O2 homoexchange on metals

This reaction is generally very fast and can occur at sub-ambient temperature on most group 8 metals. Table 1 gives typical values of the intrinsic rate of H2/ H2 equilibration on Group 8 and Cu metal catalysts. 

Noble metals are the most active catalysts, while copper and iron are the least active at low temperature ( 20°C). However, Schuit and van Reijenl l showed that the activation energy varied in the reverse order of the intrinsic activity at 0°C 

It is worth noticing that the hydrogen equilibration reaction normally does not occur on metal surfaces covered with carbon deposits. This property was used by Andersson et to measure the active surface area 

FYom the data of Table 2, the relative activities in 02/ 02 equilibration at 300°C (chlorine-free precursors) would be 

Chloride ions behave as a poison for the equilibration reaction (see example 3 in Table 2). Most of these Cl ions are located on the support in the vicinity of the metal particles. However, it was shown by EXAFSl °l and by SIMSl that some Cl atoms were still bound to the metals after high-temperature oxidation or reduction, which can explain the inhibiting role of chlorine in equilibration. 

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