Catalyst deactivation and life

In general the activity of any catalyst falls off with time. Ultimately the conversion of reactants or production rate reaches an unacceptable level, or a compensatory temperature increase degrades selectivity. Long life is especially important for expensive catalytic materials deactivated precious metals may be recoverable, but the credit will only partially offset the cost of new material With fixed beds, even of cheap catalysts, considerable labour costs are incurred in discharging and recharging the reaction vessel, while loss of production during these operations will mean a loss of income (and possibly customers). 

Loss of catalytic activity can occur in several ways, but firstly we will consider simple physical loss. Particulate catalysts used in agitated liquid phases or fluidized beds are liable to both wear and fracture through collisions of the particles with each other, the vessel walls and fittings. The process is usually referred to as attrition. The finest particles formed tend to escape the main separation or filtration equipment, and continual make-up of the catalyst charge is required. 

A far more common mechanism for catalyst deactivation in high- 

As in the case of homogeneous catalysis, poisons can also lead to deactivation of heterogeneous catalysts. Soluble or volatile metal or nitrogen compounds can destroy acid sites, while carbon monoxide and sulphur compounds almost invariably poison nickel and noble metal hydrogenation catalysts by bonding strongly with surface metal atoms. These considerations often lead to the selection of less active, but more poison-resistant, catalysts for industrial use. 

Slow catalyst deactivation may also result if certain impurities, present in commercial grades of chemicals used for catalyst preparation, migrate to the catalyst surface in use. 

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