Google is your friend: the TOF (turnover frequency)  is the total number of moles transformed into the desired product by one mole of active site per hour. The larger the TOF, the more active the catalyst.  The extent of the active site can be calculated from the available surface area (and SSA = 6/D[3,2] for unit density).  See many postings on RG - for example: https://www.researchgate.net/post/How_to_calculate_the_turnover_number_of_a_catalyst