Oxygen catalyst activities temperature effects

Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts where both the active metal and ceria were dispersed onto high-surface-area Y-A1203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavy-duty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. 

The catalyst plays a crucial role in the technology. A typical modern catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on silica spherical particles of 5 mm [8]. The very fast reaction takes place inside a thin layer (egg-shell catalyst). Preferred conditions are temperatures around 150 to 160 °C and pressures 8 to 10 bar. Hot spots above 200 °C lead to permanent catalyst deactivation. The excess of ethylene to acetic acid is 2 1 to 3 1. Because of explosion danger, the oxygen concentration in the reaction mixture should be kept below 8%. Small amount of water in the initial mixture are necessary for catalyst activation. The dilution of the reaction mixture with inert gas is necessary because of high exothermic effect. Accordingly, the reactor is designed at low values of the per-pass conversions, namely 15 - 35% for the acetic acid and 8-10% for ethylene. The above elements formulate hard constraints both for design and for plantwide control. 

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