The present invention relates to improvements of fluid bed catalysts. More particularly, the present invention relates to improvements of fluid bed oxidation catalysts useful in the preparation of maleic anhydride from C.sub.4 hydrocarbons, including n-butane. The present invention includes a process of contacting a fluid bed catalyst with a reactivating agent in a catalyst standpipe. The present invention also provides a catalyst containing the mixed oxides of vanadium and phosphorus, treated using the above process. The present invention further provides a process for producing maleic anhydride, utilizing the catalyst prepared using the above process. The present invention additionally provides for a reactor standpipe, utilized for regenerating catalyst according to the above process.