Catalysts can become spent for several reasons, including:
1. Poisoning: The active sites of the catalyst can be "poisoned" by impurities or by-products that bind strongly to the surface, rendering it inactive. 2. Sintering: High temperatures can cause metal particles to agglomerate, decreasing the surface area available for reactions. 3. Coking: The formation of carbon deposits (coke) on the catalyst surface can block active sites. 4. Fouling: Accumulation of non-reactive materials on the catalyst surface can reduce its activity.
