The technique of solid electrolyte potentiometry (SEP) has been used to follow the oxygen activity in vanadium phosphate (VPO) catalysts during the catalytic oxidation of butene. Reaction products and oxygen activity were monitored concurrently in an effort to correlate selectivity for partial oxidation of butene to maleic anhydride, furan and crotonaldehyde with oxygen activity in the catalyst. It was found that partial oxidation selectivity increased with decreased oxygen activity in the catalyst. Additionally, the activity for total oxidation and oxygen activity were found to be dependent on the past history of the catalyst for a catalyst with a P/V ratio of 1.07. A high temperature reduction of the catalyst was able to freeze the catalyst into a form that was selective for partial oxidation. Models to account for the rate and oxygen activity data from this study suggest that subsurface oxygen is involved in the oxidation of butene over VPO catalysts and causes the non-selective total oxidation reaction.
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