Vapor formation, catalyst deactivation mechanism

Au/y-Al203 deactivates in CO oxidation, but it exhibits stable activity in selective CO oxidation (SCO) in the presence of H2. The activity for CO oxidation could be suppressed significantly by thermal treatment at 100°C, and the lost activity could be recovered by exposing the catalyst to water vapor at room temperature. A catalyst deactivated in CO oxidation could be regenerated by exposing it to H2 at room temperature or by running the SCO reaction over it. The results can be explained with a reaction mechanism for CO oxidation involving an active site that consists of an ensemble of metallic Au atoms and Au-hydroxyl. Deactivation in CO oxidation is due to the formation of an inactive carbonate, and deactivation by thermal treatment is due to dehydroxylation of the Au-hydroxyl. 

Ozone acts as the precursor of the key oxidants in the toluene decomposition. The presence of water vapor is also very important, as has been demonstrated for the catalytic oxidation of benzene with ozone on supported manganese Oxides catalysts [68]. It suppresses the catalyst deactivation by inhibiting the buildup of organic by-products on the catalyst surface, including formic acid and strongly bound surface formates. Scheme 18.2 proposes a general pattern mechanism for the plasma-driven total oxidation of the hydrocarbons. 

Causes of deactivation are basically three-fold chemical, mechanical or thermal— hereby six different routes of deactivation of catalyst material are described (some have been introduced before, without further explanation) poisoning (i.e. CO on Pt), fouling (i.e. coke formation during ethene hydrogenation on Pt), thermal degradation, vapor compound formation accompanied by transport, vapor-solid and/or solid-solid reactions, and attrition/crushing [162, 163]. 

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