The sensitivity of positronium to detect inner surfaces of porous material has been utilized to study model catalysts of Pt, Co, and Ni on gamma-alumina. The results indicate that a carrier material with higher surface area can be distinguished from a lower surface area carrier. The technique is very sensitive to small amounts (0.01%) of metal particles on the catalyst surface. The difference between the stare of reduction in the model catalysts is clearly visible between Pt and Co catalysts. Monte-Carlo simulations are used to model the behavior of positronium in porous material.