Operation and Reaction Mechanism

During operation of early catalysts it was usual to limit the conversion of ethylene both to achieve maximum selectivity and to control the evolution of heat. A short contact time was required and conversion was limited to about 8-10%. The ethylene content of the feed gas was close to the lower flanunability limit The reaction mechanism was suggested by Sachtler et al. to proceed by adsorption of molecular oxygen directly onto the silver surface, where it reacts with ethylene  

The adsorbed atomic oxygen that remains on the silver surface does not react with ethylene to produce more ethylene oxide but instead forms oxides of carbon  

The catalyst surface is, therefore, continuously oxidized by oxygen and then reduced by ethylene during operation. Only six out of every seven ethylene molecules form ethylene oxide, so that the maximum selectivity allowed by the Sachtler mechanism is only 85.7%. 

The Sachtler mecharusm did not take into account the critical role of chloride, and with high selectivities now being regularly and reliably reported, it clearly no longer satisfied the latest information, particularly when NOx is used as a moderator in the feed gas, and selectivities in excess of 90% are achieved. Further contributions to this debate were clearly required by the industry and academy alike. 

Van Santen proposed an alternative mecharusm in which molecular oxygen on the silver surface first dissociates to give atomic oxygen species. He suggested that only weakly bound oxygen atoms interact with ethylene to give 

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