Why, in the bromination of phenols, do you not need an acid catalyst like you do in the bromination of benzene?

Benzene's delocalised π electrons means that it is not very nucleophilic and so cannot polarise a neutral molecule and produce the required electrophile. As a result it require a catalyst to form the electrophile. In phenols, the lone pair from the oxygen delocalises into the π electrons, increasing the electron density in the ring and making it more nucleophilic. This means that it is more able to polarise a molecule and produce an electrophile meaning that a catalyst isn't required.