Size Dependence and Cooperative Behavior

Surfaces of small particles have small dimensions. Such surfaces of small dimensions are present on stepped surfaces with short terrace length. With such small dimensions, no sharply defined critical temperature for phase separation exists and an intermediate surface state with partial ordering over a wide temperature interval can be formed. 

In the previous section we noted that synchronization of reaction events that occur at different parts of a catalyst is a necessity for an oscUlatoiy time dependence of reaction kinetics. The cooperative phenomenon called self organization can then take place. When particles become so small that only one catalytic cycle takes place per particle, synchronization is lost and no such self organization can occur. 

A precondition for oscillatory behavior is the bistability of different surface phases. Under stationary conditions on Pd, the oxidation of CO to CO2 can give rise to hysteresis 

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