalysts. 

Increased sintering can also occur if the active catalyst components form volatile compounds with the reactants. An example is the sintering of copper catalysts in the presence of chlorine compounds. 

The main effect of sintering is loss of active surface area and the resulting decrease in catalyst activity. However, a change in selectivity can also occur, especially in the case of structure-sensitive reactions. Extensive investigations of sintering have been carried out on highly dispersed metals such as Pt/Al203. 

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Thermal processes and sintering of the catalyst lead to a loss of active surface area... 

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