A pretreatment method for increasing the average pore size of a catalyst support is disclosed which increases the diffusivity and effectiveness factor . The pretreatment method includes calcining the support in moisturized air at an elevated temperature sufficient to increase the average pore size. In some embodiments, the support may be treated with an acidic/basic solution prior to the calcination step. Alternatively, the calcination step may occur in a gas mixture including water/air/acidic (or basic) gases.